A common polymorphism associated with antibiotic-induced cardiac arrhythmia

Investigating the cardiotoxicity of N-n-butyl haloperidol iodide: Inhibition mechanisms on hERG channels

The human Ether-à-go-go-Related Gene (hERG) encodes a protein responsible for forming the alpha subunit of the IKr channel, which plays a crucial role in cardiac repolarization. The proper functioning of hERG channels is paramount in maintaining a normal cardiac rhythm. Inhibition of these channels can result in the prolongation of the QT interval and potentially life-threatening arrhythmias. Cardiotoxicity is a primary concern in the field of drug development. N-n-Butyl haloperidol iodide (F2), a derivative of haloperidol, has been investigated for its therapeutic potential. However, the impact of this compound on cardiac toxicity, specifically on hERG channels, remains uncertain. This study employs computational and experimental methodologies to examine the inhibitory mechanisms of F2 on hERG channels. Molecular docking and molecular dynamics simulations commonly used techniques in computational biology to predict protein-ligand complexes' binding interactions and stability. In the context of the F2-hERG complex, these methods can provide valuable insights into the potential binding modes and strength of interaction between F2 and the hERG protein. On the other hand, electrophysiological assays are experimental techniques used to characterize the extent and nature of hERG channel inhibition caused by various compounds. By measuring the electrical activity of the hERG channel in response to different stimuli, these assays can provide important information about the functional effects of ligand binding to the channel. The study's key findings indicate that F2 interacts with the hERG channel by forming hydrogen bonding, π-cation interactions, and hydrophobic forces. This interaction leads to the inhibition of hERG currents in a concentration-dependent manner, with an IC50 of 3.75 μM. The results presented in this study demonstrate the potential cardiotoxicity of F2 and underscore the significance of considering hERG channel interactions during its clinical development. This study aims to provide comprehensive insights into the interaction between F2 and hERG, which will may guid us in the safe use of F2 and in the development of new derivatives with high efficiency while low toxicity.

Genetic risk factors for drug-induced long QT syndrome: findings from a large real-world case-control study

Aim: Drug-induced long QT syndrome (diLQTS), an adverse effect of many drugs, can lead to sudden cardiac death. Candidate genetic variants in cardiac ion channels have been associated with diLQTS, but several limitations of previous studies hamper clinical utility. Materials & methods: Thus, the purpose of this study was to assess the associations of KCNE1-D85N, KCNE2-I57T and SCN5A-G615E with diLQTS in a large observational case-control study (6,083 self-reported white patients treated with 27 different high-risk QT-prolonging medications; 12.0% with diLQTS). Results: KCNE1-D85N significantly associated with diLQTS (adjusted odds ratio: 2.24 [95% CI: 1.35-3.58]; p = 0.001). Given low minor allele frequencies, the study had insufficient power to analyze KCNE2-I57T and SCN5A-G615E. Conclusion: KCNE1-D85N is a risk factor for diLQTS that should be considered in future clinical practice guidelines.

A case with a trend of QT interval prolongation due to the introduction of methadone to a pancreatic cancer patient on levofloxacin

BACKGROUND

As methadone can prevent the development of opioid resistance, it has application in alleviating cancer-related pain that proves challenging to manage with other opioids. QT interval prolongation is a serious side effect of methadone treatment, with some reported deaths. In particular, owing to the increased risk of QT interval prolongation, caution should be exercised when using it in combination with drugs that also prolong the QT interval.

CASE PRESENTATION

This study presents a case in which methadone was introduced to a patient (a man in his 60s) already using levofloxacin, which could prolong the QT interval-a serious side effect of methadone treatment-and whose QTc value tended to increase. Given that levofloxacin can increase the risk of QT interval prolongation, we considered switching to other antibacterial agents before introducing methadone. However, because the neurosurgeon judged that controlling a brain abscess was a priority, low-dose methadone was introduced with continuing levofloxacin. Owing to the risks, we performed frequent electrocardiograms. Consequently, we responded before the QTc increased enough to meet the diagnostic criteria for QT interval prolongation. Consequently, we prevented the occurrence of drug-induced long QT syndrome.

CONCLUSIONS

When considering the use of methadone for intractable cancer pain, it is important to eliminate possible risk factors for QT interval prolongation. However, as it may be difficult to discontinue concomitant drugs owing to comorbidities, there could be cases in which the risk of QT interval prolongation could increase, even with the introduction of low-dose methadone. In such cases, frequent monitoring, even with simple measurements such as those used in this case, is likely to prevent progression to more serious conditions.

The 2023 Canadian Cardiovascular Society Clinical Practice Update on Management of the Patient With a Prolonged QT Interval

A prolonged QT interval on the electrocardiogram is associated with an increased risk of the torsades de pointes form of ventricular arrhythmia resulting in syncope, sudden cardiac arrest or death, or misdiagnosis as a seizure disorder. The cause of QT prolongation can be congenital and inherited as an autosomal dominant variant, or it can be transient and acquired, often because of QT-prolonging drugs or electrolyte abnormalities. Automated measurement of the QT interval can be inaccurate, especially when the baseline electrocardiogram is abnormal, and manual verification is recommended. In this clinical practice update we provide practical tips about measurement of the QT interval, diagnosis, and management of congenital long QT syndrome and acquired prolongation of the QT interval. For congenital long QT syndrome, certain β-adrenergic-blocking drugs are highly effective, and implantable defibrillators are infrequently required. Many commonly prescribed drugs such as antidepressants and antibiotics can prolong the QT interval, and recommendations are provided on their safe use.

Rate-dependent effects of state-specific sodium channel blockers in cardiac tissue: Insights from idealized models

A common side effect of pharmaceutical drugs is an increased propensity for cardiac arrhythmias. Many drugs bind to cardiac ion-channels in a state-specific manner, which alters the ionic conductances in complicated ways, making it difficult to identify the mechanisms underlying pro-arrhythmic drug effects. To better understand the fundamental mechanisms underlying the diverse effects of state-dependent sodium (Na+) channel blockers on cellular excitability, we consider two canonical motifs of drug-ion-channel interactions and compare the effects of Na+ channel blockers on the rate-dependence of peak upstroke velocity, conduction velocity, and vulnerable window size. In the literature, both motifs are referred to as "guarded receptor," but here we distinguish between state-specific binding that does not alter channel gating (referred to here as "guarded receptor") and state-specific binding that blocks certain gating transitions ("gate immobilization"). For each drug binding motif, we consider drugs that bind to the inactivated state and drugs that bind to the non-inactivated state of the Na+ channel. Exploiting the idealized nature of the canonical binding motifs, we identify the fundamental mechanisms underlying the effects on excitability of the various binding interactions. Specifically, we derive the voltage-dependence of the drug binding time constants and the equilibrium fractions of channels bound to drug, and we then derive a formula that incorporates these time constants and equilibrium fractions to elucidate the fundamental mechanisms. In the case of charged drug, we find that drugs that bind to inactivated channels exhibit greater rate-dependence than drugs that bind to non-inactivated channels. For neutral drugs, the effects of guarded receptor interactions are rate-independent, and we describe a novel mechanism for reverse rate-dependence resulting from neutral drug binding to non-inactivated channels via the gate immobilization motif.

