Explore the vast range of titles available.

Meckel's and condylar cartilages are key to mandible development, with Meckel's cartilage acting as a template and condylar cartilage as a growth center. In achondroplasia, the most common form of genetic dwarfism, abnormalities of these cartilages lead to micrognathia, with significant functional repercussions for affected individuals. How FGFR3 overactivation in achondroplasia disrupts Meckel's and condylar cartilages is largely unknown. Our aim was to identify the pathways driving these disruptions by analyzing the genes expressed in these cartilages in a mouse model mimicking achondroplasia. Using cartilage laser-microdissection and RNA-sequencing analyses, we first compared the transcriptome of Meckel's and condylar cartilages from E16.5 embryos of control and Fgfr3Y367C/+ mice. Over 900 genes were differentially expressed, including the Dkk1 gene, which encodes an inhibitor of β-catenin-dependent Wnt signaling and was significantly overexpressed in chondrocytes of Fgfr3 mutants in both Meckel's and condylar cartilages. Immunostaining of sections of the cartilages confirmed the high expression at the protein level. Primary cultures of Meckel's cartilage chondrocytes showed that, in Fgfr3Y367C/+ mutants, activation of the canonical Wnt pathway with Wnt3a was reduced, while a Dkk1 antagonist increased Wnt activity, suggesting that Dkk1 overexpression was responsible for decreased canonical Wnt activity in mutants. In a mandible organ culture model, inhibition of Dkk1 also significantly increased the mandible size, due to an increased elongation of the condylar cartilage of mutants, as seen after tissue clearing and Sox9 immunolabeling. In this cartilage, increased proliferation and defective differentiation into hypertrophic chondrocytes was partially corrected by Dkk1 inhibition. Our data suggest that dysregulation of Wnt/β-catenin activity due to Fgfr3 gain-of function mutation constitutes an important underlying mechanism in craniofacial defects observed in achondroplasia.

A gain-of-function mutation in the fibroblast growth factor receptor 3 gene (FGFR3) results in achondroplasia (ACH), the most frequent form of dwarfism. Constitutive activation of FGFR3 impairs bone formation and elongation and many signal transduction pathways. Identification of new and relevant compounds targeting the FGFR3 signaling pathway is of broad importance for the treatment of ACH, and natural plant compounds are prime drug candidate sources. Here, we found that the phenolic compound (-)-epicatechin, isolated from Theobroma cacao, effectively inhibited FGFR3's downstream signaling pathways. Transcriptomic analysis in an Fgfr3 mouse model showed that ciliary mRNA expression was modified and influenced significantly by the Indian hedgehog and PKA pathways. (-)-Epicatechin is able to rescue mRNA expression impairments that control both the structural organization of the primary cilium and ciliogenesis-related genes. In femurs isolated from a mouse model (Fgfr3 ) of ACH, we showed that (-)-epicatechin eliminated bone growth impairment during 6 days of ex vivo culture. In vivo, we confirmed that daily subcutaneous injections of (-)-epicatechin to Fgfr3 mice increased bone elongation and rescued the primary cilium defects observed in chondrocytes. This modification to the primary cilia promoted the typical columnar arrangement of flat proliferative chondrocytes and thus enhanced bone elongation. The results of the present proof-of-principle study support (-)-epicatechin as a potential drug for the treatment of ACH.

Impairment of the tightly regulated ossification process leads to a wide range of skeletal dysplasias and deciphering their molecular bases has contributed to the understanding of this complex process. Here, we report a homozygous mutation in the mitochondria-associated granulocyte macrophage colony stimulating factor-signaling gene (MAGMAS) in a novel and severe spondylodysplastic dysplasia. MAGMAS, also referred to as PAM16 (presequence translocase-associated motor 16), is a mitochondria-associated protein involved in preprotein translocation into the matrix. We show that MAGMAS is specifically expressed in trabecular bone and cartilage at early developmental stages and that the mutation leads to an instability of the protein. We further demonstrate that the mutation described here confers to yeast strains a temperature-sensitive phenotype, impairs the import of mitochondrial matrix pre-proteins and induces cell death. The finding of deleterious MAGMAS mutations in an early lethal skeletal dysplasia supports a key role for this mitochondrial protein in the ossification process.

