Frequent methylation of estrogen receptor in prostate cancer: Correlation with tumor progression

Prior studies have shown that the estrogen receptor (ER) gene is down-regul ated in prostate cancer, but the mechanism of its inactivation is not known . We hypothesize that inactivation of the ER gene in prostate dancer is thr ough promoter methylation. To test this hypothesis, we investigated the met hylation status of the ER gene in prostate cancer cell lines, prostate canc er, and benign prostatic hyperplasia (BPH) tissues samples using the bisulf ite genomic sequencing method. Our results show that the ER gene promoter w as methylated in 100% (six of six) of the prostate cancer cell lines tested and all were accompanied by loss of ER mRNA expression. Treatment of these cell lines with demethylating agent 5-aza-2'-deoxycytidine restored ER mRN A expression in all of the ER-negative cell lines, In addition, elevated ex pression of DNA methyltransferase mRNA was found in all of the prostate can cer cell lines. Of the prostate tissue samples analyzed, 60% (6 of 10) in t he BPH samples, 80% (8 of 10) in the low-grade cancer samples (grades I and II), and 95% (20 of 21) in the high-grade cancer samples (grades m-V) exhi bited promoter methylation of the ER gene. The overall methylation levels i n the cancer samples were higher than that in the BPH samples. The differen ces between the high-grade cancer samples and BPH samples were significant at all CpG sites. Only at three CpG sites were the differences significant between the low-grade cancer samples and BPH samples. This study presents t he first evidence that ER gene is transcriptionally inactivated by DNA meth ylation in prostate cancer. Our data suggest that ER may be involved in the pathogenesis of prostate cancer, as well as BPH.