Comparative frequency of chromosome aberrations in a human lymphocyte culture depending on the mode of neutron irradiation

The study of the dose dependence of the incidence of chromosome aberrations in a human lymphocyte culture exposed at the G0 stage to pulsed neutrons of 0.7 MeV revealed no significant distinctions in the cytogenetic effect at the pulse frequency of 1 and 5 Hz, but at 100 Hz it was somewhat increased. As to the biological effectiveness, pulsed neutrons of 0.7 MeV occupied an intermediate position between 0.35 and 0.85 MeV static neutrons.     

An analysis of the cytogenetic effects of gamma and neutron irradiation in a human lymphocyte culture at the G0 and G1 stages of the mitotic cycle (a structural-functional approach)

A study was made of the dose dependence of the chromosome aberration frequency in human lymphocytes exposed to 60Co-gamma radiation and neutrons (mean energy of 0.85 MeV) at the G0 stage and in different periods of the G1 and G1/S stages of the cycle. With gamma irradiation the dose dependence for cells at the G1 and G1/S stages was at a higher level than that for cells at the G0 stage, whereas the opposite picture was observed for cells exposed to neutron radiation. The difference was also noted in the time-response curves where gamma radiation increased and neutrons, on the contrary, decreased the aberration yield in the cells that passed from G0 to G1 stage. The experimental data obtained are attributed to activation of repair system at the G1 stage which is mainly conditioned by chromatin decondensation; the activating, that is, the functional factor influences the aberration induction with gamma irradiation, while the decondensation, that is, the structural factor, with neutron irradiation.     

The quantitative patterns in the yield of chromosome aberrations in a human lymphocyte culture under fractionated gamma-neutron irradiation at different stages of the mitotic cycle. The cytogenetic effects at the G0 stage]

A study was made of the quantitative regularities of the interaction of cytogenetic damages induced, at the G0 stage of human lymphocyte culture, by fractionated gamma-neutron radiation, within a wide range of doses, delivered in the direct of reverse sequence at the intervals between fractions of 1 and 5 h. The results were compared with those obtained in experiments with single and fractionated gamma- or neutron irradiation within the same dose range.     

Effect of the time before fixation on the yield of chromosome aberrations in a culture of human lymphocytes exposed to gamma rays at different stages of the mitotic cycle

A comparative study was made of the yield of chromosome aberrations in human lymphocyte culture exposed to 60Co-gamma-rays (2 Gy) at different mitotic cycle stages the cells being fixed after 52 and 60 hr. It was shown that with the latter fixation time (60 hr) the frequency of chromosome aberrations after irradiation in G1 stage was substantially lower than that with the former one (52 hr) and, vice versa, it was higher after irradiation in S and G2 stages. The authors discuss the probable causes of the distinctions observed.     

Formation of chromosome aberrations at different stages of the mitotic cycle in a culture of human lymphocytes irradiated with neutrons of average energy 0.35 and 0.85 MeV

Incidence of chromosome aberrations at different mitotic cycle stages of a human lymphocyte culture has been studied after exposure to 60Co-gamma-quanta (2 Gy) and neutrons of 0.85 Mev (1 Gy) and 0.35 MeV (0.5 Gy). The yield of chromosome aberrations with both gamma- and neutron-radiation depends on a stage of the mitotic cycle. The S stage is the most radioresistant one with both types of ionizing radiation.     

A dose-effect relationship study for the frequency of binuclear cells with micronuclei in a human lymphocyte culture after gamma irradiation

The method of the cytokinetic blocking was used to determine, in 18 healthy male donors aged 21-35 years, the frequency of binuclear cells with micronuclei that amounted to 1.1 +/- 0.2% (0.3-2.4%) on the average. The dependence of this parameter upon radiation dose (60Co-gamma radiation, 0.05 to 2 Gy) was a linear quadratic function and was described by the following regression equation: Yx = 0.76 + 5.76 D +/- 2.61 D2. With a dose of 0.05 Gy the frequency of binuclear cells with micronuclei exceeded significantly the control level.     

