AKT1 interacts with DHX9 to mitigate R-loop-induced replication stress in ovarian cancer.

PARP inhibitor (PARPi)-resistant BRCA mutant (BRCAm) high-grade serous ovarian cancer (HGSOC) represents a new clinical challenge with unmet therapeutic needs. Here, we performed a quantitative high-throughput drug combination screen that identified the combination of an ATR inhibitor (ATRi) and an AKT inhibitor (AKTi) as an effective treatment strategy for both PARPi-sensitive and PARPi-resistant BRCAm HGSOC. The ATRi and AKTi combination induced DNA damage and R-loop-mediated replication stress (RS). Mechanistically, the kinase domain of AKT1 directly interacted with DHX9 and facilitated recruitment of DHX9 to R-loops. AKTi increased ATRi-induced R-loop-mediated RS by mitigating recruitment of DHX9 to R-loops. Moreover, DHX9 was upregulated in tumors from PARPi-resistant BRCAm HGSOC patients, and high co-expression of DHX9 and AKT1 correlated with worse survival. Together, this study reveals an interaction between AKT1 and DHX9 that facilitates R-loop resolution and identifies combining ATRi and AKTi as a rational treatment strategy for BRCAm HGSOC irrespective of PARPi resistance status.