Stem cell functionality is microenvironmentally defined during tumor expansion and therapy response in colon cancer

Solid malignancies have been speculated to depend on cancer stem cells (CSCs) for expansion and relapse after therapy. Here we report on quantitative analyses of lineage tracing data from primary colon cancer xenograft tissue to assess CSC functionality in a human solid malignancy. The temporally obtained clone size distribution data support a model in which stem cell function in established cancers is not intrinsically but entirely spatiotemporally orchestrated. Functional stem cells that drive tumour expansion predominantly reside at the tumour edge, close to cancer-associated fibroblasts (CAFs). Hence, stem cell properties change in time depending on the cell location. Furthermore, although chemotherapy enriches for cells with a CSC phenotype, also in this context functional stem cell properties are fully defined by the microenvironment. To conclude, we identified osteopontin (OPN) as a key CAF-produced factor that drives in situ clonogenicity in colon cancer. RNA-Seq was performed to compare gene expression between the edge and the centre of tumors, derived from human colorectal cancer cell lines grown as PDX in nude mice.