By CHRISTINE HAMILTON
It came up in a staff conversation just the other day: Would you test your horse for genetic diseases? Ten years ago, I might have said no and wondered why I should even consider it.
But not now. For one thing, I have thought about raising a few foals. I would want to know if any mare I owned was a carrier for any genetic diseases before I made breeding decisions.
But there are increasingly more scenarios where genetic testing can help horse owners and breeders. When the first horse genome was sequenced from a Thoroughbred mare named Twilight in 2007, it cost $15 million. Since then the process has become more affordable and, therefore, more common, causing a genetic information explosion.
“What we know about horse genetics has probably doubled in the last six or seven years,” says equine veterinarian and geneticist Molly McCue, DVM, PhD, of the University of Minnesota in St. Paul.
The resulting research has given veterinarians a better understanding of a number of equine diseases.“We know quite a lot,” she says, “but a lot of it hasn’t yet trickled down to the horse owner.”
FROM SIMPLE TO COMPLEX
Most of the known genetic conditions are “simple-trait, yes-or-no diseases,” McCue says.
“A simple trait is usually controlled by one gene, and it’s easy to predict. If you have the mutation, you have the disease,” she says. “It’s a clear, direct correlation.”
Simple-trait diseases are caused when a horse inherits either one or two copies of a mutated gene. Veterinarians often use genetic testing to diagnose problems. Say, if a stock-type horse is having problems with tying up, it’s common to test for the polysaccharide storage myopathy (PSSM) mutation and develop a treatment plan from there.
“But we are transitioning to a place of looking at complex traits that involve multiple genes,” McCue continues. “Or, looking at one gene that only causes disease when a number of other environmental factors come into play.”
Problems such as equine metabolic syndrome (EMS) and osteochondritis dissecans (OCD) are examples of diseases linked to complex traits. The presence of multiple genes together increases a horse’s risk for developing those diseases when combined with environmental factors. But if managed correctly, even horses at high genetic risk are less likely to develop the disease.
“As we begin identifying the genes that put horses at risk,” McCue says, “we can then test for them and identify high-risk individuals. When owners can make better decisions about managing them.
“So, if you knew you had a 2-year-old at risk for developing EMS later on, you would know to really monitor his weight and keep him off early spring pastures. Or, if a broodmare is a high risk for OCD, you would know to watch her foal’s nutrition and make sure it grows slowly. It’s exciting to be able to have that information and do something about it, prior to there being any disease manifestation.”
One big reason for testing is to help in making breeding decisions, to breed away from problem genes while keeping good genes.
Genes are connected to each other in surprising ways, and when horsemen breed for specific positive genes, other genes tag along. It is interesting that a number of the known stock horse genetic diseases are linked to genes affecting either coat color patterns or muscle function, specific traits that Western horsemen have long bred for and sought.
McCue points out that many coat color pattern-linked diseases are related to the function of melanocytes, the cells that produce the skin pigment called melanin. Melanocytes are thought to be a link to genetic diseases involving dierent parts of the body, such as the inner ear (deafness), the eye’s uveal tract (moon blindness), and the neural control of the gastrointestinal system (overo lethal white syndrome).
Stock horses have been bred for a certain type of muscle function, she adds, greatly influenced by the Quarter Horse and expression of the myostatin gene, which regulates muscle growth. When the gene is expressed, it inhibits muscle growth. The less it is expressed, the more muscular a horse is and the more Type 2B muscle fibers he has—the type of muscles needed for sprinting performance. Research is currently focusing on mutations of myostatin that could be linked not only with performance but possibly with disease.
“But having a genetic mutation is not necessarily a death sentence,” McCue says, “or a reason to throw the baby out with the bath water.”
So, by knowing whether or not any mare of mine is a carrier for a genetic disease, I can avoid producing that in her foal, as well as reduce the odds that the foal would be a carrier, too. And the mare can still pass on all of her good genes.
“It is just arming yourself with information,” she says. “It’s about being informed of risk, and having information that allows you to make educated decisions.”
Molly Mccue, DVM, PhD, is a veterinarian and associate professor at the University of Minnesota in St. Paul. A diplomate of the American College of Veterinary Internal Medicine, she specializes in complex equine genetic diseases and population research. She is a lifelong Quarter Horse owner.