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

What voltage-sensing phosphatases can reveal about the mechanisms of ion channel regulation by phosphoinositides

Many membrane proteins including ion channels and ion transporters are regulated by membrane phospholipids such as phosphoinositides in cell membranes and organelles. Voltage-sensing phosphatase, VSP, is a voltage-sensitive phosphoinositide phosphatase which dephosphorylates PI(4,5)P2 into PI(4)P. VSP rapidly reduces the level of PI(4,5)P2 upon membrane depolarization, thus serving as a useful tool to quantitatively study phosphoinositide-regulation of ion channels and ion transporters using a cellular electrophysiology system. In this review, we focus on the application of VSPs to Kv7 family potassium channels, which have been important research targets in biophysics, pharmacology and medicine.

Arrhythmogenic Mechanisms in Hypokalaemia: Insights From Pre-clinical Models

Potassium is the predominant intracellular cation, with its extracellular concentrations maintained between 3. 5 and 5 mM. Among the different potassium disorders, hypokalaemia is a common clinical condition that increases the risk of life-threatening ventricular arrhythmias. This review aims to consolidate pre-clinical findings on the electrophysiological mechanisms underlying hypokalaemia-induced arrhythmogenicity. Both triggers and substrates are required for the induction and maintenance of ventricular arrhythmias. Triggered activity can arise from either early afterdepolarizations (EADs) or delayed afterdepolarizations (DADs). Action potential duration (APD) prolongation can predispose to EADs, whereas intracellular Ca²⁺ overload can cause both EADs and DADs. Substrates on the other hand can either be static or dynamic. Static substrates include action potential triangulation, non-uniform APD prolongation, abnormal transmural repolarization gradients, reduced conduction velocity (CV), shortened effective refractory period (ERP), reduced excitation wavelength (CV × ERP) and increased critical intervals for re-excitation (APD-ERP). In contrast, dynamic substrates comprise increased amplitude of APD alternans, steeper APD restitution gradients, transient reversal of transmural repolarization gradients and impaired depolarization-repolarization coupling. The following review article will summarize the molecular mechanisms that generate these electrophysiological abnormalities and subsequent arrhythmogenesis.

Upgraded molecular models of the human KCNQ1 potassium channel

The voltage-gated potassium channel KCNQ1 (K_V7.1) assembles with the KCNE1 accessory protein to generate the slow delayed rectifier current, I_KS, which is critical for membrane repolarization as part of the cardiac action potential. Loss-of-function (LOF) mutations in KCNQ1 are the most common cause of congenital long QT syndrome (LQTS), type 1 LQTS, an inherited genetic predisposition to cardiac arrhythmia and sudden cardiac death. A detailed structural understanding of KCNQ1 is needed to elucidate the molecular basis for KCNQ1 LOF in disease and to enable structure-guided design of new anti-arrhythmic drugs. In this work, advanced structural models of human KCNQ1 in the resting/closed and activated/open states were developed by Rosetta homology modeling guided by newly available experimentally-based templates: X. leavis KCNQ1 and various resting voltage sensor structures. Using molecular dynamics (MD) simulations, the capacity of the models to describe experimentally established channel properties including state-dependent voltage sensor gating charge interactions and pore conformations, PIP2 binding sites, and voltage sensor–pore domain interactions were validated. Rosetta energy calculations were applied to assess the utility of each model in interpreting mutation-evoked KCNQ1 dysfunction by predicting the change in protein thermodynamic stability for 50 experimentally characterized KCNQ1 variants with mutations located in the voltage-sensing domain. Energetic destabilization was successfully predicted for folding-defective KCNQ1 LOF mutants whereas wild type-like mutants exhibited no significant energetic frustrations, which supports growing evidence that mutation-induced protein destabilization is an especially common cause of KCNQ1 dysfunction. The new KCNQ1 Rosetta models provide helpful tools in the study of the structural basis for KCNQ1 function and can be used to generate hypotheses to explain KCNQ1 dysfunction.

Is There a Role for Genetics in the Prevention of Sudden Cardiac Death?

The identification of patients at risk for sudden cardiac death (SCD) is fundamental for both acquired cardiovascular diseases (such as coronary artery diseases, CAD) and inherited arrhythmia syndromes (such as the long-QT syndrome, LQTS). Genetics may play a role in both situations, although the potential to exploit this information to reduce the burden of SCD varies among these two groups. Concerning acquired cardiovascular diseases, which affect most of the general population, preliminary data suggest an association between genetics and the risk of dying suddenly. The maximal utility, instead, is reached in inherited arrhythmia syndromes, where the discovery of monogenic diseases such as LQTS tracked the way for the first genotype-phenotype correlations. The aim of this review is to provide a general overview focusing on the current genetic knowledge and on the present and future applicability for prevention in these two populations at risk for SCD.

Structural analysis of the S4-S5 linker of the human KCNQ1 potassium channel

KCNQ1 plays important roles in the cardiac action potential and consists of an N-terminal domain, a voltage-sensor domain, a pore domain and a C-terminal domain. KCNQ1 is a voltage-gated potassium channel and its channel activity is regulated by membrane potentials. The linker between transmembrane helices 4 and 5 (S4-S5 linker) is important for transferring the conformational changes from the voltage-sensor domain to the pore domain. In this study, the structure of the S4-S5 linker of KCNQ1 was investigated by solution NMR, circular dichroism and fluorescence spectroscopic studies. The S4-S5 linker adopted a helical structure in detergent micelles. The W248 may interact with the cell membrane.

