The Sins of the Fathers and Mothers: Genomic Imprinting in Mammalian Development

Epilepsy in Angelman syndrome

American College of Medical Genetics Statement on Diagnostic Testing for Uniparental Disomy

The molecular basis of Angelman syndrome and Prader-Willi syndrome is well established, and genetic testing for these disorders is clinically available. Imprinting abnormalities account for up to 4% of patients with Angelman and Prader-Willi syndromes. Deletions of the imprinting center region are the molecular abnormality observed in a subset of Angelman and Prader-Willi syndrome cases with imprinting defects. Genetic testing of imprinting center deletions in patients with Angelman and Prader-Willi syndrome is not readily available. Such testing is important for the diagnostics of Angelman and Prader-Willi syndrome because it allows for more accurate diagnosis and recurrence risk prediction in families. Here we describe the development, validation, and implementation of a real time quantitative polymerase chain reaction (PCR)-based assay for imprinting center deletion detection in patients with Angelman and Prader-Willi syndrome, which we have incorporated into our genetic testing strategy for these disorders. To date we have tested, on a clinical basis, five patients with either Angelman or Prader-Willi syndrome in whom an imprinting center defect was implicated and found a deletion in one patient that was determined to be familial.

Prader-Willi syndrome (PWS) and Angelman syndrome (AS) are neurobehavioral disorders resulting from deficiency of imprinted gene expression from paternal or maternal chromosome 15q11-15q13, respectively. In humans, expression of the imprinted genes is under control of a bipartite cis-acting imprinting center (IC). Families with deletions causing PWS imprinting defects localize the PWS-IC to 4.3 kb overlapping with SNRPN exon 1. Families with deletions causing AS imprinting defects localize the AS-IC to 880 bp 35 kb upstream of the PWS-IC. We report two mouse mutations resulting in defects similar to that seen in AS patients with deletion of the AS-IC. An insertion/duplication mutation 13 kb upstream of Snrpn exon 1 resulted in lack of methylation at the maternal Snrpn promoter, activation of maternally repressed genes, and decreased expression of paternally repressed genes. The acquisition of a paternal epigenotype on the maternal chromosome in the mutant mice was demonstrated by the ability to rescue the lethality and growth retardation in a mouse model of a PWS imprinting defect. A second mutation, an 80-kb deletion extending upstream of the first mutation, caused a similar imprinting defect with variable penetrance. These results suggest that there is a mouse functional equivalent to the human AS-IC.

Reply to Chinnery et al.

Prader-Willi Syndrome (PWS) is a complex human genetic disease arising from a loss of paternal allele expression of imprinting genes on chromosome 15q11-q13. Normally the CpG islands at this site are heavily methylated in the maternal allele, but unmethylated in the paternal allele and therefore activated in gene expression. only the methylated allele should present in pws patients when methylation-specific pcr (msp) is analyzed. This paper reports an analysis of PWS in Thai patients using consensus diagnostic criteria based on a combination of clinical data, basic G-banding and fluorescence in situ hybridization (FISH) cytogenetics, PCR-based methylation assay, and bisulfite sequencing of the CpG islands of SNRPN to confirm 15q deletion or the methylation pattern of the SNRPN promoter and exon 1. Lack of complete clinical reports or inadequacy of the minimum laboratory support required had made it difficult to diagnose PWS, Angelman syndrome and other microdeletion disorders. Accuracy of 100% was obtained for diagnosis of the PWS study patients using the minimum requirements necessary. A total of 20 patients were diagnosed as PWS based on clinical criteria and the scoring tool for PWS, and the same approach was applied to four separate patients with some unmatched criteria but phenotypic similarity to PWS. Findings showed that 70% of those clinically diagnosed as PWS patients (14/20) had a deletion at 15q11-q13 according to FISH, while all 20 patients showed MSP positive of SNRPN gene. Six cases (30%) without a paternal deletion were confirmed to have maternal uniparental disomy (mUPD) of PWS by MSP and methylation sequencing approaches. Noteworthy, two of the six cases with mUPD were 3.5 year-old twins. None of the five cases with scores lower than the reported consensus criteria showed positive G-band, FISH or MSP results. We demonstrate here the high power of combining clinical findings, FISH and MSP in definitive diagnosis of PWS and in distinguishing between the two major different types of molecular mechanisms. No false positives or false negatives were observed in our analysis.

