DISSECTING THE ROLE OF HISTONE DEACETYLASE 3 (HDAC3) IN LEUKEMOGENESIS
Creators: name: MEHDIPOUR, PARINAZ
Publisher: name: Universit� degli Studi di Milano
Year: 2016

description: Histone deacetylases (HDACs) are epigenetic enzymes that modulate chromatin structure through the deacetylation of lysine residues in histones, playing a crucial role in cell viability, cell cycle progression and tumorigenesis. Yet the role of individual HDACs in these biological processes remains enigmatic. Inappropriate recruitment of HDACs is involved in the pathogenesis of several forms of leukemia and several lines of evidence point to a role for HDACs in tumor progression, consistent with the anti-proliferative and apoptotic effects of HDAC inhibitors (HDACi). In this regard, HDACs are considered promising targets for development of new molecules for cancer therapy. To date, some HDACi which have a broad antitumor activity and low toxicity towards normal cells, such as Romidepsin (Depsipeptide or FK228) and SAHA have been approved by U.S. food and drug administration (FDA) for the treatment of cutaneous T-cell lymphoma (CTCL). Moreover, HDACi are undergoing clinical trials for the treatment of hematological malignancies as well as for solid tumors. Most of the HDACi available at the moment are not isoform specific, being active on more than one HDAC. Thus, to design more selective HDACi for cancer therapy it is important to elucidate the role of individual HDACs. Acute Promyelocytic Leukemia (APL) is the first model disease in which the involvement of HDACs has been documented. APL is a subtype of Acute Myeloid Leukemia (AML), a cancer of blood and bone marrow, which is characterized by hyperproliferation of immature granulocytes blocked at the promyelocytic stage. It is genetically associated with a chromosomal translocation t(15;17)(q22;q21), which encodes the oncogenic fusion protein PML-RAR? found in more than 90% of APL patients. In murine models of APL that recapitulate the human disease, PML-RAR? induces a ?pre-leukemic? stage with long latency and without an overtly dramatic phenotype before full leukemic transformation. In fact, for this reason it is assumed that in addition to this oncogenic fusion protein, other genetic hits are required for clonal expansion of leukemic blasts. This fusion protein recruits a number of chromatin modifier enzymes such as HDACs and DNA methyltransferases (DNMTs) to the promoter of retinoic acid (RA) target genes and transcriptionally silence them, leading to the myeloid differentiation block. Furthermore, PML-RAR? causes the impairment of p53 pathway by deacetylation and degradation of p53 through the recruitment of HDAC- containing complexes. HDACs from class I (HDACs 1, 2 and 3) have been found associated with PML-RAR? paving the way for the use of HDACi for APL treatment. Recently, a study on APL, which has been conducted by Santoro et al., showed that among class I HDACs, HDAC1 and to a lesser extent HDAC2 have a dual role in APL development and maintenance. In fact, while they behave as oncosuppressors at the early stages, they function as oncogenes in established tumor cells. Since inhibition of HDAC1 and HDAC2 in pre-leukemic stage leads to the acceleration of the disease in murine models of leukemia, it suggests caution in the clinical utility of epidrugs that target any of these two HDAC isoforms. Moreover, it has been shown that the expression of HDAC3, which associates with nuclear hormone corepressor and silencing mediator of retinoid hormone (NCoR/SMRT) complex, is frequently increased in tumors, while Hdac3 downregulation results in reduced proliferation and survival of tumor cells. In view of these observations, in this study we functionally assessed the role of HDAC3 in the development and maintenance of APL. To achieve our goal, we have dissected the role of HDAC3 in two different phases of the disease: pre-leukemic phase and full-established leukemia. The murine model of APL, which we used, is the mCGPR/PR mouse model in which PML-RAR? is expressed under the control of the cathepsin G promoter. The mice show a very long latency (the pre-leukemic phase) associated with high penetrance (more than 90% of the mice develop APL). We characterized the role of HDAC3 through a functional knock-down approach, assessing its impact on cellular differentiation, proliferation and the ability to influence the transplantation of HSCs and APL cells. Indeed, Hdac3-KD in vitro reduced the proliferative potential of both pre-leukemic and full leukemic cells and boosted their differentiation, suggesting that HDAC3 plays the role of an oncogene in APL initiation and progression. These results were not restricted to APL, because lymphoma driven by c-myc overexpression and leukemia driven by MLL-AF9, were both impaired in cell growth upon Hdac3-KD. In vivo, inoculation of Hdac3-KD pre-leukemic cells into lethally irradiated recipient mice or inoculation of Hdac3-KD APL cells into the recipient mice did not result in leukemia development or progression, respectively. These results suggest that HDAC3 can be considered as a target for epidrugs in the treatment of hematological malignancies. Thus, we assessed this hypothesis with the treatment of pre-leukemic and leukemic cells with the HDAC3 selective inhibitor, RGFP966. Indeed, inhibition of HDAC3 enzymatic activity with RGFP966, phenocopied Hdac3-KD phenotypes in pre-leukemic and leukemic cells confirming the putative oncogenic role of HDAC3. In conclusion, my PhD project has expanded our comprehension about the role of HDAC3 in hematological malignancies and is beginning to unravel alternative views on the targets of epidrugs for the treatment of leukemic patients.