Exonic Deletions and Deep Intronic Variants of the SLC26A4 Gene Contribute to the Genetic Diagnosis of Unsolved Patients With Enlarged Vestibular Aqueduct

Enlarged vestibular aqueduct (EVA) is a frequently occurring inner ear malformation that associates with sensorineural hearing loss (SNHL), with SLC26A4 being the responsible gene. Based on multiplex PCR enrichment and sequencing of the exonic and flanking regions of the SLC26A4 gene, we developed a panel specifically for EVA and found that up to 95% of EVA patients in our Chinese cohorts carried biallelic SLC26A4 pathogenic variants (M2). In this study, we tried to investigate the genetic etiology of 13 previously undiagnosed EVA patients with monoallelic (M1) or none (M0) SLC26A4 variant using a stepwise approach, including copy number variation (CNV) analysis of multiplex PCR enrichment and next-generation sequencing data, single-molecule real-time (SMRT) sequencing of the whole SLC26A4 gene, whole exome sequencing (WES), and whole genome sequencing (WGS). CNV analysis revealed deletions in Exons 1–3, Exons 5–6, and Exons 9–10 of the SLC26A4 gene in seven patients, and SMRT sequencing identified the same heterozygous deep intronic variant (NM_000441.2:c.304+941C>T) in two patients, resulting in a final diagnosis in 9/13 patients. Notably, the variants of Exons 9–10 deletion and c.304+941C>T have not been reported previously. We further showed that the variant c.304+941C>T led to the exonization of partial AluSz6 element (126 bp) where the variant is located through sequencing of the mRNA extracted from the blood of a heterozygous variant carrier. In conclusion, our stepwise approach improved the diagnosis rate of EVA, expanded the mutational spectrum of the SLC26A4 gene, and highlighted the contribution of exonic deletions and deep intronic variants to EVA.