#LQTS #channelopathies #genetics #electrophysiology #inherited-arrhythmias

LQTS Genetics and Pathophysiology

Definition

Long QT syndrome is a repolarisation disorder caused by variants that reduce outward potassium current — IKs in LQT1, IKr in LQT2 — or increase late inward sodium current (INaLate) in LQT3, prolonging the ventricular action potential and creating the substrate for torsades de pointes. Three genes, KCNQ1, KCNH2 and SCN5A, account for most genotype-positive cases, but rare syndromal genes, common modifier variants and acquired exposures determine when the phenotype is expressed. (sources/lqts-nejm-2025, rating: very high; sources/channelopathies-jaha-2025, rating: high)

Key Concepts

Genetics

Inheritance and variant classes

Autosomal dominant inheritance predominates. Jervell and Lange-Nielsen syndrome is the exception — biallelic KCNQ1 or KCNE1 variants — and calmodulinopathy is predominantly de novo, so a negative family history does not exclude it. (sources/channelopathies-jaha-2025, rating: high; sources/arrhythmia-genetics-mgenetik-2025, rating: high)
Functional class tracks gene: KCNQ1 and KCNH2 variants are loss-of-function; SCN5A (LQT3) and CACNA1C (LQT8) variants are gain-of-function. ANK2, CAV3 and SNTA1 cause LQTS indirectly through protein trafficking or Nav1.5 complex disruption rather than a primary pore lesion. (sources/membrane-potential-physrev-2021, rating: very high; sources/channelopathies-jaha-2025)
Variant severity hierarchy: dominant-negative variants carry greater risk than haploinsufficiency, and transmembrane pore variants greater risk than other transmembrane or N/C-terminal variants. (sources/lqts-jaccep-2022, rating: high)

Major subtypes and frequencies

Gene	Subtype	Share of genotype-positive LQTS	Key points
KCNQ1	LQT1	30–50%	IKs loss-of-function; ~50% in prospective cohorts, 30–45% in registries
KCNH2	LQT2	25–40%	IKr loss-of-function; transmembrane pore variants carry the highest risk
SCN5A	LQT3	5–10%	INaLate gain-of-function; ClinGen groups it as "SCN5A-related cardiac rhythm disorder"
CALM1/2/3	Calmodulinopathy	<1%	Near-complete penetrance, extreme QTc, often de novo
CACNA1C	LQT8 (Timothy)	<1%	ICaL gain-of-function; multisystem phenotype
KCNE1	LQT5; JLN when biallelic	<1%	IKs β-subunit
KCNJ2	LQT7 (Andersen-Tawil)	<1%	IK1 loss-of-function; periodic paralysis and dysmorphism

The three major genes account for ~90% of genotype-positive cases in prospective cohorts but only ~80% in registry-based reviews; older symptomatic registries report LQT1 30–35% and LQT2 25–30%, while a current review reports 40–45% and ~40%. The gradient reflects ascertainment — prospective genetic screening captures a broader, milder spectrum. (sources/lqts-nejm-2025, rating: very high; sources/lqts-jaccep-2022, rating: high; sources/genetics-va-fcvm-2022, rating: medium)
Overall variant detection rate is 70–80% across all subtypes. (sources/arrhythmia-genetics-mgenetik-2025, rating: high)
The 2020 Primer reconciles the shares as LQT1 30–35%, LQT2 25–40% and LQT3 5–10%, with 15–20% genotype-negative despite extensive testing and 5–10% carrying multiple mutations — the latter presenting younger and more severely. (sources/inherited-arrhythmias-nrdp-2020, rating: very high)

Gene-specific natural history

LQT1: arrhythmic events cluster with exercise and swimming; resting QT prolongation is often minimal because IKs contributes little at rest. (sources/channelopathies-jaha-2025, rating: high)
LQT2: events are provoked by auditory stimuli, emotion and the postpartum period; pore variants and dominant-negative alleles carry the highest risk. (sources/channelopathies-jaha-2025; sources/lqts-jaccep-2022)
LQT3: events occur at rest and during sleep, first events are more often lethal, and onset is typically after puberty. (sources/channelopathies-jaha-2025; sources/scn5a-jaccep-2018, rating: high)

