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Progressive retinal atrophy (TTC8 gene, Golden Retriever)

Progressive retinal atrophy (TTC8 gene, Golden Retriever)

Progressive retinal atrophy is characterized by a gradual loss of vision due to degeneration of the photoreceptor cells (rods and cones), with a tendency to progress to blindness.

Symptoms

Progressive retinal atrophy is a heterogeneous disorder that usually manifests around the age of 5 years, showing a late onset of the disease. The first clinical sign observed is night blindness, caused by degeneration of the rods responsible for peripheral vision. Affected dogs experience difficulty adapting to low-light environments. Over time, the condition progresses, resulting in the gradual loss of central vision due to degeneration of the cones, and eventually culminating in blindness. This process of retinal degeneration is bilateral and progressive. In addition to visual problems, affected dogs show clinical homology with Bardet-Biedl syndrome in humans. Associated symptoms include renal and dental defects, sperm problems, short stature, anosmia and obesity.

Disease Management

The management of progressive retinal atrophy in dogs aims to improve the quality of life of the affected dog, even though there is no curative treatment for this disease. However, measures can be taken to ensure a suitable environment, such as keeping furniture and objects in place, which facilitates the dog`s adaptation to the disease.

Genetic basis

This disease follows an autosomal recessive mode of inheritance. Autosomal recessive inheritance means that the dog, regardless of sex, must receive two copies of the mutation or pathogenic variant to be at risk of developing the disease. Both parents of an affected dog must carry at least one copy of the mutation. Animals with only one copy of the mutation are not at increased risk of developing the disease, but may pass the mutation on to future generations. Breeding between dogs carrying genetic variants that can cause disease, even if they do not show symptoms, is not recommended.

Technical report

Progressive retinal atrophy encompasses a group of diseases caused by genetic variations identified in different breeds, all sharing the characteristic of causing a gradual loss of vision that usually culminates in blindness. Although dogs of the same breed are expected to share the same variant causing this condition, in the case of the Golden Retriever two variants in different genes have been identified as possible triggers of progressive retinal atrophy. In this case, the c.669delA variant in the TTC8 gene, a component of the BBSome protein complex related to primary cilia homeostasis and the development of Bardet-Biedl syndrome, is analyzed. The variant consists of a deletion that induces a change in the reading frame, resulting in loss of function of the resulting protein. This mutation accounts for approximately 30% of cases of progressive retinal atrophy in Golden Retrievers.

Most affected breeds

Golden Retriever

Bibliography

Downs LM, Wallin-Håkansson B, Bergström T, et al. A novel mutation in TTC8 is associated with progressive retinal atrophy in the golden retriever. Canine Genet Epidemiol. 2014 Apr 16;1:4.

Mäkeläinen S, Hellsand M, van der Heiden AD, et al. Deletion in the Bardet-Biedl Syndrome Gene TTC8 Results in a Syndromic Retinal Degeneration in Dogs. Genes (Basel). 2020 Sep 18;11(9):1090.