#channelopathies #inherited-arrhythmias #sudden-cardiac-death #LQTS

Long QT Syndrome (LQTS)

Overview

Long QT syndrome is the most prevalent cardiac channelopathy, characterised by prolongation of the QT interval on ECG due to abnormal cardiac repolarization. It predisposes to torsades de pointes (TdP), ventricular fibrillation, and sudden cardiac death in the absence of structural heart disease. LQTS encompasses 17 genetic subtypes; the three major forms — LQT1 (KCNQ1), LQT2 (KCNH2), and LQT3 (SCN5A) — account for ~75% of genotype-positive cases. Clinical presentation ranges from asymptomatic QT prolongation to syncope, aborted cardiac arrest, and SCD, often triggered by exercise, emotion, or auditory stimuli depending on subtype. Management is stepwise: lifestyle modification + beta-blockers as the foundation, escalating to mexiletine (LQT3), LCSD, and ICD for higher-risk patients.

Key Facts

Epidemiology

Prevalence: ~1:2500 based on ECG/genetic screening; possibly as high as 1:2000 accounting for silent carriers; genotype-based estimates suggest up to 1:80. (sources/channelopathies-jaha-2025, rating: high)
Arrhythmic risk by age: Highest in childhood; decreases with age; patients >60 years have attenuated risk. (sources/channelopathies-jaha-2025)
Sex-based risk: Males aged 10–12 have 4× higher arrhythmic risk than females. From ages 18–40, risk reverses: women 11% vs. men 2%. Sex hormones modulate IKr — oestrogen inhibits (↑QT), testosterone potentiates (↓QT). Postpartum period carries elevated risk. (sources/channelopathies-jaha-2025)
Asymptomatic carrier risk: Family members with a confirmed pathogenic LQTS variant but normal QTc carry a tenfold increased risk of cardiac events compared to non-carriers — justifying variant analysis in all first-degree relatives regardless of QTc. (sources/arrhythmia-genetics-mgenetik-2025, rating: high)
Drug-induced QTc prolongation: Only 10–15% of cases represent unmasked hidden LQTS. (sources/arrhythmia-genetics-mgenetik-2025) — See Contradictions: the AHA 2020 statement cites ~30% carrying latent LQTS variants in drug-induced TdP specifically. (sources/drug-arrhythmia-aha-2020, rating: very high)

Genetics

Major Subtypes (LQT1–LQT17)

17 subtypes defined by gene mutations; three major subtypes — LQT1 (KCNQ1, 30–35%), LQT2 (KCNH2, 25–30%), LQT3 (SCN5A, 5–10%) — account for ~75% of genotype-positive cases. (sources/channelopathies-jaha-2025, rating: high)
Overall variant detection rate: 70–80% across all LQTS subtypes. (sources/arrhythmia-genetics-mgenetik-2025)
Notable syndromal subtypes:
- Timothy syndrome (LQT8): CACNA1C gain-of-function; rare and highly malignant; associated with autism, webbed digits, and immune deficiency. (sources/channelopathies-jaha-2025)
- Jervell and Lange-Nielsen syndrome: KCNQ1 or KCNE1 homozygous/compound heterozygous; autosomal recessive; sensorineural deafness + severe LQTS; high mortality. (sources/channelopathies-jaha-2025)
- Andersen-Tawil syndrome (LQT7): KCNJ2 loss-of-function; triad of periodic paralysis + arrhythmias + dysmorphic features; see entities/Andersen-Tawil-Syndrome. (sources/channelopathies-jaha-2025)
High-risk genetic features: Transmembrane pore variants in KCNH2 carry the greatest relative cardiac event risk within LQT2. (sources/channelopathies-jaha-2025)

ClinGen Gene-Disease Validity (2026)

