Inducible responses to DNA damage

Stage-specific hypermutability of the regA locus of Volvox, a gene regulating the germ-soma dichotomy

Mutation at the regA locus confers on somatic cells of Volvox (which otherwise undergo programmed death) ability to redifferentiate as reproductive cells. Stable mutations at the regA locus, but not at other loci, were induced at high frequency when embryos at one particular stage were exposed to either UV irradiation, novobiocin, nalidixic acid, bleomycin, 4-hydroxyaminoquinoline-1-oxide, 5-bromodeoxyuridine, or 5-fluorouracil. All treatments led to some mutations that were not expressed until the second generation after treatment. The sensitive period was after somatic and reproductive cells of the next generation had been set apart, but before they had undergone cytodifferentiation. Hypermutability occurs in presumptive reproductive cells (in which regA is normally not expressed) somewhat before regA normally acts in somatic cells. We postulate that hypermutability of regA in the reproductive cells at this time reflects a change of state that the locus undergoes as it is inactivated.

Adaptation-like response to the chemical induction of sister chromatid exchanges in human lymphocytes

Experiments have been performed to determine whether human lymphocytes in primary cultures can show an "adaptive" response to the induction of cellular lesions (manifested as a production of sister chromatid exchanges, SCEs) as previously found in bacteria and established human and mammalian cell lines. Human lymphocytes were pretreated with various subtoxic concentrations (5-50 ng/ml) of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) once every 6h for 72 h, and subsequently challenged by a high dose (4 micrograms/ml) of MNNG. The lymphocytes in MNNG-challenged cultures had the lowest frequency of SCEs when pretreated with 10 ng/ml MNNG. Further cross-resistance study revealed that repeated pretreatments of lymphocytes with 10 ng/ml MNNG for 72 h can render the cells resistant to the induction of SCEs by the following challenge with a high dose of MNNG, but not of mitomycin C or ethyl nitrosourea. The data also suggest variations in the degree of the adaptation-like response among individuals.

Adaptive response of human lymphocytes to low concentrations of radioactive thymidine

When human lymphocytes were cultured with [3H]thymidine, which acts as a source of low-level chronic radiation, and then exposed to 150 rad of x-rays at 5, 7, 9, or 11 hours before fixation, the yield of chromatid aberrations was less than the sum of the yields of aberrations induced by [3H]thymidine and x-rays separately. Often fewer aberrations were found after exposure to radiation from both sources than were found after exposure to x-rays alone. At the same fixation times, nonradioactive thymidine did not affect the yield of x-ray-induced aberrations. The same phenomenon occurred at earlier fixation times, after exposure to 30 or 40 rad of x-rays and [3H]thymidine. This response is analogous to the adaptive response to alkylating agents whereby prior treatment with small doses for a long period reduces the damage occurring from large doses of similar agents given for a short time.

Proliferative kinetics and chemical-induced sister chromatid exchanges in human lymphocyte cultures

Although human lymphocyte cultures contain cells that have divided for different numbers of times after PHA stimulation, this heterogeneity of different cell divisions can be explained by a difference in the times when cells start their first DNA synthesis in responding to PHA. Cycling lymphocytes, whether they entered cycling earlier or later after stimulation, have about the same mean cell cycle times of 12-14 hr. Treatment with 3 X 10(-6) M MMC was found to induce an approximately 5-hr delay in the cell cycle. The induction of SCEs by chemical treatment depends on the stage in the cell cycle at treatment and on the persistence of the induced SCE-forming lesions. The most efficient time of treatment is the G1/S boundary in the first cell cycle of the 2 consecutive cycles before sampling. Among 3 alkylating agents tested here, EMS and 4NQO induce quite long-lived lesions that lead to SCE formation, whereas MMC-induced lesions seem to be completely removed within a cell cycle. Treatments with increasing concentrations of the chemical induce a larger increase in SCE frequency and longer delays in the cell cycle. However, with a fixed experimental regimen, treatments with relatively higher doses cause a deformity of the dose-response relationship. The data also show that longer BrdUrd treatment before fixation results in a sampling of cells that have higher SCE frequencies. Repeated pretreatments of human lymphocytes with a very low concentration of MNNG render them resistant to a following challenge with MNNG, or ENU, but not with MMC. The data at present indicate that there is a quite large variability among blood donors in this induction of resistance in lymphocytes.