Towler, O. Will

Abstract Pseudoarthroses (false joints) occur as a complication of fracture healing in the normotopic skeleton but have not been reported at ectopic sites. Fibrodysplasia ossificans progressiva (FOP) is an ultra-rare genetic disorder characterized by progressive heterotopic ossification (HO) and congenital skeletal abnormalities including developmental arthropathy in the normotopic skeleton. Here, we report ectopic pseudojoint formation in 6 patients with FOP, all occurring after painful flare-ups and HO in soft connective tissues of the knee, hip, and forearm. To our knowledge, FOP is the only human condition in which ectopic pseudojoint formation occurs. These findings support that dysregulated BMP pathway signaling from mutant ACVR1 mediates not only HO, but also ectopic pseudojoint formation, and that ectopic pseudojoints can arise from de novo musculoskeletal elements.

AbstractGenetic variants are vital in informing clinical phenotypes, aiding physical diagnosis, guiding genetic counseling, understanding the molecular basis of disease, and potentially stimulating drug development. Here we describe two families with an ultrarare ACVR1 gain‐of‐function pathogenic variant (codon 375, Arginine > Proline; ACVR1R375P) responsible for a mild nonclassic fibrodysplasia ossificans progressiva (FOP) phenotype. Both families include people with the ultrarare ACVR1R375P variant who exhibit features of FOP while other individuals currently do not express any clinical signs of FOP. Thus, the mild ACVR1R375P variant greatly expands the scope and understanding of this rare disorder.