Monitoring humans for somatic mutation in the endogenous PIG-a gene using red blood cells

Erythrocyte PIG-A mutant frequencies in cancer patients receiving cisplatin

BACKGROUND

Cisplatin is a primary chemotherapy choice for various solid tumors. DNA damage caused by cisplatin results in apoptosis of tumor cells. Cisplatin-induced DNA damage, however, may also result in mutations in normal cells and the initiation of secondary malignancies. In the current study, we have used the erythrocyte PIG-A assay to evaluate mutagenesis in non-tumor hematopoietic tissue of cancer patients receiving cisplatin chemotherapy.

METHODS

Twenty-one head and neck cancer patients undergoing treatment with cisplatin were monitored for the presence of PIG-A mutant total erythrocytes and the young erythrocytes, reticulocytes (RETs), in peripheral blood for up to five and a half months from the initiation of the anti-neoplastic chemotherapy.

RESULTS

PIG-A mutant frequency (MF) in RETs increased at least two-fold in 15 patients at some point of the monitoring, while the frequency of total mutant RBCs increased at least two-fold in 6 patients. A general trend for an increase in the frequency of mutant RETs and total mutant RBCs was observed in 19 and 18 patients, respectively. Only in one patient did both RET and total RBC PIG-A MFs did not increase at any time-point over the monitoring period.

CONCLUSION

Cisplatin chemotherapy induces moderate increases in the frequency of  PIG-A mutant erythrocytes in head and neck cancer patients. Mutagenicity measured with the flow cytometric PIG-A assay may serve as a tool for predicting adverse outcomes of genotoxic antineoplastic therapy.

The PIG-A gene mutation assay in human biomonitoring and disease

The blood cell phosphatidylinositol glycan class A (PIG-A) gene mutation assay has been extensively researched in rodents for in vivo mutagenicity testing and is now being investigated in humans. The PIG-A gene is involved in glycosyl phosphatidylinositol (GPI)-anchor biosynthesis. A single mutation in this X-linked gene can lead to loss of membrane-bound GPI anchors, which can be enumerated via corresponding GPI-anchored proteins (e.g., CD55) using flow cytometry. The studies published to date by different research groups demonstrate a remarkable consistency in PIG-A mutant frequencies. Moreover, with the low background level of mutant erythrocytes in healthy subjects (2.9-5.56 × 10-6 mutants), induction of mutation post genotoxic exposure can be detected. Cigarette smoking, radiotherapy, and occupational exposures, including lead, have been shown to increase mutant levels. Future applications of this test include identifying new harmful agents and establishing new exposure limits. This mutational monitoring approach may also identify individuals at higher risk of cancer development. In addition, identifying protective agents that could mitigate these effects may reduce baseline somatic mutation levels and such behaviors can be encouraged. Further technological progress is required including establishing underlying mechanisms of GPI anchor loss, protocol standardization, and the development of cryopreservation methods to improve GPI-anchor stability over time. If successful, this assay has the potential be widely employed, for example, in rural and low-income countries. Here, we review the current literature on PIG-A mutation in humans and discuss the potential role of this assay in human biomonitoring and disease detection.

Next Generation Sequencing Workshop at the Royal Society of Medicine (London, May 2022): how genomics is on the path to modernizing genetic toxicology

The use of error-corrected Next Generation Sequencing (ecNG) to determine mutagenicity has been a subject of growing interest and potentially a disruptive technology that could supplement, and in time, replace current testing paradigms in preclinical safety assessment. Considering this, a Next Generation Sequencing Workshop was held at the Royal Society of Medicine in London in May 2022, supported by the United Kingdom Environmental Mutagen Society (UKEMS) and TwinStrand Biosciences (WA, USA), to discuss progress and future applications of this technology. In this meeting report, the invited speakers provide an overview of the Workshop topics covered and identify future directions for research. In the area of somatic mutagenesis, several speakers reviewed recent progress made with correlating ecNGS to classic in vivo transgenic rodent mutation assays as well as exploring the use of this technology directly in humans and animals, and in complex organoid models. Additionally, ecNGS has been used for detecting off-target effects of gene editing tools and emerging data suggest ecNGS potential to measure clonal expansion of cells carrying mutations in cancer driver genes as an early marker of carcinogenic potential and for direct human biomonitoring. As such, the workshop demonstrated the importance of raising awareness and support for advancing the science of ecNGS for mutagenesis, gene editing, and carcinogenesis research. Furthermore, the potential of this new technology to contribute to advances in drug and product development and improve safety assessment was extensively explored.

