The c.IVS1+1G>A mutation intheGJB2 gene is prevalent and large deletions involving theGJB6 gene are not present in the Turkish population

The GJB2 (Cx26) Gene Variants in Patients with Hearing Impairment in the Baikal Lake Region (Russia)

The GJB2 (Cx26) gene pathogenic variants are associated with autosomal recessive deafness type 1A (DFNB1A, OMIM #220290). Direct sequencing of the GJB2 gene among 165 hearing-impaired individuals living in the Baikal Lake region of Russia identified 14 allelic variants: pathogenic/likely pathogenic-nine variants, benign-three variants, unclassified-one variant, and one novel variant. The contribution of the GJB2 gene variants to the etiology of hearing impairment (HI) in the total sample of patients was 15.8% (26 out of 165) and significantly differed in patients of different ethnicity (5.1% in Buryat patients and 28.9% in Russian patients). In patients with DFNB1A (n = 26), HIs were congenital/early onset (92.3%), symmetric (88.5%), sensorineural (100.0%), and variable in severity (moderate-11.6%, severe-26.9% or profound-61.5%). The reconstruction of the SNP haplotypes with three frequent GJB2 pathogenic variants (c.-23+1G>A, c.35delG or c.235delC), in comparison with previously published data, supports a major role of the founder effect in the expansion of the c.-23+1G>A and c.35delG variants around the world. Comparative analysis of the haplotypes with c.235delC revealed one major haplotype G A C T (97.5%) in Eastern Asians (Chinese, Japanese and Korean patients) and two haplotypes, G A C T (71.4%) and G A C C (28.6%), in Northern Asians (Altaians, Buryats and Mongols). The variable structure of the c.235delC-haplotypes in Northern Asians requires more studies to expand our knowledge about the origin of this pathogenic variant.

Global Distribution of Founder Variants Associated with Non-Syndromic Hearing Impairment

The genetic etiology of non-syndromic hearing impairment (NSHI) is highly heterogeneous with over 124 distinct genes identified. The wide spectrum of implicated genes has challenged the implementation of molecular diagnosis with equal clinical validity in all settings. Differential frequencies of allelic variants in the most common NSHI causal gene, gap junction beta 2 (GJB2), has been described as stemming from the segregation of a founder variant and/or spontaneous germline variant hot spots. We aimed to systematically review the global distribution and provenance of founder variants associated with NSHI. The study protocol was registered on PROSPERO, the International Prospective Register of Systematic Reviews, with the registration number "CRD42020198573". Data from 52 reports, involving 27,959 study participants from 24 countries, reporting 56 founder pathogenic or likely pathogenic (P/LP) variants in 14 genes (GJB2, GJB6, GSDME, TMC1, TMIE, TMPRSS3, KCNQ4, PJVK, OTOF, EYA4, MYO15A, PDZD7, CLDN14, and CDH23), were reviewed. Varied number short tandem repeats (STRs) and single nucleotide polymorphisms (SNPs) were used for haplotype analysis to identify the shared ancestral informative markers in a linkage disequilibrium and variants' origins, age estimates, and common ancestry computations in the reviewed reports. Asia recorded the highest number of NSHI founder variants (85.7%; 48/56), with variants in all 14 genes, followed by Europe (16.1%; 9/56). GJB2 had the highest number of ethnic-specific P/LP founder variants. This review reports on the global distribution of NSHI founder variants and relates their evolution to population migration history, bottleneck events, and demographic changes in populations linked with the early evolution of deleterious founder alleles. International migration and regional and cultural intermarriage, coupled to rapid population growth, may have contributed to re-shaping the genetic architecture and structural dynamics of populations segregating these pathogenic founder variants. We have highlighted and showed the paucity of data on hearing impairment (HI) variants in Africa, establishing unexplored opportunities in genetic traits.

Genetic Epidemiology of Hearing Loss in the 22 Arab Countries: A Systematic Review

BACKGROUND

Hearing loss (HL) is a heterogeneous condition that causes partial or complete hearing impairment. Hundreds of variants in more than 60 genes have been reported to be associated with Hereditary HL (HHL). The HHL prevalence is thought to be high in the Arab population; however, the genetic epidemiology of HHL among Arab populations is understudied. This study aimed to systematically analyze the genetic epidemiology of HHL in Arab countries.

METHODS

We searched four literature databases (PubMed, Scopus, Science Direct, and Web of Science) from the time of inception until January 2019 using broad search terms to capture all the reported epidemiological and genetic data related to Arab patients with HHL.

FINDINGS

A total of 2,600 citations were obtained; 96 studies met our inclusion criteria. Our search strategy yielded 121,276 individuals who were tested for HL over 52 years (1966-2018), of whom 8,099 were clinically diagnosed with HL and belonged to 16 Arab countries. A total of 5,394 patients and 61 families with HHL were genotyped, of whom 336 patients and 6 families carried 104 variants in 44 genes and were from 17/22 Arab countries. Of these variants, 72 (in 41 genes) were distinctive to Arab patients. Arab patients manifested distinctive clinical phenotypes. The incidence of HHL in the captured studies ranged from 1.20 to 18 per 1,000 births per year, and the prevalence was the highest in Iraq (76.3%) and the lowest in Jordan (1.5%).

INTERPRETATION

This is the first systematic review to capture the prevalence and spectrum of variants associated with HHL in an Arab population. There appears to be a distinctive clinical picture for Arab patients with HHL, and the range and distribution of variants among Arab patients differ from those noted in other affected ethnic groups.

Connexin Genes Variants Associated with Non-Syndromic Hearing Impairment: A Systematic Review of the Global Burden

Mutations in connexins are the most common causes of hearing impairment (HI) in many populations. Our aim was to review the global burden of pathogenic and likely pathogenic (PLP) variants in connexin genes associated with HI. We conducted a systematic review of the literature based on targeted inclusion/exclusion criteria of publications from 1997 to 2020. The databases used were PubMed, Scopus, Africa-Wide Information, and Web of Science. The protocol was registered on PROSPERO, the International Prospective Register of Systematic Reviews, with the registration number “CRD42020169697”. The data extracted were analyzed using Microsoft Excel and SPSS version 25 (IBM, Armonk, New York, United States). A total of 571 independent studies were retrieved and considered for data extraction with the majority of studies (47.8% (n = 289)) done in Asia. Targeted sequencing was found to be the most common technique used in investigating connexin gene mutations. We identified seven connexin genes that were associated with HI, and GJB2 (520/571 publications) was the most studied among the seven. Excluding PLP in GJB2, GJB6, and GJA1 the other connexin gene variants (thus GJB3, GJB4, GJC3, and GJC1 variants) had conflicting association with HI. Biallelic GJB2 PLP variants were the most common and widespread variants associated with non-syndromic hearing impairment (NSHI) in different global populations but absent in most African populations. The most common GJB2 alleles found to be predominant in specific populations include; p.Gly12ValfsTer2 in Europeans, North Africans, Brazilians, and Americans; p.V37I and p.L79Cfs in Asians; p.W24X in Indians; p.L56Rfs in Americans; and the founder mutation p.R143W in Africans from Ghana, or with putative Ghanaian ancestry. The present review suggests that only GJB2 and GJB3 are recognized and validated HI genes. The findings call for an extensive investigation of the other connexin genes in many populations to elucidate their contributions to HI, in order to improve gene-disease pair curations, globally.

