Citations

BACKGROUND: Type 2 long-QT syndrome results from KCNH2 -encoded Kv11.1 loss-of-function, but whether secondary inward-current dysfunction amplifies dysfunction remains unclear. METHODS: We characterized sodium ( SCN5A , I Na ) and L-type calcium channel ( CACNA1C , I Ca ) function in KCNH2-G604S and variant-corrected isogenic control induced pluripotent stem cell–derived cardiomyocytes (iPSC-CMs). Whole-cell patch-clamp measured action potential duration at 90% repolarization (APD90), and potassium (I Kr ), sodium (I Na ), and calcium (I Ca, L ) currents. Fluo-4 imaging quantified calcium transients. A modified Kernik-Clancy iPSC-CM model incorporated experimentally measured persistent I Na,L and I Ca,L fractions and simulated isolated and combined current defects. RESULTS: KCNH2-G604S caused severe I Kr loss-of-function (72.5% reduction). APD90 was prolonged in G604S-iPSC-CMs (765±58 ms) compared with isogenic controls (368±37 ms; P <0.005), with early afterdepolarizations in 50% of G604S cells. Nifedipine, verapamil, and mexiletine successfully shortened APD90 and abolished early afterdepolarizations. Compared with isogenic control, I Na,L (0.60±0.1% versus 0.26±0.06%; P <0.01), persistent I Ca,L (2.88±0.55% versus 1.26±0.35%; P <0.05), and calcium transient decay times ( P <0.0001) were greater in G604S iPSC-CMs. Increased persistent I Ca,L was phenocopied by E-4031 in isogenic controls and reduced by nifedipine in G604S iPSC-CMs. In silico modeling revealed that the I Kr defect alone prolonged APD90 to only 620 ms. The addition of the observed I Na,L defect had a small additive effect (+20 ms); however, incorporating persistent I Ca,L synergistically extended APD90 to 758 ms, replicating the experimental phenotype. Flux dynamics analysis indicated that this persistent Ca 2+ entry suppresses the forward-mode sodium-calcium exchanger, forcing a compensatory reliance on sarcoplasmic-endoplasmic reticulum Ca-ATPase that primes for calcium instability. CONCLUSIONS: The KCNH2-G604S variant drives a severe multihit phenotype via primary I Kr loss-of-function and secondary state-dependent gain-of-function in persistent I Ca,L . These comprehensive validations demonstrate that the remodeled persistent I Ca,L acts as the essential amplifier driving extreme action potential duration prolongation and Ca 2+ mishandling.

