Mutant activated FGFR3 impairs endochondral bone growth by preventing SOX9 downregulation in differentiating chondrocytes

Advances in the mechanism and therapies of achondroplasia

Achondroplasia (ACH), is the prevailing type of genetic dwarfism in humans, caused by mutations in fibroblast growth factor receptor 3 (FGFR3) that are inherited in an autosomal dominant manner. FGFR3 is mainly expressed in condensed mesenchyme, chondrocytes, and mature osteoblasts and osteoclasts, in which it regulates the formation, development, growth, and remodeling of the skeletal system. Mutations in FGFR3 causing ACH result in enhanced FGFR3 signaling through combined mechanisms including enhancing FGF dimerization and tyrosine kinase activity and stabilizing FGF receptors. In ACH, suppression of the proliferation and maturation of chondrocytes in the growth plate leads to a notable reduction in growth plate size, trabecular bone volume, and bone elongation through a profound enhancement of FGFR3 signaling. This review aims to comprehensively outline the cellular and molecular mechanisms contributing to the pathological process of ACH and its potential therapeutic interventions.

Syndrome-informed phenotyping identifies a polygenic background for achondroplasia-like facial variation in the general population

Human craniofacial shape is highly variable yet highly heritable with numerous genetic variants interacting through multiple layers of development. Here, we hypothesize that Mendelian phenotypes represent the extremes of a phenotypic spectrum and, using achondroplasia as an example, we introduce a syndrome-informed phenotyping approach to identify genomic loci associated with achondroplasia-like facial variation in the general population. We compare three-dimensional facial scans from 43 individuals with achondroplasia and 8246 controls to calculate achondroplasia-like facial scores. Multivariate GWAS of the control scores reveals a polygenic basis for facial variation along an achondroplasia-specific shape axis, identifying genes primarily involved in skeletal development. Jointly modeling these genes in two independent control samples, both human and mouse, shows craniofacial effects approximating the characteristic achondroplasia phenotype. These findings suggest that both complex and Mendelian genetic variation act on the same developmentally determined axes of facial variation, providing insights into the genetic intersection of complex traits and Mendelian disorders.

The IFITM5 mutation in osteogenesis imperfecta type V is associated with an ERK/SOX9-dependent osteoprogenitor differentiation defect

Osteogenesis imperfecta (OI) type V is the second most common form of OI, distinguished by hyperplastic callus formation and calcification of the interosseous membranes, in addition to the bone fragility. It is caused by a recurrent, dominant pathogenic variant (c.-14C>T) in interferon-induced transmembrane protein 5 (IFITM5). Here, we generated a conditional Rosa26-knockin mouse model to study the mechanistic consequences of the recurrent mutation. Expression of the mutant Ifitm5 in osteo-chondroprogenitor or chondrogenic cells resulted in low bone mass and growth retardation. Mutant limbs showed impaired endochondral ossification, cartilage overgrowth, and abnormal growth plate architecture. The cartilage phenotype correlates with the pathology reported in patients with OI type V. Surprisingly, expression of mutant Ifitm5 in mature osteoblasts caused no obvious skeletal abnormalities. In contrast, earlier expression in osteo-chondroprogenitors was associated with an increase in the skeletal progenitor cell population within the periosteum. Lineage tracing showed that chondrogenic cells expressing the mutant Ifitm5 had decreased differentiation into osteoblastic cells in diaphyseal bone. Moreover, mutant IFITM5 disrupted early skeletal homeostasis in part by activating ERK signaling and downstream SOX9 protein, and inhibition of these pathways partially rescued the phenotype in mutant animals. These data identify the contribution of a signaling defect altering osteo-chondroprogenitor differentiation as a driver in the pathogenesis of OI type V.

Syndrome-informed phenotyping identifies a polygenic background for achondroplasia-like facial variation in the general population

Human craniofacial shape is highly variable yet highly heritable with genetic variants interacting through multiple layers of development. Here, we hypothesize that Mendelian phenotypes represent the extremes of a phenotypic spectrum and, using achondroplasia as an example, we introduce a syndrome-informed phenotyping approach to identify genomic loci associated with achondroplasia-like facial variation in the normal population. We compared three-dimensional facial scans from 43 individuals with achondroplasia and 8246 controls to calculate achondroplasia-like facial scores. Multivariate GWAS of the control scores revealed a polygenic basis for normal facial variation along an achondroplasia-specific shape axis, identifying genes primarily involved in skeletal development. Jointly modeling these genes in two independent control samples showed craniofacial effects approximating the characteristic achondroplasia phenotype. These findings suggest that both complex and Mendelian genetic variation act on the same developmentally determined axes of facial variation, providing new insights into the genetic intersection of complex traits and Mendelian disorders.

