Catecholaminergic Polymorphic Ventricular Tachycardia Phenotype Research Articles

Prognosis and clinical management of asymptomatic family members with RYR2-mediated catecholaminergic polymorphic ventricular tachycardia: a review.

Despite its low prevalence, the potential diagnosis of catecholaminergic polymorphic ventricular tachycardia (CPVT) should be at the forefront of a paediatric cardiologists mind in children with syncope during exercise or emotions. Over the years, the number of children with a genetic diagnosis of CPVT due to a (likely) pathogenic RYR2 variant early in life and prior to the onset of symptoms has increased due to cascade screening programmes. Limited guidance for this group of patients is currently available. Therefore, we aimed to summarise currently available literature for asymptomatic patients with a (likely) pathogenic RYR2 variant, particularly the history of CPVT and its genetic architecture, the currently available diagnostic tests and their limitations, and the development of a CPVT phenotype - both electrocardiographically and symptomatic - of affected family members. Their risk of arrhythmic events is presumably low and a phenotype seems to develop in the first two decades of life. Future research should focus on this group in particular, to better understand the development of a phenotype over time, and therefore, to be able to better guide clinical management - including the frequency of diagnostic tests, the timing of the initiation of drug therapy, and lifestyle recommendations.

Precision medicine in catecholaminergic polymorphic ventricular tachycardia: Recent advances toward personalized care.

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a rare inherited cardiac ion channelopathy where the initial disease presentation is during childhood or adolescent stages, leading to increased risks of sudden cardiac death. Despite advances in medical science and technology, several gaps remain in the understanding of the molecular mechanisms, risk prediction, and therapeutic management of patients with CPVT. Recent studies have identified and validated seven sets of genes responsible for various CPVT phenotypes, including RyR2, CASQ-2, TRDN, CALM1, 2, and 3, and TECRL, providing novel insights into the molecular mechanisms. However, more data on atypical CPVT genotypes are required to investigate the underlying mechanisms further. The complexities of the underlying genetics contribute to challenges in risk stratification as well as the uncertainty surrounding nongenetic modifiers. Therapeutically, although medical management involving beta-blockers and flecainide, or insertion of an implantable cardioverter defibrillator remains the mainstay of treatment, animal and stem cell studies on gene therapy for CPVT have shown promising results. However, its clinical applicability remains unclear. Current gene therapy studies have primarily focused on the RyR2 and CASQ-2 variants, which constitute 75% of all CPVT cases. Alternative approaches that target a broader population, such as CaMKII inhibition, could be more feasible for clinical implementation. Together, this review provides an update on recent research on CPVT, highlighting the need for further investigation of the molecular mechanisms, risk stratification, and therapeutic management of this potentially lethal condition.

Calmodulin mutations affecting Gly114 impair binding to the NaV1.5 IQ-domain.

Missense variants in CALM genes encoding the Ca2+-binding protein calmodulin (CaM) cause severe cardiac arrhythmias. The disease mechanisms have been attributed to dysregulation of RyR2, for Catecholaminergic Polymorphic Ventricular Tachycardia (CPVT) and/or CaV1.2, for Long-QT Syndrome (LQTS). Recently, a novel CALM2 variant, G114R, was identified in a mother and two of her four children, all of whom died suddenly while asleep at a young age. The G114R variant impairs closure of CaV1.2 and RyR2, consistent with a CPVT and/or mild LQTS phenotype. However, the children carrying the CALM2 G114R variant displayed a phenotype commonly observed with variants in NaV1.5, i.e., Brugada Syndrome (BrS) or LQT3, where death while asleep is a common feature. We therefore hypothesized that the G114R variant specifically would interfere with NaV1.5 binding. Here, we demonstrate that CaM binding to the NaV1.5 IQ-domain is severely impaired for two CaM variants G114R and G114W. The impact was most severe at low and intermediate Ca2+ concentrations (up to 4µM) resulting in more than a 50-fold reduction in NaV1.5 binding affinity, and a smaller 1.5 to 11-fold reduction at high Ca2+ concentrations (25-400µM). In contrast, the arrhythmogenic CaM-N98S variant only induced a 1.5-fold reduction in NaV1.5 binding and only at 4µM Ca2+. A non-arrhythmogenic I10T variant in CaM did not impair NaV1.5 IQ binding. These data suggest that the interaction between NaV1.5 and CaM is decreased with certain CaM variants, which may alter the cardiac sodium current, INa. Overall, these results suggest that the phenotypic spectrum of calmodulinopathies may likely expand to include BrS- and/or LQT3-like traits.