Novel Kv7.1-phosphatidylinositol 4,5-bisphosphate interaction sites uncovered by charge neutralization scanning

Kv7.1 to Kv7.5 α-subunits belong to the family of voltage-gated potassium channels (Kv). Assembled with the β-subunit KCNE1, Kv7.1 conducts the slowly activating potassium current IKs, which is one of the major currents underlying repolarization of the cardiac action potential. A known regulator of Kv7 channels is the lipid phosphatidylinositol 4,5-bisphosphate (PIP2). PIP2 increases the macroscopic current amplitude by stabilizing the open conformation of 7.1/KCNE1 channels. However, knowledge about the exact nature of the interaction is incomplete. The aim of this study was the identification of the amino acids responsible for the interaction between Kv7.1 and PIP2. We generated 13 charge neutralizing point mutations at the intracellular membrane border and characterized them electrophysiologically in complex with KCNE1 under the influence of diC8-PIP2. Electrophysiological analysis of corresponding long QT syndrome mutants suggested impaired PIP2 regulation as the cause for channel dysfunction. To clarify the underlying structural mechanism of PIP2 binding, molecular dynamics simulations of Kv7.1/KCNE1 complexes containing two PIP2 molecules in each subunit at specific sites were performed. Here, we identified a subset of nine residues participating in the interaction of PIP2 and Kv7.1/KCNE1. These residues may form at least two binding pockets per subunit, leading to the stabilization of channel conformations upon PIP2 binding.

Cardiac potassium channel subtypes: new roles in repolarization and arrhythmia

About 10 distinct potassium channels in the heart are involved in shaping the action potential. Some of the K+ channels are primarily responsible for early repolarization, whereas others drive late repolarization and still others are open throughout the cardiac cycle. Three main K+ channels drive the late repolarization of the ventricle with some redundancy, and in atria this repolarization reserve is supplemented by the fairly atrial-specific KV1.5, Kir3, KCa, and K2P channels. The role of the latter two subtypes in atria is currently being clarified, and several findings indicate that they could constitute targets for new pharmacological treatment of atrial fibrillation. The interplay between the different K+ channel subtypes in both atria and ventricle is dynamic, and a significant up- and downregulation occurs in disease states such as atrial fibrillation or heart failure. The underlying posttranscriptional and posttranslational remodeling of the individual K+ channels changes their activity and significance relative to each other, and they must be viewed together to understand their role in keeping a stable heart rhythm, also under menacing conditions like attacks of reentry arrhythmia.

Glycine/Serine polymorphism at position 38 influences KCNE1 subunit's modulatory actions on rapid and slow delayed rectifier K+ currents

BACKGROUND

 KCNE1 encodes a modulator of KCNH2 and KCNQ1 delayed rectifier K(+) current channels. KCNE1 mutations might cause long QT syndrome (LQTS) by impairing KCNE1 subunit's modulatory actions on these channels. There are major and minor polymorphismic KCNE1 variants whose 38(th) amino acids are glycine and serine [KCNE1(38G) and KCNE1(38S) subunits], respectively. Despite its frequent occurrence, the influence of this polymorphism on the K(+) channels' function is unclear.

METHODS AND RESULTS

 Patch-clamp recordings were obtained from human embryonic kidney -293T cells. KCNH2 channel current density in KCNE1(38S)-transfected cells was smaller than that in KCNE1(38G)-transfected cells by 34%. The voltage-sensitivity of the KCNQ1 channel current in KCNE1(38S)-transfected cells was lowered compared to that in KCNE1(38G)-transfected cells, with a +13mV shift in the half-maximal activation voltage. KCNH2 channel current density or KCNQ1 channel voltage-sensitivity was not different between KCNE1(38G)-transfected cells and cells transfected with both KCNE1(38G) and KCNE1(38S). Moreover, the KCNH2 channel current in KCNE1(38S)-transfected cells was more susceptible to E4031, a QT prolonging drug and a condition with hypokalemia, than that in KCNE1(38G)-transfected cells.

CONCLUSIONS

 Homozygous inheritance of KCNE1(38S) might cause a mild reduction of the delayed rectifier K(+) currents and might thereby increase an arrhythmogenic potential particularly in the presence of QT prolonging factors. By contrast, heterozygous inheritance of KCNE1(38G) and KCNE1(38S) might not affect the K(+) currents significantly.  (Circ J 2014; 78: 610-618).

Overview and management of cardiac adverse events associated with tyrosine kinase inhibitors

BACKGROUND

Small-molecule tyrosine kinase inhibitors (TKIs) may provide an effective therapeutic option in patients with hematologic malignancies and solid tumors. However, cardiovascular (CV) events, including hypertension, heart failure, left ventricular systolic dysfunction, and QT prolongation, have emerged as potential adverse events (AEs) with TKI therapy.

PURPOSE

We review what is known about the mechanism of action of CV AEs associated with TKI use and discuss therapeutic interventions that may prevent and manage these events in clinical practice.

METHODS

References for this review were identified through searches of PubMed and Medline databases, and only papers published in English were considered. Search terms included "cardiac," "cardiovascular," "cancer," and "kinase inhibitor." Related links in the databases were reviewed, along with relevant published guidelines.

RESULTS

Although the link between rising blood pressure (BP) and CV AEs is observed but not proven, good clinical practice supports an aggressive policy on proper long-term BP management. There are insufficient data from randomized controlled clinical trials to show indisputably that aggressive or effective heart failure therapy in patients receiving TKIs will fundamentally change outcomes; however, clinical practice suggests that this is an effective long-term approach. Recognizing that QT prolongation is associated with TKI use facilitates identification of patients at high risk for this CV AE and increases awareness of the need for routine electrocardiograms and electrolyte monitoring for those receiving TKI treatment.

