Bradycardia‐induced long QT syndrome caused by a de novo missense mutation in the S2‐S3 inner loop of HERG

Long QT syndrome is a congenital disorder that presents with a defective cardiac ion channel and is either associated with prolonged action potential or, more commonly, known as an acquired form in which “torsades de pointes” type arrhythmias specifically occur after secondary causes. We report a case of a novel HERG mutation (A490T) that caused a bradycardia‐associated form of long QT syndrome. A 27‐year‐old woman exhibited recurrent syncope due to torsades de pointes associated with a disturbance of the cardiac conduction system. By using polymerase chain reaction and single strand conformational polymorphism analyses, we identified a heterozygous single nucleotide substitution of HERG (G to A at nt 1468). This mutational change was not present in 140 Japanese control individuals. Electrophysiological assays for the A490T mutant HERG channel were conducted in the heterologous expression system with COS7 cells. The mutant channel was found to reconstitute functional channel currents, suggesting the homomeric mutant channel was functional. The mutation did not change the properties of the activation gate and inward rectification, however the current density of this mutant channel was small compared with that of wild type HERG. Taken together, this mutant may cause subtle changes in HERG channel functions (I_Kr) in vivo. In this case, genetic background and unexpected bradycardia may have contributed to the development of long QT syndrome. © 2001 Wiley‐Liss, Inc.     

Bradycardia-induced long QT syndrome caused by a de novo missense mutation in the S2-S3 inner loop of HERG

Long QT syndrome is a congenital disorder that presents with a defective cardiac ion channel and is either associated with prolonged action potential or, more commonly, known as an acquired form in which "torsades de pointes" type arrhythmias specifically occur after secondary causes. We report a case of a novel HERG mutation (A490T) that caused a bradycardia-associated form of long QT syndrome. A 27-year-old woman exhibited recurrent syncope due to torsades de pointes associated with a disturbance of the cardiac conduction system. By using polymerase chain reaction and single strand conformational polymorphism analyses, we identified a heterozygous single nucleotide substitution of HERG (G to A at nt 1468). This mutational change was not present in 140 Japanese control individuals. Electrophysiological assays for the A490T mutant HERG channel were conducted in the heterologous expression system with COS7 cells. The mutant channel was found to reconstitute functional channel currents, suggesting the homomeric mutant channel was functional. The mutation did not change the properties of the activation gate and inward rectification, however the current density of this mutant channel was small compared with that of wild type HERG. Taken together, this mutant may cause subtle changes in HERG channel functions (I(Kr)) in vivo. In this case, genetic background and unexpected bradycardia may have contributed to the development of long QT syndrome.     

KCNH2 Gene Mutation: A Potential Link Between Epilepsy and Long QT-2 Syndrome

Long QT syndrome (LQTS) is closely associated with syncope, seizure, and sudden death but LQTS is frequently misdiagnosed as epilepsy. LQTS and epilepsy both belong to the group of ion channelopathies that manifest in the heart and brain. Therefore, genetic analysis of genes associated with potassium and sodium homeostasis and electrical disorders may reveal a link between epilepsy and lethal cardiac arrhythmia. Here, the authors report a young woman who suffered recurrent seizure episodes and syncopes that occurred while walking and also during rest. She showed electroencephalogram abnormalities and a pathological prolonged QTc interval in electrocardiogram. The patient and the patient's asymptomatic family members underwent genetic screening of the three genes most frequently associated with LQTS: KCNQ1, KCNH2, and SCN5A. The patient and the family members did not show DNA alterations in the genes KCNQ1 and SCN5A associated with LQT-1 and LQT-3, respectively. However, the patient showed a de novo mutation 2587T→C in exon 10 of KCNH2 gene associated with LQT-2. The mutation caused a stop codon substitution (R863X) in the HERG channel, leading to a 296–amino acid deletion. The patient's asymptomatic relatives did not show the KCNH2 gene mutation. R863X alteration in HERG channel may be involved in both prolonged QTc interval and epilepsy. This fact raises the possibility that R863X alteration in KCNH2-encoded potassium channel may confer susceptibility for epilepsy and cardiac LQT-2 arrhythmia.     