High Rates of Three Common GJB2 Mutations c.516G>C, c.-23+1G>A, c.235delC in Deaf Patients from Southern Siberia Are Due to the Founder Effect

The mutations in the GJB2 gene (13q12.11, MIM 121011) encoding transmembrane protein connexin 26 (Cx26) account for a significant portion of hereditary hearing loss worldwide. Earlier we found a high prevalence of recessive GJB2 mutations c.516G>C, c.-23+1G>A, c.235delC in indigenous Turkic-speaking Siberian peoples (Tuvinians and Altaians) from the Tyva Republic and Altai Republic (Southern Siberia, Russia) and proposed the founder effect as a cause for their high rates in these populations. To reconstruct the haplotypes associated with each of these mutations, the genotyping of polymorphic genetic markers both within and flanking the GJB2 gene was performed in 28 unrelated individuals homozygous for c.516G>C (n = 18), c.-23+1G>A (n = 6), or c.235delC (n = 4) as well as in the ethnically matched controls (62 Tuvinians and 55 Altaians) without these mutations. The common haplotypes specific for mutations c.516G>C, c.-23+1G>A, or c.235delC were revealed implying a single origin of each of these mutations. The age of mutations estimated by the DMLE+ v2.3 software and the single marker method is discussed in relation to ethnic history of Tuvinians and Altaians. The data obtained in this study support a crucial role of the founder effect in the high prevalence of GJB2 mutations c.516G>C, c.-23+1G>A, c.235delC in indigenous populations of Southern Siberia.

The Analysis of GJB2, GJB3, and GJB6 Gene Mutations in Patients with Hereditary Non-Syndromic Hearing Loss Living in Sivas

OBJECTIVES

The aim of the present study was to investigate the presence of GJB2, GJB3, and GJB6 gene mutations in non-syndromic sensorineural hearing loss (NSHL) cases living in Sivas region, to provide appropriate genetic counseling for cases who were found to have mutation, and to contribute to decrease the frequency of mutant allele in the next generation and plan treatment and rehabilitation with early diagnosis.

MATERIALS AND METHODS

The study included 53 unrelated cases that were diagnosed with congenital NSHL between June 2009 and March 2010. Multiplex ligation-dependent probe amplification method was used for genotyping of GJB2, GJB3, and GJB6 gene mutations.

RESULTS

Heterozygous 35delG variant was determined in 1.9% (n=1) of cases, homozygous 35delG in 15.1% (n=8), heterozygous IVS1+1G>A mutation in 1.9% (n=1), compound heterozygous in 3.8% (n=2), and homozygous IVS1+1G>A variant in 3.8% (n=2). None of the cases had mutation in GJB3 and GJB6 genes. Mutated allele frequencies in the present study were found to be 17.9% for 35delG and 6.6% for IVS1+1G>A.

CONCLUSION

The present study showed that 35delG mutation is the most common variant in the Sivas region, and that IVS1+1G>A mutation should be investigated in hearing loss. Another result of the present study was that genetic analyzes would allow early diagnosis of hearing impairments particularly when infants whose parents have consanguinity do not pass the newborn hearing screening.

Analyses of del(GJB6-D13S1830) and del(GJB6-D13S1834) deletions in a large cohort with hearing loss: Caveats to interpretation of molecular test results in multiplex families

BACKGROUND

Mutations involving the closely linked GJB2 and GJB6 at the DFNB1 locus are a common genetic cause of profound congenital hearing loss in many populations. In some deaf GJB2 heterozygotes, a 309 kb deletion involving the GJB6 has been found to be the cause for hearing loss when inherited in trans to a GJB2 mutation.

METHODS

We screened 2,376 probands from a National DNA Repository of deaf individuals.

RESULTS

Fifty-two of 318 heterozygous probands with pathogenic GJB2 sequence variants had a GJB6 deletion. Additionally, eight probands had an isolated heterozygous GJB6 deletion that did not explain their hearing loss. In two deaf subjects, including one proband, a homozygous GJB6 deletion was the cause for their hearing loss, a rare occurrence not reported to date.

CONCLUSION

This study represents the largest US cohort of deaf individuals harboring GJB2 and GJB6 variants, including unique subsets of families with deaf parents. Testing additional members to clarify the phase of GJB2/GJB6 variants in multiplex families was crucial in interpreting clinical significance of the variants in the proband. It highlights the importance of determining the phase of GJB2/GJB6 variants when interpreting molecular test results especially in multiplex families with assortative mating.

Genetic Epidemiology and Clinical Features of Hereditary Hearing Impairment in the Taiwanese Population

Hereditary hearing impairment (HHI) is a common but heterogeneous clinical entity caused by mutations in a plethora of deafness genes. Research over the past few decades has shown that the genetic epidemiology of HHI varies significantly across populations. In this study, we used different genetic examination strategies to address the genetic causes of HHI in a large Taiwanese cohort composed of >5000 hearing-impaired families. We also analyzed the clinical features associated with specific genetic mutations. Our results demonstrated that next-generation sequencing-based examination strategies could achieve genetic diagnosis in approximately half of the families. Common deafness-associated genes in the Taiwanese patients assessed, in the order of prevalence, included GJB2, SLC26A4, OTOF, MYO15A, and MTRNR1, which were similar to those found in other populations. However, the Taiwanese patients had some unique mutations in these genes. These findings may have important clinical implications for refining molecular diagnostics, facilitating genetic counseling, and enabling precision medicine for the management of HHI.

Selective Heterozygous Advantage of Carriers of с.-23+1G>A Mutation in GJB2 Gene Causing Autosomal Recessive Deafness 1A

We present the results of analysis of skin epidermis thickness in individuals with recessive mutation c.-23+1G>A in the GJB2 gene in comparison with individuals without this mutation living in Eastern Siberia (Yakut population). We examined 152 individuals with different genotypes by GJB2 gene mutation c.-23+1G>A. Homozygotes and heterozygotes by c.-23+1G>A have thicker epidermal layer (0.245 mm and 0.269 mm, respectively) in comparison with individuals without this mutation (0.193 mm) (p<0.05). The obtained data support the hypothesis about selective advantage of carriers of mutant GJB2 gene alleles and partly explain extremely high carrier frequency (10.3%) of c.-23+1G>A mutation in the GJB2 gene in Yakut population in Eastern Siberia.

