DNA replication fork perturbation is a major challenge to the maintenance of genome integrity. It has been suggested that processing of stalled forks might involve fork regression, in which the fork reverses and the two nascent DNA strands anneal. Here, we show that FBH1 catalyzes regression of a model replication fork in vitro and promotes fork regression in vivo in response to replication perturbation. Cells respond to fork stalling by activating checkpoint responses requiring signaling through stress-activated protein kinases. Importantly, we show that FBH1, through its helicase activity, is required for early phosphorylation of ATM substrates such as CHK2 and CtIP as well as hyperphosphorylation of RPA. These phosphorylations occur prior to apparent DNA double-strand break formation. Furthermore, FBH1-dependent signaling promotes checkpoint control and preserves genome integrity. We propose a model whereby FBH1 promotes early checkpoint signaling by remodeling of stalled DNA replication forks.

Biotech Research and Innovation Centre University of Copenhagen Ole Maaløes Vej 5 2200 Copenhagen N Denmark

Center for Healthy Aging Department of Cellular and Molecular Medicine University of Copenhagen Panum Institute Blegdamsvej 3B 2200 Copenhagen N Denmark

Institute of Molecular and Translational Medicine Faculty of Medicine and Dentistry Palacky University Olomouc Hnevotinska 5 77900 Olomouc Czech Republic

Institute of Molecular and Translational Medicine Faculty of Medicine and Dentistry Palacky University Olomouc Hnevotinska 5 77900 Olomouc Czech Republic; Danish Cancer Society Research Centre Strandboulevarden 49 2100 Copenhagen O Denmark

Institute of Molecular Cancer Research University of Zürich Winterthurerstrasse 190 8057 Zürich Switzerland

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