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Ja
oszy
ski1,a,*
1 Tsukuba Research Institute, Banyu Pharmaceutical Co. Ltd, Okubo 3, Tsukuba, Ibaraki 300-2611, Japan
2 Institute for Virus Research, Kyoto University, Shogoin-Kawaracho 53, Sakyo-ku, Kyoto, 606-8507, Japan
| Abstract |
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. This translesion DNA polymerase, belonging to the Y-family, was found to be moderately inhibited by the presence of 8-OH-G on either mouse or human templates. From our previous results, inhibition of various polymerases by 8-OH-G increases in the following order: Pol
< Pol
< Polß < Pol
, showing that major replicative and repair polymerases are more sensitive to this lesion than enzymes belonging to the Y-family. In the direct mutagenesis experiments, Pol
was found to be more mutagenic than Pol
studied previously: it inserted dAMP more efficiently than dCMP opposite 8-OH-G. Pol
was also able to cause indirect mispair (action-at-a-distance mutagenesis), this effect being more distinct on mouse templates. Two adjacent 8-OH-G residues in codon 12 inhibited Pol
moderately and induced misincorporation of dAMP. However, this effect was not comparable to the strong relaxation of the enzyme specificity, observed previously in the case of Pol
. Pol
catalyzed incorporation (and misincorporation of dAMP) much more efficiently on mouse templates, human DNA fragments being distinctly worse substrates. Interestingly, in direct mutagenesis systems, the preference for dAMP over dCMP was nearly the same on mouse and human templates. | Introduction |
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,
and
, which were shown to be required for bypass of various forms of DNA damage (Burgers et al. 2001). Polymerase
bypasses thymine-thymine cyclobutane dimers and thymine-thymine 6-4 photoproducts with low efficiency (Johnson et al. 2000a; Tissier et al. 2000), and benzo[a]pyrene as well as benzo[c]phenanthrene diol epoxides adducted to adenine (Frank et al. 2002). However, this enzyme is strongly blocked by the same diol epoxides adducted to guanine (Frank et al. 2002). Polymerase
(Pol
), encoded by the human Xeroderma pigmentosum variant (XPV) gene (Johnson et al. 1999; Masutani et al. 1999), was found to catalyze efficient and accurate replication past thymine dimers (Johnson et al. 2000b), and to play an important role in the error-free bypass of UV lesions in an in vivo yeast system (Yu et al. 2001). Moreover, Pol
bypasses platinum complexes-DNA adducts efficiently, but in an error-prone manner (Veisman et al. 2000), and also 1,N6-ethenoadenine (Levine et al. 2001). Inaccurate replication by Pol
was also shown in the case of abasic sites and benzo[a]pyrene adducts (Zhang et al. 2000a). Pol
has also been examined in several experimental systems containing 8-hydroxyguanine (8-OH-G) [also known as 7,8-dihydro-8-oxoguanine, 8-oxo-G]. In the same year, two research groups reported efficient and accurate replication past this lesion (Haracska et al. 2000) and misinsertion (Zhang et al. 2000a). Recently, we investigated the mutagenicity of this and other enzymes using synthetic human c-Ha-ras fragments containing 8-OH-G in various positions in hypermutagenic codon 12, including a tandem arrangement of two lesions (Ja
oszy
ski et al. 2003). We demonstrated that in this experimental system Pol
was much more efficient than DNA polymerases
and ß, and even two adjacent 8-OH-G residues were efficiently bypassed by the enzyme. Moreover, Pol
incorporated dAMP opposite 8-OH-G, showed a moderate action-at-a-distance mutagenic effect, being able to misread guanine 3'-flanked by 8-OH-G, and was found to be strongly mutagenic in the tandem 8-OH-G system, where all four dNTPs were incorporated; dCMP and dAMP with very high, dGMP with moderate and dTMP with low efficiency (Ja
oszy
ski et al. 2003). Polymerase
(Pol
), in turn, is known to bypass 1,N6-ethenoadenine in a relatively inefficient way, generating replication errors (Levine et al. 2001). This enzyme also catalyzes error-prone replication past acetylaminofluorene-derived lesions (Suzuki et al. 2001), abasic sites (Ohashi et al. 2000; Zhang et al. 2000b) and 8-OH-G (Zhang et al. 2000b). Pol
turned out to be unable to bypass thymine-thymine dimers (both cis-syn cyclobutane dimers and 6-4 photoproducts) (Ohashi et al. 2000; Zhang et al. 2000b), and cisplatin DNA adducts (Ohashi et al. 2000; Gerlach et al. 2001). A highly interesting finding was the arylhydrocarbon receptor-dependent transcription of Pol
(Ogi et al. 2001). This observation was subsequently followed by the discovery that Pol
was able preferentially to incorporate the correct base opposite dG-N2-benzo[a]pyrene diolepoxide (Suzuki et al. 2001).
