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Geographically isolated populations have been successfully used to localize genes for recessive inherited diseases, including non-syndromic sensorineural recessive deafness (NSRD). To date, 25 loci for NSRD have been localized on human chromosomes (DFNB loci), and six of the corresponding genes have been identified. Here, we report on the contribution of the DFNB1 locus (GJB2 gene) to NRSD in seven affected families living in three northern Tunisian geographic isolates, and we provide evidence for genetic heterogeneity within isolates. This finding challenges the classical view of a single 'founder' mutation segregating in such isolates.

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.

We report the identification of a novel CC chemokine receptor 5 (CCR5) variant that seems associated with resistance to HIV-1 infection. The V130I mutation of the CCR5 receptor is located in the intracellular loop ICL2 known as DRY box and described in the literature as a nonsynonymous mutation present in nonhuman primates group. Extensive molecular modeling and dynamics simulations were performed to elucidate the mechanism by which the V130I mutation may induce conformational change of the CCR5 folding protein and prevent the interaction with the β-arrestin protein. Our study provides new mechanistic insight into how a specific mutation in the regulatory domain of CCR5 might alter the structural folding of the DRY box and the possible ICL2 loop binding with the β-arrestin protein, as described in our previous computational study. The results from our large-scale simulations complement recent experimental results and clinical features and offer useful insights into the mechanism behind CCR5 protein folding and signal transduction. In order for HIV, the entry of the virus to the cells must fuse with the CCR5 receptor that sits on the surface of T-helper immune cells. The described V130I mutation in the gene encoding the CCR5 protein may results in a defective CCR5-Arrestin binding complex that blocks entry of the virus.

Usher syndrome (USH) is an autosomal recessive disorder characterised by hearing impairment and retinitis pigmentosa (RP). The prevalence of USH varies from one population to another, for example, 3-4.4 per 100 000 in Scandinavian and North American populations1,2 and 6.2-10 per 100 000 in the city of Birmingham, UK.3 This syndrome is clinically heterogeneous and three clinical forms have been described4: (1) USH type I (USH1) is characterised by severe to profound congenital deafness, constant vestibular dysfunction, and prepubertal onset of RP; (2) USH type II (USH2) is characterised by congenital moderate to severe deafness, absence of vestibular dysfunction, and onset of RP usually in the late second to early third decade; (3) USH type III (USH3) is characterised by postlingual progressive deafness, occasional vestibular dysfunction, and progressive RP with a variable age of onset (see also http://www.ncbi.nlm.nih.gov/omim). Usher syndrome is also genetically heterogeneous; at least six distinct loci are responsible for USH1 (USH1A-F), three for USH2 (USH2A-C), and one for USH35 (Hereditary hearing loss homepage at URL, http://dnalab.www.uia.ac.be/dnalab/hhh). Usher syndrome type II (USH2) appears to be the commonest clinical form of the disorder in the American population, accounting for more than 50% of all USH cases.3,6 This clinical form tends to be rare in other populations.7 In the Tunisian population, only two USH2 families, Us8 and Z (this work), have been identified so far. These families were ascertained from villages from the south and the north of Tunisia, respectively, where endogamous marriage is relatively common for social and cultural reasons. Of all the USH2 subtypes, USH2A seems to be the most frequent. According to a study performed in various ethnic populations, USH2A is responsible for more than 85% of USH2 cases.9 This genetic form showed considerable phenotypic …

Usher type II syndrome is defined by the association of retinitis pigmentosa, appearing in the late second to early third decade of life, with congenital moderate to severe non-progressive hearing loss. This double sensory impairment is not accompanied by vestibular dysfunction. To date, only one Usher type II locus, USH2A, at chromosome band 1q41, has been defined. Here, we demonstrate by linkage analysis, that the gene responsible for Usher type II syndrome in a Tunisian consanguineous family maps to chromosome 3 at position p23-24.2, thus providing definitive evidence for the genetic heterogeneity of the syndrome. A maximum lod score of 4.3 was obtained with the polymorphic microsatellite markers corresponding to loci D3S1578, D3S3647 and D3S3658. This maps the gene underlying USH2B to a chromosomal region which overlaps the interval defined for the non-syndromic sensorineural recessive deafness DFNB6, raising the possibility that a single gene underlies both defects. However, the audiometric features in the patients affected by USH2B and DFNB6 are very different.

Editor—Congenital deafness occurs in approximately 1 in 1000 live births and at least 50% of these cases are hereditary.1 Among the prelingual genetic forms of deafness, the autosomal recessive forms ( DFNB ) are frequent (80% of the cases) and in most cases are sensorineural and severe.1 Twenty eight loci that cause autosomal recessive non-syndromic hearing loss (ARNSHL) have been identified (http://dnalab-www.uia.ac.be./dnalab/hhh/index.html). The first locus defined for recessive deafness ( DFNB1 ) is linked to chromosome 13q12-13 and was identified by homozygosity mapping in two large consanguineous families from Tunisia.2 This initial report was followed by the identification of other consanguineous families of different ethnic origins which were linked to the DFNB1 locus and of several non-consanguineous white families in which the ARNSHL phenotype cosegregated with markers from chromosome 13q12-13.3-6Mutations in connexin26 ( Cx26 ), a gene that encodes gap junction protein beta-2 (GJB-2), have been shown to result in autosomal recessive ( DFNB1 ) and dominant ( DFNA3 ) non-syndromic sensorineural deafness.7 Mutations in the Cx26 gene have been found to be the most common cause of autosomal recessive deafness and the most frequently observed mutation is 35delG.8-12 The high prevalence of Cx26 mutations and their importance as a cause of ARNSHL have prompted the development of several different mutation detection assays to screen the single Cx26 coding exon.13-16 A rapid method to detect mutations in the GJB2 …