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Exploring Chemical and Genetic Interventions for SCN2A Neurodevelopmental Disorders using a SCN2A-deficient Mouse Model
Recent advancements in genetics have revealed that SCN2A is one of the leading genes associated with neurodevelopmental disorders including autism spectrum disorder and epilepsy. In particular, loss-of-function and truncation variants account for a majority of cases. As there are no current treatments specific for SCN2A, the neuropharmacogenomics field has strived to further elucidate the role of SCN2A in neurodevelopment to identify intervention targets. Rodent models offer in vivo, pre-clinical insight into the effects of genetic variation on behavior, biochemistry, and electrophysiology as well as the mechanisms on molecular, cellular, and circuitry levels. Due to SCN2A’s critical involvement in the initiation and propagation of action potential neuronal firing early in neurological development, full null homozygous knockout of Scn2a in mice is perinatal lethal. Furthermore, canonical heterozygous knockout of Scn2a in mice does not render phenotypes that recapitulate SCN2A deficiency in humans. Therefore my dissertation aims at developing a mouse model that better parallels the human condition, then using that pre-clinical platform to explore precision medicine.
Using the unconventional strategy of gene trapping, we generated mice with a severe reduction in Scn2a expression, resulting in significant behavioral and electrophysiological differences from neurotypical wild-type mice with full Scn2a expression, but enough residual expression that the Scn2a-deficient mice survived into adulthood. The severely decreased sociability accompanied by increased high and low order repetitive behaviors observed with the Scn2a-deficient mice suggest autism-like phenotypes. In addition, Scn2a-deficient mice also displayed other co-morbidities of neurodevelopmental disorders including atypical innate behavior, increased anxiety, increased sensitivity to stimuli, motor discoordination, and impaired learning and memory. On the electrophysiological level, these mice displayed enhanced intrinsic excitabilities of principal neurons in the prefrontal cortex and striatum, brain regions known to be involved in seizures and social behavior. This increased excitability was autonomous and reversible by the genetic restoration of Scn2a expression in adult mice. Further, RNA-sequencing revealed a downregulation of multiple potassium channels as well as differential expression of glutamate excitatory and GABA inhibitory signaling, which led to the pursuit of targeting these pathways. Indeed, the use of potassium channel openers alleviated the hyperexcitability of Scn2a-deficient neurons, thus supporting the pursuit of these targets.
Since characterization of the Scn2a-deficient mouse model revealed disruption in excitatory and inhibitory pathways, excitatory/inhibitory balance was examined further as a precision medicine target. Increasing Scn2a expression throughout the whole brain by excising the gene trap, as well as specific targeting of the striatum and the neurons that project to it using a retrograde viral vector, rescued social deficits. However the striatum-specific injection did not lead to a social rescue. This shifted the focus to the neurons that project to the striatum such as the medial prefrontal cortex. Using chemogenetics to reduce excitatory signaling in the prelimbic region of the medial prefrontal cortex, we were able to increase the social behavior in Scn2a-deficient mice. Synthesizing the results from the retrograde striatum and prelimbic-specific rescue, the next hypothesis tested was a circuity-level manipulation of the medial prefrontal cortex projections to the striatum. Retrograde control (striatum) of chemogenetics (medial prefrontal cortex) decreased the excitatory signaling in the medial prefrontal cortex neurons that project to the striatum, which also led to improved sociability. On the other side of the excitatory/inhibitory balance, increasing inhibitory signaling through acute exposure to small-molecule GABA receptor positive allosteric modulators, clonazepam and AZD7325, rescued sociability.
This dissertation opens up new avenues of research by supporting the use of a pre-clinical mouse model of Scn2a deficiency to advance the study of underlying mechanisms behind SCN2A-related neurodevelopmental disorders. Although the results of this dissertation need additional validation such as cellular support, the data and results in this dissertation can serve as a guide to further explore excitatory/inhibitory balance as a neuropharmacogenomics precision medicine target to treat SCN2A-related neurodevelopmental disorders.