KVLQT1 mutations in three families with familial or sporadic long QT syndrome

Congenital long QT syndrome (LQTS) is a heterogeneous group of heritable disorders characterized by prolongation of the QT interval on the electrocardiogram, ventricular arrhythmias and sudden death. At least four genes can, when mutated, produce this phenotype. Of these genes, the recently identified KVLQT1 potassium channel is thought to be the one most commonly responsible. In this study, we used single strand conformational polymorphism (SSCP) analysis to screen two large and nine small LQTS families for mutations of the KVLQT1 potassium channel gene. We identified a novel missense mutation in two unrelated families which substitutes a serine for a conserved glycine in the putative pore region of the KVLQT1 channel. In a third family, a new alanine to valine mutation at a CpG dinucleotide resulted in the spontaneous occurrence of the long QT syndrome in monozygotic twin offspring of unaffected parents. Mutations at this same nucleotide have been observed in eight of the 19 LQTS families determined to have KVLQT1 mutations, suggesting this is a mutational hot spot. Both of these mutations alter the amino acid sequence in, or adjacent to, the pore of the channel and many diminish the channel's ability to conduct a repolarizing potassium current. To date, all KVLQT1 mutations determined to cause the LQTS are missense mutations. These data confirm the role of KVLQT1 in the LQTS and suggest that mutant KVLQT1 proteins may exert a dominant negative effect on repolarizing potassium currents by forming multimers with normal potassium channel protein subunits, dramatically reducing the number of fully-functional KVLQT1 channels.      

Molecular genetics of the long QT syndrome: Two novel mutations of the KVLQT1 gene and phenotypic expression of the mutant gene in a large kindred

At least three different gene loci were recently shown to account for the long QT syndrome (LQTS), a monogenic disorder with altered myocardial repolarization and occurrence of life-threatening cardiac arrhythmias. We screened 44 unrelated probands for mutations of the gene encoding the cardiac potassium channel KVLQT1 using single-strand conformational polymorphism (SSCP) and subsequent DNA sequencing. Two different mutations, T182I and D188N, were identified in two separate pedigrees. Cosegregation of the mutation with the disease phenotype was evident in both families. No mutations were identified at codon 212, previously suggested to represent a mutational hot spot of the KVLQT1 channel, in any of the 44 probands. The large pedigree with the D188N mutation (30 affected and 43 nonaffected individuals) permitted an analysis of expression of the mutant gene in its documented carriers. Although the mean (± SD) Qtc interval was markedly longer in affected (484 ± 38 ms) than in nonaffected individuals (406 ± 27 ms, P < 0.001), there was a marked overlapping of individual values in these two groups. QTc values in symptomatic and asymptomatic carriers of the mutant gene were not significantly different. In conclusion, we have identified two novel mutations of the KVLQT1 component of a cardiac potassium channel. Our data support the functional significance of the pore-S6 domain of this membrane protein and emphasize the diagnostic usefulness of DNA analyses in families with LQTS. Hum Mutat 11:158–165, 1998. © 1998 Wiley-Liss, Inc.     

Single Nucleotide Deletion Mutation of KCNH2 Gene is Responsible for LQT Syndrome in a 3-Generation Korean Family

Long QT syndrome (LQTS) is characterized by the prolongation of the QT interval in ECG and manifests predisposition to life threatening arrhythmia which often leads to sudden cardiac death. We encountered a 3-generation family with 5 affected family members in which LQTS was inherited in autosomal dominant manner. The LQTS is considered an ion channel disorder in which the type and location of the genetic mutation determines to a large extent the expression of the clinical syndrome. Upon screening of the genomic sequences of cardiac potassium ion channel genes, we found a single nucleotide C deletion mutation in the exon 3 of KCNH2 gene that co-segregates with the LQTS in this family. This mutation presumably resulted in a frameshift mutation, P151fs+15X. This study added a new genetic cause to the pool of mutations that lead to defected potassium ion channels in the heart.     

