The nonhomologous end joining (NHEJ) pathway is initiated in response to the formation of DNA double-strand breaks (DSBs) induced by DNA-damaging agents, such as ionizing radiation. DNA DSBs are recognized by the MRN complex (MRE11A:RAD50:NBN), leading to ATM activation and ATM-dependent recruitment of a number of DNA damage checkpoint and repair proteins to DNA DSB sites (Lee and Paull 2005). The ATM phosphorylated MRN complex, MDC1 and H2AFX-containing nucleosomes (gamma-H2AX) serve as scaffolds for the formation of nuclear foci known as ionizing radiation induced foci (IRIF) (Gatei et al. 2000, Paull et al. 2000, Stewart et al. 2003, Stucki et al. 2005). Ultimately, both BRCA1:BARD1 heterodimers and TP53BP1 (53BP1) are recruited to IRIF (Wang et al. 2007, Pei et al. 2011, Mallette et al. 2012), which is necessary for ATM-mediated CHEK2 activation (Wang et al. 2002, Wilson et al. 2008). In G1 cells, TP53BP1 promotes NHEJ by recruiting RIF1 and PAX1IP, which displaces BRCA1:BARD1 and associated proteins from the DNA DSB site and prevents resection of DNA DSBs needed for homologous recombination repair (HRR) (Escribano-Diaz et al. 2013, Zimmermann et al. 2013, Callen et al. 2013). TP53BP1 also plays an important role in ATM-mediated phosphorylation of DCLRE1C (ARTEMIS) (Riballo et al. 2004, Wang et al. 2014). Ku70:Ku80 heterodimer (also known as the Ku complex or XRCC5:XRCC6) binds DNA DSB ends, competing away the MRN complex and preventing MRN-mediated resection of DNA DSB ends (Walker et al. 2001, Sun et al. 2012). The catalytic subunit of the DNA-dependent protein kinase (DNA-PKcs, PRKDC) is then recruited to DNA-bound Ku to form the DNA-PK holoenzyme. Two DNA-PK complexes, one at each side of the break, bring DNA DSB ends together, joining them in a synaptic complex (Gottlieb 1993, Yoo and Dynan 2000). DNA-PK complex recruits DCLRE1C (ARTEMIS) to DNA DSB ends (Ma et al. 2002). PRKDC-mediated phosphorylation of DCLRE1C, as well as PRKDC autophosphorylation, enables DCLRE1C to trim 3'- and 5'-overhangs at DNA DSBs, preparing them for ligation (Ma et al. 2002, Ma et al. 2005, Niewolik et al. 2006). The binding of inositol phosphate may additionally stimulate the catalytic activity of PRKDC (Hanakahi et al. 2000). Other factors, such as polynucleotide kinase (PNK), TDP1 or TDP2 may remove unligatable damaged nucleotides from 5'- and 3'-ends of the DSB, converting them to ligatable substrates (Inamdar et al. 2002, Gomez-Herreros et al. 2013). DNA ligase 4 (LIG4) in complex with XRCC4 (XRCC4:LIG4) is recruited to ligatable DNA DSB ends together with the XLF (NHEJ1) homodimer and DNA polymerases mu (POLM) and/or lambda (POLL) (McElhinny et al. 2000, Hsu et al. 2002, Malu et al. 2002, Ahnesorg et al. 2006, Mahajan et al. 2002, Lee et al. 2004, Fan and Wu 2004). After POLL and/or POLM fill 1- or 2-nucleotide long single strand gaps at aligned DNA DSB ends, XRCC4:LIG4 performs the ligation of broken DNA strands, thus completing NHEJ. The presence of NHEJ1 homodimer facilitates the ligation step, especially at mismatched DSB ends (Tsai et al. 2007). Depending on other types of DNA damage present at DNA DSBs, NHEJ can result in error-free products, produce dsDNA with microdeletions and/or mismatched bases, or result in translocations (reviewed by Povrik et al. 2012).
View original pathway at Reactome.
