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Molecular Oncology, Markers, Clinical Correlates |
Gene CpG Island in Primary Gastric Cancers and Gastric Cancer Cell Lines
1 Department of Pathology, Yamagata University School of Medicine, Yamagata, Japan, and 2 Laboratory of Molecular Pharmacology, National Cancer Institute, NIH, Bethesda, Maryland
| ABSTRACT |
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(SNCG) gene expression is usually highly tissue-specific and restricted to the nervous system, SNCG is expressed in advanced-stage breast and ovarian cancers. When overexpressed, SNCG stimulates cancer cell proliferation and metastasis. It is thought that the molecular mechanism of CpG island demethylation may underlie aberrant SNCG expression. To determine whether aberrant SNCG expression and demethylation play a role in gastric carcinogenesis, we examined the expression and methylation status of SNCG in primary gastric cancers, gastric cancer cell lines, and non-neoplastic gastric mucosal tissues. Experimental Design: Ten gastric cancer cell lines, 105 primary gastric cancers, and 10 non-neoplastic gastric mucosal tissues were examined. SNCG expression and methylation status were examined by reverse transcription-PCR and bisulfite-single-strand conformational polymorphism followed by direct sequencing, respectively. The relationship between SNCG methylation status and various clinicopathological factors of the primary gastric cancers was then analyzed.
Results: SNCG mRNA expression was observed in 5 of 10 cell lines. Analysis of cell lines positive for SNCG expression revealed that most of the SNCG CpGs were demethylated. SNCG mRNA was not expressed in the 10 non-neoplastic gastric mucosal tissues, although several CpGs were demethylated. Of the 105 primary gastric cancers, 40 (38.1%) showed apparent SNCG demethylation, similar to the result obtained using cell lines. SNCG demethylation was more frequent in primary gastric cancers positive for lymph node metastasis (51%; 26 of 51) than in cancers without lymph node involvement (26%; 14 of 54; P < 0.05), and also more common in stage II-IV (48%; 27 of 56) than in stage I (27%; 13 of 49) cancers (P < 0.05).
Conclusions: Aberrant SNCG gene expression can occur via CpG island demethylation, and tends to occur during the more progressive stages of gastric carcinogenesis.
| INTRODUCTION |
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The synuclein
(SNCG) gene, also known as the breast cancer specific gene 1 (BCSG1), is a member of the synuclein neuronal protein family along with synuclein
(SNCA) and synuclein ß (SNCB; Refs. 18, 19, 20
). Although the function of synuclein proteins remains largely unknown, SNCA aggregation may be important in the etiology and pathogenesis of neurodegenerative disorders such as Alzheimers disease and Parkinsons disease (21)
. SNCG is located on chromosome 10q23, transcribed as a 1Kb mRNA, and encodes a 127-amino acid polypeptide (19)
. SNCG protein is highly tissue-specific, and is expressed at presynaptic terminals in the brain and peripheral nervous system (19
, 20)
. However, this tissue specificity appears to be lost during breast and ovarian cancer disease progression. Whereas SNCG expression is normally silent in the breast and ovary, it becomes abundantly expressed in the vast majority of advanced-stage breast and ovarian cancers (22)
. SNCG expression is activated by demethylation of the SNCG CpG island in a similar manner to the oncogenes and growth-related genes described above (14, 15, 16, 17
, 22)
. These findings suggest that SNCG might act as a proto-oncogene, and play a positive role in the process of invasion and metastasis in breast and ovarian cancer (22)
. To determine whether aberrant expression and demethylation of SNCG is involved in gastric carcinogenesis, we examined the expression and methylation status of SNCG in primary gastric cancers, gastric cancer cell lines, and non-neoplastic gastric mucosal tissues.
| MATERIALS AND METHODS |
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Gastric Cancer Patients and Surgical Specimens.
