Probing citrate metabolism in tumor and immune cells in diffuse midline gliomas - PROJECT SUMMARY Diffuse midline gliomas (DMGs) are malignant primary brain tumors in children. Patient prognosis is bleak with median survival of ~9 months after initial diagnosis. Tumorigenesis in DMGs is driven by lysine to methionine mutations in histone H3 (H3K27M) that cause epigenetic dysregulation. The aggressive nature of tumor proliferation, the inability of many drugs to cross the blood brain barrier, and the lack of reliable biomarkers of response to therapy are significant hurdles in the development of novel therapies for DMG patients. In addition, the immunologically cold microenvironment that is dominated by tumor-associated macrophages and microglia (TAMs) and devoid of T cells severely limits the efficacy of immunotherapy for DMGs. Glucose metabolism shapes tumor proliferation and anti-tumor immunity. Our studies with patient-derived and syngeneic DMG models indicate that the H3K27M mutation upregulates the rate-limiting glycolytic enzyme phosphoglycerate kinase 1 (PGK1). Concomitantly, glucose metabolism via glycolysis to lactate and via the tricarboxylic acid cycle to citrate are elevated in DMGs. Mechanistic studies indicate that citrate secreted into the microenvironment is converted to acetyl CoA in TAMs and drives expression of the immunosuppressive cytokine transforming growth factor-β. Silencing or inhibiting PGK1 depletes citrate and relieves immunosuppression, thereby enhancing response to immunotherapy in vivo. Deuterium metabolic imaging is a novel, clinical stage method of imaging glucose metabolism in vivo. Our studies indicate that PGK1 inhibition downregulates glycolytic lactate production in DMG-bearing mice, an effect that can be visualized using [6,6’-2H]-glucose at early timepoints when changes cannot be observed by anatomical imaging. Based on these results, we will test the hypothesis that PGK1 inhibition disrupts tumor metabolism (Aim 1), enhances anti-tumor immunity and response to immunotherapy (Aim 2) and that [6,6’-2H]-glucose provides an early readout of DMG response to therapy at 3T (Aim 3). Our application is significant because we will delineate, therapeutically target, and non-invasively monitor metabolic mechanisms of immunosuppression in the DMG tumor microenvironment. Our proposal is innovative because we identify, to the best of our knowledge for the first time, PGK1 as a driver of immunosuppression and a druggable vulnerability in DMGs. Our studies will also validate [6,6’-2H]-glucose as an agent that provides a biologically meaningful readout of DMG response to therapy in vivo. In summary, our proposal will develop an integrated metabolic therapy and imaging strategy that has the potential to improve outcomes and quality of life for children with this devastating form of childhood cancer.
