N-methyl-N-nitrosourea-induced mutations in human cells. Effects of the transcriptional activity of the target gene

Transcription-associated mutagenesis

Transcription requires unwinding complementary DNA strands, generating torsional stress, and sensitizing the exposed single strands to chemical reactions and endogenous damaging agents. In addition, transcription can occur concomitantly with the other major DNA metabolic processes (replication, repair, and recombination), creating opportunities for either cooperation or conflict. Genetic modifications associated with transcription are a global issue in the small genomes of microorganisms in which noncoding sequences are rare. Transcription likewise becomes significant when one considers that most of the human genome is transcriptionally active. In this review, we focus specifically on the mutagenic consequences of transcription. Mechanisms of transcription-associated mutagenesis in microorganisms are discussed, as is the role of transcription in somatic instability of the vertebrate immune system.

Interactions of the human, rat, Saccharomyces cerevisiae and Escherichia coli 3-methyladenine-DNA glycosylases with DNA containing dIMP residues

In DNA, the deamination of dAMP generates 2'-deoxy-inosine 5'-monophosphate (dIMP). Hypoxanthine (HX) residues are mutagenic since they give rise to A.T-->G.C transition. They are excised, although with different efficiencies, by an activity of the 3-methyl-adenine (3-meAde)-DNA glycosylases from Escherichia coli (AlkA protein), human cells (ANPG protein), rat cells (APDG protein) and yeast (MAG protein). Comparison of the kinetic constants for the excision of HX residues by the four enzymes shows that the E.coli and yeast enzymes are quite inefficient, whereas for the ANPG and the APDG proteins they repair the HX residues with an efficiency comparable to that of alkylated bases, which are believed to be the primary substrates of these DNA glycosylases. Since the use of various substrates to monitor the activity of HX-DNA glycosylases has generated conflicting results, the efficacy of the four 3-meAde-DNA glycosylases of different origin was compared using three different substrates. Moreover, using oligo-nucleotides containing a single dIMP residue, we investigated a putative sequence specificity of the enzymes involving the bases next to the HX residue. We found up to 2-5-fold difference in the rates of HX excision between the various sequences of the oligonucleotides studied. When the dIMP residue was placed opposite to each of the four bases, a preferential recognition of dI:T over dI:dG, dI:dC and dI:dA mismatches was observed for both human (ANPG) and E.coli (AlkA) proteins. At variance, the yeast MAG protein removed more efficiently HX from a dI:dG over dI:dC, dI:T and dI:dA mismatches.

Ethylnitrosourea-induced mutation and molecular analysis of transgenic mice containing the gpt shuttle vector

Novel transgenic mice were developed in order to study the in vivo mutagenesis. The transgenic mice carried pCGK shuttle vector, which contained the Escherichia coli gpt gene as a mutational target, the kanamycin-resistant gene (Kanr) and cos region derived from bacteriophage lambda. The shuttle vector can be recovered from the transgenic mouse genome into the gpt-deficient E. coli by an in vitro packaging method and is selectable as a Kanr phenotype. Mutations induced at the gpt gene can be easily detected with a selective agent, 6-thioguanine (6-TG). In the previous study, the pCGK shuttle vector was incorporated into Chinese hamster CHL/IU cells and the resultant transgenic cell line was shown to be a useful system to study in vitro mutagenesis at the gpt gene. Therefore, an advantage of the shuttle vector is that in vivo mutational data obtained from the transgenic mouse can be compared with those of transgenic cell line in vitro. A transgenic CD-1 mouse line, designated as #128, that carried approximately 50 copies of pCGK shuttle vectors, was selected among 4 transgenic mouse lines. To investigate the sensitivity of the #128 line, the transgenic mice were treated with a single intraperitoneal injection of 250 mg/kg of N-ethyl-N-nitrosourea (ENU) or with 50 mg kg-1 day-1 of ENU for 5 consecutive days, and bone marrow, spleen and liver were dissected to investigate their mutational responses. The background mutant frequency was between 18x10(-6) and 75x10(-6) among all tissues tested. ENU induced significant increases in the mutant frequency above the background level in all three tissues at 14 days after single or 5-day treatment with the chemical. The increases in the mutant frequencies in bone marrow, spleen and liver were 6.4- to 6.8-fold, 3.0- to 5.6-fold and 3.0- to 3.3-fold, respectively. The shuttle vector DNA was recovered from the bone marrow of both spontaneous and ENU-treated mice and the gpt gene was amplified by polymerase chain reaction. The amplified DNA was subject to DNA sequence analysis. Out of 79 spontaneous and 52 ENU-induced mutants, the gpt gene could be amplified from 28 spontaneous and 46 ENU-induced mutants. DNA sequence analysis showed that predominant mutations were identified as A:T to T:A transversions (22 out of 46 sequenced mutants) and G:C to A:T transitions (9/46) in ENU-induced mutants, whereas G:C to T:A transversions (7 out of 28 sequenced mutants) were predominant in spontaneous mutants. These results demonstrate that this transgenic mouse, in combination with the transgenic CHL/IU cell line, is a useful system to study in vivo and in vitro mutational events at the same target gene.

