Methylation analysis and developmental profile of two individuals with Angelman syndrome due to mosaic imprinting defects

Angelman syndrome (AS) is a severe neurodevelopmental disorder caused by loss of expression of the maternally-inherited UBE3A on chromosome 15q11.2. In AS due to a chromosomal deletion that encompasses UBE3A, paternal uniparental disomy of chromosome 15, or imprinting defects (ImpD), the SNRPN locus is unmethylated, while in neurotypical individuals, it is ∼50% methylated. We present the developmental profile of two adults with mild AS assessed using standardized behavioral and neurodevelopmental measures. Both had intellectual disability with unusually advanced verbal communication skills compared to other individuals with AS. Methylation of the SNRPN locus was examined using Methylation Specific Quantitative Melt Analysis (MS-QMA) in different tissues at one time point for participant A (22 years) and two time points for participant B (T1: 22 years, T2: 25 years), and these levels were compared to a typical AS cohort. While participant A showed methylation levels comparable to the typical AS cohort, participant B showed methylation mosaicism in all tissues at both time points and changes in methylation levels from T1 to T2. AS should be considered in individuals with intellectual disability and verbal speech who may not have the typical symptoms of AS.