Mathematics is often used to model biological systems. In mammary gland development, mathematical modeling has been limited to acinar and branching morphogenesis and breast cancer, without reference to normal duct formation. We present a model of ductal elongation that exploits the geometrically-constrained shape of the terminal end bud (TEB), the growing tip of the duct, and incorporates morphometrics, region-specific proliferation and apoptosis rates. Iterative model refinement and behavior analysis, compared with biological data, indicated that the traditional metric of nipple to the ductal front distance, or percent fat pad filled to evaluate ductal elongation rate can be misleading, as it disregards branching events that can reduce its magnitude. Further, model driven investigations of the fates of specific TEB cell types confirmed migration of cap cells into the body cell layer, but showed their subsequent preferential elimination by apoptosis, thus minimizing their contribution to the luminal lineage and the mature duct.

Colon cancer cells frequently carry mutations that activate the β-catenin and mitogen-activated protein kinase (MAPK) signaling cascades. Yet how oncogenic alterations interact to control cellular hierarchies during tumor initiation and progression is largely unknown. We found that oncogenic BRAF modulates gene expression associated with cell differentiation in colon cancer cells. We therefore engineered a mouse with an inducible oncogenic BRAF transgene, and analyzed BRAF effects on cellular hierarchies in the intestinal epithelium in vivo and in primary organotypic culture. We demonstrate that transgenic expression of oncogenic BRAF in the mouse strongly activated MAPK signal transduction, resulted in the rapid development of generalized serrated dysplasia, but unexpectedly also induced depletion of the intestinal stem cell (ISC) pool. Histological and gene expression analyses indicate that ISCs collectively converted to short-lived progenitor cells after BRAF activation. As Wnt/β-catenin signals encourage ISC identity, we asked whether β-catenin activity could counteract oncogenic BRAF. Indeed, we found that intestinal organoids could be partially protected from deleterious oncogenic BRAF effects by Wnt3a or by small-molecule inhibition of GSK3β. Similarly, transgenic expression of stabilized β-catenin in addition to oncogenic BRAF partially prevented loss of stem cells in the mouse intestine. We also used BRAF V637E knock-in mice to follow changes in the stem cell pool during serrated tumor progression and found ISC marker expression reduced in serrated hyperplasia forming after BRAF activation, but intensified in progressive dysplastic foci characterized by additional mutations that activate the Wnt/β-catenin pathway. Our study suggests that oncogenic alterations activating the MAPK and Wnt/β-catenin pathways must be consecutively and coordinately selected to assure stem cell maintenance during colon cancer initiation and progression. Notably, loss of stem cell identity upon induction of BRAF/MAPK activity may represent a novel fail-safe mechanism protecting intestinal tissue from oncogene activation.

The mammary gland is a dynamic organ that undergoes extensive morphogenesis during the different stages of embryonic development, puberty, estrus, pregnancy, lactation and involution. Systemic and local cues underlie this constant tissue remodeling and act by eliciting an intricate pattern of responses in the mammary epithelial and stromal cells. Decades of studies utilizing methods such as transplantation and lineage tracing have identified a complex hierarchy of mammary stem cells, progenitors and differentiated epithelial cells that fuel mammary epithelial development. Importantly, these studies have extended our understanding of the molecular crosstalk between cell types, and signaling pathways maintaining normal homeostasis that often are deregulated during tumorigenesis. While several questions remain, this research has many implications for breast cancer. Fundamental among these are the identification of the cells of origin for the multiple subtypes of breast cancer and the understanding of tumor heterogeneity. A deeper understanding of these critical questions will unveil novel breast cancer drug targets and treatment paradigms. In this review, we provide a current overview of normal mammary development and tumorigenesis from a stem cell perspective.

Cellular hierarchies and signals that govern stemness and differentiation of intestinal adenoma cells are not well defined. In this study, we used organotypic culture to investigate the impact of b-catenin and BMP signals in cells that form intestinal adenoma in the mouse. We found that activation of b-catenin signaling by loss of APC or transgenic induction of oncogenic mutant b-catenin (Ctnnb1 mut ) initiates the conversion of untransformed intestinal cells to tumor cells. These tumor cells display cancer stem cell (CSC) traits such as increased expression of the CSC markers Cd133 and Cd44, a high capacity for self-renewal and unlimited proliferative potential. Subsequent inactivation of transgenic Ctnnb1 mut results in the reversion of tumor cells to normal intestinal stem cells, which immediately reinstall the cellular hierarchy of the normal intestinal epithelium. Our data demonstrate that oncogenic activation of b-catenin signaling initiates the early steps of intestinal cellular transformation in the absence of irreversible genetic or epigenetic changes. Interestingly, we found that tumor cells in culture and in adenoma produce BMP4, which counteracts CSC-like traits by initiating irreversible cellular differentiation and loss of self-renewal capacity. We conclude that the opposition of stemness-maintaining oncogenic b-catenin signals and autocrine differentiating BMP signals within the adenoma cell provides a rationale for the formation of cellular hierarchies in intestinal adenoma and may serve to limit adenoma growth.In the intestine, recurring oncogenic mutations play key roles in tumor initiation and progression. 1 Such mutations often target components of conserved signaling pathways that underlie the maintenance of a cellular hierarchy in the intestinal epithelium. For instance, activity of the Wnt/b-catenin signaling pathway is in the normal intestine restricted to the stem cell compartment and provides cues for stem cell maintenance and cellular proliferation. 2 In intestinal adenoma, which is a benign precursor of carcinoma, b-catenin signaling is generally activated by oncogenic mutations in APC, which controls b-catenin stability, or in CTNNB1 (coding for b-catenin) itself.3,4 Mutations in Apc or Ctnnb1 also induce formation of intestinal adenoma in mice, such as in Apc Min mice. 5-8Active BMP signaling, on the other hand, antagonizes Wnt/b-catenin and is normally restricted to the differentiated cell compartment of the intestine, and excluded from the stem cell niche. [9][10][11] Inactivating mutations in components of the BMP signaling pathway are often found in sporadic human carcinoma and in Juvenile Polyposis. Studies in mice show that loss of BMP signals can initiate intestinal tumorigenesis and contribute to tumor progression (for review, Ref. 9).In contrast to cellular hierarchies of the normal intestine, the perturbed hierarchies formed by mutated intestinal tumor cells have not been well defined. Morphological and gene expression heterogeneities, such as unequal distribution of cell ...