Poinsignon C, de Chasseval R, Soubeyrand S, Moshous D, Fischer A, Haché RJ, de Villartay JP.; ''Phosphorylation of Artemis following irradiation-induced DNA damage.''; PubMedEurope PMCScholia
Nick McElhinny SA, Snowden CM, McCarville J, Ramsden DA.; ''Ku recruits the XRCC4-ligase IV complex to DNA ends.''; PubMedEurope PMCScholia
Dynan WS, Yoo S.; ''Interaction of Ku protein and DNA-dependent protein kinase catalytic subunit with nucleic acids.''; PubMedEurope PMCScholia
Hsu HL, Yannone SM, Chen DJ.; ''Defining interactions between DNA-PK and ligase IV/XRCC4.''; PubMedEurope PMCScholia
Tsai CJ, Kim SA, Chu G.; ''Cernunnos/XLF promotes the ligation of mismatched and noncohesive DNA ends.''; PubMedEurope PMCScholia
Sibanda BL, Critchlow SE, Begun J, Pei XY, Jackson SP, Blundell TL, Pellegrini L.; ''Crystal structure of an Xrcc4-DNA ligase IV complex.''; PubMedEurope PMCScholia
Chen L, Morio T, Minegishi Y, Nakada S, Nagasawa M, Komatsu K, Chessa L, Villa A, Lecis D, Delia D, Mizutani S.; ''Ataxia-telangiectasia-mutated dependent phosphorylation of Artemis in response to DNA damage.''; PubMedEurope PMCScholia
Chan DW, Chen BP, Prithivirajsingh S, Kurimasa A, Story MD, Qin J, Chen DJ.; ''Autophosphorylation of the DNA-dependent protein kinase catalytic subunit is required for rejoining of DNA double-strand breaks.''; PubMedEurope PMCScholia
Ding Q, Reddy YV, Wang W, Woods T, Douglas P, Ramsden DA, Lees-Miller SP, Meek K.; ''Autophosphorylation of the catalytic subunit of the DNA-dependent protein kinase is required for efficient end processing during DNA double-strand break repair.''; PubMedEurope PMCScholia
Zimmermann M, Lottersberger F, Buonomo SB, Sfeir A, de Lange T.; ''53BP1 regulates DSB repair using Rif1 to control 5' end resection.''; PubMedEurope PMCScholia
Ciccia A, Elledge SJ.; ''The DNA damage response: making it safe to play with knives.''; PubMedEurope PMCScholia
Stucki M, Clapperton JA, Mohammad D, Yaffe MB, Smerdon SJ, Jackson SP.; ''MDC1 directly binds phosphorylated histone H2AX to regulate cellular responses to DNA double-strand breaks.''; PubMedEurope PMCScholia
Yoo S, Dynan WS.; ''Geometry of a complex formed by double strand break repair proteins at a single DNA end: recruitment of DNA-PKcs induces inward translocation of Ku protein.''; PubMedEurope PMCScholia
Li Y, Chirgadze DY, Bolanos-Garcia VM, Sibanda BL, Davies OR, Ahnesorg P, Jackson SP, Blundell TL.; ''Crystal structure of human XLF/Cernunnos reveals unexpected differences from XRCC4 with implications for NHEJ.''; PubMedEurope PMCScholia
Lee JH, Paull TT.; ''ATM activation by DNA double-strand breaks through the Mre11-Rad50-Nbs1 complex.''; PubMedEurope PMCScholia
Wilson KA, Stern DF.; ''NFBD1/MDC1, 53BP1 and BRCA1 have both redundant and unique roles in the ATM pathway.''; PubMedEurope PMCScholia
Heo J, Li J, Summerlin M, Hays A, Katyal S, McKinnon PJ, Nitiss KC, Nitiss JL, Hanakahi LA.; ''TDP1 promotes assembly of non-homologous end joining protein complexes on DNA.''; PubMedEurope PMCScholia
Zhou T, Akopiants K, Mohapatra S, Lin PS, Valerie K, Ramsden DA, Lees-Miller SP, Povirk LF.; ''Tyrosyl-DNA phosphodiesterase and the repair of 3'-phosphoglycolate-terminated DNA double-strand breaks.''; PubMedEurope PMCScholia
Valerie K, Povirk LF.; ''Regulation and mechanisms of mammalian double-strand break repair.''; PubMedEurope PMCScholia
Critchlow SE, Bowater RP, Jackson SP.; ''Mammalian DNA double-strand break repair protein XRCC4 interacts with DNA ligase IV.''; PubMedEurope PMCScholia
Lee JW, Blanco L, Zhou T, Garcia-Diaz M, Bebenek K, Kunkel TA, Wang Z, Povirk LF.; ''Implication of DNA polymerase lambda in alignment-based gap filling for nonhomologous DNA end joining in human nuclear extracts.''; PubMedEurope PMCScholia
Gu J, Lu H, Tippin B, Shimazaki N, Goodman MF, Lieber MR.; ''XRCC4:DNA ligase IV can ligate incompatible DNA ends and can ligate across gaps.''; PubMedEurope PMCScholia
Hanakahi LA, Bartlet-Jones M, Chappell C, Pappin D, West SC.; ''Binding of inositol phosphate to DNA-PK and stimulation of double-strand break repair.''; PubMedEurope PMCScholia
