The N588K-HERG K+ channel mutation in the ‘short QT syndrome’: Mechanism of gain-in-function determined at 37°C

The Therapeutic Potential of hERG1 K+ Channels for Treating Cancer and Cardiac Arrhythmias

hERG potassium channels present pharmacologists and medicinal chemists with a dilemma. On the one hand hERG is a major reason for drugs being withdrawn from the market because of drug induced long QT syndrome and the associated risk of inducing sudden cardiac death, and yet hERG blockers are still widely used in the clinic to treat cardiac arrhythmias. Moreover, in the last decade overwhelming evidence has been provided that hERG channels are aberrantly expressed in cancer cells and that they contribute to tumour cell proliferation, resistance to apoptosis, and neoangiogenesis. Here we provide an overview of the properties of hERG channels and their role in excitable cells of the heart and nervous system as well as in cancer. We consider the therapeutic potential of hERG, not only with regard to the negative impact due to drug induced long QT syndrome, but also its future potential as a treatment in the fight against cancer.

Ranolazine inhibition of hERG potassium channels: drug-pore interactions and reduced potency against inactivation mutants

The antianginal drug ranolazine, which combines inhibitory actions on rapid and sustained sodium currents with inhibition of the hERG/I_Kr potassium channel, shows promise as an antiarrhythmic agent. This study investigated the structural basis of hERG block by ranolazine, with lidocaine used as a low potency, structurally similar comparator. Recordings of hERG current (I_hERG) were made from cell lines expressing wild-type (WT) or mutant hERG channels. Docking simulations were performed using homology models built on MthK and KvAP templates. In conventional voltage clamp, ranolazine inhibited I_hERG with an IC₅₀ of 8.03 μM; peak I_hERG during ventricular action potential clamp was inhibited ~ 62% at 10 μM. The IC₅₀ values for ranolazine inhibition of the S620T inactivation deficient and N588K attenuated inactivation mutants were respectively ~ 73-fold and ~ 15-fold that for WT I_hERG. Mutations near the bottom of the selectivity filter (V625A, S624A, T623A) exhibited IC₅₀s between ~ 8 and 19-fold that for WT I_hERG, whilst the Y652A and F656A S6 mutations had IC₅₀s ~ 22-fold and 53-fold WT controls. Low potency lidocaine was comparatively insensitive to both pore helix and S6 mutations, but was sensitive to direction of K⁺ flux and particularly to loss of inactivation, with an IC₅₀ for S620T-hERG ~ 49-fold that for WT I_hERG. Docking simulations indicated that the larger size of ranolazine gives it potential for a greater range of interactions with hERG pore side chains compared to lidocaine, in particular enabling interaction of its two aromatic groups with side chains of both Y652 and F656. The N588K mutation is responsible for the SQT1 variant of short QT syndrome and our data suggest that ranolazine is unlikely to be effective against I_Kr/hERG in SQT1 patients.

•

hERG K⁺ channels regulate cardiac action potential repolarization.

•

The molecular basis of hERG block by ranolazine and structurally related lidocaine was studied.

•

S6 Y652A and F656A mutations affected greatly ranolazine but not lidocaine binding.

•

T623 and S624 residues may directly interact with ranolazine but not lidocaine.

•

N588K and S620T attenuated inactivation mutants had reduced sensitivity to both drugs.

Modification by KCNE1 variants of the hERG potassium channel response to premature stimulation and to pharmacological inhibition

human Ether-à-go-go-Related Gene (hERG) encodes the pore-forming subunit of cardiac rapid delayed rectifier K(+) current (I Kr) channels, which play important roles in ventricular repolarization, in protecting the myocardium from unwanted premature stimuli, and in drug-induced Long QT Syndrome (LQTS). KCNE1, a small transmembrane protein, can coassemble with hERG. However, it is not known how KCNE1 variants influence the channel's response to premature stimuli or if they influence the sensitivity of hERG to pharmacological inhibition. Accordingly, whole-cell patch-clamp measurements of hERG current (I hERG) were made at 37°C from hERG channels coexpressed with either wild-type (WT) KCNE1 or with one of three KCNE1 variants (A8V, D76N, and D85N). Under both conventional voltage clamp and ventricular action potential (AP) clamp, the amplitude of I hERG was smaller for A8V, D76N, and D85N KCNE1 + hERG than for WT KCNE1 + hERG. Using paired AP commands, with the second AP waveform applied at varying time intervals following the first to mimic premature ventricular excitation, the response of I hERG carried by each KCNE1 variant was reduced compared to that with WT KCNE1 + hERG. The I hERG blocking potency of the antiarrhythmic drug quinidine was similar between WT KCNE1 and the three KCNE1 variants. However, the I hERG inhibitory potency of the antibiotic clarithromycin and of the prokinetic drug cisapride was altered by KCNE1 variants. These results demonstrate that naturally occurring KCNE1 variants can reduce the response of hERG channels to premature excitation and also alter the sensitivity of hERG channels to inhibition by some drugs linked to acquired LQTS.

In vitro-in vivo extrapolation of drug-induced proarrhythmia predictions at the population level

Drug cardiotoxicity is a serious issue for patients, regulators, pharmaceutical companies and health service payers because they are all affected by its consequences. Despite the wide range of data they generate, existing approaches for cardiac safety testing might not be adequate and sufficiently cost-effective, probably as a result of the complexity of the problem. For this reason, translational tools (based on biophysically detailed, mathematical models) allowing for in vitro-in vivo extrapolation are gaining increasing interest. This current review describes approaches that can be used for cardiac safety assessment at the population level, by accounting for various sources of variability including kinetics of the compound of interest.

Pharmacologic Approach to Defective Protein Trafficking in the E637K-hERG Mutant with PD-118057 and Thapsigargin

Background

Treatment of LQT2 is inadequate. Many drugs which can pharmacologically rescue defective protein trafficking in LQT2 also result in potent blockade of HERG current, negating their therapeutic benefit. It is reported that PD-118057 and thapsigargin can rescue LQT2 without hERG channel blockade, but the precise mechanism of action is unknown. Furthermore, the effect of PD-118057 and thapsigargin on the dominant negative E637K-hERG mutant has not been previously investigated.

Objective

In this study, we investigated: (a) the effect of PD-118057 and thapsigargin on the current amplitudes of WT-hERG and WT/E637K-hERG channels; (b) the effect of PD-118057 and thapsigargin on the biophysical properties of WT-hERG and WT/E637K-hERG channels; (c) whether drug treatment can rescue channel processing and trafficking defects of the WT/E637K-hERG mutant.

Methods

The whole-cell Patch-clamp technique was used to assess the effect of PD-118057 and thapsigargin on the electrophysiological characteristics of the rapidly activating delayed rectifier K⁺ current (I_kr) of the hERG protein channel. Western blot was done to investigate pharmacological rescue on hERG protein channel function.

