Susan F. Leitman and Charles D. Bolan
EPIDEMIOLOGY
Classic hereditary hemochromatosis, also known as HFE-hemochromatosis, is an autosomal recessive disorder caused by inappropriate dietary absorption of iron and abnormal iron cycling. It is characterized by progressive accumulation of iron in tissues, particularly the liver, pancreas, heart, endocrine organs, and skin, which may lead to end-stage organ damage, usually during or after middle age.1-3 It is one of the most common single-gene disorders in Caucasians of northern European descent, with an incidence of 1 in 200 and a carrier rate of 1 in 10 persons. However, the clinical penetrance of the disorder is highly variable, and only a minority of affected persons develops severe or life-threatening organ dysfunction.4,5
Genetic Basis for Classic Hemochromatosis: HFE Mutations
Mutations in HFE, an MHC class-I like gene on chromosome 6, are found in nearly 90% of people with the clinical phenotype and 100% of affected people with a strong family history of the disorder. 6,7
Substitution of tyrosine for cysteine at amino acid 282 of the HFE gene product (C282Y) is the founder mutation; by linkage disequilibrium analysis, the mutation originated recently, within the past 2000 years. C282Y occurs with highest frequency in northwestern European populations, reaching 14% in areas of Great Britain (Table 25.1). Allele frequency decreases in a north-to-south and west-to-east direction across Europe, and the ancestral haplotype may have been of Viking or Celtic origin; the haplotype is extremely rare in African and Asian populations. Homozygosity for C282Y is seen in 64% to 96% of persons with clinical hemochromatosis.
A second HFE mutation, replacement of histidine by aspartate at residue 63 of the HFE protein (H63D) is frequently found on the non-C282Y-containing chromosome of individuals with clinical hemochromatosis who are heterozygous for C282Y.6 H63D is an older mutation with a wider population distribution, having an allele frequency of 5% to 14% throughout Europe and Asia. It appears to be a genetic polymorphism without much clinical impact in the absence of another genetic or environmental factor. Compound heterozygosity for C282Y/H63D is seen in 4% to 7% of persons with a hemochromatosis phenotype.
Seventeen additional polymorphisms in HFE have been described. Of these, only the S65C mutation appears to have clinical impact, and it may cause mild iron overload when the individual is compound heterozygous for C282Y or H63D.
PATHOPHYSIOLOGY
Since iron excretion in the gut is fixed at 1 mg/day, normal iron balance must be maintained by meticulous control of iron absorption in the intestine and iron release from macrophages. These are modulated in response to body iron stores and the erythropoietic demand for iron.
Hepcidin: Key Regulator of Iron Homeostasis
Hepcidin, a liver-derived peptide hormone, is a key negative regulator of iron release into the plasma by intestinal enterocytes, macrophages, hepatocytes, and placental cells.8 It binds to and causes internalization and degradation of the cell surface iron exporter, ferroportin.9 Hepcidin excess decreases intestinal iron absorption and macrophage iron release, and causes anemia. Hepcidin deficiency promotes intestinal iron absorption and leads to tissue iron overload. Hepcidin gene expression is enhanced by iron overload and inflammation, and suppressed by anemia and hypoxia. Although hepcidin is ordinarily induced by dietary iron loading, its expression is inappropriately reduced in all forms of inherited hemochromatosis.10,11
Iron Overload Disorders and Hepcidin Deficiency
Hepcidin deficiency plays a central role in the pathogenesis of the inherited hemochromatosis disorders, including those due to mutations in the HFE gene, the hemojuvelin gene (HJV), the transferrin receptor 2 gene (TfR2), and hepcidin itself (HAMP) (Table 25.2). Hemojuvelin acts as a coreceptor in the bone morphogenetic protein (BMP) pathway, interacting with BMP ligands and BMP type I and II receptors to generate an active signaling complex.10This complex activates a Smad receptor signaling cascade and translocation of a Smad complex to the nucleus, where it increases HFE transcription. HJV and HAMPmutations are critical components in the same common pathway; their negative effect on hepcidin expression is associated with severe iron loading in childhood, or juvenile hemochromatosis.
