Achondroplasia: pathogenesis and implications for future treatment

p75 neurotrophin receptor regulates craniofacial growth and morphology in postnatal development

Craniofacial abnormalities are among the most prevalent congenital defects, significantly affecting appearance, function, and quality of life. While the role of genetic mutations in craniofacial malformations is recognized, the underlying molecular mechanisms remain poorly understood. In this study, we investigate the role of p75 neurotrophin receptor (p75NTR) in craniofacial development by comparing wild-type (p75NTR+/+) mice against p75NTR-deficient (p75NTR-/-) knockout mice. We employed histology, micro-CT surface distance, volumetric analysis, and geometric morphometric analysis to assess craniofacial development and growth. On postnatal day 7 (P7), p75NTR-/- mice exhibited reduced skull length compared to wild-type controls. By P28, micro-CT analysis revealed significant reductions in calvarial bone volume and trabecular bone thickness in p75NTR-/- mice. Geometric morphometric analysis identified significant shape alterations in the nasal, parietal, and occipital regions, with p75NTR-/- mice showing a shortened cranium and tapered nasal bone morphology. These findings highlight the critical role of p75NTR in regulating postnatal craniofacial development. Disruption of p75NTR signaling impairs both the growth and morphological integrity of craniofacial structures, which may contribute to the pathogenesis of congenital craniofacial abnormalities. In the future, a better understanding of the molecular mechanisms through which p75NTR mediates craniofacial development may offer valuable insights for future targeted therapeutic strategies for craniofacial defects.

Progress in managing children with achondroplasia

INTRODUCTION

Achondroplasia is a heritable disorder of the skeleton that affects approximately 300,000 individuals worldwide. Until recently, treatment for this condition has been purely symptomatic. Efficacious treatment options for children are now approved or are in clinical trials.

AREAS COVERED

This review discusses key advances in the therapeutic management of children with achondroplasia, including vosoritide, the first approved drug, and other emerging precision therapies. These include navepegritide, a long-acting form of C-type natriuretic peptide, and infigratinib, a tyrosine kinase receptor inhibitor, summarizing trial outcomes to date.

EXPERT OPINION

The advent of the first approved precision therapy for achondroplasia in vosoritide has been a paradigm shifting advance for children affected by this condition. In addition to changing their natural growth history, it is hoped that it will decrease their medical complications and enhance functionality. These new treatment options highlight the importance of prompt prenatal identification and subsequent testing of a suspected fetus with achondroplasia and counseling of families. It is hoped that, in the near future, families will have the option to consider a range of effective targeted therapies that best suit their child with achondroplasia, starting from birth should they choose.

Bariatric Surgery in Patients with Achondroplasia, a Feasibility Study of Two Case Studies

BACKGROUND

Achondroplasia is a common skeletal dysplasia with a high prevalence of obesity in adulthood. Bariatric surgery has been shown to be effective in treating obesity and related comorbidities, but its feasibility and effectiveness in patients with achondroplasia have not been clearly established.

OBJECTIVES

The objective of this study was to evaluate the feasibility and effectiveness of bariatric surgery in patients with achondroplasia.

SETTING

This study was performed in France, and bariatric surgeons from the Société Française et Francophone de Chirurgie de l'Obésité et des Maladies Métaboliques (French Francophone Society of Surgery for Obesity or Metabolic Diseases) were asked to participate.

METHODS

Two adult women with confirmed achondroplasia and a high BMI were selected for laparoscopic sleeve gastrectomy. Preoperative data were collected, including demographic information, comorbidities, and follow-up at 1, 3, and 6 months and 1 year after surgery. Complications were monitored and recorded.

RESULTS

Both patients had good excess weight loss outcomes, with an average excess weight loss of 60.5% 1 year after surgery. One patient had a follow-up of 3 years and an excess weight loss of 44%. The surgery was well-tolerated, and no major complications were observed.

CONCLUSIONS

Bariatric surgery is feasible and effective in patients with achondroplasia, with good outcomes for excess weight loss and related comorbidities. These findings suggest that bariatric surgery should be considered a treatment option for patients with achondroplasia and obesity.

Case of twin achondroplasia and autism coexistence and literature review

Achondroplasia and autism spectrum disorder (ASD) are two genetically based disorders. The coexistence of autism with chromosomal abnormalities such as Down syndrome, monogenic syndromes such as tuberous sclerosis, Fragile X, and Rett syndrome, and microdeletion syndromes such as Phelan-McDermid syndrome helps to shed light on the genetic basis of autism spectrum disorder. The association between ASD and achondroplasia has been reported twice in the literature. In this article, we report Turkish patients who were born as identical twins from IVF pregnancy of 34 and 36-year-old parents, clinically and molecularly diagnosed with achondroplasia, and diagnosed with ASD at the age of 39 months. Our case is the first twin patient with the coexistence of achondroplasia and autism. We discuss environmental and genetic factors contributing to the development of ASD.

Meeting Report from 2nd ICCBH-ERN BOND Spinal Complications in Children and Adults with Achondroplasia Workshop, Dublin, Ireland, 2nd July 2022

A pre-meeting workshop on spinal complications in children and adults with achondroplasia was held in Dublin, Ireland at the 10th International Conference on Children's Bone Health (ICCBH) 2-5 July 2022. The pathophysiology, natural history and medical/surgical management of thoraco-lumbar kyphosis and spinal stenosis remains poorly described in the literature. The structure of the workshop consisted of lectures, a debate and an interactive round table discussion. In total over 100 delegates affiliated to over 70 institutions from 20 countries were in attendance.