Unique Mutational Spectrum of the GJB2 Gene and its Pathogenic Contribution to Deafness in Tuvinians (Southern Siberia, Russia): A High Prevalence of Rare Variant c.516G>C (p.Trp172Cys)

Mutations in the GJB2 gene are the main cause for nonsyndromic autosomal recessive deafness 1A (DFNB1A) in many populations. GJB2 mutational spectrum and pathogenic contribution are widely varying in different populations. Significant efforts have been made worldwide to define DFNB1A molecular epidemiology, but this issue still remains open for some populations. The main aim of study is to estimate the DFNB1A prevalence and GJB2 mutational spectrum in Tuvinians-an indigenous population of the Tyva Republic (Southern Siberia, Russia). Sanger sequencing was applied to analysis of coding (exon 2) and non-coding regions of GJB2 in a cohort of Tuvinian patients with hearing impairments (n = 220) and ethnically matched controls (n = 157). Diagnosis of DFNB1A was established for 22.3% patients (28.8% of familial vs 18.6% of sporadic cases). Our results support that patients with monoallelic GJB2 mutations (8.2%) are coincidental carriers. Recessive mutations p.Trp172Cys, c.-23+1G>A, c.235delC, c.299_300delAT, p.Val37Ile and several benign variants were found in examined patients. A striking finding was a high prevalence of rare variant p.Trp172Cys (c.516G>C) in Tuvinians accounting for 62.9% of all mutant GJB2 alleles and a carrier frequency of 3.8% in controls. All obtained data provide important targeted information for genetic counseling of affected Tuvinian families and enrich current information on variability of GJB2 worldwide.

Elucidation of the unique mutation spectrum of severe hearing loss in a Vietnamese pediatric population

The mutational spectrum of deafness in Indochina Peninsula, including Vietnam, remains mostly undetermined. This significantly hampers the progress toward establishing an effective genetic screening method and early customized rehabilitation modalities for hearing loss. In this study, we evaluated the genetic profile of severe-to-profound hearing loss in a Vietnamese pediatric population using a hierarchical genetic analysis protocol that screened 11 known deafness-causing variants, followed by massively parallel sequencing targeting 129 deafness-associated genes. Eighty-seven children with isolated severe-to-profound non-syndromic hearing loss without family history were included. The overall molecular diagnostic yield was estimated to be 31.7%. The mutational spectrum for severe-to-profound non-syndromic hearing loss in our Vietnamese population was unique: The most prevalent variants resided in the MYO15A gene (7.2%), followed by GJB2 (6.9%), MYO7A (5.5%), SLC26A4 (4.6%), TMC1 (1.8%), ESPN (1.8%), POU3F4 (1.8%), MYH14 (1.8%), EYA1 (1.8%), and MR-RNR1 (1.1%). The unique spectrum of causative genes in the Vietnamese deaf population was similar to that in the southern Chinese deaf population. It is our hope that the mutation spectrum provided here could aid in establishing an efficient protocol for genetic analysis of severe-to-profound hearing loss and a customized screening kit for the Vietnamese population.

A Mutational Analysis of GJB2, SLC26A4, MT-RNA1, and GJB3 in Children with Nonsyndromic Hearing Loss in the Henan Province of China

BACKGROUND

Hearing impairment is one of the most common neurosensory disorders afflicting humans. Approximately half of all cases have a genetic etiology. The distribution and frequency of genetic mutations that cause deafness differ significantly by ethnic group and geographic region.

METHODS

130 sporadic nonsyndromic hearing loss (NSHL) children from the Henan province were subjected to microarray-based mutation detection. Nine pathogenic mutations were detected in four of the most common deafness-related genes (GJB2, GJB3, SLC26A4, and MT-RNA1).

RESULTS

Fifty percent of the analyzed patients (65/130) were shown to have genetic defects known to be related to deafness. Slightly >30% (41/130) had biallelic pathogenic mutations. One patient had pathogenic mutations in their mitochondrial genes (MT-RNA1); no mutations were detected in the GJB3 gene. Twenty-three (17.69%) patients were carriers of a single mutation in a recessive gene; these findings alone, however, cannot be interpreted as a cause of hearing loss. Utilizing this molecular strategy, we were able to arrive at a conclusive diagnosis for 42 of the NSHL children.

CONCLUSION

Genetic factors play a major role in sporadic NSHL patients from the Henan province, but it is clear that our screen needs to be expanded to include additional genes and alleles. Screening of potential pathogenic genes is important for patient risk assessment.

Unique spectra of deafness-associated mutations in Mongolians provide insights into the genetic relationships among Eurasian populations

Genetic factors are an important cause of idiopathic sensorineural hearing impairment (SNHI). From the epidemiological perspective, mutations of three deafness genes: GJB2, SLC26A4, and MT-RNR1, are much more prevalent than those of other genes worldwide. However, mutation spectra of common deafness genes differ remarkably across different populations. Here, we performed comprehensive genetic examination and haplotype analyses in 188 unrelated Mongolian families with idiopathic SNHI, and compared their mutation spectra and haplotypes to those of other European and Asian cohorts. We confirmed genetic diagnoses in 18 (9.6%) of the 188 families, including 13 with bi-allelic GJB2 mutations, three with bi-allelic SLC26A4 mutations, and two with homoplasmic MT-RNR1 m.1555A>G mutation. Moreover, mono-allelic mutations were identified in 17 families (9.0%), including 14 with mono-allelic GJB2 mutations and three with mono-allelic SLC26A4 mutations. Interestingly, three GJB2 mutations prevalent in other populations, including c.35delG in Caucasians, c.235delC in East Asians, and c.-23+1G>A in Southwest and South Asians, were simultaneously detected in Mongolian patients. Haplotype analyses further confirmed founder effects for each of the three mutations, indicating that each mutation derived from its ancestral origin independently. By demonstrating the unique spectra of deafness-associated mutations, our findings may have important clinical and scientific implications for refining the molecular diagnostics of SNHI in Mongolian patients, and for elucidating the genetic relationships among Eurasian populations.

Genetic Aetiology of Nonsyndromic Hearing Loss in Moravia-Silesia

BACKGROUND AND OBJECTIVE

Hearing loss is the most common sensory deficit in humans. The aim of this study was to clarify the genetic aetiology of nonsyndromic hearing loss in the Moravian-Silesian population of the Czech Republic.

