Maternal origin of 15q11–13 deletions in Angelman syndrome suggests a role for genomic imprinting

Six persons with the classical Angelman syndrome (AS) phenotype and de novo deletions of chromosome 15q11-q13 were studied to determine the parental origin of the chromosome deletion. Four of the 6 patients had informative cytogenetic studies and all demonstrated maternal inheritance of the deletion. These findings, together with other reported cases of the origin of the chromosome 15 deletion in AS, suggest that deletion of the maternally contributed chromosome leads to the AS phenotype. This contrasts with the Prader-Willi syndrome (PWS) in which a similar deletion of the paternally contributed chromosome 15 is observed. In deletion cases, a parental gamete effect such as genomic imprinting may be the best model to explain why apparently identical 15q11-q13 deletions may develop the different phenotypes of AS or PWS.     

Linkage analysis with chromosome 15q11-13 markers shows genomic imprinting in familial Angelman syndrome.

Angelman syndrome (AS) and Prader-Willi syndrome (PWS) have become the classical examples of genomic imprinting in man, as completely different phenotypes are generated by the absence of maternal (AS) or paternal (PWS) contributions to the q11-13 region of chromosome 15 as a result of deletion or uniparental disomy. Apparently, most patients are sporadic cases. The genetic mechanism underlying familial AS has remained enigmatic for a long time. Recently, evidence has been emerging suggesting autosomal dominant inheritance of a detectable or undetectable defect in a gene or genes at 15q11-13, subject to genomic imprinting. The present report describes an unusually large pedigree with segregation of AS through maternal inheritance and apparent asymptomatic transmission through several male ancestors. Deletion and paternal disomy at 15q11-13 were excluded. However, the genetic defect is still located in this region, as we obtained a maximum lod score of 5.40 for linkage to the GABA receptor locus GABRB3 and the anonymous DNA marker D15S10, which have been mapped within or adjacent to the AS critical region at 15q11-13. The size of the pedigree allowed calculation of an odds ratio in favour of genomic imprinting of 9.25 x 10(5). This family illustrates the necessity of extensive pedigree analysis when considering recurrence risks for relatives of AS patients, those without detectable deletion or disomy in particular.     

Angelman syndrome with a chromosomal inversion 15 inv(p11q13) accompanied by a deletion in 15q11q13.

A family is described in which an inversion of chromosome 15, 15 inv(p11q13), is segregating. All family members are healthy except the proband who is a 10 year old boy with Angelman syndrome. Although the chromosomal inversion has been passed from the grandfather to both his son and his daughter with no ill effect, passage from daughter to grandson has resulted in a deletion of chromosome 15 material which is presumed to be the cause of Angelman syndrome in this boy. The probabilities of an inversion of this type being instrumental in causing the syndrome are discussed.     

Identification of novel deletions of 15q11q13 in Angelman syndrome by array-CGH: molecular characterization and genotype-phenotype correlations

Angelman syndrome (AS) is a neurodevelopmental disorder characterized by mental retardation, absent speech, ataxia, and a happy disposition. Deletions of the 15q11q13 region are found in approximately 70% of AS patients. The deletions are sub-classified into class I and class II based on their sizes of approximately 6.8 and approximately 6.0, respectively, with two different proximal breakpoints and a common distal breakpoint. Utilizing a chromosome 15-specific comparative genomic hybridization genomic microarray (array-CGH), we have identified, determined the deletion sizes, and mapped the breakpoints in a cohort of 44 cases, to relate those breakpoints to the genomic architecture and derive more precise genotype-phenotype correlations. Interestingly four patients of the 44 studied (9.1%) had novel and unusually large deletions, and are reported here. This is the first report of very large deletions of 15q11q13 resulting in AS; the largest deletion being >10.6 Mb. These novel deletions involve three different distal breakpoints, two of which have been earlier shown to be involved in the generation of isodicentric 15q chromosomes (idic15). Additionally, precise determination of the deletion breakpoints reveals the presence of directly oriented low-copy repeats (LCRs) flanking the recurrent and novel breakpoints. The LCRs are adequate in size, orientation, and homology to enable abnormal recombination events leading to deletions and duplications. This genomic organization provides evidence for a common mechanism for the generation of both common and rare deletion types. Larger deletions result in a loss of several genes outside the common Angelman syndrome-Prader-Willi syndrome (AS-PWS) critical interval, and a more severe phenotype.     

