The role of the miR-200 family in epithelial-mesenchymal transition in ovarian cancer

Ovarian cancer has the highest mortality among gynecological cancers. The lack of a screening method and being asymptomatic at an early stage cause the disease to be diagnosed in advanced stages. The main cause of deaths from ovarian cancer is distant metastasis in advanced stages. Metastasis is the transition of cells from epithelial phenotype to mesenchymal phenotype (EMT). Relatively little is known about the molecular genetics of disease onset and progression. Within the scope of the thesis, the target genes of one specific microRNA (miRNA) and the proteins whose expressions are changed by the regulation of these genes were examined. The aim is to show the relationship of miRNAs and the epithelial mesenchymal transition process in patients with epithelial ovarian cancer and to elucidate the metastasis mechanism for early diagnosis. MiRNAs are non-coding small RNAs about 22 nucleotides long, single-stranded RNA sequences that belong to the regulatory gene sequences. By binding to target mRNAs, they lead to translational suppression or decrease the stability of the mRNA. They control a variety of biological processes, including cell differentiation, cell proliferation, apoptosis, stress resistance, and fat metabolism. Some miRNAs have oncogenic or tumor suppressor activity. The miRNA expression profiles differ in different cancer types. Therefore, some miRNAs can be used as diagnostic and prognostic markers of various cancer types The transformation of cells from epithelial to mesenchymal characteristics plays an important role in embryonic development, proliferation, and metastasis of cancer cells. Cells undergoing epithelial-mesenchymal transition (EMT) lose their epithelial properties. Cell skeletons are rearranged. They acquire a motile phenotype by up and down regulation of many molecules including intercellular junction proteins and mesenchymal markers. Within the scope of this thesis, the gene expression analysis of hsa-miR-200c-3p, which is a miRNA considering to play a role in the epithelial-mesenchymal transition, has been performed using the ovarian tissues obtained from healthy individuals and advanced epithelial ovarian cancer patients. It has been shown that the expression of hsa-miR-200c-3p was significantly increased in ovarian cancer patients (p <0.05). The genes targeted by hsa-miR-200c-3p were determined by in-silico methods and gene expression analysis of one of these target genes, which is ZEB1 (Zinc finger E-box binding homeobox 1), was performed in healthy and advanced stage cancerous ovarian tissues. The expression of ZEB1decreased significantly (p <0.05) in cancerous tissues compared to healthy ones. As a result of the correlation analysis, a strong negative correlation was found between hsa-miR-200c-3p and the ZEB1 gene (r=-0,75; p<0,001). In addition, immunohistochemical analysis of ZEB1 protein and vimentin and e-cadherin cell surface antigens has been performed, in healthy and ovarian cancer tissues because they are considered to play a role in EMT, as a result of immunohistochemical staining, the staining intensity and prevalence of ZEB1 and e-cadherin proteins were higher in cancerous tissues than healthy ones (p <0.05). However, it has been showed that the intensity and extent of staining of vimentin protein decrease in cancerous tissues (p <0.05). There was no positive correlation between ZEB1 mRNA and protein. We hypothesize that hsa-miR-200c-3p binds to the promoter region of its target gene, ZEB1, and suppresses its expression. Thus, the expression of ZEB1 decreased and ZEB1 cannot sufficiently bind to the mRNA of e-cadherin coding gene, CDH1, and as a result, the expression of e-cadherin increased. The decreasing level of vimentin protein, a mesenchymal biomarker in ovarian cancer tissues, was found to be compatible with the transition from mesenchymal to epithelial. According to our results, we suggest that hsa-miR-200c-3p as a novel candidate biomarker in the diagnosis of ovarian cancer.