Breast cancer is a heterogenous disease. It is becoming increasingly apparent that in many breast cancers, a minority of neoplastic cells comprising the tumor are drivers of the malignant, metastatic seeding ability and inherent chemoresistance of a tumor. We term these cells “Tumor Propagating Cells” (TPCs), referring only to their functional activity and not to their cell of origin. The characterization of TPCs has been largely hindered by the lack of reliable methods for the isolation and purification of these cells including the current best practice of using cell surface markers. Our strategy to isolate TPCs is to develop “molecular probes” that report on the activity of pathways controlling the TPC phenotype. We have identified a transcription factor, Id1 (Inhibitor of Differentiation 1) which is expressed by a rare cell in ~ 50% of Hormone receptor negative (HR-) breast cancers. The Id family (Id1-4) are basic Helix Loop Helix (bHLH) proteins which hetero-dimerize with other bHLH transcription factors and control their transcriptional activity. Id1 has a general role in the inhibition of differentiation and lineage commitment and its expression is required for the maintenance of self renewal and multi-potency in numerous stem cells (like hematopoietic, neuronal and embryonic). Work from our lab and others have shown that in a cancer context, Id1 is required and sufficient to drive breast cancer cell proliferation, migration and invasion.
Based on their role in mammary gland development and tumorigenesis, we hypothesize that Id1 expressing cells are TPCs in the HR- subtype. Using a lentiviral vector encoding the Id1 proximal promoter upstream of the EGFP cDNA to mark Id1 + cells, we are able to prospectively isolate the rare Id1+ cellular fraction in the p53-/- mouse model of Triple Negative breast cancer. Id1+ tumour cells have high in vivo tumour reconstituting activity as well as increased in vitro self renewal capacity (tumorsphere assay). We have demonstrated that knock down of Id1 leads to diminished primary tumor formation in vivo using the 4T1 syngeneic mouse model and are investigating Id1’s role in a metastatic context. Current directions include defining the transcriptional and proteomic landscape of the rare cells which are critical for tumorigenesis and metastasis, with the ultimate aim of identifying and targeting these cells in a therapeutic context.