Reactome: A Curated Pathway Database

HDR through Single Strand Annealing (SSA)

Stable Identifier
Homo sapiens
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Homology directed repair (HDR) through single strand annealing (SSA), similar to HDR through homologous recombination repair (HRR), involves extensive resection of DNA double strand break ends (DSBs), preceded by ATM activation and formation of the so-called ionizing radiation induced foci (IRIF) at DNA DSB sites. Following ATM activation and foci formation, the two-step resection is initiated by the MRN complex (MRE11A:RAD50:NBN) and RBBP8 (CtIP) associated with BRCA1:BARD1, and completed by EXO1 or DNA2 in cooperation with DNA helicases BLM, WRN and BRIP1 (BACH1) (Sartori et al. 2007, Yun and Hiom 2009, Eid et al. 2010, Nimonkar et al. 2011, Suhasini et al. 2011, Sturzenegger et al. 2014). Long 3'-ssDNA overhangs produced by extensive resection are coated by the RPA heterotrimer (RPA1:RPA2:RPA3), triggering ATR signaling. ATR signaling is needed for SSA, probably because of the related phosphorylation of RPA2 (Zou and Elledge 2003, Anantha et al. 2007, Liu et al. 2012).

RAD52 is the key mediator of SSA. Activated ATM phosphorylates and activates ABL1, and activated ABL1 subsequently phosphorylates pre-formed RAD52 heptameric rings, increasing their affinity for ssDNA (Honda et al. 2011). Phosphorylated RAD52 binds phosphorylated RPA heterotrimers on 3'-ssDNA overhangs at resected DNA DSBs. RAD52 also binds RAD51 and prevents formation of invasive RAD51 nucleofilaments involved in HRR (Chen et al. 1999, Van Dyck et al. 1999, Parsons et al. 2000, Jackson et al. 2002, Singleton et al. 2002).

RAD52 promotes annealing of two 3'-ssDNA overhangs when highly homologous directed repeats are present in both 3'-ssDNA overhangs. Nonhomologous regions lying 3' to the annealed repeats are displaced as 3'-flaps (Parsons et al. 2000, Van Dyck et al. 2001, Singleton et al. 2002, Stark et al. 2004, Mansour et al. 2008). The endonuclease complex composed of ERCC1 and ERCC4 (XPF) is subsequently recruited to SSA sites through direct interaction between RAD52 and ERCC4, leading to cleavage of 3' flaps (Motycka et al. 2004, Al-Minawi et al. 2008). The identity of a DNA ligase that closes the remaining single strand nicks (SSBs) to complete SSA-mediated repair is not known.

SSA results in deletion of one of the annealed repeats and the intervening DNA sequence between the two annealed repeats and is thus mutagenic.

Literature References
PubMed ID Title Journal Year
9837724 Visualisation of human rad52 protein and its complexes with hRad51 and DNA. J Mol Biol 1999
18539610 Hierarchy of nonhomologous end-joining, single-strand annealing and gene conversion at site-directed DNA double-strand breaks Nucleic Acids Res. 2008
17962301 The ERCC1/XPF endonuclease is required for efficient single-strand annealing and gene conversion in mammalian cells Nucleic Acids Res. 2008
10921897 Precise binding of single-stranded DNA termini by human RAD52 protein. EMBO J 2000
12139939 Analysis of the human replication protein A:Rad52 complex: evidence for crosstalk between RPA32, RPA70, Rad52 and DNA J. Mol. Biol. 2002
12791985 Sensing DNA damage through ATRIP recognition of RPA-ssDNA complexes Science 2003
21325134 BLM-DNA2-RPA-MRN and EXO1-BLM-RPA-MRN constitute two DNA end resection machineries for human DNA break repair Genes Dev. 2011
17928296 Sequential and synergistic modification of human RPA stimulates chromosomal DNA repair J Biol Chem 2007
17965729 Human CtIP promotes DNA end resection Nature 2007
21804533 Tyrosine phosphorylation enhances RAD52-mediated annealing by modulating its DNA binding EMBO J. 2011
15485900 Genetic steps of mammalian homologous repair with distinct mutagenic consequences Mol. Cell. Biol. 2004
10212258 Radiation-induced assembly of Rad51 and Rad52 recombination complex requires ATM and c-Abl J Biol Chem 1999
14734547 Physical and functional interaction between the XPF/ERCC1 endonuclease and hRad52 J. Biol. Chem. 2004
12370410 Structure of the single-strand annealing domain of human RAD52 protein. Proc Natl Acad Sci U S A 2002
21052091 DNA end resection by CtIP and exonuclease 1 prevents genomic instability EMBO Rep. 2010
22977173 Distinct roles for DNA-PK, ATM and ATR in RPA phosphorylation and checkpoint activation in response to replication stress Nucleic Acids Res. 2012
19357644 CtIP-BRCA1 modulates the choice of DNA double-strand-break repair pathway throughout the cell cycle Nature 2009
25122754 DNA2 cooperates with the WRN and BLM RecQ helicases to mediate long-range DNA end resection in human cells J. Biol. Chem. 2014
21240188 Interaction between the helicases genetically linked to Fanconi anemia group J and Bloom's syndrome EMBO J. 2011
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