Journal Facts

Journal
BoneKEy Knowledge Environment
ISSN
1533-4368
Volume
Year
2001

Article Facts

Title
From chondrocytes to cancer: Fibroblast growth factor receptor 3 (FGFR3)
DOI
10.1138/2001016
Author

Online Date
03/06/2001

Article Links

BoneKEy-Osteovision | Commentary

From chondrocytes to cancer: Fibroblast growth factor receptor 3 (FGFR3)


DOI:10.1138/2001016

Multiple myeloma causes significant skeletal morbidity. Our understanding of the mechanisms responsible for associated bone destruction has increased exponentially in recent years, but many questions remain regarding tumor progression in multiple myeloma. This month, Chesi et al, have implicated FGFR3 in the progression of myeloma (). Here is the background that led to these findings. Many B-cell malignancies are characterized by chromosomal translocations to the immunoglobulin heavy-chain (IgH) locus on chromosome 14q32. These result in dysregulation of oncogenes such as c-myc and cyclin D1 that contribute to the pathogenesis of the respective diseases. Only recently have such translocations been identified in multiple myeloma, since they were not detectable by conventional cytogenetics. Using a specialized molecular approach, Chesi et al., demonstrated that a t(4;14)(p16;q32) translocation occurs in 20% of myeloma cells and tumors (), and this results in the dysregulation of 2 genes at the 4p16.3 locus: 1) FGFR3, by juxtaposition to the 3′ Ca enhancer on der(); 2) multiple myeloma SET domain protein/Wolf-Hirschhorn syndrome candidate gene 1 (MMSET/WHSC1), by association with the intronic enhancer on der(). Overexpression of FGFR3 in the murine B9 cell line conferred IL-6-independent growth and provided indirect evidence that the FGFR3 may play a role in the progression of multiple myeloma (). These important observations have now been substantiated by mechanistic insight into the contribution of FGFR3 in myeloma tumor progression ().

Fibroblast growth factor (FGF) was originally purified from bovine pituitary gland as a mitogen that stimulated the growth of NIH 3T3 cells. Since then, at least twenty-three FGFs and four FGF receptor (FGFR) genes have been identified. The biologic activities of FGFs include regulation of cell growth, survival, differentiation and migration. FGFR3 is one of four high-affinity tyrosine kinase receptors that mediate the effects of the FGF ligands. Alternative splicing in the third loop of the extracellular immunoglobulin-like domain generates isoforms with varying specificity for the FGF ligands. FGFR3 is expressed in a restricted, time-dependent manner in the proliferating chondrocytes of developing long bones, among other tissues. It is normally undetectable in the B-cell lineage. FGFR3 signaling inhibits endochondral bone growth as evidenced by the following: 1) FGFR3-null mice have enhanced and prolonged endochondral bone growth (); 2) targeted overexpression to growth plate chondrocytes of a constitutively active FGFR3 due to a G380R substitution in the transmembrane domain resulted in dwarfed mice (); 3) activating mutations in the transmembrane domain (G380R), extracellular ligand-binding domain (R248C), or intracellular kinase domain (K650E) of FGFR3 result in the human dwarfing conditions of achondroplastic dwarfism and lethal thanatophoric dysplasia (TD) types I and II. In these human conditions, the degree of constitutive activity of the mutant receptors correlates with the severity of disease (). Thus, FGFR3 plays an important role in skeletal development by inhibiting chondrocyte proliferation and differentiation.

How does FGFR3, normally an inhibitor of chondrocyte growth, cause tumor progression in multiple myeloma? The authors found the t(4;14) translocation in nine of thirty myeloma cell lines; eight of the nine expressed functional FGFR3. FGFR3 mutations were detected in five, four of which were activating: two of these are identical or similar to those reported in thanatophoric dysplasia (Y373C, K650E). FGFR3 was not detected in the twenty-one cell lines which lacked the translocation. In the myeloma cell lines which expressed FGFR3, either wild-type or mutant, FGF-1 induced phosphorylation of ERK1 and ERK2, proteins of the MAP kinase signaling pathway. In one primary myeloma tumor, the t(4;14) translocation occurs early, and the activating mutation of FGFR3 occurs late (during tumor progression). Activating Ras mutations were detected in 13/30 multiple myeloma cell lines; none of which had activating FGFR3 receptor mutations. Finally, the activating mutant FGFR3 identified in the myeloma lines (K650E, Y373C and stopδ) as well as the activating Ras V12 mutant, when expressed in NIH 3T3 cells, induced cellular focus formation. Wild-type FGFR3 (in the presence of ligand FGF-1) and the F384L mutant (which was not activating) did not. This FGFR3 transformation was significantly reduced by expression of dominant-negative Raf 301 and Ras N17, further implication of the MAP kinase activation, via Ras, by FGFR3. Consistent with these in vitro transformation data, the NIH 3t3 cells transfected with the K650E mutant, but not empty vector or wild-type FGFR3, were tumorigenic in nude mice. Collectively, the data lead the authors to 3 conclusions: 1) myeloma cells which have the t(4;14) translocation express functional FGFR3, which can activate MAP kinase; 2) FGFR3 activating mutations are acquired by the myeloma cells during tumor progression and are mutually exclusive with those of Ras; 3) activated FGFR3 functions as an oncogene through the Ras pathway.

