Genetic toxicity of procarbazine in bacteria and yeast

CD59-deficient bone marrow erythroid cells from rats treated with procarbazine and propyl-nitrosourea have mutations in the Pig-a gene

Procarbazine (PCZ) and N-propyl-N-nitrosourea (PNU) are rodent mutagens and carcinogens. Both induce GPI-anchored marker-deficient mutant-phenotype red blood cells (RBCs) in the flow cytometry-based rat RBC Pig-a assay. In the present study, we traced the origin of the RBC mutant phenotype by analyzing Pig-a mutations in the precursors of RBCs, bone marrow erythroid cells (BMEs). Rats were exposed to a total of 450 mg/kg PCZ hydrochloride or 300 mg/kg PNU, and bone marrow was collected 2, 7, and 10 weeks later. Using a flow cell sorter, we isolated CD59-deficient mutant-phenotype BMEs from PCZ- and PNU-treated rats and examined their endogenous X-linked Pig-a gene by next generation sequencing. Pig-a mutations consistent with the properties of PCZ and PNU were found in sorted mutant-phenotype BMEs. PCZ induced mainly A > T transversions with the mutated A on the nontranscribed strand of the Pig-a gene, while PNU induced mainly T > A transversions with the mutated T on the nontranscribed strand. The treatment-induced mutations were distributed across the protein coding sequence of the Pig-a gene. The causal relationship between BMEs and RBCs and the agent-specific mutational spectra in CD59-deicient BMEs indicate that the rat RBC Pig-a assay, scoring CD59-deficient mutant-phenotype RBCs in peripheral blood, detects Pig-a gene mutation.

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

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

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.

Flow cytometric analysis of Pig-a gene mutation and chromosomal damage induced by procarbazine hydrochloride in CD-1 mice

Procarbazine is a genotoxic carcinogen whose DNA-damaging activities are not reliably detected in vitro. We evaluated the in vivo genotoxic effects of procarbazine on hematopoietic cells of male CD-1 mice using a multi-endpoint study design that scored micronucleated reticulocyte (MN-RET) frequency and gene mutation at the Pig-a locus. CD-1 mice were treated for 3 days with procarbazine, up to 150 mg/kg/day. Blood samples collected on Day 3 exhibited robust induction of MN-RETs, with the high dose group exhibiting a mean 29-fold increase. Blood collected 15 and 30 days after treatment began was analyzed for Pig-a mutation with a dual labeling method that facilitated mutant cell frequency measurements in both total erythrocytes and the reticulocyte subpopulation. Procarbazine significantly increased mutant reticulocyte frequencies by Day 15. Mutant erythrocyte responses were also apparent, with a peak incidence observed for the high dose group on Day 30. These results demonstrate that the complex metabolism and resulting genotoxicity of procarbazine is best evaluated in intact animal models, and show that the flow cytometric methods employed offer a means to efficiently monitor both in vivo chromosomal damage and mutation.

Functional genomics in the mouse: phenotype-based mutagenesis screens

Significant progress has been made in sequencing the genomes of several model organisms, and efforts are now underway to complete the sequencing of the human genome. In parallel with this effort, new approaches are being developed for the elucidation of the functional content of the human genome. The mouse will have an important role in this phase of the genome project as a model system. In this review we discuss and compare classical genetic approaches to gene function-phenotype-based mutagenesis screens aimed at the establishment of a large collection of single gene mutations affecting a wide range of phenotypic traits in the mouse. Whereas large scale genome-wide screens that are directed at the identification of all loci contributing to a specific phenotype may be impractical, region-specific saturation screens that provide mutations within a delimited chromosomal region are a feasible alternative. Region-specific screens in the mouse can be performed in only two generations by combining high-efficiency chemical mutagenesis with deletion complexes generated using embryonic stem (ES) cells. The ability to create and analyze deletion complexes rapidly, as well as to map novel chemically-induced mutations within these complexes, will facilitate systematic functional analysis of the mouse genome and corresponding gene sequences in humans. Furthermore, as the extent of the mouse genome sequencing effort is still uncertain, we underscore a necessity to direct sequencing efforts to those chromosomal regions that are targets for extensive mutagenesis screens.

