Automation of mouse micronucleus genotoxicity assay by laser scanning cytometry

Flow cytometric method for scoring rat liver micronuclei with simultaneous assessments of hepatocyte proliferation

The current report describes a newly devised method for automatically scoring the incidence of rat hepatocyte micronuclei (MNHEP) via flow cytometry, with concurrent assessments of hepatocyte proliferation-frequency of Ki-67-positive nuclei, and the proportion of polyploid nuclei. Proof-of-concept data are provided from experiments performed with 6-week old male Crl:CD(SD) rats exposed to diethylnitrosamine (DEN) or quinoline (QUIN) for 3 or 14 consecutive days. Non-perfused liver tissue was collected 4 days after cessation of treatment in the case of 3-day studies, or 1 day after last administration in the case of 14-day studies for processing and flow cytometric analysis. In addition to livers, blood samples were collected one day after final treatment for micronucleated reticulocyte (MN-RET) measurements. Dose-dependent increases in MNHEP, Ki-67-positive nuclei, and polyploidy were observed in 3- and 14-day DEN studies. Both treatment schedules resulted in elevated %MNHEP for QUIN-exposed rats, and while cell proliferation effects were subtle, appreciable increases to normalized liver weights were observed. Whereas DEN caused markedly higher %MNHEP when exposure was extended to two weeks, QUIN-induced MNHEP were slightly increased with protracted dosing. Parallel microscopy-based MNHEP frequencies were highly correlated with flow cytometry-based measurements (four study/aggregate R²  = 0.80). No increases in MN-RET were seen in any of the four studies. Collectively, these results suggest liver micronuclei are amenable to an automated scoring technique that provides objective analyses and higher information content relative to conventional microscopy. Additional work is needed to expand the number and types of chemicals tested, identify the most advantageous treatment schedules, and test the transferability of the method. Environ. Mol. Mutagen. 59:176-187, 2018. © 2018 Wiley Periodicals, Inc.

Automatic detection of micronuclei by cell microscopic image processing

With the development and applications of ionizing radiation in medicine, the radiation effects on human health get more and more attention. Ionizing radiation can lead to various forms of cytogenetic damage, including increased frequencies of micronuclei (MNi) and chromosome abnormalities. The cytokinesis block micronucleus (CBMN) assay is widely used method for measuring MNi to determine chromosome mutations or genome instability in cultured human lymphocytes. The visual scoring of MNi is time-consuming and scorer fatigue can lead to inconsistency. In this work, we designed software for the scoring of in vitro CBMN assay for biomonitoring on Giemsa-stained slides that overcome many previous limitations. Automatic scoring proceeds in four stages as follows. First, overall segmentation of nuclei is done. Then, binucleated (BN) cells are detected. Next, the entire cell is estimated for each BN as it is assumed that there is no detectable cytoplasm. Finally, MNi are detected within each BN cell. The designed Software is even able to detect BN cells with vague cytoplasm and MNi in peripheral blood smear. Our system is tested on a self-provided dataset and is achieved high sensitivities of about 98% and 82% in recognizing BN cells and MNi, respectively. Moreover, in our study less than 1% false positives were observed that makes our system reliable for practical MNi scoring.

Main issues addressed in the 2014-2015 revisions to the OECD Genetic Toxicology Test Guidelines

The Organization for Economic Cooperation and Development (OECD) recently revised the test guidelines (TGs) for genetic toxicology. This article describes the main issues addressed during the revision process, and the new and consistent recommendations made in the revised TGs for: (1) demonstration of laboratory proficiency; (2) generation and use of robust historical control data; (3) improvement of the statistical power of the tests; (4) selection of top concentration for in vitro assays; (5) consistent data interpretation and determination of whether the result is clearly positive, clearly negative or needs closer consideration; and, (6) consideration of 3R's for in vivo assay design. The revision process resulted in improved consistency among OECD TGs (including the newly developed ones) and more comprehensive recommendations for the conduct and the interpretation of the assays. Altogether, the recommendations made during the revision process should improve the efficiency, by which the data are generated, and the quality and reliability of test results. Environ. Mol. Mutagen. 58:284-295, 2017. © 2017 Wiley Periodicals, Inc.

