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Advances in Brief |
Department of Clinical Oncology, The University of Hong Kong, Hong Kong, China [T. C-M. Y., J. S. T. S., X-Y. G.]; Department of Pathology [D. X.], and Cancer Institute [Y. F., Q-L. W.], Sun Yat-Sen University, Guangzhou, China; and State Key Laboratory of Genetic Engineering, Institute of Genetics, Fudan University, Shanghai, China [K-K. H.]
| ABSTRACT |
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19.5 cM) were used to survey the amplification status in four ovarian cancer cell lines that have been confirmed as containing amplification at 19q13.1. Two separated overlapping MARs, MAR1 (
200 kb) and MAR2 (
1.1 Mb), were identified at 19q13.1. Two candidate oncogenes, AKT2 and SEI-1, were identified in MAR2. Amplification and overexpression of these two genes in four ovarian cancer cell lines were confirmed by Southern and Northern blot analyses. The proliferation-related function of AKT2 and SEI-1 suggests that both genes are likely to be biological targets of an amplification event at 19q13.1 in ovarian cancer and to play important roles in ovarian tumorigenesis. | Introduction |
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Gene amplification and the consequent overexpression of the amplified oncogene is one of the common genetic alterations in various solid tumors that have been shown to play an important role in tumor pathogenesis, probably because the overexpression of the oncogene confers a growth advantage. Therefore, to identify commonly amplified chromosomal regions and the target oncogene within the regions it is imperative to understand the molecular mechanisms of the development and progression of cancer. In our previous study, the gain of 19q was detected in 12 (39%) of 31 primary ovarian cancers (3) . High copy-number amplification of 19q13.1-q13.2 in the form of a hsr has been observed in four ovarian cancer cell lines (UACC326, UACC1123, UACC2727, and OVCAR-3) by chromosome microdissection (4) . Although AKT2 has been implicated as a candidate oncogene at 19q13 (5) , coamplification of two or more oncogenes within one amplicon has been described previously. For example, chromosomal region 12q13-q15 has been shown to contain several genes, including MDM2, CDK4, GLI, SAS, and CHOP, that are potentially relevant to tumor growth. Coamplification and overexpression of CDK4, SAS, and MDM2 has been frequently detected in human parosteal osteosarcomas (6) . In addition, amplification of AKT2 was only detected in about 1213% of primary ovarian cancers in a previous study (5) . Therefore, it is highly conceivable that one or more flanking oncogene(s) residing on either side of AKT2 could also contribute to the development of ovarian cancer. In the present study, we have applied the physical mapping strategy by using FISH and Southern Blot analysis to identify the overlapping MARs at 19q13.1-q13.2 in the above mentioned four ovarian cancer cell lines. The results showed that there are two separate MARs at 19q13.1, each of which may contain one or more oncogenes involved in the amplification events in ovarian cancer.
| Materials and Methods |
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Probe Selection.
About 200 STS markers within the interested region of 19q13.1-q13.2 (from D19S425 to D19S907) were used as the initial starting point for probe selection. The genetic distance between D19S425 and D19S907 is about 19.5 cM (58.177.6 cM; GeneMap99, NCBI). The sequences of STS markers were obtained from GeneMap99, and BLAST search was performed to select corresponding cosmid clones for FISH study. Thirty cosmid clones were selected for FISH analysis and the distance between each other was
1 cM. All of the cosmid clones were kindly provided by the Human Genome Center at Lawrence Livermore National Laboratory as free gifts. Thirteen known genes, one EST, and five unique genomic DNA sequences within the target region were selected to fill in gaps that could not be covered by cosmid clones. All of the cosmid clones and genes used in the present study are listed in Table 1
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Southern and Northern Blot Analyses.
Southern blot hybridization was performed by a standard method. The PCR products of cDNA and unique genomic DNA sequences were labeled with [32P]dCTP using Random Labeling kit (Life Technologies, Inc., Rockville, MD). Fifteen µg of genomic DNA isolated from ovarian cancer cell lines were digested with EcoRI, fractionated on 1% agarose gel, transferred to a nylon membrane (Bio-Rad, Hercules, CA), and hybridized overnight at 42°C with 32P-probes. Ten µg of total cellular RNA, prepared by Trizol/chloroform method, were size fractionated on 1% agarose/2.2 M formaldehyde gel, transferred to a nylon membrane, and hybridized with 32P-probes.
Tissue Microarray.
A total of 200 epithelial ovarian cancer cases with 400 tissue specimens were obtained from the archives of Cancer Center, Sun Yat-sen University of Medical Sciences, Guangzhou, China. Two specimens, one from tumor tissue and the other from surrounding nontumor tissue, were selected. The tissue microarray was constructed as described previously (8)
. Briefly, tissue cylinders with a diameter of 0.6 mm were taken from the selected regions of a donor block and then precisely punched into a recipient paraffin block using a tissue-arraying instrument (Beecher Instruments, Silver Spring, MD). Five-µm consecutive sections of the microarray block were made with a microtome.
