Evaluating the interplay between estrous cyclicity and induced seizure susceptibility inScn2a^K1422Emice

Abstract

Pathogenic variants in SCN2A are associated with a range of neurodevelopmental disorders (NDD). SCN2A-related NDD show wide phenotypic heterogeneity, suggesting that modifying factors must be considered in order to properly elucidate the mechanisms of pathogenic variants. Recently, we characterized neurological phenotypes in a mouse model of the variant SCN2A-p.K1422E. We demonstrated that heterozygous Scn2aK1422Efemale mice showed a distinct, reproducible distribution of flurothyl-induced seizure thresholds. Women with epilepsy often show a cyclical pattern of altered seizure susceptibility during specific phases of the menstrual cycle which can be attributed to fluctuations in hormones and corresponding changes in neurosteroid levels. Rodent models have been used extensively to examine the relationship between the estrous (menstrual) cycle, steroid hormones, and seizure susceptibility. However, the effects of the estrous cycle on seizure susceptibility have not been evaluated in the context of an epilepsy-associated genetic variant. To determine whether the estrous cycle affects susceptibility to flurothyl-induced seizures in Scn2aK1422Efemale mice, estrous cycle monitoring was performed in mice that had undergone ovariectomy (OVX), sham surgery, or no treatment prior to seizure induction. Removing the influence of circulating sex hormones via OVX did not affect the non-unimodal distribution of flurothyl seizure thresholds observed in Scn2aK1422Efemales. Additionally, flurothyl seizure thresholds were not associated with estrous cycle stage in mice that underwent sham surgery or were untreated. These data suggest that variation in Scn2aK1422Eflurothyl seizure threshold is not significantly influenced by the estrous cycle and, by extension, fluctuations in ovarian hormones. Interestingly, untreated Scn2aK1422Efemales showed evidence of disrupted estrous cyclicity, an effect not previously described in a genetic epilepsy model. This unexpected result highlights the importance of considering sex specific effects and the estrous cycle in support of more inclusive biomedical research.

Strain-dependent effects on neurobehavioral and seizure phenotypes inScn2aK1422Emice

“…The infrequent nature of spontaneous seizures in B6.Scn2a E/+ mice motivated our investigation of susceptibility to acute, chemically-induced seizures. Previously, we showed that B6.Scn2a E/+ mice exhibit lower susceptibility to flurothyl-induced GTCS 35,46 . When considering how to approach the question of background strain and seizure susceptibility in the Scn2a K1422E mouse model, we elected to use an alternative seizure induction paradigm (KA).…”

“…Interestingly, a sex-specific effect was observed in B6.Scn2a E/+ mice using the chemoconvulsant flurothyl 35,46 . Another possibility is that this sex-difference is not specific to B6 and is instead being masked in highly resistant F1D2.Scn2a E/+ mice.…”

Neocortical and cerebellar malformations affect flurothyl-induced seizures in female C57BL/6J mice

“…One limitation of our study is that we did not monitor the estrous cycle of our female mice, which could have introduced behavioral variability in our data. However, a recent report also using flurothyl reported no difference in seizure onset time between female mice in proestrus/estrus compared to those in diestrus (Echevarria-Cooper and Kearney, 2023). Thus, future studies could examine the possible interaction between gonadal sex hormones and brain malformations in female C57BL/6J mice by monitoring and testing mice at different stages of the estrous cycle or alternatively by performing ovariectomies and hormone replacement.…”