Computational drug repurposing has emerged as a strategic response to the escalating costs, risks, and timelines associated with de novo drug development, offering a faster translational pathway by leveraging compounds with established safety and pharmacokinetic profiles. What was once largely opportunistic, driven by serendipitous clinical observations, has evolved into a systematic, disease-focused paradigm enabled by integrative computational pipelines. These pipelines rest on four methodological pillars: in silico prediction approaches such as structure-based docking and ligand similarity modeling; artificial intelligence and machine learning frameworks capable of uncovering non-obvious drug–disease associations; multi-omics integration strategies that align drug-induced molecular signatures with disease-specific perturbations; and systems or network biology models that situate drugs within disease-relevant interactomes and pathway architectures. Across cardiovascular disease, oncology, and neurodegenerative disorders, these frameworks have demonstrated how disease-contextualized modeling can prioritize candidates with mechanistic plausibility and translational potential. Yet, important challenges remain, including data heterogeneity, algorithmic bias, limited reproducibility, and structural barriers related to intellectual property, regulatory pathways, and clinical trial design. Addressing these constraints will require interoperable knowledge graphs, improved model interpretability, and alignment with precision medicine strategies that incorporate patient-specific molecular profiles. Looking forward, the convergence of large-scale biomedical data, generative design platforms, and adaptive validation systems positions computational repurposing as a scalable engine for therapeutic innovation. For biomedical and computational researchers alike, these pipelines represent a pragmatic and mechanistically grounded framework for accelerating disease-specific therapeutic discovery.

BACKGROUND:Inherited PLN (phospholamban) R14del variants cause dilated cardiomyopathy with a high burden of malignant ventricular arrhythmias. However, the single-cell electrophysiological substrate underlying arrhythmogenicity is incompletely characterized, and it is unclear whether antiarrhythmic agents validated in wild-type (WT) or long QT models retain efficacy in this genotype. We sought to define the genotype-specific electrophysiological phenotype of PLN R14del cardiomyocytes and evaluate how modulation of the transient outward potassium current and late sodium current alters arrhythmic risk. METHODS:Isogenic WT (HD.15S1) and CRISPR (clustered regularly interspaced short palindromic repeats)-edited PLN R14del human induced pluripotent stem cell-derived cardiomyocytes were studied using high-throughput optical action potential (AP) recordings. A deep learning framework classified AP morphology to quantify normal versus aberrant AP types, AP duration, and early afterdepolarization incidence at baseline and under graded concentrations of the transient outward potassium current activator NS-5806, the transient outward potassium current inhibitor acacetin, and the selective late sodium current blocker GS-967 (eleclazine). RESULTS:At baseline, PLN R14del human induced pluripotent stem cell-derived cardiomyocytes exhibited a subtle but significant arrhythmogenic phenotype, with a reduced proportion of normal APs, prolonged AP duration, and increased early afterdepolarizations compared with isogenic WT, consistent with impaired calcium handling and diminished repolarization reserve. In WT cells, NS-5806 produced a biphasic, dose-dependent response, transiently destabilizing and then restoring normal AP morphology; in PLN R14del cells, NS-5806 induced marked proarrhythmic remodeling and near-complete loss of normal APs at higher doses. GS-967 paradoxically exacerbated arrhythmic features in the mutant line, including AP duration prolongation and suppression of normal APs. Acacetin preserved stability in WT cells but failed to rescue normal AP morphology in PLN R14del cardiomyocytes. CONCLUSIONS:PLN R14del cardiomyopathy creates a distinct electrophysiological substrate that fundamentally alters antiarrhythmic drug responsiveness, such that agents beneficial in WT contexts may be ineffective or proarrhythmic in this genotype. Preclinical antiarrhythmic evaluation should incorporate genotype-specific, patient-derived human induced pluripotent stem cell models to enable precision medicine strategies in inherited cardiomyopathies.