Wang J, Aroumougame A, Lobrich M, Li Y, Chen D, Chen J, Gong Z.; ''PTIP associates with Artemis to dictate DNA repair pathway choice.''; PubMedEurope PMCScholia
Paull TT, Rogakou EP, Yamazaki V, Kirchgessner CU, Gellert M, Bonner WM.; ''A critical role for histone H2AX in recruitment of repair factors to nuclear foci after DNA damage.''; PubMedEurope PMCScholia
Ma Y, Pannicke U, Lu H, Niewolik D, Schwarz K, Lieber MR.; ''The DNA-dependent protein kinase catalytic subunit phosphorylation sites in human Artemis.''; PubMedEurope PMCScholia
Ahnesorg P, Smith P, Jackson SP.; ''XLF interacts with the XRCC4-DNA ligase IV complex to promote DNA nonhomologous end-joining.''; PubMedEurope PMCScholia
Walker JR, Corpina RA, Goldberg J.; ''Structure of the Ku heterodimer bound to DNA and its implications for double-strand break repair.''; PubMedEurope PMCScholia
Gómez-Herreros F, Romero-Granados R, Zeng Z, Alvarez-Quilón A, Quintero C, Ju L, Umans L, Vermeire L, Huylebroeck D, Caldecott KW, Cortés-Ledesma F.; ''TDP2-dependent non-homologous end-joining protects against topoisomerase II-induced DNA breaks and genome instability in cells and in vivo.''; PubMedEurope PMCScholia
Soubeyrand S, Pope L, De Chasseval R, Gosselin D, Dong F, de Villartay JP, Haché RJ.; ''Artemis phosphorylated by DNA-dependent protein kinase associates preferentially with discrete regions of chromatin.''; PubMedEurope PMCScholia
Nikjoo H, O'Neill P, Wilson WE, Goodhead DT.; ''Computational approach for determining the spectrum of DNA damage induced by ionizing radiation.''; PubMedEurope PMCScholia
Povirk LF.; ''Processing of damaged DNA ends for double-strand break repair in mammalian cells.''; PubMedEurope PMCScholia
Ma Y, Pannicke U, Schwarz K, Lieber MR.; ''Hairpin opening and overhang processing by an Artemis/DNA-dependent protein kinase complex in nonhomologous end joining and V(D)J recombination.''; PubMedEurope PMCScholia
Fan W, Wu X.; ''DNA polymerase lambda can elongate on DNA substrates mimicking non-homologous end joining and interact with XRCC4-ligase IV complex.''; PubMedEurope PMCScholia
Danielsen JR, Povlsen LK, Villumsen BH, Streicher W, Nilsson J, Wikström M, Bekker-Jensen S, Mailand N.; ''DNA damage-inducible SUMOylation of HERC2 promotes RNF8 binding via a novel SUMO-binding Zinc finger.''; PubMedEurope PMCScholia
Wang B, Matsuoka S, Ballif BA, Zhang D, Smogorzewska A, Gygi SP, Elledge SJ.; ''Abraxas and RAP80 form a BRCA1 protein complex required for the DNA damage response.''; PubMedEurope PMCScholia
Mahajan KN, Nick McElhinny SA, Mitchell BS, Ramsden DA.; ''Association of DNA polymerase mu (pol mu) with Ku and ligase IV: role for pol mu in end-joining double-strand break repair.''; PubMedEurope PMCScholia
Goodarzi AA, Yu Y, Riballo E, Douglas P, Walker SA, Ye R, Härer C, Marchetti C, Morrice N, Jeggo PA, Lees-Miller SP.; ''DNA-PK autophosphorylation facilitates Artemis endonuclease activity.''; PubMedEurope PMCScholia
Buck D, Malivert L, de Chasseval R, Barraud A, Fondanèche MC, Sanal O, Plebani A, Stéphan JL, Hufnagel M, le Deist F, Fischer A, Durandy A, de Villartay JP, Revy P.; ''Cernunnos, a novel nonhomologous end-joining factor, is mutated in human immunodeficiency with microcephaly.''; PubMedEurope PMCScholia
Leber R, Wise TW, Mizuta R, Meek K.; ''The XRCC4 gene product is a target for and interacts with the DNA-dependent protein kinase.''; PubMedEurope PMCScholia
Wang B, Matsuoka S, Carpenter PB, Elledge SJ.; ''53BP1, a mediator of the DNA damage checkpoint.''; PubMedEurope PMCScholia
Malu S, De Ioannes P, Kozlov M, Greene M, Francis D, Hanna M, Pena J, Escalante CR, Kurosawa A, Erdjument-Bromage H, Tempst P, Adachi N, Vezzoni P, Villa A, Aggarwal AK, Cortes P.; ''Artemis C-terminal region facilitates V(D)J recombination through its interactions with DNA Ligase IV and DNA-PKcs.''; PubMedEurope PMCScholia
Gottlieb TM, Jackson SP.; ''The DNA-dependent protein kinase: requirement for DNA ends and association with Ku antigen.''; PubMedEurope PMCScholia
