Autosomal recessive nonsyndromic neurosensory deafness at DFNB1 not associated with the compound-heterozygous GJB2 (connexin 26) genotype M34T/167delT

Consensus interpretation of the p.Met34Thr and p.Val37Ile variants in GJB2 by the ClinGen Hearing Loss Expert Panel

PURPOSE

Pathogenic variants in GJB2 are the most common cause of autosomal recessive sensorineural hearing loss. The classification of c.101T>C/p.Met34Thr and c.109G>A/p.Val37Ile in GJB2 are controversial. Therefore, an expert consensus is required for the interpretation of these two variants.

METHODS

The ClinGen Hearing Loss Expert Panel collected published data and shared unpublished information from contributing laboratories and clinics regarding the two variants. Functional, computational, allelic, and segregation data were also obtained. Case-control statistical analyses were performed.

RESULTS

The panel reviewed the synthesized information, and classified the p.Met34Thr and p.Val37Ile variants utilizing professional variant interpretation guidelines and professional judgment. We found that p.Met34Thr and p.Val37Ile are significantly overrepresented in hearing loss patients, compared with population controls. Individuals homozygous or compound heterozygous for p.Met34Thr or p.Val37Ile typically manifest mild to moderate hearing loss. Several other types of evidence also support pathogenic roles for these two variants.

CONCLUSION

Resolving controversies in variant classification requires coordinated effort among a panel of international multi-institutional experts to share data, standardize classification guidelines, review evidence, and reach a consensus. We concluded that p.Met34Thr and p.Val37Ile variants in GJB2 are pathogenic for autosomal recessive nonsyndromic hearing loss with variable expressivity and incomplete penetrance.

DFNB1 Non-syndromic Hearing Impairment: Diversity of Mutations and Associated Phenotypes

The inner ear is a very complex sensory organ whose development and function depend on finely balanced interactions among diverse cell types. The many different kinds of inner ear supporting cells play the essential roles of providing physical and physiological support to sensory hair cells and of maintaining cochlear homeostasis. Appropriately enough, the gene most commonly mutated among subjects with hereditary hearing impairment (HI), GJB2, encodes the connexin-26 (Cx26) gap-junction channel protein that underlies both intercellular communication among supporting cells and homeostasis of the cochlear fluids, endolymph and perilymph. GJB2 lies at the DFNB1 locus on 13q12. The specific kind of HI associated with this locus is caused by recessively-inherited mutations that inactivate the two alleles of the GJB2 gene, either in homozygous or compound heterozygous states. We describe the many diverse classes of genetic alterations that result in DFNB1 HI, such as large deletions that either destroy the GJB2 gene or remove a regulatory element essential for GJB2 expression, point mutations that interfere with promoter function or splicing, and small insertions or deletions and nucleotide substitutions that target the GJB2 coding sequence. We focus on how these alterations disrupt GJB2 and Cx26 functions and on their different effects on cochlear development and physiology. We finally discuss the diversity of clinical features of DFNB1 HI as regards severity, age of onset, inner ear malformations and vestibular dysfunction, highlighting the areas where future research should be concentrated.

GJB2 mutations: Genotypic and phenotypic correlation in a cohort of 690 hearing-impaired patients, toward a new mutation?

OBJECTIVES

To analyze the clinical features of hearing impairment and to search for correlations with the genotype in patients with GJB2 mutations.

DESIGN

Case series.

SETTING

Collaborative study in referral centers, institutional practice.

PATIENTS

A total of 690 hearing-impaired patients were genotypically and phenotypically described. The mutations of GJB2 and GJB6 were studied. Heterozygous patients were searched for another mutation by microsatellite approach.

MAIN OUTCOME MEASURES

Prevalence of GJB2 mutations, microsatellite approach, hearing-impairment.

RESULTS

In 498 patients (72,17% of the cohort), no mutation was found. Homozygotous patients were 59 (8,55%), with 51 for c.35delG, 6 for p.M34T and 2 for GJB6. Compound heterozygous were 64 (9,28%) with 56 c.35delG-others mutations. Genotypes with biallelic non sense mutations had a high risk of severe to profound hearing impairment. It was frequently milder in compound heterozygotes than in c.35delG homozygotes. Heterozygous patients were 69 (10%) with 21 c.35delG, 20 p.M34T and 28 others mutations. We selected patients with a complete historical medical file (clinical and audiometric data). Then, we performed a microsatellite approach (multiplex PCR of short DNA fragments) to localize a new pathologic allele. Seventeen heterozygous patients were studied. Six patients (35%) showed the same haplotype. They were compound heterozygous bearing a new pathologic allele.

CONCLUSION

Genotype may affect deafness severity, but environmental and other genetic factors may also modulate the severity and evolution of GJB2-GJB6 deafness. A new haplotype for GJB2 is described but the exact mutation remains unknown.

Study of Met34Thr variant in nonsyndromic hearing loss in four Portuguese families

Objective

The purpose of this work was to characterize the Met34Thr variant in a group of patients with nonsyndromic hearing loss, in order to establish a genotype-phenotype correlation.

Methods

13 cases from 4 unrelated Portuguese families were selected, in which one or more hearing-impaired members had Met34Thr variant.

Results

Met34Thr variant was identified in 11/13 cases. Two cases have an additional mutation - Val153Ile and 35delG. Hearing loss was mild in 2 patients (Met34Thr/Val153Ile; Met34Thr/Met34Thr), moderate in 3(Met34Thr/WT; Met34Thr/35delG; Met34Thr/Met34Thr), severe in 2 (2 Met34Thr/WT) and profound in 1 (Met34Thr/WT). Three individuals with Met34Thr had normal hearing thresholds.

Conclusion

The present data corroborate the hypothesis that the Met34Thr variant is associated with mild-to-severe forms of deafness and that this variant seems to segregate with a dominant hearing loss with incomplete penetrance and a variable expression of the phenotype. However, other factors are likely to also have a pathologic effect.

The controversial p.Met34Thr variant in GJB2 gene: Two siblings, one genotype, two phenotypes

INTRODUCTION

Recent advances in molecular genetics have increased the identification of genes and mutations responsible for inherited forms of hearing loss (HL), enabling early detection of these cases. Approximately, 60% of early-onset HL cases are due to genetic causes, of which 70% are non-syndromic. Of these, 75-80% are inherited in an autosomal recessive pattern (DFNB). Mutations in GJB2 gene, coding for connexin 26 (Cx26), are the major cause of autosomal recessive hereditary HL, but some GJB2 mutations are yet of unclear or controversial significance.