Modern Approaches to Chemical Toxicity Screening

Chemical toxicity has a serious impact on public health, and toxicity failures of drug candidates drive up drug development costs. Many in vitro bioassays exist for toxicity screening, and newer versions of these tend to be high throughput or high content assays, some of which rely on electrochemical detection. Toxicity very often results from metabolites of the chemicals we are exposed to, so it is important that assays feature metabolic conversion. Combining bioassays, computational predictions, and accurate chemical pathway elucidation presents our best chance for reliable toxicity prediction. Employing electrochemical and electrochemiluminescent approaches, cell-free microfluidic arrays can measure relative rates of formation of DNA-metabolite adduct formation (a measure of genotoxicity) as well as DNA oxidation levels resulting from enzyme-generated metabolites. Enzymes for several organ types can be studied simultaneously. These arrays can be used to identify the most reactive metabolites, and subsequent mechanistic details can then be investigated with high throughput LC-HPLC using enzyme/DNA-coated magnetic beads.

State-of-the-Art Metabolic Toxicity Screening and Pathway Evaluation

Routine in vitro bioassays and animal toxicity studies of drug and environmental chemical candidates fail to reveal toxicity in ∼30% of cases. This Feature article addresses research on new approaches to in vitro toxicity testing as well as our own efforts to produce high-throughput genotoxicity arrays and LC-MS/MS approaches to reveal possible chemical pathways of toxicity.

In vitro micronucleus assay for the analysis of total particulate matter in cigarette smoke: comparison of flow cytometry and laser scanning cytometry with microscopy

The possible genotoxicity of the total particulate matter (TPM) in cigarette smoke has typically been evaluated using the in vitro micronucleus assay. In recent years, automated scoring techniques have been developed to replace the manual counting process in this assay. However, these automated scoring techniques have not been applied in routine genotoxicity assays for the analysis of TPM to improve the assay efficiency. Chinese hamster ovary (CHO) cells were treated with TPM produced from 14 types of cigarettes at five concentrations (25-200μg/ml) without exogenous metabolic activation. The three following methods were used to score the micronucleus (MN) frequency: (a) flow cytometry with SYTOX and EMA dyes, which differentially stain micronuclei and apoptotic/necrotic chromatin to enhance assay reliability; (b) laser scanning cytometry with FITC and PI dyes, which is a system that combines the analytical capabilities of flow and image cytometry; and (c) visual microcopy with Giemsa dye. The test results obtained using the three methods were compared using correlation analysis. The key findings for this set of compounds include the following: (a) both flow cytometry- and laser scanning cytometry-based methods were effective for MN identification, (b) the three scoring methods could detect dose-dependent micronucleus formation for the 14 types of TPM, and (c) the MN frequencies that were measured in the same samples by flow cytometry, laser scanning cytometry, and visual microscopy were highly correlated, and there were no significant differences (p>0.05). In conclusion, both flow cytometry and laser scanning cytometry can be used to evaluate the MN frequency induced by TPM without exogenous metabolic activation. The simpler and faster processing and the high correlation of the results make these two automatic methods appropriate tools for use in in vitro micronucleus assays for the analysis of TPM using CHO cells.

HUMN project initiative and review of validation, quality control and prospects for further development of automated micronucleus assays using image cytometry systems

