Pig-a mutation: kinetics in rat erythrocytes following exposure to five prototypical mutagens

N-Nitrosodimethylamine investigations in Muta™Mouse define point-of-departure values and demonstrate less-than-additive somatic mutant frequency accumulations

The N-nitrosamine, N-nitrosodimethylamine (NDMA), is an environmental mutagen and rodent carcinogen. Small levels of NDMA have been identified as an impurity in some commonly used drugs, resulting in several product recalls. In this study, NDMA was evaluated in an OECD TG-488 compliant Muta™Mouse gene mutation assay (28-day oral dosing across seven daily doses of 0.02-4 mg/kg/day) using an integrated design that assessed mutation at the transgenic lacZ locus in various tissues and at the endogenous Pig-a gene-locus, along with micronucleus frequencies in peripheral blood. Liver pathology was determined together with NDMA exposure in blood and liver. The additivity of mutation induction was assessed by including two acute single-dose treatment groups (i.e. 5 and 10 mg/kg dose on Day 1), which represented the same total dose as two of the repeat dose treatment groups. NDMA did not induce statistically significant increases in mean lacZ mutant frequency (MF) in bone marrow, spleen, bladder, or stomach, nor in peripheral blood (Pig-a mutation or micronucleus induction) when tested up to 4 mg/kg/day. There were dose-dependent increases in mean lacZ MF in the liver, lung, and kidney following 28-day repeat dosing or in the liver and kidney after a single dose (10 mg/kg). No observed genotoxic effect levels (NOGEL) were determined for the positive repeat dose-response relationships. Mutagenicity did not exhibit simple additivity in the liver since there was a reduction in MF following NDMA repeat dosing compared with acute dosing for the same total dose. Benchmark dose modelling was used to estimate point of departure doses for NDMA mutagenicity in Muta™Mouse and rank order target organ tissue sensitivity (liver > kidney or lung). The BMD50 value for liver was 0.32 mg/kg/day following repeat dosing (confidence interval 0.21-0.46 mg/kg/day). In addition, liver toxicity was observed at doses of ≥ 1.1 mg/kg/day NDMA and correlated with systemic and target organ exposure. The integration of these results and their implications for risk assessment are discussed.

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.

Pig-a gene mutation assay study design: critical assessment of 3- versus 28-day repeat-dose treatment schedules

The in vivo Pig-a assay is being used in safety studies to evaluate the potential of chemicals to induce somatic cell gene mutations. Ongoing work is aimed at developing an Organisation for Economic Cooperation and Development (OECD) test guideline to support routine use for regulatory purposes (OECD project number 4.93). Among the details that will need to be articulated in an eventual guideline are recommended treatment and harvest schedules. With this in mind, experiments reported herein were performed with Wistar Han rats exposed to aristolochic acid I (AA), 1,3-propane sultone, chlorambucil, thiotepa or melphalan using each of two commonly used treatment schedules: 3 or 28 consecutive days. In the case of the 3-day studies, blood was collected for Pig-a analysis on days 15 or 16 and 29 or 30. For the 28-day studies blood was collected on day 29 or 30. The effect of treatment on mutant reticulocytes and mutant erythrocytes was evaluated with parametric pair-wise tests. While each of the five mutagens increased mutant phenotype cell frequencies irrespective of study design, statistical significance was consistently achieved at lower dose levels when the 28-day format was used (e.g. 2.75 vs 20 mg/kg/bw for AA). To more thoroughly investigate the dose-response relationships, benchmark dose (BMD) analyses were performed with PROAST software. These results corroborate the pair-wise testing results in that lower BMD values were obtained with the 28-day design. Finally, mutagenic potency, as measured by BMD analyses, most consistently correlated with the mutagens' tumorigenic dose 50 values when the lengthier treatment schedule was used. Collectively, these results suggest that both 3- and 28-day treatment schedules have merit in hazard identification-type studies. That being said, for the purpose of regulatory safety assessments, there are clear advantages to study designs that utilise protracted exposures.

Recommendations for conducting the rodent erythrocyte Pig-a assay: A report from the HESI GTTC Pig-a Workgroup

The rodent Pig-a assay is a flow cytometric, phenotype-based method used to measure in vivo somatic cell mutation. An Organization for Economic Co-operation and Development (OECD) test guideline is currently being developed to support routine use of the assay for regulatory purposes (OECD project number 4.93). This article provides advice on best practices for designing and conducting rodent Pig-a studies in support of evaluating test substance safety, with a focus on the rat model. Various aspects of assay conduct, including laboratory proficiency, minimum number of animals per dose group, preferred treatment and blood sampling schedule, and statistical analysis are described.

Evaluation of PIG-A-mutated granulocytes and ex-vivo binucleated micronucleated lymphocytes frequencies after breast cancer radiotherapy in humans

Although the PIG-A gene mutation frequency (MF) is considered a good proxy to evaluate the somatic MF in animals, evidence remains scarce in humans. In this study, a granulocyte PIG-A-mutant assay was evaluated in patients undergoing radiation therapy (RT) for breast cancer. Breast cancer patients undergoing adjuvant RT were prospectively enrolled. RT involved the whole breast, with (WBNRT) or without (WBRT) nodal area irradiation. Blood samples were obtained from participants before (T0) RT, and T1, T2, and T3 samples were collected 3 weeks after the initiation of RT, at the end of RT, and at least 10 weeks after RT discontinuation, respectively. The MF was assessed using a flow cytometry protocol identifying PIG-A-mutant granulocytes. Cytokinesis-blocked micronucleated lymphocyte (CBML) frequencies were also evaluated. Thirty patients were included, and five of them had received chemotherapy prior to RT. The mean (±SD) PIG-A MFs were 7.7 (±12.1) per million at T0, 5.2 (±8.6) at T1, 6.4 (±8.0) at T2 and 3.8 (±36.0) at T3. No statistically significant increases were observed between the PIG-A MF at T0 and the MFs at other times. RT significantly increased the CBML frequencies: 7.9 ‰ (±3.1‰) versus 33.6‰ (±17.2‰) (p < .0001). By multivariate analysis, the CBML frequency was correlated with age at RT initiation (p = .043) and irradiation volume at RT discontinuation (p = .0001) but not with chemotherapy. RT for breast cancer therapy failed to induce an increase in the PIG-A MF. The PIG-A assay in humans needs further evaluation, in various genotoxic exposures and including various circulating human cells.

Dextran sulfate sodium mouse model of inflammatory bowel disease evaluated for systemic genotoxicity via blood micronucleus and Pig-a gene mutation assays

Inflammatory bowel disease (IBD) is an important risk factor for gastrointestinal cancers. Inflammation and other carcinogenesis-related effects at distal, tissue-specific sites require further study. In order to better understand if systemic genotoxicity is associated with IBD, we exposed mice to dextran sulfate sodium salt (DSS) and measured the incidence of micronucleated cells (MN) and Pig-a mutant phenotype cells in blood erythrocyte populations. In one study, 8-week-old male CD-1 mice were exposed to 0, 1, 2, 3 or 4% w/v DSS in drinking water. The 4-week in-life period was divided into four 1-week intervals-alternately on then off DSS treatment. Low volume blood samples were collected for MN analysis at the end of each week, and cardiac blood samples were collected at the end of the 4-week period for Pig-a analyses. The two highest doses of DSS were observed to induce significant increases in reticulocyte frequencies. Even so, no statistically significant treatment-related effects on the genotoxicity biomarkers were evident. While one high-dose mouse showed modestly elevated MN frequencies during the DSS treatment cycles, it also exhibited exceptionally high reticulocyte frequencies (e.g. 18.7% at the end of the second DSS cycle). In a second study, mice were treated with 0 or 4% DSS for 9-18 consecutive days. Exposure was continued until rectal bleeding or morbidity was evident, at which point the treatment was terminated and blood was collected for MN analysis. The Pig-a assay was conducted on samples collected 29 days after the start of treatment. The initial blood specimens showed highly elevated reticulocyte frequencies in DSS-exposed mice (mean ± SEM = 1.75 ± 0.10% vs. 13.04 ± 3.66% for 0 vs. 4% mice, respectively). Statistical analyses showed no treatment-related effect on MN or Pig-a mutant frequencies. Even so, the incidence of MN versus reticulocytes in the DSS-exposed mice were positively correlated (linear fit R2 = 0.657, P = 0.0044). Collectively, these results suggest that in the case of the DSS CD-1 mouse model, systemic effects include stress erythropoiesis but not remarkable genotoxicity. To the extent MN may have been slightly elevated in a minority of individual mice, these effects appear to be secondary, likely attributable to stimulated erythropoiesis.

