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.     

Familial Prader-Willi syndrome: case report and a literature review

Prader-Willi syndrome (PWS) is a neurobehavioural disorder arising through a number of different genetic mechanisms. All involve loss of paternal gene expression from chromosome 15q11q13. Although the majority of cases of PWS are sporadic, precise elucidation of the causative genetic mechanism is essential for accurate genetic counselling as the recurrence risk varies according to the mechanism involved. A pair of siblings affected by PWS is described. Neither demonstrates a microscopically visible deletion in 15q11q13 or maternal disomy. Methylation studies at D15S63 and at the SNRPN locus confirm the diagnosis of PWS. Molecular studies reveal biparental inheritance in both siblings with the exception of D15S128 and D15S63 where no paternal contribution is present indicating a deletion of the imprinting centre. Family studies indicate that the father of the siblings carries the deletion which, he has inherited from his mother. The recurrence risk for PWS in his offspring is 50%.     

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.     

Cloning of the breakpoints of a submicroscopic deletion in an Angelman syndrome patient

The majority of cases of the two distinct disorders Prader–Willisyndrome (PWS) and Angelman syndrome (AS) result from cytogeneticdeletions of chromosome 15q11–q13. These deletions areexclusively of maternal origin in AS but of paternal originin PWS indicating that the 15q11–q13 region is subjectto genomic imprinting. Transmission of a submicroscopic deletionin one three generation family resulted in AS only upon maternaltransmission of the deletion with no clinical phenotype associatedwith paternal transmission (1, 2). The breakpoint of this submicroscopicdeletion has been cloned and sequenced. This is the first deletionjunction from the AS/PWS region which has been so characterized.The nucleotide sequence of the deletion junction revealed a19 bp insertion of unknown origin with no evidence of repetitiveelements. A probe from the proximal deletion breakpoint, PB11,lies within the currently defined minimum region of deletionoverlap in PWS, which contains the SNRPN and D15S63 locl. Ourresults suggest that the imprinted gene(s) responsible for thePWS phenotype are proximal of pB11 in this deletion overlapregion.     

Counselling dilemmas associated with the molecular characterisation of two Angelman syndrome families.

We report the molecular characterisation of two families with Angelman syndrome referred for prenatal diagnosis, in which atypical molecular findings resulted in counselling dilemmas. The first is a familial case of Angelman syndrome in which the two affected children have mutations which affect the imprinting mechanism, as shown by the presence of paternal DNA methylation patterns at D15S63 and SNRPN and biparental inheritance of 15q11-q13 markers. DNA prepared from a 21 week fetal blood sample detected a fetus with normal maternal and paternal DNA methylation patterns at D15S63, but inheritance of the same maternal chromosome 15q11-q13 as the two affected sibs. This is probably a result of germline mosaicism in the mother. The second is a case of Angelman syndrome with an atypical deletion of 15q11-q13, which involves both unusual proximal and distal breakpoints. The deletion was characterised in order to assess the risk of Angelman syndrome in a second pregnancy in the mother of this child.     

Clinical spectrum and molecular diagnosis of Angelman and Prader-Willi syndrome patients with an imprinting mutation

Recent studies have identified a new class of Prader-Willi syndrome (PWS) and Angelman syndrome (AS) patients who have biparental inheritance, but neither the typical deletion nor uniparental disomy (UPD) or translocation. However, these patients have uniparental DNA methylation throughout 15q11-q13, and thus appear to have a mutation in the imprinting process for this region. Here we describe detailed clinical findings of five AS imprinting mutation patients (three families) and two PWS imprinting mutation patients (one new family). All these patients have essentially the classical clinical phenotype for the respective syndrome, except that the incidence of microcephaly is lower in imprinting mutation AS patients than in deletion AS patients. Furthermore, imprinting mutation AS and PWS patients do not typically have hypopigmentation, which is commonly found in patients with the usual large deletion. Molecular diagnosis of these cases is initially achieved by DNA methylation analyses of the DN34/ZNF127, PW71 (D15S63), and SNRPN loci. The latter two probes have clear advantages in the simple molecular diagnostic analysis of PWS and AS patients with an imprinting mutation, as has been found for typical deletion or UPD PWS and AS cases. With the recent finding of inherited microdeletions in PWS and AS imprinting mutation families, our studies define a new class of these two syndromes. The clinical and molecular identification of these PWS and AS patients has important genetic counseling consequences. Am. J. Med. Genet. 68:195–206, 1997 © 1997 Wiley-Liss, Inc.     

