One-Carbon Metabolism Inhibition Depletes Purines and Results in Profound and Prolonged Ewing Sarcoma Growth Suppression.

Ewing sarcoma is the second most common primary bone malignancy in adolescents and young adults. Patients who present with localized disease have experienced a steadily improving survival rate over the years, whereas those who present with metastatic disease have the same dismal prognosis as 30 years ago, with long-term survival rates of less than 20%, despite maximal intensification of chemotherapy. Thus, novel treatment approaches are a significant unmet clinical need. Targeting metabolic differences between Ewing sarcoma and normal cells offers a promising approach to improve outcomes for these patients. One-carbon metabolism utilizes serine and folate to generate glycine and tetrahydrofolate-bound one-carbon units required for de novo nucleotide biosynthesis. Elevated expression of several one-carbon metabolism genes is significantly associated with reduced survival in patients with Ewing sarcoma. We show that both genetic inhibition and pharmacologic inhibition of a key enzyme of the mitochondrial arm of the one-carbon metabolic pathway, serine hydroxymethyltransferase 2, lead to substantial inhibition of Ewing sarcoma cell proliferation and colony-forming ability and that this effect is primarily caused by depletion of glycine and one-carbon units required for the synthesis of purine nucleotides. Inhibition of one-carbon metabolism at a different node, using the clinically relevant dihydrofolate reductase inhibitor pralatrexate, similarly yields profound growth inhibition, with depletion of thymidylate and purine nucleotides. Genetic depletion of serine hydroxymethyltransferase 2 dramatically impairs tumor growth in a xenograft model of Ewing sarcoma. Together, these data establish dependence on one-carbon metabolism as a novel and targetable vulnerability of Ewing sarcoma cells, which can be exploited for therapy. SIGNIFICANCE: Using both genetic and pharmacologic approaches, this study identifies Ewing sarcoma's dependence on one-carbon metabolism as a targetable vulnerability that can be effectively harnessed for therapy.