Single-cell RNAseq uncovers involution mimicry as an aberrant development pathway during breast cancer metastasis

Single-cell RNA-seqhas emerged as a powerful method to unravel heterogeneity of complex biological systems; this has enabled in vivo characterization of cell type compositions through unsupervised sampling and modelling of transcriptional states in single cells. Here we used the high-throughput microfluidic-based single-cell RNA-seqmethod Drop-seq to elucidate the cellular composition and functional diversity of breast tumoursduring the induction of metastatic disease in a transgenic model related to pregnancy-associated breast cancer (PABC). We characterised with unprecedented definition, how the activation of developmental programs associated to pregnancy results in the acquisition of an aggressive phenotype. We show that cancer cells are classified in a structure comparable with the lineages of the epithelial mammary gland hierarchy, revealing high dynamics and plasticity of cancer cells during disease progression. This cancer progression program is orchestrated by alveolar milk secretory cells, that in conjunction with cell types from the tumour microenvironment (TME), including cancer-associated fibroblasts (CAFs), form a multi-cellular process that resembles an aberrant involution. Finally, we analysed the interactomeof the tumour ecosystem to define a high-resolution landscape of the molecular pathways of cell-to-cell communication that underpins extra-cellular remodelling and inflammation associated with the aggressive involution mimicry. In conclusion, our study recapitulates developmental mechanisms that have gone awry during carcinogenesis in a model of PABC. We provide a large-scale single-cell transcriptional landscape of breast tumoursthat allows unprecedented understanding of breast heterogeneity and deep analysis of key molecular events that result in cancer malignancy.