Sartori, A. A.; Vivalda, F.; Gatti, M.; Manfredi, L.; Dogan, H.; Porro, A.; Collotta, G.; Ceppi, I.; von Aesch, C.; van Ackeren, V.; Wild, S.; Steger, M.; Canovas, B.; Cubillos-Rojas, M.; Riera, A.; Cejka, P.; Nebreda, A. R.; Dibitetto, D.; Rottenberg, S.

Human CtIP plays a critical role in homologous recombination (HR) by promoting the resection of DNA double-strand breaks. Moreover, CtIP maintains genome stability through protecting stalled replication forks from nucleolytic degradation. However, the upstream signaling mechanisms governing the molecular switch between these two CtIP-dependent processes remain largely elusive. Here, we show that phosphorylation of CtIP by the p38a stress kinase and subsequent PIN1-mediated CtIP cis-to-trans isomerization is required for fork stabilization but dispensable for HR. We found that stalled forks are degraded in cells expressing non-phosphorylatable CtIP or lacking PIN1-p38a activity, while expression of a CtIP trans-locked mutant overcomes the requirement for PIN1-p38a in fork protection. We further reveal that Brca1-deficient mammary tumor cells that have acquired PARPi resistance regain chemosensitivity after PIN1 or p38a inhibition. Collectively, our findings identify the PIN1-p38-CtIP signaling pathway as a critical regulator of replication fork integrity.