[Chromatid exchange and heterochromatin alteration of human chromosomes with BUdR-labelling. Demonstrated by benzimidazolfluorochrome and Giemsa stain]

Comparative analysis of sister-chromatid exchanges in plant and human chromosomes

Species specificity concerning the two main SCE characteristics, namely frequency and localization, of "spontaneous" SCEs in plant cells (root tips of Crepis capillaris) and in human lymphocytes was investigated under comparable conditions. The FPG technique was used for detection of SCEs after bifilar incorporation of BrdU into DNA (TB-BB). Data of parallel experiments showed that the frequency of SCEs in plant cells was considerably higher than that observed in human lymphocytes--13.2 and 7.3 SCEs per cell respectively. The difference was even more pronounced when the SCE frequency was estimated on the basis of DNA content/cell (pg). Analysis of SCE distribution was limited to SCEs localized in the centromere, since contradictory results most frequently concerned this chromosome region. The data of the present experiments showed that the frequency of SCEs localized in the centromere regions of plant chromosomes was considerably lower than that observed in human chromosomes. Compared with the relative sizes of the centromere regions in the two genomes, however, these frequencies proved to be quite similar. In both systems the centromeres were involved in SCE, as could be expected for random distribution, along the length of the chromosomes.

Lymphocyte proliferation and nucleoid sedimentation in a case of premature aging distinct from Werner's syndrome

Lymphocyte proliferation and nucleoid sedimentation were studied in a patient with premature aging resembling the Werner's syndrome (WS). Onset of patchy brown hyperpigmentations at the age of 9 months permitted distinction from classical WS and suggested a WS-like premature aging disease. By photometric recording of density changes during cell culture, we examined the course of cell proliferation after PHA stimulation over 7 days and compared these results to those obtained in two normal controls. Cultured cells of the patient displayed an aberrant proliferation pattern characterized by continuous growth without an initial reduction phase. The markedly reduced proliferative capacity of purified cells from the patient could in part be corrected by fetal bovine serum. The cells of the patient displayed a characteristic nucleoid sedimentation profile after ultraviolet irradiation indicating retarded DNA replication, which may be a common feature of various premature aging diseases. The absence of thermolability of cell proliferation and the presence of a high number of chromatid aberrations disclosed differences from classical WS.

Blood lymphocyte culture system: quantitative analysis of X-ray-induced chromosome aberrations in man, muntjac and cattle

Peripheral blood lymphocytes of 3 mammalian species, man, muntjac and cattle, which have various amounts of DNA and divergent karyotypes, were exposed to 100-400 rad of X-rays, and frequencies of dicentrics and other aberrations were analysed at first post-irradiation metaphases. During experiments, various preparative or physical and biological factors that could influence the yield of chromosome aberrations were taken into account. The frequency of dicentrics scored at first post-irradiation metaphases showed best fit to both linear and quadratic dose-response curves, y = a + bD and y = bD + cD2 with a high correlation coefficient of 0.98 (P less than 0.001). The frequency of dicentrics obtained at different post-irradiation fixation times did not show significant variation, indicating a homogeneous sensitivity of peripheral lymphocytes to X-irradiation. BrdU incorporation following X-irradiation showed no increase in the frequency of chromosome aberrations. The frequency of dicentrics in man, muntjac and cattle showed a close correlation with their DNA content, but no meaningful correlation was found between the yield of dicentrics and the chromosome arm number or the nuclear volume. The ratio of dicentric yields, 1.00:0.67:1.04 obtained in man, muntjac and cattle were comparable to the ratio of their DNA contents, 1.00: 0.65: 1.07. The base-line frequency of SCEs was similar in the 3 species and no significant variation in SCE frequency was noticed even after administration of 400 rad of X-rays.

Sister-chromatid exchanges: a report of the GENE-TOX program

The effects of a number of chemicals on sister-chromatid exchange (SCE) frequencies in in vivo and in vitro systems are reviewed. Standardized protocols for future SCE testing in important systems, as well as for evaluation of test results, are presented. Data reported thus far suggest that SCE analysis may prove useful, especially at a secondary level, as a test of mutagenic carcinogens. Strengths and limitations of SCE analysis are summarized as a guide for future evaluation and use of this procedure.