CONCLUSION

Regular monitoring, early recognition, and appropriate interventions for CV AEs can help more patients derive the benefit of long-term TKI therapy.

Kv7.1 ion channels require a lipid to couple voltage sensing to pore opening

Voltage-gated ion channels generate dynamic ionic currents that are vital to the physiological functions of many tissues. These proteins contain separate voltage-sensing domains, which detect changes in transmembrane voltage, and pore domains, which conduct ions. Coupling of voltage sensing and pore opening is critical to the channel function and has been modeled as a protein-protein interaction between the two domains. Here, we show that coupling in Kv7.1 channels requires the lipid phosphatidylinositol 4,5-bisphosphate (PIP2). We found that voltage-sensing domain activation failed to open the pore in the absence of PIP2. This result is due to loss of coupling because PIP2 was also required for pore opening to affect voltage-sensing domain activation. We identified a critical site for PIP2-dependent coupling at the interface between the voltage-sensing domain and the pore domain. This site is actually a conserved lipid-binding site among different K(+) channels, suggesting that lipids play an important role in coupling in many ion channels.

Determinants of incomplete penetrance and variable expressivity in heritable cardiac arrhythmia syndromes

Mutations in genes encoding ion channel pore-forming α-subunits and accessory β-subunits as well as intracellular calcium-handling proteins that collectively maintain the electromechanical function of the human heart serve as the underlying pathogenic substrate for a spectrum of sudden cardiac death (SCD)-predisposing heritable cardiac arrhythmia syndromes, including long QT syndrome (LQTS), short QT syndrome (SQTS), Brugada syndrome (BrS), and catecholaminergic polymorphic ventricular tachycardia (CPVT). Similar to many Mendelian disorders, the cardiac "channelopathies" exhibit incomplete penetrance, variable expressivity, and phenotypic overlap, whereby genotype-positive individuals within the same genetic lineage assume vastly different clinical courses as objectively assessed by phenotypic features such electrocardiographic abnormalities and number/type of cardiac events. In this Review, we summarize the current understanding of the global architecture of complex electrocardiographic traits such as the QT interval, focusing on the role of common genetic variants in the modulation of ECG parameters in health and the environmental and genetic determinants of incomplete penetrance and variable expressivity in the heritable cardiac arrhythmia syndromes most likely to be encountered in clinical practice.

Fetal Heart Rate Predictors of Long QT Syndrome

Background—

Fetal long QT syndrome (LQTS) is associated with complex arrhythmias including torsades de pointes and 2° atrioventricular block. Sinus bradycardia has also been associated with fetal LQTS, but little is known of this rhythm manifestation. Our purpose was to characterize the fetal heart rate (FHR)/gestational age (GA) profile of fetal LQTS.

Methods and Results—

We ascertained fetal LQTS subjects by family history (Group 1) or fetal arrhythmia referral (Group 2). We compared FHR in LQTS subjects versus normal fetuses. To identify FHR predictors of LQTS, we calculated a bradycardia index as % of LQTS FHR recordings either ≤110 beats per minute (obstetric standard) or ≤3 rd percentile for GA. Among 42 LQTS subjects, 26 were in Group 1 and 16 in Group 2. There were 536 normal fetuses. The bradycardia index was only 15% for FHR ≤110 beats per minute, but 66% for FHR ≤3rd percentile for GA. Ten fetuses with complex arrhythmias also had severe and sustained sinus bradycardia throughout gestation. Identifying a fetal proband in Group 2 resulted in LQTS diagnosis in 9 unsuspected members of 6 families.

Conclusions—

FHR varies by GA in both normal and LQTS fetuses. Postnatal evaluation of neonates with FHR ≤3 rd percentile for GA may improve ascertainment of LQTS in fetuses, neonates, and undiagnosed family members.

KCNE5 (KCNE1L) variants are novel modulators of Brugada syndrome and idiopathic ventricular fibrillation

BACKGROUND

Brugada syndrome (BrS) has a significantly higher incidence among the male sex. Among genes coding ion channels and their modulatory proteins, KCNE5 (KCNE1L) is located in the X chromosome and encodes an auxiliary β-subunit for K channels. KCNE5 has been shown to modify the transient outward current (I(to)), which plays a key role in determining the repolarization process in the myocardium. This study investigated whether KCNE5 mutations could be responsible for BrS and other idiopathic ventricular fibrillation (IVF).

METHODS AND RESULTS

In 205 Japanese patients with BrS or IVF who tested negative for SCN5A mutation, we conducted a genetic screen for KCNE5 variants. We identified 2 novel KCNE5 variants: p.Y81H in 3 probands and p.[D92E;E93X] in 1 proband from 4 unrelated families. Y81H was identified in 1 man and 2 women; D92E;E93X was found in a 59-year-old man. All probands received implantable cardioverter-defibrillators. Functional consequences of the KCNE5 variants were determined through biophysical assay using cotransfection with KCND3 or KCNQ1. In the experiments with KCND3, which encodes Kv4.3, I(to) was significantly increased for both KCNE5 variants compared to wild type. In contrast, there were no significant changes in current properties reconstructed by KCNQ1+ wild type KCNE5 and the 2 variants. With the simulation model, both variants demonstrated notch-and-dome or loss-of-dome patterns.

CONCLUSIONS

KCNE5 modulates I(to), and its novel variants appeared to cause IVF, especially BrS, in male patients through gain-of-function effects on I(to). Screening for KCNE5 variants is relevant for BrS or IVF.

