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

An exploratory analysis of multiple mutation spectra

The last decade has witnessed a remarkable increase in the number of mutations identified both in human disease-related genes and mutation reporter genes including those in mammalian cells and transgenic animals. This has led to the curation of a number of computerised databases, which make mutation data freely available for analysis. A primary interest of both the clinical researcher and the genetic toxicologist is determination of location and types of mutation within a gene of interest. Collections of mutation data observed for a disease-related gene or, for a gene exposed to a particular chemical, permits discovery of regions of sequence along the gene prone to mutagenesis and may provide clues to the origin of a mutation. The principal tool for visualising the distribution pattern of mutant data along a gene is the mutation spectrum: the distribution and frequency of mutations along a nucleotide sequence. In genetic toxicology, the current wealth of mutation data available allows us to construct many mutation spectra of interest to investigate the mutagenic mechanisms and mutational sites for one or a group of mutagens. Using the multivariate statistical methods principal components analysis (PCA) and cluster analysis (CA) we have tested the ability of these methods to establish the underlying patterns within and between 60 UV-induced, mitomycin C-induced and spontaneous mutations in the supF gene. The spectra were derived from human, monkey and mouse cells including both repair efficient and repair deficient cell lines. We demonstrate and support the successful application of multivariate statistical methods for exploring large sets of mutation spectra to reveal underlying patterns, groupings and similarities. The methods clearly demonstrate how different patterns of spontaneous and UV-induced supF mutation spectra can result from variation in plasmid, culture medium, species origin of cell line and whether mutations arose in vivo or in vitro.

Use of an infectious Simian virus 40-based shuttle vector to analyse UV-induced mutagenesis in monkey cells

SV40 based shuttle vectors able to be packaged as pseudovirions have been used either as naked DNA or as pseudovirus to analyse the mutation frequency and the UV-induced mutation spectra obtained after transfection or infection of COS7 monkey cells. The frequency of supF spontaneous mutants was similar whatever the state of the vector, indicating that the transfection step is not responsible for the high spontaneous mutation frequency when using shuttle vectors. Nevertheless the UV-induced mutation frequency of the supF gene was higher when transfected DNA was replicated into COS7 cells than when pseudovirus infection was performed. The UV induced mutation spectra was basically similar in both situations but a new hot-spot at nucleotide 110 was obtained after pseudovirus infection. UV-pretreated and control COS7 cells were infected with untreated or UV-damaged pi SVPC7 shuttle virus and the survival and the supF mutation frequency were analysed in the progeny. The survival of UV-damaged pseudovirus replicated in 10 J/m2 UV-pretreated cells was 2-fold higher than in untreated cells. This increase in the survival was accompanied by a slight enhancement in the number of supF mutants.

8-Methoxypsoralen photoinduced plasmid-chromosome recombination in Saccharomyces cerevisiae using a centromeric vector

The characterization of a new system to study the induction of plasmid-chromosome recombination is described. Single-stranded and double-stranded centromeric vectors bearing 8-methoxypsoralen photoinduced lesions were used to transform a wild-type yeast strain bearing the leu2-3,112 marker. Using the SSCP methodology and DNA sequencing, it was demonstrated that repair of the lesions in plasmid DNA was mainly due to conversion of the chromosomal allele to the plasmid DNA.

Ultraviolet B light-induced mutagenesis of p53 hotspot codons 248 and 249 in human skin fibroblasts

Mutations in the p53 tumor suppressor gene are detected in approximately half of non-melanoma skin cancers. The type of base-pair changes observed strongly suggests solar radiation as the causative mutagen. Mutations are distributed nonrandomly and form moderate hotspots. We studied the capacity of ultraviolet B light (UVB, 280-320 nm) to induce base-pair changes into the p53 exon 7 sequence extending from nt 14067 to 14075 in human skin fibroblasts. This sequence contains hotspot codon 248. UVB induced mostly C-->A and G-->T transversions. The base-pair change with the highest relative abundance was C-->A in the first position of codon 250 (CCC-->ACC), followed by (in diminishing relative abundance) G-->T in the third position of codon 249 (AGG-->AGT), C-->A in the first position of codon 248 (CGG-->AGG), and C-->A in the third position of codon 247 (AAC-->AAA). The C-->T transition in the third position of codon 247 (AAC-->AAT) occurred with moderate efficiency. These base-pair changes are compatible with pyrimidine photodimers as premutagenic lesions, but they could also form opposite 8-hydroxyguanine, which is the major oxidation product of guanine. No evidence was obtained for the presence of tandem double CC-->TT transitions in the untranscribed strand at codons 247/248 and 250. The relative abundance of mutations induced by UVB in the p53 sequence extending from codon 247 to 250 in human fibroblasts does not correlate with mutations observed in the DNA from non-melanoma skin cancer. This lack of correlation suggests that the mutability of this p53 sequence at the DNA level plays only a minor role in the pathogenesis of non-melanoma skin cancer in humans.

