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Neuromuscul Disord. 2009 Jan;19(1):37-43. doi: 10.1016/j.nmd.2008.10.001. Epub 2008 Dec 3.

Polysaccharide storage myopathy phenotype in quarter horse-related breeds is modified by the presence of an RYR1 mutation.

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  • 1Department of Veterinary Population Medicine, University of Minnesota College of Veterinary Medicine, 225 VMC, 1365 Gortner Avenue, St. Paul, MN 55108, USA.


In this study we examined a family of Quarter Horses with Polysaccharide Storage Myopathy (PSSM) with a dominant mutation in the skeletal muscle glycogen synthase (GYS1) gene. A subset of horses within this family had a more severe and occasionally fatal PSSM phenotype. The purpose of this study was to identify a modifying gene(s) for the severe clinical phenotype. A genetic association analysis was used to identify RYR1 as a candidate modifying gene. A rare, known equine RYR1 mutation, associated with malignant hyperthermia (MH), was found to segregate in this GYS1 PSSM family. Retrospective analysis of patient records (n=179) demonstrated that horses with both the GYS1 and RYR1 mutations had a more severe clinical phenotype than horses with the GYS1 mutation alone. A treadmill trial (n=8) showed that serum creatine kinase activity was higher and exercise intolerance greater in horses with both mutations compared to the GYS1 mutation alone.

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