Shared neural circuitry for female and male sexual behaviours in Drosophila

Objective:Prader-Willi (PWS) and Angelman syndromes (AS) are genomic imprinting diseases with intellectual disability. In approximately 70 % of the cases, there is a cytogenetic deletion involving the chromosome 15q11-q13 inherited from patient's father (in PWS) and from patient's mother (in AS). In approximately 20-25% and 3-7% of the cases, there is an uniparental disomy (UPD) of chromosome 15 in PWS and AS patients, respectively. The mutation ratio in AS patients is approximately 10 %, while the ratio is two percent in PWS patients.

Prader–Willi and Angelman syndromes are often referred to as a sister pair of neurodevelopmental disorders, resulting from different genetic and epigenetic alterations to the same chromosomal region, 15q11-q13. Some of the primary phenotypes of the two syndromes have been suggested to be opposite to one another, but this hypothesis has yet to be tested comprehensively, and it remains unclear how opposite effects could be produced by changes to different genes in one syndrome compared to the other. We evaluated the evidence for opposite effects on sleep and eating phenotypes in Prader–Willi syndrome and Angelman syndrome, and developed physiological–genetic models that represent hypothesized causes of these differences. Sleep latency shows opposite deviations from controls in Prader–Willi and Angelman syndromes, with shorter latency in Prader–Willi syndrome by meta-analysis and longer latency in Angelman syndrome from previous studies. These differences can be accounted for by the effects of variable gene dosages of UBE3A and MAGEL2, interacting with clock genes, and leading to acceleration (in Prader–Willi syndrome) or deceleration (in Angelman syndrome) of circadian rhythms. Prader–Willi and Angelman syndromes also show evidence of opposite alterations in hyperphagic food selectivity, with more paternally biased subtypes of Angelman syndrome apparently involving increased preference for complementary foods (“baby foods”); hedonic reward from eating may also be increased in Angelman syndrome and decreased in Prader–Willi syndrome. These differences can be explained in part under a model whereby hyperphagia and food selectivity are mediated by the effects of the genes SNORD-116, UBE3A and MAGEL2, with outcomes depending upon the genotypic cause of Angelman syndrome. The diametric variation observed in sleep and eating phenotypes in Prader–Willi and Angelman syndromes is consistent with predictions from the kinship theory of imprinting, reflecting extremes of higher resource demand in Angelman syndrome and lower demand in Prader–Willi syndrome, with a special emphasis on social–attentional demands and attachment associated with bedtime, and feeding demands associated with mother-provided complementary foods compared to offspring-foraged family-type foods.

Response to Epstein et al.

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.

Snapshots of ARF1: Implications for Mechanisms of Activation and Inactivation

Siesta-time is in the genes.

Prader-Willi syndrome (PWS) and Angelman syndrome (AS) are distinct mental retardation disorders caused by paternal deficiency (PWS) or maternal deficiency (AS) of gene(s) in 15qll.2-ql3. We have constructed a 3.5 Mb yeast artificial chromosome (YAC) contig of the PWS/AS region and cosmid contigs of selected YACs at D15S13, SNRPN, S10, and S113. Cosmid clones have been used for fluorescence in situ hybridization (FISH) detection of deletions in PWS and AS patients. In addition, a total of 28 short tandem repeat polymorphisms (STRs) have been mapped to specific YACs in the contig, providing a highly informative set of markers for detection of deletion or uniparental disomy (UPD) in PWS and AS patients. Use of the 3 most informative markers in this region (S542, S128, and ASSCA-1) plus 3 markers distal on 15q (S123, S125, and S131) provide an efficient diagnostic strategy for UPD15.A combination of FISH and STR analysis has identified small deletions in one sporadic and one familial case of PWS (family O). Both deletions involve all or part of the SNRPN gene but do not extend telomeric to PAR-5 or PAR-1, two novel transcripts expressed exclusively from the paternal chromosome. However, expression of SNRPN, PAR-5, and PAR-1 is lost in both cases, implying the presence of an imprinting control region near SNRPN. The smallest deletion in family O is estimated at approximately 30-40 kb in size and involves a newly identified CpG island at the 5′ end of SNRPN which is methylated on the maternal chromosome. This small deletion in two PWS affected siblings was present in the father and the paternal grandmother, both of whom were phenotypically normal.

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