Novel animal model of soft tissue tumor due to aberrant hedgehog signaling activation in pericyte lineage

Pericytes are pluripotent cells that enclose the endothelium of small blood vessels in the whole body. These cells are thought to play a limited role in vascular development and blood pressure regulation; however, current evidence from numerous studies suggests several significant biologic aspects of pericytes in animals. One viewpoint is that pericytes are also known as potential cellular origin of multiple soft tissue tumors. Experimental evidence of the cellular origin of pericytic tumors is still insufficient, however, and their molecular pathogenesis is poorly understood. Here, we used a conditional constitutively active Smoothened allele (Rosa-SmoM2) and Cre recombinase mice to activate hedgehog (Hh) signaling, exclusively in the monocyte/macrophage and osteoclast lineage (LysMcre) or in RANK expressing cells (RANKcre) that are recognized as osteoclast precursor cells. Mice conditionally expressing SmoM2 with LysMcre displayed no significant skeletal phenotype; surprisingly, however, RANKcre; Rosa-SmoM2 mice frequently developed progressive soft tissue tumors in regions of the leg. Genetic lineage tracing analysis uncovered a new domain of RANKcre-expressing cells in the skeletal muscle interstitial cells that display markers consistent with vascular pericytes. Neoplasms arising from these cells showed increased expression of Matrix metalloproteinases (MMPs) that are molecular indicators of malignancy. Moreover, the tumors displayed strong bone invasive potency associated with osteoclastic bone resorption. Thus, these findings provide a novel insight into tumor pathology: Hh signal activated-pericytes can be a potential cellular origin of multiple soft tissue tumors.