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10.1245/ASO.2005.07.020
Annals of Surgical Oncology 12:354-363 (2005)
© 2005 Society of Surgical Oncology
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Original Article

Tumor Progression Through Epigenetic Gene Silencing of O6–Methylguanine-DNA Methyltransferase in Human Biliary Tract Cancers

Yasuo Koga, MD1, Yoshihiko Kitajima, MD, PhD1, Atsushi Miyoshi, MD1, Ken Sato, MD1, Kenji Kitahara, MD, PhD1, Hidenobu Soejima, MD, PhD2 and Kohji Miyazaki, MD, PhD1

1 Department of Surgery, Saga University Faculty of Medicine, Nabeshima 5-1-1, Saga, 849-8501, Japan
2 Department of Biomolecular Sciences, Division of Molecular Biology & Genetics, Saga University Faculty of Medicine, Nabeshima 5-1-1, Saga, 849-8501, Japan

Correspondence: Address correspondence and reprint requests to: Kohji Miyazaki, MD, PhD; E-mail: miyazak2{at}post.saga-med.ac.jp


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Background: We previously demonstrated in an immunohistochemical study that reduced expression of O6–methylguanine-DNA methyltransferase (MGMT) correlated with a poorer prognosis in patients with biliary tract cancers. The purpose of this study was to clarify how MGMT deficiency leads to a poor outcome in biliary tract cancer. Thus, we examined epigenetic (promoter methylation) and genetic (gene mutation) alterations in biliary tract cancer.

Methods: We examined 37 biliary tract cancer specimens from patients who underwent surgical resection. Promoter methylation was determined by one-step or two-step methylation-specific polymerase chain reaction. Gene mutation was identified by direct sequencing. The expression of MGMT protein in paraffin-embedded tissue was examined by immunohistochemistry.

Results: Frequencies of promoter methylation were 70% for p16/INK4a, 49% for MGMT, 46% for hMLH1, 41% for E-cadherin, and 32% for DAPK genes. MGMT methylation status was closely correlated with the MGMT protein expression determined by immunohistochemistry (P < .001). Although this was not statistically significant, biliary tract cancer tumors with MGMT methylation expressed multigene methylation more frequently than tumors without MGMT methylation (P = .071). A total of 33 mutations were identified in 4 cancer-related genes: p53, K-ras, ß-catenin, and p16/INK4a genes. The most common mutation was GC to AT transitions (58%), which were significantly associated with MGMT promoter methylation (P = .011). These findings suggest that loss of MGMT expression by promoter methylation results in accumulation of GC to AT gene mutations.

Conclusions: Reduced MGMT expression may increase the malignant potential of biliary tract cancer through both epigenetic and genetic mechanisms.

Key Words: DNA alkylation • DNA repair gene • MGMT • Biliary tract cancer • Multigene methylation • Gene mutation


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Biliary tract cancer, which includes gallbladder and extrahepatic bile duct cancer, is relatively uncommon in Japan. However, the incidence of biliary tract cancer has markedly increased over the past several decades, and biliary tract cancer now ranks as the sixth leading cause of cancer death in Japan.1 The prognosis of patients with biliary tract cancer is poor because biliary tract cancer is inherently aggressive and has often reached an advanced stage at the time of diagnosis.2,3 An understanding of the molecular mechanisms that underlie the development and progression of biliary tract cancer will aid in diagnosing this disease, screening high-risk patients, and developing new treatments.4

Alkylating agents, such as N-nitroso compounds, are highly carcinogenic in human tissue.5 The biological effects of these agents are attributed to alkylation at the O6 position of guanine in DNA.5 Once O6-methylguanine is formed, it can pair with thymine during DNA replication, resulting in a GC to AT transitional mutation.6 O6-Methylguanine-DNA methyltransferase (MGMT), a cellular DNA repair protein, provides protection from the biological effects of alkylating agents.7 MGMT repairs premutagenic bases by transferring and accepting methyl groups from O6-methylguanine. MGMT knock-out mice develop thymic lymphoma or lung adenoma after exposure to alkylating agents, thus indicating that DNA alkylation–induced tumorigenesis occurs in the absence of MGMT.8

Alkylating agents are principally metabolized and activated in hepatocytes and then are released into the bile duct and stored in the gallbladder.9 Thus, high exposure to alkylating agents occurs in the epithelium of the gallbladder and extrahepatic bile duct. In a previous immunohistochemical study, we found that approximately 60% of gallbladder and extrahepatic bile duct cancers had deficient MGMT expression.10 Additionally, deficient MGMT expression correlated with poor prognosis in both types of cancer. We suggested that deficient MGMT expression contributed to DNA alkylation–induced carcinogenesis in the biliary tract and the subsequent progression of biliary tract cancer. However, the molecular mechanism by which deficient MGMT expression occurs and leads to a poor outcome in patients with biliary tract cancer remains largely undetermined.

