Role of histone kinase VprBP in gene silencing: mechanisms, targets, and regulation

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PROJECT SUMMARY The proper regulation of gene expression in human cells is achieved by signals emanating from a distinct histone modification occurring in chromatin architecture. Studies of gene regulation mechanisms by histone modification may thus aid in the understanding and treatment of ailments caused by abnormal transcription regulation. VprBP is a nuclear protein that was originally identified on the basis of its ability to interact with HIV-1 viral protein R. Although VprBP has been studied mainly in connection with Cul4 E3 ubiquitin ligase activity, we recently discovered that VprBP is a transdominant inhibitor of the p53 tumor suppressor and counteracts p53 transactivation, apoptosis, and growth suppression functions. High-level expression of VprBP in a wide range of human tumor samples and cancer cell lines, but low to undetectable expression of VprBP in their normal counterparts, suggests that VprBP possesses oncogenic properties. Our finding that VprBP-depleted cancer cells grow very slow and do not produce tumor xenografts also supports the idea that VprBP facilitates tumorigenesis. Our studies indicated that VprBP interacts with histone H3 tails protruding from nucleosomes and that this interaction facilitates VprBP recruitment and subsequent gene silencing in cancer cells. Unexpectedly, more recent work from our laboratory uncovered the presence of kinase activity specific for threonine 120 (T120) of histone H2A in VprBP. Our functional studies demonstrated that H2A-T120 phosphorylation (H2A-T120p) is essential for VprBP-driven gene silencing in cancer cells. Based on the available evidence, VprBP is the only kinase that is responsible for H2A-T120p occurring in human cancer cells. Importantly, our development of a highly selective inhibitor to manipulate the oncogenic VprBP kinase activity sets the stage for a more detailed analysis of VprBP function in abnormal gene silencing in cancer cells. The long-term goal of the proposed research is to understand the biological processes that VprBP controls and the molecular basis of its action as a mediator of tumorigenesis. The overall objectives are to determine the mechanisms of VprBP-mediated inactivation of the genes that regulate cell proliferation and to develop a set of molecular tools for controlling the magnitude of H2A-T120p at VprBP target loci in a precise manner. Our hypothesis is that VprBP establishes and maintains the silent state of key growth regulatory genes by a two-step mechanism wherein it is recruited to target genes via interaction with gene- specific transcription factors, and phosphorylates H2A-T120 as a mark for the recruitment of additional factors involved in gene repression and cell transformation. In the first Aim, we will employ the RNA-guided CRISPR-Cas9 system in which we can manipulate H2A-T120p at specific loci, and identify the genes that are directly regulated by VprBP-mediated H2AT-120p and critical for VprBP-promoted oncogenic events. In the second Aim, we will investigate the molecular mechanisms underlying the role of H2A-T120p in maintaining the silent state of target genes by identifying and characterizing factors that selectively recognize H2A-T120p. In the third Aim, we will develop bisubstrate analogue inhibitors with higher potency toward VprBP as novel molecular tools to control H2A-T120p and block growth and proliferation of cancer cells. VprBP-mediated H2A-T120p is clearly a very important new causative mechanism for cancer development, which our lab discovered, and we have developed tools and expertise that put us in the best position to advance research on this critical subject.