PATIENTS AND METHODS

This study included 200 patients (93 males, 107 females, mean age 16.9 years, ranging from 4 months to 62 years) with nonsyndromic sensorineural hearing loss. We screened all patients for mutations in GJB2 and the large deletion del(GJB6-D13S1830). We performed further screening for additional genes (SERPINB6, TMIE, COCH, ESPN, ACTG1, KCNQ4, and GJB3) with Sanger sequencing on a subset of patients that were negative for GJB2 mutations.

RESULTS

We detected biallelic GJB2 mutations in 44 patients (22%). Among these patients, 63.6%, 9.1% and 2.3% exhibited homozygous c.35delG, p.Trp24*, and p.Met34Thr mutations, respectively. The remaining 25% of these patients exhibited compound heterozygous c.35delG, c.-23+1G>A, p.Trp24*, p.Val37Ile, p.Met34Thr, p.Leu90Pro, c.235delC, c.313_326del14, p.Ser139Asn, and p.Gly147Leu mutations. We found a monoallelic GJB2 mutation in 12 patients (6.6%). We found no pathogenic mutations in the other tested genes. Conclusions: One fifth of our cohort had deafness related to GJB2 mutations. The del(GJB6-D13S1830), SERPINB6, TMIE, COCH, ESPN, ACTG1, GJB3, and KCNQ4 mutations were infrequently associated with deafness in the Moravian-Silesian population. Therefore, we suggest that del(GJB6-D13S1830) testing should be performed only when patients with deafness carry the monoallelic GJB2 mutation.

Research of genetic bases of hereditary non-syndromic hearing loss

AIM

Hearing loss is the most common sensory disorder that affects approximately one per 1000 live births. With this project, we aimed to identify gene variants that were common causes of hearing loss in Turkey to contribute to the planning of genetic screening programs for hearing loss, as well as to improve genetic counseling to affected families.

MATERIAL AND METHODS

Twenty-one families with at least two affected individuals and parental consanguinity who presented with non-syndromic severe-to-profound sensorineural hearing loss were included in this study. We first screened for mutations in GJB2 and mitochondrial DNA 12S RNA genes. Subsequently, we genotyped the TMIE c.250C>T and SNP markers flanking the SLC26A4, MYO7A, MYO15A, OTOF, CDH23, TMIE, TECTA, PCDH15, TMC1, TMPRSS3, TMHS genes in the remaining twelve families without mutations in GJB2.

RESULTS

Screening for mutations in GJB2 gene showed c.[35delG];[35delG] mutation in four families, c.[35delG];[507C>A] mutation in two families, c.[35delG];[-23+1G>A] mutation in one family, and c.457G>A heterozygous mutation in one family. Genotyping SNP markers showed the c.[250C>T];[250C>T] mutation in TMIE in one family. A homozygous region with SNP genotypes was detected with the OTOF gene in one family, the TMPRSS3 gene in another family, and also a homozygous region was detected with TMHS, OTOF, and TMPRSS3 genes in another family.

CONCLUSIONS

Further research will be required to determine the genetic bases of hearing loss in families with non-syndromic hearing loss.

Opinions of hearing parents about the causes of hearing impairment of their children with biallelic GJB2 mutations

Hereditary hearing impairment (HI) caused by recessive GJB2 mutations is a frequent sensory disorder. The results of the molecular-based studies of HI are widely used in various genetic test systems. However, the ethical aspects are less described than the genetic aspects. The concerns expressed by individuals from groups with genetic risks must be included in the counseling of patients and their families. For evaluation of subjective opinions of hearing parents about the presumed causes of HI of their children, we analyze the cohort of parents having children with confirmed hereditary HI caused by biallelic recessive GJB2 mutations (in a homozygous or a compound heterozygous state). This study included 70 deaf children with HI due to mutations in the GJB2 gene and 91 questionnaires about the presumed causes of their deafness filled by their parents. Most of the parents at 78% (CI 68.4-85.4%) attributed their children's HI to "non-hereditary" causes and 22% (CI 14.7-31.6%) to "hereditary" causes (p < 0.05). Therefore, the prior opinions of the parents did not correspond to positive GJB2 genetic testing results. The subjective opinions of parents are probably partly based on family history, since respondents with deaf relatives in their pedigree more likely supposed hereditary causes for HI in their children than the respondents without deaf relatives (p < 0.001).

Spectrum and Frequency of the GJB2 Gene Pathogenic Variants in a Large Cohort of Patients with Hearing Impairment Living in a Subarctic Region of Russia (the Sakha Republic)

Pathogenic variants in the GJB2 gene, encoding connexin 26, are known to be a major cause of hearing impairment (HI). More than 300 allelic variants have been identified in the GJB2 gene. Spectrum and allelic frequencies of the GJB2 gene vary significantly among different ethnic groups worldwide. Until now, the spectrum and frequency of the pathogenic variants in exon 1, exon 2 and the flanking intronic regions of the GJB2 gene have not been described thoroughly in the Sakha Republic (Yakutia), which is located in a subarctic region in Russia. The complete sequencing of the non-coding and coding regions of the GJB2 gene was performed in 393 patients with HI (Yakuts—296, Russians—51, mixed and other ethnicities—46) and in 187 normal hearing individuals of Yakut (n = 107) and Russian (n = 80) populations. In the total sample (n = 580), we revealed 12 allelic variants of the GJB2 gene, 8 of which were recessive pathogenic variants. Ten genotypes with biallelic recessive pathogenic variants in the GJB2 gene (in a homozygous or a compound heterozygous state) were found in 192 out of 393 patients (48.85%). We found that the most frequent GJB2 pathogenic variant in the Yakut patients was c.-23+1G>A (51.82%) and that the second most frequent was c.109G>A (2.37%), followed by c.35delG (1.64%). Pathogenic variants с.35delG (22.34%), c.-23+1G>A (5.31%), and c.313_326del14 (2.12%) were found to be the most frequent among the Russian patients. The carrier frequencies of the c.-23+1G>A and с.109G>A pathogenic variants in the Yakut control group were 10.20% and 2.80%, respectively. The carrier frequencies of с.35delG and c.101T>C were identical (2.5%) in the Russian control group. We found that the contribution of the GJB2 gene pathogenic variants in HI in the population of the Sakha Republic (48.85%) was the highest among all of the previously studied regions of Asia. We suggest that extensive accumulation of the c.-23+1G>A pathogenic variant in the indigenous Yakut population (92.20% of all mutant chromosomes in patients) and an extremely high (10.20%) carrier frequency in the control group may indicate a possible selective advantage for the c.-23+1G>A carriers living in subarctic climate.

Unraveling of Enigmatic Hearing-Impaired GJB2 Single Heterozygotes by Massive Parallel Sequencing: DFNB1 or Not?

