Standardized cell sources and recommendations for good cell culture practices in genotoxicity testing

Are hydroxyapatite-based biomaterials free of genotoxicity? A systematic review

Hydroxyapatite (HA) is a biomaterial widely used in clinical applications and pharmaceuticals. The literature on HA-based materials studies is focused on chemical characterization and biocompatibility. Generally, biocompatibility is analyzed through adhesion, proliferation, and differentiation assays. Fewer studies are looking for genotoxic events. Thus, although HA-based biomaterials are widely used as biomedical devices, there is a lack of literature regarding their genotoxicity. This systematic review was carried out following the PRISMA statement. Specific search strategies were developed and performed in four electronic databases (PubMed, Science Direct, Scopus, and Web of Science). The search used "Hydroxyapatite OR Calcium Hydroxyapatite OR durapatite AND genotoxicity OR genotoxic OR DNA damage" and "Hydroxyapatite OR Calcium Hydroxyapatite OR durapatite AND mutagenicity OR mutagenic OR DNA damage" as keywords and articles published from 2000 to 2022, after removing duplicate studies and apply include and exclusion criteria, 53 articles were identified and submitted to a qualitative descriptive analysis. Most of the assays were in vitro and most of the studies did not show genotoxicity. In fact, a protective effect was observed for hydroxyapatites. Only 20 out of 71 tests performed were positive for genotoxicity. However, no point mutation-related mutagenicity was observed. As the genotoxicity of HA-based biomaterials observed was correlated with its nanostructured forms as needles or rods, it is important to follow their effect in chronic exposure to guarantee safe usage in humans.

Comparative analysis of micronucleus induction and DNA damage biomarkers in TK6 and A375 cells using flow cytometry

Previously, we introduced an alternative adherent A375 cell line for clastogenicity and aneugenicity testing using a high content imaging platform. To further characterize the performance of A375 cells, we investigated the sensitivity and specificity of A375 and TK6 cells by directly comparing micronucleus (MN) induction, cytotoxicity (relative cell counts, viability, and apoptosis), clastogenicity (γH2AX), and aneuploidy markers (pH 3, MPM-2, and polyploidy) using flow cytometric methods. We evaluated 14 compounds across different mechanisms (non-genotoxic apoptosis inducers, clastogens, and aneugens with either tubulin binding or aurora kinase inhibiting phenotypes) at 4-h and 24-h post treatment. Both aneugens and clastogens tested positive for micronucleus induction in both cell lines. Apoptosis continued to be a confounding factor for flow cytometry-based micronuclei assessment in TK6 cells as evidenced by positive responses by the three cytotoxicants. Conversely, A375 cells were not affected by apoptosis-related false positive signals and did not produce a positive response in the in vitro micronucleus assay. Benchmark dose response (BMD) analysis showed that the induction of micronuclei and biomarkers occurred at similar concentrations in both cell lines for clastogens and aneugens. By showing that A375 cells have similar sensitivity to TK6 cells but a greater specificity, these results provide additional support for A375 cells to be used as an alternative adherent cell line for in vitro genetic toxicology assessment.

Longitudinal characterization of TK6 cells sequentially adapted to animal product-free, chemically defined culture medium: considerations for genotoxicity studies

Introduction: In vitro approaches are an essential tool in screening for toxicity of new chemicals, products and therapeutics. To increase the reproducibility and human relevance of these in vitro assessments, it is advocated to remove animal-derived products such as foetal bovine serum (FBS) from the cell culture system. Currently, FBS is routinely used as a supplement in cell culture medium, but batch-to-batch variability may introduce inconsistency in inter- and intra-lab assessments. Several chemically defined serum replacements (CDSR) have been developed to provide an alternative to FBS, but not every cell line adapts easily and successfully to CDSR-supplemented medium, and the long-term effect on cell characteristics remains uncertain. Aim: The aim of this study was to adapt the TK6 cell line to animal-product free CDSR-supplemented medium and evaluate the long-term effects on cell health, growth, morphology, phenotype, and function. This included a provisional assessment to determine the suitability of the transitioned cell line for standardised genotoxicity testing using the "in vitro mammalian cell micronucleus test" (OECD TG 487). Materials and methods: Gradual adaptation and direct adaptation methodologies were compared by assessing the cell proliferation, size and viability every passage until the cells were fully adapted to animal-free CDSR. The metabolic activity and membrane integrity was assessed every 4-8 passages by PrestoBlue and CytoTox-ONE™ Homogeneous Membrane Integrity Assay respectively. A detailed morphology study by high content imaging was performed and the expression of cell surface markers (CD19 and CD20) was conducted via flow cytometry to assess the potential for phenotypic drift during longer term culture of TK6 in animal-free conditions. Finally, functionality of cells in the OECD TG 487 assay was evaluated. Results: The baseline characteristics of TK6 cells cultured in FBS-supplemented medium were established and variability among passages was used to set up acceptance criteria for CDSR adapted cells. TK6 were adapted to CDSR supplemented medium either via direct or gradual transition reducing from 10% v/v FBS to 0% v/v FBS. The cell growth rate was compromised in the direct adaptation and therefore the gradual adaptation was preferred to investigate the long-term effects of animal-free CDSR on TK6 cells. The new animal cells showed comparable (p > 0.05) viability and cell size as the parent FBS-supplemented cells, with the exception of growth rate. The new animal free cells showed a lag phase double the length of the original cells. Cell morphology (cellular and nuclear area, sphericity) and phenotype (CD19 and CD20 surface markers) were in line (p > 0.05) with the original cells. The new cells cultured in CDSR-supplemented medium performed satisfactory in a pilot OECD TG 487 assay with compounds not requiring metabolic activation. Conclusion: TK6 cells were successfully transitioned to FBS- and animal product-free medium. The new cell cultures were viable and mimicked the characteristics of FBS-cultured cells. The gradual transition methodology utilised in this study can also be applied to other cell lines of interest. Maintaining cells in CDSR-supplemented medium eliminates variability from FBS, which in turn is likely to increase the reproducibility of in vitro experiments. Furthermore, removal of animal derived products from cell culture techniques is likely to increase the human relevance of in vitro methodologies.

Chemical and in vitro toxicological comparison of emissions from a heated tobacco product and the 1R6F reference cigarette

It has previously been found that, compared with cigarette smoke, the aerosols generated by heated tobacco products contain fewer and lower harmful and potentially harmful constituents (HPHCs) and elicit lower biological activity in in vitro models and lower smoking-related exposure biomarker levels in clinical studies. It is important to accumulate such scientific evidences for heated tobacco products with a novel heating system, because different heating system may affect the quantitative aspect of the amount of HPHCs and the qualitative aspect of the biological activity of the aerosol generated. Here, the chemical properties of, and toxicological responses to aerosols emitted by DT3.0a, a new heated tobacco product with a novel heating system, and cigarette smoke (CS) were compared, using chemical analyses, in vitro battery (standardized genotoxicity and cytotoxicity) assays, and mechanistic (ToxTracker and two-dimensional cell culture) assays. Regular- and menthol-flavored DT3.0a and standard 1R6F reference cigarettes were tested. Selected HPHC yields were lower in DT3.0a aerosol than 1R6F CS. The genotoxicity-related assays indicated that DT3.0a aerosol was not genotoxic, regardless of metabolic activation. The other biological assays indicated that less cytotoxicity induction and oxidative stress response were elicited by DT3.0a aerosol compared with 1R6F CS. Similar results were found for both regular and menthol DT3.0a. Like previous reports for heated tobacco products with other heating systems, the results of this study indicated that DT3.0a aerosols have chemical and biological properties less likely to be harmful than 1R6F CS.

