Clinical Perspective. Molecular genetics of inherited long QT syndromes. The long QT syndrome. Introduction. Clinical aspects/diagnostic criteria

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1 European Heart Journal (1998) 19, Article No. hj Clinical Perspective Molecular genetics of inherited long QT syndromes Introduction Unravelling the molecular cause of QT prolongation is one of the most exciting contributions of molecular biology to clinical cardiology in the last decade. Not only have mutations in four different genes been described, at least two additional chromosomal loci have been identified harbouring genes with an as yet unknown DNA sequence and protein function. Simultaneously cellular electrophysiology has provided the tools to study the effect of single base mutations on protein function. The development of molecular genetics in concert with molecular electrophysiology has provided substantial proof of the causative relationship among DNA mutation, amino acid change and changes in ion channel characteristics. The physiological in vivo experiments in transgenic or gene targeted animals, showing prolonged repolarization and potential arrhythmias, will complete the evidence in the near future. The long QT syndrome, defined as the association of prolonged QT intervals and life threatening polymorphic ventricular tachycardias (torsade de pointes), can be inherited or acquired. Not all family members who inherit a defective gene will develop symptoms. Mutation carriers only develop symptoms in specific circumstances. Whether these circumstances are related to sympathetic stimulation or genetic factors is the subject of ongoing research. Identification of the mutation and recognition of the changed ion channel function can have therapeutic consequences. Pre-symptomatic diagnosis of individuals at risk is generally poor and medical treatment may increase the risk of arrhythmias. Therefore identification of individuals at risk, and identification of the gene defect involved, is very important. In the development of cardiac arrhythmias both genetic and acquired factors are involved, with genetic factors accounting for half or more of the arrhythmias. In this review we will discuss the major group of inherited arrhythmias: the long QT syndromes. Revision submitted 8 December 1997, and accepted 20 December Correspondence: Dr J. L. M. C. Geelen, Division of Genetics, University Maastricht, Joseph Bechlaan 113, 6229 GR Maastricht, The Netherlands X/98/ $18.00/0 The long QT syndrome Clinical aspects/diagnostic criteria The long QT syndrome can be a genetic disorder primarily characterized by a prolonged QT interval and life-threatening tachyarrhythmias, particularly in association with emotional or physical stress. In order to diagnose the long QT syndrome in cases of borderline QT prolongation and/or absence of symptoms a diagnostic set of criteria was proposed by Schwartz in 1985 [1]. Major factors are QT prolongation (QTc>440 ms), stress-induced syncope and family members with the long QT syndrome. Minor factors are congenital deafness, episodes of T-wave alternans, low heart rate (in children) and abnormal ventricular repolarization. To account for differences between sexes in QT values the criteria were refined in 1993 [2]. In these criteria, relative points are assigned to different ECG, clinical, and familial findings, where QT prolongation, torsade de pointes, and syncopes are the most important. Genetic aspects Familial transmission of prolongation of the QT interval has been observed in approximately 50% of patients [3,4]. The most important genetic entities are the Romano Ward syndrome (autosomal dominant, a prevalence of approximately 1: : and a penetrance of 0 9) [5] and the Jervell and Lange-Nielsen syndrome (autosomal recessive; very rare) [6 8]. Both syndromes show similar repolarization defects. In the Romano Ward syndrome the symptoms are limited to cardiac problems, while in the Jervell and Lange-Nielsen syndrome the cardiac symptoms accompany sensorineural hearing loss. The age of onset of cardiac problems in the Jervell and Lange Nielsen syndrome varies, but is usually at a young age. Cases with syncopal attacks occurring between the ages of 2 and 4 months and cases of sudden death before the age of 10 years have been described [9]. Three of the four affected siblings described by Jervell and Lange-Nielsen (1957) died before the age of 10 years [8] The European Society of Cardiology

