JOURNAL TRANSCRIPT

---

PHYSIOLOGY IN MEDICINE In collaboration with The American Physiological Society, Thomas E. Andreoli, MD, Editor

Molecular Biology and the Prolonged QT Syndromes Jeffrey A. Towbin, MD, Matteo Vatta, PhD 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. Am J Med. 2001;110:385–398. 䉷2001 by Excerpta Medica, Inc.

T

CLINICAL FEATURES

he long QT sydrome (LQTS) was first formally described by Jervell and Lange-Nielsen (1) in 1957, but scattered case reports date back to at least 1856 when Meissner (2) described a young deaf girl who experienced sudden cardiac death while being admonished at school. This young girl had two brothers who had previously died suddenly after episodes of fright and rage. Romano et al (3) in 1963 and Ward in 1964 (4) reported families with similar cardiac history but without deafness, suggesting a similar but different syndrome. The full picture of the clinical disorders, however, required careful analysis using such diagnostic tools as the electrocardiogram in order to define the disease criteria. More recently, the use of molecular genetic tools has also allowed for a basic understanding of the underlying mechanisms of disease at the level of the genes and the proteins they encode. The purpose of this article is to describe the clinical features, inheritance patterns, diagnostic approaches, molecular genetic insights, and therapeutic options for children and adults with the long QT syndromes.

From the Department of Pediatrics (Cardiology) (JAT, MV) and the Department of Molecular and Human Genetics (JAT), Baylor College of Medicine, Houston, Texas. Requests for reprints should be addressed to Jeffrey A. Towbin, MD, Pediatric Cardiology, Baylor College of Medicine, One Baylor Plaza, Room 333E, Houston, Texas 77030. 䉷2001 by Excerpta Medica, Inc. All rights reserved.

The typical clinical presentation of LQTS is the occurrence of syncope or cardiac arrest precipitated by emotional or physical stress in a young, usually healthy individual. Females are affected more commonly than males, with a ratio of approximately 2:1 (5,6), but this is age dependent. An equal number of males and females are diagnosed between infancy and age 15 years, after which time there is a sharp increase in the female-to-male ratio (6,7). Importantly, the age at initial syncopal event is younger in males (11 ⫾ 11 years vs 16 ⫾ 13 years), and by age 15 years many more males than females (80% vs 52%) have had their first event. The syncopal episodes in LQTS are the result of torsades de pointes or polymorphic ventricular tachycardia, which often degenerates into ventricular fibrillation (8). The syncopal episodes are commonly associated with sudden increases in sympathetic activity, such as during strong emotional outpouring (ie, fright, anger, sudden surprise), or physical activity (8,9). Swimming appears to be commonly associated with syncope and sudden death in individuals with LQTS (10). Additional associations with syncope or sudden death include sudden awakening from sleep, as occurs with the ringing of the alarm clock or telephone, or a thunderstorm (10). In some children, the presentation of syncope appears as a seizure (11,12), and these children may go for years without appropriate therapy for the underlying LQTS and ventricular arrhythmias, instead receiving an0002-9343/01/$–see front matter 385 PII S0002-9343(00)00715-4

Molecular Biology and the Prolonged QT Syndromes/Towbin and Vatta

Figure 1. Fifteen-lead surface electrocardiogram of a 12-year-old female with long QT syndrome. Apparent in lead II is a deformation in the T wave. This is an example of “pseudo” 2:1 atrioventricular block in a patient with a QTc of 500 milliseconds.

ticonvulsant medications. A less common presentation of sudden cardiac death in individuals with LQTS is cardiac arrest during sleep or at rest (12). It is unclear whether the same underlying pathogenic mechanisms are at play in these patients as in those having symptoms associated with stress.

ELECTROCARDIOGRAPHIC FEATURES The classical electrocardiographic (ECG) features of LQTS include prolongation of the rate-corrected QT interval (QTc; Figure 1) as measured by Bazett’s formula (QTc ⫽ QT/公RR) (13). The ECG changes in LQTS include considerably more than just the simple prolongation of the QT interval. Indeed, most of the characteristic ECG findings are consistent with the concept that repolarization in LQTS displays substantial spatial and temporal heterogeneity. Some of these important ECG features include T- and U-wave abnormalities, including Twave alternans (Figure 2), relative bradycardia, and episodic polymorphous ventricular tachycardia or tor-

sades de pointes (Figure 3), which may degenerate into ventricular fibrillation.

DISEASE CLASSIFICATION LQTS occurs either as an inherited disorder, sporadic disorder, or it may be acquired. The clinical presentation is similar in all forms of LQTS, but minor variations should be noted. Two inherited forms of LQTS have been described thus far and include the Romano-Ward syndrome (3) and the Jervell and Lange-Nielsen syndrome (JLNS) (1).

Romano-Ward Syndrome This is the most common of the inherited forms of LQTS and appears to be transmitted as an autosomal dominant trait (3). In this disorder, gene carriers are expected to be clinically affected and have a 50% likelihood of passing the disease-causing gene to their offspring. Males and females are expected to be affected equally and to the same degree, although in clinical practice this does not appear

Figure 2. One-day-old female infant with Romano-Ward long QT syndrome and a QTc of 560 milliseconds. A zoomed view of lead V6 shows classic T-wave alternans with a change in T-wave axis on alternating beats. 386

April 1, 2001

THE AMERICAN JOURNAL OF MEDICINE威 Volume 110

Molecular Biology and the Prolonged QT Syndromes/Towbin and Vatta

Figure 3. Two-day-old male infant with long QT syndrome. Polymorphic ventricular tachycardia with a more classic appearance of torsades de pointes is seen. Note the obvious rapid changing axis over one beat in the first three strips. Again, the arrhythmia self terminates.

to be the case, with females being more commonly diagnosed than males by a 2:1 margin (6,7). Individuals with Romano-Ward syndrome have the pure clinical syndrome of prolonged QT interval on electrocardiogram with the associated symptom complex of syncope, sudden death, and in some patients, seizures (12). Occasionally, other noncardiac abnormalities, such as diabetes mellitus (14), asthma (15), or syndactyly (16), may also occur. LQTS may also be involved in some cases of sudden infant death syndrome (SIDS) (17).

Jervell and Lange-Nielsen Syndrome Initially described in 1957 (1), this is a relatively uncommon inherited form of LQTS with apparent autosomal recessive transmission. These patients have a clinical presentation identical to those with Romano-Ward syndrome but also have associated sensorineural deafness (1,18,19). Individuals with JLNS usually have longer QT intervals as compared with individuals with RomanoWard syndrome and also have a more malignant course. The suggestion that JLNS is transmitted in an autosomal recessive fashion was first suggested by the initial report in which four deaf children with the clinical syndrome had two unaffected siblings and normal, nonconsanguineous parents. Confirmation of the recessive pattern of inheritance came with reports in which consanguinity was present among unaffected parents of affected children (20,21). In autosomal recessive inheritance, both parents are heterozygous gene carriers but are unaffected clinically. Because both alleles must be abnormal in order to express the disease phenotype, only 25% of offspring are likely to become affected, but another 50% of the offspring will be gene carriers.

Sporadic LQTS Individuals with sporadic LQTS present as the first member of a family to have the diagnostic features of LQTS (5,8,15,22). Family history is negative for evidence of LQTS in any relatives, including SIDS or other unexpected sudden cardiac death. Screening electrocardiograms in family members must be normal to consider the affected individual as a sporadic case. If genetic testing is performed, the parents and siblings must be normal, while the affected individual carries an identifiable gene mutation. Individuals with sporadic LQTS have a clinical syndrome identical to those with the inherited forms of LQTS (although usually without deafness).

