Novel Calmodulin Mutation (CALM3-A103V) Associated with CPVT Syndrome Activates Arrhythmogenic Ca Waves and Sparks

Abstract

Introduction: Calmodulin (CaM) is an essential Ca2+ signaling protein in a wide range of biological processes. In the dyadic cleft CaM binds to RyR2, Cav1.2 and CaMKII. CaM mutations are associated with an autosomal dominant syndrome of sudden death that can present with clinical features of catecholaminergic polymorphic ventricular tachycardia (CPVT) or long QT syndrome (LQTS). CPVT-linked CaM mutations activate ryanodine receptor (RyR2) Ca release channels; whereas LQTS-CaMs have no effects on RyR2 channels but prolong the action potential by impairing L-type Ca current (LTCC) inactivation. Here we report a novel CaM mutation – A103V – in CALM3, in a 10 year old female with episodes of exertion-induced syncope and normal QT interval consistent with CPVT. Her mother is mutation positive and also had a positive exercise stress test.Methods: CaM-A103V Ca binding affinity was measured by spectrofluorometry. Ca handling and LTCC were tested in mouse ventricular myocytes dialyzed with WT or the mutant CaM and compared to a published CPVT CaM mutant (N54I).Results: A103V modestly lowered CaM Ca binding affinity (3-fold reduction vs WT-CaM). In permeabilized myocytes, A103V-CaMs at a physiological intracellular concentration (100nM) promoted significantly higher spontaneous Ca wave and spark activity, a typical cellular phenotype of CPVT. Even a 1:4 mixture of A103V-CaM:WT-CaM activated Ca waves, demonstrating functional dominance. In voltage-clamped cardiomyocytes dialyzed with WT or mutant CaM, CaM-A103V had significantly less effects on LTCC inactivation compared to published CaM mutants that cause LQTS.Conclusion: A103V CaM shares functional characteristics with the established CPVT-CaM N54I. A small proportion of A103V-CaM is sufficient to evoke arrhythmogenic Ca disturbances, which explains the autosomal dominant inheritance.