Arrhythmogenic Calmodulin Mutations Disrupt Intracellular Cardiomyocyte Ca2+ Regulation by Distinct Mechanisms

BACKGROUND Calmodulin (CaM) mutations have been identified recently in subjects with congenital long QT syndrome (LQTS) or catecholaminergic polymorphic ventricular tachycardia (CPVT), but the mechanisms responsible for these divergent arrhythmia-susceptibility syndromes in this context are unknown. We tested the hypothesis that LQTS-associated CaM mutants disrupt Ca2+ homeostasis in developing cardiomyocytes possibly by affecting either late Na current or Ca2+-dependent inactivation of L-type Ca2+ current. METHODS AND RESULTS We coexpressed CaM mutants with the human cardiac Na channel (NaV1.5) in tsA201 cells, and we used mammalian fetal ventricular cardiomyocytes to investigate LQTS- and CPVT-associated CaM mutations (LQTS- and CPVT-CaM). LQTS-CaM mutants do not consistently affect L-type Na current in heterologous cells or native cardiomyocytes, suggesting that the Na channel does not contribute to LQTS pathogenesis in the context of CaM mutations. LQTS-CaM mutants (D96V, D130G, F142L) impaired Ca2+-dependent inactivation, whereas the CPVT-CaM mutant N54I had no effect on Ca2+-dependent inactivation. LQTS-CaM mutants led to loss of Ca2+-transient entrainment with the rank order from greatest to least effect: CaM-D130G~CaM-D96V>>CaM-F142L. This rank order follows measured Ca2+-CaM affinities for wild-type and mutant CaM. Acute isoproterenol restored entrainment for CaM-130G and CaM-D96V but caused irreversible cytosolic Ca2+ overload for cells expressing a CPVT-CaM mutant. CONCLUSIONS CaM mutations associated with LQTS may not affect L-type Na+ current but may evoke defective Ca2+-dependent inactivation of L-type Ca2+ current.