An E321G MYH1 mutation is strongly associated with nonexertional rhabdomyolysis in Quarter Horses

Background An E321G mutation in MYH1 was recently identified in Quarter Horses (QH) with immune‐mediated myositis (IMM) defined by a phenotype of gross muscle atrophy and myofiber lymphocytic infiltrates. Hypothesis/Objectives We hypothesized that the MYH1 mutation also was associated with a phenotype of nonexertional rhabdomyolysis. The objective of this study was to determine the prevalence of the MYH1 mutation in QH with exertional (ER) and nonexertional (nonER) rhabdomyolysis. Animals Quarter Horses: 72 healthy controls, 85 ER‐no atrophy, 56 ER‐atrophy, 167 nonER horses selected regardless of muscle atrophy. Methods Clinical and histopathologic information and DNA was obtained from a database for (1) ER > 2 years of age, with or without atrophy and (2) nonER creatine kinase (CK) ≥ 5000 U/L,