SCN5A
Details
SCN5A encodes Nav1.5, the α-subunit of the primary cardiac voltage-gated sodium channel. Located on chromosome 3p21–p24 with 28 exons, it is the single most clinically important gene in cardiac channelopathies. The same gene produces entirely distinct syndromes depending on whether the mutation is gain- or loss-of-function — and even within a single mutation, simultaneous gain and loss of function can occur at different phases of the action potential (overlap syndromes). Nav1.5 is preferentially expressed at the intercalated disc, where it interacts with β-subunits, desmosomal proteins (PKP2), gap junction proteins (Cx43), and intracellular scaffolding proteins. Mutations in any of these Nav1.5-interacting proteins can produce SCN5A-like phenotypes.
Key Facts
Cardiac Sodium Channel Physiology
- Nav1.5 drives phase 0 depolarization of atrial, ventricular, and Purkinje myocytes; essential for fast impulse propagation. (sources/scn5a-jaccep-2018, rating: high)
- Channel gating: S4 voltage sensors drive fast activation → Na⁺ influx; DIII–DIV intracellular linker (IFM motif) drives fast inactivation; slow inactivation requires longer recovery. (sources/scn5a-jaccep-2018)
- A small fraction (<1%) of Nav1.5 remains open or reactivates during repolarization → late sodium current (INaL) and window current. Both are normally very small; SCN5A gain-of-function mutations can pathologically amplify them. (sources/scn5a-jaccep-2018)
- The final phenotypic outcome depends on which gating property is altered and in which direction. (sources/scn5a-jaccep-2018)
LQT3 (Gain-of-Function)
- SCN5A gain-of-function mutations → pathological increase in INaL or window current → prolonged plateau → delayed repolarization → QT prolongation. (sources/scn5a-jaccep-2018, sources/channelopathies-jaha-2025)
- LQT3 accounts for 5–10% of genotype-positive LQTS (behind LQT1 ~35%, LQT2 ~30%). (sources/scn5a-jaccep-2018)
- Clinical hallmarks distinguishing LQT3 from LQT1/LQT2:
- Marked resting bradycardia; QT interval prolongs more at slow heart rates, normalizes at faster rates (explains why INaL/window current amplifies at long plateau durations) (sources/scn5a-jaccep-2018)
- Arrhythmic events occur predominantly at rest or sleep, not during exercise or emotional stress (sources/scn5a-jaccep-2018)
- First cardiac event is more likely to be lethal in LQT3 vs LQT1/LQT2 (sources/scn5a-jaccep-2018)
- Phenotypic onset typically after puberty (vs. childhood in LQT1) (sources/scn5a-jaccep-2018)
- Downstream consequences: Persistent Na⁺ influx → intracellular Na⁺ overload → reverse-mode Na⁺/Ca²⁺ exchanger activation → intracellular Ca²⁺ overload → impaired contraction, relaxation, and increased oxygen consumption — a pathway also central to heart failure pathophysiology. (sources/scn5a-jaccep-2018)
- Caveolin-3, β4-subunit, and α1-syntrophin mutations produce LQT3-like phenotypes by increasing INa or INaL in vitro. (sources/scn5a-jaccep-2018)
LQT3 Management
- Beta-blockers: Initially considered ineffective or even detrimental in LQT3 (early clinical and preclinical data); later studies demonstrated a clear benefit — beta-blockers are now recommended for LQT3. (sources/scn5a-jaccep-2018)
- Mexiletine (INaL blocker): Class I (ESC 2022). Short-term studies with mexiletine, flecainide, and ranolazine all demonstrate QT-shortening ability. Long-term mexiletine registry (n=34, median follow-up 36 months): significant reduction in all arrhythmic events. (sources/scn5a-jaccep-2018, sources/VA-SCD-ESC-2022)
- ICD: Indicated in symptomatic patients with QTc >500 ms; also after resuscitated VF or haemodynamically compromised VT. (sources/scn5a-jaccep-2018)
- Risk stratification: Primarily symptomatic status + QTc duration; asymptomatic with QTc >500 ms = high risk. (sources/scn5a-jaccep-2018)
Brugada Syndrome (Loss-of-Function)
