Control of BEK and K-SAM splice sites in alternative splicing of the fibroblast growth factor receptor 2 pre-mRNA

Expansion of the complex genotypic and phenotypic spectrum of FGFR2-associated neurocutaneous syndromes

The fibroblast growth factor receptors comprise a family of related but individually distinct tyrosine kinase receptors. Within this family, FGFR2 is a key regulator in many biological processes, e.g., cell proliferation, tumorigenesis, metastasis, and angiogenesis. Heterozygous activating non-mosaic germline variants in FGFR2 have been linked to numerous autosomal dominantly inherited disorders including several craniosynostoses and skeletal dysplasia syndromes. We report on a girl with cutaneous nevi, ocular malformations, macrocephaly, mild developmental delay, and the initial clinical diagnosis of Schimmelpenning-Feuerstein-Mims syndrome, a very rare mosaic neurocutaneous disorder caused by postzygotic missense variants in HRAS, KRAS, and NRAS. Exome sequencing of blood and affected skin tissue identified the mosaic variant c.1647=/T > G p.(Asn549=/Lys) in FGFR2, upstream of the RAS signaling pathway. The variant is located in the tyrosine kinase domain of FGFR2 in a region that regulates the activity of the receptor and structural mapping and functional characterization revealed that it results in constitutive receptor activation. Overall, our findings indicate FGFR2-associated neurocutaneous syndrome as the accurate clinical-molecular diagnosis for the reported individual, and thereby expand the complex genotypic and phenotypic spectrum of FGFR-associated disorders. We conclude that molecular analysis of FGFR2 should be considered in the genetic workup of individuals with the clinical suspicion of a mosaic neurocutaneous condition, as the knowledge of the molecular cause might have relevant implications for genetic counseling, prognosis, tumor surveillance and potential treatment options.

The Role of Fibroblast Growth Factors in Tooth Development and Incisor Renewal

The mineralized tissue of the tooth is composed of enamel, dentin, cementum, and alveolar bone; enamel is a calcified tissue with no living cells that originates from oral ectoderm, while the three other tissues derive from the cranial neural crest. The fibroblast growth factors (FGFs) are critical during the tooth development. Accumulating evidence has shown that the formation of dental tissues, that is, enamel, dentin, and supporting alveolar bone, as well as the development and homeostasis of the stem cells in the continuously growing mouse incisor is mediated by multiple FGF family members. This review discusses the role of FGF signaling in these mineralized tissues, trying to separate its different functions and highlighting the crosstalk between FGFs and other signaling pathways.

FGF signaling: diverse roles during cochlear development

Mammalian inner ear comprises of six sensory organs; cochlea, utricle, saccule, and three semicircular canals. The cochlea contains sensory epithelium known as the organ of Corti which senses sound through mechanosensory hair cells. Mammalian inner ear undergoes series of morphogenesis during development beginning thickening of ectoderm nearby hindbrain. These events require tight regulation of multiple signaling cascades including FGF, Wnt, Notch and Bmp signaling. In this review, we will discuss the role of newly emerging signaling, FGF signaling, for its roles required for cochlear development. [BMB Reports 2017; 50(10): 487-495].

Revisiting Crouzon syndrome: reviewing the background and management of a multifaceted disease

PURPOSE

Crouzon syndrome is a complex craniosynostosis of autosomal dominant transfer, with a highly variable phenotypic appearance. Some patients are afflicted with a mild form of the disease and able to live a fully functional lifestyle, whereas many patients suffer from a severe form of the disease causing a significant impact on their quality of life. Although several case reports and genetic studies have been performed, there has been no recent review of the literature to collate the information on this disorder. In this paper, we seek to unify the findings of this disorder to provide the pediatric provider with a succinct, but complete discussion of this disease.

METHODS

Articles from 1994 to 2014 were queried on PubMed and reviewed for utility by all three researchers involved in the project. Further literature review was done on relevant articles found within references of selected articles.

RESULTS

Crouzon syndrome, although of variable penetrance, is thought to be caused in part by a mutation in the fibroblast growth factor receptor-2 (FGFR2) on chromosome 10. Aside from craniofacial malformations, the disease can also cause hearing loss and airway challenges due to malformations in the nasal cavity and nasopharyngeal airway. Management options from the perspective of the otorhinolaryngologist are diverse and revolve around craniosynostectomy, offsetting midfacial hypoplasia, addressing obstructive sleep apnea and a compromised airway, psychosocial and esthetic interventions, and ultimately requiring a multidisciplinary team approach.

CONCLUSION

Mutations in the FGFR2 are responsible for 50 % of mutations within this multifaceted syndrome. Crouzon syndrome is an autosomal dominant disorder with a number of distinguishing characteristics, including craniosynostosis, maxillary hypoplasia, exophthalmos, and multiple other features. Early intervention, both medically and surgically, as well as disciplined follow-up with the pediatric provider are crucial to the management of this disorder. In particular, management should address cranial suture release, midfacial advancement, evaluation for hearing deficits, and obstructive sleep apnea, with expedient intervention for airway compromise and increased intracranial pressure.

Palifermin for the protection and regeneration of epithelial tissues following injury: new findings in basic research and pre-clinical models

Keratinocyte growth factor (KGF) is a paracrine-acting epithelial mitogen produced by cells of mesenchymal origin, that plays an important role in protecting and repairing epithelial tissues. Pre-clinical data initially demonstrated that a recombinant truncated KGF (palifermin) could reduce gastrointestinal injury and mortality resulting from a variety of toxic exposures. Furthermore, the use of palifermin in patients with hematological malignancies reduced the incidence and duration of severe oral mucositis experienced after intensive chemoradiotherapy. Based upon these findings, as well as the observation that KGF receptors are expressed in many, if not all, epithelial tissues, pre-clinical studies have been conducted to determine the efficacy of palifermin in protecting different epithelial tissues from toxic injury in an attempt to model various clinical situations in which it might prove to be of benefit in limiting tissue damage. In this article, we review these studies to provide the pre-clinical background for clinical trials that are described in the accompanying article and the rationale for additional clinical applications of palifermin.

Activating somatic FGFR2 mutations in breast cancer

It is known that FGFR2 gene variations confer a risk for breast cancer. FGFR2 and FGF10, the main ligand of FGFR2, are both overexpressed in 5–10% of breast tumors. In our study, we sequenced the most important coding regions of FGFR2 in somatic tumor tissue of 140 sporadic breast cancer patients and performed MLPA analysis to detect copy number variations in FGFR2 and FGF10. We identified one somatic heterozygous missense mutation, p.K660N (c.1980G>C), within the tyrosine kinase domain of FGFR2 in tumor tissue of a sporadic breast cancer patient, which is likely mediated by the FGFR2-IIIb isoform. The presence of wild type and mutated alleles in equal quantities suggests that the mutation has driven clonal amplification of mutant cells. We have analyzed the tyrosine kinase activity of p.K660N and another recently described somatic breast cancer mutation in FGFR2, p.R203C, after expression in HEK293 cells and demonstrated that the intrinsic tyrosine kinase activity of both mutant proteins is strongly increased resulting in elevated phosphorylation and activity of downstream effectors. To our knowledge, this is the first report of functional analysis of somatic breast cancer mutations in FGFR2 providing evidence for the activating nature of FGFR2-mediated signalling in the pathogenesis of breast cancer.

