Physiological genomics identifies genetic modifiers of long QT syndrome type 2 severity.

Congenital long QT syndrome (LQTS) is an inherited channelopathy associated with life-threatening arrhythmias. LQTS type 2 (LQT2) is caused by mutations in KCNH2, which encodes the potassium channel hERG. We hypothesized that modifier genes are partly responsible for the variable phenotype severity observed in some LQT2 families. Here, we identified contributors to variable expressivity in an LQT2 family by using induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) and whole exome sequencing in a synergistic manner. We found that iPSC-CMs recapitulated the clinical genotype-phenotype discordance in vitro. Importantly, iPSC-CMs derived from the severely affected LQT2 patients displayed prolonged action potentials compared with cells from mildly affected first-degree relatives. The iPSC-CMs derived from all patients with hERG R752W mutation displayed lower IKr amplitude. Interestingly, iPSC-CMs from severely affected mutation-positive individuals exhibited greater L-type Ca2+ current. Whole exome sequencing identified variants of KCNK17 and the GTP-binding protein REM2, providing biologically plausible explanations for this variable expressivity. Genome editing to correct a REM2 variant reversed the enhanced L-type Ca2+ current and prolonged action potential observed in iPSC-CMs from severely affected individuals. Thus, our findings showcase the power of combining complementary physiological and genomic analyses to identify genetic modifiers and potential therapeutic targets of a monogenic disorder. Furthermore, we propose that this strategy can be deployed to unravel myriad confounding pathologies displaying variable expressivity.

Voltage-gated sodium channels assemble and gate as dimers.

Fast opening and closing of voltage-gated sodium channels are crucial for proper propagation of the action potential through excitable tissues. Unlike potassium channels, sodium channel α-subunits are believed to form functional monomers. Yet, an increasing body of literature shows inconsistency with the traditional idea of a single α-subunit functioning as a monomer. Here we demonstrate that sodium channel α-subunits not only physically interact with each other but they actually assemble, function and gate as a dimer. We identify the region involved in the dimerization and demonstrate that 14-3-3 protein mediates the coupled gating. Importantly we show conservation of this mechanism among mammalian sodium channels. Our study not only shifts conventional paradigms in regard to sodium channel assembly, structure, and function but importantly this discovery of the mechanism involved in channel dimerization and biophysical coupling could open the door to new approaches and targets to treat and/or prevent sodium channelopathies.

KChIP2 is a core transcriptional regulator of cardiac excitability.

Arrhythmogenesis from aberrant electrical remodeling is a primary cause of death among patients with heart disease. Amongst a multitude of remodeling events, reduced expression of the ion channel subunit KChIP2 is consistently observed in numerous cardiac pathologies. However, it remains unknown if KChIP2 loss is merely a symptom or involved in disease development. Using rat and human derived cardiomyocytes, we identify a previously unobserved transcriptional capacity for cardiac KChIP2 critical in maintaining electrical stability. Through interaction with genetic elements, KChIP2 transcriptionally repressed the miRNAs miR-34b and miR-34c, which subsequently targeted key depolarizing (INa) and repolarizing (Ito) currents altered in cardiac disease. Genetically maintaining KChIP2 expression or inhibiting miR-34 under pathologic conditions restored channel function and moreover, prevented the incidence of reentrant arrhythmias. This identifies the KChIP2/miR-34 axis as a central regulator in developing electrical dysfunction and reveals miR-34 as a therapeutic target for treating arrhythmogenesis in heart disease.

Brugada syndrome disease phenotype explained in apparently benign sodium channel mutations.

BACKGROUND: Brugada syndrome (BrS) is an arrhythmogenic disorder that has been linked to mutations in SCN5A, the gene encoding for the pore-forming α-subunit of the cardiac sodium channel. Typically, BrS mutations in SCN5A result in a reduction of sodium current with some mutations even exhibiting a dominant-negative effect on wild-type (WT) channels, thus leading to an even more prominent decrease in current amplitudes. However, there is also a category of apparently benign (atypical) BrS SCN5A mutations that in vitro demonstrates only minor biophysical defects. It is therefore not clear how these mutations produce a BrS phenotype. We hypothesized that similar to dominant-negative mutations, atypical mutations could lead to a reduction in sodium currents when coexpressed with WT to mimic the heterozygous patient genotype. METHODS AND RESULTS: WT and atypical BrS mutations were coexpressed in Human Embryonic Kidney-293 cells, showing a reduction in sodium current densities similar to typical BrS mutations. Importantly, this reduction in sodium current was also seen when the atypical mutations were expressed in rat or human cardiomyocytes. This decrease in current density was the result of reduced surface expression of both mutant and WT channels. CONCLUSIONS: Taken together, we have shown how apparently benign SCN5A BrS mutations can lead to the ECG abnormalities seen in patients with BrS through an induced defect that is only present when the mutations are coexpressed with WT channels. Our work has implications for risk management and stratification for some SCN5A-implicated BrS patients.

