X-linked serotonin 2C receptor is associated with a non-canonical pathway for sudden unexpected death in epilepsy

Abstract

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.

Serotonin receptor expression in hippocampus and temporal cortex of temporal lobe epilepsy patients by postictal generalized electroencephalographic suppression duration

“…12,13 In animal models, elevated serotonin reduced seizure frequency, seizures reduced serotonergic firing, low serotonin or serotonin receptor deletion (5HT1A, 5HT2C, 5HT4, 5HT7) promoted seizures, and 5HT1A overexpression resulted in sporadic autonomic dysfunction and death. [14][15][16][17][18][19][20][21] The 5HT2 receptor family generally facilitates excitatory effects 17 ; thus, 5HT2A antagonists may provide improvement of epilepsy. 10,22 In high-risk SUDEP patients, hippocampal serotonin transporter (SERT) protein expression was increased.…”

“…In temporal lobe epilepsy (TLE) patients, magnetic resonance imaging and positron emission tomographic imaging revealed decreased 5HT1A receptor binding 12,13 . In animal models, elevated serotonin reduced seizure frequency, seizures reduced serotonergic firing, low serotonin or serotonin receptor deletion (5HT1A, 5HT2C, 5HT4, 5HT7) promoted seizures, and 5HT1A overexpression resulted in sporadic autonomic dysfunction and death 14–21 . The 5HT2 receptor family generally facilitates excitatory effects 17 ; thus, 5HT2A antagonists may provide improvement of epilepsy 10,22 .…”

Loss of functional System x-c uncouples aberrant postnatal neurogenesis from epileptogenesis in the hippocampus of Kcna1-KO mice

“…RNAScope was performed according to Massey, et al 50 . Tissue sections containing hippocampus were cryo-sectioned using a CM1950 cryostat (Leica Biosystems, Buffalo Gove, IL) at 15 μm thick sections and mounted onto Thermofisher Superfrost Plus slides, then stored at −80C until further use.…”

Chronic evoked seizures in young pre-symptomatic APP/PS1 mice induce serotonin changes and accelerate onset of Alzheimer’s disease-related neuropathology

“…Primary SUDEP risk factors include frequent generalized tonic-clonic seizures (GTCS), polytherapy, and early onset, long duration of epilepsy history [10][11][12][13] . The etiology of SUDEP is poorly understood but may involve dysregulation of the cardiac, respiratory, and autonomic nervous systems because of chronic seizures 10, [14][15][16] . Traditional preclinical SUDEP models reflect genetic risk-factors associated with epilepsy, including Scn1a +/or KCN1a null models 16 .…”