Acquired LQTS LQTS is considered to be acquired under conditions of exposure to certain drugs, electrolyte abnormalities, intracranial disease, dietary deficiencies, and such heart diseases as congenital heart disease, myocardial infarction, dilated cardiomyopathy, and mitral valve prolapse (23– 32). We have also seen severely malnourished and anorexic patients present with bradycardia and prolongation of the QTc interval. Pharmacologic agents are probably the most common cause of acquired LQTS. Agents that block potassium (K⫹) channels in general (23,24), and selectively block the rapidly activating component of the cardiac delayed rectifier K⫹ channel current (IKr) in particular, have been associated with acquired LQTS (25,26). In addition to antiarrhythmic agents, especially class ⌱A or class III antiarrhythmics (27–29), other LQTS-causing drugs include certain antihistamines (ie, Seldane) (30), macrolide April 1, 2001

THE AMERICAN JOURNAL OF MEDICINE威

Volume 110 387

Molecular Biology and the Prolonged QT Syndromes/Towbin and Vatta

Table 1. Causes of Acquired Long QT Syndrome Type/Name of Drug

Chemical Name

Anesthetics/asthma Adrenaline Antihistamines Seldane Hismanol Benadryl Antibiotics E-Mycin, EES, EryPeds, PCE, etc. Bactrim, Septra Pentam intravenous Heart medications Heart rhythm drugs Quinidine, Quinidex, Duraquin, Quinaqlute, etc. Pronestyl Norpace Betapace Lipid lowering drugs Lorelco Antianginal drugs Vascor Gastrointestinal Propulsid Antifungal drugs Nizoral Diflucan Sporanox Psychotropic drugs Elavil, Norpramin, Viractil, Compazine, Stelazine, Thorazine, Mellaril, Etrafon, Trilafon, others Haldol Risperdal Orap Diuretics Lozol Potassium loss

antibiotics (erythromycin) (31), cisapride (32), and others (Table 1). The fact that only segments of the population are at risk for acquired LQTS suggests a genetic predisposition toward excessive drug response, and the link to a genetic defect has been suggested (33,34) although not yet proven. It is possible that the drug can “unmask” an otherwise unexpressed genetic disorder leading to symptoms.

EPIDEMIOLOGY LQTS was considered to be a very rare disorder until recently. Early in the course of studying LQTS there was speculation that LQTS was “undoubtedly more unrecognized than rare” (22), and this has recently been shown to

388

April 1, 2001

THE AMERICAN JOURNAL OF MEDICINE威 Volume 110

Epinephrine Terfenadine Astemizole Diphenhydramine Erythromycin Trimethoprim & Sulfamethoxazole Pentamidine

Quinidine Procainamide Disopyramide Sotalol Probucol Bepridil Cisapride Ketoconazole Fluconazole Itraconazole Amitriptyline (Tricyclics) Haloperidol Risperidone Pimozide Indapamide

be true as many new cases became identified. This new ability to identify patients was the result of improved physician education and awareness, the new molecular genetic information recently obtained, and the computerbased advertising by the LQTS support groups. The incidence of LQTS in the general population is not generally known, but recent speculations suggest LQTS to be as common as cystic fibrosis. The incidence of JLNS, which appears to account for less than 10% of the total cases of LQTS, has been estimated to be approximately 2 to 3 per 1,000 among congenitally deaf individuals (20,21,33). Fraser et al (20,21) estimated the prevalence of JLNS to be between 1.6 and 6 per million population in children aged 4 to 14 years in England, Wales, and Ireland.

Molecular Biology and the Prolonged QT Syndromes/Towbin and Vatta

Table 2. Diagnostic Criteria in Long QT Syndrome Clinical Finding Electrocardiographic findings* QTc † ⬎480 ms1/2 460–470 ms1/2 450 (male) ms1/2 Torsades de pointes‡ T-wave alternans Notched T wave in three leads Low heart rate for age§ Clinical history Syncope‡ With stress Without stress Congenital deafness Family history** Family members with definite LQTS¶ Unexplained sudden cardiac death below age 30 among immediate family members

Points

3 2 1 2 1 1 0.5

2 1 0.5 1 0.5

* In the absence of medications or disorders known to affect these ECG features. † QTc calculated by Bazett’s formula, where QTc ⫽ QT/公RR. ‡ Mutually exclusive. § Resting heart rate below the second percentile for age. ** The same family member cannot be counted twice. ¶ Definite LQTS is defined by an LQTS score ⬎4. Scoring: ⬍1 point ⫽ low probability of LQTS; 2 to 3 points ⫽ intermediate probability of LQTS; ⬎4 points ⫽ high probability of LQTS. Source: Circulation (35). LQTS ⫽ long QT syndrome.

DIAGNOSTIC CRITERIA In those patients with LQTS in which classical, clear-cut evidence of disease is apparent, the diagnosis can be made rapidly by simply taking a history and evaluating the electrocardiogram. As with many other clinical disorders, however, many cases are borderline or are confusing based on the lack of clinical correlation between history and the classical diagnostic methods. Another problem that existed until recently was that this disorder was believed to be rare and many physicians were unaware of the disease. For these reasons, Schwartz et al developed a set of diagnostic criteria in 1985 (8,10), in order to more consistently and broadly allow for the correct diagnosis of LQTS to be entertained. These criteria included major criteria (prolonged QTc greater than 0.44 seconds, stressinduced syncope, family members with LQTS) and minor criteria (congenital deafness, episodes of T-wave alternans, low heart rate in children, abnormal ventricular repolarization), with either two major criteria or one major and two minor criteria required for diagnosis. Since 1985, new information accumulated that enabled clinicians to be more certain of the diagnosis of LQTS, and for

this reason, Schwartz et al updated the diagnostic criteria in 1993 (Table 2) (35). In this iteration of the diagnostic criteria, a point system was used to distinguish the likelihood of a patient having the disease. The criteria take into account the ECG findings, clinical history, and family history and rank the findings by points based on the “importance” of the finding (ie, QTc greater than 0.48 seconds ⫽ 3 points; low heart rate for age ⫽ 0.5 points). In familial cases in which genetic linkage analysis can be performed, an additional set of criteria is also used. The criteria, first described by Keating et al (36), use the data obtained by electrocardiogram and by history. In these cases, a QTc greater than 0.46 seconds is diagnostic of affected status, a QTc less than 0.41 seconds is consistent with an unaffected status, and QTc 0.41 to 0.46 seconds carries uncertain status. However, episodes of syncope or other concerning symptoms, as well as a family history of sudden death, syncope, or LQTS, may also be used to determine the clinical likelihood of disease. The use of these criteria better enables the molecular geneticist to perform linkage analysis.

PROVOCATIVE TESTS The diagnosis of LQTS can be quite difficult in some patients using the surface electrocardiogram alone. While some ECG abnormalities, such as obvious QTc prolongation and T-wave abnormalities such as T-wave alternans, are diagnostic of LQTS, many patients have borderline QT interval prolongation or normal QT intervals with either symptoms (ie, syncope), torsades de pointes, or a family history of LQTS. In these patients, provocative tests may be useful. The tests may include determining the extent of QT interval shortening with treadmill exercise or evaluation of the presence of U waves or arrhythmias with epinephrine or isoproterenol infusion (37,38). However, exercise under controlled conditions rarely provokes torsades de pointes, whereas spontaneous episodes often occur with combinations of exercise and emotional stress. Infusion of epinephrine or isoproterenol may elicit an arrhythmic response but may be difficult to interpret (38).

