Recommendations and quality criteria for micronucleus studies with humans

Micronucleus (MN) analyses in peripheral blood lymphocytes and exfoliated cells from different organs (mouth, nose, bladder and cervix) are at present the most widely used approaches to detect damage of genetic material in humans. MN are extranuclear DNA-containing bodies, which can be identified microscopically. They reflect structural and numerical chromosomal aberrations and are formed as a consequence of exposure to occupational, environmental and lifestyle genotoxins. They are also induced as a consequence of inadequate intake of certain trace elements and vitamins. High MN rates are associated with increased risk of cancer and a range of non-cancer diseases in humans. Furthermore, evidence is accumulating that measurements of MN could be a useful tool for the diagnosis and prognosis of different forms of cancer and other diseases (inflammation, infections, metabolic disorders) and for the assessment of the therapeutic success of medical treatments. Recent reviews of the current state of knowledge suggest that many clinical studies have methodological shortcomings. This could lead to controversial findings and limits their usefulness in defining the impact of exposure concentrations of hazardous chemicals, for the judgment of remediation strategies, for the diagnosis of diseases and for the identification of protective or harmful dietary constituents. This article describes important quality criteria for human MN studies and contains recommendations for acceptable study designs. Important parameters that need more attention include sufficiently large group sizes, adequate duration of intervention studies, the exclusion of confounding factors which may affect the results (sex, age, body mass index, nutrition, etc.), the evaluation of appropriate cell numbers per sample according to established scoring criteria as well as the use of proper stains and adequate statistical analyses.

High Yields of DNA-Transfer into Mouse L-Cells by Electropermeabilization

DNA transfection in mouse L-cells was performed by means of the electropermeabilization technique (U. Zimmermann, G. Pilwat, and F. Riemann, Z. Naturforsch. 29, 304 (1974)). The plasmid pSV 2-neo used leads to neomycin-resistance in stably transfected L-cells. Optimized conditions resulted in high yields of clones at relatively low DNA concentration. The influence of temperature during pulse application and during the subsequent resealing process as well as the field parameters and medium com­position are discussed.

Terahertz radiation induces spindle disturbances in human-hamster hybrid cells

The aim of this study was to investigate and quantify the production of spindle disturbances in A(L) cells, a human-hamster hybrid cell line, by 0.106 THz radiation (continuous wave). Monolayer cultures in petri dishes were exposed for 0.5 h to 0.106 THz radiation with power densities ranging from 0.043 mW/cm(2) to 4.3 mW/cm(2) or were kept under sham conditions (negative control) for the same period. As a positive control, 100 µg/ml of the insecticide trichlorfon, which is an aneuploidy-inducing agent, was used for an exposure period of 6 h. During exposure, the sample containers were kept at defined environmental conditions in a modified incubator as required by the cells. Based on a total of 6,365 analyzed mitotic cells, the results of two replicate experiments suggest that 0.106 THz radiation is a spindle-acting agent as predominately indicated by the appearance of spindle disturbances at the anaphase and telophase (especially lagging and non-disjunction of single chromosomes) of cell divisions. The findings in the present study do not necessarily imply disease or injury but may be important for evaluating possible underlying mechanisms.

Aldosterone causes DNA strand breaks and chromosomal damage in renal cells, which are prevented by mineralocorticoid receptor antagonists

Epidemiological studies exploring the connection between hypertension and cancer incidence find a higher cancer mortality in hypertensive patients, particularly elevated in hypertension associated with a stimulation of the renin-angiotensin-aldosterone system. Primary aldosteronism, with plasma aldosterone levels between 0.5 and 1 nM (18-36 ng/dL) and local aldosterone levels up to 500 nM (18,000 ng/dL), is now recognised as a more common cause for hypertension. We recently found angiotensin II to be genotoxic due to its induction of oxidative stress. Since aldosterone in higher concentrations also has oxidative effects, its potential genotoxic action in pig LLC-PK1 cells with properties of proximal tubules was analysed. DNA damage was evaluated by two test systems: the comet assay, and the micronucleus frequency test. The results showed that aldosterone concentrations starting from 10 nM (360 ng/dL) caused a significant increase of DNA damage monitored with the comet assay in LLC-PK1, while there was no change in cell vitality and proliferation. The micronucleus frequency test revealed that 10 nM aldosterone also leads to the formation of micronuclei. Furthermore, the formation of superoxide radicals in the cells by this aldosterone concentration could be detected with the superoxide-specific stain dihydroethidium. Further evidence for oxidative stress-induced DNA damage was its reversibility by the antioxidants tempol and catalase. Addition of the steroidal mineralocorticoid receptor antagonist spironolactone or the novel selective nonsteroidal antagonist (R)-BR-4628 reduced the DNA damage and the amount of superoxide radicals indicating a receptor-dependent process.

Homocysteine exerts genotoxic and antioxidative effects in vitro

INTRODUCTION

Patients with end-stage renal disease suffer from increased genomic damage and cancer incidence. One possible reason is the accumulation of uremic toxins such as homocysteine (Hcy). Elevated Hcy levels--usually indicative of cardiovascular events--correlated with the genomic damage in cross-sectional studies. Therefore we investigated the genotoxic effects of Hcy in vitro.

METHODS

To analyse the genomic damage, micronucleus tests and the comet-assay were performed in L5178Y and HL60 cells. Additionally, the influence of Hcy on cell cycle progression, DNA-cytosine-methylation, oxidative stress levels and on the cellular glutathione content were determined.

RESULTS

Low millimolar concentrations of Hcy-induced micronuclei in both cell lines but did not enhance the DNA damage observed with the comet-assay. Cell cycle progression was inhibited in S-phase, while DNA-cytosine-methylation remained unchanged. Furthermore, Hcy protected cells challenged with H(2)O(2) from oxidative stress. This was accompanied by an increased cellular glutathione level.

CONCLUSION

Since the genotoxic effect was limited to high Hcy concentrations, a contribution of Hcy to the enhanced genomic damage in end-stage renal disease patients would only be conceivable upon local Hcy accumulation. Whether the detected antioxidant capacity of Hcy is relevant for any situation in patients remains to be elucidated.

Relation between Different Treatment Modalities and Genomic Damage of End-Stage Renal Failure Patients

BACKGROUND

Patients with end-stage renal disease display enhanced genomic damage. We investigated the relation between genomic damage and different treatment modalities.

