Influence of culture time on the frequency and contents of human lymphocyte micronuclei with and without cytochalasin B

In vivo micronuclei in uncultured T-lymphocytes of male railroad transit workers and referents

In the biomonitoring of human genotoxic effects, micronuclei (MN) usually are scored in phytohaemagglutinin-stimulated cultured lymphocytes. MN also can be examined in uncultured lymphocytes, which facilitates the analysis of genotoxic damage incurred in vivo. Characterization of MN in cultured lymphocytes by fluorescence in situ hybridization (FISH) has shown a clear over-representation of the X and Y chromosomes in the MN of males. However, it is not known if this phenomenon also occurs in vivo. The purpose of the present study was to assess the frequency and composition of MN formed in vivo from immunomagnetically isolated uncultured T-lymphocytes of men. To evaluate the possible effects of genotoxic exposure on in vivo MN, we examined 17 railroad workers occupationally exposed to complex chemical mixtures and 14 referents, all nonsmokers. The results showed similar total frequencies of micronucleated cells among the exposed workers and the referents. When the MN were characterized by FISH, there were no significant differences between the exposed and referents with regards to the frequency of centromere-positive or centromere-negative MN. Centromeric label was observed in 69% of all MN, indicating that most of the MN contained whole chromosomes (or chromatids). 80% of the centromere-positive MN harbored autosomes, 12% Y chromosomes, and 8% X chromosomes. The occurrence of the Y- and X-chromosomes in MN was, respectively, 5.5- and 3.8-times greater than would be expected assuming an equal contribution by all chromosomes. Thus, sex chromosomes appear to be over-represented in lymphocyte MN of men in vivo, confirming previous results obtained in vitro.

Merotelic kinetochores in mammalian tissue cells

Merotelic kinetochore attachment is a major source of aneuploidy in mammalian tissue cells in culture. Mammalian kinetochores typically have binding sites for about 20-25 kinetochore microtubules. In prometaphase, kinetochores become merotelic if they attach to microtubules from opposite poles rather than to just one pole as normally occurs. Merotelic attachments support chromosome bi-orientation and alignment near the metaphase plate and they are not detected by the mitotic spindle checkpoint. At anaphase onset, sister chromatids separate, but a chromatid with a merotelic kinetochore may not be segregated correctly, and may lag near the spindle equator because of pulling forces toward opposite poles, or move in the direction of the wrong pole. Correction mechanisms are important for preventing segregation errors. There are probably more than 100 times as many PtK1 tissue cells with merotelic kinetochores in early mitosis, and about 16 times as many entering anaphase as the 1% of cells with lagging chromosomes seen in late anaphase. The role of spindle mechanics and potential functions of the Ndc80/Nuf2 protein complex at the kinetochore/microtubule interface is discussed for two correction mechanisms: one that functions before anaphase to reduce the number of kinetochore microtubules to the wrong pole, and one that functions after anaphase onset to move merotelic kinetochores based on the ratio of kinetochore microtubules to the correct versus incorrect pole.

Micronucleus incidence and their chromosomal origin related to therapy in acute lymphoblastic leukemia (ALL) patients: detection by micronucleus and FISH techniques

Micronucleus assay and dual color-fluorescence in situ hybridization (DC-FISH), using centromere-specific and whole chromosome-specific painting probes, are considered a useful screening test to determine the incidence of micronucleus, their origin and contents. The patients with acute lymphoblastic leukemia (ALL), who had undergone chemotherapy, were analysed before and after treatment with vincristine, methotrexate, daunomycin, prednisone, and asparaginase. The incidence of micronuclei after the antileukemic agent treatment was significantly higher than before the treatment. Application of DC-FISH using a combination of whole chromosome-specific painting probes and the same chromosome-specific alpha-satellite centromeric probe showed that there were no significant differences in the micronucleus incidence for any specific chromosome (chromosomes 7, 8, 11, 17, X, and Y). There were no significant differences between the incidence of centromere-positive micronuclei and the incidence of centromere-negative micronucleus. We concluded that antileukemic agents induced the somatic genetic damage but this damage is not related to any specific chromosome studied.

