Hypokalemia-induced long QT syndrome with an underlying novel missense mutation in S4-S5 linker of KCNQ1

Congenital long QT syndrome (LQTS) is caused by mutations in at least five genes coding for cardiac potassium or sodium channels that regulate the duration of ventricular action potentials. Acquired LQTS often is associated with drugs or metabolic abnormalities. A 47-year-old woman who presented with marked QT prolongation (QTc = 620 msec(1/2)) and repeated episodes of torsades de pointes associated with hypokalemia (2.6 mEq/L) was screened for mutations in LQTS genes using polymerase chain reaction/single-strand conformation polymorphism (PCR/SSCP). We identified a novel missense mutation in the intracellular linker of S4-S5 domains of KCNQ1, resulting in an amino acid substitution of cysteine for arginine at position 259 (R259C). Whole cell, patch clamp experiments were conducted on COS7 cells transfected with wild-type and/or R259C KCNQ1 with or without KCNE1. Functional analyses of the mutant KCNQ1 subunit on COS7 cells revealed its functional channels in the homozygous state, producing a significantly smaller current than the KCNQ1 channels and a less severe dominant-negative effect on I(Ks). The novel KCNQ1 mutation R259C is the molecular basis for I(Ks) dysfunction underlying an apparently sporadic case of hypokalemia-induced LQTS, consistent with a mild mutation likely to disclose the clinical manifestation of LQTS in a context of severe hypokalemia. Our findings suggest that gene carriers with such mild mutations might not be so rare as commonly expected in patients with acquired LQTS, and stress the importance of mutational analysis for detecting either "silent" forms of congenital LQTS or de novo mutations.     

Biophysical characterization of KCNQ1 P320 mutations linked to long QT syndrome 1

Hereditary long QT syndrome (LQTS) is a cardiovascular disorder characterized by prolongation of the QT interval on the surface ECG and a high risk for arrhythmia-related sudden death. Mutations in a cardiac voltage-gated potassium channel, KCNQ1, account for the most common form of LQTS, LQTS1. The objective of this study was the characterization of a novel KCNQ1 mutation linked to LQTS. Electrophysiological properties and clinical features were determined and compared to characteristics of a different mutation at the same position. Single-strand conformation polymorphism analysis followed by direct sequencing was performed to screen LQTS genes for mutations. A novel missense mutation in the KCNQ1 gene, KCNQ1 P320H, was identified in the index patient presenting with recurrent syncope and aborted sudden death triggered by physical stress and swimming. Electrophysiological analyses of KCNQ1 P320H and the previously reported KCNQ1 P320A mutation indicate that both channels are non-functional and suppress wild type I_Ks in a dominant-negative fashion. Based on homology modeling of the KCNQ1 channel pore region, we speculate that the proline residue at position 320 limits flexibility of the outer pore and is required to maintain the functional architecture of the selectivity filter/pore helix arrangement. Our observations on the KCNQ1 P320H mutation are consistent with previous studies indicating that pore mutations in potassium channel α-subunits are associated with more severe electrophysiological and clinical phenotypes than mutations in other regions of these proteins. This study emphasizes the significance of mutation screening for diagnosis, risk-assessment, and mutation-site specific management in LQTS patients.     

New KCNQ1 mutations leading to haploinsufficiency in a general population; Defective trafficking of a KvLQT1 mutant

OBJECTIVE: KCNQ1 mutations lead to the long QT syndrome (LQTS), characterized by a prolonged QT interval, syncopes and sudden death. However, some mutations are associated with non-penetrant phenotype (no symptoms, QTc normal or borderline). The objective of this study was to determine whether KCNQ1 variants are associated with borderline QTc prolongation in a general population and to evaluate the frequency of carriers. METHODS: We selected 2008 unrelated and untreated healthy individuals from a non-patient population. The KCNQ1 gene was screened by denaturing high-performance liquid chromatography (dHPLC) in 50 men and 50 women presenting the longest QTc intervals (403 to 443 ms). RESULTS: We identified a nonsense mutation, Y148X, and an in-frame deletion of the serine residue 276 (DeltaS276), in S2 and S5 transmembrane domains, respectively. DeltaS276 KvLQT1 channels expressed in COS-7 cells failed to conduct any K+ current in the homozygous state. Besides, a slight reduction in channel activity was observed when coexpressed with WT KvLQT1 and IsK. Confocal microscopy performed on transfected COS-7 cells revealed that DeltaS276 KvLQT1 was retained in the endoplasmic reticulum, whereas WT KvLQT1 was localized in the cell membrane. The two mutation carriers presented borderline QTc interval prolongation at slow heart rate but a 24-h ECG recording revealed a marked QTc prolongation at higher heart rate for the Y148X carrier. CONCLUSIONS: In this population, two subjects with borderline QTc prolongations (438 and 443 ms) were carriers of KCNQ1 mutations leading to haploinsufficiency and are potentially at risk of developing drug-induced arrhythmia. The study provides the first demonstration of a defective cell surface localization of a KvLQT1 mutant missing one amino acid in a transmembrane domain.     

