Uniparental disomy (UPD1) is a fascinating and important pathogenetic mechanism, even though it is the basis of only a small number of well-defined clinical conditions. At the outset, we may list these five syndromes:
· Prader-Willi syndrome
· Angelman syndrome
· Beckwith-Wiedemann syndrome
· Silver-Russell syndrome
· Transient neonatal diabetes
Prader-Willi syndrome, Angelman syndrome, and Beckwith-Wiedemann syndrome can be due to other genetic causes in addition to UPD, and for convenience we include a discussion of these other causes in this chapter. In addition to the above five conditions, certain other UPDs can be the cause of abnormality. These may manifest, in various combinations, the following traits: intrauterine and postnatal growth retardation, intellectual deficit, congenital malformations, and dysmorphic features. In a category by itself, UPD can be the cause of homozygosity for an autosomal recessive gene. The foregoing notwithstanding, however, the fact remains that most UPDs appear to be without any phenotypic consequence, and a number of syndromes that had seemed fair candidates turned out not to be due to UPD (Kotzot, 2002).
BIOLOGY
A distinction should be made between UPD in which both chromosomes are identical (uniparental isodisomy [UPID]) and that in which they are different (uniparental heterodisomy [UPHD]) (Fig. 20-1a). Uniparental disomy is normally demonstrable only at the molecular level: typically, although not invariably, the UPD pair of chromosomes are cytogenetically normal, and the karyotype is normal, 46,XX or 46,XY. The pattern of polymorphic DNA markers shows that both chromosomes have the same haplotype as one of the chromosomes from one of the parents (isodisomy), or the two chromosomes have the same haplotypes as the chromosome pair from one of the parents (heterodisomy). For example, the chromosome 1 haplotypes from parents and child set out in Figure 20-1b show that the child has two identical copies of one of the father's pair, thus paternal uniparental isodisomy. This UPD was discovered fortuitously, when the child was being investigated for a clinical diagnosis of congenital insensitivity to pain, an autosomal recessive disorder (Miura et al., 2000). He proved to be homozygous for the appropriate gene (TRKA, located at 1q21–q22), and his father carried the gene but his mother did not. This scenario, a child with a recessive disorder for which only one parent is heterozygous, is commonly the circumstance behind the discovery of UPIDs that would otherwise have been without clinical effect (discussed further below). The other typical route to recognition of harmless UPDs is an incidental discovery in the course of DNA marker analysis being done for other reasons.
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Figure 20-1. (a) The distinction between uniparental heterodisomy and uniparental isodisomy. The four parental homologs are shown in different patterns. In the child with heterodisomy, the two homologs are different. In isodisomy, they are identical. Meiotic crossing-over can lead to segmental iso/heterodisomy, and the pattern can reveal whether the initial nondisjunction had been at meiosis I or II (see text). (b) The molecular picture of a child with paternal uniparental isodisomy 1. The markers run from D1S468 at the top of chromosome 1 down to D1S2836 at the bottom. Both of the child's chromosome 1 haplotypes are the same, and the same as one of his father's no. 1 chromosomes. He has no chromosome 1 from his mother. (The arrow points to the position of the TRKA locus. Homozygosity for an abnormal TRKA allele was the cause of his having the recessive condition congenital insensitivity to pain, which had led to his ascertainment.) (From Miura et al. (2000), courtesy Y. Indo, and with the permission of Springer-Verlag.) |
The state of iso- or heterodisomy can allow an inference as to the site of the initial chromosomal error. Isodisomy typically reflects a meiosis II nondisjunction or a mitotic error, while heterodisomy is due to nondisjunction at meiosis I. Partial heterodisomy and partial isodisomy can coexist on the same chromosome. For example, a crossover at meiosis I in, say, the distal long arm, followed by meiosis I nondisjunction, could lead to a disomic gamete isodisomic for distal long arm, and heterodisomic for proximal long arm (Fig. 20-1a, lower right). If the nondisjunction were at meiosis II, the isodisomy and heterodisomy would be the other way around, involving the proximal and distal segments, respectively (Fig. 20-1a, lower left).
Epigenetics
An epigenetic effect can determine that the same genotype—the same DNA sequence— will produce different phenotypes. Through genomic imprinting, a chromosomal segment receives an “epigenetic mark” according to the sex of the parent contributing that chromosomal segment. The physical basis of this can be, among other mechanisms, methylation of the DNA (that is, a methyl group attached to cytosine bases). There are certain chromosome segments (in sum, only a small fraction of the whole genome) that are subject to imprinting.2 A segment of chromosome, or perhaps just a single locus, is genetically active or not active (“silent”), according to whether it was transmitted from the mother or from the father, thus a parent-of-origin effect. Imprintable segments (or loci) function monoallelically. That is to say, it is only the segment of maternal origin, or only the segment of paternal origin, as the case may be, which is genetically active. If both segments originate from one parent, there will be either double the amount (biallelic) of expression or no expression, according to the contributing parent. It is this functional imbalance that is the root cause of the phenotypic defect in the UPD syndromes (Kotzot, 1999, 2001). If a chromosome is not subject to imprinting, UPD does not of itself cause abnormality, other things being equal. The only other factor due to UPD, and specifically UPID, which can lead to defect is homozygosity for a recessive gene (isozygosity).
Uniparental Disomy for the Entire Chromosomal Complement (Uniparental Diploidy)
Paternal uniparental disomy (UPDpat) for the full diploid complement, in which all 46 chromosomes are of paternal origin, produces the condition of complete hydatidiform mole. When in addition to a double set of paternally derived chromosomes there is also a haploid maternal set (triploidy, with a total chromosome count of 69), a partial hydatidiform mole results. Hydatidiform mole is discussed in more detail in Chapter 21. Maternal uniparental disomy (UPDmat) for the full diploid complement causes benign cystic ovarian teratoma, an unusual tumor of the ovary in which several embryonic tissues may be represented. It arises in an unovulated germ cell following failure of a premeiotic or of a meiotic cell division (Miura et al., 1999).
In a unique case of partially parthenogenetic chimerism, a 46,XX/46,XY male child with growth asymmetry had complete maternal isodisomy in the 46,XX cell line and biparental inheritance in the 46,XY line (Strain et al., 1995). These authors suggest that an ovum had completed a mitosis on its own, and then one of its daughter cells received the sperm (for the 46,XY line) while the other underwent diploidization (for the 46,XX,upd(mat) line).
Uniparental Disomy for a Complete Chromosome
In UPD for a complete and intact chromosome, both members of a homologous pair come from the one parent. Four routes leading to this state are the following (and see Fig. 20-2):
· Gametic complementation
· Trisomic rescue
· Monosomic rescue
· Mitotic error
Gametic complementation is mentioned first, as the simplest and classic example, but in truth UPD is hardly ever the consequence of a meiotic error happening coincidentally in both parents (Park et al., 1998; Shaffer et al., 1998).
Trisomy rescue or correction3 is the mechanism behind most UPD. The cause of the trisomy is a typical meiotic nondisjunction. The rescue process takes place in a cell of the trisomic conceptus at an early postzygotic stage (possibly even in the zygote), with one of the trisomic chromosomes being discarded, perhaps because of anaphase lag. This enables a cell line within the conceptus to be restored to disomy, but it is the “wrong” chromosome that has been discarded. Purely by chance, the discarded chromosome happens to be the one coming from the normal gamete, and so the remaining two are from the same parent. These two will comprise one of each of the homologs of that parent; thus, uniparental heterodisomy. This would be expected to happen on one-third of occasions, biparental inheritance being maintained in the other two-thirds (close to these ratios was observed in a large study of UPD16; Yong et al., 2002). The 46-chromo-some cell with UPD that results from this process may be the progenitor of the cells that produce the inner cell mass, which in turn gives rise to the embryo. Any remaining trisomic cells may go on to form the placenta, leading to confined placental mosaicism, or they may also contribute to the inner cell mass, leading to trisomy/normal mosaicism of the embryo. Thus, the phenotypes in some UPD states are complicated by the additional effects of compromised placental function and of fetal trisomy mosaicism.
It is interesting that only a single correcting event seems to take place, a conclusion reached from the observation that UPD children show no more than two alleles at tested loci (H. R. Slater, pers. comm., 2002). This might seem surprising: if an embryo still contains trisomic cells, why doesn't a second correction take place? It may be that the conditions obtaining in the first few mitoses, before the inner cell mass has established itself, provide an environment conducive to correction, which is soon lost. The observation that trisomy in postnatal individuals more often exists in the nonmosaic than in the mosaic state—for example, in Down syndrome—indicates that conversion of trisomic tissue to disomy during embryogenesis proper is infrequent. The kinetochore proteins are not transcribed until the 8–16 cell stage, and this might be the basis of an early infidelity of the process of mitosis (and cf. the similar suggestion about the genesis of chaotic mosaicism, p. 387).
Monosomic rescue also comes into play following a nondisjunctional event. If a nullisomic gamete is generated at meiosis, then the conceptus will be monosomic (assuming a normal gamete from the other parent). Mitotic correction then takes place, and this is achieved by replication of the single, normal homolog from the other parent. In this case, the UPD will be an isodisomy (Engel and Antonarakis, 2002). The replication could take the form of an isochromosome (Robinson et al., 1994; Tonk et al., 1996).
