Cell-cycle dependent micronucleus formation and mitotic disturbances induced by 5-azacytidine in mammalian cells

DNA hypomethylation leads to cGAS-induced autoinflammation in the epidermis

DNA methylation is a fundamental epigenetic modification, important across biological processes. The maintenance methyltransferase DNMT1 is essential for lineage differentiation during development, but its functions in tissue homeostasis are incompletely understood. We show that epidermis-specific DNMT1 deletion severely disrupts epidermal structure and homeostasis, initiating a massive innate immune response and infiltration of immune cells. Mechanistically, DNA hypomethylation in keratinocytes triggered transposon derepression, mitotic defects, and formation of micronuclei. DNA release into the cytosol of DNMT1-deficient keratinocytes activated signaling through cGAS and STING, thus triggering inflammation. Our findings show that disruption of a key epigenetic mark directly impacts immune and tissue homeostasis, and potentially impacts our understanding of autoinflammatory diseases and cancer immunotherapy.

Micronuclei as a biological endpoint for genotoxicity: A minireview

The in vitro micronucleus assay has now been applied in many laboratories. This endpoint is useful in biomonitoring or ecotoxicology, as a sensitivity measure of human cells in cancer treatment and also to replace or supplement other in vitro genotoxicity assays. Learning more about the mechanisms of micronucleus formation allows conclusions about its biological significance. It has been demonstrated that disturbance of the mitotic apparatus (spindle, kinetochores) as well as impaired function of topoisomerase II can be involved in micronucleus formation. In addition, the roles of changes in DNA-conformation that are induced by alterations in the status of cytosine-methylation and of the cellular DNA repair capacity have been shown. The fate of micronucleus-containing cells is not known: the cells may theoretically be cytostatic and micronucleus-formation may therefore be a way of the organism to eliminate genetic damage or the cells may survive the loss of that chromosomal material and develop into transformed cells. Published data and ideas of selected areas within this field are reviewed.

Age-related changes in radiation-induced micronuclei among healthy adults

The aim of the present study was to establish the extent of in vitro radioresponse of lymphocytes among 62 healthy adults of both genders and to estimate the distribution of baseline micronuclei and radiosensitivity among individuals of the study population using the cytochalasin block micronucleus test. A younger study group consisted of 10 males (mean age, 22.4 years; range, 21-27) and 12 females (mean age, 24.8 years; range, 20-29), whereas an older study group consisted of 18 males (mean age, 35.1 years; range, 30-44) and 22 females (mean age, 38.5 years; range, 30-48). For evaluation of radiosensitivity blood samples were irradiated in vitro using 60Co gamma-ray source. The radiation dose employed was 2 Gy, the dose rate 0.45 Gy/min. The study revealed a significant gender effect on baseline micronuclei favoring females (Z = 3.25, P < 0.001), while yields of radiation-induced micronuclei did not differ significantly (Z = 0.56, P < 0.56) between genders. The distribution of baseline micronuclei among the individuals tested followed Poisson distribution in both study groups and in both genders, whereas the distribution of radiosensitivity among individuals of the older study group did not fulfill Poisson expectations (Kolmogorov-Smirnof test, P < 0.01). In contrast to a nonsignificant difference in radiosensitivity between males and females of the same age group (Z = 1.97, P < 0.56), a statistically significant difference in radiosensitivity between younger and older group for both genders was found (Z = 3.03, P < 0.03). Since the individuals tested were healthy, the observed variability in radiation response is considered to be an early effect of ageing.

DNA methylation influences the decatenation activity of topoisomerase II

Methylation of cytosine in CpG sequences of the DNA in mammalian cells is involved in the regulation of events like gene expression, genomic imprinting, transcription, and DNA replication. Changes in the DNA methylation pattern influence the DNA conformation. Therefore, certain proteins are disturbed in their interaction with DNA. In this paper, we investigate the influence on the decatenation activity of topoisomerase II, an essential enzyme that modulates DNA topology. We compare the decatenation activity of topoisomerase II for a mitochondrial parasite DNA, that we had methylated to different degrees, and found the decatenation to be methylation dependent. Thus, changes in the methylation pattern are a mechanism for the disturbance of certain topoisomerase II activities and may, therefore, be an event in topoisomerase II related mutagenicity and carcinogenicity.

