Sheltie Progressive Retinal Atrophy (CNGA1-PRA)
Sheltie Progressive Retinal Atrophy is an inherited disorder that belongs in group of eye disorders known as progressive retinal atrophy. Progressive retinal atrophy (PRA) includes autosomal recessively inherited diseases that lead to degeneration of retinal photoreceptor cells in dogs and other pets. It is a canine equivalent to retinitis pigmentosa (RP), an inherited eye disorder affecting human beings. The first case of PRA was observed in England more than hundred years ago.
In general, PRA is characterized by disturbance of vision in dark, visual field defects, and abnormalities in the electroretinogram. It appears in both eyes simultaneously. The age of onset and rate of retinal degeneration varies between the different forms of the conditions. Some forms of PRA are common to multiple dog breeds, while others are recognized in just a single breed. Until now, it has been identified in more than 100 dog breeds. Almost all forms of PRA are inherited in a recessively manner, with exceptions in some breeds such as Old English Mastiffs, Bullmastiffs, Siberian Husky and Samoyed where forms of PRA are inherited as dominant or X-linked disorders.
There are two types of photoreceptors in the eye, rods and cones. Rods have an important role for vision in dim light and also night vision. PRA causes rode degeneration and in this way leads to night blindness. Researchers estimated that PRA causes death of around 95% of the dog’s photoreceptors. Dog owners or breeders recognize PRA by ”glow” or ”increased shine” in the eyes. The disease is progessive. Initial stage is characterized by night blindness, while advanced stage of PRA can cause full blindness in the dog. After the appearance of first symptoms, it usually takes one year for the disorder to progress from initial to the advanced stange, causing severe and irreverisble damage to the dog’s eye.
Sheltie Progressive Retinal Atrophy (CNGA1-PRA) is a form of PRA with an adult onset, with average of developing first symptoms at 5 years of age.
Sheltie Progressive Retinal Atrophy (CNGA1-PRA) is an inherited autosomal recessive disease. It is caused by a mutation in CNGA1 (cyclic nuceotide gated channel alpha 1) gene. This gene encodes for protein expressed in the outer segment of rod photoreceptors in the retina. The mutation is a 4 base pair deletion, which results in premature stop codon and expression of truncated protein.
Dog can be clear, carrier or affected. Carriers of the gene are heterozygous and do not develope the disease’s symptoms. When mating two carrier dogs, each future cub has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier.
Karlstam et al., A slowly progressive retinopathy in the Shetland Sheepdog. Vet Ophthalmol. 2011 Jul; 14(4):227-38. retinal degeneration called Slow Progressing Retinopathy ( SPR ) that progresses more slowly and does not cause such obvious visual impairment as does PRA.