Multiple synostoses syndrome is due to a missense mutation in exon 2 of FGF9 gene

Recent research on the growth plate: Advances in fibroblast growth factor signaling in growth plate development and disorders

Skeletons are formed through two distinct developmental actions, intramembranous ossification and endochondral ossification. During embryonic development, most bone is formed by endochondral ossification. The growth plate is the developmental center for endochondral ossification. Multiple signaling pathways participate in the regulation of endochondral ossification. Fibroblast growth factor (FGF)/FGF receptor (FGFR) signaling has been found to play a vital role in the development and maintenance of growth plates. Missense mutations in FGFs and FGFRs can cause multiple genetic skeletal diseases with disordered endochondral ossification. Clarifying the molecular mechanisms of FGFs/FGFRs signaling in skeletal development and genetic skeletal diseases will have implications for the development of therapies for FGF-signaling-related skeletal dysplasias and growth plate injuries. In this review, we summarize the recent advances in elucidating the role of FGFs/FGFRs signaling in growth plate development, genetic skeletal disorders, and the promising therapies for those genetic skeletal diseases resulting from FGFs/FGFRs dysfunction. Finally, we also examine the potential important research in this field in the future.

Role of FGF/FGFR signaling in skeletal development and homeostasis: learning from mouse models

Fibroblast growth factor (FGF)/fibroblast growth factor receptor (FGFR) signaling plays essential roles in bone development and diseases. Missense mutations in FGFs and FGFRs in humans can cause various congenital bone diseases, including chondrodysplasia syndromes, craniosynostosis syndromes and syndromes with dysregulated phosphate metabolism. FGF/FGFR signaling is also an important pathway involved in the maintenance of adult bone homeostasis. Multiple kinds of mouse models, mimicking human skeleton diseases caused by missense mutations in FGFs and FGFRs, have been established by knock-in/out and transgenic technologies. These genetically modified mice provide good models for studying the role of FGF/FGFR signaling in skeleton development and homeostasis. In this review, we summarize the mouse models of FGF signaling-related skeleton diseases and recent progresses regarding the molecular mechanisms, underlying the role of FGFs/FGFRs in the regulation of bone development and homeostasis. This review also provides a perspective view on future works to explore the roles of FGF signaling in skeletal development and homeostasis.

Achondroplasia with multiple-suture craniosynostosis: a report of a new case of this rare association

We report on a female patient with an exceedingly rare combination of achondroplasia and multiple-suture craniosynostosis. Besides the specific features of achondroplasia, synostosis of the metopic, coronal, lambdoid, and squamosal sutures was found. Series of neurosurgical interventions were carried out, principally for acrocephaly and posterior plagiocephaly. The most common achondroplasia mutation, a p.Gly380Arg in the fibroblast growth factor receptor 3 (FGFR3) gene, was detected. Cytogenetic and array CGH analyses, as well as molecular genetic testing of FGFR1, 2, 3 and TWIST1 genes failed to identify any additional genetic alteration. It is suggested that this unusual phenotype is a result of variable expressivity of the common achondroplasia mutation.

Autoinhibitory mechanism for the mutation-induced impaired FGF9 signaling

Fibroblast growth factor 9 (FGF9), an important member of the fibroblast growth factor (FGF) family, can bind with high affinity to FGFR3 in a heparin-dependent approach. In humans, the deletions and mutations resulting in dysfunction of the FGF9 signaling can cause human skeletal dysplasia and cancers. A mutation (S99N) in this protein has been identified to be associated with significantly impaired FGF signaling considered as a potential cause of synostoses syndrome. However, the detailed mechanism for this observation still remains unknown. In this study, we used molecular dynamics simulations and free energy calculations to study the interactions of FGF9(WT/S99N), FGFR3c, and heparin, with an aim of providing atomic sights into the detailed mechanism for the impaired FGF signaling caused by the S99N mutation. We found that the S99N mutation has a well-ordered C-terminal structure, which can reduce its homodimerization ability so as to break the monomer-dimer equilibrium in the FGF signaling, which is considered as a key factor to regulate extracellular matrix affinity and tissue diffusion in the FGF signaling pathway. The FGF9(WT) monomer can preferentially form a homodimer owing to its comparatively favorable binding free energy. In contrast, the FGF9(S99N) monomer is preferred to bind with the FGFR3c receptor to form an inactive complex, leading to impair FGF signaling. To support our computational findings, we also performed biochemical experiments, which confirm the computational results mentioned above. The impaired FGF signaling is believed to be a potential cause of human synostoses syndrome, implicating an important role for FGF9 in normal joint development.

