Box 2 | Repair of DNA double-strand breaks

The repair of DNA double-strand breaks^{10, 11, 49, 50, 64} (DSBs) is carried out by two different mechanisms: a rapid, error-prone mechanism dubbed 'non-homologous end-joining' (NHEJ) that quickly seals the breaks at the expense of creating local microdeletions, and a high-fidelity repair process based on homologous recombination (HR) between sister chromatids. These highly structured processes stem from the concerted action of several multi-protein complexes.

The predominant repair mode in mammalian cells is NHEJ. The exposed ends of the DNA strands are detected by the KU70–KU80 heterodimer that recruits the catalytic subunit of the DNA-dependent protein kinase (DNA-PKcs). DNA-PKcs, in turn, might recruit additional proteins to the damaged site and probably phosphorylates some of them. The XRCC4–ligase IV heterodimer finally seals the breaks. Despite the disadvantage of its low fidelity, this pathway can act quickly, as required of an emergency mechanism, and, unlike HR, it does not depend on sister DNA molecules, which exist in the cells only after DNA replication. NHEJ is also important in sealing the breaks formed during V(D)J recombination, in which the T-cell receptor and immunoglobulin genes are rearranged¹⁶⁹.

The high-fidelity, HR-based pathway is mediated by the RAD51-associated proteins that include several RAD51 paralogues, RAD52, the RAD54 helicase and the BRCA2 tumour suppressor. Following initial end resection and binding of RAD52 to the single-stranded end, RAD51 forms a nucleoprotein filament on the exposed strand. This process is probably initiated by a RAD51–BRCA2 complex^{170, 171} and is essential for the main step in this pathway: strand invasion and strand displacement. This is mediated by RAD51 and RAD54, and allows use of the undamaged sister molecule as a template for the resynthesis of the missing portions in the broken molecule.

The MRE11–RAD50–NBS1 (MRN) complex (reviewed by D'Amours and Jackson⁶⁴) seems to be essential for the HR pathway¹⁷², probably by carrying out the initial processing of the DSB ends, and is also involved in meiotic recombination, telomere maintenance and checkpoint signalling. Following DSB induction, the MRN complex rapidly forms prominent foci at the damaged sites. These foci include additional players in the DSB response, such as RAD51 and BRCA1. MRE11 is a nuclease; RAD50 is an ATPase; and the NBS1 protein, which is essential for the assembly of the complex, interacts with histone H2AX¹⁷³ and is further involved specifically in the HR pathway¹⁷².