Development of an in vivo gene mutation assay using the endogenous Pig-A gene: I. Flow cytometric detection of CD59-negative peripheral red blood cells and CD48-negative spleen T-cells from the rat

Genotoxic mode of action and threshold exploration of 2-methyl furan under 120-day sub-chronic exposure in male Sprague-Dawley rats

2-Methylfuran (2-MF) is an important member of the furan family generated during food thermal processing. An in-vivo multiple endpoint genotoxicity assessment system was applied to explore the genotoxic mode of action and threshold of 2-MF. Male Sprague-Dawley rats received 2-MF by oral gavage at doses of 0.16, 0.625, 2.5, and 10 mg/kg.bw/day for 120 days. An additional 15 days were granted for recovery. The Pig-a gene mutation frequency of RET and RBC showed significant increases among the 2-MF groups on day 120. After a 15-day recovery period, the Pig-a gene mutation frequency returned to levels similar to those in the vehicle control. The tail intensity (TI) values of peripheral blood cells at a dose of 10 mg/kg.bw/day significantly increased from day 4 and remained at a high level after the recovery period. No statistical difference was found in the micronucleus frequency of peripheral blood between any 2-MF dose group and the corn oil group at any timepoint. 2-MF may not induce the production of micronuclei, but it could cause DNA breakage. It could not be ruled out that 2-MF may accumulate in vivo and cause gene mutations. Hence, DNA, other than the spindle, may be directly targeted. The mode of action of 2-MF may be that it was metabolized by EPHX1 to more DNA-active metabolites, thus leading to oxidative and direct DNA damage. The point of departure (PoD) of 2-MF-induced genotoxicity was derived as 0.506 mg/kg bw/day.

Human genetic risk of treatment with antiviral nucleoside analog drugs that induce lethal mutagenesis: The special case of molnupiravir

This review considers antiviral nucleoside analog drugs, including ribavirin, favipiravir, and molnupiravir, which induce genome error catastrophe in SARS-CoV or SARS-CoV-2 via lethal mutagenesis as a mode of action. In vitro data indicate that molnupiravir may be 100 times more potent as an antiviral agent than ribavirin or favipiravir. Molnupiravir has recently demonstrated efficacy in a phase 3 clinical trial. Because of its anticipated global use, its relative potency, and the reported in vitro "host" cell mutagenicity of its active principle, β-d-N4-hydroxycytidine, we have reviewed the development of molnupiravir and its genotoxicity safety evaluation, as well as the genotoxicity profiles of three congeners, that is, ribavirin, favipiravir, and 5-(2-chloroethyl)-2'-deoxyuridine. We consider the potential genetic risks of molnupiravir on the basis of all available information and focus on the need for additional human genotoxicity data and follow-up in patients treated with molnupiravir and similar drugs. Such human data are especially relevant for antiviral NAs that have the potential of permanently modifying the genomes of treated patients and/or causing human teratogenicity or embryotoxicity. We conclude that the results of preclinical genotoxicity studies and phase 1 human clinical safety, tolerability, and pharmacokinetics are critical components of drug safety assessments and sentinels of unanticipated adverse health effects. We provide our rationale for performing more thorough genotoxicity testing prior to and within phase 1 clinical trials, including human PIG-A and error corrected next generation sequencing (duplex sequencing) studies in DNA and mitochondrial DNA of patients treated with antiviral NAs that induce genome error catastrophe via lethal mutagenesis.

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.

Pig-a gene mutations in bone marrow granulocytes of procarbazine-treated F344 rats

It was previously demonstrated that procarbazine (PCZ) is positive in the rat erythrocyte Pig-a gene mutation assay. However, since mammalian erythrocytes lack genomic DNA, it was necessary to analyze nucleated bone-marrow erythroid precursor cells to confirm that PCZ induces mutations in the Pig-a gene (Revollo et al., Environ Mol Mutagen, 2020). In this study, the association between Pig-a mutation and loss of GPI anchors was further strengthened and the genesis of Pig-a mutation in PCZ-dosed rats was evaluated by analyzing bone-marrow granulocytes. Erythrocytes and granulocytes both originate from myeloid progenitor cells, but granulocytes contain DNA throughout their developmental stages. F344 rats were treated with three doses of 150 mg/kg PCZ; 2 weeks later, CD48-deficient mutant phenotype bone-marrow granulocytes (BMGs [CD11b⁺ ]) were isolated by flow-cytometric sorting. Sequencing data showed that the CD48-deficient mutant phenotype BMGs contained mutations in the Pig-a gene while wild-type BMGs did not. PCZ-induced mutations included missense, nonsense and splice site variants; the majority of mutations were A > T, A > C, and A > G, with the mutated A on the nontranscribed DNA strand. The PCZ-induced mutational analysis in BMGs supports the association between the phenotype measured in the Pig-a assay and mutation in the Pig-a gene. Also, PCZ mutation spectra were similar in bone-marrow erythroids and BMGs, but none of the mutations detected in BMGs were the same as the erythroid precursor cell mutations from the same rats. Thus, mutations induced in the Pig-a assay appear to be induced after commitment of myeloid progenitor cells to either the granulocyte or erythroid pathway.

