Polycomb and Cellular Hierarchy in the Brain

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DESCRIPTION (provided by applicant): Gliomas display a hierarchy of differentiation states within the tumor, similar to normal brain development in which stem cells maintain a dynamic balance between the state of self-renewal and differentiation. Indeed, molecular signals that initiate and maintain gliomas commonly overlap with those involved in normal brain development. Stem cell programs can be a critical requirement for cancers to promote their malignancy, which has been an underlying concept of the cancer stem cell (CSC) model. Some cancers may not follow the CSC model and CSCs remain controversial; however, rigorous functional studies from multiple laboratories have identified and characterized CSCs in gliomas. Glioblastoma multiforme (GBM) is the most common and most lethal form of gliomas with current therapy providing only palliation. As GBM stem cells (GSCs) are resistant to conventional therapy, it is important to understand mechanisms involved in GSC maintenance and devise new strategies to target them. We are interested in the Polycomb pathway, as it is a critical epigenetic regulator responsible for embryonic development, fate decision of neural stem cells, and GBM malignancy. Enhancer of Zeste Homolog 2 (EZH2), a histone lysine methyl transferase of Polycomb complex, mediates transcriptional repression of pro-differentiation genes in both normal and neoplastic brain stem cells. We previously showed that EZH2 is preferentially expressed in GSCs and that EZH2-mediated transcriptional silencing promotes GSCs. In preliminary studies, we have identified that the EZH2 protein directly binds to and activates Signal Transducer and Activator of Transcription 3 (STAT3), suggesting a novel role for the Polycomb pathway. The central goals of this proposal are to investigate the hypothesis that EZH2 not only mediates transcriptional silencing but also contributes to the activation of STAT3 signaling in GBM, and to translate this knowledge to the development of new therapeutic approaches. To test this hypothesis, we will first interrogate the molecular mechanisms through which EZH2 activates STAT3, by utilizing a series of biochemical and genetic approaches.)Second, we will evaluate in vivo biological effects of this newly proposed function of EZH2 in human GBM xenografts and mouse gliomas. Finally, we will perform preclinical studies to evaluate the therapeutic efficacy of targeting the EZH2-STAT3 interaction, and identify biomarkers that predict maximal therapeutic responses.)We anticipate that completion of these proposed studies will yield a new paradigm for Polycomb biology and a novel therapeutic approach to target STAT3, which may lead to the translation into improved therapies.