No abstract

Acetaldehyde, a major metabolite of ethanol, reacts with dG residues in DNA, resulting in the formation of the N(2)-ethyl-2'-deoxyguanosine (N(2)-Et-dG) adduct. This adduct has been detected in lymphocyte DNA of alcohol abusers. To explore the miscoding property of the N(2)-Et-dG DNA adduct, phosphoramidite chemical synthesis was used to prepare site-specifically modified oligodeoxynucleotides containing a single N(2)-Et-dG. These N(2)-Et-dG-modified oligodeoxynucleotides were used as templates for primer extension reactions catalyzed by the 3' --> 5' exonuclease-free (exo(-)) Klenow fragment of Escherichia coli DNA polymerase I. The primer extension was retarded one base prior to the N(2)-Et-dG lesion and opposite the lesion; however, when the enzyme was incubated for a longer time or with increased amounts of this enzyme, full extension occurred. Quantitative analysis of the fully extended products showed the preferential incorporation of dGMP and dCMP opposite the N(2)-Et-dG lesion, accompanied by a small amounts of dAMP and dTMP incorporation and one- and two-base deletions. Steady-state kinetic studies were also performed to determine the frequency of nucleotide insertion opposite the N(2)-Et-dG lesion and chain extension from the 3' terminus from the dN.N(2)-Et-dG (N is C, A, G, or T) pairs. These results indicate that the N(2)-Et-dG DNA adduct may generate G --> C transversions in living cells. Such a mutational spectrum has not been detected with other methylated dG adducts, including 8-methyl-2'-deoxyguanosine, O(6)-methyl-2'-deoxyguanosine, and N(2)-methyl-2'-deoxyguanosine. In addition, N(2)-ethyl-2'-deoxyguanosine triphosphate (N(2)-Et-dGTP) was efficiently incorporated opposite a template dC during DNA synthesis catalyzed by the exo(-) Klenow fragment. The utilization of N(2)-Et-dGTP was also determined by steady-state kinetic studies. N(2)-Et-dG DNA adducts are also formed by the incorporation of N(2)-Et-dGTP into DNA and may cause mutations, leading to the development of alcohol- and acetaldehyde-induced human cancers.

Aryl radicals and arenediazonium ions are suspected to react with cellular DNA, resulting in C8-arylguanine adducts. 8-Phenyl-2'-deoxyguanosine (8-PhdG) was synthesized as a model adduct by reacting dG with benzenediazonium chloride and incorporated into oligodeoxynucleotides using phosphoramidite techniques. A site-specifically modified oligodeoxynucleotide containing a single 8-PhdG was then used as a template for primer extension reactions catalyzed by the intact (exo+) or 3'-->5' exonuclease-free (exo-) Klenow fragment of Escherichia coli DNA polymerase I and mammalian DNA polymerase alpha (pol alpha). Although primer extensions catalyzed by the Klenow fragments were retarded at the position of 8-PhdG, most of the primer extension passed the lesion to form the fully extended products. In contrast, primer extensions catalyzed by pol alpha were strongly blocked opposite the lesion. The fully extended products formed during DNA synthesis were analyzed to quantify the miscoding specificities of 8-PhdG. The exo- Klenow fragment incorporated primarily dCMP, the correct base, opposite 8-PhdG, along with small amounts of incorporation of dAMP. Two-base deletions were also observed. In contrast, the exo+ Klenow fragment incorporated dCMP opposite the lesion. When pol alpha was used, 8-PhdG promoted small amounts of misincorporation of dAMP and dGMP as well as one- and two-base deletions. The duplex containing 8-PhdG.dG was thermally and thermodynamically more stable than dG.dG. The duplex containing 8-PhdG.dA was thermodynamically more stable than dG.dA. We conclude that 8-PhdG is a weak miscoding lesion, capable of generating G-->T and G-->C transversions and deletions in cells.