Differentiated oxidation modes of guanine between CpG and ^5mCpG by a photoactivatable Pt(IV) anticancer prodrug

CpG and its cytosine-methylated counterpart (^5mCpG) are a unique reversible pair of sequences in regulating the expression of genes epigenetically. As DNA is the potential target of Pt-based anticancer metallodrugs, herein, we comparatively investigate the interactions of 5'-CpG and 5'-^5mCpG with a photoactivatable anticancer Pt(IV) prodrug, trans,trans,trans-[Pt^IV(N₃)₂(OH)₂(py)₂] (1; py = pyridine), to explore the effects of methylation on the platination and ROS-induced oxidation of the CpG motif. Mono-platinated dinucleotides were demonstrated by ESI-MS to be the main products for both 5'-CpG and 5'-^5mCpG with the bound Pt moiety as [Pt^II(N₃)(py)₂] generated by the photodecomposition of complex 1 under irradiation with blue light, accompanied by the formation of less abundant di-platinated adducts. G-N7 and C-N3/^5mC-N3 were shown to be the major and minor platination sites, respectively, with G-N1 as the third and weakest platination site, in particular, in di-platinated products. Moreover, platinated dinucleotides associated with guanine and/or cytosine oxidation were also observed. Apart from 8-oxo-guanine (^oxG) and N-formylamidoiminohydantoin (RedSp) reported previously, novel oxidation adducts 5-guanidinohydantoin (Gh) derived from guanine and 1-carbamoyl-4,5-dihydroxy-2-oxoimidazolidine (ImidCyt) derived from cytosine in CpG, and diimino imidazole (DIz) and 2,5-diaminoimidazol-4-one (imidazolone, Iz) derived from guanine and Imid^5mCyt derived from ^5mC in ^5mCpG were proposed according to MS information. These results showed that methylation exerted little effects on the platination modes of CpG, but triggered distinct oxidation pathways of CpG, perhaps causing discriminated DNA damage to CpG-rich genes. This work provides novel insights into the role of the anticancer photoactivatable Pt(IV) prodrug through damaging the epigenetically modified DNA sequences.