Breakage in the SNRPN locus in a balanced 46,XY,t(15;19) Prader-Willi syndrome patient

Genetic diversity, demographic history, and selective signatures of Silkie chicken

BACKGROUND

Silkie is a traditional Chinese chicken breed characterized by its unique combination of specialized morphological traits. While previous studies have focused on the genetic basis of these traits, the overall genomic characteristics of the Silkie breed remain largely unexplored. In this study, we employed whole genome resequencing data to examine the genetic diversity, selective signals and demographic history of the Silkie breed through comparative analyses with seven other Chinese indigenous breeds (IDGBs), a commercial breed, and the wild ancestor Red Jungle Fowl.

RESULTS

In total, 20.8 million high-quality single nucleotide polymorphisms and 86 large structural variations were obtained. We discovered that Silkie exhibits a relatively high level of inbreeding and is genetically distinct from other IDGBs. Furthermore, our analysis indicated that Silkie has experienced a stronger historical population bottleneck and has a smaller effective population size compared with other IDGBs. We identified 45 putatively selected genes that are enriched in the melanogenesis pathway, which probably is related to the feather color. Among these genes, LMBR1 and PDSS2 have been previously associated with the extra toe and the hookless feathers, respectively. Six of the selected genes (KITLG, GSK3B, SOBP, CTBP1, ELMO2, SNRPN) are known to be associated with neurodevelopment and mental diseases in human, and are possibly related to the distinct behavior of Silkie. We further identified structural variants in Silkie and found previously reported variants linked to hyperpigmentation (END3), muff and beard (HOXB8), and Rose-comb phenotype (MNR2). Additionally, we found a 0.61 Mb inversion overlapping with the GMDS gene, which was previously linked to neurodevelopmental defects in zebrafish and humans. This may also be related to the behavior distinctiveness of Silkie.

CONCLUSIONS

Our study revealed that Silkie is genetically distinct and relatively highly inbred compared to other IDGB chicken populations, possibly attributed to more prolong population bottlenecks and selective breeding practice. These results enhance our understanding of how domestication and selective breeding have shaped the genome of Silkie. These findings contribute to the broader field of domestication and avian genomics, and have implications for the future conservation and breeding efforts.

N-Acetylglucosamine Kinase-Small Nuclear Ribonucleoprotein Polypeptide N Interaction Promotes Axodendritic Branching in Neurons via Dynein-Mediated Microtubule Transport

N-acetylglucosamine kinase (NAGK) has been identified as an anchor protein that facilitates neurodevelopment with its non-canonical structural role. Similarly, small nuclear ribonucleoprotein polypeptide N (SNRPN) regulates neurodevelopment and cognitive ability. In our previous study, we revealed the interaction between NAGK and SNRPN in the neuron. However, the precise role in neurodevelopment is elusive. In this study, we investigate the role of NAGK and SNRPN in the axodendritic development of neurons. NAGK and SNRPN interaction is significantly increased in neurons at the crucial stages of neurodevelopment. Furthermore, overexpression of the NAGK and SNRPN proteins increases axodendritic branching and neuronal complexity, whereas the knockdown inhibits neurodevelopment. We also observe the interaction of NAGK and SNRPN with the dynein light-chain roadblock type 1 (DYNLRB1) protein variably during neurodevelopment, revealing the microtubule-associated delivery of the complex. Interestingly, NAGK and SNRPN proteins rescued impaired axodendritic development in an SNRPN depletion model of Prader-Willi syndrome (PWS) patient-derived induced pluripotent stem cell neurons. Taken together, these findings are crucial in developing therapeutic approaches for neurodegenerative diseases.

Atypical 15q11.2-q13 Deletions and the Prader-Willi Phenotype

Background: Prader-Willi syndrome (PWS) is a rare genetic disorder resulting from the lack of expression of the PWS region (locus q11-q13) on the paternally derived chromosome 15, as a result of a type I or II paternal deletion (50%), maternal uniparental disomy (43%), imprinting defect (4%) or translocation (<1%). In very rare cases, atypical deletions, smaller or larger than the typical deletion, are identified. These patients may have distinct phenotypical features and provide further information regarding the genotype−phenotype correlation in PWS. Methods: A prospective study in eight patients (six males and two females) with an atypical deletion in the PWS region accompanies an overview of reported cases. Results: All patients had hypotonia (100%) and many had typical PWS facial characteristics (75%), social and emotional developmental delays (75%), intellectual disabilities (50%), neonatal feeding problems and tube feeding (63%), history of obesity (50%), hyperphagia (50%) and scoliosis (50%). All males had cryptorchidism. Two patients had two separate deletions in the PWS critical region. Conclusions: Our findings provide further insight into PWS genotype−phenotype correlations; our results imply that inclusion of both SNURF-SNPRN and SNORD-116 genes in the deletion leads to a more complete PWS phenotype. A larger deletion, extending further upstream and downstream from these genes, does not cause a more severe phenotype. Conventional PWS methylation testing may miss small deletions, which can be identified using targeted next generation sequencing. PWS’s phenotypic diversity might be caused by differentially methylated regions outside the 15q11.2 locus.

The Diagnostic Journey of a Patient with Prader-Willi-Like Syndrome and a Unique Homozygous SNURF-SNRPN Variant; Bio-Molecular Analysis and Review of the Literature

Prader–Willi syndrome (PWS) is a rare genetic condition characterized by hypotonia, intellectual disability, and hypothalamic dysfunction, causing pituitary hormone deficiencies and hyperphagia, ultimately leading to obesity. PWS is most often caused by the loss of expression of a cluster of genes on chromosome 15q11.2-13. Patients with Prader–Willi-like syndrome (PWLS) display features of the PWS phenotype without a classical PWS genetic defect. We describe a 46-year-old patient with PWLS, including hypotonia, intellectual disability, hyperphagia, and pituitary hormone deficiencies. Routine genetic tests for PWS were normal, but a homozygous missense variant NM_003097.3(SNRPN):c.193C>T, p.(Arg65Trp) was identified. Single nucleotide polymorphism array showed several large regions of homozygosity, caused by high-grade consanguinity between the parents. Our functional analysis, the ‘Pipeline for Rapid in silico, in vivo, in vitro Screening of Mutations’ (PRiSM) screen, showed that overexpression of SNRPN-p.Arg65Trp had a dominant negative effect, strongly suggesting pathogenicity. However, it could not be confirmed that the variant was responsible for the phenotype of the patient. In conclusion, we present a unique homozygous missense variant in SNURF-SNRPN in a patient with PWLS. We describe the diagnostic trajectory of this patient and the possible contributors to her phenotype in light of the current literature on the genotype–phenotype relationship in PWS.

