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

Role of the natriuretic peptide system in normal growth and growth disorders

The C-type natriuretic peptide (CNP) and its receptor (NPR-B) are recognized as important regulators of longitudinal growth. Animal models involving CNP or NPR-B genes (Nppc or Npr2) support the fundamental role of CNP/NPR-B for endochondral ossification. Studies with these animals allow the development of potential drug therapies for dwarfism. Polymorphisms in two genes related to the CNP pathway have been implicated in height variability in healthy individuals. Biallelic loss-of-function mutations in NPR-B gene (NPR2) cause acromesomelic dysplasia type Maroteux, a skeletal dysplasia with extremely short stature. Heterozygous mutations in NPR2 are responsible for nonsyndromic familial short stature. Conversely, heterozygous gain-of-function mutations in NPR2 cause tall stature, with a variable phenotype. A phase 2 multicenter and multinational trial is being developed to evaluate a CNP analog treatment for achondroplasia. Pediatricians and endocrinologists must be aware of growth disorders related to natriuretic peptides, although there is still much to be learned about its diagnostic and therapeutic use.

Novel approaches to short stature therapy

Besides growth hormone, several pharmaceutical products have been investigated for efficacy and safety in increasing short term growth or adult height. Short-term treatment with testosterone esters in boys with constitutional delay of growth and puberty is efficacious in generating secondary sex characteristics and growth acceleration. The addition of oxandrolone to growth hormone (GH) in Turner syndrome has an additive effect on adult height gain. Treatment with GnRH analogs is the established treatment of central precocious puberty, and its addition to GH therapy appears effective in increasing adult height in GH deficient children, and possibly short children born SGA or with SHOX deficiency, who are still short at pubertal onset. Aromatase inhibitors appear effective in several rare disorders, but their value in increasing adult height in early pubertal boys with GH deficiency or idiopathic short stature is uncertain. A trial with a C-natriuretic peptide analog offers hope for children with achondroplasia.

The Fibroblast Growth Factor signaling pathway

The signaling component of the mammalian Fibroblast Growth Factor (FGF) family is comprised of eighteen secreted proteins that interact with four signaling tyrosine kinase FGF receptors (FGFRs). Interaction of FGF ligands with their signaling receptors is regulated by protein or proteoglycan cofactors and by extracellular binding proteins. Activated FGFRs phosphorylate specific tyrosine residues that mediate interaction with cytosolic adaptor proteins and the RAS-MAPK, PI3K-AKT, PLCγ, and STAT intracellular signaling pathways. Four structurally related intracellular non-signaling FGFs interact with and regulate the family of voltage gated sodium channels. Members of the FGF family function in the earliest stages of embryonic development and during organogenesis to maintain progenitor cells and mediate their growth, differentiation, survival, and patterning. FGFs also have roles in adult tissues where they mediate metabolic functions, tissue repair, and regeneration, often by reactivating developmental signaling pathways. Consistent with the presence of FGFs in almost all tissues and organs, aberrant activity of the pathway is associated with developmental defects that disrupt organogenesis, impair the response to injury, and result in metabolic disorders, and cancer. For further resources related to this article, please visit the WIREs website.

A pathway to bone: signaling molecules and transcription factors involved in chondrocyte development and maturation

Decades of work have identified the signaling pathways that regulate the differentiation of chondrocytes during bone formation, from their initial induction from mesenchymal progenitor cells to their terminal maturation into hypertrophic chondrocytes. Here, we review how multiple signaling molecules, mechanical signals and morphological cell features are integrated to activate a set of key transcription factors that determine and regulate the genetic program that induces chondrogenesis and chondrocyte differentiation. Moreover, we describe recent findings regarding the roles of several signaling pathways in modulating the proliferation and maturation of chondrocytes in the growth plate, which is the 'engine' of bone elongation.

