The induction of hypoxanthine-guanine-phosphoribosyl-transferase (HPRT) mutations and the survival rate of Chinese hamster cells were investigated after exposure to accelerated He-4 and C-12 ions with varying linear energy transfer (LET) (20-360 keV/mum) and to gamma rays. The power law was found for the mutation induction rate as a function of the radiation dose with LET up to 50 keV/mum. The dependence transforms into a linear one at higher LET values. The RBE of heavy charged ions increases with the LET of ions to maximal values at 80-100 keV/mum. The RBE values for the induction of mutations and chromosomal aberrations amount to 4.8-5.0 and are twice as high as the values corresponding to the cell-survival criterion (2.3-2.9). A high heterogeneity and chromosomal instability in aneuploidy and chromosomal-aberration levels were observed in spontaneous mutants and in those induced by protons (LET of 0.218 keV/mum), N-14 ions (LET of similar to77 keV/mum), and gamma rays. The chromosomal instability was highest for spontaneous mutants and decreased with increasing LET. The metabolic hypothesis for the chromosomal instability is discussed. The regularities in the formation of unstable chromosomal aberrations in human-blood lymphocytes were studied after exposure to protons, 12C, Mg-24, and N-14 ions (LET of 0.218, similar to12, 42.7, and similar to77 keV/mum, respectively), and gamma rays. The rate of aberrations increased with the dose and radiation LET. The RBE values were 1.0. 1.2-1.3, 1.4-1.7, and 2.0-2.2 for protons, C-12, Mg-24, and N-14 ions, respectively. The FISH analysis of stable chromosomal aberrations (translocations) revealed a high efficiency of N-14 ions (RBE of similar to3.0). According to all cytogenetic tests, there were no essential differences between the efficiency of protons and gamma rays. The dose dependence of the rate of production of cells with chromosomal aberrations after exposure of human lymphocytes, Chinese hamster cells, and human melanoma cells to gamma rays and their adaptive response were studied in the dose range of 0.01-1 Gy. For all types of cells, a nonlinear dose-effect dependence was revealed for cells with chromosomal aberrations. This dependence is characterized by a hypersensitivity of cells at doses of 0.01-0.2 Gy and an induced radioresistance at doses above 0.5 Gy. The possible mechanisms involved in this phenomenon are discussed.

PURPOSE:To detect the frequencies of interchanges among 11 chromosomes in lymphocytes irradiated with gamma-rays and to find out whether these frequencies reflect the proximity of some of these chromosomes within the interphase nucleus. MATERIAL AND METHODS:Exchange aberrations were detected in the first mitosis after irradiation of human lymphocytes with 3 and 5 Gy gamma-rays of 60Co. Two-colour repeated FISH with two differently chemically modified probes in each hybridization was applied. The microscope stage positions of each mitosis were recorded after the first hybridization and used for the automatic scanning of images after all successive experiments. Five images were obtained for each mitosis differing in visualized pairs of chromosomes. Comparing these images, exchanges among 10 chromosomes could be detected. Painting of the p arm of chromosome 21 with the painting probe for chromosome 22 also made it possible to detect exchanges of this chromosome with other chromosomes of the selected group. RESULTS:Frequencies of exchange aberrations induced in chromosomes of the selected group as well as interchanges between many pairs of chromosomes of this group were roughly proportional to the DNA content of chromosomes. Higher frequencies of interchanges than expected according to the model of linear proportionality were found between several chromosomes involved in translocations frequent in different subtypes of leukaemia. CONCLUSIONS:Frequencies of interchanges among 11 chromosomes of human lymphocytes induced by gamma-rays do not indicate as clearly as fast neutrons the non-random arrangement of chromosomes in the cell nucleus. The interaction of a large number of chromosomes in exchange aberrations suggests that the chromatin in the territory of one chromosome is accessible for several other chromosomes.

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.

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).

A new model of DNA single-strand break (SSB) and double-strand break (DSB) induction by radiations of different LET has been developed. Utilizing quadratic dependence of the dose that delta-electrons depart in the track of heavy particles the fraction of heavy particle energy deposited in the target of DNA dimensions has been calculated. SSBs arise from energy depositions in one strand of DNA, direct DSBs arise from two SSBs on opposite strands of DNA in the track of one particle. It is concluded that DSBs induced by gamma-radiation are mostly of enzymatic origin, meanwhile DSBs induced by high-LET radiation are direct DSBs. The dependence of the radiosensitivity D0-1 on LET (L) for isogenic mutants of E. coli with different sensitivity to gamma-radiation has been determined on the bases of the model and considering microscopic energy fluctuations. The shape of D0-1 (L) function is formed both by physical characteristics of radiation and by the ability of cells to repair some types of DNA damage. The model provides a basis for further investigation.

Basic types of DNA damage produced by gamma-radiation in cells are reviewed. Different DNA injuries are related to various levels of DNA repair processes, established in the case of Escherichia coli cells. The role of the balance of repair enzyme activities is considered in connection with the induction of enzymatic DNA double-strand breaks (DSBs). The concept of "metastable sites" has been introduced. "Metastable sites" are formed from great nucleolytic gaps. They are measured as DSBs although they can be repaired as single-strand breaks (SSBs). A simple mathematical model of the inactivation of different mutants of E. coli cells has been constructed on the basis of available experimental data. Kinetic equations of the model have been solved and some parameters estimated for both sensitive and resistant mutants.

A bacterial biodetection system has been developed for rapid detection of environmental genotoxins. This cellular bioassay is based on the receptor reporter principle with the SOS system as receptor sensitive to DNA damage and the biolu­ minescence system as rapid optical reporter. Since the bioassay combines the SOS system with the bioluminescence system, we have termed it sos lux test. For the SOS lux test, a recombinant plasmid pPLSI (DSM 10333) was constructed in which the promoterless operon of bioluminescence of Photo bacterium leiognathi 54DI0 (lux C,D,A,B,F,E) (Krasnojarsk Institute of Biophysics Collection) is under control of a SOS promoter (part of the cda gene of the plasmid ColD, which carries a strong SOS promoter). This plasmid pPLSI can be used to transform any Escherichia coli recA+ system or other microorganisms with a SOS system, suitable for the detection of a