Achondroplasia (ACH) and hypochondroplasia (HCH), the two most common types of dwarfism, are each caused by FGFR3 gain-of-function mutations that result in increased FGFR3 signaling, which disrupts chondrogenesis and osteogenesis, resulting in disproportionately shortened long bones. In this study, TYRA-300, a potent and selective FGFR3 inhibitor, was evaluated in 3 genetic contexts: wild-type mice, the Fgfr3 Y367C/+ mouse model of ACH, and the Fgfr3 N534K/+ mouse model of HCH. In each model, TYRA-300 treatment increased nasoanal length and tibia and femur length. In the two FGFR3-altered models, TYRA-300–induced growth partially restored the disproportionality of long bones. Histologic analysis of the growth plate in Fgfr3 Y367C/+ mice revealed that TYRA-300 mechanistically increased both proliferation and differentiation of chondrocytes. Importantly, children with ACH can experience medical complications due to foramen magnum stenosis, and TYRA-300 significantly improved the size and shape of the skull and foramen magnum in Fgfr3 Y367C/+ mice. Spinal stenosis is also a frequent complication, and TYRA-300 increased the lumbar vertebrae length and improved the shape of the intervertebral discs in both models. Taken together, these studies demonstrate that the selective FGFR3 inhibitor TYRA-300 led to a significant increase in bone growth in two independent FGFR3-driven preclinical models as well as in wild-type mice.

Activating mutations in the fibroblast growth factor receptor 3 (FGFR3) or inactivating mutations in guanylyl cyclase–B (GC-B), also known as NPR-B or Npr2, cause short-limbed dwarfism. FGFR3 activation causes dephosphorylation and inactivation of GC-B, but the contribution of GC-B dephosphorylation to achondroplasia (ACH) is unknown. GC-B 7E/7E mice that express a glutamate-substituted version of GC-B that cannot be inactivated by dephosphorylation were bred with mice expressing FGFR3-G380R, the most common human ACH mutation, to determine if GC-B dephosphorylation is required for ACH. Crossing GC-B 7E/7E mice with FGFR3 G380R/G380R mice increased naso-anal and long (tibia and femur), but not cranial, bone length twice as much as crossing GC-B 7E/7E mice with FGFR3 WT/WT mice from 4 to 16 weeks of age. Consistent with increased GC-B activity rescuing ACH, long bones from the GC-B 7E/7E /FGFR3 G380R/G380R mice were not shorter than those from GC-B WT/WT /FGFR3 WT/WT mice. At 2 weeks of age, male but not female FGFR3 G380R/G380R mice had shorter long bones and smaller growth plate hypertrophic zones, whereas female but not male GC-B 7E/7E mice had longer bones and larger hypertrophic zones. In 2-week-old males, crossing FGFR3 G380R/G380R mice with GC-B 7E/7E mice increased long bone length and hypertrophic zone area to levels observed in mice expressing WT versions of both receptors. We conclude that preventing GC-B dephosphorylation rescues reduced axial and appendicular skeleton growth in a mouse model of achondroplasia.

Hypochondroplasia (HCH) is a mild dwarfism caused by missense mutations in fibroblast growth factor receptor 3 (FGFR3), with the majority of cases resulting from a heterozygous p.Asn540Lys gain-of-function mutation. Here, we report the generation and characterization of the first mouse model ( Fgfr3 Asn534Lys/+ ) of HCH to our knowledge. Fgfr3 Asn534Lys/+ mice exhibited progressive dwarfism and impairment of the synchondroses of the cranial base, resulting in defective formation of the foramen magnum. The appendicular and axial skeletons were both severely affected and we demonstrated an important role of FGFR3 in regulation of cortical and trabecular bone structure. Trabecular bone mineral density (BMD) of long bones and vertebral bodies was decreased, but cortical BMD increased with age in both tibiae and femurs. These results demonstrate that bones in Fgfr3 Asn534Lys/+ mice, due to FGFR3 activation, exhibit some characteristics of osteoporosis. The present findings emphasize the detrimental effect of gain-of-function mutations in the Fgfr3 gene on long bone modeling during both developmental and aging processes, with potential implications for the management of elderly patients with hypochondroplasia and osteoporosis.

Activating mutations in fibroblast growth factor receptor 3 (FGFR3) and inactivating mutations in the natriuretic peptide receptor 2 (NPR2) guanylyl cyclase both result in decreased production of cyclic GMP in chondrocytes and severe short stature, causing achondroplasia (ACH) and acromesomelic dysplasia, type Maroteaux, respectively. Previously, we showed that an NPR2 agonist BMN-111 (vosoritide) increases bone growth in mice mimicking ACH ( Fgfr3 Y367C/+ ). Here, because FGFR3 signaling decreases NPR2 activity by dephosphorylating the NPR2 protein, we tested whether a phosphatase inhibitor (LB-100) could enhance BMN-111–stimulated bone growth in ACH. Measurements of cGMP production in chondrocytes of living tibias, and of NPR2 phosphorylation in primary chondrocytes, showed that LB-100 counteracted FGF-induced dephosphorylation and inactivation of NPR2. In ex vivo experiments with Fgfr3 Y367C/+ mice, the combination of BMN-111 and LB-100 increased bone length and cartilage area, restored chondrocyte terminal differentiation, and increased the proliferative growth plate area, more than BMN-111 alone. The combination treatment also reduced the abnormal elevation of MAP kinase activity in the growth plate of Fgfr3 Y367C/+ mice and improved the skull base anomalies. Our results provide a proof of concept that a phosphatase inhibitor could be used together with an NPR2 agonist to enhance cGMP production as a therapy for ACH.

