The results of our systematic review showed that RARβ2 methylation in prostate cancer was associated with tumor risk as either detected in urine, serum or tissue by MSP or QMSP. However, the RARβ2 methylation was not associated with increased risk for developing pathological stage or Gleason score of prostate cancer in comparison between RARβ2 methylated bladder cancer patients and unmethylated patients.
Hypermethylations of the RARβ2 gene having been reported in many studies declared that the frequency of RARβ methylation was found to be significantly higher in patients group compared with controls 
. Previous reports also demonstrated that genetic variations of RARβ affect prostate cancer susceptibility
.To further confirm RARβ2 promoter methylation status in prostate cancer patient' s diagnosis, we carried out a meta-analysis of 12 studies involving 777 cases and 404 controls to derive a more precise estimation of the association. The analysis showed that RARβ2 methylation in prostate cancer patients, compared to non-cancer controls, was 17.62 times higher than that in non-cancer people after the trim-and-fill method which further confirmed RARβ2 methylation was a potential risk factor for prostate cancer as detected both in urine, serum and tumor tissues. MSP is a nonquantitative nonfluorometric PCR method to investigate promoter methylation. This method may fail to detect low concentrations of methylated alleles, unlike QMSP which can detect up to 1/1000 methylated alleles
. In this meta-analysis, both MSP and QMSP have the same effect in RASSF1A methylation detection.
However, Carmen Jerónimo's study suggested that RARβ2 methylation levels correlated with pathological tumor stage but not with Gleason score
. R. Dumache's study demonstrated that RARβ2 methylation was correlated not only pathological tumor stage but also Gleason score
. But in Woodson K's study, methylation of RARβ2 was proved to correlate with tumor grade but not pathological stage
. To resolve the conflicting results, we carried out a meta-analysis which indicated that the RARβ2 methylation status did not correlate with either the pathological stage or Gleason score of prostate cancer patients, suggested that inactivation of RARβ2 may be an early event in prostate carcinogenesis.
Early diagnosis of prostate cancer currently relied on trans-rectal ultrasound guided needle biopsy (10 to 12 cores) in men with increased total PSA (greater than 4.0 ng/ml) and/or abnormal DRE findings
. However, 65% to 70% of men with total PSA in the 4.0 to 10.0 ng/ml range had a negative prostate biopsy result
. In addition, more than 20% of men with PSA in the 2.0 to 4.0 ng/ml range were found to have cancer when evaluated by prostate biopsy
. More troublesome, there was no conclusive evidence that screening based on PSA decreases prostate cancer mortality
. Early detection of prostate cancer may be made more effective and efficient as RARβ2 may be an early biomarker in prostate carcinogenesis diagnosis.
Cancer is not a single cell disease. Aberrant cancer cells and their interactive microenvironment are needed for cancer to progress to androgen independence and distant metastasis
. Tumor heterogeneity in methylation patterns may be influenced by response to the microenvironment and local expression of genes, hormones, oxidative stress, or some other factor that can modulate methylation. Tsuyoshi Nakayama'study indicated that three CpG sites (numbers 20 to 22) near the βRARE region were consensus regions of methylation in PCa, which might be critical for the silencing of the gene by blocking access of liganded RAR/RXR heterodimers and other cis-acting transcription factors to their binding sequences
RARb2 might be silenced not only by DNA methylation but also by histone deacetylation. Acetylation and deacetylation on lysine residues of histone amino-terminal tails had profound effects on gene transcription
. The RARβ promoter was under the control of a high-affinity retinoic acid response element itself. Thus, once silencing of RARβ had occurred, the lack of RAR-beta might reinforce the inactive silent state at its own promoter probably promoting methylation as a secondary repression mechanism
. Fuks F's study demonstrated that RARβ methylation-negative cells (LNCaP, PC3, and DU145) were hypo-acetylated at both H3 and H4. Combined TSA and all- trans retinoic acid treatment after 5-azacytidine treatment increases the accumulation of acetylated histones, leading to reactivation of the methylated RARβ promoter and subsequently the expression of RARβ
In conclusion, our meta-analysis suggested that detection of RARβ2 methylation might be a potential biomarker diagnostic tool in prostate cancer. The detection of RARβ2 methylation in urine or serum is a potential non-invasive diagnostic tool in prostate cancer. It is necessary to conduct large sample size studies of the association between RARβ2 methylation and prostate cancer risk, eventually leading to our better understanding.