IDENTIFICATION OF NEW MYC DEPENDENCIES AMONG RNA-BINDING PROTEINS

Increased expression and activity of the MYC protein is a widespread cancer hallmark and renders tumor cells addicted to sustained activation of a variety of other gene products. Identification of those dependencies can offer new therapeutic approaches against MYC-driven tumors. Previous studies showed that RNA processing events have a critical role in MYC-induced tumorigenesis and survival. Moreover, we and others observed that multiple genes encoding RNABinding Proteins (RBPs) were positively regulated upon MYC activation in various cell types. Hence, we hypothesized that the activity of specific RBPs could become rate-limiting for the growth and/or survival of MYC-overexpressing cells. Toward the identification of those RBPs, we set up high-throughput genetic dropout screens, involving both RNAi and CRISPR/Cas9 technologies. We designed lentiviral shRNA and sgRNA libraries targeting 730 RBPs, which we transduced in cell lines allowing controlled super-activation of MYC, in order to identify genes whose expression was specifically required in this condition. A series of candidates emerged from our screens, including UPF1 and XRN1, two RBPs involved in mRNA turnover and in particular in nonsense-mediated mRNA decay (NMD). Biological validation in different systems confirmed our screening results and allowed us to extend our observations to other NMD factors, thus identifying NMD as a critical pathway in MYC-overexpressing cells. Addressing the mechanisms underlying the synthetic lethality between MYC and NMD shall not only allow us to unravel this unexpected crosstalk, but shall also pave the way toward the development of new therapeutic opportunities against MYC-dependent tumors.