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J Thorac Cardiovasc Surg 2005;130:1371
© 2005 The American Association for Thoracic Surgery
General Thoracic Surgery |
a Division of Thoracic Surgery, Tri-Service General Hospital, National Defense Medical Center, Taipei, Taiwan, Republic of China.
c Department of Surgery, Tri-Service General Hospital, National Defense Medical Center, Taipei, Taiwan, Republic of China.
f Department of Pathology, Tri-Service General Hospital, National Defense Medical Center, Taipei, Taiwan, Republic of China.
b Division of Thoracic Surgery, Veterans General Hospital, Taipei, Taiwan, Republic of China.
d Department of Life Sciences, National Taiwan Normal University, Taipei, Taiwan, Republic of China.
e Department of Humanity and Social Studies, National Defense Medical Center, Taipei, Taiwan, Republic of China.
Read at the Eighty-fifth Annual Meeting of The American Association for Thoracic Surgery, San Francisco, Calif, April 10-13, 2005.
Received for publication March 25, 2005; revisions received May 15, 2005; accepted for publication June 7, 2005. * Address for reprints: Shih-Chun Lee, MD, Division of Thoracic Surgery, National Defense Medical Center, 325, Section 2, Cheng Gong Road, Nei Hu, Taipei, Taiwan 114, Republic of China. (Email: ylc116{at}yahoo.com.tw).
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Abstract |
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METHODS: We analyzed promoter methylation of hMLH1 using methylation-specific polymerase chain reaction and hMLH1 and hMSH2 protein by using immunohistochemistry in 60 resected tumor specimens. The Pearson 
2 test was used to compare expression of hMLH1 and hMSH2 protein among patients with different clinicopathologic parameters. Concordance analysis was performed between hMLH1 methylation and its protein expression.
RESULTS: Loss of hMLH1 and hMSH2 protein was found in 43 (72%) and 39 (65%, P = .06) of 60 resected specimens, respectively. hMLH1 protein correlated well with tumor staging (P < .0001), depth of tumor invasion (P = .008), and nodal involvement (P < .0001) but not with distant metastasis, whereas hMSH2 did not show correlation with any of these parameters. A concordance rate of 83.3% was present between expression of hMLH1 protein and its promoter methylation (P < .001).
CONCLUSIONS: Aberrant expression of hMLH1 and hMSH2 protein is frequently associated with the presence of esophageal squamous cell carcinoma, and expression of hMLH1 protein is a better prognostic predictor than is expression of hMSH2 protein. Promoter methylation is one of the mechanisms responsible for loss of hMLH1 protein in esophageal squamous cell cancer.
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Introduction |
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Promoter methylation is one of the most important mechanisms leading to inactivation of gene expression and has been known to cause aberrant expression of some important tumor suppressor genes, such as p16,
1
E-cadherin,
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and the von Hippel Lindau gene.
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Promoter methylation has been implicated in the carcinogenic process in many cancers, including gastric cancer, colorectal cancer, breast cancer, lung cancer, ovarian cancer, bladder cancer, and oral cancers.
4,5
Mismatch repair (MMR) is an important mechanism by which cells correct errors in DNA replication during proliferation.
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Cells with MMR defects show mutation rates up to 1000-fold greater than those observed in normal cells.
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Two MMR genes, human mutL homolog 1 (hMLH1) and human mutS homolog 2 (hMSH2), have been cloned and demonstrated to participate in DNA MMR.
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The mutant phenotype has been detected in tumors from patients with hereditary nonpolyposis colorectal cancer,
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as well as in lung,
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ovarian,
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head and neck,
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and gastrointestinal cancers, including gastric cancer
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and adenocarcinoma of the esophagus.
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The loss of corresponding protein is seen in almost all of the tumors with germline MMR gene mutation.
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The loss of hMLH1 expression has been shown to correlate with cytosine methylation of CpG islands regions in its promoter region in colon cancer cell lines and tissues.
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Promoter methylation associated with loss of the hMLH1 protein has been found in some MMR-defective colorectal and endometrial cancer cell lines in which mutations in the MMR genes, including hMSH2, hMLH1, hPMS2, and hPMS1, were absent.
