Molecular epidemiology of DFNB1 deafness in France

BACKGROUND: Mutations in the GJB2 gene have been established as a major cause of inherited non syndromic deafness in different populations. A high number of sequence variations have been described in the GJB2 gene and the associated pathogenic effects are not always clearly established. The prevalence of a number of mutations is known to be population specific, and therefore population specific testing should be a prerequisite step when molecular diagnosis is offered. Moreover, population studies are needed to determine the contribution of GJB2 variants to deafness. We present our findings from the molecular diagnostic screening of the GJB2 and GJB6 genes over a three year period, together with a population-based study of GJB2 variants. METHODS AND RESULTS: Molecular studies were performed using denaturing High Performance Liquid Chromatograghy (DHPLC) and sequencing of the GJB2 gene. Over the last 3 years we have studied 159 families presenting sensorineural hearing loss, including 84 with non syndromic, stable, bilateral deafness. Thirty families were genotyped with causative mutations. In parallel, we have performed a molecular epidemiology study on more than 3000 dried blood spots and established the frequency of the GJB2 variants in our population. Finally, we have compared the prevalence of the variants in the hearing impaired population with the general population. CONCLUSION: Although a high heterogeneity of sequence variation was observed in patients and controls, the 35delG mutation remains the most common pathogenic mutation in our population. Genetic counseling is dependent on the knowledge of the pathogenicity of the mutations and remains difficult in a number of cases. By comparing the sequence variations observed in hearing impaired patients with those sequence variants observed in general population, from the same ethnic background, we show that the M34T, V37I and R127H variants can not be responsible for profound or severe deafness

Doubly mutant mice, deficient in connexin32 and -43, show normal prenatial development of organs where the two gap juction proteins are expressed in the same cells

The connexins are a family of proteins that form the intercellular membrane channels of gap junctions. Genes encoding 13 different rodent connexins have been cloned and characterized to date. Connexins vary both in their distribution among adult cell types and in the properties of the channels that they form. In order to explore the functional significance of connexin diversity, several mouse connexin-encoding genes have been disrupted by homologous recombination in embryonic stem cells. Although those experiments have illuminated specific physiological roles for individual connexins, the results have also raised the possibility that connexins may functionally compensate for one another in cells where they are coexpressed. In the present study, we have tested this hypothesis by interbreeding mice carrying null mutations in the genes (Gjb1 and Gja1) encoding connexin32 (β1 connexin) and connexin43 (α1 connexin), respectively. We found that fetuses lacking both connexins survive to term but, as expected, the pups die soon thereafter from the cardiac abnormality caused by the absence of connexin43. A survey of the major organ systems of the doubly mutant fetuses, including the thyroid gland, developing teeth, and limbs where these two connexins are coexpressed, failed to reveal any morphological abnormalities not already seen in connexin43 deficient fetuses. Furthermore, the production of thyroxine by doubly mutant thyroids was confirmed by immunocytochemistry. We conclude that, at least as far as the prenatal period is concerned, the normal development of those three organs in fetuses lacking connexin43 cannot simply be explained by the additional presence of connexin32 and vice-versa. Either gap junctional coupling is dispensable in embryonic and fetal cells in which these two connexins are coexpressed, or coupling is provided by yet another connexin when both are absent. Dev. Genet. 24:5–12, 1999

Two molecular assays for the rapid and inexpensive detection of GJB2 and GJB6 mutations

The hypoacusia can be classified in two clinical forms: Syndromic (SHL) and Nonsyndromic (NSHL). In particular, the NSHL describes the 70–80% of hypoacusia cases and it is mainly due to genetic factors, which are causative of the deafness at the birth. The genetic hypoacusia presents different inheritance patterns: autosomal dominant (20%), autosomal recessive (80%), X-linked (1%), and mitochondrial (1%), respectively. To date, about 35 deafness-causative genes have been identified and most of them codify for connexin transmembrane proteins. Approximately 1:2500 children with NSHL carries mutations in the GJB2 and GJB6 (13q12) genes, which code for connexin 26 (Cx26) and connexin 30 (Cx30), respectively. In the Caucasian population, the most common mutations are 35delG, M34T and 167delT, and D13S1830. Given the frequency distribution of the four mutations in the Caucasian population and the pathogenic connection with NSHL, the development of accurate, rapid, and “low-cost” molecular assays should be strongly encouraged. To this purpose, we set up two different molecular assays (namely the Cx26 and Cx26-30 molecular assays) for the fast and inexpensive detection of 35delG, M34T, 167delT, and D13S1830 mutations. Both the molecular approaches showed to be accurate, sensitive, reproducible, and “low-cost” alternatives for the proper evaluation of the GJB2 and GJB6 genes, which are causative ofNSHL. In conclusion, the Cx26 and Cx26-30 molecular assays can be applied to individual, preconception, prenatal, or postnatal screening for the causative-mutations of NSHL