In vitro micronucleus assay: Method for assessment of nanomaterials using cytochalasin B

The in vitro micronucleus (MNvit) assay is used to evaluate the aneugenic and clastogenic potential of a test material based upon its ability to induce micronuclei in the cells. This protocol is provided for testing of nanomaterials (NM) with standard cell lines in the absence of metabolic activation. The use of cytochalasin B (CytoB) and the analysis of binucleated cells in the cytokinesis-block version of the micronucleus assay ensures that cells analyzed have undergone cell division, which is required for expression of DNA damage and micronucleus formation. Issues specific to NM that were problematic with standard test methods are addressed, including test system choice, dose selection, test material exposures, CytoB timing, cytotoxicity determination, and DNA damage expression time. A step-by-step protocol for in vitro micronucleus assessment of NM is provided.

Impact of experimental design factors on the potency of genotoxicants in in vitro tests

Previous studies have shown that differences in experimental design factors may alter the potency of genotoxic compounds in in vitro genotoxicity tests. Most of these studies used traditional statistical methods based on the lowest observed genotoxic effect levels, whereas more appropriate methods, such as the benchmark dose (BMD) approach, are now available to compare genotoxic potencies under different test conditions. We therefore investigated the influence of two parameters, i.e. cell type and exposure duration, on the potencies of two known genotoxicants [aflatoxin B1 and ethyl methanesulfonate (EMS)] in the in vitro micronucleus (MN) assay and comet assay (CA). Both compounds were tested in the two assays using two cell types (i.e. CHO-K1 and TK6 cells). To evaluate the effect of exposure duration, the genotoxicity of EMS was assessed after 3 and 24 h of exposure. Results were analyzed using the BMD covariate approach, also referred to as BMD potency ranking, and the outcome was compared with that of more traditional statistical methods based on lowest observed genotoxic effect levels. When comparing the in vitro MN results obtained in both cell lines with the BMD covariate approach, a difference in potency was detected only when EMS exposures were conducted for 24 h, with TK6 cells being more sensitive. No difference was observed in the potency of both EMS and aflatoxin B1 in the in vitro CA using both cell lines. In contrast, EMS was more potent after 24 h exposure compared with a 3 h exposure under all tested conditions, i.e. in the in vitro MN assay and CA in both cell lines. Importantly, for several of the investigated factors, the BMD covariate method could not be used to confirm the differences in potencies detected with the traditional statistical methods, thus highlighting the need to evaluate the impact of experimental design factors with adequate approaches.

The expression of Phase II drug-metabolizing enzymes in human B-lymphoblastoid TK6 cells

In vitro genotoxicity testing plays an important role in chemical risk assessment. The human B-lymphoblastoid cell line TK6 is widely used as a standard cell line for regulatory safety evaluations. Like many other mammalian cell lines, TK6 cells have limited metabolic capacity; therefore, usually require a source of exogenous metabolic activation for use in genotoxicity testing. Previously, we developed a set of TK6-derived cell lines that individually express one of fourteen cytochrome P450s (CYPs). In the present study, we surveyed a panel of major Phase II drug-metabolizing enzymes to characterize their baseline expression in TK6 cells. These results may serve as a reference enzymatic profile of this commonly used cell line.

Effects of antioxidant capacity on micronucleus induction by cigarette smoke in mammalian cells

We have compared micronucleus (MN) induction by cigarette smoke in the L5178Y, TK6, and CHL/IU cell lines. The test sample was total particulate matter of 3R4F reference cigarette smoke, suspended in DMSO. After 3-h treatment, with or without a rat liver S9 metabolic activation system, followed by 24-h recovery, dose-dependent MN increases were seen in all cell lines. However, CHL/IU and TK6 cells were more resistant than L5178Y cells (comparison by Benchmark Doses with PROAST software). 3R4F smoke generates reactive oxygen species (ROS). Therefore, we explored the relationship between the sensitivities to 3R4F smoke and the antioxidant capacities of the cell lines. While the total antioxidant capacities were not significantly different among the cell lines, cellular glutathione (GSH) was higher in CHL/IU cells than in L5178Y cells. Pretreatment of CHL/IU cells with a GSH precursor, N-acetylcysteine (NAC), reduced the genotoxicity/cytotoxicity of 3R4F, whereas an inhibitor of GSH biosynthesis, buthionine sulfoximine (BSO), enhanced it. The effects of NAC and BSO were also seen after treatment with allyl isothiocyanate, a ROS-generating chemical, but not with mitomycin C, a ROS-independent genotoxicant. Pretreatment with NAC increased cellular thiol levels. From the present results, the genotoxicity and cytotoxicity of cigarette smoke differs among these cell lines in a manner that may be related to their antioxidant thiol levels.

Genotoxic activity of endocrine disrupting compounds commonly present in paper mill effluents

In the present study we evaluated cytotoxic and genotoxic activities of endocrine disrupting compounds (EDCs), including dimethyl phthalate (DMP), diethyl phthalate (DEP), dibutyl phthalate (DBP), benzyl butyl phthalate (BBP), di(2-ethylhexyl) phthalate (DEHP), bisphenol A (BPA), and nonylphenol (NP), which have been previously identified in effluents from two paper mills with different paper production technologies (virgin or recycled fibres). Moreover, we evaluated genotoxic activity of the effluents from these two paper mills and compared it to the activity of artificial complex mixtures consisting of the seven EDCs at concentrations detected in corresponding paper mill effluents. None of the EDCs was genotoxic in Salmonella typhimurium (SOS/umuC assay), while all induced DNA damage in human hepatocellular carcinoma (HepG2) cells (comet assay). After 4 h of exposure genotoxic effects were determined at concentrations ≥ 1 μg/L for BBP and DEHP, ≥10 μg/L for DMP, DEP, DBP, and BPA, and ≥100 μg/L for NP, while after 24 h of exposure DNA damage occurred at ≥10 μg/L for DBP, BPA and NP, and ≥100 μg/L for DMP, DEP, BBP and DEHP. The effluents and corresponding artificial mixtures of EDCs from paper mill that uses virgin fibres did not induce DNA damage in HepG2 cells, while the effluents and corresponding artificial mixtures for the paper mill that uses recycled fibres were genotoxic. Genotoxic activity of effluents was significantly higher compared to corresponding artificial mixtures suggesting the presence of further unknown compounds contributing to the effect. Wastewater monitoring based on chemical analysis is limited to determination of targeted compounds and does not take into account possible interactions between chemicals in mixtures. Therefore, it alone cannot provide an adequate information on potential toxic effects required for the assessment of genotoxic activity of real environmental samples and their potential threats to the environment and human health.

Repair pathways for radiation DNA damage under normoxic and hypoxic conditions: Assessment with a panel of repair-deficient human TK6 cells

Various types of DNA lesions are produced when cells are exposed to ionizing radiation (IR). The type and yield of IR-induced DNA damage is influenced by the oxygen concentration. Thus, different DNA repair mechanisms may be involved in the response of normoxic and hypoxic cells to irradiation with IR. However, differences between the repair mechanisms of IR-induced DNA damage under normoxic versus hypoxic conditions have not been clarified. Elucidating the relative contribution of individual repair factors to cell survival would give insight into the repair mechanisms operating in irradiated normoxic and hypoxic cells. In the present study, we used a panel of repair-deficient human TK6 cell lines that covered seven repair pathways. Cells were irradiated with X-rays under normoxic and hypoxic conditions, and the sensitivities of each mutant relative to the wild-type (i.e. relative sensitivity) were determined for normoxic and hypoxic conditions. The sensitivity of cells varied depending on the type of repair defects. However, for each repair mutant, the relative sensitivity under normoxic conditions was comparable to that under hypoxic conditions. This result indicates that the relative contribution of individual repair pathways to cell survival is comparable in normoxic and hypoxic cells, although the spectrum of IR-induced DNA damage in hypoxic cells differs from that of normoxic cells.

