Constitutive activation of fibroblast growth factor receptor-2 by a point mutation associated with Crouzon syndrome

Morphology of the Occipital Bones and Foramen Magnum Resulting From Premature Minor Suture Fusion in Crouzon Syndrome

To identify skull-base growth patterns in Crouzon syndrome, we hypothesized premature minor suture fusion restricts occipital bone development, secondarily limiting foramen magnum expansion.

Skull-base suture closure degree and cephalometric measurements were retrospectively studied using preoperative computed tomography (CT) scans and multiple linear regression analysis.

Evaluation of multi-institutional CT images and 3D reconstructions from Wake Forest's Craniofacial Imaging Database (WFCID).

Sixty preoperative patients with Crouzon syndrome under 12 years-old were selected from WFCID. The control group included 60 age- and sex-matched patients without craniosynostosis or prior craniofacial surgery.

None.

2D and 3D cephalometric measurements.

3D volumetric evaluation of the basioccipital, exo-occipital, and supraoccipital bones revealed decreased growth in Crouzon syndrome, attributed solely to premature minor suture fusion. Spheno-occipital (β = -398.75; P < .05) and petrous-occipital (β = -727.5; P < .001) suture fusion reduced growth of the basioccipital bone; lambdoid suture (β = -14 723.1; P < .001) and occipitomastoid synchondrosis (β = -16 419.3; P < .001) fusion reduced growth of the supraoccipital bone; and petrous-occipital suture (β = -673.3; P < .001), anterior intraoccipital synchondrosis (β = -368.47; P < .05), and posterior intraoccipital synchondrosis (β = -6261.42; P < .01) fusion reduced growth of the exo-occipital bone. Foramen magnum morphology is restricted in Crouzon syndrome but not directly caused by early suture fusion.

Premature minor suture fusion restricts the volume of developing occipital bones providing a plausible mechanism for observed foramen magnum anomalies.

Craniosynostosis: current conceptions and misconceptions

Cranial bones articulate in areas called sutures that must remain patent until skull growth is complete. Craniosynostosis is the condition that results from premature closure of one or more of the cranial vault sutures, generating facial deformities and more importantly, skull growth restrictions with the ability to severely affect brain growth. Typically, craniosynostosis can be expressed as an isolated event, or as part of syndromic phenotypes. Multiple signaling mechanisms interact during developmental stages to ensure proper and timely suture fusion. Clinical outcome is often a product of craniosynostosis subtypes, number of affected sutures and timing of premature suture fusion. The present work aimed to review the different aspects involved in the establishment of craniosynostosis, providing a close view of the cellular, molecular and genetic background of these malformations.

Ocular biometric features of pediatric patients with fibroblast growth factor receptor-related syndromic craniosynostosis

Ametropia is reported as a common ophthalmic manifestation in craniosynostosis. We retrospectively compared childhood refractive error and ocular biometric features of fibroblast growth factor receptor (FGFR)-related syndromic craniosynostosis patients with those of non-syndromic craniosynostosis and control subjects. Thirty-six eyes (18 patients) with FGFR-related syndromic craniosynostosis, 76 eyes (38 patients) with non-syndromic craniosynostosis, and 114 eyes (57 patients) of intermittent exotropes were included in the analysis. Mean age at examination was 7.82 ± 2.51 (range, 4–16) years and mean spherical equivalent was -0.09 ± 1.46 Diopter. Mean age and refractive error were not different between groups, but syndromic craniosynostosis patients had significantly longer axial length, lower corneal power, and lower lens power than other groups (p < 0.01, p < 0.01, and p < 0.01, respectively). Axial length was positively correlated and keratometry and lens power were negatively correlated with age in non-syndromic craniosynostosis and controls, while these correlations between age and ocular biometric parameters were not present in the FGFR-related syndromic craniosynostosis. In conclusion, ocular biometric parameters in FGFR-related syndromic craniosynostosis differed from those of non-syndromic craniosynostosis and age-matched controls, and did not show the relations with age, suggesting this cohort may have abnormal refractive growth.

Targeting Aberrant FGFR Signaling to Overcome CDK4/6 Inhibitor Resistance in Breast Cancer

Breast cancer (BC) is the most common cause of cancer-related death in women worldwide. Therapies targeting molecular pathways altered in BC had significantly enhanced treatment options for BC over the last decades, which ultimately improved the lives of millions of women worldwide. Among various molecular pathways accruing substantial interest for the development of targeted therapies are cyclin-dependent kinases (CDKs)-in particular, the two closely related members CDK4 and CDK6. CDK4/6 inhibitors indirectly trigger the dephosphorylation of retinoblastoma tumor suppressor protein by blocking CDK4/6, thereby blocking the cell cycle transition from the G1 to S phase. Although the CDK4/6 inhibitors abemaciclib, palbociclib, and ribociclib gained FDA approval for the treatment of hormone receptor (HR)-positive, human epidermal growth factor receptor 2 (HER2)-negative BC as they significantly improved progression-free survival (PFS) in randomized clinical trials, regrettably, some patients showed resistance to these therapies. Though multiple molecular pathways could be mechanistically responsible for CDK4/6 inhibitor therapy resistance, one of the most predominant ones seems to be the fibroblast growth factor receptor (FGFR) pathway. FGFRs are involved in many aspects of cancer formation, such as cell proliferation, differentiation, and growth. Importantly, FGFRs are frequently mutated in BC, and their overexpression and/or hyperactivation correlates with CDK4/6 inhibitor resistance and shortened PFS in BC. Intriguingly, the inhibition of aberrant FGFR activity is capable of reversing the resistance to CDK4/6 inhibitors. This review summarizes the molecular background of FGFR signaling and discusses the role of aberrant FGFR signaling during cancer development in general and during the development of CDK4/6 inhibitor resistance in BC in particular, together with other possible mechanisms for resistance to CDK4/6 inhibitors. Subsequently, future directions on novel therapeutic strategies targeting FGFR signaling to overcome such resistance during BC treatment will be further debated.

