INCREASED CALVARIA CELL-DIFFERENTIATION AND BONE-MATRIX FORMATION INDUCED BY FIBROBLAST GROWTH-FACTOR RECEPTOR 2 MUTATIONS IN APERT-SYNDROME

Apert syndrome, associated with fibroblast growth factor receptor (FGF R) 2 mutations, is characterized by premature fusion of cranial suture s. We analyzed proliferation and differentiation of calvaria cells der ived from Apert infants and fetuses with FGFR-2 mutations. Histologica l analysis revealed premature ossification, increased extent of subper iosteal bone formation, and alkaline phosphatase-positive preosteoblas tic cells in Apert fetal calvaria compared with age-matched controls. Preosteoblastic calvaria cells isolated from Apert infants and fetuses showed normal cell growth in basal conditions or in response to exoge nous FGF-2, In contrast, the number of alkaline phosphatase-positive c alvaria cells was fourfold higher than normal in mutant fetal calvaria cells with the most frequent Apert FGFR-2 mutation (Ser252Trp), sugge sting increased maturation rate of cells in the osteoblastic lineage. Biochemical and Northern blot analyses also showed that the expression of alkaline phosphatase and type 1 collagen were 2-10-fold greater th an normal in mutant fetal calvaria cells. The in vitro production of m ineralized matrix formed by immortalized mutant fetal calvaria cells c ultured in aggregates was also increased markedly compared with contro l immortalized fetal calvaria cells. The results show that Apert FGFR- 2 mutations lead to an increase in the number of precursor cells that enter the osteogenic pathway, leading ultimately to increased subperio steal bone matrix formation and premature calvaria ossification during fetal development, which establishes a connection between the altered genotype and cellular phenotype in Apert sqndromic craniosynostosis.