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.     

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.     

Comparison of high resolution chromosome banding and fluorescence in situ hybridization (FISH) for the laboratory evaluation of Prader-Willi syndrome and angelman syndrome

The development of probes containing segments of DNA from chromosome region 15q11-q13 provides the opportunity to confirm the diagnosis of Prader-Willi syndrome (PWS) and Angelman syndrome (AS) by fluorescence in situ hybridization (FISH). We have evaluated FISH studies and high resolution chromosome banding studies in 14 patients referred to confirm or rule out PWS and five patients referred to confirm or rule out AS. In four patients (three from the PWS category and 1 from the AS group) chromosome analysis suggested that a deletion was present but FISH failed to confirm the finding. In one AS group patient, FISH identified a deletion not detectable by high resolution banding. Review of the clinical findings in the discrepant cases suggested that the FISH results were correct and high resolution findings were erroneous. Studies with a chromosome 15 alpha satellite probe (D15Z) on both normal and abnormal individuals suggested that incorrect interpretation of chromosome banding may occasionally be attributable to alpha satellite polymorphism but other variation of 15q11-q13 chromosome bands also contributes to misinterpretation. We conclude that patients who have been reported to have a cytogenetic deletion of 15q11-q13 and who have clinical findings inconsistent with PWS and AS should be reevaluated by molecular genetic techniques. © 1994 Wiley-Liss, Inc.     

Difficulties of genetic counseling and prenatal diagnosis in a consanguineous couple segregating for the same translocation (14;15) (q11;q13) and at risk for Prader-Willi and Angelman syndromes

Prader-Willi syndrome (PWS) and Angelman syndrome (AS) are associated with a loss of function of imprinted genes in the 15q11-q13 region mostly due to deletions or uniparental disomies (UPD). These anomalies usually occur de novo with a very low recurrence risk. However, in rare cases, familial translocations are observed, giving rise to a high recurrence risk. We report on the difficulties of genetic counseling and prenatal diagnosis in a family segregating for a translocation (14;15)(q11;q13) where two consanguineous parents carry the same familial translocation in this chromosome 15 imprinting region. Both children of the couple inherited a chromosomal anomaly leading to PWS. However, a paternal 15q11-q13 deletion was responsible for PWS in the first child, whereas prenatal diagnosis demonstrated that PWS was associated with a maternal 15q11-q13 UPD in the fetus. This report demonstrates that both conventional and molecular cytogenetic parental analyses have to be performed when a deletion is responsible for PWS or AS in order not to overlook a familial translocation and to insure reliable diagnosis and genetic counseling.     

The mechanisms involved in formation of deletions and duplications of 15q11-q13.

Haplotype analysis was undertaken in 20 cases of 15q11-q13 deletion associated with Prader-Willi syndrome (PWS) or Angelman syndrome (AS) to determine if these deletions arose through unequal meiotic crossing over between homologous chromosomes. Of these, six cases of PWS and three of AS were informative for markers on both sides of the deletion. For four of six cases of paternal 15q11-q13 deletion (PWS), markers on both sides of the deletion breakpoints were inferred to be of the same grandparental origin, implying an intrachromosomal origin of the deletion. Although the remaining two PWS cases showed evidence of crossing over between markers flanking the deletion, this was not more frequent than expected by chance given the genetic distance between proximal and distal markers. It is therefore possible that all PWS deletions were intrachromosomal in origin with the deletion event occurring after normal meiosis I recombination. Alternatively, both sister chromatid and homologous chromosome unequal exchange during meiosis may contribute to these deletions. In contrast, all three cases of maternal 15q11-q13 deletion (AS) were associated with crossing over between flanking markers, which suggests significantly more recombination than expected by chance (p = 0.002). Therefore, there appears to be more than one mechanism which may lead to PWS/AS deletions or the resolution of recombination intermediates may differ depending on the parental origin of the deletion. Furthermore, 13 of 15 cases of 15q11-q13 duplication, triplication, or inversion duplication had a distal duplication breakpoint which differed from the common distal deletion breakpoint. The presence of at least four distal breakpoint sites in duplications indicates that the mechanisms of rearrangement may be complex and multiple repeat sequences may be involved.     

