• DNA damage;
  • G2 checkpoint;
  • recovery;
  • checkpoint-kinase 1 (Chk1);
  • senescence;
  • mitotic catastrophe


Besides the well-understood DNA damage response via establishment of G2 checkpoint arrest, novel studies focus on the recovery from arrest by checkpoint override to monitor cell cycle re-entry. The aim of this study was to investigate the role of Chk1 in the recovery from G2 checkpoint arrest in HCT116 (human colorectal cancer) wt, p53–/– and p21–/– cell lines following H2O2 treatment. Firstly, DNA damage caused G2 checkpoint activation via Chk1. Secondly, overriding G2 checkpoint led to (i) mitotic slippage, cell cycle re-entry in G1 and subsequent G1 arrest associated with senescence or (ii) premature mitotic entry in the absence of p53/p21WAF1 causing mitotic catastrophe. We revealed subtle differences in the initial Chk1-involved G2 arrest with respect to p53/p21WAF1: absence of either protein led to late G2 arrest instead of the classic G2 arrest during checkpoint initiation, and this impacted the release back into the cell cycle. Thus, G2 arrest correlated with downstream senescence, but late G2 arrest led to mitotic catastrophe, although both cell cycle re-entries were linked to upstream Chk1 signalling. Chk1 knockdown deciphered that Chk1 defines long-term DNA damage responses causing cell cycle re-entry. We propose that recovery from oxidative DNA damage-induced G2 arrest requires Chk1. It works as cutting edge and navigates cells to senescence or mitotic catastrophe. The decision, however, seems to depend on p53/p21WAF1. The general relevance of Chk1 as an important determinant of recovery from G2 checkpoint arrest was verified in HT29 colorectal cancer cells.