Robert H. Heflich

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

The product of the phosphatidylinositol glycan complementation group A gene (Pig‐A) is involved in the synthesis of glycosylphosphatidylinositol (GPI) anchors that link various protein markers to the surface of several types of mammalian cells, including hematopoietic cells. Previous observations indicate that Pig‐A mutation results in the lack of GPI synthesis and the absence of GPI‐anchored proteins on the cell surface. As a first step in designing a rapid assay for measuring Pig‐A mutation in the rat, we developed flow cytometry (FCM) strategies for detecting GPI‐negative cells in rat peripheral blood and spleen. Anti‐CD59 was used to detect GPI‐anchored proteins on red blood cells (RBCs), and anti‐CD48 was used to detect GPI‐anchored proteins on spleen T‐cells. The spontaneous frequency of CD59‐negative RBCs in five male F344 rats ranged from 1 × 10−6 to 27 × 10−6. In contrast, treatment of five rats with three doses of 40 mg/kg N‐ethyl‐N‐nitrosourea (ENU) increased the frequency of CD59‐negative RBCs to 183 × 10−6 to 249 × 10−6 at 2 weeks and to 329 × 10−6 to 413 × 10−6 at 4 weeks after dosing. In the same 4‐week posttreatment rats, the frequency of CD48‐negative T‐cells was 11 × 10−6 to 16 × 10−6 in control rats and 194 × 10−6 to 473 × 10−6 in ENU‐treated rats. The frequencies of GPI‐deficient cells were similar for RBCs and spleen T‐cells. These results indicate that FCM detection of GPI‐linked markers may form the basis for a rapid in vivo mutation assay. Although RBCs may be useful for a minimally invasive assay, T‐cells are a promising tissue for both detecting GPI‐deficient cells and confirming that Pig‐A gene mutation is the cause of the phenotype. Environ. Mol. Mutagen., Published 2008 Wiley‐Liss, Inc.

The in vivo pig‐a gene mutation assay, a potential tool for regulatory safety assessment

The Pig‐a (phosphatidylinositol glycan, Class A) gene codes for a catalytic subunit of the N‐acetylglucosamine transferase complex involved in an early step of glycosylphosphatidyl inositol (GPI) cell surface anchor synthesis. Pig‐a is the only gene involved in GPI anchor synthesis that is on the X‐chromosome, and research into the origins of an acquired genetic disease involving GPI anchor deficiency (paroxysmal nocturnal hemoglobinuria) indicates that cells lacking GPI anchors, or GPI‐anchored cell surface proteins, almost always have mutations in the Pig‐a gene. These properties of the Pig‐a gene and the GPI anchor system have been exploited in a series of assays for measuring in vivo gene mutation in blood cells from humans, rats, mice, and monkeys. In rats, flow cytometric measurement of Pig‐a mutation in red blood cells requires microliter volumes of blood and data can be generated in hours. Spontaneous mutant frequencies are relatively low (<5 × 10−6) and rats treated with multiple doses of the potent mutagen, N‐ethyl‐N‐nitrosourea, display Pig‐a mutant frequencies that are close to the sum of the frequencies produced by the individual exposures. A general observation is that induced mutant frequencies are manifested earlier in reticulocytes (about 2 weeks after treatment) than in total red blood cells (about 2 months after exposure). Based on data from a limited number of test agents, the assay shows promise for regulatory applications, including integration of gene mutation measurement into repeat‐dose toxicology studies. Environ. Mol. Mutagen., 2010. Published 2010 Wiley‐Liss, Inc.

Accumulation and persistence of Pig-A mutant peripheral red blood cells following treatment of rats with single and split doses of N-ethyl-N-nitrosourea.

We previously reported the development of an in vivo gene mutation assay using the phosphatidylinositol glycan complementation group A gene (Pig-A) as an endogenous reporter. The assay quantifies mutation in rat peripheral red blood cells (RBCs) by flow cytometric detection of cells negative for glycosylphosphatidyl inositol (GPI)-anchored protein surface markers. In this study, we examined the accumulation and persistence of Pig-A mutant RBCs in rats treated with N-ethyl-N-nitrosourea (ENU) using two dosing schedules. Male F344 rats were given single i.p. injections of 8.9, 35.6, or 142.4 mg/kg ENU or four equal weekly doses totaling 35.6 or 142.4 mg/kg ENU (8.9 mg/kgx4 or 35.6 mg/kgx4; split-dose groups). Before the treatment and through 26 weeks after the single dose or beginning the split-dose regimen, peripheral RBCs were collected and Pig-A mutant frequencies measured as RBCs negative for the GPI-anchored protein, CD59. Mean CD59-negative RBC frequencies in negative control rats ranged from 3.9 x 10(-6) to 28.7 x 10(-6) and displayed no time-related trend. With single ENU doses, CD59-negative RBC frequencies increased in a time- and dose-related manner. Maximum responses were observed beginning at 6 weeks post-treatment (57.3 x 10(-6) in the 8.9 mg/kg group; 186.9 x 10(-6) in the 35.6 mg/kg group; 759.2 x 10(-6) in the 142.4 mg/kg group), and these elevated mutant frequencies persisted to the last sampling time. In addition, splitting the dose of ENU into four weekly doses produced nearly the same mutant frequency as when given as a single dose: the maximum responses after four weekly doses of 8.9 or 35.6 mg/kg were 176.8 x 10(-6) and 683.3 x 10(-6), respectively. These results indicate that ENU-induced Pig-A mutant RBCs accumulate in a near additive fashion in rats, and once present in the peripheral blood, persist for at least 6 months. These characteristics of Pig-A mutation could be important for detecting weak mutagens by repeated or subchronic/chronic dosing protocols.

