Progressive retinal atrophy in Papillons and Phalens (Pap-PRA 1)

The progressive retinal atrophy (PRA) is the most common cause of complete blindness both in humans and dogs. PRA is characterized by a very rapid loss of function of rods followed by a loss of cone function. The primary clinical sign is often vision impairment in a dim light and progressive loss of vision up to complete blindness. There are many forms of PRAs and many gene mutations causing PRA disease in a number of breeds have been described.

In 2013, Ahonen and its team published finding of causal indel mutation (1bp deletion followed by 6 bp insertions) in CNGB1 gene that is responsible for PRA in Papillon and Phalène breeds – this specific form of disease is known as Pap-PRA 1. In an occurrence study, 145 dogs of Papillon and Phalène breeds were tested and the occurrence of carriers was 17.2 %. This specific mutation was found neither in any of 334 healthy control dogs of 10 different breeds nor in 121 PRA-affected dogs of 44 different breeds (Ahonen at al., 2013).

The mutation causing Pap-PRA 1 is inherited as an autosomal recessive genetic disorder. The disease occurs in dogs that inherit the mutant gene from each of the parents. These dogs are designated P/P (positive homozygotes). The molecular-genetic test can be used to confirm the diagnosis. The carriers of the mutant gene designated N/P (heterozygotes) have only one changed gene from one parent and exhibit no clinical signs. However, they pass the mutation to their offsprings. A cross of two heterozygotes (N/P) will theoretically yield 25% of healthy offspring, 50% of the offspring will be carriers of the mutation and 25% of the offspring will inherit the mutant gene from their parents and will be affected by Pap-PRA 1.

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Reference:

Ahonen SJ, Arumilli M, Lohi H (2013) A CNGB1 Frameshift Mutation in Papillon and Phalène Dogs with Progressive Retinal Atrophy. PLoS ONE 8(8): e72122. doi:10.1371/journal.pone.0072122