Activation of PI3K and deletion of p53 in keratin 15-expressing mouse mammary cells induces tumor heterogeneity and plasticity modeling metaplastic breast cancer.

Although relatively rare, metaplastic breast cancer responds poorly to traditional therapies compared to other subtypes. Accordingly, there is a need for novel animal models to understand its pathogenesis and plasticity. Since alterations in PIK3CA and TP53 genes are common in metaplastic breast cancer, we generated a mouse model of metaplastic breast cancer by driving Pik3ca activation and Trp53 loss in keratin 15-expressing mammary cells. In this model, male and female mice developed spontaneous mammary lesions, with malignancy reliant on loss of both Trp53 alleles. Importantly, tumors of this model are heterogeneous and resemble the mixed histology of metaplastic breast cancer by exhibiting squamous cell carcinoma, carcinosarcoma, and sarcoma features. We developed mammary cell lines from mouse tumors representing these different histological subtypes. These Pik3ca-activated tumor cells were more sensitive to alpelisib, a p110α-selective inhibitor approved by the FDA for the treatment of some PIK3CA mutant cancers, compared to Pik3ca WT cells. Additionally, some of these cell lines expressed the androgen receptor, a hormone receptor targeted in prostate cancer and currently under investigation as a therapeutic target in breast cancer. Transplantation of these cell lines into recipient mice maintained histological heterogeneity. Additionally, transplantation of either Epcam+ or Epcam- sorted cells, representing epithelial cell-like and nonepithelial cell-like, respectively, from a carcinosarcoma cell line, initiated tumor formation. Both sorted populations formed tumors with mixed histologic features, demonstrating plasticity arising from different tumor-initiating components. These new models of metaplastic breast cancer from relevant genetic drivers serve as a platform for identifying mechanisms driving plasticity that could inform therapeutic strategies based on histology and reveal how plasticity alters treatment efficacy. © 2025 The Author(s). The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.