Cellular basis for the electrocardiographic and arrhythmic manifestations of Timothy syndrome: Effects of ranolazine

doi:10.1016/j.hrthm.2006.12.046

Heart Rhythm

Volume 4, Issue 5, May 2007, Pages 638-647

https://doi.org/10.1016/j.hrthm.2006.12.046 Get rights and content

Background

Timothy syndrome is a multisystem disorder associated with QT interval prolongation and ventricular cardiac arrhythmias. The syndrome has been linked to mutations in Ca_V1.2 resulting in gain of function of the L-type calcium current (I_Ca,L). Ranolazine is an antianginal agent shown to exert an antiarrhythmic effect in experimental models of long QT syndrome.

Objective

The purpose of this study was to develop and characterize an experimental model of Timothy syndrome by using BayK8644 to mimic the gain of function of I_Ca,L and to examine the effects of ranolazine.

Methods

Action potentials from epicardial and M regions and a pseudo-electrocardiogram (ECG) were simultaneously recorded from coronary-perfused left ventricular wedge preparations, before and after addition of BayK8644 (1 μM).

Results

BayK8644 preferentially prolonged action potential duration of the M cell, leading to prolongation of the QT interval and an increase in transmural dispersion of repolarization (from 44.3 ± 7 ms to 86.5 ± 25 ms). Stimulation at cycle lengths of 250–500 ms led to ST-T wave alternans due to alternation of the plateau voltage of the M cell action potential as well as development of delayed afterdepolarizations in epicardial and M cell action potentials. Ventricular extrasystoles and tachycardia (monomorphic, bidirectional, or torsades de pointes) developed spontaneously or after rapid pacing. Peak and late I_Na were unaffected by BayK8644. Clinically relevant concentrations of ranolazine (10 μM) suppressed all actions of BayK8644.

Conclusion

A left ventricular wedge model of long QT syndrome created by augmentation of I_Ca,L recapitulates the ECG and arrhythmic manifestations of Timothy syndrome, which can be suppressed by ranolazine.

Introduction

Timothy syndrome, also referred to as syndactyly-associated long QT syndrome (LQTS) or LQT8, is a multisystem disorder characterized by developmental defects causing dysmorphic facial features including round face, flat nasal bridge, receding upper jaw, thin upper lip, and webbing of the toes and fingers (syndactyly).¹ The disorder also is associated with prolongation of the QT interval, development of ventricular arrhythmias, and sudden cardiac death. The syndrome has recently been linked to a missense mutation in Ca_V1.2, which encodes for the α subunit of the L-type calcium channel, resulting in a gain of function of the L-type calcium current (I_Ca,L).¹

Calcium loading has been shown to contribute to the development of both the trigger [early afterdepolarizations [EADs] and delayed afterdepolarizations (DADs)] and substrate (transmural dispersion of repolarization [TDR]) for torsades de pointes (TdP). Thus, the gain of function in I_Ca,L is expected to be associated with a high risk for TdP in this form of LQTS.

Ranolazine is a novel antianginal agent capable of producing anti-ischemic effects at plasma concentrations of approximately 1.5–8 μM with minimal or no changes in heart rate or blood pressure.² In addition to its anti-ischemic effects, ranolazine has been shown to be effective in suppressing arrhythmogenesis in experimental models of LQTS, particularly LQT2 and LQT3.3, 4, 5 The antiarrhythmic efficacy of ranolazine has been attributed to its potent block of late sodium channel current (late I_Na).⁴ Inhibition of late I_Na lessens the prolongation of the action potential (AP) of the M cell, the cell type in which late I_Na is most prominent, thus limiting the increase in TDR and the development of EADs.

In the present study, we used the calcium channel agonist BayK8644 to mimic the gain of function of I_Ca,L in an attempt to create an experimental model of Timothy syndrome and to elucidate the cellular basis for the ECG features and arrhythmias responsible for sudden cardiac death in this variant of LQTS. We also tested the hypothesis that ranolazine can effectively suppress the arrhythmias observed in the BayK8644 model of Timothy syndrome.

