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. Author manuscript; available in PMC: 2011 Sep 6.
Published in final edited form as: Cancer Control. 2010 Oct;17(4):245–255. doi: 10.1177/107327481001700405

Promoter Hypermethylation in Prostate Cancer

Jong Y Park 1
PMCID: PMC3167691  NIHMSID: NIHMS317596  PMID: 20861812

Abstract

Background

The prostate gland is the most common site of cancer and the second leading cause of cancer mortality in American men. It is well known that epigenetic alterations such as DNA methylation within the regulatory (promoter) regions of genes are associated with transcriptional silencing in cancer. Promoter hypermethylation of critical pathway genes could be potential biomarkers and therapeutic targets for prostate cancer.

Methods

This review discusses current information on methylated genes associated with prostate cancer development and progression.

Results

Over 30 genes have been investigated for promoter methylation in prostate cancer. These methylated genes are involved in critical pathways, such as DNA repair, metabolism, and invasion/metastasis. The role of hypermethylated genes in regulation of critical pathways in prostate cancer is reviewed.

Conclusions

These findings may provide new information of the pathogenesis of prostate cancer. Certain epigenetic alterations in prostate tumors are being translated into clinical practice for therapeutic use.

Introduction

Prostate cancer is the most common type of cancer (other than the skin) and the second leading cause of cancer mortality in American men. One man in 6 will develop prostate cancer during his lifetime, and 1 man in 34 will die of the disease.1 In 2010 in the United States, an estimated 217,730 new cases will be diagnosed, and 32,050 men will die of the disease.2 Although prostate cancer can be found early through PSA screening, this test is not 100% accurate, and false-positive results can lead to unnecessary prostate biopsy tests. However, a low mortality rate from prostate cancer suggests that public awareness of early detection and advanced treatments of prostate cancer have begun to affect prostate cancer outcomes.

The probability of developing prostate cancer sharply increases in the sixth decade of life (7%) and further increases after age 70 years (13%). These numbers contrast significantly with the probability of 0.01% among men under 40 years of age and 2.5% among those 40 to 59 years of age.2 The aging of the current population means that the disease will become an even greater public health problem in the future.

In some patients with prostate cancer, the disease progresses relatively slow. In these cases, patients often die with prostate cancer rather than of prostate cancer. However, some cases grow aggressively and metastasize through the bloodstream and the lymphatic system to other parts of the body. There are two important clinical challenges. The first challenge is the early detection of prostate cancer. Currently, digital rectal examination and serum prostate-specific antigen (PSA) screening are two main clinical diagnostic tools. However, due to their limited accuracies, these methods cannot reliably identify early-stage prostate cancer. Therefore, the identification of biomarkers that can facilitate the diagnosis of prostate cancer at the early stages could improve the current standard of treatments. The second challenge is to determine if a patient is presenting with aggressive or indolent prostate cancer. This is critically important information, given the significant morbidity associated with treatment interventions, and could eventually help distinguish men who need intensive treatment from those who may be better served by watchful waiting. Currently, the level of PSA, the clinical stage, and the grade of tumor (Gleason score) are used to estimate prognosis and determine treatment modalities. Although they are useful, additional biomarkers are needed to better predict the outcome of prostate cancer. Therefore, molecular biomarkers should help in determining who may need a prostate biopsy, which treatments a patient will undergo, and who may have a recurrence.

Role of DNA Methylation in Prostate Cancer

Tumorigenesis and progression of prostate cancer are results of the accumulation of genetic and epigenetic alterations. Although genetic changes are involved in the inactivation of genes with important anticancer functions (eg, tumor suppressor and DNA repair genes), DNA methylation in a promoter region is an important epigenetic mechanism for the downregulation (silencing) of expression of these genes. DNA methylation in the promoter region of tumor suppressor genes appears to occur at early stages of carcinogenesis and arises with various frequencies. Therefore, epigenetic changes have the potential to be a new generation of biomarkers. Several types of epigenetic changes have been reported for prostate cancer including DNA hypermethylation, loss of imprinting, and altered histone modification patterns.

