Reticulocyte and micronucleated reticulocyte responses to gamma irradiation: dose-response and time-course profiles measured by flow cytometry

Characterization of benzo[a]pyrene and colchicine based on an in vivo repeat-dosing multi-endpoint genotoxicity quantitative assessment platform

Two prototypical genotoxicants, benzo[a]pyrene (B[a]P) and colchicine (COL), were selected as model compounds to deduce their quantitative genotoxic dose–response relationship at low doses in a multi-endpoint genotoxicity assessment platform. Male Sprague-Dawley rats were treated with B[a]P (2.5–80 mg/kg bw/day) and COL (0.125–2 mg/kg bw/day) daily for 28 days. The parameters included were as follows: comet assay in the peripheral blood and liver, Pig-a gene mutation assay in the peripheral blood, and micronucleus test in the peripheral blood and bone marrow. A significant increase was observed in Pig-a mutant frequency in peripheral blood for B[a]P (started at 40 mg/kg bw/day on Day 14, started at 20 mg/kg bw/day on Day 28), whereas no statistical difference for COL was observed. Micronucleus frequency in reticulocytes of the peripheral blood and bone marrow increased significantly for B[a]P (80 mg/kg bw/day on Day 4, started at 20 mg/kg bw/day on Days 14 and 28 in the blood; started at 20 mg/kg bw/day on Day 28 in the bone marrow) and COL (started at 2 mg/kg bw/day on Day 14, 1 mg/kg bw/day on Day 28 in the blood; started at 1 mg/kg bw/day on Day 28 in the bone marrow). No statistical variation was found in indexes of comet assay at all time points for B[a]P and COL in the peripheral blood and liver. The dose–response relationships of Pig-a and micronucleus test data were analyzed for possible point of departures using three quantitative approaches, i.e., the benchmark dose, breakpoint dose, and no observed genotoxic effect level. The practical thresholds of the genotoxicity of B[a]P and COL estimated in this study were 0.122 and 0.0431 mg/kg bw/day, respectively, and our results also provided distinct genotoxic mode of action of the two chemicals.

Analysis of Biomarkers of DNA Damage and Mutagenicity in Mice Exposed to Acrylonitrile

Acrylonitrile (ACN), which is a widely used industrial chemical, induces cancers in the mouse via unresolved mechanisms. For this report, complementary and previously described methods were used to assess in vivo genotoxicity and/or mutagenicity of ACN in several mouse models, including (i) female mice devoid of cytochrome P450 2E1 (CYP2E1), which yields the epoxide intermediate cyanoethylene oxide (CEO), (ii) male lacZ transgenic mice, and (iii) female (wild-type) B6C3F1 mice. Exposures of wild-type mice and CYP2E1-null mice to ACN at 0, 2.5 (wild-type mice only), 10, 20, or 60 (CYP2E1-null mice only) mg/kg body weight by gavage for 6 weeks (5 days/week) produced no elevations in the frequencies of micronucleated erythrocytes, but induced significant dose-dependent increases in DNA damage, detected by the alkaline (pH >13) Comet assay, in one target tissue (forestomach) and one nontarget tissue (liver) of wild-type mice only. ACN exposures by gavage also caused significant dose-related elevations in the frequencies of mutations in the hypoxanthine-guanine phosphoribosyltransferase (Hprt) reporter gene of T-lymphocytes from spleens of wild-type mice; however, Hprt mutant frequencies were significantly increased in CYP2E1-null mice only at a high dose of ACN (60 mg/kg) that is lethal to wild-type mice. Similarly, drinking water exposures of lacZ transgenic mice to 0, 100, 500, or 750 ppm ACN for 4 weeks caused significant dose-dependent elevations in Hprt mutant frequencies in splenic T-cells; however, these ACN exposures did not increase the frequency of lacZ transgene mutations above spontaneous background levels in several tissues from the same animals. Together, the Comet assay and Hprt mutant frequency data from these studies indicate that oxidative metabolism of ACN by CYP2E1 to CEO is central to the induction of the majority of DNA damage and mutations in ACN-exposed mice, but ACN itself also may contribute to the carcinogenic modes of action via mechanisms involving direct and/or indirect DNA reactivity.