The fourth possibility is a mitotic error in an initially normal conception, leading to either trisomy or monosomy. In the case of a trisomy, this is followed shortly thereafter by loss in this cell line of the “wrong” trisomic chromosome. In the case of a monosomy, the remaining homolog is subsequently duplicated.
Note that each of these four scenarios requires that there be two separate abnormal events, occurring either simultaneously (the first scenario) or sequentially (the latter three). These errors can be both meiotic (the first), meiotic followed by mitotic (second and third), or both mitotic (the fourth). In whichever case, the original abnormality will practically always have been a sporadic event, with no discernible increased risk of recurrence due to having had one affected child. Indeed, as yet not one instance is known of a recurrence of UPD in the setting of normal parental karyotypes.
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Figure 20-2. Mechanisms whereby complete uniparental disomy (UPD) may be generated. (a) Gametic complementation, with one parent producing a disomic gamete, and the other a nullisomic gamete. (b) Meiotic nondisjunction in one parent to produce a disomic gamete, with a trisomic conceptus following fertilization, and subsequent mitotic loss of the homolog from the other parent. This is uniparental heterodisomy, from the parent in whom the nondisjunction had taken place. (c) Meiotic nondisjunction in one parent to produce a nullisomic gamete, with monosomic conceptus following fertilization, and subsequent mitotic reduplication of the homolog from the other parent. This is uniparental isodisomy, from the parent who contributed the normal gamete. The reduplication may produce a free homolog, or an isochromosome. Since most meiotic nondisjunction occurs in maternal gametogenesis, the asterisked gametes in (b) and (c) can be imagined to be oocytes, with UPD(mat) and UPD(pat) resulting accordingly. (d) Two sequential mitotic errors. |
One risk factor is known, and this is increasing maternal age. The link here is that meiotic nondisjunction, the root cause of most UPD, is more prevalent in women of older childbearing age. The meiotic errors noted above as leading to trisomic rescue and monosomic rescue are typically of maternal origin. Ginsburg et al. (2000) have shown that maternal age is higher in the subset of patients with Prader-Willi, Angelman, and Russell-Silver syndromes due to UPD than in those due to other causes. A reduced level of recombination is seen in UPD 15 (Robinson et al., 1998), an observation also made in the classic disorder with a maternal age association, namely Down syndrome. It is worth noting that paternal UPD also has a maternal age effect, a seemingly contradictory statement that can be appreciated when considering the mechanism of monosomic rescue that is the usual cause of this type of UPD.
Rare mechanisms to generate complete UPD include the following:
· Correction of interchange trisomy
· Correction of interchange monosomy
· Isochromosome formation
· Correction of imbalance due to extra structurally abnormal chromosome(s) (ESAC)
If one parent carries a reciprocal or Robertsonian translocation, asymmetric segregation of the chromosomes may lead to an interchange trisomy (p. 79) at conception, in which the translocation chromosomes or chromosome, plus one of the normal homologs, are transmitted. Postzygotic correction by the loss of one homolog restores disomy, but if it is the other parent's chromosome that is lost, UPD is the consequence. Or, if a nullisomic gamete meets a normal gamete, the normal gamete may replicate the homolog in question, to restore disomy (just as in monosomy rescue, above). In the case of an acrocentric chromosome with a Robertsonian translocation parent, it could replicate as an isochromosome (Berend et al., 2000a; McGowan et al., 2002). Kotzot (2001)recorded 22 examples of UPD associated with a reciprocal or Robertsonian translocation, involving UPDs for chromosomes 7, 13, 14, 15, and 16. Complementary isochromosomes (p. 140), of which only a sin-gle-digit number have ever been described, can even allow the circumstance of “contraposed UPD”—that is, there may be UPD of the p arm from one parent, and UPD of the q arm from the other. Finally, a 47,+ESAC karyotype may have a coexisting UPD for the same chromosome from which the ESAC was derived. It may be that the zygote from a 23,ESAC + 23,N → 46,ESAC conception attempted to correct the imbalance, or at least lessen it, by replication of the normal homolog (James et al., 1995).
Segmental Uniparental Disomy
Segmental UPD may arise as the consequence of a postzygotic somatic recombination, between the maternal and paternal homolog (Fig. 20-3). The UPD segment lies distally, the rest of the chromosome having a normal biparental disomy. The karyotype is normal. An alternative mechanism is the following sequence: meiotic nondisjunction produces a disomic gamete, a trisomic conception occurs, a mitotic crossing-over takes place between a maternal and a paternal chromatid, and, finally, one of the chromosomes that had come with the disomic gamete is lost (Kotzot, 2001). Segmental UPD can have an effect if the particular chromosomal segment incorporates loci subject to imprinting. If the recombination occurs in a cell after the formation of the inner cell mass (which gives rise to the embryo), the segmental UPD will involve only some cells; in other words, there is mosaic segmental UPD. Beckwith-Wiedemann syndrome (BWS), Silver-Russell syndrome (SRS), and transient neonatal diabetes mellitus (TNDM) are conditions in which segmental UPD may apply.
A partial trisomy might have different abnormal phenotypic effects according to the parental origin of the duplicated segment if that segment is subject to imprinting. Trisomy for distal 14q provides an example. A similar picture of dysmorphology and psychomotor deficit is seen in either paternally or maternally originating 14q trisomy. But low birth weight, sometimes less than 2000 grams for a full-term baby, is a specific observation when the duplicated 14q segment comes from the mother (Georgiades et al., 1998).
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Figure 20-3. A mechanism whereby segmental uniparental (iso)disomy may be generated. In one cell of the early conceptus, the paternal (pat) and maternal (mat) homologs of a chromosome pair (a) undergo somatic recombination between the short arms (b,c). Segregation at mitosis (d) produces daughter cells with segmental UPD: in one (e), the short arm distal segments of both chromosomes are now of paternal origin, and in the other (f), they are both of maternal origin. These cells can then be the source of segmentally UPD tissue in a part of the conceptus.10 |
Aberrant Imprinting
A normally imprinted chromosomal segment may loose its imprint and become active: this is “relaxation” (or erasure) of the imprint effect. Consider BWS. In some BWS cases with normal biparental inheritance of the no. 11 chromosomes, the IGF2 (insulin-like growth factor 2) locus on distal 11p shows biallelic expression; normally, only the paternal allele should be functional. This overexpression of a growth factor is a likely contributor to the overgrowth that is characteristic of the syndrome. Prader-Willi syndrome due to upd(15)mat provides a possible example of the same process operating in the other direction. A degree of relaxation of imprinting may allow some small expression of the otherwise imprinted 15q segment, and, consequently, the phenotype is somewhat mitigated (mentioned also below). An iatrogenic cause of aberrant imprinting may in theory relate to pregnancy following assisted reproductive technology (De Rycke et al., 2002), and associations with Beckwith-Wiedemann syndrome and Angelman syndrome are noted below.
UNIPARENTAL DISOMY PHENOTYPES
Uniparental disomy has been observed for most chromosomes, with only chromosomes 12, 18, and 19 yet awaiting recognition. For some cases, probably most, there is no apparent phenotypic consequence. For others, there may be some resultant disorder, and we list below some of the proposed syndromes of UPD. The reader seeking more detail is referred to Engel and Antonarakis (2002). Some conditions, such as Cornelia de Lange syndrome and Sotos syndrome, in which UPD had been thought of as a fair possibility have turned out not to be (see Chapter 17). In the case of UPD arising from failed trisomic rescue, trisomy of the placenta and/or a residual low-level trisomy of the fetus may also contribute to the eventual phenotype. De Pater et al. (1997) note that
a fetal trisomic cell line may not be detected unless the possibility of mosaicism is painstakingly pursued, and Benn (1998) uses the expression “occult mosaicism” to denote an unprovable suspicion. Because mosaicism can never be completely excluded, nor can homozygosity for an unknown gene, one should generally suppose that there is an absence of any UPD effect when instances of both normal and abnormal phenotypes are known, or when the observed abnormalities are inconsistent (Kotzot, 1999). The abnormal phenotypes will more likely be due to non-UPD mechanisms.
Certain clinical groups might be considered candidates to harbor cases of UPD. Intrauterine growth retardation (IUGR) is an obvious category. Moore et al. (1997) addressed the question in a study of a cohort of 35 severely affected babies with a 46,N karyotype. Two instances of UPD (5% of the total) were identified, both with upd(16), and in each a coexisting placental 47,+16/46 mosaicism was shown. Neither baby survived. Kotzot et al. (2000a), in checking chromosomes 2, 6, 14, 16, 20, and 22, found no instances of UPD in a series of 23 cases of IUGR, using a broader definition of birth weight and/or length below the 10th centile. Eggermann et al. (2001a) studied 21 patients with pre- and postnatal growth retardation, choosing chromosomes 2, 7, 9, 14, 16, and 20 for analysis, and identified one with upd(14)mat and one with upd(20)mat. Except for the upd(20)mat, which may or may not have been the cause of the child's “minor features,” it should be noted that only known UPDs (for 7, 14, 16) were identified in these and other such surveys.
