Abstracts

Rationale: Deletion or mutation of the maternally inherited UBE3A allele, which encodes the ubiquitin protein ligase E3A (UBE3A), results in EEG abnormalities and severe epilepsy which are highly penetrant and often intractable in individuals with Angelman syndrome (AS). Deletion or mutation of the orthologous maternal Ube3a allele in mice is also sufficient to enhance seizure susceptibility. However, in both mouse and man, little is known about the cellular and circuit-level mechanisms of epilepsy in the absence of UBE3A. Such knowledge has hampered rational strategies for preventing epilepsy in individuals with AS. Methods: Here, we characterized both ictogenic and epileptogenic properties in AS model mice and genetically dissected cell type-specific requirements for UBE3A toward the development of balanced neural circuits. Specifically, using the flurothyl kindling model of epileptogenesis, we compared the consequences of selective Ube3a deletion from either glutamatergic or GABAergic neurons, gaining insight into how each neuron type contributes to the development of circuit hyperexcitability in AS. Moreover, by spatio-temporally profiling the post-ictal expression of immediate-early genes, we will trace the specific neural circuits that are most vulnerable to UBE3A loss. Results: Compared to control mice (n=11), AS model (Ube3am-/p+) mice (n=10) exhibited reduced myoclonic and generalized (p < 0.05) seizure threshold. We next tested flurothyl sensitivity in mice with selective loss of maternal Ube3a from either glutamatergic (Ube3aFLOX/p+::NEX-Cre) or GABAergic (Ube3aFLOX/p+::Gad2-Cre) neurons. Ube3aFLOX/p+::Gad2-Cre mice (n=11) exhibited an approximately 30% reduction compared to controls (n=13) in latency to myoclonic and generalized seizures (p < 0.001). In contrast, seizure thresholds were similar between controls (n=12) and Ube3aFLOX/p+::NEX-Cre mice (n=7). Ube3aFLOX/p+::Gad2-Cre mice also exhibited enhanced lethality to repeated (once daily) exposures to flurothyl. Conclusions: Loss of Ube3a in GABAergic neurons renders mice particularly susceptible to flurothyl-induced seizures, while loss of Ube3a in glutamatergic neurons has no effect on seizure susceptibility. This study elucidates cellular mechanism of intractable epilepsy in AS and may reveal therapeutic targets. Funding: American Epilepsy Society Postdoctoral Research Fellowship (2016)