Cardiac conduction diseases: understanding the molecular mechanisms to uncover targets for future treatments.

INTRODUCTION: The cardiac conduction system (CCS) is crucial for maintaining adequate cardiac frequency at rest and modulation during exercise. Furthermore, the atrioventricular node and His-Purkinje system are essential for maintaining atrioventricular and interventricular synchrony and consequently maintaining an adequate cardiac output. AREAS COVERED: In this review article, we examine the anatomy, physiology, and pathophysiology of the CCS. We then discuss in detail the most common genetic mutations and the molecular mechanisms of cardiac conduction disease (CCD) and provide our perspectives on future research and therapeutic opportunities in this field. EXPERT OPINION: Significant advancement has been made in understanding the molecular mechanisms of CCD, including the recognition of the heterogeneous signaling at the subcellular levels of sinoatrial node, the involvement of inflammatory and autoimmune mechanisms, and the potential impact of epigenetic regulations on CCD. However, the current treatment of CCD manifested as bradycardia still relies primarily on cardiovascular implantable electronic devices (CIEDs). On the other hand, an If specific inhibitor was developed to treat inappropriate sinus tachycardia and sinus tachycardia in heart failure patients with reduced ejection fraction. More work is needed to translate current knowledge into pharmacologic or genetic interventions for the management of CCDs.

An observational, retrospective, comprehensive pharmacovigilance analysis of hydroxychloroquine-associated cardiovascular adverse events in patients with and without COVID-19.

BACKGROUND: Coronavirus disease 2019 (COVID-19) is a global pandemic. Hydroxychloroquine (HCQ)-associated cardiovascular adverse events (CVAEs) have been increasingly reported. AIM: This study aimed to present an observational, retrospective, and comprehensive pharmacovigilance analysis of CVAE associated with HCQ in patients with and without COVID-19 using the US Food and Drug Administration Adverse Events Reporting System (FAERS) data from January 2020 to December 2020. METHOD: We identified 3302 adverse event reports from the FAERS database in the year 2020 and divided them into COVID-19 and non-COVID-19 groups, respectively. Then we analyzed whether there were differences in CVAEs between the two groups. RESULTS: We found that CVAE was higher in cases with COVID-19 compared to those without COVID-19, odds ratio (OR) of 1.26 and a 95% confidence interval (95% CI) of 1.02-1.54. Cases with COVID-19 treated with HCQ exhibited relatively higher proportions of torsade de points (TdP) and QT prolongation (OR 3.10, 95% CI 2.24-4.30), shock-associated TdP (OR 2.93, 95% CI 2.13-4.04), cardiac arrhythmias (OR 2.07, 95% CI 1.60-2.69), cardiac arrhythmia terms (including bradyarrhythmias and tachyarrhythmias) (OR 2.15, 95% CI 1.65-2.80), bradyarrhythmias (including conduction defects and disorders of sinus node function) (OR 2.56, 95% CI 1.86-3.54), and conduction defects (OR 2.56, 95% CI 1.86-3.54). CONCLUSION: Our retrospective observational analysis suggested that the proportion of CVAE associated with HCQ, especially TdP and QT prolongation, was higher in patients with COVID-19. Understanding the effects of COVID-19 on the cardiovascular system is essential to providing comprehensive medical care to patients receiving HCQ treatment.

Management of atrial fibrillation: two decades of progress - a scientific statement from the European Cardiac Arrhythmia Society.

