Studies of the reaction of acetaldehyde with deoxynucleosides

Removal of acetaldehyde from saliva by a slow-release buccal tablet of L-cysteine

High alcohol intake is an independent risk factor for upper gastrointestinal (GI)-tract cancers. There is increasing evidence that acetaldehyde, the first metabolite of ethanol, might be responsible for ethanol-associated carcinogenesis. Especially among Asian heavy drinkers with the ALDH2-deficiency gene, i.e., a genetic inability to remove acetaldehyde, the risk of digestive tract cancers is markedly increased. Local acetaldehyde production from ethanol either by oral microbes, mucosal cells or salivary glands is a plausible carcinogenic agent in the saliva. The aim of our study was to examine whether is it possible to bind carcinogenic acetaldehyde from saliva with L-cysteine, which is slowly released from a special buccal tablet. Nine healthy male volunteers took part in our study, and each subject served as his own control. A placebo or L-cysteine-containing tablet was fastened under the upper lip. Thereafter the volunteers ingested 0.8 g/kg of body weight of 10% (v/v) ethanol, and saliva samples were collected at 20 min intervals for 320 min. Salivary acetaldehyde and ethanol levels were analysed by headspace gas chromatography. The mean reduction of acetaldehyde concentration of the saliva with the L-cysteine tablet compared to placebo was 59% (CL(95%) 43%, 76%). Area under the curve (AUC(0-320min)) with the L-cysteine and placebo tablet were 54.3 +/- 11 microM x hr and 162 +/- 34.2 microM x hr (mean +/- SEM), respectively (p = 0.003). After alcohol intake, up to two-thirds of carcinogenic acetaldehyde can be removed from saliva with a slow-releasing buccal L-cysteine drug formulation. Thus, a buccal cysteine tablet could potentially be used to prevent upper GI-tract cancers, especially among high-risk individuals.

Mutational spectrum induced by acetaldehyde in the HPRT gene of human T lymphocytes resembles that in the p53 gene of esophageal cancers

As the primary metabolite of alcohol, acetaldehyde (AA) may be responsible for many pathological effects related to consumption of alcohol, such as esophageal cancer. The spectrum of p53 mutations in esophageal tumors is indicative of the involvement of exogenous agents, such as tobacco smoke. There is, however, no experimental proof for the involvement of alcohol as data on mutational spectrum induced by AA in human genes is completely lacking. The aim of this study is to investigate whether AA leaves mutational fingerprint in the HPRT reporter gene in human peripheral T cells. Pre-existing in vivo HPRT mutants were removed from PHA-stimulated T lymphocytes before in vitro treatment with 2.4 mM AA for 24 h. Following cell growth to allow mutation expression, independent 6-thioguanine-resistant mutants were selected from large numbers of subcultures showing a 3-fold induction of mutant frequency on average. A total of 73 induced and 36 spontaneous mutants were found to carry a missense, nonsense, frameshift or splice mutation. Base substitutions were identified in the coding or splicing sequences of 55 induced and 26 control mutants. The induced base changes were mainly G > A transition (40%, G on non-transcribed strand) followed by A > T transversions (14.5%, A on non-transcribed strand). The control mutants had significantly (P = 0.04) less G > A transition (15.4%) and completely lacked A > T transversions. We also identified 5'-AGG-3' or 5'-AAG-3' as potential target sequences for AA-induced G > A transitions. This specific mutational spectrum induced by AA is consistent with the known formation and persistency of N(2)-ethyl-2'-guanosine adduct and with the predominance of G > A transitions and mutations at A:T base pairs in the p53 gene of esophageal tumors. We conclude that AA may be involved in the pathogenesis of esophageal cancer.

