Abstract PR06: Contribution of mutant microenvironment to hereditary cancer: Single-cell gene expression profiling of a genetically engineered mouse model of human hereditary BRCA1-related breast cancer

Carriers of germline mutations in tumor suppressor genes such as BRCA1 suffer from highly increased cancer risk with few effective measures for prevention and treatment. Development of better medical management options requires a more comprehensive understanding of the underlying mechanism of these hereditary cancers. Recent studies suggest that this cancer predisposition, most specifically in the breast and ovary, cannot be sufficiently explained by the DNA-damage repair defect alone in BRCA1-mutant cells, suggesting that additional factors are at play. Further bolstering this argument is the recent finding that BRCA1-heterozygous mutant cells show striking haplo-insufficient phenotypes. Based on these observations as well as preliminary studies in our laboratory, we propose that the heterozygous BRCA1-mutant mammary tissue microenvironment in germline BRCA1-mutation carriers may significantly contribute to breast cancer development by creating a pro-tumorigenic niche. To functionally interrogate this question, we have recently developed a genetically engineered mouse (GEM) model to mimic breast tumor predisposition in human carriers of germline BRCA1 mutation. This model combines Cre/Lox and CRISPR/Cas9 technologies to allow temporal-spatial conditional inactivation of Brca1 as well as other frequently co-mutated genes (e.g., Trp53 and Pten) to initiate breast tumors. Importantly, the surrounding cells in the tissue microenvironment uniformly harbor constitutive heterozygous Brca1 mutation, thereby recapitulating the tissue microenvironment in the human hereditary disease. Use of this mouse model allows us to systematically investigate the aberrations that occur in the heterozygous Brca1-mutant mammary tissue microenvironment, while avoiding confounding variations in age, parity, menstrual cycle, and obesity, which are nearly impossible to eliminate in human samples. Using this GEM model, we have performed high-throughput single-cell RNA-sequencing analysis to identify protumorigenic signaling molecules that are aberrantly upregulated in Brca1-heterozygous cells in the mammary microenvironment. Furthermore, functional in vitro experiments using human cells have demonstrated that Brca1-heterozygous cells create a proliferation-favoring environment compared to wild type, suggesting a role in promoting tumor development. Ongoing in vivo studies using our GEM model as well as in vitro experiments using its derivative organoids will further shed light on the detailed mechanisms that mediate this microenvironmental effect caused by heterozygous Brca1 mutation. Overall, this work may provide significant insights into our fundamental understanding of Brca1-hereditary breast cancer and thereby inform more effective strategies for its prevention, prognosis, and treatment. Furthermore, insights from this work may also be relevant to sporadic cancers with microenvironments altered due to physiologic stress. Thus, our findings may have important implications for both hereditary and sporadic cancers. This abstract is also being presented as Poster B16. Citation Format: Carman M. Li, Yaara Oren, Aviv Regev, Joan S. Brugge. Contribution of mutant microenvironment to hereditary cancer: Single-cell gene expression profiling of a genetically engineered mouse model of human hereditary BRCA1-related breast cancer [abstract]. In: Proceedings of the AACR Special Conference: Advances in Modeling Cancer in Mice: Technology, Biology, and Beyond; 2017 Sep 24-27; Orlando, Florida. Philadelphia (PA): AACR; Cancer Res 2018;78(10 Suppl):Abstract nr PR06.