长QT综合征KCNH2基因S4区新移码突变L539fs/47的研究

目的 对1个先天性长QT综合征家系进行分子遗传学分析.方法 应用短串联重复序列(short tandem repeat,STR)连锁分析确定突变基因的位点,聚合酶链反应-单链构象多态性结合测序的方法 筛选KCNH2基因的突变.结果 先证者KCNH2基因在第7外显子存在19 bp的缺失,位于KCNH2基因编码序列1619～1637之间,同时突变基因的下游存在1个A1692G(CTA→CTG,L564L)多态位点,引起L539fs/47移码突变.突变基因来源于父亲,其兄弟为致病基因的携带者但未出现临床症状.结论 KCNH2基因的L539fs/47移码突变是新突变点,是引起本家系临床症状的原因.     

一个先天性长QT综合征家系KCNH2基因新突变的确定及其编码蛋白的二级结构预测

目的 确定一个先天性长QT综合征家系的基因突变位点,并对突变所引起的编码蛋白的结构改变进行预测.方法 应用聚合酶链反应和直接测序分析先证者,找到突变位点后合成位点特异性引物,应用多重聚合酶链反应对长QT家系成员进行筛查;利用网络分析软件对突变所引起的编码蛋白结构进行预测.结果 发现了1个KCNH2基因新错义突变,即跨膜片段S2的F463L突变(GenBank接受序列号EU218526);突变没有引起预测跨膜区的改变,但编码蛋白的疏水性及二级结构,突变基因最小自由能二级结构都发生了变化.结论 作者所发现的突变点丰富了长QT综合征离子通道突变的基因库资料,用软件分析基因突变可能引起编码蛋白二级结构的改变有利于理解引起长QT综合征的结构基础.     

A Case of Long QT Syndrome Type 3 Aggravated by Beta-Blockers and Alleviated by Mexiletine: The Role of Epinephrine Provocation Test

Long QT syndrome (LQTs) is an uncommon genetic disease causing sudden cardiac death with Torsade de Pointes (TdP). The first line drug treatment has been known to be β-blocker. We encountered a 15-year-old female student with LQTs who had prolonged QTc and multiple episodes of syncope or agonal respiration during sleep. Although her T wave morphology in surface electrocardiography resembled LQTs type 1, her clinical presentation was unusual. During the epinephrine test, TdP was aggravated during β-blocker medication, but alleviated by sodium channel blocker (mexiletine). Therefore, she underwent implantable cardioverter defibrillator implantation.     

A de novo missense mutation (R1623Q) of the SCN5A gene in a Japanese girl with sporadic long QT syndrome

Two missense mutations and a nine-nucleotide deletion of the cardiac sodium channel (SCN5A) gene have been shown to cause long QT syndrome (LQTS) in several familial cases. We identified a novel missense mutation (R1623Q) of the SCN5A gene in a Japanese girl with sporadic LQTS. We used polymerase chain reaction, single-strand conformation polymorphism analysis and DNA sequence analysis to identify a mutation of the SCN5A gene in the patient. A single nucleotide substitution of guanine to adenine, in codon 1623, changed the coding sense of the SCN5A from arginine to glutamine (R1623Q) in the S4 segment of domain IV which is a highly conserved region of the SCN5A. This mutation was not identified in the unaffected biological parents and brother of the patient, and 100 normal, unrelated individuals. This finding is the first evidence of a de novo mutation in SCN5A associated with LQTS. Hum Mutat 11:481, 1998. © 1998 Wiley-Liss, Inc.     

先天性长QT综合征一家系的突变分析

目的对1个先天性长QT综合征(long QT syndrome，LQTS)家系的突变进行检测与分析。方法根据先证者的临床表现及心电图先对其致病基因初步判断，然后采用聚合酶链反应以及直接测序法对其进行KCNQ1基因检测以发现致病突变。结果根据先证者的临床表现及心电图将其致病基因初步定位于KCNQ1基因上，且在KCNQ1基因上发现一错义突变940(G—A)(G314S)，此突变在欧美及日本人群中均有发现，为长QT综合征的突变热点。结论在中国长QT综合征患者中有与国外患者同样的突变热点。     

Clinical and electrophysiological characterization of a novel mutation R863X in HERG C-terminus associated with long QT syndrome

We have found a novel nonsense mutation in the C-terminus of HERG in a four-generation Chinese family with long QT syndrome and investigated the molecular mechanism of this mutation in vitro. Six family members, including the proband, were clinically affected. Syncope and ventricular tachycardia of torsades de pointes were triggered by startling or emotional stress, and -adrenergic blockade treatment was ineffective. Haplotype analysis showed that only LQT₂ markers cosegregated with the disease, and sequence analysis revealed a substitution of T with C at nucleotide position 2770 of the HERG gene (U04270), which creates a stop codon at amino acid position 863 (R863X) of the HERG protein, leading to a deletion of 296 amino acids. Whole cell patch clamp studies showed that the R863X HERG could not induce time-dependent current. Coexpression of R863X with wild-type HERG showed reduced current densities and accelerated voltage-dependent inactivation of HERG channels. Subcellular localization of R863X-EGFP revealed that the mutant did not traffic to the cell surface. These data suggest that R863X failed to form functional HERG channels, contributing to a prolongation of the QT interval and long QT syndrome with a dominant phenotype. These findings provide new insights into the structure-function relationships of the HERG C-terminus.     

