Resurgent and Gating Pore Currents Induced byDe Novo SCN2AEpilepsy Mutations

Abstract

Over 150 mutations in the SCN2A gene, which encodes the neuronal Nav1.2 protein, have been implicated in human epilepsy cases. Of these, R1882Q and R853Q are two of the most commonly reported mutations. This study utilized voltage-clamp electrophysiology to characterize the biophysical effects of the R1882Q and R853Q mutations on the hNav1.2 channel, including their effects on resurgent current and gating pore current, which are not typically investigated in the study of Nav1.2 channel mutations. HEK cells transiently transfected with DNA encoding either wild-type (WT) or mutant hNav1.2 revealed that the R1882Q mutation induced a gain-of-function phenotype, including slowed fast inactivation, depolarization of the voltage dependence of inactivation, and increased persistent current. In this model system, the R853Q mutation primarily produced loss-of-function effects, including reduced transient current amplitude and density, hyperpolarization of the voltage dependence of inactivation, and decreased persistent current. The presence of a Navβ4 peptide (KKLITFILKKTREK-OH) in the pipette solution induced resurgent currents, which were increased by the R1882Q mutation and decreased by the R853Q mutation. Further study of the R853Q mutation in Xenopus oocytes indicated a reduced surface expression and revealed a robust gating pore current at negative membrane potentials, a function absent in the WT channel. This not only shows that different epileptogenic point mutations in hNav1.2 have distinct biophysical effects on the channel, but also illustrates that individual mutations can have complex consequences that are difficult to identify using conventional analyses. Distinct mutations may, therefore, require tailored pharmacotherapies in order to eliminate seizures.

Differential Inhibition of Human Nav1.2 Resurgent and Persistent Sodium Currents by Cannabidiol and GS967

“…For example, patients with epilepsy that is driven by SCN2A mutations that enhance persistent and/or resurgent current, such as the R1882Q mutation, may benefit from this compound. This mutation depolarizes the voltage dependence of inactivation, slows inactivation, and enhances both persistent and resurgent currents [12]. CBD will likely selectively inhibit the aberrantly enhanced resurgent currents in this mutant form of the hNav1.2 channel.…”

“…However, the involvement of resurgent currents in the pathogenicity of epilepsy mutations of SCN2A has only recently begun to be investigated. Though there is not yet evidence that the enhancement of resurgent current is a primary pathogenic mechanism of SCN2A epilepsy mutations, one recent study [12] showed that enhancement of Nav1.2-mediated resurgent currents was among the effects of the R1882Q mutation, which has been reported as likely pathogenic in many epilepsy patients, and which was predicted by Berecki et al [46] to enhance neuronal excitability. Kearney et al [47] demonstrated that a mutation that enhances Nav1.2 persistent currents in mouse neurons induces seizures and a strong epileptic phenotype.…”

“…This channel isoform is predominantly expressed in glutamatergic neurons in the brain; and epilepsy mutations in this protein are generally believed to cause a net augmentation of the channel function, leading to hyperexcitability and inappropriate action potential firing. Some of these mutations enhance channel function through the augmentation of resurgent and/or persistent currents (examples of raw data shown in Figure 1) [8][9][10][11][12]. Resurgent currents occur when there is an intracellular blocking molecule present, which binds to the central pore upon depolarization, blocking the transient current and hindering the interaction of the inactivation particle with the central pore.…”

Epilepsy-Related Voltage-Gated Sodium Channelopathies: A Review

“…(Heron et al, 2002;Scalmani et al, 2006;Misra et al, 2008) L1563V DIV Missense BFNIS Increase in neuronal excitability Accelerated recovery from fast inactivation (Heron et al, 2002;Scalmani et al, 2006;Xu et al, 2007;Misra et al, 2008;Berecki et al, Epileptic seizure with pale, atonic periods followed by a spasm-like out-throwing of both arms Predominantly right-sided epileptiform activity (EEG) (Davidsson et al, 2008) *Non genetic origin mutations reported: Mutations described through clinical diagnosis, but the mutation type (Mendelian or de novo) were not reported, mainly due to the lack of parents to perform genotyping and difficulty in contacting the family. Normally, LoF SCN2A gene mutations for epilepsy are related to late-onset epilepsy; however, the mechanism of action is unclear (Mason et al, 2019). In some cases, NaV1.2 seizures are not controlled not even by various antiepileptic drugs, as with the patient described by Syrbe and colleagues (2016).…”

Functional and Pharmacological Evaluation of a NovelSCN2AVariant Linked to Early-onset Epilepsy

“…These experiments were motivated by a prior report indicating that the epilepsy-associated mutation SCN2A-R1882Q affecting a nearby residue had greater resurgent current than WT channels. 14 We recorded resurgent current in WT and M1879T expressing cells elicited by the voltage protocol shown in Figure 3A This difference was evident across several tested voltages with comparisons made using either current density (Figure 3B) or the level of resurgent current measured as a percentage of the peak current elicited at -10 mV (Figure 3C). Enhanced resurgent current exhibited by M1879T channels may contribute to the gain of sodium conductance conferred by this mutation.…”