Apert syndrome mutations in fibroblast growth factor receptor 2 exhibit increased affinity for FGF ligand

Dominantly acting mutations of the fibroblast growth factor (FGF) receptor 2 (FGFR2) gene have been implicated in various craniosynostosis syndromes. Apert syndrome, characterized in addition by syndactyly of the limbs, involves specific mutations at two adjacent residues, Ser252Trp and Pro253Arg, predicted to lie in the linker region between IgII and IgIII of the FGFR2 ligand-binding domain. We have analysed the interaction of FGF ligands with wild-type and Apert- type mutant FGFR2 ectodomains in solution. Wild-type and Apert-type receptors form a complex with FGF ligands with a stoichiometry of 2:2 (ligand:receptor). The kinetics and specificity of ligand binding to wild-type and Apert mutant receptors have been analysed using surface plasmon resonance techniques. This reveals that Apert mutations, compared with wild-type, exhibit a selective decrease in the dissociation kinetics of FGF2, but not of other FGF ligands examined. In contrast, the substitution Ser252Leu in FGFR2, previously observed in several asymptomatic individuals, exhibited wild-type kinetics. These findings indicate that Apert syndrome arises as a result of increased affinity of mutant receptors for specific FGF ligands which leads to activation of signalling under conditions where availability of ligand is limiting.      

Biochemical analysis of pathogenic ligand-dependent FGFR2 mutations suggests distinct pathophysiological mechanisms for craniofacial and limb abnormalities

Gain-of-function missense mutations in FGF receptor 2 (FGFR2) are responsible for a variety of craniosynostosis syndromes including Apert syndrome (AS), Pfeiffer syndrome (PS) and Crouzon syndrome (CS). Unlike the majority of FGFR2 mutations, S252W and P253R AS mutations and a D321A PS mutation retain ligand-dependency and are also associated with severe limb pathology. In addition, a recently identified ligand-dependent S252L/A315S double mutation in FGFR2 was shown to cause syndactyly in the absence of craniosynostosis. Here, we analyze the effect of the canonical AS mutations, the D321A PS mutation and the S252L/A315S double mutation on FGFR2 ligand binding affinity and specificity using surface plasmon resonance. Both AS mutations and the D321A PS mutation, but not the S252L/A315S double mutation, increase the binding affinity of FGFR2c to multiple FGFs expressed in the cranial suture. Additionally, all four pathogenic mutations also violate FGFR2c ligand binding specificity and enable this receptor to bind FGF10. Based on our data, we propose that an increase in mutant FGFR2c binding to multiple FGFs results in craniosynostosis, whereas binding of mutant FGFR2c to FGF10 results in severe limb pathology. Structural and biophysical analysis shows that AS mutations in FGFR2b also enhance and violate FGFR2b ligand binding affinity and specificity, respectively. We suggest that elevated AS mutant FGFR2b signaling may account for the dermatological manifestations of AS.     

Mild isolated craniosynostosis due to a novel FGFR3 mutation, p.Ala334Thr

Craniosynostosis is the premature fusion of one or more sutures of the skull, which can be syndromic or isolated. Mutations in FGFR1, FGFR2, or FGFR3, among others, are often responsible for these syndromic cases. The associated of FGFR3 mutations with craniosynostosis has been restricted to three mutations, the common p.Pro250Arg in Muenke syndrome, p.Ala391Glu in Crouzon syndrome with acanthosis nigricans, and p.Pro250Leu identified in a family with isolated craniosynostosis. Other FGFR3 mutations result in various skeletal dysplasias: achondroplasia, hypochondroplasia, and thanatophoric dysplasia. Here, we report a novel mutation in exon 8 (IIIc) of FGFR3, p.Ala334Thr, in a young boy with mild craniosynostosis. The mutation segregated with mild craniosynostosis in the family and was absent in 188 normal controls. Alanine 334 is evolutionarily conserved in vertebrates and is located at the amino terminus of the βF loop in the FGFR3c isoform. The mutation is predicted to alter the protein tertiary structure which may impair its binding to its ligand, FGF1. The identification of a mutation in these clinically heterogeneous disorders can aid recurrence risk assessments. Although the implementation of a stepwise screening strategy is useful in diagnostics, mutations in unscreened regions of genes associated with craniosynostosis may explain a small proportion of craniosynostosis cases.     

