Cell cycle stage-specific roles of Rad18 in tolerance and repair of oxidative DNA damage

The E3 ubiquitin ligase Rad18 mediates tolerance of replication fork-stalling bulky DNA lesions, but whether Rad18 mediates tolerance of bulky DNA lesions acquired outside S-phase is unclear. Using synchronized cultures of primary human cells, we defined cell cycle stage-specific contributions of Rad18 to genome maintenance in response to ultraviolet C (UVC) and H ₂O₂-induced DNA damage. UVC and H₂O₂ treatments both induced Rad18-mediated proliferating cell nuclear antigen mono-ubiquitination during G0, G1 and S-phase. Rad18 was important for repressing H₂O₂-induced (but not ultraviolet-induced) double strand break (DSB) accumulation and ATM S1981 phosphorylation only during G1, indicating a specific role for Rad18 in processing of oxidative DNA lesions outside S-phase. However, H₂O₂-induced DSB formation in Rad18-depleted G1 cells was not associated with increased genotoxin sensitivity, indicating that back-up DSB repair mechanisms compensate for Rad18 deficiency. Indeed, in DNA LigIV-deficient cells Rad18-depletion conferred H ₂O₂-sensitivity, demonstrating functional redundancy between Rad18 and non-homologous end joining for tolerance of oxidative DNA damage acquired during G1. In contrast with G₁-synchronized cultures, S-phase cells were H₂O₂-sensitive following Rad18-depletion. Weconclude that although Rad18 pathway activation by oxidative lesions is not restricted to S-phase, Rad18-mediated trans-lesion synthesis by Polg is dispensable for damage-tolerance in G1 (because of back-up non-homologous end joining-mediated DSB repair), yet Rad18 is necessary for damage tolerance during S-phase.