The association of Angelman''s syndrome with deletions within 15q11-13.

The inheritance of Angelman's syndrome, a disorder characterised by mental retardation, epilepsy, ataxia, and a happy disposition, is debated because affected sibs occur less frequently than expected with autosomal recessive inheritance. After discovering two unrelated patients with a small deletion of the proximal long arm of chromosome 15, 10 further patients with Angelman's syndrome were reassessed. Five had apparently normal karyotypes, four had a deletion within 15q11-13, and one had a pericentric inversion, inv(15)(p11q13) involving the same chromosomal region. In the latter case, the healthy mother had the same pericentric inversion, indicating that the patient also had a submicroscopic mutation on his other chromosome 15. These data map the Angelman locus to 15q11-13 and suggest that de novo visible deletions (associated with a low recurrence risk) and autosomal recessively inherited cases combine to give an overall sib recurrence risk of less than 25%.     

Angelman syndrome associated with a maternal 15q11-13 deletion of less than 200 kb

Angelman syndrome (AS) is a neurogenetic disorder arising froma lack of genetic contribution from the maternal chromosome15q11–13. To date, the AS critical region has been definedby an inherited deletion of approximately 1.5Mb, spanning the3–21 (D15S10), LS6–1 (D15S113) and GABRB3 loci.We have Identified an individual with the typical features ofAS who has a deletion of the maternal chromosome which encompassesLS6–1, but does not extend to either flanking marker.This deletion, initially detected by (CA)n repeat analysis,was further characterised by fluorescence In situ hybridisation(FISH) using cosmids derived from a 260 kb LS6–1 yeastartificial chromosome (YAC). Neither end cosmid from this YACclone falls within the deletion, suggesting that the minimalAS region Is less than 200 kb. We also studied three loci within15q11–13 which detect parent-of-origin specific DNA methylationimprints, and found that both normal maternal and paternal patternswere present in this patient.     

Familial translocations involving 15q11-q13 can give rise to interstitial deletions causing Prader-Willi or Angelman syndrome.

A de novo interstitial deletion of 15q11-q13 is the major cause of Prader-Willi syndrome (PWS) and Angelman syndrome (AS). Here we describe two unrelated PWS patients with a typical deletion, whose fathers have a balanced translocation involving the PWS/AS region. Microsatellite data suggest that the deletion is the result of an unequal crossover between the derivative chromosome 15 and the normal chromosome 15. We conclude that familial translocations involving 15q11-q13 can give rise to interstitial deletions causing PWS or AS and that prenatal diagnosis in such families should include fluorescence in situ hybridisation or microsatellite studies or both.     

Is Angelman syndrome an alternate result of del(15)(q11q13)?

Two unrelated females, age 15 and 5 years respectively, were studied cytogenetically because of severe mental retardation, seizures and ataxia-like incoordination. A similar deletion of the proximal long arm of chromosome 15 was found in both patients. Re-evaluation showed no voracious appetite or obesity; normal size of hands and feet, minimal to no hypotonia by history or examination and facial features not typical of the Prader-Willi syndrome. However, the facial appearance of the girls was similar to each other with mild hypertelorism. The similarity of these girls and dissimilarity to Prader-Willi syndrome suggest a different syndrome, perhaps the result of deletion of a different segment of 15q. The findings of ataxic-like movements, frequent, unprovoked and prolonged bouts of laughter and facial appearance are more compatible with the diagnosis of Angelman syndrome.     

Further evidence for dominant inheritance at the chromosome 15q11-13 locus in familial Angelman syndrome.

