The Mixed Lineage Leukemia (MLL) gene is essential for embryonic hematopoietic stem cell (HSC) development, but its role during adult hematopoiesis is unknown. Using an inducible knockout model, we demonstrate that Mll is essential for the maintenance of adult HSCs and progenitors, with fatal bone marrow failure occurring within 3 weeks of Mll deletion. Mll-deficient cells are selectively lost from mixed bone marrow chimeras, demonstrating their failure to self-renew even in an intact bone marrow environment. Surprisingly, HSCs lacking Mll exhibit ectopic cell-cycle entry, resulting in the depletion of quiescent HSCs. In contrast, Mll deletion in myelo-erythroid progenitors results in reduced proliferation and reduced response to cytokine-induced cell-cycle entry. Committed lymphoid and myeloid cells no longer require Mll, defining the early multipotent stages of hematopoiesis as Mll dependent. These studies demonstrate that Mll plays selective and independent roles within the hematopoietic system, maintaining quiescence in HSCs and promoting proliferation in progenitors.

SUMMARYDespite correlations between histone methyltransferase (HMT) activity and gene regulation, direct evidence that HMT activity is responsible for gene activation is sparse. We address the role of the HMT activity for MLL1, a histone H3 lysine 4 (H3K4) methyltransferase critical for maintaining hematopoietic stem cells (HSCs). Here, we show that the SET domain, and thus HMT activity of MLL1, is dispensable for maintaining HSCs and supporting leukemogenesis driven by the MLL-AF9 fusion oncoprotein. Upon Mll1 deletion, histone H4 lysine 16 (H4K16) acetylation is selectively depleted at MLL1 target genes in conjunction with reduced transcription. Surprisingly, inhibition of SIRT1 is sufficient to prevent the loss of H4K16 acetylation and the reduction in MLL1 target gene expression. Thus, recruited MOF activity, and not the intrinsic HMT activity of MLL1, is central for the maintenance of HSC target genes. In addition, this work reveals a role for SIRT1 in opposing MLL1 function.

The histone methyltransferase Mixed Lineage Leukemia (MLL) is essential to maintain hematopoietic stem cells and is a leukemia protooncogene. Although clustered homeobox genes are well-characterized targets of MLL and MLL fusion oncoproteins, the range of Mll -regulated genes in normal hematopoietic cells remains unknown. Here, we identify and characterize part of the Mll -dependent transcriptional network in hematopoietic stem cells with an integrated approach by using conditional loss-of-function models, genomewide expression analyses, chromatin immunoprecipitation, and functional rescue assays. The Mll -dependent transcriptional network extends well beyond the previously appreciated Hox targets, is comprised of many characterized regulators of self-renewal, and contains target genes that are both dependent and independent of the MLL cofactor, Menin. Interestingly, PR-domain containing 16 emerged as a target gene that is uniquely effective at partially rescuing Mll -deficient hematopoietic stem and progenitor cells. This work highlights the tissue-specific nature of regulatory networks under the control of MLL/Trithorax family members and provides insight into the distinctions between the participation of MLL in normal hematopoiesis and in leukemia.

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Objective To investigate the role of acyl-CoA:cholesterol acyltransferase 1 (ACAT1) in hematopoiesis. Approach and Results ACAT1 converts cellular cholesterol to cholesteryl esters for storage in multiple cell types and is a potential drug target for human diseases. In mouse models for atherosclerosis, global Acat1 knockout causes increased lesion size; bone marrow (BM) transplantation experiments suggest that the increased lesion size might be caused by ACAT1 deficiency in macrophages. However, BM contains hematopoietic stem cells (HSCs) which give rise to cells in myeloid and lymphoid lineages; these cell types affect atherosclerosis at various stages. Here, we test the hypothesis that global Acat1-/- may affect hematopoiesis, rather than affecting macrophage function only, and show that Acat1-/- mice contain significantly higher numbers of myeloid cells and other cells than wild type mice. Detailed analysis of BM cells demonstrated that Acat1-/- causes a higher proportion of the stem cell-enriched LSK population (Lin-Sca1+ckit+) to proliferate, resulting in higher numbers of myeloid progenitor cells. In addition, we show that Acat1-/- causes higher monocytosis in Apoe-/- mouse during atherosclerosis development. Conclusion ACAT1 plays important roles in hematopoiesis in normal mouse and in Apoe-/- mouse during atherosclerosis development.