Variable Expression of Long QT Syndrome Among Gene Carriers from Families with Five Different HERG Mutations

Objectives: This study assessed the phenotypic variability of LQTS in carriers with the same and with different mutations in the LQT2 gene. Background: Mutations of ion‐channel genes are known to cause the long QT syndrome (LQTS), a disorder associated with distinctive genotypic‐specific electrocardiographic patterns and variable clinical expression. Methods: Clinical and electrocardiographic characteristics were assessed in five large LQTS families, each with a different mutation of the HERG gene (LQT2; n = 469, 69% genotyped, 102 carriers). One mutation was located on the N‐terminus and the other four on the C‐terminus of the HERG channel protein. Results: The QTc duration and the frequency of cardiac events (syncope and LQTS‐related cardiac arrest/deatht were similar among carriers with the five HERG mutations. QTc was as variable in carriers of the same mutation as it was among carriers with different HERG mutations (P = 0.19). Qualitative assessment of the electrocardiograms revealed extensive intra‐and interfamilial variability in T‐vvave morphology. Among carriers with multiple electrocardiograms extending over 2 to 7 years, variation in QTc over time was minimal. A strong association was found between QTc and the occurrence of cardiac events in carriers of all five mutations. Conclusions: The clinical expression of LQTS was equally variable in carriers from families with the same or different HERG mutations. These findings highlight the complexity of the clinical phenotype in this Mendelian dominant disorder and suggest that one or more modifier genes contribute to the variable expression of this syndrome. A.N.E. 2002;7(1):40–46     

Novel Compound Heterozygous Mutations in the KCNQ1 Gene Associated with Autosomal Recessive Long QT Syndrome (Jervell and Lange‐Nielsen syndrome)

Background: The Jervell and Lange‐Nielsen syndrome (JLNS) is the autosomal recessive form of long QT syndrome (LQTS)—a familial cardiac disorder that causes syncope, seizures, and sudden death from ventricular arrhythmias, specifically torsade de pointes. JLNS is associated with sensorineural deafness and has been shown to occur with homozygous mutations in KCNQ1 or KCNE1 in JLNS families in which QTc prolongation is inherited as a dominant trait. This study investigated the molecular pathology of a family with clinical evidence of JLNS. Methods and Results: Single‐strand conformation polymorphism, denaturing high performance liquid chromatography, and DNA sequencing analyses were used to screen for KCNQ1 mutations. Novel compound heterozygous nonsense mutations R518X/Q530X in the C‐terminus of KCNQ1 were identified in both affected dizygotic twins; both the parents and a sibling each carried only one of the mutant alleles and were asymptomatic with modestly prolonged QTc intervals (0.46, 0.50, and 0.45 seconds, respectively). These two nonsense mutations lead to premature termination of C‐terminus with truncation of the postulated assembly domain. Conclusion: Novel compound heterozygous nonsense mutations in C‐terminus of KCNQ1 can cause JLNS. A.N.E 2003;8(3):246‐250     

Pressure rinsing of coronary stents immediately before implantation reduces inflammation and neointimal hyperplasia