Pattern of chromosomal replication in synchronized lymphocytes. I. Evaluation and application of methotrexate block

The BrdUrd-Hoechst 33258-Giemsa technique was employed to study patterns of chromosomal replication in human lymphocytes synchronized by Methotrexate (MTX). It is proposed that in the presence of MTX, a major portion of the cell population is blocked in an advanced stage of the S-phase and not in the G1/S border of the cell cycle. At this point, the replication of the chromosomal segments corresponding to the R-bands is terminated, and the replication of the G-bands and the inactive X-chromosomes is being initiated. The use of this method in the study of higher resolution patterns of chromosomal replications is proposed.

Simultaneous staining of sister chromatid exchanges and Q-bands in human chromosomes after treatment with methyl methane sulphonate, quinacrine mustard, and quinacrine

Human peripheral lymphocyte chromosomes were stained simultaneously for sister chromatid exchanges (SCEs) and Q-banding. No effect of treatment with MMS, QM, and Q on the distribution of SCEs in chromosomes was found compared with controls. The SCEs were distributed between chromosomes roughly according to metaphase length, with the shorter chromosomes underrepresented. The majority of SCEs were located to pale bands, while a few occurred in bright bands and at interfaces between pale and bright bands. A greater frequency than expected of SCEs had occurred at identical sites in homologous chromosomes. This frequency was significantly increased after treatment with MMS.

Sister chromatid exchanges in human leukocyte chromosomes: spontaneous and induced frequencies in early- and late-proliferating cells in vitro

Human leukocyte cultures were pulse-treated with the trifunctional alkylating mutagen trenimon in a final concentration of 10(-7) M for 15--20 h after culture start, i.e., in the G1 phase of the cell cycle. At 24 h after culture start bromodeoxyuridine (BUdR) was added to the trenimon-treated cultures and to several untreated cultures running in parallel. The series treated with BUdR only and the series treated with BUdR+trenimon were each used to prepare two cultures at different culture times. Mitoses were collected during consecutive intervals of 12 h from 30 h up to 102 h after culture initiation by colcemid. For all preparation times (42 h, 54 h, 66 h, 78 h, 90 h, and 102 h) the frequencies of first, second, and third and further mitoses were determined in the BUdR- and in the BUdR+trenimon-treated series. In the trenimon-treated series a clear cell cycle delay was detected as compared with the normal distribution of different types of mitoses found in series treated with BUdR only. Spontaneous and trenimon-induced sister chromatid exchange (SCE) frequencies were determined in second mitoses occurring at 66 h, 78 h, 90 h, and 102 h after culture start. For all these preparation times about six SCE per metaphase were consistently found in BUdR-treated, and about 19 SCE per metaphase in BUdR+-trenimon-treated series, indicating a homogeneous sensitivity of early- and late-proliferating cells with respect to the induction of SCE.

Sister chromatid exchanges in cultured peripheral lymphocytes from twins

Sister chromatid exchange points (SCE points) on individual chromosomes were studied in cultured lymphocytes from 11 monozygotic (MZ) and nine dizygotic (DZ) same-sexed pairs by means of sequential Q-banding and BUdR-Giemsa techniques. No statistically significant variation between unrelated individuals with respect to SCE points on specific chromosomes was found. Intrapair differences in the number of SCE points on specific chromosomes were not significantly smaller between MZ twin partners as compared with DZ partners. The results suggest that genetic factors do not play any major role in the frequency and distribution of SCE in normal subjects.

The rate of sister chromatid exchange in normal human bone marrow cells

Sister chromatid exchange (SCE) was studied in bone marrow cells of six healthy individuals. Compared with peripheral blood lymphocytes, a remarkably low and constant rate of SCEs was observed, ranging from 3.64 to 4.65 per metaphase. The lower incidence of SCE can be related only partly to the higher contraction of bone marrow cells and the shorter exposure time to BUdR. Rather, a cell-specific phenomenon is suggested.

The mutagenic effect of benzene, toluene and xylene studied by the SCE technique

In experiments in vitro, neither benzene, toluene nor xylene changed the number of sister-chromatid exchanges (SCEs) or the number of chromosomal aberrations in human lymphocytes. Toluene and xylene caused a significant cell growth inhibition which was not observed with benzene in the same concentrations.

Factors influencing the frequency of mitomycin C-induced sister-chromatid exchanges in 5-bromodeoxyuridine-substituted human lymphocytes in culture

Newly developed techniques for the detection of sister-chromatid exchanges (SCE) require the substitution of 5-bromodeoxyuridine (BrdU) for thymidine in DNA. We investigated the possibility of interactions between BrdU and one mutagen--carcinogen, mitomycin C (MMC) for the induction of both chromosomal aberrations and SCE in human peripheral lymphocytes in culture. No effect on aberration yield was found. Neither comparisons between the yields of SCE by BrdU substitution and differential staining and those detected by tritiated thymidine incorporation and autoradiography nor between the yields of SCE for different levels of BrdU incorporation provided any evidence of synergism. It was found, however, that MMC persists in cultures and continues to increase SCE frequencies for about 30 h. It was also observed that some MMC-induced DNA lesions persist long enough so that some of those present prior to S phase of the first cell cycle cause additional SCE in the third cycle.