Results

In our study, PD-118057 was shown to significantly enhance both the maximum current amplitude and tail current amplitude, but did not alter the gating and kinetic properties of the WT-hERG channel, with the exception of accelerating steady-state inactivation. Additionally, thapsigargin shows a similar result as PD-118057 for the WT-hERG channel, but with the exception of attenuating steady-state inactivation. However, for the WT/E637K-hERG channel, PD-118057 had no effect on either the current or on the gating and kinetic properties. Furthermore, thapsigargin treatment did not alter the current or the gating and kinetic properties of the WT/E637K-hERG channel, with the exception of opening at more positive voltages.

Conclusion

Our findings illustrate that neither PD-118057 nor thapsigargin play a role in correcting the dominant-negative effect of the E637K-hERG mutant.

Action potential clamp and pharmacology of the variant 1 Short QT Syndrome T618I hERG K⁺ channel

BACKGROUND

The familial Short QT Syndrome (SQTS) is associated with an increased risk of cardiac arrhythmia and sudden death. Gain-of-function mutations in the hERG K(+) channel protein have been linked to variant 1 of the SQTS. A hERG channel pore (T618I) mutation has recently been identified in families with heritable SQTS. This study aimed to determine effects of the T618I-hERG mutation on (i) hERG current (I(hERG)) elicited by ventricular action potentials; (ii) the sensitivity of I(hERG) to inhibition by four clinically used antiarrhythmic drugs.

METHODS

Electrophysiological recordings of I(hERG) were made at 37°C from HEK 293 cells expressing wild-type (WT) or T618I hERG. Whole-cell patch clamp recording was performed using both conventional voltage clamp and ventricular action potential (AP) clamp methods.

RESULTS

Under conventional voltage-clamp, WT I(hERG) peaked at 0-+10 mV, whilst for T618I I(hERG) maximal current was right-ward shifted to ∼ +40 mV. Voltage-dependent activation and inactivation of T618I I(hERG) were positively shifted (respectively by +15 and ∼ +25 mV) compared to WT I(hERG). The I(hERG) 'window' was increased for T618I compared to WT hERG. Under ventricular AP clamp, maximal repolarising WT I(hERG) occurred at ∼ -30 mV, whilst for T618I hERG peak I(hERG) occurred earlier during AP repolarisation, at ∼ +5 mV. Under conventional voltage clamp, half-maximal inhibitory concentrations (IC(50)) for inhibition of I(hERG) tails by quinidine, disopyramide, D-sotalol and flecainide for T618I hERG ranged between 1.4 and 3.2 fold that for WT hERG. Under action potential voltage clamp, T618I IC(50)s ranged from 1.2 to 2.0 fold the corresponding IC(50) values for WT hERG.

CONCLUSIONS

The T618I mutation produces a more modest effect on repolarising I(hERG) than reported previously for the N588K-hERG variant 1 SQTS mutation. All drugs studied here appear substantially to retain their ability to inhibit I(hERG) in the setting of the SQTS-linked T618I mutation.

RNA interference targeting E637K mutation rescues hERG channel currents and restores its kinetic properties

BACKGROUND

Long QT syndrome (LQTS) is a monogenic proarrhythmic disorder that predisposes affected individuals to sudden death from tachyarrhythmia. As an inherited disease, LQTS cannot be completely cured by conventional treatment modalities. Individualized gene therapy is a promising therapeutic approach.

OBJECTIVE

The purpose of this study was to investigate the role of small interference RNA (siRNA) on expression of E637K-hERG (human ether-a-go-go-related gene) mutant and whether it can be used to rescue the mutant's dominant-negative suppressive effects on hERG protein channel function.

METHODS

Western blot was performed to select the most sensitive siRNAs to target E637K-hERG mutant knockdown. Confocal laser scanning microscope was performed to monitor cellular localization of wild-type (WT)-hERG and E637K-hERG with or without siRNA. Patch-clamp technique was used to assess the effect of siRNA on the electrophysiologic characteristics of the rapidly activating delayed rectifier K(+) current I(Kr) of the hERG protein channel.

RESULTS

siRNA led to a significant decrease in the level of E637K-hERG protein but did not affect the level of WT-hERG protein. WT-hERG localization in cells coexpressing E637K-hERG mutant was restored to the membrane by siRNA. The siRNA-mediated inhibition of E637K-hERG mutant restored the maximum current and tail current amplitudes. Furthermore, siRNA treatment rescued the kinetic properties of WT/E637K-hERG protein channel to a level comparable to that of WT-hERG protein channel.

CONCLUSION

Our findings illustrated that siRNA can effectively inhibit E637K-hERG protein expression and rescue the dominant-negative effect of this mutation by restoring the kinetic properties of hERG protein channel. It has potential clinical implications with regard to the possibility of using siRNA in the treatment of LQTS.

High potency inhibition of hERG potassium channels by the sodium-calcium exchange inhibitor KB-R7943

BACKGROUND AND PURPOSE

KB-R7943 is an isothiourea derivative that is used widely as a pharmacological inhibitor of sodium-calcium exchange (NCX) in experiments on cardiac and other tissue types. This study investigated KB-R7943 inhibition of hERG (human ether-à-go-go-related gene) K(+) channels that underpin the cardiac rapid delayed rectifier potassium current, I(Kr) .

EXPERIMENTAL APPROACH

Whole-cell patch-clamp measurements were made of hERG current (I(hERG) ) carried by wild-type or mutant hERG channels and of native rabbit ventricular I(Kr) . Docking simulations utilized a hERG homology model built on a MthK-based template.

KEY RESULTS

KB-R7943 inhibited both I(hERG) and native I(Kr) rapidly on membrane depolarization with IC(50) values of ∼89 and ∼120 nM, respectively, for current tails at -40 mV following depolarizing voltage commands to +20 mV. Marked I(hERG) inhibition also occurred under ventricular action potential voltage clamp. I(hERG) inhibition by KB-R7943 exhibited both time- and voltage-dependence but showed no preference for inactivated over activated channels. Results of alanine mutagenesis and docking simulations indicate that KB-R7943 can bind to a pocket formed of the side chains of aromatic residues Y652 and F656, with the compound's nitrobenzyl group orientated towards the cytoplasmic side of the channel pore. The structurally related NCX inhibitor SN-6 also inhibited I(hERG) , but with a markedly reduced potency.

CONCLUSIONS AND IMPLICATIONS

KB-R7943 inhibits I(hERG) /I(Kr) with a potency that exceeds that reported previously for acute cardiac NCX inhibition. Our results also support the feasibility of benzyloxyphenyl-containing NCX inhibitors with reduced potential, in comparison with KB-R7943, to inhibit hERG.