17,18
Interestingly, these results demonstrate that the promoter of hMSH2 is rarely methylated, indicating that aberrant methylation of the hMLH1 promoter is potentially a very important mechanism in the inactivation of the MMR system in human cancers.
17,18
Little is known about whether promoter methylation of hMLH1 is associated with loss of hMLH1 protein and the role of promoter methylation in tumorigenesis of squamous cell carcinoma of the esophagus. There are no correlative studies of aberrant expression of these MMR genes and the prognosis of esophageal squamous cell cancer. We examined promoter methylation of hMLH1 and expression of hMLH1 and hMSH2 protein in squamous cell carcinoma of the esophagus in relation to prognosis.
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Methods |
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for esophageal cancer. Survival and follow-up data were obtained from the Institutional Cancer Registry.
DNA Extraction and Methylation-specific Polymerase Chain Reaction Assay for hMLH1
Serial 5-µm sections were cut from formalin-fixed, paraffin-embedded tissue blocks. All slides were stained with hematoxylin and eosin, and one of the slides was used as a guide to localize tumor regions. The tumor cells were microdissected from 3 slides of serial sections. After deparaffinization in xylene, genomic DNA of recovered tumor cells was prepared by using proteinase K digestion and phenol-chloroform extraction, followed by ethanol precipitation. The promoter methylation status of the hMLH1 gene of all tumor samples and their normal mucosa control samples was determined by means of chemical treatment with sodium bisulfite and subsequent methylation-specific polymerase chain reaction (PCR) analysis, as described by Herman and colleagues.
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For bisulfite treatment of DNA used for methylation-specific PCR, extracted DNA (1 µg) in a volume of 50 µL was denatured with NaOH (final concentration, 0.2 mol/L) for 10 minutes at 37°C for samples with nanogram quantities of human DNA, and 1 µg of salmon sperm DNA (Sigma-Aldrich Corporation, St Louis, Mo) was added as a carrier before modification. Thirty microliters of 10 mmol/L hydroquinone (Sigma-Aldrich) and 520 µL of 3 mol/L sodium bisulfite (Sigma-Aldrich) at pH 5, both freshly prepared, were added and mixed, and samples were incubated under mineral oil at 50°C for 16 hours. Modified DNA was purified by using the Wizard DNA purification resin according to the manufacturer (Promega Corp, Madison, Wis) and eluted into 50 µL of water. Modification was completed by NaOH (final concentration, 0.3 mol/L) treatment for 5 minutes at room temperature, followed by ethanol preditation. DNA was resuspended in water and stored at –20°C until used for PCR amplification. Primer sequences of hMLH1 for the unmethylated reaction were 5'-TTTTGATGTAGATGTTTTATTAGGGTTGT-3' (sense) and 5'-ACCACCTCATCATAACTACCCACA-3' (antisense), and for the methylated reaction, they were 5'-ACGTAGACGTTTTATTAGGGTCGC-3' (sense) and 5'-CCTCATCGT AAC-TACCCGCG-3' (antisense).
9
Paraffin-embedded samples were amplified first with flanking PCR primers that amplify bisulfite-modified DNA but that would not preferentially amplify methylated or unmethylated DNA. The primers used were 5'-GAGTAGTTTTTTTTTTAGGAGTGAAG-3'27 (sense) and 5'-AAAAACTATAAAACCCTATACCTAATCTA-3' (antisense).
9
PCR was performed for 40 cycles at an annealing temperature of 65°C and 62°C for unmethylated and methylated reactions, respectively, by using 50 ng of bisulfite-modified DNA. All PCRs were performed with positive controls for both unmethylated and methylated alleles and no DNA control. A ratio of greater than 0.5 was defined as aberrant methylation.
