Genetic mutations and arrhythmia: simulation from DNA to electrocardiogram

Available online at www.sciencedirect.com

Journal of Electrocardiology 40 (2007) S47 – S50 www.jecgonline.com

Genetic mutations and arrhythmia: simulation from DNA to electrocardiogram Zheng I. Zhu, PhD, a Colleen E. Clancy, PhD* a

Department of Physiology and Biophysics, Institute for Computational Biomedicine, Weill Medical College of Cornell University, New York, NY, USA Received 18 April 2007; accepted 14 May 2007

Abstract

In the past two decades, mutations in cardiac ion channels have been shown to underlie a number of rare inherited cardiac arrhythmias. Defects in cardiac Na+ channels can disrupt channel gating and cause electrical abnormalities that increase susceptibility to cardiac arrhythmia. Dozens of mutations have been identified in the gene SCN5A, which encodes the α subunit of the cardiac Na+ channel, and have been causally linked to a wide spectrum of cardiac arrhythmic disorders. An important step in understanding genetically based arrhythmias is to clarify the relationship between molecular defects and the disruption of the delicate balance of dynamic interactions at the cell, tissue, and organ levels. Here, we provide an overview of cardiac Na+ channel mutations that are associated with inherited arrhythmia syndromes. We also address pros and cons of current methodologies used to understand how specific genetic defects disrupt channel-gating kinetics and underlie cardiac arrhythmia. Finally, we discuss effects of mutations on predictability and efficacy of treatment with Na+ channel–blocking drugs. © 2007 Elsevier Inc. All rights reserved.

Keywords:

Arrhythmia; Cardiac ion channel mutation; Na+ channel blockers

Introduction

Mutations increase susceptibility to arrhythmia

Voltage-gated Na+ channels are critical for generating the rapid excitatory depolarization that underlies cardiac excitation. Depolarization of the membrane due to cardiac Na + channel activation simultaneously triggers rapid inactivation of the channels, which results in a “turning off” of the Na+ current. Subsequent repolarization of the cell membrane (due to K+ efflux) allows for recovery of inactivated channels to available resting states, where they are available to open again and initiate another action potential. The timing of these discrete processes is essential for the normal coupling of excitation and contraction of cardiac tissue, which results in a balance between sufficient systolic blood pressure and adequate time for filling of the ventricles. Mutations in the Na+ channel can disrupt the necessarily precise timing of electrical signals in the heart and lead to sudden cardiac death.1-8

A number of mutations have been identified in the gene SCN5A, which encodes the α subunit of the cardiac Na+ channel, and have been causally linked to a wide spectrum of cardiac arrhythmic disorders including long QT syndrome (LQTs), Brugada syndrome (BrS), isolated cardiac conduction disease, sick sinus syndrome, or a combination of these syndromes (Fig. 1).1-9 The study of naturally occurring mutations in Na+ channels provides valuable information for understanding how the channels function normally and contribute to the overall electrical activity of the heart, as well as provides information about the structural determinants of the channel molecule that coordinate for channel gating.7,10,11 Brugada syndrome results in ST-segment elevation in the right precordial electrocardiogram leads and right bundle branch block. Mutations in the gene SCN5A have been linked to BrS, and they all cause loss of channel function.1,5,8,12-15 Examples of identified mutations include the R1432G mutation in the P segment of channel DIII, which abolishes the Na+ current,16 the T1620M mutation that increases the rate of channel inactivation and slows channel recovery from inactivation,12,17,18 and the Cterminal mutation (A1924T)19,20 that shifts the steady-state

⁎ Corresponding author. Department of Physiology and Biophysics, Institute for Computational Biomedicine, Weill Medical College of Cornell University, New York, NY 10021, USA. Tel.: +1 646 962 6374; fax: 212 746 6226. E-mail address: [email protected] 0022-0736/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jelectrocard.2007.05.033

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dominantly autosomal dominant, although less common autosomal recessive forms exist and typically result in more severe phenotypes.7 All of the inherited arrhythmia syndromes share one commonality—predisposition of affected patients to lethal arrhythmias. An important step is to begin to consider appropriate pharmacologic interventions for treating patients with mutations to reduce arrhythmia incidence. Mutations affect pharmacologic outcomes +