A specific spectrum of p53 mutations in lung cancer from smokers: review of mutations compiled in the IARC p53 database

Mutations in the p53 gene are common in lung cancer. Using data from the the International Agency for Research on Cancer p53 mutation database (R1), we have analyzed the distribution and nature of p53 mutations in 876 lung tumors described in the literature. These analyses confirm that G to T transitions are the predominant type of p53 mutation in lung cancer from smokers. The most frequently mutated codons include 157, 158, 179, 248, 249, and 273, and several of them (157, 248, and 273) have been shown to correspond to sites of in vitro DNA adduct formation by metabolites of polycyclic aromatic hydrocarbons (PAHs) such as benzo(a)pyrene. Furthermore, most of the base changes at codons 248, 249, and 273 in lung cancer differ from those commonly observed at these codons in other cancers reported in the database. Thus, lung cancer from smokers shows a distinct, unique p53 mutation spectrum that is not observed in lung cancer from nonsmokers. These results further strengthen the association between active smoking, exposure to PAHs, and lung cancer. They also indicate that a different pattern of mutations occurs in nonsmokers, and this observation may help to identify other agents causally involved in lung cancer in nonsmokers.

Increased transcription decreases the spontaneous mutation rate at the thymidine kinase locus in human cells

Transcription increases DNA repair efficiency and modulates the distribution of certain types of DNA damage. Furthermore, increased transcription level stimulates spontaneous mutation rate in yeast. We explored whether transcription level affects spontaneous mutation rate in human cells. We first developed two thymidine kinase (tk) inducible human cell lines using the Gal4-Estrogen receptor system. In our TK6i-G3 and G9 tk heterozygous cell lines, the active tk allele is linked to an inducible promoter element. Tk mRNA is induced following treatment with estrogen. Spontaneous mutation rate was significantly decreased in human cell lines after induction in contrast to the report in yeast. Thus, humans may have evolved different or additional mechanisms to deal with transcription related spontaneous mutagenesis.

DNA repair of UV photoproducts and mutagenesis in human mitochondrial DNA

The induction and repair of DNA photolesions and mutations in the mitochondrial (mt) DNA of human cells in culture were analysed after cell exposure to UV-C light. The level of induction of cyclobutane pyrimidine dimers (CPD) in mitochondrial and nuclear DNA was comparable, while a higher frequency of pyrimidine (6-4) pyrimidone photoproducts (6-4 PP) was detected in mitochondrial than in nuclear DNA. Besides the known defect in CPD removal, mitochondria were shown to be deficient also in the excision of 6-4 PP. The effects of repair-defective conditions for the two major UV photolesions on mutagenesis was assessed by analysing the frequency and spectrum of spontaneous and UV-induced mutations by restriction site mutation (RSM) method in a restriction endonuclease site, NciI (5'CCCGG3') located within the coding sequence of the mitochondrial gene for tRNALeu. The spontaneous mutation frequency and spectrum at the NciI site of mitochondrial DNA was very similar to the RSM background mutation frequency (approximately 10(-5)) and type (predominantly GC>AT transitions at G1 of the NciI site). Conversely, an approximately tenfold increase over background mutation frequency was recorded after cell exposure to 20 J/m2. In this case, the majority of mutations were C>T transitions preferentially located on the non-transcribed DNA strand at C1 and C2 of the NciI site. This mutation spectrum is expected by UV mutagenesis. This is the first evidence of induction of mutations in mitochondrial DNA by treatment of human cells with a carcinogen.

