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Molecular Biology and Genetics |
Laboratory of Molecular Medicine, Human Genome Center, Institute of Medical Science, The University of Tokyo, Tokyo 108-8639 [H. O., S. S., T. K., O. K., R. Y., Y. F., Y. N.], Department of Gastroenterological Surgery, Kyoto University Graduate School of Medicine, Kyoto 606-8507 [H. O., T. K., Y. Y.], and Laboratory for Medical Informatics, SNP Research Center, Riken (Institute of Physical and Chemical Research), Tokyo 108-8639 [T. T.], Japan
| ABSTRACT |
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| INTRODUCTION |
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cDNA microarray technology, which enables investigators to obtain comprehensive data with respect to gene-expression profiles, is progressing rapidly. Several studies have already demonstrated the usefulness of this technique for identifying novel cancer-related genes and for classifying human cancers at the molecular level (4 , 5) .
In this paper, we report the identification of genes the expression of which has been altered during hepatocarcinogenesis through the use of a genome-wide cDNA microarray containing 23,040 genes. Expression profiles of these genes in 20 primary HCCs fell into three categories that correlated well with the infection status and type of hepatitis virus. Analyses of these profiles along with clinicopathological data also facilitated identification of genes associated with tumor differentiation and vessel invasiveness. This large body of information not only furthers an understanding of the mechanisms of hepatocarcinogenesis but also reveals novel features of known genes and identifies additional biological factors involved in liver cancer.
| MATERIALS AND METHODS |
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cDNA Microarrays.
We fabricated a "genome-wide" cDNA microarray with 23,040 cDNAs selected from the UniGene database of the National Center for Biotechnology Information. The cDNAs were amplified by reverse transcription-PCR using poly(A)+RNA isolated from various human organs as templates; lengths of the amplicons ranged from 200 to 1100 bp without repetitive or poly(A) sequences. The PCR products were spotted in duplicate on type-7 glass slides (Amersham) using an Array Spotter Generation III (Amersham). Each slide contained 52 housekeeping genes, to normalize the signal intensities of the different fluorescent dyes.
RNA Preparation, Hybridization, and Acquisition of Data.
Frozen specimens were serially sectioned in 10-µm slices and stained with H&E to define the analyzed regions. To avoid cross-contamination of cancer and noncancerous cells, we prepared these two populations by laser-captured microdissection. Total RNA was extracted from each population and then amplified using Ampliscribe T7 Transcription Kit (Epicentre Technologies). The preparation of probes, hybridization, and scanning was performed as described previously (6)
. The fluorescence intensities of Cy5 (nontumor) and Cy3 (tumor) for each target spot were adjusted so that the mean Cy5 and Cy3 intensities of 52 housekeeping genes for each slide were equal.
Validation of Data.
To assess the reproducibility of the normalized intensity ratios, we compared the log2(Cy3:Cy5 intensity ratio) of the 52 housekeeping genes between different slide sets. When the difference between normalized logarithmic ratios from two experiments was less than 1.0, we defined the data as reproducible. The reproducibility was more than 90% when the intensities of Cy3 and Cy5 were both above 25,000.
Classification of 20 HCCs According to Gene Expression Profiles.
We applied the hierarchical clustering method to both genes and samples. To obtain reproducible clusters, we used only selected genes that passed the cutoff filter (both Cy3 and Cy5 signals greater than 25,000 in more than 80% cases examined). The analysis was performed using web-available software ("Cluster" and "TreeView") written by M. Eisen.4
Before applying the clustering algorithm, the fluorescence ratio for each spot was first log-transformed; then the data for each sample were centered to remove experimental biases.
Identification of Genes Responsible for Clinicopathological Factors.
We first arranged the relative expression of each gene (Cy3:Cy5 intensity ratio) into one of four categories: up-regulated (ratio, >2.0), down-regulated (ratio, <0.5), unchanged (ratio, between 0.5 and 2.0), and not expressed (or slight expression but under the cutoff level for detection). We used these categories to detect changes in expression that were common among samples as well as specific to a certain subgroup. To detect differentially expressed genes, we recorded the number of samples in each category within each subgroup, for each gene. Then we calculated the U values of Mann-Whitney tests, which measured how the sample distributions between subgroups overlap. The number of samples within each group is counted and, according to the order of the category, the number of overlapped samples is incorporated into the U value. A small U shows that the sample distribution of the two groups is clearly separated, e.g., commonly up-regulated in the HBV group and down-regulated in the HCV group. We applied a hierarchical clustering algorithm to all of the selected genes using hamming distance (edit distance).
| RESULTS AND DISCUSSION |
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-tubulin) and CBX1 participate in centrosome formation (7
, 8)
; CKS1 and PCTK1, encoding cdc2/cdc28 kinases, are essential for activation of the anaphase-promoting complex. PSMD8 (26S proteasome subunit p31) is reportedly responsible for activation of these kinases (9)
. Others have reported that CSE1L, TTK, and PLK1 are associated with formation of the mitotic spindle (7
, 10)
and that PLK1 can affect the number of centrosomes when exogenously expressed (11)
; overexpression of PLK1 has been correlated with poor prognosis in a subset of human cancers (12)
. Our comprehensive expression data for these genes may account for a high incidence of chromosomal instability in HCC, and they suggest that promotion of the mitotic process is generally involved in hepatocarcinogenesis. Therefore, regulation of these mitosis-associated genes either by chemotherapeutic agents or by gene delivery might be an effective therapeutic strategy for HCCs.