IsK and KvLQT1: mutation in either of the two subunits of the slow component of the delayed rectifier potassium channel can cause Jervell and Lange-Nielsen syndrome

The Jervell and Lange-Nielsen syndrome (JLNS) comprises profound congenital sensorineural deafness associated with syncopal episodes. These are caused by ventricular arrhythmias secondary to abnormal repolarisation, manifested by a prolonged QT interval on the electrocardiogram. Recently, in families with JLNS, Neyroud et al. reported homozygosity for a single mutation in KVLQT1 , a gene which has previously been shown to be mutated in families with dominantly inherited isolated long QT syndrome [Neyroud et al . (1997) Nature Genet ., 15, 186-189]. We have analysed a group of families with JLNS and shown that the majority are consistent with mutation at this locus: five families of differing ethnic backgrounds were homozygous by descent for markers close to the KVLQT1 gene and a further three families from the same geographical region were shown to be homozygous for a common haplotype and to have the same homozygous mutation of the KVLQT1 gene. However, analysis of a single small consanguineous family excluded linkage to the KVLQT1 gene, establishing genetic heterogeneity in JLNS. The affected children in this family were homozygous by descent for markers on chromosome 21, in a region containing the gene IsK . This codes for a transmembrane protein known to associate with KVLQT1 to form the slow component of the delayed rectifier potassium channel. Sequencing of the affected boys showed a homozygous mutation, demonstrating that mutation in the IsK gene may be a rare cause of JLNS and that an indistinguishable phenotype can arise from mutations in either of the two interacting molecules.      

KCNQ1 and KCNH2 mutations associated with long QT syndrome in a Chinese population

The long QT syndrome (LQTS) is a cardiac disorder characterized by prolongation of the QT interval on electrocardiograms (ECGs), syncope and sudden death caused by a specific ventricular tachyarrhythmia known as torsade de pointes. LQTS is caused by mutations in ion channel genes including the cardiac sodium channel gene SCN5A, and potassium channel subunit genes KCNQ1, KCNH2, KCNE1, and KCNE2. Little information is available about LQTS mutations in the Chinese population. In this study, we characterized 42 Chinese LQTS families for mutations in the two most common LQTS genes, KCNQ1 and KCNH2. We report here the identification of four novel KCNQ1 mutations and three novel KCNH2 mutations. The KCNQ1 mutations include L191P in the S2-S3 cytoplasmic loop, F275S and S277L in the S5 transmembrane domain, and G306V in the channel pore. The KCNH2 mutations include L413P in transmembrane domain S1, E444D in the extracellular loop between S1 and S2, and L559H in domain S5. The location and character of these mutations expand the spectrum of KCNQ1 and KCNH2 mutations causing LQTS. Excitement, exercises, and stress appear to be the triggers for developing cardiac events (syncope, sudden death) for LQTS patients with KCNQ1 mutations F275S, S277L, and G306V, and all three KCNH2 mutations L413P, E444D and L559H. In contrast, cardiac events for an LQTS patient with KCNQ1 mutation L191P occurred during sleep or awakening from sleep. KCNH2 mutations L413P and L559H are associated with the bifid T waves on ECGs. Inderal or propanolol (a beta blocker) appears to be effective in preventing arrhythmias and syncope for an LQTS patient with the KCNQ1 L191P mutation.     