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.

Emerging therapies for the treatment of rare pediatric bone disorders

In recent years, new therapies for the treatment of rare pediatric bone disorders have emerged, guided by an increasing understanding of the genetic and molecular etiology of these diseases. Herein, we review three such disorders, impacted by debilitating deficits in bone mineralization or cartilage ossification, as well as the novel disease-modifying drugs that are now available to treat these conditions. Specifically, we discuss asfotase alfa, burosumab-twza, and vosoritide, for the treatment of hypophosphatasia, X-linked hypophosphatemia and achondroplasia, respectively. For each skeletal disorder, an overview of the clinical phenotype and natural history of disease is provided, along with a discussion of the clinical pharmacology, mechanism of action and FDA indication for the relevant medication. In each case, a brief review of clinical trial data supporting drug development for each medication is provided. Additionally, guidance as to drug dosing and long-term monitoring of adverse events and pediatric efficacy is presented, to aid the clinician seeking to utilize these novel therapies in their practice, or to become familiar with the healthcare expectations for children receiving these medications through specialized multidisciplinary clinics. The availability of these targeted therapies now significantly augments treatment options for conditions in which past therapy has relied upon less specific, symptomatic medical and orthopedic care.

Evaluation of FGFR inhibitor ASP5878 as a drug candidate for achondroplasia

Achondroplasia is caused by gain-of-function mutations in FGFR3 gene and leads to short-limb dwarfism. A stabilized analogue of C-type natriuretic peptide (CNP) is known to elongate bone by interacting with FGFR3 signals and thus is a promising drug candidate. However, it needs daily administration by percutaneous injection. FGFR inhibitor compounds are other drug candidates for achondroplasia because they directly fix the mutant protein malfunction. Although FGFR inhibitors elongate the bone of model mice, their adverse effects are not well studied. In this study, we found that a new FGFR inhibitor, ASP5878, which was originally developed as an anti-cancer drug, elongated the bone of achondroplasia model male mice at the dose of 300 μg/kg, which confers an AUC of 275 ng·h/ml in juvenile mice. Although ASP5878 was less effective in bone elongation than a CNP analogue, it is advantageous in that ASP5878 can be administered orally. The AUC at which minimal adverse effects were observed (very slight atrophy of the corneal epithelium) was 459 ng·h/ml in juvenile rats. The positive discrepancy between AUCs that brought efficacy and minimal adverse effect suggests the applicability of ASP5878 to achondroplasia in the clinical setting. We also analyzed effects of ASP5878 in a patient-specific induced pluripotent stem cell (iPSC) model for achondroplasia and found the effects on patient chondrocyte equivalents. Nevertheless, cautious consideration is needed when referring to safety data obtained from its application to adult patients with cancer in clinical tests.

Analogs of C-type natriuretic peptide as a potential new non-surgical treatment strategy in knee osteoarthritis

Osteoarthritis (OA) is a common, chronic, progressive, and multifactorial musculoskeletal system disease affecting millions of people around the world. Despite the use of several treatment modalities, the search for a disease modifying drug continuous. Recent evidence suggest involvement of C-type natriuretic peptide (CNP) signaling in induction of chondroprotective pathways. A CNP analog (BMN 111) with an extended plasma half-life due to its neutral-endopeptidase resistance has shown to be pharmacologically active in achondroplasia enabling to hypothesize that BMN 111 may also be used as a treatment strategy in OA, in which CNP signaling has been suggested to be protective and/or reparative.

TransCon CNP, a Sustained-Release C-Type Natriuretic Peptide Prodrug, a Potentially Safe and Efficacious New Therapeutic Modality for the Treatment of Comorbidities Associated with Fibroblast Growth Factor Receptor 3-Related Skeletal Dysplasias

TransCon CNP is a C-type natriuretic peptide (CNP-38) conjugated via a cleavable linker to a polyethylene glycol carrier molecule, designed to provide sustained systemic CNP levels upon weekly subcutaneous administration. TransCon CNP is in clinical development for the treatment of comorbidities associated with achondroplasia. In both mice and cynomolgus monkeys, sustained exposure to CNP via TransCon CNP was more efficacious in stimulating bone growth than intermittent CNP exposure. TransCon CNP was well tolerated with no adverse cardiovascular effects observed at exposure levels exceeding the expected clinical therapeutic exposure. At equivalent dose levels, reductions in blood pressure and/or an increase in heart rate were seen following single subcutaneous injections of the unconjugated CNP-38 molecule or a daily CNP-39 molecule (same amino acid sequence as Vosoritide, USAN:INN). The half-life of the daily CNP-39 molecule in cynomolgus monkey was estimated to be 20 minutes, compared with 90 hours for CNP-38, released from TransCon CNP. C max for the CNP-39 molecule (20 µg/kg) was approximately 100-fold higher, compared with the peak CNP level associated with administration of 100 µg/kg CNP as TransCon CNP. Furthermore, CNP exposure for the daily CNP-39 molecule was only evident for up to 2 hours postdose (lower limit of quantification 37 pmol/l), whereas TransCon CNP gave rise to systemic exposure to CNP-38 for at least 7 days postdose. The prolonged CNP exposure and associated hemodynamically safe peak serum concentrations associated with TransCon CNP administration are suggested to improve efficacy, compared with short-lived CNP molecules, due to better therapeutic drug coverage and decreased risk of hypotension. SIGNIFICANCE STATEMENT: The hormone C-type natriuretic peptide (CNP) is in clinical development for the treatment of comorbidities associated with achondroplasia, the most common form of human dwarfism. The TransCon Technology was used to design TransCon CNP, a prodrug that slowly releases active CNP in the body over several days. Preclinical data show great promise for TransCon CNP to be an effective and well-tolerated drug that provides sustained levels of CNP in a convenient once-weekly dose, while avoiding high systemic CNP bolus concentrations that can induce cardiovascular side effects.

