Targeting anti-cancer drug resistance in mouse models of breast cancer

Resistance to anti-cancer drugs is one of the biggest challenges in clinical oncology. In contrast to the success of local therapy (e.g. surgery or radiotherapy), the treatment of disseminated cancers using classical DNA-damaging chemotherapeutic agents and novel specific inhibitors frequently fails due to the presence of drug resistance. We have studied intrinsic and acquired resistance to poly(ADP-ribose) polymerase (PARP) inhibitors (PARPi) in genetically engineered mouse models of BRCA1/2-associated hereditary breast cancer. Due to their defect in the error-free DNA repair by homologous recombination (HR), BRCA1/2-deficient cells are highly sensitive to inhibitors of PARP. We have shown that BRCA1-deficient mouse mammary tumors were sensitive to the PARPi olaparib as single agent, but eventually acquired resistance, which was mostly caused by up-regulation of the drug efflux transporter P-glycoprotein (Pgp). In Pgp-deficient Brca1-/-;p53-/- mouse mammary tumors resistance to PARP inhibition was explained by the loss of 53BP1 in a quarter of the cases. 53BP1 is a key regulator of non-homologous end joining and loss of 53BP1 partially restores homologous recombination, which abolishes the synthetic lethality of PARP inhibition. In a mouse model for BRCA2-deficient breast cancer, we identified a subgroup of tumors with a sarcomatoid, epithelial-to-mesenchymal transition (EMT)-like morphology that were upfront resistant to multiple drugs, including PARP inhibitors, despite their HR deficiency. We found that in these tumors expression of Pgp contributes to the multi-drug resistance phenotype and that EMT correlates with Pgp expression in several other mouse models of breast cancer. In addition to drug resistance, EMT has been linked to an increase in tumor-initiating cells (TICs). These TICs are characterized by stem cell-like properties and may be responsible for drug resistance and tumor re-growth. In several cancers, including breast cancer, TICs have been identified as a sub-fraction of tumor cells characterized by specific cell surface markers. Nevertheless, there is an ongoing debate whether this TIC model is applicable to all cancer (sub)types. Here, we show that the use of stem cell markers for the identification of TICs is context-dependent and can differ between tumor subtypes. Since patients with tumors that have a defect in the HR pathway may substantially benefit from DNA-damaging therapy or PARP inhibitors, it would be useful to have a reliable tool to identify these patients upfront. We have used a proteomic approach to identify proteins that are differentially expressed between BRCA1-deficient and -proficient mouse mammary tumors and that may thus identify BRCA1-related breast cancers. We also used proteomics to study the short-term response of sensitive and resistant tumors to treatment with cisplatin. Up-regulation of fatty acid metabolism was found as a candidate predictive marker for platinum-resistance when analysed shortly after treatment. In conclusion, this thesis describes our work on several aspects of drug resistance in genetically engineered mouse models of breast cancer. This includes acquired and intrinsic resistance to PARP inhibition, and biomarker discovery for anti-cancer drugs against HR-deficient breast cancer