Chemical probes and possible targets for the induction of aneuploidy

Threshold-mediated mechanisms in mutagenesis: implications in the classification and regulation of chemical mutagens

Chemical mutagens are currently regulated and labelled on the basis of their hazardous properties defined in hazard classification schemes. The strength and type of experimental evidence is used as the only criterion for classification in categories which express different levels of concern for the possibility of adverse effects - notably transmissible genetic alterations - in humans. Differently from the classification of carcinogens, no consideration is given to potency, nor to the mechanism of action. The rationale of such hazard based classification is that the hazardous property of a chemical is an intrinsic feature, which is expressed independently of dosing. Changing of dose level results in a mere change in the probability to observe an adverse effect, but not in its potential occurrence. The lack of theoretical threshold underlying this approach can be envisaged, in principle, for stochastic processes such as DNA damage, which can be triggered by single molecular interactions. On the other hand, indirect mechanisms of genotoxicity, involving multiple interactions with non-DNA targets, are expected to show a threshold. At variance to DNA reactive agents, chemicals acting with threshold-mediated mechanism do change also qualitatively their toxic properties depending on the dose level. Possible problems arising in the application of hazard based schemes for the evaluation of chemicals with threshold-mediated mechanism of action are discussed, using the spindle poisons benzimidazole fungicides as an example.

Evaluation of thresholds for benomyl- and carbendazim-induced aneuploidy in cultured human lymphocytes using fluorescence in situ hybridization

Threshold mechanisms of activity for mutagenic agents have been debated for some time, especially for those substances which induce aneuploidy by inhibiting mitotic spindle function. No observed effect levels (NOELs) or "practical thresholds" have been demonstrated for several aneugens both in vitro and in vivo generally by either counting chromosomes in metaphase preparations or by observing micronuclei. Recently, fluorescence in situ hybridization (FISH) has proven to be a sensitive and useful technique for the assessment of aneuploidy at low concentrations. Using binucleate human lymphocytes coupled with FISH, we have been able to characterize a threshold mechanism of action for two spindle inhibitors, benomyl and its active metabolite, carbendazim. Test chemicals were added 24 h following culture initiation. After a further 20 h, cytochalasin B was added, and cells were harvested 28 h later (72 h post initiation). The distribution of chromosomes between the nuclei of binucleate cells was evaluated by fluorescence microscopy for the simultaneous detection of centromeres labeled with FITC (green) or Cy-3 (red). Six human chromosomes were investigated in pairs (1 and 8, 11 and 18, and X and 17). Abnormalities were classified as chromosome loss (including centromeric positive micronuclei), chromosome gain, non-disjunction, or polyploidy. Dose-response data were generated over a range of closely spaced concentrations at 100 ng/ml intervals. The threshold, defined as the lowest "effect" concentration using statistical methods, was determined for each chromosome. Non-disjunction proved to be the most sensitive endpoint for the detection of aneuploidy occurring at higher frequencies and lower concentrations. Results for the six chromosomes demonstrated similar dose-response data which included a series of concentrations with no statistically significant increase above background, followed by a second range of higher concentrations with a statistically significant, concentration-dependent increase. Nearly equimolar threshold concentrations were determined for benomyl- and carbendazim-induced non-disjunction.

Cell cycle dependent aneuploidy induction by X-rays in vitro in human lymphocytes

Although ionising radiation mainly induces DNA strand breaks leading to chromosomal aberrations, there are indications that it also might induce numerical chromosome aberrations (aneuploidy). The existing data, however, do not provide evidence for a mechanism. To assess the relative sensitivity of the G1 vs. G2 cellular targets, whole blood cultures of lymphocytes were irradiated in vitro with different doses of X-rays (0.5, 1 and 2 Gy). The lymphocytes were harvested after cytochalasin-B blockade to allow the selective study of binucleated cells, having undergone only one division in culture. Harvesting was performed at different sampling times (70, 74, and 78 hours). To evaluate the micronuclei, regarding whole chromosomes or acentric fragments, an oligonucleotide probe that recognises the centromeric region of all human chromosomes was used. The relative percentage of centromere-positive micronuclei ranged from 5 up to 18% depending on the cell cycle stage and on the received dose. Cells exposed during the G1 phase exhibited a slightly higher frequency of centromere-positive micronuclei than cells that were in G2 at the time of exposure. G1 exposure induced a centromere-positive micronuclei dose-effect relationship that was not observed after G2 exposure. The observed difference in response of both phases on the centromere-positive micronuclei yields may be due to the involvement of different targets.

