Sunday before last, we published a blog entitled, "When Is It Time?" Two days later, it was time for me to take my DM-affected boxer Bobby on his last trip to my vet's office. Unlike his sire Max, who was affected later in his life than Bobby but maintained his cheerful, animated demeanor to the end, Bobby seemed far less resilient in the face of the rapid loss of his mobility. Perhaps because Bobby also had cardiomyopathy and was exhausted by the effort of going through what used to be the routine motions of living in this oppressive
heat? I don't know, but Bobby was very subdued – uninterested in even his favorite foods -- for the last week or so of his life. Florida
Although I thought I had resigned myself to the inevitable, when I got to the vet's office I found I wasn't such a tough cookie after all, and neither was my vet or her staff. I drove home vowing that I would never again breed a litter that might contain an at risk puppy. I suspect that everyone who has just lost a dog to DM says the same thing.
And of course, it’s an easy thing for me to say: I’ve been breeding and showing boxers since 1973, and at this point will probably breed only one or two more litters in my lifetime. But how easy is it going to be for the next generation of breeders to avoid breeding At-Risk puppies while trying to consider ARVC, SAS and myriad other health concerns, along with temperament, head type, conformation and movement? Not easy at all when you consider this update from Dr Joan Coates (taken from the October 2010 ABCF Messenger at www.abcfoundation.org):
“Results on genetic testing for the Boxer breed so far…
As of 10/1/2010, we have tested 1987 Boxers. The genotype total includes 251 clears (12.6%), 608 carriers (30.6%), and 1128 (57%) AT-RISK. The allelic frequency takes into account the number of chromosomes with the mutation. The allelic frequency is 72% in the population of Boxers tested.”
I also took a look at the information sheet that was included with the OFA DM certificate I just received for one of my dogs, and under Explanation of results this was the explanation for At-Risk:
“AT-RISK (A/A): This dog is homozygous A/A, with two mutated copies of the gene, and is at risk for developing Degenerative Myelopathy (DM). The research has shown that all dogs in the research study with confirmed DM have had A/A DNA test results; however, not all dogs testing as A/A have shown clinical signs of DM. DM is typically a late onset disease and dogs testing as A/A that are clinically normal may still begin to show signs of the disease as they age…Research is ongoing to estimate what percentage of dogs testing as A/A will develop DM within their lifespan. At this point, the mutation can only be interpreted as being at risk of developing DM within the animal’s life.” [Emphasis mine.]
Then under Guidelines for Breeding, Dr Coates wrote:
“…The “A” (mutated) allele appears to be very common in some breeds [over 70% of boxers have the allele]. In these breeds, an overly aggressive breeding program to eliminate dogs testing A/A or A/N might be devastating to the breed as a whole because it would eliminate a large fraction of the high quality dogs that would otherwise contribute desirable qualities to the breed....A realistic approach when considering which dogs to select for breeding would be to treat the test results as one would treat any other undesirable trait or fault. Dogs testing At-Risk (A/A) should be considered to have a more serious fault than those testing as Carriers (A/N)….the test result should be one factor among many in a balanced breeding program.” Good, conservative advice: Don’t throw the baby out with the bathwater.
Now go back up to Dr Coates’ explanation of At-Risk and reread the parts in bold print. While I was writing this blog, I was emailing back and forth with geneticist and boxer breeder Dr Bruce Cattanach. Due to my own personal experience with DM – so far, 100% of my At-Risk dogs and their At-Risk close relatives have developed DM – I have always been of the opinion that if an At-Risk boxer lived long enough, he or she was going to develop DM. But after reading the following in one of Dr Cattanach’s emails, I had a “light bulb” moment, and finally understood what Dr Coates has been saying about At-Risk, and why her breeding guidelines are so cautious and conservative.
Here’s what Dr Cattanach wrote:
“OK, here is something factual. 57% of the breed is homozygous At Risk. But does over half the breed die of DM – no they die of cancer or ARVC, etc, as well. But of those that don’t die of these other things, do more than half get DM? I am fairly sure this will not be true. Hence, as Joan Coates says, there is a question of penetrance. Only a proportion of these gets DM. Do 80% get the disease? Or, say, only 20%? This would make a big difference to me about how I thought about breeding from or producing DM homozygotes.”
I hope Dr Coates’ research will give us the answer to the question of penetrance one day, but in the meantime, I’d be very interested to hear your answers to this question: How many of the breeders and owners who are reading this have DM At-Risk boxers that are 10 years old or older, but have not yet developed DM? Unfortunately, I know what the penetrance rate is for my own dogs so far, but I would be very curious to know if my dogs are typical. This is not a formal survey, so names of dogs and owners are not important. Just email me privately at email@example.com, or if you prefer, comment on the blog. I will publish the results of the survey in a couple of weeks’ time. Thanks in advance for your responses.