Mutagenesis - recent issues

Confounding experimental considerations in nanogenotoxicology

2009-06-25

The development of novel nanomaterials with unique physico-chemical properties is increasing at a rapid rate, with potential applications across a broad range of manufacturing industries and consumer products. Nanomaterial safety is therefore becoming an increasingly contentious issue that has intensified over the past 4 years, and in response, a steady stream of studies focusing on nanotoxicology are emerging. However, it is becoming increasingly evident that nanomaterials cannot be treated in the same manner as chemical compounds with regards to their safety assessment, as their unique physico-chemical properties are also responsible for unexpected interactions with experimental components that generate misleading data-sets. In this report, we focus on nanomaterial interactions with colorimetric and fluorometric dyes, components of cell culture growth medium and genotoxicity assay components, and the resultant consequences on test systems are demonstrated. Thus, highlighting some of the potential confounding factors that need to be considered in order to ensure that in vitro genotoxicity assays report true biological impacts in response to nanomaterial exposure.

State of the art survey of the buccal micronucleus assay--a first stage in the HUMNXL project initiative

2009-06-25

The study of DNA damage in exfoliated buccal cells is a minimally invasive method for monitoring populations for exposure to genotoxic agents. The presence of micronuclei (MN) and other nuclear anomalies within these cells has been shown to be associated with genetic defects in genome maintenance, accelerated ageing, genotoxic damage and some degenerative diseases. To identify important information gaps regarding these biomarkers, a new initiative was launched within the framework of the HUman MicroNucleus (HUMN) collaborative programme, the HUMN_XL project (‘XL’ designating eXfoLiated cell). An invitation to join the project was sent out together with a questionnaire to all laboratories that have published on the buccal micronucleus assay. Overall, 188 messages were delivered and 58 laboratories from 25 countries agreed to participate (43 contributing data). The questionnaire was designed to collect methodological information regarding the laboratory's performance of the assay and to assess the extent and type of epidemiological data that are routinely collected. The results provide an overview of the most commonly used methods for buccal cell collection and preparation, slide preparation, staining, scoring criteria and an evaluation of epidemiological data, including demographics, genetic background, gender, health status, occupation, exposure, lifestyle and dietary habit. According to this survey, a potential base of 15 103 subjects can be included in future pooled analyses. A number of protocol discrepancies emerged, implying that method standardization is a major priority. The results of this survey will contribute to (i) identify technical and epidemiological key variables that impact on buccal MN frequency in human populations, (ii) drive the design of future intra- and interlaboratory validation studies and (iii) determine the role of MN frequency and other biomarkers, in monitoring genomic damage and predicting cancer and other degenerative diseases.

Is habitual alcohol drinking associated with reduced electrophoretic DNA migration in peripheral blood leukocytes from ALDH2-deficient male Japanese?

Lu, Y., Morimoto, K. — 2009-06-25

Alcohol drinking-derived acetaldehyde is believed to cross-link DNA and induce sister chromatid exchanges in peripheral blood lymphocytes. However, little population data are available to illustrate effects of alcohol-derived acetaldehyde on DNA migration as assayed by the comet assay in peripheral lymphocytes. In the present study, we investigated lifestyle behaviours, including alcohol consumption, in 150 Japanese males by questionnaire, determined their aldehyde dehydrogenase 2 (ALDH2) family genotypes by polymerase chain reaction and measured the DNA migration in peripheral blood leukocytes by the alkaline comet assay. The results showed that habitual alcohol drinking is significantly negatively associated with DNA migration in peripheral blood leukocytes (r = –0.321, P = 0.005) of ALDH2-deficient, but not of ALDH2-proficient genotypes (r = 0.048, P = 0.683). The amount of pure alcohol consumed per time by the subjects showed a similar phenomenon (r = –0.257, P = 0.025 for the ALDH2-deficient, but r = –0.061, P = 0.606 for the ALDH2-proficient genotype). Further stepwise multiple regression analysis showed that alcohol drinking frequency was a significant predictor of DNA migration for subjects with ALDH2-deficient genotype, but not for subjects with ALDH2-proficient genotype. In summary, the present result suggests that frequent alcohol drinking is significantly associated with a reduced electrophoretic DNA migration in peripheral blood leukocytes from ALDH2-deficient male Japanese subjects.

The Xpc gene markedly affects cell survival in mouse bone marrow

2009-06-25

The XPC protein (encoded by the xeroderma pigmentosum Xpc gene) is a key DNA damage recognition factor that is required for global genomic nucleotide excision repair (G-NER). In contrast to transcription-coupled nucleotide excision repair (TC-NER), XPC and G-NER have been reported to contribute only modestly to cell survival after DNA damage. Previous studies were conducted using fibroblasts of human or mouse origin. Since the advent of Xpc–/– mice, no study has focused on the bone marrow of these mice. We used carboplatin to induce DNA damage in Xpc–/– and strain-matched wild-type mice. Using several independent methods, Xpc–/– bone marrow was ~10-fold more sensitive to carboplatin than the wild type. Importantly, 12/20 Xpc–/– mice died while 0/20 wild-type mice died. We conclude that G-NER, and XPC specifically, can contribute substantially to cell survival. The data are important in the context of cancer chemotherapy, where Xpc gene status and G-NER may be determinants of response to DNA-damaging agents including carboplatin. Additionally, altered cell cycles and altered DNA damage signalling may contribute to the cell survival end point.

Enhancement of the recombinagenic and mutagenic activities of bleomycin in yeast by intercalation of acridine compounds into DNA

Hoffmann, G. R., Ronan, M. V., Sylvia, K. E., Tartaglione, J. P. — 2009-06-25

Strain D7 of Saccharomyces cerevisiae was used to measure the induction by bleomycin (BLM) of mitotic recombination at the trp5 locus and point mutations at ilv1 in the presence and absence of acridine compounds. BLM is a potent mutagen and recombinagen in the D7 assay. The acridines vary, some being mutagenic or recombinagenic and others not. Combined treatments were used to distinguish whether a genetically inactive acridine has no effect on the genetic activity of BLM or modulates its action. When an acridine is itself genetically active, combined treatments were used to determine whether its effects are additive with those of BLM or whether there is interaction between the two compounds. Acridine compounds that share the ability to intercalate between the base pairs of DNA but differ in their mutagenic specificity owing to the presence of different substituent groups were analysed. Clear potentiation and synergistic interactions were detected in combined treatments with BLM and aminoacridines, nitroacridines or an acridine mustard. Potentiation and synergy were also observed in sequential exposures in which the yeast were grown in the presence of acridine compounds and then treated with BLM in the absence of free acridine. The results are consistent with an increase in BLM susceptibility conferred by acridine intercalation. It is likely that the intercalating agents increase the access of BLM to the minor groove of DNA, where it abstracts a hydrogen from the 4' position of deoxyribose, creating a free radical that is processed into strand breaks.

