Accomodation of S-cis-tamoxifen-N2-guanine adduct within a bent and widened DNA minor groove

The non-steroidal anti-estrogen tamoxifen [TAM] has been in clinical use over the last two decades as a potent adjunct chemotherapeutic agent for treatment of breast cancer. It has also been given prophylactically to women with a strong family history of breast cancer. However, tamoxifen treatment has also been associated with increased endometrial cancer, possibly resulting from the reaction of metabolically activated tamoxifen derivatives with cellular DNA. Such DNA adducts can be mutagenic and the activities of isomeric adducts may be conformation-dependent. We therefore investigated the high resolution NMR solution conformation of one covalent adduct (cis-isomer, S-epimer of [TAM]G) formed from the reaction of tamoxifen [TAM] to N2-of guanine in the d(C-[TAM]G-C)·d(G-C-G) sequence context at the 11-mer oligonucleotide duplex level. Our NMR results establish that the S-cis [TAM]G lesion is accomodated within a widened minor groove without disruption of the Watson-Crick [TAM]G·C and flanking Watson-Crick G·C base-pairs. The helix axis of the bound DNA oligomer is bent by about 30° and is directed away from the minor groove adduct site. The presence of such a bulky [TAM]G adduct with components of the TAM residue on both the 5′- and the 3′-side of the modified base could compromise the fidelity of the minor groove polymerase scanning machinery.