Cone Rode Dystrophy 1 (CRD1)
Cone Rode Dystrophy 1 (CRD1) is a genetic eye disorder affecting the American Staffordshire Terriers. CRD1 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. CRD1 shares same symptoms as CRD2 appearing in American Pitbull Terriers, but it is caused by a different mutation.
Characteristics and Symptoms
The symptoms start develop already at an early age. At 11-weeks of age, the outer nuclear layer was 6 to 8 nuclei thick, in comparison to 12 to 15 nuclei of thickness in a healthy dog. Photoreceptors of inner segment and outer segment were distinctly distorted, and inner segment rods were more severely affected than cones. When the puppy was 20 months old, the retina was in an advanced state of degeneration, with less than 2 or 3 outer nuclear layer cells. Before 1 year of age, visual impairment in the affected dog can be noticed, which progresses into severe blindness in the early adulthood.
Cone Rode Dystrophy 1 is caused by a mutation in PDE6B gene. The PDR6B gene is known to have a great importance in process of phototransduction. CRD1 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 CRD1 is caused by a different mutation than CRD2, 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