- SCN5A loss-of-function → reduced peak INa via decreased Nav1.5 surface expression, non-functional channels, or gating changes (delayed activation, accelerated inactivation). LOF mutations account for ~20% of BrS (only gene significantly over-represented in BrS vs. healthy controls). (sources/scn5a-jaccep-2018, sources/channelopathies-jaha-2025)
- BrS is increasingly regarded as part of a cardiomyopathy spectrum with structural RVOT changes on CMR (enlarged ventricular volumes); arrhythmogenic substrate may reside predominantly in the RVOT. (sources/scn5a-jaccep-2018)
- SCN5A-based BrS is characterised by prolonged conduction intervals throughout the heart at all cardiac levels — more so than non-SCN5A BrS. (sources/scn5a-jaccep-2018)
- BrS has an oligogenic architecture; SCN5A is the strongest genetic factor but additional common variants (GWAS) contribute; family members without the SCN5A variant may have phenotypic BrS. (sources/scn5a-jaccep-2018)
- SCN5A loss-of-function in BrS is associated with additional conduction disease: 1st-degree AV block, RBBB, sick sinus syndrome. (sources/channelopathies-jaha-2025)
- Mutations in Nav1.5 channel-interacting proteins also associated with BrS phenotypes. (sources/channelopathies-jaha-2025)
SCN5A in Dilated Cardiomyopathy
- DCM was first linked to SCN5A (D1275N mutation) in a large 5-generation family in 2004; >20 different SCN5A mutations now associated with DCM, >90% missense. (sources/scn5a-jaccep-2018)
- Age-dependent penetrance with phenotypic expression increasing with age. (sources/scn5a-jaccep-2018)
- Conduction defects precede DCM by 15–20 years — first/second-degree AV block, LBBB, RBBB are nearly universal and are the key clinical clue to SCN5A aetiology in unexplained IDCM. (sources/scn5a-jaccep-2018)
- Arrhythmias in >90% of SCN5A-DCM: atrial fibrillation, sick sinus syndrome, PVCs, ventricular tachycardias. (sources/scn5a-jaccep-2018)
- Mutation mechanisms in DCM: ~50% are loss-of-function (reduced membrane expression, adverse gating); ~33% are gain-of-function (increased window current); some cause both; S4-segment missense mutations can create an aberrant cation-leak pore. (sources/scn5a-jaccep-2018)
- Three hypotheses for the DCM mechanism:
- Direct: Nav1.5 interacts with cytoskeletal/intercalated disc proteins → structural deformation; or cation leak/Na⁺–Ca²⁺ overload → intracellular acidification or Ca²⁺ overload → impaired contraction
- Conduction-defect-mediated (indirect): LOF → conduction slowing → dyssynchrony → DCM; supported by timing (conduction precedes DCM by 15–20 y) and 90%-SCN5A-knockdown mouse models developing progressive DCM-like changes
- Arrhythmia-mediated: GOF → long-lasting arrhythmias → tachycardia-induced CMP; supported by reversible DCM cases after arrhythmia suppression (especially MEPPC/R222Q)
- (sources/scn5a-jaccep-2018, rating: high)
- SCN5A-DCM management (ClinGen confirmed Definitive, 03/04/2026): Standard HF therapy; avoid sodium channel blockers (risk of worsening conduction); amiodarone for arrhythmias. In selected patients with high PVC burden, no significant conduction defects, and no CMR fibrosis — hydroquinidine or flecainide may paradoxically be considered (preferably after LV normalisation with HF therapy/amiodarone). (sources/scn5a-jaccep-2018)
- Genetic testing indicated in IDCM with frequent ventricular ectopy or severe conduction defects. (sources/scn5a-jaccep-2018)
MEPPC (Multifocal Ectopic Premature Purkinje-Related Complexes)
- Caused by SCN5A R222Q gain-of-function (also R222P, I141V at the same nucleotide): increased window current → premature action potentials during Purkinje cell repolarization. (sources/scn5a-jaccep-2018)
- ECG: High-burden narrow complex polymorphic PVCs; reported in several unrelated families worldwide. (sources/scn5a-jaccep-2018)
- Reversible DCM: Some patients develop DCM that normalises after pharmacological suppression of the ectopy burden (hydroquinidine, flecainide, amiodarone) — supports the arrhythmia-mediated DCM hypothesis. (sources/scn5a-jaccep-2018)
- See concepts/MEPPC for full detail.