Standard protocol for the PIGRET assay, a high-throughput reticulocyte Pig-a assay with an immunomagnetic separation, used in the interlaboratory trial organized by the Mammalian Mutagenicity Study Group of the Japanese Environmental Mutagen and Genome Society

The PIGRET assay is one of the Pig-a assays targeting reticulocytes (RETs), an in vivo genotoxicity evaluation method using flow cytometry with endogenous reporter glycosylphosphatidylinositol anchor protein. The PIGRET assay with RETs selectively enriched with anti-CD71 antibodies has several desirable features: high-throughput assay system, low background frequency of mutant cells, and early detection of mutation. To verify the potential and usefulness of the PIGRET assay for short-term testing, an interlaboratory trial involving 16 laboratories organized by the Mammalian Mutagenicity Study Group of the Japanese Environmental Mutagen and Genome Society was conducted. The collaborating laboratories assessed the mutagenicities of a total of 24 chemicals in rats using a single-treatment design and standard protocols for conducting the Pig-a assay on the total red blood cell assay and the PIGRET assay. Here the standard protocol for the PIGRET assay was described in detail.

Application of counter-selectable marker PIGA in engineering designer deletion cell lines and characterization of CRISPR deletion efficiency

The CRISPR/Cas9 system is a technology for genome engineering, which has been applied to indel mutations in genes as well as targeted gene deletion and replacement. Here, we describe paired gRNA deletions along the PIGA locus on the human X chromosome ranging from 17 kb to 2 Mb. We found no compelling linear correlation between deletion size and the deletion efficiency, and there is no substantial impact of topologically associating domains on deletion frequency. Using this precise deletion technique, we have engineered a series of designer deletion cell lines, including one with deletions of two X-chromosomal counterselectable (negative selection) markers, PIGA and HPRT1, and additional cell lines bearing each individual deletion. PIGA encodes a component of the glycosylphosphatidylinositol (GPI) anchor biosynthetic apparatus. The PIGA gene counterselectable marker has unique features, including existing single cell level assays for both function and loss of function of PIGA and the existence of a potent counterselectable agent, proaerolysin, which we use routinely for selection against cells expressing PIGA. These designer cell lines may serve as a general platform with multiple selection markers and may be particularly useful for large scale genome engineering projects such as Genome Project-Write (GP-write).

PIG-A gene mutation as a genotoxicity biomarker in human population studies: An investigation in lead-exposed workers

The rodent Pig-a gene mutation assay has demonstrated remarkable sensitivity in identifying in vivo mutagens, while much less is known about the value of the human PIG-A assay for risk assessment. To obtain more evidence of its potential as a predictive biomarker for carcinogen exposure, we investigated PIG-A mutant frequencies (MFs), along with performing the Comet assay and micronucleus (MN) test, in 267 workers occupationally exposed to lead. Multivariate Poisson regression showed that total red blood cell PIG-A MFs were significantly higher in lead-exposed workers (10.90 ± 10.7 × 10^-6 ) than in a general population that we studied previously (5.25 ± 3.6 × 10^-6 ) (p < .0001). In contrast, there was no increase in lymphocyte MN frequency or in DNA damage as measured by percentage comet tail intensity in whole blood cells. Current year worker blood lead levels (BLL), an exposure biomarker, were elevated (232.6 ± 104.6 μg/L, median: 225.4 μg/L); a cumulative blood lead index (CBLI) also was calculated based on a combination of current and historical worker BLL data. Chi-square testing indicated that PIG-A MFs were significantly related to CBLI (p = .0249), but independent of current year BLL (p = .4276). However, % comet tail intensity and MN frequencies were better associated with current year BLL than CBLI. This study indicates that the PIG-A assay could serve as biomarker to detect the genotoxic effects of lead exposure and demonstrates that a battery of genotoxicity biomarkers having mechanistic complementarity may be useful for comprehensively monitoring human carcinogenic risk.

