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Molecular Oncology, Markers, Clinical Correlates |
Department of Surgery, Medical Institute of Bioregulation, Kyushu University, Beppu 874-0838 [H. I., A. M., K. M., M. M.], and Department of Surgery, Oita Prefectural Hospital, Oita 870-0855 [H. U.], Japan
| ABSTRACT |
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Experimental Design: RNA was extracted from tumor/normal paired samples of 43 patients with gastric cancer, and cDNA microarray hybridization was performed.
Results: We selected 78 genes that were differentially expressed between aggressive and nonaggressive groups with respect to five conventional pathological factors. Next, we determined a coefficient for each gene. Thereafter a prognostic score was calculated by summing-up the value for each gene. It ranged from -47 to 201 with a median of 114. There were two peaks in its distribution. Ten of 11 patients who were alive with no evidence of recurrence >5 years after the operation showed a score of <100 points, whereas all 19 patients who died of disease showed >100 points. In 13 patients who were alive but the follow-up time was <5 years, 2 of the 3 patients with >100 points revealed recurrent disease during the follow-up.
Conclusions: These findings demonstrate that such a system with cDNA microarray can contribute to the comprehensive analysis of malignant behavior of the tumor and may provide accurate information on prognosis.
| INTRODUCTION |
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Several interesting molecules have been reported with respect to the prognosis of gastric cancer patients. These include cell cycle regulation factors such as p27 or proliferating cell nuclear antigen, matrix proteinases, cell adhesion molecules such as E-cadherin, autocrine motility factors, angiogenic factors, growth factors, oncogenes, tumor suppressor genes such as p53, and several others (1, 2, 3, 4, 5, 6) . The expression of these molecules has been studied at the protein level by immunohistochemistry or at the RNA level by molecular methodologies such as Northern blot or reverse transcription-PCR. The interpretation of the immunohistochemical results is often difficult and sometimes different between investigators. On the other hand, it is more quantitative based on Northern blot or reverse transcription-PCR analysis. However, these methods cannot evaluate many genes at once. In addition, only one or a few selected molecules will not define the whole characteristics of a tumor. Thus, it has been recommended to study the malignant potential of a tumor from the viewpoint of the total expression profile of many genes.
Recent excellent advances in the cDNA microarray technique that can investigate gene expression systematically enable us to visualize gene expression profiles in human tumors (7, 8, 9) . This technique can show another interesting possibility; it may be equivalent to multiple competitive Northern blot analyses to evaluate the tumor:normal ratio of many genes simultaneously. The purpose of this study was to establish a prognostic scoring system for gastric cancer using cDNA microarray. Our strategy was to: (a) select the differentially expressed genes between two different groups for each of the five conventional pathological factors; (b) determine a coefficient of expression for each gene; and (c) calculate the prognostic score. Our prognostic scoring system was revealed to be very useful in the determination of an individuals prognosis.
| MATERIALS AND METHODS |
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RNA Isolation and cDNA Microarray.
The total RNA was extracted from the specimens as described previously (10)
. Poly(A)+ mRNA was prepared from total RNA using oligotex TM-MAG mRNA purification kit (TaKaRa Shuzo Co., Ltd., Shiga, Japan). The fluorescently labeled cDNA probes were prepared as described. One-µg aliquots of RNA from cancer tissue and normal corresponding tissue were labeled using the RNA Fluorescence Labeling Core Kit (TaKaRa Shuzo Co., Ltd.) with Cy3-dUTP and Cy5-dUTP (Amersham), respectively, in each paired case.
We used the commercially available cDNA microarray (IntelliGene Human Cancer chip; TaKaRa Shuzo Co., Ltd.) that contains 425 selected known human genes that are considered to be cancer related. These include oncogenes, tumor suppressor genes, growth factors, apoptosis-related genes, matrix proteinase genes, angiogenesis-related genes, drug-resistant genes, and so on. The internal control genes included 14 genes such as ß-actin, ATP synthase, or glyceraldehyde-3-phosphate dehydrogenase. Labeled probes were mixed with hybridization solution (6x SSC, 0.2% SDS, 5x Denhardts solution, and 0.1 mg/ml denatured salmon sperm DNA). After hybridization for 14 h at 42°C, the slides were twice washed in 2x SSC and 0.1% SDS for 30 min at 55°C, washed in 2x SSC and 0.1% SDS for 5 min at 65°C, and washed in 0.05x SSC for 5 min at room temperature. The slides were scanned using the Affymetrix 418 (Affymetrix).
Data Analysis.
The signal intensity of hybridization was evaluated photometrically by the ImaGene computer program (BioDiscovery) and normalized to the averaged signals of housekeeping genes. A cutoff value for each expression level was automatically calculated according to the background fluctuation. The fluctuation can be estimated as the variance of the log ratio of Cy3:Cy5 minus the variance of the log ratio of Cy3:Cy5 of highly expressed genes.
We selected five conventional prognostic factors (6
, 11)
and divided the cases into two groups for each factor as shown in Table 1
. The average expression intensity of each gene was calculated in each group and compared between the two groups. Then we picked up the differentially expressed genes with a significant difference that was defined by the Students t test as mentioned below. The representative selected genes related to the depth of invasion and lymph node metastasis are shown in Table 2
. The frequency of pick-up of each gene was evaluated as a coefficient. This means that if the gene was selected as differentially in all five prognostic factors, its coefficient was evaluated as five. If it was picked up in relation to three prognostic factors, its coefficient was three. If the gene expression was inversely correlated with the malignant potential, a minus point was given in this case.
