Abstracts

Rationale: Seizure disorders caused by sodium channel mutations are often accompanied by co-morbidities, including impaired cognition and movement. It is not clear whether these comorbidities are products of the seizures themselves or independent consequences of the mutations. Dominant gain-of-function mutations of SCN8A are one cause of developmental and epileptic encephalopathy. We used a mouse model expressing the pathogenic SCN8A variant N1768D to investigate cell type-specific changes in gene expression before and after seizure onset to identify changes that could contribute to the observed comorbidities._x000D_ In a previous study of bulk brain RNA, we did not detect changes in gene expression prior to seizure onset. Within 24 hours after the first seizure, there was increased expression of genes associated with astrocytosis and neuroprotection (Sprissler et al. Exp Neurol. 2016). However, cell type-specific changes may have been obscured in the bulk RNA analysis. _x000D_
Methods: Single nucleus RNA-sequencing (snRNA-seq) was carried out on isolated hippocampus from 3 Scn8a^N1768/D/+ mice and 3 wildtype littermates at 50 days of age, prior to seizure onset. Additional analysis of samples collected one month after seizure onset is in progress.

Results: We identified 57 clusters of cells. Prior to seizure onset, we observed a small increase in the proportion of oligodendrocytes and oligodendrocyte precursor cells (OPCs) in the hippocampus of Scn8a^N1768D/+ mice compared with wildtype littermates (oligodendrocytes: 1.24-fold; oligodendrocyte precursor cells: 1.13-fold). Within the OPC population, we observed small (< 1.4-fold) changes in expression of genes involved in the regulation of neurite outgrowth, including Grip1, Tenm1, Dcx, Enpp2, and Trpc5. _x000D_ We also observed a 1.25-fold increase in overall expression of Ttr encoding transthyretin in Scn8a^N1768D/+ hippocampus. This effect was greatest in microglia (1.27-fold). Ttr is thought to play a protective role in neurological disease and has been identified in previous RNA-sequencing studies of epilepsy models._x000D_
Conclusions: Prior to seizure onset, elevated activity of the sodium channel Nav1.6 does not cause major changes in neuronal gene expression. There is also a small increase in the number of oligodendrocytes and OPCs in Scn8a^N1768D/+ hippocampus prior. Comparison with post-seizure results will be presented.

Funding: Supported by NINDS R01 NS34509