Inhibition of EZH2 transactivation function sensitizes solid tumors to genotoxic stress

Abstract

Drugs that block the activity of the methyltransferase EZH2 are in clinical development for the treatment of non-Hodgkin lymphomas harboring EZH2 gain-of-function mutations that enhance its polycomb repressive function. We have previously reported that EZH2 can act as a transcriptional activator in castration-resistant prostate cancer (CRPC). Now we show that EZH2 inhibitors can also block the transactivation activity of EZH2 and inhibit the growth of CRPC cells. Gene expression and epigenomics profiling of cells treated with EZH2 inhibitors demonstrated that in addition to derepressing gene expression, these compounds also robustly down-regulate a set of DNA damage repair (DDR) genes, especially those involved in the base excision repair (BER) pathway. Methylation of the pioneer factor FOXA1 by EZH2 contributes to the activation of these genes, and interaction with the transcriptional coactivator P300 via the transactivation domain on EZH2 directly turns on the transcription. In addition, CRISPR-Cas9–mediated knockout screens in the presence of EZH2 inhibitors identified these BER genes as the determinants that underlie the growth-inhibitory effect of EZH2 inhibitors. Interrogation of public data from diverse types of solid tumors expressing wild-type EZH2 demonstrated that expression of DDR genes is significantly correlated with EZH2 dependency and cellular sensitivity to EZH2 inhibitors. Consistent with these findings, treatment of CRPC cells with EZH2 inhibitors dramatically enhances their sensitivity to genotoxic stress. These studies reveal a previously unappreciated mechanism of action of EZH2 inhibitors and provide a mechanistic basis for potential combination cancer therapies.

A cryptic transactivation domain of EZH2 binds AR and AR’s splice variant, promoting oncogene activation and tumorous transformation

“…The above observations highlighted the existence of both classic EZH2:PRC2 sites and non-canonical EZH2-solo targets in prostate cancer, in agreement with previous reports that EZH2 forms interactions with AR, mediating target gene activation instead of repression ( 17 , 18 , 20 , 43 ). Consistently, a significant portion of EZH2-solo sites overlapped binding of AR and/or AR-V7 (Figure 1D ), both expressed in 22Rv1 cells ( 28 ), whereas AR/AR-V7 binding at those canonical PRC2 sites was found to be generally minimal (Figure 1E ).…”

“…The biochemical basis underlying EZH2:AR/variant interaction, however, remains murky. Recently, a cryptic transactivation domain of EZH2 (EZH2 TAD ) was reported to interact directly with cMyc and co-activators (such as p300) for promoting target gene activation ( 23 , 24 , 43 ). We thus queried whether or not EZH2 TAD also interacts with AR and AR variant in prostate cancer.…”

A cryptic transactivation domain of EZH2 binds AR and AR’s splice variant promoting oncogene activation and tumorous transformation

“…The above observations highlighted the existence of both classic EZH2:PRC2 sites and noncanonical EZH2-solo targets in prostate cancer, in agreement with previous reports that EZH2 forms interactions with AR, mediating target gene activation instead of repression (17,18,20,28). Consistently, a significant portion of EZH2-solo sites overlapped binding of AR and/or AR-V7 (Figure 1D), both expressed in 22Rv1 cells (27), whereas AR/AR-V7 binding at those canonical PRC2 sites was found to be generally minimal (Figure 1E).…”

“…The biochemical basis underlying such EZH2:AR/variant interactions, however, remains murky. Recently, a cryptic transactivation domain of EZH2 (EZH2 TAD ) was reported to interact directly with cMyc and coactivators (such as p300) for promoting target gene activation (23,24,28). We thus queried whether or not EZH2 TAD also interacts with AR/variants in prostate cancer.…”

Landscape of immune-related signatures induced by targeting of different epigenetic regulators in melanoma: implications for immunotherapy.

“…The drugs also affected the expression of genes encoding proteolytic enzymes (cathepsins), and their inhibitors (serpins), implicating epigenetic drugs in the regulation of different cancer-related processes, controlled by cathepsins, such as proliferation, angiogenesis, metastasis and invasion. These results extend previous knowledge on the non-immuno-related anti-tumor activities of epigenetic inhibitors, such as reactivation of tumor suppressor genes by demethylating agents [47], promotion of cell death and suppression of angiogenesis by HDAC inhibitors [48], inhibition of proto-oncogenes MYC and BCL2 by BET inhibitors [49] and downregulation of DNA repair genes by EZH2 inhibitors [50].…”