Achondroplasia

FGF and ROR2 receptor tyrosine kinase signaling in human skeletal development

Skeletal malformations are among the most frequent developmental disturbances in humans. In the past years, progress has been made in unraveling the molecular mechanisms that govern skeletal development by the use of animal models as well as by the identification of numerous mutations that cause human skeletal syndromes. Receptor tyrosine kinases have critical roles in embryonic development. During formation of the skeletal system, the fibroblast growth factor receptor (FGFR) family plays major roles in the formation of cranial, axial, and appendicular bones. Another player of relevance to skeletal development is the unusual receptor tyrosine kinase ROR2, the function of which is as interesting as it is complex. In this chapter, we review the involvement of FGFR signaling in human skeletal disease and provide an update on the growing knowledge of ROR2.

Growth hormone treatment, final height, insulin-like growth factors, ghrelin, and adiponectin in four siblings with Seckel syndrome

OBJECTIVE

To report on the effect of growth hormone (GH) treatment on final height (FH) and to describe the insulin-like growth factor (IGF) system, ghrelin, and adiponectin (ADPN) in children with Seckel syndrome.

SUBJECTS AND RESULTS

Four severely growth-retarded Iraqi siblings (two girls and two boys) with Seckel syndrome were referred at ages 16.5, 14.4, 12.4, and 10.4 years. They were born at term, but their growth was retarded and birth weight ranged between 1 and 1.5 kg. The children were healthy and had a normal response to GH provocative test. Long-term GH treatment of the youngest brother and sister increased the FH by 7.2 and 3.4 cm, respectively, compared with their older brother and sister. At FH, body mass index standard deviation scores (BMISDS) ranged from -3.0 to -3.9. Serum levels of immunoreactive IGF-1, bioactive IGF-1, and IGF-binding protein 3 were all within normal to high range before GH treatment and increased after GH treatment. Fasting plasma ghrelin remained severely reduced. Despite low BMISDS, plasma ADPN was moderately reduced and showed an almost complete absence of the low-molecular-weight subform.

CONCLUSION

This is the first report on the effect of GH treatment on FH in children with Seckel syndrome. GH may have increased FH. In addition to growth defects and reduced BMISDS, patients with Seckel syndrome are characterized by low fasting ghrelin levels, low total ADPN, and near deficiency of the low-molecular-weight ADPN subform. The possible significance of the hormonal changes requires further investigations.

Genetic regulation of the growth plate

The epiphyseal growth plate consists of a layer of cartilage present only during the growth period and vanishes soon after puberty in long bones. It is divided to three well-defined zones, from epiphyses; resting, proliferative, and hypertrophic zones. Chondrocyte proliferation and differentiation and subsequent bone formation in this cartilage are controlled by various endocrine, autocrine, and paracrine factors which finally results into elimination of the cartilaginous tissue and promotion of the epiphyseal fusion. As chondrocytes differentiate from round, quiescent, and single structure to flatten and proliferative and then large and terminally differentiated, they experience changes in their gene expression pattern which allow them to transform from cartilaginous tissue to bone. This review summarizes the literature in this area and shortly describes different factors that affect growth plate cartilage both at the local and systemic levels. This may eventually help us to develop new treatment strategies of different growth disorders.

The extracellular domain of fibroblast growth factor receptor 3 inhibits ligand-independent dimerization

Dysregulation of the ligand-independent dimerization of receptor tyrosine kinases (RTKs), which is the first step in the activation of RTKs, leads to various pathologies. A mechanistic understanding of the dimerization process is lacking, and this lack of basic knowledge is one bottleneck in the development of effective RTK-targeted therapies. For example, the roles and relative contributions of the different domains of RTKs to receptor dimerization are unknown. Here, we used quantitative imaging Förster resonance energy transfer (QI-FRET) to determine the contribution of the extracellular domain of fibroblast growth factor receptor 3 (FGFR3) to the dimerization of the receptor. We provide evidence that the contribution of the extracellular domain of FGFR3 to dimerization is repulsive in the absence of ligand and is on the order of ~1 kcal/mol. The repulsive contribution of the extracellular domain is similar in magnitude, but opposite in sign, to the contribution of pathogenic single-amino acid mutations to RTK signaling, and is therefore likely to be important for biological function. Together, these results highlight the fine balance in the domain interactions that regulate RTK dimerization and signaling.

