The α,β-unsaturated aldehydes (enals) acrolein, crotonaldehyde, and trans-4-hydroxynonenal (4-HNE) are products of endogenous lipid peroxidation, arising as a consequence of oxidative stress. The addition of enals to dG involves Michael addition of the N2-amine to give N 2-(3-oxopropyl)-dG adducts, followed by reversible cyclization of N1 with the aldehyde, yielding 1,N2-dG exocyclic products. The 1,N 2-dG exocyclic adducts from acrolein, crotonaldehyde, and 4-HNE exist in human and rodent DNA. The enal-induced 1,N2-dG lesions are repaired by the nucleotide excision repair pathway in both Escherichia coli and mammalian cells. Oligodeoxynucleotides containing structurally defined 1,N 2-dG adducts of acrolein, crotonaldehyde, and 4-HNE were synthesized via a postsynthetic modification strategy. Site-specific mutagenesis of enal adducts has been carried out in E. coli and various mammalian cells. In all cases, the predominant mutations observed are G→T transversions, but these adducts are not strongly miscoding. When placed into duplex DNA opposite dC, the 1,N2-dG exocyclic lesions undergo ring opening to the corresponding N2-(3-oxopropyl)-dG derivatives. Significantly, this places a reactive aldehyde in the minor groove of DNA, and the adducted base possesses a modestly perturbed Watson-Crick face. Replication bypass studies in vitro indicate that DNA synthesis past the ring-opened lesions can be catalyzed by pol η, pol ι, and pol κ. It also can be accomplished by a combination of Rev1 and pol ζ acting sequentially. However, efficient nucleotide insertion opposite the 1,N2-dG ring-closed adducts can be carried out only by pol ι and Rev1, two DNA polymerases that do not rely on the Watson-Crick pairing to recognize the template base. The N 2-(3-oxopropyl)-dG adducts can undergo further chemistry, forming interstrand DNA cross-links in the 5′-CpG-3′ sequence, intrastrand DNA cross-links, or DNA-protein conjugates. NMR and mass spectrometric analyses indicate that the DNA interstand cross-links contain a mixture of carbinolamine and Schiff base, with the carbinolamine forms of the linkages predominating in duplex DNA. The reduced derivatives of the enal-mediated N2-dG:N 2-dG interstrand cross-links can be processed in mammalian cells by a mechanism not requiring homologous recombination. Mutations are rarely generated during processing of these cross-links. In contrast, the reduced acrolein-mediated N2-dG peptide conjugates can be more mutagenic than the corresponding monoadduct. DNA polymerases of the DinB family, pol IV in E. coli and pol κ in human, are implicated in error-free bypass of model acrolein-mediated N2-dG secondary adducts, the interstrand cross-links, and the peptide conjugates.
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