Decreased proliferation and altered differentiation in osteoblasts from genetically and clinically distinct craniosynostotic disorders

Craniosynostoses are a heterogeneous group of disorders characterized by pr emature fusion of cranial sutures. Mutations in fibroblast growth factor re ceptors (FGFRs) have been associated with a number of such conditions. Neve rtheless, the cellular mechanism(s) involved remain unknown. We analyzed ce ll proliferation and differentiation in osteoblasts obtained from patients with three genetically and clinically distinct craniosynostoses: Pfeiffer s yndrome carrying the FGFR2 C342R substitution, Apert syndrome with FGFR2 P2 53R change, and a nonsyndromic craniosynostosis without FGFR canonic mutati ons, as compared with control osteoblasts. Osteoblasts from craniosynostoti c patients exhibited a lower proliferation rate than control osteoblasts. P 253R and nonsyndromic craniosynostosis osteoblasts showed a marked differen tiated phenotype, characterized by high alkaline phosphatase activity, incr eased mineralization and expression of noncollagenous matrix proteins, asso ciated with high expression and activation of protein kinase C alpha and pr otein kinase C epsilon isoenzymes, By contrast, the low proliferation rate of C342R osteoblasts was not associated with a differentiated phenotype, Al though they showed higher alkaline phosphatase activity than control, C342R osteoblasts failed to mineralize and expressed low levels of osteopontin a nd osteonectin and high protein kinase C zeta levels. Stimulation of prolif eration and inhibition of differentiation were observed in all cultures on FGF2 treatment. Our results suggest that an anticipated proliferative/diffe rentiative switch, associated with alterations of the FGFR transduction pat hways, could be the causative common feature in craniosynostosis and that m utations in distinct FGFR2 domains are associated with an in vitro heteroge neous differentiative phenotype.