SRP19 and the protein secretion machinery is a targetable vulnerability in cancers with APC loss

Loss of the tumor suppressor gene (TSG) Adenomatous Polyposis Coli (APC) is a hallmark event in colorectal cancers. Since it is not possible to directly target a TSG, no treatment options are available for these patients. Here, we identify SRP19 and the protein secretion machinery as a unique vulnerability in cancers with heterozygous APC loss. SRP19 is located 15 kb from APC and is almost always codeleted in these tumors. Heterozygous APC/SRP19 loss leads to lower levels of SRP19 mRNA and protein. Consequently, cells with APC/SRP19 loss are vulnerable to partial suppression of SRP19. Moreover, we show that SRP19 is rate limiting for the formation of the Signal Recognition Particle, a complex that mediates ER-protein translocation, and thus, heterozygous SRP19 loss leads to less protein secretion and higher levels of ER-stress. As a result, low-dose arsenic trioxide induces ER-stress and inhibits proliferation in cultured cell lines and animal models. Our work identifies a strategy to treat cancers with APC deletion and provides a framework for identifying and translating vulnerabilities associated with loss of a TSG.