METHODS

In a longitudinal study two groups of patients were analyzed in monthly intervals. We assessed the initiation of hemodialysis in 5 conservatively treated patients, and a switch from hemodialysis to hemodiafiltration in 7 patients. DNA damage was investigated in peripheral blood lymphocytes by micronucleus frequency and by comet assay analysis. With regard to potential genotoxicity of advanced glycation end products (AGEs), levels of imidazolone A and AGE-associated fluorescence (AGE-FL) were determined.

RESULTS

The initiation of hemodialysis did not alter the genomic damage. In patients who switched from hemodialysis to hemodiafiltration, a small but significant reduction in the comet assay but not in the micronucleus frequency was observed. Elevated plasma levels of imidazolone A and AGE-FL were not influenced by the treatment modalities.

CONCLUSION

In our small patient group no major reduction of the elevated genomic damage could be reached. Disease factors not influenced by altered dialysis modalities may have contributed considerably in our patient group. The persisting high levels of DNA damage suggest a need for further improvement. Inhibiting AGE formation may be one promising way for the future.

Genotoxicity of phytoestrogens

Plant extracts containing phytohormones are very popular as 'alternative' medicine for many kinds of diseases. They are especially favored by women who enter menopause and are concerned about the side effects of hormone replacement therapy. However, adverse health effects of phytoestrogens have often been ignored. This review examines the literature on genotoxicity and apoptotic effects of phytohormones. Genistein, coumestrol, quercetin, zearalenone, and resveratrol exerted genotoxic effects in in vitro test systems. Other phytoestrogens such as lignans, the isoflavones daidzein and glycetein, anthocyanidins, and the flavonol fisetin exhibited only weak or no effects in vitro. However, some metabolites of daidzein showed a genotoxic activity in vitro. Practically all of the phytoestrogens exhibit pro-apoptotic effects in some cell systems. Further investigations regarding dose-response-relationships and other aspects relevant for extrapolation to human exposure seem necessary. Until then, care may be advised in taking concentrated phytohormones. Nevertheless, the intake of substantial amounts of plant-food in a normal diet constitutes an important, individual contribution to cancer prevention.

Pilot study for comparison of reticulocyte-micronulei with lymphocyte-micronuclei in human biomonitoring

Biomonitoring tries to determine the consequences for humans of exposures to environmental or pharmaceutical agents. Different end points have been employed to assess the burden of genomic damage. This is the first report comparing a recently introduced new end point, the reticulocyte-micronuclei analyzed by flow cytometry with the widely used lymphocyte-micronucleus assay, applied to two exposure scenarios leading to enhanced genomic damage. Radioiodine therapy was chosen to represent a short time exposure and hemodialysis treatment in end-stage renal failure was chosen to represent a chronic exposure. The results show that iodine radiation induced measurable genomic damage in the lymphocyte-micronucleus assay as well as in the reticulocyte-micronucleus test. Of two groups of patients under hemodialysis treatment, a reduced genomic damage was found with the lymphocyte-micronucleus test, but not with the reticulocyte-micronucleus test in the group undergoing daily hemodialysis, which removes uremic toxins more efficiently as compared to conventional hemodialysis, the treatment applied in the other group. The limited life-span of reticulocytes may make them less suitable for accumulation of chronic low level damage than lymphocytes. In conclusion, the lymphocyte-micronucleus test may be applicable to more exposure situations (including low chronic exposure), but the reticulocyte-micronucleus assay may be easier to perform in a clinical setting. The latter reflects a more rapid reduction of genomic damage after an acute exposure.

Anti-genotoxicity of coffee against N-methyl-N-nitro-N-nitrosoguanidine in mouse lymphoma cells

The main aim of this work was to test the hypothesis that instant coffee, a commonly consumed polyphenolic beverage with antioxidant activity, can protect mammalian cells against genotoxic effects in vitro. For this purpose, the L5178Y mouse lymphoma cell line was selected to assess modulatory effects of coffee on the genotoxicity of N-methyl-N-nitro-N-nitrosoguanidine (MNNG). We initiated the work with a set of preliminary experiments in which the cytokinesis-block micronucleus test was performed. Results obtained from these experiments demonstrated a dose-related decrease in genotoxicity following co-treatment of mouse lymphoma cells with three doses of caffeinated instant coffee. Both pre-treatment and co-treatment showed significant antigenotoxic effects against MNNG. Caffeinated and decaffeinated instant coffee samples inhibited genotoxicity. There was no significant change in the antigenotoxic effect of caffeinated instant coffee after filtration using a 0.2 microm filter. Similar in vitro experiments demonstrated antigenotoxic effects against MNNG when boiled coffee was used instead of instant coffee. On the basis of the findings from the above preliminary experiments, further work was carried out to evaluate the possible protective effects of caffeinated instant coffee against MNNG-induced DNA damage, mutation and chromosomal damage. Results from three or five independent experiments demonstrated significant protective effects of caffeinated instant coffee against MNNG-induced DNA damage in the comet assay, mutation at the Tk locus and chromosomal damage in the cytokinesis-block micronucleus test.

Genotoxic activity of newly synthesized derivatives of cyano-pyridone in murine cells in vivo and in vitro

Possible genotoxic activity of two newly synthesized cyanopyridone compounds [4-(N-methyl-phalimidyl-3)-3-cyano-4-methyl-pyridone-2 (MPhCMP) and 1-(4-hydroxyphenyl)-3-cyano-4-methyl-pyridone-2 (HCMP)] with in vitro antitumor activity was studied both in in vitro and in vivo murine test systems. In L5178Y mouse lymphoma cells, HCMP did not induce micronuclei (MN) at the highest available (because of toxicity) concentration (100 microg/ml), while MPhCMP at dose of 50 microg/ml induced 2.6-fold, and at dose of 100 microg/ml 3.95-fold increase of number of the cells with MN. The concentration of 100 microg/ml is a threshold of toxicity of MPhCMP. In experiments on possible DNA damaging activity (the comet assay) of both substances using the same doses as in in vitro mutagenesis assay, we did not reveal any evidence of DNA damage. The acute toxicity of compounds was studied on male Swiss albino mice. LD50 values of MPhCMP and HCMP were 177.5 and 288 mg/kg, respectively. MPhCMP was more potent MN inductor than HCMP (2.5-fold at doses equivalent to 1/2 of LD50). Both substances possessing in vitro antitumor activity along with weak genotoxicity have a good chance for successful in vivo antitumor studies in rodents.

