Abstract
Patients with class I V600EBRAF-mutant (MT) colorectal cancer (CRC) have a poor prognosis and their response to combined anti-BRAF/EGFR inhibition remains limited. There is clearly an unmet need in further understanding the biology of V600EBRAFMT CRC. We have used differential gene expression of BRAFWT and MT CRC cells to identify pathways underpinning BRAFMT CRC. We tested a panel of molecularly/genetically subtyped CRC cells for their sensitivity to the Unfolded Protein Response (UPR) activator BOLD-100. To identify novel combination strategies for BOLD-100, we performed RNA sequencing and high-throughput drug screening. Pathway enrichment analysis identified that the UPR and DNA repair pathways were significantly enriched in BRAFMT CRC. We found that oncogenic BRAF plays a crucial role in mediating response to BOLD-100. Using a systems biology approach, we identified V600EBRAFMT-dependent activation of the replication stress response kinase ATR as a key mediator of resistance to BOLD-100. Further analysis identified acute increases in BRAFMT-dependent-reactive oxygen species (ROS) levels following treatment with BOLD-100 that was demonstrated to promote ATR/CHK1 activation and apoptosis. Furthermore, activation of ROS/ATR/CHK1 following BOLD-100 was found to be mediated through the AHR transcription factor and CYP1A1. Importantly, pharmacological blockade of this resistance pathway with ATR inhibitors synergistically increased BOLD-100-induced apoptosis and growth inhibition in BRAFMT models. These results unveil possible novel therapeutic opportunity for BRAFMT CRC. Implications: BOLD-100 induces BRAFMT-dependent replication stress, and targeted strategies against replication stress (eg. by using ATR inhibitors) in combination with BOLD-100 may serve as a potential novel therapeutic strategy for clinically aggressive BRAFMT CRC.