Polycomb protein histone methyltransferase enhancer of Zeste homologe 2 (EZH2) is frequently overexpressed in human malignancy and is implicated in cancer cell proliferation and invasion. However, it is largely unknown whether EZH2 has a role in modulating DNA damage response. Here, we show that EZH2 is an important determinant of cell fate decision in response to genotoxic stress. EZH2 depletion results in abrogation of both cell cycle G1 and G2/M checkpoints, directing DNA damage response toward predominant apoptosis in both p53-proficient and p53-deficient cancer cells, but not in normal cells. Mechanistically, EZH2 regulates DNA damage response in p53 wild-type cells mainly through transcriptional repression of FBXO32, which binds to and directs p21 for proteasome-mediated degradation, whereas it affects p53-deficient cells through regulating Chk1 activation by a distinct mechanism. Furthermore, pharmacological depletion of EZH2 phenocopies the effects of EZH2 knockdown on cell cycle checkpoints and apoptosis. These data unravel a crucial role of EZH2 in determining the cancer cell outcome following DNA damage and suggest that therapeutic targeting oncogenic EZH2 might serve as a strategy for improving conventional chemotherapy in a given malignancy. Cell Death and Differentiation (2011) 18, 1771-1779; doi:10.1038/cdd.2011.48; published online 6 May 2011Following DNA damage, mammalian cells activate cell cycle checkpoint mechanisms to induce cell cycle arrest and protect cells from apoptosis. 1 The interconnections between the pathways regulating the cell cycle checkpoints and apoptosis dictate the cellular outcome to DNA damage, 2 but whether these pathways are differentially regulated by oncogenic lesions in tumor cells to allow cancer-specific perturbation is poorly understood. Inhibition of key cell cycle checkpoint regulators such as cyclin-dependent kinase inhibitor p21 and Chk1/2 have been shown to increase the sensitivity to DNA damage in p53-proficient or p53-deficient cancer cells, respectively. 3-14 A treatment strategy to simultaneously abrogate both G1 and G2/M checkpoint and thus sensitizing both p53 wild-type and mutant tumors has yet to be developed.Polycomb protein enhancer of Zeste homologe 2 (EZH2) is a histone methyltransferase that is frequently overexpressed in a wide variety of human malignancies, 15,16 and is implicated in cell proliferation, invasion and metastasis. [17][18][19][20][21][22][23][24][25] Mechanistically, the oncogenic function of EZH2 has been attributed to associated histone H3 with trimethylated lysine 27 (H3K27Me3), leading to transcriptional repression of tumor suppressor genes, including p16(INK4a) and p19(ARF), 26 E-cadherin, 19 adrenergic receptor-b2, 27 RUNX3, 20 p57 (also called CDKN1C), 21 Bim 28 and DAB2IP. 23,24 As such, EZH2 is emerging as a crucial regulator of cell fate by affecting multiple signaling pathways. It is not clear, however, whether EZH2 overexpression in cancer cells has a role in affecting cellular response to DNA damage. This study investi...