Stereocilia defects in waltzer (Cdh23), shaker1 (Myo7a) and double waltzer/shaker1 mutant mice.

Mutations in myosin VIIa (Myo7a) and cadherin 23 (Cdh23) cause deafness in shaker1 (sh1) and waltzer (v) mouse mutants respectively. In humans, mutations in these genes cause Usher's syndrome type 1B and D respectively, as well as certain forms of non-syndromic deafness. Examination of the organ of Corti from shaker1 and waltzer mice has shown that these genes are required for the proper organisation of hair cell stereocilia. Here we show that at embryonic day 18.5, the outer hair cells of Cdh23(v) homozygote mutant mice appear immature, projecting fewer recognisable stereocilia than heterozygote controls, and by post-natal day (P) 4 their stereocilia are arranged in a disorganised pattern rather than in the regular 'V'-shape seen in heterozygotes. Inner hair cell stereocilia are also disorganised in Cdh23(v) mutant homozygotes. Myo7a was expressed normally in the hair cells of P0 Cdh23(v2J) mutants demonstrating that cadherin 23 is not required for Myo7a expression at this stage. No stereocilia defects were observed in P4 Cdh23(v)/Myo7a(4626SB) double heterozygotes (+/Cdh23(v) +/Myo7a(4626SB)) and neither the Cdh23(v) nor Myo7a(4626SB) homozygote phenotypes were affected by the presence of one mutant copy of Myo7a or Cdh23 respectively. The hair cell phenotype of double homozygote mutant mice did not differ from single Myo7a(4626SB) homozygote mutants. Finally, we found no significant correlation between loss of hearing and double heterozygosity for mutations in Cdh23 and Myo7a in mice aged between 7.5 and 10 months. These findings suggest that Cdh23 and Myo7a are both required for establishing and/or maintaining the proper organisation of the stereocilia bundle and that they do not genetically interact to affect this process nor to cause age-related hearing loss.