Genetic background dependent modifiers of craniosynostosis severity

Craniosynostosis severity varies in patients with identical genetic mutations. To understand causes of this phenotypic variation, we backcrossed the FGFR2^+/C342Y mouse model of Crouzon syndrome onto congenic C57BL/6 and BALB/c backgrounds. Coronal suture fusion was observed in C57BL/6 (88% incidence, p < .001 between genotypes) but not in BALB/c FGFR2^+/C342Y mutant mice at 3 weeks after birth, establishing that that the two models differ in phenotype severity. To begin identifying pre-existing modifiers of craniosynostosis severity, we compared transcriptome signatures of cranial tissues from C57BL/6 vs. BALB/c FGFR2^+/+ mice. We separately analyzed frontal bone with coronal suture tissue from parietal bone with sagittal suture tissues because the coronal suture but not the sagittal suture fuses in FGFR2^+/C342Y mice. The craniosynostosis associated Twist and En1 transcription factors were down-regulated, while Runx2 was up-regulated, in C57BL/6 compared to BALB/c tissues, which could predispose to craniosynostosis. Transcriptome analyses under the GO term MAPK cascade revealed that genes associated with calcium ion channels, angiogenesis, protein quality control and cell stress response were central to transcriptome differences associated with genetic background. FGFR2 and HSPA2 protein levels plus ERK1/2 activity were higher in cells isolated from C57BL/6 than BALB/c cranial tissues. Notably, the HSPA2 protein chaperone is central to craniofacial genetic epistasis, and we find that FGFR2 protein is abnormally processed in primary cells from FGFR2^+/C342Y but not FGFR2^+/+ mice. Therefore, we propose that differences in protein quality control responses may contribute to genetic background influences on craniosynostosis phenotype severity.

Cranial Neural Crest Cells and Their Role in the Pathogenesis of Craniofacial Anomalies and Coronal Craniosynostosis

Craniofacial anomalies are among the most common of birth defects. The pathogenesis of craniofacial anomalies frequently involves defects in the migration, proliferation, and fate of neural crest cells destined for the craniofacial skeleton. Genetic mutations causing deficient cranial neural crest migration and proliferation can result in Treacher Collins syndrome, Pierre Robin sequence, and cleft palate. Defects in post-migratory neural crest cells can result in pre- or post-ossification defects in the developing craniofacial skeleton and craniosynostosis (premature fusion of cranial bones/cranial sutures). The coronal suture is the most frequently fused suture in craniosynostosis syndromes. It exists as a biological boundary between the neural crest-derived frontal bone and paraxial mesoderm-derived parietal bone. The objective of this review is to frame our current understanding of neural crest cells in craniofacial development, craniofacial anomalies, and the pathogenesis of coronal craniosynostosis. We will also discuss novel approaches for advancing our knowledge and developing prevention and/or treatment strategies for craniofacial tissue regeneration and craniosynostosis.

Viral delivery of tissue nonspecific alkaline phosphatase diminishes craniosynostosis in one of two FGFR2C342Y/+ mouse models of Crouzon syndrome

Craniosynostosis is the premature fusion of cranial bones. The goal of this study was to determine if delivery of recombinant tissue nonspecific alkaline phosphatase (TNAP) could prevent or diminish the severity of craniosynostosis in a C57BL/6 FGFR2^C342Y/+ model of neonatal onset craniosynostosis or a BALB/c FGFR2^C342Y/+ model of postnatal onset craniosynostosis. Mice were injected with a lentivirus encoding a mineral targeted form of TNAP immediately after birth. Cranial bone fusion as well as cranial bone volume, mineral content and density were assessed by micro CT. Craniofacial shape was measured with calipers. Alkaline phosphatase, alanine amino transferase (ALT) and aspartate amino transferase (AST) activity levels were measured in serum. Neonatal delivery of TNAP diminished craniosynostosis severity from 94% suture obliteration in vehicle treated mice to 67% suture obliteration in treated mice, p<0.02) and the incidence of malocclusion from 82.4% to 34.7% (p<0.03), with no effect on cranial bone in C57BL/6 FGFR2^C342Y/+ mice. In contrast, treatment with TNAP increased cranial bone volume (p< 0.01), density (p< 0.01) and mineral content (p< 0.01) as compared to vehicle treated controls, but had no effect on craniosynostosis or malocclusion in BALB/c FGFR2^C342Y/+ mice. These results indicate that postnatal recombinant TNAP enzyme therapy diminishes craniosynostosis severity in the C57BL/6 FGFR2^C342Y/+ neonatal onset mouse model of Crouzon syndrome, and that effects of exogenous TNAP are genetic background dependent.

[Clinical and genetic characteristics of craniosynostosis]

Craniosynostosis is caused by premature fusion of one or more cranial sutures leading to deformity of the cranium. Depending on the type and number of the sutures involved and the order of their fusion, different forms of deformities may develop. Two main types of craniosynostosis can be distinguished: non-syndromic (isolated) and syndromic forms. In the latter group the cranial deformity is usually associated with dysmorphic features, limb anomalies and other symptoms while in non-syndromic form the cranial deformity can be seen only. The type and severity of associated anomalies in the syndromic form are different. Early fusion of sutures can be caused by both environmental and genetic factors. In the present paper the authors aim to review the clinical features and genetic background of craniosynostosis focusing on some common syndromes.

Neural crest cell signaling pathways critical to cranial bone development and pathology

Neural crest cells appear early during embryogenesis and give rise to many structures in the mature adult. In particular, a specific population of neural crest cells migrates to and populates developing cranial tissues. The ensuing differentiation of these cells via individual complex and often intersecting signaling pathways is indispensible to growth and development of the craniofacial complex. Much research has been devoted to this area of development with particular emphasis on cell signaling events required for physiologic development. Understanding such mechanisms will allow researchers to investigate ways in which they can be exploited in order to treat a multitude of diseases affecting the craniofacial complex. Knowing how these multipotent cells are driven towards distinct fates could, in due course, allow patients to receive regenerative therapies for tissues lost to a variety of pathologies. In order to realize this goal, nucleotide sequencing advances allowing snapshots of entire genomes and exomes are being utilized to identify molecular entities associated with disease states. Once identified, these entities can be validated for biological significance with other methods. A crucial next step is the integration of knowledge gleaned from observations in disease states with normal physiology to generate an explanatory model for craniofacial development. This review seeks to provide a current view of the landscape on cell signaling and fate determination of the neural crest and to provide possible avenues of approach for future research.

