UC Davis School of Veterinary Medicine Veterinary Genetics Laboratory

Cone Degeneration

Day-blindness, affecting German Shorthaired Pointer, Alaskan Malamute, Siberian Husky, Alaskan Sled Dog and Miniature Australian Shepherd breeds

Introduction

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 that 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.

The VGL offers a DNA test for the two CNGB3 cone degeneration mutations that affect the German Shorthaired Pointer, Alaskan Malamute, Miniature Australian Shepherd, Siberian Husky and Alaskan Sled Dogs breeds. Australian Shepherds may also be at risk. Genetic testing is recommended for all of these breeds.

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Results reported as:

N/N No copies of CNGB3 mutation; dog is normal.
N/CD1 1 copy of CNGB3 mutation; dog is normal but is a carrier.
N/CD2 1 copy of CNGB3 mutation; dog is normal but is a carrier.
CD1/CD2 2 copies of CNGB3 mutations; dog has day-blindness.
CD1/CD1 2 copies of CNGB3 mutations; dog has day-blindness.
CD2/CD2 2 copies of CNGB3 mutations; dog has day-blindness.

Note: This test is specific for the CNGB3 deletion and CNGB3 SNP known to cause day- blindness in German Shorthaired Pointer, Alaskan Malamute, Miniature Australian Shepherd, Siberian Husky and Alaskan Sled Dog breeds.

References:

Sadjanin DJ, Lowe JK, McElsee JL, Milne BS, Phippen TM, Sargan DR, Aguirre GD, Acland GM, Ostrander EA. 2002. Canine CNGB3 mutations establish cone degeneration as orthologous to the human achromatopsia locus ACHM3. Human Molecular Genetics 11(16):1823-1833.

Yeh CY, Goldstein O, Kukekova AV, Holley D, Knollinger AM, Huson HJ, Pearce-Kelling SE, Acland GM, Komáromy AM. 2013. Genomic deletion of CNGB3 is identical by descent in multiple canine breeds and causes achromatopsia. BMC Genetics. Apr 20;14(1):27.

 
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