Human sister chromatid exchange caused by methylazoxymethanol acetate

The incidence of sister chromatid exchange was determined in human leucocyte cultures treated with methylazoxymethanol acetate. In all individuals examined, treated cultures manifested a significantly higher frequency of sister chromatid exchanges than controls. Two concentrations of MAM AC were tested in blood cultures from nine individuals. The concentrations varied from individual to individual since they were determined by means of individual dose-response curves, which involved [3H]-thymidine incorporation in PHA-stimulated short-term lymphocyte cultures versus MAM AC contraction. The lower concentration was less than the TD50 dose. Compared to control cultures, the lower concentration caused a higher incidence of sister chromatid exchange in eight of nine individuals. The cumulative mean value for all control cultures was 5.32 exchanges per cell while that for cultures treated with the higher concentration was 10.73.

Methods for determining the proliferation kinetics of cells by means of 5-bromodeoxyuridine

After treatment of cells with 5-bromodeoxyuridine (BUdR), the percentage of completely BUdR-labelled interphase nuclei is greater the longer the BUdR treatment. The labelling effect is visible after staining with the fluorochrome 33258 Hoechst and with Giemsa. Various formulae and a nomogram are presented by means of which the percentage of cells in S period, duration of the S period and the whole cell cycle can be determined by examination of a single preparation or by comparison of several preparations. The methods are tested using cell cultures of Microtus agrestis and compared with autoradiographic methods after labeling with 3H-thymidine.

The incidence of sister chromatid exchanges in cultured human lymphocytes

The incidence of sister chromatid exchanges (SCE) in cultured human lymphocytes from 50 normal individuals was studied using the BUdR-Giemsa technique. The mean SCE frequency per metaphase was 7.9 with a standard deviation of 1.36 and a range of 1-21. The incidence of exchanges was not influenced by the age of the donor nor did the exchange rate differ between sexes. The results of this study are compared with those of previous reports and reasons for the wide variation between results discussed.

BrdU-33258 Hoechst analysis of DNA replication in human lymphocytes with supernumerary or structurally abnormal X chromosomes

BrdU-33258 Hoechst techniques have been used to characterize DNA replication patterns in lymphocytes from hunam females with supernumerary or structurally abnormal X chromosomes. Fluorescence analysis permits identification of late replicating X chromosomes in a very high proportion of cells and affords a high resolution method for determining the interchange points of X-X and X-autosome translocations. Asynchrony among terminal replication patterns of multiple late replicating X chromosomes within an individual cell can occasionally be demonstrated. The arms of isochromosomes usually exhibit symmetrical fluorescence patterns, with replication terminating in bands Xq21 and Xq23 (predominant pattern) or in bands Xq25 and Xq27 (alternative pattern) in both arms. In the vast majority of lymphocytes containing a balanced X-13 or X-19 translocation, the normal X is late replicating. However, DNA synthesis in the translocation products occasionally appears somewhat delayed relative to that expected for an early replicating X, consistent with possible position effects on replication kinetics.

Mapping of sister-chromatid exchanges in human chromosomes using G-banding and autoradiography

Lumphocytes were pulse-labelled with [3H] thymidine. Following G-banding, the cells were autoradiographed and 46 in their third post-labelling division selected. The locations of 611 sister-chromatid exchanges (SCE's) which had occurred in the previous two cell cycles were recorded as label discontinuities along identified chromosomes. Between particular chromosomes, SCE frequency was proportional to chromosome length. SCE frequency distributions within particular chromosomes fitted Poisson expectations. There was no over-representation of exchanges in centromeric regions, or in the C-banded regions of chromosomes 1, 9 and 16. A trend of increased frequency of SCE in darkly G-banded regions and in relatively darkly banded chromosomes was evident. The apparent excess of SCE in dark G-bands could be considered to be a consequence of the more condensed state of the DNA in these regions in the interphase nucleus relative to the DNA in pale G-band regions. Such compaction could result in an enhanced probability of SCE and a reduced probability of gross inter- or intra-change involving these regions. In contrast, the more extended interphase state of the DNA in pale G-banded regions would allow non-homologous exchange and account for the preferred location of X-ray-induced exchange events to pale G-bands.