Craniosynostosis-associated Fgfr2(C342Y) mutant bone marrow stromal cells exhibit cell autonomous abnormalities in osteoblast differentiation and bone formation

We recently reported that cranial bones of Fgfr2^C342Y/+ craniosynostotic mice are diminished in density when compared to those of wild type mice, and that cranial bone cells isolated from the mutant mice exhibit inhibited late stage osteoblast differentiation. To provide further support for the idea that craniosynostosis-associated Fgfr mutations lead to cell autonomous defects in osteoblast differentiation and mineralized tissue formation, here we tested bone marrow stromal cells isolated from Fgfr2^C342Y/+ mice for their ability to differentiate into osteoblasts. Additionally, to determine if the low bone mass phenotype of Crouzon syndrome includes the appendicular skeleton, long bones were assessed by micro CT. Fgfr2^C342Y/+ cells showed increased osteoblastic gene expression during early osteoblastic differentiation but decreased expression of alkaline phosphatase mRNA and enzyme activity, and decreased mineralization during later stages of differentiation, when cultured under 2D in vitro conditions. Cells isolated from Fgfr2^C342Y/+ mice also formed less bone when allowed to differentiate in a 3D matrix in vivo. Cortical bone parameters were diminished in long bones of Fgfr2^C342Y/+ mice. These results demonstrate that marrow stromal cells of Fgfr2^C342Y/+ mice have an autonomous defect in osteoblast differentiation and bone mineralization, and that the Fgfr2^C342Y mutation influences both the axial and appendicular skeletons.

Novel molecular pathways elicited by mutant FGFR2 may account for brain abnormalities in Apert syndrome

Apert syndrome (AS), the most severe form craniosynostosis, is characterized by premature fusion of coronal sutures. Approximately 70% of AS patients carry S252W gain-of-function mutation in FGFR2. Besides the cranial phenotype, brain dysmorphologies are present and are not seen in other FGFR2-asociated craniosynostosis, such as Crouzon syndrome (CS). Here, we hypothesized that S252W mutation leads not only to overstimulation of FGFR2 downstream pathway, but likewise induces novel pathological signaling. First, we profiled global gene expression of wild-type and S252W periosteal fibroblasts stimulated with FGF2 to activate FGFR2. The great majority (92%) of the differentially expressed genes (DEGs) were divergent between each group of cell populations and they were regulated by different transcription factors. We than compared gene expression profiles between AS and CS cell populations and did not observe correlations. Therefore, we show for the first time that S252W mutation in FGFR2 causes a unique cell response to FGF2 stimulation. Since our gene expression results suggested that novel signaling elicited by mutant FGFR2 might be associated with central nervous system (CNS) development and maintenance, we next investigated if DEGs found in AS cells were also altered in the CNS of an AS mouse model. Strikingly, we validated Strc (stereocilin) in newborn Fgfr2^S252W/+ mouse brain. Moreover, immunostaining experiments suggest a role for endothelial cells and cerebral vasculature in the establishment of characteristic CNS dysmorphologies in AS that has not been proposed by previous literature. Our approach thus led to the identification of new target genes directly or indirectly associated with FGFR2 which are contributing to the pathophysiology of AS.

Tissue-specific responses to aberrant FGF signaling in complex head phenotypes

BACKGROUND

The role of fibroblast growth factor and receptor (FGF/FGFR) signaling in bone development is well studied, partly because mutations in FGFRs cause human diseases of achondroplasia and FGFR-related craniosynostosis syndromes including Crouzon syndrome. The FGFR2c C342Y mutation is a frequent cause of Crouzon syndrome, characterized by premature cranial vault suture closure, midfacial deficiency, and neurocranial dysmorphology. Here, using newborn Fgfr2c(C342Y/+) Crouzon syndrome mice, we tested whether the phenotypic effects of this mutation go beyond the skeletal tissues of the skull, altering the development of other non-skeletal head tissues including the brain, the eyes, the nasopharynx, and the inner ears.

RESULTS

Quantitative analysis of 3D multimodal imaging (high-resolution micro-computed tomography and magnetic resonance microscopy) revealed local differences in skull morphology and coronal suture patency between Fgfr2c(C342Y/+) mice and unaffected littermates, as well as changes in brain shape but not brain size, significant reductions in nasopharyngeal and eye volumes, and no difference in inner ear volume in Fgfr2c(C342Y/+) mice.

CONCLUSIONS

These findings provide an expanded catalogue of clinical phenotypes in Crouzon syndrome caused by aberrant FGF/FGFR signaling and evidence of the broad role for FGF/FGFR signaling in development and evolution of the vertebrate head.

The Fgfr2 W290R mouse model of Crouzon syndrome

PURPOSE

This study aimed to review and discuss the utility of the Fgfr2 (W290R) mouse mutant as a model of human Crouzon syndrome.

METHODS

A review of current and past scientific literature on Fibroblast Growth Factor Receptor-2 (FGFR2) protein domain structure, FGFR mutations associated with human Crouzon syndrome, and phenotypic and molecular changes combined with recent observations and experimental data of the Fgfr2 (W290R) mouse mutant was conducted. A comparison of the Fgfr2 (W290R) mouse mutant with another mouse model of Crouzon syndrome, Fgfr2 (C342R) mouse mutant, was also performed. Finally, possible future research directions using the Fgfr2 (W290R) mutant mice were discussed.

RESULTS

The Fgfr2 (W290R) heterozygous mouse exhibits defects characteristic of human Crouzon syndrome. At the molecular level, the defects observed in the mouse mutant are due to the dysregulation of signaling of both the IIIb and IIIc isoforms of Fgfr2. The involvement of the IIIb isoform of FGFR2 in the etiopathology of Crouzon syndrome is a novel finding in the craniosynostosis literature field. Dysregulated signaling of both IIIb and IIIc isoforms causes a broad spectrum of changes that explain some of the defects observed clinically in humans. Several of the defects observed in the Fgfr2 (W290R) homozygous mouse mutant are attributable to a loss-of-function mechanism in contrast to the frequently reported gain-of-function receptor function associated with mutated FGF receptors in craniosynostosis.

CONCLUSIONS

The Fgfr2 ( W290R ) mouse model can be used as a model system to further investigate the cellular, molecular, and biochemical mechanisms of Crouzon syndrome.

p38 Inhibition ameliorates skin and skull abnormalities in Fgfr2 Beare-Stevenson mice

Beare-Stevenson cutis gyrata syndrome (BSS) is a human genetic disorder characterized by skin and skull abnormalities. BSS is caused by mutations in the FGF receptor 2 (FGFR2), but the molecular mechanisms that induce skin and skull abnormalities are unclear. We developed a mouse model of BSS harboring a FGFR2 Y394C mutation and identified p38 MAPK as an important signaling pathway mediating these abnormalities. Fgfr2+/Y394C mice exhibited epidermal hyperplasia and premature closure of cranial sutures (craniosynostosis) due to abnormal cell proliferation and differentiation. We found ligand-independent phosphorylation of FGFR2 and activation of p38 signaling in mutant skin and calvarial tissues. Treating Fgfr2+/Y394C mice with a p38 kinase inhibitor attenuated skin abnormalities by reversing cell proliferation and differentiation to near normal levels. This study reveals the pleiotropic effects of the FGFR2 Y394C mutation evidenced by cutis gyrata, acanthosis nigricans, and craniosynostosis and provides a useful model for investigating the molecular mechanisms of skin and skull development. The demonstration of a pathogenic role for p38 activation may lead to the development of therapeutic strategies for BSS and related conditions, such as acanthosis nigricans or craniosynostosis.

