The time-course of micronucleated polychromatic erythrocytes in mouse bone marrow and peripheral blood

The 28 + 28 day design is an effective sampling time for analyzing mutant frequencies in rapidly proliferating tissues of MutaMouse animals

The Organisation for Economic Co-Operation and Development Test Guideline 488 (TG 488) uses transgenic rodent models to generate in vivo mutagenesis data for regulatory submission. The recommended design in TG 488, 28 consecutive daily exposures with tissue sampling three days later (28 + 3d), is optimized for rapidly proliferating tissues such as bone marrow (BM). A sampling time of 28 days (28 + 28d) is considered more appropriate for slowly proliferating tissues (e.g., liver) and male germ cells. We evaluated the impact of the sampling time on mutant frequencies (MF) in the BM of MutaMouse males exposed for 28 days to benzo[a]pyrene (BaP), procarbazine (PRC), isopropyl methanesulfonate (iPMS), or triethylenemelamine (TEM) in dose-response studies. BM samples were collected + 3d, + 28d, + 42d or + 70d post exposure and MF quantified using the lacZ assay. All chemicals significantly increased MF with maximum fold increases at 28 + 3d of 162.9, 6.6, 4.7 and 2.8 for BaP, PRC, iPMS and TEM, respectively. MF were relatively stable over the time period investigated, although they were significantly increased only at 28 + 3d and 28 + 28d for TEM. Benchmark dose (BMD) modelling generated overlapping BMD confidence intervals among the four sampling times for each chemical. These results demonstrate that the sampling time does not affect the detection of mutations for strong mutagens. However, for mutagens that produce small increases in MF, sampling times greater than 28 days may produce false-negative results. Thus, the 28 + 28d protocol represents a unifying protocol for simultaneously assessing mutations in rapidly and slowly proliferating somatic tissues and male germ cells.

Measurement of micronuclei and internal dose in mice demonstrates that 3-monochloropropane-1,2-diol (3-MCPD) has no genotoxic potency in vivo

In this study 3-monochloropropane-1,2-diol (3-MCPD), a compound that appears as contaminant in refined cooking oils, has been studied with regard to genotoxicity in vivo (mice) with simultaneous measurement of internal dose using state-of-the-art methodologies. Genotoxicity (chromosomal aberrations) was measured by flow cytometry with dual lasers as the frequency of micronuclei in erythrocytes in peripheral blood from BalbC mice intraperitoneally exposed to 3-MCPD (0, 50, 75, 100, 125 mg/kg). The internal doses of 3-MCPD in the mice were calculated from N-(2,3-dihydroxypropyl)-valine adducts to hemoglobin (Hb), quantified at very low levels by high-resolution mass spectrometry. Convincing evidence for absence of genotoxic potency in correlation to measured internal doses in the mice was demonstrated, despite relatively high administered doses of 3-MCPD. The results are discussed in relation to another food contaminant that is formed as ester in parallel to 3-MCPD esters in oil processing, i.e. glycidol, which has been studied previously by us in a similar experimental setup. Glycidol has been shown to be genotoxic, and in addition to have ca. 1000 times higher rate of adduct formation compared to that observed for 3-MCPD. The conclusion is that at simultaneous exposure to 3-MCPD and glycidol the concern about genotoxicity would be glycidol.

Dose and Radioadaptive Response Analysis of Micronucleus Induction in Mouse Bone Marrow

Enhanced cellular DNA repair efficiency and suppression of genomic instability have been proposed as mechanisms underlying radio-adaptive responses following low-dose radiation exposures. We previously showed that low-dose γ irradiation does not generate radio-adaptation by lowering radiation-induced cytogenetic damage in mouse spleen. Since radiation may exert tissue-specific effects, we extended these results here by examining the effects of γ radiation on cytogenetic damage and proliferative index in bone marrow erythrocytes of C57BL/6 and BALB/c mice. In C57BL/6 mice, the induction of micronuclei in polychromatic erythrocytes (MN-PCE) was observed at radiation doses of 100 mGy and greater, and suppression of erythroblast maturation occurred at doses of >500 mGy. A linear dose-response relationship for MN-PCE frequencies in C57BL/6 mice was established for radiation doses between 100 mGy and 1 Gy, with departure from linearity at doses of >1 Gy. BALB/c mice exhibited increased MN-PCE frequencies above baseline following a 20 mGy radiation exposure but did not exhibit radio-sensitivity relative to C57BL/6 mice following 2 Gy exposure. Radio-adaptation of bone marrow erythrocytes was not observed in either strain of mice exposed to low-dose priming γ irradiation (single doses of 20 mGy or 100 mGy or multiple 20 mGy doses) administered at various times prior to acute 2 Gy irradiation, confirming the lack of radio-adaptive response for induction of cytogenetic damage or suppression or erythrocyte proliferation/maturation in bone marrow of these mouse strains.

Fluoranthene and phenantrene, two predominant PAHs in heat-prepared food, do not influence the frequency of micronucleated mouse erythrocytes induced by other PAHs

Polycyclic aromatic hydrocarbons (PAHs) occur in complex mixtures present in the human environment. Because of the carcinogenic properties of some of these PAHs, they raise concerns regarding health and food safety. Because the occurrence of benzo[a]pyrene, chrysene, benz[a]anthracene, and benzo[b]fluoranthene (PAH4) are considered markers for other genotoxic PAHs in foodstuffs, the European Union has put a maximum level of PAH4 in some foodstuffs. Fluoranthene (Flu) and phenanthrene (Phe), two other PAHs, are not classified as genotoxic and are abundant at rather high concentrations in food. Inasmuch as PAH4, Flu, and Phe are metabolized by the same cytochrome P450 pathway system, it is important to clarify whether Phe and Flu influence the genotoxicity of PAH4. We have analyzed the genotoxic response of Phe and Flu, separately and together, as well as in combination with different low doses of PAH4. In all experiments we used the flow cytometer-based micronucleus test in vivo. Phe and Flu, when administered separately, did not show any dose-related effect on the frequency of micronucleated polychromatic erythrocytes (fMNPCE). Nor did a mixture of Phe and Flu change the fMNPCEs. Phe and Flu did not significantly change the fMNPCE of PAH4-exposed FVB and BALB/c mice.

