GJB2 mutations in Iranians with autosomal recessive non-syndromic sensorineural hearing loss

Hereditary hearing loss (HHL) is an extremely common disorder. About 70% of HHL is non-syndromic, with autosomal recessive forms accounting for approximately 85% of the genetic load. Although very heterogeneous, the most common cause of HHL in many different world populations is mutations of GJB2, a gene that encodes the gap junction protein connexin 26 (Cx26). This study investigates the contribution of GJB2 to the autosomal recessive non-syndromic deafness (ARNSD) load in the Iranian population. One hundred sixty eight persons from 83 families were studied. GJB2-related deafness was diagnosed in 9 families (4, 35delG homozygotes; 3, 35delG compound heterozygotes; 1, W24X homozygote; 1, non-35delG compound heterozygote). The carrier frequency of the 35delG allele in this population was approximately 1% (1/83). Because the relative frequency of Cx26 mutations is much less than in the other populations, it is possible that mutations in other genes play a major role in ARNSD in Iran.     

A novel C202F mutation in the connexin26 gene (GJB2) associated with autosomal dominant isolated hearing loss

Mutations in the GJB2 gene encoding connexin26 (CX26) account for up to 50% of cases of autosomal recessive hearing loss. In contrast, only one GJB2 mutation has been reported to date in an autosomal dominant form of isolated prelingual hearing loss. We report here a novel heterozygous 605G→T mutation in GJB2 in all affected members of a large family with late childhood onset of autosomal dominant isolated hearing loss. The resulting C202F substitution, which lies in the fourth (M4) transmembrane domain of CX26, may impair connexin oligomerisation. Finally, our study suggests that GJB2 should be screened for heterozygous mutations in patients with autosomal dominant isolated hearing impairment, whatever the severity of the disease.


Keywords: C202F mutation; connexin26 gene (GJB2); autosomal dominant hearing loss     

Identification of mutations in the connexin 26 gene that cause autosomal recessive nonsyndromic hearing loss

Mutations in the Cx26 gene have been shown to cause autosomal recessive nonsyndromic hearing loss (ARNSHL) at the DFNB1 locus on chromosome 13q12. Using direct sequencing, we screened the Cx26 coding region of affected and nonaffected members from seven ARNSHL families either linked to the DFNB1 locus or in which the ARNSHL phenotype cosegregated with markers from chromosome 13q12. Cx26 mutations were found in six of the seven families and included two previously described mutations (W24X and W77X) and two novel Cx26 mutations: a single base pair deletion of nucleotide 35 resulting in a frameshift and a C-to-T substitution at nucleotide 370 resulting in a premature stop codon (Q124X). We have developed and optimized allele-specific PCR primers for each of the four mutations to rapidly determine carrier and noncarrier status within families. We also have developed a single stranded conformational polymorphism (SSCP) assay which covers the entire Cx26 coding region. This assay can be used to screen individuals with nonsyndromic hearing loss for mutations in the CX26 gene. Hum Mutat 11:387–394, 1998. © 1998 Wiley-Liss, Inc.     

Three novel GJB2 (connexin 26) variants associated with autosomal dominant syndromic and nonsyndromic hearing loss

Connexin 26 (Cx26), encoded by the GJB2 gene, is a key protein involved in the formation of gap junctions in epithelial organs including the inner ear and palmoplantar epidermis. Pathogenic variants in GJB2 are responsible for approximately 50% of inherited sensorineural deafness. The majority of these variants are associated with autosomal recessive inheritance; however, rare reports of dominantly co‐segregating variants have been published. Since we began offering GJB2 testing in 2003, only about 2% of detected GJB2 variants from our laboratory have been classified as dominant. Here we report three novel dominant GJB2 variants (p.Thr55Ala, p.Gln57_Pro58delinsHisSer, and p.Trp44Gly); two associated with syndromic sensorineural hearing loss and one with nonsyndromic hearing loss. In the kindred with the p.Thr55Ala variant, the proband and his father present with only leukonychia as a cutaneous finding of their syndromic hearing loss. This phenotype has been previously documented in conjunction with palmoplantar hyperkeratosis, but isolated leukonychia is a novel finding likely associated with the unique threonine to alanine change at codon 55 (other variants at this codon have been reported in cases of nonsyndromic hearing loss). This report contributes to the short list of GJB2 variants associated with autosomal dominant hearing loss, highlights the variability of skin and nail findings associated with such cases, and illustrates the occurrence of both syndromic and nonsyndromic presentations with changes in the same gene.     