Analysis of Protein Expression: Immunohistochemistry Assay
Paraffin blocks of tumors were cut into 5-µm slices and then processed by using standard deparaffinization and rehydration techniques. After antigen retrieval with microwave heating, all tissue sections on slides were immunostained according to instructions from the antibody suppliers. The monoclonal antibody used was G168-728 (1:250; PharMingen, San Diego, Calif) for the hMLH1 protein and FE11 (1:50; Oncogen Science, Cambridge, Mass) for the hMSH2 protein, respectively. The binding of primary antibody was visualized with a detection system (DAKO LSAB Kit K675; DakoCytomation California Inc, Carpinteria, Calif). The normal staining pattern for hMLH1 and hMSH2 is nuclear. Tumor cells that exhibited an absence of nuclear staining in the presence of nonneoplastic cells or infiltrating lymphocytes with nuclear staining were considered to have an abnormal pattern. Staining results were examined by 2 observers blinded to the status of the molecular analyses.
Statistical Analysis
The Pearson 2 test was used to compare expression of hMLH1 and hMHS2 protein and methylation status of hMLH1 among patients undergoing esophagectomy for esophageal squamous cell carcinoma with different clinicopathologic characteristics, including age, stage, and overall TNM category. Comparison of age between patients with and without alterations was made by using the 2-sample t test. The Kaplan-Meier method was used to estimate the probability of survival as a function of time and the median survival.
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The log-rank test was used to assess the significance of difference between pairs of survival probabilities.
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Discussion |
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and is believed to be involved in carcinogenesis in some human cancers.
9-14
Promoter methylation is an important mechanism that causes inactivation of expression of DNA MMR genes, such as hMLH1, in human cancers.
10,12,13
Little is known about whether promoter methylation of hMLH1 is associated with loss of hMLH1 protein and whether aberrant expression of hMLH1 protein is involved in tumorigenesis of squamous cell carcinoma of the esophagus. Furthermore, there are no correlative studies of aberrant expression of these MMR genes and the prognosis of esophageal squamous cell cancer. To address the above questions, we designed this study to investigate promoter methylation of hMLH1 and expression of hMLH1 and hMSH2 protein in squamous cell carcinoma of the esophagus in relation to prognosis. Aberrant expression of hMLH1 protein was observed in a considerable number of our patients within the early and late stages, suggesting that loss of hMLH1 protein is an early event in tumorigenesis of esophageal squamous cell cancer, and it persists as tumor progresses. Loss of hMSH2 protein was also observed in a considerable number of patients, but it did not show significant correlation between any of the patients' clinicopathologic parameters. In contrast, aberrant expression of hMLH1 protein correlated well with patients' overall stage, T status, and N status. These results suggest that expression of hMLH1 protein is a better prognostic predictor than expression of hMSH2 protein. Intriguingly, survival analysis did not show significant correlation between patients with normal and aberrant expression of hMLH1 or hMSH2 protein. We speculated that absence of correlation between survival and expression of hMLH1 or hMSH2 protein might be attributed to the small sample size studied.
A concordance of 83.3% observed between promoter methylation of hMLH1 and its protein expression implies that promoter methylation is an important mechanism that is responsible for the inactivation of protein expression of hMLH1. Within the remaining 16.7% of discordance, 8 (13.3%) tumor samples presented with unmethylated promoter of hMLH1 but aberrant expression of hMLH1 protein, suggesting that there might be mechanisms other than promoter methylation leading to inactivation of gene expression, such as loss of heterozygosity or splicing of mRNA transcript, as reported previously.
9,23
Our results support previous results that suggest promoter methylation of the hMLH1 as a major mechanism responsible for silencing of its gene expression and an active player in carcinogenesis for some human cancers.
8-12
The majority of previous studies did not correlate results from molecular analyses with patients' clinicopathologic parameters or outcome, as we did in the current study.
Promoter methylation with aberrant expression of hMLH1, but not hMSH2, has been reported in gastrointestinal cancers, including colorectal and gastric cancers.
5,17
There are limited reports discussing the role of promoter methylation and altered expression of hMLH1 or hMSH2 protein in esophageal cancer. Evans and coworkers
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demonstrated loss of both hMLH1 and hMSH2 protein in 78% (21/27) of resected esophageal adenocarcinomas, but they concluded that protein expression was not associated with tumor grade, pathologic stage, or patient survival. Promoter methylation was not investigated in their study. In comparison, we also demonstrated a high frequency of aberrant protein expression of both hMLH1 and hMSH2 and promoter methylation of hMLH1. Unlike their results, a good correlation between aberrant protein expression of hMLH1 and tumor stage and individual T and N status was observed in our study. In addition, we showed a significant correlation between promoter methylation of hMLH1 with patients' stage and N status. Differences in these 2 results might be attributed to different types of cancers, different ethnic groups, and different sample sizes studied or to patient selection.
