Project Summary/Abstract
Heterotopic ossification (HO) refers to extraskeletal pathological bone formation that occurs as a common
complication after injury or as a manifestation of particular genetic disorders. However, limited by poor
understanding of the underlying cellular and molecular mechanisms, there is currently no effective treatment and
surgical eviction often results in recurrence. The overall goal of the project is to investigate cellular and molecular
mechanisms underlying HO initiation and expansion and identify potential treatment targets for genetic and
acquired HO. Our preliminary data suggests that in mouse models of progressive osseous heteroplasia (POH),
a human disease caused by null mutations in GNAS that encodes Gαs, progressive expansion of ectopic bone
was due to recruitment of surrounding wild-type cells. Mechanistically, Gnas-/- mesenchymal cells differentiate
into osteoblasts and recruit wild-type cells to form bone by activating YAP, a transcription factor that regulates
expression of Sonic hedgehog (Shh), which encodes a secreted ligand in the Hh family. Secreted SHH further
induces YAP activation, thereby resulting in additional Shh expression and osteoblast differentiation in
surrounding wild type cells. In human samples with acquired HO, upregulated YAP and HH signaling were also
found, indicating the clinical importance of our findings. In our unpublished preliminary studies, we found
upregulated Wnt5a expression in Gnas-/- SMPs in vitro and in HO lesions of POH mouse models in vivo. WNT5a
not only regulates stem cell proliferation and enhances repopulation, but also plays an essential role in long bone
and cartilage development during embryogenesis. Importantly, we found that Wnt5a expression was also
controlled by YAP activation and was required for HO in both genetic and acquired HO. These findings lead to
a central hypothesis that YAP and WNT5a are activated in a positive feedback loop to recruit progenitor cells for
osteoblast differentiation in soft tissues, thereby promoting HO formation in both genetic and acquired HO mouse
models. We will test these hypotheses in three aims. The first aim is to clarify the relationship between YAP and
Wnt5a expression in ectopic bone formation in POH and test whether YAP and WNT5a interact in a positive
feedback loop. The second aim is to illuminate the molecular mechanism in response to YAP and WNT5a in
ectopic bone formation. The third aim is to investigate the contribution of WNT5a to ectopic bone formation in
fibrodysplasia ossificans progressiva (FOP) and trauma-induced HO models. Taken together, these proposed
studies will yield novel insights into a common cellular and molecular mechanism underlying initiation and
expansion of distinct HO forms and highlight the importance of rare genetic disease studies in identifying a
shared core pathological mechanism underlying a class of diseases.