OBJECTIVES

The aim of the present study was to identify the etiology of hearing loss, and correlate genotype-phenotype, in two Portuguese siblings with profound and moderate non-syndromic sensorineural bilateral HL.

MATERIAL AND METHODS

The affected subjects and their parents underwent audiological and genetic study. Molecular analysis of GJB2 gene was performed, searching for mutations in the coding region and receptor splicing site by automated sequencing.

RESULTS

The onset and the degree of HL were different in the two affected subjects. However, the same GJB2 genotype [p.Met34Thr]+[p.Arg184Pro] was identified in both siblings. The c.551G>C (p.Arg184Pro) and c.101T>C (p.Met34Thr) missense variants were inherited from the father and mother, respectively, both heterozygous carriers of these variants.

CONCLUSION

The clinical and genetic data here presented suggest that the non-syndromic sensorineural HL of these two Portuguese siblings might be due to the presence of p.Met34Thr and p.Arg184Pro variants in compound heterozygosity. If so, p.Met34Thr variant could have function as a hypomorphic allele that may cause HL depending on the opposing GJB2 allele. The observed phenotypic variability may not, however, be solely explained by variable expression of this genotype. A putative modifier gene or mutations in another HL-associated gene could probably be contributing to the severe HL in one of the siblings.

A genotype-phenotype correlation in Sicilian patients with GJB2 biallelic mutations

The aim of this work was to study the genotype distribution of Sicilian patients with biallelic GJB2 mutations; to correlate genotype classes and/or specific mutations of GJB2 gene (35delG-non-35delG) with audiologic profiles. A total of 10 different mutations and 11 different genotypes were evidenced in 73 SNHL subjects; 35delG (90.36 % of cases) and IVS1+1 (13.69 %) were the most common mutations found in the cohort with a significant difference in the distribution between North and South Sicily. Audiological evaluation revealed a severe (16/73) to profound (47/73) hearing loss (HL) in 86.13 % of cases without significant difference between the degree of HL and the province of origin of the subjects (P = 0.727). The homozygous truncating (T/T) genotype was the most widespread (89.04 % of cases), with a severe-to-profound hearing impairment in 90.36 % of T/T class with respect to truncating/non-truncating (T/NT) and non-truncating/non-truncating (NT/NT) genotypes (P = 0.012). From the comparison of homozygous 35delG and 35delG/non-35delG genotypes, a more profound HL in the homozygous 35delG than in compound heterozygous 35delG/non-35delG (p < 0.0001) resulted. This study confirms that 35delG is the most common mutation in the Mediterranean area with a heterogeneous distribution of the genotypes between North and South Sicily; probands homozygotes for 35delG or presenting a T/T genotype are more apt to have a severe-to-profound HL.

Significance of heterozygosis M34T mutation of GJB2 gene in non-syndromic congenital deafness. Retrospective analysis of 12,472 samples of amniotic fluid

OBJECTIVE

to determinate the role of heterozygosis of M34T mutation of GJB2 gene in non syndromic congenital deafness.

METHODS

retrospective study between March 2010 and June 2013. Molecular screening for 35delG and M34T mutations of the GJB2 gene was offered to all women undergoing to second trimester genetic amniocentesis. Patients were excluded from the study group if one of the following conditions were present: infections, fetal abnormalities, family history for congenital deafness, diagnosis of chromosomal abnormalities, and consanguinity between parents.

RESULTS

a total of 12.472 Caucasian women gave informed consent for this test. Seventy-seven cases were excluded. From the 12.395 amniotic fluid analysis remained, the following was found: 2 cases of 35delG homozygosis and 352 cases of heterozygous carriers (42 M34T mutation, 298 35delG mutation, 12 double heterozygosis M34T/35delG). The follow up in first year of life in the 42 newborns with heterozygosis for M34T mutation showed a mild deafness in 23 cases.

CONCLUSIONS

in our series, presence of heterozygosis M34T mutation is associated in more than 50% of cases to mild congenital deafness.

A study of GJB2 and delGJB6-D13S1830 mutations in Brazilian non-syndromic deaf children from the Amazon region

Objective

Method

Results

Conclusion

Connexin 26 mutations and nonsyndromic hearing impairment in Northern Finland

OBJECTIVE

The aims of the present study were to evaluate the role of the gap junction protein beta-2 gene (GJB2), encoding connexin 26 (Cx26), in children with moderate to profound prelingual nonsyndromic sensorineural hearing impairment (HI) and to investigate the carrier frequencies of the GJB2 gene mutations in a control population in Northern Finland.

METHODS

Mutation analysis was performed by direct sequencing and carrier detection by conformation sensitive gel electrophoresis further confirmed by direct sequencing.

RESULTS

Cx26 mutations were found in 15 of 71 (21.1%) (67 families) children with HI. Homozygosity for the mutation 35delG was shown to be the cause of HI in 13 of 15 (86.7%) children. Homozygosity for the M34T genotype was found in one child, and compound heterozygosity for the M34T/V37I genotype was found in another. Five families of those with suspected familial HI (29.4%) and six families out of those with sporadic HI (12.0%) had a homozygous or compound heterozygous mutation. The carrier frequency for the mutation 35delG was 1 of 78 (4 of 313) and that for the M34T was 1 of 26 (12 of 313).

CONCLUSION

35delG/35delG genotype was found to be a significant cause of moderate to profound prelingual nonsyndromic sensorineural HI in Northern Finland. M34T/M34T genotype was seen in only one child, but the carrier frequency of the M34T allele was about three times higher than that of the 35delG mutation.

Prevalence of c.35delG and p.M34T mutations in the GJB2 gene in Estonia

OBJECTIVE

The purpose of this study was to determine the prevalence of c.35delG and p.M34T mutations in the GJB2 gene among children with early onset hearing loss and within a general population of Estonia.

METHODS

Using an arrayed primer extension assay, we screened 233 probands with early childhood onset hearing loss for 107 different mutations in the GJB2 gene. We then looked for the two most common mutations, c.35delG and p.M34T, in a population of 998 consecutively born Estonian neonates to determine the frequency of these mutations in the general population.

