Repair of DNA adducts in asynchronous CHO cells and the role of repair in cell killing and mutation induction in synchronous cells treated with 7-bromomethylbenz[a]anthracene

Molecular analysis of ERCC2 mutations in the repair deficient hamster mutants UVL-1 and V-H1

The cDNA sequence of the Chinese hamster ERCC2 nucleotide excision repair and transcription gene from the UVL-1 Chinese hamster ovary (CHO) mutant cell line and the V-H1 Chinese hamster V79 mutant line was analyzed. ERCC2 encodes a presumed ATP-dependent DNA helicase and is single copy in CHO lines due to the structural hemizygosity of chromosome 9. Both UVL-1 and V-H1 have intermediate levels of (6-4) photoproduct repair but are as highly UV sensitive as the group 2 mutants that have no detectable repair. Deficiency in cyclobutane dimer removal has also been shown for V-H1. In UVL-1, a single base substitution resulting in an Arg75-->Trp substitution in helicase domain Ia was identified. The equivalent amino acid position is also Arg in the human, mouse, Xiphophorus maculatus, Saccharomyces cerevisiae, and Schizosaccharomyces pombe homologs. In V-H1, a single base substitution resulting in a Thr46-->Ile substitution in helicase domain I (the ATP-binding domain) was identified in both alleles. The equivalent amino acid position is also Thr in the five homologs. Analysis of three V-H1 partial revertants revealed that they still have the original V-H1 mutation in both alleles, indicating that these are second site reversion events. Site-specific mutagenesis was used to introduce the Thr46-->Ile, Arg75-->Trp, and Lys48-->Arg (helicase domain I) mutations into a hamster ERCC2 expression plasmid. These plasmids each failed to confer UV resistance to group 2 mutant cells, further demonstrating that the changes identified are the causative mutations in V-H1 and UVL-1. Correlations between specific mutations, biochemical activities, and repair phenotype are discussed.

Molecular analysis of CXPD mutations in the repair-deficient hamster mutants UV5 and UVL-13

The cDNA sequence of the Chinese hamster xeroderma pigmentosum group D (CXPD) nucleotide excision repair gene was analyzed from three Chinese hamster ovary (CHO) cell lines: repair proficient strain AA8 and repair deficient, UV complementation group 2 strains UV5 and UVL-13. CXPD encodes a presumed ATP-dependent DNA helicase and is single copy in CHO lines due to the hemizygosity of chromosome 9. Comparison of the deduced wild-type AA8 CXPD protein sequence with that of the Chinese hamster V79 lung-derived cell line revealed two amino acid polymorphisms. Position 285 is glutamine in AA8 and arginine in V79, and position 298 is alanine in AA8 and threonine in V79. Comparison with the human XPD, Saccharomyces cerevisiae RAD3, and Schizosaccharomyces pombe rad15 homologs shows variability at these positions. Analysis of the CXPD sequence in the repair deficient CHO lines UV5 and UVL-13 revealed, in each case, a single base substitution resulting in an amino acid substitution. Position 116 is tyrosine in UV5 and cysteine in AA8, and the corresponding positions of XPD, RAD3, and rad15 are cysteine. Position 615 is glutamic acid in UVL-13 and glycine in AA8, and the corresponding positions of XPD, RAD3, and rad15 are glycine. In both UV5 and UVL-13, positions 285 and 298 are glutamine and alanine, respectively, as seen in AA8. These results suggest that cysteine 116 and glycine 615 are critical to the repair function of CXPD.

DNA strand-specific repair of (+-)-3 alpha,4 beta-dihydroxy-1 alpha,2 alpha-epoxy-1,2,3,4-tetrahydrobenzo[c]phenanthrene adducts in the hamster dihydrofolate reductase gene

We evaluated the formation and removal of (+-)-3 alpha,4 beta-dihydroxy-1 alpha,2 alpha-epoxy-1,2,3,4- tetrahydrobenzo[c]phenanthrene (BcPHDE)-DNA adducts in two Chinese hamster ovary (CHO) cell lines. One line of repair-proficient cells (MK42) carries a stable 150-fold amplification of the dihydrofolate reductase (DHFR) locus. The other line of repair-deficient cells (UV-5) is diploid for this gene and is defective in excision of bulky DNA lesions. Two methods were used to quantitate adduct levels in treated cells: Escherichia coli UvrABC excision nuclease cleavage and 32P-postlabeling. DNA repair was examined in the actively transcribed DHFR gene, in an inactive region located 25 kilobases downstream, and in the overall genome. Between 8 and 24 hr after BcPHDE exposure, preferential repair of the DHFR gene compared to the noncoding region was apparent in MK42 cells. This gene-specific repair was associated with adduct removal from the DHFR transcribed strand. However, UV-5 cells showed no lesion reduction from this strand of the gene. By both quantitation methods, regions accessible to repair in MK42 cells showed a 2-fold reduction in DNA adduct levels by 24 hr. That the decline in adducts reflects genomic repair was demonstrated by the constant damage level remaining in UV-5 cells. Since BcPHDE-induced mutations in DHFR apparently arise from adducted purines on the nontranscribed strand, results from the present study support the idea that a consequence of strand-specific repair is strand-biased mutations.

