Structural basis for reduced FGFR2 activity in LADD syndrome: Implications for FGFR autoinhibition and activation

Elucidating the Role of Wildtype and Variant FGFR2 Structural Dynamics in (Dys)Function and Disorder

The fibroblast growth factor receptor 2 (FGFR2) gene is one of the most extensively studied genes with many known mutations implicated in several human disorders, including oncogenic ones. Most FGFR2 disease-associated gene mutations are missense mutations that result in constitutive activation of the FGFR2 protein and downstream molecular pathways. Many tertiary structures of the FGFR2 kinase domain are publicly available in the wildtype and mutated forms and in the inactive and activated state of the receptor. The current literature suggests a molecular brake inhibiting the ATP-binding A loop from adopting the activated state. Mutations relieve this brake, triggering allosteric changes between active and inactive states. However, the existing analysis relies on static structures and fails to account for the intrinsic structural dynamics. In this study, we utilize experimentally resolved structures of the FGFR2 tyrosine kinase domain and machine learning to capture the intrinsic structural dynamics, correlate it with functional regions and disease types, and enrich it with predicted structures of variants with currently no experimentally resolved structures. Our findings demonstrate the value of machine learning-enabled characterizations of structure dynamics in revealing the impact of mutations on (dys)function and disorder in FGFR2.

An Assessment of Dispersion-Corrected DFT Methods for Modeling Nonbonded Interactions in Protein Kinase Inhibitor Complexes

Accurate modeling of nonbonded interactions between protein kinases and their small molecule inhibitors is essential for structure-based drug design. Quantum chemical methods such as density functional theory (DFT) hold significant promise for quantifying the strengths of these key protein-ligand interactions. However, the accuracy of DFT methods can vary substantially depending on the choice of exchange-correlation functionals and associated basis sets. In this study, a comprehensive benchmarking of nine widely used DFT methods was carried out to identify an optimal approach for quantitative modeling of nonbonded interactions, balancing both accuracy and computational efficiency. From a database of 2139 kinase-inhibitor crystal structures, a diverse library of 49 nonbonded interaction motifs was extracted, encompassing CH-π, π-π stacking, cation-π, hydrogen bonding, and salt bridge interactions. The strengths of nonbonded interaction energies for all 49 motifs were calculated at the advanced CCSD(T)/CBS level of theory, which serve as references for a systematic benchmarking of BLYP, TPSS, B97, ωB97X, B3LYP, M062X, PW6B95, B2PLYP, and PWPB95 functionals with D3BJ dispersion correction alongside def2-SVP, def2-TZVP, and def2-QZVP basis sets. The RI, RIJK, and RIJCOSX approximations were used for selected functionals. It was found that the B3LYP/def2-TZVP and RIJK RI-B2PLYP/def2-QZVP methods delivered the best combination of accuracy and computational efficiency, making them well-suited for efficient modeling of nonbonded interactions responsible for molecular recognition of protein kinase inhibitors in their targets.

Comprehensive functional network analysis and screening of deleterious pathogenic variants in non-syndromic hearing loss causative genes

Hearing loss (HL) is a significant public health problem and causes the most frequent congenital disability in developed societies. The genetic analysis of non-syndromic hearing loss (NSHL) may be considered as a complement to the existent plethora of diagnostic modalities available. The present study focuses on exploring more target genes with respective non-synonymous single nucleotide polymorphisms (nsSNPs) involved in the development of NSHL. The functional network analysis and variant study have successfully been carried out from the gene pool retrieved from reported research articles of the last decade. The analyses have been done through STRING. According to predicted biological processes, various variant analysis tools have successfully classified the NSHL causative genes and identified the deleterious nsSNPs, respectively. Among the predicted pathogenic nsSNPs with rsIDs rs80356586 (I515T), rs80356596 (L1011P), rs80356606 (P1987R) in OTOF have been reported in NSHL earlier. The rs121909642 (P722S), rs267606805 (P722H) in FGFR1, rs121918506 (E565A) and rs121918509 (A628T, A629T) in FGFR2 have not been reported in NSHL yet, which should be clinically experimented in NSHL. This also indicates this variant's novelty as its association in NSHL. The findings and the analyzed data have delivered some vibrant genetic pathogenesis of NSHL. These data might be used in the diagnostic and prognostic purposes in non-syndromic congenitally deaf children.

