Fibrodysplasia ossificans progressiva: a blueprint for metamorphosis.

The most important milestone in understanding a genetic disease is the identification of the causative mutation. However, such knowledge is often insufficient to decipher the pathophysiology of the disorder or to effectively treat those affected. Fibrodysplasia ossificans progressiva (FOP) is a rare, disabling, genetic disease of progressive heterotopic endochondral ossification (HEO) enabled by missense mutations that promiscuously and provisionally activate ACVR1/ALK2, a bone morphogenetic protein (BMP) type I receptor, in all affected individuals. While activating mutations of the ACVR1/ALK2 receptor are necessary, disease activity and progression also depend on altered cell and tissue physiology. Recent findings identify inflammatory and immunological factors, vascular-derived mesenchymal stem cells, and a hypoxic lesional microenvironment that trigger, promote, and enable episodic progression of FOP in the setting of the genetic mutation. Effective therapies for FOP will need to consider these seminal pathophysiologic interactions.