Drosophila IRBP/Ku p70 corresponds to the mutagen-sensitive mus309 gene and is involved in P-element excision in vivo

The P family of transposable elements in Drosophila transpose by a cut-and-paste mechanism involving double-strand gap repair. We report here that a Drosophila mutagen-sensitive mutant, mus3O9, contains a mutation in IRBP (inverted repeat binding protein), the Drosophila homolog of the mammalian Ku p70 gene. We show that the repair of double-strand DNA breaks after P-element excision is severely reduced in mus3O9 mutants using an in vivo assay for P-element transposase activity. In addition, excision products recovered from mus3O9 mutant embryos by use of a plasmid-based P-element mobility assay contain large deletions, suggesting that IRBP is involved in the repair of double-strand DNA breaks. Our findings provide the first demonstration that a mutation in the IRBP gene affects double-strand DNA break repair and suggest that DNA repair functions are conserved between Drosophila and mammals.

Cloning and analysis of the dec-1 female-sterile locus, a gene required for proper assembly of the Drosophila eggshell

Female-sterile mutations at the dec-1 (defective chorion-1) locus of Drosophila severely disrupt the organization of the eggshell late in oogenesis. Previous characterization of dec-1 mutations has correlated the defects with failure to accumulate an early eggshell protein that undergoes proteolytic cleavage during choriogenesis. To enable further study of the regulation and processing of dec-1 products, we have molecularly cloned the locus and characterized its transcripts. Chromosome jumping was used to isolate a deficiency breakpoint within the locus. Overlapping genomic clones from a wild-type library were then obtained, and a region including the dec-1 locus was identified by hybridization to cDNA probes complementary to RNA from stage 9-10 egg chambers. Analysis of genomic rearrangements associated with the locus verified its identity. Two transcripts from the locus have been identified and characterized using cDNA clones, RNase protection, and primer extension analyses. A 4.0-kb transcript accumulates maximally in stages 9-10, when the primary follicle cell protein associated with dec-1 mutations is synthesized. A second transcript of 5.8 kb, generated by alternative splicing, accumulates during stages 11-12. These results are discussed in light of previous analysis of dec-1 mutations.

Evaluation of the L5178Y mouse lymphoma cell mutagenesis assay: interlaboratory reproducibility and assessment

The L5178Y mouse lymphoma cell mutagenesis assay is used to detect the mutagenic activity of chemicals in a mammalian cell system. To evaluate this assay we compared the results of assays performed independently on 63 chemicals by laboratories at SRI International and Litton Bionetics, Inc. The two laboratories used similar protocols. The solvent and positive control mutant frequencies and cloning efficiencies obtained by the two laboratories were similar, which justified the use of the same quality-control criteria and analytical procedures for analyzing the results from both laboratories. The rate of concordance between the two laboratories was 92% for tests in the absence of S9 activation and 95% for tests in its presence. The results of the assays agreed for 57 of the 63 chemicals; three chemicals could not be compared because there were questionable calls in at least one of the laboratories; the results disagreed for the three remaining chemicals. The concordance rate for these overall assay evaluations was 95%. The interlaboratory concordance rates were similar to concordance rates for replicate experiments within the laboratories (96% at LBI, 94% at SRI). The mouse lymphoma cell mutagenicity results are concordant with the rodent chronic assay results in 78% of 50 chemicals and with the Salmonella assay results in 79% of 56 chemicals. Fifteen carcinogens were examined for genotoxic effects in mouse lymphoma, Salmonella, Chinese hamster ovary (CHO) chromosomal aberration, and CHO sister chromatid exchange assay. Eight of these were positive in all four assays. Of the seven noncarcinogens that were tested in these four assays, none was negative in all four. The main conclusion to be drawn from this study is that the mouse lymphoma cell forward mutation assay, as performed and evaluated in this study, detects chemical mutagenicity in a manner that is highly consistent with other genetic endpoints as well as rodent carcinogenicity studies. Thus the assay quality control and response criteria established in this study led not only to a high degree of reproducibility but also to an apparently reliable detection of mutagenic activity.