GENETICS OF LONG QT SYNDROME As previously noted, three forms of inherited LQTS have been described, including autosomal dominant (Romano-Ward syndrome), autosomal recessive (JLNS), and sporadic cases. Over the past decade, the genetic aspects of all three forms of LQTS have been unraveled. In 1991, Keating et al (36) identified genetic linkage to the short arm of chromosome 11 (11p15.5) in several families with Romano-Ward syndrome. Shortly thereafter, we

April 1, 2001

THE AMERICAN JOURNAL OF MEDICINE威

Volume 110 389

Molecular Biology and the Prolonged QT Syndromes/Towbin and Vatta

Figure 4. Genetics of ventricular arrhythmias. Chromosomal location and ion channel topology, as well as some of the original mutations identified in each channel, are demonstrated.

demonstrated genetic heterogeneity, and this was confirmed by several laboratories subsequently (39,40). In a collaborative effort (41), linkage was shown for several families to two new loci, the long arm of chromosome 7 (7q35–36) and the short arm of chromosome 3 (3p21). The three loci were later termed LQT1 (11p15.5), LQT2 (7q35–36), and LQT3 (3p21). A fourth locus (LQT4) on chromosome 4q (4q25–27) was later described as well (42). Most recently, two other genes, both located on chromosome 21q22 (LQT5, LQT6), were identified (Figure 4) (33,43). Penetrance in Romano-Ward syndrome is reduced, and in some families Romano-Ward syndrome appears to occur in a recessively inherited pattern (44).

GENE IDENTIFICATION IN ROMANOWARD SYNDROME KVLQT1 or KCNQ1: The LQT1 Gene The first of the genes mapped for LQTS, termed LQT1, required 5 years from the time that mapping to chromosome 11p15.5 was first reported to gene cloning. This gene, originally named KVLQT1, but more recently called KCNQ1, is a novel potassium channel gene that consists of 16 exons, spans approximately 400 kb, and is widely expressed in human tissues, including heart, inner ear, kidney, lung, placenta, and pancreas, but not in skeletal muscle, liver, or brain. In the original report, 11 different mutations (deletion and missense mutations) were identified in 16 families with LQTS, establishing KVLQT1 390

April 1, 2001

THE AMERICAN JOURNAL OF MEDICINE威 Volume 110

as LQT1. To date, more than 100 families with KVLQT1 mutations have been described. Although most of the mutations are “private” (ie, seen in only one family), there is at least one frequently mutated region (called a “hot spot”) of KVLQT1 (45). Analysis of the predicted amino acid sequence of KVLQT1 suggests that it encodes a potassium channel ␣ subunit with a conserved potassium-selective pore-signature sequence flanked by six membrane-spanning segments similar to shaker-type channels (Figure 4) (46). A putative voltage sensor is found in the fourth membranespanning domains (S4), and the selective pore loop is between the fifth and sixth membrane-spanning domains (S5, S6). Biophysical characterization of the KVLQT1 protein confirmed that KVLQT1 is a voltage-gated potassium channel protein subunit that requires coassembly with a ␤ subunit called minK to function properly (Figure 4) (47,48). Expression of either KVLQT1 or minK alone results in inefficient (or no) current development. When minK and KVLQT1 are coexpressed in either mammalian cell lines or Xenopus oocytes, however, the slowly activating potassium current (IKs) is developed in cardiac myocytes. Combination of normal and mutant KVLQT1 subunits forms abnormal IKs channels, and these mutations are believed to act through a dominant-negative mechanism (the mutant form of KVLQT1 interferes with the function of the normal wild-type form through a “poison pill”–type mechanism) or a loss-of-function mechanism (only the mutant form loses activity) (49,50,51).

Molecular Biology and the Prolonged QT Syndromes/Towbin and Vatta

Because KVLQT1 and minK form a unit, mutations in minK could also be expected to cause LQTS (47,48), and this fact was subsequently demonstrated (see “minK: The LQT5 Gene”). The vast majority of mutations in KVLQT1 and minK are heterozygous mutations in patients with Romano-Ward syndrome (52,53). KVLQT1 appears to be the most commonly mutated gene in LQTS.

HERG: The LQT2 Gene After the initial localization of LQT2 to chromosome 7q35–36 by Jiang et al (41), candidate genes (ie, genes encoding proteins that could cause repolarization abnormalities if mutated, such as ion channels, modulators of ion channels, members of the sympathetic nervous system) in this chromosomal region were analyzed. HERG (human ether-a-go-go-related gene), a cardiac potassium channel gene originally cloned from a brain cDNA library (54), and which is expressed in neural crest-derived neurons, microglia, a wide variety of tumor cell lines, and the heart (55), was found to be mutated in patients with clinical evidence of LQTS (55). Six LQTSassociated mutations were initially identified in HERG, including missense mutations, intragenic deletions, and a splicing mutation. Later, Schulze-Bahr et al (56) confirmed HERG as the LQT2 gene, identifying mutations in other families. Currently, this gene is thought to be the second most common gene mutated in LQTS (second to KVLQT1). As with KVLQT1, “private” mutations that are scattered throughout the entire gene without preferential clustering are seen (Figure 4). The HERG gene consists of 16 exons and spans 55 kb of genomic sequence (55). The predicted topology of HERG is shown in Figure 4 and is similar to KVLQT1. Unlike KVLQT1, HERG has extensive intracellular amino- and carboxyl-termini, with a region in the carboxyl-terminal domain having sequence similarity to nucleotide binding domains (NBDs). Electrophysiologic and biophysical characterization of HERG expressed in Xenopus oocytes established that HERG encodes the rapidly activating delayed-rectifier potassium current IKr (34,57) and electrophysiologic studies of LQTS-associated mutations demonstrated a loss-of-function or a dominant-negative mechanism of action (58). In addition, protein trafficking abnormalities have been shown to occur (59,60). This channel has been shown to coassemble with ␤ subunits for normal function, similar to that seen in IKs. McDonald et al initially suggested that the complexing of HERG with minK is needed to regulate the IKr potassium current (61). Bianchi et al (62) provided confirmatory evidence that minK is involved in regulation of both IKs and IKr. Most recently, Abbott et al (43) identified MiRP1 (Figure 4) as a ␤ subunit for HERG (see “MiRP1: The LQT6 Gene”).

SCN5A: The LQT3 Gene Use of the candidate gene approach established that the gene responsible for chromosome 3–linked LQTS (LQT3) (63) is the cardiac sodium channel gene SCN5A (64). SCN5A is highly expressed in human myocardium and brain, but not in skeletal muscle, liver, or uterus (64,65). It consists of 28 exons that span 80 kb and encodes a protein of 2,016 amino acids with a putative structure that consists of four homologous domains (DI to DIV), each of which contains six membrane-spanning segments (S1 to S6) similar to the structure of the potassium channel ␣ subunits (Figure. 4) (66). Mutation analysis identified three mutations, one 9 – base pair (bp) intragenic deletion (⌬K1505P1506Q1507) and two missense mutations (R1644H and N1525S) in six families with LQTS (64); when expressed in Xenopus oocytes, all mutations generated a late phase of inactivation-resistant, mexiletine- and tetrodotoxin-sensitive whole-cell current by means of different mechanisms (67,68). Two of the three mutations showed dispersed reopening after the initial transient, but the other mutation showed both dispersed reopening and long-lasting bursts. These results suggested that SCN5A mutations act through a gain-offunction mechanism (the mutant channel functions normally, but with altered properties, such as delayed inactivation) and that the mechanism of chromosome 3–linked LQTS is persistent nonactivated sodium current in the plateau phase of the action potential. An et al (69) also showed that not all mutations in SCN5A are associated with persistent current and demonstrated that SCN5A interacted with ␤ subunits. Furthermore, mutations in SCN5A have been shown to result in widely different clinical phenotypes and different responses to medications (70). Novel mutations in SCN5A were identified by our laboratory in patients with Brugada syndrome and idiopathic ventricular fibrillation (71), disorders with normal QT interval but ST segment elevation in the right precordial leads. Electrophysiologically, these mutations result in more rapid recovery from inactivation of the mutant channels or loss of function causing the Brugada syndrome–type phenotype. In addition, mutations in this gene cause conduction system disease (72) and SIDS (17).

