Two long QT syndrome loci map to chromosomes 3 and 7 with evidence for further heterogeneity

Genetics of cardiac arrhythmias and sudden cardiac death

This presentation deals with the molecular substrates of the inherited diseases leading to genetically determined cardiac arrhythmias and sudden death. In the first part of this article the current knowledge concerning the molecular basis of cardiac arrhythmias will be summarized. Second, we will discuss the most recent evidence showing that the picture of the molecular bases of cardiac arrhythmias is becoming progressively more complex. Thanks to the contribution of molecular genetics, the genetic bases, pathogenesis, and genotype-phenotype correlation of diseases--such as the long QT syndrome, the Brugada syndrome, progressive cardiac conduction defect (Lenegre disease), catecholaminergic polymorphic ventricular tachycardia, and Andersen syndrome--have been progressively unveiled and shown to have an extremely high degree of genetic heterogeneity. The evidence supporting this concept is outlined, with particular emphasis on the growing complexity of the molecular pathways that may lead to arrhythmias and sudden death, in terms of the relationships between genetic defect(s) and genotype(s), as well as gene-to-gene interactions. The current knowledge is reviewed, focusing on the evidence that a single clinical phenotype may be caused by different genetic substrates and, conversely, a single gene may cause very different phenotypes acting through different pathways.

Inherited arrhythmia syndromes: applying the molecular biology and genetic to the clinical management

Thanks to the contribution of molecular genetics, the genetic bases, the pathogenesis and genotype-phenotype correlation of diseases such as the Long QT syndrome, the Brugada Syndrome, the Progressive cardiac conduction defect (Lenegre disease), the Catecholaminergic Polymorphic Ventricular Tachycardia and Andersen Syndrome have been progressively unveiled and show an extremely high degree of genetic heterogeneity. The evidences supporting this concept are outlined with a particular emphasis on the growing complexity of the molecular pathways that may lead to arrhythmias and sudden death, in term of the relationships between genetic defect(s) and genotype(s) as well as gene-to gene interactions. The current knowledge is reviewed, focusing on the evidence that a single clinical phenotype may be caused by different genetic substrates and, conversely, a single gene may cause very different phenotypes acting through different pathways.

Pathological changes of the heart in sudden infant death

There are more than 120 different theories on the possible causes of sudden infant death (SID). In particular, dysfunctions of the central nervous system, cardiorespiratory insufficiency due to infections including atypical immune reactions, and cardiac dysregulation have been discussed during the previous decade. Reports on disturbances of the cardiac rhythmogenic function due to LQTS were among the most speculative. Based on gross histological, immunohistochemical and molecular genetic investigations of SID cases, the most important and most frequent findings of the heart are shown. The significance of different types of myocarditis, hypoxia-related changes, disturbances of the rhythmogenic function, cardiomyopathy, and other changes is discussed with regard to the cause of death. In conclusion, most of the changes reported in the literature are not sufficient to explain the cause of death. Problems in the diagnosis are shown which influence the classification of these disturbances as well as the classification of SID.

Effects of sex on the pharmacokinetic and pharmacodynamic properties of quinidine

AIMS

To investigate the source of the apparent increased susceptibility of women to develop QT interval prolongation and torsade de pointes after the administration of drugs that delay cardiac repolarization.

METHODS

Plasma quinidine concentrations and electrocardiographic changes (QRS and QT intervals) were measured over 24 h following the administration of single oral doses of the QT prolonging drug quinidine (3 mg kg(-1)) and compared between 27 male and 21 female healthy volunteers.

RESULTS

There were no significant differences between males and females in plasma quinidine concentrations or in calculated pharmacokinetic variables. Maximum quinidine concentrations in males and females were 997 +/- 56 and 871 +/- 57 ng ml(-1), respectively (mean difference (-125, 95% confidence intervals (CI) -239, 11 ng ml(-1), P = NS). Quinidine lengthened actual (QTa) and corrected (QTc) QT intervals and the QRS interval to a greater extent in females than males (P < 0.001 for each), but there were no significant sex differences detected in the effects of quinidine on the heart rate corrected JT interval. Maximum prolongation of QTc interval was observed 2 h after quinidine and was significantly greater in women (33 +/- 16 vs 24 +/- 17 ms, mean difference 9 +/- 20 ms, 95% CI 3, 15, P = 0.037). At this time mean differences (95% CI) were 1.0 min(-1) (-2.5, 4.4, P = NS) for heart rate, 5.5 ms (3.5, 7.6, P = 0.05) for the QRS and 3.4 ms (-2.5, 9.3, P = NS) for the JTc intervals.

CONCLUSIONS

Quinidine-induced increases in QTc were larger in females, but no sex differences in quinidine pharmacokinetics were found. The disparity in prolongation of cardiac repolarization is thus due to a pharmacodynamic difference which appears more complex than simply an increase in repolarization delay in females.

Cardiac sodium channel diseases

In the last few years, a very active line of research took place after the first identification of SCN5A mutations associated with an inherited form of cardiac arrhythmias and sudden death, the LQT3 variant of the long QT syndrome. Subsequently, two allelic diseases additional to LQT3 were shown to be due to mutations in the same gene, the Brugada syndrome (BrS) and the Lev-Lenegre syndrome (progressive cardiac conduction defect). Genotype-phenotype correlation and in vitro expression studies provide evidence that structure-function relationships of the SCN5A protein are much more complex than initially anticipated. The biophysical characterization of the sodium channel defects associated with different phenotypes and the genotype-phenotype correlation studies brought to the attention of the scientific community a plethora of mechanisms by which even a single amino acid substitution may remarkably affect cardiac excitability. Finally, the evidence of patients harboring an SCN5A mutation and overlapping clinical presentations creates a need for a revision of the traditional classification of the above mentioned diseases. It is now appropriate to consider the "sodium channel syndrome" as a unique clinical entity that may manifest itself with a spectrum of possible phenotypes.

