A critical comparison of the micronucleus yield from high and low LET irradiation of plateau-phase cell populations

Radiation therapy induced micronuclei in cervical cancer—does it have a predictive value for local disease control?

OBJECTIVE

To evaluate the predictive value of serial changes in micronuclei induction during external radiotherapy (EXRT) in cervical cancer with respect to local response at the end of EXRT and local disease free survival (LDFS).

METHODS

Twenty-five patients of squamous cell cancer of the cervix were treated by 50 Gy of EXRT delivered over 5 weeks followed by intracavitary brachytherapy. Serial cytological smears were taken from cervical growth at weekly intervals during the course of EXRT and stained by Giemsa and May-Grunwald's stain. Micronuclei induction were scored as (a) number of cells expressing micronuclei (MN), and (b) total number of micronuclei (TMN) in 1000 tumor cells from each of the serial smears.

RESULTS

A significant rise in micronuclei count was seen for both MN and TMN from pretreatment (week 0) to successive weeks of EXRT. For those having a near total tumor regression by end of EXRT, a significant rise in micronuclei was evident even at the end of first week of EXRT (MN: P = 0.05, TMN: P = 0.04). A superior LDFS was observed in patients showing greater than 50% increment in MN value in the first week (median survival for <50% vs. > or =50% rise: 5 months vs. not reached, P = 0.21), while it reached significance for a similar rise of TMN (median survival <50% vs. > or =50% rise: 5 months vs. not reached, P = 0.04).

CONCLUSIONS

The significant rise of micronuclei at the end of first week of EXRT in cervical cancers as observed from serial cytological smears could predict for a better local response and LDFS.

Diesel exhaust particulate matter dispersed in a phospholipid surfactant induces chromosomal aberrations and micronuclei but not 6-thioguanine-resistant gene mutation in V79 cells

Diesel exhaust particulate material (DPM) was assayed for induction of chromosomal aberrations (CA), micronucleus (MN) formation, and 6-thioguanine-resistant (TG9 gene mutation in V79 cells as a dispersion in dipalmitoyl phosphatidylcholine (DPPC) in physiological saline, a simulated pulmonary surfactant. Filter-collected automobile DPM provided for the study was not organic solvent extracted, but was directly mixed into DPPC in saline dispersion as a model of pulmonary surfactant conditioning of a soot particle depositing in a lung alveolus. A statistically significant difference was found between treated and control groups at all concentrations tested in a CA assay. Assay for MN induction also gave a positive response: Above 50 microg/ml, the frequencies of micronucleated cells (MNC) were about 2 times higher than those in the control group. The forward gene mutation assay did not show a positive response when cells were treated with up to 136 microg DPM/ml for 24 h, as dispersion in DPPC in saline. Some comparison assays were run on direct dispersions of the DPM into dimethyl sulfoxide, with results equivalent to those seen with a DPPC-saline preparation: DPM in dimethyl sulfoxide (DMSO) was positive for MN induction but was negative for forward gene mutation in V79 cells. The positive clastogenicity results are consistent with other studies of DPM dispersed into DPPC-saline surfactant that have shown activity in mammalian cells for sister chromatid exchange, unscheduled DNA synthesis, and MN induction. The forward gene mutation negative results are consistent with studies of that assay applied to V79 cells challenged with DPM solvent extract.

Response of quiescent and total tumor cells in solid tumors to neutrons with various cadmium ratios

PURPOSE

Response of quiescent (Q) and total tumor cells in solid tumors to neutron irradiation with three different cadmium (Cd) ratios was examined. The role of Q cells in tumor control was also discussed.