Definitive genes for LQTS: KCNQ1 (LQT1), KCNH2 (LQT2), and CALM1/CALM2/CALM3 (calmodulinopathy, definitive since 09/25/2018). CACNA1C has definitive evidence for Timothy syndrome (LQT8, 04/14/2023). (sources/clingen-summary-2026-05-09, rating: high)
Strong evidence: TRDN (triadin) has strong evidence for autosomal recessive LQTS (04/24/2020) — an important gene in recessive unexplained neonatal arrhythmia (also classified under CPVT5). (sources/clingen-summary-2026-05-09)
Disputed / should not be labelled P/LP for LQTS: SCN4B (LQT10, disputing 09/25/2018) — variants found on panels should not be classified P/LP for LQTS without extraordinary functional evidence. (sources/clingen-summary-2026-05-09)
Limited evidence: CAV3 (LQT9, limited 12/15/2020) and KCNJ2 (LQT7, limited 12/15/2020) — relevant primarily for Andersen-Tawil syndrome, not isolated LQTS. (sources/clingen-summary-2026-05-09)
SCN5A — broad reclassification: ClinGen now uses the umbrella designation "SCN5A-related cardiac rhythm disorder" (definitive 10/08/2025) rather than listing SCN5A as definitively causing LQTS separately. This encompasses LQT3, Brugada syndrome, DCM, and conduction disease under one entity. (sources/clingen-summary-2026-05-09)
KCNE2 removed: Disputed in the 2020 Adler et al. ClinGen reappraisal; no longer a supported LQTS gene. (sources/arrhythmia-genetics-mgenetik-2025)
Short QT crossover: KCNQ1 has separate strong evidence for Short QT syndrome (10/27/2020); KCNH2 has definitive evidence for Short QT (08/03/2020). (sources/clingen-summary-2026-05-09)
See concepts/ClinGen-Gene-Disease-Validity for full framework.

Calmodulinopathy LQTS

CALM1/2/3 mutations impair Ca²⁺-dependent inactivation of KCNQ1/IKs, KCNH2/IKr, and the L-type Ca²⁺ channel → extreme QTc prolongation with neonatal onset and very high mortality. (sources/arrhythmia-genetics-mgenetik-2025)
Neonatal LQTS with QTc >600 ms and no identifiable cause should prompt calmodulin gene sequencing. (sources/clingen-summary-2026-05-09)

Modifier Genes and Polygenic Risk

Incomplete penetrance and variable expressivity in LQTS — including between family members sharing an identical mutation — is partially explained by modifier genes. (sources/modifier-genes-scd-ehj-2018, rating: high)
NOS1AP: The most validated LQTS modifier gene (GWAS-derived). Two common noncoding NOS1AP variants were associated with elevated life-threatening arrhythmia risk in South African LQT1 and Netherlands LQT2 cohorts. Also associated with drug-induced LQTS and SCD risk in AMI. See entities/NOS1AP. (sources/modifier-genes-scd-ehj-2018)
KCNH2-K897T: Common coding variant that can convert latent LQT2 to symptomatic disease; also aggravates LQT1 (longer QTc at maximal exercise). Associated with post-AMI life-threatening arrhythmias. See entities/KCNH2. (sources/modifier-genes-scd-ehj-2018)
KCNE1-D85N: Common KCNE1 coding variant predisposing to congenital and drug-induced LQTS; sex-specific modifier (prolongs QTc in male but not female LQT1 carriers). Avoid IKr-blocking drugs even when this variant is the only finding. (sources/modifier-genes-scd-ehj-2018)
KCNQ1-rs2074238 (protective intronic modifier): Minor T allele associated with lower arrhythmic risk and shorter QTc; validated in 336 LQT1 subjects from South African and Finnish cohorts. (sources/modifier-genes-scd-ehj-2018)
iPSC-CM modifier discovery: Chai et al. identified a protective KCNK17 GOF variant and an aggravating REM2 variant (enhanced ICa,L) explaining intrafamilial phenotypic discordance in LQT2, using WES + iPSC-CM electrophysiology + CRISPR correction. See concepts/Modifier-Genes. (sources/modifier-genes-scd-ehj-2018)
Common SNP PRS: Common SNPs account for ~15% of variance in LQTS susceptibility (Lahrouchi 2020); genotype-negative LQTS patients carry a higher PRS than genotype-positive patients — suggesting polygenic architecture in mutation-negative cases. PRS modulates QTc in known LQTS carriers: higher PRS quartile → higher QTc and higher proportion of QTc ≥480 ms across LQT1/2/3 probands. (sources/repolarisation-jaccep-2023, rating: high)
Nauffal 2022 QT-PRS: Top PRS decile → QTc approximately 8.7 ms longer than population mean — comparable to some monogenic LQTS variants. (sources/gwas-arrhythmias-cmp-genes-2025, rating: high)
diLQT PRS (Simon 2024): Explains ~30% of QTc variance during drug exposure; OR 1.34/SD for drug-induced LQTS case status — common variants are a major determinant of pharmacogenomic QT response. (sources/gwas-arrhythmias-cmp-genes-2025)
75% of marked QTc prolongation is non-genetic: Even among patients with "unexplained" QTc prolongation, ~75% lack both a high PRS and an identifiable rare variant — drugs, electrolytes, and autonomic tone dominate. (sources/gwas-arrhythmias-cmp-genes-2025)
See concepts/Polygenic-Risk-Score and concepts/GWAS-Cardiac-Genetics for methodology.

