Abstract A20: Depletion of replication factor MCM7 is synthetically lethal to oncogenic KRAS expression

Abstract

Abstract Colorectal cancer (CRC) is a substantial cause of death in the western world. With KRAS being one of the most frequently altered oncogenes in CRC, it is an obvious target for cancer therapy. But despite enormous efforts over the past three decades to target mutated RAS, not a single drug has made it to the clinic. In order to find vulnerabilities of KRAS mutant cells, we performed a shRNA-based screen. We used CaCo2 cells (wild type for KRAS/BRAF/PIK3CA) with doxycycline (DOX) inducible expression of the oncogenic KRASG12V as a model system. The custom-designed shRNA library comprised 121 selected genes encoding signaling proteins and transcription factors, which we have previously identified to be strongly up- or downregulated after treatment with MEK inhibitors. We found that CaCo2 cells expressing KRASG12V are highly sensitive to the depletion of the DNA replication licensing factor Minichromosome Maintenance Complex Component 7 (MCM7), whereas wild-type CaCo2 cells are resistant to MCM7 depletion. The screening result was verified with 3 independent shRNAs under anchorage dependent and independent conditions. Similar results were obtained with NRASG12D and in DLD1 (KRASwt/KRASG13D) and DLD1 (KRASwt/-) cells. The proposed molecular mechanism is RAS-induced genotoxic stress that cannot be resolved after MCM7 reduction. MCM7-containing complexes prime DNA for replication by binding to replication origins. However, only a few of the licensed origins are later used for DNA replication, while most origins stay dormant. The dormant origins are essential for resolving stalled replication forks after replicative stress. Unresolved stalled forks lead to DNA damage, which drives the cell into apoptosis. Further experiments will show the extent of DNA damage upon RAS induction and knock down of MCM7. Since DNA replication licensing is only required in actively dividing cells and cells with physiological RAS signaling seem to be resistant to partial depletion of MCM7, it might be an ideal target for treating RAS mutated cancers. Citation Format: Bastian Gastl, Kathleen Klotz-Noack, Bertram Klinger, Johannes Zuber, Nils Blüthgen, Reinhold Schäfer, Christine Sers. Depletion of replication factor MCM7 is synthetically lethal to oncogenic KRAS expression [abstract]. In: Proceedings of the AACR Precision Medicine Series: Opportunities and Challenges of Exploiting Synthetic Lethality in Cancer; Jan 4-7, 2017; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2017;16(10 Suppl):Abstract nr A20.