Integrated molecular analysis indicates undetectable change in DNA damage in mice after continuous irradiation at ~ 400-fold natural background radiation

BACKGROUND

In the event of a nuclear accident, people are exposed to elevated levels of continuous low dose-rate radiation. Nevertheless, most of the literature describes the biological effects of acute radiation.

OBJECTIVES

DNA damage and mutations are well established for their carcinogenic effects. We assessed several key markers of DNA damage and DNA damage responses in mice exposed to low dose-rate radiation to reveal potential genotoxic effects associated with low dose-rate radiation.

METHODS

We studied low dose-rate radiation using a variable low dose-rate irradiator consisting of flood phantoms filled with 125Iodine-containing buffer. Mice were exposed to 0.0002 cGy/min (~ 400-fold background radiation) continuously over 5 weeks. We assessed base lesions, micronuclei, homologous recombination (HR; using fluorescent yellow direct repeat mice), and transcript levels for several radiation-sensitive genes.

RESULTS

We did not observe any changes in the levels of the DNA nucleobase damage products hypoxanthine, 8-oxo-7,8-dihydroguanine, 1,N6-ethenoadenine, or 3,N4-ethenocytosine above background levels under low dose-rate conditions. The micronucleus assay revealed no evidence that low dose-rate radiation induced DNA fragmentation, and there was no evidence of double strand break-induced HR. Furthermore, low dose-rate radiation did not induce Cdkn1a, Gadd45a, Mdm2, Atm, or Dbd2. Importantly, the same total dose, when delivered acutely, induced micronuclei and transcriptional responses.

CONCLUSIONS

These results demonstrate in an in vivo animal model that lowering the dose-rate suppresses the potentially deleterious impact of radiation and calls attention to the need for a deeper understanding of the biological impact of low dose-rate radiation.

Sublethal radiation injury uncovers a functional transition during erythroid maturation

OBJECTIVE

Clastogenic injury of the erythroid lineage results in anemia, reticulocytopenia, and transient appearance of micronucleated reticulocytes. However, the micronucleated reticulocyte dose-response in murine models is only linear to 2 Gy total body irradiation and paradoxically decreases at higher exposures, suggesting complex radiation effects on erythroid intermediates. To better understand this phenomenon, we investigated the kinetics and apoptotic response of the erythron to sublethal radiation injury.

MATERIALS AND METHODS

We analyzed the response to 1 and 4 Gy total body irradiation of erythroid progenitors and precursors using colony assays and imaging flow cytometry, respectively. We also investigated cell cycling and apoptotic gene expression of the steady-state erythron.

RESULTS

After 1 Gy total body irradiation, erythroid progenitors and precursors were partially depleted. In contrast, essentially all bone marrow erythroid progenitors and precursors were lost within 2 days after 4 Gy irradiation. Imaging flow cytometry analysis revealed preferential loss of phenotypic erythroid colony-forming units and proerythroblasts immediately after sublethal irradiation. Furthermore, these populations underwent radiation-induced apoptosis, without changes in steady-state cellular proliferation, at much higher frequencies than later-stage erythroid precursors. Primary erythroid precursor maturation is associated with marked Bcl-xL upregulation and Bax and Bid downregulation.

CONCLUSIONS

Micronucleated reticulocyte loss after higher sublethal radiation exposures results from rapid depletion of erythroid progenitors and precursors. This injury reveals that erythroid colony-forming units and proerythroblasts constitute a particularly proapoptotic compartment within the erythron. We conclude that the functional transition of primary proerythroblasts to later-stage erythroid precursors is characterized by a shift from a proapoptotic to an antiapoptotic phenotype.

Time-course of micronucleated erythrocytes in response to whole-body gamma irradiation in a model mammalian species, the bank vole (Clethrionomys glareolus, Schreber)

The time course of the formation of micronucleated polychromatic (MNPCEs) and normochromatic erythrocytes (MNNCEs) in the bone marrow of the bank vole (Clethrionomys glareolus, Schreber), a model mouse-like species, was studied using the standard micronucleus test at 0, 6, 12, 18, 24, 30, 36 and 48 hr following whole-body acute γ-irradiation at a dose of 0.5 Gy. Based on the existing literature on laboratory mice, it was suggested that such a dose will not have significant effect on erythroid cell proliferation in the bank vole and hence on the time course of the rise of micronucleated cells. In total, ∼905,000 polychromatic (PCEs) and normochromatic erythrocytes (NCEs) from 82 adult bank voles were analyzed. Although the mean frequencies of MNNCEs were too low to allow for the correct assessment of their time course, an analysis of PCEs showed an increasing rate of MNPCE appearance at 6 hr that reached a maximum at 18-24 hr after irradiation and subsequently decreased. Because the kinetics of MNPCEs reflects the process of erythropoiesis, the current results regarding the time points of appearance of radiation-induced MNPCEs provide the first information on the prolongation of one of the terminal stages of erythrocyte formation in bank vole specimens, namely the stage of maturation of PCEs from erythroblasts. Moreover, the observed time-course data, as well as the low-background frequencies of MNPCEs and characteristic level of PCEs response to radiation, showed similarities between the two model species: bank vole (this study) and laboratory mice (literature data).

