Identification and Characterisation of New Mouse Models for Hearing Loss

Abstract

Mouse N-ethyl-N-Nitrosourea (ENU) mutagenesis programmes have been successfully employed for the identification of models of human disease, and allowed the discovery of novel gene associations. This thesis describes the characterization of two new ENU-induced mouse models of hearing loss, goya and Boycie, identified at the MRC Harwell Institute. The goya mice carry a splice site mutation in the mitogen-activated protein kinase Map3k1 gene. In addition to an eyes-open-at-birth phenotype, Map3k1goya/goya mice initially develop supernumerary cochlear outer hair cells (OHCs) that subsequently degenerate, and a progressive profound hearing loss is observed by 9-weeks of age. Interestingly, heterozygote mice also develop supernumerary OHCs, but no cellular degeneration or hearing loss is observed. The extra OHCs were found not to arise from aberrant control of proliferation via p27KIP1. Expression of MAP3K1 is present in a number of inner ear cell types (inner and outer hair cells, spiral ganglion and stria vascularis), and investigation of targets downstream of MAP3K1 identified increased p38 phosphorylation in these cells. The goya mutant reveals a signalling molecule involved with hair cell development and survival. Boycie mice were identified as exhibiting a high frequency progressive hearing loss with a semi-dominant mode of inheritance. Ultrastructural and morphological assessment of these mice identified interesting pathological findings, including: a consistent pattern of OHC loss in the basal and mid cochlear turns of both heterozygote and homozygote mice; thinning of the stria vascularis in similar cochlear regions; and, spiral ganglion neuron degeneration in the apical turn. The Boycie mutation was mapped to a ~3 Mb candidate interval on Chromosome 5 containing only one ENU-induced coding lesion, which is in the Orai1 gene that codes for a component of the calcium activated calcium entry (CRAC) channel. These two novel mouse models highlight the important role for ENU mutagenesis in disease gene discovery.