Transcriptomics reveals the molecular regulation of Chinese medicine formula on improving bone quality in broiler

Skeletal disorder is of concern to the poultry industry as it affects animal welfare and production performance. Traditional Chinese medicine could improve bone quality and reduce the incidence of bone disease, but the molecular regulation of Chinese medicine formula (CMF) on improving bone quality in broilers is still unclear. This study was performed to research the effects of CMF on skeletal performance of Cobb broilers and reveal the molecular regulation. A total of 120 one-day-old Cobb broilers were randomly allocated into 4 equal groups of 30 chickens, with 5 replicates and 6 chickens in each replicate. The control (CON) group was fed a diet without CMF, while the CMF1, CMF2, and CMF3 groups were supplemented with different CMF at 6,000 mg/kg diet, respectively. The broilers were raised to 60 d of age, then bone tissues were collected for biomechanical properties, micro-CT detection and transcriptomic sequencing analysis. The results showed that CMF3 improved the biomechanical properties of broiler tibia, via increasing the elastic modulus (P < 0.05), yield strength (P > 0.05), maximum stress (P < 0.05) and fracture stress (P < 0.05) of the tibia. Micro-CT analysis indicated that CMF3 increased the bone mineral density (BMD), bone volume/total volume (BV/TV), bone surface density (BS/TV), trabecular number (Tb.N), trabecular thickness (Tb.Th), and decreased the trabecular separation (Tb.Sp) of femur cancellous bone (P < 0.05). RNA-seq analysis revealed 2,177 differentially expressed genes (DEGs) (|log2FoldChange| ≥ 1, FDR < 0.05) between the CMF3 group and CON group. Kyoto Encyclopedia of Genes and Genomes pathway (KEGG) analysis showed 13 pathways mostly associated with bone growth and development and bone metabolism, and we identified 39 bone-related DEGs. This study suggests that CMF3 could improve bone strength and bone microstructure of broilers, and showed a positive effect on bone performance. Our research could provide a theoretical reference for the development of pollution-free feed additives to improve the skeletal performance of broilers, which could help promote healthy farming of chickens.

Embryonic cranial cartilage defects in the Fgfr3Y367C /+ mouse model of achondroplasia

Achondroplasia, the most common chondrodysplasia in humans, is caused by one of two gain of function mutations localized in the transmembrane domain of fibroblast growth factor receptor 3 (FGFR3) leading to constitutive activation of FGFR3 and subsequent growth plate cartilage and bone defects. Phenotypic features of achondroplasia include macrocephaly with frontal bossing, midface hypoplasia, disproportionate shortening of the extremities, brachydactyly with trident configuration of the hand, and bowed legs. The condition is defined primarily on postnatal effects on bone and cartilage, and embryonic development of tissues in affected individuals is not well studied. Using the Fgfr3Y367C/+ mouse model of achondroplasia, we investigated the developing chondrocranium and Meckel's cartilage (MC) at embryonic days (E)14.5 and E16.5. Sparse hand annotations of chondrocranial and MC cartilages visualized in phosphotungstic acid enhanced three-dimensional (3D) micro-computed tomography (microCT) images were used to train our automatic deep learning-based 3D segmentation model and produce 3D isosurfaces of the chondrocranium and MC. Using 3D coordinates of landmarks measured on the 3D isosurfaces, we quantified differences in the chondrocranium and MC of Fgfr3Y367C/+ mice relative to those of their unaffected littermates. Statistically significant differences in morphology and growth of the chondrocranium and MC were found, indicating direct effects of this Fgfr3 mutation on embryonic cranial and pharyngeal cartilages, which in turn can secondarily affect cranial dermal bone development. Our results support the suggestion that early therapeutic intervention during cartilage formation may lessen the effects of this condition.

Candidate genes for obstructive sleep apnea in non-syndromic children with craniofacial dysmorphisms - a narrative review

