Cone Degeneration Alaskan Malamute Type (CD)
Cone degeneration Alaskan malamute type (CD) is genetic disorder affecting the eyes in Alaskan malamutes. Cone degeneration until now has been identified in German shorthaired pointer and miniature Australian Shepherd. Alaskan malamute is the breed in which this disorder was observed for the first time in the 1960’s and the strain of dogs in which it was observed were inbred. Cone degeneration shares same phenotypical characteristics and similar genetic background as 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
There are two types of photoreceptors in the eye, cones and rods. 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 day blindness and photophobia, which are obvious only in presence of bright light, while there are no symptoms in dim light 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. Electroretinograph is the test used to detect cone degeneration and 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 Alaskan malamute type (CD) is a deletion of the entire 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.