Women with mutations in breast cancer (BrCa) gene BRCA1 are highly predisposed to BrCa. In these families, cancer develops almost exclusively in females and often at very young ages. The molecular pathogenic steps, especially the earliest ones that drive the transition of normal mammary epithelial cells (BRCA1mut/+) to tumor cells, are largely unknown. Mouse model-based studies have not successfully addressed this question, in part because, unlike women with BRCA1 (B1) mutations, B1 heterozygous mice do not form mammary tumors any faster than the wild-type (wt) mice. A mouse model that can biologically mimic what is happening in the mammary tissue of B1 mutation carriers would be an invaluable tool to study B1 mutant BrCa. Furthermore, most of the efforts to study breast tumor-associated gene changes have been based on GWAS studies or RNA-seq based analysis of the breast tumor samples. Such studies have shown that the transcriptional network is altered in the breast tumors compared to wild-type tissue. However, whether those changes are tumor initiating, i.e., “driver” changes, or are “passenger” mutations/changes is still not clear. Given the role of B1 in suppressing replication stress (RS), a tumor-promoting event, and the increased RS observed in B1 heterozygous cells, we have established an RS-driven B1 BrCa mouse model to study the early genetic changes that drive B1 mutant cancer. We find that intraductal injections of 4-nitroquinoline-1-oxide (4NQO1, a replication stress-inducing agent) leads to breast adenocarcinoma in B1flx/wt;Trp53flx/flx,K14Cre mice on average 84 days post injections unlike B1 wt mice that do not make tumors. This is the first RS-induced BrCa model that can successfully recapitulate B1 mutant BrCa. The fast timeline of tumor formation provides significant advantage for studying tumor formation, progression, and response to therapy. Furthermore, single-cell RNA sequencing analysis of the breast adenocarcinoma and tissue collected early during injections in these mice has revealed presence of a highly proliferative “transdifferentiated population” that shares properties of both luminal and basal cells (e.g., K8+K18+) and also exists in the hyperplastic tissue. No such population was found in control PBS injected mammary fat pad of B1flx/wt;Trp53flx/flx,K14Cre mice, indicating that this population might be dependent on RS. We also find that this population is enriched very early during mammary tumorigenesis and some of the top hits include cancer stem cell markers. We have also confirmed the presence of this population by immune histochemistry-based experiments. Finally, our study provides evidence for a BrCa mouse model where basal-like tumor formation is driven by RS. We also found evidence for an early precancerous transdifferentiated population that is enriched upon RS and have identified early markers that can potentially drive breast tumorigenesis in B1 mutant BrCa. Identification of such candidate genes is critical in the design of future preventive and/or therapeutic strategies.

Citation Format: Joshua Rivera, Delan Khalid, Stevenson Tran, Kemmie Kibaja, Shailja Pathania. Tracking the path of breast tumorigenesis in BRCA1 mutant breast cancer [abstract]. In: Proceedings of the AACR Special Conference on the Evolving Landscape of Cancer Modeling; 2020 Mar 2-5; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2020;80(11 Suppl):Abstract nr B36.