METHODS AND MATERIALS

C3H/He mice bearing SCC VII tumors received continuous administration of 5-bromo-2'-deoxyuridine (BrdU) for 5 days using implanted mini-osmotic pumps to label all proliferating (P) cells. Thirty minutes after intraperitoneal injection of sodium borocaptate-10B (BSH), or 3 h after oral administration of dl-p-boronophenylalanine-10B (BPA), the tumors were irradiated with neutrons, or those without 10B-compounds were irradiated with gamma rays. This neutron irradiation was performed using neutrons with three different cadmium (Cd) ratios. The tumors were then excised, minced, and trypsinized. The tumor cell suspensions were incubated with cytochalasin-B (a cytokinesis-blocker), and the micronucleus (MN) frequency in cells without BrdU labeling (Q cells) was determined using immunofluorescence staining for BrdU. The MN frequency in total (P + Q) tumor cells was determined from tumors that were not pretreated with BrdU. The sensitivity to neutrons was evaluated in terms of the frequency of induced micronuclei in binuclear tumor cells (MN frequency).

RESULTS

Without 10B-compounds, the MN frequency in Q cells was lower than that in the total cell population. The sensitivity difference between total and Q cells was reduced by neutron irradiation. Relative biological effectiveness (RBE) of neutrons compared with gamma rays was larger in Q cells than in total cells, and the RBE values for low-Cd-ratio neutrons tended to be larger than those for high-Cd-ratio neutrons. With 10B-compounds, MN frequency for each cell population was increased, especially for total cells. This increase in MN frequency was marked when high-Cd-ratio neutrons were used. BPA increased the MN frequency for total tumor cells more than BSH. Nevertheless, the sensitivity of Q cells treated with BPA was lower than that in BSH-treated Q cells. This tendency was clearly observed in high-Cd-ratio neutrons.

CONCLUSION

From the viewpoint of enhancing the Q-cell sensitivity, tumors should be irradiated with high-Cd-ratio neutrons after BSH administration. However, normal tissue reaction remains to be examined because of its low tumor-to-normal tissue and tumor-to-blood biodistribution ratios.

Inhibition of methotrexate-induced chromosomal damage by folinic acid in V79 cells

Methotrexate (MTX), an anticancer compound, is widely used in the treatment of leukemia. It induces cytogenetic damage as well as cytostatic effects on a variety of cell systems. Folinic acid (Leucovorin) is generally administered along with MTX as a rescue agent to decrease MTX-induced toxicity. However, information regarding the inhibitory effect of folinic acid against cytogenetic damage caused by MTX is limited. This study was conducted to assess the cytogenetic effect of MTX and its inhibition by folinic acid (FA) using the micronucleus and chromosomal aberration assays concurrently. Exponentially growing V79 cells were treated with MTX at five different concentrations (5-100 micrograms ml-1) with S9 microsomal fraction for 6 h and post-treated with two concentrations of FA (5 or 50 micrograms) for 40 h. Results indicate that MTX alone induced a concentration-related increase in % micronucleated binucleated cells (MNBN) and % aberrant cells (Abs). There was a decrease in nuclear division index (NDI) with increase in MTX concentration. Similarly, the mitotic index (MI) also decreased in all concentrations of MTX tested. The addition of FA at 50 micrograms ml-1 significantly reduced % MNBN (40-68%) and % Abs (36-77%). Inhibition was also seen at 5 micrograms FA (12 to 54% for MNBN and 20 to 61% for Abs). These results indicate that FA is capable of reducing the cytogenetic damage induced by MTX and appears to be an anticlastogenic agent.

Protective effect of vanillin on radiation-induced micronuclei and chromosomal aberrations in V79 cells

