Arrhythmogenesis in a catecholaminergic polymorphic ventricular tachycardia mutation that depresses ryanodine receptor function

Significance Catecholaminergic polymorphic ventricular tachycardia (CPVT) is an arrhythmogenic syndrome characterized by life-threatening cardiac arrhythmias triggered by physical exercise or emotional stress. Although patients with CPVT have no abnormalities in cardiac structure, they present ventricular fibrillation or sudden death as the first symptom. Most CPVT cases are linked to mutations in cardiac ryanodine receptor (RyR2), an intracellular Ca ²⁺ channel that provides the majority of Ca ²⁺ that enables heart contraction. The current mechanism for CPVT arrhythmogenesis requires RyR2 affected by gain-of-function mutations. However, loss-of-function mutations of RyR2 have also been found in CPVT patients. We generated an animal model of CPVT that harbors a loss-of-function mutation that was originally described in humans and elucidated novel mechanisms by which hypoactive RyR2 channels trigger malignant arrhythmias. Current mechanisms of arrhythmogenesis in catecholaminergic polymorphic ventricular tachycardia (CPVT) require spontaneous Ca ²⁺ release via cardiac ryanodine receptor (RyR2) channels affected by gain-of-function mutations. Hence, hyperactive RyR2 channels eager to release Ca ²⁺ on their own appear as essential components of this arrhythmogenic scheme. This mechanism, therefore, appears inadequate to explain lethal arrhythmias in patients harboring RyR2 channels destabilized by loss-of-function mutations. We aimed to elucidate arrhythmia mechanisms in a RyR2-linked CPVT mutation (RyR2-A4860G) that depresses channel activity. Recombinant RyR2-A4860G protein was expressed equally as wild type (WT) RyR2, but channel activity was dramatically inhibited, as inferred by [ ³H]ryanodine binding and single channel recordings. Mice heterozygous for the RyR2-A4860G mutation (RyR2-A4860G ⁺/⁻) exhibited basal bradycardia but no cardiac structural alterations; in contrast, no homozygotes were detected at birth, suggesting a lethal phenotype. Sympathetic stimulation elicited malignant arrhythmias in RyR2-A4860G ⁺/⁻ hearts, recapitulating the phenotype originally described in a human patient with the same mutation. In isoproterenol-stimulated ventricular myocytes, the RyR2-A4860G mutation decreased the peak of Ca ²⁺ release during systole, gradually overloading the sarcoplasmic reticulum with Ca ²⁺. The resultant Ca ²⁺ overload then randomly caused bursts of prolonged Ca ²⁺ release, activating electrogenic Na ⁺-Ca ²⁺ exchanger activity and triggering earl