Response kinetics of radiation-induced micronucleated reticulocytes in human bone marrow culture

The frequency of micronucleated reticulocytes (MN-RETs) in the bone marrow or peripheral blood is a sensitive indicator of cytogenetic damage. While the kinetics of MN-RET induction in rodent models following irradiation has been investigated and reported, information about MN-RET induction of human bone marrow after radiation exposure is sparse. In this report, we describe a human long-term bone marrow culture (LTBMC), established in three-dimensional (3D) bioreactors, which sustains long-term erythropoiesis. Using this system, we measured the kinetics of human bone marrow red blood cell (RBC) and reticulocyte (RET) production, as well as the kinetics of human MN-RET induction following radiation exposure up to 6Gy. Human bone marrow established in the 3D bioreactor demonstrated an average percentage of RBCs among total viable cells peaking at 21% on day 21. The average percentage of RETs among total viable cells reached a maximum of 11% on day 14, and remained above 5% by day 28, suggesting that terminal erythroid differentiation was still active. Time- and dose-dependent induction of MN-RET by gamma radiation was observed in the human 3D LTBMC, with peak values occurring at approximately 3 days following 1Gy irradiation. A trend towards delayed peak to 3-5 days post-radiation was observed with radiation doses ≥2Gy. Our data reveal valuable information on the kinetics of radiation-induced MN-RET of human bone marrow cultured in the 3D bioreactor, a synthetic bioculture system, and suggest that this model may serve as a promising tool for studying MN-RET formation in human bone marrow, thereby providing opportunities to study bone marrow genotoxicity testing, mitigating agent effects, and other conditions that are not ordinarily feasible to experimental manipulation in vivo.

Cyanobacterial extracts and microcystin-LR are inactive in the micronucleus assay in vivo and in vitro

Cyanobacteria are sometimes widespread in lakes and can produce potent toxins, which can be dangerous for animals that drink the water, e.g. cattle and dogs. If the toxins are taken up by fish and other organisms in the food chain, or occur in drinking-water, they may pose a problem also for humans. Microcystin-LR, a hepatotoxic cyclic peptide, is one of the most frequently found cyanobacterial toxins. Data on the genotoxic potential of microcystin-LR and other cyanobacterial toxins are contradictory. Here we report results of the micronucleus assay carried out in vivo and in vitro with these toxins. To increase the sensitivity, we used the flow cytometry-based micronucleus assay in the mouse. In this study both pure microcystin-LR and cyanobacterial extracts originating from four different lakes in Sweden were analysed. Although doses up to near lethality were used and an average of 200,000 young erythrocytes, polychromatic erythrocytes, were analysed from each animal, no genotoxic effect was observed, nor could any effect be shown in the in vitro micronucleus study, using human lymphocytes. These results show that the low concentration of microcystins that now and then occur in drinking-water does not increase the cancer risk through chromosome breaks or mal-distribution of chromosomes.

Validating high-throughput micronucleus analysis of peripheral reticulocytes for radiation biodosimetry: benchmark against dicentric and CBMN assays in a mouse model

Automation of radiation biodosimetry is one of the top priority tasks considered by the Office of Science and Technology Policy and the Homeland Security Council in preparation for the nation's readiness for a possible radionuclear terrorist attack. The Center for Biophysical Assessment and Risk Management Following Irradiation, a consortium of researchers and institutions centered at the University of Rochester, has been investigating automated scoring of radiation-induced micronucleus formation in reticulocytes for high-throughput radiation biodosimetry. The collaborative project is based on a commercially-available product by Litron Laboratories in Rochester, New York. The study was designed to validate the flow-cytometry based analysis of micronucleated reticulocyte expression for radiation biodosimetry by benchmarking against the standard lymphocyte-based biodosimetry methods in a mouse model. C57B1/6 mice and C3H mice were exposed to Cs total-body radiation from 0-3 Gy. Blood samples were subsequently analyzed for CD71+ micronucleated reticulocyte and reticulocyte frequencies by flow cytometry. Results showed a linear dose-response of MN-RET up to 1 Gy for C57B1/6 and 2 Gy for C3H mice. On the other hand, robust and good dose-response curves were obtained with lymphocyte-based dicentric assay and cytokinesis-block micronucleus assay up to 3 Gy. High-throughput, automated analyses of micronucleated reticulocytes is a sensitive and reproducible method for detecting recent radiation exposure. In mice, the dose range of detection is useful up to 1 Gy (C57Bl/6) and 2 Gy (C3H) but not reliable beyond these dose limits. The utilization of this automated analysis for human radiation biodosimetry is currently under investigation.

Effect of gene polymorphisms and ethanol consumption on micronucleus frequency in human reticulocytes: a preliminary study

OBJECTIVE

Results from previous studies suggest that alcohol consumption can be genotoxic on peripheral lymphocytes. The aim of our study was to examine the association of alcohol consumption and its genotoxic effect on hematopoietic stem cells in vivo.

METHODS

We investigated 156 healthy Japanese males in a cross-sectional study. Lifestyles, including alcohol drinking behavior and cigarette smoking status, were investigated by means of a self-completed questionnaire. Polymorphisms of ADH1B and ALDH2 were identified by PCR-restriction fragment length polymorphism (RFLP) analysis. The presence of micronuclei in transferrin-positive reticulocytes (MN-RET) was detected with a single-laser flow cytometer. Associations between the genetic polymorphisms, lifestyle factors, and MN-RET frequency were statistically analyzed.

RESULTS

We found a significant difference in the mean frequencies of MN-RET between habitual drinkers and non-habitual drinkers (P = 0.043), and between the ALDH2*1/*1 and ALDH2*2/*2 genotype (P = 0.015). The ADH1B*2 and ALDH2*2 haplotype was estimated to have a significantly higher influence on MN-RET frequency than the ADH1B*2 and ALDH2*1 haplotype (P = 0.00035), and the frequency of alcohol consumption played a significant role in MN-RET frequency on the background of the ADH1B*2 and ALDH2*1 haplotype (P = 0.012).

CONCLUSION

The results of our study suggest a possible association between the ADH1B and ALDH2 polymorphism and the genotoxic effects of alcohol drinking on hematopoietic stem cells.