“Unclassified congenital developmental defects” was the criterion for entry to the study of Ginsburg et al. (2000), comprising a cohort of 50 individuals whose mothers were 35 years of age or older at the time of delivery. This sort of patient is, of course, very familiar to the genetic counselor. Four turned out to have a UPD. The specific UPDs were maternal heterodisomy 14 in a woman with short stature and early puberty; paternal isodisomy 15 in a boy with previously undiagnosed (but retrospectively apparent) Angelman syndrome; upd(16)mat along with a partial 16p trisomy; and a child with Silver-Russell syndrome having a upd(7)mat. Four out of 50 (8%) may be a higher fraction than could usually be expected, and it is easy to be wise after the event that two of the cases were not really unclassifiable. A lower fraction, namely 0%, was observed in a larger study of 120 children with two or more malformations, developmental or growth retardation, and a normal routine karyotype (Rosenberg et al., 2001). To investigate a possible contribution of UPD to spontaneous abortion, Shaffer et al. (1998) studied 18 cytogenetically normal cases from first-trimester miscarriage, but none showed UPD. In a more extensive study, Fritz et al. (2001a)analyzed products of conception along with parental blood samples, which included some 77 spontaneous abortions with normal karyotypes. Of these, only two showed UPD: one case of maternal UPHD 9, and one of paternal UPID 21. Thus, from these two reports, only 2% of karyotypically normal spontaneous abortion is associated (whether or not this be causally) with UPD.
One particular potential phenotypic effect is due to the reduction to homozygosity of a recessive gene, as noted earlier and as shown in Figure 20-1b. According to a standard scenario, this would require the coincidence of a series of events such as the following: one parent is heterozygous for a certain recessive gene; that parent produces a disomic gamete, isodisomic for the segment of chromosome containing the gene; the chromosome from the other parent is lacking in the gamete or is discarded after fertilization. Perhaps more often, the initiating event is a meiotic nondisjunction in the parent who is not heterozygous, with a nullisomic gamete being produced. The gamete from the other (heterozygous) parent then replicates itself, as in monosomic rescue described above. Engel and Antonarakis (2002) list 22 reports in which this sort of mechanism has been identified, including one notable example in which a child had both cystic fibrosis and Kartagener syndrome, the loci for these two separate recessive disorders lying on chromosome 7. The most extraordinary case is that of a couple, both normal homozygotes, whose child had maple syrup urine disease due to fresh mutation in oogenesis, with meiosis II nondisjunction then producing an isodisomic ovum (Lebo et al., 2000).
Chromosome 1
Maternal UPD of chromosome 1 may have of itself no effect (provided no recessive genes are unmasked, as exemplified by Miura et al., 2000, and illustrated in Fig. 20-1b). Field et al. (1998) made the serendipitous discovery of UPD 1 in a normal diabetic adult in the course of a genetic study of diabetes, as did Miyoshi et al. (2001) in their investigation of two normal persons with anomalous Rh blood grouping results. In the former, upd(1)mat was found; in the latter, mosaicism for paternal isodisomy 1. Unmasking of recessive genes, rather than an effect of imprinting, may have been the basis of phenotypic abnormality in a unique case of upd(1)pat described in Chen et al. (1999c). A woman of normal intelligence had a myopathy, short stature, sterility, and deafness. In this case, there was a paternal isodisomy, with the no. 1 elements present in the form of two isochromosomes, i(1)(p10) and i(1)(q10).
Chromosome 2
It is still unclear whether a maternal UPD 2 syndrome exists, but there is a tentative case (Engel and Antonarakis, 2002). Confined placental mosaicism for trisomy 2 may be a cause of IUGR (Wolstenholme et al., 2001b).
Chromosome 3
There is a single case from the earlier literature (Betz et al., 1974) of a retarded girl who was homozygous for a rare cytogenetic polymorphism that was carried by only one parent, which might possibly be an example of upd(3).
Chromosome 4
Only one case of complete upd(4)mat is on record, associated with trisomy 4 confined placental mosaicism and presumed postzygotic correction of trisomy in the embryo (Kuchinka et al., 2001). Trisomy 4 was discovered at direct chorionic villus analysis, with a 46,XX karyotype apparent on long-term villus culture and at amniocentesis. Growth retardation occurred in the latter part of the pregnancy, and the baby was stillborn at 30 weeks. It cannot be determined whether the placental trisomy or the fetal uniparental disomy, or both, had been the cause.
Chromosome 5
Paternal UPD 5 may be of no effect, other than that due to isozygosity (Engel and Antonarakis, 2002).
Chromosome 6
The defining feature of TNDM is hyperglycemia requiring treatment with insulin, with a gradual resolution to normal glucose metabolism in the first few months of life, but a subsequent risk for non–insulin-dependent diabetes in adult life. In at least a substantial fraction of cases, TNDM is due to upd(6)pat, with 6q23–q24 being the crucial region. A methylation defect within the critical region, or 6q23–q24 duplication of paternal origin, this latter accounting for all familial cases, are other routes whereby TNDM may arise (Temple and Shield, 2002). But upd(6)pat can also be without apparent effect, as shown in an otherwise normal girl with thalassemia whose family was being studied to find a donor for marrow transplantation, and who turned out to have paternal UPID 6 (Bittencourt et al., 1997). Barely a dozen cases have been reported (Kotzot, 2001; Engel and Antonarakis, 2002). One example due to a familial insertion involving the segment 6q22–q23 is mentioned on p. 169.
Chromosome 7
Silver-Russell syndrome has as its major features intrauterine and postnatal growth retardation, limb asymmetry, and a degree of cognitive compromise. Maternal UPD 7 is recognized as the cause in a minority of cases, about 5%–10% (Eggermann et al., 1997b; Russo et al., 2000). The clinical picture may differ a little from non-UPD cases of SRS (Hannula et al., 2001). Both distal 7q (q31qter) and 7p12-p14 have been invoked as the putative “SRS region”; alternatively, each segment may, separately, be a SRS region (Eggermann et al., 2001b; Hitchins et al., 2001; Bentley et al., 2003). There is a maternal age association: very few SRS children born to mothers under age 35 have UPD, but around half of those born to mothers 35 or over are due to upd(7)mat, and this may reflect a maternal meiosis I error as the underlying cause (Ginsburg et al., 2000). A concomitant heterozygosity for a recessive gene on the maternal chromosome 7 due to isodisomy 7 may cause the child to have both SRS and the particular recessive disease (Hehr et al., 2000). One case is recorded of SRS in the setting of a maternal reciprocal translocation involving chromosome 7: the conception was probably an interchange trisomy, 47,+7,t(7;16) (q21;q24), with subsequent loss of the paternal chromosome 7 producing the balanced state but with a maternal UPHD 7 (Dupont et al., 2002).
As for paternal UPID 7, in two cases only have the effects of isozygosity been recognized (Engel and Antonarakis, 2002). One woman had normal linear growth congruent with that of her sisters, and a normal intellect, attested by her being a student at a business school. It was only because she had a recessive condition with its locus on chromosome 7 (congenital chloride diarrhea) that she had been investigated (Höglund et al., 1994).
Chromosome 8
Complete paternal isodisomy 8 is apparently without any phenotypic effect, and one may suppose that this reflects a lack of imprinted genes on this chromosome. Benlian et al. (1996) made the fortuitous discovery of upd(8)pat in an otherwise normal child with lipoprotein lipase deficiency, a recessive condition for which the locus maps to 8p22. Similarly, Karanjawala et al. (2000) discovered maternal isodisomy 8 by chance in a man participating in a diabetes research study. He was himself nondiabetic, although he did have the unusual history of a neuroendocrine gut tumor (carcinoid) at an atypically young age.
Chromosome 9
Maternal UPD 9 appears to be without effect (Björck et al., 1999; Engel and Antonarakis, 2002). Homozygosity at the SURF-1 locus due to isodisomy 9 is documented in twins with Leigh syndrome (Tiranti et al., 1999).
Chromosome 10
Maternal UPD 10 appears to be without effect (Jones et al., 1995).
Chromosome 11
Mosaic segmental upd(11p)pat is the basis of about 20% of sporadically occurring cases of BWS (Li et al., 1998a; Engel et al., 2000; Maher and Reik, 2000). In BWS, the striking clinical picture is that of overgrowth of tissues and organs. There are growth regulation loci in 11p15 that are considered to be “BWS genes,” and some of these are under the aegis of putative control elements that exert their influence epigenetically. These loci include the interacting and reciprocally imprinted insulin-like growth factor 2 (IGF2) and H19 genes (which are normally expressed from paternal and maternal chromosomes respectively), the p57kip2 (also known as CDKN1C) gene (maternally expressed), and the LIT1 (also KCNQ1OT) antisense transcript of the KVLQT1 gene (paternally expressed). The IGF2 gene promotes growth, while the H19 and p57kip2 genes have growth suppressor qualities. Biallelic expression of the IGF2 gene or nulliallelic expression of the H19 and/or the p57kip2 genes is a plausible pathogenetic mechanism for the excessive growth, this being the consequence of each of the two distal 11p chromosomal segments functioning paternally. Hemihyperplasia is a clinical indicator of this category, and those tissues with the greater fraction of UPD 11p cells may show a greater degree of overgrowth. Itoh et al. (2000) describe a child with BWS having a normal adrenal gland on the right side and a very enlarged one on the left: 30% of cells in the right gland had upd(11)pat, compared with 88% on the left. Epigenetic mechanisms exist due to other than UPD, noted in the Genetic Counseling section below, and Figure 20-4 outlines the different causes of BWS.
