Characterization of the distribution of sister chromatid exchange frequencies: implications for research design

Sister-chromatid exchanges in cattle: breed, sex and BrdU dose effects

The spontaneous incidence of sister-chromatid exchanges (SCE) was investigated in a group of cattle, composed of 21 animals of both sexes and from two different breeds (Fleckvieh and Pirenaica). Peripheral lymphocytes of these animals were cultured in three different bromodeoxyuridine (BrdU) concentrations: 5, 15 and 30 micrograms/ml. The work was carried out following a randomized block design. Among the analyzed sources of variability, group, breed and BrdU dose factors had significant effects on the SCE frequency. No differences between sexes were found. Comparisons of the BdrU doses showed that the 5 micrograms/ml dose differed from both the 15 and 30 micrograms/ml doses, whereas the 15 and 30 micrograms/ml doses did not differ from each other. The results indicate that the breed of cattle as well as the BrdU dose chosen for the analysis must be considered when the SCE test is used for the biomonitoring of environmental mutagens.

Sources of bias, effect of confounding in the application of biomarkers to epidemiological studies

This article addresses some methodological aspects of the application of biomarkers of exposure, effect and susceptibility to cancer epidemiology. The application of biomarkers to cancer epidemiology should enhance the validity of exposure and outcome measurement, and strengthen the statistical association between exposure and diseases, thus reducing the possibility of random and systematic error. However, the use of biomarkers provides in turn new opportunities for bias and confounding. Small sample size is a limitation of many molecular epidemiology studies. Three major types of bias are recognized: selection bias, information bias (measurement error), and confounding. An important aspect of confounding is that biomarkers can be seen both as exposure and outcome. A second aspect of confounding lies in the role of the biological marker in the causal pathway between exposure and disease. Molecular epidemiology offers better opportunity for the elucidation of interactions between genetic and environmental factors. Genetic susceptibility studies should demonstrate the highest disease risk for exposed susceptible groups and the lowest risk for non-susceptible unexposed groups.

Sister-chromatid exchanges in lymphocytes are increased in relation to longitudinally measured occupational exposure to low concentrations of styrene

A longitudinal investigation of styrene exposure was conducted among 48 workers employed at a reinforced plastic boat manufacturing facility. 8-h time-weighted average (TWA) exposures to styrene and concentrations of styrene in the breath were determined for each individual on 7 randomly chosen days during 1 year. Peripheral blood lymphocytes from each subject were analyzed for sister chromatid exchanges (SCEs) 2 times and micronuclei (MN) 4 times during this period. Individual mean SCEs ranged from 4.7 to 9.5 SCEs per cell with a population mean of 6.4 +/- 0.2 SCEs per cell. SCEs were found to be significantly increased with an overall observed increase of 11.7% related to increasing exposure to styrene (mean air concentration 64.2 mg/m3 +/- 71.5; range 0.88-235 mg/m3) and with cigarette smoking. Examination of the relative contribution of each variable to regression of SCEs showed that smoking contributed about 62% and styrene exposure contributed about 25% of the total variability. Intra-individual lymphocyte MN frequencies did not vary significantly over time nor was a gradient toward increased MN observed with styrene exposure. However, significant inter-individual differences in MN frequencies were observed. Females had significantly higher MN frequencies than did males; MN were also increased with age. This study is highly unusual in that it illustrates the ability to separately quantify the relative contribution of each of two variables--smoking and styrene exposure--to an increase in SCEs in lymphocytes of an exposed human population.

Synergistic effects of azathioprine and ultraviolet light detected by sister chromatid exchange analysis

The immunosuppressant azathioprine and longwave ultraviolet (UV) light have been postulated to have a synergistic effect on DNA resulting in carcinogenic change. This study investigated the in vitro effect of UV light on renal transplant recipients (RTRs) immunosuppressed with azathioprine and prednisolone to prevent rejection and patients on azathioprine for skin conditions and normal controls on no drug therapy. The results show clearly that there is an increase in sister chromatid exchanges (SCEs) in both patient groups in response to UV light with the greatest increase shown by the RTRs. Both patient groups without exposure to UV also showed a significant increase, as compared with normal controls, with the RTRs again showing the highest level. Both patient groups also showed disruption of the cell cycle in response to UV light as measured by the proliferative rate index (PRI) but the controls did not. These findings support the hypothesis of a synergistic effect between azathioprine and UV light and correlate well with the clinical pattern of carcinogenesis observed in these patients, with RTRs having a well-documented increased risk of neoplasia, particularly skin carcinomas in light-exposed areas.

Lack of sister-chromatid exchange induction by polymethyl methacrylate bone cement in human lymphocytes cultured in vitro

Human lymphocytes cultured in vitro were used to assess the ability of polymethyl methacrylate (PMMA), currently used in orthopedic surgery as bone cement, to induce sister-chromatid exchanges (SCE). Under the conditions used in this study, PMMA bone cement did not produce any significant increase in SCEs in phytohemagglutinin-stimulated human lymphocytes. However, a significant decline (p less than 0.05) in the proliferation rate index was observed, which might be an indicator of the cytotoxic effect of PMMA.