Vanillin (VA), an anticlastogen, has been demonstrated to inhibit gene mutations in both bacterial and mammalian cells. However, the data on its effect against radiation-induced cytogenetic damage are limited. The aim of this study was to investigate the protective effect of VA on radiation-induced chromosomal damage in V79 cells. Exponentially growing cells were exposed to five doses of X-rays (1-12 Gy) and UV radiation (50-800 microJ x 10(2) cm-2 and posttreated with 3 concentrations of VA (5, 50 or 100 micrograms ml-1 for 16 h for micronucleus (MN) and 18 h for structural chromosomal aberration (SCA) analyses. MN and SCA assays were performed concurrently according to standard procedures. Results indicate that there was a dose related increase in the percent of micronucleated binucleated cells (MNBN) (5.6 to 79.6) and percent of aberrant cells (Abs) (12 to 98) with X-ray treatment alone. Inhibition studies showed that the addition of VA at 100 micrograms ml-1 significantly reduced the percent of MNBN (21 to 48) induced by X-ray at 1, 2, and 4 Gy. There was a slight decrease in percent MNBN at 5 and 50 micrograms VA ml-1. All three concentrations of VA decreased percent Abs (15.7 to 57.1) induced by X-rays at all doses. UV radiation alone significantly increased percent MNBN (3.5 to 14.8) and percent Abs (17 to 29). Addition of 50 or 100 micrograms VA ml-1, significantly decreased percent MNBN (31.7 to 86.2) and percent Abs (54.5 to 90.9) at all doses of UV radiation. A decrease in percent MNBN (2.8 to 72.4) and percent Abs (34.8 to 66.7) was also noted at 5 micrograms VA ml-1. These data clearly indicate the protective effect of VA on radiation-induced chromosomal damage, suggesting that VA is an anticlastogenic agent.

Modification of the response of a quiescent cell population within a murine solid tumour to boron neutron capture irradiation: studies with nicotinamide and hyperthermia

C3H/He mice bearing SCC VII tumours received 5-bromo-2'-deoxyuridine (BrdU) continuously for 5 days via implanted mini-osmotic pumps, to label all proliferating (P) cells. 20 min after intraperitoneal injection of sodium borocaptate-10B (BSH), or 3 h after oral administration of dl-p-boronophenylalanine-10B (BPA), the tumours were irradiated with thermal neutrons. To modify the uptake dose of 10B, nicotinamide (NA) was intraperitoneally injected 60 min before the administration of 10B-compounds and/or the tumours were heated to 41.5 degrees C for 20 min immediately before irradiation. After irradiation, the tumours were excised, minced and trypsinized. The tumour cell suspensions were then incubated with cytochalasin-B (a cytokinesis-blocker). The micronucleus (MN) frequency in cells not BrdU-labelled (quiescent (Q) cells) was determined using immunofluorescence staining for BrdU. With or without the administration of 10B-compounds, the sensitivity of Q cells was lower than that of total (P + Q) tumour cells. With thermal neutron irradiation in the presence of either BPA or BSH, the MN frequency in each cell population was increased. A greater increase in the MN frequency of total tumour cells was observed after thermal neutron irradiation in the presence of BPA than in the presence of BSH. The distribution of 10B from BPA into tumour cells was thought to be more dependent on the uptake ability of the tumour cells than that from BSH. Sufficient quantity of 10B from these two 10B-compounds to cause a highly lethal event inside the cancer cell with thermal neutron irradiation could not be delivered to Q cells. When NA and/or heat treatment were combined with 10B-compound administration, NA increased MN frequency in the BSH treated total cells, and heat treatment elevated MN frequency in Q cells. From the viewpoint of cell kill effect, the combined treatment with nicotinamide and heat treatment was more useful than treatment with either nicotinamide or heat treatment alone, not only in the total tumour cells but also in the Q cells.

Serial cytological assay of micronucleus induction: a new tool to predict human cancer radiosensitivity

BACKGROUND AND PURPOSE

The micronucleus test, generally done in cultured tumour cells irradiated in vitro, has not gained wide acceptance in predicting human cancer radiosensitivity. The purpose of this study was to see if micronucleus assay by serial scrape smear cytology can predict oral cancer radiosensitivity.

MATERIALS AND METHODS

Forty nine oral cancer patients given radiotherapy (60 Gy/25 fractions/5 weeks) form the study population. Serial scrape smears were taken from their tumours before treatment and after delivery of 2, 5, 8 and 12 fractions, stained by Giemsa and the number of micronucleated cells (MNC) noted. The patients were grouped to those who developed tumour recurrence ('Resistant') and those who did not ('Sensitive'), and the pattern of micronucleus induction compared.

