Enantiomer-specific and paracrine leukemogenicity of mutant IDH metabolite 2-hydroxyglutarate.

Canonical mutations in IDH1 and IDH2 produce high levels of the R-enantiomer of 2-hydroxyglutarate (R-2HG), which is a competitive inhibitor of α-ketoglutarate (αKG)-dependent enzymes and a putative oncometabolite. Mutant IDH1 collaborates with HoxA9 to induce monocytic leukemia in vivo. We used two mouse models and a patient-derived acute myeloid leukemia xenotransplantation (PDX) model to evaluate the in vivo transforming potential of R-2HG, S-2HG and αKG independent of the mutant IDH1 protein. We show that R-2HG, but not S-2HG or αKG, is an oncometabolite in vivo that does not require the mutant IDH1 protein to induce hyperleukocytosis and to accelerate the onset of murine and human leukemia. Thus, circulating R-2HG acts in a paracrine manner and can drive the expansion of many different leukemic and preleukemic clones that may express wild-type IDH1, and therefore can be a driver of clonal evolution and diversity. In addition, we show that the mutant IDH1 protein is a stronger oncogene than R-2HG alone when comparable intracellular R-2HG levels are achieved. We therefore propose R-2HG-independent oncogenic functions of mutant IDH1 that may need to be targeted in addition to R-2HG production to exploit the full therapeutic potential of IDH1 inhibition.

Constitutive IRF8 expression inhibits AML by activation of repressed immune response signaling.

Myeloid differentiation is blocked in acute myeloid leukemia (AML), but the molecular mechanisms are not well characterized. Meningioma 1 (MN1) is overexpressed in AML patients and confers resistance to all-trans retinoic acid-induced differentiation. To understand the role of MN1 as a transcriptional regulator in myeloid differentiation, we fused transcriptional activation (VP16) or repression (M33) domains with MN1 and characterized these cells in vivo. Transcriptional activation of MN1 target genes induced myeloproliferative disease with long latency and differentiation potential to mature neutrophils. A large proportion of differentially expressed genes between leukemic MN1 and differentiation-permissive MN1VP16 cells belonged to the immune response pathway like interferon-response factor (Irf) 8 and Ccl9. As MN1 is a cofactor of MEIS1 and retinoic acid receptor alpha (RARA), we compared chromatin occupancy between these genes. Immune response genes that were upregulated in MN1VP16 cells were co-targeted by MN1 and MEIS1, but not RARA, suggesting that myeloid differentiation is blocked through transcriptional repression of shared target genes of MN1 and MEIS1. Constitutive expression of Irf8 or its target gene Ccl9 identified these genes as potent inhibitors of murine and human leukemias in vivo. Our data show that MN1 prevents activation of the immune response pathway, and suggest restoration of IRF8 signaling as therapeutic target in AML.