Paternal UPD 11 for the whole chromosome, following postzygotic rescue of trisomy 11, has been associated with severe intrauterine growth retardation (Webb et al., 1995). A single case of mosaic complete paternal UPD 11 has been seen with typical BWS (Dutly et al., 1998b). As for maternal UPD 11, the only recorded case, which involved mosaic upd(11q13–qter), is that of Kotzot et al. (2001b), concerning a dysmorphic and severely retarded woman, in whom the picture was complicated by a concomitant partial trisomy 11q13–qter mosaicism.
Chromosome 13
Neither maternal nor paternal UPD 13, iso- or heterodisomy, appears to have any effect on the phenotype (Berend et al., 1999; Soler et al., 2000). A unique example of familial UPD 13, paternal and maternal, emphasizes this point: a normal mother with presumed 45,XX, i(13q)pat had a normal child with 45,XY, i(13q)mat (Slater et al., 1995). She may have been the result of monosomic rescue, and her son, of trisomic rescue!
Chromosome 14
UPD 14 produces different syndromes according to the paternal or maternal basis of the disomy (Sutton and Shaffer; 2000; Engel and Antonarakis, 2002; Kamnasaran and Cox, 2002; Kurosawa et al., 2002; McGowan et al., 2002). Either may be seen in the setting of a normal karyotype or with a Robertsonian translocation (or Robertsonian isochromosome). A balanced 45,der(13;14) Robertsonian translocation may reflect correction of an initially 46,der(13;14), 14 conception, while the 45,der(14;14) case might in fact result from a 45,14 conception which then corrected by reduplication of the single chromosome 14 to give an i(14q) with isodisomy. Paternal UPD 14 is the more severe, with obstetric complication (polyhydramnios and premature labor), a particular pattern of malformation, growth retardation, and major functional neurological compromise. Survival is poor. Maternal UPD 14 produces a syndrome of pre- and postnatal growth retardation, a characteristic facies (to which the effects of an arrested hydrocephalus may contribute), and intellectual development that may be low–normal to normal. Other possible contributors to the abnormal phenotype include a concomitant low-level trisomy 14 mosaicism and isozygosity for a recessive gene. With respect to the latter, Kamnasaran and Cox (2002) note that the similarity of phenotypes between uphd(14) and upid(14) favors a role solely for the UPD of itself.
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Figure 20-4. The no. 11 chromosomes in different chromosomal bases of Beckwith-Wiedemann syndrome (BWS). The maternal homolog is shown open, the paternal homolog is dotted, and the BWS critical region at 11p15 is shown cross-hatched. Another chromosome is shown gray. (a) The normal state of biparental inheritance of intact no. 15 chromosomes. (b) Paternal (pat) duplication of distal 11p. (c) Maternal (mat) translocation disrupting the BWS critical region. (d) The two cell lines in mosaic segmental paternal UPD of 11p. |
Chromosome 15
Prader-Willi syndrome (PWS) and Angelman syndrome (AS) are the two UPD 15 syndromes. PWS is due to absent activity of a set of genes under the control of the complex SNRPN gene.
Two candidates for this set are MAGEL2, which is expressed in the brain, and necdin, a gene associated with appetite control (Lee et al., 2000). The key component within the SNRPN locus may be a 120 kb segment that includes sequences coding for certain small nucleolar RNAs (Gallagher et al., 2002). AS is due to absent activity of the UBE3A gene. These several genes all reside in the segment 15q11–q13. The absence of activity is due to either the loss or the nonfunctioning of this region on one chromosome 15. Loss is most commonly due to a simple interstitial deletion (classical deletion). Certain sequences on either side of this region are prone to come together, and in doing so they set the stage for the looping out and deletion of the interstitial segment, after which the breakpoints unite (Ji et al., 2000). Whether the phenotype comes to be PWS or AS depends on which parent contributed the deleted chromosome. Nonfunctioning of structurally normal genes within 15q11–q13 is due the imprint status. This is most commonly the consequence of UPD 15, with the phenotype determined according to the parent of origin of the disomic pair of chromosomes. A rare cause is failure of, or damage to, the chromosome 15 imprinting center (IC). Study of these IC-damaged cases—only about 140 worldwide—has cast much light on the processes of molecular pathogenesis in PWS and AS, and so the length of their commentaries below is quite out of proportion to their frequencies. In the case of AS, mutation in the UBE3A gene is a third category of mechanism.
The 15q11–q13 Imprinting Center. Normal persons have one paternally imprinted no. 15 and one maternally imprinted chromosome 15. The imprinting state of a chromosome 15is set and reset as it is transmitted down the generations, according to the sex of the transmitting parent. This resetting, an “epigenetic modification,” is dictated during gametogenesis from the cis-acting IC, with methylation of genes comprising, in large part at least, the crux of the process. The IC is bipartite, with a centromeric element, the AS-IC, and 35 kb distant a telomeric element, the PWS-IC; this latter part includes exon 1 of SNRPN. Interaction between these two elements directs the process. In maternal gametogenesis, the AS-IC has responsibility for initiating a paternal → maternal switch on the chromosome 15 that the mother herself received from her father. The chromosome15 she got from her mother retains (or is restored to) a maternal imprint. With an active AS-IC, the UBE3A gene, lying about 1 Mb distant, is free to function. Paternal gametogenesis serves to effect a maternal → paternal switch, or to retain a paternal status. Consequently, a number of genes under its aegis are able to function in part at least by being demethylated. The UBE3A gene's activity is prevented. These epigenetic modifications operate only in cis, and so the maternal and paternal chromosomes continue to function autonomously, with their different repertoires of expression, during the life of the individual.
A scheme for the various molecular defects of PWS and AS is presented in Figure 20-5; the different genetic categories of AS have been designated types I through V. An algorithm for a logical testing procedure for clinically suspected AS is set out in Clayton-Smith and Laan (2003).
Classical Deletion. This is the most frequent basis of the two syndromes, accounting for about 70% of both PWS and AS. The deletion removes about 4 Mb within 15q11–q13, encompassing the PWS and the AS structural genes and their regulatory sequences, including the IC. Most deletions are of about the same size. There are three common deletion breakpoint regions, and these map to the regions of an END duplicon, illegitimate recombination between these duplicons being the underlying cause of the deletions.4 If the deletion occurs on a paternally originating chromosome, it will cause the PWS phenotype to develop5; and a maternal deletion produces AS. In a sense, there is an “unmasking of the silent alleles” on the other chromosome. In addition to the crucial alleles in PWS (the “client genes” under SNRPN's control) and AS (UBE3A and possibly others), a number of other loci may be deleted, and so the expression “contiguous gene syndrome” is not inappropriate, although it has a somewhat different sense from its usage elsewhere in this book. One of the least important of these other loci is the P gene that contributes to normal pigmentation, and so children with PWS and AS due to classical deletion typically have fairer complexions than do their siblings.6 Mosaicism may lead to a milder phenotype (Golden et al., 1999; Tekin et al., 2000).
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Figure 20-5. An outline of the different genetic forms of Prader-Willi syndrome (PWS) and Angelman syndrome (AS). The PWS/AS critical region of chromosome 15 is depicted. A bipartite imprinting center (IC) with AS and PWS components (AIC and PIC) controls, in cis, the activity of a set of PWS genes and the UBE3A gene. A switched-on IC and an actively functioning gene are shown in unbroken outline; a switched-off IC and an unactivated gene are shown in dashed outline. A mutated UBE3A gene is asterisked, with a dotted outline. (1) Normally, the UBE3A gene is transcribed only from the maternal chromosome (mat), and the PWS genes only from the paternal chromosome (pat), with each chromosome thus functioning appropriately for its parent of origin. In PWS there is non-functioning of the PWS genes because of one of the following: (2) the PWS genes have been removed by a typical large deletion from the paternal chromosome; (3) both chromosomes are of maternal origin; or (4) a microdeletion of the PIC has fixed a maternal imprint status on the paternal chromosome. In AS there is nonfunctioning of the UBE3A gene because of one of the following: (5) the UBE3A gene has been removed by a typical large deletion from the maternal chromosome; (6) both chromosomes are of paternal origin; (7) a microdeletion of the AIC has fixed a paternal imprint status on the maternal chromosome; or (8) there is a mutation in the UBE3A gene on the maternal chromosome. A further category (9) is not shown, comprising the 10%–15% in which no genetic defect can be shown. Approximate percentages of each PWS/AS category are indicated; in another ~10% of AS no genetic defect can be identified. matP, a paternally functioning chromosome of maternal origin; patM, a maternally functioning chromosome of paternal origin; UPD, uniparental disomy. |
Prader-Willi and Angelman Syndromes due to Deletion, Associated with Uncommon Rearrangement. Loss of the PW/AS region can be due to transmission of an unbalanced translocation or an inversion involving chromosome 15. The male carrier of a balanced reciprocal translocation in which one breakpoint is in the region of 15q13 can transmit an unbalanced complement to produce a deletion PWS child (Hultén et al., 1991; Smeets et al., 1992), and the female carrier can have a child with deletion AS (Stalker and Williams, 1998). There may be an additional effect from the concomitant imbalance involving the other chromosome of a translocation, such as the case in Wenger et al. (1997)of a child with AS and 8p monosomy due to tertiary monosomy from a maternal t(8;15). Meiotic recombination in the carrier can produce a rec chromosome (as distinct from a der) in which the PWS/AS region is deleted (Chan et al., 1993; Clayton-Smith et al., 1993; Horsthemke et al., 1996). A de novo unbalanced translocation having a deletion in 15q11–q13 would cause PWS, if of paternal origin, and AS, if of maternal origin (Fig. 20-6). A more extensive deletion of paternal origin, involving the no. 15 or the other chromosome, could worsen the clinical picture in PWS (Smith et al., 2000a); contrariwise, a very small deletion might be associated with an attenuated phenotype (and study of which cases enables demarcation of the borders of the actual PWS-determining region) (Gallagher et al., 2002). A handful of PWS cases have been due to a Y;15 translocation with breakpoints in Yp and at 15q12–q13, deleting the PWS region, having the karyotype 45,X, der(Y),t(Y;15) (Vickers et al., 1994).