One hundred and five gastric cancer samples were obtained at surgery. Patients ranged in age from 43 to 89 years (mean, 67 years). All of the patients received a median of 37.5 months of follow-up care (range, 377 months). All of the samples were snap-frozen and stored at 80°C until processing. The tumors consisted of 40 early and 65 advanced tumors, and were identified as 69 differentiated and 36 undifferentiated tumor samples. Lymph node metastasis was positive in 51 cases, and 49 cases were stage I (Table 1)
. Genomic DNA was extracted from each sample using standard procedures.
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Reverse Transcription-PCR.
RNA isolated from cell lines and non-neoplastic gastric mucosal tissues was reverse transcribed and amplified using the ONE-STEP reverse transcription-PCR System (Life Technologies, Inc., Gaithersburg, MD). Primer sequences used were sense 5'-CAA GAA GGG CTT CTC CAT CGC CAA GG-3' and antisense 5'-CCT CTT TCT CTT TGG ATG CCA CAC CC-3' for SNCG (318 bp; Ref. 22
). A 587-bp ß-actin fragment was amplified as a control. After initial activation of the enzyme at 94°C for 2 min, PCR reactions were exposed to 38 cycles of 94°C for 30 s, 60°C for 30 s, and 72°C for 30 s.
5-Aza-2'-Deoxycytidine Treatment.
To examine SNCG expression in response to treatment with 5-aza-2'-deoxycytidine (Sigma), cell lines were incubated for 96 h with 0.2 or 1 µM 5-aza-2'-deoxycytidine, and then harvested for RNA extraction and reverse transcription-PCR.
Bisulfite-Single-Strand Conformation Polymorphism (SSCP) and Sequencing Analyses.
DNA samples were treated with bisulfite to convert all of the unmethylated cytosines to uracils, while leaving methylated cytosines unaffected. Briefly, 2 µg of aliquots of genomic DNA were denatured by treatment with NaOH and modified by sodium bisulfite. DNA samples were then purified using Wizard DNA purification resin (Promega, Madison, WI), treated with NaOH, precipitated with ethanol, and resuspended in 30 µl. DNA methylation status was determined by bisulfite-SSCP with some modifications (23)
. After treatment of DNA with sodium bisulfite, a 415-bp fragment encompassing the CpG island, and exon 1 of the SNCG gene was amplified using primers 5'-GGT TGA GTT AGT AGG AGT TTA-3' and 5'-CCT ACC ATA CCC CAC TTA CCC-3' (22)
. The PCR mix contained 1x PCR buffer [15 mM Tris-HCl (pH 8.0) and 50 mM KCl], 1.5 mM MgCl2, 1 µM each primer, 0.2 mM deoxynucleoside triphosphates, 0.5 units of Taq polymerase (AmpliTag Gold DNA Polymerase; PE Applied Biosystems, Foster City, CA), 5% DMSO, and 2.5 µCi [
-32P]dCTP (Amersham, Buckinghamshire, England), and genomic DNA (100 ng) in a final volume of 10 µl. After activation of the Taq polymerase at 95°C for 10 min, PCR was performed in a GeneAmp PCR System 9700 (PE Applied Biosystems) for 38 cycles of denaturation at 94°C for 30 s, annealing at 55°C for 45 s, and extension at 72°C for 60 s, followed by a final 7 min extension at 72°C. PCR products were diluted 1:10 in denaturing loading buffer [95% formamide, 10 mM EDTA (pH 8.0), 0.02% xylene cyanol FF, and 0.02% bromphenol blue], heated at 95°C for 4 min, placed on ice, and 2 µl aliquots subjected to electrophoresis on 6% polyacrylamide gels. Gels were dried and exposed to hyperfilm MP autoradiography film (Amersham) for 416 h. Direct sequencing was performed using small pieces of the gel containing the shifted band detected by bisulfite-SSCP. Gel slices were immersed in 50 µl of water, heated to 95°C, and then applied to PCR under the conditions described above for SSCP, except that PCR was carried out in a volume of 50 µl. The PCR products were then purified using the QIA quick PCR purification kit (Qiagen, Tokyo, Japan) and sequenced using the Big Dye Terminator Cycle Sequencing Ready Reaction kit (PE Applied Biosystems). Gel electrophoresis, data collection, and analysis were performed on a Genetic Analyzer (model 310; PE Applied Biosystems).
