Large-mutation spectra induced at hemizygous loci by low-LET radiation: evidence for intrachromosomal proximity effects

A mathematical model is used to analyze mutant spectra for large mutations induced by low-LET radiation. The model equations are based mainly on two-break misrejoining that leads to deletions or translocations. It is assumed, as a working hypothesis, that the initial damage induced by low-LET radiation is located randomly in the genome. Specifically, we analyzed data for two hemizygous loci: CD59- mutants, mainly very large-scale deletions (>3 Mbp), in human-hamster hybrid cells, and data from the literature on those HPRT- mutants which involve at least deletion of the whole gene, and often of additional flanking markers (approximately 50-kbp to approximately 4.4-Mbp deletions). For five data sets, we estimated f, the probability that two given breaks on the same chromosome will misrejoin to make a deletion, as a function of the separation between the breaks. We found that f is larger for nearby breaks than for breaks that are more widely separated; i.e., there is a "proximity effect". For acute irradiation, the values of f determined from the data are consistent with the corresponding break misrejoining parameters found previously in quantitative modeling of chromosome aberrations. The value of f was somewhat smaller for protracted irradiation than for acute irradiation at a given total dose; i.e., the mutation data show a decrease that was smaller than expected for dose protraction by fractionation or low dose rate.

An aberrant form of CD59 derived from HeLa cells

We isolated a CD59 cDNA from a HeLa cell library which encoded a mutated form of CD59, having a single base substitution (G to T) that changed Arg55 to Met. Since this mutation occurred in the vicinity of the putative active site of CD59, we expressed the aberrant form of the protein in Chinese hamster ovary cells in order to test for effects upon function. We found that the mutation did not influence complement inhibitory activity of CD59. However, the epitopes recognised by the function-blocking CD59 monoclonal antibodies BRIC229 and YTH 53.1 were significantly affected. The G to T substitution caused loss of an Mnl I restriction site which permitted PCR-RFLP analysis. All of 52 human subjects studied, and our in-house HeLa cells, were homozygous for the normal CD59 sequence, indicating that the altered sequence was not due to normal variation in the general population. Therefore this mutation probably arose spontaneously in the HeLa cell line used to generate the commercially obtained cDNA library.

Induction of oxyradicals by arsenic: implication for mechanism of genotoxicity

Although arsenic is a well-established human carcinogen, the mechanisms by which it induces cancer remain poorly understood. We previously showed arsenite to be a potent mutagen in human-hamster hybrid (A(L)) cells, and that it induces predominantly multilocus deletions. We show here by confocal scanning microscopy with the fluorescent probe 5',6'-chloromethyl-2',7'-dichlorodihydrofluorescein diacetate that arsenite induces, within 5 min after treatment, a dose-dependent increase of up to 3-fold in intracellular oxyradical production. Concurrent treatment of cells with arsenite and the radical scavenger DMSO reduced the fluorescent intensity to control levels. ESR spectroscopy with 4-hydroxy-2,2,6,6-tetramethyl-1-hydroxypiperidine (TEMPOL-H) as a probe in conjunction with superoxide dismutase and catalase to quench superoxide anions and hydrogen peroxide, respectively, indicates that arsenite increases the levels of superoxide-driven hydroxyl radicals in these cells. Furthermore, reducing the intracellular levels of nonprotein sulfhydryls (mainly glutathione) in A(L) cells with buthionine S-R-sulfoximine increases the mutagenic potential of arsenite by more than 5-fold. The data are consistent with our previous results with the radical scavenger DMSO, which reduced the mutagenicity of arsenic in these cells, and provide convincing evidence that reactive oxygen species, particularly hydroxyl radicals, play an important causal role in the genotoxicity of arsenical compounds in mammalian cells.

A versatile image analysis approach for simultaneous chromosome identification and localization of FISH probes