Intra- and inter-laboratory reproducibility of the rat blood Pig-a gene mutation assay

The in vivo Pig-a assay is being used in safety studies to evaluate the potential of chemicals to induce somatic cell gene mutations. Ongoing work is aimed at developing an Organization for Economic Cooperation and Development (OECD) test guideline to support routine use for regulatory purposes (OECD project number 4.93). Among the requirements for OECD approval are demonstrations of assay reliability, including reproducibility within and among laboratories. Experiments reported herein address the reproducibility of the rat blood Pig-a assay using the reference mutagens chlorambucil and melphalan. These agents were evaluated for their ability to induce Pig-a mutant erythrocytes in three separate studies conducted across two laboratories. Each of the studies utilized a common treatment schedule: 28 consecutive days of exposure via oral gavage. Whereas one laboratory studied Crl:CD(SD) rats, the other laboratory used Wistar Han rats. One or two days after cessation of treatment blood samples were collected for mutant reticulocyte and mutant erythrocyte measurements that were accomplished with the same analytical technique whereby samples were depleted of wildtype erythrocytes via immunomagnetic separation followed by flow cytometric enumeration of mutant phenotype cells (MutaFlow®). Dunnett's test results showed similar qualitative outcomes within and between laboratories, that is, each chemical and each study demonstrated statistically significant, dose-related increases in mutant reticulocyte and erythrocyte frequencies. Benchmark dose analysis (PROAST software) provided a means to quantitatively analyze the results, and the relatively tight, overlapping benchmark dose confidence intervals observed for each of the two chemicals indicate that within and between laboratory reproducibility of the Pig-a assay are high, adding further support for the development of an OECD test guideline.

Assessment of Pig-a, Micronucleus, and Comet Assay Endpoints in Tg.RasH2 Mice Carcinogenicity Study of Aristolochic Acid I

A newly developed in vivo Pig-a gene mutation assay displays great potential for integration into genotoxicity tests. To obtain more evidence for application of the Pig-a assay, we integrated this assay, micronucleus test in peripheral blood (MN-pb test) and bone marrow (MN-bm test), as well as a Comet assay into a transgenic RasH2 mice carcinogenicity study. Fourteen male RasH2 mice and five wild-type (WT) mice were treated with a strong mutagen aristolochic acid I at a dose of 5 mg/kg/day for 4 consecutive weeks. Mice recovered in 5 weeks. Peripheral bloods were collected for Pig-a assay, MN-pb test, and Comet assay at several time points, while bone marrow and target organs were harvested for the MN-bm test and pathological diagnosis after mice were euthanized. Finally, 13 of the 14 RasH2 mice developed squamous cell carcinomas in the forestomach, while there were no carcinomas in the WT mice. Pig-a mutant frequencies (MFs) consecutively increased throughout the study to a maximum value of approximately 63-fold more than background. These frequencies were relative to the incidence, size, and malignant degree of tumors. Micronucleated reticulocytes increased from Day 1 to Day 49, before returning to background levels. No positive responses were observed in either the MN-bm test or the Comet assay. Results suggested that, when compared with the other two tests, the Pig-a assay persistently contributed to sustaining MFs, enhanced detection sensitivity due to the accumulation of Pig-a mutations, and demonstrated better predictability for tumorigenicity. Environ. Mol. Mutagen. 61:266-275, 2020. © 2019 Wiley Periodicals, Inc.

Detection of Pig-a Mutant Erythrocytes in the Peripheral Blood of Rats and Mice

The endogenous X-linked phosphatidyl inositol glycan class A gene (Pig-a) can be used as a reporter of in vivo somatic cell mutation in rats and mice. Pig-a mutant cells are deficient in specific protein surface markers and can be identified and quantified by immunofluorescent staining followed by high-throughput flow cytometry. Pig-a mutation detection is commonly performed with red blood cells (RBCs) because: (1) the low volumes of blood required for determining mutant frequencies in RBCs allow multiple samplings on small laboratory animals over extended periods of time; (2) the execution of the RBC assay is easy and the interpretation of the results is straightforward; and (3) RBC Pig-a mutant frequencies are known within hours of sample collection. Two endpoints are determined in the assay: the frequency of mutant total RBCs and the frequency of mutant reticulocytes. When Pig-a mutation is used to assess the in vivo mutagenic potential of suspect hazards, the frequency of mutant reticulocytes is an early indicator of mutagenic potential, while the mutant frequency in total RBCs can be measured more rapidly and with greater precision.

3Rs friendly study designs facilitate rat liver and blood micronucleus assays and Pig-a gene mutation assessments: Proof-of-concept with 13 reference chemicals

Regulatory guidance documents stress the value of assessing the most appropriate endpoints in multiple tissues when evaluating the in vivo genotoxic potential of chemicals. However, conducting several independent studies to evaluate multiple endpoints and/or tissue compartments is resource intensive. Furthermore, when dependent on visual detection, conventional approaches for scoring genotoxicity endpoints can be slow, tedious, and less objective than the ideal. To address these issues with current practices we attempted to (1) devise resource sparing treatment and harvest schedules that are compatible with liver and blood micronucleus endpoints, as well as the Pig-a gene mutation assay, and (2) utilize flow cytometry-based methods to score each of these genotoxicity biomarkers. Proof-of-principle experiments were performed with 4-week-old male and female Crl:CD(SD) rats exposed to aristolochic acids I/II, benzo[a]pyrene, cisplatin, cyclophosphamide, diethylnitrosamine, 1,2-dimethylhydrazine, dimethylnitrosamine, 2,6-dinitrotoluene, hydroxyurea, melphalan, temozolomide, quinoline, or vinblastine. These 13 chemicals were each tested in two treatment regimens: one 3-day exposure cycle, and three 3-day exposure cycles. Each exposure, blood collection, and liver harvest was accomplished during a standard Monday-Friday workweek. Key findings are that even these well-studied, relatively potent genotoxicants were not active in both tissues and all assays (indeed only cisplatin was clearly positive in all three assays); and whereas the sensitivity of the Pig-a assay clearly benefitted from three versus one treatment cycle, micronucleus assays yielded qualitatively similar results across both study designs. Collectively, these results suggest it is possible to significantly reduce animal and other resource requirements while improving assessments of in vivo genotoxicity potential by simultaneously evaluating three endpoints and two important tissue compartments using fit-for-purpose study designs in conjunction with flow cytometric scoring approaches. Environ. Mol. Mutagen., 60:704-739, 2019. © 2019 Wiley Periodicals, Inc.

