Christopher B. Buck and Lee Ratner
PRINCIPLES OF TUMOR VIROLOGY
Viral infections are estimated to play a causal role in at least 11% of all new cancer diagnoses worldwide.1 A vast majority of cases (>85%) occur in developing countries, where poor sanitation, high rates of cocarcinogenic factors such as HIV/AIDS, and lack of access to vaccines and cancer screening all contribute to increased rates of virally induced cancers. Even in developed countries, where effective countermeasures are widely available, cancers attributable to viral infection account for at least 4% of new cases.2,3
Viruses thought to cause various forms of human cancer come from six distinct viral families with a range of physical characteristics (Table 5.1). All known human cancer viruses are capable of establishing durable, long-term infections and cause cancer only in a minority of persistently infected individuals. The low penetrance of cancer induction is consistent with the idea that a virus capable of establishing a durable productive infection would not benefit from inducing a disease that kills the host.4 The slow course of cancer induction (typically over a course of many years after the initial infection) suggests that viral infection alone is rarely sufficient to cause human malignancy and that virally induced cancers arise only after additional oncogenic “hits” have had time to accumulate stochastically.
In broad terms, viruses can cause cancer through either (or both) of two broad mechanisms: direct or indirect. Direct mechanisms, in which the virus-infected cell ultimately becomes malignant, are typically driven by the effects of viral oncogene expression or through direct genotoxic effects of viral gene products. In most established examples of direct viral oncogenesis, the cancerous cell remains “addicted” to viral oncogene expression for ongoing growth and viability.
A common feature of DNA viruses that depend on host cell DNA polymerases for replication (e.g., papillomaviruses, herpesviruses, and polyomaviruses) is the expression of viral gene products that promote progression into the cell cycle. A typical mechanism of direct oncogenic effects is through the inactivation of tumor suppressor proteins, such as the guardian of the genome, p53, and retinoblastoma protein (pRB). This effectively primes the cell to express the host machinery necessary for replicating the viral DNA. The study of tumor viruses has been instrumental in uncovering the existence and function of key tumor suppressor proteins, as well as key cellular proto-oncogenes, such as Src and Myc.
In theory, viruses could cause cancer via direct hit-and-run effects. In this model, viral gene products may serve to preserve cellular viability and promote cell growth in the face of otherwise proapoptotic genetic damage during the early phases of tumor development. In principle, the precancerous cell might eventually accumulate enough additional genetic hits to allow for cell growth and survival independent of viral oncogene expression. This would allow for stochastic loss of viral nucleic acids from the nascent tumor, perhaps giving a growth advantage due to the loss of “foreign” viral antigens that might otherwise serve as targets for immune-mediated clearance of the nascent tumor. Although hit-and-run effects have been observed in animal models of virally induced cancer,5 these effects are extremely difficult to address in humans. Currently, there are no clearly established examples of hit-and-run effects in human cancer.
In indirect oncogenic mechanisms, the cells that give rise to the malignant tumor have never been infected by the virus. Instead, the viral infection is thought to lead to cancer by attracting inflammatory immune responses that, in turn, lead to accelerated cycles of tissue damage and regeneration of noninfected cells. In some instances, virally infected cells may secrete paracrine signals that drive the proliferation of uninfected cells. At a theoretical level, it may be difficult to distinguish between indirect carcinogenesis and hit-and-run direct carcinogenesis, because, in both cases, the metastatic tumor may not contain any viral nucleic acids.
A variety of hunting approaches have been used to uncover etiologic roles for viruses in human cancer. The first clues that high-risk human papillomaviruses (HPVs), Epstein-Barr virus (EBV), Kaposi’s sarcoma–associated herpesvirus (KSHV), and Merkel cell polyomavirus (MCPyV) might be carcinogenic were based on the detection of virions, viral DNA, or viral RNA in the tumors these viruses cause. A common feature of known virally induced cancers is that they are more prevalent in immunosuppressed individuals, such as individuals suffering from HIV/AIDS or patients on immunosuppressive therapy after organ transplantation. This is thought to reflect the lack of immunologic control over the cancer-causing virus. Studies focused on AIDS-associated cancers provided the first evidence for the carcinogenic potential of KSHV and MCPyV. A theoretical limitation of this approach is that some virally induced cancers may not occur at dramatically elevated rates in all types of immunosuppressed subjects, particularly if the virus causes only a fraction of cases (e.g., HPV-induced head and neck cancers). Fortunately, the unbiased analysis of nucleic acid sequences found in tumors has become substantially more tractable as deep-sequencing methods have continued to fall in price. In the coming years, it should be increasingly possible to search for viral sequences without making the starting assumption that all virally induced tumors are associated with immunosuppression.6
One limitation of tumor sequencing approaches is that they might miss undiscovered divergent viral species within viral families known to have extensive sequence diversity7 and could miss viral families that have not yet been discovered.8 Tumor-sequencing approaches might also miss viruses that cause cancer by hit-and-run or indirect mechanisms. It is conceivable that this caveat could be addressed by focusing on sequencing early precancerous lesions thought to ultimately give rise to metastatic cancer.
An additional successful approach to hunting cancer viruses involves showing that individuals who are infected with a particular virus have an increased long-term risk of developing particular forms of cancer. This approach was successful for identifying and validating the carcinogenic roles of high-risk HPV types, hepatitis B virus (HBV), hepatitis C virus (HCV), KSHV, and human T-lymphotropic virus 1 (HTLV-1). Although viruses that are extremely prevalent, such as EBV and MCPyV, are not amenable to this approach per se, it may still be possible to draw connections between cancer risk and either unusually high serum antibody titers against viral antigens or unusually high viral load. Relatively high serologic titers reflect either comparatively poor control of the viral infection in at-risk individuals or expression of viral antigens in tumors or tumor precursor cells.9,10
The finding that a virus causes cancer is good news, in the sense that it can suggest possible paths to clinical intervention. These can include the development of vaccines or antiviral agents that prevent, attenuate, or eradicate the viral infection and thereby prevent cancer; the development of methods for early detection or diagnosis of cancer based on assays for viral nucleic acids or gene products; or the development of drugs or immunotherapeutics that treat cancer by targeting viral gene products. Unfortunately, establishing the carcinogenicity of a given viral species is an arduous process that must inevitably integrate multiple lines of evidence.11 The demonstration that the virus can transform cells in culture and/or cause cancer in animal models provides circumstantial evidence of the oncogenic potential of a virus. All known human cancer viruses meet this criterion. However, it is important to recognize that viruses can theoretically coevolve to be noncarcinogenic in their native host (e.g., humans) and cause cancer only in the dysregulated environment of a nonnative host animal. This caveat may apply to human adenoviruses.
Finding that viral DNA is clonally integrated in a primary tumor and its metastatic lesions helps address the caveat that the virus might merely be a hitchhiker that finds the tumor cell a conducive environment in which to replicate (as opposed to playing a causal carcinogenic role). This caveat is also addressed by the observation that, in most instances, viruses found in tumors have lost the ability to exit viral latency and are functionally unable to produce new progeny virions. An unfortunate consequence of this is that vaccines or antiviral agents that target virion proteins (e.g., vaccines against high-risk HPVs or HBV) or gene products expressed late in the viral life cycle (e.g., herpesvirus thymidine kinase, which is the target of drugs such as ganciclovir) are rarely effective for treating existing virally induced tumors.
Demonstrating that a vaccine or antiviral agent targeting the virus either prevents or treats human cancer is by far the strongest form of evidence that a given virus causes human cancer. This type of proof has fully validated the causal role of HBV in human liver cancer. Compelling clinical trial data also show that antiherpesvirus therapeutics can prevent KSHV- or EBV-associated lymphoproliferative disorders, and that vaccination against HPV can prevent the development of precancerous lesions on the uterine cervix.
PAPILLOMAVIRUSES
History
The idea that cancer of the uterine cervix might be linked to sexual behavior was first proposed in the mid 19th century by Dominico Rigoni-Stern, who observed that nuns rarely contracted cervical cancer, whereas prostitutes suffered from cervical cancer more often than the general populace.12 Another major milestone in cervical cancer research was Georgios Papanikolaou’s development of the so-called Pap smear for early cytologic diagnosis of precancerous cervical lesions.13 This form of screening, which allows for surgical intervention to remove precancerous lesions, has saved many millions of lives in developed countries, where public health campaigns have made testing widely available.
Although observations in the early 1980s suggested the possibility of a hit-and-run carcinogenic role for herpes simplex viruses in cervical cancer,14 this hypothesis was abandoned in light of studies led by Harald zur Hausen. Low-stringency hybridization approaches revealed the presence of two previously unknown papillomavirus types, HPV16 and HPV18, in various cervical cancer cell lines, including the famous HeLa cell line.15,16 There is now overwhelming evidence that a group of more than a dozen sexually transmitted HPV types, including HPV16 and HPV18, play a causal role in essentially all cases of cervical cancer. HPVs associated with a high risk of cancer also cause about half of all penile cancers, 88% of anal cancers, 43% of vulvar cancers, 70% of vaginal cancers,2 and an increasing fraction of head and neck cancers (see the following). In 2008, zur Hausen was awarded the Nobel Prize for his groundbreaking work establishing the link between HPVs and human cancer.
The viral family Papillomaviridae is named for the benign skin warts (papillomas) that some members of the family cause. In the early 1930s, Richard Edwin Shope and colleagues demonstrated viral transmission of papillomas in a rabbit model system.17 Using this system, Peyton Rous and others showed that cottontail rabbit papillomavirus-induced lesions can progress to malignant skin cancer.18,19 This was the first demonstration of a cancer-causing virus in mammals, building on Rous’ prior work demonstrating a virus capable of causing cancer in chickens (the Rous sarcoma retrovirus).
Tissue Tropism and Gene Functions
Although papillomaviruses can achieve infectious entry into a wide variety of cell types in vitro and in vivo, the late phase of the viral life cycle, during which the viral genome undergoes vegetative replication and the L1 and L2 capsid proteins are expressed, is strictly dependent on host cell factors found only in differentiating keratinocytes near the surface of the skin or mucosa. Interestingly, a majority of HPV-induced cancers appear to arise primarily at zones of transition between stratified squamous epithelia and the single-layer (columnar) epithelia of the endocervix, the inner surface of the anus, and tonsillar crypts. It is thought that the mixed phenotypic milieu in cells at squamocolumnar transition zones may cause dysregulation of the normal coupling of the HPV life cycle to keratinocyte differentiation.
There are nearly 200 known HPV types.20 In general, each papillomavirus type is a functionally distinct serotype, meaning that serum antibodies that neutralize one HPV type do not robustly neutralize other HPV types. Various HPV types preferentially infect different skin or mucosal surfaces. Different types tend to establish either transient infections that may be cleared over the course of months, or stable infections where virions are chronically shed from the infected skin surface for the lifetime of the host. HPV infections may or may not be associated with the formation of visible warts or other lesions. High-risk HPV types, with clearly established causal links to human cancer, are preferentially tropic for the anogenital mucosa and the oral mucosa, are usually transmitted by sexual contact, rarely cause visible warts, and usually establish only transient infections in a great majority of exposed individuals. The lifetime risk of sexual exposure to a high-risk HPV type has been estimated to be >70%. Individuals who fail to clear their infection with a high-risk HPV type and remain persistently infected are at much greater risk of developing cancer. Polymerase chain reaction (PCR)-based screening for the presence of high-risk HPV types thus serves as a useful adjunct to, or even a replacement for, the traditional Pap test.21
A consequence of the strict tissue-differentiation specificity of the papillomavirus life cycle is that HPVs do not replicate in standard monolayer cell cultures. Papillomaviruses also seem to be highly species restricted, and there are no known examples of an HPV type capable of infecting animals.22 Thus, the investigation of key details of papillomavirus biology has relied almost entirely on modern recombinant DNA and molecular biologic analyses.
Papillomavirus genomes are roughly 8 kb, double-stranded, closed-circular DNA molecules (essentially reminiscent of a plasmid). During the normal viral life cycle, the genome does not adopt a linear form, does not integrate into the host cell chromosome, and remains as an extrachromosomal episome or minichromosome. All the viral protein-coding sequences are arranged on one strand of the genome. The expression of various proteins is regulated by differential transcription and polyadenylation, as well as effects at the level of RNA splicing, export from the nucleus, and translation. In addition to the late half of the viral genome, which encodes the L1 and L2 capsid proteins, all papillomaviruses encode six key early region genes: E1, E2, E4, E5, E6, and E7.