Non-disjunction and chromosome loss in gamma-irradiated human lymphocytes: a fluorescence in situ hybridization analysis using centromere-specific probes

Centromere-specific DNA probes for chromosomes 4, 7 and 18 were used to simultaneously analyze chromosome loss, non-disjunction, breaks within the labeled region, and nucleoplasmic bridges induced by gamma rays in binucleated human lymphocytes. The doses used were 0, 1, 2 and 4 Gy, and approximately 1000 cells were scored per dose. Micronucleus frequency increased in a linear-quadratic fashion. For chromosome loss, significant increases were observed at 2 and 4 Gy, whereas for non-disjunction significant increases were observed at 1 Gy; thus non-disjunction allowed us to detect the effects of radiation at a lower dose than chromosome loss. The use of centromere-specific probes allowed discrimination between the clastogenic and aneugenic effects of ionizing radiation. The analysis of chromosome loss, not taking fragmented signals into account, ensures the detection of an aneugenic effect, which was not possible using pancentromeric probes. The frequency of chromosome breakage within the labeled regions was higher in nuclei than in micronuclei, suggesting an increase in the engulfment of chromosomal material by nuclei as a consequence of the presence of cytochalasin B in the cultures. Chromatin filaments connecting main nuclei, the so-called nucleoplasmic bridges, were observed in irradiated samples, and are a manifestation of rearranged chromosomes producing anaphase bridges.

Increased nondisjunction of chromosome 21 with age in human peripheral lymphocytes

Fluorescence in situ hybridization (FISH) on binucleated cells with chromosome-specific DNA probes provides a convenient way to visualize reciprocal segregation patterns in daughter nuclei, and overcomes most problems related to the artefactual loss or gain of chromosomes that flaw chromosome preparations. In this study, FISH was employed to evaluate age- and sex-effects on spontaneous malsegregation, nondisjunction and loss of chromosome 21 in human lymphocytes after the first division in culture. A total of 68 healthy nonsmokers and nondrinkers of alcohol (37 males and 31 females) were grouped by age as Group I (0-10 years), Group II (20-30 years), Group III (40-50 years) and Group IV (60-70 years), with at least seven subjects per group and sex. FISH with a pericentric chromosome 21 specific DNA probe was carried out on binucleated lymphocytes, cytokinesis-blocked by cytochalasin B (6 microg/ml for 26 h) at 44 h after initiation of cultures. Linear regression analyses demonstrated a significant age-related increase in the frequency of micronuclei without chromosome 21 (MN-21)(r=0.73, p<0.001 in females; r=0.69, p<0.001 in males) in all binucleated cells, with a steeper slope in females (0.1758) than in males (0. 1241). Analysis using the 2x2 chi-square (chi(2)) test on the frequencies of MN-21 showed significant age-related differences in both males and females, except males in Group III and Group IV (p>0. 05). A significant sex-related difference was found only in subjects over 60 years (p<0.05), with females having more MN-21 (12.57 per thousand vs. 8.43 per thousand) than males. Loss of chromosome 21, occurring at mean levels of 0.38 per thousand in all binucleated cells and 0.24 per thousand in binucleated cells containing four FISH signals, was shown not to be age- or sex-related. A positive age-related increase in nondisjunction of chromosome 21 was shown in males (r=0.50, p<0.01), females (r=0.61, p<0.001) and all subjects (r=0.55, p<0.001) by linear regression analysis. An age effect was found only between children and adults (p<0.01 for females, p<0.05

Differences in malsegregation rates obtained by scoring ana-telophases or binucleate cells

In this work we have applied in situ hybridization with alphoid centromeric probes specific to chromosomes 7 and 11 to ana-telophase cells from human primary fibroblasts. The aim was to visualize the events leading to aneuploidy directly during anaphase, analyse the induction of aneuploidy during this mitotic stage and compare the frequencies of chromosome malsegregation observed in ana-telophases with the estimated malsegregation obtained in binucleate cells after a short cytochalasin B treatment. Significantly higher frequencies of chromosome loss and chromosome non-disjunction were observed in fibroblasts undergoing ana-telophase during recovery from a nocodazole-induced mitotic arrest compared with binucleate cells obtained by a further 30 min incubation with cytochalasin B. Using the same experimental schedule, analysis of hybridization signals in mononucleate cells showed higher frequencies of polyploid nuclei in cytochalasin B-treated cultures, indicating that part of the ana-telophases observed after release from the nocodazole-induced mitotic arrest may give rise to polyploid mononucleate cells instead of binucleate ones. A reduced distance between spindle poles was also measured in cells undergoing ana-telophase in the presence of cytochalasin B. Our study suggests that in nocodazole and cytochalasin B-treated cultures the shorter pole-to-pole distance may favour the reformation of a single membrane around telophase chromosomes, especially when several lagging chromosomes lie between the two future daughter nuclei. This would give rise to polyploid mononucleate cells at the ensuing interphase.

