Abstract
Radiation-induced DNA double strand breaks are well known to initiate ATM-dependent DNA damage checkpoint pathway. The factors involving this pathway form discrete foci at the sites of DNA double strand breaks, which amplify DNA damage signals. DNA double strand breaks are repaired by two major repair pathways, non-homologous end joining (NHEJ) and homologous recombination (HR). Although NBS1 involving HR has been shown to form foci, which is important for ATM-dependent signallin, little is know about a role of RAD51 foci. Present study examined spatiotemporal relationship between ATM foci and RAD51 foci.
We successfully detected RAD51 foci in normal human diploid cells 30 minutes after X-irradiation with 0.5 Gy. These foci were mainly observed in the S phase cells, and most of the foci were colocalized with those of phosphorylated ATM. Interestingly, significant change in size of phosphorylated ATM was observed thereafter, and grown foci were colocalized with NBS1 and phosphorylated BRCA1. Three dimensional analysis revealed that RAD51 foci were included in a part of large colocalized foci, indicating that RAD51 foci detected chromatin regions processing HR.
These results suggest that DNA damage checkpoint pathway is activated at the sites of HR, and the secondary changes in chromatin structure coupled with HR may be involved in the growth of foci of DNA damage checkpoint factors.
