Cell cycle and growth response of CHO cells to X-irradiation: Threshold-free repair at low doses

Purpose: To test the hypothesis of a threshold for induced repair of DNA da mage (IR) and, secondarily, of hyperradiosensitivity (HRS) to low-dose X-ir radiation, Methods and Materials: Exponentially growing Chinese hamster ovary cells (C HO) were X-irradiated with doses from 0.2 to 8 Gy, Survival data were estab lished by conventional colony-forming assay and flow-cytometric population counting. The early cell cycle response to radiation was studied based on D NA-profiles and bromodeoxyuridine pulse-labeling experiments. Results: Colony-forming data were consistent with HRS, However, these data were of low statistic significance. Population counting provided highly rep roducible survival curves that were in perfect accord with the linear-quadr atic (LQ) model. The dominant cell cycle reaction was a dose-dependent dela y of G2 M and late S-phase, Conclusion: There was no evidence for a threshold of IR and for low-dose HR S in X-irradiated CHO cells. It is suggested that DNA damage repair activit y is constitutively expressed during S-phase and is additionally induced in a dose-dependent and threshold-free manner in late S-phase and G2, The res ulting survival is precisely described by the LQ model. (C) 2001 Elsevier S cience Inc.