Kv Channel Ancillary Subunits: Where Do We Go from Here?

Voltage-gated potassium (Kv) channels each comprise four pore-forming α-subunits that orchestrate essential duties such as voltage sensing and K+ selectivity and conductance. In vivo, however, Kv channels also incorporate regulatory subunits-some Kv channel specific, others more general modifiers of protein folding, trafficking, and function. Understanding all the above is essential for a complete picture of the role of Kv channels in physiology and disease.

The genetics of drug-induced QT prolongation: evaluating the evidence for pharmacodynamic variants

Drug-induced long QT syndrome (diLQTS) is an adverse effect of many commonly prescribed drugs, and it can increase the risk for lethal ventricular arrhythmias. Genetic variants in pharmacodynamic genes have been associated with diLQTS, but the strength of the evidence for each of those variants has not yet been evaluated. Therefore, the purpose of this review was to evaluate the strength of the evidence for pharmacodynamic genetic variants associated with diLQTS using a novel, semiquantitative scoring system modified from the approach used for congenital LQTS. KCNE1-D85N and KCNE2-T8A had definitive and strong evidence for diLQTS, respectively. The high level of evidence for these variants supports current consideration as risk factors for patients that will be prescribed a QT-prolonging drug.

Rare Variation in Drug Metabolism and Long QT Genes and the Genetic Susceptibility to Acquired Long QT Syndrome

BACKGROUND

Acquired long QT syndrome (aLQTS) is a serious unpredictable adverse drug reaction. Pharmacogenomic markers may predict risk.

METHODS

Among 153 aLQTS patients (mean age 58 years [range, 14-88], 98.7% White, 85.6% symptomatic), computational methods identified proteins interacting most significantly with 216 QT-prolonging drugs. All cases underwent sequencing of 31 candidate genes arising from this analysis or associating with congenital LQTS. Variants were filtered using a minor allele frequency <1% and classified for susceptibility for aLQTS. Gene-burden analyses were then performed comparing the primary cohort to control exomes (n=452) and an independent replication aLQTS exome sequencing cohort.

RESULTS

In 25.5% of cases, at least one rare variant was identified: 22.2% of cases carried a rare variant in a gene associated with congenital LQTS, and in 4% of cases that variant was known to be pathogenic or likely pathogenic for congenital LQTS; 7.8% cases carried a cytochrome-P450 (CYP) gene variant. Of 12 identified CYP variants, 11 (92%) were in an enzyme known to metabolize at least one culprit drug to which the subject had been exposed. Drug-drug interactions that affected culprit drug metabolism were found in 19% of cases. More than one congenital LQTS variant, CYP gene variant, or drug interaction was present in 7.8% of cases. Gene-burden analyses of the primary cohort compared to control exomes (n=452), and an independent replication aLQTS exome sequencing cohort (n=67) and drug-tolerant controls (n=148) demonstrated an increased burden of rare (minor allele frequency<0.01) variants in CYP genes but not LQTS genes.

CONCLUSIONS

Rare susceptibility variants in CYP genes are emerging as potentially important pharmacogenomic risk markers for aLQTS and could form part of personalized medicine approaches in the future.

Functionalizing Fibrin Hydrogels with Thermally Responsive Oligonucleotide Tethers for On-Demand Delivery

There are a limited number of stimuli-responsive biomaterials that are capable of delivering customizable dosages of a therapeutic at a specific location and time. This is especially true in tissue engineering and regenerative medicine applications, where it may be desirable for the stimuli-responsive biomaterial to also serve as a scaffolding material. Therefore, the purpose of this study was to engineer a traditionally non-stimuli responsive scaffold biomaterial to be thermally responsive so it could be used for on-demand drug delivery applications. Fibrin hydrogels are frequently used for tissue engineering and regenerative medicine applications, and they were functionalized with thermally labile oligonucleotide tethers using peptides from substrates for factor XIII (FXIII). The alpha 2-plasmin inhibitor peptide had the greatest incorporation efficiency out of the FXIII substrate peptides studied, and conjugates of the peptide and oligonucleotide tethers were successfully incorporated into fibrin hydrogels via enzymatic activity. Single-strand complement oligo with either a fluorophore model drug or platelet-derived growth factor-BB (PDGF-BB) could be released on demand via temperature increases. These results demonstrate a strategy that can be used to functionalize traditionally non-stimuli responsive biomaterials suitable for on-demand drug delivery systems (DDS).

Clinical Genetics of Inherited Arrhythmogenic Disease in the Pediatric Population

Sudden death is a rare event in the pediatric population but with a social shock due to its presentation as the first symptom in previously healthy children. Comprehensive autopsy in pediatric cases identify an inconclusive cause in 40-50% of cases. In such cases, a diagnosis of sudden arrhythmic death syndrome is suggested as the main potential cause of death. Molecular autopsy identifies nearly 30% of cases under 16 years of age carrying a pathogenic/potentially pathogenic alteration in genes associated with any inherited arrhythmogenic disease. In the last few years, despite the increasing rate of post-mortem genetic diagnosis, many families still remain without a conclusive genetic cause of the unexpected death. Current challenges in genetic diagnosis are the establishment of a correct genotype-phenotype association between genes and inherited arrhythmogenic disease, as well as the classification of variants of uncertain significance. In this review, we provide an update on the state of the art in the genetic diagnosis of inherited arrhythmogenic disease in the pediatric population. We focus on emerging publications on gene curation for genotype-phenotype associations, cases of genetic overlap and advances in the classification of variants of uncertain significance. Our goal is to facilitate the translation of genetic diagnosis to the clinical area, helping risk stratification, treatment and the genetic counselling of families.

