The role of neuronal sumoylation in the pathogenesis of the Angelman Syndrome

The Angelman Syndrome (AS) is a rare neurodevelopmental disorder caused by the loss of the maternally expressed UBE3A gene, which encodes an E3 ubiquitin ligase. Although considerable efforts have been put to dissect UBE3A function in the brain, the pathogenic mechanisms remain largely unknown and effective treatments are not available yet. Being an E3 ubiquitin ligase, defective ubiquitination, which may occur in either nuclear or cytosolic compartments, is thought to be a primary mechanism underlying synaptic and circuit dysfunction in AS. Increasing evidence indicates ubiquitination is not only crucial for brain pathophysiology per se, but it also functionally interplays with other post-translational modifications (PTMs). Among them, sumoylation is a ubiquitin-related PTM consisting in the covalent conjugation of Small Ubiquitin-like MOdifier (SUMO) to target proteins, and it shares several structural and functional features with ubiquitination. In the brain, sumoylation is finely regulated during neurodevelopment, where it modulates synaptic and extrasynaptic pathways that are fundamental to neuronal circuit assembly and function. In this project, we test the hypothesis that altered ubiquitination in AS, due to the loss of UBE3A, might lead to aberrant sumoylation, ultimately contributing to disease pathogenesis. To explore this possibility, we assessed the impact of UBE3A ablation on developmental trajectories of nuclear and cytosolic sumoylation using in vivo animal models and human neurons derived from AS individuals or engineered by genome editing. Our data reveal that both SUMO1- and SUMO2/3-conjugation is impaired at early postnatal developmental stages in the nucleus and cytosol of AS cortices. To molecularly dissect the underlying mechanisms, we characterized neuronal sumoylomes using proteomics and found that impaired sumoylation affects proteins involved in RNA metabolism, trafficking and processing, epigenetics and splicing. Finally, we functionally validated these data integrating further proteomics and transcriptomics analyses. Taken together, all these data indicated that sumoylation is unbalanced in AS during early postnatal development, leading to impairments of fundamental pathways for neuronal homeostasis (i.e. epigenetics, RNA splicing and processing). Our findings uncovered a potential novel pathogenic mechanism of AS and provide the rationale to develop new therapeutic strategies to treat patients.