Misincorporation rate and type on the leading and lagging strands of UV-damaged DNA

We have examined the fidelity of replication of the leading and lagging strands of UV-irradiated DNA by using an EBV-derived shuttle vector system which contains as marker gene for mutation analysis the bacterial gpt gene in both orientations relative to the EBV oriP. Human cells stably transformed with this vector were UV irradiated and gpt mutation rate and type were analysed. An increased mutagenicity associated with UV irradiation was observed, but the average error frequency was unaffected by the direction of replication of the target gene. Some variability by position and sequence context of leading and lagging strand errors was detected, suggesting that the different architecture of the replication complex for the two strands might, to some extent, affect mutation spectra. The comparable fidelity of translesion replication on the leading and lagging strands is in agreement with the current model for eukaryotic replication that postulates the simultaneous synthesis of both strands by a DNA polymerase with a proof-reading exonuclease.

Molecular dosimetry of chemical mutagens. Relationship between DNA adduct formation and genetic changes analyzed at the molecular level

This is a review of the work carried out by 16 collaborating institutes within a project which was part of the European Programme: Science and Technology for Environmental Protection (STEP). The purpose of the project was to investigate the relationship between the exposure to genotoxic chemicals and the induction of DNA damage and genetic effects as determined in in vitro and in vivo assays under laboratory conditions. Two types of investigation were performed: (i) determination of the relationship between the extent of exposure to a genotoxic chemical and the frequency of DNA adducts formed in the test organism and (ii) identification of those DNA adducts which are responsible for the biological effects of genotoxic chemicals. The research was carried out with a series of alkylating agents which all induce similar types of DNA damage but for which the proportions of the different types of adducts vary. The frequency of this type of DNA damage was also modulated by base excision repair processes. In addition, a number of genotoxic agents which cause DNA damage recognized by nucleotide excision repair were investigated. The consequences of DNA adduct formation, i.e., the induction of gene mutations, were analyzed at the DNA sequence level, generating mutational spectra. These investigations of the mutational specificities of carcinogens contributed to our understanding of the molecular mechanisms which are involved in cancer induction by genotoxins.

Binding and repair of O6-ethylguanine in double-stranded oligodeoxynucleotides by recombinant human O6-alkylguanine-DNA alkyltransferase do not exhibit significant dependence on sequence context

Double-stranded (ds) oligodeoxynucleotides (29mers) containing an O6-ethylguanine (O6-EtGua) flanked 5' and 3' by different bases (5'..TGT..3'; 5'..CGG..3', 5'..GGT..3'; 5'..GGG..3'; 5'..GGA..3') were synthesized to investigate the binding and repair characteristics of recombinant human O6-alkylguanine-DNA alkyltransferase (AT) in vitro. The apparent association constant (KA(app)) of AT to the oligomers and the repair rate constant for O6-EtGua (k) respectively, were determined by gel retardation and a monoclonal antibody-based filter binding assay. When ds- or single-stranded (ss) oligomers with or without O6-EtGua were used, no major differences in KA(app) values were observed with either substrate: KA(app) values for native AT were 7.1 and 8.4 x 10(5) M(-1) respectively, for unmodified and [O6-EtGua]-containing ds-oligomers. The corresponding values for ss-oligomers were 1.0 and 4.9 x 10(5) M(-1). The N-terminal first 56 amino acids of AT only exert a limited influence on DNA binding; the KA(app) values for an N-terminally truncated AT protein (1.1 x 10(5) M(-1)) and native AT were of the same order. Moreover, KA(app) was hardly affected by Cys(145)-methylated AT (2.0 x 10(5) M(-1)). The k-values (6.5-11.5 x 10(6) M(-1)s(-1)) were not significantly dependent on nucleotide sequence. k-values of 5.3 and 4.0 x 10(6) M(-1)s(-1) respectively, were obtained with the N-terminally truncated AT protein and for repair of the postreplicative mispair [O6-EtGua]: T by native AT. The low KA(app), the negligible influence on O6 of ethylation, and the minor modulation KA(app) and k by varying the bases flanking O6-EtGua, all indicate that the binding of AT to DNA is non-specific and mediated mainly by ionic interactions [reduced KA(app) and k-values at increased ionic strength]. Surplus DNA reduces the rate of O6-EtGua repair in ds-oligomers by competitive binding of AT molecules. The reaction mechanism of AT with DNA in vivo requires further investigation.

