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G-quadruplex ligands break DNA through trapping Topoisomerase 2 on site

In this study published in the journal eLife, IPBS scientists unveil the mechanism of cytotoxicity of two ligands of particular structures adopted by DNA, the G-quadruplexes (G4). Using original genetic and molecular approaches they demonstrate that two ligands stabilizing these structures cause DNA breaks via trapping Topoisomerase 2 alpha at G4 sites.

 

DNA can adopt non-canonical structures, different from the double helix. Among these structures, G-quadruplexes (G4) are four-stranded structures formed by guanine-rich sequences. G4s control replication, oncogene transcription and telomerase activity, three key processes in tumorigenesis. Thus, G4s are considered as potential anti-cancer therapeutic targets. They have been the subject of particular interest since the beginning of the 2000s, notably thanks to the development of small molecules, G4 ligands, which bind and stabilize them within cells.  It was known that some G4 ligands induce DNA breaks, explaining their cytotoxic effects, but the mechanisms of formation of these breaks remained enigmatic.

A recent study from IPBS has clarified how G4 ligands induce DNA breaks. It is based on the selection and RNA-sequencing of cells resistant to the cytotoxic activity of G4 ligands. This approach revealed that G4 ligand-induced breaks are transcription-dependent and are induced by topoisomerase 2 alpha (TOP2A). TOP2A is an essential protein, overexpressed in many cancers, which resolves DNA supercoiling and catenation provoked by transcription and DNA replication. During this process, TOP2A cuts the DNA tangles by inducing a transient double-strand break that is masked and immediately repaired once the DNA is unraveled. The scientists found that G4 ligands block TOP2A activity at the stage of closure of these transient breaks, acting as a topoisomerase "poison" in the same way as other molecules currently used in cancer therapy. However, unlike the latter, G4 ligands selectively trap TOP2A at the level of G4 structures, acting by a dual mechanism. This work has thus elucidated the activity of the molecule CX-5461, currently under clinical evaluation as an anticancer agent.

A parallel study, from the same team reveals that the NELF complex associates with G4. This complex is normally involved in the transient arrest of transcription at so-called "pause" sites. Suspecting that this association with G4 had a functional impact, the authors found that the NELF complex is required for the induction of DNA breaks by G4 ligands, indicating that these breaks are related to the arrest of the transcription machinery at break sites.

These two studies therefore provide a better understanding of the mechanisms that determine the anticancer efficacy of G4 ligands and could help guide potential therapeutic strategies.

Stabilization of the G-quadruplex structures formed during transcription causes DNA double-strand breaks induced by TOP2A activity by a mechanism that involves inhibition of its religation activity. This high-resolution microscopy image shows the co-localization (yellow) of TOPO2A cleavage complexes (red) with G-quadruplex structures (green) within human cells. © Dennis Gomez

Reference

Transcription-associated topoisomerase 2α activity is a major effector of cytotoxicity induced by G-quadruplex ligands.

Bossaert M., Pipier A., Riou J.F., Noirot C., Nguyễn L-T., Serre R-F., Bouchez O., Defrancq E., Calsou P., Britton S., Gomez D.

eLife June 28 2021; DOI: 10.7554/eLife.65184

Contact

Researchers IPBS: Dennis Gomez | Dennis.Gomez@ipbs.fr |  +33 (0)5 61 17 59 60

Sébastien Britton | Sebastien.Britton@ipbs.fr | +33 (0)5 61 17 59 07
Press IPBS: Francoise Viala | communication@ipbs.fr | +33 (0)5 61 17 52 59