Integrated In Vivo Genotoxicity Assessment of Procarbazine Hydrochloride Demonstrates Induction of Pig-a and LacZ Mutations, and Micronuclei, in MutaMouse Hematopoietic Cells

Procarbazine hydrochloride (PCH) is a DNA-reactive hematopoietic carcinogen with potent and well-characterized clastogenic activity. However, there is a paucity of in vivo mutagenesis data for PCH, and in vitro assays often fail to detect the genotoxic effects of PCH due to the complexity of its metabolic activation. We comprehensively evaluated the in vivo genotoxicity of PCH on hematopoietic cells of male MutaMouse transgenic rodents using a study design that facilitated assessments of micronuclei and Pig-a mutation in circulating erythrocytes, and lacZ mutant frequencies in bone marrow. Mice were orally exposed to PCH (0, 6.25, 12.5, and 25 mg/kg/day) for 28 consecutive days. Blood samples collected 2 days after cessation of treatment exhibited significant dose-related induction of micronuclei in both immature and mature erythrocytes. Bone marrow and blood collected 3 and 70 days after cessation of treatment also showed significantly elevated mutant frequencies in both the lacZ and Pig-a assays even at the lowest dose tested. PCH-induced lacZ and Pig-a (immature and mature erythrocytes) mutant frequencies were highly correlated, with R2 values ≥0.956, with the exception of lacZ vs. Pig-a mutants in mature erythrocytes at the 70-day time point (R2 = 0.902). These results show that PCH is genotoxic in vivo and demonstrate that the complex metabolism and resulting genotoxicity of PCH is best evaluated in intact animal models. Our results further support the concept that multiple biomarkers of genotoxicity, especially hematopoietic cell genotoxicity, can be readily combined into one study provided that adequate attention is given to manifestation times. Environ. Mol. Mutagen. 60:505-512, 2019. © 2018 Her Majesty the Queen in Right of Canada.

In vivo genotoxicity testing strategies: Report from the 7th International workshop on genotoxicity testing (IWGT)

The working group reached complete or majority agreement on many issues. Results from TGR and in vivo comet assays for 91 chemicals showed they have similar ability to detect in vivo genotoxicity per se with bacterial mutagens and Ames-positive carcinogens. TGR and comet assay results were not significantly different when compared with IARC Group 1, 2 A, and unclassified carcinogens. There were significantly more comet assay positive responses for Group 2B chemicals, and for IARC classified and unclassified carcinogens combined, which may be expected since mutation is a sub-set of genotoxicity. A liver comet assay combined with the bone marrow/blood micronucleus (MNviv) test would detect in vivo genotoxins that do not exhibit tissue-specific or site-of-contact effects, and is appropriate for routine in vivo genotoxicity testing. Generally for orally administered substances, a comet assay at only one site-of-contact GI tract tissue (stomach or duodenum/jejunum) is required. In MNviv tests, evidence of target tissue exposure can be obtained in a number of different ways, as recommended by ICH S2(R1) and EFSA (Hardy et al., 2017). Except for special cases the i.p. route is inappropriate for in vivo testing; for risk evaluations more weight should be given to data from a physiologically relevant administration route. The liver MN test is sufficiently validated for the development of an OECD guideline. However, the impact of dosing animals >6 weeks of age needs to be evaluated. The GI tract MN test shows promise but needs more validation for an OECD guideline. The Pig-a assay detects systemically available mutagens and is a valuable follow-up to in vitro positive results. A new freeze-thaw protocol provides more flexibility. Mutant reticulocyte and erythrocyte frequencies should both be determined. Preliminary data are available for the Pig-a assay in male rat germ cells which require validation including germ cell DNA mutation origin.

Assessment of the Pig-a, micronucleus, and comet assay endpoints in rats treated by acute or repeated dosing protocols with procarbazine hydrochloride and ethyl carbamate

The utility and sensitivity of the newly developed flow cytometric Pig-a gene mutation assay have become a great concern recently. In this study, we have examined the feasibility of integrating the Pig-a assay as well as micronucleus and Comet endpoints into acute and subchronic general toxicology studies. Male Sprague-Dawley rats were treated for 3 or 28 consecutive days by oral gavage with procarbazine hydrochloride (PCZ) or ethyl carbamate (EC) up to the maximum tolerated dose. The induction of CD59-negative reticulocytes and erythrocytes, micronucleated reticulocytes in peripheral blood, micronucleated polychromatic erythrocytes in bone marrow, and Comet responses in peripheral blood, liver, kidney, and lung were evaluated at one, two, or more timepoints. Both PCZ and EC produced positive responses at most analyzed timepoints in all tissue types, both with the 3-day and 28-day treatment regimens. Furthermore, comparison of the magnitude of the genotoxicity responses indicated that the micronucleus and Comet endpoints generally produced greater responses with the higher dose, short-term treatments in the 3-day study, while the Pig-a assay responded better to the cumulative effects of the lower dose, but repeated subchronic dosing in the 28-day study. Collectively, these results indicate that integration of several in vivo genotoxicity endpoints into a single routine toxicology study is feasible and that the Pig-a assay may be particularly suitable for integration into subchronic dose studies based on its ability to accumulate the mutations that result from repeated treatments. This characteristic may be especially important for assaying lower doses of relatively weak genotoxicants. Environ. Mol. Mutagen. 60:56-71, 2019. © 2018 Wiley Periodicals, Inc.

Suitability of Long-Term Frozen Rat Blood Samples for the Interrogation of Pig-a Gene Mutation by Flow Cytometry

The rodent blood Pig-a assay has been undergoing international validation for use as an in vivo hematopoietic cell gene mutation assay, and given the promising results an Organization for Economic Co-operation and Development (OECD) Test Guideline is currently under development. Enthusiasm for the assay stems in part from its alignment with 3Rs principles permitting combination with other genotoxicity endpoint(s) and integration into repeat-dose toxicology studies. One logistical requirement and experimental design limitation has been that blood samples required antibody labeling and flow cytometric analysis within one week of collection. In the current report, we describe the performance of freeze-thaw reagents that enable storage and subsequent labeling and analysis of rat blood samples for at least seven months. Data generated from three laboratories are presented that demonstrate rat erythrocyte recoveries in the range of 80-90%. Despite some loss of erythrocytes, Pearson coefficients and Bland-Altman analyses based on fresh blood vs. frozen/thawed matched pairs indicate that mutant cell and reticulocyte frequencies are not significantly affected, as the measurements are highly correlated and exhibit low bias. Collectively, these data support the effectiveness and suitability of a freeze-thaw procedure that endows the assay with several new advantageous characteristics that include: flexibility in scheduling personnel/instrumentation; reliability when shipping samples from in-life facilities to analytical sites; 3Rs-friendly, as blood from positive control animals can be stored frozen to serve as analytical controls; and ability to defer a decision to generate Pig-a data until more toxicological information becomes available on a test substance. Environ. Mol. Mutagen. 60:47-55, 2019. © 2018 Wiley Periodicals, Inc.