In Vitro PIG-A Gene Mutation Assay in Human B-Lymphoblastoid TK6 Cells

The X-linked PIG-A gene is involved in the biosynthesis of glycosylphosphatidylinositol (GPI) anchors. PIG-A mutant cells fail to synthesize GPI and to express GPI-anchored protein markers (e.g., CD59 and CD55). In recent years, in vitro PIG-A assay has been established based on the high conservation of PIG-A/Pig-a loci among different species and the large data from the in vivo system. The purpose of this study was to extend the approach for PIG-A mutation assessment to in vitro human B-lymphoblastoid TK6 cells by detecting the loss of GPI-linked CD55 and CD59 proteins. TK6 cells were treated with three mutagens 7,12-dimethylbenz[a]anthracene (DMBA), N-ethyl-N-nitrosourea (ENU), etoposide (ETO), and two nonmutagens: cadmium chloride (CdCl2) and sodium chloride (NaCl). The mutation rate of PIG-A gene within TK6 cells was determined on the 11th day with flow cytometry analysis for the negative frequencies of CD55 and CD59. The antibodies used in this production were APC mouse-anti-human CD19 antibody, PE mouse anti-human CD55 antibody, PE mouse anti-human CD59 antibody, and nucleic acid dye 7-AAD. An immunolabeling method was used to reduce the high spontaneous level of preexisting PIG-A mutant cells. Our data suggested that DMBA-, ENU-, and ETO-induced mutation frequency of PIG-A gene was increased by twofold compared with the negative control, and the effects were dose-dependent. However, CdCl2 and NaCl did not significantly increase the mutation frequency of PIG-A gene, with a high cytotoxicity at a dose of 10 mmol/L. Our study suggested that the novel in vitro PIG-A gene mutation assay within TK6 cells may represent a complement of the present in vivo Pig-a assay, and may provide guidance for their potential use in genotoxicity even in cells with a significant deficiency of GPI anchor.

Bisphenol A-induced DNA damages promote to lymphoma progression in human lymphoblastoid cells through aberrant CTNNB1 signaling pathway

Lymphoma is a group of blood cancers that develop from the immune system, and one of the main risk factors is associated with exposure to environmental chemicals. Bisphenol A (BPA) is a common chemical used in the manufacture of materials in polycarbonate and epoxy plastic products and can interfere with the immune system. BPA is considered to possibly induce lymphoma development by affecting the immune system, but its potential mechanisms have not been well established. This study performed a gene-network analysis of microarray data sets in human lymphoma tissues as well as in human cells with BPA exposure to explore module genes and construct the potential pathway for lymphomagenesis in response to BPA. This study provided evidence that BPA exposure resulted in disrupted cell cycle and DNA damage by activating CTNNB1, the initiator of the aberrant constructed CTNNB1-NFKB1-AR-IGF1-TWIST1 pathway, which may potentially lead to lymphomagenesis.

Follow-up genotoxicity assessment of Ames-positive/equivocal chemicals using the improved thymidine kinase gene mutation assay in DNA repair-deficient human TK6 cells

Genotoxicity testing plays an important role in the safety assessment of pharmaceuticals, pesticides, and chemical substances. Among the guidelines for various genotoxicity tests, the in vitro genotoxicity test battery comprises the bacterial Ames test and mammalian cell assays. Several chemicals exhibit conflicting results for the bacterial Ames test and mammalian cell genotoxicity studies, which may stem from the differences in DNA repair capacity or metabolism, between different cell types or species. For better understanding the mechanistic implications regarding conflict outcomes between different assay systems, it is necessary to develop in vitro genotoxicity testing approaches with higher specificity towards DNA-damaging reagents. We have recently established an improved thymidine kinase (TK) gene mutation assay (TK assay) that is deficient in DNA excision repair system using human lymphoblastoid TK6 cells lacking XRCC1 and XPA (XRCC1 -/-/XPA -/-), the core factors of base excision repair and nucleotide excision repair, respectively. This DNA repair-deficient TK6 cell line is expected to specifically evaluate the genotoxic potential of chemical substances based on the DNA damage. We focused on four reagents, N-(1-naphthyl)ethylenediamine dihydrochloride (NEDA), p-phenylenediamine (PPD), auramine, and malachite green (MG) as the Ames test-positive chemicals. In our assay, assessment using XRCC1 -/-/XPA -/- cells revealed no statistically significant increase in the mutant frequencies after treatment with NEDA, PPD, and MG, suggesting the chemicals to be non-genotoxic in humans. The observations were consistent with that of the follow-up in vivo studies. In contrast, the mutant frequency was markedly increased in XRCC1 -/-/XPA -/- cells after treatment with auramine. The results suggest that auramine is the genotoxic reagent that preferentially induces DNA damages resolved by BER and/or NER in mammals. Taken together, BER/NER deficient cell-based genotoxicity testing will contribute to elucidate the mechanism of genotoxicity and therefore play a pivotal role in the accurate safety assessment of chemical substances.

Genotoxicity of multi-walled carbon nanotube reference materials in mammalian cells and animals

Carbon nanotubes (CNTs) were the first nanomaterials to be evaluated by the International Agency for Research on Cancer (IARC). The categorization as possibly carcinogenic agent to humans was only applicable to multi-walled carbon nanotubes called MWCNT-7. Other types of CNTs were not classifiable because of missing data and it was not possible to pinpoint unique CNT characteristics that cause cancer. Importantly, the European Commission's Joint Research Centre (JRC) has established a repository of industrially manufactured nanomaterials that encompasses at least four well-characterized MWCNTs called NM-400 to NM-403 (original JRC code). This review summarizes the genotoxic effects of these JRC materials and MWCNT-7. The review consists of 36 publications with results on cell culture experiments (22 publications), animal models (9 publications) or both (5 publications). As compared to the publications in the IARC monograph on CNTs, the current database represents a significant increase as there is only an overlap of 8 publications. However, the results come mainly from cell cultures and/or measurements of DNA strand breaks by the comet assay and the micronucleus assay (82 out of 97 outcomes). A meta-analysis of cell culture studies on DNA strand breaks showed a genotoxic response by MWCNT-7, less consistent effect by NM-400 and NM-402, and least consistent effect by NM-401 and NM-403. Results from other in vitro tests indicate strongest evidence of genotoxicity for MWCNT-7. There are too few observations from animal models and humans to make general conclusions about genotoxicity.

A comparative analysis of the mutagenicity of platinum-containing chemotherapeutic agents reveals direct and indirect mutagenic mechanisms

Platinum-based drugs are a mainstay of cancer chemotherapy. However, their mutagenic effect can increase tumour heterogeneity, contribute to the evolution of treatment resistance and also induce secondary malignancies. We coupled whole genome sequencing with phenotypic investigations on two cell line models to compare the magnitude and examine the mechanism of mutagenicity of cisplatin, carboplatin and oxaliplatin. Cisplatin induced significantly more base substitution mutations than carboplatin or oxaliplatin when used at equitoxic concentrations on human TK6 or chicken DT40 cells, and also induced the highest number of short insertions and deletions. The analysis of base substitution spectra revealed that all three tested platinum drugs elicit both a direct mutagenic effect at purine dinucleotides, and an indirect effect of accelerating endogenous mutagenic processes, whereas the direct mutagenic effect appeared to correlate with the level of DNA damage caused as assessed through histone H2AX phosphorylation and single-cell agarose gel electrophoresis, the indirect mutagenic effects were equal. The different mutagenicity and DNA-damaging effect of equitoxic platinum drug treatments suggest that DNA damage independent mechanisms significantly contribute to their cytotoxicity. Thus, the comparatively high mutagenicity of cisplatin should be taken into account in the design of chemotherapeutic regimens.