Comparison of genetic variation of breast cancer susceptibility genes in Chinese and German populations

Genome-wide association studies (GWAS) identified several genetic risk factors for breast cancer, however, most of them were validated among women of European ancestry. This study examined single-nucleotide polymorphisms (SNPs) contributing to breast cancer in Chinese (984 cases and 2206 controls) and German (311 cases and 960 controls) populations. Eighteen SNPs significantly associated with breast cancer, previously identified in GWAS were genotyped. Twelve SNPs passed quality control and were subjected to statistical analysis. Seven SNPs were confirmed to be significantly associated with breast cancer in the Chinese population, reflecting three independent loci (ESR1, FGFR2, TOX3) and five of these were also confirmed in the German population. The strongest association was identified for rs2046210 in the Chinese (odds ratio (OR)=1.42, 95% confidence interval (CI)=1.28-1.59, P=1.9 × 10(-10)) and rs3803662 in the German population (OR=1.43, 95% CI=1.17-1.74, P=4.01 × 10(-4)), located upstream of the ESR1 and TOX3 gene, respectively. For the first time, rs3757318 at 6q25.1, located next to the gene encoding estrogen receptor α (ESR1) was found to be strongly associated with breast cancer (OR=1.33, 95% CI=1.18-1.49, P=1.94 × 10(-6)) in the Chinese population. The frequency of this variant was markedly lower in the German population and the association was not significant. Despite the genetic differences, essentially the same risk loci were identified in the Chinese and the German populations. Our study suggested the existence of common genetic factors as well as disease susceptibility differences for breast cancer in both populations and highlighted the importance of performing comparison analyses for disease susceptibility within ethnic populations.

Delivery of growth factors for tissue regeneration and wound healing

Growth factors are soluble secreted proteins capable of affecting a variety of cellular processes important for tissue regeneration. Consequently, the self-healing capacity of patients can be augmented by artificially enhancing one or more processes important for healing through the application of growth factors. However, their application in clinics remains limited due to lack of robust delivery systems and biomaterial carriers. Interestingly, all clinically approved therapies involving growth factors utilize some sort of a biomaterial carrier for growth factor delivery. This suggests that biomaterial delivery systems are extremely important for successful usage of growth factors in regenerative medicine. This review outlines the role of growth factors in tissue regeneration, and their application in both pre-clinical animal models of regeneration and clinical trials is discussed. Additionally, current status of biomaterial substrates and sophisticated delivery systems such as nanoparticles for delivery of exogenous growth factors and peptides in humans are reviewed. Finally, issues and possible future research directions for growth factor therapy in regenerative medicine are discussed.

Alternative Splicing of Fibroblast Growth Factor Receptor IgIII Loops in Cancer

Alternative splicing of the IgIII loop of fibroblast growth factor receptors (FGFRs) 1–3 produces b- and c-variants of the receptors with distinctly different biological impact based on their distinct ligand-binding spectrum. Tissue-specific expression of these splice variants regulates interactions in embryonic development, tissue maintenance and repair, and cancer. Alterations in FGFR2 splicing are involved in epithelial mesenchymal transition that produces invasive, metastatic features during tumor progression. Recent research has elucidated regulatory factors that determine the splice choice both on the level of exogenous signaling events and on the RNA-protein interaction level. Moreover, methodology has been developed that will enable the in depth analysis of splicing events during tumorigenesis and provide further insight on the role of FGFR 1–3 IIIb and IIIc in the pathophysiology of various malignancies. This paper aims to summarize expression patterns in various tumor types and outlines possibilities for further analysis and application.

Splicing reporter mice revealed the evolutionally conserved switching mechanism of tissue-specific alternative exon selection

Since alternative splicing of pre-mRNAs is essential for generating tissue-specific diversity in proteome, elucidating its regulatory mechanism is indispensable to understand developmental process or tissue-specific functions. We have been focusing on tissue-specific regulation of mutually exclusive selection of alternative exons because this implies the typical molecular mechanism of alternative splicing regulation and also can be good examples to elicit general rule of “splice code”. So far, mutually exclusive splicing regulation has been explained by the outcome from the balance of multiple regulators that enhance or repress either of alternative exons discretely. However, this “balance” model is open to questions of how to ensure the selection of only one appropriate exon out of several candidates and how to switch them. To answer these questions, we generated an original bichromatic fluorescent splicing reporter system for mammals using fibroblast growth factor-receptor 2 (FGFR2) gene as model. By using this splicing reporter, we demonstrated that FGFR2 gene is regulated by the “switch-like” mechanism, in which key regulators modify the ordered splice-site recognition of two mutually exclusive exons, eventually ensure single exon selection and their distinct switching. Also this finding elucidated the evolutionally conserved “splice code,” in which combination of tissue-specific and broadly expressed RNA binding proteins regulate alternative splicing of specific gene in a tissue-specific manner. These findings provide the significant cue to understand how a number of spliced genes are regulated in various tissue-specific manners by a limited number of regulators, eventually to understand developmental process or tissue-specific functions.

Combined use of MS2 and PP7 coat fusions shows that TIA-1 dominates hnRNP A1 for K-SAM exon splicing control

Splicing of the FGFR2 K-SAM exon is repressed by hnRNP A1 bound to the exon and activated by TIA-1 bound to the downstream intron. Both proteins are expressed similarly by cells whether they splice the exon or not, so it is important to know which one is dominant. To answer this question, we used bacteriophage PP7 and bacteriophage MS2 coat fusions to tether hnRNP A1 and TIA-1 to distinct sites on the same pre-mRNA molecule. hnRNP A1 fused to one coat protein was tethered to a K-SAM exon containing the corresponding coat protein's binding site. TIA-1 fused to the other coat protein was tethered to the downstream intron containing that coat protein's binding site. This led to efficient K-SAM exon splicing. Our results show that TIA-1 is dominant for K-SAM exon splicing control and validate the combined use of PP7 and MS2 coat proteins for studying posttranscriptional events.

