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

Abstract

Word Count 222Word Count 3326 ABSTRACT ObjectiveWe identified a novel de novo SCN2A variant (M1879T) associated with infantile-onset epilepsy that responded dramatically to sodium channel blocker antiepileptic drugs. We analyzed the functional and pharmacological consequences of this variant to establish pathogenicity, and to correlate genotype with phenotype and clinical drug response. MethodsThe clinical and genetic features of an infant boy with epilepsy are presented. We investigated the effect of the variant using heterologously expressed recombinant human Na V 1.2 channels.We performed whole-cell patch clamp recording to determine the functional consequences and response to carbamazepine. ResultsThe M1879T variant caused disturbances in channel inactivation including substantially depolarized voltage-dependence of inactivation, slower time course of inactivation, and enhanced resurgent current that collectively represent a gain-of-function. Carbamazepine partially normalized the voltage-dependence of inactivation and produced use-dependent block of the variant channel at high pulsing frequencies. Carbamazepine also suppresses resurgent current conducted by M1879T channels, but this effect was explained primarily by reducing the peak transient current. Molecular modeling suggests that the M1879T variant disrupts contacts with nearby residues in the C-terminal domain of the channel. InterpretationOur study demonstrates the value of conducting functional analyses of SCN2A variants of unknown significance to establish pathogenicity and genotype-phenotype correlations. We also show concordance of in vitro pharmacology using heterologous cells with the drug response observed clinically in a case of SCN2A-associated epilepsy. Adney, et al, page 3Adney, DeKeyser, Abramova, and Thompson declare no conflicts of interest with the work described herein.

Hyperexcitability and Pharmacological Responsiveness of Cortical Neurons Derived from Human iPSCs Carrying Epilepsy-Associated Sodium Channel Nav1.2-L1342P Genetic Variant

“…This type of analysis is informative regarding the specific biophysical properties of a particular variant (Ben-Shalom et al, 2017;Berecki et al, 2018;Mason et al, 2019). Moreover, biophysical analysis can also provide information regarding the "gain-of-function" versus the "loss-of-function" feature of a particular variant of interest (Adney et al, 2020;Thompson et al, 2020). Determining a gain versus loss-of-function is critically important, both for establishing genotype-phenotype correlations and informing clinical practice.…”

Sodium channel Nav1.2-L1342P variant displaying complex biophysical properties renders hyperexcitability of cortical neurons derived from human iPSCs

“…This type of analysis is informative regarding the specific biophysical features of a particular variant (Ben-Shalom et al, 2017;Berecki et al, 2018;Mason et al, 2019). Moreover, this biophysical analysis can also provide information regarding "the gain-of-function" versus the "loss-of-function" feature of that particular variant (Adney et al, 2020) . Determining the gain versus loss-of-function is critically important, both for establishing genotype-phenotype correlations and for informing clinical practice.…”

Channelopathy of Dravet Syndrome and Potential Neuroprotective Effects of Cannabidiol

“…58 GoF variants (such as M1879T or R1882Q) in SCN2A are thought to cause early-onset epilepsy (onset before 3 months of age) by promoting excitability of cortical neurons during the developmental stage when Nav1.2 predominates in the axon initial segment (AIS). 59 However, LoF SCN2A gene mutations for epilepsy are related to late-onset epilepsy. 60 Interactions between genetic variants of SCN2A and KCNQ2 in the mouse and variants of SCN1A and SCN9A in patients provide models of potential genetic modifier effects in the more common human polygenic epilepsies.…”