Abstract Background Calcium release channel deficiency syndrome (CRCDS) arises from loss-of-function variants in the RYR2-encoded type 2 ryanodine receptor (RyR2), predisposing patients to sudden cardiac arrest/death (SCA/SCD) despite normal exercise testing. Previously, a novel homozygous RYR2 duplication causing ~ 80% RyR2 reduction was associated with exertion-related SCD. However, how Ca2+ homeostasis is maintained despite RyR2-loss remains unknown, as RYR2-/- animals are non-viable. Purpose To generate a RYR2-knockout (RYR2-KO-/-) induced pluripotent stem cell-derived cardiomyocyte (iPSC-CM) model to dissect compensatory mechanisms sustaining Ca2+ homeostasis and identify arrhythmogenic pathways predisposing to SCD. Methods CRISPR/Cas9 editing was used to introduce a homozygous c.163delT variant into a wild-type iPSC line (control), generating a RYR2-KO-/- model. A patient-derived RYR2-duplication (RYR2-DUP+/+) iPSC line was used for comparison. RyR2 expression was assessed by western blot (WB) and immunofluorescence (IF). Proteome and phospho-proteome were assessed using omics approaches. Intracellular Ca2+ handling was evaluated by Fluo-4 AM imaging (0.5 Hz and long-burst–long-pause [LBLP] protocol). Electrophysiological properties (action potential [AP], and sarcolemmal currents [ICaL, INa and INCX]) were assessed by patch-clamp. Results Despite complete RyR2-loss in RYR2-KO-/- iPSC-CMs as confirmed by WB and IF, contractility and contractile machinery expression was preserved. Ca2+ transient amplitude (ΔF/F0) was reduced markedly in RYR2-KO (0.14±0.01, p&lt;0.01) and RYR2-DUP (0.27±0.2, p&lt;0.01) compared to control (0.52±0.02). Proteomics revealed a significant NCX1 up-regulation and IP3R-interacting protein (ITPRIP) down-regulation in RYR2-KO compared to control, with pathway-analyses showing significant up-regulation of IP3-related pathways. Pharmacological IP3Rs block (2-APB, 10μM) abolished Ca2+ transient generation in RYR2-KO iPSC-CMs (Amplitude: baseline: 0.15±0.01, 2-APB: 0.06±0.01, p&lt;0.01). RyR2-ablation induced electrical remodeling (pA/pF), including ICaL reduction, INCX up-regulation and hyperpolarization of the INa window current in RYR2-KO (ICaL [at 0mV]: -8.5±1.6; INCX [at -110mV]: -4.1±1.1) as compared to control (ICaL:-7.5±3.7, p&lt;0.01; INCX:-1.5±0.5, p=0.02). RYR2-KO CMs demonstrated increased delayed after-depolarizations and beat-to-beat variability as compared to control. Despite Ca2+ sparks abolishment, RYR2-KO CMs displayed higher erratic beating (at 0.5Hz) and arrhythmogenic propensity than control, showing ventricular tachycardia-like events during LBLP stimulation. Conclusions Genetic RYR2-ablation disrupts physiological Ca2+ cycling and triggers electrical remodeling, including INCX up-regulation, overall promoting arrhythmogenesis. Compensatory IP3Rs activation sustains Ca2+ transient generation, revealing a mechanism preserving excitation-contraction coupling that may paradoxically promote arrhythmogenesis in CRCDS.RYR2-KO: Generation and CharacterizationElectrophysiological Assessment

Abstract Background Type 8 long QT syndrome (LQT8) is characterised by prolonged QT interval and a high burden of ventricular repolarisation abnormalities including sudden cardiac death. Purpose Patients and clinicians face uncertainties about the daily risk of cardiac arrest, particularly during physical performance. Therefore, we aim to establish a risk assessment based on systematically collected data. Methods We analysed eight patients genetically confirmed to carry CACNA1C variants, of whom three had cardiac arrests in early childhood, while others were discovered incidentally. One patient met criteria for cardiac-only Timothy syndrome. All patients performed cardiopulmonary exercise testing (CPET) with several having multiple studies over time, resulting in a total of 19 CPETs. Echocardiograms were available for all patients, and Holter monitor results for all but one patient. All CPETs were conducted on beta-blockade. Results On resting ECG the average heart rate was 66.5 ± 10.7 beats per minute (bpm). Average QT at diagnosis was 476.8 ± 47.8 ms, with a corrected QTc of 487.9 ± 37.2 ms. Two patients had sinus arrhythmia and three were atrially paced at rest. No ventricular arrhythmias were observed at rest. On initial CPET, average peak oxygen uptake was 29.3 ± 8.5 ml/kg/min (82 ± 25% predicted) at a peak heart rate of 141.6 ± 24.3 bpm (73 ± 13% predicted). Peak respiratory exchange ratio averaged 1.09 ± 0.13. On initial CPET, four patients developed premature ventricular contractions (PVCs), but only in one patient were more than 5 per minute detected. That patient had a high PVC burden with multiple runs of non-sustained ventricular tachycardia. There were no episodes of syncope or cardiac arrest across all 19 CPETs. Conclusions Our cases demonstrate the safety and utility of exercise testing in CACNA1C-related disease. Longitudinal follow-up revealed diverse electrophysiological findings, including QT prolongation and reduced chronotropic response. From our data it is suggested that CPET is safe to perform on appropriate therapy and exercise should be encouraged.

