rs397508072 — KCNQ1 Q356X

KCNQ1 Q356X — When the Heart's Repolarization Brake Is Cut

Every heartbeat ends with a carefully timed electrical recovery — the repolarization phase¹¹ repolarization phase
Phase 3 of the ventricular action potential, lasting roughly 250-350 ms, during which potassium ions flow out of cardiomyocytes to restore the negative resting membrane potential. The speed of repolarization determines the QT interval on the ECG that resets the cardiac muscle for the next beat. The KCNQ1 gene encodes the pore-forming subunit of the IKs potassium channel — a key contributor to that electrical reset. The Q356X variant (NM_000218.3:c.1066C>T) introduces a premature stop codon at position 356, cutting the 676-amino-acid protein roughly in half. The truncated mRNA is degraded by nonsense-mediated decay²² nonsense-mediated decay
A cellular quality-control mechanism that degrades mRNA transcripts containing premature stop codons, preventing production of potentially toxic truncated proteins, producing a loss-of-function through haploinsufficiency rather than a dominant-negative mechanism. The result: insufficient IKs current, impaired ventricular repolarization, a prolonged QT interval on the ECG, and a substrate for life-threatening arrhythmia.

The variant is classified Pathogenic in ClinVar VCV000045950³³ ClinVar VCV000045950
ClinVar Variation ID 45950; multiple submitters, no conflicts, 2-star review status and is associated with both Romano-Ward LQT syndrome type 1 (autosomal dominant, heterozygous) and the more severe Jervell and Lange-Nielsen syndrome type 1 (autosomal recessive, homozygous or compound heterozygous, with congenital deafness).

The Mechanism

The IKs channel is a hetero-octamer: four KCNQ1 α-subunits form the central potassium-conducting pore, flanked by regulatory KCNE1 (MinK) β-subunits. Together they generate the slow delayed rectifier current that activates during sustained depolarization and provides the repolarization reserve that becomes especially critical during exercise, when heart rate and sympathetic tone increase. The Q356X truncation falls in the intracellular C-terminus of KCNQ1, beyond the S6 transmembrane helix. Because the truncated protein undergoes nonsense-mediated mRNA decay, heterozygous carriers produce approximately 50% of normal IKs current — a haploinsufficient mechanism⁴⁴ haploinsufficient mechanism
Haploinsufficiency occurs when one functional copy of a gene is insufficient to maintain normal function; in contrast, dominant-negative mutations produce a defective protein that actively poisons the remaining normal copies, typically causing more severe channel dysfunction. IKs haploinsufficiency prolongs the cardiac action potential and QT interval, particularly under adrenergic stress. When IKs reserve is absent, a triggered premature beat can induce [Torsades de pointes | A polymorphic ventricular tachycardia that on the ECG appears to twist around the isoelectric line; it often self-terminates but can degenerate into ventricular fibrillation and cardiac arrest] — a rapid, disorganized ventricular arrhythmia that can lead to syncope, cardiac arrest, or sudden death.

Homozygous or compound heterozygous Q356X carriers lack functional IKs entirely. KCNQ1 is also expressed in the stria vascularis of the cochlea, where IKs maintains the endocochlear potential required for hair-cell mechanotransduction. Complete IKs ablation causes profound bilateral sensorineural deafness alongside the severe cardiac phenotype of Jervell and Lange-Nielsen syndrome.

The Evidence

The Q356X variant's pathogenicity derives from multiple lines of evidence. At the population level, the T allele is essentially absent from gnomAD (approximately 10 observed alleles across 1.4 million), consistent with strong negative selection.

Mutation spectrum data: Splawski et al. 2000⁵⁵ Splawski et al. 2000
Spectrum of mutations in LQT genes, 262 unrelated patients; KCNQ1 accounted for 42% of identified mutations; stop-gain and frameshift mutations as a class represent 5-7% of KCNQ1 LQT1 variants established that nonsense mutations in KCNQ1 consistently segregate with disease in affected families.

Mutation-type risk stratification: The landmark Moss et al. 2007 Circulation⁶⁶ Moss et al. 2007 Circulation
600 LQT1 patients across 101 families from three international registries; independent predictors of cardiac events assessed through age 40 study established that truncating mutations (which cause haploinsufficiency) carry a meaningfully lower clinical event risk than dominant-negative missense mutations (HR 2.26 for dominant-negative vs haploinsufficiency), though both are clinically significant.

Stop-codon-specific data: Ruwald et al. 2015 Heart Rhythm⁷⁷ Ruwald et al. 2015 Heart Rhythm
1,090 LQT1 patients from the International Long QT Registry; stop-codon mutations specifically vs other mutation types found that KCNQ1 stop-codon carriers had a 27% cumulative cardiac event rate by age 40 versus 44% for non-C-loop missense carriers (HR 0.57, p=0.035). This reduced — but still substantial — risk reflects the haploinsufficiency mechanism. Importantly, only 1 aborted cardiac arrest occurred among stop-codon carriers during the follow-up period.

JLNS severity: Homozygous biallelic KCNQ1 loss-of-function produces a far more severe phenotype. Per GeneReviews, more than 50% of untreated JLNS patients die before age 15, and 50% have a cardiac event before age 3. Beta-blockers provide only partial protection in JLNS (51% of patients still experience events despite therapy).

Practical Actions

Romano-Ward (CT heterozygotes): The trigger profile of LQT1 is distinctive — 62% of life-threatening events occur during exercise or emotional arousal, with swimming particularly implicated. Beta-blockers (nadolol or propranolol preferred over selective agents) reduce cardiac events by approximately 50-80% in LQT1. Unsupervised swimming and competitive athletics should be restricted until formal cardiac evaluation and, if applicable, ICD placement are completed.

JLNS (TT homozygotes): Management requires urgent pediatric cardiology and audiology referral. Beta-blockers are first-line but have limited efficacy (51% event rate despite therapy). ICD is strongly recommended given the extreme arrhythmia burden and early onset. Left cardiac sympathetic denervation (LCSD) may reduce events in ICD-ineligible or refractory cases. Cochlear implantation addresses the deafness component.

Interactions

KCNQ1 Q356X belongs to the class of truncating KCNQ1 variants that cause loss of function through haploinsufficiency. KCNE1 (MinK), the regulatory β-subunit that assembles with KCNQ1 to form the IKs channel, is also a cause of LQT syndrome (LQT5, OMIM 613695) and JLNS type 2 when mutated. Patients who carry both a KCNQ1 variant and a KCNE1 variant (compound digenic heterozygosity) can exhibit more severe IKs impairment than either alone. Other LQT genes — KCNH2 (LQT2), SCN5A (LQT3) — affect independent ion channels; concurrent variants in these genes (digenic LQTS) are associated with more severe QT prolongation and higher sudden-death risk than single-gene LQTS.