minK: The LQT5 Gene The minK (IsK or KCNE1) gene was initially localized to chromosome 21 (21q22.1) and found to consist of three exons that span approximately 40 kb (73). It encodes a short protein consisting of 130 amino acids and has only one transmembrane-spanning segment with small extracellular and intercellular regions (Figure 4). When expressed in Xenopus oocytes, it produces potassium current that closely resembles the slowly activating delayedrectifier potassium current IKs in cardiac cells (47,48). The fact that the minK clone was expressed only in XenoApril 1, 2001

THE AMERICAN JOURNAL OF MEDICINE威

Volume 110 391

Molecular Biology and the Prolonged QT Syndromes/Towbin and Vatta

Figure 5. Cardiac action potential. Note that the time course of the cardiac action potential can be divided into five phases: upstroke of rapid depolarization (phase 0), which is mostly the result of rapid inflow of sodium (SCN5A); rapid repolarization after the peak (phase 1) is primarily the result of an outward repolarizing chloride current; the plateau (phase 2) where there is a balance of the inward currents caused by calcium and sodium and outward currents caused by chloride and potassium (IKr); rapid repolarization after the plateau (phase 3), predominantly caused by outward potassium current (IKs); and the period between the maximum negativity (maximum diastolic potential) and the upstroke of the next action potential (phase 4) caused by the balance between slow inward sodium current and outward potassium current.

pus oocytes and not in mammalian cell lines raised the question whether minK is a human channel protein. With the cloning of KVLQT1 and coexpression of KVLQT1 and minK in both mammalian cell lines and Xenopus oocytes, it became clear that KVLQT1 interacts with minK to form the cardiac slowly activating delayed-rectifier IKs current (47,48); minK alone cannot form a functional channel but induces the IKs current by interacting with endogenous KVLQT1 protein in Xenopus oocytes and mammalian cells. Bianchi et al (62) also showed that mutant minK results in abnormalities of IKs and IKr and in protein trafficking abnormalities. McDonald et al (61) showed that minK also interacts with HERG, regulating IKr. Splawski et al (74) demonstrated that minK mutations cause LQT5 by identifying mutations in two families with LQTS. In both cases, missense mutations (S74I; .D76N) were identified that reduced IKs by shifting the voltage dependence of activation and accelerating channel deactivation. This was later confirmed by others (53,75) and further supported by the fact that a mouse model with mutant minK (76) developed a phenotype (which included deafness). The functional consequences of these mutations include delayed cardiac repolarization and, hence, an increased risk of dysrhythmias (52, 53,76,77).

mapped this KCNE2 gene to chromosome 21q22.1, within 79 kb of KCNE1 (minK) and arrayed in opposite orientation (Figure 4). The open reading frames of these two genes share 34% identity, and both are contained in a single exon, suggesting that they are related through gene duplication and divergent evolution. Three missense mutations associated with dysrhythmias were identified in KCNE2 by Abbott et al (43) (Figure 4), and biophysical analysis demonstrated that these mutants form channels that open slowly and close rapidly, thus diminishing potassium currents. In one case, the missense mutation, a C-to-G transversion at nucleotide 25 that produced a glutamine (Q) to glutamic acid (E) substitution at codon 9 (Q9E) in the putative extracellular domain of MiRP1, led to the development of torsades de pointes and ventricular fibrillation after intravenous clarithromycin infusion (ie, drug induced). Therefore, like minK, this channel protein acts as a ␤ subunit but, by itself, leads to risk of ventricular arrhythmia when mutated. These similar channel proteins (ie, minK and MiRP1) suggest that there is a family of channels that regulate ion channel ␣ subunits. The specific role of this subunit and its stoichiometry remain unclear and are currently hotly debated.

MiRP1: The LQT6 Gene

Other Ion Channel Genes and Romano-Ward LQTS

The MiRP1 gene (the minK-related peptide 1 or KCNE2 gene) is a novel potassium channel gene cloned and characterized by Abbott et al (43) This small integral membrane subunit protein assembles with HERG (LQT2) to alter its function and enable full development of the IKr current. MiRP1 is a 123–amino acid channel protein with a single predicted transmembrane segment similar to that described for minK. Chromosomal localization studies 392

April 1, 2001

THE AMERICAN JOURNAL OF MEDICINE威 Volume 110

Identification of SCN5A, HERG, and KVLQT1 as LQTScausing genes has led to the identification of three critical electrical currents involved in the cardiac action potential (Figure 5) (33). The sodium current (SCN5A, INa) is responsible for generating the cardiac action potential (upstroke or phase 0) and contributes some current to the plateau phase (phase 2). The IKr potassium current en-

Molecular Biology and the Prolonged QT Syndromes/Towbin and Vatta

Figure 6. Electrocardiograms from patients with mutations in KVLQT1, HERG, and SCN5A, respectively. Note the broad T waves associated with KVLQT1 mutations, whereas the electrocardiogram from a HERG-mutated patient has low amplitude T waves. The SCN5A-associated electrocardiogram shows high amplitude and long delay in the onset of the T wave. (Reprinted with permission from reference 101.)

coded by HERG acts at the plateau phase and the rapid repolarization phase after the plateau (phase 3). The IKs potassium current encoded by KVLQT1 and minK acts at the rapid repolarization after the plateau phase (phase 3). There are still more currents for which responsible genes have not been identified, however. Likely candidates include a rapidly activating outward potassium current (ITo) and a chloride current (ICl) acting at the phase 1 repolarization after the peak; a calcium current (ICa) acting at the plateau phase; and an inward rectifier potassium current (IK1) that contributes to the maintenance of resting potential. These electrical currents are likely to be eventually identified and should contribute to the understanding of basic biology of the human heart (33).

AGE AND GENDER INFLUENCE ON QTC: RELATIONSHIP OF GENOTYPE It has been appreciated for several years that women are more susceptible to the development of torsades de pointes in the setting of QT prolongation. Compared with men, women also exhibit a longer QTc interval (6,7). It was assumed that this gender difference in the length of the QT interval and the propensity to torsades de pointes was the result of a QT-prolonging influence of female hormones or other modifying agent. However, using genotyped families, Lehmann et al (78) showed that KVLQT1 and HERG gene carrier males have a shorter QTc than women; in SCN5A-carriers, however, men had longer QTc intervals than women carrying this gene. On the other hand, non– gene carrier blood relatives in KVLQT1- or HERG-linked families demonstrated that men had shorter QTc intervals compared with either

women or children. These findings led Lehmann et al (78) to speculate that factors in men (such as androgens) act to shorten the QTc and exert a protective effect against the occurrence of torsades de pointes, the opposite of the view previously entertained.

