GRIN2D Recurrent De Novo Dominant Mutation Causes a Severe Epileptic Encephalopathy Treatable with NMDA Receptor Channel Blockers

Abstract

N-methyl-D-aspartate receptors (NMDARs) are ligand-gated cation channels that mediate excitatory synaptic transmission. Genetic mutations in multiple NMDAR subunits cause various childhood epilepsy syndromes. Here, we report a de novo recurrent heterozygous missense mutation-c.1999G>A (p.Val667Ile)-in a NMDAR gene previously unrecognized to harbor disease-causing mutations, GRIN2D, identified by exome and candidate panel sequencing in two unrelated children with epileptic encephalopathy. The resulting GluN2D p.Val667Ile exchange occurs in the M3 transmembrane domain involved in channel gating. This gain-of-function mutation increases glutamate and glycine potency by 2-fold, increases channel open probability by 6-fold, and reduces receptor sensitivity to endogenous negative modulators such as extracellular protons. Moreover, this mutation prolongs the deactivation time course after glutamate removal, which controls the synaptic time course. Transfection of cultured neurons with human GRIN2D cDNA harboring c.1999G>A leads to dendritic swelling and neuronal cell death, suggestive of excitotoxicity mediated by NMDAR over-activation. Because both individuals' seizures had proven refractory to conventional antiepileptic medications, the sensitivity of mutant NMDARs to FDA-approved NMDAR antagonists was evaluated. Based on these results, oral memantine was administered to both children, with resulting mild to moderate improvement in seizure burden and development. The older proband subsequently developed refractory status epilepticus, with dramatic electroclinical improvement upon treatment with ketamine and magnesium. Overall, these results suggest that NMDAR antagonists can be useful as adjuvant epilepsy therapy in individuals with GRIN2D gain-of-function mutations. This work further demonstrates the value of functionally evaluating a mutation, enabling mechanistic understanding and therapeutic modeling to realize precision medicine for epilepsy.

GRIN2Bencephalopathy: novel findings on phenotype, variant clustering, functional consequences and treatment aspects

“…9 Less frequent findings of GRIN2B encephalopathy include generalised cerebral volume loss, CVI, hyperkinetic movement disorders (dystonia, dyskinesia, chorea) and developmental regression. These features have been repeatedly observed in other GRIN-associated encephalopathies, 451012192627 suggesting a shared phenotypic spectrum, with differences likely reflecting variant class, location and effect in the various NMDAR subunits. However, we did not observe EEG patterns of continuous spikes and waves during sleep or centrotemporal spikes in our GRIN2B cohort, which appears to reflect a milder end of the GRIN spectrum that is so far predominantly associated with GRIN2A .…”

“…[3][4][5][6][7][8][9][10][11][12][13] To delineate the phenotypic spectrum of GRIN2B encephalopathy, we reviewed previously published and newly diagnosed patients with pathogenic/likely pathogenic de novo variants in GRIN2B. We evaluated the functional consequences of 16 variants in Xenopus laevis oocytes, investigated in vitro responses to memantine for six potential gain-of-function variants and aimed for translation of these results into personalised therapeutic approaches.…”

Computationally Discovered Potentiating Role of Glycans on NMDA Receptors

“…Mutation V667I in GluN2D changes EC50 for glycine and glutamate by factors of 1.7 and 1.5, respectively. This mutation was found in patients with epileptic encephalopathy and global developmental delay46. Mutations G815R and F817L in GluN1 change EC50 for glutamate by factors of 4.4 and 4.2, respectively (data for glycine not reported).…”

GRIN1 mutation associated with intellectual disability alters NMDA receptor trafficking and function

“…For example, several GRIN2A mutations have been associated with increased NMDAR-mediated charge transfer, which could increase neuronal excitability and potentially promote neuronal death. 7–13,17,19 The phenotypes associated with this hyperexcitability include disorders along the epilepsy-aphasia spectrum, as well as other epileptic encephalopathies associated with severe neurodevelopmental delays. 5,11,12 To date, many of these pathologic syndromes have been associated with GluN2A or GluN2B.…”