Mechanism of hearing loss

The cochlea comprises three fluid-filled compartments known as scala tympani, scala media, and scala vestibule. The scala tympani and scala vestibule contain a fluid (perilymph) with a high concentration of Na⁺ and low concentration of K⁺; whereas, scala media is filled with fluid (endolymph) which contain high concentrations of K⁺ and low concentration of Na⁺. The endolymph in scala media also includes a high positive endocochlear potential equal to 110 to 120 mV. This potential drives the K⁺ ion present in endolymph through the mechano-transduction in the hair's cells. Expelled out of K⁺ ions in the extracellular space is necessary to restore the cell polarization. This extra expelled K⁺ around hair cells needs to be removed to avoid K⁺ ion toxicity and maintain hair cell function. According to the K⁺ recycling hypothesis, these K⁺ ions are sunken by neighboring cells and transported back to endolymph through gap junction. [11]During auditory transduction in the cochlea of the inner ear cochlea, GJB2 (Gap junction beta-2 protein) maintains potassium homeostasis [5]. The six connexins 26 (Cx26) molecules assemble and form a single connexon in each cochlea cell, where Cx26 is mostly expressed. A gap junction between two connexons of the neighboring cochlea cells is formed, which serves as a path to maintain the K⁺ ion concentration balance between the different fluid compartments of the inner ear. Mutations in the GJB2 gene disrupt the communication channels between these cells, cause an imbalance of the potassium ion exchange and affect hearing function [12].

GJB2 gene involvement in hearing loss

GJB2 gene is one of the most common causes of non-syndromic hearing loss, and almost 300 different variants have been reported. The most common hereditary type of hearing impairment is ARNSHL, and more than 700 different causative mutations have been identified in over 80 DFNB loci. DFNB1 locus on chromosome 13q12.11 contains the most common ARNSHL gene (GJB2) with a single coding-exon, and more than 50% ARNSHL is linked with GJB2 mutations. More than 100 GJB2 mutations responsible for ARNSHL have been documented so far [13].

Prevalence of GJB2 gene mutation in Pakistani population

The prevalence of GJB2 mutations is different in different areas. According to the WHO, 2012, the prevalence rate of HL in Pakistan was 2.4% higher than the world's prevalence of 1.7% [13]. Compared to other populations, it is one of the most common mutations in Pakistan[13].  Consanguineous marriages increase the rate of recessive genetic disorders. High consanguinity in any population may lead to genetic diseases like HL. In Pakistan, consanguineous marriages are high; therefore, the population is unique to study recessive genetic disorders such as HL. The prevalence of profound bilateral HL in Pakistan is 1.6 per 1000 population, and 70% of the disorder arises in consanguineous families [7]. The studies conducted on the Pakistani population revealed a variable prevalence of GJB2 ranging from 4.2% to 53.33% [14-16].

In the Pakistani population, the contribution of GJB2 to NSHL is varied from 4.2 to 53.33%. In 2005, Santos et al. showed that only 6.1% of the studied families in Pakistan had GJB2 linked HL [17]. The most common mutations found in this report were p.Trp* and p.Trp77*,  p.Arg2His, and p.Leu90Pro mutations were also detected. The 70% studied families belonged to the Punjab province, and another 30% belonged to other Pakistan provinces.  In 2013, Bukhari et al. found a low prevalence (4.28%) of GJB2 mutation in the Hazara division, KPK province, compared to other provinces. Only three mutations, p.Trp24*, p. Ile35Ser, and p.Gly11Leufs24*, were found after sequencing the DNAs of 70 deaf patients (18). However, in 2014, Anjum et al. showed that the frequency of GJB2 mutation is higher (50%) in deaf patients from different areas of Pakistan [18]. The reported mutations were Trp24*, Gly12Valfs*2, Gln124*, Trp77*, Val153Ile, and Thr8Met. Similarly, in 2014 Shafique et al. revealed that the prevalence of GJB2 gene mutations in deaf patients of Punjab province was 53% [14]. The most common mutations were p.Trp24* and p.Trp77*. Other identified mutations were p.Gly200Arg p.Gly11Leufs24*, p.Glu147Lys, and p.R127Cfs*85. In 2017, Wang et al. revealed only mutation Gly11Leufs24* (35 del G). Similarly, in 2017, Shaikh et al. studied GJB2 mutations in a population of Sindh province, and prevalence was 6 % (9/150). The mutations found during the study were c.71G> A (p. Trp24*),  c.231G> A (p.Trp77*), C35delG (Gly11Leufs24*), c.233delC (p.Leu79Cysfs3*), c.310del14 (p.Leu79Cysfs3*) , c.380G>A (p.Arg127His) as mention in table 01 and table 02. In 2019, mutations found in GJB2, were c.1055C>T (p.Pro352Leu), c.6202A>C p.(Thr2068Pro), c.2496_2496delC p.(Tyr832*) and c.355G>T p.(Glu119*) [13].