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RUBICAN Report Summary

Project ID: 293438
Funded under: FP7-IDEAS-ERC
Country: Israel

Final Report Summary - RUBICAN (RNF20 and H2B ubiquitination: linking chromatin dynamics, transcriptional control and cancer)

Chromatin modifications are emerging as major regulators of gene expression. Notably, numerous chromatin modifications have been found to be deregulated in cancer, and some of them have been causally related to cancer progression.
In our study, we focused on one particular chromatin modification: monoubiquitylation of Histone H2B (abbreviated as H2Bub1). The main tool through which we manipulated H2Bub1 levels, in cultured cells as well as in mice, is downregulation or overexpression of RNF20, the main E3 ligase enzyme responsible for carrying out this modification. Thus, by depleting RNF20, we can dramatically reduce the cellular levels of H2Bub1 and assess the functional consequences. Employing this approach, we could show that H2Bub1 is tightly coupled with ongoing gene expression, and particularly with the elongation step in RNA synthesis. Importantly, we could show that RNF20 depletion has differential effects on different groups of genes, and the nature of those genes provides insights into its impact on cell biology and cancer. As one example, we found that depletion of RNF20 and H2Bub1 augments the expression of genes that drive inflammatory processes. These experiments were initially performed in cultured human cells, where we could also elaborate the underlying molecular mechanism and show that the levels of H2Bub1, and presumably the chromatin state changes dictated by them, affect differentially the binding of activating versus repressive forms of the NF-kB transcription factor. Importantly, by generating mice that harbor decreased levels of RNF20 and H2Bub1 (RNF20 knockout mice), we could show that such mice are prone to chronic inflammation, and develop a disease that is highly resembling human inflammatory bowel disease (IBD). Furthermore, we showed that this predisposes to accelerated development of colorectal cancer, with pathological features resembling very closely human IBD-associated cancer. Importantly, we obtained evidence suggesting that people with constitutively lower RNF20 levels may be excessively predisposed to chronic inflammatory diseases, and perhaps also to cancers such as colorectal cancer. Thus, levels of H2Bub1 in normal human tissue may be an indicator for risk of chronic inflammation and inflammation-associated cancer.
However, the picture is not that simple, and reduction in H2Bub1 is not always promoting cancer, as predicted by the definition of RNF20 as a putative tumor suppressor. Indeed, we could show that RNF20/H2Bub1 play opposing roles in different subtypes of human breast cancer. While, in agreement with the above designation of a putative tumor suppressor, H2Bub1 levels are indeed reduced in the highly aggressive triple negative/basal-like breast tumors, where lower H2Bub1 correlates with bad prognosis, the opposite is true in the less aggressive but more common estrogen receptor positive/luminal breast tumors. Thus, in the latter type of cancer, high H2Bub1 may actually promote, rather than inhibit, the proliferation of the cancer cells. We further identified the molecular mechanisms that are responsible for these opposite effects in the different cancer subtypes, showing that H2Bub1 downregulation supports pathways that drive basal-like cancer, most notably the NF-kB pathway, whereas H2Bub1 is actually required for optimal activity of the estrogen receptor pathway, which is the key driver of ER+/luminal cancer proliferation.
In an additional arm of this project, we identified particular microRNAs (small regulatory RNAs), specifically members of the so-called Let7 family, as positive regulators of H2B ubiquitination. We further established that this is due to the ability of Let7 microRNAs to reduce the levels of enzymes (deubiquitinases) that remove ubiquitin from H2B. This offers a new explanation to the well documented ability of Let7 microRNAs to act as tumor suppressors, and to the fact that the levels of those microRNAs are frequently reduced in many human cancers.
Being deposited and removed by specific enzymes, H2Bub1 can potentially be targeted for therapy (“druggable”), particularly cancer therapy. We believe that our findings may provide better understanding of what should be attempted in that regard, and under what conditions.

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