Abstract 19541: Impaired Calcium Release, Prolonged Action Potentials and Early Afterdepolarizations in Ventricular Myocytes from Mice Harboring a Ryanodine Receptor 2 Loss-of-Function Mutation Associated with Catecholaminergic Polymorphic Ventricular Tachycardia

Abstract

Catecholaminergic polymorphic ventricular tachycardia (CPVT) patients harboring the cardiac ryanodine receptor (RyR2) mutation RyR2-A4860G exhibit sympathetically-driven idiopathic ventricular fibrillation without cardiac structural defects. In sharp contrast to other CPVT mutations characterized to date, RyR2-A4860G fails to activate in response to increasing luminal [Ca 2+ ] and, when expressed in HEK293 or HL-1 cells, it exhibits attenuated store overload-induced Ca 2+ release. Thus, RyR2-A4860G is the first loss-of-function mutation linked to CPVT, but the arrhythmogenic mechanisms are unknown. We used homologous recombination to generate a mouse harboring the RyR2-A4860G mutation and used isolated ventricular myocytes to investigate the cellular mechanisms by which this mutation generates ventricular arrhythmias. Mice heterozygous for the mutation (RyR2-A4860G +/- ) display no apparent cardiac structural defects. Whole cell current-clamp combined with confocal Ca 2+ imaging to simultaneously record actions potential (AP) and intracellular Ca 2+ transient ([Ca 2+ ] i ) revealed that AP duration at 90% repolarization (APD 90 ) in RyR2-A4860G +/- was longer than in WT cardiomyocytes at 1 Hz (209±51 vs. 118±37 ms, n=12 and 7, respectively). As a result of prolonged APs, RyR2-A4860G +/- cells exhibited early afterdepolarizations (EADs) at significantly greater frequency than WT (50% vs 14%, respectively). Ca 2+ imaging of RyR2-A4860G +/- cells showed that during the longer APs, Ca 2+ release events are often interspaced between two successive AP-triggered Ca 2+ transients. Since the [Ca 2+ ] i amplitude in RyR2-A4860G +/- cells is significantly lower than in WT (3.5±0.6 , 2.5±0.3, and 2.5±0.2 vs. 3.8±0.3, 4.3±0.5, and 4.4±0.7, at 1, 2 and 3 Hz, respectively), the presence of Ca 2+ release suggests decreased Ca 2+ -dependent inactivation, and subsequent re-activation, of L-type Ca 2+ current during the AP plateau. In conclusion, our results demonstrate that RyR2-A4860G mutation is accompanied by profound impairment of SR Ca 2+ release, which leads to AP prolongation and EADs. This is a novel mechanism of arrhythmias in CPVT, distinct from delayed afterdepolarizations that are commonly detected in all gain-of-function RyR2 mutations.