Large-scale Characterization of Orthotopic Cell Line–Derived Xenografts Identifies TGFβ Signaling as a Key Regulator of Breast Cancer Morphology and Aggressiveness

Breast cancer is a heterogeneous disease with diverse morphologic and molecular subtypes. Preclinical models that recapitulate the heterogeneity of human breast cancer are needed to advance our fundamental understanding of what makes breast cancer an aggressive disease. To study mechanisms underlying breast cancer progression, we generated orthotopic cell line-derived xenograft (CDX) models from 20 different human breast cancer cell lines using both mammary intraductal injections and fat pad transplantations. The resulting mammary intraductal CDX and fat pad transplantation CDX models covered the full spectrum of disease progression, from in situ disease to metastatic growth. Pathologic analysis revealed two distinct tumor growth morphologies, flat versus nodular, and transcriptomic analysis identified the TGFβ pathway as a potential regulator of these two phenotypes in primary breast cancer. Indeed, knockout of SMAD4 converted nodular-growing tumors to a more confined disease, whereas constitutively active TGFβ receptor I renders lesions more aggressive. This research not only offers insights into the factors driving breast cancer morphology and aggressiveness but also establishes a comprehensive and valuable resource of well-characterized orthotopic CDX models for breast cancer research. SIGNIFICANCE: Establishment and investigation of the largest collection of fully characterized breast cancer cell line xenograft models identifies key determinants of tumor morphology and aggressiveness and enables informed preclinical modeling for further research.