Cardiac rhythm is fundamental to cardiovascular health, ensuring synchronized electrical impulses that maintain effective heartbeats and blood circulation. Central to this process are electrolytes—sodium, potassium, calcium, magnesium, and chloride—which regulate the generation and propagation of action potentials across cardiac cell membranes. Each electrolyte plays a distinct role in cardiac electrophysiology: sodium drives rapid depolarization, potassium facilitates repolarization, calcium modulates contraction, magnesium stabilizes ion channels, and chloride maintains ionic balance. Electrolyte imbalances, such as hyperkalemia, hypokalemia, hypernatremia, and hypocalcemia, are critical contributors to arrhythmias, contractility issues, and cardiomyopathies. For instance, hyperkalemia actually depresses the upstroke of the action potential by partially depolarizing the resting membrane (inactivating Na+ channels), slowing impulse conduction. Similarly, hypercalcemia shortens action potential duration, while hypocalcemia compromises cardiac contractility. Clinically, maintaining electrolyte homeostasis is critical to mitigating arrhythmic risk and improving outcomes in conditions such as atrial fibrillation and heart failure. Advances in therapeutic interventions, including electrolyte supplementation, ion channel modulators, and precision medicine approaches, offer new opportunities for improving cardiac care. Furthermore, understanding the interplay between electrolytes, myocardial ultrastructure, and systemic comorbidities like hypertension and diabetes is critical for developing targeted therapies. This review highlights the pivotal roles of electrolytes in maintaining cardiac rhythm and provides insights into their clinical and therapeutic implications for managing electrolyte-driven cardiac diseases.

Background: Hypertrophic cardiomyopathy (HCM) is often caused by mutations such as p.R943X in the Myosin Binding Protein C3 (MYBPC3), leading to early and delayed afterdepolarizations, myofibrillar disarray, and calcium dysfunction, which may induce lethal arrhythmias. Dilated cardiomyopathy (DCM) can also result from mutations, such as the p.R14del mutation in the phospholamban (PLN) protein, which is crucial for calcium signaling regulation, leading to arrhythmogenesis when provoked. This study explores the arrhythmogenic potential in human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) carrying the HCM MYBPC3 p.R943X mutation and the DCM PLN p.R14del mutation. Methods: We utilized a 384-well optical physiological system to record action potentials (APs) in hiPSC-CMs carrying the MYBPC3 p.R943X mutation and the DCM PLN p.R14del mutation, including their isogenic controls. Action potential metrics were measured under spontaneous and electrically paced conditions. The effects of Acacetin, NS-5806, nifedipine, and GS-967 on the recorded AP types were analyzed to understand the underlying channelopathies and arrhythmogenic potential. Special attention was given to the variability between the HCM MYBPC3 p.R943X and DCM PLN p.R14del cell lines' susceptibility to arrhythmogenesis and their isogenic controls. Results: The HCM MYBPC3 p.R943X hiPSC-CMs exhibited a significant increase in early afterdepolarizations, non-sustained and sustained ventricular tachycardia, and AP notches compared to their isogenic control. AP notches and arrhythmogenic events were notably provoked by NS-5806 in both the HCM MYBPC3 p.R943X and DCM PLN p.R14del hiPSC-CMs. However, these were significantly ameliorated (p<0.05) by Acacetin administration (5 µM), highlighting its potential as an effective antiarrhythmic in managing both HCM- and DCM-induced arrhythmias. GS-967 also showed efficacy in reducing early afterdepolarizations (EADs) in the HCM model. Conclusions: This study provides novel insights into the arrhythmogenic potential of hiPSC-CMs carrying MYBPC3 p.R943X and PLN p.R14del mutations, underscoring the significant role of Acacetin in attenuating these effects. It paves the way for developing targeted therapies that address the complex molecular mechanisms underlying arrhythmogenesis in cardiomyopathies. The findings suggest that Acacetin could be a promising candidate for the pharmacologic treatment of cardiomyopathies characterized by arrhythmogenic risks.

Hypertrophic cardiomyopathy (HCM) can be caused by a truncation mutant, p.R943X, in the Myosin Binding Protein C3 (MYBPC3). Early and delayed afterdepolarizations are common action potential (AP) features seen in early stages of HCM; however, the myofibrillar disarray and calcium dysfunction may induce other channelopathies and AP morphologies. The presence of an AP notch also predisposes to lethal forms of arrhythmias, culminating in sudden cardiac arrest.

