Marked differences in base selectivity between DNA and the free nucleotides upon adduct formation from Bay- and Fjord-region diol epoxides

Inherent stereospecificity in the reaction of aflatoxin B(1) 8,9-epoxide with deoxyguanosine and efficiency of DNA catalysis

Kinetic analysis of guanine alkylation by aflatoxin B(1) exo-8,9-epoxide, the reactive form of the hepatocarcinogen aflatoxin B(1), shows the reaction to be >2000 times more efficient in DNA than in aqueous solution, that is, with free 2'-deoxyguanosine. Thermodynamic analysis reveals AFB(1) exo-8,9-epoxide intercalation as the predominant source of the observed DNA catalytic effect. However, the known exo > endo epoxide stereospecificity of the DNA alkylation is observed even with free deoxyguanosine (ratio >20:1 determined by LC-MS and NMR measurements), as predicted by theoretical calculations [ Bren , U. , et al. ( 2007 ) Chem. Res. Toxciol. 20 , 1134 - 1140 ].

3'-Intercalation of a N2-dG 1R-trans-anti-benzo[c]phenanthrene DNA adduct in an iterated (CG)3 repeat

The conformation of the 1 R,2 S,3 R,4 S-benzo[ c]phenanthrene- N (2)-dG adduct, arising from trans opening of the (+)-1 S,2 R,3 R,4 S- anti-benzo[ c]phenanthrene diol epoxide, was examined in 5'- d(ATCGC XCGGCATG)-3'.5'-d(CATGCCG CGCGAT)-3', where X = 1 R,2 S,3 R,4 S-B[ c]P- N (2)-dG. This duplex, derived from the hisD3052 frameshift tester strain of Salmonella typhimurium, contains a (CG) 3 iterated repeat, a hotspot for frameshift mutagenesis. NMR experiments showed a disconnection in sequential NOE connectivity between X (4) and C (5), and in the complementary strand, they showed another disconnection between G (18) and C (19). In the imino region of the (1)H NMR spectrum, a resonance was observed at the adducted base pair X (4) x C (19). The X (4) N1H and G (18) N1H resonances shifted upfield as compared to the other guanine imino proton resonances. NOEs were observed between X (4) N1H and C (19) N (4)H and between C (5) N (4)H and G (18) N1H, indicating that base pairs X (4) x C (19) and C (5) x G (18) maintained Watson-Crick hydrogen bonding. No NOE connectivity was observed between X (4) and G (18) in the imino region of the spectrum. Chemical shift perturbations of greater than 0.1 ppm were localized at nucleotides X (4) and C (5) in the modified strand and G (18) and C (19) in the complementary strand. A total of 13 NOEs between the protons of the 1 R-B[ c]Ph moiety and the DNA were observed between B[ c]Ph and major groove aromatic or amine protons at base pairs X (4) x C (19) and 3'-neighbor C (5) x G (18). Structural refinement was achieved using molecular dynamics calculations restrained by interproton distances and torsion angle restraints obtained from NMR data. The B[ c]Ph moiety intercalated on the 3'-face of the X (4) x C (19) base pair such that the terminal ring of 1 R-B[ c]Ph threaded the duplex and faced into the major groove. The torsion angle alpha' [X (4)]-N3-C2-N2-B[ c]Ph]-C1 was calculated to be -177 degrees, maintaining an orientation in which the X (4) exocyclic amine remained in plane with the purine. The torsion angle beta' [X (4)]-C2-N2-[B[ c]Ph]-C1-C2 was calculated to be 75 degrees. This value governed the 3'-orientation of the B[ c]Ph moiety with respect to X (4). The helical rise between base pairs X (4) x C (19) and C (5) x G (18) increased and resulted in unwinding of the right-handed helix. The aromatic rings of the B[ c]Ph moiety were below the Watson-Crick hydrogen-bonding face of the modified base pair X (4) x C (19). The B[c]Ph moiety was stacked above nucleotide G (18), in the complementary strand.

Dose-dependent benzo(a)pyrene [B(a)P]-DNA adduct levels and persistence in F-344 rats following subchronic dietary exposure to B(a)P

In order to investigate the relationship between BaP-DNA adduct formation and long-term exposure to benzo(a)pyrene (BaP), DNA adduct levels in liver and lung tissues of male and female F-344 rats subchronically exposed to BaP were determined. Doses of 0, 5, 50, and 100mg/kg BaP, representing control, low, intermediate, and high doses, respectively, were administered in the animal diet over a 90-day period. After dosing, animals were sacrificed, liver and lung tissues were removed, DNA was isolated and analyzed for BaP-induced DNA adducts by the (32)P-postlabeling method using a four-directional thin-layer chromatography system. At low and intermediate BaP doses, DNA adduct levels in the tissues were significantly correlated with exposure. However, at high BaP doses, the dose-DNA adduct relationship became non-linear. Similarly, the relative DNA adducts persistence at intermediate and high doses were significantly higher than that measured at low dose. The low and intermediate dose linearity and high dose non-linearity may be due to saturation of metabolic activation and detoxification enzymes, and DNA repair processes.