The S4-S5 linker of KCNQ1 channels forms a structural scaffold with the S6 segment controlling gate closure

In vivo, KCNQ1 α-subunits associate with the β-subunit KCNE1 to generate the slowly activating cardiac potassium current (I(Ks)). Structurally, they share their topology with other Kv channels and consist out of six transmembrane helices (S1-S6) with the S1-S4 segments forming the voltage-sensing domain (VSD). The opening or closure of the intracellular channel gate, which localizes at the bottom of the S6 segment, is directly controlled by the movement of the VSD via an electromechanical coupling. In other Kv channels, this electromechanical coupling is realized by an interaction between the S4-S5 linker (S4S5(L)) and the C-terminal end of S6 (S6(T)). Previously we reported that substitutions for Leu(353) in S6(T) resulted in channels that failed to close completely. Closure could be incomplete because Leu(353) itself is the pore-occluding residue of the channel gate or because of a distorted electromechanical coupling. To resolve this and to address the role of S4S5(L) in KCNQ1 channel gating, we performed an alanine/tryptophan substitution scan of S4S5(L). The residues with a "high impact" on channel gating (when mutated) clustered on one side of the S4S5(L) α-helix. Hence, this side of S4S5(L) most likely contributes to the electromechanical coupling and finds its residue counterparts in S6(T). Accordingly, substitutions for Val(254) resulted in channels that were partially constitutively open and the ability to close completely was rescued by combination with substitutions for Leu(353) in S6(T). Double mutant cycle analysis supported this cross-talk indicating that both residues come in close contact and stabilize the closed state of the channel.

The rate-dependent biophysical properties of the LQT1 H258R mutant are counteracted by a dominant negative effect on channel trafficking

The long QT syndrome (LQTS) is a cardiac disorder caused by a prolonged ventricular repolarization. The co-assembly of the pore-forming human KCNQ1 alpha-subunits with the modulating hKCNE1 beta-subunits generates I(Ks)in vivo, explaining why mutations in the hKCNQ1 gene underlie the LQT1 form of congenital LQT. Here we describe the functional defects of the LQT1 mutation H258R located in the S4-S5 linker, a segment important for channel gating. Mutant subunits with this arginine substitution generated no or barely detectable currents in a homotetrameric condition, but did generate I(Ks)-like currents in association with hKCNE1. Compared to the WT hKCNQ1/hKCNE1 complex, the H258R/hKCNE1 complex displayed accelerated activation kinetics, slowed channel closure and a hyperpolarizing shift of the voltage-dependence of activation, thus predicting an increased K(+) current. However, current density analysis combined with subcellular localization indicated that the H258R subunit exerted a dominant negative effect on channel trafficking to the plasma membrane. The co-expression hKCNQ1/H258R/hKCNE1, mimicking the heterozygous state of a patient, displayed similar properties. During repetitive stimulation the mutant yielded more current compared to WT at 1 Hz but this effect was counteracted by the trafficking defect at faster frequencies. These rate-dependent effects may be relevant given the larger contribution of I(Ks) to the "repolarization reserve" at higher action potential rates. The combination of complex kinetics that counteract the trafficking problem represents a particular mechanism underlying LQT1.

Ischemic, genetic and pharmacological origins of cardiac arrhythmias: the contribution of the Quebec Heart Institute

Research in the field of basic electrophysiology at the Quebec Heart Institute (Laval Hospital, Quebec City, Quebec) has evolved since its beginning in the 1990s. Interests were focused on cardiac arrhythmias induced by drugs, allelic variants and metabolic factors produced during ischemia. The results have contributed to the creation of new standards in drug development, more specifically, testing all new drugs for their potential effects on cardiac potassium currents, which could produce life-threatening proarrhythmic effects. In a French-Canadian population, three heterozygous single nucleotide polymorphisms in hK(v)1.5, a gene encoding for a major atrial repolarizing current, were found. These variants affect the expression level of the hK(v)1.5 channel and change the inactivation process in the presence of its accessory beta subunit. Because these effects could shorten atrial action potential, their presence was tested in postcoronary bypass patients and a higher prevalence was found in patients with postoperative atrial fibrillation. Finally, three potentially proarrhythmic factors characteristic of ischemia were identified: pH decrease; oxygen free radicals, which both increase the flow of K(+) ions through human ether-a-go-go-related gene and hK(v)1.5, producing a reduction in action potential duration, frequently leading to cardiac arrhythmias; and lysophosphatidylcholine, a metabolite involved in the production of cardiac arrhythmias early during ischemia that was shown to be a major cause of electrical uncoupling. Over the past decade, the Quebec Heart Institute has provided a significant amount of original data in the field of basic cardiac electrophysiology, specifically concerning arrhythmias originating from pharmacological agents, genetic background and cardiac ischemia.

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 alpha and beta 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.

The perfect storm: Torsades de Pointes in a child with leukemia

Torsades de Pointes (TdP) is a life-threatening ventricular arrhythmia that can be associated with metabolic abnormalities, exposure to arrhythmogenic medications, and congenital long-QT syndrome. This report describes a patient with ALL and multiple complications of therapy who developed TdP. The patient had no evidence of congenital long-QT syndrome, but a constellation of factors appears to have led to QT prolongation, ventricular ectopy, and TdP. Although the patient suffered cardiac arrest, rapid recognition of TdP and prompt defibrillation resulted in an excellent outcome.