Background: The J-wave syndromes (JWS), comprised of Brugada (BrS) and early repolarization syndromes (ERS), increase the risk of life-threatening ventricular arrhythmias in patients resulting in sudden cardiac death. There are limited therapeutic strategies to treat these life-threatening conditions. Present management is via an implantable cardioverter defibrillator and/or medication, including quinidine. However, these treatments may have serious side-effects. Thus, novel therapies are needed to treat JWS. In this study, we investigate the effects of ARumenamide-787 (AR-787) in suppressing the electrocardiographic and arrhythmic manifestations of JWS and hypothermia. Methods: We studied the effects of AR-787 on sodium current (I Na ) and the delayed-rectifier potassium current (I Kr ) in HEK-293 cells stably expressing the α- and β1-subunits of the cardiac (Na V 1.5) sodium channel or the hERG channel. In addition, we studied its effect on the transient outward potassium current (I to ) and calcium current (I Ca ) from dissociated canine ventricular myocytes, along with action potentials and ECG from coronary-perfused right (RV) and left (LV) ventricular wedge preparations. The I to agonist, NS5806, I Ca blocker, verapamil, and sodium current (I Na ) blocker, ajmaline, to induce the electrocardiographic and arrhythmic manifestations of JWS (prominent J waves/ST segment elevation, phase 2 reentry, and polymorphic VT/VF) in right and left canine ventricular wedge preparations. Results: AR-787 exerted pleiotropic effects on the cardiac ion channels. The predominant effect was an inhibition of I to and enhancement of I Na , with subtle effects to inhibit the I Kr and augment I Ca . AR-787 diminished the electrocardiographic J wave and prevented and/or suppressed all arrhythmic activity in canine RV and LV experimental models of BrS, ERS and hypothermia. We observed no arrhythmogenic effects of AR-787 on I Kr . Conclusions: Our findings point to AR-787 as a promising candidate for the pharmacologic treatment of JWS and hypothermia. AR-787 suppresses the electrocardiographic and arrhythmic manifestations of JWS and hypothermia without significant prolongation of the QT interval, as with the classic BrS drug, quinidine.

Background: Most therapeutics targeting cardiac voltage-gated sodium channels (Nav1.5) attenuate the sodium current (INa) conducted through the pore of the protein. Whereas these drugs may be beneficial for disease states associated with gain-of-function (GoF) in Nav1.5, few attempts have been made to therapeutically treat loss-of-function (LoF) conditions. The primary impediment to designing efficacious therapies for LoF is a tendency for drugs to occlude the Nav1.5 central pore. We hypothesized that molecular candidates with a high affinity for the fenestrations would potentially reduce pore block. Methods and Results: Virtual docking was performed on 21 compounds, selected based on their affinity for the fenestrations in Nav1.5, which included a class of sulfonamides and carboxamides we identify as ARumenamide (AR). Six ARs, AR-051, AR-189, AR-674, AR-802, AR-807 and AR-811, were further docked against Nav1.5 built on NavAb and rNav1.5. Based on the virtual docking results, these particular ARs have a high affinity for Domain III-IV and Domain VI-I fenestrations. Upon functional characterization, a trend was observed in the effects of the six ARs on INa. An inverse correlation was established between the aromaticity of the AR’s functional moieties and compound block. Due to its aromaticity, AR-811 blocked INa the least compared with other aromatic ARs, which also decelerated fast inactivation onset. AR-674, with its aliphatic functional group, significantly suppresses INa and enhances use-dependence in Nav1.5. AR-802 and AR-811, in particular, decelerated fast inactivation kinetics in the most common Brugada Syndrome Type 1 and Long-QT Syndrome Type 3 mutant, E1784K, without affecting peak or persistent INa. Conclusion: Our hypothesis that LoF in Nav1.5 may be therapeutically treated was supported by the discovery of ARs, which appear to preferentially block the fenestrations. ARs with aromatic functional groups as opposed to aliphatic groups efficaciously maintained Nav1.5 availability. We predict that these bulkier side groups may have a higher affinity for the hydrophobic milieu of the fenestrations, remaining there rather than in the central pore of the channel. Future refinements of AR compound structures and additional validation by molecular dynamic simulations and screening against more Brugada variants will further support their potential benefits in treating certain LoF cardiac arrhythmias.