XRCC1 Arg399Gln, Arg194Trp and Arg280His polymorphisms in breast cancer risk: a meta-analysis

Huang, Y., Li, L., Yu, L. — 2009-06-25

X-ray repair cross-complementing group 1 (XRCC1) plays an important role in base excision and single-strand break repair, as a scaffold protein that brings together proteins of the DNA repair complex, and appears to be a candidate for cancer risk. However, studies on the association between polymorphisms in this protein and cancer have yielded conflicting results. We performed a meta-analysis to investigate the association between the breast cancer and the XRCC1 polymorphisms Arg194Trp (9411 cases and 9783 controls), Arg399Gln (22 481 cases and 23 905 controls) and Arg280His (6062 cases and 5864 controls) in different inheritance models. Our analysis suggested that Arg399Gln was associated with a trend of increased breast cancer risk when using both dominant [odds ratio (OR) = 1.06, 95% confidence interval (CI): 1.00–1.13] and recessive models (OR = 1.12, 95% CI: 1.02–1.23) to analyse the data. In ethnic subgroups and using recessive model analysis: Arg399Gln increased breast cancer risk in Asians (OR = 1.26, 95% CI: 0.96–1.64) and Africans (OR = 1.80, 95% CI: 0.97–3.32), and also while only slightly increasing the breast cancer risk in Caucasians (OR = 1.08, 95% CI: 0.95–1.22). However, Arg194Trp (recessive model, OR = 0.95, 95% CI: 0.75–1.20) and Arg280His (recessive model, OR = 1.28, 95% CI: 0.64–2.55) did not appear to be risk factors for breast cancer. Larger scale primary studies are required to further evaluate the interaction of XRCC1 polymorphisms and breast cancer risk in specific populations.

Mutagenicity testing for chemical risk assessment: update of the WHO/IPCS Harmonized Scheme

2009-06-25

Since the publication of the International Programme on Chemical Safety (IPCS) Harmonized Scheme for Mutagenicity Testing, there have been a number of publications addressing test strategies for mutagenicity. Safety assessments of substances with regard to genotoxicity are generally based on a combination of tests to assess effects on three major end points of genetic damage associated with human disease: gene mutation, clastogenicity and aneuploidy. It is now clear from the results of international collaborative studies and the large databases that are currently available for the assays evaluated that no single assay can detect all genotoxic substances. The World Health Organization therefore decided to update the IPCS Harmonized Scheme for Mutagenicity Testing as part of the IPCS project on the Harmonization of Approaches to the Assessment of Risk from Exposure to Chemicals. The approach presented in this paper focuses on the identification of mutagens and genotoxic carcinogens. Selection of appropriate in vitro and in vivo tests as well as a strategy for germ cell testing are described.

Micronuclei in peripheral blood from patients after cytostatic therapy mainly arise ex vivo from persistent damage

2009-06-25

The micronucleus test (MNT) is a well-established assay in genotoxicity testing and human biomonitoring. The cytokinesis-block micronucleus test (CBMNT) is the preferred method for measuring MN in cultured human lymphocytes from human subjects exposed to genotoxins. It is, however, unclear to what extent mutagen exposure either leads to the formation of MN already in vivo or to the formation of MN ex vivo during cell culture as a consequence of persisting DNA damage. To address this question, we investigated peripheral blood of 22 patients who had received cytostatic therapies including drugs with clastogenic and aneugenic effects. We also performed the MNT with blood samples from 13 healthy controls without relevant mutagen exposure. The incidence of MN was studied 24, 48 and 72 h after the start of the culture in mononuclear lymphocytes in cultures without cytochalasin B and also at 72 h in binucleated lymphocytes in the standard CBMNT. The mean frequency of binuclear cells with MN in the CBMNT was clearly increased in blood samples from patients (29.3 versus 10.2 per 1000 in controls). In contrast, mononuclear lymphocytes analysed 24 or 48 h after start of the cultures only revealed a marginal increase in MN frequencies in comparison to controls. These results suggest that mutagen exposure in vivo mainly leads to the formation of MN during ex vivo proliferation of lymphocytes as a consequence of persistent damage. Characterization of MN in binuclear lymphocytes from patients by fluorescence in situ hybridization (FISH) with a pan-centromeric probe indicated that MN arose by clastogenic and aneugenic mechanisms. A high portion of MN was relatively large and exhibited several centromere signals. If the results of this study with patients exposed to cytostatic drugs also apply to other kinds of mutagen exposure, increased MN frequencies in the CBMNT can only be expected for exposures leading to persistent damage in peripheral lymphocytes and MN formation during ex vivo lymphocyte culture.

Comparison of the Ames II and traditional Ames test responses with respect to mutagenicity, strain specificities, need for metabolism and correlation with rodent carcinogenicity

Kamber, M., Fluckiger-Isler, S., Engelhardt, G., Jaeckh, R., Zeiger, E. — 2009-06-25

The Ames II Salmonella mutagenicity assay procedure was used to test 71 chemicals, and the results were compared with those from the traditional Ames Salmonella test using the NTP database as the reference. All Ames II tests were performed using a fluctuation procedure in microplate format, using TAMix for the detection of base pair substitutions and TA98 to detect frameshift mutations. There was 84% agreement between the two procedures in identifying mutagens and non-mutagens, which is equivalent to the intra- and interlaboratory reproducibility of 87% for the traditional test. The two tests also performed similarly in their predictions of rodent carcinogenicity.