Derivation of point of departure (PoD) estimates in genetic toxicology studies and their potential applications in risk assessment

Genetic toxicology data have traditionally been employed for qualitative, rather than quantitative evaluations of hazard. As a continuation of our earlier report that analyzed ethyl methanesulfonate (EMS) and methyl methanesulfonate (MMS) dose–response data (Gollapudi et al., 2013), here we present analyses of 1‐ethyl‐1‐nitrosourea (ENU) and 1‐methyl‐1‐nitrosourea (MNU) dose–response data and additional approaches for the determination of genetic toxicity point‐of‐departure (PoD) metrics. We previously described methods to determine the no‐observed‐genotoxic‐effect‐level (NOGEL), the breakpoint‐dose (BPD; previously named Td), and the benchmark dose (BMD10) for genetic toxicity endpoints. In this study we employed those methods, along with a new approach, to determine the non‐linear slope‐transition‐dose (STD), and alternative methods to determine the BPD and BMD, for the analyses of nine ENU and 22 MNU datasets across a range of in vitro and in vivo endpoints. The NOGEL, BMDL10 and BMDL1SD PoD metrics could be readily calculated for most gene mutation and chromosomal damage studies; however, BPDs and STDs could not always be derived due to data limitations and constraints of the underlying statistical methods. The BMDL10 values were often lower than the other PoDs, and the distribution of BMDL10 values produced the lowest median PoD. Our observations indicate that, among the methods investigated in this study, the BMD approach is the preferred PoD for quantitatively describing genetic toxicology data. Once genetic toxicology PoDs are calculated via this approach, they can be used to derive reference doses and margin of exposure values that may be useful for evaluating human risk and regulatory decision making. Environ. Mol. Mutagen. 55:609–623, 2014.

Genotoxicity of silver nanoparticles evaluated using the Ames test and in vitro micronucleus assay

Silver nanoparticles (AgNPs) have antimicrobial properties, which have contributed to their widespread use in consumer products. A current issue regarding nanomaterials is the extent to which existing genotoxicity assays are useful for evaluating the risks associated with their use. In this study, the genotoxicity of 5 nm AgNPs was assessed using two standard genotoxicity assays, the Salmonella reverse mutation assay (Ames test) and the in vitro micronucleus assay. Using the preincubation version of the Ames assay, Salmonella strains TA102, TA100, TA1537, TA98, and TA1535 were treated with 0.15-76.8 μg/plate of the AgNPs. Toxicity limited the doses that could be assayed to 2.4-38.4 μg/plate; no increases in mutant frequency over the vehicle control were found for the concentrations that could be assayed. Human lymphoblastoid TK6 cells were treated with 10-30 μg/ml AgNPs, and additional cells were treated with water and 0.73 gy X-rays as vehicle and positive controls. Micronucleus frequency was increased by the AgNP treatment in a dose-dependent manner. At a concentration of 30 μg/ml (with 45.4% relative population doubling), AgNPs induced a significant, 3.17-fold increase with a net increase of 1.60% in micronucleus frequency over the vehicle control, a weak positive response by our criteria. These results demonstrate that the 5 nm AgNP are genotoxic in TK6 cells. Also, the data suggest that the in vitro micronucleus assay may be more appropriate than the Ames test for evaluating the genotoxicity of the AgNPs.

