Abnormal propagation of calcium waves and ultrastructural remodeling in recessive catecholaminergic polymorphic ventricular tachycardia.

Abstract

The recessive form of catecholaminergic polymorphic ventricular tachycardia is caused by mutations in the cardiac calsequestrin-2 gene; this variant of catecholaminergic polymorphic ventricular tachycardia is less well characterized than the autosomal-dominant form caused by mutations in the ryanodine receptor-2 gene. We characterized the intracellular Ca²⁺ homeostasis, electrophysiological properties, and ultrastructural features of the Ca²⁺ release units in the homozygous calsequestrin 2-R33Q knock-in mouse model (R33Q) R33Q knock-in mouse model. We studied isolated R33Q and wild-type ventricular myocytes and observed properties not previously identified in a catecholaminergic polymorphic ventricular tachycardia model. As compared with wild-type cells, R33Q myocytes (1) show spontaneous Ca²⁺ waves unable to propagate as cell-wide waves; (2) show smaller Ca²⁺sparks with shortened coupling intervals, suggesting a reduced refractoriness of Ca²⁺ release events; (3) have a reduction of the area of membrane contact, of the junctions between junctional sarcoplasmic reticulum and T tubules (couplons), and of junctional sarcoplasmic reticulum volume; (4) have a propensity to develop phase 2 to 4 afterdepolarizations that can elicit triggered beats; and (5) involve viral gene transfer with wild-type cardiac calsequestrin-2 that is able to normalize structural abnormalities and to restore cell-wide calcium wave propagation. Our data show that homozygous cardiac calsequestrin-2-R33Q myocytes develop spontaneous Ca²⁺ release events with a broad range of intervals coupled to preceding beats, leading to the formation of early and delayed afterdepolarizations. They also display a major disruption of the Ca²⁺ release unit architecture that leads to fragmentation of spontaneous Ca²⁺ waves. We propose that these 2 substrates in R33Q myocytes synergize to provide a new arrhythmogenic mechanism for catecholaminergic polymorphic ventricular tachycardia.