Generation of two induced pluripotent stem cell lines from catecholaminergic polymorphic ventricular tachycardia patients carrying RYR2 mutations

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a congenital arrhythmic syndrome caused by the RYR2 gene encoded ryanodine receptor. Mutations on RYR2 are commonly associated with ventricular tachycardia after adrenergic stimulation, leading to lethal arrhythmias and sudden cardiac death. We generated two human induced pluripotent stem cell (iPSC) lines from CPVT affected patients carrying single missense heterozygote RYR2 mutations, c.1082 G > A and c.100 A > C. Pluripotency and differentiation capability into derivatives of three germ layers were evaluated along with karyotype stability in the report. The generated patient-specific iPSC lines provide a reliable tool to investigate the CPVT phenotype and understand underlaying mechanisms.

PO-04-090 2 HIT WONDER: AUTOSOMAL RECESSIVE-CPVT WITH CONDUCTION DISEASE

Catecholaminergic Polymorphic Ventricular Tachycardia (CPVT) is a primary arrhythmogenic syndrome with clinical manifestations of syncope and sudden death following exercise or agitation. Present familial cases of autosomal recessive CPVT due to homozygous variants in RYR2 with associated conduction disease. The case was reviewed in detail as below. A 14 year-old girl presented following out of hospital cardiac arrest at a trampoline park. AED strip demonstrated ventricular fibrillation that terminated to sinus rhythm with electrical cardioversion. Workup was notable for a resting ECG with a rate of 52 bpm, a PR of 238 and a right bundle branch block with a QRS duration of 145ms . Exercise stress test resulted in frequent premature ventricular beats and bidirectional couplets. Genetic testing demonstrated homozygosity of a novel VUS in RYR2 (c.9392A>C (p.Tyr3131Ser)). Analysis by REVEL demonstrated a score of (0.763), predicting pathogenicity. Parents were found to be heterozygous asymptomatic carriers of the same variant and one sibling was homozygous for the variant. This sibling had a prolonged PR interval and intraventricular conduction delay and was phenotypically positive during exercise stress test. Two siblings who were heterozygous for the variant had normal ECG and exercise stress tests. The proband was started on nadalol and flecainide and a dual chamber ICD was placed for secondary prevention and significant sinus bradycardia prior to initiation of antiarrhythmics. On follow up EST the patient was noted to have acute onset high grade AV block during exercise resulting in initiation of ventricular pacing as well as runs of atrial tachycardia. Baseline ECG did not demonstrate QRS widening and flecainide level was within limits. The affected sibling was started on nadolol. We report two cases of patients homozygous for a VUS in RYR2 manifesting phenotypically as CPVT with associated conduction disease. Further study of this distinct phenotype of CPVT is warranted.

Abstract 12554: Hipsc Derived Cardiomyocytes for Mechanistic, Patient Tailored Treatment in a Novel Mutation Linked to Cpvt

Background: Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a disease related to altered ryanodine receptor (RyR) mediated SR Ca 2+ leak and associated with sudden cardiac death in young adults. We recently identified a family with a mixed phenotype of CPVT and long-QT-syndrome (CPVT3*) related to a novel mutation in the trans-2.3-enyol-CoA reductase-like (TECRL) gene. Ventricular arrhythmia burden varies greatly between individual CPVT3* patients. Methods/Results: We generated human induced pluripotent stem cell derived cardiomyocytes (iPSc-CM) from a control and CPVT3* patient to explore underlying mechanisms and individualize pharmacological treatment. iPSc-CM were studied using confocal Ca 2+ imaging (fluo-4). Ca 2+ signaling during excitation-contraction coupling was studied in the presence of different clinically available modulators of RyR mediated SR Ca 2+ leak. In particular, cardiomyocyte function and arrythmias were tested upon addition of the class Ic antiarrhythmic flecainide (10μM), the RyR stabilizer dantrolene (1μM), the betablocker R-Carvedilol (1μM), the kinase inhibitor Rimacalib (10μM) and the RyR-Calstabin interaction stabilizer S107 (10μM). Under baseline conditions, CPVT3* iPSc-CM showed significantly decreased Ca 2+ transient (CaT) amplitudes and increased CaT duration. Upon addition of the beta-agonist isoproterenol (ISO, 100nM), CaT amplitudes and pro-arrhythmogenic Ca 2+ release (i.e. Ca 2+ sparks) significantly increased in CPVT3* as compared to control. All pharmaceuticals altered Ca 2+ signaling in CPVT3* in a substance dependent manner and pro-arrhythmogenic Ca 2+ release was particularly mitigated upon addition of flecainide. Treatment of the CPVT3* patient with flecainide significantly reduced ventricular premature beats as determined by 24 hour holter-ECG monitoring. Conclusion: We quantitatively assessed drugs targeting RyR mediated SR Ca 2+ leak in a CPVT3* patient with a novel TECRL mutation using iPSc-CM. Favorable in-vitro results of flecainide were mirrored in a significant reduction of ventricular arrhythmia in-vivo. Our results underscore the potential of iPSc-CM for the development of patient-tailored treatment in genetic arrhythmia syndromes.