Advances in Skeletal Dysplasia Genetics

Skeletal dysplasias result from disruptions in normal skeletal growth and development and are a major contributor to severe short stature. They occur in approximately 1/5,000 births, and some are lethal. Since the most recent publication of the Nosology and Classification of Genetic Skeletal Disorders, genetic causes of 56 skeletal disorders have been uncovered. This remarkable rate of discovery is largely due to the expanded use of high-throughput genomic technologies. In this review, we discuss these recent discoveries and our understanding of the molecular mechanisms behind these skeletal dysplasia phenotypes. We also cover potential therapies, unusual genetic mechanisms, and novel skeletal syndromes both with and without known genetic causes. The acceleration of skeletal dysplasia genetics is truly spectacular, and these advances hold great promise for diagnostics, risk prediction, and therapeutic design.

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.

Meclozine promotes longitudinal skeletal growth in transgenic mice with achondroplasia carrying a gain-of-function mutation in the FGFR3 gene

Achondroplasia (ACH) is one of the most common skeletal dysplasias causing short stature owing to a gain-of-function mutation in the FGFR3 gene, which encodes the fibroblast growth factor receptor 3. We found that meclozine, an over-the-counter drug for motion sickness, inhibited elevated FGFR3 signaling in chondrocytic cells. To examine the feasibility of meclozine administration in clinical settings, we investigated the effects of meclozine on ACH model mice carrying the heterozygous Fgfr3(ach) transgene. We quantified the effect of meclozine in bone explant cultures employing limb rudiments isolated from developing embryonic tibiae from Fgfr3(ach) mice. We found that meclozine significantly increased the full-length and cartilaginous primordia of embryonic tibiae isolated from Fgfr3(ach) mice. We next analyzed the skeletal phenotypes of growing Fgfr3(ach) mice and wild-type mice with or without meclozine treatment. In Fgfr3(ach) mice, meclozine significantly increased the body length after 2 weeks of administration. At skeletal maturity, the bone lengths including the cranium, radius, ulna, femur, tibia, and vertebrae were significantly longer in meclozine-treated Fgfr3(ach) mice than in untreated Fgfr3(ach) mice. Interestingly, meclozine also increased bone growth in wild-type mice. The plasma concentration of meclozine during treatment was within the range that has been used in clinical settings for motion sickness. Increased longitudinal bone growth in Fgfr3(ach) mice by oral administration of meclozine in a growth period suggests potential clinical feasibility of meclozine for the improvement of short stature in ACH.

Fibroblast growth factor (FGF) signaling in development and skeletal diseases

Fibroblast growth factors (FGF) and their receptors serve many functions in both the developing and adult organism. Humans contain 18 FGF ligands and four FGF receptors (FGFR). FGF ligands are polypeptide growth factors that regulate several developmental processes including cellular proliferation, differentiation, and migration, morphogenesis, and patterning. FGF-FGFR signaling is also critical to the developing axial and craniofacial skeleton. In particular, the signaling cascade has been implicated in intramembranous ossification of cranial bones as well as cranial suture homeostasis. In the adult, FGFs and FGFRs are crucial for tissue repair. FGF signaling generally follows one of three transduction pathways: RAS/MAP kinase, PI3/AKT, or PLCγ. Each pathway likely regulates specific cellular behaviors. Inappropriate expression of FGF and improper activation of FGFRs are associated with various pathologic conditions, unregulated cell growth, and tumorigenesis. Additionally, aberrant signaling has been implicated in many skeletal abnormalities including achondroplasia and craniosynostosis. The biology and mechanisms of the FGF family have been the subject of significant research over the past 30 years. Recently, work has focused on the therapeutic targeting and potential of FGF ligands and their associated receptors. The majority of FGF-related therapy is aimed at age-related disorders. Increased understanding of FGF signaling and biology may reveal additional therapeutic roles, both in utero and postnatally. This review discusses the role of FGF signaling in general physiologic and pathologic embryogenesis and further explores it within the context of skeletal development.

