Abstract
BRCA2 interacts with TREX-2 subunits and is required to prevent R-loop–associated DNA damage.
Major finding: BRCA2 interacts with TREX-2 subunits and is required to prevent R-loop–associated DNA damage.
Concept: R-loop formation is enhanced by improper RNA biogenesis and export and is increased in cells lacking BRCA2.
Impact: Failure to resolve R-loops may represent a significant source of genomic instability in cancer.
R-loops are DNA–RNA hybrid structures that impede transcriptional elongation and have been linked to DNA replication fork stalling and genomic instability. The mechanisms that govern R-loop resolution are not well understood; however, mRNA processing and export factors, such as the THO complex, have been found to suppress R-loop formation and prevent transcription-associated genomic instability. The TREX-2 RNA export complex shares similar functions with THO and is required for genomic maintenance in yeast, prompting Bhatia and colleagues to examine whether the protective function of TREX-2 is conserved in humans. Silencing of the TREX-2 subunits PCID2 and DSS1 in human cancer cells induced DNA damage that is linked to altered DNA replication fork dynamics. Consistent with previous findings that DSS1 binds BRCA2, the authors showed that PCID2 binds BRCA2 in situ; however, unlike DSS1, PCID2 suppression did not affect BRCA2-dependent recruitment of RAD51. Labeling of R-loops in vivo using a fusion protein generated from the DNA–RNA hybrid-binding (HB) domain of the R-loop resolution factor RNH1 and GFP revealed that suppression of DSS1 and BRCA2, but not PCID2, increased R-loop formation. Interestingly, BRCA2 was found to spontaneously associate with DNA–RNA hybrids, and silencing of BRCA2, but not PCID2 or RAD51, promoted R-loop accumulation at actively transcribed genes in replicating and non-replicating cells, suggesting that BRCA2 mediates R-loop–associated genomic instability independently of its known role in homologous recombination. In line with these findings, activation of the DNA damage response due to HB-GFP–induced DNA replication blockade was enhanced following BRCA2 depletion, whereas suppression of R-loops via RNH1 overexpression reduced endogenous DNA damage in BRCA2-mutant cancer cells. Together, these results highlight R-loop processing as a previously unrecognized tumor-suppressive function of BRCA2 and provide evidence that DNA–RNA hybrid accumulation drives genomic instability in cancer.
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