Lipotoxic disruption of NHE1 interaction with PI(4,5)P2 expedites proximal tubule apoptosis.

Chronic kidney disease progression can be predicted based on the degree of tubular atrophy, which is the result of proximal tubule apoptosis. The Na+/H+ exchanger NHE1 regulates proximal tubule cell survival through interaction with phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2], but pathophysiologic triggers for NHE1 inactivation are unknown. Because glomerular injury permits proximal tubule luminal exposure and reabsorption of fatty acid/albumin complexes, we hypothesized that accumulation of amphipathic, long-chain acyl-CoA (LC-CoA) metabolites stimulates lipoapoptosis by competing with the structurally similar PI(4,5)P2 for NHE1 binding. Kidneys from mouse models of progressive, albuminuric kidney disease exhibited increased fatty acids, LC-CoAs, and caspase-2-dependent proximal tubule lipoapoptosis. LC-CoAs and the cytosolic domain of NHE1 directly interacted, with an affinity comparable to that of the PI(4,5)P2-NHE1 interaction, and competing LC-CoAs disrupted binding of the NHE1 cytosolic tail to PI(4,5)P2. Inhibition of LC-CoA catabolism reduced NHE1 activity and enhanced apoptosis, whereas inhibition of proximal tubule LC-CoA generation preserved NHE1 activity and protected against apoptosis. Our data indicate that albuminuria/lipiduria enhances lipotoxin delivery to the proximal tubule and accumulation of LC-CoAs contributes to tubular atrophy by severing the NHE1-PI(4,5)P2 interaction, thereby lowering the apoptotic threshold. Furthermore, these data suggest that NHE1 functions as a metabolic sensor for lipotoxicity.

The variant hERG/R148W associated with LQTS is a mutation that reduces current density on co-expression with the WT.

BACKGROUND: A variant of the ether-à-go-go related channel (hERG), p.Arg148Trp (R148W) was found at heterozygous state in two infants who died from sudden infant death syndrome (SIDS), one with documented prolonged QTc and Torsade de Pointes (TdP), and in an adult woman with QTc >500 ms, atrioventricular block and TdP. This variant was previously reported in cases of severe ventricular arrhythmia but very rarely in control subjects. Its classification as mutation or polymorphism awaited electrophysiological characterization. METHODS: The properties of this N-terminal, proximal domain, hERG variant were explored in Xenopus oocytes injected with the same amount of RNA encoding for either hERG/WT or hERG/R148W or their equimolar mixture. The human ventricular cell (TNNP) model was used to test the effects of changes in hERG current. RESULTS: R148W alone produced a current similar to the WT (369 ± 76 nA (mean ± SEM), n=13 versus 342 ± 55 nA in WT, n=13), while the co-expression of 1/2 WT+1/2 R148W lowered the current by 29% versus WT (243 ± 35 nA, n=13, p<0.05). The voltage dependencies of steady-state activation and inactivation were not changed in the variant alone or in co-expression with the WT. The time constants of fast recovery from inactivation and of fast and slow deactivation analyzed between -120 and +20 mV were not changed. The voltage-dependent distribution of the current amplitudes among fast-, slow- and non-deactivating fractions was unaltered. A 6.6% increase in APD90 from 323.5 ms to 345 ms was observed using the human cardiac ventricular myocyte model. CONCLUSIONS: Such a decrease in hERG current as evidenced here when co-expressing the hERG/R148W variant with the WT may have predisposed to the observed long QT syndrome and associated TdP. Therefore, the heterozygous carriers of hERG/R148W may be at risk of cardiac sudden death.

Ubiquitination-dependent quality control of hERG K+ channel with acquired and inherited conformational defect at the plasma membrane.