Delineating the Effects of Ionizing Radiation on Erythropoietic Lineage-Implications for Radiation Biodosimetry

The overall lethality/morbidity of ionizing radiation exposure involves multiple forms of inhibitory or cytotoxic effects that may manifest in different tissues with a varying dose and time response. One of the major systemic effects leading to lethality of radiation includes its suppressive effect on hematopoiesis, which could be observed even at doses as low as 1-2 Gy, whereas effects on gastrointestinal and nervous systems appear at relatively higher doses in the same order. This article reviews the effects of radiation on the three distinct stages of erythropoiesis-formation of erythroid progenitor cells, differentiation of erythroid precursor cells, and terminal maturation. During these stepwise developmental processes, erythroid progenitor cells undergo rapid expansion to form terminally differentiated red blood cells that are continuously replenished from bone marrow into the circulating peripheral blood stream. Cellular radiation response depends upon many factors such as cell lineage, rate of proliferation, and differentiation status. Therefore, we discuss radiation-induced alterations during the progenitor, precursor, and terminal maturation stages and the implications thereof. Since biomarkers of ionizing radiation exposure in human populations are of great interest for assessing normal tissue injury as well as for biodosimetry in the event of accidental or incidental radiation exposures, we also highlight blood-based biomarkers that have potential utility for medical management.

The Dependence of the Mutagenic Effect on the Dose of X-Ray Irradiation in an In Vivo Experiment on Female (CBA×C57Bl/6)F1 Mice

We studied the mutagenic effect of X-ray irradiation in doses of 0.5, 1, and 1.5 Gy on female (CBA×C57Bl/6)F1 mice. The mutagenic effect (assessed by the parameter "frequency of bone marrow polychromatophilic erythrocytes with micronuclei") linearly depended on the dose of X-ray irradiation in the range of up to 1 Gy and reached the plateau at 1.5 Gy. The fraction of polychromatophilic erythrocytes was 45, 45, and 46% under control conditions (without exposure) and exposure to the irradiation in the doses of 0.5 and 1 Gy, respectively. Irradiation in a dose of 1.5 Gy induced a slight inhibition of erythropoiesis. These data confirm the hypothesis on possible death of highly aberrant erythrocyte precursors after irradiation in high doses.

Integration of micronucleus tests with a gene mutation assay in F344 gpt delta transgenic rats using benzo[a]pyrene

Reduction of the number of animals used in in vivo genotoxicity tests is encouraged. For this purpose, we conducted integrated toxicity tests combining gene mutation assays with multiple-organ micronucleus (MN) tests (peripheral blood, bone marrow, liver, and colon) in F344 gpt delta transgenic (Tg) rats. Seven-week-old male F344 gpt delta rats were orally administered 62.5 or 125 mg/kg/day benzo[a]pyrene (B[a]P) for 28 days. One day after the final day of treatment (day 29) and three days after the final treatment (day 31), bone marrow, liver, and colon samples were collected, and mutation assays and MN tests were performed. The gpt mutant frequency (MF) significantly increased in bone marrow, liver and colon but MN induction was only significant in bone marrow but not in liver and colon. Similarly MN induction was only observed in bone marrow in non-Tg F344 rats. In peripheral blood obtained on day 4, 15, 29, 31, a time-dependent increase was observed in reticulocyte MN frequency during the treatment. Thus, our integrated method successfully detected both gene mutations and MN induction caused by B[a]P. In addition, no significant differences were observed between sampling times (day 29 versus 31), suggesting that sampling on day 29 is also valid to evaluate gene mutations. On the other hand, MN results in bone marrow and peripheral blood were different depending on the sampling day. An appropriate sampling day should be designated according to which assays are integrated. We confirmed that integration of the MN test with a gene mutation assay using F344 gpt delta Tg rats is useful to evaluate different endpoints related to genotoxicity using the same animals and to reduce animal use.

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.

