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Division of Human Cancer Genetics, Department of Molecular Virology, Immunology and Medical Genetics, Ohio State University, Columbus, Ohio
Requests for reprints: Rami I. Aqeilan, Division of Human Cancer Genetics, Ohio State University, Biomedical Research Tower, Room 1088, 460 West 12th Avenue, Columbus, OH 43210. Phone: 614-292-5906; Fax: 614-292-4097; E-mail: rami.aqeilan{at}osumc.edu.
| Abstract |
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| Introduction |
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Esophageal squamous cell carcinoma (SCC) is an important cause of morbidity and mortality worldwide (8). Nutritional deficiency and exposure to carcinogens, such as N-nitrosamines, are associated with an increased risk of esophageal SCC (9). The mouse forestomach has a squamous-epithelial lining analogous to the human distal esophagus. The murine squamocolumnar junction (SCJ), a zone of transition between squamous-epithelial and glandular tissue, is analogous to the human esophagogastric junction. These structures are commonly studied as model systems for the distal esophagus in human (10). N-nitrosomethylbenzylamine (NMBA), an environmental carcinogen (11), has been extensively used to induce esophageal and forestomach tumors in rodents. Following bioactivation, NMBA produces an electrophilic methylating agent that produces the mutagenic adduct O6-methylguanine in DNA. Indeed, human esophageal epithelial DNA isolated from patients in areas with a high risk for esophageal SCC showed elevated levels of O6-methyldeoxyguanosine (9, 11). To understand the role of the WWOX gene in esophageal cancer, we used this established model to investigate whether inactivation of the Wwox gene renders the mouse more susceptible to NMBA-induced esophageal/forestomach carcinogenesis (12).
| Materials and Methods |
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NMBA-induced forestomach carcinogenesis study. This study was approved by the Ohio State University Animal Facility Institutional Animal Care and Use Committee and conducted according to NIH guidelines.
Thirty-four Wwox+/+ mice and 26 Wwox+/ mice (1012 weeks old) were given six intragastric doses of NMBA (Ash Stevens) for a period of 3 weeks, at 2 mg/kg body weight, twice weekly. After 15 weeks, all animals were sacrificed and analyzed for tumor incidence. Whole esophagi and stomachs were excised and opened longitudinally. The number of animals bearing tumors (
0.5 mm in diameter) in the forestomach and SCJ with the glandular stomach was scored. Tissues were fixed in 10% buffered formalin and embedded in paraffin. Sections (4 µm) were either stained with H&E for histopathology or left unstained for immunohistochemical studies.
Immunohistochemistry. Tissue sections were dewaxed, rehydrated, and pretreated with H2O2 followed by citrate buffer (pH 6.0) and Target Retrieval Solution (Dako) for a total of 25 min in a pressure cooker. To detect proliferating cell nuclear antigen (PCNA), cytokeratin 14 (KRT14), and Wwox protein expression, tissues were incubated overnight in a humidified chamber at room temperature with a monoclonal mouse anti-PCNA (Neomarkers) at a 1:350 dilution or mouse anti-cytokeratin 14 monoclonal antiserum (clone LL002; Novocastra Lab) at 1:100 dilution or rabbit anti-WWOX polyclonal antiserum (13) at 1:8,000 dilution followed by incubations with a biotinylated goat anti-mouse or anti-rabbit antibody serum (Dako). Proteins were visualized with 3,3'-diaminobenzidine tetrahydrochloride (Dako). Slides were counterstained with hematoxylin, dehydrated, and coverslipped. For PCNA visualization, slides were incubated with 3-amino-9-ethylcarbazole substrate chromogen system (Dako). Cells with a red reaction product in the nucleus were defined as positive for PCNA, an endogenous marker for cell proliferation.
Statistical analysis. Tumor incidence differences were analyzed by two-tailed Fisher's exact test (Biostate).1 Multiplicity (tumor/mice) was analyzed using one-way ANOVA.
| Results |
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7-fold higher in Wwox+/ mice than in wild-type mice (3.2 ± 0.34 versus 0.47 ± 0.17; P < 0.0001). Furthermore, Wwox+/ mice showed a higher incidence of large tumors (
1 mm) than wild-type controls. The incidence of large tumors among Wwox+/ mice was
5-fold higher than in wild-type mice (P = 1 x 105) and the number of these tumors per mouse was
8-fold higher than in wild-type littermates (Fig. 2
; P < 0.0001). These data suggest that inactivation of Wwox led to the formation of more aggressive tumors as compared with wild-type mice.
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Progression to malignancy in Wwox+/ mice. Histopathologic examination of forestomach tumors revealed that 27% (7 of 26) of Wwox+/ mice had invasive SCC in the forestomach (Fig. 1B, d and g), as compared with none of the Wwox+/+ mice (Table 1; P = 0.002). In addition, forestomachs from Wwox+/ mice showed an array of deep downgrowths, dysplasia, squamous papillomas, focal ulcerated lesions, and glandular metaplasia (data not shown). By contrast, forestomachs from Wwox+/+ mice typically displayed a thickened epithelium with basal cell hyperplasia (Fig. 1B, a) and occasional presence of squamous papillomas (data not shown). These data show that inactivation of the Wwox gene leads to tumor progression towards malignancy.
