Abstract 4189: Cancer drugs on the fly: whole-animal chemical screening in Drosophila identifies unexpected role for EGFR in RAF intestinal stem cell tumors

A major challenge in cancer therapeutics is to identify drugs that target cancer stem cells. This presents a problem for traditional cell-based screening approaches because the complex cellular interactions underlying stem cell biology are difficult to recapitulate in vitro. Conversely, mice, which are arguably among the best animal models for cancer—are not amenable to high-throughput screening. However, many aspects of stem cell initiated tumorigenesis can be modeled in the fruit fly Drosophila melanogaster, which due to its small size is well suited for large-scale screens. The Drosophila intestine, in particular, presents a compelling model to study stem-cell derived intestinal tumors because Drosophila intestinal stem cells share many features with their mammalian counterparts including a propensity to form tumors in response to mutations in WNT, NOTCH, or EGFR signaling. We therefore developed an in vivo screening pipeline using adult Drosophila, to identify compounds that can suppress the growth of stem cell initiated tumors. In previous work we showed that ectopically expressing a constitutively active human c-RAF transgene, RAF(gof) in Drosophila intestinal stem cells results in fast growing tumors that are sensitive to several FDA approved chemotherapeutics. These results establish the clinical relevance of using Drosophila to model stem cell cancers. Here we present results from a screen of known bioactives, that reveal an unexpected role of the EGF receptor, EGFR, in RAF(gof) intestinal stem cell tumors. Based on the linear order of genes in the canonical RAF pathway—EGFR→RAS→RAF→MEK→ERK—we expected that drugs that inhibit genes acting downstream of RAF would score as hits in the screen, whereas genes that act upstream of RAF would not. Indeed, that same logic is applied when assessing whether to give human patients certain pathway specific drugs: patients with activating mutations in RAS or RAF are typically not given drugs that target the upstream EGFR. Consistent with our expectation, we found that drugs that inhibit genes downstream of RAF, effectively stopped Raf(gof) tumor growth. However, to our surprise, we also found that inhibitors of EGFR blocked the growth of RAF(gof) tumors. Using RNAi against EGFR, we genetically validated the chemical results. Our preliminary evidence suggests that EGFR is required in a parallel pathway, upstream of AKT. These findings are significant because they suggest that in some cases, human patients with RAF activated tumors may benefit from drug therapies that target EGFR. Importantly, our results demonstrate that taking an unbiased approach in assessing which drug therapies to apply to a given cancer, may yield unexpectedly good results. Citation Format: Michele Markstein, Hannah Dayton, Samantha Dettorre. Cancer drugs on the fly: whole-animal chemical screening in Drosophila identifies unexpected role for EGFR in RAF intestinal stem cell tumors. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4189.