A common founder for the 35delG GJB2 gene mutation in connexin 26 hearing impairment

Fifty to eighty percent of autosomal recessive congenital severe to profound hearing impairment result from mutations in a single gene, GJB2, that encodes the protein connexin 26. One mutation of this gene, the 35delG allele, is particularly common in white populations. We report evidence that the high frequency of this allelic variant is the result of a founder effect rather than a mutational hot spot in GJB2, which was the prevailing hypothesis. Patients homozygous for the 35delG mutation and normal hearing controls originating from Belgium, the UK, and the USA were genotyped for different single nucleotide polymorphisms (SNPs). Four SNPs mapped in the immediate vicinity of GJB2, while two were positioned up to 76 kb from it. Significant differences between the genotypes of patients and controls for the five SNPs closest to GJB2 were found, with nearly complete association of one SNP allele with the 35delG mutation. For the most remote SNP, we could not detect any association. We conclude that the 35delG mutation is derived from a common, albeit ancient founder.


Keywords: connexin 26; GJB2; 35delG; founder effect     

A quantitative cSMART assay for noninvasive prenatal screening of autosomal recessive nonsyndromic hearing loss caused by GJB2 and SLC26A4 mutations

PurposeThe aim of this study was to assess the performance of a noninvasive prenatal screening (NIPS) assay for accurate fetal genotyping of pregnancies at genetic risk for autosomal recessive nonsyndromic hearing loss (ARNSHL).MethodsA total of 80 pregnant couples carrying known mutations in either the GJB2 or SLC26A4 genes associated with a risk for ARNSHL were recruited to the study. Fetal amniocyte samples were genotyped by invasive prenatal screening (IPS), whereas the cell-free fetal DNA present in maternal plasma samples was genotyped using a novel NIPS method based on circulating single-molecule amplification and resequencing technology (cSMART).ResultsIPS of the 80 at-risk pregnancies identified 20 normal homozygote, 42 heterozygote, 5 affected homozygote, and 13 affected compound heterozygote fetuses. Benchmarking against IPS, 73 of 80 fetuses (91.3%) were correctly genotyped by the cSMART NIPS assay. A low fetal DNA fraction (<6%) was identified as the main contributing factor in five of seven discordant NIPS results. At fetal DNA fractions >6%, the sensitivity and specificity of the cSMART assay for correctly diagnosing ARNSHL were 100 and 96.5%, respectively.ConclusionBased on key performance indicators, the cSMART NIPS assay has clinical potential as an alternative to traditional IPS of ARNSHL.     

High prevalence of the W24X mutation in the gene encoding connexin-26 (GJB2) in Spanish Romani (gypsies) with autosomal recessive non-syndromic hearing loss

Molecular testing for mutations in the gene encoding connexin-26 (GJB2) at the DFNB1 locus has become the standard of care for genetic diagnosis and counseling of autosomal recessive non-syndromic hearing impairment (ARNSHI). The spectrum of mutations in GJB2 varies considerably among the populations, different alleles predominating in different ethnic groups. A cohort of 34 families of Spanish Romani (gypsies) with ARNSHI was screened for mutations in GJB2. We found that DFNB1 deafness accounts for 50% of all ARNSHI in Spanish gypsies. The predominating allele is W24X (79% of the DFNB1 alleles), and 35delG is the second most common allele (17%). An allele-specific PCR test was developed for the detection of the W24X mutation. By using this test, carrier frequencies were determined in two sample groups of gypsies from different Spanish regions (Andalusia and Catalonia), being 4% and 0%, respectively. Haplotype analysis for microsatellite markers closely flanking the GJB2 gene revealed five different haplotypes associated with the W24X mutation, all sharing the same allele from marker D13S141, suggesting that a founder effect for this mutation is responsible for its high prevalence among Spanish gypsies.     

A connexin 26 mutation causes a syndrome of sensorineural hearing loss and palmoplantar hyperkeratosis (MIM 148350)

We report a missense mutation in the connexin 26 gene (GJB2) in a family with an autosomal dominant syndrome of hearing loss and hyperkeratosis. The affected family members have high frequency, slowly progressive, bilateral, sensorineural hearing loss and palmoplantar hyperkeratosis. The mutation causes an amino acid substitution (G59A), which may disrupt a reverse turn in the first extracellular loop of connexin 26. Connexin 26 mutations have been reported in syndromes of deafness and palmoplantar keratoderma. These data provide additional evidence for the role of connexin 26 in syndromes of this type.     