Syndromal and recessive forms

Timothy syndrome (LQT8, CACNA1C): rare, highly malignant; autism, webbed digits and immune deficiency alongside severe QT prolongation. (sources/channelopathies-jaha-2025, rating: high)
Jervell and Lange-Nielsen syndrome (KCNQ1/KCNE1 biallelic): autosomal recessive, sensorineural deafness, severe LQTS and high mortality — 35% SCD risk by age 40 despite beta-blockade. (sources/lqts-jaccep-2022, rating: high)
Andersen-Tawil syndrome (LQT7, KCNJ2): periodic paralysis, arrhythmia and dysmorphic features. See entities/Andersen-Tawil-Syndrome. (sources/channelopathies-jaha-2025)
TRDN: strong evidence for autosomal recessive LQTS/CPVT5 — important in unexplained neonatal arrhythmia. (sources/clingen-summary-2026-05-09, rating: high)

ClinGen gene-disease validity

Definitive: KCNQ1, KCNH2, CALM1/2/3 and CACNA1C. SCN5A is definitive under the broader "SCN5A-related cardiac rhythm disorder" designation (10/08/2025), which encompasses LQT3, Brugada syndrome, DCM and conduction disease rather than a separate LQTS entity. (sources/clingen-summary-2026-05-09, rating: high)
Disputed or limited: SCN4B (LQT10) is disputed and must not be reported as P/LP for LQTS; KCNE2 was disputed and removed (Adler 2020); CAV3 (LQT9) and KCNJ2 (LQT7) have limited evidence for isolated LQTS and are primarily relevant to Andersen-Tawil syndrome. See concepts/ClinGen-Gene-Disease-Validity. (sources/clingen-summary-2026-05-09; sources/arrhythmia-genetics-mgenetik-2025)
The 2020 Primer's ClinGen reading is stricter on the accessory genes: disputed for KCNE2, KCNJ5, SCN4B, SNTA1, AKAP9 and ANK2; limited for CAV3, KCNE1 and KCNJ2; strong for TRDN; definitive for CALM1/2/3. Its KCNE1 designation is phenotype-specific — biallelic KCNE1 remains an established cause of Jervell and Lange-Nielsen syndrome. (sources/inherited-arrhythmias-nrdp-2020, rating: very high)

Calmodulinopathy

Phenotype: near-complete penetrance despite heterozygosity, extreme QTc (often >600 ms), neonatal or early-childhood onset and high SCD risk; predominantly de novo, so a negative family history does not exclude it. Neonatal LQTS with QTc >600 ms unexplained by SCN5A/KCNQ1/KCNH2 warrants CALM1/2/3 sequencing. (sources/arrhythmia-genetics-mgenetik-2025, rating: high; sources/calm-fcvm-2018, rating: high)
Mechanism and target: EF-hand III/IV mutations reduce C-lobe Ca²⁺ affinity, impair Ca²⁺-dependent inactivation of Cav1.2 and produce ICaL gain-of-function → APD prolongation → early afterdepolarisations. Verapamil reverses QT prolongation in hiPSC-CM models; sustained ICaL blockade is the mechanistic target. Full detail: concepts/Calmodulinopathy. (sources/calm-fcvm-2018)
Why heterozygosity is enough: CALM1/2/3 share 85% nucleotide homology but encode an identical 149-amino-acid protein, so one mutant allele of six still produces a substantial fraction of mutant calmodulin in the pre-bound pool. The International Calmodulinopathy Registry (74 patients) found the variant de novo in 93% of screened families and germline mosaicism in a parent in the remainder. (sources/inherited-arrhythmias-nrdp-2020, rating: very high)

