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

Induction of sister chromatid exchanges in Chinese hamster ovary cells by organophosphate insecticides and their oxygen analogs

Induction of sister chromatid exchanges (SCEs) in cultures of Chinese hamster ovary cells by 10 anticholinesterase organophosphate insecticides was investigated. The insecticides were two phosphates (dichlorvos and dicrotophos), four sulfur-containing organophosphates (malathion, parathion, leptophos, and diazinon), and four oxygen analogs of the latter (malaoxon, paraoxon, leptophosoxon, and diazoxon). All of the compounds except diazinon induced statistically significant increases in SCE frequencies at concentrations between 0.03 and 1.0 mM. These results suggest that SCE induction is a common property of organophosphate insecticides. Compared to the sulfur-containing organophosphates, the oxygen analogs consistently produced higher SCE frequencies and had stronger antiproliferative activity. Compared to two known genotoxicants, doxorubicin and ethyl methanesulfonate, the ability of organophosphates to produce SCEs is much weaker.

The role of chromosomal proteins in the induction of a differential staining of sister chromatids by light

Fixed chromosomes of human lymphocytes, cultured in the presence of bromodeoxyuridine (BrdU) during two cell cycles, were exposed to near-ultraviolet irradiation, stained with Giemsa, and after destaining, were subjected to either Coomassie Blue or Feulgen--Schiff staining. A differential reaction of sister chromatids was first revealed by Coomassie Blue staining. Differential staining with Giemsa required a longer irradiation time. This appeared to be reduced after the addition of dithiodipyridine to the cells during their last few hours of culture. The differential pattern obtained after Coomassie Blue staining was the inverse of that obtained after Giemsa staining. From these findings we concluded that the induction of sister chromatid differentiation by light in BrdU-substituted DNA containing chromosomes occurs primarily via chromosomal proteins, presumably by differential breakage of their disulphide bonds. The results of the Feulgen--Schiff staining indicated that differential depurination of BrdU-containing DNA could occur, although only after very prolonged irradiation. A faint though distinctly differential Feulgen--Schiff pattern of sister chromatid staining, resulting from differential removal of DNA, was observed after photosensitization by specific DNA-binding dyes. Thus, DNA seems to be affected only under more extreme conditions.

Culture media and species-related variations in the requirement of 5-bromodeoxyuridine for differential sister-chromatid staining

Various concentrations of 5-bromodeoxyuridine (BdrU) ranging from 0.01 to 10.0 microgram/ml were tried for finding the minimal concentration required for differential staining of sister chromatids in lymphocytes of man, muntjac and cattle grown in three commonly used culture media, namely TC 199, Dulbecco's MEM and Ham's F-10. The lymphocytes grown in TC 199 required the lowest concentration of BrdU whereas it was highest for the lymphocytes grown in F-10. The minimal concentration varied for the 3 species studied, and it was not related to their DNA content. The differing amounts of thymidine and folic acid present in the various culture media seemed to account for the difference in the quantity of BdrU required for eliciting differential staining. Staining may also have depended on the intracellular nucleotide pool and/or on the difference in the substitutable dT sites of the genomes.

Enhanced frequency of chromosome aberrations in lymphocytes of male compared with female muntjacs after X-ray irradiation in vitro

Recently, there has been considerable interest in the study of radiation-induced dicentrics and other chromosome aberrations in phytohaemagglutinin (PHA)-stimulated blood lymphocytes of various mammals, including man, with the aim of establishing a proper biological dosimetry for the assessment of genetic radiation hazards in human. These studies have revealed that the radiosensitivity of lymphocytes differs between species and even between individuals of the same species, but the cause(s) of these variations still remains unclear. The life-shortening response to whole-body X-ray irradiation is influenced by both age and sex of the individuals, but although age has been shown to have effects on chromosome aberration yields, in vitro, the influence of sex on the sensitivity of lymphocyte chromosomes to radiation-induced damage has not been reported. It is generally thought that sex does not influence the aberration yield within the limitation of measurements used in such studies. However, we report here a study of X-ray induced chromosome aberration in lymphocytes of Indian muntjac, in which the yield of dicentrics was found to be consistently greater in male than in female lymphocytes.