A large body of evidence supports the hypothesis that reactive oxygen species take part in activation of Ha-ras genes by inducing single substitutions in particular codons. High frequencies of G
T transversion were observed in hypermutagenic codons 12 and 61 of the c-Ha-ras gene in some types of cancer, e.g. in squamous cell and basal cell carcinomas (White & Balmain 1988; van der Schoef et al. 1990; Pierceall et al. 1991). This particular substitution is the most frequent mutation caused by 8-OH-G (Shibutani et al. 1991; Cheng et al. 1992), a marker of DNA oxidation (Kamiya 2003). Indeed, a ras gene containing 8-OH-G in codon 12 was found to have transforming activity in NIH3T3 cells (Kamiya et al. 1992a). Therefore, systems containing 8-OH-G residues in mutation hotspots of ras genes have been established and widely used to study the bases of mutagenesis and proto-oncogene activating events (Kamiya et al. 1992b, 1995a,b; Le Page et al. 1995; Tan et al. 1999; Ja
oszy
ski et al. 2003). In the previous studies (Ja
oszy
ski et al. 2003), we applied an experimental system containing a single moiety of 8-OH-G in various positions of codon 12 of the human c-Ha-ras gene, or two lesions in a tandem arrangement, to characterize in detail the mutagenicity of Pol
. Comparison with other DNA polymerases suggested the possible role of Pol
in error-prone translesion DNA synthesis past 8-OH-G when two 8-OH-Gs were present in codon 12. This enzyme was not blocked or even inhibited by such a double lesion, in contrast to polymerases
and ß (Ja
oszy
ski et al. 2003). The present study explores in detail kinetics of translesion DNA synthesis past 8-OH-G by DNA polymerase
, so far not investigated in a quantitative manner. We used the ras-gene experimental system for systematic study of the effects of 8-OH-G on DNA synthesis catalyzed by Pol
. As mentioned above, this interesting enzyme was found to generate replication errors when its substrate contains a single 8-OH-G (Zhang et al. 2000b). However, the kinetics of this reaction, especially with DNA sequences corresponding to proto-oncogenes, remained unstudied. To estimate sequence-context effects, we compared the mutagenic potentials of two sequences: the previously used human c-Ha-ras and newly synthesized mouse Ha-ras1 fragments, differing in the sequence of the critical codon 12, but both being sensitive to oxidation of guanine. We analyzed four experimental systems: (i) direct mutagenesis system, the first replicated nucleotide was 8-OH-G 3'-flanked by G; (ii) action-at-a-distance mutagenesis system, the first replicated nucleotide was G 3'-flanked by 8-OH-G; (iii) tandem lesion system, the first replicated nucleotide was 8-OH-G 3'-flanked by 8-OH-G; and (iv) control system with non-modified templates. We demonstrated that Pol
was capable of an action-at-a-distance mutagenesis, and this enzyme appeared to be the most mutagenic among Y-polymerases.
| Results |
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To compare reaction rates between human and mouse ras sequences, we used the same experimental conditions (i.e., enzyme activity, substrate concentrations, reaction time and temperature) for both c-Ha-Ras and Ha-ras1 fragments. The sequences used and typical insertion reaction results are shown, respectively, in Figs 1 and 2.
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catalyzed insertion of dCMP opposite guanine on the control template M0 (Figs 2A and 3A, Table 1) with nearly 1200 times higher efficiency (measured as a ratio kcat/Km) than the incorporation of dGMP, indicating the high accuracy of the reaction. Traces of misincorporated dTMP were also observed, but the amounts were under the detection limits (i.e., bands were clearly visible on strongly overexposed phosphorimaging plates, when primer bands were saturated). When the first replicated nucleotide was 8-OH-G (template M1; Figs 2B and 3B, Table 1) typical lesion mutagenicity was observed. dAMP was misincorporated at over 3 times higher efficiency than correct dCMP. Traces of dGMP were also misincorporated, but with a very low efficiency (kcat/Km = 0.91). The presence of 8-OH-G inhibited the incorporation of dCMP over 21 times, compared to that of the control template M0. In the template M2, when the first replicated nucleotide was unmodified guanine 3'-flanked by the lesion (Figs 2C and 3C, Table 1) a similar, though slightly stronger inhibition of dCMP insertion was observed (over 31 times lower efficiency than to the control template M0). The interesting action-at-a-distance mutagenic effect appeared relatively strong when the ratio kcat/Km was analyzed. This effect turned out to occur at over 1.5 times higher efficiency than the correct insertion of dCMP. The presence of two adjacent 8-OH-G residues (template M3, Figs 2D and 3D, Table 1) in codon 12 decreased the efficiency of incorporation of dCMP, resulting in strong inhibition. Also in this case, misincorporation of dAMP was about 2.6 times more efficient than insertion of dCMP.
