TopBP1 plays important roles in chromosome replication, DNA damage response, and other cellular regulatory functions in vertebrates. Although the roles of TopBP1 have been studied mostly in cancer cell lines, its physiological function remains unclear in mice and untransformed cells. We generated conditional knock-out mice in which exons 5 and 6 of the TopBP1 gene are flanked by loxP sequences. Although TopBP1-deficient embryos developed to the blastocyst stage, no homozygous mutant embryos were recovered at E8.5 or beyond, and completely resorbed embryos were frequent at E7.5, indicating that mutant embryos tend to die at the periimplantation stage. This finding indicated that TopBP1 is essential for cell proliferation during early embryogenesis. Ablation of TopBP1 in TopBP1 flox/flox mouse embryonic fibroblasts and 3T3 cells using Cre recombinase-expressing retrovirus arrests cell cycle progression at the G 1 , S, and G 2 /M phases. The TopBP1-ablated mouse cells exhibit phosphorylation of H2AX and Chk2, indicating that the cells contain DNA breaks. The TopBP1-ablated mouse cells enter cellular senescence. Although RNA interference-mediated knockdown of TopBP1 induced cellular senescence in human primary cells, it induced apoptosis in cancer cells. Therefore, TopBP1 deficiency in untransformed mouse and human primary cells induces cellular senescence rather than apoptosis. These results indicate that TopBP1 is essential for cell proliferation and maintenance of chromosomal integrity.TopBP1 5 is conserved in eukaryotes and contains repeats of BRCA1 carboxyl-terminal motifs, which are found in proteins involved in DNA repair and regulation of cell cycle checkpoints (1-6). Recent findings indicate that TopBP1 participates in the loading of Cdc45 for the assembly of the preinitiation complex that is necessary for chromosomal DNA replication (7-11). Knockdown of TopBP1 in human cancer cells showed that TopBP1 is necessary for the activation of cyclin E/CDK2 and Cdc7/Dbf4 kinases as well as the loading of DNA polymerase onto chromatin (12). In response to replication fork stalling or DNA damage, TopBP1 functions as an activator of the ATR⅐ATRIP complex by binding to Rad9 of the Rad9⅐Hus1⅐Rad1 complex (13,14). The activated ATR phosphorylates Chk1, Nbs1, Smc1, and H2AX (15-18). TopBP1 also mediates ATR-mediated Chk1 activation by facilitating the interaction of claspin and Chk1 (19,20). Furthermore, association of TopBP1 with NBS1 is involved in double-stranded DNA break-induced homologous recombination repair (21). Therefore, TopBP1 plays crucial roles in the maintenance of genomic integrity.TopBP1 regulates gene expression. Binding of TopBP1 to E2F1 represses E2F1 proapoptotic activity by recruiting the Brg1⅐Brm chromatin remodeling complex (22). TopBP1 also binds to the DNA binding domain of p53 and inhibits the promoter binding activity of this protein (23). Therefore, TopBP1 depletion derepresses the proapoptotic activity of p53 and E2F1 and induces cellular apoptosis. In addition, TopBP1 represses Miz-1 express...

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