Usher syndrome is an autosomal recessive disorder manifesting hearing loss, retinitis pigmentosa and vestibular dysfunction, and having three clinical subtypes. Usher syndrome type 1 is the most severe subtype due to its profound hearing loss, lack of vestibular responses, and retinitis pigmentosa that appears in prepuberty. Six of the corresponding genes have been identified, making early diagnosis through DNA testing possible, with many immediate and several long-term advantages for patients and their families. However, the conventional genetic techniques, such as direct sequence analysis, are both time-consuming and expensive. Targeted exon sequencing of selected genes using the massively parallel DNA sequencing technology will potentially enable us to systematically tackle previously intractable monogenic disorders and improve molecular diagnosis. Using this technique combined with direct sequence analysis, we screened 17 unrelated Usher syndrome type 1 patients and detected probable pathogenic variants in the 16 of them (94.1%) who carried at least one mutation. Seven patients had the MYO7A mutation (41.2%), which is the most common type in Japanese. Most of the mutations were detected by only the massively parallel DNA sequencing. We report here four patients, who had probable pathogenic mutations in two different Usher syndrome type 1 genes, and one case of MYO7A/PCDH15 digenic inheritance. This is the first report of Usher syndrome mutation analysis using massively parallel DNA sequencing and the frequency of Usher syndrome type 1 genes in Japanese. Mutation screening using this technique has the power to quickly identify mutations of many causative genes while maintaining cost-benefit performance. In addition, the simultaneous mutation analysis of large numbers of genes is useful for detecting mutations in different genes that are possibly disease modifiers or of digenic inheritance.

Genetic modifiers can be detected in mice by looking for strain background differences in inheritance or phenotype of a mutation. They can be mapped by analyses of appropriate linkage crosses and congenic lines, and modifier genes of large effect can be identified by positional-candidate gene testing. Inbred strains of mice vary widely in onset and severity of age-related hearing loss (AHL), an important consideration when assessing hearing in mutant mice. At least 8 mapped loci and a mitochondrial variant (mt-Tr) are known to contribute to AHL in mouse strains; one locus (ahl) has been identified as a variant of the cadherin 23 gene (Cdh23(753A/G)). This variant also was shown to modify hearing loss associated with the Atp2b2(dfw-2J) and Mass1(frings) mutations. The hearing modifier (Moth1) of tubby (Tub(tub)) mutant mice was shown to be a strain variant of the Mtap1a gene. Human hearing modifiers include DFNM1, which suppresses recessive deafness DFNB26, and a nuclear gene that modulates the severity of hearing loss associated with a mitochondrial mutation. Recently, a variant of the human ATP2B2 gene was shown to exacerbate hearing loss in individuals homozygous for a CDH23 mutation, similar to the Atp2b2(dfw-2J)-Cdh23(753A/G) interaction affecting hearing in mice. Because modifier genes and digenic inheritance are not always distinguishable, we also include in this review several examples of digenic inheritance of hearing loss that have been reported in both mice and humans.