Growth of the normal skull vault and its alteration in craniosynostosis: insights from human genetics and experimental studies

A model for the pharmacological treatment of crouzon syndrome

OBJECTIVE

Crouzon syndrome is caused by mutations in fibroblast growth factor receptor 2 (FGFR2) leading to constitutive activation of receptors in the absence of ligand binding. The syndrome is characterized by premature fusion of the cranial sutures that leads to abnormal cranium shape, restricted brain growth, and increased intracranial pressure. Surgical remodeling of the cranial vault is currently used to treat affected infants. The purpose of this study was to develop a pharmacological strategy using tyrosine kinase inhibition as a novel treatment for craniosynostotic syndromes caused by constitutive FGFR activation.

METHODS

Characterization of cranial suture fusion in Fgfr2 mutant mice, which carry the most common Crouzon mutation, was performed using micro-computed tomographic analysis from embryogenesis through maturation. Whole calvarial cultures from wild-type and Fgfr2 mice were established and cultured for 2 weeks in the presence of dimethyl sulfoxide control or PD173074, an FGFR tyrosine kinase inhibitor. Paraffin sections were prepared to show suture morphology and calcium deposition.

RESULTS

In untreated Fgfr2 cultures, the coronal suture fused bilaterally with loss of overlap between the frontal bone and parietal bone. Calvaria treated with PD173074 (2 micromol/L) showed patency of the coronal suture and were without evidence of any synostosis.

CONCLUSION

We report the successful use of PD173074 to prevent in vitro suture fusion in a model for Crouzon syndrome. Further studies are underway to develop an in vivo treatment protocol as a novel therapeutic modality for FGFR associated craniosynostotic syndromes.

Fibroblast growth factor receptor 1 signaling in the osteo-chondrogenic cell lineage regulates sequential steps of osteoblast maturation

Mutations in fibroblast growth factor receptors (Fgfrs) 1-3 cause skeletal disease syndromes in humans. Although these Fgfrs are expressed at various stages of chondrocyte and osteoblast development, their function in specific skeletal cell types is poorly understood. Using conditional inactivation of Fgfr1 in osteo-chondrocyte progenitor cells and in differentiated osteoblasts, we provide evidence that FGFR1 signaling is important for different stages of osteoblast maturation. Examination of osteogenic markers showed that inactivation of FGFR1 in osteo-chondro-progenitor cells delayed osteoblast differentiation, but that inactivation of FGFR1 in differentiated osteoblasts accelerated differentiation. In vitro osteoblast cultures recapitulated the in vivo effect of FGFR1 on stage-specific osteoblast maturation. In immature osteoblasts, FGFR1 deficiency increased proliferation and delayed differentiation and matrix mineralization, whereas in differentiated osteoblasts, FGFR1 deficiency enhanced mineralization. Furthermore, FGFR1 deficiency in differentiated osteoblasts resulted in increased expression of Fgfr3, a molecule that regulates the activity of differentiated osteoblasts. Mice lacking Fgfr1, either in progenitor cells or in differentiated osteoblasts, showed increased bone mass as adults. These data demonstrate that signaling through FGFR1 in osteoblasts is necessary to maintain the balance between bone formation and remodeling through a direct effect on osteoblast maturation.

Recent advances in craniofacial morphogenesis

Craniofacial malformations are involved in three fourths of all congenital birth defects in humans, affecting the development of head, face, or neck. Tremendous progress in the study of craniofacial development has been made that places this field at the forefront of biomedical research. A concerted effort among evolutionary and developmental biologists, human geneticists, and tissue engineers has revealed important information on the molecular mechanisms that are crucial for the patterning and formation of craniofacial structures. Here, we highlight recent advances in our understanding of evo-devo as it relates to craniofacial morphogenesis, fate determination of cranial neural crest cells, and specific signaling pathways in regulating tissue-tissue interactions during patterning of craniofacial apparatus and the morphogenesis of tooth, mandible, and palate. Together, these findings will be beneficial for the understanding, treatment, and prevention of human congenital malformations and establish the foundation for craniofacial tissue regeneration.

Parietal bone agenesis with gracile bones and splenic hypoplasia/aplasia: Clinico-pathologic report and differential diagnosis with review of cranio-gracile bone syndromes, “osteocraniostenosis” and Kleeblattschädel

The findings in a newborn male with agenesis of parietal bones, gracile long bones, and hypoplasia of the spleen are presented. Although parietal agenesis is unique, the findings are compatible with 18 previously reported cases characterized by cranial hypomineralization, Kleeblatschädel, gracile bones, and splenic aplasia/hypoplasia, the nomenclature for which has been descriptive: gracile bone disorders, "osteocraniostenosis," "osteocraniosplenic syndrome." The term "osteocraniostenosis" may be inappropriate in that craniostenosis has been infrequently reported, a feature also of importance with respect to the pathogenesis of the Kleeblatschädel. The group likely reflects heterogeneous etiology and pathogenesis. Precedent, however, including animal models, justifies consideration of genetic aberrations, in particular, mutations in homeobox genes. (c) 2006 Wiley-Liss, Inc.

Clinical dividends from the molecular genetic diagnosis of craniosynostosis

A dozen years have passed since the first genetic lesion was identified in a family with craniosynostosis, the premature fusion of the cranial sutures. Subsequently, mutations in the FGFR2, FGFR3, TWIST1, and EFNB1 genes have been shown to account for approximately 25% of craniosynostosis, whilst several additional genes make minor contributions. Using specific examples, we show how these discoveries have enabled refinement of information on diagnosis, recurrence risk, prognosis for mental development, and surgical planning. However, phenotypic variability can present a significant challenge to the clinical interpretation of molecular genetic tests. In particular, the difficulty of analyzing the complex interaction of genetic background and prenatal environment in determining clinical features, limits the value of identifying low penetrance mutations.