Eleven patients with Angelman syndrome (AS) and their parents from 5 families have been studied with high resolution chromosome analysis and molecular probes from region 15q11-13 in an attempt to elucidate the mode of inheritance in familial AS. No deletions were detected. All families were informative with a combination of different short arm cytogenetic markers. All sets of sibs inherited the same maternal chromosome 15, but in 3 families sibs inherited different paternal 15s. Analysis of 6 polymorphic DNA markers supported the conclusion that AS sibs inherit the same maternal 15, but often different paternal 15s. These data make autosomal recessive inheritance at a 15q11-13 locus very unlikely and support the hypothesis that familial AS is due to maternal transmission of a mutation within 15q11-13.     

Supernumerary inv dup(15) in a patient with Angelman syndrome and a deletion of 15q11–q13

We have studied a patient with Angelman syndrome (AS) and a 47,XY,+inv dup(15) (pter→q11::q11→pter) karyotype. Molecular cytogenetic studies demonstrated that one of the apparently normal 15s was deleted at loci D15S9, GABRB3, and D15S12. There were no additional copies of these loci on the inv dup (15). The inv dup (15) contained only the pericentromeric sequence D15Z1. Quantitative DNA analysis confirmed these findings and documented a standard large deletion of sequences from 15q11-q13, as usually seen in patients with AS. DNA methylation testing at D15S63 showed a deletion of the maternally derived chromosome. AS in this patient can be explained by the absence of DNA sequences from chromosome 15q11-q13 on one of the apparently cytogenetically normal 15s, and not by the presence of an inv dup (15). This is the fourth patient with an inv dup (15) and AS or Prader Willi syndrome, who has been studied at the molecular level. In all cases an additional alteration of chromosome 15 was identified, which was hypothesized to be the cause of the disease. Patients with inv dup (15)s may be at increased risk for other chromosome abnormalities involving 15q11-q13. © 1995 Wiley-Liss, Inc.     

Incidence of 15q deletions in the Angelman syndrome: a survey of twelve affected persons

Prometaphase chromosome study of 12 persons with an established diagnosis of the Angelman syndrome demonstrated that 5 had a 15q12 deletion appearing similar to that commonly observed in the Prader-Willi syndrome. Phenotype-karyotype correlation did not show any obvious clinical differences between those with and those without the deletion and no clinical overlap between Angelman and Prader-Willi syndrome was apparent. Our survey suggests that 15q12 deletions are frequent in Angelman syndrome but presence of the deletion does not appear to distinguish different clinical phenotypes. Experience with the cytogenetic study of Prader-Willi syndrome predicts that considerable complexity will emerge between the presence of 15 chromosome abnormalities and clinical expression of Angelman syndrome.     

Large genomic duplicons map to sites of instability in the Prader-Willi/Angelman syndrome chromosome region (15q11-q13).

The most common etiology for Prader-Willi syndrome and Angelman syndrome is de novo interstitial deletion of chromosome 15q11-q13. Deletions and other recurrent rearrangements of this region involve four common 'hotspots' for breakage, termed breakpoints 1-4 (BP1-BP4). Construction of an approximately 4 Mb YAC contig of this region identified multiple sequence tagged sites (STSs) present at both BP2 and BP3, suggestive of a genomic duplication event. Interphase FISH studies demonstrated three to five copies on 15q11-q13, one copy on 16p11.1-p11.2 and one copy on 15q24 in normal controls, while analysis on two Class I deletion patients showed loss of approximately three signals at 15q11-q13 on one homolog. Multiple FISH signals were also observed at regions orthologous to both human chromosomes 15 and 16 in non-human primates, including Old World monkeys, suggesting that duplication of this region may have occurred approximately 20 million years ago. A BAC/PAC contig for the duplicated genomic segment (duplicon) demonstrated a size of approximately 400 kb. Surprisingly, the duplicon was found to contain at least seven different expressed sequence tags representing multiple genes/pseudogenes. Sequence comparison of STSs amplified from YAC clones uniquely mapped to BP2 or BP3 showed two different copies of the duplicon within BP3, while BP2 comprised a single copy. The orientation of BP2 and BP3 are inverted relative to each other, whereas the two copies within BP3 are in tandem. The presence of large duplicated segments on chromosome 15q11-q13 provides a mechanism for homologous unequal recombination events that may mediate the frequent rearrangements observed for this chromosome.     