Role of C-type natriuretic peptide signalling in maintaining cartilage and bone function

C-type natriuretic peptide (CNP) has been demonstrated in human and mouse models to play critical roles in cartilage homeostasis and endochondral bone formation. Indeed, targeted inactivation of the genes encoding CNP results in severe dwarfism and skeletal defects with a reduction in growth plate chondrocytes. Conversely, cartilage-specific overexpression of CNP was observed to rescue the phenotype of CNP deficient mice and significantly enhanced bone growth caused by growth plate expansion. In vitro studies reported that exogenous CNP influenced chondrocyte differentiation, proliferation and matrix synthesis with the response dependent on CNP concentration. The chondroprotective effects were shown to be mediated by natriuretic peptide receptor (Npr)2 and enhanced synthesis of cyclic guanosine-3',5'-monophosphate (cGMP) production. Recent studies also showed certain homeostatic effects of CNP are mediated by the clearance inactivation receptor, Npr3, highlighting several mechanisms in maintaining tissue homeostasis. However, the CNP signalling systems are complex and influenced by multiple factors that will lead to altered signalling and tissue dysfunction. This review will discuss the differential role of CNP signalling in regulating cartilage and bone homeostasis and how the pathways are influenced by age, inflammation or sex. Evidence indicates that enhanced CNP signalling may prevent growth retardation and protect cartilage in patients with inflammatory joint disease.

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.

Recent research on the growth plate: Recent insights into the regulation of the growth plate

For most bones, elongation is driven primarily by chondrogenesis at the growth plates. This process results from chondrocyte proliferation, hypertrophy, and extracellular matrix secretion, and it is carefully orchestrated by complex networks of local paracrine factors and modulated by endocrine factors. We review here recent advances in the understanding of growth plate physiology. These advances include new approaches to study expression patterns of large numbers of genes in the growth plate, using microdissection followed by microarray. This approach has been combined with genome-wide association studies to provide insights into the regulation of the human growth plate. We also review recent studies elucidating the roles of bone morphogenetic proteins, fibroblast growth factors, C-type natriuretic peptide, and suppressor of cytokine signaling in the local regulation of growth plate chondrogenesis and longitudinal bone growth.

Recent research on the growth plate: Advances in fibroblast growth factor signaling in growth plate development and disorders

Skeletons are formed through two distinct developmental actions, intramembranous ossification and endochondral ossification. During embryonic development, most bone is formed by endochondral ossification. The growth plate is the developmental center for endochondral ossification. Multiple signaling pathways participate in the regulation of endochondral ossification. Fibroblast growth factor (FGF)/FGF receptor (FGFR) signaling has been found to play a vital role in the development and maintenance of growth plates. Missense mutations in FGFs and FGFRs can cause multiple genetic skeletal diseases with disordered endochondral ossification. Clarifying the molecular mechanisms of FGFs/FGFRs signaling in skeletal development and genetic skeletal diseases will have implications for the development of therapies for FGF-signaling-related skeletal dysplasias and growth plate injuries. In this review, we summarize the recent advances in elucidating the role of FGFs/FGFRs signaling in growth plate development, genetic skeletal disorders, and the promising therapies for those genetic skeletal diseases resulting from FGFs/FGFRs dysfunction. Finally, we also examine the potential important research in this field in the future.

Optimal management of complications associated with achondroplasia

Achondroplasia is the most common form of skeletal dysplasia, resulting in disproportionate short stature, and affects over 250,000 people worldwide. Individuals with achondroplasia demonstrate a number of well-recognized anatomical features that impact on growth and development, with a complex array of medical issues that are best managed through a multidisciplinary team approach. The complexity of this presentation, whereby individual impairments may impact upon multiple activity and participation areas, requires consideration and discussion under a broad framework to gain a more thorough understanding of the experience of this condition for individuals with achondroplasia. This paper examines the general literature and research evidence on the medical and health aspects of individuals with achondroplasia and presents a pictorial model of achondroplasia based on The International Classification of Functioning, Disability, and Health (ICF). An expanded model of the ICF will be used to review and present the current literature pertaining to the musculoskeletal, neurological, cardiorespiratory, and ear, nose, and throat impairments and complications across the lifespan, with discussion on the impact of these impairments upon activity and participation performance. Further research is required to fully identify factors influencing participation and to help develop strategies to address these factors.