Does aneuploidy per se cause developmental abnormalities?

The following questions are addressed: (A) What are aneuploidogens and how do they act? (B) Is there any evidence that aneuploidy per se causes malformations? (C) What examples are there of abnormalities, apparently attributable to aneuploidogens acting as teratogens? (D) Do abnormalities of cell division cause both teratogenesis and aneuploidy? Considerable research has addressed question (A), but there is little which addresses the other three questions. The question of whether aneuploidy per se causes malformations remains open. Some suggestions for further research are made.

Analysis of DES-induced micronuclei in binucleated rat fibroblasts: comparison between FISH with a rat satellite I probe and immunocytochemical staining with CREST serum

The usefulness of fluorescence in situ hybridization (FISH) with rat satellite I DNA was compared with immunocytochemical staining with CREST serum for the analysis of the content of micronuclei from primary rat fibroblasts. We analyzed micronuclei induced in vitro by the aneugenic compound diethylstilbestrol (DES) or the clastogenic compound mitomycin C (MMC). Since a centromeric probe was not available for the rat, we isolated rat satellite I DNA by PCR with primers designed on the basis of the known rat satellite I DNA sequence. The PCR products obtained as well as the cloned PCR products showed hybridization to the centromeric regions of a large number of chromosomes, but not of chromosome 1, 19, 20, X and Y. Clone 18-5 was further analyzed and was shown to contain at least 4 repeats of the rat satellite I family. This probe, which hybridizes in the centromeric region of 34 of the 42 chromosomes, was used throughout the study as a probe for the FISH analysis of the micronuclei. For the immunocytochemical staining, the commonly used commercial anti-centromeric antibodies could not be used because of the weakness of the fluorescent signals given. Consequently, CREST serum of a single patient was used, which showed bright and distinct signals on the kinetochores of each chromosome. After treatment of the cells with the aneugen DES an increase in centromere (FISH) and kinetochore (CREST) positive micronuclei was found, whereas after treatment with the clastogen MMC, the percentage of centromere-positive micronuclei was similar to that observed in controls. Analysis of a large number of DES-induced micronuclei showed that the immunocytochemical method is equally as or slightly less sensitive for the detection of chromosomes in micronuclei and we therefore recommend FISH with probe 18-5 for the detection of chromosome loss in rat cells.

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

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

Categorization of micronuclei by size and measurement of each ratio in cytokinesis-block and conventional cultures of human lymphocytes exposed to mitomycin C and colchicine

Micronuclei (MN) assays are very useful tests for monitoring human exposure to mutagens and carcinogens. We investigated the effects of the culture method (either conventional or cytokinesis-block) and exposure time (48 or 72hr) on the frequency and size distribution of MN in human peripheral lymphocytes exposed to mitomycin C (MMC) or colchicine. To quantitatively analyze the effects of the agents, methods and exposure times, we categorized MN by size into small (MN-1), medium (MN-2), and large (MN-3). MN-1 were less than one fifth, MN-2 one fifth to one third, and MN-3 larger than one third of the diameter of the main nucleus.Both MMC and colchicine induced dose-related increases in the frequency of MN. The number and distribution of the size-categorized MN were influenced by the agent, method and exposure time. The conventional culture method was useful for detecting the induction of MN-1 by MMC, whereas the cytokinesis-block method was useful for detecting the induction of MN-1 by colchicine. The ratios of MN in the various size categories reflect the different mechanisms of MN induction by MMC and colchicine.These findings suggest that categorization of MN by size can allow one to differentiate between a clastogen and an aneuploidogen, and that the ratios of MN in the three size categories may provide a good index for estimating the type of MN induction for human monitoring.