HFE Localization and Function
HFE is highly expressed in Kupffer cells of the liver and in tissue macrophages. Binding to β2 microglobulin (β2m) allows expression of HFE/β2m on the cell surface,12 where it forms a stable complex with transferrin receptor 1 (TfR1). The C282Y mutation prevents formation of a disulfide bond in HFE, disabling β2m binding, and preventing cell surface expression. Disruption of the HFE/β2m/TfR1 complex, and mutations in TfR2, are associated with adult-onset iron overload. HFE and TfR2 may regulate hepcidin expression by enhanced iron transport into the cell (endocytosis of diferric transferrin), by upstream regulation of hepcidin, or as weak coreceptors for BMP-SMAD signaling. According to this model, it might be possible to treat hemochromatosis by hepcidin replacement.
The most common form of secondary hemochromatosis is transfusional iron overload: 1 mL of red cells contains about 1 mg of iron. Inappropriate absorption of iron in the gut may also occur in association with ineffective erythropoiesis. In this case, the erythropoietic stimulus to decrease hepcidin levels overrides the effect of iron overload on increasing hepcidin expression.
Iron Homeostasis
The distribution of body iron is shown in Table 25.3, with a comparison of iron stores in the normal state and in subjects with hemochromatosis.
Excess iron and tissue injury. When the capacity for iron storage is exceeded, excess tissue iron causes cellular damage by catalyzing the formation of oxyradicals.13 Oxidative damage to lipids, proteins, carbohydrates, and DNA may lead to widespread impairment in cell function and integrity. In particular, lipid peroxidation may result in impaired membrane-dependent mitochondrial and lysosomal function. Oxidative injury to DNA, particularly in hepatocytes, may predispose to mutagenesis and cancer.
Nontransferrin-bound iron (NTBI): This represents “free iron in serum.” NTBI enters cells freely, independent of receptor-mediated uptake. NTBI levels are low or undetectable at transferrin saturation (TS) below 40%, and increase linearly with TS levels above 40% to 50%. NTBI and its intracellular labile iron counterpart may be the direct mediators of oxidant stress. 13
CLINICAL FEATURES AND DIAGNOSIS OF HFE-HEMOCHROMATOSIS
Prior to the availability of biochemical and genetic screening tests, HFE-hemochromatosis was identified by damage to the liver, pancreas, heart, and joints, and diagnosed by demonstrating increased iron stores on liver biopsy. The “classic triad” of cirrhosis, diabetes, and skin pigmentation appeared in many publications and textbooks.1 Patients typically presented with the following symptoms:
Severe liver disease due to hepatic fibrosis or cirrhosis
Cardiac failure and refractory arrhythmias
Polyendocrine failure: insulin-dependent diabetes and hypogonadotrophic hypogonadism
Debilitating symmetric polyarthritis
Grayish skin pigmentation
It is now recognized that this severe clinical phenotype is relatively rare and only develops in 1% to 4% of untreated C282Y homozygotes over their lifetime.4 Between 40% and 60% of C282Y homozygote males and 60% to 80% of homozygote females will remain asymptomatic or have minimal clinical manifestations throughout their lives; of the 40% to 50% who do develop symptoms that affect quality of life, arthritis, fatigue, and sexual dysfunction are the most common complaints (Table 25.4).14,15
New Diagnostic Definition
In the current era of molecular testing, recognizing that clinical penetrance can be highly variable, the diagnosis of hemochromatosis is established by the detection of two mutated HFE alleles. This definition does not require active symptoms or signs of illness or the presence of iron overload. Four stages of the disorder are recognized16:
Common Clinical Presentation
The most common clinical presentation of hereditary hemochromatosis is with nonspecific symptoms, and therefore practitioners should have a low threshold for ordering serum TS and ferritin studies in patients with unexplained chronic fatigue, arthralgia, or arthritis, sexual dysfunction, hepatomegaly, or elevated liver function values (alanine aminotransferase [ALT]). Since symptoms are easily overlooked, the single most common event currently leading to a diagnosis of hemochromatosis is the incidental detection of an abnormal laboratory test result, either an elevated TS, serum ferritin, or ALT. In hemochromatosis subjects diagnosed with fatigue on presentation, screening laboratory tests to evaluate possible concomitant thyroid disease should be obtained.