RESULTS

In 115 (49%) of the patients with early onset hearing loss, we found a mutation in at least one allele of the GJB2 gene. Seventy-three (31%) were homozygous for the c.35delG mutation, seven (3%) were homozygous for the p.M34T mutation, and five (2%) had c35delG/p.M34T compound heterozygosity. Other six identified mutations in GJB2 gene occurred rarely. Among the 998 anonymous newborn samples, we detected 45 who were heterozygous for c.35delG, 2 individuals homozygous for c.35delG, and 58 who were heterozygous for p.M34T. Additionally, we detected two c.35delG/p.M34T compound heterozygotes.

CONCLUSION

The most common GJB2 gene mutations in Estonian children with early onset hearing loss were c.35delG and p.M34T, with c.35delG accounting for 75% of GJB2 alleles. The carrier frequency for c.35delG and p.M34T in a general population of Estonia was 1 in 22 and 1 in 17, respectively, and was higher than in most other countries.

Sequence Variations and Haplotypes of the GJB2 Gene Revealed by Resequencing of 192 Chromosomes from the General Population in Korea

Objectives

Hearing impairment (HI) is the most common sensory deficit in human. The Gap Junction Protein, Beta-2 (GJB2) gene encodes the protein connexin 26, and this gene accounts for up to half of the cases of autosomal recessive nonsyndromic HI. This study was conducted to obtain a set of sequence variations (SVs) of the GJB2 gene among Koreans from the general population for making molecular genetic diagnoses and performing genetic counseling.

Methods

We resequenced the GJB2 gene in 192 chromosomes from 96 adult individuals of Korean descent and who were without a history of hearing difficulty. The data of the SVs was obtained and the haplotypes were reconstructed from the data.

Results

Five SVs were observed, including a novel one (c.558G>A; p.T186T), with the allele frequencies ranging from 0.5% (1/192) to 41% (79/192). The linkage disequilibrium study and haplotype construction showed that some of the SVs are in tight linkage, resulting in a limited number of haplotypes.

Conclusion

We observed SVs of the GJB2 gene with different allele frequencies, and a limited number of haplotypes were constructed. The data from this study can be used as reference data for GJB2-related hearing genetic studies, including studies on the founder effect and population genetics, and this data is particularly relevant to people of East Asian decent.

Diverse deafness mechanisms of connexin mutations revealed by studies using in vitro approaches and mouse models

Mutations in connexins (Cxs), the constitutive protein subunits of gap junction (GJ) intercellular channels, are one of the most common human genetic defects that cause severe prelingual non-syndromic hearing impairments. Many subtypes of Cxs (e.g., Cxs 26, 29, 30, 31, 43) and pannexins (Panxs) are expressed in the cochlea where they contribute to the formation of a GJ-based intercellular communication network. Cx26 and Cx30 are the predominant cochlear Cxs and they co-assemble in most GJ plaques to form hybrid GJs. The cellular localization of specific Cx subtypes provides a basis for understanding the molecular structure of GJs and hemichannels in the cochlea. Information about the interactions among the various co-assembled Cx partners is critical to appreciate the functional consequences of various types of genetic mutations. In vitro studies of reconstituted GJs in cell lines have yielded surprisingly heterogeneous mechanisms of dysfunction caused by various Cx mutations. Availability of multiple lines of Cx-mutant mouse models has provided some insight into the pathogenesis processes in the cochlea of deaf mice. Here we summarize recent advances in understanding the structure and function of cochlear GJs and give a critical review of current findings obtained from both in vitro studies and mouse models on the mechanisms of Cx mutations that lead to cell death in the cochlea and hearing loss.

Molecular diagnosis of hearing loss

This unit discusses an approach to identifying a genetic cause in an individual with nonsyndromic hearing loss. Two protocols are presented, including a full-gene sequencing assay to identify mutations in the GJB2 gene encoding the connexin 26 protein. Mutations in the GJB2 gene represent the most common cause of congenital hearing loss. In addition, a protocol to detect the presence of a 342-kb deletion that includes a portion of the GJB6 gene is presented. The GJB6-D13S1830 deletion, in homozygosity or in combination with a single GJB2 mutation, causes hearing loss. In addition to the two protocols presented, the Strategic Planning section presents a discussion of a decision-making process that can be used to begin determining which gene(s) to test for in a patient presenting with nonsyndromic hearing loss. This task can be quite challenging, with the suspected involvement of over 90 genes.

DNA sequence analysis of GJB2, encoding connexin 26: observations from a population of hearing impaired cases and variable carrier rates, complex genotypes, and ethnic stratification of alleles among controls

Mutations in GJB2 are associated with hereditary hearing loss. DNA sequencing of GJB2 in a cohort of hearing impaired patients and a multi-ethnic control group is reported. Among 610 hearing impaired cases, 43 DNA sequence variations were identified in the coding region of GJB2 including 24 mutations, 8 polymorphisms, 3 unclassified variants (G4D, R127C, M163V), 1 controversial variant (V37I), and 7 novel variants (G12C, N14D, V63A, T86M, L132V, D159, 592_600delinsCAGTGTTCATGACATTC). Sixteen non-coding sequence variations were also identified among cases including the IVS1+1A>G mutation, 2 polymorphisms, and 13 novel variants. A diagnosis of GJB2-associated hearing loss was confirmed for 63 cases (10.3%). Heterozygous mutations were found in 39 cases (6.4%). Eleven cases carrying novel or unclassified variants (1.8 %) and 18 cases carrying the controversial V37I variant were identified (3%). In addition, 294 control subjects from 4 ethnic groups were sequenced for GJB2. Thirteen sequence variations in the coding region of GJB2 were identified among controls including 2 mutations, 6 polymorphisms, 2 unclassified variants (G4D, T123N), 1 controversial variant (V37I), and 2 novel variants (R127L, V207L). Nine sequence variations were identified among controls in the non-coding regions in and around GJB2 exon 2. Of particular interest among controls were the variability in carrier rates and ethnic stratification of alleles, and the complex genotypes among Asians, 47% of whom carried two to four sequence variations in the coding region of GJB2. These data provide new information about carrier rates for GJB2-based hearing loss in various ethnic groups and contribute to evaluation of the pathogenicity of the controversial V37I variant.