Quantifying murine bone marrow and blood radiation dose response following (18)F-FDG PET with DNA damage biomarkers

The purpose of this study was to quantify the poorly understood radiation doses to murine bone marrow and blood from whole-body fluorine 18 ((18)F)-fluorodeoxyglucose (FDG) positron emission tomography (PET), by using specific biomarkers and comparing with whole body external low dose exposures. Groups of 3-5 mice were randomly assigned to 10 groups, each receiving either a different activity of (18)F-FDG: 0-37MBq or whole body irradiated with corresponding doses of 0-300mGy X-rays. Blood samples were collected at 24h and at 43h for reticulocyte micronucleus assays and QPCR analysis of gene expression in peripheral blood leukocytes. Blood and bone marrow dose estimates were calculated from injected activities of (18)F-FDG and were based on a recommended ICRP model. Doses to the bone marrow corresponding to 33.43mGy and above for internal (18)F-FDG exposure and to 25mGy and above for external X-ray exposure, showed significant increases in radiation-induced MN-RET formation relative to controls (P<0.05). Regression analysis showed that both types of exposure produced a linear response with linear regression analysis giving R(2) of 0.992 and 0.999 for respectively internal and external exposure. No significant difference between the two data sets was found with a P-value of 0.493. In vivo gene expression dose-responses at 24h for Bbc3 and Cdkn1 were similar for (18)F-FDG and X-ray exposures, with significant modifications occurring for doses over 300mGy for Bbc3 and at the lower dose of 150mGy for Cdkn1a. Both leucocyte gene expression and quantification of MN-RET are highly sensitive biomarkers for reliable estimation of the low doses delivered in vivo to, respectively, blood and bone marrow, following (18)F-FDG PET.

The influence of Trp53 in the dose response of radiation-induced apoptosis, DNA repair and genomic stability in murine haematopoietic cells

Apoptotic and DNA damage endpoints are frequently used as surrogate markers of cancer risk, and have been well-studied in the Trp53+/- mouse model. We report the effect of differing Trp53 gene status on the dose response of ionizing radiation exposures (0.01-2 Gy), with the unique perspective of determining if effects of gene status remain at extended time points. Here we report no difference in the dose response for radiation-induced DNA double-strand breaks in bone marrow and genomic instability (MN-RET levels) in peripheral blood, between wild-type (Trp53+/+) and heterozygous (Trp53+/-) mice. The dose response for Trp53+/+ mice showed higher initial levels of radiation-induced lymphocyte apoptosis relative to Trp53+/- between 0 and 1 Gy. Although this trend was observed up to 12 hours post-irradiation, both genotypes ultimately reached the same level of apoptosis at 14 hours, suggesting the importance of late-onset p53-independent apoptotic responses in this mouse model. Expected radiation-induced G1 cell cycle delay was observed in Trp53+/+ but not Trp53+/-. Although p53 has an important role in cancer risk, we have shown its influence on radiation dose response can be temporally variable. This research highlights the importance of caution when using haematopoietic endpoints as surrogates to extrapolate radiation-induced cancer risk estimation.

Radiation-induced DNA damage and the relative biological effectiveness of 18F-FDG in wild-type mice

Clinically, the most commonly used positron emission tomography (PET) radiotracer is the glucose analog 2-[(18)F] fluoro-2-deoxy-D-glucose ((18)F-FDG), however little research has been conducted on the biological effects of (18)F-FDG injections. The induction and repair of DNA damage and the relative biological effectiveness (RBE) of radiation from (18)F-FDG relative to 662 keV γ-rays were investigated. The study also assessed whether low-dose radiation exposure from (18)F-FDG was capable of inducing an adaptive response. DNA damage to the bone marrow erythroblast population was measured using micronucleus formation and lymphocyte γH2A.X levels. To test the RBE of (18)F-FDG, mice were injected with a range of activities of (18)F-FDG (0-14.80 MBq) or irradiated with Cs-137 γ-rays (0-100 mGy). The adaptive response was investigated 24h after the (18)F-FDG injection by 1 Gy in vivo challenge doses for micronucleated reticulocyte (MN-RET) formation or 1, 2 and 4 Gy in vitro challenges doses for γH2A.X formation. A significant increase in MN-RET formation above controls occurred following injection activities of 3.70, 7.40 or 14.80 MBq (P < 0.001) which correspond to bone marrow doses of ~35, 75 and 150 mGy, respectively. Per unit dose, the Cs-137 radiation exposure induced significantly more damage than the (18)F-FDG injections (RBE = 0.79 ± 0.04). A 20% reduction in γH2A.X fluorescence was observed in mice injected with a prior adapting low dose of 14.80 MBq (18)F-FDG relative to controls (P < 0.019). A 0.74 MBq (18)F-FDG injection, which gives mice a dose approximately equal to a typical human PET scan, did not cause a significant increase in DNA damage nor did it generate an adaptive response. Typical (18)F-FDG injection activities used in small animal imaging (14.80 MBq) resulted in a decrease in DNA damage, as measured by γH2A.X formation, below spontaneous levels observed in control mice. The (18)F-FDG RBE was <1.0, indicating that the mixed radiation quality and/or low dose rate from PET scans is less damaging than equivalent doses of gamma radiation.

