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Cone Degeneration German Shorthaired Pointer Type (CD)
| Acronym: | CD |
| Gene: | CNGB3 |
| Mutation: | c.784G>A |
| Inheritance: | Autosomal recessive |
| Sample type: | CHS (Cheek Swab), WBE (Whole Blood EDTA) |
Genetics and characteristics
Cone degeneration German shorthaired pointer type (CD) is an inherited disorder affecting the German shorthaired pointer breed. Except for the German shorthaired pointers, cone degeneration has until now been identified also in Alaskan malamutes and miniature Australian Shepherd. The disorder has been identified for the first time in the 1960s in a strain of inbred Alaskan malamute dogs. Phenotypically, as well as genetically, characteristics of cone degeneration are similar to 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.
Cones and rods are two main photoreceptors in the eye. 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 recognizable only in the daylight, and they are day blindness and photophobia. In the dim light, the symptoms are not obvious 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. For detection of cone degeneration test called electroretinograph is used. It is efficient for CD detection only at a very young age for 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 German shorthaired pointer type (CD) is a point mutation in the 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.
Yeh, C., Y., (2013): Genomic deletion of CNGB3 is identical by descent in multiple canine breeds and causes achromatopsia. BMC genetics 2013, 14:27.