The genotoxicity potential of luteolin is enhanced by CYP1A1 and CYP1A2 in human lymphoblastoid TK6 cells

Luteolin (5,7,3',4'-tetrahydroxyflavone) belongs to the flavone subclass of flavonoids. Luteolin and its glycosides are present in many botanical families, including edible plants, fruits, and vegetables. While the beneficial properties of luteolin have been widely studied, fewer studies have investigated its toxicity. In the present study, using human lymphoblastoid TK6 cells and our newly developed TK6-derived cell lines that each stably express a single human cytochrome P450 (CYP1A1, 1A2, 1B1, 2A6, 2B6, 2C8, 2C18, 2C9, 2C19, 2D6, 2E1, 3A4, 3A5, and 3A7), we systematically evaluated luteolin-induced cytotoxicity and genotoxicity, and the role of specific CYPs in the bioactivation of luteolin. Treatments with luteolin for 4-24 h induced cytotoxicity, apoptosis, DNA damage, and chromosome damage in a concentration-dependent manner. Subsequently, we observed that luteolin-induced cytotoxicity and genotoxicity, measured by the high-throughput micronucleus assay, were significantly increased in TK6 cells transduced with CYP1A1 and 1A2. In addition, key apoptosis and DNA damage biomarkers, including cleaved PARP-1, cleaved caspase-3, and phosphorylated histone 2AX (γH2A.X), were all significantly increased in the CYP1A1- and 1A2-expressing cells compared with the empty vector controls. Analysis by LC-MS/MS revealed that TK6 cells biotransformed the majority of luteolin into diosmetin, a less toxic O-methylated flavone, after 24 h; the presence of CYP1A1 and 1A2 partially reversed this process. Altogether, these results indicate that metabolism by CYP1A1 and 1A2 enhanced the toxicity of luteolin in vitro. Our results further support the utility of our TK6 cell system for identification of the specific CYPs responsible for chemical bioactivation and toxicity potential.

The effect of chronic dosing and p53 status on the genotoxicity of pro-oxidant chemicals in vitro

In this study, we have studied the cytotoxicity and genotoxic potency of 3 pro-oxidants; H2O2, menadione and KBrO3 in different dosing scenarios, namely acute (1-day dosing) and chronic (5-days). For this purpose, relative population doubling (RPD%) and mononucleated micronucleus (MN) test were used. TK6 cells and NH32 were employed in in vitro experiments. In the study, the total acute dose was divided into 5 days for each prooxidant chemicals by dose fractionation (1/5th per day) method. Acute dosing was compared to chronic dosing. The oxidative stress caused by the exposure of cells with pro-oxidant chemicals to the cells was determined by an optimized 2',7'-dichlorofluorescein diacetate (DCFHDA) test method. The antioxidant levels of the cell lines were altered with buthionine sulfoxide (BSO) and N-acetyl cysteine (NAC), and the effect of antioxidant capacity on the MN formation in the cells was observed with this method. In the case of H2O2 and menadione, fractional dosing has been observed to result in lower toxicity and lower genotoxicity. But in the case of KBrO3, unlike the other 2 pro-oxidants, higher MN induction was observed with fractionated doses. DCFHDA test clearly demonstrated ROS induction with H2O2 and menadione but not with KBrO3. Unexpectedly, DCFHDA test demonstrated that KBrO3 did not cause an increase ROS levels in both acute and chronic dosing, suggesting an alternative ROS induction mechanism. It was also observed that, treatment with BSO and NAC, caused increasing and decreasing of MN fold change respectively, allowing further ROS specific mechanisms to be explored. Hence, dose fractionation expectedly caused less MN, cytotoxicity and ROS formation with H2O2 and menadione exposure, but not with KBrO3. This implies a unique mechanism of action for KBrO3 induced genotoxicity. Chronic dosing in vitro may be a valuable approach allowing better understanding of how chemicals damage DNA and pose human hazards.

Novel Psychoactive Phenethylamines: Impact on Genetic Material

Psychedelic and stimulating phenethylamines belong to the family of new psychoactive substances (NPS). The acute toxicity framework has begun to be investigated, while studies showing genotoxic potential are very limited or not available. Therefore, in order to fill this gap, the aim of the present work was to evaluate the genotoxicity by treating TK6 cells with 2C-H, 2C-I, 2C-B, 25B-NBOMe, and the popular 3,4-Methylenedioxymethylamphetamine (MDMA). On the basis of cytotoxicity and cytostasis results, we selected the concentrations (6.25–35 µM) to be used in genotoxicity analysis. We used the micronucleus (MN) as indicator of genetic damage and analyzed the MNi frequency fold increase by an automated flow cytometric protocol. All substances, except MDMA, resulted genotoxic; therefore, we evaluated reactive oxygen species (ROS) induction as a possible mechanism at the basis of the demonstrated genotoxicity. The obtained results showed a statistically significant increase in ROS levels for all genotoxic phenethylamines confirming this hypothesis. Our results highlight the importance of genotoxicity evaluation for a complete assessment of the risk associated also with NPS exposure. Indeed, the subjects who do not have hazardous behaviors or require hospitalization by using active but still “safe” doses could run into genotoxicity and in the well-known long-term effects associated.

TK6 genome profile compared with WIL2-NS: Reference data to improve the reproducibility of genotoxicity studies

The TK6 cell line has been widely used for genotoxicity screening by taking advantage of the TK1 mutations. A number of publications have employed TK6 as a tool for the assay; however, its characterization is limited. Because genomes of cell lines are often changed during cell culture, differences in TK6 genomes could be observed between laboratories, which would cause potential problems with reproducibility using TK6. In this study, TK6(IVGT) (JCRB1435), registered with the JCRB Cell Bank as the standard for TK6, has been characterized by karyotyping, SNP microarray and targeted sequencing, and were then compared to WIL2-NS (JCRB9063), a subline derived from a common ancestor with TK6. Changes at the chromosome level were quantitatively assessed by the microarray data, which were shown by DNA sizes and Scales of Genomic Alterations (SGA). An extinct common ancestral cell line, WI-L2 originated from hereditary spherocytosis, has been extrapolated from analysis of genomic signatures shared between TK6 and WIL2-NS, revealing a point mutation in SPTA1, related to the disease. DNA size is calculated to be 102.6 %, 103.1 % and 103.9 % for WI-L2, TK6(IVGT) and WIL2-NS respectively, with the reference values of a normal diploid genome, and each genome shows SGA of 2.8 %, 4.5 % and 4.2 % respectively. Differences between TK6(IVGT) and WIL2-NS are due to regions and sizes of gains, implying that genomes of both cells tend to increase their DNA size independently. Our data provide fundamental genomic features of TK6 and serve as a reference profile of the standardized TK6 cell line, which leads to an increase in robustness of assays using the TK6 cells.

Direct Determination of Pseudouridine in RNA by Mass Spectrometry Coupled with Stable Isotope Labeling

Pseudouridine (Ψ) is the only "mass-silent" nucleoside produced by post-transcriptional RNA modification. We developed a mass spectrometry (MS)-based technique coupled with in vivo deuterium (D) labeling of uridines for direct determination of Ψs in cellular RNA and applied it to the comprehensive analysis of post-transcriptional modifications in human ribosomal RNAs. The method utilizes human TK6/mouse FM3A cells deficient in uridine monophosphate synthase using a CRISPR-Cas9 technique to turn off de novo uridine synthesis and fully labels uridines with D at uracil positions 5 and 6 by cultivating the cells in a medium containing uridine-5,6-D₂. The pseudouridylation reaction in those cells results in the exchange of the D at the C5 of uracil with hydrogen from solvent, which produces a -1 Da mass shift, thus allowing MS-based determination of RNA Ψs. We present here the experimental details of this method and show that it allows the identification of all Ψs in human major nuclear and nucleolar RNAs, including several previously unknown Ψs. Because the method allows direct determination of Ψs at the femtomole level of RNA, it will serve as a useful tool for structure/function studies of a wide variety of noncoding RNAs.