Regulation of intraocular pressure by soluble and membrane guanylate cyclases and their role in glaucoma

Glaucoma is a progressive optic neuropathy characterized by visual field defects that ultimately lead to irreversible blindness (Alward, 2000; Anderson et al., 2006). By the year 2020, an estimated 80 million people will have glaucoma, 11 million of which will be bilaterally blind. Primary open-angle glaucoma (POAG) is the most common type of glaucoma. Elevated intraocular pressure (IOP) is currently the only risk factor amenable to treatment. How IOP is regulated and can be modulated remains a topic of active investigation. Available therapies, mostly geared toward lowering IOP, offer incomplete protection, and POAG often goes undetected until irreparable damage has been done, highlighting the need for novel therapeutic approaches, drug targets, and biomarkers (Heijl et al., 2002; Quigley, 2011). In this review, the role of soluble (nitric oxide (NO)-activated) and membrane-bound, natriuretic peptide (NP)-activated guanylate cyclases that generate the secondary signaling molecule cyclic guanosine monophosphate (cGMP) in the regulation of IOP and in the pathophysiology of POAG will be discussed.

Postnatal soluble FGFR3 therapy rescues achondroplasia symptoms and restores bone growth in mice

Achondroplasia is a rare genetic disease characterized by abnormal bone development, resulting in short stature. It is caused by a single point mutation in the gene coding for fibroblast growth factor receptor 3 (FGFR3), which leads to prolonged activation upon ligand binding. To prevent excessive intracellular signaling and rescue the symptoms of achondroplasia, we have developed a recombinant protein therapeutic approach using a soluble form of human FGFR3 (sFGFR3), which acts as a decoy receptor and prevents FGF from binding to mutant FGFR3. sFGFR3 was injected subcutaneously to newborn Fgfr3(ach/+) mice-the mouse model of achondroplasia-twice per week throughout the growth period during 3 weeks. Effective maturation of growth plate chondrocytes was restored in bones of treated mice, with a dose-dependent enhancement of skeletal growth in Fgfr3(ach/+) mice. This resulted in normal stature and a significant decrease in mortality and associated complications, without any evidence of toxicity. These results describe a new approach for restoring bone growth and suggest that sFGFR3 could be a potential therapy for children with achondroplasia and related disorders.

Protective effects of C-type natriuretic peptide on linear growth and articular cartilage integrity in a mouse model of inflammatory arthritis

Objective

The C-type natriuretic peptide (CNP) signaling pathway is a major contributor to postnatal skeletal growth in humans. This study was undertaken to investigate whether CNP signaling could prevent growth delay and cartilage damage in an animal model of inflammatory arthritis.

Methods

We generated transgenic mice that overexpress CNP (B6.SJL-Col2a1-NPPC) in chondrocytes. We introduced the CNP transgene into mice with experimental systemic inflammatory arthritis (K/BxN T cell receptor [TCR]) and determined the effect of CNP overexpression in chondrocytes on the severity of arthritis, cartilage damage, and linear growth. We also examined primary chondrocyte cultures for changes in gene and protein expression resulting from CNP overexpression.

Results

K/BxN TCR mice exhibited linear growth delay (P < 0.01) compared to controls, and this growth delay was correlated with the severity of arthritis. Diminished chondrocyte proliferation and matrix production was also seen in K/BxN TCR mice. Compared to non–CNP-transgenic mice, K/BxN TCR mice with overexpressed CNP had milder arthritis, no growth delay, and less cartilage damage. Primary chondrocytes from mice overexpressing CNP were less sensitive to inflammatory cytokines than wild-type mouse chondrocytes.