Impaired Dynamic Sarcoplasmic Reticulum Ca Buffering in Autosomal Dominant CPVT2.

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a potentially lethal cardiac arrhythmia syndrome triggered by catecholamines released during exercise, stress, or sudden emotion. Variants in the calsequestrin-2 gene (CASQ2), encoding the major calcium (Ca) binding protein in the sarcoplasmic reticulum (SR), are the second most common cause of CPVT. Recently, several CASQ2 gene variants, such as CASQ2-K180R, have been linked to an autosomal dominant form of Casq2-linked CPVT (CPVT2), but the underlying mechanism is not known. A K180R mouse model was generated using CRIPSR/Cas9. Heterozygous and homozygous K180R mice were studied using telemetry ECG recordings in vivo. Ventricular cardiomyocytes were isolated and studied using fluorescent Ca indicators and patch clamp. Expression levels and localization of SR Ca-handling proteins were evaluated using Western blotting and immunostaining. Intra-SR Ca kinetics were quantified using low-affinity Ca indicators. K180R mice exhibit an autosomal dominant CPVT phenotype following exercise or catecholamine stress. Upon catecholamine stress, K180R ventricular cardiomyocytes exhibit increased spontaneous SR Ca release events, triggering delayed afterdepolarizations and spontaneous beats. K180R had no effect on levels of Casq2, Casq2 polymers, or other SR Ca-handling proteins. Intra-SR Ca measurements revealed that K180R impaired dynamic intra-SR Ca buffering, resulting in a more rapid rise of free Ca in the SR during diastole. Steady-state SR Ca buffering and total SR Ca content were not changed. Consistent with the reduced dynamic intra-SR buffering, K180R causes reduced SR Ca release refractoriness. CASQ2-K180R causes CPVT2 via a heretofore unknown mechanism that differs from CASQ2 variants associated with autosomal recessive CPVT2. Unlike autosomal recessive CASQ2 variants, K180R impairs the dynamic buffering of Ca within the SR without affecting total SR Ca content or Casq2 protein levels. Our data provide insight into the molecular mechanism underlying autosomal dominant CPVT2.

Abstract P1083: Calcium-dependent Proteolysis Of Mitochondrial Opa1 By Calpain Contributes To Arrhythmogenesis In Catecholaminergic Polymorphic Ventricular Tachycardia

