Mouse genetic mosaic model for basal-like breast cancer reveals disruption of basal-myoepithelial architecture at premalignancy.

In early cancer evolution, mutant cells must escape tissue-architectural constraints before progressing to malignancy. In the case of invasive breast cancers, loss of integrity of the wildtype basal-myoepithelial layer that wraps around premalignant luminal cells precedes the formation of an invasive tumor mass. The spatiotemporal details of these early events have remained elusive. Here, we used the Mosaic Analysis with Double Markers (MADM) mouse model of basal-like breast cancer to generate rare, GFP-labeled, premalignant luminal cells within an otherwise colorless, normal mouse, and pinpointed hyperalveolarization from the ducts as a key phase of progression, several months before malignant transformation. Using a whole-mount tissue clearing-staining-imaging workflow to examine the colorless basal-myoepithelial cell layer that surrounds GFP-labeled premalignant luminal cells, we observed a gradual disruption of the basal-myoepithelial layer during the hyperalveolarization phase. Increased cell size, altered cell morphology, and elevated proliferation of mutant cells occurred independently of basal disruption. In contrast, expression of the basal marker Keratin 14 in premalignant luminal cells, a feature that contributes to the "basal-like" nomenclature of this cancer, was strongly associated with basal disruption, suggesting that basal cell-mediated containment may suppress a critical transition in premalignant cell state. This work identifies disruption of the basal cell layer as an early hallmark of premalignant progression, supporting a chronology in which architectural remodeling of the mammary epithelium precedes malignant progression.