Pediatric obstructive sleep apnea (POSA) is a complex disease with multifactorial etiopathogenesis. The presence of craniofacial dysmorphisms influencing the patency of the upper airway is considered a risk factor for POSA development. The craniofacial features associated with sleep-related breathing disorders (SRBD) - craniosynostosis, retrognathia and micrognathia, midface and maxillary hypoplasia - have high heritability and, in a less severe form, could be also found in non-syndromic children suffering from POSA. As genetic factors play a role in both POSA and craniofacial dysmorphisms, we hypothesize that some genes associated with specific craniofacial features that are involved in the development of the orofacial area may be also considered candidate genes for POSA. The genetic background of POSA in children is less explored than in adults; so far, only one genome-wide association study for POSA has been conducted; however, children with craniofacial disorders were excluded from that study. In this narrative review, we discuss syndromes that are commonly associated with severe craniofacial dysmorphisms and a high prevalence of sleep-related breathing disorders (SRBD), including POSA. We also summarized information about their genetic background and based on this, proposed 30 candidate genes for POSA affecting craniofacial development that may play a role in children with syndromes, and identified seven of these genes that were previously associated with craniofacial features risky for POSA development in non-syndromic children. The evidence-based approach supports the proposition that variants of these candidate genes could lead to POSA phenotype even in these children, and, thus, should be considered in future research in the general pediatric population.

History and highlights of the teratological collection in the Narrenturm, Vienna (Austria)

The collection of the Narrenturm in Vienna houses and maintains more than 50,000 objects including approximately 1200 teratological specimens; making it one of the biggest collections of specimens from human origin in Europe. The existence of this magnificent collection-representing an important resource for dysmorphology research, mostly awaiting contemporary diagnoses-is not widely known in the scientific community. Here, we show that the Narrenturm harbors a wealth of specimens with (exceptionally) rare congenital anomalies. These museums can be seen as physical repositories of human malformation, covering hundreds of years of dedicated collecting and preserving, thereby creating unique settings that can be used to expand our knowledge of developmental conditions that have to be preserved for future generations of scientists.

Phase 1 safety, tolerability, pharmacokinetics and pharmacodynamics results of a long-acting C-type natriuretic peptide prodrug, TransCon CNP

AIM

TransCon CNP is a novel prodrug designed to provide sustained release of C-type natriuretic peptide (CNP) for once-weekly therapy, addressing the pathology leading to aberrant skeletal development in achondroplasia. This phase 1 trial was initiated to assess the safety, tolerability, pharmacodynamics (PD) and pharmacokinetics (PK) of TransCon CNP.

METHODS

This randomized, placebo-controlled, single-ascending dose phase 1 trial was performed at two sites in Australia and enrolled 45 healthy adult males. Subjects received placebo or TransCon CNP (single-ascending dose cohorts [3, 10, 25, 75 or 150 μg CNP/kg]). The primary endpoint was frequency of adverse events and other safety outcomes. Other endpoints included PK and PD measured by cyclic guanosine-monophosphate (cGMP) and amino-terminal propeptide of CNP (NTproCNP).

RESULTS

TransCon CNP provided continuous systemic exposure to CNP over at least 7 days post-dose. Plasma and urine levels of cGMP were significantly increased in subjects administered TransCon CNP at 75-150 μg CNP/kg, indicating target engagement of active CNP at the natriuretic peptide receptor-B (NPR-B) for at least 1 week post-dose. TransCon CNP was well-tolerated, with no serious treatment-emergent adverse events or discontinuations. Extensive cardiac safety assessments did not reveal any clinically relevant effects on electrocardiogram parameters, including heart rate, PR, QRS and QTcF intervals.

CONCLUSIONS

Safety and PD data from this phase 1 trial support that TransCon CNP is well tolerated, with a PK profile compatible with a once-weekly dosing regimen. Further studies are ongoing to evaluate the potential of TransCon CNP to positively impact abnormal endochondral ossification in children with achondroplasia.

Expanding horizons of achondroplasia treatment: current options and future developments

Activating mutations in the FGFR3 receptor tyrosine kinase lead to most prevalent form of genetic dwarfism in humans, the achondroplasia. Many features of the complex function of FGFR3 in growing skeleton were characterized, which facilitated identification of therapy targets, and drove progress toward treatment. In August 2021, the vosoritide was approved for treatment of achondroplasia, which is based on a stable variant of the C-natriuretic peptide. Other drugs may soon follow, as several conceptually different inhibitors of FGFR3 signaling progress through clinical trials. Here, we review the current achondroplasia therapeutics, describe their mechanisms, and illuminate motivations leading to their development. We also discuss perspectives of curing achondroplasia, and options for repurposing achondroplasia drugs for dwarfing conditions unrelated to FGFR3.

