Abstract PR07: Elucidating mechanisms of p53-deficient breast cancer development via lineage tracing and clonal analysis

TP53 is the most frequently mutated gene in human breast cancers and its mutations have been found in all breast cancer subtypes (e.g., luminal, basal-like, claudin-low). Genome-sequencing studies predict that TP53 mutation is an early event in breast cancer development. However, how p53-deficiency contributes to breast cancer initiation, leading to development of different subtypes of breast cancer, remains largely elusive. To address this, we developed a novel approach to trace development of breast cancer from luminal mammary epithelial cells (MECs), which may represent cells of origin for most breast cancers. Our approach is based on induced loss of p53 in luminal MECs at the level of single cells, via intraductal injection of Cre-expressing adenovirus under the control of the pan-luminal Keratin 8 promoter (Ad-K8-Cre). This genetic approach permits evolution of single mutant luminal MECs in their native habitat through interaction with the microenvironment (e.g., neighboring p53-wild type MECs, stromal cells, immune cells), and allows us to dissect contributions from genetic, epigenetic, and environmental factors to breast tumorigenesis. Lineage and clonal analyses revealed that induced loss of p53 alone in luminal MECs triggered their clonal expansion without affecting their original luminal identity, leading to formation of a p53-deficient luminal premalignant field. Molecular analysis of luminal MECs in this premalignant field revealed potential involvement of environmental factors (e.g., cyclic ovarian hormones, immune cells in the mammary gland microenvironment) in its formation. Claudin-low mammary tumors eventually emerged from this p53-deficient luminal premalignant field with 100% penetrance, apparently due to acquisition of spontaneous, recurrent mutations in p53-deficient MECs. These data support that p53-deficiency alone does not affect the fate of luminal MECs directly, but it facilitates development of mammary tumors with loss of the luminal identity after a long latency. As deficiencies of both p53 and BRCA1 in luminal MECs predispose them to development of basal-like breast cancer (rather than claudin-low cancer), we studied whether induced loss of BRCA1 in p53-deficient luminal MECs would lead to their cell fate alteration. In fact, when we induced loss of both p53 and BRCA1 in luminal MECs, we observed luminal-to-basal cell fate alteration only after a short latency. Together, our data suggest that loss of p53 in single luminal MECs leads to their clonal dominance over wild-type neighbors without directly impairing their luminal fate; it is the cooperating oncogenic events (e.g., loss of BRCA1, or acquisition of recurrent secondary mutations) that drive development of breast cancer from p53-deficient luminal cells toward different subtypes. Our results also predict that loss of p53 in luminal MECs of BRCA1 mutation carriers would not only lead to their clonal dominance, but also cause luminal-to-basal change within the mutant clones (as an early event), and ultimately development of basal-like breast cancer from these aberrant clones. This abstract is also being presented as Poster A35. Citation Format: Dongxi Xiang, Luwei Tao, Zhe Li. Elucidating mechanisms of p53-deficient breast cancer development via lineage tracing and clonal analysis [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 PR07.