Mechanisms of Abnormal Calcium Homeostasis in Mutations Responsible for Catecholaminergic Polymorphic Ventricular Tachycardia

Abstract

Abstract-Catecholaminergic polymorphic ventricular tachycardia is a heritable arrhythmia unmasked by exertion or stress and is characterized by triggered activity and sudden cardiac death. In this study, we simulated mutations in 2 genes linked to catecholaminergic polymorphic ventricular tachycardia, the first located in calsequestrin (CSQN2) and the second in the ryanodine receptor (RyR2). The aim of the study was to investigate the mechanistic basis for spontaneous Ca 2ϩ release events that lead to delayed afterdepolarizations in affected patients. Sarcoplasmic reticulum (SR) luminal Ca 2ϩ sensing was incorporated into a model of the human ventricular myocyte, and CSQN2 mutations were modeled by simulating disrupted RyR2 luminal Ca 2ϩ sensing. In voltage-clamp mode, the mutant CSQN2 model recapitulated the smaller calcium transients, smaller time to peak calcium transient, and accelerated recovery from inactivation seen in experiments. In current clamp mode, in the presence of ␤ stimulation, we observed delayed afterdepolarizations, suggesting that accelerated recovery of RyR2 induced by impaired luminal Ca 2ϩ sensing underlies the triggered activity observed in mutant CSQN2-expressing myocytes. In current-clamp mode, in a model of mutant RyR2 that is characterized by reduced FKBP12.6 binding to the RyR2 on ␤ stimulation, the impaired coupled gating characteristic of these mutations was modeled by reducing cooperativity of RyR2 activation. In current-clamp mode, the mutant RyR2 model exhibited increased diastolic RyR2 open probability that resulted in formation of delayed afterdepolarizations. In conclusion, these minimal order models of mutant CSQN2 and RyR2 provide plausible mechanisms by which defects in RyR2 gating may lead to the cellular triggers for arrhythmia, with implications for the development of targeted therapy. (Circ Res. 2007;100:e22-e31.) Key Words: catecholaminergic polymorphic ventricular tachycardia Ⅲ delayed afterdepolarization Ⅲ ryanodine receptor Ⅲ calsequestrin Ⅲ calcium handling Ⅲ simulation C atecholaminergic polymorphic ventricular tachycardia (CPVT) is a heritable arrhythmia unmasked by exertion or stress. Mutations in the cardiac ryanodine receptor 2 (RyR2, gene RYR2) 1-3 and the calcium buffer calsequestrin 2 (CSQN2, gene CASQ2) 4,5 have been reported in families affected by the disorder. More than 40 mutations have been identified thus far, with many additional mutants constantly being added to the spectrum of disease-linked gene products. 6 -8 The rate of discovery of newly identified CPVT mutations has outpaced the experimental characterization of the function of the mutant gene products. Indeed, much controversy still exists regarding the cellular mechanisms by which these mutations cause arrhythmias. Hypotheses have been advanced on the basis of recent experimental characterizations in transgenic animal models, 9 in ventricular myocytes, 10,11 and in lipid bilayers. 12 Evidence suggests that the RyR2 exists in a supramolecular release complex along with CSQN2 at junctio...

Calsequestrin 2 (CASQ2) mutations increase expression of calreticulin and ryanodine receptors, causing catecholaminergic polymorphic ventricular tachycardia

“…Although Mg 2+ may affect several ion channels, Mg 2+ inhibition of RyR2 channel activity (24, 35-37) appeared to be sufficient to account for its beneficial effects on CASQ2-deficient myocytes. A recent simulation model of CPVT similarly supported this conclusion: addition of other currents (e.g., the sodium-Ca 2+ exchanger) was unnecessary to recapitulate experimental data (38). In CASQ2-deficient mice, Mg 2+ also significantly decreased the frequency of catecholamineinduced sustained ventricular arrhythmias ( Table 2).…”

Ca2+-induced delayed afterdepolarizations are triggered by dyadic subspace Ca2+ affirming that increasing SERCA reduces aftercontractions

“…The general mechanism (besides stretch) has been assumed to be based on an intracellular Ca 2ϩ storage (endoplasmic reticulum) rapidly unloading its content into the cytosolic space, while reuptake has shown a much longer time constant, making the system prone to selfsustained intracellular Ca 2ϩ oscillations. Despite the large number of experimental results, and the range of mathematical models available, only a limited number of modeling studies have been conducted to reproduce DADs based on biophysical mechanisms (24,34,64,81). Using 37 mathematical models of atrial, Purkinje, and ventricular cells of various species, we found that 23 of them could be used to reproduce DADs (see Table 3).…”

“…Mathematical models (Shannon04 and Iyer07) have been used in the past to confirm the principal plausibility of this hypothesis (34,81), but alternative mechanisms were not quantitatively assessed. Our results show that both of these models can be used to reproduce DADs in the absence of a dependence of the RyR open probability on Ca SR levels (see Table 3 and Fig.…”

“…(64) studied the ability of the Earm and Noble (17) atrial cell model to produce DADs under simulated conditions of Na ϩ and thus Ca 2ϩ overload (Na ϩ -K ϩ pump block increased intracellular Na ϩ ) and were able to find DADs, but only within a narrow parameter space. A few other modeling studies addressed the appearance of DADs, but always using only one specific model (24,34,81). These approaches did not reflect the fact that DADs are a general phenomenon of atrial, Purkinje, and ventricular cells across species, as shown by the respective findings in the mouse, rat, guinea pig, ferret, rabbit, feline, canine, calf, sheep, and human (40, 56, 69, 77, 84, 87, 94 -96, 103, 105).…”

Chaste

“…The models include representation of the main mechanisms of ionic transport across the cell membrane and between subcellular compartments. From a mathematical point of view, the models typically consist of systems of ordinary differential equations (ODEs), with the most detailed ones (such as Iyer et al (2007)) having over 60 ODEs. These models allow representation of the effect of mutations, drugs and disease on ion channel function.…”