Cell Cycle Effect on the Induction of DNA Double-Strand Breaks by X Rays

Filter elution was used to compare X-ray-induced DNA single- and double-strand breaks in proliferating (P) and quiescent (Q) cells of the 66 and 67 mouse mammary tumor lines. There was no difference either between cell type or between growth states in the amount of single-strand breaks as defined by elution at pH 12.2. In contrast, Q cells appeared to sustain a much larger amount of double-strand break damage per Gray than P cells, when the damage was measured by elution at either pH 7.2 or pH 9.6. Experiments which combined centrifugal elutriation with pH 7.2 elution demonstrated that ${\rm G}_{1}\text{-}{\rm P}$ cells were similar to Q (≥95% G1) cells in the induction of elution-detectable double-strand breaks, while the S-phase enriched fractions sustained less damage than ${\rm G}_{1}\text{-}{\rm P}$, Q, or asynchronous P populations. Studies in which P populations were pulse labeled with [14 C]thymidine confirmed this finding. Mathematical analysis of the elution kinetics of irradiated P, Q, and S-phase cells supports a model in which the complex elution profiles observed for P cells could be explained as the sum of the one-component exponential elution profiles of ${\rm G}_{1}\text{-}$ and S-phase subpopulations. Also, the correlation between damage measured by pH 7.2 elution and cell survival was tested by examining the dose response for stimulated 66 cells (${\rm St}_{4}$), which like Q cells are ≥95% in G1 but are more resistant to X-ray-induced cytotoxicity than are the 66 Q cells. However, the induction of double-strand breaks in ${\rm St}_{4}$ cells was identical to that in Q cells. Thus we conclude that there is not necessarily a correlation between the amount of elution-detectable X-ray-induced double-strand breaks and cell survival.