Membrane trafficking in concert with the peripheral quality control machinery plays a critical role in preserving plasma membrane (PM) protein homeostasis. Unfortunately, the peripheral quality control may also dispose of partially or transiently unfolded polypeptides and thereby contribute to the loss-of-expression phenotype of conformational diseases. Defective functional PM expression of the human ether-a-go-go-related gene (hERG) K(+) channel leads to the prolongation of the ventricular action potential that causes long QT syndrome 2 (LQT2), with increased propensity for arrhythmia and sudden cardiac arrest. LQT2 syndrome is attributed to channel biosynthetic processing defects due to mutation, drug-induced misfolding, or direct channel blockade. Here we provide evidence that a peripheral quality control mechanism can contribute to development of the LQT2 syndrome. We show that PM hERG structural and metabolic stability is compromised by the reduction of extracellular or intracellular K(+) concentration. Cardiac glycoside-induced intracellular K(+) depletion conformationally impairs the complex-glycosylated channel, which provokes chaperone- and C-terminal Hsp70-interacting protein-dependent polyubiquitination, accelerated internalization, and endosomal sorting complex required for transport-dependent lysosomal degradation. A similar mechanism contributes to the down-regulation of PM hERG harboring LQT2 missense mutations, with incomplete secretion defect. These results suggest that PM quality control plays a determining role in the loss-of-expression phenotype of hERG in certain hereditary and acquired LTQ2 syndromes.

Ionic bases for electrical remodeling of the canine cardiac ventricle.

Emerging evidence suggests that ventricular electrical remodeling (VER) is triggered by regional myocardial strain via mechanoelectrical feedback mechanisms; however, the ionic mechanisms underlying strain-induced VER are poorly understood. To determine its ionic basis, VER induced by altered electrical activation in dogs undergoing left ventricular pacing (n = 6) were compared with unpaced controls (n = 4). Action potential (AP) durations (APDs), ionic currents, and Ca(2+) transients were measured from canine epicardial myocytes isolated from early-activated (low strain) and late-activated (high strain) left ventricular regions. VER in the early-activated region was characterized by minimal APD prolongation, but marked attenuation of the AP phase 1 notch attributed to reduced transient outward K(+) current. In contrast, VER in the late-activated region was characterized by significant APD prolongation. Despite marked APD prolongation, there was surprisingly minimal change in ion channel densities but a twofold increase in diastolic Ca(2+). Computer simulations demonstrated that changes in sarcolemmal ion channel density could only account for attenuation of the AP notch observed in the early-activated region but failed to account for APD remodeling in the late-activated region. Furthermore, these simulations identified that cytosolic Ca(2+) accounted for APD prolongation in the late-activated region by enhancing forward-mode Na(+)/Ca(2+) exchanger activity, corroborated by increased Na(+)/Ca(2+) exchanger protein expression. Finally, assessment of skinned fibers after VER identified altered myofilament Ca(2+) sensitivity in late-activated regions to be associated with increased diastolic levels of Ca(2+). In conclusion, we identified two distinct ionic mechanisms that underlie VER: 1) strain-independent changes in early-activated regions due to remodeling of sarcolemmal ion channels with no changes in Ca(2+) handling and 2) a novel and unexpected mechanism for strain-induced VER in late-activated regions in the canine arising from remodeling of sarcomeric Ca(2+) handling rather than sarcolemmal ion channels.

A small molecule agonist of EphA2 receptor tyrosine kinase inhibits tumor cell migration in vitro and prostate cancer metastasis in vivo.

During tumor progression, EphA2 receptor can gain ligand-independent pro-oncogenic functions due to Akt activation and reduced ephrin-A ligand engagement. The effects can be reversed by ligand stimulation, which triggers the intrinsic tumor suppressive signaling pathways of EphA2 including inhibition of PI3/Akt and Ras/ERK pathways. These observations argue for development of small molecule agonists for EphA2 as potential tumor intervention agents. Through virtual screening and cell-based assays, we report here the identification and characterization of doxazosin as a novel small molecule agonist for EphA2 and EphA4, but not for other Eph receptors tested. NMR studies revealed extensive contacts of doxazosin with EphA2/A4, recapitulating both hydrophobic and electrostatic interactions recently found in the EphA2/ephrin-A1 complex. Clinically used as an α1-adrenoreceptor antagonist (Cardura®) for treating hypertension and benign prostate hyperplasia, doxazosin activated EphA2 independent of α1-adrenoreceptor. Similar to ephrin-A1, doxazosin inhibited Akt and ERK kinase activities in an EphA2-dependent manner. Treatment with doxazosin triggered EphA2 receptor internalization, and suppressed haptotactic and chemotactic migration of prostate cancer, breast cancer, and glioma cells. Moreover, in an orthotopic xenograft model, doxazosin reduced distal metastasis of human prostate cancer cells and prolonged survival in recipient mice. To our knowledge, doxazosin is the first small molecule agonist of a receptor tyrosine kinase that is capable of inhibiting malignant behaviors in vitro and in vivo.

