Ames Waltzer deaf mice have reduced electroretinogram amplitudes and complex alternative splicing of Pcdh15 transcripts.
PURPOSE
Mutations of PCDH15, the gene encoding protocadherin 15, cause either nonsyndromic deafness DFNB23 or Usher syndrome type 1F (USH1F) in humans and deafness with balance problems in Ames waltzer (av) mice. Persons with USH1 usually begin to exhibit signs of retinitis pigmentosa (RP) in early adolescence, but av mice are reported to have functional retinas. In this study, the auditory, visual and molecular biological phenotype of Pcdh15av-5J and Pcdh15av-Jfb mice is characterized, and their usefulness as animal models of USH1 is evaluated.


METHODS
Hearing thresholds of mice between 6 and 10 weeks of age were measured by auditory brain stem response (ABR). Immunohistochemistry and histology were used to examine the effect of homozygosity of Pcdh15av-5J on stereocilia bundles of inner ear hair cells and on the photoreceptor cells of the retina. Scotopic and photopic Ganzfeld ERGs were recorded from homozygous Pcdh15av-5J and Pcdh15av-Jfb mice at different ages. Heterozygous littermates served as control subjects. Measurements of the width of the outer nuclear layer (ONL) and the length of rod photoreceptor outer segment (ROS) were made.


RESULTS
Homozygous Pcdh15av-5J mice have profound hearing loss and disorganized stereocilia bundles of inner ear hair cells. Compared with heterozygous littermates, homozygous Pcdh15av-5J and Pcdh15av-Jfb mutant mice had scotopic ERG amplitudes consistently reduced by approximately 40% at all light intensities. The b-to-a-wave ratio confirmed that the a- and b-waves were reduced proportionally in homozygous mutant mice. Histologic measurements of retinal sections revealed no significant differences in either the ONL width or the ROS length as a function of genotype. The protocadherin 15 labeling pattern with antisera PB303 in the retina of both heterozygous and homozygous Pcdh15av-5J mice was indistinguishable from the wild type. Wild-type Pcdh15 have many alternatively spliced isoforms. A novel isoform was found in the retina of homozygous Pcdh15av-5J mice, which appears to circumvent the effect of the mutant allele (IVS14-2A-->G), which causes skipping of exon 14, a shift in the translation reading frame and a premature stop codon in exon 15.


CONCLUSIONS
Pcdh15(av-5J) and Pcdh15(av-Jfb) mice do not faithfully mimic the RP found in USH1 due to mutations of PCDH15, but have significantly attenuated ERG function in the absence of histologic change. The decline in ERG amplitude with a preserved b-to-a-wave ratio suggests a role for Pcdh15 in retinal function and/or generation of the ERG potentials. Understanding the molecular mechanism by which av mice circumvent degeneration of the retina might offer insights into potential therapies for USH1.