Micronuclei induced in round spermatids of mice after stem-cell treatment with chloral hydrate: evaluations with centromeric DNA probes and kinetochore antibodies

The chromosomal effects of chloral hydrate (CH) on germ cells of male mice were investigated using two methods to detect and characterize spermatid micronuclei (SMN); (a) anti-kinetochore immunofluorescence (SMN-CREST) and (b) multicolor fluorescence in situ hybridization with DNA probes for centromeric DNA and repetitive sequences on chromosome X (SMN-FISH). B6C3F1 mice received single intraperitoneal (i.p.) injections of 82.7, 165.4, or 413.5 mg/kg and round spermatids were sampled at three time intervals representing cells treated in late meiosis, early meiosis, or as spermatogonial stem cells. No increases in the frequencies of SMN were detected for cells treated during meiosis using either SMN-CREST or SMN-FISH methods. After spermatogonial stem-cell treatment, however, elevated frequencies of SMN were detected by both methods. With SMN-FISH, dose trends were observed both in the frequencies of spermatids containing micronuclei and in the frequency of spermatids carrying centromeric label. These findings corroborate the recent report by Allen and colleagues [Allen JW et al.(1994): Mutat. Res. 323:81-88] that CH treatment of spermatogenic stem cells induced SMN. Furthermore, our findings suggest that chromosomal malsegregation or loss may occur in spermatids long after CH treatment of stem cells. Further studies are needed to understand the mechanism of action of the CH effect on stem cells and to determine whether similar effects are induced in human males treated with CH.

Pattern and frequency of nocodazole induced meiotic nondisjunction in oocytes of mice carrying the 'tobacco mouse' metacentric Rb(16.17)7Bnr

Oocytes from (C3H/HeH x 101/H)F1 and Rb(16.17)7Bnr homozygous females were exposed to a range of doses of nocodazole in vitro. The spindle poison caused a dose dependent increase in metaphase I (MI) arrest and hyperploidy. A concentration of 0.03 microgram/ml was found to induce a maximum hyperploid frequency of 3.1% and 11.6% respectively without a high level of MI arrest. Between 0.03 and 0.05 microgram/ml MI arrest increased substantially and reached a frequency of approximately 90%. In a further experiment oocytes from Rb7 homozygous, heterozygous and 3H1 females were exposed to 0.03 microgram/ml nocodazole 4, 6 or 8 h after the onset of maturation. The phase at which the spindle was inhibited resulted in a specific pattern of nondisjunction which in turn was dependent on whether the female carried an Rb metacentric. 3H1 oocytes gave a normally distributed pattern of increase in aneuploid frequency (over the spontaneous value) centering around a 6 h application. This was thought to be due to the interaction of chromosomes with the microtubules of the spindle during attachment and/or alignment. In contrast both Rb homozygotes and heterozygotes gave the same biphasic response, with a high frequency of aneuploidy in the oocytes when nocodazole was applied 4 and 8 h after the onset of maturation. In Rb homozygotes we demonstrated that the Rb bivalent underwent nondisjunction more frequently than the average acrocentric, when nocodazole was administered early.(ABSTRACT TRUNCATED AT 250 WORDS)

Indications for a threshold of chemically-induced aneuploidy in vitro in human lymphocytes

