Mutational specificity of benzo[a]pyrene diolepoxide in monkey cells

The role of polycyclic aromatic hydrocarbon-DNA adducts in inducing mutations in mouse skin

Polycyclic aromatic hydrocarbons (PAH) form stable and depurinating DNA adducts in mouse skin to induce preneoplastic mutations. Some mutations transform cells, which then clonally expand to establish tumors. Strong clues about the mutagenic mechanism can be obtained if the PAH-DNA adducts can be correlated with both preneoplastic and tumor mutations. To this end, we studied mutagenesis in PAH-treated early preneoplastic skin (1 day after exposure) and in the induced papillomas in SENCAR mice. Papillomas were studied by PCR amplification of the H-ras gene and sequencing. For benzo[a]pyrene (BP), BP-7,8-dihydrodiol (BPDHD), 7,12-dimethylbenz[a]anthracene (DMBA) and dibenzo[a,l]pyrene (DB[a,l]P), the codon 13 (GGC to GTC) and codon 61 (CAA to CTA) mutations in papillomas corresponded to the relative levels of Gua and Ade-depurinating adducts, despite BP and BPDHD forming significant amounts of stable DNA adducts. Such a relationship was expected for DMBA and DB[a,l]P, as they formed primarily depurinating adducts. These results suggest that depurinating adducts play a major role in forming the tumorigenic mutations. To validate this correlation, preneoplastic skin mutations were studied by cloning H-ras PCR products and sequencing individual clones. DMBA- and DB[a,l]P-treated skin showed primarily A.T to G.C mutations, which correlated with the high ratio of the Ade/Gua-depurinating adducts. Incubation of skin DNA with T.G-DNA glycosylase eliminated most of these A.T to G.C mutations, indicating that they existed as G.T heteroduplexes, as would be expected if they were formed by errors in the repair of abasic sites generated by the depurinating adducts. BP and its metabolites induced mainly G.C to T.A mutations in preneoplastic skin. However, PCR over unrepaired anti-BPDE-N(2)dG adducts can generate similar mutations as artifacts of the study protocol, making it difficult to establish an adduct-mutation correlation for determining which BP-DNA adducts induce the early preneoplastic mutations. In conclusion, this study suggests that depurinating adducts play a major role in PAH mutagenesis.

Mutation spectra in supF: approaches to elucidating sequence context effects

Shuttle vectors carrying the supF suppressor tRNA gene were originally developed for mutagenesis experiments in primate and human cells. Since then, the supF gene has been used as a mutation reporter in other mammalian cells, yeast, Escherichia coli, and transgenic mice. The widespread use of the vector for studies of many DNA reactive agents has produced a large database of mutation spectra. These provide primary information on the kinds and distribution of mutations provoked by many agents and, in many instances, allow comparisons between related agents or the same agent in different cell backgrounds. In this review we will discuss some of these data with a primary focus on the interpretation of UV mutation spectra. We will also describe our development and application of custom supF marker genes as an approach to studying the effect of sequence context on mutation hotspots and cold spots. Our studies suggest that C-C photoproducts are not mutagenic in certain sequence contexts in which T-C photoproducts are mutation hotspots. In addition, we have found several examples of sequence context effects acting as much as 80 bases away from the site of mutation. We will consider some of the problems raised by these studies and the possible resolution of some of them offered by the newly discovered family of damage bypass DNA polymerases.

The tumor promoter TPA enhances benzo[a]pyrene and benzo[a]pyrene diolepoxide mutagenesis in Big Blue mouse skin

