Defective Brca2 influences topoisomerase I activity in mammalian cells.

Abstract

The Chinese hamster cell mutant V-C8 is defective in the Brca2 gene (Kraakman-van der Zwet et al., 2002, Cell Biol.; 22: 669). Here we report that V-C8 cells were 10-fold more sensitive to camptothecin, an inhibitor of topoisomerase I, than the parental V79 cells. The level of the relaxation activity of topoisomerase I in nuclear extracts was also lower (4-fold) in V-C8 than V79 cells, in spite of the fact that the level of the topoisomerase I protein was the same in these cells. The survival of V-C8 cells in the presence of camptothecin, the sensitivity of V-C8 topoisomerase I to camptothecin, and the level of the relaxation activity in V-C8 nuclear extract were almost completely restored by transfection of V-C8 cells with the murine Brca2 gene or by the transfer of human chromosome 13 providing the BRCA2 gene. These results indicate that the observed changes in the topoisomerase I activity in V-C8 are due to the defective function of the Brca2 gene.

Repair of and checkpoint response to topoisomerase I-mediated DNA damage

“…Because of the diversity of the p53 downstream targets that either induce apoptosis or cell cycle arrest or enhance DNA repair [140], it is likely that the outcome of p53 deficiencies is conditional on the cellular context. BRCA1-and BRCA2-deficient cells are hypersensitive to camptothecins [244,245], which is logical considering the key roles of BRCA1 and BRCA2 in DNA repair, HR (see Section 4.4), checkpoint response [132], and genomic stability. BRCA1 is probably one of the human functional analogs of Rad9 in budding yeast.…”