Foramen magnum stenosis and midface hypoplasia in C-type natriuretic peptide-deficient rats and restoration by the administration of human C-type natriuretic peptide with 53 amino acids

C-type natriuretic peptide (CNP)-knockout (KO) rats exhibit impaired skeletal growth, with long bones shorter than those in wild-type (WT) rats. This study compared craniofacial morphology in the CNP-KO rat with that in the Spontaneous Dwarf Rat (SDR), a growth hormone (GH)-deficient model. The effects of subcutaneous administration of human CNP with 53 amino acids (CNP-53) from 5 weeks of age for 4 weeks on craniofacial morphology in CNP-KO rats were also investigated. Skulls of CNP-KO rats at 9 weeks of age were longitudinally shorter and the foramen magnum was smaller than WT rats. There were no differences in foramen magnum stenosis and midface hypoplasia between CNP-KO rats at 9 and 33 weeks of age. These morphological features were the same as those observed in CNP-KO mice and activated fibroblast growth factor receptor 3 achondroplasia-phenotype mice. In contrast, SDR did not exhibit foramen magnum stenosis and midface hypoplasia, despite shorter stature than in control rats. After administration of exogenous CNP-53, the longitudinal skull length and foramen magnum size in CNP-KO rats were significantly greater, and full or partial rescue was confirmed. The synchondrosis at the cranial base in CNP-KO rats is closed at 9 weeks, but not at 4 weeks of age. In contrast, synchondrosis closure in CNP-KO rats treated with CNP-53 was incomplete at 9 weeks of age. Administration of exogenous CNP-53 accelerated craniofacial skeletogenesis, leading to improvement in craniofacial morphology. As these findings in CNP-KO rats are similar to those in patients with achondroplasia, treatment with CNP-53 or a CNP analog may be able to restore craniofacial morphology and foramen magnum size as well as short stature.

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.

Proposal of patient-specific growth plate cartilage xenograft model for FGFR3 chondrodysplasia

OBJECTIVE

FGFR3 chondrodysplasia is caused by a gain-of-function mutation of the FGFR3 gene. The disease causes abnormal growth plate cartilage and lacks effective drug treatment. We sought to establish an in vivo model for the study of FGFR3 chondrodysplasia pathology and drug testing.

DESIGN

We created cartilage from human induced pluripotent stem cells (hiPSCs) and transplanted the cartilage into the subcutaneous spaces of immunodeficient mice. We then created cartilage from the hiPSCs of patients with FGFR3 chondrodysplasia and transplanted them into immunodeficient mice. We treated some mice with a FGFR inhibitor after the transplantation.

RESULTS

Xenografting the hiPSC-derived cartilage reproduced human growth plate cartilage consisting of zones of resting, proliferating, prehypertrophic and hypertrophic chondrocytes and bone in immunodeficient mice. Immunohistochemistry of xenografts using anti-human nuclear antigen antibody indicated that all chondrocytes in growth plate cartilage were human, whereas bone was composed of human and mouse cells. The pathology of small hypertrophic chondrocytes due to up-regulated FGFR3 signaling in FGFR3 skeletal dysplasia was recapitulated in growth plate cartilage formed in the xenografts of patient-specific hiPSC-derived cartilage. The mean diameters of hypertrophic chondrocytes between wild type and thanatophoric dysplasia were significantly different (95% CI: 13.2-26.9; n = 4 mice, one-way analysis of variance (ANOVA)). The pathology was corrected by systemic administration of a FGFR inhibitor to the mice.

CONCLUSION

The patient-specific growth plate cartilage xenograft model for FGFR3 skeletal dysplasia indicated recapitulation of pathology and effectiveness of a FGFR inhibitor for treatment and warrants more study for its usefulness to study disease pathology and drug testing.

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.

Arthroscopic Treatment of Discoid Lateral Meniscus Tears in Children With Achondroplasia

BACKGROUND

Achondroplasia is the most common form of skeletal dysplasia that presents to the pediatric orthopaedist. More than half of achondroplasia patients are affected with knee pain. It is thought that the majority of this pain may be due to spinal stenosis, hip pathology, or knee malalignment. Discoid menisci can be a source of lateral knee joint pain in skeletally immature patients in general. We present the first case series of patients with achondroplasia who had symptomatic discoid lateral menisci treated with arthroscopic knee surgery.

METHODS

The charts of 6 patients (8 knees) with achondroplasia who underwent arthroscopic knee surgery for symptomatic discoid lateral menisci were collected. History and physical examination data, magnetic resonance imaging findings, and operative reports were reviewed. Meniscal tear configuration and treatment type (meniscectomy vs. repair) were noted.