A newborn screening pilot study using methylation-sensitive high resolution melting on dried blood spots to detect Prader-Willi and Angelman syndromes

Prader-Willi (PWS) and Angelman (AS) syndromes are two clinically distinct imprinted disorders characterized by genetic abnormalities at 15q11-q13. Early diagnosis of both syndromes provides improved treatment and accurate genetic counseling. Whole blood (WB) is the most common DNA source of many methodologies to detect PWS and AS, however, the need of WB makes a massive screening difficult in newborns due to economic and technical limitations. The aim of this study was to adapt a Methylation-sensitive High-Resolution Melting (MS-HRM) approach from dried blood spot (DBS) samples, assessing the different DNA isolation techniques and diagnostic performance. Over a 1-year period, we collected 125 DBS cards, of which 45 had already been diagnosed by MS-HRM (20 PWS, 1 AS, and 24 healthy individuals). We tested three different DBS-DNA extraction techniques assessing the DNA concentration and quality, followed by MS-HRM and statistical comparison. Each DBS-DNA extraction method was capable of accuracy in detecting all PWS and AS individuals. However, the efficiency to detect healthy individuals varied according to methodology. In our experience, DNA extracted from DBS analyzed by the MS-HRM methodology provides an accurate approach for genetic screening of imprinting related disorders in newborns, offering several benefits compared to traditional whole blood methods.

A rapid and accurate methylation-sensitive high-resolution melting analysis assay for the diagnosis of Prader Willi and Angelman patients

Background

Prader Willi (PWS) and Angelman (AS) syndromes are rare genetic disorders characterized by deletions, uniparental disomy, and imprinting defects at chromosome 15. The loss of function of specific genes caused by genetic alterations in paternal allele causes PWS while the absence in maternal allele results AS. The laboratory diagnosis of PWS and AS is complex and demands molecular biology and cytogenetics techniques to identify the genetic mechanism related to the development of the disease. The DNA methylation analysis in chromosome 15 at the SNURF‐SNRPN locus through MS‐PCR confirms the diagnosis and distinguishes between PWS and AS. Our study aimed to establish the MS‐PCR technique associated with High‐Resolution Melting (MS‐HRM) in PWS and AS diagnostic with a single pair of primers.

Methods

We collected blood samples from 43 suspected patients to a cytogenetic and methylation analysis. The extracted DNA was treated with bisulfite to perform comparative methylation analysis.

Results

MS‐HRM and MS‐PCR agreed in 100% of cases, identifying 19(44%) PWS, 3(7%) AS, and 21(49%) Normal. FISH analysis detected four cases of PWS caused by deletions in chromosome 15.

Conclusion

The MS‐HRM showed good performance with a unique pair of primers, dispensing electrophoresis gel analysis, offering a quick and reproducible diagnostic.

Translocation breakpoint disrupting the host SNHG14 gene but not coding genes or snoRNAs in typical Prader-Willi syndrome

Prader-Willi syndrome (PWS) is a well-known imprinting disorder arising from a loss of paternally imprinted gene(s) at 15q11.2-q13. We report a typical PWS patient with a balanced reciprocal translocation, 46, XY, t(15;19)(q11.2;q13.3). After Illumina whole-genome sequencing, we used BreakDancer-1.45 software to predict candidate breakpoints and manually investigated via the Integrated Genome Viewer. Breakpoint PCR followed by Sanger sequencing determined the t(15;19) breakpoints. We investigated the expression of upstream/centromeric and downstream/telomeric genes of the 15q11.2 breakpoint by reverse transcriptase PCR, using total RNA extracted from the patient's lymphoblasts. Of note, the expression of paternally expressed genes PWAR6, SNORD109A/B, SNORD116, IPW, and PWAR1, downstream of the breakpoint, was abolished. Interestingly, the breakpoint did not destroy protein coding genes or individual snoRNAs. These results indicate that these genes may play a major role in the PWS phenotype.

A novel deletion of SNURF/SNRPN exon 1 in a patient with Prader-Willi-like phenotype

Here we report the smallest deletion involving SNURF/SNRPN that causes major symptoms of Prader-Willi syndrome (PWS), including hypotonia, dysmorphic features, intellectual disability, and obesity. A female patient with the aforementioned and additional features was referred to the Mayo Clinic Cytogenetics laboratory for genetic testing. Chromosomal microarray analysis and subsequent Sanger sequencing identified a de novo 6.4 kb deletion at 15q11.2, containing exon 1 of the SNURF gene and exon 1 of the shortest isoform of the SNRPN gene. SNURF/SNRPN exon 1, which is methylated on the silent maternal allele, is associated with acetylated histones on the expressed paternal allele. This region also overlaps with the PWS-imprinting center (IC). Subsequent molecular methylation analysis was performed using methylation-specific MLPA (MS-MLPA), which characterized that the deletion of SNURF/SNRPN exon 1 was paternal in origin, consistent with the PWS-like phenotype. Since SNURF/SNRPN gene and the PWS-IC are known to regulate snoRNAs, it is likely that the PWS-like phenotype observed in patients with paternal SNURF/SNRPN deletion is due to the disrupted expression of SNORD116 snoRNAs.

The dilemma of diagnostic testing for Prader-Willi syndrome

Although Prader-Willi syndrome (PWS) is a well-described clinical dysmorphic syndrome, DNA testing is required for a definitive diagnosis. A definitive diagnosis can be made in approximately 99% of cases using DNA testing; there are a number of DNA tests that can be used for this purpose, although there is no set standard algorithm of testing. The dilemma arises because of the complex genetic mechanisms at the basis of PWS, which need to be elucidated. To establish the molecular mechanism with a complete work up, involves at least 2 tests. Here we discuss the commonly used tests currently available and suggest a cost-effective approach to diagnostic testing.

Prader-Willi Syndrome due to an Unbalanced de novo Translocation t(15;19)(q12;p13.3)

Prader-Willi syndrome (PWS) is a complex, multisystem genetic disorder characterized by endocrine, neurologic, and behavioral abnormalities. We report the first case of an unbalanced de novo reciprocal translocation of chromosomes 15 and 19, 45,XY,-15,der(19)t(15;19)(q12;p13.3), resulting in monosomy for the PWS critical chromosome region. Our patient had several typical features of PWS including infantile hypotonia, a poor suck and feeding difficulties, tantrums, skin picking, compulsions, small hands and feet, and food seeking, but not hypopigmentation, a micropenis, cryptorchidism or obesity as common findings seen in PWS at the time of examination at 6 years of age. He had seizures noted from 1 to 3 years of age and marked cognitive delay. High-resolution SNP microarray analysis identified an atypical PWS type I deletion in chromosome 15 involving the proximal breakpoint BP1. The deletion extended beyond the GABRB3 gene but was proximal to the usual distal breakpoint (BP3) within the 15q11q13 region, and GABRA5, GABRG3, and OCA2 genes were intact. No deletion of band 19p13.3 was detected; therefore, the patient was not at an increased risk of tumors from the Peutz-Jeghers syndrome associated with a deletion of the STK11 gene.