Simple differential Giemsa staining of sister chromatids after treatment with photosensitive dyes and exposure to light and the mechanism of staining

The essential steps of the 33258 Hoechst-Giemsa method for differential chromatid staining consist of (1) 33258 Hoechst treatment, (2) exposure to light, and (3) Giemsa staining. The staining was shown to be a function of the concentration of 33258 Hoechst and the light exposure. The dye was successfully replaced by various metachromatic dyes such as thionine. Two simple methods are proposed. Failure of the pale stained chromatids to restore Giemsa affinity with urea and trypsin and the diminished Feulgen reaction after light exposure suggest that not masking proteins but photolysis of the BrdU-incorporation chromatid components in the present of photosensitive dyes play a role in the differential staining.

Induction by alkylating agents of sister chromatid exchanges and chromatid breaks in Fanconi's anemia

Sister chromatid exchanges, which may reflect chromosome repair in response to certain types of DNA damage, provide a means of investigating the increased chromosome fragility characteristic of Fanconi's anemia. By a recently developed technique using 33258 Hoechst and 5-bromodeoxyuridine, it was observed that the baseline frequency of sister chromatid exchanges in phytohemagglutinin-stimulated lymphocytes from four males with Fanconi's anemia differed little from that of normal lymphocytes. However, addition of the bifunctional alkylating agent mitomycin C (0.01 or 0.03 mug/ml) to the Fanconi's anemia cells during culture induces less than half of the increase in exchanges found in identically treated normal lymphocytes. This reduced increment in exchanges in accompanied by a partial suppression of mitosis and a marked increase in chromatid breaks and rearrangements. Many of these events occur at sites of incomplete chromatid interchange. The increase in sister chromatid exchanges induced in Fanconi's anemia lymphocytes by the monofunctional alkylating agent ethylmethane sulfonate (0.25 mg/ml) was slightly less than that in normal cells. Lymphocytes from two sets of parents of the patients with Fanconi's anemia exhibited a normal response to alkylating agents, while dermal fibroblasts from two different patients with Fanconi's anemia reacted to mitomycin C with an increase in chromatid breaks, but a nearly normal increment of sister chromatid exchanges. The results suggest that chromosomal breaks and rearrangements in Fanconi's anemia lymphocytes may result from a defect in a form of repair of DNA damage.

The rate of sister chromatid exchanges parallel to spontaneous chromosome breakage in Fanconi's anemia and to trenimon-induced aberrations in human lymphocytes and fibroblasts

The incidence of structural chromosome aberrations and the rate of sister chromatid exchanges (SCE) was investigated in lymphocyte cultures from a patient with typical Fanconi's anemia and his parents. The rate of SCEs was found to be normal. In experiments with the alkylating agent Trenimon the SCE rates proved to be a sensitive indicator for the induction of structural aberrations: in presence of an induced aberration rate half as high as the spontaneous rate in the Fanconi's anemia case, the rate of SCEs was found to be quintupled. Dose-effect relationships for the induction of SCE rates by Trenimon were studied over a wide dose range in lymphocyte and fibroblast cultures. The results reflect the same difference in sensitivity earlier observed in the induction of structural chromosome aberrations, fibroblasts being far more sensitive.

Fluorometrical detection of thymine base differences in complementary strands of satellite DNA in human metaphase chromosomes

Using the fluorochrome "Hoechst 33258", intensity of fluorescence was found to differ distinctly between the sister chromatids in the paracentric regions of chromosomes 1, 16, and 19, after one round of replication in medium containing BUdR. Thus the effect of fluorescence asymmetry is not limited to the part of the Y chromosomes that fluoresces intensely with quinacrine; it can also be determined in the weakly Q-fluorescent pericentric regions of chromosomes, which are known to be the sites where highly reiterated sequences of satellite DNA are located. However, an exception is the paracentric region of chromosome 9 which does not show the effect of lateral asymmetry. The difference of fluorescence intensity in the heterochromatic regions of the sister chromatids of human chromosome 1 is measured by densitometric tracement along the long axes of chromosomes; this is obtained from two individuals with an "uncoiler" heterchomatic block (type III) having a relative intensity of 1:1.93 in an average of the total measured blocks. This corresponds to the uneven distribution of thymine base of 22.8 and 43.2 in the two strands of the DNA double hexlix. A chromatid exchange rate of 9 in 100 metaphases per cell cycle was found within the uncoiler region of chromosome 1.