Computational modeling of the human atrial anatomy and electrophysiology

This review article gives a comprehensive survey of the progress made in computational modeling of the human atria during the last 10 years. Modeling the anatomy has emerged from simple "peanut"-like structures to very detailed models including atrial wall and fiber direction. Electrophysiological models started with just two cellular models in 1998. Today, five models exist considering e.g. details of intracellular compartments and atrial heterogeneity. On the pathological side, modeling atrial remodeling and fibrotic tissue are the other important aspects. The bridge to data that are measured in the catheter laboratory and on the body surface (ECG) is under construction. Every measurement can be used either for model personalization or for validation. Potential clinical applications are briefly outlined and future research perspectives are suggested.

Acute alteration of cardiac ECG, action potential, I(Kr) and the human ether-a-go-go-related gene (hERG) K+ channel by PCB 126 and PCB 77

Polychlorinated biphenyls (PCBs) have been known as serious persistent organic pollutants (POPs), causing developmental delays and motor dysfunction. We have investigated the effects of two PCB congeners, 3,3',4,4'-tetrachlorobiphenyl (PCB 77) and 3,3',4,4',5-pentachlorobiphenyl (PCB 126) on ECG, action potential, and the rapidly activating delayed rectifier K+ current (I(Kr)) of guinea pigs' hearts, and hERG K+ current expressed in Xenopus oocytes. PCB 126 shortened the corrected QT interval (QTc) of ECG and decreased the action potential duration at 90% (APD(90)), and 50% of repolarization (APD₅₀) (P<0.05) without changing the action potential duration at 20% (APD₂₀). PCB 77 decreased APD₂₀ (P<0.05) without affecting QTc, APD₉₀, and APD₅₀. The PCB 126 increased the I(Kr) in guinea-pig ventricular myocytes held at 36°C and hERG K+ current amplitude at the end of the voltage steps in voltage-dependent mode (P<0.05); however, PCB 77 did not change the hERG K+ current amplitude. The PCB 77 increased the diastolic Ca²⁺ and decreased Ca²⁺ transient amplitude (P<0.05), however PCB 126 did not change. The results suggest that PCB 126 shortened the QTc and decreased the APD₉₀ possibly by increasing I(Kr), while PCB 77 decreased the APD₂₀ possibly by other modulation related with intracellular Ca²⁺. The present data indicate that the environmental toxicants, PCBs, can acutely affect cardiac electrophysiology including ECG, action potential, intracellular Ca²⁺, and channel activity, resulting in toxic effects on the cardiac function in view of the possible accumulation of the PCBs in human body.

Increased vulnerability of human ventricle to re-entrant excitation in hERG-linked variant 1 short QT syndrome

The short QT syndrome (SQTS) is a genetically heterogeneous condition characterized by abbreviated QT intervals and an increased susceptibility to arrhythmia and sudden death. This simulation study identifies arrhythmogenic mechanisms in the rapid-delayed rectifier K⁺ current (I_Kr)-linked SQT1 variant of the SQTS. Markov chain (MC) models were found to be superior to Hodgkin-Huxley (HH) models in reproducing experimental data regarding effects of the N588K mutation on KCNH2-encoded hERG. These ionic channel models were then incorporated into human ventricular action potential (AP) models and into 1D and 2D idealised and realistic transmural ventricular tissue simulations and into a 3D anatomical model. In single cell models, the N588K mutation abbreviated ventricular cell AP duration at 90% repolarization (APD₉₀) and decreased the maximal transmural voltage heterogeneity (δV) during APs. This resulted in decreased transmural heterogeneity of APD₉₀ and of the effective refractory period (ERP): effects that are anticipated to be anti-arrhythmic rather than pro-arrhythmic. However, with consideration of transmural heterogeneity of I_Kr density in the intact tissue model based on the ten Tusscher-Noble-Noble-Panfilov ventricular model, not only did the N588K mutation lead to QT-shortening and increases in T-wave amplitude, but δV was found to be augmented in some local regions of ventricle tissue, resulting in increased tissue vulnerability for uni-directional conduction block and predisposing to formation of re-entrant excitation waves. In 2D and 3D tissue models, the N588K mutation facilitated and maintained re-entrant excitation waves due to the reduced substrate size necessary for sustaining re-entry. Thus, in SQT1 the N588K-hERG mutation facilitates initiation and maintenance of ventricular re-entry, increasing the lifespan of re-entrant spiral waves and the stability of scroll waves in 3D tissue.

Sudden cardiac death may arise in individuals with diseased heart tissue, or in apparently healthy subjects who suffer from genetic defects in ‘ion channel’ proteins, which increase cardiac arrhythmia risk and are associated with significant morbidity and mortality. One rare, though serious, genetic condition is the ‘short QT syndrome’ (SQTS). Although it is now known that the KCNH2-encoded N588K-hERG mutation is associated with the main (SQT1) variant of the SQTS, the mechanisms by which ventricular arrhythmia is initiated and sustained are still unclear due to lack of genotypically accurate experimental models. In this study, we used sophisticated multi-scale computer models of human ventricles in order to investigate the pro-arrhythmic effects of the N588K hERG mutation. It was found that the mutation accelerated the ventricular repolarization process, produced augmented electrical heterogeneity in some local regions of the tissue, leading to increased risk of arrhythmia genesis. It was also found that accelerated ventricular repolarization reduced the substrate size of the tissue required to sustain re-entrant circuits in both two and three dimensions. This study provides new mechanistic insight into understanding of how changes to hERG channel function in SQT1 lead to exacerbated ventricular arrhythmia risk in this inherited arrhythmia syndrome.

Molecular determinants of hERG potassium channel inhibition by disopyramide

The Class Ia antiarrhythmic drug disopyramide (DISO) causes QT interval prolongation that is potentially dangerous in acquired Long QT Syndrome but beneficial in short QT syndrome, through inhibition of the hERG-encoded channels responsible for rapid delayed rectifier K(+) current (I(Kr)). In this study, alanine mutants of hERG S6 and pore helix residues and MthK-based homology modelling and ligand docking were used to investigate molecular determinants of DISO binding to hERG. Whole-cell hERG current (I(hERG)) recordings were made at 37°C from HEK-293 cells expressing WT or mutant hERG channels. WT outward I(hERG) tails were inhibited with an IC(50) of 7.3μM, whilst inward I(hERG) tails in a high [K(+)](e) of 94mM were blocked with an IC(50) of 25.7μM. The IC(50) for the Y652A mutation was ~55-fold that of WT I(hERG); this mutation also abolished a leftward shift in voltage-dependent I(hERG) activation present for WT hERG. The IC(50) for F656A I(hERG) was ~51 fold its corresponding WT control. In contrast to previously studied methanesulphonanilide hERG inhibitors, neither the G648A S6 nor the T623A and S624A pore helical mutations modified DISO IC(50). Computational docking with the hERG model showed that DISO did not exhibit a single unique binding pose; instead several low energy binding poses at the lower end of the pore cavity favoured interactions with Y652 and F656. In the WT hERG model DISO did not interact directly with residues at the base of the pore helix, consistent with the minimal effect of mutation of these residues on drug block.

