Sunday, November 13, 2011

DM Again

Recently on the Facebook group, Boxers – Form & Function, a group member asked whether it would be possible to eradicate the DM gene from the breed in just a few generations, or whether trying to do that might result in unfortunate unintended consequences. Naturally, everyone who joined the discussion had his or her own opinion on whether or not we should always? sometimes? it depends? avoid making breeding combinations that might produce at risk puppies. It was a very interesting discussion and is still ongoing. If you’re interested, go to FB and sign on to the group. Because Dr Joan Coates is still researching unanswered questions about DM, she has wisely avoided making hard and fast breeding recommendations and instead has urged caution and that we take into consideration ALL our dogs’ virtues and faults when making breeding decisions. Below, UK geneticist Dr Bruce Cattanach offers his own opinion, previously published in a post to the SB-L. Dr Cattanach is currently engaged in research on ARVC and JRD.


I too have concerns about DM and these are mainly centered on the lack of information available to help breeders.  The 45% homozygote [at risk], 38% heterozygote [carrier], and 17% clear figures offered may be skewed, but testing has been conducted for a number of years now so I suggest the incidence of the gene in the breed must indeed be very high.

Now, clearly, this does not mean that the incidence of affected dogs [at risk dogs that actually develop DM] is anywhere near 45%.  There is the penetrance problem; only a proportion of homozygotes develop the disease.  I think it is time some figure is offered to breeders to get the problem into perspective.  If the penetrance is about 50%, then based on the above information, about 20% of Boxers might become affected, and I think this is still far too high to be believable.  So, how about a 10% penetrance, such that maybe only 4% of homozygotes would develop the disease?  This is slightly more credible but would still represent a mammoth problem.  I therefore think breeders need to have some idea of the magnitude of the risk so that they can deal with the problem more appropriately – not so severely as to wipe out essential breeding stock, but still in a way that exerts some selection pressure to reduce the incidence of the gene.  Caution perhaps should be the watchword for the moment.

And then there is no information whatever on the distribution of the gene across the breed.  Some information on this could help understanding.  Some sectors might be free of the gene but I suspect these may be few and far between.  There is no official DM testing in the UK but from the very few tests that I have heard about I know the gene is around in UK dogs too – but on enquiring at the major vet schools, I have not found any evidence of the disease being present.  Here we have the penetrance issue again. Should UK breeders be testing and selecting against the gene when we have not seen any cases?  This is not an easy question to answer, but I am sure most UK breeders would offer an answer in a flash.

Complicating matters further, Joan Coates told us in a recent talk at our genetics lab [Harwell MRC Mammalian Genetics Unit, UK] that the gene is to be found in quite a wide number of breeds but this is far less evident when one considers the disease. Pembroke Corgis, GSDs and Ridgebacks, with Boxers tagging along, seem to be the affected breeds, but I am sure Joan said that the disease incidence across breeds did not relate well to the gene incidence.  Thus I think there is one breed with a near 100% incidence of the gene (homozygotes?) and yet without any trace of the disease.  What can be expected of breeders in this situation?

The bottom line is that we should be cautious until we know more.  Selection is still possible, and justifiable, without going to extreme.  A simple approach would be to test the better pups in litters to help choose which pup(s) one should keep.  The DM status of stud dogs is what everyone wants to know, but even if the incidence of the gene was even half right the exclusion of all homozygotes for breeding might decimate the breed.

A cautious approach until one knows more seems essential, but please don't just ignore.

Bruce Cattanach

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