Pathophysiology

Ion Channel Mechanisms by Subtype

LQT1 and LQT2: Loss-of-function in KCNQ1 (IKs) and KCNH2 (IKr) respectively reduces repolarizing K⁺ currents → delayed repolarization → prolonged QT. (sources/channelopathies-jaha-2025, rating: high)
LQT3: SCN5A gain-of-function → persistent (late) Na⁺ influx (INaLate) and/or increased window current during plateau → prolonged QT. Both residual currents amplify at slow heart rates (longer plateau) — hallmark: bradycardia-dependent QT prolongation, normalising at faster rates. Arrhythmic events predominantly at rest/sleep; first events in LQT3 more likely lethal; onset typically after puberty. (sources/channelopathies-jaha-2025, sources/scn5a-jaccep-2018)
LQT4 (ANKB): ANK2 (ankyrin-B) loss-of-function disrupts subcellular targeting of NCX1, Na⁺/K⁺-ATPase, and IP3 receptor → mislocalisation of NCX1 away from t-tubular Ca²⁺ release sites → altered Ca²⁺ handling → APD prolongation. One of few non-channel proteins causing LQTS. (sources/membrane-potential-physrev-2021, rating: very high)
LQT5 (KCNE1) and LQT6 (KCNE2): KCNE1 (minK) and KCNE2 (MiRP1) are β-subunits co-assembling with KCNQ1 (IKs) and KCNH2 (IKr) respectively. LOF in either reduces the associated current; KCNE2 (LQT6) disputed by ClinGen 2020. (sources/membrane-potential-physrev-2021)
LQT8 (Timothy syndrome, CACNA1C): GOF in Cav1.2 → increased inward ICa,L → prolonged plateau → QT prolongation; impaired inactivation → Ca²⁺ overload → severe multisystem phenotype (autism, bradycardia, webbed digits). (sources/channelopathies-jaha-2025)
LQT9 (CAV3) and LQT12 (SNTA1): Mutations in caveolin-3 or α1-syntrophin → indirect INaLate augmentation via disrupted Nav1.5 macromolecular complex → same functional consequence as LQT3 GOF. (sources/membrane-potential-physrev-2021)
Genotype-specific T-wave morphology: IKs (KCNQ1) predominates in subepicardium and RV; nearly absent at rest → LQT1 shows minimal baseline QT prolongation that unmasks with exercise. IKr (KCNH2) prominent at low heart rates → LQT2 prolonged at baseline; IKr reduction causes greater LV APD prolongation → characteristic bifid T-wave. SCN5A GOF prolongs phase 2 → delayed T-wave onset with long ST-segment in LQT3 regardless of heart rate. (sources/repolarisation-jaccep-2023, rating: high)

Final Common Pathway (TdP Generation)