Mutations in the gene for connexin 26 (GJB2) that cause hearing loss have a dominant negative effect on connexin 30

Mutations in the gene (GJB2) encoding connexin 26 (cx26) have been linked to sensorineural hearing loss either alone or as part of a syndrome. Here we compare the properties of four cx26 mutants derived from point mutations associated with dominantly inherited hearing loss, either non-syndromic (W44S, R75W) or with various skin disorders (G59A, D66H, R75W). Since cx26 and cx30 are co-localized within the inner ear the effect of the dominant cx26 mutations on both of these wild-type proteins was determined. Communication-deficient HeLa cells were transiently transfected with the various cDNA constructs by microinjection. Dye transfer studies using the gap junction permeant tracer Cascade Blue demonstrated a disruption to the intercellular coupling for all four of the mutant proteins. Immunostaining of the transfected cells revealed that for the G59A and D66H mutants this correlated with impaired intracellular trafficking and targeting to the plasma membrane, as both proteins had a perinuclear localization. The impaired trafficking was rescued by oligomerization both with cx26 and with cx30, suggesting that cx26 and cx30 can form heteromeric connexons. Significantly reduced dye transfer rates were observed between cells co-expressing either cx26 or cx30 together with W44S or R75W compared with the wild-type proteins alone. The dominant actions of the G59A and D66H mutants were only on cx30 and cx26, respectively. We suggest that cx26 and cx30 form heteromeric connexons in vivo, within the inner ear, with particular properties essential for hearing. Disruption of these heteromeric channels by certain mutations may underlie the non-syndromic nature of the deafness.     

连接蛋白基因一个新致聋突变体p.Y155X及功能分析

目的 观察连接蛋白(connexin 26,CX26)基因的一个新致聋突变c.465T→A,P.Y155X,在体外表达细胞功能的改变,以探讨其致聋的可能机理.方法 常染色体隐性遗传耳聋家系的先证者外周血抽提DNA,DNA直接测序法分析CX26基因突变.将在该家系发现的突变c.465T→A,P.Y155X和野生型CX26(wtCX26)定向克隆到pEGFP-N1质粒,构建CX26 p.Y155X-EGFP及wtCX26-EGFP融合蛋白表达载体,转染HeLa细胞,Western印迹分析蛋白的表达,共聚焦显微镜观察突变蛋白和野生型CX26在HeLa细胞的定位及有无间隙连接斑形成,染料转移实验分析间隙连接的功能.结果 在该耳聋家系发现CX26基因一个新的致聋突变:c.465T→A,P.Y155X.CX26 P.Y155X突变体在HeLa细胞表达的突变蛋白的分子量小于野生型蛋白分子量;突变蛋白在细胞质表达,不能分布到细胞膜和形成间隙连接,无染料转移.野生型表达于细胞膜并形成间隙连接斑,能转移染料.结论 CX26 P.Y155X突变体在翻译后不能从细胞内转运到细胞膜,不能形成间隙连接通道.CX26基因c.465T→A,P.Y155X导致常染色体隐性遗传性聋.     

The novel R75Q mutation in the GJB2 gene causes autosomal dominant hearing loss and palmoplantar keratoderma in a Turkish family

Dominant mutations in the GJB2 gene encoding connexin 26 (Cx26) can cause non-syndromic hearing impairment alone or in association with palmoplantar keratoderma (PPK). We have identified the novel G224A (R75Q) mutation in the GJB2 gene in a four-generation family from Turkey with autosomal dominant inherited hearing impairment and PPK. The age of onset and progression of hearing loss were found to be variable among affected family members, but all of them had more severe impairment at higher hearing frequencies. Interestingly, the novel R75Q mutation affects the same amino acid residue as described recently in a small family (R75W) with profound prelingual hearing loss and PPK. However, the R75W mutation was also observed in a control individual without PPK and unknown hearing status. Therefore, the nature of the R75W mutation remains ambiguous. Our molecular findings provide further evidence for the importance of the conserved R75 in Cx26 for the physiological function of the inner ear and the epidermal cells of the skin.     