Stewart GS, Wang B, Bignell CR, Taylor AM, Elledge SJ.; ''MDC1 is a mediator of the mammalian DNA damage checkpoint.''; PubMedEurope PMCScholia
Mallette FA, Mattiroli F, Cui G, Young LC, Hendzel MJ, Mer G, Sixma TK, Richard S.; ''RNF8- and RNF168-dependent degradation of KDM4A/JMJD2A triggers 53BP1 recruitment to DNA damage sites.''; PubMedEurope PMCScholia
Escribano-Díaz C, Orthwein A, Fradet-Turcotte A, Xing M, Young JT, Tkáč J, Cook MA, Rosebrock AP, Munro M, Canny MD, Xu D, Durocher D.; ''A cell cycle-dependent regulatory circuit composed of 53BP1-RIF1 and BRCA1-CtIP controls DNA repair pathway choice.''; PubMedEurope PMCScholia
Inamdar KV, Pouliot JJ, Zhou T, Lees-Miller SP, Rasouli-Nia A, Povirk LF.; ''Conversion of phosphoglycolate to phosphate termini on 3' overhangs of DNA double strand breaks by the human tyrosyl-DNA phosphodiesterase hTdp1.''; PubMedEurope PMCScholia
Lee KJ, Jovanovic M, Udayakumar D, Bladen CL, Dynan WS.; ''Identification of DNA-PKcs phosphorylation sites in XRCC4 and effects of mutations at these sites on DNA end joining in a cell-free system.''; PubMedEurope PMCScholia
Gatei M, Young D, Cerosaletti KM, Desai-Mehta A, Spring K, Kozlov S, Lavin MF, Gatti RA, Concannon P, Khanna K.; ''ATM-dependent phosphorylation of nibrin in response to radiation exposure.''; PubMedEurope PMCScholia
Riballo E, Kühne M, Rief N, Doherty A, Smith GC, Recio MJ, Reis C, Dahm K, Fricke A, Krempler A, Parker AR, Jackson SP, Gennery A, Jeggo PA, Löbrich M.; ''A pathway of double-strand break rejoining dependent upon ATM, Artemis, and proteins locating to gamma-H2AX foci.''; PubMedEurope PMCScholia
Jiang W, Crowe JL, Liu X, Nakajima S, Wang Y, Li C, Lee BJ, Dubois RL, Liu C, Yu X, Lan L, Zha S.; ''Differential phosphorylation of DNA-PKcs regulates the interplay between end-processing and end-ligation during nonhomologous end-joining.''; PubMedEurope PMCScholia
Pei H, Zhang L, Luo K, Qin Y, Chesi M, Fei F, Bergsagel PL, Wang L, You Z, Lou Z.; ''MMSET regulates histone H4K20 methylation and 53BP1 accumulation at DNA damage sites.''; PubMedEurope PMCScholia
Niewolik D, Pannicke U, Lu H, Ma Y, Wang LC, Kulesza P, Zandi E, Lieber MR, Schwarz K.; ''DNA-PKcs dependence of Artemis endonucleolytic activity, differences between hairpins and 5' or 3' overhangs.''; PubMedEurope PMCScholia
Nick McElhinny SA, Havener JM, Garcia-Diaz M, Juárez R, Bebenek K, Kee BL, Blanco L, Kunkel TA, Ramsden DA.; ''A gradient of template dependence defines distinct biological roles for family X polymerases in nonhomologous end joining.''; PubMedEurope PMCScholia
Callen E, Di Virgilio M, Kruhlak MJ, Nieto-Soler M, Wong N, Chen HT, Faryabi RB, Polato F, Santos M, Starnes LM, Wesemann DR, Lee JE, Tubbs A, Sleckman BP, Daniel JA, Ge K, Alt FW, Fernandez-Capetillo O, Nussenzweig MC, Nussenzweig A.; ''53BP1 mediates productive and mutagenic DNA repair through distinct phosphoprotein interactions.''; PubMedEurope PMCScholia
Davis BJ, Havener JM, Ramsden DA.; ''End-bridging is required for pol mu to efficiently promote repair of noncomplementary ends by nonhomologous end joining.''; PubMedEurope PMCScholia
Baumann P, West SC.; ''DNA end-joining catalyzed by human cell-free extracts.''; PubMedEurope PMCScholia
Douglas P, Sapkota GP, Morrice N, Yu Y, Goodarzi AA, Merkle D, Meek K, Alessi DR, Lees-Miller SP.; ''Identification of in vitro and in vivo phosphorylation sites in the catalytic subunit of the DNA-dependent protein kinase.''; PubMedEurope PMCScholia
Sun J, Lee KJ, Davis AJ, Chen DJ.; ''Human Ku70/80 protein blocks exonuclease 1-mediated DNA resection in the presence of human Mre11 or Mre11/Rad50 protein complex.''; PubMedEurope PMCScholia
DNA double strand break (DSB) response involves sensing of DNA DSBs by the MRN complex which triggers ATM activation. ATM phosphorylates a number of proteins involved in DNA damage checkpoint signaling, as well as proteins directly involved in the repair of DNA DSBs. For a recent review, please refer to Ciccia and Elledge, 2010.
