Progressive Retinal Atrophy (PRA) is a generalized disorder in which the cells in the retina of a dog degenerate and die, in most cases eventually leading to complete blindness. PRA is the dog equivalent of Retinitis Pigmentosa in humans. Different forms of PRA have been reported in over 100 dog breeds and can be genetically heterogeneous both between and within breeds.
The retina is a membrane located in the back of the eye that contains two types of cells known as photoreceptors. These cells take light coming into the eyes and relay it back to the brain as electrical impulses. These impulses are interpreted by the brain as vision. In dogs suffering from PRA, the photoreceptors begin to degenerate, causing an inability to interpret changes in light resulting in a loss of vision. Rod cells, which normally function in low-light environments, begin to degenerate first. This leads to night-blindness. The cone cells, which normally function in bright-light or daytime conditions, will deteriorate next. This often leads to complete blindness over time.
A deletion in the DNA code of the CNGA1 gene (which produces an important transport protein) is associated with the Shetland Sheepdog form of PRA (PRA-CNGA1). Not all PRA diseases in Shelties are caused by this mutation. Additional forms of PRA are present in Shelties.
Because Shetland Sheepdog PRA is a recessive disorder, a dog must have two copies of the mutation in order for the disease to manifest. This means that a dog can have one copy of the mutation and not experience any signs or symptoms of PRA-CNGA1. This dog would be known as a carrier. The carrier can then pass on either the normal gene or the mutated gene to any offspring. If two carriers are bred, there is a 25% per puppy that they will develop symptoms of PRA-CNGA1.
Wiik AC, Ropstad EO, Ekesten B, Karlstam L, Wade CM, Lingaas F. Progressive retinal atrophy in Shetland sheepdog is associated with a mutation in the CNGA1 gene. Anim Genet. 2015 Oct;46(5):515-21. [PubMed: 26202106]
|P/P||Affected: Dog has two copies of the CNGA1-PRA mutation and will be affected. The mutation will always be passed on to every offspring.|
|n/P||Carrier: Dog carries one copy of the mutation associated with CNGA1-PRA. Dog will not be affected by CNGA1-PRA but may pass the mutation to offspring.|
|n/n||Clear: Dog is negative for the mutation associated with CNGA1-PRA.|