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| RESULTS |
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Table 3
shows a coefficient for each gene ranging from plus 5 to minus 3. Interestingly, 347 of 425 evaluated genes were not differentially expressed between the two groups in any factor, and a coefficient was therefore 0. There was a significantly different expression in the remaining 78 genes between the two groups. Four genes were regarded as plus 5 because they appeared in all of the five factors; the next eight genes were regarded as plus 4 because they appeared in four factors. Similarly, 19 genes were scored as plus 3, 9 genes were plus 2, 20 genes were plus 1, 15 genes were minus 1, and 3 genes were minus 3. There was no gene scored as minus 2. To calculate the prognostic score using the 78 differentially expressed genes, the following formula was considered:
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-fetoprotein and have a very poor prognosis (12)
. Group B included 3 patients with >100 points and 10 with <100 points. Two of the 3 with >100 points showed recurrent disease; one stage II patient recurred in the paraaortic lymph nodes 3 years after operation, and the other stage III patient recurred in the liver 2.5 years after operation. None of the 10 patients with <100 points showed recurrent disease at the time of this writing. | DISCUSSION |
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Our study demonstrates another useful application of cDNA microarray in clinical care. It has been difficult to predict the recurrence, metastasis, or prognosis of the individual gastric cancer patient, especially those with an intermediate stage, by the conventional staging system. Thus, it is desirable to estimate the grade of malignancy of the tumor as a score. Our study demonstrates that it is possible to do so by cDNA microarray. The prognostic score in this case demonstrated that patients whose score was >100 points showed a poor prognosis, whereas those whose score was <100 points showed a good prognosis. In addition, this scoring system was able to evaluate a patient who showed an unexpected outcome; one patient with stage I disease died of liver metastasis 37 months after operation. The score of this patient was 137, indicating a poor prognosis, although his stage of disease was early. As shown in Fig. 1B
, the prognostic scoring system could differentiate high-risk patients in an intermediate stage of II or III. Two of three patients in group B (Fig. 1B)
showed recurrent disease, and the tumors of these patients showed a score of >100. These patients were difficult to identify using the conventional staging system.
The strategy for our scoring system was: (a) to select the differentially expressed genes between two different groups for each of the five conventional pathological factors; (b) to determine a coefficient for each gene; and (c) to calculate the prognostic score. The methods described here may be considered tentative and may be further refined. Variables include which conventional pathologic factors should be selected and what the most suitable method to determine a coefficient for each selected gene is. These problems will be resolved, and the most suitable method or analysis will be optimized by further studies in the near future.
The DNA chip used in this study contained 425 cancer-related genes that have been reported in the literature. Our study demonstrated that
80% of these genes were not differentially expressed between aggressive and nonaggressive groups of gastric cancer. This means that not a large number of genes appear to participate in determining the grade of malignancy. Microdissection of exclusively tumor elements could, however, modify this conclusion. There were four genes that were selected in all five pathological factors. Of these, three genes such as matrix metalloproteinase-7 (13)
, secreted protein, acidic, cysteine-rich (osteonectin) (14)
, and transforming growth factor-ß3 (15)
have been reported to be correlated with tumor progression or metastasis in several cancers including gastric cancer. Especially we have demonstrated that matrix metalloproteinase-7 is a good indicator of aggressive behavior of the tumor (13
, 16
, 17)
, and this study confirmed our hypothesis. The molecules that are known to correlate with tumor progression or metastasis, such as fibronectin precursor, carcinoembryonic antigen-related cell adhesion molecule 6, IGF binding protein 3, or proliferating cell nuclear antigen were selected in four factors. On the other hand, thrombospondin 2 is an angiostatic factor reported to be suppressed in highly metastatic cancers; its expression was inversely correlated with tumor progression in colon or lung cancers (18
, 19)
. Thus, the selection of thrombospondin 2 in this study of gastric cancer needs further study. However, recently, Lee et al. (20)
reported similar results to those of our study in that it was overexpressed in gastric cancer compared with normal gastric tissue.
In conclusion, this study demonstrates that the prognosis of individual gastric cancer patients can be predicted by cDNA microarray technique. Such analysis will be important to both patients and their doctors in terms of life planning and treatment planning, respectively.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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1 This work was supported in part by the Grant-in-Aid for Scientific Research on Priority Areas of Cancer (12215116, 11671251, 12218227), the Grant-in-Aid for Scientific Research (B) (12557100, 12470241) and (C) (12213101, 12671232, 12670166), and the Grant-in-Aid for Exploratory Research (13877188), the Ministry of Education, Culture, Sports, Science and Technology of Japan. This work was also supported by the Uehara Memorial Foundation, Naito Foundation, Casio Science Promotion Foundation, Foundation for Promotion of Cancer Research in Japan, and Sagawa Foundation for Promotion of Cancer Research. ![]()
2 To whom requests for reprints should be addressed, at Department of Surgery, Medical Institute of Bioregulation, Kyushu university, 4546 Tsurumibaru, Beppu 874-0838, Japan. ![]()
Received 12/26/01; revised 7/15/02; accepted 7/16/02.
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