Abstract 1853: Inhibition of mTOR downregulates expression of DNA repair proteins and is highly efficient against BRCA2-mutated breast cancer when combined to PARP inhibition
Authors: Florence Coussy, Rania El Botty, Rana Hatem, Franck Assayag, Ahmed Dahmani, Marine Huppé, Sophie Chateau Joubert, Jean -Luc Servely, Virginie Bernard, Sophie Vacher, Berangere Ouine, Aurelie Cartier, Leanne De Koning, Paul Cottu, Ivan Bieche, Elisabetta Marangoni
Journal: Cancer Research (2017)

Abstract Purpose: DNA repair deficiencies and activation of PI3K/AKT/mTOR pathway are common events in breast cancer. BRCA1/2 mutations have been associated with sensitivity to PARP1 inhibitors (synthetic lethality), while alterations in component of the PI3K/AKT/mTOR pathway might confer sensitivity to PI3KCA and mTOR inhibitors. Here we explored the therapeutic benefit of combining a mTOR and a PARP inhibitor in BRCA2-mutated patient-derived xenografts (PDX) with alterations in the PI3K/AKT/mTOR pathway. Experimental design: the combination of the mTOR inhibitor everolimus and the PARP inhibitor olaparib was tested in two BRCA2-mutated PDX established from a luminal B tamoxifen-resistant and a basal-like breast cancers. Both carried alterations in the PI3K/AKT/mTOR pathway: PIK3R1 mutation and PTEN loss for the luminal B PDX and PTEN loss for the basal-like PDX. To identify putative crosstalk events between mTOR and DNA repair, a Reverse Phase Protein Array (RPPA) analysis of multiple signaling pathways and DNA repair processes was performed on untreated and treated xenografts. Gene and protein expression changes were confirmed by RT-PCR and Western Blot analyses. The capacity to repair DNA damage was measured by P-H2AX immunostaining. Results: in both PDX, everolimus and olaparib showed marked anti-tumor activity with a growth inhibition comprised between 78% and 86% in the monotherapy setting and 96% in the combination arm, where 100% of mice showed tumor regressions. In the luminal B tumor this combination was more efficient than the combination of everolimus and endocrine therapies (fulvestrant or tamoxifen). In both PDX, the fraction of P-H2AX (marker of unrepaired DNA double-strand breaks) positive cells was increased after everolimus treatment, suggesting a link between mTOR and DNA damage, and strongly increased in the combination setting. RPPA analysis of tumors treated by everolimus alone revealed a marked downregulation of different proteins involved in DNA repair, including FANCD2, RAD50 and SUV39H1, a chromatin compactor factor essential in homologous recombination. In the combination setting, expression of these proteins was almost completely abolished, suggesting convergence of PARP and mTOR in downregulation of DNA repair components. Conclusions: our results suggest that combining mTOR and DNA repair inhibitors could be a successful strategy to treat a subset of breast cancer with BRCA2 mutation and alterations in the PI3K/AKT/mTOR pathway. Further experiments with mTOR and PARP inhibitors combinations are ongoing in sporadic breast cancer PDX showing a BRCAness phenotype. Citation Format: Florence Coussy, Rania El Botty, Rana Hatem, Franck Assayag, Ahmed Dahmani, Marine Huppé, Sophie Chateau Joubert, Jean -Luc Servely, Virginie Bernard, Sophie Vacher, Berangere Ouine, Aurelie Cartier, Leanne De Koning, Paul Cottu, Ivan Bieche, Elisabetta Marangoni. Inhibition of mTOR downregulates expression of DNA repair proteins and is highly efficient against BRCA2-mutated breast cancer when combined to PARP inhibition [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1853. doi:10.1158/1538-7445.AM2017-1853