The zebrafish as a novel animal model to study the molecular mechanisms of mechano-electrical feedback in the heart.

Altered mechanical loading of the heart leads to hypertrophy, decompensated heart failure and fatal arrhythmias. However, the molecular mechanisms that link mechanical and electrical dysfunction remain poorly understood. Growing evidence suggest that ventricular electrical remodeling (VER) is a process that can be induced by altered mechanical stress, creating persistent electrophysiological changes that predispose the heart to life-threatening arrhythmias. While VER is clearly a physiological property of the human heart, as evidenced by "T wave memory", it is also thought to occur in a variety of pathological states associated with altered ventricular activation such as bundle branch block, myocardial infarction, and cardiac pacing. Animal models that are currently being used for investigating stretch-induced VER have significant limitations. The zebrafish has recently emerged as an attractive animal model for studying cardiovascular disease and could overcome some of these limitations. Owing to its extensively sequenced genome, high conservation of gene function, and the comprehensive genetic resources that are available in this model, the zebrafish may provide new insights into the molecular mechanisms that drive detrimental electrical remodeling in response to stretch. Here, we have established a zebrafish model to study mechano-electrical feedback in the heart, which combines efficient genetic manipulation with high-precision stretch and high-resolution electrophysiology. In this model, only 90 min of ventricular stretch caused VER and recapitulated key features of VER found previously in the mammalian heart. Our data suggest that the zebrafish model is a powerful platform for investigating the molecular mechanisms underlying mechano-electrical feedback and VER in the heart.

Circadian rhythms govern cardiac repolarization and arrhythmogenesis.

Sudden cardiac death exhibits diurnal variation in both acquired and hereditary forms of heart disease, but the molecular basis of this variation is unknown. A common mechanism that underlies susceptibility to ventricular arrhythmias is abnormalities in the duration (for example, short or long QT syndromes and heart failure) or pattern (for example, Brugada's syndrome) of myocardial repolarization. Here we provide molecular evidence that links circadian rhythms to vulnerability in ventricular arrhythmias in mice. Specifically, we show that cardiac ion-channel expression and QT-interval duration (an index of myocardial repolarization) exhibit endogenous circadian rhythmicity under the control of a clock-dependent oscillator, krüppel-like factor 15 (Klf15). Klf15 transcriptionally controls rhythmic expression of Kv channel-interacting protein 2 (KChIP2), a critical subunit required for generating the transient outward potassium current. Deficiency or excess of Klf15 causes loss of rhythmic QT variation, abnormal repolarization and enhanced susceptibility to ventricular arrhythmias. These findings identify circadian transcription of ion channels as a mechanism for cardiac arrhythmogenesis.

Molecular determinants of pentamidine-induced hERG trafficking inhibition.

Pentamidine is an antiprotozoal compound that clinically causes acquired long QT syndrome (acLQTS), which is associated with prolonged QT intervals, tachycardias, and sudden cardiac arrest. Pentamidine delays terminal repolarization in human heart by acutely blocking cardiac inward rectifier currents. At the same time, pentamidine reduces surface expression of the cardiac potassium channel I(Kr)/human ether à-go-go-related gene (hERG). This is unusual in that acLQTS is caused most often by direct block of the cardiac potassium current I(Kr)/hERG. The present study was designed to provide a more complete picture of how hERG surface expression is disrupted by pentamidine at the cellular and molecular levels. Using biochemical and electrophysiological methods, we found that pentamidine exclusively inhibits hERG export from the endoplasmic reticulum to the cell surface in a heterologous expression system as well as in cardiomyocytes. hERG trafficking inhibition could be rescued in the presence of the pharmacological chaperone astemizole. We used rescue experiments in combination with an extensive mutational analysis to locate an interaction site for pentamidine at phenylalanine 656, a crucial residue in the canonical drug binding site of terminally folded hERG. Our data suggest that pentamidine binding to a folding intermediate of hERG arrests channel maturation in a conformational state that cannot be exported from the endoplasmic reticulum. We propose that pentamidine is the founding member of a novel pharmacological entity whose members act as small molecule antichaperones.

