Oxidized purines: From RNA interference to DNA cross-links.

摘要

The low redox potentials of guanosine (G) and adenosine (A) make them especially susceptible to oxidation. The most common oxidized lesion, 7,8-dihydro-8--oxoguanosine (8-OxoG or OG), has been widely studied and has an estimated frequency of one in every 106 Gs in normal cellular DNA. 7,8-Dihydro-8-oxoadenosine (8-OxoA or OA) is less common, but is still found in healthy cells. The abundance of OA in the cell increases with age and particularly high levels are found in prostate and breast cancer cells.;During the course of studying OA oxidation, an interesting result was obtained. After investigation it was shown that in certain sequences OA forms both inter- and intrastrand DNA cross-links under oxidative conditions. DNA interstrand cross-links (ICLs) are extremely harmful to cells due to their ability to block replication and transcription of DNA. In fact, a single unrepaired ICL is enough to kill a eukaryotic cell. The discovery that OA is able to form such cross-links in significant yields is of great biological significance.;The ability of OG to form both a Watson-Crick base pair with cytosine or a Hoogsteen base pair with adenosine can be exploited by using OG as a base pair switch to avoid off-target effects in RNAi. An N 2 steric blockade can be flipped from the minor to the major groove by using appropriate base pairing, thus avoiding interactions with proteins that are not part of the RNAi pathway. Preliminary results in this area are the first example of efficient RNAi knockdown using a non-Watson-Crick base pair and show that an OG:A base pair can be accommodated in several locations, including adjacent to the mRNA cleavage site and in the seed region. The immunostimulatory effects of OG-containing duplexes are also investigated.;The redox potentials of OG and OA are even lower than those of their parent nucleobases, allowing for further oxidation to occur at these lesions. The reactive nature of OG and OA can have both positive and negative consequences. During oxidation, OA can react with solvent water or with added amines. The addition of polyamines at C2 of OA would result in adducts that could prove useful in therapeutic RNA interference (RNAi).