DNA methylation in the promoter of a number of genes occurs frequently in prostate tumors but rarely in normal prostate tissues. CpG islands are CpG-rich areas of 200 base-pairs to several kilo bases in length, usually located near the promoters of highly expressed genes, and they are the sites of almost all hypermethylation in human tumors,3 including the prostate. A common molecular feature associated with tumorigenesis is hypermethylation of cytosines 5′ to guanosines (CpG) within the regulatory (promoter) region of suppressor gene genomic DNA.4–8 5-Methyl cytosine is unstable and mutates to thymine, and methylated CpG sites degrade to TpG/CpA. In tumors, many CpG islands exhibit aberrant hypermethylation, resulting in gene silencing (Figure). Many of the silenced genes encode proteins that are tumor suppressor genes involved in tumorigenesis and progression.

Figure.

Figure

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Role of DNA methylation in expression. Unmethylated and methylated CpG sites are indicated by yellow and black circles, respectively. Gene 1 and gene 2 are rarely methylated and therefore expressed. Densely hypermethylated CpG islands in the promoter region of a tumor suppressor gene (TSG) in tumor inhibit expression. Hypomethylation in the promoter region of oncogene in tumor reactivates a transcription process.

DNA Methylation Detection Methods

Multiple molecular biology techniques are available for detecting the DNA methylation pattern in genomic DNA.9 Based on the type of technique used, two major groups of detection methods are available.

Hybridization Method

This method is a combination of Southern blot and methylation-sensitive restriction enzymes treatment together with polymerase chain reaction (PCR). Since some restriction enzymes are methylation-sensitive, these enzymes cannot digest methylated target sequence. Combined with Southern blot technique, the hybridization method can assess the overall methylation status of target CpG sites. Major limitations of this technique are requirement of a large amount of genomic DNA and limited information of promoter methylation. At least 5 µg of genomic DNA is needed to analyze the methylation status. Results from this technique provide information for target CpG sites of methylation-sensitive restriction enzymes.9

PCR Method

In order to detect methylated CpG sites, DNA samples are modified by sodium bisulfite. Sodium bisulfite deaminates cytosine and transforms into uracil.10 Methylated cytosine, however, is not transformed by bisulfite treatment. Currently, methylation-specific PCR (MSP) and quantitative real-time MSP are two major techniques detecting methylation with the use of bisulfite-modified DNA. These PCR methods can be performed in a short time without a large amount of DNA sample. Also, they provide specific and sensitive results, especially using quantitative real-time MSP, which has sufficient sensitivity to detect methylation of 0.1% of alleles. However, like other PCR-based techniques, these methods may provide false-positive results.11

Hypermethylated Genes in Prostate Tumor

The majority of previous publications in epigenetic research in prostate cancer focused on DNA hypermethylation. Indeed, gene-silencing is more common by DNA hypermethylation in the promoter region than by DNA mutations in carcinogenesis. Numerous studies on various hypermethylated genes in different cancers suggest a key part of the carcinogenesis and progression of cancer.12

Currently, over 30 genes have been investigated for their frequencies of hypermethylation and for their potential role in prostate cancer (Table). Many of these hypermethylated genes are tumor suppressor genes that are coded for the proteins that regulate the cell cycle and/or promote apoptosis. The functions of tumor suppressor genes in prostate cancer fall into five major categories: DNA repair, apoptosis, cell cycle, corticosteroid hormonal response, and invasion/metastasis. Defected function of these genes by promoter hypermethylation can contribute to the carcinogenesis and progression of prostate cancer.

Table.

