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Background Consanguineous marriage, a common practice in South India, increase the incidence of autosomal recessive diseases such as nonsyndromic hearing loss (NSHL) in offspring. This trend was noted in the children with hearing impairment (HI) who received cochlear implants (CI) at our University hospital in Porur, Chennai, India. To ascertain the genetic etiology of HI in these patients, we performed multiplex ligation-dependent probe amplification (MLPA) analysis. Methods A total of 25 families who had a child with NSHL were included in the study. MLPA screening of GJB2, GJB6, and GJB3 was performed for all the recruited individuals. Results The pathogenic p.W24X* mutation of GJB2 was detected in 2 patients; both of their parents were heterozygous carriers. Both families had a second-degree consanguineous marriage. Conclusion This study has important implications for molecular-diagnosis strategy and genetic counseling for families with HI in South India.

Clouston syndrome (CS; also termed hidrotic ectodermal dysplasia) is a rare autosomal dominant genetic skin disorder, characterized by alopecia, nail dystrophy, and palmoplantar hyperkeratosis. Mutations in the GJB6 gene, which encodes the gap junction protein connexin 30, have been shown to cause this disorder. To date, four mutations of GJB6 have been found in patients with CS: G11R, V37E, D50N and A88V. Mutations in GJA1 (V41L) and GJB2 (R127H) are also related to CS. We found a novel missense mutation, N14S, in GJB6 and the previously identified F191L mutation in GJB2 (Cx26) in a proband with CS in a Han Chinese pedigree; these mutations were not found in 200 ethnically matched nonconsanguineous Han Chinese controls.

Mutations in the GJB2 and GJB6 genes, respectively, coding for connexin26 (Cx26) and connexin30 (Cx30) proteins, are the most common cause for prelingual non-syndromic deafness in humans. In the inner ear, Cx26 and Cx30 are expressed in different non-sensory cell types, where they largely co-localize and may form heteromeric gap junction channels. Here, we describe the generation and characterization of a mouse model for human bilateral middle/high-frequency hearing loss based on the substitution of an evolutionarily conserved threonine by a methionine residue at position 5 near the N-terminus of Cx30 (Cx30T5M). The mutation was inserted in the mouse genome by homologous recombination in mouse embryonic stem cells. Expression of the mutated Cx30T5M protein in these transgenic mice is under the control of the endogenous Cx30 promoter and was analysed via activation of the lacZ reporter gene. When probed by auditory brainstem recordings, Cx30(T5M/T5M) mice exhibited a mild, but significant increase in their hearing thresholds of about 15 dB at all frequencies. Immunolabelling with antibodies to Cx26 or Cx30 suggested normal location of these proteins in the adult inner ear, but western blot analysis showed significantly down-regulated the expression levels of Cx26 and Cx30. In the developing cochlea, electrical coupling, probed by dual patch-clamp recordings, was normal. However, transfer of the fluorescent tracer calcein between cochlear non-sensory cells was reduced, as was intercellular Ca(2+) signalling due to spontaneous ATP release from connexin hemichannels. Our findings link hearing loss to decreased biochemical coupling due to the point-mutated Cx30 in mice.

Mutations in four members of the connexin gene family have been shown to underlie distinct genetic forms of deafness, including GJB2 [connexin 26 (Cx26)], GJB3 (Cx31), GJB6 (Cx30) and GJB1 (Cx32). We have found that alterations in a fifth member of this family, GJA1 (Cx43), appear to cause a common form of deafness in African Americans. We identified two different GJA1 mutations in four of 26 African American probands. Three were homozygous for a Leu-->Phe substitution in the absolutely conserved codon 11, whereas the other was homozygous for a Val-->Ala transversion at the highly conserved codon 24. Neither mutation was detected in DNA from 100 control subjects without deafness. Cx43 is expressed in the cochlea, as is demonstrated by PCR amplification from human fetal cochlear cDNA and by RT-PCR of mouse cochlear tissues. Immunohistochemical staining of mouse cochlear preparations showed immunostaining for Cx43 in non-sensory epithelial cells and in fibrocytes of the spiral ligament and the spiral limbus. To our knowledge this is the first alpha connexin gene to be associated with non-syndromic deafness. Cx43 must also play a critical role in the physiology of hearing, presumably by participating in the recycling of potassium to the cochlear endolymph.

Prelingual non-syndromic (isolated) deafness is the most frequent hereditary sensory defect. In >80% of the cases, the mode of transmission is autosomal recessive. To date, 14 loci have been identified for the recessive forms (DFNB loci). For two of them, DFNB1 and DFNB2, the genes responsible have been characterized; they encode connexin 26 and myosin VIIA, respectively. In order to evaluate the extent to which the connexin 26 gene (Cx26) contributes to prelingual deafness, we searched for mutations in this gene in 65 affected Caucasian families originating from various countries, mainly tunisia, France, New Zealand and the UK. Six of these families are consanguineous, and deafness was shown to be linked to the DFNB1 locus, 10 are small non consanguineous families in which the segregation of the trait has been found to be compatible with the involvement of DFNB1, and in the remaining 49 families no linkage analysis has been performed. A total of 62 mutant alleles in 39 families were identified. Therefore, mutations in Cx26 represent a major cause of recessively inherited prelingual deafness since according to the present results they would underlie approximately half of the cases. In addition, one specific mutation, 30delG, accounts for the majority (approximately 70%) of the Cx26 mutant alleles. It is therefore one of the most frequent disease mutations so far identified. Several lines of evidence indicate that the high prevalence of the 30delG mutation arises from a mutation hot spot rather than from a founder effect. Genetic counseling for prelingual deafness has been so far considerably impaired by the difficulty in distinguishing genetic and non genetic deafness in families presenting with a single deaf child. Based on the results presented here, the development of a simple molecular test could be designed which should be of considerable help.

One of the differences between a mere accumulation of cells and a tissue or even an organ is that in organized forms (tissue, organ) cells typically communicate with each other. They coordinate their function, growth, and differentiation. In addition to cell adhesion molecules, increted hormones and local mediators, direct intercellular communication via gap junction channels plays a key role in this process. In cardiac tissue, these channels also provide the basis for the propagation of the action potential from cell to cell. Gap junction channels are dodecameric proteins consisting of connexin subunits of one (homomeric channels) or more isoforms (heteromeric channels). The connexins belong to an evolutionarily old protein family comprising 21 members in the mouse and 20 in man.1 In the mammalian heart typically three connexins are expressed: Cx43, encoded by the gene Gja1, is the most abundant; Cx40, encoded by Gja5, is confined to atria and the specific conduction system; Cx45, encoded by Gjc1, is found in early stages of development and in the specific conduction system, including sinus nodal cells. In recent years, murine Cx30.2, encoded by Gjd3, (the murine orthologue of human Cx31.9, encoded by Gjd3 Ref here?) and now Cx30, encoded by Gjb6, (see the report by Gros et al .2) have been found in the murine heart. Cx30 expression was limited to a narrow epicardial layer of the sinus node. Since Cx30-deficient … *Corresponding author. Tel: +49 341 865 1651, Fax: +49 341 865 1452, Email: dhes{at}medizin.uni-leipzig.de