OBJECTIVES: We took advantage of the genetic isolate of Finns to characterize a common long QT syndrome (LQTS) mutation, and to estimate the prevalence of LQTS. BACKGROUND: The LQTS is caused by mutations in different ion channel genes, which vary in their molecular nature from family to family. METHODS: The potassium channel gene KCNQ1 was sequenced in two unrelated Finnish patients with Jervell and Lange-Nielsen syndrome (JLNS), followed by genotyping of 114 LQTS probands and their available family members. The functional properties of the mutation were studied using a whole-cell patch-damp technique. RESULTS: We identified a novel missense mutation (G589D or KCNQ1-Fin) in the C-terminus of the KCNQ1 subunit. The voltage threshold of activation for the KCNQ1-Fin channel was markedly increased compared to the wild-type channel. This mutation was present in homozygous form in two siblings with JLNS, and in heterozygous form in 34 of 114 probands with Romano-Ward syndrome (RWS) and 282 family members. The mean (+/- SD) rate-corrected QT intervals of the heterozygous subjects (n = 316) and noncarriers (n = 423) were 460 +/- 40 ms and 410 +/- 20 ms (p < 0.001), respectively. CONCLUSIONS: A single missense mutation of the KCNQ1 gene accounts for 30% of Finnish cases with LQTS, and it may be associated with both the RWS and JLNS phenotypes of the syndrome. The relative enrichment of this mutation most likely represents a founder gene effect. These circumstances provide an excellent opportunity to examine how genetic and nongenetic factors modify the LQTS phenotype.     

先天性QT延长综合征亚型JLN综合征的基因突变研究

目的　对一先天性QT延长综合征亚型JLN综合征 (JLNS)家系进行基因突变检测 ,以期发现中国人特有的JLNS突变。方法　采用聚合酶链反应及直接测序法对一JLNS家系进行KCNQ1及KCNE1的基因检测。结果　在先证者及其姐姐的KCNQ1基因的第 1 5外显子发现一错义突变 :2 0 37(G→A)、G6 4 3S ,还有另一突变是KCNQ1基因外显子 2a的第 2 2 7位核苷酸C被T代替 ,其编码的苏氨酸被异亮氨酸所代替。而这两突变分别来自其表型正常的父母亲。结论　JLNS可由KCNQ1基因上复合的杂合突变所引起。在中国QT延长综合征患者中发现两个JLNS的新的突变。     

End-recovery QTc: a useful metric for assessing genetic variants of unknown significance in long-QT syndrome

QTc for Assessing Significance of LQT Variants. Introduction: Genetic variants represent benign single‐nucleotide polymorphisms, disease causing mutations or variants of unknown significance (VUS). Resting, exercise, and recovery QTc intervals have been utilized to detect long‐QT syndrome (LQTS) mutations. We sought to provide clinical data that may assist in classifying the presented VUS as disease causing/benign and to determine whether resting and/or end‐recovery QT parameters can evaluate the significance of VUS. Methods and Results: Twenty‐six patients with a VUS in genes associated with LQTS (15 females, age 38 ± 16 years) and 26 age and gender matched controls (age 37 ± 20 years) were included. There were 10 VUS (5 KCNQ1, 4 KCNH2, 1 KCNE1) in 12 families. All but 1 VUS was associated with sudden cardiac death (SCD), aborted SCD or Torsade de pointes. A Schwartz score of ≥3.5 was observed in at least 1 family member with each VUS. Resting QTc was marginally longer in VUS patients compared with controls (458 ± 48 vs 437 ± 25, P = 0.052). A prolonged resting QTc (>470 ms males, >480 ms females) identified 6 VUS carriers and 1 control. VUS carriers had a substantially longer end‐recovery QTc (502 ± 68 vs 427 ± 17, P < 0.01) with an end‐recovery QTc > 445 ms in 20/26 VUS patients compared to 2/26 controls (P < 0.01). The area under the receiver operating characteristic curve for resting QTc was 0.68 (95% CI, 0.53–0.83, P = 0.03) compared to the end‐recovery QTc of 0.88 (95% CI, 0.76–0.99, P < 0.0001). Conclusion: Variants in the current study appear to be disease causing. The end‐recovery QTc is a useful metric when interpreting LQT VUS. (J Cardiovasc Electrophysiol, Vol. 23, pp. 637–642, June 2012)     

Epinephrine Bolus Test in Detecting Long QT Syndrome Mutation Carriers with Indeterminable Electrocardiographic Phenotype