Three siblings with Prader–Willi syndrome caused by imprinting center microdeletions and review

Prader–Willi syndrome (PWS) is a complex genetic imprinting disorder characterized by childhood obesity, short stature, hypogonadism/hypogenitalism, hypotonia, cognitive impairment, and behavioral problems. Usually PWS occurs sporadically due to the loss of paternally expressed genes on chromosome 15 with the majority of individuals having the 15q11‐q13 region deleted. Examples of familial PWS have been reported but rarely. To date 13 families have been reported with more than one child with PWS and without a 15q11‐q13 deletion secondary to a chromosome 15 translocation, inversion, or uniparental maternal disomy 15. Ten of those 13 families were shown to carry microdeletions in the PWS imprinting center. The microdeletions were found to be of paternal origin in nine of the ten cases in which family studies were carried out. Using a variety of techniques, the microdeletions were identified in regions within the complex SNRPN gene locus encompassing the PWS imprinting center. Here, we report the clinical and genetic findings in three adult siblings with PWS caused by a microdeletion in the chromosome 15 imprinting center inherited from an unaffected father that controls the activity of genes in the 15q11‐q13 region and summarize the 13 reported cases in the literature.

     

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.     

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.      

Modification of 15q11 -- q13 DNA methylation imprints in unique Angelman and Prader -- Willi patients

The clearest example of genomic Imprinting in humans comes fromstudies of the Angelman (AS) and Prader—Wil (PWS) syndromes.Although these are clinically distinct disorders, both typicallyresult from a loss of the same chromosomal region, 15q11 - q13.AS usually results from either a maternal deletion of this region,or paternal uniparental disomy (UPD; both chromosomes 15 Inheritedfrom the father). PWS results from paternal deletion of 15q11- q13 or maternal UPD of chromosome 15. We have recently describeda parent-specific DNA methylation imprint in a gene at the D15S9locus (new gene symbol, ZNF 127), within the 15q11 - q13 region,that identifies AS and PWS patients with either a deletion orUPD. Here we describe an AS sibship and three PWS patients inwhich chromosome 15 rearrangements alter the methylation stateat ZNF127, even though this locus is not directly involved inthe rearrangement. Parent-specific DNA methylation imprintsare also altered at ZNF127 and D15S63 (another locus with aparent-specific methylation imprint) in an AS sibship whichhave no detectable deletion or UPD of chromosome 15. These uniquepatients may provide insight into the imprinting process thatoccurs in proximal chromosome 15 in humans.     

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.     

The necdin gene is deleted in Prader-Willi syndrome and is imprinted in human and mouse

Human chromosome 15q11-q13 contains genes that are imprinted and expressed from only one parental allele. Prader-Willi syndrome (PWS) is due to the loss of expression of one or more paternally expressed genes on proximal human chromosome 15q, most often by deletion or maternal uniparental disomy. Several candidate genes and a putative imprinting centre have been identified in the deletion region. We report that the human necdin-encoding gene (NDN) is within the centromeric portion of the PWS deletion region, between the two imprinted genes ZNF127 and SNRPN. Murine necdin is a nuclear protein expressed exclusively in differentiated neurons in the brain. Necdin is postulated to govern the permanent arrest of cell growth of post-mitotic neurons during murine nervous system development. We have localized the mouse locus Ndn encoding necdin to chromosome 7 in a region of conserved synteny with human chromosome 15q11-q13, by genetic mapping in an interspecific backcross panel. Furthermore, we demonstrate that expression of Ndn is limited to the paternal allele in RNA from newborn mouse brain. Expression of NDN is detected in many human tissues, with highest levels of expression in brain and placenta. NDN is expressed exclusively from the paternally inherited allele in human fibroblasts. Loss of necdin gene expression may contribute to the disorder of brain development in individuals with PWS.      

