Replication ofN2,3-Ethenoguanine by DNA Polymerases

Abstract

DNA is prone to attack by physical and chemical agents generated endogenously and exogenously, producing modified DNA bases (i.e. DNA adducts/lesions), abasic sites, and inter- and intrastrand DNA crosslinks. DNA adducts, if not properly repaired, can lead to blocked replication, misincorporation, and mutation, potentially causing gene deregulation and cancer. Etheno (e) DNA adducts are exocyclic adducts that, in addition to their use as fluorescent nucleotidfe derivatives,[1] were first recognized as reaction products of DNA with reactive metabolites of the occupational carcinogen vinyl chloride (VC).[2] Endogenous etheno-DNA adducts, arising from lipid peroxidation-derived DNA damage, were also detected in rats[3] and humans[4] without VC exposure. VC is a known carcinogen that induces hepatic angiosarcomas.[5] The major DNA adduct formed by VC, N7-(2-oxoethyl)guanine,[3, 6] is generally not considered to be mutagenic, because in vitro experiments showed that it did not cause detectable miscoding in an assay with modified poly(GC).[7] However, etheno adducts formed by VC (e.g. 1,N6-ethenoadenine, 3,N4-ethenocytosine, N2,3-ethenoguanine (N2,3-eG), and 1,N2-ethenoguanine (1,N2-eG)) have all been shown to be mutagenic in vitro and in bacteria (see N2,3-eG and 1,N2-eG structures in Figure 1a).[8] N2,3-eG is the most abundant endogenous etheno adduct, with levels estimated to be approximately 36 N2,3-eG lesions/cell in livers of untreated rats or humans.[9] A common assumption is that N2,3-eG is highly mutagenic; N2,3-eG is considered to contribute to the carcinogenesis of VC and inflammation-driven malignancies.[10] The dominance of a GC to AT transition in five of six K-ras (oncogene) tumors from VC workers[9] suggests the importance of a G adduct, but the misincorporation characteristics of 1,N2-eG are not consistent with this transition.[8a,f] Little repair of N2,3-eG occurs in VC-exposed rats, since the half-life of this lesion in rat liver and lung (150 days) and in rat kidney (75 days) is quite long.[11] The lability of the glycosidic bond of N2,3-e-deoxyguanosine (N2,3-e-dG) makes it difficult to adequately discern its mutagenic potential.[12] Both C and T were incorporated opposite N2,3-eG in a polyribo(G/N2,3-eG) template by avian myeloblastosis virus (AMV) reverse transcriptase.[13] N2,3-e-Deoxyguanosine triphosphate is inserted opposite T by several polymerases (pols).[8d]