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.
Breed-Specific Information for the Field Spaniel
The Mutation of the RPGRIP1 gene associated with progressive retinal Atrophy, cone-Rod dystrophy 4 has been identified in Field Spaniels, although its overall frequency in this breed is unknown. It is also unknown if Field Spaniels inheriting two copies of this mutation develop the clinical signs of progressive retinal atrophy, cone-rod dystrophy 4.
Genetic testing of the RPGRIP1 gene in Field Spaniels 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. Field Spaniels 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.
Busse C, Barnett KC, Mellersh CS, Adams VJ. Ophthalmic and cone derived electrodiagnostic findings in outbred Miniature Long-haired Dachshunds homozygous for a RPGRIP1 mutation. Vet Ophthalmol. 2011 May;14(3):146-52. [Pubmed:21521437]
Mellersh CS, Boursnell ME, Pettitt L, Ryder EJ, Holmes NG, Grafham D, Forman OP, Sampson J, Barnett KC, Blanton S, Binns MM, Vaudin M. Canine RPGRIP1 mutation establishes cone-rod dystrophy in miniature longhaired dachshunds as a homologue of human Leber congenital amaurosis. Genomics. 2006 Sep; 88(3):293-301.
Miyadera K, Kato K, Aguirre-Hernández J, Tokuriki T, Morimoto K, Busse C, Barnett K, Holmes N, Ogawa H, Sasaki N, Mellersh CS, Sargan DR. Phenotypic variation and genotype-phenotype discordance in canine cone-rod dystrophy with an RPGRIP1 mutation. Mol Vis. 2009 Nov 11; 15:2287-305.
Narfstrom K, Jeong M, Hyman J, Madsen RW, Bergstrom TF. Assessment of hereditary retinal degeneration in the English springer spaniel dog and disease relationship to an RPGRIP1 mutation. Stem Cells Int. 2012;2012:685901.