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

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.