@article {Teo497313, author = {Wee S. Teo and Holly Holliday and Nitheesh Karthikeyan and Aur{\'e}lie S. Cazet and Daniel L. Roden and Kate Harvey and Christina Valbirk Konrad and Reshma Murali and Binitha Anu Varghese and Archana P. T. and Chia-Ling Chan and Andrea McFarland and Simon Junankar and Sunny Ye and Jessica Yang and Iva Nikolic and Jaynish S. Shah and Laura A. Baker and Ewan K.A. Millar and Mathew J. Naylor and Christopher J. Ormandy and Sunil R. Lakhani and Warren Kaplan and Albert S. Mellick and Sandra A. O{\textquoteright}Toole and Alexander Swarbrick and Radhika Nair}, title = {Id proteins promote a cancer stem cell phenotype in triple negative breast cancer via negative regulation of Robo1}, elocation-id = {497313}, year = {2020}, doi = {10.1101/497313}, publisher = {Cold Spring Harbor Laboratory}, abstract = {Breast cancers display phenotypic and functional heterogeneity and several lines of evidence support the existence of cancer stem cells (CSCs) in certain breast cancers, a minor population of cells capable of tumor initiation and metastatic dissemination. Identifying factors that regulate the CSC phenotype is therefore important for developing strategies to treat metastatic disease. The Inhibitor of Differentiation Protein 1 (Id1) and its closely related family member Inhibitor of Differentiation 3 (Id3) (collectively termed Id) are expressed by a diversity of stem cells and are required for metastatic dissemination in experimental models of breast cancer. In this study, we show that ID1 is expressed in rare neoplastic cells within ER-negative breast cancers. To address the function of Id1 expressing cells within tumors, we developed two independent murine models of Triple Negative Breast Cancer (TNBC) in which a genetic reporter permitted the prospective isolation of Id1+ cells. Id1+ cells are enriched for self-renewal in tumorsphere assays in vitro and for tumor initiation in vivo. Conversely, depletion of Id1 and Id3 in the 4T1 murine model of TNBC demonstrates that Id1/3 are required for cell proliferation and self-renewal in vitro, as well as primary tumor growth and metastatic colonization of the lung in vivo. Using combined bioinformatic analysis, we have defined a novel mechanism of Id protein function via negative regulation of the Roundabout Axon Guidance Receptor Homolog 1 (Robo1) leading to activation of a Myc transcriptional programme.CSCCancer stem cellTNBCTriple Negative Breast CancerIdId1 and Id3}, URL = {https://www.biorxiv.org/content/early/2020/03/11/497313}, eprint = {https://www.biorxiv.org/content/early/2020/03/11/497313.full.pdf}, journal = {bioRxiv} }