RESULTS

Each patient was found to have a tear of the discoid meniscus. All menisci were treated with saucerization. In addition, meniscal repair was performed in 2 cases, partial meniscectomy in 3 cases, and subtotal meniscectomy in 3 cases. Two patients had bilateral discoid meniscal tears which were treated. Average follow-up was 2.4 years (range, 1 to 4.5 y) and the average pediatric International Knee Documentation Committee (pedi-IKDC) score was 85.3% (range, 75% to 95.4%). At final follow-up, all patients were pain free and able to return to full activities.

CONCLUSIONS

Discoid meniscus tears may be a source of lateral joint line pain in patients with achondroplasia. These injuries can be successfully treated with arthroscopic surgery in this patient population. Future studies need to be done to determine the exact incidence of discoid menisci in achondroplasia patients and also to determine whether there is a genetic relationship between the 2 conditions.

LEVEL OF EVIDENCE

Level IV-case series.

Modeling craniofacial and skeletal congenital birth defects to advance therapies

Craniofacial development is an intricate process of patterning, morphogenesis, and growth that involves many tissues within the developing embryo. Genetic misregulation of these processes leads to craniofacial malformations, which comprise over one-third of all congenital birth defects. Significant advances have been made in the clinical management of craniofacial disorders, but currently very few treatments specifically target the underlying molecular causes. Here, we review recent studies in which modeling of craniofacial disorders in primary patient cells, patient-derived induced pluripotent stem cells (iPSCs), and mice have enhanced our understanding of the etiology and pathophysiology of these disorders while also advancing therapeutic avenues for their prevention.

Achondroplasia: Really rhizomelic?

Achondroplasia is the most common form of short limb dwarfism in humans. The shortening of the limb lengths in achondroplasia is widely described as "rhizomelic." While this appearance may be convincing clinically, the description is not necessarily true or helpful radiologically. The aims of this study, were therefore, to determine whether rhizomelic shortening is a true feature of achondroplasia at diagnosis in infancy. Humeral, radial, femoral, and tibial diaphyseal lengths were recorded by two independent observers from 22 skeletal surveys of infants with achondroplasia and compared with 150 normal age-matched control subjects. Upper and lower limb bone length ratios (radial/humeral and tibial/femoral lengths, respectively) in both groups were compared using an unpaired t-test. Mean upper limb length ratios were statistically higher within the achondroplasia group at 0.87 ± 0.04 (n = 22, mean age 70 ± 94 days) compared to normal controls at 0.79 ± 0.02 (n = 150, mean age 113 days ± 88 days; P < 0.0001). Lower limb length ratios were not significantly different between groups (0.84 ± 0.04 vs. 0.83 ± 0.02, P = 0.46). There was good inter-observer agreement of limb length measurements, with an average measurement difference of 0.1 ± 1.4 mm. In conclusion, infants with achondroplasia demonstrate statistically significant rhizomelic shortening within the upper limbs, but not lower limbs at diagnosis, compared to normal controls. The term "rhizomelic shortening" in relation to achondroplasia should be reserved when describing upper limb proportions. © 2016 Wiley Periodicals, Inc.

The future of growth-promoting therapy

Growth hormone (GH) has been in use for 50 years in children with short stature. Recent developments suggest that our traditional approaches to growth-promoting therapy will be challenged in the following areas:1)Diagnostic: The diagnosis of GH deficiency has always been problematic, largely due to limitations inherent in GH stimulation tests and related biochemical measures. The development of new tools for diagnosis of genetic etiologies of hormonal deficiencies and insensitivity, as well as the growing availability of such methodologies, will greatly strengthen existing diagnostic strategies. 2)Therapeutic: Long-acting GH preparations are already in clinical trials. IGF-I therapy is approved for treatment of IGF deficiency. Novel approaches to select skeletal dysplasias show promise and will potentially expand our therapeutic armamentarium. 3) Monitoring: Traditional weight-based dosing of GH will be supplemented by IGF-based and auxology-based strategies, allowing greater individualization of therapy. 4) Safety: Growth-promoting therapies will continue to require careful monitoring of safety. Recent consensus workshops have advocated life-time surveillance programs. 5) Ethics: Questions will continue to be raised concerning ethical issues related to growth-promoting therapy in children. The availability of new tools for diagnosis and monitoring will address some, but not all, of these issues.

Discoid Meniscus Associated With Achondroplasia

Achondroplasia is the most common skeletal dysplasia. This form of dwarfism is caused by a point mutation in the fibroblast growth factor receptor 3 (FGFR3) gene, leading to inhibition of endochondral ossification for these patients. This results in a normal trunk height but shortened limbs. The discoid meniscus may be an important associated finding to better understand the common complaints of leg pain for these patients. Although the incidence for a discoid meniscus is between 3% and 5% for the general population, it is unknown with achondroplasia. This case series includes 4 patients, with ages ranging from adolescence to early adulthood, with symptoms of knee pain that were not attributable to some of the more common findings seen in this patient population. Typically, patients with achondroplasia who experience knee pain are evaluated for more common and well-known etiologies such as genu varum, ligamentous instability, and neurogenic claudication. However, the authors propose that symptomatic discoid lateral meniscus should be added to the differential diagnosis for lower-extremity pain in the achondroplasia population. A thorough history and physical examination, in combination with magnetic resonance imaging, can aid in making the diagnosis. Treatment with arthroscopic debridement, saucerization of the meniscus, and repair for unstable injuries has yielded good outcomes for this patient population. [Orthopedics. 2016; 39(3):e498-e503.].