Application of the in vivo Pig-a gene mutation assay to test the potential genotoxicity of p-phenylenediamine

Currently, it remains controversial whether p-phenylenediamine (PPD) is genotoxic. In this study, we evaluated the potential genotoxicity of PPD using the newly-developed Pig-a gene mutation assay. The results of three classical genetic toxicity tests (bacterial reverse mutation assay, mammalian cell chromosomal aberration test, and mammalian erythrocyte micronucleus test) are all positive, suggesting that PPD is potentially genotoxic. In Pig-a assay, Sprague-Dawley rats are orally administered with PPD for 28 consecutive days at three doses (12.5, 25, and 50 mg/kg/day). Our result shows that PPD (25 and 50 mg/kg/day) dose-dependently increases RET^CD59- value over controls on Day 8. RET^CD59- keeps increasing to the maximum on Day 15 and then decreases until Day 29. PPD also dose-dependently increase RBC^CD59- value on Day 15, which keeps elevating until Day 29. The time-course of RET^CD59- and RBC^CD59- induced by PPD are similar with that induced by N-ethyl-N-nitrosourea (ENU) treatment for 3 days. Our data suggests that PPD has potential genotoxic effects, and the Pig-a assay is sensitive to assess mutagenicity. However, further investigation of the changes of RET^CD59- and RBC^CD59- induced by hair dyes containing PPD should be detected by Pig-a assay in occupational exposure population to confirm the safety of PPD usage.

Trimethoprim-sulfamethoxazole treatment increases the Pig-a mutant frequency in peripheral blood from severely malnourished rats

Severe malnutrition is a complex condition that increases susceptibility to infections. Thus, drugs are extensively used in malnutrition cases. In the present study, we assessed the mutagenic effects of combined trimethoprim and sulfamethoxazole (TMP-SMX) treatment in undernourished (UN) and well-nourished (WN) rats. Six-week-old UN and WN Han-Wistar rats were treated with TMP-SMX at a daily dose of 10 mg/kg/d TMP and 50 mg/kg/d SMX for 5 or 10 days. Blood was collected from the tail vein one day before (day -1) and 15, 30, and 45 days after TMP-SMX administration. The Pig-a mutant frequencies (MFs) in peripheral blood reticulocytes (RETs) and erythrocytes (RBCs) were measured through flow cytometry. Severe malnutrition increased the basal MFs in RETs (RET CD59-) and RBC (RBCs CD59-). These findings support the hypothesis that severe malnutrition is mutagenic even in the absence of exposure to an exogenous mutagen. UN and WN rats treated for 5 or 10 consecutive days with TMP-SMX had significantly increased and sustained Pig-a mutant frequencies, demonstrating the mutagenic effects of this drug.

A regenerative erythropoietic response does not increase the frequency of Pig-a mutant reticulocytes and erythrocytes in Sprague-Dawley rats

The in vivo rodent Pig-a mutation assay is a sensitive test to identify exposure to mutagenic substances, and has been proposed as an assay for the identification of impurities for pharmaceuticals. Red blood cells (RBCs) and reticulocytes (RETs) are analyzed by flow cytometry after exposure to potentially mutagenic chemicals for cells deficient in the cell surface anchored protein CD59, representing mutation in the X-linked Pig-a gene. The full potential of the assay as well as its limitations are currently being explored. The current study investigated the effects of regenerative erythropoietic bone marrow responses on the frequency of Pig-a mutated reticulocytes (RET^CD59- ) and erythrocytes (RBC^CD59- ). We hypothesized that a robust regenerative erythropoietic response would not increase the basal frequency of RET^CD59- or RBC^CD59- cells. Two groups of six male Sprague-Dawley rats either had 2 mL of blood sampled each day via an indwelling catheter over a period of 5 days or were minimally sampled for hematology and used as controls. Blood was also then collected and evaluated 5, 18, and 49 days after the initial bleed period for the number of Pig-a mutant cells in either the RET or RBC population. Despite the expected decrease in hematocrit and the correlative increase in reticulocytes after bleeding, no increase in the number of Pig-a mutant cells was observed in male Sprague-Dawley rats that were bled for five consecutive days. These results indicate that changes in erythropoiesis and hematology parameters in rats appear to have no effect on the background levels of Pig-a mutated RETs and RBCs. Environ. Mol. Mutagen. 59:91-95, 2018. © 2017 Wiley Periodicals, Inc.

In vivo pig-a and micronucleus study of the prototypical aneugen vinblastine sulfate

The Pig-a assay is being used in regulatory studies to evaluate the potential of agents to induce somatic cell gene mutations and an OECD test guideline is under development. A working group involved with establishing the guideline recently noted that representative aneugenic agents had not been evaluated, and to help fill this data gap Pig-a mutant phenotype and micronucleated reticulocyte frequencies were measured in an integrated study design to assess the mutagenic and cytogenetic damage responses to vinblastine sulfate exposure. Male Sprague Dawley rats were treated for twenty-eight consecutive days with vinblastine dose levels from 0.0156 to 0.125 mg/kg/day. Micronucleated reticulocyte frequencies in peripheral blood were determined at Days 4 and 29, and mutant cell frequencies were determined at Days -4, 15, 29, and 46. Vinblastine affected reticulocyte frequencies, with reductions noted during the treatment phase and increases observed following cessation of treatment. Micronucleated reticulocyte frequencies were significantly elevated at Day 4 in the high dose group. Although a statistically significant increase in mutant reticulocyte frequencies were found for one dose group at a single time point (Day 46), it was not deemed biologically relevant because there was no analogous finding in mutant RBCs, it occurred at the lowest dose tested, and only 1 rat exceeded an upper bound tolerance interval established with historical negative control rats. Therefore, whereas micronucleus induction reflects vinblastine's well-established aneugenic effect on hematopoietic cells, the lack of a Pig-a response indicates that this tubulin-binding agent does not cause appreciable mutagenicity in this same cell type. Environ. Mol. Mutagen. 59:30-37, 2018. © 2017 Wiley Periodicals, Inc.

Applying the erythrocyte Pig-a assay concept to rat epididymal sperm for germ cell mutagenicity evaluation

The Pig-a assay, a recently developed in vivo somatic gene mutation assay, is based on the identification of mutant erythrocytes that have an altered repertoire of glycosylphosphatidylinositol (GPI)-anchored cell surface markers. We hypothesized that the erythrocyte Pig-a assay concept could be applied to rat cauda epididymal spermatozoa (sperm) for germ cell mutagenicity evaluation. We used GPI-anchored CD59 as the Pig-a mutation marker and examined the frequency of CD59-negative sperm using flow cytometry. A reconstruction experiment that spiked un-labeled sperm (mutant-mimic) into labeled sperm at specific ratios yielded good agreement between the detected and expected frequencies of mutant-mimic sperm, demonstrating the analytical ability for CD59-negative sperm detection. Furthermore, this methodology was assessed in F344/DuCrl rats administered N-ethyl-N-nitrosourea (ENU), a prototypical mutagen, or clofibrate, a lipid-lowering drug. Rats treated with 1, 10, or 20 mg/kg body weight/day (mkd) ENU via daily oral garage for five consecutive days showed a dose-dependent increase in the frequency of CD59-negative sperm on study day 63 (i.e., 58 days after the last ENU dose). This ENU dosing regimen also increased the frequency of CD59-negative erythrocytes. In rats treated with 300 mkd clofibrate via daily oral garage for consecutive 28 days, no treatment-related changes were detected in the frequency of CD59-negative sperm on study day 85 (i.e., 57 days after the last dose) or in the frequency of CD59-negative erythrocytes on study day 29. In conclusion, these data suggest that the epidiymal sperm Pig-a assay in rats is a promising method for evaluating germ cell mutagenicity. Environ. Mol. Mutagen. 58:485-493, 2017. © 2017 Wiley Periodicals, Inc.

