Abstract A065: Single cell transcriptomics reveals putative tumor promoting subpopulation in Brca1 mutant breast cancer mouse model
Authors: Pathania Shailja , Rivera Joshua , Tran Stevension , Li Carman Man Chung , Brugge Joan , Zarringhalam Kourosh 
Published: February 2024

Abstract
Women with BRCA1 (B1) mutations have an exceptionally high risk of developing breast cancer. The only effective preventive strategy currently offered to these women is the life altering prophylactic mastectomy. Considering the limited treatment options available, it is critical that new preventive strategies be identified. Design of such strategies requires an understanding of early events in the breast cells that drive tumorigenesis. B1 heterozygous mouse models can help us identify these early changes however, despite the well-established association between B1 heterozygosity and cancer predisposition in humans, there are currently no such B1 mouse models that faithfully recapitulate this high risk of tumor formation. We have now established a mouse model that induces mammary tumors in B1 heterozygous (Brca1wt/flx,Trp53flx/flx,K14cre) mice upon replication stress (RS). Our approach is based on our published work that reveals haploinsufficiency for RS suppression in B1 heterozygous cells. We find that increasing RS in B1 heterozygous mice results in accelerated mammary tumorigenesis. RS in this mouse model was delivered by injecting DNA-adduct forming 4-nitroquinoline-1-oxide (4NQO) via mammary intraductal injections over a course of 4 weeks. RS served as an efficient and abnormally rapid driver of tumor formation (30 days post completion of injection regimen) in B1 heterozygous mice. B1 heterozygous mice formed mammary adenocarcinoma. These tumors exhibit a triple negative breast cancer phenotype similar to that found in human BRCA1 mutant breast cancer. Our scRNAseq-based analysis of 42,000 cells from mouse mammary tissue collected at different time points during tumorigenesis has identified early transcriptomic changes that occur in different mammary cell types (e.g., luminal and basal) as they respond to RS. We find that in response to RS, a population of trans-differentiated cells accumulates in the mammary tissue. This population is bi-potent (expresses both luminal progenitor and basal epithelial markers), is specifically enriched in Brca1 heterozygous mammary tissue and expresses certain prognostic markers with a strong correlation to poor outcome in human breast cancer. This population is also enriched for proliferation markers. We suspect that this transdifferentiated population, which is primarily enriched in B1 heterozygous mammary tissue undergoing RS, marks the earliest cancer promoting cell population. Finally, pseudotime reconstruction-based lineage analysis reveals that alveolar luminal progenitor cells are the most likely cell of origin for this putative tumor promoting cell population in the breast. Altogether, our integrative approach reveals that B1 heterozygosity in combination with RS leads to accumulation and proliferation of a specific mammary cell population with a unique transcriptomic profile, that contributes to breast tumorigenesis. Identification of such early drivers is critical for the design of effective preventive and therapeutic strategies for women with BRCA1 mutation.
Citation Format: Shailja Pathania, Joshua Rivera, Stevension Tran, Carman Man Chung Li, Joan Brugge, Kourosh Zarringhalam. Single cell transcriptomics reveals putative tumor promoting subpopulation in Brca1 mutant breast cancer mouse model [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Advances in Breast Cancer Research; 2023 Oct 19-22; San Diego, California. Philadelphia (PA): AACR; Cancer Res 2024;84(3 Suppl_1):Abstract nr A065.