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Achondroplasia is the most common of the skeletal dysplasias that result in marked short stature (dwarfism). Although its clinical and radiologic phenotype has been described for more than 50 years, there is still a great deal to be learned about the medical issues that arise secondary to this diagnosis, the manner in which these are best diagnosed and addressed, and whether preventive strategies can ameliorate the problems that can compromise the health and well being of affected individuals. This review provides both an updated discussion of the care needs of those with achondroplasia and an exploration of the limits of evidence that is available regarding care recommendations, controversies that are currently present, and the many areas of ignorance that remain.

Osteoarthritis and rheumatoid arthritis are common cartilage and joint diseases that globally affect more than 200 million and 20 million people, respectively. Several transcription factors have been implicated in the onset and progression of osteoarthritis, including Runx2, C/EBPβ, HIF2α, Sox4, and Sox11. Interleukin-1 β (IL-1β) leads to osteoarthritis through NF-ĸB, IκBζ, and the Zn2+-ZIP8-MTF1 axis. IL-1, IL-6, and tumor necrosis factor α (TNFα) play a major pathological role in rheumatoid arthritis through NF-ĸB and JAK/STAT pathways. Indeed, inhibitory reagents for IL-1, IL-6, and TNFα provide clinical benefits for rheumatoid arthritis patients. Several growth factors, such as bone morphogenetic protein (BMP), fibroblast growth factor (FGF), parathyroid hormone-related protein (PTHrP), and Indian hedgehog, play roles in regulating chondrocyte proliferation and differentiation. Disruption and excess of these signaling pathways cause genetic disorders in cartilage and skeletal tissues. Fibrodysplasia ossificans progressive, an autosomal genetic disorder characterized by ectopic ossification, is induced by mutant ACVR1. Mechanistic target of rapamycin kinase (mTOR) inhibitors can prevent ectopic ossification induced by ACVR1 mutations. C-type natriuretic peptide is currently the most promising therapy for achondroplasia and related autosomal genetic diseases that manifest severe dwarfism. In these ways, investigation of cartilage and chondrocyte diseases at molecular and cellular levels has enlightened the development of effective therapies. Thus, identification of signaling pathways and transcription factors implicated in these diseases is important.

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.

Achondroplasia is a genetic skeletal condition that results in disproportionately short stature and medical complications throughout life. Infigratinib is an orally bioavailable FGFR1-3 selective tyrosine kinase inhibitor in development for achondroplasia. In this phase 2 dose-finding study, we evaluated the safety and efficacy of oral infigratinib in children with achondroplasia between the ages of 3 and 11 years. A total of 72 children were enrolled in five sequential cohorts to receive daily infigratinib at doses of 0.016 mg per kilogram of body weight (cohort 1), 0.032 mg per kilogram (cohort 2), 0.064 mg per kilogram (cohort 3), 0.128 mg per kilogram (cohort 4), and 0.25 mg per kilogram (cohort 5) for 6 months, followed by 12 months of extended treatment in which the dose in cohorts 1 and 2 could be escalated to the next ascending level at months 6 and 12. The primary safety outcome was the incidence of adverse events that led to a decrease in the dose or discontinuation of infigratinib. The primary efficacy outcome was the change from baseline in the annualized height velocity. During treatment, all the children had at least one adverse event, most of which were mild or moderate in severity; none resulted in treatment discontinuation. In cohort 5, an increased annualized height velocity was observed, which persisted throughout the duration of the study, with a mean change from baseline at 18 months of 2.50 cm per year (95% confidence interval [CI], 1.22 to 3.79; P = 0.001). The mean change from baseline in height z score was 0.54 (95% CI, 0.35 to 0.72) relative to an untreated achondroplasia reference population at 18 months; the mean change from baseline in the upper-to-lower body segment ratio was -0.12 (95% CI, -0.18 to -0.06). The administration of oral infigratinib did not result in any apparent major safety signal and increased the annualized height velocity and z score and decreased the upper-to-lower body segment ratio at 18 months of treatment in cohort 5. (Funded by BridgeBio Pharma; PROPEL2 ClinicalTrials.gov number, NCT04265651.).

Achondroplasia, the most prevalent short-stature disorder, is caused by missense variants overactivating the fibroblast growth factor receptor 3 (FGFR3). As current surgical and pharmaceutical treatments only partially improve some disease features, we sought to explore a genetic approach. We show that an enhancer located 29 kb upstream of mouse Fgfr3 ( –29E ) is sufficient to confer a transgenic mouse reporter with a domain of expression in cartilage matching that of Fgfr3 . Its CRISPR/Cas9-mediated deletion in otherwise WT mice reduced Fgfr3 expression in this domain by half without causing adverse phenotypes. Importantly, its deletion in mice harboring the ortholog of the most common human achondroplasia variant largely normalized long bone and vertebral body growth, markedly reduced spinal canal and foramen magnum stenosis, and improved craniofacial defects. Consequently, mouse achondroplasia is no longer lethal, and adults are overall healthy. These findings, together with high conservation of –29E in humans, open a path to develop genetic therapies for people with achondroplasia.

Achondroplasia is a rare genetic disorder caused by mutations in the Fibroblast Growth Factor receptor 3 (FGFR3). These mutations lead to aberrant increase of inhibitory signaling in proliferating chondrocytes at the growth plate. Recifercept is a potential treatment for this disease using a decoy approach to sequester FGFR3 ligands subsequently normalizing activation of the mutated FGFR3 receptor. Recifercept binds to FGF isoforms in vitro and in cellular model systems and reduces FGFR3 signaling. In addition, in a transgenic mouse model of achondroplasia, Recifercept restores reduced body weight and long bone growth in these mice. These data suggest that Recifercept treatment could lead to clinical benefits in children treated with this molecule.