The use of micronucleus (MN) assays in in vitro genetic toxicology testing, radiation biodosimetry and population biomonitoring to study the genotoxic impacts of environment gene-interactions has steadily increased over the past two decades. As a consequence there has been a strong interest in developing automated systems to score micronuclei, a biomarker of chromosome breakage or loss, in mammalian and human cells. This paper summarises the outcomes of a workshop on this topic, organised by the HUMN project, at the 6th International Conference on Environmental Mutagenesis in Human Populations at Doha, Qatar, 2012. The aim of this paper is to summarise the outcomes of the workshop with respect to the set objectives which were: (i) Review current developments in automation of micronucleus assays by image cytometry; (ii) define the performance characteristics of automated MN scoring using image cytometry and methods of assessment for instrument validation and quality control and (iii) discuss the design of inter-laboratory comparisons and standardisation of micronucleus assays using automated image cytometry systems. It is evident that automated scoring of micronuclei by automated image cytometry using different commercially available platforms [e.g. Metafer (MetaSystems), Pathfinder™ (IMSTAR), iCyte(®) (Compucyte)], particularly for lymphocytes, is at a mature stage of development with good agreement between visual and automated scoring across systems (correlation factors ranging from 0.58 to 0.99). However, a standardised system of validation and calibration is required to enable more reliable comparison of data across laboratories and across platforms. This review identifies recent progress, important limitations and steps that need to be taken into account to enable the successful universal implementation of automated micronucleus assays by image cytometry.

Metabolic toxicity screening using electrochemiluminescence arrays coupled with enzyme-DNA biocolloid reactors and liquid chromatography-mass spectrometry

New chemicals or drugs must be guaranteed safe before they can be marketed. Despite widespread use of bioassay panels for toxicity prediction, products that are toxic to a subset of the population often are not identified until clinical trials. This article reviews new array methodologies based on enzyme/DNA films that form and identify DNA-reactive metabolites that are indicators of potentially genotoxic species. This molecularly based methodology is designed in a rapid screening array that utilizes electrochemiluminescence (ECL) to detect metabolite-DNA reactions, as well as biocolloid reactors that provide the DNA adducts and metabolites for liquid chromatography-mass spectrometry (LC-MS) analysis. ECL arrays provide rapid toxicity screening, and the biocolloid reactor LC-MS approach provides a valuable follow-up on structure, identification, and formation rates of DNA adducts for toxicity hits from the ECL array screening. Specific examples using this strategy are discussed. Integration of high-throughput versions of these toxicity-screening methods with existing drug toxicity bioassays should allow for better human toxicity prediction as well as more informed decision making regarding new chemical and drug candidates.

Laser scanning cytometry: principles and applications-an update

Laser scanning cytometer (LSC) is the microscope-based cytofluorometer that offers a plethora of unique analytical capabilities, not provided by flow cytometry (FCM). This review describes attributes of LSC and covers its numerous applications derived from plentitude of the parameters that can be measured. Among many LSC applications the following are emphasized: (a) assessment of chromatin condensation to identify mitotic, apoptotic cells, or senescent cells; (b) detection of nuclear or mitochondrial translocation of critical factors such as NF-κB, p53, or Bax; (c) semi-automatic scoring of micronuclei in mutagenicity assays; (d) analysis of fluorescence in situ hybridization (FISH) and use of the FISH analysis attribute to measure other punctuate fluorescence patterns such as γH2AX foci or receptor clustering; (e) enumeration and morphometry of nucleoli and other cell organelles; (f) analysis of progeny of individual cells in clonogenicity assay; (g) cell immunophenotyping; (h) imaging, visual examination, or sequential analysis using different probes of the same cells upon their relocation; (i) in situ enzyme kinetics, drug uptake, and other time-resolved processes; (j) analysis of tissue section architecture using fluorescent and chromogenic probes; (k) application for hypocellular samples (needle aspirate, spinal fluid, etc.); and (l) other clinical applications. Advantages and limitations of LSC are discussed and compared with FCM.

Laser scanning cytometry for automation of the micronucleus assay

Laser scanning cytometry (LSC) provides a novel approach for automated scoring of micronuclei (MN) in different types of mammalian cells, serving as a biomarker of genotoxicity and mutagenicity. In this review, we discuss the advances to date in measuring MN in cell lines, buccal cells and erythrocytes, describe the advantages and outline potential challenges of this distinctive approach of analysis of nuclear anomalies. The use of multiple laser wavelengths in LSC and the high dynamic range of fluorescence and absorption detection allow simultaneous measurement of multiple cellular and nuclear features such as cytoplasmic area, nuclear area, DNA content and density of nuclei and MN, protein content and density of cytoplasm as well as other features using molecular probes. This high-content analysis approach allows the cells of interest to be identified (e.g. binucleated cells in cytokinesis-blocked cultures) and MN scored specifically in them. MN assays in cell lines (e.g. the CHO cell MN assay) using LSC are increasingly used in routine toxicology screening. More high-content MN assays and the expansion of MN analysis by LSC to other models (i.e. exfoliated cells, dermal cell models, etc.) hold great promise for robust and exciting developments in MN assay automation as a high-content high-throughput analysis procedure.