Mechanisms of imprinting of the Prader-Willi/Angelman region

Prader-Willi syndrome (PWS) and Angelman syndrome (AS) are two distinct neurodevelopmental disorders, each caused by several genetic and epigenetic mechanisms involving the proximal long arm of chromosome 15. Lack of a functional paternal copy of 15q11-q13 causes PWS; lack of a functional maternal copy of UBE3A, a gene within 15q11-q13, causes AS. This region of chromosome 15 contains a number of imprinted genes that are coordinately regulated by an imprinting center (PWS/AS-IC) that contains two functional elements, the PWS-SRO and the AS-SRO. A chromosome lacking the PWS-SRO has the maternal state of gene activity and epigenetic modification after either maternal or paternal transmission; a chromosome lacking the AS-SRO but containing the PWS-SRO has the paternal state of gene activity and epigenetic modification after either maternal or paternal transmission. The maternal state of chromosome 15q11-q13 is associated with methylation of the PWS-SRO, while the paternal state is associated with lack of methylation of the PWS-SRO. Although most models of PWS/AS region imprinting assume that the PWS-SRO is methylated during oogenesis and that this methylation of the maternal PWS-SRO is maintained after fertilization, several lines of evidence suggest that the maternal PWS-SRO is in fact not methylated until after fertilization. Imprinting defects affecting the PWS/AS region can arise from failure to demethylate the PWS-SRO in the male germ line, from failure to methylate the maternal PWS-SRO, or from failure to maintain PWS-SRO methylation after fertilization.     

Array-based comparative genomic hybridization analysis of recurrent chromosome 15q rearrangements

Genomic rearrangements of chromosome 15q11-q13 cause diverse phenotypes including autism, Prader-Willi syndrome (PWS), and Angelman syndrome (AS). This region is subject to genomic imprinting and characterized by complex combinations of low copy repeat elements. Prader-Willi and Angelman syndrome are caused primarily by 15q11-13 deletions of paternal and maternal origin, respectively. Autism is seen with maternal, but not paternal, interstitial duplications. Isodicentric 15q, most often of maternal origin, is associated with a complex phenotype often including autistic features. Limitations of conventional cytogenetic tests preclude a detailed analysis in most patients with 15q rearrangements. We have developed a microarray for comparative genomic hybridization utilizing 106 genomic clones from chromosome 15q to characterize this region. The array accurately localized all breakpoints associated with gains or losses on 15q. The results confirmed the location of the common breakpoints associated with interstitial deletions and duplications. The majority of idic(15q) chromosomes are comprised of symmetrical arms with four copies of the breakpoint 1 to breakpoint 5 region. Patients with less common breakpoints that are not distinguished by routine cytogenetic methods were more accurately characterized by array analysis. This microarray provides a detailed characterization for chromosomal abnormalities involving 15q11-q14 and is useful for more precise genotype-phenotype correlations for autism, PWS, AS, and idic(15) syndrome.     

Imprinting center analysis in Prader-Willi and Angelman syndrome patients with typical and atypical phenotypes

Prader-Willi syndrome (PWS) and Angelman syndrome (AS) are genetic disorders caused by a deficiency of imprinted gene expression from the paternal or maternal chromosome 15, respectively. This deficiency is due to the deletion of the 15q11-q13 region, parental uniparental disomy of the chromosome 15, or imprinting defect (ID). Mutation of the UBE3A gene causes approximately 10% of AS cases. In this present study, we describe the molecular analysis and phenotypes of two PWS patients and four AS patients with ID. One of the PWS patients has a non-familial imprinting center (IC) deletion and displayed a severe phenotype with an atypical PWS appearance, hyperactivity and psychiatric vulnerability. The other PWS and AS patients did not present genetic abnormalities in the IC, suggesting an epimutation as the genetic cause. The methylation pattern of two AS patients showed a faint maternal band corresponding to a mosaic ID. One of these mosaic patients displayed a mild AS phenotype while the other displayed a PWS-like phenotype.     

FISH detection of chromosome 15 deletions in Prader-Willi and Angelman syndromes

We have evaluated fluorescence in situ hybridization (FISH) analysis for the clinical laboratory detection of the 15q11-q13 deletion seen in Prader-Willi syndrome (PWS) and Angelman syndrome (AS) using probes for loci D15S11, SNRPN, D15S10, and GABRB3. In a series of 118 samples from patients referred for PWS or AS, 29 had deletions by FISH analysis. These included two brothers with a paternally transmitted deletion detectable with the probe for SNRPN only. G-banding analysis was less sensitive for deletion detection but useful in demonstrating other cytogenetic alterations in four cases. Methylation and CA-repeat analyses of 15q11-q13 were used to validate the FISH results. Clinical findings of patients with deletions were variable, ranging from newborns with hypotonia as the only presenting feature to children who were classically affected. We conclude that FISH analysis is a rapid and reliable method for detection of deletions within 15q11-q13 and whenever a deletion is found, FISH analysis of parental chromosomes should also be considered. © 1996 Wiley-Liss, Inc.     