Modern cytogenetic techniques, such as comparative genomic hybridization (CGH) and the multi-color fluorescence in situ hybridization (FISH) techniques of multiplex fluorescence in situ hybridization (M-FISH) and spectral karyotyping (SKY), require a coordinated banding analysis to maximize their usefulness. All of the methods currently used, including Giemsa (G-) banding, Alu banding, and 4',6-diamidino-2-phenyl-indole (DAPI) banding, have serious drawbacks. A simple and effective method to band chromosomes concurrently with FISH is needed. To address this problem, we stained chromosomes with DAPI and chromomycin A3, and then used an image analysis program to generate banding by dividing the image taken with a DAPI excitation filter by the image taken with a chromomycin A3 excitation filter. The result was a metaphase spread in which the chromosomes possessed a banding pattern characteristic of R-banding. The image analysis program was then used to generate linescans of pixel intensity versus relative position along the length of chromosomes that were banded using this technique, which we have called D/C R-banding. Each chromosome in a genome was represented by a characteristic scan profile, which was unaffected by FISH signals. Reference linescans were prepared by karyotyping D/C R-banded chromosomes for a given species, and then drawing lines along the length of the known chromosomes. The linescans were combined into a spreadsheet database, which was linked by dynamic data exchange to the image analysis program and normalized for length and intensity. The linescan of an unknown chromosome was then transferred to the spreadsheet, where it was normalized for length and intensity and overlaid on the linescans of each chromosome in the genome. Unknown chromosomes were identified by comparison of their graphs with graphs in the standardized reference genome. We have used this approach to create reference linescan karyotypes of several species, and to identify chromosomes on which FISH was performed.

Mutagenicity of arsenic in mammalian cells: role of reactive oxygen species

Arsenite, the trivalent form of arsenic present in the environment, is a known human carcinogen that lacked mutagenic activity in bacterial and standard mammalian cell mutation assays. We show herein that when evaluated in an assay (AL cell assay), in which both intragenic and multilocus mutations are detectable, that arsenite is in fact a strong dose-dependent mutagen and that it induces mostly large deletion mutations. Cotreatment of cells with the oxygen radical scavenger dimethyl sulfoxide significantly reduces the mutagenicity of arsenite. Thus, the carcinogenicity of arsenite can be explained at least in part by it being a mutagen that depends on reactive oxygen species for its activity.

Analysis of mutant quantity and quality in human-hamster hybrid AL and AL-179 cells exposed to 137Cs-gamma or HZE-Fe ions

We measured the number of mutants and the kinds of mutations induced by 137Cs-gamma and by HZE-Fe (56Fe [600 MeV/amu, LET = 190 KeV/micrometer) in standard AL human hamster hybrid cells and in a new variant hybrid, AL-179. We found that HZE-Fe was more mutagenic than 137Cs-gamma per unit dose (about 1.6 fold), but was slightly less mutagenic per mean lethal dose, DO, at both the S1 and hprt- loci of AL cells. On the other hand, HZE-Fe induced about nine fold more complex S1- mutants than 137Cs-gamma rays, 28% vs 3%. 137Cs-gamma rays induced about twice as many S1- mutants and hprt-mutants in AL-179 as in AL cells, and about nine times more of the former were complex, and potentially unstable kinds of mutations.

The use of human repetitive DNA to target selectable markers into only the human chromosome of a human-hamster hybrid cell line (AL)

We used the plasmid BLUR-8 that contains an 800-base pair (bp) sequence of human repetitive Alu DNA in a cotransfection protocol to target the plasmids pSV2neo or EBO-pcD-leu-2 (hygro) into a single site of the sole human chromosome, number 11, of a Chinese hamster-human hybrid cell line (AL). The neo and hygro plasmids confer resistance to the antibiotics G418 and hygromycin, respectively. Of the 33 cotransfected clones with single-site insertions, 1/13 without BLUR-8 and 6/20 with BLUR-8 were only in human chromosome 11. A frequency of insertion of 1/13 is not different than expected by chance (rho = 0.3512). On the other hand, the probability that 6/20 insertions, as seen with BLUR-8, occurred by chance is low (rho = 0.0003). We suggest that the human DNA sequences contained in BLUR-8 targeted insertions into only the human chromosome.

Detection of direct mutagenicity of cigarette smoke condensate in mammalian cells

Mutagenicity of cigarette smoke condensate (CSC) and the acidic, basic and neutral fractions of CSC was examined in the AL hybrid cell, a Chinese hamster ovary cell containing one human chromosome 11. Since the human chromosome 11 is not necessary for survival of the AL cells, mutations involving large deletions and chromosomal loss by non-dysjunction are non-lethal events that are detectable by loss of human cell surface antigens (a1, a2 and a3) encoded by genes on chromosome 11p (a1 and a3) and 11q (a2) through an antibody-complement lysis assay. Exposure of AL cells to CSC without exogenous metabolic activation caused a dose-dependent cytotoxicity and mutagenicity. Mutagenicity also increased with time of incubation up to 3 h with a maximum of 300 a1- mutants/10(5) survivors (250% above background; P less than 0.0005) after incubation with 100 micrograms/ml CSC. Cytotoxicity and mutagenicity of CSC were inversely proportional to cell density. Fifty percent lethal doses for the acidic, basic and neutral fractions of CSC after 3 h of incubation were 30, 100 and 240 micrograms/ml respectively, and the acidic fraction at a concentration of 25 micrograms/ml induced 350 a1- mutants/10(5) survivors (230% above background; P less than 0.0005); the basic and neutral fractions were less mutagenic. These results indicate that CSC and fractions of CSC can directly produce a spectrum of mutations, through both deletional and non-dysjunctional mechanisms of a kind known to lead to inactivation of tumor suppressor genes.

