Identification of two major loci that suppress hearing loss and cochlear dysmorphogenesis in Eya1bor/bor mice

The Eya1(bor) mutant hypomorph contains an intracisternal A particle insertion in intron 7 of the Eya1 gene that results in a 50% reduction in wild-type mRNA levels. The homozygous mutants have middle and inner ear defects and variable kidney abnormalities. The severity of the disorder is affected by genetic background. In contrast to complete deafness and cochlear agenesis in the C3HeB/FeJ strain, F2 Eya1(bor/bor) mutants from an intercross between C3HeB/FeJ-Eya1(bor/+) and C57BL/6J showed variable auditory brain-stem responses and cochlear coiling. In this study, using these F2 Eya1(bor/bor) mutants, we have identified two major loci, Mead1 (modifier of Eya1-associated deafness 1) and Mead2, that are responsible for suppression of the original phenotypes. We have narrowed these two loci to 5.4 and 4.4 cM, respectively, in congenic lines. Quantitative PCR demonstrated that this modifying effect did not result from an increase in wild-type Eya1 mRNA, suggesting Mead1 and Mead2 are interacting directly or indirectly with Eya1 during inner ear development.