Non-canonical splice site variants are increasingly recognized as a relevant cause of the USH2A-associated diseases, non-syndromic autosomal recessive retinitis pigmentosa and Usher syndrome type 2. Many non-canonical splice site variants have been reported in public databases, but an effect on pre-mRNA splicing has only been functionally verified for a subset of these variants. In this study, we aimed to extend the knowledge regarding splicing events by assessing a selected set of USH2A non-canonical splice site variants and to study their potential pathogenicity. Eleven non-canonical splice site variants were selected based on four splice prediction tools. Ten different USH2A constructs were generated and minigene splice assays were performed in HEK293T cells. An effect on pre-mRNA splicing was observed for all 11 variants. Various events, such as exon skipping, dual exon skipping and partial exon skipping were observed and eight of the tested variants had a full effect on splicing as no conventionally spliced mRNA was detected. We demonstrated that non-canonical splice site variants in USH2A are an important contributor to the genetic etiology of the associated disorders. This type of variant generally should not be neglected in genetic screening, both in USH2A-associated disease as well as other hereditary disorders. In addition, cases with these specific variants may now receive a conclusive genetic diagnosis.
Keywords: USH2A; Usher syndrome; minigene splice assay; non-canonical splice site; pre-mRNA splicing; retinitis pigmentosa.