In Vitro Characterization of KCNT1 Mutations from Pediatric Patients

Background/Purpose: The KCNT1 gene encodes for sodium-gated potassium channels and is mainly expressed in brain and heart. Gain-of-function mutations in KCNT1 are associated with autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE) and epilepsy of infancy with migrating focal seizures (EIMFS). Both epilepsy syndromes can be associated with mental retardation, behavioral abnormalities and epileptic encephalopathy. Drug treatment with quinidine has been proposed as a novel therapeutic option, and clinical improvement has been reported in single cases. However, it remains controversial if the changes in potassium conductance resulting from individual mutations are correlated with either clinical phenotype or response to quinidine treatment.

Methods: KCNT1 mutations were analyzed by heterologous expression in Xenopus oocytes and functional analysis using 2-electrode voltage clamp.

Results: Heterologous expression of six out of seven mutations in KCNT1 associated with epilepsy resulted in up to fivefold increase in potassium currents. Coexpression of mutations and KCNT1 wildtype channels resulted in an intermediate phenotype with two- to threefold increase in potassium current amplitudes. No potassium currents could be recorded following expression of the mutation I335N. Coexpression of I335N and KCNT1 wildtype channels, however, resulted in increased potassium currents compared with wildtype alone. Quinidine sensitivity was comparable for all mutations, independent of alterations of potassium conductance and clinical phenotype.

Conclusion: Functional analysis of epilepsy-associated mutations in KCNT1 potassium channels showed gain-of function for all mutations following coexpression with KCNT1 wildtype subunits. Quinidine sensitivity was comparable for all mutations and does therefore not explain different clinical responses to treatment in distinct patients.