Decreased Proliferation and Altered Differentiation in Osteoblasts from Genetically and Clinically Distinct Craniosynostotic Disorders

Craniosynostoses are a heterogeneous group of disorders characterized by premature fusion of cranial sutures. Mutations in fibroblast growth factor receptors (FGFRs) have been associated with a number of such conditions. Nevertheless, the cellular mechanism(s) involved remain unknown. We analyzed cell proliferation and differentiation in osteoblasts obtained from patients with three genetically and clinically distinct craniosynostoses: Pfeiffer syndrome carrying the FGFR2 C342R substitution, Apert syndrome with FGFR2 P253R change, and a nonsyndromic craniosynostosis without FGFR canonic mutations, as compared with control osteoblasts. Osteoblasts from craniosynostotic patients exhibited a lower proliferation rate than control osteoblasts. P253R and nonsyndromic craniosynostosis osteoblasts showed a marked differentiated phenotype, characterized by high alkaline phosphatase activity, increased mineralization and expression of noncollagenous matrix proteins, associated with high expression and activation of protein kinase Calpha and protein kinase Cepsilon isoenzymes. By contrast, the low proliferation rate of C342R osteoblasts was not associated with a differentiated phenotype. Although they showed higher alkaline phosphatase activity than control, C342R osteoblasts failed to mineralize and expressed low levels of osteopontin and osteonectin and high protein kinase Czeta levels. Stimulation of proliferation and inhibition of differentiation were observed in all cultures on FGF2 treatment. Our results suggest that an anticipated proliferative/differentiative switch, associated with alterations of the FGFR transduction pathways, could be the causative common feature in craniosynostosis and that mutations in distinct FGFR2 domains are associated with an in vitro heterogeneous differentiative phenotype.     

Craniosynostosis associated with FGFR3 pro250arg mutation results in a range of clinical presentations including unisutural sporadic craniosynostosis.

Several mutations involving the fibroblast growth factor receptor (FGFR) gene family have been identified in association with phenotypically distinct forms of craniosynostosis. One such point mutation, resulting in the substitution of proline by arginine in a critical region of the linker region between the first and second immunoglobulin-like domains, is associated with highly specific phenotypic consequences in that mutation at this point in FGFR1 results in Pfeiffer syndrome and analogous mutation in FGFR2 results in Apert syndrome. We now show that a much more variable clinical presentation accompanies analogous mutation in the FGFR3 gene. Specifically, mental retardation, apparently unrelated to the management of the craniosynostosis, appears to be a variable clinical consequence of this FGFR3 mutation.     

Muenke syndrome (FGFR3-related craniosynostosis): expansion of the phenotype and review of the literature

Muenke syndrome is an autosomal dominant disorder characterized by coronal suture craniosynostosis, hearing loss, developmental delay, carpal and tarsal fusions, and the presence of the Pro250Arg mutation in the FGFR3 gene. Reduced penetrance and variable expressivity contribute to the wide spectrum of clinical findings in Muenke syndrome. To better define the clinical features of this syndrome, we initiated a study of the natural history of Muenke syndrome. To date, we have conducted a standardized evaluation of nine patients with a confirmed Pro250Arg mutation in FGFR3. We reviewed audiograms from an additional 13 patients with Muenke syndrome. A majority of the patients (95%) demonstrated a mild-to-moderate, low frequency sensorineural hearing loss. This pattern of hearing loss was not previously recognized as characteristic of Muenke syndrome. We also report on feeding and swallowing difficulties in children with Muenke syndrome. Combining 312 reported cases of Muenke syndrome with data from the nine NIH patients, we found that females with the Pro250Arg mutation were significantly more likely to be reported with craniosynostosis than males (P < 0.01). Based on our findings, we propose that the clinical management should include audiometric and developmental assessment in addition to standard clinical care and appropriate genetic counseling.     

