Sixteen years and counting: the current understanding of fibroblast growth factor receptor 3 (FGFR3) signaling in skeletal dysplasias

Probing phosphorylation events in biological membranes: The transducer function

The extracellular environment is sensed by receptors in the plasma membrane. Some of these receptors initiate cytoplasmic signaling cascades involving phosphorylation: the addition of a phosphate group to a specific amino acid, such as tyrosine, in a protein. Receptor Tyrosine Kinases (RTKs) are one large class of membrane receptors that can directly initiate signaling cascades through their intracellular kinase domains, which both catalyze tyrosine phosphorylation and get phosphorylated. In the first step of signaling, the ligands stabilize phosphorylation-competent RTK dimers and oligomers, which leads to the phosphorylation of specific tyrosine residues in the activation loop of the kinases. Here we discuss quantitative measurements of tyrosine phosphorylation efficiencies for RTKs, described by the "transducer function". The transducer function links the phosphorylation (the response) and the binding of the activating ligand to the receptor (the stimulus). We overview a methodology that allows such measurements in direct response to ligand binding. We discuss experiments which demonstrate that EGF is a partial agonist, and that two tyrosines in the intracellular domain of EGFR, Y1068 and Y1173, are differentially phosphorylated in the EGF-bound EGFR dimers.

Exploring FGFR3 Mutations in the Male Germline: Implications for Clonal Germline Expansions and Paternal Age-Related Dysplasias

Delayed fatherhood results in a higher risk of inheriting a new germline mutation that might result in a congenital disorder in the offspring. In particular, some FGFR3 mutations increase in frequency with age, but there are still a large number of uncharacterized FGFR3 mutations that could be expanding in the male germline with potentially early- or late-onset effects in the offspring. Here, we used digital polymerase chain reaction to assess the frequency and spatial distribution of 10 different FGFR3 missense substitutions in the sexually mature male germline. Our functional assessment of the receptor signaling of the variants with biophysical methods showed that 9 of these variants resulted in a higher activation of the receptor´s downstream signaling, resulting in 2 different expansion behaviors. Variants that form larger subclonal expansions in a dissected postmortem testis also showed a positive correlation of the substitution frequency with the sperm donor's age, and a high and ligand-independent FGFR3 activation. In contrast, variants that measured high FGFR3 signaling and elevated substitution frequencies independent of the donor's age did not result in measurable subclonal expansions in the testis. This suggests that promiscuous signal activation might also result in an accumulation of mutations before the sexual maturation of the male gonad with clones staying relatively constant in size throughout time. Collectively, these results provide novel insights into our understanding of the mutagenesis of driver mutations and their resulting mosaicism in the male germline with important consequences for the transmission and recurrence of associated disorders.

Cell-based screen identifies porphyrins as FGFR3 activity inhibitors with therapeutic potential for achondroplasia and cancer

Overactive fibroblast growth factor receptor 3 (FGFR3) signaling drives pathogenesis in a variety of cancers and a spectrum of short-limbed bone dysplasias, including the most common form of human dwarfism, achondroplasia (ACH). Targeting FGFR3 activity holds great promise as a therapeutic approach for treatment of these diseases. Here, we established a receptor/adaptor translocation assay system that can specifically monitor FGFR3 activation, and we applied it to identify FGFR3 modulators from complex natural mixtures. An FGFR3-suppressing plant extract of Amaranthus viridis was identified from the screen, and 2 bioactive porphyrins, pheophorbide a (Pa) and pyropheophorbide a, were sequentially isolated from the extract and functionally characterized. Further analysis showed that Pa reduced excessive FGFR3 signaling by decreasing its half-life in FGFR3-overactivated multiple myeloma cells and chondrocytes. In an ex vivo culture system, Pa alleviated defective long bone growth in humanized ACH mice (FGFR3ACH mice). Overall, our study presents an approach to discovery and validation of plant extracts or drug candidates that target FGFR3 activation. The compounds identified by this approach may have applications as therapeutics for FGFR3-associated cancers and skeletal dysplasias.

Measurement of FGFR3 signaling at the cell membrane via total internal reflection fluorescence microscopy to compare the activation of FGFR3 mutants

Fibroblast growth factor receptors (FGFRs) initiate signal transduction via the RAS/mitogen-activated protein kinase pathway by their tyrosine kinase activation known to determine cell growth, tissue differentiation, and apoptosis. Recently, many missense mutations have been reported for FGFR3, but we only know the functional effect for a handful of them. Some mutations result in aberrant FGFR3 signaling and are associated with various genetic disorders and oncogenic conditions. Here, we employed micropatterned surfaces to specifically enrich fluorophore-tagged FGFR3 (monomeric GFP [mGFP]-FGFR3) in certain areas of the plasma membrane of living cells. We quantified receptor activation via total internal reflection fluorescence microscopy of FGFR3 signaling at the cell membrane that captured the recruitment of the downstream signal transducer growth factor receptor-bound 2 (GRB2) tagged with mScarlet (GRB2-mScarlet) to FGFR3 micropatterns. With this system, we tested the activation of FGFR3 upon ligand addition (fgf1 and fgf2) for WT and four FGFR3 mutants associated with congenital disorders (G380R, Y373C, K650Q, and K650E). Our data showed that ligand addition increased GRB2 recruitment to WT FGFR3, with fgf1 having a stronger effect than fgf2. For all mutants, we found an increased basal receptor activity, and only for two of the four mutants (G380R and K650Q), activity was further increased upon ligand addition. Compared with previous reports, two mutant receptors (K650Q and K650E) had either an unexpectedly high or low activation state, respectively. This can be attributed to the different methodology, since micropatterning specifically captures signaling events at the plasma membrane. Collectively, our results provide further insight into the functional effects of mutations to FGFR3.

Growth parameters in children with achondroplasia: A 7-year, prospective, multinational, observational study

PURPOSE

This study was undertaken to collect baseline growth parameters in children with achondroplasia who might enroll in interventional trials of vosoritide, and to establish a historical control.

METHODS

In this prospective, observational study, participants (≤17 years) underwent a detailed medical history and physical examination and were followed every 3 months until they finished participating in the study by enrolling in an interventional trial or withdrawing.

RESULTS

A total of 363 children were enrolled (28 centers, 8 countries). Mean (SD) follow up was 20.4 (15.0) months. In participants <1 year, mean annualized growth velocity (AGV) was 11.6 cm/year for girls and 14.6 cm/year for boys. By age 1 year, mean AGV decreased to 7.4 cm/year in girls and 7.1 cm/year in boys. By age 10 years, mean AGV decreased to 3.6 cm/year for both sexes. Mean height z-score in participants <1 year was -2.5 for girls and -3.2 for boys and decreased up to the age 5 years (-5.3 for girls; -4.6 for boys). Girls and boys had a disproportionate upper-to-lower body segment ratio. Mean ratio was highest in participants aged <1 year (2.9 for girls; 2.8 for boys) and decreased gradually to approximately 2 in both sexes from 4 years of age onward.

CONCLUSION

This study represents one of the largest datasets of prospectively collected medical and longitudinal growth data in children with achondroplasia. It serves as a robust historical control to measure therapeutic interventions against and to further delineate the natural history of this condition.