The hERG K(+) channel S4 domain L532P mutation: characterization at 37°C

hERG (human Ether-à-go-go Related Gene) is responsible for ion channels mediating rapid delayed rectifier potassium current, I_Kr, which is key to cardiac action potential repolarization. Gain-of-function hERG mutations give rise to the SQT1 variant of the Short QT Syndrome (SQTS). Reggae mutant zebrafish, with a S4 zERG mutation (Leucine499Proline; L499P), display arrhythmic features analogous to those seen in the SQTS. The affected S4 domain ERG residue is highly conserved. This study was executed to determine how the homologous hERG mutation (L532P) influences channel function at 37 °C. Whole-cell measurements of current (I_hERG) were made from HEK 293 cells expressing WT or L532P hERG. The half maximal activation voltage (V_0.5) of L532P I_hERG was positively shifted by ~+36 mV compared to WT I_hERG; however at negative voltages a pronounced L532P I_hERG was observed. Both activation and deactivation time-courses were accelerated for L532P I_hERG. The inactivation V_0.5 for L532P I_hERG was shifted by ~+32 mV. Under action potential (AP) voltage-clamp, L532P I_hERG exhibited a dome-shaped current peaking at ~+16 mV, compared to ~−31 mV for WT-I_hERG. The L532P mutation produced an ~ 5-fold increase in the IC₅₀ for dronedarone inhibition of I_hERG. Homology modeling indicated that the L532 residue within the S4 helix lies closely apposed to the S5 region of an adjacent hERG subunit. Alterations to the S4 domain structure and, potentially, to interactions between adjacent hERG subunits are likely to account for the functional effects of this mutation.

Novel characteristics of a trafficking-defective G572R-hERG channel linked to hereditary long QT syndrome

BACKGROUND

The congenital long QT syndrome is a heterogeneous genetic disease associated with delayed cardiac repolarization, prolonged QT intervals, the development of ventricular arrhythmias and sudden death. Type 2 congenital long QT syndrome (LQT2) results from KCNH2 or hERG gene mutations. hERG encodes the K(v)11.1 alpha subunit of the rapidly activating delayed rectifier K(+) current in the heart. Studies of mutant hERG channels indicate that most LQT2 missense mutations generate trafficking-deficient K(v)11.1 channels.

OBJECTIVE

To identify the mechanism underlying G572R-hERG by using molecular and electrophysiological analyses.

METHODS AND RESULTS

To elucidate the electrophysiological properties of the G572R-hERG mutant channels, mutant hERG subunits were heterologously expressed in HEK293 cells alone or in combination with wild-type (WT)-hERG subunits. Patch-clamp techniques were used to record currents, and double immunofluorescence protein tagging and Western blotting were performed to examine the cellular trafficking of mutant subunits. When expressed alone, G572R-hERG subunits were not present in the cell membrane and did not produce detectable currents. When coexpressed with WT-hERG subunits, G572R-hERG decreased current density and altered gating properties of the WT-hERG channel.

CONCLUSION

The hERG-associated missense mutation G572R, like most LQT2 missense mutations, generates a trafficking-deficient phenotype. Furthermore, G572R-hERG causes a loss of function in hERG by a strong dominant negative effect on the WT-hERG channel.

Voltage-dependent inactivation gating at the selectivity filter of the MthK K+ channel

Voltage-dependent K⁺ channels can undergo a gating process known as C-type inactivation, which involves entry into a nonconducting state through conformational changes near the channel’s selectivity filter. C-type inactivation may involve movements of transmembrane voltage sensor domains, although the mechanisms underlying this form of inactivation may be heterogeneous and are often unclear. Here, we report on a form of voltage-dependent inactivation gating observed in MthK, a prokaryotic K⁺ channel that lacks a canonical voltage sensor and may thus provide a reduced system to inform on mechanism. In single-channel recordings, we observe that Po decreases with depolarization, with a half-maximal voltage of 96 ± 3 mV. This gating is kinetically distinct from blockade by internal Ca²⁺ or Ba²⁺, suggesting that it may arise from an intrinsic inactivation mechanism. Inactivation gating was shifted toward more positive voltages by increasing external [K⁺] (47 mV per 10-fold increase in [K⁺]), suggesting that K⁺ binding at the extracellular side of the channel stabilizes the open-conductive state. The open-conductive state was stabilized by other external cations, and selectivity of the stabilizing site followed the sequence: K⁺ ≈ Rb⁺ > Cs⁺ > Na⁺ > Li⁺ ≈ NMG⁺. Selectivity of the stabilizing site is weaker than that of sites that determine permeability of these ions, suggesting that the site may lie toward the external end of the MthK selectivity filter. We could describe MthK gating over a wide range of positive voltages and external [K⁺] using kinetic schemes in which the open-conductive state is stabilized by K⁺ binding to a site that is not deep within the electric field, with the voltage dependence of inactivation arising from both voltage-dependent K⁺ dissociation and transitions between nonconducting (inactivated) states. These results provide a quantitative working hypothesis for voltage-dependent, K⁺-sensitive inactivation gating, a property that may be common to other K⁺ channels.

Emerging concepts in the pharmacogenomics of arrhythmias: ion channel trafficking

Continuous, rhythmic beating of the heart requires exquisite control of expression, localization and function of cardiac ion channels - the foundations of the cardiac myocyte action potential. Disruption of any of these processes can alter the shape of the action potential, predisposing to cardiac arrhythmias. These arrhythmias can manifest in a variety of ways depending on both the channels involved and the type of disruption (i.e., gain or loss of function). As much as 1% of the population of developed countries is affected by cardiac arrhythmia each year, and a detailed understanding of the mechanism of each arrhythmia is crucial to developing and prescribing the proper therapies. Many of the antiarrhythmic drugs currently on the market were developed before the underlying cause of the arrhythmia was known, and as a result lack specificity, causing side effects. The majority of the available drugs target the conductance of cardiac ion channels, either by blocking or enhancing current through the channel. In recent years, however, it has become apparent that specific targeting of ion channel conductance may not be the most effective means for treatment. Here we review increasing evidence that suggests defects in ion channel trafficking play an important role in the etiology of arrhythmias, and small molecule approaches to correct trafficking defects will likely play an important role in the future of arrhythmia treatment.