Connexin 26 variants and auditory neuropathy/dys-synchrony among children in schools for the deaf

Genetic and auditory studies of 731 children with severe-to-profound hearing loss in US schools for the deaf and 46 additional children receiving clinical services for hearing loss ranging from moderate to profound demonstrated that mutations in the connexin 26 (GJB2) and connexin 30 (GJB6) genes explain at least 12% of those with nonsyndromic sensorineural deafness. Otoacoustic emissions (OAEs) testing to detect functional outer hair cells indicated that 76 of the children had emissions and therefore may have (as yet unconfirmed) auditory neuropathy/dys-synchrony (AN/AD). Five of these children with OAEs were GJB2 homozygotes or compound heterozygotes with the genotypes 35delG/35delG, W77X/W77X, 35delG/360delGAG, 35delG/V95M, and V84M/M34T. In particular, unilateral AN/AD was confirmed in a child with moderate hearing loss and the 35delG/V95M genotype. Detecting OAEs in individuals with GJB2 mutations suggests that lack of functional gap junctions as a result of GJB2 mutations does not necessarily destroy all outer hair cell function.

Ethnicity and mutations in GJB2 (connexin 26) and GJB6 (connexin 30) in a multi-cultural Canadian paediatric Cochlear Implant Program

OBJECTIVE

To determine the relationship between ethnicity and mutations in the GJB2 and GJB6 genes in multi-cultural patients enrolled in a Canadian paediatric Cochlear Implant Program.

METHODS

Blood was analyzed from 65 paediatric cochlear implant users by direct sequencing of the coding region and intron/exon boundaries of the GBJ2 gene. Individuals heterozygous for one mutation in GJB2 or in whom mutations in GJB2 were not detected were analyzed for the common 342 kb deletion mutation D13S1830 in the GJB6 gene. Information regarding ethnicity of patients' families was obtained from patient records and/or interview.

RESULTS

GJB2 mutations were found in 36.9% of paediatric cochlear implant users tested. Nine different GJB2 mutations were identified among individuals from 14 different countries of origin. Seventy-eight percent of all identified pathogenic GJB2 mutations were 35delG. Biallelic GJB2 mutations were found in 16 cochlear implant users (66.7% of GJB2 mutations). Three novel GJB2 sequence changes were identified: (1) a missense mutation T107C (L36P) in an individual of African decent; (2) a missense mutation G475T (D159Y) in an individual of Caribbean decent; (3) a regulatory region change 1-34C to T in an individual of African decent. GJB6-D13S1830 mutations were not found in any of the patients tested. Individuals of African, Caribbean and East Indian decent had different GJB2 mutations than the remainder of individuals tested. Patients of Asian, Italian, Spanish, Polish and Armenian decent were not found to carry mutations in GJB2 or the common GJB6-D13S1830 mutation.

CONCLUSIONS

This study represents the largest number of biallelic GJB2 mutations isolated in a group of paediatric cochlear implant users to date. Numerous and diverse GJB2 mutations were found in this multi-cultural group of children. Even though GJB2 mutations have been widely reported in the literature, this discussion represents the first report of GJB2 mutations in a multi-ethnic population (Canadian), as compared with previous studies that investigated fairly homogeneous populations. The diversity of GJB2 mutations identified reinforces the importance of testing for changes in GJB2 by direct sequencing of the entire coding region rather than testing only for common mutations.

GJB2 mutations and degree of hearing loss: a multicenter study

Hearing impairment (HI) affects 1 in 650 newborns, which makes it the most common congenital sensory impairment. Despite extraordinary genetic heterogeneity, mutations in one gene, GJB2, which encodes the connexin 26 protein and is involved in inner ear homeostasis, are found in up to 50% of patients with autosomal recessive nonsyndromic hearing loss. Because of the high frequency of GJB2 mutations, mutation analysis of this gene is widely available as a diagnostic test. In this study, we assessed the association between genotype and degree of hearing loss in persons with HI and biallelic GJB2 mutations. We performed cross-sectional analyses of GJB2 genotype and audiometric data from 1,531 persons, from 16 different countries, with autosomal recessive, mild-to-profound nonsyndromic HI. The median age of all participants was 8 years; 90% of persons were within the age range of 0-26 years. Of the 83 different mutations identified, 47 were classified as nontruncating, and 36 as truncating. A total of 153 different genotypes were found, of which 56 were homozygous truncating (T/T), 30 were homozygous nontruncating (NT/NT), and 67 were compound heterozygous truncating/nontruncating (T/NT). The degree of HI associated with biallelic truncating mutations was significantly more severe than the HI associated with biallelic nontruncating mutations (P<.0001). The HI of 48 different genotypes was less severe than that of 35delG homozygotes. Several common mutations (M34T, V37I, and L90P) were associated with mild-to-moderate HI (median 25-40 dB). Two genotypes--35delG/R143W (median 105 dB) and 35delG/dela(GJB6-D13S1830) (median 108 dB)--had significantly more-severe HI than that of 35delG homozygotes.

Prevalence of GJB2 mutations and the del(GJB6-D13S1830) in Argentinean non-syndromic deaf patients

Genetically caused congenital deafness is a common trait affecting 1 in 2000 children and it is predominantly inherited in an autosomal recessive fashion. Several mutations in the GJB2 gene and a deletion of 342 kb in GJB6 (delGJB6-D13S1830) have been identified worldwide in patients with hearing impairment. The aim of this study was to determine the prevalence of these mutations in Argentina. Non-syndromic 46 probands (17 familial and 29 sporadic cases) were genetically evaluated. Mutations in GJB2 and/or delGJB6-D13S1830 were found in 19 patients, accounting for 41.3% of the sample. Of the 46 patients investigated in this study, 12 (26.1%) were diagnosed to carry sequence variations in both alleles; all but one, were considered causative for hearing impairment in those patients. In 7 out of 46 patients (15.2%) only one mutant allele was detected. Of their 38 chromosomes, 71% resulted with mutations in the GJB2 gene and 11% in GJB6. The most frequent mutation in GJB2 (24%) was c.35delG (11% homozygous and 13% heterozygous and compound heterozygous). In addition, 11 sequence variations different from c.35delG, were identified in the coding region of the GJB2 gene: T8M, V27I, M34T, E47X, R75W, W77R, I82M, L90P, E129K, V153I, M163V. The delGJB6-D13S1830 mutation was found in 4 patients (9%), 3 of them associated with GJB2 mutations, resulting in compound heterozygous for the DFNB1 locus. The present study demonstrates that mutations in the GJB2 gene and the delGJB6-D13S1830 are prevalent in the Argentinean population.

