A self-amplifying loop of YAP and SHH drives formation and expansion of heterotopic ossification.

Abstract

Heterotopic ossification (HO) occurs as a common complication after injury or in genetic disorders. The mechanisms underlying HO remain incompletely understood, and there are no approved prophylactic or secondary treatments available. Here, we identify a self-amplifying, self-propagating loop of Yes-associated protein (YAP)-Sonic hedgehog (SHH) as a core molecular mechanism underlying diverse forms of HO. In mouse models of progressive osseous heteroplasia (POH), a disease caused by null mutations in <i>GNAS</i>, we found that <i>Gnas^-/-</i> mesenchymal cells secreted SHH, which induced osteoblast differentiation of the surrounding wild-type cells. We further showed that loss of <i>Gnas</i> led to activation of YAP transcription activity, which directly drove <i>Shh</i> expression. Secreted SHH further induced YAP activation, <i>Shh</i> expression, and osteoblast differentiation in surrounding wild-type cells. This self-propagating positive feedback loop was both necessary and sufficient for HO expansion and could act independently of <i>Gnas</i> in fibrodysplasia ossificans progressiva (FOP), another genetic HO, and nonhereditary HO mouse models. Genetic or pharmacological inhibition of YAP or SHH abolished HO in POH and FOP and acquired HO mouse models without affecting normal bone homeostasis, providing a previously unrecognized therapeutic rationale to prevent, reduce, and shrink HO.