The Pig-a Gene Mutation Assay in Mice and Human Cells: A Review

This MiniReview describes the principle of mutation assays based on the endogenous Pig-a gene and summarizes results for two species of toxicological interest, mice and human beings. The work summarized here largely avoids rat-based studies, as are summarized elsewhere. The Pig-a gene mutation assay has emerged as a valuable tool for quantifying in vivo and in vitro mutational events. The Pig-a locus is located at the X-chromosome, giving the advantage that one inactivated allele can give rise to a mutated phenotype, detectable by multicolour flow cytometry. For in vivo studies, only minute blood volumes are required, making it easily incorporated into ongoing studies or experiments with limited biological materials. Low blood volumes also allow individuals to serve as their own controls, providing temporal information of the mutagenic process, and/or outcome of intervention. These characteristics make it a promising exposure marker. To date, the Pig-a gene mutation assay has been most commonly performed in rats, while reports regarding its usefulness in other species are accumulating. Besides its applicability to in vivo studies, it holds promise for genotoxicity testing using cultured cells, as shown in recent studies. In addition to safety assessment roles, it is becoming a valuable tool in basic research to identify mutagenic effects of different interventions or to understand implications of various gene defects by investigating modified mouse models or cell systems. Human blood-based assays are also being developed that may be able to identify genotoxic environmental exposures, treatment- and lifestyle-related factors or endogenous host factors that contribute to mutagenesis.

[The PIG-A gene as a new biomarker of mutagenesis: proof of concept and technical specifications]

Gene mutations are not directly detected by current genotoxicity assays and most of them need a cell culture step. The whole blood PIG-A assay consists in the detection of the mutation frequency within the PIG-A sentinel gene by identification of glycosyl-phosphatidyl-inositol (GPI-) deficient cells. PIG-A mutated/GPI-deficient cells can be detected by flow cytometry as they no longer express surface fluorescence for GPI-linked markers. The last researches have focused on cell enrichment techniques leading to increased throughput and sensitivity. The results of this new and promising biomarker of mutagenesis, performed in humans or rodents, are now available within 2 hours after blood collection.

In Vivo Rat T-Lymphocyte Pig-a Assay: Detection and Expansion of Cells Deficient in the GPI-Anchored CD48 Surface Marker for Analysis of Mutation in the Endogenous Pig-a Gene

The Pig-a assay is being developed as an in vivo gene mutation assay for regulatory safety assessments. The assay is based on detecting mutation in the endogenous Pig-a gene of treated rats by using flow cytometry to measure changes in cell surface markers of peripheral blood cells. Here we present a methodology for demonstrating that phenotypically mutant rat T-cells identified by flow cytometry contain mutations in the Pig-a gene, an important step for validating the assay. In our approach, the mutant phenotype T-cells are sorted into individual wells of 96-well plates and expanded into clones. Subsequent sequencing of genomic DNA from the expanded clones confirms that the Pig-a assay detects exactly what it claims to detect-cells with mutations in the endogenous Pig-a gene. In addition, determining the spectra of Pig-a mutations provides information for better understanding the mutational mechanism of compounds of interest. Our methodology of combining phenotypic antibody labeling, magnetic enrichment, sorting, and single-cell clonal expansion can be used in genotoxicity/mutagenicity studies and in other general immunotoxicology research requiring identification, isolation, and expansion of extremely rare subpopulations of T-cells.

Moderate malnutrition in rats induces somatic gene mutations

The relationship between malnutrition and genetic damage has been widely studied in human and animal models, leading to the observation that interactions between genotoxic exposure and micronutrient status appear to affect genomic stability. A new assay has been developed that uses the phosphatidylinositol glycan class A gene (Pig-a) as a reporter for measuring in vivo gene mutation. The Pig-a assay can be employed to evaluate mutant frequencies (MFs) in peripheral blood reticulocytes (RETs) and erythrocytes (RBCs) using flow cytometry. In the present study, we assessed the effects of malnutrition on mutagenic susceptibility by exposing undernourished (UN) and well-nourished (WN) rats to N-ethyl-N-nitrosourea (ENU) and measuring Pig-a MFs. Two week-old UN and WN male Han-Wistar rats were treated daily with 0, 20, or 40mg/kg ENU for 3 consecutive days. Blood was collected from the tail vein one day before ENU treatment (Day-1) and after ENU administration on Days 7, 14, 21, 28, 35, 42, 49, 56 and 63. Pig-a MFs were measured in RETs and RBCs as the RET(CD59-) and RBC(CD59-) frequencies. In the vehicle control groups, the frequencies of mutant RETs and RBCs were significantly higher in UN rats compared with WN rats at all sampling times. The ENU treatments increased RET and RBC MFs starting at Day 7. Although ENU-induced Pig-a MFs were consistently lower in UN rats than in WN rats, these differences were not significant. To understand these responses, further studies should use other mutagens and nucleated surrogate cells and examine the types of mutations induced in UN and WN rats.

Comparison of integrated genotoxicity endpoints in rats after acute and subchronic oral doses of 4-nitroquinoline-1-oxide

During interlaboratory validation trials for the Pig-a gene mutation assay we assessed the genotoxicity of 4-nitroquinoline-1-oxide (4NQO) across endpoints in multiple tissues: induction of Pig-a mutant red blood cells (RBCs) and reticulocytes (RETs); micronucleated RETs (MN RETs); and DNA damage in blood and liver via the alkaline Comet assay (%tail intensity [TI]). In a previous subchronic toxicity study with 28 daily doses, biologically meaningful increases were observed only for Pig-a mutant RBCs/RETs while marginal increases in the frequency of MN RET were observed, and other clastogenic endpoints were negative. Follow up acute studies were performed using the same cumulative doses (0, 35, 70, 105, and 140 mg/kg) administered in a bolus, or split over three equal daily doses, with samples collected up to 1 month after the last dose. Both of the acute dosing regimens produced similar results, in that endpoints were either positive or negative, regardless of 1 or 3 daily doses, but the three consecutive daily dose regimen yielded more potent responses in TI (in liver and blood) and Pig-a mutant frequencies. In these acute studies the same cumulative doses of 4NQO induced positive responses in clastogenic endpoints that were negative or inconclusive using a subchronic study design. Additionally, a positive control group using combination doses of cyclophosphamide and ethyl methanesulfonate was employed to assess assay validity and potentially identify a future positive control treatment for integrated genetic toxicity studies.

Human erythrocyte PIG-A assay: an easily monitored index of gene mutation requiring low volume blood samples

This laboratory has previously described a method for scoring the incidence of rodent blood Pig-a mutant phenotype erythrocytes using immunomagnetic separation in conjunction with flow cytometric analysis (In Vivo MutaFlow®). The current work extends this approach to human blood. The frequencies of CD59- and CD55-negative reticulocytes (RET(CD59-/CD55-)) and erythrocytes (RBC(CD59-/CD55-)) serve as phenotypic reporters of PIG-A gene mutation. Immunomagnetic separation was found to provide an effective means of increasing the number of reticulocytes and erythrocytes evaluated. Technical replicates were utilized to provide a sufficient number of cells for precise scoring while at the same time controlling for procedural accuracy by allowing comparison of replicate values. Cold whole blood samples could be held for at least one week without affecting reticulocyte, RET(CD59-/CD55-) or RBC(CD59-/CD55-) frequencies. Specimens from a total of 52 nonsmoking, self-reported healthy adult subjects were evaluated. The mean frequency of RET(CD59-/CD55-) and RBC(CD59-/CD55-) were 6.0 × 10(-6) and 2.9 × 10(-6), respectively. The difference is consistent with a modest selective pressure against mutant phenotype erythrocytes in the circulation, and suggests advantages of studying both populations of erythrocytes. Whereas intra-subject variability was low, inter-subject variability was relatively high, with RET(CD59-/CD55-) frequencies differing by more than 30-fold. There was an apparent correlation between age and mutant cell frequencies. Taken together, the results indicate that the frequency of human PIG-A mutant phenotype cells can be efficiently and reliably estimated using a labeling and analysis protocol that is well established for rodent-based studies. The applicability of the assay across species, its simplicity and statistical power, and the relatively non-invasive nature of the assay should benefit myriad research areas involving DNA damage, including studies of environmental factors that modify "spontaneous" mutation frequencies.

