An inducible BRCA1 expression system with in vivo applicability uncovers activity of the combination of ATR and PARP inhibitors to overcome therapy resistance

Poly(ADP-ribose) polymerase inhibitors (PARPi) have transformed cancer therapy for patients harbouring homologous recombination repair (HRR) deficiencies, notably BRCA1/2 mutations. However, resistance to PARPi remains a clinical challenge, with restoration of BRCA1 function via hypomorphic variants representing an understudied scenario. Here, we engineered a doxycycline-inducible BRCA1 expression system in the BRCA1-mutant, triple-negative breast cancer cell line MDAMB436, permitting controlled analysis of functionally distinct BRCA1 hypomorphs in vitro and in vivo. Among multiple BRCA1 variants generated--including RING, coiled-coil, and BRCT domain mutants--only overexpression of the {Delta}exon11 hypomorph robustly conferred resistance to olaparib and carboplatin, with drug sensitivity correlating to {Delta}exon11 expression levels. While {Delta}exon11 BRCA1 mediated HRR restoration, its efficiency was consistently lower than full-length BRCA1, as measured by RAD51 foci formation and interaction with repair partners such as PALB2. In vivo, tumours expressing {Delta}exon11 BRCA1 exhibited only partial resistance to olaparib compared to those expressing full-length BRCA1. Importantly, the combination of olaparib and the ATR inhibitor, ceralasertib, overcame {Delta}exon11-mediated resistance, impairing RAD51 foci formation in {Delta}exon11-expressing cells. Our findings identify a dose-dependent, hypomorphic HRR restoration by {Delta}exon11 BRCA1, help explain the variable resistance observed in BRCA1-mutant pre-clinical models expressing this hypomorph and propose ATR inhibition in combination with PARPi as a clinical strategy to counteract therapeutic resistance mediated by {Delta}exon11 BRCA1 hypomorphs. Statement of significanceThis work demonstrates that ATR inhibition can overcome PARP inhibitor resistance mediated by BRCA1 {Delta}exon 11 hypomorphs, supporting combination therapy as a promising strategy for PARPi-resistant BRCA1-mutant cancers.