Sequence specificity in the reaction of benzopyrene diol epoxide with DNA

Repair analysis of promutagenic (+)-anti-BPDE DNA adduct in transcriptionally active sequences of plasmid DNA in Escherichia coli

The extent of formation and repair of promutagenic (+)-anti-BPDE-N2-dG in transcriptionally active thymidine kinase (tk) gene insert and vector DNA fragments was assessed in the (+)-anti-BPDE treated plasmid p220-tk within the Escherichia coli hosts of varying repair potential. Polyclonal antibody (BP1), specific for (+)-anti-BPDE DNA adduct, was utilized for quantitative estimation of this bulky lesion in nanograms amounts of membrane transblotted DNA fragments. A carcinogen dose-dependent quantitative antibody binding response, due to selective recognition of the major (+)-anti-BPDE adduct, was seen with various DNA fragments separated by gel electrophoresis. The sensitivity of the immunodetection at 0.2 fmol (+)-anti-BPDE DNA adduct, allowed a linear detection in the range of modification level of 0.64 x 10(-7) to 86 x 10(-7) adducts per nucleotide in plasmid DNA. Based on this sensitivity, detection of 0.07 and 0.46 (+)-anti-BPDE DNA adducts in respective tk and vector DNA fragments was achieved upon immunoanalysis of the in vitro modified DNA. Adduct concentration dependent antibody binding was independent of size of the vector or insert fragments. Antibody binding response, to DNA modified in vivo, was dependent upon the dose of (+/-)-anti-BPDE to plasmid DNA replicating within bacterial hosts. The repair of (+)-anti-BPDE DNA adducts was determined as the loss of antibody binding sites in the specific fragments of plasmid DNA within host E. coli. About 50% of the initial DNA damage was repaired from the individual fragments during 15 min post-incubation in the repair-proficient (wild-type) E. coli cells. Complete adduct removal occurred in approx. 60 min of post-incubation period. A significant (91%) decrease in the survival of mutant (uvrA- recA-) cells was observed at 4 microM (+/-)-anti-BPDE treatment without any reduction in the colony forming units in the wild-type cells. On the contrary, no repair was seen in the excision repair-deficient (uvrA-) E. coli cells. The results indicate (1) the selectivity of the immunological method and the unique ability of the (+)-anti-BPDE specific antibodies to monitor the direct loss of this promutagenic base lesion from the in vivo modified DNA (2) the role of host excision repair pathway in efficient removal of adducts from bacterial genome determines the survival of the bacterial cells and (3) the repair of (+)-anti-BPDE DNA adducts in episomally replicating, transcriptionally active sequences occur at a rapid rate presumably due to the ease of accessibility of repair enzymes to lesions within DNA.

DNA adducts: biological markers of exposure and potential applications to risk assessment

DNA adducts have been investigated extensively during the past decade. This research has been advanced, in part, by the development of ultrasensitive analytical methods, such as 32P-postlabeling and mass spectrometry, that enable detection of DNA adducts at concentrations as low as one adduct per 10(9) to 10(10) normal nucleotides. Studies of mutations in activated oncogenes such as ras, inactivated tumor suppressor genes such as p53, and surrogate genes such as hprt provide linkage between DNA adducts and carcinogenesis. The measurement of DNA adducts, or molecular dosimetry, has important applications for cancer risk assessment. Cancer risk assessment currently involves estimating the probable effects of carcinogens in humans based on results of animal bioassays. Estimates of risk are then derived from mathematical models that fit data of tumor incidence at the high animal exposures and extrapolate to probable human exposures that may be orders of magnitude lower. Molecular dosimetry could extend the observable range of mechanistic data several orders of magnitude lower than can be achieved in carcinogenesis bioassays. This measurement also compensates automatically for individual and species differences in toxicokinetic factors, as well as any nonlinearities that affect the quantitative relationships between exposure and molecular dose. As a result, molecular dosimetry can provide a basis for conducting high- to low-dose, route-to-route, and interspecies extrapolations. The incorporation of such data into risk assessment promises to reduce uncertainties and produce more accurate estimates of risk compared to current methods.

DNA repair in human lymphocytes treated in vitro with (+)-anti- and (+/-)-syn-benzo[a]pyrene diolepoxide

Human PBL were treated in vitro with the ultimate reactive metabolites of BaP anti- and syn-BaPDE and DNA damage and repair were measured. The incorporation of radioactivity into DNA due to UDS was higher after treatment with anti-BaPDE. Radioactive DNA adduct dosimetry applied to PBL treated with tritiated syn- and anti-BaPDE demonstrated that anti-BaPDE gave more DNA adducts, which were more efficiently removed than syn adducts in the 24 h following the treatment. HPLC analysis of deoxynucleosides obtained from the enzymatic digestion of DNA showed that in treated PBL the major DNA adduct involved deoxyguanosine. DNA strand breaks, detected by FADU, were induced at comparable levels by anti- and syn-BaPDE (0.1-0.4 micrograms/ml), and persisted after 20 h of post-treatment incubation. Only in the case of syn-BaPDE did the percentage of double-stranded DNA tend to increase with time after the treatment.