Atypical Crouzon Syndrome with a Novel Cys62Arg Mutation in FGFR2 Presenting with Sagittal Synostosis

The management of a 1-year-old boy with Crouzonoid features is presented with a description of molecular genetic investigations that revealed a previously unreported mutation of the fibroblast growth factor receptor 2 (FGFR2) gene encoding the amino acid substitution p.Cys62Arg within the immunoglobin-like (IgI) domain. The patient presented in atypical fashion with severe sagittal synostosis but only mild exorbitism and hypertelorism. Owing to the progressively increasing size of the cranial occipital bullet, a total calvarial modeling procedure was performed at 8 months of age to correct the craniofacial deformity.

Standard genetic testing of the major mutational “hotspots” associated with craniosynostosis was initially negative. However, further testing for atypical sites of mutation revealed a heterozygous nucleotide substitution (c.184T>C) in exon 3 of FGFR2. This mutation has not been previously reported and is only the second to be identified in the IgI domain; it was not present in either parent, indicating that it had arisen de novo. The child remains well 6 months postoperatively but will be monitored more closely compared with the usual protocol for nonsyndromic sagittal synostosis owing to the potential for increased risk of secondary complications.

Key learning points from this case include the need for careful phenotypic evaluation of children presenting with apparently isolated sagittal synostosis and genetic testing for atypical mutations if the usual hotspots are negative.

Leucine-rich repeat, immunoglobulin-like and transmembrane domain 3 (LRIT3) is a modulator of FGFR1

Fibroblast growth factor receptors (FGFRs) play critical roles in craniofacial and skeletal development via multiple signaling pathways including MAPK, PI3K/AKT, and PLC-?. FGFR-mediated signaling is modulated by several regulators. Proteins with leucine-rich repeat (LRR) and/or immunoglobulin (IG) superfamily domains have been suggested to interact with FGFRs. In addition, fibronectin leucine-rich repeat transmembrane protein 3 (FLRT3) has been shown to modulate the FGFR-mediated signaling via the fibronectin type III (FNIII) domain. Therefore proteins with LRR, IG, and FNIII are candidate regulators of the FGFRs. Here we identify leucine-rich repeat, immunoglobulin-like and transmembrane domain 3 (LRIT3) as a regulator of the FGFRs.

Regulation of non-classical FGF1 release and FGF-dependent cell transformation by CBF1-mediated notch signaling

FGF1, a widely expressed proangiogenic factor involved in tissue repair and carcinogenesis, is released from cells through a non-classical pathway independent of endoplasmic reticulum and Golgi. Although several proteins participating in FGF1 export were identified, genetic mechanisms regulating this process remained obscure. We found that FGF1 export and expression are regulated through Notch signaling mediated by transcription factor CBF1 and its partner MAML. The expression of a dominant negative (dn) form of CBF1 in 3T3 cells induces transcription of FGF1 and sphingosine kinase 1 (SphK1), which is a component of FGF1 export pathway. dnCBF1 expression stimulates the stress-independent release of transduced FGF1 from NIH 3T3 cells and endogenous FGF1 from A375 melanoma cells. NIH 3T3 cells transfected with dnCBF1 form colonies in soft agar and produce rapidly growing highly angiogenic tumors in nude mice. The transformed phenotype of dnCBF1 transfected cells is efficiently blocked by dn forms of FGF receptor 1 and S100A13, which is a component of FGF1 export pathway. FGF1 export and acceleration of cell growth induced by dnCBF1 depend on SphK1. Similar to dnCBF1, dnMAML transfection induces FGF1 expression and release, and accelerates cell proliferation. The latter effect is strongly decreased in FGF1 null cells. We suggest that the regulation of FGF1 expression and release by CBF1-mediated Notch signaling can play an important role in tumor formation.

Genetic basis of potential therapeutic strategies for craniosynostosis

Craniosynostosis, the premature fusion of one or more cranial sutures, is a common malformation of the skull that can result in facial deformity and increased intracranial pressure. Syndromic craniosynostosis is present in ∼15% of craniosynostosis patients and often is clinically diagnosed by neurocranial phenotype as well as various other skeletal abnormalities. The most common genetic mutations identified in syndromic craniosynostosis involve the fibroblast growth factor receptor (FGFR) family with other mutations occurring in genes for transcription factors TWIST, MSX2, and GLI3, and other proteins EFNB1, RAB23, RECQL4, and POR, presumed to be involved either upstream or downstream of the FGFR signaling pathway. Both syndromic and nonsyndromic craniosynostosis patients require early diagnosis and intervention. The premature suture fusion can impose pressure on the growing brain and cause continued abnormal postnatal craniofacial development. Currently, treatment options for craniosynostosis are almost exclusively surgical. Serious complications can occur in infants requiring either open or endoscopic repair and therefore the development of nonsurgical techniques is highly desirable although arguably difficult to design and implement. Genetic studies of aberrant signaling caused by mutations underlying craniosynostosis in in vitro calvarial culture and in vivo animal model systems have provided promising targets in designing genetic and pharmacologic strategies for systemic or adjuvant nonsurgical treatment. Here we will review the current literature and provide insights to future possibilities and limitations of therapeutic applications.

Dura in the pathogenesis of syndromic craniosynostosis: fibroblast growth factor receptor 2 mutations in dural cells promote osteogenic proliferation and differentiation of osteoblasts