Both K⁺ LOF and Na⁺/Ca²⁺ GOF converge on prolonged APD → M-cell APD prolongation disproportionate to subepicardium → transmural dispersion of repolarization (TDR) → EAD in mid-myocardium → triggered beat on T-wave → re-entry into dispersed substrate → TdP. (sources/channelopathies-jaha-2025, sources/membrane-potential-physrev-2021)
Repolarization reserve: Normal repolarization depends on multiple redundant ion currents (IKr, IKs, IK1, INa-L). When one mechanism is already perturbed by a latent LQTS variant, the addition of an IKr-blocking drug depletes repolarization reserve and unmasks TdP risk — explaining why ~10–20% of drug-induced TdP patients carry LQTS gene variants. (sources/repolarisation-jaccep-2023)

Sex Hormones and Circadian Variation

Oestradiol inhibits IKr and potentiates ICa,L → net APD prolongation → female QT ~10–12 ms longer than male baseline, with higher arrhythmic risk at ages 18–40. Testosterone upregulates IKr and IKs → shorter APD in males; pubescent males experience a QTc shortening absent in females, explaining the cross-over in arrhythmic risk around puberty. Progesterone upregulates IKs and has antiarrhythmic properties — its postpartum fall explains heightened LQT2 arrhythmic risk in the 9-month postpartum period. (sources/membrane-potential-physrev-2021, sources/lqts-pregnancy-medicina-2022, rating: medium)
Circadian APD amplification (LQT2): Kcnh2 is circadian-regulated (BMAL1/CLOCK). Wild-type Kv11.1 protein t½ ~12 h blunts protein oscillation amplitude. KCNH2 mutations can shorten Kv11.1 t½ to <6 h → two simultaneous consequences: (1) lower steady-state Kv11.1 protein → less IKr → longer baseline APD; and (2) larger circadian APD swing → potential clustering of arrhythmic events at specific times of day. (sources/circadian-scd-jmcc-2025, rating: high)

Diagnosis

ESC 2022 Class I diagnostic criteria (any one of):
- QTc ≥480 ms on repeated 12-lead ECGs
- Schwartz Score ≥3.5 points
- Pathogenic variant in an LQTS gene, irrespective of QTc duration
(sources/VA-SCD-ESC-2022, rating: very high)
Clinical QTc context: In 1,710 LQTS cases, mean QTc was 471±45 ms; 47% of LQT1, 36% of LQT2, and 35% of LQT3 patients had QTc <460 ms — confirming QTc alone is insufficient for exclusion. (sources/arrhythmia-genetics-mgenetik-2025)
Diagnostic workup: 12-lead ECG, Schwartz Score, exercise treadmill test (QTc paradoxically shortens in LQT1 on exercise; fails to shorten in LQT2), 24-hr ambulatory ECG; genetic testing for high clinical suspicion with negative workup. (sources/channelopathies-jaha-2025)
Genetic testing: Class I per EHRA/HRS/APHRS/LAHRS consensus; a pathogenic genetic result is part of the diagnostic score. (sources/arrhythmia-genetics-mgenetik-2025)
AHA 2020 specific testing thresholds: Genetic testing recommended for strong clinical index of suspicion for LQTS; also for idiopathic QT prolongation with QTc >480 ms (prepuberty) or >500 ms (adults), even when asymptomatic. (sources/genetic-test-aha-2020, rating: high)
Epinephrine challenge: NOT recommended (Class III) for routine diagnosis. EPS: NOT recommended (Class III). (sources/VA-SCD-ESC-2022)
AI-ECG: Up to 40% of LQTS patients have QTc <450 ms at baseline. Deep learning models have achieved AUC 0.741 for identifying concealed LQTS from the 12-lead ECG and can discriminate LQT1 from LQT2. Input occlusion reveals mechanistic T-wave footprints corresponding to predicted IKr inhibition effects. (sources/repolarisation-jaccep-2023)

