Is it possible that both Allie and her littermate acquired DCM from a secondary cause? That they were both deficient in taurine, or shared some other reason despite living on opposite sides of the continent?
If Allie’s DCM is not due to secondary causes, did she and her littermate inherit DCM from their parents? If so, was it from one parent, or both? I consulted with several veterinarians and quickly discovered understanding genetics isn’t straightforward! I have respect for breeders who are able to juggle the genetic aspects of breeding both physically and mentally healthy dogs.
Genetics in a Nutshell
In people, DCM is usually inherited in an autosomal dominant pattern. According to Laboklin Laboratories (1), in dogs it can be either autosomal dominant or autosomal recessive inheritance patterns.
Autosomal Recessive: Sufferers must inherit two alleles (different versions of the same gene) to have the disorder. Some generations will be carriers, and some will be affected.
Autosomal Dominant: Sufferers must inherit only one allele to have the disorder. The dominant pattern generally does not skip generations. This is seen in the Doberman Pinscher, for example.
In Dr. Casal’s article for the Royal Spaniel (2) she describes a rare situation in the autosomal dominant pattern. Is this a possibility of what happened with Allie and her littermate?:
In autosomal dominant disorders that are either severely deleterious or that would be selected against by breeders, most of the cases observed in a population will represent new mutations. These will occur as rare sporadic cases with no prior evidence of their occurrence in related animals.
If English Cocker Spaniels are known to be Taurine deficient, is that an inherited defect? What are the chances that they’re ALL afflicted by some other dietary, environment or secondary health reason?
Now that I know Taurine supplementation can improve the symptoms, but isn’t known to be curative, does that mean some other genetic defect is at play?
A dog can be either clear, a carrier, or affected. A dog that is a carrier can be physically healthy, but if it’s bred to an affected dog, or a dog that is also a carrier, it can pass on the disorder to the offspring. For me, nothing explains this better than a visual chart. This chart represents an autosomal recessive genetic pattern (image from Glenbern.ca)
Then there is the question of penetrants. This is a definition from the Oxford dictionary:
Penetrance: the extent to which a particular gene or set of genes is expressed in the phenotypes of individuals carrying it, measured by the proportion of carriers showing the characteristic phenotype.
For instance, if there is 95% penetrance for a particular mutation in a gene responsible for a disease, then 95% of individuals with that mutation will be affected, whereas 5% will not show any trait.
It’s also important to note that an echocardiogram is the only valid way to diagnose DCM, and the diagnosis must come from a cardiologist. A dog that is clinically healthy, is not necessarily clear. They could still be a carrier.
There’s so much more to it, though! I came across this gem by Beverly Biology. This is an excellent video which explains how autosomal dominant and autosomal recessive inheritance patterns work:
Breeding Without Testing
If DCM is genetic in English Cocker Spaniels, breeding dogs that are clear or carriers has been going on for generations, resulting in healthy dogs. There is a valid argument that by excluding all carriers, breeders would drastically narrow the gene pool, resulting in more problems. On the other hand, without any way of knowing if a dog is clear or a carrier, there is no way of knowing if two carriers might be bred, possibly resulting in an affected puppy.
If breeders have not thought of DCM as being genetic, there would be no reason for them to pay attention to which ones were affected or not. It’s possible they think when it does happen, that other factors are involved in creating the disease (diet, environment, secondary health issues etc.)
DCM can significantly shorten a dog’s life. There are ethical questions here regarding the affected pups, the lives of the people who purchase them from breeders, and future breeding decisions. If feasible, the development of a test for DCM in English Cocker Spaniels would help to alleviate the concern over unknown factors.
Questions regarding genetics and other issues will be addressed in future posts, summarizing conversations with well known researchers.
References:
(1) “LABOKLIN (UK)| Genetic Diseases | Dogs| Dilated Cardiomyopathy ( DCM / JDCM).” Laboklin.co.uk, 2023, www.laboklin.co.uk/laboklin/showGeneticTest.jsp?testID=8764. Accessed 21 Aug. 2024.
(2) Casal PhD, Dipl, ECAR, Dr. Margret L. “Basic Genetics for Reputable Breeders: | LynWood Cavalier King Charles Spaniels | Linda Baird.” Royalspaniel.com, 2019, www.royalspaniel.com/health-testing/basic-genetics-for-reputabl.html. Accessed 20 Aug. 2024.