Development of an in vivo gene mutation assay using the endogenous Pig-A gene: II. Selection of Pig-A mutant rat spleen T-cells with proaerolysin and sequencing Pig-A cDNA from the mutants†‡

We previously reported that rat spleen T‐cells and peripheral red blood cells that are deficient in glycosylphosphatidylinositol (GPI) synthesis [presumed mutants for the phosphatidylinositol glycan complementation group A gene (Pig‐A)] could be detected by flow cytometry (FCM) as cells negative for GPI‐linked markers (CD48 and CD59, respectively). To establish this procedure as a rapid in vivo gene mutation assay, we have examined the Pig‐A gene of GPI‐deficient rat spleen T‐cells for DNA sequence alterations. Splenocytes were isolated from male F344 rats, primed with ionomycin and phorbol‐12‐myristate‐13‐acetate, and seeded at limiting‐dilution into 96‐well plates. To select for GPI‐deficient T‐cells, the cells were cultured for 10 days in a medium containing rat T‐STIM® and 2 nM proaerolysin (ProAER). The frequency of ProAER‐resistant (ProAERr) spleen T‐cells from control rats ranged from 1.3 × 10−6 to 4.8 × 10−6, while administration of three doses of 40 mg/kg N‐ethyl‐N‐nitrosourea increased the frequency of ProAERr T‐cells 100‐fold at 4 weeks after dosing. FCM analysis of the cells in ProAERr clones revealed that they were CD48‐negative, and thus presumably GPI‐deficient. Sequencing of Pig‐A cDNA from six ProAERr clones indicated that they all contained alterations in the Pig‐A protein coding sequence; five had base pair substitutions and one had multiple exons deleted. These results indicate that GPI‐deficient spleen T‐cells are Pig‐A gene mutants and support the use of FCM analysis of GPI‐deficient cells as a rapid assay for measuring in vivo gene mutation. Environ. Mol. Mutagen., 2008. Published 2008 Wiley‐Liss, Inc.

Improvement of in vivo genotoxicity assessment: Combination of acute tests and integration into standard toxicity testing

A working group convened at the 2009 5th IWGT to discuss possibilities for improving in vivo genotoxicity assessment by investigating possible links to standard toxicity testing. The working group considered: (1) combination of acute micronucleus (MN) and Comet assays into a single study, (2) integration of MN assays into repeated-dose toxicity (RDT) studies, (3) integration of Comet assays into RDT studies, and (4) requirements for the top dose when integrating genotoxicity measurements into RDT studies. The working group reviewed current requirements for in vivo genotoxicity testing of different chemical product classes and identified opportunities for combination and integration of genotoxicity endpoints for each class. The combination of the acute in vivo MN and Comet assays was considered by the working group to represent a technically feasible and scientifically acceptable alternative to conducting independent assays. Two combination protocols, consisting of either a 3- or a 4-treament protocol, were considered equally acceptable. As the integration of MN assays into RDT studies had already been discussed in detail in previous IWGT meetings, the working group focussed on factors that could affect the results of the integrated MN assay, such as the possible effects of repeated bleeding and the need for early harvests. The working group reached the consensus that repeated bleeding at reasonable volumes is not a critical confounding factor for the MN assay in rats older than 9 weeks of age and that rats bled for toxicokinetic investigations or for other routine toxicological purposes can be used for MN analysis. The working group considered the available data as insufficient to conclude that there is a need for an early sampling point for MN analysis in RDT studies, in addition to the routine determination at terminal sacrifice. Specific scenarios were identified where an additional early sampling can have advantages, e.g., for compounds that exert toxic effects on hematopoiesis, including some aneugens. For the integration of Comet assays into RDT studies, the working group reached the consensus that, based upon the limited amount of data available, integration is scientifically acceptable and that the liver Comet assay can complement the MN assay in blood or bone marrow in detecting in vivo genotoxins. Practical issues need to be considered when conducting an integrated Comet assay study. Freezing of tissue samples for later Comet assay analysis could alleviate logistical problems. However, the working group concluded that freezing of tissue samples can presently not be recommended for routine use, although it was noted that results from some laboratories look promising. Another discussion topic centred around the question as to whether tissue toxicity, which is more likely observed in RDT than in acute toxicity studies, would affect the results of the Comet assay. Based on the available data from in vivo studies, the working group concluded that there are no clear examples where cytotoxicity, by itself, generates increases or decreases in DNA migration. The working group identified the need for a refined guidance on the use and interpretation of cytotoxicity methods used in the Comet assay, as the different methods used generally lead to inconsistent conclusions. Since top doses in RDT studies often are limited by toxicity that occurs only after several doses, the working group discussed whether the sensitivity of integrated genotoxicity studies is reduced under these circumstances. For compounds for which in vitro genotoxicity studies yielded negative results, the working group reached the consensus that integration of in vivo genotoxicity endpoints (typically the MN assay) into RDT studies is generally acceptable. If in vitro genotoxicity results are unavailable or positive, consensus was reached that the maximum tolerated dose (MTD) is acceptable as the top dose in RDT studies in many cases, such as when the RDT study MTD or exposure is close (50% or greater) to an acute study MTD or exposure. Finally, the group agreed that exceptions to this general rule might be acceptable, for example when human exposure is lower than the preclinical exposure by a large margin.

