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Advances in Brief |
Departments of Surgery II [N. I., M. O., N. M., T. T., T. O., N. T., A. T.] and Bioregulatory Function [N. I.], Yamaguchi University School of Medicine, Yamaguchi 755-8505; Department of Computer Science and Systems Engineering, Faculty of Engineering, Yamaguchi University, Yamaguchi 755-8611 [T. M., S. U., Y. H.]; and Department of Oncology, Nippon Roche Research Center, Kanagawa 247-8530 [H. Y-O., K. H., H. N.], Japan
| ABSTRACT |
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| Introduction |
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Recent development of DNA microarray technology, a type of high-throughput analysis for gene expression, has opened a new era in medical sciences (4, 5, 6) . Supervised learning and unsupervised learning methods have been introduced into gene expression analysis of DNA microarray data (7 , 8) . Using hierarchical cluster analysis, an unsupervised learning method, Honda et al. (9) showed different gene expression profiles in the hepatic lesions of chronic hepatitis associated with HBV and HCV and suggested that the molecular mechanisms responsible for the pathogenesis of HCC differ between HBV and HCV infections. Several studies compared gene expression between nontumorous liver and HCC and revealed gene expression patterns that are rather specific to HCC (10, 11, 12, 13, 14) . However, there is only one study that compared gene expression patterns between HCC with HBV infection (B-type HCC) and HCC with HCV infection (C-type HCC; 14 ), and only a limited number of specimens were analyzed. Therefore, additional studies are needed to understand molecular mechanisms involved in the development and progression of virus-induced HCCs. In this study, we investigated gene expression patterns of 45 HCC samples using high-density oligonucleotide microarrays and the supervised learning method to gain additional insight into hepatocarcinogenesis or cancer progression related to HBV or HCV infection. The results of this study provide additional markers and molecular targets for the diagnosis and treatment of B- and C-type HCCs.
| Materials and Methods |
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Samples and RNA Extraction.
Total RNA was extracted with Sepasol-RNAI (Nacalai Tesque, Tokyo, Japan) and purified with the RNeasy Mini kit (Qiagen, Tokyo, Japan) according to the manufacturers instructions. Quality of the total RNA was judged from the ratio between 28S and 18S RNase after agarose gel electrophoresis.
cDNA Synthesis and in Vitro Translation for Labeled cRNA Probe.
cDNA was synthesized with the reverse SuperScript Choice System (Invitrogen Life Technologies, Carlsbad, CA) according to the manufacturers instructions. cRNA was synthesized from the cDNA template by use of the MEGAscript T7 kit (Ambion, Austin, TX) according to the manufacturers instructions. Mononucleotides and short oligonucleotides were removed by column chromatography on a CHROMA SPIN +STE-100 column (Clontech, Palo Alto, CA).
Gene Expression Analysis by Means of High-density Oligonucleotide Arrays.
Gene expression patterns were examined by high-density oligonucleotide arrays (HuGeneFL Array; Affymetrix, Santa Clara, CA). After the cRNA was fragmented at 95°C for 35 min, hybridization was performed in 200 µl of buffer containing 0.1 M 2-(N-morpholino)ethanesulfonic acid (pH 6.7), 1 M NaCl, 0.01% Triton X-100, 20 µg of herring sperm DNA, 100 µg of acetylated BSA, 10 µg of the fragmented cRNA, and biotinylated control oligonucleotides at 45°C for 12 h. To increase hybridization signals, the washed chips were further hybridized with biotinylated antistreptavidin Ab and stained with streptavidin R-phycoerythrin (Molecular Probes, Eugene, OR) as described in the instruction manual (Affymetrix). The intensity of each pixel was detected by laser scanner (Affymetrix), and expression levels of each cDNA and reliability (Present/Absent call) were calculated with software (Affymetrix GeneChip version 3.3 and Affymetrix Microarray Suite version 4.0).
Procedure for Gene Selection.
To filter genes out of the
6000, we first investigated all genes for which mean average differences were >2-fold between B- and C-type HCCs. Of the filtered genes, we selected D genes that had average expression levels of >20 (arbitrary units by Affymetrix) in both types of HCCs.
We used the Fisher ratio to evaluate separability between B- and C-type HCCs. The Fisher ratio for gene i is given by
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2BT(i) are the sample mean and sample variance, respectively, of the expression levels of gene i for the samples in B-type HCC (BT). P(BT) is the priori probability of BT. As a final step, we ranked D selected genes by the Fisher ratio as F(i1) > F(i2) > ··· > F(iD). To investigate how many genes should be considered, we performed the random permutation test according to the method by Luo et al. (16) . In the test, samples labels were randomly permuted among the two types of HCCs, and the Fisher ratio for each gene was again computed. This random permutation of sample labels was repeated 1000 times. The Fisher ratios generated from the actual data were then assigned Ps based on the distribution of the Fisher ratios from randomized data.