Sensitivity and single-strand DNA break repair in Chinese hamster mutants exposed to the carcinogen aflatoxin B1 epoxide and its dichloride model

Aflatoxin B1 (AFB1) is a potent carcinogen and mutagen. It requires metabolic activation to be converted to the DNA-binding product aflatoxin B1 epoxide (AFB1-epoxide). A model of this epoxide is aflatoxin B1 dichloride (AFB1Cl2). Both react at the N7 position of guanine to form large adducts. The major adduct formed can either be rapidly removed to leave an apurinic site or can undergo ring opening of the imidazole ring to form a chemically stable adduct. A number of Chinese hamster DNA repair-deficient mutants have been screened for their sensitivity to AFB1-epoxide and AFB1Cl2. Some of the mutants screened belong to different UV complementation groups. Human genes involved in nucleotide excision-repair correct deficiencies found in these complementation groups. The mutants which were found to be most sensitive to AFB1 (V-C4 and V-H1) were further investigated. Alkaline elution was used to measure AFB1-induced DNA single-strand break repair in the mutants. V-H1 repaired completely in 24 h whereas V-C4 displayed only partial repair.

The repair of large DNA adducts in mammalian cells

This paper describes experiments involving the measurement of DNA damage and repair after treatment with 4-nitroquinoline 1-oxide (4NQO) or aflatoxin B1 (AFB1) epoxide in a number of mammalian cell cultures primarily associated with defects in the excision repair of UV-induced DNA damage. The results with transformed derivatives of XP cells belonging to different complementation groups showed that the extent of repair of 4NQO adducts at the N2 or C8 of guanosine did not correlate to the extent of repair reported by others after UV-irradiation. An examination of 4NQO repair in rodent UV-sensitive cell lines from different ERCC groups indicated that again there was little correlation between the extent of 4NQO and UV repair. However, regardless of complementation group those mutants that were defective in the repair of pyrimidine dimers and 6,4-photoproducts did exhibit a reduced ability to repair the 4NQO N2 guanosine adduct, whereas those mutants defective in pyrimidine dimer repair alone were able to repair this lesion as normal. In all of these cell lines there was a normal capacity to repair the 4NQO C8 guanosine adduct. Less extensive experiments involving AFB1 epoxide showed an XPC-transformed cell line was able to repair 40% of lesions after 6 h, whereas only 20% of repair is seen after UV. The rodent mutant V-C4 which belongs to the same ionising radiation group as irs2, was partially defective in repairing AFB1-induced damage. These experiments highlight the fact that although there are many commonalities between the repair of UV damages and lesions classed as large DNA adducts differences clearly exist, the most striking example here being the repair of the C8 guanosine 4NQO adduct which rarely correlates with a defect in UV repair.

Introduction of cytochrome P450IA2 metabolic capability into cell lines genetically matched for DNA repair proficiency/deficiency

We introduced into the CHO cell line the cDNA of the mouse cytochrome P3450 (P450IA2) gene, which oxidizes aromatic amines. A cDNA clone of P3450 was transfected into mutant UV5 cells, which is defective in nucleotide excision repair. Expression of the P3450 cDNA was measured using 9000 x g supernatant (S9) fractions from CHO cells to evaluate Salmonella TA1538 mutagenicity with the mutagen 2-amino-3-methylimidazo[4,5-f]quinoline (IQ). The P3450-expressing clone UV5P3 was reverted to repair proficiency using ethyl methanesulfonate to obtain the UV-resistant clone 5P3R2, which maintained the same level of P3450 protein activity as UV5P3. These genetically similar cell lines were compared for toxicity and mutation induction at the aprt locus. With 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (the most prevalent mutagen found in fried beef) the differential sensitivity due to repair deficiency/proficiency was approximately 40-fold, and with IQ there were smaller, but significant, differences in sensitivity. These genotoxic effects occurred at doses that were approximately 10 times lower than those that previously gave similar effects in S9-mediated assays. Thus, these cell lines should be valuable for genotoxicity analysis as well as important for assessing DNA repair when evaluating compounds that undergo metabolic activation.