Gamma-radiation induces micronucleated reticulocytes in 3D bone marrow bioreactors in vitro

Radiation injury to the bone marrow is potentially lethal due to the potent DNA-damaging effects on cells of the hematopoietic system, including bone marrow stem cell, progenitor, and the precursor cell populations. Investigation of radiation genotoxic effects on bone marrow progenitor/precursor cells has been challenged by the lack of optimal in vitro surrogate organ culture systems, and the overall difficulty to sustain lineage-specific proliferation and differentiation of hematopoiesis in vitro. We report the investigation of radiation genotoxic effects in bone marrow cultures of C57Bl/6 mice established in 3D bioreactors, which sustain long-term bone marrow cultures. For these studies, genotoxicity is measured by the induction of micronucleated reticulocytes (MN-RETs). The kinetics and dose-response relationship of MN-RET induction in response to gamma-radiation of bioreactor-maintained bone marrow cultures are presented. Our data showed that 3D long-term bone marrow cultures had sustained erythropoiesis capable of generating reticulocytes up to 8 weeks. The peak time-interval of viable cell output and percentage of reticulocytes increased steadily and reached the initial peak between the 14th and 21st days after inoculations. This was followed by a rebound or staying relatively constant until week 8. The percentage of MN-RET reached the maximum between 24 h and 32 h post 1 Gy gamma-ray. There was a near linear MN-RET induction by gamma-radiation from 0 Gy to 1.0 Gy, followed by an attenuated increase to 1.5-2.0 Gy. The MN-RET response showed a downtrend beyond 2 Gy. Our data suggest that bone marrow culture in the 3D bioreactor may be a useful organ culture system for the investigation of radiation genotoxic effect in vitro.

Micronucleus frequency in erythrocytes of mice after long-term exposure to radiofrequency radiation

PURPOSE

The aim of the study was to investigate genotoxicity of long-term exposure to radiofrequency (RF) electromagnetic fields by measuring micronuclei in erythrocytes. The blood samples were collected in two animal studies evaluating possible cocarcinogenic effects of RF fields.

METHODS

In study A, female CBA/S mice were exposed for 78 weeks (1.5 h/d, 5 d/week) to either a continuous 902.5 MHz signal similar to that emitted by analog NMT (Nordic Mobile Telephone) phones at a whole-body specific absorption rate (SAR) of 1.5 W/kg, or to a pulsed 902.4 MHz signal similar to that of digital GSM (Global System for Mobile Communications) phones at 0.35 W/kg. A third group was sham-exposed, and a fourth group served as cage controls. All but the cage control animals were exposed to 4 Gy of x-rays during three first weeks of the experiment. In study B, female transgenic mice (line K2) and their nontransgenic littermates were exposed for 52 weeks (1.5 h/d, 5 d/week). Two digital mobile phone signals, GSM and DAMPS (Digital Advanced Mobile Phone System), were used at 0.5 W/kg. All but the cage-control animals were exposed 3 times per week to an ultraviolet radiation dose of 1.2 MED (minimum erythema dose).

RESULTS AND CONCLUSIONS

The results did not show any effects of RF fields on micronucleus frequency in polychromatic or normochromatic erythrocytes. The results were consistent in two mouse strains (and in a transgenic variant of the second strain), after 52 or 78 weeks of exposure, at three SAR levels relevant to human exposure from mobile phones, and for three different mobile signals.

Radio-adaptive response to environmental exposures at Chernobyl

The genetic consequences resulting from environmental exposure to ionizing radiation have a significant impact on both radiation regulatory policies and the comprehension of the human health risks associated with radiation exposure. The primary objectives of the study were to assess 1) genotoxicity of exposure to radiation as a function of absorbed dose and dose rate, and 2) induction of a radio-adaptive response following a priming dose at varying dose rates. Results demonstrated that sub-acute environmental exposures of 10cGy gamma radiation resulted in indistinguishable levels of chromosomal damage as compared to controls. A radio-adaptive response was observed in all experimental groups, exposed to a subsequent acute challenge dose of 1.5 Gy, demonstrating that low dose rates of low energy transfer (LET) radiation are effective in reducing genetic damage from a subsequent acute low-LET radiation exposure. Furthermore, the data presented herein demonstrate a potential beneficial effect of sub-chronic exposure to low levels of low-LET radiation in an environmental setting and do not support the Linear No Threshold (LNT) hypothesis.

Evaluation of the potential genotoxicity of chromium picolinate in mammalian cells in vivo and in vitro

Chromium picolinate (CrPic) is a synthetic nutritional supplement primarily used for weight loss and muscle building. Recent studies have indicated that CrPic might be genotoxic and these findings together with the wide-spread consumer use, have increased the concern about its safety. In the present study we investigated the potential genotoxicity of CrPic in mice given a single intraperitoneal injection (up to 3 mg/kgb.wt.) by evaluating the frequency of micronucleated polychromatic erythrocytes (fMNPCE) in peripheral blood, and DNA damage in lymphocytes and hepatocytes. The fMNPCE was evaluated after 42 h and DNA damage after 16 h. Using the Comet assay DNA damage was also monitored in extended-term cultures of human lymphocytes and in L5178Y mouse lymphoma cells that had been exposed for 3h to 500 microM CrPic under different exposure conditions. A slight, but significant CrPic-induced increase in DNA damage (P<0.001) was observed in the human lymphocytes, but only when these cells were exposed in the absence of serum. In all other experiments CrPic was found to be without genotoxic effects, both in vivo and in vitro. Taken together, our results suggest that a high concentration of CrPic might be DNA damaging, but only under non-physiological conditions.

