β-Adrenergic modulation of arrhythmogenesis and identification of targeted sites of antiarrhythmic therapy in Timothy (LQT8) syndrome: a theoretical study

Multiple beta cell-independent mechanisms drive hypoglycemia in Timothy syndrome

The canonical G406R mutation that increases Ca2+ influx through the CACNA1C-encoded CaV1.2 Ca2+ channel underlies the multisystem disorder Timothy syndrome (TS), characterized by life-threatening arrhythmias. Severe episodic hypoglycemia is among the poorly characterized non-cardiac TS pathologies. While hypothesized from increased Ca2+ influx in pancreatic beta cells and consequent hyperinsulinism, this hypoglycemia mechanism is undemonstrated because of limited clinical data and lack of animal models. We generated a CaV1.2 G406R knockin mouse model that recapitulates key TS features, including hypoglycemia. Unexpectedly, these mice do not show hyperactive beta cells or hyperinsulinism in the setting of normal intrinsic beta cell function, suggesting dysregulated glucose homeostasis. Patient data confirm the absence of hyperinsulinism. We discover multiple alternative contributors, including perturbed counterregulatory hormone responses with defects in glucagon secretion and abnormal hypothalamic control of glucose homeostasis. These data provide new insights into contributions of CaV1.2 channels and reveal integrated consequences of the mutant channels driving life-threatening events in TS.

Transmural APD heterogeneity determines ventricular arrhythmogenesis in LQT8 syndrome: Insights from Bidomain computational modeling

Long QT Syndrome type 8 (LQT8) is a cardiac arrhythmic disorder associated with Timothy Syndrome, stemming from mutations in the CACNA1C gene, particularly the G406R mutation. While prior studies hint at CACNA1C mutations' role in ventricular arrhythmia genesis, the mechanisms, especially in G406R presence, are not fully understood. This computational study explores how the G406R mutation, causing increased transmural dispersion of repolarization, induces and sustains reentrant ventricular arrhythmias. Using three-dimensional numerical simulations on an idealized left-ventricular model, integrating the Bidomain equations with the ten Tusscher-Panfilov ionic model, we observe that G406R mutation with 11% and 50% heterozygosis significantly increases transmural dispersion of repolarization. During S1-S4 stimulation protocols, these gradients facilitate conduction blocks, triggering reentrant ventricular tachycardia. Sustained reentry pathways occur only with G406R mutation at 50% heterozygosis, while neglecting transmural heterogeneities of action potential duration prevents stable reentry, regardless of G406R mutation presence.

Inhibition of Voltage-Gated Na+ Currents Exerted by KB-R7943 (2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea), an Inhibitor of Na+-Ca2+ Exchanging Process

KB-R7943, an isothiourea derivative, has been recognized as an inhibitor in the reverse mode of the Na⁺-Ca²⁺ exchanging process. This compound was demonstrated to prevent intracellular Na⁺-dependent Ca²⁺ uptake in intact cells; however, it is much less effective at preventing extracellular Na⁺-dependent Ca²⁺ efflux. Therefore, whether or how this compound may produce any perturbations on other types of ionic currents, particularly on voltage-gated Na⁺ current (I_Na), needs to be further studied. In this study, the whole-cell current recordings demonstrated that upon abrupt depolarization in pituitary GH₃ cells, the exposure to KB-R7943 concentration-dependently depressed the transient (I_Na(T)) or late component (I_Na(L)) of I_Na with an IC₅₀ value of 11 or 0.9 μM, respectively. Likewise, the dissociation constant for the KB-R7943-mediated block of I_Na on the basis of a minimum reaction scheme was estimated to be 0.97 μM. The presence of benzamil or amiloride could suppress the I_Na(L) magnitude. The instantaneous window Na⁺ current (I_Na(W)) activated by abrupt ascending ramp voltage (V_ramp) was suppressed by adding KB-R7943; however, subsequent addition of deltamethrin or tefluthrin (Tef) effectively reversed KB-R7943-inhibted I_Na(W). With prolonged duration of depolarizing pulses, the I_Na(L) amplitude became exponentially decreased; moreover, KB-R7943 diminished I_Na(L) magnitude. The resurgent Na⁺ current (I_Na(R)) evoked by a repolarizing V_ramp was also suppressed by adding this compound; moreover, subsequent addition of ranolazine or Tef further diminished or reversed, respectively, its reduction in I_Na(R) magnitude. The persistent Na⁺ current (I_Na(P)) activated by sinusoidal voltage waveform became enhanced by Tef; however, subsequent application of KB-R7943 counteracted Tef-stimulated I_Na(P). The docking prediction reflected that there seem to be molecular interactions of this molecule with the hNa_V1.2 or hNa_V1.7 channels. Collectively, this study highlights evidence showing that KB-R7943 has the propensity to perturb the magnitude and gating kinetics of I_Na (e.g., I_Na(T), I_Na(L), I_Na(W), I_Na(R), and I_Na(P)) and that the Na_V channels appear to be important targets for the in vivo actions of KB-R7943 or other relevant compounds.

