Abstract
There are at least two pathways, in which DNA double strand breaks (DSBs) are rejoined: non-homologous end joining and homologous recombination (HR). Defect of HR can result in genomic instability and cancer predisposition. Previously, we have demonstrated using NBS1-deficient DT40 cells that NBS1, protein responsible for radiation sensitive disorder Nijmegen breakage syndrome, functions in HR repair. We also showed that hMRE11 is important for NBS1-dependent HR activity, while the ATM kinase is dispensable. Here, we investigate the relationship between NBS1 and BRCA1/BRCA2, which are known to function in HR. Cells expressing either a tandem BRCT domain of BRCA1 or BRC4 repeats, binding region to Rad51, of BRCA2, inhibit radiation-induced foci formation of endogenous respective BRCA1 and Rad51, due to dominant-negative effect. When homologous recombination was assayed by DR-GFP reporter gene, these cell lines with dominant-negative effect showed the reduced HR frequency regardless the presence of NBS1 protein. This additive effect of HR suggests that NBS1 functions in HR independently of BRCA1 and BRCA2. To confirm these results, we are also investigating the dominant-negative effect of BRCA1, BRCA2 on MMS-induced lethality and chromosome instability in NBS cells and the complemented cells.