Prolonged QT syndrome in children: an uncommon but potentially fatal entity

Prolonged QT syndrome may be either congenital, as in Jervell and Lange-Nielsen or Romano-Ward syndromes, or acquired in nature. Affected children are at risk for syncope, seizures, dysrhythmias and sudden death. Physicians should consider long QT syndrome (LQTS) in all patients who present with syncope. A thorough personal and family history should be documented, with particular attention to prior syncopal episodes, congenital deafness, and unexplained sudden death. Syncope that is either recurrent or induced by exercise or stress is concerning and also should be noted. An electrocardiogram with manual calculation of the QT interval should be performed on all patients with a suggestive history. Furthermore, the diagnosis of LQTS warrants evaluation of all other family members. With recognition and appropriate treatment of affected patients, the potentially fatal consequences of LQTS may be prevented.

Beta blockers normalize QT hysteresis in long QT syndrome

OBJECTIVES

This study was performed to evaluate the impact of beta blockers on QT adaptation to heart rate during the exercise and recovery phases of exercise testing in long QT syndrome.

BACKGROUND

Long QT syndrome is characterized by familial syncope and sudden death in the context of sudden heart rate changes. QT hysteresis has been proposed as a phenotypic marker of long QT syndrome, suggesting altered QT adaptation to changes in heart rate.

METHODS

Fourteen patients with long QT syndrome (aged 26 +/- 16 years, 6 male) and 10 healthy volunteers (aged 37 +/- 11 years, 9 male) underwent graded exercise testing with continuous lead II electrocardiographic monitoring. Long QT patients underwent repeat assessment after 1 month of beta blockade. QT intervals at matching heart rates were compared during exercise and recovery to determine the effect of beta blockade on QT hysteresis, defined as the recovery QT peak interval subtracted from the exercise QT peak interval.

RESULTS

In the 14 long QT syndrome patients, beta blockers slowed the resting heart rate without affecting the corrected QT interval (502 +/- 52 ms baseline vs 481 +/- 40 ms beta blocker, P =.17). The increase in heart rate with exercise was similar in the 3 groups (P =.73). Exaggerated hysteresis of the QT interval was seen in the patients with long QT syndrome at baseline compared with controls (46 +/-19 ms vs 19 +/- 11 ms 1 minute into recovery, P =.006). Beta blockers had minimal effect on the QT interval but markedly reduced hysteresis with minimal separation of the exercise and recovery QT/RR curves (25 +/- 35 ms 1 minute into recovery, P =.027). The combined curve separation at all 6 time points analyzed was 165 +/- 95 ms in patients with long QT syndrome at baseline, 40 +/- 131 ms after beta blockade, and 29 +/- 30 ms in control subjects (P =.002). Comparison of the beta blocker effect on hysteresis in the 2 genotypes suggested a greater reduction in hysteresis in the 3 patients with long QT syndrome 1 compared with the 11 patients with long QT syndrome 2.

CONCLUSIONS

Beta blockers reduce QT hysteresis in patients with long QT syndrome to values seen in normal patients. This improved QT adaptation to changes in heart rate may explain the clinical benefit of beta blockers in long QT syndrome.

Molecular mechanisms underlying the long QT syndrome

Recent studies of the molecular basis of the long QT syndrome (LQTS) have advanced our understanding of the mechanisms responsible for the abnormal prolongation of ventricular repolarization and revealed associations between LQTS and other primary electrical diseases of the heart such as Brugada syndrome. The role of DNA single nucleotide polymorphisms in acquired LQTS and differences between the Romano-Ward and Jervell-Lange-Nielsen forms of congenital LQTS are gradually coming into focus. In this brief review, our goal is to summarize the molecular mechanisms proposed to underlie the susceptibility to arrhythmias in LQTS and discuss the direction of current and future research.

ABCs of molecular cardiology and the impact of the Human Genome Project on clinical cardiology

The last decade was marked by a revolution in molecular biology, culminating with the Human Genome Project. This revolution has changed the classic practice of clinical cardiology in many ways, increasing our awareness of inheritance of defective genes and their impact on health and disease, and providing new diagnostic and therapeutic tools. On the other hand, identification of new diseases in the clinical setting has triggered research into previously unexplored areas of molecular biology. As a result of this interaction, both fields underwent major paradigm shifts. This article presents a primer of molecular biology for the cardiologist, followed by a discussion of the impact the Human Genome Project will have on the clinical practice of cardiology.

Molecular genetic basis of sudden cardiac death

In this review, the up-to-date understanding of the molecular basis of disorders causing sudden death will be described. Two arrhythmic disorders causing sudden death have recently been well described at the molecular level, the long QT syndromes (LQTS) and Brugada syndrome, and in this article we will review the current scientific knowledge of each disease. A third disorder, hypertrophic cardiomyopathy (HCM), a myocardial disorder causing sudden death, has also been well studied. Finally, a disorder in which both myocardial abnormalities and rhythm abnormalities coexist, arrhythmogenic right ventricular dysplasia (ARVD) will also be described. The role of the pathologist in these studies will be highlighted.

Heart rate variability in patients with congenital long QT syndrome

BACKGROUND

The congenital long QT syndrome (LQTS) affecting myocardial repolarization is caused by mutations in different cardiac potassium or sodium channel genes. Adrenergic triggers are known to initiate life-threatening torsade de pointes ventricular tachycardias in LQTS patients, and anti-adrenergic therapy has been shown to be effective in many cases. Despite this well-documented adrenergic component, the data about autonomic modulation of the heart rate in LQTS, as described by heart rate variability (HRV) analysis, are very limited.

METHODS

Conventional time- and frequency-domain and newer nonlinear measures of HRV were compared in resting conditions among 27 LQTS patients with gene mutations at the LQT1 (n = 8), LQT2 (n = 10) or LQT3 (n = 9) loci and 34 LQTS noncarrier family members.

RESULTS

None of the conventional time- or frequency-domain or newer nonlinear measures of HRV differed significantly between the LQTS carriers and LQTS noncarriers or between the LQT1, LQT2, and LQT3 carriers.

