PP2A Inactivation Mediated by <i>PPP2R4</i> Haploinsufficiency Promotes Cancer Development

Abstract Protein phosphatase 2A (PP2A) complexes counteract many oncogenic kinase pathways. In cancer cells, PP2A function can be compromised by several mechanisms, including sporadic mutations in its scaffolding A and regulatory B subunits or more frequently through overexpression of cellular PP2A inhibitors. Here, we identify a novel genetic mechanism by which PP2A function is recurrently affected in human cancer, involving haploinsufficiency of PPP2R4, a gene encoding the cellular PP2A activator PTPA. Notably, up to 70% of cancer patients showed a heterozygous deletion or missense mutations in PPP2R4. Cancer-associated PTPA mutants exhibited decreased abilities to bind the PP2A-C subunit or activate PP2A and failed to reverse the tumorigenic phenotype induced by PTPA suppression, indicating they function as null alleles. In Ppp2r4 gene-trapped (gt) mice showing residual PTPA expression, total PP2A activity and methylation were reduced, selectively affecting specific PP2A holoenzymes. Both PTPAgt/gt and PTPA+/gt mice showed higher rates of spontaneous tumors, mainly hematologic malignancies and hepatocellular adenomas and carcinomas. These tumors exhibited increased c-Myc phosphorylation and increased Wnt or Hedgehog signaling. We observed a significant reduction in lifespan in PTPA+/gt mice compared with wild-type mice. In addition, chemical-induced skin carcinogenesis was accelerated in PTPA+/gt compared with wild-type mice. Our results provide evidence for PPP2R4 as a haploinsufficient tumor suppressor gene, defining a high-penetrance genetic mechanism for PP2A inhibition in human cancer. Cancer Res; 77(24); 6825–37. ©2017 AACR.