The Big Blue mouse was used to investigate the role of cell proliferation in mutation fixation in the mouse back skin model of carcinogenesis. Phorbol 12-myristate 13 acetate (TPA) was applied to the dorsum of Big Blue mice to manipulate cell proliferation, and benzo[a]pyrene (BaP) or BaP-diolepoxide (BPDE) was applied to produce premutagenic DNA damage. Mutations in the lacI transgene of skin DNA were measured. BaP and BPDE elevated mutant frequency, DNA adducts, and cell damage over untreated and acetone-treated mice. BPDE-DNA adducts peaked within 30 min of exposure and DNA adducts, formed after application of both BaP and BPDE, declined rapidly with time. As the dose of BaP increased (4 to 64 microg), DNA adducts, mutant frequency, and cell damage increased in a dose-dependent manner. TPA applied after BaP and BPDE further increased mutant frequency, DNA adducts, and cell damage, while variably affecting mitotic index and other measures of cell proliferation. TPA became less effective at increasing mitotic index as the dose of BaP increased, although all measures of cell proliferation, taken together, increased. The most effective production of DNA adducts and mutations occurred when the carcinogen was applied simultaneously with or within 1 hr of TPA. Mutations induced by BPDE were predominantly base substitutions: of these base substitutions, 35% were G:C --> A:T transitions, and 36% were G:C --> T:A and 29% G:C --> C:G transversions. Approximately 88% of all mutations and 100% of base substitutions were at G:C sites; 60% of all mutations and 70% of the base substitution mutations occurred at CpG sites. A:T --> G:C transitions were not found. All of the single-base deletions were at G:C base pairs.

A comparison of mutational specificity of mutations induced by S9-activated B[a]P and benzo[a]pyrene-7,8-diol-9,10-epoxide at the endogenous aprt gene in CHO cells

We have determined the mutational specificity of S9-activated benzo[a]pyrene (B[a]P) at the endogenous aprt locus in a hemizygous Chinese hamster ovary cell line. The aprt gene of recovered mutants was amplified using the polymerase chain reaction (PCR) and directly sequenced. This spectrum was then compared to mutations recovered following treatment with the B[a]P metabolite, benzo[a]pyrene diol-epoxide (BPDE). No significant difference between the two spectra in the types of mutations produced, or their distribution was observed. This observation supports the hypothesis that BPDE is the reactive metabolite of B[a]P, responsible for the significant biological effects caused by this ubiquitous polycyclic aromatic hydrocarbon. The major mutation recovered was the G:C-->T:A transversion, and mutations were primarily localized within runs of guanines. We also confirmed our previous finding that mutation by B[a]P is non-random, targeting events in runs of guanines flanked by adenine residues. This same target hotspot region is found in codon 61 of the human c-Ha-ras1 proto-oncogene. This may help explain the selective activation of this codon by BPDE.

Mutational specificity in a shuttle vector replicating in chromium(VI)-treated mammalian cells

An SV40-based shuttle vector, pZ189, was used to characterize the mutation specificity and to explore the mechanism of chromium mutagenesis in mammalian cells. We showed previously that mutagenic DNA damage is induced by the treatment of plasmid with chromium(VI) plus glutathione in vitro. The induced mutation pattern suggested that chromium mutagenesis can be induced by the generation of reactive oxygen intermediates. To further investigate the mechanism of chromium mutagenesis, we treated cultured mammalian cells containing normal pZ189 vector with chromium(VI). Mutations were induced by Cr(VI) in a dose-dependent manner. The majority of base substitution mutations were widely distributed across the supF mutation target gene and occurred mainly at GC basepairs. Overall, the mutation spectra were not significantly different from each other except for a mutation hot spot at position 43, observed only in plasmids from Cr(VI)-treated cells. The characteristics of Cr(VI)-induced mutations were similar to those observed in the mutation spectra induced by H2O2 treatment of the pZ189 plasmid or plasmid-containing cells. These results are consistent with the hypothesis that induction of mutations by chromium in cultured cells occurs through the generation of oxidative DNA damage.

Genotoxicity and mutagenicity of the alpha, beta-unsaturated carbonyl compound crotonaldehyde (butenal) on a plasmid shuttle vector