Prader-Willi syndrome: a review of clinical, genetic, and endocrine findings

INTRODUCTION

Prader-Willi syndrome (PWS) is a multisystemic complex genetic disorder caused by lack of expression of genes on the paternally inherited chromosome 15q11.2-q13 region. There are three main genetic subtypes in PWS: paternal 15q11-q13 deletion (65-75 % of cases), maternal uniparental disomy 15 (20-30 % of cases), and imprinting defect (1-3 %). DNA methylation analysis is the only technique that will diagnose PWS in all three molecular genetic classes and differentiate PWS from Angelman syndrome. Clinical manifestations change with age with hypotonia and a poor suck resulting in failure to thrive during infancy. As the individual ages, other features such as short stature, food seeking with excessive weight gain, developmental delay, cognitive disability and behavioral problems become evident. The phenotype is likely due to hypothalamic dysfunction, which is responsible for hyperphagia, temperature instability, high pain threshold, hypersomnia and multiple endocrine abnormalities including growth hormone and thyroid-stimulating hormone deficiencies, hypogonadism and central adrenal insufficiency. Obesity and its complications are the major causes of morbidity and mortality in PWS.

METHODS

An extensive review of the literature was performed and interpreted within the context of clinical practice and frequently asked questions from referring physicians and families to include the current status of the cause and diagnosis of the clinical, genetics and endocrine findings in PWS.

CONCLUSIONS

Updated information regarding the early diagnosis and management of individuals with Prader-Willi syndrome is important for all physicians and will be helpful in anticipating and managing or modifying complications associated with this rare obesity-related disorder.

Small mosaic deletion encompassing the snoRNAs and SNURF-SNRPN results in an atypical Prader-Willi syndrome phenotype

Genetic analyses were performed in a male patient with suspected Prader-Willi syndrome who presented with hypogonadism, excessive eating, central obesity, small hands and feet and cognition within the low normal range. However, he had no neonatal hypotonia or feeding problems during infancy. Chromosome analysis showed a normal male karyotype. Further analysis with array-CGH identified a mosaic 847 kb deletion in 15q11-q13, including SNURF-SNRPN, the snoRNA gene clusters SNORD116 (HBII-85), SNORD115, (HBII-52), SNORD109 A and B (HBII-438A and B), SNORD64 (HBII-13), and NPAP1 (C15ORF2). MLPA confirmed the deletion and the results were compatible with a paternal origin. Metaphase-FISH verified the mosaicism with the deletion present in 58% of leukocytes analyzed. Three smaller deletions in this region have previously been reported in patients with Prader-Willi syndrome phenotype. All three deletions included SNORD116, but only two encompassed parts of SNURF-SNRPN, implicating SNORD116 as the major contributor to the Prader-Willi phenotype. Our case adds further information about genotype-phenotype correlation and supports the hypothesis that SNORD116 plays a major role in the pathogenesis of Prader-Willi syndrome. Furthermore, it examplifies diagnostic difficulties in atypical cases and illustrates the need for additional testing methods when Prader-Willi syndrome is suspected.

Novel Tandem Duplication in Exon 1 of the SNURF/SNRPN Gene in a Child with Transient Excessive Eating Behaviour and Weight Gain

A deletion in 15q11.2 involving the SNURF/SNRPN gene is the typical finding in patients with Prader-Willi syndrome. Apart from translocations disrupting this gene, no other mutation types have been described so far. We report a patient in whom a small duplication in exon 1 of the SNURF/SNRPN gene was diagnosed which is predicted to interrupt only SNURF expression. The patient was investigated due to overgrowth, increased appetite and developmental delay in childhood. This duplication was inherited from her father who carries the duplication on his paternal chromosome 15 and also had transient excessive eating behaviour as an adolescent. RNA studies showed that the duplication introduces a premature stop codon in SNURF.

Allelic dropout can cause false-positive results for Prader-Willi and Angelman syndrome testing

The diagnosis of many genetic disorders relies on a combination of clinical suspicion and confirmatory genetic testing. Our laboratory uses a standard methylation-sensitive PCR (MSP) to target the differentially methylated SNRPN gene to test for Prader-Willi syndrome (PWS) and Angelman syndrome. One patient, a 27-month-old female, who lacked the classical clinical features of PWS, but had a molecular diagnosis of PWS by MSP by another laboratory, had repeat testing in our laboratory. Testing by MSP in our laboratory also identified an apparent loss of the unmethylated paternal allele, consistent with a diagnosis of PWS. Confirmatory testing using Southern blot analysis with a methylation-sensitive restriction enzyme showed a normal pattern of methylation, detecting both the methylated maternal and unmethylated paternal alleles. To investigate these discrepant results, we amplified and sequenced the SNRPN locus in this patient and identified a single nucleotide change within the binding site for the unmethylated DNA-specific primer. These results indicate this nucleotide change led to allelic dropout in the MSP analysis, yielding the false-positive result. Subsequently, MSP analysis using an alternate primer set that was developed by our laboratory detected both methylated and unmethylated alleles. These findings illustrate that allelic dropout due to the presence of rare polymorphisms can cause false-positive results in commonly used MSP assays and lead to molecular misdiagnosis.

