A 5-year-old white girl with Prader-Willi syndrome and a submicroscopic deletion of chromosome 15q11q13

We report on a 5-year-old white girl with Prader-Willi syndrome (PWS) and a submicroscopic deletion of 15q11q13 of approximately 100–200 kb in size. High resolution chromosome analysis was normal but fluorescence in situ hybridization (FISH), Southern hybridization, and microsatellite data from the 15q11q13 region demonstrated that the deletion was paternal in origin and included the SNRPN, PAR-5, and PAR-7 genes from the proximal to distal boundaries of the deletion segment. SNRPN and PW71B methylation studies showed an abnormal pattern consistent with the diagnosis of PWS and supported the presence of a paternal deletion of 15q11q13 or an imprinting mutation. Biparental (normal) inheritance of PW71B (D15S63 locus) and a deletion of the SNRPN gene were observed by microsatellite, quantitative Southern hybridization, and/or FISH analyses. Our patient met the diagnostic criteria for PWS, but has no reported behavior problems, hyperphagia, or hypopigmentation. Our patient further supports SNRPN and possibly other genomic sequences which are deleted as the cause of the phenotype recognized in PWS patients. © 1996 Wiley-Liss, Inc.     

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.     

Maternal disomy and Prader-Willi syndrome consistent with gamete complementation in a case of familial translocation (3;15) (p25;q11.2)

Maternal uniparental disomy (UPD) for chromosome 15 is responsible for an estimated 30% of cases of Prader-Willi syndrome (PWS). We report on an unusual case of maternal disomy 15 in PWS that is most consistent with adjacent-1 segregation of a paternal t(3;15)(p25;q11.2) with simultaneous maternal meiotic nondisjunction for chromosome 15. The patient (J.B.), a 17-year-old white male with PWS, was found to have 47 chromosomes with a supernumerary, paternal der(15) consisting of the short arm and the proximal long arm of chromosome 15, and distal chromosome arm 3p. The t(3;15) was present in the balanced state in the patient's father and a sister. Fluorescent in situ hybridization analysis demonstrated that the PWS critical region resided on the derivative chromosome 3 and that there was no deletion of the PWS region on the normal pair of 15s present in J.B. Methylation analysis at exon alpha of the small nuclear ribonucleoprotein-associated polypeptide N (SNRPN) gene showed a pattern characteristic of only the maternal chromosome 15 in J.B. Maternal disomy was confirmed by polymerase chain reaction analysis of microsatellite repeats at the gamma-aminobutyric acid receptor beta3 subunit (GABRB3) locus. A niece (B.B.) with 45 chromosomes and the derivative 3 but without the der(15) demonstrated a phenotype consistent with that reported for haploinsufficiency of distal 3 p. Uniparental disomy associated with unbalanced segregation of non-Robertsonian translocations has been reported previously but has not, to our knowledge, been observed in a case of PWS. Furthermore, our findings are best interpreted as true gamete complementation resulting in maternal UPD 15 and PWS. Am. J. Med. Genet. 78:134–139, 1998. © 1998 Wiley-Liss, Inc.     

Prader-Willi syndrome in a child with mosaic trisomy 15 and mosaic triplo-X: a molecular analysis.

A 3.3 year old girl with Prader-Willi syndrome (PWS) and mosaicism for two aneuploidies, 47,XXX and 47,XX,+15, is presented. The triplo-X cell line was found in white blood cells and fibroblasts, the trisomy 15 cell line in 50% of the fibroblasts. Using methylation studies of the PWS critical region and by polymorphic microsatellite analysis, the existence of uniparental maternal heterodisomy for chromosome 15 was shown in white blood cells. This provided a molecular explanation for the PWS in this child. In fibrolasts, an additional paternal allele was detected for markers on chromosome 15, which is in agreement with the presence of mosaicism for trisomy 15 in these cells. This example provides direct evidence for trisomic rescue by reduction to disomy as a possible basis for PWS. Whereas the trisomy 15 was caused by a maternal meiosis I error, the triplo-X resulted from a postzygotic gain of a maternal X chromosome, as shown by the finding of two identical maternal X chromosomes in the 47,XXX cell line. Because the triplo-X and the trisomy 15 were present in different cell lines, gain of an X chromosome occurred either in the same cell division as the trisomy 15 rescue or shortly before or after.     