DNA extraction during Giemsa differentiation of chromatids singly and doubly substituted with BrdU

Human lymphocytes were cultured in 3H-labelled BrdU. Cells were pretreated to induce differentiation, autoradiographed and Giemsa-stained. DNA extraction was deduced if grain counts were lower in differentiated mitoses compared with untreated controls.--The differentiation method involved sequential pretreatments with short wave UV and 2 x SSc at 60 degrees C. This removed 34% of label from first division cells (with TB.TB chromosomes) but relatively more (53%) from second division (TB.BB chromosomes). In second division cells, about two thirds of label was lost from pale (BB) chromatids but only one third from dark (TB) chromatids. The UV and SSC pretreatments acted in collaboration, since neither alone reduced grain counts significantly.--On testing other methods, similar preferential DNA extraction was obtained with Perry and Wolff's FPG method, and with the hot salt pretreatment of Korenberg and Freedlender. However, good Giemsa differentiation could also be obtained using Hoechst 33258 and light pretreatments without any DNA loss. Reverse differentiation patterns (TB pale, BB dark) induced by warm acids resulted in extraction of nearly two thirds of 3H-BrdU label, but relative loss was the same from pale and dark chromatin. Direct reverse staining using alkaline Giemsa did not result in any loss of label.--Thus preferential DNA loss from pale stained chromatin underlies differentiation methods using light plus hot salt pretreatments, but it is not obligatory for good differentiation using other techniques.

X-ray sensitization of chromatids with unifilarly and bifilary substituted DNA

When cells are grown for two rounds of DNA replication in the presence of the thymidine analogue 5-bromodeoxyuridine, chromosomes containing one chromatid with unifilarly substituted DNA and one with bifilarly substituted DNA are found. These can be distinguished by harlequin staining techniques that stain one chromatid dark and one light. When the degree of substitution is 60% or greater, 3 times as many X-ray-induced chromatid breaks are produced as in unsubstituted chromatids. This represents maximal sensitization. The unifilarly substituted (dark) chromatid is as sensitive as its bifilarly substituted (light) sister chromatid. If cells are grown in low concentrations of 5-bromo-deoxyuridine (BrdUrd), then the amount of substitution is less and the bifilarly substituted chromatid is more sensitive than the unifilarly substituted one. When large numbers of cells are grown in very low concentrations of BrdUrd, the analogue is almost completely depleted during the first round of replication leading to harlequin chromosomes containing one unsubstituted (dark) and one unifilarly substituted (light) chromatid. Under these conditions a maximal sensitization between light-staining and dark-staining chromatids can occur. This can be confused with the differential sensitivity between unifilarly and bifilarly substituted chromatids. The apparent discrepant results obtained by different investigators are most likely caused by the use of very low levels of BrdUrd in some of the experiments.

Diethylstilbestrol-diphosphate induces chromosomal aberrations but not sister chromatid exchanges in murine bone marrow cells in vivo

Diethylstilbestrol diphosphate (DES-dp) clastogenesis was examined in the bone marrow of C57Bl/6 male and female mice. Significant and sex-related dose effects were observed for the induction of chromatid-type chromosomal aberrations and for the inhibition of cellular proliferation. Females were more sensitive to the effects of DES-dp than males when assessed for either induced chromosomal aberrations or proliferative inhibition. Contrary to other published results, we did not observe either an increase in sister chromatid exchanges or an increased incidence of aneuploidy. Ovariectomy reduced the ability of DES-dp to inhibit cellular proliferation and decreased the high degree of variability between animals at high doses of DES-dp. The results of our studies show that DES is a clastogenic agent in vivo which may relate to its carcinogenicity.

