One of the most fascinating aspects of the development of multi-cellular organisms is the fidelity with which so many unique cell types are generated during development. For example, hematopoietic stem cells (HSCs) undergo self-renewing divisions in addition to dividing to yield progeny that differentiate into cell types as diverse as red blood cells, platelets, B- and T-lymphocytes and macrophages. The mechanisms that control the balance between self-renewing and differentiating HSCs 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.
Current Ernst lab projects revolve around the function of the Mixed Lineage Leukemia (MLL or MLL1) gene. This gene, located on human chromosome 11q23, is altered by chromosomal translocation in human leukemia. Chromosomal translocations involving the MLL1 locus generally produce chimeric fusion oncoproteins with altered properties relative to the wild-type MLL1. The normal version of the MLL1 protein is essential for HSC maintenance during late fetal development and in adult bone marrow.1 MLL1 normally functions as an epigenetic regulator, modifying chromatin to regulate its downstream target genes. These target genes define the MLL1 molecular pathway and many of them play critical roles in stem cell self-renewal and in leukemogenesis.
Development of hematopoietic stem cells. Our work has shown that the murine Mll1 gene is essential for the development of HSCs in the aorta-gonad-mesonephros region where some of the earliest HSCs are functionally identifiable.2 Fetal liver HSCs lacking Mll1 can develop, but fail to acquire functional properties of adult HSCs.3 Furthermore, unique vascular defects are observed in germline Mll1 knockout embryos. These developmental studies illustrate the critical roles of this pathway in vascular/hematopoietic development, which are being further pursued by identifying cellular functions and molecular pathways regulated by Mll1 during embryogenesis.
Mll1 function in adult hematopoiesis and mechanisms of gene regulation. Using conditional targeted approaches, we identified Mll1-dependent processes within the adult hematopoietic system, which include HSC self-renewal and B-cell differentiation.4,5 We have used ChIP-seq/PCR and expression profiling to identify direct target genes of MLL1 and have also explored the role of Menin as a co-factor for maintaining the expression of these target genes.5,6 Despite identifying some jointly regulated target genes, our work demonstrates that Menin and MLL1 regulate largely distinct gene sets. Furthermore, we have used several genetic models to illustrate that the MLL1 protein complex likely regulates important HSC genes through its association with a histone acetyltransferase, MOF, rather than through its own histone methyltransferase activity. These studies also led to the discovery that Sirt1 can repress MLL1 target genes selectively through de-acetylation of histone H4, lysine 16 around the promoters of MLL1 target genes.7 Ongoing efforts will determine the specificity of these opposing pathways in different cell types and whether their pharmacologic manipulation can influence MLL1 function in diverse tissues.
Our work establishes that a subset of the genes that are naturally regulated by MLL1 in HSCs are hijacked by MLL fusion oncoproteins to result in refractory leukemia. Interestingly some HSC-specific target genes are exempt from the up-regulation of MLL1 target genes such as Hoxa9, Meis1, Evi1 and Eya1. Understanding the distinct molecular pathways that are regulated by MLL1 versus its oncogenic variants will allow us to 1) develop methods to enhance MLL1 activity to expand HSCs and 2) identify targeted therapeutics with specificity toward the oncogenic activity of MLL fusion oncoproteins rather than the HSC self-renewal program.
- Li BE and P Ernst. Two decades of leukemia oncoprotein epistasis: the Mixed Lineage Leukemia paradigm for epigenetic deregulation in leukemia. Experimental Hematology, 2014 Dec;42(12):995-1012.
- Ernst P, Fisher J, Avery W, Wade S, Foy D, and Korsmeyer SJ. Definitive Hematopoiesis Requires the Mixed Lineage Leukemia Gene. Developmental Cell 2004 Mar;6(3):437-43.
- Gan T*, Jude CD*, Zaffuto KM and P Ernst. Developmentally regulated removal of MLL reveals a selective requirement during adult but not fetal hematopoiesis, Leukemia, 2010 Oct;24(10):1732-41. [*equal contribution]
- Jude CD, Climer L, Xu D, Artinger E, Fisher JK, and P Ernst. Unique and independent roles for MLL in adult hematopoietic stem cells and progenitors. Cell Stem Cell, 2007 Sep 13;1(3):324-37.
- Li BE, Gan T, Meyerson M, Rabbitts T and P Ernst. Distinct pathways regulated by Menin and by MLL1 in hematopoietic stem cells and B-cells. Blood, 2013 Sep 19;122(12):2039-46
- Artinger EL, Mishra BP, Zaffuto KM, Li BE, Chung EKY, Moore AW, Chen Y, Cheng C and P Ernst. An MLL-dependent program sustains hematopoiesis. PNAS, 2013 Jul 16;110(29):12000-5.
- Mishra BP, Zaffuto KM, Artinger EL, Org T, Mikkola HKA, Cheng C, Djabali M and P Ernst. The histone methyltransferase activity of MLL1 is dispensable for hematopoiesis and leukemogenesis. Cell Reports, 2014 May 22;7(4):1239-47.