The master transcriptional regulator E2 serves as a transcriptional repressor, and loss of E2 expression (typically through integration of the viral episome into the host cell DNA) results in the upregulation of early gene expression. The most extensively studied early region proteins are the E6 and E7 oncogenes of HPV16 and HPV18. The E6 protein of high-risk HPV types triggers the destruction of p53 by recruiting a host cell ubiquitin–protein ligase, E6AP.23–25 Another important oncogenic function of E6 is the activation of cellular telomerase.26 A wide variety of additional high-risk E6 activities that do not involve p53 have been identified.27
Most E7 proteins, including those of many low-risk HPV types, contain a conserved LXCXE motif that mediates interaction with pRB and the related “pocket” proteins p107 and p130.28 Interestingly, the LXCXE motif is present in a wide variety of other oncogenes, most notably the T antigens of polyomaviruses and the E1A oncogenes of adenoviruses. The interaction of E7 with pRB disrupts the formation of a complex between pRB and E2F transcription factors, thereby blocking the ability of pRB to trigger cell cycle arrest.29 The E7 proteins of high-risk HPVs can also contribute to chromosomal mis-segregation and aneuploidy, which may in turn contribute to malignant progression.30 Like E6, E7 interacts with a wide variety of additional cellular targets, the spectrum of which seems to vary with different HPV types.27
Some papillomavirus types express an E5 oncogene, which functions as an agonist for cell surface growth factor receptors such as platelet-derived growth factor beta (PDGF-β) and epidermal growth factor (EGF) receptor.31Because E5 expression is uncommon in cervical tumors, it is uncertain whether the protein plays a key role in human cancer.
Human Papilloma Virus Vaccines
Two preventive vaccines against cancer-causing HPVs, trade named Gardasil (Merck) and Cervarix (GSK), are currently marketed worldwide for the prevention of cervical cancer. Both vaccines contain recombinant L1 capsid proteins based on HPV16 and HPV18 that are assembled in vitro into virus-like particles (VLPs). Together, HPV16 and HPV18 cause about 70% of all cases of cervical cancer worldwide. Gardasil also includes VLPs based on HPV types 6 and 11, which rarely cause cervical cancer but together cause about 90% of all genital warts. The VLPs contained in the vaccines are highly immunogenic in humans, eliciting high-titer serum antibody responses against L1 that are capable of neutralizing the infectivity of the cognate HPV types represented in the vaccine. It appears that the current HPV vaccines may confer lifelong immunity against new infection with the HPV types represented in the vaccine.32 The vaccines elicit lower titer cross-neutralizing responses against a subset of cancer-causing HPV types that are closely related to HPV16 and HPV18.33 Although these cross-neutralizing responses can at least partially protect vaccinees against a new infection with additional high-risk types, such as HPV31 and HPV45, it remains unclear how durable the lower level cross-protection will be.33
Because L1 is not expressed in latently infected keratinocyte stem cells residing on the epithelial basement membrane, current HPV vaccines are very unlikely to eradicate existing infections.34,35 Like keratinocyte stem cells, cervical cancers and precursor lesions rarely or never express L1. Thus, the existing L1-based vaccines seem unlikely to serve as therapeutic agents for treating cervical cancer.
Three types of next-generation HPV vaccines are currently in human clinical trials. Merck has recently announced that a newer version of Gardasil, which contains VLPs based on a total of nine different HPV types, remained highly effective against HPV16 and HPV18 and also prevented 97% of precancerous cervical lesions caused by a wider variety of high-risk HPV types.36 Another class of second-generation vaccines targets the papillomavirus minor capsid protein L2. An N-terminal portion of L2 appears to represent a highly conserved “Achilles’ heel”, which contains conserved protein motifs required for key steps of the infectious entry process.37 Anti-L2 antibodies can neutralize a broad range of different human and animal HPV types, and thus, L2 vaccines are hoped to offer protection against all HPVs that cause cervical cancer, all low-risk HPV types that cause abnormal Pap smear results, as well as the full range of HPV types that cause skin warts. Finally, a wide variety of vaccines that seek to elicit cell-mediated immune responses against the E6 and E7 oncoproteins are aimed at a therapeutic intervention for the treatment of cervical cancer.38
Oropharyngeal Cancer
It is well established that tobacco products and alcohol cause head and neck cancer. In the late 1990s, Maura Gillison and colleagues noted a surprising number of new cases of tonsillar cancer in nonsmokers.39Many of the tumors found in nonsmokers were found to have wild-type p53 genes, raising the possibility that the tumor might be dependent on a p53-suppressing viral oncogene (as seen in cervical cancer). Gillison and colleagues went on to show that nearly half of all tonsillar cancers contain HPV DNA, most commonly HPV16. Interestingly, HPV-positive oropharyngeal cancers tend to be less lethal than tobacco-associated HPV-negative tumors. This finding has important implications for treatment of HPV-positive head and neck cancers.40
Although the incidence of tobacco-associated head and neck cancer has been declining in recent decades due to decreased tobacco use, recent studies suggest an ongoing increase in the incidence of HPV-associated cancers of the tonsils and the base of the tongue. By 2025, the number of new HPV-induced head and neck cancer cases in the United States is expected to roughly equal the number of new cervical cancer cases.39 Based in part on these observations, the U.S. Centers for Disease Control and Prevention recommends that boys, in addition to girls, should be vaccinated against high-risk HPVs.
Nonmelanoma Skin Cancer
Epidermodysplasia verruciformis (EV) is a rare immunodeficiency that is characterized by the appearance of numerous flat, wartlike lesions across wide areas of skin. The lesions typically contain genus betapapillomaviruses, such as HPV5 or HPV8. EV patients frequently develop squamous cell carcinomas (SCC) in sun-exposed skin areas (suggesting that ultraviolet [UV] light exposure is a cofactor). It is also well established that other immunosuppressed individuals, such as organ transplant recipients and HIV-infected individuals, are at increased risk of developing SCC.41,42 Although the E6 and E7 proteins of betapapillomaviruses appear to exert a different spectrum of effects than the E6 and E7 proteins of HPV types associated with cervical cancer,43–45 Betapapillomavirus oncogenes can transform cells in vitro.46 Although these circumstantial lines of evidence suggest that infectious agents, such as Betapapillomaviruses, might play a causal role in SCC, recent deep sequencing studies have observed few or no viral sequences in SCC tumors.47 Although the results argue against durable direct oncogenic effects of any known viral species in SCC, an animal model system using bovine papillomavirus type 4 strongly suggests that papillomaviruses can cause cancer by hit and run mechanisms.5 Thus, the question of whether hit-and-run or indirect oncogenic effects of HPVs may be at play in human SCC remains open.
POLYOMAVIRUSES
History
In the early 1950s, Ludwik Gross showed that a filterable infectious agent could cause salivary gland cancer in laboratory mice.48 Later work by Bernice Eddy and Sarah Stewart showed that the murine polyoma (Greek for “many tumors”) virus caused many different types of cancer in experimentally infected mice.49 The discovery that murine polyomavirus could be grown in cell culture helped rekindle research interest in tumor virology and interest in the question of whether viruses might cause human cancer.
Like papillomaviruses, polyomaviruses have a nonenveloped capsid assembled from 72 pentamers of a single major capsid protein (VP1). Both viral families also carry circular dsDNA genomes. These physical similarities initially led to the classification of both groups into a single family, Papovaviridae. When sequencing studies ultimately revealed that polyomaviruses have a unique genome organization (with early and late genes being arranged on opposing strands of the genome) and almost no sequence homology to papillomaviruses, the two groups of viruses were divided into separate families.
In the early 1960s, Bernice Eddy, Maurice Hilleman, and Benjamin Sweet reported the discovery of simian vacuolating virus 40 (SV40), a previously unknown polyomavirus that was found as a contaminant in vaccines against poliovirus.50,51 SV40 was derived from the rhesus monkey kidney cells used to amplify poliovirus virions in culture.52 SV40 rapidly became an important model polyomavirus, and studies of its major and minor tumor antigens (large T [LT]and small t [ST], respectively) have played an important role in understanding various aspects of carcinogenesis. Despite significant alarm about the possible risk SV40 might pose to exposed individuals, a comprehensive, decades long series of studies have failed to uncover compelling evidence that SV40 exposure is causally associated with human cancer.53
Two naturally human-tropic polyomaviruses, BK virus (BKV) and John Cunningham virus (JCV), were first reported in back-to-back publications in 1971.54,55 BKV and JCV are known to cause kidney disease and a lethal brain disease called progressive multifocal leukoencephalopathy, respectively, in immunosuppressed individuals. Although both viruses can cause cancer in experimentally exposed animals, it remains unclear whether either virus plays a causal role in human cancer. Although BKV LT expression can frequently be observed in the inflammatory precursor lesions that are thought to give rise to prostate cancer,56 there is no evidence for the persistence of BKV DNA in malignant prostate tumors.57 There have been case studies finding BKV T-antigen expression in bladder cancer,58 and some reports have indicated the presence of JCV DNA in colorectal tumors. The long history of conflicting evidence concerning possible roles for BKV or JCV in human cancer is reviewed elsewhere.59,60
Merkel Cell Polyomavirus
In 2008, Yuan Chang and Patrick Moore reported their lab’s discovery of the fifth known human polyomavirus species, which they named Merkel cell polyomavirus (MCV or MCPyV) based on its presence in Merkel cell carcinoma (MCC).61 The discovery used an RNA deep sequencing approach called digital transcriptome subtraction. Using classic Southern blotting, this report demonstrated the clonal integration of MCPyV in an MCC tumor and its distant metastases. Many other labs worldwide have independently confirmed the presence of MCPyV DNA in about 80% of MCC tumors.11
MCC is a rare but highly lethal form of cancer that typically presents as a fast-growing lesion on sun-exposed skin surfaces (Fig. 5.1).62 The risk of MCC is dramatically higher in HIV/AIDS patients, offering an initial clue that MCC might be a virally induced cancer.63 Although MCC tumors express neuroendocrine markers associated with sensory Merkel cells of the epidermis, one recent report has shown that some MCC tumors also express B-cell markers, including rearranged antibody loci.64 Currently, there is no clear evidence for the involvement of MCPyV in other tumors with neuroendocrine features.
In 2012, the International Agency for Research on Cancer (IARC) concluded that MCPyV is a class 2A carcinogen (probably carcinogenic to humans).10,53 It should be noted that IARC evaluations rely heavily on animal carcinogenicity studies, and the 2A designation was assigned prior to a recent report showing that MCV-positive MCC lines are tumorigenic in a mouse model system.65
A great majority of healthy adults have serum antibodies specific for the MCPyV major capsid protein VP1. A majority also shed MCPyV virions from apparently healthy skin surfaces, and there is a strong correlation between individual subjects’ serologic titer against VP1 and the amount of MCPyV DNA they shed.66–68 Interestingly, MCC patients tend to have exceptionally strong serologic titers against VP1.69MCC tumors do not express detectable amounts of VP1, so this is unlikely to reflect direct exposure to the tumor and instead likely represents a history of a high MCPyV load in MCC patients. A recent study of archived serum samples shows that unusually high serologic titers against MCPyV VP1 often precede the development of MCC by many years.70
Like the LT protein of SV40 (and the E7 proteins of high-risk HPVs), an N-terminal portion of the MCPyV LT protein contains an LXCXE motif that mediates inactivation of pRB function. In contrast to SV40 LT, which carries a p53-inactivation domain that overlaps the C-terminal helicase domain, MCPyV LT does not appear to inactivate p53 function.71 Instead, the MCPyV LT helicase domain activates DNA damage responses and induces cell cycle arrest in cultured cell lines.72 This may explain why the LT genes found in MCC tumors essentially always carry mutations that truncate LT upstream of the helicase domain. siRNA experiments indicate that most (although possibly not all) MCC tumors are “addicted” to the expression of MCPyV T antigens.73–75 Interestingly, patients with higher levels of MCPyV DNA in their tumors, stronger T-antigen expression, and tumors that have been infiltrated by CD8+ T cells appear to have better prognoses.76 This is consistent with the idea that cell-mediated immunity can help clear MCC tumors that express MCPyV antigens.