Control of Biophysical and Pharmacological Properties of Potassium Channels by Ancillary Subunits

Potassium channels facilitate and regulate physiological processes as diverse as electrical signaling, ion, solute and hormone secretion, fluid homeostasis, hearing, pain sensation, muscular contraction, and the heartbeat. Potassium channels are each formed by either a tetramer or dimer of pore-forming α subunits that co-assemble to create a multimer with a K⁺-selective pore that in most cases is capable of functioning as a discrete unit to pass K⁺ ions across the cell membrane. The reality in vivo, however, is that the potassium channel α subunit multimers co-assemble with ancillary subunits to serve specific physiological functions. The ancillary subunits impart specific physiological properties that are often required for a particular activity in vivo; in addition, ancillary subunit interaction often alters the pharmacology of the resultant complex. In this chapter the modes of action of ancillary subunits on K⁺ channel physiology and pharmacology are described and categorized into various mechanistic classes.

Importance of evaluating protein glycosylation in pluripotent stem cell-derived cardiomyocytes for research and clinical applications

Proper protein glycosylation is critical to normal cardiomyocyte physiology. Aberrant glycosylation can alter protein localization, structure, drug interactions, and cellular function. The in vitro differentiation of human pluripotent stem cells into cardiomyocytes (hPSC-CM) has become increasingly important to the study of protein function and to the fields of cardiac disease modeling, drug testing, drug discovery, and regenerative medicine. Here, we offer our perspective on the importance of protein glycosylation in hPSC-CM. Protein glycosylation is dynamic in hPSC-CM, but the timing and extent of glycosylation are still poorly defined. We provide new data highlighting how observed changes in hPSC-CM glycosylation may be caused by underlying differences in the protein or transcript abundance of enzymes involved in building and trimming the glycan structures or glycoprotein gene products. We also provide evidence that alternative splicing results in altered sites of glycosylation within the protein sequence. Our findings suggest the need to precisely define protein glycosylation events that may have a critical impact on the function and maturation state of hPSC-CM. Finally, we provide an overview of analytical strategies available for studying protein glycosylation and identify opportunities for the development of new bioinformatic approaches to integrate diverse protein glycosylation data types. We predict that these tools will promote the accurate assessment of protein glycosylation in future studies of hPSC-CM that will ultimately be of significant experimental and clinical benefit.

KCNE2 gene mutation and Brugada syndrome

KCNE2 gene mutations have been associated with atrial fibrillation, long QT syndrome, Brugada syndrome and unexplained sudden cardiac death. Herein, we describe a case of Brugada syndrome carrying an heterozygous variant in the KCNE2 gene [NM_172201.2:c.161 T > C, p.(Met54Thr, M54T)]. Gain of function of the Ito current possibly explains the Brugada ECG phenotype in this case.

Long QT syndrome - Bench to bedside

Long QT syndrome (LQTS) is a cardiovascular disorder characterized by an abnormality in cardiac repolarization leading to a prolonged QT interval and T-wave irregularities on the surface electrocardiogram. It is commonly associated with syncope, seizures, susceptibility to torsades de pointes, and risk for sudden death. LQTS is a rare genetic disorder and a major preventable cause of sudden cardiac death in the young. The availability of therapy for this lethal disease emphasizes the importance of early and accurate diagnosis. Additionally, understanding of the molecular mechanisms underlying LQTS could help to optimize genotype-specific treatments to prevent deaths in LQTS patients.

In this review, we briefly summarize current knowledge regarding molecular underpinning of LQTS, in particular focusing on LQT1, LQT2, and LQT3, and discuss novel strategies to study ion channel dysfunction and drug-specific therapies in LQT1, LQT2, and LQT3 syndromes.

Cardiac transmembrane ion channels and action potentials: cellular physiology and arrhythmogenic behavior

Cardiac arrhythmias are among the leading causes of mortality. They often arise from alterations in the electrophysiological properties of cardiac cells and their underlying ionic mechanisms. It is therefore critical to further unravel the pathophysiology of the ionic basis of human cardiac electrophysiology in health and disease. In the first part of this review, current knowledge on the differences in ion channel expression and properties of the ionic processes that determine the morphology and properties of cardiac action potentials and calcium dynamics from cardiomyocytes in different regions of the heart are described. Then the cellular mechanisms promoting arrhythmias in congenital or acquired conditions of ion channel function (electrical remodeling) are discussed. The focus is on human-relevant findings obtained with clinical, experimental, and computational studies, given that interspecies differences make the extrapolation from animal experiments to human clinical settings difficult. Deepening the understanding of the diverse pathophysiology of human cellular electrophysiology will help in developing novel and effective antiarrhythmic strategies for specific subpopulations and disease conditions.

A computational model of induced pluripotent stem-cell derived cardiomyocytes for high throughput risk stratification of KCNQ1 genetic variants

In the last decade, there has been tremendous progress in identifying genetic anomalies linked to clinical disease. New experimental platforms have connected genetic variants to mechanisms underlying disruption of cellular and organ behavior and the emergence of proarrhythmic cardiac phenotypes. The development of induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) signifies an important advance in the study of genetic disease in a patient-specific context. However, considerable limitations of iPSC-CM technologies have not been addressed: 1) phenotypic variability in apparently identical genotype perturbations, 2) low-throughput electrophysiological measurements, and 3) an immature phenotype which may impact translation to adult cardiac response. We have developed a computational approach intended to address these problems. We applied our recent iPSC-CM computational model to predict the proarrhythmic risk of 40 KCNQ1 genetic variants. An I_Ks computational model was fit to experimental data for each mutation, and the impact of each mutation was simulated in a population of iPSC-CM models. Using a test set of 15 KCNQ1 mutations with known clinical long QT phenotypes, we developed a method to stratify the effects of KCNQ1 mutations based on proarrhythmic markers. We utilized this method to predict the severity of the remaining 25 KCNQ1 mutations with unknown clinical significance. Tremendous phenotypic variability was observed in the iPSC-CM model population following mutant perturbations. A key novelty is our reporting of the impact of individual KCNQ1 mutant models on adult ventricular cardiomyocyte electrophysiology, allowing for prediction of mutant impact across the continuum of aging. This serves as a first step toward translating predicted response in the iPSC-CM model to predicted response of the adult ventricular myocyte given the same genetic mutation. As a whole, this study presents a new computational framework that serves as a high throughput method to evaluate risk of genetic mutations based-on proarrhythmic behavior in phenotypically variable populations.