Frequencies of Hypermethylated Genes in Prostate Cancer

Gene Common Name Frequency (Methylated/N) Reference
APC Adenomatous polyposis coli 27% (27/101) Maruyama et al17
90% (66/73) Yegnasubramanian et al19
57% (21/37) Kang et al22
100% (118/118) Jerónimo et al20
78% (88/113) Florl et al28
82% (59/72) Tokumaro et al116
64% (109/170) Enokida et al117
3.0* Rosenbaum et al48
48% (25/52)** Hoque et al27
83% (44/53) Bastian et al45
73% (131/179) Cho et al66
27% (21/79) Henrique et al115
83% (65/78) Bastian et al55
40% (182/459) Richiardi et al118
AR Androgen receptor 13% (2/15) Kinoshita et al78
25% (6/24) Nakayama et al80
8% (3/38) Sasaki et al79
15% (16/109) Yamanaka et al18
39% (30/76) Reibenwein et al81
CAV1 Caveolin-1 91% (20/22) Cui et al130
CCND2 Cyclin D2 32% (32/101) Padar et al42
99% (117/118) Henrique et al43
CD44 CD44 antigen 78% (31/40) Lou et al56
68% (27/40) Kito et al46
32% (36/111) Woodson et al41
72% (58/81) Singal et al39
CDH1 E-cadherin 54% (19/35) Li et al131
27% (27/101) Maruyama et al17
0% (0/111) Woodson et al41
69% (70/101) Padar et al42
24% (22/90) Woodson et al40
0% (0/73) Yegnasubramanian et al19
4% (5/114) Florl et al28
61% (49/81) Singal et al39
77% (40/52)** Hoque et al27
30% (6/20) Yao et al91
CDH13 H-cadherin 31% (31/101) Maruyama et al17
45% (68/151) Alumkal et al132
CDKN2A Cyclin-dependent kinase inhibitor 2A (p16) 13% (3/24) Jarrard et al25
70% (21/30) Gu et al26
73% (8/11) Nguyen et al24
3% (3/101) Maruyama et al17
66% (21/32) Konishi et al21
6% (4/73) Yegnasubramanian et al19
77% (91/118) Jerónimo et al20
4% (5/113) Florl et al28
37% (19/52) Hoque et al27
CDKN2A Cyclin-dependent kinase inhibitor 2A (p14) 22% (2/9) Nguyen et al24
3% (1/32) Konishi et al21
6% (1/16) Konishi et al34
4% (5/118) Jerónimo et al20
0% (0/73) Yegnasubramanian et al19
37% (19/52)* Hoque et al27
6% (6/95)* Rouprêt et al35
DAPK Death-associated protein kinase 1% (1/101) Maruyama et al17
36% (39/109) Yamanaka et al18
0% (0/73) Yegnasubramanian et al19
28% (27/95)** Rouprêt et al35
EDNRB Endothelin receptor type B 70% (23/35) Nelson et al57
83% (40/48) Jerónimo et al58
49% (36/73) Yegnasubramanian et al19
72% (58/81) Singal et al39
100% (80/80) Ellinger et al59
50% (9/18)** Bastian et al113
EPHA7 EPH receptor A7 42% (20/48) Guan et al135
ER-α Estrogen receptor alpha 90% (28/31) Li et al89
19% (14/73) Yegnasubramanian et al19
ER-β Estrogen receptor beta 83% (19/23) Nojima et al90
65% (13/20) Yao et al91
FHIT Fragile histidine triad 15% (15/101) Maruyama et al17
GSTP1 Glutathione S-transferase P1 100% (20/20) Lee et al96
91% (52/57) Lee et al99
75% (24/32) Santourlidis et al100
94% (16/17) Goessl et al101
44% (4/9)** Suh et al107
72% (23/32)** Goessl et al106
91% (63/69) Jerónimo et al102
79% (22/28) Cairns et al98
85% (89/105) Jerónimo et al104
36% (36/101) Maruyama et al17
75% (24/32) Konishi et al21
58% (7/12) Gonzalgo et al103
71% (43/61) Harden et al97
88% (96/109) Yamanaka et al18
84% (99/118) Woodson et al41
100% (18/18) Köllermann et al105
95% (69/73) Yegnasubramanian et al19
87% (32/37) Kang et al22
95% (112/118) Jerónimo et al20
72% (58/81) Singal et al39
79% (89/113) Florl et al28
48% (25/52)** Hoque et al27
83% (79/95)** Rouprêt et al35
HIC1 Hypermethylated in cancer 1 99% (108/109) Yamanaka et al18
100% (73/73) Yegnasubramanian et al19
89% (N/A) Kekeeva et al60
LPL Lipoprotein lipase 38% (21/56) Kim et al49
MGMT O6-methylguanine DNA methyltransferase 25% (8/32) Konishi et al21
0% (0/101) Maruyama et al17
2% (2/109) Yamanaka et al18
19% (22/118) Jerónimo et al20
76% (28/37) Kang et al22
1% (1/73) Yegnasubramanian et al19
19% (10/52)** Hoque et al27
15% (14/95)** Rouprêt et al35
PITX2 Paired-like homeodomain 2 3.4* Weiss et al52
NA Vanaja et al54
PTGS2 Prostaglandin-endoperoxide synthase 2 88% (64/73) Yegnasubramanian et al19
71% (38/53) Bastian et al45
65% (51/78) Bastian et al55
68% (54/80) Ellinger et al59
RARβ Retinoic acid receptor, beta 79% (11/14) Nakayama et al93
53% (54/101) Maruyama et al17
78% (85/109) Yamanaka et al18
84% (42/50) Zhang et al94
70% (79/113) Florl et al28
40% (32/81) Singal et al39
35% (18/52)** Hoque et al27
RASSF1A Ras association domain family 1 isoform A 71% (37/52) Liu et al37
53% (54/101) Maruyama et al17
99% (117/118) Jerónimo et al20
49% (40/81) Singal et al39
78% (88/113) Florl et al28
96% (70/73) Yegnasubramanian et al19
84% (31/37) Kang et al22
73% (38/52)** Hoque et al27
74% (97/131) Kawamoto et al38
RBP1 Retinol-binding protein 1 81% (96/118) Jerónimo et al20
47% (17/36) Jerónimo et al109
S100A2 S100 calcium-binding protein A2 99% (117/118) Jerónimo et al20
94% (32/34) Rehman et al134
SLC5A8 Solute carrier family 5, member 8 70% (7/10) Park et al11
SLC18A2 Vesicular monoamine transporter 2 88% (15/17) Sørensen et al63
TIG1 Tazarotene-induced gene 1 55% (17/31) Tokumaro et al110
53% (26/50) Zhang et al94
70% (43/61) Topaloglu et al111
70% (125/179) Cho et al66
10% (16/168)† Ellinger et al112
96% (77/80) Ellinger et al59
TIMP-2 Tissue inhibitor of metalloproteinase-2 60% (25/42) Pulukuri et al123
TIMP-3 Tissue inhibitor of metalloproteinase-3 6% (7/109) Yamanaka et al18
97% (114/118) Jerónimo et al20
0% (0/73) Yegnasubramanian et al19
37% (19/52)** Hoque et al27
41% (37/91)** Rouprêt et al35
TNFRSF10C TNF receptor superfamily, member 10c 50% (25/50) Shivapurkar et al64
65% (117/180) Cho et al66
78% (46/59) Cheng et al67
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*

