Abstract
Objective
Duplication of the maternal chromosome 15q11.2-q13.1 region causes Dup15q syndrome, a highly penetrant neurodevelopmental disorder characterized by severe autism and refractory seizures. Although UBE3A, the gene encoding the ubiquitin ligase E3A, is thought to be the main driver of disease phenotypes, the cellular and molecular mechanisms that contribute to the development of the syndrome are yet to be determined. We previously established the necessity of UBE3A overexpression for the development of cellular phenotypes in human Dup15q neurons, including increased action potential firing and increased inward current density, which prompted us to further investigate sodium channel kinetics.
Methods
We use a Dup15q patient-derived induced pluripotent stem cell line that was CRISPR-edited to remove the supernumerary chromosome and create an isogenic control line. We perform whole-cell patch clamp electrophysiology on Dup15q and corrected control neurons at two time points of in vitro development.
Results
Compared to corrected neurons, Dup15q neurons show increased sodium current density and a depolarizing shift in steady-state inactivation. Moreover, onset of slow inactivation is delayed, and a faster recovery from both fast and slow inactivation processes is observed in Dup15q neurons. A fraction of sodium current in Dup15q neurons (~15%) appears to be resistant to slow inactivation. Not unexpectedly, a higher fraction of persistent sodium current is also observed in Dup15q neurons. These phenotypes were modulated by the anticonvulsant drug rufinamide.
Significance
Sodium channels play a crucial role in the generation of action potentials, and sodium channelopathies have been uncovered in multiple forms of epilepsy. For the first time, our work identifies in Dup15q neurons dysfunctional inactivation kinetics, which have been previously linked to multiple forms of epilepsy. Our work can also guide therapeutic approaches to epileptic seizures in Dup15q patients and emphasize the role of drugs that modulate inactivation kinetics, such as rufinamide.
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