Functions of Ku and WRN in DNA repair
Half of cancer patients receive radiotherapy treatment at some point. One of the main effects of this treatment is to generate
DNA double-strand breaks in irradiated cells,
toxic damage that can lead to cell death.
Understanding the
molecular mechanisms of repair of this damage is
essential for designing specific inhibitors. The "Nuclear envelope, Telomeres and DNA repair" team at
I2BC has been studying the
Ku70/80 heterodimer (Ku), the first factor to bind to DNA breaks to initiate their repair (Non-homologous end-joining, NHEJ). In collaboration with teams from the University of Leicester and IPBS (Toulouse), they have characterized super-complexes formed during these early stages of NHEJ repair (DNA repair machinery more flexible thanks to PAXX?).
The
Werner protein or WRN is also involved in these steps. It is a
helicase-type enzyme: it is capable of separating the two strands of a DNA double helix, a bit like a zipper.
WRN allows the creation of certain replication forks, especially when they are blocked. It is
the only human helicase to possess a 3' -> 5' exonuclease activity, meaning it is capable of degrading DNA from its 3' end. Dysfunction of the WRN protein is responsible for
Werner syndrome, a rare genetic disease characterized by premature aging associated with a high risk of cancer and reduced life expectancy.
WRN interacts with the Ku70/80 heterodimer (Ku). This interaction stimulates WRN's exonuclease activity, but the molecular basis of the interaction is unknown.
Structural model of Ku:ADN:WRN complex
In a
recent study published in Nature Communications, the teams from I2BC and the University of Leicester present a
high-resolution cryo-EM structure (3.3Å resolution) of human Ku bound to DNA in complex with the WRN exonuclease domain. This structure reveals several interaction sites between WRN and the Ku:DNA complex.
Surprisingly, the researchers highlight that
a domain of Ku70 (SAP) is stabilized within this complex, and they identify specific contacts mediating this interaction. These
interaction surfaces have been validated (through collaboration with IPBS and ISS (Rome, Italy)) by evaluating the impact of point mutations at the Ku-WRN interfaces:
- By biochemical studies
measuring the exonuclease activity of purified recombinant proteins;
-
By cellular studies on the recruitment of WRN to nuclear sites damaged by laser biphoton micro-irradiation.
Finally, the researchers demonstrate that disruption of the WRN-Ku70 interaction leads to aberrant resection of blocked replication forks.
The prospects of this work include extending the characterization of the molecular mechanisms of the NHEJ repair pathway by including other actors and characterizing mini-proteins that inhibit this pathway.
Contacts at Frédéric-Joliot Institute for Life sciences / I2BC :
Jean-Baptiste Charbonnier (jb.charbonnier@i2bc.paris-saclay.fr)
This text was translated
with the assistance of Mistral AI.