Large-scale characterisation of cell line xenografts identifies SMAD4 as key regulator of breast cancer morphology and aggressiveness

Breast cancer (BC) is a complex and heterogeneous disease with diverse molecular subtypes, tumour growth dynamics, varying morphologies and high metastatic potential. Many advances have progressed our understanding of the disease, yet fundamental understanding of what makes breast cancer an aggressive entity is lacking. While the majority of available studies rely on a limited set of models, failing to recapitulate the full spectrum of breast cancer, an extensive resource of well characterized xenograft models may aid in gaining solid and reproducible insights into BC biology. In this study, we generated mouse models from 26 different human BC cell lines, each with unique characteristics, using mammary intraductal (MIND) and fat pad transplantation (FPT) methodologies. Our models faithfully recapitulate human disease and cover the full spectrum of disease progression, from in situ disease to metastatic growth, while retaining distinct growth characteristics and molecular subtype. Comprehensive pathological analysis revealed two distinct tumour growth morphologies: indolent vs aggressive. Transcriptomics analysis identified the TGF-beta pathway as a potential regulator of these two phenotypes. Strikingly, we were able to completely reverse aggressive nodular-growing tumours to a more indolent disease by knockout of SMAD4. This research provides a valuable resource of fully characterized in vivo cell line xenograft models and reveals novel insights into the factors driving BC tumour growth behaviour and aggressiveness. It therefore serves as a basis for understanding the underlying mechanisms of tumour development and progression and enables accurate choice of model for future studies.