Enhancing the sensitivity of the thymidine kinase assay by using DNA repair-deficient human TK6 cells

The OECD guidelines define the bioassays of identifying mutagenic chemicals, including the thymidine kinase (TK) assay, which specifically detects the mutations that inactivate the TK gene in the human TK6 lymphoid line. However, the sensitivity of this assay is limited because it detects mutations occurring only in the TK gene but not any other genes. Moreover, the limited sensitivity of the conventional TK assay is caused by the usage of DNA repair-proficient wild-type cells, which are capable of accurately repairing DNA damage induced by chemicals. Mutagenic chemicals produce a variety of DNA lesions, including base lesions, sugar damage, crosslinks, and strand breaks. Base damage causes point mutations and is repaired by the base excision repair (BER) and nucleotide excision repair (NER) pathways. To increase the sensitivity of TK assay, we simultaneously disrupted two genes encoding XRCC1, an important BER factor, and XPA, which is essential for NER, generating XRCC1 ^-/- /XPA ^-/- cells from TK6 cells. We measured the mutation frequency induced by four typical mutagenic agents, methyl methane sulfonate (MMS), cis-diamminedichloro-platinum(II) (cisplatin, CDDP), mitomycin-C (MMC), and cyclophosphamide (CP) by the conventional TK assay using wild-type TK6 cells and also by the TK assay using XRCC1 ^-/- /XPA ^-/- cells. The usage of XRCC1 ^-/- /XPA ^-/- cells increased the sensitivity of detecting the mutagenicity by 8.6 times for MMC, 8.5 times for CDDP, and 2.6 times for MMS in comparison with the conventional TK assay. In conclusion, the usage of XRCC1 ^-/- /XPA ^-/- cells will significantly improve TK assay.

Development and Application of TK6-derived Cells Expressing Human Cytochrome P450s for Genotoxicity Testing

Metabolism plays a key role in chemical genotoxicity; however, most mammalian cells used for in vitro genotoxicity testing lack effective metabolizing enzymes. We recently developed a battery of TK6-derived cell lines that individually overexpress 1 of 8 cytochrome P450s (CYP1A1, 1A2, 1B1, 2A6, 2B6, 2C9, 2C19, and 3A4) using a lentiviral expression system. The increased expression and metabolic function of each individual CYP in each established cell line were confirmed using real-time PCR, Western blotting, and mass spectrometry analysis; the parental TK6 cells and empty vector (EV) transduced cells had negligible CYP levels. Subsequently, we evaluated these cell lines using 2 prototypical polyaromatic hydrocarbon mutagens, 7,12-dimethylbenz[a]anthracene (DMBA) and benzo[a]pyrene (B[a]P), that require metabolic activation to exert their genotoxicity. DMBA-induced cytotoxicity, phosphorylation of histone H2A.X, and micronucleus formation were significantly increased in TK6 cells with CYP1A1, 1B1, 2B6, and 2C19 expression as compared with EV controls. B[a]P significantly increased cytotoxicity, DNA damage, and chromosomal damage in TK6 cells overexpressing CYP1A1 and 1B1 when compared with EV controls. B[a]P also induced micronucleus formation in TK6 cells expressing CYP1A2. These results suggest that our CYP-expressing TK6 cell system can be used to detect the genotoxicity of compounds requiring metabolic transformation.

Cytotoxic and genotoxic effects of e-liquids and their potential associations with nicotine, menthol and phthalate esters

In this study, we determined DNA damage and chromosome breakage (indicators of genotoxicity) and cell viability (an indicator of cytotoxicity) in human lymphoblastoid TK6 and Chinese hamster ovary (CHO) cells treated with 33 e-liquids using in vitro single cell gel (comet), micronucleus (MN), and trypan blue assays, respectively. We also measured the contents of nicotine, five phthalate esters, and DL-menthol in the e-liquids to examine their effects on DNA damage, chromosome breakage, and cell viability. Our chemical analyses showed that: (1) six e-liquids had nicotine ≥2-fold higher than the manufacture's label claim (2-3.5 mg); (2) both dimethyl- and dibutyl-phthalate levels were >0.1 μg/g, i.e., their threshold limits as additives in cosmetics; and (3) the DL-menthol contents ranged from 0.0003 to 85757.2 μg/g, with those of two e-liquids being >1 mg/g, the threshold limit for trigging sensory irritation. Though all the e-liquids induced DNA damage in TK6 cells, 20 resulted in cell viabilities ≤75%, indicating cytotoxicity, yet the inverse relationship between cell viability and DNA damage (r = -0.628, p = 0.003) might reflect their role as pro-apoptotic and DNA damage inducers. Fifteen e-liquids induced MN% in TK6 cells ≥3-fold that of untreated cells. Some of the increase in %MN might be false due to high cytotoxicity, yet six brands showed acceptable cell viabilities (59-71%), indicating chromosome damage. DNA damage and %MN increased when the TK6 cells were exposed to metabolic activation. The CHO cells were less sensitive to the genotoxic effects of the e-liquids than the TK6 cells. DL-menthol was found to be associated with decreased cell viability and increased DNA damage, even at low levels. We cannot dismiss the presence of other ingredients in e-liquids with cytotoxic/genotoxic properties since out of the 63 different flavors, 47 induced DNA damage (≥3-folds), and 26 reduced cell viability (≤75%) in TK6 cells.

Image analysis of mechanistic protein biomarkers for the characterization of genotoxicants: Aneugens, clastogens, and reactive oxygen species inducers

The early detection of genotoxicity contributes to cutting-edge drug discovery and development, requiring effective identification of genotoxic hazards posed by drugs while providing mode of action (MoA) information in a high throughput manner. In other words, there is a need to complement standard genotoxicity testing according to the test battery given in ICH S2(R1) with new in vitro tools, thereby contributing to a more in-depth analysis of genotoxic effects. Here, we report on a proof-of-concept MoA approach based on post-translational modifications of proteins (PTMs) indicative of clastogenic and aneugenic effects in TK6 cells using imaging technology (with automated analysis). Cells were exposed in a 96-well plate format with a panel of reference (geno)toxic compounds and subsequently analyzed at 4 and 24 hr to detect dose-dependent changes in PTMs, relevant for mechanistic analysis. All tested compounds that interfere with the spindle apparatus yielded a BubR1 (S640) (3/3) and phospho-histone H3 (S28) (7/9) positive dose-response reflecting aneugenicity, whereas compounds inducing DNA double-strand-breaks were associated with positive FANCD2 (S1404) and 53BP1 (S1778) responses pointing to clastogenicity (2/3). The biomarker p53 (K373) was able to distinguish genotoxicants from non-genotoxicants (2/4), while the induction of reactive oxygen species (ROS), potentially causing DNA damage, was associated with a positive Nrf2 (S40) response (2/2). This work demonstrates that genotoxicants and non-genotoxicants induce different biomarker responses in TK6 cells which can be used for reliable classification into MoA groups (aneugens/clastogens/non-genotoxicants/ROS inducers), supporting a more in-depth safety assessment of drug candidates.

