(P 251) TARGETING SOFT TISSUE SARCOMA CANCER STEM CELLS IMPROVES DOXORUBICIN SENSITIVITY IN VITRO

Objective: Soft tissue sarcomas (STS) are rare tumors encompassing over 70 distinct histopathological subtypes that share a common treatment strategy comprising surgical resection, radiation, and chemotherapy in certain situations. Disease progression and failure to respond to anthracycline based chemotherapy, a standard first-line agent, is associated with poor outcomes. Recurrence and chemo-resistance represent significant barriers to improving patient survival. We are interested in the contribution of STS cancer stem cells (STS-CSCs) to the phenomenon of chemo-resistance to doxorubicin. We hypothesized the presence of a common genetic signature across unique STS subtypes involved in CSC-regulation that could be targeted to improve the efficacy of existing treatment regimens.

Methods: STS-CSCs were profiled by flow cytometry using the Aldeflour assay. This is a well-established technique to measure the aldehyde dehydrogenase activity of cells which is high in the stem cell population. The assay fluorescently labels Aldefluor bright and dim cells as CSCs and non-CSCs, respectively. The abundance of the CSC population in several STS cell lines modeling dedifferentiated liposarcoma, leiomyosarcoma, and undifferentiated pleomorphic sarcoma were assessed by Aldeflour assay. In order to gain insight into the molecular pathways active in STS-CSCs, Aldeflour-bright and -dim populations were isolated by FACs and analyzed by RNA-sequencing.

Results: Gene-set enrichment analysis of genes upregulated in STS-CSCs identified a signature for the histone methyltransferase Enhancer of Zeste homolog 2 (EZH2), part of the polycomb repressive complex 2 (PRC2) responsible for H3K27 methylation. As an epigenetic modulator, increased EZH2 expression and activity potentiates decreased activity of genes involved in growth suppression and thereby has oncogenic activity. EZH2 can be inhibited with small molecules such as Tazemetostat, an approved treatment for metastatic and locally advanced epithelioid sarcoma. To test the effects of EZH2 inhibition on STS-CSCs, we first generated doxorubicin resistance STS cell lines by serial selection with increasing concentrations of doxorubicin. We identified a positive correlation between CSC abundance and doxorubicin IC50 in the resistant cell lines. Tazemetostat had little effect on cell viability of sarcoma cells, but specifically inhibits colony formation by CSCs in soft agar assays. Finally, co-treatment of doxorubicin and tazemetostat was synergistic in doxorubicin-naïve soft tissue sarcoma cells, and rescued chemosensitivity in doxorubicin resistant lines.

Conclusion: Our data confirm the presence of shared genetic programs across distinct subtypes of STS that are unique to CSCs and amenable to therapeutic targeting.