Typical findings related to the most common clinical signs and symptoms of hemochromatosis are shown in Table 25.5. It is difficult to assign a frequency to these symptoms since there is a continuum of increasing frequency with increasing age, and with male versus female sex.16 Arthritis is the clinical feature with the greatest impact on quality of life.17 In contrast to significant cardiac abnormalities described in hemochromatosis patients who presented with very high iron prior to the advent of more frequent screening and the availability of a genetic test, heart disease now is generally absent or clinically insignificant in newly diagnosed, asymptomatic subjects.18
The considerable variability in clinical penetrance of C282Y homozygosity, both in rate of accumulation of iron stores and appearance of organ dysfunction, may be due to environmental, lifestyle, and genetic factors (Table 25.6).
LABORATORY TESTING
Once the clinical suspicion of hemochromatosis is raised, the diagnosis should be confirmed by laboratory testing, including those listed below:
Confirmatory Laboratory Tests
Serum iron, transferrin, and TS: Transferrin is the major iron transport protein in plasma. Several assay methods for TS exist: the most accurate is direct colorimetric analysis of serum iron (SI) combined with nephelometric assay of transferrin, wherein TS = molar concentration of iron divided by twice the molar concentration of transferrin. Less expensive but also less robust methods include chemical analyses of total serum iron binding capacity (TIBC) and unbound iron capacity (UIBC). Saturation of serum iron binding capacity is measured by dividing the serum iron by either TIBC (SI/TIBC) or by the sum of iron and UIBC [(SI)/(SI+UIBC)]. Normal range for TS is 15% to 45%.
Serum ferritin: This is a major intracellular iron storage protein and is measured immunologically. This estimates the degree of iron overload and size of mobilizable iron stores (1 μg/L ferritin = 7–8 mg stored iron; for example, 1,000 μg/L ferritin = 7,000–8,000 mg stored iron). It is used to determine pace of initial phlebotomy therapy. Normal levels are < 350 μg/L in men and < 120 μg/L in women.
HFE genotype: Definitive diagnostic test, assesses predisposition to serious illness, useful for family counseling.
Ancillary Laboratory Tests
ALT: to assess degree of liver injury
Complete Blood Count: obtain baseline hemoglobin and red cell mean corpuscular volume (MCV), which can be monitored during therapy (decrease in MCV is an indicator of iron-limited erythropoiesis).
Blood glucose and electrolytes
Total and free testosterone: as indicated by symptoms
Thyroid function tests: as indicated by symptoms
Alpha fetoprotein: as baseline for subsequent monitoring for liver cancer
Serologic tests for exposure to hepatitis B and C (HBsAg and anti-HCV): active viral hepatitis worsens liver injury; useful to guide vaccine administration
Role of Liver Biopsy
Liver biopsy is generally not required for diagnosis. Although it previously served as the “gold standard” for both diagnosis and prognosis, the diagnosis is now more safely and reliably made with use of the HFE genotype.16
Indications for Biopsy
Histologic Findings
Radiographic and Other Tests
Skeletal films: performed to evaluate symptomatic joints
Liver ultrasound: useful in workup of non-hemochromatosis causes of elevated ferritin; may show steatosis. Important in surveillance for liver cancer.
CT and/or MRI of liver: not indicated diagnostically. Useful for suspected liver cancer.
Superconducting quantum interference device (SQUID) assessment: provides most sensitive noninvasive quantitative assessment of iron stores; has limited availability.
POPULATION SCREENING
The clinical course of hemochromatosis meets the definition of a disorder for which population screening should be performed2,3:
Thus, the costs of widespread testing and preventive treatment are considered favorable (more effective and less expensive) than delaying until development of late symptoms—particularly as the early, presenting symptoms are nonspecific, often unrecognized as being caused by hemochromatosis, and are associated with a 5 to 10 year delay until accurate diagnosis.
Laboratory Screening
Transferrin Saturation: The single best screening test is the serum TS: it is inexpensive, widely available, and highly sensitive and specific for the presence of the C282Y HFE allele.19 The decision threshold at which confirmatory testing should be initiated ranges from TS values of 45% to 62%, depending on whether sensitivity or specificity is preferred (Table 25.7). Since TS is affected by diet and diurnal variation, an elevated value should be confirmed by a second TS after an overnight fast, in the absence of oral iron supplements. Phenotype screening with TS is not advised until age 20 to 30, as iron burdens are generally low below this age.16 An algorithm for workup of persons detected through screening programs is shown in Figure 25.1.
Ferritin screening: Ferritin is an acute phase reactant; levels rise with inflammation, infection, and non-hemochromatosis liver disease. Lack of sensitivity and specificity make it a less reliable screening test.