Mutations in fibroblast growth factor receptor 2 (FGFR2), a transmembrane receptor expressed in suture mesenchyme, osteogenic fronts, and dura, have been implicated in the etiopathogenesis of craniosynostosis syndromes. The C278F- and P253R-FGFR2 mutations result in Crouzon and Apert syndromes, respectively. The dura mater plays a critical role in the formation and maintenance of cranial sutures. However, its role in syndromic craniosynostosis remains unclear. This study examines the influence of FGFR2 mutations in dural cells on osteoblast proliferation and differentiation. Primary cultures of dural cells and osteoblasts were established, and adenoviral-FGFR2 constructs were prepared by subcloning mutant (C278F and P253R) FGFR2 constructs into adenovirus. Dural cells were infected with adenovirus, and dural protein expression was confirmed by immunostaining. Infected dural cells were cocultured with osteoblasts using a transwell system for 7 days. Dural cells infected with null adenovirus served as the negative control. In separate cultures, osteoblasts were directly infected with the adenoviral-FGFR2 constructs. Osteoblast proliferation was analyzed by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, and differentiation was analyzed by alkaline phosphatase assay, histochemical staining, and gene expression studies. Osteoblasts directly infected with the Crouzon (C278F-FGFR2) mutation demonstrated an increase in cell proliferation (P < 0.05). Osteoblasts directly infected with the Apert (P253R-FGFR2) mutation demonstrated an increase in alkaline phosphatase activity. In coculture experiments, osteoblasts cocultured with Crouzon-transformed dural cells demonstrated increased cell proliferation (P < 0.05), whereas osteoblasts cocultured with Apert-transformed dural cells showed an increase in alkaline phosphatase activity (P < 0.05). In addition, osteogenic gene expression (alkaline phosphatase, osteopontin, and runx2) were up-regulated in osteoblasts cocultured with Apert-expressing dural cells. These experiments suggest that FGFR2 mutations in dural cells alter normal dural signaling. Apert mutations promote osteodifferentiation, whereas Crouzon mutations result in enhanced cell proliferation. These mutations may induce craniosynostosis in part through the influence of mutation-induced constitutive signaling in the dura, with subsequent enhancement of dural-mediated osteogenesis.

Conditional expression of Spry1 in neural crest causes craniofacial and cardiac defects

Background

Growth factors and their receptors are mediators of organogenesis and must be tightly regulated in a temporal and spatial manner for proper tissue morphogenesis. Intracellular regulators of growth factor signaling pathways provide an additional level of control. Members of the Sprouty family negatively regulate receptor tyrosine kinase pathways in several developmental contexts. To gain insight into the role of Spry1 in neural crest development, we analyzed the developmental effects of conditional expression of Spry1 in neural crest-derived tissues.

Results

Here we report that conditional expression of Spry1 in neural crest cells causes defects in craniofacial and cardiac development in mice. Spry1;Wnt1-Cre embryos die perinatally and exhibit facial clefting, cleft palate, cardiac and cranial nerve defects. These defects appear to be the result of decreased proliferation and increased apoptosis of neural crest and neural crest-derived cell populations. In addition, the domains of expression of several key transcription factors important to normal craniofacial and cardiac development including AP2, Msx2, Dlx5, and Dlx6 were reduced in Spry1;Wnt1-Cre transgenic embryos.

Conclusion

Collectively, these data suggest that Spry1 is an important regulator of craniofacial and cardiac morphogenesis and perturbations in Spry1 levels may contribute to congenital disorders involving tissues of neural crest origin.

Discovery of novel fibroblast growth factor receptor 1 kinase inhibitors by structure-based virtual screening

Fibroblast growth factors (FGFs) play important roles in embryonic development, angiogenesis, wound healing, and cell proliferation and differentiation. In search of inhibitors of FGFR1 kinase, 2.2 million compounds were docked into the ATP binding site of the protein. A co-crystal structure, which shows two alternative conformations for the nucleotide binding loop, is reported. Docking was performed on both conformations and, ultimately, 23 diverse compounds were purchased and assayed. Following hit validation, two compounds 10 and 16, a benzylidene derivative of pseudothiohydantoin and a thienopyrimidinone derivative, respectively, were discovered that inhibit FGFR1 kinase with IC(50) values of 23 and 50 microM. Initial optimization of 16 led to the more unsaturated 40, which has significantly enhanced potency, 1.9 microM. The core structures represent new structural motifs for FGFR1 kinase inhibitors. The study also illustrates complexities associated with the choice of protein structures for docking, possible use of multiple kinase structures to seek selectivity, and hit identification.

Mutation of the bone morphogenetic protein GDF3 causes ocular and skeletal anomalies

Ocular mal-development results in heterogeneous and frequently visually disabling phenotypes that include coloboma and microphthalmia. Due to the contribution of bone morphogenetic proteins to such processes, the function of the paralogue Growth Differentiation Factor 3 was investigated. Multiple mis-sense variants were identified in patients with ocular and/or skeletal (Klippel-Feil) anomalies including one individual with heterozygous alterations in GDF3 and GDF6. These variants were characterized, individually and in combination, through integrated biochemical and zebrafish model organism analyses, demonstrating appreciable effects with western blot analyses, luciferase based reporter assays and antisense morpholino inhibition. Notably, inhibition of the zebrafish co-orthologue of GDF3 accurately recapitulates patient phenotypes. By demonstrating the pleiotropic effects of GDF3 mutation, these results extend the contribution of perturbed BMP signaling to human disease and potentially implicate multi-allelic inheritance of BMP variants in developmental disorders.

The FGF family: biology, pathophysiology and therapy

The family of fibroblast growth factors (FGFs) regulates a plethora of developmental processes, including brain patterning, branching morphogenesis and limb development. Several mitogenic, cytoprotective and angiogenic therapeutic applications of FGFs are already being explored, and the recent discovery of the crucial roles of the endocrine-acting FGF19 subfamily in bile acid, glucose and phosphate homeostasis has sparked renewed interest in the pharmacological potential of this family. This Review discusses traditional applications of recombinant FGFs and small-molecule FGF receptor kinase inhibitors in the treatment of cancer and cardiovascular disease and their emerging potential in the treatment of metabolic syndrome and hypophosphataemic diseases.

Advances in the molecular pathogenesis of craniofacial conditions

The impact that the understanding of fibroblast growth factor receptor (FGFR) biology and its relevance to the pathogenesis of the craniosynostoses has made cannot be underestimated. As the genetic and molecular pathology of other conditions become increasingly understood, there is much hope that robust and relevant animal models of these conditions may be generated. From these models-and in conjunction with laboratory studies in vitro-comes a real hope of improved therapeutic strategies. The future lies in increased cooperation between clinicians working in high-volume centers and basic scientists. This article decribes the results of a decade of research in which the molecular pathology of the craniosynostoses was unravelled. The understanding of the importance of FGFR mutations to the genetic etiology of craniosynostosis opened up novel studies in developmental biology in various tissues. Such studies describe the functional effects of FGFR mutations. Investigations of FGFR expression in human craniofacial development have related functional molecular studies to human craniosynostosis syndromes, which provides a link between the gene mutation and the affected child.

