CDH1 loss remodels gene expression and lineage identity in human mammary epithelial cells

Invasive lobular carcinoma (ILC) is a common subtype of breast cancer, molecularly defined by genetic loss of CDH1, and subsequent loss of cell adhesion protein E-cadherin, in ~95% of ILC. Though CDH1 loss occurs early in ILC oncogenesis, it is unclear how this facilitates transformation. We modeled early CDH1 loss using "normal" human mammary epithelial cells (HMEC), i.e., finite lifespan cells reflecting early hyperplasia, and targeted E-cadherin signaling using antibodies versus causing genetic CDH1 loss using siRNA or CRISPR/Cas9-knockout. Transcriptome analysis across four HMEC models showed that the mode of E-cadherin targeting 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 CDH1 loss - as in ILC oncogenesis - repressed EMT signatures, and instead remodeled gene expression toward a luminal epithelial phenotype. RNA-seq, single cell transcriptomics, flow cytometry, microscopy, and ATAC-seq analyses support that CDH1 loss induces lineage remodeling to a luminal state, which is mirrored in transcriptomic analysis of clinical ILC precursor lesions. By isolating luminal versus basal cells prior to CDH1 knockout, we found that CDH1 loss led to remodeling of lineage identity in both populations, converging on a new lineage homeostasis with a luminal progenitor-like phenotype. Consistent with the shift to a luminal progenitor phenotype, CDH1 loss enhanced proliferative capacity over the finite lifespan of the HMECs, highlighting a feature of early CDH1 loss that may contribute to clonal advantage during tumor initiation. Moreover, CDH1 loss enhanced anoikis resistance, a defining feature of ILC cells. Our findings support that genetic loss of CDH1 in mammary epithelial cells induces transcriptional and phenotypic changes consistent with lineage identity remodeling toward a luminal progenitor-like state, which may underpin the mechanism by which early CDH1 loss mediates ILC oncogenesis.