Fig. 1. The cardiac Na channel is involved in multiple arrhythmogenic syndromes. Shown is a schematic representation of the voltage-gated Na+ channel (NaV1.5), in which mutations can lead to the LQT3 form of LQTS, BrS, and isolated cardiac conduction disorder (ICCD) or mixed combinations of disorders. Reproduced from Zhu and Clancy.9

inactivation to more negative membrane potentials and reduces the availability of channels. There are 2 suggested mechanisms by which loss of Na+ current leads to the Brugada phenotype. The first is that loss of channels selectively hasten repolarization in the epicardium where there are large repolarizing K+ currents, leading to a loss of the epicardial action potential plateau phase. The observed ST-segment elevation in patients results from the dispersion of action potential plateau potentials. The other recently proposed mechanism underlying the BrS phenotype is that loss of Na+ current, observed as a depolarizing shift of the channel activation curve, may result in sufficiently slow conduction that some areas of the myocardium are undergoing repolarization while others are still depolarizing.21 Long QT syndrome manifests as prolongation of QT interval and increases susceptibility to polymorphic ventricular tachycardia and results from “gain-of-function” mutations in SCN5A, such as the deletion of 3 residues in the IIIIV linker (ΔKPQ)3 and mutations in the C terminus including E1784K22 and I1768V,23,24 which evoke a small, persistent current during the action potential plateau that delays repolarization. Inherited forms of LQTS are pre-

Block of cardiac Na+ channels for the intended management of cardiac arrhythmia has been widely used.25-27 Despite the prospective therapeutic value of the inherent voltage- and use-dependent properties of channel block by these drugs in the treatment of tachyarrhythmias, it is precisely these complex effects that have led to unpredictable and unforeseen toxic side effects, most notably arrhythmia induction.28,29 There are many factors that influence drug block of voltage-gated Na+ channels. Pharmacologic agents vary in conformation, charge, and affinity.30-37 The effect of ion channel block is not simply to reduce current. Cellular electrical activity derives from a complex nonlinear coupled system and, as a result, the effect of a drug that interacts and alters Na+ channel kinetics will necessarily affect membrane potential and other voltagedependent channels and processes. Hence, predicting the effects of Na+ channel blockers on the cardiac action potential is difficult. The prediction of drug effects is further complicated when naturally occurring mutations that alter channel kinetics are present in the channel drug receptor. For example, studies have shown that Na+ channel blockade by flecainide can reduce QT prolongation in carriers of some Na+ channel–linked LQTS type 3 (LQT3) mutations.38-40 Moreover, the same drug treatment can evoke ST-segment elevation, a hallmark of the BrS, in patients with a predisposition to the disease.40 Thus, in the case of LQT3, flecainide has potential therapeutic applica-

Fig. 2. A, Naturally occurring mutations can lead to complex drug effects and reduced predictability of drug actions. Shown is a schematic example depicting open-state drug (mexilitine) binding in the Markov model framework for a wild-type Na+ channel and a ΔKPQ mutant channel. B, Simulated effects of mexilitine on the cardiac ventricular action potential in the presence of ΔKPQ mutation. A small dose of mexilitine abolishes EAD and normalizes AP morphology by preferentially blocking the late Na+ current induced by the mutation. C, Simulated effect of mexilitine on a 1-dimension tissue containing ΔKPQ mutation. Panels A and B from Clancy et al.55

