Abstract 2269: The spindle assembly checkpoint as a therapeutic vulnerability in RIT1-mutant lung adenocarcinoma
Authors: Riley Amanda , Vichas Athea , Nkinsi Naomi T. , Parrish Phoebe C. , Kamlapurkar Shriya , Berger Alice H. 
Published: July 2021

Abstract
Standard care of lung cancer is moving away from chemotherapy in favor of personalized approaches based on specific mutations in each tumor. Genome-sequencing studies have identified somatic mutations in the small GTPase RIT1 (Ras-like in all tissues) in lung adenocarcinoma patients. Of the identified mutations, M90I is the most recurrent. Thousands of patients per year are diagnosed with RIT1-driven cancer, but treatment options are limited. A targeted therapy for RIT1-driven disease could greatly improve patient outcomes. Little is known about how RIT1 drives cellular transformation. To genetically dissect signaling pathways downstream of RIT1, we performed a genome-wide CRISPR/Cas9 screen in isogenic PC9 lung adenocarcinoma cells in which cell survival is dependent on expression of RIT1M90I. We found that RIT1-mutant cells were highly dependent on components of the Spindle Assembly Checkpoint (SAC), including the Aurora kinases (A and B). The SAC is a surveillance mechanism that ensures proper chromosome segregation during mitosis. We hypothesized that RIT1M90I weakened the SAC and rendered cells vulnerable to loss of mitotic regulators. To explore this, we performed time-lapse imaging in HeLa H2B-GFP cells stably expressing RIT1M90I. In parental cells, the median duration of mitosis was 70.5 min (95% CI =  63 – 82 min), while in RIT1M90I-mutant cells, this was reduced to 48 min (95% CI = 45 – 51 min). Mitotic index was unaffected, suggesting that RIT1M90I does not regulate mitotic entry. This difference in mitotic timing was eliminated by treatment with reversine, an inhibitor that abolishes the SAC, demonstrating that RIT1M90I perturbs mitotic timing at the level of the SAC. If RIT1M90I weakens the SAC, we would expect higher prevalence of chromosomal abnormalities (such as chromosome bridges) in RIT1M90I-mutant cells. Indeed, analysis of fixed-cell populations indicated that RIT1M90I-mutant cells showed higher prevalence of mitotic abnormalities (79% of RIT1M90I-mutant cells compared to 46% of parental cells, 95% CI= 15 – 53). These data support the hypothesis that RIT1M90I weakens the SAC and imply that further SAC perturbation could be lethal. To explore this, we performed a small molecule screen of 160 clinically-relevant inhibitors in PC9-RIT1M90I and PC9-KRASG12V cells. Intriguingly, RIT1M90I-mutant cells were more sensitive than RAS-mutant cells to alisertib and barasertib, inhibitors of Aurora kinase A and B, respectively. Furthermore, alisertib or barasertib treatment abrogated soft agar colony formation in RIT1M90I-mutant cells but not in RAS-mutant cells. Together with our functional genomic analysis, we propose a model whereby expression of RIT1M90I weakens the SAC, thereby accelerating mitosis, increasing the abundance of mitotic abnormalities, and rendering cells vulnerable to genetic knockdown of SAC genes or small molecule inhibition of Aurora kinases A/B.
Citation Format: Amanda Riley, Athea Vichas, Naomi T. Nkinsi, Phoebe C. Parrish, Shriya Kamlapurkar, Alice H. Berger. The spindle assembly checkpoint as a therapeutic vulnerability in RIT1-mutant lung adenocarcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2269.