In the last decade, there has been tremendous progress in identifying genetic mutations linked to clinical diseases, such as cardiac arrhythmia. Many experimental platforms have been developed to study this link, including induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs). IPSC-CMs are patient-derived cardiac cells which allow for the study of genetic variants within a patient-specific context. However, experimentally iPSC-CMs have certain limitations, including: (1) they exhibit variability in behavior within cells that are apparently genetically identical, and (2) they are immature compared to adult cardiac cells. In our study, we have developed a computational approach to model 40 genetic variants in the KCNQ1 gene and predict the proarrhythmic risk of each variant. To do this, we modeled the ionic current determined by KCNQ1, I_Ks, to fit experimental data for each mutation. We then simulated the impact of each mutation in a population of iPSC-CMs, incorporating variability across the population. We also simulated each variant in an adult cardiac cell model, providing a link between iPSC-CM response to mutants and adult cardiac cell response to the same mutants. Overall, this study provides a new computational framework to evaluate risk of genetic mutations based-on proarrhythmic behavior diverse populations of iPSC-CM models.

Hypochlorhydria reduces mortality in heart failure caused by Kcne2 gene deletion

Heart failure (HF) is an increasing global health crisis, affecting 40 million people and causing 50% mortality within 5 years of diagnosis. A fuller understanding of the genetic and environmental factors underlying HF, and novel therapeutic approaches to address it, are urgently warranted. Here, we discovered that cardiac-specific germline deletion in mice of potassium channel β subunit-encoding Kcne2 (Kcne2^CS-/- ) causes dilated cardiomyopathy and terminal HF (median longevity, 28 weeks). Mice with global Kcne2 deletion (Kcne2^Glo-/- ) exhibit multiple HF risk factors, yet, paradoxically survived over twice as long as Kcne2^CS-/- mice. Global Kcne2 deletion, which inhibits gastric acid secretion, reduced the relative abundance of species within Bacteroidales, a bacterial order that positively correlates with increased lifetime risk of human cardiovascular disease. Strikingly, the proton-pump inhibitor omeprazole similarly altered the microbiome and delayed terminal HF in Kcne2^CS-/- mice, increasing survival 10-fold at 44 weeks. Thus, genetic or pharmacologic induction of hypochlorhydria and decreased gut Bacteroidales species are associated with lifespan extension in a novel HF model.

A current understanding of drug-induced QT prolongation and its implications for anticancer therapy

The QT interval, a global index of ventricular repolarization, varies among individuals and is influenced by diverse physiologic and pathophysiologic stimuli such as gender, age, heart rate, electrolyte concentrations, concomitant cardiac disease, and other diseases such as diabetes. Many drugs produce a small but reproducible effect on QT interval but in rare instances this is exaggerated and marked QT prolongation can provoke the polymorphic ventricular tachycardia 'torsades de pointes', which can cause syncope or sudden cardiac death. The generally accepted common mechanism whereby drugs prolong QT is block of a key repolarizing potassium current in heart, IKr, generated by expression of KCNH2, also known as HERG. Thus, evaluation of the potential that a new drug entity may cause torsades de pointes has relied on exposure of normal volunteers or patients to drug at usual and high concentrations, and on assessment of IKr block in vitro. More recent work, focusing on anticancer drugs with QT prolonging liability, is defining new pathways whereby drugs can prolong QT. Notably, the in vitro effects of some tyrosine kinase inhibitors to prolong cardiac action potentials (the cellular correlate of QT) can be rescued by intracellular phosphatidylinositol 3,4,5-trisphosphate, the downstream effector of phosphoinositide 3-kinase. This finding supports a role for inhibition of this enzyme, either directly or by inhibition of upstream kinases, to prolong QT through mechanisms that are being worked out, but include enhanced inward 'late' sodium current during the plateau of the action potential. The definition of non-IKr-dependent pathways to QT prolongation will be important for assessing risk, not only with anticancer therapies but also with other QT prolonging drugs and for generating a refined understanding how variable activity of intracellular signalling systems can modulate QT and associated arrhythmia risk.

Structures Illuminate Cardiac Ion Channel Functions in Health and in Long QT Syndrome

The cardiac action potential is critical to the production of a synchronized heartbeat. This electrical impulse is governed by the intricate activity of cardiac ion channels, among them the cardiac voltage-gated potassium (Kv) channels KCNQ1 and hERG as well as the voltage-gated sodium (Nav) channel encoded by SCN5A. Each channel performs a highly distinct function, despite sharing a common topology and structural components. These three channels are also the primary proteins mutated in congenital long QT syndrome (LQTS), a genetic condition that predisposes to cardiac arrhythmia and sudden cardiac death due to impaired repolarization of the action potential and has a particular proclivity for reentrant ventricular arrhythmias. Recent cryo-electron microscopy structures of human KCNQ1 and hERG, along with the rat homolog of SCN5A and other mammalian sodium channels, provide atomic-level insight into the structure and function of these proteins that advance our understanding of their distinct functions in the cardiac action potential, as well as the molecular basis of LQTS. In this review, the gating, regulation, LQTS mechanisms, and pharmacological properties of KCNQ1, hERG, and SCN5A are discussed in light of these recent structural findings.

AKT and ERK1/2 activation via remote ischemic preconditioning prevents Kcne2-dependent sudden cardiac death

Sudden cardiac death (SCD) is the leading global cause of mortality. SCD often arises from cardiac ischemia reperfusion (IR) injury, pathologic sequence variants within ion channel genes, or a combination of the two. Alternative approaches are needed to prevent or ameliorate ventricular arrhythmias linked to SCD. Here, we investigated the efficacy of remote ischemic preconditioning (RIPC) of the limb versus the liver in reducing ventricular arrhythmias in a mouse model of SCD. Mice lacking the Kcne2 gene, which encodes a potassium channel β subunit associated with acquired Long QT syndrome were exposed to IR injury via coronary ligation. This resulted in ventricular arrhythmias in all mice (15/15) and SCD in 5/15 mice during reperfusion. Strikingly, prior RIPC (limb or liver) greatly reduced the incidence and severity of all ventricular arrhythmias and completely prevented SCD. Biochemical and pharmacological analysis demonstrated that RIPC cardioprotection required ERK1/2 and/or AKT phosphorylation. A lack of alteration in GSK‐3β phosphorylation suggested against conventional reperfusion injury salvage kinase (RISK) signaling pathway protection. If replicated in human studies, limb RIPC could represent a noninvasive, nonpharmacological approach to limit dangerous ventricular arrhythmias associated with ischemia and/or channelopathy‐linked SCD.