Homology directed repair (HDR) of replication-independent DNA double strand breaks (DSBs) via homologous recombination repair (HRR) or single strand annealing (SSA) requires the activation of ATM followed by ATM-mediated phosphorylation of DNA repair proteins. ATM coordinates the recruitment of DNA repair and signaling proteins to DSBs and formation of the so-called ionizing radiation induced foci (IRIF). While IRIFs include chromatin regions kilobases away from the actual DSB, this Reactome pathway represents simplified foci and shows events that happen at the very ends of the broken DNA.
For both HRR and SSA to occur, the ends of the DNA DSB must be processed (resected) to generate lengthy 3' ssDNA tails, and the resulting ssDNA coated with RPA complexes, triggering ATR activation and signaling.
After the resection step, BRCA2 and RAD51 trigger HRR, a very accurate process in which the 3'-ssDNA overhang invades a sister chromatid, base pairs with the complementary strand of the sister chromatid DNA duplex, creating a D-loop, and uses the complementary sister chromatid strand as a template for DNA repair synthesis that bridges the DSB.
The SSA is triggered when 3'-ssDNA overhangs created in the resection step contain highly homologous direct repeats. In a process involving RAD52, the direct repeats in each 3'-ssDNA overhang become annealed, the unannealed 3'-flaps excised, and structures then processed by DNA repair synthesis. SSA results in the loss of one of the annealed repeats and the DNA sequence between the two repeats. Therefore, SSA is error-prone and is probably used as a backup for HRR, with RAD52 loss-of-function mutations being synthetically lethal with mutations in HRR genes, such as BRCA2 (reviewed by Ciccia and Elledge 2010).
RIF1 binds TP53BP1 phosphorylated by ATM at DNA double strand breaks (DSBs). RIF1 binding interferes with the accumulation of BRCA1:BARD1 heterodimers and associated proteins at DNA DSBs. Therefore, TP53BP1-mediated recruitment of RIF1 prevents RBBP8 (CtIP) binding to BRCA1:BARD1 and the subsequent resection of DNA DSBs. The action of RIF1 and TP53BP1 promotes non-homologous end joining (NHEJ) of DNA DSBs during G1 phase of the cell cycle, when sister chromatids are not available for homologous recombination-mediated repair (Zimmermann et al. 2013, Escribano-Diaz et al. 2013).
Similar to RIF1, PAX1IP (PTIP) is also recruited to DNA DSBs through interaction with ATM-phosphorylated TP53BP1. Since RIF1 and PAX1IP interact with different phosphorylated sites on TP53BP1, they can simultaneously bind TP53BP1 and colocalize in the majority of TP53BP1 foci. PAX1IP contributes to inhibition of DNA DSB resection mediated by BRCA1-recruited RBBP8 (CtIP) (Callen et al. 2013).
Activated ATM phosphorylates DCLRE1C (ARTEMIS) at serine residue S645. S645 phosphorylation is necessary for the function of DCLRE1C in non-homologous end joining (NHEJ) (Riballo et al. 2004, Poinsignon et al. 2004, Chen et al. 2005).
TP53BP1 is required for the recruitment of DCLRE1C (ARTEMIS) to DNA double strand breaks (DSBs) and ATM-mediated phosphorylation of DCLRE1C (Riballo et al. 2004). DCRLE1C directly interacts with the TP53BP1-binding protein PAX1IP (PTIP) (Wang et al. 2014).
