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Molecular Biology and Genetics |
University of Cincinnati Department of Surgery, Division of Surgical Oncology, Cincinnati, Ohio 45219 [W. M. C., A. S., A. M. L.]; Howard Hughes Medical Institute and the University of Cincinnati Department of Molecular Genetics, Biochemistry, and Microbiology, Cincinnati, Ohio 45267 [W. M. C., A. S., J. M., J. G.]; University of Cincinnati Department of Pathology and Laboratory Medicine, Cincinnati, Ohio 45267 [J. W., O. J. K., C. F-P.]; and Department of Hematology and Oncology, St. Vincents Comprehensive Cancer Center, New York, New York [W. M. C., A. S., J. M., J. G.]
| ABSTRACT |
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| INTRODUCTION |
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, as well as gastrin, connexin 43, WISP1, and WISP2 (4, 5, 6, 7, 8, 9, 10)
. Several lines of evidence implicate the Wnt signaling pathway as a contributor to gastric carcinogenesis. Persons with germ-line mutation of the APC tumor suppressor gene have a 10-fold increased risk of developing gastric cancer as compared with normal persons (11) . Mutations in the APC gene have also been found in sporadic gastric cancers. Nakatsuru et al. (12) reported APC mutations in 12 of 46 gastric cancers. Ten of the 12 mutations were noted in intestinal and signet-ring carcinomas. Other investigators noted lower rates of APC mutation in diffuse-type gastric cancers. There have been five studies examining the incidence of ß-catenin mutation in gastric cancer. The initial report by Candidus et al. (13) reported no mutations in 16 diffuse- and 5 intestinal-type tumors. Park et al. (14) reported mutations in 7 of 26 intestinal-type gastric cancers, but no mutations in 17 diffuse tumors. Woo et al. (15) described ß-catenin nuclear staining in 81 of 303 tumors (27%). Seventy-seven tumors with nuclear ß-catenin were analyzed, and 4 (5%) ß-catenin mutations were detected. Another recent report examined diffuse tumors only and noted nuclear staining for ß-catenin in 23%; however, no mutations were detected from a total of 35 specimens (16) . Thus, although ß-catenin mutations have been detected in gastric cancer, the incidence has varied greatly among studies. The incidence of Wnt activation in intestinal- versus diffuse-type tumors remains unclear.
The goals of this study were to determine: (a) the incidence of ß-catenin nuclear localization in gastric cancer; (b) the incidence of ß-catenin mutations in tumors with and without nuclear ß-catenin; and (c) the incidence of ß-catenin mutation in diffuse- versus intestinal-type gastric cancers. We report that ß-catenin nuclear localization occurs in approximately one-third of gastric tumors, that the presence of ß-catenin nuclear localization is a useful indicator of Wnt pathway activation in gastric cancer, and that ß-catenin mutations occur in both diffuse- and intestinal-type gastric cancer at a higher rate than has been recognized previously.
| MATERIALS AND METHODS |
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LCM3
and DNA Extraction.
All tumors that displayed nuclear staining for ß-catenin were selected for LCM. Normal-appearing lymphocytes within the same tissue sections were used as internal controls. An additional 19 randomly selected tumors that were negative for nuclear ß-catenin were also subjected to LCM. Using the ß-catenin-stained slides as road maps, serial sections were stained with H&E and left without coverslips. LCM was then used to microdissect the cells in the region of the nuclear staining cells (Arcturus, Mountain View, CA). All cases were dissected by a dedicated pathologist (O. J. K.). The microdissected tissues were placed in a 0.5 ml microcentrifuge tube and treated with digestion buffer [10 mM Tris-HCl (pH 8.0), 1 mM EDTA, and 1.0% Tween 20] containing 400 µg/ml proteinase K at 55°C for 12 h. Samples were then heated to 95°C for 10 min to inactivate the proteinase K and used for PCR.
PCR Amplification and Sequence Analysis.
After LCM and DNA extraction, exon 3 of ß-catenin was amplified by PCR. The primers used to amplify exon 3 of ß-catenin were forward primer 5'-TAACATTTCCAATCTACTAATGC-3' and reverse primer 5'-AGCTACTTGTTCTTGAGTGAAG-3'. The PCR reaction was performed in a 12.5-µl reaction volume containing 1 µl of template DNA, 6 mM of each primer, 200 µM deoxynucleotide triphosphates, 2.0 mM MgCl2, 4% DMSO, 0.6 unit of AmpliTaq DNA polymerase, and 1.25 µl of 10x PCR buffer containing 100 mM Tris-HCl (pH 8.3) and 500 mM KCl (Applied Biosystems, Forest City, CA). The reaction mixture was preincubated for 5 min at 95°C and then incubated for 35 cycles of 94°C for 1 min, annealing at 54°C for 2 min, and extension at 72°C for 3 min on a MJ systems thermal cycler (MJ Research). The final extension was continued at 72°C for 7 min. An aliquot of the PCR product was electrophoresed on a 1.5% Seakem GTG agarose gel in TBE buffer (FMC Bioproducts, Rockland, ME). The PCR products were visualized with ethidium bromide and purified using the Qiaquick PCR purification kit (Qiagen, Valencia, CA). Sequence analysis was performed using the USB ThermoSequenase Radiolabeled Terminator Cycle Sequencing kit (USB, Cleveland, OH) in both the sense and antisense directions using the same primers as for the PCR amplification. All mutations were verified by repeat sequence analysis in the sense and antisense directions.
| RESULTS |
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| DISCUSSION |
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(a) Our strategy of immunohistochemical screening for nuclear ß-catenin was successful in focusing our analysis on those tumors more likely to harbor mutations in Wnt pathway genes. The hypothesis that nuclear ß-catenin predicts Wnt pathway activation is especially strengthened by our finding that no mutations were present in 19 tumors negative for nuclear ß-catenin.