Computational models in cardiology

The treatment of individual patients in cardiology practice increasingly relies on advanced imaging, genetic screening and devices. As the amount of imaging and other diagnostic data increases, paralleled by the greater capacity to personalize treatment, the difficulty of using the full array of measurements of a patient to determine an optimal treatment seems also to be paradoxically increasing. Computational models are progressively addressing this issue by providing a common framework for integrating multiple data sets from individual patients. These models, which are based on physiology and physics rather than on population statistics, enable computational simulations to reveal diagnostic information that would have otherwise remained concealed and to predict treatment outcomes for individual patients. The inherent need for patient-specific models in cardiology is clear and is driving the rapid development of tools and techniques for creating personalized methods to guide pharmaceutical therapy, deployment of devices and surgical interventions.

Computational Cardiac Modeling Reveals Mechanisms of Ventricular Arrhythmogenesis in Long QT Syndrome Type 8: CACNA1C R858H Mutation Linked to Ventricular Fibrillation

Functional analysis of the L-type calcium channel has shown that the CACNA1C R858H mutation associated with severe QT interval prolongation may lead to ventricular fibrillation (VF). This study investigated multiple potential mechanisms by which the CACNA1C R858H mutation facilitates and perpetuates VF. The Ten Tusscher-Panfilov (TP06) human ventricular cell models incorporating the experimental data on the kinetic properties of L-type calcium channels were integrated into one-dimensional (1D) fiber, 2D sheet, and 3D ventricular models to investigate the pro-arrhythmic effects of CACNA1C mutations by quantifying changes in intracellular calcium handling, action potential profiles, action potential duration restitution (APDR) curves, dispersion of repolarization (DOR), QT interval and spiral wave dynamics. R858H “mutant” L-type calcium current (I_CaL) augmented sarcoplasmic reticulum calcium content, leading to the development of afterdepolarizations at the single cell level and focal activities at the tissue level. It also produced inhomogeneous APD prolongation, causing QT prolongation and repolarization dispersion amplification, rendering R858H “mutant” tissue more vulnerable to the induction of reentry compared with other conditions. In conclusion, altered I_CaL due to the CACNA1C R858H mutation increases arrhythmia risk due to afterdepolarizations and increased tissue vulnerability to unidirectional conduction block. However, the observed reentry is not due to afterdepolarizations (not present in our model), but rather to a novel blocking mechanism.

Pro-arrhythmogenic effects of CACNA1C G1911R mutation in human ventricular tachycardia: insights from cardiac multi-scale models

Mutations in the CACNA1C gene are associated with ventricular tachycardia (VT). Although the CACNA1C mutations were well identified in patients with cardiac arrhythmias, mechanisms by which cardiac arrhythmias are generated in such genetic mutation conditions remain unclear. In this study, we identified a novel mechanism of VT resulted from enhanced repolarization dispersion which is a key factor for arrhythmias in the CACNA1C G1911R mutation using multi-scale computational models of the human ventricle. The increased calcium influx in the mutation prolonged action potential duration (APD), produced steepened action potential duration restitution (APDR) curves as well as augmented membrane potential differences among different cell types during repolarization, increasing transmural dispersion of repolarization (DOR) and the spatial and temporal heterogeneity of cardiac electrical activities. Consequentially, the vulnerability to unidirectional conduction block in response to a premature stimulus increased at tissue level in the G1911R mutation. The increased functional repolarization dispersion anchored reentrant excitation waves in tissue and organ models, facilitating the initiation and maintenance of VT due to less meandering rotor tip. Thus, the increased repolarization dispersion caused by the G1911R mutation is a primary factor that may primarily contribute to the genesis of cardiac arrhythmias in Timothy Syndrome.

