Abstract 265: Reconstructing the early steps of breast tumor initiation

Abstract Breast cancer screening programs, detect not only the early stages of the disease but also various preneoplastic lesions which are very difficult to predict if and when they may develop into cancer. This uncertainty can lead to over-diagnosis, over-treatment and stress for millions of screened women. As consequence, discovering reliable biomarkers of this evolution could make it possible to monitor these patients appropriately and propose an effective prevention strategy. To meet this challenge, we urgently need to better understand the mechanisms that regulate human mammary homeostasis during early steps of tumor initiation. The hierarchy and the composition of human mammary epithelium, composed of stem, myoepithelial, luminal progenitors and mature luminal cells, are complex. This complexity raises the question of whether a given mammary epithelial cell is equally likely to be the cell-of-origin of breast cancer. Interestingly, increasing evidences show that disruption of lineage integrity can be a precursor of breast tumor initiation, and the basal and the luminal lineages respond differently to a given oncogenic stress and may each follow their own rule towards malignant transformation. These results strongly suggest that each lineage may have its own pliancy to be transformed by a given oncogene, hijacking the inter-tumoral heterogeneity observed in breast cancers. The objectives of this study are to develop a model to reconstruct the early steps of breast tumor initiation and understand the interplay between the initiating oncogenic event and the cell-of-origin. To answer these questions, we used a human mammary epithelial cell lines (HME) that maintains a cellular hierarchy with mammary stem cells (MaSC) and luminal cells (LC). Upon oncogenic expression of HRASG12V or c-Myc, we observed different modifications of cell homeostasis, with altered proportions of the lineages populations in the cell line. Depending on the oncogenic stress induced, we also demonstrated a modification in the morphology and the cellular composition of HME organoids, as well as in the capacity to form tumors. HRASG12V induces increase of MaSC compartment, formation of more branched organoids and generate aggressive basal-like tumors. Conversely, c-MYC leads to an almost loss of MaSC and formation of more proliferative unbranched organoid and generate tumors with limited growth. To address more precisely how cell-of-origin and lineage plasticity influence malignant transformation, we are developing for the first time a lineage tracing model in human mammary epithelium with HME. Moreover, a single cell RNA-seq was performed at different early time points during HME cellular differentiation in homeostasis and upon oncogenic stress to decipher the molecular mechanisms that underpin the very earliest steps of breast tumor initiation. Citation Format: Anaïs Grandon, Shuheng Lin, Martin Castagné, Julien Wicinski, Olivier Rosnet, Christophe Ginestier, Emmanuelle Charafe-Jauffret. Reconstructing the early steps of breast tumor initiation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 265.