A comparison of schizophrenia-risk GRIN2A gene variants Q655* and R695Q on brain and monoaminergic and GABAergic neurotransmitter system function

Tse, Joyce and Dawson, Neil and Hawkes, Cheryl (2026) A comparison of schizophrenia-risk GRIN2A gene variants Q655* and R695Q on brain and monoaminergic and GABAergic neurotransmitter system function. Masters thesis, Lancaster University.

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Abstract

Dysfunctional neurotransmitter systems contribute to the pathophysiology of schizophrenia. Mutations in the GRIN2A gene, which encodes the GluN2A subunit of the N methyl-D-aspartate receptor (NMDAR), have been suggested to contribute towards aberrant NMDAR functioning and are associated with an increased risk of schizophrenia. In this study, I investigated the effect of ultra-rare variants in GRIN2A associated with schizophrenia, including the protein truncating Grin2a.Q655* (Grin2a.QX) and missense mutation Grin2a.R695Q (Grin2a.RQ), on brain and neurotransmitter system function. I used 14C-2-deoxyglucose (14C-2DG) functional brain imaging to investigate cerebral metabolism in schizophrenia-related brain regions in Grin2a.QX mutants, under baseline conditions and after challenge with the monoamine neurotransmitter releasing drug d-amphetamine. Quantitative PCR (qPCR) was used to examine mRNA expression levels of genes involved in monoaminergic and GABAergic neurotransmitter systems, and Western blotting used to evaluate protein expression of important receptors implicated in schizophrenia pathophysiology. Gene expression markers of neurogenesis in the hippocampal dentate gyrus (DG) were also characterized. The Grin2a.QX mutation induced hypermetabolism in the DG, but did not alter metabolism in any other brain region characterized, including in subregions of the prefrontal cortex (PFC) and basal ganglia. In the PFC, markers for Sst expressing GABAergic cells were altered in Grin2a.QX mice, but GABAergic cells remained unaltered in the hippocampus. In addition, markers for early radial glial cells (RGL) involved in DG neurogenesis, Zic5 and Glis3, were upregulated, but markers of later stages of DG neuronal maturation were unaltered, in hippocampal samples from Grin2a.QX mice. Interestingly, the metabolic response to d-amphetamine was unaltered in Grin2a.QX mutant mice, suggesting that monoaminergic system function was not grossly altered in these animals. In addition, monoaminergic transporters and receptors involved in the d-amphetamine response were largely found to be unaltered at the mRNA (Htr2a, Vmat2) and protein level (D2R, 5-HT2AR), further suggesting Grin2a.QX mutation has a limited impact on monoaminergic system function. In contrast to Grin2a.QX mutants, the Grin2a.RQ mutants displayed more widely altered GABAergic (Lamp5 and Vip) and serotonergic (Htr2a) gene expression in the PFC, with limited effects seen in the hippocampus. This aligns with previous work showing that the Grin2a.RQ mutation impacted cerebral metabolism in the PFC but not the hippocampus. Overall, this study found that the two Grin2a variants associated with schizophrenia have differential impacts on brain and neurotransmitter system function, despite the mutations being in the same gene. The Grin2a.QX mutation primarily affected the hippocampus, specifically the DG, while the Grin2a.RQ mutation affected the PFC. Further work is needed to elucidate the mechanisms through which these differential effects occur.