Cytotoxic effects of protease inhibitors on human cells. 1. High sensitivity of xeroderma pigmentosum cells to antipain

Antipain had little effect on UV-survival and UV-induced sister chromatid exchanges in normal and xeroderma pigmentosum (XP) cells, suggesting that it may not affect DNA repair. Antipain itself produced a small, but significant, amount of sister chromatid exchanges. XP cells showed very high sensitivity to the cytotoxic effect of antipain, but an SV40-transformed XP cell strain did not demonstrate high sensitivity.

Effects of inhibitors of DNA synthesis on spontaneous and ultraviolet light-induced sister-chromatid exchanges in Chinese hamster cells

Effects of inhibitors of DNA synthesis on spontaneous and ultraviolet light (UV)-induced sister-chromatid exchanges (SCE) were examined in a Chinese hamster cell line, V79 B-1. The inhibitors used were hydroxyurea (HU), 1-beta-D-arabinofuranosylcytosine (ara-C), aphidicolin (APC), 2',3'-dideoxythymidine triphosphate (ddTTP), neocarzinostatin (NCS), novobiocin (NB) and cycloheximide (CHX). HU, ara-C, and APC increased spontaneous SCE frequency, and had a synergistic effect on UV-induced SCE frequency. DdTTP, NCS and NB failed to show any statistically significant effect on either spontaneous or UV-induced SCE frequencies, though NCS and NB did slightly increase both spontaneous and UV-induced SCE frequencies. On the contrary, CHX decreased spontaneous SCE frequency, and more drastically, also UV-induced SCE frequency. These results are interpreted with respect to the replicating fork of DNA, a structure postulated to be involved in the formation of spontaneous and UV-induced SCE. A new model for SCE formation is proposed.

Bloom's syndrome in a Japanese boy with lymphoma

A 14-year-old boy with malignant lymphoma was identified as having Bloom's syndrome after he was shown to display a characteristically elevated frequency of spontaneous sister-chromatid exchanges. Frequencies of sister-chromatid exchanges per nucleus were 80.7 in blood lymphocytes and 29.9 in dermal fibroblasts. Clinical features, although suggestive of Bloom's syndrome, were not clear enough to permit the diagnosis. This is the first confirmed case of this rare genetic disorder in Japan, and in Asia.

DETERMINATION OF CELL CYCLE IN PLANTS USING BrdU-FPG

The BrdU-FPG method for differential staining of sister chromatids was employed to estimate the cell cycle time in two plants species. The protocol is based upon the ability to differentiate cytologically between chromosomes which have completed two or three DNA synthesis phases. Vicia faba L. which has a well established cell cycle time was used to verify the technique. Utilizing this method the cell cycle time of Zebrina pendula Schnizl. root tip meristem cells was determined to be 17.0 hours.

Cultured mouse embryos metabolize benzo[a]pyrene during early gestation: genetic differences detectable by sister chromatid exchange

Mouse embryos explanted at 7 1/2 or 8 1/2 days of gestation were cultured in medium containing benzo[a]pyrene and supplemented with 5-bromodeoxyuridine to allow detection of sister chromatid exchanges. The murine Ah locus regulates the inducible metabolism of polycyclic hydrocarbons such as benzo[a]pyrene. A high frequency of sister chromatid exchange was induced by benzo[a]pyrene in embryos from three Ah-"responsive" inbred strains (BALB/cDub, C3H/AnfCum, and C57BL/6N); there was little or no increase in two Ah-"nonresponsive" inbred strains (AKR/J and DBA/2J). Benzo[a]pyrene also induced sister chromatid exchanges in the Ah-responsive recombinant inbred line B6NXAKN-12 but not in the Ah-nonresponsive recombinant inbred line B6NXAKN-3. Sister chromatid exchange in cultured Ah-responsive mouse embryos was thus shown to be a sensitive assay. These data provide direct evidence that genetically responsive mouse embryos (early postimplantation stage) possess the subcellular processes necessary for induction of enzymes that metabolize benzo[a]pyrene to its chemically active forms(s). Both the Ah regulatory gene product (a cytoslic receptor) and the structural gene product (inducible cytochrome P1-450) therefore appear to be functional at an early embryonic age. Furthermore, this metabolic capacity may play an important role in the damage to embryonic cells by polycyclic hydracarbons.

