Abstracts

Rationale: Mutations in the KCNT1 gene cause a variety of epilepsy syndromes ranging from autosomal dominant nocturnal frontal lobe epilepsy to intractable epilepsies of infancy and childhood including malignant migrating partial seizures of infancy (MMPSI) and Ohtahara syndrome. KCNT1 encodes the Slack sodium-activated potassium channel. Slack channels ensure accurate timing of action potentials (for example, in the timing circuits of the auditory system). Additionally, they interact with Fragile X mental retardation protein (FMRP) to control translation of proteins important for neuronal development. The precise mechanism by which KCNT1 mutations lead to epilepsy is not yet clear.Methods: Mutations identified in epilepsy patients were introduced into KCNT1, and mutant channels were expressed in Xenopus oocytes. Oocytes were evaluated by two electrode voltage clamp and single channel patch clamp recordings. Single channel kinetics were used to develop a stochastic model of channel gating that incorporated interactions between channels. KCNT1 knockout mice were evaluated for seizures with EEG, maximum electroshock, and pentylenetetrazole induction. The mice also underwent behavioral testing including open field, rotarod, and Lashley maze to screen for neurodevelopmental abnormalities.Results: All of the epilepsy-causing mutations studied to date, including those tested in this study, have been associated with increased channel conductance. These mutations occurred most commonly in the C terminus, which contains regulatory elements for channel opening, but mutations were also identified at other sites including the pore and N terminal encoding domains. At the single channel level, increased opening was due to cooperative gating between adjacent channels, as confirmed by comparison to a simulated kinetic model. KCNT1 knockout mice had no overt behavioral changes and mild abnormalities on testing including decreased exploratory movement, impaired motor learning, and a slight impairment in spatial learning. They did not have spontaneous seizures, and there was a slight reduction in electroshock seizure threshold.Conclusions: The limited phenotype of Slack knockout mice suggests that the increased potassium current in the epilepsy mutations is likely to be a crucial factor in the neurodevelopmental consequences of epilepsy-causing mutations, rather than only the loss of FMRP interactions. The mutations increase channel conductance by altering inter-channel interactions (cooperative gating), so interrupting these interactions may provide a specific therapy for these disorders.