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tion, whereas for BrS it has proven useful as diagnostic tool.38,39 However, in some cases, flecainide has been reported to provoke BrS symptoms (ST-segment elevation) in patients harboring LQT3 mutations.38,40,41 Furthermore, flecainide preferentially blocks some LQT3- or BrS-linked mutant Na+ channels42-45 Investigation of the drug interaction with these and other LQT3- and BrS-linked mutations may indicate the usefulness of Na+ channel blockers in the detection and management of these disorders. Despite attempts to reveal the underlying ionic mechanisms of genetic arrhythmias and to connect the underlying genotypic changes to alterations in protein structure and resulting protein level function, understanding the causes and mechanisms of cardiac abnormalities has been extremely difficult.15,24,46 Approaches to understand effects of mutations and drugs One reason that prediction of the effects of mutations and pharmacologic intervention has been so challenging is that most approaches for understanding mechanisms of cardiac arrhythmias address one functional level: gene, cell, or system. But arrhythmia is an integrative disorder, and common single-scale approaches typically fail to reveal the most sought after information: how disruption in protein function due to mutations and/or drugs and, consequently, through complex interactions and behaviors of cells, lead to loss of synchronization and arrhythmogenic rhythms in tissue to cause failure of coordinated contraction. Hence, a fundamental challenge remains: to find a way to use and integrate obtained information gathered at individual system scales—to put the pieces back together—to understand how genetically based arrhythmias arise. A systems approach is warranted when properties of a system cannot be observed or readily predicted through study of the constituent elements of a system. Such is the case when mutations in ion channel genes result in complex arrhythmia phenotypes or when drugs exhibit complex pharmacodynamics and alter tissue properties in the heart.47 The only viable experimental approach for connecting genes to systems behavior is to use transgenic animals. However, genetic perturbations in mice may fail to reproduce phenotypes observed in humans because of the vastly different mouse and human cardiac physiologies.48 Mice may also display such complex phenotypes that the link between a mutation and the functional manifestation of the defect is as difficult to understand in mice as it is in humans.48-50 Alternative approaches clearly must be developed. The sophistication of genetic techniques permits investigation of cloned human proteins, often removed from the environment where they function physiologically. However, if the reductionism approach used to study functioning of human proteins is combined with tissue- and systems-level experiments in animals with physiology more like human cardiac physiology (ie, rabbits, dogs, sheep, etc), such a bidirectional approach is likely to yield information about relationships between system scales.

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Another complementary approach that is becoming increasingly used is to develop simulation methods to build models based directly on experimental data obtained at single-system scales and then to use high-performance computing to simulate system dynamics.51-54 Models can help to identify when a set of components is insufficient to reproduce a set of emergent behaviors and may suggest properties of missing components that the experimentalist can then attempt to identify. New components can then be added to the model to test for emergence of a particular behavior again. Models can also be used to test how perturbations, like mutations and drugs, affect system behavior (Fig. 2). They also allow us to follow the propagation of a perturbation over multiple scales and keep track of system parameters and their interactions. By doing so, we can identify the precise origin of, for example, arrhythmogenic triggers and reveal the specific players in complex cellular and tissue environments that cause the behavior.55 Theoretical studies have revealed cellular-level abnormalities that result from mutations and have, in some cases, connected these cellular findings with predicted body surface measurements by computing “pseudo” electrocardiograms.56 These studies have resulted in improved understanding of arrhythmia mechanisms in general and have been valuable as examples of the importance of combined clinical observations (electrophysiology studies), genetics (positional cloning and genotyping), molecular biology (cloning), cellular physiology (electrophysiology), pharmacology, and theoretical modeling to lend new insight into the mechanisms by which defective channels elicit variable phenotypic syndromes. Conclusions Clearly, a combined interdisciplinary approach among physicians, biologists, and mathematicians is required for a true systems approach to understanding both normal and abnormal cardiac function and the complex effects of mutations and pharmacologic intervention. A systems approach is likely to lead to a deeper understanding of mechanisms of arrhythmia, which will allow for new and genotype-targeted treatment development. References 1. Grant AO, Carboni MP, Neplioueva V, et al. Long QT syndrome, Brugada syndrome, and conduction system disease are linked to a single sodium channel mutation. J Clin Invest 2002;110:1201. 2. Benson DW, Wang DW, Dyment M, et al. Congenital sick sinus syndrome caused by recessive mutations in the cardiac sodium channel gene (SCN5A). J Clin Invest 2003;112:1019. 3. Bennett PB, Yazawa K, Makita N, George Jr AL. Molecular mechanism for an inherited cardiac arrhythmia. Nature 1995;376:683. 4. Tan HL, Bink-Boelkens MTE, Bezzina CR, et al. A sodium-channel mutation causes isolated cardiac conduction disease. Nature 2001;409: 1043. 5. Veldkamp MW, Viswanathan PC, Bezzina C, Baartscheer A, Wilde AAM, Balser JR. Two distinct congenital arrhythmias evoked by a multidysfunctional Na+ channel. Circ Res 2000;86:E91. 6. Viswanathan PC, Veldkamp MW, Connie C, Wilde AAM, Balser JR. A molecular mechanism for class IA drug intolerance in the Brugada syndrome. Circulation 2000;102:264.

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