Cyclobutane-pyrimidine dimer excision in UV-sensitive CHO mutants and the effect of the human ERCC2 repair gene

Using a radiochromatographic assay, we have examined cis-syn cyclobutane-pyrimidine dimer removal after ultraviolet irradiation in cell lines representative of the first 6 complementation groups of Chinese hamster ovary DNA nucleotide excision repair mutants. AA8, the CHO cell line from which these mutants were derived, consistently showed normal dimer excision for a rodent cell. The mutants uniformly exhibited no significant dimer excision within the limits of determination. Additionally, V-H1, a mutant belonging to complementation group 2 and derived from V79 hamster cells, exhibited no dimer excision. Two UV5 derived transformants that carry the complementing human ERCC2 repair gene showed a capacity for dimer excision comparable to the AA8 wild-type cells.

Molecular cloning and biological characterization of a human gene, ERCC2, that corrects the nucleotide excision repair defect in CHO UV5 cells

The UV-sensitive Chinese hamster ovary (CHO) cell line UV5, which is defective in the incision step of nucleotide excision repair, was used to identify and clone a complementing human gene, ERCC2, and to study the repair process. Genomic DNA from a human-hamster hybrid cell line was sheared and cotransferred with pSV2gpt plasmid DNA into UV5 cells to obtain five primary transformants. Transfer of sheared DNA from one primary transformant resulted in a secondary transformant expressing both gpt and ERCC2. The human repair gene was identified with a probe for Alu-family repetitive sequences. For most primary, secondary, and cosmid transformants, survival after UV exposure showed a return to wild-type levels of resistance. The levels of UV-induced mutation at the aprt locus for secondary and cosmid transformants varied from 50 to 130% of the wild-type level. Measurements of the initial rate of UV-induced strand incision by alkaline elution indicated that, whereas the UV5 rate was 3% of the wild-type level, rates of cosmid-transformed lines were similar to that of the wild type, and the secondary transformant rate was about 165% of the wild-type rate. Analysis of overlapping cosmids determined that ERCC2 is between 15.5 and 20 kilobases and identified a closely linked gpt gene. Cosmids were obtained with functional copies of both ERCC2 and gpt. ERCC2 corrects only the first of the five CHO complementation groups of incision-defective mutants.

Molecular cloning and biological characterization of a human gene, ERCC2, that corrects the nucleotide excision repair defect in CHO UV5 cells

The UV-sensitive Chinese hamster ovary (CHO) cell line UV5, which is defective in the incision step of nucleotide excision repair, was used to identify and clone a complementing human gene, ERCC2, and to study the repair process. Genomic DNA from a human-hamster hybrid cell line was sheared and cotransferred with pSV2gpt plasmid DNA into UV5 cells to obtain five primary transformants. Transfer of sheared DNA from one primary transformant resulted in a secondary transformant expressing both gpt and ERCC2. The human repair gene was identified with a probe for Alu-family repetitive sequences. For most primary, secondary, and cosmid transformants, survival after UV exposure showed a return to wild-type levels of resistance. The levels of UV-induced mutation at the aprt locus for secondary and cosmid transformants varied from 50 to 130% of the wild-type level. Measurements of the initial rate of UV-induced strand incision by alkaline elution indicated that, whereas the UV5 rate was 3% of the wild-type level, rates of cosmid-transformed lines were similar to that of the wild type, and the secondary transformant rate was about 165% of the wild-type rate. Analysis of overlapping cosmids determined that ERCC2 is between 15.5 and 20 kilobases and identified a closely linked gpt gene. Cosmids were obtained with functional copies of both ERCC2 and gpt. ERCC2 corrects only the first of the five CHO complementation groups of incision-defective mutants.

Phenotypic heterogeneity within the first complementation group of UV-sensitive mutants of Chinese hamster cell lines

A DNA-repair mutant was characterized that has the extraordinary and interesting properties of extreme sensitivity to UV killing combined with a high level of nucleotide excision repair. The mutant V-H1 isolated from the V79 Chinese hamster cell line appeared very stable, with a reversion frequency of about 3.5 X 10(-7). Genetic complementation analysis indicates that V-H1 belongs to the first complementation group of UV-sensitive Chinese hamster ovary (CHO) mutants described by Thompson et al. (1981). This corresponds with data on cross-sensitivity and mutation induction after UV irradiation published by this group. Surprisingly, the mutant V-H1 shows only slightly reduced (to approximately 70%) unscheduled DNA synthesis (UDS) after UV exposure, while the other two mutants of this complementation group are deficient in UDS after UV. In agreement with the high residual UDS, in V-H1 also the amount of repair replication in response to UV treatment is relatively high (approximately 50%). It has also been shown that the incision step of the nucleotide excision pathway takes place in V-H1 (with a lower rate than observed in wild-type cells), whereas another mutant (UV5) of the same complementation group is deficient in incision. This heterogeneity within the first complementation group indicates that the repair gene of this complementation group may have more than one functionally important domain or that the gene is not involved in the incision per se but is involved in e.g. preferential repair of active genes.

