Sister-chromatid exchange analyses in rodent maternal, embryonic and extra-embryonic tissues: transplacental and direct mutagen exposures

Levels of 5-Bromo-2′-deoxyuridine and Its Metabolites During Continuous Infusion Paradigms in a Transplacental System

The levels of 5-bromo-2'-deoxyuridine (BrdUrd) (a teratogen and mutagen) and its metabolites were measured using the hplc in maternal and fetal blood at 20–21 days of gestation in Sprague-Dawley derived CD rats during continuous infusion paradigms which are used in visualization of sister chromatid exchanges. The time course of the maternal values following a dose of 1 g/kg body weight was determined by sampling with an aortic catheter. Fetal levels were taken after a 2-hour intraperitoneal infusion of BrdUrd into the mother. It was found that the maternal level of BrdUrd plateaued at 0.4 mg%, while the level of metabolites gradually increased with time for both techniques. Also, although the overall amount of radioactivity indicated that approximately 40% of the total maternal dose reached the fetus, the fetal concentration of BrdUrd was less than one-seventh the maternal one.

Sister-chromatid exchange: second report of the Gene-Tox Program

This paper reviews the ability of a number of chemicals to induce sister-chromatid exchanges (SCEs). The SCE data for animal cells in vivo and in vitro, and human cells in vitro are presented in 6 tables according to their relative effectiveness. A seventh table summarizes what is known about the effects of specific chemicals on SCEs for humans exposed in vivo. The data support the concept that SCEs provide a useful indication of exposure, although the mechanism and biological significance of SCE formation still remain to be elucidated.

Transplacental genotoxicity of triethylenemelamine, benzene, and vinblastine in mice

Transplacental cytogenetic effects of triethylenemelamine (TEM), benzene, and vinblastine on maternal mice and their fetuses have been investigated using micronucleus and sister chromatid exchange (SCE) as genetic endpoints. CD-1 mice were treated on day 14 and 15 of gestation with TEM (0.125, 0.25, and 0.5 mg/kg), benzene (439,878, and 1,318 mg/kg), and vinblastine (0.5, 1, and 2 mg/kg) by intraperitoneal injection at 24 hr intervals, and sacrificed 40 hr after the first injection. Erythrocytic precursor cells in maternal bone marrow and fetal livers (2-4) from each pregnant mouse were used for the micronucleus and/or the SCE analyses. Significant dose-related increases in both micronuclei and SCE were found in maternal bone marrow and fetal liver following TEM treatment. Benzene at the highest dose (1,318 mg/kg) also caused a significant increase in micronuclei and SCE in both maternal bone marrow and fetal liver cells. The embryonic genotoxic effect of TEM was much higher than that of benzene for both genetic endpoints, and the frequency of micronuclei induced by benzene was higher in fetal liver than in maternal bone marrow cells. Vinblastine, a spindle poison, induced micronuclei but not SCE. Micronuclei induction by vinblastine was 7 fold greater in maternal bone marrow than in fetal liver cells. All three chemicals were cytotoxic in maternal bone marrow cells, but not in fetal liver cells except for TEM, which showed a weak cytotoxicity in fetal liver cells in the micronucleus assay. These results indicate that TEM, benzene, and vinblastine are transplacental genotoxicants in mice.

Advantages of a combined testing protocol (CTP) with multiple endpoints: the reverse tier approach

A multi-endpoint CTP approach provides an efficient procedure to investigate the hazards of chemicals in our environment. Genotoxicity to multiple tissues is assessed after an exposure of limited duration by using a variety of sensitive short-term tests. Resources are thus conserved and results obtained without long delays. Additional information helpful in understanding the observed genotoxic effects is readily obtained in the same experiment by inclusion of additional nongenotoxic endpoints. Numbers of animals required to test chemicals are substantially decreased by increasing the amount of information obtained from each animal. Flexibility is a basic asset of a CTP approach. Therefore, pharmacokinetics of the test chemical or mixture and concerns about subpopulations of exposed humans can guide all aspects of the design of CTP protocol including the animal species, the exposure protocol and the endpoints. The procedures selected for evaluating potential mutagens should be based primarily on the usefulness of the information generated and secondarily on the cost and resources required to perform the procedure. The most expensive procedure is that procedure which does not generate useful information, regardless of the initial cost. The CTP as presently discussed is an attempt to maximize useful information in the most efficient manner possible. Although considerably more expensive than in vitro procedures, the protocol is far less expensive than conducting individual short-term animal procedures, and a fraction of the cost and time required to perform a typical cancer bioassay. Chemicals that are of major concern, for example as in the case of extensive human exposure, would be a prime candidate for initial testing by this methodology.