CONCLUSIONS

These findings suggest that baseline cardiac autonomic modulation of the heart rate measured in resting conditions by traditional or newer nonlinear measures of HRV is not altered in LQTS patients. Furthermore, no differences are observed in HRV parameters between LQTS patients with potassium (KvLQT1, HERG), and sodium (SCN5A) ion channel gene mutations. HRV analysis in resting conditions does not improve phenotypic characterization of LQTS patients.

Abrupt rate accelerations or premature beats cause life-threatening arrhythmias in mice with long-QT3 syndrome

Deletion of amino-acid residues 1505-1507 (KPQ) in the cardiac SCN5A Na(+) channel causes autosomal dominant prolongation of the electrocardiographic QT interval (long-QT syndrome type 3 or LQT3). Excessive prolongation of the action potential at low heart rates predisposes individuals with LQT3 to fatal arrhythmias, typically at rest or during sleep. Here we report that mice heterozygous for a knock-in KPQ-deletion (SCN5A(Delta/+)) show the essential LQT3 features and spontaneously develop life-threatening polymorphous ventricular arrhythmias. Unexpectedly, sudden accelerations in heart rate or premature beats caused lengthening of the action potential with early afterdepolarization and triggered arrhythmias in Scn5a(Delta/+) mice. Adrenergic agonists normalized the response to rate acceleration in vitro and suppressed arrhythmias upon premature stimulation in vivo. These results show the possible risk of sudden heart-rate accelerations. The Scn5a(Delta/+) mouse with its predisposition for pacing-induced arrhythmia might be useful for the development of new treatments for the LQT3 syndrome.

Evaluation and treatment of pediatric patients with congenital or acquired long QT interval syndromes

The long QT syndrome should be considered when evaluating patients with syncope. Prolongation of the QT interval and abnormalities of T wave morphology due to abnormal ventricular repolarization characterize the syndrome. In the past decade, molecular genetics has revealed that abnormal repolarization is the result of gene mutations encoding integral ion channels that generate the cardiac action potential. Eight subgroups of long QT syndrome associated with five genes have been described to date. The explosion in research in this area has led to a greater understanding of the clinical expression of autosomal dominant (Romano-Ward syndrome), autosomal recessive (Jervell and Lange-Nielsen syndrome) and acquired forms of the disease. This has also led to investigation in the area of genotype-specific therapy. The purpose of this review is to outline the strides made in the field of molecular genetics and update the reader on the recent advances in diagnosis and treatment of the long QT syndrome.

Pharmacological and device approach to therapy of inherited cardiac diseases associated with cardiac arrhythmias and sudden death

A genetic origin in diseases like the long QT syndrome, the Brugada syndrome, or hypertrophic cardiomyopathy have been identified over the past years. These diseases have in common that they may result in sudden cardiac death of the patient. Recognition of patients based on their phenotype and application in clinical practice of the knowledge acquired on the genetic basis may have a major impact on how we approach them. In the long QT syndrome several mutations have been identified both in the sodium and in the potassium channels. The different electrophysiological effects of the mutations lead to a common phenotype: prolongation of the QT interval; but also to a common clinical impact: occurrence of malignant ventricular arrhythmias. Genetics should help us in treating in a more rational way our patients depending on the type of mutation. In the Brugada syndrome, mutations affecting the sodium channel have been so far identified. The results are electrophysiologically opposite to the ones observed in the long QT syndrome. Thus different mutations in the same gene lead to different functional consequences. Again, identification and study of the right mutation may lead to a more rational treatment directed to correct the malfunction of the channel.

Genetics of brugada, long QT, and arrhythmogenic right ventricular dysplasia syndromes

This article outlines the up-to-date understanding of the molecular basis of primary ventricular arrhythmias. Two disorders have recently been well described at the molecular level, the long QT syndromes and Brugada syndrome, and this article reviews the current scientific knowledge of each disease. A third disorder, arrhythmogenic right ventricular dysplasia, which is on the cusp of understanding, will also be described.

Molecular biology and the prolonged QT syndromes

The prolonged QT syndromes are characterized by prolongation of the QT interval corrected for heart rate (QTc) on the surface electrocardiogram associated with T-wave abnormalities, relative bradycardia, and ventricular tachyarrhythmias, including polymorphic ventricular tachycardia and torsades de pointes. These patients tend to present with episodes of syncope, seizures, or sudden death typically triggered by exercise, emotion, noise, or, in some cases, sleep. These disorders of cardiac repolarization are commonly inherited, with the autosomal dominant form, Romano-Ward syndrome, most common. A rare autosomal recessive form associated with sensorineural deafness, Jervell and Lange-Nielsen syndrome, in which the cardiac disorder is autosomal dominant and deafness is a recessive trait, also occurs. The underlying genetic causes of these forms of prolonged QT interval syndromes are heterogeneous, with at least seven genes responsible for the clinical syndromes. All of the five genes identified to date encode ion channel proteins, suggesting this to be an ion channelopathy. In this review, the genetic basis of the prolonged QT interval syndromes will be discussed, genotype-phenotype correlations identified, and the approaches to genetic testing and treatments will be outlined.

Prolonged QT interval and sudden infant death--report of two cases

In the two cases where infants died suddenly and unexpectedly the electrocardiogram (ECG) of a younger sibling (case 1) and of a living twin (case 2) led to the suspicion that the two infants could have died from long QT syndrome (LQTS). In case 1, a His bundle (HB) dispersion and a pronounced hypoplasia of the right external nucleus arcuatus were detected. In case 2, a severe interstitial pneumonia and an accompanying mild myocarditis were found by histology. Molecular genetic investigations of the coding regions of the genes, HERG, KVLQT1 and SCN5A gave no indication for the mutations, thus, affecting related myocardial ion channels as possible sources of inhomogeneity of repolarisation. Since a molecular genetic deviation could not yet be elaborated the possible role of related disturbance remains unknown.

