Several modifier genes have been localised that modify the phenotype of carriers of known deafness genes. In humans, a dominant modifier has been reported to suppress deafness in people homozygous for the DFNB26 recessive deafness locus (Riazuddin et al., 2000). Two modifiers have been proposed to influence the hearing loss observed in carriers of the A1555G mitochondrial mutation associated with progressive hearing loss, a locus on chromosome 8 near D8S277 and a locus on chromosome 6 near the TFB1M gene (Bykhovskaya et al., 2000; 2004). In the mouse, three modifiers of deafness genes have been identified. The first is moth1, which has a major effect upon the hearing of tubby mutant homozygotes (tub/tub), and has been identified as a variant of the Mtap1 gene. A second is mdfw, modifier of deaf waddler, which largely determines whether heterozygous carriers of a deaf waddler mutation (+/Atp2b2dfw2J) show progressive hearing loss or not. This modifier is most likely to be a synonymous single nucleotide polymorphism that leads to in-frame skipping of exon 7 of the Cdh23 gene (Noben-Trauth et al., 2003). This variant of Cdh23 is also known as Ahl because it was originally mapped as a locus involved in age-related hearing loss in a number of common mouse strains. The third mouse modifier is a mitochondrial mutation that affects hearing impairment in mice homozygous for the susceptible allele of Ahl (ahl/ahl) and has been identified as a single nucleotide insertion in the tRNA-Arg gene.
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