A Genetic Approach in the Evaluation of Short Stature

Short stature is considered a condition in which the height is 2 standard deviations below the mean height of a given age, sex, and population group. Human height is a polygenic and heterogeneous characteristic, and its heritability is reported to be approximately 80%. More than 600 variants associated with human growth were detected in the genome-wide association studies. Rare and common variants concurrently affect human height. The rare variations that play a role in human height determination and have a strong impact on protein functions lead to monogenic short stature phenotypes, which are a highly heterogeneous group. With rapidly developing technologies in the last decade, molecular genetic tests have begun to be used widely in clinical genetics, and thus, the genetic etiology of several rare diseases has been elucidated. Identifying the genetic etiology underlying idiopathic short stature which represents phenotypically heterogeneous group of diseases ranging from isolated short stature to severe and syndromic short stature has promoted the understanding of the genetic regulation of growth plate and longitudinal bone growth. In cases of short stature, definite molecular diagnosis based on genetic evaluation enables the patient and family to receive genetic counseling on the natural course of the disease, prognosis, genetic basis, and recurrence risk. The determination of the genetic etiology in growth disorders is essential for the development of novel targeted therapies and crucial in the development of mutation-specific treatments in the future.

A registry of achondroplasia: a 6-year experience from the Czechia and Slovak Republic

Background

Achondroplasia (ACH) is one of the most prevalent genetic forms of short-limbed skeletal dysplasia, caused by gain-of-function mutations in the receptor tyrosine kinase FGFR3. In August 2021, the C-type natriuretic peptide (CNP) analog vosoritide was approved for the treatment of ACH. A total of six other inhibitors of FGFR3 signaling are currently undergoing clinical evaluation for ACH. This progress creates an opportunity for children with ACH, who may gain early access to the treatment by entering clinical trials before the closure of their epiphyseal growth plates and cessation of growth. Pathophysiology associated with the ACH, however, demands a long observational period before admission to the interventional trial. Public patient registries can facilitate the process by identification of patients suitable for treatment and collecting the data necessary for the trial entry.

Results

In 2015, we established the prospective ACH registry in the Czechia and the Slovak Republic (http://www.achondroplasia-registry.cz). Patient data is collected through pediatric practitioners and other relevant specialists. After informed consent is given, the data is entered to the online TrialDB system and stored in the Oracle 9i database. The initial cohort included 51 ACH children (average age 8.5 years, range 3 months to 14 years). The frequency of selected neurological, orthopedic, or ORL diagnoses is also recorded. In 2015–2021, a total of 89 measurements of heights, weights, and other parameters were collected. The individual average growth rate was calculated and showed values without exception in the lower decile for the appropriate age. Evidence of paternal age effect was found, with 58.7% of ACH fathers older than the general average paternal age and 43.5% of fathers older by two or more years. One ACH patient had orthopedic limb extension and one patient received growth hormone therapy. Low blood pressure or renal impairment were not found in any patient.

Conclusion

The registry collected the clinical information of 51 pediatric ACH patients during its 6 years of existence, corresponding to ~ 60% of ACH patients living in the Czechia and Slovak Republic. The registry continues to collect ACH patient data with annual frequency to monitor the growth and other parameters in preparation for future therapy.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13023-022-02374-x.

Clinical management and emerging therapies of FGFR3-related skeletal dysplasia in childhood

Skeletal dysplasia is a diverse group of disorders that affect bone development and morphology. Currently, approximately 461 different genetic skeletal disorders have been identified, with over 430 causative genes. Among these, fibroblast growth factor receptor 3 (FGFR3)-related skeletal dysplasia is a relatively common subgroup of skeletal dysplasia. Pediatric endocrinologists may encounter a suspected case of skeletal dysplasia in their practice, especially when evaluating children with short stature. Early and accurate diagnosis of FGFR3-related skeletal dysplasia is essential for timely management of complications and genetic counseling. This review summarizes 5 representative and distinct entities of skeletal dysplasia caused by pathogenic variants in FGFR3 and discusses emerging therapies for FGFR3-related skeletal dysplasias.

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.

Differentiation of Induced Pluripotent Stem Cells Into Chondrocytes: Methods and Applications for Disease Modeling and Drug Discovery

Induced pluripotent stem cell (iPSC) technology allows pathomechanistic and therapeutic investigation of human heritable disorders affecting tissue types whose collection from patients is difficult or even impossible. Among them are cartilage diseases. Over the past decade, iPSC-chondrocyte disease models have been shown to exhibit several key aspects of known disease mechanisms. Concurrently, an increasing number of protocols to differentiate iPSCs into chondrocytes have been published, each with its respective (dis)advantages. In this review we provide a comprehensive overview of the different differentiation approaches, the hitherto described iPSC-chondrocyte disease models and mechanistic and/or therapeutic insights that have been derived from their investigation, and the current model limitations. Key lessons are that the most appropriate differentiation approach is dependent upon the cartilage disease under investigation and that further optimization is still required to recapitulate the in vivo cartilage. © 2022 American Society for Bone and Mineral Research (ASBMR).

Molecular signaling in temporomandibular joint osteoarthritis

Objective

Temporomandibular joint (TMJ) osteoarthritis (OA) is a type of TMJ disorders with clinical symptoms of pain, movement limitation, cartilage degeneration and joint dysfunction. This review article is aiming to summarize recent findings on signaling pathways involved in TMJ OA development and progression.

Methods

Most recent findings in TMJ OA studies have been reviewed and cited.

Results

TMJ OA is caused by inflammation, abnormal mechanical loading and genetic abnormalities. The molecular mechanisms related to TMJ OA have been determined using different genetic mouse models. Recent studies demonstrated that several signaling pathways are involved in TMJ OA pathology, including Wnt/β-catenin, TGF-β and BMP, Indian Hedgehog, FGF, NF-κB, and Notch pathways, which are summarized in this review article. Alterations of these signaling pathways lead to the pathological changes in TMJ tissues, affecting cartilage matrix degradation, catabolic metabolism and chondrocyte apoptosis.

Conclusion

Multiple signaling pathways were involved in the pathological process of TMJ OA. New therapeutic strategies, such as stem cell application, gene editing and other techniques may be utilized for TMJ OA treatment.

The translational potential of this article

TMJ OA is a most important subtype of TMJ disorders and may lead to substantial joint pain, dysfunction, dental malocclusion, and reduced health-related quality of life. This review article summarized current findings of signaling pathways involved in TMJ OA, including Wnt/β-catenin, TGF-β and BMP, Indian Hedgehog, FGF, NF-κB, and Notch pathways, to better understand the pathological mechanisms of TMJ OA and define the molecular targets for TMJ OA treatment.

Novel Insights Into the Genetic Causes of Short Stature in Children

Short stature is a common reason for consulting a growth specialist during childhood. Normal height is a polygenic trait involving a complex interaction between hormonal, nutritional and psychosocial components. Genetic factors are becoming very important in the understanding of short stature. After exclusion of the most frequent causes of growth failure, clinicians need to evaluate whether a genetic cause might be taken into consideration. In fact, genetic causes of short stature are probably misdiagnosed during clinical practice and the underlying cause of short stature frequently remains unknown, thus classifying children as having idiopathic short stature (ISS). However, over the past decade, novel genetic techniques have led to the discovery of novel genes associated with linear growth and thus to the ability to define new possible aetiologies of short stature. In fact, thanks to the newer genetic advances, it is possible to properly re-classify about 25–40% of children previously diagnosed with ISS. The purpose of this article is to describe the main monogenic causes of short stature, which, thanks to advances in molecular genetics, are assuming an increasingly important role in the clinical approach to short children.