Paternally inherited microdeletion at 15q11.2 confirms a significant role for the SNORD116 C/D box snoRNA cluster in Prader-Willi syndrome

Prader-Willi syndrome (PWS) is a neurobehavioral disorder manifested by infantile hypotonia and feeding difficulties in infancy, followed by morbid obesity secondary to hyperphagia. It is caused by deficiency of paternally expressed transcript(s) within the human chromosome region 15q11.2. PWS patients harboring balanced chromosomal translocations with breakpoints within small nuclear ribonucleoprotein polypeptide N (SNRPN) have provided indirect evidence for a role for the imprinted C/D box containing small nucleolar RNA (snoRNA) genes encoded downstream of SNRPN. In addition, recently published data provide strong evidence in support of a role for the snoRNA SNORD116 cluster (HBII-85) in PWS etiology. In this study, we performed detailed phenotypic, cytogenetic, and molecular analyses including chromosome analysis, array comparative genomic hybridization (array CGH), expression studies, and single-nucleotide polymorphism (SNP) genotyping for parent-of-origin determination of the 15q11.2 microdeletion on an 11-year-old child expressing the major components of the PWS phenotype. This child had an ∼236.29 kb microdeletion at 15q11.2 within the larger Prader-Willi/Angelman syndrome critical region that included the SNORD116 cluster of snoRNAs. Analysis of SNP genotypes in proband and mother provided evidence in support of the deletion being on the paternal chromosome 15. This child also met most of the major PWS diagnostic criteria including infantile hypotonia, early-onset morbid obesity, and hypogonadism. Identification and characterization of this case provide unequivocal evidence for a critical role for the SNORD116 snoRNA molecules in PWS pathogenesis. Array CGH testing for genomic copy-number changes in cases with complex phenotypes is proving to be invaluable in detecting novel alterations and enabling better genotype-phenotype correlations.

Balanced translocations in mental retardation

Over the past few decades, the knowledge on genetic defects causing mental retardation has dramatically increased. In this review, we discuss the importance of balanced chromosomal translocations in the identification of genes responsible for mental retardation. We present a database-search guided overview of balanced translocations identified in patients with mental retardation. We divide those in four categories: (1) balanced translocations that helped to identify a causative gene within a contiguous gene syndrome, (2) balanced translocations that led to the identification of a mental retardation gene confirmed by independent methods, (3) balanced translocations disrupting candidate genes that have not been confirmed by independent methods and (4) balanced translocations not reported to disrupt protein coding sequences. It can safely be concluded that balanced translocations have been instrumental in the identification of multiple genes that are involved in mental retardation. In addition, many more candidate genes were identified with a suspected but (as yet?) unconfirmed role in mental retardation. Some balanced translocations do not disrupt a protein coding gene and it can be speculated that in the light of recent findings concerning ncRNA's and ultra-conserved regions, such findings are worth further investigation as these potentially may lead us to the discovery of novel disease mechanisms.

A paternal deletion of MKRN3, MAGEL2 and NDN does not result in Prader-Willi 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.

Prader-Willi phenotype caused by paternal deficiency for the HBII-85 C/D box small nucleolar RNA cluster

Prader-Willi syndrome (PWS) is caused by deficiency for one or more paternally expressed imprinted transcripts within chromosome 15q11-q13, including SNURF-SNRPN and multiple small nucleolar RNAs (snoRNAs). Balanced chromosomal translocations that preserve expression of SNURF-SNRPN and centromeric genes but separate the snoRNA HBII-85 cluster from its promoter cause PWS. A microdeletion of the HBII-85 snoRNAs in a child with PWS provides, in combination with previous data, effectively conclusive evidence that deficiency of HBII-85 snoRNAs causes the key characteristics of the PWS phenotype, although some atypical features suggest that other genes in the region may make more subtle phenotypic contributions.

Methylation-specific multiplex ligation-dependent probe amplification analysis of subjects with chromosome 15 abnormalities

Prader-Willi syndrome (PWS) and Angelman syndrome (AS) are neurodevelopmental disorders caused by loss of expression of imprinted genes from the 15q11-q13 region. They arise from similar defects in the region but differ in parent of origin. There are two recognized typical 15q11-q13 deletions depending on size and several diagnostic assays are available but each has limitations. We evaluated the usefulness of a methylation-specific multiplex ligation-dependent probe amplification (MLPA) kit consisting of 43 probes to detect copy number changes and methylation status in the region. We used the MLPA kit to genotype 82 subjects with chromosome 15 abnormalities (62 PWS, 10 AS and 10 individuals with other chromosome 15 abnormalities) and 13 with normal cytogenetic findings. We developed an algorithm for MLPA probe analysis which correctly identified methylation abnormalities associated with PWS and AS and accurately determined copy number in previously assigned genetic subtypes including microdeletions of the imprinting center. Furthermore, MLPA analysis identified copy number changes in those with distal 15q deletions and ring 15s. MLPA is a relatively simple, cost-effective technique found to be useful and accurate for methylation status, copy number and analysis of genetic subtype in PWS and AS, as well as other chromosome 15 abnormalities.

Genomic analysis of the chromosome 15q11-q13 Prader-Willi syndrome region and characterization of transcripts for GOLGA8E and WHCD1L1 from the proximal breakpoint region

BACKGROUND

Prader-Willi syndrome (PWS) is a neurobehavioral disorder characterized by neonatal hypotonia, childhood obesity, dysmorphic features, hypogonadism, mental retardation, and behavioral problems. Although PWS is most often caused by a paternal interstitial deletion of a 6-Mb region of chromosome 15q11-q13, the identity of the exact protein coding or noncoding RNAs whose deficiency produces the PWS phenotype is uncertain. There are also reports describing a PWS-like phenotype in a subset of patients with full mutations in the FMR1 (fragile X mental retardation 1) gene. Taking advantage of the human genome sequence, we have performed extensive sequence analysis and molecular studies for the PWS candidate region.

RESULTS

We have characterized transcripts for the first time for two UCSC Genome Browser predicted protein-coding genes, GOLGA8E (golgin subfamily a, 8E) and WHDC1L1 (WAS protein homology region containing 1-like 1) and have further characterized two previously reported genes, CYF1P1 and NIPA2; all four genes are in the region close to the proximal/centromeric deletion breakpoint (BP1). GOLGA8E belongs to the golgin subfamily of coiled-coil proteins associated with the Golgi apparatus. Six out of 16 golgin subfamily proteins in the human genome have been mapped in the chromosome 15q11-q13 and 15q24-q26 regions. We have also identified more than 38 copies of GOLGA8E-like sequence in the 15q11-q14 and 15q23-q26 regions which supports the presence of a GOLGA8E-associated low copy repeat (LCR). Analysis of the 15q11-q13 region by PFGE also revealed a polymorphic region between BP1 and BP2. WHDC1L1 is a novel gene with similarity to mouse Whdc1 (WAS protein homology region 2 domain containing 1) and human JMY protein (junction-mediating and regulatory protein). Expression analysis of cultured human cells and brain tissues from PWS patients indicates that CYFIP1 and NIPA2 are biallelically expressed. However, we were not able to determine the allele-specific expression pattern for GOLGA8E and WHDC1L1 because these two genes have highly related sequences that might also be expressed.