Recent work has shown that the pRB interacting domain of LT mediates increased expression of the cellular gene survivin. The knockdown of survivin using siRNAs results in MCC tumor cell death and YM155, a small molecule inhibitor of survivin expression, protects mice from MCC tumors in a xenograft challenge system.77,78
In contrast to SV40, where LT appears to be the dominant oncogene, the MCPyV ST protein appears to play a key role in cell transformation. In addition to modifying the signaling functions of the cellular proto-oncogene PP2A, ST triggers the phosphorylation of eukaryotic translation initiation factor 4E binding protein 1.79 This results in dysregulation of cap-dependent translation and cellular transformation.
Although there is an intriguing epidemiologic correlation between MCC and chronic lymphocytic leukemia (CLL),80 there are conflicting reports concerning the presence of MCPyV in CLL and other lymphocytic cancers.81–83
Other Human Polyomaviruses
In recent years, the number of known human polyomaviruses has expanded dramatically. Of the 12 currently known HPyV species, only MCPyV has been clearly linked to human cancer. One new HPyV, trichodysplasia spinulosa polyomavirus (TSV or TSPyV) has been found in association with abnormal spiny growths on the facial skin of a small number of immunocompromised individuals.
EPSTEIN-BARR VIRUS
History
In 1958, Denis Burkitt provided the first clear clinical description of an unusual B-cell–derived tumor that frequently affects the jawbones of children in equatorial Africa.84 After hearing Burkitt give a 1961 lecture entitled “The Commonest Children’s Cancer in Tropical Africa – A Hitherto Unrecognized Syndrome,” Michael Epstein became interested in the idea that an insect vector-borne infection might account for the high incidence of Burkitt lymphoma in tropical Africa. Epstein, together with then PhD candidate Yvonne Barr, began examining tumor samples sent to them by Burkitt. Electron micrographs of lymphoid cells that grew out of the tumors in culture revealed viral particles with a morphology strikingly similar to herpes simplex viruses.85 It was soon shown that Epstein-Barr herpesvirus (EBV, later designated human herpesvirus 4 [HHV-4]) can transform cultured B cells and is the agent responsible for infectious mononucleosis.86–88
Although the initial conjecture that tropically endemic Burkitt lymphoma depends on a geographically restricted infectious agent ultimately proved correct, it was quickly established that the EBV infection is not restricted to the tropics. It instead appears likely that the malaria parasite Plasmodium falciparum is a key geographically restricted cocarcinogen responsible for endemic Burkitt lymphoma.53 In areas where children suffer repeated malaria infections, it appears that the parasite triggers abnormal B-cell responses, as well as weakened cell-mediated immune function, and these effects of recurring malaria infection in turn promote or allow the development of EBV-induced Burkitt tumors.11
Epstein-Barr Virus Life Cycle
EBV chronically infects nearly all humans. In a great majority of individuals, the infection is initially established in early childhood and is never associated with any noticeable symptoms. The infection is typically transmitted when virions, shed in the saliva of a chronically infected individual, come in contact with the oropharyngeal epithelium of a naïve individual. Although infected epithelial cells, such as keratinocytes, might serve to amplify the virus in some circumstances,89 the establishment of chronic infection is ultimately dependent on mature B cells, as subjects with X-linked agammaglobulinemia (who lack mature B cells) appear to be immune to stable EBV infection.90Individuals who escape infection during childhood and instead first become infected during adolescence or adulthood often develop mononucleosis, which is associated with fevers and extreme fatigue lasting for weeks or sometimes months. Interestingly, late-infected individuals who experience mononucleosis and high EBV viral load are at increased risk of developing EBV-positive Hodgkin lymphoma.91
EBV-infected B cells can either go on to produce new virions, which are typically associated with cell lysis, or the virus can enter a nonproductive state known as latency. Viral latency is defined as a condition in which the virus expresses few (or possibly no) gene products but can, under some conditions, “reawaken” to express the full range of viral gene products and produce new progeny virions. Latently infected cells are highly resistant to immune clearance.
There are three recognized forms of EBV latency. In latency I, EBV nuclear antigen-1 (EBNA1), which is required for the stable maintenance of the circularized viral DNA minichromosome, is the only viral protein expressed. EBV-derived microRNAs (miRs) may also be expressed. At the other end of the spectrum, latency III is characterized by the expression of EBNA1–6, several latent membrane proteins (LMP1, 2A, and 2B), two noncoding RNAs (EBER1 and 2), the BCL-2 homolog BHRF1, BARF0, and multiple miRs. Although the initial discovery of EBV involved the visualization of virions, indicating that the virus had exited latency and entered the productive lytic phase of the life cycle, viral gene expression in EBV-induced cancers generally follows one of the three latent patterns. The oncogenic activities of various EBV gene products have recently been reviewed.87,88
In a great majority of healthy individuals, EBV exists almost exclusively in a latent state, with the occasional asymptomatic shedding of virions in the saliva. The infection is controlled, at least in part, by CD8+ T cells specific for various latency proteins. EBV, like other herpesviruses, expresses a variety of proteins that interfere with cell-mediated immune responses. Intriguingly, results from mouse model systems suggest that the chronic immunostimulatory effects of persistent gammaherpesvirus emergence (or abortive emergence) from latency in healthy hosts can nonspecifically boost immunity to other infections.92
Lymphomas
In addition to endemic Burkitt lymphoma, EBV is often present in sporadic cases of Burkitt lymphoma in individuals who have not been exposed to malaria. Although nearly all cases of endemic Burkitt’s lymphoma contain EBV DNA in the tumor (typically in a latency I–like state), only about 20% of sporadic cases arising in immunocompetent individuals contain EBV. Rates of Burkitt lymphoma are elevated in HIV-infected individuals, and HIV-associated Burkitt lymphomas contain EBV in about 30% of cases.
A common hallmark of all types of Burkitt’s lymphomas is deregulation of the cellular Myc proto-oncogene. A classic mutation involves chromosomal translocation of the Myc gene to the antibody heavy chain locus. Burkitt’s lymphoma tumors that lack detectable EBV DNA tend to carry multiple additional mutations in host cell genes, raising the possibility that an originally EBV-positive precursor cell ultimately accumulated mutations that rendered it independent of viral genes.88,93
In addition to Burkitt lymphoma, EBV is associated, to varying extents, with a histologically diverse range of other lymphoid cancers, including Hodgkin lymphoma, natural killer (NK)/T-cell lymphoma, primary central nervous system (CNS) lymphoma, and diffuse large B-cell lymphoma. The incidence of these various forms of lymphoma is significantly increased both in AIDS patients as well as in iatrogenically and congenitally immunosuppressed individuals.88 In particular, the essentially universal presence of EBV in CNS lymphomas in AIDS patients makes it possible to diagnose the disease with a PCR test for EBV that, together with radiologic findings, can obviate the need for a brain biopsy.
EBV is almost invariably associated with lymphoproliferative disorders, such as plasmacytic hyperplasia and polymorphic B cell hyperplasia, which are often observed in organ transplant recipients. These polyclonal lymphoproliferative responses can, in some instances, progress to oligoclonal or monoclonal lymphomas of various types. The occurrence of EBV-associated lymphoproliferative disease in immunosuppressed patients is generally heralded by the increased detection of EBV DNA in the peripheral blood and the oral cavity. This presumably reflects the failure of cellular immune responses to drive the virus into full latency and perhaps also a failure of cell-mediated immune responses targeting latency-associated EBV gene products present in the nascent tumor.
Carcinomas
In Southern China, NPC affects 25 out of 100,000 people, accounting for 18% of all cancers in China as a whole.94 Most other world regions have a 25- to 100-fold lower rate of NPC. EBV is present in nearly all cases of NPC, both in endemic and nonendemic regions. Although there is support for the idea that dietary intake of salted fish and other preserved foods is a factor in endemic NPC, it remains possible that genetic traits or as yet unidentified environmental cocarcinogenic factors may play a role as well. Individuals with rising or relatively high IgA antibody responses to EBNA1, DNase, and/or EBV capsid antigens have a dramatically increased risk of developing NPC, offering an early detection method for at-risk individuals.87
EBV is also present in a small percentage (5% to 15%) of gastric adenocarcinomas and over 90% of gastric lymphoepithelioma-like carcinomas. In contrast to NPC, the prevalence of EBV-associated gastric cancer is similar in all world regions. As with NPC, elevated antibody responsiveness to EBV antigens may offer a method for identifying individuals at greater risk of gastric cancer.
Prevention and Treatment
The reduction of immunosuppression in response to increasing EBV loads is a standard approach to preventing EBV diseases in T-cell immunosuppressed individuals. Another approach to the prevention of EBV disease relies on ganciclovir (or related antiherpesvirus drugs), which can trigger the death of cells that express the EBV thymidine kinase gene. Pretreating at-risk individuals, such as organ transplant recipients, with ganciclovir has been shown to effectively prevent the development of EBV-induced lymphoproliferative disorders.95 However, it is important to note that thymidine kinase is only expressed in the lytic phase of the viral life cycle, and drugs of this class are not generally effective for treating existing tumors, presumably due to the fact that EBV gene expression in tumors is typically of a latent type.
Although a recently developed vaccine targeting the EBV gp350 virion surface antigen did not provide sterilizing immunity to EBV infection, vaccinees did experience lower peak EBV viral loads upon infection.96 Given the strong correlation between high EBV loads and the development of EBV diseases, it is hoped that the vaccine’s ability to merely blunt the acute infection may offer significant protection against disease.
Most forms of EBV-associated lymphoid cancers express the B-cell marker CD20, making rituximab (an anti-CD20 mAb) a potentially effective adjunct therapy.97,98 An emerging treatment approach that has recently entered clinical trials involves stimulating T cells ex vivo against peptides based on EBV antigens or against autologous EBV-transformed B cells.
KAPOSI’S SARCOMA HERPESVIRUS
History and Epidemiology
In the late 19th century, Hungarian dermatologist Moritz Kaposi’s described a relatively rare type of indolent pigmented skin sarcoma affecting older men.99 Kaposi’s sarcoma (KS) was later found to be more prevalent in the Mediterranean region and in eastern portions of sub-Saharan Africa.100 An early clue to the emergence of the HIV/AIDS pandemic in the early 1980s was a dramatic increase in the incidence of highly aggressive forms of KS, particularly in gay men who were much younger than typical KS patients. After the discovery of HIV, it was briefly hypothesized that HIV might be a direct cause of KS. However, this hypothesis failed to explain the existence of KS long prior to the HIV pandemic and the low incidence of KS in individuals who became infected with HIV via blood products. This latter observation was more easily explained by the existence of a sexually transmitted cofactor other than HIV.101
Using a subtractive DNA hybridization approach known as representational difference analysis, Yuan Chang, Patrick Moore, and colleagues discovered the presence of a previously unknown herpesvirus in KS tumors.102 The newly founded field of research rapidly established key lines of evidence supporting the conclusion that KSHV (later designated human herpesvirus-8 [HHV-8]) is a causal factor in KS.11
It is now clear that the rate of KSHV infection varies greatly in different world regions.11,103 In North America and Western Europe, KSHV seroprevalence in the general population ranges from 1% to 7%. Seroprevalence among gay men in these regions is substantially higher (25% to 60%), suggesting a possible link to sexual transmission. KSHV infection is much more prevalent in the general population in central and eastern Africa, where seroprevalence ranges from 23% to 70%. In endemic areas, up to 15% of children are seropositive, suggesting either vertical transmission or transmission via nonsexual casual contact (presumably via saliva). In endemic regions, KS is estimated to be the third most common cancer among adults.104
Kaposi’s Sarcoma-Associated Herpesvirus in Kaposi’s Sarcoma
KS tumors are complex on a number of levels. In contrast to most other forms of cancer, where it is often clear that a single cell type has proliferated out of control, KS tumors are composed of cells from multiple lineages (Fig. 5.2). KSHV-infected cells in the tumor often have a spindle-shaped morphology. Interestingly, spindle cells do not exhibit a highly transformed phenotype and tend to show relatively little chromosomal instability. In a culture, the cells are highly dependent on exogenous cytokines and other factors present in the tumor microenvironment in vivo. Although spindle cells express a number of markers of the endothelial lineage, it is uncertain whether they are derived from mature endothelial cells, the early precursor cells that give rise to smooth muscle and vascular endothelial cells, or cells of the lymphatic endothelial lineage. KS tumors also contain infiltrating lymphocytes and monocytes, as well as aberrant neovascular spaces lined with infected and uninfected endothelial cells. The aberrant blood vessels in KS lesion vessels rupture easily and leak red blood cells, giving KS tumors their classic dark red, brown, or purple color.