Crotonaldehyde is an alpha,beta-unsaturated carbonyl compound and an important environmental and industrial toxic substance. Its mutagenic and carcinogenic properties are related to its reactivity to DNA, where it forms different guanine adducts. In order to study the mutagenic consequences of this agent in intact human cells, we treated the shuttle vector plasmid pZ189 with different doses of crotonaldehyde at 37 degrees C for 2 h and then transfected the such damaged plasmid into the normal human lymphoblast cell line GM0621. Within these host cells the guanine adducts are repaired and the plasmids replicated by cellular enzymes. After 2.5 days replicated plasmids were purified from the cells and plasmid survival was quantitated by transformation ability. With increasing doses of crotonaldehyde, we found a significant decline of plasmid survival, reflecting a pronounced genotoxicity of crotonaldehyde-induced DNA damage in intact human cells. Using the plasmid encoded mutagenesis marker gene supF, we were able to screen for mutants and determine mutation frequency in recovered plasmids. A significant increase in mutation frequency with increasing doses of crotonaldehyde reflects mutagenicity of crotonaldehyde-induced DNA damage. Base sequence analysis of recovered mutants revealed 39% point mutations, 46% deletions, and 15% insertions and inversions. Most of the point mutations (82%) were located at G:C base pairs, which is well explained by the DNA damage profile of crotonaldehyde. Among deletions we found a frequent reoccurrence of two hot spot deletions, representing 62% of all deletions. The sites of breakpoints of these deletions hot spots and of other deletions within the plasmid were also found to be sites of DNA breaks, directly induced by crotonaldehyde, as seen in an endlabeled plasmid fragment, treated with crotonaldehyde. Further analysis of the flanking sequences around the deletion breakpoints revealed a high frequency of four different kinds of short sequence homologies of up to eight base pairs.

Molecular characterization of mutation and comparison of mutation profiles in the hprt gene of Chinese hamster ovary cells treated with benzo[a]pyrene trans-7,8-diol-anti-9,10-epoxide, 1-nitrobenzo[a]pyrene trans-7,8-diol-anti-9,10-epoxide, and 3-nitrobenzo[a]pyrene trans-7,8- diol-anti-9,10-epoxide

Both 1- and 3-nitrobenzo[a]pyrene (nitro-BaP) are environmental contaminants, potent mutagens in Salmonella, and moderate mutagens in Chinese hamster ovary (CHO) cells. The mutagenicity of their oxidized metabolites,trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10-epoxy -7,8,9,10-tetrahydro-1-nitrobenzo[a]pyrene (1-nitro-BaP-DE) and trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10-tetrahydro-3-nitrobenzo[a]- pyrene (3-nitro-BaPDE), together with trans-7,8-dihydroxy-anti-9, 10-ep- oxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BaP-DE), was determined in CHO-K1 cells, and the resulting mutations at the hprt locus were characterized by polymerase chain reaction (PCR) amplification of reverse-transcribed hprt mRNA, followed by DNA sequence analysis. The mutant frequencies, in mutants/10(6) clonable cells, at 30 and 100 ng/ml, were BaP-DE, 248 and 456; 1-nitro-BaP-DE, 68 and 260; 3-nitro-BaP-DE, 81 and 232, respectively. In general, the three diolepoxides exhibited similar mutational spectra: 1) 64% (23/36 sequenced mutants) of BaP-DE, 53% (19/36) of 1-nitro-BaP-DE, and 64% (23/36) of 3-nitro-BaP-DE mutants resulted from simple base pair substitution, with the predominant mutation being G-->T transversion; 2) 90%, 100%, and 100% of mutations at G:C had the mutated dG on the nontranscribed DNA strand; and 3) about one quarter of the mutants produced by each mutagen had one or more PCR products with partial or complete exon deletions. The mutagens induced few frameshifts or complex mutations. Among the differences in mutational specificity for the three diolepoxides, the proportion of substituted dGs with 3' purines was significant (P < 0.05) for BaP-DE (16/19, 84%) and 3-nitro-BaP-DE (17/20, 85%), but not significant for 1-nitro-BaP-DE-induced mutants (11/17, 65%, P > 0.05). Also, high proportions of BaP-DE and 3-nitro-BaP-DE base pair substitutions at G:C occurred in DNA sequence contexts of 5'-GG-3', 5'-GGA-3', and 5'-TGGA-3', while the proportions of 1-nitro-BaP-DE mutants in these contexts were often lower. The results indicate that nitro substitution at C1 or C3 of BaP-DE reduces mutational potency in CHO cells and appears to have only subtle effects upon the mutational pattern in the hprt gene.

