Large-Scale Characterization of Orthotopic Cell Line-Derived Xenografts Identifies TGF-β Signaling as a Key Regulator of Breast Cancer Morphology and Aggressiveness

Breast cancer (BC) is a heterogeneous disease with diverse morphological and molecular subtypes. Preclinical models that recapitulate the heterogeneity of human BC are needed to advance our fundamental understanding of what makes BC an aggressive disease. To study mechanisms underlying BC progression, we generated orthotopic cell line-derived xenograft (CDX) models from 20 different human BC cell lines using both mammary intraductal (MIND) injections and fat-pad transplantations (FPT). The resulting MIND-CDX and FPT-CDX models covered the full spectrum of disease progression, from in situ disease to metastatic growth. Pathological analysis revealed two distinct tumor growth morphologies, flat vs. nodular, and transcriptomics analysis identified the TGF-β pathway as a potential regulator of these two phenotypes in primary BC. Indeed, knockout of SMAD4 converted nodular-growing tumors to a more confined disease, while constitutively active TGFBR1 renders lesions more aggressive. This research not only offers insights into the factors driving BC morphology and aggressiveness but also establishes a comprehensive and valuable resource of well-characterized orthotopic CDX models for BC research.