Three cosmid clones, R31396 (mapped within MAR1), F23149 (within MAR2), F19514 (between the two MARs), were selected as FISH probes. The FISH reaction was performed as described previously (7) with some modifications. Briefly, the tissue array section was deparaffinized, treated with protenase K (400 µg/ml) at 37°C for 45 min, denatured at 75°C in 70% formamide, 2x SSC for 5 min, and hybridized with biotin-labeled cosmid probe at 37°C for 2 days. Washing condition and posthybridization treatment were identical as described above.
| Results |
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207 kb (between 36,677 and 36,884 kb) and MAR2 is
1124 kb (between 40,434 and 41,568 kb). The distance between MAR1 and MAR2 is
3.5 Mb. The results were summarized in Table 1
Overexpression of AKT2 and SEI-1.
Candidate oncogenes within the MARs were screened through database search with NCBI Map Viewer. Several known genes were found in both MARs, including SUPT5H, SEI-1, GMFG, and AKT2 in MAR2. Amplification of these genes had been detected by Southern blot hybridization, and examples of amplification of AKT2 and SEI-1 are shown in Fig. 2A
. Among these genes, AKT2 and SEI-1 have been associated with cell proliferation and cell cycle control. Therefore, the RNA expression levels of these two genes were studied by Northern blot analysis to determine whether they were overexpressed in the tested ovarian cancer cell lines. The result showed that both AKT2 and SEI-1 were indeed overexpressed in all four of the ovarian cancer cell lines comparing with the total RNA from normal ovary (Fig. 2B)
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| Discussion |
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In this study, four ovarian cancer cell lines containing amplicon of 19q13.1-q13.2 in the form of hsr were characterized to define the overlapping MAR. Forty-nine cosmid clones and DNA markers within a 13.9-Mb region (average distance between two markers is
280 kb) at 19q13.1 were selected to detect overlapping MARs. The resolution of markers selected in the critical region from stSG53555 to stSG12762 (36,33342,426 kb; NCBI Map Viewer) is
170 kb. The result showed that amplicons in all four cell lines were not continuous, and two separate overlapping MARs were identified. Our tissue microarray study also supported the observation that the amplification frequencies of MAR1 and MAR2 were obviously higher than those in the region between MAR1 and MAR2. The fact that a hsr can be composed of two or more separated DNA sequences from the same or different chromosomes has been demonstrated by chromosome microdissection in breast cancer (11)
. In nine breast cell lines, 12 of 15 hsr markers were demonstrated to be composed of multiple chromosome components (11)
. Other studies also observed extensive DNA rearrangements in N-myc amplicons in neuroblastoma (12)
. These results indicate that the mechanism of hsr formation is very complex and might be associated with genomic instability.
The coamplification of two separated MARs in all four tested ovarian cancer cell lines suggests that the coamplification is not random. It is highly possible that the target genes located in these two regions may provide a stronger selective advantage to ovarian cancer progression. There is evidence that the existence of four independent amplified regions within 11q13 in breast cancer may provide potential coselection and synergistic role of different amplified genes (13) . Using interphase FISH technique, Tanner et al. (14) found three frequently coamplified regions at 20q11, 20q12, and 20q13.2 in breast cancer. Several candidate oncogenes have been considered as the biological target of amplification events, including ASC-2 (AIB 3) at 20q11 (15) , AIB1 at 20q12 (16) , ZNF217 and NABC1 at 20q13.2 (17) .
As expected, AKT2 was amplified in all four of the ovarian cancer cell lines and mapped in the overlapping MAR2. AKT2 is a serine-threonine kinase gene and frequently amplified in ovarian cancer (5) . The PI3K/AKT2 pathway has been demonstrated to be important in malignant transformation (18) . Another interesting candidate oncogene in MAR2 is SEI-1. Amplification and overexpression of SEI-1 has been demonstrated by Southern and Northern blot analyses. SEI-1 was recently isolated by yeast two-hybrid screening using p16INK4a as bait (19) . The SEI-1 gene encodes a Mr 34,000 protein (p34SEI-1), a CDK4-binding protein, which renders the activity of cyclin D/CDK4 resistant to the inhibitory effect of p16INK4a (19) . It is well known that the tumor-suppressing function of p16INK4a is through its binding to, and subsequent inhibition of, cyclin D 1/CDK4 kinase activity. Another study found that p34SEI-1 (also named TRIP-Br1) interacts with KRIP-1 (TIF1ß) and functionally contacts DP-1, stimulating E2F/DP-1 transcriptional activity (20) . These features suggest that SEI-1 is a candidate oncogene, and that its amplification may play an important role in the development and progression of ovarian cancer. Further study of genes within these two MARs may lead to the isolation of the oncogene(s) that is the biological target of the amplification event at 19q13.1 in ovarian cancer. In addition, the results in this study provide a useful basis for future studies of this region for understanding the molecular mechanisms of gene amplification and amplicon evolution in solid tumors.
| FOOTNOTES |
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1 Supported in part by the Leung Kwok Tze Foundation and Chinese Visiting Scholar Foundation of Key Lab in University. ![]()
2 To whom requests for reprints should be addressed, at Department of Clinical Oncology, The University of Hong Kong, Room 109, School of Chinese Medicine, 10 Sassoon Road, Hong Kong, China; Fax: 852-28169126; E-mail: xyguan{at}hkucc.hku.hk ![]()
3 The abbreviations used are: CGH, comparative genomic hybridization; hsr, homogeneously staining region; MAR, minimal amplified region; FISH, fluorescence in situ hybridization; DAPI, 4',6-diamidino-2-phenylindole; STS, sequence tagged site; NCBI, National Center for Biotechnology Information. ![]()
Received 7/15/02. Accepted 10/30/02.
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