Ligand-independent activation of fibroblast growth factor receptors by point mutations in the extracellular, transmembrane, and kinase domains

Genomic organization of the human fibroblast growth factor receptor 2 (FGFR2) gene and comparative analysis of the human FGFR gene family

The human fibroblast growth factor receptor (FGFR) genes play important roles in normal vertebrate development. Mutations in the human FGFR2 gene have been associated with many craniosynostotic syndromes and malformations, including Crouzon, Pfeiffer, Apert, Jackson-Weiss, Beare-Stevenson cutis gyrata, and Antley-Bixler syndromes, and Kleeblaatschadel (cloverleaf skull) deformity. The mutations identified to date are concentrated in the previously characterized region of FGFR2 that codes for the extracellular IgIII domain of the receptor protein. The search for mutations in other regions of the gene, however, has been hindered by lack of knowledge of the genomic structure. Using a combination of genomic library screening, long-range PCR, and genomic walking, we have characterized the genomic structure of nearly the entire human FGFR2 gene, including a delineation of the organization and size of all introns and exons and determination of the DNA sequences at the intron/exon boundaries. Comparative analysis of the human FGFR gene family reveals that the genomic organization of the FGFRs is relatively conserved. Moreover, alignment of the amino acid sequences shows that the four corresponding proteins share 46% identity overall, with up to 70% identity between individual pairs of FGFR proteins. However, the FGFR2 gene contains an additional exon not found in other members of the family, and it also has much larger intronic sequences throughout the gene. Remarkable similarities in genomic organization, intron/exon boundaries, and intron sizes are found between the human and mouse FGFR2 genes. Knowledge gained from this study of the human FGFR2 gene structure may prove useful in future screening studies designed to find additional mutations associated with craniosynostotic syndromes, and in understanding the molecular and cell biology of this receptor family.

Control of Bone Growth by Fibroblast Growth Factors

Fibroblast growth factors (FGFs) and their receptors (FGFRs) negatively regulate longitudinal bone growth. Activating FGFR3 mutations impair growth, causing human skeletal dysplasias, whereas inactivating mutations stimulate growth. Systemic administration of FGF-2 to mice stimulates bone growth at low doses but inhibits growth at high doses. In organ culture, FGF-2 inhibits growth by decreasing growth plate chondrocyte proliferation, hypertrophy and cartilage matrix synthesis. Local FGF-2 infusion accelerates ossification of growth plate cartilage. Thus, FGFs may regulate both growth plate chondrogenesis and ossification.

A novel skeletal dysplasia with developmental delay and acanthosis nigricans is caused by a Lys650Met mutation in the fibroblast growth factor receptor 3 gene

We have identified a novel fibroblast growth factor receptor 3 (FGFR3) missense mutation in four unrelated individuals with skeletal dysplasia that approaches the severity observed in thanatophoric dysplasia type I (TD1). However, three of the four individuals developed extensive areas of acanthosis nigricans beginning in early childhood, suffer from severe neurological impairments, and have survived past infancy without prolonged life-support measures. The FGFR3 mutation (A1949T: Lys650Met) occurs at the nucleotide adjacent to the TD type II (TD2) mutation (A1948G: Lys650Glu) and results in a different amino acid substitution at a highly conserved codon in the kinase domain activation loop. Transient transfection studies with FGFR3 mutant constructs show that the Lys650Met mutation causes a dramatic increase in constitutive receptor kinase activity, approximately three times greater than that observed with the Lys650Glu mutation. We refer to the phenotype caused by the Lys650Met mutation as "severe achondroplasia with developmental delay and acanthosis nigricans" (SADDAN) because it differs significantly from the phenotypes of other known FGFR3 mutations.

De novo alu-element insertions in FGFR2 identify a distinct pathological basis for Apert syndrome

Apert syndrome, one of five craniosynostosis syndromes caused by allelic mutations of fibroblast growth-factor receptor 2 (FGFR2), is characterized by symmetrical bony syndactyly of the hands and feet. We have analyzed 260 unrelated patients, all but 2 of whom have missense mutations in exon 7, which affect a dipeptide in the linker region between the second and third immunoglobulin-like domains. Hence, the molecular mechanism of Apert syndrome is exquisitely specific. FGFR2 mutations in the remaining two patients are distinct in position and nature. Surprisingly, each patient harbors an Alu-element insertion of approximately 360 bp, in one case just upstream of exon 9 and in the other case within exon 9 itself. The insertions are likely to be pathological, because they have arisen de novo; in both cases this occurred on the paternal chromosome. FGFR2 is present in alternatively spliced isoforms characterized by either the IIIb (exon 8) or IIIc (exon 9) domains (keratinocyte growth-factor receptor [KGFR] and bacterially expressed kinase, respectively), which are differentially expressed in mouse limbs on embryonic day 13. Splicing of exon 9 was examined in RNA extracted from fibroblasts and keratinocytes from one patient with an Alu insertion and two patients with Pfeiffer syndrome who had nucleotide substitutions of the exon 9 acceptor splice site. Ectopic expression of KGFR in the fibroblast lines correlated with the severity of limb abnormalities. This provides the first genetic evidence that signaling through KGFR causes syndactyly in Apert syndrome.

