ABSTRACT
Figure 2. Schematic diagram of double-strand break repair mechanisms. (A) In nonhomologous end-joining, NHEJ, double-strand breaks (DSBs) are recognized by the heterodimer Ku70/Ku80 (Ku). These DNA end binding proteins recruit the protein kinase DNA-PKcs, which in turn phosphorylates and recruits other proteins. When the DNA ends are incompatible for ligation, exo-or endonucleases are recruited to modify the ends; shown here are WRN, FEN1, Artemis and TDP1. Next, a DNA polymerase fi lls in any recessed ends,then LIG4 in complex with XRCC4 and XLF seals the nick. NHEJ is the predominate DSB repair pathway used in human cells and is available in G1, S and G2 of the cell cycle. (B) Alternative NHEJ, Alt-NHEJ, is employed when NHEJ is compromised. DSBs are thought to be recognized by PARP1 and/or the MRN complex. The distinguishing feature of this repair pathway is that end resectioning occurs until short stretches of homology (5-25 nucleotides) are found. The fl aps are removed, DNA synthesis fi lls in missing nucleotides and then Ligase 3 (LIG3) seals the nick. (C) In homologous recombination (HR), the major damage recognition player is the MRN complex. Among other functions, BRCA1 activates the DNA damage response to induce cell cycle arrest following DSB formation. MRN, CtIP, EXO1, DNA2 and BLM may all function to resect the DNA and generate 3’ single-stranded tails. These tails are then bound by RPA and RAD51 fi laments. RAD51 recombinase searches for homology within another homologous strand of DNA, preferentially its sister chromatid. RAD52 and RAD54 promote these processes. DNA synthesis copies the DNA off the sister chromatid, then Holliday junctions (HJ) are resolved using proteins like GEN1, Mus81-EME1 or the BTR complex, consisting of BLM, topoisomerase 3α, RMI1 and RMI2. Finally, DNA ligase, LIG1, ligates the DNA ends to restore DNA integrity. HR is only operable during S and G2 phases of the cell cycle.
