The pendulum of the Ku-Ku clock.

Area of Science:

• Molecular Biology
• Genetics
• Biochemistry

Background:

• Canonical DNA non-homologous end-joining (c-NHEJ) and homologous recombination (HR) are the primary pathways for repairing DNA double-strand breaks (DSBs).
• Ku protein, abundant in human cells, binds DSBs and typically blocks end resection, a process crucial for HR.
• The interplay between Ku binding, end resection, and DSB repair pathway choice remains an area of active investigation.

Purpose of the Study:

• To review the current understanding of how DNA end resection occurs at DSBs despite the presence of end-binding proteins like Ku.
• To elucidate the role of Ku in determining the choice between c-NHEJ and HR pathways.
• To highlight similarities and differences in resection processes that facilitate optimal repair pathway selection.

Main Methods:

• Review of existing literature on DNA double-strand break repair mechanisms.
• Analysis of studies involving DSBs generated by Spo11 or TOP2, which create protein-bridged lesions.
• Examination of the roles of key proteins such as Ku, MRE11-RAD50-NBS1 (MRN) complex, CtIP, and EXO1 in resection and repair.

Main Results:

• Ku binding to DSBs can act as a block to resection, influencing repair pathway choice.
• Resection-dependent c-NHEJ repairs a subset of DSBs, challenging the traditional view of c-NHEJ as resection-independent.
• HR requires end resection for repairing DSBs, particularly one-ended DSBs arising from replication stress.
• Proteins like MRN, CtIP, and EXO1 are critical for removing end-blocking lesions, including those caused by Ku or protein-bridged DSBs, to enable resection.

Conclusions:

• Ku plays an active role in directing DSBs to either c-NHEJ or HR pathways.
• The process of resection is more nuanced than previously thought, occurring even in the presence of Ku and enabling different repair outcomes.
• Understanding these mechanisms is key to comprehending genome stability and developing targeted therapies.