Hazard ratio

**

Urine samples

†

Serum DNA

N/A = not available

DNA Repair Gene

Although the specific causes of prostate cancer are not known, androgens, estrogens, inflammation, and DNA repair capacity have been implicated. DNA is constantly damaged by endogenous oxygen free radicals and exogenous chemicals. DNA mutations are estimated to spontaneously occur 20,000 to 40,000 times every day.13,14 The DNA repair process is important to the survival of the cell. Therefore, different repair pathways are available to reverse the different types of DNA damage. More than 150 DNA repair enzymes participate in this process.15 Defects in these DNA repair pathways may increase persistent mutations in daughter cell generations, genomic instability, and ultimately a prostate cancer risk.

Among several distinct DNA repair pathways, the direct reversal repair pathway may be important in carcinogenesis in the prostate. Methylguanine DNA methyltransferase (MGMT), the only known enzyme in the direct reversal repair pathway, leads to the direct restoration of the natural chemical composition of DNA without the need for genomic reconstruction. Therefore, defective MGMT activity is associated with an increased mutation.16 Reports regarding MGMT methylation in prostate tumor tissues have been inconsistent. While three studies reported a low frequency of MGMT promoter hypermethylation (0% to 2%) in prostate tumor tissues,17–19 others observed higher prevalence of hypermethylation (19% to 76%).20–22 Two investigator groups reported 15% and 19% MGMT hypermethylation frequencies in urine sediment samples collected from prostate cancer patients.27,35 These data suggest that MGMT promoter methylation can be a potential biomarker for early detection and surveillance of prostate cancer. However, larger studies will be necessary to resolve these inconsistent results.

