loss remodels gene expression and lineage identity in human mammary epithelial cells.

Invasive lobular carcinoma (ILC) is a common subtype of breast cancer that is defined in part by genetic loss of caused by mutation or deletion, leading to loss of cell adhesion protein E-cadherin in >90% of ILC. Genetic loss of is an early event in ILC oncogenesis, yet the mechanisms by which E-cadherin acts as a tumor suppressor are not well understood. To study how early loss drives ILC oncogenesis, we used a series of non-transformed human mammary epithelial cell (HMEC) models to target /E-cadherin, inhibiting extracellular E-cadherin signaling using antibodies versus modeling genetic loss using siRNA or knockout via CRISPR/Cas9. Through transcriptome analyses across four HMEC models, we found that the mode of E-cadherin loss or suppression is critical for the subsequent phenotype. Antibody-mediated inhibition of cell-cell contacts induced gene signatures of epithelial-mesenchymal transition (EMT), consistent with the role of E-cadherin suppression during the EMT process. Conversely, genetic loss - as in ILC oncogenesis - repressed EMT signatures, and instead remodeled gene expression toward a luminal epithelial phenotype. Using single cell transcriptomics and flow cytometry analyses of cell lineage markers, we found that genetic loss of reprogrammed cells to a luminal progenitor-like phenotype. By isolating luminal versus basal cells prior to knockout, we found that loss led to remodeling of lineage identity in both populations, converging on a new lineage homeostasis with a luminal progenitor-like phenotype. Consistent with increased progenitor features, loss enhanced proliferative capacity over the finite lifespan of the HMECs, highlighting a feature of early loss that may contribute to clonal advantage during tumor initiation. Our findings support that inhibition of E-cadherin results in different transcriptional response compared to loss, with the latter driving a transcriptional and phenotypic state characteristic of a luminal progenitor-like population, which offers new insight into early events in ILC oncogenesis.