Human chromosome 5 complements the DNA double-strand break-repair deficiency and gamma-ray sensitivity of the XR-1 hamster variant

XR-1 is a Chinese hamster ovary (CHO) cell mutant which is unusually sensitive to killing by gamma rays in the G1 portion of the cell cycle but has nearly normal resistance to gamma-ray damage in late S phase. The cell-cycle sensitivity correlates with the mutant's inability to repair DNA double-strand breaks (DSBs) produced by ionizing radiation and restriction enzymes. We have previously shown in somatic cell hybrids of XR-1 cells and human fibroblasts that the XR-1 mutation is a recessive mutation. In this study, using somatic cell hybrids formed between XR-1 and human fibroblasts, we map the human complementing gene to chromosome 5 by chromosome-segregation analysis. This gene biochemically restores the hamster defect to wild-type levels of gamma-ray and bleomycin resistance as well as restoring its proficiency to repair DNA DSBs, suggesting that a single gene is responsible for the XR-1 phenotype. We have tentatively assigned the name XRCC4 (X-ray-complementing Chinese hamster gene 4) to this human gene until its biochemical function in repair is discovered.

Analysis of mutant frequency curves and survival curves applied to the AL hybrid cell system

A model is presented for the statistical analysis of survival curves and mutant frequency curves for a hybrid cell system. The derivation of the model is given in the Appendix, and depends on simple assumptions about the distribution of insults, their repair, and the loss of a marker that is not rescued. A single formula (5) is found which relates a survival curve to the mutant frequency curve, i. e., the response curve for production of mutants per 10(5) survivors induced by a mutagen. The analysis is applied to loss of the a1 gene in AL-J1 hybrid cells submitted to Cesium gamma-rays. Previous experimental data using X-rays was reported by Waldren et al. (1986: Proc. Natl. Acad. Sci, USA 83, 4839.) Also, a derived formula (10), which predicts the probability that in a surviving cell a marker is lost and not rescued, will form the basis for testing the validity of the model in the future using new experimental data.

Measurement of low levels of x-ray mutagenesis in relation to human disease

We previously demonstrated that conventional methods for measurement of mutagenesis in mammalian cells are subject to serious error that causes underestimation of environmental contributions to cancer and genetic disease. This error has been corrected by use of somatic cell hybrids containing a single human chromosome on which the marker genes are carried and by using doses of mutagenic agents so low that little cell killing occurs. This method permits direct measurement of the effects of low doses of radiation and other mutagens without resort to the controversial extrapolation procedure customarily used to estimate effects of doses in the neighborhood of actual human exposures. The new data demonstrate that the true mutagenesis efficiency at the low doses of ionizing radiation that approximate human exposures is more than 200 times greater than those obtained with conventional methods. This methodology also permits evaluation of localized mutations, large and small chromosomal deletions, and nondisjunctional processes and can be used for mutagens that need metabolic activation as well as for cooperatively acting agents. The two opposing classical views that in mammalian cells extrapolation to low doses of x-radiation is linear, on the one hand, or involves a threshold, on the other, are both demonstrated to be incorrect at least for the conditions here considered. The actual curve exhibits a downward concavity so that the mutational efficiency is maximal at low doses. These data may have important implications for human health.