Cell Cycle Genes

The cell cycle pathway regulates cell growth. One of the distinguishing characteristics of tumor cells is uncontrolled growth. Many genes act as checkpoints that regulate the cell cycle. Defective cell cycle genes may lead to the carcinogenesis and progression of prostate cancer.23

The tumor suppressor gene CDKN2 is one of the cyclin-dependent kinase inhibitors (CDKIs). CDKN2A (p16INK4), a key protein in the signaling pathway, can be damaged by a variety of genetic and epigenetic changes including hypermethylation in prostate cancer. Aberrant CDKI expression is observed in many tumor tissues including prostate.20,21,24 Results regarding the frequency of CDKN2A promoter methylation are inconsistent in prostate tumors, ranging from 3% to 77%.17,19–21,24–28 Perhaps these inconsistent results are due to different detection methods and/or different targets of methylated loci. Since Herman et al29 first reported inactivation of CDKN2A by DNA methylation in prostate cancer, other researchers have investigated the role of hypermethylated CDKN2A in the carcinogenesis and progression of prostate cancer.17,19–21,24–28 Although there was no significant association between CDKN2A low expression and increased CDKN2A exon 2 methylation, the exon 2 methylation may be a potential biomarker for prostate tumor.24 These results were confirmed by other investigators. Konishi et al21 observed that methylation occurred in the promoter region in 9% of samples and in exon 2 in 66% of tumors. Jerónimo et al20 found that the CDKN2A gene was frequently methylated in tumor tissue (77%) and in benign prostatic hyperplasia (BPH). These data support p16 methylation as a potential biomarker for an early detection of prostate cancer.

Another CDKI, the CDKN2A (p14ARF) promoter, has been methylated in various cancers,30–33 including prostate cancer.19–21,24,27,34,35 Based on seven publications, frequencies of p14ARF methylation ranges from 0% to 37%.19–21,24,27,34,35 Without two outliers,24,27 most published studies reported low methylation rates that ranged from 0% to 6%.19,21,24,34,35 Thus, the p14 is not a good candidate for a biomarker.

The RAS family of proto-oncogenes plays a key role in signal transduction pathways involved in cellular proliferation and survival, interacting with other regulatory circuits of cell growth and death. RAS association domain family protein 1 isoform A (RASSF1A) is known as a tumor suppressor gene. The RASSF1 protein was known to be associated with the DNA repair proteins and with the apoptotic effect.36 Inactivation by methylation of RASSF1A may deregulate the DNA repair pathway and cell cycle control in the tumor. The RASSF1A gene is silenced by aberrant methylation of the promoter in a large fraction of various cancers including prostate.37 In prostate tumors, RASSF1A promoter methylation is a common event, occurring in 49% to 99% of tumor tissues.17,19,20,22,27,28,37–39 RASSF1A promoter methylation is also associated with aggressive prostate cancer.17,22,37

Others cell cycle genes — CD44, cyclin D2 (CCND2), lipoprotein lipase (LPL), endothelin B receptor (EDNRB), hypermethylated in cancer 1 (HIC1), paired-like homeodomain transcription factor 2 (PITX2), and prostaglandin-endoperoxidase synthase 2 (PTGS2) — are often have a lower expression in prostate tumor tissues than in adjacent normal tissues. These low expressions are significantly correlated with promoter methylation level.40–45 Furthermore, expression of these genes and their promoter methylation may correlate with the tumorigenesis, progression, and clinicopathological features of prostate cancer.46–55 Many studies observed relatively high frequencies of promoter methylation in these genes: CD44 (32% to 78%),39,41,46,56 cyclin D2 (32% to 99%),42,43 LPL (38%),49 EDNRB (49% to 100%),19,39,57–59 HIC1, (89% to 100%),18,19,60 PITX2,52,54 and PTGS2 (65% to 88%).19,45,55,59 The frequencies of methylation of these genes, with the exception of EDNRB and HIC1, were significantly higher in prostate tumors than in normal tissues.42,43,49,55,57 Together, promoter methylation of these genes is a good candidate as a useful prostate cancer biomarker for the identification of the more aggressive prostate cancer that might benefit from different therapeutic modalities. However, the methylation status of EDNRB and HIC1 in prostate tumors parallels the respective normal tissue, although a high proportion of tumors are methylated.18,19,39,58–60 Therefore, DNA methylation sites in EDNRB and HIC1 are not good candidates for a marker for prognostic marker for prostate cancer progression and an intervention target for prostate cancer.