Background: In long QT syndrome (LQTS), prolonged and heterogeneous ventricular repolarization predisposes to serious arrhythmias. We examined how QT intervals are modified by epinephrine bolus in mutation carriers of three major LQTS subtypes with indefinite QT interval. Methods: Genotyped, asymptomatic subjects with LQTS type 1 (LQT1; n = 10; four different KCNQ1 mutations), type 2 (LQT2; n = 10; three different HERG mutations), and type 3 (LQT3; n = 10; four different SCN5A mutations), and healthy volunteers (n = 15) were examined. Electrocardiogram was recorded with body surface potential mapping system. After an epinephrine 0.04 μg/kg bolus QT end, QT apex, and T‐wave peak‐to‐end (Tpe) intervals were determined automatically as average of 12 precordial leads. Standard deviation (SD) of the 12 channels was calculated. Results: Heart rate increased 26 ± 10 bpm with epinephrine bolus, and similarly in all groups. QT end interval lengthened, and QT apex interval shortened in LQTS and normals, leading to lengthening of Tpe interval. However, the lengthening in Tpe was larger in LQTS than in normals (mean 32 vs 18 ms; P < 0.05) and SD of QT apex increased more in LQTS than in normals (mean 23 vs 7 ms; P < 0.01). The increase in Tpe was most pronounced in LQT2, and in SD of QT apex in LQT1 and LQT2. Conclusions: Abrupt adrenergic stimulation with a moderate dose of exogenous epinephrine affects ventricular repolarization in genotype‐specific fashion facilitating distinction from normals. This delicate modification may help in diagnosing electrocardiographically silent mutation carriers when screening LQTS family members. Ann Noninvasive Electrocardiol 2011;16(2):172–179     

Homozygosity for a HERG potassium channel mutation causes a severe form of long QT syndrome: identification of an apparent founder mutation in the Finns

OBJECTIVES

We studied the clinical characteristics and molecular background underlying a severe phenotype of long QT syndrome (LQTS).

BACKGROUND

Mutations of cardiac ion channel genes cause LQTS, manifesting as increased risk of ventricular tachycardia and sudden death.

METHODS

We studied two siblings showing prolonged QT intervals corrected for heart rate (QTc), their asymptomatic parents with only marginally prolonged QTc intervals and their family members. The potassium channel gene HERG was screened for mutations by deoxyribonucleic acid sequencing, and the electrophysiologic consequences of the mutation were studied in vitro using the whole-cell patch-clamp technique.

RESULTS

A novel missense mutation (L552S) in the HERG channel, present in the homozygous state in the affected siblings and in the heterozygous state in their parents, as well as in 38 additional subjects from six LQTS families, was identified. One of the homozygous siblings had 2:1 atrioventricular block immediately after birth, and died at the age of four years after experiencing unexplained hypoglycemia. The other sibling had an episode of torsade de pointes at the age of two years. The mean QTc interval differed significantly (p < 0.001) between heterozygous symptomatic mutation carriers (500 ± 59 ms), asymptomatic mutation carriers (452 ± 34 ms) and noncarriers (412 ± 23 ms). When expressed in vitro, the HERG-L552S formed functional channels with increased activation and deactivation rates.

CONCLUSIONS

Our data demonstrate that homozygosity for a HERG mutation can cause a severe cardiac repolarization disorder without other phenotypic abnormalities. Absence of functional HERG channels appears to be one cause for intrauterine and neonatal bradycardia and 2:1 atrioventricular block.     

长QT综合征家系KCNQ1 S145L和KCNH2 Y475 C基因新突变

目的研究中国遗传性长QT综合征(LQTS)患者的临床特点及LQTS最常见的基因KCNQ1和KCNH2突变.方法应用聚合酶链反应和测序分析77个遗传性LQTS家系,筛查了LQTS致病基因KCNQ1和KCNH2,观察临床表现和心电图改变.结果77例先证者心电图表现为LQT1者24例、LQT2者42例、LQT3者3例,8例心电图表现不典型.年龄(27.6±16.4)岁.QTc(561±70)ms,发病年龄(17.6±14.7)岁.晕厥触发因素包括运动、情绪激动和铃声刺激等.目前已经发现了4KCNQ1突变和7 KCNH2突变,其中6个为首次发现.结论LQT2为中国最常见的LQTS;本组发现KCNQ1和KCNH2各1个新突变;中国LQTS患者心电图表现和临床特点与欧美LQT患者有所不同.     