Genotoxicity of several clinically used topoisomerase II inhibitors

DNA topoisomerase II is an essential nuclear enzyme that modulates DNA topology during multiple cellular processes such as DNA replication and chromosome segregation. Several important clinical antitumor drugs and antibiotics act through inhibition of topoisomerase II. There are a number of different steps in the action of topoisomerase II, all of which are potential targets for inhibition through drugs and also for cellular and genetic toxicity as well as for mutagenesis. We have investigated and compared the genotoxicity and mutagenicity of the mechanistically different topoisomerase II inhibitors m-amsacrine, mitoxantrone, etoposide, genistein, ICRF 193, and berenil using the in vitro micronucleus test, single cell gelelectrophoresis (comet assay) and the mutation assay (tk-locus) in L5178Y mouse lymphoma cells. All six compounds induced micronuclei and all except berenil were mutagenic. M-amsacrine, mitoxantrone, etopside and genistein induced DNA migration in the comet assay, whereas ICRF 193 was only weakly positive and berenil was negative in this test. Our results are in good agreement with the compounds' proposed mechanisms of interaction with topoisomerase II.

5-Azacytidine- and Hoechst-induced aneuploidy in Indian muntjac

Hoechst 33258 (bis-benzimidazole) and 5-azacytidine (5-AC) cause decondensation of the pericentric heterochromatin in mouse and aberrations in the sequence of centromere separation apparently by different mechanisms. We treated the male Indian muntjac cells (2n=7), which do not undergo decondensation of the pericentric heterochromatin, to study if these chemicals would result in induction of aneuploidy limited to the Y(2) chromosome. This paper reports that both agents result in aneuploidy primarily limited to one chromosome, the Y(2). It is likely that other chromosomes are not tolerated in aneuploid condition because every chromosome carries some household genes including those essential for mitotic progression. The loss/gain of the Y(2) chromosome is tolerated because it is the smallest chromosome and is almost entirely composed of constitutive heterochromatin. Since Indian muntjac has only three pairs of large chromosomes comprising its basic genome, which can be clearly viewed under high dry objective, these cells are very suitable for the preliminary analysis of aneuploidy-inducing ability of various chemicals.

Interaction effects of 5-azacytidine with topoisomerase II inhibitors on CHO cells, as detected by cytogenetic analysis

Different cell treatment protocols with the hypomethylating agent 5 azacytidine (5-aza C) were used in exponentially growing Chinese hamster ovary (CHO) cells in order to test its influence on the induction of chromosomal aberrations (CAs) induced by topoisomerase II inhibitors, ellipticine (EPC) and teniposide (VM-26). Cells pre-treated with 1 microg/ml 5-aza C for 1 h during the S-phase and post-treated in the last 2 h of incubation with 0.6 microg/ml EPC or 0.04 microg/ml VM-26 showed a reduction of 48% and 45%, respectively, in the frequencies of CAs as compared to the sum value of the frequencies obtained for each drug alone. 5-aza C added to the cultures for the last 2 h before cell fixation after a 30-min pulse treatment with EPC or VM-26 caused a 38% and 28% reduction, respectively. Simultaneous treatments with 5-aza C plus EPC, or 5-aza C plus VM-26 during the last 2 h of incubation (G2-phase), showed a significant effect of CA reduction (24%) only for the combination of 5-aza C + EPC. Preliminary assays with 5-aza C alone added to the cultures at different times demonstrated its effectiveness in inducing chromosome damage during the S-phase. Since S-phase-treated CHO cells showed a higher degree of reduction in the frequencies of CAs induced by EPC and VM-26, we suggest that 5-aza C incorporation into DNA may change the topo II cleavage sites, protecting the DNA from the induction of damage, or that the hypomethylation induced by incorporation of 5-aza C into DNA may change the chromatin structure facilitating the access to DNA repair enzymes. An alternative possibility is that 5-azaC can reactivate methylated genes involved in the repair of DNA double-strand breaks induced by topo II inhibitors.

Radiation-induced micronuclei formation in human breast cancer cells: dependence on serum and cell cycle distribution

Micronuclei (MN) formation was defined as a form of radiation-induced damage in MCF-7 cells. MN appeared post-mitosis and were scored in bi-nucleated cells of cytochalasin B treated cultures. MN were surrounded by an envelope composed of inner and outer membranes, and contained fragmented chromosomes. However, typical features of apoptosis, such as chromatin margination or condensation were not observed. Reducing serum concentration resulted in a decreased MN formation, suggesting that serum factors directly affected MN formation and/or that serum depletion decreased the availability of radiation sensitive MN-forming cells for mitosis. Irradiation of G1 and S phase enriched populations revealed that S phase cells were more prone to MN formation than G1 cells. Radiation-induced chromosomal aberration can therefore be modulated by altering serum level and cell cycle distribution.

