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.