Canine Disease Profile – PRA-Type-1 Papillons
Written by Dr. David Silversides, DVM
Progressive Retinal Atrophy (PRA) refers to a group of genetic diseases affecting the retina and involving the progressive degeneration of the rod and cone light receptor cells.
Rod receptor cells detect weak light and are useful for night vision as well as for peripheral vision. Cone receptor cells detect colour vision and are useful for day vision. PRA-Type 1 of the Papillon is a recessive genetic disease of the retina where clinical signs first appear around the age of 4 to 6 years.
Clinical signs of PRA-Type-1
Because the rod receptor cells are the first to be affected, dogs that are double mutant (M/M) will start to lose their night and peripheral vision. The development of the disease is slow but progressive; eventually the cone receptor cells are affected and dogs now begin to lose their day vision. An affected dog can retain limited day vision for up to several years, but the final outcome is usually blindness.
PRA-Type-1, a new mutation
Papillons are a small spaniel described by the American Kennel Club as friendly, alert and happy. Although Papillons are an old breed, PRA-Type-1 of Papillons is a newly recognized disease, first described in the scientific literature in 1995.
It is an example of a genetic disease seen only in one breed. Genetically, PRA-Type-1 behaves as a typical autosomal recessive genetic disease, where dogs with the disease are M/M double mutant, and dogs without the disease are either N/N clear or M/N carrier.
Proactive Papillon Breed Clubs
PRA-Type-1 is caused by a rather complicated mutation within the CNGB1 gene that is a combination of both a deletion and an insertion and which was described in 2013. The Papillon breed clubs in North America are particularly active in trying to remove the mutation from their breeding stock.
At Labgenvet, we perform our tests by DNA sequencing, making the N/N, M/N and M/M animals easy to identify. Since 2016, Labgenvet has tested over 80 samples for this disease sent in by Papillon breeders.
DNA Sequencing Profiles and Frequencies
Here are representative DNA sequencing profiles and the mutation frequencies seen at Labgenvet:
Although the probability is low of having an animal that is M/M double mutant at risk of having the retinal degeneration, the probability of having an animal that is M/N carrier is actually quite high. These probabilities will be reduced over time as the Papillon breeders continue to use the DNA test to help them with their mating decisions.
For additional genetic information and references regarding PRA-Type1 (Papillons), please refer to the following link: https://labgenvet.ca/en/disease/pra-type-i-papillon-type/.
A comprehensive list of simple genetic diseases in dogs can be found at: https://labgenvet.ca/en/dog-genetic-disease-search/.
© 2018 David W. Silversides