EZH2 methyltransferase and H3K27 methylation in breast cancer

Abstract

Histone modifications are thought to control the regulation of genetic programs in normal physiology and cancer. Methylation (mono-, di-, and tri-methylation) on histone H3 lysine (K) 27 induces transcriptional repression, and thereby participates in controlling gene expression patterns. Enhancer of zeste (EZH) 2, a methyltransferase and component of the polycomb repressive complex 2 (PRC2), plays an essential role in the epigenetic maintenance of the H3K27me3 repressive chromatin mark. Abnormal EZH2 expression has been associated with various cancers including breast cancer. Here, we discuss the contribution of EZH2 and the PRC2 complex in controlling the H3K27 methylation status and subsequent consequences on genomic instability and the cell cycle in breast cancer cells. We also discuss distinct molecular mechanisms used by EZH2 to suppress BRCA1 functions.

Keywords: BRCA1; Breast cancer; EZH2; H3K27 methylation.

Figure 1

Transcriptional regulation through H3K27 methylation. (A) Transcriptional repression regulated by the polycomb repressive complex 2 (PRC2) including EZH1 or EZH2. PRC2 complexes, which include EED, SUZ12, RbAP46/48 and EZH2, catalyze H3K27 di- and tri-methylation. This in turn leads to a more condensed chromatin state and transcriptional repression. PRC2 complexes containing EZH1 can also catalyze H3K27 methylation, or compact chromatin through other mechanism. G9a has H3K27 methyltransferase activity and also affects gene silencing. (B) Transcriptional activation regulated by UTX and JMJD3. UTX and JMJD3 are demethylases that form complexes with MLL, RbBP5 and WDR5. The removal of methyl groups from H3K27me3 leads to transcriptionally active chromatin.

Figure 2

The biology of EZH2 in breast cancer. EZH2 gene expression is regulated by the HIF, E2F, and RAF/ERK/Elk pathways. Increased levels of EZH2 have been linked to H3K27 trimethylation (H3K27me3) on promoter regions of several genes (RAD51, RUNX3, CDKN1C (p57 ^KIP2), FOXC1, and CDH1 (E-cadherin)) whose expression is decreased. Reduced levels of RAD51 lead to the activation of Raf1/ERK and β-catenin signaling. Transcription of the CDKN1A gene is under RUNX3 control and lower concentrations of RUNX3 result in lower p21^WAF/Cip1 levels, which fail to fully block the cell cycle. The CDKN1C gene encodes the cyclin-dependent kinase inhibitor p57^KIP2, a strong inhibitor of cyclinE1 and CDK2 complexes. Expression of the CDKN1C gene is regulated through H3K27me3 of its promoter. Reduction of p57^KIP2 leads to accelerated G1-S transition, which has been suggested to support breast cancer progression. Accumulation of H3K27me3 in the promoters of the FOXC1 gene, a fork head family transcription factor used in developmental programs, and the CDH1 gene, that encodes E-cadherin, has been observed in breast cancer cell lines. A decrease of FOXC1 and E-cadherin would be in agreement with enhanced cell invasion and metastasis.

Figure 3

Molecular interplay between EZH2 and BRCA1 in basal like breast cancer. In ER-negative breast cancer cells, elevated EZH2 concentrations block the phosphorylation of BRCA1 (Ser1423). Once the tyrosine phosphatase Cdc25C is phosphorylated, it dephosphorylates the cyclinB-bound Cdc2 (CDK1) and thereby triggers entry into mitosis. Accumulation of Cdc2 and cyclinB1 results in increased cell proliferation. In addition to its canonical function as a H3K27 methyltransferase, EZH2 most likely also has non-canonical functions. EZH2 can phosphorylate Akt-1 (Ser473), but not Akt-2 and -3, and interact with Akt-1. It leads to reduced nuclear retention of BRCA1. In breast cancer cell lines this leads to increased centrosome numbers and genome instability. The EZH2 inhibitor DZNep is more effective on BRCA1-deficient cells, and can thus serve as one arm in a therapeutic regimen.