Integration of Pig-a, micronucleus, chromosome aberration and comet assay endpoints in a 28-day rodent toxicity study with urethane

As part of the international Pig-a validation trials, we examined the induction of Pig-a mutant reticulocytes and red blood cells (RET(CD59-) and RBC(CD59-), respectively) in peripheral blood of male Sprague Dawley(®) rats treated with urethane (25, 100 and 250mg/kg/day) or saline by oral gavage for 29 days. Additional endpoints integrated into this study were: micronucleated reticulocytes (MN-RET) in peripheral blood; chromosome aberrations (CAb) and DNA damage (%tail intensity via the comet assay) in peripheral blood lymphocytes (PBL); micronucleated polychromatic erythrocytes (MN-PCE) in bone marrow; and DNA damage (comet) in various organs at termination (the 29th dose was added for the comet endpoint at sacrifice). Ethyl methanesulfonate (EMS; 200mg/kg/day on Days 3, 4, 13, 14, 15, 27, 28 and 29) was evaluated as the concurrent positive control (PC). All animals survived to termination and none exhibited overt toxicity, but there were significant differences in body weight and body weight gain in the 250-mg/kg/day urethane group, as compared with the saline control animals. Statistically significant, dose-dependent increases were observed for urethane for: RET(CD59-) and RBC(CD59-) (on Days 15 and 29); MN-RET (on Days 4, 15 and 29); and MN-PCE (on Day 29). The comet assay yielded positive results in PBL (Day 15) and liver (Day 29), but negative results for PBL (Days 4 and 29) and brain, kidney and lung (Day 29). No significant increases in PBL CAb were observed at any sample time. Except for PBL CAb (likely due to excessive cytotoxicity), EMS-induced significant increases in all endpoints/tissues. These results compare favorably with earlier in vivo observations and demonstrate the utility and sensitivity of the Pig-a in vivo gene mutation assay, and its ability to be easily integrated, along with other standard genotoxicity endpoints, into 28-day rodent toxicity studies.

Rat Pig-a mutation assay responds to the genotoxic carcinogen ethyl carbamate but not the non-genotoxic carcinogen methyl carbamate

Determination of the mode of action of carcinogenic agents is an important factor in risk assessment and regulatory practice. To assess the ability of the erythrocyte-based Pig-a mutation assay to discriminate between genotoxic and non-genotoxic modes of action, the mutagenic response of Sprague Dawley rats exposed to methyl carbamate (MC) or ethyl carbamate (EC) was investigated. EC, a potent carcinogen, is believed to induce DNA damage through the formation of a DNA-reactive epoxide group, whereas the closely structurally related compound, MC, cannot form this epoxide and its weaker carcinogenic activity is thought to be secondary to inflammation and promotion of cell proliferation. The frequency of Pig-a mutant phenotype cells was monitored before, during, and after 28 consecutive days of oral gavage exposure to either MC (doses ranging from 125 to 500 mg/kg/day) or EC (250 mg/kg/day). Significant increases in the frequency of mutant reticulocytes were observed from Days 15 through 43, with a peak mean frequency of 19.9×10(-6) on Day 29 (i.e. 24.9-fold increase relative to mean vehicle control across all four sampling times). As expected, mutant erythrocyte responses lagged behind mutant reticulocyte responses, with a maximal mean frequency of 8.2×10(-6) on Day 43 (i.e. 16.4-fold increase). No mutagenic effects were observed with MC. A second indicator of in vivo genotoxicity, peripheral blood micronucleated reticulocytes, was also studied. This endpoint was responsive to EC (3.3-fold mean increase), but not to MC. These results support the hypothesis that genotoxicity contributes to the carcinogenicity of EC but not of MC, and illustrates the value of the Pig-a assay for discriminating between genotoxic and non-genotoxic modes of action.

Comparison of male versus female responses in the Pig-a mutation assay

Validation of the Pig-a gene mutation assay has been based mainly on studies in male rodents. To determine if the mutagen-induced responses of the X-linked Pig-a gene differ in females compared to males, 7- or 14-week old male and female Sprague Dawley rats were exposed to N-ethyl-N-nitrosourea (ENU). In the study with the 7-week old rats, exposure was to 0, 1, 5 or 25mg ENU/kg/day for three consecutive days (study Days 1-3). Pig-a mutant phenotype reticulocyte (RET(CD59-)) and mutant phenotype erythrocyte (RBC(CD59-)) frequencies were determined on study Days -4, 15, 29 and 46 using immunomagnetic separation in conjunction with flow cytometric analysis (In Vivo MutaFlow®). Additionally, blood samples collected on Day 4 were analysed for micronucleated reticulocyte (MN-RET) frequency (In Vivo MicroFlow®). The percentage of reticulocytes (%RET) was markedly higher in the 7-week old males compared to females through Day 15 (2.39-fold higher on Day -4). At 25mg/kg/day, ENU reduced Day 4 RET frequencies in both sexes, and the two highest dose levels resulted in elevated MN-RET frequencies, with no sex or treatment × sex interaction. The two highest dose levels significantly elevated the frequencies of mean RET(CD59-) and RBC(CD59-) in both sexes from Day 15 onward. RET(CD59-) and RBC(CD59-) frequencies were somewhat lower for females compared to males at the highest dose level studied, and differences in RET(CD59-) resulted in a statistically significant interaction effect of treatment × sex. In the study with 14-week old rats, treatment was for 3 days with 0 or 25mg ENU/kg/day. RET frequencies differed to a lesser degree between the sexes, and in this case there was no evidence of a treatment × sex interaction. These results suggest that the slightly higher response in younger males than in the younger females may be related to differences in erythropoiesis function at that age. In conclusion, while some quantitative differences were noted, there were no qualitative differences in how males and females responded to a prototypical mutagen, and support the contention that both sexes are equally acceptable for Pig-a gene mutation studies.

Evaluation of the in vivo mutagenicity of isopropyl methanesulfonate in acute and 28-day studies

Understanding the mutagenic dose response could prove beneficial in the management of pharmaceutically relevant impurities. For most alkyl ester impurities, such as isopropyl methanesulfonate (IPMS), little in vivo mutagenicity data exist for dose analysis. The likelihood of a sublinear dose response for IPMS was assessed by comparing the Swain Scott constant, the SN 1/SN 2 reaction mechanism and the O(6) :N(7) guanine adduct ratio to that of more well-known alkyl esters. Based on available information, IPMS was predicted to have a mutagenic profile most like ethyl nitrosourea. To test this hypothesis, mature male Wistar Han rats were administered IPMS using acute (single administration at 3.5 to 56 mg/kg) or subchronic (28 days at 0.125 to 2 mg/kg/day) exposures. The in vivo Pig-a mutation assay was used to identify mutant phenotype reticulocyte (Ret) and red blood cell (RBC) populations. The maximum mutant response occurred approximately 15 and 28 days after the last dose administration in the mutant Ret and RBC populations respectively in the acute study and on Day 29 and 56 in the mutant Ret and RBC populations, respectively, in the subchronic study. A comparison of RBC mutant frequencies from acute and subchronic protocols suggests a sublinear response; however, this was not substantiated by statistical analysis. A No Observed Effect Level (NOEL) of 0.25 mg/kg/day resulted in a Permitted Daily Exposure equivalent to the Threshold of Toxicological Concern. An estimate of the NOEL based on the previously mentioned factors, in practice, would have pre-empted further investigation of the potent mutagen IPMS.

