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Progressive Generalized Retinal Atrophy (CCDC66 gene, Schapendoes)

Progressive Generalized Retinal Atrophy (CCDC66 gene, Schapendoes)

Generalized 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. This condition, observed in the Schapendoes breed, presents a late onset and slow progression.

Symptoms

Progressive generalized retinal atrophy in Schapendoes manifests between 2 and 5 years of age, so the onset of the disease is late and the progression is gradual. The first clinical indicator is night blindness, resulting from degeneration of the rods responsible for peripheral vision. Affected dogs find it difficult to adapt to low-light environments. Over time, the condition progresses, leading to gradual loss of central vision due to cone degeneration, and eventually culminating in blindness. The involvement of both types of photoreceptors may extend over a period of up to 2 years.

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 variations in different genes identified in different breeds, sharing the characteristic of causing a gradual loss of vision that usually leads eventually to blindness. In the specific case of progressive generalized retinal atrophy in Schapendoes, the association with the CCDC66 gene, whose sequence is highly conserved in vertebrates, has been identified. The genetic variant detected involves the insertion of an adenine (c.521_522insA) in exon 6, generating a premature stop codon and giving rise to a truncated protein (p.Asn174LysfsX) prone to rapid degradation. This insertion mutation has not been found in other affected dogs from 29 different breeds, suggesting its unique presence in the Schapendoes breed, although larger studies are required to confirm this observation.

Most affected breeds

Schapendoes

Bibliography

Dekomien G, Vollrath C, Petrasch-Parwez E, et al. Progressive retinal atrophy in Schapendoes dogs: mutation of the newly identified CCDC66 gene. Neurogenetics. 2010 May;11(2):163-74.

Lippmann T, Jonkisz A, Dobosz T, et al. Haplotype-defined linkage region for gPRA in Schapendoes dogs. Mol Vis. 2007 Feb 7;13:174-80. PMID: 17327822; PMCID: PMC2533032.