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Tumor Biology |
Department of Clinical Oncology, Queen Mary Hospital, The University of Hong Kong [X-Y. G., J. S. T. S., T. C-M. T.]; Cancer Institute, Sun Yat-sen University of Medical Sciences, Guangzhou, Peoples Republic of China [F. Y.]; State Key Laboratory of Genetic Engineering Institute of Genetics, Fudan University, Shanghai, Peoples Republic of China [K-K. H.]; and Arizona Cancer Center, The University of Arizona, Tucson, Arizona [J-M. Y.]
| ABSTRACT |
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| INTRODUCTION |
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Identification of commonly amplified chromosomal regions and corresponding oncogenes within the region is imperative to understand the molecular mechanism of tumorigenesis. Recently, PIK3CA which encodes a catalytic subunit of phosphatidylinositol 3-kinase has been implicated as a candidate oncogene at 3q26 (8) . Since only low-level amplification of PIK3CA was detected in OV-4, OV-27, and an ovarian cancer cell line UACC-1598 that contains high-level amplification at 3q26 in the form of double minutes (9) , there is likelihood of another oncogene residing in this region. In this report, we used a hybrid selection method to rapidly isolate a novel expressed gene from the microdissected DNA of 3q26.
| MATERIALS AND METHODS |
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Chromosome Microdissection.
Chromosome microdissection and PCR amplification of microdissected DNA were performed as described previously (10)
. Briefly, five copies of the 3q26 band were dissected from G-banding normal metaphase chromosomes. The dissected DNA fragments were treated with topoisomerase I and then amplified by PCR with UN1 primer (5'-CCGACTCGAGNNNNNNATGTGG-3').
cDNA Synthesis and Library Construction.
A random primed cDNA library was constructed in bacteriophage
GEM4 according to the manufacturers protocol (RiboClone cDNA Synthesis System; Promega, Madison, WI). For hybrid selection, random primed cDNA was prepared from OV-4 mRNA using a SMART PCR cDNA Library Construction kit (Clontech Laboratories, Palo Alto, CA).
Hybrid Selection.
Hybrid selection was performed as described previously (11)
. The method was to apply random primed cDNA from ovarian cancer OV-4 containing 3q26 amplification to select coding region sequences from microdissected DNA fragments of 3q26. Briefly, 5 µg of random primed cDNAs prepared from the OV-4 cells were immobilized on a 7 mm in diameter nylon membrane and then hybridized with PCR-amplified microdissected DNA fragments from the 3q26 region in 200 µl of hybridization solution (5x SSC, 5x Denhardts solution, 0.1% SDS, 100 µg/ml salmon sperm DNA, and 100 µg/ml human Cot-1 DNA) at 65°C with gentle rotation overnight. After a stringent wash, the hybridized microdissected DNA was eluted and recovered by PCR.
cDNA Library Screen.
PCR-recovered, selected DNA fragments (100 ng) were labeled with 32P by random priming (Life Technologies, Inc., Rockville, MD) and then purified over Sephadex G50. The probe was prehybridized with 100 µg of human Cot-1 DNA at 65°C for 1 h and then hybridized to cDNA library plaque lifts by the standard method.
Chromosome Mapping.
A BAC clone selected through BLAST database searching was labeled with Spectrum Orange-dUTP by nick translation (Life Technologies, Inc.). The labeled probe was then hybridized to prebanded normal lymphocyte metaphase chromosomes by FISH according to the method described previously (11)
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Southern and Northern Blot Analyses.
Genomic DNA from human placenta, primary ovarian cancer specimens, and ovarian cancer cell lines was isolated using the SDS/phenol/chloroform method. DNA was digested with EcoRI, fractionated on 1% agarose gel, transferred to a nylon membrane (Zeta-probe; Bio-Rad, Hercules, CA), and hybridized overnight at 42°C with 32P-labeled probes for the eIF-5A2 and PIK3CA genes. Total cellular RNA was prepared using the TRIzol/chloroform method, size fractionated on 1% agarose/2.2 M formaldehyde gel, transferred to a nylon membrane, and hybridized with 32P-labeled probes for the eIF-5A2 and PIK3CA genes.
| RESULTS |
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Amplification of eIF-5A2 in Ovarian Cancer.