Rearrangement of chromosome 15 in the region q11.2----q12 in an individual with obesity syndrome and her normal mother

Rearrangement of the proximal long arm of chromosome 15 have been found in most patients with the Prader-Willi syndrome (PWS) and in some with Angelman syndrome. We present an individual with syndromic obesity and her normal mother, who both have an abnormal chromosome 15. The proposita is a 26-year-old women with marked obesity, acanthosis, nigricans, short fingers, and severe cone degeneration of the retina. She has high plasma insulin levels, hypothyroidism, and an empty sella on CT scan. High-resolution chromosome banding demonstrated an increase in band 15q12. Further analysis showed the same abnormal 15 in her normal mother but not in her normal sister. This case and recent reports in the literature indicate that duplication of chromosome 15q in the PWS region may be associated with a syndrome of obesity, acanthosis nigricans, empty sella, and rodcore dystrophy as well as with a normal phenotype. Whether normal individuals with such a duplication carry increased risk of having offspring with an obesity syndrome is yet to be determined.     

Rearrangement of chromosome 15 in the region q11.2→q12 in an individual with obesity syndrome and her normal mother

Rearrangements of the proximal long arm of chromosome 15 have been found in most patients with the Prader-Willi syndrome (PWS) and in some with Angelman syndrome. We present an individual with syndromic obesity and her normal mother, who both have an abnormal chromosome 15. The proposita is a 26-year-old woman with marked obesity, acanthosis nigricans, short fingers, and severe cone degeneration of the retina. She has high plasma insulin levels, hypothyroidism, and an empty sella on CT scan. High-resolution chromosome banding demonstrated an increase in band 15q12. Further analysis showed the same abnormal 15 in her normal mother but not in her normal sister. This case and recent reports in the literature indicate that duplications of chromosome 15q in the PWS region may be associated with a syndrome of obesity, acanthosis nigricans, empty sella, and rodcore dystrophy as well as with a normal phenotype. Whether normal individuals with such a duplication carry increased risk of having offspring with an obesity syndrome is yet to be determined.     

A case of Prader – Willi syndrome arising as a result of familial unbalanced translocation t(11;15)(q25;q13)

We report on a case of Prader-Willi syndrome (PWS) with a true reciprocal unbalanced translocation, 45,XX,-15,der(11)t(11;15)pat. The proposita was diagnosed clinically as having severe PWS. Molecular studies revealed loss of the paternal methylation pattern at locus D15S63 and a deletion encompassing the loci from at least D15S10 to D15S97 of paternal chromosome 15. FISH studies confirmed the deletion of 15q11-q13 region and the presence of two telomeres on all chromosomes. The proposita's father, the father's sister and their mother are all carriers of the same balanced translocation t(11;15)(q25;q13). By genomic imprinting we would expect that if the father's sister were to give birth to a child with the same unbalanced translocation as the proband, it would be affected by Angelman syndrome.
To date, a similar familial unbalanced translocation due to loss of the small chromosome 15 derivative has not been described.     

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.     

Prader-Willi syndrome: current understanding of cause and diagnosis

Prader-Willi syndrome (PWS) is characterized by hypotonia, obesity, hypogonadism, short stature, small hands and feet, mental deficiency, a characteristic face, and an interstitial deletion of the proximal long arm of chromosome 15 in about one-half of the patients. The incidence is estimated to be about 1 in 25,000, and PWS is the most common syndromal cause of human obesity. DNA abnormalities, usually deletions or duplications of chromosome 15, have been identified in individuals with PWS with or without recognizable chromosome 15 deletions. Paternal origin of the chromosome 15 deletion by cytogenetic and DNA studies has been found in nearly all PWS individuals studied. No cytogenetic evidence for chromosome breakage has been identified, although an environmental cause (e.g., paternal hydrocarbon-exposed occupations) of the chromosome 15 abnormality has been proposed. PWS patients with the chromosome 15 deletion are more prone to hypopigmentation compared with PWS individuals with normal chromosomes, but no other clinical differences are consistently identified between those with and without the chromosome deletion. Anthropometric, dermatoglyphic, and other clinical findings indicate homogeneity of PWS patients with the chromosome deletion and heterogeneity of the nondeletion patients. A review of our current understanding of the major clinical, cytogenetic, and DNA findings is presented, and clinical manifestations and cytogenetic abnormalities are summarized from the literature.     