Cell signaling regulation in salivary gland development

The mammalian salivary gland develops as a highly branched structure designed to produce and secrete saliva. This review focuses on research conducted on mammalian salivary gland development, particularly on the differentiation of acinar, ductal, and myoepithelial cells. We discuss recent studies that provide conceptual advances in the understanding of the molecular mechanisms of salivary gland development. In addition, we describe the organogenesis of submandibular glands (SMGs), model systems used for the study of SMG development, and the key signaling pathways as well as cellular processes involved in salivary gland development. The findings from the recent studies elucidating the identity of stem/progenitor cells in the SMGs, and the process by which they are directed along a series of cell fate decisions to form functional glands, are also discussed. Advances in genetic tools and tissue engineering strategies will significantly increase our knowledge about the mechanisms by which signaling pathways and cells establish tissue architecture and function during salivary gland development, which may also be conserved in the growth and development of other organ systems. An increased knowledge of organ development mechanisms will have profound implications in the design of therapies for the regrowth or repair of injured tissues. In addition, understanding how the processes of cell survival, expansion, specification, movement, and communication with neighboring cells are regulated under physiological and pathological conditions is critical to the development of future treatments.

Comparison of tyrosine kinase domain properties for the neurotrophin receptors TrkA and TrkB

The tropomyosin-related kinase (Trk) family consists of three receptor tyrosine kinases (RTKs) called TrkA, TrkB, and TrkC. These RTKs are regulated by the neurotrophins, a class of secreted growth factors responsible for the development and function of neurons. The Trks share a high degree of homology and utilize overlapping signaling pathways, yet their signaling is associated with starkly different outcomes in certain cancers. For example, in neuroblastoma, TrkA expression and signaling correlates with a favorable prognosis, whereas TrkB is associated with poor prognoses. To begin to understand how activation of the different Trks can lead to such distinct cellular outcomes, we investigated differences in kinase activity and duration of autophosphorylation for the TrkA and TrkB tyrosine kinase domains (TKDs). We find that the TrkA TKD has a catalytic efficiency that is ∼2-fold higher than that of TrkB, and becomes autophosphorylated in vitro more rapidly than the TrkB TKD. Studies with mutated TKD variants suggest that a crystallographic dimer seen in many TrkA (but not TrkB) TKD crystal structures, which involves the kinase-insert domain, may contribute to this enhanced TrkA autophosphorylation. Consistent with previous studies showing that cellular context determines whether TrkB signaling is sustained (promoting differentiation) or transient (promoting proliferation), we also find that TrkB signaling can be made more transient in PC12 cells by suppressing levels of p75NTR. Our findings shed new light on potential differences between TrkA and TrkB signaling, and suggest that subtle differences in signaling dynamics can lead to substantial shifts in the cellular outcome.

Lacrimo-auriculo-dento-digital syndrome: A novel mutation in a Korean family and review of literature

Background

Lacrimo‐auriculo‐dento‐digital (LADD) syndrome is a rare autosomal dominant disorder caused by mutations in one of the three genes: fibroblast growth factor receptor 2 (FGFR2), FGFR3, or FGF10. Affected patients have hypoplasia/aplasia of lacrimal ducts/glands, hypoplasia/aplasia of salivary glands, dental anomalies, ear anomalies, hearing loss, and digital anomalies.

Case Presentation

Proband was an 11‐year‐old male with xerostomia, xerophthalmia, and a referring diagnosis of Sjogren syndrome. He presented with microdontia, hypodontia, low‐set/cupped ear auricles, and hearing loss in the left ear.

Methods

Whole exome sequencing (WES) was performed on proband. Variations and segregation within the family were verified using Sanger sequencing.

Results

Molecular studies revealed a novel heterozygous missense mutation in exon 11 of FGFR2: c.1547C>T (p.Ala516Val), compatible with LADD syndrome.

Conclusion

To the best of our knowledge, this is the first report of a family with LADD syndrome in Korea. The combination of xerostomia and xerophthalmia, seen in patients with LADD syndrome, may be misdiagnosed as Sjogren syndrome. WES may be a useful clinical tool in ascertaining the affected gene in patients with suspected genetic disorders. Here, a literature review and summary of 23 case reports/series of LADD syndrome are presented, which may help to identify patients with this condition.

Structure-based Identification of Endocrine Disrupting Pesticides Targeting Breast Cancer Proteins

Endocrine disrupting pesticides (EDPs) are exogenous compounds that disrupt endocrine activity. Human exposure to EDPs can occur through occupational contact, and through the consumption of food, milk and water with trace amounts of these pollutants. Several EDPs are epidemiologically linked to breast cancer or are considered as possible carcinogens. However, current evidence is not fully conclusive and their mechanisms of action remain unknown. Thus, the potential interactions between 262 EDPs and 189 proteins associated with breast cancer were evaluated by using a virtual high-throughput screening approach, with AutoDock Vina 1.1.1. The molecular coordinates were previously downloaded from Protein Data Bank and EDCs DataBank, and used for preparation and optimization in Sybyl X-2.0. The best affinity score (-11.0 kcal/mol) was obtained for flucythrinate with the nuclear receptor for vitamin D (VDR). This synthetic pyrethroid, along with other EDPs, such as fluvalinate, bifenthrin, cyhalothrin and cypermethrin, are proposed as multi-target ligands of several proteins related to breast cancer. In addition, the validation of our protocol showed a good accuracy in terms of binding pose prediction and affinity estimation. This study provides a guide to prioritize EDPs for which further in vitro and in vivo analysis could be done to evaluate the risk and possible mechanisms of action of these contaminants and their potential association with breast cancer.

