Interpretation of mutation induction by accelerated heavy ions in bacteria.

In this report, a quantitative interpretation of mutation induction cross sections by heavy charged particles in bacterial cells is presented. The approach is based on the calculation of the fraction of energy deposited by indirect hits in the sensitive structure. In these events the particle does not pass through the sensitive volume, but this region is hit by delta rays. Four track structure models, developed by Katz (in Quantitative Mathematical Models in Radiation Biology, pp. 57-83, Springer-Verlag, 1988). Chatterjee et al. (Radiat. Res. 54, 479-494, 1973), Kiefer and Straaten (Phys. Med. Biol. 31, 1201-1209, 1982) and Kudryashov et al. (Proceedings of the First Soviet Congress on Microdosimetry, Atomizdat, Moscow, 1973), respectively, were used for the calculations. With the latter two models, very good agreement of the calculations with experimental results on mutagenesis in bacteria was obtained. Depending on the linear energy transfer (LET infinity) of the particles, two different modes of mutagenic action of heavy ions are distinguished: "delta-ray mutagenesis," which is related to those radiation qualities that preferentially kill the cells in direct hits (LET infinity > or = 100 keV/microns), and "track core mutagenesis," which arises from direct hits and is observed for lighter ions or ions with high energy (LET infinity < or = 100 keV/microns).

Induction of lacI- mutations in Escherichia coli cells after single and split-dose irradiation.

In the lacI system of Escherichia coli, X-ray mutagenesis follows a linear-quadratic curve with suppression; the survival curve is exponential. Dose fractionation leads to nearly complete repair of premutational lesions during an incubation interval of 3.5 h. Repair starts with a delay of 1.5-2 h, suggesting the involvement of an inducible repair/mutation fixation system. The dose-dependence of mutagenesis is described by a simple model assuming two hits being required. A probable explanation might be that the premutagenic lesions consist of two closely spaced lesions on the opposite strands of the DNA molecule.

Mutation induction by accelerated heavy ions in bacteria.

Induction of lacI- forward mutations in Escherichia coli Ymcl and his(-)-->his+ reversions in Salmonella typhimurium TA102 was investigated after irradiation with heavy ions in the range of Z = 1-36. Particle specific energies (E) were in the range of 1-600 MeV/u. A strong dependence of the mutation induction cross-section (sigma m) on both particle energy and LETinfinity was observed. The results suggest that two different ranges of LETinfinity can be distinguished. In the range of high LETinfinity (> 100 keV/micron) sigma m increases with increasing specific particle energy if LETinfinity is kept constant (Fe ions as compared with carbon ions or alpha-particles). In the range of low LETinfinity (< 100 keV/micron) sigma m decreases with increasing energy (Ne ions as compared with He ions).

Influence of repeated lyophilization on the survival of Deinococcus proteolyticus, Micrococcus luteus and Escherichia coli.

Repeated lyophilization of Deinococcus proteolyticus, Micrococcus luteus and Escherichia coli cells results in a successive decrease of their survival. The survival curve is exponential with E. coli and M. luteus, and sigmoidal with a broad shoulder with D. proteolyticus both after repeated lyophilization and after UV- or gamma-irradiation. When cells were subjected to gamma-irradiation after a 20-fold freeze-drying, the corresponding survival curve became exponential without the shoulder. Hence we assume that irradiation and repeated lyophilization afflict the same cellular structures and/or functions.

Induction of SOS repair by ionizing radiation. Results from experiments at accelerators.

beta-galactosidase and alkaline phosphatase activities of Escherichia coli strain PQ37 carrying the fusion gene of sulA and lacZ treated with different types of ionizing radiation were examined. The induction factor (ratio of beta-galactosidase to alkaline phosphatase activity), reflecting the SOS-induction potency, increases significantly with radiation dose. Maximum effectiveness to induce SOS-response has been found for deuterium and helium ions in comparison to gamma-rays, carbon or krypton ions. Increased energy of helium ions leads to greater SOS-induction potency of radiation.

Cell inactivation, mutation and DNA strand-break induction by gamma-rays at very low temperatures.

