Authors: Massey, Cory A; Thompson, Samantha J; Ostrom, Ryan W; Drabek, Janice; Sveinsson, Olafur A; Tomson, Torbjörn; Haas, Elisabeth A; Mena, Othon J; Goldman, Alica M; Noebels, Jeffrey L
Issue: Brain Commun. 2021 Jul 9;3(3):fcab149.
Sudden Unexpected Death in Epilepsy (SUDEP) is a leading cause of epilepsy-related mortality, and the analysis of mouse SUDEP models is steadily revealing a spectrum of inherited risk phenotypes based on distinct genetic mechanisms. Serotonin (5-HT) signaling enhances post-ictal cardiorespiratory drive and, when elevated in the brain, reduces death following evoked audiogenic brainstem seizures in inbred mouse models. However, no gene in this pathway has yet been linked to a spontaneous epilepsy phenotype, the defining criterion of SUDEP. Most monogenic models of SUDEP invoke a failure of inhibitory synaptic drive as a critical pathogenic step. Accordingly, the G protein-coupled, membrane serotonin receptor 5-HT2C inhibits forebrain and brainstem networks by exciting GABAergic interneurons, and deletion of this gene lowers the threshold for lethal evoked audiogenic seizures. Here we characterize epileptogenesis throughout the lifespan of mice lacking X-linked, 5-HT2C receptors (loxTB Htr2c). We find that loss of Htr2c generates a complex, adult-onset spontaneous epileptic phenotype with a novel progressive hyperexcitability pattern of absences, non-convulsive, and convulsive behavioral seizures culminating in late onset sudden mortality predominantly in male mice. RNAscope localized Htr2c mRNA in subsets of Gad2+ GABAergic neurons in forebrain and brainstem regions. To evaluate the contribution of 5-HT2C receptor-mediated inhibitory drive, we selectively spared their deletion in GAD2+ GABAergic neurons of pan-deleted loxTB Htr2c mice, yet unexpectedly found no amelioration of survival or epileptic phenotype, indicating that expression of 5-HT2C receptors in GAD2+ inhibitory neurons was not sufficient to prevent hyperexcitability and lethal seizures. Analysis of human SUDEP and epilepsy genetic databases identified an enrichment of HTR2C non-synonymous variants in SUDEP cases. Interestingly, while early lethality is not reflected in the mouse model, we also identified variants mainly among male Sudden Infant Death Syndrome patients. Our findings validate HTR2C as a novel, sex-linked candidate gene modifying SUDEP risk, and demonstrate that the complex epilepsy phenotype does not arise solely from 5-HT2C-mediated synaptic disinhibition. These results strengthen the evidence for the serotonin hypothesis of SUDEP risk in humans, and advance current efforts to develop gene-guided interventions to mitigate premature mortality in epilepsy.