Background Brugada (BrS) and early repolarization syndromes (ERS), the so-called J wave syndromes (JWS), are associated with life-threatening ventricular arrhythmias. Pharmacologic approaches to therapy are currently limited. In this study, we examine the effects of ARumenamide-787 (AR-787) to suppress the electrocardiographic and arrhythmic manifestations of JWS and hypothermia. Methods We studied the effects of AR-787 on I Na and I Kr in HEK-293 cells stably expressing the α- and β1-subunits of the cardiac (Na V 1.5) sodium channel and hERG channel, respectively. In addition, we studied its effect on I to , I Na and I Ca in dissociated canine ventricular myocytes along with action potentials and ECG from coronary-perfused right (RV) and left (LV) ventricular wedge preparations. The I to agonist, NS5806 (5–10 μM), I Ca blocker, verapamil (2.5 μM), and I Na blocker, ajmaline (2.5 μM), were used to mimic the genetic defects associated with JWS and to induce the electrocardiographic and arrhythmic manifestations of JWS (prominent J waves/ST segment elevation, phase 2 reentry and polymorphic VT/VF) in canine ventricular wedge preparations. Results AR-787 (1, 10 and 50 μM) exerted pleiotropic effects on cardiac ion channels. The predominant effect was inhibition of the transient outward current (I to ) and enhancement of the sodium channel current (I Na ), with lesser effects to inhibit I Kr and augment calcium channel current (I Ca ). AR-787 diminished the electrocardiographic J wave and prevented and/or suppressed all arrhythmic activity in canine RV and LV experimental models of BrS, ERS and hypothermia. Conclusions Our findings point to AR-787 as promising candidate for the pharmacologic treatment of JWS and hypothermia.

Hypertrophic cardiomyopathy (HCM) can be caused by a truncation mutant, p.R943X, in the Myosin Binding Protein C3 (MYBPC3). Early and delayed afterdepolarizations are common action potential (AP) features seen in early stages of HCM; however, the myofibrillar disarray and calcium dysfunction may induce other channelopathies and AP morphologies. The presence of an AP notch also predisposes to lethal forms of arrhythmias, culminating in sudden cardiac arrest. Using human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs), we characterized baseline arrhythmia type and incidence in hiPSC-CMs carrying the MYBPC3 p.R943x and its isogenic control. AP metrics were measured using the voltage probe, Fluovolt, from spontaneously beating and electrically paced cells. Acacetin, NS-5806, nifedipine, and GS-967 were administered to probe the channelopathies underlying the AP types recorded. We evaluated whether a dysregulated Ca2+-CaMKIId-INaL circuit is a cellular mechanism that predisposes HCM cardiomyocytes to arrhythmias. We established a high throughput, 384-well optical physiological system revealing baseline arrhythmogenic APs characterized in HCM hiPSC-CMs carrying MYBPC3 p.R943X. The APs clearly show a marked increase in early afterdepolarizations, non-sustained ventricular tachycardia, sustained ventricular tachycardia, and AP notches compared to isogenic control hiPSC-CMs.Understanding the mechanisms underlying heightened AP heterogeneity in HCM hiPSC-CMs is pivotal in administering or discovering precise therapy. This study is the first to characterize cellular inputs into an array of AP phenotypes in MYBPC3 p.R943X. The use of hiPSC-CMs lines unambiguously associates the signaling pathways and intracellular circuitries involved in sustaining arrhythmogenesis. Results may incentivize the discovery of antiarrhythmics with pleiotropic profiles suited to target multiple ion channel dysfunctions. This comprehensive approach will establish many novel molecular mechanisms and reveal points of therapeutic intervention.