The effects of short- and long-term exposure of chick embryos to neutral red on the frequency of sister-chromatid exchange

The chick embryo was used to study the effects of neutral red (NR) on the frequency of sister-chromatid exchange (SCE) in specific tissues exposed to this mutagen for short and long periods as development proceeded. In short-term trials, aqueous NR at doses of 10, 25 and 100 micrograms was injected in 3-day and 6-day embryos. In each case, embryos were also treated with 5-bromodeoxyuridine (BrdU) for a 24-h period (two cell cycles) and harvested at 4 days and 7 days, resp. A long-term exposure (about 8 cell cycles) was achieved by exposing embryos to NR from day 3 to day 7 of incubation. At a NR dose of 25 micrograms, the chronic exposure resulted in a doubling of the rate of SCE (11.4/cell) over that observed in embryos exposed for only 24 h at either days 3-4 (6.0/cell) or days 6-7 (6.0/cell). At 100 micrograms of NR, the same relationship held with SCE rates of 14.2/cell for the chronic exposure versus rates of 8.0/cell (3-4 days) and 6.9/cell (6-7 days). At 10 micrograms of NR, no such accumulation of SCE occurred upon long-term treatment. These results show an enhanced SCE response upon growth of embryonic cells in the presence of NR for several days. This may be the result of the persistence of past lesions with the addition of more lesions upon continued exposure to NR.

Sister chromatid differential staining by direct staining in Na2HPO4-Giemsa solution and the mechanism involved

The direct staining of BrdU-substituted Chinese hamster chromosomes in a Na2HPO4-Giemsa solution without any pretreatments resulted in a B-dark type SCD istituted (TB) chromatids stained light. Detailed examinations of the staining process suggested that the Na2HPO4 solution acts to collapse chromosomes whereas the Giemsa dye works to reconstruct the collapsed chromosomes, and that during the reconstruction process preferential binding of the Giemsa dye to the BB-chromatids occurs to produce the B-dark SCD. It was revealed that not only the time but the temperature at which chromosome preparations are kept prior to use considerably affect the occurrence of SCD.

Sister-chromatid exchanges in oral contraceptive users

The effect of the use of an oral contraceptive on the frequency of sister-chromatid exchanges (SCEs) was investigated. Oral contraceptive users showed significantly higher mean SCE per cell compared with both normal and pregnant women. This result obviously means an increased mutagenic environment in these cells--due either to the pill itself or to a metabolite(s).

Mutagenic activity of anticancer agent cis-dichlorodiammine platinum-II

cis-Dichlorodiamminoplatinum-II (cis-DDP) has been widely used as an anticancer chemotherapeutic agent. The mutagenicity of cis-DDP was investigated in vitro and in vivo using sister-chromatid exchange analysis and the analysis of chromosomal aberrations. Parallel human lymphocyte cultures were incubated with and without the addition of BrdU at 4 concentrations of cis-DDP. Significant increases in SCE rate were observed at 0.25 micrograms/ml and higher, showing a clear dose-response relation between SCE rate and cis-DDP concentration. A significant increase in chromosome breakage and tetraradial figures was observed in BrdU free cultures treated with cis-DDP again showing a dose dependency. Analysis of the distribution of cells in the first, second and third division in cis-DDP treated cultures demonstrated the depressing effect of the drug on mitotic activity. In vivo analysis of SCE and chromosome aberrations in mouse showed that 13.85 mg/kg i.p. of cis-DDP produces significant increases in the rate of SCE and chromosome aberrations in bone-marrow cells.