BACKGROUND: Atrial fibrillation (AF) is the most common sustained arrhythmia encountered in clinical practice. The aim of this review was to evaluate the progress made in the management of AF over the two last decades. RESULTS: Clinical classification of AF is usually based on the presence of symptoms, the duration of AF episodes and their possible recurrence over time, although incidental diagnosis is not uncommon. The majority of patients with AF have associated cardiovascular diseases and more recently the recognition of modifiable risk factors both cardiovascular and non-cardiovascular which should be considered in its management. Among AF-related complications, stroke and transient ischaemic accidents (TIAs) carry considerable morbidity and mortality risk. The use of implantable devices such as pacemakers and defibrillators, wearable garments and subcutaneous cardiac monitors with recording capabilities has enabled to access the burden of "subclinical AF". The recent introduction of non-vitamin K antagonists has led to improve the prevention of stroke and peripheral embolism. Agents capable of reversing non-vitamin K antagonists have also become available in case of clinically relevant major bleeding. Transcatheter closure of left atrial appendage represents an option for patients unable to take oral anticoagulation. When treating patients with AF, clinicians need to select the most suitable strategy, i.e. control of heart rate and/or restoration and maintenance of sinus rhythm. The studies comparing these two strategies have not shown differences in terms of mortality. If an AF episode is poorly tolerated from a haemodynamic standpoint, electrical cardioversion is indicated. Otherwise, restoration of sinus rhythm can be obtained using intravenous pharmacological cardioversion and oral class I or class III antiarrhythmic is used to prevent recurrences. During the last two decades after its introduction in daily practice, catheter ablation has gained considerable escalation in popularity. Progress has also been made in AF associated with heart failure with reduced or preserved ejection fraction. CONCLUSIONS: Significant progress has been made within the past 2 decades both in the pharmacological and non-pharmacological managements of this cardiac arrhythmia.

Role of chronic continuous intravenous lidocaine in the clinical management of patients with malignant type 3 long QT syndrome.

BACKGROUND: Type 3 long QT syndrome (LQT3) is caused by pathogenic, gain-of-function variants in SCN5A leading to a prolonged action potential, ventricular ectopy, and torsades de pointes. Treatment options include pharmacotherapy, cardiac denervation, and/or device therapy. Rarely, patients with malignant LQT3 require cardiac transplantation. OBJECTIVE: The purpose of this study was to evaluate the role of chronic continuous intravenous (IV) lidocaine as a therapeutic option for select patients with LQT3 refractory to standard therapy. METHODS: We performed a retrospective review of patients evaluated and treated at Mayo Clinic and identified 4 of 161 patients with LQT3 (2.5%) who were refractory to standard therapies and therefore treated with IV lidocaine. RESULTS: There were 4 patients (2 female [50%]). The median age at first IV lidocaine infusion was 2 months (interquartile range 1.5-4.8 months), and the median cumulative duration on IV lidocaine was 11.5 months (interquartile range 8.7-17.8 months). The main indication for IV lidocaine in all patients was persistent ventricular arrhythmias. Before IV lidocaine, all patients received an implantable cardioverter-defibrillator, and while on intermittent IV lidocaine, all patients underwent bilateral cardiac sympathetic denervation. Additionally, 2 (50%) patients had cardiac ablation for premature ventricular complexes. In all patients, lidocaine infusion resulted in a significant reduction of LQT3-triggered cardiac events. The main side effects of IV lidocaine observed were dizziness (n = 2, 50%) and seizures (n = 2, 50%). During follow-up, 3 of 4 (75%) patients underwent orthotopic cardiac transplantation. The remaining patient continues to receive IV lidocaine bolus for rescue as needed. CONCLUSION: For patients with LQT3 who are refractory to standard treatment, chronic IV lidocaine infusion can be used as a potential "bridge to transplant."

Molecular Pharmacology of K2P Potassium Channels.

Potassium channels of the tandem of two-pore-domain (K2P) family were among the last potassium channels cloned. However, recent progress in understanding their physiological relevance and molecular pharmacology revealed their therapeutic potential and thus these channels evolved as major drug targets against a large variety of diseases. However, after the initial cloning of the fifteen family members there was a lack of potent and/or selective modulators. By now a large variety of K2P channel modulators (activators and blockers) have been described, especially for TASK-1, TASK-3, TREK-1, TREK2, TRAAK and TRESK channels. Recently obtained crystal structures of K2P channels, alanine scanning approaches to map drug binding sites, in silico experiments with molecular dynamics simulations (MDs) combined with electrophysiological studies to reveal the mechanism of channel inhibition/activation, yielded a good understanding of the molecular pharmacology of these channels. Besides summarizing drugs that were identified to modulate K2P channels, the main focus of this article is on describing the differential binding sites and mechanisms of channel modulation that are utilized by the different K2P channel blockers and activators.