Apoptosis Genes

Programmed cell death (apoptosis) is a critical process for carcinogenesis in human. Typical morphological characteristics of apoptosis are damages of the plasma membrane, condensation and fragmentation of the nucleus, and DNA fragmentation.61 A major component of the apoptosis pathway is the caspase family. However, other genes, including death-associated protein kinase (DAPK), fragile histidine triad (FHIT), solute carrier family 5A8 (SLC5A8), vesicular monoamine transporter 2 (SLC18A2), and tumor necrosis factor receptor superfamily, member 10C (TNFRSF10C), are also involved in this pathway. A repressed expression of these genes by hypermethylation in the promoter region has been shown for prostate cancer.17–19,35,62–65 However, DAPK and FHIT may have a limited value due to a persistently low frequency of methylation in tumors and normal tissues.17–19,35 SLC5A8, SLC18A2, and TNFRSF10C were found to be hypermethylated in 50% to 88% of prostate cancers and significantly downregulated in tumor compared with normal prostate tissues.11,62–64,66,67 Expression of SLC18A2 and TNFRSF10C is negatively associated with biochemical recurrence after radical prostatectomy.63,68

Corticosteroid Hormonal Response Genes

The specific causes of prostate cancer are not known, but multiple etiological factors, including genetics, hormones, diet, infection, and environmental exposures, are thought to play significant roles. Although the precise role of androgens and their receptors in the carcinogenesis and progression of prostate cancer has not been fully investigated, previous studies suggest that these genes are important.69,70 Differences in the activities of these enzymes are determined to a large extent by genetic and epigenetic changes in the genes encoding them.

It is known that androgens stimulate the growth of prostate cells through the androgen receptor (AR).71 While silencing of AR expression decreases growth and induces apoptosis in vitro,72–74 overexpression of the AR also induces growth inhibition and apoptosis.75 In addition to prostatectomy and radiation therapy, androgen deprivation is one of the most effective treatments for prostate cancer. However, many advanced prostate cancer cells can survive in a low androgen environment due to a high expression of the androgen receptor.76 AR is one of the most frequently overexpressed proteins in the androgen-independent cases.77 Feldman and Feldman76 suggested five different possible pathways that lead to development of androgen-independent status. Several groups found AR promoter methylation in 8% to 39% of the prostate tumor tissues.18,78–81 Frequencies of AR promoter methylation are higher in androgen-independent cases than in primary prostate tumor tissues.78,80

The role of estrogen in the carcinogenesis of prostate tissues is not clear. However, a loss of expression of the estrogen receptor (ER)-β was induced by promoter methylation during the development of prostate cancer.82 The biological actions of estrogens are meditated by the ER. Two ERs are highly homologous DNA-binding domains but different N-terminus and ligand-binding domains. Both ERs, ER-α and ER-β, are downregulated in prostate tumor tissues.83,84 Promoter methylation is the primary mechanism responsible for low expression of ERs.79,85,86 ER-α expression is associated with a poor prognosis for hormonal therapy.87 ER-β is the main subtype in the prostate tissue and may serve as a tumor suppressor gene since ER-β protects against uncontrolled cell proliferation in normal prostate cells.86 However, high expression of ER-β in prostate tumors is associated with increased risk for recurrence and distant metastasis.84,88 Therefore, ER-β may have multiple roles in carcinogenesis and progression. The frequency of ER promoter methylation ranges from 19% to 90% in prostate tumors.19,89–91 The extent of ER promoter methylation is significantly higher in prostate tumors than in the BPH samples.89,90