Phosphoinositide binding differentially regulates NHE1 Na+/H+ exchanger-dependent proximal tubule cell survival.

Tubular atrophy predicts chronic kidney disease progression, and is caused by proximal tubular epithelial cellcaused by proximal tubular epithelial cell (PTC) apoptosis. The normally quiescent Na(+)/H(+) exchanger-1 (NHE1) defends against PTC apoptosis, and is regulated by PI(4,5)P(2) binding. Because of the vast array of plasma membrane lipids, we hypothesized that NHE1-mediated cell survival is dynamically regulated by multiple anionic inner leaflet phospholipids. In membrane overlay and surface plasmon resonance assays, the NHE1 C terminus bound phospholipids with low affinity and according to valence (PIP(3) > PIP(2) > PIP = PA > PS). NHE1-phosphoinositide binding was enhanced by acidic pH, and abolished by NHE1 Arg/Lys to Ala mutations within two juxtamembrane domains, consistent with electrostatic interactions. PI(4,5)P(2)-incorporated vesicles were distributed to apical and lateral PTC domains, increased NHE1-regulated Na(+)/H(+) exchange, and blunted apoptosis, whereas NHE1 activity was decreased in cells enriched with PI(3,4,5)P(3), which localized to basolateral membranes. Divergent PI(4,5)P(2) and PI(3,4,5)P(3) effects on NHE1-dependent Na(+)/H(+) exchange and apoptosis were confirmed by selective phosphoinositide sequestration with pleckstrin homology domain-containing phospholipase Cδ and Akt peptides, PI 3-kinase, and Akt inhibition in wild-type and NHE1-null PTCs. The results reveal an on-off switch model, whereby NHE1 toggles between weak interactions with PI(4,5)P(2) and PI(3,4,5)P(3). In response to apoptotic stress, NHE1 is stimulated by PI(4,5)P(2), which leads to PI 3-kinase activation, and PI(4,5)P(2) phosphorylation. The resulting PI(3,4,5)P(3) dually stimulates sustained, downstream Akt survival signaling, and dampens NHE1 activity through competitive inhibition and depletion of PI(4,5)P(2).

Molecular coupling in the human ether-a-go-go-related gene-1 (hERG1) K+ channel inactivation pathway.

Emerging evidence suggests that K(+) channel inactivation involves coupling between residues in adjacent regions of the channel. Human ether-a-go-go-related gene-1 (hERG1) K(+) channels undergo a fast inactivation gating process that is crucial for maintaining electrical stability in the heart. The molecular mechanisms that drive inactivation in hERG1 channels are unknown. Using alanine scanning mutagenesis, we show that a pore helix residue (Thr-618) that points toward the S5 segment is critical for normal inactivation gating. Amino acid substitutions at position 618 modulate the free energy of inactivation gating, causing enhanced or reduced inactivation. Mutation of an S5 residue that is predicted to be adjacent to Thr-618 (W568L) abolishes inactivation and alters ion selectivity. The introduction of the Thr-618-equivalent residue in Kv1.5 enhances inactivation. Molecular dynamic simulations of the Kv1.2 tetramer reveal van der Waals coupling between hERG1 618- and 568-equivalent residues and a significant increase in interaction energies when threonine is introduced at the 618-equivalent position. We propose that coupling between the S5 segment and pore helix may participate in the inactivation process in hERG1 channels.

A novel strategy using cardiac sodium channel polymorphic fragments to rescue trafficking-deficient SCN5A mutations.

BACKGROUND: Brugada syndrome (BrS) is associated with mutations in the cardiac sodium channel (Na(v)1.5). We previously reported that the function of a trafficking-deficient BrS Na(v)1.5 mutation, R282H, could be restored by coexpression with the sodium channel polymorphism H558R. Here, we tested the hypothesis that peptide fragments from Na(v)1.5, spanning the H558R polymorphism, can be used to restore trafficking of trafficking-deficient BrS sodium channel mutations. METHODS AND RESULTS: Whole-cell patch clamping revealed that cotransfection in human embryonic kidney (HEK293) cells of the R282H channel with either the 40- or 20-amino acid cDNA fragments of Na(v)1.5 containing the H558R polymorphism restored trafficking of this mutant channel. Fluorescence resonance energy transfer suggested that the trafficking-deficient R282H channel was misfolded, and this was corrected on coexpression with R558-containing peptides that restored trafficking of the R282H channel. Importantly, we also expressed the peptide spanning the H558R polymorphism with 8 additional BrS Na(v)1.5 mutations with reduced currents and demonstrated that the peptide was able to restore significant sodium currents in 4 of them. CONCLUSIONS: In the present study, we demonstrate that small peptides, spanning the H558R polymorphism, are sufficient to restore the trafficking defect of BrS-associated Na(v)1.5 mutations. Our findings suggest that it might be possible to use short cDNA constructs as a novel strategy tailored to specific disease-causing mutants of BrS.