Section snippets

Methods

Dogs weighing 20–35 kg were anticoagulated with heparin (180 IU/kg) and anesthetized with pentobarbital (35 mg/kg IV). The chest was opened via left thoracotomy, and the heart was excised and placed in a cold cardioplegic solution ([K⁺]_o = 8 mmol/L, 4°C). Fourteen animals were used in this study. Nine of 14 wedges were included in the study. Five wedge preparations displayed persistent ST-segment depression indicating the presence of ischemia somewhere in the preparation and were discarded. A

Results

The calcium channel agonist BayK8644 (1 μM, 60 minutes of exposure) produced an increase in APD of epicardial (Epi) and M cells in the coronary-perfused LV wedge preparations (Figure 1). The preferential prolongation of the M cell AP induced by BayK8644 led to QT prolongation and an increase in TDR. Also noteworthy is the more prominent plateau and rectangular form of the M cell AP. Ranolazine (10 μM, 30 minutes of exposure) in the continued presence of BayK8644 (1 μM) caused little change in

Discussion

Timothy syndrome, or syndactyly-associated LQTS or LQT8, is characterized by multisystem dysfunction, including prolongation of the QT interval, development of ventricular arrhythmias, and sudden cardiac death. Splawski et al1, 9 reported linkage of Timothy syndrome to missense mutations in the Ca_V1.2 gene, resulting in a gain of function of the L-type calcium current (I_Ca,L). A second variant of the Timothy syndrome is not associated with syndactyly but is linked to a missense mutation in Ca_V

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Cited by (78)