Evidence That Oxymorphone-Induced Increases in Micronuclei Occur Secondary to Hyperthermia

Oxymorphone is a potent opioid analgesic. Oral administration of oxymorphone to rats at doses >or= 20 mg/kg and mice at 500 mg/kg produced an increase in micronucleated polychromatic erythrocytes (MPCEs). Oxymorphone does not produce chromosome aberrations in vitro, suggesting that the increased MPCEs in vivo may involve indirect mechanisms. Opioids are known to affect thermoregulatory mechanisms. Changes in body temperature can increase the incidence of MPCEs in rodents. Studies were conducted to examine the relationship between increased MPCEs in rats given oxymorphone and changes in body temperature. Single oral doses of oxymorphone associated with increased MPCEs (20 and 40 mg/kg) also produced a marked, rapid increase in body temperature. When animals were pretreated with sodium salicylate, peak body temperature was lower and returned to baseline more quickly than when oxymorphone was given alone. MPCEs were evaluated in rats after administration of oxymorphone (40 mg/kg) alone or following pretreatment with an oral dose of sodium salicylate. Oxymorphone alone produced a statistically significant increase in the incidence of MPCEs (3.6 per 1000 polychromatic erythrocytes vs. 0.4 in controls). The number of MPCEs in animals pretreated with sodium salicylate was similar to controls. Sodium salicylate alone had no effect on the number of MPCEs. Systemic oxymorphone exposure was not affected by sodium salicylate pretreatment; maximum plasma concentration (C(max)) and area-under-the-curve values were similar after administration of oxymorphone alone or following pretreatment with sodium salicylate. These results indicate that the increased incidence of MPCEs following oxymorphone administration is directly related to increased body temperature.

Proton-induced genetic damage in lacZ transgenic mice

The plasmid-based lacZ transgenic mouse model system was used to evaluate the mutagenic and genotoxic potential of 250 MeV/nucleon proton radiation by evaluating the frequency of micronucleated polychromatic reticulocytes in peripheral blood and bone marrow and the mutant frequencies of the lacZ reporter transgene in spleen and brain, respectively. Doses of 0.1-2 Gy produced dose- and time-dependent changes in the frequency of micronucleated polychromatic reticulocytes within 48 h, with peak induction up to sixfold above control levels. The frequency of micronucleated polychromatic reticulocytes returned to control levels within 1 week after exposure. With doses of 4 Gy, the elevation in the frequency of micronucleated polychromatic reticulocytes was delayed up to 1 week after exposure, but complete recovery to control levels was observed at 16 weeks postirradiation. Significant increase in mutant frequencies in brain tissue was observed at 8 week after proton exposure at doses as low of 0.1 Gy. Mutant frequencies in spleen increased up to twofold above spontaneous mutant frequencies at 8 weeks after exposure to 0.5-1 Gy. These effects appeared saturated at doses >1 Gy for both tissues, possibly due to elimination of damaged cells from the tissue systems. These in vivo results highlight the importance of considering tissue specificity, dose and temporal dependence when assessing radiation effects.

Flow cytometric analysis of micronucleated reticulocytes: Time- and dose-dependent response of known mutagens in mice, using multiple blood sampling

According to the current Organization of Economic Cooperation and Development (OECD) and International Committee on Harmonization (ICH) guidelines for the mammalian erythrocyte micronucleus (MN) test, analysis of peripheral blood reticulocytes (RETs) for the presence of micronuclei can be performed using flow cytometry. The MicroFlow PLUS method (Litron Laboratories, Rochester, NY) for MN analysis by flow cytometry is based on the binding of FITC-labeled antibodies to the CD71 transferrin receptor of immature RETs, on parallel RNA degradation, and on propidium iodide staining of DNA present as micronuclei. The objective of this study was to assess the sensitivity of this flow cytometry method to detect time- and dose-dependent induction of micronuclei in mouse peripheral blood RETs after treatment with nine chemical agents. Five known clastogens, two known aneugens, and two compounds previously reported to be inactive in the mouse bone marrow MN test were evaluated at three dose levels. Multiple blood sampling of the same animal before and at two time points after treatment was conducted. All known mutagens produced a dose-dependent increase in micronucleated reticulocytes (MN-RETs); the compounds previously shown to be inactive in the in vivo MN test were also negative using the present methodology. The highest frequency of MN-RETs was observed at 48 hr after treatment, except for 5-fluorouracil, which had its peak response at 72 hr. The results indicate that micronuclei can be measured by multiple blood sampling of the same animal before and after treatment without altering the sensitivity of the assay. The results confirm that the flow cytometric assessment of MN-RETs in mouse peripheral blood using MicroFlow PLUS is a sensitive method with high analysis throughput, and robust quality control.

Evaluation of the reticulocyte micronucleus assay in patients treated with radioiodine for thyroid cancer

In the case of accidental radiation exposure, biological dosimetry has an important role. Previous studies have indicated that the flow cytometric micronucleus (MN) assay in human transferrin receptor positive reticulocytes (Tf-Ret) in blood could be a sensitive biomarker for chromosome damage. In the present investigation, the utility and sensitivity of this method was studied in 44 young patients from Belarus, who were treated with 131I for thyroid cancer. Red marrow (RM) is the critical organ in radioiodine therapy (RIT). In our patients, it was exposed to 100-700 mSv low-dose rate irradiation within 2-4 days. About 3 days after 131I administration, the frequency of micronucleated-Tf-Ret (f(MN-Tf-Ret)) increases within 1 day to a maximum and declines in the following 2-5 days to its value before treatment. A total dose of 100 mSv was easily detectable. The sensitivity of the assay after acute irradiation may be 50 mSv. The method should be useful for monitoring individuals after a radiation accident, provided blood samples can be obtained within a few days after exposure. The time-course of f(MN-Tf-Ret) is interpreted using a model, which considers the exponential exposure of red marrow in RIT as well as the kinetics of erythroblast maturation and reticulocyte migration into the peripheral circulation. Similar modelling was done on published data of MN in immature mouse erythrocytes. Striking similarities in the kinetic and in the yield of MN-induction were found between these two species. This lends support for the use of the mouse as a model for the MN-induction in humans.