Conclusion

CNP overexpression in chondrocytes can prevent endochondral growth delay and protect against cartilage damage in a mouse model of inflammatory arthritis. Pharmacologic or biologic modulation of the CNP signaling pathway may prevent growth retardation and protect cartilage in patients with inflammatory joint diseases, such as juvenile idiopathic arthritis.

FGFR3 mutation causes abnormal membranous ossification in achondroplasia

FGFR3 gain-of-function mutations lead to both chondrodysplasias and craniosynostoses. Achondroplasia (ACH), the most frequent dwarfism, is due to an FGFR3-activating mutation which results in impaired endochondral ossification. The effects of the mutation on membranous ossification are unknown. Fgfr3(Y367C/+) mice mimicking ACH and craniofacial analysis of patients with ACH and FGFR3-related craniosynostoses provide an opportunity to address this issue. Studying the calvaria and skull base, we observed abnormal cartilage and premature fusion of the synchondroses leading to modifications of foramen magnum shape and size in Fgfr3(Y367C/+) mice, ACH and FGFR3-related craniosynostoses patients. Partial premature fusion of the coronal sutures and non-ossified gaps in frontal bones were also present in Fgfr3(Y367C/+) mice and ACH patients. Our data provide strong support that not only endochondral ossification but also membranous ossification is severely affected in ACH. Demonstration of the impact of FGFR3 mutations on craniofacial development should initiate novel pharmacological and surgical therapeutic approaches.

Meclozine facilitates proliferation and differentiation of chondrocytes by attenuating abnormally activated FGFR3 signaling in achondroplasia

Achondroplasia (ACH) is one of the most common skeletal dysplasias with short stature caused by gain-of-function mutations in FGFR3 encoding the fibroblast growth factor receptor 3. We used the drug repositioning strategy to identify an FDA-approved drug that suppresses abnormally activated FGFR3 signaling in ACH. We found that meclozine, an anti-histamine drug that has long been used for motion sickness, facilitates chondrocyte proliferation and mitigates loss of extracellular matrix in FGF2-treated rat chondrosarcoma (RCS) cells. Meclozine also ameliorated abnormally suppressed proliferation of human chondrosarcoma (HCS-2/8) cells that were infected with lentivirus expressing constitutively active mutants of FGFR3-K650E causing thanatophoric dysplasia, FGFR3-K650M causing SADDAN, and FGFR3-G380R causing ACH. Similarly, meclozine alleviated abnormally suppressed differentiation of ATDC5 chondrogenic cells expressing FGFR3-K650E and -G380R in micromass culture. We also confirmed that meclozine alleviates FGF2-mediated longitudinal growth inhibition of embryonic tibia in bone explant culture. Interestingly, meclozine enhanced growth of embryonic tibia in explant culture even in the absence of FGF2 treatment. Analyses of intracellular FGFR3 signaling disclosed that meclozine downregulates phosphorylation of ERK but not of MEK in FGF2-treated RCS cells. Similarly, meclozine enhanced proliferation of RCS cells expressing constitutively active mutants of MEK and RAF but not of ERK, which suggests that meclozine downregulates the FGFR3 signaling by possibly attenuating ERK phosphorylation. We used the C-natriuretic peptide (CNP) as a potent inhibitor of the FGFR3 signaling throughout our experiments, and found that meclozine was as efficient as CNP in attenuating the abnormal FGFR3 signaling. We propose that meclozine is a potential therapeutic agent for treating ACH and other FGFR3-related skeletal dysplasias.

Regulatory mechanisms for the development of growth plate cartilage

In vertebrates, most of the skeleton is formed through endochondral ossification. Endochondral bone formation is a complex process involving the mesenchymal condensation of undifferentiated cells, the proliferation of chondrocytes and their differentiation into hypertrophic chondrocytes, and mineralization. This process is tightly regulated by various factors including transcription factors, soluble mediators, extracellular matrices, and cell-cell and cell-matrix interactions. Defects of these factors often lead to skeletal dysplasias and short stature. Moreover, there is growing evidence that epigenetic and microRNA-mediated mechanisms also play critical roles in chondrogenesis. This review provides an overview of our current understanding of the regulators for the development of growth plate cartilage and their molecular mechanisms of action. A knowledge of the regulatory mechanisms underlying the proliferation and differentiation of chondrocytes will provide insights into future therapeutic options for skeletal disorders.