2 1428 Clinical Perspective Genetic heterogeneity in the Romano Ward syndrome Linkage analysis is used to identify chromosomal regions involved in genetic diseases. By linkage analysis using polymorphic markers, Keating et al. [3] identified the first gene involved in the long QT syndrome in a single large Utah family on chromosome 11p15.5 at or near the H-Ras-1 locus (the LQT1 locus). This observation was confirmed by analysing six additional families [4]. Evidence for genetic heterogeneity was found by Curran et al. [10] and Dean et al. [11], who found no linkage with chromosome 11p15.5 in a total of four families. Studies, using similar sets of markers demonstrated a clear lack of linkage to the LQT1 locus in 8/23 families and 4/13 families, respectively [12,13]. In the past few years, the extent of heterogeneity has been further clarified. Three additional long QT syndrome loci LQT2, LQT3 and LQT4 have been mapped to chromosome 7q35 36, chromosome 3p21 24 and chromosome 4q25 27, respectively. In the study by Jiang et al. [14] nine families showed linkage to chromosome 7q35 36, three families to chromosome 3p21 24 and seven to chromosome 11p15.5. Subsequent studies by Schott et al. [15] identified the LQT4 locus on chromosome 4 in a single large family. Recently a fifth long QT locus was identified on chromosome 21 [16]. The major long QT loci are LQT1 and LQT2, with an approximately equal representation [17]. Additional genes for the Romano Ward long QT syndrome must exist, since a number of families have been identified in which no linkage was established with either the LQT1, LQT2, LQT3, LQT4 or LQT5 locus [14,18]. Identification of long QT genes To identify genes responsible for the long QT syndrome two approaches can be used: the candidate gene approach and the positional cloning approach. In the candidate gene approach, a gene is considered a candidate, if dysfunction of the protein encoded by a specific gene could theoretically result in the disease. The syndrome is associated with a prolonged QT interval, a sign of abnormal cardiac repolarization. Therefore, genes coding for or regulating ionchannels are important candidate genes, provided, they are localized in the correct chromosomal region. In the positional cloning approach the chromosomal region of interest is narrowed down as far as possible by linkage analysis, followed by an analysis of coding sequences in that area. After identifying a gene of interest as the potential gene responsible for the disorder, mutation analysis is required for final proof. Mutations should be found only in affected individuals and not in the healthy population. To date, the genes linked to the LQT1, LQT2, LQT3 and LQT5 loci have been identified (discussed below). LQT1 By linkage analysis, it was shown that the LQT1 gene was located on chromosome 11 near the H-ras-1 locus [3,4]. As the H-ras-1 gene product is an important protein in signal transduction, it was considered as candidate gene for LQT1. However, based on sequence analysis and the occurrence of recombination events between the H-ras-1 gene and the LQT1 locus, it was shown that the H-ras-1 gene could not be the gene involved [19]. Two potassium channel genes (KCNA4 and KCNC1) are located on the short arm of chromosome 11, but were also excluded by linkage analysis [20,21]. Therefore, a positional cloning approach was used. Analysis of the coding sequence in the LQT1 area resulted in the identification of a coding sequence, which showed 53% homology to potassium channel proteins. Screening of cdna libraries using a probe based on this sequence resulted in the identification of the K v LQT1 gene by Wang et al. [21]. The coding region of the gene predicted a protein with six hydrophobic transmembrane regions (S1 S6) and a typical potassium channel pore region. To screen for mutations in the K v LQT1 gene, single strand conformation polymorphism analysis (a technique which detects sequence-dependent differences in electrophoretic mobility based on molecular shape in non-denaturing gels) was used, followed by sequencing of aberrant fragments. A list of the mutations detected and the positions within the gene is shown in Table 1. LQT2 To identify the LQT2 gene, located on chromosome 7q35 36, the candidate gene approach was followed. Since the long QT is associated with abnormal cardiac repolarization, genes encoding ion channels are candidate genes. In 1994, Warmke and Ganetzky [22] identified a cdna encoding the human ether-a-go-go related gene (HERG). This gene is located on chromosome 7 and the protein shows amino acid sequence homology with potassium channels. To test the candidacy of HERG, its localization was refined by mapping to a yeast artificial chromosome library specific for chromosome 7. The

