Abstracts

Rationale: KCNT1 (Slack) is a Na⁺-activated K⁺ channel that has been implicated in multiple epilepsy syndromes including malignant migrating partial seizures of infancy (MMPSI) and autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE). Patients with mutations in KCNT1 develop focal or multifocal seizures with neurodevelopmental or psychiatric comorbidities. Previous studies demonstrated that epilepsy-associated KCNT1 mutations paradoxically display increased potassium channel conductance, and that the channel interacts with proteins necessary for normal neuronal development and plasticity, particularly fragile X mental retardation protein (FMRP). The mechanisms by which these mutations lead to an increased channel conductance and ultimately to epilepsy and cognitive deficits are poorly understood. Methods: KCNT1 mutations were identified in patients with a variety of epilepsy syndromes including MMPSI, ADNFLE, Ohtahara syndrome, and infantile spasms. Whole cell and single channel properties of these mutant channels were evaluated in an oocyte expression system to evaluate for any genotype-phenotype correlations and for biophysical mechanisms of increased conductance. KCNT1^-/- mice were evaluated with EEG, electrical and chemical seizure induction, and behavioral testing to determine how channel dysfunction contributes to epilepsy and neurodevelopmental and cognitive abnormalities. Results: Increased channel conductance was seen in all mutations tested. There was no clear correlation between the degree of conductance increase and either phenotypic severity or location in the protein sequence. Single channel recordings demonstrated that the mutations could lead to increased channel conductance through multiple mechanisms, the most prominent of which was cooperative gating. KCNT1^-/- mice did not have spontaneous seizures. Maximum electroshock thresholds were reduced (22%) in these mice as compared to wild type (p = 0.048, N = 14). There was no difference in latency to pentylenetetrazole-induced seizures (p = 0.271, N = 14). On behavioral testing, KCNT1^-/- mice diplayed an excess of anxiety-like behavior on open field testing (initial movement reduced 38%, p < 0.001, N = 22) and impaired learning on a rotarod task (second day success reduced 13%, p = 0.012, N=22). Conclusions: Increased channel conductance was common to all epilepsy-causing KCNT1 mutations tested, suggesting that the current gain is intricately involved in epileptogenesis. The lack of correlation between the conductance increase and phenotypic severity, however, suggests that alterations in non-conducting functions may be important as well. This hypothesis is supported by the fact that loss of the channel produces behavioral changes in an animal model.