Purpose: To describe the course of photoreceptor disease in canine X-linked retinal degeneration.
Methods: Retinas from 55 dogs (44 males, 8 carrier females, 3 homozygous females) were obtained by enucleation under general anesthesia. After fixation and dehydration, tissues were embedded in epoxy resin, sectioned at 1 microm for light microscopy and stained with azure II/methylene blue and a paraphenylenediamine counterstain. For electron microscopy, regions identified by light microscopy were selected and cut at 60 nm. Sections were stained with uranyl acetate-lead citrate. Electroretinography from an additional group of normal males, affected males, and carrier females was performed and the rod and cone responses evaluated.
Results: The earliest lesion detectable by electron microscopy was vesiculation of rod discs, followed by disruption of outer segments and death of rods. Loss of cones and progressive atrophy of inner retinal layers followed. Lesions were most severe in the peripheral retina and advanced toward the optic disc with disease progression. Significant variation in disease severity was present in males despite the presence of the same disease allele in all affected dogs. Carrier females displayed generalized reduction in photoreceptor density as well as multifocal areas of complete rod loss. The electroretinogram (ERG) findings were compatible with the histopathologic abnormalities. Homozygous females had lesions similar to those seen in affected males.
Conclusions: X-linked retinal degeneration is characterized by initial degeneration of rod photoreceptors, followed by loss of cones and progressive atrophy of the inner retina. Carrier females display a phenotype consistent with random X-chromosome inactivation. Variation in genetic background may alter expression of the disease allele in affected animals, thus accounting for variation in phenotypic expression of the disease.