The benzene metabolite, hydroquinone and etoposide both induce endoreduplication in human lymphoblastoid TK6 cells

Ji, Z., Zhang, L., Guo, W., McHale, C. M., Smith, M. T. — 2009-06-25

Both occupational exposure to the leukemogen benzene and in vitro exposure to its metabolite hydroquinone (HQ) lead to the induction of numerical and structural chromosome changes. Several studies have shown that HQ can form DNA adducts, disrupt microtubule assembly and inhibit DNA topoisomerase II (topo II) activity. As these are potential mechanisms underlying endoreduplication (END), a phenomenon that involves DNA amplification without corresponding cell division, we hypothesized that HQ could cause END. We measured END in the human lymphoblastoid cell line, TK6, treated with HQ (0–20 µM) and etoposide (0–0.2 µM) for 48 h. Etoposide was used as a positive control as it is a topo II poison and established human leukemogen that has previously been shown to induce END in Chinese hamster ovary cells. Both HQ and etoposide significantly induced END in a dose-dependent manner (P_trend < 0.0001 and P_trend = 0.0003, respectively). Since END may underlie the acquisition of high chromosome numbers by tumour cells, it may play a role in inducing genomic instability and subsequent carcinogenesis from HQ and etoposide. In order to further explore the cytogenetic effects of HQ and etoposide, we also examined specific structural changes. HQ did not induce translocations of chromosome 11 [t(11;?)] but significantly induced translocations of chromosome 21 [t(21;?)] and structural chromosome aberrations (SCA) (P_trend = 0.0415 and P_trend < 0.0001, respectively). Etoposide potently induced all these structural changes (P_trend < 0.0001). The lack of an effect of HQ on t(11;?) and the reduced ability of HQ to induce t(21;?) and SCA, compared with etoposide, further suggests that HQ acts primarily as a topo II catalytic inhibitor rather than as a topo II poison in intact human cells.

Systematic random sampling of the comet assay

McArt, D. G., Wasson, G. R., McKerr, G., Saetzler, K., Reed, M., Howard, C. V. — 2009-06-25

The comet assay is a technique used to quantify DNA damage and repair at a cellular level. In the assay, cells are embedded in agarose and the cellular content is stripped away leaving only the DNA trapped in an agarose cavity which can then be electrophoresed. The damaged DNA can enter the agarose and migrate while the undamaged DNA cannot and is retained. DNA damage is measured as the proportion of the migratory ‘tail’ DNA compared to the total DNA in the cell. The fundamental basis of these arbitrary values is obtained in the comet acquisition phase using fluorescence microscopy with a stoichiometric stain in tandem with image analysis software. Current methods deployed in such an acquisition are expected to be both objectively and randomly obtained. In this paper we examine the ‘randomness’ of the acquisition phase and suggest an alternative method that offers both objective and unbiased comet selection. In order to achieve this, we have adopted a survey sampling approach widely used in stereology, which offers a method of systematic random sampling (SRS). This is desirable as it offers an impartial and reproducible method of comet analysis that can be used both manually or automated. By making use of an unbiased sampling frame and using microscope verniers, we are able to increase the precision of estimates of DNA damage. Results obtained from a multiple-user pooled variation experiment showed that the SRS technique attained a lower variability than that of the traditional approach. The analysis of a single user with repetition experiment showed greater individual variances while not being detrimental to overall averages. This would suggest that the SRS method offers a better reflection of DNA damage for a given slide and also offers better user reproducibility.

In vitro comet assay for DNA repair: a warning concerning application to cultured cells

Azqueta, A., Lorenzo, Y., Collins, A. R. — 2009-06-25

The comet assay (single-cell gel electrophoresis) is a sensitive and simple method for measuring DNA damage. An early modification of the assay involved the application of specific repair endonucleases to convert lesions to breaks; thus, for example, endonuclease III was used to measure oxidized pyrimidines. This concept has now been extended to produce an in vitro assay for DNA repair activity in a cell-free extract, for example from lymphocytes. The extract is incubated with substrate DNA containing specific base damage, and repair incision is detected as breaks in this DNA. We have recently been studying effects of phytochemicals in cultured cells, whether as antioxidants or as potential modulators of DNA repair. We realized that there is a need to check that observed effects that appear as an enhancement of repair (i.e. increased breaks in substrate DNA) are not simply due to a direct damaging effect of the phytochemical or to induction of non-specific nucleases. Here, we describe a rigorous approach to testing for this possibility, which we recommend to anyone carrying out similar experiments.

Transcription-associated recombination in eukaryotes: link between transcription, replication and recombination

Gottipati, P., Helleday, T. — 2009-04-29

Homologous recombination (HR) is an important DNA repair pathway and is essential for cellular survival. It plays a major role in repairing replication-associated lesions and is functionally connected to replication. Transcription is another cellular process, which has emerged to have a connection with HR. Transcription enhances HR, which is a ubiquitous phenomenon referred to as transcription-associated recombination (TAR). Recent evidence suggests that TAR plays a role in inducing genetic instability, for example in the THO mutants (Tho2, Hpr1, Mft1 and Thp2) in yeast or during the development of the immune system leading to genetic diversity in mammals. On the other hand, evidence also suggests that TAR may play a role in preventing genetic instability in many different ways, one of which is by rescuing replication during transcription. Hence, TAR is a double-edged sword and plays a role in both preventing and inducing genetic instability. In spite of the interesting nature of TAR, the mechanism behind TAR has remained elusive. Recent advances in the area, however, suggest a link between TAR and replication and show specific genetic requirements for TAR that differ from regular HR. In this review, we aim to present the available evidence for TAR in both lower and higher eukaryotes and discuss its possible mechanisms, with emphasis on its connection with replication.

