Identification of a novel homozygous mutation in transmembrane channel like 1 (TMC1) gene, one of the second-tier hearing loss genes after GJB2 in India

Background & objectives: Hearing impairment is a common and heterogeneous sensory disorder in humans. Among about 90 genes, which are known to be associated with hearing impairment, mutations in the GJB2 (gap junction protein beta 2) gene are the most prevalent in individuals with hereditary hearing loss. Contribution of the other deafness-causing genes is relatively poorly understood. Here, we present our findings on two families with transmembrane channel like 1 (TMC1) gene variants of the 47 families with nonsyndromic hearing loss (NSHL) studied. Methods: Forty seven families including 26 consanguineous families with at least two hearing impaired children and one normal hearing child and 21 non-consanguineous families having at least three hearing impaired children and one normal hearing child were enrolled for this study. Genetic linkage studies were carried out in 41 families that were GJB2 (Connexin 26) negative. Seven polymorphic short tandem repeat markers at the DFNB7/11 locus were studied employing fluorescently labelled markers. Results: A novel homozygous missense mutation c.1283C>A (p.Ala428Asp) was identified co-segregating with hearing loss. This change results in substitution of a highly conserved polar alanine to a charged aspartic acid and is predicted to be deleterious. In addition, a previously reported nonsense mutation, p.R34X in TMC1, was found. Interpretation & conclusions: While mutations in TMC1 are not as common a cause of NSHL as those in GJB2, TMC1 should be considered for diagnostic investigations in cases of NSHL in GJB2-negative families.