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1 Molecular Oncology and Aging Group, Molecular Biology and Biochemistry Research Center (CIBBIM), Valle Hebron Hospital Research Institute, Barcelona, Spain; 2 Department of Medical Genetics, Biomedicum Helsinki, University of Helsinki, Helsinki, Finland; 3 Institute of Molecular Pathology/Applied Tumor Biology, University of Heidelberg, Heidelberg, Germany; 4 Montefiore Medical Center, Albert Einstein Cancer Center, Bronx, New York; and 5 First Department of Internal Medicine, Sapporo Medical University, Sapporo, Japan
Requests for reprints: Diego Arango, Molecular Oncology and Aging Group, Molecular Biology and Biochemistry Research Center (CIBBIM), Valle Hebron Hospital Research Institute, Passeig Vall d'Hebron 119-129, Barcelona 08035, Spain. Phone: 34-93-489-4058; Fax: 34-93-489-4040; E-mail: darango{at}vhebron.net.
| Abstract |
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| Introduction |
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Approximately 15% of the tumors of the colon and the rectum display a microsatellite unstable phenotype (MSI). This is observed as frequent insertions and deletions within short repetitive sequences known as microsatellites. Mutations within coding regions of the target genes result in frameshifts that can disrupt protein function. Mutations that confer a growth advantage to the cells are selected and can be found in a significant percentage of colorectal tumors with an MSI phenotype. Genes frequently targeted by these mutation include those involved in molecular mechanisms important in the development of colorectal tumors, including the transforming growth factor-ß pathway, Wnt signaling, and DNA damage repair and apoptosis pathways (6). Because loss of EPHB2 activity is an important step in tumor progression, and because this receptor contains an A9 track in exon 17 that could be a target for mutation in MSI tumors, we screened this region for alterations in MSI tumor cell lines as well as in primary adenomas and carcinomas with MSI.
Hypermethylation of cytosines located within CpG islands in the promoter of tumor suppressor genes is emerging as an important mechanism of gene silencing in both microsatellite stable (MSS) and unstable colorectal tumors, and has been reported to disrupt important pathways in colorectal tumorigenesis, including the TP53 pathway (p14ARF), the WNT signaling pathway (APC, E-cadherin), DNA repair (MGMT, hMLH1, BRCA1), apoptosis (DAPK), and metastasis (E-cadherin, TIMP3; ref. 7). We identified a CpG island spanning the proximal EPHB2 promoter and the first exon, and investigated the possible contribution of aberrant methylation of this region in the regulation of EPHB2 expression.
In this study, we investigated the mechanisms of inactivation of EPHB2 in colorectal tumors and found that microsatellite unstable tumors have frequent mutations in the A9 repeat in exon 17 of EPHB2. Moreover, a CpG island in the proximal promoter region of this gene was hypermethylated in most of the colorectal tumors studied.
| Materials and Methods |
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EPHB2 mutation screening. The A9 repeat in exon 17 of the larger EPHB2 transcript (accession no., NM_017449) and flanking genomic DNA sequence were PCR-amplified in the 246 MSI tumor samples entered in this study (primer sequence and PCR conditions available upon request). Mutation screening in the amplified PCR fragments was done by direct automated sequencing (ABI 3100 capillary sequencer), fragment analysis (GeneScan Software, Applied Biosystems, Foster City, CA) and/or single-stranded conformational polymorphism, as previously described (9, 13, 14).
Methylation-specific PCR. DNA methylation status of an EPHB2 promoter-associated CpG island (from 537 to +836) was determined in colorectal tumor samples and cell lines included in the study by bisulfite conversion of unmethylated, but not methylated, cytosine to uracil as previously described (15). PCR reactions using primers specific for either the methylated or the modified unmethylated DNA (methylation-specific PCR) were carried out to determine the methylation profile of each sample. Primers were designed using MethPrimer 1.1 software. EPHB2 primers sequences for the methylated sequence were 5'-TTGTTTTTGTTAGTCGCGTTAGAC-3' (sense) and 5'-CAATAATCTCTCCCGACGCT-3' (antisense), and for the unmethylated sequence 5'-TGTTTTTGTTAGTTGTGTTAGATGT-3' (sense) and 5'-CAACAATAATCTCTCCCAACACT-3' (antisense). PCR amplification was done using EcoStar DNA polymerase (Ecogen, Barcelona, Spain) under the following conditions: 95°C for 10 minutes, 35 cycles of 30 seconds at 95°C, 30 seconds at 58°C and 30 seconds at 72°C, and 4 minutes of final extension at 72°C. In vitro methylated DNA (CpG Genome Universal Methylated DNA; Chemicon International, Temecula, CA) was used as a positive control for methylated alleles, whereas DNA from normal lymphocytes and normal colon tissues were used as negative controls. Each PCR product was directly loaded onto 2% agarose gels, stained with ethidium bromide, and visualized under UV illumination.
