Sixteen years and counting: the current understanding of fibroblast growth factor receptor 3 (FGFR3) signaling in skeletal dysplasias

Abstract

In 1994, the field of bone biology was significantly advanced by the discovery that activating mutations in the fibroblast growth factor receptor 3 (FGFR3) receptor tyrosine kinase (TK) account for the common genetic form of dwarfism in humans, achondroplasia (ACH). Other conditions soon followed, with the list of human disorders caused by FGFR3 mutations now reaching at least 10. An array of vastly different diagnoses is caused by similar mutations in FGFR3, including syndromes affecting skeletal development (hypochondroplasia [HCH], ACH, thanatophoric dysplasia [TD]), skin (epidermal nevi, seborrhaeic keratosis, acanthosis nigricans), and cancer (multiple myeloma [MM], prostate and bladder carcinoma, seminoma). Despite many years of research, several aspects of FGFR3 function in disease remain obscure or controversial. As FGFR3-related skeletal dysplasias are caused by growth attenuation of the cartilage, chondrocytes appear to be unique in their response to FGFR3 activation. However, the reasons why FGFR3 inhibits chondrocyte growth while causing excessive cellular proliferation in cancer are not clear. Likewise, the full spectrum of molecular events by which FGFR3 mediates its signaling is just beginning to emerge. This article describes the challenging journey to unravel the mechanisms of FGFR3 function in skeletal dysplasias, the extraordinary cellular manifestations of FGFR3 signaling in chondrocytes, and finally, the progress toward therapy for ACH and cancer.

Figure 1

A summary of FGFR3 mutations discussed in the text. (A) A summary of human FGFR3 mutations discussed in the text. Mutations in available murine models are also indicated. TD - thanatophoric dysplasia; ACH- achondroplasia; HCH - hypochondroplasia; SADDAN -severe achondroplasia with developmental delay and acanthosis nigricans; PLSD-SD - platys-pondylic lethal skeletal dysplasia, San Diego type. (B) An overview of human FGFR3 protein structure with indicated positions of the mutations discussed in text. Ig1–3 extracellular immunoglobulin-like domains of FGFR3; TM - transmembrane domain; TK1–2 intracellular tyrosine kinase domains.

Figure 2

Histologic appearance of human control and thanatophoric dysplasia (TD) growth plate cartilage. Histologic appearance of resting, proliferative, and hypertrophic zone of the femoral growth plate of a control 23 weeks of gestation fetus (left panel) compared to a 24 weeks of gestation TD fetus (R248C-FGFR3; right panel). Note the increased amount of spindle-shaped cells in the TD resting cartilage (arrows), loss of proliferating cells in the TD proliferative zone, and an increased cell-to-matrix ratio in both the TD proliferative and hypertrophic zones. Also note the overall disorganization of both the proliferative and hypertrophic zones as well as the mesenchymal tissue invading the hypertrophic zone in TD (arrows). Sections were stained with Goldner’s trichrome. CJ - chondro-osseous junction. Scale bar: 100 μm.

Figure 3

Molecular mechanisms of FGFR3 signaling in cartilage. Aberrant activation of FGFR3 alters chondrocyte behaviour by inducing premature growth arrest, loss of extracellular matrix, altered differentiation and changes in cell shape. Together, these cellular phenotypes (grey arrows) contribute to profound disruption of the growth plate cartilage resulting in skeletal dysplasia. At the molecular level, the growth arrest phenotype is mediated by induction of several inhibitors of the cell cycle, belonging to cip/kip family (p21) or INK4 family (p16, p18, p19), whereas the loss of the extracellular matrix originates from both inhibition of production of major matrix components (collagen type II and aggrecan), and active matrix degradation, mediated by several members of matrix metalloproteinase family (MMP). Expression of two important physiological regulators of chondrocyte differentiation, Indian hedgehog (Ihh) and parathyroid hormone related protein (PTHrP), is inhibited by FGFR3 in cartilage, likely contributing to impaired chondrocyte hypertrophic differentiation. ERK MAP kinase is a major pathway for growth arrest, extracellular matrix loss and impaired chondrocyte differentiation. FGFR3 causes prolonged activation of the ERK signaling module (RAS-RAF-MEK-ERK), mediated by adapter (GAB1, FRS2 and SHC)-driven recruitment of SHP2-GRB2-SOS1 complexes to the cell membrane, where they activate RAS. In addition, SNAIL1 transcription factor is involved in regulation of FGFR3-mediated ERK activity, although the exact nature of this regulation is not presently clear (question marks). The FGFR3-mediated activation of the ERK pathway is antagonized by CNP signaling, which inhibits ERK pathway by inactivation of RAF kinase, via inhibitory phosphorylation mediated by cGMP-activated protein kinase (PKG). Some FGFR3 mutants also activate STAT1, possibly via direct phosphorylation at Y701. It is, however, currently unclear whether activated STAT1 or other STATs induce cell cycle inhibitor expression in cartilage, thereby contributing to the FGFR3-mediated growth arrest (question mark). Finally, chronic activation of FGFR3 leads to inhibition of canonical cytokine-STAT signaling in chondrocytes, via both induction of SOCS inhibitors of cytokine signaling and inhibition of expression of receptors for IL6 (IL6Rα) or LIF (LIFR). As the latter cytokines represent positive regulators of chondrocyte proliferation, their inhibition might contribute to the pathological effects of FGFR3. NPR-B - natriuretic peptide receptor B/guanylyl cyclase B; GTP - guanosine-5′-triphosphate; cGMP - cyclic guanosine monophosphate; gp130 - glycoprotein 130.