Overexpression of glutamate-cysteine ligase protects human COV434 granulosa tumour cells against oxidative and {gamma}-radiation-induced cell death

Cortes-Wanstreet, M. M., Giedzinski, E., Limoli, C. L., Luderer, U. — 2009-04-29

Ionizing radiation is toxic to ovarian follicles and can cause infertility. Generation of reactive oxygen species (ROS) has been implicated in the toxicity of ionizing radiation in several cell types. We have shown that depletion of the antioxidant glutathione (GSH) sensitizes follicles and granulosa cells to toxicant-induced apoptosis and that supplementation of GSH is protective. The rate-limiting reaction in GSH biosynthesis is catalysed by glutamate–cysteine ligase (GCL), which consists of a catalytic subunit (GCLC) and a regulatory subunit (GCLM). We hypothesized that overexpression of Gclc or Gclm to increase GSH synthesis would protect granulosa cells against oxidant- and radiation-induced cell death. The COV434 line of human granulosa tumour cells was stably transfected with vectors designed for the constitutive expression of Gclc, Gclm, both Gclc and Gclm or empty vector. GCL protein and enzymatic activity and total GSH levels were significantly increased in the GCL subunit-transfected cells. GCL-transfected cells were resistant to cell killing by treatment with hydrogen peroxide compared to control cells. Cell viability declined less in all the GCL subunit-transfected cell lines 1–8 h after 0.5 mM hydrogen peroxide treatment than in control cells. We next examined the effects of GCL overexpression on responses to ionizing radiation. ROS were measured using a redox-sensitive fluorogenic dye in cells irradiated with 0, 1 or 5 Gy of -rays. There was a dose-dependent increase in ROS within 30 min in all cell lines, an effect that was significantly attenuated in Gcl-transfected cells. Apoptosis, assessed by terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labelling and activated caspase-3 immunoblotting, was significantly decreased in irradiated Gclc-transfected cells compared to irradiated control cells. Suppression of GSH synthesis in Gclc-transfected cells reversed resistance to radiation. These findings show that overexpression of GCL in granulosa cells can augment GSH synthesis and ameliorate various sequelae associated with exposure to oxidative stress and irradiation.

The effect of selenium, as selenomethionine, on genome stability and cytotoxicity in human lymphocytes measured using the cytokinesis-block micronucleus cytome assay

Wu, J., Lyons, G. H., Graham, R. D., Fenech, M. F. — 2009-04-29

A supranutritional intake of selenium (Se) may be required for cancer prevention, but an excessively high dose could be toxic. Therefore, the effect on genome stability of seleno-L-methionine (Se-met), the most important dietary form of Se, was measured to determine its bioefficacy and safety limit. Peripheral blood lymphocytes were isolated from six volunteers and cultured with medium supplemented with Se-met in a series of Se concentrations (3, 31, 125, 430, 1880 and 3850 µg Se/litre) while keeping the total methionine (i.e. Se-met + L-methionine) concentration constant at 50 µM. Baseline genome stability of lymphocytes and the extent of DNA damage induced by 1.5-Gy -ray were investigated using the cytokinesis-block micronucleus cytome assay after 9 days of culture in 96-microwell plates. High Se concentrations (≥1880 µg Se/litre) caused strong inhibition of cell division and increased cell death (P < 0.0001). Baseline frequency of nucleoplasmic bridges and nuclear buds, however, declined significantly (P trend < 0.05) as Se concentration increased from 3 to 430 µg Se/litre. Se concentration (≤430 µg Se/litre) had no significant effect on baseline frequency of micronuclei and had no protective effect against genome damage induced by exposure to 1.5-Gy -ray irradiation. In conclusion, Se, as Se-met, may improve genome stability at concentrations up to 430 µg Se/litre, but higher doses may be cytotoxic. Therefore, a cautious approach to supplementation with Se-met is required to ensure that optimal genome health is achieved without cytotoxic effects.

Radioprotective effects of hesperidin against genotoxicity induced by {gamma}-irradiation in human lymphocytes

2009-04-29

The radioprotective effect of hesperidin against genotoxicity induced by -irradiation has been investigated in vivo/in vitro in cultured blood lymphocytes from human volunteers. Peripheral blood samples were collected at 0 (10 min before) and at 1, 2 and 3 h after a single oral ingestion of 250 mg hesperidin. At each time point, the whole blood was exposed in vitro to 150 cGy of ⁶⁰Co -irradiation and then the lymphocytes were cultured with mitogenic stimulation to determine the micronuclei in cytokinesis-blocked binucleated cells. For each volunteer, the results showed a significant increase in the incidence of micronuclei after exposure of cells to -irradiation as compared to control samples. The lymphocytes in the blood samples collected at 1 h after hesperidin ingestion and exposed in vitro to -rays exhibited a significant decrease in the incidence of micronuclei, compared with similarly irradiated lymphocytes from blood samples collected at 0 h. The maximum protection and decrease in frequency of micronuclei (33%) was observed at 1 h after ingestion of hesperidin. These data have important application for the protection of human lymphocytes from the genetic damage and side effects induced by -irradiation in patients undergoing radiotherapy.

DNA demethylation protects from cleavable complex stabilization and DNA strand breakage induced by the topoisomerase type I inhibitor camptothecin

Orta, M. L., Mateos, S., Cortes, F. — 2009-04-29

Methylation of cytosine in CpG sequences of the DNA in mammalian cells is an epigenetic feature regulated very exactly that bears importance for events like gene expression, DNA replication, transcription and genetic imprinting. Changes in the DNA methylation pattern, both hypermethylation and hypomethylation, have been observed in the carcinogenic process. These changes, in general, influence the DNA conformation in such a way that certain proteins are disturbed in their interactions with the molecule. In this paper, we investigated in cultured Chinese hamster ovary cells the influence of hypomethylation induced by the substitution of 5-aza-2'-deoxycytidine for cytidine in DNA on topoisomerase type I (topo I) function, measured as the capacity of the enzyme inhibitor camptothecin (CPT) to stabilize the topoisomerase–DNA complexes and to induce DNA strand breakage. Our results demonstrate that the degree of methylation in DNA correlates with the effectiveness of CPT to stabilize the topo I–DNA complexes and to induce DNA cleavage. A protective effect of hypomethylation, as a whole, has been observed.