Statistical Analysis.
Statistical comparisons were performed using Students t test, Fishers exact test, or
2 test, as appropriate. Ps < 0.05 were considered statistically significant. Kaplan-Meier analysis was performed to estimate a survival function over time for individual covariates. Survival estimates between cancers exhibiting SNCG methylation or demethylation were compared by log-rank test.
| RESULTS |
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mRNA Expression and Methylation Status of SNCG in Non-Neoplastic Gastric Mucosal Tissues.
SNCG mRNA was not expressed in any of the 10 non-neoplastic gastric mucosal tissues. However, a few tissues exhibited faint bands with mobility shifts by bisulfite-SSCP (data not shown), and sequencing of these bands revealed partial demethylation of SNCG CpGs (Fig. 3C)
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Correlation between Methylation Status and Clinicopathological Factors of Primary Gastric Cancers.
Clinicopathological features of primary gastric cancers showing SNCG demethylation were analyzed. Demethylation of SNCG was more frequent in lymph node metastasis-positive primary gastric cancers (51%; 26 of 51) than in metastasis-negative cancers (26%; 14 of 54; P < 0.05), and in stage II-IV cancers (48%; 27 of 56) than in stage I cancers (27%; 13 of 49; P < 0.05; Table 1
).
SNCG Demethylation and Patient Prognosis.
Kaplan-Meier estimates were used to examine the relationship between SNCG demethylation and patient survival. Although not statistically significant, there was a tendency toward patients with a poor prognosis showing SNCG demethylation (P = 0.076; Fig. 5
).
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| DISCUSSION |
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We also analyzed the clinicopathological features of gastric cancers that showed SNCG demethylation. Previous studies have suggested that SNCG acts as an oncogene in breast and ovarian cancer cells (22 , 24 , 25) . A stage-specific expression pattern of SNCG mRNA has been demonstrated by in situ hybridization that varied from virtually no detectable expression in normal or benign breast tissues to low level or partial expression in low-grade ductal carcinoma in situ, through to high expression in advanced infiltrating carcinomas (24) . Similarly, immunohistochemical studies have shown that SNCG protein is not expressed in normal breast tissues but is expressed in a high percentage of stage III/IV breast carcinomas (25) . SNCG is also expressed in ovarian cancers (25) . In the present study, SNCG demethylation was more frequent in primary gastric cancers positive for lymph node metastasis (51%; 26 of 51) than in metastasis-negative cancers (26%; 14 of 54; P < 0.05), and was also more frequent in stage II-IV cancers (48%; 27 of 56) than in stage I cancers (27%; 13 of 49; P < 0.05). Our results are in agreement with those obtained for breast and ovarian cancers (25) . Although not statistically significant, there was an increased tendency for gastric cancer patients with poor prognoses to show SNCG demethylation compared with gastric cancer patients with normal methylation (P = 0.076).
Although additional studies on gene methylation, protein expression, and localization in normal and transformed tissues are required to establish that the methylation status of synuclein
CpG islands regulates mRNA, and ultimately protein expression, a distinct correlation between mRNA levels and CpG methylation is established by the present study. Our study indicated that SNCG was frequently expressed, via CpG demethylation, during the more progressive stages of gastric carcinogenesis. SNCG, a potential oncoprotein, may play a role in the progression and metastasis of common human malignancies.
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| FOOTNOTES |
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Requests for reprints: Gen Tamura, Department of Pathology, Yamagata University School of Medicine, 2-2-2 Iida-nishi, Yamagata 990-9585, Japan. Phone: 81-23-628-5243; Fax: 81-23-628-5245; E-mail: gtamura{at}med.id.yamagata-u.ac.jp
Received 8/28/03; revised 12/18/03; accepted 1/ 2/04.
| REFERENCES |
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-synuclein gene. Hum Genet, 103: 106-12, 1998.[CrossRef][Medline]
-synuclein in neurodegenerative disorders. Am J Pathol, 152: 367-72, 1998.[Abstract]
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