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Progressive Retinal Atrophy (Swedish Vallhund Type)

Progressive Retinal Atrophy (PRA) in Swedish Vallhunds is a genetic eye condition leading to the degeneration of retinal photoreceptors and eventual blindness.

Affected Genes: MERTK

Inheritance: Autosomal Recessive

Variant(canFam6):
chr17:36107888-36107889: 6 kb LINE insertion with duplication of 16 bp AAAATATTTTCTTTC

Breed: Swedish Vallhund

General Information: Progressive Retinal Atrophy (PRA) in Swedish Vallhunds is a devastating inherited condition characterized by the gradual deterioration of both rod and cone photoreceptor cells in the retina, crucial for vision in various lighting conditions. Signs of the disease can be detected as early as 2 months of age during veterinary eye exams, although noticeable vision loss typically does not manifest until the dog is between 1 to 12 years old. Initially, affected dogs may experience night blindness due to the early loss of rod cell function, followed by the decline in cone cell function, leading to complete vision loss, potentially as early as 9 years of age. This condition is marked by observable changes in the reflectivity and appearance of the tapetum—a structure behind the retina—during eye exams.

How to Read Your Dog's Test Results for this Genetic Variant:

Two Variants Detected: Dog Likely Affected

One Variant Detected: Dog Unlikely Affected

No Variants Detected: No Effect

Gene / Testing Information: Genetic testing for Progressive Retinal Atrophy (PRA) in Swedish Vallhunds targets the MERTK gene, which can identify carriers of this autosomal recessive disease. Dogs must inherit two mutated genes, one from each parent, to exhibit symptoms of PRA, while carriers with only one copy do not show signs of the disease. Breeding decisions should be informed by genetic testing to avoid mating carriers, which poses a 25% risk per offspring inheriting the disease and a 50% chance of being a carrier. Since symptoms often appear later in life, pre-breeding genetic screening is crucial to prevent the propagation of this mutation within the breed. To reduce the incidence of PRA, it is advised not to breed carriers together. Non-carrier dogs are at no risk of producing affected offspring. It is also important to note that a normal result in the MERTK gene does not exclude the possibility of PRA due to other genetic variations, and there are instances of dogs with two copies of the mutation not developing PRA, suggesting possible variable expressivity or incomplete penetrance of this gene.

References:
Ahonen S. Canine Models of Human Vision Disorders: Identification of New Loci and Genes for Glaucoma and Retinal Degeneration [published doctoral dissertation]. 2014. Univ. of Helsinki. http://urn.fi/URN:ISBN:978-951-51-0303-1

Everson R, Pettit L, Forman OP, Dower-Tylee O, McLaughlin B, Ahonen S, Kaukonen M, Komaromy AM, Lohi H, Mellersh CS, Sansom J, Ricketts SL. An intronic LINE-1 insertion is MERTK is strongly associated with retinopathy in Swedish Vallhund dogs. PLoS One 2017 12(8):e0183021.