Progressive Retinal Atrophy Golden Retriever Type 2 (GR-PRA2)
Progressive retinal atrophy Golden Retriever type 2 (GR2-PRA) is one form of progressive retinal atrophy (PRA) affecting Golden Retrievers. Until now, in Golden Retrievers there have been identified three different forms of PRA caused by 3 different mutations.
Progressive retinal atrophy (PRA) comprises autosomal recessively inherited diseases that lead to degeneration of retinal photoreceptor cells in dogs and other pets. There are more than 20 mutations responsible for specific forms of PRA, and some breeds, including Golden Retrievers, can be affected by more than only one form of PRA. Other than that, some forms of PRA are common to multiple dog breeds, while others are recognized in just a single breed. It has been identified in most dog breeds, but also in mixed breed dogs.
In general, forms of PRA are characterized by disturbance of dark vision, visual field defects, and abnormalities in the electroretinogram. The electroretinogram (ERG) is a diagnostic test used to measure the electrical activity of cells in the retina in response to a light stimulation. PRA is a progressive disorder and can progress into blindness. It appears in both eyes simultaneously. The age of onset and rate of retinal degeneration varies between the different forms of the conditions. Almost all PRA disorders are recessively inherited, with exceptions of dominant and X-linked PRA inheritance (XLPRA) in few breeds, such as Old English Mastiffs, Bullmastiffs, Siberian Husky and Samoyed.
Vision in dim light and also night vision is possible due to normal functioning rods. PRA causes rode degeneration, which leads to night blindness. Researches estimated that PRA causes death of around 95% of the dog’s photoreceptors. Dog owners or breeders can recognize PRA by ”glow” or ”increased shine” in the eyes. The disease is progressive. Initial stage of PRA is night blindness, while in severe cases advanced stage of PRA causes secondary cataracts or full blindness in the dog. The advanced stage of PRA usually occurs within one year, since the appearance of the first symptoms.
After the death of rods, oxygen is still being delivered to the dead roads, which they cannot use. This excessive oxygen is toxic and it causes oxidative damage and consequently, cone death. The death of retinal tissue causes release of toxic products of cells. They are being absorbed by the lens, which causes lens damage and also cataract development.
Progressive retinal atrophy Golden Retriever type 2 (GR2-PRA) is a late-onset form of PRA affecting the Labrador retriever breed. The clinical signs occur in affected dog around 5 years of age. As other forms of PRA, the first symptoms of progressive retinal atrophy Golden Retriever type 2 (GR2-PRA) are poor vision in dim light, which eventually progresses into complete blindness. As previously mentioned, GR2-PRA is one of 3 forms of PRA affecting Golden Retrievers. Other forms are GR2-PRA and PRA-PRCD, while mutation responsible for PRA-PRCD appears to be very rare among Golden Retriever breed.
Mutation responsible for GR2-PRA is a newly discovered deletion in TTC8 gene. Interestingly, mutation in human TTC8 gene has been associated with Retinitis Pigmentosa, human equivalent of PRA. The disorder is inherited in an autosomal recessive manner and it affects females and males equally. Considering the mutated gene, dog can be clear, carrier or affected. Heterozygotes are carriers and they have no symptoms. When mating two carriers, future cubs have a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being clear (unaffected and not a carrier).
There is no cure for GR2-PRA, and only way to prevent it, is to breed dogs which are not carriers of the mutation. Defected genes for autosomal recessive disease can be passed for many generations without affected individuals occurring until two carriers are bred to one another. The only way to find out if there is a chance of getting an affected puppy is to do progressive retinal atrophy Golden retriever type 2 genetic testing.
Downs LM, Wallin-Håkansson B, Boursnell M, , Bergström T, Mellersh CS. (2014.): A novel mutation in TTC8 is associated with progressive retinal atrophy in the golden retriever. Canine Genetics and Epidemiology 1:4