Mutations In Cardiac Ion Channels Research Articles

Genetic mutations and arrhythmia: simulation from DNA to electrocardiogram

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.

Diversity in cardiac sodium channel disease phenotype in transgenic mice carrying a single SCN5A mutation

Lethal ventricular arrhythmias are increasingly considered an important cause of sudden death in relatively young individuals. A genetic predisposition has been recognised in many cases, and research in the last decade has focused on underlying inherited mutations in cardiac ion channels.

Clues or Miscues?

The congenital long-QT syndrome (LQTS) is an inherited condition associated with a prolongation of the QT interval on the ECG characterized by syncope, seizures, and sudden death secondary to ventricular arrhythmia, specifically torsade de pointes.1 LQTS is particularly notable because it affects seemingly healthy children and young adults, often with no apparent serious health issues. Unfortunately, LQTS may present dramatically with sudden cardiac death in 10% of children and syncope or seizures in 30% to 40% as the first symptom.2 Importantly, although LQTS has a high mortality when not diagnosed and treated, the currently available therapies are very effective in preventing sudden cardiac death.3 Thus, making the correct diagnosis in as many individuals with LQTS as possible before life-threatening symptoms emerge is critical. Article p 2613 The condition now designated as LQTS was first described as the Jervell Lange-Nielson syndrome (LQTS with hearing deficit) in 1957.4 A few years later, the Romano Ward syndrome was described (LQTS with normal hearing).5,6 In 1979, the International LQTS Registry was established. Much of the information about LQTS, correlations between genotype and phenotype, risk stratification, and treatment of LQTS stems from information gleaned from this registry and other large cohorts.7–11 Identification of the gene mutations in cardiac ion channels in the 1990s led to further understanding of LQTS. At least 9 genes have been identified that encode for proteins that modulate ion channel structure, function, and signaling or trafficking; alter cardiac repolarization; and increase the risk for ventricular arrhythmias.3,12 The identified genes are thought to represent 70% of the mutations that cause LQTS, with others yet to be identified or their location on identified genes yet to be found. The authors13 of “Diagnostic Miscues in Congenital Long-QT Syndrome” in this issue of Circulation have illustrated many …

The Congenital Long QT Syndrome and Implications for Young Athletes

The congenital long QT syndrome (LQTS) is caused by cardiac ion channel mutations, which predispose young individuals to sudden cardiac death often related to exercise. The issue of LQTS and sports participation has received significant publicity due to reports of sudden death in young competitive athletes. This article reviews the pathophysiology, clinical characteristics, and management of LQTS in the physically active and athletic population.

Sodium channels as macromolecular complexes: Implications for inherited arrhythmia syndromes

Mutations in cardiac ion channels and their auxiliary subunits can lead to life-threatening cardiac arrhythmias. In recent years it has become apparent that ion channels are part of large, multi-protein complexes, comprising not only the ion channels and their auxiliary subunits, but also components of the cytoskeleton, regulatory kinases and phosphatases, trafficking proteins, extracellular matrix proteins, and possibly even other ion channels. Disruption of any member of a particular ion channel complex has the potential to disrupt the function of the associated channels, resulting in paroxysmal disease. Understanding the molecular composition of individual ion channel signaling complexes in heart may yield important insights into the molecular basis of cardiac arrhythmias and may suggest novel therapeutic approaches to treatment of these life-threatening conditions.

Inherited cardiac arrhythmia syndromes: What have they taught us about arrhythmias and anti‐arrhythmic therapy?

1. In recent years, the identification of the gene defects in a vast array of monogenic disorders has revolutionized our understanding of the basic mechanisms underlying numerous disease processes. 2. Mutations in cardiac ion channels have been identified as the basis of a wide range of inherited arrhythmia syndromes, including the congenital long QT syndromes, Brugada syndrome, Lenegre syndrome, Andersen's disease and familial atrial fibrillation. 3. Identification of mutations in the human-ether-a-go-go-related gene (HERG) K(+) channel as the molecular basis of congenital long QT syndrome type 2 also led to the discovery that HERG is the molecular target for the vast majority of drugs (both cardiac and non-cardiac) that cause drug-induced arrhythmias. This has had profound implications not only for the development of anti-arrhythmic agents, but also for drug development in general. 4. The sequencing of the human genome in a sense represents the pinnacle of the reductionist era of molecular medicine. The great challenge now is to re-integrate the information gathered during the 'reductionist era' to provide a better understanding of the intact organism. Computer modelling is likely to be a key component of that re-integration process.

Computational physiology and the Physiome Project.