Testing of acetaminophen in support of the international multilaboratory in vivo rat Pig-a assay validation trial

Acetaminophen, a nonmutagenic compound as previously concluded from bacteria, in vitro mammalian cell, and in vivo transgenic rat assays, presented a good profile as a nonmutagenic reference compound for use in the international multilaboratory Pig-a assay validation. Acetaminophen was administered at 250, 500, 1,000, and 2,000 mg·kg-1 ·day-1 to male Sprague Dawley rats once daily in 3 studies (3 days, 2 weeks, and 1 month with a 1-month recovery group). The 3-Day and 1-Month Studies included assessments of the micronucleus endpoint in peripheral blood erythrocytes and the comet endpoint in liver cells and peripheral blood cells in addition to the Pig-a assay; appropriate positive controls were included for each assay. Within these studies, potential toxicity of acetaminophen was evaluated and confirmed by inclusion of liver damage biomarkers and histopathology. Blood was sampled pre-treatment and at multiple time points up to Day 57. Pig-a mutant frequencies were determined in total red blood cells (RBCs) and reticulocytes (RETs) as CD59-negative RBC and CD59-negative RET frequencies, respectively. No increases in DNA damage as indicated through Pig-a, micronucleus, or comet endpoints were seen in treated rats. All positive controls responded as appropriate. Data from this series of studies demonstrate that acetaminophen is not mutagenic in the rat Pig-a model. These data are consistent with multiple studies in other nonclinical models, which have shown that acetaminophen is not mutagenic. At 1,000 mg·kg-1 ·day-1 , Cmax values of acetaminophen on Day 28 were 153,600 ng/ml and 131,500 ng/ml after single and repeat dosing, respectively, which were multiples over that of clinical therapeutic exposures (2.6-6.1 fold for single doses of 4,000 mg and 1,000 mg, respectively, and 11.5 fold for multiple dose of 4,000 mg) (FDA 2002). Data generated were of high quality and valid for contribution to the international multilaboratory validation of the in vivo Rat Pig-a Mutation Assay.

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.

Erythrocyte-Based Pig-a Gene Mutation Assay

In addition to chromosomal damage, assessment of gene mutation is an important part of genotoxicity testing employed during preclinical safety testing. The Pig-a gene mutation assay is based on the loss of function of the Pig-a gene, which results in a lack of cell surface expression of specific proteins that are targeted to the surface by GPI anchors. This cell surface phenotype is readily assessed by flow cytometric analysis of red blood cells. This chapter describes a procedure for the collection, processing, and analysis of peripheral blood samples using materials supplied in MutaFlow^® kits and a common benchtop flow cytometer.

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.

Pig-a gene mutation database

Mutations in the X-linked phosphatidylinositol glycan, class A gene (Pig-a) lead to loss of glycosylphosphatidylinositol (GPI) anchors and GPI-anchored proteins from the surface of erythrocytes and other mammalian cells. The Pig-a gene mutation assay quantifies in vivo gene mutation by immunofluorescent labeling and flow cytometry to detect the loss of GPI-anchored proteins on peripheral blood erythrocytes. As part of the regulatory acceptance of the assay, a public database has been created that provides detailed information on Pig-a gene mutation assays conducted in rats and mice. A searchable version of the database is available through a website designed and hosted by the University of Maryland School of Pharmacy. Currently, the database contains only mouse and rat data, but it is anticipated that it will expand to include data from other species, including humans. A major purpose in developing the database was to aid in the preparation of a Retrospective Performance Analysis and Detailed Review Paper required for Organisation for Economic Co-operation and Development Test Guideline acceptance. We anticipate, however, that it also will be useful for accessing and comparing Pig-a data to data from other assays and for conducting quantitative assessments of Pig-a gene mutation responses. Environ. Mol. Mutagen., 60:759-762, 2019. © 2019 Wiley Periodicals, Inc.

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.

In vitro mammalian cell mutation assays based on the Pig-a gene: A report of the 7th International Workshop on Genotoxicity Testing (IWGT) Workgroup

Pig-a gene mutation assays enumerate cells with the glycosylphosphatidylinositol (GPI) anchor-deficient phenotype as a reporter of mutation in the endogenous Pig-a gene. Methods for measuring mutation in this gene are quite well established for in vivo systems. This approach to mutagenicity assessment has now been extended to in vitro mammalian cell-based systems. An expert workgroup from the 7th International Workshop on Genotoxicity Testing tasked with assessing the status of in vitro mammalian cell mutation assays has investigated the merits and limitations of in vitro Pig-a gene mutation assays. A review of the current status of these developing methodologies and the formation of consensus statements on the utility and application of these assays were performed to provide guidance for their potential use in genotoxicity hazard identification and risk assessment.