Nuclear and mitochondrial genes mutated in nonsyndromic impaired hearing

Half of the cases with congenital impaired hearing are hereditary (HIH). HIH may occur as part of a multisystem disease (syndromic HIH) or as disorder restricted to the ear and vestibular system (nonsyndromic HIH). Since nonsyndromic HIH is almost exclusively caused by cochlear defects, affected patients suffer from sensorineural hearing loss. One percent of the total human genes, i.e. 300-500, are estimated to cause syndromic and nonsyndromic HIH. Of these, approximately 120 genes have been cloned thus far, approximately 80 for syndromic HIH and 42 for nonsyndromic HIH. In the majority of the cases, HIH manifests before (prelingual), and rarely after (postlingual) development of speech. Prelingual, nonsyndromic HIH follows an autosomal recessive trait (75-80%), an autosomal dominant trait (10-20%), an X-chromosomal, recessive trait (1-5%), or is maternally inherited (0-20%). Postlingual nonsyndromic HIH usually follows an autosomal dominant trait. Of the 41 mutated genes that cause nonsyndromic HIH, 15 cause autosomal dominant HIH, 15 autosomal recessive HIH, 6 both autosomal dominant and recessive HIH, 2 X-linked HIH, and 3 maternally inherited HIH. Mutations in a single gene may not only cause autosomal dominant, nonsyndromic HIH, but also autosomal recessive, nonsyndromic HIH (GJB2, GJB6, MYO6, MYO7A, TECTA, TMC1), and even syndromic HIH (CDH23, COL11A2, DPP1, DSPP, GJB2, GJB3, GJB6, MYO7A, MYH9, PCDH15, POU3F4, SLC26A4, USH1C, WFS1). Different mutations in the same gene may cause variable phenotypes within a family and between families. Most cases of recessive HIH result from mutations in a single locus, but an increasing number of disorders is recognized, in which mutations in two different genes (GJB2/GJB6, TECTA/KCNQ4), or two different mutations in a single allele (GJB2) are involved. This overview focuses on recent advances in the genetic background of nonsyndromic HIH.

Spectrum and frequencies of mutations in the GJB2 (Cx26) gene among 156 Czech patients with pre-lingual deafness

Mutations in the gene gap junction beta 2 (GJB2), the gene for the connexin 26, are the most common cause of pre-lingual deafness worldwide. The mutation 35delG within GJB2 is prevalent in Europe. To date, there are no data about GJB2 mutation spectrum and frequencies from the Czech population. We investigated and report here the spectrum and frequencies of mutations in the GJB2 gene among 156 unrelated, congenital deafness Czech patients. Allele-specific polymerase chain reaction, together with fluorescent fragment analysis, were used for the detection of the 35delG mutation. The entire coding region of the GJB2 was directly sequenced in all patients who were not homozygous for the 35delG. No pathogenic mutation was detected in 51.9% of patients. At least one pathogenic mutation was found in 48.1% of patients, and both pathogenic mutations were detected in 37.8% of patients. Single mutations in a heterozygous state were detected in 10.3% of patients. The mutation 35delG accounts for 82.8% of detected disease mutations, Trp24stop accounts for 9.7% of pathogenic alleles and was found in patients with gypsy heritage. Mutation 313del14 accounts for 3.7% of pathogenic alleles. The frequency of 35delG heterozygotes in the Czech Republic is 1 : 29.6. Testing for only the three most common mutations would detect over 96% of all pathogenic alleles in the Czech Republic.

Hearing impairment in Dutch patients with connexin 26 (GJB2) and connexin 30 (GJB6) mutations

OBJECTIVE

Despite the identification of mutations in the connexin 26 (GJB2) gene as the most common cause of recessive nonsyndromic hearing loss, the pattern of hearing impairment with these mutations remains inconsistent. Recently a deletion encompassing the GJB6 gene was identified and hypothesized to also contribute to hearing loss. We hereby describe the hearing impairment in Dutch patients with biallelic connexin 26 (GJB2) and GJB2+connexin 30 (GJB6) mutations.

METHODS

The audiograms of patients who were screened for GJB2 and GJB6 mutations were analysed retrospectively. Standard statistical testing was done for symmetry and shape, while repeated measurement analysis was used to assess the relation between mutation and severity. Progression was also studied via linear regression analysis.

RESULTS

Of 222 hearing-impaired individuals, 35 exhibited sequence variations; of these 19 had audiograms for study. Hearing loss in patients with biallelic "radical" (i.e. deletions, nonsense and splice site) mutations was significantly worse than in the wild type and heterozygotes (SAS proc GENMOD, p=0.013). The presence of at least one missense mutation in compound heterozygotes tends to lead to better hearing thresholds compared to biallelic radical mutations (p=0.08). One patient with the [35delG]+[del(GJB6-D13S1830)] genotype was severely impaired. Non-progressive hearing impairment was demonstrated in five 35delG homozygotes in individual longitudinal analyses. However a patient with the [299A>C]+[416G>A] genotype showed significant threshold progression in the lower frequencies. Findings on asymmetry and shape were inconclusive.

CONCLUSIONS

Our data support the hypothesis that severity is a function of genotype and its effect on the amino acid sequence. A bigger cohort is required to establish non-progressivity more definitively.

Connexin disorders of the ear, skin, and lens

Gap junctions provide coupled cells with a direct pathway for sharing ions, nutrients, and small metabolites, thus helping to maintain homeostasis in various tissues. Abnormal function and/or expression of specific connexin genes has been linked to several diseases, including genetic deafness, skin disease, peripheral neuropathies, and cataracts. Research has provided significant insight into the function of gap junction proteins in both in vitro and in vivo models; however, questions regarding the exact mechanisms by which connexin related diseases occur in mammalian systems remain. Here, we discuss the disease states that are related to three human connexin genes, Cx26 (GJB2), Cx46 (GJA3) and Cx50 (GJA8), and recent scientific evidence characterizing those diseases in various experimental models.