Repolarization of the cardiac action potential. Does an increase in repolarization capacity constitute a new anti-arrhythmic principle?

The cardiac action potential can be divided into five distinct phases designated phases 0-4. The exact shape of the action potential comes about primarily as an orchestrated function of ion channels. The present review will give an overview of ion channels involved in generating the cardiac action potential with special emphasis on potassium channels involved in phase 3 repolarization. In humans, these channels are primarily K(v)11.1 (hERG1), K(v)7.1 (KCNQ1) and K(ir)2.1 (KCNJ2) being the responsible alpha-subunits for conducting I(Kr), I(Ks) and I(K1). An account will be given about molecular components, biophysical properties, regulation, interaction with other proteins and involvement in diseases. Both loss and gain of function of these currents are associated with different arrhythmogenic diseases. The second part of this review will therefore elucidate arrhythmias and subsequently focus on newly developed chemical entities having the ability to increase the activity of I(Kr), I(Ks) and I(K1). An evaluation will be given addressing the possibility that this novel class of compounds have the ability to constitute a new anti-arrhythmic principle. Experimental evidence from in vitro, ex vivo and in vivo settings will be included. Furthermore, conceptual differences between the short QT syndrome and I(Kr) activation will be accounted for.

Drug-induced QT interval shortening: potential harbinger of proarrhythmia and regulatory perspectives

ATP-dependent potassium channel openers such as pinacidil and levcromakalim have long been known to shorten action potential duration and to be profibrillatory in non-clinical models, raising concerns on the clinical safety of drugs that shorten QT interval. Routine non-clinical evaluation of new drugs for their potential to affect cardiac repolarization has revealed that drugs may also shorten QT interval. The description of congenital short QT syndrome in 2000, together with the associated arrhythmias, suggests that drug-induced short QT interval may be proarrhythmic, and an uncanny parallel is evolving between our appreciation of the short and the long QT intervals. Epidemiological studies report an over-representation of short QT interval values in patients with idiopathic ventricular fibrillation. Therefore, as new compounds that shorten QT interval are progressed further into clinical development, questions will inevitably arise on their safety. Arising from the current risk-averse clinical and regulatory environment and concerns on proarrhythmic safety of drugs, together with our lack of a better understanding of the clinical significance of short QT interval, new drugs that substantially shorten QT interval will likely receive an unfavourable regulatory review unless these drugs fulfil an unmet clinical need. This review provides estimates of parameters of QT shortening that may be of potential clinical significance. Rufinamide, a recently approved anticonvulsant, illustrates the current regulatory approach to drugs that shorten QT interval. However, to further substantiate or confirm the safety of these drugs, their approval may well be conditional upon large-scale post-marketing studies with a focus on cardiac safety.

Delayed rectifier K(+) currents and cardiac repolarization

The two components of the cardiac delayed rectifier current have been the subject of numerous studies since firstly described. This current controls the action potential duration and is highly regulated. After identification of the channel subunits underlying IKs, KCNQ1 associated with KCNE1, and IKr, HERG, their involvement in human cardiac channelopathies have provided various models allowing the description of the molecular mechanisms of the KCNQ1 and HERG channels trafficking, activity and regulation. More recently, studies have been focusing on the unveiling of different partners of the pore-forming proteins that contribute to their maturation, trafficking, activity and/or degradation, on one side, and on their respective expression in the heterogeneous cardiac tissue, on the other side. The aim of this review is to report and discuss the major works on IKs and IKr and the most recent ones that help to understand the precise function of these currents in the heart.

hERG1a/1b heteromeric currents exhibit amplified attenuation of inactivation in variant 1 short QT syndrome

Potassium channels encoded by hERG (human ether-à-go-go-related gene) underlie the cardiac rapid delayed rectifier K⁺ current (I_Kr) and hERG mutations underpin clinically important repolarization disorders. Virtually all electrophysiological investigations of hERG mutations have studied exclusively the hERG1a isoform; however, recent evidence indicates that native I_Kr channels may be comprised of hERG1a together with the hERG1b variant, which has a shorter N-terminus. Here, for the first time, electrophysiological effects were studied of a gain-of-function hERG mutation (N588K; responsible for the ‘SQT1’ variant of the short QT syndrome) on current (I_hERG1a/1b) carried by co-expressed hERG1a/1b channels. There were no significant effects of N588K on I_hERG1a/1b activation or deactivation, but N588K I_hERG1a/1b showed little inactivation up to highly positive voltages (⩽+80 mV), a more marked effect than seen for hERG1a expressed alone. I_hERG1a/1b under action potential voltage-clamp, and the effects on this of the N588K mutation, also showed differences from those previously reported for hERG1a. The amplified attenuation of I_hERG inactivation for the N588K mutation reported here indicates that the study of co-expressed hERG1a/1b channels should be considered when investigating clinically relevant hERG channel mutations, even if these reside outside of the N-terminus region.

Action potential clamp and chloroquine sensitivity of mutant Kir2.1 channels responsible for variant 3 short QT syndrome

Recently identified genetic forms of short QT syndrome (SQTS) are associated with an increased risk of arrhythmia and sudden death. The SQT3 variant is associated with an amino-acid substitution (D172N) in the KCNJ2-encoded Kir2.1 K⁺ channel. In this study, whole-cell action potential (AP) clamp recording from transiently transfected Chinese Hamster Ovary cells at 37 °C showed marked augmentation of outward Kir2.1 current through D172N channels, associated with right-ward voltage-shifts of peak repolarizing current during both ventricular and atrial AP commands. Peak outward current elicited by ventricular AP commands was inhibited by chloroquine with an IC₅₀ of 2.45 μM for wild-type (WT) Kir2.1, of 3.30 μM for D172N-Kir2.1 alone and of 3.11 μM for co-expressed WT and D172N (P > 0.05 for all). These findings establish chloroquine as an effective inhibitor of SQT3 mutant Kir2.1 channels.

Role of intracellular domains in the function of the herg potassium channel

The functional role of the large intracellular regions (which include the cyclic nucleotide binding domain, cNBD, and the Per-Arnt-Sim domain, PAS) in the herg channel is not well understood. We have studied possible interactions of the cNBD with other parts of the channel protein using lysine mutations to disrupt such interactions. Some lysine mutations caused significant right shifts in the voltage dependence of inactivation; almost all the mutants caused speeding up of deactivation time course. In a homology model of the cNBD, lysine mutations that affected both inactivation and deactivation lie in a hydrophobic band on the surface of the structure of this domain. Some known mutations in the Long QT Syndrome type 2, with effects on deactivation, are located at residues close to hydrophobic bands on the cNBD and the PAS domains. Such bands of residues in these intracellular domains may play an important part in channel function.