Evaluation of the DNA repair host-mediated assay. II. Presence of genotoxic factors in various organs of mice treated with chemotherapeutic agents

The DNA repair host-mediated assay was further calibrated by testing 7 chemotherapeutic agents known to possess carcinogenic activity, namely bleomycin (BLM), cis-diamminedichloroplatinum-II (cis-Pt), cyclophosphamide (CP), diethylstilboestrol (DES), isonicotinic acid hydrazide (isoniazid, INH), natulan (NAT) and mitomycin C (MMC). Differential survival of wild-type and uvrB/recA E. coli strains served as a measure of genotoxic activity. In in vitro assays, BLM, cis-Pt and MMC exhibited high genotoxic activity. The other 4 compounds had no measurable effect on the survival of the two strains, either with or without mouse liver preparations. In the host-mediated assays BLM, cis-Pt, MMC and also NAT induced strong killing of the DNA repair-deficient bacteria recovered from liver, spleen, lungs, kidneys and the blood of treated mice compared to the wild-type strain. The results are not indicative of large organ-specific differences in genotoxically active amounts of the drugs immediately after their application to the host animals. CP, INH and DES did not show geneotix activity in these assays even at very high exposure levels. To compare the genetic endpoint measured in the DNA repair assays, i.e. induction of repairable DNA damage, with the induction of gene mutations, the ability of the 7 drugs to induce valine-resistant (VALr) mutants in E. coli was measured in host-mediated assays under identical treatment conditions. INH showed considerable mutagenic activity in E. coli cells recovered from liver and spleen, while BLM and MMC induced a 3-4-fold increase in VALr mutants above spontaneous levels. The other compounds showed no mutagenic activity under these in vivo conditions. From these results it can be concluded that the type of primary DNA lesions produced by these chemotherapeutic agents (cross-links by MMC and cis-Pt, and strand breaks by BLM and possibly by NAT; base alkylation by INH) appears to determine whether a compound will be highly positive in the DNA repair assay as in the case of BLM, cis-Pt, MMC and NAT, and less effective in inducing mutations under similar conditions, or whether the opposite will occur, as in the case of INH; DES and CP probably do not interact sufficiently with bacterial DNA to show an effect in either of the genetic endpoints; and the present DNA repair host-mediated assay may represent a sensitive, rapid and economic method for monitoring genotoxic factors in various organs of experimental animals which have been treated with cytostatic drugs.

Testing of chemicals for genetic activity with Saccharomyces cerevisiae: a report of the U.S. Environmental Protection Agency Gene-Tox Program

The yeast Saccharomyces cerevisiae is a unicellular fungus that can be cultured as a stable haploid or a stable diploid . Diploid cultures can be induced to undergo meiosis in a synchronous fashion under well-defined conditions. Consequently, yeasts can be used to study genetic effects both in mitotic and in meiotic cells. Haploid strains have been used to study the induction of point mutations. In addition to point mutation induction, diploid strains have been used for studying mitotic recombination, which is the expression of the cellular repair activities induced by inflicted damage. Chromosomal malsegregation in mitotic and meiotic cells can also be studied in appropriately marked strains. Yeast has a considerable potential for endogenous activation, provided the tests are performed with appropriate cells. Exogenous activation has been achieved with S9 rodent liver in test tubes as well as in the host-mediated assay, where cells are injected into rodents. Yeast cells can be recovered from various organs and tested for induced genetic effects. The most commonly used genetic end point has been mitotic recombination either as mitotic crossing-over or mitotic gene conversion. A number of different strains are used by different authors. This also applies to haploid strains used for monitoring induction of point mutations. Mitotic chromosome malsegregation has been studied mainly with strain D6 and meiotic malsegregation with strain DIS13 . Data were available on tests with 492 chemicals, of which 249 were positive, as reported in 173 articles or reports. The genetic test/carcinogenicity accuracy was 0.74, based on the carcinogen listing established in the Gene-Tox Program. The yeast tests supplement the bacterial tests for detecting agents that act via radical formation, antibacterial drugs, and other chemicals interfering with chromosome segregation and recombination processes.

