Functional defects of Cx26 resulting from a heterozygous missense mutation in a family with dominant deaf-mutism and palmoplantar keratoderma

Abstract

Mutations in GJB2 encoding the gap junction protein connexin-26 (Cx26) have been established as the basis of autosomal recessive non-syndromic hearing loss. The involvement of GJB2 in autosomal dominant deafness has also been proposed, although the putative mutation identified in one family with both deafness and palmoplantar keratoderma has recently been suggested to be merely a non-disease associated polymorphism. We have observed a similar phenotype in an Egyptian family that segregated with a heterozygous missense mutation of GJB2, leading to a non-conservative amino acid substitution (R75W). The deleterious dominant-negative effect of R75W on gap channel function was subsequently demonstrated in the paired oocyte expression system. Not only was R75W alone incapable of inducing electrical conductance between adjacent cells, but it almost completely suppressed the activity of co-expressed wildtype protein. The Cx26 mutant W77R, which has been implicated in autosomal recessive deafness, also failed to form functional gap channels by itself but did not significantly interfere with the function of wildtype Cx26. These data provide compelling evidence for the serious functional consequences of Cx26 mutations in dominant and recessive deafness.

Functional consequences of novel connexin 26 mutations associated with hereditary hearing loss

“…167delT and 235delC, which are the most common mutations in the Ashkenazi Jewish populations and Japanese populations respectively, were not observed in our study. Our results indicate that the R75W mutation is most likely associated with the non-syndromic form of hearing loss rather than the syndromic hearing loss as reported earlier, 21 although the effect of genetic background cannot be ruled out. Identification of 18 mutations and seven polymorphisms in this study and a large spectrum of mutations and polymorphisms identified by earlier studies on Cx26, suggest that this gene is subject to a relaxed selection.…”

“…Two individuals were heterozygous for R75W, which was previously reported to be associated with dominant hearing loss and palmoplantar keratoderma. 21 Richard et al 21 found R75W in a heterozygous condition in an individual from the control group without skin disorder. Hearing levels of the control group individuals in this study were unknown, and individuals affected with hearing loss were likely to be present in the control group they studied.…”

Defective channels lead to an impaired skin barrier

“…These skin-disease-associated connexin mutants appear to either suppress wild-type gap junction channel activity by exerting a dominant-negative effect on wild-type connexins or a transdominant effect on other connexin proteins (Rouan et al, 2001), or represent gain-of-function variants (Richard et al, 1998;Bakirtzis et al, 2003;Essenfelder et al, 2004;Gerido et al, 2007;Lee et al, 2009). For example, mutant forms of Cx31 can traffic incorrectly and accumulate in organelles such as the ER, causing ER stress and activation of the unfolded protein response (UPR), which results in cell death (Tattersall et al, 2009).…”

Human oligodendrocytes express Cx31.3: Function and interactions with Cx32 mutants

“…Expression of the Golgiretained R142W Cx32 mutation in myelinating Schwann cells of transgenic mice resulted in decreased levels of wild type mouse Cx32 and caused demyelination (Jeng et al 2006). Furthermore, dominant Cx26 mutations that cause hearing loss suppressed channel activity of coexpressed wild type Cx26, but only those mutants additionally causing skin disease inhibited intercellular conductance of co-expressed wild type Cx43 (Richard et al 1998;Rouan et al 2001). Here we describe the expression of Cx31.3, the human ortholog of Cx29 (Altevogt et al 2002), and its relation to the other oligodendrocyte GJ proteins Cx32 and Cx47.…”