Decreased expression of EZH2 is associated with ESR1 upregulation and response to anti-estrogens.

Abstract

Abstract Abstract #6127 Background: In prostate and breast cancer high levels of Enhancer of Zeste Homolog 2 (EZH2) are associated with tumor progression. EZH2, a histone H3 methyl transferase, is part of the polycomb complex. Together with histone deacetylases (HDACs) EZH2 regulates “genome wide” gene transcription silencing. The aims of this study are a) to correlate EZH2 expression with endocrine therapy response in patients with recurrent disease treated with first-line tamoxifen monotherapy and b) to determine the role of EZH2 in endocrine therapy response using in vitro cell line models.
 Material and Methods: EZH2 mRNA levels were measured with quantitative real-time PCR (qRT-PCR) in 297 retrospectively collected hormone receptor positive (HR+) primary breast tumor specimens of patients with recurrent disease who did respond (N=110) or were resistant (N=187) to first-line tamoxifen monotherapy. In vitro, EZH2 and estrogen receptor (ESR1) expression was downregulated with siRNAs in the human breast cancer cell line MCF7 and assessed for their sensitivity to the selective estrogen receptor degrader ICI164.384 after 96hrs treatment (N=3). To establish therapy response in vitro, cell number counts were determined. All p-values are two-sided and significant if P<0.05.
 Results: In 297 HR+ breast tumors, EZH2 as continuous variable, associated significantly with poor response (OR= 0.67 [0.49-0.91]; P=0.001) and a shorter progression-free survival (PFS) (HR=1.28 [1.11-1.47], P<0.001). In the multivariate model with traditional predictive factors, the tertile with highest EZH2 levels was independently related with response (OR= 0.50 [0.26-0.98]; P=0.045) and PFS (HR=1.82 [1.33-2.49], P<0.001).
 In vitro, EZH2 downregulation in MCF7 with siRNAs showed a significant decrease in cell number compared to the mock silenced cell line (40%). Moreover, ICI treatment of EZH2 silenced MCF7 cells resulted in a 70% cell number decrease versus 23% decrease in the controls (P<0.001). In addition, EZH2 downregulation is associated with a twofold upregulation with ESR1 (P<0.001). Conversely, silencing of ESR1 in MCF7 resulted did not alter EZH2 levels but, as expected, increases HER2 protein levels.
 Conclusion: In primary breast tumors, high EZH2 mRNA levels are associated with poor outcomes after tamoxifen therapy. These results suggest that EZH2 may identify patients at risk for tamoxifen therapy failure. In vitro studies show that downregulation of EZH2 inhibits cell proliferation and on the other hand results in upregulation of ESR1 levels. The latter may explain the increased sensitivity to ICI164.384 of EZH2 silenced cells. Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 6127.