Abstracts

Rationale: SCN8A epileptic encephalopathy (EE) is a severe epilepsy syndrome resulting from de novo gain-of-function mutations in the SCN8A gene, encoding the sodium channel Nav1.6. Patients with SCN8A mutations have seizure onset between birth and 12 months of age and additionally exhibit cognitive and motor dysfunction. Patients also have a notable risk for sudden unexpected death in epilepsy (SUDEP) which increases significantly if seizures are not controlled. Nav1.6 is expressed in both excitatory and inhibitory neurons, and the function of inhibitory interneurons is critical to constrain activity of excitatory neurons. Inhibitory neuron dysfunction has been linked to various genetic epilepsy syndromes, indicating that their characterization is critical in SCN8A EE. Previous work has shown that somatostatin-positive (SST) interneurons have abnormal physiology and contribute to seizures. However, parvalbumin-positive (PV) interneurons, another inhibitory interneuron population critical for balancing network excitability, have yet to be studied in the context of SCN8A EE.
_x000D_ Methods: Using patient-derived mouse models of SCN8A epileptic encephalopathy (Scn8aD/+ and Scn8aW/+), electrophysiology experiments were performed in addition to seizure confirmation via video/EEG recording. Brain slices were prepared and recordings were taken using the whole-cell patch clamp technique. Recordings of membrane properties and cell excitability were obtained from WT, Scn8aD/+, and Scn8aW/+ mice.
_x000D_ Results: In this study, we show that PV interneurons in Scn8aD/+ and Scn8aW/+ mice are initially hyperexcitable. Despite initial hyperexcitability, PV interneurons experience premature depolarization block, a state of action potential failure, leading to potential overall hypoexcitability. Furthermore, expression of the SCN8A mutation R1872W selectively in PV interneurons (Scn8aW/+-PV) led to the development of both audiogenic induced seizures and spontaneous seizures.

Conclusions: These findings indicate a disruption within the PV inhibitory network in SCN8A EE, leading to potential disinhibition and unchecked excitation. Understanding the contributions of these interneurons in SCN8A EE is critical to recognizing the physiological consequences of increased Nav1.6 activity in not only excitatory neurons, but also in inhibitory interneurons, particularly when developing mechanistically-based treatments.

Funding: NIH RO1 NS103090