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Progressive Retinal Atrophy (PRA-PRCD)
| Acronym: | PRCD, PRA-PRCD |
| Gene: | PRCD |
| Mutation: | c.5G>A |
| Inheritance: | Autosomal recessive |
| Sample type: | CHS (Cheek Swab), WBE (Whole Blood EDTA) |
Genetics and characteristics
PRA-PRCD is an eye disorder, which belongs to the progressive retinal atrophy group of disorders. Progressive retinal atrophy (PRA) comprises autosomal recessively inherited diseases that lead to degeneration of retinal photoreceptor cells in dogs and other animals. In general, these diseases are characterized by disturbance of night vision, visual field defects, and abnormalities in the electroretinogram, which can progress to blindness. It appears in both eyes simultaneously. The age of onset and rate of retinal degeneration varies between the different forms of the conditions. Some forms of PRA are common to multiple dog breeds, while others are recognized in just a single breed. PRA appears in most dog breeds, but also in mixed breed dogs. Almost all PRA disorders are recessively inherited, with exceptions of dominant and X-linked PRA inheritance in a few breeds, such as Old English Mastiffs, Bullmastiffs, Siberian Husky, and Samoyed.
Rods are important for vision in dim light, as well as night vision, and PRA with its rode degeneration leads to night blindness. It is estimated that PRA causes the death of around 95% of the dog’s photoreceptors. For dog owners or breeders, PRA is recognized by ”glow” or ”increased shine” in the eyes. The disease can progress, from the initial stage of night blindness to the advanced stage of PRA which causes secondary cataracts or full blindness in the dog. The advanced stage of PRA usually occurs within one year after the appearance of the first symptoms. After the death of rods, oxygen is still delivered to the dead rods, which they cannot use. This excessive oxygen is toxic to surrounding cells - it causes oxidative damage and, consequently, cone death. The death of retinal tissue causes the release of toxic cell products. They are absorbed by the lens, which causes lens damage and cataract development.
PRA-PRCD stands for progressive rod-cone degeneration. It has been identified in various breeds, and its development varies among breeds. Early-onset PRA-PRCD occurs during the postnatal retinal differentiation period, which is 2 to 6 weeks of age. It is characterized by rapid progression of retinal degeneration. In contrast to early-onset, late-onset PRA-PRCD first symptoms occur after normal development and maturation of the retina and can become clinically evident not until later in life. Progression of retinal degeneration is much slower than in early-onset PRA-PRCD.
The mutated gene was mapped to the centromeric region of canine chromosome 9 (CFA9). The researchers have discovered that an identical mutation causes PRA-PRCD in dogs and Retinitis Pigmentosa in humans. PRA-PRCD is inherited in an autosomal recessive pattern. In case of showing the signs of PRA-PRCD, the healthy parents of an affected cub 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. There is no cure for PRA-PRCD, and the only way to prevent it is to breed dogs that are not carriers of the mutation. Defected genes for autosomal recessive disease can be passed for many generations without affected individuals occurring until two carriers are bred to one another. The only way to find out if there is a chance of getting an affected puppy is to do PRA-PRCD genetic testing.
Results Reported As
Test Result |
Interpretation of test result |
CLEAR |
Tested mutation was not detected in animal with „clear“ result. Animal tested as clear has wild-type allele in homozygous state (i.e. two pairs of healthy alleles). It will not develop disease caused by tested mutation.* It will pass only wild-type allele to its offspring. |
CARRIER |
Tested mutation was detected in animal with „carrier“ result. Animal tested as carrier has one wild-type and one mutation allele, it is in heterozygous state. It will not develop disease caused by tested mutation.* It can pass wild-type or mutation allele to its offspring. |
AFFECTED |
Tested mutation was detected in animal with „affected“ result. Animal tested as affected has two copies of mutation alleles affecting the gene. It is likely the animal will experience a genetic disorder due to this mutation.** It will pass only mutation allele to its offspring. |
*Test excludes only tested mutation but not possible unknown mutations or factors that can lead to similar condition/symptoms.
** Potential unknown mutations or multiple other factors can possibly affect the likelihood of experiencing a genetic disorder.
References:
Dostal, J. (2011.): Progressive rod-cone degeneration (PRCD) in selected dog breeds and variability in its phenotypic expression. Veterinari Medicina, 56, 2011 (5): 243-247.
Miyadera, K. (2012): Genetic and phenotypic variations of inherited retinal diseases in dogs: the power of within- and across-breed studies. Mamm Genome, 23: 40-61.
Zangerl, B. (2006.): Identical Mutation in a Novel Retinal Gene Causes Progressive Rod-Cone Degeneration (prcd) in Dogs and Retinitis Pigmentosa in Man. Genomics 88 (5): 551-563.