Micronuclei in blood lymphocytes and genetic polymorphism for GSTM1, GSTT1 and NAT2 in pesticide-exposed greenhouse workers

The frequency of micronuclei (MN) in cultured peripheral lymphocytes was used as a biomarker of genotoxic effects in 34 Italian pesticide-exposed greenhouse workers and 33 unexposed referents matched with the exposed workers for age and smoking habits. The possible influence of the genetic polymorphisms of xenobiotic metabolizing enzymes glutathione S-transferase M1 (GSTM1), T1 (GSTT1), and N-acetyltransferase 2 (NAT2) was also evaluated. To restrict the analysis primarily to cells that have divided once in vitro, MN were scored only in cells showing label after a 42-h incubation with bromodeoxyuridine (BrdU), as detected by immunofluorescence (anti-BrdU technique). Two different concentrations of BrdU (0.5 and 1 microg/ml) were compared. Individual frequencies of micronucleated cells (MNCs) obtained with the two concentrations of BrdU significantly correlated with each other (r=0.55, P<0.001). Higher mean MNCs frequencies (per 1000 cells) were detected among exposed smokers (9.0 at 0.5 microg/ml BrdU and 7.8 at 1 microg/ml BrdU) than in smoking referents (6.3 and 5.9, respectively). In multiple regression analysis controlling for age, sex, smoking and genotypes, a significant elevation of MNC frequency (P=0.004 at 1 microg/ml BrdU; P=0.052 at 0.5 microg/ml BrdU) was observed in greenhouse workers with a work history of extensive pesticide spraying (n=17). Increased MNC frequencies were also associated with ageing at 0.5 microg/ml BrdU, with the GSTM1-positive genotype at both 1 (P=0.028) and 0.5 (P=0.056) microg/ml BrdU in all subjects, and with the NAT2 fast acetylator genotype in smokers at 0.5 microg/ml BrdU (P=0.043). The results indicate that MN rates are increased in greenhouse workers, especially in those involved in pesticide spraying. The GSTM1 positive and NAT2 fast genotypes appear to be associated with elevated MNC frequencies, which contradicts with earlier results on elevated chromosomal aberration rates in GSTM1 null smokers and NAT2 slow subjects.

Age-dependent inclusion of sex chromosomes in lymphocyte micronuclei of man

Two-color centromeric FISH was used to study the inclusion of the X and Y chromosomes in micronuclei of cultured lymphocytes from 10 men representing two age groups (21-29 years and 51-55 years). In addition, pancentromeric FISH was separately performed to identify any human chromosomes in micronuclei. One hundred micronuclei per probe were examined from each donor. A higher mean frequency of Y-positive micronuclei was observed in the older men than in the younger men. In both age groups, the X chromosome was micronucleated clearly more often than expected by chance, and the Y chromosome was overrepresented in micronuclei among the older men but not among the younger men. In lymphocytes of four women, X-positive micronuclei were more frequent than they were in men, even after the fact that women have two X chromosomes was taken into account. Similar results were obtained in first-division lymphocytes identified by cytochalasin-B-induced cytokinesis block. In comparison with normal cells, these binucleate cells showed a higher frequency (per 1,000 nuclei) of X-positive micronuclei (in the older men) but a lower frequency of micronuclei harboring autosomes or acentric fragments. In conclusion, the results show that both the X chromosome and the Y chromosome are preferentially micronucleated in male lymphocytes, the Y chromosome only in older subjects. Although the X chromosome has a general tendency to be included in micronuclei, it is micronucleated much more often in women than in men, which is probably the main reason for the high micronucleus frequency in women that has been documented in many previous studies.