Baseline effects of non-flavored e-liquids in the in vitro micronucleus assay

Electronic nicotine delivery systems (ENDSs; e.g. e-cigarettes) are being developed as potentially reduced-risk alternatives to the continued use of conventional tobacco products. They typically comprise a device that heats an e-liquid to generate inhalable vapor. E-liquids and ENDS-derived vapor have been the focus of toxicological assessment; in particular, their DNA-damaging potential has been investigated with varying outcomes and conclusions. In vitro genetic toxicology assays have formed a part of these assessments. However, they are susceptible to producing misleading or false positive results, especially under extreme conditions. In the present study, we evaluated a series of six neat (non-vaporized) non-flavored e-liquids (NFEL-A to F) in a flow cytometry version of the in vitro micronucleus assay in order to characterize their baseline effects on Chinese hamster ovary cells under hazard identification conditions. The NFELs induced cytotoxicity universally despite differing in propylene glycol (PG), vegetable glycerin (VG), and nicotine content. In addition, significant genotoxic responses were also detected with the PG-predominant e-liquids NFEL-A, D, and F but not with NFEL-B, C, or E, which contained higher proportions of VG. All six NFELs induced extreme cell culture conditions (i.e. increases in pH and osmolality) at the concentrations assessed. They also exhibited nonbiologically relevant effects on the mechanistic endpoints (i.e. cell cycle and phosphorylated histones H2AX and H3). In conclusion, although the PG component of the NFELs drove micronucleus formation in the assay, data on the complementary mechanistic endpoints suggest that this apparent DNA damage is potentially misleading and of negligible biological relevance as a risk for DNA integrity. In future assessments, any adverse changes (such as signatures of micronuclei induction, G2M arrest, and increases in γH2AX) relative to this reference data set might indicate a possible genotoxic hazard and would prompt further investigations for exploring the extent of risk.

Mode-of-action analysis of the effects induced by nicotine in the in vitro micronucleus assay

Nicotine's genotoxic potential has been extensively studied in vitro. While the results of mammalian cell-based studies have inferred that it can potentially damage chromosomes, in general and with few exceptions, adverse DNA effects have been observed primarily at supraphysiological concentrations in nonregulatory assays that provide little information on its mode-of-action (MoA). In this study, a modern-day regulatory genotoxicity assessment was conducted using a flow cytometry-based in vitro micronucleus (MN) assay, Good Laboratory Practice study conditions, Chinese hamster ovary cells of known provenance, and acceptance/evaluation criteria from the current OECD Test Guideline 487. Nicotine concentrations up to 3.95 mM had no effect on background levels of DNA damage; however, concentrations above the point-of-departure range of 3.94-4.54 mM induced increases in MN and hypodiploid nuclei, indicating a possible aneugenicity hazard. Follow-up experiments designed to elucidate nicotine's MoA revealed cellular vacuolization, accompanying distortions in microtubules, inhibition of tubulin polymerization, centromere-positive DNA, and multinucleate cells at MN-inducing concentrations. Vacuoles likely originated from acidic cellular compartments (e.g., lysosomes). Remarkably, genotoxicity was suppressed by chemicals that raised the luminal pH of these organelles. Other endpoints (e.g., changes in phosphorylated histones) measured in the study cast doubt on the biological relevance of this apparent genotoxicity. In addition, three major nicotine metabolites, including cotinine, had no MN effects but nornicotine induced a nicotine-like profile. It is possible that nicotine's lysosomotropic properties drive the genotoxicity observed in vitro; however, the potency and mechanistic insights revealed here indicate that it is likely of minimal physiological relevance for nicotine consumers. Environ. Mol. Mutagen. 2019. © 2019 The Authors. Environmental and Molecular Mutagenesis published by Wiley Periodicals, Inc. on behalf of Environmental Mutagen Society.

Hazard Assessment of Polymeric Nanobiomaterials for Drug Delivery: What Can We Learn From Literature So Far

The physicochemical properties of nanobiomaterials, such as their small size and high surface area ratio, make them attractive, novel drug-carriers, with increased cellular interaction and increased permeation through several biological barriers. However, these same properties hinder any extrapolation of knowledge from the toxicity of their raw material. Though, as suggested by the Safe-by-Design (SbD) concept, the hazard assessment should be the starting point for the formulation development. This may enable us to select the most promising candidates of polymeric nanobiomaterials for safe drug-delivery in an early phase of innovation. Nowadays the majority of reports on polymeric nanomaterials are focused in optimizing the nanocarrier features, such as size, physical stability and drug loading efficacy, and in performing preliminary cytocompatibility testing and proving effectiveness of the drug loaded formulation, using the most diverse cell lines. Toxicological studies exploring the biological effects of the polymeric nanomaterials, particularly regarding immune system interaction are often disregarded. The objective of this review is to illustrate what is known about the biological effects of polymeric nanomaterials and to see if trends in toxicity and general links between physicochemical properties of nanobiomaterials and their effects may be derived. For that, data on chitosan, polylactic acid (PLA), polyhydroxyalkanoate (PHA), poly(lactic-co-glycolic acid) (PLGA) and policaprolactone (PCL) nanomaterials will be evaluated regarding acute and repeated dose toxicity, inflammation, oxidative stress, genotoxicity, toxicity on reproduction and hemocompatibility. We further intend to identify the analytical and biological tests described in the literature used to assess polymeric nanomaterials toxicity, to evaluate and interpret the available results and to expose the obstacles and challenges related to the nanomaterial testing. At the present time, considering all the information collected, the hazard assessment and thus also the SbD of polymeric nanomaterials is still dependent on a case-by-case evaluation. The identified obstacles prevent the identification of toxicity trends and the generation of an assertive toxicity database. In the future, in vitro and in vivo harmonized toxicity studies using unloaded polymeric nanomaterials, extensively characterized regarding their intrinsic and extrinsic properties should allow to generate such database. Such a database would enable us to apply the SbD approach more efficiently.

Genotoxicity Assessment of Nanomaterials: Recommendations on Best Practices, Assays, and Methods

Nanomaterials (NMs) present unique challenges in safety evaluation. An international working group, the Genetic Toxicology Technical Committee of the International Life Sciences Institute's Health and Environmental Sciences Institute, has addressed issues related to the genotoxicity assessment of NMs. A critical review of published data has been followed by recommendations on methods alterations and best practices for the standard genotoxicity assays: bacterial reverse mutation (Ames); in vitro mammalian assays for mutations, chromosomal aberrations, micronucleus induction, or DNA strand breaks (comet); and in vivo assays for genetic damage (micronucleus, comet and transgenic mutation assays). The analysis found a great diversity of tests and systems used for in vitro assays; many did not meet criteria for a valid test, and/or did not use validated cells and methods in the Organization for Economic Co-operation and Development Test Guidelines, and so these results could not be interpreted. In vivo assays were less common but better performed. It was not possible to develop conclusions on test system agreement, NM activity, or mechanism of action. However, the limited responses observed for most NMs were consistent with indirect genotoxic effects, rather than direct interaction of NMs with DNA. We propose a revised genotoxicity test battery for NMs that includes in vitro mammalian cell mutagenicity and clastogenicity assessments; in vivo assessments would be added only if warranted by information on specific organ exposure or sequestration of NMs. The bacterial assays are generally uninformative for NMs due to limited particle uptake and possible lack of mechanistic relevance, and are thus omitted in our recommended test battery for NM assessment. Recommendations include NM characterization in the test medium, verification of uptake into target cells, and limited assay-specific methods alterations to avoid interference with uptake or endpoint analysis. These recommendations are summarized in a Roadmap guideline for testing.