Aberrant gating, but a normal expression pattern, underlies the recessive phenotype of the deafness mutant Connexin26M34T

Mutations in the gene GJB2, encoding the gap junction protein Connexin26 (Cx26), are the most prevalent cause of inherited hearing loss, and Cx26M34T was one of the first mutations linked to deafness (Kelsell et al., 1997; Nature 387, 80-83). We report the first characterization of the gating properties of M34T, which had previously been reported to be nonfunctional. Although homotypic mutant channels did not produce detectable currents, heterotypic pairings with wtCx26 confirmed that M34T formed intercellular channels, although the gating properties were altered. Cx26M34T displayed an inverted response to transjunctional voltage (Vj), mediating currents that activate in a time- and Vj-dependent manner. These characteristics suggest that the channel population is only partially open at rest, consistent with previous reports that dye transfer in M34T-expressing cells is reduced or abolished (e.g., Thonnissen et al., Human Genet. 111, 190-197). To investigate the controversial recessive/dominant behavior of this mutant, we coexpressed M34T with wtCx26 RNA at equimolar levels, mimicking the situation in heterozygotic individuals. Under these conditions, M34T did not significantly reduce Cx26/Cx26 coupling, or alter the electrophysiological properties of the wt channels, consistent with the recessive nature of the allele. Overexpression of the mutant did have some inhibitory effects on conductance, possibly explaining some of the previous reports in exogenous expression systems and some patients. Consistent with its electrophysiological behavior, we also show that M34T localizes to cell junctions in both transfected HeLa cells and patient-derived tissue.

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.

Clinical evidence of the nonpathogenic nature of the M34T variant in the connexin 26 gene

Mutations in GJB2 are the most common cause of congenital nonsyndromic hearing loss. The controversial allele variant M34T has been hypothesized to cause autosomal dominant or recessive nonsyndromic hearing impairment and some in vitro data has been consistent with this hypothesis. In this report, we present the clinical and genotypic study of 11 families (seven familial forms of nonsyndromic sensorineural hearing loss (NSSNHL) and four sporadic cases) in which the M34T GJB2 variant has been identified. The M34T mutation did not segregate with the deafness in six of the seven familial forms of NSSNH. Eight persons with normal audiogram presented a heterozygous M34T variation and five normal hearing individuals were composite heterozygous for M34T and another GJB2 mutation. Four normal hearing individuals with a documented audiogram were M34T/35delG and one was M34T/(GJB6-D13S1830)del. Screening a French control population of 116 subjects we have found an M34T allele frequency of 1.72%. This percentage was not significatively different from the prevalence of the M34T allele in the deaf population, which was 2.12%. All these data suggest that the M34T variant is not clinically significant in human and is a frequent polymorphism in France.

GJB2 Mutations in the Swiss Hearing Impaired

OBJECTIVE

Mutations in the GJB2 gene encoding connexin 26 (Cx26) protein are a major cause for nonsyndromic autosomal recessive and sporadic deafness. However, its contribution to hearing impairment in Switzerland remains undefined. To determine the frequency and type of GJB2 mutations in the Swiss hearing-impaired population diagnosed under the age of 2 yr and at 2 yr and older and to assess the effectiveness of denaturing high-performance liquid chromatography (DHPLC) in screening for mutation in GJB2.

METHODS

Thirty-four patients with hearing impairment underwent mutation screening of the single coding exon of GJB2 with DHPLC followed by bidirectional sequencing to identify sequence alterations.

RESULTS

GJB2 mutations were more common in children diagnosed with hearing impairment under the age of 2 yr compared to the group 2 yr and older. In patients under age 2 yr, 9 of 20 (45%) harbored 13 GJB2 mutations including a common 313del14nt mutation; four of these patients were homozygous or compound heterozygous for GJB2 mutations. In contrast, 2 of 14 patients in the 2 yr and older group (14%) had a single mutation in GJB2. The 35delG mutation was exclusively found in 5 patients under the age of 2 yr. DHPLC for mutation screening was 100% sensitive and 83% specific for detecting sequence alterations in GJB2.

CONCLUSIONS

In Switzerland, GJB2 mutations are a major cause of nonsyndromic hearing impairment in children under the age of 2. Similar to other populations, GJB2 mutations are uncommon in the affected Swiss patients identified after 2 yr. Although 35delG mutation is common in the hearing-impaired children under the age of 2, it was absent in patients diagnosed with hearing impairment after the age of 2. DHPLC is a highly sensitive tool for detection of GJB2 mutations.

HUMANNONSYNDROMICSENSORINEURALDEAFNESS

Given the unique biological requirements of sound transduction and the selective advantage conferred upon a species capable of sensitive sound detection, it is not surprising that up to 1% of the approximately 30,000 or more human genes are necessary for hearing. There are hundreds of monogenic disorders for which hearing loss is one manifestation of a syndrome or the only disorder and therefore is nonsyndromic. Herein we review the supporting evidence for identifying over 30 genes for dominantly and recessively inherited, nonsyndromic, sensorineural deafness. The state of knowledge concerning their biological roles is discussed in the context of the controversies within an evolving understanding of the intricate molecular machinery of the inner ear.

Frequency and distribution of GJB2 (connexin 26) and GJB6 (connexin 30) mutations in a large North American repository of deaf probands

PURPOSE

Profound hearing loss occurs with a frequency of 1 in 1000 live births, half of which is genetic in etiology. The past decade has witnessed rapid advances in determining the pathogenesis of both syndromic and nonsyndromic deafness. The most significant clinical finding to date has been the discovery that mutations of GJB2 at the DFNB1 locus are the major cause of profound prelingual deafness in many countries. 1 More recently, GJB2 mutations have been shown to cause deafness when present with a deletion of the GJB6 gene. We report on the prevalence of GJB2 and GJB6 mutations in a large North American Repository of DNA from deaf probands and document the profound effects of familial ethnicity and parental mating types on the frequency of these mutations in the population.

METHODS

Deaf probands were ascertained through the Annual Survey of Deaf and Hard of Hearing Children and Youth, conducted at the Research Institute of Gallaudet University. Educational, etiologic, and audiologic information was collected after obtaining informed consent. DNA studies were performed for the GJB2 and GJB6 loci by sequencing and PCR methods.

RESULTS

GJB2 mutations accounted for 22.2% of deafness in the overall sample but differed significantly among Asians, African-Americans and Hispanics and for probands from deaf by deaf and deaf by hearing matings, as well as probands from simplex and multiplex sibships of hearing parents. In our sample, the overall incidence of GJB2/GJB6 deafness was 2.57%.

CONCLUSION

GJB2 mutations account for a large proportion of deafness in the US, with certain mutations having a high ethnic predilection. Heterozygotes at the GJB2 locus should be screened for the GJB6 deletion as a cause of deafness. Molecular testing for GJB2 and GJB6 should be offered to all patients with nonsyndromic hearing loss.

