Excision of hydrocarbon-DNA adducts and consequent cell survival in normal and repair defective human cells

Interspecies Differences in the Removal of DNA Adducts

Numerous physical and chemical agents damage cellular DNA in vivo. Such damage has been associated with various biochemical, physiological, and pathological dysfunctions including: alterations in gene expression, cell death, mutation, birth defects, cancer, and aging. Cells and organisms unable to prevent the induction of DNA damage or to repair such damage once it is induced, are predisposed to one or more of these pathologies. Over the course of evolution, living systems have developed various mechanisms to cope with such damage including enzymatic repair, information redundancy and, in extreme situations, cellular replacement. Enzymatic repair can be divided into two general categories: prereplication and post-replication repair. Each of these categories includes many “repair” systems which differ with the size of the repaired region, the nature of the enzymes involved in the repair process, the type of agent inducing the repair process or form of lesion removed. Over the last decade, numerous methods have been developed to measure DNA damage, both directly as well as indirectly, but few studies exist comparing the results of these methods with one another. Little is known as to whether these methods are measuring the same or different endpoints. Interspecies, intertissue, and interorgan comparisons can only be made when comparable techniques have been utilized. From such studies, it is now apparent that significant differences in DNA repair exist among species, within species, and between organs. Further, it is now a reasonable speculation that such differences may, in part, account for differences in organ susceptibility and risk per cell per unit time for spontaneous malignant transformation observed between species.

Comparison of DNA-incising capacities in fibroblast strains from the Mannheim XP collection after treatment with N-acetoxy-2-acetylaminofluorene and UV light

The DNA-incising capacity was determined in 8 normal and 23 XP fibroblast strains of the Mannheim XP collection using the alkaline elution technique after treatment with both UV light and the "UV-like" carcinogen (Ac)2ONFln. Experimental conditions were chosen to allow for selective monitoring of repair-specific enzyme-catalyzed breaks. In order to compare DNA-incising capacities of the various cell strains after UV irradiation with those after treatment with (Ac)2ONFln, dose-response experiments including up to 8 dose levels were performed. The elution curves were analyzed by linear regression analysis. Elution velocities (in terms of DNA single-strand breaks per 10(6) nucleotides) were plotted against the square root of the doses. The slope of the resulting regression line yielded a characteristic term, designated EO, for the DNA-incising capacity of each cell strain. In contrast to normal fibroblasts, EO was found to be reduced in all XP cell strains belonging to the complementation groups A, C, D, E, F (or G) and I investigated, after treatment with both UV light or (Ac)2ONFln. Surprisingly, XP variant strains also exhibited lower EO values. A comparison of post-UV with post-(Ac)2ONFln DNA-incising capacities revealed that reduction in the EO values was very similar in all XP cell strains tested. These data suggest that the sensitivity of XP cells towards UV light or (Ac)2ONFln is due to the same enzymatic defect, namely impaired incision of DNA containing pyrimidine dimers or (Ac)2ONFln-DNA adducts.

Excision repair of UV- or benzo[a]pyrene diol epoxide-induced lesions in xeroderma pigmentosum variant cells is 'error free'

