Inhibiting KRAS and the DNA damage response in pancreatic cancer

Presently, treatment options for pancreatic ductal adenocarcinoma (PDAC) are limited to surgical resection, conventional chemotherapy, or radiation. Activating mutations of the KRAS oncogene occurs in 95% of PDAC, but there are currently no FDA-approved direct KRAS inhibitors available for this cancer. Among the numerous attempts to indirectly target KRAS, one strategy has focused on targeting the DNA damage response (DDR), which is comprised of cell cycle checkpoints and DNA damage repair pathways. This approach can take advantage of KRAS oncogene-induced replication stress and/or DNA damage repair deficiencies already present in subsets of cancer patients. The first part of my dissertation focuses on expanding our understanding of the role of the checkpoint kinase 1 (CHK1) serine/threonine kinase in facilitating cell cycle checkpoints and on evaluating CHK1 as a therapeutic target in KRAS-mutant PDAC. I observed two compensatory mechanisms that emerge in response to pharmacologic CHK1 inhibition: upregulation of the KRAS effector ERK MAPK and of the metabolic process of autophagy, a nutrient scavenging pathway essential for PDAC growth. We determined that concurrent pharmacologic inhibition of ERK and/or autophagy with CHK1 induced synergistic growth inhibition in KRAS-mutant PDAC. To identify new potential DDR targets for KRAS-mutant PDAC, in the second portion of my dissertation I performed a loss-of-function screen utilizing a comprehensive and DDR-focused 504-gene CRISPR-Cas9 library. This screen identified VCP as a potential new target. VCP encodes the valosin-containing protein (VCP), a pleiotropic ATPase that exhibits diverse cellular functions including roles in DNA repair, protein degradation, and intracellular membrane function. I validated VCP as an essential protein in KRAS-mutant PDAC cell growth in vitro and determined that VCP inhibition resulted in accumulation of unrepaired DNA damage and polyubiquitinated proteins. I also observed increased dependence on autophagy upon VCP inhibition. Concurrent inhibition of VCP and autophagy was synergistic in KRAS-mutant PDAC cell lines. These results suggest that regulators of the DDR, such as CHK1 and VCP, may be promising therapeutic strategies for the treatment of KRAS-mutant PDAC.