MOLECULAR BASIS OF THE CARDIAC ACTION POTENTIAL AND THE EFFECT ON THE ELECTROCARDIOGRAM The cardiac action potential is mediated by a delicate balance between inward (ie, INa, ICa) and outward (IKr, IKs) currents described above (33). Persistent noninactivated inward sodium current in the plateau phase of the action potential or reduced IKr and IKs outward currents could prolong cardiac repolarization and the cardiac action potential, leading to prolongation of the QTc on the electrocardiogram. Excessive prolongation of the cardiac action potential could result in re-activation of L-type calcium or sodium channels, leading to early afterdepolarization (EAD), a possible in vivo mechanism underlying torsades de pointes (33). Using the information gained from identification of genes responsible for LQTS (LQT1, LQT2, LQT3), Moss et al identified differences in the electrocardiographic patterns, particularly in the T wave, based on the defective gene (79). The authors found that the electrocardiograms obtained from patients with KVLQT1 mutations (LQT1) had broad T waves, those from patients with HERG defects (LQT2) had low amplitude T waves, and SCN5A mutations (LQT3) resulted in electrocardiograms with high amplitude and long delays in the T-wave onset (Figure 6) (79). April 1, 2001

THE AMERICAN JOURNAL OF MEDICINE威

Volume 110 393

Molecular Biology and the Prolonged QT Syndromes/Towbin and Vatta

GENETICS OF JERVELL AND LANGENIELSEN SYNDROME Neyroud et al (80) reported the first molecular abnormality in patients with JLNS when they reported on two families in which three children were affected by JLNS, finding a novel homozygous deletion-insertion mutation of KVLQT. A deletion of 7 bp and an insertion of 8 bp at the same location led to premature termination at the C-terminal end of the KVLQT1 channel. At the same time, Splawski et al (81) identified a homozygous insertion of a single nucleotide that caused a frameshift in the coding sequence after the second putative transmembrane domain (S2) of KVLQT1. Together, these data strongly suggested that at least one form of JLNS is caused by homozygous mutations in KVLQT1. This has been confirmed by others (53,75,82– 84). As a general rule, heterozygous mutations in KVLQT1 cause Romano-Ward syndrome (LQTS only), whereas homozygous (or compound heterozygous) mutations in KVLQT1 cause JLNS (LQTS and deafness). The hypothetical explanation suggests that although heterozygous KVLQT1 mutations act by a dominant-negative mechanism (49,50,85), some functional KVLQT1 potassium channels still exist in the stria vascularis of the inner ear. Therefore, congenital deafness is averted in patients with heterozygous KVLQT1 mutations. For patients with homozygous KVLQT1 mutations, no functional KVLQT1 potassium channels can be formed. It has been shown by in situ hybridization that KVLQT1 is expressed in the inner ear (80), suggesting that homozygous KVLQT1 mutations can cause the dysfunction of potassium secretion in the inner ear and lead to deafness (76). However, it should be noted that incomplete penetrance exists and not all heterozygous or homozygous mutations follow this rule (86). As with Romano-Ward syndrome, if KVLQT1 mutations can cause the phenotype, it could be expected that minK mutations could also be causative of the phenotype (JLNS). Schulze-Bahr et al (83), in fact, showed that mutations in minK result in JLNS syndrome as well, and this was confirmed subsequently (53,84). Hence, abnormal IKs current, whether it occurs as a result of homozygous or compound heterozygous mutations in KVLQT1 or minK, results in LQTS and deafness.

GENOTYPE-PHENOTYPE CORRELATIONS Moss et al (79) showed that the ECG manifestations of LQTS were in great part determined by the channel mutated. Different T-wave patterns were clearly evident when comparing tracings from patients with mutations in LQT1, LQT2, and LQT3. More recently, Zareba et al 394

April 1, 2001

THE AMERICAN JOURNAL OF MEDICINE威 Volume 110

(87) showed that the mutated gene may result in a specific clinical phenotype with different triggers and may predict outcome. For instance, these authors suggested that mutations in LQT1 and LQT2 result in early symptoms (ie, syncope) but the risk of sudden death was relatively low for any event. In contrast, mutations in LQT3 resulted in a paucity of symptoms, but when symptoms occurred, they were associated with a high likelihood of sudden death. Ackerman et al (88), Moss et al (89), and Hajj et al (90) showed that swimming is a common trigger for symptoms in patients with LQT1, whereas Wilde et al (91) have found auditory triggers to be common in LQT2. LQT3, on the other hand, appears to be associated with sleep-associated symptoms. Coupled with the findings by Moss et al, it could be suggested that understanding the underlying cause of LQTS in any individual could be used to improve survival by prevention and gene-specific therapy.

GENETIC TESTING Currently, five LQTS-causing genes have been identified with more than 50 mutations described to date. This genetic heterogeneity makes genetic testing more difficult than if a single gene defect were responsible for the disease. However, under certain conditions genetic testing can be performed. In large families in which linkage analysis may be performed, identification of the gene of interest (if the linkage is to one of the known genes) can be discerned rapidly, and mutations can be screened. Once a mutation is identified in one affected family member (usually the proband), the remaining family members can be screened for this mutation quickly. In small families or sporadic cases, mutation screening for all known genes must be initiated. Usually, KVLQT1 (LQT1) mutations are screened initially, because this appears to be the most common disease-causing gene. If no mutation is uncovered in KVLQT1, HERG and SCN5A are then analyzed followed by minK and MiRP1. If no mutation is found in any of these three genes, one cannot conclude that the subject does not have LQTS, because other disease-causing genes remain to be discovered.

MANAGEMENT OF LQTS At present, there are three classical modalities for treatment of LQTS that have withstood the test of time: ␤ blockers (22,92), pacemakers (93–95), and left cervicothoracic sympathetic ganglionectomy (96). The mortality of untreated symptomatic patients with LQTS exceeds 20% in the year after their first syncopal episode and approaches 50% within 10 years of initial presentation (10). With institution of the classical therapy, this can be reduced to 3% to 4% in 5 years after initial presentation

Molecular Biology and the Prolonged QT Syndromes/Towbin and Vatta

(10). Despite the lack of a placebo-controlled, randomized clinical trial, strong evidence supports the use of antiadrenergic interventions as the mainstay of therapy for most patients. The trigger for many life-threatening events appears to involve sudden increases in sympathetic activity (ie, emotional or physical stress), and therefore, antiadrenergic therapy makes physiological sense. The ␤-adrenergic blocking agents prevent new syncopal episodes in approximately 75% of patients. Suppression of complex ventricular arrhythmias, that is, couplets and ventricular tachycardia, seems desirable. The addition of a class IB agent (mexiletine) to ␤-blocker therapy may be helpful. High-risk patients with drugresistant, symptomatic ventricular tachycardia are referred for left cardiac sympathetic denervation, which apparently provides additional protection (96). In addition, the use of cardiac pacing as an adjunct to ␤ blockers appears to be most rational in patients with evidence of pause-dependent or bradycardia-dependent arrhythmias (93–95). Symptomatic bradycardia, induced by ␤-blocker therapy, should also be considered an indication for elective pacing. Beta-blocker therapy is monitored with treadmill exercise testing with the desired result a blunting of the heart rate response to exercise. Unfortunately, in some patients, this comes at the expense of excessive sinus bradycardia at rest and with minimal levels of exertion. Excessive fatigue, inattentiveness, and irritability may result in discontinuance of therapy by the patient. Compliance, especially in the adolescent population, may be enhanced by returning the patient to a relatively normal lifestyle by the elimination of chronotropic incompetence with a pacemaker. Other less time-tested therapies are also available. In some rare cases, torsades de pointes persists despite therapy with the classical modalities. The implantable cardioverter-defibrillator has been used successfully in this setting (97). It had not been considered to be first-line therapy, because shocks from the device can precipitate further emotional stress and set off a circuitous response of persistent malignant arrhythmias. However, the Multicenter Automatic Defibrillator Implantation Trial (MADIT) (98), which demonstrated dramatic superiority of therapy with automatic implantable defibrillators over “best conventional therapy” in patients with coronary disease at high risk for ventricular arrhythmias, has made this therapeutic approach somewhat appealing. Since this report, automatic implantable defibrillators have been used more commonly in patients with LQTS. Another new approach to treating patients with LQTS is the so-called “gene- specific” approach. With the identification of the precise molecular defect in some patients with LQTS, specific mechanism-based therapies have been devised and small therapeutic trials performed. Schwartz et al (99) were the first to use this approach when they used the sodium channel blocker mexiletine in

Table 3. Drugs Causing Torsade de Pointes Antiarrhythmic agents Quinidine Disopyramide Procainamide (N-Acetyl-procainamide) Sotalol Ibutilide Amiodarone Calcium-channel blocking agents Bepridil Lidoflazine Central nervous system–active agents Thioridazine Tricyclic antidepressants Pimozide Antibiotics Macrolides (eg, Erythromycin) Pentamidine Trimethoprim-sulfa Antihistamines Terfenadine Astemizole Miscellaneous Terodiline Liquid protein diets Organophosphorous insecticide poisoning Ketanserin Cisapride Probucol

patients with mutations in the sodium channel gene SCN5A (LQT3). In these patients the QTc was dramatically shortened in a statistically significant manner. Patients with potassium channel mutations (HERG, LQT2) treated with mexiletine had no change in the QTc. However, no data currently exist that demonstrate clinical efficacy of this approach in either decreasing the number of syncopal events or improving survival. Although the data are intriguing, use of sodium channel blockers alone for patients with SCN5A mutations should still be considered experimental. Other gene-specific trials have also been performed with similar results. Compton et al (100) used intravenous potassium to elevate the serum potassium to greater than 4.8 in patients with LQT2 and found significant shortening of the QTc. Again, no data on survival or symptom improvement exist for this therapy. Other gene-specific therapeutic approaches are currently being developed.