Cell inactivation, mutation and DNA strand-break induction by gamma-radiation have been investigated at very low temperatures (-78 degrees C, -196 degrees C, and -268 degrees C). In Escherichia coli Ymel, lacI+-->lacI- and Salmonella typhimurium TA102, his--->his+ dose-modifying factors determined for low radiation doses are similar for both mutation induction and cell inactivation. The sensitivity of repair-deficient strains E. coli polA- and E. coli recA- was also reduced at low temperature to a comparable extent. This suggests that the lesions which are responsible for cell inactivation and mutagenesis could be strongly mutually related and/or that different types of lesions which are responsible for cell inactivation and mutation induction in bacteria are reduced at low temperature to the same or similar extent. Likewise, a lower yield of DNA strand breaks in plasmids irradiated at low temperature was observed.

Distribution of ABL and BCR genes in cell nuclei of normal and irradiated lymphocytes.

Using dual-color fluorescence in situ hybridization (FISH) combined with two-dimensional (2D) image analysis, the locations of ABL and BCR genes in cell nuclei were studied. The center of nucleus-to-gene and mutual distances of ABL and BCR genes in interphase nuclei of nonstimulated and stimulated lymphocytes as well as in lymphocytes stimulated after irradiation were determined. We found that, after stimulation, the ABL and BCR genes move towards the membrane, their mutual distances increase, and the shortest distance between heterologous ABL and BCR genes increases. The distribution of the shortest distances between ABL and BCR genes in the G0 phase of lymphocytes corresponds to the theoretical distribution calculated by the Monte-Carlo simulation. Interestingly, the shortest ABL-BCR distances in G1 and S(G2) nuclei are greater in experiment as compared with theory. This result suggests the existence of a certain regularity in the gene arrangement in the G1 and S(G2) nuclei that keeps ABL and BCR genes at longer than random distances. On the other hand, in about 2% to 8% of lymphocytes, the ABL and BCR genes are very close to each other (the distance is less than approximately 0.2 to 0.3 microm). For comparison, we studied another pair of genes, c-MYC and IgH, that are critical for the induction of t(8;14) translocation that occurs in the Burkitt's lymphoma. We found that in about 8% of lymphocytes, c-MYC and IgH are very close to each other. Similar results were obtained for human fibroblasts. gamma-Radiation leads to substantial changes in the chromatin structure of stimulated lymphocytes: ABL and BCR genes are shifted to the nuclear center, and mutual ABL-BCR distances become much shorter in the G1 and S(G2) nuclei. Therefore, we hypothesize that the changes of chromatin structure in the irradiated lymphocytes might increase the probability of a translocation during G1 and S(G2) stages of the cell cycle. The fact that the genes involved in the t(8;14) translocation are also located close together in a certain fraction of cells substantiates the hypothesis that physical distance plays an important role in the processes leading to the translocations that are responsible for oncogenic transformation of cells.

Localisation and distance between ABL and BCR genes in interphase nuclei of bone marrow cells of control donors and patients with chronic myeloid leukaemia.

Quantitative measurements of the nuclear localisation of the ABL and BCR genes and the distance between them were performed in randomly oriented bone marrow cells of control donors and patients with chronic myeloid leukaemia (CML). Most ABL and BCR genes (75%) are located at a distance of 20-65% of the local radius from the nuclear centre to the nuclear membrane. A chimeric BCR-ABL gene located on a derivative chromosome 22 resulting from t(9;22)(q34;q11) [the so-called Philadelphia (Ph) chromosome] as well as the intact ABL and BCR genes of patients suffering from chronic myeloid leukaemia are also located mostly in this region, which has a mean thickness of 2 microns in bone marrow cells. We have not found any significant differences in the location of the two genes in the G1 and G2 phases of the cell cycle, nor between bone marrow cells and stimulated lymphocytes. Irradiation of lymphocytes with a dose of 5 Gy of gamma-rays results in a shift of both genes to the central region of the nucleus (0-20% of the radius distant from the nuclear centre) in about 15% of the cells. The minimum distance between one ABL and one BCR gene is less than 1 micron in 47.5% of bone marrow cells of control donors. Such a small distance is found between homologous ABL and between homologous BCR genes in only 8.1% and 8.4% of cells, respectively. It is possible that the relative closeness of nonhomologous ABL and BCR genes in interphase nuclei of bone marrow cells could facilitate translocation between these genes. In 16.4% of bone marrow cells one ABL and one BCR gene are juxtaposed (the distance between them varies from 0-0.5 micron) and simulate the Ph chromosome. This juxtaposition is the result of the projection of two genes located one above another into a plane, as follows from the probability calculation.