FGFR3 biology and skeletal disease

Fibroblast Growth Factor Receptor 3 (FGFR3) is one of four high-affinity receptors for canonical FGF ligands. It acts in many tissues and plays a special role in skeletal development, especially post-embryonic bone growth, where it inhibits chondrocyte proliferation and differentiation. Gain of function mutations cause the most common forms of dwarfism in humans, and they are also detected in cancer. Triggered by ligand binding or in some cases mutation, FGFR3 activation involves dimerization of receptor monomers, phosphorylation of specific tyrosine residues in the receptor's kinase domain and in the tightly linked scaffold protein Fibroblast Receptor Factor Substrate 2 (FRS2). Signaling molecules recruited to these phosphorylation sites propagate signals through cascades that are subject to modulation. Signal output is also regulated by the fate of the receptor and the interval between its activation and degradation. Trafficking pathways have been identified for both lysosomal and proteasomal degradation, as well as, an alternative fate that involves intramembrane cleavage that produces an intracellular domain fragment capable of nuclear transport and potential function.

Sheathless transradial coronary angioplasty in an achondroplasic patient with ST elevation myocardial infarction

We present a case of a 50-year-old man with achondroplasia, who presented with anterior ST segment myocardial infarction (STEMI). He was taken urgently to the catheterisation laboratory. Owing to his short stature, it was not possible to insert a radial artery sheath so he underwent a sheathless primary percutaneous coronary intervention (PCI) through the right radial artery, with no complication. He was shifted to the coronary care unit and had an uneventful hospital course. He was discharged in stable condition and follows up regularly as an outpatient.

Multikinase activity of fibroblast growth factor receptor (FGFR) inhibitors SU5402, PD173074, AZD1480, AZD4547 and BGJ398 compromises the use of small chemicals targeting FGFR catalytic activity for therapy of short-stature syndromes

Activating mutations in the fibroblast growth factor receptor 3 (FGFR3) cause the most common genetic form of human dwarfism, achondroplasia (ACH). Small chemical inhibitors of FGFR tyrosine kinase activity are considered to be viable option for treating ACH, but little experimental evidence supports this claim. We evaluated five FGFR tyrosine kinase inhibitors (TKIs) (SU5402, PD173074, AZD1480, AZD4547 and BGJ398) for their activity against FGFR signaling in chondrocytes. All five TKIs strongly inhibited FGFR activation in cultured chondrocytes and limb rudiment cultures, completely relieving FGFR-mediated inhibition of chondrocyte proliferation and maturation. In contrast, TKI treatment of newborn mice did not improve skeletal growth and had lethal toxic effects on the liver, lungs and kidneys. In cell-free kinase assays as well as in vitro and in vivo cell assays, none of the tested TKIs demonstrated selectivity for FGFR3 over three other FGFR tyrosine kinases. In addition, the TKIs exhibited significant off-target activity when screened against a panel of 14 unrelated tyrosine kinases. This was most extensive in SU5402 and AZD1480, which inhibited DDR2, IGF1R, FLT3, TRKA, FLT4, ABL and JAK3 with efficiencies similar to or greater than those for FGFR. Low target specificity and toxicity of FGFR TKIs thus compromise their use for treatment of ACH. Conceptually, different avenues of therapeutic FGFR3 targeting should be investigated.

Advances in treatment of achondroplasia and osteoarthritis

Achondroplasia (ACH) is the prototype and most common of the human chondrodysplasias. It results from gain-of-function mutations that exaggerate the signal output of the fibroblast growth factor receptor 3 (FGFR3), a receptor tyrosine kinase that negatively regulates growth plate activity and linear bone growth. Several approaches to reduce FGFR3 signaling by blocking receptor activation or inhibiting downstream signals have been proposed. Five show promise in preclinical mouse studies. Two candidate therapies target the extracellular domain of FGFR3. The first is a decoy receptor that competes for activating ligands. The second is a synthetic blocking peptide that prevents ligands from binding and activating FGFR3. Two established drugs, statins and meclozine, improve growth of ACH mice. The strongest candidate therapy employs an analog of C-type natriuretic peptide (CNP), which antagonizes the mitogen-activated-protein (MAP) kinase pathway downstream of the FGFR3 receptor and may also act independently in the growth plate. Only the CNP analog has reached clinical trials. Preliminary results of Phase 2 studies show a substantial increase in growth rate of ACH children after six months of therapy with no serious adverse effects. A challenge for drug therapy in ACH is targeting agents to the avascular growth plate. The application of gene therapy in osteoarthritis offers insights because it faces similar technical obstacles. Major advances in gene therapy include the emergence of recombinant adeno-associated virus as the vector of choice, capsid engineering to target vectors to specific tissues, and development of methods to direct vectors to articular chondrocytes.

Total knee arthroplasty in patients with skeletal dysplasia

INTRODUCTION

Dwarfism is a challenge in arthroplasty. The anatomical features provide a lot of pitfalls. The aim of this study was to follow-up growth-restricted patients after endoprosthetic treatment.

MATERIALS AND METHODS

138 knee arthroplasties in patients with a height ≤150 cm between January 1, 2000 and May 5, 2013 at our institution were enrolled in this study. 124 cases were available for 1-year follow-up. Out of these, 43 cases were available for 5-year follow-up so far. 14 patients were lost to follow-up.