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Classification of HCCs by Gene Expression Profiles.
We further investigated whether clinical HCCs could be classified into groups on the basis of their gene-expression profiles. For this purpose, we used the hierarchical clustering method. To obtain reproducible clusters, we selected 4,531 genes that passed the cutoff filter (both cy3 and cy5 signals greater than 25,000). The overall expression patterns across 20 HCC samples are shown in Fig. 1
. The analyses resulted in the clustering of identical genes spotted on different positions into adjacent rows, indicating the reliability of the expression data. The 20 HCCs examined fell into three groups, as the dendrogram shows.
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Identification of Genes Related to HBV-positive or HCV-positive Status.
To identify genes responsible for the differences between HBV-positive and HCV-positive tumors, we performed Mann-Whitney tests and found that 19 known genes and 21 ESTs showed significantly different expression patterns between these two groups. Among the 19 known genes (Fig. 2)
, seven (GPX2, CYP2E, EPHX1, AKR1C4, FMO3, UGT1A1, and UGT2B10) encode key molecules for activating chemotherapeutic drugs or detoxifying xenobiotic carcinogens.
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On the other hand, expression of UGT1A1, UGT2B10, and GPX2 was preferentially repressed in HBV-positive HCCs (UGT1A1 was reduced in 8 of 10 HBV-positive HCCs examined), but expression levels of these genes were unchanged in most HCV-positive HCCs. In accordance with our observations, Strassburg et al. (21) have shown decreased expression of UGT1A1 in HCCs as well as in hepatic adenomas, implicating UGT1A1 in an early step of hepatocarcinogenesis. UGT1A1 and UGT2B10 catalyze Phase II conjugation reactions, which are frequently related to detoxification of the active forms of carcinogens. GPX2, a major form of glutathione peroxidase in liver, functions as an antioxidant, and decreased glutathione peroxidase activity in HCCs has been reported elsewhere (22) . Hence, reduced activities of these enzymes may reflect enhanced exposure of hepatocytes to activated carcinogens or radicals. Our results suggest that decreased expression of detoxification enzymes may be involved especially in the mechanisms of HBV-specific hepatocarcinogenesis. Furthermore, because UGT1A1 also catalyzes glucuronidation of SN-38, an active form of irinotecan (23) , HBV-positive HCCs may show greater sensitivity to irinotecan than do HCV-positive HCCs. Different expression patterns among detoxification enzymes should provide information for optimizing the choice and/or the dosage of anticancer drugs for treating HCC patients on an individual basis.
Results of comparing expression profiles between HBV-positive and HCV-positive HCCs implied that hepatitis viruses affect expression of dozens of genes in HCC in a type-specific manner, invoking partly different mechanisms of carcinogenesis. Consequently, identification of genes defining virus-type-specific expression profiles would contribute to our ability to develop virus-type-dependent treatment regimens.
Identification of Genes Related to HCC Progression.
As in the multistep model of adenoma-to-carcinoma sequence accepted for colorectal tumors, HCCs are considered to develop as well-differentiated tumors and then progress to moderately-to-poorly differentiated states (24)
. A comparison of expression profiles between well-differentiated tumors (Edmondson grade I; n = 7) and moderately to poorly differentiated tumors (Edmondson grade II or III; n = 13; Fig. 3A
) by means of Mann-Whitney test identified a total of 321 genes (including 193 ESTs) that showed different expression patterns between the two histologically divided groups. In addition to the genes encoding liver-specific proteins, they included genes associated with apoptosis and the immune system. Apoptosis-related genes including TNFSF10, TNFSF14, GADD34, CFLAR, CLU, CASP6, and phosphatidylserine receptor (25
, 26)
were preferentially reduced in moderately-to-poorly differentiated tumors, implying that a reduced rate of apoptosis is a major characteristic of tumor progression. Genes associated with immune systems included MAGEC1, one of the tumor antigens recognized by CTLs, whose expression was also repressed only in moderately-to-poorly differentiated tumors. Reduced expression of genes encoding immune targets may confer a growth advantage by allowing tumor cells to escape from immune surveillance.
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The genes and their products represented by the numerous ESTs of unknown function that we classified in the same clusters as genes associated with apoptosis or immunity may be useful as novel targets for drug discovery or tumor markers. Accumulation of data with respect to expression profiles of cancer specimens, clinicopathological data, sensitivity to treatment, and prognosis will not only help us to understand the precise mechanisms of carcinogenesis but also yield practical information for identifying optimized therapeutic modalities and novel therapeutic targets.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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1 Supported in part by Research for the Future Program Grant 96L00102 from the Japan Society for the Promotion of Science. ![]()
2 To whom requests for reprints should be addressed, at Laboratory of Molecular Medicine, Human Genome Center, Institute of Medical Science, The University of Tokyo, 4-6-1 Shirokanedai, Minato-ku, Tokyo 108-8639, Japan. Phone: 81-3-5449-5372; Fax: 81-3-5449-5433; E-mail: yusuke{at}ims.u-tokyo.ac.jp ![]()
3 The abbreviations used are: HCC, hepatocellular carcinoma; HBV, hepatitis B virus; HCV, hepatitis C virus; EST, expressed sequence tag. ![]()
4 Internet address: http://www.microarrays.org/software. ![]()
Received 11/27/00. Accepted 1/ 4/01.
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