Role of Connexin 26 (GJB2) & Mitochondrial Small Ribosomal RNA (Mt 12S Rrna) Genes in Sporadic & Aminoglycoside-Induced Non Syndromic Hearing Impairment (Report)

Role of Connexin 26 (GJB2) & Mitochondrial Small Ribosomal RNA (Mt 12S Rrna) Genes in Sporadic & Aminoglycoside-Induced Non Syndromic Hearing Impairment (Report)

Hearing loss is the most common sensory deficit in humans. It affects approximately 10 per cent of the world population (1), which is significant enough to compromise the development of normal language skills and social development. It can appear at any age with varying degrees of severity. In India, one in every 600 children has hearing impairment (2). Hearing loss can be classified based on age at onset (pre or post-lingual), type of ear defect (conductive, sensorineural or mixed), degree of hearing loss (mild, moderate, severe and profound), and can be syndromic/non syndromic (3). Congenital/pre-lingual forms of deafness are always of sensorineural type, of which half are due to environmental factors (ototoxic drugs like aminoglycosides, cisplatin; bacterial/viral infections and acoustic trauma) and the remaining due to genetic causes (4). Seventy per cent of genetic cases are classified as non syndromic and 30 per cent are syndromic. Among the non syndromic, autosomal dominant (DFNA) contributes 22 per cent, autosomal recessive (DFNB)–77 per cent, X-linked (DFN)–1 per cent and mitochondrial (1-20%) (5). Over the past one decade, remarkable progress has been made in identifying and cloning the genes for hearing loss. More than 100 genes that are involved in syndromic hearing impairment, have been mapped. To date, about 132 hearing impairment loci have been mapped for non syndromic hearing impairment, of which 54 gene loci are associated with autosomal dominant mode of inheritance, 67 gene loci with autosomal recessive mode of inheritance, eight are X chromosome linked, one is Y-linked, and 2 are mitochondrial gene loci6. Fifty nuclear genes [(22 genes for autosomal dominant (DFNA), 28 for autosomal recessive (DFNB)], one X-linked (DFN) gene and two mitochondrial genes for non syndromic deafness have been characterized (6). In some instances, different mutations at the same locus have been found to cause both syndromic and non syndromic forms of deafness. For example, the DFNB1 locus is shown to cause both syndromic (Palmoplantar Keratoderma–PPK, Keratitis Ichthyosis deafness–KID) as well as non syndromic hearing impairment. In autosomal dominant, DFNA9 (COCH gene) locus is the most common one whereas in X-linked, DFN3 locus (POU3F4 gene), and in mitochondria, 12S rRNA gene are the most common ones involved in hearing impairment.

Role of Connexin 26 (GJB2) & Mitochondrial Small Ribosomal RNA (Mt 12S Rrna) Genes in Sporadic & Aminoglycoside-Induced Non Syndromic Hearing Impairment (Report)



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