Abstract A77: RNA helicase DDX3 – A novel therapeutic target in sarcoma

High grade sarcomas are among the most common solid tumors in childhood. Although intensive chemotherapy has dramatically improved the outcome of children with localized osteosarcoma, Ewing sarcoma, and rhabdomyosarcoma, the prognosis for children with metastatic or recurrent disease remains dismal, with no substantial improvement since the introduction of multi-agent chemotherapy regimens 30 years ago. The identification of novel therapeutic targets, and the development of drugs to exploit these targets, is therefore an urgent need. RNA helicases are ubiquitous enzymes, aberrantly expressed in a variety of solid and hematologic malignancies, that are involved in all aspects of RNA metabolism, including transcription, translation, mRNA splicing, and regulation of other protein-RNA interactions. The RNA helicase DDX3 is essential for translation of key cell cycle regulatory mRNA transcripts including cyclin A, cyclin E1, TGFB1, STAT1 and others. Expression of DDX3 is dramatically upregulated in mammary epithelial cells exposed to BPDE, a toxin found in cigarette smoke, and overexpression of DDX3 in breast cancer cell lines correlates with invasiveness and aggressive activity. MCF10a mammary epithelial cells induced to overexpress DDX3 undergo epithelial-mesenchymal transition (EMT) with increased invasive behavior, motility, and colony-forming activity, consistent with neoplastic transformation. Novel DDX3 mutations have recently been identified in medulloblastoma. These data suggest DDX3 may play a key role in the development and progression of a wide variety of solid tumors. We therefore investigated DDX3 expression in sarcomas. We found that sarcomas, both cell lines and primary samples, commonly express high levels of DDX3. Knock down of DDX3 expression in cell lines impaired proliferation, clonogenic activity in soft agar, sphere formation when grown under nonadherent conditions, and tumorigenicity in immune deficient mice. We have developed a novel DDX3 inhibitor, known as RK-33, that binds to the ATP binding domain and impairs helicase activity in a DNA unwinding assay. We found that RK-33 is selectively cytotoxic to sarcoma cell lines, compared with normal mesenchymal cells, and that sensitivity correlates with DDX3 expression. Importantly, Ewing sarcoma cells with a stem-like phenotype, characterized by high expression of aldehyde dehydrogenase, are as sensitive to RK-33 as the bulk population, despite being resistant to standard cytotoxic agents. Finally, we found that RK-33 suppresses the growth of human sarcomas in a murine xenograft model, and that sensitivity correlates with DDX3 expression. Thus, we have evidence that the RNA helicase DDX3 is an important mediator of sarcoma proliferation and tumorigenesis, and that inhibition of DDX3 with the small molecule RK-33 is a novel way to target chemotherapy-resistant sarcomas. Citation Format: David M. Loeb, Breelyn A. Wilky, Catherine Kim, Elizabeth Montgomery, Venu Raman. RNA helicase DDX3 – A novel therapeutic target in sarcoma. [abstract]. In: Proceedings of the AACR Special Conference on Pediatric Cancer at the Crossroads: Translating Discovery into Improved Outcomes; Nov 3-6, 2013; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2013;74(20 Suppl):Abstract nr A77.