3 Clinical Perspective 1429 Table 1 Summary of K v LQT1 mutations Table 2 Summary of mutations in HERG Codon Coding effect Mutation Region deletion F167W/G168d S2 168 missense G168R S bp insertion frameshift S2 S3 174 missense R174C S2 S3 178 missense A178T S2 S3 178 missense A178P S2 S3 189 missense G189R S2 S3 190 missense R190Q S2 S3 254 missense V254M S4 S5 273 missense L273F S5 269 missense G269D S5 306 missense G306R pore 310 missense T310R pore 312 missense T312I pore 313 missense I1313M pore 314 missense G314S pore 315 missense Y315S pore 317 missense D317N pore 320 missense P320A pore 325 missense G325R S6 341 missense A341E S6 341 missense A341V S6 342 missense L342F S6 344 missense A344V S6 345 missense G345E S6 345 missense G345R S6 366 missense R366P distal to S6 378 missense A378T distal to S6 539 missense R539W distal to S6 555 missense R555C distal to S6 References: Wang et al. [21] ; Russell et al. [18] ; De Jager et al. [47] ; Tanaka et al. [48] ; Van den Berg et al. [49] ; Chouabe et al. [29] ; Wollnik et al. [37]. Numbering of codons based on the kidney isoform of K v LQT1 [29]. gene proved to be localized on the same yeast artificial chromosome as a marker that was tightly linked to LQT2. Final identification of HERG and LQT2 resulted from mutation analysis: six mutations were associated with LQT2 [23]. The LQT2 gene encodes a cardiac protein with six transmembrane domains, a typical potassium channel pore region and nucleotide binding domain. A list of the mutations and their location is shown in Table 2. LQT3 A candidate gene approach was used for identification of the LQT3 gene on chromosome 3p In 1995, the human cardiac sodium channel SCN5A was mapped to chromosome 3p21, making it a candidate gene for LQT3 [24]. Using genotype analysis it was shown by Wang et al. that SCN5A was closely linked to the LQT3 locus in three families with no recombinations [5]. Final identification of SCN5A as the LQT3 gene was based on mutational analysis: the same intragenic deletion was found in two unrelated Codon Coding effect Mutation Region 470 missense N470D S2 474 missense T474I S2 S3 561 missense A561V S5 561 missense A561Thr S5 593 missense I593R S5-pore 611 missense Y611H S5-pore 614 missense A614V pore 628 missense G628S pore 630 missense V630L pore 822 missense V822M NBD Nucleotide Coding effect Region del 1261 frameshift, truncated protein S1 del bp deletion S3 G to C splice donor intron III NBD References: Curran et al. [23] ; Tanaka et al. [48] ; Benson et al. [50] ; Dause et al. [51] ; Satler et al. [52]. Table 3 Summary of mutations in SCN5A Codon Change Region Frequency del KPQ inactivation loop R1644H S4, DIV N1325 S4 S5, DIII 1 References: Wang et al. [6,26]. families [5]. The deleted amino acids (KPQ) are located in the cytoplasmic peptide linker between domain III and IV, a region that is responsible for fast inactivation of the ion channel activity [25]. Other mutations detected in the LQT3 gene are missense mutations at codon 1325 (N1325S) and codon 1644 (R1644H) [26]. A list of the mutations and deletions are shown in Table 3. LQT5 The gene involved in LQT5 was identified using a functional approach. Recently it was shown that the LQT1 gene co-assembles with the mink (KCNE1, IsK) gene to form the slowly activating delayed rectifier K + current (I Ks ). Therefore it was hypothesized, that mutations in mink also could cause the long QT syndrome. This hypothesis was confirmed by the detection of two mutations in mink (S74L, D76N) which affected the correct functioning of the I Ks channel [16]. Jervell and Lange-Nielsen syndrome To identify the gene involved in the Jervell and Lange- Nielsen syndrome a genome-wide search with polymorphic satellite markers was undertaken in combination

4 1430 Clinical Perspective with a candidate gene approach. If the parents in the families studied were first-degree cousins, advantage could be taken of consanguity to search for homozygosity at the disease locus [9]. The cardiac symptoms of the Jervell and Lange- Nielsen syndrome are comparable to the Romano Ward syndrome, with an onset of cardiac problems usually at a young age. In addition hearing loss is observed. Therefore genes coding for ion-channels were considered as candidate genes. At first, the ion channel genes HERG and SCN5A and the locus for LQT4 were excluded. By linkage analysis, a JLN locus was localized on chromosome 11p in the same region as K v LQT1. Therefore both the S2 to S6 region (previously shown to be involved in LQT1 linked Romano Ward syndrome) and the C-terminal part of K v LQT1 were screened. No mutations were observed in