Extensive molecular analyses have established the multistep processes involved in genetic and epigenetic alterations that contribute to the development and progression of several types of solid tumors in humans.11 Genetic abnormalities that have been identified in biliary tract cancer include point mutations in K-ras and ß-catenin proto-oncogenes and alterations in p53, p16/INK4a, APC, and DPC4 tumor-suppressor genes,12–15 in combination with chromosomal deletions or mutations and infrequent microsatellite instability.4,15,16 We previously examined K-ras and ß-catenin point mutations in gallbladder cancer tissues, but the mutation rates were lower than those in a previous report.17

Aberrant methylation of CpG islands within the promoter region of tumor-suppressor and DNA repair genes has been identified in several human cancers.18 Aberrant methylation is a novel mechanism for gene silencing that is involved in carcinogenesis and cancer progression. We recently reported that MGMT gene silencing in hepatocellular carcinoma may be caused by promoter methylation.19 Several reports indicate that the hypermethylation of promoter regions in MGMT20–22 p16/INK4a,23 E-cadherin24 tumor-suppressor genes, the hMLH125 mismatch repair gene, and the DAPK26 gene (associated with apoptosis) is a fundamental part of tumorigenesis in a variety of cancers. Recent reports indicate a novel pathway, termed CpG island methylator phenotype, in several tumors; it is characterized by simultaneous methylation of multiple CpG islands, including cancer-related genes such as p16/INK4a, THBS1, and hMLH1.27,28 Furthermore, some reports indicate that this methylator phenotype is associated with advanced tumor stage and poor prognosis in esophageal adenocarcinoma29 and pancreatic endocrine neoplasms.30 Methylation of multiple genes has been also demonstrated in gallbladder cancer31 and intrahepatic cholangiocarcinoma32. However, correlations with the clinical outcomes of these patients were not apparent.

In this study, we examined two hypotheses that explain how deficient MGMT expression leads to poor outcomes in patients with biliary tract cancer. One hypothesis was that MGMT deficiency leads to an accumulation of GC to AT mutations in some critical molecules, such as oncogenes and tumor-suppressor genes. These mutations then increase the progression of biliary tract cancer. The second hypothesis was that MGMT deficiency allows the promoter regions of tumor-suppressor genes and other repair genes to become simultaneously hyper-methylated, resulting in poor outcomes for patients with biliary tract cancer. To determine which of these two molecular pathways is associated with cancer progression in biliary tract cancer, we studied mutations and the promoter methylation status of several cancer-related genes in surgically resected cancer tissues. First, we analyzed MGMT promoter methylation status and MGMT protein expression in 37 biliary tract cancer tissues. Then, we compared the MGMT methylation status with clinicopathologic factors and genetic and epigenetic alterations of cancer-related genes. The genes studied were K-ras, p53, p16/INK4a, and ß-catenin for mutation analysis, on which mutations have been focused mainly in biliary tract cancer, and p16/INK4a, hMLH1, E-cadherin, and DAPK for methylation analysis, which were well characterized as to epigenetic silencing.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Tissue Samples
Cancer specimens were obtained from patients with biliary tract cancer who had surgical resections in the Department of Surgery at Saga University Hospital (Saga, Japan) from May 1994 through September 2003. Informed consent for the use of the specimens was obtained from all patients. A total of 37 specimens of patients with 22 gallbladder cancers (59%) and 15 extrahepatic bile duct cancers (41%) were studied. There were 15 (41%) men and 22 (59%) women. The mean age was 64.3 years (range, 45–82 years). A total of 37 patients underwent surgical resection with curative intent. Concomitant hepatic resection or pancreaticoduodenectomy was performed with 18 (49%) and 15 (41%) of the resections, respectively. Concomitant lymphadenectomy was performed in all resections. A margin-negative resection was accomplished in 35 (95%) patients.

