Angelman syndrome: A novel UBE3A splice variant

Abstract

Introduction: Angelman syndrome (AS) is a neurodevelopmental disorder characterised by moderate to severe developmental delay, absent or near absent speech, gait ataxia, microcephaly and seizures. Deficient expression or function of the maternally inherited UBE3A allele results in AS. Pathogenic UBE3A sequence variants (predominantly loss-of-function) account for ~11% of Angelman syndrome cases. A maternally inherited c.1900G>C (p.(Val634Leu)) variant of uncertain significance was identified in a sevenyear-old male with clinical features consistent with Angelman syndrome (although no seizures noted). This variant affects the first nucleotide of UBE3A exon 7 (transcript NM-130838.2) and in silico splicing tools predict weakening of the canonical 3'-splice site. Methods: RT-PCR and Sanger sequencing of gel-purified amplicons was performed on mRNA isolated from the proband and maternal peripheral blood mononuclear cells (PBMCs). To detect aberrantly spliced transcripts targeted by nonsense-mediated mRNA decay (NMD), PBMCs were cultured in the presence of cycloheximide (CHX) to inhibit NMD. Results: The UBE3A:c.1900G>C variant causes aberrant splicing and use of a cryptic 3'splice site (r.1899-1900ins [1900-38-1900-1]) resulting in a frameshift: p.(Val634-Phefs∗19). Cycloheximide inhibition of transcripts demonstrated an increase in the relative abundance of abnormally spliced transcripts, suggesting targeting by NMD. The c.1900G>C variant was re-classified as a pathogenic. Conclusions: The maternally inherited UBE3A: c.1900G>C variant induces abnormal splicing of the predominant UBE3A isoforms expressed in blood and brain, introducing a frameshift consistent with the pathogenesis of Angelman syndrome. Therefore, UBE3A splicing outcomes observed in mRNA isolated from blood can inform the molecular consequences of cryptic splice variants for diagnosis and genetic counselling of Angelman syndrome.L6340667572021-02-08 <br />