the S2 to S6 region. However, in the C-terminal part a homozygous deletion insertion mutation was found (seven nucleotides deleted, insertion of eight nucleotides) resulting in a frameshift and a premature stop [27]. Recently, additional mutations were identified in K v LQT1 linked Jervell and Lange- Nielsen syndrome: a homozygous insertion of a single nucleotide (G), resulting in a frameshift after the S2 region [28], a 5 bp deletion in the S2 S3 region and a missense mutation in the pore region [29,30]. However, in some Jervell and Lange-Nielsen syndrome families, no linkage was found to the K v LQT1 locus, indicating that the Jervell and Lange-Nielsen syndrome is genetically heterogeneous. Analysis of these families identified a region of homozygosity on chromosome 21, in a region where mink, a protein that interacts with K v QT1 is located. Sequence analysis resulted in the detection of homozygous mutations in the mink gene, demonstrating that IsK may be a cause of Jervell and Lange-Nielsen syndrome [30,31]. Therefore the Jervell and Lange-Nielsen syndrome can arise from mutations in either of the two interacting molecules (mink and K v LQT1). Molecular pathology of the long QT syndrome The long QT syndrome is associated with abnormal cardiac repolarization. Myocardial repolarization, following an action potential, is a complex process involving coordinated action of a number of ion channels. Mutations in SCN5A are the likely cause of LQT3 linked long QT syndrome. SCN5A, the cardiac voltage-gated sodium channel subunit, is a large protein of 2016 amino acids and consists of four domains (D1 to DIV) each of which contains six membrane-spanning regions. This channel initiates myocellular action potentials, then closes and remains closed (inactivated) for the remainder of the action potential. The mutations described, affect regions which are involved in inactivation. The three amino acid deletion KPQ is located in the inactivation loop. The missense mutation at codon 1644 (R1644H) affects the voltage sensor located in DIV-S4. Mutations in this area of the skeletal muscle sodium channel gene SCN4A in patients with paramyotonia congenita have been associated with delayed inactivation [26]. The missense mutation in codon 1335 (N1335S) in the intracellular domain between DIII-S4 and DIII-S5 apparently also affects inactivation, as mutations in this region in SCN4A result in a delayed inactivation [32]. Therefore mutations associated with the cardiac gene SCN5A, delay channel inactivation or change the voltage dependence of channel inactivation, leading to a prolonged depolarizing current. This results in a prolonged action potential duration and a prolonged QT interval. Delayed-rectifier K + channels are specifically involved in the repolarization of the cells following an action potential. Two types of delayed-rectifier K + channels have been described: one resulting in a rapidly activating current, I Kr, and the other resulting in a slowly activating current, I ks. The K + channel α-subunit, responsible for I Kr, has been identified as the product of the HERG gene, the gene that maps to the LQT2 locus [33]. Potassium channels are formed of four α-subunits. Therefore the combination of mutant and normal proteins can result in abnormal channels and a dominant negative effect due to interference of the mutant protein on normal protein function. For the HERG protein, a number of mutations, scattered over the gene, have been described. These mutations include missense mutations and mutations affecting the size of the protein. The effect of a number of mutations on the activity of normal HERG has recently been evaluated by in vitro expression studies [33,34]. Proteins encoded by the intragenic deletion mutants del 1261 (single base pair deletion, resulting in premature stop codon) and del 1498 (27 bp deletion) (regions S1 and S3) of HERG did not form functional channels and did not affect the activity of normal HERG, indicating the absence of coassembly of mutant and normal proteins. The mutants A561V (S5) and G628S (pore) did not form functional channels, but did block normal function, indicating coassembly. In contrast to the A561V and the G628S mutants, the N470D (S2) mutant expressed functional channels, but with altered kinetic properties. Co-expression of the N470D mutant with normal HERG resulted in a small decrease of the I Kr compared to normal HERG. The negative effect of the mutations characterized to date is the most pronounced for the G628S mutation, located in the pore region, and the least pronounced for the N470D mutation, located in the S2 region.