Two Iranian families with a novel mutation in GJB2 causing autosomal dominant nonsyndromic hearing loss

Mutations in GJB2, encoding connexin 26 (Cx26), cause both autosomal dominant and autosomal recessive nonsyndromic hearing loss (ARNSHL) at the DFNA3 and DFNB1 loci, respectively. Most of the over 100 described GJB2 mutations cause ARNSHL. Only a minority has been associated with autosomal dominant hearing loss. In this study, we present two families with autosomal dominant nonsyndromic hearing loss caused by a novel mutation in GJB2 (p.Asp46Asn). Both families were ascertained from the same village in northern Iran consistent with a founder effect. This finding implicates the D46N missense mutation in Cx26 as a common cause of deafness in this part of Iran mandating mutation screening of GJB2 for D46N in all persons with hearing loss who originate from this geographic region.     

Low prevalence of Connexin 26 (GJB2) variants in Pakistani families with autosomal recessive non-syndromic hearing impairment

The Pakistani population has become an important resource for research on autosomal recessive non-syndromic hearing impairment (ARNSHI) due to the availability of large extended and highly consanguineous pedigrees. Here is presented the first report on the prevalence of gap junction beta-2 (GJB2) variants in Pakistan. One hundred and ninety-six unrelated Pakistani families with ARNSHI were recruited for a study on the genetics of NSHI. DNA sequencing of the GJB2 coding region was done on two affected individuals per family. Evolutionary conservation and predicted effect on the protein product were studied in order to hypothesize whether or not a variant was potentially deleterious. Homozygous putatively functional GJB2 variants were identified in 6.1% of families. None of the putatively functional GJB2 variants were observed in the compound heterozygous state. The six putatively causative variants noted were 231G > A(W77X), 71G > A(W24X), 167delT, 95G > A(R32H), 358-360delGAG(delE120), and 269T > C(L90P), with 231G > A(W77X) and 71G > A(W24X) being the most common. In addition, five benign polymorphisms, 380G > A(R127H), 457G > A(V153I), 493C > T(R165W), 79G > A(V27I), and 341 A > G(E114G), were identified within this population. In a few individuals, benign polymorphisms were observed to occur on the same haplotype, namely [457G > A(V153I); 493C > T(R165W)] and [79G > A(V27I); 341 A > G(E114G)]. The spectrum of GJB2 sequence variants in Pakistan may reflect shared origins of hearing impairment alleles within the Indian subcontinent. The high degree of consanguinity within Pakistan may have maintained the GJB2 prevalence at a much lower rate than within India and other populations.     

A deleterious mutation in the LOXHD1 gene causes autosomal recessive hearing loss in Ashkenazi Jews

Autosomal recessive nonsyndromic sensorineural hearing loss (ARNSHL) in Ashkenazi Jews, is mainly caused by mutations in the GJB2 and GJB6 genes. Here we describe a novel homozygous mutation of the LOXHD1 gene resulting in a premature stop codon (R1572X) in nine patients of Ashkenazi Jewish origin who had severe-profound congenital non-progressive ARNSHL and benefited from cochlear implants. Upon screening for the mutation among 719 anonymous Ashkenazi-Jews we detected four carriers, indicating a carrier rate of 1:180 Ashkenazi Jews. This is the second reported mutation in the LOXHD1 gene, and its homozygous presence in two of 39 Ashkenazi Jewish families with congenital ARNSHL suggest that it could account for some 5% of the familial cases in this community.     

GJB2 (connexin 26) mutations are not a major cause of hearing loss in the Indonesian population

Although hereditary hearing loss is a very heterogeneous disorder, variants in one gene, GJB2 (connexin 26), account for up to 50% of autosomal recessive nonsyndromal sensorineural hearing loss in most populations. This study investigates the contribution of GJB2 to autosomal recessive nonsyndromal hearing loss in the Indonesian population. We performed DNA sequence analysis in 120 patients with profound early childhood nonsyndromal hearing loss and in 100 control individuals and identified three novel variations resulting in amino acid substitutions (p.Gly4Asp, p.Thr5Ala, and p.Gly160Arg). Although we proved that p.Gly4Asp was not disease-causing, the pathological nature of p.Thr5Ala and p.Gly160Arg could not be determined. No recurrent disease-causing mutation could be detected in this Indonesian population. These findings are in contrast with the results obtained in other populations where GJB2 is a major cause of congenital recessive hearing loss.