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

Abstract

Fibrodysplasia ossificans progressiva (FOP) is a disabling genetic disorder of progressive heterotopic ossification (HO). Here, we report a patient with an ultra-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 GS activation domain as the engineered constitutively active (c.a.) variant p.Q207D. It was predicted that both mutations at residue 207 have similar functional effects by introducing a negative charge. Transgenic p.Q207D-c.a. mice have served as a model for FOP HO in several in vivo studies. However, we found that the engineered ACVR1(Q207D-c.a.) is significantly more active than the classic FOP mutation ACVR1(R206H) when overexpressed in chicken limbs and in differentiation assays of chondrogenesis, osteogenesis and myogenesis. Importantly, our studies reveal that the ACVR1(Q207E) resembles the classic FOP receptor in these assays, not the engineered ACVR1(Q207D-c.a.). Notably, reporter gene assays revealed that both naturally occurring FOP receptors (ACVR1(R206H) and ACVR1(Q207E)) were activated by BMP7 and were sensitive to deletion of the ligand binding domain, whereas the engineered ACVR1(Q207D-c.a.) exhibited ligand independent activity. We performed an in silico analysis and propose a structural model for p.Q207D-c.a. that irreversibly relocates the GS domain into an activating position, where it becomes ligand independent. We conclude that the engineered p.Q207D-c.a. mutation has severe limitations as a model for FOP, whereas the naturally occurring mutations p.R206H and p.Q207E facilitate receptor activation, albeit in a reversible manner.

Peripheral Blood Mononuclear Cell Immunophenotyping in Fibrodysplasia Ossificans Progressiva Patients: Evidence for Monocyte DNAM1 Up‐regulation

“…Patients 1, 2, 3, 4, and 6 were previously described (8,25). Patient 5, never reported before, showed a classical FOP presentation with bilateral malformation of the great toes, cervical stiffness and first clinical manifestation at 6 years as ectopic ossification of a knee, first diagnosed as exostosis.…”

The immunophilin FKBP12 inhibits hepcidin expression by binding the BMP type I receptor ALK2 in hepatocytes

“…17 These mutations are partially resistant to the suppressive effect of FKBP12. 26 Both R206H and Q207E have defective binding to FKBP12, and R258S fails to interact with the immunophilin (Figure 3B). To further investigate the FKBP12 binding ability of ALK2, Hep3B cells, transfected with the HAMP-Luc vector and WT or mutants ALK2, were treated with increasing concentration of tacrolimus to displace FKBP12.…”

Clinical Utility Gene Card for: Fibrodysplasia ossificans progressiva

“…Only recently a new heterozygous ACVR1 mutation at codon 207 (c.619C4G, p.(Gln207Glu)) located in a codon adjacent to the c.617G4A, p.(Arg206His) of the ACVR1 was reported in two FOP patients with the classical phenotype. 4 Among patients with FOP-like heterotopic ossification and/or toe malformation, there are patients with clinical features unusual for FOP. These atypical FOP patients form two classes: FOP-plus (classic defining features of FOP plus one or more atypical features, predominantly associated with the classical p.(Arg206His) mutation) and FOP variants (major variations in one or both of the two classic defining features of FOP, associated with non-Arg206His mutations within the ACVR1 receptor).…”

“…Novel ACVR1 mutations occur mainly in FOP variants and some cases of FOP plus. [4][5][6] A public list of disease causing variants is not available yet.…”