ACUTE MANAGEMENT OF TORSADES DE POINTES Once torsades de pointes is recognized, all QT-prolonging drugs should be withdrawn (if acquired LQTS) and drugs potentially causative of torsades de pointes (Table April 1, 2001

THE AMERICAN JOURNAL OF MEDICINE威

Volume 110 395

Molecular Biology and the Prolonged QT Syndromes/Towbin and Vatta

3) should be discontinued. In addition, even modest hypokalemia should be corrected and the serum level of potassium kept within the high normal range. Potassium not only shortens the QT interval but also can decrease the potency of QT-prolonging drugs. If the episode of torsades de pointes persists for a long period, cardioversion therapy should be used.

SUMMARY During the past decade, breakthroughs in the clinical and molecular genetic understanding of the long QT syndromes have occurred. However, much remains to be learned. Collaborative interactions between clinicians and basic scientists have enabled many of the new findings to be discovered, and continued close working relationships should provide the incentive necessary to continue this growth in knowledge. It is hoped that the dawn of the twenty-first century will bring with it better diagnostic and therapeutic capabilities for this potentially devastating disease.

REFERENCES 1. Jervell A, Lange-Nielsen F. Congenital deaf-mutism, function heart disease with prolongation of the Q-T interval and sudden death. Am Heart J. 1957;54:59 – 68. 2. Meissner FL. Taubstummheit und Taubstummenbildung. Leipzig and Heidelberg. Winter 1856:119 –120. 3. Romano C, Gemme G, Pongiglione R. Antmie cardiache rare in eta pediatrica. Clin Pediatr. 1963;45:656 – 683. 4. Ward OC. A new familial cardiac syndrome in children. J Ir Med Assoc. 1964;54:103–106. 5. Moss AJ, Schwartz PJ, Crampton R, et al. The long QT syndrome: prospective longitudinal study of 328 families. Circulation. 1991; 84:1136 –1144. 6. Hashiba K. Sex differences in phenotypic manifestation and gene transmission in the Romano-Ward syndrome. Ann NY Acad Sci. 1992;644:142–156. 7. Rautaharju PM, Zhou SH, Wong S, et al. Differences in the evolution of the electrocardiographic QT interval with age. Can J Cardiol. 1992;8:690 – 695. 8. Schwartz PJ. Idiopathic long syndrome. progress and questions. Am Heart J. 1985;109:399 – 411. 9. Schwartz PJ, Zaza A, Locati E, Moss AJ. Stress and sudden death. The case of the long QT syndrome. Circulation. 1991;83:II71–II80. 10. Schwartz PJ, Locati EH, Napolitano C, Priori SG. The long QT syndrome. In: Zipes DP, Jalife J, eds. Cardiac Electrophysiology: From Cell to Bedside. Philadelphia: WB Saunders Company, 1996: 788 – 811. 11. Horn CA, Beckman RH, Dick M II, Lacina SJ. The congenital long QT syndrome: an unusual cause of childhood seizures. Am J Dis Child. 1986;140:659 – 661. 12. Ratshin RA, Hunt D, Russell RO Jr, Rackley CE. QT-interval prolongation, paroxysmal ventricular arrhythmias, and convulsive syncope. Ann Intern Med. 1971;75:19 –24. 13. Bazett HC. An analysis of the time relations of electrocardiogram. Heart. 1918;7:353–370. 14. Ewing DJ, Boland O, Neilson JMM, Cho CG, Clarke BF. Autonomic neuropathy, QT interval lengthening, and unexpected deaths in male diabetic patients. Diabetologia. 1991;34:182–185. 396

April 1, 2001

THE AMERICAN JOURNAL OF MEDICINE威 Volume 110

15. Weintraub RG, Gow RM, Wilkinson JL. The congenital long QT syndromes in children. J Am Coll Cardiol. 1990;16:674 – 680. 16. Marks ML, Trippel DL, Keating MT. Long QT syndrome associated with syndactyly identified in females. Am J Cardiol. 76;10: 744 –745. 17. Schwartz PJ, Priori SG, Dumaine R, et al. A molecular link between the sudden infant death syndrome and the long-QT syndrome. N Engl J Med. 2000;343:262–267. 18. Jervell A. Surdocardiac and related syndromes in children. Adv Intern Med. 1971;17:425– 438. 19. Fraser GR, Froggatt P, Murphy T. Genetical aspects of the cardioauditory syndrome of Jervell and Lange-Nielsen (congenital deafness and electrocardiographic abnormalities). Am Hum Genet. 1964;28:133–156. 20. Fraser GR, Froggatt P, James TN. Congenital deafness associated with electrocardiographic abnormalities, fainting attacks and sudden death. A recessive syndrome. QJ Med. 1964;33:361–385. 21. Schwartz PJ, Periti M, Malliani A. The long Q-T syndrome. Am Heart J. 1975;89:378 –390. 23. Fermini B, Jurkiewicz NK, Jow B, et al. Use-dependent effects of the class III antiarrhythmic agent NE-10064 (Azimilide) on cardiac repolarization: block of delayed rectifier potassium and Ltype calcium currents. J Cardiovasc Pharmacol. 1995;26:259 –271. 24. Lipka LJ, Dizon JM, Reiffel JA. Desired mechanisms of drugs for ventricular arrhythmia; class III antiarrhythmic agents. Am Heart J. 1995;130:632– 640. 25. Sedgwick ML, Lip G, Rae AP, Cobbe SM. Chem cardioversion of atrial fibrillation with intravenous dofetilide. Int J Cardiol. 1995; 49:159 –166. 26. Sedgwick ML, Rasmussen HS, Cobbe SM. Effects of the class III antiarrhythmic drug dofetilide on ventricular monophasic action potential duration and QT interval dispersion in stable angina pectoris. Am J Cardiol. 1992;70:1432–1437. 27. Vaughan Williams EM. A classification of antiarrhythmic action reassessed after a decade of new drugs. J Clin Pharmacol. 1984;24: 129 –147. 28. Roden DM, Bennett PB, Snyders DJ, Balser JR, Hondeghem LM. Quinidine delays IK activation in guinea pig ventricular myocytes. Circ Res. 1988;62:1055–1058. 29. Carlsson L, Abrahamsson C, Andersson B, Duker G, Schiller-Linhardt G. Proarrhythmic effects of the class III agent almokalant: importance of infusion rate, QT dispersion, and early afterdepolarizations. Cardiovasc Res. 1993;27:2186 –2193. 30. Woosley RL, Chen Y, Freiman JP, Gillis RA. Mechanism of the cardiotoxic actions of terfenadine. J Am Med Assoc. 1993;269: 1532–1536. 31. Echizen H, Kawasaki H, Chiba K, Tani M, Ishizaki T. A potent inhibitory effect of erythromycin and other macrolide antibiotics on the mono-N-dealkylation metabolism on disopyramide with human liver microsomes. J Pharmacol Exp Ther. 1993;264:1425– 1431. 32. Lewin MB, Bryant RM, Fenrick AL, Grifka RG. Cisapride-induced long QT interval. J Pediatr. 1996;128:279 –281. 33. Roden DM, Spooner PM, and the NIH Working Group. Inherited long QT syndromes: a paradigm for understanding arrhythmogenesis. J Cardiovasc Electrophysiol. 2000;10:1664 –1683. 34. Sanguinetti MC, Jiang C, Curran ME, Keating MT. A mechanistic link between an inherited and an acquired cardiac arrhythmia: HERG encodes the IKr potassium channel. Cell. 1995;81:299 –307. 35. Schwartz PJ, Moss AJ, Vincent GM, Crampton RS. Diagnostic criteria for the long QT syndrome: an update. Circulation. 1993; 88:782–784. 36. Keating MT, Atkinson D, Dunn C, Timothy K, Vincent GM, Leppert M. Linkage of a cardiac arrhythmia, the long QT syndrome, and the Harvey ras-1 gene. Science. 1991;252:704 –706.

