Background
The ovarian cancer pathogenesis is still unknown. According to recent data, it is possible that cancer stem cells are responsible for formation, progression and tumor repetition after the treatment has been completed. Same as embryonic stem cells, cancer stem cells express markers of pluripotency. NANOG is a key transcription factor that regulates self-renewal and differentiation in embryonic stem cells; its expression was also detected in different tumors, one of them being ovarian carcinoma. The purpose of our study was to assess NANOG expression in different ovarian serous tumors, and in case of ovarian serous carcinoma to evaluate its significance for prognosis of disease.
Patients and methods
The first part of the study was retrospective, and the second part was prospective. In the retrospective part NANOG expression was immunohistochemically analyzed in the ovarian tissue microarrays. The microarrays were constructed from archived ovarian tissue of 109 ovarian serous carcinoma patients, 30 borderline ovarian serous tumor patients, and 20 benign ovarian serous tumor patients. Immunohistochemical reaction in tumor tissue samples was scored based on percentage of positive cells and signal intensity of tumor cells. According to acquired scores we classified tumor samples into four groups: NANOG – negative group (0), NANOG - slightly positive group (+1), NANOG - moderately positive group (+2) and NANOG - strongly positive group (+3). For ovarian serous carcinoma patients clinical data was acquired, and correlation between NANOG expression and clinical parameters was analyzed. 20 tissue samples of ovarian serous carcinoma were further immunohistochemically stained for markers of pluripotency SOX2, SSEA-4 and marker of mesenchymally-derived cells vimentin. In prospective part of our study Western blot and immunohistochemistry were performed for assessment of NANOG expression in 17 intraoperatively obtained tissue samples of ovarian serous tumors and the correlation between the results were analyzed.
Results
Out of 159 ovarian serous tumors, NANOG was positive in 78 (49.1%) cases. All NANOG positive tumors were high grade carcinomas (p < 0.001). All low grade carcinomas, all borderline tumors and all benign tumors were NANOG negative. In terms of high grade carcinoma group 78 (73.6%) were NANOG positive: 24 (22.6%) slightly positive, 29 (27.4%) moderately positive and 25 (23.6%) strongly positive. All cases, where NANOG was positively expressed presented high grade morphology. NANOG expression showed no significant correlation with clinical parameters such as age of patients, stage of disease, tumor residue after radical surgery, chemotherapy efficiency and survival. The only independent prognostic factors were stage of disease (p = 0.023) and tumor residue (p < 0.001). We proved immunohistochemistry to be reliable method for NANOG assessment in case of ovarian serous tumors as significant correlation (p < 0.001) between the results of Western blot and immunohistochemistry were confirmed. Using light microscope, changes similar to epithelial-mesenchymal transition were observed in ovarian surface epithelium of patients suffering from ovarian serous carcinoma. These changes might generate putative cancer stem cells positive for markers of pluripotency and vimentin.
Conclusions
Immunohistochemistry was proved to be an adequate method for NANOG expression assessment. NANOG expression in ovarian serous tumor cells statistically significantly correlated with high grade ovarian serous carcinoma. Our results showed no statistically significant correlation between NANOG expression and clinical parameters. Changes similar to epithelial-mesenchymal transition and generation of putative cancer stem cells observed by light microscope need further research and clarification.
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