Cone Rode Dystrophy 2 (CRD2)
Cone Rode Dystrophy 2 (CRD2) is a genetic eye disorder affecting the Pit Bull Terriers. CRD2 is a part of a wider group of disorders, the hereditary retinal degenerations or progressive retinal atrophies, whose various forms are known to affect numerous different dog breeds. In general, these diseases are characterized by the 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. CRD2 shares same symptoms as CRD1 appearing in American Staffordshire Terriers, but it is caused by a different mutation.
Characteristics and Symptoms
The symptoms start to develop already at an early age. At 3-weeks of age, the retina was developed to a similar stage as in a healthy dog. Also, the number of cells in the outer nuclear layer appeared normal. However, the inner segment layer was less developed than in a healthy dog of the same age. By 12 weeks of age, the outer nuclear layer of the affected dog was comprised of only 5 to 7 layers, compared to usual 10 to 11 layers thick ONL in healthy dogs. In the inner and outer segment, the rods and cones were present but were abnormal, disorganized and reduced in size and number. By the 20 months of age, there was no inner and outer segment present in the affected dog’s retina and the outer nuclear layer was dramatically thinned. At first, visual impairment in the affected dog can be noticed, which progresses into severe blindness in the early adulthood.
Cone Rode Dystrophy 2 in Pit Bull Terriers is caused by a mutation in the IQCB1 gene. CRD2 is inherited in an autosomal recessive pattern. A dog can be clear, carrier (heterozygote) or affected (homozygote). 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.
Since CRD2 is caused by a different mutation than CRD1, mating a CRD1 carrier with a CRD2 carried does not produce affected offspring.
Goldstein O, Mezey JG, Schweitzer PA, et al. IQCB1 and PDE6B mutations cause similar early onset retinal degeneration in two closely related Terrier dog breeds. Invest Ophthalmol Vis Sci. 2013;54:7005–7019. DOI:10.1167/ iovs.13-12915