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.     

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 characterization of small supernumerary marker chromosomes in two Prader-Willi patients

A small supernumerary chromosome was observed in two Prader-Willi syndrome (PWS) patients. The clinical diagnosis of PWS was confirmed by the ascertainment of the deletion of region 15q11-13 in one case and uniparental disomy (UPD) of the same region in the other. The markers were negative for dystamycinA/DAPI banding, did not contain NOR-positive satellites, and had an appearance consistent with a very small ring chromosome. Fluorescent in situ hybridization (FISH) analysis with the “all human centromere” probe indicated the presence of centromeric sequences in both markers. Chromosomal in situ suppression hybridization with chromosome specific libraries demonstrated that the small markers in the deleted and UPD patient originated from chromosome 15 and X, respectively. To the best of our knowledge these are the only PWS patients reported with a supernumerary marker chromosome other than inv dup(15) characterized by FISH. Am. J. Med. Genet. 68:99–104, 1997 © 1997 Wiley-Liss, Inc.     

De novo paracentric inversion (X)(q26q28) with features mimicking Prader-Willi syndrome

Different genetic and non-genetic disorders, including several chromosomal abnormalities, may mimic Prader-Willi syndrome (PWS). We report on an 11-year-old girl with features reminiscent of PWS due to an unreported de novo paracentric inversion Xq26q28. Microdeletion 15q11-q13 and maternal uniparental disomy 15 were ruled out. The importance of chromosomal studies in addition to molecular analysis on patients with features suggestive of PWS is stressed.     

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.     

Somatic mosaicism for maternal uniparental disomy 15 in a girl with Prader-Willi syndrome: confirmation by cell cloning and identification of candidate downstream genes

Although uniparental disomy often results from the postzygotic rescue of a meiotic non-disjunction event, mosaicism is usually confined to the placenta. We describe a girl with Prader-Willi syndrome (PWS) who is mosaic for normal cells and cells with maternal uniparental disomy 15 [upd(15)mat] in blood and skin. Somatic mosaicism was confirmed by cloning and genotyping of skin fibroblasts. X inactivation studies indicated that upd occurred prior to X inactivation. RNA samples from the cloned cells were used in DNA microarray experiments to study the effect of upd(15)mat on the gene expression pattern of fibroblasts. Proof of principle was obtained by detecting several chromosome 15 genes known to be imprinted. We did not obtain any evidence for novel 15q genes showing imprinted expression in fibroblasts. Differentially expressed genes on other chromosomes are candidates for downstream genes regulated by an imprinted gene and may play a role in the pathogenesis of PWS. The finding of strongly reduced mRNA levels in upd(15)mat cells of the gene encoding secretogranin II (SCG2), which is a precursor of the dopamine releasing factor secretoneurin, raises the question whether hyperphagia in patients with PWS might be due to a defect in dopamine-modulated food reward circuits.     

Mosaicism for maternal uniparental disomy 15 in a boy with some clinical features of Prader–Willi syndrome

