rs28937317 — SCN5A N1325S

SCN5A N1325S — When the Heart's Electrical Off-Switch Stays On

Every heartbeat begins with a precisely timed electrical impulse. Millions of sodium channels along heart muscle membranes snap open, flood the cell with sodium ions, and trigger the rapid depolarization that generates a contraction. Then — just as critically — they close. The cardiac sodium channel Nav1.5¹¹ Nav1.5
The protein encoded by SCN5A; "Na" = sodium, "v" = voltage-gated, "1.5" = the cardiac isoform. Nav1.5 is responsible for the fast inward sodium current that initiates the cardiac action potential must inactivate completely within milliseconds to allow the cell to repolarize and prepare for the next beat.

The N1325S variant (ClinVar VCV000009370, Pathogenic/Likely pathogenic, 4-star review status, multiple submitters without conflicts) disrupts this inactivation. Instead of fully closing, mutant Nav1.5 channels continue to leak a small but persistent current — the "late sodium current" — long into the repolarization phase. The electrical result is a prolonged QT interval on the ECG and the substrate for lethal cardiac arrhythmias. The clinical diagnosis is Long QT syndrome type 3²² Long QT syndrome type 3
LQT3 is one of approximately 17 genetic subtypes of LQTS; it is specifically caused by gain-of-function variants in SCN5A and accounts for roughly 8-10% of all genotype-positive LQTS cases.

The Mechanism

The asparagine-to-serine substitution at position 1324 (MANE Select transcript NM_000335.5) sits in the Domain III-IV linker region³³ Domain III-IV linker region
The cytoplasmic loop connecting transmembrane domains III and IV of Nav1.5; this region acts as the inactivation particle — it physically plugs the channel pore after opening. Mutations here are a common mechanism for gain-of-function channel defects of Nav1.5. Disrupting the asparagine residue impairs the fast inactivation gate, allowing channels to flicker between conducting and partially inactivated states. The result is a persistent inward sodium current (I_NaL) that outlasts the normal channel-open window by tens to hundreds of milliseconds.

Excess late sodium entry has two critical downstream consequences. First, it directly prolongs the action potential duration (APD), manifesting as a prolonged QTc interval⁴⁴ prolonged QTc interval
QTc = QT interval corrected for heart rate. Normal upper limit: 440 ms in men, 460 ms in women. Each 10-ms increase above baseline in LQT3 patients is associated with a 19% increase in cardiac event risk on the surface ECG. Second, the persistent sodium influx secondarily activates CaMKII⁵⁵ CaMKII
Calmodulin-dependent protein kinase II — a calcium/sodium-sensitive kinase that, when chronically activated, phosphorylates ryanodine receptors and other calcium-handling proteins, promoting calcium overload and arrhythmogenic spontaneous depolarizations. This creates a pathological feedback loop where Nav1.5 dysfunction feeds into calcium dysregulation, multiplying arrhythmia risk.

The N1325S variant is classified as a gain-of-function⁶⁶ gain-of-function
Unlike loss-of-function SCN5A variants that cause Brugada syndrome by reducing peak sodium current, N1325S increases total sodium entry through the persistent late current. This distinction matters for drug selection: sodium channel blockers that suppress late current (mexiletine, ranolazine) are therapeutic; those that indiscriminately block all sodium current (Class I antiarrhythmics like flecainide in high doses) require careful dosing variant — the mechanistic opposite of Brugada syndrome SCN5A variants.

The Evidence

Tian et al., 2004⁷⁷ Tian et al., 2004
Tian XL et al. Cardiovascular Research 2004 — transgenic mice homozygous for N1325S had action potential duration at 90% repolarization of 69 ± 5.9 ms versus 46.7 ± 4.8 ms in controls; 52 of 156 transgenic mice died from spontaneous arrhythmia during the study established the in vivo pathogenicity of N1325S using transgenic mice, demonstrating spontaneous polymorphic ventricular tachycardia and fibrillation frequently causing sudden cardiac death. Mexiletine treatment in these mice suppressed arrhythmias and normalized action potential duration — the first evidence that this variant is pharmacologically targetable.

