Homologous recombination (HR) is critical for maintaining
genome stability through precise repair of
DNA double-strand breaks (DSBs) and restarting stalled or collapsed DNA replication forks. HR is regulated by many
proteins through distinct mechanisms. Some
proteins have direct enzymatic roles in HR reactions, while others act as accessory factors that regulate HR enzymatic activity or coordinate HR with other cellular processes such as the cell cycle. The
breast cancer susceptibility gene BRCA2 encodes a critical accessory
protein that interacts with the
RAD51 recombinase and this interaction fluctuates during the cell cycle. We previously showed that a BRCA2- and p21-interacting
protein, BCCIP, regulates BRCA2 and RAD51 nuclear focus formation,
DSB-induced HR and cell cycle progression. However, it has not been clear whether BCCIP acts exclusively through BRCA2 to regulate HR and whether BCCIP also regulates the alternative
DSB repair pathway, non-homologous end joining. In this study, we found that BCCIP fragments that interact with BRCA2 or with p21 each inhibit
DSB repair by HR. We further show that transient down-regulation of BCCIP in human cells does not affect non-specific integration of transfected
DNA, but significantly inhibits homology-directed gene targeting. Furthermore, human HT1080 cells with constitutive down-regulation of BCCIP display increased levels of spontaneous
single-stranded DNA (ssDNA) and DSBs. These data indicate that multiple BCCIP domains are important for HR regulation, that BCCIP is unlikely to regulate non-homologous end joining, and that BCCIP plays a critical role in resolving spontaneous DNA damage.