Cell cycle and genetic background dependence of the effect of loss of BRCA2 on ionizing radiation sensitivity

Abstract

Carriers of mutations in the BRCA2 gene are at a highly elevated risk of breast and other cancers. The BRCA2 gene encodes a very large protein thought to play a role in DNA repair. To examine the effect of mutation of BRCA2 on sensitivity to ionizing radiation, we used a previously described mouse model system (Brca2^Tr) in which the Brca2 open reading frame is truncated. Mouse embryo fibroblasts carrying this mutation have a proliferative defect, which we show here can be substantially rescued by genetic ablation of p53. Proliferating Brca2^Tr/Tr/p53^−/− cells, like Brca2^Tr/Tr cells, show genomic instability. We used the clonogenic survival assay, which depends on the ability of cells to proliferate, to examine the cell cycle dependence of radiation sensitivity of Brca2^Tr/Tr/p53^−/− compared to p53^−/− and wild-type cells. This showed that the Brca2 mutation had little effect on cells irradiated in quiescence but sensitized proliferating cells to ionizing radiation on a p53^−/− background. These results suggest that the major role of Brca2 in mediating cell survival after irradiation is in the S and G₂ phases of the cell cycle.