The relation between chemically induced sister-chromatid exchanges and chromatid breakage

Studies of classical chromosome aberrations and sister-chromatid exchanges (SCES) suggest independent mechanisms for the two events despite some common features. Examination of chromosome breakage caused by X-rays, visible light, and viruses has shown that few chromatid breaks are accompanied by SCEs at the sites of breaks. No similar observations were available for chemically induced breaks, but it has been reported that rat chromosomes exposed to dimethylbenzanthracene (DMBA) contained a preponderance of both aberrations and SCEs in certain specific regions, implicating a common process in their formation. These conclusions were drawn from a comparison of breaks induced in vivo with SCEs induced in vitro. However, we used 7 chemical mutagens to induce both chromatid breaks and SCEs in "harlequin" chromosomes of cultured rat and Chinese hamster ovary (CHO) cells and found that 25% of the 914 breaks scored were associated with SCEs. The proportion of breaks accompanied by SCEs is related to the overall SCE frequency and falls into the range predicted on the basis that breaks and SCEs occur independently. The reported association between sites for SCEs and aberrations also reflects secondary factors, such as induction of SCEs and aberrations during DNA synthesis in late replicating regions of the chromosomes.

Effect of levamisole on the mitosis of murine thymocytes in culture

The effect of levamisole on murine thymocytes stimulated by concanavalin A in culture was measured as the change in number of cells in the DNA synthetic phase (S-phase) and in number of cells undergoing mitosis (M-phase). Addition of levamisole to Con A-stimulated lymphocytes led to an approximately tenfold increase in the frequency of mitosis and a fivefold increase in the frequency of cells heavily labeled with thymidine. Thus, levamisole leads to an increased number of cells undergoing proliferation. Determination of cell viability excluded the possibility that the effect of levamisole was simply due to enhanced numbers of viable cells in culture. Examining the effect of levamisole on the proliferating cells alone, it was found that they had a shorter cell cycle time. It was therefore concluded that levamisole affects thymocyte proliferation in vitro in two ways: enhancement of the number of proliferating cells and shortening the cell cycle time of proliferating cells. A similar effect in vivo might be important in the mechanism of action of levamisole as an immunotherapeutic agent.

Induction of sister-chromatid exchanges in cultured human cells by an organophosphorous insecticide: malathion

Because malathion is a widely used organophosphorous insecticide, the effects of non-toxic concentrations (2.5--40 micrograms/ml) on sister-chromatid exchange (SCE) frequencies were determined. Human fetal fibroblasts were exposed once or twice to malathion, with 20 h between exposures. A single exposure to a concentration of 40 micrograms/ml resulted in a highly significant increase in the number of SCEs. After a double exposure, a concentration of 20 micrograms/ml induced an even greater increase in SCE frequencies. Comparison of Sce frequencies after single and double exposures indicated a cumulative effect; the number of exchanges at concentrations of 5 micrograms/ml or higher was significantly greater after the double exposure. An analysis of SCEs by chromosome group showed that exchanges were distributed approximately according to chromosome length.

Induction of sister chromatid exchanges in the central mudminnow following in vivo exposure to mutagenic agents

Sister-chromatid exchange (SCE) studies using the central mudminnow, Umbra limi, demonstrated its usefulness as a model in vivo system for the detection of genetically active substances (mutagens and/or carcinogens). 5 days following the injection of 500 micrograms/g of 5-bromodeoxyuridine (BrdU), SCE rates were found to be quite low in the gills (2.0--3.3 SCEs/metaphase), kidneys (2.6--3.4 SCEs/metaphase), and intestines (3.7--4.5 SCEs/metaphase). However, after i.p. exposure to microgram quantities of methyl methanesulfonate (MMS) or cyclophosphamide (CP), large linear dose-dependent increases in SCE rates were observed in all tissues examined. amined. On a microgram/g basis, CP was found to be a 2--4 times more potent inducer of SCE than was MMS. Studies involving the addition of neutral red dye (NR) to the fish's aquarium water revealed that the mudminnow could concentrate the dye in its gill and kidney tissues. This dye was found to cause significant increases in SCE rates at water levels of less than 0.1 ppm. However, the concentration of dye in the tissues did not show a correlation with the SCE rate. Possible explanations for this observation are presented in the text. These findings demonstrate the feasibility of using a fish such as the mudminnow to investigate cytogenetic consequences of aquatic pollution.