Relative biological effectiveness of fission neutrons for induction of micronucleus formation in mouse reticulocytes in vivo

Following whole-body irradiation of ICR mice with various doses of fission neutrons or X-rays, the frequency of micronuclei (MNs) in peripheral blood reticulocytes was measured at 12 h intervals beginning immediately after irradiation and ending at 72 h after irradiation. The resulting time-course curve of MN frequency had a clear peak 36 h after irradiation, irrespective of the type of radiation applied and the dose used. The MN frequency, averaged as the unweighted mean over the experimental time course, showed a linear increase with increasing dose of either fission neutrons or X-rays. The linear response to X-rays supports reported conclusion that induction of MN formation in reticulocytes is a dose-rate independent phenomenon. The relative biological effectiveness (RBE) of fission neutrons to X-rays for MN induction was estimated to be 1.9 +/- 0.3. This value is considerably lower than the RBE value of 4.6 +/- 0.5 reported for the same fission neutrons for induction of lymphocyte apoptosis in the thymus of ICR mice that represents dose-rate independent, one-track event. Based on these results, we propose that MNs increased in reticulocytes after irradiation mostly represent acentric fragments caused by single chromosome breaks, and that some confounding factor is operating in erythroblasts for the formation of aberrations from non-rejoining DNA double-strand breaks more severely after high-LET radiation than after low-LET radiation.

Semicarbazide is not genotoxic in the flow cytometry-based micronucleus assay in vivo

Semicarbazide, mainly produced for research purposes, has been known for various reasons within foods. It occurs as one of the break-down products of nitrofurans, a group of widely used veterinary drugs, and as a break-down product of azodicarbonamide used as a flour treatment agent to improve baking properties. Furthermore, it occurs in carrageenan, a food additive (thickener). Semicarbazide was tested for genotoxicity. In this study, we present results from the flow cytometer-based micronucleus assay in peripheral blood of mice. Male CBA and Balb/C mice were intraperitoneally (i.p.) injected with semicarbazide (hydrochloride). The doses were: 0, 40, 80, 120 mg/kg body weight. Blood samples were collected and the frequency of micronucleated polychromatic erythrocytes, fMPCE, and the cell proliferation, %PCE, was determined. In order to further increase the sensitivity of the test for agents with an aneugenic property, also the fMPCE restricted to micronuclei containing a high mean DNA content, was measured. Neither of the different analyses disclosed any adverse effect in either of the two mouse strains.

A comparison of genotoxicity between three common heterocyclic amines and acrylamide

Heterocyclic amines (HCAs), a group of genotoxic compounds formed during the heating of proteinaceous food items, have been known since the late 1970s. However, the genotoxic effect of these compounds in the low dose region has not yet been thoroughly studied. Here we used a sensitive flow cytometer-based micronucleus assay in mice to determine the frequency of micronucleated erythrocytes (fMPCE) of the three common HCAs, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx), and 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), in the low dose region. We especially looked for any deviation from linearity of the dose-response curves. Male Balb/C mice were intra peritoneally injected with different doses of either PhIP (0-36 mg/kg b.w.), MeIQx (0-90 mg/kg b.w.) or IQ (0-40 mg/kg b.w.). In the case of PhIP, we found a significant dose-response relationship, while MeIQx and IQ did not display an increased fMPCE level. This flow cytometer method allows for determination of the DNA content of micronuclei. All three HCAs tested here yielded a low DNA content of micronuclei, indicating that they do not possess aneugenic effects. A comparison between the HCAs and acrylamide (AA), another heat induced genotoxic compound, revealed that the slope of the dose-response curve is about 10 times steeper for PhIP than AA. In spite of this, AA probably constitutes a higher human risk than HCAs since the intake is about a 100- to 1000-fold higher than the intake of HCAs.

Adenomatous polyposis coli influences micronuclei induction by PhIP and acrylamide in mouse erythrocytes

Micronucleus (MN) induction in erythrocytes of multiple intestinal neoplasia (Min) mice with heterozygous Apc mutation was measured after s.c. injections of acrylamide, glycidamide, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) and colchicine, and compared with wild-type (wt) mice. Since Apc influences microtubule dynamics, we wanted to test whether Min-mice were more sensitive to the production of MN than wild-type mice. We also examined the effect of pre-treatment with cytosine beta-D-arabinofuranoside (Ara C) and hydroxyurea, which inhibit ligation of DNA strand breaks in the repair of DNA adducts. All compounds induced a significant increase in MN in both strains of mice with the following potencies: acrylamide<glycidamide<PhIP. No difference in the induction of MN was seen between Min-mice and wt-mice exposed to acrylamide, glycidamide or colchicine without pre-treatment. However, in Min-mice, PhIP treatment induced much less MN than in wt-mice, with about four- and six-fold increase in MN in Min-mice and wt-mice, respectively. A reduced ability to repair PhIP adducts may be the reason for the lower induction of MN in Min-mice. Treatment with Ara C and hydroxyurea, to increase sensitivity, gave more than a four-fold increase in MN, but strongly reduced proliferation. Pre-treatment with Ara C and hydroxyurea made the Min-mice slightly more sensitive to MN induction by glycidamide compared to wt-mice. We conclude that Min-mice are less sensitive than wt-mice to MN induction by PhIP that forms bulky DNA adducts, while Min-mice and wt-mice are equally sensitive to MN induction by acrylamide and glycidamide that form DNA base adducts.

The dose-response relationship at very low doses of acrylamide is linear in the flow cytometer-based mouse micronucleus assay

Acrylamide (AA) is genotoxic and has been classified as a probable human carcinogen. Human exposure to AA may be high by the consumption of starch-based food that has been treated at high temperature, e.g. potato chips and crisps. For risk assessment, extrapolation to the expected low doses to humans will be more reliable when data from low experimental doses can be used. We have registered the effects of a series of low doses in the sensitive flow cytometer-based micronucleus assay in mice, paying special attention to deviations from the expected linear dose-response function. Two experiments were performed with CBA mice, injected i.p. with different doses of AA. In one experiment the effects of 22 doses (two mice per dose) ranging from 0 to 100 mg/kg b.w. were studied. In the second experiment seven doses (five mice per dose) ranging from 0 to 30 mg/kg b.w. were used. In both experiments, a clear increase of the frequency of micronucleated erythrocytes was seen, already at the lowest doses used. The dose-response function was found to be linear with a tendency to have a steeper rise at the lowest doses. The low DNA content of the micronuclei indicated an absence of whole chromosomes, i.e. no aneugenic effect of AA.

