A previously unrecognised phenotype characterised by obesity, muscular hypotonia, and ability to speak in patients with Angelman syndrome caused by an imprinting defect

The clinical features of Angelman syndrome (AS) comprise severe mental reta rdation, postnatal microcephaly, macrostomia and prognathia, absence of spe ech, ataxia, anal a happy disposition. We report on seven patients who lack most sf these features, but presented with obesity, muscular hypotonia and mild mental retardation, Based on the latter findings, the patients were i nitially suspected of having Prader-Willi syndrome, DNA methylation analysi s of SNRPN and D15S63, however, revealed an AS pattern, ie the maternal ban d was faint or absent. Cytogenetic studies and microsatellite analysis demo nstrated apparently normal chromosomes 15 of biparental inheritance, We con clude that these patients have an imprinting defect and a previously unreco gnised form of AS. The mild phenotype may be explained by an incomplete imp rinting defect or by cellular mosaicism.