Coffee-mediated protective effects against directly acting genotoxins and gamma-radiation in mouse lymphoma cells

The cytokinesis-block micronucleus test was performed using L5178Y mouse lymphoma cells to ascertain whether or not standard (caffeinated) instant coffee, the commonly consumed polyphenolic beverage with antioxidant activity can protect against chromosomal damage induced by the directly acting agents N-methyl-N-nitro-N-nitrosoguanidine (MNNG), mitomycin C (MMC), methyl methanesulfonate (MMS) and gamma radiation. Our results demonstrated significant reductions in the in vitro genotoxic effects of MNNG, MMC, and MMS following co-treatment of mouse lymphoma cells with standard instant coffee. Subsequently, the comet assay was carried out to assess the effect of coffee co-treatment on the level of DNA damage induced by MMS in mouse lymphoma cells. The results demonstrated a significant reduction in MMS-induced DNA damage following co-treatment with standard instant coffee. Protective effects were observed in mouse lymphoma cells which were treated with coffee immediately after exposure to gamma radiation (1 and 2 Gy). Another experiment showed protection when the mammalian cells were irradiated (0.5 and 1 Gy) midway (at 2 h) during a 4 h coffee treatment. However, the protective effect against the lower dose (0.5 Gy) was not significant. In addition we assessed the modulatory effect of coffee on MNNG-induced apoptotic frequency by flow cytometry. The results revealed only a minor influence of coffee on the frequency of apoptotic cells induced by the test compounds, rendering an increase in sensitivity for apoptosis as a reason for the reduced genomic damage an unlikely or at least incomplete explanation.

Genotoxic activity of four newly synthesized pyrrolin-2-one derivatives

PURPOSE

To study the genotoxic activity of 4 newly synthesized derivatives of pyrrolin-2-one by means of the micronucleus (MN) assay both in vivo and in vitro, and the DNA-damaging activity of these substances by means of the comet assay on murine cells in vitro.

MATERIALS AND METHODS

The following compounds were studied: [3-(imidazolyl-1)-propylamide-4,5,5-trimethyl- pyrrolin-2-one] (IPA-TP); [3-cyclohexylamide-4,5,5 -trimethyl- pyrrolin-2-one](CH-TP); [3-piperonylamide-4,5,5- trimethyl-pyrrolin-2-one] (PA-TP); and [3-phenethylamide- 4,5,5-trimethyl-pyrrolin-2-one] (PHA-TP). L5178Y mouse lymphoma cells were used to study the activity of the compounds by the MN and the comet assays. The acute toxicity and MN-inducing activity of the 4 compounds was assessed on Swiss albino mice.

RESULTS

IPA-TP, PA-TP, and PHA-TP were very weak MN inducers in mouse lymphoma cells, which induced MN only at toxic for lymphoma cells concentrations. No doseeffect relationship was registered. CH-TP was tested at low concentration because of bad solubility and was not MNinducer. IPA-TP and CH-TP were not active in the comet assay, while both PA-TP and PHA-TP were active. The study of acute toxicity showed the following results: LD(50) of IPATP, CH-TP, PA-TP and PHA-TP were 460 mg/kg, 650 mg/kg, 370 mg/kg, and 350 mg/kg, respectively. The substances were studied using the MN assay on mouse bone marrow polychromatic erythrocytes (PCEs), and all of them were active only at doses equal to 1/2 of LD(50). The increase of bone marrow cells with MN was 2.5-5.8-fold compared with the background MN level. Lower doses (1/5 of LD(50)) of all substances were not effective.

CONCLUSION

A good agreement between in vivo and in vitro genotoxicity was obtained. IPA-TP, PA-TP, PHA-TP with potential antitumor activity, comparatively low acute toxicity and genotoxicity are good candidates for in vivo studies of antitumor activity.

Estrogenic activity of naturally occurring anthocyanidins

Anthocyanins, which are natural plant pigments from the flavonoid family, represent substantial constituents of the human diet. Because some other bioflavonoids are known to have estrogenic activity, the aim of this study was to determine the estrogenic activity of the anthocyanine aglycones. Binding affinity to the estrogen receptor-alpha was 10,000- to 20,000-fold lower than that of the endogenous estrogen estradiol. In the estrogen receptor-positive cell line MCF-7, the anthocyanidins induced expression of a reporter gene. The tested anthocyanidins showed estrogen-inducible cell proliferation in two cell lines (MCF-7 and BG-1), but not in the receptor-negative human breast cancer cell line MDA-MB-231. The phytoestrogen-induced cell proliferation could be blocked by addition of the receptor antagonist 4-hydroxytamoxifen. Combination treatments with the endogenous estrogen estradiol resulted in a reduction of estradiol-induced cell proliferation. Overall, the tested anthocyanidins exert estrogenic activity, which might play a role in altering the development of hormone-dependent adverse effects.

Hormonal and genotoxic activity of resveratrol

Resveratrol (RES) is a natural polyphenol present in red wines and various human food items. The estrogenic activity of RES was demonstrated in two in vitro assay systems, i.e. binding to human estrogen receptor alpha and stimulation of MCF-7 cell proliferation. To investigate the inhibition of cell proliferation observed at high concentrations of RES, we analyzed the compound for genotoxic potential. RES induced cellular toxicity, micronuclei, and metaphase chromosome displacement in L5178Y mouse lymphoma cells. Likewise, the induction of micronuclei was observed in Chinese hamster V79 cells. Determination of kinetochore signals in micronuclei and cell cycle analysis suggested that RES did not cause a direct disturbance of mitosis. In support of this notion, cell-free tubulin polymerization studies indicated no direct effect of RES on microtubule assembly. According to an estimation of daily intake and bioavailability, concentrations that were found genotoxic in vitro might be reached in human exposure. On the other side, the estrogenic acitivity might be beneficial. Therefore, further investigations of mechanisms, possibly including animal models, would be desirable to clarifiy a potential risk for humans.

[Incidence and spectrum of malignant disease among dialysis patients in North Bavaria]

BACKGROUND AND OBJECTIVE

In end-stage renal failure the incidence of cancer is increased. With regard to frequency and pattern of distribution of the tumors, there are substantial regional differences. Since this topic has to date received only minimal attention in Germany, we undertook a multi-centric analysis (8 dialysis centres) in North Bavaria in order to address the occurrence of malignant diseases in end-stage renal failure.

PATIENTS AND METHODS

Of a total of 2228 patients, who underwent hemodialysis in the period from 1990 - 99 as a consequence of end-stage renal failure, the medical records of 1727 persons were analysed. Only those patients were considered, whose malignancy was diagnosed in the course of the dialysis. The Saarland cancer register served as a comparative age- and sex-matched population, with which we calculated the expected frequency of the various cancers as well as the standard incidence ratio (SIR) for the dialysis patients.