QTc Prolongation and Risk of Torsades de Pointes in Hospitalized Pediatric Oncology Patients

OBJECTIVE

To evaluate the prevalence of torsades de pointes and to identify risk factors associated with QTc prolongation of ≥500 milliseconds in hospitalized pediatric oncology patients. A QTc prolongation of ≥500 milliseconds is associated with higher mortality in hospitalized adults but has not been demonstrated in pediatrics.

STUDY DESIGN

A single-center, retrospective review of all hospitalized oncology patients ≤21 years of age was performed from 2014 to 2016. Patients with long/short QT syndrome or a QRS interval of ≥120 ms were excluded. Rapid response events were reviewed to determine the prevalence of torsades. In patients with ECGs for review, data were compared between patients with a QTc of <500 and ≥500 ms via logistic regression.

RESULTS

There were 1934 hospitalized patients included. Rapid response events occurred in 90 patients (4.7%) with 2 torsades events (0.1%). There were 1412 electrocardiograms performed in 287 unique patients (10.6 ± 6.3 years of age; 43% female). The mean QTc was 448 ± 31 ms; 25 patients (8.7%) had ≥1 ECG with a QTc of ≥500 ms. The prevalence of torsades was greater in patients with a QTc of ≥500 ms (8% vs 0%; P<.01). In multivariate analysis, factors associated with a QTc of ≥500 ms included female sex, (OR 2.95) and ≥2 QT-prolonging medications (OR, 2.95).

CONCLUSIONS

The prevalence of torsades in hospitalized pediatric oncology patients was low (0.1%), although the risk was significantly greater in patients with a QTc of ≥500 ms. Routine monitoring of electrocardiograms and electrolytes is essential in patients with risk factors predisposing to QTc prolongation.

A Rare Case of the Digenic Inheritance of Long QT Syndrome Type 2 and Type 6

We report a 37-year-old woman with an out-of-hospital cardiac arrest caused by ventricular fibrillation due to digenic inheritance of long QT syndrome type 2 (KCNH2 gene) and type 6 (KCNE2 gene). During hospitalization, prolonged QTc intervals and frequent episodes of ventricular tachyarrhythmias manifested. Genetic testing identified a mutation of the KCNH2 gene and an unclassified variant, most likely pathogenic, of the KCNE2 gene. This digenic inheritance is extremely rare.

Disease-linked mutations alter the stoichiometries of HCN-KCNE2 complexes

The four hyperpolarization-activated cylic-nucleotide gated (HCN) channel isoforms and their auxiliary subunit KCNE2 are important in the regulation of peripheral and central neuronal firing and the heartbeat. Disruption of their normal function has been implicated in cardiac arrhythmias, peripheral pain, and epilepsy. However, molecular details of the HCN-KCNE2 complexes are unknown. Using single-molecule subunit counting, we determined that the number of KCNE2 subunits in complex with the pore-forming subunits of human HCN channels differs with each HCN isoform and is dynamic with respect to concentration. These interactions can be altered by KCNE2 gene-variants with functional implications. The results provide an additional consideration necessary to understand heart rhythm, pain, and epileptic disorders.

Predictors of cardiac arrhythmic events in non coronary artery disease patients

Arrhythmic sudden cardiac death (SCD) represents a major worldwide public health problem accounting for 15–20% of deaths. Risk stratification to identify patients at risk of SCD is crucial in order to implement preventive measures in the general population. Several biomarkers have been tested exploring different pathophysiological mechanisms of cardiac conditions. Conflicting results have been described limiting so far their use in clinical practice. The use of new biomarkers such as microRNAs and sex hormones and the emerging role of genetic on risk prediction of SCD is a current research topic showing promising results.

This review outlines the role of plasma biomarkers to predict ventricular arrhythmias and SCD in non coronary artery disease with a special focus on their relationship with the genetic biomarkers.

KCNQ1 rescues TMC1 plasma membrane expression but not mechanosensitive channel activity

Transmembrane channel-like protein isoform 1 (TMC1) is essential for the generation of mechano-electrical transducer currents in hair cells of the inner ear. TMC1 disruption causes hair cell degeneration and deafness in mice and humans. Although thought to be expressed at the cell surface in vivo, TMC1 remains in the endoplasmic reticulum when heterologously expressed in standard cell lines, precluding determination of its roles in mechanosensing and pore formation. Here, we report that the KCNQ1 Kv channel forms complexes with TMC1 and rescues its surface expression when coexpressed in Chinese Hamster Ovary cells. TMC1 rescue is specific for KCNQ1 within the KCNQ family, is prevented by a KCNQ1 trafficking-deficient mutation, and is influenced by KCNE β subunits and inhibition of KCNQ1 endocytosis. TMC1 lowers KCNQ1 and KCNQ1-KCNE1 K⁺ currents, and despite the surface expression, it does not detectably respond to mechanical stimulation or high salt. We conclude that TMC1 is not intrinsically mechano- or osmosensitive but has the capacity for cell surface expression, and requires partner protein(s) for surface expression and mechanosensitivity. We suggest that KCNQ1, expression of which is not thought to overlap with TMC1 in hair cells, is a proxy partner bearing structural elements or a sequence motif reminiscent of a true in vivo TMC1 hair cell partner. Discovery of the first reported strategy to rescue TMC1 surface expression should aid future studies of the TMC1 function and native partners.

Screening for germline KCNQ1 and KCNE2 mutations in a set of somatotropinoma patients

OBJECTIVE

Recently, mutations in KCNQ1, a potassium channel gene usually linked to long QT syndrome, were reported to cause maternally inherited gingival fibromatosis and growth hormone deficiency (GHD). Expression of the mutated KCNQ1 with the auxiliary potassium channel subunit KCNE2 was shown to reduce pituitary hormone secretion in functional experiments. Here, we investigated if germline mutations in KCNQ1 and KCNE2 were present in patients with somatotropinomas, which represent a model of growth hormone excess.

DESIGN AND METHODS

KCNQ1 and KCNE2 were screened for germline mutations in 53 patients with acromegaly by Sanger sequencing. Effects of the variants were predicted by in silico tools.