Achondroplasia (ACH) is the most frequent form of dwarfism and is caused by gain-of-function mutations in the fibroblast growth factor receptor 3-encoding (FGFR3-encoding) gene. Although potential therapeutic strategies for ACH, which aim to reduce excessive FGFR3 activation, have emerged over many years, the use of tyrosine kinase inhibitor (TKI) to counteract FGFR3 hyperactivity has yet to be evaluated. Here, we have reported that the pan-FGFR TKI, NVP-BGJ398, reduces FGFR3 phosphorylation and corrects the abnormal femoral growth plate and calvaria in organ cultures from embryos of the Fgfr3Y367C/+ mouse model of ACH. Moreover, we demonstrated that a low dose of NVP-BGJ398, injected subcutaneously, was able to penetrate into the growth plate of Fgfr3Y367C/+ mice and modify its organization. Improvements to the axial and appendicular skeletons were noticeable after 10 days of treatment and were more extensive after 15 days of treatment that started from postnatal day 1. Low-dose NVP-BGJ398 treatment reduced intervertebral disc defects of lumbar vertebrae, loss of synchondroses, and foramen-magnum shape anomalies. NVP-BGJ398 inhibited FGFR3 downstream signaling pathways, including MAPK, SOX9, STAT1, and PLCγ, in the growth plates of Fgfr3Y367C/+ mice and in cultured chondrocyte models of ACH. Together, our data demonstrate that NVP-BGJ398 corrects pathological hallmarks of ACH and support TKIs as a potential therapeutic approach for ACH.

Abstract Disclosure: J.H. Starrett: None. C. Lemoine: None. M. Guillo: None. C. Fayad: None. N. Kaci: None. M. Neal: None. E. Pettitt: None. M. Pache: None. Q. Ye: None. M.S. Stalvey: None. R. Charlton: None. R.V. Swanson: None. L. Legeai-Mallet: None. Achondroplasia (ACH) and hypochondroplasia (HCH), the two most common types of dwarfism, are each caused by gain-of-function alterations in FGFR3. FGFR3 is expressed in growth plate chondrocytes where it negatively regulates endochondral bone growth. Mutations such as G380R, which causes approximately 99% of ACH, increase FGFR3 signaling, disrupting chondrogenesis and osteogenesis resulting in disproportionately shortened long bones. Beyond the functional challenges related to short stature, people with ACH can experience serious medical complications and require multiple surgeries throughout their lives. TYRA-300, a potent and selective FGFR3 inhibitor, was evaluated in the Fgfr3Y367C/+ mouse model of ACH and the Fgfr3N534K/+ mouse model of HCH. In both models, TYRA-300 treatment increased naso-anal length, tibia and femur length, and partially restored the disproportionality of the long bones. Histologic analysis of the growth plate in Fgfr3Y367C/+ mice revealed that mechanistically TYRA-300 increased both proliferation and differentiation of chondrocytes. Micro-CT analysis of Fgfr3Y367C/+ mice revealed that TYRA-300 treatment increased bone mineral density and bone volume to tissue volume ratio of the femoral metaphyses, suggesting that TYRA-300 improved bone quality and strength in this model. Importantly, children with ACH can experience critical foramen magnum stenosis, and TYRA-300 significantly improved the size and shape of the skull and foramen magnum in Fgfr3Y367C/+ mice. Spinal stenosis is also a frequent complication, and TYRA-300 increased the lumbar vertebrae length and improved the shape of the intervertebral discs in both models. TYRA-300 is currently being evaluated in a Phase 2 trial in children with ACH. BEACH301 is a Phase 2, multicenter, open-label, dose-escalation/dose-expansion study evaluating TYRA-300 in children with ACH, ages 3 to 10, with open growth plates. The primary objectives of this study will be to assess safety and tolerability in children with ACH and evaluate change from baseline in annualized growth velocity to determine the dose(s) for further development. Secondary objectives will include evaluating change from baseline in height z-score, proportionality and pharmacokinetics. Presentation: Sunday, July 13, 2025