Etiology and management of sustained ventricular tachycardia

Ventricular tachyarrhythmias secondary to a variety of underlying cardiovascular problems pose a therapeutic challenge to the clinician. The initial presentation may be as sudden cardiac death, which underlies its public health problem. The underlying conditions predisposing to this arrhythmia include ischemic heart disease, dilated cardiomyopathy, hypertrophic cardiomyopathy, arrhythmiogenic right ventricle dysplasia and certain postoperative states including corrective surgery for tetralogy of Fallot and valve replacement. Other causes include prolonged QT syndrome, idiopathic right and left ventricle tachycardia and bundle branch re-entry tachycardia. Ischaemic heart disease is the most common cause of ventricular tachycardia and therapy has evolved considerably over the past two decades. The development of and refinements in the implantable cardioverter-defibrillator (ICD) have introduced a new dimension in therapeutic options and markedly improved survival in these patients. Insights in the dichotomy between arrhythmia suppression and total mortality have reoriented drug therapy with a decrease in the use of sodium channel blockers. beta-blockers have emerged as antiarrhythmic drugs in their own right and their synergistic effects with amiodarone have strengthened the antiarrhythmic drug arm. The role of these drugs in patients with hemodynamically stable ventricular tachycardia, especially in relatively preserved ventricles needs to be explored. Catheter ablation techniques have provided curative therapy in patients with idiopathic and bundle branch reentry tachycardia. Further advances in radiofrequency ablation, including use of newer mapping techniques, promise a greater role for ablation of ischemic ventricular tachycardia in the future. A hybrid approach consisting of drugs, catheter ablation and/ or ICD may provide effective therapeutic approach in some situations. Further innovations and technologic developments promise a further reorientation in therapy towards identification and treatment of the underlying arrhythmogenic substrate.

Genotype and severity of long QT syndrome

OBJECTIVES

To describe the state of the art of our understanding of the long QT syndromes and to provide the genetic correlation of clinical severity of patients with this disorder.

DATE SOURCES

In this review, we outline data that were obtained from work in our laboratory, as well as information reported in the literature.

STUDY SELECTION

The information in this review spans the last decade; data were obtained from the studies that had the most impact, as well as from recent work at our laboratory.

DATA EXTRACTION

The data reported herein were extracted from the world literature on sudden death and the clinical aspects of long QT syndrome. The genes identified to date, mutations in these genes, and the biophysical perturbations in the mutated ion channels, as well as the severity of disease, are detailed.

DATA SYNTHESIS

The extracted data are described as a state-of-the-art review.

CONCLUSIONS

The long QT syndromes, genetically heterogeneous disorders due to mutations in genes encoding ion channels, are relatively common causes of syncope and sudden death. The affected genes, along with the genetic background of individuals, determine the clinical severity of disease. An understanding of the mechanisms responsible for long QT syndrome is expected to enable development of specific therapies.

Spectrum of mutations in long-QT syndrome genes. KVLQT1, HERG, SCN5A, KCNE1, and KCNE2

BACKGROUND

Long-QT Syndrome (LQTS) is a cardiovascular disorder characterized by prolongation of the QT interval on ECG and presence of syncope, seizures, and sudden death. Five genes have been implicated in Romano-Ward syndrome, the autosomal dominant form of LQTS: KVLQT1, HERG, SCN5A, KCNE1, and KCNE2. Mutations in KVLQT1 and KCNE1 also cause the Jervell and Lange-Nielsen syndrome, a form of LQTS associated with deafness, a phenotypic abnormality inherited in an autosomal recessive fashion.

METHODS AND RESULTS

We used mutational analyses to screen a pool of 262 unrelated individuals with LQTS for mutations in the 5 defined genes. We identified 134 mutations in addition to the 43 that we previously reported. Eighty of the mutations were novel. The total number of mutations in this population is now 177 (68% of individuals).

CONCLUSIONS

KVLQT1 (42%) and HERG (45%) accounted for 87% of identified mutations, and SCN5A (8%), KCNE1 (3%), and KCNE2 (2%) accounted for the other 13%. Missense mutations were most common (72%), followed by frameshift mutations (10%), in-frame deletions, and nonsense and splice-site mutations (5% to 7% each). Most mutations resided in intracellular (52%) and transmembrane (30%) domains; 12% were found in pore and 6% in extracellular segments. In most cases (78%), a mutation was found in a single family or an individual.

The long QT syndromes: genetic basis and clinical implications

It is becoming clear that mutations in the KVLQT1, human "ether-a-go-go" related gene, cardiac voltage-dependent sodium channel gene, minK and MiRP1 genes, respectively, are responsible for the LQT1, LQT2, LQT3, LQT5 and LQT6 variants of the Romano-Ward syndrome, characterized by autosomal dominant transmission and no deafness. The much rarer Jervell-Lange-Nielsen syndrome (with marked QT prolongation and sensorineural deafness) arises when a child inherits mutant KVLQT1 or minK alleles from both parents. In addition, some families are not linked to the known genetic loci. Cardiac voltage-dependent sodium channel gene encodes the cardiac sodium channel, and long QT syndrome (LQTS) mutations prolong action potentials by increasing inward plateau sodium current. The other mutations cause a decrease in net repolarizing current by reducing potassium currents through "dominant negative" or "loss of function" mechanisms. Polymorphic ventricular tachycardia (torsade de pointes) is thought to be initiated by early after-depolarizations in the Purkinje system and maintained by reentry in the myocardium. Clinical presentations vary with the specific gene affected and the specific mutation. Nevertheless, patients with identical mutations can also present differently, and some patients with LQTS mutations may have no manifest baseline phenotype. The question of whether the latter situation is one of high risk for administration of QT prolonging drugs or during myocardial ischemia is under active investigation. More generally, the identification of LQTS genes has provided tremendous new insights for our understanding of normal cardiac electrophysiology and its perturbation in a wide range of conditions associated with sudden death. It seems likely that the approach of applying information from the genetics of uncommon congenital syndromes to the study of common acquired diseases will be an increasingly important one in the next millennium.

