The World Health Organization has estimated that 360 million people worldwide have disabling HL (http://www.who.int/mediacentre/factsheets/fs300/en/) and that as the population ages, the global burden of diseases attributable to deafness will increase [14]. Linkage analysis was once regarded as the most powerful and widely used method for linking critical intervals to identify disease-causing genes in large pedigrees; however, this method is not appropriate for small families. Recently, with the marked advancements in sequencing technology, WES, which involves targeted sequencing of the protein-coding subset of the human genome, has been used as a convenient, rapid method for identification of new genes. Next-generation sequencing has advantages of small samples quantity, minimal cost, high-throughput sequencing, and low requirement for family size [15]. In this study, WES was used to find the disease-causing gene of a Chinese family with HL, and we identified a novel TMC1 missense mutation in exon 20, c.1979C>T, p.P660L.
TMC1 on chromosome 9q21 contains 24 exons that make up a coding region of 2283 nucleotides. The gene sequence is highly conserved, which suggests strong selective pressure throughout animal evolution. Mutations in TMC1 are a common cause of autosomal recessive nonsyndromic deafness, particularly in India, Pakistani, Turkish, and Tunisian families [2]. Thirty-five reported homozygous recessive mutations in TMC1, found in over 60 families worldwide, have been identified in different parts of TMC1 and cause different structural and functional disparity in intracellular and extracellular domains (Fig. 4).
TMC1 topology is predicted to include six membrane-spanning domains with three extracellular loops, a large intracellular loop settled between transmembrane (TM) domains four and five, a long intracellular N-terminus, and a short intracellular C-terminus. The structure suggests that the protein may function as a receptor, transporter, pump, or channel [16]. The mutation found in this study, p.P660L, was located within a predicted third extracellular loop situated between the fifth and sixth TM domains (Fig. 4). Pro660 is located in a highly conserved glycosyltransferase domain, which is highly conserved among many species. To the best of our knowledge, this is the first report describing a missense mutation in the glycosyltransferase domain of TMC1, supporting the importance of the conserved role of P660L in human TMC1 function.
TMC1 expression is constant in mature cochlear and vestibular hair cells, as shown in dn and Bth mutant mice carrying recessive (Tmc1dn) and dominant (Tmc1bth) TMC1 alleles. In the DFNA36 models, heterozygous mice (Tmc1
Bth/+) showed progressive hair cell degeneration [17], while homozygous mice (Tmc1
Bth/Bth) exhibited profound deafness [5]. Models of human recessive deafness, i.e., DFNB7/11, TMC1
dn/dn mice, do not have cochlear responses to sound stimuli and show several physiological deficits in hair cell maturation [5]. Kim et al. [9] concluded that TMC1/2 double mutant mice lacked conventional mechanotransduction, leading them to hypothesize that TMC1 is required for targeting the MET channel to the tips of the stereocilia, where they can interact with other constituents of the transduction complex, including the tip link. Although the specific function of TMC1 is unknown, recent studies have revealed that TMC1 is necessary for MET in cochlear and vestibular hair cells. Additionally, TMC1 is thought to be a component of the mechanotransduction channel in hair cells of the mammalian inner ear [3].
Caenorhabditis elegans
tmc1 has been shown to encode a sodium-sensitive ion channel. The predicted structure of TMC1 is similar to that of the α-subunit of voltage-dependent K+ channels, which have six TM segments and intracellular N- and C-termini [18], and TMC1 has been predicted to function as an ion channel or transporter mediating K+ homeostasis in the inner ear [19]. Ion channels serve many functions, including the transport of ions and water, control of electrical excitability, and regulation of ion homeostasis. The first four TM domains of the K+ channel α-subunit act as voltage sensors for activation gating [20], whereas the intervening segment between TM5 and TM6 appears to confer channel function [18]. TMC1 c.1979C>T, p.P660L is located in a predicted third extracellular loop located extracellularly between TM5 and TM6, and mutations in this loop may affect ion channel function in hair cells.