Histone H2A Mono-Ubiquitination in Neurodevelopmental Disorders: Molecular Insights from Rare Genetic Variants

De novo dominant ASXL3 frameshift variants are the genetic basis of Bainbridge-Ropers Syndrome (BRS) and syndromic autism spectrum disorder (ASD), emphasizing the importance of this gene in development. ASXL3 is a component of the Polycomb repressive deubiquitination (PR-DUB) complex that is critical for Polycomb-mediated transcriptional repression. PR-DUB catalyzes the removal of ubiquitin from Histone 2A (H2AUb1), a repressive histone modification catalyzed by Polycomb repressive complex 1 (PRC1). The molecular mechanism of H2AUb1 and the cellular processes it regulates during normal development and disease remain largely unexplored. To investigate the role of ASXL3 in development we generated a mouse model that carries a clinically relevant Asxl3 frameshift variant (Asxl3fs). Genetic inactivation of Asxl3 leads to perinatal lethality, multi-organ developmental defects, and increased levels of H2AUb1. Asxl3fs/fs mice exhibit highly penetrant congenital heart defects, primarily hypoplastic right ventricles. Our structural, molecular, and transcriptomic analysis of cardiogenesis revealed that defects in extracellular matrix (ECM) composition and associated signaling underlie the Asxl3fs dependent heart defects. Within the developing cerebral cortex, loss of ASXL3 perturbs the composition of excitatory neurons and fidelity of cortical layer deposition. We carried out single-cell RNA sequencing at three developmental stages during neurogenesis to characterize the cellular composition and transcriptomic changes. The emerging pathogenic model based on analysis of multiple cell types, at sequential developmental timepoints, implicates overactivation of Notch signaling that alters NPCs proliferation and timing of differentiation. These early developmental defects lead to altered composition of excitatory neurons with aberrant expression of proneural genes responsible for layer specificity at later timepoints. Across cortical development, dysregulated genes were enriched for high confidence ASD risk genes, implicating a convergent pathological mechanism. Together our findings underscore the importance of ASXL3 in Polycomb transcriptional repression during development and provide insight into developmental mechanisms altered by ASD risk genes.PhDCellular & Molecular BiologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/169973/1/btmcg_1.pd