Modifier genes and polygenic risk

NOS1AP is the most validated modifier: common noncoding variants raise life-threatening arrhythmia risk in LQT1 and LQT2, and modulate drug-induced LQTS and post-MI SCD risk. See entities/NOS1AP. (sources/modifier-genes-scd-ehj-2018, rating: high)
Actionable coding modifiers: KCNE1-D85N predisposes to congenital and drug-induced LQTS and prolongs QTc in male LQT1 carriers — avoid IKr-blocking drugs even when it is the sole finding. KCNH2-K897T can convert latent LQT2 to symptomatic disease and aggravates LQT1 at maximal exercise. (sources/modifier-genes-scd-ehj-2018)
Polygenic contribution: common SNPs explain ~15% of LQTS susceptibility variance; the top PRS decile adds ~8.7 ms to QTc. A drug-induced LQTS PRS explains ~30% of QTc variance during drug exposure (OR 1.34 per SD). (sources/repolarisation-jaccep-2023, rating: high; sources/gwas-arrhythmias-cmp-genes-2025, rating: high)
75% of marked unexplained QTc prolongation is non-genetic — drugs, electrolytes and autonomic tone dominate; most patients have neither a high PRS nor a rare variant. (sources/gwas-arrhythmias-cmp-genes-2025)

Genetic testing and counselling

Genetic testing is Class I in suspected or definite LQTS (ESC 2022). AHA 2020 additionally recommends testing for idiopathic QT prolongation with QTc >480 ms prepuberty or >500 ms in adults, even when asymptomatic. (sources/VA-SCD-ESC-2022, rating: very high; sources/genetic-test-aha-2020, rating: high)
First-degree relatives should be offered variant analysis regardless of QTc: family members carrying a confirmed pathogenic variant but with normal QTc have a tenfold higher cardiac-event risk than non-carriers. (sources/arrhythmia-genetics-mgenetik-2025)
Interpretation depends on the panel: reported yield is 70–80%, but variants in ClinGen-disputed genes (SCN4B, KCNE2) must not be reclassified as P/LP simply because they were found. Panel composition therefore changes the clinical meaning of a "positive" result. See concepts/Genetic-Testing-in-Arrhythmias. (sources/arrhythmia-genetics-mgenetik-2025; sources/clingen-summary-2026-05-09)

Pathophysiology

Ion channel mechanisms by subtype

Subtype	Gene / current	Mechanism	Signature
LQT1	KCNQ1 / IKs loss-of-function	Reduced phase 3 outward K⁺ current	Minimal resting QT prolongation; unmasks with sympathetic activation
LQT2	KCNH2 / IKr loss-of-function	Reduced IKr; greater LV than RV APD prolongation	Resting QT prolongation; bifid T-wave
LQT3	SCN5A / INaLate gain-of-function	Persistent late Na⁺ influx and increased window current	Long ST-segment, delayed T-wave onset, bradycardia-dependent
LQT4	ANK2 / ankyrin-B loss-of-function	Mislocalises NCX1, Na⁺/K⁺-ATPase and IP3 receptor	Altered Ca²⁺ handling; non-channel mechanism
LQT5/6	KCNE1 / KCNE2 β-subunits	Reduce IKs and IKr respectively	KCNE2 disputed by ClinGen 2020
LQT8	CACNA1C / ICaL gain-of-function	Increased inward Ca²⁺, impaired inactivation	Timothy syndrome; Ca²⁺ overload and multisystem disease
LQT9/12	CAV3 / SNTA1	Disrupt the Nav1.5 macromolecular complex → INaLate augmentation	Functionally equivalent to LQT3

(sources/channelopathies-jaha-2025, rating: high; sources/membrane-potential-physrev-2021, rating: very high)

