Characterization of two de novo KCNT1 mutations in children with malignant migrating partial seizures in infancy

The KCNT1 gene encodes for subunits contributing to the Na+-activated K+ current (KNa), expressed in many cell types. Mutations in KCNT1 have been found in patients affected with a wide spectrum of early-onset epilepsies, including Malignant Migrating Partial Seizures in Infancy (MMPSI), a severe early-onset epileptic encephalopathy characterized by pharmacoresistant focal seizures migrating from one brain region or hemisphere to another and neurodevelopment arrest or regression, resulting in profound disability. In the present study we report identification by whole exome sequencing (WES) of two de novo, heterozygous KCNT1 mutations (G288S and, not previously reported, M516V) in two unrelated MMPSI probands. Functional studies in a heterologous expression system revealed that channels formed by mutant KCNT1 subunits carried larger currents when compared to wild-type KCNT1 channels, both as homo- and heteromers with KCNT1 subunits. Both mutations induced a marked leftward shift in homomeric channel activation gating. Interestingly, the KCNT1 blockers quinidine (3-1000 μM) and bepridil (0.03-10 μM) inhibited in a concentration-dependent manner wild-type and mutant KCNT1 currents, with mutant channels showing higher sensitivity to blockade. This latter result suggests two genotype-tailored pharmacological strategies to specifically counteract the dysfunction of KCNT1 activating mutations in MMPSI affected patients.