RESULTS

In total 125 malignant diseases were documented. The cancer incidence was highest in the first year of treatment and was clearly lower in the subsequent periods. Of great importance was the age of the patients. The highest SIR scores were found for patients of middle age (35 - 50 years). An enhanced risk for cancer of the kidney, bladder, prostate, liver, oral cavity and the pharynx and larynx, as well as of the lymphatic and hemopoetic systems was found, while there was no or only a slight increase in the frequency of carcinoma of the mammary gland, stomach, colon-sigma-rectum and bronchial systems.

CONCLUSION

The high incidence of cancer in end-stage renal failure should be given greater attention. Particularly in the high-risk group of younger dialysis patients, a regular screening - especially for tumors of the kidney, bladder and liver - appears justified.

In vitro radiosensitivity measured in lymphocytes and fibroblasts by colony formation and comet assay: comparison with clinical acute reactions to radiotherapy in breast cancer patients

PURPOSE

To compare colony-forming and comet assays on fibroblasts and lymphocytes of 32 breast cancer patients irradiated after breast-conserving operations and to correlate the results with acute clinical radiation reactions in the skin.

MATERIAL AND METHODS

Skin fibroblasts were isolated and cultivated before radiotherapy and lymphocytes were drawn prior to the first and directly after the final external irradiation. The colony-forming assay was performed with fibroblasts and the comet assay with lymphocytes and fibroblasts of breast cancer patients according to standard protocols. The clinical radiation reactions of the patients were graded according to the RTOG system.

RESULTS

No significant correlation (p =0.09) was detected between clinical acute skin reactions and the in vitro clonogenic data in fibroblasts. Results of the comet assay in lymphocytes, however, showed a significant correlation (p <0.05) with the clinical data when patients were divided into two groups with average and elevated acute reactions. Apart from initial damage, fibroblasts did not show significant differences between the two patient groups. Repeated comet assays in lymphocytes of the same patient drawn before treatment and before and after external radiotherapy demonstrated good reproducibility of the test and no significant impact of preceding radiation treatment. There was a good correlation (r =0.65) between the comet assay results in fibroblasts and lymphocytes of the same individual.

CONCLUSIONS

In this cohort of patients, a significant correlation between the in vitro results of the comet assay in lymphocytes and clinical acute reactions was detected. The results of the comet assay and of fibroblast colony formation did not correlate with in vitro radiosensitivity.

Assaying the estrogenicity of phytoestrogens in cells of different estrogen sensitive tissues

There is currently much concern that a wide range of both synthetic and naturally occurring environmental chemicals may act as endocrine disruptors (ED), and may adversely affect humans and wildlife. We examined the estrogenic effects of the phytoestrogens daidzein (DAI), equol (EQU) and O-desmethylangolensin (O-DMA), two metabolites of DAI, in three different assays. Binding affinity to the estrogen receptor alpha was 1000-10,000-fold lower compared with the endogenous estrogen estradiol. In the receptor positive cell line MCF-7 the phytoestrogens induced the expression of a reporter gene. The E-SCREEN is based on the estrogen-receptor binding induced proliferation of the human breast cancer cell line MCF-7. We also adapted the E-SCREEN for the estrogen-receptor positive human ovarian cancer cell line BG-1. The tested phytoestrogens induced cell proliferation in both cell lines, but not in the receptor negative human breast cancer cell line MDA-MB-231. The phytoestrogen-induced cell proliferation could be blocked by addition of the receptor antagonist 4-hydroxytamoxifen (OHT). Combination treatments with the endogenous estrogen estradiol showed competitive effects in MCF-7 cells. These studies demonstrated that the tested phytoestrogens exerted estrogenic responses in cells derived from two different tissues, breast and ovary. Furthermore, we demonstrated that BG-1 cells are a suitable additional cell system to investigate estrogenicity of test compounds.

Comet-assay analysis identifies genomic damage in lymphocytes of uremic patients

This study investigates genomic damage in peripheral lymphocytes from patients with moderate to severe chronic renal insufficiency and those on long-term maintenance hemodialysis (MHD) and hemodiafiltration therapy. As a measure for genomic damage, the comet assay (single-cell gel electrophoresis) was applied. This test detects single- and double-strand breaks and alkali labile sites through electrophoretic mobility of the resulting fragments. The average damage (percentage of DNA in the tail region of the comet) observed in cells of the control group of 21 healthy subjects was 10.5% +/- 0.8%. There was a significant increase to 14.7% +/- 3.5% in cells of 23 patients with chronic renal failure, and a further increase to 17.1% +/- 3.5% in the subgroup of 12 patients with serum creatinine values greater than 6 mg/dL. Damage was 16.7% +/- 4.2% in cells of the MHD group (26 patients) and 20.1% +/- 3.0% in the subgroup with MHD therapy longer than 10 years (8 patients). Cellular DNA damage in the group of 15 maintenance hemodiafiltration patients was 15.6% +/- 2.1%, ranging between predialysis and MHD patients, and did not seem to increase with treatment time. These results, together with previously observed elevated frequencies of micronuclei, decreased DNA repair, and increased cancer incidence described for these patient groups, emphasize the need to further optimize the current therapy for reducing the degree of genomic damage.

Increased formation of micronuclei after hormonal stimulation of cell proliferation in human breast cancer cells

The carcinogenicity of sex hormones is considered to be the result of a combination of genotoxic and epigenetic modes of action. For estrogens, genotoxic activities include DNA damage by reactive metabolites and indirect genotoxicity by redox cycling and production of reactive oxygen species. Here, we present data on the induction of micronuclei in estrogen receptor-positive (MCF-7) and -negative (MDA) human breast cancer cell lines treated with estradiol to support an additional mechanism of chromosomal damage. MCF-7 cells, but not MDA cells, treated with estradiol in the picomolar concentration range showed an increase in micronucleus formation which correlated with the estradiol-induced cell proliferation. Addition of the specific estradiol-receptor antagonist hydroxytamoxifen suppressed the estradiol-induced formation of micronuclei in MCF-7 cells. Increased frequencies were also seen after normalization of the data to the number of cell divisions by additional treatment of the cells with cytochalasin B. Thus, formation of micronuclei was not due to the chromosomal damaging activity of estradiol. The induced genomic damage may be explained by a hormone-specific forcing of responsive cells through the cell cycle, thereby overriding checkpoints operating under homeostatic control of the cell cycle.

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.