The molecular etiology of nonsyndromic sensorineural hearing loss (SNHL) in subjects with only one detectable autosomal recessive GJB2 mutation is unclear. Here, we report GJB2 single heterozygotes with various final genetic diagnoses and suggest appropriate diagnostic strategies. A total of 160 subjects with SNHL without phenotypic markers were screened for GJB2 mutations. Single-nucleotide variants or structural variations within the DFNB1 locus or in other deafness genes were examined by Sanger sequencing, breakpoint PCR, and targeted exome sequencing (TES) of 129 deafness genes. We identified 27 subjects with two mutations and 10 subjects with only one detectable mutation in GJB2. The detection rate of the single GJB2 mutation among the 160 SNHL subjects in the present study (6.25%) was higher than 2.58% in normal hearing controls in Korean. The DFNB1 was clearly excluded as a molecular etiology in four (40%) subjects: other recessive deafness genes (N = 3) accounted for SNHL and the causative gene for the other non-DFNB1 subject (N = 1) was not identified. The etiology of additional two subjects was potentially explained by digenic etiology (N = 2) of GJB2 with MITF and GJB3, respectively. The contribution of the single GJB2 mutation in the four remaining subjects is unclear. Comprehensive diagnostic testing including TES is prerequisite for understanding GJB2 single heterozygotes.

[The audiological analysis in the patients homozygous for the c.-23+1G>A mutation in the GJB2 gene presenting with the loss of hearing in Yakutiya]

In the course of previous investigations carried out in the Republic of Sakha (Yakutiya), we have identified the main molecular-genetic factor responsible for the hereditary impairment of hearing among the indigenous population (mostly the Yakuts).The disease was shown to be attributable to the c.-23+1G>A mutation localized in the splice donor site (exon 1) of the GJB2 (Cx26) gene. The present study involved the comprehensive audiological analysis of the patients homozygous for the c.-23+1G>A mutation in the GJB2 gene based on the results of the study of a large sample of the patients residing in Yakutiya. All individuals with the GJB2 genotype c.-23+1G>A/c.-23-1G>A (n=108) at the mean age of 14.32±4.7 years (all ethnic Yakuts)were examined with the use oftonal threshold audiometry for air conduction testing at the frequencies of 0.25, 0.5, 1.0, 2.0, 4.0, and 8.0 kHz and bone conduction testing at the frequencies of 0.25, 0.5, 1.0, and 4.0 with a step of 5.0 dB.The results of the ASSR test were used whenever tonal threshold audiometry proved impracticable The data obtained in the study characterize the allelic form of the disease associated with the GJB2 genotype c.-23+1G>A/c.-23-1G>A as the congenital bilateral symmetric (90.1%), sensorineural (90.1%) form of hearing impairment of variable severity (from grade 1 to complete deafness) with the «flat» audiological profile (median slope not more than 5.0 dB in the extended frequency range (EFR) of 0.5, 1.0, 2.0, and 4.0, kHz). It is concluded that the results of the audiological analysis performed in the present study give evidence of relatively homogeneous but variable in terms of severity impairment of hearing in the patients homozygous for the c.-23+1G>A mutation in the GJB2 (Cx26) gene. It may serve as a positive prognostic sign to be used in the development and prescription of hearing aids.

Finding mutation within non-coding region of GJB2 reveals its importance in genetic testing of hearing loss in Iranian population

OBJECTIVE

Hereditary hearing loss is the most common neurosensory disorder in humans. Half of the cases have genetic etiology with extraordinary genetic heterogeneity. Mutations in one gene, GJB2, are the most common cause for autosomal recessive non-syndromic hearing loss (ARNSHL) in many different populations. GJB2 encodes a gap junction channel protein (connexin 26), and is located on DFNB1 locus on chromosome 13q12.11 which also involve another connexin gene, GJB6. Mutation screening of GJB2 revealed that a high number of patients with deaf phenotype have heterozygous genotype and carry only one mutant allele. As the first comprehensive study in Iran, we have targeted GJB2-related Iranian heterozygotes, looking for second mutant allele which leads to hearing impairment. They bear first mutation in their coding exon of GJB2.

METHOD

Using PCR-based direct sequencing, we assessed 103 patients with ARNSHL for variants in non-coding exon and promoter region of this gene, for the first time in Iran.

RESULT

We have identified the second mutant allele in splice site of exon-1 of GJB2 which is known as IVS1+1G>A in 17 probands. We found no mutation in promoter region of GJB2.

CONCLUSION

Our findings reveal that IVS1+1G>A mutation in noncoding exon of GJB2 is the most common mutation after 35delG within multi ethnical Iranian heterozygote samples. It emphasizes to approach exon1 of GJB2 in case of ARNSHL genetic diagnosis.

Genetics of non-syndromic hearing loss in the Middle East

Hearing impairment is the most common sensory disorder, present 1 in every 500 newborns. About 80% of genetic HL is classified as non-syndromic deafness. To date, over 115 non-syndromic loci have been identified of which fifty associated with autosomal recessive non-syndromic hearing loss (ARNSHL). In this review article, we represent the 40 genes function and contribution to genetic deafness in different Middle Eastern populations as well as gene frequencies and mutation spectrum. The wide variety of mutations have so far detected in 19 countries reflects the heterogeneity of the genes involved in HL in this region. The deafness genes can cause dysfunction of cochlear homeostasis, cellular organization, neuronal transmission, cell growth, differentiation, and survival, some coding for tectorial membrane-associated proteins, and the remaining with unknown functions. Non-syndromic deafness is highly heterogeneous and mutations in the GJB2 are responsible for almost 30-50% in northwest to as low as 0-5% in south and southeast of the Middle East, it remain as major gene in ARNSHL in Middle East. The other genes contributing to AR/ADNSHL in some countries have been determined while for many other countries in the Middle East have not been studied or little study has been done. With the advancement of next generation sequencing one could expect in next coming year many of the remaining genes to be determine and to understand their function in the inner ear.

Spectrum and frequency of GJB2 mutations causing deafness in the northwest of Iran

OBJECTIVE

Mutations in GJB2 and GJB6 which comprise DFNB1 locus cause up to half of all cases of the prelingual autosomal recessive non-syndromic hearing loss (ARNSHL) worldwide. This study has intended to assess the spectrum and frequency of GJB2/GJB6 mutations in northwest of Iran.