RESULTS

Only deep intronic and synonymous polymorphisms were detected in KCNQ1. These findings were likely insignificant based on in silico predictions and the variants' frequencies in the general population. In KCNE2, a heterozygous c.22A>G, p.(Thr8Ala) mutation with unknown significance was found in three patients. It was present in the database controls with a frequency of 0.0038.

CONCLUSIONS

KCNQ1 or KCNE2 mutations do not appear to account for somatotropinoma formation, although larger patient series are needed to validate the findings.

Postmortem Genetic Testing for Cardiac Ion Channelopathies in Stillbirths

BACKGROUND

Although stillbirth is a significant health problem worldwide, the definitive cause of death remains elusive in many cases, despite detailed autopsy. In this study of partly explained and unexplained stillbirths, we used next-generation sequencing to examine an extended panel of 35 candidate genes known to be associated with ion channel disorders and sudden cardiac death.

METHODS AND RESULTS

We examined tissue from 242 stillbirths (≥22 weeks), including those where no definite cause of death could be confirmed after a full autopsy. We obtained high-quality DNA from 70 cases, which were then sequenced for a custom panel of 35 genes, 12 for inherited long- and short-QT syndrome genes (LQT1-LQT12 and SQT1-3), and 23 additional candidate genes derived from genome-wide association studies. We examined the functional significance of a selected variant by patch-clamp electrophysiological recording. No predicted damaging variants were identified in KCNQ1 (LQT1) or KCNH2 (LQT2). A rare putative pathogenic variant was found in KCNJ2(LQT7) in 1 case, and several novel variants of uncertain significance were observed. The KCNJ2 variant (p. R40Q), when assessed by whole-cell patch clamp, affected the function of the channel. There was no significant evidence of enrichment of rare predicted damaging variants within any of the candidate genes.

CONCLUSIONS

Although a causative link is unclear, 1 putative pathogenic and variants of uncertain significance variant resulting in cardiac channelopathies was identified in some cases of otherwise unexplained stillbirth, and these variants may have a role in fetal demise.

CLINICAL TRIAL REGISTRATION

URL: https://www.clinicaltrials.gov. Unique identifier: NCT01120886.

The impact of recent advances in genetics in understanding disease mechanisms underlying the long QT syndromes

Long QT syndrome refers to a characteristic abnormality of the electrocardiogram and it is associated with a form of ventricular tachycardia known as torsade-de-pointes and sudden arrhythmic death. It can occur as part of a hereditary syndrome or can be acquired usually because of drug administration. Here we review recent genetic, molecular and cellular discoveries and outline how they have furthered our understanding of this disease. Specifically we focus on compound mutations, genome wide association studies of QT interval, modifier genes and the therapeutic implications of this recent work.

Drug-induced fatal arrhythmias: Acquired long QT and Brugada syndromes

Since the early 1990s, the concept of primary "inherited" arrhythmia syndromes or ion channelopathies has evolved rapidly as a result of revolutionary progresses made in molecular genetics. Alterations in genes coding for membrane proteins such as ion channels or their associated proteins responsible for the generation of cardiac action potentials (AP) have been shown to cause specific malfunctions which eventually lead to cardiac arrhythmias. These arrhythmic disorders include congenital long QT syndrome, Brugada syndrome, catecholaminergic polymorphic ventricular tachycardia, short QT syndrome, progressive cardiac conduction disease, etc. Among these, long QT and Brugada syndromes are the most extensively studied, and drugs cause a phenocopy of these two diseases. To date, more than 10 different genes have been reported to be responsible for each syndrome. More recently, it was recognized that long QT syndrome can be latent, even in the presence of an unequivocally pathogenic mutation (silent mutation carrier). Co-existence of other pathological conditions in these silent mutation carriers may trigger a malignant form of ventricular arrhythmia, the so called torsade de pointes (TdP) that is most commonly brought about by drugs. In analogy to the drug-induced long QT syndrome, Brugada type 1 ECG can also be induced or unmasked by a wide variety of drugs and pathological conditions; so physicians may encounter patients with a latent form of Brugada syndrome. Of particular note, Brugada syndrome is frequently associated with atrial fibrillation whose therapeutic agents such as Vaughan Williams class IC drugs can unmask the dormant and asymptomatic Brugada syndrome. This review describes two types of drug-induced arrhythmias: the long QT and Brugada syndromes.

Pharmacogenetics of Potassium Channel Blockers

The QT interval on surface electrocardiograms provides a model of a multicomponent integrated readout of many biological systems, including ion channels, modulatory subunits, signaling systems that modulate their activity, and mechanisms that regulate the expression of their responsible genes. The problem of drug exposure causing exaggerated QT interval prolongation and torsades de pointes highlights the multicomponent nature of cardiac repolarization and the way in which simple perturbations can yield exaggerated responses. Future directions will involve cellular approaches coupled to evolving technologies that can interrogate multicellular systems and provide a sophisticated view of mechanisms in this previously idiosyncratic drug reaction.

Molecular Pathophysiology of Congenital Long QT Syndrome

Ion channels represent the molecular entities that give rise to the cardiac action potential, the fundamental cellular electrical event in the heart. The concerted function of these channels leads to normal cyclical excitation and resultant contraction of cardiac muscle. Research into cardiac ion channel regulation and mutations that underlie disease pathogenesis has greatly enhanced our knowledge of the causes and clinical management of cardiac arrhythmia. Here we review the molecular determinants, pathogenesis, and pharmacology of congenital Long QT Syndrome. We examine mechanisms of dysfunction associated with three critical cardiac currents that comprise the majority of congenital Long QT Syndrome cases: 1) IKs, the slow delayed rectifier current; 2) IKr, the rapid delayed rectifier current; and 3) INa, the voltage-dependent sodium current. Less common subtypes of congenital Long QT Syndrome affect other cardiac ionic currents that contribute to the dynamic nature of cardiac electrophysiology. Through the study of mutations that cause congenital Long QT Syndrome, the scientific community has advanced understanding of ion channel structure-function relationships, physiology, and pharmacological response to clinically employed and experimental pharmacological agents. Our understanding of congenital Long QT Syndrome continues to evolve rapidly and with great benefits: genotype-driven clinical management of the disease has improved patient care as precision medicine becomes even more a reality.