Prader-Willi and Angelman syndromes: sister imprinted disorders

Prader-Willi syndrome (PWS) and Angelman syndrome (AS) are clinically distinct complex disorders mapped to chromosome 15q11-q13. They both have characteristic neurologic, developmental, and behavioral phenotypes plus other structural and functional abnormalities. However, the cognitive and neurologic impairment is more severe in AS, including seizures and ataxia. The behavioral and endocrine disorders are more severe in PWS, including obsessive-compulsive symptoms and hypothalamic insufficiency. Both disorders can result from microdeletion, uniparental disomy, or an imprinting center defect in 15q11-q13, although the abnormality is on the paternally derived chromosome 15 for PWS and the maternally derived 15 for AS because of genomic imprinting. Although the same gene may control imprinting for both disorders, the gene(s) causing their phenotypes differ. AS results from underexpression of a single gene, UBE3A, which codes for E6-AP, a protein that functions to transfer small ubiquitin molecules to certain target proteins, to enable their degradation. The genes responsible for PWS are not determined, although several maternally imprinted genes in 15q11-q13 are known. The most likely candidate is SNRPN, which codes for a small nuclear ribonucleoprotein, a ribosome-associated protein that controls gene splicing and thus synthesis of critical proteins in the brain. Animal models exist for both disorders. The genetic relationship between PWS and AS makes them unique and potentially highly instructive disorders that contribute substantially to the population burden of cognitive impairment.     

Interstitial 6q deletion and Prader-Willi-like phenotype

A third case of an interstitial deletion of the long arm of chromosome 6 with clinical features mimicking Prader-Willi syndrome (PWS) is presented. Although preliminary clinical evaluation in each case suggested PWS, further review revealed that the features in all three cases are not completely compatible with the characteristic findings in Prader-Willi syndrome. Furthermore, the deletions in the three cases do not show a consistent region of overlap. Consequently, no particular band or region in 6q can be defined as associated widi obesity. However, our findings confirm the suggestion of Villa et al. in 1995, that individuals with a PWS phenotype who are cytogenetically and molecularly negative for a deletion of 15q11-q13 should be examined for a deletion of 6q.     

Angelman and Prader-Willi syndromes share a common chromosome 15 deletion but differ in parental origin of the deletion

Many Prader-Willi syndrome (PWS) and Angelman syndrome (AS) patients have a cytogenetic deletion of 15q11q13. While AS and PWS share a similar cytogenetic anomaly, they have very different clinical phenotypes. DNAs from 4 AS patients were examined using 5 chromosome 15q11q13-specific cloned DNA segments. With the present level of resolution, the molecular deletions between AS and those previously reported for PWS did not appear to differ. However, in contrast to the paternal inheritance of the deleted chromosome 15 observed in the majority of PWS patients, maternal inheritance of the deleted chromosome 15 was demonstrated in the AS patients by restriction fragment length polymorphisms (RFLPs).     

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.     

Genomic imprinting: potential function and mechanisms revealed by the Prader-Willi and Angelman syndromes

The Prader-Willi (PWS) and Angelman (AS) syndromes are two clinically distinct syndromes which result from lack of expression of imprinted genes within chromosome 15q11-q13. These two syndromes result from 15q11-q13 deletions, chromosome 15 uniparental disomy (UPD), imprinting centre mutations and, for AS, probable mutations in a single gene. The differential phenotype results from a paternal genetic deficiency in PWS patients and a maternal genetic deficiency in AS patients. Within 15q11-q13, four genes (SNRPN, IPW, ZNF127, FNZ127) and two expressed sequence tags (PAR1 and PAR5) have been found to be expressed only from the paternally inherited chromosome, and therefore all must be considered candidate genes involved in the pathogenesis of PWS. A candidate AS gene (UBE3A) has very recently been identified. The mechanisms of imprinted gene expression are not yet understood, but it is clear that DNA methylation is involved in both somatic cell expression and inheritance of the imprint. The presence of DNA methylation imprints that distinguish the paternally and maternally inherited alleles is a common characteristic of all known imprinted genes which have been studied extensively, including SNRPN and ZNF127. Recently, several PWS and AS patients have been found that have microdeletions in a region upstream of the SNRPN gene referred to as the imprinting centre, or IC. Paternal IC deletions in PWS patients and maternal IC deletions in AS patients result in uniparental DNA methylation and uniparental gene expression at biparentally inherited loci. The IC is a novel genetic element which controls initial resetting of the parental imprint in the germline for all imprinted gene expression over a 1.5-2.5 Mb region within chromosome 15q11-q13.      