Enhancement of cytogenetic damage by chlorpromazine in human lymphocytes treated with alkylating antineoplastics and caffeine

In cultured human lymphocytes chlorpromazine (CPZ) was found to induce cell division delays and to have no effect on sister-chromatid exchanges (SCEs) or on mitotic indices (MIs). CPZ induces cytotoxic effects in combination with caffeine (CAF) and alkylating agents. In combination with CAF it induced cell division delays and suppression of MIs. In combination with melphalan (MEL) and CAF, CPZ synergistically induced SCEs, caused cell division delay and suppressed MIs. In combination with chlorambucil (CBC) and CAF, CPZ produced synergism on induction of SCEs, enhanced cell division delays and reduced MIs.

Sister-chromatid exchange (SCE) rate in normal and abnormal sexual development in males and females

Thirty-three patients with abnormal sexual development (9 male hypogonads, 20 females with primary amenorrhea and 4 cases of ambiguous genitalia), 10 normal males and 8 normal females (below the age of 30 years) were evaluated for SCE/cell and for SCE distribution according to chromosome groups (A to G). Smokers and alcoholics and subjects under medication were excluded from the study. The average rates of SCE/cell in male hypogonads, primary amenorrhea and ambiguous genitalia were 4.23 +/- 1.51, 4.02 +/- 0.90 and 4.33 +/- 1.34, respectively, whereas in normal males and females the average rates were 4.27 +/- 0.69 and 4.49 +/- 0.87, respectively. The SCE data followed a Poisson distribution. Chi-square testing showed a statistically significant difference only in B-group chromosomes when male hypogonads were compared with normal males (p less than 0.02) and females with primary amenorrhea were compared with normal females (p less than 0.02), suggesting the importance of the study of SCE frequency distribution at chromosome group level to bring out the differences otherwise concealed in average rates.

Sister chromatid exchanges and dementia of the Alzheimer type

Sister chromatid exchange (SCE) analysis was carried out on ten patients with the clinical diagnosis of dementia of the Alzheimer type (DAT) and 11 cognitively-intact controls of a comparable age. There was no significant difference between patient and control groups in either the mean baseline SCE frequency or the SCE frequency following in vitro exposure to mitomycin-C. The present results, based on the largest sample (n = 21) published to date, fail to confirm the single positive report in the literature of an increased baseline SCE frequency in DAT patients compared to controls.

Sister-chromatid exchanges in human lymphocytes exposed to 1-p-(3-methyltriazeno)benzoic acid potassium salt

The 1-p-(3-methyltriazeno) benzoic acid potassium salt (MTBA) is a triazeno analogue of dacarbazine, an antineoplastic agent capable of mediating the appearance of new antigenic specificities on cancer cells in mice, a phenomenon described as 'chemical xenogenization' (CX). Recently we reported the clastogenic potential of MTBA on human lymphocytes. Since sister-chromatid exchange (SCE) assay is more sensitive than clastogenic tests, at least at low drug concentrations, we assessed SCE frequencies induced by MTBA on human lymphocytes stimulated by PHA. Drug treatment at 2-500 micrograms/ml was performed in vitro prior to or after PHA addition. SCE values increased significantly in a dose-dependent manner up to 200 micrograms/ml. However, SCE frequencies, as well as chromosome breaks, did not increase dramatically. These data indicate that MTBA concentrations used for CX do not cause severe cytogenetic damage to immune cells at least in vitro.

Induction of sister-chromatid exchanges by d(42 MeV)-Be neutrons in unstimulated human-blood lymphocytes

The ability of d(42 Mev)-Be neutrons and 250 kV X rays to produce sister-chromatid exchanges (SCE) has been re-examined using unstimulated (G0) human-blood lymphocytes. Contrary to a previous report, the neutrons produced a significant and dose-dependent increase in SCE. X rays, as previously, produced no measurable increase at any absorbed dose. The relative biological effectiveness (RBE) for this end-point is therefore undefined and effectively infinite. In contrast to the findings of many workers, the between-cell distributions of SCE were, in most cases, underdispersed. This could imply that the SCE burden of a cell was not selectively neutral in these experiments.