Molecular Biology and the Prolonged QT Syndromes/Towbin and Vatta 37. Jackman WM, Szabo B, Friday DJ, et al. Ventricular tachyarrhythmias related to early afterdepolarizations and triggered firing: relationship to QT interval prolongation and potential therapeutic role for calcium channel blocking agents. J Cardiovasc Electrophysiol. 1990;1:170 –195. 38. Shimizu W, Ohe T, Kurita T, Tokuda T, Shimomura K. Epinephrine-induced ventricular premature complexes due to early afterdepolarizations and effects of verapamil and propranolol in a patient with congenital long QT syndrome. J Cardiovasc Electrophysiol. 1994;5:438 – 444. 39. Towbin JA, Li H, Taggart T, et al. Evidence of genetic heterogeneity in Romano-Ward long QT syndrome: analysis of 23 families. Circulation. 1994;90:2635–2644. 40. Benhorin J, Kalman YM, Madina A, et al. Evidence of genetic heterogeneity in the long QT syndrome. Science. 1993;260:1960 – 1962. 41. Jiang C, Atkinson D, Towbin JA, et al. Two long QT syndrome loci map to chromosome 3 and 7 with evidence for further heterogeneity. Nature Genet. 1994;8:141–147. 42. Schott J, Charpentier F, Peltier S, et al. Mapping of a gene for long QT syndrome to chromosome 4q25–27. Am J Hum Genet. 1995; 57:1114 –1122. 43. Abbott GW, Sesti F, Splawski I, et al. MiRP1 forms IKr potassium channels with HERG, and is associated with cardiac arrhythmia. Cell. 1999;97:175–187. 44. Priori SG, Schwartz PJ, Napolitano C, et al. A recessive variant of the Romano-Ward long-QT syndrome. Circulation. 1998;97: 2420 –2425. 45. Li H, Chen Q, Moss AJ, et al. New mutations in the KVLQT1 potassium channel that cause long QT syndrome. Circulation. 1998;97:1264 –1269. 46. Wang Q, Curran ME, Splawski I, et al. Positional cloning of a novel potassium channel gene: KVLQT1 mutations cause cardiac arrhythmias. Nature Genet. 1996;12:17–23. 47. Barhanin J, Lesage F, Guillemare E, Finc M, Lazdunski M, Romey G. KVLQT1 and IsK (minK) proteins associate to form the Iks cardiac potassium current. Nature. 1996;384:78 – 80. 48. Sanguinetti MC, Curran ME, Zou A, et al. Coassembly of KvLQT1 and minK (IsK) proteins to form cardiac Iks potassium channel. Nature. 1996;384:80 – 83. 49. Demolombe S, Baro I, Pereon Y, et al. A dominant negative isoform of the long QT syndrome 1 gene product. J Biol Chem. 1998; 273:6837– 6843. 50. Shalaby FY, Levesque PC, Yang WP, et al. Dominant-negative KvLQT1 mutations underlie the LQT1 form of long QT syndrome. Circulation. 1997;96:1733–1736. 51. Wollnick B, Schreeder BC, Kubish C, Esperer HD, Wieacker P, Jensch TJ. Pathophysiological mechanisms of dominant and recessive KVLQTI K⫹ channel mutations found in inherited cardiac arrhythmias. Human Molec Genet. 1997;6:1943–1949. 52. Chouabe C, Neyroud N, Richard PK, et al. 2000 Novel mutations in KVLQT1 that affect IKs activation through interactions with IsK. Cardiovasc Res. 45:971–980. 53. Duggal P, Vesely MR, Wattanasirichaigoon D, Villafane J, Kaushik V, Beggs AH. Mutation of the gene for IsK associated with both Jervell and Lange-Nielsen and Romano-Ward forms of long-QT syndrome. Circulation. 1998;97:142–146. 54. Warmke JE, Ganetzky B. A family of potassium channel genes related to Eag in Drosophila and mammals. Proc Natl Acad Sci USA. 1994;91:3438 –3442. 55. Curran ME, Splawski I, Timothy KW, Vincent GM, Green ED, Keating MT. A molecular basis for cardiac arrhythmia: HERG mutations cause long QT syndrome. Cell. 1995;80:795– 803. 56. Schulze-Bahr E, Haverkamp W, Funke H. The long-QT syndrome. N Engl J Med. 1995;333:1783–1784.

57. Trudeau MC, Warmke J, Ganetzky B, Robertson G. HERG, a human inward rectifier in the voltage-gated potassium channel family. Science. 1995;269:92–95. 58. Sanguinetti MC, Curran ME, Spector PS, Keating MT. Spectrum of HERG K⫹-channel dysfunction in an inherited cardiac arrhythmia. Proc Natl Acad Sci USA. 1996;93:2208 –2212. 59. Furutani M, Trudeau MC, Hagiwara N, et al. Novel mechanism associated with an inherited cardiac arrhythmia. Defective protein trafficking by the mutant HERG (G601S) potassium channel. Circulation. 1999;99:2290 –2294. 60. Zhou Z, Gong Q, Miles L, et al. HERG channel dysfunction in human long QT syndrome. J Biol Chem. 1998;263:21061–21066. 61. McDonald TV, Yu Z, Ming Z, et al. A MinK-HERG complex regulates the cardiac potassium current IKr. Nature. 1997;388:289 – 292. 62. Bianchi L, Shen Z, Dennis AT, et al. Cellular dysfunction of LQT5minK mutants: abnormalities of IKs, IKr and trafficking in long QT syndrome. Hum Molec Genet. 1999;8:1499 –1507. 63. George AL, Varkony TA, Drakin HA, et al. Assignment of the human heart tetrodotoxin-resistant voltage-gated Na channel ␣-subunit gene (SCN5A) to band 3p21. Cytogenet Cell Genet. 1995;68:67–70. 64. Wang Q, Shen J, Splawski I, et al. SCN5A mutations associated with an inherited cardiac arrhythmia, long QT syndrome. Cell. 1995;80:805– 811. 65. Hartmann HA, Colom LV, Sutherland ML, Noebels JL. Selective localization of cardiac SCN5A sodium channels in limbic regions of rat brain. Nature Neurosci. 1999;2:593–595. 66. Gellens M, George AL, Chen L, Chahine M, Horn R, Barchi R, Kallen R. Primary structure and functional expression of the human cardiac tetrodotoxin-insensitive voltage-dependent sodium channel. Proc Natl Acad Sci USA. 1992;89:554 –558. 67. Bennett PB, Yazawa K, Makita N, George AL Jr. Molecular mechanism for an inherited cardiac arrhythmia. Nature. 1995;376:683– 685. 68. Dumaine R, Wang Q, Keating MT, et al. Multiple mechanisms of sodium channel-linked long QT syndrome. Circ Res. 1996;78: 916 –924. 69. An RH, Wang XL, Kerem B, et al. Novel LQT-3 mutation affects Na⫹ channel activity through interactions between alpha and beta 1 subunits. Circ Res. 1998;83:141–146. 70. Benhorin J, Taub R, Goldmit M, et al. Effects of flecainide in patients with new SCN5A mutation. Mutation-specific therapy for long-QT syndrome? Circulation. 2000;101:1698 –1706. 71. Chen Q, Kirsch GE, Zhang D, et al. Genetic basis and molecular mechanism for idiopathic ventricular fibrillation. Nature. 1998; 392:293–296. 72. Schott J-J, Alshinawi C, Kyndt F, et al. Cardiac conduction defects associated with mutations in SCN5A. Nature Genet. 1999;23:20 – 21. 73. Honore E, Attali B, Heurteaux C, et al. Cloning, expression, pharmacology and regulation of a delayed rectifier K⫹ channel in mouse heart. EMBO J. 1991;10:2805–2811. 74. Splawski I, Tristani-Firouzi M, Lehmann MH, Sanguinetti MC, Keating MT. Mutations in the hminK gene cause long QT syndrome and suppress IKs function. Nature Genet. 1997;17:338 –340. 75. Chouabe C, Neyroud N, Guichency P, Lazdunski M, Romey G, Barhanin J. Properties of KVLQT1 K⫹ channel mutations in Romano-Ward and Jervell and Lange-Nielsen inherited cardiac arrhythmias. EMBO J. 1997;16:5472–5479. 76. Vetter DE, Mann JR, Wangemann P, et al. Inner ear defects induced by null mutation of the IsK gene. Neuron. 1996;17:1251– 1264. 77. Franqueza L, Lin M, Splawski I, Keating MT, Sanguinetti MC. Long QT syndrome-associated mutations in the S4-S5 linker of April 1, 2001

