A common FGFR3 gene mutation is present in achondroplasia but not in hypochondroplasia

Optimizing CRISPR/Cas9 technology for precise correction of the Fgfr3-G374R mutation in achondroplasia in mice

CRISPR/Cas9 is a powerful technology widely used for genome editing, with the potential to be used for correcting a wide variety of deleterious disease-causing mutations. However, the technique tends to generate more indels (insertions and deletions) than precise modifications at the target sites, which might not resolve the mutation and could instead exacerbate the initial genetic disruption. We sought to develop an improved protocol for CRISPR/Cas9 that would correct mutations without unintended consequences. As a case study, we focused on achondroplasia, a common genetic form of dwarfism defined by missense mutation in the Fgfr3 gene that results in glycine to arginine substitution at position 374 in mice in fibroblast growth factor receptor 3 (Fgfr3-G374R), which corresponds to G380R in humans. First, we designed a GFP reporter system that can evaluate the cutting efficiency and specificity of single guide RNAs (sgRNAs). Using the sgRNA selected based on our GFP reporter system, we conducted targeted therapy of achondroplasia in mice. We found that we achieved higher frequency of precise correction of the Fgfr3-G374R mutation using Cas9 protein rather than Cas9 mRNA. We further demonstrated that targeting oligos of 100 and 200 nucleotides precisely corrected the mutation at equal efficiency. We showed that our strategy completely suppressed phenotypes of achondroplasia and whole genome sequencing detected no off-target effects. These data indicate that improved protocols can enable the precise CRISPR/Cas9-mediated correction of individual mutations with high fidelity.

A Track Record on SHOX: From Basic Research to Complex Models and Therapy

SHOX deficiency is the most frequent genetic growth disorder associated with isolated and syndromic forms of short stature. Caused by mutations in the homeobox gene SHOX, its varied clinical manifestations include isolated short stature, Léri-Weill dyschondrosteosis, and Langer mesomelic dysplasia. In addition, SHOX deficiency contributes to the skeletal features in Turner syndrome. Causative SHOX mutations have allowed downstream pathology to be linked to defined molecular lesions. Expression levels of SHOX are tightly regulated, and almost half of the pathogenic mutations have affected enhancers. Clinical severity of SHOX deficiency varies between genders and ranges from normal stature to profound mesomelic skeletal dysplasia. Treatment options for children with SHOX deficiency are available. Two decades of research support the concept of SHOX as a transcription factor that integrates diverse aspects of bone development, growth plate biology, and apoptosis. Due to its absence in mouse, the animal models of choice have become chicken and zebrafish. These models, therefore, together with micromass cultures and primary cell lines, have been used to address SHOX function. Pathway and network analyses have identified interactors, target genes, and regulators. Here, we summarize recent data and give insight into the critical molecular and cellular functions of SHOX in the etiopathogenesis of short stature and limb development.

Identification of a novel nonsense mutation and a missense substitution in the AGPAT2 gene causing congenital generalized lipodystrophy type 1

Congenital generalized lipodystrophy (CGL) is an autosomal recessive disease characterized by the generalized scant of adipose tissue. CGL type 1 is caused by mutations in gene encoding 1-acylglycerol-3-phosphate O-acyltransferase-2 (AGPAT2). A clinical and molecular genetic investigation was performed in affected and unaffected members of two families with CGL type 1. The AGPAT2 coding region was sequenced in index cases of the two families. The presence of the identified mutations in relevant parents was tested. We identified a novel nonsense mutation (c.685G>T, p.Glu229*) and a missense substitution (c.514G>A, p.Glu172Lys). The unaffected parents in both families were heterozygous carrier of the relevant mutation. The results expand genotype–phenotype spectrum in CGL1 and will have applications in prenatal and early diagnosis of the disease. This is the first report of Persian families identified with AGPAT2 mutations.