Intracellular Ca 2+ cycling in ventricular myocytes (VMs) is tightly linked to mitochondrial metabolic output, and disturbances in this coupling contribute to cardiac arrhythmias. We recently demonstrated in the heritable arrhythmia syndrome catecholaminergic polymorphic ventricular tachycardia (CPVT), the arrhythmogenic phenotype is revealed by oxidation of the cardiac ryanodine receptor (RyR2) calcium release channel by mitochondrial reactive oxygen species (mito-ROS), secondary to causative genetic mutation. However, exact mechanisms linking RyR2 dysfunction to mitochondrial injury remain unclear. We hypothesized that RyR2 hyperactivity activates the intermembrane space (IMS)-residing mitochondrial Ca 2+ -dependent protease , driving proteolysis of OPA1, a protein responsible for cristae flattening. This results in altered mitochondrial cristae diameter and mito-ROS production, exacerbating the arrhythmogenic CPVT phenotype. To test this, we created a unique CPVT RyR2-S2222L (+/-) rat model. Electron microscopy studies revealed a significant increase in cristae diameters in Langendorff-perfused CPVT rat hearts. Western blot analysis showed significantly reduced OPA1 expression in isolated CPVT VMs, suggesting that Ca 2+ -dependent proteolysis of OPA1 could underlie these ultrastructural changes. Pilot experiments suggest specific inhibition of via adeno-associated viral injection of IMS-targeted calpastatin (CAST, 10 12 VP, 4 weeks) can reduce ventricular tachycardia incidence of CPVT rats in vivo. Adenoviral expression of IMS-CAST in isolated VMs restored OPA1 expression levels and significantly improved intracellular Ca 2+ cycling in CPVT, suggesting a marked improvement in RyR2 function. Furthermore, genetic probes mtRCamp1h and HyPer-7 revealed that IMS- inhibition significantly improves mitochondrial matrix Ca 2+ homeostasis and mito-ROS in CPVT VMs, respectively. Importantly, these beneficial effects were lost in VMs with shRNA-mediated knockdown of OPA1. Our data suggest that enhanced activity of RyR2 promotes IMS- -mediated degradation of OPA1, resulting in mitochondrial structural damage that contributes to proarrhythmic remodeling in CPVT.

How does flecainide impact RyR2 channel function?

Flecainide, a cardiac class 1C blocker of the surface membrane sodium channel (NaV1.5), has also been reported to reduce cardiac ryanodine receptor (RyR2)-mediated sarcoplasmic reticulum (SR) Ca2+ release. It has been introduced as a clinical antiarrhythmic agent for catecholaminergic polymorphic ventricular tachycardia (CPVT), a condition most commonly associated with gain-of-function RyR2 mutations. Current debate concerns both cellular mechanisms of its antiarrhythmic action and molecular mechanisms of its RyR2 actions. At the cellular level, it targets NaV1.5, RyR2, Na+/Ca2+ exchange (NCX), and additional proteins involved in excitation-contraction (EC) coupling and potentially contribute to the CPVT phenotype. This Viewpoint primarily addresses the various direct molecular actions of flecainide on isolated RyR2 channels in artificial lipid bilayers. Such studies demonstrate different, multifarious, flecainide binding sites on RyR2, with voltage-dependent binding in the channel pore or voltage-independent binding at distant peripheral sites. In contrast to its single NaV1.5 pore binding site, flecainide may bind to at least four separate inhibitory sites on RyR2 and one activation site. None of these binding sites have been specifically located in the linear RyR2 sequence or high-resolution structure. Furthermore, it is not clear which of the inhibitory sites contribute to flecainide's reduction of spontaneous Ca2+ release in cellular studies. A confounding observation is that flecainide binding to voltage-dependent inhibition sites reduces cation fluxes in a direction opposite to physiological Ca2+ flow from SR lumen to cytosol. This may suggest that, rather than directly blocking Ca2+ efflux, flecainide can reduce Ca2+ efflux by blocking counter currents through the pore which otherwise limit SR membrane potential change during systolic Ca2+ efflux. In summary, the antiarrhythmic effects of flecainide in CPVT seem to involve multiple components of EC coupling and multiple actions on RyR2. Their clarification may identify novel specific drug targets and facilitate flecainide's clinical utilization in CPVT.

EP News: Basic and Translational

Blackwell et al (JCI Insight January 6, 2022;doi:10.1172/jci.insight.151893, PMID 34990403) investigated the sources of ventricular arrhythmia triggered by delayed afterdepolarizations (DADs) in catecholaminergic polymorphic ventricular tachycardia (CPVT) syndrome by using cardiac calsequestrin knockout mice. The authors demonstrated that loss of calsequestrin in only ventricular myocytes produced a full-blown CPVT phenotype whereas mice with loss of calsequestrin in only Purkinje cells were comparable to wild-type mice.

The Purkinje-myocardial junction is the anatomic origin of ventricular arrhythmia in CPVT.

