Practice guidelines for the molecular analysis of Prader-Willi and Angelman syndromes

BACKGROUND: Prader-Willi syndrome (PWS) and Angelman syndrome (AS) are clinically distinct neurodevelopmental genetic disorders that map to 15q11-q13. The primary phenotypes are attributable to loss of expression of imprinted genes within this region which can arise by means of a number of mechanisms. The most sensitive single approach to diagnosing both PWS and AS is to study methylation patterns within 15q11-q13; however many techniques exist for this purpose. Given the diversity of techniques available, there is a need for consensus testing and reporting guidelines. METHODS: Testing and reporting guidelines have been drawn up and agreed in accordance with the procedures of the UK Clinical Molecular Genetics Society and the European Molecular Genetics Quality Network. RESULTS: A practical set of molecular genetic testing and reporting guidelines has been developed for these two disorders. In addition, advice is given on appropriate reporting policies, including advice on test sensitivity and recurrence risks. In considering test sensitivity, the possibility of differential diagnoses is discussed. CONCLUSION: An agreed set of practice guidelines has been developed for the diagnostic molecular genetic testing of PWS and AS

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

Establishment of the first WHO international genetic reference panel for Prader Willi and Angelman syndromes

Prader Willi and Angelman syndromes are clinically distinct genetic disorders both mapping to chromosome region 15q11-q13, which are caused by a loss of function of paternally or maternally inherited genes in the region, respectively. With clinical diagnosis often being difficult, particularly in infancy, confirmatory genetic diagnosis is essential to enable clinical intervention. However, the latter is challenged by the complex genetics behind both disorders and the unmet need for characterised reference materials to aid accurate molecular diagnosis. With this in mind, a panel of six genotyping reference materials for Prader Willi and Angelman syndromes was developed, which should be stable for many years and available to all diagnostic laboratories. The panel comprises three Prader Willi syndrome materials (two with different paternal deletions, and one with maternal uniparental disomy (UPD)) and three Angelman syndrome materials (one with a maternal deletion, one with paternal UPD or an epigenetic imprinting centre defect, and one with a UBE3A point mutation). Genomic DNA was bulk-extracted from Epstein–Barr virus-transformed lymphoblastoid cell lines established from consenting patients, and freeze-dried as aliquots in glass ampoules. In total, 37 laboratories from 26 countries participated in a collaborative study to assess the suitability of the panel. Participants evaluated the blinded, triplicate materials using their routine diagnostic methods against in-house controls or externally sourced uncertified reference materials. The panel was established by the Expert Committee on Biological Standardization of the World Health Organization as the first International Genetic Reference Panel for Prader Willi and Angelman syndromes

Angelman syndrome (AS, MIM 105830)

Angelman syndrome (AS) is a distinct neurogenetic syndrome, first described in 1965. The phenotype is well known in infancy and adulthood, but the clinical features may change with age. The main clinical characteristics include severe mental retardation, epileptic seizures and EEG abnormalilties, neurological problems and distinct facial dysmorphic features. Behavioural problems such as hyperactivity and sleeping problems are reported, although these patients present mostly a happy personality with periods of inappropriate laughter. Different underlying genetic mechanisms may cause AS, with deletion of chromosome 15 as the most frequent cause. Other genetic mechanisms such as paternal uniparental disomy, imprinting defect and mutation in the UBE3A gene are present in smaller groups of patients with AS. As the recurrence risk can be up to 50%, the clinical diagnosis of AS should be confirmed by laboratory tesing, and genetic counselling should be provided. Treatment of seizures, physical therapy or other intervention strategies are helpful to ameliorate the symptoms