Abstracts

Rationale: SCN8A encephalopathy (EIEE13, OMIM #614558) is a severe, early-onset seizure disorder with developmental delay, cognitive impairment, and elevated risk of SUDEP (sudden unexpected death in epilepsy). Most cases are caused by de novo missense mutations of SCN8A that result in elevated activity of the sodium channel Nav1.6. Anti-epileptic drugs that target sodium channels provide transient improvement in some patients, but seizure control is usually incomplete. To identify novel therapeutic targets and secondary changes that might contribute to comorbidities and SUDEP, we examined global gene expression before and after seizure onset in a mouse model of SCN8A encephalopathy. Methods: We prepared RNA from mice heterozygous for the knock-in mutation p.Asn1768Asp (Scn8a-N1768D/+), identified in an individual with SCN8A encephalopathy and SUDEP. The mice recapitulate the seizure disorder, with 50% percent of Scn8a-N1768D/+ mice developing severe seizures and sudden death before 6 months of age. RNA was isolated from forebrain (cerebral cortex plus hippocampus), cerebellum, and brainstem at three ages: pre-seizure (6 weeks of age), post-seizure (within 24 hrs of seizure onset, at 3 to 4 months of age), and non-seizure (8 months of age). RNA-seq analysis was carried out and transcript abundance in mutant mice and wildtype littermates was compared by differential expression analysis. Results: Altered transcript profiles in Scn8a-N1768D/+ mice were observed only in the forebrain region and only after seizure onset. The abundance of 50 transcripts was increased ?-3-fold after seizures; these included two anti-convulsant neuropeptides and more than a dozen genes involved in the response to neuronal damage. The abundance of 15 transcripts decreased ?-3-fold in response to seizures. Considerable overlap was observed between the genes affected in this genetic model of epilepsy and those altered by chemically induced seizures, traumatic brain injury, ischemia and inflammation. The overlap includes several transcripts that are elevated in reactive astrocytes. Follow-up immunostaining revealed reactive astrocytosis in the hippocampus of mutant mice after seizures. Conclusions: Seizures have a direct effect on gene expression in this mouse model of epileptic encephalopathy. Transcript levels were altered only after seizures, demonstrating that the abnormal SCN8A genotype was not sufficient to change gene expression. Elevated expression of neuropeptides and hippocampal astrocytosis are prominent components of the response to seizures. Our data suggest that seizures could contribute to comorbidities in epileptic encephalopathy by a transcriptional mechanism. Funding: NIH R01 NS34509 to MHM Postdoctoral fellowship from the Dravet Syndrome Foundation to JLW