The 9-1-1 complex protects ssDNA gaps in BRCA2-deficient cancer.

A hallmark of BRCA2-deficient cells is the generation of single-stranded DNA (ssDNA) gaps during replication, which are considered key lesions that trigger genomic instability and synthetic lethality. In the classic study that established the paradigm of “replication gaps dictate PARP inhibitor synthetic lethality,” gaps were recognized as a feature distinguishing BRCA-deficient cells from cells with other repair defects; exposed gaps exacerbate drug toxicity, whereas gap reduction is associated with resistance ^[2]. However, in contrast to the double-strand break repair pathways, which have been systematically characterized ^[3], it has remained unclear what mechanisms protect and repair these gaps in BRCA-deficient cells. The clamp loader checkpoint complex 9-1-1 (RAD9A-HUS1-RAD1) had previously come into view mainly for its role in mitosis-dependent microhomology-mediated end joining ^[5], and whether and how it participates in gap maintenance was unknown.

This new study conducted genome-wide CRISPR screens in homologous BRCA2 proficient and deficient cancer cells and found that the 9-1-1 complex is essential for the survival of BRCA2-deficient cells, independently of ATR—an unexpected result because 9-1-1 has long been regarded as an upstream factor in DNA damage checkpoint activation. Mechanistically, after 9-1-1 loss, PRIMPOL-dependent gaps in BRCA2-deficient cells cannot undergo post-replicative repair, undergo pathological expansion, and cause increased DNA damage. The driver of excessive expansion is EXO1-mediated degradation: EXO1 knockdown can rescue this phenotype. The 9-1-1 complex also participates in POLζ-dependent gap filling. Based on these findings, the authors proposed a model in which, once ssDNA gaps exceed a critical length, translesion synthesis (TLS)-mediated repair can no longer access them, making repair entirely dependent on homologous recombination. Because BRCA2-deficient cells precisely lack this backup mechanism, gap protection becomes essential for survival.

This work advances the field’s understanding. Previously, TLS was known as the primary tolerance mechanism for bypassing replication-blocking damage through catalysis by specialized polymerases ^[1], and the REV1-POLζ axis recruits proteins that can be disrupted by the small molecule JH-RE-06 to sensitize chemotherapy ^[4]. The present study links the gap-filling function of TLS POLζ with 9-1-1-mediated control of gap length, showing that gap “length” determines “who repairs.” It also liberates 9-1-1 from its established framework as a checkpoint/mitotic repair factor ^[5], assigning it an ATR-independent role in genome maintenance and explaining why targeting 9-1-1 may selectively kill BRCA2-deficient tumors. In addition, evidence from other work indicates that EXO1- and DNA2-mediated gap expansion is required in BRCA1-deficient cells but not in BRCA2-deficient cells ^[6], suggesting that the requirement for gap metabolism enzymes may vary with the specific type of homologous recombination deficiency and warranting distinction across different BRCA-deficient contexts.

Of course, numerous questions remain unresolved: how the “critical value” for gap length is defined at the molecular level, the specific biochemical step at which 9-1-1 inhibition suppresses PRIMPOL-dependent gap expansion, and whether targeting 9-1-1 can translate into a therapeutic window for BRCA2-deficient tumors in vivo all require further investigation.