Formation of 1,4-dioxo-2-butene-derived adducts of 2'-deoxyadenosine and 2'-deoxycytidine in oxidized DNA

Radiation-mediated formation of complex damage to DNA: a chemical aspect overview

During the last three decades, a considerable amount of work has been undertaken to determine the nature, the mechanism of formation and the biological consequences of radiation-induced DNA lesions. Most of the information was obtained via the development of chemical approaches, including theoretical, analytical and organic synthesis methods. Since it is not possible to present all the results obtained in this review article, we will focus on recent data dealing with the formation of complex DNA lesions produced by a single oxidation event, as these lesions may play a significant role in cellular responses to ionizing radiation and also to other sources of oxidative stress. Through the description of specific results, the contribution of different chemical disciplines in the assessment of the structure, the identification of the mechanism of formation and the biological impacts in terms of repair and mutagenicity of these complex radiation-induced DNA lesions will be highlighted.

Repair of the major lesion resulting from C5'-oxidation of DNA

Oxidation of the C5'-position of DNA results in direct strand scission. The 3'-fragments produced contain DNA lesions at their 5'-termini. The major DNA lesion contains an aldehyde at its C5'-position, but its nucleobase is unmodified. Excision of the lesion formed from oxidation of thymidine (T-al) is achieved by strand displacement synthesis by DNA polymerase β (Pol β) in the presence or absence of flap endonuclease 1 (FEN1). Pol β displaces T-al and thymidine with comparable efficiency, but less so than a chemically stabilized abasic site analogue (F). FEN1 cleaves the flaps produced during strand displacement synthesis that are two nucleotides or longer. A ternary complex containing T-al is also a substrate for the bacterial UvrABC nucleotide excision repair system. The sites of strand scission are identical in ternary complexes containing T-al, thymidine, or F. UvrABC incision efficiency of these ternary complexes is comparable as well but significantly slower than a duplex substrate containing a bulky substituted thymidine. However, cleavage occurs only on the 5'-fragment and does not remove the lesion. These data suggest that unlike many lesions the redundant nature of base excision and nucleotide excision repair systems does not provide a means for removing the major damage product produced by agents that oxidize the C5'-position. This may contribute to the high cytotoxicity of drugs that oxidize the C5'-position in DNA.

Comparison of the in vitro replication of the 7-(2-oxoheptyl)-1,N2-etheno-2'-deoxyguanosine and 1,N2-etheno-2'-deoxyguanosine lesions by Sulfolobus solfataricus P2 DNA polymerase IV (Dpo4)

Oligonucleotides were synthesized containing the 7-(2-oxoheptyl)-etheno-dGuo adduct, which is derived from the reaction of dGuo and the lipid peroxidation product 4-oxo-2-nonenal. The in vitro replication of 7-(2-oxoheptyl)-etheno-dGuo by the model Y-family polymerase Sulfolobus solfataricus P2 DNA Polymerase IV (Dpo4) was examined in two sequences. The extension products were sequenced using an improved LC-ESI-MS/MS protocol developed in our laboratories, and the results were compared to that of the 1,N(2)-etheno-dGuo adduct in the same sequence contexts. Both etheno adducts were highly miscoding when situated in 5'-TXG-3' local sequence contexts with <4% of the extension products being derived from error-free bypass. The major extension products resulted from the misinsertion of Ade opposite the adduct and a one-base deletion. The major extension products from replication of the etheno lesions in a 5'-CXG-3' local sequence context were the result of misinsertion of Ade, a one-base deletion, and error-free bypass. Other minor extension products were also identified. The 7-(2-oxoheptyl)-etheno-dGuo lesion resulted in a larger frequency of misinsertion of Ade, whereas the 1,N(2)-etheno-dGuo gave more of the one-base deletion product. Conformational studies of duplex DNA containing the 7-(2-oxoheptyl)-etheno-dGuo in a 5'-TXG-3' sequence context by NMR indicated the presence of a pH-dependent conformational transition, likely involving the glycosyl bond at the adducted guanosine; the pK(a) for this transition was lower than that observed for the 1,N(2)-epsilon-dGuo lesion. However, the 7-(2-oxoheptyl)-etheno-dGuo lesion, the complementary Cyt, and both flanking base pairs remained disordered at all pH values, which is attributed to the presence of the hydrophobic heptyl group of the 7-(2-oxoheptyl)-etheno-dGuo lesion. The altered pK(a) value and the structural disorder at the 7-(2-oxoheptyl)-etheno-dGuo lesion site, as compared to the same sequence containing the 1,N(2)-etheno-dGuo, may contribute to higher frequency of misinsertion of Ade.