Non-GH Agents and Novel Therapeutics in the Management of Short Stature

Short stature is one of the most common reasons for referral to pediatric endocrinologists. The vast majority of short children do not have growth hormone (GH) deficiency or another pathologic process that is interfering with normal growth. While GH has been approved in the US for several etiologies of non-GH deficient short stature, its high cost and need for daily injections represent barriers for many families. Alternative agents for the management of short stature include the use of gonadotropin releasing hormone analogs (GnRHas) to delay puberty, and aromatase inhibitors (AIs) in boys to postpone epiphyseal fusion. The results of studies employing GnRHas as either monotherapy or combined with GH are mixed, and there is a dearth of rigorously designed clinical trials that have followed patients to adult height. While AIs have been found to result in modest increases in adult height in some studies, important questions about their long-term safety exist. The C-type natriuretic peptide analog vosoritide is an experimental agent that is emerging as a potential treatment for a few specific conditions including achondroplasia, although its efficacy in attenuating disproportionality is as yet unproven. While each of these therapeutic strategies holds promise, none are currently considered standard of care and several important questions remain. These include the impact of these interventions on quality of life as well as long-term outcomes.

High-Resolution Melting Analysis for Rapid Detection of Mutations in Patients with FGFR3-Related Skeletal Dysplasias

Background: Mutations in the fibroblast growth factor receptor 3 (FGFR3) gene are related to skeletal dysplasias (SDs): acondroplasia (ACH), hypochodroplasia (HCH) and type I (TDI) and II (TDII) tanatophoric dysplasias. This study was designed to standardize and implement a high-resolution melting (HRM) technique to identify mutations in patients with these phenotypes. Methods: Initially, FGFR3 gene segments from 84 patients were PCR amplified and subjected to Sanger sequencing. Samples from 29 patients positive for mutations were analyzed by HRM. Results: Twelve of the patients FGFR3 mutations had ACH (six g.16081 G > A, three g.16081 G > C and three g.16081 G > A + g.16002 C > T); thirteen of patients with HCH had FGFR3 mutations (eight g.17333 C > A, five g.17333 C > G and five were negative); and four patients with DTI had FGFR3 mutations (three g.13526 C > T and one g.16051G > T and two patients with DTII (presented mutation g.17852 A > G). When analyzing the four SDs altogether, an overlap of the dissociation curves was observed, making genotyping difficult. When analyzed separately, however, the HRM analysis method proved to be efficient for discriminating among the mutations for each SD type, except for those patients carrying additional polymorphism concomitant to the recurrent mutation. Conclusion: We conclude that for recurrent mutations in the FGFR3 gene, that the HRM technique can be used as a faster, reliable and less expensive genotyping routine for the diagnosis of these pathologies than Sanger sequencing.

Cilia kinases in skeletal development and homeostasis

Primary cilia are dynamic compartments that regulate multiple aspects of cellular signaling. The production, maintenance, and function of cilia involve more than 1000 genes in mammals, and their mutations disrupt the ciliary signaling which manifests in a plethora of pathological conditions-the ciliopathies. Skeletal ciliopathies are genetic disorders affecting the development and homeostasis of the skeleton, and encompass a broad spectrum of pathologies ranging from isolated polydactyly to lethal syndromic dysplasias. The recent advances in forward genetics allowed for the identification of novel regulators of skeletogenesis, and revealed a growing list of ciliary proteins that are critical for signaling pathways implicated in bone physiology. Among these, a group of protein kinases involved in cilia assembly, maintenance, signaling, and disassembly has emerged. In this review, we summarize the functions of cilia kinases in skeletal development and disease, and discuss the available and upcoming treatment options.

Pharmacokinetics and Exposure-Response of Vosoritide in Children with Achondroplasia

BACKGROUND AND OBJECTIVE

Vosoritide, an analog of C-type natriuretic peptide, has been developed for the treatment of children with achondroplasia. The pharmacokinetics of vosoritide and relationships between plasma exposure and efficacy, biomarkers, and safety endpoints were evaluated in a phase II, open-label, dose-escalation study (N = 35 patients aged 5-14 years who received daily subcutaneous injections for 24 months) and a phase III, double-blind, placebo-controlled study (N = 60 patients aged 5-18 years randomized to receive daily subcutaneous injections for 52 weeks).

METHODS

Pharmacokinetic parameters for both studies were obtained from non-compartmental analysis. Potential correlations between vosoritide exposure and changes in annualized growth velocity, collagen type X marker (CXM; a biomarker of endochondral ossification), cyclic guanosine monophosphate (cGMP; a biomarker of pharmacological activity), heart rate, and systolic and diastolic blood pressures were then evaluated.

RESULTS

The exposure-response relationships for changes in both annualized growth velocity and the CXM biomarker saturated at 15 μg/kg, while systemic pharmacological activity, as measured by urinary cGMP, was near maximal or saturated at exposures obtained at the highest dose studied (i.e. 30 μg/kg). This suggested that the additional bioactivity was likely in tissues not related to endochondral bone formation. In the phase III study, following subcutaneous administration at the recommended dose of 15 μg/kg to patients with achondroplasia aged 5-18 years, vosoritide was rapidly absorbed with a median time to maximal plasma concentration (C_max) of 15 minutes, and cleared with a mean half-life of 27.9 minutes after 52 weeks of treatment. Vosoritide exposure (C_max and area under the concentration-time curve [AUC]) was consistent across visits. No evidence of accumulation with once-daily dosing was observed. Total anti-vosoritide antibody (TAb) responses were detected in the serum of 25 of 60 (42%) treated patients in the phase III study, with no apparent impact of TAb development noted on annualized growth velocity or vosoritide exposure. Across the exposure range obtained with 15 µg/kg in the phase III study, no meaningful correlations between vosoritide plasma exposure and changes in annualized growth velocity or CXM, or changes from predose heart rate, and systolic or diastolic blood pressures were observed.

CONCLUSIONS

The results support the recommended dose of vosoritide 15 µg/kg for once-daily subcutaneous administration in patients with achondroplasia aged ≥ 5 years whose epiphyses are not closed.

CLINICAL TRIALS REGISTRATION

NCT02055157, NCT03197766, and NCT01603095.

An RNA aptamer restores defective bone growth in FGFR3-related skeletal dysplasia in mice

Achondroplasia is the most prevalent genetic form of dwarfism in humans and is caused by activating mutations in FGFR3 tyrosine kinase. The clinical need for a safe and effective inhibitor of FGFR3 is unmet, leaving achondroplasia currently incurable. Here, we evaluated RBM-007, an RNA aptamer previously developed to neutralize the FGFR3 ligand FGF2, for its activity against FGFR3. In cultured rat chondrocytes or mouse embryonal tibia organ culture, RBM-007 rescued the proliferation arrest, degradation of cartilaginous extracellular matrix, premature senescence, and impaired hypertrophic differentiation induced by FGFR3 signaling. In cartilage xenografts derived from induced pluripotent stem cells from individuals with achondroplasia, RBM-007 rescued impaired chondrocyte differentiation and maturation. When delivered by subcutaneous injection, RBM-007 restored defective skeletal growth in a mouse model of achondroplasia. We thus demonstrate a ligand-trap concept of targeting the cartilage FGFR3 and delineate a potential therapeutic approach for achondroplasia and other FGFR3-related skeletal dysplasias.

