May 8, 2002 ITHACA, N.Y. -- Cornell University researchers say the discovery of the two different mutations for X-linked progressive retinal atrophy (XLPRA1 and XLPRA2) in dogs, as reported in the May 1, 2002, issue of Human Molecular Genetics (Vol. 11, No. 9), provides a new animal "model" for studying causes and testing treatments for inherited human blindness.
Dogs' health and well-being will benefit, too, according to scientists at the Baker Institute for Animal Health in Cornell's College of Veterinary Medicine, because new gene-screening tests will help detect disease-prone animals in breeding programs.
The journal report, "DifferentRPGR exon ORF15 Mutations inCanids Provide Insights Into Photoreceptor Cell Degeneration," is the work of Qi Zhang, Gregory M. Acland, Wen X. Wu, Jennifer L. Johnson, Sue Pearce-Kelling and Gustavo D. Aguirre, all at Cornell; Brian Tulloch and Alan F. Wright at the MRC Human Genetics Unit of Western General Hospital in Edinburgh, Scotland; and Raf Vervoort at the University of Leuven in Belgium.
Both XLPRA1 and XLPRA2 are part of the family of inherited retinal degenerations in dogs, called progressive retinal atrophy (PRA). PRA is the equivalent of retinitis pigmentosa (RP) in humans, and X-linked RP is one of the major causes of human retinal blindness.
The potential for X-linked diseases is genetically transmitted, from one generation to the next, by female organisms (which have two X chromosomes, while males have one X and one Y chromosome). Animals -- either human or canine -- with the potential for X-linked disease are considered "normal" if they have none of the defective genes. Females with one defective gene are "carriers" and can transmit potential for the disease. "Affected" individuals have only the disease gene and no normal gene.
The disease XLPRA1 is present, veterinary researchers say, in both the Siberian husky and Samoyed breeds, and the mutations are the same, suggesting that the mutation may have been present in a common canine ancestor before the two breeds developed. Both the canine and human inherited vision disorders involve the loss of photoreceptor cells in the eye's retina responsible for black-and-white (rods) and color (cones) vision. The genetic defect identified by Cornell researchers in the recent study is a result of a complex mutation pattern in the RPGR gene, which compromises the function and viability of the photoreceptor cells. Since this mutation pattern in the RPGR gene is the same in humans and dogs, the study provides new information on the mechanism of disease that exists in human patients, with RP3, the most prevalent form of X-linked retinitis pigmentosa.
"This research holds promise for dogs and humans alike," says Cornell's Aguirre. "Discoveries like this one show the value of the canine model and of comparative genomic analysis to aid in our understanding of gene function and disease, which are of importance to humans and other species."
According to a recent report by the National Eye Institute, a primary funder of the study, "the number of blind or visually impaired Americans is likely to double over the next 30 years as the giant baby boom generation ages." Many of these patients will suffer from macular degeneration, the leading cause of blindness in the aging population. Others, however, will suffer from RP, of which XLRP is one of the most common forms of the disease. Funding for the Cornell research team also was provided by the Foundation Fighting Blindness and the Van Sloun Fund for Canine Genetic Research.
"This disease has very severe and devastating consequences in people as in dogs," says Aguirre. "Having a model that so closely mimics human retinal disease will allow us not only to study the mechanism of the disease, but also explore gene therapy as a cure in both human patients and dogs."
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