Adducts to nuclear DNA and mitochondrial DNA as biomarkers in chemoprevention.

DNA adducts are biomarkers evaluating the biologically effective dose of carcinogens, which reflects, more realistically than the external exposure dose, an enhanced risk of developing cancer. Likewise, inhibition of DNA adduct formation can be assumed as an indicator of decreased risk. Molecular dosimetry techniques can be exploited in anticarcinogenicity studies in animal models as well as in Phase II clinical chemoprevention trials. We have extensively used these end points in animal studies using individual carcinogens and complex mixtures. As assessed by 32P-postlabelling, DNA adducts were formed in the liver of rats fed a diet supplemented with 2-acetylaminofluorene. DNA adducts were detected by synchronous fluorescence spectrophotometry (SFS) in rat liver, lung, heart and testis following intratracheal (l.t) instillations of benzo[a]pyrene. The whole-body exposure of rats to mainstream cigarette smoke resulted in the appearance of DNA adducts in lung, heart, aorta and kidney, whereas adducts were not detected by SFS in liver, brain, oesophagus and testis. Moreover, typical diagonal radioactive zones and multiple DNA adducts were revealed by 32P-postlabelling in the tracheal epithelium, nasal mucosa, aorta and testis of smoke-exposed rats. Formation of adducts to lung DNA, as assessed by both 32P-postlabelling and SFS, also occurred in rats receiving i.t. instillations of air particulate extracts from polluted urban and rural areas. The oral administration of the thiol N-acetylcysteine (NAC) significantly inhibited the formation of DNA adducts in all organs of the rats exposed to the aforementioned carcinogens, which correlated with the parallel inhibition of biochemical, cytogenetic and histopathological alterations as well as with inhibition of preneoplastic and neoplastic lesions in rodents. Our working hypothesis is that DNA adducts in trachea/lung, heart and aorta may be associated with lung cancer, cardiomyopathies and atherosclerosis, respectively. DNA adducts were consistently detectable in the DNA of smooth muscle cells from abdominal aorta specimens taken at surgery from atherosclerotic patients. Even broader are the consequences of mitochondrial (mt) DNA impairment, which has been associated with aging, cancer, and other degenerative diseases. Our data show that adduct levels are consistently higher in mtDNA than in the nuclear DNA in different organs of rats exposed either to benzo[a]pyrene, 2-acetylaminofluorene or cigarette smoke. NAC significantly decreased the formation of adducts to mtDNA when administered with drinking-water. Inhibition of adducts to nuclear DNA is one of the end points evaluated in ongoing Phase II chemoprevention trials in high-risk individuals.