DNA double-strand breaks originating from diverse causes in eukaryotic cells are accompanied by the formation of phosphorylated H2AX (␥H2AX) foci. Here we show that ␥H2AX formation is also a cellular response to topoisomerase I cleavage complexes known to induce DNA double-strand breaks during replication. In HCT116 human carcinoma cells exposed to the topoisomerase I inhibitor camptothecin, the resulting ␥H2AX formation can be prevented with the phosphatidylinositol 3-OH kinase-related kinase inhibitor wortmannin; however, in contrast to ionizing radiation, only camptothecin-induced ␥H2AX formation can be prevented with the DNA replication inhibitor aphidicolin and enhanced with the checkpoint abrogator 7-hydroxystaurosporine. This ␥H2AX formation is suppressed in ATR (ataxia telangiectasia and Rad3-related) deficient cells and markedly decreased in DNA-dependent protein kinase-deficient cells but is not abrogated in ataxia telangiectasia cells, indicating that ATR and DNA-dependent protein kinase are the kinases primarily involved in ␥H2AX formation at the sites of replication-mediated DNA doublestrand breaks. Mre11-and Nbs1-deficient cells are still able to form ␥H2AX. However, H2AX؊/؊ mouse embryonic fibroblasts exposed to camptothecin fail to form Mre11, Rad50, and Nbs1 foci and are hypersensitive to camptothecin. These results demonstrate a conserved ␥H2AX response for double-strand breaks induced by replication fork collision. ␥H2AX foci are required for recruiting repair and checkpoint protein complexes to the replication break sites.Compact chromatin can be a structural barrier for DNA processing during replication, transcription, recombination, and DNA repair. Following DNA damage, chromatin must be modified to permit the access of repair proteins to the DNA lesions (1). Homologous recombination and non-homologous end joining are the main repair pathways for DNA doublestrand breaks (2). Both processes are assumed to require chromatin alterations for DNA end processing, strand invasion, branch migration, DNA synthesis, and ligation as well as for recruiting checkpoint proteins (3).The fundamental unit of chromatin is the nucleosome, which consists of an octamer of the four core histones, H2A, H2B, H3, and H4, around which the DNA is bound. Histone H2A includes three subfamilies whose members contain characteristic sequence elements that have been conserved independently throughout eukaryotic evolution: H2A1-H2A2, H2AZ, and H2AX (4 -6). In mammals, H2AX represents 2-25% total H2A, whereas in yeast, the major H2A is the H2AX ortholog (7,8).H2AX phosphorylation (at its C terminus on serine 139) has been found at the sites of double-strand breaks in chromosomal DNA (9). This phosphorylated form of H2AX has been named ␥H2AX 1 (8). ␥H2AX is rapidly formed in cells treated with ionizing radiation (IR) but also during V(D)J and class-switch recombination and apoptosis (8 -13). Because ␥H2AX appears within minutes after IR, ␥H2AX focus formation is considered to be a sensitive and selective signal for the existe...

Topoisomerase I (Top1) catalyzes two transesterification reactions: single-strand DNA cleavage and religation that are normally coupled for the relaxation of DNA supercoiling in transcribing and replicating chromatin. A variety of endogenous DNA modifications, potent anticancer drugs and carcinogens uncouple these two reactions, resulting in the accumulation of Top1 cleavage complexes. Top1 cleavage complexes damage DNA and kill cells by generating replication-mediated DNA double-strand breaks (DSBs) and by stalling transcription complexes. The repair of Top1-mediated DNA lesions involves integrated pathways that are conserved from yeasts to humans. Top1-mediated DNA damage and cell cycle checkpoint responses can be studied biochemically and genetically in yeast and human cells with known genetic defects. Defects in these repair/checkpoint pathways, which promote tumor development, explain, at least in part, the selectivity of camptothecins and other Top1 inhibitors for cancer cells.

The Mre11⅐Rad50⅐Nbs1 (MRN) complex binds DNA double strand breaks to repair DNA and activate checkpoints. We report MRN deficiency in three of seven colon carcinoma cell lines of the NCI Anticancer Drug Screen. To study the involvement of MRN in replication-mediated DNA double strand breaks, we examined checkpoint responses to camptothecin, which induces replication-mediated DNA double strand breaks after replication forks collide with topoisomerase I cleavage complexes. MRN-deficient cells were deficient for Chk2 activation, whereas Chk1 activation was independent of MRN. Chk2 activation was ataxia telangiectasia mutated (ATM)-dependent and associated with phosphorylation of Mre11 and Nbs1. Mre11 complementation in MRNdeficient HCT116 cells restored Chk2 activation as well as Rad50 and Nbs1 levels. Conversely, Mre11 down-regulation by small interference RNA (siRNA) in HT29 cells inhibited Chk2 activation and down-regulated Nbs1 and Rad50. Proteasome inhibition also restored Rad50 and Nbs1 levels in HCT116 cells suggesting that Mre11 stabilizes Rad50 and Nbs1. Chk2 activation was also defective in three of four MRN-proficient colorectal cell lines because of low Chk2 levels. Thus, six of seven colon carcinoma cell lines from the NCI Anticancer Drug Screen are functionally Chk2-deficient in response to replication-mediated DNA double strand breaks. We propose that Mre11 stabilizes Nbs1 and Rad50 and that MRN activates Chk2 downstream from ATM in response to replication-mediated DNA double strand breaks. Chk2 deficiency in HCT116 is associated with defective S-phase checkpoint, prolonged G 2 arrest, and hypersensitivity to camptothecin. The high frequency of MRN and Chk2 deficiencies may contribute to genomic instability and therapeutic response to camptothecins in colorectal cancers.