Gly341Arg mutation indicating malignant hyperthermia susceptibility: specific cause of chronically elevated serum creatine kinase activity.

We report on three families with the Gly341Arg ryanodine receptor gene (RYR1) mutation. Thirteen individuals were heterozygote carriers of the Gly341Arg mutation and had clearly positive in vitro contracture tests, indicating malignant hyperthermia susceptibility. Nine Gly341Arg mutation positive individuals from two families had elevated serum creatine kinase (CK) activity at rest (up to six times the normal upper limit). Their clinical and neurological examinations as well as detailed muscle histology were normal. The third family did not show increased CK activity. These findings indicate that the Gly341Arg mutation can be a specific cause of chronically elevated serum CK activity in asymptomatic individuals.