G2 repair and the formation of chromosomal aberrations

Abstract

The effects of hydroxy urea (HU) and caffeine, alone and in combination, on X-ray-induced chromosome damage and mitotic delay were studied in root tips of Viciafaba. In addition, some experiments were carried out in which X-ray-irradiated roots were post-treated with 5-fluorodeoxyuridine (FdUrd). Post-treatments with all three agents were given the last few hours before fixation. Both HU and FdUrd strongly increased the frequency of X-ray-induced gaps, breaks and isochromatid breaks of the NU-type, but only when the damage was induced in replicating (S) or replicated (G2) chromosomes. The magnitude of the potentiation was independent of the initial degree of chromosome damage. Potentiation by HU was obtained in cells that were exposed to the inhibitor 2 to 5 hours before they reached metaphase. The potentiating effect of FdUrd on X-ray-induced chromosome damage was completely reversed by thymidine. Caffeine was able to potentiate induced chromosome damage only if the cells were X-ray-irradiated in the G2 phase and if the post-treatment with caffeine was given directly after irradiation. The experimental results are compared with results of analogous experiments performed in mammalian cells. Possible molecular mechanisms involved in the action of HU, FdUrd and caffeine on the formation of X-ray-induced chromatid aberrations are discussed.