p53 Mediates Permanent Arrest over Multiple Cell Cycles in Response to γ-Irradiation

Abstract

A new technique that monitors cell cycle progression over multiple cycles was used to gain insight into how p53 limits the emergence of variants with structural chromosome alterations following γ-irradiation. G₀-synchronized, p53⁺ (with a functional p53 pathway) normal human fibroblast and epithelial strains underwent a dose-dependent permanent arrest in the initial G₀–G₁ phase after irradiation. The dose-response curves indicate that a single event, such as an irreparable DNA break, may be sufficient to induce arrest. p53⁺ cells that escaped the initial G₀–G₁ phase after irradiation entered S phase in at least two waves. However, many of these cells underwent long-term arrest in subsequent phases. In contrast, virtually all of the cells in isogenic p53^- (with a nonfunctional p53 pathway) strains escaped from the first G₀–G₁ phase without delay, regardless of the dose. p53^- cells were also eliminated in subsequent phases but at significantly lower frequencies. Consistent with these findings, the reproductive viability of p53^- cells was higher than p53⁺ cells. The nonclonogenic fraction appeared to be eliminated within three cycles for both cell types. In addition, artificial holding in G₀ after irradiation, which allows for the repair of potentially lethal damage, led to similar increases in survival in p53⁺ and p53^- cells. These data are inconsistent with the hypothesis that the primary function of p53-dependent G₀–G₁ arrest in response to γ-irradiation is to allow additional time for DNA repair. Rather, they indicate that p53 helps maintain genetic stability by eliminating cells with damaged chromosomes from the reproductively viable population.