Impact of pacing frequency in amiodarone interaction with cardiomyocytes near physiological temperature in health and disease conditions.

Long QT syndrome type 3 (LQT-3) is a disease related to abnormal cardiac sodium channel function (Nav 1.5), usually due to augmented late sodium current (INaL ), and may lead to ventricular fibrillation. Amiodarone is approved for ventricular fibrillation. Thus, we investigated whether pacing frequency impacts the ability of amiodarone to reverse the arrhythmic phenotype observed in LQT-3. Anemone neurotoxin 2 (ATX-II, here named only ATX) was used to enhance INaL in mice left ventricular myocytes (LVM). A video detector system monitored sarcomere shortening. At 1 Hz, amiodarone attenuated sarcomere shortening only at 10 µmol/L; at 3 and 5 Hz, 0.1 and 1 µmol/L amiodarone also reduced sarcomere shortening. However, no effect of amiodarone was observed on time to 50% of sarcomere contraction and relaxation. In LVM exposed to ATX (10 nmol/L), an arrhythmic phenotype was observed, and it was more severe when cells were paced at 1 Hz. Amiodarone failed to reverse the ATX induced phenotype at different pacing frequencies. Thus, our results suggest that amiodarone's ability to reverse arrhythmias induced by augmentation of INaL is limited. These findings suggest further experimentation will be required to clarify whether a clinical effect can be ascribed to an effect of amiodarone on other ion channels in LQT-3.

The Citrus Flavonoid Hesperetin Has an Inadequate Anti-Arrhythmic Profile in the ΔKPQ NaV1.5 Mutant of the Long QT Type 3 Syndrome.

Type 3 long QT syndromes (LQT3) are associated with arrhythmogenic gain-of-function mutations in the cardiac voltage-gated Na+ channel (hNaV1.5). The citrus flavanone hesperetin (HSP) was previously suggested as a template molecule to develop new anti-arrhythmic drugs, as it blocks slowly-inactivating currents carried by the LQT3-associated hNaV1.5 channel mutant R1623Q. Here we investigated whether HSP also has potentially beneficial effects on another LQT3 hNaV1.5 channel variant, the ΔKPQ, which is associated to lethal ventricular arrhythmias. We used whole-cell patch-clamp to record Na+ currents (INa) in HEK293T cells transiently expressing hNaV1.5 wild type or ΔKPQ mutant channels. HSP blocked peak INa and the late INa carried by ΔKPQ mutant channels with an effective concentration of ≈300 µM. This inhibition was largely voltage-independent and tonic. HSP decreased the rate of inactivation of ΔKPQ channels and, consequently, was relatively weak in reducing the intracellular Na+ load in this mutation. We conclude that, although HSP has potential value for the treatment of the R1623Q LQT3 variant, this compound is inadequate to treat the LQT3 associated to the ΔKPQ genetic variant. Our results underscore the precision medicine rationale of better understanding the basic pathophysiological and pharmacological mechanisms to provide phenotype- genotype-directed individualization of treatment.

Effects of amiodarone on rodent ventricular cardiomyocytes: Novel perspectives from a cellular model of Long QT Syndrome Type 3.

AIMS: Amiodarone (AMIO) is currently used in medical practice to reverse ventricular tachycardia. Here we determine the effects of AMIO in the electromechanical properties of isolated left ventricle myocyte (LVM) from mice and guinea pig and in a cellular model of Long QT Syndrome Type 3 (LQTS-3) using anemone neurotoxin 2 (ATX II), which induces increase of late sodium current in LVM. MAIN METHODS AND KEY FINDINGS: Using patch-clamp technique, fluorescence imaging to detect cellular Ca2+ transient and sarcomere detection systems we evaluate the effect of AMIO in healthy LVM. AMIO produced a significant reduction in the percentage of sarcomere shortening (0.1, 1 and 10 µM) in a range of pacing frequencies, however, without significant attenuation of Ca2+ transient. Also, 10 µM of AMIO caused the opposite effect on action potential repolarization of mouse and guinea pig LVM. When LVM from mouse and guinea pig were paced in a range of pacing frequencies and exposed to ATX (10 nM), AMIO (10 µM) was only able to abrogate electromechanical arrhythmias in LVM from guinea pig at lower pacing frequency. SIGNIFICANCE: AMIO has negative inotropic effect with opposite effect on action potential waveform in mouse and guinea pig LVM. Furthermore, the antiarrhythmic action of AMIO in LQTS-3 is species and frequency-dependent, which indicates that AMIO may be beneficial for some types of arrhythmias related to late sodium current.