Translesional synthesis on a DNA template containing N2-methyl-2'-deoxyguanosine catalyzed by the Klenow fragment of Escherichia coli DNA polymerase I

Formaldehyde is produced in most living systems and is present in the environment. Evidence that formaldehyde causes cancer in experimental animals infers that it may be a carcinogenic hazard to humans. Formaldehyde reacts with the exocyclic amino group of deoxyguanosine, resulting in the formation of N2-methyl-2'-deoxyguanosine (N2-Me-dG) via reduction of the Schiff base. The same reaction is likely to occur in living cells, because cells contain endogenous reductants such as ascorbic acid and gluthathione. To explore the miscoding properties of formaldehyde-derived DNA adducts a site-specifically modified oligodeoxynucleotide containing a N2-Me-dG was prepared and used as the template in primer extension reactions catalyzed by the Klenow fragment of Escherichia coli DNA polymerase I. The primer extension reaction was slightly stalled one base before the N2-Me-dG lesion, but DNA synthesis past this lesion was readily completed. The fully extended products were analyzed to quantify the miscoding specificities of N2-Me-dG. Preferential incorporation of dCMP, the correct base, opposite the lesion was observed, along with small amounts of misincorporation of dTMP (9.4%). No deletions were detected. Steady-state kinetic studies indicated that the frequency of nucleotide insertion for dTMP was only 1.2 times lower than for dCMP and the frequency of chain extension from the 3'-terminus of a dT:N2-Me-dG pair was only 2.1 times lower than from a dC:N2-Me-dG pair. We conclude that N2-Me-dG is a miscoding lesion capable of generating G-->A transition mutations.

Specific tandem GG to TT base substitutions induced by acetaldehyde are due to intra-strand crosslinks between adjacent guanine bases

Acetaldehyde is present in tobacco smoke and automotive exhaust gases, is produced by the oxidation of ethanol, and causes respiratory organ cancers in animals. We show both the types and spectra of acetaldehyde-induced mutations in supF genes in double- and single-stranded shuttle vector plasmids replicated in human cells. Of the 101 mutants obtained from the double-stranded plasmids, 63% had tandem base substitutions, of which the predominant type is GG to TT transversions. Of the 44 mutants obtained from the single-stranded plasmids, 39% had tandem mutations that are of a different type than the double-stranded ones. The GG to TT tandem substitutions could arise from intra-strand crosslinks. Our data indicate that acetaldehyde forms intra- as well as inter-strand crosslinks between adjacent two-guanine bases. Based upon the following observations: XP-A protein binds to acetaldehyde-treated DNA, DNA excision repair-deficient xeroderma pigmentosum (XP) cells were more sensitive to acetaldehyde than the repair-proficient normal cells, and a higher frequency of acetaldehyde-induced mutations of the shuttle vectors was found in XP cells than in normal cells, we propose that the DNA damage caused by acetaldehyde is removed by the nucleotide excision repair pathway. Since treatment with acetaldehyde yields very specific GG to TT tandem base substitutions in DNA, such changes can be used as a probe to identify acetaldehyde as the causal agent in human tumors.

Formation of DNA adducts in human buccal epithelial cells exposed to acetaldehyde and methylglyoxal in vitro

Acetaldehyde (AA) and methylglyoxal (MG) are reactive, ubiquitous aldehydes, present in the environment and endogenously formed in animals and humans. They have both been shown to readily form DNA adducts under simulated physiological conditions. We report here on the use of cultured normal and SV40T antigen-immortalized human buccal epithelial cells as model systems for aldehyde exposure of the oral epithelium, occurring through the ingestion of alcoholic beverages and brewed coffee, as well as by inhalation of tobacco smoke and automobile exhaust. By the application of recently developed 32P-postlabeling methods, the presence of both endogenous and induced AA and MG DNA adducts was demonstrated in cultured human epithelial cells. Furthermore, these DNA adducts were formed in a dose-dependent manner at aldehyde concentrations that were relatively nontoxic to the cells.

Structure and mechanism of formation of a new adduct from formaldehyde and guanosine

Reactions between formaldehyde and guanosine in the range pH 4-10 gave N2-hydroxymethylguanosine 1, bis-N2-guanosinylmethane 2 and 7-[7-(beta-D-ribofuranosyl)-purin-6(7H)- on-2-yl]-3-(beta-D-ribofuranosyl)-5,6,7,8-tetrahydro-sym-triazino[ 1,2-alpha] purin-10(3H)-one 3a. The latter is a new adduct, the formation of which occurs in neutral solution, is favoured at higher pH and can be rationalised by a sequence of condensations involving two molecules each of guanosine and formaldehyde.