Multi-scale disease modeling of SCN2A-related epilepsy due to gain-of-function variants

Epilepsy affects up to 1% of the population, and 3 million in the United States alone. A growing proportion of pediatric epilepsies are tied to causative variants in ion channel genes, including the voltage-gated sodium channel gene SCN2A. Next-generation sequencing approaches are readily utilized in the clinic to find an ever-expanding array of variants in many epilepsy-associated genes, including SCN2A. Among SCN2A variants, those that manifest with loss-of-function are associated with severe neurodevelopmental disorders and late-onset epilepsy. On the other hand, gain-of-function SCN2A variants predominantly have a phenotype of early-onset epilepsy. Additionally, patients with epilepsy due to gain-of-function variants benefit more from therapy with sodium channel blocking anti-seizure medications (ASMs). The designation of loss-of-function or gain-of-function is based on biophysical properties of channels expressed in heterologous cells, which is then extrapolated to effects on neuronal excitability. However, these extrapolations of SCN2A variants have not been directly tested in neurons. Additionally, while mouse models of SCN2A haploinsufficiency (loss-of-function) are available, gain-of-function SCN2A variant models are rarely available. In this proposal, I will exploit two different variants of SCN2A that have a convergent clinical phenotype yet disparate biophysical mechanisms. This variants will allow me to define how an epilepsy-associated sodium channel mutation impacts multiple levels: single neurons, synaptic balance, and hippocampal network output. In Aim 1, I will functionally analyze iPSC-derived neurons with the M1879T and E430A variants, defining how the different variants impact excitability and thus converge toward an epileptic phenotype. Furthermore, I will define changes in population firing in excitatory/inhibitory neuron co-culture using a novel approach with an optical voltage reporter. In Aim 2, I will analyze changes in excitability, synaptic signaling, and network output in the hippocampus of genome-edited mice heterozygous for the Scn2a-E430A variant. This project will help shed light on the role of sodium channel gain-of-function variants in excitatory neurons at multiple levels, and identify changes in epileptic circuits underlying early-onset genetic epilepsy. In this proposal, I will develop new expertise in disease modeling with iPSCs, phenotyping of epileptic animal models, and slice-patch electrophysiology. This expertise will enable me to exploit native neuronal model systems in future research investigating the variant-specific contribution to epilepsy severity. The combination of diverse mentors in neuronal physiology and in vivo genetic epilepsy combined with the collaborative institutional environment will provide an excellent opportunity for career development and propel the candidate towards independent research.