Presence of the Apert canonical S252W FGFR2 mutation in a patient without severe syndactyly.

Apert syndrome, characterised by craniosynostosis, craniofacial anomalies, and symmetrical syndactyly of the digits (cutaneous and bony fusion), has been associated with two canonical mutations in the FGFR2 gene (S252W, P253R) in the great majority of cases. Since these two alterations have been observed exclusively among these patients, it has been suggested that the S252W and P253R changes may play an important role in the occurrence of syndactyly. In order to verify whether the mutations S252W and P253R could also cause a milder phenotype, without involvement of the limbs, we have screened 22 patients with clinical characteristics compatible with Crouzon or Pfeiffer syndrome for these two particular changes. Surprisingly, we identified a Pfeiffer-like patient with the mutation S252W, and therefore we have shown for the first time the occurrence of one of the canonical Apert mutations without severe abnormalities of the upper and lower extremities.     

Rare mutations of FGFR2 causing apert syndrome: identification of the first partial gene deletion, and an Alu element insertion from a new subfamily

Apert syndrome (AS) is a severe disorder, characterized by craniosynostosis and complex syndactyly of the hands and feet. Two heterozygous gain-of-function substitutions (Ser252Trp and Pro253Arg) in exon IIIa of fibroblast growth factor receptor 2 (FGFR2) are responsible for >98% of cases. Here we describe two novel mutations in FGFR2 in the two patients in whom a mutation had not previously been found in our cohort of 227 AS cases. The first is a 1.93-kb deletion, removing exon IIIc and substantial portions of the flanking introns. This is the first large FGFR2 deletion described in any individual with craniosynostosis. The other mutation is a 5' truncated Alu insertion into exon IIIc. This is the third Alu insertion identified in AS; all have occurred within an interval of only 104 bp, representing an enrichment of over a million-fold compared to the background genomic rate. We show that the inserted Alu element belongs to a small subfamily, not previously known to be mobile, which we term Alu Yk13. Both the deletion and insertion are likely to act by a similar gain-of-function mechanism in which disruption of exon IIIc leads to illegitimate mesenchymal expression of an FGFR2 spliceform containing the alternatively spliced exon IIIb. All the AS-associated Alu insertions have arisen in the paternal germline; we propose that their enrichment in FGFR2 is driven by positive selection of the mutant spermatogonial progenitors, a mechanism analogous to that explaining why the canonical AS nucleotide substitutions also reach exceptionally high levels in sperm.     

FGF signaling in craniofacial biological control and pathological craniofacial development

Fibroblast growth factor receptors comprise a family of four evolutionarily conserved transmembrane proteins (FGFR1, FGFR2, FGFR3 and FGFR4) known to be critical for the normal development of multiple organ systems. In this review we will primarily focus upon the role of FGF/FGFR signaling as it influences the development of the craniofacial skeleton. Signaling by FGF receptors is regulated by the tissue-specific expression of FGFR isoforms, receptor subtype specific fibroblast growth factors and heparin sulfate proteoglycans. Signaling can also be limited by the expression of endogenous inhibitors. Gain-of-function mutations in FGFRs are associated with a series of congenital abnormality syndromes referred to as the craniosynostosis syndromes. Craniosynostosis is the clinical condition of premature cranial bone fusion and patients who carry craniosynostosis syndrome-associated mutations in FGFRs commonly have abnormalities of the skull vault in the form of craniosynostosis. Patients may also have abnormalities in the facial skeleton, vertebrae and digits. In this review we will discuss recent in vitro and in vivo studies investigating biologic mechanisms by which signaling through FGFRs influences skeletal development and can lead to craniosynostosis.     