A novel dominant missense mutation--D179N--in the GJB2 gene (Connexin 26) associated with non-syndromic hearing loss

Mutations of the GJB2 gene, encoding Connexin 26, are the most common cause of hereditary congenital hearing loss in many countries, and account for up to 50% of cases of autosomal-recessive non-syndromic deafness. By contrast, only a few GJB2 mutations have been reported to cause an autosomal-dominant form of non-syndromic deafness. We report on a family from southern Italy in whom dominant, non-syndromic, post-lingual hearing loss is associated with a novel missense mutation in the GJB2 gene. Direct sequencing of the gene showed a heterozygous G-->A transition at nucleotide 535, resulting in an aspartic acid to asparagine amino acid substitution at codon 179 (D179N). This mutation occurred in the second extracellular domain (EC2), which would seem to be very important for connexon-connexon interaction.

Roles of Met-34, Cys-64, and Arg-75 in the assembly of human connexin 26. Implication for key amino acid residues for channel formation and function

Connexins form a family of membrane proteins that assemble into communication channels and directly connect the cytoplasms of adjoining cells. Malfunctioning of connexin channels often cause disease, such as the mutations M34T and R75W in human connexin 26, which are associated with hereditary deafness. Another residue known to be essential for normal channel activity in the connexin is Cys-64. To obtain structural and functional insights of connexin 26, we studied the roles of these three residues by expressing mutant connexins in insect Sf9 and HeLa cells. The M34T and M34A mutants both formed gap junction plaques, but dye transfer assays showed that the M34A mutant had a significantly reduced permeability, suggesting that for proper channel function a side chain of adequate size is required at this position. We propose that Met-34 is located in the innermost helix of the channel, where it ensures a fully open channel structure via interactions with other transmembrane helices. Gap junction channels formed by the R75W and R75D mutants dissociated upon solubilization in dodecyl maltoside, whereas the R75A mutant remained hexameric. All gap junctions formed by Arg-75 mutants also showed only negligible activity in dye transfer experiments. These results suggest that residue Arg-75 plays a role in subunit interactions needed to retain a functional and stable connexin hexamer. The C64S mutant was suggested to be defective in oligomerization and/or protein folding even in the presence of wild-type connexin.

Otoacoustic emissions and brainstem evoked potentials in compound carriers of connexin 26 mutations

This study compares the effects of mutations in the gap junction protein connexin 26 (Cx26), on outer hair cells (OHCs), inner hair cells (IHCs) and auditory nerve/brainstem among carriers of these mutations. One hundred and twenty eight individuals, from a village with widespread consanguinity and congenital deafness, due to three Cx26 mutations, were selected among relatives of deaf persons, and divided into non-carriers, carriers of one mutation, homozygous to one mutation, or compound heterozygous carriers of two different mutations. Distortion product otoacoustic emissions (DPOAEs), auditory brainstem responses (ABRs) and audiometric evaluation were compared in these genetic groups. Hearing loss among homozygotes and compound heterozygotes was comparable and ranged from mild to profound. Most ABRs from these groups showed no responses or partial responses (peaks III, V) with prolonged latencies, but some individuals had all peaks at normal latencies. DPOAEs were absent, except sporadic responses. Carriers of one mutation had significantly smaller DPOAEs compared to non-carriers, although normal pure tone audiograms and ABRs were found in these groups. In conclusion, based on DPOAEs, Cx26 mutations may impact OHC function among carriers of one or two Cx26 mutations. IHC/nerve impairment among homozygotes and compound heterozygotes is variable. OHCs may be more susceptible to Cx26 mutations compared to IHCs and the auditory nerve and brainstem pathway activated by them.

Non-syndromic autosomal-dominant deafness

Non-syndromic deafness is a paradigm of genetic heterogeneity. More than 70 loci have been mapped, and 25 of the nuclear genes responsible for non-syndromic deafness have been identified. Autosomal-dominant genes are responsible for about 20% of the cases of hereditary non-syndromic deafness, with 16 different genes identified to date. In the present article we review these 16 genes, their function and their contribution to deafness in different populations. The complexity is underlined by the fact that several of the genes are involved in both dominant and recessive non-syndromic deafness or in both non-syndromic and syndromic deafness. Mutations in eight of the genes have so far been detected in only single dominant deafness families, and their contribution to deafness on a population base might therefore be limited, or is currently unknown. Identification of all genes involved in hereditary hearing loss will help in the understanding of the basic mechanisms underlying normal hearing, will facilitate early diagnosis and intervention and might offer opportunities for rational therapy.

Effectiveness of sequencing connexin 26 (GJB2) in cases of familial or sporadic childhood deafness referred for molecular diagnostic testing

PURPOSE

Hearing loss is a common congenital disorder that is frequently associated with mutations in the GJB2 gene encoding the connexin 26 protein (Cx26). We sought to evaluate the effectiveness of direct DNA sequencing for detection of Cx26 mutations as a clinical diagnostic test.

METHODS

We designed a clinical assay using a three-step polymerase chain reaction (PCR)-based DNA sequencing strategy to detect all possible mutations in the open reading frame and flanking sequences of Cx26. The results of the first 324 cases of childhood deafness referred for diagnostic testing were analyzed.

RESULTS

A total of 127 of the 324 (39.2%) cases had at least one mutant Cx26 allele (36.1% of sporadic cases, 70% of familial cases). Of these 127 case, 57 (44.8%) were homozygotes or compound heterozygotes. Thirty-four different mutations were identified, including 10 novel mutations, 6 of which (T8M, K15T, R32L, M93I, N206S, and 511-512insAACG) may be pathogenic. We also provide new evidence on the pathogenicity or nonpathogenicity of 12 previously reported mutations, and clarify the confusing nomenclature of the 313-326del14 mutation.

CONCLUSION

A simple and rigorous method for efficient PCR-based sequence analysis of Cx26 is a sensitive clinical assay for evaluating deaf children. Its widespread use is likely to identify additional pathogenic mutations and lead to a better understanding of the clinical significance of previously identified mutations.

GJB2 (connexin 26) variants and nonsyndromic sensorineural hearing loss: a HuGE review

Despite the enormous heterogeneity of genetic hearing loss, variants in one locus, Gap Junction Beta 2 or GJB2 (connexin 26), account for up to 50% of cases of nonsyndromic sensorineural hearing loss in some populations. This article reviews genetic epidemiology studies of the alleles of GJB2, prevalence rates, genotype-phenotype relations, contribution to the incidence of hearing loss, and other issues related to the clinical validity of genetic testing for GJB2. This review focuses primarily on three alleles: 167 Delta T, 35 Delta G, and 235 Delta C. These alleles are recessive for nonsyndromic prelingual sensorineural hearing loss, and the evidence suggests complete penetrance but variable expressivity.