Duodenogastric reflux and foregut carcinogenesis: analysis of duodenal juice in a rodent model of cancer

The incidence of esophageal adenocarcinoma is increasing rapidly. In rats, surgically induced duodenoesophageal reflux is carcinogenic. One proposed mechanism of carcinogenesis is based on the reaction of physiological bile acids with nitrite to produce carcinogenic N:-nitroso amides. To test this hypothesis, duodenal juice was analyzed for endogenously formed N:-nitroso bile acids and its genotoxicity was determined. Esophagojejunostomy was performed on 15 Sprague-Dawley rats to produce duodeno-esophageal reflux. At the time of surgery and 2 and 6 weeks later, duodenal contents were aspirated and analyzed immediately. High performance liquid chromatography coupled to tandem mass spectrometry was used to detect bile acids and their nitroso derivates. Genotoxicity was assessed using a micronucleus test. The characteristic pattern of bile acid derivatives, with taurocholic acid (TCA) and glycocholic acid (GCA) as the predominant conjugates, was detected in all samples. However, even selective reaction monitoring experiments failed to demonstrate the presence of any N:-nitroso-TCA or N:-nitroso-GCA. In addition, other nitroso derivatives could not be detected in any of the samples by neutral loss experiments monitoring the loss of nitric oxide (detection limit 0.1% of the concentration of TCA). All samples were cytotoxic, but neither the preoperative nor the postoperative samples were genotoxic. Duodenal juice was cytotoxic but not genotoxic. Tumorigenesis of esophageal adenocarcinoma in the rodent model could not be linked to a specific carcinogen, especially not to nitroso bile acids. Chronic inflammation is likely to be the mechanism of carcinogenesis by duodenogastric reflux.

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.

In vitro micronucleus assay with Chinese hamster V79 cells - results of a collaborative study with in situ exposure to 26 chemical substances

A collaborative study with 10 participating laboratories was conducted to evaluate a test protocol for the performance of the in vitro micronucleus (MN) test using the V79 cell line with one treatment and one sampling time only. A total of 26 coded substances were tested in this study for MN-inducing properties. Three substances were tested by all 10 laboratories and 23 substances were tested by three or four laboratories in parallel. Six aneugenic, 7 clastogenic and 6 non-genotoxic chemicals were uniformly recognised as such by all laboratories. Three chemicals were tested uniformly negative by three laboratories although also clastogenic properties have been reported for these substances. Another set of three clastogenic substances showed inconsistent results and one non-clastogenic substance was found to be positive by one out of three laboratories. Within the study, the applicability of the determination of a proliferation index (PI) as an internal cytotoxicity parameter in comparison with the determination of the mitotic index (MI) was also evaluated. Both parameters were found to be useful for the interpretation of the MN test result with regard to the control of cell cycle kinetics and the mode of action for MN induction. The MN test in vitro was found to be easy to perform and its results were mainly in accordance with results from chromosomal aberration tests in vitro.

Supra-additive genotoxicity of a combination of gamma-irradiation and ethyl methanesulfonate in mouse lymphoma L5178Y cells

While testing for genotoxicity is usually performed on single chemicals, exposure of humans always comprises a number of genotoxic agents. The investigation of potentially synergistic effects of combinations therefore is an important issue in toxicology. Combinations of 511 keV gamma-radiation with the chemical alkylating agent ethyl methane-sulfonate were investigated in the in vitro micronucleus test in mouse lymphoma L5178Y cells. With combinations in the low dose linear effect range for the individual agents (0. 25-2 Gy and 0.8-3.2 mM, respectively), supra-additivity by 34-86% was seen. The synergism was more pronounced at the higher dose levels. Supra-additivity was confirmed in experiments using cytochalasin B and analyzing binucleate cells only, to control for putative effects on the cell cycle. Statistical significance was shown by a 2-factor analysis of variance with interaction. The results indicate that damage to DNA by gamma-radiation and alkylation could affect different rate limiting steps in the formation of micronuclei. Further investigations will have to show whether the observations are of general validity, in particular, whether other end-points of genotoxicity produce the same results and whether the degree of supra-additivity is always dose dependent. The latter would have a strong impact on risk assessment for mixtures at low doses.

Helicobacter pylori induces DNA damage in vitro

A close association between Helicobacter pylori infection and the development of gastric adenocarcinoma in humans has been demonstrated. Therefore, the direct induction of DNA damage by H. pylori was investigated here using the in vitro micronucleus assay. After 5 days of incubation with bacterial lysate a dose-dependent formation of micronuclei was found, which was not limited to cytotoxic protein concentrations and was not observed after treatment with Escherichia coli lysate (control). This induction of DNA damage may be a link between chronic H. pylori infection and development of adenocarcinoma of the stomach.

Aflatoxin B1-induced mitotic recombination in L5178Y mouse lymphoma cells

Aflatoxin B1 is a human hepatocarcinogen. It is also a known point mutagen in bacteria and mammalian cells. This mutagenic activity may be at least partly responsible for its carcinogenic activity. However, recent studies show that aflatoxin B1 induces mitotic recombination in the yeast Saccharomyces cerevisiae. Because numerous reports have implicated mitotic recombination in mechanisms leading to carcinogenesis and because no one has shown that aflatoxin B1 induces recombination in mammalian cells, we decided to examine the ability of aflatoxin B1 to induce recombination in a mammalian cell line. We used a combination of methods, analysis for loss of heterozygosity and whole chromosome in situ hybridization, to identify mechanisms of chromosome mutation, including mitotic recombination in the mammalian L5178Y mouse lymphoma cell system. Our experiments revealed that mitotic recombination caused approximately 60% or more of the aflatoxin B1-induced mutagenic lesions in this cell system. Thus, mitotic recombination plays an important role in aflatoxin B1-induced mutagenesis in mammalian cells and possibly in chemically induced mutagenesis and carcinogenesis. This work suggests that multiple genetic lesions may be involved in aflatoxin B1-induced pathology.

Combination of paclitaxel and radiation: genotoxicity in vitro in four mammalian cell lines

Paclitaxel is an antimicrotubular agent that blocks the cells in the G2/M phase of the cell cycle. Due to this action, it is presumed that this drug could function as a radiation sensitizer. We studied the genotoxic effects of a combination of paclitaxel and radiation in four mammalian cell lines in the micronucleus assay. The results do not show a clear radiation-sensitizing effect. In the three cell lines, L5178Y, V79 and HeLa, the micronucleus frequencies varied around a theoretical additive effect of the single treatments (paclitaxel or radiation alone). Only the human breast cancer cell line MCF-7 showed consistently lower than additive micronucleus frequencies, although the deviation was not significant. Overall, it remains inconclusive whether paclitaxel exerts a radiosensitizing effect and, if so, whether this effect depends on the cell type or other characteristics of tumor biology.