FGFs in endochondral skeletal development

The mammalian skeleton developments and grows through two complementary pathways: membranous ossification, which gives rise to the calvarial bones and distal clavicle, and endochondral ossification, which is responsible for the bones of the limbs, girdles, vertebrae, face and base of the skull and the medial clavicle. Fibroblast growth factors (FGFs) and their cognate FGF receptors (FGFRs) play important roles in regulating both pathways. However, the details of how FGF signals are initiated, propagated and modulated within the developing skeleton are only slowly emerging. This prospect will focus on the current understanding of these events during endochondral skeletal development with special attention given to concepts that have emerged in the past few years.

Common skeletal growth retardation disorders resulting from abnormalities within the mesenchymal stem cells reservoirs in the epiphyseal organs pertaining to the long bones

Among the objectives in writing the current chapter were the curiosity and the interest in allocating the sites and routes of migration of the reservoirs of the mesenchymal precartilaginous stem cells of the developing limbs in health and in disease. We chose to emphasize the events believed to initiate in these regions of stem cells, which may lead to growth retardation disorders. Thus, this narrow niche touches an enlarged scope of developmental biology angles and fields. The enclosed coverage sheds light on part of the musculoskeletal system, skeletogenesis, organogenesis of mobile structures and organs, the limbs, joints and digits (arthrology). It appears that the key role of the cartilage-bone regions is their responsibility to replenish the physis with committed chondrocytes, during the developmental, maturation and puberty periods. We shall start by outlining the framework of normal limb formation, the modalities, signals and the agents participating in this biological creation and regulation, illustrating potential sites that might deviate from normal development during the growth periods.

Achondroplasia: pathogenesis and implications for future treatment

PURPOSE OF REVIEW

Although the genetic defect underlying achondroplasia has been known for over a decade, no effective therapies to stimulate bone growth have emerged. Here we review the recent literature and summarize the molecular mechanisms underlying disease pathology and examine their potential as therapeutic targets. Currently used preclinical models are discussed in the context of recent advances with a special focus on C-type natriuretic peptide.

RECENT FINDINGS

Research on the mutation in Fibroblast Growth Factor Receptor 3 (FGFR3) that causes achondroplasia suggests that disease results from increased signal transduction from the mutant receptor. Thus, current therapeutic strategies have focused on reducing signals emanating from FGFR3. First-generation therapies directly targeting FGFR3, such as kinase inhibitors and neutralizing antibodies, designed for targeting FGFR3 in cancer, are still in the preclinical phase and have yet to translate into the management of achondroplasia. Counteracting signal transduction pathways downstream of FGFR3 holds promise with the discovery that administration of C-type natriuretic peptide to achondroplastic mice ameliorates their clinical phenotype. However, more research into long-term effectiveness and safety of this strategy is needed. Direct targeting of therapeutic agents to growth plate cartilage may enhance efficacy and minimize side effects of these and future therapies.

SUMMARY

Current research into the pathogenesis of achondroplasia has expanded our understanding of the mechanisms of FGFR3-induced disease and has increased the number of approaches that we may use to potentially correct it. Further research is needed to validate these approaches in preclinical models of achondroplasia.

Co-occurrence of achondroplasia and Down syndrome: Genotype/phenotype association

BACKGROUND

This report describes the sixth case of an unusual association: Down syndrome with achondroplasia. It also analyzes the effects of both of these disorders on patient phenotype.

METHODS

A male infant was evaluated for Down syndrome. His appearance also suggested a diagnosis of achondroplasia. The child was evaluated by physical examination, radiography, cytogenetic study, and mutation analysis.

RESULTS

Chromosome analysis showed a karyotype of 47,XY,+21 in all 30 cells analyzed. Radiographic examination showed typical findings of achondroplasia, such as disproportionately large skull, shortening of limb segments, and lumbar lordosis. FGFR3 screening showed a heterozygous G1138A mutation.

