Which Holds the Key to BRCAness: Inability to Repair the Break, Protect the Fork, or Prevent the Gap?

Defects in genes crucial for the process of DNA repair by homology-directed DNA repair (HDR), such as BRCA1 and BRCA2, are well-known contributors to cancer pathogenesis as well as an Achilles' heel that can be exploited therapeutically. BRCA1/2-deficient cells are exquisitely sensitive to agents that stall replication forks, such as PARP inhibitors and platinating drugs, presumably due to the inability to repair double-stranded breaks that form as a consequence of replication fork collapse. BRCA1/2 also promote tolerance to DNA replication stress by protecting replication forks from nucleolytic degradation. Both biological endpoints involve the deposition of RAD51 onto single-stranded DNA (ssDNA) for homology searching and strand exchange during HDR repair, as well as protection of newly synthesized DNA from nucleolytic degradation (i.e., replication fork protection). In this issue of Cancer Research, Panzarino and colleagues performed multiple separation-of-function studies and identify the lesion most associated with intolerance to replication stress in BRCA1/2-deficient cells is persistent ssDNA gaps in newly synthesized DNA, resulting from a failure to restrain DNA replication. Mechanisms that suppress gap formation are closely associated with chemoresistance, and the authors' findings challenge the paradigm that lack of HR repair or fork protection underlie the phenotype known as BRCAness.See related article by Panzarino et al., p. 1388.