Application of pharmacokinetic/pharmacodynamic and stochastic modelling to 6-mercaptopurine micronucleus induction in mouse bone marrow erythrocytes

This study investigates the kinetics of bone marrow micronucleated polychromatic erythrocytes and some mechanistic aspects of micronuclei induction using mathematical models. Female mice were administered a single intraperitoneal injection of the purine antagonist 6-mercaptopurine at 50 mg kg(-1). The time course evolution of the drug concentrations in the plasma and the micronucleated polychromatic erythrocyte kinetic rate in bone marrow were observed. Two mathematical models were developed for this study. The first model was built from a simultaneous pharmacokinetic/pharmacodynamic approach, but was invalidated after comparing its predictions to experimental data. The second model was a stochastic model based on some biological hypotheses involved in micronuclei induction. This model predicted a wavy kinetic rate of micronucleated polychromatic erythrocytes that was confirmed by a second data set obtained from a specifically built experimental design. The biological hypotheses were then discussed. It turned out from this work that mathematical modelling could be used as a tool to explore the cellular mechanisms of toxicity of the compound: for instance, the assumptions that 6-mercaptopurine induced micronuclei mainly in cells entering the S phase, and not only during the last cell cycle but during one of the earlier cycles preceding the extrusion of the main nucleus, were confirmed. Moreover, the use of the stochastic model would help to schedule more accurately the bone marrow or blood harvesting times in the in vivo rodent micronucleus test.

Macrocyclic musk compounds—an absence of genotoxicity in the Ames test and the in vivo Micronucleus assay

Three synthetic macrocyclic musks, ethylene dodecanedioate, ethylene brassylate, and cyclopentadecanolide, which are widely used as ingredients of perfume fragrances, were tested for genotoxicity. In this report we present results from two different studies, the flow-cytometer-based micronucleus assay in peripheral blood of mice and the Salmonella/microsome test with TA 97, TA 98, and TA 100. Female NMRI and male CD 1 mice were intraperitoneally injected with one of the three macrocyclic musk compounds. Three different doses (0.1-1.6 g/kg bw) of each of the compounds were tested. Blood samples were collected on two occasions from each mouse and the frequency of micronucleated polychromatic erythrocytes (fMPCE) was determined. Neither of the compounds caused a significant difference from the control fMPCE. No mutagenic effect with and without S9 mix in the tested Salmonella strains was observed. The presence of S9 mix reduced the killing effect of high doses.

Micronucleus formation induced by the combination of low doses of X-rays and antineoplastic drugs in bone marrow of male mice

People are widely exposed during their lifetime to many biological, chemical, and physical agents in the environment and at work. In this paper the effects of combined exposures of nonmutagenic doses of X-rays and anticancer agents (cyclophosphamide, mitomycin C, and vinblastine) have been investigated on the induction of micronuclei in the bone marrow of laboratory mice. The combination of X-rays and anticancer drugs enhanced the frequency of micronuclei in some cases. The strongest effects were found after the combination of X-rays and cyclophosphamide at 24 h and 72 h. The combined treatment of X-rays and mitomycin C enhanced the mutagenic effect at 72 h. The combination of X-rays + vinblastine slightly potentiated the mutagenic effect at 24 h and 48 h.

Stochastic modeling in toxicokinetics. Application to the in vivo micronucleus assay

A stochastic model for the in vivo micronucleus assay is presented. This model describes the kinetic of the rate of micronucleated polychromatic erythrocytes induced by the administration of a mutagenic compound. For this, biological assumptions are made both on the erythropoietic system and on the mechanisms of action of the compound. Its pharmacokinetic profile is also taken into account and it is linked to the induced toxicological effect. This model has been evaluated by analyzing the induction of micronuclei is mice bone marrow by a mutagenic compound, 6-mercaptopurine (6-mp). This analysis enabled to make interesting remarks about the induction of micronuclei by 6-mp and to put to light an unsuspected wavy kinetic by optimizing the experimental design of the in vivo micronucleus assay.

Impact of p53 status on heavy-ion radiation-induced micronuclei in circulating erythrocytes

Transgenic mice that differed in their p53 genetic status were exposed to an acute dose of highly charged and energetic (HZE) iron particle radiation. Micronuclei (MN) in two distinct populations of circulating peripheral blood erythrocytes, the immature reticulocytes (RETs) and the mature normochromatic erythrocytes (NCEs), were measured using a simple and efficient flow cytometric procedure. Our results show significant elevation in the frequency of micronucleated RETs (%MN-RETs) at 2 and 3 days post-radiation. At 3 days post-irradiation, the magnitude of the radiation-induced MN-RET was 2.3-fold higher in the irradiated p53 wild-type animals compared to the unirradiated controls, 2.5-fold higher in the p53 hemizygotes and 4.3-fold higher in the p53 nullizygotes. The persistence of this radiation-induced elevation of MN-RETs is dependent on the p53 genetic background of the animal. In the p53 wild-type and p53 hemizygotes, %MN-RETs returned to control levels by 9 days post-radiation. However, elevated levels of %MN-RETs in p53 nullizygous mice persisted beyond 56 days post-radiation. We also observed elevated MN-NCEs in the peripheral circulation after radiation, but the changes in radiation-induced levels of MN-NCEs appear dampened compared to those of the MN-RETs for all three strains of animals. These results suggest that the lack of p53 gene function may play a role in the iron particle radiation-induced genomic instability in stem cell populations in the hematopoietic system.