Molecular genetics of hearing loss

Hereditary isolated hearing loss is genetically highly heterogeneous. Over 100 genes are predicted to cause this disorder in humans. Sixty loci have been reported and 24 genes underlying 28 deafness forms have been identified. The present epistemic stage in the realm consists in a preliminary characterization of the encoded proteins and the associated defective biological processes. Since for several of the deafness forms we still only have fuzzy notions of their pathogenesis, we here adopt a presentation of the various deafness forms based on the site of the primary defect: hair cell defects, nonsensory cell defects, and tectorial membrane anomalies. The various deafness forms so far studied appear as monogenic disorders. They are all rare with the exception of one, caused by mutations in the gene encoding the gap junction protein connexin26, which accounts for between one third to one half of the cases of prelingual inherited deafness in Caucasian populations.

Hereditary deafness and phenotyping in humans

Hereditary deafness has proved to be extremely heterogeneous genetically with more than 40 genes mapped or cloned for non-syndromic dominant deafness and 30 for autosomal recessive non-syndromic deafness. In spite of significant advances in the understanding of the molecular basis of hearing loss, identifying the precise genetic cause in an individual remains difficult. Consequently, it is important to exclude syndromic causes of deafness by clinical and special investigation and to use all available phenotypic clues for diagnosis. A clinical approach to the aetiological investigation of individuals with hearing loss is suggested, which includes ophthalmology review, renal ultrasound scan and neuro-imaging of petrous temporal bone. Molecular screening of the GJB2 (Connexin 26) gene should be undertaken in all cases of non-syndromic deafness where the cause cannot be identified, since it is a common cause of recessive hearing impairment, the screening is straightforward, and the phenotype unremarkable. By the same token, mitochondrial inheritance of hearing loss should be considered in all multigeneration families, particularly if there is a history of exposure to aminoglycoside antibiotics, since genetic testing of specific mitochondrial genes is technically feasible. Most forms of non-syndromic autosomal recessive hearing impairment cause a prelingual hearing loss, which is generally severe to profound and not associated with abnormal radiology. Exceptions to this include DFNB2 (MYO7A), DFNB8/10 (TMPRSS3) and DFNB16 (STRC) where age of onset may sometimes be later on in childhood, DFNB4 (SLC26A4) where there may be dilated vestibular aqueducts and endolymphatic sacs, and DFNB9 (OTOF) where there may also be an associated auditory neuropathy. Unusual phenotypes in autosomal dominant forms of deafness, include low frequency hearing loss in DFNA1 (HDIA1) and DFNA6/14/38 (WFS1), mid-frequency hearing loss in DFNA8/12 (TECTA), DFNA13 (COL11A2) and vestibular symptoms and signs in DFNA9 (COCH) and sometimes in DFNA11 (MYO7A). Continued clinical evaluation of types and course of hearing loss and correlation with genotype is important for the intelligent application of molecular testing in the next few years.

GJB2 (connexin 26) mutations and childhood deafness in Thailand

HYPOTHESIS

The purpose of this study was to elucidate whether GJB2 mutations are responsible for childhood deafness in Southeast Asia.

BACKGROUND

GJB2 mutations are responsible for a large part of childhood deafness in many countries. In Whites, there is a common mutation (35delG) that accounts for about 70 to 80% of the GJB2 mutations. Previously, we and others reported a common GJB2 mutation (235delC) in Japanese patients with prelingual deafness. The association of the 235delC mutation with a single haplotype suggested a founder effect of the mutation.

METHODS

We analyzed the GJB2 gene in 17 deaf patients from 12 unrelated families in Thailand. Genomic DNA was extracted from peripheral lymphocytes of each patient and the entire coding region of the GJB2 gene was sequenced.

RESULTS

GJB2 mutations were found in 4 patients in 3 families. Patient 1 was a homozygote of 235delC. Patient 2 was a compound heterozygote of 235delC and W24X (71G --> A). Patient 3A and 3B (in 1 family) were heterozygotes of a novel mutation M34L (100A --> T).

CONCLUSION

The 235delC mutation may be widely distributed in Asian countries outside of Japan.

W44C mutation in the connexin 26 gene associated with dominant non-syndromic deafness

Although more than 50% of recessive non-syndromic deafness is attributed to mutations in the connexin 26 (Cx26) gene, only a few reported families have shown dominant transmission of the trait. The W44C mutation was originally reported in two families from the same geographic region of France, which exhibited dominant non-syndromic hearing loss. In this report, we describe a third family with early-onset severe-to-profound non-syndromic hearing loss segregating with the W44C mutation. Our observation places W44C among recurrent mutations in the Cx26 gene and emphasizes the importance of screening for this as well as other Cx26 mutations in autosomal dominant families.

Genetic analysis of the connexin-26 M34T variant: identification of genotype M34T/M34T segregating with mild-moderate non-syndromic sensorineural hearing loss

Mutations in the human gap junction beta-2 gene (GJB2) that encodes connexin-26 have been shown to cause non-syndromic sensorineural hearing loss (NSSNHL) at the DFNB1 locus on 13q11. Functional and genetic data regarding the disease causing potential of one particular GJB2 sequence variant, 101 T-->C (M34T), have proven contradictory. In this study, we found the prevalence of the M34T allele in a cohort of white sib pairs and sporadic cases with NSSNHL from the United Kingdom and Ireland to be 3.179% of chromosomes screened. Significantly, we identified the first M34T/M34T genotype cosegregating in a single family with mid to high frequency NSSNHL. Screening a control population of 630 subjects we identified 25 M34T heterozygotes; however, no M34T homozygotes were detected. Surprisingly, the majority of M34T alleles (88%) were in cis with a 10 bp deletion in the 5' non-coding sequence. This non-coding deletion was also homozygous in the homozygous M34T subjects. Microsatellite analysis of flanking loci in M34T heterozygotes and controls does not define an extensive ancestral haplotype but preliminary data suggest two common alleles in subjects with the M34T allele. In summary, we provide data that support M34T acting as a recessive GJB2 allele associated with mild-moderate prelingual hearing impairment.