The application of discovery toxicology and pathology towards the design of safer pharmaceutical lead candidates

Toxicity is a leading cause of attrition at all stages of the drug development process. The majority of safety-related attrition occurs preclinically, suggesting that approaches to identify 'predictable' preclinical safety liabilities earlier in the drug development process could lead to the design and/or selection of better drug candidates that have increased probabilities of becoming marketed drugs. In this Review, we discuss how the early application of preclinical safety assessment--both new molecular technologies as well as more established approaches such as standard repeat-dose rodent toxicology studies--can identify predictable safety issues earlier in the testing paradigm. The earlier identification of dose-limiting toxicities will provide chemists and toxicologists the opportunity to characterize the dose-limiting toxicities, determine structure-toxicity relationships and minimize or circumvent adverse safety liabilities.

Evaluation of an automated in vitro micronucleus assay in CHO-K1 cells

In this paper, we describe the evaluation of an automated in vitro micronucleus assay using CHO-K1 cells in 96-well plates. CHO-K1 cells were pre-loaded with a cell dye that stains the cytoplasm, after which the cells were treated with the test compounds for either 3h (for the +S9 condition) or 24h (for the -S9 condition). A total of 10 concentrations were tested, of which the top five concentrations were scored (limited by either cytotoxicity or solubility). At the end of the incubation period the cells were fixed and their DNA was stained with Hoechst. The visualization and scoring of the cells was done using an automated fluorescent microscope coupled with proprietary automated image analysis software provided by Cellomics (Pittsburg, PA). A total of 46 compounds were used in this evaluation, including 8 aneugens and 25 clastogens with varied mechanisms of action. Thirteen non-genotoxic compounds were also included. The automated scoring had a sensitivity of 88% and a specificity of 100%, with a predictive value positive of 100% and a predictive value negative of 76%, compared to data from the literature that was obtained with manual scoring. We also describe the incorporation of a metabolic activation system using rat liver S9 homogenates, and the use of cell number counts as a cytotoxicity index which is complementary to the CBPI- (cytokinesis-block proliferation index) based index. Finally, we also discuss the potential for artefactual findings due to fluorescent precipitate, which should be carefully monitored to prevent false positive results. In conclusion, the automated in vitro micronucleus scoring is a valid alternative to the manual scoring of slides, and it has the advantage of generating data in a rapid and consistent manner, and with low compound requirements, which makes it well suited as a screening assay in the early stages of compound development.

In vitro erythropoiesis from bone marrow-derived progenitors provides a physiological assay for toxic and mutagenic compounds

The goal of this study was to create an in vitro cell culture system that captures essential features of the in vivo erythroid micronucleus (MN) genotoxicity assay, thus enabling increased throughput and controlled studies of the hematopoietic DNA damage response. We show that adult bone marrow (BM) cultures respond to erythropoietin, the principal hormone that stimulates erythropoiesis, with physiological erythropoietic proliferation, differentiation, and enucleation. We then show that this in vitro erythropoietic system clearly signals exposure to genotoxicants through erythroid MN formation. Furthermore, we determined that DNA repair-deficient (MGMT(-/-)) BM displayed sensitivity to genotoxic exposure in vivo compared with WT BM and that this phenotypic response was reflected in erythropoietic cultures. These findings suggest that this in vitro erythroid MN assay is capable of screening for genotoxicity on BM in a physiologically reflective manner. Finally, responses to genotoxicants during erythroid differentiation varied with exposure time, demonstrating that this system can be used to study the effect of DNA damage at specific developmental stages.

