GRIN2D/GluN2D NMDA receptor: Unique features and its contribution to pediatric developmental and epileptic encephalopathy

Abstract

N-methyl-d-aspartate receptors (NMDARs), a subset of ligand-gated ionotropic glutamate receptors, are critical for learning, memory, and neuronal development. However, when NMDAR subunits are mutated, a host of neuropathological conditions can occur, including epilepsy. Recently, genetic variation within the GRIN2D gene, which encodes the GluN2D subunit of the NMDAR, has been associated with a set of early-onset neurological diseases, notably developmental and epileptic encephalopathy (DEE). Importantly, patients with GRIN2D variants are largely refractory to conventional anti-epileptic drug (AED) treatment, highlighting the need to further understand the distinctive characteristics of GluN2D in neurological and pathological functions. In this review, we first summarize GluN2D's unique spatial and temporal expression patterns, electrophysiological profiles, and contributions to both pre- and postsynaptic signaling. Next, we review thirteen unique case studies from DEE patients harboring ten different causal GRIN2D variants. These patients are highly heterogenous, manifesting multiple seizure types, electroencephalographic recordings, and neurological and developmental outcomes. Lastly, this review concludes by highlighting the difficulty in treating patients with DEE-associated GRIN2D variants, and stresses the need for selective therapeutic agents delivered within a precise time window.

Keywords: Autism; Channelopathy; Developmental and epileptic encephalopathy; Developmental delay; Epilepsy; Glutamate receptor; Intellectual disability; Precision medicine; Translational study.

Figure-1.. Distinct functional features of GluN2D-containing NMDARs.

A, Space-filled ribbon structure of a homology model of human GluN1/GluN2D receptor generated from GluN1/GluN2B crystallographic data (PDB-4PE5; Karakas and Furukawa, 2014; Lee et al., 2014). ATD: amino terminal domain, ABD: agonist-binding domain, TMD: transmembrane domain, comprised by M1, M2, M3, and M4. B, Concentration-response curves for glutamate (in the presence of 30 μM glycine) for GluN1/GluN2A (Blue) and GluN1/GluN2D (Red) generated from two-electrode voltage-clamp current recordings at V_HOLD −40 mV. C, Representative traces of whole cell voltage clamp current recordings in response to 1 mM glutamate from human GluN1/GluN2A- and GluN1/GluN2D-transfected HEK293 cells at 60 mV. D, Representative unitary currents (activated by 1 mM glutamate and 50 μM glycine) from outside-out single channel patch clamp recordings from human GluN1/GluN2A- and GluN1/GluN2D-transfected HEK293 cells at V_HOLD −80 mV. C, close; O, open. H.Yuan, unpublished data.

Figure-2.. Developmental and epileptic encephalopathy (DEE)-associated human GRIN2D/GluN2D missense variants.

A,B, Locations of DEE-associated GRIN variants (indicated by BOLD/PURPLE). S1,S2 comprise the ABD; TMD-link: transmembrane domains (comprised by M1, M2, M3, and M4) and linker regions (S1-M1 linker, M1-M2 linker, M2-M3 linker, M3-S2 linker, and S2-M4 linker); see Fig-1 for domain organization; References are in the text. C, Box-and-whisker plot for the age of onset of 12 patients harboring 9 unique DEE-associated GRIN2D variants. Mean age of onset is 11 ± 3.6 months, with a median age of onset of 6.5 months. Onset data for Patient #8 was not available. D, Different types of electroencephalographic (EEG) patterns were observed from 12 patients harboring 9 unique DEE-associated GRIN2D variants. EEG type was not available for Patient #8. The most common EEG recording was multifocal discharges (5/12), followed by hysparrhymia (4/12), focal spike-and-wave discharges (2/12), and paroxysmal fast activity (1/12). E, Various clinical features observed in DEE-associated GRIN2D variant-containing patients. Besides the unifying phenotypes of DEE (epilepsy and developmental delay/intellectual disability), 69% of patients are hypotonic, 31% of patients display autistic-like features and behaviors, 45% of patients experienced cortical atrophy in brain MRI, and 75% are refractory to typical AED therapy. Magnetic resonance imaging (MRI) data was not available for Patient #4 and Patient #8. Patient #2 has not started any formal drug therapy and was not included in the calculation.