Use of ranolazine as rescue therapy in a patient with Timothy syndrome type 2
2022, Revista Espanola de Cardiologia
Long-term follow-up of a patient with type 2 Timothy syndrome and the partial efficacy of mexiletine
2021, Gene
Citation Excerpt :
The patient appeared to present less ventricular arrhythmia with Mex than with Nad alone, but the number of shocks remained high (1 shock every 5 months vs. 1 shock every 3.7 months). Ranolazine inhibits INa,L and ICa,L and its in vitro efficacy was reported in 2007 (Sicouri et al., 2007). A case report was later published by Shah et al. (2012); in which a type 2 TS patient with the p.(Gly402Ser) mutation was successfully treated with ranolazine.
We report a detailed case of type 2 TS due to a p.(Gly402Ser) mutation in exon 8 of the CACNA1C gene. The patient shows a marked prolongation of repolarization with a mean QTc of 540 ms. He shows no structural heart disease, syndactyly, or cranio-facial abnormalities. However, he shows developmental delays, without autism, and dental abnormalities. The cardiac phenotype is very severe, with a resuscitated cardiac arrest at 2.5 years of age, followed by 26 appropriate shocks during nine years of follow-up. Adding mexiletine to nadolol resulted in a reduction of the QTc and a slight decrease in the number of appropriate shocks.
Timothy Syndrome
2018, Cardiac Electrophysiology: From Cell to Bedside: Seventh Edition
Mexiletine prevents recurrent torsades de pointes in acquired long QT syndrome refractory to conventional measures
2015, JACC: Clinical Electrophysiology
The purpose of this study was to examine the role of mexiletine, a late sodium current (I_Na-L) blocker, in acute termination of torsades de pointes (TdP) refractory to conventional therapy in acquired long QT syndromes (LQTS).
Long QT interval can predispose to TdP and is therefore associated with significant mortality. Currently, there is no available pharmacotherapy to target directly the ionic basis of most LQTS for the acute termination of TdP. Earlier evidence highlighted the role of I_Na-L in the pathophysiology of long QT and TdP, particularly in patients with congenital LQTS.
Twelve patients with TdP caused by acquired LQTS were treated with mexiletine after failure of conventional treatment including discontinuation of QT-prolonging drugs, intravenous administration of magnesium, and correction of serum electrolyte abnormalities.
No recurrence of TdP occurred within 2 h after initiation of treatment with mexiletine in all 12 patients. Macro T-wave alternans accompanied by QT prolongation, an electrocardiographic precursor of TdP that was seen in 3 patients, was also abolished by mexiletine. Treatment with mexiletine shortened the QTc interval from 599 ± 27 ms to 514 ± 16 ms (p = 0.001). The interval from the peak to the end of the T-wave (T_p-e interval) decreased from 145 ± 18 ms to 106 ± 9 ms (p = 0.005). The T_p-e/QT ratio decreased from 0.27 ± 0.02 to 0.23 ± 0.018 (p = 0.01). Mexiletine had no significant effect on QRS complex duration.
I_Na-L blockade with mexiletine may be an effective treatment approach to terminate refractory TdP from several acquired causes of LQTS.
Electronic "expression" of the inward rectifier in cardiocytes derived from human-induced pluripotent stem cells
2013, Heart Rhythm
Citation Excerpt :
Similarly, our data obtained with BayK-8644, in conjunction with the studies of Jonsson et al8, suggest that IK1 and a normal resting potential are key to understanding the role of ICa inactivation defects in ventricular repolarization.8 BayK-8644 mimics the molecular changes in ICa seen in TS, and our results suggest a possible basis for the putative difference between TS in the working myocardium, in which it is thought to primarily work through generation of EADs,39–41 and the findings in h-iPSC–derived cardiomyocytes from patients with TS, in which the main cellular arrhythmogenic event was an increase in DADs.34 The electronic expression of IK1 qualitatively alters the mechanisms of repolarization of h-iPSC–derived cardiac myocytes to make them more physiological.
Human-induced pluripotent stem cell (h-iPSC)–derived cardiac myocytes are a unique model in which human myocyte function and dysfunction are studied, especially those from patients with genetic disorders. They are also considered a major advance for drug safety testing. However, these cells have considerable unexplored potential limitations when applied to quantitative action potential (AP) analysis. One major factor is spontaneous activity and resulting variability and potentially anomalous behavior of AP parameters.
To demonstrate the effect of using an in silico interface on electronically expressed I_K1, a major component lacking in h-iPSC–derived cardiac myocytes.
An in silico interface was developed to express synthetic I_K1 in cells under whole-cell voltage clamp.
Electronic I_K1 expression established a physiological resting potential, eliminated spontaneous activity, reduced spontaneous early and delayed afterdepolarizations, and decreased AP variability. The initiated APs had the classic rapid upstroke and spike and dome morphology consistent with data obtained with freshly isolated human myocytes as well as the readily recognizable repolarization attributes of ventricular and atrial cells. The application of 1 µM of BayK-8644 resulted in anomalous AP shortening in h-iPSC–derived cardiac myocytes. When I_K1 was electronically expressed, BayK-8644 prolonged the AP, which is consistent with the existing results on native cardiac myocytes.
The electronic expression of I_K1 is a simple and robust method to significantly improve the physiological behavior of the AP and electrical profile of h-iPSC–derived cardiac myocytes. Increased stability enables the use of this preparation for a controlled quantitative analysis of AP parameters, for example, drug responsiveness, genetic disorders, and dynamic behavior restitution profiles.
Antiarrhythmic effects of the highly selective late sodium channel current blocker GS-458967
2013, Heart Rhythm
Citation Excerpt :
The IC50 for the inhibition of ATX-II-induced late INa in isolated rabbit ventricular myocytes is much greater for GS967 than for flecainide (0.13 μM vs 3.4 μM). Our data indicate that 0.1–0.3 μM is sufficient to inhibit arrhythmias induced in canine PFs and PV and SVC sleeve preparations whereas 5–10 μM of ranolazine is needed to suppress EAD- or DAD-induced triggered activity in experimental models of arrhythmias.34,35 Thus, GS967 is a more than 10-fold more potent and more than 70-fold more selective inhibitor of late INa compared to previously described late INa inhibitors.
Previous studies have shown that late sodium channel current (I_Na) blockers such as ranolazine can exert antiarrhythmic effects by suppressing early and delayed afterdepolarization (EAD and DAD)-induced triggered activity.
To evaluate the electrophysiological properties of GS-458967 (GS967), a potent and highly selective late I_Na blocker, in canine Purkinje fibers (PFs) and pulmonary vein (PV) and superior vena cava (SVC) sleeve preparations.
Transmembrane action potentials were recorded from canine PFs and PV and SVC sleeve preparations by using standard microelectrode techniques. The rapidly activating delayed rectifier potassium channel current blocker E-4031 (2.5–5 µM) and the late I_Na agonist ATX-II (10 nM) were used to induce EADs in PFs. Isoproterenol (1 µM), high calcium ([Ca²⁺]_o = 5.4 mM), or their combination was used to induce DADs and triggered activity.
In PFs, GS967 (10–300 nM) caused a significant concentration-dependent reduction in action potential duration without altering the maximum rate of rise of the action potential upstroke, action potential amplitude, or resting membrane potential at any rate studied (basic cycle lengths of 1000, 500, and 300 ms) or concentration evaluated (n = 5; P < .05). GS967 (30–100 nM) abolished EADs and EAD-induced triggered activity elicited in PFs by exposure to E-4031 (n = 4) or ATX-II (n = 4). In addition, GS967 reduced or abolished DADs and suppressed DAD-induced triggered activity elicited in PFs (n = 4) and PV (n = 4) and SVC (n = 3) sleeve preparations by exposure to isoproterenol, high calcium, or their combination.
Our data suggest that the selective inhibition of late I_Na with GS967 can exert antiarrhythmic effects by suppressing EAD- and DAD-mediated extrasystolic activity in PFs and PV and SVC sleeve preparations.

