Autosomal dominant hearing loss resulting from p.R75Q mutation in the GJB2 gene: nonsyndromic presentation in a South Indian family

A systematic review of the monogenic causes of Non-Syndromic Hearing Loss (NSHL) and discussion of Current Diagnosis and Treatment options

Hearing impairment is one of the most widespread inheritable sensory disorder affecting at least 1 in every 1000 born. About two-third of hereditary hearing loss (HHL) disorders are non-syndromic. To provide comprehensive update of monogenic causes of non-syndromic hearing loss (NSHL), literature search has been carried out with appropriate keywords in the following databases-PubMed, Google Scholar, Cochrane library, and Science Direct. Out of 2214 papers, 271 papers were shortlisted after applying inclusion and exclusion criterion. Data extracted from selected papers include information about gene name, identified pathogenic variants, ethnicity of the patient, age of onset, gender, title, authors' name, and year of publication. Overall, pathogenic variants in 98 different genes have been associated with NSHL. These genes have important role to play during early embryonic development in ear structure formation and hearing development. Here, we also review briefly the recent information about diagnosis and treatment approaches. Understanding pathogenic genetic variants are helpful in the management of affected and may offer targeted therapies in future.

Case Report: A Novel GJB2 Missense Variant Inherited From the Low-Level Mosaic Mother in a Chinese Female With Palmoplantar Keratoderma With Deafness

Dominant variants in the gap junction beta-2 (GJB2) gene may lead to various degrees of syndromic hearing loss (SHL) which is manifest as sensorineural hearing impairment and hyperproliferative epidermal disorders, including palmoplantar keratoderma with deafness (PPKDFN). So far, only a few GJB2 dominant variants causing PPKDFN have been discovered. Through the whole-exome sequencing (WES), a Chinese female patient with severe palmoplantar hyperkeratosis and delayed-onset hearing loss has been identified. She had a novel heterozygous variant, c.224G>C (p.R75P), in the GJB2 gene, which was unreported previously. The proband’s mother who had a mild phenotype was suggested the possibility of mosaicism by WES (∼120×), and the ultra-deep targeted sequencing (∼20,000×) was used for detecting low-level mosaic variants which provided accurate recurrence-risk estimates and genetic counseling. In addition, the analysis of protein structure indicated that the structural stability and permeability of the connexin 26 (Cx26) gap junction channel may be disrupted by the p.R75P variant. Through retrospective analysis, it is detected that the junction of extracellular region-1 (EC1) and transmembrane region-2 (TM2) is a variant hotspot for PPKDFN, such as p.R75. Our report reflects the important and effective diagnostic role of WES in PPKDFN and low-level mosaicism, expands the spectrum of the GJB2 variant, and furthermore provides strong proof about the relevance between the p.R75P variant in GJB2 and PPKDFN.

Genetics Landscape of Nonsyndromic Hearing Loss in Indian Populations

Congenital nonsyndromic hearing loss (NSHL) has been considered as one of the most prevalent chronic disorder in children. It affects the physical and mental conditions of a large children population worldwide. Because of the genetic heterogeneity, the identification of target gene is very challenging. However, gap junction β-2 ( GJB2 ) is taken as the key gene for hearing loss, as its involvement has been reported frequently in NSHL cases. This study aimed to identify the association of GJB2 mutants in different Indian populations based on published studies in Indian population. This will provide clear genetic fundamental of NSHL in Indian biogeography, which would be helpful in the diagnosis process.

Functional Evaluation of a Rare Variant c.516G>C (p.Trp172Cys) in the GJB2 (Connexin 26) Gene Associated with Nonsyndromic Hearing Loss

Mutations in the GJB2 gene encoding transmembrane protein connexin 26 (Cx26) are the most common cause for hearing loss worldwide. Cx26 plays a crucial role in the ionic and metabolic homeostasis in the inner ear, indispensable for normal hearing process. Different pathogenic mutations in the GJB2 gene can affect all stages of the Cx26 life cycle and result in nonsyndromic autosomal recessive (DFNB1) or dominant (DFNA3) deafness and syndromes associating hearing loss with skin disorders. This study aims to elucidate the functional consequences of a rare GJB2 variant c.516G>C (p.Trp172Cys) found with high frequency in deaf patients from indigenous populations of Southern Siberia (Russia). The substitution c.516G>C leads to the replacement of tryptophan at a conserved amino acid position 172 with cysteine (p.Trp172Cys) in the second extracellular loop of Cx26 protein. We analyzed the subcellular localization of mutant Cx26-p.Trp172Cys protein by immunocytochemistry and the hemichannels permeability by dye loading assay. The GJB2 knockout HeLa cell line has been generated using CRISPR/Cas9 genome editing tool. Subsequently, the HeLa transgenic cell lines stably expressing different GJB2 variants (wild type and mutations associated with hearing loss) were established based on knockout cells and used for comparative functional analysis. The impaired trafficking of mutant Cx26-p.Trp172Cys protein to the plasma membrane and reduced hemichannels permeability support the pathogenic effect of the c.516G>C (p.Trp172Cys) variant and its association with nonsyndromic hearing loss. Our data contribute to a better understanding of the role of mutations in the second extracellular loop of Cx26 protein in pathogenesis of deafness.

Role of DFNB1 mutations in hereditary hearing loss among assortative mating hearing impaired families from South India

Background

DFNB1, the first locus to have been associated with deafness, has two major genes GJB2 & GJB6, whose mutations have played vital role in hearing impairment across many ethnicities in the world. In our present study we have focused on the role of these mutations in assortative mating hearing impaired families from south India.