PIG-A gene mutation as a mutagenicity biomarker among coke oven workers

PIG-A gene mutations can be detected in humans, and PIG-A assays can potentially predict the risk of exposure to carcinogens. However, extensive, population-based studies to validate this are lacking. We studied a cohort of occupational coke oven workers with chronic high exposure to carcinogenic polycyclic aromatic hydrocarbons, which are well-studied genotoxins classified by the IARC as carcinogenic to humans. Peripheral blood erythrocytes of workers were assessed for gene mutations using a PIG-A assay, and chromosome damage using the cytokinesis-block micronucleus test with lymphocytes. Two sample populations from a non-industrialized city and new employees in industrial plants were selected as controls. We observed a significantly elevated PIG-A mutation frequency (MF) and increased frequencies of micronuclei (MN) and nuclear buds (NBUDs) in coke oven workers, compared with levels in the control groups. We found that the coke oven workers with different lengths of service had a relatively high mutation frequency. Overall, the study findings showed that occupational exposure of coke oven workers increases the genetic damage and the PIG-A MF could be a potential biomarker for risk assessment of carcinogen exposure.

Benchmark dose estimation among benzene-exposed workers in China: Based on quantitative multi-endpoint genotoxicity assessments

Based on previous exposure studies, benzene (BZ) has been classified as a human carcinogen and occupational exposure limit (OELs) to BZ has been set to be about 1 ppm around the world. However, health hazards have still been reported with exposure below the OEL. Thus, the OEL needs to be updated to reduce health risk. The overall aim of our study was therefore to generate new OEL for BZ via a benchmark dose (BMD) approach and based on quantitative and multi-endpoint genotoxicity assessments. Genotoxicities were determined using the novel human PIG-A gene mutation assay, the micronucleus (MN) test and the COMET assay in benzene-exposed workers. Among the 104 workers with below current OELs, they exhibited significantly higher PIG-A mutant frequencies (MFs) (15.96 ± 14.41 × 10^-6) and MN frequencies (11.55 ± 6.83‰) than among the general subjects (PIG-A MFs: 5.45 ± 4.56 × 10^-6, MN frequencies: 4.51 ± 1.58‰), but no difference in the COMET assay. A significant association was also observed between BZ exposures and PIG-A MFs and MN frequencies (P < 0.001). Our results indicate that health hazards were induced among workers with below OEL exposures. Based on results from the PIG-A and MN assays, the lower confidence limit of the BMD (BMDL) were calculated to be 8.71 mg/m³-year and 0.44 mg/m³-year, respectively. Based on these calculations, the OEL for BZ was determined to be lower than 0.07 ppm. This value can be considered by regulatory agencies to set new exposure limits and to better protect workers.

PIG-A gene mutation as a genotoxicity biomaker in polycyclic aromatic hydrocarbon-exposed barbecue workers

Background

The PIG-A gene mutation assay is a valuable tool for measuring in vivo gene mutations in blood cells. The human PIG-A assay, used as a potential genotoxicity biomarker, is minimally invasive, sensitive, and cost-efficient; however, the relationship between carcinogen exposure and PIG-A mutations is not well understood.

Methods

We investigated the genotoxic effect of red blood cells using PIG-A assay and lymphocyte cytokinesis-block micronucleus test in barbecue restaurant workers (N = 70) exposed to polycyclic aromatic hydrocarbons (PAHs) and self-identified healthy control subjects (N = 56). Urinary PAH metabolites were measured to evaluate internal exposure levels.

Results

Multivariate Poisson regression showed that the PAH-exposed workers exhibited significantly higher PIG-A mutant frequency (MF) (8.04 ± 6.81 × 10^− 6) than did the controls (5.56 ± 5.26 × 10^− 6) (RR = 0.707, 95% CI: 0.615–0.812, P < 0.001). These results indicate that PAH exposure is a risk factor for elevated PIG-A MF. The frequencies of micronuclei (MN) and nuclear buds (NBUD) in the PAH-exposed workers (MN: 3.06 ± 2.07 ‰, NBUD: 1.38 ± 1.02 ‰) were also significantly higher than in the controls (MN: 1.46 ± 0.64 ‰, P < 0.001; NBUD: 0.70 ± 0.60 ‰, P < 0.001). Additionally, PIG-A MFs showed better associations with several urinary hydroxylated PAH metabolites (P_2-OH-Flu = 0.032, r_2-OH-Flu = 0. 268; P_2-OH-Phe = 0.022, r_2-OH-Phe = 0.286; P_3-OH-Phe = 0.0312, r_3-OH-Phe = 0.270; P_4-OH-Phe = 0.018, r_4-OH-Phe = 0.296), while the increase in MN, NPB, and NBUD frequencies was not associated with any OH-PAH metabolites; and high-PAH-exposed workers showed the highest PIG-A MFs. Furthermore, there was a significant association between PIG-A MF and PAH exposure levels (Chi-square test for trend, P = 0.006).

Conclusions

Our results indicate that an increase in PIG-A MF in barbecue workers could reflect the response to PAH exposure, providing evidence of its potential as a genotoxicity biomarker in human risk assessment.

Supplementary Information

The online version contains supplementary material available at 10.1186/s41021-021-00230-1.

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.

