DISSECTING THE ROLE OF LYSINE-SPECIFIC DEMETHYLASE1 (LSD1): IDENTIFICATION OF MARKERS/EFFECTORS OF SENSITIVITY TO LSD1 INHIBITORS IN CANCER.

Post-translational modification of histone tails plays a critical role in chromatin regulation, gene activity and nuclear architecture. The addition or removal of post-translational modifications from histone tails is fairly dynamic and is achieved by a number of different histone modifying enzymes. Given the fundamental roles of histone modifications in gene regulation and expression, it is not surprising that aberrant patterns of histone marks are found in cancer. Such modifications include histone lysine methylation, which can either promote or repress gene activity depending on the extent of methylation and its context. Histone lysine methylation is maintained by dynamic opposition of methyltransferase and demethylase enzymes, both of which are implicated in normal embryonic development and tumorigenesis. LSD1 is a flavin-containing amine oxidase that, by reducing the cofactor FAD, demethylates H3K4me1/2 and H3K9me1/2 at target loci in a context-dependent manner. LSD1 can act as either a transcriptional co-repressor, as a part of several chromatin complexes such as CoREST and NuRD, or as a co-activator in association with androgen and estrogen receptor. In cancer cells, it has been shown that LSD1 is required for the development and maintenance of acute myeloid leukemia (AML) and cooperate with the oncogenic fusion protein MLL-AF9 to sustain leukemic stem cells (LSCs). LSD1 inhibition impaired the proliferation potential of murine and human AML cells and was accompanied by induction of differentiation. Moreover, LSD1 inhibitors unlocked the ATRA-driven therapeutic response in AML by increasing H3K4me2 level and reactivating the retinoic acid signaling pathway. LSD1 could be an attractive target for cancer therapy because of its deregulation in a number of cancers, including lung, breast, melanoma and hematological malignancies. Despite recent diagnostic and technological improvements, cancer continues to retain its heavyweight status as one of the most challenging diseases to treat. It is a heterogeneous disease that often results in different clinical outcomes for patients with the same affected tissue. And as such, the disparateness of this disease makes it extremely difficult to fight. The ability to anticipate the clinical behavior of cancers is essential in determining the most suitable therapeutic interventions. Considering that cancer is so diverse and clinical outcome predictions often vary from patient to patient, a considerable amount of effort is being invested to discover molecular biomarkers that can categorize cancer patients with distinct clinical outcomes to expand prognostic capabilities. Given the unsatisfactory clinical outcome associated with standard chemotherapy in acute myeloid leukemia (AML) and melanoma treatment, there is an essential need for new targets. Recently LSD1 have gained great interest for their use as anticancer therapeutics. However, the efficacy of LSD1 inhibitors is limited to a substantial subset of cancer cells. Thus, identification of good predictive biomarkers for sensitivity to treatment with LSD1 inhibitors will be of great value in determining the most suitable therapeutic setting. Two lines of evidence have provoked our interest in LSD1. First, LSD1 inhibition impaired the proliferation potential of a subset of solid tumors and AML cells. second, LSD1 inhibitors unlocked the ATRA-driven therapeutic response in AML cells. Our lab, in collaboration with prof. Antonello Mai and prof. Andrea Mattevi, previously developed a new compound working as an LSD1 specific inhibitor, MC2580. By taking advantage of this inhibitor, we have previously shown that LSD1 inhibition sensitizes NB4 cells to retinoic acid (RA) treatment and induces cell growth arrest and differentiation when combined with a physiological concentration of RA (RA low). Starting from these observations, we hypothesized that LSD1 inhibition sensitize UF1 cells, that were established from a patient who was clinically resistant to RA treatment and harbor a point mutation in ligand binding domain (LBD) of RAR\u3b1 moiety. Surprisingly LSD1 inhibition in UF1 cells led to cell growth inhibition, induced cell differentiation and promoted G1 phase arrest, as a single agent. we performed a genome-wide expression analysis comparing gene expression profiling of the two cell lines (NB4 vs UF1) which differently response to LSD1 inhibitor, before and after MC treatment. We found that p21 highly expressed in UF1 cells and MC-treatment led to further upregulation of p21 in UF1 cells but not in NB4. High level of p21 in UF1 cells, is consistent with the fact that UF1 cells are in higher percentage in G1 phase and lower growth rate. We also showed that induction of p21 by HDAC inhibitors sensitized resistant cells (NB4) to LSD1 inhibitor which further confirmed our observation. Knockdown of p21, rescued UF1 cells from cell growth inhibition, cell differentiation and G1 phase arrest mediated by LSD1 inhibitor. Similar to APL cells, Knock-down of p21 in non-APL AML, SCLC and melanoma cells, rescued cells from the effects of MC. Furthermore, we observed that p21 by binding to CDK leads to G1 cell cycle arrest and sensitizes resistant cells to LSD1 inhibitor. Given modest efficacy of LSD1 inhibitors against a subset of cancer cells, combination therapy with LSD1 inhibitors will be a critical approach for therapeutic intervention. In this study we showed that forced cell cycle inhibition either with p21 induction by HDAC inhibitors or directly by CDK inhibitors (Palbociclib) presents a promising therapeutic strategy in solid and hematologic cancers. In conclusion: \u2022 Inhibition of LSD1 suppresses G1 to S phase transition and cell proliferation in a p21-dependent manner. \u2022 Loss of p21 enables progression of cell cycle and rescues the LSD1 inhibitor phenotypes. \u2022 P21 provoked by LSD1 inhibitor could serves as a biomarker to verify pharmacological activity and a prognostic tool reflecting responsiveness to LSD1 inhibitor. \u2022 Forced cell cycle inhibition either with p21 induction by HDAC inhibitors or directly by CDK inhibitors sensitized tumor cells to LSD1 inhibition