Abstracts

Rationale: SCN8A encephalopathy (EIEE13) is caused by gain-of-function mutations resulting in hyperactivity of the voltage-gated sodium channel Nav1.6. This channel is concentrated at the axon initial segment and nodes of Ranvier and plays a crucial role in the initiation and propagation of action potentials (APs). Clinical features of patients with SCN8A encephalopathy include seizure onset between 0-18 months of age, intellectual disability and motor impairment (Meisler et al, Epilepsia 2016). Seizures are often refractory to treatment with anti-epileptic drugs, and sudden unexpected death in epilepsy (SUDEP) has been reported in approximately 10% of patients. Recently a knock-in mouse model carrying a SCN8A mutation (p.Asn1768Asp; N1768D) was generated and shown to exhibit many pathological phenotypes seen in human patients, including spontaneous seizures and sudden death (Wagnon et al, Hum. Molec. Genet. 2015). Mice homozygous for this mutation (D/D) have seizure onset at 3 weeks of age, multiple seizures and sudden death within 24 hrs. In contrast, only 50% of mice heterozygous for the mutation (D/+) exhibit seizures, with onset at 2-4 months and sudden death within 1 month. In this study, we electrophysiologically characterized layer II neurons of the medial entorhinal cortex (mEC), a major input region to the hippocampal circuitry, in D/D and D/+ mice and WT littermates. Methods: All recordings were done at 3 weeks of age. Seizures in D/D mice were confirmed by video recording. Brain slices were prepared and electrophysiology recordings taken using the whole-cell patch clamp electrophysiology technique. Recordings of membrane properties and persistent (INaP) and resurgent (INaR) sodium channel currents were obtained. Results: Increased AP firing frequencies and abnormal AP waveforms were recorded in the mEC of both D/+ and D/D mice compared to WT. In mEC layer II neurons, at a current injection step of 110 pA, AP firing was increased from 2.2 1.2 Hz (n=20) in WT neurons to 7.6 1.1 Hz (n=20; P < 0.005) in D/+ neurons and further increased to 9.6 1.0 Hz in D/D neurons (n=27; P < 0.001). In WT mEC neurons, brief stimulation within the deep layers resulted in single synaptically evoked APs, and increasing the amplitude of the current to 2X and 3X the threshold current did not lead to bursting. In contrast, increasing the stimulus intensity in both D/+ and D/D mEC neurons led to the initiation of AP bursts. INaR current amplitudes were significantly (P < 0.001) increased in both D/+ (-1522 21 pA; n=9) and D/D (-1936 34 pA; n= 8) mEC neurons compared to WT (-847 27 pA; n= 8). INaP currents were also enhanced in D/+ (-834 31 pA; n=9) and D/D (-1037 11 pA; n=10) mEC neurons compared to WT (-247 14 pA; n=10) (P < 0.001). Conclusions: The N1768D mouse model of SCN8A encephalopathy exhibits hyperexcitability of mEC neurons and altered Na channel dynamics. These alterations would increase neuronal excitability and are likely to contribute to the development of spontaneous seizures and eventual SUDEP. Funding: NIH R01 NS075157 (MKP) and RO1 NS 034509 (MHM)