Progressive Retinal Atrophy Papillon & Phalène Type (PAP-PRA 1)
Progressive retinal atrophy Papillon and Phalène type (PAP-PRA) is a form PRA disorder. PRA stands for progressive retinal athrophy, a group of inherted eye disorders. Progressive retinal atrophy is characterized by a progressive rod degeneration, followed by a cone degeneration. PRA is a leading cause of blindness among dogs and shares similar symptoms to retinis pigmentosa, an eye disease in humans which is also the leading cause of blidness among humans. There are many different forms of PRA and it has been identified in number of dog breeds. Some forms of PRA are specific for a certain breed. PAP-PRA is such form of PRA, specific for the Papillon breed.
According to FCI, Papillon breed is separated into two different breeds, Papillon and Phalène. Their segregation is based on morphological features. The American Kennel Club recognizes both breeds simply as Papillon. Based on same origin, Papillon and Phalène share genetic cause of PRA.
Characteristics and Symptoms
Most typical form of PRA causes primary rod degeneration. Rods are light sensitive photoreceptors and their degeneration results in night blindness. PRA appears graduadly and is of progressive nature. It can progress from beggining stage of night blindness until severe form of PRA which includes complete blindness. Onset of clinical symptoms in affected Papillom and Phalène dogs is at 5 to 6 years of age. Examination of affected dogs with electroretinogram (ERG) reveals primary loss of the rod photoreceptor cells, which is afterwards followed by cone cells degradation.
Progressive retinal atrophy Papillon and Phalène type (PAP-PRA) is caused by a frameshit mutation in the cyclic nucleotide gated channel beta 1 (CNGB1) gene. Research revealed high percentage of the affected gene carriers. Among 145 tested Papillon and Phalene dogs, occurrence of carriers was 17.2%. PAP-PRA is inherited in an autosomal recessive pattern. Healthy parents of an affected dog are obligate heterozygotes, and therefore carry one mutant allele. Heterozygotes have no symptoms. At conception, each cub has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier.