Although there exists compelling genetic evidence for a homologous recombination-independent pathway for repair of interstrand cross-links (ICLs) involving translesion synthesis (TLS), biochemical support for this model is lacking. To identify DNA polymerases that may function in TLS past ICLs, oligodeoxynucleotides were synthesized containing site-specific ICLs in which the linkage was between N 2 -guanines, similar to crosslinks formed by mitomycin C and enals. Here, data are presented that mammalian cell replication of DNAs containing these lesions was ϳ97% accurate. Using a series of oligodeoxynucleotides that mimic potential intermediates in ICL repair, we demonstrate that human polymerase (pol) not only catalyzed accurate incorporation opposite the cross-linked guanine but also replicated beyond the lesion, thus providing the first biochemical evidence for TLS past an ICL. The efficiency of TLS was greatly enhanced by truncation of both the 5 and 3 ends of the nontemplating strand. Further analyses showed that although yeast Rev1 could incorporate a dCTP opposite the cross-linked guanine, no evidence was found for TLS by pol or a pol /Rev1 combination. Because pol was able to bypass these ICLs, biological evidence for a role for pol in tolerating the N 2 -N 2 -guanine ICLs was sought; both cell survival and chromosomal stability were adversely affected in pol -depleted cells following mitomycin C exposure. Thus, biochemical data and cellular studies both suggest a role for pol in the processing of N 2 -N 2 -guanine ICLs.

ConspectusSignificant levels of the 1,N 2 -γ-hydroxypropano-dG adducts of the α,β-unsaturated aldehydes acrolein, crotonaldehyde, and 4-hydroxy-2E-nonenal (HNE) have been identified in human DNA, arising from both exogenous and endogenous exposure. They yield interstrand DNA cross-links between guanines in the neighboring C•G and G•C base pairs located in 5′-CpG-3′ sequences, as a result of opening of the 1,N 2 -γ-hydroxypropano-dG adducts to form reactive aldehydes that are positioned within the minor groove of duplex DNA. Using a combination of chemical, spectroscopic, and computational methods, we have elucidated the chemistry of cross-link formation in duplex DNA. NMR spectroscopy revealed that, at equilibrium, the acrolein and crotonaldehyde cross-links consist primarily of interstrand carbinolamine linkages between the exocyclic amines of the two guanines located in the neighboring C•G and G•C base pairs located in 5′-CpG-3′ sequences, that maintain the Watson-Crick hydrogen bonding of the cross-linked base pairs. The ability of crotonaldehyde and HNE to form interstrand cross-links depends upon their common relative stereochemistry at the C6 position of the 1,N 2 -γ-hydroxypropano-dG adduct. The stereochemistry at this center modulates the orientation of the reactive aldehyde within the minor groove of the doublestranded DNA, either facilitating or hindering the cross-linking reactions; it also affects the stabilities of the resulting diastereoisomeric cross-links. The presence of these cross-links in vivo is anticipated to interfere with DNA replication and transcription, thereby contributing to the etiology of human disease. Reduced derivatives of these cross-links are useful tools for studying their biological processing. IntroductionThe α,β-unsaturated aldehydes (enals) acrolein, crotonaldehyde, and 4-hydroxynonenal (4-HNE) (Scheme 1) are endogenous byproducts of lipid peroxidation, arising as a consequence of oxidative stress. [1][2][3][4] Acrolein and crotonaldehyde exposures also occur from exogenous sources, e.g., cigarette smoke 5 and automobile exhaust. 6 Enals react with DNA nucleobases to give exocyclic adducts; they also react with proteins. 7 Addition of enals to dG involves Michael addition of the N 2 -amine to give N 2 -(3-oxopropyl)-dG adducts (1, 3-8), followed by * Michael P. Stone telephone, 615-322-2589; fax, 615-322-7591; michael.p The lipid peroxidation product 4-HNE afforded related dGadducts (13-16). 14 Identification of acrolein adducts of other nucleosides followed. 15,16 The principal acrolein adduct is γ-OH-PdG (9), 10,12 although the regioisomeric 6-hydroxypyrimido[1,2-a]purin-10(3H)-one (α-OH-PdG, 10) has also been observed. 12,17 The γ-OH-PdG adduct (9) exists as a mixture of C8-OH epimers. With crotonaldehyde, addition at N 2 -dG creates a stereocenter at C6. Of four possible products, the two with the trans relative configurations at C6 and C8 (11,12) are observed. 12,18 These are also formed through the reaction of dG with two equivalents of acetaldehyde. 5,19,20 The cor...