DCLRE1C (ARTEMIS) forms a stable complex with PRKDC (DNA-PKcs), even in the absence of DNA ends (Ma et al. 2002). Autophosphorylation of PRKDC as well as ATM-mediated phosphorylation of DCLRE1C are not prerequisites for the interaction of PRKDC and DCLRE1C (Ding et al.2003).
PRKDC (DNA-PKcs) phosphorylates DCLRE1C (ARTEMIS) at least on serine residue S516, and this phosphorylation is necessary for the activation of DCLRE1C endonucleolytic activity (Ma et al. 2002, Ma et al. 2005, Soubeyrand et al. 2006) as it relieves an autoinhibitory conformation of DCLRE1C (Niewolik et al. 2006).
DNA polymerases mu (POLM) and lambda (POLL) facilitate non-homologous end joining (NHEJ) of DNA double strand breaks (DSBs) by filling single strand (ss) gaps (usually 1- or 2- nucleotide gaps) present at DNA DSB ends positioned for ligation in the synaptic complex containing XRCC5:XRCC6 (Ku), PRKDC (DNA-PKcs), DCLRE1C (ARTEMIS), XRCC4:LIG4 and NHEJ1 (XLF) (Mahajan et al. 2002, Lee et al. 2004, Fan and Wu 2004, McElhinny et al. 2005, Davis et al. 2008).
DCLRE1C (ARTEMIS) possesses an intrinsic 5' to 3' exonuclease activity. Upon binding to PRKDC (DNA-PKcs) at DNA double strand breaks (DSBs) and undergoing PRKDC-mediated phosphorylation, DCLRE1C acquires endonucleolytic activity towards 5' and 3' overhangs at DNA double strand breaks, and it also acquires a hairpin opening activity (Ma et al. 2002, Ma et al. 2005). Autophosphorylation of PRKDC, although not required for the kinase activity of PRKDC, is needed for the activation of the endonucleolytic activity of DCLRE1C, probably by inducing a conformational change in PRKDC that provides DCLRE1C with access to DNA ends (Goodarzi et al. 2006, Niewolik et al. 2006, Jiang et al. 2015).
A complex consisting of XRCC4 homodimer and DNA ligase IV (LIG4) (Sibanda et al. 2001) is recruited to the synaptic complex consisting of PRKDC (DNA-PKcs), XRCC5, XRCC6, DCLRE1C (ARTEMIS) and ligatable DNA double strand break (DSB) ends (Critchlow and Jackson 1997, Leber et al. 1998, Malu et al. 2012). XRCC4 directly interacts with XRCC5:XRCC6 (McElhinny et al. 2000, Hsu et al. 2002), while LIG4 directly interacts with PRKDC (Hsu et al. 2002) and DCLRE1C (Malu et al. 2012). NHEJ1 (XLF) homodimer binds XRCC4 and is recruited to DNA DSBs together with XRCC4 and LIG4, where it acts as a facilitator of LIG4 activity (Ahnesorg et al. 2006, Buck et al. 2006, Tsai et al. 2007, Li et al. 2008). DNA polymerases mu (POLM) or lambda (POLL) are recruited to DNA DSBs through interaction with the Ku complex (XRCC5:XRCC6) and XRCC4 (Mahajan et al. 2002, Lee et al. 2004, Fan and Wu 2004).
Autophosphorylation of DNA-PKcs (PRKDC) is required for NHEJ in vivo, especially for endonucleolytic processing of DNA double strand break ends, which makes them suitable for ligation (Chan et al, 2002; Ding et al, 2003). In vivo, PRKDC autophosphorylates at threonine residues T2609, T2638 and T2647, and serine residue S2612 (Douglas et al. 2002).
Free radical-induced DNA double strand breaks (DSBs) frequently have unligatable 3'-phosphoglycolate termini, while topoisomerase II (TOP2) inhibition produces unligatable 5'-ends, with a 5'-phosphotyrosil bond between the DNA DSB 5'-end and TOP2 (reviewed by Povirk 2012). Tyrosyl-DNA phosphodiesterase TDP1 is able to remove 3'-phosphoglycolate and plays an important role in non-homologous end joining (NHEJ) (Inamdar et al. 2002, Zhou et al. 2005, Zhou et al. 2009, Heo et al. 2015). Tyrosil-DNA phosphodiesterase TDP2 removes 5'-phosphotyrosine and is also involved in NHEJ (Gomez-Herreros et al. 2013).