DNA sequence amplification of eIF-5A2 in 30 primary ovarian cancers was studied by Southern blot analysis. Amplification of eIF-5A2 was detected in 15 of 30 cases and high-copy-number amplification was observed in 4 cases (OV-4, OV-7, OV-13, and OV-27; Fig. 4A
). High-copy-number amplification of eIF-5A2 was also detected in the ovarian cancer cell line UACC-1598, which contains 3q26 amplification in the form of double minutes (Fig. 4A)
. Expression level of eIF-5A2 in four primary ovarian cancers (OV-4, OV-7, OV-24, and OV-27) and ovarian cancer cell line UACC-1598 was analyzed by Northern blot and overexpression of eIF-5A2 was observed in all cases (Fig. 4B)
. To confirm whether the double minutes contain eIF-5A2, metaphase prepared from UACC-1598 was hybridized with the BAC clone containing eIF-5A2 by FISH. The result demonstrated that the BAC clone hybridized to all double minutes (Fig. 5A)
. Amplification of eIF-5A2 was also observed in another ovarian cancer cell line (OVCAR3) by FISH (Fig. 5B)
. According to our CGH results, a minimal overlapping amplicon at 3q26.1-26.2 was defined and eIF-5A2 is mapped exactly within the region (Fig. 5C)
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| DISCUSSION |
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It has been suggested that eIF-5A plays a role in the translation initiation; however, the initiation of protein synthesis in eIF-5A-depleted yeast cells was not significantly affected (14) . Although the precise function of eIF-5A is unclear, the necessity for the hypusine-containing eIF-5A for cell proliferation is well studied (14 , 15) . It has been demonstrated that complete intracellular depletion of eIF-5A by gene deletion resulted in inhibition of cell growth (13) . Other studies indicated that the inhibition of deoxyhypusine synthase, the enzyme involved in the hypusination reaction of eIF-5A, inhibited Chinese hamster ovary cell proliferation (16) , suppressed the growth of HeLa cells and v-src-transformed NIH3T3 cells (17) . Hanauske-Abel et al. (18) suggested that hypusine containing eIF-5A may directly affect the expression of a selective set of genes involved in the G1 to S transition of the cell cycle in the eukaryotic cells since the cell cycle could be arrested at the G1-S boundary by inhibition of eIF-5A hypusine modification. Recently, Tome et al. (19) reported that suppression of eIF-5A hypusine modification by excess putrescine accumulation or treated cells with diaminoheptane is one mechanism by which cells may be induced to undergo apoptosis. A recent study showed that the hypusine formation activity is serum responsive and significantly increased by >30-fold in Ras oncogene-transformed NIH3T3 cells (20) . All of these studies strongly suggested the association between eIF-5A and cancer development.
In the present study, amplification of eIF-5A2 was detected in 15 of 30 primary ovarian cancers and several ovarian cancer cell lines, including UACC-1598 which contains high-copy-number amplification of the 3q26 region. Overexpression of eIF-5A2 was observed in all four tested primary ovarian cancers with 3q26 amplification. In addition, a BAC clone containing eIF-5A2 was hybridized to double minutes in the ovarian cancer cell line UACC-1598. Therefore, based on its chromosomal location, its amplification status in ovarian cancer, and its possible proliferation-related function, eIF-5A2 is considered as the putative oncogene within the minimal overlapping amplicon at 3q26.2 in ovarian cancer.
The amplified region of 3q26 may contain more than one important gene, including PIK3CA and eIF-5A2 which are the biological targets of amplification events in ovarian cancer. eIF-5A2 is telomeric to PIK3CA and the genomic distance between them is about 7cM. Further studies of the amplification and expression of eIF-5A2 in a large set of ovarian cancers using tissue microarray as well as determination of its function will be necessary to address the relationship between eIF-5A2 and ovarian cancer.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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1 Supported in part by the Leung Kwok Tze Foundation. ![]()
2 To whom requests for reprints should be addressed, at Department of Clinical Oncology, Queen Mary Hospital, The University of Hong Kong, Room 129, Professorial Block, Pokfulam Road, Hong Kong. E-mail: xyguan{at}hkucc.hku.hk ![]()
3 The abbreviations used are: CGH, comparative genomic hybridization; BAC, bacterial artificial chromosome; FISH, fluorescence in situ hybridization. ![]()
4 X-Y. Guan, T. C-M. Tang, K-F. Huo, J. S. T. Sham, Y. Fang, and J-M. Yang, unpublished data. ![]()
Received 7/10/00. Accepted 3/ 1/01.
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