Drug repurposing is the use of a given therapeutic agent for indications other than that for which it was originally designed or intended. The concept is appealing because of potentially lower development costs and shorter timelines than are needed to produce a new drug. To date, drug repurposing for cardiovascular indications has been opportunistic and driven by knowledge of disease mechanisms or serendipitous observation rather than by systematic endeavours to match an existing drug to a new indication. Innovations in two areas of personalized medicine - computational approaches to associate drug effects with disease signatures and predictive model systems to screen drugs for disease-modifying activities - support efforts that together create an efficient pipeline to systematically repurpose drugs to treat cardiovascular disease. Furthermore, new experimental strategies that guide the medicinal chemistry re-engineering of drugs could improve repurposing efforts by tailoring a medicine to its new indication. In this Review, we summarize the historical approach to repurposing and discuss the technological advances that have created a new landscape of opportunities. Drugs can be repurposed for new therapeutic indications. In this Review, Mercola and colleagues summarize the latest techniques for systematic drug repurposing and re-engineering, which could increase the pace, efficiency and cost-effectiveness of drug discovery for the treatment of cardiovascular disease.

Background: Long-QT syndrome (LQTS) is primarily an electrical disorder characterized by a prolonged myocardial action potential. The delay in cardiac repolarization leads to electromechanical (EM) abnormalities, which adds a diagnostic value for LQTS. Prolonged left ventricular (LV) contraction was identified as a potential risk for arrhythmia. The aim of this meta-analysis was to assess the best predictor of all EM parameters for cardiac events (CEs) in LQTS patients. Methods: We systematically searched all electronic databases up to March 2020, to select studies that assessed the relationship between echocardiographic indices—contraction duration (CD), mechanical dispersion (MD), QRS onset to peak systolic strain (QAoC), and the EM window (EMW); and electrical indices— corrected QT interval (QTC), QTC dispersion, RR interval in relation to CEs in LQTS. This meta-analysis included a total of 1041 patients and 373 controls recruited from 12 studies. Results: The meta-analysis showed that LQTS patients had electrical and mechanical abnormalities as compared to controls—QTC, WMD 72.8; QTC dispersion, WMD 31.7; RR interval, WMD 91.5; CD, WMD 49.2; MD, WMD 15.9; QAoC, WMD 27.8; and EMW, WMD −62.4. These mechanical abnormalities were more profound in symptomatic compared to asymptomatic patients in whom disturbances were already manifest, compared to controls. A CD ≥430 ms had a summary sensitivity (SS) of 71%, specificity of 84%, and diagnostic odds ratio (DOR) >19.5 in predicting CEs. EMW and QTC had a lower accuracy. Conclusions: LQTS is associated with pronounced EM abnormalities, particularly prolonged LV myocardial CD, which is profound in symptomatic patients. These findings highlight the significant role of EM indices like CD in managing LQTS patients.

Aims:To clarify the clinical characteristics and outcomes of children with SCN5A-mediated disease and to improve their risk stratification.Methods and results:A multicentre, international, retrospective cohort study was conducted in 25 tertiary hospitals in 13 countries between 1990 and 2015. All patients ≤16 years of age diagnosed with a genetically confirmed SCN5A mutation were included in the analysis. There was no restriction made based on their clinical diagnosis. A total of 442 children {55.7% boys, 40.3% probands, median age: 8.0 [interquartile range (IQR) 9.5] years} from 350 families were included; 67.9% were asymptomatic at diagnosis. Four main phenotypes were identified: isolated progressive cardiac conduction disorders (25.6%), overlap phenotype (15.6%), isolated long QT syndrome type 3 (10.6%), and isolated Brugada syndrome type 1 (1.8%); 44.3% had a negative electrocardiogram phenotype. During a median follow-up of 5.9 (IQR 5.9) years, 272 cardiac events (CEs) occurred in 139 (31.5%) patients. Patients whose mutation localized in the C-terminus had a lower risk. Compound genotype, both gain- and loss-of-function SCN5A mutation, age ≤1 year at diagnosis in probands and age ≤1 year at diagnosis in non-probands were independent predictors of CE.Conclusion:In this large paediatric cohort of SCN5A mutation-positive subjects, cardiac conduction disorders were the most prevalent phenotype; CEs occurred in about one-third of genotype-positive children, and several independent risk factors were identified, including age ≤1 year at diagnosis, compound mutation, and mutation with both gain- and loss-of-function.