Retinoic acid receptor β (RARβ) is known as a tumor suppressor gene by interacting with retinoic acid. Expression of RARβ is reported to be absent or downregulated in tumor tissues,92 and the RARβ2 promoter is aberrantly methylated in many cancers, including prostate cancer.93 Several groups reported that frequencies of methylation of the RARβ2 promoter range from 40% to 84% of primary prostate cancers but rarely in normal prostate tissues or BPH samples.17,18,28,39,93,94 Moderately high frequency of RARβ promoter methylation was observed in 35% of urine samples.27 In addition, the RARβ2 promoter is methylated in 20% of prostatic intraepithelial neoplasia (PIN) samples. Therefore, RARβ2 gene methylation may be an ideal biomarker candidate for early detection of prostate cancer.18,93

Glutathione S-transferase P1 (GSTP1) is involved in the detoxifying process and elimination of potentially genotoxic foreign compounds by conjugating glutathione into toxic chemicals. These processes protect prostate cells from DNA adducts and carcinogenesis. Thus, defective GSTP1 activity may increase DNA mutations, thereby possibly increasing the risk of prostate cancer.95 Because of its consistently frequent hypermethylation in the promoter region in prostate cancer, GSTP1 is perhaps the most studied gene in prostate cancer. Lee et al96 first reported a high frequency of GSTP1 hypermethylation in prostate tumor tissues.96 Since then, numerous studies confirmed similar results. Methylation of the GSTP1 promoter region occurs in 36% to 100% of tumor tissues.17–22,28,39,41,96–105 However, this methylation is rarely detected in normal prostate or BPH tissues. GSTP1 methylation was also detected consistently with high frequency in urine samples, blood, and ejaculates of prostate cancer patients, while either low or no methylation was detected in the samples from healthy controls.27,25,106,107 These different frequencies of GSTP1 promoter hypermethylation between tumor and normal prostate tissues make an ideal biomarker for prostate cancer.

Retinoids have an antitumorigenesis function and are involved in cell growth and differentiation. Their functional effects are mainly mediated by retinol-binding protein (RBP1). The role of RBP1 expression in carcinogenesis is not yet defined. However, the low expression of RBP1 by promoter methylation has been associated with the malignant tumor tissues, including prostate.108,109 Two studies reported that RBP1 promoter hypermethylation was found in 47% and 81% of tumors. No BPHs and normal prostate tissues were methylated.20,109

Tazarotene-induced gene 1 (TIG1) is frequently silenced in prostate tumors. This gene, also known as retinoid acid (RA) receptor-responsive 1 gene, was first identified as an RA-responsive gene. Several investigators reported that TIG1 was frequently methylated (53% to 96%) in prostate tumors, but in normal tissue or benign hyperplasia, TIG1 methylation was either absent or low.59,66,94,110,111 Zhang et al94 further found that the methylation of TIG1 and RARβ was positively correlated. Therefore, it is possible that the methylation of the retinoid response gene TIG1 occurred in response to the methylation and inactivation of RARβ. Ellinger et al112 analyzed the diagnostic and prognostic possibilities of methylation analysis in cell-free serum DNA of patients with prostate cancer. They found that hypermethylation in TIG1 was more frequent in prostate cancer patients (10%) compared to BPH (0%) and healthy individuals (0%).59 The detection of hypermethylation in cell-free serum DNA may allow the specific diagnosis of prostate cancer.113

Tumor Cell Invasion and Metastasis Genes

Metastasis is an extremely complicated process that occurs through a series of sequential steps involving invasion, transport, adhesion at a distant site, and outgrowth into a secondary organ. Although metastases are the cause of 90% of human cancer deaths, little is known about the genetic and biochemical determinants of metastasis.

The methylated adenomatous polyposis coli (APC) gene causes familial adenomatous polyposis, which is an inherited disorder characterized by extensive colon polyps and the development of colorectal cancer in early adulthood. The APC complex is known to function as a gatekeeper in the cell, preventing the transcription of gene products that promote cell proliferation and survival rather than differentiation and apoptosis.114 Hypermethylation of APC implies silencing of this gatekeeper, making the cell vulnerable to further epigenetic and genetic changes and thus progression toward the development of invasive cancer. APC promoter methylation is common in various human tumors, especially in the colon. Most studies found a prevalence of 27% to 100% in prostate cancer tissue but only 5% to 6% in non cancerous tissue.17,19,20,22,27,28,45,48,55,66,115–118 Recent studies found that methylation in APC is associated with progression of prostate cancer.48,115,118 In two small cohorts of prostate cancer patients, a 3-fold statistically significantly increased hazard ratio (HR) for promoter methylation in APC has been reported among the patients who experienced prostate-specific antigen (PSA) recurrence, metastasis, or death.48,115 Richiardi et al118 found that hypermethylation in the promoter of the APC gene is involved in prostate cancer progression using large survival analysis.