RESULTS

IKS score increased from 35 ± 16 on admission to 67 ± 22 (p < 0.001) at 1-year follow-up and 65 ± 23 (p < 0.001) at 5-year follow-up. Function Score increased from 40 ± 29 on admission to 64 ± 21 (p < 0.001) at 1-year follow-up and 63 ± 23 (p < 0.001) at 5-year follow-up. Revision surgery was required in one case (0.8 %) after 1-year follow-up, and in an additional three cases (7 %) after 5-year follow-up.

CONCLUSIONS

Knee arthroplasty can be performed in patients suffering from dwarfism with good clinical benefits. However, survival rates are worse compared to the general population.

Neutral endopeptidase-resistant C-type natriuretic peptide variant represents a new therapeutic approach for treatment of fibroblast growth factor receptor 3-related dwarfism

Achondroplasia (ACH), the most common form of human dwarfism, is caused by an activating autosomal dominant mutation in the fibroblast growth factor receptor-3 gene. Genetic overexpression of C-type natriuretic peptide (CNP), a positive regulator of endochondral bone growth, prevents dwarfism in mouse models of ACH. However, administration of exogenous CNP is compromised by its rapid clearance in vivo through receptor-mediated and proteolytic pathways. Using in vitro approaches, we developed modified variants of human CNP, resistant to proteolytic degradation by neutral endopeptidase, that retain the ability to stimulate signaling downstream of the CNP receptor, natriuretic peptide receptor B. The variants tested in vivo demonstrated significantly longer serum half-lives than native CNP. Subcutaneous administration of one of these CNP variants (BMN 111) resulted in correction of the dwarfism phenotype in a mouse model of ACH and overgrowth of the axial and appendicular skeletons in wild-type mice without observable changes in trabecular and cortical bone architecture. Moreover, significant growth plate widening that translated into accelerated bone growth, at hemodynamically tolerable doses, was observed in juvenile cynomolgus monkeys that had received daily subcutaneous administrations of BMN 111. BMN 111 was well tolerated and represents a promising new approach for treatment of patients with ACH.

Statin treatment rescues FGFR3 skeletal dysplasia phenotypes

Gain-of-function mutations in the fibroblast growth factor receptor 3 gene (FGFR3) result in skeletal dysplasias, such as thanatophoric dysplasia and achondroplasia (ACH). The lack of disease models using human cells has hampered the identification of a clinically effective treatment for these diseases. Here we show that statin treatment can rescue patient-specific induced pluripotent stem cell (iPSC) models and a mouse model of FGFR3 skeletal dysplasia. We converted fibroblasts from thanatophoric dysplasia type I (TD1) and ACH patients into iPSCs. The chondrogenic differentiation of TD1 iPSCs and ACH iPSCs resulted in the formation of degraded cartilage. We found that statins could correct the degraded cartilage in both chondrogenically differentiated TD1 and ACH iPSCs. Treatment of ACH model mice with statin led to a significant recovery of bone growth. These results suggest that statins could represent a medical treatment for infants and children with TD1 and ACH.

[Advances in bone dysplasias]

The prevalence of bone dysplasias is estimated to be one case per 1,000 inhabitants, which suggests that, at some point in the career of an orthopaedic surgeon, he will face with one of these patients. The aim of this paper is to review the general aspects of bone dysplasias and focus on those, which due to their frequency and importance, we consider most relevant (achondroplasia, multiple epiphyseal dysplasia, spondyloepiphyseal dysplasia, osteogenesis imperfecta), reviewing their fundamental features and the latest therapeutic advances. There is no cure for these diseases, so early diagnosis and appropriate therapeutic management, becomes the key to improving quality of life of these patients.

Hindlimb heating increases vascular access of large molecules to murine tibial growth plates measured by in vivo multiphoton imaging

Advances in understanding the molecular regulation of longitudinal growth have led to development of novel drug therapies for growth plate disorders. Despite progress, a major unmet challenge is delivering therapeutic agents to avascular-cartilage plates. Dense extracellular matrix and lack of penetrating blood vessels create a semipermeable "barrier," which hinders molecular transport at the vascular-cartilage interface. To overcome this obstacle, we used a hindlimb heating model to manipulate bone circulation in 5-wk-old female mice (n = 22). Temperatures represented a physiological range of normal human knee joints. We used in vivo multiphoton microscopy to quantify temperature-enhanced delivery of large molecules into tibial growth plates. We tested the hypothesis that increasing hindlimb temperature from 22°C to 34°C increases vascular access of large systemic molecules, modeled using 10, 40, and 70 kDa dextrans that approximate sizes of physiological regulators. Vascular access was quantified by vessel diameter, velocity, and dextran leakage from subperichondrial plexus vessels and accumulation in growth plate cartilage. Growth plate entry of 10 kDa dextrans increased >150% at 34°C. Entry of 40 and 70 kDa dextrans increased <50%, suggesting a size-dependent temperature enhancement. Total dextran levels in the plexus increased at 34°C, but relative leakage out of vessels was not temperature dependent. Blood velocity and vessel diameter increased 118% and 31%, respectively, at 34°C. These results demonstrate that heat enhances vascular carrying capacity and bioavailability of large molecules around growth plates, suggesting that temperature could be a noninvasive strategy for modulating delivery of therapeutics to impaired growth plates of children.