5 Clinical Perspective 1431 Mutated HERG protein can impair the I Kr by two mechanisms: (a) if the mutant protein is non functional and does not coassemble with other subunits to form channels, a reduction of the number of HERG K + channels results (haploinsufficiency), (b) if the mutant protein is non-functional or functions with altered characteristics and co-assembles with other subunits, interference with the activity of the normal protein results in an extra decrease in channel functioning (dominant negative effect). Recently it has been shown, that the I Ks channel function depends on the voltage gated K + - channel α-subunit coded for by the K v LQT1 gene, in association with the mink protein [35 37]. K v LQT1, the gene associated with LQT1 and the Jervell and Lange-Nielsen long QT syndrome, shows the characteristic features of a potassium channel α-subunit: six transmembrane domains and a typical K + -channel pore sequence [21]. However, expression of K v LQT1 alone induced a voltage dependent K + current, unlike other known cardiac K + currents [35 37]. MinK is a small glycoprotein with a single transmembrane domain, lacking homology to other cloned channels and is expressed in heart and inner ear [9,27]. Coexpression of K v LQT1 and mink induces a slowly activating delayed rectifier K + current with kinetics similar to the native I Ks. Therefore K v LQT1 is the subunit that co-assembles with mink to form functional I ks channels [35,36]. As the I Ks channel consists of a complex of mink and K v LQT1, mutations in either K v LQT1 or mink can affect the channel function, either by interfering with IsK binding or by directly resulting in non-functional potassium channels. In the Romano Ward syndrome most of the mutations in K v LQT1 are missense mutations (Table 1). These mutations are scattered over the S2 to S6 regions and a region in the C-terminal, with a possible hotspot in the S6 domain (A341E, A341V) [29]. In the Jervell and Lange- Nielsen syndrome, two homozygous frameshift mutations have been detected, resulting in a premature translation stop [9,28]. In analogy to the effect of missense mutations on the function of HERG [33], a dominant negative suppression was found for the Romano Ward related K v LQT1 mutants [29,37]. By contrast the Jervell and Lange-Nielsen related K v LQT1 mutations show a smaller or no dominant negative effect on the function of the normal protein. MinK is an essential component for the correct functioning of the channel: mutations in the C-terminal part of mink inhibit the K v LQT1+WT IsK induced current [36]. Recently a mink null mutant mouse has been established. Heterozygous mink mutant mice were apparently normal, while homozygous mutant mice showed ear abnormalities comparable to the abnormalities observed in the Jervell and Lange-Nielsen syndrome [27]. The two homozygous frameshift mutations observed in KvLQT1 associated with Jervell and Lange-Nielsen syndrome result in premature stops in the C-terminal part [9] and the S2 region, respectively [28]. When the premature stop was located in the S2 region, the heterozygous family members showed an increased QT interval and the characteristics of the Romano Ward syndrome (no deafness!). The premature stop in the C-terminal part did not affect the clinical status of the heterozygous family members. Deafness is apparently inherited as an autosomal recessive trait and the QT prolongation as a dominant trait, if the transmembrane domains are affected. As the C-terminal part of KvLQT1 is involved in the interaction with IsK, deletion of the C-terminal domain will result in loss of modulation by IsK and dysfunctioning of the KvLQT1 channels [32]. However, the exact effects of mutations in K v LQT1 or isoforms of KvLQT1 [38,39] on channel function and a possible correlation with clinical data remains to be established. Therapeutic implications The mortality of untreated symptomatic long QT syndrome patients exceeds 20% in the year after the first syncopal episode and approaches 50% within 10 years [1]. This can be reduced by therapy to 3% to 4% in 5 years. New syncopal episodes are prevented by beta-blockade in approximately 75% of the patients. Additional therapy includes cardiac sympathetic denervation and cardiac pacing in patients with evidence of pause- or bradycardia-dependent arrhythmias [40]. Four genes have been identified which are involved in the long QT syndrome. These genes encode a sodium-channel, two potassium-channels and a modulator of potassium channel function. As these channels are activated and inactivated at different time points during the process of depolarization and repolarization of the myocyte, ECG T-wave patterns are indicative of the gene involved [41]. With the identification of the gene defect involved in a specific long QT syndrome, gene-specific therapy may be developed [42 45]. In vitro assays indicate that sodium channel blockers, such as lidocaine and mexiletine may be used in patients with a defect in the sodium channel gene SCN5A [42,44,45]. Intravenous administration of lidocaine to two patients with an SCN5A-related disorder resulted in shortening of the QT interval and a normal configuration of the

