Processing of DNA double-strand break ends

Homology directed repair (HDR) through homologous recombination (HRR) or single strand annealing (SSA) requires extensive resection of DNA double-strand break (DSB) ends (Thompson and Limoli 2003, Ciccia and Elledge 2010). The resection is performed in a two-step process, where the MRN complex (MRE11A:RAD50:NBN) and RBBP8 (CtIP) bound to BRCA1 initiate the resection. This step is regulated by the complex of CDK2 and CCNA (cyclin A), ensuring the initiation of HRR during S and G2 phases of the cell cycle, when sister chromatids are available. The initial resection is also regulated by ATM-mediated phosphorylation of RBBP8 and CHEK2-mediated phosphorylation of BRCA1 (Chen et al. 2008, Yun and Hiom 2009, Buis et al. 2012, Wang et al. 2013, Davies et al. 2015, Parameswaran et al. 2015). After the initial resection, DNA nucleases EXO1 and/or DNA2 perform long-range resection, which is facilitated by DNA helicases BLM or WRN, as well as BRIP1 (BACH1) (Chen et al. 2008, Nimonkar et al. 2011, Sturzenegger et al. 2014, Suhasini et al. 2011). The resulting long 3'-ssDNA overhangs are coated by the RPA heterotrimers (RPA1:RPA2:RPA3), which recruit ATR:ATRIP complexes to DNA DSBs and, in collaboration with RAD17:RFC and RAD9:HUS1:RAD1 complexes, and TOPBP1 and RHNO1, activate ATR signaling. Activated ATR phosphorylates RPA2 and activates CHEK1 (Cotta-Ramusino et al. 2011), both of which are necessary prerequisites for the subsequent steps in HRR and SSA.

同源导向修复（HDR）通过同源重组（HRR）或单链退火（SSA）过程，需要大量切除DNA双链断裂（DSB）末端（Thompson and Limoli 2003，Ciccia and Elledge 2010）。该切除过程分为两步进行，其中MRN复合物（MRE11A:RAD50:NBN）和RBBP8（CtIP）结合BRCA1启动切除。此步骤受到CDK2和CCNA（细胞周期蛋白A）复合物的调控，确保HRR在细胞周期的S期和G2期启动，此时姐妹染色单体可供使用。初始切除还受到ATM介导的RBBP8磷酸化和CHEK2介导的BRCA1磷酸化的调控（Chen et al. 2008，Yun and Hiom 2009，Buis et al. 2012，Wang et al. 2013，Davies et al. 2015，Parameswaran et al. 2015）。在初始切除之后，DNA核酸酶EXO1和/或DNA2执行长距离切除，这一过程得到DNA解旋酶BLM或WRN以及BRIP1（BACH1）的促进（Chen et al. 2008，Nimonkar et al. 2011，Sturzenegger et al. 2014，Suhasini et al. 2011）。产生的长3'-ssDNA突出末端被RPA异源三聚体（RPA1:RPA2:RPA3）覆盖，招募ATR:ATRIP复合物至DNA DSBs，并与RAD17:RFC和RAD9:HUS1:RAD1复合物，以及TOPBP1和RHNO1协同激活ATR信号传导。激活的ATR磷酸化RPA2并激活CHEK1（Cotta-Ramusino et al. 2011），这两者均为HRR和SSA后续步骤所必需的先决条件。