Genotype screening: A 1998 consensus conference decided against widespread population screening using genetic tests. The high cost of genetic tests and variable clinical penetrance of hemochromatosis, coupled with concerns over stigmatization, discrimination, and insurability, led to rejection of this approach at the time. 20
Screening of Family Members of C282Y Homozygotes
Screening of children: The most cost-effective test is HFE genotype; biochemical screening is also acceptable; testing should be delayed until age 20 to 30. If more than two children are involved, the best approach may be genotyping of the other parent. 21
FIGURE 25.1 Decision tree for hemochromatosis population screening.
Screening of siblings: All siblings should be counseled to undergo either genetic or phenotypic screening. The most cost-effective test is HFE genotype, but phenotypic screening with combination of TS and ferritin is also acceptable. 21
TREATMENT
Phlebotomy Therapy
Phlebotomy: Periodic removal of one unit (500 mL) of whole blood. Safe, inexpensive, standard of care for the past 50 years.22 One unit of whole blood removes 200–250 mg of iron. Double red cell collection by apheresis removes 360 mL of packed red cells (400–420 mg of iron) and may be particularly useful in blood center setting.
Controversy regarding treatment indications for subjects with modest iron burdens
Patients generally desire treatment and are eager and willing to be blood donors.
Therapy is safe, accessible, and prevents late organ damage.
Referral to blood center shifts argument in favor of treatment (double benefit, to subject and to community; efficient care; no charge for procedure).
Guidelines for Phlebotomy Therapy
Phase I: Iron Depletion
Pace: Initiate phlebotomy at 1 to 4 week intervals, depending on ferritin, hemoglobin, ALT, gender, and weight. As iron depletion approaches, decrease pace to monthly.
Target of “de-ironing” therapy: several assays may be used
Ferritin < 30 μg/L
TS < 30%
Decrease in red cell MCV to 3% to 5% below prephlebotomy level.23
Monitoring parameters
Prephlebotomy fingerstick hemoglobin or hematocrit (+/− venous CBC) at each visit to avoid anemia
Ferritin every 4 to 8 weeks initially, then ferritin +/− TS every 1 to 2 treatments once ferritin <100 μg/L.
Safety guide: Threshold hemoglobin for therapeutic bleed ≥ 12.5 g/dL (hematocrit ≥ 38%). In general, do not bleed below this level; defer therapy for 1 to 4 weeks until hemoglobin recovers. Iron deficiency is not necessary during treatment; anemia should be avoided.
General guide: For initial ferritin of 500 to 1,500, patients generally require 15 to 30 bleeds to achieve iron depletion. If initial ferritin is >2,000 μg/dL, it may require more than 40 to 50 bleeds.
Phase II: Preventing Reaccumulation (Maintenance)
Pace: 500 mL removed every 8 to 26 weeks (mean 10–12 weeks), depending on gender, weight, age, and dietary habits. This is usually a lifelong requirement, although some subjects reaccumulate iron very slowly.
Goals of maintenance therapy
Ferritin 30 to 50 μg/L
TS < 50%
Hemoglobin > 12.5 g/dL
Monitoring parameters
Prephlebotomy fingerstick hemoglobin or hematocrit (+/− venous CBC) at each visit, and ferritin and/or TS every 1 to 2 treatments.
Evaluation of Anemia during Phlebotomy Therapy
Development of a hemoglobin <12.5 g/dL despite elevated ferritin levels may be due to occult bleeding, medications such as proton pump inhibitors,24 or endocrine causes such as hypothyroidism (men and women), or decreased testosterone levels (men). If concomitant disorder of erythroid production is present (thalassemia, renal insufficiency) and urgent need for phlebotomy exists, weekly erythropoietin may be helpful. Anemia may also be due to development of liver cancer.
Arthritis, Endocrine Replacement,Vaccinations, and Cancer Surveillance
Arthritis: Responds moderately well to nonsteroidal anti-inflammatory agents
Joint aspiration to exclude gout or pseudogout in acutely inflamed joints.
Orthopedic evaluation for joint replacement for severe chronic hip, knee, or ankle pain. Cumulative incidence of major joint replacement in C282Y +/+ hemochromatosis subjects is 30% by age 70.
Testosterone replacement: Consider in males with symptomatic sexual dysfunction and low testosterone levels.
HAV and HBV vaccination: Should be given as prophylaxis against future hepatic injury in unexposed patients.
