Reactions of [13C]-labelled tobacco smoke with DNA to generate selected adducts formed without metabolic activation

DNA adducts are central in the carcinogenic process because they can cause miscoding leading to permanent mutations in important genes involved in carcinogenesis. While it is known that tobacco smoking leads to increased levels of multiple DNA adducts, most DNA adducts detected to date in humans cannot be explicitly attributed to smoking but instead have various possible exogenous and endogenous sources. We plan to probe the tobacco source of DNA adducts by providing carbon-13 labelled ([¹³C]-labelled) cigarettes to smokers and analyzing [¹³C]-labelled DNA adducts in their oral cells to determine which adducts arise from smoking. Prior to conducting studies in humans, we first report here proof-of-principle machine smoking experiments to evaluate carbon isotopologues of (a) selected carbonyls and (b) DNA adducts resulting from direct exposure of cigarette smoke vapour-phase to calf-thymus DNA. The smoke of the study cigarettes, made from a 50:50 mixture of [¹³C]-labelled tobacco and a popular commercial tobacco, yielded similar concentrations of carbonyl compounds and their respective DNA adducts compared with the smoke of 1R6F reference cigarettes and the popular brand of cigarettes. We detected [¹³C]-isotopologues of DNA adducts such as 1,N⁶-etheno-dA, (8R/S)-3-(2'-deoxyribos-1-yl)-5,6,7,8-tetrahydro-8-hydroxypyrimido[1,2-a]purine-10(3H)-one (γ-OH-Acr-dG), and (6S,8S and 6R,8R)-3-(2'-deoxyribos-1-yl)-5,6,7,8-tetrahydro-8-hydroxy-6-methylpyrimido[1,2-a]purine-10(3H)-one [(6S,8S)-γ-OH-Cro-dG and (6R,8R)-γ-OH-Cro-dG], proving that they have a direct source from tobacco smoke and providing important new insights regarding their mechanisms of formation. These unique results form the basis for further studies in cell culture and in cigarette smokers to establish how carcinogens in tobacco smoke cause DNA adduct formation.