Abstract Background Hypertrophic cardiomyopathy (HCM) is a heritable cardiac disorder characterized by increased left ventricular (LV) wall thickness, often leading to heart failure (HF). The role of cardiovascular magnetic resonance (CMR) imaging in predicting new onset of HF symptoms in patients with HCM remains unknown. This study aimed to identify CMR predictors of new-onset HF symptoms in individuals with HCM. Methods This study was a single-centre retrospective cohort study of HCM patients treated at a tertiary referral centre in the USA who underwent CMR examination between 1998 and 2018, had no HF symptoms at baseline CMR, and at least 1 year of follow-up. Clinical data were collected by review of electronic medical records, and CMR images were analyzed by a blinded expert cardiac radiologis. The primary outcome was new onset of HF symptoms, defined as New York Heart Association (NYHA) class ≥ II at follow-up. Kaplan–Meier analyses and Cox proportional hazard analyses were performed. Results Of 1462 patients diagnosed with HCM who had at least 1 CMR, 276 HCM patients without HF symptoms (average age 52.7 years, 33.3% female),median maximum left ventricular (LV) wall thickness was 19 mm ([IQR] 17–22) with a and median LV ejection fraction of 71% (IQR 66–77). Late gadolinium enhancement was detected in 56.2%) patients (60.7% had mild; 30.7% moderate; 8.6% severe). During a median follow-up period of 6.3 years, 93 patients developed HF symptoms (NYHA class II in 56 (60.2%); class III in 31 (33.3%); and class IV in 6 (6.5%). Multivariable analysis adjusted for age showed that LA enlargement (HR 1.626; 95% CI 1.01–2.62; P = .045) and LV mass index (HR 1.014; 95% CI 1.007–1.022; P≤ .001) and sex (HR 1.7; 95% CI 1.074–2.691; P = .023) were independent predictors of new onset of HF symptoms in patients with HCM. Conclusions Nearly half of the patients with HCM developed HF symptoms within 6.3 years. Left atrial enlargement, LV mass index, and sex were independent predictors of new onset of HF symptoms in HCM patients. These findings emphasize the value of CMR in HF risk stratification.

Abstract Aims SCN5A-E1784K (c.5350G&gt;A) is the most common variant associated with the long QT (LQTS) and Brugada syndromes (BrS). It can manifest variably as LQTS, BrS, and/or conduction disorders. This presents a challenge for risk stratification. We aimed to describe clinical and ECG characteristics and identify risk markers that associate with arrhythmic events. Methods and results We undertook a retrospective observational multicentre study of a large cohort of 231 subjects with SCN5A-E1784K from Europe, USA, and Japan. Comprehensive demographic and clinical data, including initial presentation ECG and follow-up, were collected. ‘Lethal events’ were defined as sudden death, non-fatal cardiac arrest, and documented sustained VT or VF. ‘Cardiac events’ were defined as arrhythmic syncope plus any lethal events. Clinical characteristics and ECG parameters corrected for age were investigated for association with lethal and/or cardiac events. Fourteen (6%) subjects experienced a lethal event and 45 (19%) a cardiac event. PR interval and QRS duration were associated with lethal and cardiac events. In multivariable models, both PR interval and QRS duration were associated with lethal events, but only QRS duration was associated with cardiac events. Only age-corrected QRS (rQRS) was associated with lethal and cardiac event-free survival from birth after correction for multiple testing. Conclusion Ventricular myocardial conduction appears likely to play a role in the risk of arrhythmic events in patients with SCN5A-E1784K. This provides an important opportunity for the personalization of management and has the potential to guide preventative therapies.

ABSTRACT Introduction Cardiac myosin inhibitors (CMIs) improve hemodynamics and symptoms in hypertrophic cardiomyopathy (HCM), but recent real‐world data have raised concerns regarding an increased risk of atrial fibrillation (AF). We sought to reassess AF risk using randomized controlled trial (RCT) evidence and to contextualize discrepancies with observational reports. Methods and Results We performed a systematic review and random‐effects meta‐analysis of RCTs evaluating the effect of CMIs on incident AF in HCM. MEDLINE, Scopus, and CENTRAL were searched through November 25, 2025. Eight RCTs ( n = 1581) comparing mavacamten or aficamten with placebo or active control were included. Event counts were pooled as risk ratios (RRs) with 95% confidence intervals (CIs). CMIs were not associated with increased AF risk (RR 1.11, 95% CI 0.62–1.98; heterogeneity I 2 = 0%). No differences were observed between mavacamten and aficamten, with consistent findings across HCM subtypes (obstructive and non‐obstructive). Importantly, most trials (7/8) excluded patients with uncontrolled, persistent, or permanent AF, five excluded paroxysmal AF at screening, and one excluded any AF history. Baseline left atrial enlargement was generally in the mild to moderate range across trials, suggesting limited representation of high‐risk AF phenotypes. Conclusion Randomized evidence to date does not demonstrate an increased AF risk with CMIs. Differences between RCT populations and real‐world cohorts, including restrictive AF eligibility criteria, lower baseline atrial remodeling, and limited rhythm surveillance may explain discordant observational findings and highlight the need for longitudinal monitoring in clinical practice.