(b) The use of LCM allowed us to sample pure populations of tumor cells for mutations in the ß-catenin gene. Most other published studies have relied on the less precise method of manual microdissection. It is likely that the use of laser capture greatly decreases the incidence of false-negative results that can occur when normal, contaminating tissue DNA is amplified by PCR.
Exon 3 of the ß-catenin gene encodes serine-threonine phosphorylation sites for the glycogen synthase kinase-3ß kinase that regulate degradation of ß-catenin by the ubiquitin-proteasome pathway. Mutations in exon 3 of ß-catenin and alteration of these phosphorylation sites confer resistance to phosphorylation and lead to the accumulation of cytoplasmic and nuclear ß-catenin and subsequent changes in gene expression. Nearly all ß-catenin mutations reported in human cancers are localized to exon 3, and most occur at serine-threonine sites or on adjacent residues (19) . Although changes at phosphorylation sites directly impair phosphorylation itself, it is theorized that mutations adjacent to serine/threonine residues alter the DSG amino acid motif that serves as a recognition sequence for ubiquitination (2) . The resulting decrease in phosphorylation and alteration of the ubiquitin DSG amino acid targeting sequence leads to diminished ß-catenin degradation. In this study, 14 of 21 mutations were located at or adjacent to phosphorylation sites. Mutation at codon 37 was most common and was detected in 5 tumors. The serine to alanine mutation at codon 37 (S37A) is common in colon carcinoma and has been described in numerous other tumor types as well (19) . The significance of the S37A mutation has been documented by pulse-chase analysis that demonstrated an extended half-life of the ß-catenin protein. In addition, it has been reported that S37A mutant ß-catenin is not ubiquitinated in SKBR3 cells treated with a proteosomal inhibitor (2 , 3) .
Five of the mutations detected are predicted to affect regions not involved in phosphorylation or ubiquitination. It is less clear how these mutations might affect the phosphorylation of ß-catenin; however, such mutations have also been reported in anaplastic thyroid carcinoma (20) . It is possible that such alterations may alter protein secondary structure and thereby the interaction of ß-catenin with the APC/axin complex. Alternatively, they may, in some other manner, alter the recognition site of phosphorylation by glycogen synthase kinase-3ß or simply prevent phosphorylation in an unknown manner.
In 54 tumors, nuclear ß-catenin was noted; yet no ß-catenin mutations were found. Our study was technically biased by the limited examination of ß-catenin. We could have missed mutations in other exons of the gene or failed to detect a deletion of exon 3. It is also possible that these tumors may harbor an alteration in another Wnt pathway regulatory gene. APC is a likely candidate because mutations in gastric cancer have been described previously (12) . Hypermethylation of the APC promoter has also been described in gastric tumors and thus could potentially contribute to down-regulation of APC and subsequent Wnt activation (21) . Other recent reports have described over expression of various Wnt ligands and altered frizzled receptors in gastric cancer (22, 23, 24, 25, 26) . Axin is a known negative regulator of ß-catenin and is mutated in hepatocellular carcinomas with ß-catenin accumulation and without ß-catenin mutation (27) . A recent report noted multiple different mutations were responsible for activation of the Wnt pathway in ovarian endometrioid adenocarcinomas (28) .
Gastric cancer can be divided pathologically into two broad groups, intestinal-type and diffuse-type, as classified by Lauren (29) . The intestinal-type is well differentiated and is composed of neoplastic gland-forming cells, whereas the diffuse-type is poorly differentiated and is composed of scattered groups of loosely associated cells without much cell-to-cell adhesion. A previous study found no ß-catenin mutations in diffuse-type cancer but found that 27% of intestinal-type cancers carried a mutation in exon 3 of ß-catenin (14) . We did not identify any significant difference in the incidence of either ß-catenin nuclear staining or ß-catenin mutation between diffuse- and intestinal-type tumors. We believe this suggests that Wnt activation is likely an early event in gastric carcinogenesis and that other molecular events may play a more critical role in determining diffuse- versus intestinal-type histology.
In conclusion, Wnt pathway activation occurs in nearly one-third of gastric adenocarcinomas as defined by ß-catenin nuclear localization. ß-catenin mutation is a frequent cause of pathway activation in those tumors with nuclear ß-catenin. Our results demonstrate that ß-catenin mutation occurs in both diffuse- and intestinal-type cancers. This suggests that in the stomach as in the colon, Wnt pathway activation may be an early inciting event in multistep carcinogenesis.
| FOOTNOTES |
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1 This work was supported by NIH CA89403 (to A. M. L.), CA08411 (to C. F. P.), CA32102 (to C. F. P.), CA63507 (to J. G.), and a grant from Ohio Cancer Research Associates (to A. M. L.). J. G. is an Investigator with the Howard Hughes Medical Institute. ![]()
2 To whom correspondence should be addressed, at University of Cincinnati, Division of Surgical Oncology, Barrett Cancer Center, 234 Goodman Center, Cincinnati, OH 45219-0772. E-mail: lowyam{at}healthall.com ![]()
3 The abbreviation used is: LCM, laser capture microdissection. ![]()
Received 3/14/02. Accepted 4/18/02.
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- and ß-catenin genes in human gastric and breast carcinomas. Cancer Res., 56: 49-52, 1996.
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