ClinGen Gene-Disease Validity — SCN5A
- SCN5A-related cardiac rhythm disorder (Definitive, 10/08/2025): ClinGen's Hereditary Cardiovascular Disease GCEP created a broad "SCN5A-related cardiac rhythm disorder" designation as Definitive — recognising that a single SCN5A variant may simultaneously or sequentially produce multiple cardiac syndromes (LQT3, BrS, conduction disease, sick sinus syndrome, AF, DCM). This broad designation is clinically important for variant reporting: variants formerly reported as "pathogenic for Brugada syndrome" may now be more accurately reported under this umbrella classification, avoiding under-recognition of overlapping phenotypes. (sources/clingen-summary-2026-05-09, rating: high; ClinGen classification date: 10/08/2025)
- DCM (Definitive, 03/04/2026): ClinGen separately established SCN5A as a Definitive gene for dilated cardiomyopathy — the most recent cardiovascular classification in the 2026 dataset. This confirms SCN5A-DCM as a standalone established entity and underscores the importance of SCN5A on DCM panels (alongside TTN, LMNA, MYH7). (sources/clingen-summary-2026-05-09, rating: high; ClinGen classification date: 03/04/2026)
Other SCN5A Diseases
- Lev-Lenègre syndrome: Progressive conduction system fibrosis → progressive P-wave/PR/QRS prolongation → bundle branch blocks → complete AVB. SCN5A loss-of-function implicated. (sources/scn5a-jaccep-2018)
- Sick sinus syndrome (SSS): Familial autosomal dominant SSS at younger age; SCN5A loss-of-function most commonly involved. Mouse models with SCN5A LOF mutations mimic SSS. Sinus exit block, atrial standstill (also with connexin-43 polymorphism co-mutation). (sources/scn5a-jaccep-2018)
- Atrial fibrillation: Both LOF (intra-atrial conduction slowing) and GOF (EAD/DAD-triggered AF via prolonged APD or enhanced diastolic Na⁺ entry) mechanisms implicated in familial AF; structural atrial remodelling secondary to Nav1.5 dysfunction may also contribute. (sources/scn5a-jaccep-2018)
- SCN5A is definitively listed for familial AF by ClinGen alongside KCNA5 and TTN. (sources/arrhythmia-genetics-mgenetik-2025)
- Most common gene in familial isolated CCD in young adults: SCN5A detected in 76% of familial CCD cases <50 years; LOF predominant. (sources/conduction-disorders-jaha-2025)
SCN5A and Atrial Standstill
- Atrial standstill is the atrial-level expression of SCN5A loss of function — complete atrial inexcitability with absent atrial electrograms and failure of atrial capture at maximal pacing output. (sources/atrial-standstill-hrs-2026, rating: medium)
- The "exit block" hypothesis: because sinus node pacemaking is driven primarily by Ca²⁺ channels, SCN5A loss does not directly impair automaticity; the lesion is failure of impulse conduction from the sinus node into surrounding atrial myocardium. This remains experimentally unconfirmed. (sources/atrial-standstill-hrs-2026)
- Heterozygous loss of function is tolerated in the general population — gnomAD LOEUF for SCN5A is 0.42, above the <0.35 haploinsufficiency threshold — so clinical standstill usually requires a coinherited modifier or a second variant. (sources/atrial-standstill-hrs-2026)
- The digenic model: full phenotypic expression in the Dutch and Japanese families occurred only in individuals carrying both an SCN5A mutation and two GJA5 (connexin 40) promoter polymorphisms (c.-44G>A and c.171A>G, in complete linkage disequilibrium) that cut Cx40 promoter activity below 50%. Reported in 2 families and never independently replicated in 20 years; the homozygous Cx40 polymorphism frequency is ~5.1% in the Japanese general population, so most carriers of the at-risk combination never develop disease. (sources/atrial-standstill-hrs-2026)
- Key variants: p.Leu212Pro, p.Asp1275Asn (the canonical digenic example), p.Val1340Leu (~50% INa reduction, homozygous/consanguineous), p.Arg222Gly (autosomal dominant, progressive biatrial dilatation). (sources/atrial-standstill-hrs-2026)
- Biallelic variants are a distinct and severe disease (systematic review, 61 patients): sick sinus syndrome 65%, atrial standstill 59%, device implantation 76%, major cardiac events 50%, mortality 15% at a mean age of 9 years. Non-missense variants carry earlier onset and higher event risk. (sources/atrial-standstill-hrs-2026)
- Haemodynamic collapse can occur without arrhythmia or device failure in biallelic SCN5A disease, extending the phenotype beyond rhythm disturbance. (sources/atrial-standstill-hrs-2026)
- Cascade implications: relatives negative for atrial standstill may still carry the variant and face BrS, LQT3 or conduction disease. Screening must evaluate the full phenotype. See concepts/Atrial-Standstill.