Standard protocol for the total red blood cell Pig-a assay used in the interlaboratory trial organized by the Mammalian Mutagenicity Study Group of the Japanese Environmental Mutagen Society

The Pig-a assay, a promising tool for evaluating in vivo genotoxicity, is based on flow cytometric enumeration of red blood cells (RBCs) that are deficient in glycosylphosphatidylinositol anchor protein. Various approaches for measuring Pig-a mutant cells have been developed, particularly focusing on measuring mutants in peripheral RBCs and reticulocytes (RETs). The Pig-a assay on concentrated RETs-the PIGRET assay-has the potential to detect genotoxicity in the early stages of a study. To verify the potential and usefulness of the PIGRET assay for short-term testing, we conducted an interlaboratory trial involving 16 laboratories organized by the Mammalian Mutagenicity Study Group of the Japanese Environmental Mutagen Society (MMS/JEMS). The collaborating laboratories assessed the mutagenicity of a total of 24 chemicals in rats using a single-treatment design and standard protocols for conducting the Pig-a assay on total RBCs (the RBC Pig-a assay) and the PIGRET assay. Here, we describe the standard protocol for the RBC Pig-a assay in detail.

Pilot studies evaluating the nongenotoxic rodent carcinogens phenobarbital and clofibrate in the rat Pig-a assay

The Pig-a assay is an emerging and promising in vivo method to determine mutagenic potential of chemicals. Since its development in 2008, remarkable progress has been made in harmonizing and characterizing the test procedures, primarily using known mutagenic chemicals. The purpose of the present study was to evaluate specificity of the Pig-a assay using two nongenotoxic and well-characterized rodent liver carcinogens, phenobarbital and clofibrate, in male F344/DuCrl rats. Daily oral administration of phenobarbital or clofibrate at established hepatotoxic doses for 28 days resulted in substantial hepatic alterations, however, did not increase the frequency of Pig-a mutation markers (RET^CD59- and RBC^CD59- ) compared to vehicle control or pre-exposure (Day -5) mutant frequencies. These results are consistent with the existing literature on the nonmutagenic mode of action (MoA) of phenobarbital and clofibrate liver tumors. The present study contributes to the limited, but expanding evidence on the specificity of the Pig-a assay and further for the investigations of carcinogenic MoAs, i.e., mutagenic or nonmutagenic potential of chemicals. Environ. Mol. Mutagen. 60:42-46, 2019. © 2018 Wiley Periodicals, Inc.

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.

Analysis of mutation in the rat Pig-a assay: II. Studies with bone marrow granulocytes

The in vivo erythrocyte Pig-a gene mutation assay measures the phenotypic loss of GPI-anchored surface markers. Molecular analysis of the marker-deficient erythrocytes cannot provide direct proof that the mutant phenotype is due to mutation in the Pig-a gene because mammalian erythrocytes lack genomic DNA. Granulocytes are nucleated cells that originate from myeloid progenitor cells in bone marrow as is the case for erythrocytes, and thus analysis of Pig-a mutation in bone marrow granulocytes can provide information about the source of mutations detected in the erythrocyte Pig-a assay. We developed a flow cytometric Pig-a assay for bone marrow granulocytes and evaluated granulocyte Pig-a mutant frequencies in bone marrow from male rats treated acutely with N-ethyl-N-nitrosourea (ENU). Bone marrow cells from these rats were stained with anti-CD11b for identifying granulocytes and anti-CD48 for detecting the Pig-a mutant phenotype. The average Pig-a mutant frequency in granulocyte precursors of control rats was 8.42 × 10^-6 , whereas in ENU-treated rats it was 567.13 × 10^-6 . CD11b-positive/CD48-deficient mutant cells were enriched using magnetic separation and sorted into small pools for sequencing. While there were no Pig-a mutations found in sorted CD48-positive wild-type cells, Pig-a mutations were detected in mutant granulocyte precursors. The most frequent mutation observed was T→A transversion, followed by T→C transition and T→G transversion, with the mutated T on the nontranscribed DNA strand. While the spectrum of mutations in bone marrow granulocytes was similar to that of erythroid cells, different Pig-a mutations were found in mutant-phenotype granulocytes and erythroids from the same bone marrow samples, suggesting that most Pig-a mutations were induced in bone marrow cells after commitment to either the granulocyte or erythroid developmental pathway. Environ. Mol. Mutagen. 59:733-741, 2018. © 2018 Wiley Periodicals, Inc.