Novel KCNQ1 mutations associated with recessive and dominant congenital long QT syndromes: evidence for variable hearing phenotype associated with R518X

Congenital long QT syndrome may be transmitted as either an autosomal dominant or recessive trait. Two families with the autosomal recessive Jervell and Lange-Nielsen syndrome (JLNS), and one family with the autosomal dominant Romano-Ward syndrome (RWS) were evaluated for mutations in KCNQ1. Two different novel frameshift mutations were discovered in one of the JLNS families (1188delC) and in the RWS family (504delG). A third allele (R518X) was observed in the second JLNS family. The R518X allele was previously associated with recessive long QT syndrome without deafness, but was present in a congenitally deaf proband in our study. These data extend the range of known KCNQ1 mutations associated with both recessive and dominant forms of congenital long QT syndrome, and demonstrate that the R518X allele may be associated with or without congenital deafness.     

一个中国人Liddle综合征家系的SCNN1G基因新突变及其临床特征

目的 分析2个Liddle综合征家系上皮细胞钠通道编码基因SCNN1B及SCNN1G的基因突变.方法 收集2个临床诊断为Liddle综合征的家系,抽取先证者及其家系成员外周血基因组DNA,PCR扩增上皮细胞钠通道β亚单位编码基因SCNN1B和γ亚单位编码基因SCNN1G第13外显子,产物直接DNA测序进行基因突变检测.结果 例1 SCNN1B基因第13外显子的扩增片段经双向测序显示第564密码子存在CGA-TGA(R-X)杂合无义突变,其家系成员均未发现这一基因突变;例2 SCNN1G基因第567密码子存在CAG-TAG(Q-X)杂合无义突变,2个家系成员携带此突变基因,这一突变位点尚未在国内外报道过,50名无关正常人中未发现此突变基因.结论 对临床诊断的Liddle综合征患者及其亲属,进行基因突变检测有助于确定诊断及早期筛查出家系中的其他患者.编码人类肾小管上皮细胞钠通道γ亚单位基因SCNN1G第13外显子第567密码子CAG-TAG(Q-X)杂合无义突变可能会导致Liddle综合征.     

The G604S-hERG mutation alters the biophysical properties and exerts a dominant-negative effect on expression of hERG channels in HEK293 cells

We have recently identified a missense mutation, G604S, in the human ether-a-go-go related gene (hERG) that results in a malignant phenotype in a full pedigree of a Chinese congenital long QT syndrome (LQTS) family. The present study characterized the pathophysiological consequences of the mutation at the cellular level. Mutant G604S-hERG channels were expressed in HEK293 cells using a lipofectamine method. hERG currents were recorded using the voltage clamp technique. The expression of hERG protein was detected by Western blotting, and the subcellular location of hERG channels in cell was analyzed by confocal microscopy. We found that the G604S mutation did not lead to any expression of detectable currents, which was consistent with Western blotting analysis that the G604S-hERG mutation only expressed a band at 135 kDa. When coexpressed with wild-type (WT)-hERG, G604S-hERG exhibited strong dominant-negative current suppression resulting in decreased current density and altered gating properties of the WT-hERG channel, as well as interference with the trafficking of WT-hERG channel protein. In addition, confocal microscopy demonstrated that G604S-hERG subunits could be inserted into the cell membrane when forming heteromultimeric channels with WT-hERG channel subunits. Our results suggest that G604S mutation causes a loss of function in hERG through a strong dominant-negative effect on WT-hERG channel function that caused by impaired trafficking of WT-hERG channels, and further accentuates this suppression by forming heteromultimeric functional channels with WT-hERG subunits.     