Cell proliferation and expression of tumor marker KRT14. To study proliferation in NMBA-induced forestomach tumors, we used PCNA (an endogenous cell proliferation marker) immunohistochemistry. In wild-type mice, PCNA-positive nuclei were largely found in the basal cell layers of the forestomach (Fig. 1B, b) and in suprabasal cells in areas with mild hyperplasia (data not shown). In general, Wwox+/ forestomachs were proliferative with frequent deep downgrowths that showed abundant PCNA-positive nuclei in areas of focal hyperplastic lesions and dysplasia (data not shown). Wwox+/ forestomach SCCs that were highly proliferative exhibited numerous PCNA-positive cells in invasive tumor areas (Fig. 1B, e and h). These data showed that inactivation of Wwox led to a highly proliferative phenotype.
We next used KRT14 immunohistochemistry to examine tumor progression in Wwox+/mice. KRT14, a tumor marker for human and rodent esophageal carcinogenesis (14, 15), has been found to be useful in the diagnosis of basaloid squamous carcinoma, an aggressive variant of SCC (16). In control Wwox+/+ mice epithelium, KRT14 was found exclusively in the basal cell layer of the forestomach (Fig. 1B, c). By contrast, Wwox+/ mice forestomach tumors showed strong KRT14 expression in dysplastic epithelia and in invasive SCC (Fig. 1B, f and i). These results show the aggressive nature of the tumors formed in Wwox+/ mouse forestomach.
Wwox haploinsufficiency is cancer-predisposing. To examine whether NMBA treatment resulted in the inactivation of the remaining Wwox allele in Wwox+/ forestomach tumors, we analyzed Wwox protein expression by immunohistochemistry. Wwox expression is strong in Wwox+/+ mice forestomach epithelium (Fig. 1C, a). In heterozygous mice, normal forestomach epithelial cells showed moderately strong Wwox expression (Fig. 1C, b). Interestingly, Wwox staining in the same forestomach section was weak and diffuse in epithelial cells showing dysplasia (Fig. 1C, c), and very weak in the invasive tumor areas of SCC in Wwox+/ mice (Fig. 1C, d). This finding suggests that Wwox is haploinsufficient for tumor initiation in forestomach tissue.
| Discussion |
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Intriguingly, weak Wwox protein expression was detected in Wwox+/ forestomach carcinomas (Fig. 1C), a condition that resembles loss of heterozygosity in human tumors due to the fragility of the WWOX locus in human. In our present study, the only genetic difference between Wwox+/ and wild-type mice is the disrupted Wwox allele in Wwox+/ mice, suggesting that the second Wwox allele may have anti-growth properties in cancer transformation. It is quite possible that in more advanced malignant stages, the second allele of WWOX is lost and tumors become more invasive and probably metastatic. Indeed, our immunohistochemical analysis indicated that Wwox expression is reduced in invasive SCC cases when compared with normal epithelium of the same mouse section (Fig. 1C, bd). Together, our findings suggest that WWOX is a single hit tumor suppressor gene and that WWOX is haploinsufficient for tumor suppression and significant reduction in Wwox levels is selected during tumor progression. This observation of WWOX haploinsufficiency is in agreement with early findings showing that some tumors retain an intact copy of the WWOX gene and express reduced levels of the WWOX protein (4, 17).
The WWOX gene shares a number of similarities with the FHIT gene, which is located at 3p14.2 and spans the FRA3B common fragile site (reviewed in ref. 18). Decrease or loss of both WWOX and FHIT expression was reported in a number of common cancers (reviewed in refs. 4, 17). Interestingly, WWOX and FHIT protein expression is coordinately altered in gastric adenocarcinoma (6). Likewise, Fhit/ and Fhit+/ mice exhibit increased NMBA tumor incidence (10, 18). This data suggests that the mechanism of WWOX and FHIT inactivation could be similarly contributing to cancer development.
In summary, our data provide the first direct in vivo evidence that the WWOX gene plays a critical role in mouse forestomach, and thus, human esophageal cancer development and progression. Therefore, the Wwox-mutant mouse is a useful model for designing experiments to investigate the role of the WWOX gene in human cancers, including esophageal cancer, and to develop novel strategies to prevent and treat these cancers.
| 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.
We thank Lisa Rawahneh, Dean Marshal, Christopher Lemmon, Maureen Mork, and Mohamed Kaou for technical assistance. We are grateful to Dr. Nicola Zanesi for helpful discussion.
| Footnotes |
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Received 3/22/07. Accepted 4/25/07.
| References |
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