Use of somatic cell hybrids for quantitation of mutagenesis: reduction in background mutants by fluorescence-activated cell sorting (FACS)

Environmentally induced mutations, especially those involving large scale genetic damage such as deletions and chromosome loss, are of central importance in the production of human genetic disease and cancer. We have developed a methodology, the AL assay, that permits detection of such extensive genetic changes which often escape detection in other systems in which they are lethal. The AL assay employs a human-Chinese hamster ovary cell hybrid that retains a single human chromosome, number 11. A set of specific cell surface antigens result from genes located on opposite arms of this chromosome. Exposure to mutagens produces mutants which form colonies in the presence of complement and specific antiserum that kill nonmutant cells. The frequency and pattern of marker loss provides a measure of single gene mutation, large and small deletion, and loss of the entire chromosome 11. We have employed the indirect fluorescein conjugated isothiocyanate (FITC) technique and fluorescence-activated cell sorting (FACS) to remove spontaneous mutants from the initial population. The 100-fold reduction in background thus far achieved should allow accurate analysis of mutation by ionizing radiation at doses of less than 10 rad.

Cell-cycle kinetics and ultraviolet light survival in UV-1, a Chinese hamster ovary cell mutant defective in post-replication recovery

UV-I, an ultraviolet-sensitive mutant of CHO-KI, is abnormally slow to recover from the inhibition of DNA synthesis caused by u.v. irradiation. When synchronized UV-I cells are irradiated in G1, their movement into S phase is unaltered, but thymidine incorporation is depressed (compared with that in the parent cell similarly treated). When irradiated in S phase, again incorporation is more depressed, and S phase suffers a greater delay in UV-I than in the parent cell. UV-I and its parent have similar capacities for excision repair of u.v.-induced damage inflicted in G1, and so enter S phase with similar amounts of unrepaired damage. The single-cell survival was measured after irradiation at different points in the cell cycle. The mutant and parent cells have similar values of D0 (mean lethal dose) except in mitosis, when the parent cell shows markedly greater resistance to u.v. irradiation. Dq (quasi-threshold dose) is fairly constant for the parent cell, but in UV-I it falls to a minimum in S phase. The responses of UV-I to u.v. irradiation are generally consistent with its known defect in the process of post-replication recovery, i.e. the ability to join up the abnormally small DNA fragments synthesized on a u.v.-damaged template.

A survey of DNA repair incision activities after ultraviolet irradiation of a range of human, hamster, and hamster-human hybrid cell lines

We describe a rapid and simple assay for DNA damage and repair in mammalian cells, based on the partial unwinding of nicked DNA in alkali, and involving transfer of this DNA to nitrocellulose and the digestion of single-stranded DNA with S1 nuclease, all steps taking place on tissue-culture chamber slides. Direct breakage of DNA by ionizing radiation has been examined, and we have developed a standard procedure for measuring enzymic breakage of DNA as an index of excision-repair capacity following ultraviolet irradiation. We report a wide range of repair capacities among various hamster and human cell lines, with considerable overlap between the two species. Hybrids between hamster and human cells tend to display repair activity characteristic of the hamster parent.

Measurement of mutagenesis in mammalian cells

A method using mammalian cells in vitro for detection and quantitation of mutagenic actions that appears to be useful for screening for carcinogenesis and genetic damage by environmental agents is presented. The method involves use of stable human--Chinese hamster ovary hybrid cells that have retained a single human chromosome not necessary for cell reproduction. Forward mutations are detected in genes necessary for production of specific human cell surface antigens. Such mutants form colonies in the presence of specific antisera and complement that destroy the unmutagenized cells. Use of the method is illustrated for the action of x-irradiation, N-methyl-N'-nitro-N-nitrosoguanidine, and caffeine. The method appears to be unique in that it permits assessment of lesions that cause loss of all or most of the chromosome as well as various localized gene mutations. The former action is particularly important because of the major involvement of chromosomal lesions in an extremely important class of human genetic disease.

Isolation and characterization of temperature-sensitive mutants of Chinese hamster ovary cells after treatment with UV and x-irradiation

The isolation of ten conditionally lethal temperature-sensitive mutants of the Chinese hamster ovary cell (CHO-Kl, pro-) by the BUdR-visible light selection procedure described. Treatment with radiation at doses known to cause single gene mutation in mammalian cells increases the mutation frequency by a factor of at least 14. These mutants will grow with normal plating efficiency at 34.5 degrees but will not grow at 39.5 degrees. Complementation analysis by two independent methods indicates that all mutants are recessive and allows the assignment of the mutants to six genetically independent complementation groups. Reversion analysis indicates that the TS-mutants are stable, spontaneous revertants arising at a frequency of less than 10(-6). Preliminary chromosome analysis revealed no systematic chromasomal abnormality in the mutants. Mitotic accumulation is used to study the generation time of the parental cells and representative mutants at 34.5 degrees and 39.5 degrees. The uses of these mutants for genetic analysis of mammalian cells in culture is discussed.