Increased genomic damage in lymphocytes of patients before and after long-term maintenance hemodialysis therapy

This study investigates spontaneous genomic damage in peripheral lymphocytes of 19 patients with severe end-stage renal disease not enrolled onto a maintenance hemodialysis (MHD) program (creatinine level, 5.4 to 10.5 mg/dL) and 16 long-term MHD patients (111 to 282 months on MHD) and the possible association of genomic damage with the degree of renal insufficiency and duration of MHD. Genomic damage was assessed by evaluating the numbers of micronuclei (MN), which are cytoplasmic DNA-containing structures. The average number of MN in the control group of 23 healthy subjects was 15.3 +/- 4.7 MN/1,000 binucleate (BN) cells. The MN frequencies in the long-term MHD group were significantly greater (44.3 +/- 13.7 MN/1,000 BN) than the control frequencies. A significant increase in MN frequencies (28.2 +/- 9.4 MN/1,000 BN) was also seen in patients with advanced renal failure. The major findings of the study, high MN frequencies in long-term hemodialysis and advanced chronic renal failure patients, may result from decreased DNA repair previously shown and may contribute to the increased cancer incidence in these patients.

Characterization of the genotoxicity of anthraquinones in mammalian cells

Naturally occurring 1,8-dihydroxyanthraquinones are under consideration as possible carcinogens. Here we wanted to elucidate a possible mechanism of their genotoxicity. All three tested anthraquinones, emodin, aloe-emodin, and danthron, showed capabilities to inhibit the non-covalent binding of bisbenzimide Hoechst 33342 to isolated DNA and in mouse lymphoma L5178Y cells comparable to the topoisomerase II inhibitor and intercalator m-amsacrine. In a cell-free decatenation assay, emodin exerted a stronger, danthron a similar and aloe-emodin a weaker inhibition of topoisomerase II activity than m-amsacrine. Analysis of the chromosomal extent of DNA damage induced by these anthraquinones was performed in mouse lymphoma L5178Y cells. Anthraquinone-induced mutant cell clones showed similar chromosomal lesions when compared to the topoisomerase II inhibitors etoposide and m-amsacrine, but were different from mutants induced by the DNA alkylator ethyl methanesulfonate. These data support the idea that inhibition of the catalytic activity of topoisomerase II contributes to anthraquinone-induced genotoxicity and mutagenicity.

Radiation-induced comet-formation in human skin fibroblasts from radiotherapy patients with different normal tissue reactions

BACKGROUND

In clinical radiotherapy most patients tolerate the applied dosage with no or moderate side effects. However, 5 to 10% of all individuals show increased acute and/or late reactions. In-vitro test systems are investigated for their suitability for predictive purposes. This paper attempts a correlation between the induction and repair of DNA damage measured in the comet assay and the clinical observed reaction in order to evaluate the suitability of the comet assay for prediction of radiation sensitivity.

PATIENTS AND METHODS

Skin fibroblasts of 30 patients with average tissue reactions or acute and/or late increased side effects and cell lines of 4 individuals carrying the heritable disease ataxia telangiectasia (AT) were irradiated in vitro. The induction and repair of DNA damage was measured at different time points after irradiation in the comet assay (single cell gel electrophoresis). These results were compared to the acute and late clinical reactions classified according to the RTOG grading system.

RESULTS

The radiation induced DNA damage decreased over time reflecting DNA repair. Cells of the AT individuals showed an elevated damage induction and a reduced repair capacity compared to patients with average tissue reactions. Fibroblasts of patients with increased acute and late side effects exhibited slower DNA repair. In addition to the known lack of cell cycle control, our results indicate that AT cells show reduced DNA repair capacity.

CONCLUSIONS

The comet assay seems to be able to detect some types of increased individual radiation sensitivity. In contrast to other predictive in-vitro tests, the comet assay needs less time and fewer cells, which would be useful in a clinical setting.

Identification of ataxia telangiectasia heterozygotes, a cancer-prone population, using the single-cell gel electrophoresis (Comet) assay

Heterozygotes of ataxia telangiectasia (AT) may comprise up to 1% of the general population. Because these individuals have no clinical expression of AT but may be highly radiosensitive and strongly predisposed for several forms of cancer, identification of AT carriers represents a considerable interest in cancer epidemiology and radiotherapy. We report a new approach for the in vitro identification of AT-heterozygotes based on the evaluation of the radiosensitivity and DNA damage repair ability of peripheral blood mononuclear cells using the single-cell gel electrophoresis (Comet) assay. The assay was performed on cells isolated from four different groups of individuals: (1) apparently healthy donors (n = 10); (2) patients with breast cancer showing a normal reaction to radiotherapy (n = 10); (3) a group of obligate AT carriers (parents of AT-homozygotes, n = 20); and (4) AT-homozygotes (n = 4). Cells irradiated with 3 Gy of x-rays were assayed for three parameters: (1) the initial and (2) residual DNA damage and (3) the kinetics of DNA damage repair. Both AT-heterozygotes' and AT-homozygotes' cells were found to be highly sensitive to x-irradiation. Quantitative evaluation of the single-cell electrophoregrams revealed that the average initial DNA damage in AT-heterozygous and AT-homozygous cells was almost three times higher than that in control non-AT cells. In addition, the DNA repair process in irradiated AT carrier cells was almost three times slower, and the extent of irreparable DNA damage in these cells was three times greater than in controls. Simultaneous assessment of the three parameters enabled correct identification of all tested AT carriers. This method seems to be a sensitive and useful tool for populational studies as a rapid prescreening test for a mutated AT status. The approach can also be extended for prediction of the in vivo radiosensitivity, which would enable optimization of individual radiotherapy schedules.

Cytotoxic versus genotoxic effects of nitric oxide (NO)

The pathobiochemistry of endogenous reactive nitrogen species includes functions in inflammation and carcinogenesis. Genotoxicity has been suggested to play a major role. Two donor compounds, spermine NONOate, which can release authentic nitric oxide (NO), and 3-Morpholino-sydnonimine hydrochloride (SIN-1), which generates NO together with superoxide, possibly yielding peroxynitrite (ONOO-), were investigated in L5178Y mouse lymphoma cells for cytotoxic and genotoxic effects. As demonstrated by cell growth, 'micronucleus' and 'comet' assays NO, with and without concomitant superoxide formation, did not induce significant genotoxicity at concentrations with low cytotoxicity. Therefore, at least for the three tested parameters and the chosen time window, the pronounced cytotoxicity exhibited by NO and its oxidative metabolites most likely outweighs any genotoxic potential.