Denaturing high-performance liquid chromatography screening of the long QT syndrome-related cardiac sodium and potassium channel genes and identification of novel mutations and single nucleotide polymorphisms

Mutations in cardiac potassium and sodium channel genes are responsible for several hereditary cardiac arrhythmia syndromes. We established a denaturing high-performance liquid chromatography (DHPLC) protocol for rapid mutation screening of these genes, and reported mutations and variations identified by this method. We included 28 patients with Brugada syndrome, 4 with congenital long QT syndrome (LQTS), 11 with drug-induced LQTS, 4 with idiopathic ventricular fibrillation, and 50 normal volunteers. Polymerase chain reactions were performed to amplify the entire coding region of these genes. DHPLC was used to screen for heteroduplexes then DNA sequencing was performed. With this method, we identified the mutation(s) in all four patients with congenital LQTS (KCNQ1 A341V, KCNH2 N633D, KCNH2 2768Cdel and KCNE1 K70 N Y81C double mutations). We also identified the SCN5A A551T mutation in 1 of the 28 patients with Brugada syndrome. All the above-mentioned mutations were novel except KCNQ1 A341V. No mutations were identified in patients with drug-induced LQTS or idiopathic ventricular fibrillation. In total, 25 single nucleotide polymorphisms were identified, 10 of which were novel. In conclusion, DHPLC is a sensitive and rapid method for detection of cardiac sodium and potassium channel gene mutations.     

Use of mutant-specific ion channel characteristics for risk stratification of long QT syndrome patients

Inherited long QT syndrome (LQTS) is caused by mutations in ion channels that delay cardiac repolarization, increasing the risk of sudden death from ventricular arrhythmias. Currently, the risk of sudden death in individuals with LQTS is estimated from clinical parameters such as age, gender, and the QT interval, measured from the electrocardiogram. Even though a number of different mutations can cause LQTS, mutation-specific information is rarely used clinically. LQTS type 1 (LQT1), one of the most common forms of LQTS, is caused by mutations in the slow potassium current (I(Ks)) channel α subunit KCNQ1. We investigated whether mutation-specific changes in I(Ks) function can predict cardiac risk in LQT1. By correlating the clinical phenotype of 387 LQT1 patients with the cellular electrophysiological characteristics caused by an array of mutations in KCNQ1, we found that channels with a decreased rate of current activation are associated with increased risk of cardiac events (hazard ratio=2.02), independent of the clinical parameters usually used for risk stratification. In patients with moderate QT prolongation (a QT interval less than 500 ms), slower activation was an independent predictor for cardiac events (syncope, aborted cardiac arrest, and sudden death) (hazard ratio = 2.10), whereas the length of the QT interval itself was not. Our results indicate that genotype and biophysical phenotype analysis may be useful for risk stratification of LQT1 patients and suggest that slow channel activation is associated with an increased risk of cardiac events.     

Inhibition of cardiac delayed rectifier K+ currents by an antisense oligodeoxynucleotide against IsK (minK) and over-expression of IsK mutant D77N in neonatal mouse hearts

The IsK (minK or KCNE1) protein is known to co-assemble with the KvLQT1 (KCNQ1) protein to form a channel underlying the slowly activating delayed rectifier K+ current (IKs). Controversy remains as to whether the IsK protein assembles with ERG (the ether-a-go-go-related gene) products to form or modulate the channel-underlying the rapidly activating delayed rectifier K+ current (IKr). We investigated the effects of antisense oligodeoxynucleotides (AS-ODN) against IsK and its mutant D77N [which underlies a form of long QT syndrome (LQT5) in humans] on the delayed rectifier K+ current (IK) of neonatal mouse ventricular myocytes in primary culture. Patch-clamp experiments on these cells showed that IK consists of IKs and IKr. IK was not recorded from ventricular cells transfected with AS-ODN, while it was recorded from cells transfected with the corresponding sense oligodeoxynucleotides (S-ODN). IK was not recorded from cells transfected with the D77N mutant, and the action potential duration was much longer than in cells transfected with wild-type IsK. Furthermore, HERG could not induce currents in COS-1 cells co-expressed with the D77N mutant and HERG (the human form of ERG). These results indicate that the IsK protein associates with both KvLQT1 and ERG products to modulate IKr and IKs in cardiac myocytes.     

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.     

Sodium channel abnormalities are infrequent in patients with long QT syndrome: identification of two novel SCN5A mutations.

