Cone Degeneration CD Test Code 101
For breeds: German Shorthaired Pointer, Alaskan Malamute and Australian Shepherd
The OptiGen CD Test is a series of two DNA based tests that provide a method to unequivocally identify Cone Degeneration Disease (CD) in the German Shorthaired Pointer (GSHP), Alaskan Malamute and Australian Shepherd. The two mutations causing CD in these breeds occur in the same gene (CNGB3) but are distinct mutations and arose separately. One mutation, which is complete deletion of the CNGB3 gene, affects Malamutes and Australian Shepherds, while a different single nucleotide mutation in the same gene causes Cone Degeneration in the GSHP breed.
CD disease causes day blindness due to degeneration of the retinal “cones” – cone-shaped cells in the retina that respond primarily to bright daylight. CD can be diagnosed in the early weeks of the affected dog’s life. Between 8 and 12 weeks of age, when retinal development is normally completed in dogs, signs of vision problems are noticeable. The pups become day-blind and are photophobic – meaning that exposure to bright light is irritating or even painful. The pup will shun brightly-lit areas. Vision in dim light remains normal. The retina of the affected dog initially appears normal when examined by an ophthalmologist and initially the ERG (electroretinogram) recording is normal. However, the ERG response from the degenerating cones declines with age and is non-recordable in the mature CD-affected dog.
In contrast to PRA (Progressive Retinal Atrophy), which is the more common type of retinal disease in many dog breeds, CD does not affect night vision. A second type of cell in the retina, the “rods” – rod-shaped cells that respond primarily to dim light and detect movement – are not involved in this disease. The CD-affected dog keeps the ability to see at night or in dimly-lit areas.
Just as PRA is the canine version of human RP (retinitis pigmentosa), CD is the canine version of the human genetic disease achromatopsia – total color blindness and day-blindness. Another name for day blindness is “hemerolopia.” Exciting gene therapy research aimed at correcting this condition (in dogs as a model for human disease) is currenty being conducted at the University of Pennsylvania.
Although the disease is rare in
general, within an affected line it is important to control the gene
frequency so as to prevent producing puppies affected with the disease. Reliable identification of dogs that do not carry disease genes is the key to controlling autosomal recessive diseases. The OptiGen CD test enables 100% accurate identification of these dogs. Called "genetically clear," "noncarriers" or, more formally, "homozygous normals," such dogs can pass only the normal gene on to all their pups - which means that none of their pups can ever be affected with CD. These "clear" dogs can be bred to any mate, even to a CD-affected German Shorthaired Pointer, which may be a desirable breeding prospect for other reasons.
Homozygous means both copies of the gene in your dog are the SAME - both normal or both CD. A carrier has one normal and one CD gene.
Because the OptiGen CD test is a mutation-based gene test, it accurately and specifically identifies normal dogs, carriers (heterozygous dogs) and affecteds. Possible test results are listed in the table below.
|Possible results for the OptiGen CD test|
|Test Result*||Genotype of GSHP Tested||Significance For Breeding||Risk For Developing CD|
|N||Normal||Can be bred to any GSHP ||Will never develop CD|
|C||Carrier||Carrier of CD||Will never develop CD |
|A||Affected||Homozygous for CD ||Will develop CD|
|*This is the result that will be reported when a GSHP is tested using the OptiGen CD test.|
The following table highlights the desirable breedings that will NOT produce affected pups. These breedings include at least one parent proven "normal" by the OptiGen CD test. All other breedings are at risk of producing CD-affected pups. However, all dogs can be bred safely. It isn't necessary-or even desirable-to remove any dog from the breeding population. But when choosing pups to retain as potential breeding stock, it is important to select for dogs proven "normal" by the OptiGen CD test, and select, when possible, against dogs proven to be carriers. Pups can be tested to distinguish carriers from normals as soon as they are old enough to have a small blood sample collected.
|Expected results for breeding strategies using the OptiGen CD test|
|Parent 1 |
|Parent 2 Genotype|
|Normal||All = Normal||1/2 = Normal |
1/2 = Carriers
|All = Carriers|
|Carrier||1/2 = Normal|
1/2 = Carriers
|1/4 = Normal |
1/2 = Carriers
1/4 = Affected
|1/2 = Carriers|
1/2 = Affected
|Affected||All = Carriers||1/2 = Carriers|
1/2 = Affected
|All = Affected|
How you can participate...
The CD Test for the German Shorthaired Pointer, Alaskan Malamute or Australian Shepherds is done on a small sample of blood obtained by your veterinarian. This allows the lowest risk of contamination of the sample and added assurance of a match of the sample with the identified dog. Read and print the instructions on the Ship Sample page. Then fill out the Test Request Form. On-line submission of the Test Request Form lets you be sure accurate information and correct spellings are put in the database. And, when you've completed one Form, a second Test Request Form for another dog or for a litter is easy and saves you time.