Analysis of mutation in the rat Pig-a assay: I) studies with bone marrow erythroid cells

We have established a flow cytometry-based Pig-a assay for rat bone marrow erythroid cells (BMEs). The BME Pig-a assay uses a DNA-specific stain and two antibodies: one against the transmembrane transferrin receptor (CD71 marker) and the other against the GPI-anchored complement inhibitory protein (CD59 marker). In F344 male rats treated acutely with a total of 120 mg/kg of N-ethyl-N-nitrosourea (ENU) the frequency of CD59-deficient phenotypically mutant BMEs increased approximately 24-fold compared to the rats concurrently treated with the vehicle. Such an increase of mutant BMEs coincides with increases of CD59-deficient reticulocytes measured in rats treated with similar doses of ENU. Sequence analysis of the endogenous X-linked Pig-a gene of CD59-deficient BMEs revealed that they are Pig-a mutants. The spectrum of ENU-induced Pig-a mutations in these BMEs was consistent with the in vivo mutagenic signature of ENU: 73% of mutations occurred at A:T basepairs, with the mutated T on the nontranscribed strand of the gene. T→A transversion was the most frequent mutation followed by T→C transition; no deletion or insertion mutations were present in the spectrum. Since BMEs are precursors of peripheral red blood cells, our findings suggest that CD59-deficient erythrocytes measured in the flow cytometric erythrocyte Pig-a assay develop from BMEs containing mutations in the Pig-a gene. Thus, the erythrocyte Pig-a assay detects mutation in the Pig-a gene. Environ. Mol. Mutagen. 59:722-732, 2018. © 2018 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.

Detection of In Vivo Mutation in the Pig-a Gene of Mouse Bone Marrow Erythroids

Detection of in vivo mutation is important for evaluating the health risks associated with chemicals. The Pig-a in vivo gene mutation assay has been developed over the last decade for this purpose. Most approaches for the assay, however, measure cells with a Pig-a mutant phenotype in erythrocytes from the peripheral blood, with the mutations causing the phenotype being difficult to determine directly. This chapter describes a procedure for detecting mutations in the Pig-a gene of phenotypically mutant mouse bone marrow erythroids, the precursors of peripheral blood erythrocytes. The strategy for molecular analysis of Pig-a gene mutation includes enrichment of GPI-anchor deficient cells with a cell sorter followed by a cloning and sequencing of Pig-a cDNAs.

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.

Absence of in vivo mutagenicity of multi-walled carbon nanotubes in single intratracheal instillation study using F344 gpt delta rats

Introduction

It is known that fibrous particles of micrometer length, such as carbon nanotubes, which have same dimensions as asbestos, are carcinogenic. Carcinogenicity of nanomaterials is strongly related to inflammatory reactions; however, the genotoxicity mechanism(s) is unclear. Indeed, inconsistent results on genotoxicity of multi-walled carbon nanotubes (MWCNTs) have been shown in several reports. Therefore, we analyzed the in vivo genotoxicity induced by an intratracheal instillation of straight MWCNTs in rats using a different test system—the Pig-a gene mutation assay—that can reflect the genotoxicity occurring in the bone marrow. Since lungs were directly exposed to MWCNTs upon intratracheal instillation, we also performed the gpt assay using the lungs.

Findings

We detected no significant differences in Pig-a mutant frequencies (MFs) between the MWCNT-treated and control rats. Additionally, we detected no significant differences in gpt MFs in the lung between the MWCNT-treated and control rats.

Conclusions

Our findings indicated that a single intratracheal instillation of MWCNTs was non-mutagenic to both the bone marrow and lung of rats.

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.

In Vivo Pig-a gene mutation assay: Guidance for 3Rs-friendly implementation

The rodent Pig-a assay is an in vivo method for the detection of gene mutation, where lack of glycosylphosphatidylinositol-anchored proteins on the surface of circulating red blood cells (RBCs) serves as a reporter for Pig-a gene mutation. In the case of rats, the frequency of mutant phenotype RBCs is measured via fluorescent anti-CD59 antibodies and flow cytometry. The Pig-a assay meets the growing expectations for novel approaches in animal experimentation not only focusing on the scientific value of the assay but also on animal welfare aspects (3Rs principles), for example, amenable to integration into pivotal rodent 28-day general toxicology studies. However, as recommended in the Organisation for Economic Co-operation and Development Test Guidelines for genotoxicity testing, laboratories are expected to demonstrate their proficiency. While this has historically involved the extensive use of animals, here we describe an alternative approach based on a series of blood dilutions covering a range of mutant frequencies. The experiments described herein utilized either non-fluorescent anti-CD59 antibodies to provide elevated numbers of mutant-like cells, or a low volume blood sample from a single N-ethyl-N-nitrosourea treated animal. Results from these so-called reconstruction experiments from four independent laboratories showed good overall precision (correlation coefficients: 0.9979-0.9999) and accuracy (estimated slope: 0.71-1.09) of mutant cell scoring, which was further confirmed by Bland-Altman analysis. These data strongly support the use of reconstruction experiments for training purposes and demonstrating laboratory proficiency with very few animals, an ideal situation given the typically conflicting goals of demonstrating laboratory proficiency and reducing the use of animals. Environ. Mol. Mutagen. 57:678-686, 2016. © 2016 Wiley Periodicals, Inc.