Duplication of chromosome 15 in the region 15q11-13 in a patient with developmental delay and ataxia with similarities to Angelman syndrome.

Duplications of the proximal long arm of chromosome 15 have been seen in the Prader-Willi syndrome (PWS), and in subjects without the Prader-Willi phenotype but with other clinical features including short stature, diabetes, anal and jejunal atresia, and acanthosis nigricans. The non-PWS subjects all had different phenotypes despite the identical findings on cytogenetic analysis. A normal phenotype has also been observed in patients with similar duplications. We report a further patient with a duplication of 15q11-13 which was detected cytogenetically and confirmed on molecular genetic analysis. She has developmental delay, particularly concerning the acquisition of speech, and an ataxic gait. These are interesting clinical features in view of the association of Angelman syndrome with abnormalities of 15q11-13.     

Genomic inversions of human chromosome 15q11-q13 in mothers of Angelman syndrome patients with class II (BP2/3) deletions

Parental submicroscopic genomic inversions have recently been demonstrated to be present in several genomic disorders. These inversions are genomic polymorphisms that facilitate misalignment and abnormal recombination between flanking segmental duplications. Angelman syndrome (AS; MIM 105830) is associated with specific abnormalities of chromosome 15q11-q13, with about 70% of cases being mother-of-origin 4 Mb deletions. We present here evidence that some mothers of AS patients with deletions of the 15q11-q13 region have a heterozygous inversion involving the region that is deleted in the affected offspring. The inversion was detected in the mothers of four of six AS cases with the breakpoint 2-3 (BP2/3) 15q11-q13 deletion, but not in seven mothers of AS due to paternal uniparental disomy (UPD) 15. We have identified variable inversion breakpoints within BP segmental duplications in the inverted AS mothers, as well as in AS deleted patients. Interestingly, the BP2-BP3 region is inverted in the mouse draft genome sequence with respect to the human draft sequence. The BP2-BP3 chromosome 15q11-q13 inversion was detected in four of 44 subjects (9%) of the general population (P<0.004). The BP2/3 inversion should be an intermediate estate that facilitates the occurrence of 15q11-q13 BP2/3 deletions in the offspring.     

Molecular dissection of the Prader-Willi/Angelman syndrome region (15q11-13) by YAC cloning and FISH analysis

The authors wish to note an error in the relative order of probesPW71 and TD189-1. The order of PWS/AS probes should be revisedas follows: cen-IR39-ML34-IR4-3R-TD189-1-PW71-LS6-1-TD3-21-GABRB3-IR10-1-CMW1-tel.Thecorrected map of the PWS/AS critical region (Figure 4) summarizingprobe order from interphase FISH analysis and YAC contig informationis provided below. The reversed order of TD189-1 and PW71 wasdiscovered by analysis of YAC 71B11, a non-chimeric YAC of 700kb from the CEPH library which was identified with the STS fromIR4-3R as indicated in Table 1. This YAC was also positive foran STS from the left end of YAC 307A12, identified with theSTS from TD 189-1. In confirmation of these results, the rightend of 71B11 was positive for 307A12 by hybridization, and anSTS from the left end of 71B11 was positive for YACs 172A10and 495D1. The Alu-PCR dot-blot hybridization experiment inFigure 1a appears to represent a false positive overlap betweenYACs A156E1 (used as probe) and B58C7 (labeled 58 on the figure).This is possibly due to homologous sequences contained withinthese two YACs. All other Alu-PCR dot-blot experiments havebeen confirmed by either YAC end hybridization by STS analysis.