The latency status of KSHV in KS tumors is also complex, with the expression of gene products typical of latency (e.g., LANA) as well as lytic-phase genes (e.g., RTA/ORF50). Some of these gene products, such as the viral interleukin (IL)-6 homolog (vIL-6), trigger proliferation and secondary cytokine signaling in noninfected cells within the tumor. The tumorigenic effects of individual KSHV gene products have recently been reviewed.88,103 In contrast to EBV, where tumorigenesis is driven by latency gene expression, it appears that KS pathogenesis is often dependent on lytic phase gene expression. This may explain why ganciclovir, which is not a particularly effective treatment for EBV tumors, was found to prevent the formation of new KS lesions in HIV-positive patients.105 However, it should be noted that this outcome has more recently proven difficult to reproduce.106 At present, there are no recommended preventive therapies for individuals at risk of KS, but this is an area of active investigation.
There are a variety of possible explanations for the need for lytic-phase KSHV gene expression during tumor development. For example, infected spindle cells may lose the viral DNA during cell division and require reinfection for ongoing tumorigenicity. Alternatively, factors secreted by a small fraction of tumor cells that enter the lytic phase may be required for tumorigenesis. An important area of current research focus is the role of KSHV gene products in the regulation of angiogenesis in KS lesions107 and several current trials are investigating inhibitors of angiogenic pathways for the treatment of KS.
Lymphoproliferative Disorders
KSHV causes two forms of B-cell proliferative disorder: multicentric Castleman disease (MCD) and primary effusion lymphoma (PEL). Both diseases are most commonly found in association with HIV infection. In HIV-infected individuals, MCD tumors contain KSHV in nearly all cases, whereas in HIV-negative individuals, the tumor contains KSHV in only about 50% of cases.108 KSHV in MCD tumors exhibits periodic activation of lytic replication and the expression of lytic phase genes.109 The expression of vIL-6 during disease flare-ups appears to play a role in MCD pathogenesis, raising the possibility that tocilizumab (a mAb therapeutic that targets the IL-6 receptor) may be of therapeutic benefit.
PEL comprises about 4% of all HIV-associated non-Hodgkin lymphomas.110 Typically, PEL tumors express markers of both plasma cells (akin to multiple myeloma tumors) and immunoblasts (similar to some EBV-induced tumors). In AIDS patients, essentially all PEL tumors are infected with KSHV and a great majority are also coinfected with EBV.88 Although PEL is rare in HIV-negative individuals, PEL tumors in such individuals contain KSHV in about 50% of cases.
A common approach to the treatment of all KSHV-associated diseases is the restoration of immune function, either through antiretroviral therapy of HIV/AIDS or through a reduction of immunosuppressive therapy. The general success of immune reconstitution in many KSHV-associated diseases presumably involves an immune-mediated attack of cells expressing KSHV gene products, particularly the many lytic-phase gene products the virus can produce in various disease states.
ANIMAL AND HUMAN RETROVIRUSES
The first oncogenic retroviruses were discovered by Ellerman and Bang in 1908 and by Rous in 1911, but it was many years before the significance of these findings was appreciated.111 One reason the field was stymied was the failure to identify RNA forms of the viral genome in infected cells. This led to the discovery of the reverse transcriptase independently by Baltimore and Temin in 1970. Another major development was the finding in 1976 of viral oncogenes derived from cellular genes, with the identification by Varmus and Bishop of the first dominant oncogene, src. With the discovery of IL-2 by Gallo in 1976, it became possible to culture the first human retrovirus, HTLV-1, from a form of adult T-cell leukemia/lymphoma (ATLL) that was first recognized by Takatsuki and coworkers.112 These advances opened the door for Montagnier and colleagues’ isolation of HIV-1 in 1983, a discovery confirmed independently by Gallo and Levy. This breakthrough led to the first licensed HIV test in 1985.
Retroviruses are positive single-strand RNA viruses that utilize transcription of their RNA genome into a DNA intermediate during virus replication.111 This accounts for their name, retroviruses, because this is opposite to the normal flow of eukaryotic genetic information. They infect a wide range of vertebrate animal species and are distantly related to repetitive elements in the human genome, known as retrotransposons. Retroviruses are also related to hepadnaviruses, double-stranded DNA viruses, such as hepatitis B virus, which also undergo a reverse transcription step in their replication.
Retroviruses may be classified as endogenous or exogenous depending on whether they appear in the genome of the host species. There are approximately 100,000 endogenous retroviral elements in the human genome, making up nearly 8% of the genetic information, but their potential roles in disease are unclear.113 Retroviruses may also be classified as ecotropic, xenotropic, or polytropic depending on whether they infect cells of the same animal species from which they are derived, infect cells of a different species, or both. Amphotropic retroviruses infect cells of the species of origin without producing disease, but infect cells of other species and may produce disease.
Retroviruses that produce disease after a long incubation period are termed lentiviruses and include human, simian, feline, ovine, caprine, and bovine immunodeficiency viruses. Another group of retroviruses that are not clearly associated with disease are known as spumaviruses and include human and simian foamy viruses. HTLV-1, which is classified in the genus Delta, is the only retrovirus known to be oncogenic in humans. A member of the retroviral genus Gamma identified in 2008, designated xenotropic murine leukemia virus-related virus (XMRV), was thought to be associated with human prostate cancer; however, more recent studies showed XMRV to be a lab-derived artifact.114 A genus betaretrovirus related to the mouse mammary tumor virus has been suggested to be associated with biliary cirrhosis, but this finding requires independent validation.115
Retroviruses producing tumors in animals or birds are designated transforming viruses and may be classified as acute or chronic transforming retroviruses. Acute transforming retroviruses have acquired a mutated cellular gene, termed oncogene, and induce cancer in an animal within a few weeks. Many dominant acting proto-oncogenes in humans (e.g., ras, myc, and erbB), were first identified as retroviral oncogenes.
Chronic transforming retroviruses integrate almost randomly in the genome, but when integrated in the vicinity of specific genes disrupt their regulation and induce cell proliferation or resistance to apoptosis. Chronic transforming retroviruses induce malignancy only after many weeks to months of infection. The use of a murine leukemia virus vector for gene therapy in children with a form of severe combined immune deficiency syndrome characterized by defective expression of the common gamma chain of the IL-2 receptor resulted in T-cell acute lymphoblastic leukemia. This was found to be the result of persistent expression of the LIM domain only 2 (LMO2) gene triggered by the nearby integration of the retroviral vector.116
In addition to acute or chronic transformation mechanisms, retroviruses can transform cells through direct effects on cell physiology mediated by structural or nonstructural viral proteins. Transforming genes of HTLV-1 are nonstructural viral proteins that activate host cell signaling pathways.117 Because the oncogenic effects of HTLV-1 transforming genes generally take many years to cause cancer, the virus does not fit the precise definition of having either an acute or a chronic oncogenic mechanism.
HIV-1 infection is also associated with a variety of malignancies, but only by indirect effects of suppressing immunity to oncogenic virus infections, such as gammaherpesviruses, high-risk human papillomaviruses, and hepatitis viruses.
Human T-Cell Leukemia Virus Epidemiology
Four species of human T-cell leukemia virus have been identified. HTLV-1 was identified in 1980 as the first human retrovirus associated with cancer, and it is the focus of the remainder of this section.118HTLV-2 was discovered in 1982 and shares 70% genomic homology with HTLV-1.119 HTLV-3 and -4 were sporadically isolated from individuals who had contact with monkeys.120 HTLV-2, -3, and -4 do not appear to be associated with disease in humans.
HTLV-1 is present in 15 to 20 million individuals worldwide, most commonly in the Caribbean Islands, South America, southern Japan, and parts of Australia, Melanesia, Africa, and Iran.121 In the United States, Canada, and Europe, 0.01% to 0.03% of blood donors are infected with HTLV-1. It is most commonly found in individuals who emigrated from endemic regions or among African Americans. HTLV-1 is transmitted sexually, by contaminated cell-associated blood products, or by breast-feeding.122 Only 2% to 5% of HTLV-1–infected individuals develop disease, and ATLL only occurs in individuals who acquired HTLV-1 by breast-feeding.
Human T-Cell Leukemia Virus Molecular Biology
HTLV-1, like other retroviruses, encodes Gag, Protease, Pol, and Envelope proteins.123 Gag proteins compose the inner nucleocapsid core of the virus. The Pol proteins include the reverse transcriptase and integrase. The reverse transcriptase copies the single-stranded viral RNA into double-stranded DNA, and it is inhibited by several nucleoside analogs, but not by the nonnucleoside reverse transcriptase inhibitors approved for HIV-1.124 The integrase is responsible for inserting the linear double-stranded DNA product of reverse transcription into the host chromosomal DNA. At least one integrase inhibitor, raltegravir, now approved for HIV-1, is active against HTLV-1.125 Integration occurs throughout the human genome, but there is preference for integration into transcriptionally active genomic regions.126 The viral protease proteolytically processes Gag, Protease, and Pol precursor proteins to the mature individual proteins, but it is not affected by inhibitors of HIV-1 protease. The envelope proteins include the transmembrane protein, which anchors the surface envelope protein on the virion, which mediates binding to the viral receptor.127
The viral genome also encodes regulatory proteins, including Tax and HTLV-1 bZIP factor (HBZ).117 Tax is a transcriptional transactivator protein that functions as a coactivator to induce members of the cAMP response element-binding protein/activating transcription factor (CREB/ATF) family, nuclear factor kappa B (NF-κB), and serum response factor (SRF) pathways. Tax activation of the CREB/ATF pathway is responsible for upregulation of the viral promoter. Tax induction of NF-κB promotes cell proliferation and resistance to apoptosis. Tax also binds and activates cyclin-dependent kinases and inhibits cell cycle checkpoint proteins. Tax is important for tumor initiation, whereas HBZ may be important in tumor maintenance.128
HTLV-1 preferentially immortalizes CD4+ T lymphocytes and induces tumors in mice.129 Tax also promotes the leukemia-initiating activity of ATLL cells in mouse models.130 In immunodeficient mice reconstituted with human hematopoietic cells, HTLV-1 causes CD4+ lymphomas.131
Clinical Characteristics and Treatment of HTLV-Associated Malignancies
The diagnosis of HTLV-1 is based on serologic assays.132 HTLV-1 is associated with various inflammatory disorders, including uveitis, polymyositits, pneumonitis, Sjögren syndrome, and myelopathy. Infected patients are susceptible to certain infectious disorders (e.g. staphylococcal dermatitis) and opportunistic infections such as pneumocystis pneumonia, disseminated cryptococcosis, strongyloidiasis, or toxoplasmosis.133 Vaccines have not been developed for HTLV infections.
T-lymphocyte proliferative disorders develop in 1% to 5% of infected individuals and are generally CD2+, CD3+, CD4+, CD5+, CD25+, CD29+, CD45RO+, CD52+, HLA-DR+, T-cell receptor αβ+, and variably CD30+, and lack CD7, CD8, and CD26 expression. The virus is clonally integrated in the malignant cells. Complex karyotypes are often found, and cytogenetic analysis is rarely useful. The histologic features of lymph nodes in ATLL may be indistinguishable from those of other peripheral T-cell lymphomas.134 Circulating tumor “flower cells” are helpful in the diagnosis (Fig. 5.3).
