rs397508075 — KCNQ1 KCNQ1 Long QT Type 1 Variant 3

Pathogenic nonsense variant (Q359X) in KCNQ1 that truncates the IKs potassium channel, causing Long QT syndrome type 1 with markedly increased risk of life-threatening arrhythmias during exercise and emotional stress

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KCNQ1 Q359X — When a Premature Stop Codon Silences the Heart's Repolarization Safety Valve

The heart's electrical rhythm depends on a precisely timed cascade of ion channel openings and closings. After each beat, the muscle must rapidly repolarize — reset its electrical charge — to be ready for the next. One of the key channels driving this repolarization is the IKs current11 IKs current
The slow component of the delayed rectifier potassium current — encoded by KCNQ1 (the α subunit) in combination with KCNE1 (the β subunit) — is the dominant repolarizing current at fast heart rates and during adrenergic stimulation, encoded by KCNQ1. The rs397508075 variant introduces a premature stop codon at position 359 of the 676-amino-acid KCNQ1 protein, truncating more than 45% of the channel before it reaches its C-terminal assembly and trafficking domains.

The Mechanism

KCNQ1 is located on the plus strand of chromosome 11 (11p15.5). The c.1075C>T change converts a glutamine codon (CAG) to a stop codon (TAG) at amino acid position 359, within the third intracellular loop of the channel protein. This truncation is predicted to eliminate the protein entirely through [nonsense-mediated mRNA decay | A cellular surveillance mechanism that degrades mRNAs containing premature stop codons more than ~50–55 nucleotides upstream of the last exon-exon junction. KCNQ1 Q359X satisfies this criterion, making protein-level truncation unlikely — the mRNA itself is degraded before translation is complete], rather than producing a dominant-negative truncated protein. The result is [haploinsufficiency | Only one functional copy of KCNQ1 remains, producing roughly half the normal IKs current density]: heterozygous carriers produce approximately 50% of normal IKs current.

Under resting conditions, 50% IKs is often sufficient for normal or near-normal QT intervals — many heterozygous carriers have QTc values only modestly prolonged (460–500 ms). But during [adrenergic stimulation | Activation of the sympathetic nervous system during exercise, excitement, startle, or emotional stress triggers catecholamine release. Catecholamines activate PKA, which phosphorylates KCNQ1 and normally produces a several-fold increase in IKs current to accelerate repolarization at fast heart rates], the demand for IKs increases dramatically — and the haploinsufficient channel cannot meet it. The result is a dangerously prolonged action potential at precisely the moments when the heart is beating fastest: during a race, a swim, or a moment of sudden fright.

The Evidence

Long QT syndrome type 1 (LQT1) is the most common form of inherited LQTS, accounting for approximately 35–45% of genetically confirmed cases. The evidence base for KCNQ1 loss-of-function variants is among the most developed in cardiac genetics.

A landmark Circulation study of 216 genotyped LQT1 patients followed for a median of 10 years22 Circulation study of 216 genotyped LQT1 patients followed for a median of 10 years
Vincent et al. 2009 — high efficacy of β-blockers in LQT1 found that 75% of patients treated with beta-blockers remained asymptomatic throughout follow-up, and cardiac events were reduced by 64%. Crucially, essentially all treatment failures occurred in patients who were either non-compliant with therapy or had taken QT-prolonging drugs — not in patients faithfully on beta-blockers and avoiding triggers.

Mutation location matters for risk stratification. A 600-patient analysis of 77 KCNQ1 mutations33 600-patient analysis of 77 KCNQ1 mutations
Moss et al. 2007, Circulation — clinical aspects by location, coding type, and biophysical function found that transmembrane-domain mutations and those producing severe dominant-negative channel dysfunction carry the highest event rates; C-terminal and loss-of-function mutations including nonsense variants carry intermediate-to-lower risk.

This is supported by a 1,090-patient Heart Rhythm study of mutation coding type44 1,090-patient Heart Rhythm study of mutation coding type
Earle et al. 2016 — stop-codon mutations associated with lower risk in LQT1 which found that stop-codon mutations, including Q-type nonsense variants in KCNQ1, were associated with a hazard ratio of 0.57 (95% CI 0.34–0.96) for cardiac events versus non-c-loop missense mutations. A possible mechanism: haploinsufficient channels may partially rescue current amplitude relative to dominant-negative mutants that poison all assembled tetramers.

Despite lower relative risk, the absolute risk of untreated LQT1 remains clinically significant. Barsheshet et al. 201255 Barsheshet et al. 2012
Mutations in cytoplasmic loops and risk of life-threatening events — PMID 22456477 confirmed that even the lower-risk non-c-loop KCNQ1 mutations carry meaningful event rates, with 10–15% of carriers experiencing syncope or aborted cardiac arrest by age 40 without treatment.

Practical Actions

The cornerstone of LQT1 management is beta-blocker therapy. Nadolol is the preferred agent — it is the only beta-blocker shown to reduce events in both LQT1 and LQT2, and outperforms propranolol and atenolol in network meta-analyses. Metoprolol has shown no significant risk reduction for LQT1 in the registry data and should be avoided as the primary LQT1 agent.

Trigger avoidance is LQT1-specific: swimming and water sports carry unique danger because the combined effects of immersion (vagal activation), exertion (adrenergic activation), and sudden startle (dive reflex) create compounded arrhythmia risk. Competitive swimming is specifically contraindicated in previously symptomatic LQT1 carriers by AHA/ACC guidelines.

