SCN9A Epileptic Encephalopathy Mutations Display a Gain-of-function Phenotype and Distinct Sensitivity to Oxcarbazepine

Abstract

Genetic mutants of voltage-gated sodium channels (VGSCs) are considered to be responsible for the increasing number of epilepsy syndromes. Previous research has indicated that mutations of one of the VGSC genes, SCN9A (Nav1.7), result in febrile seizures and Dravet syndrome in humans. Despite these recent efforts, the electrophysiological basis of SCN9A mutations remains unclear. Here, we performed a genetic screen of patients with febrile seizures and identified a novel missense mutation of SCN9A (W1150R). Electrophysiological characterization of different SCN9A mutants in HEK293T cells, the previously-reported N641Y and K655R variants, as well as the newly-found W1150R variant, revealed that the current density of the W1150R and N641Y variants was significantly larger than that of the wild-type (WT) channel. The time constants of recovery from fast inactivation of the N641Y and K655R variants were markedly lower than in the WT channel. The W1150R variant caused a negative shift of the G-V curve in the voltage dependence of steady-state activation. All mutants displayed persistent currents larger than the WT channel. In addition, we found that oxcarbazepine (OXC), one of the antiepileptic drugs targeting VGSCs, caused a significant shift to more negative potential for the activation and inactivation in WT and mutant channels. OXC-induced inhibition of currents was weaker in the W1150R variant than in the WT. Furthermore, with administering OXC the time constant of the N641Y variant was longer than those of the other two SCN9A mutants. In all, our results indicated that the point mutation W1150R resulted in a novel gainof-function variant. These findings indicated that SCN9A mutants contribute to an increase in seizure, and show distinct sensitivity to OXC.

Trigeminal neuralgia: An overview from pathophysiology to pharmacological treatments

“…100 Furthermore, OXC was also able to inhibit sodium currents of hNav1.7 and caused a hyperpolarizing shift in the activation and inactivation properties of Nav1.7 channels. 101 Thus, although both CBZ and OXC are able to block Nav current, the effect can be achieved with a lower concentration with OXC. 102 It should be noted that the differences in the analgesic properties of CBZ and OXC may in part be related to actions on other voltage-gated ion channels, such as voltage-gated calcium channels (VGCCs), 103,104 and thus, their ability to modulate several neurotransmitter systems involved in pain modulation (for review, see Tomi c et al 105 ).…”

Epilepsy-Related Voltage-Gated Sodium Channelopathies: A Review

“…Pain disorder mutations with GoF are related with diseases such as erythromelalgia (EMI), small-fiber neuropathy (SFN) and paroxysmal extreme pain disorder (PEPD), and mutations with LoF are related with congenital insensitivity to pain (CIP) (Cen et al, 2017). Epilepsy studies such as Zhang S. et al (2020) showed mutations with GoF phenotype: W1150R, N641Y, and K655R mutations ( Table 5). Being that, after treatment with OXC (120 µmol/L), N641Y and K655R reduced sodium current and decreased the opening time of the channel, while W1150R did not alter that (Zhang S. et al, 2020).…”

“…Epilepsy studies such as Zhang S. et al (2020) showed mutations with GoF phenotype: W1150R, N641Y, and K655R mutations ( Table 5). Being that, after treatment with OXC (120 µmol/L), N641Y and K655R reduced sodium current and decreased the opening time of the channel, while W1150R did not alter that (Zhang S. et al, 2020). However, in a study conducted by Yang et al (2018), one of the patients presented generalized tonic-clonic Faster recovery from inactivation More susceptible to clonic and tonic seizures induced by electrical stimulation (mice) Enhanced persistent current (Singh et al, 2009;Zhang S. et al, 2020) I1901fs C-terminal…”

“…Generalized tonic-clonic seizure (Yang et al, 2018) Non genetic origin mutations reported* K655R Focal seizures with secondary generalization High-potential spike activity, paroxysmal release, and d frequency power enhancement (EEG) (Zhang S. et al, 2020) seizure with fever, treated with sodium valproic acid, and a LoF mutation I1901fs was observed (Yang et al, 2018) ( Table 5).…”

“…Variants of NaV1.7 have been related with febrile seizure or GEFS+ (Cen et al, 2017;Zhang S. et al, 2020) and even as asymptomatic (Singh et al, 2009). However, SCN9A can act as a putative modifier of NaV1.1 gene; consequently, it can elevate the severity of patients' phenotype (Guerrini et al, 2010;Parihar and Ganesh, 2013).…”