p21CDKN1A allows the repair of replication-mediated DNA double-strand breaks induced by topoisomerase I and is inactivated by the checkpoint kinase inhibitor 7-hydroxystaurosporine

This study provides evidence for the importance of p21 CDKN1A for the repair of replication-mediated DNA double-strand breaks (DSBs) induced by topoisomerase I. We report that defects of p21 CDKN1A and p53 enhance camptothecin-induced histone H2AX phosphorylation ( γ H2AX), a marker for DNA DSBs. In human colon carcinoma HCT116 cells with wild-type (wt) p53, γ H2AX reverses after camptothecin removal. By contrast, γ H2AX increases after camptothecin removal in HCT116 cells deficient for p53 (p53−/−) or p21 CDKN1A (p21−/−) as the cells reach the late-S and G2 phases. Since p21−/− cells exhibit similar S-phase arrest as wt cells in response to camptothecin and aphidicolin does not abrogate the enhanced γ H2AX formation in p21−/− cells, we conclude that enhanced γ H2AX formation in p21−/− cells is not due to re-replication. The cell cycle checkpoint abrogator and Chk1/Chk2 inhibitor 7-hydroxystaurosporine (UCN-01) also increases camptothecin-induced γ H2AX formation and inhibits camptothecin-induced p21 CDKN1A upregulation in HCT116 wt cells. TUNEL (terminal deoxynucleotidyl transferase (TdT)-mediated dUTP-biotin nick end labeling) assays demonstrate that γ H2AX formation in late S and G2 cells following CPT treatment corresponds to DNA breaks. However, these breaks are not related to apoptotic DNA fragmentation. We propose that p21 CDKN1A prevents the collapse of replication forks damaged by stabilized topoisomerase I cleavage complexes.