Genotype-specific repolarisation signatures

Regional current distribution explains the T-wave patterns. IKs predominate in subepicardium and right ventricle and are nearly absent at rest, so LQT1 produces little baseline QT prolongation that unmasks with sympathetic activation. IKr dominates at slow heart rates and its loss prolongs LV more than RV APD, producing the bifid LQT2 T-wave. SCN5A gain-of-function prolongs phase 2, giving LQT3 a long ST-segment and delayed T-wave onset regardless of rate. (sources/repolarisation-jaccep-2023, rating: high)
Trigger patterns follow the same physiology: exercise and swimming in LQT1 (catecholamine-dependent IKs recruitment), auditory stimuli and emotion in LQT2, and bradycardia or sleep in LQT3 (longer diastolic interval allows greater INaLate accumulation). (sources/channelopathies-jaha-2025; sources/repolarisation-jaccep-2023)

Final common pathway (TdP generation)

Potassium loss-of-function and sodium/calcium gain-of-function converge on prolonged APD → disproportionate M-cell prolongation → transmural dispersion of repolarisation → early afterdepolarisation in mid-myocardium → triggered beat on the T-wave → re-entry → TdP. (sources/channelopathies-jaha-2025; sources/membrane-potential-physrev-2021)
Repolarisation reserve: redundant currents (IKr, IKs, IK1, INaLate) normally protect against APD prolongation. A latent LQTS variant plus an IKr-blocking drug depletes that reserve and unmasks TdP — the mechanistic basis of acquired LQTS. (sources/repolarisation-jaccep-2023)

Sex hormones

Oestradiol inhibits IKr and potentiates ICaL → longer APD; testosterone upregulates IKr and IKs → shorter APD, explaining the pubertal QTc cross-over; progesterone upregulates IKs and is antiarrhythmic, so its postpartum fall explains the excess LQT2 risk. (sources/membrane-potential-physrev-2021, rating: very high; sources/lqts-pregnancy-medicina-2022, rating: medium)

Circadian variation

Circadian APD amplification in LQT2: Kcnh2 is BMAL1/CLOCK-regulated; wild-type Kv11.1 has a ~12 h half-life that blunts protein oscillation, but KCNH2 mutations can shorten it to <6 h → lower steady-state IKr (longer baseline APD) plus a larger circadian APD swing, potentially clustering arrhythmias at specific times of day. (sources/circadian-scd-jmcc-2025, rating: high)