Aberrant fibroblast growth factor receptor signaling in bladder and other cancers

Fibroblast growth factors (FGFs) are potent mitogens, morphogens, and inducers of angiogenesis, and FGF signaling governs the genesis of diverse tissues and organs from the earliest stages. With such fundamental embryonic and homeostatic roles, it follows that aberrant FGF signaling underlies a variety of diseases. Pathological modifications to FGF expression are known to cause salivary gland aplasia and autosomal dominant hypophosphatemic rickets, while mutations in FGF receptors (FGFRs) result in a range of skeletal dysplasias. Anomalous FGF signaling is also associated with cancer development and progression. Examples include the overexpression of FGF2 and FGF6 in prostate cancer, and FGF8 overexpression in breast and prostate cancers. Alterations in FGF signaling regulators also impact tumorigenesis, which is exemplified by the down-regulation of Sprouty 1, a negative regulator of FGF signaling, in prostate cancer. In addition, several FGFRs are mutated in human cancers (including FGFR2 in gastric cancer and FGFR3 in bladder cancer). We recently identified intriguing alterations in the FGF pathway in a novel model of bladder carcinoma that consists of a parental cell line (TSU-Pr1/T24) and two sublines with increasing metastatic potential (TSU-Pr1-B1 and TSU-Pr1-B2), which were derived successively through in vivo cycling. It was found that the increasingly metastatic sublines (TSU-Pr1-B1 and TSU-Pr1-B2) had undergone a mesenchymal to epithelial transition. FGFR2IIIc expression, which is normally expressed in mesenchymal cells, was increased in the epithelial-like TSU-Pr1-B1 and TSU-Pr1-B2 sublines and FGFR2 knock-down was associated with the reversion of cells from an epithelial to a mesenchymal phenotype. These observations suggest that modified FGF pathway signaling should be considered when studying other cancer types.

FGF2 posttranscriptionally down-regulates expression of SDF1 in bone marrow stromal cells through FGFR1 IIIc

The chemokine stromal cell-derived factor-1 (SDF-1) is constitutively expressed by bone marrow stromal cells and plays key roles in hematopoiesis. Fibroblast growth factor 2 (FGF2), a member of the FGF family that plays important roles in developmental morphogenic processes, is abnormally elevated in the bone marrow from patients with clonal myeloid disorders and other disorders where normal hematopoiesis is impaired. Here, we report that FGF2 reduces SDF-1 secretion and protein content in bone marrow stromal cells. By inhibiting SDF-1 production, FGF2 compromises stromal cell support of hematopoietic progenitor cells. Reverse-transcriptase-polymerase chain reaction (RT-PCR) analysis revealed that bone marrow stromal cells express 5 FGF receptors (FGFRs) among the 7 known FGFR subtypes. Blocking experiments identified FGFR1 IIIc as the receptor mediating FGF2 inhibition of SDF-1 expression in bone marrow stromal cells. Analysis of the mechanisms underlying FGF2 inhibition of SDF-1 production in bone marrow stromal cells revealed that FGF2 reduces the SDF-1 mRNA content by posttranscriptionally accelerating SDF-1 mRNA decay. Thus, we identify FGF2 as an inhibitor of SDF-1 production in bone marrow stromal cells and a regulator of stromal cell supportive functions for hematopoietic progenitor cells.

Growth Factor Modulation of Fibroblast Proliferation, Differentiation, and Invasion: Implications for Tissue Valve Engineering

We have previously shown that transforming growth factor-beta1 (TGF-beta1) stimulates transdifferentiation of fibroblasts into smooth muscle alpha-actin (alpha-SMA) positive myofibroblasts. However, TGF-beta, as such, is unsuitable for effective population of a heart valve matrix, because it dose-dependently inhibits growth of fibroblasts. The aim of this study was to investigate combinations of other growth factors with TGF-beta to stimulate the proliferation of suitably differentiated cells and to enhance their invasion into aortic valve matrices. Human dermal mesenchymal cells (hDMC1.1) were treated with combinations of growth factors to stimulate these cells to trans-differentiate into myofibroblasts, to proliferate, and to invade. Growth factors were chosen after expression of their respective receptors was confirmed in hDMC1.1 using reverse transcriptase polymerase chain reaction. We combined TGF-beta with several growth factors such as insulin-like growth factor (IGF-1, IGF-2), epidermal growth factor (EGF), basic fibroblast growth factor (bFGF), and platelet-derived growth factor (PDGF-AA, PDGF-BB, and PDGFAB). Nuclear Ki67 staining, MTT assay, and cell counting revealed that only EGF and bFGF were capable of overcoming TGF-beta-induced growth inhibition. However, bFGF but not EGF inhibited TGF-beta-induced alpha-SMA expression, as evidenced by immuno-cytochemistry and Western blotting. A growth factor cocktail (TGF-beta, EGF, bFGF) has been established that maintains TGF-beta-induced trans-differentiation but overcomes TGF-beta-induced growth inhibition while stimulating fibroblast proliferation and invasion.

Apert and Crouzon syndromes: clinical findings, genes and extracellular matrix

Apert and Crouzon syndromes are well known craniostenosis. In the last 10 years several studies were performed to provide a better understanding of the etiology and pathogenesis of these diseases. Both have an autosomal dominant mode of transmission, and a mutation in the gene encoding for the fibroblast growth factor receptor 2 (FGFR2) is the cause in most patients. However, the fact that the same mutation can produce a wide range of phenotypic expression makes the mechanism of anomalous development more complex. The extracellular matrix (ECM) is composed of proteins, glycosaminoglycans, and cytokines that are secreted in an autocrine and paracrine manner and are able to modify the ECM. Fibroblast growth factors are complexed with heparan sulfate, a component of the ECM, before binding the FGFR2. Data exist about different expressions of cytokines and ECM macromolecule in craniostenosis-derived fibroblasts and osteoblasts. Changes in ECM composition could explain the altered osteogenic process and account for pathologic variations in cranial development in addition to the FGFR2 mutations.

Fibroblast growth factor signaling during early vertebrate development

Fibroblast growth factors (FGFs) have been implicated in diverse cellular processes including apoptosis, cell survival, chemotaxis, cell adhesion, migration, differentiation, and proliferation. This review presents our current understanding on the roles of FGF signaling, the pathways employed, and its regulation. We focus on FGF signaling during early embryonic processes in vertebrates, such as induction and patterning of the three germ layers as well as its function in the control of morphogenetic movements.