METHODS

508 Patients with presumed ARNSHL were analyzed by applying ARMS-PCR, SSCP, PCR-RFLP and sequencing assays.

RESULTS

Seventy-five (14.7%) different homozygous and eighty-seven (17.1%) different compound heterozygous genotypes were detected in this cohort. Concerning the GJB2 gene, c.35delG was the most prevalent mutation, accounting for 16.4% of the samples. In addition 29 sequence variations other than c.35delG mutation were distinguished in GJB2; namely, delE120, Ins A 290-291, R143Q, V37I, R32H, Y155X, V27I + T123N, F154F, 167delT, 312del14, 299-300delA, T8M, W24X, E114G + V27I, 235delC, R184P, V153I, S139N, A171T, M163V (unknown mutation), G127V, E147X, R127H, 35insG, R143W, V27I, G160S, E114G and IVS1 + 1G > A. Moreover, the IVS1 + 1G > A was accounted as a second common mutation.

CONCLUSIONS

Overall, the frequency of GJB2 mutations (≥31%) is in agreement with other white population. These findings highlight the importance of the study of GJB2 gene in the diagnosis to provide early treatment and genetic counseling.

GJB2 mutations are rare in probands with hearing loss in Chinese assortative mating families

OBJECTIVE

GJB2 mutation is recognized as the prevalent causes of non-syndromic hearing loss (NSHL) worldwide. However, the mutation profiles of GJB2 are rarely reported in deafness probands of the assortative mating family. Therefore, this study aimed to characterize the frequencies of GJB2 mutations in probands with hearing loss in the assortative mating family in Hubei province, Central China.

METHODS

Genomic DNA was extracted from blood samples of 29 probands with hearing loss. The target fragments were amplified by polymerase chain reaction (PCR) and subjected to sequencing to identify sequence variations.

RESULTS

None of 29 probands harbored homozygous mutation in GJB2, while GJB2 heterozygous mutations c.134G>A, c.139G>T, and a deletion c.235delC were identified in three probands, respectively.

CONCLUSION

GJB2 mutations are rare in Chinese probands of assortative mating families. Screening for responsible genes other than GJB2 is necessary for NSHL in these probands.

A novel splice-site mutation in the GJB2 gene causing mild postlingual hearing impairment

The DFNB1 subtype of autosomal recessive, nonsyndromic hearing impairment, caused by mutations affecting the GJB2 (connection-26) gene, is highly prevalent in most populations worldwide. DFNB1 hearing impairment is mostly severe or profound and usually appears before the acquisition of speech (prelingual onset), though a small number of hypomorphic missense mutations result in mild or moderate deafness of postlingual onset. We identified a novel GJB2 splice-site mutation, c. -22-2A>C, in three siblings with mild postlingual hearing impairment that were compound heterozygous for c. -22-2A>C and c.35delG. Reverse transcriptase-PCR experiments performed on total RNA extracted from saliva samples from one of these siblings confirmed that c. -22-2A>C abolished the acceptor splice site of the single GJB2 intron, resulting in the absence of normally processed transcripts from this allele. However, we did isolate transcripts from the c. -22-2A>C allele that keep an intact GJB2 coding region and that were generated by use of an alternative acceptor splice site previously unknown. The residual expression of wild-type connection-26 encoded by these transcripts probably underlies the mild severity and late onset of the hearing impairment of these subjects.

Genetic mutations of GJB2 and mitochondrial 12S rRNA in nonsyndromic hearing loss in Jiangsu Province of China

BACKGROUND

Hearing loss is caused by several environmental and genetic factors and the proportion attributed to inherited causes is assumed at 50 ~ 60% . Mutations in GJB2 and mitochondrial DNA (mtDNA) 12S rRNA are the most common molecular etiology for nonsyndromic sensorineural hearing loss (NSHL). The mutation spectra of these genes vary among different ethnic groups.

METHODS

To add the molecular etiologic information of hearing loss in the Chinese population, a total of 658 unrelated patients with NSHL from Jiangsu Province of China were selected for mutational screening including GJB2 and mtDNA 12S rRNA genes using PCR and DNA sequencing technology. As for controls, 462 normal-hearing individuals were collected.

RESULTS

A total of 9 pathogenic mutations in the GJB2 and 7 pathogenic mutations in the 12S rRNA gene were identified. Of all patients, 70 had monoallelic GJB2 coding region mutation in the heterozygous state, 94 carried two confirmed pathogenic mutations including 79 homozygotes and 15 compound heterozygotes. The 235delC appears to be the most common deafness-causing GJB2 mutation (102/658, 15.50% ). No mutations or variants in the GJB2 exon1 and basal promoter region were found. In these patients, 4 subjects carried the m.1494C > T mutation (0.61% ) and 39 subjects harbored the m.1555A > G mutation (5.93% ) in mtDNA 12S rRNA gene. A novel sequence variant at m.1222A > G in the 12S rRNA gene was identified, which could alter the secondary structure of the 12S rRNA.

CONCLUSION

The mutation spectrum and prevalence of GJB2 and mtDNA 12S rRNA genes in Jiangsu population are similar to other areas of China. There are in total 31.46% of the patients with NSHL carry deafness-causing mutation in GJB2 or mtDNA 12S rRNA genes. Mutation in GJB2 gene is the most common factor, mtDNA 12S rRNA also plays an important part in the pathogenesis of hearing loss in Jiangsu Province areas. The m.1222A > G was found to be a new candidate mutation associated with hearing loss. Our results indicated the necessity of genetic screening for mutations of these genes in Jiangsu patients with NSHL.

Prevalence of IVS1+1G>A mutation among Iranian Azeri Turkish patients with autosomal recessive non-syndromic hearing loss (ARNSHL)

OBJECTIVE

Mutations in the GJB2 gene has been reported as a main cause for autosomal recessive non-syndromic hearing loss (ARNSHL) all over the world. IVS1+1G>A which is splice site mutation have been detected in several populations as disruptive mutation. This study has intended to assess the significance of this mutation, IVS1+1G>A, to the autosomal recessive non-syndromic genetic load among Iranian Azeri Turkish patients.

METHODS

Following our previous study, one hundred and seventy four unrelated patients with prelingual ARNSL were included in this study. Thirty nine patients had only one identified mutated allele, whereas hundred and thirty five patients were negative for coding region of GJB2. All these patients were screened for IVS1+1G>A by applying PCR-RFLP assay.

RESULTS

Among studied patients nine compound heterozygote with 35delG, delE120, 235delC were identified. Additionally, six patients were detected with only one IVS1+1G>A mutated allele. In these patients, the other mutated allele was left unidentified. One patient was identified to be homozygous for IVS1+1G>A. Further studies carried out on parents of positive cases, showed that one of the healthy parents (mother) to be homozygous for IVS1+1G>A mutation. By self-report, this person had no hearing impairment, although it is possible that she has mild or moderate hearing loss, which she is unable to detect. Her child was compound heterozygous (IVS1+1G>A/35delG) with profound deafness. The frequency of IVS1+1G>A was found to be about 4.9%, however the parental consanguinity was observed in 37.5% of IVS1+1G>A-mutated families.