Frequency of transferrin receptor positive reticulocytes (TF-Ret) in blood as an indicator of total-body radiation exposure: a pilot study in nuclear medicine patients

Approximately 3-20% of all reticulocytes in blood of healthy persons are immature and transferrin receptor positive (Tf-Ret). Tf-Ret were measured by flow cytometry in 27 patients treated with three different radiopharmaceuticals labeled with (131)I and in 25 healthy controls. Patients were chronically exposed within 6 days to blood doses from 0.18-1.89 Gy (D6). Typically, two-thirds of D6 was administered within the first day (D1). The study had to be confined to intra-subject investigations due to high biological variability of Tf-Ret counts. A significant radiation-induced decline was found in patients D1 doses that were ≥0.5 Gy. Tf-Ret frequency declined during the first 4 to 5 days of nuclear therapy to about 30-60% of its initial value, and increased in the following 3 days without reaching the initial value. At the time of nadir, the relative frequency of Tf-Ret was more depressed than that of reticulocytes and lymphocytes. The relative Tf-Ret frequency at nadir could be fitted to the equation: %-Tf-Ret=exp-(D1/D(o)). D(o) was found to be 1.0 ± 0.4 Gy (Mean ± SEM). The study shows that Tf-Ret frequency in blood might be a good parameter for estimation of the radiation dose to red marrow.

The in vivo Pig-a gene mutation assay is useful for evaluating the genotoxicity of ionizing radiation in mice

The in vivo Pig-a mutation assay has been adapted for measuring mutation in rats, mice, monkeys, and humans. To date, the assay has been used mainly to assess the mutagenicity of chemicals that are known to be powerful point mutagens. The assay has not been used to measure the biological effects associated with ionizing radiation. In this study, we modified the Pig-a gene mutation assay (Kimoto et al. [2011b]: Mutat Res 723:36-42) and used 3-color staining with fluorescently labeled anti-CD24, anti-TER-119, and anti-CD71 to detect the Pig-a mutant frequencies in total red blood cells (RBCs) and in reticulocytes (RETs) from X-irradiated mice. Single exposures to X-irradiation resulted in dose- and time-dependent increases in Pig-a mutant frequencies, and these subsequently declined over time returning to background frequencies. The same total amount of radiation, delivered either as a single dose or as four repeat doses at weekly intervals, increased Pig-a mutant frequencies to comparable levels, reaching maxima 2-3 weeks after the single dose or 2-3 weeks after the last of the repeat doses. These increased frequencies subsequently returned to background levels. Our results indicated that the 3-color Pig-a assay was useful for evaluating the in vivo genotoxicity of radiation.

Biological effects and adaptive response from single and repeated computed tomography scans in reticulocytes and bone marrow of C57BL/6 mice

