Consistent with our findings, EZH2 mRNA expression was upregulated two fold or more in over 80% of high-grade serous human EOC specimens in the newly released t
tlas (TCGA) serous ovarian cystoadenocarcinoma gene expression database (http//www.cancergenome.nih.gov
gene is located at chromosome 7q36.1. Gene amplification contributes to EZH2 overexpression in several types of cancer (14
). However, TCGA gene copy number analysis indicates that EZH2
gene amplification occurs only in a very small percentage of EOCs (less than 10% specimens show >4 copy of EZH2 gene) (http//www.cancergenome.nih.gov
), suggesting that gene amplification is not a major mechanism that leads to EZH2 upregulation in human EOCs. EZH2 is an E2F target gene (35
). A very recent study showed that VEGF stimulates EZH2 expression in human EOC cells via E2F family members, E2F1 and E2F3 (24
). However, VEGF only stimulates the expression of EZH2 mRNA up to 3 fold (24
), which is far below the level of increase in EZH2 mRNA or protein in human EOC cells compared to cultured pHOSE cells ( and data not shown), suggesting additional mechanisms contribute to EZH2 upregulation in human EOC cells. In the future, we will elucidate additional mechanisms that contribute to EZH2 upregulation in human EOCs.
EZH2 has been demonstrated as a prognostic marker for breast and prostate cancers and positively correlates with disease-free survival and overall survival in those patient populations (11
). In addition, EZH2 overexpression correlates with more advanced disease stages of breast and prostate cancers (11
). However, EZH2 expression was not a prognostic marker in other types of cancers including renal clear cell carcinoma and hepatocellular carcinomas (33
). We showed that there was no significant correlation between EZH2 expression and disease-free or overall survival in high-grade serous EOC patients (). Consistent with our findings, low expression of H3K27Me3 has been demonstrated to be a poor prognostic marker in EOC (37
). In contrast, a very recent study showed that EZH2 expression in either EOC cells or ovarian tumor vasculature is predictive of poor clinical outcome (24
). The basis for the discrepancy between our study and that of Lu et al’s is unclear. A possible reason may be that we correlated EZH2 expression with overall or disease-free survival only in high-grade serous histotype EOCs (), while the study by Lu et al. includes additional histotypes of EOCs (24
We showed that EZH2 expression positively correlated with Ki67 expression in EOCs (). There are conflicting results regarding the prognostic value of Ki67 in ovarian carcinoma (38
). A recent study by Kobel et al. suggests that differences in Ki67 index among different subtypes of EOCs confound the Ki67 survival analysis because nearly all high-grade serous EOCs have a high Ki67 index (43
). In analysis of each individual subtypes, Ki67 is no longer a prognostic marker. Consistent with this, although EZH2 correlates with Ki67 expression (), EZH2 expression was not a prognostic indicator for either overall or disease-free survival in the tested high-grade serous histotype EOC patients ().
Interestingly, when compared to normal ovarian surface epithelium, EZH2 expression is significantly upregulated (up to 23 fold) in ovarian epithelial inclusion cysts (44
), which are thought to be the precursor lesion of a subset of EOC (45
). This suggests that EZH2 overexpression is an early event during EOC development. Although ovarian surface epithelium is thought to be the cell origin of EOC (46
), there are still several histopathology-based theories that differ in their explanations about the origins of EOC (46
). Notably, recent evidence suggests that a proportion of high-grade serous EOC may arise from distal fallopian tube (48
). Therefore, it will be interesting to examine EZH2 expression in normal fallopian tube epithelium.
Multiple genes have been implicated in EZH2 inhibition induced apoptosis. For example, FBXO32 contributes to EZH2 inhibition induced apoptotis in breast cancer cells (45
), and Bim expression has been demonstrated to mediate EZH2 inhibition induced apoptosis in prostate cancer cells (49
). Likewise, E-cadherin, DAB2IP and SLIT2 have all been implicated in mediating increased invasiveness conferred by high levels of EZH2 expression (32
). Further studies are warranted to delineate the molecular mechanisms by which EZH2 overexpression promotes proliferation and invasion of human EOC cells.
In summary, the data reported here show that EZH2 is overexpressed in ~66% of primary human EOCs and its overexpression correlates with a high proliferation index and tumor grade in EOCs. Knockdown of EZH2 inhibits the growth of human EOC cells in vitro and in vivo. EZH2 knockdown induces apoptosis of human EOC cells. In addition, EZH2 knockdown suppresses the invasion of human EOC cells. Further, inhibition of the growth and invasion of human EOC cells induced by EZH2 knockdown correlates with a decrease in the levels of H3K27Me3, suggesting that EZH2 histone methyltransferase activity is critical for its function in human EOC cells. Together, our data imply that EZH2 is a potential target for developing epigenetic modifying therapeutics for EOC.