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is an arrhythmia syndrome caused by gene mutations that render RYR2 Ca release channels hyperactive, provoking spontaneous Ca release and delayed afterdepolarizations (DADs). What remains unknown is the cellular source of ventricular arrhythmia triggered by DADs: Purkinje cells in the conduction system or ventricular cardiomyocytes in the working myocardium. To answer this question, we used a genetic approach in mice to knock out cardiac calsequestrin either in Purkinje cells or in ventricular cardiomyocytes. Total loss of calsequestrin in the heart causes a severe CPVT phenotype in mice and humans. We found that loss of calsequestrin only in ventricular myocytes produced a full-blown CPVT phenotype, whereas mice with loss of calsequestrin only in Purkinje cells were comparable to WT mice. Subendocardial chemical ablation or restoration of calsequestrin expression in subendocardial cardiomyocytes neighboring Purkinje cells was sufficient to protect against catecholamine-induced arrhythmias. In silico modeling demonstrated that DADs in ventricular myocardium can trigger full action potentials in the Purkinje fiber, but not vice versa. Hence, ectopic beats in CPVT are likely generated at the Purkinje–myocardial junction via a heretofore unrecognized tissue mechanism, whereby DADs in the ventricular myocardium trigger full action potentials in adjacent Purkinje cells.

Abstract 12552: Juxta-Purkinje Ventricular Myocytes Are the Cellular Trigger of Catecholaminergic Polymorphic Ventricular Tachycardia

Introduction: Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a genetic arrhythmia syndrome caused by mutations that render intracellular calcium release channels hyperactive during adrenergic stress. Hyperactive calcium release has also been identified as a cause of ventricular arrhythmia in acquired heart diseases such as heart failure. What remains unknown is the cellular source of ventricular arrhythmia triggered by inappropriate calcium release – Purkinje cells in the cardiac conduction system or ventricular cardiomyocytes in the working myocardium. Methods: To investigate the cellular origin, we used a genetic approach in mice to knock out cardiac calsequestrin (Casq2) either in Purkinje cells or in ventricular cardiomyocytes (Panel A). Total loss of calsequestrin in the heart causes a severe CPVT phenotype in mice and humans. Immunostaining and western blotting were used to examine protein expression and arrhythmias were induced by administration of isoprenaline. Results: Loss of Casq2 only in ventricular myocytes produced a full-blown CPVT phenotype, whereas mice with loss of Casq2 only in Purkinje cells were comparable to wild-type mice (Panel B). Subendocardial chemical ablation or restoration of Casq2 expression in subendocardial cardiomyocytes neighboring Purkinje cells was sufficient to protect against catecholamine-induced arrhythmias (Panels C, D). Conclusion: Our data indicate that subendocardial ventricular myocytes juxtaposed to Purkinje fibers are the source of ventricular arrhythmias triggered by intracellular calcium release and hence identify a novel cellular target for arrhythmia prevention.

RyR2 Gain-of-Function and Not So Sudden Cardiac Death

RyR2 Gain-of-Function and Not So Sudden Cardiac Death

Rare RYR2 p.Thr85Ile variant is associated with catecholaminergic polymorphic ventricular tachycardia

Mutations in the cardiac ryanodine receptor 2 gene (RYR2) are noted in approximately 55% of the patients with catecholaminergic polymorphic ventricular tachycardia (CPVT). However, a high background rate of rare amino acid-altering variants in RYR2 [≈3% (whites) to 6% (nonwhites)] in combination with delayed onset and variable expressivity of the CPVT phenotype make attributing causality to rare RYR2 variants difficult. The proposed set of guidelines for characterization of variants include, among many different criteria, functional studies to classify variants as pathogenic versus non- pathogenic. However, in vitro data, especially for complex channelopathies like CPVT, does not always correlate well with the human phenotype; therefore, characterizing the phenotypic features of rare RYR2 variants in humans is of importance. We report a 13-year-old carrier of a rare maternally inherited variant in exon 3 of RYR2 (p.Thr85Ile, c.254C>T [rs772005577]) who was successfully resuscitated from exercise-associated sudden cardiac arrest and was subsequently found to have exercise-induced complex ventricular ectopy. His brother, who has been asymptomatic, was also found to carry the same variant and demonstrated complex ventricular ectopy on exercise stress testing. These findings suggest that this rare variant, which has previously not been reported in patients with RYR2 related conditions, is associated with pathogenicity.

Protein expression profiles in murine ventricles modeling catecholaminergic polymorphic ventricular tachycardia: effects of genotype and sex.