Murine Electrophysiological Models of Cardiac Arrhythmogenesis

Cardiac arrhythmias can follow disruption of the normal cellular electrophysiological processes underlying excitable activity and their tissue propagation as coherent wavefronts from the primary sinoatrial node pacemaker, through the atria, conducting structures and ventricular myocardium. These physiological events are driven by interacting, voltage-dependent, processes of activation, inactivation, and recovery in the ion channels present in cardiomyocyte membranes. Generation and conduction of these events are further modulated by intracellular Ca2+ homeostasis, and metabolic and structural change. This review describes experimental studies on murine models for known clinical arrhythmic conditions in which these mechanisms were modified by genetic, physiological, or pharmacological manipulation. These exemplars yielded molecular, physiological, and structural phenotypes often directly translatable to their corresponding clinical conditions, which could be investigated at the molecular, cellular, tissue, organ, and whole animal levels. Arrhythmogenesis could be explored during normal pacing activity, regular stimulation, following imposed extra-stimuli, or during progressively incremented steady pacing frequencies. Arrhythmic substrate was identified with temporal and spatial functional heterogeneities predisposing to reentrant excitation phenomena. These could arise from abnormalities in cardiac pacing function, tissue electrical connectivity, and cellular excitation and recovery. Triggering events during or following recovery from action potential excitation could thereby lead to sustained arrhythmia. These surface membrane processes were modified by alterations in cellular Ca2+ homeostasis and energetics, as well as cellular and tissue structural change. Study of murine systems thus offers major insights into both our understanding of normal cardiac activity and its propagation, and their relationship to mechanisms generating clinical arrhythmias.

Filamin C: a novel component of the KCNE2 interactome during hypoxia

Aim

KCNE2 encodes for the potassium voltage-gated channel, KCNE2. Mutations in KCNE2 have been associated with long-QT syndrome (LQTS). While KCNE2 has been extensively studied, the functions of its C-terminal domain remain inadequately described. Here, we aimed to elucidate the functions of this domain by identifying its protein interactors using yeast two-hybrid analysis.

Methods

The C-terminal domain of KCNE2 was used as bait to screen a human cardiac cDNA library for putative interacting proteins. Co-localisation and co-immunoprecipitation analyses were used for verification.

Results

Filamin C (FLNC) was identified as a putative interactor with KCNE2. FLNC and KCNE2 co-localised within the cell, however, a physical interaction was only observed under hypoxic conditions.

Conclusion

The identification of FLNC as a novel KCNE2 ligand not only enhances current understanding of ion channel function and regulation, but also provides valuable information about possible pathways likely to be involved in LQTS pathogenesis.

Drug-induced Inhibition and Trafficking Disruption of ion Channels: Pathogenesis of QT Abnormalities and Drug-induced Fatal Arrhythmias

Risk of severe and fatal ventricular arrhythmias, presenting as Torsade de Pointes (TdP), is increased in congenital and acquired forms of long QT syndromes (LQTS). Drug-induced inhibition of K+ currents, IKs, IKr, IK1, and/or Ito, delay repolarization, prolong QT, and increase the risk of TdP. Drug-induced interference with IKr is the most common cause of acquired LQTS/TdP. Multiple drugs bind to KNCH2-hERG-K+ channels affecting IKr, including antiarrythmics, antibiotics, antivirals, azole-antifungals, antimalarials, anticancer, antiemetics, prokinetics, antipsychotics, and antidepressants. Azithromycin has been recently added to this list. In addition to direct channel inhibition, some drugs interfere with the traffic of channels from the endoplasmic reticulum to the cell membrane, decreasing mature channel membrane density; e.g., pentamidine, geldalamicin, arsenic trioxide, digoxin, and probucol. Other drugs, such as ketoconazole, fluoxetine, norfluoxetine, citalopram, escitalopram, donepezil, tamoxifen, endoxifen, atazanavir, and roxitromycin, induce both direct channel inhibition and impaired channel trafficking. Although many drugs prolong the QT interval, TdP is a rare event. The following conditions increase the risk of drug-induced TdP: a) Disease states/electrolyte levels (heart failure, structural cardiac disease, bradycardia, hypokalemia); b) Pharmacogenomic variables (presence of congenital LQTS, subclinical ion-channel mutations, history of or having a relative with history of drug-induced long QT/TdP); c) Pharmacodynamic and kinetic factors (high doses, women, elderly, metabolism inhibitors, combining two or more QT prolonging drugs, drugs that prolong the QT and increase QT dispersion, and drugs with multiple actions on ion channels). Because most of these conditions are preventable, careful evaluation of risk factors and increased knowledge of drug use associated with repolarization abnormalities are strongly recommended.

Propofol inhibits hERG K+ channels and enhances the inhibition effects on its mutations in HEK293 cells

QT interval prolongation, a potential risk for arrhythmias, may result from gene polymorphisms relevant to cardiomyocyte repolarization. Another noted cause of QT interval prolongation is the administration of chemical compounds such as anesthetics, which may affect a specific type of cardiac K⁺ channel encoded by the human ether-a-go-go-related gene (hERG). hERG K⁺ current was recorded using whole-cell patch clamp in human embryonic kidney (HEK293) cells expressing wild type (WT) or mutated hERG channels. Expression of hERG K⁺ channel proteins was evaluated using western blot and confirmed by fluorescent staining and imaging. Computational modeling was adopted to identify the possible binding site(s) of propofol with hERG K⁺ channels. Propofol had a significant inhibitory effect on WT hERG K⁺ currents in a concentration-dependent manner, with a half-maximal inhibitory concentration (IC₅₀) of 60.9±6.4μM. Mutations in drug-binding sites (Y652A or F656C) of the hERG channel were found to attenuate hERG current blockage by propofol. However, propofol did not inhibit the trafficking of hERG protein to the cell membrane. Meanwhile, for the three selective hERG K⁺ channel mutant heterozygotes WT/Q738X-hERG, WT/A422T-hERG, and WT/H562P-hERG, the IC₅₀ of propofol was calculated as 14.2±2.8μM, 3.3±1.2μM, and 5.9±1.9μM, respectively, which were much lower than that for the wild type. These findings indicate that propofol may potentially increase QT interval prolongation risk in patients via direct inhibition of the hERG K⁺ channel, especially in those with other concurrent triggering factors such as hERG gene mutations.

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

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

Human Induced Pluripotent Stem Cells as a Platform for Personalized and Precision Cardiovascular Medicine

Human induced pluripotent stem cells (hiPSCs) have revolutionized the field of human disease modeling, with an enormous potential to serve as paradigm shifting platforms for preclinical trials, personalized clinical diagnosis, and drug treatment. In this review, we describe how hiPSCs could transition cardiac healthcare away from simple disease diagnosis to prediction and prevention, bridging the gap between basic and clinical research to bring the best science to every patient.