Mutagenicity monitoring in humans: Global versus specific origin of mutations

An underappreciated aspect of human mutagenicity biomonitoring is tissue specificity reflected in different assays, especially those that measure events that can only occur in developing bone marrow (BM) cells. Reviewed here are 9 currently-employed human mutagenicity biomonitoring assays. Several assays measure chromosome-level events in circulating T-lymphocytes (T-cells), i.e., traditional analyses of aberrations, translocation studies involving chromosome painting and fluorescence in situ hybridization (FISH) and determinations of micronuclei (MN). Other T-cell assays measure gene mutations. i.e., hypoxanthine-guanine phosphoriboslytransferase (HPRT) and phosphoribosylinositol glycan class A (PIGA). In addition to the T-cell assays, also reviewed are those assays that measure events in peripheral blood cells that necessarily arose in BM cells, i.e., MN in reticulocytes; glycophorin A (GPA) gene mutations in red blood cells (RBCs), and PIGA gene mutations in RBC or granulocytes. This review considers only cell culture- or cytometry-based assays to describe endpoints measured, methods, optimal sampling times, and sample summaries of typical quantitative and qualitative results. However, to achieve its intended focus on the target cells where events occur, kinetics of the cells of peripheral blood that derive at some point from precursor cells are reviewed to identify body sites and tissues where the genotoxic events originate. Kinetics indicate that in normal adults, measured events in T-cells afford global assessments of in vivo mutagenicity but are not specific for BM effects. Therefore, an agent's capacity for inducing mutations in BM cells cannot be reliably inferred from T-cell assays as the magnitude of effect in BM, if any, is unknown. By contrast, chromosome or gene level mutations measured in RBCs/reticulocytes or granulocytes must originate in BM cells, i.e. in RBC or granulocyte precursors, thereby making them specific indicators for effects in BM. Assays of mutations arising directly in BM cells may quantitatively reflect the mutagenicity of potential leukemogenic agents.

Dietary and lifestyle factors effect erythrocyte PIG-A mutant frequency in humans

It is well understood that poor diet and lifestyle choices can increase the risk of cancer. It is also well documented that cancer is a disease of DNA mutations, with mutations in key genes driving carcinogenesis. Measuring these mutations in a minimally invasive way may be informative as to which exposures are harmful and thus allow us to introduce primary preventative measures, in a bid to reduce cancer incidences. Here, we have measured mutations in the phosphatidylinositol glycan class A (PIG-A) gene in erythrocytes from healthy volunteers (n = 156) and from non-cancer patients attending the local endoscopy department (n = 144). The X-linked PIG-A gene encodes an enzyme involved in glycosylphosphatidylinositol (GPI) anchor synthesis. A silencing mutation in which leads to the absence of GPI anchors on the extracellular surface which can be rapidly assessed using flow cytometry. The background level of PIG-A mutant erythrocytes was 2.95 (95% CI: 2.59-3.67) mutant cells (10-6). Older age increased mutant cell frequency (P < 0.001). There was no difference in mutant cell levels between males and females (P = 0.463) or smokers and non-smokers (P = 0.186). In the endoscopy group, aspirin users had lower mutant frequencies (P = 0.001). Further information on diet and exercise was available for the endoscopy patient group alone, where those with a higher health promotion index score had lower mutant frequencies (P = 0.011). Higher dietary intake of vegetables reduced mutant cell levels (P = 0.022). Participants who exercised for at least 1 h a week appeared to have reduced mutant frequencies than those who did not exercise, although this was not statistically significant (P = 0.099). This low background level of mutant erythrocytes in a population makes this assay an attractive tool to monitor exposures such as those associated with lifestyles and diet, as demonstrated here.

Human blood PIG-A mutation and micronucleated reticulocyte flow cytometric assays: Method optimization and evaluation of intra- and inter-subject variation

We previously described flow cytometry-based methods for scoring the incidence of micronucleated reticulocytes (MN-RET) and PIG-A mutant phenotype reticulocytes (MUT RET) in rodent and human blood samples. The current report describes important methodological improvements for human blood analyses, including immunomagnetic enrichment of CD71-positive reticulocytes prior to MN-RET scoring, and procedures for storing frozen blood for later PIG-A analysis. Technical replicate variability in MN-RET and MUT RET frequencies based on blood specimens from 14 subjects, intra-subject variability based on serial blood draws from 6 subjects, and inter-subject variation based on up to 344 subjects age 0 to 73 years were quantified. Inter-subject variation explained most of the variability observed for both endpoints (≥77%), with much lower intra-subject and technical replicate variability. The relatively large degree of inter-subject variation is apparent from mean and standard deviation values for MN-RET (0.15 ± 0.10%) and MUT RET (4.7 ± 5.0 per million, after omission of two extreme outliers). The influences of age and sex on inter-subject variation were investigated, and neither factor affected MN-RET whereas both influenced MUT RET frequency. The lowest MUT RET values were observed for subjects <11 years old, and males had moderately higher frequencies than females. These results indicate that MN-RET and MUT RET are automation-compatible biomarkers of genotoxicity that bridge species of toxicological interest to include human populations. These data will be useful for appropriately designing future human studies that include these biomarkers of genotoxicity, and highlight the need for additional work aimed at identifying the sources of inter-individual variability reported herein.

