ACVR1 p.Q207E causes classic fibrodysplasia ossificans progressiva and is functionally distinct from the engineered constitutively active ACVR1 p.Q207D variant

Fibrodysplasia ossificans progressiva (FOP) is a disabling genetic disorder ofprogressive heterotopic ossification (HO). Here, we report a patient with anultra-rare point mutation [c.619C>G, p.Q207E] located in a codon adjacent to the most common FOP mutation [c.617G>A, p.R206H] of Activin A Receptor, type 1(ACVR1) and that affects the same intracellular amino acid position in the GSactivation domain as the engineered constitutively active (c.a.) variant p.Q207D.It was predicted that both mutations at residue 207 have similar functionaleffects by introducing a negative charge. Transgenic p.Q207D-c.a. mice haveserved as a model for FOP HO in several in vivo studies. However, we found thatthe engineered ACVR1(Q207D-c.a.) is significantly more active than the classicFOP mutation ACVR1(R206H) when overexpressed in chicken limbs and indifferentiation assays of chondrogenesis, osteogenesis and myogenesis.Importantly, our studies reveal that the ACVR1(Q207E) resembles the classic FOPreceptor in these assays, not the engineered ACVR1(Q207D-c.a.). Notably, reportergene assays revealed that both naturally occurring FOP receptors (ACVR1(R206H)and ACVR1(Q207E)) were activated by BMP7 and were sensitive to deletion of theligand binding domain, whereas the engineered ACVR1(Q207D-c.a.) exhibited ligand independent activity. We performed an in silico analysis and propose a structuralmodel for p.Q207D-c.a. that irreversibly relocates the GS domain into anactivating position, where it becomes ligand independent. We conclude that theengineered p.Q207D-c.a. mutation has severe limitations as a model for FOP,whereas the naturally occurring mutations p.R206H and p.Q207E facilitate receptoractivation, albeit in a reversible manner.