Transplacental teratogenesis and mutagenesis in mouse fetuses treated with cyclophosphamide

We studied transplacental fetotoxicity, teratogenicity, and mutagenicity in Swiss Webster mice following different doses of cyclophosphamide (CP; 0, 5, 10, 15, or 20 mg/kg), a well-known mutagen/teratogen, on day 12 of gestation. The fetal survival and weight on day 18 of gestation decreased significantly with increasing CP dose (P less than 0.01). The CP-treated fetuses were also dysmorphic (e.g., shortened limbs, digital defects, cleft palate, open eyes, and hydrocephaly) and the percentage of dysmorphology increased with increasing CP doses (P less than 0.01). To evaluate mutagenesis, a separate group of females received 5-bromodeoxyuridine tablet (50-mg) implants on day 12 of gestation and a CP treatment 8 h later. Fetal liver cells were harvested 24 h post-BrdU implant to analyze sister chromatid exchange (SCE) frequency and micronuclei. CP caused a significant increase in the SCEs per fetal liver cell from 3.4 +/- 0.02 (control) to 90.0 +/- 0.04 (20 mg/kg CP) (P less than 0.01). The increasing CP dose was also related to an increase in micronuclei. The data suggest that CP is transplacentally toxic, teratogenic, and mutagenic. Further analyses of the data suggest that the mutagenic effects of CP may in fact contribute indirectly to the CP-related teratogenic effects. Such conclusions are based on path analysis with directional causations associated with SCEs per cell and the dysmorphic features studied.

Persistence of SCE-inducing damage in mouse embryos and fetuses following superovulation

The frequency of sister-chromatid exchange (SCE) was analyzed in postimplantation embryos of superovulated Swiss and C57BL/6J Han mice. 12- and 16-day-old embryos, analyzed for SCE in vitro as whole embryo primary cell cultures, displayed an average increase in SCE of about 50%. Although an increased frequency of SCE cannot be used for the prediction of specific effects on the health of an individual, damage and possible repair of DNA which provide opportunities for modification of chromosomal structure and function may be present.

In vitro SCE discrepancy between embryonic and extraembryonic mouse tissues

In vitro sister-chromatid exchange (SCE) background levels and cytokinetics were compared in embryonic (whole embryo cell suspensions) and extraembryonic (yolk sac and amnion, placenta) cells of inbred and outbred strains at various gestational stages (days 12-17). Results indicate a tissue origin (embryonal, extraembryonal) related variation in the formation of baseline SCE frequencies and cytokinetics. The significant higher SCE levels in extraembryonic tissues (maximum increase of 2 X the background values of the embryo cells) were independent of mouse strain and gestational stage. An average of 4-5 SCEs/cell in embryo cells is contrasted by 7-9 SCEs/cell in extraembryo cells. Mitotic index was generally lower and average generation time longer (by 2-3 h) in extraembryonic tissue cells. No significant differences in SCE frequencies and no changes in cytokinetics were detected at the BrdU concentrations used (1.2-4.8 micrograms/ml). The reason for the inter-tissue differences in baseline SCE is still not clear.

A comparison of mutagenic and embryotoxic effects of cyclophosphamide on the chick embryo

The cytogenetic and embryotoxic effects of cyclophosphamide (CP) after injection on day 3 of single doses ranging from 0.1 to 30 micrograms were investigated in the chick embryo. The occurrence of embryotoxicity was checked on incubation day 8; clastogenic effects as well as mitotic indices were evaluated in primitive blood cells 6, 12, 18, 24 and 30 hr after the administration of CP. Teratogenic doses of CP induced high percentages of aberrant cells with many unstable aberrations. Mitotic activity appeared to be inhibited at the early collection intervals. Nonembryotoxic doses of CP also increased the incidence of aberrant cells without any substantial effect upon mitotic activity. Although the clastogenic effects of CP occur at lower doses than the teratogenic effects, the thresholds for both effects lie in the lower part of the embryotoxicity dose range, as estimated by the Chick Embryotoxicity Screening Test (CHEST).