In vivo genotoxicity assessment of aluminium oxide nanomaterials in rat peripheral blood cells using the comet assay and micronucleus test

2009-04-29

Advances in nanotechnology and its usage in various fields have led to the exposure of humans to engineered nanomaterials (NMs) and there is a need to tackle the potential human health effects before these materials are fully exploited. The main purpose of the current study was to assess whether aluminium oxide NMs (Al₂O₃-30 nm and Al₂O₃-40 nm) could cause potential genotoxic effects in vivo. Characterization of Al₂O₃-30 nm and Al₂O₃-40 nm was done with transmission electron microscopy, dynamic light scattering and laser Doppler velocimetry prior to their use in this study. The genotoxicity end points considered in this study were the frequency of micronuclei (MN) and the percentage of tail DNA (% Tail DNA) migration in rat peripheral blood cells using the micronucleus test (MNT) and the comet assay, respectively. Genotoxic effects were evaluated in groups of female Wistar rats (five per group) after single doses of 500, 1000 and 2000 mg/kg body weight (bw) of Al₂O₃-30 nm, Al₂O₃-40 nm and Al₂O₃-bulk. Al₂O₃-30 nm and Al₂O₃-40 nm showed a statistically significant dose-related increase in % Tail DNA for Al₂O₃-30 nm and Al₂O₃-40 nm (P < 0.05). However, Al₂O₃-bulk did not induce statistically significant changes over control values. The MNT also revealed a statistically significant (P < 0.05) dose-dependent increase in the frequency of MN, whereas Al₂O₃-bulk did not show any significant increase in frequency of MN compared to control. Cyclophosphamide (40 mg/kg bw) used as a positive control showed statistically significant (P < 0.001) increase in % Tail DNA and frequency of MN. The biodistribution of Al₂O₃-30 nm and Al₂O₃-40 nm and Al₂O₃-bulk in different rat tissues, urine and feces was also studied 14 days after treatment using inductively coupled plasma mass spectrometry. The data indicated that tissue distribution of Al₂O₃ was size dependent. Our findings suggest that Al₂O₃ NMs were able to cause size- and dose-dependent genotoxicity in vivo compared to Al₂O₃-bulk and control groups.

Removal of red light minimizes methylene blue-stimulated DNA damage in oesophageal cells: implications for chromoendoscopy

Sturmey, R. G., Wild, C. P., Hardie, L. J. — 2009-04-29

Barrett's oesophagus (BO) carries an increased risk of progression to oesophageal adenocarcinoma. Chromoendoscopy with methylene blue (MB) can be used to facilitate identification of BO and target areas for biopsy. If photoexcited, MB can generate reactive oxygen species and genotoxic photodegradation products leading to DNA damage. We have previously demonstrated that levels of DNA damage are increased in BO following MB chromoendoscopy. The aim of this study was to investigate whether DNA damage, as measured by the comet assay, can be minimized during chromoendoscopy by varying MB concentration and light wavelength using an in vitro model. OE33 cells were treated with MB (0.015–15 mM) and exposed to white light (WL). Cells were also illuminated with WL fractions (580–700, 480–580, 350–480, <575, <610 and <688 nm) in the presence of MB. At clinically relevant concentrations, WL illumination of MB (15 mM) caused significant DNA damage in vitro (P < 0.001). Illumination of MB with red light (580–700 nm) also stimulated high levels of DNA damage in OE33 cells (P < 0.001). This effect was not observed with green or blue light. Filtering WL to remove red light wavelengths (>575 nm) reduced DNA damage and apoptosis to control levels in MB-treated cells. In addition, reducing the concentration of MB 10-fold markedly reduced the DNA-damaging effect of MB in vitro. The results show that photoactivation of MB by red light is responsible for the majority of DNA damage. Simple modifications to MB chromoendoscopy, such as filtering out red light from endoscopic WL or reducing MB concentration, are likely to limit DNA damage induced by the procedure.

Multiple factors conferring high radioresistance in insect Sf9 cells

Cheng, I-C., Lee, H.-J., Wang, T. C. — 2009-04-29

Sf9, a lepidopteran cell line isolated from the fall armyworm, Spodoptera frugiperda, was shown to be significantly more resistant to growth inhibition and apoptosis induction effects of x-ray irradiation than several human cell lines of different origins. The single-cell electrophoresis technique revealed that Sf9 cells showed lower x-ray irradiation-induced DNA damage as well as better efficiency at repairing these damages. In addition, Sf9 cells were lower in both background and x-ray irradiation-induced intracellular oxidative stress, in which the higher intracellular level of reduced glutathione seemed to play a major role. The significance of oxidative stress in determining the radioresistance of Sf9 cells was confirmed by their being more resistant to hydrogen peroxide while equally susceptible to other non-reactive oxygen species of N-nitroso alkylating agents when compared with a human cell line. Although the Sf9 and human cell lines were equally susceptible to the lethal effects of N-nitroso alkylating agents, the components of DNA damage-induced and the repair enzymes involved significantly differ. This phenomenon is also discussed in this report.

High protein-high red meat versus high carbohydrate weight loss diets do not differ in effect on genome stability and cell death in lymphocytes of overweight men

Benassi-Evans, B., Clifton, P. M., Noakes, M., Keogh, J. B., Fenech, M. — 2009-04-29

The importance of diet in DNA damage prevention is well established; however, the comparison of weight loss diets with different micronutrient and macronutrient profiles on genome stability in peripheral blood lymphocytes (PBLs) has not been studied. This study tested the hypothesis that genome stability in PBLs of overweight men who consume a high protein–high red meat (HP) weight loss diet is different from that of overweight men who consume a high carbohydrate (HC) weight loss diet. Thirty-three male subjects were randomly assigned to an HP or HC isocaloric energy-restricted dietary intervention for 12 weeks intensive weight loss and weight maintenance up to 52 weeks. Blood samples were collected at 0, 12 and 52 weeks. DNA damage in PBLs was assessed using the cytokinesis-block micronucleus cytome (CBMN-Cyt) assay. Average weight loss after 12 weeks was 9.3 ± 0.7 kg for both diets, with no further change at 52 weeks. Two-way analysis of variance showed no time or diet effect on micronucleus frequency (chromosome loss/breaks). There was a significant trend with time (P = 0.03) but not diet, for reduction of nuclear buds (gene amplification). There was a positive trend with time for increased nucleoplasmic bridges (chromosome rearrangement) (P = 0.051). Necrosis and apoptosis both significantly decreased with time (P = 0.037 and P = 0.007, respectively) with no diet effect. There was no significant effect of time or diet for nuclear division index, a biomarker of immune response. The results suggest that the effect of the HP weight loss diet on DNA damage measured using the CBMN-Cyt assay in PBLs was not different from that observed for the HC weight loss diet.

