Abnormal Propagation of Calcium Waves and Ultrastructural Remodeling in Recessive Catecholaminergic Polymorphic Ventricular Tachycardia

Abstract

C atecholaminergic polymorphic ventricular tachycardia (CPVT) is an inherited arrhythmogenic disease characterized by stress-and emotion-induced life-threatening arrhythmias. Mutations in the cardiac ryanodine receptor-2 (RyR2) and cardiac calsequestrin-2 (CASQ2) genes have been associated with the autosomal-dominant and the autosomalrecessive forms of CPVT.1 Knock-in mouse models that carry gain-of-function RyR2 mutations exhibit phenotypes similar to the clinical manifestations observed in patients with CPVT, including the development of bidirectional/polymorphic VT on exposure to catecholamines.2-5 Investigations of knock-in models of dominant CPVT have demonstrated that abnormal Ca 2+ release from mutant RyR2 induces cell-wide Ca 2+ waves, delayed afterdepolarizations (DADs) and triggered activity, all of which are arrhythmogenic. 2,4,6 Recently, attention has turned to the study of the recessive form of CPVT that is caused by homozygous mutations in the CASQ2 gene. Most experimental studies on recessive CPVT have been performed using CASQ2 knockout mice, whereas only few knock-in mouse carriers of human CASQ2 mutations sparks with shortened coupling intervals, suggesting a reduced refractoriness of Ca 2+ 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. Conclusions:Our data show that homozygous cardiac calsequestrin-2-R33Q myocytes develop spontaneous Ca 2+ 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 Ca 2+ release unit architecture that leads to fragmentation of spontaneous Ca 2+ waves. We propose that these 2 substrates in R33Q myocytes synergize to provide a new arrhythmogenic mechanism for catecholaminergic polymorphic ventricular tachycardia. (Circ Res. 2013;113:142-152.)

Single Delivery of an Adeno-Associated Viral Construct to Transfer the CASQ2 Gene to Knock-In Mice Affected by Catecholaminergic Polymorphic Ventricular Tachycardia Is Able to Cure the Disease From Birth to Advanced Age

“…10,18,23 Furthermore, we observed an absence of the electron-dense CASQ2 polymer in the jSR cisternae.…”

“…Interestingly, we have shown that our gene therapy strategy is able to recover protein levels and to revert the jSR widening and fragmentation 23 seen in CRUs of R33Q. 10 It is also remarkable that electron microscopy allowed to appreciate the correct localization of the overexpressed CASQ2 in the jSR as documented by the reappearance of the chain-like polymers of CASQ2 in the jSR.…”

Stochastic initiation and termination of calcium-mediated triggered activity in cardiac myocytes

“…However, SR Ca 2+ release can also occur spontaneously, as observed in a variety of pathological conditions ranging from heart failure (8,9) to ischemia (10) to catecholaminergic polymorphic ventricular tachycardia (11)(12)(13)(14). Spontaneous Ca 2+ release (SCR) transiently elevates the cytosolic Ca 2+ concentration ([Ca] i ), which in turn increases the forward mode of the electrogenic Na + /Ca 2+ exchange (NCX) current (I NCX ).…”

Genetics of Sudden Cardiac Death

“…Thanks to the presence of several mouse models of CPVT that recapitulate the clinical phenotype of the disease, the understanding of arrhythmogenic mechanisms has advanced rapidly. 117 We now know that mutations in the RYR2 and CASQ2 genes lead to spontaneous diastolic Ca 2+ release that by increasing Ca 2+ levels in the cytosol activates the Na + / Ca 2+ antiporter. In the attempt to normalize Ca 2+ levels, the Na + /Ca 2+ antiporter extrudes Ca…”

“…117 Interestingly, the availability of animal models and of mechanistic understanding of the disease have allowed the advancement of therapeutic strategies. Watanabe et al 118 reported that flecainide is able to specifically attenuate calcium release by binding the RyR and by doing so can inhibit triggered activity.…”