A novel heterozygous mutation of three consecutive nucleotides causing Apert syndrome in a Congolese family

Apert syndrome (OMIM 101200) is a rare genetic condition characterized by craniosynostosis and syndactyly of hands and feet with clinical variability. Two single nucleotides mutations in the linker region between the immunoglobulin-like domains II and IIIa of the ectodomainin the Fibroblast Growth Factor Receptor 2 gene (FGFR2, OMIM 176943) are responsible of the vast majority of cases: c.755C > G; p.Ser252Trp (65%) and c.758C > G; p.Pro253Arg (34%. Three exceptional cases carry multiple substitutions of adjacent nucleotides in the linker region.Here we present a Congolese male patient and his mother, both affected with Apert syndrome of variable severity, carrying a previously undescribed heterozygous mutation of three consecutive nucleotides (c.756_758delGCCinsCTT) in the IgII–IgIIIa linker region. This is the fourth live-born patient to carry a multiple nucleotide substitution in the linker region and is the second alternative amino acid substitutions of the Pro253. Remarkably, this novel mutation was detected in the first Central African patient ever to be tested molecularly for the Apert syndrome. To discriminate between a hitherto unreported mutation and an ethnic specific polymorphism, we tested 105 Congolese controls, and no variation was detected.     

Description of a new mutation and characterization of FGFR1, FGFR2, and FGFR3 mutations among Brazilian patients with syndromic craniosynostoses

Dominant mutations in three fibroblast growth factor receptor genes (FGFRs1-3) cause Crouzon, Jackson-Weiss, Pfeiffer, and Apert syndromes. In the present study, 50 Brazilian patients with these four syndromes (27 Apert, 17 Crouzon, 5 Pfeiffer, and 1 Jackson-Weiss patients) were screened for mutations in the FGFR1-3 genes. Except for one, all the Apert patients had either S252W (n = 16) or P253R (n = 10) mutations. The remaining Apert case is atypical with a mutation altering the splice site of FGFR2 exon IIIc. The Pfeiffer patients had mutations in one of the FGFR genes: three in FGFR2, one in FGFR1, and one in FGFR3. In contrast, only 8 of the 17 Crouzon patients studied had a mutation in either FGFR2 (n = 7) or FGFR3 locus (n = 1). Mutations in the FGFR2 locus account for most (93%) of our syndromic craniosynostotic cases, whereas 5% had mutations in the FGFR3 locus and only 2% had mutations in the FGFR1 gene. Except for one, all the other mutations were reported previously in craniosynostotic patients from other populations. Interestingly, the mutation C278F, previously described in Crouzon and Pfeiffer cases, was here identified in a familial case with Jackson-Weiss. Also, unexpectedly, a common mutation altering the splice site of the FGFR2 exon IIIc was found in one Apert and two Pfeiffer patients. In addition, we identified a new mutation (A337P) in the FGFR2 exon IIIc associated with Crouzon phenotype. Am. J. Med. Genet. 78:237–241, 1998. © 1998 Wiley-Liss, Inc.     

FGF9 mutation causes craniosynostosis along with multiple synostoses

Craniosynostosis is commonly caused by mutations in fibroblast growth factor receptors (FGFRs), highlighting the essential role of FGF‐mediated signaling in skeletal development. We set out to identify the molecular defect in a family referred for craniosynostosis and in whom no mutation was previously detected. Using next‐generation sequencing, we identified a novel missense mutation in FGF9. Modeling based upon the crystal structure and functional studies confirmed its pathogenicity showing that it impaired homodimerization and FGFR3 binding. Only one FGF9 mutation has been previously reported in a multigeneration family with multiple synostoses (SYNS3) but no signs of craniosynostosis. In contrast, our family has a greater phenotypic resemblance to that observed in the Fgf9 spontaneous mouse mutant, elbow‐knee‐synostosis, Eks, with both multiple synostoses and craniosynostosis. We have demonstrated for the first time that mutations in FGF9 cause craniosynostosis in humans and confirm that FGF9 mutations cause multiple synostoses.     

