The goal of our research is to decipher the cellular and molecular mechanisms governing normal blood function and to unravel how they become deregulated in myeloid malignancies using the mouse as a model system. We are particularly interested in understanding how normal hematopoietic stem cells (HSCs) can become leukemia-initiating stem cells (LCSs), which promote tumor growth and disease recurrence, and to search for ways to prevent their transformation. Current projects in the laboratory study the implication of apoptosis, autophagy, immune regulation, DNA repair mechanisms and differentiation pathways in the maintenance and regulation of normal HSCs and transformed LSCs.
Stress-response mechanisms in HSC function
The hematopoietic system, like other organs in the body, must constantly contend with a broad range of cellular and environmental stresses. HSCs need to respond to such insults in a timely and appropriate manner to preserve their own functionality and genomic integrity, and ensure the continued production of blood cells. Inappropriate stress-response in HSCs can lead to deregulated blood homeostasis and to the development of blood disorders. We are interested in understanding how regulation of apoptosis, autophagy and cellular detoxification machinery normally preserve HSC function and life-long production of blood cells, and how corruption of these stress-response mechanisms contribute to HSC transformation and the development of myeloid malignancies.
Microenvironmental regulations in blood homeostasis
HSCs reside primarily in endosteal regions of the bone marrow within highly regulated niches consisting of multiple cell types, and are expose to a complex milieu of cytokine, growth factor and immune regulators. These microenvironmental cues play essential roles in regulating HSC localization, self-renewal and differentiation properties. We are interested in understanding how: 1) immune regulators control HSC activity and production of myeloid cells; and 2) how development of myeloid malignancies affects specialized stromal populations that are part of the HSC niche in the bone marrow.
Regulatory networks and LSC transformation
A large body of work has shown essential roles for two major categories of molecular networks in the regulation of self-renewal, proliferation and differentiation activity of HSCs: cell intrinsic processes such as cell cycle regulators (i.e., Bmi1, p53, p21) and the PI3-kinase signaling pathway (i.e., ATM, PTEN, mTOR, FoxO), and non-cell autonomous developmental pathways such as TGF-β, Wnt, Hedgehog and Notch. We are interested in understanding how leukemic transformation affect the activity of these molecular circuits. We are currently de-constructing the transcriptional network of HSC-transformed LSCs by exploiting both genome-wide mRNA and miRNA microarray approaches, and chromatin immunoprecipitation (ChIP) followed by high throuput sequencing (ChIP-seq) approaches.
DNA damage response and aging of the blood system
We have recently shown that HSCs are intrinsically vulnerable to mutagenesis following DNA damage. We found that their quiescent cell cycle status restrict them to the use of the error-prone nonhomologous end joining (NHEJ) repair mechanism, which render HSCs susceptible to genomic instability and transformation. This finding provides the beginning of a molecular understanding for why HSCs, despite being protected at the cellular level, are more likely than other blood cells to become transformed and initiate hematological malignancies. We are now interested in understanding how intrinsic DNA damage and the use of mutagenic DNA repair mechanisms contribute to the aging and transformation of the HSC compartment.
Pathways of myeloid differentiation and myeloid malignancies
During steady-state hematopoiesis, self-renewing HSCs give rise to non self-renewing multipotent progenitors (MPPs), which produce balanced levels of lineage-committed lymphoid and myeloid progenitors and homeostatic levels of all mature blood cells. However, this “classical” pathway of HSC differentiation fails to fully explain the massive myeloid expansion observed in myeloid disorders, which usually occurs without an accompanying increase in lymphopoiesis. We are interested in investigating alternative pathways of myeloid differentiation and to identify druggable targets for the treatment of blood disorders such as chronic inflammation and myeloid malignancies.
Flach J, Bakker ST, Mohrin M, Conroy PC, Pietras EM, Reynaud D, Alvarez S, Diolaiti ME, Ugarte F, Forsberg EC, Le Beau MM, Stohr BA, Méndez J, Morrison CG, Passegué E. Replication stress is a potent driver of functional decline in aging hematopoietic stem cells. Nature (in press).
Pietras EM, Lakshminarasimhan R, Techner JM, Fong S, Flach J, Binnewies M, Passegué E. Re-entry into quiescence protects hematopoietic stem cells from the killing effect of chronic exposure to type I interferons. J Exp Med. 2014 Feb 10;211(2):245-62.
Schepers K, Pietras EM, Reynaud D, Flach J, Binnewies M, Garg T, Wagers AJ, Hsiao EC, Passegué E. Myeloproliferative neoplasia remodels the endosteal bone marrow niche into a self-reinforcing leukemic niche. Cell Stem Cell. 2013 Sep 5;13(3):285-99.
Warr M, Binnewie M, Flach J, Reynaud D, Garg T, Malhotra R, Debnath J, Passegué E. FoxO3a directs a protective autophagy program in hematopoietic stem cells. Nature. 2013 Feb 21;494(7437):323-7.
Reynaud D, Pietras E, Barry-Holson K, Mir A, Binnewies M, Jeanne M, Sala-Torra O, Radich JP, Passegué E. IL-6 controls leukemic multipotent progenitor cell fate and contributes to chronic myelogenous leukemia development. Cancer Cell. 2011 Nov 15;20(5):661-73.
Mohrin M, Bourke E, Alexander D, Warr M, Barry-Holson K, LeBeau M, Morrison CG,Passegué E. Hematopoietic stem cell quiescence promotes error prone DNA repair and mutagenesis. Cell Stem Cell. 2010 Aug 6;7(2):174-85.
Santaguida M, Schepers K, King B, Sabnis AJ, Forsberg EC, Attema JL, Braun BS, Passegué E. JunB protects against myeloid malignancies by limiting hematopoietic stem cell proliferation and differentiation without affecting self-renewal. Cancer Cell. 2009 Apr 7;15(4):341-52.