Although the Cdk inhibitor p21 Waf1/Cip1 , one of the transcriptional targets of p53, has been implicated in the maintenance of G 2 arrest after DNA damage, its function at this stage of the cell cycle is not really understood. Here, we show that the exposure of normal human ®broblasts (NHFs) to genotoxic agents provokes permanent cell cycle exit in G 2 phase, whereas mouse embryo ®broblasts and transformed human cells progress through mitosis and arrest in G 1 without intervening cytokinesis. p21 Waf1/Cip1 exerts a key role in driving this G 2 exit both by inhibiting cyclin B1±Cdk1 and cyclin A±Cdk1/2 complexes, which control G 2 /M progression, and by blocking the phosphorylation of pRb family proteins. NHFs with compromised pRb proteins could still ef®ciently arrest in G 2 but were unable to exit the cell cycle, resulting in cell death. Our experiments show that, when under continuous genotoxic stress, normal cells can reverse their commitment to mitotic progression due to passage through the restriction point and that mechanisms involving p21 Waf1/Cip1 and pocket proteins can induce exit in G 2 and G 1 .

Ku antigen is necessary for DNA double-strand break (DSB) repair through its ability to bind DNA ends with high a nity and to recruit the catalytic subunit of DNA-PK to the DSBs. Ku-de®cient cells are hypersensitive to agents causing DSBs in DNA but also to the DNA topoisomerase II (topo II) inhibitor ICRF-193, which does not induce DSBs. This suggests a new role of Ku antigen, that is independent of DSB repair by DNA-PK. Here we characterize the basis for the hypersensitivity of Ku-de®cient cells to . Chromosome condensation and segregation, which are dependent on topo II, but also the catalytic activity of topo II in late S-G2 were inhibited to a comparable extent when ICRF-193 was applied to Ku-de®cient cells or wild-type cells. However, mutant cells arrested in G2 by ICRF-193 treatment were unable to progress into M phase upon drug removal, although drug-trapped topo II complexes were removed from DNA and the two isoforms of topo II recovered their catalytic activity as in wild-type cells. The reversibility of G2 arrest was recovered by complementation of mutant cells with a human Ku86 cDNA. Notably, chromosome condensation was abnormal in Ku-de®cient cells after suppression of the G2 arrest by ca eine, even in the absence of ICRF-193. These results re¯ect the involvement of Ku-antigen in the cellular response to topo II inhibition, more particularly in relieving G2 arrest caused by topo II inhibition in late S/G2 and the subsequent recovery of chromosome condensation.