Integrated RNA-Seq Analysis Uncovers the Potential Mechanism of the “Kidney Governing Bones” Theory of TCM

Background. As in philosophy of traditional Chinese medicine (TCM), the theory of “kidney governing bones” has been demonstrated by a series of scientific studies. Furthermore, many groups including ours have explored the molecular mechanisms related to bone development, growth, and regeneration using modern biology technologies, such as RNA sequencing (RNA-Seq) and isobaric tags for relative and absolute quantification (ITRAQ), and have demonstrated that the underlying molecular mechanisms were highly consistent with the “kidney governing bones” theory. Objective. Kidney-yang deficiency (YD), as a pathological condition, has a passive effect on the skeleton growth; more specifically, it is a state of skeletal metabolic disorder. However, the exact molecular mechanisms related to the “kidney governing bones” theory under the control of multiple organs and systems are still unknown. Methods. In this study, we performed RNA-Seq analysis to investigate and compare the gene expression patterns of six types of tissue (bone, cartilage, kidney, testicle, thyroid gland, and adrenal gland) from YD rats and normal rats and analyzed the interaction effects controlled by multiple functional genes and signaling pathways between those tissues. Results. Our results showed that, in the state of YD, the functions of bone and cartilage were inhibited. Furthermore, multiple organs involving the reproductive, endocrine, and urinary systems were also investigated, and our results showed that YD could cause dysfunctions of these systems by downregulating multiple functional genes and signaling pathways that positively regulate the homeostasis of these tissues. Conclusion. We ensure that “kidney governing bones” was not a simple change in a single gene but the changes in complex biological networks caused by functional changes in multiple genes. This also coincides with the holistic view of TCM, which holds that the human body itself is an organic whole and the functional activities of each organ coordinate with each other.

The Induced Pluripotent Stem Cells in Articular Cartilage Regeneration and Disease Modelling: Are We Ready for Their Clinical Use?

The development of induced pluripotent stem cells has brought unlimited possibilities to the field of regenerative medicine. This could be ideal for treating osteoarthritis and other skeletal diseases, because the current procedures tend to be short-term solutions. The usage of induced pluripotent stem cells in the cell-based regeneration of cartilage damages could replace or improve on the current techniques. The patient’s specific non-invasive collection of tissue for reprogramming purposes could also create a platform for drug screening and disease modelling for an overview of distinct skeletal abnormalities. In this review, we seek to summarise the latest achievements in the chondrogenic differentiation of pluripotent stem cells for regenerative purposes and disease modelling.

The hypertrophic chondrocyte: To be or not to be

Hypertrophic chondrocytes are the master regulators of endochondral ossification; however, their ultimate cell fates cells remain largely elusive due to their transient nature. Historically, hypertrophic chondrocytes have been considered as the terminal state of growth plate chondrocytes, which are destined to meet their inevitable demise at the primary spongiosa. Chondrocyte hypertrophy is accompanied by increased organelle synthesis and rapid intracellular water uptake, which serve as the major drivers of longitudinal bone growth. This process is delicately regulated by major signaling pathways and their target genes, including growth hormone (GH), insulin growth factor-1 (IGF-1), indian hedgehog (Ihh), parathyroid hormone-related protein (PTHrP), bone morphogenetic proteins (BMPs), sex determining region Y-box 9 (Sox9), runt-related transcription factors (Runx) and fibroblast growth factor receptors (FGFRs). Hypertrophic chondrocytes orchestrate endochondral ossification by regulating osteogenic-angiogenic and osteogenic-osteoclastic coupling through the production of vascular endothelial growth factor (VEGF), receptor activator of nuclear factor kappa-B ligand (RANKL) and matrix metallopeptidases-9/13 (MMP-9/13). Hypertrophic chondrocytes also indirectly regulate resorption of the cartilaginous extracellular matrix, by controlling formation of a special subtype of osteoclasts termed "chondroclasts". Notably, hypertrophic chondrocytes may possess innate potential for plasticity, reentering the cell cycle and differentiating into osteoblasts and other types of mesenchymal cells in the marrow space. We may be able to harness this unique plasticity for therapeutic purposes, for a variety of skeletal abnormalities and injuries. In this review, we discuss the morphological and molecular properties of hypertrophic chondrocytes, which carry out important functions during skeletal growth and regeneration.

Molecular Mechanisms of Chondrocyte Proliferation and Differentiation

Cartilage is a kind of connective tissue that buffers pressure and is essential to protect joint movement. It is difficult to self-recover once cartilage is damaged due to the lack of blood vessels, lymph, and nerve tissues. Repair of cartilage injury is mainly achieved by stimulating chondrocyte proliferation and extracellular matrix (ECM) synthesis. Cartilage homeostasis involves the regulation of multiple growth factors and the transduction of cellular signals. It is a very complicated process that has not been elucidated in detail. In this review, we summarized a variety of signaling molecules related to chondrocytes function. Especially, we described the correlation between chondrocyte-specific regulatory factors and cell signaling molecules. It has potential significance for guiding the treatment of cartilage injury.