THE AMERICAN JOURNAL OF MEDICINE威

Volume 110 397

Molecular Biology and the Prolonged QT Syndromes/Towbin and Vatta

78.

79.

80.

81.

82.

83.

84.

85.

86.

87.

88.

89.

398

KVLQT1 potassium channels modify gating, and interaction with minK subunits. J Biol Chem. 1999;274:21063–21070. Lehmann MH, Timothy KW, Frankovich D, et al. Age-gender influence on the rate-corrected QT interval and the QT-heart rate relation in families with genotypically characterized long QT syndrome. J Am Coll Cardiol. 1997;29:93–99. Moss AJ, Zareba W, Benhorin J, et al. Electrocardiographic Twave patterns in genetically distinct forms of the hereditary long-QT syndrome. Circulation. 1995;92:2929 –2934. Neyroud N, Tesson F, Denjoy I, et al. A novel mutation on the potassium channel gene KVLQT1 causes the Jervell and LangeNielsen cardioauditory syndrome. Nature Genet. 1997;15:186 – 189. Splawski I, Timothy KW, Vincent GM, Atkinson DL, Keating MT. Brief report: molecular basis of the long-QT syndrome associated with deafness. N Engl J Med. 1997;336:1562–1567. Chen Q., Zhang D, Gingell RL, et al. Homozygous deletion in KVLQTI associated with Jervell and Lange-Nielsen syndrome. Circulation. 1999;99:1344 –1347. Schulze-Bahr E, Wang Q, Wedekind H, et al. KCNE1 mutations cause Jervell, and Lange-Nielsen syndrome. Nature Genet. 1997; 17:267–268. Tyson J, Transbjaerg L, Bellman S, et al. IsK and KVLQT1. Mutation in either of the two subunits of the slow component or the delayed rectifier potassium channel can cause Jervell and Lange-Nielsen syndrome. Human Molec Genet. 1997;6:2179 – 2185. Mohammad-Panah R, Demolombe S, Neyroud N, et al. Mutations in a dominant-negative isoform correlate with phenotype in inherited cardiac arrhythmias. Am J Human Genet. 1999;64:1015– 1023. Priori SG, Napolitano C, Schwartz PJ. Low penetrance in the long-QT syndrome. Clinical impact. Circulation. 1999;99:529 – 533. Zareba W, Moss AJ, Schwartz PJ, et al. Influence of the genotype on the clinical course of the long-QT syndrome. N Engl J Med. 1998;339:960 –965. Ackerman MJ, Tester DJ, Porter CJ, Edwards WD. Molecular diagnosis of the inherited long-QT syndrome in a woman who died after near-drowning. N Engl J Med. 1999;341:1121–1115. Moss AJ, Robinson JL, Gessman L, et al. Comparison of clinical and genetic variables of cardiac events associated with loud noise versus swimming among subjects with the long QT syndrome. Am J Cardiol. 1999;84:876 – 879.

April 1, 2001

THE AMERICAN JOURNAL OF MEDICINE威 Volume 110

90. Hajj Ali RH, Zareba W, Moss AJ, et al. Clinical and genetic variables associated with acute arousal and nonarousal-related cardiac events among subjects with long QT syndrome. Am J Cardiol. 2000;85:457– 461. 91. Wilde AA, Jongblood RJ, Doevendans PA, et al. Auditory stimuli as a trigger for arrhythmic events differentiate HERG-related (LQTS2) patients from KVLQT1-related patients (LQTS1). J Am Coll Cardiol. 1999;33:327–332. 92. Moss AJ, Zareba W, Hall WJ, et al. Effectiveness and limitations of ␤-blocker therapy the congenital long-QT syndrome. Circulation. 2000;101:616 – 623. 93. Eldar M, Griffin JC, Abbott JA, et al. Permanent cardiac pacing in patients with the long QT syndrome. J Am Coll Cardiol. 1987;10: 600 – 607. 94. Moss AJ, Liu JE, Gottlieb S, Locati E, Schwartz PJ, Robinson JL. Efficacy of permanent pacing in the management of high risk patients with long QT syndrome. Circulation. 1991;84:1524 –1529. 95. Viskin S. Cardiac pacing in the long QT syndrome: review of available data and practical recommendations. J Cardiovasc Electrophysiol. 2000;11:593– 600. 96. Schwartz PJ, Locati E, Moss AJ, Crampton RS, Trazzi R, Ruberti U. Left cardiac sympathetic denervation in the therapy of the congenital long QT syndrome: a worldwide report. Circulation. 1991;84: 503–511. 97. Platia EV, Griffith LSC, Watkins L, Mirowski M, Mower MM, Reid PR. Management of the prolonged QT syndrome and recurrent ventricular fibrillation with an implantable cardioverter-defibrillator. Clin Cardiol. 1985;8:490 – 493. 98. Moss AJ, Hall WJ, Cannom DS, et al, for the MADIT Investigators. Improved survival with an implanted defibrillator in patients with coronary disease at high risk of ventricular arrhythmias. N Engl J Med. 1996;335:1933–1940. 99. Schwartz PJ, Priori SG, Locati EH, et al. Long QT syndrome patients with mutations of SCN5A, and HERG have differential responses to Na⫹ channel blockade, and to increase in heart rate. Circulation. 1995;92:3381–3386. 100. Compton SJ, Lux RL, Ramsey MR, et al. Genetically defined therapy of inherited long-QT syndrome. Circulation. 1996;94:1018 – 1022. 101. Roden DM, Lazzara R, Rosen M, et al. Multiple mechanisms in the long-QT syndrome. Current knowledge, gaps, and future directions. Circulation. 1996;94:1996 –2012.