P53 MEDIATES PERMANENT ARREST OVER MULTIPLE CELL-CYCLES IN RESPONSE TO GAMMA-IRRADIATION

A new technique that monitors cell cycle progression over multiple cyc les was used to gain insight into how p53 limits the emergence of vari ants with structural chromosome alterations following gamma-irradiatio n. G(0)-synchronized, p53(+) (with a functional p53 pathway) normal hu man fibroblast and epithelial strains underwent a dose-dependent perma nent arrest in the initial G(0)-G(1) 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 escap ed the initial G(0)-G(1) phase after irradiation entered S phase in at least two waves. However, many of these cells underwent long-term arr est in subsequent phases. In contrast, virtually all of the cells in i sogenic p53(-) (with a nonfunctional p53 pathway) strains escaped from the first G(0)-G(1) phase without delay, regardless of the dose. p53( -) cells were also eliminated in subsequent phases but at significantl y lower frequencies. Consistent with these findings, the reproductive viability of p53(-) cells was higher than p53(+) cells. The nonclonoge nic fraction appeared to be eliminated within three cycles for both ce ll types. Tn addition, artificial holding in G(0) after irradiation, w hich allows for the repair of potentially lethal damage, led to simila r increases in survival in p53(+) and p53(-) cells. These data are inc onsistent with the hypothesis that the primary function of p53-depende nt G(0)-G(1) arrest in response to gamma-irradiation is to allow addit ional time for DNA repair. Rather, they indicate that p53 helps mainta in genetic stability by eliminating cells with damaged chromosomes fro m the reproductively viable population.