Did the GJB2 35delG mutation originate in Iran?

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 etiology of hearing loss in Iran

Hearing loss (HL) is an etiologically heterogeneous disorder that affects around 5% of the world's population. There has been an exponential increase in the identification of genes and variants responsible for hereditary HL over recent years. Iran, a country located in the Middle East, has a high prevalence of consanguineous marriages, so heterogeneous diseases such as HL are more common. Comprehensive studies using different strategies from linkage analysis to next-generation sequencing, especially exome-sequencing, have achieved significant success in identifying possible pathogens in deaf Iranian families. About 12% of non-syndromic autosomal recessive HL genes investigated to date, were first identified in families from Iran. Variations of 56 genes have been observed in families with NSHL in Iran. Variants in GJB2, SLC26A4, MYO15A, MYO7A, CDH23, and TMC1 account for 16.5%, 16.25%, 13.5%, 9.35%, 6.9% and 4.92%, cases of NSHL, respectively. In summary, there are also different diagnostic rates between studies conducted in Iran. In the comprehensive investigations conducted by the Genetic Research Center of the University of Social Welfare and Rehabilitation Sciences over the past 20 years, the overall diagnosis rate is about 80% while there are other studies with lower diagnostic rates which could reflect differences in project designs, sampling, and accuracy and validity of the methods used. Furthermore, there are several syndromic HHLs in Iran including, Waardenburg syndrome, BOR syndrome, Brown-Vialetto-Van Laere syndrome, Wolfram syndrome, among which Pendred and Usher syndromes are well-studied. These results are of importance for further investigation and elucidation of the molecular basis of HHL in Iran.

A common founder effect of the splice site variant c.-23 + 1G > A in GJB2 gene causing autosomal recessive deafness 1A (DFNB1A) in Eurasia

Mutations in the GJB2 gene are known to be a major cause of autosomal recessive deafness 1A (OMIM 220290). The most common pathogenic variants of the GJB2 gene have a high ethno-geographic specificity in their distribution, being attributed to a founder effect related to the Neolithic migration routes of Homo sapiens. The c.-23 + 1G > A splice site variant is frequently found among deaf patients of both Caucasian and Asian origins. It is currently unknown whether the spread of this mutation across Eurasia is a result of the founder effect or if it could have multiple local centers of origin. To determine the origin of c.-23 + 1G > A, we reconstructed haplotypes by genotyping SNPs on an Illumina OmniExpress 730 K platform of 23 deaf individuals homozygous for this variant from different populations of Eurasia. The analyses revealed the presence of common regions of homozygosity in different individual genomes in the sample. These data support the hypothesis of the common founder effect in the distribution of the c.-23 + 1G > A variant of the GJB2 gene. Based on the published data on the c.-23 + 1G > A prevalence among 16,177 deaf people and the calculation of the TMRCA of the modified f2-haplotypes carrying this variant, we reconstructed the potential migration routes of the carriers of this mutation around the world. This analysis indicates that the c.-23 + 1G > A variant in the GJB2 gene may have originated approximately 6000 years ago in the territory of the Caucasus or the Middle East then spread throughout Europe, South and Central Asia and other regions of the world.

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.

Screening of 10 DFNB Loci Causing Autosomal Recessive Non-Syndromic Hearing Loss in Two Iranian Populations Negative for GJB2 Mutations

BACKGROUND

Autosomal recessive non-syndromic hearing loss (ARNSHL), one of the global public health concerns, is marked by a high degree of genetic heterogeneity. The role of GJB2, as the most common cause of ARNSHL, is only <20% in the Iranian population. Here, we aimed to determine the relative contribution of several apparently most common loci in a cohort of ARNSHL Iranian families that were negative for the GJB2 mutations.

METHODS

Totally, 80 Iranian ARNSHL families with 3 or more affected individuals from Isfahan and Hamedan provinces, Iran were enrolled in 2017. After excluding mutations in the GJB2 gene via Sanger sequencing, 60 negative samples (30 families from each province) were analyzed using homozygosity mapping for 10 ARNSHL loci.

RESULTS

Fourteen families were found to be linked to five different known loci, including DFNB4 (5 families), DFNB2 (3 families), DFNB7/11 (1 family), DFNB9 (2 families) and DFNB3 (3 families).