It is known that cells from one class of xeroderma pigmentosum (XP) patients, called XP variants, carry out excision repair of UV-induced DNA damage at a normal rate and are only slightly more sensitive than normal cells to the cytotoxic effect of UV radiation, but are much more sensitive to the mutagenic effect of UV. To see if this hypermutability were the result of an 'error-prone', excision repair process, we irradiated fibroblasts derived from an XP variant patient, XP4BE, under conditions that allowed the cells various lengths of time for excision repair before the onset of DNA synthesis (S phase) and assayed the frequency of 6-thioguanine (TG)-resistant mutants. Cells synchronized by release from confluence (G0 state) and irradiated just prior to S phase showed a dose-dependent increase in mutants at very high frequencies; cells irradiated in early G1, approximately 12 h before the onset of S phase, showed frequencies 4 times lower. Cells irradiated in the G0 state and allowed 24 h or 48 h for excision repair before the onset of S phase showed still lower frequencies. A comparison of the relative rates of decrease in mutant frequency with time for excision repair before the onset of S phase in XP variant cells and normal human fibroblasts after a dose of 4 or 6 J/m2 showed that these were equal. However, for every time point, the frequency of mutants induced per dose of UV was significantly higher in the XP variant population than in the normal, suggesting that the XP variant cells have an abnormally error-prone process of replicating DNA on a template containing unexcised lesions or normal cells are by-passing many of such lesions using an error-free process. A similar comparative study in synchronized populations of XP4BE cells and normal cells, using the anti 7,8-diol-9,10-epoxide of benzo[a]pyrene, showed that excision repair prior to the onset of S phase also decreased the frequency of mutants induced in XP variant cells by this agent. But for every dose and time point, the frequencies induced in XP4BE cells and normal cells were identical. Thus, the hypermutability of the XP4BE cells was specific to UV radiation-induced DNA lesions.

Xeroderma pigmentosum patients from the Federal Republic of Germany: decrease in post-UV colony-forming ability in 30 xeroderma pigmentosum fibroblast strains is quantitatively correlated with a decrease in DNA-incising capacity

A total of 16 normal and 46 XP fibroblast strains from the Mannheim Collection were investigated for colony-forming ability following exposure to both UV light and the "UV-like" carcinogen (Ac)2ONFln. The dose-response experiments included up to 13 dose levels. The exponential segments of the curves were analysed by linear regression and the negative reciprocal of the regression coefficient (D0) was calculated for each cell strain. For quantitating the DNA-incising capacity, DNA elution curves were determined at several UV dose levels. Plotting the initial velocities of the elution curves versus the UV dose yielded a regression line, the slope of which was used to obtain the characteristic value E0. Comparing D0 with E0 values showed that cell strains in which colony-forming ability was reduced suffered a reduction of DNA-incising capacity of the same magnitude. There were only 3 exceptional strains in which reduction of DNA-incising capacity was less pronounced than reduction of colony-forming ability. We have previously shown (Fischer et al. 1982) that D0 values from 27 XP strains of the Mannheim Collection were correlated with clinical symptoms. This correlation is now being extended by relating colony-forming ability to the magnitude of the DNA incision defect. From our data we conclude that the best quantitative biochemical denominator to explain the sun sensitivity of XP is that of a defective incision of UV-damaged DNA. A considerable similarity in sensitivity towards both UV light and (Ac)2ONFln was found in 16 normal and 46 XP strains. This seems to indicate that UV- and (Ac)2ONFln-induced DNA damage are removed to a large extent by the same pathways in human fibroblasts.

Hypersensitivity of xeroderma pigmentosum cells to dietary carcinogens

Xeroderma pigmentosum patients, in addition to ultraviolet-induced skin cancers, have an increased prevalence of neoplasms occurring in sites shielded from ultraviolet radiation. We postulated that these internal neoplasms might be related to ingestion of dietary carcinogens. As model dietary carcinogens, we studied the tryptophan pyrolysis products, 3-amino-1,4-dimethyl-5H-pyrido[4,3-b]indole (Trp-P-1) and 3-amino-1-methyl-5H-pyrido[4,3-b]indole (Trp-P-2). These dietary compounds bind to DNA and are highly mutagenic and carcinogenic. Cytotoxicity of these compounds was examined in cultured lymphoblastoid cell lines from xeroderma pigmentosum patients in complementation groups A, B, C, D and E and the variant form and from normal donors. All xeroderma pigmentosum lymphoblastoid cell lines showed a greater reduction in viable cell concentration than the 2 normal lymphoblastoid cell lines following addition of Trp-P-1 or Trp-P-2 (5 micrograms/ml) to the culture medium. Possible differences in cellular activation of these compounds were overcome by treating the cells with rat-liver microsome-activated Trp-P-2. There was a greater reduction in viable cell concentration in the xeroderma pigmentosum group A and D cells than in the normal lymphoblastoid cell lines after treatment with activated Trp-P-2. These data suggest that the xeroderma pigmentosum DNA-repair system is defective in repairing Trp-P-1 and Trp-P-2 induced DNA damage in addition to being defective in repairing ultraviolet-induced DNA damage. Thus xeroderma pigmentosum patients may be at increased risk of toxicity from some dietary carcinogens.