Uniparental Disomy. About 25% of PWS, and only 3%–5% of AS, is due to UPD. The cytogenetic study typically shows a normal 46,XX or 46,XY karyotype. The phenotype depends on the parent of origin of the disomic pair of no. 15 chromosomes.
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Figure 20-6. A (7;15) translocation in which the Prader-Willi syndrome (PWS) region is deleted; the child had PWS. The karyotype is 45,der(7)t(7;15)(q36.3;q13),-15. (Case of V. Petrovic, from Jauch et al., 1995.) Compare and contrast with the similar rcp(8;15) translocation in Figure 7-5, in which the 15q breakpoint is proximal to the PWS/AS region, and so the 45,t(8;15) carrier parent has a functionally balanced rearrangement. |
In PWS due to UPD, both chromosome 15s come from the mother, and so neither of the PWS critical regions is expressed. This functional lack causes the PWS phenotype. In most (80% or more) cases, the UPD had its origin in a maternal meiosis I nondisjunction. A maternal age effect is clear: five times as many PWS children born to mothers under age 35 have a deletion as have UPD, but the reverse applies to those born to mothers 35 or over, in whom there is a fivefold excess of those showing UPD (Ginsburg et al., 2000). The phenotype is very similar to classical deletion PWS, although the facies may be less typical with the UPD form of PWS, and some of the minor manifestations are less likely to occur. Consequently, diagnosis of PWS due to UPD may be delayed in comparison to deletion PWS (Cassidy et al., 1997; Gunay-Aygun et al., 1997). Rogan et al. (1998) actually demonstrated expression of some 15q11–q13 genes in two PWS children with maternal UPD 15 in whom the clinical picture was atypical (no excess appetite, apparently normal mental development).
A more severe form of PWS is associated with upd(15)mat, but having the additional factor of a trisomy 15 mosaicism (Olander et al., 2000). The initial error is a maternal nondisjunction leading to a trisomic 15 conception. Post-conception correction with loss of the paternal no. 15 generates a (uniparental) disomic 15 cell line, but the original trisomic line still makes a significant and deleterious contribution to the quantum of tissue giving rise to the embryo. The phenotype due to the trisomy 15 mosaicism of itself is illustrated in a case with biparental disomy in the normal cell line (Gérard-Blanluet et al., 2001).
In AS due to UPD, both chromosome 15s are from the father, and neither chromosome expresses the AS critical region. Most cases involve a postzygotic origin of the extra paternal chromosome, possibly following the correction of monosomy 15 due to a nullisomic ovum. Again, this is likely to reflect a maternal age effect, as discussed above with respect to PWS. Very few AS children born to mothers under age 35 have UPD, but those born to mothers age 35 or over have deletion and UDP in about equal numbers (Ginsburg et al., 2000). A few are due to a paternal second meiotic error (Robinson et al., 2000). In parallel with the observations in UPD PWS noted above, the phenotype in AS due to UPD is not quite as severe as in the deletion form, with these children showing a lesser frequency of seizures, and some being able to say a few words (Fridman et al., 2000). But it remains true that the handicap is severe.
Prader-Willi and Angelman Syndromes due to Uniparental Disomy, Associated with Uncommon Rearrangement. Uniparental disomy can result from a variety of rearrangements involving chromosome 15. The male carrier of a reciprocal translocation involving chromosome 15 could transmit a disomic 15 spermatocyte from 3:1 nondisjunction, with the maternal chromosome 15 then being lost, and have a child with UPD AS; his carrier sister could have a PWS child (Smeets et al., 1992; Toth-Fejel et al., 1996). Similarly, a familial nonhomologous Robertsonian translocation in which one of the component chromosomes is a no. 15 giving a trisomic 15 conception, and with postzygotic loss of the no. 15 from the other parent, would lead to upd(15) with either PWS or AS, according to the sex of the carrier parent (Fig. 6-6) (Nicholls et al., 1989; Smith et al., 1993; Toth-Fejel et al., 1996). The same thing could happen if the translocation were de novo. A maternally originating de novo homologous der(15;15) (which may actually be a 15q isochromosome), with no chromosome 15 contributed from the father, would cause PWS (Robinson et al., 1994), and, vice versa, AS would result from a paternal isochromosome 15q (Tonk et al., 1996). Smith et al. (1994) describe AS from asymmetric segregation of a paternal 8;15 translocation (Fig. 7-5). The heterozygous father passed on his der(8) and his normal chromosome 15 (thus, paternal UPD), and there was absence of a maternal chromosome 15. Some PWS children with a 47,+idic(15) karyotype may actually have UPD of the two intact no. 15s, and the idic (15) is a phenotypically irrelevant relic of the original process of abnormal chromosomal behavior (Robinson et al., 1993b).
Imprinting Center Defects. A very small group of PWS and AS patients, about 1% and 2%–4% respectively, have normal biparental inheritance and no classical deletion but a uniparental pattern of methylation and gene expression (Buiting et al., 2003). Most of these cases reflect abnormal function of the IC, while a minority, about 10%–15%, have an actual IC microdeletion. The latter category can be strongly suspected when there is a positive family history, while in the former, sporadic occurrence has been universally observed. Whether PWS or AS is seen depends on which component of the IC is deleted or nonfunctional.
Functional Imprinting Center Defect. Buiting et al. (2003) analyzed 44 PWS and 76 AS patients with a failure of IC functioning, an IC deletion or point mutation having been excluded; these aberrant epigenetic states are referred to as epimutations.7 Some shared with an unaffected sibling the 15q11–q13 haplotype on their paternal (PWS) or maternal (AS) chromosome, supporting the presumption of a de novo defect. With PWS, the structure of pedigrees suggests the error may lie in spermatogenesis, and the basis of the epimutation may be a failure to erase the maternal imprint, as an act of omission (Buiting et al., 2003). Thus, for example, the father of such a PWS child passes on his maternal chromosome 15 with its maternal imprint still in place, and the child inherits two maternally imprinted no. 15 chromosomes. In AS, the typical scenario may be the imposition of an anomalous imprint status. This can be thought of as an act of commission: the mother inappropriately applies a paternal imprint to the chromosome 15 she passes to the child, or (since some maternal epimutations are mosaic) the error may occur postzygotically.8 If the error is incomplete, a milder AS phenotype may be seen (Brockmann et al., 2002).
Microdeletion of Imprinting Center. Microdeletions of the IC, generally of kilobase size,9 remove one or the other of its major component parts, the PWS-IC or the AS-IC. The inability to reset an appropriate imprint status leads to the “fixation of an ancestral epigenotype” (Saitoh et al., 1997). Only a handful of cases have been identified worldwide (Ming et al., 2000; Buiting et al., 2000). Their particular importance to the counselor lies in the high recurrence risk: the mode of inheritance is essentially sex-influenced on the basis of autosomal dominant transmission, with a 50% risk for the heterozygous father (for PWS) or the heterozygous mother (for AS), according to which component part of the IC is deleted. Fresh mutation is recorded.
In PWS due to IC microdeletion, the father would have received the deletion on his mother's chromosome 15. He is normal, since an erased paternal imprint on his maternal chromosome is, naturally, correct. The deletion could have originated in his mother or antecedent to her, provided transmission had been exclusively matrilineal. But when he passes this chromosome 15 with its fixed maternal epigenotype to a child of his, with the maternal → paternal imprint switch unable to function, the child has a functional maternal UPD 15. Such a family is illustrated in Ming et al. (2000). Of 10 children, all of them normal and with normal karyotypes on standard cytogenetics, 4 inherited an IC microdeletion, presumably from their deceased mother (their father was proven not to have the deletion). Two of these children were male, and each went on to have a child with PWS in the next generation, an example of grandmatrilineal inheritance.