Dynamic, structural and thermodynamic basis of insulin-like growth factor 1 kinase allostery mediated by activation loop phosphorylation

Despite the importance of kinases' catalytic activity regulation in cell signaling, detailed mechanisms underlying their activity regulation are poorly understood. Herein, using insulin-like growth factor 1 receptor kinase (IGF-1RK) as a model, the mechanisms of kinase regulation by its activation loop (A-loop) phosphorylation were investigated through molecular dynamics (MD) and alchemical free energy simulations. Analyses of the simulation results and free energy landscapes determined for the entire catalytic cycle of the kinase revealed that A-loop phosphorylation affects each step in the IGF-1RK catalytic cycle, including conformational change, substrate binding/product release and catalytic phosphoryl transfer. Specifically, the conformational equilibrium of the kinase is shifted by 13.2 kcal mol-1 to favor the active conformation after A-loop phosphorylation, which increases substrate binding affinity of the activated kinase. This free energy shift is achieved primarily via destabilization of the inactive conformation. The free energy of the catalytic reaction is also changed by 3.3 kcal mol-1 after the phosphorylation and in the end, facilitates product release. Analyses of MD simulations showed that A-loop phosphorylation produces these energetic effects by perturbing the side chain interactions around each A-loop tyrosine. These interaction changes are propagated to the remainder of the kinase to modify the orientations and dynamics of the αC-helix and A-loop, and together yield the observed free energy changes. Since many protein kinases share similar interactions identified in this work, the mechanisms of kinase allostery and catalysis unraveled here can be applicable to them.

Effects of FGFR2 kinase activation loop dynamics on catalytic activity

The structural mechanisms by which receptor tyrosine kinases (RTKs) regulate catalytic activity are diverse and often based on subtle changes in conformational dynamics. The regulatory mechanism of one such RTK, fibroblast growth factor receptor 2 (FGFR2) kinase, is still unknown, as the numerous crystal structures of the unphosphorylated and phosphorylated forms of the kinase domains show no apparent structural change that could explain how phosphorylation could enable catalytic activity. In this study, we use several enhanced sampling molecular dynamics (MD) methods to elucidate the structural changes to the kinase's activation loop that occur upon phosphorylation. We show that phosphorylation favors inward motion of Arg664, while simultaneously favoring outward motion of Leu665 and Pro666. The latter structural change enables the substrate to bind leading to its resultant phosphorylation. Inward motion of Arg664 allows it to interact with the γ-phosphate of ATP as well as the substrate tyrosine. We show that this stabilizes the tyrosine and primes it for the catalytic phosphotransfer, and it may lower the activation barrier of the phosphotransfer reaction. Our work demonstrates the value of including dynamic information gleaned from computer simulation in deciphering RTK regulatory function.

Interrogation of a lacrimo-auriculo-dento-digital syndrome protein reveals novel modes of fibroblast growth factor 10 (FGF10) function

Heterozygous mutations in the gene encoding fibroblast growth factor 10 (FGF10) or its cognate receptor, FGF-receptor 2 IIIb result in two human syndromes - LADD (lacrimo-auriculo-dento-digital) and ALSG (aplasia of lacrimal and salivary glands). To date, the partial loss-of-FGF10 function in these patients has been attributed solely to perturbed paracrine signalling functions between FGF10-producing mesenchymal cells and FGF10-responsive epithelial cells. However, the functioning of a LADD-causing G138E FGF10 mutation, which falls outside its receptor interaction interface, has remained enigmatic. In the present study, we interrogated this mutation in the context of FGF10's protein sequence and three-dimensional structure, and followed the subcellular fate of tagged proteins containing this or other combinatorial FGF10 mutations, in vitro We report that FGF10 harbours two putative nuclear localization sequences (NLSs), termed NLS1 and NLS2, which individually or co-operatively promote nuclear translocation of FGF10. Furthermore, FGF10 localizes to a subset of dense fibrillar components of the nucleolus. G138E falls within NLS1 and abrogates FGF10's nuclear translocation whilst attenuating its progression along the secretory pathway. Our findings suggest that in addition to its paracrine roles, FGF10 may normally play intracrine role/s within FGF10-producing cells. Thus, G138E may disrupt both paracrine and intracrine function/s of FGF10 through attenuated secretion and nuclear translocation, respectively.

Novel FGF10 mutation in autosomal dominant aplasia of lacrimal and salivary glands

OBJECTIVE

Aplasia of lacrimal and salivary glands (ALSG) is a rare autosomal dominant inherited disease, characterized by aplasia, atresia, or hypoplasia of the lacrimal and salivary systems with variable expressivity. The purpose of this study was to identify genetic etiology of an ALSG family.