Results from studies related to methylation and protein expression of the hMLH1 gene in squamous cell cancer of the esophagus remain controversial. Nie and associates
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showed a frequency of hMLH1 methylation in 5 (20%) of 24 resected esophageal squamous cell carcinomas, but they did not describe its protein expression. On the other hand, Hayashi and colleagues
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reported a low frequency of 20% for aberrant hMLH1 protein expression in 30 resected esophageal squamous cell carcinoma samples. In that study only 7 of the 30 resected tumor samples were analyzed for methylation of the hMLH1 gene, and none of them was methylated. In contrast, Kubo and coworkers
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reported a high frequency (66.7%) of hMLH1 methylation and its protein expression in 34 esophageal carcinomas associated with other primary cancers but did not clarify their frequencies in esophageal carcinoma alone. Discrepancies in these results can be attributed to differences in patient selection, different ethnic groups of patients, and possibly different sample sizes studied. As noted, their studies did not perform an association analysis for methylation and protein expression of the hMLH1 gene, nor did they show a good correlation between gene methylation and protein expression with patients' clinicopathologic parameters, probably because of smaller sample sizes compared with that in our series.
In conclusion, our results suggest that aberrant expression of hMLH1 and hMSH2 protein is frequently associated with the presence of esophageal squamous cell carcinoma, suggesting that loss of these 2 DNA MMR proteins is involved in the tumorigenesis of this type of esophageal cancer. Protein expression of hMLH1 is a better prognostic predictor for resected squamous cell cancer of the esophagus than protein expression of hMSH2. Promoter methylation is one of the mechanisms responsible for the inactivation of hMLH1 protein expression and is a potential molecular marker for squamous cell cancer of the esophagus. Further studies with cell line or resected tissue samples to elucidate other possible mechanisms of altered expression of the hMLH1 and hMSH2, including microsatellite instability of the genes, loss of heterozygosity, and alterations in mRNA transcription, might help us to understand better the role of MMR system in the tumorigenesis of squamous cell carcinoma of the esophagus.
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Discussion |
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Over the past decade, there have been enormous efforts to understand the molecular mechanisms of defective mismatch repair in colon cancer, which have led to diagnostic tests to screen these gene mutations and have aided the oncologist to apply appropriate and specific regimens for tumor treatment. However, similar data are lacking in the esophageal cancer model.
Dr Tzao, my first question addresses your unique results of the high frequencies of hMLH1 and hMSH2 downregulation. Both are very rarely simultaneously downregulated in cancer. Either a tumor mutates or epigenetically alters hMLH1 or hMSH2, but this is one of the first reports I can find of both being downregulated with such high frequencies of 72% and 65%, respectively. These frequencies are difficult to explain from a tumor biology point of view. In colorectal cancers, in which immunohistochemical analysis of these genes was first described by Lindor in 2002, he never saw absences of both hMLH1 and hMSH2 together in more than 1100 cancers. The absence rates of immunohistochemical expression in most other cell types are less than 30%. Given your unprecedented results, what are your thoughts on the proposed mechanism of hMSH2 downregulation with hMLH1?
Dr Tzao. Thank you, Dr Yang, for the questions and important comments. What you have referred to is probably that colorectal cancer has a low incidence of hMLH1 expression. One of my collaborators, Dr Yi-Ching Wang, published an article on lung cancer lately in the Journal of Clinical Investigation, discussing both hMLH1 and hMSH2. They found a significant frequency in loss of expression of both of these 2 mismatch repair proteins. As far as I know, there are no data presenting the status of protein expression of these 2 proteins in parallel for squamous cell carcinoma in the same report. Those were what we observed. At this time, I have no good explanation as to the mechanism for loss of both of these 2 proteins. One might speculate that discrepancies could be attributed to differences in ethnic group, patient selection, or a different type of cancer studied.
