Targeting PML for therapy in leukemia-initiating cells

DESCRIPTION (provided by applicant): This proposal describes a five-year training program for the development of an academic career in Cancer Stem Cell Biology. I have four-years of experience as a cancer biologist, two-years as a stem cell biologist, and five-years as a hematologist in a university hospital. I am now planning to expand my knowledge, skills and research field to cancer-initiating cells by combining stem cell and cancer research. My goal over the next five years is to become an independent scientist in the field of cancer stem cell biology, especially in leukemia-initiating cells. I am planning to reach my goal by a two-phase plan. The first phase (first 2-3 years) will be characterized by intense bench work to construct the basis of my research projects and to acquire experience in fields that are new to me, such as the characterization of hematological malignancies in clinically relevant mouse models and contribution of clinical trials in the U.S.A. Further, I will develop/establish niche-specific Pml-deleted mouse models for my own research during independent phase. In the first part of the program, I will be mentored by Pier Paolo Pandolfi, PhD, a leading cancer biologist who has great experience in modeling leukemia and solid tumors in vivo and who has trained numerous independent investigators. The program will be enriched by the collaboration of Drs. D.G. Tenen, D.E. Avigan, T. Suda and J. Teruya-Feldstein. The first part of the award will be completed at Beth Israel Deaconess Medical Center, a scientific environment of excellence with the solid clinical resources necessary to promote the success of this Career Development Award. The second phase will be entirely devoted to establishing my career as an independent scientist. One of the main topics in this phase would be detailed functional analysis of Pml in the microenvironment in niche-specific conditional knockout mice. Five years of support are necessary to produce sufficient data, especially in consideration of the mouse models that I am planning to establish, to allow me to pursue an independent career track. Research will focus on the study of the mechanisms regulating quiescence in leukemia-initiating cells (LICs). Specifically, I am aiming at the development of LIC-specific therapy in paradigmatic hematopoietic stem cell (HSC) disease: chronic myeloid leukemia (CML). Maintenance of leukemia has been demonstrated to be dependent upon a small sub-population of cells within the bulk leukemic population that have self-renewal properties and are termed "leukemia-initiating cells" (LICs). LICs share mechanistic properties with regular stem cells including a more quiescent nature, which is thought to mediate their resistance to standard chemotherapy-based treatment. Failure to effectively target LICs can result in disease relapse. Chronic Myelogenous Leukemia (CML) is an extensively studied stem cell disorder in which the LIC pool is not always eradicated by current targeted therapy, leading to disease relapse upon drug discontinuation. I have defined the essential role of PML in the maintenance of CML-initiating-cell, and present a new therapeutic approach for targeting quiescent LICs by pharmacological inhibition of PML. I have demonstrated that expression of the Promyelocytic Leukemia (PML) tumor suppressor is surprisingly high in both regular hematopoietic stem cells (HSCs) and in CML blasts and that loss of PML expression predicts a more favorable outcome in CML. I subsequently demonstrated that PML plays a key role in maintaining the quiescence and self-renewal properties of HSCs/LICs although the exact molecular mechanisms involved are poorly understood. PML is also known to be pharmacologically inhibited by treatment of cells with arsenic, which specifically decreases the stability of the protein. Taking advantage of this, I have used arsenic-mediated ablation of Pml in a mouse model of CML to successfully target LICs. This work has the potential to have a significant impact on treatment and eradication of CML. Importantly, my findings support the notion PML-ablation by arsenic might be an effective tool to render CML-initiating cells more sensitive to anti-tumor therapy. However, the effectiveness of PML targeting both in clinically relevant mouse model and in the clinic needs to be assessed. Therefore, in order to understand the key pathways downstream PML required for LIC maintenance and to translate PML-ablative LIC targeting to the clinic, I propose the following Specific Aims: (1) to further define the mechanisms of PML-dependent cell cycle regulation in LICs;(2) to analyze the effect of PML loss in the interaction between LICs and their niche;(3) to test the efficacy of arsenic-mediated therapy in preclinical models and finally to support a clinical trial of combination arsenic + Dasatinib treatment for CML through in vitro evaluation. The Training Program outlined in this proposal will launch my independent research career. Importantly, accomplishing these aims could greatly improve treatment of patients with CML, possibly allowing discontinuation of therapy after LIC eradication. Further, understanding PML function could provide other therapeutic targets for LIC and cancer stem cell ablation. PUBLIC HEALTH RELEVANCE: I have defined the essential role of PML in the maintenance of CML-initiating-cells, and present a new therapeutic approach for targeting quiescent leukemia initiating cells (LICs) by using arsenic to inhibit the function of PML. My findings support the notion that PML-ablation by arsenic may be an effective tool to render CML-initiating cells more sensitive to anti-tumor therapy. This work has the potential to have a significant impact on the treatment of CML. Tyrosine kinase inhibitors (TKIs) are successful at suppressing the malignant clone in the majority of CML patients. However, the disease recurs upon discontinuation of therapy. In this proposal, I will evaluate the effect of combining arsenic with TKI therapy in a preclinical mouse models and will support a clinical trial, with goal of targeting the LICs in order to result in curative rather than suppressive therapy for this disease. The elucidation of the mechanisms by which PML regulates the maintenance of the LIC pool could in turn lead to novel therapeutic approaches to treat leukemia.