Abstract B01: Genome-scale shRNA screen provides insight into the role of CUL3 in adaptive mechanisms of resistance to mutant BRAF inhibition

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Abstract MAPK inhibition with targeted BRAF inhibitors has revolutionized the treatment of BRAF-mutant melanoma. However, in most cases, the effects of mutant BRAF inhibition are transient, owing to the acquisition of drug resistance. To resolve mechanisms that drive resistance to mutant BRAF inhibition, we performed a genome-scale shRNA screen capable of revealing genes whose loss of function confers resistance to vemurafenib (VEM). We validated ten of our top candidates with independent lentiviral shRNA constructs. We found that knockdown of NF1 and CUL3 confers resistance to VEM. This result is consistent with previous genome-scale loss-of-function screens that have also identified NF1 and CUL3 as drivers of resistance to mutant BRAF inhibition. NF1 encodes a protein that hydrolyzes Ras-GTP. Not surprisingly, it has been shown that loss of NF1 results in increased Ras-GTP levels that lead to reactivation of MAPK in the presence of VEM. However, little is known about the mechanistic changes that underlie CUL3-mediated resistance. Here, we show that unlike the classical reactivation of MAPK that is seen with the loss of NF1, loss of CUL3 results in increased signaling through a Rac1-dependent focal-adhesion-like process that is accompanied by sustained MAPK activity in the presence of VEM. Furthermore, we show that this mechanism of resistance can be abrogated by inhibiting the Src kinase family, a nine-member protein family that critically mediates integrin signaling. In relation to other known drivers of resistance to mutant BRAF inhibition, loss of NF1 reactivates MAPK in a manner much like RAS mutations or BRAF alterations/amplification. By contrast, loss of CUL3 leads to resistance reminiscent of a reversible therapy-resistant state that lacks the key genomic alterations in MAPK signaling proteins. Citation Format: Marion Vanneste, Eliot Zhu, Charlotte Feddersen, Afshin Varzavand, Tyler Foley, Lei Zhao, Rob Piper, Christopher Stipp, Adam Dupuy, Michael Henry. Genome-scale shRNA screen provides insight into the role of CUL3 in adaptive mechanisms of resistance to mutant BRAF inhibition [abstract]. In: Proceedings of the AACR Special Conference on Melanoma: From Biology to Target; 2019 Jan 15-18; Houston, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(19 Suppl):Abstract nr B01.