The fate of X-ray-induced chromosome aberrations in blood lymphocyte culture

The BrdU-Giemsa method which facilitates an unequivocal identification of metaphases at different cycles has been utilized to investigate the fate of X-ray-induced chromosome aberrations in the blood lymphocyte culture system of the Indian muntjac which has the lowest diploid number (2n = 6 female/7 male) and easily distinguishable large-sized chromosomes. The results demonstrate that about 50% of dicentrics and only 12% of rings were transmitted from the first cycle to the second. There were as high as 73% abnormal cells in the second cycle as against 94% in the first cycle following 4.0 Gy. However, the frequencies of dicentrics, rings and of abnormal cells were greatly reduced in the third+ cycles. The frequencies of acentric fragments per post-irradiated first, second and third+ division cell were 2.21, 0.64 and 0.24, respectively. In sharp contrast to all earlier reports, about 75% of them were retained as a single acentric fragment in the second cycle. Analysis of fragment segregation during anaphase separation supports this finding. The survival probability of dicentrics and rings was found to be more than 60% in the second and only 18% in the third+ cycle.     

Dose-response curves for the action of 6 MeV mean-energy neutrons on a human lymphocyte culture at different stages of the mitotic cycle

A linear component was predominant in the dose-response curve characterizing the radiosensitivity of human lymphocyte chromosomes after exposure to fast neutrons (E = 6 MeV) at different mitotic cycle stages. This was indicative of a single-hit mechanism of the formation of chromosome aberrations after the effect of 6 MeV neutrons. It is suggested that the plateau of the dose-response curve at the S-stage may be considered as an indication of repair of damages induced by neutrons at this stage.     

Chromosome aberrations in human lymphocytes at a various duration of cultivation after irradiation

Human peripheral blood lymphocytes were exposed to 60Co gamma-rays (a dose of 3 Gy) and cultivated during seven days in the presence of PHA and BrdU. It was shown that the metaphases of the first and second mitosises occurred during cultivation of the irradiated and unirradiated lymphocytes, being evidence about of irregularity of the coming into division of various fractions of lymphocytes. The time of cultivation did not influence a rate of aberrations in metaphases of the first and second mitosises of the irradiated lymphocytes. During the first and the subsequent mitosises the number of exchange chromosome aberrations decreased and reached a control level in metaphases of the fourth and fifth mitosises. The number of paired fragments at second and third mitosises increased a little and started to decrease only in metaphases of the fourth and fifth mitosises. The decrease in chromosome aberrations with prolongation of the cultivation of lymphocytes after irradiating is a consequence of elimination of cells with chromosome damages during sequential mitotic divisions.     

Formation of chromosome aberrations in human lymphocytes to the action of 5-fluorodeoxyuridine used separately and in combination with gamma irradiation: changes in the effect in the course of the mitotic cycle

Human lymphocytes were treated after different times of incubation, either by 60Co gamma-rays (1 Gy) followed by 5-fluorodeoxyuridine (FUdR, 2.10-7 M during 2,5 h) or by radiation and FUdR, separately. Chromosomal aberrations were studied after 51 h of incubation. When administered alone, FUdR increased the frequency of chromatid aberrations and gaps over the spontaneous level. This increase took place mainly during two periods of the mitotic cycle, namely, on the borderline between G1 and S stages and at the end of the G2 stage. FudR barely affected the frequency of chromosomal aberrations. THe effect did not depend upon the concentration of FUdR. Irradiation during the G1 stage produced chromatid aberrations and gaps with the same frequency as FUdR, whereas the frequency of chromosome aberrations was much higher. When administered after irradiation, FUdR increased the frequency of all types of aberrations; the periods of mitotic cycle when this increase was statistically significant correspond to those of "mutagenic" action of FUdR mentioned above. This pattern may be easily explained if one postulates that in our experiments FUdR exhibited the features of a "pseudomutagen" i.e. the factor which suppresses repair of primary lesions (spontaneous or radiation-induced) without giving rise to new mutational changes.     

The fate of cells with chromosome aberrations after total-body irradiation and bone marrow transplantation

Cytogenetic studies were done on bone marrow cells and peripheral lymphocytes of four patients (three with acute nonlymphocytic leukemia, one with aplastic anemia) at various intervals up to 861 days after total-body X irradiation (TBI) at doses between 4.5 and 10 Gy (450-1000 rad) followed by syngeneic or allogeneic bone marrow transplantation. Whereas no radiation-induced aberrations could be found in the bone marrow, apart from a transient finding in the patient with the lowest radiation dose, aberrant metaphases were seen in the peripheral lymphocytes of three patients in the range from 2.5 to 46% even at 861 days after the exposure. There were no demonstrable aberrations related to TBI in the only patient developing graft-versus-host disease. The dicentric yield as determined in the aberrant metaphases with 46 centromeres ranged between 3.4 +/- 1.3 and 4.9 +/- 0.4. In one patient it was demonstrated by BUdR-labeling that after 10 Gy (1000 rad) TBI the surviving and heavily damaged lymphocytes can go into cell cycle and reach at least the third mitosis. The percentage of aberrant cells diminished by about 25% at each mitotic division.     