Keratinocyte growth factor/fibroblast growth factor 7, a homeostatic factor with therapeutic potential for epithelial protection and repair

Keratinocyte growth factor (KGF) is a paracrine-acting, epithelial mitogen produced by cells of mesenchymal origin. It is a member of the fibroblast growth factor (FGF) family, and acts exclusively through a subset of FGF receptor isoforms (FGFR2b) expressed predominantly by epithelial cells. The upregulation of KGF after epithelial injury suggested it had an important role in tissue repair. This hypothesis was reinforced by evidence that intestinal damage was worse and healing impaired in KGF null mice. Preclinical data from several animal models demonstrated that recombinant human KGF could enhance the regenerative capacity of epithelial tissues and protect them from a variety of toxic exposures. These beneficial effects are attributed to multiple mechanisms that collectively act to strengthen the integrity of the epithelial barrier, and include the stimulation of cell proliferation, migration, differentiation, survival, DNA repair, and induction of enzymes involved in the detoxification of reactive oxygen species. KGF is currently being evaluated in clinical trials to test its ability to ameliorate severe oral mucositis (OM) that results from cancer chemoradiotherapy. In a phase 3 trial involving patients who were treated with myeloablative chemoradiotherapy before autologous peripheral blood progenitor cell transplantation for hematologic malignancies, KGF significantly reduced both the incidence and duration of severe OM. Similar investigations are underway in patients being treated for solid tumors. On the basis of its success in ameliorating chemoradiotherapy-induced OM in humans and tissue damage in a variety of animal models, additional clinical applications of KGF are worthy of investigation.

MAZ elements alter transcription elongation and silencing of the fibroblast growth factor receptor 2 exon IIIb

The fibroblast growth factor receptor 2 (FGFR2) gene exons IIIb and IIIc are alternatively spliced in a mutually exclusive and cell type-specific manner. FGFR2 exon choice depends on both activation and silencing. Exon IIIb silencing requires cis-acting elements upstream and downstream of the exon. To examine the influence of transcription on exon IIIb silencing, the putative RNA polymerase II (RNAPII)-pausing MAZ4 element was inserted at different positions within the FGFR2 minigene construct. MAZ4 insertions 5' to the upstream silencing elements or between exon IIIb and downstream silencing elements result in decreased silencing. An insertion 3' of the downstream silencing elements, however, has no effect on splicing. An RT-PCR elongation assay shows that the MAZ4 site in these constructs is likely to be a RNAPII pause site. Insertion of another RNAPII pause site into the minigene has a similar effect on exon IIIb silencing. Transfection of in vitro transcribed RNA demonstrates that the cell type specificity of FGFR2 alternative splicing requires co-transcriptional splicing. Additionally, changing the promoter alters both FGFR2 minigene splicing and the MAZ4 effect. We propose that RNAPII pauses at the MAZ4 elements resulting in a change in the transcription elongation complex that influences alternative splicing decisions downstream.

Expression of keratinocyte growth factor receptor (KGFR/FGFR2 IIIb) in vascular smooth muscle cells

Keratinocyte growth factor receptor (KGFR), also known as fibroblast growth factor receptor (FGFR)2 IIIb, is located in many types of epithelial cells and is activated by four known ligands (FGF-1, FGF-3, FGF-7 (also known as KGF) and FGF-10) that are predominantly synthesized by mesenchymal cells. In the early stage of atherosclerosis, vascular smooth muscle cells (VSMC) transform from a contractile to a synthetic phenotype, proliferate and migrate into the intima. Previously, FGF-7 mRNA expression was reported in VSMC, but KGFR mRNA was not detected. In the present study, we attempted to determine whether KGFR is localized in VSMC cultured from rat aorta and VSMC in human normal and atherosclerotic coronary arteries. Expression of KGFR mRNA and its protein was detected in cultured rat VSMC by reverse transcription-polymerase chain reaction and western blot analysis, respectively. Immunohistochemically, KGFR was localized in the VSMC of the outer layer of the media in normal human coronary arteries. Furthermore, it was localized in the VSMC of the media and thickened intima of atherosclerotic arteries. Recombinant FGF-7 and/or FGF-10 proteins stimulated the growth of cultured rat VSMC. These findings indicate that KGFR localized in VSMC may contribute to the proliferation of VSMC in normal and atherosclerotic arteries.

Human RPE cells express the FGFR2IIIc and FGFR3IIIc splice variants and FGF9 as a potential high affinity ligand

The expression of splice variants of FGF receptors, which differ in the third Ig domain, was investigated in retinal pigment epithelium (RPE) cells in vitro and in vivo. This region of the protein determines ligand-binding specificity. Additionally, the expression of potential ligands for these receptors was investigated. Expression of FGF receptor transcript alternative splicing was analyzed by RT-PCR/Southern analysis in RPE cells in vitro and in vivo. The expression of FGFs by RT-PCR, in situ hybridization, and immunohistochemistry in sections of the human posterior pole was also investigated. The ARPE-19 cell line expresses only the FGFR2IIIc splice variant and does not express any FGFR3 splice variants in vitro. Two in vivo samples exhibited expression of the FGFR2IIIc and FGFR3IIIc splice variants and no evidence of the corresponding IIIb splice variant. The results from previous studies for these receptors imply that FGF9 or FGF4 could act as ligands. We demonstrated that FGF9 is expressed in a subpopulation of the RPE, as well as photoreceptors and other neurons of the retina. FGF4 was not detected by RT-PCR analysis in RPE cells in vitro. These data suggest that FGF9 may be an autocrine/paracrine factor in the outer retina.

A Non-sequence-specific double-stranded RNA structural element regulates splicing of two mutually exclusive exons of fibroblast growth factor receptor 2 (FGFR2)

Alternative splicing of fibroblast growth factor receptor 2 (FGFR2) mutually exclusive exons IIIb and IIIc represents a tightly regulated and functionally relevant example of post-transcriptional gene regulation. Rat prostate cancer DT3 and AT3 cell lines demonstrate exclusive selection of either exon IIIb or exon IIIc, respectively, and have been used to characterize regulatory FGFR2 RNA cis-elements that are required for splicing regulation. Two sequences termed ISE-2 and ISAR are located in the intron between exons IIIb and IIIc and are required for cell-type specific exon IIIb. Previous studies suggest that the function of these elements involves formation of an RNA stem structure, even though they are separated by more than 700 nucleotides. Using transfected minigenes, we performed a systematic analysis of the sequence and structural components of ISE-2 and ISAR that are required for their ability to regulate FGFR2 splicing. We found that the primary sequence of these elements can be replaced by completely unrelated sequences, provided that they are also predicted to form an RNA stem structure. Thus, a nonsequence-specific double stranded RNA stem constitutes a functional element required for FGFR2 splicing; suggesting that a double-stranded RNA binding protein is a component of the splicing regulatory machinery.

IIIc isoform of fibroblast growth factor receptor 1 is overexpressed in human pancreatic cancer and enhances tumorigenicity of hamster ductal cells

BACKGROUND & AIMS

Fibroblast growth factors (FGFs) are mitogenic polypeptides that signal via FGF receptors (FGFRs). Pancreatic ductal adenocarcinomas (PDACs) overexpress multiple FGFs, implying a potential for growth modulation. In this study we investigated the importance of the IIIc splice variant of FGFR-1 (FGFR-1 IIIc) in PDAC.

