Abstract
Humans have evolved a series of DNA double-strand break (DSB) repair pathways to efficiently and accurately rejoin nascently formed pairs of double-stranded DNA ends (DSEs). In G0/G1-phase cells, non-homologous end joining (NHEJ) and alternative end joining (A-EJ) operate to support covalent rejoining of DSEs. While NHEJ is predominantly utilized and collaborates extensively with the DNA damage response (DDR) to support pairing of DSEs, much less is known about A-EJ collaboration with DDR factors when NHEJ is absent. Non-cycling lymphocyte progenitor cells use NHEJ to complete V(D)J recombination of antigen receptor genes, initiated by the RAG1/2 endonuclease which holds its pair of targeted DSBs in a synapse until each specified pair of DSEs is handed off to the NHEJ DSB sensor complex, Ku. Similar to designer endonuclease DSBs, the absence of Ku allows for A-EJ to access RAG1/2 DSEs but with random pairing to complete their repair. Here, we describe recent insights into the major phases of DSB end joining, with an emphasis on synapsis and tethering mechanisms, and bring together new and old concepts of NHEJ vs. A-EJ and on RAG2-mediated repair pathway choice.
Highlights
Division of Radiation and Cancer Biology, Department of Radiation Oncology, Abstract: Humans have evolved a series of DNA double-strand break (DSB) repair pathways to efficiently and accurately rejoin nascently formed pairs of double-stranded DNA ends (DSEs)
Chromatin-modified DNA damage signals remain despite non-homologous end joining (NHEJ) ligation until Ku is extracted by p97 [155], clearly demonstrating that Ku extraction must first occur before chromatin damage signals are reverted
Hybrid joins would seemingly be the result of impaired MR complex DNA end tethers while in association with the Recombination-Activating Gene (RAG) post-cleavage synaptic complex (PSC) and is consistent with an NHEJ functional redundancy that serves to maintain DSE pairing (Figure 2); whether such ends are repaired by aberrant NHEJ or by
Summary
As a “rapid response” DSB repair pathway [37], NHEJ can complete ligation of compatible broken ends in minutes [9]. All NHEJ reactions require the KU70 and KU80 (“Ku”) DSE sensing complex and the XRCC4/Ligase IV ligation complex, which are considered core components to the NHEJ reaction (Figure 1) Their essentiality is best evidenced by evolutionary conservation down to yeast and in some bacterial and archaeal species [11,44], the greatest sensitivity to ionizing radiation (IR) when absent in cells, and their requisite role to complete V(D)J recombination [45,46,47,48,49,50,51,52]. Binding of XLF, XRCC4 and Ligase IV induces conformation changes to strengthen synapsis of paired DSEs. Intervention: hyper trans autophosphorylation induces release of DNA-PKcs and engagement for end compatibility using a toolkit of end-processing enzymes. Ku is removed from DNA by the ATPase VCP/p97
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