Global and unique functions of H3K4 methyltransferases in hematopoiesis and leukemia

The mechanisms that control the balance between self-renewing and differentiating hematopoietic stem cell (HSC) divisions are one major interest of our group. Since these processes are often deregulated in leukemia, we study molecular pathways in both normal and leukemic cells to better understand how proliferation, self-renewal and differentiation are disrupted by leukemia-associated fusion oncogenes.

Our lab initially focused on determining functions of the Mixed Lineage Leukemia (MLL/MLL1/KMT2A) gene in normal hematopoiesis due to its discovery as a leukemia proto-oncogene. The human MLL1 gene, located on human chromosome 11q23, is altered by chromosomal translocation frequently in infant acute lymphocytic leukemia (ALL) and therapy-induced acute leukemia, as well as approximately 10% of adult myeloid leukemia. Chromosomal translocations disrupt the MLL1 gene, generally producing chimeric fusion oncoproteins with altered properties relative to the wild-type MLL1. The poor response of patients with MLL translocations to existing therapy motivates us to more deeply understand the role of MLL1 in hematopoiesis and leukemia.

Our research projects extend from the following areas:

Unique roles for the H3K4-methyltransferase MLL1 in hematpoietic development. Using diverse loss- and gain-of-function murine models, we have established that Mll1 is essential for the development of HSCs in the aorta-gonad-mesonephros (AGM) region (Ernst et al. Developmental Cell 2004). Using pluripotent cells, we have shown that MLL1 levels are critical for the emergence of multipotent hematopoietic progenitors from hemogenic endothelium (Ernst et al. Current Biology 2004, Yang et al. Stem Cell Reports 2020). Unique vascular defects observed in germline Mll1 knockout embryos also may contribute to early hematopoietic phenotypes. Hematopoietic-targeted Mll1 deletion illustrated that fetal liver HSCs develop and expand throughout gestation, but fail to acquire functional properties of adult HSCs (Gan et al. Leukmemia 2010). Using these model systems, we are defining MLL1-dependent transcriptional networks, how they support hematopoietic development, how they differ from leukemogenic programs, and whether they can be exploited to alter stem cell functions.

Mll1 function in adult hematopoiesis and mechanisms of gene regulation. Conditional knockout approaches identified HSCs and pre-B cells as cell types in which Mll1 plays an important homeostasis function (Jude et al. Cell Stem Cell 2007, Gan et al. Journal of Immunology 2018). We have employed ChIP-seq/PCR and transcriptome analyses to identify direct target genes of MLL1 and have also determined the role of Menin as a partner in regulating target genes. Our work showed that Menin and MLL1 regulate largely distinct gene sets (Li et al. Blood 2013, Chen et al. Experimental Hematology 2018). Mechanistic studies illustrate that the MLL1 protein complex likely regulates important HSC genes through its association with a histone acetyltransferases, rather than through its own histone methyltransferase activity (Mishra et al. Cell Reports 2014).

Histone methyltransferases as drug targets in leukemia. Although MLL1 does not play a role in initiating or sustaining MLL-fusion oncoprotein initiated leukemia (Chen et al. Cancer Cell 2017, Chen et al. Cell Cycle 2018) its loss can rate-limit MN-1 or Nup98-Hoxa9 initiated AML (Reidel et al. Journal of Clinical Investigation 2016; Xu et al. Cancer Cell 2016), or BCR-ABL initiated B-ALL (Gan et al. Journal of Immunology 2018). These observations suggest that therapeutic targeting of MLL1 may have value in particular settings.  In contrast, we found that the closely related MLL2 (KMT2B) controls several key survival pathways in AML that have nothing to do with MLL-fusion oncoprotein deregulated pathways, making it an attractive drug target in AML generally (Chen et al. Cancer Cell 2017, Chen et al. Experimental Hematology 2018). In addition to understanding specific versus general roles of H3K4me3 modification by particular enzymes, we are interested in the impact of these enzymes/modifications on transcriptional plasticity and evasion from therapy.