Jason L. Minkler

DNA crosslinking, sister-chromatid exchange and specific-locus mutations

Chinese hamster ovary cells were treated with the DNA-crosslinking chemicals, mitomycin C (MMC) and porfiromycin (POR), and their monofunctional derivative decarbamoyl mitomycin C (DCMMC). After exposure, the cells were studied for the induction of sister-chromatid exchanges (SCEs) and mutations at the hypoxanthine phosphoribosyltransferase and adenine phosphoribosyltransferase loci. The frequency of SCEs varied significantly in successive sampling intervals, requiring the weighting of each interval by the percentage of second-division mitosis in that interval to obtain the mean SCE frequency for each dose. All 3 compounds were potent inducers of SCEs but weakly mutagenic. All 3 chemicals by concentration were approximately equally effective in inducing SCEs or mutations. When the induced SCEs and mutations were compared at equal levels of survival, DCMMC was slightly more effective than MMC or POR in inducing SCEs and somewhat less mutagenic. These results indicate that the DNA interstrand crosslink is not the major lesion responsible for the induction of SCE or mutation by these compounds.

Induction of long-lived chromosome damage, as manifested by sister-chromatid exchange, in lymphocytes of animals exposed to mitomycin-C.

The cytogenetic effects of repeated vs. acute exposure to a chemical mutagen--carcinogen were determined with an in vivo system in which chemicals injected into rabbits induce sister-chromatid exchanges (SCEs). SCE induction can be monitored when the animals peripheral lymphocytes are cultured in the presence of bromodeoxyuridine (BrdUrd) and then scored for SCE frequency. Mitomycin-C (MMC), 0.5 mg/kg, was injected intraperitoneally once a week for 8 weeks. This treatment initially induced small increases in SCE frequency within one day of injection, followed by a return to control levels within 1 week. After the 4th injection, however, the frequency failed to return to normal. After the 5th injection, however it showed a 4-fold increase over the control which was sustained for the remaining 3 weeks of treatment and for an additional 2 weeks thereafter. The frequency then dropped to twice the control value and remained at this level for more than 4 months. All of the high SCE values after the first 4 weeks were due in part to the appearance and persistence of a population of cells with high SCE frequencies. Exposure to the same total dose given as a single injection resulted in a transient elevation in the SCE frequency and a subsequent return to lower values, with no evidence of a delayed effect such as the increase observed after 4 weeks in repeatedly exposed animals. Overall, repeated exposure is at least as effective as acute exposure in eliciting long-lived SCEs in vivo.

The development of chromosome-specific composite DNA probes for the mouse and their application to chromosome painting

The speed and ease of human cytogenetic analysis has been greatly enhanced by the technique of fluorescence in situ hybridization (FISH). Non-radioactive fluorescently tagged complex DNA probes specific for individual chromosomes can be hybridized to conventionally obtained metaphase chromosome spreads. Several chromosomes may be “painted” concurrently by using combinations of different labeled probes. Surveys of chromosome breakage and rearrangement may be performed very quickly by avoiding the time consuming process of GTG-banding. The application of FISH to mouse cytogenetics would allow large scale molecular toxicology studies to be conducted on the effects of such environmental insults as potential carcinogens, mutagens and radiation. Progress has been hampered, however, as the Mus musculus karyotype consists of 40 acrocentric chromosomes of approximately the same size, making the recognition and separation of individual chromosomes very difficult. We now describe the successful production and application of chromosome-specific composite DNA probes for M. musculus chromosomes 2 and 8. Stable Robertsonian translocated chromosomes were isolated on a flow sorter and their DNA subsequently amplified by degenerate oligonucleotide primer (DOP) PCR. Small pools (300 copies) of each chromosome were denatured at 94° C then annealed with the primer at 30°C for 15 cycles. This was followed by 20 cycles at an annealing temperature of 62° C. Additional amplification was performed at an annealing temperature of 62° C. The chromosome-specific DNA was labeled with biotin 11-dUTP by nick translation and used for FISH. The usefulness of the technique for translocation detection is demonstrated by analyzing chromosome exchanges induced in mice irradiated with 137Cs γ rays.

The relationship between sister-chromatid exchange and perturbations in DNA replication in mutant EM9 and normal CHO cells.

The majority of the high (12-fold elevated) baseline sister-chromatid exchanges (SCEs) that occur in the CHO mutant line EM9 appear to be a consequence of incorporated BrdUrd, and they arise during replication of DNA containing BrdUrd in a template strand. In normal CHO cells the alkaline elution patterns of DNA newly replicated on a BrdUrd-containing template are significantly altered compared with those seen during the replication on an unsubstituted template. The nascent DNA synthesized on such an altered template is delayed in reaching mature size, possibly because replication forks are temporarily blocked at sites occurring randomly along the template. Transient blockage of replication forks may be a prerequisite for SCE. The delay in replication on BrdUrd-substituted templates was greater in EM9 cells than in parental AA8 cells and was also greater in AA8 cells treated with benzamide, an inhibitor of poly(ADPR) polymerase, than in untreated AA8 cells. Under these conditions, treatment with benzamide also produced a 7-fold increase in SCEs in AA8. An EM9-derived revertant line that has a low baseline SCE frequency showed less delay in replication on BrdUrd-substituted templates than did EM9. However, under conditions where the template strand contained CldUrd, which was shown to produce 4-fold more SCEs than BrdUrd in AA8 cells, the replication delay in AA8 was not any greater in the CldUrd-substituted cells. Thus, other factors besides the delay appear to be involved in the production of SCEs by the template lesions resulting from incorporation of the halogen-substituted pyrimidine molecules.

