A Novel Homozygous Mutation in the EC1/EC2 Interaction Domain of the Gap Junction Complex Connexon 26 Leads to Profound Hearing Impairment

Abstract

To date, about 165 genetic loci or genes have been identified which are associated with nonsyndromal hearing impairment. In about half the cases, genetic defects in the GJB2 gene (connexin 26) are the most common cause of inner-ear deafness. The genes GJB2 and GJB6 are localized on chromosome 13q11-12 in tandem orientation. Connexins belong to the group of “gap junction” proteins, which form connexons, each consisting of six connexin molecules. These are responsible for the exchange of ions and smaller molecules between neighboring cells. Mutational analysis in genes GJB2 and GJB6 was brought by direct sequencing of the coding exons including the intron transitions. Here we show in the participating extended family a homozygous mutation c.506G>A, (TGC>TAC) p.Cys169Tyr, in the GJB2 gene, which could be proven for the first time and led to nonsyndromal severe hearing impairment in the afflicted patients. The mutation is located in the EC1/EC2 interaction complex of the gap junction connexon 26 complex and interrupts the K+ circulation and therefore the ion homeostasis in the inner ear. The homozygous mutation p.Cys169Tyr identified here provides a novel insight into the structure-function relationship of the gap junction complex connexin/connexon 26.

Frequency of GJB2 mutations in families with autosomal recessive non-syndromic hearing loss in Khuzestan province

“…The presence of this cysteine, in addition to creating proper folding, is essential for proper performance of the ionic channel. Mutation in one of the three Cys residues in the extracellular domain of the Cx26 protein disturbs the flow of K + ions, hence ionic homeostasis, in the inner ear (BIRKENHÄGER et al, 2014;MAEDA et al, 2009).…”

“…Autosomal recessive genes with more than 55 different genes (http://hereditaryhearingloss.org) have been identified so far and they are responsible for about 80% of non-syndromic HL cases (CRYNS and VAN CAMP, 2004;MAEDA et al, 2009). However, in some populations, mutations in the GJB2 (Gap junction protein, GJB2, 26kDa) gene, which is located in DFNB1 locus, are responsible for half of the ARNSHL cases (BIRKENHÄGER et al, 2014). The GJB2 gene encodes connexin26 (Cx26) protein.…”

“…However, prevalence of GJB2 mutations is different in various geographical regions or ethnicities. For example, a frame-shift mutation c.35delG mutation is the most common one in Caucasians, c.235delC in East Asians, c. 167delT in Ashkenazi Jews and c.71G>A in Indians (AZAIEZ et al, 2007;BIRKENHÄGER et al, 2014;DENOYELLE et al, 1999;MANI et al, 2009). In addition, to GJB2 coding region mutations (exon 2), two pathogenic mutations in the noncoding first exon (c.-23G>T) and donor splice site )c.IVS1 + 1G > A) have been identified (DENOYELLE et al, 1999;MANI et al, 2009).…”

The role of gene GJB2 and connexin 26 in hearing impairment

“…They form the intramolecular disulfide bridges between the cysteine residues. The disulfide bonds of the residual cysteine are between 53 and 180, 60 and 174, and also between 64 and 169 [17]. Studies on other proteins have shown that a substitution of cysteine residues would result in the destabilization of the protein structure [16].…”

“…Although the translation of protein structure is completed, the resulting protein has an abnormal structure of the extracellular domain E2 [17]. This mutation occurs in a highly conserved region, which affects one of the three cysteine residues responsible for disulfide bonds, which are extremely important for the connexon-connexon interaction.…”