G-quadruplexes in human promoters: A challenge for therapeutic applications

Background: G-rich sequences undergo unique structural equilibria to form G-quadruplexes (G4) both in vitro and in cell systems. Several pathologies emerged to be directly related to G4 occurrence at defined genomic portions. Additionally, G-rich sequences are significantly represented around transcription start sites (TSS) thus leading to the hypothesis of a gene regulatory function for G4. Thus, the tuning of G4 formation has been proposed as a new powerful tool to regulate gene expression to treat related pathologies. However, up-to date this approach did not provide any new really efficient treatment. Scope of review: Here, we summarize the most recent advances on the correlation between the structural features of G4 in human promoters and the role these systems physiologically exert. In particular we focus on the effect of G4 localization among cell compartments and along the promoters in correlation with protein interaction networks and epigenetic state. Finally the intrinsic structural features of G4 at promoters are discussed to unveil the contribution of different G4 structural modules in this complex architecture. Major conclusions: It emerges that G4s play several roles in the intriguing and complex mechanism of gene expression, being able to produce opposite effects on the same target. This reflects the occurrence of a highly variegate network of several components working simultaneously. General significance: The resulting picture is still fuzzy but some points of strength are definitely emerging, which prompts all of us to strengthen our efforts in view of a selective control of gene expression through G4 modulation