CONCLUSIONS

Our results support founder effect regarding these mutations and the presence of an ancient ancestor is strengthened in comparison to hot spot hypothesis. Also the results suggest variable expressivity of IVS1+1G>A mutation with respect to hearing loss.

GJB2 and GJB6 genes and the A1555G mitochondrial mutation are only minor causes of nonsyndromic hearing loss in the Qatari population

OBJECTIVE

This study reports results from the first survey of the genetic causes of nonsyndromic sensorineural hearing loss (NSHHL) in the Qatari population. DESIGN AND STUDY SAMPLES: Data were collected from 126 Qatari patients (58 males and 68 females) belonging to inbred families (56%), showing an autosomal recessive pattern of inheritance (96%). Fifty-three patients were less than 10 years old, 55 in the age range of 10 to 20 years, while 18 were aged between 20 and 30 years. All subjects had moderate to severe sensorineural hearing loss and were screened for GJB2 mutations, GJB6 deletion, and for A1555G mitochondrial mutation.

RESULTS

Four patients were homozygous and one was heterozygous for c.35delG; five were homozygous for the IVS1 + 1G < A, and two were heterozygous for c.229 T > C. Only 8.3% of the pathogenic alleles were detected. No patients were positive for GJB6 deletion or for A1555G .

CONCLUSIONS

These findings: (1) demonstrate that GJB2, GJB6 deletion and A1555G mutation account for a minor proportion of NSHHL in the Qatari population, (2) further strengthen the need to search for causative genes, (3) clearly contribute to establishing preventive strategies for NSHHL in Qatar and in the Gulf area.

Assessing Noncoding Sequence Variants of GJB2 for Hearing Loss Association

Involvement of GJB2 noncoding regions in hearing loss (HL) has not been extensively investigated. However, three noncoding mutations, c.-259C>T, c.-23G>T, and c.-23+1G>A, were reported. Also, c.-684_-675del, of uncertain pathogenicity, was found upstream of the basal promoter. We performed a detailed analysis of GJB2 noncoding regions in Portuguese HL patients (previously screened for GJB2 coding mutations and the common GJB6 deletions) and in control subjects, by sequencing the basal promoter and flanking upstream region, exon 1, and 3'UTR. All individuals were genotyped for c.-684_-675del and 14 SNPs. Novel variants (c.-731C>T, c.-26G>T, c.*45G>A, and c.*985A>T) were found in controls. A hearing individual homozygous for c.-684_-675del was for the first time identified, supporting the nonpathogenicity of this deletion. Our data indicate linkage disequilibrium (LD) between SNPs rs55704559 (c.*168A>G) and rs5030700 (c.*931C>T) and suggest the association of c.[*168G;*931T] allele with HL. The c.*168A>G change, predicted to alter mRNA folding, might be involved in HL.

Molecular screening of patients with nonsyndromic hearing loss from Nanjing city of China

Hearing loss is the most frequent sensory disorder involving a multitude of factors, and at least 50% of cases are due to genetic etiology. To further characterize the molecular etiology of hearing loss in the Chinese population, we recruited a total of 135 unrelated patients with nonsyndromic sensorineural hearing loss (NSHL) for mutational screening of GJB2, GJB3, GJB6, SLC26A4, SLC26A5 IVS2-2A>G and mitochondrial 12SrRNA, tRNA(Ser(UCN)) by PCR amplification and direct DNA sequencing. The carrier frequencies of deafness-causing mutations in these patients were 35.55% in GJB2, 3.70% in GJB6, 15.56% in SLC26A4 and 8.14% in mitochondrial 12SrRNA, respectively. The results indicate the necessity of genetic screening for mutations of these causative genes in Chinese population with nonsyndromic hearing loss.

Prenatal screening for the 35delG GJB2, del (GJB6-D13S1830), and del (GJB6-D13S1854) mutations in the Romanian population

OBJECTIVE

In this study, the aim of prenatal screening was to estimate the carrier frequency of the three mutations 35delG, del (GJB6-D13S1830), and del (GJB6-D13S1854), which are known to be the leading mutations of hereditary hearing loss in European populations.

METHODS

We performed a prenatal screening to assess the carrier frequency of the most common mutations at the DFNB1 gene locus in the general population. Samples of amniotic fluid (n=339) and chorionic villi (n=11) were taken from an unselected group of 350 unrelated pregnant women with normal hearing. Genomic fetal DNA was extracted and analyzed by PCR multiplex assays.

RESULTS

The rate of carriers for the 35delG GJB2 mutation was 3.14%, comparable to that of most Southeastern European populations. All samples were negative for GJB6-D13S1830 and GJB6-D13S1854 deletions. The genetic tests were considered for carrier detection and early diagnosis rather than termination of pregnancy.

CONCLUSIONS

Our study suggests a need for detecting the carriers. This is the first step for the construction of a national database and provides information for health planners and policy makers to help them in planning programs and allocation resources. The molecular testing was well received by pregnant women and appears to be feasible and highly acceptable.

Autosomal recessive deafness 1A (DFNB1A) in Yakut population isolate in Eastern Siberia: extensive accumulation of the splice site mutation IVS1+1G>A in GJB2 gene as a result of founder effect

Hereditary forms of hearing impairment (HI) caused by GJB2 (Cx26) mutations are the frequent sensory disorders registered among newborns in various human populations. In this study, we present data on the molecular, audiological and population features of autosomal recessive deafness 1A (DFNB1A) associated with the donor splicing site IVS1+1G>A mutation of GJB2 gene in Yakut population isolate of the Sakha Republic (Yakutia) located in Eastern Siberia (Russian Federation). The Yakut population exhibits high frequency of some Mendelian disorders, which are rare in other populations worldwide. Mutational analysis of GJB2 gene in 86 unrelated Yakut patients with congenital HI without other clinical features has been performed. In this study, we registered a large cohort of Yakut patients homozygous for the IVS1+1G>A mutation (70 unrelated deaf subjects in total). Detailed audiological analysis of 40 deaf subjects with genotype IVS1+1G>A/IVS1+1G>A revealed significant association of this genotype with mostly symmetrical bilateral severe to profound HI (85% severe-to-profound HI versus 15% mild-to-moderate HI, P<0.05). The highest among six investigated Eastern Siberian populations carrier frequency of the IVS1+1G>A mutation (11.7%) has been found in Yakut population. Reconstruction of 140 haplotypes with IVS1+1G>A mutation demonstrates the common origin of all mutant chromosomes found in Yakuts. The age of mutation was estimated to be approximately 800 years. These findings characterize Eastern Siberia as the region with the most extensive accumulation of the IVS1+1G>A mutation in the world as a result of founder effect.

