Tremendous advances have been made in the study of biomarkers related to carcinogenesis during the past 20 years. This perspective will briefly review improvements in methodology and instrumentation that have increased our abilities to measure the formation, repair, and consequences of DNA adducts. These biomarkers of exposure, along with surrogates such as protein adducts, have greatly improved our understanding of species differences in metabolism and effects of chemical stability and DNA repair on tissue differences in molecular dose. During this same time frame, improvements in assays for biomarkers of effect have provided better data and an improved understanding of the dose responses for both gene and chromosomal mutations. A framework analysis approach was used to examine the mode of action of genotoxic chemicals and the default assumption that cancer can be expected to be linear at very low doses. This analysis showed that biomarkers of exposure are usually linear at low doses, with the exception being when identical adducts are formed endogenously. Whereas biomarkers of exposure extrapolate down to zero, biomarkers of effect can only be interpolated back to the spontaneous or background number of mutations. The likely explanation for this major difference is that at high exposures, the biology that results in mutagenesis is driven by DNA damage resulting from the chemical exposure. In contrast, at very low exposures, the biology that results in mutagenesis is driven by endogenous DNA damage. The shapes of the dose-response curves for biomarkers of exposure and effect can be very different, with biomarkers of effect better informing quantitative estimates of risk for cancer, a disease that results from multiple mutations. It is also clear, however, that low dose data on mutagenesis are needed for many more chemicals.

Although children born to HIV-infected (HIV+) women receiving antiretroviral therapy during pregnancy show virtually no adverse clinical effects at birth, the antiretroviral nucleoside analog drugs are known to damage nuclear and mitochondrial DNA. In this study, biomarkers of mitochondrial toxicity and genotoxicity have been examined in a well-characterized sample set consisting of infants born to HIV-uninfected (HIV-) mothers (n = 30), and HIV- infants (n = 20) born to HIV-infected (HIV+) mothers who received either no antiretroviral therapy (n = 10) or zidovudine (3'-azido-3'-deoxythymidine [AZT]) during pregnancy (n = 10). DNA from cord blood leukocytes and peripheral blood leukocytes taken at 1 and 2 years of age was examined for loss of mitochondrial DNA (mtDNA) and telomere integrity. Telomere length, a measure of nuclear DNA damage, was the same in all infants at birth and at age 1 year. The quantity of mtDNA was assessed relative to nuclear DNA using a polymerase chain reaction-based chemiluminescence detection (PCR-CID) method that determined mitochondrial D Loop gene copies relative to nuclear 18S RNA gene copies by comparison with a standard curve. MtDNA quantity was expressed as a ratio of gene copy numbers. In infants of uninfected mothers (AZT-/HIV-) at the three time points, the ratios were 442 to 515, whereas in infants of untreated AZT-/HIV+ mothers the ratios were 261 to 297, and in infants of AZT-treated (AZT+/HIV+) mothers the ratios were 146 to 203. At all three time points, differences between the AZT-/HIV- group and the two HIV+ groups were statistically significant (p <.05), and differences between the AZT-/HIV+ and AZT+/HIV+ groups were also statistically significant (p <.05), demonstrating that AZT exposure causes a persistent depletion of mtDNA. The study shows that children of HIV+ mothers are at risk for mitochondrial damage that is further increased in infants of mothers receiving AZT during pregnancy.

Abstract1,3-Butadiene (BD) is an important industrial and environmental chemical classified as a human carcinogen based on epidemiologic studies in occupationally exposed workers and animal studies in laboratory rats and mice. BD is metabolically activated to three epoxides that can react with nucleophilic sites in biomolecules. Among these, 1,2,3,4-diepoxybutane (DEB) is considered the ultimate carcinogen due to its high genotoxicity and mutagenicity attributed to its ability to form DNA-DNA cross-links. Our laboratory has developed quantitative high-performance liquid chromatography-MESI + -tandem mass spectrometry methods for two DEB-specific DNA-DNA cross-links, 1,4-bis-(guan-7-yl)-2,3-butanediol (bis-N7G-BD) and 1-(guan-7-yl)-4-(aden-1-yl)-2,3-butanediol (N7G-N1A-BD). This report describes molecular dosimetry analysis of these adducts in tissues of B6C3F1 mice and F344 rats exposed to a range of BD concentrations (0-625 ppm). Much higher (4-to 10-fold) levels of DEB-DNA crosslinks were observed in mice compared with rats exposed to the same BD concentrations. In both species, bis-N7G-BD levels were 1.5-to 4-fold higher in the liver than in other tissues examined. Interestingly, tissues of female animals exposed to BD contained higher concentrations of bis-N7G-BD adducts than tissues of male animals, which is in accord with previously reported differences in tumor incidence. The molecular dosimetry data presented herein suggest that species and gender differences observed in BD-induced cancer are directly related to differences in the extent of BD metabolism to DEB. Furthermore, a rat model of sensitivity to BD may be more appropriate than a mouse model for assessing human risk associated with BD exposure, because rats and humans seem to be similar with respect to DEB formation.

1,3-Butadiene (BD) is a high-volume chemical used in the production of rubber and plastic. BD is a potent carcinogen in mice and a much weaker carcinogen in rats, and has been classified as a probable human carcinogen. Upon metabolic activation in vivo, it forms DNA-reactive metabolites, 1,2-epoxy-3-butene (EB), 1,2:3, 4-diepoxybutane (DEB), and 3,4-epoxy-1,2-butanediol (EBD). The molecular dosimetry of N-7 guanine adduct formation by these metabolites of BD in liver, lung, and kidney of B6C3F1 mice and F344 rats exposed to 0, 20, 62.5, or 625 ppm BD was studied. The adducts, racemic and meso forms of N-7-(2,3,4-trihydroxybut-1-yl)guanine (THB-Gua), N-7-(2-hydroxy-3-buten-1-yl)guanine (EB-Gua I), and N-7-(1-hydroxy-3-buten-2-yl)guanine (EB-Gua II), were isolated from DNA by neutral thermal hydrolysis, desalted on solid-phase extraction cartridges, and quantitated by LC/ESI(+)/MS/MS. The number of adducts per 10(6) normal guanine bases for a given adduct was higher in mice than rats exposed to 625 ppm BD, but generally similar at lower levels of exposure. The THB-Gua adducts were the most abundant (6-27 times higher than EB-Gua) and exhibited a nonlinear exposure-response relationship. In rats, the exposure-response curves for the formation of THB-Gua adducts reached a plateau after 62.5 ppm, suggesting saturation of metabolic activation. The number of THB-Gua adducts continued to increase in mice between 62.5 and 625 ppm BD. In contrast, the less common EB-Gua adducts had a linear exposure-response relationship in both species. Combining the information from this study with previous data on BD metabolism, we were able to estimate the number of THB-Gua that resulted from DEB and EBD, and conclude that most of the THB-Gua is formed from EBD. We hypothesize that most of the EBD arises from the immediate conversion of DEB to EBD within the endoplasmic reticulum. This study highlights the need for measurements of the levels of EBD in tissues of rats and mice and for the development of a unique biomarker for DEB that is available for binding to DNA.