A human skeletal overgrowth mutation increases maximal velocity and blocks desensitization of guanylyl cyclase-B

C-type natriuretic peptide (CNP) increases long bone growth by stimulating guanylyl cyclase (GC)-B/NPR-B/NPR2. Recently, a Val to Met missense mutation at position 883 in the catalytic domain of GC-B was identified in humans with increased blood cGMP levels that cause abnormally long bones. Here, we determined how this mutation activates GC-B. In the absence of CNP, cGMP levels in cells expressing V883M-GC-B were increased more than 20 fold compared to cells expressing wild-type (WT)-GC-B, and the addition of CNP only further increased cGMP levels 2-fold. In the absence of CNP, maximal enzymatic activity (Vmax) of V883M-GC-B was increased 15-fold compared to WT-GC-B but the affinity of the enzymes for substrate as revealed by the Michaelis constant (Km) was unaffected. Surprisingly, CNP decreased the Km of V883M-GC-B 10-fold in a concentration-dependent manner without increasing Vmax. Unlike the WT enzyme the Km reduction of V883M-GC-B did not require ATP. Unexpectedly, V883M-GC-B, but not WT-GC-B, failed to inactivate with time. Phosphorylation elevated but was not required for the activity increase associated with the mutation because the Val to Met substitution also activated a GC-B mutant lacking all known phosphorylation sites. We conclude that the V883M mutation increases maximal velocity in the absence of CNP, eliminates the requirement for ATP in the CNP-dependent Km reduction, and disrupts the normal inactivation process.

The ras-GTPase activity of neurofibromin restrains ERK-dependent FGFR signaling during endochondral bone formation

The severe defects in growth plate development caused by chondrocyte extracellular signal-regulated protein kinases 1 and 2 (ERK1/2) gain or loss-of-function suggest that tight spatial and temporal regulation of mitogen-activated protein kinase signaling is necessary to achieve harmonious growth plate elongation and structure. We provide here evidence that neurofibromin, via its Ras guanosine triphosphatase -activating activity, controls ERK1/2-dependent fibroblast growth factor receptor (FGFR) signaling in chondrocytes. We show first that neurofibromin is expressed in FGFR-positive prehypertrophic and hypertrophic chondrocytes during growth plate endochondral ossification. Using mice lacking neurofibromin 1 (Nf1) in type II collagen-expressing cells, (Nf1col2(-/-) mutant mice), we then show that lack of neurofibromin in post-mitotic chondrocytes triggers a number of phenotypes reminiscent of the ones observed in mice characterized by FGFR gain-of-function mutations. Those include dwarfism, constitutive ERK1/2 activation, strongly reduced Ihh expression and decreased chondrocyte proliferation and maturation, increased chondrocytic expression of Rankl, matrix metalloproteinase 9 (Mmp9) and Mmp13 and enhanced growth plate osteoclastogenesis, as well as increased sensitivity to caspase-9 mediated apoptosis. Using wildtype (WT) and Nf1(-/-) chondrocyte cultures in vitro, we show that FGF2 pulse-stimulation triggers rapid ERK1/2 phosphorylation in both genotypes, but that return to the basal level is delayed in Nf1(-/-) chondrocytes. Importantly, in vivo ERK1/2 inhibition by daily injection of a recombinant form of C-type natriuretic peptide to post-natal pups for 18 days was able to correct the short stature of Nf1col2(-/-) mice. Together, these results underscore the requirement of neurofibromin and ERK1/2 for normal endochondral bone formation and support the notion that neurofibromin, by restraining RAS-ERK1/2 signaling, is a negative regulator of FGFR signaling in differentiating chondrocytes.