Resistance of DNA G-quadruplex against oxidative stress

Abstract

The existence of guanine quadruplex (GQ) in the genomic regions with biological significance is closely related to the inhibition of disease progression. Studies have found that GQs at the telomere overhang can inhibit the activity of overly expressed telomerase in the cancer cells. Furthermore, the formation of GQs at promoter regions of oncogenes is capable of regulating the transcription of malignant proteins in the diseased cell lines. The vulnerability of GQs towards oxidative damage is hence interesting to know as the motif concentrates twelve guanines, the genomic oxidation hotspot, into approximately 1 nm3 space. The survivability of the GQs under malignant/physiological ROS levels can be a key towards therapeutic methods against cancer. Furthermore, the rich topological structures distinguish the biological roles of the GQs. Hence, it is of great interests to explore whether and furthermore how the GQ topology would modulate their oxidative resistance. Reactive oxygen/nitrogen species (ROS/RNS) are constantly produced from normal cellular metabolism and inflammatory processes, and are key molecular triggers in cellular signalling pathways. Under pathological conditions such as cancer and cardiovascular diseases etc, oxidative stress, a hike in ROS level, often is presented in cellular compartments. In this thesis work, the oxidative damage in two G-rich genomic sequences, human telomere (HT) and the promoter of cmyc oncogene (cMYC), are interrogated by various ROS threats under both physiological and maligant levels. The resulting DNA damage on guanines and other nucleotides are revealed by alkaline labile strand cleavage and visualized on denature PAGE gel electrophoresis. For both HT and cMYC sequences, comparable guanine damages are observed from both the duplex structures and the GQs under physiological conditions. Folding into GQs does not increase the resistance against the ROS to the genomic DNA under interest comparing to the duplex when the oxidative stress is not stringent. Instead, GQs are found to be more chemically vulnerable under malignant conditions with high ROS levels. Guanine damage and total strand break ups are more pronounced in both GQ under high concentrations of peroxynitrite and hydroxyl radicals. The high vulnerability of GQ towards oxidative stresses suggest a possibility that acceleration of strand breaks in GQ due to oxidative damage may be a key to telomeric shortening and malfunction of oncogenic promoters. The results here may reveal a new biofunction of GQs as a damage hotspot under oxidative threats, and suggests a new anticancer therapeutic strategy as to target GQ for photodynamic therapy