Protein and Ligand Interactions of MYC Promoter G-quadruplex

G-quadruplexes (G4s) are non-canonical secondary structures formed in single-stranded guanine-rich nucleic acid sequences, such as those found in oncogene promoters and telomeres. MYC, one of the most critical oncogenes, has a DNA G4 (MycG4) in its proximal promoter region that functions as a transcriptional silencer. MycG4 is very stable and the pathological activation of MYC requires its active unfolding. However, it remains unclear what drives MycG4 unfolding in cancer cells. We have studied the interactions of DDX5 with the MycG4 at both molecular and cellular levels and discovered that DDX5 actively unfolds the MycG4 and is involved in the MYC gene transcriptional regulation, which is described in the first part of this dissertation. DDX5 is extremely proficient at unfolding the MycG4 and ATP hydrolysis is not directly coupled to the G4-unfolding of DDX5. In cancer cells, DDX5 is enriched at the MYC promoter and activates MYC transcription. G4-interactive small molecules inhibit the DDX5 interaction with the MYC promoter and DDX5-mediated MYC activation. The second part of this dissertation describes the study of interactions of indenoisoquinoline anticancer drugs with MycG4. The MycG4 transcriptional silencer is a very attractive therapeutic target. Compounds that bind and stabilize the MycG4 have been shown to repress MYC gene transcription and are antitumorigenic. Indenoisoquinolines are human topoisomerase I inhibitors in clinical testing. However, some indenoisoquinolines with potent anticancer activity do not exhibit strong topoisomerase I inhibition, suggesting a separate mechanism of action. Our studies show that indenoisoquinolines strongly bind and stabilize MycG4 and lower MYC levels in cancer cells. Moreover, the analysis of indenoisoquinoline analogues for their MYC inhibitory activity, topoisomerase I inhibitory activity, and anticancer activity reveals a synergistic effect of MYC inhibition and topoisomerase I inhibition on anticancer activity. Besides the MycG4, human telomeric G4s are also attractive targets for anticancer drugs due to their ability to inhibit telomere extension in cancer cells. The last part of this dissertation reviews two recent solution structural studies on small molecule complexes with the hybrid-2 telomeric G4 and the hybrid-1 telomeric G4. Structural information of those complexes can advance the design of telomeric G4-interactive small molecules in the cancer therapeutic areas.