RESULTS

Both groups of tumours had MNC even before treatment, with statistically significant dose-related increase with radiotherapy. The sensitive group had a higher mean increase in MNC count than the resistant group (6.1 times and 3.6 times the pre-treatment value, respectively) and better correlation with dose (r = 0.54 vs. 0.43). The increase in MNC count occurred earlier in the resistant group than in the sensitive, the TMNC (time for the pre-treatment value to double) being 3.3 days and 7.6 days, respectively. Also, the resistant group showed a plateauing of the MNC count which the sensitive group lacked.

CONCLUSIONS

The higher MNC induction in the sensitive tumours suggests the usefulness of the assay as a test of radiosensitivity. The differing patterns of MNC increase suggest that differences in proliferation rate is an important cause of tumour failure. Serial cytological assay of micronucleus induction can identify both radiosensitivity and proliferation characteristics of tumours, and thus may turn out to be a useful test of radiocurability.

Inhibitory effect of folinic acid on radiation-induced micronuclei and chromosomal aberrations in V79 cells

Folinic acid (FA), clinically called leucovorin, has been widely used as a nutrient supplement in dietary intake and is capable of inhibiting cytotoxicity and chromosomal damage induced by chemicals. However, data on its antigenotoxic effect on radiation-induced chromosomal damage are limited. The present study was, therefore, performed to investigate the effect of FA on radiation-induced (X-rays and UV radiation) micronuclei (MN) and structural chromosomal aberrations (SCA) concurrently in V79 Chinese hamster lung cells. Exponentially growing cells were exposed to five doses of X-rays (1-12 Gy) and UV radiation (50-800 microJ x 10(2)/cm2) and post-treated with 5 or 50 micrograms FA/ml of culture medium for 16 h. The slides were analyzed for the presence of MN and SCA using standard procedures. The results showed that X-ray treatment alone produced dose-related cytotoxicity as measured by nuclear division index (NDI) and mitotic index (MI). X-rays produced a clear dose-related clastogenicity as measured by percent of micronucleated binucleated cells (MNBN) (5-79%) and percent of aberrant cells (11-92%). FA at 5 micrograms/ml slightly decreased X-ray induced chromosomal damage in both assays; however, the inhibition was significant (12-46% of MNBN, 14-48% in aberrant cells) only when X-ray-treated cultures were post-treated with 50 micrograms FA/ml. Post-treatment of FA had no effect on X-ray induced cytotoxicity as measured by NDI and MI. A similar a dose-related increase in % MNBN (0.5-10.3%) and percent aberrant cells (6-35%) was produced by UV radiation treatment alone. There were significant percentages of MNBN and aberrant cell inhibitions at both 5 and 50 micrograms/ml in both assays. As in the case of X-ray-treated cells, there was a clear dose-related cytotoxicity in UV-treated cells alone. No reduction in NDI or MI was found when UV-exposed cells were post-treated with 5 or 50 micrograms of FA. These data demonstrate the beneficial effect of FA in decreasing radiation-induced chromosomal damage.

Micronucleus induction and reproductive death in a human cell line exposed to low-energy argon beam

Little is known about the action of charged particles of very high linear energy transfer (LET) on human cells and, in particular, the relationship between DNA damage and reproductive death. The aim of this study was to measure the biological efficiency of a low-energy argon beam (E = 7.1 MeV/nucleon, LET = 1590 keV/micron) produced at GSI, Darmstadt, on a human melanoma cell line (CAL4) established in our Institute. Two different methods were used: the micronucleus (MN) test and the colony-forming assay. The MN test, using the cytochalasin-block method, is a measure of genotoxic damage. MN are scored in binucleate cells (BNC) and are formed from acentric fragments or whole chromosomes that have not been incorporated into daughter nuclei at mitosis. The colony-forming assay quantifies reproductive death. Parallel experiments were run with cobalt gamma-rays for comparison. After Co irradiation, the MN-free BNC dose-response curve coincided with that of the loss of colony-forming ability, suggesting the potential of the former as a predictive test of cell killing. After Ar irradiation, there was a dissociation between the two effects, especially at high doses: cell death was greater than the frequency of BNC with MN. The inactivation cross-section was 74 microns2; it was 39 microns2 for MN yield. Therefore, the relative biological effectiveness (RBE) was higher for cell killing than for MN yield (0.8 and 0.5, respectively at a Co dose of 3 Gy). The total MN count in BNC followed the same pattern of response as the fraction of BNC with MN.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative studies on radiation-induced micronuclei and chromosomal aberrations in V79 cells

