Two groups of researchers collaborating on a map of the canine genome have discovered the probable genetic correlation between the most widespread cause of inherited blindness in dogs and a similar human disease.
The scientists, working at Cornell University's James A. Baker Institute for Animal Health and the Fred Hutchinson Cancer Research Center in Seattle, report their findings in Proceedings of the National Academy of Sciences published tomorrow (March 17). They say that the genetic defect responsible for progressive rod-cone degeneration (called prcd), a form of progressive retinal atrophy known to cause blindness in at least five dog breeds, appears to be the canine version of the human gene defect producing RP17, one of the numerous forms of retinitis pigmentosa, a leading cause of familial blindness.
Before this latest research, no association had been suspected between RP17 and prcd. Now the researchers consider it likely that RP17 and prcd spring from mutations in corresponding genes in humans and dogs.
"If this is true, the identification of the prcd gene may lead not only to an unequivocal diagnostic test for dogs but also to gene therapy methods for prcd in dogs -- which eventually may be applicable to human RP17 patients," says veterinary ophthalmologist and geneticist Gregory Acland, the lead author of the report and a senior research associate in the laboratory of Gustavo Aguirre at the Baker Institute, a unit of Cornell's College of Veterinary Medicine.
"This is an important finding for the field of canine genetics as well, since it represents the first use of the map for identifying a disease locus," says Elaine Ostrander, a molecular biologist in Hutchinson's clinical research division and a co-author of the report.
"It is exciting that we now have a road map and have identified signposts indicating where to look for the defective gene causing prcd, a possibility that did not exist before," adds co-author Kunal Ray, a molecular geneticist at Cornell.
Recent advances in genetic therapy may allow doctors to treat retinitis pigmentosa (as well as rod-cone degeneration in dogs) by cloning the normal gene, inserting a copy into a transfer vector such as a harmless virus and injecting the gene-carrying vector into the back of the eye. The transfer of the normal gene to diseased retinal cells might prevent or reverse the disease.
Aguirre, Acland and Ray form the core of the Cornell team that has been responsible for most of the research to date on the underlying causes of canine progressive retinal atrophy (PRA), a group of at least seven clinically similar but separately inherited retinal diseases that Aguirre has studied since 1972. Most of the 60 or more breeds that can develop PRA are suspected of having prcd, a disease that causes dogs born with normal vision to develop to night blindness and then total blindness. The disease only becomes evident after dogs reach reproductive age.
Work to develop diagnostic tests for prcd is already underway at Cornell. The first beneficiaries of a test will be breeders and owners of the five dog breeds that have been shown through interbreeding to inherit the same defect: Labrador retrievers, Portuguese water dogs, poodles, and English and American cocker spaniels.
"To understand the magnitude of the problem," Acland says, "consider this: With an estimated affected rate of two to three percent, about 3,200 of the nearly 160,000 Labrador retriever puppies registered with the American Kennel Club in 1997 can be expected to go blind in adulthood."
Other breeds suspected of inheriting prcd, and that stand to benefit from the discovery, include Nova Scotia duck tolling retrievers, Australian cattle dogs, basenjis, English mastiffs, Chesapeake Bay retrievers, Italian greyhounds, papillons, and potentially many others.
Acland says: "I really think that there's a very good chance that prcd is widespread among a host of other breeds that are not even suspected at this point, We see a disease in mastiffs, for example, and Italian greyhounds that looks just like prcd.
If the disease in two such unrelated breeds turns out to be prcd , he says, that will suggest strongly that prcd is an old mutation that may have descended through hundreds of breeds. "While some affected breeds have only separated in the past one or two hundred years, it has been a long time since mastiffs and Italian greyhounds separated into unconnected populations," Acland notes.
Also reporting the findings are Cathryn Mellersh of the Hutchinson Center and Amelia Langston, formerly at Hutchinson and now an assistant professor at Emory University.
The prcd study at Cornell University was supported by the Morris Animal Foundation, Seeing Eye, the National Eye Institute and the Foundation for Fighting Blindness. Ostrander's work was supported by a grant from the Canine Health Foundation of the American Kennel Club.