Microdissection and DNA Extraction
Laser capture microdissection was used to isolate cancer foci, because some biliary tract cancers are extensively contaminated with normal cells. Serial 8-µm sections were cut from frozen tumor specimens and mounted on slides. Slides were stained with hematoxylin and eosin, and a coverslip was placed on one slide from each tumor. The slides with coverslips were used to localize lesions of interest for microdis-section on the other slides. Using the laser capture microdissection system (PixCell IIe Microscope, Arc-turus Engineering, Mountain View, CA) and the guidance of a pathologist, we gathered 4000 to 5000 cancer cells from 2 or 3 sections of each tumor, DNA was extracted from isolated tumor cells by using a DNA extraction kit (QIAamp DNA Micro Kit, Qiagen, Hilden, Germany).

Methylation Analysis
We examined the methylation status of MGMT and four candidate genes: p16/INK4a, hMLH1, E-cadherin, and DAPK. The promoter methylation status was determined by methylation-specific polymerase chain reaction (MSP), as previously described.33 In the preliminary experiment, MSP sensitivities of MGMT and hMLH1 were lower than those of the other genes to detect allelic hyper-methylation by single-step MSP. Therefore, the methylation status of the MGMT and hMLH1 promoter was determined by a two-step MSP method to increase the sensitivity. Briefly, 400 ng of DNA was subjected to urea/bisulfite treatment according to the method of Paulin et al.,34 in which unmethylated cytosines are converted to uracils. The modified DNA was resuspended in 20 mL of Tris-EDTA buffer and immediately subjected to polymerase chain reaction (PCR) or stored at –20°C. Step 1 primers flanked the CpG-rich promoter regions of the respective target genes. Hence, these primers did not discriminate between methylated and unmethylated nucleotides after bisulfite modification. Primer sequences used for step 1 PCR were 5'-GTTTTYGGT-TTYGTTTYGTTTTAGATTT-3' and 5'-AACTAC-CACCRTCCCRAAAAAAAAC-3' for MGMT and 5'-GGTATTTTTGTTTTTATTGGTTGG-3' and 5'-TCTAAATACTCAACGAAAATACCTT-3' for hMLH1. The annealing temperatures were 59°C and 53°C for each gene. The first PCR amplification was performed with 20 µg of bisulfite-treated DNA. Then, 1 mL from 1/1000th of the first PCR product was subjected to the second-step PCR by using primers designed to recognize bisulfite-induced uracil from unmethylated cytosines, as previously described.33 The promoter methylation status of the other three genes was assessed by conventional MSP with specific primers, as previously described.23–26 The MSP reaction volume was 20 µL and contained 1 U of Hot Start Ex Taq DNA polymerase (Takara Biochemical, Kyoto, Japan), 2 µL of 10 x Ex Taq buffer, 2 µL of deoxynucleoside triphosphate mixture, primer sets (8 pmol per reaction), and 1 µL of DNA template. PCR was performed for 3 minutes at 96°C, 30 seconds at 96°C, 30 seconds at annealing temperature, and 30 seconds at 72°C for 35 cycles, followed by 4 minutes at 72°C for all reactions. The first PCR product for MGMT and hMLH1 was sequenced directly by using the Big Dye Terminator Cycle Sequencing Ready Reaction (Perkin Elmer Applied Biosystems Division, Foster City, CA) and analyzed on an ABI Prism 310 Genetic Analyzer (Perkin Elmer). In vitro methylated DNA (Intergen, Purchase, NY) was used as a positive control for methylation, and DNA from normal lymphocytes was used as a negative control for methylation. Water was used as a negative control. Ten microliters of PCR product was analyzed with 10% Tris-borate, EDTA gel electrophoresis (Invitrogen, Carlsbad, CA). Each MSP was repeated at least three times.

Mutation Analysis
Samples were analyzed for gene mutations in exons 1 and 2 of K-ras, exons 5 through 8 of p53, exons 1 and 2 of p16/INK4a, and exon 3 of ß-catenin. DNA fragments containing the corresponding exons were amplified by PCR and sequenced by using the Big Dye Terminator Cycle Sequencing Ready Reaction (Perkin Elmer). The primers used to amplify K-ras,35 p53,36 p16/INK4a,37 and ß-catenin17 have been previously described. DNA sequences were analyzed with an ABI Prism 310 Genetic Analyzer (Perkin Elmer). Each mutation was verified in both the sense and antisense directions.