Ionizing radiation (IR) induces single-strand breaks, i.e., cleavage of the phosphodiester backbone. When two single-strand breaks occur within approximately 10 base pairs, a DNA double-strand break (DSB) results. IR-induced DSBs are complex DNA damage lesions, frequently containing base damage, 5'-OH groups, and 3'-hydroxy or phosphoglycolate groups that must be removed prior to ligation in the final step of NHEJ (Friedberg et al, 1995; Nikjoo et al, 2001; Valerie and Povirk, 2003). The Ku70/80 heterodimer (XRCC5:XRCC6) (Walker et al., 2001) binds to the ends of the double-strand break. Ku can translocate inwards from the site of the break in an ATP-independent manner (reviewed in Dynan and Yoo, 1998). Binding of XRCC5:XRCC6 to DNA DSBs competes away the MRN complex and associated proteins from the DNA DSB (Sun et al. 2012).
The DNA ligase complex composed of DNA ligase 4 (LIG4) and the XRCC4 homodimer (Sibanda et al. 2000) catalyzes ligation of DNA double strand break (DSB) ends during non-homologous end joining (NHEJ). The XRCC4:LIG4 complex is recruited to NHEJ sites through interaction of its subunits with XRCC5:XRCC6 (Ku complex), PRKDC (DNA-PKcs) and DCLRE1C (ARTEMIS) (McElhinny et al. 2000, Hsu et al. 2002, Malu et al. 2012). Phosphorylation of XRCC4 by PRKDC may regulate the activity of the ligase complex (Lee et al. 2004). XRCC4:LIG4 can ligate incompatible DNA DSB ends, and may also ligate across single nucleotide gaps (Gu et al. 2007). The presence of the accessory protein NHEJ1 (XLF) facilitates XRCC4:LIG4 ligase activity, especially at mismatched DNA DSB ends (Ahnesorg et al. 2006, Buck et al. 2006, Tsai et al. 2007). Depending on other types of DNA damage present at DNA DSBs, NHEJ can result in error-free products, produce dsDNA with microdeletions and/or mismatched bases, or result in translocations (reviewed by Povirk 2012).
DNA-PKcs (PRKDC) is recruited to the Ku70:Ku80:DNA double strand break ends complex (XRCC5:XRCC6:DNA DSBs) (Gottlieb and Jackson, 1993), causing Ku to translocate inwards (away from the break) approximately 10 bp (Yoo and Dynan, 1999). This forms the DNA-PK complex (DNA-PKcs plus Ku70/Ku80) at each end of the DSB. Two DNA-PK complexes, one on either side of the DSB, interact to bring the DNA ends together.
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DataNodes
DNA
DSBs:p-MRN:p-S1981,Ac-K3016-ATM:KAT5:K63PolyUb-K14,K16,p-S139-H2AFX,Me2K21-HIST1H4A-Nucleosome:p-5T-MDC1:p-S102-WHSC1:RNF8:Zn2+:SUMO1:p-T4827-HERC2:UBE2N:UBE2V2:RNF168:PIAS4:p-S25,S1778-TP53BP1:RIF1:PAX1IP:DCLRE1CDNA
DSBs:p-MRN:p-S1981,Ac-K3016-ATM:KAT5:K63PolyUb-K14,K16,p-S139-H2AFX,Me2K21-HIST1H4A-Nucleosome:p-5T-MDC1:p-S102-WHSC1:RNF8:Zn2+:SUMO1:p-T4827-HERC2:UBE2N:UBE2V2:RNF168:PIAS4:p-S25,S1778-TP53BP1:RIF1:PAXIP1DNA
DSBs:p-MRN:p-S1981,Ac-K3016-ATM:KAT5:K63PolyUb-K14,K16,p-S139-H2AFX,Me2K21-HIST1H4A-Nucleosome:p-5T-MDC1:p-S102-WHSC1:RNF8:Zn2+:SUMO1:p-T4827-HERC2:UBE2N:UBE2V2:RNF168:PIAS4:p-S25,S1778-TP53BP1:p-5S,2T-BRCA1-A complexHomologous Recombination (HRR) or Single Strand
Annealing (SSA)For both HRR and SSA to occur, the ends of the DNA DSB must be processed (resected) to generate lengthy 3' ssDNA tails, and the resulting ssDNA coated with RPA complexes, triggering ATR activation and signaling.
After the resection step, BRCA2 and RAD51 trigger HRR, a very accurate process in which the 3'-ssDNA overhang invades a sister chromatid, base pairs with the complementary strand of the sister chromatid DNA duplex, creating a D-loop, and uses the complementary sister chromatid strand as a template for DNA repair synthesis that bridges the DSB.
