Further Analysis of the Crouzon Mouse: Effects of the FGFR2^C342Y Mutation Are Cranial Bone–Dependent

Crouzon syndrome is a debilitating congenital disorder involving abnormal craniofacial skeletal development caused by mutations in fibroblast growth factor receptor-2 (FGFR2). Phenotypic expression in humans exhibits an autosomal dominant pattern that commonly involves premature fusion of the coronal suture (craniosynostosis) and severe midface hypoplasia. To further investigate the biologic mechanisms by which the Crouzon syndrome–associated FGFR2^C342Y mutation leads to abnormal craniofacial skeletal development, we created congenic BALB/c FGFR2^C342Y/+ mice. Here, we show that BALB/c FGFR2^C342Y/+ mice have a consistent craniofacial phenotype including partial fusion of the coronal and lambdoid sutures, intersphenoidal synchondrosis, and multiple facial bones, with minimal fusion of other craniofacial sutures. This phenotype is similar to the classic and less severe form of Crouzon syndrome that involves significant midface hypoplasia with limited craniosynostosis. Linear and morphometric analyses demonstrate that FGFR2^C342Y/+ mice on the BALB/c genetic background differ significantly in form and shape from their wild-type littermates and that in this genetic background the FGFR2^C342Y mutation preferentially affects some craniofacial bones and sutures over others. Analysis of cranial bone cells indicates that the FGFR2^C342Y mutation promotes aberrant osteoblast differentiation and increased apoptosis that is more severe in frontal than parietal bone cells. Additionally, FGFR2^C342Y/+ frontal, but not parietal, bones exhibit significantly diminished bone volume and density compared to wild-type mice. These results confirm that FGFR2-associated craniosynostosis occurs in association with diminished cranial bone tissue and may provide a potential biologic explanation for the clinical finding of phenotype consistency that exists between many Crouzon syndrome patients.