Prader–Willi syndrome (PWS) is caused by the lack of paternal expression of imprinted genes in the human chromosomal region 15q11.2–q13.2, which can be due to an interstitial deletion at 15q11.2–q13 of paternal origin (65–75%), maternal uniparental disomy (matUPD) of chromosome 15 (20–30%), or an imprinting defect (1–3%). The majority of PWS-associated matUPD15 cases represent a complete heterodisomy of chromosome 15 or a mixture of hetero- and isodisomic regions across the chromosome 15. Pure maternal isodisomy is observed in only a few matUPD15 patients. Here we report a case of an 18-year-old boy with some clinical features of Prader–Willi syndrome, such as overweight, muscular hypotonia, facial dysmorphism and psychiatric problems, but there was no reason to suspect PWS in the patient based solely on the phenotype estimation. However, chromosomal microarray analysis (CMA) revealed mosaic loss of heterozygosity of the entire chromosome 15. Methylation-specific multiplex ligation-dependant probe amplification (MS-MLPA) analysis showed hypermethylation of the SNRPN and NDN genes in the PWS/AS critical region of chromosome 15 in this patient. Taking into consideration the MS-MLPA results and the presence of PWS features in the patient, we concluded that it was matUPD15, although the patient's parents were not enrolled in the study. According to CMA and karyotyping, no trisomic or monosomic cells were present. To the best of our knowledge, only two PWS cases with mosaic maternal isodisomy 15 and without trisomic/monosomic cell lines have been reported so far.     

A paternal deletion of MKRN3, MAGEL2 and NDN does not result in Prader�CWilli syndrome

The Prader–Willi syndrome (PWS) is caused by a 5–6 Mbp de novo deletion on the paternal chromosome 15, maternal uniparental disomy 15 or an imprinting defect. All three lesions lead to the lack of expression of imprinted genes that are active on the paternal chromosome only: MKRN3, MAGEL2, NDN, C15orf2, SNURF-SNRPN and more than 70 C/D box snoRNA genes (SNORDs). The contribution to PWS of any of these genes is unknown, because no single gene mutation has been described so far. We report on two patients with PWS who have an atypical deletion on the paternal chromosome that does not include MKRN3, MAGEL2 and NDN. In one of these patients, NDN has a normal DNA methylation pattern and is expressed. In another patient, the paternal alleles of these genes are deleted as the result of an unbalanced translocation 45,X,der(X)t(X;15)(q28;q11.2). This patient is obese and mentally retarded, but does not have PWS. We conclude that a deficiency of MKRN3, MAGEL2 and NDN is not sufficient to cause PWS.     

Submicroscopic deletion in cousins with Prader-Willi syndrome causes a grandmatrilineal inheritance pattern: effects of imprinting

The Prader-Willi syndrome (PWS) critical region on 15q11-q13 is subject to imprinting. PWS becomes apparent when genes on the paternally inherited chromosome are not expressed. Familial PWS is rare. We report on a family in which a male and a female paternal first cousin both have PWS with cytogenetically normal karyotypes. Fluorescence in situ hybridization (FISH) analysis shows a submicroscopic deletion of SNRPN, but not the closely associated loci D15S10, D15S11, D15S63, and GABRB3. The cousins' fathers and two paternal aunts have the same deletion and are clinically normal. The grandmother of the cousins is deceased and not available for study, and their grandfather is not deleted for SNRPN. DNA methylation analysis of D15S63 is consistent with an abnormality of the imprinting center associated with PWS. "Grandmatrilineal" inheritance occurs when a woman with deletion of an imprinted, paternally expressed gene is at risk of having affected grandchildren through her sons. In this case, PWS does not become evident as long as the deletion is passed through the matrilineal line. This represents a unique inheritance pattern due to imprinting.     

Dermatoglyphic features in Prader-Willi syndrome with respect to chromosomal findings

Dermatoglyphic findings were compared in 38 Prader-Willi syndrome (PWS) patients and 270 normal controls. Twenty-one of the PWS patients had an interstitial deletion of the proximal long arm of chromosome 15 and seventeen PWS cases had normal chromosomes. Findings in PWS are not diagnostic but do show some consistent deviations that can be used in the clinical evaluation of PWS patients. These include a displacement of the axial triradius away from the normal proximal position, an excess of whorls primarily on the thumbs, radial termination of the palmar A mainline, and lack of arches on the big toe. Deletion PWS patients were much more homogeneous than non-deletion cases with respect to plantar patterns. The previously reported deficit of plantar pattern intensity was restricted only to deletion PWS and was characterized by a lack of plantar interdigital II-IV patterns with almost exclusively hallucal distal loops.