Transgenic Animal Models in Toxicology: Historical Perspectives and Future Outlook

Transgenic animal models are powerful tools for developing a more detailed understanding on the roles of specific genes in biological pathways and systems. Applications of these models have been made within the field of toxicology, most notably for the screening of mutagenic and carcinogenic potential and for the characterization of toxic mechanisms of action. It has long been a goal of research toxicologists to use the data from these models to refine hazard identification and characterization to better inform human health risk assessments. This review provides an overview on the applications of transgenic animal models in the assessment of mutagenicity and carcinogenicity, their use as reporter systems, and as tools for understanding the roles of xenobiotic-metabolizing enzymes and biological receptors in the etiology of chemical toxicity. Perspectives are also shared on the future outlook for these models in toxicology and risk assessment and how transgenic technologies are likely to be an integral tool for toxicity testing in the 21st century.

Further development of the rat Pig-a mutation assay: measuring rat Pig-a mutant bone marrow erythroids and a high throughput assay for mutant peripheral blood reticulocytes.

Recent studies indicate that the Pig‐a assay is a promising tool for evaluating in vivo mutagenicity. We have developed novel rat Pig‐a assays that facilitate measuring mutant frequencies in two early arising populations of blood cells, bone marrow erythroids (BMEs) and peripheral blood (PB) reticulocytes (RETs). In these assays, bone marrow cells of erythroid origin and PB red blood cells (RBCs) were identified using an antibody against rat erythroid‐specific marker HIS49. In addition, RETs were selectivity enriched from PB using magnetic separation of cells positive for CD71, a transferrin receptor expressed on the surface of BMEs and RETs, but not on the surface of mature RBCs. With magnetic enrichment, more than 1 × 106 CD71‐positive RETs could be evaluated by flow cytometry for Pig‐a mutant frequency within 5 to 8 min. CD59‐deficient RET and BME frequencies of more than 100 × 10−6 and 80 × 10−6 were detected 1 week after treating rats with 40 mg/kg N‐ethyl‐N‐nitrosourea; by comparison, the frequency of CD59‐deficient total RBCs in these rats was 13.2 × 10−6. The frequency of spontaneous Pig‐a mutant RETs and BMEs was less than 5 × 10−6 and 15 × 10−6, respectively. Since ∼98% of nucleated cells in the BME fraction were erythroblasts, it should be possible to use BMEs to determine the spectrum of CD59‐deficient Pig‐a mutations in cells of erythroid lineage. Conducting concurrent Pig‐a assays on RETs and BMEs may be useful for evaluating the in vivo mutagenicity of chemicals, especially when prolonged mutant manifestation is not feasible or when the confirmation of mutation induction is necessary.

Manifestation of Pig-a mutant bone marrow erythroids and peripheral blood erythrocytes in mice treated with N-ethyl-N-nitrosourea: direct sequencing of Pig-a cDNA from bone marrow cells negative for GPI-anchored protein expression.

Our previous rat studies indicate that the endogenous Pig-a gene is a promising reporter of in vivo mutation and potentially useful as the basis for an in vivo genotoxicity assay. The function of the Pig-a protein in the synthesis of glycosylphosphatidyl inositol (GPI) anchors is conserved in variety of eukaryotic cells, including human and rodent cells, which implies that Pig-a mutants can be measured in a similar manner in different mammalian species. In the present study, we developed a flow cytometric Pig-a assay for rapidly measuring gene mutation in the mouse. An antibody to TER-119, a specific cell-surface marker of murine erythroid lineage, was used to identify erythrocytes in peripheral blood (PB) and erythroids in bone marrow (BM). An antibody to CD24, a GPI-anchored protein, was used to identify Pig-a mutants as CD24-negative cells. CD-1 mice were administered a single dose of 100mg/kgN-ethyl-N-nitrosourea (ENU), and PB and BM were collected at 1, 2, and 4 weeks after dosing. While the Pig-a mutant frequency (MF) in PB was increased moderately at 2 and 4 weeks after ENU dosing, the Pig-a MF in BM was strongly increased starting at 1 week after the dosing, with the elevated MF persisting for at least 4 weeks after the dosing. We also used flow cytometric sorting to isolate CD24-negative erythroids from the BM of ENU-treated mice. cDNA sequencing indicated that these cells have mutations in the Pig-a gene, with base-pair substitutions typical of ENU-induced mutation spectra. The results indicate that the Pig-a mutation assay can be adapted for measuring mutation in BM erythroids and PB of mice. Taken together, the data suggest that Pig-a mutants are fixed in the BM, where they further proliferate and differentiate; erythrocytes derived from these BM Pig-a mutants transit from the BM and accumulate in PB.