Genotoxicity of 3,6-dinitrobenzo[e]pyrene, a novel mutagen in ambient air and surface soil, in mammalian cells in vitro and in vivo

2009-04-29

3,6-Dinitrobenzo[e]pyrene (3,6-DNBeP), newly identified in airborne particles and surface soil, is a potent mutagen in Salmonella typhimurium. The present study investigated the genotoxic potency of 3,6-DNBeP in vitro and in vivo using mammalian cell strains (Chinese hamster CHL/IU and human HepG2) and ICR mice, respectively. In the hprt gene mutation assay using HepG2 cells, the spontaneous mutant frequency was 61.1 per 10⁵ clonable cells, which increased to 229 per 10⁵ clonable cells after treatment with 1.0 µg/ml (3 µM) 3,6-DNBeP. Notably, in HepG2 cells with increased N-acetyltransferase 2 activity, the mutant frequency increased to 648 per 10⁵ clonable cells by treatment of 1.0 µg/ml (3 µM) 3,6-DNBeP. The sister chromatid exchange frequency increased approximately three times the control level in HepG2 cells treated with 3,6-DNBeP at a concentration of 1.0 µg/ml (3 µM). In HepG2 and CHL/IU cells, the frequency of the cells with micronuclei was 0.9 and 1.2%, and the frequencies increased to 2.3 and 7.6% after 1.0 µg/ml (3 µM) 3,6-DNBeP-treatment, respectively. The H2AX phosphorylation level increased 8-fold compared with the background level with 1.0 µg/ml (3 µM) 3,6-DNBeP-treatment in HepG2 cells. Moreover, the comet assay showed that 3,6-DNBeP produced DNA damage in the cells of liver, kidney, lung and bone marrow in ICR mice 3 h after intraperitoneal injection at 40 mg/kg (0.12 mmol/kg) body weight. These data indicate that 3,6-DNBeP is genotoxic to mammalian cells in vitro and in vivo.

Environmental exposure measurement in cancer epidemiology

Wild, C. P. — 2009-02-26

Environmental exposures, used in the broadest sense of lifestyle, infections, radiation, natural and man-made chemicals and occupation, are a major cause of human cancer. However, the precise contribution of specific risk factors and their interaction, both with each other and with genotype, continues to be difficult to elucidate. This is partially due to limitations in accurately measuring exposure with the subsequent risk of misclassification. One of the primary challenges of molecular cancer epidemiology therefore is to improve exposure assessment. Progress has been made with biomarkers such as carcinogens and their metabolites, DNA and protein adducts and mutations measured in various tissues and body fluids. Nevertheless, much remains to be accomplished in order to establish aetiology and provide the evidence base for public health decisions. This review considers some of the principles behind the application of exposure biomarkers in cancer epidemiology. It also demonstrates how the same biomarkers can contribute both to establishing the biological plausibility of associations between exposure and disease and be valuable endpoints in intervention studies. The potential of new technologies such as transcriptomics, proteomics and metabonomics to provide a step change in environmental exposure assessment is discussed. An increasing recognition of the role of epigenetic changes in carcinogenesis presents a fresh challenge as alterations in DNA methylation, histone modification and microRNA in response to environmental exposures demand a new generation of exposure biomarker. The overall importance of this area of research is brought into sharp relief by the large prospective cohort studies (e.g. UK Biobank) which need accurate exposure measurement in order to shed light on the complex gene:environment interactions underlying common chronic disorders including cancer. It is suggested that a concerted effort is now required, with appropriate funding, to develop and validate the required exposure assessment methodology before these cohorts come to maturity.

Dietary intake of meat and meat-derived heterocyclic aromatic amines and their correlation with DNA adducts in female breast tissue

Rohrmann, S., Lukas Jung, S.-U., Linseisen, J., Pfau, W. — 2009-02-26

It was the aim of this study to examine the association of the consumption of meat in general, meat prepared by different cooking methods and the dietary intake of heterocyclic aromatic amines (HCA) with the level of DNA adducts in the breast tissue of women undergoing reduction mammoplasty. Dietary intake of meat and HCA were assessed via questionnaire in 44 women undergoing reduction mammoplasty. DNA adduct analysis in breast tissue was performed by ³²P-postlabelling analysis. Spearman rank correlation coefficients (r) were calculated to examine the association of meat consumption and dietary HCA intake with tissue DNA adduct levels. A median DNA adduct level of 18.45 (interquartile range 12.81–25.65) per 10⁹ nucleotides in breast tissue was observed; median HCA intake was 40.43 ng/day (interquartile range 19.55–102.33 ng/day). Total HCA intake (r = 0.33, P = 0.03), consumption of fried meat (r = 0.39, P = 0.01), beef (r = 0.32, P = 0.03) and processed meat (r = 0.51, P = 0.0004) were statistically significantly correlated with the level of DNA adducts in breast tissue. The detected DNA adducts could not be confirmed to be specific HCA-derived DNA adducts by comparison with external standards, using the ³²P-postlabelling assay. We observed strong correlations of dietary HCA intake and consumption of fried and processed meat with DNA adduct levels in breast tissue of 44 women. Since the detected DNA adducts were not necessarily specific only for HCA, it is possible that HCA intake is a surrogate of other genotoxic substances, such as polycyclic aromatic hydrocarbons, in meat prepared at high temperatures.

Interference of cell cycle progression by zidovudine and lamivudine in NIH 3T3 cells

Fang, J.-L., McGarrity, L. J., Beland, F. A. — 2009-02-26

Zidovudine (3'-azido-3'-deoxythymidine; AZT) and lamivudine [(–)2',3'-dideoxy-3'-thiacytidine; 3TC] are nucleoside reverse transcriptase inhibitors used to treat and prevent human immunodeficiency virus-1 infections. In short-term incubations (<48 h), AZT, but not 3TC, has been shown to interfere with cell cycle progression. In the present study, we examined if these alterations persist during long-term incubations in which cells were exposed to AZT (0–1000 µM) or 3TC (0–500 µM) in continuous culture for up to 5 weeks. In addition, we investigated the reversibility of these effects upon removal of the drugs. Both drugs caused concentration- and time-dependent decreases in the number of viable cells, with the effect being more pronounced with AZT. There was only a slight increase in the number of viable cells treated with AZT for 5 weeks and then allowed a 1-week recovery period; cell viability in cells treated with 3TC returned to control levels during the recovery period. The decrease in viable cells was not due to apoptotic or necrotic cell death, but rather was associated with S and G2/M phase cell cycle arrest. Western blot analysis indicated that AZT treatment caused a decrease in checkpoint kinase 1 (Chk1) and checkpoint kinase 2 (Chk2) at all time points. Cyclin-dependent kinase 1 was decreased at later time points, while cyclin A was increased at early times. These data indicate that AZT and, to a lesser extent, 3TC interfere with cell growth by slowing cell cycle progression and that checkpoint proteins Chk1 and Chk2 may play an important role in this delay.