Novel KCNQ1 and HERG missense mutations in Dutch long-QT families

Congenital long QT syndrome (cLQTS) is electrocardiographically characterized by a prolonged QT interval and polymorphic ventricular arrhythmias (torsade de pointes). These cardiac arrhythmias may result in recurrent syncopes, seizure, or sudden death. LQTS can occur either as an autosomal dominant (Romano Ward) or as an autosomal recessive disorder (Jervell and Lange-Nielsen syndrome). Mutations in at least five genes have been associated with the LQTS. Four genes, encoding cardiac ion channels, have been identified. The most common forms of LQTS are due to mutations in the potassium-channel genes KCNQ1 and HERG. We have screened 24 Dutch LQTS families for mutations in KCNQ1 and HERG. Fourteen missense mutations were identified. Eight of these missense mutations were novel: three in KCNQ1 and five in HERG. Novel missense mutations in KCNQ1 were Y184S, S373P, and W392R and novel missense mutations in HERG were A558P, R582C, G604S, T613M, and F640L. The KCNQ1 mutation G189R and the HERG mutation R582C were detected in two families. The pathogenicity of the mutations was based on segregation in families, absence in control individuals, the nature of the amino acid substitution, and localization in the protein. Genotype-phenotype studies indicated that auditory stimuli as trigger of cardiac events differentiate LQTS2 and LQTS1. In LQTS1, exercise was the predominant trigger. In addition, a number of asymptomatic gene defect carriers were identified. Asymptomatic carriers are still at risk of the development of life-threatening arrhythmias, underlining the importance of DNA analyses for unequivocal diagnosis of patients with LQTS.

Identical twins with long QT syndrome associated with a missense mutation in the S4 region of the HERG

Familial long QT syndrome (LQTS) is caused by mutations in genes encoding ion channels important in determining ventricular repolarization. Mutations in at least five genes have been associated with the LQTS. Fire genes, KCNQ1, HERG, SCN5A, KCNE1, and KCNE2, have been identified. We have identified a missense mutation in the HERG gene in identical twins in a Japanese family with LQTS. The identical twins in our study had QT prolongation and the same missense mutation. However only the proband had a history of syncope. Although many mutations in LQT genes have been reported, there are few reports of twins with LQTS. This is the first report, to our knowledge, of identical twins with a HERG gene mutation.

Effects of flecainide in patients with new SCN5A mutation: mutation-specific therapy for long-QT syndrome?

BACKGROUND

Mutations in the cardiac sodium channel gene (SCN5A) can cause one variant of the congenital long-QT syndrome. The effects of some of these mutations on the alpha-subunit channel properties can be blocked by type Ib antiarrhythmic drugs. Recently, we have described a new SCN5A mutation (D1790G) that affects the channel properties in a manner suggesting that sodium blockers of the Ib type will be ineffective in carriers of this mutation. Hence, the ECG effects of flecainide-acetate, a type Ic sodium blocker, were evaluated in carriers of this mutation.

METHODS AND RESULTS

Eight asymptomatic mutation carriers and 5 control subjects were studied. Intravenous lidocaine was tested first in only 2 mutation carriers and had no significant effect on any ECG parameter. Flecainide significantly shortened all heart rate-corrected repolarization duration parameters only in carriers and not in control subjects: QT(c) shortened by 9.5% (from 517+/-45 to 468+/-36 ms, P=0.011), and the S-offset to T-onset interval shortened by 64.7% (from 187+/-88 to 66+/-50 ms, P=0.0092). Flecainide also normalized the marked baseline repolarization dispersion in most mutation carriers. These effects among carriers were maintained during long-term (9 to 17 months) outpatient flecainide therapy with no adverse effects.

CONCLUSIONS

This report is the first to describe SCN5A mutation carriers who significantly responded to flecainide therapy yet did not respond to lidocaine. These results have important implications for long-QT allele-specific therapeutic strategies.

Expression of distinct ERG proteins in rat, mouse, and human heart. Relation to functional I(Kr) channels

One form of inherited long QT syndrome, LQT2, results from mutations in HERG1, the human ether-a-go-go-related gene, which encodes a voltage-gated K(+) channel alpha subunit. Heterologous expression of HERG1 gives rise to K(+) currents that are similar (but not identical) to the rapid component of delayed rectification, I(Kr), in cardiac myocytes. In addition, N-terminal splice variants of HERG1 and MERG1 (mouse ERG1) referred to as HERG1b and MERG1b have been cloned and suggested to play roles in the generation of functional I(Kr) channels. In the experiments here, antibodies generated against HERG1 were used to examine ERG1 protein expression in heart and in brain. In Western blots of extracts of QT-6 cells expressing HERG1, MERG1, or RERG1 (rat ERG1) probed with antibodies targeted against the C terminus of HERG1, a single 155-kDa protein is identified, whereas a 95-kDa band is evident in blots of extracts from cells expressing MERG1b or HERG1b. In immunoblots of fractionated rat (and mouse) brain and heart membrane proteins, however, two prominent high molecular mass proteins of 165 and 205 kDa were detected. Following treatment with glycopeptidase F, the 165- and 205-kDa proteins were replaced by two new bands at 175 and 130 kDa, suggesting that ERG1 is differentially glycosylated in rat/mouse brain and heart. In human heart, a single HERG1 protein with an apparent molecular mass of 145 kDa is evident. In rats, ERG1 protein (and I(Kr)) expression is higher in atria than ventricles, whereas in humans, HERG1 expression is higher in ventricular, than atrial, tissue. Taken together, these results suggest that the N-terminal alternatively spliced variants of ERG1 (i.e. ERG1b) are not expressed at the protein level in rat, mouse, or human heart and that these variants do not, therefore, play roles in the generation of functional cardiac I(Kr) channels.

Clinical and genetic variables associated with acute arousal and nonarousal-related cardiac events among subjects with long QT syndrome

In patients with the long QT syndrome (LQTS), the occurrence of cardiac events (syncope or cardiac arrest) is frequently associated with acute arousal caused by exercise, swimming, emotion, or noise. However, cardiac events may also occur during sleep or with ordinary daily activities. The purpose of this study was to determine whether there are differential clinical, electrocardiographic, and genetic features among LQTS patients who experienced cardiac events with and without acute arousal. We identified 1,325 patients with cardiac events from the International LQTS Registry. Based on the precipitating conditions of the first event, 427 patients were classified as arousal, 345 as nonarousal, and the remaining 553 were unknown (not classifiable). Gene linkage was known in 78 of the 772 patients with classifiable first events. The age at first cardiac event was significantly younger in the arousal than the nonarousal group (11.7 vs. 15.5 years, respectively; p<0.001). The arousal-type patients had a higher rate of subsequent cardiac events during follow-up after the index event than the nonarousal-type patients (p = 0.02). Arousal-related cardiac events occurred in 85% of LQT1, 67% of LQT2, and 33% of LQT3 patients (p = 0.008). This study provides evidence that the genotype is an important determinant of the LQTS phenotype in terms of arousal and nonarousal-related cardiac events.