CONCLUSIONS

The interaction of these two distinct genetic disorders in the same patient produces a phenotype typical of each syndrome with some overlapping signs. This case represents de novo origin of two disorders that both may be parental-age related.

Achondroplasia: Craniofacial manifestations and considerations in dental management

Achondroplasia is the most common form of skeletal dysplasia dwarfism that manifests with stunted stature and disproportionate limb shortening. Achondroplasia is of dental interest because of its characteristic craniofacial features which include relative macrocephaly, depressed nasal bridge and maxillary hypoplasia. Presence of large head, implanted shunt, airway obstruction and difficulty in head control require special precautions during dental management. Craniofacial manifestations and considerations in dental management are presented in 11-year-old female patient with achondroplasia.

Physical basis behind achondroplasia, the most common form of human dwarfism

Fibroblast growth factor receptor 3 (FGFR3) is a receptor tyrosine kinase that plays an important role in long bone development. The G380R mutation in FGFR3 transmembrane domain is known as the genetic cause for achondroplasia, the most common form of human dwarfism. Despite many studies, there is no consensus about the exact mechanism underlying the pathology. To gain further understanding into the physical basis behind the disorder, here we measure the activation of wild-type and mutant FGFR3 in mammalian cells using Western blots, and we analyze the activation within the frame of a physical-chemical model describing dimerization, ligand binding, and phosphorylation probabilities within the dimers. The data analysis presented here suggests that the mutation does not increase FGFR3 dimerization, as proposed previously. Instead, FGFR3 activity in achondroplasia is increased due to increased probability for phosphorylation of the unliganded mutant dimers. This finding has implications for the design of targeted molecular treatments for achondroplasia.

FGFR3 signaling induces a reversible senescence phenotype in chondrocytes similar to oncogene-induced premature senescence

Oncogenic activation of the RAS-ERK MAP kinase signaling pathway can lead to uncontrolled proliferation but can also result in apoptosis or premature cellular senescence, both regarded as natural protective barriers to cell immortalization and transformation. In FGFR3-related skeletal dyplasias, oncogenic mutations in the FGFR3 receptor tyrosine kinase cause profound inhibition of cartilage growth resulting in severe dwarfism, although many of the precise mechanisms of FGFR3 action remain unclear. Mutated FGFR3 induces constitutive activation of the ERK pathway in chondrocytes and, remarkably, can also cause both increased proliferation and apoptosis in growing cartilage, depending on the gestational age. Here, we demonstrate that FGFR3 signaling is also capable of inducing premature senescence in chondrocytes, manifested as reversible, ERK-dependent growth arrest accompanied by alteration of cellular shape, loss of the extracellular matrix, upregulation of senescence markers (alpha-GLUCOSIDASE, FIBRONECTIN, CAVEOLIN 1, LAMIN A, SM22alpha and TIMP 1), and induction of senescence-associated beta-GALACTOSIDASE activity. Our data support a model whereby FGFR3 signaling inhibits cartilage growth via exploiting cellular responses originally designed to eliminate cells harboring activated oncogenes.

Twenty classic hand radiographs that lead to diagnosis

Most of the common skeletal dysplasias have some manifestation in the hand. Many have characteristic findings in the hand that lead to the diagnosis. Hand bones are also affected in many systemic hematologic and metabolic conditions. The diagnosis can be clinched on a single hand radiograph if characteristic findings are present. This pictorial essay illustrates characteristic findings of 20 common conditions including bone dysplasias and metabolic and hematologic abnormalities on a single hand radiograph. It also includes some common hand abnormalities without systemic skeletal abnormalities.

