Bone Abstracts

Fibrodysplasia ossificans progressiva (FOP) is a rare congenital disorder characterized by progressive heterotopic ossification. FOP patients only present great toe malformations at birth. However, as they grow older they develop soft tissue lumps as a result of flare-ups causing the irreversible replacement of skeletal muscle tissue with bone tissue leading to cumulative physical immobility. Classical FOP patients possess a mutation (c.617G>A; R206H) in the activin receptor IA (ACVR1)-encoding gene which is associated with dysregulated bone morphogenetic protein (BMP) signalling. Nonetheless, not all FOP patients with this mutation exhibit equal severity in symptom presentation or disease progression which indicates a strong contribution by environmental factors. Although inflammation is known to be the main trigger of flare-ups the molecular pathway remains largely unknown.

Our objective was to study the process of osteogenic differentiation in primary dermal fibroblasts from five FOP patients based on a novel method of growth factor-induced osteogenic transdifferentiation. In all patients the presence of the classical FOP mutation was confirmed. The osteogenic properties of the cells were evaluated by the mRNA expression of Runt-related transcription factor 2 (Runx2), alkaline phosphatase (Alp), osteocalcin (OC) and the presence of mineralization by alizarin red staining. Given the pro-inflammatory role of TGFβ, we also performed pharmacological inhibition of TGFβ signalling by the TGFβ type I receptor inhibitor GW788388.

During osteogenic trans differentiation the expression of Runx2 and Alp over time was higher in FOP cell lines compared to healthy controls (Runx2:p=0.001; Alp:p>0.05). All cell lines exhibited increase in mineralization. Addition of the TGFβ type I receptor inhibitor to the osteogenic media resulted in the attenuation of osteogenic differentiation shown by the decrease in expression of osteogenic markers in patients vs untreated cells at several time points (Runx2:p=0.045). Mineralisation was also inhibited by this inhibitor.

These data indicate that TGFβ is involved in the molecular pathway of flare-up-induced ossification. Inhibition of this pathway may limit ectopic ossification in FOP.

Disclosure: The authors declared no competing interests.