ATLL is categorized in four subtypes.135 (1) Smoldering ATLL is defined as 5% or more abnormal T lymphocytes and lactate dehydrogenase (LDH) levels up to 1.5× the upper limit of normal, with normal lymphocyte count, calcium, and no lymph node or visceral disease other than skin or pulmonary disease. (2) Chronic ATLL is characterized by lymphocytosis, LDH up to 2× the upper limit of normal, no hypercalcemia, and no CNS, bone, pleural, peritoneal, or gastrointestinal involvement, although the lymph nodes, liver, spleen, skin, or lungs may be involved. The mean survival of these forms of ATLL is 2 to 5 years.136 No intervention in these subtypes of ATLL has been defined that prevents progression to the more aggressive forms of ATLL. Although chronic or smoldering ATLL may respond to zidovudine and interferon, randomized studies have not been conducted.137 (3) Lymphoma-type ATLL is characterized by ≤1% abnormal T lymphocytes and features of non-Hodgkin lymphoma. (4) Acute-type ATLL includes the remaining patients. Even with optimal therapy, the median survival of lymphoma and acute-type ATLL is less than 1 year.138 Lymphoma and acute types of ATLL are the most common presenting subtypes. Other major prognostic factors include performance status, age, the presence of more than three involved lesions, and hypercalcemia.139
Combination chemotherapy for lymphoma or acute-type ATLL with the infusional etoposide, prednisone, vincristine, and doxorubicin (EPOCH) regimen or the LSG-15 regimen results in complete remission rates of 15% to 40%.140,141 However, responses are short lived, with <10% of patients free of disease at 4 years. The addition of anti-CCR4 antibody, mogamulizumab, may improve response rates, but studies are still underway.142 The combination of interferon and zidovudine with or without arsenic may result in the remission of acute, but not lymphoma subtypes.143 Allogenic transplantation may result in long-term, disease-free survival for patients with complete or near complete remission of disease, although infectious complications have been notable in these studies.144
HEPATITIS VIRUSES
The earliest record of an epidemic caused by a hepatitis virus was in 1885, occurring in individuals vaccinated for smallpox with lymph from other people.145 The cause of the epidemic, HBV, was not identified until 1966, when Blumberg discovered the Australian antigen now known to be the hepatitis B surface antigen (HBsAg). This was followed by the discovery of the virus particle by Dane in 1970. In the early 1980s, the HBV genome was sequenced and the first vaccines were tested. In the mid 1970s, Alter described cases of hepatitis not due to hepatitis A or B viruses, and the suspected agent was designated non-A, non-B hepatitis virus, now known as HCV.146 In 1987, Houghton used molecular cloning to identify the HCV genome and develop a diagnostic test, which was licensed in 1990.
Approximately 240 million people are chronically infected with HBV and 150 to 200 million people are infected with HCV worldwide, according to the World Health Organization (WHO). About 1 million deaths per year are attributed to the chronic diseases such as liver cirrhosis and hepatocellular carcinoma (HCC) that result from viral hepatitis infections. HBV and HCV are the leading cause of liver cancer in the world, accounting for almost 80% of the cases. In the United States, Europe, Egypt, and Japan, more than 60% of HCC cases are associated with HCV, and 20% are related to HBV and chronic alcoholism.147 In Africa and Asia, 60% of HCC is associated with HBV, 20% related to HCV, and the remainder related to other risk factors, such as alcohol and aflatoxin. HCC is the sixth most common cancer worldwide and is the third most common cause of cancer death.148
In Asia and Africa, up to 70% of individuals have serologic evidence of current or prior HBV infection, and 8% to 15% of these subjects have a chronic active infection. Rates of HCV infection of >3.5% occur in Central and East Asia, North Africa, and the Middle East. In the United States, 0.8 to 1.4 million individuals are infected with HBV, and 3.2 million with HCV. The incidence of HCC in the United States tripled between 1975 and 2005, particularly in African American and Hispanic males.149
HBV is transmitted primarily through exposure to infected blood, semen, and other body fluids, whereas HCV is transmitted primarily by contact with contaminated blood. Acute HCV infection causes mild and vague symptoms in about 15% of individuals and resolves spontaneously in 10% to 50% of cases.150 Liver enzymes are normal in 5% to 50% of individuals with chronic HCV infection.151 After 20 years of an HCV infection, the likelihood of cirrhosis is 10% to 15% for men, and 1.5% for women.152 Cofactors that increase the likelihood of cirrhosis are coinfection with both hepatitis viruses, persistently high levels of HBV or HCV viremia, HBeAg, certain viral genotypes, schistosoma, HIV, alcoholism, male gender, advanced age at the time of infection, diabetes, and obesity.153,154
Hepatitis B Virus
HBV is an enveloped DNA virus that is a member of the Hepadnaviridae family.155 HBV has a strong preference for infecting hepatocytes, but small amounts of viral DNA can also be found in kidney, pancreas, and mononuclear cells, although it is not linked to extrahepatic disease. The viral genome is a relaxed circular, partially double-stranded (ds) DNA of 3.2 kb. The genome exists as an episomal covalently closed circular dsDNA (cccDNA) molecule in the nucleus of infected cells, although chromosomal integration of viral genomic sequences can occur during cycles of hepatocyte regeneration and proliferation. In addition to 40 to 42 nm virions, HBV-infected cells also produce noninfectious 20-nm spherical and filamentous particles. The viral genome encodes four open reading frames. The presurface–surface (preS-S) region encodes three proteins from different translational initiation sites; these include the S (HBsAg), M (or pre-S2), and L (or pre-S1) proteins. The L protein is responsible for receptor binding and virion assembly. The precore–core (preC-C) region encodes the HBcAg and HBeAg. The P region encodes the viral polymerase, and the X (HBx) protein modulates host-signal transduction.
After infection, the viral genome is transcribed by host RNA polymerase II, and viral proteins are translated. Nucleocapsids assemble in the cytosol, incorporating a molecule of pregenomic RNA into the viral core, where reverse transcription occurs to produce the dsDNA viral genome. Viral cores are enveloped with intracellular membranes and viral L, M, and S surface antigens, which are exported from the cells.
HBV replication is not cytotoxic. Instead, liver injury is due to the host immune response, primarily T-cell and proinflammatory cytokine responses. Chronic HBV carriers exhibit an attenuated virus-specific T-cells response, although a vigorous humoral response is still evident. About 5% of infections in adults and up to 90% of infections in neonates result in a persistent infection, which may or may not be associated with symptoms and elevated serum aminotransferase levels. About 20% of such individuals develop cirrhosis. Immunosuppressed individuals also have a higher likelihood of a persistent infection.
With acute infection, viral titers of 109 to 1010 virions per mililiter are present, whereas levels of 107 to 109 virions per mililiter and HBsAg, and in some cases, HBeAg are present in the blood of individuals with a persistent infection. The resolution of infection, which is associated with declining viral DNA titers, is observed at a rate of 5% to 10% per year in persistently infected individuals. However, even subjects who have resolved the infection continue to have very low levels of viral DNA (103 to 105 copies per mililiter) for most of their lives.
HBV infection can be managed with alpha interferon or nucleos(t)ide analogs that inhibit the viral polymerase, such as lamivudine, telbivudine, entecavir, adefovir, and tenofovir.156 Entecavir and tenofovir are both effective at inducing viral suppression, and may be used in combination in patients with high HBV DNA load or multidrug resistance. Because these agents are all associated with some toxicity, current guidelines recommend therapy only when liver disease is clinically apparent, with continued treatment for 6 to 12 months after clearance of HBeAg or HBsAg. Although these drugs effectively control HBV, they typically fail to cure the infection due to the long-term persistence of the cccDNA form of the viral genome. Other nucleos(t)ide analogs are currently in clinical trials, as well as a novel form of interferon (IFN-λ) and an inhibitor of virus release.157
Hepatitis D virus (HDV) occurs only in individuals coinfected with HBV. HDV is composed a single-stranded circular viral RNA genome of 1,679 nucleotides, a central core of HDAg, and an outer coat with all three HBV envelope proteins. HDV infection results in more severe complications than infection with HBV alone, with a higher likelihood and more rapid progression to cirrhosis and HCC.
Hepatits C Virus
HCV is an enveloped RNA virus associated with cancer, primarily HCC and, rarely, splenic marginal zone lymphoma.158 HCV is a positive-sense, single-stranded RNA virus of the Flaviviridae family.159There are seven genotypes of HCV; in the United States, about 70% of infections are caused by genotype 1.160 HCV replicates in the cytoplasm and does not integrate into the host cell genome. The viral RNA is 9.6 kb and encodes a single polyprotein of 3,010 amino acids that is proteolytically processed into structural and nonstructural proteins. In addition to the structural roles of the core (C) protein, it has also been reported to affect various host cell functions. The envelope glycoproteins E1 and E2 mediate infectious entry through tetraspanin CD81 and other receptors on hepatocytes and B lymphocytes.
HCV non structural proteins NS2, NS3, NS4A, NS4B, NS5A, NS5B, and p7 are required for virus replication and assembly. NS2 is a membrane-associated cysteine protease. NS3 is a helicase and NTPase that unwinds RNA and DNA substrates. The complex of NS3 with NS4A forms a serine protease. NS4B induces the formation of a membranous web associated with the viral RNA replicase. NS5A is an RNA-binding phosphoprotein, whereas NS5B is the RNA-dependent RNA polymerase. The p7 protein forms a cation channel in infected cells that has a role in particle maturation and release.
Treating an HCV infection typically utilizes 24 to 48 weeks of pegylated IFN-α and ribavirin.161 Treatment with IFN and ribavirin alone produces sustained virologic responses in 70% to 80% of subjects with genotype 2 or 3 infections. Recently approved inhibitors of the NS3-4A protease (e.g., telaprevir, boceprevir, or simeprevir) may be included in IFN-based regimens, particularly if the patient has failed prior therapy. Protease inhibitors are currently approved for use in IFN/ribavirin combination therapy for HCV genotype 1 or 4 infection. Sofosbuvir, a nucleoside analog inhibitor of the viral NS5B polymerase, has recently been approved for use in combination with ribavirin alone for genotypes 2 or 3, or in triple therapy for genotypes 1 and 4. Recently, IFN-free regimens have also been approved. Additional protease and polymerase inhibitors are currently in development. A recent meta-analysis of eight randomized controlled trials comparing antiviral therapy with placebo suggested that antiviral therapy resulted in a 50% reduced risk of HCC.162
Hepatitis Virus Pathogenesis
HBV and HCV depress innate immune responses by inhibiting Toll-like receptor signaling through effects of HBx and NS3-4A.147 In addition, HCV C inhibits the Janus kinase (JAK)-signal transducer and activator of transcription (STAT) signaling, and NS5A and E2 inhibit IFN signaling. Through an undefined mechanism, HBV can inhibit JAK-STAT signaling as well.
HBV and HCV induce HCC by direct and indirect mechanisms.147 Both HBV and HCV encode proteins that have pro- and antiapoptotic properties. High levels of HBx block activation of the NF-κB pathway, whereas HCV C and NS5A block apoptosis by the activation of AKT and NF-κB, respectively. The C and NS5A proteins may also induce epithelial–mesenchymal transition (EMT), which is important for liver fibrosis, through effects on transforming growth factor β and Src signaling. Mice transgenic for NS5A develop steatosis and HCC.
HBx and HCV C are associated with mitochondria, where they trigger oxidative stress that induces apoptosis. In addition, HBs and HBx and NS3-4A alter calcium signaling and increase reactive oxygen species, which trigger endoplasmic reticulum (ER) stress, an unfolded protein response, and the production of proinflammatory cytokines that induce collagen synthesis, which drives the development of fibrosis. Autophagy is triggered by both viruses to restore ER integrity, which promotes cell survival and viral persistence.
HBV and HCV also disrupt tumor suppressor proteins. HCV NS5B recruits an ubiquitin ligase protein to modify pRB and induce its degradation, whereas HBx and HCV C proteins both inhibit p16INK4a and p21 cell cycle inhibitors, which leads to the inactivating phosphorylation of pRB. The HBx and HCV C, NS3, and NS5A proteins deregulate p53 tumor suppressor activity, by compromising p53-mediated DNA repair. HBV and HCV also induce alterations in micro-RNAs that are partially responsible for cell cycle effects.
Although not part of the normal virus replication cycle, the tendency of HBV genomic DNA sequences to integrate within the host cell chromosomes also contributes to the pathogenesis of HBV-associated HCC. In most hepatoma cells, HBV replication is extinguished, and integration at certain sites provides a growth or survival advantage, leading to tumors that are clonal with respect to viral integration. Whole-genome sequencing studies have identified a number of cellular loci, including TERT and MLL, where HBV integration is associated with HCC.163,164
Both HBV and HCV promote characteristics of cancer stem cells. HBx promotes the expression of Nanog, Kruppel-like factor 4, octamer-binding transcription factor 4, and Myc. These markers are also induced by HBV and HCV-induced hypoxia and hypoxia-induced factors.