Disorders of Cardiac Repolarization Long QT and Short QT Syndromes

The long and short QT syndromes are heterogeneous diseases characterized by abnormal ventricular repolarization and episodes of syncope and/or life-threatening cardiac arrhythmias. Several disease-causing genes have been identified, including those encoding cardiac ion channel-composing proteins. The clinical determination of genotype offers a striking benefit: diagnosis, prediction of clinical phenotype, risk stratification, clinical and genetic counseling, and introduction of therapy. Genetic testing is of special importance for the genotyped patient's family members to prevent unexpected cardiac death. By means of recently advanced methodology in molecular genetics and electrophysiology it is expected that novel genes responsible for these disease entities will be identified.

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.

Role of an S4-S5 linker lysine in the trafficking of the Ca(2+)-activated K(+) channels IK1 and SK3

We have investigated the role of the S4-S5 linker in the trafficking of the intermediate (human (h) IK1) and small (rat SK3) conductance K(+) channels using a combination of patch-clamp, protein biochemical, and immunofluorescence-based techniques. We demonstrate that a lysine residue (Lys(197)) located on the intracellular loop between the S4 and S5 domains is necessary for the correct trafficking of hIK1 to the plasma membrane. Mutation of this residue to either alanine or methionine precluded trafficking of the channel to the membrane, whereas the charge-conserving arginine mutation had no effect on channel localization or function. Immunofluorescence localization demonstrated that the K197A mutation resulted in a channel that was primarily retained in the endoplasmic reticulum, and this could not be rescued by incubation at 27 degrees C. Furthermore, immunoblot analysis revealed that the K197A mutation was overexpressed compared with wild-type hIK1 and that this was due to a greatly diminished rate of channel degradation. Co-immunoprecipitation studies demonstrated that the K197A mutation did not preclude multimer formation. Indeed, the K197A mutation dramatically suppressed expression of wild-type hIK1 at the cell surface. Finally, mutation of this conserved lysine in rat SK3 similarly resulted in a channel that failed to correctly traffic to the plasma membrane. These results are the first to demonstrate a critical role for the S4-S5 linker in the trafficking and/or function of IK and SK channels.

KCNH2-K897T is a genetic modifier of latent congenital long-QT syndrome

BACKGROUND

Clinical heterogeneity among patients with long-QT syndrome (LQTS) sharing the same disease-causing mutation is usually attributed to variable penetrance. One potential explanation for this phenomenon is the coexistence of modifier gene alleles, possibly common single nucleotide polymorphisms, altering arrhythmia susceptibility. We demonstrate this concept in a family segregating a novel, low-penetrant KCNH2 mutation along with a common single nucleotide polymorphism in the same gene.

METHODS AND RESULTS

The proband is a 44-year-old white woman with palpitations associated with presyncope since age 20, who presented with ventricular fibrillation and cardiac arrest. Intermittent QT prolongation was subsequently observed (max QTc, 530 ms), and LQT2 was diagnosed after the identification of a missense KCNH2 mutation (A1116V) altering a conserved residue in the distal carboxyl-terminus of the encoded HERG protein. The proband also carried the common KCNH2 polymorphism K897T on the nonmutant allele. Relatives who carried A1116V without K897T were asymptomatic, but some exhibited transient mild QTc prolongation, suggesting latent disease. Heterologous expression studies performed in cultured mammalian cells and using bicistronic vectors linked to different fluorescent proteins demonstrated that coexpression of A1116V with K897T together resulted in significantly reduced current amplitude as compared with coexpression of either allele with WT-HERG. Thus, the presence of KCNH2-K897T is predicted to exaggerate the IKr reduction caused by the A1116V mutation. These data explain why symptomatic LQTS occurred only in the proband carrying both alleles.

CONCLUSIONS

We have provided evidence that a common KCNH2 polymorphism may modify the clinical expression of a latent LQT2 mutation. A similar mechanism may contribute to the risk for sudden death in more prevalent cardiac diseases.

Cellular and ionic mechanism for drug-induced long QT syndrome and effectiveness of verapamil

OBJECTIVES

We examined the cellular and ionic mechanism for QT prolongation and subsequent Torsade de Pointes (TdP) and the effect of verapamil under conditions mimicking KCNQ1 (I(Ks) gene) defect linked to acquired long QT syndrome (LQTS).

BACKGROUND

Agents with an I(Kr)-blocking effect often induce marked QT prolongation in patients with acquired LQTS. Previous reports demonstrated a relationship between subclinical mutations in cardiac K+ channel genes and a risk of drug-induced TdP.

METHODS

Transmembrane action potentials from epicardial (EPI), midmyocardial (M), and endocardial (ENDO) cells were simultaneously recorded, together with a transmural electrocardiogram, at a basic cycle length of 2,000 ms in arterially perfused feline left ventricular preparations.

RESULTS

The I(Kr) block (E-4031: 1 micromol/l) under control conditions (n = 5) prolonged the QT interval but neither increased transmural dispersion of repolarization (TDR) nor induced arrhythmias. However, the I(Kr) blocker under conditions with I(Ks) suppression by chromanol 293B 10 micromol/l mimicking the KCNQ1 defect (n = 10) preferentially prolonged action potential duration (APD) in EPI rather than M or ENDO, thereby dramatically increasing the QT interval and TDR. Spontaneous or epinephrine-induced early afterdepolarizations (EADs) were observed in EPI, and subsequent TdP occurred only under both I(Ks) and I(Kr) suppression. Verapamil (0.1 to 5.0 micromol/l) dose-dependently abbreviated APD in EPI more than in M and ENDO, thereby significantly decreasing the QT interval, TDR, and suppressing EADs and TdP.

