Progressive hearing loss and increased susceptibility to noise-induced hearing loss in mice carrying a Cdh23 but not a Myo7a mutation.

Exposure to intense noise can damage the stereocilia of sensory hair cells in the inner ear. Since stereocilia play a vital role in the transduction of sound from a mechanical stimulus into an electrical one, this pathology is thought to contribute to noise-induced hearing loss. Mice homozygous for null mutations in either the myosin VIIa ( Myo7a) or cadherin 23 ( Cdh23) genes are deaf and have disorganized stereocilia bundles. We show that mice heterozygous for a presumed null allele of Cdh23 ( Cdh23(v)) have low- and high-frequency hearing loss at 5-6 weeks of age, the high-frequency component of which worsens with increasing age. We also show that noise-induced hearing loss in 11-12-week-old Cdh23(v) heterozygotes is two times greater than for wild-type littermates. Interestingly, these effects are dependent upon the genetic background on which the Cdh23(v) mutation is carried. Noise-induced hearing loss in 11-12-week-old mice heterozygous for a null allele of Myo7a ( Myo7a(4626SB)) is not significantly different from wild-type littermates. CDH23 is the first gene known to cause deafness in the human population to be linked with predisposition to noise-induced hearing loss.

Cdh23 mutations in the mouse are associated with retinal dysfunction but not retinal degeneration.

Mutations in the cadherin 23 gene (CDH23) cause Usher syndrome type 1D in humans, a disease that results in retinitis pigmentosa and deafness. Cdh23 is also mutated in the waltzer mouse. In order to determine if the retina of the waltzer mouse undergoes retinal degeneration and to gain insight into the function of cadherin 23 in the retina, we have characterized the anatomy and physiology of retinas of waltzer mouse mutants. Three mutant alleles of Cdh23 were examined by histology and electroretinography (ERG). ERGs of the three Cdh23 mutant groups revealed two of them to have abnormal retinal function. One allele had a- and b-waves that were only approximately 80% of Cdh23 heterozygotes. Another allele had a significantly faster implicit time for both the a- and b-waves of the ERG. No anatomical abnormality was detected in any of the Cdh23 mutants by light microscopy. Because the mutant Cdh23 phenotype was found to be similar to the previously reported retinal phenotype of Myo7a mutant mice, the orthologue of another Usher syndrome (type 1B) gene, we generated mice that carried mutations in both genes to test for genetic interaction in the retina. No functional interaction between cadherin 23 and myosin VIIa was detected by either microscopy or ERG.

Defects in whirlin, a PDZ domain molecule involved in stereocilia elongation, cause deafness in the whirler mouse and families with DFNB31.

The whirler mouse mutant (wi) does not respond to sound stimuli, and detailed ultrastructural analysis of sensory hair cells in the organ of Corti of the inner ear indicates that the whirler gene encodes a protein involved in the elongation and maintenance of stereocilia in both inner hair cells (IHCs) and outer hair cells (OHCs). BAC-mediated transgene correction of the mouse phenotype and mutation analysis identified the causative gene as encoding a novel PDZ protein called whirlin. The gene encoding whirlin also underlies the human autosomal recessive deafness locus DFNB31. In the mouse cochlea, whirlin is expressed in the sensory IHC and OHC stereocilia. Our findings suggest that this novel PDZ domain-containing molecule acts as an organizer of submembranous molecular complexes that control the coordinated actin polymerization and membrane growth of stereocilia.

Elongation of hair cell stereocilia is defective in the mouse mutant whirler.

The recessive mouse mutant whirler (wi) shows no response to sound and exhibits circling and head-tossing behaviour, indicative of both auditory and vestibular dysfunction. The wi mutation maps genetically to mouse chromosome 4. We examined the organ of Corti of whirler mutants to explore the possibility that the wi mutation affects sensory hair cells. Scanning electron microscopy (SEM) reveals that the specialised microvilli (stereocilia) that are projected by the sensory hair cells and are vital for sound transduction are abnormal in wi homozygotes. Specifically, wi homozygous inner hair cell (IHC) stereocilia are approximately half the length of equivalent stereocilia in heterozygous littermates. They are arranged normally into ranks, but the gradation in height and width of stereocilia in adjacent ranks is less prominent in wi homozygotes. Analysis of IHC stereocilia during the course of their development shows that, by embryonic day 18.5, mutant stereocilia are already significantly shorter than those in controls. Mutant stereocilia elongate at a normal rate, at least until postnatal day 1, but prematurely stop elongating between postnatal days 1 and 4. Stereocilia length then decreases. At postnatal day 15, outer hair cell (OHC) stereocilia in wi homozygotes appear short and are arranged in a rounded, "U" shape rather than the normal "W" or "V" shape. Eventually, both IHCs and OHCs degenerate. We show that the whirler locus encodes a protein(s) required for the elongation and maintenance of IHC and OHC stereocilia.

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.