Cranial suture biology and dental development: genetic and clinical perspectives

Premature fusion of the calvarial bones at the sutures, or craniosynostosis (CS), is a relatively common birth defect (1:2000-3000) frequently associated with limb deformity. Patients with CS may present oral defects, such as cleft soft palate, hypodontia, hyperdontia, and delayed tooth eruption, but also unusual associations of major dental anomalies such as taurodontism, microdontia, multiple dens invaginatus, and dentin dysplasia. The list of genes that are involved in CS includes those coding for the different fibroblast growth factor receptors and a ligand of ephrin receptors, but also genes encoding transcription factors, such as MSX2 and TWIST. Most of these genes are equally involved in odontogenesis, providing a pausible explanation for clinical associations of CS with dental agenesis or tooth malformations. On the basis of the present knowledge on genes and transcription factors that are involved in craniofacial morphogenesis, and from dental clinics of CS syndromes, the molecular mechanisms that control suture formation and suture closure are expected to play key roles in patterning events and development of teeth. The purpose of this article is to review and merge the recent advances in the field of suture research at the genetic and cellular levels with those of tooth development, and to apply them to the dental clinics of CS syndromes. These new perspectives and future challenges in the field of both dental clinics and molecular genetics, more in particular the identification of possible candidate genes involved in both CS and dental defects, are discussed.

[Genetic origin of non-syndromic cleft lip and palate. TWIST, a candidate gene? Research protocol]

Non syndromic cleft lip and palate (CLP) is the most frequent human malformation. CLP is of complex inheritance and at least twenty contributing chromosomal regions have been identified by linkage studies. On the other hand, mutations in several genes such as TWIST and FGFR2 result in syndromic cranio-facial abnormalities of highly variable range. It is our hypothesis that some mutations at TWIST might contribute to CLP in absence of other dysmorphic features. Thus, DNA biopsies of patients with non syndromic CLP are collected and prepared to search for allelic variations or mutations at TWIST. This study should contribute to improve the classification of facial malformations relative to gene, to help to a better understanding of the inheritance pattern of this pathology, to help to genetic counselling for some cases aiming at the prevention of genetic disease. This project is based on a close cooperation between the Orthodontic Department, the Paediatric Surgery Department and the Center for Clinical Investigation (University Hospital in Strasbourg), in a joint project with an academic research laboratory, expert in molecular biology and genetics.

An 11q11-q13.3 duplication, including FGF3 and FGF4 genes, in a patient with syndromic multiple craniosynostoses

Interstitial duplications of 11q are very rare and seldom reported. In this paper we describe the first case of a duplication involving bands 11q11 and 11q12. This newly described patient has multiple craniosynostoses, congenital heart defect and developmental delay, and is a carrier of a mosaic duplication: 46,XY,dup(11)(q11-->q13.3)(29)/46,XY(6). The breakpoints were further delimited by comparative genomic hybridization microarray. We also performed fluorescent in situ hybridization analysis to determine the extension of the duplication in a patient described earlier with a duplication 11q13.5-q21. An overlapping region of less than 1.2 Mb was identified and included the duplication of genes FGF3 and FGF4 in both individuals. We discuss the possible implications of dosage effects of these genes in the onset of craniosynostosis.

FGF2 effects in periosteal fibroblasts bearing the FGFR2 receptor Pro253 Arg mutation

AIM

A growing number of mutations mapped in the receptor gene for fibroblast growth factor have been implicated in several cranial development disorders including the Apert and Crouzon syndromes. The present paper investigated cellular mechanisms underlying Apert phenotype, by analyzing the effects of FGF2 in primary cultures of Apert periosteal fibroblasts carrying the FGFR2 Pro253Arg mutation.

RESULTS

FGF2 administration significantly decreased extracellular matrix production in mutant cells by stimulating degradative enzymatic activities. Gene expression analysis revealed that decorin and biglycan, two proteoglycans involved in collagen fibrillogenesis, were more expressed in mutant cells and down-regulated by FGF2. FGF2 receptor binding showed little differences in high affinity receptor counts between mutant and wild-type cells, while we showed for the first time that low affinity receptors are significantly fewer in mutant cells. Differences were found in Crouzon syndrome, where both high and low affinity receptor counts were up-regulated.

CONCLUSIONS

The different mutation and low affinity receptor regulation in mutant receptors support the hypothesis that the impact on the activity of the ligand-receptor complex could allow distinct modes of FGF2 activation in Apert and Crouzon syndromes, which interfere with the FGFR2 signalling cascade.

Intracellular retention, degradation, and signaling of glycosylation-deficient FGFR2 and craniosynostosis syndrome-associated FGFR2C278F

Fibroblast growth factors (FGFs) and their receptors (FGFRs) are known to play a critical role in a variety of fundamental processes, including wound healing, angiogenesis, and development of multiple organ systems. Mutations in the FGFR gene family have been linked to a series of syndromes (the craniosynostosis syndromes) whose primary phenotype involves aberrant development of the craniofacial skeleton. Craniosynostosis syndrome-linked FGFR mutations have been shown to be gain of function in terms of receptor activation and have been presumed to result in increased levels of FGF/FGFR signaling. Unfortunately, studies attempting to link expression of mutant FGFRs with changes in cellular phenotype have yielded conflicting results. In an effort to better understand the biochemical consequences of these mutations on receptor function, here we have investigated the effect of the FGFR2C278F mutation of Crouzon craniosynostosis syndrome on receptor trafficking, ubiquitination, degradation, and signaling. We find that FGFR2C278F exhibits diminished glycosylation, increased degradation, and limited cellular sublocalization in the osteoblastic cell line, MC3T3E1(C4). Additionally, we show that trafficking and autoactivation of wild type FGFR2 is glycosylation-dependent. Both FGFR2C278F and unglycosylated wild type FGFR2 signal through phospholipase Cgamma in a ligand-independent manner as well as exhibit dramatically increased binding to the adaptor protein, Frs2. These findings suggest that autoactive FGFR2 can signal from intracellular compartments. Based upon our results, we propose that the functional signaling of craniosynostosis mutant, autoactive receptors is limited in some cell types by protective cellular responses, such as increased trafficking to lysosomes and proteasomes for degradation.