The micronucleus test in rat erythrocytes from bone marrow, spleen and peripheral blood: the response to low doses of ionizing radiation, cyclophosphamide and vincristine determined by flow cytometry

The frequency of micronucleated polychromatic erythrocytes (fMPCE) was determined in samples from bone marrow, spleen and peripheral blood of rats exposed to low doses of X-rays, cyclophosphamide or vincristine. The fMPCE values were lower in the peripheral blood than in bone marrow or spleen. This is due to the elimination of MPCE from the circulating blood, which was confirmed by the results from prolonged exposure of rats to gamma-radiation. When the analysis was restricted to the youngest PCE in peripheral blood, the sensitivity of the assay was considerably improved. This can be reproducibly achieved with the flow cytometric analysis.

Pharmacokinetic parameters determined from the clastogenic activity of ethylnitrosourea and dimethylnitrosamine in mice in vivo

The latency period (LP) and the time of effective activity (TEA) of ethylnitrosourea (ENU) and dimethylnitrosamine (DMN) were inferred by comparing their kinetics of micronucleated polychromatic erythrocytes (MN-PCE) formation with the kinetics induced by radiation. The results indicate that LP and TEA vary between ENU and DMN. For ENU, these parameters are very similar to radiation indicating a rapid distribution, reaction and elimination. DMN presents a very long LP which agrees with the requirement of mutagen activation. The kinetics of MN-PCE production caused by DMN showed two peaks; this could be due to the presence of two different metabolites, two types of lesions in DNA or two mechanisms of MN-PCE formation. These hypotheses do not exclude each other. The data presented here support the conclusion that the comparison of MN-PCE-formation kinetics induced by chemical agents with that caused by radiation permits one to estimate the LP and the TEA, and provide information on the possible mechanism of action of chemical mutagens.

Erythropoiesis and the induction of micronuclei in mouse spleen determined by flow cytometry

Erythrocytes from the spleen of CBA mice have been prepared for analyses by flow cytometry. About 80% of the polychromatic erythrocytes (PCE) in the spleen originate from erythropoiesis in the spleen, while the remaining 20% come from the peripheral blood. Analyses of the RNA content of PCE revealed that splenic PCE do not mature into normochromatic erythrocytes (NCE) in the spleen but leave the organ at a more immature stage. A considerable part of the PCE from bone marrow also mature into NCE in the bone marrow. The rate of RNA breakdown in PCE follows an exponential function. Time-courses for the appearance of micronucleated PCE (MPCE) from spleen and from bone marrow were determined by analysis of samples taken with short intervals after an acute dose of 0.1 Gy X-rays. The time-courses were identical for MPCE from the spleen and the bone marrow. The frequency of MPCE (fMPCE) starts to increase at about 10 h after irradiation and reaches its maximum after about another 20 h upon which fMPCE returns to control level. The first induced MPCE in peripheral blood appear at about 20 h after irradiation. The effects of the carcinogen DMBA, 9,10-dimethyl-1,2-benzanthracene, at low doses were determined in PCE from spleen and bone marrow. The sensitivity was found to be about the same for erythroblasts in the spleen and the bone marrow. Protracted exposure to gamma-irradiation at a very low dose rate (44 mGy/day) gave a similar increase of fMPCE in bone marrow and spleen. The suitability of using splenic erythrocytes in the micronucleus test is discussed.

Spontaneous and radiation-induced micronuclei in erythrocytes from four species of wild rodents: a comparison with CBA mice

Almost 100 animals of 4 different species of small wild rodents (bank vole, Clethrionomys glareolus; field vole, Microtus agrestis; yellow-necked mouse, Apodemus flavicollis; and wood mouse, Apodemus sylvaticus) were trapped in central Sweden and used in experiments to determine the spontaneous and radiation-induced frequencies of polychromatic (fMPCE) and normochromatic erythrocytes (fMNCE) from bone marrow (bm) and peripheral blood (pb) using flow cytometric analysis. The results were compared with those from similar experiments with CBA mice. The saving of time and labour by the use of the flow cytometer-based analysis was a prerequisite for this study in which about 135 million PCE were analysed. The two species of voles had a mean background fMPCE (bm) of about the same value as CBA mice, while the yellow-necked mice had about five times higher fMPCE (bm). Wood mice had more than twice the fMPCE (bm) compared to CBA mice. Between individual animals in each of the 4 species, the background fMPCE (bm) varied more than between individual CBA mice, and the elimination of micronucleated erythrocytes was considerable. When exposed to ionizing radiation, the voles did not show a significant response. The response of the two Apodemus species was similar to that of the CBA mice, although it varied between individual animals and was not correlated to their background fMPCE. This study indicates that bank voles and field voles are unsuitable testing objects in the in vivo micronucleus assay. On the other hand, yellow-necked mice and wood mice seem to be useful in this test. Since the variation between individuals is considerable in wild Apodemus mice, large groups will be needed for obtaining statistically significant results when exposure to a genotoxic agent is low. Alternatively, repeated samples can be taken from individual wild mice to study the effect of a decreased exposure after keeping the animals for a period of time in an uncontaminated environment.

Ascorbic acid is not clastogenic and does not modify the effect of extended low-dose-rate gamma-irradiation in mouse bone marrow

Ascorbic acid was given to CBA mice in drinking water (5%) a week before and during 35-day exposure to gamma-radiation from 137 Cs at a very low dose-rate (44 mGy/day). The frequency of micronucleated normochromatic erythrocytes (fMNCE) in peripheral blood was monitored by repeated sampling during the exposure. The analyses were made with flow cytometry giving a high resolution because of the large number of cells analysed, about 10(6) for each dose group and sampling occasion. Ascorbic acid in the drinking water did not modify the increase of fMNCE in the gamma-irradiated groups of mice, nor did ascorbic acid influence the fMNCE in the non-irradiated groups of mice.