The largest clinical study of LQT3 patients comes from Wilde et al., 2016⁸⁸ Wilde et al., 2016
Wilde AAM et al. Circulation 2016 — 406 LQT3 patients with 51 distinct SCN5A mutations; 391 analyzed for outcomes; mean follow-up not stated but multicenter international registry over decades, an international multicenter registry of 391 LQT3 patients: 30% experienced cardiac events (syncope, aborted arrest, or sudden death); 20% had life-threatening events. Every 10-ms increase in QTc above baseline was associated with a 19% increase in cardiac events. Beta-blocker therapy reduced events by 83% in women but showed no clear benefit in men — a sex-specific drug response that carries direct management implications.

For treatment, Mazzanti et al., 2016⁹⁹ Mazzanti et al., 2016
Mazzanti A et al. JACC 2016 — 34 LQT3 patients, 56% male, median age 22 years, treated with mexiletine at 8 mg/kg/day; median follow-up 36 months; the reduction in event rates was highly statistically significant (p=0.0097 for annual rate) provided the strongest clinical evidence for gene-specific mexiletine therapy in LQT3. Mexiletine shortened QTc by a mean of 63 ms and reduced annual arrhythmic event rates from 10.3% to 0.7%. For patients who cannot tolerate mexiletine, van den Berg et al., 2014¹⁰¹⁰ van den Berg et al., 2014
van den Berg MP et al. Int J Cardiol 2014 — single case report of N1325S patient successfully managed with ranolazine + beta-blocker after inadequate response to beta-blocker alone; ranolazine is a late sodium current inhibitor with a more favorable side-effect profile for some patients documented successful management with ranolazine plus beta-blocker in a confirmed N1325S carrier.

Li et al., 2020¹¹¹¹ Li et al., 2020
Li G et al. Front Pharmacol 2020 — N1325S and R1623Q were identified as mexiletine-sensitive mutations based on their gating properties; M1652R was not; the study explains why not all LQT3 variants respond equally to mexiletine specifically confirmed N1325S as a mexiletine-sensitive mutation at the biophysical level, demonstrating that its gating properties (particularly window current expansion) predict a favorable clinical drug response — providing a mechanistic rationale for gene-specific therapy.

Practical Actions

LQT3 caused by N1325S requires cardiological management. Three priorities dominate: (1) quantify the QTc interval and arrhythmia burden at baseline; (2) initiate gene-specific pharmacotherapy with mexiletine, which specifically targets the mechanism of this variant; (3) assess ICD candidacy for high-risk individuals. QT-prolonging drugs — including many common antibiotics, antipsychotics, and antidepressants — must be reviewed and substituted where possible.

LQT3 arrhythmias characteristically occur at slower heart rates and during sleep or rest, unlike LQT1 (exercise-triggered) or LQT2 (auditory/startle- triggered). This has practical implications: the risk period is not exercise but rather nocturnal bradycardia. Avoiding fever (which accelerates sodium channel dysfunction) and certain medications is also critical.

Interactions

N1325S interacts with the broader SCN5A channelopathy landscape. Other pathogenic SCN5A gain-of-function variants cause LQT3 through the same mechanism — all produce late sodium current — but vary in their mexiletine sensitivity based on gating kinetics. Loss-of-function SCN5A variants (rs45620037 and related Brugada syndrome variants) cause the mechanistic opposite phenotype; compound heterozygosity for gain- and loss-of-function SCN5A variants can produce overlap syndromes with both prolonged QT and Brugada-pattern ECG changes. Other inherited arrhythmia genes — KCNQ1 (LQT1), KCNH2 (LQT2), KCNE1, KCNE2 — can compound QT prolongation risk when co-inherited with SCN5A variants.