Hearing loss in a mouse model of Muenke syndrome

The heterozygous Pro250Arg substitution mutation in fibroblast growth factor receptor 3 (FGFR3), which increases ligand-dependent signalling, is the most common genetic cause of craniosynostosis in humans and defines Muenke syndrome. Since FGF signalling plays dosage-sensitive roles in the differentiation of the auditory sensory epithelium, we evaluated hearing in a large group of Muenke syndrome subjects, as well as in the corresponding mouse model (Fgfr3(P244R)). The Muenke syndrome cohort showed significant, but incompletely penetrant, predominantly low-frequency sensorineural hearing loss, and the Fgfr3(P244R) mice showed dominant, fully penetrant hearing loss that was more severe than that in Muenke syndrome individuals, but had the same pattern of relative high-frequency sparing. The mouse hearing loss correlated with an alteration in the fate of supporting cells (Deiters'-to-pillar cells) along the entire length of the cochlear duct, with the most extreme abnormalities found at the apical or low-frequency end. In addition, there was excess outer hair cell development in the apical region. We conclude that low-frequency sensorineural hearing loss is a characteristic feature of Muenke syndrome and that the genetically equivalent mouse provides an excellent model that could be useful in testing hearing loss therapies aimed at manipulating the levels of FGF signalling in the inner ear.     

FGFR2 mutation in a patient with Apert syndrome associated with humeroradial synostosis

ABSTRACT  Most cases of Apert syndrome are due to S252W or P253R mutations in the fibroblast growth factor receptor 2 (FGFR2) gene. Differences in the effects of S252W and P253R mutations on the clinical features of Apert syndrome have been studied, but little is known about the type of FGFR2 mutation in Apert syndrome with humeroradial synostosis. To study a correlation between the FGFR2 mutations and the clinical complications, we examined the FGFR2 gene in a patient with Apert syndrome associated with humeroradial synostosis, and found that the mutation was S252W. This report suggested that S252W mutation in FGFR2 may cause humeroradial synostosis in Apert syndrome.     

Trigonocephaly in Muenke syndrome

Saethre-Chotzen syndrome is caused by mutations in the TWIST gene on chromosome 7p21.2. However, Muenke et al. [(1997); Am J Hum Genet 91: 555-564] described a new subgroup carrying the Pro250Arg mutation in the fibroblast growth factor receptor (FGFR) 3 gene on chromosome 4p16. Uni or bicoronal synostosis appears to be the main clinical finding in both syndromes. We observed trigonocephaly as a new manifestation in Muenke syndrome. As a consequence we advise to routinely perform mutation analysis of the FGFR1, 2, and 3 genes in children with non-syndromic trigonocephaly.     

Saethre-Chotzen syndrome caused by TWIST 1 gene mutations: functional differentiation from Muenke coronal synostosis syndrome

The Saethre-Chotzen syndrome (SCS) is an autosomal dominant craniosynostosis syndrome with uni- or bilateral coronal synostosis and mild limb deformities. It is caused by loss-of-function mutations of the TWIST 1 gene. In an attempt to delineate functional features separating SCS from Muenke's syndrome, we screened patients presenting with coronal suture synostosis for mutations in the TWIST 1 gene, and for the Pro250Arg mutation in FGFR3. Within a total of 124 independent pedigrees, 39 (71 patients) were identified to carry 25 different mutations of TWIST 1 including 14 novel mutations, to which six whole gene deletions were added. The 71 patients were compared with 42 subjects from 24 pedigrees carrying the Pro250Arg mutation in FGFR3 and 65 subjects from 61 pedigrees without a detectable mutation. Classical SCS associated with a TWIST 1 mutation could be separated phenotypically from the Muenke phenotype on the basis of the following features: low-set frontal hairline, gross ptosis of eyelids, subnormal ear length, dilated parietal foramina, interdigital webbing, and hallux valgus or broad great toe with bifid distal phalanx. Functional differences were even more important: intracranial hypertension as a consequence of early progressive multisutural fusion was a significant problem in SCS only, while mental delay and sensorineural hearing loss were associated with the Muenke's syndrome. Contrary to previous reports, SCS patients with complete loss of one TWIST allele showed normal mental development.     

