Mechanisms of neuronal pathogenicity in SPTBN1 syndrome

PROJECT SUMMARY βII-spectrin, a key component of the cytoskeleton, forms a submembrane lattice that organizes micron-scale domains of transmembrane proteins. βII-spectrin also facilitates organelle transport and likely provides structural support and flexibility to neurons. Loss of neuronal βII-spectrin in mice causes early postnatal lethality and significant alterations in cortical development and axonal connectivity. Heterozygous variants in SPTBN1, the gene encoding βII-spectrin, cause an early-onset neurodevelopmental syndrome characterized by speech and motor delays, comorbid with intellectual disability, autism spectrum disorder, ADHD, epilepsy, dysmorphisms, and cortical deficits, collectively known as SPTBN1 syndrome. Previous structure-function evaluations of disease variants in heterologous cells and rescue studies in βII-spectrin null murine cortical neurons indicate that pathogenic mechanisms involve βII-spectrin instability and aberrant binding to key molecular partners. My preliminary data shows that missense and nonsense SPTBN1 syndrome variants differentially impact βII- spectrin protein levels and subcellular localization. These findings imply that different groups of βII-spectrin variants impart neuronal dysfunction through divergent mechanisms. However, the molecular pathways through which these variants affect the total βII-spectrin pool and downstream βII-spectrin-dependent processes in human neurons remain unexplored. In this study, I will evaluate the premise that nonsense and missense SPTBN1 syndrome variants disrupt βII- spectrin function in human cortical neurons (hCNs) through molecularly distinct pathological changes in expression and subcellular distribution. I will first study nonsense SPTBN1 variants in hCNs to determine the molecular and functional consequences of pathogenic βII-spectrin haploinsufficiency compared to βII-spectrin heterozygous knockout hCNs (Aim 1). I will also interrogate missense SPTBN1 syndrome variants that distinctly change βII-spectrin levels and distribution, uncover mechanistic basis for these effects, and explore methods to ameliorate βII-spectrin misexpression (Aim 2). These experiments will leverage the power of endogenous expression of disease-linked SPTBN1 variants at heterozygous levels in hCNs to uncover mechanistic insights into SPTBN1 syndrome pathogenesis. Ultimately, this work will guide the future design of therapeutics for SPTBN1 syndrome and other spectrinopathies of the nervous system.