The effect of chemical radiation protectors on cell cycle progression after gamma or neutron irradiation

Abstract. The effects of two chemical radiation protectors, WR-1065 and WR-151326, were characterized in V79 Chinese hamster cells after either cobalt-60 (⁶⁰Co) gamma or fission spectrum neutron irradiation. Each protector was administered at a concentration of 4 mM to exponentially growing cultures for 30 min prior to and during irradiation with either ⁶⁰Co gamma or JANUS fission spectrum neutrons. After irradiation the cells were either plated immediately for survival or returned to the incubator and assayed for cell progression. Aliquots of cells were removed at selected times, counted, fixed and stained with 4'6-diamidino-2-phenylindole (DAPI). Analysis of DNA histograms indicate that the presence of the protector during irradiation reduced the division delay experienced at the G₂-M interface. Implications of these effects are discussed.     

Dose-response relationship of the yield of chromosome aberrations in a human lymphocyte culture following gamma-irradiation at high doses

A study was made of the yield of chromosome aberrations in a human lymphocyte culture at the G0 stage after 60Co-gamma-irradiation with doses of 5-12 Gy. It was shown that a linear-quadratic dependence of the aberration frequency observed with median doses became purely linear at high doses.     

Sodium fluoride-induced chromosome aberrations in different stages of the cell cycle: a proposed mechanism

In an attempt to clarify the controversy about sodium fluoride (NaF) clastogenicity, the induction of chromosome aberrations in Chinese hamster ovary cells (CHO) by NaF was investigated. Following a protocol used for screening chemicals for clastogenic activity, significant increases of aberrant cells were observed when cells were exposed to NaF for 4 h and harvested 8 h later. Cell-cycle kinetic studies demonstrated most cells were exposed in G2 of the cell cycle. Smaller increases in aberrant cells were observed when cells were harvested 20 h later (most cells were exposed in G1/S). The sensitivity of G2 cells to NaF was investigated further, along with the induction of aberrations at low doses. The results indicated that G2 cells are sensitive to NaF and the percent of aberrant cells increased with dose and length of exposure. With a 3-h exposure until harvest, no statistically significant increase in aberrant cells was observed at doses below 10 micrograms/ml NaF. These data are consistent with a threshold for NaF-induced clastogenicity around 10 micrograms/ml, as has been proposed previously (Scott and Roberts, 1987). It thus may be predicted that clastogenic effects would not occur in humans exposed to the levels of fluoride that are present in drinking water or dentifrices. An understanding of the mechanism of NaF-induced clastogenicity would help to clarify this point. It has previously been reported that NaF inhibits DNA synthesis/repair. The types of aberrations, mostly deletions and gaps, the induction of endoreduplicated cells, the cell-cycle delay and the sensitivity of G2 cells to NaF observed are similar to that reported in the literature for DNA synthesis/repair inhibitors like aphidicolin (APC). Similarities in the induction of aberrations by NaF and APC were confirmed in experiments with G2 cells. Based on these results and those previously reported for NaF and APC, it is proposed that NaF-induced aberrations may occur by an indirect mechanism involving the inhibition of DNA synthesis/repair.     

The cell cycle of spermatogonial colony forming stem cells in the CBA mouse after neutron irradiation

In the CBA mouse testis about 10% of the stem cell population is highly resistant to neutron irradiation (D0, 0.75 Gy). Following a dose of 1.50 Gy these cells rapidly increase their sensitivity towards a second neutron dose and progress fairly synchronously through their first post-irradiation cell cycle. From experiments in which neutron irradiation was combined with hydroxyurea it appeared that in this cycle the S-phase is less radiosensitive (D0, 0.43 Gy) than the other phases of the cell cycle (D0, 0.25 Gy). From experiments in which hydroxyurea was injected twice after irradiation the speed of inflow of cells in S and the duration of S and the cell cycle could be calculated. Between 32 and 36 hr after irradiation cells start to enter the S-phase at a speed of 30% of the population every 12 hr. At 60 hr 50% of the population has already passed the S-phase while 30% is still in S. The data point to a cell cycle time of about 36 hr, while the S-phase lasts 12 hr at the most.