# An Inducible BRCA1 Expression System with In Vivo Applicability Uncovers Activity of the Combination of ATR and PARP Inhibitors to Overcome Therapy Resistance

## TL;DR

This study shows that combining ATR and PARP inhibitors can overcome resistance in BRCA1-mutant cancers caused by a specific BRCA1 variant.

## Contribution

A novel inducible BRCA1 system reveals that ATR inhibition can counteract resistance caused by the ∆exon11 BRCA1 hypomorph.

## Key findings

- The ∆exon11 BRCA1 variant partially restores HRR and causes resistance to PARP and platinum drugs.
- Combining olaparib with the ATR inhibitor ceralasertib overcomes ∆exon11-mediated resistance in vivo.
- Resistance levels correlate with ∆exon11 expression and are less than those seen with full-length BRCA1.

## Abstract

Poly(ADP-ribose) polymerase inhibitors (PARPi) benefit cancers with homologous recombination repair (HRR) defects, yet resistance emerges. Using a doxycycline-inducible system in BRCA1-mutant MDA-MB-436 cells, we tested different BRCA1 hypomorphs. Among all the variants tested, only the ∆exon11 BRCA1 isoform conferred robust, dose-dependent resistance to olaparib and carboplatin. BRCA1 ∆exon11 partially restored HRR—less efficiently than full-length BRCA1—based on RAD51 foci and PALB2 interaction. In vivo, BRCA1 ∆exon11 yielded partial resistance to olaparib. Notably, combining olaparib with the ATR inhibitor ceralasertib abrogated resistance by suppressing RAD51 foci, supporting PARPi–ATR inhibitor combinations to overcome BRCA1 ∆exon11-mediated resistance.

Background: 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. Methods: 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. Results: Among multiple BRCA1 variants generated—including RING, coiled-coil, and BRCT domain mutants—only overexpression of the ∆exon11 hypomorph robustly conferred resistance to olaparib and carboplatin, with drug sensitivity correlating to ∆exon11 expression levels. While ∆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 Δexon11 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 ∆exon11-mediated resistance, impairing RAD51 foci formation in ∆exon11-expressing cells. Conclusions: Our findings identify a dose-dependent, hypomorphic HRR restoration by ∆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 ∆exon11 BRCA1 hypomorphs.