Risk Stratification

1-2-3 LQTS Risk Calculator: Estimates arrhythmic risk before therapy initiation; recommended prior to ICD decisions: Class IIa (ESC 2022). (sources/VA-SCD-ESC-2022, rating: very high)
ICD in asymptomatic high-risk LQTS (per 1-2-3 LQTS Risk calculator) in addition to genotype-specific therapies: Class IIb. (sources/VA-SCD-ESC-2022)
Trigger patterns inform risk stratification: LQT1 events triggered by exercise/swimming; LQT2 by auditory stimuli/emotion/postpartum; LQT3 at rest/during sleep. (sources/channelopathies-jaha-2025)

Management

Lifestyle modification: Avoid QT-prolonging drugs (CredibleMeds list); avoid competitive sport in high-risk subtypes; correct hypokalaemia/hypomagnesaemia. (sources/channelopathies-jaha-2025, rating: high)
Beta-blockers (non-selective): Class I — nadolol preferred (long-acting, once-daily, superior compliance evidence); propranolol as alternative. Metoprolol is inferior due to shorter duration of action. (sources/VA-SCD-ESC-2022)
Mexiletine (Na⁺ channel blocker) in LQT3 with prolonged QTc: Class I (upgraded from IIa in ESC 2022). Long-term mexiletine registry (n=34 LQT3, median 36 months): significant reduction in all arrhythmic events. Flecainide and ranolazine also shorten QTc in short-term studies. (sources/VA-SCD-ESC-2022, sources/scn5a-jaccep-2018)
ICD + beta-blockers after cardiac arrest: Class I. ICD if symptomatic on beta-blockers + genotype-specific therapies: Class I. (sources/VA-SCD-ESC-2022)
LCSD (left cardiac sympathetic denervation): Class I — when ICD is contraindicated/declined, OR patient on full therapy with ICD still has multiple shocks or syncope from VA. Post-LCSD QTc <500 ms predicts success; QTc >500 ms persistent = consider ICD. See concepts/Left-Cardiac-Sympathetic-Denervation. (sources/VA-SCD-ESC-2022)

Precision Therapy in Practice (Mayo Clinic 20-Year Cohort, n=1,304)

Real-world expert LQTS management requires far more than 3 standard modalities — up to 18 distinct configurations were used for LQT3 alone. Phenotypic expression ultimately outweighs genotype in guiding treatment; regimens can differ even within the same family. (sources/precision-lqts-tcm-2024, rating: high)

Intentional nontherapy (18% of all patients): Reserved for post-pubertal, asymptomatic patients with normal resting QTc and normal QTc recovery on stress testing, who show phenotypic regression or historically mislabelled events. Precautionary measures (QT-drug avoidance, electrolyte correction, fever control) replace active therapy. Zero lethal events; zero non-lethal events at mean 7.5±4.3 years follow-up. (sources/precision-lqts-tcm-2024)
LCSD monotherapy (5% of patients; 42 LQT1 + 18 LQT2): BB intolerance was the primary indication in 90%. BCE rate 5% (non-lethal); zero lethal events. See concepts/Left-Cardiac-Sympathetic-Denervation. (sources/precision-lqts-tcm-2024)
Propranolol preferred over nadolol in LQT3: Propranolol directly inhibits the late sodium current (INaLate) — the primary pathological current in LQT3; nadolol does not share this property. This mechanistic advantage makes propranolol the BB of choice in LQT3 despite less favourable dosing frequency. (sources/precision-lqts-tcm-2024)
Mexiletine and IPAP for high-risk LQT2: Although mexiletine is considered a LQT3 drug, it reduced mean QTc by 65±45 ms in a high-risk LQT2 subset (mean QTc 543 ms, 47% prior BCE); zero BCEs at 6.5 years. Intentional permanent atrial pacing (IPAP, target ≥80 bpm) reduced BCE rate from 1.01 to 0.02/year (p=0.003) in 35 LQT2 patients (mean QTc 501 ms). Atrial pacemaker without defibrillation capability may be sufficient for select LQT2. (sources/precision-lqts-tcm-2024)
Flecainide for p.Ile1768Val-SCN5A (LQT3): This unique variant increases Nav1.5 availability via accelerated recovery from inactivation. Flecainide was chosen to target this specific biophysical mechanism — an example of variant-level precision prescribing. (sources/precision-lqts-tcm-2024)
RCSD (right cardiac sympathetic denervation) in refractory LQT3: Added incrementally on top of LCSD + ICD after continued BCE despite multi-drug therapy; never as a primary or bilateral-first procedure. Used in 5 LQT3 patients (3%). (sources/precision-lqts-tcm-2024)
Cardiac transplant (4 LQT3 patients, 2.6%): Most aggressive endpoint; used for intractable arrhythmia with multiple ICD shocks. Continuous IV lidocaine served as bridge. High-risk variants included p.Arg1623Gln-SCN5A, p.Phe1486Leu-SCN5A, p.Leu618Phe-SCN5A (two with LQT3-MEPPC overlap). (sources/precision-lqts-tcm-2024)
ICD device selection: Transvenous vs SQ-ICD vs epicardial ICD decided via shared decision-making. SQ-ICD rarely used in LQT2 due to T-wave oversensing and inappropriate shock risk. Epicardial ICD used for athletes in high-velocity collision sports. ICD must not be implanted solely to enable sports participation — device indication must be based on phenotypic risk alone. (sources/precision-lqts-tcm-2024)
ICD over-implantation: In the Mayo Clinic cohort, 119/1,304 (9.1%) carried an ICD before first specialist evaluation; over half underwent ICD extraction after shared-decision-making reassessment. This illustrates systematic ICD over-implantation in LQTS at non-specialist centres. Most LQTS patients can be managed successfully without an ICD. (sources/precision-lqts-tcm-2024)