Cytochrome P-450 metabolic activity in embryonic and extraembryonic tissue lineages of mouse embryos

Mouse morulae, blastocysts, and embryonic and extraembryonic tissue layers were examined for benzo[a]-pyrene metabolism by cytochrome P-450, using the sister chromatid exchange assay. Benzo[a]pyrene exposure in vitro increased sister chromatid exchanges in blastocysts of all genetically responsive mice examined [BALB/cDub, C3H/AnfCum, and outbred Dub:(ICR) strains] but not blastocysts of the nonresponsive AKR/J strain. Benzo[a]pyrene treatment of responsive 7 1/2- and 8 1/2-day (postimplantation-stage) embryos, either intact or as separate tissue layers, increased sister chromatid exchanges in tissues of both embryonic and extraembryonic lineages--i.e., in the embryo proper, in isolated embryonic ectoderm, and in yolk sac, chorion, extraembryonic ectoderm, and extraembryonic endoderm layers. These results indicate that cytochrome P-450 is active in most or all tissues of the early mammalian embryo. It could metabolize xenobiotic molecules reaching the conceptus near the onset of morphogenesis and organogenesis, or it could have another as yet undefined role in normal development.

A method for analysing sister-chromatid exchange in mouse preimplantation embryos

Analysis of sister-chromatid exchange (SCE) has been shown to be a sensitive and reproducible method for detecting the action of mutagens and carcinogens. We have succeeded in establishing a reliable technique which allows to perform SCE in preimplantation embryos in order to make the pre-uterine stages of development accessible to routine detection of DNA damage. Using the mouse strain and technique described, approximately 30-40% of mice will mate successfully after synchronization and spontaneous ovulation. From 3 pregnant females, about 30 four- to eight-cell embryos will be obtained, representing one experimental group providing approximately 50-80 two-S-phase labelled metaphases with a SCE frequency baseline below 6 exchanges.

Stage-specific induction of sister-chromatid exchanges in utero

In order to correlate the induction of sister-chromatid exchanges (SCE) to biological endpoints, and elucidate aspects of this relationships, 7,12-dimethylbenz[a]anthracene (DMBA) and methyl methanesulfonate (MMS), chemicals with different biological actions at different stages in development, have been evaluated for their ability to induce SCE at different gestational ages in the Sprague-Dawley rat. Transplacental exposure to these agents was accomplished by a recently developed intraperitoneal infusion technique to replenish metabolically degraded 5-bromo-2'-deoxyuridine used for SCE visualization. Maternal bone marrow and whole fetal tissue, fetal liver and fetal brain were compared. Day-9 embryo was found to be very sensitive to the effects of both agents, with the ability to induce SCE declining in development in whole fetus and fetal organs. The embryotoxic effects of the agents seem to be ones best correlated with the capacity of the agents to induce SCE. Also, fetal liver is more sensitive than fetal brain to the effects of DMBA compared with MMS, suggesting fetal metabolic activation may have occurred. Measurement of the amount of radiolabelled DMBA reaching the fetal tissue used to estimate SCE indicates that the amount of chemical reaching the fetus does not account for the increased sensitivity, especially at Day 9. Some factor(s) in development, such as differentiation stage, rather than the fetal accessibility to chemical, seem to be important in the induction of SCE in utero.

Metabolic activation of cyclophosphamide by yolk sac endodermal cells of the early chick embryo

Endodermal cells isolated from the yolk sacs of day 3 chick embryos were able to activate metabolically cyclophosphamide. This was demonstrated by the cytotoxicity of cyclophosphamide to the endodermal cells themselves as well as by the ability of endodermal tissue to mediate a cytotoxic response in coculture with KB cells, a human tumor cell line unable to activate cyclophosphamide. Yolk sac endodermal cells from day 3 embryos were sensitive to cyclophosphamide when the drug was added immediately after the start of culture, but not when the drug was added after 24 hr of culture. The ability to metabolize cyclophosphamide by the day 3 embryo appeared limited to the endodermal cells of the yolk sac as cells derived from neither the embryo proper nor yolk sac mesoderm-ectoderm tissue were positive in these tests. Using whole blastoderms, cyclophosphamide activation was detected as early as 12 hr of egg incubation.

