ABCA Health & Education Foundation, Inc. — September 2019
The mutation that causes EAOD has not been discovered yet. The scientific team working on the research project that ABCA HEF has been funding — headed by Dr. Hannes Lohi at the University of Finland and Dr. James Mickelson at the University of Minnesota — has been working intensively to identify this causative mutation for the last three years, and has greatly narrowed the region in which it is expected to be found, but their work is still ongoing.
In the meantime, they have shared information with Genoscoper Laboratories in Europe, and Genoscoper’s parent company Wisdom (Mars) in North America, to enable those testing labs to offer what is called a “marker test” for EAOD. A marker test is not a test for the causative mutation. Rather, it is a test for markers — in this case a set of four markers — that are so closely linked to one another in the dog’s genetic material that they are always inherited together. There is good reason to believe that the not-yet-discovered causative mutation is also tightly linked to those markers, because so far all the dogs with EAOD that have been tested have carried two copies of the marker set. But the reverse of that is not true — all the dogs that carry two copies of the marker set do not suffer from early adult deafness.
The ABCA HEF does not endorse this marker test, nor do we encourage people to test at this time, before the causative mutation is found. But at the same time, we are not telling people not to test. The test does have some significant benefits, especially for someone whose dogs have deafness somewhere in their pedigree and who must make a breeding decision in the immediate future or who is considering buying or beginning the training of a young dog with deafness in its pedigree. The presence of deafness in a pedigree greatly increases the chances that the marker set will be associated with a causative mutation. The decision whether to purchase the test at this stage is yours alone, but in making that decision here are some factors we think you should consider.
We believe the current data strongly suggest that a CLEAR result on the test means that the tested dog does not carry the causative mutation for EAOD.
However, we believe there is not enough evidence to conclude that a CARRIER result, or an AT RISK result, means that the tested dog does carry the causative mutation. Remember, this is not a test for the mutation itself, as the CEA test is. In the CEA test, an Affected result means that your dog is carrying two copies of the mutation that causes CEA, and will pass one copy on to its offspring. In the EAOD test, an At Risk result does not mean that your dog is carrying two copies of the mutation that causes early deafness. It only means that your dog is carrying two copies of a set of markers that do not cause EAOD but have been shown to be associated with some increased risk that the dog who carries them will have EAOD. How great is that increased risk? We have no idea. In fact, it is entirely possible, from the current state of our knowledge, that your At Risk dog is not carrying the causative mutation at all.
Based on statistics from dogs in the research database, estimates have been published that up to 35% of Border Collies may carry one copy of the marker set, and close to 8% may carry two copies. This is notably higher than would be expected based on the percentage of dogs that display EAOD. It may simply be that the marker set is frequently found without the causative mutation. However, it may also be that the sample groups used were not representative of the Border Collie population as a whole, but were skewed by the disproportionate inclusion of dogs already suspected of deafness. It could also be that expression of the causative mutation is affected by the actions of yet-to-be-discovered modifier genes which may, for example, act to delay
the onset of deafness to an age where old-age deafness would be expected or beyond the lifetime of the dog.
But whether or not these prevalence figures are inaccurate to some extent, they illustrate the vital importance of not removing so-called Carrier dogs from breeding. If this test were to cause breeders not to breed “Carrier” dogs, we would be removing 1/3 of our population from our gene pool, without regard to working ability, and in a situation where for all we know they may not be carrying the causative mutation at all. That loss of good working lines and genetic diversity would be devastating, and is our biggest fear regarding use of the marker test. Carrier results from the currently available tests should not exclude dogs from being used for breeding. If your dog’s test results come back “Carrier,” we strongly recommend that you simply use the test to make sure the dog you breed him/her to is Clear. Do not just exclude your dog from breeding.
We have been asked whether the researchers have submitted their results for peer review and publication. The answer is that so far they have not. We have a written agreement with them that they will do so, but the agreement does not specify at what stage of the research this must be done. It is unlikely they will do it before the causal mutation itself is found, since that is what the project was designed to accomplish and that will be the culmination of their work.
We have also been asked why Genoscoper/Wisdom only offers this test as part of a panel of tests, rather than as a stand-alone, less expensive test. In all likelihood they are doing this (a) because it is better for their research goals, since this way they can test for the presence of many variants of interest across a large number of dogs and breeds, which can be very informative for researchers, and (b) because it is better for their marketing goals if people come to view them as a one-stop shop for all dog testing. Whatever we may think of this, ABCA HEF has no control over it. We do have assurance that the results of the study will be published once the causative mutation is found, and one of the purposes of insisting on that was so that other testing companies can develop and offer their own tests, and competition among them should bring prices down and offer dog owners a variety of options.
And we have been asked if Genoscoper/Wisdom will provide those who buy the test now with updated results at no additional cost, once the causative mutation has been found. This is a question we have asked several times, and never received an answer. This leads us to believe that they will not do so, which is a reason why dog owners who have no immediate need for testing might want to put off having their dogs tested for the present.
In addition to the Genoscoper/Wisdom test, an Australian testing company called Orivet is also offering a DNA marker test for EAOD. The test Orivet is offering is a stand-alone test for a single marker, one of the variants discussed in the 2012 PLOS publication by Yokoyama, et al. According to Dr. Lohi, this is not one of the four markers being used in the Genoscoper/Wisdom test, and it does not correlate well with their marker set. Dr. Lohi’s team has found dogs that do not carry the marker set Genoscoper/Wisdom is using, but are heterozygous for the marker Orivet is using. Such a dog would be classified as Clear under the Genoscoper/Wisdom test, but as Carrier by the Orivet test. Again, neither the Orivet test nor the Genoscoper/Wisdom test is a test for the causative mutation for EAOD and, therefore, some At Risk or Carrier results may be false positives for EAOD.