Clinical,Genetic, and Electrophysiologic Characteristics of a New Pas‐Domain HERG Mutation (M124R) Causing Long QT Syndrome

Objectives: To describe the clinical, genetic, and electrophysiologic characteristics of a new PAS‐domain HERG mutation (M124R) that has been identified in a single large Jewish family with Long QT syndrome (LQTS). Background: Many previously reported HERG mutations causing LQTS are located either in the C‐terminus, or in the pore region. Relatively fewer clinical data are available on N‐terminus (PAS‐domain) mutation carriers. Methods: Clinical data were available in 76 family members (aged 1–93 years, 69 alive) over 18 years of follow‐up, while electrocardiographic data were available in 57, and genetic data in 45 family members. Cellular electrophysiology was assessed in transfected Chinese Hamster Ovary (CHO) cells using the whole‐cell patch‐clamp technique. Results: Thirty‐six family members were phenotypically categorized as nonaffected, 3 as equivocal, and 20 as affected. Mean QTc was 410 ± 23, 440 ± 10, and 498 ± 41 ms, respectively, in these three subgroups. Eight out of 20 affected family members were symptomatic: five had only syncope, two had aborted cardiac arrest, and one sudden death. Genetic analyses identified the M124R point mutation in all affected members tested (n = 16), while all those tested with nonaffected (n = 26) and equivocal (n = 3) phenotype did not carry the mutation. The M124R mutation reduced the HERG tail‐current density by 65%, significantly accelerated the deactivation kinetics, and caused a negative shift in the voltage dependence of activation. Conclusions: A new PAS‐domain HERG mutation (M124R) was identified as causing LQTS in a large Jewish family, with high penetrance and frequent disease‐related symptoms. This mutation markedly decreased the tail‐current density and accelerated the deactivation kinetics of the HERG channel in transfected CHO cells.     

A double-point mutation in the selectivity filter site of the KCNQ1 potassium channel results in a severe phenotype, LQT1, of long QT syndrome

Introduction: Slowly activating delayed-rectifier potassium currents in the heart are produced by a complex protein with α and β subunits composed of the potassium voltage-gated channel KQT-like subfamily, member 1 (KCNQ1) and the potassium voltage-gated channel Isk-related family, member 1 (KCNE1), respectively. Mutations in KCNQ1 underlie the most common type of hereditary long QT syndrome (LQTS). Like other potassium channels, KCNQ1 has six transmembrane domains and a highly conserved potassium selectivity filter in the pore helix called "the signature sequence." We aimed to investigate the functional consequences of a newly identified mutation within the signature sequence.
Methods and Results: Potassium channel genomic DNA from a family with clinical evidence of LQTS was amplified by polymerase chain reaction (PCR), and the resulting products were then sequenced. Three family members had a double-point mutation in KCNQ1 at nucleotides 938 (T-to-A) and 939 (C-to-A), resulting in an isoleucine-to-lysine change at amino acid position 313. These patients displayed prolonged QTc intervals (629, 508, and 500 ms^1/2, respectively) and repetitive episodes of syncope, but no deafness. Three-dimensional structure modeling of KCNQ1 revealed that this mutation is located at the center of the channel pore. COS-7 cells displayed a lack of current when transfected with a plasmid expressing the mutant. In addition, the mutant displayed a dominant negative effect on current but appeared normal with respect to plasma membrane integration.
Conclusion: An I313K mutation within the selectivity filter of KCNQ1 results in a dominant-negative loss of channel function, leading to a long QT interval and subsequent syncope.     

Re‐evaluating pathogenicity of variants associated with the long QT syndrome

1 Introduction

Genetic testing for congenital long QT syndrome (LQTS) has become common. Recent studies have shown that some variants labelled as pathogenic might be misclassified due to sparse case reports and relatively common allele frequencies (AF) in the general population. This study aims to evaluate the presence of LQTS‐associated variants in the Genome Aggregation Database (gnomAD) population, and assess the functional impact of these variants.