A critical evaluation of centromeric labeling to distinguish micronuclei induced by chromosomal loss and breakage in vitro

The in vitro micronucleus assay in conjunction with CREST-staining and fluorescence in situ hybridization (FISH) with centromere-specific DNA probes is being increasingly utilized for the detection of clastogenic and aneuploidy-inducing agents. Although potentially powerful techniques, both methods have unique characteristics that can influence sample processing and the interpretation of results. In this article, the use of the CREST and the FISH modifications of the in vitro micronucleus assay have been used to characterize the origin of the micronuclei induced by cyclophosphamide, 4,4'-methylene-bis(2-chloroaniline), 4-nitroquinoline N-oxide and ionizing radiation in metabolically competent MCL-5 cells or a derived cell line lacking metabolic activation. Using these results and our previous experiences with these techniques, a detailed comparison including the strengths and limitations of each technique as well as potential problems in performing each assay and in analyzing the data is discussed. In spite of their limitations, our results to date indicate that CREST-staining as well as FISH with centromere-specific DNA probes can be used to accurately distinguish micronuclei formed from chromosome loss from those originating from chromosome breakage and that these techniques can be valuable complements to the in vitro micronucleus assay.

The in vitro micronucleus test: a multi-endpoint assay to detect simultaneously mitotic delay, apoptosis, chromosome breakage, chromosome loss and non-disjunction

Genotoxicity testing aims to detect a large range of genetic damage endpoints and evaluate such results in context of cell survival. The cytokinesis block micronucleus test offers the advantage to provide simultaneously information on both cell cycle progression and chromosome/genome mutations. Indeed, 1. frequencies of cytokinesis-blocked binucleated cells (and polynucleated) are good estimators of the mitotic rate; 2. frequencies of apoptotic figures in mononucleated and binucleated cells provide a measure for cell death before or after cell division; 3. combination of fluorescence in situ hybridization (FISH) for centromere/telomeres and micronucleus scoring allows discrimination between clastogenic and aneugenic events; 4. detection of FISH signals for chromosome specific sequences in both macronuclei and micronuclei, discriminates between aneuploidy due to chromosome non-disjunction or to chromosome loss. The cytokinesis block in vitro micronucleus test is thus a cytogenetic multi-test providing mechanistic information with a simple, rapid, objective, microscopical analysis.

The genetics of the repair of 5-azacytidine-mediated DNA damage in the fission yeast Schizosaccharomyces pombe

We have recently demonstrated that Schizosaccharomyces pombe cells treated with the nucleoside analogue 5-azacytidine (5-azaC) require previously characterised G2 checkpoint mechanisms for survival. Here we present a survey of known DNA repair mutations which defines those genes required for survival in the presence of 5-azaC. Using a combination of single-mutant and epistasis analyses we find that the excision, mismatch and recombinational repair pathways are all required in some degree for the repair of 5-azaC-mediated DNA damage. There are distinct differences in the epistatic interactions of several of the repair mutations with respect to 5-azaC-mediated DNA damage relative to UV-mediated DNA damage.

5-Azacytidine-induced 6-thioguanine resistance at the gpt locus in AS52 cells: cellular response

Treatment of AS52 cells with 5-azacytidine resulted in an induction of 6-thioguanine-resistant [6TG] colonies, which reached a maximum by an expression time of 9 days. Dose responses for both cytotoxicity and mutation induction were determined following treatment with 5-azacytidine. At 20 microM treatment, 5-azacytidine exposure resulted in about 50% survival. Mutant frequency reached a maximum of 10 microM. At concentrations between 10 and 20 microM, 5-azacytidine was a potent mutagen but did not exhibit a dose response. Although many compounds both induce cell death and affect the growth rate of cells, 5-azacytidine specifically induced cell death and did not affect the doubling time of the surviving treated cell population.

Regulatory Genotoxicity Testing: A Critical Appraisal

This review considers current approaches to regulatory genotoxicity testing, focusing on how the use of animals can be further replaced, reduced and refined. The complementary roles of in vitro and in vivo testing, and the justification for using animals, are discussed in detail. Recommendations are made for improvements and further work, in the light of the considerable current controversy surrounding the composition and deployment of testing strategies, and the interpretation of the data generated, particularly for carcinogenicity prediction. The major problems are the oversensitivity of in vitro tests and the insensitivity of in vivo assays. On the basis of an analysis of some published databases, it is concluded that there is insufficient support for using in vivo genotoxicity assays for screening. Also, it is questionable whether the scientific benefits of using such assays always outweigh the costs to the animals involved. The considerable efforts being made to harmonise in vivo protocols and to develop improved methods for detecting genotoxicity are discussed. It is recommended that more emphasis be placed on characterising genotoxins in vitro, especially for mechanisms of activity, to optimise the benefits of any confirmatory animal tests.. Also, regulatory agencies are urged to require better-designed and more-scientifically sound protocols, in which animal numbers are minimised and data interpretation, particularly that of negative results, is facilitated. Lastly, in the development and validation of transgenic rodent systems, emphasis should be placed on developing protocols in which other acute toxicity and metabolism endpoints can be measured simultaneously with in vivo mutagenesis, while minimising animal numbers.