CONCLUSION

We have presented an updated version of a sequence-based physical map for a complex chromosomal region, and we raise the possibility of polymorphism in the genomic orientation of the BP1 to BP2 region. The identification of two new proteins GOLGA8E and WHDC1L1 encoded by genes in the 15q11-q13 region may extend our understanding of the molecular basis of PWS. In terms of copy number variation and gene organization, this is one of the most polymorphic regions of the human genome, and perhaps the single most polymorphic region of this type.

Reciprocal translocations: a trap for cytogenetists?

We report four cases of subjects with phenotypic abnormalities and mental retardation associated with apparently balanced translocations, two inherited and two de novo, which showed, by molecular analysis, a hidden complexity. All the cases have been analyzed with different molecular techniques, including array-CGH, and in two of them the translocation breakpoints have been defined at the level of base pairs via studies in somatic hybrids containing single derivative chromosomes. We demonstrated that all the translocations were in fact complex rearrangements and that an imbalance was present in three of them, thus accounting for the phenotypic abnormalities. In one case, a Prader-Willi subject, we were not able to determine the molecular cause of his phenotype. This study, while confirming previous data showing unexpected complexity in translocations, further underscores the need for molecular investigations before taking for granted an apparently simple cytogenetic interpretation.

Molecular breakpoint cloning and gene expression studies of a novel translocation t(4;15)(q27;q11.2) associated with Prader-Willi syndrome

BACKGROUND

Prader-Willi syndrome (MIM #176270; PWS) is caused by lack of the paternally-derived copies, or their expression, of multiple genes in a 4 Mb region on chromosome 15q11.2. Known mechanisms include large deletions, maternal uniparental disomy or mutations involving the imprinting center. De novo balanced reciprocal translocations in 5 reported individuals had breakpoints clustering in SNRPN intron 2 or exon 20/intron 20. To further dissect the PWS phenotype and define the minimal critical region for PWS features, we have studied a 22 year old male with a milder PWS phenotype and a de novo translocation t(4;15)(q27;q11.2).

METHODS

We used metaphase FISH to narrow the breakpoint region and molecular analyses to map the breakpoints on both chromosomes at the nucleotide level. The expression of genes on chromosome 15 on both sides of the breakpoint was determined by RT-PCR analyses.

RESULTS

Pertinent clinical features include neonatal hypotonia with feeding difficulties, hypogonadism, short stature, late-onset obesity, learning difficulties, abnormal social behavior and marked tolerance to pain, as well as sticky saliva and narcolepsy. Relative macrocephaly and facial features are not typical for PWS. The translocation breakpoints were identified within SNRPN intron 17 and intron 10 of a spliced non-coding transcript in band 4q27. LINE and SINE sequences at the exchange points may have contributed to the translocation event. By RT-PCR of lymphoblasts and fibroblasts, we find that upstream SNURF/SNRPN exons and snoRNAs HBII-437 and HBII-13 are expressed, but the downstream snoRNAs PWCR1/HBII-85 and HBII-438A/B snoRNAs are not.

CONCLUSION

As part of the PWCR1/HBII-85 snoRNA cluster is highly conserved between human and mice, while no copy of HBII-438 has been found in mouse, we conclude that PWCR1/HBII-85 snoRNAs is likely to play a major role in the PWS- phenotype.

SNURF-SNRPN and UBE3A transcript levels in patients with Angelman syndrome

The imprinted domain on human chromosome 15 consists of two oppositely imprinted gene clusters, which are under the control of an imprinting center (IC). The paternally expressed SNURF-SNRPN gene hosts several snoRNA genes and overlaps the UBE3A gene, which is encoded on the opposite strand, expressed - at least in brain cells - from the maternal chromosome only, and affected in patients with Angelman syndrome (AS). In contrast to SNURF-SNRPN, imprinted expression of UBE3A is not regulated by a 5' differentially methylated region. Here we report that splice forms of the SNURF-SNRPN transcript overlapping UBE3A in an antisense orientation are present in brain but barely detectable in blood. In contrast, splice forms that do not overlap with UBE3A are of similar abundance in brain and blood. The tissue distribution of the splice forms parallels that of the snoRNAs encoded in the respective parts of the SNURF-SNRPN transcript. Using a quantitative PCR assay, we have found that the ratio of SNURF-SNRPN/UBE3A transcript levels is increased in blood cells of AS patients with an imprinting defect, but not in AS patients with a UBE3A mutation or an unknown defect. Our findings are compatible with the assumption that imprinted UBE3A expression is regulated through the SNURF-SNRPN sense- UBE3A antisense transcript.

Imprinting disorders: non-Mendelian mechanisms affecting growth

Most autosomal genes are expressed from both maternal and paternal alleles. However, imprinted genes are an example of non-Mendelian genetics, in which only one member of the gene pair is expressed and expression is determined by the parent of origin. Imprinted genes may account for 0.1-1% of all mammalian genes. At least 50 imprinted genes have been identified in humans, and imprinted genes frequently cluster under the control of an imprinting center. Many imprinted genes contribute to growth, either as growth factors, such as insulin-like growth factors (IGF2 in Beckwith-Wiedemann syndrome), or as growth inhibitors, such as the GRB10 gene in Russell-Silver syndrome. Imprinted genes have evolved over time in mammals to fine-tune the growth of the fetus. Paternally expressed genes generally enhance growth, whereas maternally expressed genes appear to suppress growth. In addition, normal and abnormal genomic imprinting and loss of heterozygosity contribute to a wide range of malignancies. A common process for controlling gene activity is methylation, which can be changed during male or female gametogenesis. Examples of classic human disorders related to genomic imprinting are Beckwith-Wiedemann syndrome (chromosome 11), Prader-Willi/Angelman syndromes (chromosome 15), Russell-Silver syndrome (chromosome 7), and Albright hereditary osteodystrophy (chromosome 20). Several of these disorders are discussed and illustrated.

