Abstract 4941: Identification and isolation of ovarian cancer stem cell populations using a novel functional reporter may provide new insights into ovarian cancer cell dynamics and drug resistance
Authors: Gilbert Samuel F. , Robinson Mikella , Alexander Logan A. , Lujano-Olazaba Omar , Lara Jacqueline M. , Waters Jennifer A. , Patrus Omid , Horkowitz Alex , House Carrie D. 
Published: September 2020

Abstract
Background: Ovarian cancer is the most lethal gynecologic malignancy mainly due to a high prevalence of chemotherapy-resistant recurrent disease. This mechanism is unknown but recent studies have pointed to a small population of chemoresistant cancer stem cells (CSC) as the driver of these recurrences. Traditional surface markers of cancer stem cells (CD44, ALDH, CD117, etc.) have shown mixed success in identifying this subpopulation, with different cell lines and patient samples exhibiting dissimilar profiles. These markers are often related to the cell of origin and are not necessarily specific to CSCs. As such, a reporter that responds to functional features of stem cells would be beneficial in identifying the CSC subpopulation regardless of cell line and patient sample. Here we demonstrate that using a reporter to detect SOX2 and OCT4 overexpression (called SORE6) can identify the CSC subpopulation within ovarian cancer cell lines.
Methods: The SORE6 reporter was stably transduced into OV90 and ACI23 ovarian cancer cell lines. The subpopulation of cells having high SOX2/OCT4 expression (SORE6+) were isolated using fluorescence assisted cell sorting (FACs) and evaluated for CSC characteristics relative to SORE6- cells. Gene expression, viability, spheroid formation capacity, and tumorigenicity was assessed for both SORE6+ and SORE6- cells. In vivo limiting dilution analyses were performed on athymic nude female mice using subcutaneous injection.
Results: Compared to the bulk population which has low levels of SOX2/OCT4 expression (SORE6-), SORE6+ cells showed a 1.5-fold increase in SOX2/OCT4/NANOG transcript levels in ACI23 cells and 2 to 15-fold increase in these transcript levels in the OV90 cells. SORE6+ cells showed enhanced chemoresistance when treated with conventional chemotherapies for ovarian cancer, carboplatin and paclitaxel, although this difference was more dramatic in the OV90 cells (four-fold) relative to ACI23 (two-fold). These findings indicate that SOX2/OCT4 expression could contribute to drug resistance and clinical recurrence. SORE6+ cells also showed an increased capacity to form spheroids in low-serum conditions and again this difference was more dramatic in the OV90 cells. In vivo limiting dilution studies are underway but appear to show significant differences in tumorigenicity at low dilutions of ≤5,000 cells/mouse.
Conclusion: The SORE6 reporter construct identifies CSC populations within ovarian cancer cell lines and may be a reliable tool for isolating CSCs that depend on SOX2 and OCT4 expression. Use of this reporter will enhance our understanding of mechanisms that support chemoresistance and enable the study of CSC dynamics within the tumor microenvironment, both of which have significant implications for ovarian cancer progression and treatment.
Citation Format: Samuel F. Gilbert, Mikella Robinson, Logan A. Alexander, Omar Lujano-Olazaba, Jacqueline M. Lara, Jennifer A. Waters, Omid Patrus, Alex Horkowitz, Carrie D. House. Identification and isolation of ovarian cancer stem cell populations using a novel functional reporter may provide new insights into ovarian cancer cell dynamics and drug resistance [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 4941.