Compensatory erythropoiesis has no impact on the outcome of the in vivo Pig-a mutation assay in rats following treatment with the haemolytic agent 2-butoxyethanol

The Pig-a assay has rapidly gained international interest as a useful tool for assessing the mutagenic potential of compounds in vivo. Although a large number of compounds, including both mutagens and non-mutagens, have been tested in the rat Pig-a assay in haematopoietic cells, there is limited understanding of how perturbations in haematopoiesis affect assay performance. Of particular concern is the possibility that regenerative haematopoiesis alone, without exposure to a genotoxic agent, could result in elevated Pig-a mutant cell frequencies. To address this concern, Wistar-Han rats were dosed by oral gavage with a non-genotoxic haemolytic agent, 2-butoxyethanol (2-BE). Dose levels ranging from 0 to 450 mg/kg were tested using both single administration and 28-day treatment regimens. Haematology parameters were assessed at minimum within the first 24h of treatment and 8 days after the final administration. Pig-a mutant frequencies were assessed on Days 15 and ~30 for both treatment protocols and also on Days 43 and 57 for the 28-day protocol. Even at doses of 2-BE that induced marked intravascular lysis and strong compensatory erythropoiesis, the average Pig-a mutant phenotype red blood cell and reticulocyte frequencies were within the historical vehicle control distribution. 2-BE therefore showed no evidence of in vivo mutagenicity in these studies. The data suggest that perturbations in haematopoiesis alone do not lead to an observation of increased mutant frequency in the Pig-a assay.

Induction of Pig-a mutant erythrocytes in male and female rats exposed to 1,3-propane sultone, ethyl carbamate, or thiotepa

Validation of the Pig-a gene mutation assay has been based mainly on studies in male rodents. To determine if the mutagen-induced responses of the X-linked Pig-a gene differ in females compared to males, groups of five male and female Sprague Dawley rats were exposed to the mutagens 1,3-propane sultone (80mg/kg/day), ethyl carbamate (600mg/kg/day), or thiotepa (7.5mg/kg/day) for three consecutive days (study days 1-3). Pig-a mutant phenotype reticulocyte (RET(CD59-)) and mutant phenotype erythrocyte (RBC(CD59-)) frequencies were determined on study days -4, 15, 30 and 46 using immunomagnetic separation in conjunction with flow cytometric analysis (In Vivo MutaFlow(®)). While the percentage of reticulocytes (%RET) was markedly higher for pre-treatment blood samples from males compared to females (6.6% vs. 3.5%), this sex effect was slight or nonexistent at later time points. Treatment-related effects to %RET were generally modest owing to the 12-day interval between last exposure and blood sampling. Mean RET(CD59-) and RBC(CD59-) frequencies were consistently low in vehicle control animals of both sexes, with 77% of samples exhibiting mutant cell frequencies ≤1×10(-6) over study days 15-46. Treatment with each mutagen caused significant increases to mean RET(CD59-) and RBC(CD59-) frequencies. Whereas genotoxicant-induced RET(CD59-) values were maximal on day 15, induced RBC(CD59-) frequencies were highest at the last sampling time. Sex did not affect 1,3-propane sultone- or thiotepa-induced mutant cell frequencies. While ethyl carbamate-exposed females exhibited higher mean mutant cell frequencies compared to like-treated males, statistical significance was achieved only for RBC(CD59-) at one time point (7.6±1.0×10(-6) compared to 4.7±0.6×10(-6) on day 30). Thus, while some quantitative differences were evident, there were no qualitative differences in how males and females responded to three diverse mutagens. These data support the use of both sexes for Pig-a gene mutation studies.

Assessment of 5-fluorouracil and 4-nitroquinoline-1-oxide in vivo genotoxicity with Pig-a mutation and micronucleus endpoints

Genotoxicity assessments were conducted on male Sprague Dawley rats treated with 5-fluorouracil (5-FU) and 4-nitroquinoline-1-oxide (4NQO) as part of an international validation trial of the Pig-a mutant phenotype assay. Rats were orally exposed to 0, 11.5, 23, or 46 mg/kg/day 5-FU for three consecutive days (Days 1-3); blood was sampled on Days -1, 4, 15, 29, and 45. Pig-a mutant phenotype reticulocyte (RET(CD59-)) and mutant phenotype erythrocyte (RBC(CD59-)) frequencies were determined on Days -1, 15, 29, and 45, and percent micronucleated reticulocytes (%MN-RET) were measured on Day 4. Rats were treated with 4NQO for 28 consecutive days by oral gavage, at doses of 1.5, 3, or 6 mg/kg/day. RBC(CD59-) and RET(CD59-) frequencies were determined on Days -1, 15, and 29, and MN-RET were quantified on Day 29. Whereas 5-FU was found to increase %MN-RET, no significant increases were observed for RBC(CD59-) or RET(CD59-) at any of the time points studied. The high dose of 4NQO (6 mg/kg/day) was observed to markedly increase RBC(CD59-) and RET(CD59-) frequencies, and this same dose level caused a weak but significantly elevated increase in MN-RET (approximately twofold). Collectively, the results provide additional support for the combination of Pig-a mutation and MN-RET into acute and 28-day repeat-dose studies.

The in vitro PIG-A gene mutation assay: mutagenicity testing via flow cytometry based on the glycosylphosphatidylinositol (GPI) status of TK6 cells

The X-linked PIG-A gene is involved in the biosynthesis of the cell surface anchor GPI, and its inactivation may serve as a new marker for mutagenicity. The in vivo PIG-A gene mutation assay is currently being validated by several groups. In this study, we established a corresponding in vitro variant of the PIG-A assay applying B-lymphoblastoid TK6 cells. PE-conjugated antibodies against the GPI-anchored proteins CD55 and CD59 were used to determine the GPI status via multicolor flow cytometry. Mutant spiked TK6 cell samples were analyzed, and mutants were quantified with even small numbers being quantitatively recovered. To validate our approach, mutant spiked cell samples were analyzed by flow cytometry and proaerolysin selection in parallel, yielding a high correlation. Further, we developed a procedure to reduce the background level of preexisting mutant cells to lower than 20 in 10(6) cells to increase the sensitivity of the assay. Spontaneous rate of GPI deficiency was investigated being 0.76 × 10(-6)/cell/generation for TK6 cells. The optimal phenotype expression time after ethyl methanesulfonate treatment was found to be 10 days. We applied the in vitro PIG-A assay to demonstrate the mutagenicity of ethyl methanesulfonate, 4-nitroquinoline 1-oxide and UV-C irradiation in a dose-dependent and statistically significant manner. Pyridine and cycloheximide were included as negative controls providing negative test results up to 10 mM. These data suggest that the in vitro PIG-A assay could complement the in vivo PIG-A assay with some distinct advantages compared to other in vitro mammalian mutagenicity tests.