Skeletal muscle channelopathies, many of which are inherited as autosomal dominant mutations, include myotonia and periodic paralysis. Myotonia is defined by a delayed relaxation after muscular contraction, whereas periodic paralysis is defined by episodic attacks of weakness. One sub-type of periodic paralysis, known as hypokalemic periodic paralysis (hypoPP), is associated with low potassium levels. Interestingly, the P1158S missense mutant, located in the third domain S4-S5 linker of the "skeletal muscle", Nav1.4, has been implicated in causing both myotonia and hypoPP. A common trigger for these conditions is physical activity. We previously reported that Nav1.4 is relatively insensitive to changes in extracellular pH compared to Nav1.2 and Nav1.5. Given that intense exercise is often accompanied by blood acidosis, we decided to test whether changes in pH would push gating in P1158S towards either phenotype. Our results suggest that, unlike in WT-Nav1.4, low pH depolarizes the voltage-dependence of activation and steady-state fast inactivation, decreases current density, and increases late currents in P1185S. Thus, P1185S turns the normally pH-insensitive Nav1.4 into a proton-sensitive channel. Using action potential modeling we predict a pH-to-phenotype correlation in patients with P1158S. We conclude that activities which alter blood pH may trigger the noted phenotypes in P1158S patients.

E1784K is the most common mixed syndrome SCN5a mutation underpinning both Brugada syndrome type 1 (BrS1) and Long-QT syndrome type 3 (LQT3). The charge reversal mutant enhances the late sodium current (I Na ) passed by the cardiac voltage-gated sodium channel (Na V 1.5), delaying cardiac repolarization. Exercise-induced triggers, like elevated temperature and cytosolic calcium, exacerbate E1784K late I Na . In this study, we tested the effects of Ranolazine, the late I Na blocker, on voltage-dependent and kinetic properties of E1784K at elevated temperature and cytosolic calcium. We used whole-cell patch clamp to measure I Na from wild type and E1784K channels expressed in HEK293 cells. At elevated temperature, Ranolazine attenuated gain-of-function in E1784K by decreasing late I Na , hyperpolarizing steady-state fast inactivation, and increasing use-dependent inactivation. Both elevated temperature and cytosolic calcium hampered the capacity of Ranolazine to suppress E1784K late I Na . In-silico action potential (AP) simulations were done using a modified O’Hara Rudy (ORd) cardiac model. Simulations showed that Ranolazine failed to shorten AP duration, an effect augmented at febrile temperatures. The drug-channel interaction is clearly affected by external triggers, as reported previously with ischemia. Determining drug efficacy under various physiological states in SCN5a cohorts is crucial for accurate management of arrhythmias.

