ABL1

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

Activated ATM phosphorylates the NBN (NBS1) subunit of the MRN complex (MRE11A:RAD50:NBN) (Gatei et al. 2000), as well as the nucleosome histone H2AFX (H2AX) on serine residue S139, producing gamma-H2AFX (gamma-H2AX) containing nucleosomes (Rogakou et al. 1998, Burma et al. 2001). H2AFX is phosphorylated on tyrosine 142 (Y142) under basal conditions (Xiao et al. 2009). After ATM-mediated phosphorylation of H2AFX on S139, tyrosine Y142 has to be dephosphorylated by EYA family phosphatases in order for the DNA repair to proceed and to avoid apoptosis induced by DNA DSBs (Cook et al. 2009). Gamma-H2AFX recruits MDC1 to DNA DSBs (Stucki et al. 2005). After ATM phosphorylates MDC1 (Liu et al. 2012), the MRN complex, gamma-H2AFX nucleosomes, and MDC1 serve as a core of the nuclear focus and a platform for the recruitment of other proteins involved in DNA damage signaling and repair (Lukas et al. 2004, Soutoglou and Misteli 2008).

RNF8 ubiquitin ligase binds phosphorylated MDC1 (Kolas et al. 2007) and, in cooperation with HERC2 and RNF168 (Bekker-Jensen et al. 2010, Campbell et al. 2012), ubiquitinates H2AFX (Mailand et al. 2007, Huen et al. 2007, Stewart et al. 2009, Doil et al. 2009) and histone demethylases KDM4A and KDM4B (Mallette et al. 2012).

Ubiquitinated gamma-H2AFX recruits UIMC1 (RAP80), promoting the assembly of the BRCA1-A complex at DNA DSBs. The BRCA1-A complex consists of RAP80, FAM175A (Abraxas), BRCA1:BARD1 heterodimer, BRCC3 (BRCC36), BRE (BRCC45) and BABAM1 (MERIT40, NBA1) (Wang et al. 2007, Wang and Elledge 2007)

Ubiquitin mediated degradation of KDM4A and KDM4B allows TP53BP1 (53BP1) to associate with histone H4 dimethylated on lysine K21 (H4K20Me2 mark) by WHSC1 at DNA DSB sites (Pei et al. 2011).

Once recruited to DNA DSBs, both BRCA1:BARD1 heterodimers and TP53BP1 are phosphorylated by ATM (Cortez et al. 1999, Gatei et al. 2000, Kim et al. 2006, Jowsey et al. 2007), which triggers recruitment and activation of CHEK2 (Chk2, Cds1) (Wang et al. 2002, Wilson and Stern 2008, Melchionna et al. 2000).

Depending on the cell cycle stage, BRCA1 and TP53BP1 competitively promote either homology directed repair (HDR) or nonhomologous end joining (NHEJ) of DNA DSBs. HDR through homologous recombination repair (HRR) or single strand annealing (SSA) is promoted by BRCA1 in association with RBBP8 (CtIP), while NHEJ is promoted by TP53BP1 in association with RIF1 (Escribano-Diaz et al. 2013)