Functional characterization of a distinct ryanodine receptor mutation in human malignant hyperthermia-susceptible muscle.

Malignant hyperthermia is an inherited autosomal disorder of skeletal muscle in which certain volatile anesthetics and depolarizing muscle relaxants trigger an abnormally high release of Ca2+ from the intracellular Ca2+ store, the sarcoplasmic reticulum. In about 50% of cases, malignant hyperthermia susceptibility is linked to the gene encoding the skeletal muscle ryanodine receptor/Ca2+ release channel (RYR1). To date, eight point mutations have been identified in human RYR1. Although these mutations are thought to lead to an increased caffeine and halothane sensitivity in the contractile response of skeletal muscle, their functional consequences have not been investigated on the molecular level. In the present study, we provide the first functional characterization of a point mutation located in the central part of RYR1, Gly2434 --> Arg. Using high affinity [3H]ryanodine binding as the experimental approach, we show that this mutation enhances the sensitivity of RYR1 to activating concentrations of Ca2+ and to the exogenous and diagnostically used ligands caffeine and 4-chloro-m-cresol. In parallel, the sensitivity to inhibiting concentrations of Ca2+ and calmodulin was reduced, transferring the mutant Ca2+ release channel into a hyperexcitable state.