A distinct molecular mechanism by which phenytoin rescues a novel long QT 3 variant.

BACKGROUND: Genetic variants in SCN5A can result in channelopathies such as the long QT syndrome type 3 (LQT3), but the therapeutic response to Na+ channel blockers can vary. We previously reported a case of an infant with malignant LQT3 and a missense Q1475P SCN5A variant, who was effectively treated with phenytoin, but only partially with mexiletine. Here, we functionally characterized this variant and investigated possible mechanisms for the differential drug actions. METHODS: Wild-type or mutant Nav1.5 cDNAs were examined in transfected HEK293 cells with patch clamping and biochemical assays. We used computational modeling to provide insights into altered channel kinetics and to predict effects on the action potential. RESULTS: The Q1475P variant in Nav1.5 reduced the current density and channel surface expression, characteristic of a trafficking defect. The variant also led to positive shifts in the voltage dependence of steady-state activation and inactivation, faster inactivation and recovery from inactivation, and increased the "late" Na+ current. Simulations of Nav1.5 gating with a 9-state Markov model suggested that transitions from inactivated to closed states were accelerated in Q1475P channels, leading to accumulation of channels in non-inactivated closed states. Simulations with a human ventricular myocyte model predicted action potential prolongation with Q1475P, compared with wild type, channels. Patch clamp data showed that mexiletine and phenytoin similarly rescued some of the gating defects. Chronic incubation with mexiletine, but not phenytoin, rescued the Nav1.5-Q1475P trafficking defect, thus increasing mutant channel expression. CONCLUSIONS: The gain-of-function effects of Nav1.5-Q1475P predominate to cause a malignant long QT phenotype. Phenytoin partially corrects the gating defect without restoring surface expression of the mutant channel, whereas mexiletine restores surface expression of the mutant channel, which may explain the lack of efficacy of mexiletine when compared to phenytoin. Our data makes a case for experimental studies before embarking on a one-for-all therapy of arrhythmias.

Long-term prognosis of patients withJ-wave syndrome.

OBJECTIVE: Limited data are currently available regarding the long-term prognosis of patients with J-wave syndrome (JWS). The aim of this study was to investigate the long-term prognosis of patients with JWS and identify predictors of the recurrence of ventricular fibrillation (VF). METHODS: This was a multicentre retrospective study (seven Japanese hospitals) involving 134 patients with JWS (Brugada syndrome (BrS): 85; early repolarisation syndrome (ERS): 49) treated with an implantable cardioverter defibrillator. All patients had a history of VF. All patients with ERS underwent drug provocation testing with standard and high intercostal ECG recordings to rule out BrS. The impact of global J waves (type 1 ECG or anterior J waves and inferolateral J waves in two or more leads) on the prognosis was evaluated. RESULTS: During the 91±66 months of the follow-up period, 52 (39%) patients (BrS: 37; ERS: 15) experienced recurrence of VF. Patients with BrS and ERS with global J waves showed a significantly higher incidence of VF recurrence than those without (BrS: log-rank, p=0.014; ERS: log-rank, p=0.0009). The presence of global J waves was a predictor of VF recurrence in patients with JWS (HR: 2.16, 95% CI 1.21 to 3.91, p=0.0095), while previously reported high-risk electrocardiographic parameters (high-amplitude J waves ≥0.2 mV and J waves associated with a horizontal or descending ST segment) were not predictive of VF recurrence. CONCLUSIONS: This multicentre long-term study showed that the presence of global J waves was associated with a higher incidence of VF recurrence in patients with JWS.

The role of mexiletine in the management of long QT syndrome.