MATERIALS AND METHODS

We recruited a Turkish family with ALSG and performed a mutational analysis, based on the candidate gene approach, to clarify the molecular genetic etiology.

RESULTS

The candidate gene sequencing of the FGF10 gene identified a novel heterozygous nonsense mutation (c.237G > A, p.Trp79*) in the exon 1.

CONCLUSION

The identified novel mutation would result in a haploinsufficiency of the FGF10, because of nonsense-mediated mRNA decay caused by a premature stop codon. This report further confirms that ALSG is caused by the haploinsufficiency of functional FGF10.

CLINICAL RELEVANCE

Identification of the genetic etiology of the ALSG will help both the family members and dentist understand the nature of the disorder. Therefore, it will positively motivate oral health care to avoid further destruction of the tooth due to the lack of salivary production.

Altered FGF signalling in congenital craniofacial and skeletal disorders

The fibroblast growth factor (FGF) signalling pathway has been the focus of intense genetic and functional research for several decades. The emerging data implicate FGF signalling in diverse regulatory processes, both in the developing embryo as well as in the adult organism. Alterations in this tightly regulated pathway can lead to a number of pathological conditions, ranging from well-recognized congenital disorders to cancer. In order to mediate their cellular processes, FGFs signal through a subfamily of tyrosine kinase receptors, called FGF receptors (FGFRs). In humans, four FGFRs are described, and, to date, mutations in FGFR1, FGFR2, and FGFR3 have been shown to underlie human developmental disorders. FGFs/FGFRs are known to be key players in both endochondral and intramembranous bone development. In this review, we focus on the major developmental craniofacial and skeletal disorders which result from altered FGF signalling.

Receptor tyrosine kinase mutations in developmental syndromes and cancer: two sides of the same coin

Receptor tyrosine kinases (RTKs) are a family of ligand-binding cell surface receptors that regulate a wide range of essential cellular activities, including proliferation, differentiation, cell-cycle progression, survival and apoptosis. As such, these proteins play an important role during development and throughout life; germline mutations in genes encoding RTKs cause several developmental syndromes, while somatic alterations contribute to the pathogenesis of many aggressive cancers. This creates an interesting paradigm in which mutation timing, type and location in a gene leads to different cell signaling and biological responses, and ultimately phenotypic outcomes. In this review, we highlight the roles of RTKs in developmental disorders and cancer. The multifaceted roles of these receptors, their genetic signatures and their signaling during developmental morphogenesis and oncogenesis are discussed. Additionally, we propose that comparative analysis of RTK mutations responsible for developmental syndromes may shed light on those driving tumorigenesis.

The Fibroblast Growth Factor signaling pathway

The signaling component of the mammalian Fibroblast Growth Factor (FGF) family is comprised of eighteen secreted proteins that interact with four signaling tyrosine kinase FGF receptors (FGFRs). Interaction of FGF ligands with their signaling receptors is regulated by protein or proteoglycan cofactors and by extracellular binding proteins. Activated FGFRs phosphorylate specific tyrosine residues that mediate interaction with cytosolic adaptor proteins and the RAS-MAPK, PI3K-AKT, PLCγ, and STAT intracellular signaling pathways. Four structurally related intracellular non-signaling FGFs interact with and regulate the family of voltage gated sodium channels. Members of the FGF family function in the earliest stages of embryonic development and during organogenesis to maintain progenitor cells and mediate their growth, differentiation, survival, and patterning. FGFs also have roles in adult tissues where they mediate metabolic functions, tissue repair, and regeneration, often by reactivating developmental signaling pathways. Consistent with the presence of FGFs in almost all tissues and organs, aberrant activity of the pathway is associated with developmental defects that disrupt organogenesis, impair the response to injury, and result in metabolic disorders, and cancer. For further resources related to this article, please visit the WIREs website.

Functions of Fibroblast Growth Factor Receptors in cancer defined by novel translocations and mutations

The four receptor tyrosine kinases (RTKs) within the family of Fibroblast Growth Factor Receptors (FGFRs) are critical for normal development but also play an enormous role in oncogenesis. Mutations and/or abnormal expression often lead to constitutive dimerization and kinase activation of FGFRs, and represent the primary mechanism for aberrant signaling. Sequencing of human tumors has revealed a plethora of somatic mutations in FGFRs that are frequently identical to germline mutations in developmental syndromes, and has also identified novel FGFR fusion proteins arising from chromosomal rearrangements that contribute to malignancy. This review details approximately 200 specific point mutations in FGFRs and 40 different fusion proteins created by translocations involving FGFRs that have been identified in human cancer. This review discusses the effects of these genetic alterations on downstream signaling cascades, and the challenge of drug resistance in cancer treatment with antagonists of FGFRs.