Long QT syndrome (LQTS) is a heterogeneous disorder caused by mutations of at least five different loci. Three of these, LQT1, LQT2, and LQT5, encode potassium channel subunits. LQT3 encodes the cardiac-specific sodium channel, SCN5A. Previously reported LQTS-associated mutations of SCN5A include a recurring three amino acid deletion (DeltaKPQ1505-1507) in four different families, and four different missense mutations. We have examined the SCN5A gene in 88 index cases with LQTS, including four with Jervell and Lange-Nielsen syndrome and the remainder with Romano-Ward syndrome. Screening portions of DIII-DIV, where mutations have previously been found, showed that none of these patients has the three amino acid deletion, DeltaKPQ1505-1507, or the other four known mutations. We identified a novel missense mutation, T1645M, in the DIV; S4 voltage sensor immediately adjacent to the previously reported mutation R1644H. We also examined all of the additional pore-forming regions and voltage-sensing regions and discovered another novel mutation, T1304M, at the voltage-sensing region DIII; S4. Neither T1645M nor T1304M were seen in a panel of unaffected control individuals. Five of six T1304M gene carriers were symptomatic. In contrast to previous studies, QT(onset-c) was not a sensitive indicator of SCN5A-associated LQTS, at least in this family. These data suggest that mutations of SCN5A are responsible for only a small proportion of LQTS cases.     

Spectrum of pathogenic mutations and associated polymorphisms in a cohort of 44 unrelated patients with long QT syndrome

Long QT syndrome (LQTS) is a rare and clinically heterogeneous inherited disorder characterized by a long QT interval on the electrocardiogram, increased risk of syncope and sudden death caused by arrhythmias. This syndrome is mostly caused by mutations in genes encoding various cardiac ion channels. The clinical heterogeneity is usually attributed to variable penetrance. One of the reasons for this variability in expression could be the coexistence of common single nucleotide polymorphisms (SNPs) on LQTS-causing genes and/or unknown genes. Some synonymous and nonsynonymous exonic SNPs identified in LQTS-causing genes may have an effect on the cardiac repolarization process and modulate the clinical expression of a latent LQTS pathogenic mutation. We report the molecular pattern of 44 unrelated patients with LQTS using denaturing high-performance liquid chromatography analysis of the KCNQ1, KCNH2, SCN5A, KCNE1 and KCNE2 genes. Forty-five disease-causing mutations (including 24 novel ones) were identified in this cohort. Most of our patients (84%) showed complex molecular pattern with one mutation (and even two for four patients) associated with several SNPs located in several LQTS genes.     

Classification of the long-QT syndrome based on discriminant analysis of T-wave morphology

The long QT syndrome (LQTS) is a genetic disorder, typically characterized by a prolonged QT interval in the ECG due to abnormal cardiac repolarization. LQTS may lead to syncopal episodes and sudden cardiac death. Various parameters based on T-wave morphology, as well as the QT interval itself have been shown to be useful discriminators, but no single ECG parameter has been sufficient to solve the diagnostic problem. In this study we present a method for discrimination among persons with a normal genotype and those with mutations in the KCNQ1 (KvLQT1 or LQT1) and KCNH2 (HERG or LQT2) genes on the basis of parameters describing T-wave morphology in terms of duration, asymmetry, flatness and amplitude. Discriminant analyses based on 4 or 5 parameters both resulted in perfect discrimination in a learning set of 36 subjects. In both cases cross-validation of the resulting classifiers showed no misclassifications either.Patent pending—EPO 03029363.3—2305     

KCNQ1 mutations in patients with a family history of lethal cardiac arrhythmias and sudden death