METHODS

Expression of FGFR-1 IIIc was determined by a ribonuclease protection assay in pancreatic cancer cell lines and in tissues. In situ hybridization was used to localize FGFR-1 IIIc messenger RNA (mRNA) in pancreatic tissues. A cDNA encoding FGFR-1 IIIc was stably transfected into the well-differentiated TAKA-1 pancreatic ductal cell line that is not responsive to FGF5 and does not express FGFR-1.

RESULTS

FGFR-1 IIIc was expressed in 5 of 7 pancreatic cancer cell lines and in the majority of the cancer cells in 4 of 7 PDAC samples. In vitro, TAKA-1 cells stably transfected with FGFR-1 IIIc exhibited increased basal growth; enhanced basal tyrosine phosphorylation of FGFR substrate-2 (FRS2), Shc, and phospholipase Cgamma; and increased activation of mitogen-activated protein kinase (MAPK). PD98059, an inhibitor of MAPK, suppressed the basal growth of parental and transfected clones, but the effect was more marked in clones expressing FGFR-1 IIIc. In vivo, tumor formation in nude mice was dramatically enhanced with FGFR-1 IIIc transfected (20 of 20) in comparison with sham transfected (0 of 10) cells.

CONCLUSIONS

Our data indicate that FGFR-1 IIIc is expressed in human pancreatic cancer cells, promotes mitogenic signaling via the FRS2-MAPK pathway, and has the potential to enhance pancreatic ductal cell transformation.

Association of the signaling adaptor FRS2 with fibroblast growth factor receptor 1 (Fgfr1) is mediated by alternative splicing of the juxtamembrane domain

Fibroblast growth factor receptors (FGFRs) are a family of transmembrane tyrosine kinases involved in signaling via interactions with the family of fibroblast growth factors. Alternative splicing of the juxtamembrane region of FGFR1-3 leads to the inclusion or exclusion of two amino acids, valine and threonine, the VT site. The presence or absence of VT (VT+ or VT-, respectively) affects the signaling potential of the receptor. The VT+ receptor isoform is required for Erk2 phosphorylation, a component of the mitogen-activated protein kinase signaling pathway. FRS2 is an adaptor protein that links FGFRs to the mitogen-activated protein kinase signaling pathway. FRS2 interacts with a region of the juxtamembrane domain of FGFR1 that includes the alternatively spliced VT site. We investigated the interaction of FRS2 with murine Fgfr1 juxtamembrane domain. We showed the alternatively spliced VT motif, at the juxtamembrane domain of Fgfr1 is required for FRS2 interaction with Fgfr1. Activation of signaling pathways from FRS2 is likely to be regulated by controlling the Fgfr1/FRS2 interaction through alternative splicing of the VT motif of Fgfr1.

Polypyrimidine tract-binding protein represses splicing of a fibroblast growth factor receptor-2 gene alternative exon through exon sequences

The fibroblast growth factor receptor (FGFR)-2 gene contains two mutually exclusive exons, K-SAM and BEK. We made a cell line designed to become drug-resistant on repression of BEK exon splicing. One drug-resistant derivative of this line carried an insertion within the BEK exon of a sequence containing at least two independent splicing silencers. One silencer was a pyrimidine-rich sequence, which markedly increased binding of polypyrimidine tract-binding protein to the BEK exon. The BEK exon binds to polypyrimidine tract-binding protein even in the silencer's absence. Several exonic pyrimidine runs are required for this binding, and they are also required for overexpression of polypyrimidine tract-binding protein to repress BEK exon splicing. These results show that binding of polypyrimidine tract-binding protein to exon sequences can repress splicing. In epithelial cells, the K-SAM exon is spliced in preference to the BEK exon, whose splicing is repressed. Mutation of the BEK exon pyrimidine runs decreases this repression. If this mutation is combined with the deletion of a sequence in the intron upstream from the BEK exon, a complete switch from K-SAM to BEK exon splicing ensues. Binding of polypyrimidine tract binding protein to the BEK exon thus participates in the K-SAM/BEK alternative splicing choice.

Co-transcriptional splicing of pre-messenger RNAs: considerations for the mechanism of alternative splicing

Nascent transcripts are the true substrates for many splicing events in mammalian cells. In this review we discuss transcription, splicing, and alternative splicing in the context of co-transcriptional processing of pre-mRNA. The realization that splicing occurs co-transcriptionally requires two important considerations: First, the cis-acting elements in the splicing substrate are synthesized at different times in a 5' to 3' direction. This dynamic view of the substrate implies that in a 100 kb intron the 5' splice site will be synthesized as much as an hour before the 3' splice site. Second, the transcription machinery and the splicing machinery, which are both complex and very large, are working in close proximity to each other. It is therefore likely that these two macromolecular machines interact, and recent data supporting this notion is discussed. We propose a model for co-transcriptional pre-mRNA processing that incorporates the concepts of splice site-tethering and dynamic exon definition. Also, we present a dynamic view of the alternative splicing of FGF-R2 and suggest that this view could be generally applicable to many regulated splicing events.

The nonsense-mediated decay pathway and mutually exclusive expression of alternatively spliced FGFR2IIIb and -IIIc mRNAs

Exons IIIb and IIIc of the FGFR2 gene are alternatively spliced in a mutually exclusive manner in different cell types. A switch from expression of FGFR2IIIb to FGFR2IIIc accompanies the transition of nonmalignant rat prostate tumor epithelial cells (DTE) to cells comprising malignant AT3 tumors. Here we used transfection of minigenes with and without alterations in reading frame and with and without introns to examine how translation affects observed FGFR2 splice products. We observed that nonsense mutations in other than the last exon led to a dramatic reduction in mRNA that is abrogated by removal of downstream introns in both DTE and AT3 cells. The mRNA, devoid of both IIIb and IIIc exons (C1-C2), is a major splice product from minigenes lacking an intron downstream of the second common exon C2. From these observations, we suggest that repression of exon IIIc and activation of exon IIIb inclusion in DTE cells lead to the generation of both C1-IIIb-C2 and C1-C2 products. However, the C1-C2 product from the native gene is degraded due to a frameshift and a premature termination codon caused by splicing C1 and C2 together. Derepression of exon IIIc and repression of exon IIIb lead to the generation of both C1-IIIc-C2 and C1-C2 products in AT3 cells, but the C1-C2 product is degraded. The C1-IIIb-IIIc-C2 mRNA containing a premature termination codon in exon IIIc was present, but at apparently trace levels in both cell types. The nonsense-mediated mRNA decay pathway and cell type-dependent rates of inclusion of exons IIIb and IIIc result in the mutually exclusive expression of FGFR2IIIb and IIIc.

