Abstract 2958: Chromatin repressor complex as a converge point of RAS and PP2A activities in lung cancer

Abstract Protein phosphatase 2A (PP2A) is a tumor suppressor while RAS proteins are potent oncogenes in different cancer types. It is well established that RAS-mediated oncogenic transformation requires simultaneous inhibition of PP2A, but the molecular basis of their interaction is poorly understood. To address the target mechanisms co-regulated by RAS and PP2A we performed a phosphoproteomics screen upon RAS or PP2A manipulation. Overall the results indicated that RAS and PP2A regulate overlapping cellular processes, and that their activities might in particular converge at epigenetic machineries. To study the RAS and PP2A synergy on transcriptional and epigenome regulation we performed multi-omics analysis of cancer cells in which RAS and PP2A activities were modulated. PP2A inhibition resulted in global gene activation while RAS inhibition lead predominantly to gene repression. Use of GFP reporter indicative of epigenetic silencing further confirmed the role of PP2A as a transcriptional repressor and RAS as a transcriptional activator. To investigate the DNA methylation changes and chromatin remodeling effects upon PP2A modulation we used RRBS (Reduced representation bisulfite sequencing) & ATAC-seq (Assay for Transposase-Accessible Chromatin using sequencing) respectively. PP2A inhibition resulted in global DNA hypomethylation and an open state of chromatin confirming its role as a global repressor of epigenetic processes. To delineate the molecular mechanism behind the transcriptional regulation by RAS and PP2A we monitored the chromatin recruitment of transcriptional repressors upon PP2A or RAS manipulation. Either pharmacological or siRNA mediated PP2A or RAS modulation significantly affected gene repressor complex recruitment to the chromatin. Role of phosphorylation as a critical switch for chromatin repressor complex recruitment was demonstrated by CRISPR/Cas9-mediated mutagenesis of PP2A target phosphorylation site, which also led to a clear phenotype in in vivo tumorigenesis and oncogenic transcription. Collectively these results identify a novel phosphorylation switch on chromatin repressor complex as a converge point of KRAS and PP2A activities in lung cancer. Generally the results provide first indications to global importance of PP2A-mediated phosphorylation regulation in epigenetic gene regulation in cancer. Citation Format: Mukund Sharma, Anna Aakula, Francesco Tabaro, Karolina Pavic, Riikka Huhtaniemi, Matti Nykter, Jukka Westermarck. Chromatin repressor complex as a converge point of RAS and PP2A activities in lung cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2958.