Expression and clinical significance of fibroblast growth factor 1 in gastric adenocarcinoma

Background

The clinical significance of fibroblast growth factor 1 (FGF1) has been revealed in several cancers, including ovarian cancer, breast cancer, and bladder cancer. However, the clinical significance of FGF1 in gastric adenocarcinoma has not been explored.

Patients and methods

In our experiments, we systematically evaluated FGF1 expression in 178 cases of gastric adenocarcinoma with immunohistochemistry, and subsequently analyzed the correlation between FGF1 expression and clinicopathologic features. Moreover, FGF1 expression in tumor tissue and corresponding adjacent tissue was detected and compared by real-time polymerase chain reaction. The Kaplan–Meier method and the Cox-regression model were used with univariate and multivariate analysis, respectively, to evaluate the prognostic value of FGF1 in gastric adenocarcinoma.

Results

Higher FGF1 expression rate is 56.7% (101/178) in gastric adenocarcinoma. FGF1 expression in gastric adenocarcinoma was significantly higher than adjacent tissue (P<0.0001). Expression of FGF1 is significantly associated with lymph node invasion (P<0.001), distant metastasis (P=0.013), and differentiation (P=0.015). Moreover, FGF1 overexpression was closely related to unfavorable overall survival rate (P=0.021), and can be identified to be an independent unfavorable prognostic factor (P=0.004).

Conclusion

FGF1 is an independent prognostic factor, indicating that FGF1 could be a potential molecular drug target in gastric adenocarcinoma.

Bent bone dysplasia syndrome reveals nucleolar activity for FGFR2 in ribosomal DNA transcription

Fibroblast growth factor receptor 2 (FGFR2) promotes osteoprogenitor proliferation and differentiation during bone development, yet how the receptor elicits these distinct cellular responses remains unclear. Analysis of the FGFR2-skeletal disorder bent bone dysplasia syndrome (BBDS) demonstrates that FGFR2, in addition to its canonical signaling activities at the plasma membrane, regulates bone formation from within the nucleolus. Previously, we showed that the unique FGFR2 mutations that cause BBDS reduce receptor levels at the plasma membrane and diminish responsiveness to extracellular FGF2. In this study, we find that these mutations, despite reducing canonical signaling, enhance nucleolar occupancy of FGFR2 at the ribosomal DNA (rDNA) promoter. Nucleolar FGFR2 activates rDNA transcription via interactions with FGF2 and UBF1 by de-repressing RUNX2. An increase in the nucleolar activity of FGFR2 in BBDS elevates levels of ribosomal RNA in the developing bone, consequently promoting osteoprogenitor cell proliferation and decreasing differentiation. Identifying FGFR2 as a transcriptional regulator of rDNA in bone unexpectedly reveals a nucleolar route for FGF signaling that allows for independent regulation of osteoprogenitor cell proliferation and differentiation.

The role of protein structural analysis in the next generation sequencing era

Proteins are macromolecules that serve a cell’s myriad processes and functions in all living organisms via dynamic interactions with other proteins, small molecules and cellular components. Genetic variations in the protein-encoding regions of the human genome account for >85% of all known Mendelian diseases, and play an influential role in shaping complex polygenic diseases. Proteins also serve as the predominant target class for the design of small molecule drugs to modulate their activity. Knowledge of the shape and form of proteins, by means of their three-dimensional structures, is therefore instrumental to understanding their roles in disease and their potentials for drug development. In this chapter we outline, with the wide readership of non-structural biologists in mind, the various experimental and computational methods available for protein structure determination. We summarize how the wealth of structure information, contributed to a large extent by the technological advances in structure determination to date, serves as a useful tool to decipher the molecular basis of genetic variations for disease characterization and diagnosis, particularly in the emerging era of genomic medicine, and becomes an integral component in the modern day approach towards rational drug development.

Quantification of shape and cell polarity reveals a novel mechanism underlying malformations resulting from related FGF mutations during facial morphogenesis

Fibroblast growth factor (FGF) signaling mutations are a frequent contributor to craniofacial malformations including midfacial anomalies and craniosynostosis. FGF signaling has been shown to control cellular mechanisms that contribute to facial morphogenesis and growth such as proliferation, survival, migration and differentiation. We hypothesized that FGF signaling not only controls the magnitude of growth during facial morphogenesis but also regulates the direction of growth via cell polarity. To test this idea, we infected migrating neural crest cells of chicken embryos with  replication-competent avian sarcoma virus expressing either FgfR2(C278F), a receptor mutation found in Crouzon syndrome or the ligand Fgf8. Treated embryos exhibited craniofacial malformations resembling facial dysmorphologies in craniosynostosis syndrome. Consistent with our hypothesis, ectopic activation of FGF signaling resulted in decreased cell proliferation, increased expression of the Sprouty class of FGF signaling inhibitors, and repressed phosphorylation of ERK/MAPK. Furthermore, quantification of cell polarity in facial mesenchymal cells showed that while orientation of the Golgi body matches the direction of facial prominence outgrowth in normal cells, in FGF-treated embryos this direction is randomized, consistent with aberrant growth that we observed. Together, these data demonstrate that FGF signaling regulates cell proliferation and cell polarity and that these cell processes contribute to facial morphogenesis.