Alternative splicing of Xenopus alphafast-tropomyosin pre-mRNA during development: identification of determining sequences

The Xenopus alphafast-tropomyosin gene contains in its central part a set of mutually exclusive exons, designated 6A and 6B, which are incorporated into mRNA encoding, respectively, nonmuscle and muscle tropomyosins. In this study, we show that usage of both exons is strictly regulated during development, exon 6A being used in the oocyte and nonmuscle tissues of the embryo, while exon 6B is used in muscle tissues. An approach of transient embryo transgenesis was developed to study the mechanisms involved in the splice site choice during development. We demonstrate that a-tropomyosin minigenes driven by tissue-specific promoters that target gene expression in nonmuscle and muscle tissues recapitulate the splicing pattern of the endogenous gene. A mutational analysis showed that regulation occurred at both exons 6A and 6B in muscle and nonmuscle tissues. In this context, we have identified an element located in the intron downstream of 6A that participates in the recognition of the weak 5' splice site of exon 6A and the repression of exon 6B in nonmuscle cells.

An important role for the IIIb isoform of fibroblast growth factor receptor 2 (FGFR2) in mesenchymal-epithelial signalling during mouse organogenesis

The fibroblast growth factor receptor 2 gene is differentially spliced to encode two transmembrane tyrosine kinase receptor proteins that have different ligand-binding specificities and exclusive tissue distributions. We have used Cre-mediated excision to generate mice lacking the IIIb form of fibroblast growth factor receptor 2 whilst retaining expression of the IIIc form. Fibroblast growth factor receptor 2(IIIb) null mice are viable until birth, but have severe defects of the limbs, lung and anterior pituitary gland. The development of these structures appears to initiate, but then fails with the tissues undergoing extensive apoptosis. There are also developmental abnormalities of the salivary glands, inner ear, teeth and skin, as well as minor defects in skull formation. Our findings point to a key role for fibroblast growth factor receptor 2(IIIb) in mesenchymal-epithelial signalling during early organogenesis.

Mapping ligand binding domains in chimeric fibroblast growth factor receptor molecules. Multiple regions determine ligand binding specificity

Fibroblast growth factors (FGFs) mediate essential cellular functions by activating one of four alternatively spliced FGF receptors (FGFRs). To determine the mechanism regulating ligand binding affinity and specificity, soluble FGFR1 and FGFR3 binding domains were compared for activity. FGFR1 bound well to FGF2 but poorly to FGF8 and FGF9. In contrast, FGFR3 bound well to FGF8 and FGF9 but poorly to FGF2. The differential ligand binding specificity of these two receptors was exploited to map specific ligand binding regions in mutant and chimeric receptor molecules. Deletion of immunoglobulin-like (Ig) domain I did not effect ligand binding, thus localizing the binding region(s) to the distal two Ig domains. Mapping studies identified two regions that contribute to FGF binding. Additionally, FGF2 binding showed positive cooperativity, suggesting the presence of two binding sites on a single FGFR or two interacting sites on an FGFR dimer. Analysis of FGF8 and FGF9 binding to chimeric receptors showed that a broad region spanning Ig domain II and sequences further N-terminal determines binding specificity for these ligands. These data demonstrate that multiple regions of the FGFR regulate ligand binding specificity and that these regions are distinct with respect to different members of the FGF family.

Anabolic effect of aminoterminally truncated fibroblast growth factor 4 (FGF4) on bone

Fibroblast growth factor 4 (FGF4), a member of the FGF family, plays several important roles in bone development during embryogenesis. Systemic administration of FGF4 increases bone mass in rats, which suggests the potential therapeutic usefulness of this growth factor in treatment for osteopenia and in bone regeneration. We investigated the length of FGF4 required to exert its anabolic effects, because this information may be useful in developing new molecules to mimic the effects of FGF4. Because the active site of FGF family molecules is in the carboxylterminal region, we produced aminoterminally truncated recombinant human FGF4s (rhFGF4s) of different sizes. Human FGF4 cDNA containing almost the full length of the coding region (573 bp, 191 amino acid residues) was inserted into pUC18 vector and then deleted from the 5' end using the ExoIII system. Each of the deleted FGF4 cDNAs was subcloned into a pET29(+) expression vector. Differently sized recombinant proteins were expressed in the BL21(DE3)pLysS Escherichia coli strain and then purified. The growth-stimulative effects on NIH3T3 cells of each recombinant protein were examined by means of MTT colorimetric assay. Full-length and the shortened recombinant proteins, which stimulated NIH3T3 cell growth, were then subcutaneously administered into male ddY mice (6 weeks old) every day for 2 weeks. Bone mineral density (BMD) was measured using dual-energy X-ray absorptiometry (DEXA) and peripheral quantitative computed tomography (pQCT). The rhFGF4 of 134 amino acid residues, the region homologous to other members of the FGF family, exerted a growth-stimulative effect on NIH3T3 cells comparable to the full-length version of FGF4; however, the shortest version, with 111 amino acid residues, showed a limited growth-stimulative effect. Systemic administration of the rhFGF4 of 134 amino acid residues increased the bone mineral density (BMD) of femurs at a dose of 0.1 mg/kg, which was comparable to that of the full-length rhFGF4. DEXA analysis, pQCT analysis, soft X-ray photos, and contact microradiographs revealed an increase in femoral trabecular bone in FGF4-treated animals; an increase in bone formation was also evident upon histomorphometric analysis. These results indicate that the region of FGF4 that is homologous to other FGF family members provides a sufficient anabolic effect in bone and that this recombinant protein is potentially useful as a therapeutic agent in bone.

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.

Cell type-specific inhibition of keratinocyte collagenase-1 expression by basic fibroblast growth factor and keratinocyte growth factor. A common receptor pathway

Collagenase-1 is invariantly expressed by migrating basal keratinocytes in all forms of human skin wounds, and its expression is induced by contact with native type I collagen. However, net differences in enzyme production between acute and chronic wounds may be modulated by soluble factors present within the tissue environment. Basic fibroblast growth factor (bFGF, FGF-2) and keratinocyte growth factor (KGF, FGF-9), which are produced during wound healing, inhibited collagenase-1 expression by keratinocytes in a dose-dependent manner. However, KGF was >100-fold more effective than bFGF at inhibiting collagenase-1 expression, suggesting that this differential signaling is transduced via an FGF receptor that binds these ligands with different affinities. Reverse transcriptase-polymerase chain reaction analysis of human keratinocyte mRNA for fibroblast growth factor receptors (FGFRs) revealed expression of only FGFR-2 IIIb, the KGF-specific receptor, which also binds bFGF with low affinity, and FGFR-3 IIIb, which does not bind bFGF or KGF. FGFRs that bind bFGF with high affinity were not detected. Our results suggest that bFGF and KGF inhibit collagenase-1 expression through the KGF cell-surface receptor (FGFR-2 IIIb). Because bFGF induces collagenase-1 in most cell types, cell-specific expression of FGFR family members may dictate the regulation of matrix metalloproteinases in a tissue-specific manner.