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

CHO cells of normal or UV-sensitive phenotypes were analyzed for their ability to remove DNA adducts produced by the carcinogen 7-BrMeBA. At a dose of 0.1 microM, which reduced the survival of the normal AA8 cells to approximately 90% and the mutant UV5 cells to approximately 20%, the frequency of adducts was 5-6 per 10(6) nucleotides for both cell types, and AA8 cells removed approximately 30% of the adducts in 8 h and approximately 55% in 24 h. In contrast, UV5 and mutants from four other genetic complementation groups had no significant removal. Binding of 7-BrMeBA did not vary through the cell cycle in synchronous cultures. At a dose of mutagen (0.07 microM) resulting in approximately 25% survival of asynchronous UV5, the survival of synchronous cultures rose about threefold from early G1 to early S phase and then decreased somewhat in late S/G2. At a dose (0.28 microM) producing similar survival of asynchronous cultures, AA8 cells differed qualitatively in that survival decreased progressively by 5- to 10-fold between early G1 and the early part of S, and rose steeply through late S/G2 to give a 10- to 20-fold increase. We conclude that DNA repair is the major determinant of variations in survival through the cycle in normal cells. The patterns observed are consistent with a mechanism of killing in AA8 cells in which adducts disrupt DNA replication, while in UV5 cells transcriptional blocks or other effects may govern lethality. Induced mutations at the aprt and hprt loci showed changes through the cycle in both AA8 and UV5 cells, and the patterns were not readily explainable by the action of repair.

Repair of 4-nitroquinoline-1-oxide-induced DNA damage in normal human cells and cells from classical and variant xeroderma pigmentosum

The effect of 4-nitroquinoline-1-oxide (4NQO) upon 3 fibroblast cell lines derived from normal and xeroderma pigmentosum subjects have been compared. Excision-deficient XP cells (XP2BI), complementation group G, are nearly 200-fold more sensitive than normal cells to the lethal effect of 4NQO while XP variants (XP7TA), are 2-fold more sensitive. This cytotoxicity correlates with the levels of unscheduled DNA synthesis performed by the 3 cell lines. 4NQO causes a dose-related inhibition of DNA replication in all cell lines. However, newly replicated DNA synthesised immediately after treatment of cells with 4NQO is slightly smaller in XP7TA variant cells than in normal cells receiving the same dose of 4NQO, but DNA fragments in excision-deficient XP2BI are 50% smaller. It is likely that replicon elongation and joining together of newly replicated DNA fragments is dependent upon the excision of certain 4NQO-induced lesions, possibly normally repaired by a 'short-patch' repair process defective in XP2BI.

The use of DNA-repair-deficient mutants of Chinese hamster ovary cells in studying mutagenesis mechanisms and testing for environmental mutagens

Our laboratory has taken a somatic-cell-genetics approach to the study of mutagenesis by utilizing mutant strains of Chinese hamster ovary (CHO) cells that are deficient in DNA repair processes. From more than 150 UV-sensitive strains tested, five complementation classes were identified, and representative mutants were found to be defective at, or before, the incision step of excision repair. A representative mutant, strain UV-5, was compared with the parental strain in terms of cytotoxicity and dose-response curves for mutation induction after treatment with UV and several chemicals that are known to produce large adducts in DNA. Excision repair in normal CHO cells protects against both cytotoxicity and mutagenesis, but the degree of protection depends on both the agent and the genetic marker used for detecting mutations. Upon treatment with low doses (100% cell survival) of the polyaromatic hydrocarbon 7-bromomethylbenz(a)anthracene, repair-deficient UV-5 cells had linear responses for mutation induction to thioguanine resistance or azaadenine resistance, whereas the normal repair-proficient cells showed curvilinear responses in which the slope increased with dose. This behavior suggests that in the normal cells the repair system acting on potentially mutagenic lesions becomes saturated at doses that produce cytotoxicity. In no instance was a lower mutation frequency induced in UV-5 cells than the parental cells, at a given dose of mutagen, suggesting that the excision repair system is error-free in normal CHO cells.

