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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.

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.

Background Fibroblast growth factor receptor 3 (FGFR3) gain-of-function mutations play a crucial role in achondroplasia (ACH), thanatophoric dysplasia (TD), and hypochondroplasia (HCH). HCH is a less severe form of dwarfism than ACH, but similarly is caused by gain-of-function mutations in the FGFR3 gene. HCH is characterized by a disproportionate short stature and a growth deficit affecting both endochondral and intramembranous ossification. While multiple therapeutic strategies are being tested for ACH, currently there are no approved therapeutic options for individuals with HCH. We tested the hypothesis that the oral, selective FGFR tyrosine kinase inhibitor (TKI) infigratinib (BGJ398) could improve the HCH phenotype and improve endochondral and intramembranous ossification in a preclinical mouse model of HCH Fgfr3N534K/+. Methods The first Hch mouse model studied expresses the most frequent human mutation p.Asn540Lys (Fgfr3Asn534Lys/+), and exhibits a mild dwarfism and most of the hallmarks of the human pathology. Fgfr3N534K/+ mice received subcutaneous injections of infigratinib or vehicle control every 3 days (1 mg/kg) or daily (1 mg/kg) for 15 days (post-natal day [PND] 4–19) or 21 days (PND 3–24), respectively. Results Fgfr3N534K/+ mice treated with 1 mg/kg infigratinib every 3 days did not show obvious and significant modification of the dwarf phenotype. In contrast, Fgfr3N534K/+ mice treated with 1 mg/kg infigratinib daily for a total of 21 days showed a statistically significant increase in appendicular and axial skeletal measures. Length of the long bones was statistically significantly increased in Fgfr3N534K/+ mice compared with Fgfr3+/+ mice (tibia +3.18%, femur +3.16%, humerus +3.04%, ulna +2.94%, radius +3.01%). Treatment also modified the skull shape (skull width, skull height, nasal bone length and naso-occipital length), the length of the mandible and skull base, as demonstrated by measurement of the foramen magnum (foramen magnum length +3.72%). Infigratinib treatment modified the cartilage growth plate organization, in particular the hypertrophic chondrocyte area. Finally, the high activation of the MAP kinase pathway due to the HCH missense FGFR3 mutation was reduced by treatment, as revealed by the immunolabelling of phosphorylated Erk1/2 proteins. Conclusions Treatment with daily 1 mg/kg infigratinib improved the length and weight of Fgfr3N534K/+ mice and significantly modified the skull and the axial and appendicular skeleton. We demonstrated in Fgfr3N534K/+ mice that infigratinib is able to counteract the constitutive activation of FGFR3 due to the heterozygous N540K mutation localized in the tyrosine kinase 1 domain of the protein. These results provide a rationale for targeting FGFR3 with a specific TKI for the treatment of children with HCH. Presentation: Monday, June 13, 2022 12:30 p.m. - 2:30 p.m.