Immunohistochemical Analysis
Immunohistochemical staining of MGMT protein was performed on formalin-fixed, paraffin-embedded tissue sections from biliary tract tumors, as previously described.10 Rabbit polyclonal anti-MGMT antibody (clone G168-728; PharMingen, San Diego, CA) was used as the primary antibody. Antirabbit immunoglobulin G conjugated to peroxidase-labeled dextran polymer (EnVision+, Daco, Carpinteria, CA) was used for the following reaction. Positive staining was identified by the presence of brown staining in the nucleus. MGMT expression was evaluated as positive if the distribution of stained cells was to >10% of cancer cells. Normal epithelium and lymphocytes within the tumor section were used as positive internal controls.

Statistical Analysis
Differences in means were evaluated by Students t-test, and differences in frequencies were analyzed with Fisher’s exact test or the {chi}2 test, Disease-specific survival distributions were estimated by the Kaplan-Meier method and were compared by using the log-rank test. All reported P values are derived from two-sided statistical tests. P values <.05 were considered statistically significant.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Expression of MGMT Protein
Figure 1Go shows representative results of MGMT immunohistochemical staining in biliary tract cancer specimens. Eighteen (49%) of 37 patients had MGMT-positive cancer cells (Fig. 1AGo). In these patients, MGMT expression was localized in the nuclei of neoplastic cells. Nineteen (51%) of 37 patients had MGMT-negative cancer cells (Fig. 1BGo). MGMT was detected in normal cells, including epithelial mucosa, smooth muscle cells, lymphocytes, and inflammatory cells.


Figure 1
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  		FIG. 1. Immunohistochemical staining of O6-methylguanine-DNA methyltransferase (MGMT) protein in biliary tract cancer (original magnification, x 400). (A) Strongly positive immunoreactivities for MGMT are seen in the nuclei of neoplastic cells. (B) The cancer cells in this specimen were MGMT negative, but the lymphocytes and endothelial cells were MGMT positive.

 
Methylation Profile
Figure 2Go shows representative results of MSP for the promoter regions of MGMT and four cancer-related genes. When PCR products for the methylation-specific primers were observed, the promoter was judged to be methylated. Figure 3Go summarizes the gene promoter methylation status in the 37 biliary tract cancer specimens. We detected aberrant promoter methylation of at least 1 gene in 35 (95%) of the 37 cases. Frequencies of promoter methylation were 70% for p16/INK4a, 49% for MGMT, 46% for hMLH1, 41% for E-cadherin, and 32% for DAPK. Aberrant promoter methylation was not found in 2 (5%) of 37 cases. MGMT methylation status was closely correlated with MGMT protein expression determined by immunohistochemistry (P < .001). Sixteen (80%) of 20 tumors with loss of MGMT expression had methylated MGMT, whereas only 2 (12%) of 17 tumors with normal MGMT expression had methylated MGMT.


Figure 2
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  		FIG. 2. Amplified products after nested methylation-specific polymerase chain reaction (MSP) for the DNA repair genes O6-methylguanine-DNA methyltransferase (MGMT) and hMLH1. Amplified products are shown after one-step MSP for the tumor-suppressor genes p16/INK4a, E-cadherin, and DAPK. Lanes marked U and M contain products derived from unmethylated and methylated alleles, respectively. Results from representative patients with extrahepatic bile duct tumors are shown. In vitro methylated DNA (IVD) was used a positive control for methylation, and DNA from normal lymphocytes (NL) was used as a negative control for methylation. Water was used as a negative control for polymerase chain reaction. TBC, tissue of primary biliary tract cancer.

 

Figure 3
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  		FIG. 3. Methylation profile of 5 gene promoter regions (p16/INK4a, O6-methylguanine-DNA methyltransferase (MGMT), hMLH1, E-cadherin, and DAPK) in 22 gallbladder cancers (top) and 15 extrahepatic bile duct cancers (bottom). Filled boxes represent positive methylation. Open boxes denote no detectable methylation.

 
We compared the promoter methylation status of MGMT and four cancer-related genes; however, no significant relationship was identified. Multigene methylation, defined as three or more genes (not including MGMT) concurrently methylated in a tumor, was found in 11 (30%) of 37 cases. Biliary tract tumors with MGMT methylation exhibited multigene methylation more frequently than tumors without MGMT methylation (P = .071), although this difference was not statistically significant (Table 1Go).


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  		TABLE 1. MGMT promoter methylation and multigene methylation
 
Mutation Profile
We directly sequenced four cancer-related genes to identify mutations in biliary tract cancer tumors. Figure 4Go presents representative mutation results (Fig. 4AGo, point mutation at codon 175 in p53; Fig. 4BGo, point mutation at codon 45 in ß-catenin). Mutation results for all four genes are listed in Table 2Go. All mutations are missense mutations leading to amino acid substitutions. Of 37 biliary tract cancers, 23 (62%) had at least 1 gene mutation, whereas 14 (38%) had no mutations. Twenty-three (62%) tumors had mutated p53, five (14%) tumors had mutated K-ras, three (8%) tumors had mutated ß-catenin, and two (5%) tumors had mutated p16/INK4a. Seven (19%) of 37 tumors contained 2 or more mutated cancer-related genes.


