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Genomic Imprinting and Uniparental Disomy in Medicine: Clinical and Molecular Aspects

Eric Engel, Stylianos E. Antonarkis

Copyright # 2002 Wiley-Liss, Inc. ISBNs: 0-471-35126-1 (Hardback); 0-471-22193-7 (Electronic)

Chapter 7

The Angelman

Syndrome (AS)

The study and understanding of the Angelman syndrome that, to use a comparison with chemistry, is genomically ‘‘isomeric’’ to the Prader-Willi syndrome have always been somewhat lagging, relative to PWS. This may not be too surprising, considering that PWS started its clinical course in 1956 from Switzerland and AS in 1965 from England.

Thus, in succession, it was in 1987 that the first 15q11-q13 deletions in AS were observed (versus 1981 in PWS); in 1989 that the deletion appeared to preferentially involve the maternal chromosome (versus 1983 for preferential paternal 15 involvement in PWS); in 1991 that paternal UPD 15 became recognized as a cause of AS (versus 1989, for maternal UPD 15, as a cause of PWS).

CLINICAL PROFILE

In 1965 Harry Angelman reported the cases of ‘‘three children with flat heads, jerky movements, protruding tongues and bouts of laughter giving them a superficial resemblance to puppets, an unscientific name, but one which may provide for easy identification’’ (Angelman, 1965).

The presentation of the clinical picture (Zori et al., 1992; Clayton-Smith and Pembrey, 1992; Magenis et al., 1990) in the first year of life is somewhat similar to that of PWS and hardly considered typical of the diagnosis. Pregnancy and delivery are usually uneventful, with a birthweight slightly less than that of other siblings.

Clinical signs mostly consist of failure to thrive, feeding problems, seizures, and developmental difficulties. The feeding problem consists of frequent spitting or food refusal, which, only infrequently, results from inappropriate motor control of

187

188 THE ANGELMAN SYNDROME (AS)

chewing and swallowing. Seizures are often present by 6 months, although, in some cases, they may develop later but mostly within the first 2 years. They could be of several types: myoclonic, motor generalized, or with absences and drop attacks. The epilepsy is initially severe and difficult to control in half the cases, but tends to become milder and more manageable in later childhood. Abnormalities of the EEG consist of diffuse sharp and slow waves of the 2–4 Hz types. No consistent CNS anomaly is seen on neuroimaging; cortical atrophy, leukomalacia, and demyelinization may be documented. Mental retardation is profound. By 1 year, in most cases, a notable cheerfulness is accompanied by outbursts of laughter. The first abnormalities noted in these children are delayed gross motor milestones, muscular hypotonia, and speech delay. There is also hypermotoric activity, usually without aggressive behavior. AS children are also overexcitable with short attention spans. Their limited need for sleep is quite disruptive to their familial environment.

Affected children are tongue-thrusting, mouthing, hand-flapping and have a rigid, jerky ataxia with stiff, hypertonic legs and hypotonic trunk, causing a ‘‘puppet-like’’ demeanor as they walk. They become ambulatory, often between 2 and 5 years of age. Verbal communication is generally limited to a very few words and the children display better nonverbal than verbal communications, using gestures to communicate. Microbrachycephaly (usually below the 25th centile) is noted and the face presents deep-set eyes, thin upper lip, and midface hypoplasia, along with macrognathia, macrosomia, and tongue protrusion, a clinical picture quite different from that of PWS.

Occular and cutaneous hypopigmentation is often noted and eye and ear problems are common, consisting of strabismus, optic atrophy, and keratoconus. Hearing difficulties are related to middle-ear diseases.

The genetic=molecular causes of the syndrome, which are heterogenous, are discussed below (Figure 1 of this chapter and Table 3 of Chapter 5).

The differential diagnosis includes PWS, in the first year of life, and Rett syndrome, because of developmental stagnation, seizures, a jerky walk, and a

Figure 1 Classes of AS according to their molecular etiology.

PATERNAL UNIPARENTAL DISOMY 15 IN AS

189

behavior that can also be pleasant and demonstrates sleep disturbances (Zori et al., 1992; Clayton-Smith and Pembrey, 1992; Williams et al., 1995). Other differential diagnoses include the ATR-X syndrome, an X-linked thalassemia with mental retardation.