A novel regulatory mechanism for Fgf18 signaling involving cysteine-rich FGF receptor (Cfr) and delta-like protein (Dlk)

Fibroblast growth factors (FGFs) transduce signals through FGF receptors (FGFRs) and have pleiotropic functions. Besides signal-transducing FGFRs, cysteine-rich FGF receptor (Cfr; Glg1) is also known to bind some FGFs, although its physiological functions remain unknown. In this study, we generated Cfr-deficient mice and found that some of them die perinatally, and show growth retardation, tail malformation and cleft palate. These phenotypes are strikingly similar to those of Fgf18-deficient mice, and we revealed interaction between Cfr and Fgf18 both genetically and physically, suggesting functional cooperation. Consistently, introduction of Cfr facilitated Fgf18-dependent proliferation of Ba/F3 cells expressing Fgfr3c. In addition, we uncovered binding between Cfr and delta-like protein (Dlk), and noticed that Cfr-deficient mice are also similar to Dlk-transgenic mice, indicating that Cfr and Dlk function in opposite ways. Interestingly, we also found that Dlk interrupts the binding between Cfr and Fgf18. Thus, the Fgf18 signaling pathway seems to be finely tuned by Cfr and Dlk for skeletal development. This study reveals a novel regulatory mechanism for Fgf18 signaling involving Cfr and Dlk.

Achondroplasia and cervical laminoplasty

Achondroplasia is associated with short pedicles that predispose individuals with this trait to develop symptomatic spinal canal stenosis. Laminoplasty is an excellent means of treating cervical myelopathy due to stenosis in selected individuals. Laminoplasty preserves segmental motion and stability, both of which are of benefit to all individuals. The authors report the successful surgical treatment of an achondroplastic adult woman with laminoplasty. This procedure alleviated her symptoms, and she was doing well at 2-year follow-up.

Disorders of the growth plate

PURPOSE OF REVIEW

To summarize the recent advances in our understanding of the majors genes involved in chondrogenesis and their molecular mechanisms.

RECENT FINDINGS

Disorders of the growth plate and the resulting skeletal dysplasias are a consequence of defects in genes involved in various stages of the chondrocyte proliferation and differentiation process. Recent identification of disease genes has provided insights into the pathophysiology of many skeletal dysplasias.

SUMMARY

This knowledge enhances our understanding of the physiology and pathophysiology of the growth plate. Many skeletal dysplasias can now be characterized at the molecular level, allowing clinicians to provide accurate molecular diagnoses and counseling. Further research in this area will likely provide insights into possible therapeutic options for disorders of the growth plate.

Prenatal diagnosis of achondroplasia: new specific signs

BACKGROUND

Achondroplasia is one of the most common forms of short limb dwarfism. It is usually suspected on third trimester routine ultrasound because of very shortened long bones. We have described two new prenatal sonographic signs of achondroplasia visible at the proximal femoral metaphysis.

METHODS

Over 5 years, five fetuses were diagnosed with achondroplasia at the Grenoble Prenatal Diagnosis Centre. Ultrasound and tomographic examinations were performed by specialists. To study the proximal metaphysis, the ultrasound transducer was positioned at a 45 degree angle to the diaphyseal axis. Postnatal diagnosis was confirmed.

RESULTS

On computed tomography and postnatal X-ray, proximal femoral metaphysis appeared rounded, with poor, uneven ossification. Connexion to diaphysis was abnormal, with relative overgrowth of the periosteum, creating a new diagnostic sign that we called the 'collar hoop' sign. On ultrasound, all fetuses had a very rounded metaphyseal-epiphyseal interface, with an angle connexion to diaphysis wider than expected. The 'collar hoop' sign was obvious on four of the fetuses. During the same period, proximal femoral metaphyses appeared normal in 653 healthy fetuses, suggesting high specificity of those two new signs.

CONCLUSION

Metaphysis examination is recommended if fetal femur length measures below the fifth percentile, as normal aspect may rule out achondroplasia.

Sleep-related respiratory abnormalities and arousal pattern in achondroplasia during early infancy

OBJECTIVE

To assess sleep-disordered breathing (SDB), sleep architecture, and arousal pattern in infants with achondroplasia and to evaluate the relationship between foramen magnum size and the severity of SDB.

STUDY DESIGN

A retrospective review of polysomnographic recordings and medical records was performed in infants with achondroplasia and in aged-matched control subjects. All studies were re-scored with the emphasis on respiratory events, sleep state, and arousals. In addition, the neuroimaging study of the brain (magnetic resonance imaging) was reviewed to evaluate foramen magnum diameters and to assess their relationship to SDB.