Molecular biology of arrhythmic syndromes

In this review, the up-to-date understanding of the molecular basis of primary ventricular arrhythmias will be outlined. Two disorders have recently been well described at the molecular level, the long QT syndromes and Brugada syndrome, and in this paper we review the current scientific knowledge of each disease.

Long-term follow-up of patients with long-QT syndrome treated with beta-blockers and continuous pacing

BACKGROUND

The long-QT syndrome is associated with sudden cardiac death. Combination of beta-blocker and pacing therapy has been proposed for treatment of drug-resistant patients. The purpose of this study was to summarize our long-term experience with combined therapy in patients with long-QT syndrome.

METHODS AND RESULTS

A total of 37 patients with idiopathic long-QT syndrome were treated with combined therapy consisting of continuous cardiac pacing and maximally tolerated beta-blocker therapy and followed up for 6.3+/-4. 6 years (mean+/-SD). The group consisted of 32 female and 5 male patients with a mean age of 31.6 years. The mean paced rate was 82+/-7 bpm (range, 60 to 100 bpm). On follow-up, recurrent symptoms caused by pacemaker malfunction were documented in 3 patients. Four patients died during the follow-up period: 2 adolescents stopped beta-blocker therapy, 1 patient died suddenly while treated with combined therapy, and 1 patient died of unrelated causes. In addition, 3 patients had resuscitated cardiac arrest while on combined therapy, and 1 patient had repeated, appropriate implantable cardioverter-defibrillator discharges on follow-up.

CONCLUSIONS

Because 28 of 37 patients remain without symptoms with beta-blocker therapy and continuous pacing, combined therapy appears to provide reasonable, long-term control for this high-risk group. However, the incidence of sudden death and aborted sudden death (24% in all patients and 17% in compliant patients) strongly suggests the use of a "back-up" defibrillator, particularly in noncompliant adolescent patients. Implantable cardioverter-defibrillator therapy, however, may be associated with recurrent shocks in susceptible patients.

Voltage-gated ion channels and hereditary disease

By the introduction of technological advancement in methods of structural analysis, electronics, and recombinant DNA techniques, research in physiology has become molecular. Additionally, focus of interest has been moving away from classical physiology to become increasingly centered on mechanisms of disease. A wonderful example for this development, as evident by this review, is the field of ion channel research which would not be nearly as advanced had it not been for human diseases to clarify. It is for this reason that structure-function relationships and ion channel electrophysiology cannot be separated from the genetic and clinical description of ion channelopathies. Unique among reviews of this topic is that all known human hereditary diseases of voltage-gated ion channels are described covering various fields of medicine such as neurology (nocturnal frontal lobe epilepsy, benign neonatal convulsions, episodic ataxia, hemiplegic migraine, deafness, stationary night blindness), nephrology (X-linked recessive nephrolithiasis, Bartter), myology (hypokalemic and hyperkalemic periodic paralysis, myotonia congenita, paramyotonia, malignant hyperthermia), cardiology (LQT syndrome), and interesting parallels in mechanisms of disease emphasized. Likewise, all types of voltage-gated ion channels for cations (sodium, calcium, and potassium channels) and anions (chloride channels) are described together with all knowledge about pharmacology, structure, expression, isoforms, and encoding genes.

Splicing mutations in KCNQ1: a mutation hot spot at codon 344 that produces in frame transcripts

BACKGROUND

Long-QT syndrome is a monogenic disorder that produces cardiac arrhythmias and can lead to sudden death. At least 5 loci and 4 known genes exist in which mutations have been shown to be responsible for the disease. The potassium channel gene KCNQ1, previously named KVLQT1, on chromosome 11p15.5 is one of these.

METHODS AND RESULTS

We initially analyzed one family using microsatellite markers and found linkage to KCNQ1. Mutation detection showed a G to C change in the last base of exon 6 (1032 G-->C) that does not alter the coded alanine. Restriction digest analysis in the family showed that only affected individuals carried the mutation. A previous report suggested that a G to A substitution at the same position may act as a splice mutation in KCNQ1, but no data was given to support this hypothesis nor was the transcription product identified. We have shown by reverse-transcription polymerase chain reaction that 2 smaller bands were produced for the KCNQ1 gene transcripts in addition to the normal-sized transcripts when lymphocytes of affected individuals were analyzed. Sequencing these transcripts showed a loss of exon 7 in one and exons 6 and 7 in the other, but an in-frame transcript was left in each instance. We examined other families in whom long-QT syndrome was diagnosed and found another unreported splice-site mutation, 922-1 G-->C, in the acceptor site of intron 5, and 2 of the previously reported 1032 G-->A mutations. All these showed a loss of exons 6 and 7 in the mutant transcripts, validating the proposal that a consensus sequence is affected in the exonic mutations and that the integrity of the base at position 1032 is essential for correct processing of the transcript.

CONCLUSIONS

The 6 cases already reported in the literature with the 1032 G-->A transition, the novel 1032 G-->C transversion, and a recent G-->T transversion at the same base show that codon 344 is the second most frequently mutated after codon 341, suggesting at least two hotspots for mutations in KCNQ1.

Prenatal diagnosis of long QT syndrome using fetal magnetocardiography

We describe the detection of congenital long QT syndrome in a fetus at 37 weeks' gestation using magnetocardiography (MCG). The prenatal diagnosis was confirmed by standard electrocardiography (ECG) performed after birth. This is the first case report of fetal long QT syndrome detected by MCG. Fetal MCG may be useful in the prenatal diagnosis of congenital cardiac disease with abnormal ECG findings.