Congenital long QT syndrome (LQTS) is a hereditary cardiac disorder characterized by QT-interval prolongation and T-wave abnormalities on electrocardiogram (ECG), and is associated with an increased risk of torsade de pointes and sudden cardiac death. Beta-blocker medication is effective in most patients except those with a very slow heart rate. Increased late sodium currents (INa-L) can result in bradycardia-dependent QT prolongation. Mexiletine, an inhibitor of INa-L, is not only effective in treating type-3 LQTS, but also shows the promise in managing LQTS patients of other genotypes with markedly prolonged QT interval at slow heart rates.

Heart Rate-corrected QT Interval Duration in Rheumatoid Arthritis and Its Reduction with Treatment with the Interleukin 6 Inhibitor Tocilizumab.

OBJECTIVE: Individuals with rheumatoid arthritis (RA) are at a heightened risk of sudden cardiac death, an outcome increased in those with prolongation of the corrected electrocardiographic QT interval (QTc). We compared QTc between patients with RA and demographically matched controls and studied the change in QTc after treatment with the interleukin 6 inhibitor tocilizumab (TCZ). METHODS: Standard 12-lead electrocardiograms were obtained and QTc was measured in patients with RA at baseline and after 24 weeks of TCZ treatment, then compared with non-RA controls who were frequency-matched on age and sex. Indicators of the baseline QTc and predictors of change in QTc were studied using multivariable linear regression. RESULTS: A total of 94 RA and 42 non-RA controls were studied. The average baseline QTc was 10 ms longer in the RA group vs controls (422 vs 412 ms, respectively; p < 0.001) and decreased to an average of 406 ms with treatment (p < 0.001). Baseline QTc was significantly and independently higher among those with anticyclic citrullinated peptide antibodies seropositivity, higher swollen joint counts, and higher levels of C-reactive protein (CRP) and matrix metalloproteinase 3. Each log unit decrease in CRP with treatment was associated with an average reduction in QTc of 2.9 ms (p = 0.002) after adjusting for age and baseline QTc. Clinical response measures were not associated with the change in QTc. CONCLUSION: The marked normalization of QTc observed with TCZ treatment, and its close parallel with CRP reduction, support the premise that systemic inflammation contributes to cardiac repolarization abnormalities in RA that may be amenable to treatment.

Intra- and interatrial conduction abnormalities: hemodynamic and arrhythmic significance.

Alterations of normal intra- and interatrial conduction are a common outcome of multiple cardiovascular conditions. They arise most commonly in the context of advanced age, cardiovascular risk factors, organic heart disease, atrial fibrosis, and left atrial enlargement. Interatrial block (IAB), the most frequent and extensively studied atrial conduction disorder, affects up to 20% of the general primary care population. IAB can be partial (P wave duration ≥ 120 ms on any of the 12 ECG leads) or advanced (P wave ≥ 120 ms and biphasic morphology (positive-negative) in inferior leads). Advanced IAB is an independent risk factor for supraventricular tachyarrhythmias and embolic stroke in a variety of clinical settings. Advanced IAB is a cause of left atrial electromechanical dysfunction and left atrioventricular dyssynchrony and has been associated with left ventricular diastolic dysfunction. P wave duration is associated with cardiovascular and all-cause mortality in the general population. Atrial conduction abnormalities should be identified as markers of atrial remodeling, prognostic indicators, and, in the case of advanced IAB, a true arrhythmologic syndrome. IAB and other P wave abnormalities should prompt the search for associated conditions, the treatment of which may partially reverse atrial remodeling or prevent it if administered upstream. Future studies will help define the role of preventive therapeutic interventions in high-risk patients, including antiarrhythmic drug therapy and oral anticoagulation. Implications for the treatment of heart failure and for pacing should also be further investigated.

Mexiletine rescues a mixed biophysical phenotype of the cardiac sodium channel arising from the SCN5A mutation, N406K, found in LQT3 patients.