A single site-specific trans-opened 7,8,9,10-tetrahydrobenzo[a]pyrene 7,8-diol 9,10-epoxide N2-deoxyguanosine adduct induces mutations at multiple sites in DNA

Site-specific mutagenicity of trans-opened adducts at the exocyclic N(2)-amino group of guanine by the (+)-(7R,8S,9S,10R)- and (-)-(7S,8R,9R,10S)-enantiomers of a benzo[a]pyrene 7,8-diol 9,10-epoxide (7-hydroxyl and epoxide oxygen are trans, BPDE-2) has been determined in Chinese hamster V79 cells and their repair-deficient counterpart, V-H1 cells. Four vectors containing single 10S-BPDE-dG or 10R-BPDE-dG adducts positioned at G(0) or G(-1) in the analyzed 5'-ACTG(0)G(-1)GA sequence of the non-transcribed strand were separately transfected into the cells. Mutations at each of the seven nucleotides were analyzed by a novel primer extension assay using a mixture of one dNTP complementary to the mutated nucleotide and three other ddNTPs and were optimized to quantify levels of a mutation as low as 1%. Only G --> T mutations were detected at the adducted sites; the 10S adduct derived from the highly carcinogenic (+)-diol epoxide was 40-50 and 75-140% more mutagenic than the 10R adduct in V79 and V-H1 cells, respectively. Importantly, the 10S adducts, but not the 10R adducts, induced separate non-targeted mutations at sites 5' to the G(-1) and G(0) lesions (G(0) --> T and C --> T, respectively) in both cell lines. Neither the T 5' to G(0) nor sites 3' to the lesions showed mutations. Non-targeted mutations may enhance overall mutagenicity of the 10S-BPDE-dG lesion and contribute to the much higher carcinogenicity and mutagenicity of (+)-BPDE-2 compared with its (-)-enantiomer. Our study reports a definitive demonstration of mutations distal to a site-specific polycyclic aromatic hydrocarbon adduct.

Molecular topology of polycyclic aromatic carcinogens determines DNA adduct conformation: a link to tumorigenic activity

We report below on the solution structures of stereoisomeric "fjord" region trans-anti-benzo[c]phenanthrene-N2-guanine (designated (BPh)G) adducts positioned opposite cytosine within the (C-(BPh)G-C).(G-C-G) sequence context. We observe intercalation of the phenanthrenyl ring with stereoisomer-dependent directionality, without disruption of the modified (BPh)G.C base-pair. Intercalation occurs to the 5' side of the modified strand for the 1S stereoisomeric adduct and to the 3' side for the 1R stereoisomeric adduct, with the S and R-trans-isomers related to one another by inversion in a mirror plane at all four chiral carbon atoms on the benzylic ring. Intercalation of the fjord region BPh ring into the helix without disruption of the modified base-pair is achieved through buckling of the (BPh)G.C base-pair, displacement of the linkage bond from the plane of the (BPh)G base, adaptation of a chair pucker by the BPh benzylic ring and the propeller-like deviation from planarity of the BPh phenanthrenyl ring. It is noteworthy that intercalation without base-pair disruption occurs from the minor groove side for S and R-trans-anti BPh-N2-guanine adducts opposite C, in contrast to our previous demonstration of intercalation without modified base-pair disruption from the major groove side for S and R-trans-anti BPh-N6-adenine adducts opposite T. Further, these results on fjord region 1S and 1R-trans-anti (BPh)G adducts positioned opposite C are in striking contrast to earlier research with "bay" region benzo[a]pyrene-N2-guanine (designated (BP)G) adducts positioned opposite cytosine, where 10S and 10R-trans-anti stereoisomers were positioned with opposite directionality in the minor groove without modified base-pair disruption. They also are in contrast to the 10S and 10R-cis-anti stereoisomers of (BP)G adducts opposite C, where the pyrenyl ring is intercalated into the helix with directionality, but the modified base and its partner on the opposite strand are displaced out of the helix. These results are especially significant given the known greater tumorigenic potential of fjord region compared to bay region polycyclic aromatic hydrocarbons. The tumorigenic potential has been linked to repair efficiency such that bay region adducts can be readily repaired while their fjord region counterparts are refractory to repair. Our structural results propose a link between DNA adduct conformation and repair-dependent mutagenic activity, which could ultimately translate into structure-dependent differences in tumorigenic activities. We propose that the fjord region minor groove-linked BPh-N2-guanine and major groove-linked BPh-N6-adenine adducts are refractory to repair based on our observations that the phenanthrenyl ring intercalates into the helix without modified base-pair disruption. The helix is therefore minimally perturbed and the phenanthrenyl ring is not available for recognition by the repair machinery. By contrast, the bay region BP-N2-G adducts are susceptible to repair, since the repair machinery can recognize either the pyrenyl ring positioned in the minor groove for the trans-anti groove-aligned stereoisomers, or the disrupted modified base-pair for the cis-anti base-displaced intercalated stereoisomers.