Mutations in epigenetic regulators are common in relapsed pediatric acute lymphoblastic leukemia (ALL). Here, we uncovered the mechanism underlying the relapse of ALL driven by an activating mutation of the histone methyltransferase (p.E1099K). Using high-throughput drug screening, we found that -mutant cells were specifically resistant to glucocorticoids. Correction of this mutation restored glucocorticoid sensitivity. The transcriptional response to glucocorticoids was blocked in -mutant cells due to depressed glucocorticoid receptor (GR) levels and the failure of glucocorticoids to autoactivate GR expression. Although H3K27me3 was globally decreased by p.E1099K, H3K27me3 accumulated at the (GR) promoter. Pretreatment of p.E1099K cell lines and patient-derived xenograft samples with PRC2 inhibitors reversed glucocorticoid resistance and . PRC2 inhibitors restored autoactivation by glucocorticoids, increasing GR levels and allowing GR binding and activation of proapoptotic genes. These findings suggest a new therapeutic approach to relapsed ALL associated with mutation. SIGNIFICANCE: NSD2 histone methyltransferase mutations observed in relapsed pediatric ALL drove glucocorticoid resistance by repression of the GR and abrogation of GR gene autoactivation due to accumulation of K3K27me3 at its promoter. Pretreatment with PRC2 inhibitors reversed resistance, suggesting a new therapeutic approach to these patients with ALL. .