In vivo sister-chromatid exchange: a sensitive measure of DNA damage

A variety of chemical agents and X-irradiation were examined for their abilities to induce sister-chromatid exchanges (SCE) in vivo. In addition to demonstrating that several known mutagens and carcinogens are capable of inducing SCE in vivo, our studies indicate that the suspected carcinogen, tris-bromophosphate, can significantly elevate SCE levels. Comparison of the effects of these agents on SCE levels, chromosomal-aberration frequencies and cell-replication kinetics reveals that no consistent relationship exists between SCE levels and other indicators of cellular DNA damage. It is proposed that analysis of SCE induction in vivo may provide a useful technique for the screening of mutagenic and carcinogenic compounds.

Vitamin C is positive in the DNA synthesis inhibition and sister-chromatid exchange tests

Ascorbate caused a dose-dependent increase in sister-chromatid exchanges (SCEs) in Chinese hamster ovary (CHO) cells and in human lymphocytes. Moreover, in the DNA synthesis inhibition test with HeLa cells, ascorbate gave results typical of DNA-damaging chemicals. Catalase reduced SCE induction by ascorbate, prevented its cytotoxicity in CHO cells, and prevented its effect on HeLa DNA synthesis. Ascorbate reduced induction of SCE in CHO cells by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) by direct inactivation of MNNG.

Sister chromatid exchange in chromosomes of cattle (Bos taurus)

Lymhocytes from clinically healthy Holstein animals (8 males and 23 females) were cultured in media containing 5-bromodeoxyuridine (10 microgram/ml) for two cycles of deoxyribonucleic acid replication (48 h). The exchange of sister chromatids per cell varied from 1 to 16 with a mean of 5.4 and a standard deviation of 2.1 in 603 differentially stained metaphase chromosome spreads. The major fraction of exchanges in the X chromosome were located in the region of the subcentromeric G negative bands q1 and q2. When the cells were pulse labeled with tritium labeled thymidine (1 muCi/ml) for the last 6 h of the cell cycle, the rate of exchange was higher in the late replicating X chromosome (facultative heterochromatin) in comparison to its homologue (active X). This study characterizes the yield of sister chromatid exchange in lymphocytes exposed to a given amount of 5-bromodeoxyuridine in a population of normal individuals so that it may be used as a standard for 1) a diagnostic tool in pathological conditions and 2) as an assay of chromosome stability in relation to environmental hazards.

Chromosome breaks and sister chromatid exchanges in albinos in Nigeria

The prevalence of squamous cell carcinoma of the skin in albinos reaches approximately 90% in patients over 20 years of age in the vicinity of Enugu, Nigeria. Chromosome breaks and sister chromatid exchanges (SCE) were evaluated in tyrosinase-positive oculocutaneous albinos and pigmented controls of Ibo extraction who were life-long residents of Nigeria. No difference in the frequency of chromosomal breaks in 14 albinos compared to 6 pigmented controls, and no differences in the frequency of SCE in 9 albinos compared to 3 controls could be detected. Increased chromosomal abnormalities in lymphocytes do not appear to be assoicated with albinism or fulminating skin cancer present in albinos in the tropics.

BrdU-dye characterization of late replication and meiotic recombination in Armenian hamster germ cells

In vivo BrdU-dye methodology is used to reveal patterns of replication kinetics and meiotic recombination in male germ cells of the Armenian hamster. Analogue substitution over a fraction of the DNA synthesis period results in distinctive 33258 Hoechst fluorescence staining of late replicating chromatin, detectable at spermatogonial or spermatocyte I and II stages. Spermatogonial cells which are extensively substituted with BrdU over the penultimate synthesis period reveal sister chromatid differentiation in all chromosomes of primary or secondary spermatocytes. In chiasmatic regions, exchanges between unlike-stained non-sister chromatids are indicated by isolatelled segments, while those occurring between like-stained non-sister chromatids are not directly detected. In sex chromosomes from young animals, these alternative images occur in a ratio of 1:1, which supports the concept that homologue non-sister chromatid regions are randomly broken and re-united in the process of chiasma formation. Deviations from randomness appeared to occur in older animals. Sex bivalent chiasmata are either coincident with points of visible exchange, or they appear to have variable degrees of terminalization. Secondary spermatocytes display sharp chromatid contrast, and aid in mapping the positions and frequencies of homologue exchanges.

