Hippo Inactivation in the Mammary Epithelium Drives the Evolution of a Tumor-Associated Niche

Basal-like breast cancers exhibit distinct intratumor cell heterogeneity that contributes to disease pathology. In this study we use a genetic mouse model of basal-like breast cancer driven by epithelial-specific inactivation of the Hippo pathway-regulating Lats1 and Lats2 kinases to elucidate epithelial-stromal interactions in the basal-like tumor niche. We demonstrate that basal-like carcinoma initiation is accompanied by the accumulation of distinct cancer-associated fibroblasts, tumor-associated macrophage recruitment, and dramatic extracellular matrix remodeling, phenocopying the stromal diversity observed in human triple-negative breast tumors.Epithelial-stromal signaling dysregulation was observed, including increased TGF-β, PDGF, and CSF intercellular communication. Autonomous activation of the transcriptional effector TAZ was observed in Lats1/2-deleted cells along with non-autonomous activation within the evolving tumor niche. We further demonstrate that small molecule inhibition of the YAP/TAZ-associated TEAD family of transcription factors can block the development of the carcinomas and tumor-associated microenvironment. These observations demonstrate that carcinomas resulting from Hippo pathway dysregulation in the mammary epithelium are sufficient to drive cellular events that promote a basal-like tumor-associated niche and suggest that targeting dysregulated YAP/TAZ-TEAD activity provides an opportunity for therapeutic intervention of basal-like mammary tumors. We utilized a genetic mouse model allowing conditional deletion of Lats1/2 in Krt8+cells to study the impact of epithelial-specific Hippo pathway inactivation on mammary epithelial and stromal cell populations. These mice included Lats1 and Lats2-floxed alleles, a Tamoxifen-inducible Cre recombinase under the control of the Krt8 promoter, and a loxP-stop-loxP(lsl)-EYFP lineage trace to mark Cre+ cells (Lats1/2ff;lsl-EYFP; Krt8CreERT2). After administration of Tamoxifen, mammary glands from these and control (Lats1/2ff;lsl-EYFP) mice were dissociated and live cells were subsequently sorted for single-cell RNA-sequencing. Two mice were used for each condition.