Analysis of the rdd Mutant, a Pigmentary Retinopathy in Chickens

Abstract

Abstract: : Purpose: To map the locus responsible for the blind mutation rdd (retinal dysplasia and degeneration) in chickens and to further characterise the rdd phenotype. Methods: The eyes of blind and sighted birds were subjected to ophthalmic, morphometric and histopathological examination to confirm and extend published observations. Electroretinography was used to determine age of onset. Birds were crossed to create sibships suitable for genetic mapping. DNA samples were obtained and subjected to a linkage search. Results: Measurement of IOP, axial length, corneal diameter and eye weight revealed no gross morphological changes in the rdd eye. However on ophthalmic examination, rdd homozygotes have a sluggish pupillary response, atrophic pecten and widespread pigmentary disturbance which becomes more pronounced with age. Older birds also have posterior subcapsular cataracts. At three weeks of age homozygotes have a flat ERG indicating severe loss of visual function. Pathological examination shows thinning of the RPE, ONL, photoreceptors and INL and attenuation of the ganglion cell layer. The rdd mutation was shown to be sex-linked and not autosomal as previously described. Linkage analysis mapped the rdd locus to a small region of the chicken Z chromosome with homologies to human chromosomes 5q and 9p. Conclusions: Ophthalmic, histopathologic and electrophysiological observations suggest rdd is a pigmentary retinopathy similar to human recessive retinitis pigmentosa. Linkage mapping places rdd in a region homologous to human chromosomes 9p and 5q. Candidate disease genes/loci include PDE6A, WGN1 and USH2C.

Keywords: animal model • retinal degenerations: hereditary • linkage analysis