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1 The Male Urological Cancer Research Centre, Institute of Cancer Research, Surrey; 2 Department of Musculoskeletal Pathology, The Royal Orthopedic Hospital National Health Service Trust, Birmingham; and 3 Department of Histopathology, The Royal Marsden National Health Service Trust, London, United Kingdom
| ABSTRACT |
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| Introduction |
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1% of all cancers and is associated with a substantial mortality rate of
50%, which is related in part to its propensity for metastasis (1
, 2) . The clinical behavior of soft tissue sarcomas is highly variable, but few reliable determinants of outcome have been identified (2
, 3)
. New markers that predict clinical outcome, in particular the propensity of primary tumors to develop metastatic tumors, are urgently needed and would be of great clinical use, allowing for more selective treatment strategies. In this study, we have chosen leiomyosarcoma as a model to assess the relationship between gene expression profiles determined on cDNA microarray and the clinical outcome of metastasis in soft tissue sarcomas. | Materials and Methods |
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1. We used the GenePix Pro 3.0.6 software (Axon Instruments) to determine ratios of fluorescent intensities (Cy5:Cy3) for individual cDNA after subtraction of background. We had previously established the reliability of these microarray procedures; 12 of 12 genes showing alterations in expression in microarray exhibited similar alterations when examined by Northern blot analyses (5)
.
Analysis of Microarray Data.
The scanned image was analyzed with the GenePix Pro 3.0.6 software (Axon Instruments). Fluorescent signals for both channels of the spots were determined. A local background in each channel was also determined for each spot, which is the median fluorescence of pixels in a halo surrounding the same array spot. Spots or areas of array with defects were flagged bad and excluded from subsequent analysis. To enhance the reliability of the expression data, another round of quality filtering was done. Spots with fluorescent spot intensity in each channel, which were >1.4 times the local background (medians) of that channel, were considered well measured (6)
, and the data were additionally filtered to include only these spots. The median background intensity was subtracted from the median spot intensity to generate the background-corrected signal intensity for use in additional analysis.
We used the GeneSpring software (Silicon Genetics, Redwood City, CA) to carry out additional microarray analyses. BRB-ArrayTools (Biometric Research Branch, National Cancer Institute, Bethesda, MD) was used for class comparison analyses (univariate F test/two-sample t test). Fluorescent intensity ratios of Cy5:Cy3 for individual spots of the filtered data were determined by dividing the background-corrected intensity for the Cy5 by that of the Cy3 channel. These ratios were then normalized by making the median of all measurements in each sample to be 1. Genes that have expression data in less than half of the samples were filtered out before the class comparison analysis. The samples were log2-transformed, and we compared the gene expressions of the 20 primary tumors (P) and 7 metastatic tumors (M) to find genes differentially expressed between the two classes using supervised class comparison analysis with a univariate F test (two-sample t test) with randomized variance model and multivariate permutation tests to control the number of false discoveries (based on 1000 random permutations of the class labels of the experiments and controlling the number of false positive to be 30 genes 50% of the time, univariate P equals 0.0122).
Two-dimensional hierarchical clustering was then applied to the log-transformed data with average-linkage clustering with Pearson correlation around zero as the similarity metric for the 335 genes identified as differentially expressed between primary and metastatic sarcomas. This analysis divided thirty nonmetastatic tumors (P, PM, and LR) into two categories (groups 1 and 2). It is considered unlikely that exposure to chemotherapy would influence our analysis because only a single patient received chemotherapy that finished 5 weeks before surgery.
We refined the 335 differentially expressed gene list with two different supervised learning methods to find a reduced set of discriminating genes best for distinguishing the two groups (groups 1 and 2) of tumors. In one approach, to select genes for use in the classifier, all genes are examined individually and ranked on their power to discriminate the two classes (groups 1 and 2). For each gene, different cutoff points on the gene expression level for that gene are considered to predict class membership either above or below that cutoff. Genes are scored on the basis of the best prediction point for that class. The score function is the negative natural logarithm of the P value for the Fishers exact test of predicted versus actual class membership for group 1 versus group 2. To additionally check that the top-ranked discriminating genes could distinguish the two groups of tumors, we carried out supervised class prediction with the k-nearest-neighbor method and a leave-one-out cross-validation (to avoid overestimating the performance of the classifier) with the top-ranked discriminating genes. A selected number of top-ranked discriminating genes were used as the classifier for assigning the class membership of the left-out samples (using 1-, 3-, and 5-nearest-neighbor method with similarity measured by Euclidean distance between the samples). In a second approach, two-sample t tests were used to identify the genes that showed the most differential expression between the two prognostic groups of tumors.
Other Statistical Analyses.
Other statistical analyses, including Fishers exact test and Kaplan-Meier analysis, were performed with SPSS (SPSS, Inc., Chicago, IL). Fishers exact test was used for assessing the significance of association between categorical variables where appropriate. For the Kaplan-Meier analysis, metastasis was used as end points. Log-rank test was used to compare cluster groups.
| Results |
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We were interested in whether the 335 genes associated with primary and metastatic distinction would be useful in classifying nonmetastatic tumors into groups that have different potential to develop metastasis and that would allow us to predict the future development of metastasis of primary tumors. We would only expect this to happen if the gene expression profile associated with metastasis is already present in some form in the bulk of cells in the primary tumors. The expression of the 335 genes was therefore used in hierarchical clustering to classify a group of 30 nonmetastatic tumors (P, PM, and LR; Fig. 1
). The tumors were clustered into two distinct groups, with the expression profile of the two groups (groups 1 and 2) highly correlating with the original primary tumor versus metastatic tumor distinction, with group 1 having a more metastatic gene expression profile (Fishers exact test P < 0.001, Table 1
). The distribution of the various tumor types (P, PM, and LR) in the two cluster groups is shown in Table 2
. We predicted that the tumors with a more metastatic gene expression profile (group 1) would have a worse prognosis on disease progression to metastasis. Indeed, of the primary tumors (P) where follow-up data were available, all six primary tumors in cluster group 1 developed metastases, whereas only 3 of 11 tumors in group 2 developed metastases. Kaplan-Meier analysis on primary tumors (P) revealed a significant difference in the time to develop metastases in the two groups (log-rank test, P = 0.001; Fig. 2
), with primary tumors in group 1 developing metastases much more rapidly than group 2 (mean time to develop metastases = 0.95 years in group 1 versus 5.18 years in group 2).