Molecular mechanisms of fibroblast growth factor signaling in physiology and pathology

Fibroblast growth factors (FGFs) signal in a paracrine or endocrine fashion to mediate a myriad of biological activities, ranging from issuing developmental cues, maintaining tissue homeostasis, and regulating metabolic processes. FGFs carry out their diverse functions by binding and dimerizing FGF receptors (FGFRs) in a heparan sulfate (HS) cofactor- or Klotho coreceptor-assisted manner. The accumulated wealth of structural and biophysical data in the past decade has transformed our understanding of the mechanism of FGF signaling in human health and development, and has provided novel concepts in receptor tyrosine kinase (RTK) signaling. Among these contributions are the elucidation of HS-assisted receptor dimerization, delineation of the molecular determinants of ligand-receptor specificity, tyrosine kinase regulation, receptor cis-autoinhibition, and tyrosine trans-autophosphorylation. These structural studies have also revealed how disease-associated mutations highjack the physiological mechanisms of FGFR regulation to contribute to human diseases. In this paper, we will discuss the structurally and biophysically derived mechanisms of FGF signaling, and how the insights gained may guide the development of therapies for treatment of a diverse array of human diseases.

Bent bone dysplasia-FGFR2 type, a distinct skeletal disorder, has deficient canonical FGF signaling

Fibroblast growth factor receptor 2 (FGFR2) is a crucial regulator of bone formation during embryonic development. Both gain and loss-of-function studies in mice have shown that FGFR2 maintains a critical balance between the proliferation and differentiation of osteoprogenitor cells. We have identified de novo FGFR2 mutations in a sporadically occurring perinatal lethal skeletal dysplasia characterized by poor mineralization of the calvarium, craniosynostosis, dysmorphic facial features, prenatal teeth, hypoplastic pubis and clavicles, osteopenia, and bent long bones. Histological analysis of the long bones revealed that the growth plate contained smaller hypertrophic chondrocytes and a thickened hypercellular periosteum. Four unrelated affected individuals were found to be heterozygous for missense mutations that introduce a polar amino acid into the hydrophobic transmembrane domain of FGFR2. Using diseased chondrocytes and a cell-based assay, we determined that these mutations selectively reduced plasma-membrane levels of FGFR2 and markedly diminished the receptor's responsiveness to extracellular FGF. All together, these clinical and molecular findings are separate from previously characterized FGFR2 disorders and represent a distinct skeletal dysplasia.

A novel mode of protein kinase inhibition exploiting hydrophobic motifs of autoinhibited kinases: discovery of ATP-independent inhibitors of fibroblast growth factor receptor

Protein kinase inhibitors with enhanced selectivity can be designed by optimizing binding interactions with less conserved inactive conformations because such inhibitors will be less likely to compete with ATP for binding and therefore may be less impacted by high intracellular concentrations of ATP. Analysis of the ATP-binding cleft in a number of inactive protein kinases, particularly in the autoinhibited conformation, led to the identification of a previously undisclosed non-polar region in this cleft. This ATP-incompatible hydrophobic region is distinct from the previously characterized hydrophobic allosteric back pocket, as well as the main pocket. Generalized hypothetical models of inactive kinases were constructed and, for the work described here, we selected the fibroblast growth factor receptor (FGFR) tyrosine kinase family as a case study. Initial optimization of a FGFR2 inhibitor identified from a library of commercial compounds was guided using structural information from the model. We describe the inhibitory characteristics of this compound in biophysical, biochemical, and cell-based assays, and have characterized the binding mode using x-ray crystallographic studies. The results demonstrate, as expected, that these inhibitors prevent activation of the autoinhibited conformation, retain full inhibitory potency in the presence of physiological concentrations of ATP, and have favorable inhibitory activity in cancer cells. Given the widespread regulation of kinases by autoinhibitory mechanisms, the approach described herein provides a new paradigm for the discovery of inhibitors by targeting inactive conformations of protein kinases.

A FGF3 mutation associated with differential inner ear malformation, microtia, and microdontia

OBJECTIVES/HYPOTHESIS

Analysis of association between genotype and phenotype.

STUDY DESIGN

Prospective genetic study in a family.

