Compartmentalization of signals generated by receptor tyrosine kinase (RTK) endocytosis has emerged as a major determinant of various cell functions. Here, using tumour-associated Met-activating mutations, we demonstrate a direct link between endocytosis and tumorigenicity. Met mutants exhibit increased endocytosis/recycling activity and decreased levels of degradation, leading to accumulation on endosomes, activation of the GTPase Rac1, loss of actin stress fibres and increased levels of cell migration. Blocking endocytosis inhibited mutants' anchorage-independent growth, in vivo tumorigenesis and metastasis while maintaining their activation. One mutant resistant to inhibition by a Met-specific tyrosine kinase inhibitor was sensitive to endocytosis inhibition. Thus, oncogenicity of Met mutants results not only from activation but also from their altered endocytic trafficking, indicating that endosomal signalling may be a crucial mechanism regulating RTK-dependent tumorigenesis.

One of the barriers for breast cancer prevention and treatment is our poor understanding of the dynamic cellular shifts that naturally occur within the breast and how these changes contribute to tumour initiation. In this study we report the use of single cell RNA sequencing (scRNAseq) to compile a Human Breast Cell Atlas (HBCA) assembled from 55 donors that had undergone reduction mammoplasties or risk reduction mammoplasties. The data from more than 800,000 cells identified 41 cell subclusters distributed across the epithelial, immune, and stromal compartments. We found that the contribution of these different clusters varied according to the natural history of the tissue. Breast cancer risk modulating factors such as age, parity, and germline mutation affected the homeostatic cellular state of the breast in different ways however, none of the changes observed were restricted to any one cell type. Remarkably, we also found that immune cells from BRCA1/2 carriers had a distinct gene expression signature indicative of potential immune exhaustion. This suggests that immune escape mechanisms could manifest in non-cancerous tissues during very early stages of tumour initiation. Therefore, the Atlas presented here provides the research community with a rich resource that can be used as a reference for studies on the origins of breast cancer which could inform novel approaches for early detection and prevention.

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