Project summary
Polycomb group (PcG) complexes are multi-protein, evolutionarily conserved epigenetic machineries that
regulate stem cell fate decisions and early development. Mutation and abnormal expression of Polycomb
Repressive Complex 1 (PRC1) subunits are also implicated in human disease. PRC1 is a protein complex that
was classically thought to mediate transcriptional repression of genes. We and others recently showed that
specific PRC1 complexes, defined by the presence of one of the six mammalian PCGF paralogs, can also
facilitate gene transcription in stem and cancer cells to regulate cell fate decisions and oncogenes. How PRC1
is recruited to chromatin and mediates transcriptional activation is poorly understood. We recently demonstrated
that PRC1 and the estrogen receptor alpha (ER¿) co-occupy transcriptionally active genes and enhancers to
regulate proliferation of breast cancer cells. ER¿ is functionally activated by the steroid hormone estrogen. Once
activated, ER¿ binds to chromatin and recruits co-activators to regulate fundamental biological processes such
as development, reproduction, metabolism, and cancer. Our new preliminary data show that estrogen induces
rapid and dynamic recruitment to chromatin of RING1B, the core subunit of all PRC1 complexes, to activate
gene transcription in a PRC1-indepenent fashion. Moreover, we found that R-loop formation and ER¿ might be
important for RING1B recruitment to chromatin, suggesting that RING1B is tethered to chromatin by different
mechanisms. Importantly, RING1B is also required for ER¿ recruitment, gene and enhancer transcriptional
activation as well as chromatin organization. Our preliminary 3D chromatin architecture data also revealed that
RING1B/ER¿-containing enhancers physically interact with estrogen-responsive genes. These results uncover
potential new mechanisms for transcriptional activation mediated by RING1B upon estrogen administration.
Finally, we found that RING1B is overexpressed in metastatic endocrine resistant breast cancer and that
endocrine resistant cells are addicted to RING1B. We hypothesize that RING1B is a novel epigenetic regulator
of estrogen-mediated gene regulation and chromatin architecture, and that RING1B depletion and chemical
inhibition will decrease metastatic endocrine-resistant breast cancer. Here, we propose: to determine how
RING1B is recruited to estrogen-responsive genes (Aim 1), the role of RING1B in enhancer-promoter
interactions during estrogen administration (Aim 2), and the role of RING1B in endocrine resistance (Aim 3). Hi-
C experiments, coupled to genome-wide studies (including gene expression profiles, chromatin accessibility
assays), proteomics, and xenografts and PDX models will be used to address these central questions.
Fundamentally, these findings will expand our understanding of the estrogen response and Polycomb implicated
in broad biological processes including development, cellular metabolism and cancer.