METHODS

Auditory investigations, computer tomography, and genetic sequencing of the fibroblast growth factor 3 (FGF3) gene were performed on a Somali family presenting with autosomal recessive, hearing impairment, microdontia, and outer ear morphologies ranging from normal auricle development to microtia assessed as type 1 Weerda dysplasia in affected individuals.

RESULTS

Computed tomography imaging identified differential inter- and intraindividual malformations of the inner ear including labyrinth aplasia, development of a common cavity to the presence of a cochlear with 1.5 windings (Mondini malformation) in affected individuals, symptoms similar to those described as labyrinth aplasia, microtia, and microdontia (LAMM) syndrome, caused by mutations in FGF3. Genetic sequencing revealed the presence of a novel p.R95W missense mutation in FGF3 segregating with pathology. The p.R95W mutation substitutes a positively charged arginine for a polar tryptophan in the highly conserved RYLAM consensus of the beta 6 sheet of FGF3 that interacts with FGFR2.

CONCLUSIONS

These findings describe, for the first time, variable inner ear malformations and outer ear dysplasia in the presence of constant microdontia, associated with homozygous inheritance of the p.R95W mutation in FGF3, mirroring phenotypes observed in mouse models ablating FGF3/FGFR2 signaling.

Molecular pathology of the fibroblast growth factor family

The human fibroblast growth factor (FGF) family contains 22 proteins that regulate a plethora of physiological processes in both developing and adult organism. The mutations in the FGF genes were not known to play role in human disease until the year 2000, when mutations in FGF23 were found to cause hypophosphatemic rickets. Nine years later, seven FGFs have been associated with human disorders. These include FGF3 in Michel aplasia; FGF8 in cleft lip/palate and in hypogonadotropic hypogonadism; FGF9 in carcinoma; FGF10 in the lacrimal/salivary glands aplasia, and lacrimo-auriculo-dento-digital syndrome; FGF14 in spinocerebellar ataxia; FGF20 in Parkinson disease; and FGF23 in tumoral calcinosis and hypophosphatemic rickets. The heterogeneity in the functional consequences of FGF mutations, the modes of inheritance, pattern of involved tissues/organs, and effects in different developmental stages provide fascinating insights into the physiology of the FGF signaling system. We review the current knowledge about the molecular pathology of the FGF family.

Identification of FGFR4-activating mutations in human rhabdomyosarcomas that promote metastasis in xenotransplanted models

Rhabdomyosarcoma (RMS) is a childhood cancer originating from skeletal muscle, and patient survival is poor in the presence of metastatic disease. Few determinants that regulate metastasis development have been identified. The receptor tyrosine kinase FGFR4 is highly expressed in RMS tissue, suggesting a role in tumorigenesis, although its functional importance has not been defined. Here, we report the identification of mutations in FGFR4 in human RMS tumors that lead to its activation and present evidence that it functions as an oncogene in RMS. Higher FGFR4 expression in RMS tumors was associated with advanced-stage cancer and poor survival, while FGFR4 knockdown in a human RMS cell line reduced tumor growth and experimental lung metastases when the cells were transplanted into mice. Moreover, 6 FGFR4 tyrosine kinase domain mutations were found among 7 of 94 (7.5%) primary human RMS tumors. The mutants K535 and E550 increased autophosphorylation, Stat3 signaling, tumor proliferation, and metastatic potential when expressed in a murine RMS cell line. These mutants also transformed NIH 3T3 cells and led to an enhanced metastatic phenotype. Finally, murine RMS cell lines expressing the K535 and E550 FGFR4 mutants were substantially more susceptible to apoptosis in the presence of a pharmacologic FGFR inhibitor than the control cell lines expressing the empty vector or wild-type FGFR4. Together, our results demonstrate that mutationally activated FGFR4 acts as an oncogene, and these are what we believe to be the first known mutations in a receptor tyrosine kinase in RMS. These findings support the potential therapeutic targeting of FGFR4 in RMS.

Convergent signalling through Fgfr2 regulates divergent craniofacial morphogenesis

Fibroblast growth factor receptor 2 (Fgfr2) has two splice variants IIIb and IIIc, which are unique in function and localization. Signalling through Fgfr2IIIb controls epithelial-mesenchymal interactions, which regulate morphogenesis during the development of several organs including the palate and tooth. In this study, we confirm that molar tooth development in Fgfr2IIIb(-/-) mice is arrested early in development and that the molar teeth of Fgf10(-/-) mice develop through all the normal stages of morphogenesis. We show that the molar phenotype of Fgfr2IIIb(-/-) mice is, in part, owing to reduced cell proliferation in both epithelial and mesenchymal compartments. We also show that the developing molar teeth of Fgf10(-/-) mice exhibit reduced cell proliferation. However, this reduction is not sufficient to arrest molar development. Recent evidence has indicated that Fgfr2IIIb/Fgf10 signalling is active in the calvaria in some pathological situations as heterozygous deletion of Fgfr2 exon IIIc in mice leads to ectopic expression of Fgfr2IIIb in the calvarial bones and causes craniosynostosis. Here, we investigate the mRNA expression of Fgfr2IIIb and Fgfr2IIIc as well as their ligands Fgf3, -7 and -10 in the developing murine tooth, palate and calvaria. We show that Fgf7 is expressed in the calvarial mesenchyme adjacent to the developing frontal bone and Fgf10 is expressed by osteoprogenitors in the developing frontal bone condensation. Taken together, we highlight the overlapping roles of Fgfr2IIIb/Fgf10 signalling in controlling epithelial-mesenchymal interactions during normal palate and tooth morphogenesis and how elevated signalling through Fgfr2IIIb/Fgf10 solely within the mesenchyme can result in abnormal calvarial morphogenesis.