Near-clinical applications of laser scanning cytometry

Biological samples from human tissues are characterized by complexity and heterogeneity. The ability to make rapid, reliable, quantitative fluorochromatic measurements on clinical samples allows the development of new and practical assays that could influence diagnosis and treatment in a variety of clinical applications. Laser scanning cytometry (LSC) is a very versatile and adaptable technology that allows for the quantitative analysis of cell samples that are unsuitable for flow cytometry by virtue of their presentation and context. Crucially, it allows the direct visualization of cells and rare events and the correlation of imagery with fluorochromatic measurements. In this chapter, we describe early experiments in the study of cytotoxic drug uptake and resistance in human tumor cells and in the study of sputum cells from asthmatic patients, which harness the specific capabilities of LSC to practical clinical problems.

Laser scanning cytometry: principles and applications

The laser scanning cytometer (LSC) is the microscope-based cytofluorometer that offers a plethora of analytical capabilities. Multilaser-excited fluorescence emitted from individual cells is measured at several wavelength ranges, rapidly (up to 5000 cells/min), with high sensitivity and accuracy. The following applications of LSC are reviewed: (1) identification of cells that differ in degree of chromatin condensation (e.g., mitotic or apoptotic cells or lymphocytes vs granulocytes vs monocytes); (2) detection of translocation between cytoplasm vs nucleus or nucleoplasm vs nucleolus of regulatory molecules such as NF-kappaB, p53, or Bax; (3) semiautomatic scoring of micronuclei in mutagenicity assays; (4) analysis of fluorescence in situ hybridization; (5) enumeration and morphometry of nucleoli; (6) analysis of phenotype of progeny of individual cells in clonogenicity assay; (7) cell immunophenotyping; (8) visual examination, imaging, or sequential analysis of the cells measured earlier upon their relocation, using different probes; (9) in situ enzyme kinetics and other time-resolved processes; (10) analysis of tissue section architecture; (11) application for hypocellular samples (needle aspirate, spinal fluid, etc.); (12) other clinical applications. Advantages and limitations of LSC are discussed and compared with flow cytometry.

Genotoxicity screening: the slow march to the future

Genetic toxicology testing in drug discovery and development is slowly moving into the age of high-throughput screening (HTS). This has been helped by the development of new tools, as well as validation studies and data analysis to support their use in hit-to-lead or lead optimisation decisions. This review provides an overview of the current genetic toxicology methodologies and a few HTS methodologies. Comparisons are made between the predictivity of carcinogenesis that can be achieved in screening strategies as well as by the battery of regulatory tests. The importance of false-positive and false-negative calls at different stages in development is considered. There is a good prospect that in genetic toxicology, as in other areas of ADME-Tox, HTS will reduce the growing costs of carrying compounds with undesirable characteristics too far along the drug development process.

Clinical applications of laser scanning cytometry

This study reviews existing and potential clinical applications of laser scanning cytometry (LSC) and outlines possible future developments. LSC provides a technology for solid phase cytometry. Fluorochrome-labeled specimens are immobilized on microscopic slides that are placed on a conventional epifluorescence microscope and analyzed by one or two lasers. Data comparable to flow cytometry are generated. In addition, the position of each event is recorded, a feature that allows relocalization and visualization of each measured event. The major advantage of LSC compared with other cytometric methods is the combination of two features: (a) the minimal clinical sample volume needed and (b) the connection of fluorescence data and morphological information for the measured event. Since the introduction of LSC, numerous methods have been established for the analysis of cells, cellular compartments, and tissues. Although most cytometric methods use only two or three colors, the characterization of specimens with up to five fluorochromes is possible. Most clinical applications have been designed to determine ploidy and immunophenotype; other applications include analyses of tissue biopsies and sections, fluorescence in situ hybridization, and the combination of vital and nonvital information on a single-cell basis. With the currently available assays, LSC has proven its wide spectrum of clinical applicability in slide-based cytometry and can be introduced as a standard technology in multiple clinical settings.