PP2A inhibition instructs spliceosome phosphorylation patterns to create splicing vulnerability in colon adenocarcinoma

Protein phosphatase 2A (PP2A) is a promiscuous enzyme affecting cellular physiology. It is unclear however, which of the cellular processes are most perturbed upon PP2A inhibition and how such perturbations could be exploited therapeutically. Here, we report an unanticipated sensitivity of the splicing machinery to phosphorylation changes in response to PP2A inhibition by LB-100 in colorectal adenocarcinoma. We observe enrichment for differentially phosphorylated sites within cancer-critical splicing nodes of U2 snRNP, SRSF and hnRNP proteins. Changed phosphorylation endows LB-100 treated colorectal adenocarcinoma cells with differential splicing patterns. Over 1000 exon skipping and intron retention events in PP2A-inhibited cells affect predominantly regulators of genomic integrity. Finally, LB-100-evoked alternative splicing is predicted to be a source of neoantigens that can improve cancer treatment responses to immune modulators. Our findings provide a potential explanation for the pre-clinical and clinical observations that PP2A inhibition sensitizes cancer cells to immune checkpoint blockade and genotoxic agents. Experiment was performed with two cell lines of human colorectal adenocarcinoma, SW-480 (harboring Kras mutation) and HT-29 (Braf mutated), treated for 12 hrs with PP2A inhibitor, LB-100 in 4 uM concentration.