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.