Factors that influence formation of sister chromatid exchanges in human blood lymphocytes

Sister chromatid exchange (SCE) reflects an interchange of DNA sequences between helices in a replicating chromosome. This was initially accomplished by Taylor and colleagues (1957) using tritiated thymidine incorporation followed by autoradiography. The development of an elegant technique for differential staining of sister chromatids by incorporating a thymidine analog, 5-bromodeoxyuridine (BrdU) has greatly simplified the detection of SCEs in metaphase chromosomes. In recent years, the analysis of SCE has been considered to be a highly sensitive and additional (i.e., with chromosome aberrations) end point for measuring mutagenic/carcinogenic potential of various environmental agents and is increasingly being used to detect and differentiate among chromosome fragility human diseases that predispose to neoplasia. Attention has been focused to see if the induction of SCEs in lymphocyte cultures can be used as a reliable "biological dosimeter" for genetic risk assessment and to monitor the exposed populations. Several physical or preparatory as well as biological factors that modify the response and formation of SCEs make the monitoring difficult. The purpose of this article is to review and analyze these factors to facilitate an effective development of a standard protocol for SCE testing and for appropriate evaluation of test results. This may also provide clues to understand the yet unknown molecular mechanism(s) and biological significance of SCE formation.

Variation in sister-chromatid exchange among 106 members of the general U.K. population

SCE scores of lymphocytes from 106 people revealed that the majority of background variation in SCE was between cells within individuals. Highly significant differences existed between individuals. Lesser, but still highly significant differences also existed between replicate cultures. Inter-individual variation was contributed to by each person's sex and their smoking habits. SCE frequency was not influenced by any of the other factors considered, age, drinking habits and diagnostic X-ray exposure of persons or lymphocyte number and proliferation rate in cultures.

Induction of sister chromatid exchanges in human and rat hepatoma cell lines by cyclophosphamide and phosphoramide mustard and the effects of cytochrome P-450 inhibitors

The activity of the cytochrome P-450-associated metabolic pathway in human (HepG2) and rat (H4-II-E) hepatoma cells was examined. The genotoxic activities of cyclophosphamide and its direct acting metabolite, phosphoramide mustard, were studied in the hepatoma cells as cyclophosphamide is known to be metabolized by phenobarbital-inducible cytochrome P-450-associated metabolic activity. HepG2 and H4-II-E demonstrated the capacity to activate cyclophosphamide to forms capable of inducing sister chromatid exchanges in concentration-dependent fashion. Phosphoramide mustard induced a similar pattern of sister chromatid exchanges at concentrations three orders of magnitude lower than cyclophosphamide. The cytochrome P-450-associated enzyme inhibitors, SKF-525A and metyrapone, were found to reduce the level of cyclophosphamide-induced sister chromatid exchanges in HepG2 and H4-II-E, suggesting that cyclophosphamide was activated by this pathway in both hepatoma lines. Direct evidence for the presence of mRNA transcript coding for a phenobarbital-inducible cytochrome P-450 was demonstrated in HepG2 cells by Northern blot analysis. Comparison of genotoxic responses in human and rat hepatoma cells may allow for an evaluation of responses by different species to potentially mutagenic chemicals.

Induction of sister chromatid exchanges by benzidine in rat and human hepatoma cell lines and inhibition by indomethacin

The genotoxic activity of benzidine was studied in two cell lines derived from rat (H4) and human (HepG2) hepatomas which have been shown to be capable of activating certain promutagens. The responses were compared to results in two lung-derived fibroblast lines (IMR-90 and V79) which appear to have little or no metabolizing capability. Benzidine was found to induce sister chromatid exchanges in the two liver-derived cell lines in a dose-dependent fashion but failed to induce sister chromatid exchanges in the fibroblast lines. Since one proposed pathway for benzidine activation involves prostaglandin-mediated metabolism, we tested the effect of pretreatment with indomethacin, an inhibitor of this metabolic pathway. Indomethacin was highly effective in inhibiting benzidine-induced sister chromatid exchanges in both H4 and HepG2 cells. These results suggest that some DNA damage may occur in the livers of fast acetylating species such as the rat without prior N-acetylation and that some amount of DNA damage may occur in the livers of slow acetylating species, even when the liver is not the target organ for carcinogenesis.

Measurements of chemical carcinogen-induced sister-chromatid exchanges in a whole organ in vitro

Sister-chromatid exchange (SCE) was measured in epithelial cells of mammary organs exposed to 3 different carcinogens in vitro. 7,12-Dimethylbenz[alpha]anthracene (DMBA), N-nitrosodiethylamine (DENA) and N-methyl-N-nitrosourea (MNU) are capable of inducing a significant enhancement of SCE over the basal level in the mammary cells. Analysis of SCE caused by the mutagenic and/or carcinogenic agents in the organ culture presents the advantages that: (1) the culture procedure does not require enzymatic dissociation of the tissue and (2) the medium does not need to be supplemented with serum or exogenous mitogenic agents thereby minimizing the basal level of SCE. Moreover, the assay does not require utilization of exogenous microsomal enzymes needed for activation of certain carcinogenic chemicals. SCE measurements in the organ culture system also should provide clues about the mechanism of action of the chemopreventive agents, particularly the antioxidants, which are under investigation for their possible influence on the processes of neoplastic transformation.