A Genomic Approach to Delineating the Occurrence of Scoliosis in Arthrogryposis Multiplex Congenita

Arthrogryposis multiplex congenita (AMC) describes a group of conditions characterized by the presence of non-progressive congenital contractures in multiple body areas. Scoliosis, defined as a coronal plane spine curvature of ≥10 degrees as measured radiographically, has been reported to occur in approximately 20% of children with AMC. To identify genes that are associated with both scoliosis as a clinical outcome and AMC, we first queried the DECIPHER database for copy number variations (CNVs). Upon query, we identified only two patients with both AMC and scoliosis (AMC-SC). The first patient contained CNVs in three genes (FBN2, MGF10, and PITX1), while the second case had a CNV in ZC4H2. Looking into small variants, using a combination of Human Phenotype Ontogeny and literature searching, 908 genes linked with scoliosis and 444 genes linked with AMC were identified. From these lists, 227 genes were associated with AMC-SC. Ingenuity Pathway Analysis (IPA) was performed on the final gene list to gain insight into the functional interactions of genes and various categories. To summarize, this group of genes encompasses a diverse group of cellular functions including transcription regulation, transmembrane receptor, growth factor, and ion channels. These results provide a focal point for further research using genomics and animal models to facilitate the identification of prognostic factors and therapeutic targets for AMC.

Fibroblast growth factor receptor 3 activates a network of profibrotic signaling pathways to promote fibrosis in systemic sclerosis

Aberrant activation of fibroblasts with progressive deposition of extracellular matrix is a key feature of systemic sclerosis (SSc), a prototypical idiopathic fibrotic disease. Here, we demonstrate that the profibrotic cytokine transforming growth factor β selectively up-regulates fibroblast growth factor receptor 3 (FGFR3) and its ligand FGF9 to promote fibroblast activation and tissue fibrosis, leading to a prominent FGFR3 signature in the SSc skin. Transcriptome profiling, in silico analysis and functional experiments revealed that FGFR3 induces multiple profibrotic pathways including endothelin, interleukin-4, and connective tissue growth factor signaling mediated by transcription factor CREB (cAMP response element-binding protein). Inhibition of FGFR3 signaling by fibroblast-specific knockout of FGFR3 or FGF9 or pharmacological inhibition of FGFR3 blocked fibroblast activation and attenuated experimental skin fibrosis in mice. These findings characterize FGFR3 as an upstream regulator of a network of profibrotic mediators in SSc and as a potential target for the treatment of fibrosis.

Deciphering the mutational signature of congenital limb malformations

Congenital limb malformations (CLMs) affect 1 in 500 live births. However, the value of exome sequencing (ES) for CLM is lacking. The purpose of this study was to decipher the mutational signature of CLM on an exome level. We enrolled a cohort of 66 unrelated probands (including 47 families) with CLM requiring surgical correction. ES was performed for all patients and available parental samples. A definite molecular diagnosis was achieved in 21 out of 66 (32%) patients. We identified 19 pathogenic or likely pathogenic single-nucleotide variants and three copy number variants, of which 11 variants were novel. We identified four variants of uncertain significance. Additionally, we identified RPL9 and UBA2 as novel candidate genes for CLM. By comparing the detailed phenotypic features, we expand the phenotypic spectrum of diastrophic dysplasia and chromosome 6q terminal deletion syndrome. We also found that the diagnostic rate was significantly higher in patients with a family history of CLM (p = 0.012) or more than one limb affected (p = 0.034). Our study expands our understanding of the mutational and phenotypic spectrum of CLM and provides novel insights into the genetic basis of these syndromes.

An induced pluripotent stem cell line (GZHMCi004-A) derived from a fetus with heterozygous G380R mutation in FGFR3 gene causing achondroplasia

Achondroplasia (ACH; MIM #100800) is an autosomal dominant genetic disease caused by gain-of-function mutations in FGFR3 gene and results in short-limb dwarfism. Here, we generated an induced pluripotent stem cell line GZHMCi004-A derived from umbilical cord blood mononuclear cells (UCBMCs) of a fetus with heterozygous G380R mutation in FGFR3 gene. This iPSC line is a valuable in vitro model to study the pathological mechanism and the treatment of ACH.

Pubertal growth in height, sitting height and leg length in achondroplasia

Background: Children with achondroplasia (ACH) appear to lack a pubertal growth spurt in height.Aim To explore the growth spurt in height and its segments sitting height and leg length, in a large sample of ACH cases using growth curve modelling.Subjects and methods: Height and sitting height were measured longitudinally in ACH children, and the data were analysed using the SITAR (SuperImposition by Translation and Rotation) growth model, which estimates a mean growth curve and random effects for individuals defining differences in size, pubertal timing and intensity.Results: Out of 402 ACH children, 85 boys and 75 girls aged 7-20 years had respectively 529 and 454 measurements of height and sitting height, with leg length calculated by difference. SITAR analysis identified peaks in mean height velocity at 13.3 and 11.3 years in boys and girls, with peak velocities of 4.3 and 4.4 cm/year. Mean peak velocity for sitting height was 3.0 cm/year, but leg length showed no peak. The SITAR models explained 92% to 99% of the cross-sectional variance.Conclusion: ACH children do experience a growth spurt in puberty, but only half that of control children. The spurt is due entirely to sitting height, with no leg length spurt.

Recessive multiple epiphyseal dysplasia and Stargardt disease in two sisters

BACKGROUND

The rapid spread of genome-wide next-generation sequencing in the molecular diagnosis of rare genetic disorders has produced increasing evidence of multilocus genomic variations in cases with a previously well-characterized molecular diagnosis. Here, we describe two patients with a rare combination of skeletal abnormalities and retinal dystrophy caused by variants in the SLC26A2 and ABCA4 genes, respectively, in a family with parental consanguinity.

METHODS

Next-generation sequencing and Sanger sequencing were performed to obtain a molecular diagnosis for the retinal and skeletal phenotypes, respectively.

RESULTS

Genetic testing revealed that the sisters were homozygous for the p.(Cys653Ser) variant in SLC26A2 and heterozygous for the missense p.(Pro68Leu) and splice donor c.6386+2C>G variants in ABCA4. Segregation analysis confirmed the carrier status of the parents.

CONCLUSION

Despite low frequency of occurrence, the detection of multilocus genomic variations in a single disease gene-oriented approach can provide accurate diagnosis even in cases with high phenotypic complexity. A targeted sequencing approach can detect relationships between observed phenotypes and underlying genotypes, useful for clinical management.

Rationale, design, and methods of a randomized, controlled, open-label clinical trial with open-label extension to investigate the safety of vosoritide in infants, and young children with achondroplasia at risk of requiring cervicomedullary decompression surgery

Achondroplasia causes narrowing of the foramen magnum and the spinal canal leading to increased mortality due to cervicomedullary compression in infants and significant morbidity due to spinal stenosis later in adulthood. Vosoritide is a C-natriuretic peptide analogue that has been shown to improve endochondral ossification in children with achondroplasia. The objective of this trial is to evaluate the safety of vosoritide and whether vosoritide can improve the growth of the foramen magnum and spinal canal in children that may require decompression surgery. An Achondroplasia Foramen Magnum Score will be used to identify infants at risk of requiring decompression surgery. This is a 2-year open label randomized controlled trial of vosoritide in infants with achondroplasia ages 0 to ≤12 months. Approximately 20 infants will be randomized 1:1 to either open label once daily subcutaneous vosoritide combined with standard of care or standard of care alone. The primary and secondary aims of the study are to evaluate the safety and efficacy of vosoritide in children with cervicomedullary compression at risk of requiring decompression surgery. The trial will be carried out in specialized skeletal dysplasia treatment centers with well established multidisciplinary care pathways and standardized approaches to the neurosurgical management of cervicomedually compression. After 2 years, infants randomized to standard of care alone will be eligible to switch to vosoritide plus standard of care for an additional 3 years. This pioneering trial hopes to address the important question as to whether treatment with vosoritide at an early age in infants at risk of requiring cervicomedullary decompression surgery is safe, and can improve growth at the foramen magnum and spinal canal alleviating stenosis. This in turn may reduce compression of surrounding structures including the neuraxis and spinal cord, which could alleviate future morbidity and mortality.Trial registrations: ClinicalTrials.gov, NCT04554940; EudraCT number, 2020-001055-40.