View all citing articles on Scopus

: Supported by Grant HL47678 from the National Heart, Lung, and Blood Institute to Dr. Antzelevitch and grants from CV Therapeutics and NYS and Florida Grand Lodges F. & A.M.

View full text

Original-experimentalCellular basis for the electrocardiographic and arrhythmic manifestations of Timothy syndrome: Effects of ranolazine

Background

Objective

Methods

Results

Conclusion

Introduction

Section snippets

Methods

Results

Discussion

Cell

J Am Coll Cardiol

J Am Coll Cardiol

Heart Rhythm

Trends Pharmacol Sci

Heart Rhythm

Antiarrhythmic effects of ranolazine in a guinea pig in vitro model of long-QT syndrome

J Pharmacol Exp Ther

Electrophysiologic properties and antiarrhythmic actions of a novel anti-anginal agent

J Cardiovasc Pharmacol Ther

Antagonism by ranolazine of the pro-arrhythmic effects of increasing late INa in guinea pig ventricular myocytes

J Cardiovasc Pharmacol

Ionic bases for electrophysiological distinctions among epicardial, midmyocardial, and endocardial myocytes from the free wall of the canine left ventricle

Circ Res

INaCa and ICl(Ca) contribute to isoproterenol-induced delayed afterdepolarizations in midmyocardial cells

Am J Physiol

Effects of dihydropyridine calcium channel modulators on cardiac sodium channels

Am J Physiol Heart Circ Physiol

Severe arrhythmia disorder caused by cardiac L-type calcium channel mutations

Proc Natl Acad Sci U S A

Cardiac L-type channel mutations caused the longest QT interval compared to the common genotypes of long QT syndrome

Circulation

Syndactyly and long QT syndrome (Ca(V)1.2 missense mutation G406R) is associated with hypertrophic cardiomyopathy

Heart Rhythm

Idiopathic long QT syndrome in a black infant

Cardiovasc J S Afr

Efficacy of permanent pacing in the management of high-risk patients with long QT syndrome

Circulation

Cellular mechanisms underlying the long QT syndrome

Curr Opin Cardiol

Sodium channel block with mexiletine is effective in reducing dispersion of repolarization and preventing torsade de pointes in LQT2 and LQT3 models of the long-QT syndrome

Circulation

Original-experimental
Cellular basis for the electrocardiographic and arrhythmic manifestations of Timothy syndrome: Effects of ranolazine

Antagonism by ranolazine of the pro-arrhythmic effects of increasing late I_Na in guinea pig ventricular myocytes

I_NaCa and I_Cl(Ca) contribute to isoproterenol-induced delayed afterdepolarizations in midmyocardial cells