Evaluation for a mutagenicity of aristolochic acid by Pig-a and PIGRET assays in rats

The Pig-a assay, which uses the endogenous phosphatidylinositol glycan, class A gene (Pig-a) as a reporter of mutation, has been developed as a method for evaluating in vivo mutagenicity. Pig-a gene mutation can be detected by identifying the presence of CD59, the glycosylphosphatidylinositol anchor protein, on the surface of erythrocytes (RBC Pig-a assay) and reticulocytes (PIGRET assay). The International Workshop on Genotoxicity Testing (IWGT) showed the usefulness of the RBC Pig-a assay through the evaluation of several compounds. Aristolochic acid (AA), one of the evaluated compounds in the IWGT workgroup, is a carcinogenic plant toxin that is a relatively strong gene mutagen both in vitro and in vivo, but a weak inducer of micronuclei in vivo. In the present study, we examined the mutagenicity of AA in the peripheral blood of rats treated orally with a single dose of AA using Pig-a assays. Furthermore, we evaluated the advantages of the PIGRET assay compared with the RBC Pig-a assay. The results showed that a statistically significant increase in mutant frequency of the Pig-a gene was detected at day 28 by the RBC Pig-a assay, and at days 7, 14 and 28 by the PIGRET assay. In addition, the mutant frequency by the PIGRET assay was higher than that by the RBC Pig-a assay. These results indicate that the mutagenicity of AA can be detected using the Pig-a assays, as reported by the IWGT, and the PIGRET assay can detect Pig-a mutants at an early time point compared with the RBC Pig-a assay.

Detection of Pig-a gene mutants in rat peripheral blood following a single urethane treatment: A comparison of the RBC Pig-a and PIGRET assays

The rat red blood cell (RBC) Pig-a assay has been recommended by an expert working group of the International Workshop on Genotoxicity Testing as a potential new method to evaluate in vivo gene mutations in regulatory genotoxicity risk assessments. In a collaborative study in Japan, an improved Pig-a assay using reticulocytes (PIGRET assay) with magnetic enrichment of CD71-positive cells was evaluated, and it was revealed that this assay could detect the mutagenicity of chemicals earlier than the RBC Pig-a assay could. To verify further the suitability of the PIGRET assay for an in vivo short-term genotoxicity screening test, a joint research study was conducted by the Japanese Environmental Mutagen Society, and 24 compounds were evaluated. One of the compounds evaluated in this study was urethane, a multi-organ rodent carcinogen. Urethane (250, 500, and 1000mg/kg body weight) was orally administered once to 8-week-old male Crl:CD (SD) rats. Blood samples were collected at 1, 2, and 4 weeks after the administration and processed for the RBC Pig-a and PIGRET assays. In the PIGRET assay, the Pig-a mutant frequency (MF) significantly increased at both 2 and 4 weeks after the treatment of 1000mg/kg of urethane. However, in the RBC Pig-a assay, a significant increase in the Pig-a MF was observed only at 1 week after the treatment with 500mg/kg, but the MF value was within our historical control range; therefore, it was judged to be negative. These results suggest that the PIGRET assay might be useful for evaluating the in vivo mutagenicity more clearly than the RBC Pig-a assay after a single treatment of test compounds.

Evaluation of single-dose RBC Pig-a and PIGRET assays in detecting the mutagenicity of thiotepa in rats

The Pig-a assay, which uses reticulocytes (PIGRET assay) as target cells, is anticipated to detect mutagenicity at earlier time points than the RBC Pig-a assay, which uses all red blood cells as target cells. As part of a collaborative study conducted by the Mammalian Mutagenicity Study (MMS) Group, we evaluated the PIGRET and RBC Pig-a assays to detect Pig-a gene mutations induced by the carcinogen thiotepa. A single dose of thiotepa at 7.5, 15, and 30mg/kg was administered to 8-week-old male Sprague-Dawley rats by oral gavage. PIGRET and RBC Pig-a assays were performed using peripheral blood collected from rats 7, 14, and 28days after thiotepa administration (Day 0 as the day of administration), and the resulting Pig-a mutant frequencies (MFs) were compared. Increased Pig-a MF was observed from Day 7 onwards using the PIGRET assay. Pig-a MF remained fairly constant thereafter until Day 28 in the 30mg/kg group, whereas it peaked on Day 14 in the 7.5 and 15mg/kg groups. Using the RBC Pig-a assay, on the other hand, no significant increase in MF was observed at any of the dosages on Days 7, 14, or 28. These findings show that Pig-a gene mutations following a single dose of thiotepa were detected using the PIGRET assay but not the RBC Pig-a assay, which suggests that PIGRET assay is more suitable than RBC Pig-a assay for evaluating the in vivo mutagenicity by a single dose.