Mutagenic potency at the Na+/K+ ATPase locus correlates with cycle-dependent killing of 10T1/2 cells

Perturbation of DNA replication by chemical-DNA adducts produced by exposure to mutagenic/carcinogenic chemicals results in mutagenic or cytotoxic damage in the DNA. Demonstration of a correlation between cell cycle dependency of cytotoxicity and point mutation at the Na+/K+ ATPase gene could suggest that the two consequences of chemical exposure are caused by the same damage in the template DNA and that both are mediated through DNA replication-associated mechanisms. N-methyl-N'-nitro-N-nitrosoguanidine, N-ethyl-N'-nitro-N-nitrosoguanidine, 4-nitroquinoline-1-oxide, and benzo(a)pyrene-trans-7,8-dihydrodiol-9,10-epoxide demonstrated cell cycle-related patterns of cytotoxicity in 10T1/2 cells, with maximal cell killing produced by exposure in early S phase, and were highly efficient mutagens of the Na+/K+ ATPase gene relative to their cytotoxic potential. In contrast, methyl methanesulfonate and N-acetoxy-N-2-fluorenylacetamide were maximally cytotoxic in cell populations exposed in early G1 phase and were weak mutagens of the Na+/K+ ATPase gene at comparable levels of cytotoxicity. These data suggest that mutagenic/carcinogenic chemicals that are effective at producing mutations by misreplication kill cells by a related mechanism that may be associated with the perturbation of DNA replication.

Assignment of a human DNA-repair gene associated with sister-chromatid exchange to chromosome 19

The Chinese hamster ovary (CHO) cell mutant, EM9, is defective in rejoining strand breaks, hypersensitive to chlorodeoxyuridine (CldUrd), and has a high frequency of sister-chromatid exchange (SCE). Somatic cell hybrids constructed from fusion of EM9 cells with normal human lymphocytes and fibroblasts, and selected in CldUrd, extensively segregate human chromosomes but preferentially retain markers of human chromosome 19. The SCE frequency in the hybrid clones is low as in normal CHO cells, but in CldUrd-sensitive subclones, which lose the human chromosome 19 markers, SCE frequencies return to mutant levels. We therefore assign a human gene designated repair complementing defective repair in Chinese-hamster (RCC) to chromosome 19. Since this is the second (of two) human genes complementing repair-deficiency mutations in CHO cells assigned to the 19, the assignment and organization of DNA-repair genes is discussed in the light of hemizygosity in CHO cells and the evolutionary conservation of mammalian linkage groups.

Different genetic alterations underlie dual hypersensitivity of CHO mutant UV-1 to DNA methylating and cross-linking agents

CHO mutant UV-1, isolated on the basis of hypersensitivity to UV radiation (254 nm), was further characterized with respect to sensitivity to classes of DNA damaging agents in a differential cytotoxicity (DC) assay. Compared to its parental strain, Gly- A, UV-1 was dramatically (10- to 100-fold) hypersensitive to both DNA methylating and cross-linking agents. In addition, UV-1 was moderately (two- to fourfold) hypersensitive to several other classes of mutagens. DNA isolated from UV-1 or Gly- A after exposure to 14C-labeled methylnitrosourea (MNU) contained similar amounts of label, thus ruling out differences in uptake or binding. Three phenotypic revertants of UV-1 were resistant to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and other methylating agents but retained hypersensitivity to cross-linking agents. Moreover, fusion of UV-1 with two different UV-sensitive CHO mutants also having hypersensitivity to cross-link and methylation damage produced hybrids resistant to mitomycin C (MMC) but not to methyl methane sulfonate (MMS). Since the methylation and cross-link sensitivities were uncoupled in both genetic tests, the complex phenotype of UV-1 is likely due to more than one genetic alteration.