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The insertion of dCMP catalyzed by Pol
on the control template H0 (Fig. 3E, Table 1) was not accompanied by any misincorporation, but the efficiency of this reaction turned out to be over 31 times lower than the corresponding value obtained in the mouse system. Qualitatively, the effects observed in the case of sequences containing 8-OH-G were similar to those observed in the mouse fragments, but the reaction efficiencies were much lower. Gradually increasing inhibition of incorporation of dCMP was noticed, from kcat/Km = 5.32 in the case of the control template H0, through 1.31 and 1.15, respectively, in the direct mutagenesis system (template H1; Fig. 3F, Table 1) and action-at-a-distance system (template H2; Fig. 3G, Table 1), up to 0.60 for two 8-OH-G residues in a tandem arrangement (template H3; Fig. 3H, Table 1). Direct mutagenesis (Fig. 3F), appearing as exclusive misincorporation of dAMP, was nearly 2.8 times more efficient than the correct insertion of dCMP. This is comparable to the three-fold difference obtained in the mouse system. Induction of mutagenesis by 8-OH-G 3'-flanking replicated, unmodified guanine (Fig. 3G) was barely within detection limits; misincorporation of dAMP was slightly over 10 times less efficient than that of dCMP. The misincorporation rate slightly increased when the first replicated nucleotide was 8-OH-G and the lesion was also present in the 3'-flanking position (Fig. 3H).
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| Discussion |
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, were strongly inhibited by the presence of 8-OH-G, whereas Pol
appeared to be capable of catalyzing efficient translesion DNA synthesis (Ja
oszy
ski et al. 2003). Therefore, when Pol
or Polß encounters 8-OH-G during DNA synthesis, efficient bypass of the lesion may require dissociation of the polymerase from damaged template, and insertion of one or more nucleotides by an enzyme resistant to the blocking effect of the lesion. On the basis of our previous study, Pol
was suggested to be a candidate enzyme participating in this process (Ja
oszy
ski et al. 2003). Since Pol
was proved to possess strong mutagenic properties, and a uniquely relaxed specificity when the DNA lesion consisted of two adjacent 8-OH-G residues (Ja
oszy
ski et al. 2003), a role of this polymerase in hypermutability of codon 12 of the ras gene seemed very plausible. In the present study, we analyzed DNA polymerase
, another enzyme belonging to the same Y-family. Having a well-established experimental system consisting of synthetic ras fragments, we were able to measure kinetic parameters of insertion of a single nucleotide catalyzed by this enzyme and compare them to previously described data concerning Pol
, Polß and Pol
. Moreover, it was possible to compare the properties of mouse and human DNA sequences. Pol
turned out to be moderately inhibited by the presence of 8-OH-G on both mouse and human templates. From previous reports, the sensitivity of DNA polymerases toward the inhibitory effect of 8-OH-G in codon 12 of the human c-Ha-ras gene (mouse sequences have not been studied so far) increases in the following order: Pol
< Pol
< Polß < Pol
. This proves that the major replicative and repair DNA polymerases are more sensitive to 8-OH-G than enzymes belonging to the Y-family and strongly supports postulated role of the latter in bypass of particular DNA lesions.
We observed a remarkable difference in the reaction efficiencies with the mouse and human sequences. Pol
catalyzed incorporation much more efficiently on mouse templates, whereas human DNA fragments were distinctly worse substrates. There was only a four-base difference between the studied sequences. However, the mouse Ha-ras1 as compared to human c-Ha-ras has A instead of C in the third position of the codon 12, and this critical codon is 5'-flanked by T, instead of C. This remarkably increases the content of pyrimidines in the closest proximity of the first replicated nucleotide, and breaks the long sequence of purines present in the human fragment. Overall increase in the insertion efficiency observed in the mouse sequence may be due to the pyrimidine-rich neighborhood of the reaction site. However, the actual reason of such a difference in the insertion efficiency remains unknown. Regardless the differences, in the direct mutagenesis systems, the preference for dAMP over dCMP was nearly the same both on the mouse and human template, i.e., 3-fold and 2.8-fold, respectively. 8-OH-G in this system seems to be mutagenic to a similar degree, regardless of the overall efficiency of incorporation.
