Role of DNA polymerase alpha and delta in radiation clastogenesis

Aphidicolin induces 6-thioguanine resistant mutants in human diploid fibroblasts

Cytotoxic and mutagenic effects of aphidicolin (APC), an inhibitor of DNA polymerases alpha and delta, were studied in human diploid VH-10 fibroblasts. The cells were treated (2 or 4h) with APC at concentration ranges of 10-40 microM. The effect of APC on cell survival after 4 h treatment was significantly higher than after 2 h treatment. The mutagenicity of APC was investigated at the HPRT locus, and the frequency of HPRT mutants was estimated by selection in medium containing 6-thioguanine (6-TG). Treatment of fibroblast cells with 20 microM of APC for 2 or 4 h resulted approximately in 5 or 10 times increase of 6-TG resistant mutant frequencies, respectively, compared to untreated control cells. The cell cycle analyses performed during the expression time (9-12 days) have shown that after 2 and 4h treatment with APC the cells were blocked in G2 phase during the majority of the expression period, compared to control cells. Four days after the treatment, the amount of cells in G2 phase increased about two-fold (28.6-31.8% compared to 13.5% in the untreated cells). The mode of cell death during the expression time was via necrosis, rather than apoptosis, which was demonstrated by fluorescein-diacetate (FDA)-staining and terminal dUTP nick end labeling (TUNEL)-method.

Telomeric association in women with breast and uterine cervix cancer

This study compares the frequency of telomeric associations in the peripheral blood of women suffering breast and cervix uterine cancer with a healthy control group. Two kinds of cultures were developed for each individual: with and without aphidicolin. In the normal cultures, the number of telomeric associations observed was 95.5 times higher in individuals affected by breast cancer and 41.3 times higher in those affected by cervix uterine cancer when compared to the control group (p < 0.001). In the cultures with aphidicolin, higher numbers of altered metaphases were observed in both groups as compared to the control groups (p < 0.001). Statistically significant differences (p < 0.001) could also be observed when comparing telomeric associations between the two types of cancer in both cultures. When we compared individuals affected by breast cancer in both types of cultures statistical differences were found (p < 0.05), and similar results were found in individuals affected by uterine cervix cancer (p < 0.001). The findings suggest that telomeric associations may be reflecting chromosome instability observed in cancer and that this instability behaves differently for various types of cancer.

Influence of ara A on the formation of dicentrics in irradiated quiescent CHO cells

CHO K1-cells, arrested in G0/G1 phase were irradiated with graded doses of 150-kV X-rays, and the dicentric chromosome aberrations in the first metaphase after reincubation in full growth medium were evaluated. When 500 microM ara A was administered at least 1 h before irradiation, maximal enhancement of the dicentric yield was obtained under 'immediate plating' conditions as well as under 'delayed plating' conditions, and under both conditions only the linear component alpha D of the aberration yield was affected. Since ara A had no effect upon the number of induced dsbs, as measured with PFGE at doses up to 100 Gy, the action pathway of ara A is suggested to be the repair/misrepair process manifested in the linear component of the dicentric yield. By varying the time at which either ara A at 37 degrees C was administered before irradiation or ara A at 4 degrees C was added after irradiation, it was observed that the repair step sensitive to ara A was completed within about 5 min after irradiation. This is in agreement with other observations of ara A action upon a fast repair step. The conversion of DNA single-strand breaks or base damage sites into double-strand breaks as a consequence of polymerase beta inhibition by ara A would explain the present results.

Lack of effect of inhibitors of DNA synthesis/repair on the ionizing radiation-induced chromosomal damage in G2 stage of ataxia telangiectasia cells

The relationship between the repair processes occuring at the G2 phase of the cell cycle and cytogenetic damage in ataxia telangiectasia (AT) cells was studied. Lymphoblastoid cells derived from normal, heterozygote AT (HzAT) and three AT patients were exposed to X-rays or fission neutrons and post-treated with inhibitors of DNA synthesis/repair, such as inhibitors of DNA polymerases alpha, delta and epsilon (cytosine arabinoside, ara-C; aphidicolin, APC; buthylphenylen-guanine, BuPdG) or ribonucleotide reductase (hydroxyurea, HU). A strong increase of radiation-induced chromosomal aberrations was observed in normal and HzAT cells post-treated with ara-C, APC and HU, but not in the presence of BuPdG. No enhancing effect was observed in cells derived from AT patients, except for HU post-irradiation treatment. These results suggest that the enzymes that can be inhibited by these agents are not directly involved in the repair of radiation damage induced in G2 cells from AT patients, indicating that probably the AT cells that we used lack the capability to transform the primary DNA lesions into reparable products, or that AT cells might contain a mutated form of DNA polymerase resistant to the inhibitors.

Time sequence of events leading to chromosomal aberration formation

Investigations have been carried out which have measured the influence of the repair polymerases on the yield of different types of chromosomal aberrations. The studies were mainly concerned with the effect of inhibiting the polymerases on the yield of aberrations. The polymerases fill in single strand regions, and the fact that their inhibition affects the yield of aberrations suggests that single strand lesions are influential in aberration formation. The results indicate that-- 1. There are two actions of polymerases in clastogenesis. One is in their involvement in a G2 repair system, in which the pair of chromatids is concerned, and which does not yield aberrations unless the inhibition is still operating when the cells enter mitosis. The second also operates in G1 and S, and is such that when repair is inhibited, further damage accrues. 2. The second action is affected by inhibiting polymerase but operates even when the repair enzymes are active. 3. The production of chromosomal exchanges involves a series of reactions, some of which are reversible. 4. The time span over which the reactions occur is much longer than has been envisaged previously (e.g., most of a cell cycle).

The influence of growth medium on the yield of X-ray-induced chromatid exchanges in the presence and absence of aphidicolin

The frequency of exchanges in JU56 cells irradiated in the G2 phase in the presence and absence of the polymerase inhibitor aphidicolin (APC), and in the presence of a range of concentrations of cysteine was measured. It was found that in the absence of cysteine, incubation for 2 h with APC had no effect on the yield. Addition of cysteine at concentrations of 50-250 mg/l reduced the frequency of exchanges, and at these concentrations the frequency was increased by incubation with APC. At higher concentrations, the yield was reduced and incubation with APC did not elevate it. In following experiments, it was found that incubation with cysteine for a period of longer than 10 minutes was necessary before APC affected the yield of exchanges.