C-terminal HERG mutations: the role of hypokalemia and a KCNQ1-associated mutation in cardiac event occurrence

BACKGROUND

The long-QT syndrome (LQTS) is a genetically heterogeneous disease in which 4 genes encoding ion-channel subunits have been identified. Most of the mutations have been determined in the transmembrane domains of the cardiac potassium channel genes KCNQ1 and HERG. In this study, we investigated the 3' part of HERG for mutations.

METHODS AND RESULTS

New specific primers allowed the amplification of the 3' part of HERG, the identification of 2 missense mutations, S818L and V822 M, in the putative cyclic nucleotide binding domain, and a 1-bp insertion, 3108+1G. Hypokalemia was a triggering factor for torsade de pointes in 2 of the probands of these families. Lastly, in a large family, a maternally inherited G to A transition was found in the splicing donor consensus site of HERG, 2592+1G-A, and a paternally inherited mutation, A341E, was identified in KCNQ1. The 2 more severely affected sisters bore both mutations.

CONCLUSIONS

The discovery of mutations in the C-terminal part of HERG emphasizes that this region plays a significant role in cardiac repolarization. Clinical data suggests that these mutations may be less malignant than mutations occurring in the pore region, but they can become clinically significant in cases of hypokalemia. The first description of 2 patients with double heterozygosity associated with a dramatic malignant phenotype implies that genetic analysis of severely affected young patients should include an investigation for >1 mutation in the LQT genes.

Homozygous deletion in KVLQT1 associated with Jervell and Lange-Nielsen syndrome

BACKGROUND

Long-QT (LQT) syndrome is a cardiac disorder that causes syncope, seizures, and sudden death from ventricular arrhythmias, specifically torsade de pointes. Both autosomal dominant LQT (Romano-Ward syndrome) and autosomal recessive LQT (Jervell and Lange-Nielsen syndrome, JLNS) have been reported. Heterozygous mutations in 3 potassium channel genes, KVLQT1, KCNE1 (minK), and HERG, and the cardiac sodium channel gene SCN5A cause autosomal dominant LQT. Autosomal recessive LQT, which is associated with deafness, has been found to occur with homozygous mutations in KVLQT1 and KCNE1 in JLNS families in which QTc prolongation was inherited as a dominant trait.

METHODS AND RESULTS

An Amish family with clinical evidence of JLNS was analyzed for mutations by use of single-strand conformation polymorphism and DNA sequencing analyses for mutations in all known LQT genes. A novel homozygous 2-bp deletion in the S2 transmembrane segment of KVLQT1 was identified in affected members of this Amish family in which both QTc prolongation and deafness were inherited as recessive traits. This deletion represents a new JLNS-associated mutation in KVLQT1 and has deleterious effects on the KVLQT1 potassium channel, causing a frameshift and the truncation of the KVLQT1 protein. In contrast to previous reports in which LQT was inherited as a clear dominant trait, 2 parents in the JLNS family described here have normal QTc intervals (0.43 and 0.44 seconds, respectively).

CONCLUSIONS

A novel homozygous KVLQT1 mutation causes JLNS in an Amish family with deafness that is inherited as an autosomal recessive trait.

Genetic and molecular basis of cardiac arrhythmias: impact on clinical management parts I and II

Genetic approaches have succeeded in defining the molecular basis of an increasing array of heart diseases, such as hypertrophic cardiomyopathy and the long-QT syndromes, associated with serious arrhythmias. Importantly, the way in which this new knowledge can be applied to managing patients and to the development of syndrome-specific antiarrhythmic strategies is evolving rapidly because of these recent advances. In addition, the extent to which new knowledge represents a purely research tool versus the extent to which it can be applied clinically is also evolving. The present article represents a consensus report of a meeting of the European Working Group on Arrhythmias. The current state of the art of the molecular and genetic basis of inherited arrhythmias is first reviewed, followed by practical advice on the role of genetic testing in these and other syndromes and the way in which new findings have influenced current understanding of the molecular and biophysical basis of arrhythmogenesis.

Low penetrance in the long-QT syndrome: clinical impact

BACKGROUND

It is still currently held that most patients affected by the long-QT syndrome (LQTS) show QT interval prolongation or clinical symptoms. This is reflected by the assumption in linkage studies of a penetrance of 90%. We had previously suggested that a larger-than-anticipated number of LQTS patients might be affected without showing clinical signs. We have now exploited the availability of molecular diagnosis to test this hypothesis.

METHODS AND RESULTS

We identified 9 families with "sporadic" cases of LQTS, ie, families in which, besides the proband, none of the family members had clinical signs of the disease. Mutation screening by conventional single-strand conformational polymorphism and sequencing was performed on DNA of probands and family members to identify mutation carriers. Of 46 family members considered on clinical grounds to be nonaffected, 15 (33%) were found instead to be gene carriers. Penetrance was found to be 25%. In these families, conventional clinical diagnostic criteria had a sensitivity of only 38% in correctly identifying carriers of the genetic defect.

CONCLUSIONS

This study demonstrates that in some families, LQTS may appear with a very low penetrance, a finding with multiple clinical implications. The family members considered to be normal and found to be silent gene carriers are unexpectedly at risk of generating affected offspring and also of developing torsade de pointes if exposed to either cardiac or noncardiac drugs that block potassium channels. It is no longer acceptable to exclude LQTS among family members of definitely affected patients on purely clinical grounds. Conversely, it now appears appropriate to perform molecular screening in all family members of genotyped patients.

Long-term (subacute) potassium treatment in congenital HERG-related long QT syndrome (LQTS2)

INTRODUCTION

Congenital long QT syndrome (LQTS) is subdivided according to the underlying gene defect. In LQTS2, an aberrant HERG gene that encodes the potassium channel IKr leads to insufficient IKr activity and delayed repolarization, causing ECG abnormalities and torsades de pointes (TdP). Increasing serum potassium levels by potassium infusion normalizes the ECG in LQTS2 because IKr activity varies with serum potassium levels.