Effect of fibroblast growth factor 2 on degenerative endplate chondrocyte: From anabolism to catabolism

BACKGROUND

Endplate degeneration is characterized by an unbalance between the anabolism and catabolism of endplate chondrocyte (CH). Fibroblast growth factor 2 (FGF2) has been shown to promote cartilage repair by increasing articular CH anabolic activity. We aimed to explore the effect of FGF2 on the metabolism of endplate CH to elucidate whether FGF2 could be used as a therapy to delay the endplate degeneration.

METHODS

We collected the endplate tissue from the patients and tested the collagen II mRNA level as the anabolic marker and the MMP-13 and TIMP-4 expression as the catabolic markers. The FGF2, FGF receptor 1 (FGFR1), and FGFR3 mRNA expression of the endplate tissue were also analyzed. Besides, we treated the CHs with exogenic FGF2 protein, measured the markers mentioned above, the proliferation and the apoptosis of the CHs. To compare the effect of FGF2 on the CHs with or without degeneration, we also induced CHs degeneration by interleukin-1β (IL-1β) stimulation and used the FGF2 protein to treat the degenerative CHs.

RESULTS

Severely degenerative endplate had a lower collagen II and TIMP-4 mRNA level, but it expressed a more massive amount of MMP-13, FGF2, and FGFR1. FGF2 supplement upregulated the FGFR1/FGFR3, TIMP-4, collagen II expression, and promoted the CHs proliferation. In the first 24 h of IL-1β treatment, the FGF2 mRNA expression was suppressed, but it significantly increased 48 h later. Meanwhile, the FGFR1 was upregulated, and FGFR3 was inhibited by IL-1β treatment. Interestingly, the FGF2 protein supplement accelerated the degenerative CHs catabolism by decreasing collagen II and TIPM-4 expression but increasing MMP-13. However, the FGF2 could promote the anabolism process in case of the blocking of FGFR1. The FGF2 supplement could also promote the proliferation and inhibited the apoptosis of degenerative CHs, which could be magnified by FGFR1 blocking.

CONCLUSIONS

The results demonstrate that FGF2 is upregulated in the highly degenerative endplate. The supplement of FGF2 contributes to the anabolism in the early phase of endplate degeneration. In the later stage of endplate degeneration, FGF2 turns to accelerate the catabolism, which can be rejected by the reasonable use of FGFR1 inhibitors.

FGFR3 Alterations in the Era of Immunotherapy for Urothelial Bladder Cancer

FGFR3 is a prognostic and predictive marker and is a validated therapeutic target in urothelial bladder cancer. Its utility as a marker and target in the context of immunotherapy is incompletely understood. We review the role of FGFR3 in bladder cancer and discuss preclinical and clinical clues of its effectiveness as a patient selection factor and therapeutic target in the era of immunotherapy.

A new domestic cat genome assembly based on long sequence reads empowers feline genomic medicine and identifies a novel gene for dwarfism

The domestic cat (Felis catus) numbers over 94 million in the USA alone, occupies households as a companion animal, and, like humans, suffers from cancer and common and rare diseases. However, genome-wide sequence variant information is limited for this species. To empower trait analyses, a new cat genome reference assembly was developed from PacBio long sequence reads that significantly improve sequence representation and assembly contiguity. The whole genome sequences of 54 domestic cats were aligned to the reference to identify single nucleotide variants (SNVs) and structural variants (SVs). Across all cats, 16 SNVs predicted to have deleterious impacts and in a singleton state were identified as high priority candidates for causative mutations. One candidate was a stop gain in the tumor suppressor FBXW7. The SNV is found in cats segregating for feline mediastinal lymphoma and is a candidate for inherited cancer susceptibility. SV analysis revealed a complex deletion coupled with a nearby potential duplication event that was shared privately across three unrelated cats with dwarfism and is found within a known dwarfism associated region on cat chromosome B1. This SV interrupted UDP-glucose 6-dehydrogenase (UGDH), a gene involved in the biosynthesis of glycosaminoglycans. Importantly, UGDH has not yet been associated with human dwarfism and should be screened in undiagnosed patients. The new high-quality cat genome reference and the compilation of sequence variation demonstrate the importance of these resources when searching for disease causative alleles in the domestic cat and for identification of feline biomedical models.

The practice of genomic medicine is predicated on the availability of a high quality reference genome and an understanding of the impact of genome variation. Such resources have lead to countless discoveries in humans, however by working exclusively within the framework of human genetics, our potential for understanding diseases biology is limited, as similar analyses in other species have often lead to novel insights. The generation of Felis_catus_9.0, a new high quality reference genome for the domestic cat, helps facilitate the expansion of genomic medicine into the Felis lineage. Using Felis_catus_9.0 we analyze the landscape of genomic variation from a collection of 54 cats within the context of human gene constraint. The distribution of variant impacts in cats is correlated with patterns of gene constraint in humans, indicating the utility of this reference for identifying novel mutations that cause phenotypes relevant to human and cat health. Moreover, structural variant analysis revealed a novel variant for feline dwarfism in UGDH, a gene that has not been associated with dwarfism in any other species, suggesting a role for UGDH in cases of undiagnosed dwarfism in humans.

FGF/FGFR signaling in health and disease

Growing evidences suggest that the fibroblast growth factor/FGF receptor (FGF/FGFR) signaling has crucial roles in a multitude of processes during embryonic development and adult homeostasis by regulating cellular lineage commitment, differentiation, proliferation, and apoptosis of various types of cells. In this review, we provide a comprehensive overview of the current understanding of FGF signaling and its roles in organ development, injury repair, and the pathophysiology of spectrum of diseases, which is a consequence of FGF signaling dysregulation, including cancers and chronic kidney disease (CKD). In this context, the agonists and antagonists for FGF-FGFRs might have therapeutic benefits in multiple systems.

Novel Compound Heterozygous Mutations in CRTAP Cause Rare Autosomal Recessive Osteogenesis Imperfecta

Whole-exome sequencing (WES) has advantages over the traditional molecular test by screening 20,000 genes simultaneously and has become an invaluable tool for genetic diagnosis in clinical practice. Here, we reported a family with a child and a fetus presenting undiagnosed skeletal dysplasia phenotypes, while the parents were asymptomatic. WES was applied to the parents and affected fetus to identify the genetic cause of the phenotypes. We identified novel compound heterozygous mutations consisting of a single-nucleotide variant (SNV) and a large deletion in the CRTAP gene (NM_006371.4:c.1153-3C > G/hg19 chr3:g.32398837_34210906del). Genetic alterations of CRTAP are known to cause osteogenesis imperfecta (OI) in an autosomal recessive manner. Further examination of the proband’s elder sibling who was diagnosed as OI after birth found that she shares the inherited compound heterozygous mutations of CRTAP; thus, the findings support the disease-causing role of CRTAP mutations. Through the in vitro molecular test and in silico analysis, the deleterious effects of the splicing-altering SNV in CRTAP (c.1153-3C > G) on gene product were confirmed. Collectively, our WES-based pathogenic variant discovery pipeline identifies the SNVs and copy number variation to delineate the genetic cause on the proband affected with OI. The data not only extend the knowledge of mutation spectrum in patients with skeletal dysplasia but also demonstrate that WES holds great promise for genetic screening of rare diseases in clinical settings.