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5 or >5 cm), Fishers exact test P = 0.602; site (superficial or deep), Fishers exact test P = 0.333, and grade (low or high), Fishers exact test, P = 1.000 Supplementary Table 2); or the following demographic and clinical characteristics [age, Mann-Whitney test, P = 0.262; sex, Fishers exact test, P = 1.000; tumor site (retroperitoneal or nonretroperitoneal), and Fishers exact test, P = 0.722]. Hence, other clinicopathologic parameters could not account for these observed outcome differences. We were interested in identifying the genes best for distinguishing the two groups of tumors with different prognosis. We hypothesized that not all of the genes in the subset of 335 genes contributed to the distinction between groups 1 and 2, and we applied two different supervised learning methods in an attempt to identify the genes best associated with distinction between groups 1 and 2. Genes were scored on the basis of best cutoff point for discrimination by Fishers exact test of predicted versus actual class membership. To confirm that the top-ranked discriminating genes could discriminate between the two prognostic groups of tumors, we carried out supervised class prediction with the k-nearest-neighbor method and a leave-one-out cross-validation with the topranked discriminating genes. A model with the top 80 discriminating genes (Supplementary Table 3 and Supplementary Fig. 1) accurately assigned the left-out sample to the right group, with either 1- or 5-nearest-neighbor methods (30 of 30 samples in leave-one-out cross-validation test). This gene list could be additionally reduced to 20 genes with a slight loss of accuracy (29 of 30 samples in leave-one-out cross-validation test with a 1-, 3- or 5-nearest-neighbor methods). Additional reduction of the gene number to <20 resulted in additional loss of accuracy. We also additionally verified the gene selection with a second approach, where we carried out two-sample t tests to identify the genes that showed the most differential expression between the two prognostic groups. This gives rise to 99 genes with significant different expression (P < 0.001). In fact, we found that the genes selected with the two methods were in very good agreement; as many as 77 genes of the 80 most discriminating genes from method 1 were also present in this 99 significant gene list.
All except one of the 80 discriminating genes (MVP) had higher expression in group 1 than group 2 tumors. The 80 most discriminating genes included genes encoding proteins involved in biological processes associated with tumor development and invasion such as controlling cell growth and transition through the cell cycle (BMP2, PDAP1, CDC27, and CDK2AP1), signal transduction (IFNAR2, RIT1, GPSM1, GRB7, MAPKAPK2, and PAK2), apoptosis (BCL2A1), and nucleotide metabolism (GMPS).
| Discussion |
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Both vant Veer et al. (12) and Ramaswamy et al. (13) have provided microarray data showing that an expression signature determining the propensity to metastasize can be detected in the primary epithelial tumor. However, there are several interpretations of these observations. Ramaswamy et al. (13) favor the view that the propensity to metastasize is a bulk property of the primary tumor, thus challenging the notion that metastases arise from rare cells within a primary tumor that have the ability to metastasize; and Bernards and Weinberg (16) have suggested that it is the combination of early oncogenic alterations within a tumor rather than some late alterations specific for metastasis that determine metastatic potential. Fidler and Kripke (17) have argued that these microarray-based observations simply reflect the heterogeneity of the primary tumor and do not prove that all of the genetic changes required for metastasis are present in individual tumor cells; hence, it may still be only the rare cells that have completed all of the steps in the metastatic process that give rise to a metastasis. It has also been suggested that host genetic background is a major determinant both of metastatic ability and of the metastasis-related expression profile within the primary tumor (18) . Our microarray data capture the average gene expression of the tumor mass under study and do not theoretically exclude any of the above interpretations. Nonetheless, our microarray data do provide important information on tumor prognosis and suggest that it is also possible to predict metastasis with the average gene expression profile found in the bulk of the primary mesenchymal tumor.
A number of the 80 discriminating genes or their closely related counterparts have been previously reported to be associated with metastasis, e.g., bone morphogenetic protein 2 (BMP2) had higher expression in a highly metastatic breast cancer cell line (19) , and growth factor receptor-bound protein 7 (GRB7) signal transduction protein has been reported to contribute to the metastatic potential of cancer cells (20) . In conclusion, we have identified a gene expression signature in leiomyosarcomas that is predictive of metastatic outcome for tumors at time of presentation. These findings could have important applications in the clinic, where the choice of how aggressively the patient should be treated is affected by the predicted outcome of disease and demonstrated the importance of genomics research in medicine.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Note: Additional minimum information about microarray experiment (MIAME) compliant data will be available at ArrayExpress (www.ebi.ac.uk/arrayexpress) at a later date.
Requests for reprints: Yin-Fai Lee, The Male Urological Cancer Research Centre, Institute of Cancer Research, 15 Cotswold Road, Belmont, Sutton, Surrey SM2 5NG, United Kingdom. E-mail: Yin-Fai.Lee{at}icr.ac.uk
Received 5/12/04. Revised 8/20/04. Accepted 8/31/04.
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