Methods

One hundred and six assortatively mating hearing impaired (HI) families of south Indian origin comprising of two subsets: 60 deaf marrying deaf (DXD) families and 46 deaf marrying normal hearing (DXN) families were recruited for this study. In the 60 DXD families, 335 members comprising of 118 HI mates, 63 other HI members and 154 normal hearing members and in the 46 DXN families, 281 members comprising of 46 HI and their 43 normal hearing partners, 50 other HI members and 142 normal hearing family members, participated in the molecular study. One hundred and sixty five (165) healthy normal hearing volunteers were recruited as controls for this study. All the participating members were screened for variants in GJB2 and GJB6 genes and the outcome of gene mutations were compared in the subsequent generation in begetting deaf offspring.

Results

The DFNB1 allele frequencies for DXD mates and their offspring were 36.98 and 38.67%, respectively and for the DXN mates and their offspring were 22.84 and 24.38%, respectively. There was a 4.6% increase in the subsequent generation in the DXD families, while a 6.75% increase in the DXN families, which demonstrates the role of assortative mating along with consanguinity in the increase of DFNB1 mutations in consecutive generations. Four novel variants, p.E42D (in GJB2 gene), p.Q57R, p.E101Q, p.R104H (in GJB6 gene) were also identified in this study.

Conclusion

This is the first study from an Indian subcontinent reporting novel variants in the coding region of GJB6 gene. This is perhaps the first study in the world to test real-time, the hypothesis proposed by Nance et al. in 2000 (intense phenotypic assortative mating mechanism can double the frequency of the commonest forms of recessive deafness [DFNB1]) in assortative mating HI parental generation and their offspring.

Absence of KCNQ4 mutation in Bengali families with ADNSHL originated from West Bengal, India

OBJECTIVE

Autosomal Dominant Non-Syndromic Hearing Loss (ADNSHL) is extremely heterogeneous in nature. More than 60 loci with 30 different genes have been identified linked to ADNSHL. Mutation in KCNQ4 is considered as one of the most common causative factor responsible for ADNSHL. No study focused on the genetic alteration of KCNQ4 gene among hearing loss patients in India. The present study for the first time was carried out to determine the mutation spectrum of KCNQ4 gene in ADNSHL patients of West Bengal state, India.

METHOD

Twenty nine individuals from 10 independent ADNSHL family (with two or more generation affected) were studied both clinically and genetically. Most of the patients showed moderate progressive sensorineural hearing loss. Mutation analysis was conducted for KCNQ4 gene using polymerase chain reaction followed by direct sequencing.

RESULTS

Neither any reported nor a novel pathogenic mutation in KCNQ4was detected in our studied group, in contrast to the findings among East Asians.

CONCLUSION

The result of the present study suggests that mutations in KCNQ4 gene are unlikely to be a major causative factor of ADNSHL in our studied patients from West Bengal, India, pointing to other genes might be responsible for ADNSHL in our studied patients.

Rare compound heterozygosity involving dominant and recessive mutations of GJB2 gene in an assortative mating hearing impaired Indian family

Connexin 26 (Cx-26), a gap junction protein coded by GJB2 gene, plays a very important role in recycling of potassium ions, one of the vital steps in the mechanotransduction process of hearing. Mutations in the GJB2 gene have been associated with both autosomal recessive as well as dominant nonsyndromic hearing loss. As Cx-26 is linked with skin homeostasis, mutations in this gene are sometimes associated with syndromic forms of hearing loss showing skin anomalies. We report here a non consanguineous assortatively mating hearing impaired family with one of the hearing impaired partners, their hearing impaired sibling and hearing impaired offspring showing compound heterozygosity in the GJB2 gene, involving a dominant mutation p.R184Q and two recessive mutations p.Q124X and c.IVS 1+1G>A in a unique triallelic combination. To the best of our knowledge, this is the first report from India on p.R184Q mutation in the GJB2 gene associated with rare compound heterozygosity showing nonsyndromic presentation.

Update of the spectrum of GJB2 gene mutations in 152 Moroccan families with autosomal recessive nonsyndromic hearing loss

Deafness is one of the most common genetic diseases in humans and is subject to important genetic heterogeneity. The most common cause of non syndromic hearing loss (NSHL) is mutations in the GJB2 gene. This study aims to update and evaluate the spectrum of GJB2 allele variants in 152 Moroccan multiplex families with non syndromic hearing loss. Seven different mutations were detected: c.35delG, p.V37I, p.E47X, p.G200R, p.Del120E, p.R75Q, the last three mutations were described for the first time in Moroccan deaf patients, in addition to a novel nonsense mutation, the c.385G>T which is not referenced in any database. Sixty six families (43.42%) have mutations in the coding region of GJB2, while the homozygous c.35delG mutation still to date the most represented 51/152 (33.55%). The analysis of the geographical distribution of mutations located in GJB2 gene showed more allelic heterogeneity in the north and center compared to the south of Morocco. Our results showed that the GJB2 gene is a major contributor to non syndromic hearing loss in Morocco. Thus, this report of the GJB2 mutations spectrum all over Morocco has an important implication for establishing a suitable molecular diagnosis.

Intra-familial phenotypic variability in a Moroccan family with hearing loss and palmoplantar keratoderma (PPK)

Mutations in the GJB2 gene encoding connexin 26 are the main cause of hereditary hearing impairment. These mutations generate mainly autosomal recessive and rarely autosomal dominant deafness. Dominant mutations in GJB2 can be responsible for isolated deafness as well as syndromic hearing loss associated with various skin abnormalities. Until now few papers discuss dominant mutations in the GJB2 gene. In this work we report a rare case about a Moroccan family with a compound heterozygous mutation (the dominant p.R75Q and the recessive c.35delG alleles) in the GJB2 gene with intra-familial phenotypic variability. This study reinforces the involvement of p.R75Q mutation of GJB2 in syndromic deafness associated with dermatological diseases the palmoplantar keratoderma.