Clinical Characteristics and Treatment of Hepatitis Virus-Associated Malignancies
HBV and HCV infections are diagnosed by serologic assays, and/or antigen assays in the case of HBV.153 Quantitative HBV DNA and HCV RNA polymerase chain reactions are utilized to measure virus load. No vaccine has been identified that protects against HCV because infections consist of a genetically heterogenous “swarm” of virus particles, some of which escape neutralization. However, a vaccine, which now utilizes a recombinant HBsAg produced in yeast cells, has been available for HBV prevention for more than 30 years. The HBV vaccine reduces the risk of infection by more than 70%.157 Factors associated with HBV vaccination failure in adults include increased age, obesity, smoking, diabetes, end-stage renal disease, HIV infection, alcoholism, or recipients of liver or kidney transplantation. There have been recent suggestions that emerging HBV strains may be evolving to escape neutralizing antibodies elicited by the current vaccine.165 Novel vaccine adjuvants are currently in clinical trials, as well as studies of a therapeutic HBV vaccine.
Because an early diagnosis of HCC is key to a successful treatment, there has been extensive research on surveillance techniques in HBV- and HCV-infected individuals.166 The U.S. Centers for Disease Control and Prevention has recently recommended that all individuals born between 1945 and 1965 be tested for HCV infection. The American Association for the Study of Liver Diseases, as well as the European and Asian Pacific Associations for the Study of the Liver, endorse surveillance in HCV-infected individuals with cirrhosis using ultrasound every 6 months. Viral eradication does not fully eliminate the risk of HCC, and thus, continued surveillance is still recommended in cirrhotic patients.
Therapeutic options for HCC are determined not only by the number and size of HCC nodules as well as the presence or absence of vascular invasion and metastases, but also by liver function and the presence or absence of portal hypertension.167 HCC amenable to liver transplantation is usually defined as either one tumor measuring ≤50 mm in diameter or two to three tumors measuring ≤30 mm in diameter without vascular extension or metastasis (Milan criteria).168 Up to 30% of all cases of HCC present with multiple nodules of HCC, suggesting a field carcinogenesis effect of HBV and HCV.169 HBV- and HCV-infected patients may have a lower survival than noninfected patients after liver transplantation.170 Hepatitis B immune globulin and nucleos(t)ide analogs are recommended for reinfection prophylaxis in the posttransplant period for HBV-infected individuals.171 Studies are underway to examine the appropriate use of antiviral therapy for HCV-infected patients undergoing liver transplantation.
Reactivation of HCV can occur with chemotherapy or monoclonal antibody-based immunosuppressive therapies, but is less frequent as compared to HBV infection.172 Individuals who appear to have cleared an HBV infection and who have an undetectable viral load can experience HBV reactivation on rituximab therapy. Monitoring hepatic function and virus load is indicated during chemoimmunotherapy of HBV- or HCV-positive patients.173 Although there is controversy regarding the role of virus screening for patients undergoing chemotherapy, antiviral therapy is recommended for high-risk HBV-infected patients undergoing chemoimmunotherapy, such as rituximab-based chemotherapy regimens.174
An association between HCV and B-cell non-Hodgkin lymphoma (NHL) has also been demonstrated in highly endemic geographic areas.175 Lymphoproliferation has been linked to type II mixed cryoglobulinemia in many of these individuals. In addition to diffuse large B-cell lymphoma, marginal zone lymphomas and lymphoplasmacytic lymphomas are the histologic subtypes most frequently associated with HCV infection. Antiviral treatment with IFNα with or without ribavirin has been effective in the treatment of HCV-infected patients with indolent lymphoma, but rarely in individuals with aggressive lymphomas.
CONCLUSION
Oncogenic viruses are important causes of cancer, especially in less industrialized countries and in immunosuppressed individuals. They are common causes of anogenital cancers, lymphomas, oral and hepatocellular carcinomas and are associated with a variety of other malignancies. Vaccines and antiviral agents play an important role in the prevention of virus-induced cancers. Studies of virus pathogenesis will continue to establish paradigms that are critical to our understanding of cancer etiology in general.
REFERENCES
1. de Martel C, Ferlay J, Franceschi S, et al. Global burden of cancers attributable to infections in 2008: a review and synthetic analysis. Lancet Oncol 2012;13(6):607–615.
2. Schiller JT, Lowy DR. Virus infection and human cancer: an overview. Recent Results Cancer Res 2014;193:1–10.
3. Chen CJ, Hsu WL, Yang HI, et al. Epidemiology of virus infection and human cancer. Recent Results Cancer Res 2014;193:11–32.
4. Virgin HW, Wherry EJ, Ahmed R. Redefining chronic viral infection. Cell 2009;138(1):30–50.
5. Campo MS, O’Neil BW, Barron RJ, et al. Experimental reproduction of the papilloma-carcinoma complex of the alimentary canal in cattle. Carcinogenesis 1994;15(8):1597–1601.
6. Khoury JD, Tannir NM, Williams MD, et al. Landscape of DNA virus associations across human malignant cancers: analysis of 3,775 cases using RNA-Seq. J Virol 2013;87(16):8916–8926.
7. zur Hausen H, de Villiers EM. TT viruses: oncogenic or tumor-suppressive properties? Curr Top Microbiol Immunol 2009;331:109–116.
8. Mizutani T, Sayama Y, Nakanishi A, et al. Novel DNA virus isolated from samples showing endothelial cell necrosis in the Japanese eel, Anguilla japonica. Virology 2011;412(1):179–187.
9. Paulson KG, Carter JJ, Johnson LG, et al. Antibodies to merkel cell polyomavirus T antigen oncoproteins reflect tumor burden in merkel cell carcinoma patients. Cancer Res 2010;70:8388–8397.
10. International Agency for Research on Cancer (IARC) Working Group. IARC Monographs on the Evalation of Carcinogenic Risks to Humans. Malaria and Some Polyomaviruses (SV40, BK, JC, and Merkel Cell Viruses), Vol. 104. Lyon, France: IARC; 2013.
11. Moore PS, Chang Y. The conundrum of causality in tumor virology: the cases of KSHV and MCV. Semin Cancer Biol 2013;26C:4–12.
12. Rigoni-Stern D. Fatti statistici relativi alle malattie cancrose. Giornale Service Progr Pathol Terap Ser 1842;2:507–517.
13. Lowy DR. History of papillomavirus research. In: Garcea RL, DiMaio D, eds. The Papillomaviruses. New York, NY: Springer; 2007:13–28.
14. zur Hausen H. Herpes simplex virus in human genital cancer. Int Rev Exp Pathol 1983;25:307–326.
15. Durst M, Gissmann L, Ikenberg H, et al. A papillomavirus DNA from a cervical carcinoma and its prevalence in cancer biopsy samples from different geographic regions. Proc Natl Acad Sci U S A 1983;80(12):3812–3815.
16. Boshart M, Gissmann L, Ikenberg H, et al. A new type of papillomavirus DNA, its presence in genital cancer biopsies and in cell lines derived from cervical cancer. Embo J 1984;3(5):1151–1157.
17. Christensen ND. Cottontail rabbit papillomavirus (CRPV) model system to test antiviral and immunotherapeutic strategies. Antivir Chem Chemother 2005;16(6):355–362.
18. Rous P, Beard J. The progression to carcinoma of virus induced rabbit papillomas (Shope). J Exp Med 1935;62:523–545.
19. Syverton JT, Berry GP. Carcinoma in the cottontail rabbit following spontaneous virus papilloma (Shope). Proc Soc Exp Biol Med 1935;33:399–400.
20. de Villiers EM. Cross-roads in the classification of papillomaviruses. Virology 2013;445(1–2):2–10.
21. Bosch FX, Broker TR, Forman D, et al. Comprehensive control of human papillomavirus infections and related diseases. Vaccine 2013;31 Suppl 8:I1–I31.
22. Van Doorslaer K. Evolution of the papillomaviridae. Virology 2013;445(1–2):11–20.
23. Scheffner M, Werness BA, Huibregtse JM, et al. The E6 oncoprotein encoded by human papillomavirus types 16 and 18 promotes the degradation of p53. Cell 1990;63(6):1129–1136.
24. Scheffner M, Huibregtse JM, Vierstra RD, Howley PM. The HPV-16 E6 and E6–AP complex functions as a ubiquitin-protein ligase in the ubiquitination of p53. Cell 1993;75(3):495–505.
25. Huibregtse JM, Scheffner M, Howley PM. Cloning and expression of the cDNA for E6–AP, a protein that mediates the interaction of the human papillomavirus E6 oncoprotein with p53. Mol Cell Biol 1993;13(2):775–784.
26. Klingelhutz AJ, Foster SA, McDougall JK. Telomerase activation by the E6 gene product of human papillomavirus type 16. Nature 1996;380(6569):79–82.
27. White EA, Howley PM. Proteomic approaches to the study of papillomavirus-host interactions. Virology 2013;435(1):57–69.
28. Dyson N, Howley PM, Munger K, et al. The human papilloma virus-16 E7 oncoprotein is able to bind to the retinoblastoma gene product. Science 1989;243(4893):934–937.
29. Munger K, Howley PM. Human papillomavirus immortalization and transformation functions. Virus Res 2002;89(2):213–228.
30. Duensing S, Lee LY, Duensing A, et al. The human papillomavirus type 16 E6 and E7 oncoproteins cooperate to induce mitotic defects and genomic instability by uncoupling centrosome duplication from the cell division cycle. Proc Natl Acad Sci U S A 2000;97(18):10002–10007.
31. DiMaio D, Petti LM. The E5 proteins. Virology 2013;445(1–2):99–114.
32. Schiller JT, Lowy DR. Understanding and learning from the success of prophylactic human papillomavirus vaccines. Nat Rev Microbiol 2012;10(10):681–692.
33. Kemp TJ, Safaeian M, Hildesheim A, et al. Kinetic and HPV infection effects on cross-type neutralizing antibody and avidity responses induced by Cervarix((R)). Vaccine 2012;31(1):165–170.
34. Haupt RM, Wheeler CM, Brown DR, et al. Impact of an HPV6/11/16/18 L1 virus-like particle vaccine on progression to cervical intraepithelial neoplasia in seropositive women with HPV16/18 infection. Int J Cancer 2011;129(11):2632–2642.
35. Kreuter A, Wieland U. Lack of efficacy in treating condyloma acuminata and preventing recurrences with the recombinant quadrivalent human papillomavirus vaccine in a case series of immunocompetent patients. J Am Acad Dermatol 2013;68(1):179–180.
36. Joura E, Team V-S, eds. Abstract SS 8–4: Efficacy and immunogenicity of a novel 9-valent HPV L1 virus-like particle vaccine in 16- to 26-year-old women. Paper presented at: Eurogin 2013 International Multidisciplinary Congress; 2013; Florence, Italy.
37. Wang JW, Roden RB. L2, the minor capsid protein of papillomavirus. Virology 2013;445(1–2):175–186.
38. Ma B, Maraj B, Tran NP, et al. Emerging human papillomavirus vaccines. Expert Opin Emerg Drugs 2012;17(4):469–492.
39. Scudellari M. HPV: sex, cancer and a virus. Nature 2013;503(7476):330–332.
40. Gillison ML, Alemany L, Snijders PJ, et al. Human papillomavirus and diseases of the upper airway: head and neck cancer and respiratory papillomatosis. Vaccine 2012;30 Suppl 5:F34–54.
41. Silverberg MJ, Leyden W, Warton EM, et al. HIV infection status, immunodeficiency, and the incidence of non-melanoma skin cancer. J Natl Cancer Inst 2013;105(5):350–360.
42. Kempf W, Mertz KD, Hofbauer GF, et al. Skin cancer in organ transplant recipients. Pathobiology 2013;80(6):302–309.
43. Wallace NA, Gasior SL, Faber ZJ, et al. HPV 5 and 8 E6 expression reduces ATM protein levels and attenuates LINE-1 retrotransposition. Virology 2013;443(1):69–79.
44. White EA, Kramer RE, Tan MJ, et al. Comprehensive analysis of host cellular interactions with human papillomavirus E6 proteins identifies new E6 binding partners and reflects viral diversity. J Virol 2012;86(24):13174–13186.
45. White EA, Sowa ME, Tan MJ, et al. Systematic identification of interactions between host cell proteins and E7 oncoproteins from diverse human papillomaviruses. Proc Natl Acad Sci U S A 2012;109(5):E260–267.
46. Caldeira S, Zehbe I, Accardi R, et al. The E6 and E7 proteins of the cutaneous human papillomavirus type 38 display transforming properties. J Virol 2003;77(3):2195–2206.
47. Arron ST, Ruby JG, Dybbro E, et al. Transcriptome sequencing demonstrates that human papillomavirus is not active in cutaneous squamous cell carcinoma. J Invest Dermatol 2011;131(8):1745–1753.