Prevalence of the GJB2 IVS1+1G >A mutation in Chinese hearing loss patients with monoallelic pathogenic mutation in the coding region of GJB2

Background

Mutations in the GJB2 gene are the most common cause of nonsyndromic recessive hearing loss in China. In about 6% of Chinese patients with severe to profound sensorineural hearing impairment, only monoallelic GJB2 mutations known to be either recessive or of unclear pathogenicity have been identified. This paper reports the prevalence of the GJB2 IVS1+1G>A mutation in a population of Chinese hearing loss patients with monoallelic pathogenic mutation in the coding region of GJB2.

Methods

Two hundred and twelve patients, screened from 7133 cases of nonsyndromic hearing loss in China, with monoallelic mutation (mainly frameshift and nonsense mutation) in the coding region of GJB2 were examined for the GJB2 IVS1+1G>A mutation and mutations in the promoter region of this gene. Two hundred and sixty-two nonsyndromic hearing loss patients without GJB2 mutation and 105 controls with normal hearing were also tested for the GJB2 IVS1+1G>A mutation by sequencing.

Results

Four patients with monoallelic mutation in the coding region of GJB2 were found carrying the GJB2 IVS1+1G>A mutation on the opposite allele. One patient with the GJB2 c.235delC mutation carried one variant, -3175 C>T, in exon 1 of GJB2. Neither GJB2 IVS1+1G>A mutation nor any variant in exon 1 of GJB2 was found in the 262 nonsyndromic hearing loss patients without GJB2 mutation or in the 105 normal hearing controls.

Conclusion

Testing for the GJB2 IVS 1+1 G to A mutation explained deafness in 1.89% of Chinese GJB2 monoallelic patients, and it should be included in routine testing of patients with GJB2 monoallelic pathogenic mutation.

High level of intrafamilial phenotypic variability of non-syndromic hearing loss in a Lur family due to delE120 mutation in GJB2 gene

Hearing loss is the most common sensory defect in the world. The genetic basis of this condition is very complex. Molecular variations in GJB2 gene are the common cause of hearing impairment in Caucasians. One expects that affected members of a family with same mutation have similar phenotype. Here, we report phenotypic variability in hearing loss among the members of a Lur family. Two brothers from a Lur family from Lurestan province in western Iran with variable degrees of nonsyndromic sensorineural hearing loss were evaluated for genetic counseling. Clinical examinations, audiological tests and molecular studies including GJB2 gene sequencing and detection of Delta(GJB6-D13S1830) deletion were performed. Sequencing analysis of GJB2 gene revealed delE120 mutation in both brothers in homozygous form. Since one of them was profoundly deaf and the other was mild hearing loss and had normal conversation, we were expecting different genotypes or other causative effects. Delta(GJB6-D13S1830) was not found. Phenotypic variability between members of different families with the same type of mutation can be expected which may be due to the role of different modifying factors, unrecognized gap junction isoforms, or polymorphism effects.

GJB2 mutations in Mongolia: complex alleles, low frequency, and reduced fitness of the deaf

We screened the GJB2 gene for mutations in 534 (108 multiplex and 426 simplex) probands with non-syndromic sensorineural deafness, who were ascertained through the only residential school for the deaf in Mongolia, and in 217 hearing controls. Twenty different alleles, including four novel changes, were identified. Biallelic GJB2 mutations were found in 4.5% of the deaf probands (8.3% in multiplex, 3.5% in simplex). The most common mutations were c.IVS1 + 1G > A (c.-3201G > A) and c.235delC with allele frequencies of 3.5% and 1.5%, respectively. The c.IVS1 + 1G > A mutation appears to have diverse origins based on associated multiple haplotypes. The p.V27I and p.E114G variants were frequently detected in both deaf probands and hearing controls. The p.E114G variant was always in cis with the p.V27I variant. Although in vitro experiments using Xenopus oocytes have suggested that p.[V27I;E114G] disturbs the gap junction function of Cx26, the equal distribution of this complex allele in both deaf probands and hearing controls makes it a less likely cause of profound congenital deafness. We found a lower frequency of assortative mating (37.5%) and decreased genetic fitness (62%) of the deaf in Mongolia as compared to the western populations, which provides an explanation for lower frequency of GJB2 deafness in Mongolia.

A new large deletion in the DFNB1 locus causes nonsyndromic hearing loss

Mutations in the GJB2 gene encoding the gap junction protein connexin 26 are responsible for up to 30% of all cases of autosomal recessive nonsyndromic hearing impairment (HI) with prelingual onset in most populations. The corresponding locus DFNB1, located on chromosome 13q11-q12, is also affected by three distinct deletions. These deletions extended distally to GJB2, which remains intact. We report a novel large deletion in DFNB1 observed in a patient presenting profound prelingual HI. This deletion was observed in trans to a GJB2 mutated allele carrying the p.Val84Met (V84M) mutation and was shown to be associated with hearing loss. The deletion caused a false homozygosity of V84M in the proband. Quantification of alleles by quantitative fluorescent multiplex PCR (QFM-PCR) enabled us to study the breakpoints of the deletion. The deleted segment extended through at least 920kb and removed the three connexin genes GJA3, GJB2 and GJB6. The distal breakpoint inside intron 2 of CRYL1 gene differed from the breakpoints of the known DFNB1 deletions. This case highlights the importance of screening for large deletions in molecular studies of GJB2.

Genetic epidemiological studies of congenital/prelingual deafness in Turkey: Population structure and mating type are major determinants of mutation identification

In order to evaluate the genetic epidemiology of deafness in Turkey, we first analyzed the pedigree data obtained from 2,169 families whose children were students of the schools for hearing loss/deafness in 31 cities of Turkey. Single major locus segregation analysis was performed after families were grouped according to hearing status of the parents. The results showed that sporadic phenocopies, autosomal dominant, and autosomal recessive transmission account for 18.2%, 4.9%, and 76.9% of the cases respectively, after exclusion of probands with unequivocal evidence for environmental etiologies. The high frequency of autosomal recessive transmission of this study differs from those of previous ones in Western populations. We subsequently analyzed the data from a subset of 574 unrelated families that were evaluated clinically, including mutation analysis of the GJB2 gene in 406 probands. Biallelic mutations were detected in 22.4% of all probands. They were present in 68.8% of probands whose parents were both deaf, yet in only 9.3% when both parents were hearing and consanguineous without a family history of deafness. Our study shows that GJB2 is the major gene for deafness in Turkey and was amplified in deaf by deaf matings, since assortative mating preferentially affects common genes. Deafness in the remaining families appears to result from mutations at many loci that are less frequent causes of deafness, because consanguinity has a proportionally greater effect on rare genes. Conclusions of this study may be relevant to other populations where consanguineous or assortative mating is present with various frequencies.