These and previous studies, which indicate that 20-30% of multiple myeloma contain the t(4;14) translocation, coupled to the evidence that ~50% of these also have activating mutations, suggest that this mechanism is responsible for tumor progression in 10-15% of myeloma. In comparison, activating Ras mutations may account for tumor progression in another 30%. These data suggest heterogeneity of pathogenesis.

Activating mutations of FGFR3 have also been reported in bladder and cervical cancer (), but the data presented by Chesi et al, are the first to show a direct involvement of FGFR3 signaling in tumorigenesis. The data indicate that the t(4;14) translocation and subsequent expression of functional FGFR3 alone is not sufficient to induce tumorigenesis, but that an activating mutation must also occur. Based on these and other studies, the authors propose a model of progressive genetic events in the establishment and progression of multiple myeloma. Chromosomal translocation into an IgH switch region, causing dysregulation of FGFR3 and MMSET/WHSC1 in the case of t(4;14), causes immortalization of a plasma cell which would ordinarily die within 30 days after isotype switching. Overexpression of FGFR3 may provide a survival or proliferative signal through its stimulation by FGF ligands expressed in the bone microenvironment, although this possibility is not supported by the data in this report. What is clear is that point mutations causing constitutive activation of FGFR3 contribute to tumor progression. In this context, the consequences of FGFR3 signaling appear to be cell type-specific. In the normal chondrocyte, FGFR3 signaling results in growth inhibition, while in the myeloma cell, it contributes to tumor progression. The exact role of MMSET/WHSC1 is this scenario remains unclear.

Despite the exciting nature of these findings, one must use caution when extrapolating these data to the clinical situation in myeloma. Much of the data were obtained from myeloma cell lines, which may not accurately represent the pathogenesis in primary tumor progression. This is suggested by Ho et al who showed the t(4;14) translocation to be present in 57% of primary tumor, but found no correlation to disease progression (). However, the Ho study did not look for activating mutations of FGFR3, the presence of which appears to be necessary for tumor progression. Fracchiolla et al found that activating mutations of FGFR3 were a rare event in primary myeloma ().

Caution aside, these studies raise other questions about the role of FGFR3 in multiple myeloma. 1) Does FGFR3 signaling stimulate the production of osteolytic factors and contribute to the hallmark of myeloma: bone destruction? For example, myeloma production of PTHrP has been implicated in the bone destruction. An activating FGFR3 mutation, expressed in normal chondrocytes, results in growth inhibition and suppression of PTHrP (). Since such a mutant causes cell growth when ectopically expressed in myeloma cells, one would predict that it might also stimulate myeloma production of PTHrP, especially since activating Ras mutations have been shown to result in constitutive PTHrP production by some solid tumors. Indirect evidence for this is reported by Nakazawa et el, who show that the t(4;14) translocation correlates to osteolytic bone disease (). 2) Does FGFR3 signaling alter myeloma cell expression of integrins, which mediate important interactions between the myeloma cell and the bone microenvironment? Evidence from the chondrocyte suggest that the answer is yes. The G380R activating FGFR3 mutant, expressed in CFK2 chondrocytes, caused altered expression of integrin subunits, which led to a change in substrate preference from fibronectin to type II collagen (). 3) Are patients with achondroplastic dwarfism, and germ-line activating mutations in FGFR3, more likely to develop B-cell malignancies? Since no such association has been reported, it seems likely that such a mutation alone is insufficient to cause malignant transformation if the receptor is not overexpressed in the B-cell, as it is in the presence of the t(4;14) translocation.

The role of FGFR3 in multiple myeloma provides us with yet another example of a regulator of skeletal development that, when dysregulated, is associated with skeletal complication of malignancy. This is somewhat reminiscent of the PTHrP story, which started in cancer and led to the chondrocyte. Originally identified because of its dysregulation in cancer, which resulted in hypercalcemia, PTHrP has since been shown to have a major role in skeletal development by virtue of its local effects on chondrocyte growth and differentiation (). Both constitutive activation of the PTH/PTHrP receptor as well as function loss of such receptors result in the human conditions of Jansen-type metaphyseal () and Blomstrand () chondrodysplasias, respectively. Thus, the biology of skeletal development continues to teach us lessons about skeletal complications of malignancy and visa versa.

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