Role for the Werner syndrome protein (WRN) in replication fork regression

Abstract

Loss of function of human RecQ family member WRN causes the premature aging disease Werner syndrome. WRN has weak helicase and 3′ to 5′ exonuclease activities that preferentially act on complex DNA structures. Like many RecQ helicases, WRN also helps anneal complementary DNA strands. Combined unwinding and annealing likely promotes branch migration and strand exchange by WRN. In general, RecQ members act to suppress illegitimate recombination events. Specifically, WRN‐deficient cells show chromosomal instability, hypersensitivity to hydroxyurea and certain genotoxins, and replication abnormalities including asymmetric fork progression, suggesting a role for WRN in overcoming replication blocks. Resolution of such blockage may be initiated by fork regression–i.e., nascent daughter strand pairing and parental strand re‐annealing to form Holliday junctions. Since regression involves coordinated unwinding and annealing, we examined WRN's action on several model replication forks. We find that WRN readily regresses replication forks to form Holliday junctions. Further, WRN's exonuclease activity enhances regression of some fork substrates by digesting the leading daughter strand. Thus, the multiple enzymatic activities of WRN cooperate to catalyze efficient fork regression, theoretically as part of a pathway that accurately overcomes replication blocks. Supported by NCI grant R01‐CA113371.