Key points SCN5a mutations may express gain‐of‐function (Long QT Syndrome‐3), loss‐of‐function (Brugada Syndrome 1) or both (mixed syndromes), depending on the mutation and environmental triggers. One such trigger may be an increase in cytosolic calcium, accompanying exercise. Many mixed syndromes mutants, including ∆KPQ, E1784K, 1795insD and Q1909R, are found in calcium‐sensitive regions. Elevated cytosolic calcium attenuates gain‐of‐function properties in ∆KPQ, 1795insD and Q1909R, but not in E1784K. By contrast, elevated cytosolic calcium further exacerbates gain‐of‐function in E1784K by destabilizing slow inactivation. Action potential modelling, using a modified O'Hara Rudy model, suggests that elevated heart rate rescues action potential duration in ∆KPQ, 1795insD and Q1909R, but not in E1784K. Action potential simulations suggest that E1784K carriers have an increased intracellular sodium‐to‐calcium ratio under bradycardia and tachycardia conditions. Elevated cytosolic calcium, which is common during high heart rates, ameliorates or exacerbates the mixed syndrome phenotype depending on the genetic signature. Abstract Inherited arrhythmias may arise from mutations in the gene for SCN5a, which encodes the cardiac voltage‐gated sodium channel, Na V 1.5. Mutants in Na V 1.5 result in Brugada Syndrome (BrS1), Long‐QT Syndrome (LQT3) or mixed syndromes (an overlap of BrS1/LQT3). Exercise is a potential arrhythmogenic trigger in mixed syndromes. We aimed to determine the effects of elevated cytosolic calcium, which is common during exercise, in mixed syndrome Na V 1.5 mutants. We used whole‐cell patch clamp to assess the biophysical properties of Na V 1.5 wild‐type (WT), ∆KPQ, E1784K, 1795insD and Q1909R mutants in human embryonic kidney 293 cells transiently transfected with the Na V 1.5 α subunit (WT or mutants), β1 subunit and enhanced green fluorescent protein. Voltage‐dependence and kinetics were measured at cytosolic calcium levels of approximately 0, 500 and 2500 n m . In silico , action potential (AP) model simulations were performed using a modified O'Hara Rudy model. Elevated cytosolic calcium attenuates the late sodium current in ∆KPQ, 1795insD and Q1909R, but not in E1784K. Elevated cytosolic calcium restores steady‐state slow inactivation (SSSI) to the WT‐form in Q1909R, but depolarized SSSI in E1784K. Our AP simulations showed a frequency‐dependent reduction of AP duration in ∆KPQ, 1795insD and Q1909R carriers. In E1784K, AP duration is relatively prolonged at both low and high heart rates, resulting in a sodium overload. Cellular perturbations during exercise may affect BrS1/LQT3 patients differently depending on their individual genetic signature. Thus, exercise may be therapeutic or may be an arrhythmogenic trigger in some SCN5a patients.

Sudden cardiac death (SCD) from inherited channelopathies disproportionately affects those under the age of 40. The E1784K mutant in the cardiac sodium channel causes two diseases: long QT syndrome type 3 (LQT3) and Brugada syndrome type 1 (BrS1). Cardiac voltage-gated sodium channels pass a transient inward sodium current responsible for depolarizing ventricular cardiomyocytes. Mutant channels have either decreased transient current amplitude or an increased fraction of persistent current, the underlying mechanisms of BrS1 and LQT3, respectively, and both of which may cause ventricular arrhythmia. Despite carrying mutant channels from birth, patients may live for many years and millions of heart beats without incident, suggesting the presence of external triggers. We investigated whether physiological perturbations during exercise can exacerbate the effects of sodium channel mutants and trigger SCD. We studied the effects of increased temperature, decreased extracellular pH, and increased cytosolic calcium on wildtype and mutant sodium channels. We find that decreased peak sodium currents in the E1784K mutant are exacerbated by low extracellular pH. Mutant-induced increases in the persistent sodium current are further increased both by low pH and by high temperature. In some LQT3 mutants, such as ΔKPQ and 1795insD, increases in intracellular calcium provide a protective role by decreasing the persistent current. In contrast, the persistent sodium current in E1784K is insensitive to changes in intracellular calcium. Also, when intracellular calcium is elevated, persistent current in the E1784K mutant is not sensitive to Ranolazine, a selective persistent sodium current blocker. Our results suggest the pro-arrhythmic effects of the E1784K mutant in cardiac sodium channels are exacerbated by physiological changes associated with exercise. Increases in temperature and blood acidosis further decrease peak sodium currents and increase persistent sodium currents; both these changes are potentially arrhythmogenic. Additionally, E1784K persistent currents are not sensitive to exercise-induced increases in intracellular calcium, which mitigates the arrhythmogenicity of other sodium channel mutants. Overall our results suggest that factors associated with exercise may act as mutant-specific arrhythmogenic triggers underlying SCD.