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is associated with mutations in the cardiac ryanodine receptor (RyR2). These result in stress-induced ventricular arrhythmic episodes, with clinical symptoms and prognosis reported more severe in male than female patients. Murine homozygotic RyR2-P2328S (RyR2S/S ) hearts replicate the proarrhythmic CPVT phenotype of abnormal sarcoplasmic reticular Ca2+ leak and disrupted Ca2+ homeostasis. In addition, RyR2S/S hearts show decreased myocardial action potential conduction velocities (CV), all features implicated in arrhythmic trigger and substrate. The present studies explored for independent and interacting effects of RyR2S/S genotype and sex on expression levels of molecular determinants of Ca2+ homeostasis (CASQ2, FKBP12, SERCA2a, NCX1, and CaV 1.2) and CV (NaV 1.5, Connexin (Cx)-43, phosphorylated-Cx43, and TGF-β1) in mice. Expression levels of Ca2+ homeostasis proteins were not altered, hence implicating abnormal RyR2 function alone in disrupted cytosolic Ca2+ homeostasis. Furthermore, altered NaV 1.5, phosphorylated Cx43, and TGF-β1 expression were not implicated in the development of slowed CV. By contrast, decreased Cx43 expression correlated with slowed CV, in female, but not male, RyR2S/S mice. The CV changes may reflect acute actions of the increased cytosolic Ca2+ on NaV 1.5 and Cx43 function.

Co-Phenotype of Left Ventricular Non-Compaction Cardiomyopathy and Atypical Catecholaminergic Polymorphic Ventricular Tachycardia in Association With R169Q, a Ryanodine Receptor Type 2 Missense Mutation.

Left ventricular non-compaction (LVNC) is a cardiomyopathy characterized by prominent trabeculae and intertrabecular recesses. We present the cases of 3 girls with the sameryanodine receptor type 2(RYR2) mutation who had phenotypes of both catecholaminergic polymorphic ventricular tachycardia (CPVT) and LVNC .Methods and Results:Clinical characteristics and genetic background of the 3 patients were analyzed retrospectively. Age at onset was 5, 6, and 7 years, respectively. Clinical presentation included syncope during exercise in all 3 patients and cardiac arrest in 2 patients. LVNC diagnosis was confirmed on echocardiography according to previously defined criteria. Exercise stress testing provoked ventricular arrhythmia in two of the patients. Beta-blockers (n=3) and flecainide (n=2) were given, and an implantable cardioverter defibrillator was used in 1 patient. Genotyping identified the sameRYR2-R169Q missense mutation and no other CPVT- or LVNC-related gene mutations. Functional analysis of the mutation using HEK293 cells with single-cell Ca2+imaging and [3H]ryanodine binding analysis, indicated a gain of function: a reduced threshold for overload-induced Ca2+release from the sarcoplasmic reticulum and increased fractional Ca2+release. The rare association of LVNC and CPVT phenotypes withRYR2mutations is less likely to be coincidental. Screening for life-threatening arrhythmias using exercise or pharmacologic stress tests is recommended in LVNC patients to prevent sudden cardiac death in those with preserved LV function.

BASIC AND CLINICAL INSIGHTS IN CATECHOLAMINERGIC (FAMILIAL) POLYMORPHIC VENTRICULAR TACHYCARDIA.

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a potentially lethal disease, whose characteristic ventricular tachycardias are adrenergic-dependent. Although rare, CPVT should be considered in the differential diagnosis of young individuals with exercise-induced syncope. Mutations in five different genes (RYR2, CASQ2, CALM1, TRDN, and TECRL) are associated with the CPVT phenotype, although RYR2 missense mutations are implicated in up to 60 % of all CPVT cases. Genetic testing has an essential role in the diagnosis, management, pre-symptomatic diagnosis, counseling, and treatment of the proband; furthermore, genetic information can be useful for offspring and relatives. By expert consensus, CPVT gene testing is a Class I recommendation for patients with suspected CPVT. Beta-adrenergic and calcium-channel blockers are the cornerstones of treatment due to the catecholaminergic dependence of the arrhythmias. Unresponsive patients are treated with an implantable cardioverter-defibrillator to reduce the risk of sudden cardiac death. In the present article, a brief review of the genetic and molecular mechanisms of this intriguing disease is provided.