Evidence for the role of PWCR1/HBII-85 C/D box small nucleolar RNAs in Prader-Willi syndrome

Prior work has suggested that loss of expression of one or more of the many C/D box small nucleolar RNAs (snoRNAs) encoded within the complex, paternally expressed SNRPN (small nuclear ribonuclear protein N) locus may result in the phenotype of Prader-Willi syndrome (PWS). We suggest that the minimal critical region for PWS is approximately 121 kb within the >460-kb SNRPN locus, bordered by a breakpoint cluster region identified in three individuals with PWS who have balanced reciprocal translocations and by the proximal deletion breakpoint of a familial deletion found in an unaffected mother, her three children with Angelman syndrome, and her father. The subset of SNRPN-encoded snoRNAs within this region comprises the PWCR1/HBII-85 cluster of snoRNAs and the single HBII-438A snoRNA. These are the only known genes within this region, which suggests that loss of their expression may be responsible for much or all of the phenotype of PWS. This hypothesis is challenged by findings in two individuals with PWS who have balanced translocations with breakpoints upstream of the proposed minimal critical region but whose cells were reported to express transcripts within it, adjacent to these snoRNAs. By use of real-time quantitative reverse-transcriptase polymerase chain reaction, we reassessed expression of these transcripts and of the snoRNAs themselves in fibroblasts of one of these patients. We find that the transcripts reported to be expressed in lymphoblast-somatic cell hybrids are not expressed in fibroblasts, and we suggest that the original results were misinterpreted. Most important, we show that the PWCR1/HBII-85 snoRNAs are not expressed in fibroblasts of this individual. These results are consistent with the hypothesis that loss of expression of the snoRNAs in the proposed minimal critical region confers much or all of the phenotype of PWS.

The SNRPN promoter is not required for genomic imprinting of the Prader-Willi/Angelman domain in mice

In mice and humans, the locus encoding the gene for small nuclear ribonucleoprotein N (SNRPN/Snrpn), as well as other loci in the region are subject to genomic imprinting. The SNRPN promoter is embedded in a maternally methylated CpG island, is expressed only from the paternal chromosome and lies within an imprinting center that is required for switching to and/or maintenance of the paternal epigenotype. We show here that a 0.9-kb deletion of exon 1 of mouse Snrpn did not disrupt imprinting or elicit any obvious phenotype, although it did allow the detection of previously unknown upstream exons. In contrast, a larger, overlapping 4.8-kb deletion caused a partial or mosaic imprinting defect and perinatal lethality when paternally inherited.

Disease associated balanced chromosome rearrangements: a resource for large scale genotype-phenotype delineation in man

Disease associated balanced chromosomal rearrangements (DBCRs), which truncate, delete, or otherwise inactivate specific genes, have been instrumental for positional cloning of many disease genes. A network of cytogenetic laboratories, Mendelian Cytogenetics Network (MCN), has been established to facilitate the identification and mapping of DBCRs. To get an estimate of the potential of this approach, we surveyed all cytogenetic archives in Denmark and southern Sweden, with a population of approximately 6.6 million. The nine laboratories have performed 71 739 postnatal cytogenetic tests. Excluding Robertsonian translocations and chromosome 9 inversions, we identified 216 DBCRs ( approximately 0.3%), including a minimum estimate of 114 de novo reciprocal translocations (0.16%) and eight de novo inversions (0.01%). Altogether, this is six times more frequent than in the general population, suggesting a causal relationship with the traits involved in most of these cases. Of the identified cases, only 25 (12%) have been published, including 12 cases with known syndromes and 13 cases with unspecified mental retardation/congenital malformations. The remaining DBCRs were associated with a plethora of traits including mental retardation, dysmorphic features, major congenital malformations, autism, and male and female infertility. Several of the unpublished DBCRs defined candidate breakpoints for nail-patella, Prader-Willi, and Schmidt syndromes, ataxia, and ulna aplasia. The implication of the survey is apparent when compared with MCN; altogether, the 292 participating laboratories have performed >2.5 million postnatal analyses, with an estimated approximately 7500 DBCRs stored in their archives, of which more than half might be causative mutations. In addition, an estimated 450-500 novel cases should be detected each year. Our data illustrate that DBCRs and MCN are resources for large scale establishment of phenotype-genotype relationships in man.

Prader-Willi Syndrome: Clinical and Genetic Findings

Since the initial medical description by Prader, Labhart and Willi in 1956 of individuals with overlapping features, the Prader-Willi syndrome has become recognized as a classical but sporadic genetic syndrome. Prader-Willi syndrome is the most common genetic cause of life-threatening obesity in humans. It is estimated that there are 350,000-400,000 people with this syndrome worldwide. Prader-Willi Syndrome Association USA knows of more than 3,400 persons with Prader-Willi syndrome in the USA out of an approximate 17,000-22,000. Prader-Willi syndrome with an incidence of 1 in 10,000 to 25,000 individuals and Angelman syndrome, an entirely different clinical condition, were the first examples in humans of genetic imprinting. Genetic imprinting or the differential expression of genetic information depending on the parent of origin plays a significant role in other conditions including malignancies.

Genetic abnormalities in Prader-Willi syndrome and lessons from mouse models

Prader-Willi syndrome is a multigenic disorder with developmental and neurobehavioural abnormalities. There are multiple genetic causes, although all ultimately involve the loss of paternally derived gene expression of chromosome region 15q11-q13. Multiple imprinted genes expressed only from the paternal allele have been identified in the specific region of human chromosome 15q associated with Prader-Willi syndrome and in the syntenic mouse chromosome 7C region, including a novel polycistronic gene (SNURF-SNRPN) that encodes two independent proteins. The latter genetic locus may play a key role in Prader-Willi syndrome and the evolution of imprinting in this domain, because it is uniquely involved with mutations in the imprinting process and balanced translocations in this syndrome. Indeed, based on the co-localization of SNURF and SNRPN within the imprinting control region critical to Prader-Willi syndrome, evolutionary arguments would suggest that this genetic locus is a prime candidate for mutations producing the failure-to-thrive phenotype of neonates with this syndrome and of corresponding mouse models. Hence, the SNURF-SNRPN gene may encode a paternally derived postnatal growth factor.

Chromosome breakage in the Prader-Willi and Angelman syndromes involves recombination between large, transcribed repeats at proximal and distal breakpoints

Prader-Willi syndrome (PWS) and Angelman syndrome (AS) are distinct neurobehavioral disorders that most often arise from a 4-Mb deletion of chromosome 15q11-q13 during paternal or maternal gametogenesis, respectively. At a de novo frequency of approximately.67-1/10,000 births, these deletions represent a common structural chromosome change in the human genome. To elucidate the mechanism underlying these events, we characterized the regions that contain two proximal breakpoint clusters and a distal cluster. Novel DNA sequences potentially associated with the breakpoints were positionally cloned from YACs within or near these regions. Analyses of rodent-human somatic-cell hybrids, YAC contigs, and FISH of normal or rearranged chromosomes 15 identified duplicated sequences (the END repeats) at or near the breakpoints. The END-repeat units are derived from large genomic duplications of a novel gene (HERC2), many copies of which are transcriptionally active in germline tissues. One of five PWS/AS patients analyzed to date has an identifiable, rearranged HERC2 transcript derived from the deletion event. We postulate that the END repeats flanking 15q11-q13 mediate homologous recombination resulting in deletion. Furthermore, we propose that active transcription of these repeats in male and female germ cells may facilitate the homologous recombination process.

