RADX Promotes Genome Stability and Modulates Chemosensitivity by Regulating RAD51 at Replication Forks

Abstract

SUMMARY RAD51 promotes homology-directed repair (HDR), replication fork reversal, and stalled fork protection. Defects in these functions cause genomic instability and tumorigenesis, but also generate hypersensitivity to cancer therapeutics. Here we describe the identification of RADX as an RPA-like, single-strand DNA binding protein. RADX is recruited to replication forks where it prevents fork collapse by regulating RAD51. When RADX is inactivated, excessive RAD51 activity slows replication elongation and causes double-strand breaks. In cancer cells lacking BRCA2, RADX deletion restores fork protection without restoring HDR. Furthermore, RADX inactivation confers chemotherapy and PARP inhibitor resistance to cancer cells with reduced BRCA2/RAD51 pathway function. By antagonizing RAD51 at forks, RADX allows cells to maintain a high capacity for HDR while ensuring that replication functions of RAD51 are properly regulated. Thus, RADX is essential to achieve the proper balance of RAD51 activity to maintain genome stability.

Functions of SMARCAL1, ZRANB3, and HLTF in maintaining genome stability

“…Consistent with this idea, SMARCAL1 is required for replication through telomeres; however, silencing HLTF or ZRANB3 did not yield telomere instability (Poole et al , 2015). Furthermore, only silencing SMARCAL1 in unstressed caused spontaneous double-strand breaks (Dungrawala et al , 2017). …”

“…Similarly, the checkpoint inhibits fork reversal in S. cerevisiae (Sogo et al , 2002). Furthermore, failure to regulate fork reversal in cells deficient for the recently described RADX protein also yields double-strand breaks (Dungrawala et al , 2017), and fork reversal can facilitate nascent strand degradation (Kolinjivadi et al , 2017). Thus, inappropriate, excessive, or persistent fork reversal can be deleterious.…”

“…However, SMARCAL1 is also required to address endogenous sources of replication stress. Cells without SMARCAL1 display high levels of DNA damage in the absence of any added drugs (Bansbach et al , 2009, Ciccia et al , 2009, Yuan et al , 2009, Bansbach et al , 2010, Poole et al , 2015) and some SMARCAL1 protein is observed at elongating replication forks in untreated cells (Betous et al , 2012, Dungrawala et al , 2017). Thus, it is important to determine what endogenous sources of replication stress require SMARCAL1 for resolution.…”

“…These proteins are not functioning redundantly since depleting both further sensitizes cells to replication stress compared to individual depletions (Ciccia et al , 2012). Unlike SMARCAL1, ZRANB3 deficiency does not result in fork breakage in the absence of replication stress, and ZRANB3 has not been linked to telomere replication (Poole et al , 2015, Dungrawala et al , 2017). …”

“…For example, loss of SMARCAL1 increases the frequency of double-strand breaks in replicating cells (Yuan et al , 2009, Bansbach et al , 2010, Dungrawala et al , 2017). Loss of either SMARCAL1 or ZRANB3 reduces the ability of stalled forks to recover from replication stress challenges (Bansbach et al , 2009, Ciccia et al , 2009, Ciccia et al , 2012, Weston et al , 2012, Yuan et al , 2012).…”