Comparative study of cytotoxic effects induced by environmental genotoxins using XPC- and CSB-deficient human lymphoblastoid TK6 cells

Background

The human genome is constantly exposed to numerous environmental genotoxicants. To prevent the detrimental consequences induced by the expansion of damaged cells, cellular protective systems such as nucleotide excision repair (NER) exist and serve as a primary pathway for repairing the various helix-distorting DNA adducts induced by genotoxic agents. NER is further divided into two sub-pathways, namely, global genomic NER (GG-NER) and transcription-coupled NER (TC-NER). Both NER sub-pathways are reportedly involved in the damage response elicited by exposure to genotoxins. However, how disruption of these sub-pathways impacts the toxicity of different types of environmental mutagens in human cells is not well understood.

Results

To evaluate the role of NER sub-pathways on the cytotoxic effects of mutagens, we disrupted XPC and CSB to selectively inactivate GG-NER and TC-NER, respectively, in human lymphoblastoid TK6 cells, a standard cell line used in genotoxicity studies. Using these cells, we then comparatively assessed their respective sensitivities to representative genotoxic agents, including ultraviolet C (UVC) light, benzo [a] pyrene (B(a)P), 2-amino-3,8-dimethylimidazo [4,5-f] quinoxaline (MeIQx), 2-amino-1-methyl-6-phenylimidazo [4,5-b] pyridine (PhIP), γ-ray, and 2-acetylaminofluorene (2-AAF). CSB^−/− cells exhibited a hyper-sensitivity to UVC, B(a)P, and MeIQx. On the other hand, XPC^−/− cells were highly sensitive to UVC, but not to B(a)P and MeIQx, compared with wild-type cells. In contrast with other genotoxins, the sensitivity of XPC^−/− cells against PhIP was significantly higher than CSB^−/− cells. The toxicity of γ-ray and 2-AAF was not enhanced by disruption of either XPC or CSB in the cells.

Conclusions

Based on our findings, genetically modified TK6 cells appear to be a useful tool for elucidating the detailed roles of the various repair factors that exist to combat genotoxic agents, and should contribute to the improved risk assessment of environmental chemical contaminants.

Genetic instability of in vitro cell lines: Implications for genetic toxicity testing

Cell line-based in vitro testing has been widely used as an important component of the genotoxicity testing battery; however, the use of cell lines is constrained by several limitations, including the genetic drift and variability. A study recently reported in the literature comprehensively examined genomic changes in a large number of cell lines and reported extensive genetic variations within the same cell lines across passage numbers and laboratories, even for single-cell derived subclones. The primary objective of this communication is to raise awareness and stimulate discussion within the genotoxicity testing community of the extent of genetic variability of cell lines in general and how these variables could potentially influence the results and reproducibility of genotoxicity testing. Meanwhile, some recommendations for good cell culture practices are highlighted to mitigate, at least to some extent, the concern about genetic variation. Environ. Mol. Mutagen. 60:559-562, 2019. © 2019 Wiley Periodicals, Inc.

The development and prevalidation of an in vitro mutagenicity assay based on MutaMouse primary hepatocytes, Part I: Isolation, structural, genetic, and biochemical characterization

To develop an improved in vitro mammalian cell gene mutation assay, it is imperative to address the known deficiencies associated with existing assays. Primary hepatocytes isolated from the MutaMouse are ideal for an in vitro gene mutation assay due to their metabolic competence, their "normal" karyotype (i.e., neither transformed nor immortalized), and the presence of the MutaMouse transgene for rapid and reliable mutation scoring. The cells were extensively characterized to confirm their utility. Freshly isolated cells were found to have a hepatocyte-like morphology, predominantly consisting of binucleated cells. These cells maintain hepatocyte-specific markers for up to 3 days in culture. Analyses revealed a normal murine hepatocyte karyotype with a modal ploidy number of 4n. Fluorescence in situ hybridization analysis confirmed the presence of the lambda shuttle vector on chromosome 3. The doubling time was determined to be 22.5 ± 3.3 h. Gene expression and enzymatic activity of key Phase I and Phase II metabolic enzymes were maintained for at least 8 and 24 h in culture, respectively. Exposure to β-naphthoflavone led to approximately 900- and 9-fold increases in Cyp1a1 and Cyp1a2 gene expression, respectively, and approximately twofold induction in cytochrome P450 (CYP) 1A1/1A2 activity. Exposure to phenobarbital resulted in an approximately twofold increase in CYP 2B6 enzyme activity. Following this characterization, it is evident that MutaMouse primary hepatocytes have considerable promise for in vitro mutagenicity assessment. The performance of these cells in an in vitro gene mutation assay is assessed in Part II. Environ. Mol. Mutagen. 60:331-347, 2019. © 2018 The Authors. Environmental and Molecular Mutagenesis published by Wiley Periodicals, Inc. on behalf of Environmental Mutagen Society.

The development and prevalidation of an in vitro mutagenicity assay based on MutaMouse primary hepatocytes, Part II: Assay performance for the identification of mutagenic chemicals

As demonstrated in Part I, cultured MutaMouse primary hepatocytes (PHs) are suitable cells for use in an in vitro gene mutation assay due to their metabolic competence, their "normal" phenotype, and the presence of the MutaMouse transgene for reliable mutation scoring. The performance of these cells in an in vitro gene mutation assay is evaluated in this study, Part II. A panel of 13 mutagenic and nonmutagenic compounds was selected to investigate the performance of the MutaMouse PH in vitro gene mutation assay. The nine mutagens represent a range of classes of chemicals and include mutagens that are both direct-acting and requiring metabolic activation. All the mutagens tested, except for ICR 191, elicited significant, concentration-dependent increases in mutant frequency (MF) ranging from 2.6- to 14.4-fold over the control. None of the four nonmutagens, including two misleading, or "false," positives (i.e., tertiary butylhydroquinone [TBHQ] and eugenol), yielded any significant increases in MF. The benchmark dose covariate approach facilitated ranking of the positive chemicals from most (i.e., 3-nitrobenzanthrone [3-NBA], benzo[a]pyrene [BaP], and aflatoxin B₁ [AFB1]) to least (i.e., N-ethyl-N-nitrosourea [ENU]) potent. Overall, the results of this preliminary validation study suggest that this assay may serve as a complimentary tool alongside the standard genotoxicity test battery. This study, alongside Part I, illustrates the promise of MutaMouse PHs for use in an in vitro gene mutation assay, particularly for chemicals requiring metabolic activation. Environ. Mol. Mutagen. 60:348-360, 2019. © 2019 The Authors. Environmental and Molecular Mutagenesis published by Wiley Periodicals, Inc. on behalf of Environmental Mutagen Society.

2'-O-(2-Methoxyethyl) Nucleosides Are Not Phosphorylated or Incorporated Into the Genome of Human Lymphoblastoid TK6 Cells

Nucleoside analogs with 2'-modified sugar moieties are often used to improve the RNA target affinity and nuclease resistance of therapeutic oligonucleotides in preclinical and clinical development. Despite their enhanced nuclease resistance, oligonucleotides could slowly degrade releasing nucleoside analogs that have the potential to become phosphorylated and incorporated into cellular DNA and RNA. For the first time, the phosphorylation and DNA/RNA incorporation of 2'-O-(2-methoxyethyl) (2'-O-MOE) nucleoside analogs have been investigated. Using liquid chromatography/tandem mass spectrometry, we showed that enzymes in the nucleotide salvage pathway including deoxycytidine kinase (dCK) and thymidine kinase (TK1) displayed poor reactivity toward 2'-O-MOE nucleoside analogs. On the other hand, 2'-fluoro (F) nucleosides, regardless of the nucleobase, were efficiently phosphorylated to their monophosphate forms by dCK and TK1. Consistent with their efficient phosphorylation by dCK and TK1, 2'-F nucleoside analogs were incorporated into cellular DNA and RNA while no incorporation was detected with 2'-O-MOE nucleoside analogs. In conclusion, these data suggest that the inability of dCK and TK1 to create the monophosphates of 2'-O-MOE nucleoside analogs reduces the risk of their incorporation into cellular DNA and RNA.

