Progressive retinal atrophy (PRA) is the most common form of inherited disease affecting the retina in dogs. Genetically different forms of PRA, caused by mutations in different genes, affect many breeds of dog with each form usually affecting one or a small number of breeds. PRA is characterised by progressive degeneration of the retina at the back of the eye and leads to vision loss and blindness.
This specific form of PRA is caused by a mutation in a gene called IMPG2 and is indistinguishable from other forms of PRA in other breeds. The average age of onset of clinical signs is around 7 years, but can be anything between 1 and 12 years. In humans, mutations in IMPG2 are associated with a condition known as Retinitis Pigmentosa, symptoms of which include loss of peripheral vision and the ability to see at night. There is no cure for this form of PRA, but using the DNA test to identify dogs that carry the mutation in IMPG2 will prevent further spread of this blinding condition in this lovely breed.
The LINE-1 insertion in the gene called IMPG2 that causes Progressive retinal atrophy in Lhasa Apso is recessive. This means that dogs that carry two copies of the mutation (homozygotes) will almost certainly develop PRA during their lives. Dogs that carry a single copy of the mutation (also known as carriers or heterozygotes) will not develop PRA as a result of the IMPG2 mutation, but they will pass the mutation onto about half of any offspring they have. Breeding dogs that will not develop PRA should be the breeder’s priority, with a reduction in mutation frequency within the whole breed being the secondary, longer-term target.
Carriers can be bred from safely, provided they are mated to a dog that has also been tested and is clear of the IMPG2 mutation (i.e. carry no copies of the mutation). If a carrier is mated to a clear dog approximately half of the resulting puppies will also be carriers, so should be tested themselves prior to breeding. Breeding carriers to tested, clear dogs is safe, in terms of avoiding dogs affected with PRA, and will help to maintain the genetic diversity of a breed. It is therefore encouraged, particularly in the first few generations following the availability of a new genetic test, so that other desirable characteristics and traits can be preserved before the frequency of the PRA mutation within the breed is gradually reduced.