Pleiotropic features of syndromic craniosynostoses correlate with differential expression of fibroblast growth factor receptors 1 and 2 during human craniofacial development

Mutations in FGFR1, -2, and -3 are linked to five human craniosynostosis syndromes. In addition to premature fusion of cranial sutures, nonskeletal manifestations in skin, and teeth together with CNS abnormalities, reflect widespread effects of these mutations. To understand this pleiotropy, we have assessed craniofacial FGFR1 and -2 expression in the human embryo from 6 wk postfertilization. We found that both genes are expressed in sheets of condensed mesenchyme before overt chondrogenic differentiation and that distinct patterns of expression are established by 8 wk. Thus, FGFR2(BEK) is expressed evenly throughout developing cartilage and bone, whereas FGFR1 transcripts predominate in perichondria and periostea. Complementary patterns of FGFR1 and FGFR2(BEK and KGFR) expression are also observed in the enamel epithelium and papilla mesenchyme of the tooth germ, at a stage when morphogenetic tissue interactions ensue. Both genes are expressed in the cortical layer of the brain, but expression levels vary significantly within the choroid plexus and wall of the fourth ventricle. Similarly, tissue-specific differences in receptor expression are found in both the skin and salivary glands. These expression data are consistent with the pleiotropic manifestations of syndromic craniosynostoses and provide the basis for a new paradigm to explain the associated CNS problems.

Effect of transmembrane and kinase domain mutations on fibroblast growth factor receptor 3 chimera signaling in PC12 cells. A model for the control of receptor tyrosine kinase activation

The effect of six point mutations causing various human skeletal dysplasias, occurring in the transmembrane (TM) and kinase domains (KD) of fibroblast growth factor receptor 3, were introduced into a chimera composed of the extracellular domain of human platelet-derived growth factor beta and the TM and intracellular domains of hFGFR3. Stable transfectants in rat PC12 cells showed distinct differences in the two classes of mutations. The cells containing TM mutants displayed normal expression and activation but higher responsiveness to lower doses of ligand. The KD mutants showed significantly altered expression patterns. Normal amounts of a lower Mr receptor (p130) reflecting incomplete glycosylation, but only greatly decreased amounts of the mature (p170) form, were observed. However, the latter material showed normal ligand-dependent activation. In contrast, the p130 form, which is regularly observed in the expression of both native and chimeric receptors, exhibits strong ligand-independent tyrosine phosphorylation, particularly with the K650E mutation. Expression of two of the KD mutants (K650M and K650E), under control of an inducible metallothionein promoter, indicated that this receptor was sufficiently autoactivated to produce at least partial differentiation and, in the case of the K650E mutation, to induce ligand-independent neurite outgrowth. A model is presented that suggests that the low Mr (p130) KD mutants can, under the right conditions, signal intracellularly, but when they are fully glycosylated and move to the cell surface they adopt a normal, inhibited conformation, in the form of ligand-independent dimers, that neutralizes the effects of the mutations. When ligands bind, these dimeric receptors are activated in a normal manner. This model suggests that unliganded dimers may be a common intermediate in receptor tyrosine kinase signaling.

Achondroplasia, hypochondroplasia and thanatophoric dysplasia: clinically related skeletal dysplasias that are also related at the molecular level

This is the second of three articles on modern genetic concepts of a number of syndromes and disorders. Three short limb skeletal dysplasias with additional abnormalities of the skull are discussed. All are caused by mutations on fibroblast growth factor receptor 3 (FGFR3). A pathogenetic hypothesis is proposed to explain differences in the severity of short stature, midface deficiency, and craniosynostosis.

Fibroblast growth factors as multifunctional signaling factors

The fibroblast growth factor (FGF) family consists of at least 15 structurally related polypeptide growth factors. Their expression is controlled at the levels of transcription, mRNA stability, and translation. The bioavailability of FGFs is further modulated by posttranslational processing and regulated protein trafficking. FGFs bind to receptor tyrosine kinases (FGFRs), heparan sulfate proteoglycans (HSPG), and a cysteine-rich FGF receptor (CFR). FGFRs are required for most biological activities of FGFs. HSPGs alter FGF-FGFR interactions and CFR participates in FGF intracellular transport. FGF signaling pathways are intricate and are intertwined with insulin-like growth factor, transforming growth factor-beta, bone morphogenetic protein, and vertebrate homologs of Drosophila wingless activated pathways. FGFs are major regulators of embryonic development: They influence the formation of the primary body axis, neural axis, limbs, and other structures. The activities of FGFs depend on their coordination of fundamental cellular functions, such as survival, replication, differentiation, adhesion, and motility, through effects on gene expression and the cytoskeleton.