Alpha fetoprotein and liver ultrasound: Surveillance for hepatocellular cancer. Repeat every 6 months if cirrhosis is documented by biopsy.
Dietary and Lifestyle Counseling
Avoid oral iron supplements.
Limit alcohol intake to protect the liver.
Red meat in moderation, but major change in dietary habits is not required. Iron stores are most efficiently controlled by adjusting frequency of bleeds rather than reducing intake of iron-rich foods.
Avoid raw shellfish until iron depletion is achieved (avoid Vibrio vulnificus).
If ALT elevated
Discontinue alcohol intake until iron depletion completed and ALT normal
Consider discontinuation of medications with potential hepatic toxicity.
PROGNOSIS AND RESPONSE TO THERAPY
If cirrhosis is not present, long-term survival is unchanged from the general population.25 If cirrhosis is present, the risk of hepatic cancer is increased and persists for life: 18.5% of subjects with cirrhosis will develop liver cancer, which may not be detected until 5 to 10 years after iron depletion. Overall, the incidence of hepatic cancer is 100-fold greater in hemochromatosis than non-hemochromatosis subjects and accounts for 10% to 30% of hemochromatosis-related deaths. Progression of cirrhosis due to hemochromatosis is slower than in other types of cirrhosis (alcoholic, viral); however, hemochromatosis subjects undergoing liver transplant for end-stage liver disease or liver cancer have a higher than average peritransplant mortality.
The response to phlebotomy varies by tissue site (Table 25.8).
HEMOCHROMATOSIS SUBJECTS AS BLOOD DONORS
Regulatory issues. FDA allows blood centers to obtain a “variance” from federal code to permit blood from hemochromatosis subjects to be made available for transfusion into others, even if collected more frequently than 56-day interval.
FDA requirements. Phlebotomy must be performed:
Under a physician’s direction
Without charge regardless of whether subjects qualify as donors
With periodic laboratory monitoring
Logistics and safety23
Seventy-five percent of all hemochromatosis subjects meet allogeneic donor eligibility criteria
Fifty-five percent of hemochromatosis subjects were blood donors prior to knowledge of their diagnosis
Potential hemochromatosis-donor contribution estimated at 1 to 2 million red cell units per year in the United States
Recent rapid increase in number of US blood centers with FDA-approved variances to allow hemochromatosis subjects to be blood donors (81 centers, May 2007)
Hemochromatosis subjects documented to be safe, reliable donors
Advantages of phlebotomy care in the blood center
Treatment is free, consistent, accessible, and convenient
Increased patient satisfaction: avoid frustration of knowing blood will be discarded
Alleviate national blood shortages
NON-HFE IRON OVERLOAD
Ferroportin disease is the most common hereditary, non-HFE cause of iron overload26; it arises from an autosomal-dominant mutation in a gene coding for ferroportin, which is the main iron export protein in mammals.27 This condition is not restricted to Caucasians and is characterized by elevated ferritin levels despite low normal TS, iron accumulation in organs, and reticuloendothelial macrophages, and marginal anemia with poor tolerance of phlebotomy therapy.
Nonalcoholic fatty liver disease is a common acquired cause of iron overload, often associated with features of the metabolic syndrome such as obesity, hypertension, hypercholesterolemia, and elevated fasting glucose levels or type II diabetes. Subjects present with elevated liver transaminases and ferritin levels, without the degree of elevation in TS that accompanies classic HFE-associated hemochromatosis. Liver ultrasound may demonstrate findings consistent with fatty infiltration; diagnostic histopathologic features of this disorder are present on liver biopsy. Judicious phlebotomy therapy may decrease ferritin and transaminase levels, but the emphasis of patient management should be directed toward the patient’s underlying medical conditions.
FUTURE CHALLENGES
The process of molecular discovery is rapidly leading to a more comprehensive understanding of the role of HFE protein in iron homeostasis. At the same time, the availability of a genetic test has focused increased public and medical attention on hemochromatosis. Robust population screening studies are currently in progress to more accurately determine clinical penetrance, both for early as well as late complications. Increased emphasis on educational campaigns to foster prompt recognition of early symptoms by primary care providers will complement or perhaps even alleviate the need for targeted screening programs. Better appreciation of the advantages of referral to the blood center may improve the quality and accessibility of care and also confer a benefit to the general public health.
References