Radical-induced purine lesion formation is dependent on DNA helical topology

Terzidis M.A., Prisecaru A., Molphy Z., Barron N., Randazzo A., Dumont E., Krokidis M.G., Kellett A., Chatgilialoglu C. 

Free Radical Research, 50, S91-S101, 2016.

Abstract: Herein we report the quantification of purine lesions arising from gamma-radiation sourced hydroxyl radicals (HO•) on tertiary dsDNA helical forms of supercoiled (SC), open circular (OC) and linear (L) conformation, along with single-stranded folded and non-folded sequences of guanine rich DNA in selected G-quadruplex structures. We identify that DNA helical topology and folding plays major, and unexpected, roles in the formation of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxo-dG) and 8-oxo-7,8-dihydro-2'-deoxyadenosine (8-oxo-dA), along with tandem-type purine lesions 5′,8-cyclo-2′-deoxyguanosine (5′,8-cdG) and 5′,8-cyclo-2′-deoxyadenosine (5′,8-cdA). SC, OC, and L dsDNA conformers together with folded and non-folded G-quadruplexes d[TGGGGT]4 (TG4T), d[AGGG(TTAGGG)3] (Tel22) and the mutated tel24 d[TTGGG(TTAGGG)3A] (mutTel24) were exposed to HO• radicals and purine lesions were then quantified via stable isotope dilution LC-MS/MS analysis. Purine oxidation in dsDNA follows L > OC ≫ SC indicating greater damage toward the extended B-DNA topology. Conversely, G-quadruplex sequences were significantly more resistant toward purine oxidation in their unfolded states as compared with G-tetrad folded topologies; this effect is confirmed upon comparative analysis of Tel22 (∼50% solution folded) and mutTel24 (∼90% solution folded). In an effort to identify the accessibly of hydroxyl radicals to quadruplex purine nucleobases, G-quadruplex solvent cavities were then modelled at 1.33 Å with evidence suggesting that folded G-tetrads may act as potential oxidant traps to protect against chromosomal DNA damage.