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.

The potential application of human PIG-A assay on azathioprine-treated inflammatory bowel disease patients

The rodent Pig-a assay has been used extensively as a potential regulatory assay for evaluating the in vivo mutagenicity of test substances. Although the assay can be conducted in different mammalian species, there have been only a few reports describing its use in humans, and rarely in genotoxicant-exposed human populations. In this study, PIG-A mutation frequencies (MFs) were evaluated in 36 azathioprine (AZA; human carcinogen)-treated inflammatory bowel disease (IBD) patients and 36 healthy volunteers. IBD patients exhibited a slight but statistically higher MF (6.10 ± 4.44 × 10^-6 ) than healthy volunteers (4.97 ± 2.74 × 10^-6 ) (P = 0.0489). The estimated relative risk for the exposed patients was 1.22 which indicated that AZA is a risk factor for inducing PIG-A mutation. However, the PIG-A MF showed no associations with AZA treatment duration or total AZA exposure. In addition, we performed the cytokinesis-block micronucleus test on the same samples. The frequencies of micronuclei (MN) and nuclear buds (NBUD) in IBD patients (MN: 4.70 ± 2.86‰; NBUD: 1.89 ± 0.95‰) were significantly higher than in healthy volunteers (MN: 1.47 ± 0.77‰, P < 0.001; NBUD: 0.90 ± 0.58‰, P = 0.004). MN frequency also had significant correlations with AZA treatment duration (P = 0.011) and total AZA exposure (P = 0.018). Our findings indicate that AZA-treated IBD patients have only a marginally significant increase in PIG-A MF; in contrast, a much stronger AZA-associated increase in genotoxicity was detected with the lymphocyte MN assay. Environ. Mol. Mutagen. 2019. © 2019 Wiley Periodicals, Inc.

Quantitative Interpretation of Genetic Toxicity Dose-Response Data for Risk Assessment and Regulatory Decision-Making: Current Status and Emerging Priorities

The screen-and-bin approach for interpretation of genotoxicity data is predicated on three false assumptions: that genotoxicants are rare, that genotoxicity dose-response functions do not contain a low-dose region mechanistically characterized by zero-order kinetics, and that genotoxicity is not a bona fide toxicological endpoint. Consequently, there is a need to develop and implement quantitative methods to interpret genotoxicity dose-response data for risk assessment and regulatory decision-making. Standardized methods to analyze dose-response data, and determine point-of-departure (PoD) metrics, have been established; the most robust PoD is the benchmark dose (BMD). However, there are no standards for regulatory interpretation of mutagenicity BMDs. Although 5-10% is often used as a critical effect size (CES) for BMD determination, values for genotoxicity endpoints have not been established. The use of BMDs to determine health-based guidance values (HBGVs) requires assessment factors (AFs) to account for interspecies differences and variability in human sensitivity. Default AFs used for other endpoints may not be appropriate for interpretation of in vivo mutagenicity BMDs. Analyses of published dose-response data showing the effects of compensatory pathway deficiency indicate that AFs for sensitivity differences should be in the range of 2-20. Additional analyses indicate that the AF to compensate for short treatment durations should be in the range of 5-15. Future work should use available data to empirically determine endpoint-specific CES values; similarly, to determine AF values for BMD adjustment. Future work should also evaluate the ability to use in vitro dose-response data for risk assessment, and the utility of probabilistic methods for determination of mutagenicity HBGVs. Environ. Mol. Mutagen. 61:66-83, 2020. © 2019 Her Majesty the Queen in Right of Canada.

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.

Molecular analysis of glycosylphosphatidylinositol anchor deficient aerolysin resistant isolates in gulf war i veterans exposed to depleted uranium

During the First Gulf War (1991) over 100 servicemen sustained depleted uranium (DU) exposure through wound contamination, inhalation, and shrapnel. The Department of Veterans Affairs has a surveillance program for these Veterans which has included genotoxicity assays. The frequencies of glycosylphosphatidylinositol anchor (GPIa) negative (aerolysin resistant) cells determined by cloning assays for these Veterans are reported in Albertini RJ et al. (2019: Environ Mol Mutagen). Molecular analyses of the GPIa biosynthesis class A (PIGA) gene was performed on 862 aerolysin-resistant T-lymphocyte recovered isolates. The frequencies of different types of PIGA mutations were compared between high and low DU exposure groups. Additional molecular studies were performed on mutants that produced no PIGA mRNA or with deletions of all or part of the PIGA gene to determine deletion size and breakpoint sequence. One mutant appeared to be the result of a chromothriptic event. A significant percentage (>30%) of the aerolysin resistant isolates, which varied by sample year and Veteran, had wild-type PIGA cDNA (no mutation). As described in Albertini RJ et al. (2019: Environ Mol Mutagen), TCR gene rearrangement analysis of these isolates indicated most arose from multiple T-cell progenitors (hence the inability to find a mutation). It is likely that these isolates were the result of failure of complete selection against nonmutant cells in the cloning assays. Real-time studies of GPIa resistant isolates with no PIGA mutation but with a single TCR gene rearrangement found one clone with a PIGV deletion and several others with decreased levels of GPIa pathway gene mRNAs implying mutation in other GPIa pathway genes. Environ. Mol. Mutagen. 60:470-493, 2019. © 2019 Wiley Periodicals, Inc.

