Abstract 2646: Ewing sarcoma-derived iPS cell lines are resistant to chemotherapy but exhibit EWS-FLI1-independent, increased rapamycin sensitivity.

Ewing sarcoma (EWS) is a highly invasive, undifferentiated tumor of unknown histogenic origin and is the second most common type of primary bone malignancy in children and young adolescents. In approximately 85% of patients, tumor development is associated with the expression of the EWS-FLI1 fusion protein generated by the t(11;22)(q24;q12) chromosomal translocation. Although EWS tumors initially respond to aggressive combination cytotoxic chemotherapy, many tumors recur and metastasize. Disease relapse is thought to result from the outgrowth of a subpopulation of chemotherapy-resistant cancer stem cells (CSCs). Putative EWS CSCs have been identified in EWS-related cell lines and human xenografts as chemotherapy-resistant stem-like subpopulations expressing high ALDH. However, CSC isolation has proven to be challenging due to the low frequency of CSCs in vivo, the absence of specific cell surface markers for sorting, and the harsh conditions required to isolate CSCs from tumor tissues. Methods have been developed to induce the reprogramming of somatic tissues into a stem-like physiologic state similar to that of embryonic stem (ES) cells. Such strategies have been utilized to successfully generate induced pluripotent stem cells (iPSCs) from malignant tissues and have revealed significant similarities in the molecular and phenotypic characteristics of in vitro-derived iPSCs and in vivo-isolated CSCs. Thus, iPSCs have begun to serve as a prospective model for the CSC. We have generated iPSCs from the CHLA10 drug-resistant metastatic EWS cell line. These EWS iPSCs express characteristic ES cell surface markers, exhibit comparable DNA methylation patterns to previously established ES and iPS cell lines, and possess the capacity to differentiate into embryonic tissues in vivo. EWS iPSCs maintain expression of the EWS-FLI1 mutant transcript, show elevated ALDH expression, and exhibit increased drug resistance to vincristine, etoposide, and actinomycin D compared to the parent EWS cell line. Notably, EWS iPSCs are more sensitive to rapamycin than parent cells despite the persistent expression of the oncogenic EWS-FLI1 transcript. These results demonstrate that EWS iPSCs and EWS CSCs share numerous molecular and phenotypic characteristics and suggest that the mTOR signaling network participates in the regulation of iPSC maintenance and viability despite the presence of EWS-FLI1. This work highlights EWS iPSC as a potential model for the EWS CSC and provides a molecular foundation for the use of rapamycin, alone or in combination with other signal transduction pathway inhibitors, in targeting CSCs in Ewing sarcoma. Supported by Department of Defense Grant W81XWH-07-1-0580. RJA is supported by the endowed King Fahd Chair in Pediatric Oncology. Citation Format: Joseph B. Moore, David W. Lee, Michael Barbato, Timothy Triche, Daniel Wai, Robert J. Arceci. Ewing sarcoma-derived iPS cell lines are resistant to chemotherapy but exhibit EWS-FLI1-independent, increased rapamycin sensitivity. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2646. doi:10.1158/1538-7445.AM2013-2646