Analysis of sister chromatid exchange and chromosome replication kinetics using Brd-U-dye techniques

The basis of BrdU-dye techniques for detecting DNA synthesis in cytological preparations is reviewed. General procedural modifications are described for applying this approach for the study of chromosome replication kinetics and sister chromatid exchange formation. Implications of sister chromatid exchange analysis for detecting mutagen-carcinogens and for differentiating human chromosome fragility diseases predisposing to neoplasia are discussed.

Sister-chromatid exchanges induced in rabbit lymphocytes by 2-aminofluorene and 2-acetylaminofluorene after in vitro and in vivo metabolic activation

Because short in vitro treatments of Chinese hamster cells with 2-aminofluorene and 2-acetylaminofluorene did not increase SCEs even in the presence of a metabolic activation system, experiments were carried out with rabbits to see if in vivo activation occurred. Rabbits injected with 2-AF could activate the compound and a transient dose-dependent increase in SCEs was found in peripheral lymphocytes cultured at various times after the injection. With 2-AAF, however, the response was more variable: some rabbits showed an increase immediately, but one showed an increase only after a subsequent injection. This indicated that among rabbits differences exist in their ability to detoxify 2-AAF. Because rabbits could activate the compounds, in vitro experiments were carried out to see if their lymphocytes responded differently from Chinese hamster cells and to see if the metabolic changes brought about by PHA stimulation affect the ability of the cells to activate the chemicals. The addition of PHA and the consequent metabolic stimulation do affect the induction of enzymes involved in the activation of 2-AF and 2-AAF. Benzo[alpha]pyrene, in contrast, can be activated by rabbit lymphocytes independently of PHA stimulation.

Isolabeling of the long arm of the human Y chromosome demonstrated by the FPG technique

Isolabeling segments were found in the distal region of the long arm of Y chromosomes derived from human leukocytes grown through two replication cycles in medium containing BrdU and stained by the FPG technique. Three main types of Y chromosome staining patterns were demonstrated: I-Y chromosome with typical SCD, II-Y chromosome with weakly stained distal regions of long arms (isolabeling segments), III-Y chromosome with both terminal regions displaying SCD interrupted by one isolabeled segment. The existence of different types of Y chromosome staining patterns was explained on the basis of the previously described hypothesis of unequal distribution of thymine residues between two DNA polynucleotide chains in the distal part of the long arms of human Y chromosomes.

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.

Caffeine inhibition of prereplication repair of mitomycin C-induced DNA damage in human peripheral lymphocytes

Caffeine increases the number of sister-chromatid exchanges (SCE) induced by mitomycin C (MMC) in human peripheral lymphocytes in culture. This enhancement decreases when the treated cells are held in medium before phytohemagglutinin (PHA) stimulation, or when caffeine is added to cultures some time after PHA stimulation but prior to DNA synthesis. There thus appears to be a caffeine-sensitive prereplication repair system, presumably an excision mechanism, capable of repairing a fraction of the MMC-induced DNA lesions.

Factors involved in differential Giemsa-staining of sister chromatids

Microspectrophotometric evaluation of differentially stained sister chromatids made it possible to analyse precisely the factors involved in the Giemsa methods. The concentration of Hoechst 33258, pH of the mounting medium temperature during UV-exposure and the quality (wavelength)of UV-light influenced the differential staining. Exposure of blacklight of 10(-5) M Hoechst 33528-stained brdU-labeled chromosome specimens mounted in McIlvaine buffer (pH 8.0) at 50 degrees C reproducibly allowed differential staining of sister chromatids within 15 min. On the other hand, Korenberg-Freedlender's method using no Hoechst 33258 was also UV-light-dependent. Thus, photolysis of BrdU-substituted DNA was considered the basic mechanism of the Giemsa methods where the photosensitive Hoechst 33258 played a role as a sensitizer.