Independent Generation of 5-(2‘-Deoxycytidinyl)methyl Radical and the Formation of a Novel Cross-Link Lesion between 5-Methylcytosine and Guanine

Abstract

Reactive oxygen species (ROS) can damage DNA. Although a number of single nucleobase lesions induced by ROS have been structurally characterized, only a few intrastrand cross-link lesions have been identified and characterized, and all of them involve adjacent thymine and guanine or adenine. In mammalian cells, the cytosines at CpG sites are methylated. On the basis of the similar reactivity of 5-methylcytosine and thymine toward hydroxyl radical and the similar orientation of adjacent thymine guanine (TG) and 5-methylcytosine guanine (mCG) in B-DNA, we predict that the cross-link lesion, which was identified in TG and has a covalent bond formed between the 5-methyl carbon atom of T and the C8 carbon atom of G, should also form at mCG site. Here, we report for the first time the independent generation of 5-(2'-deoxycytidinyl)methyl radical, and our results demonstrate that this radical can give rise to the predicted novel intrastrand cross-link lesion in dinucleoside monophosphates d(mCG) and d(GmC). Furthermore, we show that the cross-link lesion can also form in d(mCG) from gamma irradiation under anaerobic conditions.

Identification and Quantification of a Guanine−Thymine Intrastrand Cross-Link Lesion Induced by Cu(II)/H2O2/Ascorbate

“…This is consistent with previous observations showing that intrastrand crosslink lesions are formed at much higher yield at 5′-purine·pyrimidine-3′ than at 5′-pyrimidine·purine-3′ sites (13)(14)(15)17,50). In this context, it is worth noting that the product-ion spectra of the [M + H] + ions of d(T^G), which was obtained from the 254-nm irradiation of d(XG), and d(G^T) are distinctive ( Figure S16).…”

“…Compounds 2-4 (Scheme 2)-These three compounds were synthesized starting from compound 1 following previously described procedures for the preparations of the corresponding unlabeled compounds (15). …”

“…For instance, a G^T intrastrand crosslink lesion, where the C8 of guanine is covalently bonded to the methyl carbon of its neighboring thymine, was shown to be induced in calf thymus DNA upon γ irradiation (13). These previous studies also revealed that the intrastrand crosslink lesions are initiated from a single pyrimidine radical (9)(10)(11)13,(15)(16)(17). In this regard, hydroxyl radical ( · OH) can either be added to the C5 and C6 carbon atoms of cytosine, thymine and 5-methylcytosine, or abstract a hydrogen atom from the 5-methyl group of the latter two pyrimidine bases (21).…”

“…In this regard, hydroxyl radical ( · OH) can either be added to the C5 and C6 carbon atoms of cytosine, thymine and 5-methylcytosine, or abstract a hydrogen atom from the 5-methyl group of the latter two pyrimidine bases (21). Both processes lead to the formation of pyrimidine base-centered secondary radicals, which may attack their neighboring purine bases to yield intrastrand crosslink lesions (9)(10)(11)13,(15)(16)(17).…”

Facile Photocyclization Chemistry of 5-Phenylthio-2‘-deoxyuridine in Duplex DNA

“…Moreover, further fragmentation of the ion of m/z 219 observed in the MS/MS for the 54.9-min fractions gives the same product-ion spectra (MS 3, Figure 4e&f). In this regard, we observed facile losses of NH 3 and HNCO from the modified nucleobase component. These two neutral losses have been previously found for the fragmentation of protonated uracil 14 .…”