Effects of benzene in a micronucleus test on peripheral blood utilizing acridine orange-coated slides

ATSDR evaluation of health effects of benzene and relevance to public health

As part of its mandate, the Agency for Toxic Substances and Disease Registry (ATSDR) prepares toxicological profiles on hazardous chemicals found at Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) National Priorities List (NPL) sites that have the greatest public health impact. These profiles comprehensively summarize toxicological and environmental information. This article constitutes the release of portions of the Toxicological Profile for Benzene. The primary purpose of this article is to provide public health officials, physicians, toxicologists, and other interested individuals and groups with an overall perspective on the toxicology of benzene. It contains descriptions and evaluations of toxicological studies and epidemiological investigations and provides conclusions, where possible, on the relevance of toxicity and toxicokinetic data to public health.

Genotoxicity of benzene and its metabolites

The potential role of genotoxicity in human leukemias associated with benzene (BZ) exposures was investigated by a systematic review of over 1400 genotoxicity test results for BZ and its metabolites. Studies of rodents exposed to radiolabeled BZ found a low level of radiolabel in isolated DNA with no preferential binding in target tissues of neoplasia. Adducts were not identified by 32P-postlabeling (equivalent to a covalent binding index <0.002) under the dosage conditions producing neoplasia in the rodent bioassays, and this method would have detected adducts at 1/10,000th the levels reported in the DNA-binding studies. Adducts were detected by 32P-postlabeling in vitro and following high acute BZ doses in vivo, but levels were about 100-fold less than those found by DNA binding. These findings suggest that DNA-adduct formation may not be a significant mechanism for BZ-induced neoplasia in rodents. The evaluation of other genotoxicity test results revealed that BZ and its metabolites did not produce reverse mutations in Salmonella typhimurium but were clastogenic and aneugenic, producing micronuclei, chromosomal aberrations, sister chromatid exchanges and DNA strand breaks. Rodent and human data were compared, and BZ genotoxicity results in both were similar for the available tests. Also, the biotransformation of BZ was qualitatively similar in rodents, humans and non-human primates, further indicating that rodent and human genotoxicity data were compatible. The genotoxicity test results for BZ and its metabolites were the most similar to those of topoisomerase II inhibitors and provided less support for proposed mechanisms involving DNA reactivity, mitotic spindle poisoning or oxidative DNA damage as genotoxic mechanisms; all of which have been demonstrated experimentally for BZ or its metabolites. Studies of the chromosomal translocations found in BZ-exposed persons and secondary human leukemias produced by topoisomerase II inhibitors provide some additional support for this mechanism being potentially operative in BZ-induced leukemia.

Mutational analysis of a function of xeroderma pigmentosum group A (XPA) protein in strand-specific DNA repair

To analyze the function of the xeroderma pigmentosum group A (XPA) protein in strand-specific DNA repair, we examined repair of UV-induced cyclobutane pyrimidine dimer (CPD) in transcribed and non-transcribed strands of the dihydrofolate reductase gene of xeroderma pigmentosum group A (XP-A) cell line (XP12ROSV) which was transfected with various types of mutant XPA cDNA. The transfectant overexpressing mutant XPA with a defect in the interaction with either ERCC1, replication protein A (RPA), or general transcription factor TFIIH, showed more or less decreased repair of CPD in each strand in parallel, while in the transfectant overexpressing R207G (Arg207to Gly) mutant XPA derived from XP129, a UV-resistant XP12ROSV revertant, the rate of CPD repair was almost normal in each strand. We also examined the dose responses of the XPA protein on CPD repair in each strand by the modulation of the expression levels of wild-type or R207G mutant XPA using an inducible expression system, LacSwitchtrade mark promoter. There were good correlations between the rate of CPD repair in each strand and the amount of XPA protein produced in these Lac cells. Our results indicate that the XPA protein is equally important for the CPD repair in both transcribed and non-transcribed strands and that the R207G mutation found in XP129 may not be responsible for a selective defect in CPD repair in the non-transcribed strand in XP129.

An automated new technique for scoring the rodent micronucleus assay: computerized image analysis of acridine orange supravitally stained peripheral blood cells

We developed an automated image analysis system to obtain objective data for the rodent peripheral blood micronucleus assay with acridine orange (AO) supravital staining. The system was able to identify micronucleated reticulocytes (MNRETs) and to evaluate inhibition of bone marrow cell proliferation by measuring the reticular area of reticulocytes (RETs). We also developed automated equipment to produce homogeneous acridine orange-coated glass slides. This study was designed to compare automated scoring with manual scoring using 4 model clastogens and 2 mouse strains. The MNRET incidence induced by each clastogen was similar for automated and manual scoring, and there was good correlation (r=0.92) between the methods. In addition, an index of bone marrow toxicity based on the reticular area of RETs was compared to the conventional index (% of polychromatic erythrocytes (PCE) to total erythrocytes; PCE ratio) and was similar. The results indicated that our technique for computer-assisted image analysis for the micronucleus assay with AO supravitally stained peripheral blood RETs was comparable to conventional microscopic scoring, and it was superior in objectivity and statistical power.