An Oxidized Abasic Lesion as an Intramolecular Source of DNA Adducts

5'-(2-Phosphoryl-1,4-dioxobutane) (DOB) is a lesion produced in DNA via a variety of damaging agents. The DOB lesion spontaneously generates cis- and trans-but-2-en-1,4-dial (1) via β-elimination. Cis- and trans-but-2-en-1,4-dial forms exocyclic adducts with nucleosides. We used chemically synthesized DNA containing tritiated DOB incorporated at defined sites to examine the reactivity of cis- and trans-but-2-en-1,4-dial. Although the local DNA sequence does not appear to influence the distribution of nucleoside adducts, we find that DOB generates relatively high yields of cis- and trans-but-2-en-1,4-dial nucleoside adducts that likely are promutagenic.

The biological and metabolic fates of endogenous DNA damage products

DNA and other biomolecules are subjected to damaging chemical reactions during normal physiological processes and in states of pathophysiology caused by endogenous and exogenous mechanisms. In DNA, this damage affects both the nucleobases and 2-deoxyribose, with a host of damage products that reflect the local chemical pathology such as oxidative stress and inflammation. These damaged molecules represent a potential source of biomarkers for defining mechanisms of pathology, quantifying the risk of human disease and studying interindividual variations in cellular repair pathways. Toward the goal of developing biomarkers, significant effort has been made to detect and quantify damage biomolecules in clinically accessible compartments such as blood and and urine. However, there has been little effort to define the biotransformational fate of damaged biomolecules as they move from the site of formation to excretion in clinically accessible compartments. This paper highlights examples of this important problem with DNA damage products.

Inhibition of short patch and long patch base excision repair by an oxidized abasic site

5'-(2-Phosphoryl-1,4-dioxobutane) (DOB) is an oxidized abasic lesion that is produced by a variety of DNA damaging agents, including several antitumor antibiotics. DOB efficiently and irreversibly inhibits DNA polymerase β, an essential base excision repair enzyme in mammalian cells. The generality of this mode of inhibition by DOB is supported by the inactivation of DNA polymerase λ, which may serve as a possible backup for DNA polymerase β during abasic site repair. Protein digests suggest that Lys72 and Lys84, which are present in the lyase active site of DNA polymerase β, are modified by DOB. Monoaldehyde analogues of DOB substantiate the importance of the 1,4-dicarbonyl component of DOB for efficient inactivation of Pol β and the contribution of a freely diffusible electrophile liberated from the inhibitor by the enzyme. Inhibition of DNA polymerase β's lyase function is accompanied by inactivation of its DNA polymerase activity as well, which prevents long patch base excision repair of DOB. Overall, DOB is highly refractory to short patch and long patch base excision repair. Its recalcitrance to succumb to repair suggests that DOB is a significant source of the cytotoxicity of DNA damaging agents that produce it.

Irreversible inhibition of DNA polymerase beta by an oxidized abasic lesion

DNA damage is a source of carcinogenicity and is also the source of the cytotoxicity of gamma-radiolysis and antitumor agents, such as the enediynes. The dioxobutane lesion (DOB) is produced by a variety of DNA-damaging agents, including the aforementioned. Repair of DOB is important for maintaining the integrity of the genome as well as counteracting therapeutic agents that target DNA. We demonstrate that the DOB lesion efficiently and irreversibly inhibits repair by DNA polymerase beta (Pol beta), an integral enzyme in base-excision repair. Irreversible inhibition of Pol beta by DOB suggests that this lesion provides a chemical explanation for the cytotoxicity of drugs that produce it and explains previously unexplained observations in the literature concerning abasic lesions that are not repaired efficiently. Finally, these observations provide the impetus for the design of a new family of inhibitors of Pol beta.

DNA interstrand cross-link formation by the 1,4-dioxobutane abasic lesion

The oxidized abasic lesion 5'-(2-phosphoryl-1,4-dioxobutane) (DOB) is produced concomitantly with a single-strand break by a variety of DNA-damaging agents that abstract a hydrogen atom from the C5'-position. Independent generation of the DOB lesion in DNA reveals that it reversibly forms interstrand cross-links (ICLs) selectively with a dA opposite the 3'-adjacent nucleotide. Product studies and the use of monoaldehyde models suggest that ICL formation involves condensation of the dialdehyde with the exocyclic amine. Mechanistic studies and inspection of molecular models indicate that the local DNA environment and proximity of the exocyclic amine determine the selectivity for reaction with dA. Proximity control of the electrophile's reactivity is distinct from that of structurally similar freely diffusing molecules. ICL formation by a DOB lesion that is adjacent to a single-strand break is potentially significant because the product constitutes a "clustered" or "complex" lesion. Clustered lesions can lead to highly deleterious double-strand breaks upon nucleotide excision repair.