Strand bias of ultraviolet light-induced mutations in a transcriptionally active gene in human cells

Ultraviolet (UV)-induced repair and mutational spectra were analyzed in an inducible marker gene, the metallothionein-l/guamine-xanthine phosphoribosyl transferase (gpt) fusion gene, carried by an Epstein-Barr virus-derived shuttle vector episomically maintained in human cells. The repair rate of UV photodimers from the shuttle-vector molecules was typical of transcriptionally active sequences, 70% of the dimers being removed within 8 h after irradiation. The spectrum obtained under basal gene transcription was compared with that obtained under induced transcription. In both cases, base substitutions at dipyrimidine sequences predominated. Multiple mutations and deletions probably due to recombinational events induced by UV damage were also observed. Most of the UV-mutated dipyrimidine sites were located in the transcribed strand and were independent of the transcriptional activity of the target gene. In contrast, the distribution of mutations throughout the coding region of the gpt gene was affected by transcription, with a preferential clustering of mutations occurring in the 3' half of the gene after transcription induction. The strand bias observed in the UV spectra most likely reflects selection for nonfunctional gpt protein.

Association of increased spontaneous mutation rates with high levels of transcription in yeast

Complex processes such as transcription, replication, repair, and recombination require changes in chromatin structure and the interactions of numerous trans-acting factors with DNA sequences, raising the possibility that these processes may be interrelated. Here the effect of transcription on the rate of spontaneous mutation in the yeast Saccharomyces cerevisiae was examined. With the use of a lys2 frameshift allele under the control of a highly inducible promoter, the rate of spontaneous reversion was shown to increase when the mutant gene was highly transcribed. Thus, transcriptionally active DNA and enhanced spontaneous mutation rates are associated in yeast.

Epstein-Barr virus replication studies and their application to vector design

Vectors containing elements of the Epstein-Barr virus genome are used primarily to maintain cloned DNA inserts as plasmids in mammalian cells. However, Epstein-Bar-virus-based vectors have also been valuable tools in the hands of those studying the life cycle of Epstein-Barr virus. In this article, we discuss those characteristics of Epstein-Barr virus and its life cycle that have been used in vector construction and describe methods that are particularly applicable to the use of Epstein-Barr-virus-based vectors.

Novel electrophoretic protocol for collection of mutations in the lambda light chain immunoglobulin gene in a human B-lymphoblastoid cell strain

Spontaneous and chemically induced mutation was examined in the lambda light chain immunoglobulin gene in a human B-lymphoblastoid cell strain (T5-1). The hemizygous lambda gene is a unique mutational target gene which codes for a protein that is both expressed on the cell membrane and secreted. Mutations in the lambda gene were detected by analysis of western blots of isoelectric focusing gel electrophoresis of T5-1 cell conditioned culture medium. None of 5,841 individual clones established from vehicle-exposed populations had detectable variations in the isoelectric banding pattern of the constitutively secreted lambda immunoglobulin protein. In contrast, 113 of 6,128 clonal populations established from N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)-exposed populations exhibited stable variations in expression of the lambda immunoglobulin: isoelectric variants (n = 3) and non-secretors (n = 110). MNNG-induced mutations in the lambda gene, which resulted in lambda immunoglobulin proteins with altered isoelectric points (pIs), occurred at a frequency of no less than 4.9 x 10(-4) mutations/cell, indicating the mature rearranged lambda immunoglobulin gene is comparably sensitive to carcinogen induced mutation as other human autosomal target genes. Approximately one-half of the MNNG-induced non-secretor mutant clones lacked lambda mRNA while one-half maintained constitutive transcription and expression of the lambda immunoglobulin on the cell surface, demonstrating that carcinogen damage interdicted gene function at multiple points.

Specificities of human, rat and E. coli O6-methylguanine-DNA methyltransferases towards the repair of O6-methyl and O6-ethylguanine in DNA

The behaviour of highly purified bacterial expressed rat O6-methylguanine-DNA methyltransferase (MGMT) towards the repair of CGCm6GAGCTCGCG and CGCe6GAGCTCGCG (km6G/ke6G = 1.45, where k is the second order repair rate constant determined, m6G and e6G are O6-methyl and O6-ethylguanine) is similar to that of E. coli 39kD Ada protein (km6G/ke6G = 1.6). However, the human MGMT is very different (km6G/ke6G = 163). The preferential repair of O6-ethylguanine lesion by the rat MGMT appears not to be related to the lack of the initiator methionine in the expressed protein since similar results were obtained from N-terminal Glutathione-S-transferase (GST) fused protein (GSTMGMT) which retains the methionine. The possible relationship between these findings and the differences observed in the primary amino acid sequence of these proteins is discussed. In addition the preferential repair of O6-ethylguanine substrate by the 39kD Ada protein as compared to the catalytic C-terminus alone (different by 134 times) suggests that the N-terminus plays a crucial role in the repair of O6-ethylguanine. This is in contrast to the minor effects of the GST domain when fused to the N-terminus of mammalian MGMT.

