Abstract 14157: Calmodulin Mutations in Human Genetic Arrhythmia Disorders Exhibit Divergent Functional Effects on Ryanodine Receptor Ca Release Channels
Background: Recent genetic studies identified mutations in CALM1 or CALM2, 2 of the 3 human genes encoding calmodulin (CaM), in both catecholaminergic polymorphic ventricular tachycardia (CPVT) and long QT syndrome (LQTS). CPVT is commonly caused by mutations in sarcoplasmic reticulum genes that increase diastolic Ca leakage through ryanodine receptor (RyR2) Ca relase channels, whereas LQTS is usually caused by dysfunctional plasma membrane ion channels. How mutant CaM causes either CPVT or LQTS is unknown.
Objective: To gain mechanistic insight into how CaM mutations cause divergent human arrhythmia phenotypes.
Methods and Results: We prepared recombinant wild-type (WT) and mutant CaM proteins associated with either CPVT (N54I, N98S) or LQTS ( F142L, D130G). LQTS CaM mutations drastically reduce Ca binding affinity to CaM, whereas CPVT mutations have either no effect (N54I) or slightly reduce Ca binding affinity (N98S). At physiological free CaM [100 nM] and Ca [120 nM], CPVT CaMs significantly increase spontaneous Ca wave activity in permeabilized mouse myocytes, analogous to the effect seen in calsequestrin knockout (Casq2KO) myocytes, an established genetic CPVT model (Figure). In contrast, LQTS CaMs either had no effect or reduced Ca waves (Figure). Moreover, CPVT CaMs increased Ca spark frequency under experimental conditions that prevent CaMKII activation, another important CaM target. We also assessed binding of CaM mutants to RyR2 in myocytes and SR vesicles. Whereas CPVT CaMs show normal or greater RyR2 binding affinity, LQTS CaMs exhibit reduced RyR2 binding affinity. Our findings explain the divergent effects of these arrhythmogenic human CaM mutants.
Conclusion: Enhanced mutant CaM binding to and activation of RyR2 (vs. WT CaM) may explain CPVT caused by CaM mutations. LQTS mutant CaM does not bind to RyR2, and may thus be more available to bind to other ion channel targets where the mutant CaM disrupts function.
- © 2013 by American Heart Association, Inc.