Special Populations

Pregnancy and Postpartum

Risk paradox: Pregnancy is relatively protective (physiological tachycardia shortens QT), but the 9-month postpartum period carries a 2.7-fold increased cardiac event risk and 4.1-fold life-threatening event risk versus prepregnancy baseline (Seth 2007). Risk reverts to baseline after 9 months postpartum. (sources/lqts-pregnancy-medicina-2022, rating: medium)
LQT2 highest postpartum risk: Postpartum cardiac events are disproportionately reported in LQT2, attributable to the postpartum fall in progesterone, which has direct antiarrhythmic properties via reduction of polymorphic VT in preclinical models. (sources/lqts-pregnancy-medicina-2022)
Beta-blockers: Class I throughout pregnancy and ≥40 weeks postpartum (ESC 2018; AHA/ACC/HRS 2017). Non-selective agents (propranolol, nadolol) superior to metoprolol. Postpartum event rate reduced from 3.7% → 0.8% with beta-blocker use. (sources/lqts-pregnancy-medicina-2022)
Amiodarone: contraindicated in pregnancy (QT prolongation, fetal toxicity). Mexiletine and ranolazine can be used as add-on in LQT3. (sources/lqts-pregnancy-medicina-2022)
Delivery risk stratification (ESC 2018 / Roston 2020): Low risk (QTc ≤470 ms, no events) → Level 1 standard obstetric; medium risk (QTc ≥470 ms or remote events) → Level 2, tertiary centre; high risk (recent events on therapy) → Level 3, Caesarean in cardiac theatre. (sources/lqts-pregnancy-medicina-2022)
ICD before pregnancy preferred in high-risk women; safe to implant during pregnancy if new indication arises (after 8 weeks gestation). (sources/lqts-pregnancy-medicina-2022)
See concepts/LQTS-Pregnancy-Management for full detail.