RESULTS

Twenty-four infants met the criteria for entry into analysis, 12 infants with achondroplasia (A) and 12 control infants (C). There was no significant difference in age or sex. Infants with achondroplasia had a significant increase in total respiratory disturbance index (RDI; A, 13.9 +/- 10.8 versus C, 2.0 +/- 0.9; P < .05). However, there was no significant difference in percentages of active sleep, quiet sleep, or sleep efficiency. Analysis of arousals demonstrated that infants with achondroplasia had a significant decrease in both spontaneous arousal index (A, 10.5 +/- 3.5/hr versus C, 18.6 +/- 2.7; P < .0001) and respiratory arousals (A, 10.3% +/- 6.3% versus C, 27.5 +/- 9.5%; P < .0001). Evaluation of foramen magnum dimensions demonstrated smaller foramen magnum size, but there were no significant correlations between anteroposterior or transverse diameters and RDI.

CONCLUSION

Infants with achondroplasia have significant SDB during early infancy. SDB in infants with achondroplasia is not associated with alteration in sleep architecture, possibly because of attenuation of the arousal response. We speculate that the concomitant increased apneic events and decreased arousal response will lead to vulnerability in these infants and may underlie the pathophysiologic mechanism of sudden unexpected death in this population.

Increased p21 expression in chondrocytes of achondroplasic children independently from the presence of the G380R FGFR3 mutation

BACKGROUND

Achondroplasia (ACH) represents the major cause of dwarfism and is due to mutations in the fibroblast growth factor receptor 3 (FGFR3) gene. The cellular mechanisms involved in the reduced growth have been mainly described for in vitro or in vivo models, but few data have been obtained for humans.

METHODS

Thirteen children with ACH were enrolled in the study; the presence of FGFR3 mutations was determined by restriction fragment length polymorphism analysis and sequencing, whereas protein expression in cartilage biopsy was assessed by immunohistochemistry.

RESULTS

Chondrocytes in cartilage biopsies of ACH children were characterized by the presence of growth arrest mediated by STAT activation (both STAT1 and STAT5) and increased expression of p21 and cyclin D1, whereas no expression of either p53 or cyclin D3 could be detected. This mechanism was present in ACH children carrying the G380R mutation but also in a patient in whom no mutation could be detected in the entire coding region of the FGFR3 gene.

CONCLUSIONS

These data thus demonstrate the presence of a common final mechanism involving p21 and possibly leading to a block in chondrocyte proliferation.

FGFs in endochondral skeletal development

The mammalian skeleton forms and grows through two developmental pathways: membranous ossification, which gives rise to calvarial bones and the distal clavicle, and endochondral ossification, which is responsible for the bones of the limbs, girdles, vertebrae, face, base of the skull and the medial clavicle. The regulation of both pathways is extremely complex, and the rules that govern it are still emerging. However, it has become clear that fibroblast growth factors (FGFs) and their cognate receptors (FGFRs) play essential roles. This review focuses on the roles of FGFs and FGFRs in endochondral skeletal development, with special attention given to concepts that have emerged in the past few years.

Understanding what determines the frequency and pattern of human germline mutations

Surprising findings about human germline mutation have come from applying new technologies to detect rare mutations in germline DNA, from analysing DNA sequence divergence between humans and closely related species, and from investigating human polymorphic variation. In this Review we discuss how these approaches affect our current understanding of the roles of sex, age, mutation hot spots, germline selection and genomic factors in determining human nucleotide substitution mutation patterns and frequencies. To enhance our understanding of mutation and disease, more extensive molecular data on the human germ line with regard to mutation origin, DNA repair, epigenetic status and the effect of newly arisen mutations on gamete development are needed.

