Cone Degeneration

There are 2 known mutations of the CNGB3 gene that cause canine cone degeneration (CD) day-blindness in dogs. One affects Miniature Australian Shepherd, Alaskan sled dogs, and related breeds; the other is found in German Shorthaired Pointers and Alaskan sled dogs.

One form of cone degeneration (CD1) or CNGB3-achromatopsia results from the deletion of a 400,000 bases long genomic segment that includes the CNGB3 gene. The loss of CNGB3, a key component for normal vision, causes loss of function of the cones in the eye. This can only be confirmed by electroretinography; ophthalmic exams of affected dogs are not informative as results remain normal. This disease is inherited in an autosomal recessive fashion with both sexes being equally affected. The loss of cone function results in day-blindness and decreased visual acuity, with signs of the defect appearing between 8 and 12 weeks when retinal development is completed in dogs. Typically, affected dogs become increasingly photophobic as exposure to bright light is irritating and painful. Vision in low light conditions remains normal. Dogs with one normal and one deleted gene have normal vision. This defect is found in Alaskan Sled Dogs, Alaskan Malamute, Miniature Australian Shepherd, and Siberian Husky breeds.

Another form of CNGB3 (CD2) is due to a different mutation, a nucleotide change (SNP) in exon 6 of CNGB3, and is found in German Shorthaired Pointers and Alaskan Sled Dogs. This mutation is also an inherited as autosomal recessive and the disease progression and phenotype are similar to CD1. Dogs with 1 copy each of the 2 mutations (compound heterozygotes) are affected.

Genetic testing is recommended for German Shorthaired Pointer, Alaskan Malamute, Miniature Australian Shepherd, Siberian Husky, and Alaskan Sled Dogs breeds. Australian Shepherds may also be at risk.