The Role of UBE3A Isoforms in AS-associated Seizure Susceptibility

Abstract Recurring and debilitating seizures affect ~90% of individuals with Angelman syndrome (AS), a neurodevelopmental disorder further characterized by intellectual disability, sleep disruption, and ataxia. These seizures are often refractory to commonly prescribed antiepileptic drugs, whose side effects can exacerbate other AS symptoms. As a result, AS-associated seizures are consistently ranked as one of the most challenging aspects of AS by caretakers and physicians. AS is caused by a loss of the maternally inherited UBE3A allele because the paternally inherited UBE3A allele is epigenetically silenced in neurons, resulting in a complete loss of UBE3A protein in all neurons. Previous preclinical therapeutic strategies and current clinical trials have shown some promise by re-expressing UBE3A in neurons, but these efforts face several delivery and toxicity challenges that may limit their clinical development. Therefore, it is necessary to identify other novel treatment strategies to mitigate seizures in AS. UBE3A is an E3 ubiquitin ligase that targets substrates for proteasomal degradation and consists of two main isoforms – short-UBE3A and long-UBE3A. Dysregulation of UBE3A isoform substrates underlies AS symptoms. Indeed, each isoform may have distinct cellular and phenotypic contributions to AS symptoms due to their unique N-termini, protein levels, and subcellular localizations. However, how each UBE3A isoform and their respective substrates contributes to AS epileptogenesis remains unclear. Such knowledge could suggest novel therapeutic avenues for treating refractory AS-associated epileptogenesis. Accordingly, in the current proposal I will use two seizure models to establish the contribution of each UBE3A isoform to AS seizure susceptibility (Aim 1), and I will use a proximity ligation assay to define isoform-specific UBE3A substrates in a cell type that drives seizure susceptibility in AS model mice (Aim 2). My work will thus identify the contribution of each UBE3A isoform and their respective substrates to epilepsy in AS, enabling future pharmacological targeting that may overcome the limitations of current therapeutic efforts.