The laboratory demonstration of an IC deletion is complex, and one or some of SNRPN methylation analysis, FISH on metaphase chromosomes, and gene sequencing can be applied (Buiting et al., 2003). In IC-deletion PWS, SNRPN methylation shows absence of a paternal (unmethylated) band in the affected child. The diagnosis of an inherited IC defect is confirmed if the father, and possibly some of his siblings and other normal relatives through the maternal line, show absence of a maternal band (which has no untoward effect in these persons, since a maternally originating chromosome 15 would in any event have its SNRPN gene inactivated). There are traps in interpretation, as witness the need to recognize a rare neutral variant (Buiting et al., 1999a). A harmless deletion of this sort was discovered on one chromosome of a transmitting father in a PWS family, but who carried a pathogenic IC deletion of ~200 kb on his other chromosome 15 (Buiting et al., 2000). Imprinting center analysis is for aficionados!
In AS due to IC microdeletion, the scenario is essentially the obverse of the above. A microdeletion on the maternal chromosome 15 removes the AS-IC (one case involved a molecular inversion [Buiting et al., 2001]). The defect may have arisen de novo from the maternal grandfather of the AS child, or alternatively, there could have been patrilineal transmission of the mutation, harmlessly, for any number of previous generations. Transmission from the grandfather to the mother would be without phenotypic consequence, since a paternally originating chromosome 15 would in any event have its AS-IC inactivated. But in oogenesis in the mother, the normal paternal → maternal switch on the abnormal chromosome can not be effected (thus, fixation of the ancestral paternal epigenotype). If the child receives this chromosome 15 from the mother, both homologs carry a paternal imprint. Consequently, the child has AS. In terms of molecular pathogenesis, the assumption is that the microdeletion is responsible for failure of activation of the UBE3A gene (Buiting et al., 1999b).
Prader-Willi and Angelman Syndromes due to Imprinting Center Abnormality, Associated with Uncommon Rearrangement. Disruption of the SNRPN gene can cause PWS, albeit possibly in a somewhat milder form. Kuslich et al. (1999) describe a de novo translocation t(4;15)(q27;q11.2) of paternal derivation having its chromosome 15 breakpoint between exons 2 and 3 of SNRPN, and note three other similar cases. SNRPN was expressed only from exons 1 and 2. Presumably, all the PWS genes that SNRPNinfluences are present, some on the der(15) and some on the der(4). Those on the der(4) are no longer in linear contiguity with the IC and are thus beyond its control. Intactness of these individual genes is apparently insufficient of itself to prevent PWS; a normally functioning SNRPN region is necessary. Concerning AS, one reported child had an apparently balanced para-centric inversion 15(q11.2q24.3)mat, and possible pathogenetic mechanisms include direct disruption of an AS gene; separation of an AS gene(s) from the influence of the IC, or apposition of an AS gene(s) to an element at 15q24.3 which blocks its expression (Greger et al., 1997). Presumably the inversion chromosome was of paternal origin in the AS child's mother, and so the damage/silencing of the AS gene(s) in her was without effect.
Angelman Syndrome Due to Structural Gene Mutation. Classical mutation, as yet unknown in PWS, is an important contributor to AS etiology (Kishino et al., 1997; Malzac et al., 1998). The UBE3A (ubiquitin protein ligase 3A) gene is expressed from both parental chromosomes in some tissues but, in the brain, from only the maternal chromosome. The (normal) paternal allele does not function in embryonic brain, or at least in particular parts of the brain. Thus, if the maternal gene is mutated, there is no UBE3A expression, and consequently the developing brain is damaged (Rougeulle and Lalande, 1998). In a mouse knockout model, Ube3a expression was compromised in certain cells of the hippocampus, a crucial structure in learning and memory, and of the cerebellum, which may have a role in learning as well as its classic role in coordination (Albrecht et al., 1997). The human situation is quite likely to be similar. (Mouse knockout models for PWS are lethal.)
About 70% of inherited non-deletion non-UPD non-IC AS is due to UBE3A mutation of maternal origin. As for sporadic AS, and in those patients with normal methylation results, a UBE3A mutation is seen in about 30%. The phenotype in the mutation form differs little from AS with a deletion (Lossie et al., 2001). Multigenerational transmission may be seen, with the revealing observation that AS children are born only to carrier daughters of carrier males (Fig. 20-7). The mutation transmitted by the father has no effect in his child, since this chromosome 15 region would in any event carry a paternal imprint and be silent. Fung et al. (1998) report an instructive case in which a UBE3A “mutation” in an atypically affected child turned out to be a neutral polymorphism, shared by the normal sib.
Angelman Syndrome with No Deletion, No Uniparental Disomy, No Imprinting Mutation, and No UBE3A Mutation. In some 10%–15% of AS, no genetic defect can be found (Baumer et al., 1999; Lossie et al., 2001). There is a normal karyotype, with no deletion demonstrable on FISH, normal methylation analysis (at least on the sampled tissues), biparental inheritance, and an apparently intact UBE3A gene. There may be an epigenetic influence whereby a normal UBE3Agene on the maternal chromosome fails to activate normally during embryogenesis. Or, some other AS genetic basis, as yet unknown, may be the cause.
Table 20-1 sets out the test results for the different types of PWS and AS.
Chromosome 16
This is one of the more commonly seen UPDs, and is almost always due to correction of trisomy 16 of maternal meiotic origin. Thus, it is typically a maternal UPD. A wide range of malformation may be seen, and intrauterine growth retardation is very frequent (Yong et al., 2002). It has been difficult to separate out the effects of the UPD and those of placental insufficiency due to confined placental mosaicism, which rather frequently accompanies the UPD; a residual occult fetal trisomy mosaicism always remains as a potential confounder. However, since the clinical syndrome associated with upd(16)mat is reasonably consistent and the prenatal course is more abnormal in UPD than in biparental inheritance, and given the fact that the phenotype is considerably more severe when mosaic trisomy of the child is actually identified, it may be reasonable to conclude that the maternal UPD is the basis of a specific syndrome (Engel and Antonarakis, 2002). Consistent with this interpretation, Yong et al. (2002) showed in a large series of mosaic trisomy 16 discovered at prenatal diagnosis that the degree of fetal growth restriction was greater in those with upd(16)mat than in those with biparental inheritance, although there might also be an effect of a UPD cell line having a lesser proliferative advantage over trisomic cells than would cells with biparental disomy. The issue is not entirely settled, and an important role for placental dysfunction cannot yet be discounted (Benn, 1998; Hsu et al., 1998; Kotzot, 1999). As for paternal UPD 16, it seems probable that it has no clinical consequences (Engel and Antonarakis, 2002).
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Figure 20-7. A family with inherited Angelman syndrome, due to a UBE3A mutation, reported in Moncla et al. (1999). Open symbol, normal; filled symbol, Angelman syndrome; bull's eye symbol, mutation carrier, demonstrated or inferred; N, demonstrated noncarrier. Note that all the affected children are born to carrier mothers, but that these mothers are related to each other through the male line. Some normal children have been proven to be noncarriers with molecular testing (N in symbol), but the reader can also determine that any unaffected child of a potential carrier mother, such as IV:1 and 2, the children of III:4, or V:9, the sibling of an affected child, can not be carriers. An inherited imprinting center mutation could present a similar pedigree. |
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Table 20.1. Assessment of Genetic Categories of Prader-Willi and Angelman Syndromes According to Results of Cytogenetic and Molecular Testing |
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Chromosome 17
A single case of complete upd(17)mat has been described, and the 46,XY child was normal. Ascertainment was via the discovery of trisomy 17 mosaicism at amniocentesis (Genuardi et al., 1999). The abnormal phenotype in a child with a segmental upd(17)mat, involving 17q25.3, may be due to some other factor (Rio et al., 2001).
Chromosome 20
A prima facie case exists for a UPD 20 syndrome. Eggermann et al. (2001a) review three reported cases of upd(20), one paternal and two maternal, with a major malformation phenotype in the former, and growth retardation the common observation in the latter two. A causal link is possible, as is also true concerning two abnormal children initially ascertained because of trisomy 20 mosaicism at amniocentesis (Salafsky et al., 2001; Velissariou et al., 2002, and see p. 414). A single sporadic case of pseudohypoparathyroidism in association with a paternal upd(20)pat (UPD for the long arm) might more firmly be considered to reflect a true effect of the UPD, since the causative gene (GNAS1) on 20q is known to display a parent-of-origin effect (Bastepe et al., 2001). This abnormality probably arose as a mitotic event, according to the scenario as set out in Figure 20-3d. Further support for this view comes from Aldred et al. (2002), who studied two children with deletions of 20q, one of maternal and the other of paternal origin, in whom the different patterns of parathyroid endocrinology were consistent with what is understood of the complex behavior of the GNAS1 gene.
Chromosome 21
UPD 21, maternal or paternal, appears to be without effect (Engel and Antonarakis, 2002).
Chromosome 22
Maternal UPD 22 has generally not been associated with any defect (Kotzot, 1999; Engel and Antonarakis, 2002). Intrauterine growth retardation, if present, may more likely reflect the influence of a trisomic 22 placenta, or low-level occult mosaicism of the fetus (Balmer et al., 1999; Bryan et al., 2002).