Gene mutations in cardiac arrhythmias: a review of recent evidence in ion channelopathies

Over the past 15 years, molecular genetic studies have linked gene mutations to many inherited arrhythmogenic disorders, in particular, “ion channelopathies”, in which mutations in genes encode functional units of ion channels and/or their transporter-associated proteins in patients without primary cardiac structural abnormalities. These disorders are exemplified by congenital long QT syndrome (LQTS), short QT syndrome, Brugada syndrome (BrS) and catecholaminergic polymorphic ventricular tachycardia (CPVT). Functional and pathophysiological studies have led to better understanding of the clinical spectrum, ion channel structures and cellular electrophysiology involving dynamics of intracellular calcium cycling in many subtypes of these disorders and more importantly, development of potentially more effective pharmacological agents and even curative gene therapy. In this review, we have summarized (1) the significance of unveiling mutations in genes encoding transporter-associated proteins as the cause of congenital LQTS, (2) the technique of catheter ablation applied at the right ventricular outflow tract may be curative for severely symptomatic BrS, (3) mutations with channel function modulated by protein Kinase A-dependent phosphorylation can be the culprit of CPVT mimicry in Andersen-Tawil syndrome (LQT7), (4) ablation of the ion channel anchoring protein may prevent arrhythmogenesis in Timothy syndrome (LQT8), (5) altered intracellular Ca2+ cycling can be the basis of effective targeted pharmacotherapy in CPVT, and (6) the technology of induced pluripotent stem cells is a promising diagnostic and research tool as it has become a new paradigm for pathophysiological study of patient- and disease-specific cells aimed at screening new drugs and eventual clinical application of gene therapy. Lastly, we have discussed (7) genotype-phenotype correlation in relation to risk stratification of patients with congenital LQTS in clinical practice.

Theoretical study of L-type Ca(2+) current inactivation kinetics during action potential repolarization and early afterdepolarizations

Sarcoplasmic reticulum (SR) Ca(2+) release mediates excitation–contraction coupling (ECC) in cardiac myocytes. It is triggered upon membrane depolarization by entry of Ca(2+) via L-type Ca(2+) channels (LTCCs), which undergo both voltage- and Ca(2+)-dependent inactivation (VDI and CDI, respectively). We developed improved models of L-type Ca(2+) current and SR Ca(2+) release within the framework of the Shannon-Bers rabbit ventricular action potential (AP) model. The formulation of SR Ca(2+) release was modified to reproduce high ECC gain at negative membrane voltages. An existing LTCC model was extended to reflect more faithfully contributions of CDI and VDI to total inactivation. Ba(2+) current inactivation included an ion-dependent component (albeit small compared with CDI), in addition to pure VDI. Under physiological conditions (during an AP) LTCC inactivates predominantly via CDI, which is controlled mostly by SR Ca(2+) release during the initial AP phase, but by Ca(2+) through LTCCs for the remaining part. Simulations of decreased CDI or K(+) channel block predicted the occurrence of early and delayed after depolarizations. Our model accurately describes ECC and allows dissection of the relative contributions of different Ca(2+) sources to total CDI, and the relative roles of CDI and VDI, during normal and abnormal repolarization.

Targeting intracellular calcium cycling in catecholaminergic polymorphic ventricular tachycardia: a theoretical investigation