Management of obstructive sleep apnea in children with achondroplasia: Outcomes of surgical interventions

INTRODUCTION

Children with achondroplasia are predisposed to obstructive sleep apnea (OSA), however little is known regarding surgical interventions and outcomes for this condition. The aim of this study was to evaluate the severity of OSA in children with achondroplasia and report outcomes of surgical interventions using polysomnography (PSG) parameters.

METHODS

Retrospective chart review of children with achondroplasia with documented OSA from 2002 to 2018 that had pre- and post-operative PSG results. Additional data extracted included age, gender, and type of surgical interventions. The primary outcome was change in postoperative obstructive apnea hypopnea index (OAHI).

RESULTS

Twenty-two children with achondroplasia were identified that underwent formal PSG before and after confirmed OSA. The median age was 12 months (range 4 days-15.3 years, IQR 2 years) at time of initial PSG evaluation. The majority (72.7%) of patients had severe OSA with a median preoperative OAHI of 14.25 (IQR 9.4). The most common surgical intervention was adenotonsillectomy (n = 15). Multilevel surgical intervention was required in 9 (41.0%) patients. Post-operatively, 16 (72.7%) children experienced a reduction in OAHI, of which four (18.2%) had complete OSA resolution. OAHI scores increased in six (27.3%) children. Patients with the most severe OSA at baseline had greater improvements in post-operative OAHI (P < 0.01). Neither type nor number of surgical interventions was associated with improved outcomes (P = 0.51, P = 0.89 respectively).

CONCLUSIONS

Treatment of OSA in children with achondroplasia remains challenging. Although reduction of OAHI is possible, caregivers should be counseled about the likelihood of persistent OSA and the potential for multilevel airway surgery.

Deciphering the potential pharmaceutical mechanism of Guzhi Zengsheng Zhitongwan on rat bone and kidney based on the "kidney governing bone" theory

BACKGROUND

Guzhi Zengsheng Zhitongwan (GZZSZTW) is an effective Chinese medicinal formulation for the treatment of osteoarthritis (OA) designed according to the "kidney governing bone" theory, which has been widely used as a golden guide for treating bone and cartilage diseases in traditional Chinese medicine. The aim of this study was to explore the molecular mechanism underlying its effects on the bone and kidney.

METHODS

Preparation and quality control were performed as previously described. Since GZZSZTW is orally administered in the form of pills prepared in boiled water, the Chinese materia medica (CMM) mixture of this formula was extracted with distilled water by a reflux method and was then filtered through a 0.45-μm Hollow Fiber Cartridge (GE Healthcare, USA). The filtrate was freeze-dried by a Heto PowerDry LL3000 Freeze Dryer (Thermo, USA) and stored at - 80 °C. The effects of GZZSZTW on gene expression and regulation of both kidney and bone tissues were investigated using a state-of-the-art RNA-seq technology.

RESULTS

We demonstrated that GZZSZTW could enhance kidney function and suppress bone formation and resorption by modulating the activities of osteoblast and osteoclast, and might subsequently contribute to the inhibition of osteophyte formation during the process of OA. These effects might be achieved by the synergistic interactions of various herbs and their active components in GZZSZTW, which increased the expression levels of functional genes participating in kidney function, regulation, and repair, and then decreased the expression levels of genes involved in bone formation and resorption. Thus, our findings were consistent with the "kidney governing bone" theory, which has been widely used as a guide in clinical practice for thousands of years.

CONCLUSIONS

This study has deepened the current knowledge about the molecular effects of GZZSZTW on bone and kidney regulation. Furthermore, this study might be able to provide possible strategies to further prevent and treat joint diseases by using traditional Chinese medicinal formulations following the "kidney governing bone" theory.

Congenital Spinal Canal Stenosis with Ossification of the Ligamentum Flavum in an Achondroplastic Patient: A Case Report and Literature Review

Achondroplasia has an effect on intracartilaginous ossification during the development of the spine resulting in a narrow spinal canal. This abnormal anatomy could make an achondroplastic patient tend to have spinal canal stenosis. We reported a case of congenital spinal canal stenosis with achondroplasia combined with ossified ligamentum flavum (OLF) at the thoracolumbar and lumbar spine, which was treated by decompressive surgery. We reported a 52-year-old Thai male with achondroplasia presented with progressive myelopathy and neurogenic claudication due to spinal canal stenosis. Spinal canal stenosis was observed at T10/11 and L1-L5 and OLF at T10/11 through L5 varying in size. Laminectomy and removal of the OLF were performed at T11 and L1-L5. The patient's neurological symptom improved after the surgery. He could walk with a walker at the time of 6-month follow-up postoperatively. In this report, we describe a rare case of achondroplasia with OLF presenting with progressive myelopathy and claudication symptoms from multiple levels of spinal canal stenosis. Laminectomy, removal of the ossified ligament, and fusion with instrumentation resulted in the improvement of the patient's neurological symptoms and function.

