What the latest genetic research can tell you
Embark Testing Lab July 14, 2020
How do you best apply an Embark health variant test result to your breeding program or to an individual dog’s health care? When our veterinary geneticists and professional services team at Embark field questions like these, our answers are always based on the scientific research behind the health variant in question, as well as the breed of dog tested. We asked our experts to share answers to common questions about the tested variant for Degenerative Myelopathy (SOD1A).
What is Degenerative Myelopathy (DM)?
A disease of mature dogs, DM is a progressive degenerative disorder of the spinal cord that can cause muscle wasting and gait abnormalities. Affected dogs do not usually show signs until they are at least 8 years old, where the first signs of neural degeneration appear in the nerves that innervate the hind limbs. An affected dog may scuff the tops of their hind paws or walk with a hesitant, exaggerated gait.
In advanced cases, lower motor neurons are also affected leading to weakness and muscle wasting. This variant is reported to have incomplete penetrance, meaning not all dogs with two copies of the variant will go on to develop clinical signs and other genetic and environmental factors will contribute to whether a dog develops DM. Furthermore, this variant is only known to increase the risk of DM in certain breeds. Other breeds where this variant occurs but is not associated with DM risk likely have genetic factors protecting them from this disease.
My dog has two copies of this variant and is listed as having increased risk for DM; now what?
First, it is important to remember that the SOD1A variant is incompletely penetrant, so even in breeds where DM is a problem, many dogs testing at-risk from the variant will live long lives and never develop the disease.
If your dog is considered at-risk, this should open the door to a discussion with your veterinarian. As DM is a late-onset condition and genetic risk does not mean that clinical signs are guaranteed, drastic measures should not be an immediate concern. However, there are supplements and lifestyle modifications that may be of benefit to your dog.
Some non-medical things that can help with mobility and secondary injury to the feet include a harness (Help ‘Em Up Harness is a popular option) as well as using booties to minimize damage to nails from scuffing. Be sure to take the booties off when not in use as they can trap moisture and lead to infection.
My dog has two copies of this variant but is not listed as having increased risk for DM; now what?
For some breeds, research indicates that the SOD1A variant is not likely to increase the risk that a dog will develop DM. The SOD1A variant is found in many breeds of dogs, but researchers have only observed histopathological (microscopic) changes with this variant in a small number of breeds.
When determining whether or not a variant is expected to have a clinical impact for a breed, we have taken into account research either published, internal, or otherwise presented by a subject matter authority as our primary criteria. For breeds where clinical risk from this variant is not likely, this genetic result should not be the primary factor in breeding decisions.
What does it mean if my dog is a carrier?
As DM has an autosomal recessive mode of inheritance, dogs with one copy of the variant would be termed “carriers” and not be expected to be at clinical risk. However, in breeds where the SOD1A mutation is associated with DM risk, carriers should not be bred to other carriers (or at-risk dogs) as this will lead to the production of at-risk puppies. You can visit our website and search for your breed. If the “Embark Recommended” icon appears next to Degenerative Myelopathy (SOD1A), then there is a known risk based on the current literature, and this result may need to be considered in breeding decisions.
For all other breeds, while it is still important to track the incidence of the variant and clinical disease within lines to inform on future research, it is not recommended that this variant be used as the primary factor in breeding decisions, at this time. (Also, please note that the OFA will register DM results for any breed, so an OFA icon only indicates that it can be registered with the OFA and does not confer impact to the breed.)
Guidelines for Breeding Dogs Who Are Carriers or At-Risk for Degenerative Myelopathy (DM)
Owners with dogs testing as Carriers (A/N), or At-Risk (A/A) are strongly encouraged to share these results with their attending veterinarian and seek genetic counseling when making breeding decisions.
The “A” (mutated) allele appears to be very common in some breeds. 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. Nonetheless, DM should be taken seriously. It is a fatal disease with devastating consequences for the dog and can be a trying experience for the owners that care for them. 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). Incorporating this information into their selection criteria, breeders can then proceed as conscientious breeders have always done: make their breeding selections based on all the dog’s strengths and all the dog’s faults. Using this approach and factoring the DM test results into the breeding decisions should reduce the prevalence of Degenerative Myelopathy in the subsequent generations while continuing to maintain and improve upon positive, sought-after traits.
We recommend that breeders take into consideration the DM test results as they plan their breeding programs; however, they should not over-emphasize the test results. Instead, the test result should be one factor among many in a balanced breeding program.