Induction of mutagenic DNA damage by chromium (VI) and glutathione

Certain chromium (Cr) compounds are known to be carcinogenic in humans and mutagenic in cell culture. However, the mechanism of Cr mutagenesis is not well understood. It appears that intracellular reduction of Cr by agents such as glutathione plays a role in the induction of DNA damage. We have used a simian virus 40-based shuttle vector to investigate the relationship between chromium-induced DNA damage and Cr mutagenicity. The treatment of the plasmid pZ189 with Cr(VI) plus glutathione (GSH) induced DNA strand breaks and reduced the plasmid biological activity, whereas Cr(III) treatment with or without GSH did not give rise to such DNA damage. When Cr(VI)/GSH- or Cr(III)/GSH-treated pZ189 was replicated in mammalian cells, a dose-dependent increase in mutant frequency was observed with Cr(VI)/GSH-treated pZ189, but not with Cr(III)/GSH-treated plasmid. About 43% of the mutants from Cr(VI)/GSH-treated pZ189 were deletion mutants. The remainder were base substitution mutants, mostly GC-->AT transitions and GC-->TA transversions. This pattern of mutagenesis is similar to that observed with other agents that cause oxidative DNA damage such as ionizing radiation and H2O2. These results support the hypothesis that Cr mutagenesis can be induced by the generation of reactive oxygen intermediates during the reduction of Cr(VI) by glutathione.

Analysis of mutagenesis in UV-sensitive mouse lymphoma L5178Y-R cells with a polyomavirus-based shuttle vector

A newly-developed polyomavirus-based shuttle vector, pPySLPT-2, was used to analyze mutations induced by UV radiation in the two related mouse lymphoma cell lines, L5178Y-R (LY-R) and L5178Y-S (LY-S). These well-studied cell lines differ in sensitivity to UV radiation, apparently due to differences in DNA repair capacity. Consistent with these differences, we found that replication of UV-irradiated vector is inhibited to a greater extent in the UV sensitive LY-R cells than in the LY-S cells. Mutations were induced in the vector's supF target gene at a higher frequency in the LY-R cells than in the LY-S cells, but the sequence characteristics of the base substitution mutations were similar in the two cell lines. The pattern of UV-induced mutations in the supF target gene of this polyomavirus-based vector, replicated in the mouse cells, was very similar to the pattern observed in the same target gene in the analogous simian virus 40-based vector, pZ189, replicated in monkey and human cells.

Translesional synthesis on a DNA template containing a single stereoisomer of dG-(+)- or dG-(-)-anti-BPDE (7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene)

Oligodeoxynucleotides modified site-specifically with dG-(+)-trans- and dG-(+)-cis-anti-BPDE (7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene) or dG-(-)-trans- and dG-(-)-cis-anti-BPDE were used as templates in primer extension reactions catalyzed by the Klenow fragment of Escherichia coli DNA polymerase I. The primer could be extended past the dG-(-)-trans-BPDE adduct with small amounts of dAMP incorporated opposite the lesion. A small amount of base deletions was also observed while, with the dG-(-)-cis-BPDE adduct, one- and two-base deletions predominated. When templates containing dG-(+)-trans-BPDE were used, small amounts of products containing one-base deletions were observed; with dG-(+)-cis-BPDE, substitution of dAMP opposite the lesion was also detected. The frequency of nucleotide insertion for dAMP opposite dG-(-)-trans-BPDE and the frequency of extension from the primer terminus containing the dA:dG-(-)-trans-BPDE pair were much higher than those observed with the other, stereochemically different BPDE adducts. Kinetic studies were in agreement with the results of the primer extension study. When the base flanking the 5' side of dG-BPDE was changed from dC to dT, the frequency of one-base deletions increased. We conclude that the trans- or cis-addition product of dG-(-)-anti-BPDE has a higher miscoding potential than dG-(+)-anti-BPDE in our model system and that G-->T transversions and deletions predominate. These observations are consistent with the types of mutations observed in vivo.