Developing a blood-based gene mutation assay as a novel biomarker for oesophageal adenocarcinoma

The Phosphatidylinositol glycan class A (PIG-A) gene mutation assay phenotypically measures erythrocyte mutations, assessed here for their correlation to neoplastic progression in the gastro-oesophageal reflux disease (GORD)-Barrett’s metaplasia (BM)-oesophageal adenocarcinoma (OAC) model. Endoscopy patients underwent venipuncture and erythrocytes fluorescently stained for glycosyl phosphatidylinositol (GPI)–anchored proteins; CD55 and CD59. Using flow cytometry, GPI–anchor negative erythrocytes (mutants) were scored and compared amongst groups. The study enlisted 200 patients and 137 healthy volunteers. OAC patients had a three–fold increase in erythrocyte mutant frequency (EMF) compared to GORD patients (p < 0.001) and healthy volunteers (p < 0.001). In OAC patients, higher EMF was associated with worsening tumour staging (p = 0.014), nodal involvement (p = 0.019) and metastatic disease (p = 0.008). Chemotherapy patients demonstrated EMF’s over 19–times higher than GORD patients. Patients were further classified into groups containing those with non-neoplastic disease and those with high-grade dysplasia/cancer with 72.1% of cases correctly classified by high EMF. Within the non-neoplastic group, aspirin users had lower EMF (p = 0.001) and there was a positive correlation between body mass index (p = 0.03) and age (p < 0.001) and EMF. Smokers had EMF’s over double that of non-smokers (p = 0.011). Results suggest this test could help detect OAC and may be a useful predictor of disease progression.

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.

A statistical approach for analyzing data from the in vivo Pig-a gene mutation assay

The in vivo Pig-a gene mutation assay serves to evaluate the genotoxic potential of chemicals. In the rat blood-based assay, the lack of CD59 on the surface of erythrocytes is quantified via fluorophore-labeled antibodies in conjunction with flow cytometric analysis to determine the frequency of Pig-a mutant phenotype cells. The assay has achieved regulatory relevance as it is suggested as an in vivo follow-up test for Ames mutagens in the recent ICH M7 [25] step 4 document. However, very little work exists regarding suitable statistical approaches for analyzing Pig-a data. In the current report, we present a statistical strategy based on a two factor model involving 'treatment' and 'time' incl. their interaction and a baseline covariate for log proportions to compare treatment and vehicle data per time point as well as in time. In doing so, multiple contrast tests allow us to discover time-related changes within and between treatment groups in addition to multiple treatment comparisons to a control group per single time point. We compare our proposed strategy with the results of classical Dunnett and Wilcoxon-Mann-Whitney tests using two data sets describing the mode of action of Chlorambucil and Glycidyl methacrylate both analyzed in a 28-day treatment schedule.

The development of an in vitro Pig-a assay in L5178Y cells

A recent flow cytometry-based in vivo mutagenicity assay involves the hemizygous phosphatidylinositol class A (Pig-a) gene. Pig-a forms the catalytic subunit of N-acetylglucosaminyltransferase required for glycophosphatidylinositol (GPI) anchor biosynthesis. Mutations in Pig-a prevent GPI-anchor synthesis resulting in loss of cell-surface GPI-linked proteins. The aim of the current study was to develop and validate an in vitro Pig-a assay in L5178Y mouse lymphoma cells. Ethyl methanesulfonate (EMS)-treated cells (186.24-558.72 µg/ml; 24 h) were used for method development and antibodies against GPI-linked CD90.2 and stably expressed CD45 were used to determine GPI-status by flow cytometry. Antibody concentration and incubation times were optimised (0.18 µg/ml, 30 min, 4 °C) and Zombie Violet™ (viability marker; 0.5%, 30 min, RT) was included. The optimum phenotypic expression period was 8 days. The low background mutation frequency of GPI-deficiency [GPI(-)] in L5178Y cells (0.1%) constitutes a rare event, thus flow cytometry acquisition parameters were optimised; 10⁴ cells were measured at medium flow rate to ensure a CV ≤ 30%. Spiking known numbers of GPI(-) cells into a wild-type population gave high correlation between measured and spiked numbers (R² 0.999). We applied the in vitro Pig-a assay to a selection of well-validated genotoxic and non-genotoxic compounds. EMS, N-ethyl-N-nitrosourea and 4-nitroquinoline-N-oxide dose dependently increased numbers of GPI(-) cells, while etoposide, mitomycin C, and a bacterial-specific mutagen did not. Cycloheximide and sodium chloride were negative. Sanger sequencing revealed Pig-a mutations in the GPI(-) clones. In conclusion, this in vitro Pig-a assay could complement the in vivo version, and follow up weak Ames positives and late-stage human metabolites or impurities.