The interstrand N 2 ,N 2 -dG DNA crosslinking chemistry of the acrolein-derived γ-OH-1,N 2 -propanodeoxyguanosine (γ-OH-PdG) adduct in the 5'-CpG-3' sequence was monitored within a dodecamer duplex by NMR spectroscopy, in situ, using a series of site-specific 13 C-and 15 N-edited experiments. At equilibrium 40% of the DNA was crosslinked, with the carbinolamine form of the crosslink predominating. The crosslink existed in equilibrium with the non-crosslinked N 2 -(3-oxopropyl)-dG aldehyde and its geminal diol hydrate. The ratio of aldehyde:diol increased at higher temperatures. The 1,N 2 -dG cyclic adduct was not detected. Molecular modeling suggested that the carbinolamine linkage should be capable of maintaining Watson-Crick hydrogen bonding at both of the tandem C•G base pairs. In contrast, dehydration of the carbinolamine crosslink to an imine (Schiff base) crosslink, or cyclization of the latter to form a pyrimidopurinone crosslink, was predicted to require disruption of Watson-Crick hydrogen bonding at one or both of the tandem crosslinked C•G base pairs. When the γ-OH-PdG adduct contained within the 5'-CpG-3' sequence was instead annealed into duplex DNA opposite T, a mixture of the 1,N 2 -dG cyclic adduct, the aldehyde, and the diol, but no crosslink, was observed. With this mismatched duplex, reaction with the tetrapeptide KWKK formed DNA-peptide crosslinks efficiently. When annealed opposite dA, γ-OH-PdG remained as the 1,N 2 -dG cyclic adduct although transient epimerization was detected by trapping with the peptide KWKK. The results provide a rationale for the stability of interstrand crosslinks formed by acrolein and perhaps other α,β-unsaturated aldehydes. These sequence-specific carbinolamine crosslinks are anticipated to interfere with DNA replication and contribute to acroleinmediated genotoxicity.

Current data suggest that DNA-peptide crosslinks are formed in cellular DNA as likely intermediates in the repair of DNA-protein crosslinks. In addition, a number of naturally occurring peptides are known to efficiently conjugate with DNA, particularly through the formation of Schiff-base complexes at aldehydic DNA adducts and abasic DNA sites. Since the potential role of DNA-peptide crosslinks in promoting mutagenesis is not well elucidated, here we report on the mutagenic properties of Schiff-base-mediated DNA-peptide crosslinks in mammalian cells. Site-specific DNA-peptide crosslinks were generated by covalently trapping a lysine-tryptophan-lysine-lysine peptide to the N(6) position of deoxyadenosine (dA) or the N(2) position of deoxyguanosine (dG) via the aldehydic forms of acrolein-derived DNA adducts (gamma-hydroxypropano-dA or gamma-hydroxypropano-dG, respectively). In order to evaluate the potential of DNA-peptide crosslinks to promote mutagenesis, we inserted the modified oligodeoxynucleotides into a single-stranded pMS2 shuttle vector, replicated these vectors in simian kidney (COS-7) cells and tested the progeny DNAs for mutations. Mutagenic analyses revealed that at the site of modification, the gamma-hydroxypropano-dA-mediated crosslink induced mutations at only approximately 0.4%. In contrast, replication bypass of the gamma-hydroxypropano-dG-mediated crosslink resulted in mutations at the site of modification at an overall frequency of approximately 8.4%. Among the types of mutations observed, single base substitutions were most common, with a prevalence of G to T transversions. Interestingly, while covalent attachment of lysine-tryptophan-lysine-lysine at gamma-hydroxypropano-dG caused an increase in mutation frequencies relative to gamma-hydroxypropano-dG, similar modification of gamma-hydroxypropano-dA resulted in decreased levels of mutations. Thus, certain DNA-peptide crosslinks can be mutagenic, and their potential to cause mutations depends on the site of peptide attachment. We propose that in order to avoid error-prone replication, proteolytic degradation of proteins covalently attached to DNA and subsequent steps of DNA repair should be tightly coordinated.