Reverse Transcriptase PCR Analysis for IGF-2 Gene.
To validate our microarray results and to further clarify a difference in expression pattern for IGF-2,4
we carried out semiquantitative reverse transcriptase PCR using RNA stock of 9 B-type and 12 C-type HCC samples that were subjected to microarray study. Reverse transcriptase step was performed as described previously (17)
. Five µl of cDNA solution (equivalent to the cDNA from 100 ng of initial RNA) were amplified in 45 µl of PCR mixture (17)
containing 25 pmol of each primer for IGF-2 and ß-actin genes. PCR was performed for 26 cycles for IGF-2 and 24 cycles for ß-actin. Each cycle consisted of denaturation at 94°C for 1 min, annealing at 60°C for 45 s, and elongation at 72°C for 2 min. The primers used in this study were as follows: IGF-2, 5'-ctggtggacaccctccagttc-3' (sense) and 5'-gcccacggggtatctggggaa-3' (antisense); and ß-actin, 5'-CCAGAGCAAGAGAGGTAT-3' (sense) and 5'-CTGTGGTGGTGAAGCTGTAG-3' (antisense). The expected sizes were 235 and 436 bp for IGF-2 and ß-actin genes, respectively. PCR products were separated by electrophoresis on 1.5% agarose gels and visualized under UV light after ethidium bromide staining. We determined the mean band densities using NIH Image 1.62 software, and we calculated levels of IGF-2 relative to ß-actin gene.
Statistical Analysis.
Clinicopathological factors pertaining to B- and C-type HCCs were compared, and differences were analyzed by
2 test, Fishers exact test, Students t test, or Mann-Whitneys U test (Table 1)
. P < 0.05 was accepted for statistical significance. Pearsons correlation coefficient (r) was calculated to examine the relation between microarray and reverse transcriptase PCR results. r2 > 0.16 and P < 0.05 were considered significant. Calculations were done with Statview 5.0 (Abacus Concepts, Berkeley, CA) on a Macintosh computer (Apple Computers, Inc., Cupertino, CA).
| Results and Discussion |
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Selection of the Top 83 Genes Linked to B- or C-type HCC.
Many studies have successfully identified gene subsets (i.e., gene clusters) linked to various states of many diseases by unsupervised learning such as hierarchical clustering (5
, 7, 8, 9)
. However, one cannot effectively use information on the category label of sample data by unsupervised learning (18)
. Application of the supervised learning method by the Fisher ratio to the analysis of DNA microarray data makes identification of disease-related genes easier and more precise (19)
. We therefore used the Fisher ratio to select appropriate genes for this study. Of an approximate 6000 genes, we first identified 169 for which expression differed between B- and C-type HCCs. We then ranked these 169 genes in the order of decreasing magnitude of the Fisher ratio. Next, we performed the random permutation test to assess the statistical significance of the Fisher ratios. From the distribution of the Fisher ratios based on the randomized data, 83 genes with the Fisher ratio > 0.4 were determined to be statistically significant (P < 0.05) in expression between B- and C-type HCCs. Therefore, we selected the top 83 genes of the 169 (Fig. 1
; Table 2
).
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Genes with the Larger Fisher Ratio.
The Fisher ratio measures the difference between two means normalized by the average variance. Thus, the Fisher ratio represents the ability of a gene to discriminate the two types of HCCs. Among the top 83 genes, ACP yielded the largest Fisher ratio, and RPL39L and TACSTD1 yielded the second and third largest Fisher ratio, respectively (Table 2)
. All 3 genes were up-regulated in B-type HCC in comparison to C-type HCC. ACP is known to play a role in the differentiation of normal human monocytes to macrophages (20)
but its role in the development of HCC remains unclear.Using microarray, Xu et al. found that many ribosome-related genes such as RPL family genes were up-regulated in HCC, suggesting the activation of protein translation in HCC (11)
.TACSTD1, which was identified as a gastrointestinal cancer Ag, plays an important role in cellular adhesion and its overexpression has been reported in certain other types of cancers (21)
. Consistent with these reports, RPL39L and TACSTD1 mRNA levels were also higher in our B-type HCC than in our nontumorous liver tissue. Thus, it seems that the 3 genes are potential molecular targets for the treatment of B-type HCC rather than C-type HCC.
Imprinted Genes.
We found that imprinted genes H19 and IGF2, which are located close together on chromosome 11p15.5, were up-regulated in B-type HCC in comparison to both C-type HCC and nontumorous liver. There was a positive correlation in gene expression levels between H19 and IGF2 (r = 0.517, P < 0.0001; data not shown). Although these genes are reported to be coordinately up-regulated in HCC (22)
, this is the first study to show up-regulation of these genes specifically in B-type HCC. H19 is an untranslated gene, and the biological function remains unclear. IGF2 is known to be an autocrine growth factor in many malignant tumors (22)
. Expression levels of the IGF2 mRNA were further confirmed by our semiquantitative reverse transcriptase PCR; the result of the DNA microarray was reproduced even by reverse transcriptase PCR (P = 0.0075, r = 0.558; Fig. 2a and b
). This result suggests for the first time that up-regulation of the IGF-2 pathway may play an important role in the pathogenesis of B-type HCC but not C-type HCC.