Establishing a novel Pig-a gene mutation assay in L5178YTk+/- mouse lymphoma cells

The X-linked Pig-a gene encodes an enzyme required for the biosynthesis of glycosyl phosphatidylinositol (GPI) anchors. Pig-a mutant cells fail to synthesize GPI and to express GPI-anchored protein markers (e.g., CD90) on their surface. Marker deficiency serves as a phenotypic indicator of Pig-a mutation in various in vivo assays. Here, we describe an in vitro Pig-a mutation assay in L5178YTk^+/- mouse lymphoma cells, in which mutant-phenotype cells are measured by flow cytometry using a fluorescent anti-CD90 antibody. Increased frequencies of CD90-deficient mutants were detected in cells treated with benzo[a]pyrene (B[a]P), N-ethyl-N-nitrosourea (ENU), ethyl methanesulphonate, and 7,12-dimethylbenz[a]anthracene, with near maximum mutant frequencies measured eight days after treatment. The CD90 deficiency in mutant cells quantified by flow cytometry was shown to be due to loss of GPI anchors in a limiting-dilution cloning assay using proaerolysin selection. Individual CD90-deficient cells from cultures treated with ENU, B[a]P, and vehicle were sorted and clonally expanded for molecular analysis of their Pig-a gene. Pig-a mutations with agent-specific signatures were found in nearly all clones that developed from sorted CD90-deficient cells. These results indicate that a Pig-a mutation assay can be successfully conducted in L5178YTk^+/- cells. The assay may be useful for mutagenicity screening of environmental agents as well as for testing hypotheses in vitro before committing to in vivo Pig-a assays. Environ. Mol. Mutagen. 59:4-17, 2018. Published 2017. This article is a US Government work and is in the public domain in the USA.

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.

Development of an in vitro PIG-A gene mutation assay in human cells

Mutagens can be carcinogens, and traditionally, they have been identified in vitro using the Salmonella 'Ames' reverse mutation assay. However, prokaryotic DNA packaging, replication and repair systems are mechanistically very different to those in the humans we inevitably seek to protect. Therefore, for many years, mammalian cell line genotoxicity assays that can detect eukaryotic mutagens as well as clastogens and aneugens have been used. The apparent lack of specificity in these largely rodent systems, due partly to their mutant p53 status, has contributed to the use of animal studies to resolve data conflicts. Recently, silencing mutations at the PIG-A locus have been demonstrated to prevent glycophosphatidylinositol (GPI) anchor synthesis and consequentially result in loss of GPI-anchored proteins from the cell's extracellular surface. The successful exploitation of this mutant phenotype in animal studies has triggered interest in the development of an analogous in vitro PIG-A mutation screening assay. This article describes the development of a robust assay design using metabolically active human cells. The assay includes viability and cell membrane integrity assessment and conforms to the future ideas of the 21st-century toxicology testing.

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.

A population study using the human erythrocyte PIG-A assay

Erythrocyte-based PIG-A assay is sensitive and reliable in detecting exposure to mutagenetic agents in animal studies, but there are few data from human populations. In this study, we employed a method for detecting CD59 phenotypic variants, resulting from mutation in the PIG-A gene, in human red blood cells (RBCs), and determined the CD59-deficient RBC (RBC^CD59- ) frequencies in 217 subjects from general population. The majority of subjects had a relatively low mutant frequencies (MFs) (average, 5.25 ± 3.6 × 10^-6 , median, 4.38 × 10^-6 , for all subjects), but with males having a significantly greater MFs (5.97 ± 4.0 × 10^-6 ) than females (4.19 ± 2.5 ×10^-6 ). There was no correlation between MFs and age. In addition, MFs showed no difference between smoker and nonsmoker, and also no association with smoke duration in male subjects. However, there was a significant correlation between cigarette-pack-years which indicated that the MF was only slightly elevated with the increase of cigarette-pack-years. Moreover, intraindividual variations were investigated in three volunteer subjects over 300 days, and the MFs were relatively stable and repeatable. Furthermore, a pilot study by using white blood cell (WBC) assay based on labeling with FLAER was performed in volunteer subjects. The MFs of FLAER-deficient WBC (WBC^FLAER- ) and RBC^CD59- were consistently elevated in two subjects. Our findings provide baseline data that will be helpful in designing further studies using the PIG-A assay to monitor the genotoxic effects of carcinogens in human populations. Environ. Mol. Mutagen. 57:589-604, 2016. © 2016 Wiley Periodicals, Inc.

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.