6 1432 Clinical Perspective Table 4 LQT locus Genes related to long QT syndromes Location Responsible gene Ion channel Department of Cardiology, Cardiovascular Research Institute Maastricht, Academie Hospital Maastricht, Maastricht, The Netherlands LQT1 11p15.5 K v LQT1 potassium LQT2 7q35 HERG potassium LQT3 3p21 SCN5A sodium LQT4 4q25 27?? LQT5 21q22 mink potassium* JLNS1 11p15.5 K v LQT1 potassium JLNS2 21q22 mink potassium* *mink interacts with K v LQT1 to form functional channels. T-wave [46]. A second example is the effect of the serum potassium level on the QT interval in chromosome 7-linked long QT syndrome (potassium channel gene HERG). A significant shortening of the QT interval and correction of the T-wave morphology was observed upon increase of the serum potassium level in a group of 7 HERG linked long QT subjects [43]. Further clinical studies are required in defined subsets of patients, with criteria being syncope and sudden death, to establish the benefits of lidocaine or potassium therapy. Genotyping will be crucial to prevent unnecessary treatment in non-affected family members of long QT patients. Furthermore, knowledge of molecular defects can guide the cardiologist in advising patients as to their life pattern. Conclusion Inherited long QT syndrome is a genetic heterogeneous disorder, in which more than five chromosomal loci are implicated. Four genes associated with the long QT syndrome have been identified (Table 4). One coding for a sodium channel (SCN5A), two for potassium channels (HERG and K v LQT1) and one modulator of potassium channel function (IsK). Knowledge about the specific genes and the gene defects involved is of clinical importance: the defects result in a gain of function (SCN5A) or a loss of function (HERG, K v LQT1) and therefore may indicate a possible therapy. J. L. M. C. GEELEN* P. A. DOEVENDANS R. J. E. JONGBLOED* H. J. J. WELLENS J. P. M. GERAEDTS* *Division of Genetics, University Maastricht, Maastricht, The Netherlands References [1] Schwartz PJ. Idiopathic long QT syndrome: progress and questions. Am Heart J 1985; 111: [2] Schwartz PJ, Moss AJ, Vincent GM, Crampton RS. Diagnostic criteria for the long QT syndrome. An update. Circulation 1993; 88: [3] Keating M, 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: [4] Keating M, Dunn C, Atkinson D, Timothy K, Vincent GM, Lepper M. Consistent linkage of the long QT syndrome to the Harvey ras-1 locus on chromosome 11. Am J Hum Genet 1991; 49: [5] Wang Q, Shen J, Splawski I et al. SCN5A mutations associated with an inherited cardiac arrhythmia, long QT syndrome. Cell 1995; 80: [6] Romano C, Gemme G, Pongiglione R. Aritmie cardiache rare dell eta pediatrica. Pediatrica 1963; 45: [7] Ward OC. New familiar cardiac syndrome in children. J Irish Med Assoc 1964; 54: [8] Jervell A, Lange-Nielsen F. Congenital deaf-mutism, functional heart disease with prolongation of the QT interval and sudden death. Am Heart J 1957; 54: [9] Neyroud N, Tesson F, Denjoy I et al. A novel mutation in the potassium channel gene K v LQT1 causes the Jervell and Lange-Nielsen cardioauditory syndrome. Nature Genet 1997; 15: [10] Curran C, Atkinson D, Timothy K et al. Locus heterogeneity of autosomal dominant long QT syndrome. J Clin Invest 1993; 93: [11] Dean JCS, Cross S, Jennings K. Evidence of genetic and phenotypic heterogeneity in the Romano Ward syndrome. J Med Genet 1993; 30: [12] Towbin JA, Li H, Taggart RT, Lehmann MH, Schwartz PJ. Evidence of genetic heterogeneity in Romano Ward long QT syndrome. Analysis of 23 families. Circulation 1994; 90: [13] Tanaka T, Nakahara KI, Kato N et al. Genetic linkage analyses of Romano Ward syndrome (RWS) in 13 Japanese families. Hum Genet 1994; 94: [14] 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: [15] Schott JJ, Charpentier F, Peltier S et al.mappingofagenefor long QT syndrome to chromosome 4q Am J Hum Genet 1995; 57: [16] Splawski I, Tristani-Firouzi M, Lehmann MH, Sanguinetti MC, Keating MT. Mutations in the hmink gene causing long QT syndrome and suppress I Ks function. Nature Genet 1997; 17: [17] Russell MW, MacDonald D. The molecular genetics of the congenital long QT syndromes. Curr Opin Cardiol 1996; 11: [18] Russell MW, Macdonald D, Collins FS, Brody LC. KVLQT1 mutations in three families with familial or sporadic long QT syndrome. Hum Mol Genet 1996; 5: [19] Roy N, Kahlem P, Dausse E et al. Exclusion of HRAS from long QT locus. Nature Genet 1994; 8: [20] Wymore RS, Korenberg JR, Kinoshita KD et al. Genomic organization, nucleotide sequence, biophysical properties, and localization of the voltage-gated K-channel gene KCN4A/ Kv1.4 to mouse chromosome2/human 11p14 and mapping of

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