Mutagenic specificity of the (+)anti-diol epoxide of dibenz[a,j]anthracene in the supF gene of an Escherichia coli plasmid

This study was designed to examine the mutagenic specificity of (+)anti-dibenz[a,j]anthracene 3,4-diol-1,2-epoxide ((+)anti-DB[a,j]A-DE) in SOS-induced repair-proficient Escherichia coli ES87 (delta pro-lac, strA)/F' (pro+, lac1Q, lac1am26, lacZ delta M15). The plasmid pUB3, which contains the mutation target gene, supF, was modified with (+)anti-DB[a,j]A-DE in vitro (two to five adducts/plasmid) and then transformed into bacteria by electroporation. The spontaneous mutation frequency for unmodified pUB3 in uninduced cells was about 2 x 10(-6) and for SOS-induced cells, about 8 x 10(-6). The spontaneous supF- mutations were primarily insertions, deletions, and frameshifts. The mutation frequency for (+)anti-DB[a,j]A-DE-modified pUB3 was about 8 x 10(-6) and about 32 x 10(-6) for uninduced cells and SOS-induced cells, respectively. (+)anti-DB[a,j]A-DE induced primarily point mutations in supF in SOS-induced cells. GC-->AT transitions were the major mutations observed in SOS-induced cells (37%). GC-->TA (21%) and GC-->CG (8.6%) transversion mutations were also observed, whereas mutations at AT base pairs were rare (1.9%). Furthermore, a large number of tandem GC/GC-->AT/AT transition mutations were also observed (about 15% of all mutations in SOS-induced cells). Taken together, single and tandem GC-->AT mutations accounted for slightly over half (about 51%) of the mutations observed in SOS-induced cells. These results demonstrated that (+)anti-DB[a,j]A-DE was mutagenic in repair-proficient E. coli; however, unlike other polycyclic aromatic hydrocarbons that induce primarily transversion mutations, (+)anti-DB[a,j]A-DE caused mostly GC-->AT transitions.

Spontaneous and ultraviolet-induced mutations on a single-stranded shuttle vector transfected into monkey cells

The shuttle vector plasmid PCF3A, carrying the supF target gene, can be transfected into monkey COS7 cells as single-stranded or double-stranded DNA. Single strand-derived plasmid progeny exhibited a 10-fold higher spontaneous mutation frequency than double strand-derived progeny. The location of spontaneous mutations obtained after transfection of the single-stranded vector shared similarities with that for double-stranded vectors. However, the nature of base changes was very different. Single-stranded PCF3A DNA was used to study ultraviolet-induced mutagenesis. An earlier report (Madzak and Sarasin, J. Mol. Biol., 218 (1991) 667-673) showed that single-stranded DNA exhibited a lower survival and a higher mutation frequency than double-stranded DNA after ultraviolet irradiation. In the present report, sequence analysis of mutant plasmids is presented. The use of a single-stranded vector allowed us to show the targeting of mutations at putative lesion sites and to determine the exact nature of the base implicated in each mutation. Frameshift mutations were more frequent after transfection of control or irradiated plasmid as single-stranded DNA than as double-stranded DNA. Multiple mutations, observed at a high frequency in the spontaneous and ultraviolet-induced mutation spectra following single-stranded DNA transfection, could be due to an error-prone polymerisation step acting on a single-stranded template.

Mutational spectrometry: a general approach for hot-spot point mutations in selectable genes

To observe point mutational spectra with a high degree of precision, independent large cultures of human lymphoblastoid cells were treated with a mutagen, benzo[a]pyrene diol epoxide, and mutants at the HPRT gene were selected en masse by 6-thioguanine resistance. An average of 1.6 x 10(4) 6-thioguanine-resistant mutants were created per experiment and the kinds, positions, and numbers of the most frequent mutations were examined in exon 3 of the HPRT gene by using a high-fidelity polymerase chain reaction and denaturing gradient gel electrophoresis. Sixteen exon 3-specific mutations were found to be predominantly G----T transversions and corresponded to an average of 3500 induced mutants per experiment. Of these mutations, 6 occurred within a run of 6 guanines and 5 occurred in the sequence 5'-GAAGAG-3'. The variation among independent experiments is consistent with the numerical expectation that all 16 mutations fulfill reasonable statistical criteria for mutational hot spots. The agreement with data from various systems using clone-by-clone analysis shows that the protocol reported herein can be a useful tool to study hot-spot point mutational spectra for DNA sequences for which phenotypic selection systems exist.