Long QT syndrome (LQTS) is the prototype of the cardiac ion channelopathies which cause syncope and sudden death. LQT1, due to mutations of KCNQ1 (KVLQT1), is the most common form. This study describes the genotype-phenotype characteristics in 10 families with mutations of KCNQ1, including 5 novel mutations. One hundred and two families with a history of lethal cardiac events, 55 LQTS, 9 Brugada syndrome, 18 idiopathic ventricular fibrillation (IVF), and 20 acquired LQTS, were studied by single-strand conformational polymorphism (SSCP) and DNA sequence analyzes. Families found to have KCNQ1 mutations were phenotyped using ECG parameters and cardiac event history, and genotype-phenotype correlation was performed. No mutations were found in Brugada syndrome, IVF, or acquired LQTS families. Ten out of 55 LQTS families had KCNQ1 mutations and 62 carriers were identified. Mutations included G269S in domain S5; W305X, G314C, Y315C, and D317N in the pore region; A341E and Q357R in domain S6; and 1338insC, G568A and T587M mutations in the C-terminus. W305X, G314C, Q357R, 1338insC, and G568A, appeared to be novel mutations. Gene carriers were 26 +/- 19 years (32 females). Baseline QTc was 0.47 +/- 0.03 s (range 0.40-0.57 s) and 40% had normal to borderline QTc (< or = 0.46 s). Typical LQT1 T wave patterns were present in at least one affected member of each family, and in 73% of all affected members. A history of cardiac events was present in 19/62 (31%), 18 with syncope, 2 with aborted cardiac arrest (ACA) and six with sudden death (SD). Two out of 6 SDs (33%) occurred as the first symptom. No difference in phenotype was evident in pore vs. non-pore mutations. KCNQ1 mutations were limited to LQTS families. All five novel mutations produced a typical LQT1 phenotype. Findings emphasize (1) reduced penetrance of QTc and symptoms, resulting in diagnostic challenges, (2) the problem of sudden death as the first symptom (33% of those who died), and (3) genetic testing is important for identification of gene carriers with reduced penetrance, in order to provide treatment and to prevent lethal cardiac arrhythmias and sudden death.     

A novel long-QT 5 gene mutation in the C-terminus (V109I) is associated with a mild phenotype

Mutations in the human minK gene KCNE1 have been linked to autosomal dominant and autosomal recessive long-QT (LQT) syndrome, a cardiac condition predisposing to ventricular arrhythmias. minK and KvLQT1, the LQT1 gene product, form a native cardiac K+ channel that regulates the slowly delayed rectifier potassium current I(Ks). We used single-strand conformation polymorphism and sequencing techniques to identify novel KCNE1 mutations in patients with a congenital LQT syndrome of unknown genetic origin. In 150 unrelated index patients a missense mutation (V109I) was identified that significantly reduced the wild-type I(Ks) current amplitude (by 36%) when coexpressed with KvLQT1 in Xenopus oocytes. Other biophysical properties of the I(Ks) channel were not altered. Since we observed incomplete penetrance (only one of two mutation carriers could be diagnosed by clinical criteria), and the family's history was unremarkable for sudden cardiac death, the 109I allele most likely causes a mild phenotype. This finding may have implications for the occurrence of "acquired" conditions for ventricular arrhythmias and thereby the potential cardiac risk for asymptomatic mutation carriers still remains to be determined.     

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.     

Dependence of I Ks biophysical properties on the expression system

The delayed rectifier potassium current I(Ks) is important for repolarization of the cardiac action potential. In heart I(Ks) is a heteromeric channel composed of KCNQ1 (KvLQT1) and minK (KCNE1, IsK). Here we show that the KCNQ1/minK interaction is influenced by the expression system. Co-expression of KCNQ1 and minK in Xenopus oocytes resulted in potassium currents comparable to endogenous guinea pig cardiac I(Ks) in terms of temperature dependency and activation kinetics. In contrast, heterologous expression of I(Ks) in CHO cells revealed currents with a markedly different biophysical behavior. The sensitivity to the extracellular potassium concentration, temperature dependency and kinetics differ qualitatively. Potentially there is an endogenous component that affects I(Ks) which does not appear in all expression systems.     

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.