CD48-deficient T-lymphocytes from DMBA-treated rats have de novo mutations in the endogenous Pig-a gene

A major question concerning the scientific and regulatory acceptance of the rodent red blood cell-based Pig-a gene mutation assay is the extent to which mutants identified by their phenotype in the assay are caused by mutations in the Pig-a gene. In this study, we identified T-lymphocytes deficient for the glycosylphosphatidylinositol-anchored surface marker, CD48, in control and 7,12-dimethylbenz[a]anthracene (DMBA)-treated rats using a flow cytometric assay and determined the spectra of mutations in the endogenous Pig-a gene in these cells. CD48-deficient T-cells were seeded by sorting at one cell per well into 96-well plates, expanded into clones, and exons of their genomic Pig-a were sequenced. The majority (78%) of CD48-deficient T-cell clones from DMBA-treated rats had mutations in the Pig-a gene. The spectrum of DMBA-induced Pig-a mutations was dominated by mutations at A:T, with the mutated A being on the nontranscribed strand and A → T transversion being the most frequent change. The spectrum of Pig-a mutations in DMBA-treated rats was different from the spectrum of Pig-a mutations in N-ethyl-N-nitrosourea (ENU)-treated rats, but similar to the spectrum of DMBA mutations for another endogenous X-linked gene, Hprt. Only 15% of CD48-deficient mutants from control animals contained Pig-a mutations; T-cell biology may be responsible for a relatively large fraction of false Pig-a mutant lymphocytes in control animals. Among the verified mutants from control rats, the most common were frameshifts and deletions. The differences in the spectra of spontaneous, DMBA-, and ENU-induced Pig-a mutations suggest that the flow cytometric Pig-a assay detects de novo mutation in the endogenous Pig-a gene.

Both PIGA and PIGL mutations cause GPI-a deficient isolates in the Tk6 cell line

Molecular analysis of proaerolysin selected glycosylphosphatidylinositol anchor (GPI-a) deficient isolates in the TK6 cell line was performed. Initial studies found that the expected X-linked PIGA mutations were rare among the spontaneous isolates but did increase modestly after ethyl methane sulfate (EMS) treatment (but to only 50% of isolates). To determine the molecular bases of the remaining GPI-a deficient isolates, real-time analysis for all the 25 autosomal GPI-a pathway genes was performed on the isolates without PIGA mutations, determining that PIGL mRNA was absent for many. Further analysis determined these isolates had several different homozygous deletions of the 5' region of PIGL (17p12-p22) extending 5' (telomeric) through NCOR1 and some into the TTC19 gene (total deletion >250,000 bp). It was determined that the TK6 parent had a hemizygous deletion in 17p12-p22 (275,712 bp) extending from PIGL intron 2 into TTC19 intron 7. Second hit deletions in the other allele in the GPI-a deficient isolates led to the detected homozygous deletions. Several of the deletion breakpoints including the original first hit deletion were sequenced. As strong support for TK6 having a deletion, a number of the isolates without PIGA mutations nor homozygous PIGL deletions had point mutations in the PIGL gene. These studies show that the GPI-a mutation studies using TK6 cell line could be a valuable assay detecting point and deletion mutations in two genes simultaneously.

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.

New approaches to advance the use of genetic toxicology analyses for human health risk assessment

Genetic toxicology testing has a crucial role in the safety assessment of substances of societal value by reducing human exposure to potential somatic and germ cell mutagens.

Single cell gel electrophoresis (SCGE) and Pig-a mutation assay in vivo-tools for genotoxicity testing from a regulatory perspective: a study of benzo[a]pyrene in Ogg1(-/-) mice

The OECD has developed test guidelines (TG) to identify agents with genotoxic effects. The in vivo alkaline single cell gel electrophoresis (SCGE) assay is currently being prepared to become such a TG. The performance of a combined SCGE/Pig-a gene mutation study was evaluated with the prototypical genotoxicant benzo[a]pyrene (BaP) at an exposure level known to induce germ cell mutation. We aimed to better understand (i) the strengths and weaknesses of the two methods applied in blood and their potential to predict germ cell mutagenicity, and (ii) the involvement of reactive oxygen species (ROS) following in vivo BaP-exposure. To explore the involvement of ROS on BaP genotoxicity, we utilised a mouse model deficient in a DNA glycosylase. Specifically, C57BL/6 mice (Ogg1(+/+) and Ogg1(-/-)) were treated for three consecutive days with 50 mg BaP/kg/day. DNA damage in nucleated blood cells was measured four hours after the last treatment with the SCGE assay, with and without formamidopyrimidine DNA glycosylase (Fpg). Pig-a mutant phenotype blood erythrocytes were analysed two and four weeks after treatment. BaP-induced DNA lesions were not significantly increased in either version of the SCGE assay. The phenotypic mutation frequencies for immature and mature erythrocytes were significantly increased after two weeks. These effects were not affected by genotype, suggesting oxidative damage may have a minor role in BaP genotoxicity, at least in the acute exposure situation studied here. While both assays are promising tools for risk assessment, these results highlight the necessity of understanding the limitations regarding each assay's ability to detect chemicals' genotoxic potential.