Mutation spectrum of 4-nitroquinoline 1-oxide-damaged single-stranded shuttle vector DNA transfected into monkey cells

4-Nitroquinoline 1-oxide (4NQO) is a potent mutagen and carcinogen which induces two main guanine adducts at positions C8 and N2. We recently determined the mutation spectrum induced by the ultimate metabolite of 4NQO, acetoxy-4-aminoquinolone 1-oxide in the M13lacZ'/E. coli lacZ delta M15 alpha-complementation assay. Our data suggested that dGuo-C8-AQO induces (per se or via AP sites) G to Pyr transversions. Here we report our study on 4NQO mutagenesis in monkey cells. 4NQO lesions were induced in vitro on a single-stranded (ss) DNA shuttle vector carrying the supF tRNA gene. This vector was able to replicate both in mammalian cells and in bacteria. The mutations induced in monkey cells were screened by the white/blue beta-galactosidase activity assay in E. coli. We took advantage of the peculiar feature of ss supF DNA in which the extent of secondary structure may be a function of the temperature, with the dependence of the 4NQO-specific adduct spectrum on DNA secondary structure. We reasoned that mutational spectra derived from damage induced in the presence (20 degrees C) or absence (70 degrees C) of DNA secondary structure should be different. The result of sequencing a total of 89 induced and spontaneous mutants confirmed that the spectra are statistically different. These data suggest that the two 4NQO guanine adducts may induce different mutations.

Replication of UV-irradiated DNA in human cell extracts: evidence for mutagenic bypass of pyrimidine dimers

We have examined the efficiency and fidelity of simian virus 40-origin-dependent replication of UV-irradiated double-stranded DNA in extracts of human cells. Using as a mutational target the alpha-complementation domain of the Escherichia coli lacZ gene in bacteriophage M13mp2 DNA, replication of undamaged DNA in HeLa cell extracts was highly accurate, whereas replication of DNA irradiated with UV light (280-320 nm) was both less efficient and less accurate. Replication was inhibited by irradiation in a dose-dependent manner. Nonetheless, covalently closed, monomer-length circular products were generated that were resistant to digestion by Dpn I, showing that they resulted from semiconservative replication. These products were incised by T4 endonuclease V, whereas the undamaged replication products were not, suggesting that pyrimidine dimers were bypassed during replication. When replicated, UV-irradiated DNA was used to transfect an E. coli alpha-complementation host strain to score mutant M13mp2 plaques, the mutant plaque frequency was substantially higher than that obtained with either unirradiated, replicated DNA, or unreplicated, UV-irradiated DNA. Both the increased mutagenicity and the inhibition of replication associated with UV irradiation were reversed by treatment of the irradiated DNA with photolyase before replication. Sequence analysis of mutants resulting from replication of UV-irradiated DNA demonstrated that most mutants contained C-->T transition errors at dipyrimidine sites. A few mutants contained 1-nt frameshift errors or tandem double CC-->TT substitutions. The data are consistent with the interpretation that pyrimidine dimers are bypassed during replication by the multiprotein replication apparatus in human cell extracts and that this bypass is mutagenic primarily via misincorporation of dAMP opposite a cytosine (or uracil) in the dimer.

Implication of uracil in spontaneous mutagenesis on a single-stranded shuttle vector replicated in mammalian cells

Almost all spontaneous point mutations found on a single-stranded shuttle vector after its transfection and replication in monkey cells were located at cytosine residues. In order to understand this very specific type of targeting we have studied the possible implication of uracil residues in the induction of these spontaneous mutations. The single-stranded shuttle vector pCF3A carrying the supF tRNA gene as a mutagenesis target has been allowed to replicate in mammalian COS7 cells, mutations being screened in bacteria using the beta-galactosidase assay. Progenies from untreated DNA and DNA treated with the uracil-DNA glycosylase prior to transfection were analyzed to determine the amount and classes of mutations. While spontaneous mutation frequency was 9.7 x 10(-4) for control DNA, single-stranded vector treated with the E. coli uracil-DNA glycosylase exhibited a reduced mutation frequency of about 30%. The abolished mutations were mainly confined to the cytosine to thymine transitions for which a decrease by a factor of 5 was indeed observed. This finding fits well with the fact that it is usually admitted that uracil pairs with adenine, indicating therefore that approximately 30% of spontaneous mutations observed in our experimental conditions and 80% of C to T transitions may be due to the presence of uracil instead of cytosine.