48. Gross L. A filterable agent, recovered from Ak leukemic extracts, causing salivary gland carcinomas in C3H mice. Proc Soc Exp Biol Med 1953;83(2):414–421.
49. Eddy BE, Stewart SE. Characteristics of the SE polyoma virus. Am J Public Health Nations Health 1959;49:1486–1492.
50. Sweet BH, Hilleman MR. The vacuolating virus, S.V. 40. Proc Soc Exp Biol Med 1960;105:420–427.
51. Eddy BE, Borman GS, Grubbs GE, et al. Identification of the oncogenic substance in rhesus monkey kidney cell culture as simian virus 40. Virology 1962;17:65–75.
52. Dang-Tan T, Mahmud SM, Puntoni R, et al. Polio vaccines, simian virus 40, and human cancer: the epidemiologic evidence for a causal association. Oncogene 2004;23(38):6535–6540.
53. Bouvard V, Baan RA, Grosse Y, et al. Carcinogenicity of malaria and of some polyomaviruses. Lancet Oncol 2012;13(4):339–340.
54. Gardner SD, Field AM, Coleman DV, et al. New human papovavirus (B.K.) isolated from urine after renal transplantation. Lancet.1971;1(7712):1253–1257.
55. Padgett BL, Walker DL, ZuRhein GM, et al. Cultivation of papova-like virus from human brain with progressive multifocal leucoencephalopathy. Lancet 1971;1(7712):1257–1260.
56. Das D, Wojno K, Imperiale MJ. BK virus as a cofactor in the etiology of prostate cancer in its early stages. J Virol 2008;82(6):2705–2714.
57. Akgul B, Pfister D, Knuchel R, et al. No evidence for a role of xenotropic murine leukaemia virus-related virus and BK virus in prostate cancer of German patients. Med Microbiol Immunol 2012;201(2):245–248.
58. Alexiev BA, Randhawa P, Vazquez Martul E, et al. BK virus-associated urinary bladder carcinoma in transplant recipients: report of 2 cases, review of the literature, and proposed pathogenetic model. Human Pathol 2013;44(5):908–917.
59. Abend JR, Jiang M, Imperiale MJ. BK virus and human cancer: innocent until proven guilty. Semin Cancer Biol 2009;19(4):252–260.
60. Maginnis MS, Atwood WJ. JC virus: an oncogenic virus in animals and humans? Semin Cancer Biol 2009;19(4):261–269.
61. Feng H, Shuda M, Chang Y, et al. Clonal integration of a polyomavirus in human Merkel cell carcinoma. Science 2008;319(5866):1096–1100.
62. Hodgson NC. Merkel cell carcinoma: changing incidence trends. J Surg Oncol 2005;89(1):1–4.
63. Engels EA, Frisch M, Goedert JJ, et al. Merkel cell carcinoma and HIV infection. Lancet 2002;359(9305):497–498.
64. Zur Hausen A, Rennspiess D, Winnepenninckx V, et al. Early B-cell differentiation in Merkel cell carcinomas: clues to cellular ancestry. Cancer Res 2013;73(16):4982–4987.
65. Guastafierro A, Feng H, Thant M, et al. Characterization of an early passage Merkel cell polyomavirus-positive Merkel cell carcinoma cell line, MS-1, and its growth in NOD scid gamma mice. J Virol Methods 2013;187(1):6–14.
66. Schowalter RM, Pastrana DV, Pumphrey KA, et al. Merkel cell polyomavirus and two previously unknown polyomaviruses are chronically shed from human skin. Cell Host Microbe 2010;7(6):509–515.
67. Faust H, Pastrana DV, Buck CB, et al. Antibodies to Merkel cell polyomavirus correlate to presence of viral DNA in the skin. J Infect Dis 2011;203(8):1096–1100.
68. Pastrana DV, Wieland U, Silling S, et al. Positive correlation between Merkel cell polyomavirus viral load and capsid-specific antibody titer. Med Microbiol Immunol 2011;201(1):17–23.
69. Pastrana DV, Tolstov YL, Becker JC, et al. Quantitation of human seroresponsiveness to Merkel cell polyomavirus. PLoS Pathog 2009;5(9):e1000578.
70. Faust H, Andersson K, Ekstrom J, et al. Prospective study of Merkel cell polyomavirus and risk of Merkel cell carcinoma. Int J Cancer 2014;134(4):844–848.
71. Cheng J, Rozenblatt-Rosen O, Paulson KG, et al. Merkel cell polyomavirus large T antigen has growth-promoting and inhibitory activities. J Virol 2013;87(11):6118–6126.
72. Li J, Wang X, Diaz J, et al. Merkel cell polyomavirus large T antigen disrupts host genomic integrity and inhibits cellular proliferation. J Virol 2013;87(16):9173–9188.
73. Houben R, Shuda M, Weinkam R, et al. Merkel cell polyomavirus-infected Merkel cell carcinoma cells require expression of viral T antigens. J Virol 2010;84(14):7064–7072.
74. Houben R, Grimm J, Willmes C, et al. Merkel cell carcinoma and Merkel cell carcinoma polyomavirus: evidence for hit-and-run oncogenesis. J Invest Dermatol 2012;132(1):254–256.
75. Shuda M, Chang Y, Moore PS. Merkel cell polyomavirus positive Merkel cell carcinoma requires viral small T antigen for cell proliferation. J Invest Dermatol 2013 [Epub ahead of print].
76. Paulson KG, Iyer JG, Tegeder AR, et al. Transcriptome-wide studies of merkel cell carcinoma and validation of intratumoral CD8+ lymphocyte invasion as an independent predictor of survival. J Clin Oncol 2011;29(12):1539–1546.
77. Arora R, Shuda M, Guastafierro A, et al. Survivin is a therapeutic target in Merkel cell carcinoma. Sci Transl Med 2012;4(133):133ra56.
78. Dresang LR, Guastafierro A, Arora R, et al. Response of merkel cell polyomavirus-positive merkel cell carcinoma xenografts to a survivin inhibitor. PloS One 2013;8(11):e80543.
79. Shuda M, Kwun HJ, Feng H, et al. Human Merkel cell polyomavirus small T antigen is an oncoprotein targeting the 4E-BP1 translation regulator. J Clin Invest 2011;121(9):3623–3634.
80. Howard RA, Dores GM, Curtis RE, et al. Merkel cell carcinoma and multiple primary cancers. Cancer Epidemiol Biomarkers Prev 2006;15(8):1545–1549.
81. Pantulu ND, Pallasch CP, Kurz AK, et al. Detection of a novel truncating Merkel cell polyomavirus large T antigen deletion in chronic lymphocytic leukemia cells. Blood 2010;116(24):5280–5284.
82. Tolstov YL, Arora R, Scudiere SC, et al. Lack of evidence for direct involvement of Merkel cell polyomavirus (MCV) in chronic lymphocytic leukemia (CLL). Blood 2010;115(23):4973–4974.
83. Cimino PJ, Jr., Bahler DW, Duncavage EJ. Detection of Merkel cell polyomavirus in chronic lymphocytic leukemia T-cells. Exp Mol Pathol 2013;94(1):40–44.
84. Burkitt D. A sarcoma involving the jaws in African children. Br J Surg 1958;46(197):218–223.
85. Epstein MA, Achong BG, Barr YM. Virus particles in cultured lymphoblasts from Burkitt’s lymphoma. Lancet 1964;1(7335):702–703.
86. Henle G, Henle W, Diehl V. Relation of Burkitt’s tumor-associated herpes-ytpe virus to infectious mononucleosis. Proc Natl Acad Sci U S A 1968;59(1):94–101.
87. Longnecker RM, Kieff E, Cohen JI. Epstein-Barr virus. In: Knipe DM, Howley PM, eds. Fields Virology. 6th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2013.
88. Cesarman E. Gammaherpesviruses and lymphoproliferative disorders. Annu Rev Pathol 2014;9:349–372.
89. Shannon-Lowe C, Rowe M. Epstein-Barr virus infection of polarized epithelial cells via the basolateral surface by memory B cell-mediated transfer infection. PLoS Pathog 2011;7(5):e1001338.
90. Faulkner GC, Burrows SR, Khanna R, et al. X-Linked agammaglobulinemia patients are not infected with Epstein-Barr virus: implications for the biology of the virus. J Virol 1999;73(2):1555–1564.
91. Hjalgrim H, Smedby KE, Rostgaard K, et al. Infectious mononucleosis, childhood social environment, and risk of Hodgkin lymphoma. Cancer Res 2007;67(5):2382–2388.
92. Barton ES, White DW, Cathelyn JS, et al. Herpesvirus latency confers symbiotic protection from bacterial infection. Nature 2007;447(7142):326–329.
93. Giulino-Roth L, Cesarman E. Molecular biology of Burkitt lymphoma. In: Robertson E, ed. Burkitt’s Lymphoma. New York, NY: Springer; 2013: 211–226.
94. Chang ET, Adami HO. The enigmatic epidemiology of nasopharyngeal carcinoma. Cancer Epidemiol Biomarkers Prev 2006;15(10):1765–1777.
95. Murukesan V, Mukherjee S. Managing post-transplant lymphoproliferative disorders in solid-organ transplant recipients: a review of immunosuppressant regimens. Drugs 2012;72(12):1631–1643.
96. Cohen JI, Mocarski ES, Raab-Traub N, et al. The need and challenges for development of an Epstein-Barr virus vaccine. Vaccine 2013;31(Suppl 2):B194–196.
97. Choquet S, Leblond V, Herbrecht R, et al. Efficacy and safety of rituximab in B-cell post-transplantation lymphoproliferative disorders: results of a prospective multicenter phase 2 study. Blood 2006;107(8):3053–3057.
98. Barnes JA, Lacasce AS, Feng Y, et al. Evaluation of the addition of rituximab to CODOX-M/IVAC for Burkitt’s lymphoma: a retrospective analysis. Ann Oncol 2011;22(8):1859–1864.
99. Kaposi M. Idiopathisches multiples Pigmentsarkom der Haut. Archiv für Dermatologie und Syphilis 1872;4(2):265–273.
100. Antman K, Chang Y. Kaposi’s sarcoma. N Engl J Med 2000;342(14):1027–1038.
101. Beral V, Peterman TA, Berkelman RL, et al. Kaposi’s sarcoma among persons with AIDS: a sexually transmitted infection? Lancet 1990;335(8682):123–128.
102. Chang Y, Cesarman E, Pessin MS, et al. Identification of herpesvirus-like DNA sequences in AIDS-associated Kaposi’s sarcoma. Science 1994;266(5192):1865–1869.
103. Damania BA, Cesarman E. Kaposi’s sarcoma-associated herpesvirus. In: Knipe DM, Howley PM, eds. Fields Virology. 6th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2013.
104. Cook-Mozaffari P, Newton R, Beral V, et al. The geographical distribution of Kaposi’s sarcoma and of lymphomas in Africa before the AIDS epidemic. Br J Cancer 1998;78(11):1521–1528.
105. Martin DF, Kuppermann BD, Wolitz RA, et al. Oral ganciclovir for patients with cytomegalovirus retinitis treated with a ganciclovir implant. Roche Ganciclovir Study Group. N Engl J Med 1999;340(14):1063–1070.
106. Krown SE, Dittmer DP, Cesarman E. Pilot study of oral valganciclovir therapy in patients with classic Kaposi sarcoma. J Infect Dis 2011;203(8):1082–1086.
107. Sakakibara S, Tosato G. Regulation of angiogenesis in malignancies associated with Epstein-Barr virus and Kaposi’s sarcoma-associated herpes virus. Future Microbiol 2009;4(7):903–917.
108. Soulier J, Grollet L, Oksenhendler E, et al. Kaposi’s sarcoma-associated herpesvirus-like DNA sequences in multicentric Castleman’s disease. Blood 1995;86(4):1276–1280.
109. Polizzotto MN, Uldrick TS, Wang V, et al. Human and viral interleukin-6 and other cytokines in Kaposi sarcoma herpesvirus-associated multicentric Castleman disease. Blood 2013;122(26):4189–4198.
110. Simonelli C, Spina M, Cinelli R, et al. Clinical features and outcome of primary effusion lymphoma in HIV-infected patients: a single-institution study. J Clin Oncol 2003;21(21):3948–3954.
111. Coffin JM, Hughes SH, Varmus HE, eds. The interactions of retroviruses and their hosts. Retroviruses. Cold Spring Harbor, NY: Cold Spring Harbor Laboratory Press; 1997.
