Progressive retinal Atrophy, cone-Rod dystrophy 4 - PRA-crd4

Progressive retinal atrophy (PRA) is a general term for eye diseases characterised by progressive loss of vision with age. Due to the gradual death of photoreceptors in the retina, PRA often leads to complete blindness. Cord1-PRA/crd4 (cone-rod dystrophy-PRA) is a genetic form of PRA that is caused by a change in the RPGRIP1 gene. This gene is very important for the function of the photoreceptors in the eye. Many different PRA eye diseases are known in dogs, most of which are breed-specific. The mutation for cord1-PRA/crd4, however, occurs in many different dog breeds. In cord1-PRA/crd4, the light receptor cells (rods) of the retina, which are responsible for the perception of light/dark, are affected first. Secondarily, the colour sensory cells (cones) can also be affected.

If dogs have the mutation on both gene copies, they are at risk of developing cord1-PRA. However, due to the so-called incomplete penetrance, not all dogs that carry two altered copies are necessarily affected. This indicates that additional genetic or environmental factors may have an influence on the course of the disease. Carriers of this disease who only have one gene copy with the mutation are not at risk of developing the disease due to recessive inheritance, but can pass this mutation on to their offspring. The age at which a dog at risk first shows symptoms of cord1-PRA can vary greatly. A change in the retina can occur as early as 6 months of age, but the average age is usually around 5 years. A few dogs only show signs of visual impairment from the age of 10 years. The enormous differences in age at which dogs develop cord1-PRA/crd4 are still unexplained.