长Q-T间期综合征的临床实践指南

长Q-T间期综合征(Long Q-T syndrome,LQTS)是一种单基因遗传性心脏离子通道病,以QT间期延长、T波异常、尖端扭转型室速(torsade de pointes,TdP)为心电图表现,反复发作晕厥、抽搐、甚至猝死为临床特征。尽管LQTS的总体患病率不高,但由于高发心源性猝死,已引起心血管医师的密切关注。目前国内缺少针对性的LQTS临床实践指南。本指南的编写参考了国内外本领域的基础研究、临床研究和其他国家的相关指南共识,对LQTS的临床表现、遗传学机制、诊断标准、治疗与预后、遗传咨询等方面进行总结,以期促进和规范其临床诊疗实践。     

A de novo missense mutation (R1623Q) of the SCN5A gene in a Japanese girl with sporadic long QT sydrome. Mutations in brief no. 140. Online

Two missense mutations and a nine-nucleotide deletion of the cardiac sodium channel (SCN5A) gene have been shown to cause long QT syndrome (LQTS) in several familial cases. We identified a novel missense mutation (R1623Q) of the SCN5A gene in a Japanese girl with sporadic LQTS. We used polymerase chain reaction, single-strand conformation polymorphism analysis and DNA sequence analysis to identify a mutation of the SCN5A gene in the patient. A single nucleotide substitution of guanine to adenine, in codon 1612, changed the coding sense of the SCN5A from arginine to glutamine (R1623Q) in the S4 segment of domain IV which is a highly conserved region of the SCN5A. This mutation was not identified in the unaffected biological parents and brother of the patient, and 100 normal, unrelated individuals. This finding is the first evidence of a de nova mutation in SCN5A associated with LQTS.     

Identification of a novel KCNQ1 mutation in a large Saudi family with long QT syndrome: clinical consequences and preventive implications

Congenital long QT syndrome (LQTS) is an inherited potentially fatal arrhythmogenic disorder that is characterized by prolonged corrected QT (QTc) interval. Mutations in three genes (KCNQ1, KCNH2, and SCN5A) account for the majority of the cases. However, 10 other genes are now known to be implicated in LQTS. In this work, we describe the clinical and molecular analysis in a large Saudi family with LQTS. Screening KCNQ1, KCNH2, and SCN5A genes in the proband, who presented with syncope, led to the identification of a heterozygous mutation (p.H258P) in KCNQ1. An extended clinical and genetic screening of the family identified 11 other members who were carriers for this mutation. All identified carriers had prolonged QTc intervals; yet, only two were symptomatic. Screening the family members for three LQTS modifiers (rs4657139 and rs16847548 in NOS1AP and KCNE1‐D85N) did not reveal a correlation with symptoms or QTc intervals. The electrocardiographic and molecular analysis stratified seven carriers at high risk of a cardiac event as they had a QTc of ≥500 ms and were carriers of a KCNQ1 mutation. Our work illustrates the importance of extended family screening in LQTS to identify silent carriers and hence adopt the most appropriate therapeutic and preventive intervention.     

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

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

Introduction into Cav2.1 of the homologous mutation of Cav1.2 causing the Timothy syndrome questions the role of V421 in the phenotypic definition of P-type Ca2+ channel

The Timothy syndrome is a multisystem disorder associated with the mutation of a Gly residue (G402 or G406) in the Ca_v1.2 Ca²⁺ channel. G406 is localized at the end of the IS6 segment and just before the intracellular I–II loop, which is important for the regulation of channel inactivation and the binding of the Ca_vβ subunit. This Gly residue is conserved in all Ca_v1 and Ca_v2 channels, and the G to R exchange produces a strong decrease of inactivation not only in Ca_v1.2 but also in Ca_v2.3. Here, we show that the mutation into Arg or Glu of the homologous Gly residue in Ca_v2.1 (G363) produces also a slowing of inactivation. However, the G-to-A exchange that decreases the inactivation rate in Ca_v1.2 and Ca_v2.3 increases inactivation in Ca_v2.1. Each mutation affects specifically the gating properties of Ca_v2.1 that remain nevertheless modulated by the co-expressed β subunit as with wild-type channel. The strong decrease of inactivation produced by the G363R or G363E mutations was reminiscent to that previously described for a specific splice variant of Ca_v2.1 that contains a single Val residue inserted in the I–II loop (V421). We unexpectedly found that the V421 insertion does not affect the inactivation rate of Ca_v2.1 and that the effects previously attributed to this insertion, including those on G-protein regulation, can be reproduced by the G363E mutation. Altogether, our results highlight the role of G363 in gating properties, inactivation kinetics, and G-protein regulation of Ca_v2.1 and the lack of effect of V421 insertion on inactivation. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Novel Brugada syndrome‐causing mutation in ion‐conducting pore of cardiac Na+ channel does not affect ion selectivity properties