Mixed lineage leukemia 1 (MLL1) is a gene that is disrupted by chromosomal translocation characteristically in a large proportion of infant leukemia and also in a fraction of childhood and adult leukemia. MLL1 encodes a chromatin regulatory protein related to the Drosophila Trithorax protein, a well-studied epigenetic factor that functions during development to maintain expression of its target genes. Although tremendous progress has been made understanding the downstream targets of MLL1 fusion oncoproteins and how manipulation of those targets impacts leukemogenesis, very little is known regarding how the initial expression of an MLL1 fusion protein impacts on that cell's behavior, particularly how the cell cycle is affected. Here, we focused on the function of endogenous MLL1 in the stem and progenitor cell types that are likely to be transformed upon MLL1 translocation. Our studies reveal a differential response of stem or progenitor populations to acute loss of MLL1 on proliferation and survival. These data suggest that the effects of MLL1 fusion oncoproteins will initiate the leukemogenic process differentially depending on the differentiation state of the cell type in which the translocation occurs.

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SCI-28 Epigenetic regulation of gene expression plays a central role in normal hematopoietic stem cell (HSC) maintenance and leukemogenesis. The histone methyltransferase, MLL1, is essential for the maintenance of HSCs and is a common target of chromosomal translocations that result in acute leukemia. To discover genetic networks regulated by MLL1 in HSCs, we identified genes that were acutely deregulated upon Mll1 loss in HSCs, using a conditional knockout approach and lineage-negative, c-Kit+, Sca-1+, CD48-negative (LSK/CD48neg) cells. The majority of genes that changed were proliferation-associated genes, upregulated in Mll1−/− LSK/CD48neg cells. This reflected the fact that Mll1-deficient HSCs exhibit increased proliferation in vivo, a phenotype previously documented using the Mx1-cre inducible model. To determine whether the increased proliferation was cell-intrinsic, we performed single cell proliferation studies in serum-free medium containing SCF, IL-11, and Flt3L. We found that Mll1−/− LSK/CD48neg single cells entered the cell cycle earlier and that each cell cycle was shorter than wild-type controls. Evidence for failure to suppress lineage-specific gene expression was also observed; specifically, five percent of the upregulated genes encoded erythroid-specific proteins. These included erythroid transcriptional regulators such as GATA1 and KLF1, but also structural proteins such as spectrin, KEL, and EpoR. The relationship between erythroid-lineage genes and Mll1 was unique, since no other lineage-specific programs were upregulated in Mll1−/− LSK/CD48neg cells. Among the genes downregulated upon Mll1 loss, the largest category was comprised of transcriptional regulators, including Mecom, Pbx1, and Prdm16, which are known to control HSC self-renewal and quiescence. As observed in many other tissues, Mll1−/− LSK/CD48neg cells also exhibited reduced Hoxa9 expression. Interestingly, not all identified MLL1 target genes required menin, a cofactor thought to participate in directing MLL1 to particular genomic loci in vivo, and not all targets were Mll1-dependent in nonhematopoietic tissues. Chromatin immunoprecipitation and functional studies suggest that the identified genes act within a series of parallel pathways as direct transcriptional targets of MLL1. Interestingly, reexpression of Prdm16 alone could rescue Mll1-deficient cells from rapid attrition in bone marrow chimeras. Furthermore, Prdm16 corrected the hyperproliferation phenotype of Mll1−/− LSK/CD48neg cells. These data demonstrate that MLL1 coordinately regulates proliferation, lineage-specific gene expression programs, and self-renewal. By elucidating the normal MLL1-dependent transcriptional network within HSCs, we show that this pathway is overlapping but distinguishable from the leukemogenic pathway, suggesting that targeted therapy with minimal side effects on hematopoiesis will be feasible. Disclosures: No relevant conflicts of interest to declare.