Figure 4
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  		FIG. 4. Direct DNA sequence analysis of gene mutations in biliary tract cancer. Representative mutations leading to amino acid substitutions are shown. (A) Point mutation at codon 286 in p53. (B) Point mutation at codon 45 in ß-catenin.

 

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  		TABLE 2. Mutation profile
 
We compared the methylation status with the overall mutation spectrum in the four genes (Table 3Go). Among 33 mutations, 19 (58%) were GC to AT transitions, and 14 (42%) were transversion mutations or deletions. GC to AT transitional mutations were significantly associated with MGMT promoter methylation (P = .011). Fourteen (74%) of 19 tumors with GC to AT transitions had methylated MGMT, whereas only 4 (29%) of 14 tumors with another type of mutation had methylated MGMT (Table 3Go). Significant differences in MGMT methylation status were also observed between the GC to AT transition group and the wild-type group (P = .029).


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  		TABLE 3. Analysis of MGMT methylation status and mutational spectrum
 
Analysis of Clinicopathologic Features and Genetic and Epigenetic Alterations
Table 4Go shows an analysis of clinicopathologic features and MGMT promoter methylation status in patients with biliary tract cancer. Although not statistically significant, MGMT methylation status seemed to correlate with tumor depth, hepatic invasion, and lymphatic invasion. Patient age and sex, as well as tumor size, differentiation, and stage, were not correlated with MGMT promoter methylation status. In addition, multigene methylation and mutational status in cancer-related genes were not correlated with clinicopathologic features in biliary tract cancers (data not shown).


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  		TABLE 4. Analysis of MGMT methylation status and clinicopathologic features
 
Clinical Outcome and Genetic and Epigenetic Alterations
To determine whether genetic and epigenetic alterations have any prognostic significance, we followed up 37 patients for 4.0 years. The median patient survival time was 2.2 years (95% confidence interval, 16.7–35.6 years). Kaplan-Meier survival analysis showed that the 5-year survival rate was 58% for patients with unmethylated MGMT and 29% for patients with methylated MGMT (Fig. 5Go). Log-rank tests showed that methylated MGMT was associated with a high risk of dying from biliary tract cancer when compared with unmethylated MGMT (P = .024). When we analyzed it separately in each cancer type, MGMT methylation had a strong correlation with poor prognosis in extrahepatic bile duct cancer (P = .014), but it only had trend of correlation with poor prognosis in gallbladder cancer (P = .11; data not shown). No significant associations were observed between survival and the promoter methylation status of the other individual genes, although there was a trend toward an association between hMLH1 methylation and poor survival (P = .077; log-rank test; data not shown). Kaplan-Meier survival estimates did not show any associations between survival and multigene methylation, with or without MGMT methylation status. Additionally, the mutation status in cancel-related genes did not show any prognostic significance, with or without MGMT methylation status (data not shown).


Figure 5
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  		FIG. 5. Kaplan-Meier survival analysis of patients with biliary tract cancer according to O6-methylguanine–DNA methyltransfer-ase (MGMT) promoter methylation status. Patients with MGMT methylation--positive tumors had an unfavorable prognosis compared with patients who had MGMT methylation–negative tumors (log-rank test; P = .024).

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Tumor progression proceeds through a multistep process by the accumulation of sequential genetic and epigenetic alterations.11 However, these events remain largely unknown for biliary tract neoplasms. We previously found that MGMT expression was reduced in several types of human neoplasms, including biliary tract cancer, hepatocellular cancer, gastric cancer, and breast cancer, and that reduced MGMT expression correlated with a poor prognosis.10,38,39 In particular, reduced MGMT expression occurred in approximately 60% of gallbladder and extrahepatic bile tract cancers and correlated with hepatic invasion and poor prognosis in both types of tumors.10

Methylation of CpG islands in the MGMT promoter region is associated with silencing MGMT and the loss of MGMT protein expression in primary human neoplasms,19,22 In this study, MGMT promoter methylation occurred in 49% of biliary tract cancers and was closely correlated with reduced protein expression (as determined by immunohistochemistry). Additionally, we found that MGMT promoter methylation was associated with a poor prognosis, thus indicating that it could be an important biomarker in patients with biliary tract cancer. MGMT methylation status did not correlate with tumor size or stage. Although not statistically significant, MGMT methylation status tended to correlate with tumor depth, hepatic invasion, and lymphatic invasion. These findings indicate that biliary tract cancer with MGMT methylation may exhibit highly invasive properties and result in a poor prognosis.

