DNA double strand break repair via non-homologous end-joining

Abstract

DNA double-stranded breaks (DSB) are among the most dangerous forms of DNA damage. Unrepaired DSBs results in cells undergoing apoptosis or senescence whereas mis-processing of DSBs can lead to genomic instability and carcinogenesis. One important pathway in eukaryotic cells responsible for the repair of DSBs is non-homologous end-joining (NHEJ). In this review we will discuss the interesting new insights into the mechanism of the NHEJ pathway and the proteins which mediate this repair process. Furthermore, the general role of NHEJ in promoting genomic stability will be discussed.

Keywords: DNA double strand breaks; DNA-Ligase IV; DNA-PKcs; Ku70/80; XLF; XRCC4; non-homologous end-joining.

Figure 1

DNA end recognition and stabilization of the core NHEJ factors. A. DNA double strand break (DSB) is induced; B. Ku quickly binds to the ends of the broken DNA molecule; C. Ku serves as a scaffold to recruit the core NHEJ machinery to the DNA DSB. DNA-PKcs, XRCC4, XLF, DNA Ligase IV, and APLF are independently recruited to the Ku-DNA complex; D. The core NHEJ factors interact with each other to form a stable complex at the DSB

Figure 2

DNA end processing. If required, a number of different processing enzymes can be utilized for making the DNA ends compatible for ligation for the terminal step of the NHEJ pathway; A. XRCC4, likely in conjunction with the Ku heterodimer, may serve as a scaffold required for the recruitment of specific DNA end processing enzymes. The XRCC4-Ku scaffold likely plays a role as a tool-belt where it can choose which enzyme is required depending on the nature of the DSB; B. Complex DNA damage may be processed via the DNA-PKcs-mediated recruitment of the nuclease Artemis and the processing of the complex damage may require ATM; C. Processing of the DNA ends makes them compatible for ligation

Figure 3

Bridging of the DNA ends and promotion of end stability. A. Ku binding to the DSB protects and stabilizes the DNA ends; B. DNA-PKcs can tether DNA ends via formation of a synaptic complex; C. XRCC4 and XLF can produce a filament which can bridge the two portions of the broken DNA molecule and possibly stabilize them; D. XRCC4-XLF filament possibly in complex with DNA-PKcs and Ku to produce a tightly regulated DNA end protection complex

Figure 4

Ligation of the DNA ends and dissolution of the NHEJ complex. A. Autophosphorylation and/or phosphorylation by ATM mediate a conformational change of DNA-PKcs resulting in the opening of the pincers of DNA-PKcs resulting in its release from the DSB. It is not known if this occurs before or after the terminal ligation step; B. Ligation of the DNA ends via DNA-Ligase IV. The terminal ligation step is aided by XRCC4, XLF, and likely APLF; C. Ku is likely stuck on the DNA molecule following the ligation step. Ku may be released from the repaired DSB via ubiquitylation via either Rnf8 or the SCF complex of Skp1-Cul1-Fbx112 and results in degradation of Ku; D. The DNA DSB is repaired