Slightly more than two decades have passed since the discovery of the first SCN5a mutation, DKPQ, associated with inherited arrhythmias. Ever since, a plethora of SCN5a mutations have been discovered and linked to syndromes such as Brugada Syndrome type 1 (BrS1) and Long-QT syndrome type 3 (LQT3). Interestingly, some sodium channel mutants may cause a mixed syndrome of both BrS1 and LQT3, with the most common being E1784K. Although E1784K may express a phenotypic overlap of both BrS1 and LQT3, the LQT3 phenotype is more common, especially in the Okinawa islands, where over 80% of LQT genotype positive children carry this mutation. SCN5a encodes the cardiac sodium channel, NaV1.5, which passes the transient inward sodium current underlying depolarization in ventricular myocytes. The E1784K mutant occurs in the NaV1.5 C-terminus, near a paired EF-hand like domain. The Cterminus in sodium channels is an important modulator of channel activation, fast inactivation, and slow inactivation. This modulation is in part due to a calcium-calmodulin dependent interaction between the C-terminus and the Domain III-IV linker, the sodium channel fast inactivation “particle”. The charge-reversal mutant, E1784K, may disrupt the overall integrity of the C-terminus by disrupting the electrostatic forces connecting the EF-hand domain and the IQ motif. These interactions in the C-terminus are crucial for proper inactivation. Structural and biophysical studies show that a disassembly in the C-terminus correlates with perturbed inactivation voltage-dependence and the presence of persistent sodium currents. The E1784K channotype shares common biophysical perturbations with a number of other C-terminal LQT3 and BrS1 mutants, hyperpolarizing the voltage-dependence of steady-state fast inactivation (SSFI), depolarizing the voltage-dependence of activation (GV), and increasing the late sodium current. To better understand the pathophysiology of E1784K, we studied several physiological triggers associated with exercise that are known to unmask arrhythmias. We have shown that increases in body temperature increase the fraction of non-inactivating channels passing late sodium current in E1784K. We also showed that E1784K is sensitive to changes in extracellular protons. Decreases in extracellular pH greatly decreased peak sodium currents and increased the fraction of non-inactivating E1784K sodium channels. Action potential simulations based on the ten T€ usscher model of the ventricular action potential suggested that E1784K would cause a large transmural dispersion of repolarization with increases in temperature or decreases in extracellular pH. Our pH results with WT and E1784K channels allowed us to generate a novel model, the Peters-Ruben model of sodium channel gating. This model confirms the E1784K channotype arises from defects in fast inactivation, and predicts the mutant disrupts interactions between the C-terminus and the voltage sensor in Domain 4, translocation of which has been shown to be linked to fast inactivation. We recently reported the effects of increasing cytosolic calcium on E1784K and other sodium channel mutants. Calcium-sensitivity is differentially affected

BACKGROUND:Whole-exome sequencing has transformed gene discovery and diagnosis in rare diseases. Translation into disease-modifying treatments is challenging, particularly for intellectual developmental disorder. However, the exception is inborn errors of metabolism, since many of these disorders are responsive to therapy that targets pathophysiological features at the molecular or cellular level.METHODS:To uncover the genetic basis of potentially treatable inborn errors of metabolism, we combined deep clinical phenotyping (the comprehensive characterization of the discrete components of a patient's clinical and biochemical phenotype) with whole-exome sequencing analysis through a semiautomated bioinformatics pipeline in consecutively enrolled patients with intellectual developmental disorder and unexplained metabolic phenotypes.RESULTS:We performed whole-exome sequencing on samples obtained from 47 probands. Of these patients, 6 were excluded, including 1 who withdrew from the study. The remaining 41 probands had been born to predominantly nonconsanguineous parents of European descent. In 37 probands, we identified variants in 2 genes newly implicated in disease, 9 candidate genes, 22 known genes with newly identified phenotypes, and 9 genes with expected phenotypes; in most of the genes, the variants were classified as either pathogenic or probably pathogenic. Complex phenotypes of patients in five families were explained by coexisting monogenic conditions. We obtained a diagnosis in 28 of 41 probands (68%) who were evaluated. A test of a targeted intervention was performed in 18 patients (44%).CONCLUSIONS:Deep phenotyping and whole-exome sequencing in 41 probands with intellectual developmental disorder and unexplained metabolic abnormalities led to a diagnosis in 68%, the identification of 11 candidate genes newly implicated in neurometabolic disease, and a change in treatment beyond genetic counseling in 44%. (Funded by BC Children's Hospital Foundation and others.).