Competitive Sports (AHA/ACC 2025)

Concealed variant-positive LQTS (gene+, QTc <460 ms at rest): Competitive sports participation reasonable. In the largest LQTS sports cohort (n=494 athletes), zero deaths and 0.3 events/100 patient-years were observed. Avoid QT-prolonging drug exposures; consider prophylactic beta-blocker. (sources/competitive-sports-aha-2025, rating: very high)
Phenotypic LQTS (QTc ≥460 ms prepuberty / ≥470 ms male / ≥480 ms female): Competitive sports reasonable with shared decision-making (SDM) under expert supervision after risk assessment and guideline-directed therapy. Zero deaths reported; 1.16 events/100 athlete-years in largest cohort. Requires non-selective beta-blockers, emergency action plan (EAP) with AED access. (sources/competitive-sports-aha-2025)
LQT1 swimming/diving: Can consider with SDM and appropriate precautions — supervision by CPR-trained individual, preference for pool over open water, AED on site. Most prior SCA cases were in undiagnosed/untreated patients. (sources/competitive-sports-aha-2025)
Clinical stability requirement: No breakthrough arrhythmias for ≥3 months before resuming competitive sports. (sources/competitive-sports-aha-2025)
ICD for the sole purpose of competitive sports participation: should NOT be performed — ~5%/year inappropriate shock risk + ~4%/year ICD-related complications. (sources/competitive-sports-aha-2025)
See concepts/Sports-Cardiology-SDM for full SDM framework.

Emerging Therapies

SupRep gene therapy (LQT1): AAV9-delivered shRNA suppresses mutant KCNQ1 + shRNA-immune replacement cDNA. Normalised QTi/APD90 and restored β-adrenergic response in transgenic rabbits (Bains 2024; Dotzler 2023). (sources/gene-therapy-arrhythmia-2025, rating: high)
SupRep gene therapy (LQT2): Normalised QTc from 470 → 414 ms; suppressed EADs and TdP inducibility in rabbit model (Bains 2023). (sources/gene-therapy-arrhythmia-2025)
LQT3 base editing: ABE8e-SpRY via dual AAV9 corrected SCN5A-M1875T in mice with 54% editing efficiency; normalised QTc and APD90; reduced late INa by 66%; ~20% editing sufficient to prevent arrhythmias via electrotonic coupling (Qi 2024). (sources/gene-therapy-arrhythmia-2025)
Lumacaftor (CFTR potentiator): Phase II trial (NCT04581408) to rescue LQT2 phenotype by improving KCNH2 channel trafficking. (sources/channelopathies-jaha-2025)
Allele-specific RNAi is limited by mutational heterogeneity — SupRep's mutation-agnostic approach overcomes this. High-throughput patch-clamp for VUS evaluation; iPSC-CM + CRISPR for variant-specific drug testing. (sources/gene-therapy-arrhythmia-2025)

