Heterozygous mutation of Ush1g/Sans in mice causes early-onset progressive hearing loss, which is recovered by reconstituting the strain-specific mutation in Cdh23.

Most clinical reports have suggested that patients with congenital profound hearing loss have recessive mutations in deafness genes, whereas dominant alleles are associated with progressive hearing loss (PHL). Jackson shaker (Ush1gjs) is a mouse model of recessive deafness that exhibits congenital profound deafness caused by the homozygous mutation of Ush1g/Sans on chromosome 11. We found that C57BL/6J-Ush1gjs/+ heterozygous mice exhibited early-onset PHL (ePHL) accompanied by progressive degeneration of stereocilia in the cochlear outer hair cells. Interestingly, ePHL did not develop in mutant mice with the C3H/HeN background, thus suggesting that other genetic factors are required for ePHL development. Therefore, we performed classical genetic analyses and found that the occurrence of ePHL in Ush1gjs/+ mice was associated with an interval in chromosome 10 that contains the cadherin 23 gene (Cdh23), which is also responsible for human deafness. To confirm this mutation effect, we generated C57BL/6J-Ush1gjs/+, Cdh23c.753A/G double-heterozygous mice by using the CRISPR/Cas9-mediated Cdh23c.753A>G knock-in method. The Cdh23c.753A/G mice harbored a one-base substitution (A for G), and the homozygous A allele caused moderate hearing loss with aging. Analyses revealed the complete recovery of ePHL and stereocilia degeneration in C57BL/6J-Ush1gjs/+ mice. These results clearly show that the development of ePHL requires at least two mutant alleles of the Ush1g and Cdh23 genes. Our results also suggest that because the SANS and CDH23 proteins form a complex in the stereocilia, the interaction between these proteins may play key roles in the maintenance of stereocilia and the prevention of ePHL.

Phenotypic and expression analysis of a novel spontaneous myosin VI null mutant mouse.

In humans, hearing is a major factor in quality of life. Mouse models are important tools for the discovery of genes responsible for genetic hearing loss, often enabling analysis of the processes that regulate the onset of deafness in humans. Thus far, at least 400 deafness mutants have been discovered in laboratory mouse populations and used in the study of deafness. Here we report the discovery of a new spontaneous recessive Rinshoken shaker/waltzer (rsv) mutant derived from our in-house C57BL/6J stock, which exhibits circling and/or head-tossing behaviour and complete lack of auditory brain response to any sound pressure. The hearing and balance phenotypes are associated with structural defects, in particular, disorganisation and fusion of stereocilia in the inner ear hair cells. Two sets of intersubspecific N(2) mice were generated for the positional cloning of the rsv mutation. The mutant locus was mapped to a 4.8-Mb region of chromosome 9, which contains myosin VI (Myo6), a gene responsible for deafness in humans and Snell's waltzer mutation in mice. The rsv mutant showed reduced expressions of Myo6 mRNA and MYO6 protein in the inner ear. Moreover, no immunoreactivity was observed in the cochlear and vestibular hair cells in the rsv mutant mice. We sequenced the genomic region (30,154 bp) of Myo6, including all coding exons, a non-coding exon, UTRs and the Myo6 promoter; however, no mutation was discovered in these regions. We therefore speculate that loss of MYO6 expression might cause shaker/waltzer behaviour and deafness in the rsv mutant; also, loss of MYO6 expression might be the result of mutations in an unidentified regulatory region(s) of the gene.