Occurrence of emodin, chrysophanol and physcion in vegetables, herbs and liquors. Genotoxicity and anti-genotoxicity of the anthraquinones and of the whole plants

1,8-Dihydroxyanthraquinones, present in laxatives, fungi imperfecti, Chinese herbs and possibly vegetables, are in debate as human carcinogens. We screened a variety of vegetables (cabbage lettuce, beans, peas), some herbs and herbal-flavoured liquors for their content of the 'free' anthraquinones emodin, chrysophanol and physcion. For qualitative and quantitative analysis, reversed-phase HPLC (RP-LC), gas chromatography-mass spectrometry (GC-MS) and RP-LC-MS were used. The vegetables showed a large batch-to-batch variability, from 0.04 to 3.6, 5.9 and 36 mg total anthraquinone per kg fresh weight in peas, cabbage lettuce, and beans, respectively. Physcion predominated in all vegetables. In the herbs grape vine leaves, couch grass root and plantain herb, anthraquinones were above the limit of detection. Contents ranged below 1 mg/kg (dry weight). All three anthraquinones were also found in seven of 11 herbal-flavoured liquors, in a range of 0.05 mg/kg to 7.6 mg/kg. The genotoxicity of the analysed anthraquinones was investigated in the comet assay, the micronucleus test and the mutation assay in mouse lymphoma L5178Y tk+/- cells. Emodin was genotoxic, whereas chrysophanol and physcion showed no effects. Complete vegetable extract on its own did not show any effect in the micronucleus test. A lettuce extract completely abolished the induction of micronuclei by the genotoxic anthraquinone danthron. Taking into consideration the measured concentrations of anthraquinones, estimated daily intakes, the genotoxic potency, as well as protective effects of the food matrix, the analysed constituents do not represent a high priority genotoxic risk in a balanced human diet.

Photohemolysis sensitized by the furocoumarin imperatorin and its oxyfunctionalized derivatives

The dark and photosensitized (366 nm) hemolytic effects of imperatorin and its photooxidation products, the hydroperoxides I and II as well as the corresponding alcohol of the hydroperoxide I (imperatorin alcohol), were studied on human erythrocytes. Imperatorin was shown to photosensitize hemolysis, its fluence (D) dependence of the rate of photohemolysis (V) followed the equation V = V0 + aD2 + bD1/2, in which V0 is the dark hemolysis rate and a and b are constants. At fluences below 200 kJ/m2, the main hemolytic contribution derives from the bD1/2 component, which is due to the in situ formation of the imperatorin hydroperoxides, while at fluences higher than 200 kJ/m2, the main contribution corresponds to the aD2 component due to the two-photon damage of cell membranes. Hydroperoxides I and II induce oxyhemoglobin cross-linking, as well as its conversion to methemoglobin and hemichrome. These reactions involve hydroxyl and alkoxy radicals, as the hemolysis and oxyhemoglobin conversion could be inhibited by t-butanol and butylated hydrotoluene. For comparison, the dark hemolytic effect of the imperatorin alcohol was approximately 10-fold less than of the hydroperoxides.

Are topoisomerase II inhibitor-induced micronuclei in vitro a predictive marker for the compounds' ability to cause secondary leukemias after treatment?

The inhibition of topoisomerase II (topo II) is a very powerful principle of chemotherapy and topo II inhibiting drugs are the backbone of most chemotherapeutic strategies. However, secondary malignomas can occur after treatment. Typically, secondary acute myeloid leukemia (t-AML) after treatment with topo II inhibitors has a shorter latency period than t-AML following alkylator therapy. Fragments originating from chromosome breakage as well as whole chromosomes which are not correctly distributed during mitosis give rise to micronuclei in the next interphase. Micronucleus formation has become an important endpoint in genotoxicity testing. In an effort to test the suitability of the micronucleus assay for predictive purposes, we have analyzed three human tumor cell lines for cell growth as well as micronucleus induction after treatment with four clinically used topo II inhibitors. Micronuclei were induced at levels of low toxicity by etoposide, mitoxantrone, daunorubicin and idarubicin. The induction of micronuclei was a more sensitive indicator of drug effects than reduction in cell growth. Thus, micronucleus induction may assist in the prediction of the potency of a chemotherapeutic agent for induction of secondary malignomas.

Cytotoxic and genotoxic effects of paclitaxel (Taxol) and radiation in a squamous cell carcinoma cell line of the larynx

Paclitaxel (Taxol) is an antimicrotubular agent which blocks the cells in the G2/M phase of the cell cycle. Because of this mechanism it is presumed that this drug could function as a radiation sensitizer. The cytotoxic and genotoxic effects of paclitaxel and a combination of paclitaxel and radiation were studied in the human laryngeal carcinoma cell line HLac 79. The growth of the cells was significantly reduced at concentrations of paclitaxel as low as 10 nM. Flow cytometry data showed a G2/M block after exposure to paclitaxel. Radiation at 12 and 24 h after drug treatment exerted an additive but no radiation sensitizing effect. As genotoxic effect paclitaxel induced multinucleated cells, possibly in a synergistic manner, at low concentrations (10 nM) and radiation doses up to 3 Gy.

Biotransformation of the anthraquinones emodin and chrysophanol by cytochrome P450 enzymes. Bioactivation to genotoxic metabolites

The studies presented here were designed to elucidate the enzymes involved in the biotransformation of naturally occurring 1, 8-dihydroxyanthraquinones and to investigate whether biotransformation of 1,8-dihydroxyanthraquinones may represent a bioactivation pathway. We first studied the metabolism of emodin (1, 3,8-trihydroxy-6-methylanthraquinone), a compound present in pharmaceutical preparations. With rat liver microsomes, the formation of two emodin metabolites, omega-hydroxyemodin and 2-hydroxyemodin, was observed. The rates of formation of omega-hydroxyemodin were not different with microsomes from rats that had been pretreated with inducers for different cytochrome P450 enzymes. Thus, the formation of omega-hydroxyemodin seems to be catalyzed by several cytochrome P450 enzymes at low rates. The formation of 2-hydroxyemodin was increased in liver microsomes from 3-methylcholanthrene-pretreated rats and was inhibited by alpha-naphthoflavone, by an anti-rat cytochrome P450 1A1/2 antibody, and, to a lesser degree, by an anti-rat cytochrome P450 1A1 antibody. These data suggest the involvement of cytochrome P450 1A2 in the formation of this metabolite. However, other cytochrome P450 enzymes also seem to catalyze this reaction. The anthraquinone chrysophanol (1,8-dihydroxy-3-methylanthraquinone) is transformed, in a cytochrome P450-dependent oxidation, to aloe-emodin (1, 8-dihydroxy-3-hydroxymethylanthraquinone) as the major product formed. The mutagenicity of the parent dihydroxyanthraquinones and their metabolites was compared in the in vitro micronucleus test in mouse lymphoma L5178Y cells. 2-Hydroxyemodin induced much higher micronucleus frequencies, compared with emodin. omega-Hydroxyemodin induced lower micronucleus frequencies, compared with emodin. Aloe-emodin induced significantly higher micronucleus frequencies than did chrysophanol. These data indicate that the cytochrome P450-dependent biotransformation of emodin and chrysophanol may represent bioactivation pathways for these compounds.

