Polysaccharide storage myopathy (PSSM) is a metabolic muscle problem found in at least 20 breeds, including Quarter Horses, Paints, Appaloosas, and drafts. An affected horse suffers bouts of exertional rhabdomyolysis (tying-up), muscle tremors, and/or gait abnormalities. At the 2008 American Association of Equine Practitioners convention, held Dec. 6-10 in San Diego, Calif., Molly E. McCue, DVM, MS PhD, Dipl. ACVIM, an assistant professor in the Department of Veterinary Population Medicine at the University of Minnesota, presented information about genetic testing for one PSSM type.

Dr. Molly McCue discusses research into muscle disorder PSSM.
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Researchers at the university discovered a gene mutation that causes unregulated glycogen formation in the muscles of horses with PSSM. This mutation is a gene called GYS1 on chromosome 10, which encodes the skeletal muscle glycogen synthase enzyme responsible for glycogen formation. Because this enzyme is constantly active in affected horses, they accumulate excess glycogen in the cells