Activating mutations in the extracellular domain of the fibroblast growth factor receptor 2 function by disruption of the disulfide bond in the third immunoglobulin-like domain

Multiple human skeletal and craniosynostosis disorders, including Crouzon, Pfeiffer, Jackson-Weiss, and Apert syndromes, result from numerous point mutations in the extracellular region of fibroblast growth factor receptor 2 (FGFR2). Many of these mutations create a free cysteine residue that potentially leads to abnormal disulfide bond formation and receptor activation; however, for noncysteine mutations, the mechanism of receptor activation remains unclear. We examined the effect of two of these mutations, W290G and T341P, on receptor dimerization and activation. These mutations resulted in cellular transformation when expressed as FGFR2/Neu chimeric receptors. Additionally, in full-length FGFR2, the mutations induced receptor dimerization and elevated levels of tyrosine kinase activity. Interestingly, transformation by the chimeric receptors, dimerization, and enhanced kinase activity were all abolished if either the W290G or the T341P mutation was expressed in conjunction with mutations that eliminate the disulfide bond in the third immunoglobulin-like domain (Ig-3). These results demonstrate a requirement for the Ig-3 cysteine residues in the activation of FGFR2 by noncysteine mutations. Molecular modeling also reveals that noncysteine mutations may activate FGFR2 by altering the conformation of the Ig-3 domain near the disulfide bond, preventing the formation of an intramolecular bond. This allows the unbonded cysteine residues to participate in intermolecular disulfide bonding, resulting in constitutive activation of the receptor.

New overgrowth syndrome and FGFR3 dosage effect

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The heparan sulfate-fibroblast growth factor family: diversity of structure and function

The fibroblast growth factor (FGF) receptor complex is a ubiquitous regulator of development and adult tissue homeostasis that bridges the peri-cellular matrix and the intracellular environment. Diverse members of the FGF polypeptide family, the FGF receptor tyrosine kinase (FGFRTK) family and the FGF receptor heparan sulfate proteoglycan (FGFRHS) family combine to result in active and specific FGFR signal transduction complexes. Regulated alternate splicing and combination of variant subdomains give rise to diversity of FGFRTK monomers. Divalent cations cooperate with the FGFRHS to conformationally restrict FGFRTK trans-phosphorylation, which causes depression of kinase activity and facilitates appropriate activation of the FGFR complex by FGF. Diffusional and conformational molecular models of the oligomeric FGFR complex are presented to explain how different point mutations in the FGFRTK commonly cause craniofacial and skeletal abnormalities of graded severity by graded increases in FGF-independent activity of total FGFR complexes. The role of the FGF family in liver growth and function and in prostate tumor progression is discussed.

Expression of fibroblast growth factor receptors (FGFR1, FGFR2, FGFR3) in the developing head and face

Fibroblast growth factors may play an important role in the differential growth of the skull, brain, and facial prominences. In order to understand the role of FGFs in vivo, we have analyzed the competency of head mesenchyme to respond to FGFs via expression of the high affinity receptors FGFR1, 2, and 3. Receptor transcripts, especially those of FGFR2 and FGFR3, were localized to specific regions of the head. We raise the possibilities of particular receptor-ligand combinations and the possible functions of these interactions in the morphogenesis of the head, face, and brain. Finally, we discuss the relationship between FGF receptor expression in the chicken and the phenotypes of FGF receptor mutations in humans.

Fgfr2 and osteopontin domains in the developing skull vault are mutually exclusive and can be altered by locally applied FGF2

Mutations in the human fibroblast growth factor receptor type 2 (FGFR2) gene cause craniosynostosis, particularly affecting the coronal suture. We show here that, in the fetal mouse skull vault, Fgfr2 transcripts are most abundant at the periphery of the membrane bones; they are mutually exclusive with those of osteopontin (an early marker of osteogenic differentiation) but coincide with sites of rapid cell proliferation. Fibroblast growth factor type 2 (FGF2) protein, which has a high affinity for the FGFR2 splice variant associated with craniosynostosis, is locally abundant; immunohistochemical detection showed it to be present at low levels in Fgfr2 expression domains and at high levels in differentiated areas. Implantation of FGF2-soaked beads onto the fetal coronal suture by ex utero surgery resulted in ectopic osteopontin expression, encircled by Fgfr2 expression, after 48 hours. We suggest that increased FGF/FGFR signalling in the developing skull, whether due to FGFR2 mutation or to ectopic FGF2, shifts the cell proliferation/differentiation balance towards differentiation by enhancing the normal paracrine down-regulation of Fgfr2.

Skeletal disorders associated with fibroblast growth factor receptor mutations

Mutations in three fibroblast growth factor receptor loci underlie several autosomal dominant skeletal disorders; these include dwarfism and various craniosynostosis syndromes affecting limb and craniofacial bone patterning. A functional analysis of several of these mutations has demonstrated that a constitutive activation of the receptor kinase is a common theme.