Role of FGFR2-signaling in the pathogenesis of acne

It is the purpose of this review to extend our understanding of the fibroblast growth factor (FGF) receptor-2b-signaling network in the pathogenesis of acne. A new concept of the role of FGFR2b-signaling in dermal-epithelial interaction for skin appendage formation, pilosebaceous follicle homeostasis, comedogenesis, sebaceous gland proliferation and lipogenesis is presented. The FGFR2-gain-of-function mutations in Apert syndrome and unilateral acneiform nevus are most helpful model diseases pointing the way to androgen-dependent dermalepithelial FGFR2-signaling in acne. Androgen-mediated upregulation of FGFR2b-signaling in acne-prone skin appears to be involved in the pathogenesis of acne vulgaris. In organotypic skin cultures, keratinocyte-derived interleukin-1alpha stimulated fibroblasts to secrete FGF7 which stimulated FGFR2b-mediated keratinocyte proliferation. Postnatal deletion of FGFR2b in mice resulted in severe sebaceous gland atrophy. The importance of FGFR2b in sebaceous gland physiology is further supported by the mode of action of anti-acne agents which have been proposed to attenuate FGFR2b-signaling. Downregulation of FGFR2b-signaling by isotretinoin explains its therapeutic effect in acne. Downregulation of FGFR2b-signaling during the first trimester of pregnancy disturbs branched morphogenesis and explains retinoid embryotoxicity. Insulin-like growth factor-1 (IGF-1), the mediator of growth hormone during puberty, intracts with androgen-dependent FGFR2b-signaling and links androgen- and FGF-mediated signal transduction important in sebaceous gland homeostasis. The search for a follicular defect in the dermalepithelial regulation of growth factor-signaling in acne-prone skin appears to be a most promising approach to clarify the pathogenesis of acne.

Loss-of-function fibroblast growth factor receptor-2 mutations in melanoma

We report that 10% of melanoma tumors and cell lines harbor mutations in the fibroblast growth factor receptor 2 (FGFR2) gene. These novel mutations include three truncating mutations and 20 missense mutations occurring at evolutionary conserved residues in FGFR2 as well as among all four FGFRs. The mutation spectrum is characteristic of those induced by UV radiation. Mapping of these mutations onto the known crystal structures of FGFR2 followed by in vitro and in vivo studies show that these mutations result in receptor loss of function through several distinct mechanisms, including loss of ligand binding affinity, impaired receptor dimerization, destabilization of the extracellular domains, and reduced kinase activity. To our knowledge, this is the first demonstration of loss-of-function mutations in a class IV receptor tyrosine kinase in cancer. Taken into account with our recent discovery of activating FGFR2 mutations in endometrial cancer, we suggest that FGFR2 may join the list of genes that play context-dependent opposing roles in cancer.

The precise sequence of FGF receptor autophosphorylation is kinetically driven and is disrupted by oncogenic mutations

Autophosphorylation of the tyrosine kinase domain of fibroblast growth factor receptor 1 (FGFR1) is mediated by a sequential and precisely ordered three-stage autophosphorylation reaction. First-stage autophosphorylation of an activation loop tyrosine leads to 50- to 100-fold stimulation of kinase activity and is followed by second-stage phosphorylation of three additional tyrosine residues, which are binding sites for signaling molecules. Finally, third-stage phosphorylation of a second activation loop tyrosine leads to an additional 10-fold stimulation of FGFR1 catalytic activity. In this report, we show that sequential autophosphorylation of five tyrosines in the FGFR1 kinase domain is under kinetic control, mediated by both the amino acid sequence surrounding the tyrosines and their locations within the kinase structure, and, moreover, that phosphoryl transfer is the rate-limiting step. Furthermore, the strict order of autophosphorylation is disrupted by a glioblastoma-derived, oncogenic FGFR1 point mutation in the kinase domain. We propose that disrupted stepwise activation of tyrosine autophosphorylation caused by oncogenic and other activating FGFR mutations may lead to aberrant activation of and assembly of signaling molecules by the activated receptor.