2 Methods and results

Variants associated with LQTS from the Human Gene Mutation Database were extracted and matched to the gnomAD to evaluate population‐based AF. We used MetaSVM to predict the function of LQTS variants. Allele distribution by protein topology in KCNQ1, KCNH2, and SCN5A was compared between gnomAD (n = 123,136) and a cohort of LQTS patients aggregated from eight published studies (n = 2,683). Among the 1,415 LQTS‐associated single nucleotide variants in 30 genes, 347 (25%) are present in gnomAD; 24% of the 347 variants were predicted as functionally tolerated compared with 4% of variants not present in gnomAD (P < 0.001). Of the 347 pathogenic variants in gnomAD, seven (2%) had an AF of ≥ 0.001 and 65 (19%) variants had an AF of ≥ 0.0001. In KCNQ1, KCNH2, and SCN5A, allele distribution by protein functional region was significantly different with gnomAD alleles appearing less frequently in highly pathogenic domains than case alleles.

3 Conclusion

A significant number of LQTS variants have insufficient evidence for pathogenicity and relatively common AF in the general population. Caution should be used when ascribing pathogenicity to these variants.     

Cardiac potassium channel dysfunction in sudden infant death syndrome

Life-threatening arrhythmias have been suspected as one cause of the sudden infant death syndrome (SIDS), and this hypothesis is supported by the observation that mutations in arrhythmia susceptibility genes occur in 5-10% of cases. However, the functional consequences of cardiac potassium channel gene mutations associated with SIDS and how these alleles might mechanistically predispose to sudden death are unknown. To address these questions, we studied four missense KCNH2 (encoding HERG) variants, one compound KCNH2 genotype, and a missense KCNQ1 mutation all previously identified in Norwegian SIDS cases. Three of the six variants exhibited functional impairments while three were biophysically similar to wild-type channels (KCNH2 variants V279M, R885C, and S1040G). When co-expressed with WT-HERG, R273Q and K897T/R954C generated currents resembling the rapid component of the cardiac delayed rectifier current (I_Kr) but with significantly diminished amplitude. Action potential modeling demonstrated that this level of functional impairment was sufficient to evoke increased action potential duration and pause-dependent early afterdepolarizations. By contrast, KCNQ1-I274V causes a gain-of-function in I_Ks characterized by increased current density, faster activation, and slower deactivation leading to accumulation of instantaneous current upon repeated stimulation. Action potential simulations using a Markov model of heterozygous I274V-I_Ks incorporated into the Luo-Rudy (LRd) ventricular cell model demonstrated marked rate-dependent shortening of action potential duration predicting a short QT phenotype. Our results indicate that certain potassium channel mutations associated with SIDS confer overt functional defects consistent with either LQTS or SQTS, and further emphasize the role of congenital arrhythmia susceptibility in this syndrome.     

Functionally Aberrant Mutant KCNQ1 With Intermediate Heterozygous and Homozygous Phenotypes

Background

Deleterious mutations in KCNQ1 may lead to an autosomal dominant form of long QT syndrome (LQTS) (Romano-Ward) or autosomal recessive form (Jervell and Lange-Nielsen). Both are associated with severe ventricular tachyarrhythmias due to the reduction of the slowly activating delayed rectifier K⁺ current (I_Ks). Our objective was to investigate the functional consequences of KCNQ1-R562S mutation in an atypical form of KCNQ1-linked LQTS.

Patient with Syncope and LQTS Carrying a Mutation in the PAS Domain of the hERG1 Channel

We report the case of a woman with syncope and persistently prolonged QTc interval. Screening of congenital long QT syndrome (LQTS) genes revealed that she was a heterozygous carrier of a novel KCNH2 mutation, c.G238C. Electrophysiological and biochemical characterizations unveiled the pathogenicity of this new mutation, displaying a 2‐fold reduction in protein expression and current density due to a maturation/trafficking‐deficient mechanism. The patient's phenotype can be fully explained by this observation. This study illustrates the importance of performing genetic analyses and mutation characterization when there is a suspicion of congenital LQTS. Identifying mutations in the PAS domain or other domains of the hERG1 channel and understanding their effect may provide more focused and mutation‐specific risk assessment in this population. Ann Noninvasive Electrocardiol 2011;16(2):213–218     

Repolarization Dynamics During Exercise Discriminate Between LQT1 and LQT2 Genotypes