Dr Yang. In your article you provide an explanation of why hMLH1 has reduced protein expression but provide no explanation for hMSH2. Did you look at hMSH2 methylation? Is squamous cell carcinoma unique in being deficient in DNA mismatch repair activity?
Dr Tzao. I am sorry, can you rephrase that?
Dr Yang. Well, the first part of that question—you explained why hMLH1 has reduced protein expression, but you did not do that for hMSH2.
Dr Tzao. hMSH2 methylation has been previously studied in other types of cancer, and it is rarely methylated, as reported by the majority of those published works. That is one of the reasons that we did not do the methylation assay for hMSH2. Another reason for not performing this study actually is because our system was not working out well at the time of this report, but we will keep working on that to find out possible mechanisms that are responsible for aberrant expression of hMSH2 protein.
Dr Yang. Finally, I have a clinical question. How many of the patients in your cohort received adjuvant or neoadjuvant therapy? If they did, how does this affect your survival curves?
Dr Tzao. We were expecting to see a separate curve for the hMLH1 group, and actually, before 30 months, it appeared to separate out, but after that, it seemed to converge together, making the overall survival not significantly different. Our speculation to this observation is probably because of the selection of our patients because many of our patients were in the late stage, beyond stage IIB, and were generally given postoperative adjuvant therapy but did not improve in terms of survival. This might obscure a potential significant difference in survival. I believe that we need a larger sample size, including a more even distribution in early and late stages, to see whether there is a significant difference.
Dr Yang. Correct. That was one of the questions on that. On the left-hand panel, there was a divergence in the first 15 months.
Dr Tzao. Yes, in the first 30 months of follow-up.
Dr Yang. Therefore you cannot explain that by stage or other clinical factors, correct?
Dr Tzao. The explanation to your question, as I just mentioned, is probably because of our selection of patients.
Dr Yang. Again, I want to thank the Association for the privilege of discussing this article and congratulate you again on your findings.
Dr Tzao. Thank you very much.
Dr King F. Kwong (Baltimore, Md). Dr Tzao, I congratulate you and your colleagues on a fine foray into a very difficult area.
As you know, many genetic alterations and changes do not always correlate in terms of expression, and being able to look at the protein end of things really starts to get at the true mechanisms of how these genetic changes work.
I have 2 very quick questions. First, as you know, the incidence of adenocarcinoma is much more prevalent in our country, here as well as in some western countries, which is very contradistinct from in your country and in some of Asia. Have you looked at some of these changes—protein expression—and these genetic changes in adenocarcinoma? As you know, the difference between adenocarcinoma and squamous cell carcinoma is perhaps very different tumor biologies at play.
Dr Tzao. Thank you for the question.
In our country, it is interesting to tell you that we have more than 80% of patients with squamous cell carcinoma. Very rarely do we see esophageal adenocarcinoma. I have no explanation for why there is a rarity of adenocarcinoma at this time. This is why we focused on our patient population in which squamous cell carcinoma predominates.
Dr Kwong. So you have not looked at this in adenocarcinoma?
Dr Tzao. No. There has been a study by Evans and associates, and I think that is the only study on adenocarcinoma. In that study, they do not look at the promoter methylation for hMLH1 but only the protein expression in this type of esophageal cancer.
Dr Kwong. Thank you.
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Acknowledgments |
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Footnotes |
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This article has been cited by other articles:
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  C. Tzao, G.-H. Sun, H.-J. Tung, H.-S. Hsu, W.-H. Hsu, Y.-C. Wang, Y.-L. Cheng, and S.-C. Lee Reduced Acetylated Histone H4 is Associated With Promoter Methylation of the Fragile Histidine Triad Gene in Resected Esophageal Squamous Cell Carcinoma Ann. Thorac. Surg., August 1, 2006; 82(2): 396 - 401. [Abstract] [Full Text] [PDF]
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 S. Demokan, Y. Suoglu, D. Demir, M. Gozeler, and N. Dalay Microsatellite instability and methylation of the DNA mismatch repair genes in head and neck cancer Ann. Onc., June 1, 2006; 17(6): 995 - 999. [Abstract] [Full Text] [PDF]
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