Spontaneous allelic variant inUsh1gresulting in an expanded phenotype

Abstract

Strategies to reveal the discovery of the relationships between novel phenotypic behaviors and specific genetic alterations can be achieved via either target-specific, directed mutagenesis or phenotypic selection following random chemical mutagenesis. As an alternative approach, one can exploit deficiencies in DNA repair pathways that are responsible for the maintenance of genetic integrity in response to spontaneously-induced damage. In the genetic background of mice deficient in the DNA glycosylase NEIL1, elevated numbers of spontaneous mutations arise from translesion DNA synthesis past unrepaired, oxidatively-induced base damage. Several litters ofNeil1knockout mice included animals that were distinguished by their backwards-walking behavior in open-field environments, while maintaining frantic forward movements in their home cage environment. Other phenotypic manifestations included swim test failures, head tilting, and circling. Mapping of the mutation that conferred these behaviors revealed the introduction of a stop codon at amino acid 4 of theUsh1ggene; the allele wasUsh1gbw, reflecting the backwards-walking phenotype.Ush1gbw/bwnull mice displayed auditory and vestibular defects that are commonly seen with mutations affecting inner-ear hair-cell function, including a complete lack of auditory brainstem responses and vestibular-evoked potentials. As in other Usher syndrome type I mutant mouse lines, hair-cell phenotypes included disorganized and split hair bundles, as well as altered distribution of proteins for stereocilia that localize to the tips of row 1 or row 2. Disruption to the bundle and kinocilium displacement suggested that USH1G is essential for forming the hair cell’s kinocilial links. Due to the vestibular dysfunction, however, visual behavior as measured with optokinetic tracking could not be assessed inUsh1gbw/bwmice. Consistent with other Usher type 1 models, however,Ush1gbw/bwmice had no substantial retinal degeneration compared toUsh1gbw/+controls out to six months. In contrast to previously-describedUsh1galleles, this new allele provides the first knockout model for this gene.