112. Gallo RC. History of the discovery of the first human retroviruses: HTLV-1 and HTLV-2. Oncogene 2005;24:5926–5930.
113. Smit AF, Riggs AD. Tiggers and DNA transposon fossils in the human genome. Proc Natl Acad Sci U S A 1996;93:1443–1448.
114. Delviks-Frankenberry K, Paprotka T, Cingöz O, et al. Generation of multiple replication-competent retroviruses through recombination between PreXMRV-1 and PreXMRV-2. J Virol 2013;87:11525–11537.
115. Mason AL, Zhang G. Linking human beta retrovirus infection with primary biliary cirrhosis. Gastroenterol Clin Biol 2010;34:359–366.
116. Hacein-Bey-Abina S, VonKalle C, Schmidt M, et al. LMO2-associated clonal T cell proliferation in two patients after gene therapy for SCID-X1. Science 2003;302:415–419.
117. Matsuoka M, Jeang K-T. Human T-cell leukaemia virus type 1 (HTLV-1) infectivity and cellular transformation. Nat Rev Cancer 2007;7:270–280.
118. Poiesz BJ, Ruscetti FW, Mier JW, et al. T-cell lines established from human T-lymphocytic neoplasias by direct response to T-cell growth factor. Proc Natl Acad Sci U S A 1980;77:6815–6819.
119. Kalyanaraman VS, Sarngadharan MG, Robert-Guroff M, et al. A new subtype of human T-cell leukemia virus (HTLV-II) associated with a T-cell variant of hairy cell leukemia. Science 1982;218:571–573.
120. Wolfe ND, Heneine W, Carr JK, et al. Emergence of unique primate T-lymphotropic viruses among central African bushmeat hunters. Proc Natl Acad Sci U S A 2005;102:7994–7999.
121. Goncalves DU, Prioietti FA, Ribas JGR, et al. Epidemiology, treatment, and prevention of human T-cell leukemia virus type 1-associated diseases. Clin Microbiol Rev 2010;23:577–589.
122. Hino S, Sugiyama H, Doi H, et al. Breaking the cycle of HTLV-1 transmission via carrier mothers’ milk. Lancet Oncol 1987;2:158–159.
123. Kannian P, Green PL. Human T lymphotropic virus type 1 (HTLV-1): molecular biology and oncogenesis. Viruses 2010;2:2037–2077.
124. Hill SA, Lloyd PA, McDonald S, et al. Susceptibility of human T cell leukemia virus type I to nucleoside reverse transcriptase inhibitors. J Infec Dis 2003;188:424–427.
125. Seegulam ME, Ratner L. Integrase inhibitors effective against human T-cell leukemia virus type 1. Antimicrob Agents Chemother 2011;55:2011–2017.
126. Derse D, Crise B, Li Y, et al. Human T-cell leukemia virus type 1 integration target sites in the human genome: comparison with those of other retroviruses. J Virol 2007;81:6731–6741.
127. Jones KS, Lambert S, Bouttieer M, et al. Molecular aspects of HTLV-1 entry: functional domains of the HTLV-1 surface subunit (SU) and their relationships to the entry receptors. Viruses 2011;3:794–810.
128. Matsuoka M, Green PL. The HBZ gene, a key player in HTLV-1 pathogenesis. Retrovirology 2009;6:71.
129. Grossman WJ, Kimata JT, Wong FH, et al. Development of leukemia in mice transgenic for the tax gene of human T-cell leukemia virus type I. Proc Natl Acad Sci U S A 1995;92:1057–1061.
130. El Hajj H, El-Sabban M, Hasegawa H, et al. Therapy-induced selective loss of leukemia-initiating activity in murine adult T cell leukemia. J Exp Med 2010;207:2785–2792.
131. Villaudy J, Wencker M, Gadot N, et al. HTLV-1 propels thymic human T cell development in “human immune system” Rag2-/-IL-2Rgammac-/-mice. PLoS Pathog 2011;7:e1002231.
132. Costa EAS, Magri MC, Caterino-de-Arujo A. The best algorithm to confirm the diagnosis of HTLV-1 and HTLV-2 in at-risk individuals from Sao Paulo, Brazil. J Virol Methods 2011;173:280–286.
133. Barros N, Woll F, Watanabe L, et al. Are increased Foxp3+ regulatory T cells responsible for immunosuppression during HTLV-1 infection? Case reports and review of the literature. BMJ Case Rep 2012;bcr2012006574.
134. Cook LB, Rowan AG, Melamed A, et al. HTLV-1-infected T cells contain a single integrated provirus in natural infection. Blood 2012;120:3488–3490.
135. Shimoyama M. Diagnostic criteria and classification of clinical subtypes of adult T-cell leukemia-lymphoma: a report from the Lymphoma Study Group. Br J Hematol 1991;79:426–437.
136. Takasaki Y, Iwanaga M, Imaizumi Y, et al. Long-term study of indolent adult T-cell leukemia-lymphoma. Blood 2010;115:4337–4343.
137. Bazarbachi A, Plumelle Y, Ramos JC, et al. Meta-analysis on the use of zidovudine and interferon-alfa in adult T-cell leukemia/lymphoma showing improved survival in the leukemic subtypes. J Clin Oncol 2010;28:4177–4183.
138. Katsuya H, Yamanka T, Ishitsuka K, et al. Prognostic index for acute- and lymphoma-type adult T-cell leukemia/lymphoma. J Clin Oncol 2012;30:1635–1640.
139. Tsukasaki K, Hermine O, Bazarbachi A, et al. Definition, prognostic factors, treatment, and reponse criteria of adult T-cell leukemia-lymphoma: a proposal from an international consensus meeting. J Clin Oncol 2009;27:453–459.
140. Yamada Y, Tomonaga M, Fukuda H, et al. A new G-CSF supported combination chemotherapy, LSG15, for adult T-cell leukaemia-lymphoma: Japan Clinical Oncology Group Study 9303. Br J Hematol 2001;113:375–382.
141. Ratner L, Harrington W, Feng X, et al. Human T cell leukemia virus reactivation with progression of adult T-cell leukemia-lymphoma. AIDS Malignancy Consortium. PLoS One 2009;4:e4420.
142. Tatsuro J, Ishida T, Takemoto S, et al., eds. Randomized phase II study of mogamulizumab (KW-0761) plus VCAP-AMP-VECP (mLSG15) versus mLSG15 alone for newly diagnosed aggressive adult T-cell leukemia-lymphoma (ATL). Paper presented at: 2013 ASCO Annual Meeting; 2013; Chicago, IL.
143. Bazarbachi A, Suarez F, Fields P, et al. How I treat adult T-cell leukemia/lymphoma. Blood 2011;118:1736–1745.
144. Utsonomiya A, Miyazaki Y, Takasuka Y, et al. Improved outcome of adult T cell leukemia/lymphoma with allogeneic hematopoietic stem cell transplanation. Bone Marrow Transplant 2001;27:15–20.
145. Blumberg BS. The discovery of the hepatitis B virus and the intervention of the vaccine: a scientific memoir. J Gastroenterol Hepatol 2002;17(Supplement s4):S502–S503.
146. Houghton M. Discovery of the hepatitis C virus. Liver Int 2009;29(Supplement 1):82–88.
147. Arzumanyan A, Reis HM, Feitelson MA. Pathogenic mechanisms in HBV- and HCV-associated hepatocellular carcinoma. Nat Rev Cancer 2013;13:123–135.
148. Soerjomataram I, Lortet-Tieulent J, Parkin DM, et al. Global burden of cancer in 2008: a systematic analysis of disability-adjusted life-years in 12 world regions. Lancet 2012;380:1840–1850.
149. Altekruse SF, McGlynn KA, Reichman ME. Hepatocellular carcinoma incidence, mortality, and survival trends in the United States from 1975 to 2005. J Clin Oncol 2009;27:1485–1491.
150. Shiffman ML, ed. Chronic Hepatitis C Virus: Advances in Treatment, Promise for the Future. New York: Springer Verlag; 2011.
151. Nicot F, Nassim K, Lionel R, et al. Occult hepatitis C virus infection: Where are we now? Liver Biopsy in Modern Med 2004;307–334.
152. Freeman AJ, Dore GJ, Law MG, et al. Estimating progression to cirrhosis in chronic hepatitis C virus infection. Hepatology 2001;34:809–816.
153. Wilkins T, Malcom JK, Raina D, et al. Hepatitis C: diagnosis and treatment. Am Fam Physician 2010;81:1351–1357.
154. Fallot G, Neuveut C, Buendia M-A. Diverse roles of hepatitis B virus in liver cancer. Curr Opin Virol 2012;2:467–473.
155. Ganem D, Prince AM. Hepatitis B virus infection - natural history and clinical consequences. N Engl J Med 2004;350:1118–1129.
156. Tujios SR, Lee WM. Update in the management of chronic hepatitis B. Curr Opin Gastroenterol 2013;29:250–256.
157. Seto W-K, Fung J, Yuen M-F, et al. Future prevention and treatment of chronic hepatitis B infection. J Clin Gastroenterol 2012;46:725–734.
158. Wang WK, Levy S. Hepatitis C virus (HCV) and lymphomagenesis. Leuk Lymphoma 2003;44:1113–1120.
159. Fernandez-Garcia M-D, Mazzon M, Jacobs M, et al. Pathogenesis of flavivirus infections: using and abusing the host cell. Cell Host Microbe 2009;318:318–328.
160. Moradpour D, Penin F, Rice CM. Replication of hepatitis c virus. Nat Rev Microbiol 2007;5:453–463.
161. Liang TJ, Ghany MG. Current and future therapies for hepatitis C virus infection. N Engl J Med 2013;368:1907–1917.
162. Kimer N, Dahl EK, Gluud LL, et al. Antiviral therapy for prevention of hepatocellular carcinoma in chronic hepatitis C: systematic review and meta-analysis of randomised controlled trials. BMJ Open 2012;2:e001313.
163. Fujimoto A, Totoki Y, Abe T, et al. Whole-genome sequencing of liver cancers identifies etiological influences on mutation patterns and recurrent mutations in chromatin regulators. Nat Genet 2012;44:760–764.
164. Sung WK, Zheng H, Li S, et al. Genome-wide survey of recurrent HBV integration in hepatocellular carcinoma. Nat Genet 2012;44:765–769.
165. Devi U, Locarnini S. Hepatitis B antivirals and resistance. Curr Opin Virol 2013;3:495–500.
166. Aghemo A, Colombo M. Hepatocellular carcinoma in chronic hepatitis C: from bench to bedside. Semin Immunopathol 2013;35:111–120.
167. Bruix J, Sherman M. Management of hepatocellular carcinoma: an update. Hepatology 2011;53:1020–1022.
168. Mazzaferro V, Regalia E, Doci R, et al. Liver transplantation for the treatment of small hepatocellular carcinomas in patients with cirrhosis. N Engl J Med 1996;334:693–699.
169. Mino M, Lauwers GY. Pathologic spectrum and prognostic significance of underlying liver disease in hepatocellular carcinoma. Surg Oncol Clin N Am 2003;12:13–24.
170. Burton JR, Everson GT. Management of the transplant recipient with chronic hepatitis C. Clin Liver Dis 2013;17:73–91.
171. Beckebaum S, Kabar I, Cicinnati VR. Hepatitis B and C in liver transplantation: new strategies to combat the enemies. Rev Med Virol 2012;23:172–193.
172. Torres HA, Davila M. Reactivation of hepatitis B virus and hepatitis C virus in patients with cancer. Nat Rev Clin Oncol 2012;9:156–166.
173. Huang Y-H, Hsaio L-T, Hong Y-C, et al. Randomized controlled trial of entecavir prophylaxis for rituximab-associated hepatitis B virus reactivation in patients with lymhoma and resolved hepatitis. J Clin Oncol 2013;31:2765–2772.
174. Artz AS, Somerfield MR, Feld JJ, et al. American Society of Clinical Oncology provisional clinical opinion: chronic hepatitis B virus infection screening in patients receiving cytotoxic chemotherapy for treatment of malignant diseases. J Clin Oncol 2010;28:3199–3202.
175. Forghieri F, Luppi M, Barozzi P, et al. Pathogenetic mechanisms of hepatitits C virus-induced B-cell lymphomagenesis. Clin Dev Immunol 2012;2012:807351.