Sprouty genes are expressed in osteoblasts and inhibit fibroblast growth factor-mediated osteoblast responses

Fibroblast growth factors (FGFs) and fibroblast growth factor receptors (FGFRs) are major regulators of skeletal growth and development. Signal transduction via FGFRs is complex and mediates proliferation, differentiation, or migration depending upon the cellular context. Members of the Spry gene family antagonize the FGFR signal transduction pathway and inhibit lung morphogenesis, angiogenesis, and chondrogenesis. We examined the expression of Spry2 in the osteoblastic MC3T3-E1 cell line. MC3T3-E1 cells express Spry2 in response to FGF1 stimulation. Treatment of MC3T3-E1 cells with FGF1 results in the expression of Spry2 in a manner consistent with an early response gene. Pharmacological inhibitors of mitogen-activated protein kinase activation inhibit FGF1-induced expression of Spry2 mRNA. Transient overexpression of Spry2 in MC3T3-E1 resulted in decreased FGF1-mediated extracellular signal-regulated kinase phosphorylation and FGF1-stimulated osteopontin promoter activity. Furthermore, we show that Spry2 interacts with Raf-1 in a glutathione-S-transferase pulldown assay and that this interaction may involve multiple sites. Finally, Spry2 expression precedes the onset of the expression of osteoblast differentiation markers in an in vitro assay of primary osteoblast differentiation. Taken together, these results indicate that Spry2 expression is an early response to stimulation by FGF1 in MC3T3-E1 cells and acts as a feedback inhibitor of FGF1-induced osteoblast responses, possibly through interaction with Raf1.

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

The mammalian skull vault is constructed principally from five bones: the paired frontals and parietals, and the unpaired interparietal. These bones abut at sutures, where most growth of the skull vault takes place. Sutural growth involves maintenance of a population of proliferating osteoprogenitor cells which differentiate into bone matrix-secreting osteoblasts. Sustained function of the sutures as growth centres is essential for continuous expansion of the skull vault to accommodate the growing brain. Craniosynostosis, the premature fusion of the cranial sutures, occurs in 1 in 2500 children and often presents challenging clinical problems. Until a dozen years ago, little was known about the causes of craniosynostosis but the discovery of mutations in the MSX2, FGFR1, FGFR2, FGFR3, TWIST1 and EFNB1 genes in both syndromic and non-syndromic cases has led to considerable insights into the aetiology, classification and developmental pathology of these disorders. Investigations of the biological roles of these genes in cranial development and growth have been carried out in normal and mutant mice, elucidating their individual and interdependent roles in normal sutures and in sutures undergoing synostosis. Mouse studies have also revealed a significant correspondence between the neural crest-mesoderm boundary in the early embryonic head and the position of cranial sutures, suggesting roles for tissue interaction in suture formation, including initiation of the signalling system that characterizes the functionally active suture.

Structural basis for fibroblast growth factor receptor activation

FGF signaling plays a ubiquitous role in human biology as a regulator of embryonic development, homeostasis and regenerative processes. In addition, aberrant FGF signaling leads to diverse human pathologies including skeletal, olfactory, and metabolic disorders as well as cancer. FGFs execute their pleiotropic biological actions by binding, dimerizing and activating cell surface FGF receptors (FGFRs). Proper regulation of FGF-FGFR binding specificity is essential for the regulation of FGF signaling and is achieved through primary sequence variations among the 18 FGFs and seven FGFRs. The severity of human skeletal syndromes arising from mutations that violate FGF-FGFR specificity is a testament to the importance of maintaining precision in FGF-FGFR specificity. The discovery that heparin/heparan sulfate (HS) proteoglycans are required for FGF signaling led to numerous models for FGFR dimerization and heralded one of the most controversial issues in FGF signaling. Recent crystallographic analyses have led to two fundamentally different models for FGFR dimerization. These models differ in both the stoichiometry and minimal length of heparin required for dimerization, the quaternary arrangement of FGF, FGFR and heparin in the dimer, and in the mechanism of 1:1 FGF-FGFR recognition and specificity. In this review, we provide an overview of recent structural and biochemical studies used to differentiate between the two crystallographic models. Interestingly, the structural and biophysical analyses of naturally occurring pathogenic FGFR mutations have provided the most compelling and unbiased evidences for the correct mechanisms for FGF-FGFR dimerization and binding specificity. The structural analyses of different FGF-FGFR complexes have also shed light on the intricate mechanisms determining FGF-FGFR binding specificity and promiscuity and also provide a plausible explanation for the molecular basis of a large number craniosynostosis mutations.

A further mutation of the FGFR2 tyrosine kinase domain in mild Crouzon syndrome

We report a family heterozygous for a newly identified mutation in the tyrosine kinase I domain of the FGFR2 gene (1576A > G, encoding the missense substitution Lys526Glu), associated with variable expressivity of Crouzon syndrome, including clinical nonpenetrance. Our observations expand both the clinical and molecular spectrum of this unusual subset of FGFR2 mutations.

Sef interacts with TAK1 and mediates JNK activation and apoptosis

Sef was recently identified as a negative regulator of fibroblast growth factor (FGF) signaling in a genetic screen of zebrafish and subsequently in mouse and humans. By inhibiting FGFR1 tyrosine phosphorylation and/or Ras downstream events, Sef inhibits FGF-mediated ERK activation and cell proliferation as well as PC12 cell differentiation. Here we show that Sef and a deletion mutant of Sef lacking the extracellular domain (SefIC) physically interact with TAK1 (transforming growth factor-beta-associated kinase) and activate JNK through a TAK1-MKK4-JNK pathway. Sef and SefIC overexpression also resulted in apoptotic cell death, while dominant negative forms of MKK4 and TAK1 blocked Sef-mediated JNK activation and attendant 293T cell apoptosis. These investigations reveal a novel activating function of Sef that is distinct from its inhibitory effect on FGF receptor signaling and ERK activation.

Kinematics of cranial ontogeny: Heterotopy, heterochrony, and geometric morphometric analysis of growth models

In this paper, we examine the relationship between the classical concepts of heterotopy, heterochrony and ontogenetic allometry as descriptive and as explanatory categories in the investigation of evolutionary developmental novelty in the hominid skull. We use concepts of kinematic analysis of locomotion to propose a methodological framework for the kinematic analysis of cranial form change during ontogeny. We argue that a combination of geometric-morphometric methods with graphics visualization tools currently represents the most adequate means to analyze the kinematics of ontogeny. Using cranial growth models, we simulate how evolutionary modifications of developmental processes impinge on morphological patterns of ontogeny, and explore how differences in ontogenetic patterns can tentatively be traced back to underlying process differences. Our analyses indicate that minor alterations in growth parameters elicit complex patterns of ontogenetic modification that are difficult to describe with the standard repertoire of heterochronic terminology. The proposed kinematic and model-based approach is used in a comparative analysis of cranial ontogeny in Neanderthals and anatomically modern humans, indicating that early ontogenetic modification of a small set of growth parameters is a major source of evolutionary novelty during hominid evolution.