Matrix metalloproteinases (MMPs) are proteolytic enzymes that degrade of the extracellular matrix and the basement membrane. High expressions of these enzymes have been associated with tumor growth, invasion, and tumor-induced angiogenesis.119 These pathways are controlled by the balance between the levels of the MMPs and tissue inhibitors of metalloproteinases (TIMPs).120 TIMP-2 and TIMP-3 are two of the frequently investigated members of this family because of their involvement in cancer progression and metastasis in a variety of human cancers.121–127 Pulukuri et al123 observed that 25 (60%) of 42 prostate tumors were methylated in TIMP-2 compared with 5 (16%) of 32 normal prostate samples. However, these results were not confirmed by a previous study.124 Ross et al124 found that TIMP-2 was expressed in a majority of prostate tumors and correlates with clinical stages. Contrary to the earlier study that indicated antitumor effects, TIMP-2 expression appears to have a tumor-promoting role in prostate cancer and warrants further investigation.124 High expression of TIMP-3 reduces metastasis, induces apoptosis, increases drug sensitivity, and in hibits tumor growth.125–127 A low expression by promoter methylation of TIMP-3 has been associated with poor outcomes.128 The promoter region of TIMP-3 was found to be methylated in 97% of prostate tumors.20 However, other studies reported low (6% and 0%) frequencies of TIMP-3 methylation.18,19 Two studies found TIMP-3 promoter methylation in 37% and 41% of urine sediments from prostate cancer patients.27,35 As a diagnostic marker in urine DNA, TIMP-3 may be limited by a persistent low frequency of methylation in normal controls.

Others tumor metastasis genes — Caveolin-1 (CAV1), E-cadherin (CDH1), H-cadherin (CDH13), EPHA7, and S100A2 — are often downregulated in prostate tumor tissues than in adjacent normal tissues due to methylation.17,19,27,28,39–42,91,129–135 Gene silencing of CAV1, CDH1, and CDH13 is associated with clinical features of prostate cancer.131,133,136,137 These data suggest that the methylation status of CAV1 and CDH1 not only is a potential biomarker for prostate cancer, but also may be a predictive marker of outcome.136 However, two studies reported that methylation of CDH1 promoter could not be detected in prostate cancer samples.19,41 S100A2 methylation was seen in 75% of cases of nonmalignant tissues and in 100% of cases of BPH.134

Conclusions

Although a few large-scale genome-wide analyses of epigenetic variations are currently ongoing, most published studies are small-scale with a retrospective design. Therefore, meta-analyses or large studies should be performed to obtain the complete extent and pattern of differential DNA methylation in the promoter region in the critical genes. Since epigenetic changes are involved in the carcinogenesis and progression of prostate cancer, information of these epigenetic changes may provide a clue for better diagnostic, prognostic, and predictive modalities than existing options. The ultimate goals of these epigenetic studies are to improve patient outcomes and enhance quality of life. A number of clinical trials and therapies are targeting methylated genes. Unlike DNA somatic mutations, DNA methylations are reversible. Thus, hypermethylated tumor-suppressor genes can be reactivated with drugs. Several demethylating agents such as 5-azacytidine (Vidaza) and 5-aza-2′-deoxycytidine (decitabine) have been approved as treatments for myelodysplastic syndrome (MDS) and leukemia.138–140 Some MDS patients treated with 5-azacytidine showed a significant survival benefit.141 However, a major limitation of these therapies is their nonspecific target approach, which may induce unintended side effects. Therefore, not only tumor suppressor genes but also silenced oncogenes by methylation can be reactivated. Future studies should focus on developing drugs that can target specific genes.

Footnotes

No significant relationship exists between the author and the companies/organizations whose products or services may be referenced in this article.

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