Genomic organization of the human fibroblast growth factor receptor 3 (FGFR3) gene and comparative sequence analysis with the mouse Fgfr3 gene

Fibroblast growth factor receptor 3 (FGFR3) is a developmentally regulated transmembrane protein. Three other FGFRs (1, 2, and 4) in conjunction with FGFR3 are part of the receptor tyrosine kinase super-family. Mutations in three of these genes (FGFR1, 2, and 3) have been determined to be the cause of human growth and developmental disorders. We have characterized a 22-kb DNA fragment containing the human FGFR3 gene and determined 11 kb of its nucleotide sequence. The gene consists of 19 exons and 18 introns spanning 16.5 kb, and the boundaries between exons and introns follow the GT/AG rule. The translation initiation and termination sites are located in exon 2 and exon 19 respectively. The sequence of the 5'-flanking region (1.5 kb) lacks the typical TATA or CAAT boxes. However, several putative binding sites for transcription factors SP1, AP2, Krox 24, IgHC.4, and Zeste are present. The 0.77-kb region from position -889 (5'-flanking region) to -119 (intron 1) contains a CpG island. A comparative sequence analysis of the human and mouse FGFR3 genes indicates that the overall genomic structure and organization of the human gene are nearly identical to those of its mouse counterpart. Furthermore, there is a striking similarity in the promoter regions of both genes, and several of the putative transcription factor-binding sites are conserved across species, suggesting a definitive role of these factors in the transcriptional regulation of these genes.

Sequence requirements for regulated RNA splicing of the human fibroblast growth factor receptor-1 alpha exon

Progression of astrocytes from a benign to a malignant phenotype is accompanied by a change in the RNA processing of the fibroblast growth factor receptor 1 (FGFR-1) gene. The level of a high affinity form of the FGFR-1 is dramatically elevated as a result of alpha-exon skipping during RNA splicing. In this paper we have been able to duplicate this tumor-specific RNA processing pathway by transfection of a chimeric minigene containing a 4-kilobase fragment of the human FGFR-1 gene (including the alpha-exon) into a variety of cell lines. In a transfected human astrocytoma cell line, alpha-exon skipping was consistently observed for RNA transcripts derived from both the chimeric minigene and endogenous gene expression. This exon skipping phenotype was dependent on the size of the flanking intron as deletions which reduced the introns to less than approximately 350 base pairs resulted in enhanced alpha-exon inclusion. Increased exon inclusion was not sequence-specific as exon skipping could be restored with insertion of nonspecific sequence. Cell-specific exon recognition was maintained with a 375-nucleotide sequence inclusive and flanking the alpha-exon, provided that intron size was maintained. These results identify the minimal cis-regulatory sequence requirements for exclusion of FGFR-1 alpha-exon in astrocytomas.

Cloning of multiple chicken FGF1 mRNAs and their differential expression during development of whole embryo and of the lens

Five different 5' untranslated regions (5' UTRs) of FGF1 mRNAs were cloned in chicken. The structure of these transcripts suggests that, as in mammals, distinct 5' untranslated exons are spliced to the first coding exon via alternative splicing and alternative promoter usage. In an attempt to correlate the expression of specific transcripts to distinct biological activities, the distribution of these transcripts in different tissues and during the development of both the whole embryo and the lens was studied. In tissues, we have shown a differential, but not exclusive, expression of these transcripts. In the whole embryo, the expression of one transcript correlates with later developmental processes. In the lens, only two transcripts were detected that are both differently expressed and distributed. These results suggest that the biological properties of FGF1 depend on the expression of specific FGF1 mRNAs. Because these transcripts only differ in their 5' UTRs, they could be involved in distinct translational controls.

Keratinocyte growth factor is highly overexpressed in inflammatory bowel disease

Recently we demonstrated an important function of keratinocyte growth factor (KGF) in wound re-epithelialization. As KGF is mitogenic for various epithelial cells, we speculated about a role of KGF in epithelial repair processes of other organs as seen in a variety of inflammatory diseases. Here we demonstrate a strikingly increased expression of KGF in surgical specimens from patients suffering from Crohn's disease and ulcerative colitis. The levels of KGF expression strongly correlated with the degree of inflammation as assessed by histological analysis of adjacent tissue and expression analysis of the pro-inflammatory cytokine interleukin-1 beta. The highest levels of KGF mRNA and protein were found in mesenchymal cells of the lamina propria, particularly in highly inflamed areas. As the KGF receptor is expressed in intestinal epithelial cells, KGF seems to act in a paracrine manner to stimulate proliferation of these cells. These data suggest a crucial role of KGF in epithelial repair after injury caused by inflammatory processes.

Receptor specificity of the fibroblast growth factor family

Fibroblast growth factors (FGFs) are essential molecules for mammalian development. The nine known FGF ligands and the four signaling FGF receptors (and their alternatively spliced variants) are expressed in specific spatial and temporal patterns. The activity of this signaling pathway is regulated by ligand binding specificity, heparan sulfate proteoglycans, and the differential signaling capacity of individual FGF receptors. To determine potentially relevant ligand-receptor pairs we have engineered mitogenically responsive cell lines expressing the major splice variants of all the known FGF receptors. We have assayed the mitogenic activity of the nine known FGF ligands on these cell lines. These studies demonstrate that FGF 1 is the only FGF that can activate all FGF receptor splice variants. Using FGF 1 as an internal standard we have determined the relative activity of all the other members of the FGF family. These data should serve as a biochemical foundation for determining developmental, physiological, and pathophysiological processes that involve FGF signaling pathways.

Vascular remodelling and molecular biology: new concepts and therapeutic possibilities

1. Over the past decade major advances in molecular cell biology have greatly increased our understanding of the way in which many growth factor genes are expressed and regulated. This knowledge is currently being translated into investigations of the cardiovascular system. 2. Two growth factor families appear to play particularly important roles, the fibroblast growth factors and the transforming growth factors-beta. These are multifunctional growth factors capable of remodelling the vasculature through their effects on cell migration, proliferation and matrix formation. 3. An understanding of their regulation, properties and nature of their receptors is providing novel insights into the physiology and pathobiology of the vasculature. It is also providing highly specific targets for future therapy.

