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Cancer Prevention Studies Branch [H. S., N. H., M. J. R., C. W., S. M. D., P. R. T.], Biometric Research Branch [J. S.], Laboratory of Population Genetics [Y. H.], Radiation Oncology Branch [E. Y. C.], Genetic Epidemiology Branch [A. M. G.], and Laboratory of Pathology [M. R. E-B.], National Cancer Institute, NIH, Bethesda, Maryland 20892; Shanxi Cancer Hospital and Institute, Taiyuan, 030013, Peoples Republic of China [Q-H. W., T. D.]; and Information Management Service, Inc., Silver Spring, Maryland 20904 [C. G.]
| ABSTRACT |
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2-fold change) between tumor and matched normal samples (13 overexpressed and 28 underexpressed). Hierarchical clustering showed consistent molecular profiles across patients. Multidimensional scaling plots visually distinguished cases by family history status, which was confirmed statistically using a global permutation test (P = 0.007); we then identified 152 genes of which the expression differed in tumors from family history positive versus negative cases (55 overexpressed and 97 underexpressed at P < 0.001). These data indicate that molecular profiles in esophageal squamous cell carcinoma are highly consistent and that expression patterns in familial cases differ from those in sporadic cases. | Introduction |
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To complement the genomic studies of ESCC, we are also analyzing tumor gene expression profiles. Array technologies are comprehensive and relatively accurate ways to simultaneously analyze the expression of thousands of genes, and these technologies have been used to clarify gene expression changes in many human malignancies, including nine published studies of squamous cell carcinoma and/or adenocarcinoma of the esophagus (9, 10, 11, 12, 13, 14, 15, 16, 17) . However, at least half of the microarray expression studies of ESCC published to date have relied on cell culture systems (9 , 10 , 12 , 14) . Cell lines differ from tumor cells, as they have been removed from their in vivo environment and are selected for growth characteristics in culture, thus bringing into question the clinical relevance of these findings. In this study, tumor and matched normal tissue from 19 ESCC patients were analyzed using cDNA microarrays containing 7680 genes to evaluate gene expression differences in ESCC patients from a high-risk area in China.
| Materials and Methods |
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Microarray Fabrication
The microarrays used for the experiments (NCI ROSP 8K Human Array)4
contained 7680 human cDNA clones and were prepared from the Research Genetics Named Genes set (Huntsville, AL). These cDNA clones are all known genes and can be classified into several groups based on their biological functions, such as stress proteins, cell cycle control proteins, signal transduction proteins, apoptosis, transcription factors, DNA repair and replication proteins, cytokines, and so forth. All of the 7.6K cDNAs were spotted onto poly-L-lysine-coated slides using an OmniGrid arrayer (GeneMachines, San Carlos, CA) according to Eisen and Brown (18)
.
Sample Preparation and Chip Hybridization
Total RNA Isolation.
Total RNA was extracted from frozen tumor and matched normal tissue by using TRIzol reagent (Invitrogen, Carlsbad, CA) following the protocol of the manufacturer. The integrity of total RNA was checked on 1.2% denaturing agarose gel electrophoresis (visual presence of 28S and 18S bands).
mRNA Amplification.
One to 3 µg total RNA was used in each mRNA amplification. Antisense RNAs were generated by two rounds of amplification as described previously by Wang et al. (19)
.
Labeling and Hybridization in the Dye Swap Design.
Three µg of antisense RNA from the tumor tissue and the matched normal tissue were labeled with Cy3-dUTP and Cy5-dUTP (Amersham Pharmacia Biotech, Piscataway, NJ), respectively, and called the forward group. Reciprocal labeling was also performed for each case (i.e., tumor labeled with Cy5-dUTP and normal tissue-labeled with Cy3-dUTP) and called the reverse group. The labeled probes for the forward group were hybridized with the 7.6K chip in parallel with that of the reverse group in a hybridization buffer for 1416 h at 65°C. After hybridization, the slides were washed in 2x SCC with 0.1% SDS, 1x SCC, 0.2x SCC, and 0.05x SCC, sequentially for 1 min each, and then spin dried.
Microarray Image Analysis
Hybridized arrays were scanned at 10 µm resolution on a GenePix 4000A scanner (Axon Instruments, Inc., Foster City, CA) at variable PMT voltage settings to obtain maximal signal intensities with <0.1% probe saturation. The Cy5-labeled cDNAs were scanned at 635 nm, and the Cy3-labeled cDNA samples were scanned at 532 nm. The resulting TIFF images were analyzed by GenePix Pro 3.0 software (Axon Instruments, Inc.). Both digitized images were overlaid to form a pseudo-colored image, and a detection method was then used to determine the actual target region based on the information from both red (Cy5) and green (Cy3) pixel values. The ratios of the sample intensity to the reference intensity (red:green) for all of the targets were determined, and ratio normalization was performed to normalize the center of the ratio distribution to 1.0. The intensities of each hybridization signal were evaluated by the NCI microarray database.4
Data Analysis
Missing Data.
Log2-ratios of median local background subtracted intensity levels were analyzed. Those array elements with intensity <100 in one channel were truncated at 100, and those array elements that were flagged as bad spots during image analysis or that had intensities <100 in both channels were treated as missing. Three percent of the array elements were found to be missing in this way. Those patients with missing values for a particular array element were excluded from all of the analyses involving that array element.
Normalization and Adjustment for Dye Bias.
Log2-ratios for each microarray were normalized by the locally robust smoother Lowess (20)
. Residual dye bias after normalization was removed by taking the average of forward (green-tumor/red-normal) and reverse (red-tumor/green-normal) fluorescent log2-ratios for each of the samples where both forward and reverse labeling were done. The spot-specific dye bias was estimated from these samples by taking half of the average difference between forward and reverse log2-ratios. For some samples, reverse labeling was not successful, so the estimate of the dye bias was subtracted from the log2-ratio.