CONCLUSION

Despite the high heterogeneity of ARNSHL, the genetic causes were determined in 23.5% of the studied families using homozygosity mapping. This data gives an overview of the ARNSHL etiology in the center and west of Iran, used to establish a diagnostic gene panel including most common loci for hearing loss diagnostics.

An update of spectrum and frequency of GJB2 mutations causing hearing loss in the south of Iran: A literature review

OBJECTIVE

Mutations in the GJB2 gene are a major cause of autosomal recessive non-syndromic HL (ARNSHL) in many populations. Previous studies have estimated the average frequency of GJB2 mutations to be between 16 and 18% in Iran, but would vary among different ethnic groups. Here, we have taken together and reviewed results from our three previous publications and data from search other published mutation reports to provide a comprehensive collection of data for GJB2 mutations and HL in the south of Iran.

METHODS

In all, 447 unrelated families were included and analyzed for the prevalence and type of the GJB2 gene mutations.

RESULTS

Totally, the frequency of GJB2 mutations was found to be 11.5% in the southern provinces studied which is significantly lower than that identified in Northern populations of Iran, and also a southwest to southeast Iranian gradient in the frequency of GJB2 mutations is suggested.

CONCLUSIONS

This study highlights the importance of establishing prevalence, based on the local population for screening and diagnostic programs of live births in Iran.

Updated carrier rates for c.35delG (GJB2) associated with hearing loss in Russia and common c.35delG haplotypes in Siberia

Background

Mutations in GJB2 gene are a major causes of deafness and their spectrum and prevalence are specific for various populations. The well-known mutation c.35delG is more frequent in populations of Caucasian origin. Data on the c.35delG prevalence in Russia are mainly restricted to the European part of this country. We aimed to estimate the carrier frequency of c.35delG in Western Siberia and thereby update current data on the c.35delG prevalence in Russia. According to a generally accepted hypothesis, c.35delG originated from a common ancestor in the Middle East or the Mediterranean ~ 10,000–14,000 years ago and spread throughout Europe with Neolithic migrations. To test the c.35delG common origin hypothesis, we have reconstructed haplotypes bearing c.35delG and evaluated the approximate age of c.35delG in Siberia.

Methods

The carrier frequency of c.35delG was estimated in 122 unrelated hearing individuals living in Western Siberia. For reconstruction of haplotypes bearing c.35delG, polymorphic D13S141, D13S175, D13S1853 flanking the GJB2 gene, and intragenic rs3751385 were genotyped in deaf patients homozygous for c.35delG (n = 24) and in unrelated healthy individuals negative for c.35delG (n = 67) living in Siberia.

Results

We present updated carrier rates for c.35delG in Russia complemented by new data on c.35delG carrier frequency in Russians living in Western Siberia (4.1%). Two common D13S141-c.35delG-D13S175-D13S1853 haplotypes, 126-c.35delG-105-202 and 124-c.35delG-105-202, were reconstructed in the c.35delG homozygotes from Siberia. Moreover, identical allelic composition of the two most frequent c.35delG haplotypes restricted by D13S141 and D13S175 was established in geographically remote regions: Siberia and Volga-Ural region (Russia) and Belarus (Eastern Europe).

Conclusions

Distribution of the c.35delG carrier frequency in Russia is characterized by pronounced ethno-geographic specificity with a downward trend from west to east. Comparative analysis of the c.35delG haplotypes supports a common origin of c.35delG in some regions of Russia (Volga-Ural region and Siberia) and in Eastern Europe (Belarus). A rough estimation of the c.35delG age in Siberia (about 4800 to 8100 years ago) probably reflects the early formation stages of the modern European population (including the European part of the contemporary territory of Russia) since the settlement of Siberia by Russians started only at the end of sixteenth century.

Electronic supplementary material

The online version of this article (10.1186/s12881-018-0650-5) contains supplementary material, which is available to authorized users.