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a malignant arrhythmogenic disorder linked to mutations in the cardiac ryanodine receptor (RyR2) and calsequestrin, predisposing the young to syncope and cardiac arrest. To define the role of β-adrenergic stimulation (BAS) and to identify potential therapeutic targeted sites relating to intracellular calcium cycling, we used a Luo-Rudy dynamic ventricular myocyte model incorporated with interacting Markov models of the L-type Ca2+ channel ( ICa,L) and RyR2 to simulate the heterozygous state of mouse RyR2 R4496C mutation (RyR2R4496C+/−) comparable with CPVT patients with RyR2 R4497C mutation. Characteristically, in simulated cells, pacing at 4 Hz or faster or pacing at 2 Hz under BAS with effects equivalent to those of isoproterenol at ≥0.1 μM could readily induce delayed afterdepolarizations (DADs) and DAD-mediated triggered activity (TA) in RyR2R4496C+/− but not in the wild-type via enhancing both ICa,L and sarcoplasmic reticulum (SR) Ca2+ ATPase ( IUP). Moreover, with the use of steady state values of isolated endocardial (Endo), mid-myocardial (M), and epicardial (Epi) cells as initial data for conducting single cell and one-dimensional strand studies, the M cell was more vulnerable for developing DADs and DAD-mediated TA than Endo and Epi cells, and the gap junction coupling represented by diffusion coefficient ( D) of ≤0.000766*98 cm2/ms was required for generating DAD-mediated TA in RyR2R4496C+/−. Whereas individual reduction of Ca2+ release channel of SR and Na-Ca exchanger up to 50% was ineffective, 30% or more reduction of either ICa,L or IUP could totally suppress the inducibility of arrhythmia under BAS. Of note, 15% reduction of both ICa,L and IUP exerted a synergistic antiarrhythmic efficacy. Findings of this model study confirm that BAS facilitates induction of ventricular tachyarrhythmias via its action on intracellular Ca2+ cycling and a pharmacological regimen capable of reducing ICa,L could be an effective adjunctive to β-adrenergic blockers for suppressing ventricular tachyarrhythmias during CPVT.

Possible targets of therapy for catecholaminergic polymorphic ventricular tachycardia. - Insight from a theoretical model -

BACKGROUND

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a serious disease with a high mortality but its management is limited. The aim of this study was to investigate specific target sites for therapy in order to find potential management strategies for CPVT.

METHODS AND RESULTS

The mutant Ryanodine receptor 2 (RyR2) with reduced stored-overloaded-induced Ca²⁺ release (SOICR) threshold was incorporated into the Luo-Rudy dynamic (LRd) cell model to elucidate the underlying pathologies of CPVT. The simulations reveal that β-adrenergic stimulation increased the Ca²⁺ load in cardiac myocyte, which facilitates spontaneous SR Ca²⁺ leakage, resulting in triggered arrhythmias. Varied blockade (from 0% to 90%) in specific ion channels, including the Na⁺/Ca²⁺ exchanger (I(NaCa)), fast Na⁺ channel (I(Na)), RyR2 receptor (I(rel)), Ca²⁺-ATPase (SERCA) (I(up)) or L-type Ca²⁺channel (I(Ca(L))),was performed to simulate the action of specific drugs on target sites. Blockade of the I(NaCa) (≤ 10% blockade), in contrast to the I(up) (≤ 30% blockade), I(Ca(L)) and I(Na) (≤40% blockade), and followed by I(rel) (≤ 80% blockade), was most effective in suppressing the triggered arrhythmias in CPVT. Specifically, dual blockade of I(Ca(L))/I(up), I(Na)/I(rel) or I(Ca(L))/I(rel) had a synergistic effect in CPVT management.

CONCLUSIONS

Blockade of I(NaCa) appears to be the most efficacious target for CPVT management. Dual blockade of I(Ca(L))/I(up), I(Na)/I(rel) or I(Ca(L))/I(rel) has a synergistic effect in CPVT treatment.

Genetics and cardiac channelopathies

Sudden cardiac death is a major contributor to mortality in industrialized nations; in fact, it is the cause of more deaths than acquired immune deficiency syndrome, lung and breast cancer, and stroke together. Frequently, the autopsy becomes the principal diagnostic tool because macroscopic and microscopic analyses reveal the underlying cause of death. However, a significant number of sudden cardiac deaths remain unexplained. These cases are referred to as "natural" or arrhythmogenic. In the young, in up to 50% of sudden cardiac death cases, sudden death is the first and only clinical manifestation of an inherited cardiac disease that had remained undetected by conventional clinical investigations. To improve diagnosis, genetic testing has recently been added to these clinical tools. During the last two decades, there has been considerable progress in the understanding about genetics of sudden cardiac death. With that new information, the probands and their family members can make an informed decision regarding their care and know whether and to what extent they are at risk of suffering from the disease. Thus, genetic technology and expertise have become essential for the diagnosis of some forms of inherited cardiac diseases and to provide a basis for subsequent prevention strategies. This review focuses on recent advances in the understanding of cardiopathies owing to genetic investigations.