METHODS AND RESULTS

In an LQTS2 patient who presented with TdP, we attempted to achieve a long-term (subacute) elevation of serum potassium by increased potassium intake and potassium-sparing drugs. However, due to renal potassium homeostasis, it was impossible to achieve a long-lasting rise of serum potassium above 4.0 mmol/L.

CONCLUSION

Although raising serum potassium reverses the ECG abnormalities in LQTS2, a long-lasting rise of serum potassium is only partially achievable because in the presence of normal renal function, potassium homeostasis limits the amount of serum potassium increase.

Temperature dependence of early and late currents in human cardiac wild-type and long Q-T DeltaKPQ Na+ channels

Na+ current (INa) through wild-type human heart Na+ channels (hH1) is important for normal cardiac excitability and conduction, and it participates in the control of repolarization and refractoriness. INa kinetics depend strongly on temperature, but INa for hH1 has been studied previously only at room temperature. We characterized early INa (the peak and initial decay) and late INa of the wild-type hH1 channel and a mutant channel (DeltaKPQ) associated with congenital long Q-T syndrome. Channels were stably transfected in HEK-293 cells and studied at 23 and 33 degreesC using whole cell patch clamp. Activation and inactivation kinetics for early INa were twofold faster at higher temperature for both channels and shifted activation and steady-state inactivation in the positive direction, especially for DeltaKPQ. For early INa (<24 ms), DeltaKPQ decayed faster than the wild type for voltages negative to -20 mV but slower for more positive voltages, suggesting a reduced voltage dependence of fast inactivation. Late INa at 240 ms was significantly greater for DeltaKPQ than for the wild type at both temperatures. The majority of late INa for DeltaKPQ was not persistent; rather, it decayed slowly, and this late component exhibited slower recovery from inactivation compared with peak INa. Additional kinetic changes for early and peak INa for DeltaKPQ compared with the wild type at both temperatures were 1) reduced voltage dependence of steady-state inactivation with no difference in midpoint, 2) positive shift for activation kinetics, and 3) more rapid recovery from inactivation. This study represents the first description of human Na+ channel kinetics near physiological temperature and also demonstrates complex gating changes in the DeltaKPQ that are present at 33 degreesC and that may underlie the electrophysiological and clinical phenotype of congenital long Q-T Na+ channel syndromes.

Inhibition of cardiac delayed rectifier K+ current by overexpression of the long-QT syndrome HERG G628S mutation in transgenic mice

Mutations in the HERG gene are linked to the LQT2 form of the inherited long-QT syndrome. Transgenic mice were generated expressing high myocardial levels of a particularly severe form of LQT2-associated HERG mutation (G628S). Hearts from G628S mice appeared normal except for a modest enlargement seen only in females. Ventricular myocytes isolated from adult wild-type hearts consistently exhibited an inwardly rectifying E-4031-sensitive K+ current resembling the rapidly activating cardiac delayed rectifier K+ current (Ikr) in its time and voltage dependence; this current was not found in cells isolated from G628S mice. Action potential duration was significantly prolonged in single myocytes from G628S ventricle (cycle length=1 second, 26 degrees C) but not in recordings from intact ventricular strips studied at more physiological rates and temperature (200 to 400 bpm, 37 degrees C). ECG intervals, including QT duration, were unchanged, although minor aberrancies were noted in 20% (16/80) of the G628S mice studied, primarily involving the QRS complex and, more rarely, T-wave morphology. The aberrations were more commonly observed in females than males but could not be correlated with sex-based differences in action potential duration. These results establish the presence of IKr in the adult mouse ventricle and demonstrate the ability of the G628S mutation to exert a dominant negative effect on endogenous IKr in vivo, leading to the expected LQT2 phenotype of prolonged repolarization at the single cell level but not QT prolongation in the intact animal. The model may be useful in dissecting repolarization currents in the mouse heart and as a means of examining the mechanism(s) by which the G628S mutation exerts its dominant negative effect on native cardiac cells in vivo.

The molecular basis of long QT syndrome and prospects for therapy

Long QT syndrome (LQT) is a cardiac disorder that causes sudden death from ventricular tachyarrhythmias, specifically torsade de pointes. Two types of LQT have been reported, autosomal-dominant LQT (Romano-Ward syndrome) and autosomal-recessive LQT (Jervell and Lange-Nielsen syndrome); Jervell and Lange-Nielsen syndrome is also associated with deafness. Four LQT genes have been identified for autosomal-dominant LQT: K+ channel genes KVLQT1 on chromosome 11p15.5, HERG on 7q35-36 and minK on 21q22, and the cardiac Na+ channel gene SCN5A on chromosome 3p21-24. Two genes, KVLQT1 and minK, have been identified for Jervell and Lange-Nielsen syndrome. Genetic testing and gene-specific therapies are available for some LQT patients.

Novel mechanism of HERG current suppression in LQT2: shift in voltage dependence of HERG inactivation

In a Xenopus oocyte heterologous expression system, we characterized the electrophysiology of 3 novel missense mutations of HERG identified in Japanese LQT2 families: T474I (within the S2-S3 linker), A614V, and V630L (in the outer mouth of pore-forming region). For each of the 3 mutations, injection of mutant cRNA alone did not express detectable currents. Coinjection of wild-type (WT) along with each mutant cRNA (T474I/WT, A614V/WT, and V630L/WT) suppressed HERG current in a dominant-negative manner, and the order of magnitude of current suppression was V630L/WT>A614V/WT>T474I/WT. In addition to decreases in slope conductance for all 3 mutants, the voltage dependence of steady-state inactivation was shifted to negative potentials for V630L/WT and A614V/WT. Consequently, channel availability at positive potentials was diminished, and inward rectification was enhanced for these 2 mutants. Thus, missense mutations of HERG caused dominant-negative suppression through multiple mechanisms. The shift in voltage dependence of HERG inactivation and the resulting enhanced inward rectification in A614V/WT and V630L/WT provide a novel mechanism for suppression of the HERG current carrying outward current during the repolarization phase of the action potential.