Factors affecting the genotoxic potency ranking of natural anthraquinones in mammalian cell culture systems

We had reported that the plant-derived 1,8-dihydroxyanthraquinone derivatives, emodin and danthron, were clearly genotoxic in mouse lymphoma L5178Y cells, whereas chrysophanol was only weakly genotoxic and physcion not at all. Danthron was more potent than emodin. Furthermore, we had found that these compounds bound non-covalently to DNA and inhibited topoisomerase II activity. Interestingly, in these systems emodin was more potent than danthron. This inversion of the ranking prompted us to investigate the underlying mechanism. Since emodin shows a high serum-protein binding affinity, horse serum used as a media-supplement in the mouse lymphoma genotoxicity assays was analyzed for a potential selective scavenging of emodin. Non-covalent DNA-binding in mouse lymphoma L5178Y cells was investigated in the absence or presence of serum. In the presence of 10% serum, the DNA-binding potency of emodin was markedly reduced and was lower than that of danthron. We also applied mutation assays with mouse lymphoma cells and AS52 cells and varied the serum concentration used. In the absence of serum emodin showed slightly higher mutagenicity in AS52 cells than danthron. At reduced serum concentration (0.5%) emodin was strongly cytotoxic to the mouse lymphoma cells. For chrysophanol and physcion, a considerable reduction of the non-covalent DNA-binding potency in intact cells was found when compared to danthron, in concordance with their lower genotoxic potency. Overall, these data support the understanding that the genotoxicity of anthraquinones is, at least in part, mediated by non-covalent DNA-binding.

Radiation-induced micronucleus formation in human skin fibroblasts of patients showing severe and normal tissue damage after radiotherapy

PURPOSE

Treatment schedule and total dosage in radiation therapy is based on the tumoricidal doses and the tolerance dose of the perifocal normal tissue. Since large-scale variations occur between the patients concerning the side effects, one of the major goals of radiation research recently has been the development of a predictive in vitro assay. This paper is a contribution to that effort.

MATERIALS AND METHODS

Skin fibroblasts of patients with normal-tissue reactions or acute and/or late increased side effects and cell lines of four individuals carrying the heritable disease ataxia telangiectasia were irradiated in vitro. The formation of micronuclei was chosen as biological endpoint and the results were compared with the clinically observed side effects.

RESULTS

In general, a radiation dose-dependent increase in micronucleus frequency was found. The cells of the majority of the sensitive patients, as well as those of homozygous and heterozygous ataxia telangiectasia individuals, showed a higher micronucleus induction than the average of the donors with normal sensitivity.

CONCLUSIONS

The micronucleus test seems to be a very promising tool in the evaluation of radiation sensitivity prior to therapy. However, larger studies are needed to confirm these findings and to optimize the methodology, and it is presumed that a final predictive assay will consist of a combination with other test systems.

Evaluation of the in vitro micronucleus test as an alternative to the in vitro chromosomal aberration assay: position of the GUM Working Group on the in vitro micronucleus test. Gesellschaft für Umwelt-Mutations-forschung

In order to license a pharmaceutical or chemical, a compound has to be tested for several genotoxicity endpoints, including the induction of chromosomal aberrations in vitro. A working group within the GUM has evaluated published data on the in vitro micronucleus test with the aim of judging its suitability as a replacement for the in vitro chromosomal aberration test. After strict rejection criteria were applied, a database including 96 publications and 34 compounds was obtained. For 30 of these compounds, data on both tests were available. For 24 of the 30, concordant results in both test systems were obtained (80% correlation). The discordant results in 6 compounds can be explained by a known or suspected aneugenic potential of these compounds. Considering that cell types and test protocols were extremely heterogeneous, this correlation is rather encouraging. Comparison of the different protocols, and experience established within the working group yielded several recommendations for the routine use of the in vitro micronucleus test. Although many cell lines are suitable, those most often used in genotoxicity testing (e.g. CHL, CHO, V79, human lymphocytes, L5178Y mouse lymphoma cells) are recommended. Cytochalasin B may be used in the case of human lymphocytes; however, the possibility of its interaction with aneugenic test compounds should be considered. For continuously dividing cell lines, cytochalasin B is not recommended by the working group. Although, there seems to be flexibility in the choice of treatment and sampling times, the average generation time of the chosen cell line of choice should be taken into account when determining sampling time, and treatment of cells for at least one cell cycle duration is recommended. The use of appropriate cytotoxicity tests is strongly recommended. Although studies on some parameters of the test protocol may be useful, the introduction of the in vitro micronucleus test into genotoxicity testing and guidelines should not be delayed. Even in its present state, the in vitro micronucleus is a reliable genotoxicity test. Compared with the chromosomal aberration test, it detects aneugens more reliably, it is faster and easier to perform, and it has more statistical power and the possibility of automation.

Micronuclei containing whole chromosomes harbouring the selectable gene do not lead to mutagenesis

Loss of heterozygosity is one genetic change observed in many tumours. We do not know whether the loss of chromosomal material through micronucleus formation is a viable mechanism associated with, and possibly leading to, genetic disease. Previously, we treated L5178Y mouse lymphoma cells with four aneugens. Although these aneugens induced micronuclei containing predominantly whole chromosomes, they did not induce mutations at Tk1, the selectable gene, under the same non-toxic conditions in which they induced micronuclei. This suggested that the induction of micronuclei containing whole chromosomes was not an early event leading to phenotypically expressed mutations in these cells under the conditions used. However, it is possible that chromosome 11, on which Tk1 resides, may be under-represented in the micronucleus population. To find out the frequency of induction of micronuclei containing chromosome 11, we applied fluorescence in situ hybridization using a chromosome 11 paint to micronuclei induced by colcemid and vinblastine. We found that the numbers of micronuclei containing chromosome 11 are more than sufficient to be detectable as mutations if these micronuclei lead to viable mutants. We conclude that the formation of micronuclei containing whole chromosomes does not lead to viable, dividing mutants in this system.