Abstract Aims Calcium release channel deficiency syndrome (CRCDS) results from loss-of-function (LOF) variants in the RYR2-encoded type 2 ryanodine receptor (RyR2), predisposing patients to sudden cardiac arrest/death (SCA/SCD) without abnormalities on a stress electrocardiogram (ECG). Undetected CRCDS may underlie idiopathic ventricular fibrillation (IVF) and sudden unexplained death in the young (SUDY). We aimed to determine the prevalence of potential CRCDS-causative RYR2 variants in IVF and SUDY. Methods and results We reviewed clinical evaluation and RYR2 genetic analysis of 169 IVF patients and 279 SUDY victims. Only ultra-rare (&lt;0.005% in gnomAD) nonsynonymous RYR2 variants were considered potentially pathogenic. Among IVF patients, 6/169 (3%) overall—and 6/67 (9%) with exertion-related SCA—harboured an RYR2 variant and represent potential CRCDS cases. All exhibited normal resting and stress ECGs. Genetic analysis revealed six distinct RYR2 variants, two previously characterized as LOF. In SUDY, 31/279 victims (11%) had a RYR2 variant (30 unique variants), predominantly observed in exertion-related SCD 20/83 (24%) vs. rest-related 11/196 (6%). Of the 14 SUDY victims with functionally characterized RYR2 variants, five (2% of total cohort) had a LOF variant; among the 56 exertion-related SUDY cases, four (7%) had a LOF variant. Conclusion CRCDS may account for 3% of IVF overall and 9% of exertion-related SCA in IVF. Ultra-rare RYR2 variants may underlie up to 11% of SUDY, with 65% of RYR2-positive cases occurring during exertion. LOF-RYR2 variants may contribute to ≥7% of exercise-associated SUDY. Accurate identification of the underlying ryanodinopathy is essential for clinical management of affected patients.

ABSTRACT J‐wave syndromes (JWS)—comprising Brugada syndrome (BrS) and early repolarization syndrome (ERS)—are important causes of malignant ventricular arrhythmias and sudden cardiac death in patients whose hearts appear structurally normal. Since the 2016 consensus, advances in genetics, pathophysiology, and therapy have redefined both understanding and management. BrS, once viewed as a purely electrical disorder, is now recognized along a microstructural–electrical continuum, with sodium‐channel dysfunction and subtle epicardial fibrosis of the right ventricular outflow tract as key contributors. Likewise, ERS—historically considered benign—carries significant risk when inferolateral J‐waves coexist with arrhythmic events. Genetically, SCN5A remains the sole gene with definitive disease association, while polygenic susceptibility materially modulates risk, underscoring complex inheritance. Risk stratification remains challenging: patients with prior cardiac arrest or arrhythmic syncope are highest risk, whereas asymptomatic individuals warrant multiparametric assessment integrating clinical features, ECG markers, electrophysiologic studies, and genetics. For decades, treatment centered on implantable cardioverter‐defibrillators and quinidine, both limited by availability, tolerance, and device complications. More recently, epicardial substrate ablation has emerged as a transformative therapy, with large registries and randomized trials demonstrating durable suppression of ventricular fibrillation and acceptable safety. This APHRS‐organized international consensus updates and extends the 2016 Expert Consensus and the 2022 ESC Guidelines, providing contemporary diagnostic frameworks, pragmatic risk‐stratification tools, and treatment algorithms for BrS and ERS. It emphasizes JWS as a microstructural–electrical disease spectrum and elevates substrate ablation as a major therapeutic advance, while outlining priorities for genetics, risk‐stratification and treatment algorithms.