Chromosome X
Neither upd(X)mat nor upd(X)pat appears to have any consequence in the 46,XX person, with the usual exception of homozygosity for a recessive gene (Quan et al., 1997). However, there may be a subtly different neuropsychological phenotype according to the parent of origin, at least for monosomy X (self-evidently a uniparental condition). In a British study, 80 girls with Turner syndrome underwent behavioral evaluation; 55 were 45,XM and 25 were 45,XP. The 45,XP girls were more socially adept and more articulate than the 45,XM girls. Speculatively, this may represent the effect of an imprintable X-borne “locus for social cognition” that is functional on the X chromosome transmitted from a father and nonfunctional on the X from a mother (Skuse et al., 1997). Autism, which is a male-susceptible condition, is associated with 45,XM in the case of autistic girls with Turner syndrome (Donnelly et al., 2000). In terms of response to growth hormone, it makes no difference whether the child is 45,XM or 45,XP (Tsezou et al., 1999).
Upd(X)pat offers the intriguing scenario of father-to-son transmission of an X-linked gene. A 24,XY gamete from a hemizygous father could produce a 47,XXY zygote that could subsequently lose the maternally contributed X; or, the ovum could be nullisomic X, with sex chromosomal complementation producing 46,XY (Vidaud et al., 1989). UPID X, due to a presumed maternal meiosis II error, was the basis of a unique female XXY patient, who was homozygous for a mutation in the androgen receptor gene on the X chromosome and thus manifested androgen insensitivity syndrome (p. 296). Her gonads were removed at surgery, and proved to be degenerated testes (Uehara et al., 1999b).
Rare Complexity
We know of one case of double UPD, a child with 47,XXY/46,XX,upd(X)mat,upd(16)mat, nonmosaic 47,XX,+16 having been shown at chorionic villus sampling (CVS) and 47,XXY/46,XX mosaicism at amniocentesis, and presenting as a newborn with facial dysmorphism and true hermaphroditism (J. Ryan, pers. comm., 2002). Plausibly, the conception was 48,XXY,16 due to coincidental maternal nondisjunctions, with postzygotic events producing the different lineages.
GENETIC COUNSELING
Uniparental Disomy for the Entire Chromosomal Complement
Uniparental disomy for the entire paternal chromosome set (hydatidiform mole) is associated with an increased recurrence risk; this is discussed in detail in Chapter 21 (p. 359). There is no discernibly increased risk for the recurrence of UPD for the entire maternal chromosome set (ovarian teratoma).
Uniparental Disomy for Individual Chromosomes
No instance of recurrence of full UPD for a particular chromosome, with a 46,XX or 46,XY karyotype, is known, and we assume there to be no discernibly increased recurrence risk. The association with increasing childbearing age is to be noted, but in reality the increase in risk for older mothers would be very small.
Segmental Uniparental Disomy
We presume segmental UPD arising postzygotically, and which is karyotypically 46,XX or 46,XY, would imply no increased risk. Uniparental disomy due to rearrangement would have a risk according to the nature of the specific rearrangement.
Syndromes with More Than One Genetic Basis
Beckwith-Wiedemann Syndrome
The considerable majority (about 85%) of BWS occurs sporadically, including the two more common categories of epigenetic error and UPD 11. The other categories that may have an important recurrence risk are recognized by an abnormal cytogenetic report and/or by a positive family history.
Epigenetic Error. In sporadic BWS with biparental disomy, the underlying cause is assumed to be an epigenetic error (epimutation) affecting the ovum or early conceptus, with consequent inappropriate activation status of certain genes within the BWS domain. Aberrant methylation on the maternal chromosome of either the IGF2/H19 gene pair, or of the region of the KVLQT1 gene (associated with the LIT1 transcript), identifies this category (in the former case showing an increased association with cancer, and in the latter with midline ab-dominal-wall defects and macrosomia [Bliek et al., 2001; DeBaun et al., 2002]). LIT1 dysfunction, with biallelic expression, is associated with IVF (DeBaun et al., 2003; Maher et al., 2003). In the setting of a subsequent naturally achieved pregnancy, whether the index case had been a natural or an IVF conception, according to current understanding no increased risk for recurrence would apply.
Uniparental Disomy 11. About a fifth of sporadic cases are due to mosaic segmental paternal UPD of 11p. This category of BWS can be suspected clinically if there is hemihyperplasia, and is supported on laboratory evidence of aberrant methylation status of both H19 and LIT1 (Bliek et al., 2001). No increased recurrence risk applies in the setting of segmental UPD and a normal karyotype.
Balanced 11p Rearrangement. The maternal transmission of a reciprocal translocation or an inversion with one breakpoint in distal 11p is a rare cause of BWS (Li et al., 1998a). Speculatively, the rearrangement may remove the BWS gene or genes from the influence of an 11p15 IC. The carrier will have an even risk of transmitting the normal chromosome or the balanced rearrangement; if the latter, and the mother being the carrier, the child would have BWS. Imbalanced possible outcomes need to be assessed individually, as for any rearrangement.
Distal 11p15 Duplication. If the duplication chromosome is of paternal origin, double expression of an 11p15 locus or loci brings about the BWS phenotype (Li et al., 1998a). Functional trisomy of nonimprinted 11p segments may otherwise contribute to the phenotype. A de novo abnormality seen in a single case is that of a t(11p;14p), in which the der(14) looked at first glance, like a normal 14p variant (p. 236). The phenotype in the case of maternal origin of the segment is quite different (Fisher et al., 2002). The recurrence risk for these various circumstances will depend on the nature of the rearrangement and the parental karyotypes.
Dominant Mutation. Autosomal dominant BWS accounts for about 15% of cases, and is recognized from the pedigree structure (vertical transmission, only the offspring of female heterozygotes are affected, normal cytogenetics) (Moutou et al., 1992). Careful review of the pedigree in the maternal line is necessary to identify mildly affected individuals, bearing in mind the amelioration of phenotype with time (Elliott et al., 1994; Hunter and Allanson, 1994). One locus is the cell cycle control factor p57kip2, and others are presumed to exist (Maher and Reik, 2000). The probability of transmitting a BWS gene is 50%, with the risk for an affected child applying only in the case of maternal transmission.
Prader-Willi Syndrome
A summary of the different genetic forms of PWS, and the associated risks of recurrence, is set out in Table 20-2.
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Table 20.2. Approximate Relative Frequencies and Recurrence Risks for the Different Categories of Prader-Willi and Angelman Syndromes |
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Classical Deletion 15q11–q13. About 70% of PWS is due to a de novo interstitial deletion 46,del(15)(q11–q13), in which 3–4 Mb of DNA is removed (Nicholls, 1993). The deletion is of paternal origin. No case of recurrence of PWS due to a classical deletion is on record. This empiric observation of zero recurrences out of some thousands of trials underscores the considerable unlikelihood of significant paternal gonadal mosaicism for the deletion observed in the PWS child. This is the basis of the substantial optimism that can be offered to parents in terms of any further pregnancies. A figure of around 0.1% may be a fair one to offer for the risk of recurrence. Nevertheless, the theoretical possibility of paternal gonadal mosaicism obliges acknowledgement that the risk is not zero. If prenatal diagnosis is pursued, CVS can be offered using the SNRPN methylation test (Buiting et al., 1998).
Uniparental Disomy 15. About 25% of PWS is due to maternal UPD 15, from a mother who is 46,N. We know of no recorded instance of recurrence of upd(15)mat PWS in a 46,N couple, and would otherwise assume, on theoretical grounds, any increased risk in a future pregnancy to be practically negligible, the modest maternal age effect notwithstanding.
Functional Defect of Prader-Willi Syndrome Imprinting Center. These extremely rare cases of IC defects will require individual expert advice. They can be suspected if a child is true to type clinically but there is neither classical deletion nor UPD demonstrable. All cases of functional IC deficiency have so far been sporadic (but very few are known). Since some IC microdeletions (which convey a high risk; see below) may not be recognized as such, it would be prudent to offer prenatal diagnosis with SNRPN methylation testing in the setting of any suspected IC defect.
Prader-Willi Syndrome PWS Imprinting Center Microdeletion. The recognition of these cases will require referral to a specialist laboratory. A positive family history, if observed, would oblige the assumption of this category, unless or until proven otherwise. Assuming the father carries the genetic defect, the recurrence risk is high, namely 50%. SNRPN methylation testing on CVS can identify an affected pregnancy. The father's brothers would have a 50% likelihood to be heterozygous (making the assumption that their mother would carry the mutation), and, if so, these brothers would also have a 50% risk of having a PWS child. Equally, his sisters could be carriers, but their children would all be unaffected, and it would only be their sons who might, in the next generation, have the risk for a PWS child. The siblings of the affected child would themselves have no different genetic risk than that of the general population. The reader should work through the reasoning behind these various risk assessments, even though most counselors will never encounter this actual circumstance in the clinic.
Uncommon Cytogenetically Detectable Rearrangement. The nature of the rearrangement (see the Biology section) and the parental karyotypes will determine the recurrence risk in each type. An apparently 46,N or balanced translocation karyotype at the cytogenetic level may yet harbor a de novo deletion, whose generation may have been due to crossing-over within the parental abnormal chromosome 15, and prenatal diagnosis for the detection of a deletion or of UPD may require FISH and molecular methodology (Horsthemke et al., 1996).
Angelman Syndrome
A summary of the different genetic forms of AS, and the associated risks of recurrence, is set out in Table 20-2, the five major categories noted as types I–V. More detail is available in the reviews of Stalker and Williams (1998) and Clayton-Smith and Laan (2003).
