Differences in repair of radiation induced damage in two human tumor cell lines as measured by cell survival and alkaline DNA unwinding

We studied the relationship between the repair of radiation induced DNA strand breaks and cellular repair kinetics in two human tumor cell lines, NB-100 (neuroblastoma) and HN-1 (squamous cell carcinoma). Damage was quantified using the fluorometric analysis of DNA unwiding (FADU) for DNA damage, and cell survival was assessed using a clonogenic assay. In plateau phase cells repair of sublethal damage was virtually absent in NB-100 after 4 Gy (recovery ratio 1.0), whereas HN-1 cells did show sublethal damage repair (recovery ratio 1.4). Repair of potentially lethal damage was more pronounced in NB-100 cells (recovery ratio 2.3) than in HN-1 cells (recovery ratio 1.7) after 4 Gy. Graded doses of X-rays induced comparable levels of DNA damage in both tumor cell lines. However, in HN-1 cells more DNA strand breaks were repaired after 4 Gy, leaving about 25% of the initial damage unrepaired, whereas in NB-100 about 50% was unrepaired. This higher fraction of unrepaired DNA damage correlated well with the degree of sublethal damage repair which was lower in NB-100 than in HN-1 cell, but it did not correlate with the repair of potentially lethal damage, which was higher in NB-100 than in HN-1. Since the level of damage remaining post-irradiation may be the critical variable for survival, the FADU technique can contribute in elucidating the relationship between radiosensitivity and DNA damage repair capacity.