Phenotype profile of a genetic mouse model for Muenke syndrome

PURPOSE

The Muenke syndrome mutation (FGFR3 (P250R)), which was discovered 15 years ago, represents the single most common craniosynostosis mutation. Muenke syndrome is characterized by coronal suture synostosis, midface hypoplasia, subtle limb anomalies, and hearing loss. However, the spectrum of clinical presentation continues to expand. To better understand the pathophysiology of the Muenke syndrome, we present collective findings from several recent studies that have characterized a genetically equivalent mouse model for Muenke syndrome (FgfR3 (P244R)) and compare them with human phenotypes.

CONCLUSIONS

FgfR3 (P244R) mutant mice show premature fusion of facial sutures, premaxillary and/or zygomatic sutures, but rarely the coronal suture. The mice also lack the typical limb phenotype. On the other hand, the mutant mice display maxillary retrusion in association with a shortening of the anterior cranial base and a premature closure of intersphenoidal and spheno-occipital synchondroses, resembling human midface hypoplasia. In addition, sensorineural hearing loss is detected in all FgfR3 (P244R) mutant mice as in the majority of Muenke syndrome patients. It is caused by a defect in the mechanism of cell fate determination in the organ of Corti. The mice also express phenotypes that have not been previously described in humans, such as reduced cortical bone thickness, hypoplastic trabecular bone, and defective temporomandibular joint structure. Therefore, the FgfR3 (P244R) mouse provides an excellent opportunity to study disease mechanisms of some classical phenotypes of Muenke syndrome and to test novel therapeutic strategies. The mouse model can also be further explored to discover previously unreported yet potentially significant phenotypes of Muenke syndrome.

The effect of co-culturing costal chondrocytes and dental pulp stem cells combined with exogenous FGF9 protein on chondrogenesis and ossification in engineered cartilage

Dental pulp stem cells (DPSCs), which arise from cranial neural crest cells, are multipotent, making them a candidate for use in tissue engineering that may be especially useful for craniofacial tissues. Costal chondrocytes (CCs) can be easily obtained and demonstrate higher initial cell yields and expansion than articular chondrocytes. CCs have been found to retain chondrogenic capacity that can effectively repair articular defects. In this study, human CCs were co-cultured with human DPSCs, and the results showed that the CCs were able to supply a chondro-inductive niche that promoted the DPSCs to undergo chondrogenic differentiation and to enhance the formation of cartilage. Although CCs alone could not prevent the mineralization of chondro-differentiated DPSCs, CCs combined with exogenous FGF9 were able to simultaneously promote the chondrogenesis of DPSCs and partially inhibit their mineralization. Furthermore, FGF9 may activate this inhibition by binding to FGFR3 and enhancing the phosphorylation of ERK1/2 in DPSCs. Our results strongly suggest that the co-culture of CCs and DPSCs combined with exogenous FGF9 can simultaneously enhance chondrogenesis and partially inhibit ossification in engineered cartilage.

The Muenke syndrome mutation (FgfR3P244R) causes cranial base shortening associated with growth plate dysfunction and premature perichondrial ossification in murine basicranial synchondroses

Muenke syndrome caused by the FGFR3(P250R) mutation is an autosomal dominant disorder mostly identified with coronal suture synostosis, but it also presents with other craniofacial phenotypes that include mild to moderate midface hypoplasia. The Muenke syndrome mutation is thought to dysregulate intramembranous ossification at the cranial suture without disturbing endochondral bone formation in the skull. We show in this study that knock-in mice harboring the mutation responsible for the Muenke syndrome (FgfR3(P244R)) display postnatal shortening of the cranial base along with synchondrosis growth plate dysfunction characterized by loss of resting, proliferating and hypertrophic chondrocyte zones and decreased Ihh expression. Furthermore, premature conversion of resting chondrocytes along the perichondrium into prehypertrophic chondrocytes leads to perichondrial bony bridge formation, effectively terminating the postnatal growth of the cranial base. Thus, we conclude that the Muenke syndrome mutation disturbs endochondral and perichondrial ossification in the cranial base, explaining the midface hypoplasia in patients.