Bioengineering analyses of physiological systems use the computational solution of physical conservation laws on anatomically detailed geometric models to understand the physiological function of intact organs in terms of the properties and behaviour of the cells and tissues within the organ. By linking behaviour in a quantitative, mathematically defined sense across multiple scales of biological organization--from proteins to cells, tissues, organs and organ systems--these methods have the potential to link patient-specific knowledge at the two ends of these spatial scales. A genetic profile linked to cardiac ion channel mutations, for example, can be interpreted in relation to body surface ECG measurements via a mathematical model of the heart and torso, which includes the spatial distribution of cardiac ion channels throughout the myocardium and the individual kinetics for each of the approximately 50 types of ion channel, exchanger or pump known to be present in the heart. Similarly, linking molecular defects such as mutations of chloride ion channels in lung epithelial cells to the integrated function of the intact lung requires models that include the detailed anatomy of the lungs, the physics of air flow, blood flow and gas exchange, together with the large deformation mechanics of breathing. Organizing this large body of knowledge into a coherent framework for modelling requires the development of ontologies, markup languages for encoding models, and web-accessible distributed databases. In this article we review the state of the field at all the relevant levels, and the tools that are being developed to tackle such complexity. Integrative physiology is central to the interpretation of genomic and proteomic data, and is becoming a highly quantitative, computer-intensive discipline.

Denaturing high-performance liquid chromatography quickly and reliably detects cardiac ion channel mutations in long QT syndrome.

Multiple mutations in several ion channel genes (KCNQ1, KCNH2, SCN5A, KCNE1, KCNE2, and KCNJ2) have been shown to cause autosomal dominant long QT syndrome (LQTS), a familial cardiac disorder that causes syncope, seizures, and sudden death. Due to their multiple loci and considerable size, mutation detection in these genes represents a challenge that is only partially met by the conventional screening method of single-stranded conformational polymorphism (SSCP). The recently introduced denaturing high-performance liquid chromatography (dHPLC) offers a promising new method for a fast and sensitive analysis of PCR-amplified DNA fragments. To test the applicability of dHPLC in the molecular diagnosis of LQTS, we first assessed a cohort of 192 patients from our International LQTS Registry for 14 previously identified mutations (including 10 different missense mutations, 1-bp, 2-bp, 3-bp, and 9-bp deletion mutations), and 2 polymorphisms in the LQTS potassium and sodium channel genes. Applying empirically determined exon-specific melting profiles, all mutations (including four previously undetectable by SSCP) were readily identified by dHPLC. We conclude that the dHPLC technology is a highly sensitive and efficient method for the molecular analysis of LQTS, and the same PCR amplicons developed for SSCP testing can be directly used for dHPLC assay.

Long QT syndrome and anaesthesia

Long QT syndrome (LQTS) is an arrhythmogenic cardiovascular disorder resulting from mutations in cardiac ion channels. LQTS is characterized by prolonged ventricular repolarization and frequently manifests itself as QT interval prolongation on the electrocardiogram (ECG). The age at presentation varies from in utero to adulthood. The majority of symptomatic events are related to physical activity and emotional stress. Although LQTS is characterized by recurrent syncope, cardiac arrest, and seizure-like episodes, only about 60% of patients are symptomatic at the time of diagnosis. 3 The clinical features of LQTS result from a peculiar episodic ventricular tachyarrhythmia called ‘torsade de pointes’. ‘Twisting of the points’ describes the typical sinusoidal twisting of the QRS axis around the isoelectric line of the ECG. Usually torsade de pointes start with a premature ventricular depolarization, followed by a compensatory pause. The next sinus beat often has a markedly prolonged QT interval and abnormal T wave. This is followed by a ventricular tachycardia that is characterized by variation in the QRS morphology, and a constantly changing R-R interval (Fig. 1). The ‘short-long-short’ cycle length sequence heralding torsade de pointes is a hallmark of LQTS. Commonly, the episode of torsade de pointes is self-terminating, producing a syncopal episode or pseudoseizure, secondary to the abrupt decrease in cerebral blood flow. The majority of episodes of sudden death in LQTS result from ventricular fibrillation triggered by torsade de pointes, although the mechanism of this deterioration is unknown. Traditionally, LQTS has been classified as either familial (inherited) or acquired. However, it is likely that many patients with previously labelled acquired LQTS carry a silent mutation in one of the genes responsible for congenital LQTS. 22 119 The evidence for this hypothesis has been gradually emerging over the past few years. It is important for anaesthetists to be aware of this concept, as it means that a much higher proportion of the general population may be affected by asymptomatic mutations in genes encoding cardiac ion channels than was thought previously. The prevalence of LQTS in developed countries may be as high as 1 per 1100‐3000 of the population. 32 119 About 30% of congenital LQTS carriers have an apparently normal phenotype, and thus a normal QT interval, and remain undiagnosed until an initiating event. 105 Fatal arrhythmias associated with primary electrical disease of the heart such as the Brugada and LQTS, probably account for 19% of sudden deaths in children between 1 and 13 yr of age, and 30% of sudden deaths that occur between 14 and 21 yr of age. 10 Furthermore, there is a strong association between prolonged corrected QT interval (QTc) in the first week of life and risk of sudden infant death syndrome. 86

