Abstract
In cells that do not undergo apoptosis, unrapairable DNA double stand breaks (dsb), once generated by irradiation, might perpetuate for a long time. We wanted to see the dynamics of such unrepairable dsb. For this purpose, we used quiescent and exponentially growing normal human diploid fibroblasts (NHDFs) that were X-irradiated, and measured formations and clearance of gamma-H2AX/53BP1/ATM foci (repair centers). In the NHDFs, the repair centers were immediately formed in irradiated nuclei and faded away in a time-dependent manner within a few days, however, a few of them remained and perpetuated even in a long-term culture up to 8 weeks. Formations of such H2AX foci (unrepairable dsbs) increased with X-ray doses and they continued to retain 53BP1 and ATM proteins, indicating that the repair and check point signalings have been kept activated. The relationship between the number of unrepairable foci and X-ray doses seemed well correlated with cell clonogenic survival, which implies that the unrepairable damages indeed play important role in cell survival.
