Modeling Therapy Resistance in <i>BRCA1/2</i> -Mutant Cancers

Abstract

Although PARP inhibitors target <i>BRCA1</i>- or <i>BRCA2</i>-mutant tumor cells, drug resistance is a problem. PARP inhibitor resistance is sometimes associated with the presence of secondary or "revertant" mutations in <i>BRCA1</i> or <i>BRCA2</i> Whether secondary mutant tumor cells are selected for in a Darwinian fashion by treatment is unclear. Furthermore, how PARP inhibitor resistance might be therapeutically targeted is also poorly understood. Using CRISPR mutagenesis, we generated isogenic tumor cell models with secondary <i>BRCA1</i> or <i>BRCA2</i> mutations. Using these in heterogeneous <i>in vitro</i> culture or <i>in vivo</i> xenograft experiments in which the clonal composition of tumor cell populations in response to therapy was monitored, we established that PARP inhibitor or platinum salt exposure selects for secondary mutant clones in a Darwinian fashion, with the periodicity of PARP inhibitor administration and the pretreatment frequency of secondary mutant tumor cells influencing the eventual clonal composition of the tumor cell population. In xenograft studies, the presence of secondary mutant cells in tumors impaired the therapeutic effect of a clinical PARP inhibitor. However, we found that both PARP inhibitor-sensitive and PARP inhibitor-resistant <i>BRCA2</i> mutant tumor cells were sensitive to AZD-1775, a WEE1 kinase inhibitor. In mice carrying heterogeneous tumors, AZD-1775 delivered a greater therapeutic benefit than olaparib treatment. This suggests that despite the restoration of some <i>BRCA1</i> or <i>BRCA2</i> gene function in "revertant" tumor cells, vulnerabilities still exist that could be therapeutically exploited. <i>Mol Cancer Ther; 16(9); 2022-34. ©2017 AACR</i>.