Quantifying the strength of heterointeractions among receptor tyrosine kinases from different subfamilies: Implications for cell signaling

Receptor tyrosine kinases (RTKs) are single-pass membrane proteins that control vital cell processes such as cell growth, survival, and differentiation. There is a growing body of evidence that RTKs from different subfamilies can interact and that these diverse interactions can have important biological consequences. However, these heterointeractions are often ignored, and their strengths are unknown. In this work, we studied the heterointeractions of nine RTK pairs, epidermal growth factor receptor (EGFR)-EPH receptor A2 (EPHA2), EGFR-vascular endothelial growth factor receptor 2 (VEGFR2), EPHA2-VEGFR2, EPHA2-fibroblast growth factor receptor 1 (FGFR1), EPHA2-FGFR2, EPHA2-FGFR3, VEGFR2-FGFR1, VEGFR2-FGFR2, and VEGFR2-FGFR3, using a FRET-based method. Surprisingly, we found that RTK heterodimerization and homodimerization strengths can be similar, underscoring the significance of RTK heterointeractions in signaling. We discuss how these heterointeractions can contribute to the complexity of RTK signal transduction, and we highlight the utility of quantitative FRET for probing multiple interactions in the plasma membrane.

Fgfr3 mutation disrupts chondrogenesis and bone ossification in zebrafish model mimicking CATSHL syndrome partially via enhanced Wnt/β-catenin signaling

CATSHL syndrome, characterized by camptodactyly, tall stature and hearing loss, is caused by loss-of-function mutations of fibroblast growth factor receptors 3 (FGFR3) gene. Most manifestations of patients with CATSHL syndrome start to develop in the embryonic stage, such as skeletal overgrowth, craniofacial abnormalities, however, the pathogenesis of these phenotypes especially the early maldevelopment remains incompletely understood. Furthermore, there are no effective therapeutic targets for this skeleton dysplasia. Methods: We generated fgfr3 knockout zebrafish by CRISPR/Cas9 technology to study the developmental mechanisms and therapeutic targets of CATSHL syndrome. Several zebrafish transgenic lines labeling osteoblasts and chondrocytes, and live Alizarin red staining were used to analyze the dynamical skeleton development in fgfr3 mutants. Western blotting, whole mount in situ hybridization, Edu labeling based cell proliferation assay and Wnt/β-catenin signaling antagonist were used to explore the potential mechanisms and therapeutic targets. Results: We found that fgfr3 mutant zebrafish, staring from early development stage, showed craniofacial bone malformation with microcephaly and delayed closure of cranial sutures, chondroma-like lesion and abnormal development of auditory sensory organs, partially resembling the clinical manifestations of patients with CATSHL syndrome. Further studies showed that fgfr3 regulates the patterning and shaping of pharyngeal arches and the timely ossification of craniofacial skeleton. The abnormal development of pharyngeal arch cartilage is related to the augmented hypertrophy and disordered arrangement of chondrocytes, while decreased proliferation, differentiation and mineralization of osteoblasts may be involved in the delayed maturation of skull bones. Furthermore, we revealed that deficiency of fgfr3 leads to enhanced IHH signaling and up-regulated canonical Wnt/β-catenin signaling, and pharmacological inhibition of Wnt/β-catenin could partially alleviate the phenotypes of fgfr3 mutants. Conclusions: Our study further reveals some novel phenotypes and underlying developmental mechanism of CATSHL syndrome, which deepens our understanding of the pathogenesis of CATSHL and the role of fgfr3 in skeleton development. Our findings provide evidence that modulation of Wnt/β-catenin activity could be a potential therapy for CATSHL syndrome and related skeleton diseases.

LncRNA KCNQ1OT1 accelerates fracture healing via modulating miR-701-3p/FGFR3 axis

Emerging evidence highlights the role of the long noncoding RNA (lncRNA) KCNQ1OT1 in fracture healing. Osteoblast proliferation, migration, and survival are pivotal during this process. In this study, we aimed to improve our understanding of the regulatory role of lncRNA KCNQ1OT1 during osteoblast proliferation, migration, and survival. We searched the gene expression omnibus databases and LncBase Experimental V.2 to identify key microRNAs (miRNAs) targets of KCNQ1OT1. MiR-701-3p was selected as a differentially expressed miRNA and RNA immunoprecipitation assays were performed to verify its interaction with KCNQ1OT1. Fibroblast growth factor receptor 3 (FGFR3) was also identified as a target of miR-701-3p. We further identified KCNQ1OT1 as a competing endogenous RNA of miR-701-3p that could influence osteoblast proliferation, migration, and apoptosis in vitro and in vivo. Taken together, our results indicate that the KCNQ1OT1/miR-701-3p/FGFR3 axis is an important regulator of osteoblast proliferation, migration, and apoptosis, and provide a new therapeutic avenue for fracture healing.

Proceedings of the 2019 Santa Fe Bone Symposium: New Concepts in the Care of Osteoporosis and Rare Bone Diseases

The 20th annual Santa Fe Bone Symposium was held August 9-10, 2019, in Santa Fe, New Mexico, USA. This is an annual meeting devoted to clinical applications of recent advances in skeletal research that impact the care of patients with osteoporosis, metabolic bone diseases, and inherited bone diseases. Participants included practicing and academic physicians, fellows, advanced practice providers, fracture liaison service (FLS) coordinators, clinical researchers, and bone density technologists. The symposium consisted of lectures, case presentations, and panel discussions, with an emphasis on learning through interaction of all attendees. Topics included new approaches in the use of anabolic agents for the treatment osteoporosis, a review of important events in skeletal health over the past year, new and emerging treatments for rare bone diseases, the use of genetic testing for bone diseases in clinical practice, medication-associated causes of osteoporosis, new concepts in the use of estrogen therapy for osteoporosis, new Official Positions of the International Society for Clinical Densitometry, skeletal consequences of bariatric surgery, and update on the progress and potential of Bone Health TeleECHO, a virtual community of practice using videoconferencing technology to link healthcare professionals for advancing the care of osteoporosis worldwide. Sessions on rare bone diseases were developed in collaboration with the Rare Bone Disease Alliance. Symposium premeetings included an FLS workshop by the National Osteoporosis Foundation and others devoted to the use of new therapeutic agents for the care of osteoporosis and related disorders.

Recent Insights into Long Bone Development: Central Role of Hedgehog Signaling Pathway in Regulating Growth Plate

The longitudinal growth of long bone, regulated by an epiphyseal cartilaginous component known as the “growth plate”, is generated by epiphyseal chondrocytes. The growth plate provides a continuous supply of chondrocytes for endochondral ossification, a sequential bone replacement of cartilaginous tissue, and any failure in this process causes a wide range of skeletal disorders. Therefore, the cellular and molecular characteristics of the growth plate are of interest to many researchers. Hedgehog (Hh), well known as a mitogen and morphogen during development, is one of the best known regulatory signals in the developmental regulation of the growth plate. Numerous animal studies have revealed that signaling through the Hh pathway plays multiple roles in regulating the proliferation, differentiation, and maintenance of growth plate chondrocytes throughout the skeletal growth period. Furthermore, over the past few years, a growing body of evidence has emerged demonstrating that a limited number of growth plate chondrocytes transdifferentiate directly into the full osteogenic and multiple mesenchymal lineages during postnatal bone development and reside in the bone marrow until late adulthood. Current studies with the genetic fate mapping approach have shown that the commitment of growth plate chondrocytes into the skeletal lineage occurs under the influence of epiphyseal chondrocyte-derived Hh signals during endochondral bone formation. Here, we discuss the valuable observations on the role of the Hh signaling pathway in the growth plate based on mouse genetic studies, with some emphasis on recent advances.