Soluble form of FGFR2 with S252W partially prevents craniosynostosis of the apert mouse model

Background: Apert syndrome (AS) is characterized by craniosynostosis, midfacial hypoplasia, and bony syndactyly. It is an autosomal dominantly inherited disease caused by point mutations (S252W or P253R) in fibroblast growth factor receptor (FGFR) 2. These mutations cause activation of FGFR2 depending on ligand binding. Recently, an AS mouse model, Fgfr2^+/S252W, showed phenotypes similar to those of AS patients. We previously reported that the soluble form of FGFR2^S252W (sFGFR2IIIc^S252W) efficiently inhibits enhanced osteoblastic differentiation caused by FGFR2 activation in AS in vitro, presumably because FGFs binding to FGFRs is interrupted. In this study, we developed Fgfr2^+/S252W (Ap) mice expressing the sFGFR2IIIc^S252W protein, and we investigated the effects of sFGFR2IIIc^S252W on AS‐like phenotypes. Results: In Ap mice, the coronal suture (CS) was fused prematurely at P1. In addition, the mice exhibited a widened interfrontal suture (IFS) with ectopic bone and thickened cartilage formation. In Fgfr2^+/S252W sFGFR2IIIc^S252W (Ap/Sol) mice, the CS was similar to that of wild‐type mice. Ap/Sol mice did not show any ectopic bone or cartilage formation in the IFS, but showed a wider IFS than that of the wild‐type mice. Conclusions: sFGFR2IIIc^S252W may partially prevent craniosynostosis in the Apert mouse model by affecting the CS and IFS in vivo. Developmental Dynamics 243:560–567, 2014. © 2013 The Authors Developmental Dynamics published by Wiley Periodicals, Inc. on behalf of American Association of Anatomists.     

Dynamic morphological changes in the skulls of mice mimicking human Apert syndrome resulting from gain-of-function mutation of FGFR2 (P253R)

Apert syndrome is caused mainly by gain-of-function mutations of fibroblast growth factor receptor 2. We have generated a mouse model (Fgfr2^+/P253R) mimicking human Apert syndrome resulting from fibroblast growth factor receptor 2 Pro253Arg mutation using the knock-in approach. This mouse model in general has the characteristic skull morphology similar to that in humans with Apert syndrome. To characterize the detailed changes of form in the overall skull and its major anatomic structures, euclidean distance matrix analysis was used to quantitatively compare the form and growth difference between the skulls of mutants and their wild-type controls. There were substantial morphological differences between the skulls of mutants and their controls at 4 and 8 weeks of age (P<0.01). The mutants showed shortened skull dimensions along the rostrocaudal axis, especially in their face. The width of the frontal bone and the distance between the two orbits were broadened mediolaterally. The neurocrania were significantly increased along the dorsoventral axis and slightly increased along the mediolateral axis, and also had anteriorly displayed opisthion along the rostrocaudal axis. Compared with wild-type, the mutant mandible had an anteriorly displaced coronoid process and mandibular condyle along the rostrocaudal axis. We further found that there was catch-up growth in the nasal bone, maxilla, zygomatic bone and some regions of the mandible of the mutant skulls during the 4–8-week interval. The above-mentioned findings further validate the Fgfr2^+/P253R mouse strain as a good model for human Apert syndrome. The changes in form characterized in this study will help to elucidate the mechanisms through which the Pro253Arg mutation in fibroblast growth factor receptor 2 affects craniofacial development and causes Apert syndrome.