Anti-cancer therapy: resistance to DNA damage in renewable tissues

At the base of resistance to DNA damage in renewable tissues there is an increment of EZH2 levels, whose turnover is controlled by p53 and MDM2 proteins. In this way, are uncovered the epigenetic mechanisms responsible of cellular sensitivity [1]. In the fight against cancer, radio- and chemotherapy are certainly the most commonly used methods. Despite their success, they could be harmful to healthy cells and, in particular, to renewable tissues that are the most sensitive to DNA damage [2]. The purpose of the Kuser-Abali and colleagues’ work [1] has been designed to unveil an epigenetic mechanism to enhance healthy cells resistance to radiation and antitumoral drug-induced DNA damage. They analyzed three fundamental cellular components: p53, EZH2 and the MDM2/MDMX complex. It’s already known how, in renewable tissues, p53 levels must be downregulated to ensure the cellular proliferation because p53 acts as pro-apoptotic protein and oncosoppressor. For this reason, p53 levels are kept in a range that allows proliferation without interfering with sensitivity to DNA damage. This regulation is mediated by the combined action of MDM2 (Mouse double minute homolog 2) and MDMX (Mouse double minute homolog 4), that are the two chief regulators of p53 [3]. The other key component, EZH2 (Enhancer of Zeste homolog 2), is the catalytic subunit of PRC2 (Polycomb Repressive Complex 2) and it acts as histone-lysine N-methyltransferase enzyme that catalyzes the addition of methyl groups to histone H3 at lysine 27 (H3K27me3). The action of this complex promotes the formation of heterochromatin [4]: many studies demonstrated how modification of chromatin architecture is associated with development of DNA damage resistant phenotype [5].

Thus, p53 is a determinant of sensitivity to cellular damage and, in Kuser-Abali et al.’s study, is demonstrated how MDM2/MDMX-mediated regulation of p53 modulates this sensitivity by controlling EZH2 turnover. To this end, the authors used genetic and pharmacologic approaches to uncover the epigenetic mechanisms behind modulation of sensitivity to DNA damage. For the experiments are used a tamoxifen-inducible Cre knock-in mouse model mdmx C462.  These mice show a single aminoacidic substitution on MDMX which prevents the formation of the complex with MDM2 maintaining, instead, the interaction with p53 [6, 7]. The prediction was that, after the exposure to ionizing radiation (IR) and to administration of doxorubicin in mdmx C462A/wt mice there was a greater p53 induction due to a higher tissue injury compared with the wild-type counterpart. In contrast, western blot analysis demonstrated a smaller increase of p53 in mutants related with a decrease of DNA damage and apoptosis, observable by using the nuclear marker γH2AX and TUNEL assay. Taken together, the data show how p53 responds in a quantitative manner to DNA damage [1].

It’s interesting to note how within these mutant mice there is an increment of chromatin condensation degree and how this effect is related to a higher DNA trimethylation (H3K27me3). Responsible of this histone modification is EZH2, highly expressed in bone marrow, spleen, thymus and duodenum. Observing how the resistance of these mice was related to the elevated methylation degree of the chromatin, the authors analyzed the regulator mechanisms behind EZH2 action. Because EZH2 is a protein involved in transcriptional repression, it must be dynamically regulated to permits the transcriptional activation in proliferative cells. The authors demonstrated that the EZH2 turnover requires the MDM2/MDMX complex formation that acts on it by ubiquitination.