Evidence for nontargeted mutagenesis in a monkey kidney cell line and analysis of its sequence specificity using a shuttle-vector plasmid

Intact pZ189 DNA was allowed to replicate in monkey kidney vero cells that had been pretreated with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). The E. coli MBM7070 was transfected with replicated plasmid, and those with mutations in the supF gene were identified. The frequency of mutants that did not contain recognizable changes in the electrophoretic mobility of the plasmid DNA was scored. The frequency of such mutants was 12.2 x 10(-4) (43/35376) and 6.2 x 10(-4) (22/35712) in mutants derived from cells pretreated with 0.2 mumoles/l and 2 mumoles/l MNNG respectively; these values represent an increase of 5.8- and 2.9-fold over the spontaneous mutation frequency of 2.1 x 10(-4) (10/47741) (p < 0.01). Sequence analysis of the supF genes of these mutants showed that 89% (24/27) of base substitutions occurred at G.C base pairs; 59% of the base substitutions (16/27) were transversions, and 41% (11/27) were transitions. The types of base substitutions were predominantly G.C-->T.A and G.C-->A.T. 48% of base substitutions occurred at 6 sites of the supF gene; 4 of these sites consist of 5'-TTNN where N is G or C. Base substitutions never previously reported were found, namely, T-->C at 61, G-->T at 70, G-->T at 99, and G-->C at 103 were found; these have never been reported up to now. In addition, 2 of the 5 frameshifts occurred in the region 99-105 of the supF gene (GGTGGGG), suggesting that this region is a hot spot for nontargeted frameshifts. These results strongly suggest that nontargeted mutagenesis can occur in mammalian cells and shows that the spectrum of mutations induced differs from that of spontaneous and targeted mutations.

O6-methylguanine in DNA inhibits replication in vitro by human cell extracts

To study the effects of methylation damage on DNA replication in vitro, the plasmid pSVori containing the SV40 origin of replication was reacted with N-methyl-N-nitrosourea and used as a substrate for SV40 T antigen dependent replication by HeLa cell extracts. The plasmid was methylated with a range of N-methyl-N-nitrosourea concentrations that introduced an average of 0.3-2.5 O6-methylguanine and equal amounts of 3-methyladenine lesions per DNA molecule. When methylated plasmid was incubated with extract of Mex-HeLaMR cells under conditions favoring DNA replication, an impairment of replication was observed as the accumulation of incompletely replicated form II plasmid molecules. These extracts simultaneously performed a T antigen independent, DpnI-sensitive DNA repair synthesis that increased with increasing DNA damage. Subtraction of this repair DNA synthesis revealed that methylation inhibited overall replication. At low levels of methylation (< or = 1 O6-methylguanine and < or = 1 3-methyladenine lesion per plasmid), inhibition was transient, while more extensive damage resulted in apparently irreversible inhibition of replication. Removal of O6-methylguanine by pretreatment of the methylated plasmid with purified human O6-methylguanine-DNA methyltransferase restored replication to almost normal levels. When the methylated plasmid was replicated by extracts of Mex+ HeLaS3 cells proficient in the repair of O6-methylguanine, a lower level of inhibition and less repair DNA synthesis was observed. The inhibition of DNA synthesis and the stimulation of repair DNA synthesis are thus both largely due to the presence of O6-methylguanine in DNA.(ABSTRACT TRUNCATED AT 250 WORDS)

Fine tuning of DNA repair in transcribed genes: mechanisms, prevalence and consequences

Cells fine-tune their DNA repair, selecting some regions of the genome in preference to others. In the paradigm case, excision of UV-induced pyrimidine dimers in mammalian cells, repair is concentrated in transcribed genes, especially in the transcribed strand. This is due both to chromatin structure being looser in transcribing domains, allowing more rapid repair, and to repair enzymes being coupled to RNA polymerases stalled at damage sites; possibly other factors are also involved. Some repair-defective diseases may involve repair-transcription coupling: three candidate genes have been suggested. However, preferential excision of pyrimidine dimers is not uniformly linked to transcription. In mammals it varies with species, and with cell differentiation. In Drosophila embryo cells it is absent, and in yeast, the determining factor is nucleosome stability rather than transcription. Repair of other damage departs further from the paradigm, even in some UV-mimetic lesions. No selectivity is known for repair of the very frequent minor forms of base damage. And the most interesting consequence of selective repair, selective mutagenesis, normally occurs for UV-induced, but not for spontaneous mutations. The temptation to extrapolate from mammalian UV repair should be resisted.