INTRODUCTION: Individual mutations in the SCN5A-encoding cardiac sodium channel α-subunit usually cause a single cardiac arrhythmia disorder, some cause mixed biophysical or clinical phenotypes. Here we report an infant, female patient harboring a N406K mutation in SCN5A with a marked and mixed biophysical phenotype and assess pathogenic mechanisms. METHODS AND RESULTS: A patient suffered from recurrent seizures during sleep and torsades de pointes with a QTc of 530 ms. Mutational analysis identified a N406K mutation in SCN5A. The mutation was engineered by site-directed mutagenesis and heterologously expressed in HEK293 cells. After 48 hours incubation with and without mexiletine, macroscopic voltage-gated sodium current (INa) was measured with standard whole-cell patch clamp techniques. SCN5A-N406K elicited both a significantly decreased peak INa and a significantly increased late INa compared to wide-type (WT) channels. Furthermore, mexiletine both restored the decreased peak INa of the mutant channel and inhibited the increased late INa of the mutant channel. CONCLUSION: SCN5A-N406K channel displays both "gain-of-function" in late INa and "loss-of-function" in peak INa density contributing to a mixed biophysical phenotype. Moreover, our finding may provide the first example that mexiletine exerts a dual rescue of both "gain-of-function" and "loss-of-function" of the mutant sodium channel.

Long-term flecainide therapy in type 3 long QT syndrome.

Aims: Type 3 long QT syndrome (LQT3) is caused by gain-of-function mutations in the cardiac sodium channel gene (SCN5A). Previous reports on the long-term use of sodium channel blockers in LQT3 are sparse. The objective of the current study was to evaluate the long-term safety and efficacy of flecainide therapy in patients with LQT3 who carry the D1790G SCN5A mutation. Methods and results: The study population comprised 30 D1790G carriers who were treated with flecainide and followed for 1-215 months (mean 145 ± 54 months, median 140 months). The mean baseline (off-drug) QTc was 522 ± 45 ms, and shortened to 469 ± 36 ms with flecainide therapy, a mean decrease of 53 ms [10.1%] (P < 0.01). A QTc longer than 500 ms was evident in 53% of carriers at baseline, and only in 13% on flecainide. All carriers while being compliant with flecainide therapy had no cardiac events during an average follow up of 83 ± 73 months. Twenty carriers stopped flecainide after an average follow up of 40 ± 42 months without symptoms. Six of them (30%) had cardiac events 1-11 months after stopping flecainide. Flecainide induced the appearance of Brugada pattern in six carriers (20%, 5 males), was stopped in three and was not associated with arrhythmia. Sinus-node dysfunction was evident in six carriers (20%) and was fully corrected by flecainide in three. Conclusions: These data suggest that long-term flecainide therapy is relatively safe and effective among LQT3 patients who carry the D1790G SCN5A mutation.

Eleclazine exhibits enhanced selectivity for long QT syndrome type 3-associated late Na+ current.

BACKGROUND: Eleclazine (GS-6615) is a sodium channel blocker designed to improve the selectivity for cardiac late Na+ current (INa) over peak INa. OBJECTIVES: The goals of this study were to investigate the inhibition of late INa by eleclazine using a sample of long QT syndrome type 3 (LQT3) and overlap LQT3/Brugada syndrome mutant channels; to compare the apparent binding rates for eleclazine with those for other class 1 antiarrhythmic agents; and to investigate the binding site. METHODS: Wild-type human cardiac voltage-gated sodium channel (hNaV1.5) and 21 previously reported variants were studied using patch clamp recordings from a heterologous expression system. RESULTS: Eleclazine inhibited anemone toxin II-enhanced late INa from wild-type hNaV1.5 with a drug concentration that causes 50% block of 0.62 ± 0.12 µM (84-fold selectivity over peak INa). The drug concentration that causes 50% block of eleclazine to inhibit the enhanced late INa from LQT3 mutant channels ranged from 0.33 to 1.7 µM. At predicted therapeutic concentrations, eleclazine and ranolazine inhibited peak INa to a similar degree as assessed with 4 overlap LQT3/Brugada syndrome mutations. Eleclazine was found to interact with hNaV1.5 significantly faster than ranolazine and 6 other class 1 antiarrhythmic agents. Engineered mutations (F1760A/Y1767A) located within the local anesthetic binding site decreased the inhibition of late INa and peak INa by eleclazine. CONCLUSION: At predicted therapeutic concentrations, eleclazine elicits potent inhibition of late INa across a cohort of NaV1.5 mutant channels. These properties are consistent with a class 1b antiarrhythmic agent that associates with unusually rapid binding/unbinding rates.

