PRCD

Testing for prcd-PRA in Australian Cattle Dogs

Reliable identification of dogs that do not carry disease genes is the key to eliminating autosomal recessive diseases, like prcd-PRA. The OptiGen prcd test for ACDs  provides almost 100% identification of these dogs. Called "genetically clear", "noncarriers" or, more formally, "homozygous normals," such dogs pass the normal gene on to all their pups with a very high probability - which means that their pups have a very low risk of being affected with prcd. These "clear" dogs can be bred to any mate - even to a prcd-affected dog that 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 prcd. A carrier has one normal and one prcd gene.
The OptiGen prcd test is a marker test rather than a mutation-based gene test. This means that it identifies dogs by a "fingerprint" on chromosome 9 near the prcd gene locus, rather than by detecting the prcd mutation itself. The result of the test is a genotype or pattern - a fingerprint - that allows the separation of dogs into three groups: pattern/genotypes A, B and C. Due to the properties of the DNA markers, there is a low risk of recombination - or dissociation - between these markers and the prcd gene. Therefore, this marker test classifies ACDs  into three risk groups as shown in this Table.
Possible results using the OptiGen prcd test
Pattern/
Genotype*
Risk Group
Significance For Breeding
Risk For Developing prcd
A
I. Normal
Can be bred to any dog, with 99.5% confidence of producing no affected pups
Not expected to develop prcd, risk is less than 0.0025%
B
II. Nonaffected
Probably a carrier of prcd (99.0% confidence)and should be bred only to Pattern A
Not expected to develop prcd, risk is less than 0.5%
C
III. High Risk
Probably homozygous for prcd (99.5% confidence)and should be bred only to Pattern A
Probably will develop prcd, with only 0.5% chance of not being affected.
* Probability risks are based on estimated frequency of recombination between the prcd markers and the disease gene.

The next table shows that all the desirable breedings include at least one parent with Pattern A genotype from the OptiGen prcd test. All other breedings are at risk of producing pups of Pattern C, with a high probability of developing prcd. However, all dogs can be bred with a high degree of safety. It isn't necessary - or even desirable - to remove dogs from the breeding population. But when choosing pups to retain as potential breeding stock, it is important to select for dogs with Pattern A genotype - and select against dogs with Pattern C genotype.
Expected results for breeding strategies using the OptiGen prcd test
Parent 1
Genotype
Parent 2 Genotype
A
B
C
A
All = Pattern A
1/2 = Pattern A
1/2 = Pattern B
All = Pattern B
B
1/2 = Pattern A
1/2 = Pattern B
1/4 = Pattern A
1/2 = Pattern B
1/4 = Pattern C
1/2 = Pattern B
1/2 = Pattern C
C
All = Pattern B
1/2 = Pattern B
1/2 = Pattern C
All = Pattern C
The OptiGen prcd test can be done reliably at any age, and the test result will never change with age. Since dogs are affected with other eye diseases, we recommend that yearly eye examinations by a board certified veterinary ophthalmologist be continued, for example, through the CERF program in the U.S. or similar program elsewhere. A normal exam in a young dog cannot rule out PRA.
Also keep in mind that not all retinal disease is PRA and not all PRA is the prcd form of PRA. Accurate diagnosis is essential. A dog can test as Pattern A or B, yet have a different type of PRA. Although other types of retinal degeneration occur, by far the most common is prcd. Even though the only PRA disease that has been fully defined and reported for ACDs  so far is the prcd form of PRA, we are interested in knowing about other retinal diseases, since it permits us to examine the samples in the research lab to determine if other genes are also involved.

Australian Cattle Dogs – more information:
      Is there more than one type of PRA in ACDs?
      The predominant form of PRA in ACDs is prcd-PRA. There are a couple rare cases that appear to have “typical” PRA but do not test affected with the OptiGen test. They do not appear to be inheriting their disease as a prcd allele. They might have inherited a rare type of PRA that is not prcd. Or, the most likely explanation is the dogs have an acquired (non-inherited) form of retinal degeneration. Acquired retinal disease appears to be frequent in working dogs and could lead to mis-diagnosis of prcd-PRA on a clinical basis.

What is the usual age at diagnosis?
      ACDs have been diagnosed with prcd-PRA over a very wide age range – as young as 3 years and through 8 years or older. Although the typical age of diagnosis is 4 to 6 years, a dog cannot be considered free of prcd-PRA until at least 8 years of age with a clear eye exam. As more dogs are examined, it’s likely that even younger and older dogs will be discovered showing first signs of prcd-PRA.

How frequent is PRA in ACDs?
     There isn’t a good estimate of frequency of PRA in ACDs yet, but the carrier frequency is expected to be quite high, possibly approaching 50%. More about this will be learned as testing goes on. Since there could be a high carrier rate, owners should choose to test key dogs in their lines first.

Are there any proven cases of false negative in this test?
     Two dogs out of 250 research dogs did not show as “affected” Pattern C using the prcd-PRA test for ACDs, even though they appeared to have PRA upon exam by Dr. Acland. Upon further pedigree research, it is believed that these dogs do not have prcd-PRA. If this is accurate, they are not false negative for prcd. Rather, they are “positive” for another disorder and do not have prcd-PRA. They may have another, rare form of PRA or an acquired retinal degenerative disorder. OptiGen will follow-up on any ACDs that do not test as Pattern C but subsequently are diagnosed with PRA.

Are there any proven cases of false positive alleles in this test similar to other breeds?
      So far there is no known case of a false positive allele in ACDs. There is no experience so far that a Pattern B dog might actually be Pattern A, nor that a Pattern C dog might actually be Pattern B or even A. This situation is very different than for the initial prcd-PRA test in other breeds where the rate of false positives was substantial. In ACDs, the risk of a false positive result is estimated to be less than 0.5%, based on the theoretical possibility of recombination between the prcd gene and the prcd markers. With more extensive testing of new pedigrees, it is possible that the issue of false positives in Pattern B or C might need to be reconsidered.

What ACDs were used in the research to develop their prcd-PRA test?
      Several lines of ACDs from the U.S., Australia and Europe were studied to develop and validate this test. To the best of our experience so far, the test can be used on purebred, registered ACDs worldwide.
GGenetic Registries – ACDs: According to the policy of the ACDCA, the Orthopedic Foundation for Animals (OFA) will serve as database administrator for the Australian Cattle Dog Health Registry, including the database for prcd-PRA results. OptiGen will send OFA results on all U.S. owned ACDs. OFA will post the results on OFA’s public database. Pattern A results for samples received at OptiGen up to and including July 15, 2002 will be released automatically; OFA will contact the owners of all Pattern B and C dogs to request written permission to release their results. For samples received July 16, 2002 and thereafter, OFA will release automatically all results in accordance with the revised ACDCA registry policy. The $15 fee for this registry must be ADDED to the payment to OptiGen and is passed entirely on to OFA.
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Information used with permission from OptiGen.