Contradictions / Open Questions

ClinGen revalidation removed many published LQTS genes: the 2020 reappraisal and later curations leave KCNQ1, KCNH2, CALM1/2/3 and CACNA1C definitive, with SCN5A under a broad rhythm-disorder designation. KCNE2 was disputed and removed, SCN4B remains disputed, and CAV3/KCNJ2 have limited isolated-LQTS evidence. Reporting variants in these genes as P/LP for LQTS risks misclassification. (sources/clingen-summary-2026-05-09, rating: high; sources/arrhythmia-genetics-mgenetik-2025)
Subtype frequencies differ by ascertainment: prospective cohorts report ~90% for the three major genes (LQT1 ~50%, LQT2 ~40%, LQT3 ~10%), whereas registry-based reviews report ~80% (LQT1 40–45%, LQT2 ~40%, LQT3 5–10%) and older symptomatic registries 30–35%/25–30%. LQT1 and LQT2 together account for 70–85% of genotype-positive LQTS regardless of source. (sources/lqts-nejm-2025; sources/lqts-jaccep-2022; sources/genetics-va-fcvm-2022)
NOS1AP biology paradox: risk alleles associated with longer QT and higher arrhythmic risk in humans correlate with higher NOS1AP expression, yet NOS1AP overexpression shortens APD in guinea-pig and rat myocytes. hiPSC-CM work implicates NOS1 activity, but the directional discrepancy with animal models remains unreconciled. (sources/modifier-genes-scd-ehj-2018, rating: high; sources/gwas-arrhythmias-cmp-genes-2025)
Reported detection yield versus actionable yield: a 70–80% variant detection rate is quoted across all subtypes, but panels that include ClinGen-disputed genes will report variants that should not be classified P/LP. The proportion of "positive" results that actually establish a molecular diagnosis is therefore lower than the headline yield, and is not reported consistently. (sources/arrhythmia-genetics-mgenetik-2025; sources/clingen-summary-2026-05-09)
QTc magnitude predicts risk in LQTS but not in SQTS: QTc strata stratify arrhythmic risk in LQTS, whereas the severity of QTc shortening does not predict arrhythmia risk in short QT syndrome. The same surface metric therefore carries different prognostic weight in the two repolarisation disorders, without a mechanistic explanation. (sources/lqts-jaccep-2022, rating: high; sources/repolarisation-jaccep-2023, rating: high)
The M-cell/transmural-dispersion model rests mainly on preclinical data: the standard explanation for TdP initiation — longest APD in mid-myocardial M cells, producing transmural dispersion and a mid-myocardial EAD — derives largely from wedge preparations and isolated myocytes. Direct human confirmation is limited, and the model is used as if settled. (sources/membrane-potential-physrev-2021, rating: very high)
Circadian mechanism versus clinical event clustering: Kcnh2 circadian regulation and mutation-dependent Kv11.1 half-life are established in model systems, but whether this translates into time-of-day clustering of arrhythmias in patients is described as potential and has not been demonstrated clinically. (sources/circadian-scd-jmcc-2025, rating: high)
Sex-hormone mechanism versus clinical risk: the oestrogen/testosterone/progesterone effects on IKr, IKs and ICaL are mechanistic, while the pubertal risk cross-over and postpartum LQT2 excess are observational. No trial has tested hormone-modulated therapy, so causation remains inferred. (sources/membrane-potential-physrev-2021; sources/lqts-pregnancy-medicina-2022, rating: medium)
The IKs-at-rest model does not fully explain LQT1: IKs is described as nearly absent at rest, which predicts minimal resting QT prolongation, yet many LQT1 patients have resting QTc prolongation and 47% have QTc <460 ms. The resting phenotype is therefore incompletely captured by the current regional-current model. (sources/repolarisation-jaccep-2023; sources/arrhythmia-genetics-mgenetik-2025)
KCNE1 classification is phenotype-dependent. The 2020 Primer marks KCNE1 as limited evidence for isolated LQTS, but biallelic KCNE1 is a well-established cause of Jervell and Lange-Nielsen syndrome. A single gene can be "limited" for one phenotype and definitive for another, so curations cannot be read without their phenotype qualifier. (sources/inherited-arrhythmias-nrdp-2020, rating: very high; sources/clingen-summary-2026-05-09)
The A341V spectrum proves modifiers but the causal variants remain mostly unknown. The same KCNQ1 founder mutation produces QTc values spanning the normal range, which is direct evidence of modifying factors — yet the specific modifiers are only beginning to be identified, so the observation is stronger than the explanation. (sources/inherited-arrhythmias-nrdp-2020; sources/modifier-genes-scd-ehj-2018)
The TdP initiation model is largely wedge-derived. Spatial dispersion of repolarisation and the M-cell/EAD account of TdP initiation come mostly from canine wedge preparations whose interpretation is not uniform, so the human pathway is inferred rather than directly demonstrated. (sources/inherited-arrhythmias-nrdp-2020)

Connections

Part of entities/Long-QT-Syndrome — the parent overview
Genes: entities/KCNQ1 · entities/KCNH2 · entities/SCN5A · entities/NOS1AP
Related syndromes: entities/Andersen-Tawil-Syndrome · entities/CPVT · entities/Short-QT-Syndrome · entities/Brugada-Syndrome
Genetics concepts: concepts/Calmodulinopathy · concepts/ClinGen-Gene-Disease-Validity · concepts/Genetic-Testing-in-Arrhythmias · concepts/Modifier-Genes · concepts/Polygenic-Risk-Score · concepts/GWAS-Cardiac-Genetics
Electrophysiology: concepts/Cardiac-Action-Potential · concepts/Cardiac-Repolarization · concepts/Torsades-de-Pointes · concepts/Electrical-Remodeling · concepts/Circadian-Rhythm-Cardiac-Electrophysiology
Clinical: concepts/LQTS-Pregnancy-Management · concepts/Precision-Medicine-LQTS