Achondroplasia, associated with gain-of-function mutations in FGFR3 , causes growth plate cartilage dysfunction, resulting in short-limb dwarfism. However, its precise molecular and cellular mechanisms remain unclear. To address this, we aimed to generate knock-in mice ( Fgfr3 Ach ) harboring the achondroplasia mutation (p.Gly380Arg). In addition to previously reported abnormalities, we observe an expansion of the resting zone. EdU labeling and lineage tracing analyses indicate that disruption of turnover and impairment of stem cell-like behavior of resting zone chondrocytes results in accumulation of cells in the resting zone. Single-cell RNA-seq and immunohistochemical analysis identify a cell cluster that corresponds to the expanded resting zone. Pathway analysis and functional experiments reveal that CREB disrupts stem cell-like properties in resting zone chondrocytes and contributes to dwarfism. Administration of CREB inhibitor 666-15 restores growth plate pathology and bone length. These findings demonstrate that excess FGFR3 signaling disrupts resting zone chondrocyte properties and suggest potential therapeutic targets for achondroplasia. Achondroplasia, associated with FGFR3 gain-of-function mutations, is a chondrodysplasia characterized by short-limb dwarfism. Here, Horike et al. discover that excess FGFR3 signaling is mediated by CREB and disrupts resting zone chondrocyte properties in a knock-in mouse model harboring the achondroplasia mutation.

Pseudoachondroplasia (PSACH) and multiple epiphyseal dysplasia (MED) are rare, autosomal dominant skeletal dysplasias characterised by disproportionate short stature, joint deformities, and early-onset osteoarthritis. These conditions result from mutations in key cartilage extracellular matrix (ECM) components, including cartilage oligomeric matrix protein (COMP), matrilin-3, and type IX collagen. Although genetically and clinically heterogeneous, PSACH and MED share convergent pathogenic mechanisms. Misfolded mutant ECM proteins are retained within the endoplasmic reticulum (ER) of growth plate chondrocytes, triggering chronic ER stress and impairing chondrocyte proliferation, differentiation, and survival. Moreover, some of the mutant protein is secreted and incorporated into the matrix, leading to altered collagen fibrillogenesis, disrupted proteoglycan distribution, and compromised biomechanical integrity. These alterations extend beyond cartilage, impacting tendons, ligaments, and muscle–tendon junctions, contributing to joint laxity, impaired force transmission, and mild myopathy. This review discusses the structural and functional consequences of ECM disorganisation in PSACH and MED, highlighting its central role in disease progression and emphasising the importance of considering ECM abnormalities when developing therapeutic strategies for rare short stature-associated skeletal disorders.

Introduction Achondroplasia (ACH) is a common skeletal dysplasia associated with short-limbed short stature caused by gain-of-function mutations in the fibroblast growth factor receptor 3 ( FGFR3 ) gene. Meclozine was found to inhibit FGFR3 signaling using a drug repositioning strategy. In some countries, growth hormone (GH) has been employed to ameliorate short stature in children with ACH. This study aims to investigate the effects of meclozine and GH on bone growth and quality using an experimental model of ACH. Materials and methods Meclozine (2 mg/kg/day) and/or GH (0.35 mg/kg/day) were administered to a mouse model of ACH from the age of 7 to 56 days. Body length and body weight of each mouse were measured during these treatments. At the end of treatments, these mice were subjected to micro-computed tomography scans to measure the lengths of long bones and bone mineral density (BMD). The width of the growth plate was quantified by histological analysis. Results The body and bone length of transgenic mice significantly increased after treatment with meclozine and GH, although there was no additive effect of the combination therapy on promoting bone growth. In contrast, BMD was additively increased by the combination therapy. The width of the growth plate in transgenic mice was significantly increased by both treatments, although the hypertrophic zone was enlarged by meclozine but not by GH. Conclusion Meclozine or GH may be an option for treating children with ACH to ameliorate bone length and quality, but the additive effect would be limited.

The intracellular retention of misfolded extracellular matrix proteins is a common disease mechanism in various rare skeletal diseases. This discovery has driven the study of ER stress and the unfolded protein response (UPR) as a promising therapeutic target in several skeletal dysplasias. In the case of COL10A1 mutations, targeting the UPR resulted in a clinical trial of the repurposed drug carbamazepine; however, for other closely related skeletal disorders, treatment with carbamazepine was ineffective, indicating the need for suitable markers for in vitro screenings of potential drug treatments. Mutations in cartilage oligomeric matrix protein (COMP), a cartilage structural protein, cause both multiple epiphyseal dysplasia (MED) and pseudoachondroplasia (PSACH); together referred to as the COMPopathies, which result from the intracellular retention of mutant COMP to varying degrees. In contrast to other closely related skeletal disorders, caused by mutations in cartilage structural proteins, the involvement of the UPR is less clear, and so far, no common COMPopathy marker has been identified. Here, using cell models of COMPopathies, we identified MMP9 upregulation as a common feature of six pathogenic COMP variants that do not induce a prominent UPR. We further show that the archetypal p.V194D matrilin-3 MED variant (which causes MED) does not induce MMP9 expression, suggesting that MMP9 upregulation could serve as a specific marker of COMPopathies in vitro.