SCN5A Overlap Syndromes
- Prototype: 1795insD mutation in a large Dutch family — simultaneous LOF features (SSS, bradycardia, conduction disease, Brugada-pattern ECG) and GOF feature (LQT3). (sources/scn5a-jaccep-2018)
- Mechanistic resolution: LOF is manifest during phase 0 (reduced peak INa); GOF is manifest during phases 2–3 (increased INaL and window current). The biophysical effects are dissociated across different phases of the same action potential. (sources/scn5a-jaccep-2018)
- Phenotypic modifiers: Male sex → BrS predominance; female sex → conduction disorders; advancing age → worsening conduction defects; SCN5A intronic SNPs as genetic modifiers. (sources/scn5a-jaccep-2018)
- Most common clinical overlap combination: BrS + cardiac conduction defects. (sources/scn5a-jaccep-2018)
ACM and Desmosomal Interaction
- SCN5A loss-of-function accounts for ~2% of ACM cases (up to 5–10% with LMNA overlap) — produces arrhythmogenic DCM with broad ECG abnormalities. (sources/acm-hrs-2019)
- Nav1.5 co-precipitates with desmosomal protein PKP2 and AJ protein N-cadherin; PKP2 mutations reduce INa amplitude and kinetics; SCN5A mutations can reduce N-cadherin cluster size at the intercalated disc. (sources/acm-hrs-2019)
Gene Therapy Constraints
- SCN5A coding sequence ~6,048 bp approaches/exceeds standard AAV packaging capacity (~4.7 kb) — direct replacement is infeasible with current vectors. (sources/gene-therapy-arrhythmia-2025)
- BrS workaround: AAV9-mediated MOG1 overexpression enhances Nav1.5 trafficking to the cell surface, compensating for trafficking-deficient LOF mutations. Reversed arrhythmic phenotype in SCN5A-G1746R knock-in mice. (sources/gene-therapy-arrhythmia-2025)
- A second, complementary route — SCN10A-short. An intronic SCN10A enhancer governs expression of a small protein from the last seven exons, and it is this protein rather than Nav1.5 that accounts for the INa increase linked to SCN10A intronic SNPs in GWAS. AAV9-SCN10A-short restored upstroke velocity, peak INa density and conduction velocity to wild type in SCN5A haploinsufficient mice, and separately reduced reentrant VT after ischaemia-reperfusion. MOG1 corrects trafficking; SCN10A-short increases current amplitude — useful where the defect is gating rather than localisation. (sources/gene-therapy-arrhythmia-cjca-2026, rating: high)
- LQT3 base editing: ABE8e-SpRY split across dual AAV9 corrected SCN5A-M1875T in mice: 54% editing efficiency, QTc normalised, late INa reduced 66%; ~20% editing sufficient to prevent arrhythmias (source-sink electrotonic coupling through gap junctions). (sources/gene-therapy-arrhythmia-2025)
Contradictions / Open Questions
- SCN5A bidirectional phenotype — opposite syndromes, opposite drug responses, same gene: GOF (LQT3) requires late Na⁺ channel blockade (mexiletine); LOF (BrS) is worsened by sodium channel blockers (flecainide, procainamide — which are both diagnostic provocateurs in BrS). A patient with a novel SCN5A VUS prescribed a sodium channel blocker for another indication could be harmed if the variant is loss-of-function. Functional characterisation of variant direction is critical before pharmacological decisions. (sources/channelopathies-jaha-2025)
- LQT3 beta-blocker reversal: Early clinical and preclinical data suggested beta-blockers were ineffective or potentially detrimental in LQT3; later studies demonstrated clear benefit. This reversal illustrates how mechanistic assumptions (LQT3 events at rest/bradycardia → beta-blockers should worsen) can mislead clinical practice until long-term outcome data mature. (sources/scn5a-jaccep-2018)