Extent of excision repair before DNA synthesis determines the mutagenic but not the lethal effect of UV radiation

Excision repair-proficient diploid fibroblasts from normal persons (NF) and repair-deficient cells from a xeroderma pigmentosum patient (XP12BE, group A) were grown to confluence and allowed to enter the G0 state. Autoradiography studies of cells released from G0 after 72 h and replated at lower densities (3-9 x 10(3) cells/cm2) in fresh medium containing 15% fetal bovine serum showed that semi-conservative DNA synthesis (S phage) began approximately 24 h after the replating. To determine whether the time available for DNA excision repair between ultraviolet irradiation (254 nm) and the onset of DNA synthesis was critical in determining the cytotoxic and/or mutagenic effect of UV in human fibroblasts, we released cultures of NF or XP12BE cells from G0, allowed them to reattach at lower densities, irradiated them in early G1 (approximately 18 h prior to the onset of S) or just prior to S phase, and assayed the frequency of mutations to 6-thioguanine resistance and the survival of colony-forming ability. The XP12BE cells, which are virtually incapable of excising UV-induced DNA lesions, showed approximately the same frequency of mutations and survival regardless of the time of UV irradiation. In NF cells, the slope of the dose response for mutations induced in cells irradiated just prior to S was about 7-fold steeper than that of cells irradiated 18 h earlier. However, the two sets of NF cells showed no significant difference in survival. Neither were there significant differences in the survival of NF cells released from G0, plated at cloning densities and irradiated as soon as they had attached and flattened out. (approximately 20 h prior to S) or 4, 8, 12, 16, 20, or 24 h later. We conclude that the frequency of mutations induced by UV is dependent upon the number of unexcised lesions remaining at the time of semi-conservative DNA replication. However, the amount of time available for excision of potentially cytotoxic lesions is not determined primarily by the period between irradiation and the onset of S phase.

Relationship between excision repair and the cytotoxic and mutagenic effect of the 'anti' 7,8-diol-9,10-epoxide of benzo[a]pyrene in human cells

The cytotoxic and mutagenic effect of (+/-)-7 beta, 8 alpha-dihydroxy-9 alpha, 10 alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (anti BPDE) in normally excising diploid human cells treated just prior to onset of S was compared with that of cells allowed approximately 16 h for excision repair before onset of S and with that observed in excision-deficient xeroderma pigmentosum (XP12BE) cells. The cells were synchronized by release from density inhibition of cell replication. DNA synthesis began approximately 22 h after the cells were plated at lower density (i.e., 1.4 x 10(4) cells/cm2). The frequency of thioguanine-resistant mutants induced in normal cells treated just prior to onset of S was approximately 12- to 16-fold higher than that observed in cells treated in early G1 or treated in G0 (confluence) and then plated at lower density. The frequency approximated that expected for XP12BE cells from extrapolation of data obtained at lower doses. The frequency of mutants measured in normal cells treated in exponential growth was also much higher than that in the cells treated in early G1 or in G0. No such difference could be seen in XP12BE cells treated in exponential growth or in G0. In contrast to the mutagenicity data in the normal cells, there was no significant difference in the slope of the survival curve of normal cells treated at various times prior to S phase at low densities. However, normal cells treated even at the onset of S exhibited survival equal to XP12BE cells given a 4- to 5-fold lower dose. The data support the hypothesis that DNA synthesis is the cellular event which converts unexcised DNA lesions into mutations. However, they indicate that S is not the event primarily responsible for translating DNA damage into cell death. Accompanying studies on the rate of excision of anti BPDE adducts from the normal cells during the period prior to S support the conclusions.