Aim: Brugada syndrome is an inherited cardiac disease with an increased risk of sudden cardiac death. Thus far Brugada syndrome has been linked only to mutations in SCN5A, the gene encoding the α‐subunit of cardiac Na⁺ channel. In this study, a novel SCN5A gene mutation (D1714G) is reported, which has been found in a 57‐year‐old male patient. Since the mutation is located in a segment of the ion‐conducting pore of the cardiac Na⁺ channel, which putatively determines ion selectivity, it may affect ion selectivity properties. Methods: HEK‐293 cells were transfected with wild‐type (WT) or D1714G α‐subunit and β‐subunit cDNA. Whole‐cell configuration of the patch‐clamp technique was used to study biophysical properties at room temperature (21 °C) and physiological temperature (36 °C). This study represents the first measurements of human Na⁺ channel kinetics at 36 °C. Ion selectivity, current density, and gating properties of WT and D1714G channel were studied. Results: D1714G channel yielded nearly 80% reduction of Na⁺ current density at 21 and 36 °C. At both temperatures, no significant changes were observed in V_1/2 values and slope factors for voltage‐dependent activation and inactivation. At 36 °C, but not at 21 °C, D1714G channel exhibited more slow inactivation compared with WT channel. Ion selectivity properties were not affected by the mutation at both temperatures, as assessed by either current or permeability ratio. Conclusion: This study shows no changes in ion selectivity properties of D1714G channel. However, the profoundly decreased current density associated with the D1714G mutation may explain the Brugada syndrome phenotype in our patient.     

Somatic mosaicism contributes to phenotypic variation in Timothy syndrome

Timothy syndrome type 1 (TS-1) is a rare disorder that affects multiple organ systems and has a high incidence of sudden death due to profound QT prolongation and resultant ventricular arrhythmias. All previously described cases of TS-1 are the result of a missense mutation in exon 8A (p.G406R), an alternatively spliced variant of the L-type calcium channel gene (Ca(v)1.2, CACNA1C). Most patients reported in the literature represent highly affected individuals who present early in life with severe cardiac and neurological manifestations. Here, we describe somatic mosaicism in TS-1 patients with less severe manifestations than the typical TS-1 patient. These findings suggest that the TS prognosis may not be as dismal as previously reported. Moreover, our findings have implications for genetic counseling in that previously described de novo TS mutations may represent cases of parental mosaicism and warrant careful genotyping of parental tissue other than peripheral blood lymphocytes.     

Novel KCNE3 mutation reduces repolarizing potassium current and associated with long QT syndrome

Long QT syndrome (LQTS) is an inherited disease involving mutations in the genes encoding a number of cardiac ion channels and a membrane adaptor protein. Among the genes that are responsible for LQTS, KCNE1 and KCNE2 are members of the KCNE family of genes, and function as ancillary subunits of Kv channels. The third KCNE gene, KCNE3, is expressed in cardiac myocytes and interacts with KCNQ1 to change the channel properties. However, KCNE3 has never been linked to LQTS. To investigate the association between KCNE3 and LQTS, we conducted a genetic screening of KCNE3 mutations and single nucleotide polymorphisms (SNPs) in 485 Japanese LQTS probands using DHPLC‐WAVE system and direct sequencing. Consequently, we identified two KCNE3 missense mutations, located in the N‐ and C‐terminal domains. The functional effects of these mutations were examined by heterologous expression systems using CHO cells stably expressing KCNQ1. One mutation, p.R99λH was identified in a 76‐year‐old woman who suffered torsades de pointes (TdP) after administration of disopyramide. Another mutation, p.T4A was identified in a 16‐year‐old boy and 67‐year‐old woman. Although the boy carried another KCNH2 mutation, he was asymptomatic. On the other hand, the woman suffered from hypokalemia‐induced TdP. In a series of electrophysiological analyses, the KCNQ1(Q1)+KCNE3(E3)‐R99λH channel significantly reduced outward current compared to Q1+E3‐WT, though the current density of the Q1+E3‐T4A channel displayed no statistical significance. This is the first report of KCNE3 mutations associated with LQTS. Screening for variants in the KCNE3 gene is of clinical importance for LQTS patients. Hum Mutat 30, 557–563, 2009. © 2009 Wiley‐Liss, Inc.