Dysregulation of replication stress-induced ROS in transformed cell lines: a vicious circle at cancer initiation

The canonical DNA damage response (DDR) maintains genome stability, involving DNA synthesis/cell cycle arrest. However, unchallenged cells proliferate when they are continually exposed to low-level/endogenous replication stress. We previously discovered and characterized a noncanonical cellular response that is specific to nonblocking replication stress, i.e., low-level stress (LoL-DDR), in primary cells. Although this response generates replication stress-induced reactive oxygen species (RIR), it triggers a program that prevents the accumulation of premutagenic 8-oxo-guanine (8-oxoG). Primary cells control RIR production via NADPH oxidases. Increasing the severity of replication stress above a precise threshold triggers the canonical DDR, leading to cell cycle arrest and RIR suppression, resulting in a peak-shaped dose response for RIR production. Here, we show that the LoL-DDR is dysregulated in cancer cell lines, which exhibit the following differences compared with primary cells: 1-RIR are not detoxified under high-level stress conditions, resulting in a continuous increase in the dose-response curve of RIR production; 2-RIR are not produced by NADPH oxidases; and 3-replication stress favors the accumulation of the premutagenic 8-oxoG. Moreover, using an in vitro breast cancer progression model, we show that LoL-DDR dysregulation occurs at an early stage of cancer progression. Since, conversely, ROS trigger replication stress this establishes a "vicious circle" involving replication-stress and ROS that continuously jeopardizes genome integrity that should fuel and amplify tumorigenesis.