Progressive retinal Atrophy, golden retriever 2 (GR-PRA2) is a late-onset inherited eye disease affecting golden retrievers which is often accompanied by a host of other clinical signs (described below) resembling a condition in humans called Bardet-Biedl syndrome. Affected dogs begin showing clinical symptoms related to retinal degeneration at around 4 to 5 years of age on average, though age of onset can vary. Initial clinical signs of progressive retinal atrophy involve changes in reflectivity and appearance of a structure behind the Retina called the Tapetum that can be observed on a veterinary eye exam. Progression of the disease leads to thinning of the retinal blood vessels, signifying decreased blood flow to the retina. Affected dogs initially have vision loss in dim light (night blindness) and loss of peripheral vision, progressing to complete blindness in most affected dogs.
In addition to progressive retinal atrophy, affected dogs may show other clinical signs of variable severity including cataracts, obesity, an abnormally broad muzzle, flat forehead, poor sense of smell, short stature, overgrowth of gum tissue (gingival hyperplasia), increased distance between teeth, chronic kidney disease, heart valve degeneration, irregular heat cycles, small testicles, low sperm count, sperm cell defects, and decreased male libido. However, each affected dog may develop a unique combination and severity of clinical signs.
Genetic testing of the TTC8 gene will reliably determine whether a dog is a genetic Carrier of GR-PRA2. GR-PRA2 is inherited in an Autosomal Recessive manner in dogs meaning that they must receive two copies of the mutated gene (one from each parent) to develop the disease. In general, carrier dogs do not have features of the disease but when bred with another carrier of the same Mutation, there is a risk of having affected pups. Each pup that is born to this pairing has a 25% chance of inheriting the disease and a 50% chance of inheriting one copy and being a carrier of the TTC8 gene mutation. Reliable genetic testing is important for determining breeding practices. Because symptoms do not appear until adulthood, genetic testing should be performed before breeding. In order to eliminate this mutation from breeding lines and to avoid the potential of producing affected pups, breeding of known carriers to each other is not recommended. Dogs that are not found to have this specific mutation have no increased risk of having affected pups. However, because there are multiple types of progressive retinal Atrophy caused by mutations in other genes, a normal result in TTC8 does not exclude progressive retinal atrophy in a pedigree.
There may be other causes of this condition in dogs and a normal result does not exclude a different mutation in this gene or any other gene that may result in a similar genetic disease or trait.
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Downs LM, Wallin-Håkansson B, Bergström T, Mellersh CS. A novel mutation in TTC8 is associated with progressive retinal atrophy in the golden retriever. Canine Genet Epidemiol. 2014 Apr 16;1:4.
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