Antidepressant-induced ubiquitination and degradation of the cardiac potassium channel hERG.

The most common cause for adverse cardiac events by antidepressants is acquired long QT syndrome (acLQTS), which produces electrocardiographic abnormalities that have been associated with syncope, torsade de pointes arrhythmias, and sudden cardiac death. acLQTS is often caused by direct block of the cardiac potassium current I(Kr)/hERG, which is crucial for terminal repolarization in human heart. Importantly, desipramine belongs to a group of tricyclic antidepressant compounds that can simultaneously block hERG and inhibit its surface expression. Although up to 40% of all hERG blockers exert combined hERG block and trafficking inhibition, few of these compounds have been fully characterized at the cellular level. Here, we have studied in detail how desipramine inhibits hERG surface expression. We find a previously unrecognized combination of two entirely different mechanisms; desipramine increases hERG endocytosis and degradation as a consequence of drug-induced channel ubiquitination and simultaneously inhibits hERG forward trafficking from the endoplasmic reticulum. This unique combination of cellular effects in conjunction with acute channel block may explain why tricyclic antidepressants as a compound class are notorious for their association with arrhythmias and sudden cardiac death. Taken together, we describe the first example of drug-induced channel ubiquitination and degradation. Our data are directly relevant to the cardiac safety of not only tricyclic antidepressants but also other therapeutic compounds that exert multiple effects on hERG, as hERG trafficking and degradation phenotypes may go undetected in most preclinical safety assays designed to screen for acLQTS.

hERG K+ channel-associated cardiac effects of the antidepressant drug desipramine.

Cardiac side effects of antidepressant drugs are well recognized. Adverse effects precipitated by the tricyclic drug desipramine include prolonged QT intervals, torsade de pointes tachycardia, heart failure, and sudden cardiac death. QT prolongation has been primarily attributed to acute blockade of hERG/I(Kr) currents. This study was designed to provide a more complete picture of cellular effects associated with desipramine. hERG channels were expressed in Xenopus laevis oocytes and human embryonic kidney (HEK 293) cells, and potassium currents were recorded using patch clamp and two-electrode voltage clamp electrophysiology. Ventricular action potentials were recorded from guinea pig cardiomyocytes. Protein trafficking and cell viability were evaluated in HEK 293 cells and in HL-1 mouse cardiomyocytes by immunocytochemistry, Western blot analysis, or colorimetric MTT assay, respectively. We found that desipramine reduced hERG currents by binding to a receptor site inside the channel pore. hERG protein surface expression was reduced after short-term treatment, revealing a previously unrecognized mechanism. When long-term effects were studied, forward trafficking was impaired and hERG currents were decreased. Action potential duration was prolonged upon acute and chronic desipramine exposure. Finally, desipramine triggered apoptosis in cells expressing hERG channels. Desipramine exerts at least four different cellular effects: (1) direct hERG channel block, (2) acute reduction of hERG surface expression, (3) chronic disruption of hERG trafficking, and (4) induction of apoptosis. These data highlight the complexity of hERG-associated drug effects.

A common SCN5A polymorphism modulates the biophysical defects of SCN5A mutations.