Evaluation of the in vivo mutagenicity of melamine by the RBC Pig-a assay and PIGRET assay

The Pig-a assay is a new in vivo genotoxicity test for detecting mutagens in the bodies of animals, using the endogenous Pig-a gene as the target. There are two types of Pig-a assays: the red blood cell (RBC) Pig-a assay, which uses RBCs, and the PIGRET assay, which uses reticulocytes. The Japanese Environmental Mutagen Society-Mammalian Mutagenicity Study Group collaborative study of the Pig-a assay was carried out to investigate the usefulness of the PIGRET assay. The mutagenicity of melamine was evaluated as part of this study. Eight-week-old male Crl:CD (SD) rats were administered a single gavage dose of melamine as a non-genotoxic bladder carcinogen. Blood samples were collected at the first, second and fourth weeks after administration, and the RBC Pig-a assay and PIGRET assays were conducted using these samples. Three dose levels were used in the study: the highest dose was 2000mg/kg, which is generally used as the maximum dose in in vivo genotoxicity testing, and 1000 and 500mg/kg were also used. As a positive control, a group of rats was administered a single dose of N-nitroso-N-ethylurea (ENU) by gavage at 40mg/kg. The Pig-a mutant frequencies (Pig-a MFs) did not increase in any of the melamine groups throughout the experimental period in either the RBC Pig-a assay or the PIGRET assay. Both the RBC Pig-a and PIGRET assays revealed significant increases in the Pig-a MFs in the ENU group, starting at day 7 after a single administration. Therefore, these two assays, when evaluated after a single administration, can be used to determine that melamine is non-mutagenic.

Evaluation of the PIGRET assay in rats by single oral dosing with azidothymidine

In vivo phosphatidylinositol glycan, class A (Pig-a) gene mutation assay using peripheral blood is known to be a novel and useful tool to evaluate the mutagenicity of compounds. Recently, the rat PIGRET assay which is an improved method for measuring Pig-a mutant cells in reticulocytes with magnetic enrichment of CD71 positive cells has been developed. Several reports showed that the PIGRET assay could detect the increase of Pig-a mutant frequency earlier than the Pig-a assay in total red blood cells (RBC Pig-a assay). Therefore, as part of a collaborative study by the Mammalian Mutagenicity Study (MMS) Group of the Japanese Environmental Mutagen Society, the usefulness of the PIGRET assay in comparison to the RBC Pig-a assay has been assessed for 24 compounds with various mechanisms of action. In the present study, we performed the PIGRET assay and RBC Pig-a assay with a nucleoside analogue, azidothymidine (AZT), and compared the results in these assays. We administered a single dose of AZT to rats by oral gavage up to 2000mg/kg and examined Pig-a mutant frequencies at days 7, 14 and 28 by PIGRET and RBC Pig-a assays. No significant increases in mutant frequency were observed after administration of AZT in both the RBC Pig-a and PIGRET assays and comparable to the previous results of the International Workshop on Genotoxicity Testing (IWGT) workgroup. AZT has been thought to induce not only DNA chain termination as a pharmacological effect but also a large deletion on the genome DNA. The Pig-a assays may be less sensitive to compounds such as AZT which induce large deletions on the genome DNA.

Evaluation of the RBC Pig-a and PIGRET assays using single doses of hydroxyurea and melphalan in rats

To evaluate the suitability of the rat Pig-a assay on reticulocytes (PIGRET assay) as a short-term test, red blood cell (RBC) Pig-a and PIGRET assays after single doses with hydroxyurea (HU) and melphalan (L-PAM) were conducted and the results of both assays were compared. HU was administered once orally to male SD rats at 250, 500 and 1000mg/kg, and both assays were conducted using peripheral blood withdrawn from the jugular vein at 1, 2 and 4 weeks after dosing. L-PAM was administered at 1.25, 2.5 and 5mg/kg in the same manner. L-PAM produced significant dose-dependent increases in mutant frequencies in the PIGRET assay after single oral doses, but did not produce dose-dependent increases in mutant frequencies in the RBC Pig-a assay. These results suggest that the PIGRET assay is more sensitive for the evaluation of the mutagenic potential of L-PAM than the RBC Pig-a assay. In contrast, HU, a clastogenic but not DNA-reactive compound, gave negative results in both assays. The results with these 2 chemicals indicate that the single-dose PIGRET assay in rats has the potential to properly detect DNA-reactive compounds that directly cause DNA damage in a short-term assay.