Scope and mechanism of interstrand cross-link formation by the C4'-oxidized abasic site

The C4'-oxidized abasic site (C4-AP) is a commonly formed DNA lesion, which generates two types of interstrand cross-links (ICLs). The kinetically favored cross-link consists of two full length strands and forms reversibly and exclusively with dA. Cross-link formation is attributed to condensation of C4-AP with the N6-amino group of dA. Formation of the thermodynamic ICL involves cleavage of the strand containing C4-AP on the 3'-side of the lesion. The ratios and yields of the ICLs are highly dependent upon the local sequence. Product analysis of enzyme-digested material reveals that the ICL with dA is a cyclic adduct. Formation of the thermodynamically favored cross-link is catalyzed by the surrounding DNA sequence and occurs favorably with dC and dA but not with dG or dT. Mechanistic studies indicate that beta-elimination from C4-AP is the rate-limiting step in the formation of the thermodynamic ICL and that the local DNA environment determines the rate constant for this reaction. The efficiency of ICL formation, the stability of the thermodynamic products, and their possible formation in cells (Regelus, P.; et al. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 14032) suggest that these lesions will be deleterious to the biological system in which they are produced.

GC/MS methods to quantify the 2-deoxypentos-4-ulose and 3'-phosphoglycolate pathways of 4' oxidation of 2-deoxyribose in DNA: application to DNA damage produced by gamma radiation and bleomycin

DNA oxidation plays a substantive role in the pathophysiology of human diseases, such as cancer. While the chemistry of nucleobase lesions has dominated studies of DNA damage, there is growing evidence that the oxidation of 2-deoxyribose in DNA plays a critical role in the genetic toxicology of oxidative stress. As part of an effort to define the spectrum of 2-deoxyribose oxidation products arising in vitro and in vivo, we now describe methods for quantifying products arising from 4' oxidation of 2-deoxyribose in DNA. The chemistry of 4' oxidation partitions between either of two pathways to form either a 2-deoxypentos-4-ulose abasic site (oxAB) or a strand break comprised of a 3'-phosphoglycolate (3PG) residue and a 5'-phosphate, with the release of either malondialdehyde and free base or a base propenal. Highly sensitive gas chromatography/mass spectrometry (GC/MS) methods were developed to quantify both lesions. The abasic site was converted to a 3'-phosphoro-3-pyridazinylmethylate derivative by treatment of the damaged DNA with hydrazine, which was released from DNA as 3-hydroxymethylpyridazine (HMP) by enzymatic hydrolysis. Similarly, 3PG was released as 2-phosphoglycolic acid (PG) by enzymatic hydrolysis. Following HPLC prepurification, both PG and HMP were silylated and quantified by GC/MS, with limits of detection of 100 and 200 fmol and sensitivities of 2 and 4 lesions per 10(6) nucleotides (nt) in 250 microg of DNA, respectively. Following validation of the methods with oligodeoxynucleotides containing the two lesions, the methods were applied to DNA damage produced by bleomycin and gamma radiation. As expected for an agent known to produce only 4' oxidation of DNA, the quantities of 3PG and oxAB accounted for all 2-deoxyribose oxidation events, as indicated by slopes of 0.8 and 0.3, respectively, in plots of the lesion frequency against total 2-deoxyribose oxidation events, with the latter determined by a plasmid-nicking assay. 3PG residues and oxAB were produced at the rate of 32 and 12 lesions per 10(6) nt per microM, respectively. For gamma radiation, on the other hand, 4' oxidation was found to comprise only 13% of 2-deoxyribose oxidation chemistry, with 3% oxAB (4 per 10(6) nt per Gy) and 10% 3PG (13 per 10(6) nt per Gy).

Oxidation of the sugar moiety of DNA by ionizing radiation or bleomycin could induce the formation of a cluster DNA lesion

Bleomycin, a radiomimetic drug currently used in human cancer therapy, is a well known carcinogen. Its toxicity is mostly attributed to its potentiality to induce DNA double strand breaks likely arising from the formation of two vicinal DNA strand breaks, initiated by C4-hydrogen abstraction on the 2-deoxyribose moiety. In this work we demonstrate that such a hydrogen abstraction reaction is able to induce the formation of a clustered DNA lesion, involving a 3' strand break together with a modified sugar residue exhibiting a reactive alpha,beta-unsaturated aldehyde that further reacts with a proximate cytosine base. The lesion thus produced was detected as a mixture of four isomers by HPLC coupled to tandem mass spectrometry subsequent to DNA extraction and enzymatic digestion. The modified nucleosides that constitute new types of cytosine adducts were identified as the likely two pairs of diastereomers of 6-(2-deoxy-beta-D-erythro-pentofuranosyl)-2-hydroxy-3(3-hydroxy-2-oxopropyl)-2,6-dihydroimidazo[1,2-c]-pyrimidin-5(3H)-one as inferred from mass spectrometry and NMR analyses of the chemically synthesized nucleosides. We demonstrate that bleomycin, and to a minor extent ionizing radiation, are able to induce significant amounts of the cytosine damage in cellular DNA. In addition, the repair kinetic of the lesion in a human lymphocyte cell line is rather slow, with a half-life of 10 h. The 2'-deoxycytidine adducts thus characterized that represent the first example of complex DNA lesions isolated and identified in cellular DNA upon one radical hit are likely to play an important role in the toxicity of bleomycin.