CONCLUSIONS

Subclinical I(Ks) dysfunction could be a risk of drug-induced TdP. Verapamil is effective in decreasing the QT interval and TDR and in suppressing EADs, thus preventing TdP in the model of acquired LQTS.

QT prolongation with antimicrobial agents: understanding the significance

Cardiac toxicity has been relatively uncommon within the antimicrobial class of drugs, but well described for antiarrhythmic agents and certain antihistamines. Macrolides, pentamidine and certain antimalarials were traditionally known to cause QT-interval prolongation, and now azole antifungals, fluoroquinolones and ketolides can be added to the list. Over time, advances in preclinical testing methods for QT-interval prolongation and a better understanding of its sequelae, most notably torsades de pointes (TdP), have occurred. This, combined with the fact that five drugs have been removed from the market over the last several years, in part because of QT-interval prolongation-related toxicity, has elevated the urgency surrounding early detection and characterisation methods for evaluating non-antiarrhythmic drug classes. With technological advances and accumulating literature regarding QT prolongation, it is currently difficult or overwhelming for the practising clinician to interpret these data for purposes of formulary review or for individual patient treatment decisions. Certain patients are susceptible to the effects of QT-prolonging drugs. For example, co-variates such as gender, age, electrolyte derangements, structural heart disease, end organ impairment and, perhaps most important, genetic predisposition, underlie most if not all cases of TdP. Between and within classes of drugs there are important differences that contribute to delayed repolarisation (e.g. intrinsic potency to inhibit certain cardiac ion currents or channels, and pharmacokinetics). To this end, a risk stratification scheme may be useful to rank and compare the potential for cardiotoxicity of each drug. It appears that in most published cases of antimicrobial-associated TdP, multiple risk factors are present. Macrolides in general are associated with a greater potential than other antimicrobials for causing TdP from both a pharmacodynamic and pharmacokinetic perspective. The azole antifungal agents also can be viewed as drugs that must be weighed carefully before use since they also have both pharmacodynamic and pharmacokinetic characteristics that may trigger TdP. The fluoroquinolones appear less likely to be associated with TdP from a pharmacokinetic perspective since they do not rely on cytochrome P450 (CYP) metabolism nor do they inhibit CYP enzyme isoforms, with the exception of grepafloxacin and ciprofloxacin. Nonetheless, patient selection must be carefully made for all of these drugs. For clinicians, certain responsibilities are assumed when prescribing antimicrobial therapy: (i) appropriate use to minimise resistance; and (ii) appropriate patient and drug selection to minimise adverse event potential. Incorporating information learned regarding QT interval-related adverse effects into the drug selection process may serve to minimise collateral iatrogenic toxicity.

Probucol aggravates long QT syndrome associated with a novel missense mutation M124T in the N-terminus of HERG

Patients with LQTS (long QT syndrome) with a mutation in a cardiac ion channel gene, leading to mild-to-moderate channel dysfunction, may manifest marked QT prolongation or torsade de pointes only upon an additional stressor. A 59-year-old woman had marked QT prolongation and repeated torsade de pointes 3 months after initiation of probucol, a cholesterol-lowering drug. We identified a single base substitution in the HERG gene by genetic analysis. This novel missense mutation is predicted to cause an amino acid substitution of Met(124)-->Thr (M124T) in the N-terminus. Three other relatives with this mutation also had QT prolongation and one of them had a prolonged QT interval and torsade de pointes accompanied by syncope after taking probucol. We expressed wild-type HERG and HERG with M124T in Xenopus oocytes and characterized the electrophysiological properties of these HERG channels and the action of probucol on the channels. Injection of the M124T mutant cRNA into Xenopus oocytes resulted in expression of functional channels with markedly smaller amplitude. In both HERG channels, probucol decreased the amplitude of the HERG tail current, decelerated the rate of channel activation, accelerated the rate of channel deactivation and shifted the reversal potential to a more positive value. The electrophysiological study indicated that QT lengthening and cardiac arrhythmia in the two present patients were due to inhibition of I(Kr) (rapidly activating delayed rectifier K(+) current) by probucol, in addition to the significant suppression of HERG current in HERG channels with the M124T mutation.

Mutation site-specific differences in arrhythmic risk and sensitivity to sympathetic stimulation in the LQT1 form of congenital long QT syndrome: multicenter study in Japan

OBJECTIVES

We sought to compare the arrhythmic risk and sensitivity to sympathetic stimulation of mutations located in transmembrane regions and C-terminal regions of the KCNQ1 channel in the LQT1 form of congenital long QT syndrome (LQTS).

BACKGROUND

The LQT1 syndrome is frequently manifested with variable expressivity and incomplete penetrance and is much more sensitive to sympathetic stimulation than the other forms.

METHODS

Sixty-six LQT1 patients (27 families) with a total of 19 transmembrane mutations and 29 patients (10 families) with 8 C-terminal mutations were enrolled from five Japanese institutes.

RESULTS

Patients with transmembrane mutations were more frequently affected based on electrocardiographic (ECG) diagnostic criteria (82% vs. 24%, p < 0.0001) and had more frequent LQTS-related cardiac events (all cardiac events: 55% vs. 21%, p = 0.002; syncope: 55% vs. 21%, p = 0.002; aborted cardiac arrest or unexpected sudden cardiac death: 15% vs. 0%, p = 0.03) than those with C-terminal mutations. Patients with transmembrane mutations had a greater risk of first cardiac events occurring at an earlier age, with a hazard ratio of 3.4 (p = 0.006) and with an 8% increase in risk per 10-ms increase in corrected Q-Tend. The baseline ECG parameters, including Q-Tend, Q-Tpeak, and Tpeak-end intervals, were significantly greater in patients with transmembrane mutations than in those with C-terminal mutations (p < 0.005). Moreover, the corrected Q-Tend and Tpeak-end were more prominently increased with exercise in patients with transmembrane mutations (p < 0.005).

