The installation, interpretation, and removal of histone post-translational modifications (PTMs) by distinct classes of proteins represents a crucial mode of chromatin regulation. Lysine methylation (Kme) is one of the most abundant and better studied chromatin modifications, and depending upon its location and degree of methylation, can be implicated in both active and repressed chromatin states. Proteins that interpret this mark are therefore crucial signaling nodes, as they often participate in and recruit multi-subunit protein complexes that elicit varying effects on chromatin structure. Polycomb Repressive Complex 1 (PRC1) is one such complex whose Kme reader function is reliant on the chromodomain of a Chromobox (CBX) subunit (CBX2, -4, -6, - 7, and -8). Polycomb-mediated transcriptional repression is essential for establishing and maintaining cellular identity, and deregulation of Polycomb signaling is implicated in many cancers. The development of a peptidomimetic chemical probe for Polycomb chromodomains, UNC3866, is capable of disrupting the surface groove binding mode of the chromodomain, and confirmed that peptide-like ligands are a potent and selective way to target this family of Kme readers. However, UNC3866 displays suboptimal cellular efficacy which has precluded its use as an in vivo probe. Here we report our second-generation inhibitor, UNC4976, which was initially found to display much improved efficacy over UNC3866 in a reporter cell line. Additionally, pulldown studies indicate that UNC4976 interacts with intact PRC1 in cell lysates, and chromatin immunoprecipitation (ChIP) experiments demonstrate displacement of CBX7 from chromatin at known sites of PRC1 occupancy. Current efforts are directed towards employing a nanoparticle-based delivery system to increase cellular and in vivo activity of UNC4976. We will utilize nanoparticle-formulated compound in a mouse xenograft model to assess the ability of UNC4976 to inhibit tumor growth of an ovarian clear cell adenocarcinoma line that is dependent on CBX7 for progression.

Towards the development of an in vivo chemical probe for Polycomb chromodomains

256th National Meeting (2018)

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