BRCA1 deficient embryonic stem cells display a decreased homologous recombination frequency and an increased frequency of non-homologous recombination that is corrected by expression of a Brca1 transgene

BRCA1 is a nuclear phosphoprotein that has been classified as a tumor suppr essor based on the fact that women carrying a mutated copy of the BRCA1 gen e are at increased risk of developing breast and ovarian cancer. The associ ation of BRCA1 with RAD51 has led to the hypothesis that BRCA1 is involved in DNA repair. We describe here the generation and analysis of murine embry onic stem (ES) cell lines in which both copies of the murine homologue of t he human BRCA1 gene have been disrupted by gene targeting. We show that exo genous DNA introduced into these BRCA1 deficient cells by electroporation i s randomly integrated into the genome at a significantly higher rate than i n wild type ES cells. In contrast, integration of exogenous DNA by homologo us recombination occurs in BRCA1 deficient cells at a significantly lower r ate than in wild type controls. When BRCA1 expression is re-established at 5-10% of normal levels by introduction of a Brca1 transgene into BRCA1 defi cient ES cells, the frequency of random integration is reduced to wild type levels, although the frequency of homologous recombination is not signific antly improved. These results suggest that BRCA1 plays a role in determinin g the response of cells to double stranded DNA breaks.