Pharmacogenetics of Drug-Induced QT Interval Prolongation: An Update

A prolonged QT interval is an important risk factor for ventricular arrhythmias and sudden cardiac death. QT prolongation can be caused by drugs. There are multiple risk factors for drug-induced QT prolongation, including genetic variation. QT prolongation is one of the most common reasons for withdrawal of drugs from the market, despite the fact that these drugs may be beneficial for certain patients and not harmful in every patient. Identifying genetic variants associated with drug-induced QT prolongation might add to tailored pharmacotherapy and prevent beneficial drugs from being withdrawn unnecessarily. In this review, our objective was to provide an overview of the genetic background of drug-induced QT prolongation, distinguishing pharmacokinetic and pharmacodynamic pathways. Pharmacokinetic-mediated genetic susceptibility is mainly characterized by variation in genes encoding drug-metabolizing cytochrome P450 enzymes or drug transporters. For instance, the P-glycoprotein drug transporter plays a role in the pharmacokinetic susceptibility of drug-induced QT prolongation. The pharmacodynamic component of genetic susceptibility is mainly characterized by genes known to be associated with QT interval duration in the general population and genes in which the causal mutations of congenital long QT syndromes are located. Ethnicity influences susceptibility to drug-induced QT interval prolongation, with Caucasians being more sensitive than other ethnicities. Research on the association between pharmacogenetic interactions and clinical endpoints such as sudden cardiac death is still limited. Future studies in this area could enable us to determine the risk of arrhythmias more adequately in clinical practice.

Mechanism of Proarrhythmic Effects of Potassium Channel Blockers

Any disturbance of electrical impulse formation in the heart and of impulse conduction or action potential (AP) repolarization can lead to rhythm disorders. Potassium (K(+)) channels play a prominent role in the AP repolarization process. In this review we describe the causes and mechanisms of proarrhythmic effects that arise as a response to blockers of cardiac K(+) channels. The largest and chemically most diverse groups of compound targets are Kv11.1 (hERG) and Kv7.1 (KvLQT1) channels. Finally, the proarrhythmic propensity of atrial-selective K(+) blockers inhibiting Kv1.5, Kir3.1/3.4, SK, and K2P channels is discussed.

Clinical association between pharmacogenomics and adverse drug reactions

Adverse drug reactions (ADRs) are a major public health concern and cause significant patient morbidity and mortality. Pharmacogenomics is the study of how genetic polymorphisms affect an individual's response to pharmacotherapy at the level of a whole genome. This article updates our knowledge on how genetic polymorphisms of important genes alter the risk of ADR occurrence after an extensive literature search. To date, at least 244 pharmacogenes identified have been associated with ADRs of 176 clinically used drugs based on PharmGKB. At least 28 genes associated with the risk of ADRs have been listed by the Food and Drug Administration as pharmacogenomic biomarkers. With the availability of affordable and reliable testing tools, pharmacogenomics looks promising to predict, reduce, and minimize ADRs in selected populations.

Molecular basis and drug sensitivity of the delayed rectifier (IKr) in the fish heart

Fishes are increasingly used as models for human cardiac diseases, creating a need for a better understanding of the molecular basis of fish cardiac ion currents. To this end we cloned KCNH6 channel of the crucian carp (Carassius carassius) that produces the rapid component of the delayed rectifier K(+) current (IKr), the main repolarising current of the fish heart. KCNH6 (ccErg2) was the main isoform of the Kv11 potassium channel family with relative transcript levels of 98.9% and 99.6% in crucian carp atrium and ventricle, respectively. KCNH2 (ccErg1), an orthologue to human cardiac Erg (Herg) channel, was only slightly expressed in the crucian carp heart. The native atrial IKr and the cloned ccErg2 were inhibited by similar concentrations of verapamil, terfenadine and KB-R7943 (P>0.05), while the atrial IKr was about an order of magnitude more sensitive to E-4031 than ccErg2 (P<0.05) suggesting that some accessory β-subunits may be involved. Sensitivity of the crucian carp atrial IKr to E-4031, terfenadine and KB-R7943 was similar to what has been reported for the Herg channel. In contrast, the sensitivity of the crucian carp IKr to verapamil was approximately 30 times higher than the previously reported values for the Herg current. In conclusion, the cardiac IKr is produced by non-orthologous gene products in fish (Erg2) and mammalian hearts (Erg1) and some marked differences exist in drug sensitivity between fish and mammalian Erg1/2 which need to be taken into account when using fish heart as a model for human heart.

Manipulation of KCNE2 expression modulates action potential duration and Ito and IK in rat and mouse ventricular myocytes

In heterologous expression systems, KCNE2 has been demonstrated to interact with multiple α-subunits of voltage-dependent cation channels and modulate their functions. However, the physiological and pathological roles of KCNE2 in cardiomyocytes are poorly understood. The present study aimed to investigate the effects of bidirectional modulation of KCNE2 expression on action potential (AP) duration (APD) and voltage-dependent K+ channels in cardiomyocytes. Adenoviral gene delivery and RNA interference were used to either increase or decrease KCNE2 expression in cultured neonatal and adult rat or neonatal mouse ventricular myocytes. Knockdown of KCNE2 prolonged APD in both neonatal and adult myocytes, whereas overexpression of KCNE2 shortened APD in neonatal but not adult myocytes. Consistent with the alterations in APD, KCNE2 knockdown decreased transient outward K+ current ( Ito) densities in neonatal and adult myocytes, whereas KCNE2 overexpression increased Ito densities in neonatal but not adult myocytes. Furthermore, KCNE2 knockdown accelerated the rates of Ito activation and inactivation, whereas KCNE2 overexpression slowed Ito gating kinetics in neonatal but not adult myocytes. Delayed rectifier K+ current densities were remarkably affected by manipulation of KCNE2 expression in mouse but not rat cardiomyocytes. Simulation of the AP of a rat ventricular myocyte with a mathematical model showed that alterations in Ito densities and gating properties can result in similar APD alterations in KCNE2 overexpression and knockdown cells. In conclusion, endogenous KCNE2 in cardiomyocytes is important in maintaining cardiac electrical stability mainly by regulating Ito and APD. Perturbation of KCNE2 expression may predispose the heart to ventricular arrhythmia by prolonging APD.