The transition model of RTK activation: A quantitative framework for understanding RTK signaling and RTK modulator activity

Here, we discuss the transition model of receptor tyrosine kinase (RTK) activation, which is derived from biophysical investigations of RTK interactions and signaling. The model postulates that (1) RTKs can interact laterally to form dimers even in the absence of ligand, (2) different unliganded RTK dimers have different stabilities, (3) ligand binding stabilizes the RTK dimers, and (4) ligand binding causes structural changes in the RTK dimer. The model is grounded in the principles of physical chemistry and provides a framework to understand RTK activity and to make predictions in quantitative terms. It can guide basic research aimed at uncovering the mechanism of RTK activation and, in the long run, can empower the search for modulators of RTK function.

Genomic Profiling Identifies Association of IDH1/IDH2 Mutation with Longer Relapse-Free and Metastasis-Free Survival in High-Grade Chondrosarcoma

PURPOSE

Chondrosarcomas are the second most common primary malignant bone tumors. Although histologic grade is the most important factor predicting the clinical outcome of chondrosarcoma, it is subject to interobserver variability. Isocitrate dehydrogenase 1 (IDH1) and IDH2 hotspot mutations were recently found to be frequently mutated in central chondrosarcomas. However, a few published articles have been controversial regarding the association between IDH1/IDH2 mutation status and clinical outcomes in chondrosarcomas.

EXPERIMENTAL DESIGN

We performed hotspot sequencing of IDH1 and IDH2 genes in 89 central chondrosarcomas and targeted next-generation sequencing in 54 of them, and then correlated the IDH1/IDH2 mutation status with the patient's clinical outcome.

RESULTS

Although no association was discovered between IDH mutation status and the patient's overall survival, IDH1/IDH2 mutation was found to be associated with longer relapse-free and metastasis-free survival in high-grade chondrosarcomas. Genomic profiling reveals TERT gene amplification and ATRX mutation, for the first time, in addition to TERT promoter mutation in a subset (6/30, 20%) of high-grade and dedifferentiated chondrosarcomas. These abnormalities in telomere genes are concurrent with IDH1/IDH2 mutation and with CDKN2A/2B deletion or TP53 mutation, suggesting a possible association and synergy among these genes in chondrosarcoma progression. We found 21% of patients with chondrosarcoma also had histories of second malignancies unrelated to cartilaginous tumors, suggesting possible unknown genetic susceptibility to chondrosarcoma.

CONCLUSIONS

IDH1/IDH2 mutations are associated with longer relapse-free and metastasis-free survival in high-grade chondrosarcomas, and they tend to co-occur with TERT mutations and with CDKN2A/2B and TP53 alterations in a subset of high-grade chondrosarcomas.

Clinical value of genetic analysis in prenatal diagnosis of short femur

Background

Fetal femur length (FL) is an important biometric index in prenatal screening. The etiology of short femur is diverse, with some pathogenic causes leading to adverse outcomes. To improve the accuracy and practicability of diagnosis, we investigated the value of genetic analysis in prenatal diagnosis of short femur.

Methods

We examined chromosomal microarray analysis (CMA) (64 fetuses) and karyotyping (59 fetuses) data retrospectively for short femur without fetal growth restriction (FGR). Genetic testing was conducted for 15 fetuses.

Results

Karyotyping and CMA detected chromosomal aberrations at rates of 13.6% and 27.2%, respectively. Among fetuses with other abnormalities, detection rates were 21.0% higher with CMA than karyotyping. CMA identified chromosomal abnormalities in 36.4% of cases with a FL 2–4 standard deviations (SDs) below the gestational age (GA) mean. Abnormality detection by CMA reached 38.5% in the second trimester. Duplication of 12p, 16p13.1 deletion, and uniparental disomy 16 were identified by CMA in three cases of short femur. Gene sequencing detected clinically notable mutations in 12/15 fetuses, among which 9/12 fetuses had FLs >4 SDs below the GA mean.

Conclusions

CMA yielded a higher detection value than karyotyping in fetuses with other abnormalities or a FL 2–4 SDs below the GA mean during the second trimester. Gene sequencing should be performed when FL is >4 SDs below the mean.

Clinical features and molecular genetic analysis of thanatophoric dysplasia type I in a neonate with a de novo c.2419 T > C (p. Ter807Arg) (X807R) mutation in FGFR3

We present a case report that entails prenatal ultrasonography, postnatal characteristics, and molecular genetic analysis of a newborn who presented with thanatophoric dysplasia type I (TDI) with a mutation in the fibroblast growth factor receptor 3 gene (FGFR3). A malformed newborn with tachypnea, delivered by caesarean at the gestational age of 39 weeks, was the first child of nonconsanguineous parents by a spontaneous pregnancy. Features in prenatal ultrasonography and postnatal radiography were consistent with the diagnosis of TDI, presenting with short body length (38 cm, <3rd percentile), redundant skin folds, a narrow thorax with a bust of 29.5 cm (3-5th percentile), and macrocephaly with a head circumference of 36 cm (>97th percentile). The proposita had postnatal dyspnea and unfortunately died of respiratory failure at the age of 13 days. Molecular genetic analysis revealed a mutation of c.2419 T > C (p. Ter807Arg) (X807R) in FGFR3. Live-born infants with TDI are exceedingly rare, and we hereby report a newborn with a c.2419 T > C mutation in FGFR3, emphasizing phenotype with clinical characteristics and ultrasonographic and X-ray findings, to raise awareness about the heterogeneous patterns of TD.

C-Type Natriuretic Peptide Analogue Therapy in Children with Achondroplasia

BACKGROUND

Achondroplasia is a genetic disorder that inhibits endochondral ossification, resulting in disproportionate short stature and clinically significant medical complications. Vosoritide is a biologic analogue of C-type natriuretic peptide, a potent stimulator of endochondral ossification.

METHODS

In a multinational, phase 2, dose-finding study and extension study, we evaluated the safety and side-effect profile of vosoritide in children (5 to 14 years of age) with achondroplasia. A total of 35 children were enrolled in four sequential cohorts to receive vosoritide at a once-daily subcutaneous dose of 2.5 μg per kilogram of body weight (8 patients in cohort 1), 7.5 μg per kilogram (8 patients in cohort 2), 15.0 μg per kilogram (10 patients in cohort 3), or 30.0 μg per kilogram (9 patients in cohort 4). After 6 months, the dose in cohort 1 was increased to 7.5 μg per kilogram and then to 15.0 μg per kilogram, and in cohort 2, the dose was increased to 15.0 μg per kilogram; the patients in cohorts 3 and 4 continued to receive their initial doses. At the time of data cutoff, the 24-month dose-finding study had been completed, and 30 patients had been enrolled in an ongoing long-term extension study; the median duration of follow-up across both studies was 42 months.

RESULTS

During the treatment periods in the dose-finding and extension studies, adverse events occurred in 35 of 35 patients (100%), and serious adverse events occurred in 4 of 35 patients (11%). Therapy was discontinued in 6 patients (in 1 because of an adverse event). During the first 6 months of treatment, a dose-dependent increase in the annualized growth velocity was observed with vosoritide up to a dose of 15.0 μg per kilogram, and a sustained increase in the annualized growth velocity was observed at doses of 15.0 and 30.0 μg per kilogram for up to 42 months.

CONCLUSIONS

In children with achondroplasia, once-daily subcutaneous administration of vosoritide was associated with a side-effect profile that appeared generally mild. Treatment resulted in a sustained increase in the annualized growth velocity for up to 42 months. (Funded by BioMarin Pharmaceutical; ClinicalTrials.gov numbers, NCT01603095, NCT02055157, and NCT02724228.).