The effects of diagnostic ultrasound on sister chromatid exchange frequencies: a review of the recent literature

The precise nature and significance of SCEs are still not understood. It is clear that SCE analysis is a sensitive means of detecting genotoxic agents, and that SCEs are correlated with mutation, in vitro transformation, and cancer. SCEs, as all other short-term assays, are subject to false positives and false negatives. Therefore, it is possible that even if ultrasound induces SCE, the effects may not be deleterious, although the reverse could be true as well. The majority of studies have not observed SCE induction after exposure to diagnostic ultrasound, however two positive responses have been reported. Such discrepancies cannot be explained until investigators adopt a uniform protocol for ultrasound exposure. The currently widespread use of ultrasound in antepartum diagnosis, and the rate at which its use increases, argue strongly for additional research into its potential for genetic toxicity.

In vitro development of the mammalian embryo

Normal growth and differentiation of mammalian embryos in vitro during the preimplantation period appear to be dependent upon the availability of appropriate metabolic substrates. For preimplantation embryos, defined conditions of culture have been achieved only in a few laboratory species. There is now evidence that differentiation factors isolated from fetal calf serum and human placental cord serum may promote further development of blastocysts. Postimplantation rat and mouse embryos can be cultured during the organogenesis period with rat or human sera in roller bottles. The embryonic differentiation of the rat at this stage of development is progressively retarded in such cultures with male rat serum. The embryonic development is not improved, even in sera obtained from rats at different days of gestation (12, 15-16, and 20-21). Inability to grow placental tissues simultaneously with embryos, accumulation of unfavorable substances, and rapid depletion of nutrients contribute to the retardation of embryonic growth. To improve growth and differentiation of conceptuses, a continuous culture system with the possibility of infusion of increasing concentrations of oxygen in the roller bottle gas atmosphere has been developed. This improved method allows considerable continuous growth and differentiation from the neurula stage with development of numerous primary organs. Utilizing these in vitro culture methods during pre- and postimplantation periods, it is now possible to assess embryotoxic or teratogenic potential of drugs and chemical agents. The postimplantation culture procedure allows a more precise assessment of mechanisms associated with anomalous embryonic differentiation. Bioactivation of teratogens and effects of active toxic metabolites on organ primordium differentiation have been shown by combining embryo culture with a hepatic microsomal activating system. Microinjection of teratogens and cells into conceptus compartments is being used to elucidate specific anomalous differentiation processes.

Short-term tests for transplacentally active carcinogens. Induction of sister-chromatid exchanges in foetal brain, lung and blood-forming cells by procarbazine and cyclophosphamide

The induction of sister-chromatid exchanges (SCEs) by cyclophosphamide (CP) and procarbazine (PC) in mouse granulocyte-macrophage precursor cells (GM cells) and erythroblasts from foetal liver, and cells from foetal brain and foetal lung has been measured. Agents were administered in vivo, and cells explanted into BrdUrd-containing medium for 2 cell cycles in vitro (using specific growth-promoting substances where necessary) to determine SCE frequency. Tissue and cell-type differences in responses were observed, and it is concluded that the in vivo/in vitro transplacental SCE technique is a useful indicator of genotoxic effects of agents which are potential transplacental carcinogens.

Short-term tests for transplacentally active carcinogens. A comparison of sister-chromatid exchange and the micronucleus test in mouse foetal liver erythroblasts