Glucocorticoids Decrease Longitudinal Bone Growth in Pediatric Kidney Transplant Recipients by Stimulating the FGF23/FGFR3 Signaling Pathway

Renal transplantation (RTx) is an effective therapy to improve clinical outcomes in pediatric patients with terminal chronic kidney disease. However, chronic immunosuppression with glucocorticoids (GCs) reduces bone growth and BMD. The mechanisms causing GC-induced growth impairment have not been fully clarified. Fibroblast growth factor 23 (FGF23) is a peptide hormone that regulates phosphate homeostasis and bone growth. In pathological conditions, FGF23 excess or abnormal FGF receptors (FGFR) activity leads to bone growth impairment. Experimental data indicate that FGF23 expression is induced by chronic GC exposure. Therefore, we hypothesize that GCs impair bone growth by increasing FGF23 expression, which has direct effects on bone growth plate. In a post hoc analysis of a multicentric randomized clinical trial of prepubertal RTx children treated with early GC withdrawal or chronic GC treatment, we observed that GC withdrawal was associated with improvement in longitudinal growth and BMD, and lower plasma FGF23 levels as compared with a chronic GC group. In prepubertal rats, GC-induced bone growth retardation correlated with increased plasma FGF23 and bone FGF23 expression. Additionally, GC treatment decreased FGFR1 expression whereas it increased FGFR3 expression in mouse tibia explants. The GC-induced bone growth impairment in tibiae explants was prevented by blockade of FGF23 receptors using either a pan-FGFR antagonist (PD173074), a C-terminal FGF23 peptide (FGF23180-205) which blocks the binding of FGF23 to the FGFR-Klotho complex or a specific FGFR3 antagonist (P3). Finally, local administration of PD173074 into the tibia growth plate ameliorated cartilage growth impairment in GC-treated rats. These results show that GC treatment partially reduces longitudinal bone growth via upregulation of FGF23 and FGFR3 expression, thus suggesting that the FGF23/Klotho/FGFR3 axis at the growth plate could be a potential therapeutic target for the management of GC-induced growth impairment in children.

SOX9 in cartilage development and disease

SOX9 is a pivotal transcription factor in chondrocytes, a lineage essential in skeletogenesis. Its mandatory role in transactivating many cartilage-specific genes is well established, whereas its pioneer role in lineage specification, which along with transactivation defines master transcription factors, remains to be better defined. Abundant, but yet incomplete evidence exists that intricate molecular networks control SOX9 activity during the multi-step chondrogenesis pathway. They include a highly modular genetic regulation, post-transcriptional and post-translational modifications, and varying sets of functional partners. Fully uncovering SOX9 actions and regulation is fundamental to explain mechanisms underlying many diseases that directly or indirectly affect SOX9 activities and to design effective disease treatments. We here review current knowledge, highlight recent discoveries, and propose new research directions to answer remaining questions.

Suppressing UPR-dependent overactivation of FGFR3 signaling ameliorates SLC26A2-deficient chondrodysplasias

Background

Mutations in the SLC26A2 gene cause a spectrum of currently incurable human chondrodysplasias. However, genotype-phenotype relationships of SLC26A2-deficient chondrodysplasias are still perplexing and thus stunt therapeutic development.

Methods

To investigate the causative role of SLC26A2 deficiency in chondrodysplasias and confirm its skeleton-specific pathology, we generated and analyzed slc26a2^−/− and Col2a1-Cre; slc26a2^fl/fl mice. The therapeutic effect of NVP-BGJ398, an FGFR inhibitor, was tested with both explant cultures and timed pregnant females.

Findings

Two lethal forms of human SLC26A2-related chondrodysplasias, achondrogenesis type IB (ACG1B) and atelosteogenesis type II (AO2), are phenocopied by slc26a2^−/− mice. Unexpectedly, slc26a2^−/− chondrocytes are defective for collagen secretion, exhibiting intracellular retention and compromised extracellular deposition of ColII and ColIX. As a consequence, the ATF6 arm of the unfolded protein response (UPR) is preferentially triggered to overactivate FGFR3 signaling by inducing excessive FGFR3 in slc26a2^−/− chondrocytes. Consistently, suppressing FGFR3 signaling by blocking either FGFR3 or phosphorylation of the downstream effector favors the recovery of slc26a2^−/− cartilage cultures from impaired growth and unbalanced cell proliferation and apoptosis. Moreover, administration of an FGFR inhibitor to pregnant females shows therapeutic effects on pathological features in slc26a2^−/− newborns. Finally, we confirm the skeleton-specific lethality and pathology of global SLC26A2 deletion through analyzing the Col2a1-Cre; slc26a2^fl/fl mouse line.

