P5-06-02: Response and Acquired Resistance of BRCA1-Deficient Triple-Negative Breast Cancer Xenografts to Alkylators or PARP Inhibitors.

Abstract Background. BRCA1 mutated breast tumor cells are defective in DNA repair by homologous recombination and therefore especially sensitive to treatment with DNA double-strand break (DSB) inducing agents, such as alkylators or PARP inhibitors. However, such tumors can eventually develop therapy resistance. Understanding therapy resistance mechanisms may help in designing treatment strategies to overcome therapy resistance. Methods. We have developed BRCA1-deficient triple-negative breast cancer (TNBC) xenograft models by implantating fresh human breast tumor pieces and subsequent serial passaging. These models show epigenetic loss of BRCA1 due to promoter hypermethylation or genetic inactivation of BRCA1 due to a frameshift mutation (c.2210delC) resulting in a premature stop codon (p.Thr737LeufsX15). We have used these BRCA1-deficient TNBC models to study response and acquisition of resistance to alkylating therapy (cisplatin, melphalan) and the clinical PARP inhibitor olaparib. Results. Treated tumors were sensitive to the alkylators cisplatin or melphalan or the PARP inhibitor olaparib, in some cases resulting in complete remission with no palpable tumor left. However, relapses did occur in most cases and repeated treatment of recurrent tumors eventually led to aquired resistance. Since restoration of BRCA1 function has been suggested as a mechanism of therapy resistance (Swisher et al., Cancer Res 2008; 68: 2581), we determined BRCA1 expression in therapy-sensitive and -resistant tumors by Western blot analysis. While no full length BRCA1 protein could be detected in the therapy-sensitive tumors, expression of full length BRCA1 protein was found in the majority of cisplatin resistant and olaparib resistant tumors. BRCA1 reexpression in the therapy-resistant BRCA1-c.2210delC tumors was caused by genetic restoration of the reading frame due to reversion to the wildtype sequence or additional deletions near the c.2210delC mutation. BRCA1-re-expression in the therapy-resistant TNBC xenografts with epigenetic loss of BRCA1 was caused either by loss of BRCA1 promoter methylation or by complex rearrangements at the BRCA1 locus. Conclusion. Although BRCA1-deficient TNBC xenografts are initially very sensitive to alkylating agents and olaparib, resistance to treatment develops in almost all treated tumors. This acquired resistance is frequently associated with re-expression of BRCA1 due to secondary mutations or loss of promoter methylation. Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P5-06-02.