GRIN2A Variants Associated With Idiopathic Generalized Epilepsies.

Xiao-Rong Liu,Si-Mei Lin,Yi-Wu Shi,Xing-Xing Xu,Bing-Mei Li,Bin Tang,Ting-Ting Ye,Wei-Ping Liao,Jian-Hong Luo,Cui-Ying Fan,Yong-Hong Yi,Tao Su

doi:10.3389/fnmol.2021.720984

Abstract

Objective: The objective of this study is to explore the role of GRIN2A gene in idiopathic generalized epilepsies and the potential underlying mechanism for phenotypic variation.Methods: Whole-exome sequencing was performed in a cohort of 88 patients with idiopathic generalized epilepsies. Electro-physiological alterations of the recombinant N-methyl-D-aspartate receptors (NMDARs) containing GluN2A mutants were examined using two-electrode voltage-clamp recordings. The alterations of protein expression were detected by immunofluorescence staining and biotinylation. Previous studies reported that epilepsy related GRIN2A missense mutations were reviewed. The correlation among phenotypes, functional alterations, and molecular locations was analyzed.Results: Three novel heterozygous missense GRIN2A mutations (c.1770A > C/p.K590N, c.2636A > G/p.K879R, and c.3199C > T/p.R1067W) were identified in three unrelated cases. Electrophysiological analysis demonstrated R1067W significantly increased the current density of GluN1/GluN2A NMDARs. Immunofluorescence staining indicated GluN2A mutants had abundant distribution in the membrane and cytoplasm. Western blotting showed the ratios of surface and total expression of the three GluN2A-mutants were significantly increased comparing to the wild type. Further analysis on the reported missense mutations demonstrated that mutations with severe gain-of-function were associated with epileptic encephalopathy, while mutations with mild gain of function were associated with mild phenotypes, suggesting a quantitative correlation between gain-of-function and phenotypic severity. The mutations located around transmembrane domains were more frequently associated with severe phenotypes and absence seizure-related mutations were mostly located in carboxyl-terminal domain, suggesting molecular sub-regional effects.Significance: This study revealed GRIN2A gene was potentially a candidate pathogenic gene of idiopathic generalized epilepsies. The functional quantitative correlation and the molecular sub-regional implication of mutations helped in explaining the relatively mild clinical phenotypes and incomplete penetrance associated with GRIN2A variants.

Highlights

Idiopathic generalized epilepsies (IGEs) (G40.3 in ICD-10 2016, WHO), known as genetic generalized epilepsies (GGE, OMIM# 600669), are a group of self-limited epileptic syndromes characterized by recurring generalized seizures without any underlying anatomic or neurological abnormality (Berrin et al, 2015; Scheffer et al, 2017; Collaborative, 2019)
Mutation c.1770A > C/p.K590N was identified in a case with juvenile myoclonic epilepsy (JME), mutation c.2636A > G/p.K879R in a case with juvenile absence epilepsy (JAE), and mutation c.3199C > T/p.R1067W in two individuals in a family with childhood absence epilepsy (CAE) and unclassified IGE, respectively (Table 1)
The amino acid sequence alignments showed that residues K590, K879, and R1067 were highly conserved across vertebrates (Figure 1C), indicating an important role of these residues in N-methyl-D-aspartate receptor (NMDAR) functions

Summary

Introduction

Idiopathic generalized epilepsies (IGEs) (G40.3 in ICD-10 2016, WHO), known as genetic generalized epilepsies (GGE, OMIM# 600669), are a group of self-limited epileptic syndromes characterized by recurring generalized seizures without any underlying anatomic or neurological abnormality (Berrin et al, 2015; Scheffer et al, 2017; Collaborative, 2019). IGEs were regarded as a group of genetically determined disorders (Mullen and Berkovic, 2018). Exome-based genetic screening studies have demonstrated that over twenty genes were associated with IGEs, such as CACNA1H, CACNB4, CASR, CHD4, CLCN2, EFHC1, GABRD, GABRA1, GABRG2, GABRB3, HCN2, KCC2, KCNMA1, RORB, SCN1A, SLC12A5, SLC2A1, RYR2, and THBS1 (DiFrancesco et al, 2011; Striano et al, 2012; Kahle et al, 2014; Rudolf et al, 2016; Santolini et al, 2017; Wang et al, 2017; Abou El Ella et al, 2018; Li et al, 2018; Yap and Smyth, 2019; Chan et al, 2020; Liu et al, 2021). Variants with incomplete penetrance in IGEs-associated genes are common

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Conclusion