Lambdoid synostosis and occipital plagiocephaly: clinical decision rules for surgical intervention

Lambdoid craniosynostosis has been regarded as one of the least common categories of premature fusion of the cranial sutures, yet reports have suggested the incidence may be increasing. To guide treatment decisions, the authors describe a set of rules based on radiographic indicators and clinical assessment in the child. Experience suggests that children can have abnormal-appearing cranial sutures with normal neurological status and normal-appearing sutures with neurological deficits or marked cerebral compression. Early evaluation and follow-up treatment is essential for children with suspected craniosynostosis.

Morphogenetic Movements Underlying Eye Field Formation Require Interactions between the FGF and ephrinB1 Signaling Pathways

The definitive retinal progenitors of the eye field are specified by transcription factors that both promote a retinal fate and control cell movements that are critical for eye field formation. However, the molecular signaling pathways that regulate these movements are largely undefined. We demonstrate that both the FGF and ephrin pathways impact eye field formation. Activating the FGF pathway before gastrulation represses cellular movements in the presumptive anterior neural plate and prevents cells from expressing a retinal fate, independent of mesoderm induction or anterior-posterior patterning. Inhibiting the FGF pathway promotes cell dispersal and significantly increases eye field contribution. ephrinB1 reverse signaling is required to promote cellular movements into the eye field, and can rescue the FGF receptor-induced repression of retinal fate. These results indicate that FGF modulation of ephrin signaling regulates the positioning of retinal progenitor cells within the definitive eye field.

Fibroblast growth factor signaling controlling osteoblast differentiation

Fibroblast growth factors (FGFs) play important roles in skeletal development and postnatal osteogenesis. FGF signaling controls bone formation by regulating the expression of various genes involved in osteoprogenitor cell replication, osteoblast differentiation and apoptosis. Recent genetic manipulation of FGF expression in mice and studies of the phenotype induced by gain-of-function mutations in FGF receptors in humans revealed the important role of FGF signaling in osteoblast function and differentiation. Additionally, cell biology studies allowed to identify some signaling pathways that are involved in the control of FGF actions in osteoblasts. This led to a better understanding of the functional role of FGF signaling in the control of gene expression in osteoblasts. The elucidation of molecular mechanisms by which FGF signaling promotes osteoblast gene expression and differentiation may help to find novel molecular targets and develop new therapeutic approaches to promote bone formation in human bone disorders.

FGFs, their receptors, and human limb malformations: clinical and molecular correlations

Fibroblast growth factors (FGFs) comprise a family of 22 distinct proteins with pleiotropic signaling functions in development and homeostasis. These functions are mediated principally by four fibroblast growth factor receptors (FGFRs), members of the receptor tyrosine kinase family, with heparin glycosaminoglycan as an important cofactor. Developmental studies in chick and mouse highlight the critical role of FGF-receptor signaling in multiple phases of limb development, including the positioning of the limb buds, the maintenance of limb bud outgrowth, the detailed patterning of the limb elements, and the growth of the long bones. Corroborating these important roles, mutations of two members of the FGFR family (FGFR1 and FGFR2) are associated with human disorders of limb patterning; in addition, mutations of FGFR3 and FGF23 affect growth of the limb bones. Analysis of FGFR2 mutations in particular reveals a complex pattern of genotype/phenotype correlation, which will be reviewed in detail. Circumstantial evidence suggests that the more severe patterning abnormalities are mediated by illegitimate paracrine signaling in the mesoderm, mediated by FGF10 or by a related FGF, and this is beginning to gain some experimental support. A further test of this hypothesis is provided by a unique family segregating two FGFR2 mutations in cis (S252L; A315S), in which severe syndactyly occurs in the absence of the craniosynostosis that typically accompanies FGFR2 mutations.

A Twist in fate: evolutionary comparison of Twist structure and function

The general requirement to induce mesoderm and allocate cells into different mesodermal tissues such as body muscle or heart is common in many animal embryos. Since the discovery of the twist gene, there has been great progress toward unraveling the molecular mechanisms that control mesoderm specification and differentiation. Twist was first identified in Drosophila as a gene crucial for proper gastrulation and mesoderm formation. In the fly embryo, Twist continues to play additional roles, allocating mesodermal cells into the body wall muscle fate and patterning a subset of these muscles. Twist is also required for proper differentiation of the adult musculature. Twist homologues have been identified in a great variety of organisms, which span the phylogenetic tree. These organisms include other invertebrates such as jellyfish, nematode, leech and lancelet as well as vertebrates such as frog, chick, fish, mouse and human. The Twist family shares both homology in structure across the basic helix-loop-helix domain and in expression during mesoderm and muscle development in most species. Here we review the current state of knowledge of the Twist family and consider how Twist functions during development. Moreover, we highlight experimental evidence that shows common themes that Twist employs during specification and patterning of the mesoderm among evolutionarily distant organisms. Conserved principles and the molecular mechanisms underlying them are discussed.

Genomic screening of fibroblast growth-factor receptor 2 reveals a wide spectrum of mutations in patients with syndromic craniosynostosis

It has been known for several years that heterozygous mutations of three members of the fibroblast growth-factor-receptor family of signal-transduction molecules-namely, FGFR1, FGFR2, and FGFR3-contribute significantly to disorders of bone patterning and growth. FGFR3 mutations, which predominantly cause short-limbed bone dysplasia, occur in all three major regions (i.e., extracellular, transmembrane, and intracellular) of the protein. By contrast, most mutations described in FGFR2 localize to just two exons (IIIa and IIIc), encoding the IgIII domain in the extracellular region, resulting in syndromic craniosynostosis including Apert, Crouzon, or Pfeiffer syndromes. Interpretation of this apparent clustering of mutations in FGFR2 has been hampered by the absence of any complete FGFR2-mutation screen. We have now undertaken such a screen in 259 patients with craniosynostosis in whom mutations in other genes (e.g., FGFR1, FGFR3, and TWIST) had been excluded; part of this screen was a cohort-based study, enabling unbiased estimates of the mutation distribution to be obtained. Although the majority (61/62 in the cohort sample) of FGFR2 mutations localized to the IIIa and IIIc exons, we identified mutations in seven additional exons-including six distinct mutations of the tyrosine kinase region and a single mutation of the IgII domain. The majority of patients with atypical mutations had diagnoses of Pfeiffer syndrome or Crouzon syndrome. Overall, FGFR2 mutations were present in 9.8% of patients with craniosynostosis who were included in a prospectively ascertained sample, but no mutations were found in association with isolated fusion of the metopic or sagittal sutures. We conclude that the spectrum of FGFR2 mutations causing craniosynostosis is wider than previously recognized but that, nevertheless, the IgIIIa/IIIc region represents a genuine mutation hotspot.