Disclosure: E. Muslimova: Employee; Self; QED Therapeutics. Stock Owner; Self; QED Therapeutics. B. Demuynck: None. L. Loisay: None. M. Paull: Employee; Self; QED Therapeutics. Stock Owner; Self; QED Therapeutics. L. Legeai-Mallet: None. Background: FGFR3 is a negative regulator of bone growth and gain-of-function mutations in the FGFR3 gene result in different skeletal osteochondrodysplasias, including achondroplasia (ACH) and hypochondroplasia (HCH). ACH is the most common form of rhizomelic short stature, affecting between 1 in 15,000 and 1 in 30,000 live births worldwide. The incidence of HCH is thought to be approximately the same as ACH. Currently, there is only one approved therapy for ACH and no approved therapies for HCH. Previously we demonstrated that the oral, selective FGFR 1-3 tyrosine kinase inhibitor (TKI) infigratinib has preclinical activity in a mouse model mimicking ACH (Fgfr3Y367C/+). We hypothesized that infigratinib could improve defective endochondral ossification and ameliorate the phenotype in a mouse model of HCH (Fgfr3N534K/+), which is the first and only HCH mouse model that expresses the most frequent human heterozygous mutation p.Asn540Lys.Methods:Fgfr3N534K/+ mice were given subcutaneous injections of infigratinib or vehicle control every 3 days (1 mg/kg) or daily (1 mg/kg) for 15 days (post-natal day [PND] 4-19) or 21 days (PND 3-24), respectively. Results:Fgfr3N534K/+ mice treated with 1 mg/kg infigratinib daily for a total of 21 days showed a statistically significant increase in appendicular and axial skeleton (tibia +3.18%, femur +3.16%, humerus +3.04%, ulna +2.94%, radius +3.01%). Treatment with infigratinib modified skull shape, length of the mandible, and foramen magnum length (+3.72%). Cartilage growth plate organization, in particular the hypertrophic chondrocyte area, was modified, indicating that chondrocyte differentiation is improved. These results, although of a lower magnitude, are in line with those previously reported from the Fgfr3Y367C/+ mouse model of ACH, which showed a statistically significant improvement in upper limbs (humerus +7.3%, ulna +11.1%, radius +14.2 %), lower limbs (femur +10.4%, tibia +16.8%), and foramen magnum length (+3.76%), when infigratinib was administered at a dose of 0.5 mg/kg. Conclusions: Low-dose treatment with infigratinib in the Fgfr3N534K/+ HCH mouse model ameliorated the clinical hallmarks of human pathology and significantly lengthened the axial skeleton, the appendicular skeleton and improved foramen magnum length. Development of infigratinib in ACH is currently ongoing. These latest findings with infigratinib in this mouse model of HCH support the rationale for targeting FGFR3 with a specific TKI such as infigratinib for the treatment of children with HCH. Presentation: Saturday, June 17, 2023

Bone marrow adipose tissue (BMAT) is emerging as an important regulator of bone formation and energy metabolism. Lipolysis of BMAT releases glycerol and fatty acid substrates that are catabolized by osteoblasts. Here, we investigated whether BMAT lipolysis is involved in subchondral bone formation in hand osteoarthritis (OA). Subchondral BMAT lipolysis and bone marrow adipocyte (BMAd) morphology were studied in clinical specimens of carpo-metacarpal (CMC1) ans distal interphalangeal joint (DIP) OA. BMAd size, osteoblast numbers and expression of lipolysis enzymes (ATGL, phospho-HSL, MGLL) were compared between regions of low and high bone formation. Free fatty acids, glycerol and bone biomarkers were measured in osteochondral explants. Subchondral BMAd size was positively correlated with BMI and reduced in regions of high bone formation. Osteoblast numbers were negatively correlated with BMAd size. ATGL, phoshpo-HSL and MGLL were expressed in both in BMAds and activated osteoblasts and increased in regions of high bone formation. Secreted glycerol levels, but not free fatty acids, were correlated with bone formation markers pro-collagen type I and alkaline phosphatase. Our findings reveal a previously unrecognized role of BMAT lipolysis in regulating bone formation in hand OA, which may be modulated by BMI.

ABSTRACT 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 (cGMP) in chondrocytes and severe short stature, causing achondroplasia (ACH) and acrosomelic dysplasia type Maroteaux, respectively. Previously we showed that an NPR2 agonist BMN-111 (vosoritide) increases bone growth in mice mimicking ACH (Fgfr3Y367C/+), and that in control growth plate chondrocytes, FGFR3 signaling decreases NPR2 activity by dephosphorylating the NPR2 protein. Here we tested whether a phosphatase inhibitor (LB-100) could enhance bone growth in ACH. In ex vivo imaging experiments using a FRET sensor to measure cGMP production in chondrocytes of living tibias from newborn mice, LB-100 counteracts the FGF-induced dephosphorylation and inactivation of NPR2. In ex vivo experiments with Fgfr3Y367C/+ mice, LB-100 in combination with BMN-111 increases the rate of femur growth by ∼25% vs BMN-111 alone, restores chondrocyte terminal differentiation, increases the proliferative growth plate area of the femur, and reduces the activity of the MAP kinase pathway. 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. Figure1 Figure1 * Download figure * Open in new tab Competing Interest Statement The authors have declared no competing interest.