BACKGROUND: Defects in the cardiac sodium channel gene, SCN5A, can cause a broad spectrum of inherited arrhythmia syndromes. After genotyping of a proband who presented with syncope, the SCN5A mutant P2006A and the common polymorphism H558R were identified. OBJECTIVE: The main objective of this study was to determine whether the SCN5A-H558R polymorphism could modify the defective gating kinetics observed in the P2006A mutation and therefore explain why this gain-of-function mutation has been identified in control populations. METHODS: Mutations were engineered using site-directed mutagenesis and heterologously expressed transiently in HEK293 cells. Whole-cell sodium currents were measured at room temperature using the whole-cell patch-clamp technique. RESULTS: In HEK293 cells, P2006A displayed biophysical defects typically associated with long QT syndrome by increasing persistent sodium current, producing a depolarizing shift in voltage dependence of inactivation, and hastening recovery from inactivation. Interestingly, when coexpressed either on the same or different genes, P2006A and H558R displayed currents that behaved like wild type (WT). We also investigated whether H558R can modulate the gating defects of other SCN5A mutations. The H558R polymorphism also restored the gating defects of the SCN5A mutation V1951L to the WT level. CONCLUSIONS: Our results suggest that H558R might play an important role in stabilization of channel fast inactivation and may provide a plausible explanation as to why the P2006A gain-of-function mutation has been identified in control populations. Our results also suggest that the SCN5A polymorphism H558R might be a disease-modifying gene.

Oxidative inactivation of the lipid phosphatase phosphatase and tensin homolog on chromosome ten (PTEN) as a novel mechanism of acquired long QT syndrome.

The most common cause of cardiac side effects of pharmaco-therapy is acquired long QT syndrome, which is characterized by abnormal cardiac repolarization and most often caused by direct blockade of the cardiac potassium channel human ether a-go-go-related gene (hERG). However, little is known about therapeutic compounds that target ion channels other than hERG. We have discovered that arsenic trioxide (As(2)O(3)), a very potent antineoplastic compound for the treatment of acute promyelocytic leukemia, is proarrhythmic via two separate mechanisms: a well characterized inhibition of hERG/I(Kr) trafficking and a poorly understood increase of cardiac calcium currents. We have analyzed the latter mechanism in the present study using biochemical and electrophysiological methods. We find that oxidative inactivation of the lipid phosphatase PTEN by As(2)O(3) enhances cardiac calcium currents in the therapeutic concentration range via a PI3Kα-dependent increase in phosphatidylinositol 3,4,5-triphosphate (PIP(3)) production. In guinea pig ventricular myocytes, even a modest reduction in PTEN activity is sufficient to increase cellular PIP(3) levels. Under control conditions, PIP(3) levels are kept low by PTEN and do not affect calcium current amplitudes. Based on pharmacological experiments and intracellular infusion of PIP(3), we propose that in guinea pig ventricular myocytes, PIP(3) regulates calcium currents independently of the protein kinase Akt along a pathway that includes a secondary oxidation-sensitive target. Overall, our report describes a novel form of acquired long QT syndrome where the target modified by As(2)O(3) is an intracellular signaling cascade.

Multiple mechanisms of hERG liability: K+ current inhibition, disruption of protein trafficking, and apoptosis induced by amoxapine.

The antidepressant amoxapine has been linked to cases of QT prolongation, acute heart failure, and sudden death. Inhibition of cardiac hERG (Kv11.1) potassium channels causes prolonged repolarization and is implicated in apoptosis. Apoptosis in association with amoxapine has not yet been reported. This study was designed to investigate amoxapine effects on hERG currents, hERG protein trafficking, and hERG-associated apoptosis in order to elucidate molecular mechanisms underlying cardiac side effects of the drug. hERG channels were expressed in Xenopus laevis oocytes and HEK 293 cells, and potassium currents were recorded using patch clamp and two-electrode voltage clamp electrophysiology. Protein trafficking was evaluated in HEK 293 cells by Western blot analysis, and cell viability was assessed in HEK cells by immunocytochemistry and colorimetric MTT assay. Amoxapine caused acute hERG blockade in oocytes (IC(50) = 21.6 microM) and in HEK 293 cells (IC(50) = 5.1 microM). Mutation of residues Y652 and F656 attenuated hERG blockade, suggesting drug binding to a receptor inside the channel pore. Channels were mainly blocked in open and inactivated states, and voltage dependence was observed with reduced inhibition at positive potentials. Amoxapine block was reverse frequency-dependent and caused accelerated and leftward-shifted inactivation. Furthermore, amoxapine application resulted in chronic reduction of hERG trafficking into the cell surface membrane (IC(50) = 15.3 microM). Finally, the antidepressant drug triggered apoptosis in cells expressing hERG channels. We provide evidence for triple mechanisms of hERG liability associated with amoxapine: (1) direct hERG current inhibition, (2) disruption of hERG protein trafficking, and (3) induction of apoptosis. Further experiments are required to validate a specific pro-apoptotic effect mediated through blockade of hERG channels.