Empirical analysis of BMD metrics in genetic toxicology part II: in vivo potency comparisons to promote reductions in the use of experimental animals for genetic toxicity assessment

Genotoxicity tests have traditionally been used only for hazard identification, with qualitative dichotomous groupings being used to identify compounds that have the capacity to induce mutations and/or cytogenetic alterations. However, there is an increasing interest in employing quantitative analysis of in vivo dose-response data to derive point of departure (PoD) metrics that can be used to establish human exposure limits or margins of exposure (MOEs), thereby supporting human health risk assessments and regulatory decisions. This work is an extension of our companion article on in vitro dose-response analyses and outlines how the combined benchmark dose (BMD) approach across included covariates can be used to improve the analyses and interpretation of in vivo genetic toxicity dose-response data. Using the BMD-covariate approach, we show that empirical comparisons of micronucleus frequency dose-response data across multiple studies justifies dataset merging, with subsequent analyses improving the precision of BMD estimates and permitting attendant potency ranking of seven clastogens. Similarly, empirical comparisons of Pig-a mutant phenotype frequency data collected in males and females justified dataset merging across sex. This permitted more effective scrutiny regarding the effect of post-exposure sampling time on the mutagenicity of N-ethyl-N-nitrosourea observed in reticulocytes and erythrocytes in the Pig-a assay. The BMD-covariate approach revealed tissue-specific differences in the induction of lacZ transgene mutations in Muta™Mouse specimens exposed to benzo[a]pyrene (BaP), with the results permitting the formulation of mechanistic hypotheses regarding the observed potency ranking. Lastly, we illustrate how historical dose-response data for assessments that examined numerous doses (i.e. induced lacZ mutant frequency (MF) across 10 doses of BaP) can be used to improve the precision of BMDs derived from datasets with far fewer doses (i.e. lacZ MF for 3 doses of dibenz[a,h]anthracene). Collectively, the presented examples illustrate how innovative use of the BMD approach can permit refinement of the use of in vivo data; improving the efficacy of experimental animal use in genetic toxicology without sacrificing PoD precision.

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.

Mouse Pig-a and micronucleus assays respond to N-ethyl-N-nitrosourea, benzo[a]pyrene, and ethyl carbamate, but not pyrene or methyl carbamate

This laboratory previously described a method for scoring the incidence of peripheral blood Pig-a mutant phenotype rat erythrocytes using immunomagnetic separation in conjunction with flow cytometric analysis (In Vivo MutaFlow®). The current work extends the method to mouse blood, using the frequency of CD24-negative reticulocytes (RET(CD24-)) and erythrocytes (RBC(CD24-)) as phenotypic reporters of Pig-a gene mutation. Following assay optimization, reconstruction experiments demonstrated the ability of the methodology to return expected values. Subsequently, the responsiveness of the assay to the genotoxic carcinogens N-ethyl-N-nitrosourea, benzo[a]pyrene, and ethyl carbamate was studied in male CD-1 mice exposed for 3 days to several dose levels via oral gavage. Blood samples were collected on Day 4 for micronucleated reticulocyte analyses, and on Days 15 and 30 for determination of RET(CD24-) and RBC(CD24-) frequencies. The same design was used to study pyrene, with benzo[a]pyrene as a concurrent positive control, and methyl carbamate, with ethyl carbamate as a concurrent positive control. The three genotoxicants produced marked dose-related increases in the frequencies of Pig-a mutant phenotype cells and micronucleated reticulocytes. Ethyl carbamate exposure resulted in moderately higher micronucleated reticulocyte frequencies relative to N-ethyl-N-nitrosourea or benzo[a]pyrene (mean ± SEM = 3.0 ± 0.36, 2.3 ± 0.17, and 2.3 ± 0.49%, respectively, vs. an aggregate vehicle control frequency of 0.18 ± 0.01%). However, it was considerably less effective at inducing Pig-a mutant cells (e.g., Day 15 mean no. RET(CD24-) per 1 million reticulocytes = 7.6 ± 3, 150 ± 9, and 152 ± 43 × 10(-6), respectively, vs. an aggregate vehicle control frequency of 0.6 ± 0.13 × 10(-6)). Pyrene and methyl carbamate, tested to maximum tolerated dose or limit dose levels, had no effect on mutant cell or micronucleated reticulocyte frequencies. Collectively, these results demonstrate the utility of the cross-species Pig-a and micronucleated reticulocyte assays, and add further support to the value of studying both endpoints in order to cover two distinct genotoxic modes of action.