KCNH2 ‐ L693P Causes Long QT Syndrome Type 2 Through hERG Channel Dysfunction: Functional Validation of a Variant of Uncertain Significance

Background Congenital long QT syndrome (LQTS) is an inherited arrhythmia characterized by QT prolongation and increased risk of ventricular arrhythmias. Type 2 LQTS (LQT2) results from mutations in the KCNH2 gene encoding the hERG potassium channel. With the widespread use of next‐generation sequencing, many KCNH2 variants have been identified but remain classified as variants of uncertain significance (VUS), including p.L693P, requiring functional validation. Methods A proband with recurrent syncope and prolonged QTc underwent whole‐exome and family‐based Sanger sequencing, which detected a heterozygous KCNH2 ‐L693P mutation. HEK293 cells were transiently transfected with wild‐type (WT) and/or mutant hERG plasmids. Western blotting, immunofluorescence, and whole‐cell patch clamp were performed to assess protein maturation, trafficking, and channel kinetics. Results Western blot showed reduced levels of the 155 kDa mature hERG and accumulation of the 135 kDa immature form, consistent with trafficking defects. Immunofluorescence confirmed endoplasmic reticulum (ER) retention. Electrophysiology revealed complete current loss in homozygotes and ~39% residual WT current in heterozygotes, indicating dominant‐negative‐like suppression. The mutation shifted steady‐state inactivation and delayed recovery. Conclusions The KCNH2 ‐L693P mutation impairs hERG maturation and function, supporting its reclassification from variants of uncertain significance (VUS) to pathogenic and providing evidence for improved clinical management.