Defective p53 engagement after the induction of DNA damage in cells deficient in topoisomerase 3β

The type IA topoisomerases have been implicated in the repair of dsDNA breaks by homologous recombination and in the resolution of stalled or damaged DNA replication forks; thus, these proteins play important roles in the maintenance of genomic stability. We studied the functions of one of the two mammalian type IA enzymes, Top3 beta, using murine embryonic fibroblasts (MEFs) derived from top3 beta(-/-) embryos. top3 beta(-/-) MEFs proliferated more slowly than TOP3 beta(+/+) control MEFs, demonstrated increased sensitivity to DNA-damaging agents such as ionizing and UV radiation, and had increased DNA double-strand breaks as manifested by increased gamma-H2-AX phosphorylation. However, incomplete enforcement of the G(1)-S cell cycle checkpoint was observed in top3 beta(-/-) MEFs. Notably, ataxia-telangiectasia, mutated (ATM)/ATM and Rad3-related (ATR)-dependent substrate phosphorylation after UV-B and ionizing radiation was impaired in top3 beta(-/-) versus TOP3 beta(+/+) control MEFs, and impaired up-regulation of total and Ser-18-phosphorylated p53 was observed in top3 beta(-/-) cells. Taken together, these results suggest an unanticipated role for Top3 beta beyond DNA repair in the activation of cellular responses to DNA damage.