Achondroplasia: Development, pathogenesis, and therapy

Abstract

Autosomal dominant mutations in fibroblast growth factor receptor 3 (FGFR3) cause achondroplasia (Ach), the most common form of dwarfism in humans, and related chondrodysplasia syndromes that include hypochondroplasia (Hch), severe achondroplasia with developmental delay and acanthosis nigricans (SADDAN), and thanatophoric dysplasia (TD). FGFR3 is expressed in chondrocytes and mature osteoblasts where it functions to regulate bone growth. Analysis of the mutations in FGFR3 revealed increased signaling through a combination of mechanisms that include stabilization of the receptor, enhanced dimerization, and enhanced tyrosine kinase activity. Paradoxically, increased FGFR3 signaling profoundly suppresses proliferation and maturation of growth plate chondrocytes resulting in decreased growth plate size, reduced trabecular bone volume, and resulting decreased bone elongation. In this review, we discuss the molecular mechanisms that regulate growth plate chondrocytes, the pathogenesis of Ach, and therapeutic approaches that are being evaluated to improve endochondral bone growth in people with Ach and related conditions. Developmental Dynamics 246:291-309, 2017. © 2016 Wiley Periodicals, Inc.

Keywords: FGF; FGFR3; achondroplasia; chondrogenesis; endochondral ossification; fibroblast growth factor receptor; growth plate; hypochondroplasia; skeletal dysplasia; thanatophoric dysplasia; therapy.

Figure 1. Histological organization of the postnatal growth plate

A. Histological section of the mouse proximal tibia showing growth plate chondrocytes at different stages of differentiation (resting, proliferating, prehypertrophic, and hypertrophic), perichondrium, and trabecular and cortical bone. B. Schematic of the postnatal growth plate showing progression of chondrocyte development and juxtaposition to trabecular and cortical bone, the groove of Ranvier and ring of LaCroix, and the secondary ossification center. C. Fgfr3 expression (in situ hybridization) in proliferating and prehypertrophic chondrocytes and trabecular osteoblasts in a 21-day-old mouse tibia (image courtesy of K. Karuppaiah). SOC, secondary ossification center; RC, Reserve chondrocyte zone; PC, Proliferating chondrocyte zone; PHC, Prehypertrophic chondrocyte zone; HC, Hypertrophic chondrocyte zone; TB, trabecular bone.

Figure 2. Signaling pathways in the postnatal growth plate

A. During endochondral bone development, FGF9 and FGF18, derived from the perichondrium and surrounding tissue, signal to FGFR3 in chondrocytes. The balance of chondrocyte proliferation and differentiation is controlled by crosstalk of several signaling pathways. Expression of FGFR3 is enhanced by thyroid hormone (T₃) and suppressed by PTHLH. FGFR3 signaling results in increased expression of Snail1, which is required for activation of STAT1 and MAPK signaling. Signaling from PTHLH, IHH and BMPs antagonizes the suppression of chondrocyte proliferation by FGFR3. Both FGFR3 and PTHLH function to suppress chondrocyte differentiation and antagonize the action of Wnt signaling, which promotes differentiation. FGFR3 negatively regulates the autophagy protein, ATG5. B. Activation of downstream signals, PP2a and STAT1, regulate p107, p21^Waf1/Cip1 activation, respectively, which function to suppress chondrocyte proliferation. Activation of the MAPKs, ERK1 and ERK2, regulate Sox9 expression, which functions to suppress chondrocyte terminal differentiation and endochondral ossification.

Figure 3. Clinical features of skeletal disorders resulting from activating mutations in FGFR3

A. The head of a patient with Ach is characterized by macrocephaly, frontal bossing (arrow), and hypoplasia of the midface. B. MRI (magnetic resonance imaging) showing the cervicomedullary compression at the foramen magnum (arrow). C. Rhizomelic short stature (arrow) of a patient with Ach (image courtesy of Dr. G. Finidori). D. X-rays of the lower limb (femur and tibia) of a 24 week-old normal fetus (control) and fetuses with TDI (p.Arg248Cyst) and TDII (p.Lys650Glu) FGFR3 mutations. Note the short and curved femur compared to the age-matched control.

Figure 4. The mutational spectrum of FGFR3

The relative location of gain-of-function and loss-of-function mutations causing genetic skeletal disease in humans is shown distributed over the entire FGFR3 coding region. Abbreviations for different types of genetic diseases are shown. FGF ligands are shown in blue and heparan sulfate co-factors are shown in green. Some of the mutations in FGFR3 change the affinity or specificity of the receptor for different FGF ligands, while others affect tyrosine kinase activity or receptor internalization and degradation. ECD, extracellular domain; ICD, intracellular domain; HS, heparan sulfate; I, II, III, immunoglobulin-like domains; TK, tyrosine kinase domains; TM, transmembrane domain (red).

Figure 5. Therapeutic approaches for FGFR3-related disorders

A. Schematic representation of key milestones in bone and growth plate activity during skeletal development. The location of the active growth plates and bone sutures are shown in red, according to age. As skeletal development progresses, growth plates and skull sutures fuse (green). B. Tibia intramedullary lengthening in sixteen-year-old girl with Ach using the PRECICE system (image courtesy of Dr. D. Paley). C. Schematic representation of therapeutic approaches for Ach that are currently being evaluated. 1) Soluble FGFR3 bind and sequester FGF ligands, 2) Anti-FGFR3 antibodies block ligand binding to the receptor and subsequent downstream signaling pathways. 3) Tyrosine kinase inhibitors block receptor phosphorylation of substrates. 4) Stabilized CNP (BMN-111) antagonizes RAF activation through the activation of the natriuretic peptide receptor 2 (NPR2), a guanylyl cyclase. cGMP activates cyclic GMP-dependent protein kinase II (cGKII) and p38 MAPK. 5) Meclozine, an anti-emetic drug, suppresses high ERK1/2 phosphorylation. 6) PTH(1-34) treatment leads to increased chondrocyte proliferation and suppression of Fgfr3 expression. 7) Indirect effect of r-hGH on bone growth 8) Statin promotes degradation of FGFR3.