The effectiveness of 6 chemicals (benzo[a]pyrene, (BaP), cyclophosphamide (CP), diethylnitrosamine (DEN), methyl methanesulphonate (MMS), mitomycin C (MC) and procarbazine (PC) ) as inducers of micronuclei in foetal liver and maternal bone marrow erythroblasts has been determined, and related to that of gamma-radiation. CP, DEN, MMS and PC were all more effective in the foetal liver. The induction of micronuclei and SCEs by each chemical in foetal erythroblasts after in vivo exposure was measured. When expressed as induction of sister-chromatid exchanges (SCEs) per erythroblast/induction of micronuclei per erythroblast (/microM/kg), the ratios obtained were MC 580, BaP 470, DEN 430, CP 258, MMS 140 and PC 13. The lowest doses detected as potentially genotoxic by each test in foetal liver erythroblasts are (with the exception of PC which is a relatively ineffective inducer of SCEs) similar. When isolated foetal livers were exposed in vitro, SCE dose responses to BaP, MC, MMS and PC could be directly related to those from in vivo exposure, indicating the role of the foetal liver in metabolic activation, but CP was considerably more cytotoxic. The transplacental micronucleus test, and in vivo/in vitro method for SCEs in foetal liver erythroblasts, provide sensitive, complementary assays for genotoxic effects of chemicals during prenatal life. Since foetal liver possesses greater metabolic potential than adult bone marrow, the transplacental tests respond to genotoxic agents not detected by bone-marrow systems.

New approaches to mutagenicity studies in animals for carcinogenic and mutagenic agents. II. Clastogenic effects determined in transplacentally treated mouse embryos

Cell suspensions from whole embryos were obtained on the 12th day of gestation from treated pregnant female mice. For the present experiments five model mutagens-BaP, bleomycin, mitomycin C, procarbazine, and TEM--were chosen for their different modes of action in inducing chromosomal aberrations. Time-response and dose-response were studied for chromosomal aberrations induced by transplacental treatment of mouse embryos. All five known mutagens gave a positive response in the present system. The maximum time of response varied from compound to compound and was found as early as 3h after treatment with the G2-clastogen bleomycin or as late as 18 h after treatment with the bifunctional alkylating agent TEM. Chromatid breaks and exchanges increased with dose in all instances. The correlations between clastogenic, carcinogenic, and teratogenic effects are discussed. It is concluded that the transplacental cytogenetic test may be applicable to identify chemicals that exhibit all three properties.

Sister chromatid exchange in childhood malignancies

Sister chromatid exchange studies were conducted in 53 children with malignant diseases. Children with acute lymphoid leukemia in long remission, already off chemotherapy or on maintenance therapy, had normal SCE values, whereas those receiving intensive cytostatic therapy showed elevated levels. SCE values were primarily related to the time and dose of preceding cytostatic drug therapy, and the subsequent relapse might influence the SCE frequencies, independently from the cytostatic therapy.

Transplacental and direct exposure of mouse and marmoset to ethylnitrosourea: analysis of chromosome aberrations

The effect of ethylnitrosourea (ENU) on chromosomes of mouse bone marrow cells and transplacentally exposed embryonic liver cells was investigated. Chromosome aberrations were found to be dose- and time-dependent. The maximum damage was seen 6 h after the exposure. Chromosome aberrations were also induced in bone marrow cells and lymphocytes of marmosets (Callithrix jacchus) directly exposed to ENU. Aberrations did not, however, occur in offspring whose mothers were treated with ENU before conception. Furthermore, chronic transplacentally exposed offspring have been analyzed. The frequency of chromosome aberrations was not increased in their lymphocytes and fibroblasts. The elimination of chromosome aberrations during embryogenesis is discussed.

Effect of benzo[a]pyrene on sister-chromatid exchange in fetal hamster liver exposed in utero

Benzo[a]pyrene between 50 and 125 mg/kg administered maternally caused a dose-related increase in sister-chromatid exchange in fetal hamster liver cells. There was no difference on days 11, 13 and 15 of gestation in the sensitivity of fetal liver to benzo[a]pyrene.

Effect of diesel exhaust emissions, particulates, and extract on sister chromatid exchange in transplacentally exposed fetal hamster liver

The genotoxic activity of diesel exhaust emissions, particulate matter, and an organic extract of the particulate matter was evaluated in transplacentally exposed Syrian hamster fetal liver cells. The frequency of sister chromatid exchange (SCE) was determined on day 13 of gestation. The extract of diesel particulate matter caused a dose-dependent increase in the frequency of SCE with a doubling in the incidence above 320mg/kg. The diesel particulate matter and diesel exhaust emissions did not alter the frequency of SCE. The extract and particulate matter did cause a dose-dependent decrease in the mitotic activity of the fetal liver. The in utero SCE analysis was demonstrated to be a sensitive assay for determination of the genotoxic activity of a complex mixture in transplacentally exposed fetuses.