CONCLUSIONS

In this multicenter Japanese population, LQT1 patients with transmembrane mutations are at higher risk of congenital LQTS-related cardiac events and have greater sensitivity to sympathetic stimulation, as compared with patients with C-terminal mutations.

Congenital and Acquired Long QT Syndrome

Congenital long QT syndrome (LQTS) is a rare but potentially lethal disease, characterized by prolongation of QT interval, recurrent syncope, and sudden death. In the pregenomic era (1959-1991), sympathetic imbalance was thought to be responsible for this disease. Since 1991 (postgenomic era), 7 LQTS genes have been discovered and more than 300 mutations have been identified to account for approximately 70% of patients affected. Despite the advancement in molecular genetic knowledge, diagnosis of congenital LQTS is still based on electrocardiographic and clinical characteristics. Beta-blockers remain the mainstay treatment. For high-risk patients, the implantable cardioverter-defibrillator (ICD) offer an effective therapeutic option to reduce mortality. Gene-based specific therapy is still preliminary. Further studies are required to investigate new strategies for targeting the defective genes or mutant channels. For acquired LQTS, it is generally believed that the main issue is the blockade of the slow component of the delayed rectifier K+ current (IKr). These IKr blockers have a "reverse frequency-dependent" effect on the QTc interval and increase the dispersion in repolarization. In the presence of risk factors such as female gender, slow heart rate, and hypokalemia, these IKr blockers have a high propensity to induce torsades de pointes. For patients with a history of drug-induced LQTS, care must be taken to avoid further exposure to QT-prolonging drugs or conditions. Molecular genetic analysis could be useful to unravel subclinical mutations or polymorphisms. Physicians not only need to be aware of the pharmacodynamic and pharmacokinetic interactions of various important drugs, but also need to update their knowledge.

Risk stratification and management of sudden cardiac death: a new paradigm

Risk Stratification and Management of SCD. Management of SCD is undergoing radical change in direction. It is becoming increasingly appreciated that besides depressed left ventricular systolic function and the conventional risk stratification tools, new markers for plaque vulnerability, enhanced thrombogenesis, specific genetic alterations of the autonomic nervous system, cardiac sarcolemmal and contractile proteins, and familial clustering may better segregate patients with atherosclerotic coronary artery disease who are at high risk for SCD from those who may suffer from nonfatal ischemic events. Better understanding of pathophysiologic processes, such as postmyocardial infarction remodeling, the transition from compensated hypertrophy to heart failure, and the increased cardiovascular risk of coronary artery disease in the presence of diabetes or even a prediabetic state will help to improve both risk stratification and management. The rapidly developing fields of microchips technology and proteomics may allow rapid and cost-effective mass screening of multiple risk factors for SCD. The ultimate goal is to identify novel methods for risk stratification, risk modification, and prevention of SCD that could be applied to the general public at large.

Additional Gene Variants Reduce Effectiveness of Beta-Blockers in the LQT1 Form of Long QT Syndrome

INTRODUCTION

Beta-blockers are widely used to prevent the lethal cardiac events associated with the long QT syndrome (LQTS), especially in KCNQ1-related LQTS (LQT1) patients. Some LQT1 patients, however, are refractory to this therapy.

METHODS AND RESULTS

Eighteen symptomatic LQTS patients (12 families) were genetically diagnosed as having heterozygous KCNQ1 variants and received beta-blocker therapy. Cardiac events recurred in 4 members (3 families) despite continued therapy during mean follow-up of 70 months. Three of these patients (2 families) had the same mutation [A341V (KCNQ1)]; and the other had R243H (KCNQ1). The latter patient took aprindine, which seemed to be responsible for the event. By functional assay using a heterologous mammalian expression system, we found that A341V (KCNQ1) is a loss-of-function type mutation (not dominant negative). Further genetic screening revealed that one A341V (KCNQ1) family cosegregated with S706C (KCNH2) and another with G144S (KCNJ2). Functional assay of the S706C (KCNH2) mutation was found to reduce the current density of expressed heterozygous KCNH2 channels with a positive shift (+8 mV) of the activation curve. Action potential simulation study was conducted based on the KYOTO model to estimate the influence of additional gene modifiers. In both models mimicking LQT1 plus 2 and LQT1 plus 7, the incidence of early afterdepolarization was increased compared with the LQT1 model under the setting of beta-adrenergic stimulation.

CONCLUSION

Multiple mutations in different LQTS-related genes may modify clinical characteristics. Expanded gene survey may be required in LQT1 patients who are resistant to beta-blocker therapy.

Genetic basis of drug-induced arrhythmias

Drug-induced torsade de pointes arrhythmia (TdP) is frequently seen in patients. This proarrhythmia is not restricted to anti-arrhythmics but includes a variety of drugs. A genetic predisposition is an attractive explanation for this clinical problem. In this review, we: 1) explain the arrhythmogenic mechanisms of TdP, 2) provide data for a genetic cause based upon mutations in the long QT or in cytochrome genes responsible for drug metabolism, and 3) present pathology-based electrical remodeling as an alternative explanation. It can be concluded that the current evidence for a genetic basis for drug-induced TdP is weak.