The diagnosis of Prader-Willi syndrome

The methylation test can make the diagnosis of Prader-Willi syndrome (PWS) in approximately 99% of patients and is confirmed as a reliable, robust screening test. In a patient with PWS, methylation analysis does not provide the mechanism, for which other different genetic tests are required. Appropriate tests are available in each Australian state.

Paternal deletion from Snrpn to Ube3a in the mouse causes hypotonia, growth retardation and partial lethality and provides evidence for a gene contributing to Prader-Willi syndrome

Prader-Willi syndrome (PWS) is caused by paternal deficiency of human chromosome 15q11-q13. There is conflicting evidence from human translocations regarding the direct involvement of SNRPN in the pathogenesis of PWS and it is not known if the phenotypic features result from the loss of expression of a single imprinted gene or multiple genes. In an attempt to dissect genotype/phenotype correlations for the homologous region of mouse chromosome 7C, we prepared three mutant genotypes: (i) mice with a deletion of Snrpn exon 2, which removes a portion of a small, upstream open reading frame (ORF); (ii) mice with double targeting for Snrpn exon 2 and Ube3a; (iii) mice deleted from Snrpn to Ube3a, removing coding exons for both loci and intervening genes. Mice deleted for Snrpn exon 2 have no obvious phenotypic abnormalities and switching of the genomic imprint for the region is conserved. Mice carrying the Snrpn - Ube3a deletion on the paternal chromosome showed severe growth retardation, hypotonia and approximately 80% lethality before weaning. The surviving mice were fertile and were not obese up to 14 months of age. The deletion was transmitted for multiple generations and continued to cause partial lethality when inherited paternally, but not when inherited maternally. The normal imprinted expression and methylation patterns of necdin, a gene outside the deletion region, indicate that the deletion is not an imprinting mutation. The data suggest the presence of a paternally expressed structural gene between Snrpn and Ipw whose deficiency causes lethality, although other possibilities exist, including position effects on expression of imprinted genes or that simultaneous deficiency of both ORFs of Snrpn causes lethality.

An imprinted, mammalian bicistronic transcript encodes two independent proteins

Polycistronic transcripts are common in prokaryotes but rare in eukaryotes. Phylogenetic analysis of the SNRPN (SmN) mRNA in five eutherian mammals reveals a second highly conserved coding sequence, termed SNURF (SNRPN upstream reading frame). The vast majority of nucleotide substitutions in SNURF occur in the wobble codon position, providing strong evolutionary evidence for selection for protein-coding function. Because SNURF-SNRPN maps to human chromosome 15q11-q13 and is paternally expressed, each cistron is a candidate for a role in the imprinted Prader-Willi syndrome (PWS) and PWS mouse models. SNURF encodes a highly basic 71-aa protein that is nuclear-localized (as is SmN). Because SNURF is the only protein-coding sequence within the imprinting regulatory region in 15q11-q13, it may have provided the original selection for imprinting in this domain. Whereas some human tissues express a minor SNURF-only transcript, mouse tissues express only the bicistronic Snurf-Snrpn transcript. We show that both SNURF and SNRPN are translated in normal, but not PWS, human, and mouse tissues and cell lines. These findings identify SNURF as a protein that is produced along with SmN from a bicistronic transcript; polycistronic mRNAs therefore are encoded in mammalian genomes where they may form functional operons.

Analysis of imprinted genes in subjects with Prader-Willi syndrome and chromosome 15 abnormalities

PURPOSE

To determine gene expression of five imprinted genes or transcripts from the 15q11-q13 chromosome region using reverse transcription polymerase chain reaction (RT-PCR) in a relatively large survey of Prader-Willi syndrome (PWS) and control subjects with several different chromosome 15 abnormalities.

METHODS

RT-PCR was undertaken on mRNA isolated from tissue (e.g., mostly lymphoblasts) from 38 PWS and 10 control subjects. DNA primers were used for five imprinted genes or transcripts (ZNF127, SNRPN, PAR5, IPW, and PAR1) from 15q11-q13 and fibrillin, a control gene from 15q21.

RESULTS

One PWS subject with maternal disomy 15 showed weak but detectable expression of PAR1, whereas SNRPN expression was detected in two PWS subjects [one with the 15q11-q13 deletion and one with a t(15;15) karyotype and maternal disomy 15], and the remaining typical PWS subjects showed no expression of the imprinted genes or transcripts.

CONCLUSION

No obvious clinical differences were identified in those PWS subjects with weak expression of genes compared with those showing no expression. Although the reason(s) for weak expression is unknown, possible explanations include relaxation of imprinting caused by failure to reset the imprinted genes or transcripts in the maternal germ line or by postzygotic gene expression or undetected chromosome 15 mosaicism in the deletion PWS subjects. The timing, tissue source, and other factors relating to partial expression of genes that are thought to be imprinted may play a role in clinical variability and allow for a better understanding of molecular mechanisms in PWS and other abnormalities of proximal chromosome 15q.

Genomic imprinting and chromatin insulation in Beckwith-Wiedemann syndrome

Genes are recognized as undergoing genomic imprinting when they are capable of being expressed only from the paternal or only from the maternal chromosome. The process can occur coordinately within large physical domains in mammalian chromosomes. One interesting facet of the study of genomic imprinting is that it offers insight into the regulation of large chromosomal regions. Understanding this regulation involves elucidating the cis-acting regulators of gene expression and defining the elements that maintain chromatin insulation, both required for understanding more practically applicable areas of biological research, such as efficient transgene production. This review is focused on the regulation of the imprinted domain of human chromosome 11p15.5, responsible for Beckwith-Wiedemann syndrome (BWS). Recent findings indicate that the maintenance of imprinting within this domain is critically dependent on the stable maintenance of chromatin insulation.

Prader-Willi syndrome is caused by disruption of the SNRPN gene

A Prader-Willi syndrome patient is described who has a de novo balanced translocation, (4;15)(q27;q11.2)pat, with breakpoints lying between SNRPN exons 2 and 3. Parental-origin studies indicate that there is no uniparental disomy and no apparent deletion. This patient expresses ZNF127, SNRPN exons 1 and 2, IPW, and D15S227E (PAR1) but does not express either SNRPN exons 3 and 4 or D15S226E (PAR5), as assayed by reverse transcription-PCR, of peripheral blood cells. Methylation studies showed normal biparental patterns of inheritance of loci DN34/ZNF127, D15S63, and SNRPN exon 1. Results for this patient and that reported by Sun et al. support the contention that an intact genomic region and/or transcription of SNRPN exons 2 and 3 play a pivotal role in the manifestations of the major clinical phenotype in Prader-Willi syndrome.