Physiological properties of hERG 1a/1b heteromeric currents and a hERG 1b-specific mutation associated with Long-QT syndrome

Cardiac I Kr is a critical repolarizing current in the heart and a target for inherited and acquired long-QT syndrome (LQTS). Biochemical and functional studies have demonstrated that I Kr channels are heteromers composed of both hERG 1a and 1b subunits, yet our current understanding of I Kr functional properties derives primarily from studies of homooligomers of the original hERG 1a isolate. Here, we examine currents produced by hERG 1a and 1a/1b channels expressed in HEK-293 cells at near-physiological temperatures. We find that heteromeric hERG 1a/1b currents are much larger than hERG 1a currents and conduct 80% more charge during an action potential. This surprising difference corresponds to a 2-fold increase in the apparent rates of activation and recovery from inactivation, thus reducing rectification and facilitating current rebound during repolarization. Kinetic modeling shows these gating differences account quantitatively for the differences in current amplitude between the 2 channel types. Drug sensitivity was also different. Compared to homomeric 1a channels, heteromeric 1a/1b channels were inhibited by E-4031 with a slower time course and a corresponding 4-fold shift in the IC50. The importance of hERG 1b in vivo is supported by the identification of a 1b-specific A8V missense mutation in 1/269 unrelated genotype-negative LQTS patients that was absent in 400 control alleles. Mutant 1bA8V expressed alone or with hERG 1a in HEK-293 cells dramatically reduced 1b protein levels. Thus, mutations specifically disrupting hERG 1b function are expected to reduce cardiac I Kr and enhance drug sensitivity, and represent a potential mechanism underlying inherited or acquired LQTS.

Pharmacology of the short QT syndrome N588K-hERG K+ channel mutation: differential impact on selected class I and class III antiarrhythmic drugs

BACKGROUND AND PURPOSE

The short QT syndrome (SQTS) is associated with cardiac arrhythmias and sudden death. The SQT1 form of SQTS results from an inactivation-attenuated, gain-of-function mutation (N588K) to the human ether-à-go-go-related gene (hERG) potassium channel. Pharmacological blockade of this mutated hERG channel may have therapeutic value. However, hERG-blocking potencies of canonical inhibitors such as E-4031 and D-sotalol are significantly reduced for N588K-hERG. Here, five hERG-blocking drugs were compared to determine their relative potencies for inhibiting N588K channels, and two other inactivation-attenuated mutant channels were tested to investigate the association between impaired inactivation and altered drug potency.

EXPERIMENTAL APPROACH

Whole-cell patch clamp measurements of hERG current (I(hERG)) mediated by wild-type and mutant (N588K, S631A and N588K/S631A) channels were made at 37 degrees C CHO cells.

KEY RESULTS

The N588K mutation attenuated I(hERG) inhibition in the following order: E-4031>amiodarone>quinidine>propafenone>disopyramide. Comparing the three inactivation mutants, the two single mutations, although occurring in different modules of the channel, attenuated inactivation to a nearly identical degree, whereas the double mutant caused considerably greater attenuation, permitting the titration of inactivation. Attenuation of channel inhibition was similar between the single mutants for each drug, and was significantly greater with the double mutant.

CONCLUSIONS AND IMPLICATIONS

The degree of drug inhibition of hERG channels may vary based on the level of channel inactivation. Drugs previously identified as useful for treating SQT1 have the least dependence on hERG inactivation. In addition, our findings indicate that amiodarone may warrant further investigation as a potential treatment for SQT1.

The hERG potassium channel and hERG screening for drug-induced torsades de pointes

Drug-induced torsades de pointes (TdP) arrhythmia is a major safety concern in the process of drug design and development. The incidence of TdP tends to be low, so early pre-clinical screens rely on surrogate markers of TdP to highlight potential problems with new drugs. hERG (human ether-à-go-go-related gene, alternative nomenclature KCNH2) is responsible for channels mediating the 'rapid' delayed rectifier K+ current (IKr) which plays an important role in ventricular repolarization. Pharmacological inhibition of native IKr and of recombinant hERG channels is a shared feature of diverse drugs associated with TdP. In vitro hERG assays therefore form a key element of an integrated assessment of TdP liability, with patch-clamp electrophysiology offering a 'gold standard'. However, whilst clearly necessary, hERG assays cannot be assumed automatically to provide sufficient information, when considered in isolation, to differentiate 'safe' from 'dangerous' drugs. Other relevant factors include therapeutic plasma concentration, drug metabolism and active metabolites, severity of target condition and drug effects on other cardiac ion channels that may mitigate or exacerbate effects of hERG blockade. Increased understanding of the nature of drug-hERG channel interactions may ultimately help eliminate potential hERG blockade early in the design and development process. Currently, for promising drug candidates integration of data from hERG assays with information from other pre-clinical safety screens remains essential.

hERG potassium channels and the structural basis of drug-induced arrhythmias

hERG potassium channels have a critical role in the normal electrical activity of the heart. The block of hERG channels can cause the drug-induced form of long QT syndrome, a cardiac disorder that carries an increased risk of cardiac arrhythmias and sudden death. hERG channels are extraordinarily sensitive to block by large numbers of structurally diverse drugs. In previous years, the risk of compounds causing this cardiotoxic side effect has been a common reason for the failure of compounds in preclinical safety trials. Pharmaceutical companies have successfully utilized and developed higher throughput techniques for the early detection of compounds that block hERG, and this has helped reduce the number of compounds that fail in the late stages of development. Nevertheless, this screening-based approach is expensive, consumes chemistry resources, and bypasses the problem rather than shedding light on it. Crystal structures of potassium channels have facilitated studies into the structural basis for the gating and block of hERG channels. Most drugs bind within the inner cavity, and the individual amino acids that form the drug binding site have been identified by site-directed mutagenesis approaches. Gating processes have an important influence on the drug-binding site. Recent advances in our understanding of channel activation and inactivation are providing insight into why hERG channels are more susceptible to block than other K (+) channels. Knowledge of the structure of the drug-binding site and precise nature of interactions with drug molecules should assist efforts to develop drugs without the propensity to cause cardiac arrhythmias.

