24 September 2021 Unwinding DNA ‘knots’ Back to news Researchers from the group of Puck Knipscheer discovered a new way in which cells suppress abnormal DNA structures that can cause mutations in our genetic material. Their work uncovers a detailed mechanism that ensures robust unwinding of the so-called G-quadruplex structures. This intricate mechanism involves the sequential action of two proteins, FANCJ and DHX36, and safeguards genome stability. The results were published in Science Advances on the 24th of September. The genomic DNA in the nuclei of our cells is generally folded into a double helix from which the information is read during transcription and duplicated during DNA replication. During these processes, specific guanine-rich DNA sequences can fold into abnormal four-stranded structures called G-quadruplexes or G4 structures. Our genome contains more than half a million sites that can form these structures. G4 structures resemble ‘knots’ in our genome and must be resolved because they can cause problems during DNA replication and transcription. Unsuccessful unwinding of these structures causes DNA damage and mutations, and can consequently lead to accelerated aging and cancer. Unknown mechanisms Normal cells must have very efficient mechanisms that counteract G4 structures because G4 sequences have been maintained during evolution. Consistent with this, more than ten proteins have been identified that are able to unwind these structures in purified form. However, virtually nothing is known about the mechanisms that resolve G4 structures during DNA replication or transcription. Top: 3D structure of a G-quadruplex as determined by NMR (PDBID:2LK7, Do & Phan, Chemistry, 2012). Bottom: Mechanism of DNA replication-coupled G4 unwinding. Unwinding during DNA duplication Researchers from the group of Puck Knipscheer studied the mechanism of G4 structure unwinding during DNA replication, the process that duplicates the genome before a cell can divide. They used protein extracts made from the eggs of the African clawed frog (Xenopus laevis). This is a unique model system that supports both DNA replication and G-quadruplex unwinding under physiological conditions in a test tube. This allows the researchers to dissect the molecular mechanism. They discovered that DNA replication triggers robust G4 structure unwinding. This involves two proteins, FANCJ and DHX36, that both contain the activity to unwind G4 structures. However, in this mechanism they act in a sequential manner in which only FANCJ directly unwinds the G4 structure. DHX36 instead ensures the removal of part of the DNA replication machinery before FANCJ can act and thereby couples DNA replication to G4 unwinding. Interestingly, when one of these proteins is absent, the other can take over its role, which makes this mechanism highly robust. Aid for treatments In this study, the researchers provide the first comprehensive mechanism of G4 structure unwinding. This is an important genome protective mechanism, that complements other DNA repair pathways, to keep our genetic material free of mutations and prevent cancer. Paradoxically, when stable G4 structures are formed in large excess, for example upon addition of G4 stabilizing ligands, they can induce high doses of DNA damage which can kill cells. Consistent with this, G4 ligands have been shown to selectively kill familial breast/ovarian cancer cells with a defect in DNA repair, indicating G4 stabilization is a promising target for cancer treatment. A better understanding of this mechanism would aid the development of such treatments. ~~~ Publication Sato, N. Martin-Pintado, H. Post, M. Altelaar, and P. Knipscheer. Multistep mechanism of G-quadruplex resolution during DNA replication. Science Advances (2021). Puck Knipscheer is group leader at the Hubrecht Institute and Oncode Investigator.