Occupational risk assessment of genotoxicity and oxidative stress in workers handling anti-neoplastic drugs during a working week

Rombaldi, F., Cassini, C., Salvador, M., Saffi, J., Erdtmann, B. — 2009-02-26

Twenty pharmacists and nurses handling anti-neoplastic drugs in a hospital were monitored during a working week, from Monday to Friday, in the morning (only on Monday) and afternoon (all days). Genotoxicity was analysed by the comet assay and the micronucleus (MN) test, while oxidative stress was analysed in serum by thiobarbituric acid reactive substances (TBARS) and by measurements of the antioxidant enzymes superoxide dismutase (Sod) and catalase (Cat). The exposed workers presented increased DNA damage levels by the comet assay as compared to the controls. The comet assay results have also shown significant positive correlation with the day of the week and with alcohol consumption. MN frequency was significantly higher in the exposed workers and presented noteworthy correlation with age and working time. In the oxidative stress parameters, only Cat presented a significant increase in the exposed group, considering all the samplings. However, TBARS data showed interesting results, considering the different sampling times; the exposed group presented a significant correlation with the working days and significantly higher results on Friday as compared to the controls and Monday morning. Monitoring occupational risk during a longer time, e.g. during a working week as done in this study, introduces additional aspects of risk behaviour, which can improve risk management. This study demonstrates the usefulness of evaluating oxidative stress also in genotoxic risk assessment since both events often result from the same factors.

In vivo mutagenicity of conazole fungicides correlates with tumorigenicity

Ross, J. A., Moore, T., Leavitt, S. A. — 2009-02-26

Triadimefon, propiconazole and myclobutanil are conazoles, an important class of agricultural and therapeutic fungicides. Triadimefon and propiconazole are mouse liver tumorigens, while myclobutanil is not. All three conazoles are generally inactive in short-term genotoxicity tests. We studied the in vivo mutagenicity of these three conazoles using the Big Blue® mouse assay system. Groups of mice were fed either control diet or diet containing 1800 p.p.m. triadimefon, 2500 p.p.m. propiconazole or 2000 p.p.m. myclobutanil. After 4 days of feeding, mice were immediately euthanized, livers were removed, DNA isolated and lacI genes recovered into infectious bacteriophage lambda particles by in vitro packaging. Bacteriophage with mutations in the lacI gene was detected by infecting into Escherichia coli, and mutant frequencies were determined using a colorimetric plaque assay. Propiconazole induced a 1.97-fold increase in mutant frequency compared to concurrent controls (P = 0.018) and triadimefon induced a 1.94-fold increase compared to concurrent controls (P = 0.009). Myclobutanil did not induce any change in mutant frequency (P = 0.548). These results provide the first evidence that the hepatotumorigenic conazoles are capable of inducing mutations in liver in vivo while the non-tumorigen myclobutanil is not, suggesting that mutagenicity may represent a key event in conazoles tumorigenic mode of action.

Cytogenetic and molecular analysis of MLL rearrangements in acute lymphoblastic leukaemia survivors

2009-02-26

The successful treatment of paediatric malignancies by multimodal therapy has improved outcomes for children with cancer, especially those with acute lymphoblastic leukaemia (ALL). Second malignant neoplasms, however, represent a serious complication after treatment. Depending on dosage, 2–12% of patients treated with topoisomerase II inhibitors and/or alkylating agents develop treatment-related acute myeloid leukaemia characterized by translocations at 11q23. Our goal was to study MLL rearrangements in peripheral lymphocytes using cytogenetic and molecular methods in order to evaluate the late effects of cancer therapy in patients previously treated for childhood ALL. Chromosomal rearrangements at 11q23 were analysed in cytogenetic preparations from 49 long-term ALL survivors and 49 control individuals. Patients were subdivided depending on the inclusion or omission of topoisomerase II inhibitors (VP-16 and/or VM-26) in their treatment protocol. The statistical analysis showed significant (P = 0.007) differences between the frequency of translocations observed for the groups of patients and controls. These differences were also significant (P = 0.006) when the groups of patients (independent of the inclusion of topoisomerase II inhibitors) and controls were compared (P = 0.006). The frequencies of extra signals, however, did not differ between groups of patients and controls. Several MLL translocations were detected and identified by inverse polymerase chain reaction, followed by cloning and sequencing. Thirty-five patients (81%) presented putative translocations; among those, 91% corresponded with t(4;11) (q21;q23), while the other 9% corresponded with t(11;X), t(8;11)(q23;q23) and t(11;16). Our results indicate an increase in MLL aberrations in childhood ALL survivors years after completion of therapy. The higher frequency in this cohort might be associated with therapy using anti-tumoural drugs, independent of the inclusion of topoisomerase II inhibitors. Even though the biological significance of these rearrangements needs further investigation, they demonstrate a degree of genome instability, indicating the relevance of cytogenetic and molecular studies during the follow-up of patients in complete clinical remission.

Histone H2AX phosphorylation in response to changes in chromatin structure induced by altered osmolarity

2009-02-26

DNA strand breaks trigger marked phosphorylation of histone H2AX (i.e. -H2AX). While DNA double-strand breaks (DSBs) provide a strong stimulus for this event, the accompanying structural alterations in chromatin may represent the actual signal that elicits -H2AX. Our data show that changes in chromatin structure are sufficient to elicit extensive -H2AX formation in the relative absence of DNA strand breaks. Cells subjected to hypotonic (0.05 M) treatment exhibit -H2AX levels that are equivalent to those found after the induction of 80–200 DNA DSBs (i.e. 2–5 Gy). Despite this significant increase in phosphorylation, cell survival remains relatively unaffected (<10% cytotoxicity), and there is no significant increase in apoptosis. Nuclear staining profiles indicate that -H2AX-positive cells induced under altered tonicity exhibit variable levels of staining, ranging from uniform pan staining to discrete punctate foci more characteristic of DNA strand breakage. The capability to induce significant -H2AX formation under altered tonicity in the relative absence of DNA strand breaks suggests that this histone modification evolved in response to changes in chromatin structure.

