Progressive Retinal Atrophy (rod-cone dysplasia type 4 - RCD4)

The Progressive Retinal Atrophye (PRA) in dogs is characterized by degeneration of the photoreceptor cells of the retina which causes progressive vision loss that culminates in total blindness. PRA disease affects more than 100 canine breeds and it is known to be genetically heterogeneous between the breeds.

The progressive atrophy of the retina in Gordon Setters is clinically indistinguishable from the PRA in other breeds. The onset of clinical signs is usually at the age of approx. 10 years. It is described a slow onset of the disease, when the affected dog developed night blindness in the first instance, which progressively leads to total blindness.

In 2012, Downs et al. described the mutation c.3149_3150insC in C17H2orf71 gene responsible for PRA-RCD4 disease (progressive retinal atrophy with late onset). The mutation is related with PRA-RCD4 in following breeds:

• Australian Cattle Dog
• English Setter
• Gordon Setter
• Irish Red & White Setter
• Irish Setter
• Polish Lowland Sheepdog
• Small Munsterlander
• Tibetan Terrier
• Standard Poodle

The mutation causes a shift in the reading frame of the gene leading to a premature stop codon and creation of defective protein affecting the function of the retina.
In 213, an early-onset form of PRA-RCD1 was identified in Irish Setter that is caused by the mutation in PDE6B gene (this test is also provided by Genomia).

Mutation that causes RCD4 is inherited as an autosomal recessive trait. That means the disease affects dogs with P/P (positive / positive) genotype only. The dogs with P/N (positive /negative) genotype are clinically without any symptom. They are genetically considered carriers of the disease (heterozygotes). In offspring of two heterozygous animals following genotype distribution can be expected: 25 % N/N (healthy non-carriers), 25 % P/P (affected), and 50 % N/P (healthy carriers). Because of high risk of producing affected offspring, mating of two N/P animals (carriers) can not be recommended.

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Reference: 
L. M. Downs et al.: Late-onset progressive retinal atrophy in the Gordon and Irish Setter breeds is associated with a frameshift mutation in C2orf71 , Animal Genetics 2012

Louise M. Downs, Rebekkah Hitti, Silvia Pregnolato and Cathryn S. Mellersh: Genetic screening for PRA-associated mutations in multiple dog breeds shows that PRA is heterogeneous within and between breeds; Veterinary Ophthalmology (2014) 17, 2, 126–130