Interplay between Triadin and Calsequestrin in the Pathogenesis of CPVT in the Mouse.

Recessive forms of catecholaminergic polymorphic ventricular tachycardia (CPVT) are induced by mutations in genes encoding triadin or calsequestrin, two proteins that belong to the Ca2+ release complex, responsible for intracellular Ca2+ release triggering cardiac contractions. To better understand the mechanisms of triadin-induced CPVT and to assay multiple therapeutic interventions, we used a triadin knockout mouse model presenting a CPVT-like phenotype associated with a decrease in calsequestrin protein level. We assessed different approaches to rescue protein expression and to correct intracellular Ca2+ release and cardiac function: pharmacological treatment with kifunensine or a viral gene transfer-based approach, using adeno-associated virus serotype 2/9 (AAV2/9) encoding the triadin or calsequestrin. We observed that the levels of triadin and calsequestrin are intimately linked, and that reduction of both proteins contributes to the CPVT phenotype. Different combinations of triadin and calsequestrin expression level were obtained using these therapeutic approaches. A full expression of each is not necessary to correct the phenotype; a fine-tuning of the relative re-expression of both triadin and calsequestrin is required to correct the CPVT phenotype and rescue the cardiac function. AAV-mediated gene delivery of calsequestrin or triadin and treatment with kifunensine are potential treatments for recessive forms of CPVT due to triadin mutations.

Abstract 238: Critical Role of Ryanodine Receptor Bound Calmodulin to Prevent Catecholaminergic Polymorphic Ventricular Tachycardia

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is caused by a single point mutation in the cardiac type 2 ryanodine receptor (RyR2). Using knock-in mouse (KI) model (R2474S/+), we previously reported that a single point mutation within the RyR2 sensitizes the channel to agonists, primarily mediated by defective inter-domain interaction within the RyR2 molecule and subsequent dissociation of calmodulin (CaM) from the RyR2. Here, we examined whether CPVT can be genetically rescued by enhancing the binding affinity of CaM to the RyR2. We first determined whether there is a possible amino-acid substitution at the CaM-binding domain in the RyR2 (3584-3603) that can enhancethe binding affinity of CaM, and found that V3599K substitution showed the highest binding affinity of CaM to CaM-binding domain. Hence, we generated a heterozygous KI mouse model (V3599K/+) with a single amino acid substitution in the CaM-binding domain of the RyR2, and crossbred it with the heterozygous CPVT -associated R2474S/+ KI mouse to obtain a double heterozygous R2474S/V3599K KI mouse. The CPVT phenotypes—bidirectional or polymorphic ventricular tachycardia, spontaneous Ca2+ transients, and Ca2+ sparks—were all inhibited in the R2474S/V3599K mice. Thus, enhancement of the CaM binding affinity of the RyR2 is essential to prevent CPVT-associated arrhythmogenesis.

Ryanodine receptor-bound calmodulin is essential to protect against catecholaminergic polymorphic ventricular tachycardia.

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is caused by a single point mutation in the cardiac type 2 ryanodine receptor (RyR2). Using a knockin (KI) mouse model (R2474S/+), we previously reported that a single point mutation within the RyR2 sensitizes the channel to agonists, primarily mediated by defective interdomain interaction within the RyR2 and subsequent dissociation of calmodulin (CaM) from the RyR2. Here, we examined whether CPVT can be genetically rescued by enhancing the binding affinity of CaM to the RyR2. We first determined whether there is a possible amino acid substitution within the CaM-binding domain in the RyR2 (3584-3603 residues) that can enhance its binding affinity to CaM and found that V3599K substitution showed the highest binding affinity of CaM to the CaM-binding domain. Hence, we generated a heterozygous KI mouse model (V3599K/+) with a single amino acid substitution in the CaM-binding domain of the RyR2 and crossbred it with the heterozygous CPVT-associated R2474S/+-KI mouse to obtain a double-heterozygous R2474S/V3599K-KI mouse model. The CPVT phenotypes - bidirectional or polymorphic ventricular tachycardia, spontaneous Ca2+ transients, and Ca2+ sparks - were all inhibited in the R2474S/V3599K mice. Thus, enhancement of the CaM-binding affinity of the RyR2 is essential to prevent CPVT-associated arrhythmogenesis.