The pathologic effect of a novel neomorphic Fgf9(Y162C) allele is restricted to decreased vision and retarded lens growth

Fibroblast growth factor (Fgf) signalling plays a crucial role in many developmental processes. Among the Fgf pathway ligands, Fgf9 (UniProt: P54130) has been demonstrated to participate in maturation of various organs and tissues including skeleton, testes, lung, heart, and eye. Here we establish a novel Fgf9 allele, discovered in a dominant N-ethyl-N-nitrosourea (ENU) screen for eye-size abnormalities using the optical low coherence interferometry technique. The underlying mouse mutant line Aca12 was originally identified because of its significantly reduced lens thickness. Linkage studies located Aca12 to chromosome 14 within a 3.6 Mb spanning interval containing the positional candidate genes Fgf9 (MGI: 104723), Gja3 (MGI: 95714), and Ift88 (MGI: 98715). While no sequence differences were found in Gja3 and Ift88, we identified an A→G missense mutation at cDNA position 770 of the Fgf9 gene leading to an Y162C amino acid exchange. In contrast to previously described Fgf9 mutants, Fgf9^Y162C carriers were fully viable and did not reveal reduced body-size, male-to-female sexual reversal or skeletal malformations. The histological analysis of the retina as well as its basic functional characterization by electroretinography (ERG) did not show any abnormality. However, the analysis of head-tracking response of the Fgf9^Y162C mutants in a virtual drum indicated a gene-dosage dependent vision loss of almost 50%. The smaller lenses in Fgf9^Y162C suggested a role of Fgf9 during lens development. Histological investigations showed that lens growth retardation starts during embryogenesis and continues after birth. Young Fgf9^Y162C lenses remained transparent but developed age-related cataracts. Taken together, Fgf9^Y162C is a novel neomorphic allele that initiates microphakia and reduced vision without effects on organs and tissues outside the eye. Our data point to a role of Fgf9 signalling in primary and secondary lens fiber cell growth. The results underline the importance of allelic series to fully understand multiple functions of a gene.

A comprehensive review of reported heritable noggin-associated syndromes and proposed clinical utility of one broadly inclusive diagnostic term: NOG-related-symphalangism spectrum disorder (NOG-SSD)

The NOG gene encodes noggin, a secreted polypeptide that is important for regulating multiple signaling pathways during human development, particularly in cartilage and bone. The hallmark of NOG-related syndromes is proximal symphalangism, defined by abnormal fusion of the proximal interphalangeal joints of the hands and feet. Many additional features secondary to NOG mutations are commonly but inconsistently observed, including a characteristic facies with a hemicylindrical nose, congenital conductive hearing loss due to stapes fixation, and hyperopia. The variable clinical presentations led to the designation of five different autosomal dominant syndromes, all subsequently found to have resulted from NOG mutations. These include (1) proximal symphalangism; (2) multiple synostoses syndrome 1; (3) stapes ankylosis with broad thumbs and toes; (4) tarsal-carpal coalition syndrome; and (5) brachydactyly type B2. Herein, we review the phenotypic features associated with mutations in the NOG gene, demonstrating the overlapping characteristics of these syndromes. Due to the variable phenotypic spectrum within families and among families with the same mutation, we propose a unifying term, NOG-related symphalangism spectrum disorder (NOG-SSD), to aid in the clinical recognition and evaluation of all affected individuals with these phenotypes. These NOG gene variants are available in a new locus-specific database (https://NOG.lovd.nl).

FGF and ROR2 receptor tyrosine kinase signaling in human skeletal development

Skeletal malformations are among the most frequent developmental disturbances in humans. In the past years, progress has been made in unraveling the molecular mechanisms that govern skeletal development by the use of animal models as well as by the identification of numerous mutations that cause human skeletal syndromes. Receptor tyrosine kinases have critical roles in embryonic development. During formation of the skeletal system, the fibroblast growth factor receptor (FGFR) family plays major roles in the formation of cranial, axial, and appendicular bones. Another player of relevance to skeletal development is the unusual receptor tyrosine kinase ROR2, the function of which is as interesting as it is complex. In this chapter, we review the involvement of FGFR signaling in human skeletal disease and provide an update on the growing knowledge of ROR2.