DDX5 helicase resolves G-quadruplex and is involved in MYC gene transcriptional activation

Significance

G-quadruplexes (G4) are 4-stranded nucleic acid secondary structures. MYC is a critical oncogene with a G4 in its promoter (MycG4), which acts as a transcription silencer. MycG4 is very stable and the pathological activation of MYC in cancers requires its active unfolding. We reveal herein that DDX5, a founding member of the DEAD-box RNA helicase family, can unfold DNA G4s. The unfolding mechanism of DDX5 is distinct from previously characterized G4 helicases. Importantly, DDX5 activates MYC gene transcription by proficiently unfolding the promoter MycG4. DDX5 is overexpressed in cancers and the DDX5–MycG4 interaction can be inhibited by small molecules to downregulate MYC. Therefore, our results suggest a new molecular target to suppress MYC for cancer intervention.

Abstract

G-quadruplexes (G4) are noncanonical secondary structures formed in guanine-rich DNA and RNA sequences. MYC, one of the most critical oncogenes, forms a DNA G4 in its proximal promoter region (MycG4) that functions as a transcriptional silencer. However, MycG4 is highly stable in vitro and its regulatory role would require active unfolding. Here we report that DDX5, one of the founding members of the DEAD-box RNA helicase family, is extremely proficient at unfolding MycG4-DNA. Our results show that DDX5 is a highly active G4-resolvase that does not require a single-stranded overhang and that ATP hydrolysis is not directly coupled to G4-unfolding of DDX5. The chromatin binding sites of DDX5 are G-rich sequences. In cancer cells, DDX5 is enriched at the MYC promoter and activates MYC transcription. The DDX5 interaction with the MYC promoter and DDX5-mediated MYC activation is inhibited by G4-interactive small molecules. Our results uncover a function of DDX5 in resolving DNA and RNA G4s and suggest a molecular target to suppress MYC for cancer intervention.