Evaluation of the Pig-a, micronucleus, and comet assay endpoints in a 28-day study with ethyl methanesulfonate

Ethyl methanesulfonate (EMS) was evaluated as part of the validation effort for the rat Pig-a mutation assay and compared with other well-established in vivo genotoxicity endpoints. Male Sprague-Dawley (SD) rats were given a daily dose of 0, 6.25, 12.5, 25, 50, or 100 mg/kg/day EMS for 28 days, and evaluated for a variety of genotoxicity endpoints in peripheral blood, liver, and colon. Blood was sampled pre-dose (Day 1) and at various time points up to Day 105. Pig-a mutant frequencies were determined in total red blood cells (RBCs) and reticulocytes (RETs) as RBC(CD59-) and RET(CD59-) frequencies. The first statistically significant increases in mutant frequencies were seen in RETs on Day 15 and in RBCs on Day 29 with the maximum RET(CD59-) on Day 29 and of RBC(CD59-) on Day 55. The lowest dose producing a statistically significant increase of RET(CD59-) was 12.5 mg/kg on Day 55 and 25 mg/kg for RBC(CD59-) on Day 55. EMS also induced significant increases in % micronucleated RETs (MN-RETs) in peripheral blood on Days 3, 15, and 28. No statistically significant increases in micronuclei were seen in liver or colon. Results from the in vivo Comet assay on Day 29 showed generally weak increases in DNA damage in all tissues evaluated with little evidence for accumulation of damage seen over time. The results with EMS indicate that the assessment of RBC(CD59-) and/or RET(CD59-) in the Pig-a assay could be a useful and sensitive endpoint for a repeat dose protocol and complements other genotoxicity endpoints.

Diethylnitrosamine genotoxicity evaluated in sprague dawley rats using pig-a mutation and reticulocyte micronucleus assays

Diethylnitrosamine (DEN) is a genotoxic carcinogen, but in vivo DNA-damaging activities are not usually evident in hematopoietic cells because the short-lived active metabolite is formed mainly in the liver. DEN therefore represented an interesting case for evaluating the performance characteristics of blood-based endpoints of genotoxicity that have been automated using flow cytometric analysis-frequency of micronucleated reticulocytes and Pig-a mutant phenotype reticulocytes (RET(CD59-) ) and erythrocytes (RBC(CD59-) ). Male Sprague Dawley rats were treated for 28 consecutive days with DEN at levels up to 12.5 mg/kg/day. Serial blood samples were collected and micronucleus frequencies were determined on Days 4 and 29, while RET(CD59-) and RBC(CD59-) frequencies were determined on Days 15, 29, and 42. The Pig-a analyses were conducted with an enrichment step based on immunomagnetic column separation to increase the statistical power of the assay. Modest but significant reductions to reticulocyte frequencies demonstrated that bone marrow was exposed to reactive intermediates. Even so, DEN did not affect micronucleus frequencies at any dose level tested. However, RET(CD59-) frequencies were significantly elevated in the high dose group on Day 29, and RBC(CD59-) were increased at this same dose level on Days 29 and 42. These results demonstrate that the Pig-a assay is sufficiently sensitive to evaluate chemicals for genotoxic potential, even in the case of a promutagen that has traditionally required direct assessment(s) of liver tissue for detection of DNA-damage.

Persistence of cisplatin-induced mutagenicity in hematopoietic stem cells: implications for secondary cancer risk following chemotherapy

Cisplatin is a cytostatic agent used in the treatment of many types of cancer, but its use is associated with increased incidences of secondary leukemia. We evaluated cisplatin's in vivo genotoxic potential by analyzing peripheral blood for Pig-a mutant phenotype erythrocytes and for chromosomal damage in the form of micronuclei. Mutant phenotype reticuloyte and erythrocyte frequencies, based on anti-CD59 antibody labeling and flow cytometric analysis, were determined in male Sprague Dawley rats treated for 28 consecutive days (days 1-28) with up to 0.4 mg cisplatin/kg/day, and sampled on days -4, 15, 29, and 56. Vehicle and highest dose groups were evaluated at additional time points post-treatment up to 6 months. Day 4 and 29 blood samples were also analyzed for micronucleated reticulocyte frequency using flow cytometry and anti-CD71-based labeling. Mutant phenotype reticulocytes were significantly elevated at doses ≥0.1 mg/kg/day, and mutant phenotype erythrocytes were elevated at doses ≥0.05 mg/kg/day. In the 0.4 mg/kg/day group, these effects persisted for the 6 month observation period. Cisplatin also induced a modest but statistically significant increase in micronucleus frequency at the highest dose tested. The prolonged persistence in the production of mutant erythrocytes following cisplatin exposure suggests that this drug mutates hematopoietic stem cells and that this damage may ultimately contribute to the increased incidence of secondary leukemias seen in patients cured of primary malignancies with platinum-based regimens.

Pig-a gene mutation and micronucleated reticulocyte induction in rats exposed to tumorigenic doses of the leukemogenic agents chlorambucil, thiotepa, melphalan, and 1,3-propane sultone

To evaluate whether blood-based genotoxicity endpoints can provide temporal and dose-response data within the low-dose carcinogenic range that could contribute to carcinogenic mode of action (MoA) assessments, we evaluated the sensitivity of flow cytometry-based micronucleus and Pig-a gene mutation assays at and below tumorigenic dose rate 50 (TD50) levels. The incidence of micronucleated reticulocytes (MN-RET) was used to evaluate chromosomal damage, and the frequency of CD59-negative reticulocytes (RET(CD59-) ) and erythrocytes (RBC(CD59-) ) served as phenotypic reporters of mutation at the X-linked Pig-a gene. Several leukemogenic agents with a presumed genotoxic MoA were studied. Specifically, male Sprague Dawley rats were treated via oral gavage for 28 days with chlorambucil, thiotepa, melphalan, and 1,3-propane sultone at doses corresponding to 0.33x, 1x, and 3x TD50, as well as at the maximum tolerated dose. Frequencies of MN-RET were determined at Days 4 and 29, and RET(CD59-) and RBC(CD59-) data were collected pretreatment as well as Days 15/16, 29, and 56/57. Dose-related increases were observed for each endpoint, and time to maximal effect was consistently: MN-RET < RET(CD59-)  < RBC(CD59-) . For each of the chemicals studied, the genotoxic events occurred long before tumors or preneoplastic lesions would be expected. Furthermore, in the case of Pig-a gene mutation, the responses were observed at or below the TD50 dose for three out of the four chemicals studied. These data illustrate the potential for quantitative blood-based analyses to provide dose-response and temporality information that relates genetic damage to cancer induction.

Detection of Pig-a mutant erythrocytes in the peripheral blood of rats and mice

The endogenous X-linked phosphatidyl inositol glycan class A gene (Pig-a) can be used as a reporter of in vivo somatic cell mutation in rats and mice. Pig-a mutant cells are deficient in specific protein surface markers and can be identified and quantified by immunofluorescent staining followed by high-throughput flow cytometry. Pig-a mutation detection is commonly performed with red blood cells (RBCs) because (1) the low volumes of blood required for determining mutant frequencies in RBCs allow multiple samplings on small laboratory animals over extended periods of time; (2) the execution of the RBC assay is easy and the interpretation of the results is straightforward; and (3) RBC Pig-a mutant frequencies are known within hours of sample collection. Two endpoints are determined in the assay: the frequency of mutant total RBCs and the frequency of mutant reticulocytes. When Pig-a mutation is used to assess the in vivo mutagenic potential of suspect hazards, the frequency of mutant reticulocytes is an early indicator of mutagenic potential, while the mutant frequency in total RBCs can be measured more rapidly and with greater precision.