Neurologic manifestations of achondroplasia

Achondroplasia is the best described and most common form of the congenital short-limbed dwarfing conditions. Achondroplasia is apparent at birth and has a birth prevalence of 1 in 20000-30000 live-born infants. Achondroplasia is inherited as an autosomal dominant condition, although 80% of cases occur sporadically as new events in their families. Achondroplasia is caused, in virtually all of the cases, by a G380R mutation in fibroblast growth factor receptor 3 (FGFR3). Patients with achondroplasia should be evaluated by a multidisciplinary team of clinicians including geneticists, neurologists, and orthopedists, since there are numerous bony and neurological complications. The most severe complication results from craniocervical stenosis and medullary and upper spinal cord compression, which can have devastating and even lethal sequelae during early childhood. In subsequent decades, including adolescence, spinal cord and nerve compression are more prominent. The neurological complications of achondroplasia have been recognized in adults for more than a century and are attributed to bony defects, connective tissue structures, or both. Similar neurological complications are now appreciated in infants, young children, and teenagers with achondroplasia. Defective connective tissue elements in achondroplasia frequently lead to ligamentous laxity, which can aggravate the complications associated with bony stenosis. Bony abnormalities are known to cause neurological morbidity and lead to a shortened lifespan. Neurological complications associated with achondroplasia are reviewed, including recommendations for the evaluation and management of these clinical problems.

Evaluation of the therapeutic potential of a CNP analog in a Fgfr3 mouse model recapitulating achondroplasia

Achondroplasia (ACH), the most common form of dwarfism, is an inherited autosomal-dominant chondrodysplasia caused by a gain-of-function mutation in fibroblast-growth-factor-receptor 3 (FGFR3). C-type natriuretic peptide (CNP) antagonizes FGFR3 downstream signaling by inhibiting the pathway of mitogen-activated protein kinase (MAPK). Here, we report the pharmacological activity of a 39 amino acid CNP analog (BMN 111) with an extended plasma half-life due to its resistance to neutral-endopeptidase (NEP) digestion. In ACH human growth-plate chondrocytes, we demonstrated a decrease in the phosphorylation of extracellular-signal-regulated kinases 1 and 2, confirming that this CNP analog inhibits fibroblast-growth-factor-mediated MAPK activation. Concomitantly, we analyzed the phenotype of Fgfr3(Y367C/+) mice and showed the presence of ACH-related clinical features in this mouse model. We found that in Fgfr3(Y367C/+) mice, treatment with this CNP analog led to a significant recovery of bone growth. We observed an increase in the axial and appendicular skeleton lengths, and improvements in dwarfism-related clinical features included flattening of the skull, reduced crossbite, straightening of the tibias and femurs, and correction of the growth-plate defect. Thus, our results provide the proof of concept that BMN 111, a NEP-resistant CNP analog, might benefit individuals with ACH and hypochondroplasia.

International spinal cord injury musculoskeletal basic data set

OBJECTIVES

To develop an International Spinal Cord Injury (SCI) Musculoskeletal Basic Data Set as part of the International SCI Data Sets to facilitate consistent collection and reporting of basic musculoskeletal findings in the SCI population.

SETTING

International.

METHODS

A first draft of an SCI Musculoskeletal Basic Data Set was developed by an international working group. This was reviewed by many different organizations, societies and individuals over 9 months. Revised versions were created successively.

RESULTS

The final version of the International SCI Musculoskeletal Basic Data Set contains questions on neuro-musculoskeletal history before spinal cord lesion; presence of spasticity/spasms; treatment for spasticity within the last 4 weeks; fracture(s) since the spinal cord lesion; heterotopic ossification; contracture; the location of degenerative neuromuscular and skeletal changes due to overuse after SCI; SCI-related neuromuscular scoliosis; the method(s) used to determine the presence of neuromuscular scoliosis; surgical treatment of the scoliosis; other musculoskeletal problems; if any of the musculoskeletal challenges above interfere with activities of daily living. Instructions for data collection and the data collection form are freely available on the International Spinal Cord Society (ISCoS) website (www.iscos.org.uk).

CONCLUSION

The International SCI Musculoskeletal Basic Data Set will facilitate consistent collection and reporting of basic musculoskeletal findings in the SCI population.

Intermittent PTH (1-34) injection rescues the retarded skeletal development and postnatal lethality of mice mimicking human achondroplasia and thanatophoric dysplasia

Achondroplasia (ACH) and thanatophoric dysplasia (TD) are caused by gain-of-function mutations of fibroblast growth factor receptor 3 (FGFR3) and they are the most common forms of dwarfism and lethal dwarfism, respectively. Currently, there are few effective treatments for ACH. For the neonatal lethality of TD patients, no practical effective therapies are available. We here showed that systemic intermittent PTH (1-34) injection can rescue the lethal phenotype of TD type II (TDII) mice and significantly alleviate the retarded skeleton development of ACH mice. PTH-treated ACH mice had longer naso-anal length than ACH control mice, and the bone lengths of humeri and tibiae were rescued to be comparable with those of wild-type control mice. Our study also found that the premature fusion of cranial synchondroses in ACH mice was partially corrected after the PTH (1-34) treatment, suggesting that the PTH treatment may rescue the progressive narrowing of neurocentral synchondroses that cannot be readily corrected by surgery. In addition, we found that the PTH treatment can improve the osteopenia and bone structure of ACH mice. The increased expression of PTHrP and down-regulated FGFR3 level may be responsible for the positive effects of PTH on bone phenotype of ACH and TDII mice.

