Mosaic de novo <i>SNRPN</i> gene variant associated with Prader-Willi syndrome.

Abstract

Prader-Willi syndrome (PWS) is an imprinting disorder caused by the absence of paternal expressed genes in the Prader-Willi critical region (PWCR) on chromosome 15q11.2-q13. Three molecular mechanisms have been known to cause PWS, including a deletion in the PWCR, uniparental disomy 15 and imprinting defects. We report the first case of PWS associated with a single-nucleotide <i>SNRPN</i> variant in a 10-year-old girl presenting with clinical features consistent with PWS, including infantile hypotonia and feeding difficulty, developmental delay with cognitive impairment, excessive eating with central obesity, sleep disturbances, skin picking and related behaviour issues. Whole-exome sequencing revealed a de novo mosaic nonsense variant of the <i>SNRPN</i> gene (c.73C>T, p.R25X) in 10% of DNA isolated from buccal cells and 19% of DNA from patient-derived lymphoblast cells. DNA methylation study did not detect an abnormal methylation pattern in the <i>SNRPN</i> locus. Parental origin studies showed a paternal source of an intronic single-nucleotide polymorphism within the locus in proximity to the <i>SNRPN</i> variant. This is the first report that provides evidence of a de novo point mutation of paternal origin in <i>SNRPN</i> as a new disease-causing mechanism for PWS. This finding suggests that gene sequencing should be considered as part of the diagnostic workup in patients with clinical suspicion of PWS.