Phenotypic variability in LQT3 human induced pluripotent stem cell-derived cardiomyocytes and their response to antiarrhythmic pharmacologic therapy: An in silico approach.

BACKGROUND: Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) are in vitro models with the clear advantages of their human origin and suitability for human disease investigations. However, limitations include their incomplete characterization and variability reported in different cell lines and laboratories. OBJECTIVE: The purpose of this study was to investigate in silico ionic mechanisms potentially explaining the phenotypic variability of hiPSC-CMs in long QT syndrome type 3 (LQT3) and their response to antiarrhythmic drugs. METHODS: Populations of in silico hiPSC-CM models were constructed and calibrated for control (n = 1,463 models) and LQT3 caused by INaL channelopathy (n = 1,401 models), using experimental recordings for late sodium current (INaL) and action potentials (APs). Antiarrhythmic drug therapy was evaluated by simulating mexiletine and ranolazine multichannel effects. RESULTS: As in experiments, LQT3 hiPSC-CMs yield prolonged action potential duration at 90% repolarization (APD90) (+34.3% than controls) and large electrophysiological variability. LQT3 hiPSC-CMs with symptomatic APs showed overexpression of ICaL, IK1, and INaL, underexpression of IKr, and increased sensitivity to both drugs compared to asymptomatic LQT3 models. Simulations showed that both mexiletine and ranolazine corrected APD prolongation in the LQT3 population but also highlighted differences in drug response. Mexiletine stops spontaneous APs in more LQT3 hiPSC-CMs models than ranolazine (784/1,401 vs 53/1,401) due to its stronger action on INa. CONCLUSION: In silico simulations demonstrate our ability to recapitulate variability in LQT3 and control hiPSC-CM phenotypes, and the ability of mexiletine and ranolazine to reduce APD prolongation, in agreement with experiments. The in silico models also identify potential ionic mechanisms of phenotypic variability in LQT3 hiPSC-CMs, explaining APD prolongation in symptomatic vs asymptomatic LQT3 hiPSC-CMs.

Lidocaine attenuation testing: An in vivo investigation of putative LQT3-associated variants in the SCN5A-encoded sodium channel.

BACKGROUND: Long QT syndrome type 3 (LQT3) accounts for 5%-10% of long QT syndrome and results from gain-of-function mutations in the SCN5A-encoded sodium channel. Approximately 2% of healthy individuals host rare SCN5A variants of uncertain significance (VUS). Distinction of true LQT3-causative mutations from background genetic noise is essential. OBJECTIVE: The purpose of this study was to assess the use of the lidocaine attenuation test (LAT) in evaluating patients with possible LQT3. METHODS: We reviewed the LAT results and medical records for 25 patients with a possible LQT3-associated SCN5A variant. The LAT involved a loading dose of 1 mg/kg of intravenous lidocaine followed by continuous infusion at 50 µg/(kg⋅min) for 20 minutes. If the corrected QT interval shortened by ≥30 ms, the LAT was defined as positive. RESULTS: Sixteen patients (64%) had a positive LAT, 6 of which demonstrated the E1784K variant. A positive LAT correlated in 86% of cases with abnormal in vitro channel function (mean corrected QT interval attenuation 43 ± 3 ms vs 25 ± 5 ms for wild-type variants; P = .03). Four of 5 patients (80%) with a VUS had a positive LAT (T1304M [2 patients], L1786P, and R800L). The T1304M variant demonstrated abnormal in vitro function and a positive LAT, opening the door for a potential variant promotion from VUS to likely pathogenic. CONCLUSION: The LAT may help distinguish true LQT3-causative mutations from an otherwise noncontributory VUS. Given that lidocaine acts as a late sodium current blocker, a positive LAT may enable the early identification of a pathological accentuation of the late sodium current that could be targeted therapeutically.