Horses with IMM experience rapid, widespread atrophy (wasting) of the gluteal and epaxial muscles (the latter run along either side of the spine). | Photo: iStock

A somewhat mysterious, sometimes fatal muscle-related disease in Quarter Horses is getting a little clearer. Researchers recently determined that immune-mediated myositis (IMM) is due to a genetic mutation encoding a protein that’s expressed in adult equine muscle.

Myosin heavy chain 1 (MYH1), a gene encoding the myosin protein in muscle tissue, occasionally gets passed down to offspring in a mutated form. It’s this mutated form that a team of researchers recently linked to IMM development. That team includes Carrie Finno, DVM, PhD, Dipl. ACVIM, associate professor in Veterinary Genetics and director of the Center for Equine Health at the University of California, Davis (UC Davis), and Stephanie J. Valberg, DVM, PhD, Dipl. ACVIM, ACVSMR, of the Michigan State University College of Veterinary Medicine’s Department of Large Animal Clinical Sciences, in East Lansing.

Fast-contracting muscle cells express the myosin protein to enable muscle contractions, they said, and MYH1 is one of several genes that encode forms of myosin. Like all genes, MYH1 is made up of many thousands of base pairs, which contain a code for specific amino acids that work like building blocks to create unique proteins. Finno, Valberg, and colleagues discovered that a single base pair in the MYH1 gene differed between healthy horses and those with IMM. It changed a glutamic acid amino acid to a glycine in the fast myosin of IMM horses