Phenotypic Effects of FGF4 Retrogenes on Intervertebral Disc Disease in Dogs

Two FGF4 retrogenes on chromosomes 12 (12-FGF4RG) and 18 (18-FGF4RG) contribute to short-limbed phenotypes in dogs. 12-FGF4RG has also been associated with intervertebral disc disease (IVDD). Both of these retrogenes were found to be widespread among dog breeds with allele frequencies ranging from 0.02 to 1; however, their additive contribution to disease is unknown. Surgical cases of IVDD (n = 569) were evaluated for age of onset, disc calcification, and genotypes for the FGF4 retrogenes. Multivariable linear regression analysis identified the presence of one or two copies of 12-FGF4RG associated with significantly younger age at first surgery in a dominant manner. 18-FGF4RG had only a minor effect in dogs with one copy. Multivariable logistic regression showed that 12-FGF4RG had an additive effect on radiographic disc calcification, while 18-FGF4RG had no effect. Multivariable logistic regression using mixed breed cases and controls identified only 12-FGF4RG as highly associated with disc herniation in a dominant manner (Odds Ratio, OR, 18.42, 95% Confidence Interval (CI) 7.44 to 50.26; p < 0.001). The relative risk for disc surgery associated with 12-FGF4RG varied from 5.5 to 15.1 within segregating breeds and mixed breeds. The FGF4 retrogene on CFA12 acts in a dominant manner to decrease the age of onset and increase the overall risk of disc disease in dogs. Other modifiers of risk may be present within certain breeds, including the FGF4 retrogene on CFA18.

The RTK Interactome: Overview and Perspective on RTK Heterointeractions

Receptor tyrosine kinases (RTKs) play important roles in cell growth, motility, differentiation, and survival. These single-pass membrane proteins are grouped into subfamilies based on the similarity of their extracellular domains. They are generally thought to be activated by ligand binding, which promotes homodimerization and then autophosphorylation in trans. However, RTK interactions are more complicated, as RTKs can interact in the absence of ligand and heterodimerize within and across subfamilies. Here, we review the known cross-subfamily RTK heterointeractions and their possible biological implications, as well as the methodologies which have been used to study them. Moreover, we demonstrate how thermodynamic models can be used to study RTKs and to explain many of the complicated biological effects which have been described in the literature. Finally, we discuss the concept of the RTK interactome: a putative, extensive network of interactions between the RTKs. This RTK interactome can produce unique signaling outputs; can amplify, inhibit, and modify signaling; and can allow for signaling backups. The existence of the RTK interactome could provide an explanation for the irreproducibility of experimental data from different studies and for the failure of some RTK inhibitors to produce the desired therapeutic effects. We argue that a deeper knowledge of RTK interactome thermodynamics can lead to a better understanding of fundamental RTK signaling processes in health and disease. We further argue that there is a need for quantitative, thermodynamic studies that probe the strengths of the interactions between RTKs and their ligands and between different RTKs.

A full-term infant with type II thanatophoric dysplasia

Objectives

To report a neonate with clinical findings consistent with thanatophoric dysplasia (TD). Only a few cases of this rare and lethal skeletal disorder have been reported in South-East Asia.

Case presentation

A 37-year-old Asian female, fourth gravida at 39 weeks, presented to our hospital for an elective cesarean section due to polyhydramnios, frank breech and gestational hypertension. The father was a 42-year-old Asian male. There was no history of rashes, fever, alcohol intake, substance drug abuse, smoking habit or radiation exposure. Ultrasound (US) of 34-weeks’ gestation found a baby with frontal bossing, prominent temporal lobe, clover-skull and low nasal bridge. The thoracic diameter was smaller than the abdominal diameter. Short limbs without bowing were noted in femur and humeral bones. The patient delivered a baby boy, 4115 g, APGAR scores were 5 and 7 at 1 and 5 min. The baby had a dysmorphic face, frontal bossing, low nasal bridge, low-set ears and short neck. The thorax was narrow and abdomen was protuberant. The upper and lower proximal limbs appeared short. A chest X-ray revealed short, curved ribs and opacification of both lungs. He had respiratory distress shortly after birth and had persistent severe respiratory distress despite adequate mechanical ventilation. On the third day, he had cardiac arrest; resuscitation was not done due to family request. The baby was declared deceased due to cardiopulmonary failure related to his congenital anomaly.

Conclusions

Ultrasonography could readily indicate TD prenatally. The pregnancy can continue up to late third trimester without miscarriage. Most of the neonates die in utero; those who survive are dependent on ventilator.

Genes Whose Gain or Loss-of-Function Increases Endurance Performance in Mice: A Systematic Literature Review

Endurance is not only a key factor in many sports but endurance-related variables are also associated with good health and low mortality. Twin and family studies suggest that several endurance-associated traits are ≈50% inherited. However, we still poorly understand what DNA sequence variants contribute to endurance heritability. To address this issue, we conducted a systematic review to identify genes whose experimental loss or gain-of-function increases endurance capacity in mice. We found 31 genes including two isoforms of Ppargc1a whose manipulation increases running or swimming endurance performance by up to 1800%. Genes whose gain-of-function increases endurance are Adcy5, Adcy8, Hk2, Il15, Mef2c, Nr4a3, Pck1 (Pepck), Ppard, Ppargc1a (both the a and b isoforms of the protein Pgc-1α), Ppargc1b, Ppp3ca (calcineurin), Scd1, Slc5a7, Tfe3, Tfeb, Trib3 & Trpv1. Genes whose loss-of-function increases endurance in mice are Actn3, Adrb2, Bdkrb2, Cd47, Crym, Hif1a, Myoz1, Pappa, Pknox1, Pten, Sirt4, Thbs1, Thra, and Tnfsf12. Of these genes, human DNA sequence variants of ACTN3, ADCY5, ADRB2, BDKRB2, HIF1A, PPARD, PPARGC1A, PPARGC1B, and PPP3CA are also associated with endurance capacity and/or VO₂max trainability suggesting evolutionary conservation between mice and humans. Bioinformatical analyses show that there are numerous amino acid or copy number-changing DNA variants of endurance genes in humans, suggesting that genetic variation of endurance genes contributes to the variation of human endurance capacity, too. Moreover, several of these genes/proteins change their expression or phosphorylation in skeletal muscle or the heart after endurance exercise, suggesting a role in the adaptation to endurance exercise.

Regulation of ciliary function by fibroblast growth factor signaling identifies FGFR3-related disorders achondroplasia and thanatophoric dysplasia as ciliopathies

Cilia project from almost every cell integrating extracellular cues with signaling pathways. Constitutive activation of FGFR3 signaling produces the skeletal disorders achondroplasia (ACH) and thanatophoric dysplasia (TD), but many of the molecular mechanisms underlying these phenotypes remain unresolved. Here, we report in vivo evidence for significantly shortened primary cilia in ACH and TD cartilage growth plates. Using in vivo and in vitro methodologies, our data demonstrate that transient versus sustained activation of FGF signaling correlated with different cilia consequences. Transient FGF pathway activation elongated cilia, while sustained activity shortened cilia. FGF signaling extended primary cilia via ERK MAP kinase and mTORC2 signaling, but not through mTORC1. Employing a GFP-tagged IFT20 construct to measure intraflagellar (IFT) speed in cilia, we showed that FGF signaling affected IFT velocities, as well as modulating cilia-based Hedgehog signaling. Our data integrate primary cilia into canonical FGF signal transduction and uncover a FGF-cilia pathway that needs consideration when elucidating the mechanisms of physiological and pathological FGFR function, or in the development of FGFR therapeutics.

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: a comprehensive clinical review

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.