Induction of micronuclei (MN) and structural chromosomal aberrations (SCA) by physical agents such as X-rays and UV radiation has been studied extensively in a variety of cell lines for genotoxicity assessment. However, comparative data on the relationship between these two cytogenetic endpoints are limited. This study compares SCA and MN formation in V79 Chinese hamster lung cells treated with X-rays and UV radiation. Four replicate cultures of exponentially growing cells were exposed to four doses of X-rays (100-800 rads). For two replicate cultures, cytochalasin B (3 micrograms/ml) was added and cells were harvested 16 h later for MN and nuclear division index (NDI) assessment. For the remaining two replicate cultures, colcemid (0.025 micrograms/ml) was added 16 h post-treatment and cells were harvested 2 h later for SCA and mitotic index (MI) analyses. This experiment was duplicated using four doses of UV radiation (100-800 microJ x 10(2)/cm2). In the X-ray experiment, generally, a decrease in the NDI and MI was noted with increasing dose. Also, there was a clear dose-related increase in percent micronucleated binucleated (MNBN) and aberrant cells. A similar dose response, but with lower frequencies, was observed in the UV radiation treatment. These data suggest a good correlation between chromosome damage as measured by percent MNBN and aberrant cells and cytotoxicity as measured by NDI and MI.

Differences in radiation-induced micronuclei yields of human cells: influence of ras gene expression and protein localization

Expression of ras has been correlated with increased intrinsic resistance to ionizing radiation. In this study we show that increased EJras expression in human cells is associated with a decrease in the frequency of radiation-induced micronuclei. The experimental system consisted of human osteosarcoma-derived cell lines which quantitatively vary in their EJras expression. There was a dose-dependent relationship between radiation dose and micronuclei formation in all cell lines tested. Human osteosarcoma cells, in which the ras level was undetectable, had the highest frequency of micronuclei production at all radiation doses tested. At 4 Gy the most radioresistant cells exhibited a 41.5 +/- 5% decrease in the production of micronuclei concomitant with high ras expression in comparison with the relatively radiosensitive parental cell line. Cells expressing a low amount of EJras demonstrated a 23 +/- 3% decrease in micronuclei induction compared with parental cells. Treatment of cells with lovastatin, an inhibitor of ras-encoded p21ras post-translational processing via the mevalonate pathway, markedly decreased the yield of micronuclei formation in cells transfected with ras; the drug had no effect on radiation-induced micronuclei formation in parental cells. The use of the in vitro micronuclei assay has provided a convenient way to visualize differences in the genotoxic damage induced by ionizing radiation in cells which express different amount of EJras. The results indicate that elevation of ras expression in human cells can lead to a decrease in the number of radiation-induced micronuclei formed and that this relationship is dependent on membrane association of ras-encoded p21.

