In Vitro Approaches for Assessing the Genotoxicity of Nanomaterials

Genotoxicity is associated with serious health effects and includes different types of DNA lesions, gene mutations, structural chromosome aberrations involving breakage and/or rearrangements of chromosomes (referred to as clastogenicity) and numerical chromosome aberrations (referred to as aneuploidy). Assessing the potential genotoxic properties of chemicals, including nanomaterials (NMs), is a key element in regulatory safety assessment. State-of-the-art genotoxicity testing includes a battery of assays covering gene mutations, structural and numerical chromosome aberrations. Typically various in vitro assays are performed in the first tier. It is not very likely that NMs may induce as yet unknown types of genotoxic damage beyond what is already known for chemicals. Thus, principles of genotoxicity testing as established for chemicals should be applicable to NMs as well. However, established test guidelines (i.e., OECD TG) may require adaptations for NM testing, as currently under discussion at the OECD. This chapter gives an overview of genotoxicity testing of NMs in vitro based on experiences from various research projects. We recommend a combination of a mammalian gene mutation assay (at either Tk or HPRT locus), the in vitro comet assay, and the cytokinesis-block micronucleus assay, which are discussed in detail here. In addition we also include the Cell Transformation Assay (CTA) as a promising novel test for predicting NM-induced cell transformation in vitro.

Automation of the in vitro micronucleus and chromosome aberration assay for the assessment of the genotoxicity of the particulate and gas-vapor phase of cigarette smoke

Total particulate matter (TPM) and the gas-vapor phase (GVP) of mainstream smoke from the Reference Cigarette 3R4F were assayed in the cytokinesis-block in vitro micronucleus (MN) assay and the in vitro chromosome aberration (CA) assay, both using V79-4 Chinese hamster lung fibroblasts exposed for up to 24 h. The Metafer image analysis platform was adapted resulting in a fully automated evaluation system of the MN assay for the detection, identification and reporting of cells with micronuclei together with the determination of the cytokinesis-block proliferation index (CBPI) to quantify the treatment-related cytotoxicity. In the CA assay, the same platform was used to identify, map and retrieve metaphases for a subsequent CA evaluation by a trained evaluator. In both the assays, TPM and GVP provoked a significant genotoxic effect: up to 6-fold more micronucleated target cells than in the negative control and up to 10-fold increases in aberrant metaphases. Data variability was lower in the automated version of the MN assay than in the non-automated. It can be estimated that two test substances that differ in their genotoxicity by approximately 30% can statistically be distinguished in the automated MN and CA assays. Time savings, based on man hours, due to the automation were approximately 70% in the MN and 25% in the CA assays. The turn-around time of the evaluation phase could be shortened by 35 and 50%, respectively. Although only cigarette smoke-derived test material has been applied, the technical improvements should be of value for other test substances.

The use of an automated hematology analyzer to observe cell growth in the chromosome aberration test using human lymphocytes

Human lymphocytes have been frequently used for in vitro chromosome aberration or micronucleus tests on whole-blood culture. However, it is difficult to observe or confirm the cell growth of lymphocytes just before chemical treatment compared with cultured cell lines, such as CHL or CHO cells. In order to overcome this drawback of using whole-blood culture, we investigated a possibility of using an automated hematology analyzer (AHA) (Sysmex XT-2000i, SYSMEX Corp. (Hyogo, Japan)) to measure the growth of lymphocytes applying a manual function of this apparatus. In this study, whole-blood samples were cultured for 4 days, and the growth of lymphocytes was measured once a day using a standard flow cytometer (FCM) with antibody CD3 and DNA staining solution, and by the AHA simultaneously. The results showed that growth curves produced employing the two methods coincided fairly well. Therefore, it can be concluded that the growth of lymphocytes in whole-blood culture can be measured using AHA in a straightforward and rapid way in in vitro chromosome aberration or micronucleus tests.

Low-cost metaphase finder system

In counting chromosome aberrations at low-dose radiation exposure in biological dosimetry, an automation technique has been required to process a large number of sample preparations. The metaphase finder is an automated optical microscope system, which automatically scans and finds metaphase cells on the slide glass in low magnification and relocates metaphase cells to the center of the field of view of the microscope to observe chromosomes in high magnification. The authors have constructed a cost-effective metaphase finder system by assembling commercially-available components, such as microscopes, motorized sample stages, personal computers and general-purpose image analysis software, instead of purchasing one dedicated system. The new system has high cost-effectiveness and high flexibility in adapting to the new staining methods. Mathematical morphology-based image processing to the algorithm was used to find metaphase cells. In particular, the morphology for the gray-image was used for the newest version. The performance of this metaphase finder system was tested. The new machines were distributed to six institutes of the Chromosome Network for Biodosimetry in Japan for testing the new algorithm for practical use. The scanning speed was 14.5 to 18.1 min cm2. The system was slower than several commercial metaphase finders, but there were few false positives. This system is economical and satisfactory for practical use.

New developments in automated cytogenetic imaging: unattended scoring of dicentric chromosomes, micronuclei, single cell gel electrophoresis, and fluorescence signals

The quantification of DNA damage, both in vivo and in vitro, can be very time consuming, since large amounts of samples need to be scored. Additional uncertainties may arise due to the lack of documentation or by scoring biases. Image analysis automation is a possible strategy to cope with these difficulties and to generate a new quality of reproducibility. In this communication we collected some recent results obtained with the automated scanning platform Metafer, covering applications that are being used in radiation research, biological dosimetry, DNA repair research and environmental mutagenesis studies. We can show that the automated scoring for dicentric chromosomes, for micronuclei, and for Comet assay cells produce reliable and reproducible results, which prove the usability of automated scanning in the above mentioned research fields.

