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Tumor Biology |
Department of Pathology [A. W., M. S., F. F., F. Y., J. V., Y. Q. C.] and Center for Molecular Medicine and Genetics [Y. Q. C.], Wayne State University, Detroit, Michigan 48201
| ABSTRACT |
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| INTRODUCTION |
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To better understand molecular changes associated with prostate cancer progression, we have performed a comprehensive analysis of gene expression in normal and prostate tumor tissues by SAGE. Our study provides a list of candidate genes that could be useful for the development of new diagnostic/prognostic markers for human prostate cancer.
| MATERIALS AND METHODS |
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SAGE.
SAGE analysis was performed as described previously (4)
with following modifications: ditags were PCR-amplified using biotinylated primers and digested with NlaIII enzyme (5)
. Concatemers were heated for 15 min at 65°C and chilled on ice for 10 min before being separated on an 8% polyacrylamide gel (6)
. The concatemers were then cloned into the SphI site of the pZero vector (Invitrogen). Concatenated tags were screened by PCR using M13 forward and M13 reverse primers. PCR products with inserts greater than 500 bp were isolated and sequenced with M13 forward primer on an automated 3700 DNA sequencer (Perkin-Elmer).
SAGE Data Analysis.
SAGE tags were extracted using SAGE software version 4.12 (4)
. Tags were matched to the SAGE reliable map (release 10-26-2000).4
Due to the fact that some tags map to multiple genes, and some genes have multiple tags, SAGE data were analyzed in two different ways: (a) by the exclusion method (tags that match to multiple genes were discarded, and only tags that match to a single gene were tabulated, and composite counts were analyzed for their significance); and (b) by the inclusion method (tags that match to multiple genes were counted as 100% toward each gene). All tags were tabulated, and composite counts were analyzed for their statistical significance. Lists of differentially expressed genes (P < 0.05) obtained from the exclusion and inclusion methods were compared, and finally only genes that have a P < 0.05 in both lists were considered statistically significant. The total number of genes identified was estimated by Nm + (Num - 0.1 Num)/3.5, where Nm is the number of genes matched to SAGE tags, Num is the number of SAGE tags that do not match to known genes or ESTs, 10% is the estimated sequencing error per SAGE tag, and 3.5 is the average number of tags/gene in the SAGE reliable map (release 10-26-2000).
Immunohistochemistry.
Paraffin-embedded prostate tissues were deparaffinized and rehydrated by standard procedure. The tissue sections were treated by boiling with 10 mM sodium citrate (pH 6.0) for 20 min, cooled to room temperature for 20 min to retrieve antigen, and treated with 1% hydrogen peroxide at room temperature for 10 min to inactivate endogenous peroxidase. The slides were then washed once with water and once with PBS and processed for immunohistochemical analysis. Sections were blocked in 10% normal serum for 10 min at room temperature and then incubated with primary antibody overnight at 4°C in a humidifier chamber. The section lacking primary antibody treatment served as a control. The sections were incubated for 1 h with biotinylated secondary antibody and then stained using Vectastain Elite ABC (Vector Laboratories, Inc.) avidin-biotin peroxidase complex for 30 min. Positive staining was visualized by a 5-min incubation in a solution containing 3,3'diaminobenzidine-4 substrate.
| RESULTS |
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5-fold in tumor compared to matched normal tissue (Table 3)
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E (Hs. 851), 1:9 (approximately 9-fold); (b) E2F4 (Hs. 108371), 0:5 (>5-fold); (c) cyclin D1 (Hs. 82932), 13:5 (approximately 3-fold); (d) Daxx (Hs. 180224), 12:26 (approximately 2-fold); (e) cystatin C (Hs. 135084), 15:21 (approximately 1.5-fold); (f) VHL (Hs. 174007), 3:2 (approximately 1-fold); and (g) PSA (Hs. 171995), 120:112 (approximately 1-fold). Tissues from five individual patients were used. E2F4 showed strong epithelial staining in tumor glands but weak to no staining in normal glands (Fig. 1)
E, PSA, and VHL were strongly stained, whereas cystatin C and cyclin D1 were weakly stained in epithelial cells in all five patient samples. All of these five proteins showed similar levels of staining in both normal and tumor epithelium (data not shown).
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| DISCUSSIONS |
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Finally, gene expression profiling may provide an alternative tool for human tumor classification that has thus far mainly relied on histology. A better classification could lead to a better diagnosis and/or prognosis. Such profiling has been successfully applied to the classification of human acute leukemias (13) .
To identify potential molecular markers for both epithelium and stroma, we used normal and tumor samples pooled from four patients. The tumor samples contained 8090% tumor glands and 1020% stroma. The normal samples contained 3050% benign glands and 5070% stroma. Thus, tumor samples had a 23x higher proportion of epithelial cells. This could have affected SAGE results. Although genuine differentially expressed genes such as E2F4 and Daxx were identified, integrin
-E stained epithelial cells in both normal and tumor glands with a similar intensity. Therefore, it is possible that the higher level of integrin
E found in the tumor sample is due to a higher number of epithelial cells in the tumor than in normal sample. Alternatively, the discrepancy may simply be due to a lack of correlation between the expression of mRNA and protein. Cyclin D1 also appears to lack a correlation between its mRNA and protein levels. Such mRNA-protein expression discordance is common in mammalian cells. We have observed this in a large number of androgen-regulated genes (this study). Cystatin C, VHL, and PSA protein expression seems to correlate with mRNA data.
It is believed that tumor cells in culture are different from cells in vivo. However, there is little quantitative data to confirm it. Previously, a quantitative study has been done in human colon cells by SAGE (14) . Here we have compared the gene expression profile between prostate tumor cells in vivo (tumor tissue) and a cell line in vitro (LNCaP treated with or without DHT). Among the 32,614 genes studied, approximately 2.5% of the genes (P < 0.05) were differentially expressed. This is remarkably low, considering that LNCaP cells have been maintained in culture for decades. Our data suggest that LNCaP cells should be a good model system for many aspects of studies of prostate cancer. However, we have also noticed that in between LNCaP-normal tissue and tumor tissue-normal tissue comparisons, different subsets of genes, with some overlaps, are differentially expressed under these in vivo and in vitro conditions.
Another interest of ours is to estimate the total number of genes expressed in human prostate. Despite the completion of the human genome draft, how many genes exist in Homo sapiens is still a matter of debate (15, 16, 17, 18) . Based on our SAGE data, we estimate that approximately 37,000 genes are expressed in human prostate.
| ACKNOWLEDGMENTS |
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Note Added in Proof
Data from the human genome project were published during the review of this article. Results suggested that there are approximately 40,000 genes in Homo sapiens. For our analysis described in Table 5, it will be more accurate to state that approximately 37,000 different transcripts are expressed in human prostate.
| FOOTNOTES |
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1 This study was supported in part by DAMD17-98-1-8501, R01CA74927, and a fund from Karmanos Cancer Institute. ![]()
2 To whom requests for reprints should be addressed, at Department of Pathology, Wayne State University, 540 East Canfield, Detroit, MI 48201. Phone: (313) 577-5634; Fax: (313) 577-0057; E-mail: yqchen{at}med.wayne.edu ![]()
3 The abbreviations used are: EST, expressed sequence tag; SAGE, serial analysis of gene expression; DHT, dihydrotestosterone. ![]()
4 http://www.ncbi.nlm.nih.gov/SAGE/. ![]()
Received 1/ 8/01. Accepted .
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