The Long-QT Syndrome

The heart is a pump that is stimulated by electrical signals to beat between about 50 to 180 times per minute, depending upon the individual’s level of activity and emotional state. The heart itself is made up of millions of cells that generate repetitive sequences of electrical discharges (referred to in medicine as depolarization) and recharging (repolarization) that produce each heart beat. Electrical activity of the heart cells is produced by the synchronized movement of sodium, potassium, and calcium ions into and out of the cells. ### Long-QT Syndrome Long-QT Syndrome (LQTS) is a disorder of the electrical activity of the heart that can result in blackouts (fainting, also known as syncope) or sudden cardiac death. It occurs infrequently and is caused either by an inherited genetic condition (inherited form of LQTS) or by some prescription medications (acquired form of LQTS). QT refers to the interval on ECG between the beginning of the heart’s electrical discharge through its electrical recovery (see Figure 1). Figure 1 illustrates how the QT interval is measured on the ECG. Schematic diagram of the electrical activity of one heart beat recorded by an ECG. The electrical discharge (depolarization) of the ventricle is represented by the QRS complex, with the T wave reflecting electrical recovery (repolarization). The QT is the time interval from the beginning of the QRS complex to the end of the T wave, and is generally less than 0.44 seconds in normal individuals. In subjects with the long-QT syndrome, the QT is usually longer than 0.44 seconds. Delayed electrical recovery within the heart can be associated with very dangerous, rapid, and chaotic heart rhythms (medical terms for these abnormal heart rhythms are …

Postmortem molecular analysis of SCN5A defects in sudden infant death syndrome.

Fatal arrhythmias from occult long QT syndrome may be responsible for some cases of sudden infant death syndrome (SIDS). Because patients who have long QT syndrome with sodium channel gene (SCN5A) defects have an increased frequency of cardiac events during sleep, and a recent case is reported of a sporadic SCN5A mutation in an infant with near SIDS, SCN5A has emerged as the leading candidate ion channel gene for SIDS. To determine the prevalence and functional properties of SCN5A mutations in SIDS. Postmortem molecular analysis of 93 cases of SIDS or undetermined infant death identified by the Medical Examiner's Office of the Arkansas State Crime Laboratory between September 1997 and August 1999. Genomic DNA was extracted from frozen myocardium and subjected to SCN5A mutational analyses. Missense mutations were incorporated into the human heart sodium channel alpha subunit by mutagenesis, transiently transfected into human embryonic kidney cells, and characterized electrophysiologically. Molecular and functional characterization of SCN5A defects. Two of the 93 cases of SIDS possessed SCN5A mutations: a 6-week-old white male with an A997S missense mutation in exon 17 and a 1-month old white male with an R1826H mutation in exon 28. These 2 distinct mutations occurred in highly conserved regions of the sodium channel and were absent in 400 control patients (800 alleles). Functionally, the A997S and R1826H mutant channels expressed a sodium current characterized by slower decay and a 2- to 3-fold increase in late sodium current. Approximately 2% of this prospective, population-based cohort of SIDS cases had an identifiable SCN5A channel defect, suggesting that mutations in cardiac ion channels may provide a lethal arrhythmogenic substrate in some infants at risk for SIDS.

Cyclic AMP regulates the HERG K+ channel by dual pathways

Lethal cardiac arrhythmias are a hallmark of the hereditary Long QT syndrome (LQTS), a disease produced by mutations of cardiac ion channels [1]. Often these arrhythmias are stress-induced, suggesting a relationship between beta-adrenergic activation of adenylate cyclase and cAMP-dependent alteration of one or more of the ion channels involved in LQTS. Second messengers modulate ion channel activity either by direct interaction or through intermediary kinases and phosphatases. Here we show that the second messenger cAMP regulates the K(+) channel mutated in the LQT2 form of LQTS, HERG [2], both directly and indirectly. Activation of cAMP-dependent protein kinase (PKA) causes phosphorylation of HERG accompanied by a rapid reduction in current amplitude, acceleration of voltage-dependent deactivation, and depolarizing shift in voltage-dependent activation. In a parallel pathway, cAMP directly binds to the HERG protein with the opposing effect of a hyperpolarizing shift in voltage-dependent activation. The summation of cAMP-mediated effects is a net diminution of the effective current, but when HERG is complexed with with the K(+) channel accessory proteins MiRP1 or minK, the stimulatory effects of cAMP are favored. These findings provide a direct link between stress and arrhythmia by a unique mechanism where a single second messenger exerts complex regulation of an ion channel via two distinct pathways.

Toward an Understanding of the Cause of Mitral Valve Prolapse

Toward an Understanding of the Cause of Mitral Valve Prolapse