Western blotting. SW620 cultures were treated with the DNA methyltransferase inhibitor 5-aza-2'-deoxycytidine for 72 hours (0, 2, 5, or 10 µmol/L). Twenty micrograms of total protein (radioimmunoprecipitation assay buffer lysates) were fractionated in 8% SDS-polyacrylamide gels. Proteins were transferred to a polyvinylidene difluoride membrane (Amersham, Piscataway, NJ), blocked with 10% nonfat milk for 1 hour and then probed overnight at 4°C with a 1:100 dilution of anti-EPHB2 primary antibody (Stratagene, La Jolla, CA). Membranes were washed thrice with washing buffer (PBS with 0.1% Tween 20) and then probed with a peroxidase-conjugated secondary antibody for 1 hour (1:2,000; Boehringer Mannheim, Indianapolis, IN). After washing thrice with washing buffer, the signal was detected using enhanced chemiluminescence plus (Amersham) and a Storm PhosphorImager (Molecular Dynamics, Sunnyvale, CA). The membranes were then stripped and reprobed with an anti-ß-actin antibody (clone AC74, 1:1,000; Sigma, St. Louis, MO). The signal from the ß-actin probe was used as a loading control.
| Results and Discussion |
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2 test; P = 0.03). This observation is in good agreement with earlier reports showing that EPHB2 expression was reduced or lost in colorectal carcinomas, but not in adenomas (5), and further suggests that EPHB2 inactivation may be important for the transition from adenoma to carcinoma. Frequent hypermethylation of the EPHB2 promoter in colorectal tumors. Cytosine hypermethylation in CpG dinucleotides in the regulatory region of tumor suppressor genes has been linked to reduced gene expression (7). Bisulfite treatment of genomic DNA allows precise analysis of methylation in a certain region by converting all nonmethylated cytosines into uracil, whereas methylated cytosines remain unchanged. PCR primers specific for the methylated or unmethylated sequence can then be used to investigate promoter hypermethylation (15).
The proximal promoter of EPHB2 contains a CpG island spanning 1,400 bp around the transcription start site, that could be hypermethylated and thus regulate the expression of this gene. We used a series of 60 MSI and 41 MSS colorectal tumors to investigate whether hypermethylation of the EPHB2 promoter could be a mechanism of gene inactivation in colorectal tumors. Of the 101 tumors investigated, 54 (53.4%) showed signs of EPHB2 promoter hypermethylation (Fig. 2A; Supplementary Table S1). There was no difference in the proportion of MSS and MSI tumors showing EPHB2 promoter methylation (51.2% and 55%, respectively;
2 test; P = 0.7) and no associations were found with other clinicopathologic features (patient age, sex, 5-year overall survival, tumor stage, grade and mutations in the A9 repeat in EPHB2 exon 17; Supplementary Table S3).
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Losses of large chromosomal regions are common in MSS tumors and deletions targeting important tumor suppressor genes confer a growth advantage to the cells and are clonally selected. EPHB2 is located in the short arm of chromosome 1 (1p36), one of the most commonly deleted areas in colorectal tumors (1719). The frequent inactivation of EPHB2 by mutation and promoter hypermethylation described in this study is in good agreement with the high frequency of genomic losses in this region, and given the recently shown function of EPHB2 as a tumor suppressor gene in colorectal cancer (5), this gene could be an important target for at least some of these deletions.
In conclusion, in this study, we describe for the first time the mechanisms of EPHB2 inactivation in colorectal tumors. We found frequent mutations in repetitive sequences in exon 17 in MSI adenomas and carcinomas (21% and 41%, respectively) and hypermethylation of the EPHB2 promoter in the majority of the tumors of the colon and rectum (53%).
| Acknowledgments |
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The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
| Footnotes |
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Received 7/22/05. Revised 9/ 7/05. Accepted 9/28/05.
| References |
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