A model system to study genomic imprinting of human genes

Somatic-cell hybrids have been shown to maintain the correct epigenetic chromatin states to study developmental globin gene expression as well as gene expression on the active and inactive X chromosomes. This suggests the potential use of somatic-cell hybrids containing either a maternal or a paternal human chromosome as a model system to study known imprinted genes and to identify as-yet-unknown imprinted genes. Testing gene expression by using reverse transcription followed by PCR, we show that functional imprints are maintained at four previously characterized 15q11-q13 loci in hybrids containing a single human chromosome 15 and at two chromosome 11p15 loci in hybrids containing a single chromosome 11. In contrast, three gamma-aminobutyric acid type A receptor subunit genes in 15q12-q13 are nonimprinted. Furthermore, we have found that differential DNA methylation imprints at the SNRPN promoter and at a CpG island in 11p15 are also maintained in somatic-cell hybrids. Somatic-cell hybrids therefore are a valid and powerful system for studying known imprinted genes as well as for rapidly identifying new imprinted genes.

Imprinting in Angelman and Prader-Willi syndromes

Prader-Willi syndrome (PWS) and Angelman syndrome (AS) are caused by deficiencies of gene expression from paternal or maternal chromosome 15q11-q13, respectively. Many advances have occurred during the past year. The gene for necdin was mapped in the PWS candidate region and found to be paternally expressed in mouse and human. The bisulfite method for analysis of methylation was established for genomic sequencing and diagnostics, and the methylation of Snrpn was studied in detail in the mouse. A region near the Snrpn promoter was shown to function as a silencer in Drosophila. Point mutations were found in the gene for E6-AP ubiquitin-protein ligase (UBE3A) identifying it as the AS gene, and tissue-specific imprinting (maternal expression) was shown in the human brain and in hippocampal neurons and Purkinje cells in the mouse.

Imprinted genes in the Prader-Willi deletion

Parent-of-origin-specific deletions of proximal chromosome 15q cause either the Prader-Willi syndrome (paternal deletion) or the Angelman syndrome (maternal deletion), two distinct neurodevelopmental disorders. In contrast to the Angelman syndrome, which can also be caused by mutations in a single gene (UBE3A, encoding a ubiquitin ligase), the Prader-Willi syndrome is caused by deletions in about two-thirds of cases and by maternal uniparental disomy in the remaining third. The consequence of both mechanisms, in addition to rare microdeletions or so-called 'imprinting mutations', is lack of the products of multiple genes in the region that are normally expressed only from the paternal chromosome. One gene that is consistently silent in the Prader-Willi syndrome is SNRPN, which encodes the small nuclear ribonucleoprotein particle-associated polypeptide N that forms part of the spliceosomes in the brain. A systematic search for other imprinted genes in the Prader-Willi syndrome region revealed a paternally expressed transcript (IPW, for imprinted in the Prader-Willi region) and a similarly imprinted mouse homologue (Ipw) in the conserved syntenic region on mouse chromosome 7. Ipw is highly expressed in the brain and alternatively spliced to generate different transcripts. Since there is no open reading frame that is conserved in the human and mouse IPW genes, they are postulated to function as untranslated RNAs, possibly regulating transcription in cis in the region.

A mouse model for Prader-Willi syndrome imprinting-centre mutations

Imprinting in the 15q11-q13 region involves an 'imprinting centre' (IC), mapping in part to the promoter and first exon of SNRPN. Deletion of this IC abolishes local paternally derived gene expression and results in Prader-Willi syndrome (PWS). We have created two deletion mutations in mice to understand PWS and the mechanism of this IC. Mice harbouring an intragenic deletion in Snrpn are phenotypically normal, suggesting that mutations of SNRPN are not sufficient to induce PWS. Mice with a larger deletion involving both Snrpn and the putative PWS-IC lack expression of the imprinted genes Zfp127 (mouse homologue of ZNF127), Ndn and Ipw, and manifest several phenotypes common to PWS infants. These data demonstrate that both the position of the IC and its role in the coordinate expression of genes is conserved between mouse and human, and indicate that the mouse is a suitable model system in which to investigate the molecular mechanisms of imprinting in this region of the genome.

Identification of a silencing element in the human 15q11-q13 imprinting center by using transgenic Drosophila

Prader-Willi syndrome (PWS) and Angelman syndrome are neurogenetic disorders caused by the lack of a paternal or a maternal contribution from human chromosome 15q11-q13, respectively. Deletions in the transcription unit of the imprinted SNRPN gene have been found in patients who have PWS or Angelman syndrome because of a parental imprint switch failure in this chromosomal domain. It has been suggested that the SNRPN exon 1 region, which is deleted in the PWS patients, contains an imprint switch element from which the maternal and paternal epigenotypes of the 15q11-q13 domain originate. Using the model organism Drosophila, we show here that a fragment from this region can function as a silencer in transgenic flies. Repression was detected specifically from this element and could not be observed with control human sequences. Additional experiments allowed the delineation of the silencer to a fragment of 215 bp containing the SNRPN promoter region. These results provide an additional link between genomic imprinting and an evolutionary conserved silencing mechanism. We suggest that the identified element participates in the long range regulation of the imprinted 15q11-q13 domain or locally represses SNRPN expression from the maternal allele.

The human necdin gene, NDN, is maternally imprinted and located in the Prader-Willi syndrome chromosomal region

Prader-Willi syndrome (PWS) is a neurogenetic disorder that results from the absence of a normal paternal contribution to the 15q11-13 region. The clinical manifestations of PWS are a transient severe hypotonia in the newborn period, with mental retardation, hypogonadism and obesity observed later in development. Five transcripts with exclusive expression from the paternal allele have been isolated, but none of these has been shown to be involved in PWS. In this study, we report the isolation and characterization of NDN, a new human imprinted gene. NDN is exclusively expressed from the paternal allele in the tissues analysed and is located in the PWS region. It encodes a putative protein homologous to the mouse brain-specific NECDIN protein, NDN; as in mouse, expression in brain is restricted to post-mitotic neurons. NDN displays several characteristics of an imprinted locus, including allelic DNA methylation and asynchronous DNA replication. A complete lack of NDN expression in PWS brain and fibroblasts indicates that the gene is expressed exclusively from the paternal allele in these tissues and suggests a possible role of this new gene in PWS.