Genetic effects of procarbazine in the yeast Saccharomyces cerevisiae, strain D4

Procarbazine ( PCZ ) was tested for its ability to induce mitotic gene conversions at the ade and trp loci of Saccharomyces cerevisiae, strain D4. The influence of the following factors was examined: growth phase of the yeast cells (log vs stationary phase), pH of the treatment solution, and addition of mouse S9 fractions prepared from different organs. The drug was found more toxic and mutagenic at low doses (up to 25 mg/ml) for log phase cells, and scarcely toxic but highly mutagenic, even at high doses, for stationary phase cells. PCZ activity was reduced by acidic pH, and suppressed by S9 mix. Gene conversions were also analyzed in the intrasanguineous host-mediated assay performed in mice orally administered with PCZ . In such conditions PCZ was ineffective in stimulating mitotic gene conversions, probably owing to its inactivation in the acidic environment of the gastroenteric tract.

An evaluation of the host-mediated assay and body fluid analysis. A report of the U.S. Environmental Protection Agency Gene-Tox Program

The methodologies and status of the Host-Mediated Assay were reviewed using the published literature available up to June 1980. The Working Group reviewed 274 documents, including abstracts, research articles, review articles, and publicly available contracts and grant final reports. From this group, abstracts and reviews were rejected from critical evaluation. 77 documents were accepted and reviewed by the Working Group and the test results summarized. These selected documents yielded 208 chemicals that were evaluated in th host-mediated assay. Of these chemicals, 133 were mutagenic in this assay with one or more indicators. 76 chemicals, several of which are not considered to be carcinogenic, were not detected by any of the indicators. Of the 208 chemicals, 125 had been tested in carcinogenicity assay in rodents. 90, or 71%, of the carcinogens were detected as mutagens in the Host-Mediated Assay. In several cases, those carcinogens not detected may have been negative because of improper selection of the indicator. The Working Group concluded that the Host-Mediated Assay is an important test in mutagenicity/carcinogenicity research and that, by proper selection of protocols and indicators, valuable information can be gained that otherwise would be overlooked strict, in vitro assays.

Detection of mutagenicity of procarbazine by the host-mediated assay with polychlorinated biphenyl (aroclor 1254) as enzyme inducer

Procarbazine [N-isoppropyl-alpha-(2-methylhydrazino)-p-toluamide] was tested for mutagenicity with Salmonella typhimurium G46 in the host-mediated assay by using male BALB/c mice pretreated orally with polychlorinated biphenyl (PCB, Aroclor 1254). Procarbazine was weakly mutagenic without PCB pretreatment, but the pretreatment greatly enhanced the mutagenicity of this compound. The administration of 500 mg PCB/kg 1 day before procarbazine dosage was suitable for the detection of the mutagenicity. Among PCB, 3-methylcholanthrene (3-MC) and phenobarbital sodium (PB), the former 2 inducers showed much stronger enhancing effects than PB. The pretreatment with 3-MC in combination with PB did not cause further enhancement compared with 3-MC alone.

Specific locus mutations induced in somatic cells of rats by orally and parenterally administered procarbazine

A new test, the granuloma pouch assay, was used in detecting specific locus mutations in somatic cells of rats in vivo after the animals were treated orally and parenterally with procarbazine hydrochloride, an agent used in cancer chemotherapy. The results indicate that stable intermediates are formed in the body and distributed as proximate mutagens.