Graphical Abstract Highlights d RTK-RAS-MAPK pathway members score strongly in genome-scale MEKi modifier screens d Depletion of SHOC2 potently sensitizes RAS-driven cells to MEK inhibition d SHOC2 loss impairs RTK-mediated adaptive reactivation of MAPK signaling induced by MEKi d A model of SHOC2 degradation suggests a combination therapeutic strategy with MEKi SUMMARY The mitogen-activated protein kinase (MAPK) pathway is a critical effector of oncogenic RAS signaling, and MAPK pathway inhibition may be an effective combination treatment strategy. We performed genome-scale loss-of-function CRISPR-Cas9 screens in the presence of a MEK1/2 inhibitor (MEKi) in KRAS-mutant pancreatic and lung cancer cell lines and identified genes that cooperate with MEK inhibition. While we observed heterogeneity in genetic modifiers of MEKi sensitivity across cell lines, several recurrent classes of synthetic lethal vulnerabilities emerged at the pathway level. Multiple members of receptor tyrosine kinase (RTK)-RAS-MAPK pathways scored as sensitizers to MEKi. In particular, we demonstrate that knockout, suppression, or degradation of SHOC2, a positive regulator of MAPK signaling, specifically cooperated with MEK inhibition to impair proliferation in RAS-driven cancer cells. The depletion of SHOC2 disrupted survival pathways triggered by feedback RTK signaling in response to MEK inhibition. Thus, these findings nominate SHOC2 as a potential target for combination therapy. 118 Cell Reports 29, 118-134, October 1, 2019 ª 2019 The Author(s). This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).(A) Schematic of pooled CRISPR-Cas9 screening strategy. (B-D) Genome-scale screen results in pancreatic cancer, CFPAC-1 (B), and lung cancer lines, A549 (C) and NCI-H23 (D). Red points have FDR < 0.25 (STARS algorithm). Mean trametinib sensitivity (x axis) is calculated as the difference in the log2(fold-change) in sgRNA representation between cells treated with trametinib for 14 days and the initial pool of sgRNAs. Differential sensitivity indicates the difference log2(fold-change) in sgRNA representation between the trametinib-treated and DMSO-treated arms of the screen. Scores represent the average of all guides for a given gene. (E) Venn diagram summarizes the overlap of genes that are depleted in all three screens with an FDR < 0.25.

Malignant rhabdoid tumors (MRT) are highly aggressive pediatric cancers that respond poorly to current therapies. In this study, we screened several MRT cell lines with large-scale RNAi, CRISPR-Cas9, and small-molecule libraries to identify potential drug targets specific for these cancers. We discovered MDM2 and MDM4, the canonical negative regulators of p53, as significant vulnerabilities. Using two compounds currently in clinical development, idasanutlin (MDM2-specific) and ATSP-7041 (MDM2/4-dual), we show that MRT cells were more sensitive than other p53 wild-type cancer cell lines to inhibition of MDM2 alone as well as dual inhibition of MDM2/4. These compounds caused significant upregulation of the p53 pathway in MRT cells, and sensitivity was ablated by CRISPR-Cas9-mediated inactivation of TP53. We show that loss of SMARCB1, a subunit of the SWI/SNF (BAF) complex mutated in nearly all MRTs, sensitized cells to MDM2 and MDM2/4 inhibition by enhancing p53-mediated apoptosis. Both MDM2 and MDM2/4 inhibition slowed MRT xenograft growth in vivo, with a 5-day idasanutlin pulse causing marked regression of all xenografts, including durable complete responses in 50% of mice. Together, these studies identify a genetic connection between mutations in the SWI/SNF chromatin-remodeling complex and the tumor suppressor gene TP53 and provide preclinical evidence to support the targeting of MDM2 and MDM4 in this often-fatal pediatric cancer.Significance: This study identifies two targets, MDM2 and MDM4, as vulnerabilities in a deadly pediatric cancer and provides preclinical evidence that compounds inhibiting these proteins have therapeutic potential. Analysis and interpretation of data (e.g., statistical analysis, biostatistics, computational analysis):

Highlights d p130Cas and bIII-tubulin support PDAC growth by enhancing MYC expression d p130Cas transcriptionally regulates MYC through SRC-p130Cas-DOCK1-RAC1-b-catenin d Microtubules post-translationally regulate MYC stability through calpains d Triple targeting of ERK/p130Cas/tubulin with ERKi and KX2-391 inhibits PDAC growth