- SCN5A-DCM pathomechanism uncertain: The three mechanistic hypotheses (direct structural, conduction-defect-mediated, arrhythmia-mediated) are not mutually exclusive, and their relative contributions remain unknown. The observation that conduction defects precede DCM by 15–20 years strongly supports hypothesis 2, but some patients with reversible DCM after ectopy suppression (MEPPC) support hypothesis 3. No mechanistic intervention has been validated clinically. (sources/scn5a-jaccep-2018)
- SCN5A coding sequence exceeds AAV packaging capacity: At ~6,048 bp, direct gene replacement is infeasible. The MOG1 trafficking workaround addresses only trafficking-deficient LOF variants; base editing addresses only specific point mutations. Neither strategy covers the full SCN5A mutation spectrum. (sources/gene-therapy-arrhythmia-2025)
- Every current SCN5A strategy treats a subset of variants, and none treats the gene. MOG1 requires a trafficking defect; base editing requires a known point mutation; SCN10A-short raises total INa without correcting the channel itself. There is no variant-agnostic replacement analogous to the PKP2 or CASQ2 programmes, and the cargo ceiling is the reason. (sources/gene-therapy-arrhythmia-cjca-2026, rating: high)
- SCN5A loss-of-function in ACM — management complexity: SCN5A LOF accounts for ~2% of ACM (up to 5–10% with LMNA overlap) and produces arrhythmogenic DCM. These patients may be initially evaluated with ARVC 2010 Task Force Criteria (designed for desmosomal ACM) and could receive contraindicated sodium channel blockers if the genotype is not identified. (sources/acm-hrs-2019)
- Connexin 40 or connexin 43 — the sources disagree. The SCN5A clinical review describes atrial standstill arising with a co-inherited polymorphism in connexin 43 (which is GJA1), while the atrial standstill literature and the primary Groenewegen 2003 paper identify connexin 40 (GJA5) as the modifier. Only one can be correct, and because the digenic model rests on 2 unreplicated families, the distinction has never been resolved. (sources/scn5a-jaccep-2018, rating: high; sources/atrial-standstill-hrs-2026, rating: medium)
Connections
- Related to sources/scn5a-jaccep-2018
- Related to sources/conduction-disorders-jaha-2025
- Related to sources/channelopathies-jaha-2025
- Related to sources/gene-therapy-arrhythmia-2025
- Related to sources/acm-hrs-2019
- Related to sources/VA-SCD-ESC-2022
- Related to concepts/Conduction-Disorders-in-Young-Adults
- Related to concepts/Cardiac-Action-Potential
- Related to concepts/Channelopathies
- Related to concepts/MEPPC
- Related to entities/Long-QT-Syndrome
- Related to entities/Brugada-Syndrome
- Related to entities/DCM
- Related to entities/Early-Repolarization-Syndrome
- Related to entities/ALVC
- Related to entities/ARVC
- Related to concepts/Desmosome
- Related to concepts/Final-Common-Pathway
- Related to concepts/Gene-Silencing-Therapy
- Related to concepts/Cardiogenetic-Centers
- Related to concepts/ClinGen-Gene-Disease-Validity
- Related to concepts/Atrial-Standstill — atrial-level loss of function; exit block and the digenic GJA5 model
- Related to concepts/Sinus-Node-Dysfunction — SCN5A is the commonest gene in familial sick sinus syndrome
- Related to entities/EMD · entities/LMNA — the two nuclear-envelope routes to the same atrial silence
- Related to sources/clingen-summary-2026-05-09