Hearing loss: A review on molecular genetics and epidemiologic aspects

Background and aims: Hearing loss (HL) happens due to the genetic or environmental causes or both. Risk factors include congenital infections and congenital deformities of auricle and ear duct. The present study was performed to briefly explain the genetics, molecular biology and epidemiology of HL in Middle East especially in Iran. Methods: An intense an comprehensive literature search was prformed through heading journals in the field. All data was organized using Mendeley software and incorporated to the text as required. Results: While the etiology of 25% of HL cases remains indistinct, it is estimated that at least 50% of pre lingual HL cases have a genetic cause. About 70% of genetic HL cases are non Syndromic (NSHL) without anomaly, whereas the remaining 30% are Syndromic. Autosomal recessive non-syndromic hearing loss forms (ARNSHL) are the severest forms of congenital HL with defect in cochlea. In addition to X-linked (DFNX), autosomal dominant (DFNA), autosomal recessive (DFNB) and Y-linked (DFNY) inheritance patterns, HL can be inherited through mitochondrial genes including MT-RNR1 and MT-TS. At least 120 genes have been reported to be associated with HL. Among them, mutations in connexin 26 (GJB2) have been shown to play a very important role in developing ARSNSHL in many populations depending on geographical location and ethnicity. In Caucasians and Spainish/Italian populations, 50% and 79% of HL cases have respectively been reported to be occurred due to mutations in GJB2 gene. Conclusion: In the Middle East, the prevalence seems different as an average of 14-20% of the HL in several region of Iran is due to mutation in GJB2 gene. Alternatively similar studies showed the prevalence of GJB2 mutations around 25% and 6.1% in Turkey and Pakistani populations respectively.