Several reports have demonstrated that MGMT promoter methylation is associated with clinical outcomes in human cancers.22,40,41 Esteller et al. showed that inactivation of MGMT by promoter methylation was a predictive marker for the prognosis of patients with gliomas40 and diffuse large B-cell lymphoma,41 Brabender et al,22 found that the quantitation of MGMT promoter methylation by real-time PCR identified patients at high risk for early recurrence of non--small-cell lung cancer. However, these reports did not address the mechanisms responsible for poor prognosis and tumor recurrence in patients with MGMT promoter hypermethylation.

In this study, we examined genetic and epigenetic alterations in several oncogenes, tumor-suppressor genes, and DNA repair genes that had been previously identified in biliary tract cancer tissues. In our analysis of genetic alterations, we found 33 mutations in p53, K-ras, p16/INK4a, and ß-catenin. The frequency of mutations in p53 was significantly higher than that in the other three genes. Recent studies have reported that inactivation of MGMT by promoter methylation is associated with G to A gene mutations in K-ras42 and p53 in colorectal cancer43 and p53 in non--small-cell lung cancer.44 These find-ings indicate that reduced MGMT expression induces GC to AT transitions in oncogenes and tumor-suppressor genes. Thus, we correlated MGMT methylation status with GC to AT mutations in cancer-related genes. The most frequent type of mutation that we identified was GC to AT mutations, which occurred in 19 (57.6%) of 33 patients with biliary tract cancer Furthermore, GC to AT mutations were significantly associated with MGMT promoter methylation. These findings suggest that the loss of MGMT expression by promoter methylation contributed to the GC to AT transitional mutations. However, the GC to AT mutations in these cancer-related genes did not significantly correlate with prognosis (data not shown). Thus, GC to AT mutations in the cancer-related genes that we examined may contribute to carcinogenesis rather than tumor progression in biliary tract cancer.

We next analyzed the methylation status of the promoter region in four cancer-related genes. The promoter regions of these genes had been previously shown to be methylated in biliary tract cancers.31,32 The percentage of tumors with methylated gene promoters was 70% for p16/INK4a, 46% hMLH1, 41% for E-cadherin, and 32% for DAPK. Previous studies suggested that hypermethylation of multiple promoter regions correlated with tumor progression and poor prognosis in human cancers.29,30 Brock et al.29 studied 41 patients with esophageal adenocarcinoma and found that tumors with multigene methylation resulted in significantly worse survival, although methylation of individual genes showed only a trend toward diminished survival. In the present study, multigene methylation (defined as three or more genes with methylated promoters) tended to be associated with MGMT hypermethylation, thus indicating that hypermethylation simultaneously occurs on several gene promoters together with MGMT. However, patient survival was significantly correlated with MGMT methylation, but not with multigene methylation. Analysis of more genes may reveal that multigene methylation is a significant factor in the progression of biliary tract cancer.

On the basis of the genetic and epigenetic alterations that we investigated, it is possible that reduced MGMT expression may increase the malignant potential of biliary tract cancer through both types of molecular alterations. GC to AC mutations, which are triggered by epigenetic gene silencing of MGMT, may accumulate on multiple genes involved in the progression of biliary tract cancer. Moreover, the silencing of cancer-related genes by promoter hypermethylation may also occur simultaneously with MGMT methylation. These epigenetic and genetic gene alterations may lead to poor outcomes in biliary tract cancer patients with MGMT methylation. The identification of genes that affect tumor invasion or metastasis, in which GC to AT mutations or promoter hypermethylation exists in the tumor cells, is required for elucidating molecular events in biliary tract cancer progression with MGMT methylation.

In conclusion, we demonstrated that epigenetic silencing by MGMT promoter hypermethylation might play an important role in carcinogenesis and the progression of biliary tract cancer. Assessment of the MGMT methylation status is clinically useful for identifying biliary tract cancer patients with a poor prognosis. In the future, molecular therapies that mediate demethylation of the MGMT gene may improve patient prognosis.

Received for publication July 19, 2004. Accepted for publication December 19, 2004.


   REFERENCES
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 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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