2-[2-(3,4-dichloro-phenyl)-2,3-dihydro-1H-isoindol-5-ylamino]-nicotinic acid (PD-307243) causes instantaneous current through human ether-a-go-go-related gene potassium channels

Long and short QT syndromes associated with loss and gain of human ether-a-go-go-related gene (hERG) channel activity, respectively, can cause life-threatening arrhythmias. As such, modulation of hERG channel activity is an important consideration in the development of all new therapeutic agents. In the present study, we investigated the mechanisms of action of 2-[2-(3,4-dichloro-phenyl)-2,3-dihydro-1H-isoindol-5-ylamino]-nicotinic acid (PD-307243), a known hERG channel activator, on hERG channels stably expressed in Chinese hamster ovary (CHO) cells using the patch-clamp technique. In the whole-cell recordings, the extracellular application of PD-307243 concentration-dependently increased the hERG current and markedly slowed hERG channel deactivation and inactivation. PD-307243 had no effect on the selectivity filter of hERG channels. The activity of PD-307243 was use-dependent. PD-307243 (3 and 10 muM) induced instantaneous hERG current with little decay at membrane potentials from -120 to -40 mV. At more positive voltages, PD-307243 induced an I(to)-like upstroke of hERG current. The actions of PD-307243 on the rapid component of delayed rectifier K(+) current (I(Kr)) in rabbit ventricular myocytes were similar to those observed in hERG channel-transfected CHO cells. Inside-out patch experiments revealed that PD-307243 increased hERG tail currents by 2.1 +/- 0.6 (n = 7) and 3.4 +/- 0.3-fold (n = 4) at 3 and 10 muM, respectively, by slowing the channel deactivation but had no effect on channel activation. During a voltage-clamp protocol using a prerecorded cardiac action potential, 3 muM PD-307243 increased the total potassium ions passed through hERG channels by 8.8 +/- 1.0-fold (n = 5). Docking studies suggest that PD-307243 interacts with residues in the S5-P region of the channel.

Inhibition of the HERG potassium channel by the tricyclic antidepressant doxepin

HERG (human ether-à-go-go-related gene) encodes channels responsible for the cardiac rapid delayed rectifier potassium current, I_Kr. This study investigated the effects on HERG channels of doxepin, a tricyclic antidepressant linked to QT interval prolongation and cardiac arrhythmia. Whole-cell patch-clamp recordings were made at 37 °C of recombinant HERG channel current (I_HERG), and of native I_Kr ‘tails’ from rabbit ventricular myocytes. Doxepin inhibited I_HERG with an IC₅₀ value of 6.5 ± 1.4 μM and native I_Kr with an IC₅₀ of 4.4 ± 0.6 μM. The inhibitory effect on I_HERG developed rapidly upon membrane depolarization, but with no significant dependence on voltage and with little alteration to the voltage-dependent kinetics of I_HERG. Neither the S631A nor N588K inactivation-attenuating mutations (of residues located in the channel pore and external S5-Pore linker, respectively) significantly reduced the potency of inhibition. The S6 point mutation Y652A increased the IC₅₀ for I_HERG blockade by ∼4.2-fold; the F656A mutant also attenuated doxepin's action at some concentrations. HERG channel blockade is likely to underpin reported cases of QT interval prolongation with doxepin. Notably, this study also establishes doxepin as an effective inhibitor of mutant (N588K) HERG channels responsible for variant 1 of the short QT syndrome.

The hERG potassium channel as a therapeutic target

The hERG (human ether-à-go-go-related gene) potassium channel has elicited intense scientific interest due to its counter-intuitive kinetics and its association with arrhythmia and sudden death. hERG blockade is involved in both antiarrhythmic pharmacotherapy and the pathogenesis of familial and acquired long QT syndrome (LQTS). Short QT syndrome (SQTS), muscular atrophy and many forms of cancer have also been associated with hERG as a target. Molecular models of both the channel and its blocker pharmacophores exist, revealing methods to design hERG liability out of potential drug molecules. Future developments will synthesise preclinical data on hERG with other criteria to determine net arrhythmogenic risk. Also, the molecular actions of hERG and its genetics will be elucidated in detail to allow clinical risk reduction.

hERG K+ channel blockade by the antipsychotic drug thioridazine: An obligatory role for the S6 helix residue F656

The phenothiazine antipsychotic agent thioridazine has been linked with prolongation of the QT interval on the electrocardiogram, ventricular arrhythmias, and sudden death. Although thioridazine is known to inhibit cardiac hERG K(+) channels there is little mechanistic information on this action. We have investigated in detail hERG K(+) channel current (I(hERG)) blockade by thioridazine and identified a key molecular determinant of blockade. Whole-cell I(hERG) measurements were made at 37 degrees C from human embryonic kidney (HEK-293) cells expressing wild-type and mutant hERG channels. Thioridazine inhibited I(hERG) tails at -40mV following a 2s depolarization to +20mV with an IC(50) value of 80nM. Comparable levels of I(hERG) inhibition were seen with physiological command waveforms (ventricular and Purkinje fibre action potentials). Thioridazine block of I(hERG) was only weakly voltage-dependent, though the time dependence of I(hERG) inhibition indicated contingency of blockade upon channel gating. The S6 helix point mutation F656A almost completely abolished, and the Y652A mutation partially attenuated, I(hERG) inhibition by thioridazine. In summary, thioridazine is one of the most potent hERG K(+) channel blockers amongst antipsychotics, exhibiting characteristics of a preferential open/activated channel blocker and binding at a high affinity site in the hERG channel pore.

Biophysical characterization of the new human ether-a-go-go-related gene channel opener NS3623 [N-(4-bromo-2-(1H-tetrazol-5-yl)-phenyl)-N'-(3'-trifluoromethylphenyl)urea]

Within the field of new antiarrhythmic compounds, the interesting idea of activating human ether-a-go-go-related gene (HERG1) potassium channels has recently been introduced. Potentially, drugs that increase HERG1 channel activity will augment the repolarizing current of the cardiac myocytes and stabilize the diastolic interval. This may make the myocardium more resistant to events that cause arrhythmias. We here present the compound N-(4-bromo-2-(1H-tetrazol-5-yl)-phenyl)-N'-(3'-trifluoromethylphenyl)urea (NS3623), which has the ability to activate HERG1 channels expressed in Xenopus laevis oocytes with an EC50 value of 79.4 microM. Exposure of HERG1 channels to NS3623 affects the voltage-dependent release from inactivation, resulting in a half-inactivation voltage that is rightward-shifted by 17.7 mV. Moreover, the compound affects the time constant of inactivation, leading to a slower onset of inactivation of the macroscopic HERG1 currents. We also characterized the ability of NS3623 to increase the activity of different mutated HERG1 channels. The mutants S620T and S631A are severely compromised in their ability to inactivate. Application of NS3623 to any of these two mutants did not result in increased HERG1 current. In contrast, application of NS3623 to the mutant F656M increased HERG1 current to a larger extent than what was observed with wild-type HERG1 channels. Because the amino acid F656 is essential for high-affinity inhibition of HERG1 channels, it is concluded that NS3623 has a dual mode of action, being both an activator and an inhibitor of HERG1 channels. Finally, we show that NS3623 has the ability to shorten action potential durations in guinea pig papillary muscle.