DNA repair assays as tests for environmental mutagens. A report of the U.S. EPA Gene-Tox Program

A literature review was undertaken to determine the usefulness of DNA repair assays, other than unscheduled DNA synthesis, as screening techniques for mutagenic carcinogens. 92 reports were found to contain useful data for 49 chemicals using 6 techniques, namely, (1) cesium chloride equilibrium density gradients to study repair replication, (2) benzoylated naphthoylated diethylaminoethyl cellulose columns to study repair replication, (3) 313-nm irradiation of DNA containing bromodeoxyuridine to study repair replication, (4) alkaline elution to study repair of single-strand breaks and crosslinks, (5) alkaline sucrose gradients to study repair of single-strand breaks, and (6) direct assays for removal of adducts from DNA. Almost all of the 49 chemicals studied were known mutagens or carcinogens and/or known inducers of DNA repair, 9 compounds failed to elicit DNA repair by at least 1 assay technique, and at least 3 of these were not tested by the most appropriate and sensitive method. Nevertheless, although valid for studying repair phenomena in eukaryotic cells, these assays are not considered useful for screening. They are time-consuming, expensive, and/or require highly specialized skills. Despite the high frequency of positive reports, it is obvious from the literature that repair assays will fail to detect, or will detect with low efficiency, those agents whose main action is either intercalation or induction of strand breaks. For these and other reasons, DNA repair as a basis for screening for mutagenic carcinogens is not considered to be a useful concept.

Role of DNA repair in mutagenesis of Chinese hamster ovary cells by 7-bromomethylbenz[a]anthracene

The role of DNA repair in mutagenesis was studied in normal, repair-proficient Chinese hamster ovary cells and in two mutant strains that are deficient in excision repair. By using the mutagen 7-bromomethylbenz[a]anthracene (7-BrMeBA) and the technique of alkaline elution of DNA, the mutants were found to be defective at or before the incision step of excision repair. Dose--responses were determined for cell killing, mutation induction at three loci, and sister chromatid exchanges over a survival range of 1.0--0.1 after 7-BrMeBA treatment. The mutants were 5-fold more sensitive to killing than were the normal cells, but the degree of hypersensitivity to mutation induction varied depending on the mutant strain, the genetic marker, and the dose of mutagen. In each instance, the dose--response curve for mutations was essentially linear in the repair-deficient cells. In the normal cells, however, the curves for induced resistance to thioguanine and azaadenine were complex and were curvilinear with increasing slope at low doses. This behavior may be attributable to saturation of the excision repair system. No difference was seen in the efficiency of inducing ouabain-resistant mutations in the repair-deficient cells compared to the normal cells, indicating a qualitatively different behavior of this marker. These results are consistent with excision repair of 7-BrMeBA damage being error-free in Chinese hamster ovary cells. Sister chromatid exchange, another manifestation of DNA damage, also was induced with greater efficiency in the repair-deficient cells.

DNA repair in human fibroblasts treated with a combination of chemicals

Excision repair of DNA damage was measured by the photolysis of bromodeoxy-uridine incorporated during repair in normal human and xeroderma pigmentosum group C fibroblasts (XP C) treated with a combination of the carcinogens N-acetoxy-2-acetylamino-fluorene (AAAF), and 4-nitroquinoline 1-oxide (4NQO). Repair was additive in normal and XP C cells treated with AAAF plus 4NQO, indicating that there are different rate limiting steps for removal of 4NQO and AAAF lesions.