Public Health Burden of Hearing Impairment and the Promise of Genomics and Environmental Research: A Case Study in Ghana, Africa

Hearing impairment (HI) is one of the most disabling conditions of major global health burden that contributes adversely to the social and economic development of a country, if not managed properly. A proper assessment of the nationwide burden and etiology of HI is instrumental in the prevention, treatment, and management of the condition. This article sought to perform an expert review of HI in Ghana to determine the present knowledge of its burden and possible causes of the condition. A literature search was conducted in PubMed using the following keywords: "hearing loss" OR "hearing impairment" OR "deafness" AND "Ghana." The literature was scanned until July 20, 2017, with specific inclusion of targeted landmark and background articles on HI. From the search, 18 of out 5869 articles were selected and considered for the review. The results of the search indicated that there were no extensive studies to determine the national burden of HI in Ghana. However, the few studies assessed suggested that the disease is either acquired or inherited. The burden of acquired HI was higher in adults than children, women than men, and people working in a noisy environment. Regarding the genetic cause, specific founder mutations in GJB2 gene (R143W, L79P, V178A, R184Q, A197S, I203K, and L214P) was the only identified genetic cause of HI in Ghana, but the other HI genes were not investigated. There has been some modest effort to study HI in Ghana, but comprehensive studies on the genetic and environmental etiologies (using the "multi-OMICS" approaches), classification, and burden of HI on Ghana are needed.

Heterogeneity of Hereditary Hearing Loss in Iran: a Comprehensive Review

A significant contribution to the causes of hereditary hearing impairment comes from genetic factors. More than 120 genes and 160 loci have been identified to be involved in hearing impairment. Given that consanguine populations are more vulnerable to most inherited diseases, such as hereditary hearing loss (HHL), the genetic picture of HHL among the Iranian population, which consists of at least eight ethnic subgroups with a high rate of intermarriage, is expected to be highly heterogeneous. Using an electronic literature review through various databases such as PubMed, MEDLINE, and Scopus, we review the current picture of HHL in Iran. In this review, we present more than 39 deafness genes reported to cause non-syndromic HHL in Iran, of which the most prevalent causative genes include GJB2, SLC26A4, MYO15A, and MYO7A. In addition, we highlight some of the more common genetic causes of syndromic HHL in Iran. These results are of importance for further investigation and elucidation of the molecular basis of HHL in Iran and also for developing a national diagnostic tool tailored to the Iranian context enabling early and efficient diagnosis of hereditary hearing impairment.

A comprehensive study to determine heterogeneity of autosomal recessive nonsyndromic hearing loss in Iran

Hearing loss is the most common sensory disorder worldwide and affects 1 of every 500 newborns. In developed countries, at least 50% of cases are genetic, most often resulting in nonsyndromic deafness (70%), which is usually autosomal recessive (∼80%). Although the cause of hearing loss is heterogeneous, mutations in GJB2 gene at DFNB1 locus are the major cause of autosomal recessive nonsyndromic hearing loss (ARNSHL) in many populations. Our previous study showed that mutations of GJB2 gene do not contribute to the major genetic load of deafness in the Iranian population (∼16%). Therefore, to define the importance of other genes in contributing to an ARNSHL phenotype in the Iranian population, we used homozygosity mapping to identify regions of autozygosity-by-descent in 144 families which two or more progeny had ARNSHL but were negative for GJB2 gene mutations. Using flanking or intragenic short-tandem repeat markers for 33 loci we identified 33 different homozygous variations in 10 genes, of which 9 are novel. In aggregate, these data explain ∼40% of genetic background of ARNHSL in the Iranian population.

The spectrum of GJB2 mutations in the Iranian population with non-syndromic hearing loss--a twelve year study

OBJECTIVE

Mutations in GJB2, encoding connexin 26 (CX26), are causally related to autosomal recessive form of non-syndromic hearing loss (NSHL) at the DFNB1 locus and autosomal dominant NSHL at the DFNA3 locus. In this study, we investigated the prevalence of GJB2 mutations in the Iranian deaf population.

METHODS

A total of 2322 deaf probands presenting the ethnically diverse Iranian population were screened for variants in GJB2. All persons were first screened for the c.35delG mutation, as this variant is the most prevalent GJB2-deafness causing mutation in the Iranian population. In all persons carrying zero or one c.35delG allele, exons 1 and 2 were then sequenced.

RESULTS

In total, 374 (~16%) families segregated GJB2-related deafness caused by 45 different mutations and 5 novel variants. The c.35delG mutation was most commonly identified and accounts for ~65% of the GJB2 mutations found in population studied.

CONCLUSION

Our data also show that there is a gradual decrease in the frequency of the c.35delG mutation and of GJB2-related deafness in general in a cline across Iran extending from the northwest to southeast.