FGFR3 targeting strategies for achondroplasia

Mutations that exaggerate signalling of the receptor tyrosine kinase fibroblast growth factor receptor 3 (FGFR3) give rise to achondroplasia, the most common form of dwarfism in humans. Here we review the clinical features, genetic aspects and molecular pathogenesis of achondroplasia and examine several therapeutic strategies designed to target the mutant receptor or its signalling pathways, including the use of kinase inhibitors, blocking antibodies, physiologic antagonists, RNAi and chaperone inhibitors. We conclude by discussing the challenges of treating growth plate disorders in children.

What is complex about complex disorders?

Rather than being polygenic, complex disorders probably represent umbrella terms for collections of conditions caused by rare, recent mutations in any of a large number of different genes.

A novel tyrosine kinase inhibitor restores chondrocyte differentiation and promotes bone growth in a gain-of-function Fgfr3 mouse model

Activating germline fibroblast growth factor receptor 3 (FGFR3) mutations cause achondroplasia (ACH), the most common form of human dwarfism and a spectrum of skeletal dysplasias. FGFR3 is a tyrosine kinase receptor and constitutive FGFR3 activation impairs endochondral ossification and triggers severe disorganization of the cartilage with shortening of long bones. To decipher the role of FGFR3 in endochondral ossification, we analyzed the impact of a novel tyrosine kinase inhibitor (TKI), A31, on both human and mouse mutant FGFR3-expressing cells and on the skeleton of Fgfr3(Y367C/+) dwarf mice. We found that A31 inhibited constitutive FGFR3 phosphorylation and restored the size of embryonic dwarf femurs using an ex vivo culture system. The increase in length of the treated mutant femurs was 2.6 times more than for the wild-type. Premature cell cycle exit and defective chondrocyte differentiation were observed in the Fgfr3(Y367C/+) growth plate. A31 restored normal expression of cell cycle regulators (proliferating cell nuclear antigen, KI67, cyclin D1 and p57) and allowed pre-hypertrophic chondrocytes to properly differentiate into hypertrophic chondocytes. Our data reveal a specific role for FGFR3 in the cell cycle and chondrocyte differentiation and support the development of TKIs for the treatment of FGFR3-related chondrodysplasias.

Natriuretic peptides in children: physiology and clinical utility

PURPOSE OF REVIEW

This article reviews the genetics, biochemistry, and physiology of the natriuretic polypeptide family and their receptors; their roles in cardiac, bone, and lipid metabolism in children; and pharmacological agents that utilize the natriuretic polypeptide system.

RECENT FINDINGS

Clinically, measurements of circulating levels of the natriuretic polypeptides are useful diagnostic and prognostic markers of cardiovascular disease in children. The natriuretic polypeptides also play an important role in growth and body composition. Therapeutic application of the natriuretic polypeptide system may provide new treatments for cardiac, renal, bone, and metabolic disease in children.

SUMMARY

The natriuretic polypeptide system has promising clinical utility in the care of pediatric patients with cardiac, renal, bone, and metabolic disease.

Genetic diseases of bones and joints

Genetic factors play roles in many diseases. Often these factors are ill defined and unpredictable. Other diseases are caused by specific single gene mutations and are passed to offspring in Mendelian inheritance patterns. There are over 5000 documented Mendelian disorders; over 500 of these affect bones and joints. Some of these single gene disorders affect many tissues, and the skeletal system is one of many organ systems involved. The surgical pathologist must often diagnose these disorders. Important examples are neurofibromatosis, Gaucher's disease, and alkaptonuria. Other single gene disorders almost exclusively affect the skeleton. These disorders are the skeletal dysplasias and 372 have been documented. These disorders are classified using radiographic, clinical, and molecular data. The most common dysplasias are osteogenesis imperfecta, achondroplasia, and osteopetrosis. The surgical pathologist usually does not play a role in the diagnosis of skeletal dysplasias. However, histologic studies often elucidate the pathophysiologic basis of these diseases and proper collection of tissues is important for the evolving understanding of the molecular basis of these disorders.

Fibroblast growth factor receptor 3 (FGFR3) is a strong heat shock protein 90 (Hsp90) client: implications for therapeutic manipulation

Fibroblast growth factor receptor 3 (FGFR3) is a key regulator of growth and differentiation, whose aberrant activation causes a number of genetic diseases including achondroplasia and cancer. Hsp90 is a specialized molecular chaperone involved in stabilizing a select set of proteins termed clients. Here, we delineate the relationship of Hsp90 and co-chaperone Cdc37 with FGFR3 and the FGFR family. FGFR3 strongly associates with these chaperone complexes and depends on them for stability and function. Inhibition of Hsp90 function using the geldanamycin analog 17-AAG induces the ubiquitination and degradation of FGFR3 and reduces the signaling capacity of FGFR3. Other FGFRs weakly interact with these chaperones and are differentially influenced by Hsp90 inhibition. The Hsp90-related ubiquitin ligase CHIP is able to interact and destabilize FGFR3. Our results establish FGFR3 as a strong Hsp90 client and suggest that modulating Hsp90 chaperone complexes may beneficially influence the stability and function of FGFR3 in disease.