Structural Consequences of the C8-Guanine DNA Adduct Formed by 3-Nitrobenzanthrone; an Environmental Carcinogen

Abstract

Damage to DNA results from many endogenous and exogenous sources and plays a significant role in cancer etiology. 3-Nitrobenzathrone (3-NBA) is a component of diesel exhaust that is known to form DNA adducts. Aminobenzanthrone (ABA) DNA adducts result through the reaction of electrophilic nitrenium ions formed during 3-NBA’s metabolic reduction in vivo. Furthermore, 3-NBA has been reported as being carcinogenic in animal model systems and human exposure has been demonstrated. Here I examine structural impacts of the C8-guanine-aminobenzanthrone (C8-dG-ABA) adduct, a major DNA adduct of 3-NBA, which occurs at the C8 position of guanine. This adduct produced a base-displaced intercalated structure within a DNA duplex. The ABA moiety intercalated into the duplex displacing the cytosine opposite the lesion. The adducted guanine rotated about the glycosidic bond into the syn conformation. The overall stability of the DNA duplex was reduced as demonstrated by a decrease in melting temperature of 11º C compared to the undamaged duplex. Another structure was determined for C8-dG-ABA in complex with a trans-lesion synthesis polymerase, human polymerase eta (hPol η). The conformation of the C8-dG-ABA adduct within the active site of hPol η was markedly different from the structure observed in duplex DNA. The complex was captured in a non-mutagenic insertion state. The adducted guanine was in the anti conformation and was observed to form a Watson-Crick basepair with an incoming cytosine triphosphate (dCTP). The ABA moiety was rotated out of the active site of hPol η and resided in a hydrophobic pocket. The position of the ABA moiety did not allow for a clear path for the next template base to be translocated into the active site, and may reflect a means by which the adduct blocks replication.