XLPRA2 – X Linked Progressive retinal atrophy 2
X linked progressive retinal atrophy 2 (XLPRA2) is an eye disorder, which belongs in 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 pets. In general, these diseases are characterized by disturbance of dark 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 (XLPRA) in few breeds, such as Old English Mastiffs, Bullmastiffs, Siberian Husky and Samoyed.
Rods are important for vision in dim light, or also night vision and PRA with its rode degeneration leads to night blindness. In the researches, it is estimated that PRA causes 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 initial stage of night blindness to advanced stage of PRA which causes secondary cataracts or full blindness in the dog. The advanced stage of PRA usually occurs within one year, since the appearance of the first symptoms.
After the death of rods, oxygen is still being delivered to the dead roads, which they cannot use. This excessive oxygen is toxic and it causes oxidative damage and consequently, cone death. The death of retinal tissue causes release of toxic products of cells. They are being absorbed by the lens, which causes lens damage and also cataract development.
XLPRA2 stands for X linked progressive retinal atrophy 2. First was identified XLPRA, has now been renamed to XLPRA1, to be distinguishable from the XLPRA2. XLPRA2 can be characterized as a ”early-onset” form of PRA. The first symptoms occur already after 4 weeks of age. XLPRA2 symptoms are severe and start to manifest already during retinal development, in contrast to XLPRA1.
The disorder is caused by a deletion in RPGR gene. XLPRA is inherited as a X-linked recessive disorder, and as such, it differently affects males and females. Heterozygous female with one copy of mutated gene will not develop symptoms of the XLPRA and she is the carrier of the disorder. When mated with a healthy male, all female puppies will be healthy, but there are 50% chances they will be carriers for the disease. Among male puppies, chances are 50% that they will be healthy, and 50% that they will be affected and with time show the XLPRA2 symptoms.
There is no cure for XLPRA2 and only way to prevent it, is to breed dogs which 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 XLPRA2 genetic testing.
Zhang, Q., Acland, G.M., Wu, W.X., Johnson, J.L., Pearce-Kelling, S., Tulloch, B., Vervoort, R., Wright, A.F., and Aguirre, G.D. (2002). Different RPGR exon ORF15 mutations in Canids provide insights into photoreceptor cell degeneration. Hum. Mol. Genet. 11, 993–1003.