A novel KCNC1 gain‐of‐function variant causing developmental and epileptic encephalopathy: “Precision medicine” approach with fluoxetine

Abstract Variable phenotypes, including developmental encephalopathy with (DEE) or without seizures and myoclonic epilepsy and ataxia due to potassium channel mutation, are caused by pathogenetic variants in KCNC1 , encoding for Kv3.1 channel subunits. In vitro, channels carrying most KCNC1 pathogenic variants display loss‐of‐function features. Here, we describe a child affected by DEE with fever‐triggered seizures, caused by a novel de novo heterozygous missense KCNC1 variant (c.1273G>A; V425M). Patch‐clamp recordings in transiently transfected CHO cells revealed that, compared to wild‐type, Kv3.1 V425M currents (1) were larger, with membrane potentials between −40 and +40 mV; (2) displayed a hyperpolarizing shift in activation gating; (3) failed to inactivate; and (4) had slower activation and deactivation kinetics, consistent with a mixed functional pattern with prevalent gain‐of‐function effects. Exposure to the antidepressant drug fluoxetine inhibited currents expressed by both wild‐type and mutant Kv3.1 channels. Treatment of the proband with fluoxetine led to a rapid and prolonged clinical amelioration, with the disappearance of seizures and an improvement in balance, gross motor skills, and oculomotor coordination. These results suggest that drug repurposing based on the specific genetic defect may provide an effective personalized treatment for KCNC1 ‐related DEEs.