Genotype and Exercise in LQTS . Background: Repolarization dynamics during exercise in patients with long‐QT Syndrome (LQTS) may be influenced by various factors such as a patient's genotype. We sought to systematically characterize the repolarization dynamics during exercise in patients with LQTS with a particular focus on the influence of genotype. Methods: Three groups of patients were studied on the basis of clinical status and genotype: LQT1, LQT2, and normal controls. Twenty‐five age‐ and gender‐matched patients were selected for each group. The QTc was measured during bicycle exercise testing and its dynamics were compared between the 3 groups. Results: The degree of QTc prolongation during exercise was greater in LQTS patients (LQT1 80 ± 47 ms, LQT2 64 ± 41 ms, Control 46 ± 20 ms, P = 0.02), with significant differences between LQT1 and LQT2 patients observed at heart rates ≥60% of the predicted maximum (P < 0.05). LQT1 patients demonstrated progressive or persistent QTc prolongation at higher heart rates, whereas LQT2 patients demonstrated maximum QTc prolongation at submaximal heart rates (～50% of the predicted maximum) with subsequent QTc correction toward baseline values at higher heart rates. Importantly, these observations were consistent regardless of age, gender, or exercise type in subgroup analyses. Conclusions: Reduced repolarization reserve in LQTS is genotype and heart rate specific. (J Cardiovasc Electrophysiol, Vol. 21, pp. 1242‐1246, November 2010)     

Evidence for a Single Nucleotide Polymorphism in the KCNQ1 Potassium Channel that Underlies Susceptibility to Life-Threatening Arrhythmias

INTRODUCTION: Congenital long QT syndrome (LQTS) is a genetically heterogeneous arrhythmogenic disorder caused by mutations in at least five different genes encoding cardiac ion channels. It was suggested recently that common polymorphisms of LQTS-associated genes might modify arrhythmia susceptibility in potential gene carriers. METHODS AND RESULTS: We examined the known LQTS genes in 95 patients with definitive or suspected LQTS. Exon-specific polymerase chain reaction single-strand conformation polymorphism and direct sequence analyses identified six patients who carried only a single nucleotide polymorphism in KCNQ1 that is found in approximately 11% of the Japanese population. This 1727G>A substitution that changes the sense of its coding sequence from glycine to serine at position 643 (G643S) was mostly associated with a milder phenotype, often precipitated by hypokalemia and bradyarrhythmias. When heterologously examined by voltage-clamp experiments, the in vitro cellular phenotype caused by the single nucleotide polymorphism revealed that G643S-KCNQ1 forms functional homomultimeric channels, producing a significantly smaller current than that of the wild-type (WT) channels. Coexpression of WT-KCNQ1 and G643S-KCNQ1 with KCNE1 resulted in approximately 30% reduction in the slow delayed rectifier K+ current I(Ks) without much alteration in the kinetic properties except its deactivation process, suggesting that the G643S substitution had a weaker dominant-negative effect on the heteromultimeric channel complexes. CONCLUSION: We demonstrate that a common polymorphism in the KCNQ1 potassium channel could be a molecular basis for mild I(Ks) dysfunction that, in the presence of appropriate precipitating factors, might predispose potential gene carriers to life-threatening arrhythmias in a specific population.     

Fetal Atrioventricular Block and Postpartum Augmentative QT Prolongation in a Patient With Long‐QT Syndrome With KCNQ1 Mutation

2:1 AV Block in KCNQ1. The case of a 32‐year‐old pregnant woman, who had had several syncopal episodes during swimming and running at 9 and 10 years of age and whose fetus had 2:1 AV block, is presented. The mother and baby had the same heterozygous single nucleotide substitution in KCNQ1 at T587M. After 27 weeks of gestation, the fetal 2:1 AV block disappeared, and 1:1 AV conduction resumed, with a fetal heart rate of 110–120 beats/min. The maternal electrocardiogram revealed a normal QTc interval (433 ms) without ST‐T abnormalities at gestational week 23, but the QTc was 490 and 531 ms at 1 and 2 months postpartum, with biphasic T waves in leads V2 and V3. This case is the first report of fetal 2:1 AV block with KCNQ1 mutation (T587M) and unmasked maternal QT prolongation in the postpartum period. (J Cardiovasc Electrophysiol, Vol. 21, pp. 1170‐1173)