Cone Degeneration German Shorthaired Pointer Type (CD)
Cone degeneration German shorthaired pointer type (CD) is an inherited disorder affecting German shorthaired pointer breed. Except the German shorthaired pointers, cone degeneration has until now been identified also in Alaskan malamutes and miniature Australian Shepherd. The disorder has been identified for the first time in the 1960’s in a strain of inbred Alaskan malamute dogs. Phenotypically, as well as genetically, characteristics of cone degeneration are similar to the achromatopsia, an inherited disease in human beings. This is the reason why the cone degeneration (CD) is being used as a canine model of human achromatopsia.
Characteristics and symptoms
Cones and rods are two main photoreceptors in the eye. Cones are responsible for vision in bright light and color vision. Rods enable vision in dark or in dim light. First symptoms of cone degeneration usually occur after the retinal development is normally completed, which is between 8 and 12 weeks of age in the affected dog. These symptoms are recognizable only in the day light, and they are day blindness and photophobia. In dim light, the symptoms are not obvious and vision in dim light remains normal. After the cub’s birth, cones develop normally, but with time they lose their function and their inner and outer segments start to degenerate. This events are followed with gradual loss of cones throughout the animal’s lifetime. In the adult age, dog will lack all cones. For detection of the cone degeneration test called electroretinograph is used. It is efficient for CD detection only in very young age of affected puppy, between 3 to 6 weeks of age. CD detection efficiency of electroretinography starts to drop at 6 to 12 weeks of age, and stops to be recordable in adult affected dogs. As previously mentioned, vision in dim light remains normal, which is the main difference in symptoms between cone degeneration and another common eye disorder, progressive retinal atrophy (PRA).
Mutation causing the cone degeneration German shorthaired pointer type (CD) is a point mutation in the CNGB3 gene. The disorder is inherited in an autosomal recessive pattern. Dog carrying one copy of the mutated gene is heterozygous and will not show the CD symptoms. When mating two carriers (heterozygotes) 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. Currently there is no cure for CD and the only way to avoid obtaining affected cubs 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 genetic testing.
Sidjanin, J., D., (2002): Canine CNGB3 mutations establish cone degeneration as orthologous to the human achromatopsia locus ACHM3. Oxford University Press. Human Molecular Genetic, Vol. 11, No. 16. 1823-1833.