Achievements by CSGMT/JEMS.MMS: the Collaborative Study Group for the Micronucleus Test in the Mammalian Mutagenesis Study Group of the Environmental Mutagen Society of Japan

The Collaborative Study Group for the Micronucleus Test (CSGMT) is one of the task groups in the Mammalian Mutagenesis Study Group (MMS) of the Environmental Mutagen Society of Japan (JEMS). It was established in 1982 and has made efforts to understand what the micronucleus test is, what are the advantages and disadvantages of the test as an in vivo detection system for mutagens/carcinogens, and to establish a standard protocol applicable to numerous chemicals. Members of the CSGMT have published more than 75 papers as part of collaborative studies and have contributed to the understanding of the nature of the micronucleus test and to setting guidelines for testing of medicinal and other chemicals. The CSGMT held some workshops to share up-to-date knowledge and techniques on the micronucleus test. Through workshops and collaborative studies, the CSGMT contributed to the maintaining of a high standard of knowledge and techniques among Japanese researchers of the micronucleus test. This paper reviews achievements made by the CSGMT until now.

Initial experiences and future directions for transgenic mouse mutation assays

Transgenic mice have introduced new possibilities in the field of mutation research and safety testing. Using lacZ transgenic mice (Muta Mouse), we have combined the peripheral blood micronucleus assay with the transgenic mouse mutation assay, enabling the concomitant detection of gene mutations and micronucleus induction in vivo in the same animals (Suzuki et al., 1993). Several mutagens, i.e., mitomycin C (MMC), ethyl nitrosourea (ENU), ethyl methanesulfonate (EMS) and diethyl nitrosamine (DEN), were tested in this combined assay. All of them increased the lacZ mutant frequency in bone marrow or liver, and all except DEN induced micronuclei in peripheral blood. These initial studies demonstrated that genotoxicity in vivo could be detected with these two endpoints and, more importantly, that some specificity exists among these tissues analyzed. Although transgenic mouse mutation assays have many potential applications in in vivo mutation research, several problems stand in the way of wider use. Paramount among these are cost and labor intensiveness. The color screening systems for lacZ or lacI mutation detection require large numbers of plates and tedious scoring processes. In order to make significant advances in this field, it will be necessary to use positive selection for induced mutants, such as has been described recently for the lacZ and lacI transgenic mouse models.

Micronuclei induced in peripheral blood of E mu-PIM-1 transgenic mice by chronic oral treatment with 2-acetylaminofluorene or benzene but not with diethyl-nitrosamine or 1,2-dichloroethane

Micronucleus induction in peripheral blood was examined during carcinogenicity assays of the genotoxic carcinogens 2-acetylaminofluorene (2-AAF), benzene, diethylnitrosamine (DEN) and 1,2-dichloroethane (1,2-DCE) in lymphoma prone E mu-PIM-1 transgenic mice. In both sexes, micronuclei were increased in polychromatic (PCE) and normochromatic (NCE) erythrocytes after 14 weeks of oral treatment with 75 mg/kg 2-AAF or 50 and 100 mg/kg benzene. The micronucleus frequencies induced by benzene were higher in males than in females. There was no apparent treatment related suppression of erythropoiesis by 2-AAF or by benzene. Blood micronucleus frequencies induced by benzene were similar in transgenic mice and their non-transgenic litter mates. There was no micronucleus induction or PCE suppression detected in the blood of either sex after treatment with 1 and 3 mg/kg DEN or 100 to 300 mg/kg 1,2-DCE. At 40 weeks bone marrow was sampled from mice given 100 mg/kg benzene, and it was confirmed that micronucleated PCE frequencies in blood were an accurate reflection of those induced in bone marrow. However, the spontaneous and induced frequencies of micronucleated cells in blood were slightly higher in PCE than in NCE suggesting that a small degree of selective removal of micronucleated cells occurs in this mouse strain. Control micronucleus frequencies in E mu-PIM-1 mice appeared comparable to those in other, non-transgenic mouse strains. Thus micronuclei are readily detectable in blood during chronic exposure to the bone-marrow clastogens 2-AAF and benzene, but not to DEN and 1,2-DCE, probably because active species do not reach the bone marrow in sufficient concentrations to induce increases in micronuclei.