Simultaneous analysis of chromosome damage and aneuploidy in cytokinesis-blocked V79 Chinese hamster lung cells using an antikinetochore antibody

A modified antikinetochore antibody technique was established in the V79 Chinese hamster lung cells to simultaneously analyze chromosome damage and aneuploidy induced by various agents. The method involved sequential treatment of slides with crest serum, fluoresceinated goat-antihuman and swine-antigoat antibodies, and propidium iodide. In this method, cytoplasm (green), nuclei or micronuclei (red), and kinetochores (yellow), are identified using the same filter setting under blue excitation (440-490 nm) with a barrier filter at 520 nm. Using this method, three agents, vinblastine (VB), X-rays, and methyl methanesulfonate (MMS) were tested for micronucleus/aneuploidy induction. An aneugen, VB and a clastogen, X-rays, induced predominantly kinetochore positive (K+) and negative (K-) micronucleated binucleate (MNBN) cells, respectively, in a dose-dependent fashion. An alkylating agent, MMS, produced both K+ and K- MNBN cells. These results are comparable with the results reported in the literature on these compounds using various methods and thus demonstrate the usefulness of this assay in distinguishing clastogenicity from aneugenicity.

Simultaneous micronucleus and chromosome aberration assessment in the rat

Using a cellulose column fractionation procedure to eliminate nucleated cells for micronucleus assessment, micronucleus and chromosome aberration endpoints in the same animal were compared in male and female rats following i.p. injection with cyclophosphamide (CP). Groups of 5 Wistar rats per sex were given single doses of CP at 0, 20, or 40 mg/kg. Two hours prior to sacrifice, animals were given colchicine (4 mg/kg) to arrest cells in metaphase. One femur from each animal was used for micronucleus assessment and the other for chromosome aberration assessment. In the micronucleus assessment, 2000 polychromatic erythrocytes (PCEs) per animal and in the chromosome aberration assessment, 50 metaphase cells per animal were scored. This experiment was repeated once. In both experiments, significant increases in micronucleated PCEs and chromosome aberrations were noted at both doses of CP in both sexes. In general, the clastogenic effects of CP were more pronounced in males than females. Both doses of CP caused a decrease in the proportion of PCEs and in mitotic index in both experiments, indicating toxicity of CP to the bone marrow. These results show the usefulness of this rat model for simultaneous evaluation of two cytogenetic endpoints in the same animal and indicate that assessment of MNPCE frequency in the bone marrow of male rats may be an appropriate model for screening test substances for in vivo clastogenic activity in this species.

Use of the micronucleus assay for the selective detection of radiosensitivity in BUdR-unincorporated cells after pulse-labelling of exponentially growing tumour cells

To determine the radiosensitivity of non-S-phase tumour cells in vitro, survival curves of SCC VII tumour cells were obtained after a short block with hydroxyurea. Dose-response curves of micronucleus (MN) frequency appearing in non-S-phase cells were also determined by excluding S-phase cells with immunofluorescence staining to 5-bromo-2'-deoxyuridine (BUdR). Both the dose-response curves of MN frequency and survival curves were analysed by a linear-quadratic model (surviving fraction = exp (-alpha D-beta D2), MN frequency = aD+bD2+c). A good correlation between the alpha/beta and alpha/beta ratios was observed. In both BUdR-unincorporated and asynchronous cell cultures, the regression lines between the surviving fraction and micronucleus frequency were statistically identical. Therefore, it was shown that cell survival curves, which cannot be obtained directly by the routine colony-formation technique, can be calculated using the micronucleus frequency and the regression line in asynchronous cell cultures. Therefore, it should be possible to detect the response to irradiation of quiescent cells in tumours using the immunofluorescence staining to BUdR and the MN frequency assay.

Micronucleus formation in 2-cell embryos after in vitro X-irradiation of mouse spermatozoa

Mouse spermatozoa were exposed in vitro to various X-ray doses and added to a medium containing superovulated oocytes. The percentage of fertilized oocytes was determined 24 h after sperm addition and 2-cell embryos were investigated for micronucleus formation. The fertilization rate was drastically decreased after exposure to 470 cGy whereas smaller doses had no effect. The dose-response relationship for micronucleus formation per embryo was linear-quadratic and the number of embryos with micronuclei increased linearly with dose. The distribution of micronuclei among 2-cell embryos showed a significant overdispersion relative to the Poisson distribution after sperm irradiation for doses above 188 cGy.