Fibroblast-growth-factor receptor mutations in human skeletal disorders

Fibroblast-growth-factor receptors (FGFRs), members of the tyrosine-kinase receptor family, play a crucial role in signal transduction and development. Recently, unique mutations in three human FGFR-encoding genes (FGFR1-3) have been identified as the cause of a variety of skeletal disorders. Comparison of these specific mutations with the resulting phenotypes is now providing new insight into the role of these receptors in normal and abnormal bone development.

Re-programming of expression of the KGFR and bek variants of fibroblast growth factor receptor 2 during limb regeneration in newts (Notophthalmus viridescens)

We have previously shown, by in situ hybridization, that fibroblast growth factor receptor 2 (FGFR2) is present in the basal layer of wound epithelium during limb regeneration in newts (Notophthalmus viridescens). In contrast, FGFR1 expression is observed throughout the blastema mesenchyme but is distinctly absent from the wound epithelium (Poulin et al. [1993] Development 119:353-361). Sequence analysis revealed that we have isolated both the KGFR and bek variants of FGFR2. These two variants differ only in the second half of the last of their three (or two) Ig-like domains. In this report, we show the expression patterns of FGFR2 variants during limb regeneration by in situ hybridization. During the pre-blastema stages of regeneration, FGFR2 expression was observed in the basal layer of the wound epithelium and in the cells of the periosteum. The wound epithelial hybridization was observed when the KGFR-specific probe was used while the bek-specific probe hybridized to mRNA in the cells of the periosteum. As regeneration progresses to the blastema stages, KGFR expression continued to be observed in the basal layer of the wound epithelium with additional hybridization seen in the blastema mesenchyme closely associated with the bisected bones. The bek-specific hybridization pattern observed at this stage corresponds specifically to the mesenchymal hybridization. In the differentiation stages of regeneration, the mesenchymal expression of FGFR2 becomes restricted to the cells of the condensing cartilage and later to the perichondrium. Interestingly, there appears to be a dorsoventral gradient of the expression of both KGFR and bek variants of FGFR2, which are opposite each other at the later stages of regeneration. Thus, re-programming of expression of the two FGFR2 variants is required during the initial wound closure of limb regeneration. Remarkably, the expression patterns of KGFR and bek mimic those observed in the mouse limb bud during early embryonic development (Orr-Urtreger et al. [1993] Dev. Biol. 18:475-486). Moreover, our results suggest that the two FGFR2 variants have distinct roles in limb regeneration. Further investigation regarding the potential sources of the FGF ligands will help establish the roles that FGFs and FGFRs play in limb regeneration.

Alternative splicing of amelogenins

Amelogenins comprise as much as 90% of the protein in the developing enamel matrix. Separating amelogenins by gel electrophoresis reveals a complex of polypeptides with apparent mobilities ranging from low molecular weight species on up to 28,000 Daltons. A major objective of our research is determine the extent to which alternative RNA splicing contributes to this heterogeneity. We have cloned seven alternatively spliced mouse amelogenin mRNAs. The predicted translation products of these messages are 194, 180, 156, 141, 74, 59, and 44 amino acids in length. The 194 residue amelogenin is the only mouse amelogenin to include a polypeptide segment encoded by exon 4, which has a deduced amino acid sequence of KSHSQAINTDRTAL. Antibodies were raised against synthetic exon 4 encoded polypeptides and used to immunostain histologic tooth sections. These data indicate that alternatively spliced amelogenin mRNAs are translated into protein and secreted into the enamel matrix.

Alternative splicing of the mouse amelogenin primary RNA transcript

A heterogeneous mixture of amelogenins can be extracted from developing tooth enamel matrix. In an attempt to discover the extent to which alternative splicing of the amelogenin primary RNA transcript can generate unique isoforms, we have conducted a thorough search for cDNAs amplified by reverse transcription-polymerase chain reaction (RT-PCR). Over 2400 colonies were screened by colony hybridization. Seven different alternatively spliced amelogenin mRNAs were isolated. The predicted translation products of the messages are 194, 180, 156, 141, 74, 59, and 44 amino acids in length. RT-PCR amplification products not predicted by these seven amelogenin cDNAs were characterized. The intron separating exons 5 and 6 was cloned and sequenced. Using rapid amplification of cDNA ends (RACE) techniques, the 5' ends of the amelogenin mRNAs were cloned and characterized. The finding that the same exon 1 is common to all of the cloned mRNAs indicates that mouse amelogenin is transcribed from a single promoter. The mouse amelogenin transcription and translation initiation sites, the 5' untranslated leader, and the segment encoding the signal peptide were determined. The distinctly nonamelogenin-like exon 4, first observed in human amelogenin cDNAs, has also been found in mice. Antibodies were raised to synthetic exon 4-encoded polypeptides and used to immunostain Western transfers and histologic tooth sections.

Alternative splicing of fibroblast growth factor 1 (FGF-1) transcripts: a cellular dilemma in determining exon selection and exclusion

During reverse transcription and polymerase chain reaction (RT-PCR) of kidney RNA using fibroblast growth factor 1 (FGF-1) gene-specific primers, we amplified five fragments with the sizes of 507, 410, 310, 285, and 216 bp. Cloning of the 507-, 310-, and 216-bp fragments revealed that the latter two share the same sequences to the 507-bp DNA but missing 197 and 291 bp, respectively. Characterization of the corresponding genomic DNA sequences showed that the three cDNA are alternative splicing products of the same gene. We further showed that these three transcripts are also present in brain in similar proportions albeit in a much lesser extent. Repeated attempts to clone the 410-bp fragment resulted in isolation of three additional cDNA clones; each has the inclusion of a distinct novel exon. Because each novel exon is delimited by the splicing donor and acceptor sequences, these cDNA clones are not likely to be RT-PCR artifacts. Yet their mRNA levels are extremely low, because we could not detect the corresponding PCR products on ethidium bromide-stained gels nor by Southern hybridization. These novel exons may be routinely used in different tissues. The nature of the 410- and 285-bp products was shown to be heteroduplexing among the 507-, 310-, and 216-bp PCR products. Identification of the three novel FGF-1 exons, which we designate exons -1B, -1E, and -1F, will facilitate the studies of the mechanisms of regulated alternative splicing.

Mutations in the fibroblast growth factor receptor 2 gene cause Crouzon syndrome

Crouzon syndrome is an autosomal dominant condition causing premature fusion of the cranial sutures (craniosynostosis) and maps to chromosome 10q25-q26. We now present evidence that mutations in the fibroblast growth factor receptor 2 gene (FGFR2) cause Crouzon syndrome. We found SSCP variations in the B exon of FGFR2 in nine unrelated affected individuals as well as complete cosegregation between SSCP variation and disease in three unrelated multigenerational families. In four sporadic cases, the normal parents did not have SSCP variation. Finally, direct sequencing has revealed specific mutations in the B exon in all nine sporadic and familial cases, including replacement of a cysteine in an immunoglobulin-like domain in five patients.