Cellular Detection of G-Quadruplexes by Optical Imaging Methods

Template-Assembled Synthetic G-Quartets (TASQs): multiTASQing Molecular Tools for Investigating DNA and RNA G-Quadruplex Biology

Biomimetics is defined as a "practice of making technological design that copies natural processes", with the idea that "nature has already solved the challenges we are trying to solve" (Cambridge Dictionary). The challenge we decided to address several years ago was the selective targeting of G quadruplexes (G4s) by small molecules (G4 ligands). Why? Because G4s, which are four-stranded DNA and RNA structures that fold from guanine (G)-rich sequences, are suspected to play key biological roles in human cells and diseases. Selective G4 ligands can thus be used as small-molecule modulators to gain a deep understanding of cell circuitry where G4s are involved, thus complying with the very definition of chemical biology (Stuart Schreiber) applied here to G4 biology. How? Following a biomimetic approach that hinges on the observation that G4s are stable secondary structures owing to the ability of Gs to self-associate to form G quartets, and then of G quartets to self-stack to form the columnar core of G4s. Therefore, using a synthetic G quartet as a G4 ligand represents a unique example of biomimetic recognition of G4s.We formulated this hypothesis more than a decade ago, stepping on years of research on Gs, G4s, and G4 ligands. Our approach led to the design, synthesis, and use of a broad family of synthetic G quartets, also referred to as TASQs for template-assembled synthetic G quartets (John Sherman). This quest led us across various chemical lands (organic and supramolecular chemistry, chemical biology, and genetics), along a route on which every new generation of TASQ was a milestone in the growing portfolio of ever smarter molecular tools to decipher G4 biology. As discussed in this Account, we detail how and why we successively develop the very first prototypes of (i) biomimetic ligands, which interact with G4s according to a bioinspired, like-likes-like interaction between two G quartets, one from the ligand, the other from the G4; (ii) smart ligands, which adopt their active conformation only in the presence of their G4 targets; (iii) twice-as-smart ligands, which act as both smart ligands and smart fluorescent probes, whose fluorescence is triggered (turned on) upon interaction with their G4 targets; and (iv) multivalent ligands, which display additional functionalities enabling the detection, isolation, and identification of G4s both in vitro and in vivo. This quest led us to gather a panel of 14 molecular tools which were used to investigate the biology of G4s at a cellular level, from basic optical imaging to multiomics studies.

Selectivity of Terpyridine Platinum Anticancer Drugs for G-quadruplex DNA

Guanine-rich DNA can form four-stranded structures called G-quadruplexes (G4s) that can regulate many biological processes. Metal complexes have shown high affinity and selectivity toward the quadruplex structure. Here, we report the comparison of a panel of platinum (II) complexes for quadruplex DNA selective recognition by exploring the aromatic core around terpyridine derivatives. Their affinity and selectivity towards G4 structures of various topologies have been evaluated by FRET-melting (Fluorescence Resonance Energy Transfert-melting) and Fluorescent Intercalator Displacement (FID) assays, the latter performed by using three different fluorescent probes (Thiazole Orange (TO), TO-PRO-3, and PhenDV). Their ability to bind covalently to the c-myc G4 structure in vitro and their cytotoxicity potential in two ovarian cancerous cell lines were established. Our results show that the aromatic surface of the metallic ligands governs, in vitro, their affinity, their selectivity for the G4 over the duplex structures, and platination efficiency. However, the structural modifications do not allow significant discrimination among the different G4 topologies. Moreover, all compounds were tested on ovarian cancer cell lines and normal cell lines and were all able to overcome cisplatin resistance highlighting their interest as new anticancer drugs.