G1 arrest and cell-cycle-dependent clastogenesis in UV-irradiated human fibroblasts

The demonstrations of frequent allelic deletions in lung and colon cancers have reemphasized the importance of clastogenesis in carcinogenesis. We have investigated the mechanisms of induction of chromosome aberrations in ultraviolet-irradiated diploid human fibroblasts. Cells were irradiated with UV at various times during a parasynchronous wave of cell proliferation and then harvested during the first mitosis that followed irradiation. Metaphase spreads were stained with Geimsa and the yields of chromosome aberrations were quantified. Ultraviolet irradiation induced primarily chromatid-type chromosome aberrations which included chromatid breaks and exchanges. Frequencies of aberrations displayed significant differences according to the phase of the cell cycle in which irradiation occurred and the time after irradiation when metaphases were harvested. Fibroblasts that were irradiated when in G0 and then immediately replated to stimulate cell division and cells that were at the S/G2 border when irradiated displayed the fewest numbers of aberrations. For G0-irradiated cells, the first entering mitosis carried a higher frequency of aberrations than those collected 2-4 h later. In contrast, for S/G2-irradiated cells the first into mitosis displayed fewer aberrations than subsequent fractions. Cells that were irradiated when at the G1/S border displayed the greatest numbers of aberrations with the frequencies of chromatic exchanges being significantly increased over all other times of irradiation. These studies confirm that UV is an S-phase-dependent clastogen and point to the G1/S border as a time of maximal sensitivity to clastogenesis. Irradiation of G1 cells was shown to produce a fluence-dependent reduction in the rate of entry of cells into the S-phase. There appeared to be a point late in G1 beyond which cells were resistant to irradiation and experienced less delay in S phase entry. Ataxia telangiectasia fibroblasts failed to delay entry to S phase following UV-irradiation in G1 and displayed hypersensitivity to UV-induced chromosomal aberrations. The delay in entry of damaged cells into the S phase may have the beneficial effect of providing more time for repair of potentially clastogenic DNA damage before the onset of DNA replication.