The role of MYC in bone and in the hematopoietic stem cell niche

The nuclear protein c-Myc is one of the most potent proto-oncogenes described. Its expression is found to be deregulated in more than 50 percent of human cancers, where it has been shown to control a number of biological processes including driving proliferation, cell growth and angiogenesis while inhibiting differentiation. The effects of c-Myc are highly cell type specific and include cell autonomous and non-cell autonomous processes. In order to examine the effect of excess amounts of c-Myc in bone, we have generated a mouse line expressing the tTA transactivator under the control of the mouse 2.3 kb col1a1 promoter (named Col1a1::tTA), which directs transgene expression to mature osteoblasts. By combining this transgene with the Tet-O-Myc allele, we generated a Col1a1::tTA; Tet-O-Myc double transgenic mouse model in which it is possible to inducibly express human c-MYC in osteoblasts during development and in the adult. We document that the used 'tet-off' system directs transgene expression to bones and that c-MYC can be completely repressed by treatment with doxycycline. Using this mouse model, we demonstrate that overexpression of c-MYC leads to remarkable changes in mutant bones, which depend greatly in their severity on the developmental time point of c-MYC induction. Unrestricted c-MYC expression, starting with the first emergence of mature osteoblasts in the fetal skeleton (∼E14.0), leads to lethality of the mutants at birth; their bones are shortened and thicker than in controls. Later induction of c-MYC allows survival, but leads to significant changes in bone morphology. Mutant bones develop a very irregular bone structure with signs of excessive bone formation. Surprisingly, this also correlates with definite signs of excessive osteoclast activity, which explains the formation of cavities seen in the bone matrix. Osteoblasts not only produce bone matrix, they are also very important niche cells for hematopoietic cells, especially for hematopoietic stem cells and B cells. We find that c-MYC overexpression severely impairs B lymphopoiesis and also observe signs of extramedullary hematopoiesis in the spleen, which is indicative of a compromised HSC environment in the bone marrow. Most strikingly, these animals develop frank osteosarcomas leading to a much reduced life span. Tumors arise in various bones, with the ribs and hips being the most prominent sites. These c-MYC driven tumors are very aggressive as is evident by their capacity to spontaneously metastasize to lung and liver, a rather rare phenomenon in mouse models, indicating that these tumors share important aspects of human osteosarcomas. Molecular analysis of c-MYC overexpressing osteoblasts has revealed a defect in their terminal differentiation program, rendering them incapable to progress from a relatively immature proliferative to a quiescent, calcified matrix producing state. To our knowledge this is the first report of experimental osteosarcoma formation, originating from a mature and lineage committed osteoblast population indicating that c-MYC may be able to de-differentiate more mature cells. In summary, our novel conditional mouse model showed that c-MYC expression in mature osteoblasts can lead to the formation of highly aggressive and metastatic osteosarcomas, a lethal disease in humans. This model will be the basis for further studies elucidating the molecular effects of c-MYC on de-differentiation and metastasis formation, processes, which remain poorly understood but are critical events in highly malignant cancers.