Structural and functional basis for the long QT syndrome: relevance to veterinary patients

Long QT syndrome (LQTS) is a condition characterized by prolongation of ventricular repolarization and is manifested clinically by lengthening of the QT interval on the surface ECG. Whereas inherited forms of LQTS associated with mutations in the genes that encode ion channel proteins are identified only in humans, the acquired form of LQTS occurs in humans and companion animal species. Often, acquired LQTS is associated with drug-induced block of the cardiac K+ current designated I(Kr). However, not all drugs that induce potentially fatal ventricular arrhythmias antagonize I(Kr), and not all drugs that block I(Kr), are associated with ventricular arrhythmias. In clinical practice, the extent of QT interval prolongation and risk of ventricular arrhythmia associated with antagonism of I(Kr) are modulated by pharmacokinetic and pharmacodynamic variables. Veterinarians can influence some of the potential risk factors (eg, drug dosage, route of drug administration, presence or absence of concurrent drug therapy, and patient electrolyte status) but not all (eg, patient gender/genetic background). Veterinarians need to be aware of the potential for acquired LQTS during therapy with drugs identified as blockers of HERG channels and I(Kr).

Inherited arrhythmic disorders in Japan

The clinical and genetic characteristics of inherited arrhythmic disorders in Japan are briefly summarized. The incidence of hereditary long QT syndrome (LQTS) in Japan seems comparable to that in western countries. The genotypes are mainly LQT1 and LQT2; LQT3 and other types are rare. Mutations found in Japanese LQTS families are mostly novel compared to mutations reported in other countries and in different ethnic populations. Functional assays of the mutants in heterologous expression systems have disclosed novel mechanisms of current suppression in LQT1 and LQT2, and of gain of function in LQT3. Mutations in KCNJ2 may provide a new genotype (LQT7) of LQTS. In addition, mutations or single nucleotide polymorphisms in the channel genes responsible for LQTS (KvLQT1, HERG, and SCN5A) may predispose to drug-induced LQTS. A relatively high prevalence of Brugada syndrome is suspected in the Japanese population, and 1 of approximately 2,000 asymptomatic individuals present Brugada-type ECG changes upon annual examination. Genetic screening of the symptomatic Brugada syndrome and suspected cases has revealed SCN5A mutations in only approximately 12%. Therefore, the genetic basis of the majority of cases is not known. The expressed Na+ current of SCN5A mutant channels showed the phenotype of decreased channel function commonly seen in Brugada mutations. A case of idiopathic ventricular fibrillation was found to have a novel mutation in SCN5A, in which the expressed current showed marked suppression of channel function.

Risk of proarrhythmia with class III antiarrhythmic agents: sex-based differences and other issues

Although men have a higher risk of atrial fibrillation compared with women, the absolute number of women with atrial fibrillation is greater. Congestive heart failure increases the risk of developing atrial fibrillation in women more than in men, and the prognosis for women with atrial fibrillation is worse than for men. The longer baseline corrected QT interval in women is well known. The mechanism is likely the result of increased circulating androgens, causing the QT interval to shorten in men after puberty. Female sex is associated with an increased risk of torsades de pointes in the setting of potassium antagonists. Class III antiarrhythmic drugs are frequently used for the treatment of atrial fibrillation in heart failure patients because of their neutral effect on mortality and their tolerance by patients with low ejection fractions. Although amiodarone and azimilide carry a low potential for producing torsades de pointes compared with sotalol and dofetilide, the prevalence of torsades de pointes in women is at least twice that in men for all these drugs. Careful monitoring of the QT interval and potassium level, as well as control of congestive heart failure, can help reduce the risk of proarrhythmia. Avoidance of polypharmacy with other potassium antagonists and unmonitored drug formulation changes are important in the management of all patients taking class III agents, but they are particularly crucial in women with additional risk factors for torsades de pointes.

DHPLC analysis of potassium ion channel genes in congenital long QT syndrome

Congenital long QT syndrome (LQTS) is electrocardiographically characterized by a prolonged QT interval and polymorphic ventricular arrhythmias (torsade de pointes). As a result of these arrhythmias, patients suffer from recurrent syncopes, seizures, or sudden death as the most dramatic event. Mutations in five genes, encoding cardiac ion channels, have been identified in LQTS. Two potassium-channel genes, KCNQ1 (LQT1) and KCNH2 (LQT2 or HERG), are frequently involved in LQTS. Potassium-channel defects account for approximately 50-60% of LQTS. As patients benefit from preventive medication, early detection of a genetic defect is desired to identify the family members at risk. Speed and sensitivity of mutation detection was improved by applying the denaturing high performance liquid chromatography (DHPLC) technique for analysis of the entire KCNQ1 and KCNH2 genes and the protein encoding part of the KCNE1 and KCNE2 genes. By using this methodology, seven missense mutations in the KCNQ1 gene and nine mutations (four missense, two nonsense, one insertion, and two deletions) in the KCNH2 gene have been identified in a total number of 32 index patients diagnosed with LQTS syndrome. We conclude that this method is suitable for rapid identification of LQT gene defects due to the combination of automation, high throughput, sensitivity, and short time of analysis.

J wave and ST segment elevation in the inferior leads: a latent type of variant Brugada syndrome?

In a patient referred for the evaluation of non-sustained monomorphic ventricular tachycardia on Holter recordings, ventricular fibrillation was electrically induced during electrophysiologic study. Despite the absence of structural heart diseases, his ECG revealed J wave and ST segment elevation in the inferior leads, which showed circadian variation and were augmented by the sodium channel blocker, pilsicainide. This case might lead us to notice a new concept, a 'latent' type of variant Brugada syndrome, and these ECG findings and changes might serve as its diagnostic sign.