Systemic administration of C-type natriuretic peptide as a novel therapeutic strategy for skeletal dysplasias

Skeletal dysplasias are a group of genetic disorders characterized by severe impairment of bone growth. Various forms of them add to produce a significant morbidity and mortality, yet no efficient drug therapy has been developed to date. We previously demonstrated that C-type natriuretic peptide (CNP), a member of the natriuretic peptide family, is a potent stimulator of endochondral bone growth. Furthermore, we exhibited that targeted overexpression of a CNP transgene in the growth plate rescued the impaired bone growth observed in a mouse model of achondroplasia (Ach), the most frequent form of human skeletal dysplasias, leading us to propose that CNP may prove to be an effective treatment for this disorder. In the present study, to elucidate whether or not the systemic administration of CNP is a novel drug therapy for skeletal dysplasias, we have investigated the effects of plasma CNP on impaired bone growth in Ach mice that specifically overexpress CNP in the liver under the control of human serum amyloid P component promoter or in those treated with a continuous CNP infusion system. Our results demonstrated that increased plasma CNP from the liver or by iv administration of synthetic CNP-22 rescued the impaired bone growth phenotype of Ach mice without significant adverse effects. These results indicate that treatment with systemic CNP is a potential therapeutic strategy for skeletal dysplasias, including Ach, in humans.

Anesthesia for bariatric surgery in an achondroplastic dwarf with morbid obesity

BACKGROUND AND OBJECTIVES

Achondroplasia is the most common form among the different types of osteochondrodysplasia that cause dwarfism. Dwarves develop obesity quite frequently and surgical treatment has shown greater efficacy, both for effective weight loss and long term maintenance. The objective of this report was to present the case of bariatric surgery with Y-en-Roux gastric bypass in an achondroplastic dwarf with morbid obesity. The different difficulties in the anesthetic management of this patient and the way they were dealt with were discussed in order to decrease intraoperative morbidity and mortality.

CASE REPORT

This is a 29 years old female dwarf with achondroplasia and morbid obesity since childhood. She was 123 cm tall and weighed 144 kg at the time of admission to the Bariatric Surgery service. With a body mass index (BMI) of 95.18 kg.m2, she had several associated diseases especially of the respiratory system and osteoarticular system. After a long follow-up with diet, exercises, and psychological support, her clinical condition improved and she was referred for surgery: Y-en-Roux gastroplasty using the technique of Capella-Fobi. Intubation of the awake patient under direct laryngoscopy was difficult and a bronchofibroscope had to be used. Surgery was uneventful and the patient was maintained under total intravenous anesthesia with continuous infusion of remifentanil and propofol. She was extubated at the end of the surgery still in the operating room.

CONCLUSIONS

The simultaneous comorbidities of achondroplasia and morbid obesity can hinder the anesthetic management, especially regarding the airways. A thorough pre-anesthetic evaluation is necessary to anticipate the conducts and minimize risks, therefore optimizing the evolution of anesthesia.

In vitro regulation of proliferation and differentiation within a postnatal growth plate of the cranial base by parathyroid hormone-related peptide (PTHrP)

Parathyroid hormone-related peptide (PTHrP) is known to be an important regulator of chondrocyte differentiation in embryonic growth plates, but little is known of its role in postnatal growth plates. The present study explores the role of PTHrP in regulating postnatal chondrocyte differentiation using a novel in vitro organ culture model based on the ethmoidal growth plate of the cranial base taken from the postnatal day 10 mouse. In vitro the ethmoidal growth plate continued to mineralize and the chondrocytes progressed to hypertrophy, as observed in vivo, but the proliferative zone was not maintained. Treatment with PTHrP inhibited mineralization and reduced alkaline phosphatase (ALP) activity in the hypertrophic zone in the ethmoidal growth plates grown ex vivo, and also increased the proliferation of non-hypertrophic chondrocytes. In addition, exogenous PTHrP reduced the expression of genes associated with terminal differentiation: type X collagen, Runx2, and ALP, as well as the PTH/PTHrP receptor (PPR). Activation of the protein kinase A pathway using 8-Br-cAMP mimicked some of these pro-proliferative/anti-differentiative effects of PTHrP. PTHrP and PPR were found to be expressed within the ethmoidal growth plate using semi-quantitative PCR, and in other cranial growth plates such as the spheno-occipital and pre-sphenoidal synchondroses. These results provide the first functional evidence that PTHrP regulates proliferation and differentiation within the postnatal, cranial growth plate. J. Cell. Physiol. 219: 688-697, 2009. (c) 2009 Wiley-Liss, Inc.

