Researchers believe they have found a link between a genetic mutation and horses being susceptible to developing rapid-onset muscle atrophy. • Photo by Molly Montag

Study: Genetic Mutation Linked to Quarter Horse Muscle Atrophy

Researchers believe they have identified a genetic mutation that dates back to the foundation of the Quarter Horse breed that is linked to horses developing sudden, severe muscle atrophy.

DNA testing of top money-earning Quarter Horse stallions done in conjunction with the study found the mutation more prevalent in reining and reined cow horse stallions than sires in other disciplines.

Dr. Stephanie Valberg, of Michigan State University, presented the findings about the mutation, called Myosin Heavy Chain 1 (MYH1), to the AQHA’s Stud Book And Registration Committee during the AQHA Convention in Jacksonville, Florida.

A long-time researcher into muscular diseases in Quarter Horses, Valberg and colleague Dr. Carrie Finno, of the University of California, Davis, and others published a study days after the presentation: “A missense mutation in MYH1 is associated with susceptibility to immune-mediated myositis in Quarter Horses.”

Click here to view the published study.

Valberg told committee members that researchers sought to determine why some horses exposed to a disease such as strangles or strep would develop sudden and severe muscle atrophy. Their work led them to believe disease or inflammation can trigger atrophy in some horses that have a mutation in the myosin gene, which codes for fast-twitch muscle fiber.

“You’ll see [the muscle] waste away within a matter of two or three days,” said Valberg, who noted horses can regain muscle after treatment with corticosteroids. “Their topline disappears, and their spine becomes really prominent.”

Fast-twitch muscle

Valberg and Finno identified the gene, in part, after looking closer at a group of horses afflicted by sudden muscle atrophy that also had lymphocytes, or white blood cells associated with the immune system, in their muscle fiber. A little more than 80 percent of horses in that sampling were heterozygous or homozygous for the MYH1 mutation. Comparatively, only 7.5 percent of horses in a random sampling researchers took of 147 Quarter Horses tested heterozygous for the MYH1 mutation, and none were homozygous.

Additionally, Valberg said the mutation was found in another group of equines – young horses that developed non-exertional rhabdomyolosis, or tied up without exercise.

Testing stallions

Part of Valberg’s research included voluntary testing of the DNA of top money-earning stallions in different segments of the industry for the mutation. Researchers who evaluated the samples did not know the names of the horses they were testing, but the research did provide data about which disciplines were involved.

In this case, Valberg said the mutation was most prevalent in reining, reined cow horse and halter stallions sampled.

  • 21 percent of the 37 reining stallions tested heterozygous for the mutation; one was homozygous.
  • 17 percent of the 41 reined cow horse stallions tested heterozygous; none were homozygous.
  • 16 percent of the 50 halter stallions were heterozygous. None were homozygous.

The mutation seemed to be less prevalent among the cutting and Western pleasure stallions tested, with 9 percent of 45 cutting stallions and 4 percent of Western pleasure stallions found to be heterozygous. None were homozygous.

None of the 42 barrel racing or 44 racing stallions tested as part of the research were heterozygous or homozygous for the mutation.

Future genetic testing

When Valberg was asked by a Stud Book And Registration Committee member if she felt MYH1 should be added to the AQHA’s standard five- panel genetic testing, the researcher deferred, saying her job was to do research, not to make such recommendations. However, Valberg told the member that a horse’s MYH1 status was something she would want to know when buying a horse she intended to use for breeding.

The Stud Book And Registration Committee did not forward any recommendations to the AQHA Executive Committee about whether or not the MYH1 mutation should be included on the panel.

It did recommend the Executive Committee form a task force to evaluate criteria for inclusion of additional tests in the genetic panel test.

The five-panel test currently screens for glycogen branching enzyme deficiency (GBED), hereditary equine regional dermal asthenia (HERDA), hyperkalemic periodic paralysis (HYPP), malignant hyperthermia (MH) and poly- saccharide storage myopathy (PSSM).

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* This article was updated on May 7, 2018, to specify Dr. Carrie Finno was one of the researchers that found the MYH1 mutation.