This study investigated the biological effects and adaptive responses induced by single and repeated in vivo computed tomography (CT) scans. We postulated that, through the induction of low-level oxidative stress, repeated low-dose CT scans (20 mGy, 2 days/week, 10 weeks) could protect mice (C57BL/6) from acute effects of high-dose radiation (1 Gy, 2 Gy). The micronucleated reticulocyte (MN-RET) count increased linearly after exposure to single CT scans of doses ranging from 20 to 80 mGy (P = 0.033). Ten weeks of repeated CT scans (total dose 400 mGy) produced a slight reduction in spontaneous MN-RET levels relative to levels in sham CT-scanned mice (P = 0.04). Decreases of nearly 10% in γ-H2AX fluorescence levels were observed in the repeated CT-scanned mice after an in vitro challenge dose of 1 Gy (P = 0.017) and 2 Gy (P = 0.026). Spontaneous apoptosis levels (caspase 3 and 7 activation) were also significantly lower in the repeated CT-scanned mice than the sham CT-scanned mice (P < 0.01). In contrast, mice receiving only a single CT scan showed a 19% elevation in apoptosis (P < 0.02) and a 10% increase in γ-H2AX fluorescence levels after a 2-Gy challenge (P < 0.05) relative to sham CT controls. Overall, repeated CT scans seemed to confer resistance to larger doses in mice, whereas mice exposed to single CT scans exhibited transient genotoxicity, enhanced apoptosis, and characteristics of radiation sensitization.

Sidestream tobacco smoke is a male germ cell mutagen

Active cigarette smoking increases oxidative damage, DNA adducts, DNA strand breaks, chromosomal aberrations, and heritable mutations in sperm. However, little is known regarding the effects of second-hand smoke on the male germ line. We show here that short-term exposure to mainstream tobacco smoke or sidestream tobacco smoke (STS), the main component of second-hand smoke, induces mutations at an expanded simple tandem repeat locus (Ms6-hm) in mouse sperm. We further show that the response to STS is not linear and that, for both mainstream tobacco smoke and STS, doses that induced significant increases in expanded simple tandem repeat mutations in sperm did not increase the frequencies of micronucleated reticulocytes and erythrocytes in the bone marrow and blood of exposed mice. These data show that passive exposure to cigarette smoke can cause tandem repeat mutations in sperm under conditions that may not induce genetic damage in somatic cells. Although the relationship between noncoding tandem repeat instability and mutations in functional regions of the genome is unclear, our data suggest that paternal exposure to second-hand smoke may have reproductive consequences that go beyond the passive smoker.

Exercise-induced protection of bone marrow cells following exposure to radiation

The hormetic effects of exercise training have previously been shown to enhance cellular protection against oxidative stress. Therefore, adaptations to exercise training may attenuate the harmful effects of radiation induced by oxidative stress. Flow cytometric analysis of genotoxicity (γH2AX foci and micronucleated reticulocytes (MN-RET)) and cytotoxicity (apoptosis and percentage of reticulocytes) were conducted on bone marrow cells isolated from acutely exercised (Acute EX), exercise-trained (EX), and sedentary (SED) mice following 1 and 2 Gy radiation challenges in vitro. Acute EX increased the percentage of cells with activated caspase-3 and -7 (32%, p < 0.001) and γH2AX foci formation in response to 2 Gy radiation challenge (10%, p < 0.05). Exercise training significantly attenuated γH2AX foci formation and MN-RET production in response to 1 Gy radiation challenge (18%, p < 0.05 and 22%, p < 0.05, respectively). Exercise training also significantly reduced basal percentages of cells with activated caspase-3 and -7 and in response to radiation in bone marrow cells (11%, p < 0.05). These results suggest that oxidative stress caused by acute exercise induces an adaptive response responsible for the radioprotective effects of exercise training.

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.

Flow cytometric scoring of micronucleated reticulocytes as a possible high-throughput radiation biodosimeter

Micronucleated reticulocyte (MN-RET) scoring by flow cytometry (FCM) has been used in assessment of the clastogenic effects of chemicals. However, its dose-response to acute whole body irradiation (WBI) at moderate dose rates remains to be established. We show that FCM scoring of MN-RET in peripheral blood from male ICR mice exposed to WBI X-ray doses of 0.5-5 Gy at a dose rate of 0.488 Gy/min exhibits a linear dose-response relationship 24, 48, and 72 hr following WBI. Parallel microscopic counting of micronucleated polychromatic erythrocytes (MN-PCE) in bone marrow smears from the same animals showed similar linear dose-response patterns at the same time points. Indeed, MN-RET and MN-PCE were highly correlated at all doses and time points. In view of the speed and accuracy of this method, in addition to the small blood sample size needed for the assay, the flow cytometric protocol for MN-RET scoring may provide a minimally-invasive, high throughput radiation biodosimeter.