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.     

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.     

Origin of uniparental disomy 15 in patients with Prader‐Willi or Angelman syndrome

Maternal uniparental disomy (UPD) accounts for ∼25% of Prader‐Willi patients (PWS) and paternal UPD for about 2–5% of Angelman syndrome (AS) patients. These findings and the parental origin of deletions are evidence of genomic imprinting in the cause of PWS and AS. The natural occurrence of UPD individuals allows the study of meiotic mechanisms resulting in chromosomal nondisjunction (ND). We selected patients with UPD15 from our sample of 30 PWS and 40 AS patients to study the origin of ND and the recombination along chromosome 15. These patients were analyzed with 10 microsatellites throughout the entire chromosome 15 (D15S541, D15S542, D15S11, D15S113, GABRB3, CYP19, D15S117, D15S131, D15S984, D15S115). The analysis disclosed seven heterodisomic PWS cases originating by meiosis I (MI) ND (four showed recombination and three no recombination), and one isodisomic PWS UPD15 originating by postzygotic duplication. Among the five paternal UPD15, we detected four isodisomies, three of which showed homozigosity for all markers, corresponding to a mitotic error, and one case originating from a paternal MII ND. Our results indicate that besides maternal MI and MII ND, paternal ND occurs when a PWS UPD15 patient originates from mitotic duplication of the maternal chromosome 15. ND events in AS are mainly due to mitotic errors, but paternal MII ND can occur and give origin to an AS UPD15 individual by two different mechanisms: rescue of a trisomic fetus or fertilization of a nullisomic egg with the disomic sperm, and in this case paternal and maternal ND are necessary. Am. J. Med. Genet. 94:249–253, 2000. © 2000 Wiley‐Liss, Inc.     

Genomic imprinting and uniparental disomy in Angelman and Prader-Willi syndromes: A review

Although Angelman (AS) and Prader-Willi (PWS) syndromes are human genetic disorders with distinctly different developmental and neurobehavioural phenotypes, they both have abnormalities in inheritance of chromosome 15q11–q13. Whether AS or PWS arises depends on the parental origin of a deletion or uniparental disomy (the inheritance of 2 copies of a genetic locus from only one parent) for 15q11–q13. Normal development requires a genetic contribution for this genetic region from both a male and female parent. The dependence on parental origin implies that genes in human 15q11–q13 have distinct functions depending upon epigenetic, parent-of-origin differences, known as genomic imprinting. Here, I review the role of uniparental disomy and genomic imprinting in the pathogenesis of AS and PWS, and briefly discuss phenotype-genotype correlations using candidate genes and mouse models, in particular for hypopigmentation. © 1993 Wiley-Liss, Inc.     

Congenital ichthyosis in Prader–Willi syndrome associated with maternal chromosome 15 uniparental disomy: Case report and review of autosomal recessive conditions unmasked by UPD

Prader–Willi syndrome (PWS) is a prototypic genetic condition related to imprinting. Causative mechanisms include paternal 15q11‐q13 deletion, maternal chromosome 15 uniparental disomy (UPD15), Prader–Willi Syndrome/Angelman Syndrome (PWS/AS) critical region imprinting defects, and complex chromosomal rearrangements. Maternal UPD15‐related PWS poses risks of concomitant autosomal recessive (AR) disorders when the mother carries a pathogenic variant in one of the genes on chromosome 15 associated with autosomal recessive inherited disease. Co‐occurrence of autosomal recessive conditions in the setting of UPD leads to increased complexity of the clinical phenotype, and may delay the diagnosis of PWS. We report a patient with PWS and associated congenital ichthyosis due to maternal UPD15, and a homozygous novel pathogenic variant in ceramide synthase 3 (CERS3). We also review the literature of associated disorders reported in the setting of maternal UPD15‐related PWS and provide a summary of the previously described CERS3 variants. This represents the second case of autosomal recessive congenital ichthyosis (ARCI) in the setting of PWS and UPD15. There needs to be a high index of suspicion of this genetic mechanism when there is unexpected phenotype or evolution of the clinical course in a patient with PWS.