Fidelity of replication of the leading and the lagging DNA strands opposite N-methyl-N-nitrosourea-induced DNA damage in human cells

Semi-conservative replication of double-stranded DNA in eukaryotic cells is an asymmetric process involving leading and lagging strand synthesis and different DNA polymerases. We report a study to analyze the effect of these asymmetries when the replication machinery encounters alkylation-induced DNA adducts. The model system is an EBV-derived shuttle vector which replicates in synchrony with the host human cells and carries as marker gene the bacterial gpt gene. A preferential distribution of N-methyl-N-nitrosourea (MNU)-induced mutations in the non transcribed DNA strand of the shuttle vector pF1-EBV was previously reported. The hypermutated strand was the leading strand. To test whether the different fidelity of DNA polymerases synthesizing the leading and the lagging strands might contribute to MNU-induced mutation distribution the mutagenesis study was repeated on the shuttle vector pTF-EBV which contains the gpt gene in the inverted orientation. We show that the base substitution error rates on an alkylated substrate are similar for the replication of the leading and lagging strands. Moreover, we present evidence that the fidelity of replication opposite O6-methylguanine adducts of both the leading and lagging strands is not affected by the 3' flanking base. The preferential targeting of mutations after replication of alkylated DNA is mainly driven by the base at the 5' side of the G residues.

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.

Mutagenesis by O6 meG residues within codon 12 of the human Ha-ras proto-oncogene in monkey cells

The first or/and the second guanines of the human Ha-ras codon 12 (normally GGC) were substituted by O6 meG residues and the modified sequence was subsequently introduced into an SV40-based shuttle vector able to replicate in both simian cells and bacteria. After replication in simian COS7 cells (proficient in O6-alkyl-guanine transferase), plasmid DNA was extracted and mutations were screened in E. coli DH5 alpha cells. The vast majority of the mutations induced by O6 meG were G----A transitions. The mutation frequency observed at the second guanine of codon 12 (12G2 position: 3.75% +/- 0.4) was higher than the one observed at the first guanine (12G1 position: 1.09% +/- 0.6). This difference was confirmed by the results obtained when two adjacent O6 meG residues were positioned within codon 12. The higher mutation frequency observed for the 12G2 position could be attributed to differential repair or/and variation in polymerase fidelity. These results are in agreement with animal experiments where alkylating agents gave rise to mutation on G2 position of codon 12.

Multivariate statistical analysis of mutational spectra of alkylating agents

A series of multivariate statistical methods were used to explore the current knowledge on the mutational spectra of alkylating agents (AA) in bacterial and mammalian cells. The data relative to lac I and gpt genes of Escherichia coli were considered. The analysis focused on the distribution of GC to AT transitions, which account for the majority of AA-induced mutations. The statistical analysis of 15 different mutational spectra obtained by various laboratories pointed to a number of biological factors involved in the mutational process. First of all, factor and cluster analyses demonstrated that the mutational profiles obtained in mammalian cells form a homogeneous cluster different from the cluster formed by the bacterial cell mutational spectra. SN1-type AAs give rise to classes of mutational spectra statistically different from the spectra induced by the SN2-type AAs. The analysis of the mutated sequences of both genes pointed to a correlation between mutation induction by SN1 AAs, which react through a positively charged alkylating intermediate, and the occurrence of mutations at guanines preceded 5' by a purine. Moreover, our statistical analysis showed that the distribution of AA-induced mutations is not affected by the transcriptional activity of the target gene, but is strongly determined by the sequence specificity of AA-induced mutagenesis and by the structure of the target proteins. The agreement of our results with the findings of previous studies indicates that the multivariate data analysis methods are a sensitive and reliable tool for exploring the mechanisms underlying complex biological processes. The novelty of the present results lies in their quantitative character, and in the clarity of the graphical displays. We propose the use of this methodological approach to explore the large bulk of information available on mutational spectra.