Interpretation

Our study unveils a previously unrecognized pathogenic mechanism underlying ACG1B and AO2, and supports suppression of FGFR3 signaling as a promising therapeutic approach for SLC26A2-related chondrodysplasias.

Fund

This work was supported by National Natural Science Foundation of China (81871743, 81730065 and 81772377).

Achondroplasia with SRY-positive 46, XX disorder of sex development: an extremely rare association

A 40-year-old man with achondroplasia presented with symptoms of hypogonadism, low libido and gynaecomastia. He was found to have hypergonadotropic hypogonadism, and karyotype and fluorescent in situ hybridisation analysis showed SRY-positive 46, XX disorder of sex development (DSD). He was tested to have the common activating mutation of the FGFR3 gene implicated in achondroplasia, indicating that he had the two rare conditions independently, with an extremely low incidence of 1 in 400 million. This, to the best of our knowledge, is the first report of an individual having these two rare conditions concurrently. This case highlights that individuals with achondroplasia should have normal sexual development, and in those presenting with incomplete sexual maturation or symptoms of hypogonadism should prompt further evaluation. We also propose a plausible link between achondroplasia and 46, XX DSD through the intricate interactions between the SRY, SOX9 and FGFR9 gene pathways.

Learning points

The SOX9 and FGF9 genes, which are upregulated by the SRY gene, are important in both sex determination in the embryo, as well as endochondral bone growth.Patients with achondroplasia should have normal sexual development and function in the absence of other confounding factors.Patients with achondroplasia who present with symptoms and signs of abnormal sexual development and/or hypogonadism should be appropriately investigated for other causes.

Achondroplasia: Development, pathogenesis, and therapy

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.

Chondrocyte FGFR3 Regulates Bone Mass by Inhibiting Osteogenesis

Chondrogenesis can regulate bone formation. Fibroblast growth factor receptor 3, highly expressed in chondrocytes, is a negative regulator of bone growth. To investigate whether chondrocyte FGFR3 regulates osteogenesis, thereby contributing to postnatal bone formation and bone remodeling, mice with conditional knock-out of Fgfr3 in chondrocytes (mutant (MUT)) were generated. MUT mice displayed overgrowth of bone with lengthened growth plates. Bone mass of MUT mice was significantly increased at both 1 month and 4 months of age. Histological analysis showed that osteoblast number and bone formation were remarkably enhanced after deletion of Fgfr3 in chondrocytes. Chondrocyte-osteoblast co-culture assay further revealed that Fgfr3 deficiency in chondrocytes promoted differentiation and mineralization of osteoblasts by up-regulating the expressions of Ihh, Bmp2, Bmp4, Bmp7, Wnt4, and Tgf-β1, as well as down-regulating Nog expression. In addition, osteoclastogenesis was also impaired in MUT mice with decreased number of osteoclasts lining trabecular bone, which may be related to the reduced ratio of Rankl to Opg in Fgfr3-deficient chondrocytes. This study reveals that chondrocyte FGFR3 is involved in the regulation of bone formation and bone remodeling by a paracrine mechanism.

HDAC6 deficiency or inhibition blocks FGFR3 accumulation and improves bone growth in a model of achondroplasia

Mutations that cause increased and/or inappropriate activation of FGFR3 are responsible for a collection of short-limbed chondrodysplasias. These mutations can alter receptor trafficking and enhance receptor stability, leading to increased receptor accumulation and activity. Here, we show that wildtype and mutant activated forms of FGFR3 increase expression of the cytoplasmic deacetylase HDAC6 (Histone Deacetylase 6) and that FGFR3 accumulation is compromised in cells lacking HDAC6 or following treatment of fibroblasts or chondrocytes with small molecule inhibitors of HDAC6. The reduced accumulation of FGFR3 was linked to increased FGFR3 degradation that occurred through a lysosome-dependent mechanism. Using a mouse model of Thanatophoric Dysplasia Type II (TDII) we show that both HDAC6 deletion and treatment with the small molecule HDAC6 inhibitor tubacin reduced FGFR3 accumulation in the growth plate and improved endochondral bone growth. Defective endochondral growth in TDII is associated with reduced proliferation and poor hypertrophic differentiation and the improved bone growth was associated with increased chondrocyte proliferation and expansion of the differentiation compartment within the growth plate. These findings further define the mechanisms that control FGFR3 accumulation and contribute to skeletal pathology caused by mutations in FGFR3.