The possible existence of a threshold for compounds inducing chromosomal loss was investigated for four known aneugens (colchicine, COL; carbendazim, MBC; mebendazole, MEB; nocodazole, NOC) and two clastogens (methyl methanesulfonate, MMS; mitomycin C, MMC) using the micronucleus (MN) test in human lymphocytes. The presence of a whole chromosome in the MN was studied by fluorescent in situ hybridization (FISH) using a synthetic pancentromeric oligonucleotide probe. FISH was applied on two different MN preparations: cytokinesis-blocked MN (MNCB) assay, and MN sorted by flow cytometry. At subtoxic concentrations analyzed by MNCB and FISH, COL, MEB, MBC, and NOC induced a concentration-dependent increase in centromere-positive MN (MNCen+). MMC seemed to induce an increase in both types of MN (MNCen- and MNCen+), while MMS induced only MNCen-. On the sorted micronuclei (in a wide range of low to subtoxic concentrations), the concentration-effect profile for MNCen+, with the four aneugens tested, showed a statistically nonsignificant increase over a range of concentrations, followed by a second range of high concentrations with a statistically significant increase. To analyze the existence of a threshold, a piecewise linear regression was applied to the data. The first concentration that showed a statistically significant increase in MNCen+ was chosen as a breakpoint (0.037 microM for COL, 2.62 microM for MBC, 0.27 microM for MEB, and 0.066 microM for NOC). The statistical correlation between observed and predicted values showed a high correlation (r = 0.99), indicating a clear threshold for aneuploidy induction. However, for MMS the concentration-effect profile for MNCen+ showed a continuous concentration-dependent decrease with no threshold. With the two cytotoxicity assays used (Bio-Rad and MTT), no significant reduction was detected either in the protein content or in mitochondrial succinate dehydrogenase activity with all chemicals tested for MN induction. Therefore, our data suggest that the observed thresholds were not due to indirect toxic effects but to real aneugenic effects.

Cytogenetic effects of cigarette smoke on pulmonary alveolar macrophages of the rat

To determine accurately the potential genetic damage induced by toxic inhaled agents, the cells that receive a high concentration of such agents should be analyzed. Pulmonary alveolar macrophages (PAMs) represent such cells. We compared the cytogenetic effects of cigarette smoke on PAMs of rats exposed repeatedly by different methods. This study was part of a larger investigation of the health effects resulting from different methods of exposing rats to cigarette smoke. Fischer 344/N male rats (4/group) were randomly selected from five different exposure groups: 1) nose-only sham-exposed (air) control, 2) whole-body sham-exposed control, 3) nose-only intermittent, 4) nose-only continuous, and 5) whole-body continuous. The rats were exposed 6 hr/day, 5 days/week for 22-24 days. All smoke-exposed rats received the same daily concentration x time product (600 mg.hr.m-3 for the first week, 1200 mg.hr.m-3 thereafter) of cigarette smoke. Rats were injected intraperitoneally with colchicine at the end of exposure. PAMs were obtained by lung lavage and chromosomal damage was measured. Highly significant smoke-induced differences in both structural and numerical aberrations were observed in continuously exposed rats vs. sham controls, regardless route of exposure. The structural aberrations observed were chromatid-type deletions. Both hypoploid and hyperploid cells were detected. Our data suggest that cigarette smoke is clastogenic and may disrupt spindle-fiber formation. These activities may play a role in the induction of human carcinogenesis caused by cigarette smoke exposure.

Gene dosage mutants at adenine phosphoribosyltransferase locus induced by colcemid in Chinese hamster V79-AP4 cells

Pseudodiploid Chinese hamster V79-AP4 cells, functionally diploid at the adenine phosphoribosyltransferase (aprt) locus, were treated with colcemid, a well-known aneuploidizing agent, under various experimental conditions. Aneuploid and tetraploid cells and variants resistant to 10 micrograms/ml of 2,6-diaminopurine (DAP), which selects for presumptive aprt+/- heterozygotes in the untreated cells, were induced. Many of the induced variants were hypotetraploid with three (rather than four) chromosomes carrying the aprt gene. Dot-blot and Southern analysis of the DNA of these clones confirmed that they had three copies of the aprt gene. Their APRT specific enzymatic activity was 60-80% of that of wild-type V79-AP4. The results of these and other experiments suggest that in these variants resistance to DAP is due to an altered aprt gene dosage and point to a possible genetic effect of colcemid and other aneuploidizing agents in somatic mammalian cells.