Translocations in Prader-WiIIi syndrome

The Prader-Willi Syndrome (PWS) has frequently been associated with chromosomal anomalies involving the region 15q11-q12. The first case of this syndrome associated with a de novo translocation involving chromosomes 11 and 15 is reported. The breakpoints were identified as 11q25 and 15q11 or q12[45, XX,t(11;15)(q25;q11-12)], resulting in the deletion of 15pter leads to 15q11-q12. Previously reported cases of PWS associated with translocations are reviewed in relation to the "deletion hypothesis."     

Familial translocation t(Y;15)(q12;p11) and de novo deletion of the Prader-Willi syndrome (PWS) critical region on 15q11-q13

We describe a 17-year-old girl with mild Prader-Willi syndrome (PWS) due to 15q11-q13 deletion. The deletion occurred on a paternal chromosome 15 already involved in a translocation, t(Y;15)(q12;p11), the latter being present in five other, phenotypically normal individuals in three generations. This appears to be the first case of PWS in which the causative 15q11-q13 deletion occurred on a chromosome involved in a familial translocation, but with breakpoints considerably distal to those of the familial rearrangement. The translocation could predispose to additional rearrangements occurring during meiosis and/or mitosis or, alternatively, the association of two cytogenetic anomalies on the same chromosome could be fortuitous. Am. J. Med. Genet. 70: 222–228, 1997. © 1997 Wiley-Liss, Inc.     

Genetic studies of Prader-Willi patients provide evidence for conservation of genomic architecture in proximal chromosome 15q

Prader-Willi syndrome (PWS) is a neurogenetic disorder associated with recurrent genomic recombination involving low copy repeats (LCRs) located in the human chromosome 15q11-q13. Previous studies of PWS patients from Asia suggested that there is a higher incidence of deletion and lower incidence of maternal uniparental disomy (mUPD) compared to that of Western populations. In this report, we present genetic etiology of 28 PWS patients from Taiwan. Consistent with the genetic etiology findings from Western populations, the type II deletion appears to be the most common deletion subtype. Furthermore, the ratio of the two most common deletion subtypes and the ratio of the maternal heterodisomy to isodisomy cases observed from this study are in agreement with previous findings from Western populations. In addition, we identified and further mapped the deletion breakpoints in two patients with atypical deletions using array CGH (comparative genomic hybridization). Despite the relatively small numbers of patients in each subgroup, our findings suggest that the genomic architecture responsible for the recurrent recombination in PWS is conserved in Taiwanese of the Han Chinese heritage and Western populations, thereby predisposing chromosome 15q11-q13 to a similar risk of rearrangements.     

A rapid, PCR based test for differential molecular diagnosis of Prader-Willi and Angelman syndromes.

Approximately 98% of Prader-Willi syndrome (PWS) and 80% of Angelman syndrome (AS) cases have deletions at a common region in chromosome 15q11-13, uniparental disomy for chromosomes 15 (UPD15), or mutations affecting gene expression in this region. The resulting clinical phenotype (PWS or AS) in each class of mutation depends upon the parent of origin. Both disorders are characterised at the molecular level by abnormal methylation of imprinted genes at 15q11-q13 including the small nuclear ribonucleoprotein N gene (SNRPN). Current diagnostic strategies include high resolution cytogenetics, fluorescence in situ hybridisation (FISH), Southern blot hybridisation, or microsatellite typing. We have developed a novel and rapid diagnostic test for PWS and AS based on differential digestion of expressed (paternally imprinted) SNRPN sequences by the methylation sensitive endonuclease NotI or repressed (maternally imprinted) SNRPN sequences by the methylation requiring nuclease McrBC, followed by PCR amplification of the SNRPN promoter. We have evaluated this test by blinded analysis of 60 characterised DNA samples (20 PWS, 20 AS, and 20 unaffected controls). SNRPN sequences could not be amplified from PWS patient DNA which had been digested with McrBC, nor from AS patient DNA which had been digested with NotI. We were able to make a correct diagnosis of PWS, AS, or unaffected in all 60 samples tested. This novel test is rapid and has a high specificity and sensitivity for deletion and UPD15 cases. These features make this new test suitable as the initial step in a molecular diagnostic strategy for PWS/AS.     

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.