Developmental Regulation of the Growth Plate and Cranial Synchondrosis

Long bones and the cranial base are both formed through endochondral ossification. Elongation of long bones is primarily through the growth plate, which is a cartilaginous structure at the end of long bones made up of chondrocytes. Growth plate chondrocytes are organized in columns along the longitudinal axis of bone growth. The cranial base is the growth center of the neurocranium. Synchondroses, consisting of mirror-image growth plates, are critical for cranial base elongation and development. Over the last decade, considerable progress has been made in determining the roles of the parathyroid hormone-related protein, Indian hedgehog, fibroblast growth factor, bone morphogenetic protein, and Wnt signaling pathways in various aspects of skeletal development. Furthermore, recent evidence indicates the important role of the primary cilia signaling pathway in bone elongation. Here, we review the development of the growth plate and cranial synchondrosis and the regulation by the above-mentioned signaling pathways, highlighting the similarities and differences between these 2 structures.

Tyrosine kinase inhibitor NVP-BGJ398 functionally improves FGFR3-related dwarfism in mouse model

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.

Fibroblast growth factor signaling in skeletal development and disease

Fibroblast growth factor (FGF) signaling pathways are essential regulators of vertebrate skeletal development. FGF signaling regulates development of the limb bud and formation of the mesenchymal condensation and has key roles in regulating chondrogenesis, osteogenesis, and bone and mineral homeostasis. This review updates our review on FGFs in skeletal development published in Genes & Development in 2002, examines progress made on understanding the functions of the FGF signaling pathway during critical stages of skeletogenesis, and explores the mechanisms by which mutations in FGF signaling molecules cause skeletal malformations in humans. Links between FGF signaling pathways and other interacting pathways that are critical for skeletal development and could be exploited to treat genetic diseases and repair bone are also explored.

The transcription factors SOX9 and SOX5/SOX6 cooperate genome-wide through super-enhancers to drive chondrogenesis

SOX9 is a transcriptional activator required for chondrogenesis, and SOX5 and SOX6 are closely related DNA-binding proteins that critically enhance its function. We use here genome-wide approaches to gain novel insights into the full spectrum of the target genes and modes of action of this chondrogenic trio. Using the RCS cell line as a faithful model for proliferating/early prehypertrophic growth plate chondrocytes, we uncover that SOX6 and SOX9 bind thousands of genomic sites, frequently and most efficiently near each other. SOX9 recognizes pairs of inverted SOX motifs, whereas SOX6 favors pairs of tandem SOX motifs. The SOX proteins primarily target enhancers. While binding to a small fraction of typical enhancers, they bind multiple sites on almost all super-enhancers (SEs) present in RCS cells. These SEs are predominantly linked to cartilage-specific genes. The SOX proteins effectively work together to activate these SEs and are required for in vivo expression of their associated genes. These genes encode key regulatory factors, including the SOX trio proteins, and all essential cartilage extracellular matrix components. Chst11, Fgfr3, Runx2 and Runx3 are among many other newly identified SOX trio targets. SOX9 and SOX5/SOX6 thus cooperate genome-wide, primarily through SEs, to implement the growth plate chondrocyte differentiation program.

The SOX9 upstream region prone to chromosomal aberrations causing campomelic dysplasia contains multiple cartilage enhancers

Two decades after the discovery that heterozygous mutations within and around SOX9 cause campomelic dysplasia, a generalized skeleton malformation syndrome, it is well established that SOX9 is a master transcription factor in chondrocytes. In contrast, the mechanisms whereby translocations in the –­350/–50-kb region 5′ of SOX9 cause severe disease and whereby SOX9 expression is specified in chondrocytes remain scarcely known. We here screen this upstream region and uncover multiple enhancers that activate Sox9-promoter transgenes in the SOX9 expression domain. Three of them are primarily active in chondrocytes. E250 (located at –250 kb) confines its activity to condensed prechondrocytes, E195 mainly targets proliferating chondrocytes, and E84 is potent in all differentiated chondrocytes. E84 and E195 synergize with E70, previously shown to be active in most Sox9-expressing somatic tissues, including cartilage. While SOX9 protein powerfully activates E70, it does not control E250. It requires its SOX5/SOX6 chondrogenic partners to robustly activate E195 and additional factors to activate E84. Altogether, these results indicate that SOX9 expression in chondrocytes relies on widely spread transcriptional modules whose synergistic and overlapping activities are driven by SOX9, SOX5/SOX6 and other factors. They help elucidate mechanisms underlying campomelic dysplasia and will likely help uncover other disease mechanisms.