The pathological effects of connexin 26 variants related to hearing loss by in silico and in vitro analysis.

Gap junctions (GJs) are intercellular channels associated with cell-cell communication. Connexin 26 (Cx26) encoded by the GJB2 gene forms GJs of the inner ear, and mutations of GJB2 cause congenital hearing loss that can be syndromic or non-syndromic. It is difficult to predict pathogenic effects using only genetic analysis. Using ionic and biochemical coupling tests, we evaluated the pathogenic effects of Cx26 variants using computational analyses to predict structural abnormalities. For seven out of ten variants, we predicted the variation would result in a loss of GJ function, whereas the others would completely fail to form GJs. Functional studies demonstrated that, although all variants were able to function normally as hetero-oligomeric GJ channels, six variants (p.E47K, p.E47Q, p.H100L, p.H100Y, p.R127L, and p.M195L) did not function normally as homo-oligomeric GJ channels. Interestingly, GJs composed of the Cx26 variant p.R127H were able to function normally, even as homo-oligomeric GJ channels. This study demonstrates the particular location and property of an amino acid are more important mainly than the domain where they belong in the formation and function of GJ, and will provide information that is useful for the accurate diagnosis of hearing loss.

Identification of a novel nonsynonymous mutation of EYA1 disrupting splice site in a Korean patient with BOR syndrome.

The EYA1 gene is known as the causative gene of BOR (Branchio-oto-renal) syndrome which is a genetic disorder associated with branchial cleft cysts of fistulae, hearing loss, ear malformation, and renal anomalies. Although approximately 40% of patients with BOR syndrome have mutations in the EYA1 gene and over 130 disease-causing mutations in EYA1 have been reported in various populations, only a few mutations have been reported in Korean families. In this study, genetic analysis of the EYA1 gene was performed in a Korean patient diagnosed with BOR syndrome and his parents. A de novo novel missense mutation, c.418G>A, located at the end of exon 6, changed glycine to serine at amino acid position 140 (p.G140S) and was suspected to affect normal splicing. Our in vitro splicing assay demonstrated that this mutation causes exon 6 skipping leading to frameshift and truncation of the protein to result in the loss of eyaHR. To the best of our knowledge, this is the first report revealing that a missense mutation in the exon disturbs normal splicing as a result of a substitution of the last nucleotide of an exon in EYA1.

Mutational analysis of EYA1, SIX1 and SIX5 genes and strategies for management of hearing loss in patients with BOR/BO syndrome.

BACKGROUND: Branchio-oto-renal (BOR) or branchio-otic (BO) syndrome is one of the most common forms of autosomal dominant syndromic hearing loss. Mutations in EYA1, SIX1 and SIX5 genes have been associated with BOR syndrome. In this study, clinical and genetic analyses were performed in patients with BOR/BO syndrome focusing on auditory manifestations and rehabilitation. METHODS: The audiologic manifestations were reviewed in 10 patients with BOR/BO syndrome. The operative findings and hearing outcome were analyzed in patients who underwent middle ear surgeries. The modality and outcome of auditory rehabilitation were evaluated. Genetic analysis was performed for EYA1, SIX1, and SIX5 genes. RESULTS: All patients presented with mixed hearing loss. Five patients underwent middle ear surgeries without successful hearing gain. Cochlear implantation performed in two patients resulted in significant hearing improvement. Genetic analysis revealed four novel EYA1 mutations and a large deletion encompassing the EYA1 gene. CONCLUSIONS: Auditory rehabilitation in BOR/BO syndrome should be individually tailored keeping in mind the high failure rate after middle ear surgeries. Successful outcome can be expected with cochlear implantations in patients with BOR/BO syndrome who cannot benefit from hearing aids. The novel EYA1 mutations may add to the genotypic and phenotypic spectrum of BOR syndrome in the East Asian population.

A novel synonymous mutation causing complete skipping of exon 16 in the SLC26A4 gene in a Korean family with hearing loss.

INTRODUCTION: Mutations in PDS (or SLC26A4) cause both Pendred syndrome (PS) and DFNB4, two autosomal recessive disorders that share hearing loss as a common feature. PS and DFNB4 are genetically homogeneous disorders caused by bi-allelic SLC26A4 mutations. Here, we report a novel synonymous mutation (c.1803G>A, p.Lys601Lys), that caused aberrant splicing in two Korean family members who were clinically considered to have DFNB4, along with congenital hearing loss and dilated vestibular aqueducts (DVA). METHODS: After extracting DNA from whole blood using standard procedures, the 21 exons and flanking introns of SLC26A4 were amplified with PCR. To evaluate the implication of a novel synonymous mutation (c.1803G>A), we used The Berkeley Drosophila Genome Project (BDGP) (http://www.fruitfly.org/) as a splice site prediction program and performed exon trapping analysis. RESULTS: In molecular analysis of the 21 exons of SCL26A4, we detected a known splicing mutation (c.919-2A>G, heterozygote) and a novel variant (c.1803G>A, heterozygote) in the patients (II-1 and II-2). According to in silico analysis, the novel variant (c.1803G>A) affects canonical splice donor nucleotide positioning. To define the transcript level effects of this novel 1803G>A variant, we performed exon trapping and confirmed that exon 16 is completely skipped in this variant type. CONCLUSION: We report a novel synonymous mutation (c.1803G>A) causing complete exon 16 skipping in the SLC26A4 gene in two Korean family members with hearing loss. This is the first case of a synonymous SNP (c.1803G>A) affecting vestibulocochlear organs through altering splicing accuracy by causing a complete skipping of exon 16. An important issue raised by this study is that synonymous mutations that have been previously ignored in clinical diagnoses must now be considered as potential pathogenic mutations.