A Novel KCNQ2 Gain-of-Function Variant I134N Causes Severe Developmental and Epileptic Encephalopathy.

Missense variants in the KCNQ2 gene can cause developmental and epileptic encephalopathy (DEE). While most KCNQ2-DEE cases are attributed to loss-of-function (LOF) mutations, gain-of-function (GOF) mutations have also been implicated in the disorder. This study describes the clinical features of a DEE patient with a KCNQ2 mutation in the voltage-sensing domain (VSD) and analyzes the variant's electrophysiological properties. Whole-exome sequencing was performed to identify the genetic variant. Whole-cell patch-clamp electrophysiology was used to characterize the functional effects of the mutant channel, both alone and in combination with KCNQ3 subunits at a 1:1:2 ratio to mimic the patient's allele dosage. The effect of amitriptyline (AMI) on channel activity was also evaluated. A three-year-old female with early-onset epileptic encephalopathy presented with intractable seizures, developmental regression, microcephaly, transient thyroid dysfunction, and a mixed EEG pattern of hypsarrhythmia and intermittent burst-suppression. A de novo KCNQ2 variant (c.401T>A, p.Ile134Asn) located in the conserved S2 transmembrane domain was identified and classified as likely pathogenic. Electrophysiological analysis showed that the KCNQ2-I134N mutation caused a hyperpolarizing shift in voltage-dependent activation and significantly increased current density, indicating a GOF effect. This GOF phenotype persisted when the mutant subunit was co-expressed with KCNQ3 and under a transfection ratio mimicking the patient's genotype. The hyperactivity of the mutant channel was effectively suppressed by amitriptyline. We report a novel GOF variant (I134N) in the KCNQ2 gene associated with DEE. The KCNQ blocker amitriptyline effectively suppressed mutant channel hyperactivity, suggesting its potential as a targeted therapeutic option for patients with this pathogenic variant.