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Cone Degeneration Alaskan Malamute Type (CD)
| Acronym: | CD |
| Gene: | CNGB3 |
| Mutation: | 404,820 bp deletion |
| Inheritance: | Autosomal recessive |
| Sample type: | CHS (Cheek Swab), WBE (Whole Blood EDTA) |
Genetics and characteristics
Cone degeneration Alaskan malamute type (CD) is a 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 1960s and the strain of dogs in which it was observed were inbred. Cone degeneration shares the same phenotypical characteristics and similar genetic background as achromatopsia, an inherited disease in human beings. This is the reason why cone degeneration (CD) is being used as a canine model of human achromatopsia.
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 dim light. The 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. These events are followed by the gradual loss of cones throughout the animal’s lifetime. In adult age, dogs will lack all cones. Electroretinograph is the test used to detect cone degeneration and it is efficient for CD detection only at a very young age of the 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).
The mutation causing the cone degeneration of the Alaskan malamute type (CD) is a deletion of the entire CNGB3 gene. The disorder is inherited in an autosomal recessive pattern. A 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 that 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.
Results Reported As
Test Result |
Interpretation of test result |
CLEAR |
Tested mutation was not detected in animal with „clear“ result. Animal tested as clear has wild-type allele in homozygous state (i.e. two pairs of healthy alleles). It will not develop disease caused by tested mutation.* It will pass only wild-type allele to its offspring. |
CARRIER |
Tested mutation was detected in animal with „carrier“ result. Animal tested as carrier has one wild-type and one mutation allele, it is in heterozygous state. It will not develop disease caused by tested mutation.* It can pass wild-type or mutation allele to its offspring. |
AFFECTED |
Tested mutation was detected in animal with „affected“ result. Animal tested as affected has two copies of mutation alleles affecting the gene. It is likely the animal will experience a genetic disorder due to this mutation.** It will pass only mutation allele to its offspring. |
*Test excludes only tested mutation but not possible unknown mutations or factors that can lead to similar condition/symptoms.
** Potential unknown mutations or multiple other factors can possibly affect the likelihood of experiencing a genetic disorder.
References:
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.