The frequency of the syndrome is estimated to be on the order of 1=30,000 and is poorly documented. Table 3 of Chapter 5 summarizes the current molecular subclassification of AS. These are the different classes (Jiang et al., 1999): (i) maternal deletion 15q11-q13 (65–75% of cases); (ii) complex rearrangements involving 15q11-q13 (<1%); (iii) paternal UPD15 (3–5%); (iv) imprinting mutation with or without IC deletion (3–5%); (v) UBE3A mutations (4–6%); and (vi) cases with no detectable molecular abnormality. The relative frequency of these classes of AS can also be seen in Figure 1.

PATERNAL UNIPARENTAL DISOMY 15 IN AS

Type and Frequency of Paternal UPD15 in AS

UPD is significantly less frequent than in PWS and, in general, occurs by a different nondisjunctional mechanism. The paternal UPD15, foreseen in theory by virtue of the cytogenetic features of AS as compared to PWS, was not easily discovered. An initial search (Knoll et al., 1991) failed probably because of a bias in selecting the study cases, 7 out of 10 that were familial (Engel, 1991). Shortly afterward, the same group of authors (Malcolm et al., 1991), however, presented the first two cases to result from paternal UPD15. This added strengthening evidence for genomic imprinting in humans since paternal-in-origin disomy 15 produced AS, but mater- nal-in-origin resulted in PWS. One of the above AS cases showed paternal heterodisomy and the other isodisomy; these conclusions were also substantiated by cytogenetic evaluation of chromosome 15 parental heteromorphisms.

Within a year, another case was reported (Nicholls et al., 1992), the first to be studied with several probes within the 15q11-q13 critical region, which, altogether with a telomeric probe, showed uniform homozygosity for paternal alleles, suggesting paternal isodisomy of the entire chromosome 15. This was further confirmed by cytogenetic observations of heteromorphisms, whereby both probands’ chromosome copies appeared derived from the same single paternal chromosome (Nicholls et al., 1992). Other cases were occasionally reported afterwards; they can be subdivided into those with two free chromosome 15 and those with 15;15 translocation. The frequency of paternal UPD15 in the cases of AS is between 1–5%.

Paternal UPD15 with Free Chromosomes 15

Both cases reported by Bottani et al. (1994), from a group of 14 sporadic cases of AS, had paternal isodisomy 15; a milder phenotype was suggested (Figure 2). The case of Gillessen-Kaesbach (1995) was also milder and the inherited paternal pair was considered isodisomic. Several other reports contained such cases of paternal

190 THE ANGELMAN SYNDROME (AS)

Figure 2 Angelman syndrome patient with paternal UPD15, shown at ages 8 and 10 years. Note characteristic but relatively mild facial phenotype (same patient as in Bottani et al., 1994).

UPD15 with free chromosomes 15 (Exeler et al., 1996; Fridman et al., 2000; Gyftodimou et al., 1999).

Paternal UPD15 with t(15;15) [i(15q)]

Although cases with this chromosomal complexion had been documented in the past, the case of Freeman et al. (1993) was the first one shown to derive from a potential i(15q) and to be isodisomic. The karyotype was 45,XY,t(15q;15q). Results at four different loci demonstrated that the patient had inherited chromosomal material 15 from his father only, and that he was homozygous (or hemizygous) at the three loci for which his father was heterozygous. Densitometry showed that he had indeed two identical copies of the polymorphic alleles derived from the father. It was concluded that this represented a paternal i(15q), rather than a rob t(15q;15q).

Absence of the maternal chromosome could have resulted from ovular nullisomy 15 or postzygotic loss, followed by selection of the cell line only containing the paternal i(15q). The clinical features were typical for AS.

In another case (Ramsden et al., 1996), the AS proband showed a 15;15 translocation with no visible deletion of the critical proximal area by FISH. A polymorphic microsatellite for the GABRA5 gene ruled out a maternal contribution at this locus. Parent-of-origin methylation studies at PW71 were consistent with a lack of maternal inheritance at 15q11-q13. There were no studies to determine whether a Robertsonian translocation or an isodisomic formation explained the centric fusion of chromosomes 15.

In still another case, 45,XY,t(15q;15q) (Tonk et al., 1996), FISH analysis indicated diploidy in the critical region of chromosome 15. The use of highly polymorphic microsatellite markers demonstrated paternal UPD15 for chromosome 15. In addition, all markers showed homozygosity, indicating isodisomy 15. The