Generation of the Gene List.
One-sample t tests were used to identify genes that were different at the P < 0.001 level with at least a 2-fold change between tumor and normal samples.
MDS and Difference of Gene Expression Profiles between Two Groups of Patients.
MDS is one of the dimension-reduction methods used to visualize the similarity of gene expression profiles between samples (21)
. The similarity of the gene expression profiles between samples was assessed by Euclidean distance and Pearson correlation coefficients of the log-transformed expression ratios for the 5756 genes that were without a missing value in all 19 of the cases. The MDS plot shows sample positions in a three-dimensional space. Samples with similar gene expression profiles (short distances) are placed near each other in the MDS plot and separated from other more dissimilar groups (longer distances). When MDS plots visually appeared to show differences between groups, we formally tested these differences using the global permutation test (with 1000 permutations). Differences in individual genes were then tested using a t test (P < 0.001) for groups that were significantly different in the global permutation test.
Hierarchical Clustering.
Data were evaluated using the clustering software described previously (22)
. Average linkage clustering in TreeView software (22)
was used to generate visual representations of clusters.
| Results |
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2-fold change) we identified 41 genes, including 13 overexpressed genes in tumors and 28 underexpressed genes (Table 2)
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1.5-fold changes, whereas just 6 had
2-fold changes.
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| Discussion |
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Among the underexpressed genes in ESCC we identified are several genes involved in regulation of the cell cycle and proliferation (i.e., APCL, CNN3, SPRK, FOSL1, UPK1A, EMP1, BTC, and PPL), 2 genes involved in cell adhesion and cytoskeletion functions (i.e., EVPL and ARC), and 2 genes involved in the formation and maintenance of the cornified cell envelope of stratified squamous epithelia (CSTA and CSTB). More specifically, PPL is expressed in stratified squamous epithelia (24) and EVPL (25) , a candidate gene for the tylosis esophageal cancer syndrome, is expressed exclusively in stratified squamous epithelia. Both PPL and EVPL have desmosome components and, thus, in conjunction with TGM3 and CSTA, help to maintain an intact cell surface interface (26) .
Two cases clustered differently than the other 17 (Fig. 1)
, particularly in regard to the underexpressed genes. One of these cases (SHE1190) was the only poorly differentiated tumor in the group. The other case (SHE1188) had the lowest percentage of tumor (50%) of the 19 specimens examined. No other differences, technical or otherwise, were readily evident that might explain the expression differences in these two cases.
Intermediate filaments are polymers that, together with actin and microtubules, form the cytoskeleton of cells and are critical in maintaining normal cell integrity. Epithelial cell intermediate filaments are keratins derived from a family of proteins that includes KRT14 and KRT4. The KRT14 protein is specifically expressed in mature ectodermal components of the esophagus. Missense mutations in KRT14 that perturb intermediate filament assembly result in cell degeneration, disruption of the keratin network, and cell fragility. KRT14 mutations are also associated with a group of genetic disorders known as epidermolysis bullosa simplex, a skin-fragility disorder (27)
. KRT14 was first identified as an underexpressed gene in a cDNA microarray experiment using an ESCC cell line (12)
. However, in our study of individual tumors from ESCC patients, KRT14 was overexpressed,
3-fold on average. KRT14 has also been studied as a marker of squamous differentiation in various tumors using immunohistochemistry, but only one study has reported on this technique in ESCC (28)
. Chu et al. (28)
studied 435 cases of epithelial neoplasms of various primary sites of origin (including 14 ESCCs), and found that KRT14 expression was generally restricted to squamous cell carcinomas regardless of origin and/or degree of differentiation. Thus, evidence suggests that altered KRT14 expression may play an interesting and potentially specific role in ESCC carcinogenesis. Additional studies evaluating protein, RNA, and DNA in a larger number of cases, as well as confirmatory studies using RNA from pure tumor/normal cell populations obtained by laser capture microdissection, are needed before this potential marker can be considered for use in early detection or in the evaluation of treatment response for ESCC.
A second purpose of this study was to explore how gene expression profiles in the tumors differed by demographic and clinical characteristics. We determined that expression patterns clustered by family history of UGI cancer, and that there were 152 genes of which the expression differed significantly between persons with a positive as opposed to a negative family history of UGI cancer. We and others have been interested for many years in the potential role of genetic factors in the etiology of ESCC in high-risk areas of China. Although our previous epidemiological studies found associations between a positive family history and esophageal cancer, familial aggregation of the disease in families, and higher frequency of allelic loss in ESCC cases with a positive family history (4, 5, 6) , this is the first study to provide evidence that RNA expression differs by family history status. Replication of our findings in studies with larger sample sizes is needed to confirm the association with family history.
In summary, these data indicate that molecular profiles in ESCC are highly consistent and that expression patterns in tumors from familial cases differ from those in sporadic cases. They also highlight important molecular genetic features of familial ESCC in this high-risk area of China.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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1 These authors contributed equally to this work. ![]()
2 To whom requests for reprints should be addressed, at Cancer Prevention Studies Branch, National Cancer Institute, 6116 Executive Boulevard, Room 705, Bethesda, MD 20892-8314. Phone: (301) 592-2932; Fax: (301) 435-8645; E-mail: ptaylor{at}mail.nih.gov ![]()
3 The abbreviations used are: ESCC, esophageal squamous cell carcinoma; NCI, National Cancer Institute; MDS, multidimensional scaling; UGI, upper gastrointestinal. ![]()
4 Internet address: http://nciarray.nci.nih.gov/. ![]()
Received 3/10/03. Accepted 5/22/03.
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