EXPRESSION OF THE STEM CELL MARKER OCT4 AND THE MECT1-MAML2 FUSION TRANSCRIPT IN MUCOEPIDERMOID CARCINOMA OF SALIVARY GLAND

Abstract

Mucoepidermoid carcinoma is the most common primary malignancy in the salivary glands. It accounts for 5% of all salivary gland tumors and 20% of salivary malignancies.However, the oncogenesis of MEC is poorly understood.Recently CSCs, a novel population of cancer cells, have been defined and characterized. They have stem cell characteristics that are essential for the initiation and development of human cancers. Increasing evidence suggests that human cancers are stem cell-related diseases.Poorly differentiated MEC is a lethal malignancy of human salivary gland tumors. However, whether there are CSCs in MEC and their phenotypes remains unclear. OCT4 (also known as POU5F1) is a POU-domain, octamer binding transcription factor expressed in both mouse and human ESCs and PGCs. By cooperating with other proteins, Sox2 and Nanog, OCT4 plays essential roles in the establishment and maintenance of pluripotency in ESCs.Furthermore, the expression of OCT4 has been reported in the CSCs and is related to cancer patient’s prognosis. On the other hand,one of the most important molecular discoveries in the study of MEC has been the identification of a recurrent translocation t(11;19)(q21;p13), resulting in a fusion between the MECT1 and the MAML2 genes,either as a sole abnormality or together with other structural and numerical abnormalities.The molecular consequences of the MECT1-MAML2 fusion are complex, with reported effects on several distinct signaling pathways. This study was conducted to investigate the expression of OCT4, a stem cell marker, and the presence/absence of MECT1-MAML2 fusion transcript in different grades of salivary MEC, in primary and recurrent cases, as well as their correlation with each other in relation to the different MEC grades. In this study, thirty cases of FFPEsalivary MEC were graded on the basis of the modified Healey system.Six cases were graded as low-grade, nine cases were graded as intermediate-grade and fifteen cases were graded as high-grade.A specimen of FFPE seminoma was used as a positive control, in addition, non-tumoral surface epithelium present in three cases of the studied MEC were used as a normal control.The peroxidase-antiperoxidase technique was performed for the immunohistochemical staining where each case was stained with antibody against OCT4 protein. RNA was isolated from paraffin block sections using standard optimized kits. RNA from each sample was used for cDNA synthesis by reverse transcription. This cDNA was used to assess the quantity of expressed OCT4 gene and detect the presence/absence of MECT1-MAML2 fusion transcript gene in the studied MEC cases. The positive MECT1-MAML2 fusion transcript cases were semiquantified to estimate the MECT1-MAML2 fusion transcript concentration in each case.The collected data were statistically analyzed. The PCR products of 14 cases of MEC and the seminoma case were sequenced for possible somatic mutation in OCT4 gene. All cases in the present study showed positive OCT4 expression with different quantity using quantitative real-time RT-PCR technique, while all of them were negative using IHC technique. Positive immunoreaction for OCT4 was observed in the seminoma cells and in the basal and supra-basal cells of the non-tumoral surface epithelium. Quantitative real-time RT-PCR techniqueshowed highest mean value of OCT4 expression in the high-grade MEC cases (1.72) compared with low and intermediate-grade (1.38 and 1.26 respectively).The mean values of OCT4 expression were nearly the same in primary and recurrent MEC cases (1.51 and 1.53 respectively).