Use of the cytokinesis-block method for the analysis of micronuclei in V79 Chinese hamster lung cells: results with mitomycin C and cyclophosphamide

The cytochalasin B (CYB)-blocked binucleated cell assay has been explored to analyze micronuclei and cell cycle kinetics using 2 known mutagenic carcinogens in V79 Chinese hamster lung cells. To determine the optimum time to obtain the maximum number of binucleated cells for micronucleus analysis, duplicate cultures of exponentially growing cells were treated with 3 micrograms/ml CYB for varying durations (8-48 h). A peak appearance of binucleated cells at 16 h in the presence of CYB suggested this as an optimum time for micronucleus analysis in binucleated V79 cells. To evaluate the capacity for induction of micronuclei in V79 cells, 2 mutagenic carcinogens, mitomycin C (0.125-1.0 micrograms/ml) and cyclophosphamide (2-12 micrograms/ml) were tested in duplicate cultures. Mitomycin C, a direct-acting alkylating agent, caused approximately an 18-fold increase in micronucleus frequency over controls at the highest concentration tested (1.0 micrograms/ml), and this increase occurred in a dose-related manner (r = 0.92). The concentrations of mitomycin C tested also caused a significant dose-related cell cycle delay, thus suggesting cytotoxicity to V79 cells. Cyclophosphamide, an indirect-acting alkylating agent, requiring the presence of S9 mix, caused approximately a 17-fold increase in micronucleus frequency over controls at the highest tested concentration (12 micrograms/ml), with a clear dose response (r = 0.99). The various concentrations of cyclophosphamide also caused cytotoxicity in a dose-related fashion. Thus, this study demonstrates the usefulness of the cytokinesis-block method in V79 cells as a possible screen to analyze micronucleus induction and cytotoxicity. Because this approach is much less labor intensive than conducting a structural chromosomal analysis, this assay has great potential both as an initial screen for clastogenic activity and as a tool for investigating the underlying mechanisms for clastogenicity.

The one-cell mouse embryo: cell cycle-dependent radiosensitivity and development of chromosomal anomalies in postradiation cell cycles

One-cell mouse embryos were irradiated with X-rays at different cell cycle stages. Examination of structural chromosomal anomalies and of micronucleus formation in postradiation mitoses and interphases demonstrated cell cycle-dependent radiosensitivities in the order: late G2 phase greater than G1 phase greater than S phase greater than early G2 phase greater than stage of decondensing nuclei. Comparison of the quality and quantity of chromosomal aberrations from the first to the third mitosis led to the conclusion that new chromosomal anomalies were formed in the course of postirradiation cell cycles. This hypothesis was supported by an increasing number of micronuclei from 24 to 48 h post-conception. In addition to structural chromosomal aberrations, radiation-induced chromosome loss was observed with a frequency that was obviously independent of the exposed cell cycle phase. Loss of acentric chromosome fragments and of single chromosomes contributed to the micronucleus formation.

Analysis of structural and numerical chromosomal anomalies at the first, second, and third mitosis after irradiation of one-cell mouse embryos with X-rays or neutrons

One-cell mouse embryos were irradiated with X-rays or neutrons. Analysis of the first, second, and third postradiation mitoses revealed that the yields of structural aberrations increased linearly after exposure to both radiation qualities. For X-rays the aberration frequency decreased from the first to the third mitosis, whereas after neutrons it decreased from the first to the second mitosis but then increased in the third mitosis. RBEs of 4.7, 4.8, and 7.4 were calculated for the corresponding mitoses. It was clearly demonstrated that new aberrations were produced after the first postradiation mitosis and expressed during the second and third mitosis. Chromosome loss also increased with increasing radiation dose at the second mitosis. An RBE of 2 was calculated for this effect. Comparing the presented data with previous investigations on embryonic and fetal death after prenatal irradiation, it was concluded that the high radiosensitivity of the one-cell embryo is due to the induction of structural as well as of numerical chromosome aberrations.