In vivo genotoxicity of dental bonding agents

2009-02-26

This in vivo study investigated the genotoxicity of two dental bonding agents: Adper Single Bond Plus and Prime&Bond 2.1. The somatic mutation and recombination test (SMART) in Drosophila melanogaster was applied to analyse their genotoxicity expressed as homologous mitotic recombination, as well as point and chromosomal mutation. SMART detects the loss of heterozygosity of marker genes expressed phenotypically on the fly's wings. This fruit fly has extensive genetic homology to mammals, which makes it a suitable model organism for genotoxic investigations. Adper Single Bond Plus induced statistically significant increases in the frequency of total spots at the highest concentration tested, while Prime&Bond 2.1 was positive at all concentrations tested. The mechanistic basis underlying the genotoxicity of Adper Single Bond Plus relies on mitotic recombination alone, and was different from that of Prime&Bond 2.1, which showed evidence of the contribution of both recombination and mutational events. These findings indicate that both adhesives are inducers of toxic–genetic events, with the mitotic recombination being the main mechanism of action. The clinical significance of these observations has to be interpreted with data obtained in other bioassays.

Optimization of the comet assay for the sensitive detection of PUVA-induced DNA interstrand cross-links

Wu, J. H., Wilson, J. B., Wolfreys, A. M., Scott, A., Jones, N. J. — 2009-02-26

Psoralen plus ultraviolet A (PUVA), commonly used for the treatment of hyperproliferative skin disorders, has been found to be associated with an increased risk of squamous cell cancer. Interstrand cross-link (ICL) formation by PUVA treatment is considered the major factor contributing to the carcinogenesis. However, it remains unclear how PUVA causes, or promotes cancers, in humans. As an initial step in understanding the mechanisms of mutagenesis and carcinogenesis of PUVA photochemotherapy, we have optimized and subsequently utilized a modified alkaline comet assay involving a post-lysis -irradiation at 9 Gy to sensitively measure the formation and repair of PUVA-induced ICLs in the immortalized human keratinocyte cell line HaCaT. A clear dose-dependent response of HaCaT cells to PUVA exposure was observed with a combination of a fixed UVA dose at 0.05 J/cm² and a dose of 8-methoxypsoralen ranging from 10 to 100 µM. Results also indicated that the ICL repair was concentration dependent. We have also demonstrated that PUVA-induced monoadduct formation, at an estimated ratio of 3:1 to ICLs in the present experimental conditions, does not interfere with the detection of the ICLs in the modified alkaline comet assay. Furthermore, comparison of the amount of ICL formation between the single-dose UVA treatment and a split-dose protocol was performed. The split-dose protocol was believed to generate more ICLs than the single-dose treatment, thus more effective in PUVA photochemotherapy. Our results demonstrate that comparable amounts of ICLs were formed in HaCaT cells for each dose of UVA, using either the split-dose or single-dose protocols.

Accelerated 32P-HPLC for bulky DNA adducts

Nagy, E., Cornelius, M. G., Moller, L. — 2009-02-26

Many compounds can react with DNA forming covalent modifications, so-called DNA adducts, which can influence crucial biological processes. DNA adducts from various DNA-damaging agents can act both as biomarkers and as a measurement of the actual damage of the genome. Therefore, they are of value in determining exposed or sensitive individuals or populations and in identifying agents that can induce DNA damage. A common method to measure DNA adducts is through DNA hydrolysis, adduct enrichment, ³²P-post-labelling and chromatographic separation. High-performance liquid chromatography (HPLC) with online radioactivity detection, the ³²P-HPLC (direct injection of the ³²P-labelled mixture into the HPLC with online ³²P-detection) method, gives both high sensitivity and good resolution of complex mixtures of DNA adducts. One limitation with this method is the capacity when dealing with large numbers of samples. The aim of this study was therefore to increase the analytical capacity by reducing analysis time of the ³²P-HPLC method. A change of HPLC columns to low backpressure columns and adaptation of elution conditions enabled a reduction in time per analysis from 100 to 20 min. This did not affect sensitivity but lowered chromatographic resolution, although bulky DNA adducts were still well resolved from other DNA components. This is useful when the total amount of DNA adducts is the primary interest. When high resolution is required, this can be achieved by gradient modifications, which increase the time required per analysis to 30 min. The accelerated ³²P-HPLC increases the capacity in number of samples 3- to 5-fold, depending on resolution requirements, without any negative effect on sensitivity in both in vitro and in vivo samples.

A direct view by immunofluorescent comet assay (IFCA) of DNA damage induced by nicking and cutting enzymes, ionizing 137Cs radiation, UV-A laser microbeam irradiation and the radiomimetic drug bleomycin

Grigaravicius, P., Rapp, A., Greulich, K. O. — 2009-02-26

In DNA repair research, DNA damage is induced by different agents, depending on the technical facilities of the investigating researchers. A quantitative comparison of different investigations is therefore often difficult. By using a modified variant of the neutral comet assay, where the histone H1 is detected by immunofluorescence [immunofluorescent comet assay (IFCA)], we achieve previously unprecedented resolution in the detection of fragmented chromatin and show that trillions of ultraviolet A photons (of a few eV), billions of bleomycin (BLM) molecules and thousands of quanta (of 662 keV) generate, in first order, similar damage in the chromatin of HeLa cells. A somewhat more detailed inspection shows that the damage caused by 20 Gy ionizing radiation and by a single laser pulse of 10 µJ are comparable, while the damage caused by 12 µg/ml BLM depends highly on the individual cell. Taken together, this work provides a detailed view of DNA fragmentation induced by different treatments and allows comparing them to some extent, especially with respect to the neutral comet assay.

Report on the buccal micronucleus assay workshop organized by the International Human Micronucleus (HUMN) project--Antalya, Turkey 2007

Fenech, M., Holland, N., Knasmueller, S., Burgaz, S., Bonassi, S. — 2009-02-26

This report describes the aims, discussion and outcomes of the first Human Micronucleus collaborative project workshop on the buccal micronucleus assay. It was agreed at the workshop that three activities should be initiated as soon as possible namely (i) a method for collection of databases, (ii) writing of a protocol based on the most commonly used and best validated procedures and (iii) an inter-laboratory slide-scoring exercise in this order. A follow-up workshop is planned at the 10th International Conference on Environmental Mutagens in Florence in 2009.