The clinical diagnosis of AS is sometimes easy (parents have recognized the condition in their child after seeing a television program), but at other times more challenging. Of course if accurate genetic advice is to be given, an accurate clinical diagnosis is crucial (Williams et al., 1995). The possibility of Rett syndrome may need to be considered (Scheffer et al., 1990). A proposed very simple clinical test is to gauge the response to a vibrating tuning fork held close to one ear: an AS child will laugh and turn towards it (Hall and Cadle, 2002). The counselor must take the trouble to obtain a detailed family history. A genetic defect could have been transmitted through males for some generations, only causing AS when it passed from a daughter of such a male. Figure 20-7 shows a family in which some quite distant relatives, including second cousins once removed and first cousins twice removed, had AS due to an inherited UBE3A mutation.
Classical Deletion 15q11–q13 (Type I). Similarly to PWS, about 70% of AS is due to a de novo interstitial deletion. Only one case in the world is recorded of recurrence in siblings of a typically sized deletion. This case involved, presumably, a mother with gonadal mosaicism (Kokkonen and Leisti, 2000). Thus, as for classical deletion–PWS, we presume a very low, but clearly not zero, recurrence risk. There are two recorded examples of deletion AS in cousins, which manifestly represented coincidental de novo events in these families, in that different ancestral chromosomes were involved (Connerton-Moyer et al., 1997). The comments on prenatal diagnosis in PWS (see above) apply similarly here.
Uniparental Disomy 15 (Type II). Angelman syndrome due to paternal UPD 15 is rare; as discussed above, the initial error may actually reflect a maternal age effect. Interestingly, the AS phenotype may be somewhat milder in UPD 15, and in some children it was only after an EEG showed typical findings that the diagnosis was suspected (Bottani et al., 1994). But this does not mean that some upd(15)pat AS children may not be severely affected (Prasad and Wagstaff, 1997). No recurrence is on record (Chan et al., 1993), and we assume on theoretical grounds that no usefully measurable increased risk would exist.
Angelman Syndrome Imprinting Center Microdeletion (Type IIIa). Assuming the mother carries the genetic defect, there is a high recurrence risk, namely, 50%. SNRPN methylation testing on CVS can identify an affected pregnancy. The possibility of maternal gonadal mosaicism for an IC mutation complicates the picture (Stalker et al., 1998). As noted above under PWS, but vice versa for AS, the siblings of the carrier mother could also be carriers (assuming their father is heterozygous). However, it would only be the sisters who would have the risk for an AS child.
Functional Defect of Angelman Syndrome Imprinting Center (Type IIIb). The comments above on PWS apply similarly here. All cases of AS due to a functional IC defect have so far been sporadic, but it might be prudent to offer prenatal diagnosis in a subsequent pregnancy (SNRPN methylation testing).
UBE3A Mutation (Type IV). If the mother carries the mutation, the risk for recurrence is 50%. Maternal mosaicism has been recognized (Malzac et al., 1998), and so non-demonstra-tion of the mutation in the mother does not necessarily exclude a genetic risk. Indeed, it may be that such mosaicism is not uncommon (Stalker et al., 1998). It may be appropriate to track the mutation through the patrilineal family, to be able to offer genetic counseling to female cousins who might be carriers. The reader should study the illustrative pedigree in Figure 20-7. There is the practical point that routine clinical testing for UBE3A is available in only a very few laboratories, and at some cost.
No Genetic Defect Demonstrable, Suspected Epigenetic Error (Type V). In a small fraction of AS, about 10%, no cytogenetic or molecular defect, nor UBE3A mutation, is demonstrable. Some of these cases could conceivably represent a UBE3A mutation that has not been able to be detected. (Many clinics will in fact not have access to UBE3A analysis.) The family history, if positive, may compel the assumption of a mutation, and thus imply a high recurrence risk. A negative family history might support the inference of a low risk, but would not allow a definite assumption. If a normal sibling carried the same 15q11–q13 haplotype, using DNA markers, a low-risk scenario would be probable. Expert advice should be sought.
Uncommon Cytogenetically Detectable Rearrangement. The nature of the rearrangement (see the Biology section) and the parental karyotypes will determine the recurrence risk in each type. The comments above relating to de novo deletion in PWS might, in theory, also be applicable to AS.
Gurrieri Syndrome. Some cases of this syndrome of severe mental defect with no speech, epilepsy, and skeletal dysplasia may actually have an AS genotype (Battaglia and Gurrieri, 1999), and are thus more usefully classified as a variant form of AS.
A Simplification for Angelman Syndrome
Some parents will not find it easy to come to grips with these various possible causes for their child's condition, even if, in the end, they need only consider the category that applies to themselves. It may be helpful to discuss AS and the risks of recurrence in the following terms. Let us say that AS is due simply to a lack of the UBE3A protein, a very important protein that is necessary for the brain to grow normally. The gene for UBE3A works only on the chromosome 15 from the mother, while the gene on the father's chromosome is dormant. There is a switch on the mother's chromosome that makes this gene work.
· If the bit of the maternal chromosome that contains this gene is missing (deletion), or if the mother's chromosome is replaced by another one from the father (UPD), no UBE3A protein can be made. These two types happen as one-off events.
· If the switch fails on the mother's chromosome, then the gene remains dormant, and no protein is made (IC fault). This type can happen one-off, as though the switch “gets stuck,” for no reason that we understand very well. Or, there may be a genetic fault in the actual switch, and in this case the defect could be passed to a subsequent child.
· If the UBE3A gene itself is faulty on the mother's chromosome (mutation), no protein is made, or only an abnormal protein that cannot function. The genetic risk depends on whether the faulty gene started with the child (no increased risk) or if the mother is a carrier (high risk). Note that the mother can be a carrier and still be perfectly normal, since the faulty gene would be the one she got from her father, and so in any event it would be switched off.
· Sometimes the UBE3A gene fails to work, even though the maternal chromosome is normal, and has a normal switch. We do not know why this happens (there has been a suggestion that one cause may be if the pregnancy had been IVF-conceived, but the evidence for this is very preliminary). This type is a one-off event.
A common question parents have is whether their normal children might, in the next generation, have an AS child. Or, the aunts and uncles of an AS child might want advice about risks to their future children or to their grandchildren. The answers are as follows.
· The normal siblings of an AS child have no increased risk, for any genetic category, with the possible exception of a familial translocation. Even if the AS child has (or had) a potentially heritable type of UBE3A genetic defect, the fact that the sibs themselves are normal declares that they cannot have received it. If they had received the abnormal gene, they would have AS; since they do not have AS, they cannot have the gene. The sex of the siblings is immaterial.
· Aunts and uncles have an increased risk for children or grandchildren of theirs only if a heritable type of AS is involved (IC defect, UBE3A mutation). In that case, an uncle could be a carrier, but his children would not be at risk, since the UBE3A gene would be dormant anyway. Daughters of his, however, could have an AS child. A carrier aunt would have a high risk (50%) of having an AS child. But her grandchildren, through her normal sons and daughters, would have no increased risk. Her normal children would have declared themselves, by their very normality, not to have inherited the genetic defect.
Notes
1. As a general rule, the abbreviation is in upper case (UPD, UPHD, UPID) when making broad reference to the concept of uniparental disomy, and in lower case (upd, uphd, upid), according to the rules of cytogenetic nomenclature, when attention is more focused on a specific case.
2. A little counterintuitively, imprinting refers to non-activity. An imprinted chromosome is silenced, while the nonimprinted chromosome is the active one.
3. It might be more accurate to speak of a “failed rescue,” or, better, a “foiled rescue,” since the end result is an unfortunate one.
4. There is one common distal breakpoint shared by the great majority of deletions, and two common proximal breakpoints, the two combinations giving type I and type II deletions (Amos-Landgraf et al., 1999; Ji et al., 2000).
5. An aide-mémoire: Prader-Willi due to Paternal deletion.
6. An additional copy of this gene leads to hyperpigmentation (Akahoshi et al., 2001). This is a nice example of a dosage effect: one copy of the P gene = pale skin, two copies = normal pigmentation, three copies = hyperpigmentation.
7. The word mutation is normally taken to indicate that there is a change in the DNA sequence (from the Latin mutare, to change). By definition, no such change has occurred in an epimutation. But there has been a change in the functioning of the DNA.
8. A controversial suggestion has been made that AS due to sporadic imprinting defect might be more likely in a pregnancy conceived by ICSI (intracytoplasmic sperm injection), with the epigenetic reprogramming of the early embryo in some way disturbed (Cox et al., 2002a; Ørstavik et al., 2003).
9. Thus, “microdeletion” in this context refers to a deletion that is smaller than the usual classical PWS/AS microdeletion.
10. The same mechanism of postzygotic recombination may suggest itself in the setting of somatic mosaicism for a Mendelian condition. Happle and König (1999) discuss the case of a boy with a rare skin condition (epidermolytic hyperkeratosis of Brocq) affecting most of his body, but with some areas of more severely affected skin, and some areas which were healthy. Imagining that the Brocq locus might be in the distal short arm of the chromosomes depicted in Figure 20-3 and with the black chromosome having the mutant allele, the typically affected skin would have the genotype represented in (a), the more severely affected skin would be in (f), and the normal skin, in (e).
