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.
An X-linked form of PRA (Type A PRA) was identified in the Miniature Schnauzer, but is very rare and doesn’t account for most cases of PRA. The more common PRA in the Miniature Schnauzer, Type B PRA detected by this test, is early onset with an average age at diagnosis of approximately 4 years. It is autosomal recessive and is characterised by abnormal development of the rod and cone photoreceptor cells of the retina, followed by a rapid progression of degeneration.
The intronic single nucleotide variants (SNV) in the gene called HIVEP3 that causes Progressive Retinal Atrophy in Miniature Schnauzer is autosomal recessive. This means that dogs that carry two copies of the mutation (homozygotes) will almost certainly develop Progressive Retinal Atrophy during their lives. Dogs that carry a single copy of the mutation (also known as carriers or heterozygotes) will not develop Progressive Retinal Atrophy as a result of the HIVEP3 mutation, but they will pass the mutation onto about half of any offspring they have. Breeding dogs that will not develop Progressive Retinal Atrophy 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 HIVEP3 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 Progressive Retinal Atrophy, 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 disease mutation within the breed is gradually reduced.