Effects of 2.45-GHz electromagnetic fields with a wide range of SARs on micronucleus formation in CHO-K1 cells

There has been considerable discussion about the influence of high-frequency electromagnetic fields (HFEMF) on the human body. In particular, HFEMF used for mobile phones may be of great concern for human health. In order to investigate the properties of HFEMF, we have examined the effects of 2.45-GHz EMF on micronucleus (MN) formation in Chinese hamster ovary (CHO)-K1 cells. MN formation is induced by chromosomal breakage or inhibition of spindles during cell division and leads to cell damage. We also examined the influence of heat on MN formation, since HFEMF exposure causes a rise in temperature. CHO-K1 cells were exposed to HFEMF for 2 h at average specific absorption rates (SARs) of 5, 10, 20, 50, 100, and 200 W/kg, and the effects on these cells were compared with those in sham-exposed control cells. The cells were also treated with bleomycin alone as a positive control or with combined treatment of HFEMF exposure and bleomycin. Heat treatment was performed at temperatures of 37, 38, 39, 40, 41, and 42°C.The MN frequency in cells exposed to HFEMF at a SAR of lower than 50 W/kg did not differ from the sham-exposed controls, while those at SARs of 100 and 200 W/kg were significantly higher when compared with the sham-exposed controls. There was no apparent combined effect of HFEMF exposure and bleomycin treatment. On heat treatment at temperatures from 38–42°C, the MN frequency increased in a temperature-dependent manner. We also showed that an increase in SAR causes a rise in temperature and this may be connected to the increase in MN formation generated by exposure to HFEMF.

Micronucleus induction in V79 cells after direct exposure to whole cigarette smoke

Previous investigations on the effects of cigarette smoke on cultured cells have used mainly smoke condensate dissolved in culture medium. A system has been designed which allows direct exposure of cells to fresh cigarette smoke, without an intervening layer of growth medium between the cells and the smoke. Preliminary results have been obtained which demonstrate the viability of the system. V79 cells were cultured on porous membranes (Transwell; Costar). During smoke exposure only the lower surface of each Transwell is supplied with culture medium from the bottom of the culture chambers. In this way the cells had direct contact with the atmosphere at the upper surface and could be exposed directly to the test compound. The constructed exposure system consists of a smoke generator and an exposure unit containing six Transwells, the latter contained in an incubator. Cigarette smoke was generated using a standard 2 s, 35 ml puff once per min. The puff is diluted with conditioned air from the incubator and injected into the exposure unit. Following exposure of the cells to air only for 3 h there was no effect upon V79 cell viability. However, after exposure to smoke containing between 88 and 224 mg/m3 particulate matter, an inhibition of cell proliferation and induction of micronuclei was measured. When a Cambridge filter pad was placed between the cigarette and the cell exposure system to remove particulate matter cell proliferation was also reduced and an increased frequency of micronuclei above the control value was measured.

Flow-cytometric enumeration of micronucleated polychromatic erythrocytes in mouse peripheral blood

A flow-cytometric assay is described that can be used to determine the frequency and the DNA content of micronucleated polychromatic (PCE) and normochromatic (NCE) erythrocytes in mouse peripheral blood. Thiazole orange was used for discrimination between PCEs and NCEs, while Hoechst 33342 was used to detect micronucleated PCEs and NCEs. Up to 70,000 polychromatic erythrocytes can be analyzed in less than 10 min. This corresponds to 150-3,000 micronucleated polychromatic erythrocytes, 90-95% of which are true events as determined with a fluorescence microscope after sorting. Using X-rays as the inducing agent in dose-response experiments, a significant increase can be registered at doses of 0.02 Gy. It seems possible that the method will also allow the detection of clastogenic effects of other inducing agents at lower doses than previously possible.

Micronuclei and other nuclear anomalies in buccal smears: a field test in snuff users

A revised protocol for the exfoliated cell micronucleus assay was field-tested in a population exposed to a genotoxic agent, snuff, at levels associated with a significant increase in cancer risk. The standard assay involves examination of epithelial smears to determine the prevalence of micronucleated cells, an indication of chromosome breakage or mitotic interference. The assay was revised to increase specificity and to include separate scoring of other nuclear anomalies associated with cytotoxicity and genotoxicity. The modified assay was applied to buccal smears of 38 female snuff users and 15 female nonusers recruited from a North Carolina clinic in 1987. The prevalence of micronucleation was elevated in the snuff users as compared with the nonusers (prevalence ratio = 2.4, 95% confidence interval 1.1-5.2) and, to a lesser extent, at the usual contact site as compared with a distal buccal site in the snuff users (prevalence ratio = 1.5, 95% confidence interval 0.9-2.5). The pattern of relative frequencies of several nuclear anomalies provided strong evidence of a cytotoxic effect, the prevalence ratios ranging from 2 to 13. Nuclear degenerative phenomena can be difficult to distinguish from classical micronuclei; thus, the observed association of indicators of cytotoxicity with exposure introduces the possibility of bias away from the null in micronucleus findings due to differential misclassification. Until methods to better distinguish extranuclear bodies of different origins become available, investigators should use the revised protocol and should focus on agents not thought to be cytotoxic.