PO-273 Evaluation of the role of SOX2 as cancer stem cell marker in sarcomas

Introduction Sarcomas often show a limited response to cytotoxic drugs which still remain as the most utilised agents for the treatment of these diseases. This lack of response could be explained by the existence of drug-resistant Cancer Stem Cells (CSCs), which are responsible of tumour relapse. It has been established that transformed human mesenchymal stromal/stem cell (MSCs) are the most likely cell-of-origin for many types of sarcomas. Therefore, we previously developed a model of sarcomagenesis based on transformed Bone Marrow Mesenchymal Stromal/Stem Cells (BM-MSCs). This model has proven very useful to explore the evolution of CSCs subpopulations and to search for CSC-specific markers and therapies. Here we use this model to analyse the role of SOX2 as a CSC-specific marker by isolating subpopulations presenting high SOX2-mediated transcriptional activity. Material and methods We depleted or overexpressed SOX2 in a xenograft-derived cell line (T5H-O) generated by a cell-origin-model of undifferentiated pleomorphic sarcoma developed from transformed BM-MSC. In addition, these cells were transfected with a lentiviral-based reporter system in which a composite SOX2/OCT4 response element (SORE6) coupled to a minimal cytomegalovirus (CMV) promoter controls the expression of fluorescent reporter genes. T5H-O cells expressing this system allowed us to detect and isolate viable cells expressing transcriptionally active SOX2 and/or OCT4 by flow cytometry. Results and discussions The depletion of SOX2 protein inhibited CSC related-properties of T5H-O cells (e.g. tumorsphere forming potential) whereas T5H-O cells expressing high levels of SOX2 dramatically increased their invasive properties and in vivo tumour growth potential. We found that SORE6 activity in our sarcoma model was mainly due to SOX2, rather than OCT4. Comparing to SORE6- population, SORE6 +positive cells showed enhanced tumorsphere-forming potential and increased invasion ability. Importantly, SORE6 +cells were significantly more tumorigenic than the SORE6- population, thus indicating that SOX2 activity marks a subpopulation with increased tumor-promoting ability in sarcomas. Finally, we found that EC-8042 and trabectedin, anti-tumour drugs previously reported to target CSCs in sarcoma, were able to eliminate the SORE6 +CSC subpopulation in sarcomas. Conclusion Our results indicate that SOX2 activity is a bona fide CSC-marker in sarcoma, and that SORE6 reporter system is an excellent approach for testing the effectiveness of CSC-specific treatments.