Contradictions / Open Questions

ClinGen revalidation removed many published LQTS genes: The 2020 Adler et al. ClinGen reappraisal confirmed only 3 genes as definitively causing isolated LQTS (KCNQ1, KCNH2, SCN5A via the new broad rhythm disorder designation); KCNE2 was disputed and removed; KCNJ2/KCNE1 reclassified as primarily syndromal; SCN4B disputed. Many previously published "LQTS genes" on broad panels lack sufficient curated evidence — clinicians risk misclassification by reporting variants in these genes as P/LP for LQTS. (sources/arrhythmia-genetics-mgenetik-2025, sources/clingen-summary-2026-05-09, rating: high)
ClinGen 2026 clinical implications — SCN4B, CAV3, CALM1/2/3: SCN4B (LQT10) is disputed — variants should not be labelled P/LP for LQTS. CAV3 and KCNJ2 have only limited LQTS evidence — relevant for Andersen-Tawil syndrome, not isolated LQTS interpretation. CALM1/2/3 are definitively established — neonatal LQTS or extremely prolonged QTc without other cause should always include calmodulin gene sequencing. TRDN has strong evidence for a recessive LQTS/CPVT5 phenotype — important for recessive unexplained neonatal arrhythmia. (sources/clingen-summary-2026-05-09, rating: high)
Drug-induced TdP and latent LQTS — discrepant estimates: The AHA 2020 statement reports ~30% of patients with drug-induced QT prolongation carry pathogenic variants in 1 of the 5 major LQTS genes — supporting drug-induced TdP as frequently unmasking subclinical LQTS. In contrast, the arrhythmia genetics review (2025) cites only 10–15%. The discrepancy likely reflects different patient populations, gene panels, and QT prolongation vs. TdP definitions. Neither figure currently supports routine pre-prescription genetic screening. (sources/drug-arrhythmia-aha-2020, sources/arrhythmia-genetics-mgenetik-2025)
QTc threshold inconsistency — diagnosis vs. clinical practice: ESC 2022 uses QTc ≥480 ms on repeated ECGs as a Class I diagnostic criterion. Many programmes and older guidelines use QTc >450 ms (males) or >470 ms (females). The Schwartz Score uses ≥480 ms for 3 points but overall diagnosis can be made at lower values. This creates real-world variation in who receives a diagnosis and genetic testing referral. (sources/VA-SCD-ESC-2022, sources/channelopathies-jaha-2025)
Asymptomatic LQTS — ICD threshold undefined: For asymptomatic patients on full beta-blocker therapy with persistently prolonged QTc, ICD implantation is only Class IIb (1-2-3 LQTS Risk calculator guided). No Class I or IIa threshold exists, leaving clinicians without clear guidance for the largest group of LQTS patients. (sources/VA-SCD-ESC-2022)
LQT3: gene therapy vs. Class I mexiletine — competing strategies: ESC 2022 upgraded mexiletine to Class I for LQT3 with prolonged QTc (established pharmacology; long-term registry n=34). Simultaneously, SCN5A base editing shows strong preclinical data. Both address the same INaL mechanism but compete as clinical strategies; no comparative efficacy data exist. (sources/VA-SCD-ESC-2022, sources/gene-therapy-arrhythmia-2025, sources/scn5a-jaccep-2018)
SupRep balance constraint — no validated human dosing: SupRep requires a precise suppression-to-replacement ratio: excess suppression worsens LQTS; excess replacement risks SQTS. This therapeutic window has been demonstrated in animal models but has never been established in humans. (sources/gene-therapy-arrhythmia-2025)
NOS1AP biology paradox — partially resolved: NOS1AP risk alleles (associated with longer QT and higher arrhythmic risk in humans) correlate with higher NOS1AP expression in human ventricular myocardium. However, NOS1AP overexpression in guinea pig and rat ventricular myocytes shortens APD — opposite to humans. Dababneh 2025 confirmed NOS1AP modulates repolarisation via NOS1 activity in hiPSC-CMs, establishing a mechanistic pathway, but the directional discrepancy between standard animal models and human data remains unreconciled. (sources/modifier-genes-scd-ehj-2018, sources/gwas-arrhythmias-cmp-genes-2025)
LQT3 beta-blocker initial uncertainty — reversed by evidence: Early clinical and preclinical data suggested beta-blockers were ineffective or potentially harmful in LQT3 (mechanistic reasoning: LQT3 events at rest → beta-blocker-induced bradycardia could worsen QT prolongation). Later studies demonstrated clear benefit. Caution against extrapolating mechanism-based predictions to clinical outcomes without long-term data. (sources/scn5a-jaccep-2018)
ICD overimplantation vs. underimplantation tension: The Mayo Clinic cohort shows >50% of patients referred with a pre-existing ICD had it extracted after specialist re-evaluation — suggesting systematic overimplantation at non-specialist centres. Yet ICD provides a mortality benefit in LQTS (Wang 2021 JACC). The resolution is that ICD should be individualised and precision-risk-stratified, not reflexively prescribed for any LQTS diagnosis. ICD over-prescribing creates real harms: inappropriate shocks, anxiety, PTSD, device complications. (sources/precision-lqts-tcm-2024, rating: high)
Mexiletine in LQT2 — off-label but evidence-supported: Guideline-directed therapy restricts mexiletine to LQT3. The Mayo Clinic experience shows mexiletine reduced QTc by 65 ms and eliminated BCEs in a high-risk LQT2 subset with baseline QTc ~543 ms, suggesting late sodium current plays a clinically actionable role in severe LQT2. This challenges the strict genotype-to-drug assignment and supports functional-mechanism-guided prescribing. (sources/precision-lqts-tcm-2024, sources/scn5a-jaccep-2018)

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