Cytogenetic characterization of the ionizing radiation-sensitive Chinese hamster mutant irs1.

The X-ray-sensitive mutant V79 cell line irs1 was characterized with respect to chromosomal aberrations induced by 137Cs, mitomycin C (MMC), and decarbamoyl mitomycin C (DCMMC). To measure chromosome damage induced at different cell cycle stages, irs1 and the parental V79-4 cell lines were pulse-labeled with bromodeoxyuridine (BrdUrd) at the time of exposure and harvested at various intervals corresponding to exposure in G1, S, and G2 phases of the cell cycle. Metaphase spreads were stained with an anti-BrdUrd antibody, followed by a fluorescein-conjugated second antibody. With propidium iodide as a counter stain, cells were scored for aberrations. Compared to the parental V79 cells, irs1 cells had: (1) greatly increased sensitivity to all 3 agents; (2) a high frequency of chromatid exchanges after exposure in each phase of the cell cycle; and (3) more sensitivity to the agent causing crosslinks (MMC) than its monofunctional analog (DCMMC). The finding of chromatid-type damage in cells exposed to ionizing radiation during G1 is atypical of normal cells, but is similar to observations made in several mutant rodent cell lines and in ataxia telangiectasia cells. Our results suggest that the defect in irs1 cells can manifest itself as misrepair or misreplication during all phases of the cell cycle and leads to a high incidence of chromatid exchanges and deletions.

Incorporated bromodeoxyuridine enhances the sister-chromatid exchange and chromosomal aberration frequencies in an EMS-sensitive Chinese hamster cell line

The mutant Chinese hamster cell line, EM9, is characterized by a high baseline sister-chromatid exchange (SCE) frequency, increased sensitivity to cell killing, and a defect in DNA strand-break repair. The molecular basis for this pleotrophic phenotype is not known. We examined, at the chromosomal level, the increased sensitivity of this mutant to incorporated BrdUrd. By varying the amount of BrdUrd in template DNA and measuring the frequency of SCEs and chromosomal aberrations, we demonstrated the enhanced sensitivity of EM9 to BrdUrd present in the template strand of DNA. Our results show that a 6-fold increase in SCEs occurs due to DNA replication over a BrdUrd-substituted template relative to a dThd-substituted template. With regard to aberration production in EM9, there is a significant enhancement of aberrations and a specific bias toward damage for the chromatid with Brdurd in the template strand. While these cells share some phenotypic properties with cells from patients with Blooms syndrome, the genotypic similarities have not yet been established.

The development of painting probes for dual-color and multiple chromosome analysis in the mouse

The recent development of mouse chromosome painting probes for fluorescence in situ hybridization has extended the use of this common laboratory mammal in cytogenetics. We now report the development of additional painting probes by degenerate-oligonucleotide-primed PCR on chromosomes from mouse lung fibroblast cultures, each homozygous for a single Robertsonian translocation chromosome. These probes are for Rb(1.2), Rb(1.3), Rb(4.6), and Rb(6.7). Probes were also made for the sex chromosomes by isolating shoulders from larger peaks (X) or small, clearly resolved peaks (Y) in the flow karyotype. Combinations of probes were used to paint four chromosomes simultaneously in a single color. Multicolor painting was achieved with a biotinylated Rb(1.2) probe and a digoxigenin-labeled Rb(2.8) probe. Each of the three different homologous pairs was uniquely colored by avidin-Texas Red, anti-digoxigenin-FITC, or both simultaneously. These results extend the usefulness of the mouse as a model for understanding adverse environmental exposures and genetic diseases in humans.

A long-term effect of radiation on chromosomes of cultured human fibroblasts

Abstract Six monolayer fibroblast tissue cultures of different human origin were used for this study. Portions of each culture were Co 60 -irradiated with varying doses of up to 500 rad at various times in their life history. The cultures were then maintained and observed for at least 10 months along with their normanl unirradiated controls for long-term changes in their chromosome composition. At least one of the irradiated fractions of each of the six primary cultures ultimately became pseudodiploid with one or more conspicuous markers, while the corresponding unirradiated control cultures remained diploid. In those cultures which provided favorable material for sequential observations after marker appearance, the pseudodiploid cells with the marker chromosomes soon prevailed in the culture. None of the irradiated cultures showed signs of alteration into established cell lines as of this writing. They all appear to have stopped rapid growth at approximately the same time as their normal diploid controls.