Spectrum of cis-diamminedichloroplatinum(II)-induced mutations in a shuttle vector propagated in human cells

The supF gene of the shuttle vector pZ189 was used as a target for the study of mutations induced by cis-diamminedichloroplatinum(II) (cis-DDP). Normal human repair-proficient fibroblasts and cis-DDP repair-deficient xeroderma pigmentosum (XP) cells were used as host cells to study the effect of cis-DDP on the inhibition of shuttle vector replication and mutagenesis. Transfection of cis-DDP-treated pZ189 into normal and XP cell lines resulted in a marked increase in the mutation frequency and a decrease in the replication efficiency of the vector. However, these effects were much greater for the plasmid propagated in XP cells. Atomic absorption spectroscopy showed that six to eight Pt-DNA adducts per plasmid were necessary to inhibit plasmid replication by 50% in normal cells. In contrast, only one to two Pt-DNA adducts were necessary to inhibit replication of the plasmid by 50% in XP cells. Analysis of mutation sites demonstrated that cis-DDP treatment resulted primarily in single and double mutations separated by one base and limited to a few locations within the 85-bp mature tRNA. Propagation of the cis-DDP-treated vector in either normal or XP cells led to predominantly transversion mutations at AGA, AGG, and GAG sites and a cis-DDP-associated deletion of 174 bp. Although mutations occurred at target sites for cis-DDP adduct formation, there was no correlation between sites of mutation and the most frequent sites of adduct formation.

Polyomavirus-based shuttle vectors for studying mechanisms of mutagenesis in rodent cells

We have constructed a series of polyomavirus-based shuttle vectors for analyzing mechanisms of mutagenesis in rodent cell systems. These vectors contain the supF suppressor tRNA gene which serves as the mutagenesis target; the pBR327 replication functions and ampr gene for replication and selection in bacteria; and the polyomavirus genome which permits replication in rodent cells. The polyoma genomes used in these vectors vary in their enhancer regions, causing varying efficiencies of replication in different types of rodent cells. One of the vectors (pPySLPT-2) which replicates particularly well in several different rodent cell types (i.e., Chinese hamster ovary, mouse hepatoma and mouse lymphoma) was used to compare mutation induction by UV radiation in UV repair-deficient mouse lymphoma L5178Y-R cells with mutagenesis in the related UV repair-proficient line, L5178Y-S. In both cell types, UV-induced mutants could be recovered at frequencies up to 50-fold higher than that of the spontaneous background. At a given UV fluence the L5178Y-R cells were more highly mutable than the L5178Y-S cells. Our results indicate that these new polyomavirus-based vectors should be useful for analysis of the molecular mechanisms of mutation induction in rodent cell systems, and in particular should allow detailed analysis of mutagenesis in the well characterized rodent somatic cell mutants.

Use of a simian virus 40-based shuttle vector to analyze enhanced mutagenesis in mitomycin C-treated monkey cells

When monkey cells were treated with mitomycin C 24 h before transfection with UV-irradiated pZ189 (a simian virus 40-based shuttle vector), there was a twofold increase in the frequency of mutations in the supF gene of the vector. These results suggest the existence of an enhancible mutagenesis pathway in mammalian cells. However, DNA sequence analysis of the SupF- mutants suggested no dramatic changes in the mechanisms of mutagenesis due to mitomycin C treatment of the cells.

Perspectives on the use of an endogenous gene target in studies of mutational specificity

Mutational spectra have become increasingly important tools in generating a molecular level understanding of mutagenesis in mammalian cells. The work in this field has primarily involved the use of shuttle vector systems although some work has also been reported using endogenous cellular genes as mutational targets. In this communication we discuss the relative utility of these two approaches. We specifically focus on UV light-induced mutagenesis since this agent has been studied in both types of system. We conclude that shuttle vector and endogenous gene studies of mutagenesis are highly complementary, each possessing unique advantages.