Epigenetic and genetic factors in the cellular response to radiations and DNA-damaging chemicals

DNA-damaging agents are widely used as therapeutic tools for a variety of disease states. Many such agents are considered to produce detrimental side effects. Thus, it is important to evaluate both therapeutic efficacy and potential risk. DNA-damaging agents can be so evaluated by comparison to agents whose therapeutic benefit and potential hazards are better known. We propose a framework for such comparison, demonstrating that a simple transformation of cytotoxicity-dose response patterns permits a facile comparison of variation between cells exposed to a single DNA-damaging agent or to different cytotoxic agents. Further, by transforming data from experiments which compare responses of 2 cell populations to an effects ratio, different patterns for the changes in cytotoxicity produced by epigenetic and genetic factors were compared. Using these transformations, we found that there is a wide variation (a factor of 4) between laboratories for a single agent (UVC) and only a slightly larger variation (factor of 6) between normal cell response for different types of DNA-damaging agents (x-ray, UVC, alkylating agents, crosslinking agents). Epigenetic factors such as repair and recovery appear to be a factor only at higher dose levels. Comparison in the cytotoxic effect of a spectrum of DNA-damaging agents in xeroderma pigmentosum, ataxia telangiectasia, and Fanconi's anemia cells indicates significantly different patterns, implying that the effect, and perhaps the nature, of these genetic conditions are quite different.

DNA excision repair in human cells treated with ultraviolet radiation and 7,12-dimethylbenz[a]anthracene 5,6-oxide

Excision repair was measured in normal human and xeroderma pigmentosum group C cells treated with 7,12-dimethylbenz[a]-anthracene 5,6-oxide and with ultraviolet radiation by the techniques of unscheduled DNA synthesis, repair replication, a modification of bromodeoxyuridine photolysis employing the dye Hoechst 33258 and 365 nm radiation, and endonuclease-sensitive sites assay. Radioautography and repair replication showed that in normal cells the magnitude of repair after a saturation dose of epoxide (approx. 10 microM) to be 0.1-0.2 that after a saturating ultraviolet dose (20 J/m2 at 254), though survival data showed that both doses gave nearly similar killings. Repair was of the long-patch type and repair kinetics after the epoxide treatment were similar to ultraviolet. After a combined treatment with both agents, unscheduled synthesis in normal cells was more than additive, although, considering the experimental errors, these data and those of repair replication are consistent with additivity. The epoxide did not inhibit loss of sites sensitive to the ultraviolet endonuclease. However, after a combined treatment to xeroderma pigmentosum cells there was appreciably less unscheduled synthesis than for the sum of both treatments and the epoxide inhibited the loss of nuclease-sensitive sites. We interpret the data to indicate that there are different rate-limiting steps in the removal of the ultraviolet and the epoxide damages, and that the residual repair activity in xeroderma pigmentosum cells is accomplished by different, not just fewer, enzymes than in normal cells.

Excision of DNA damage arising from chemicals of different carcinogenic potencies

Primary cultures of mouse embryo cells are more efficient in excising DNA-carcinogen adducts resulting from exposure to either 7-bromomethylbenz-[alpha]anthracene or the more carcinogen 7-bromomethyl-12-methylbenz[alpha]anthracene than are mouse L 929 cell suspension cultures. However, within each of these systems, the excisabilities of the adducts formed by either bromo-compound are similar, so differences in carcinogenic potency of the compounds cannot be attributed to differences in the excisability of their DNA-adducts.

Increased sensitivity of UV-repair-deficient human cells to DNA bound platinum products which unlike thymine dimers are not recognized by an endonuclease extracted from Micrococcus luteus

We have studied the response of human cells in culture to cis platinum[II] diammine dichloride (cis Pt[II]) induced DNA damage. The survival data, measured as a function of cis Pt[II] dose were similar in a normal cell line (Human foetal lung) compared to a UV-sensitive, thymine dimer excision repair-deficient cell line (Xeroderma pigmentosum). However, there was a marked difference between the two cell lines when binding to DNA was plotted against dose of cis Pt[II] given for 1 h. When these findings were expressed as cell survival versus binding to DNA, a 4.1--fold difference between the slopes of the survival curves for the two cell lines was obtained. These findings are consistent with the notion that normal cells are able to excise cis Pt[II] induced damage from their genome and thus increase their ability to survive as compared to excision-deficient cells. An endonuclease preparation from Micrococcus luteus is able to recognise UV damage in DNA, but did not recognise cis Pt[II] induced damage. These results possibly indicate differences in the pathways of repair of damage caused by the two agents.