Electrophysiologically Targeted Biopsies Reveal the Transcriptional Landscape of Focal Epilepsy

Abstract

Up to 30% of patients with epilepsy have intractable seizures, yet the mechanisms of focal ictogenesis remain unclear. Tissue involvement in ictal regions is heterogeneous, with different regions playing distinct roles in ictogenesis, seizure propagation, and resistance to spread. These roles are reflected in electrophysiologic differences between the seizure focus and the ictal penumbra, where evidence of synaptic spread is present but excitatory firing is constrained by largely intact inhibition. Investigation of the disruption of the normal interplay between excitatory and inhibitory activity, thought to underlie ictogenesis across a range of epilepsy etiologies, is limited by network complexity and cellular heterogeneity in human tissue samples. In this study, we relate cellular and molecular alterations to excitatory-inhibitory disruption in network dynamics defined by electrophysiologic features. This work may aid in the identification of clinically relevant tissue biomarkers and support novel preclinical therapeutic approaches for treatment-resistant focal epilepsy disorders. We developed a novel intracranial EEG guided, MRI-localized approach to sample paired biopsies from 11 patients with drug-resistant focal epilepsy with diverse etiologies, which were then studied using single-nucleus RNA sequencing (snRNAseq) and immunohistochemistry (IHC). EEG recorded from stereotactically implanted depth arrays (sEEG) was used to identify regions of epileptic involvement, based on findings from prior simultaneous clinical and microelectrode recordings. This approach addresses the intrinsic heterogeneity due to etiology and cortical architecture through paired, within-patient comparisons. We identified distinct cell-type specific transcriptional signatures that differentiate cellular populations in the seizure focus and ictal penumbra in intractable focal epilepsies. Our findings provide a link between tissue composition and gene expression that correlate with electrographic features in a heterogeneous seizure landscape. Our findings support common pathways of seizure generation and spread that are conserved across disease etiologies. Relative depletion of interneuron populations in the seizure focus supports the hypothesis of disrupted inhibition as a driver of epileptiform activity in the seizure focus. The enrichment of plasticity-associated gene signatures in the penumbra suggests a complex interaction of these regions with the seizure focus, as well as the role of the penumbra in enabling or limiting seizure expansion. This study provides a novel methodology for tissue sampling in epilepsy and uncovers biologically relevant tissue signatures that provide grounds for future work in targeting cellular and molecular alterations present in focal epilepsies.