Studies on direct and indirect effects of DNA damage on mutagenesis in monkey cells using an SV40-based shuttle vector

We are using an SV40-based shuttle vector, pZ189, to study mechanisms of mutagenesis in mammalian cells. The vector can be treated with mutagens in vitro and replicated in animal cells; resulting mutants can be selected and amplified in bacteria for DNA sequencing. This versatile vector system has allowed us to explore several different questions relating to the mutagenic process. We have studied the direct effects of template damage caused by UV or benzo[a]pyrene diolepoxide by treating vector DNA with these agents and then replicating the damaged DNA in monkey cells. Mutational mechanisms were deduced from the spectrum of mutations induced in the supF target gene of the vector DNA. To study the role of indirect effects of DNA damage on mutagenesis in mammalian cells, we have treated the cells and the vector DNA separately with DNA-damaging agents. We find that pretreatment of cells with DNA-damaging agents, or with conditioned medium from damaged cells, causes an enhancement of mutagenesis of a UV-damaged vector. Thus, DNA damage can act indirectly to enhance the mutagenic process. We also have preliminary evidence that pZ189 can be used in an in vitro DNA replication system to study the process of mutation fixation on the biochemical level. We believe that the pZ189 vector will prove to be as useful for in vitro studies of mutational mechanisms as it has been for in vivo studies.

Mutational specificities of 1'-acetoxysafrole, N-benzoyloxy-N-methyl-4-aminoazobenzene, and ethyl methanesulfonate in human cells

We have used an oriP-tk shuttle vector to determine the types of mutations induced in human cells by ethyl methanesulfonate (EMS), 1'-acetoxysafrole (AcOS), and N-benzoyloxy-N-methyl-4-aminoazobenzene (BzOMAB). Plasmid DNA was treated in vitro with mutagen and electroporated into human lymphoblastoid cells. After replication of the vector in human cells, plasmids were analyzed for mutations in the herpes simplex virus type 1 thymidine kinase gene. Ethyl methanesulfonate induced predominantly GC----AT transition mutations. Treatment of the shuttle vector with AcOS induced 5 of the 6 possible base substitution mutations, including GC----AT (32%) and AT----GC (14%) transition mutations, GC----TA (9%), GC----CG (18%), and AT----TA (14%) transversion mutations, as well as a low frequency (9%) of -1 frameshift mutations at GC base pairs. Replication in human cells of DNA modified with BzOMAB yielded a significant increase (17-fold) in the frequency of deletion mutations relative to solvent-treated DNA. A majority (94%) of the point mutations induced by BzOMAB occurred at GC base pairs and were predominantly GC----AT transitions (33%) and -1 frameshift (22%) mutations, with the remainder consisting mainly of transversions at GC base pairs (28%). The broad spectrum of base substitution mutations observed for AcOS and BzOMAB may indicate the frequent insertion of a variety of bases during replicative bypass of aralkylated bases in human cells.

Specific-locus mutations induced in eukaryotes (especially mammalian cells) by radiation and chemicals: a perspective

In the course of discovering the first mutagen (X-rays) just over 60 years ago, Herman J. Muller asked whether X-rays induced single-gene mutations and/or chromosomal (multiple-gene) mutations. To a large extent, his question has set the agenda for mutagenesis research ever since. We explore historically the answers to this question, with special emphasis on recent developments in the field of mammalian cell mutagenesis. Studies indicate that ionizing radiation and many chemical mutagens/carcinogens induce both gene and chromosomal mutations; however, only certain genetic systems permit the recovery and analysis of both classes of mutations. Few chemical mutagens induce only gene mutations in mammalian cells; instead, most mutagens appear to induce both classes of mutations, with chromosomal mutations (especially multilocus deletions) predominating at high doses. These results have implications regarding the mechanisms of mutagenesis, the role of chromosomal mutations in carcinogenesis and hereditary disease, and the type of data required for risk assessment of physical and chemical mutagens/carcinogens.

Use of a simian virus 40-based shuttle vector to analyze enhanced mutagenesis in mitomycin C-treated monkey cells

When monkey cells were treated with mitomycin C 24 h before transfection with UV-irradiated pZ189 (a simian virus 40-based shuttle vector), there was a twofold increase in the frequency of mutations in the supF gene of the vector. These results suggest the existence of an enhancible mutagenesis pathway in mammalian cells. However, DNA sequence analysis of the SupF- mutants suggested no dramatic changes in the mechanisms of mutagenesis due to mitomycin C treatment of the cells.