Mispairing of the 8,9-Dihydro-8-(N7-guanyl)-9-hydroxy-aflatoxin B₁ Adduct with Deoxyadenosine Results in Extrusion of the Mismatched dA toward the Major Groove

Abstract

The G --> T transversion is the dominant mutation induced by the cationic trans-8,9-dihydro-8-(N7-guanyl)-9-hydroxy-aflatoxin B(1) adduct. The structure of d(ACATC(AFB)GATCT).d(AGATAGATGT), in which the cationic adduct was mismatched with deoxyadenosine, was refined using molecular dynamics calculations restrained by NOE data and dihedral restraints obtained from NMR spectroscopy. Restrained molecular dynamics calculations refined structures with pairwise rmsd<1 A and a sixth root R1x factor between the refined structure and NOE data of 10.5 x 10-2. The mismatched duplex existed in a single conformation at neutral pH. The aflatoxin moiety intercalated above the 5' face of the modified (AFB)G. The mismatched dA was in the anti conformation about the glycosyl bond. It extruded toward the major groove and did not participate in hydrogen bonding with (AFB)G. The structure was compared with that of d(ACATCGATCT).d(AGATAGATGT) containing the corresponding unmodified G.A mismatch and with d(ACATC(AFB)GATCT).d(AGATCGATGT) containing the aflatoxin lesion in the correctly paired (AFB)G.C context. The correctly paired oligodeoxynucleotide exhibited Watson-Crick-type geometry at the (AFB)G.C pair. It melted at higher temperature than the mismatched (AFB)G.A duplex. The unmodified mismatched G.A duplex exhibited spectral line broadening at neutral pH, suggesting a mixture of conformations. It exhibited a lower melting temperature than did the mismatched (AFB)G.A duplex. These differences correlated with replication bypass experiments performed in vitro utilizing DNA polymerase I exo- [Johnston, D. S., and Stone, M. P. (2000) Chem. Res. Toxicol. 13, 1158-1164]. Those experiments showed that correct insertion of dC opposite (AFB)G blocked replication by the enzyme, whereas incorrect insertion of dA opposite (AFB)G allowed full-length replication of the adducted template strand.

Effect of Bulky Lesions on DNA

“…Although there are cases in which the adduct interacts with the minor (12, 13) or the major groove (14,15) of the double helix, in most cases the interaction occurs through intercalation between the surrounding base pairs (13, 16 -32). In many cases, the intercalation occurs from the major groove side (21,27,(31)(32)(33), resembling the conformation adopted by the cholesterol adducts studied in this paper. Displacement of the opposite base out of the helix is rather common, and in many cases the unpaired base is shifted toward the major groove.…”

In Vitro Recapitulating of TP53 Mutagenesis in Hepatocellular Carcinoma Associated With Dietary Aflatoxin B1 Exposure

“…AFB 1 -FAPY adducts have also displayed higher mutagenic potency and stronger blocking effects on DNA replication relative to their molecular predecessors (AFB 1 -N7-Gua adducts) 22. Nuclear magnetic resonance studies have demonstrated that AFB 1 -FAPY adducts have a more subtle effect on DNA architecture and a pronounced effect on the melting temperature of the DNA duplex than AFB 1 -N7-Gua adducts41, 42. These properties enable the AFB 1 -FAPY adducts to evade DNA repair and exhibit great mutagenic potentials 40, 43, 44.…”

Guanine Alkylation by the Potent Carcinogen Aflatoxin B1: Quantum Chemical Calculations

“…AFB 1 is among the most potent mutagens implicated in human carcinogenesis , . Moreover, it is mutagenic in bacteria and carcinogenic in fish, and it is a hepatocarcinogen in rodents , . AFB 1 has also been considered a biological/chemical weapon, e.g., in 1989 in Iraq .…”

“…AFB 1 exo-8,9-epoxide also reacts with DNA, evidently after intercalation (1). Reaction occurs with high regiospecificity at the N7 position of guanine (Gua) residues, yielding the trans-8,9-dihydro-8-(N 7 -guanyl)-9-hydroxyaflatoxin adduct (4). The proposed reaction mechanism is depicted in Figure 1.…”