Progressive retinal Atrophy, cone-Rod dystrophy 4 (PRA-crd4) is an inherited eye disease affecting dogs. PRA-crd4 occurs as a result of degeneration of both rod and cone type Photoreceptor Cells of the Retina, which are important for vision in dim and bright light, respectively. Affected dogs can show symptoms of vision loss or have signs of retinal disease on veterinary ophthalmologic exam by 3 years of age. However, age of onset varies significantly in PRA-crd4 affected dogs, and has been reported from 1 to 15 years of age. Mutations in the RPGRIP1 gene show Incomplete Penetrance, meaning that not all dogs inheriting two copies of the Mutation develop clinical disease. This suggests that other unknown genetic or environmental factors may play a role in modifying disease development and progression. Although progression tends to be relatively slow, most affected dogs (especially those with an early age of onset) will progress to complete blindness.
Genetic testing of the RPGRIP1 gene will reliably determine whether a dog is a genetic Carrier of PRA-crd4. PRA-crd4 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 being a carrier of the RPGRIP1 gene mutation. However, this disease shows Incomplete Penetrance; some dogs inheriting two copies of the mutation do not develop symptoms of PRA. Reliable genetic testing is important for determining breeding practices. 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 carriers of the mutation have no increased risk of having affected pups. However, because there are multiple types of PRA caused by mutations in multiple genes, a normal result in RPGRIP1 does not exclude PRA 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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