The SSA is triggered when 3'-ssDNA overhangs created in the resection step contain highly homologous direct repeats. In a process involving RAD52, the direct repeats in each 3'-ssDNA overhang become annealed, the unannealed 3'-flaps excised, and structures then processed by DNA repair synthesis. SSA results in the loss of one of the annealed repeats and the DNA sequence between the two repeats. Therefore, SSA is error-prone and is probably used as a backup for HRR, with RAD52 loss-of-function mutations being synthetically lethal with mutations in HRR genes, such as BRCA2 (reviewed by Ciccia and Elledge 2010).
Annotated Interactions
DNA
DSBs:p-MRN:p-S1981,Ac-K3016-ATM:KAT5:K63PolyUb-K14,K16,p-S139-H2AFX,Me2K21-HIST1H4A-Nucleosome:p-5T-MDC1:p-S102-WHSC1:RNF8:Zn2+:SUMO1:p-T4827-HERC2:UBE2N:UBE2V2:RNF168:PIAS4:p-S25,S1778-TP53BP1:RIF1:PAX1IP:DCLRE1CDNA
DSBs:p-MRN:p-S1981,Ac-K3016-ATM:KAT5:K63PolyUb-K14,K16,p-S139-H2AFX,Me2K21-HIST1H4A-Nucleosome:p-5T-MDC1:p-S102-WHSC1:RNF8:Zn2+:SUMO1:p-T4827-HERC2:UBE2N:UBE2V2:RNF168:PIAS4:p-S25,S1778-TP53BP1:RIF1:PAX1IP:DCLRE1CDNA
DSBs:p-MRN:p-S1981,Ac-K3016-ATM:KAT5:K63PolyUb-K14,K16,p-S139-H2AFX,Me2K21-HIST1H4A-Nucleosome:p-5T-MDC1:p-S102-WHSC1:RNF8:Zn2+:SUMO1:p-T4827-HERC2:UBE2N:UBE2V2:RNF168:PIAS4:p-S25,S1778-TP53BP1:RIF1:PAX1IP:DCLRE1CDNA
DSBs:p-MRN:p-S1981,Ac-K3016-ATM:KAT5:K63PolyUb-K14,K16,p-S139-H2AFX,Me2K21-HIST1H4A-Nucleosome:p-5T-MDC1:p-S102-WHSC1:RNF8:Zn2+:SUMO1:p-T4827-HERC2:UBE2N:UBE2V2:RNF168:PIAS4:p-S25,S1778-TP53BP1:RIF1:PAXIP1DNA
DSBs:p-MRN:p-S1981,Ac-K3016-ATM:KAT5:K63PolyUb-K14,K16,p-S139-H2AFX,Me2K21-HIST1H4A-Nucleosome:p-5T-MDC1:p-S102-WHSC1:RNF8:Zn2+:SUMO1:p-T4827-HERC2:UBE2N:UBE2V2:RNF168:PIAS4:p-S25,S1778-TP53BP1:RIF1:PAXIP1DNA
DSBs:p-MRN:p-S1981,Ac-K3016-ATM:KAT5:K63PolyUb-K14,K16,p-S139-H2AFX,Me2K21-HIST1H4A-Nucleosome:p-5T-MDC1:p-S102-WHSC1:RNF8:Zn2+:SUMO1:p-T4827-HERC2:UBE2N:UBE2V2:RNF168:PIAS4:p-S25,S1778-TP53BP1:RIF1:PAXIP1DNA
DSBs:p-MRN:p-S1981,Ac-K3016-ATM:KAT5:K63PolyUb-K14,K16,p-S139-H2AFX,Me2K21-HIST1H4A-Nucleosome:p-5T-MDC1:p-S102-WHSC1:RNF8:Zn2+:SUMO1:p-T4827-HERC2:UBE2N:UBE2V2:RNF168:PIAS4:p-S25,S1778-TP53BP1:RIF1:PAXIP1DNA
DSBs:p-MRN:p-S1981,Ac-K3016-ATM:KAT5:K63PolyUb-K14,K16,p-S139-H2AFX,Me2K21-HIST1H4A-Nucleosome:p-5T-MDC1:p-S102-WHSC1:RNF8:Zn2+:SUMO1:p-T4827-HERC2:UBE2N:UBE2V2:RNF168:PIAS4:p-S25,S1778-TP53BP1:p-5S,2T-BRCA1-A complexSimilar to RIF1, PAX1IP (PTIP) is also recruited to DNA DSBs through interaction with ATM-phosphorylated TP53BP1. Since RIF1 and PAX1IP interact with different phosphorylated sites on TP53BP1, they can simultaneously bind TP53BP1 and colocalize in the majority of TP53BP1 foci. PAX1IP contributes to inhibition of DNA DSB resection mediated by BRCA1-recruited RBBP8 (CtIP) (Callen et al. 2013).