Interestingly, in direct mutagenesis experiments, Pol
was found to be more mutagenic than Pol
analyzed previously. The latter enzyme in the human c-Ha-Ras system inserted dCMP more efficiently than dAMP (Ja
oszy
ski et al. 2003). In the present study, Pol
incorporated dAMP with much higher efficiency than the correct dCMP. Moreover, Pol
caused indirect mispairing (often referred to as action-at-a-distance mutagenesis (Efrati et al. 1999)). This effect was barely noticeable in the human system, but very distinct when templates were mouse Ha-ras1 gene fragments. This difference is likely caused by the template-primer slipped misalignment. The 5'-templating base in the human system is C, whereas in the mouse sequence it is T. Thus, more efficient misincorporation of dAMP on M2 template is likely induced on the way of misalignment, and incorporation of dAMP opposite the next T. Since the base 5'-flanking the first replicated nucleotide on the template H2 is C, dAMP misinsertion is not favored. The ability of Pol
to template-primer misalignment was shown by Wolfle et al. (2003). Action-at-a-distance mutagenesis was reported for the first time by Kuchino et al. (1987) and concerned the Klenow fragment of bacterial polymerase I. Later, the eukaryotic repair DNA polymerase ß was also found to show a similar effect (Efrati et al. 1999), and recently we showed that 8-OH-G stimulated mispairing at a neighboring template site when replicated by Pol
(Ja
oszy
ski et al. 2003). Pol
is the second polymerase belonging to the Y-family to show the same effect. It remains to be studied whether other enzymes of this group behave in a similar way.
Two adjacent 8-OH-G residues in codon 12, though inhibiting Pol
, also induced misincorporation. However, this effect was not comparable to the strong relaxation of enzyme specificity observed previously in the case of Pol
(Ja
oszy
ski et al. 2003). This fact represents the most important qualitative difference in the actions of these two, otherwise similar, DNA polymerases catalyzing DNA synthesis past 8-OH-G. The mechanisms of replication across such a double lesion may be significantly different for various enzymes belonging to the Y-family. The presence of two guanines sensitive to oxidation in codon 12 of the c-Ha-Ras gene was postulated as an important fact supporting the role of reactive oxygen species in activation of the gene. Taking into account the data presented herein, Pol
may rather play a role in direct mutagenic events, when a single 8-OH-G moiety is replicated and dAMP is misincorporated with high efficiency. Pol
remains the main suspect when a tandem lesion is present and other DNA polymerases may be unable to pass such a distortion in the DNA chain.
| Experimental procedures |
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protein was purified in the form of Pol
C (560 amino acids), which lacks motifs VIIa and VIIb that denote zinc clusters from the intact protein (870 amino acids), as previously described (Ohashi et al. 2000). Kinetic parameters were estimated as previously described (Ja
oszy
ski et al. 2003). Briefly, 1 nM of Pol
was incubated with 32P-labeled primer:template substrate (20 nM) and increasing concentrations (0500 µM) of a single deoxynucleotide (dCTP, dGTP, dATP or dTTP) in 40 mM Tris-HCl buffer (pH 8.0) containing 5 mM MgCl2, 10 mM dithiothreitol, 60 nM KCl, 250 µg/mL BSA and 2.5% glycerol. Reaction mixtures were incubated at 30 °C for 1 min in a total volume of 5 µL, heated to 90 °C to inactivate the enzyme and then subjected to 20% polyacrylamide gel electrophoresis. Bands in gels were visualized and their intensities were measured using a BAS 2000 scanner (Fuji, Japan). The percentage of primer extended was plotted as a function of the dNTP concentration, and the kinetic parameters were calculated from the non-linear regression fit to a rectangular hyperbola (v = (Vmax x [dNTP]/[Km + [dNTP]), as described (Creighton et al. 1995). Average values obtained from two to five independent experiments are presented.
| Acknowledgements |
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| Footnotes |
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aPresent address: Institute of Human Genetics, Polish Academy of Sciences, Strzeszyñska 32, 60-479 Pozna
, Poland | References |
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Received: 29 December 2004
Accepted: 2 March 2005
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