Human genes involved in copy number variation: mechanisms of origin, functional effects and implications for disease

Copy number variants (CNVs) overlap over 7000 genes, many of which are pivotal in biological pathways. The implications of this are profound, with consequences for evolutionary studies, population genetics, gene function and human phenotype, including elucidation of genetic susceptibility to major common diseases, the heritability of which has thus far defied full explanation. Even though this research is still in its infancy, CNVs have already been associated with a number of monogenic, syndromic and complex diseases: the development of high throughput and high resolution techniques for CNV screening is likely to bring further new insights into the contribution of copy number variation to common diseases. Amongst genes overlapped by CNVs, significant enrichments for certain gene ontology categories have been identified, including those related to immune responses and interactions with the environment. Genes in both of these categories are thought to be important in evolutionary adaptation and to be particular targets of natural selection. Thus, a full appreciation of copy number variation may be important for our understanding of human evolution.

Analysis of STAT1 activation by six FGFR3 mutants associated with skeletal dysplasia undermines dominant role of STAT1 in FGFR3 signaling in cartilage

Activating mutations in FGFR3 tyrosine kinase cause several forms of human skeletal dysplasia. Although the mechanisms of FGFR3 action in cartilage are not completely understood, it is believed that the STAT1 transcription factor plays a central role in pathogenic FGFR3 signaling. Here, we analyzed STAT1 activation by the N540K, G380R, R248C, Y373C, K650M and K650E-FGFR3 mutants associated with skeletal dysplasias. In a cell-free kinase assay, only K650M and K650E-FGFR3 caused activatory STAT1(Y701) phosphorylation. Similarly, in RCS chondrocytes, HeLa, and 293T cellular environments, only K650M and K650E-FGFR3 caused strong STAT1 activation. Other FGFR3 mutants caused weak (HeLa) or no activation (293T and RCS). This contrasted with ERK MAP kinase activation, which was strongly induced by all six mutants and correlated with the inhibition of proliferation in RCS chondrocytes. Thus the ability to activate STAT1 appears restricted to the K650M and K650E-FGFR3 mutants, which however account for only a small minority of the FGFR3-related skeletal dysplasia cases. Other pathways such as ERK should therefore be considered as central to pathological FGFR3 signaling in cartilage.

Canine fibroblast growth factor receptor 3 sequence is conserved across dogs of divergent skeletal size

BACKGROUND

Fibroblast growth factor receptor 3 (FGFR3) is expressed in the growth plate of endochondral bones and serves as a negative regulator of linear bone elongation. Activating mutations severely limit bone growth, resulting in dwarfism, while inactivating mutations significantly enhance bone elongation and overall skeletal size. Domesticated dogs exhibit the greatest skeletal size diversity of any species and, given the regulatory role of FGFR3 on growth plate proliferation, we asked whether sequence differences in FGFR3 could account for some of the size differences.

METHODS

All exons, the promoter region, and 60 bp of the 3' flanking region of the canine FGFR3 gene were sequenced for nine different dog breeds representing a spectrum of skeletal size. The resultant sequences were compared to the reference Boxer genome sequence.

RESULTS

There was no variation in sequence for any FGFR3 exons, promoter region, or 3' flanking sequence across all breeds evaluated.

CONCLUSION

The results suggest that, regardless of domestication selection pressure to develop breeds having extreme differences in skeletal size, the FGFR3 gene is conserved. This implies a critical role for this gene in normal skeletal integrity and indicates that other genes account for size variability in dogs.

FGFR3 promotes synchondrosis closure and fusion of ossification centers through the MAPK pathway

Activating mutations in FGFR3 cause achondroplasia and thanatophoric dysplasia, the most common human skeletal dysplasias. In these disorders, spinal canal and foramen magnum stenosis can cause serious neurologic complications. Here, we provide evidence that FGFR3 and MAPK signaling in chondrocytes promote synchondrosis closure and fusion of ossification centers. We observed premature synchondrosis closure in the spine and cranial base in human cases of homozygous achondroplasia and thanatophoric dysplasia as well as in mouse models of achondroplasia. In both species, premature synchondrosis closure was associated with increased bone formation. Chondrocyte-specific activation of Fgfr3 in mice induced premature synchondrosis closure and enhanced osteoblast differentiation around synchondroses. FGF signaling in chondrocytes increases Bmp ligand mRNA expression and decreases Bmp antagonist mRNA expression in a MAPK-dependent manner, suggesting a role for Bmp signaling in the increased bone formation. The enhanced bone formation would accelerate the fusion of ossification centers and limit the endochondral bone growth. Spinal canal and foramen magnum stenosis in heterozygous achondroplasia patients, therefore, may occur through premature synchondrosis closure. If this is the case, then any growth-promoting treatment for these complications of achondroplasia must precede the timing of the synchondrosis closure.

