Meta-Biol

• Pathways

DNA nucleases EXO1 and DNA2 function redundantly in yeast (Zhu et al. 2008) and humans (Nimonkar et al. 2011) in long-range resection of DNA double-strand breaks (DSBs). Both DNA nucleases act after short 3' ssDNA overhangs are created by the initial resection of DNA DSBs mediated by MRE11A and RBBP8 (CtIP). The roles of BLM (Bloom syndrome helicase) and WRN (Werner syndrome helicase) in facilitation of EXO1- or DNA2-mediated resection of DNA DSBs are also redundant.

EXO1 possesses an intrinsic 5'->3' exonuclease activity. The ATPase activity of BLM DNA helicase is not required for EXO1 catalytic activity, but BLM increases the affinity of EXO1 for DNA ends (Nimonkar et al. 2008). WRN can also positively affect EXO1 exonuclease activity, although the mechanism is not clear (Sturzenegger et al. 2014).

The DNA endonuclease DNA2 has to form a complex with either BLM (Nimonkar et al. 2011) or WRN (Sturzenegger et al. 2014) in order to perform a 5'->3' directed resection of DNA DSBs. BLM forms an evolutionarily conserved complex with TOP3A, RMI1 and RMI2, known as the STR complex in yeast (Zhu et al. 2008) and the BTB or BTRR complex in humans. The entire BTRR complex participates in the activation of DNA2-mediated resection of DNA DSBs (Sturzenegger et al. 2014).

While ATR signaling may be detectable in the absence of long-range resection of DNA DSBs by EXO1 or DNA2 (Eid et al. 2010), EXO1 or DNA2 activity may be necessary to achieve biologically meaningful level of ATR activation (Gravel et al. 2008).

BRIP1 (BACH1, FANCJ) is a DNA helicase recruited to DNA DSBs by interaction with BRCA1 (Cantor et al. 2001) and BLM (Suhasini et al. 2011). BRIP1 is necessary for BRCA1-mediated homology-directed repair of DNA DSBs, and BRIP1 loss-of-function mutations are found in familial breast cancer (Cantor et al. 2001, Litman et al. 2005). The exact role of BRIP1 in DNA repair is not completely clear. BRIP1 is needed for the successful formation of RPA foci and, subsequently, RAD51 foci (Xie et al. 2012). The available evidence suggest that it cooperates with BLM in unwinding of DNA DSBs during resection (Suhasini et al. 2011, Sarkies et al. 2012), and may be especially important for unwinding of DNA that contains oxidative damage (Suhasini et al. 2009).