Arrhythmogenesis in the short-QT syndrome associated with combined HERG channel gating defects: a simulation study

BACKGROUND

This study aimed to show the mechanism how the HERG channel gating defects causes life-threatening arrhythmia in the short-QT syndrome, using a simulation model of ventricular action potentials (APs).

METHODS AND RESULTS

To evaluate the electrophysiological consequences of the short-QT syndrome at the level of the cardiac AP, the Markov model of wild-type (WT) KCNH2 channel was modified to obtain a model of the KCNH2 channel with the N588K mutation associated with the short-QT syndrome. Two parameters (betai and betabeta) were changed to reconstruct the N588K mutant Markov model, which successfully reproduced the experimental results of voltage-clamp recordings. The WT and mutant models were then integrated into the Luo-Rudy theoretical model of the cardiac ventricular AP. Unexpectedly, 1 parameter change alone, which caused gain of function, could shorten the AP duration (APD) but failed to induce early after-depolarizations (EADs). Only the condition with the combined gating defects could lead to EAD.

CONCLUSIONS

Although the gain of function for KCNH2 shortened APD in the short-QT syndrome, this simulation study suggested that arrhythmogenesis was associated not only with gain of function, but also with accelerated deactivation of KCNH2.

Disopyramide is an effective inhibitor of mutant HERG K+ channels involved in variant 1 short QT syndrome

The recently identified idiopathic short QT syndrome (SQTS) is associated with an increased risk of arrhythmia and sudden death. The use of implantable cardioverter defibrillators helps to protect SQTS patients from ventricular fibrillation; however, pharmacological treatments to normalise the QT interval are limited: thus far only quinidine has been found to be effective in a subset of patients, with the SQT1 variant. SQT1 is associated with an amino acid substitution (N588K) in the KCNH2-encoded HERG K(+) channel that reduces HERG current (I(HERG)) inactivation and sensitivity to drug block. We demonstrate here that the N588K-HERG mutation only slightly attenuates I(HERG) blockade by the Class Ia antiarrhythmic drug disopyramide (1.5-fold elevation of IC(50)), compared to quinidine (3.5-fold elevation of IC(50)) and the Class III antiarrhythmic drug E-4031 (11.5-fold elevation of IC(50)). Thus, of the drugs studied to date, disopyramide is the one least affected by the SQT1 HERG mutation. Disopyramide is associated with QT prolongation in normal use and our findings provide a rational basis for its evaluation as a treatment for SQT1.

Temperature dependence of human ether-a-go-go-related gene K+ currents

The function of voltage-gated human ether-à-go-go related gene (hERG) K(+) channels is critical for both normal cardiac repolarization and suppression of arrhythmias initiated by premature excitation. These important functions are facilitated by their unusual kinetics that combine relatively slow activation and deactivation with rapid and voltage-dependent inactivation and recovery from inactivation. The thermodynamics of these unusual features were examined by exploring the effect of temperature on the activation and inactivation processes of hERG channels expressed in Chinese hamster ovary cells. Increased temperature shifted the voltage dependence of activation in the hyperpolarizing direction but that of inactivation in the depolarizing direction. This increases the relative occupancy of the open state and contributes to the marked temperature sensitivity of hERG current magnitude observed during action potential voltage clamps. The rates of activation and deactivation also increase with higher temperatures, but less markedly than do the rates of inactivation and recovery from inactivation. Our results also emphasize that one cannot extrapolate results obtained at room temperature to 37 degrees C by using a single temperature scale factor.

Targeted therapy for short QT syndrome

Identified in 2000, short QT syndrome is an electrical disease of the heart characterised as a channelopathy. At first considered extremely rare, families with this disease have been found in Brazil, Finland, Germany, Spain, the Netherlands, France, Turkey, Italy and the US. The focus of the paper is to present a current review of short QT syndrome, as well as providing an overview upon the potential molecular target-based strategies for management of this very deadly disease. Abnormalities in three different potassium channels have been recognised as the cause of the disease and targets for therapy will be discussed for each potassium channel individually. In addition to pharmacological strategies, gene therapy with transfer of genes coding for specific ion channel subunits or regulatory proteins are discussed.

Short QT syndrome

The idiopathic short QT syndrome (SQTS) is a recently identified condition characterized by abbreviated QT intervals (typically 300 ms or less) and in affected families is associated with an increased incidence of atrial and ventricular arrhythmias and sudden cardiac death. Genetic analysis has, to date, identified three distinct forms of the condition, involving gain-of-function mutations to three different cardiac potassium channel genes: KCNH2 (SQT1), KCNQ1 (SQT2) and KCNJ2 (SQT3). This article reviews recent advances in understanding this syndrome, discussing the basis of QT interval shortening, possible mechanisms for the associated arrhythmogenic risk in SQT1, current approaches to treatment of the SQTS (focusing on SQT1) and avenues for future investigation.

Effect of S5P alpha-helix charge mutants on inactivation of hERG K+ channels

The ether-à-go-go (EAG) family of voltage-gated K(+) channels contains three subfamilies, EAG, ether-à-go-go related (ERG) and ether-à-go-go like (ELK). The human ether-à-go-go related gene (hERG) K(+) channel has been of significant interest because loss of function in the hERG channel is associated with a markedly increased risk of cardiac arrhythmias. The hERG channel has unusual kinetics with slow activation and deactivation but very rapid and voltage-dependent inactivation. The outer pore region of the hERG K(+) channel is predicted to be different from that of other members of the voltage-gated K(+) channel family. HERG has a much longer linker between the fifth transmembrane domain (SS) and the pore helix (S5P linker) compared to other families of voltage-gated K(+) channels (43 amino acids compared to 14-23 amino acids). Further, the S5P linker contains an amphipathic alpha-helix that in hERG channels probably interacts with the mouth of the pore to modulate inactivation. The human EAG and rat ELK2 channels (hEAG and rELK2) show reduced or no inactivation in comparison to hERG channels, yet both channels are predicted to contain a similarly long S5P linker to that of hERG. In this study, we have constructed a series of chimaeric channels consisting of the S1-S6 of hERG but with the S5P alpha-helical region of either hEAG or rELK2, and one consisting of the S1-S6 of rELK2 but with the S5P alpha-helical region of hERG to investigate the role of the S5P linker in inactivation. Our studies show that charged residues on the alpha-helix of the S5P linker contribute significantly to the differences in inactivation characteristics of the EAG family channels. Further, individually mutating each of the hydrophilic residues on the S5P alpha-helix of hERG to a charged residue had significant effects on the voltage dependence of inactivation and the two residues with the greatest affect when mutated to a lysine, N588 and Q592, both lie on the same face of the S5P alpha -helix. We suggest that inactivation of hERG involves the interaction of this face of the S5P alpha-helix with a charged residue on the remainder of the outer pore domain of the channel.