Quantitative dose-response analysis of ethyl methanesulfonate genotoxicity in adult gpt-delta transgenic mice

The assumption that mutagens have linear dose-responses recently has been challenged. In particular, ethyl methanesulfonate (EMS), a DNA-reactive mutagen and carcinogen, exhibited sublinear or thresholded dose-responses for LacZ mutation in transgenic Muta™Mouse and for micronucleus (MN) frequency in CD1 mice (Gocke E and Müller L [2009]: Mutat Res 678:101-107). In order to explore variables in establishing genotoxicity dose-responses, we characterized the genotoxicity of EMS using gene mutation assays anticipated to have lower spontaneous mutant frequencies (MFs) than Muta™Mouse. Male gpt-delta transgenic mice were treated daily for 28 days with 5 to 100 mg/kg EMS, and measurements were made on: (i) gpt MFs in liver, lung, bone marrow, kidney, small intestine, and spleen; and (ii) Pig-a MFs in peripheral blood reticulocytes (RETs) and total red blood cells. MN induction also was measured in peripheral blood RETs. These data were used to calculate Points of Departure (PoDs) for the dose responses, i.e., no-observed-genotoxic-effect-levels (NOGELs), lower confidence limits of threshold effect levels (Td-LCIs), and lower confidence limits of 10% benchmark response rates (BMDL10 s). Similar PoDs were calculated from the published EMS dose-responses for LacZ mutation and CD1 MN induction. Vehicle control gpt and Pig-a MFs were 13-40-fold lower than published vehicle control LacZ MFs. In general, the EMS genotoxicity dose-responses in gpt-delta mice had lower PoDs than those calculated from the Muta™Mouse and CD1 mouse data. Our results indicate that the magnitude and possibly the shape of mutagenicity dose responses differ between in vivo models, with lower PoDs generally detected by gene mutation assays with lower backgrounds.

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.

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.

Sensitivity of the Pig-a assay for detecting gene mutation in rats exposed acutely to strong clastogens

Clastogens are potential human carcinogens whose detection by genotoxicity assays is important for safety assessment. Although some endogenous genes are sensitive to the mutagenicity of clastogens, many genes that are used as reporters for in vivo mutation (e.g. transgenes) are not. In this study, we have compared responses in the erythrocyte Pig-a gene mutation assay with responses in a gene mutation assay that is relatively sensitive to clastogens, the lymphocyte Hprt assay, and in the reticulocyte micronucleus (MN) assay, which provides a direct measurement of clastogenicity. Male F344 rats were treated acutely with X-rays, cyclophosphamide (CP) and Cis-platin (Cis-Pt), and the frequency of micronucleated reticulocytes (MN RETs) in peripheral blood was measured 1 or 2 days later. The frequencies of CD59-deficient Pig-a mutant erythrocytes and 6-thioguanine-resistant Hprt mutant T-lymphocytes were measured at several times up to 16 weeks after the exposure. All three clastogens induced strong increases in the frequency of MN RETs, with X-rays and Cis-Pt producing near linear dose responses. The three agents also were positive in the two gene mutation assays although the assays detected them with different efficiencies. The Pig-a assay was more efficient in detecting the effect of Cis-Pt treatment, whereas the Hprt assay was more efficient for X-rays and CP. The results indicate that the erythrocyte Pig-a assay can detect the in vivo mutagenicity of clastogens although its sensitivity is variable in comparison with the lymphocyte Hprt assay.

The frequency of granulocytes with spontaneous somatic mutations: a wide distribution in a normal human population

Germ-line mutation rate has been regarded classically as a fundamental biological parameter, as it affects the prevalence of genetic disorders and the rate of evolution. Somatic mutation rate is also an important biological parameter, as it may influence the development and/or the course of acquired diseases, particularly of cancer. Estimates of this parameter have been previously obtained in few instances from dermal fibroblasts and lymphoblastoid cells. However, the methodology required has been laborious and did not lend itself to the analysis of large numbers of samples. We have previously shown that the X-linked gene PIG-A, since its product is required for glycosyl-phosphatidylinositol-anchored proteins to become surface bound, is a good sentinel gene for studying somatic mutations. We now show that by this approach we can accurately measure the proportion of PIG-A mutant peripheral blood granulocytes, which we call mutant frequency, ƒ. We found that the results are reproducible, with a variation coefficient (CV) of 45%. Repeat samples from 32 subjects also had a CV of 44%, indicating that ƒ is a relatively stable individual characteristic. From a study of 142 normal subjects we found that log ƒ is a normally distributed variable; ƒ variability spans a 80-fold range, from less than 1×10⁻⁶ to 37.5×10⁻⁶, with a median of 4.9×10⁻⁶. Unlike other techniques commonly employed in population studies, such as comet assay, this method can detect any kind of mutation, including point mutation, as long as it causes functional inactivation of PIG-A gene. Since the test is rapid and requires only a small sample of peripheral blood, this methodology will lend itself to investigating genetic factors that underlie the variation in the somatic mutation rate, as well as environmental factors that may affect it. It will be also possible to test whether ƒ is a determinant of the risk of cancer.

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

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