In vitro mammalian cell mutation assays based on the Pig-a gene: A report of the 7th International Workshop on Genotoxicity Testing (IWGT) Workgroup

Pig-a gene mutation assays enumerate cells with the glycosylphosphatidylinositol (GPI) anchor-deficient phenotype as a reporter of mutation in the endogenous Pig-a gene. Methods for measuring mutation in this gene are quite well established for in vivo systems. This approach to mutagenicity assessment has now been extended to in vitro mammalian cell-based systems. An expert workgroup from the 7th International Workshop on Genotoxicity Testing tasked with assessing the status of in vitro mammalian cell mutation assays has investigated the merits and limitations of in vitro Pig-a gene mutation assays. A review of the current status of these developing methodologies and the formation of consensus statements on the utility and application of these assays were performed to provide guidance for their potential use in genotoxicity hazard identification and risk assessment.

In Vitro Mouse Lymphoma Cell (L5178Y Tk+/- -3.7.2.C) Forward Mutation Assay

The in vitro mouse lymphoma cell assay (MLA) is one of the most widely practiced assays in genetic toxicology. MLA detects forward mutations at the thymidine kinase (Tk) locus of the L5178Y (Tk^+/- -3.7.2C) cell line derived from a mouse thymic lymphoma. This assay is capable of detecting a wide range of genetic events including point mutations, deletions and multilocus, chromosomal rearrangements, mitotic recombination and nondisjunction. There are two equally accepted versions of the assay, one using soft agar cloning and the second method using liquid media cloning in 96-microwell plates. There are two morphologically distinct types of mutant colonies recovered in the MLA; small and large colony mutants. The induction of small colony mutants is associated with chemicals inducing gross chromosomal aberrations, whereas the induction of large mutant colonies is generally associated with chemicals inducing point mutations. The source and karyotype of the cell line as well as the culture conditions are important variables that could influence the assay performance. The assay when performed according to the standards recommended by the International Workshops on Genotoxicity Testing (IWGT) and the Organization of Economic Cooperation and Development Test Guideline 490 is capable of providing valuable genotoxicity hazard information as part of the overall safety assessment process of various classes of test substances.

In vitro genotoxicity assessment of an oxidized dextrin-based hydrogel for biomedical applications

Hydrogels are three-dimensional, crosslinked networks of hydrophilic polymers swollen with a large amount of water or biological fluids, without dissolving. Dextrin, a low-molecular-weight carbohydrate composed by glucose residues, has been used to develop an injectable hydrogel for biomedical applications. Dextrin was first oxidized to introduce aldehyde groups, which then reticulate with adipic acid dihydrazide, forming the dextrin-based hydrogel (HG). The HG and its components were tested for cyto- and genotoxicity according to the International Standard ISO 10993-3 on the biological evaluation of medical devices. To assess genotoxicity, a battery of in vitro genotoxicity tests employing both eukaryotic and prokaryotic models was performed: comet assay, cytokinesis-block micronucleus assay and Ames test. Our data revealed that the HG (IC₅₀  = 2.8 mg/mL) and oxidized dextrin by itself (IC₅₀  = 1.2 mg/mL) caused a concentration-dependent decrease in cellular viability of human lymphoblastoid TK6 cells after 24 hours of exposure to the test agents. However, these concentrations are unlikely to be reached in vivo. In addition, no significant increase in the DNA and chromosomal damage of TK6 cells exposed to non-cytotoxic concentrations of the HG and its isolated components was detected. Furthermore, neither the HG nor its metabolites exerted a mutagenic effect in different of Salmonella typhimurium strains and in an Escherichia coli mix. Our data demonstrated the genocompatibility of the HG (up to 3.5 mg/mL) for biomedical applications. To our best acknowledge, this is the first report with a detailed genotoxicity assessment of an aldehyde-modified polysaccharide/adipic acid dihydrazide hydrogel.

Establishing a novel Pig-a gene mutation assay in L5178YTk+/- mouse lymphoma cells

The X-linked Pig-a gene encodes an enzyme required for the biosynthesis of glycosyl phosphatidylinositol (GPI) anchors. Pig-a mutant cells fail to synthesize GPI and to express GPI-anchored protein markers (e.g., CD90) on their surface. Marker deficiency serves as a phenotypic indicator of Pig-a mutation in various in vivo assays. Here, we describe an in vitro Pig-a mutation assay in L5178YTk^+/- mouse lymphoma cells, in which mutant-phenotype cells are measured by flow cytometry using a fluorescent anti-CD90 antibody. Increased frequencies of CD90-deficient mutants were detected in cells treated with benzo[a]pyrene (B[a]P), N-ethyl-N-nitrosourea (ENU), ethyl methanesulphonate, and 7,12-dimethylbenz[a]anthracene, with near maximum mutant frequencies measured eight days after treatment. The CD90 deficiency in mutant cells quantified by flow cytometry was shown to be due to loss of GPI anchors in a limiting-dilution cloning assay using proaerolysin selection. Individual CD90-deficient cells from cultures treated with ENU, B[a]P, and vehicle were sorted and clonally expanded for molecular analysis of their Pig-a gene. Pig-a mutations with agent-specific signatures were found in nearly all clones that developed from sorted CD90-deficient cells. These results indicate that a Pig-a mutation assay can be successfully conducted in L5178YTk^+/- cells. The assay may be useful for mutagenicity screening of environmental agents as well as for testing hypotheses in vitro before committing to in vivo Pig-a assays. Environ. Mol. Mutagen. 59:4-17, 2018. Published 2017. This article is a US Government work and is in the public domain in the USA.

Analysis of negative historical control group data from the in vitro micronucleus assay using TK6 cells

The recent revisions of the Organisation for Economic Co-operation and Development (OECD) genetic toxicology test guidelines emphasize the importance of historical negative controls both for data quality and interpretation. The goal of a HESI Genetic Toxicology Technical Committee (GTTC) workgroup was to collect data from participating laboratories and to conduct a statistical analysis to understand and publish the range of values that are normally seen in experienced laboratories using TK6 cells to conduct the in vitro micronucleus assay. Data from negative control samples from in vitro micronucleus assays using TK6 cells from 13 laboratories were collected using a standard collection form. Although in some cases statistically significant differences can be seen within laboratories for different test conditions, they were very small. The mean incidence of micronucleated cells/1000 cells ranged from 3.2/1000 to 13.8/1000. These almost four-fold differences in micronucleus levels cannot be explained by differences in scoring method, presence or absence of exogenous metabolic activation (S9), length of treatment, presence or absence of cytochalasin B or different solvents used as vehicles. The range of means from the four laboratories using flow cytometry methods (3.7-fold: 3.5-12.9 micronucleated cells/1000 cells) was similar to that from the nine laboratories using other scoring methods (4.3-fold: 3.2-13.8 micronucleated cells/1000 cells). No laboratory could be identified as an outlier or as showing unacceptably high variability. Quality Control (QC) methods applied to analyse the intra-laboratory variability showed that there was evidence of inter-experimental variability greater than would be expected by chance (i.e. over-dispersion). However, in general, this was low. This study demonstrates the value of QC methods in helping to analyse the reproducibility of results, building up a 'normal' range of values, and as an aid to identify variability within a laboratory in order to implement processes to maintain and improve uniformity.

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.