Abstract 2058: Compound library screening to identify modulators of alternative splicing in non-small cell lung cancer (NSCLC)
Authors: Cooley Rachel , Powers Marissa V. , Bond Adam G. , Jensen Patrizia , Newton Gary , Scarpino Andrea , Braun Juliane , Esdar Christina , Clarke Paul A. 
Published: March 2024

Abstract
Alternative splicing is a molecular mechanism that allows a single gene to encode multiple proteins. It is a complex and highly controlled process used to regulate normal gene expression but is often dysregulated in cancer. Compounds that can modulate alternative splicing are currently being explored as a potential new class of therapeutic agent in cancer. This highlights the need for a deeper understanding of the splicing process, its regulation, and its impact. The discovery of novel and specific tool compounds that modulate splicing would therefore not only be beneficial in investigating the regulation and dysregulation of splicing in cancer but could also be exploited therapeutically.
To identify novel regulators of splicing, we generated a cell-based split luciferase screening assay based on alternative splicing of MCL-1 (myeloid cell leukemia-1 protein) pre-mRNA. The MCL1 gene usually produces an mRNA encoding a ‘long’ variant (MCL1L) that is an anti-apoptotic protein often highly expressed in cancers. However, in some circumstances, for example following genetic knockdown of splicing factors, a pro-apoptotic ‘short’ variant (MCL1S) is expressed as a result of exon-skipping.
Here we engineered the human NSCLC cell line NCI-H1299 to express a luminescent-tagged version of the MCL1S splice variant to monitor its induction upon spliceosome modulation. Using this engineered cell line, we screened an unannotated library of 12,000 compounds selected to have low molecular weights and favorable properties for cellular uptake. The screen had an average Z’ value of 0.89 over 45 microplates, indicating high assay robustness. Hits were identified as any compound with a luminescence of greater than the average + two standard deviations of the screening dataset. The compounds were validated by repetition in the screening assay, giving 34 candidate hits that were then triaged by qPCR assay to confirm splicing modulation of MCL1. One interesting hit was found to increase mRNA and cellular protein levels of MCL1S and induced altered splicing across the transcriptome that was distinct from the splicing profiles of a diverse panel of splicing modulators, including compounds targeting SF3B1, CLK, and RBM39. In addition, treatment with this hit compound resulted in accumulation of the splicing factor SC35 in cytoplasmic granules, an unusual phenotype not seen with known splicing modulators, that could explain the distinct splicing modulation we have observed.
Deconvolution and further characterization of screening hits and potentially unique new mechanisms of action could improve current understanding of alternative splicing and its regulation in NSCLC. The discovery of novel compounds to expand our current toolset of splicing modulators will be key in building the foundation for future splicing targeted drug discovery.
Citation Format: Rachel Cooley, Marissa V. Powers, Adam G. Bond, Patrizia Jensen, Gary Newton, Andrea Scarpino, Juliane Braun, Christina Esdar, Paul A. Clarke. Compound library screening to identify modulators of alternative splicing in non-small cell lung cancer (NSCLC) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 2058.