QT-prolonging drugs represent the second major modifiable trigger. The crediblemeds.org database (accessible free of charge) maintains the current list of >200 drugs in the highest-risk category. These span antibiotics (fluoroquinolones, macrolides), antihistamines, antipsychotics, antidepressants, antifungals, and several antiarrhythmics themselves.

ICD therapy is reserved for survivors of cardiac arrest, those with syncope despite adequate beta-blocker therapy, and selected very high-risk patients (QTc >550 ms, T-wave alternans). It is not the first-line approach for newly diagnosed asymptomatic carriers.

Cascade screening of first-degree relatives is essential — each has a 50% probability of inheriting the same variant, and the condition is largely manageable if identified before the first event.

Interactions

KCNQ1 assembles with KCNE1 (MinK) as β-subunit to form the complete IKs channel complex. Loss-of-function mutations in KCNE1 (rs74315445 and related KCNE1 variants, gene OMIM 176261) produce a clinically indistinguishable LQT phenotype (LQT5). Compound heterozygosity for KCNQ1 and KCNE1 variants in the same individual increases IKs impairment beyond either alone. Homozygous or compound heterozygous KCNQ1 mutations — or biallelic KCNQ1 + KCNE1 — cause Jervell and Lange-Nielsen syndrome66 Jervell and Lange-Nielsen syndrome
A recessive form of LQTS combined with congenital profound sensorineural deafness. QTc is typically >550 ms and the risk of sudden cardiac death in childhood is very high without ICD implantation, which presents with profound deafness and a much more severe cardiac phenotype (mean QTc ~557 ms, >90% cardiac event rate).

Electrolyte depletion — particularly hypokalemia and hypomagnesemia — further impairs IKs function and acutely lengthens QT in carriers. Diuretics (loop and thiazide), vomiting, diarrhea, and heat-related dehydration are practical clinical triggers that interact with the genetic predisposition.

Genotype Interpretations

What each possible genotype means for this variant.

CC Non-carrier Normal

Normal KCNQ1 Q359 — no Long QT type 1 pathogenic variant

You carry two copies of the reference C allele at rs397508075. Your KCNQ1 gene produces full-length potassium channel protein, and your IKs current is not impaired by this variant. This is the common genotype found in essentially the entire population — the T (stop-codon) allele is an ultra-rare pathogenic variant absent from large population databases. You do not carry the KCNQ1 Q359X Long QT syndrome type 1 variant.

CT LQT1 Carrier High Risk

One copy of Q359X — KCNQ1 haploinsufficiency, Long QT syndrome type 1

Long QT syndrome type 1 is the most common inherited channelopathy predisposing to sudden cardiac death. The KCNQ1 gene encodes the α-subunit of the IKs potassium channel, which assembles with KCNE1 (β-subunit) to form the complete channel complex. IKs is the dominant repolarizing current at fast heart rates and is uniquely sensitive to β-adrenergic stimulation: catecholamine signaling via PKA phosphorylation of KCNQ1 normally produces a 2–5-fold increase in IKs current amplitude, shortening the action potential to allow rapid successive beats.

The Q359X truncation is predicted to cause nonsense-mediated mRNA decay, eliminating the mutant allele's contribution at the mRNA level. Heterozygous carriers therefore have one functional KCNQ1 allele (haploinsufficiency) rather than two. Functional studies of similar KCNQ1 haploinsufficiency models show ~50% IKs current density under basal conditions. During adrenergic stimulation, the reserve current that normally protects against QT prolongation at high heart rates is halved — the channel simply cannot respond adequately to catecholamine drive.

Event risk context: Among 1,090 LQT1 patients in the International LQTS Registry, stop-codon mutations (including Q-type nonsense variants) were associated with a hazard ratio of 0.57 for cardiac events versus non-cytoplasmic-loop missense mutations. This means lower relative risk than the highest-risk mutation categories, but not zero risk — 10–15% of untreated carriers still experience syncope or aborted cardiac arrest by age 40.

Beta-blocker response: LQT1 patients respond better to beta-blockers than any other LQTS subtype. In a 216-patient Circulation study, 75% of beta-blocker-treated LQT1 patients were asymptomatic over 10 years of follow-up, with essentially all failures attributable to noncompliance or concomitant QT-prolonging drug use.

Gender and QTc: Women with LQT1 have modestly higher cardiac event rates than men, particularly in the post-pubertal period, though the difference is smaller in LQT1 than in LQT2. QTc >500 ms at baseline confers additional risk.

TT Homozygous LQT1 High Risk

Two copies of Q359X — homozygous KCNQ1 loss-of-function, Jervell and Lange-Nielsen syndrome or severe LQT1

Homozygous KCNQ1 loss-of-function eliminates essentially all IKs current from cardiac myocytes. The IKs channel's role in endolymph secretion in the cochlea also explains the associated deafness: KCNQ1 + KCNE1 are essential for K+ recycling in the inner ear's stria vascularis. Without this function, endolymph composition is disrupted and the hair cells that translate sound to nerve signals cannot function.

Clinically, JLNS1 presents at birth or in early infancy with failure to respond to sound, and subsequently with syncopal episodes and sudden cardiac death in childhood if untreated. QTc is typically >550 ms. ICD implantation is strongly recommended, often in combination with beta-blockers and sometimes left cardiac sympathetic denervation (LCSD).

If neither deafness nor markedly prolonged QTc is present, technical explanations for this genotype call should be sought before clinical escalation — true homozygous KCNQ1 Q359X is exceptionally rare.