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Achromatopsia 3 (CNGB3 gene, German short-haired Pointing Dog)

Achromatopsia 3 (CNGB3 gene, German short-haired Pointing Dog)

Achromatopsia in dogs is an autosomal recessive inherited disorder that affects the photoreceptor cells of the retina, known as cones. Although these cells remain intact, the disease prevents their proper functioning, resulting in daytime blindness and decreased visual acuity.

Symptoms

The initial symptoms of achromatopsia manifest between 8 and 12 weeks, a period when puppies have developed vision. In the presence of bright light, affected puppies experience daytime blindness and photophobia, leading them to avoid well-lit areas. Although there is a loss of cone function in the retina, the ophthalmological appearance of affected dogs remains normal. Furthermore, the degeneration does not affect the rods, preserving vision in low-light conditions. Cone function begins to deteriorate between weeks 6 and 12, and in adult dogs, retinas lack cones, except for some displaced nuclei.

Disease Management

In case your dog exhibits any symptoms, it is crucial to consult your veterinarian for a thorough evaluation. The loss of cone function can be detected through electroretinography. During interactions with your dog, it is advisable to use verbal signals instead of visual signals with the hand. Gradually facilitating your dog's adaptation to its surroundings, such as getting them accustomed to walks in low-light conditions, will contribute to making them feel more secure during brighter times of the day. Additionally, the use of a leash and harness is recommended to provide additional support.

Genetic basis

This disease follows an autosomal recessive mode of inheritance. Autosomal recessive inheritance means that the dog, regardless of sex, must receive two copies of the mutation or pathogenic variant to be at risk of developing the disease. Both parents of an affected dog must carry at least one copy of the mutation. Animals with only one copy of the mutation are not at increased risk of developing the disease, but may pass the mutation on to future generations. Breeding between dogs carrying genetic variants that can cause disease, even if they do not show symptoms, is not recommended.

Technical report

Achromatopsia, daytime blindness, or cone degeneration can be caused by mutations in the CNGA3 and CNGB3 genes, which are involved in phototransduction. Both genes encode subunits of signal-transducing ion channels in the sensory neurons of the retina. These channels are located in the cell membrane and regulate the passage of cations. Two variants in the CNGB3 gene associated with the development of achromatopsia have been identified. One of these variants, observed in the Alaskan Malamute, involves a deletion spanning the entire CNGB3 gene. In the case of the Short-haired German Pointer breed, a nonsense mutation in exon 6 (c.784G>A, variant analyzed here) has been identified, leading to the functional loss of the CNGB3 protein. The region surrounding this mutation is highly conserved in both dogs and humans.

Most affected breeds

German Short-haired Pointing Dog
Alaskan Malamute
Miniature Australian Shepherd
Siberian Husky

Bibliography

Sidjanin DJ, Lowe JK, McElwee JL, et al. Canine CNGB3 mutations establish cone degeneration as orthologous to the human achromatopsia locus ACHM3. Hum Mol Genet. 2002 Aug 1;11(16):1823-33.

Yeh CY, Goldstein O, Kukekova AV, et al. Genomic deletion of CNGB3 is identical by descent in multiple canine breeds and causes achromatopsia. BMC Genet. 2013 Apr 20;14:27.