Dysregulation of the low-level replication stress response in transformed cell lines.

The canonical DNA damage response (cDDR) maintains genome stability, involving DNA synthesis arrest. However, unchallenged cells proliferate when they are continually exposed to low-level/endogenous replication stress. We previously characterized a noncanonical response specific to nonblocking replication stress, i.e. low-level stress (LoL-DDR), in primary cells. Although LoL-DDR generates replication stress-induced ROS (RIR), it 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 threshold triggers the cDDR, 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, resulting in a continuous increase in the dose‒response curve of RIR production; (2) RIR are not produced by NADPH oxidases; (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" replication-stress/ROS that continuously jeopardizes genome integrity that should fuel and amplify tumorigenesis.