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Among detoxification-related genes, only GSTP1 was exceptionally up-regulated in B-type HCC, although its mRNA level was higher in both types of HCCs than in nontumorous liver. Experiments for hepatocarcinogenesis and a recent microarray study showed up-regulation of GST in HCC (23 , 24) . Interestingly, GST expression has been shown to be induced in a HCC cell line overexpressing HBX protein (25) . Because GST is involved in scavenging reactive oxygen intermediates that are generated by many anticancer agents, our data suggest the efficacy of these anticancer agents in treating C-type HCC and their limitations in treating B-type HCC.
Immune Response-related Genes.
The 52 genes up-regulated in C-type HCC included a number of immune response-related genes. The result that C15, IFI27, C6, and OAS1 had the larger Fisher ratios implies that C-type HCC is closely related to immune response, especially inflammation. In keeping with a previous study (14)
, we found up-regulation of natural killer receptor in C-type HCC versus B-type HCC. IFN-inducible genes (IFI27, OAS1, ISG15, and IFIT4) were also up-regulated in C-type HCC by >2- and 1.5-fold versus B-type HCC and nontumorous liver, respectively. Whereas Honda et al. demonstrated by cDNA microarray that IFN-a was commonly up-regulated in livers with chronic HBV or HCV infection (9)
, the expression levels of IFNs were more or less the same between the two types of HCCs in this study (data not shown). Because IFN is induced by double-strand RNA species, it is reasonable that up-regulation of these IFN-inducible genes is the consequence of the generation of the double-strand RNA by infection with HCV. The mechanism of IFN-
induction in B-type HCC, however, remains to be elucidated.
The time lag between HCV infection and cancer development is several decades. As a result, HCV-associated tumors arise in older patients and are almost always associated with cirrhosis. Thus, it is apparent that C-type HCC is closely related to chronic inflammation (26) , suggesting that the immune response-related genes identified here serve as molecular targets for chemoprevention and treatment of C-type HCC.
The Other Genes.
Wu et al. showed many signal transduction-related genes, including MAPK family genes to be up-regulated in B-type HCC (13)
. Up-regulation of MAPK is also suggested as a common pathway for the hepatocarcinogenesis caused by infection with HBV and HCV (14)
. In our study, MAP2K4 and MAP2K5 were up-regulated in B-type HCC versus C-type HCC; however, MAP2K5 was down-regulated in both types of HCC versus nontumorous liver. Thus, additional studies are necessary to clarify contribution of the MAPK pathway to each type of HCC.
Xu et al. reported up-regulation of liver-enriched transcription factors in HCC versus nontumorous liver (11) . We found in this study that transcription factors PPARG, SIAHBP1, and MYOG were up-regulated in B-type HCC but not in C-type HCC. These transcription factors seem to be abundant in organs other than the liver. We selected genes by focusing on differences between B- and C-type HCCs. The discrepancy, therefore, might be due partly to a difference in the gene selection method. However, our method could identify additional genes that had not been associated with these two types of HCCs thus far. Genes such as MMP9, VEGF, HMMR, TACSTD1, and MCAM that may promote metastasis, for example, were up-regulated in B-type HCC. Overall, we provide evidence that B- and C-type HCCs use different mechanisms for the promotion and suppression of metastasis. We expect the results obtained in this study to aid in understanding the molecular mechanism underlying the pathogenesis of B-type and C-type HCCs.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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1 Supported in part by a Grant-in-Aid from the Ministry of Education, Science, Sports and Culture of Japan (12671230). ![]()
2 To whom requests for reprints should be addressed, at Department of Surgery II, Yamaguchi University School of Medicine, 1-1-1 Minami-Kogushi, Ube, Yamaguchi 755-8505, Japan. Phone: 81-836-22-2262; Fax: 81-836-22-2262; E-mail: 2geka-1{at}po.cc.yamaguchi-u.ac.jp ![]()
3 The abbreviations used are: HCC, hepatocellular carcinoma; HB, hepatitis B; HBV, hepatitis B virus; HCV, hepatitis C virus; Ag, antigen; Ab, antibody; GST, glutathione S-transferase; MAPK, mitogen-activated protein kinase. ![]()
4 Gene abbreviations are used based on LocusLink at internet address: www.ncbi.nlm.nih.gov/LocusLink/. ![]()
5 Internet address: www3.ncbi.nlm.nih.gov/PubMed/. ![]()
6 Internet address: www.tigr.org/tdb/hgi/searching/reports.html. ![]()
Received 3/18/02. Accepted 5/23/02.
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