Abstract PR007: Homotypic cell-cell fusion in TNBC cells downregulates cancer testis antigen MAGEC2 and increases therapeutic sensitivity to paclitaxel and doxorubicin
Authors: Gardner Andrea , Zheng Lan , Howland Kennedy , Iloegbunam Chisom , Barrera Patrik , De Santiago Carolina , Brock Amy 
Published: May 2022

Abstract
Cell-cell fusion between cancer cells and genetically mismatched populations has been shown to generate genotypic diversity and more pathological phenotypes which may alter tumor evolution and treatment response, but little is known on how the process of cell-cell fusion itself affects progeny from genetically matched cells. To investigate the hypothesis that the process of cell-cell fusion alone leaves durable effects in daughter cell populations derived from homotypic cell-cell fusion, GFP and mCherry labeled cells from the triple negative breast cancer (TNBC) cell line HCC1806 were co-cultured and subclones were generated through single-cell FACS isolation of spontaneously generated fusion progeny (double positive cells; n=9) and control cells (single positive cells; n=9) from the same population. Subclonal populations were expanded for &gt;19 generations (1 to ~106 cells) before experimental measurements and perturbations. The morphological features, growth rate, chemotactic ability, motility, karyotype, gene expression profile, and chemotherapeutic response of each fusion and control subclone were characterized. We find no durable differences between fusion and control subclones in terms of growth rate, chemotactic ability, and response to 5-fluorouracil. However, subclonal populations generated from homotypic cell-cell fusion show remarkably consistent and statistically significant increased sensitivity to paclitaxel and doxorubicin compared to control subclones. Further, differential expression analysis of RNA sequencing data revealed loss of expression of cancer-testis antigen (CTA) MAGEC2 in fusion cells compared to controls. The MAGE family of CTAs have shown promise as an immunotherapeutic target, but this study suggests that loss or downregulation of MAGEC2 after homotypic cell-cell fusion could allow immune escape. While fusion progeny may be more protected from immune threats, fusion progeny are simultaneously sensitized to paclitaxel and doxorubicin which highlights the potential need for standard chemotherapeutics after CTA immunotherapy. Where current literature posits cell-cell fusion as a pathologically disruptive event which may increase therapeutic resistance, this is the first study to show that fusion progeny are counterintuitively more sensitive to standard chemotherapeutics and to indicate homotypic cell-cell fusion as a potential mechanism of immunotherapeutic escape.
Citation Format: Andrea Gardner, Lan Zheng, Kennedy Howland, Chisom Iloegbunam, Patrik Barrera, Carolina De Santiago, Amy Brock. Homotypic cell-cell fusion in TNBC cells downregulates cancer testis antigen MAGEC2 and increases therapeutic sensitivity to paclitaxel and doxorubicin [abstract]. In: Proceedings of the AACR Special Conference on the Evolutionary Dynamics in Carcinogenesis and Response to Therapy; 2022 Mar 14-17. Philadelphia (PA): AACR; Cancer Res 2022;82(10 Suppl):Abstract nr PR007.