PU.1Is Essential For MLL Leukemia And Exploration Of PU.1-Driven Transcriptional Program

MLL fusion proteins, resulting from the chromosomal translocations, directly activate the expression of downstream key genes such as MEIS1, HOXA9to drive an aggressive form of human leukemia. However, it is still poorly understood what additional transcriptional regulators, independent of the MLL fusion pathway, contribute to the development of MLL leukemia.Decreased expression of transcription factor PU.1, a key regulator of hematopoiesis, induces myeloid leukemia in mice, demonstrating a role of PU.1as tumor suppressor. Recent studies, however, have also suggested that PU.1is required for self-renewal capacity of normal hematopoietic stem cells (HSCs). To understand the role of PU.1in acute myeloid leukemia (AML), we explored expression profiles of AML patients with distinct genetic alterations in public databases. PU.1expression level in MLL leukemia patients was significantly higher than that of other subgroups of AML. Therefore, PU.1could act as an oncogene in the development of specific type of myeloid leukemia. Indeed, we found that PU.1is required for the growth of MLL leukemic cells via promoting cell cycle progression and inhibition of apoptosis. Importantly, PU.1expression is not under the control of MLL fusion proteins. We further identified a PU.1-governed gene signature which contains key regulators in the MEIS-HOX program (MEIS1, PBX3, FLT3, and c-KIT). PU.1directly binds to the genomic loci of its target genes in vivo, and is required to maintain active expression of those genes in both normal hematopoietic stem and progenitors and MLL leukemia. Besides, through a macrophage-specific cytokine, AIF1, PU.1also maintains an active NF-κB inflammatory pathway, providing an antiapoptotic effect for leukemic cells. Finally, the clinical significance of the identified PU.1signature was shown by its ability to predict survival in AML patients.MEIS1is a key factor in MLL leukemia pathogenesis.Through the whole genome ChIP-seq in MLL-ENL leukemic cells, we identified that MEIS1is downstream target of transcription factor PU.1.Mechanismly, we found that the activation of MEIS1by PU.1is not through the binding on promoter of MEIS1, but two distal intronic enhancers. By collaborating with other enhancer asscociated factors such as AP-1, PU.1recruited histone methytransferases to mediate the transition from poised enhancers to active enhancers. Though the details of this process were not fully understood, the binding of PU.1on these two enhancers was strongly associated with the H3K4mel/2modification.Together, our study demonstrated that PU.1independent of the MLL fusion protein, activates the MEIS/HOX and the cytokine AIF1-mediated downstream inflammatory pathways to contribute to the development of MLL leukemia. Our study links the PU.1-controlled monocyte-specific transcriptional program to the pathogenesis of human AMLs. This finding has direct translational implications for targeting the PU.1and/or AIF1-mediated inflammatory pathway as an effective therapy for MLL leukemia.