Abstracts

Rationale: GRIN2C encodes the 2C subunit of the glutamate ionotropic N-methyl-D-aspartate (NMDA) receptor. The NMDA receptor is a tetramer comprised of subunit 1 with one or more of subunits 2A-D. Pathogenic variants in the NMDA receptor subunit genes GRIN1, GRIN2A, GRIN2B and GRIN2D are associated with epilepsy, including developmental and epileptic encephalopathies (DEEs). GRIN2C, however, has not been linked to epilepsy. Here, we describe three families with a wide spectrum of epilepsies with segregation of putative pathogenic variants in GRIN2C. Methods: We phenotyped a four generation New Zealand family with 12 affected individuals. Gene panel testing did not identify a mutation in the proband with a DEE. Whole genome sequencing was then performed on 4 family members and identified a  putative pathogenic variant in GRIN2C. Sanger sequencing was performed to examine segregation of the variant. Two additional small families with putative pathogenic GRIN2C variants were subsequently identified and segregation undertaken. Results: Three families, each with a different GRIN2C variant, were identified.  Of 12 affected individuals in family 1 the variant, p.Met896Thr, segregated in all 9/9 tested affected individuals. Of 3 affected individuals in family 2 the variant, p.Asn70Lys, was identified in 2/2 tested individuals. In family 3, the variant p.Thr906Arg was identified in 1 affected individual and segregation is ongoing in 3 further  affected individuals. The GRIN2C missense variants, in cytoplasmic or extracellular domains, were all predicted to be damaging by in silico tools and not found in gnomAD. Variants p.Met896Thr and p.Thr906Arg are located in a zone with a low Missense Tolerance Ratio (MTR). Regions with low MTR are under strong purifying selection and consequently where pathogenic variants cluster.Epilepsy phenotypes of the 12 individuals (8 female) from the 3 families with GRIN2C variants included: 4 DEE (age of seizure onset 8-21 months); 3 generalized genetic epilepsies including generalized tonic clonic seizures alone in one and juvenile myoclonic epilepsy in 2 (onset 11, 12 years); 3 focal  epilepsy (onset 6-26 years)  and 2 febrile seizures. Cognitive ability ranged from normal (6/12), borderline intellect (2/12), mild intellectual disability (ID) (1/12), moderate ID (1/12) to severe ID (2/12). EEGs showed focal discharges in 7/12 and generalized discharges in 5/12. MRI in two children with DEE showed increased signal in the globus pallidus and two individuals with focal epilepsy had periventricular and subcortical nodular heterotopia. Conclusions: We report the first association of variants in GRIN2C with epilepsy. A striking phenotypic spectrum emerged ranging from DEEs to febrile seizures, and included generalized and focal epilepsies. GRIN2C joins the genes encoding other glutamatergic NMDA receptor subunits as a cause of seizure disorders. In vitro functional studies of the variants are underway, to understand the mechanism of action underpinning this phenotypic heterogeneity. Funding: Health Research Council of New Zealand, Cure Kids New Zealand, National Health and Medical Research Council of Australia