Genetic complementation between UV-sensitive CHO mutants and xeroderma pigmentosum fibroblasts

The purpose of this study was to determine the feasibility of doing complementation analysis between DNA-repair mutants of CHO cells and human fibroblasts based on the recovery of hybrid cells resistant to DNA damage. Two UV-sensitive CHO mutant lines, UV20 and UV41, which belong to different genetic complementation groups, were fused with fibroblasts of xeroderma pigmentosum in various complementation groups. Selection for complementing hybrids was performed using a combination of ouabain to kill the XP cells and mitomycin C to kill the CHO mutants. Because the frequency of viable hybrid clones was generally less than 10(-6) and the frequency of revertants of each CHO mutant was approximately 2 X 10(-7), putative hybrids required verification. The hybrid character of clones was established by testing for the presence of human DNA in a dot-blot procedure. Hybrid clones were obtained from 9 of the 10 different crosses involving 5 complementation groups of XP cells. The 4 attempted crosses with 2 other XP groups yielded no hybrid colonies. Thus, a definitive complementation analysis was not possible. Hybrids were evaluated for their UV resistance using a rapid assay that measures differential cytotoxicity (DC). All 9 hybrids were more resistant than the parental mutant CHO and XP cells, indicating that in each case complementation of the CHO repair defect by a human gene had occurred. 3 hybrids were analyzed for their UV-radiation survival curves and shown to be much more resistant that the CHO mutants but less resistant than normal CHO cells. With 2 of these hybrids, sensitive subclones, which had presumably lost the complementing gene, were found to have similar sensitivity to the parental CHO mutants. We conclude that the extremely low frequency of viable hybrids in this system limits the usefulness of the approach. The possibility remains that each of the nonhybridizing XP strains could be altered in the same locus as one of the CHO mutants.

Rapid detection of DNA-damaging agents using repair-deficient CHO cells

A screening method is introduced to detect and classify DNA-damaging agents using DNA repair-deficient strains of Chinese hamster ovary cells. Differential cytotoxicity (relative growth) of the mutant cells compared to the wild-type cells was interpreted as a measure of lethal, potentially repairable damage to DNA. The assay consists of exposing the wild-type cells and three mutant strains to the test compound in a 24-well tray and using staining intensity to estimate growth after 72 h. The battery of mutants consists of two UV-sensitive strains (UV4 and UV5) that are deficient in different aspects of nucleotide excision repair, and strain EM9, which is defective in DNA-strand-break rejoining. The assay was highly reproducible, and the magnitude of the differential cytotoxicity response compared favorably with the amount of differential killing measured by colony-formation survival curves for several chemicals. 15 direct-acting and 7 metabolism-dependent agents that were expected to produce bulky, covalent DNA adducts were tested in the assay, and all produced a differential cytotoxicity response in at least two of the mutants. UV4 and UV5 showed a response to all of the test compounds whereas EM9 showed a response to 7 of the test compounds. Thus, the pattern of mutant responses presumably reflects the types of DNA damage produced by a compound. Although this aspect is still under development, these results indicate the potential of a larger battery of mutants to classify a wide spectrum of chemicals according to the lesions they produce. 13 non-DNA damaging agents were also tested and none produced a differential cytotoxicity response, suggesting that this endpoint is specific for DNA damage. We conclude that this assay may be a cost-effective alternative or adjunct to the existing short-term tests.

Hypersensitivity to cell killing and mutation induction by chemical carcinogens in an excision repair-deficient mutant of CHO cells

A strain of Chinese hamster ovary cells that is deficient in nucleotide excision repair, strain UV5, was compared with the normal parental CHO cells in terms of cytotoxicity and mutagenesis after exposure to several chemical carcinogens that are known to produce bulky, covalent adducts in DNA. Induced mutations were measured at the hprt locus using thioguanine resistance and at the aprt locus using azaadenine resistance. The compounds tested that required metabolic activation (using rat or hamster microsomal fractions) were 7,12-dimethylbenz(a)anthracene, 3-methylcholanthrene, benzo(a)pyrene, aflatoxin B1, 2-acetylaminofluorene, and 2-naphthylamine. The direct-acting compounds (+/-)-r-7,t-8-dihydroxy-t-9,10-epoxy-7,8,9,10-tetrahydrobenzo(a)pyrene, N-acetoxy-2-acetylaminofluorene, and N-OH-2-naphthylamine were also studied. For all compounds except 2-naphthylamine and its active metabolite, the repair-deficient cells were significantly more sensitive to killing than the normal CHO cells. Mutation induction at both loci was also more efficient in UV5 cells in each instance where enhanced cytotoxicity was observed. By using tritium-labeled N-acetoxy-2-acetylaminofluorene, normal and mutant cells were shown to bind mutagen to their nuclear DNA with similar efficiency, and a greater amount of adduct removal occurred in the normal cells. From this study it is concluded that the use of excision repair-deficient CHO cells provides enhanced sensitivity for detecting mutagenesis and that a positive differential cytotoxicity response gives an indication of repairable, potentially lethal genetic damage.