Autophagy: a new phase in the maturation of growth plate chondrocytes is regulated by HIF, mTOR and AMP kinase

The overall goal of the investigation was to examine autophagy in the growth plate and to ascertain how this process was regulated. Herein, we show that in the postmitotic maturing zone of the growth plate, chondrocytes express an autophagic phenotype. This robust and particulate immunohistochemical response provides direct evidence that autophagy is a new and transient stage in the chondrocyte maturation pathway. We found that induction of autophagy was regulated by mTOR, a sensor of cellular metabolism. When mTOR was inhibited, changes in LC3 fluorescence indicated that this kinase regulated development of the autophagic state. To determine if AMP kinase was required for chondrocyte autophagy, we suppressed its expression in N1511 cells using siRNA technology. When these cells were serum starved, a condition that triggers autophagy, there was no change in LC3 distribution. This result confirmed that AMP kinase was required for the induction of the autophagic response. Based on the 2 studies described above, and our previous observation that HIF-1 is required for the induction of autophagy, we put forward the hypothesis that autophagy is regulated by the activities of AMP kinase and mTOR in a HIF-1-dependent manner. Once autophagy is activated, the postmitotic chondrocytes would be expected to remain viable in their unique microenvironment and complete their life cycle.

Achondroplasia

Achondroplasia (MIM 100800) is the most common non-lethal skeletal dysplasia. Its incidence is between one in 10,000 and one in 30,000. The phenotype is characterized by rhizomelic disproportionate short stature, enlarged head, midface hypoplasia, short hands and lordotic lumbar spine, associated with normal cognitive development. This autosomal-dominant disorder is caused by a gain-of-function mutation in the gene encoding the type 3 receptor for fibroblast growth factor (FGFR3); in more than 95% of cases, the mutation is G380R. The diagnosis is suspected on physical examination and confirmed by different age-related radiological features. Anticipatory and management care by a multidisciplinary team will prevent and treat complications, including cervical cord compression, conductive hearing loss and thoracolumbar gibbosity. Weight counselling, psychosocial guidance and professional integration programmes play an important role in the global quality of life of these patients and their families.

Age-appropriate body mass index in children with achondroplasia: interpretation in relation to indexes of height

BACKGROUND

Achondroplasia is the most common short stature skeletal dysplasia, with an estimated worldwide prevalence of 250 000. Body mass index (BMI)-for-age references are required for weight management guidance for children with achondroplasia, whose body proportions are unlike those of the average stature population.

OBJECTIVE

This study used weight and height data in a clinical setting to derive smoothed BMI-for-age percentile curves for children with achondroplasia and explored the relation of BMI with its components, weight and height.

DESIGN

This was a longitudinal observational study of anthropometric measures of children with achondroplasia from birth through 16 y of age.

RESULTS

The analysis included 1807 BMI data points from 280 children (155 boys, 125 girls) with achondroplasia. As compared with the BMI of peers of average stature, the BMI in children with achondroplasia is higher at birth, lacks a steep increase in infancy and a later nadir between 1 and 2 y of age, and remains substantially higher through 16 y of age in both sexes. Patterns of change in height and weight in children with achondroplasia are unique in that there is no overlap in the height distribution after 6 mo of age and no spike in height velocity during infancy or puberty-the 2 periods of greatest linear growth in individuals of average stature.

CONCLUSIONS

Sex- and age-specific BMI curves are available for children with achondroplasia (birth to 16 y of age) for health surveillance and future research to determine associations with health outcomes (eg, cardiovascular disease, diabetes, and indication for and outcome of surgery).