CYTOGENETIC DAMAGE AND THE RADIATION-INDUCED G(1)-PHASE CHECKPOINT

It is proposed that genomic integrity is preserved after DNA damage in a variety of ways. X irradiation induces a p53-dependent G(1)-phase c ell cycle checkpoint which putatively allows time for repair of DNA da mage. The p53 protein is also involved in the initiation of apoptosis after radiation-induced DNA damage, presumably leading to the eliminat ion of lethally damaged cells from the irradiated population. To test the hypothesis that repair occurs in the additional time provided by t he activation of the G(1)-phase checkpoint, we investigated whether th e presence of a G(1)-phase arrest modified the frequency and type of c hromosomal rearrangements at the first mitosis after irradiation. Isog enic cell lines derived from the same human glioma cell line, but diff ering in p53 status, were used. Purified G(1)-phase cells, isolated by centrifugal elutriation and X-irradiated, were studied. The wild-type p53 cell line demonstrated a dose-dependent arrest during G(1) phase, as determined by flow cytometry. These cells remained in G(1) phase a s long as 48 h after irradiation. Cells expressing a dominant-negative p53 mutation accumulated to a much lesser extent in G(1) phase after irradiation. Cells lacking the G(1)-phase checkpoint showed increased survival at all radiation doses. There were no significant differences in the type or frequency of total chromosomal aberrations in mitotic cells from either cell line after 1, 2, 4 or 6 Gy X rays, as measured by conventional cytogenetic analysis. There was an increase, however, in the number of reciprocal translocations in mitotic cells with mutan t p53 (lacking a G(1)-phase checkpoint), as measured by fluorescence i n situ hybridization with a chromosome 4-specific DNA library, but onl y after 6 Gy. The results suggest that the presence of a well-defined p53-dependent G(1)-phase arrest does not reduce chromosomal aberration s caused by low doses of ionizing radiation markedly, but may reduce t he overall degree of survival by triggering other G(1)-phase events. ( C) 1996 by Radiation Research Society