The Relevance of G-Quadruplexes for DNA Repair.

DNA molecules can adopt a variety of alternative structures. Among these structures are G-quadruplex DNA structures (G4s), which support cellular function by affecting transcription, translation, and telomere maintenance. These structures can also induce genome instability by stalling replication, increasing DNA damage, and recombination events. G-quadruplex-driven genome instability is connected to tumorigenesis and other genetic disorders. In recent years, the connection between genome stability, DNA repair and G4 formation was further underlined by the identification of multiple DNA repair proteins and ligands which bind and stabilize said G4 structures to block specific DNA repair pathways. The relevance of G4s for different DNA repair pathways is complex and depends on the repair pathway itself. G4 structures can induce DNA damage and block efficient DNA repair, but they can also support the activity and function of certain repair pathways. In this review, we highlight the roles and consequences of G4 DNA structures for DNA repair initiation, processing, and the efficiency of various DNA repair pathways.

UV-induced G4 DNA structures recruit ZRF1 which prevents UV-induced senescence.

Senescence has two roles in oncology: it is known as a potent tumor-suppressive mechanism, which also supports tissue regeneration and repair, it is also known to contribute to reduced patient resilience, which might lead to cancer recurrence and resistance after therapy. Senescence can be activated in a DNA damage-dependent and -independent manner. It is not clear which type of genomic lesions induces senescence, but it is known that UV irradiation can activate cellular senescence in photoaged skin. Proteins that support the repair of DNA damage are linked to senescence but how they contribute to senescence after UV irradiation is still unknown. Here, we unraveled a mechanism showing that upon UV irradiation multiple G-quadruplex (G4) DNA structures accumulate in cell nuclei, which leads to the recruitment of ZRF1 to these G4 sites. ZRF1 binding to G4s ensures genome stability. The absence of ZRF1 triggers an accumulation of G4 structures, improper UV lesion repair, and entry into senescence. On the molecular level loss of ZRF1 as well as high G4 levels lead to the upregulation of DDB2, a protein associated with the UV-damage repair pathway, which drives cells into senescence.