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.

The lymphocyte cytokinesis-block micronucleus cytome assay and its application in radiation biodosimetry

The cytokinesis-block micronucleus cytome (CBMN Cyt) assay is one of the best-validated methods for measuring chromosome damage in human lymphocytes. This paper describes the methodology, biology, and mechanisms underlying the application of this technique for biodosimetry following exposure to ionizing radiation. Apart from the measurement of micronuclei, it is also possible to measure other important biomarkers within the CBMN Cyt assay that are relevant to radiation biodosimetry. These include nucleoplasmic bridges, which are an important additional measure of radiation-induced damage that originate from dicentric chromosomes as well as the proportion of dividing cells and cells undergoing cell death. A brief account is also given of current developments in the automation of this technique and important knowledge gaps that need attention to further enhance the applicability of this important method for radiation biodosimetry.

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.

Lack of DNA polymerase theta (POLQ) radiosensitizes bone marrow stromal cells in vitro and increases reticulocyte micronuclei after total-body irradiation

Abstract Mammalian POLQ (pol theta) is a specialized DNA polymerase with an unknown function in vivo. Roles have been proposed in chromosome stability, as a backup enzyme in DNA base excision repair, and in somatic hypermutation of immunoglobulin genes. The purified enzyme can bypass AP sites and thymine glycol. Mice defective in POLQ are viable and have been reported to have elevated spontaneous and radiation-induced frequencies of micronuclei in circulating red blood cells. To examine the potential roles of POLQ in hematopoiesis and in responses to oxidative stress responses, including ionizing radiation, bone marrow cultures and marrow stromal cell lines were established from Polq(+/+) and Polq(-/-) mice. Aging of bone marrow cultures was not altered, but Polq(-/-) cells were more sensitive to gamma radiation than were Polq(+/+) cells. The D(0) was 1.38 +/- 0.06 Gy for Polq(+/+) cells compared to 1.27 +/- 0.16 and 0.98 +/- 0.10 Gy (P = 0.032) for two Polq(-/-) clones. Polq(-/-) cells were moderately more sensitive to bleomycin than Polq(+/+) cells and were not hypersensitive to paraquat or hydrogen peroxide. ATM kinase activation appeared to be normal in gamma-irradiated Polq(-/-) cells. Inhibition of ATM kinase activity increased the radiosensitivity of Polq(+/+) cells slightly but did not affect Polq(-/-) cells. Polq(-/-) mice had more spontaneous and radiation-induced micronucleated reticulocytes than Polq+/+ and (+/-) mice. The sensitivity of POLQ-defective bone marrow stromal cells to ionizing radiation and bleomycin and the increase in micronuclei in red blood cells support a role for this DNA polymerase in cellular tolerance of DNA damage that can lead to double-strand DNA breaks.

Reticulocyte and micronucleated reticulocyte responses to gamma irradiation: effect of age

The effect of age on the formation of radiation-induced micronucleated reticulocytes (MN-RETs) and reticulocytes (RETs) was investigated by exposing female C57BL/6J mice to graded doses of gamma rays from a (137)Cs source. Age at time of irradiation was 6, 16, or 32 weeks, and doses ranged from 0.5 to 3 Gy. A flow cytometric technique based on anti-CD71 labeling was used to measure RET and MN-RET frequencies in blood specimens collected 43 h post-irradiation. Mean RET frequencies declined in a dose-dependent manner for each age group. There was only one significant difference among the ages, that is, %RETs were not significantly reduced in the oldest animals at 0.5 Gy, whereas this dose did have a significant effect on the other age groups. MN-RET data were more complex. Age was observed to influence the baseline frequency of MN-RET, with the oldest mice exhibiting a significantly higher mean value. Each group's %MN-RETs values increased up to 1 Gy, but past this dose the frequencies plateaued or decreased. Age was observed to influence micronucleus frequency, with older mice exhibiting higher mean MN-RET values, especially at the high doses where the response was saturated (2-3 Gy). We hypothesize that these dissimilar responses can largely be explained by an age-related down-regulation of apoptosis whereby younger animals eliminate damaged bone marrow erythroid precursors with a greater efficiency compared with aged mice.