Intron retention caused by a canonical splicing variant in SSR4-related congenital disorder of glycosylation.

Congenital disorder of glycosylation type Iy (CDG-Iy) is an X-linked monogenic inherited disease caused by variants in the SSR4 gene. To date, a total of 11 variants have been identified in 14 CDG-Iy patients. Our study identified a novel canonical splicing variant, c.67+2T>C, in the SSR4 gene (according to the transcript NM_006280.3) in a Chinese CDG-Iy family. Functional analysis revealed that the c.67+2T>C variant induced the retention of the first 46 bp of intron 1 via the recognition of the downstream GC dinucleotide as a non-canonical cryptic donor splice site. This aberrant mRNA splicing resulted in the occurrence of a premature termination codon, triggered nonsense-mediated mRNA decay, and decreased the SSR4 gene expression. Our study is the first to identify aberrant mRNA processing in SSR4-related CDG-Iy and further emphasizes the activation of the non-canonical GC donor splice site in aberrant mRNA processing caused by splicing variants.