Controlled release of growth factors on allograft bone in vitro

Allografts are important alternatives to autografts for treating defects after major bone loss. Bone growth factors have both local autocrine and paracrine effects and regulate the growth, proliferation, and differentiation of osteoprogenitor cells. To study the effects of prolonged, continuous, local delivery of growth factors on bone growth, we developed a new microelectromechanical system (MEMS) drug delivery device. Bone marrow cells from mice were seeded on mouse allograft discs and cultured in osteogenic media with osteogenic protein 1 (OP-1) and/or basic fibroblast growth factor (FGF-2) delivered from MEMS devices for 6 weeks. We monitored bone formation by changes of bone volume using micro-CT scanning and release of osteocalcin using ELISA. The data suggest the MEMS devices delivered constant concentrations of OP-1 and FGF-2 to the media. Bone marrow cells grew on the allografts and increased bone volume. Addition of OP-1 increased bone formation whereas FGF-2 decreased bone formation. Local delivery of growth factors over a prolonged period modulated the differentiation of osteoprogenitor cells on allograft bone.

SHOX at a glance: from gene to protein

The Short Stature Homeobox-containing Gene SHOX was identified as the genetic cause of the short stature phenotype in patients with Turner Syndrome and in certain patients with idiopathic short stature. Shortly after, SHOX mutations were also associated with the growth failure and skeletal deformities seen in patients with Léri - Weill dyschondrosteosis and Langer mesomelic dysplasia. Today it is estimated that SHOX mutations occur with an incidence of roughly 1:1,000 in newborns, making mutations of this gene one of the most common genetic defects leading to growth failure in humans. This review summarises the involvement of SHOX in several short stature syndromes and describes recent advances in our understanding of SHOX functions and regulation. We also discuss the current evidence in the literature that points to a role of this protein in growth and bone development. These studies have improved our knowledge of the SHOX gene and protein functions, and have given insight into the etiopathogenesis of short stature. However, the exact role of SHOX in bone development still remains elusive and poses the next major challenge for researchers in this field.

Novel FGFR3 mutations creating cysteine residues in the extracellular domain of the receptor cause achondroplasia or severe forms of hypochondroplasia

Achondroplasia (ACH) and hypochondroplasia (HCH) are two autosomal-dominant skeletal disorders caused by recurrent missense FGFR3 mutations in the transmembrane (TM) and tyrosine kinase 1 (TK1) domains of the receptor. Although 98% of ACH cases are accounted for by a single G380R substitution in the TM, a common mutation (N540K) in the TK1 region is detected in only 60-65% of HCH cases. The aim of this study was to determine whether the frequency of mutations in patients with HCH was the result of incomplete mutation screening or genetic heterogeneity. Eighteen exons of the FGFR3 gene were entirely sequenced in a cohort of 25 HCH and one ACH patients in whom common mutations had been excluded. Seven novel missense FGFR3 mutations were identified, one causing ACH and six resulting in HCH. Six of these substitutions were located in the extracellular region and four of them creating additional cysteine residues, were associated with severe phenotypes. No mutations were detected in 19 clinically diagnosed HCH patients. Our results demonstrate that the spectrum of FGFR3 mutations causing short-limb dwarfism is wider than originally recognised and emphasise the requirement for complete screening of the FGFR3 gene if appropriate genetic counselling is to be offered to patients with HCH or ACH lacking the most common mutations and their families.

Occurrence of thanatophoric dysplasia type I (R248C) and hypochondroplasia (N540K) mutations in two patients with achondroplasia phenotype

We report two patients with clinical and radiological findings of achondroplasia, who had the most common FGFR3 mutation occurring in thanatophoric dysplasia type I and hypochondroplasia, respectively. Thanatophoric dysplasia is usually a lethal condition, but the patient carrying this mutation is alive and presents a medical history similar to that of patients with achondroplasia. The events leading to such a discrepancy between genotype and phenotype are unclear. These rare cases may influence an appropriate medical and genetic counseling.

The molecular and genetic basis of fibroblast growth factor receptor 3 disorders: the achondroplasia family of skeletal dysplasias, Muenke craniosynostosis, and Crouzon syndrome with acanthosis nigricans

Achondroplasia, the most common form of short-limbed dwarfism in humans, occurs between 1 in 15,000 and 40,000 live births. More than 90% of cases are sporadic and there is, on average, an increased paternal age at the time of conception of affected individuals. More then 97% of persons with achondroplasia have a Gly380Arg mutation in the transmembrane domain of the fibroblast growth factor receptor (FGFR) 3 gene. Mutations in the FGFR3 gene also result in hypochondroplasia, the lethal thanatophoric dysplasias, the recently described SADDAN (severe achondroplasia with developmental delay and acanthosis nigricans) dysplasia, and two craniosynostosis disorders: Muenke coronal craniosynostosis and Crouzon syndrome with acanthosis nigricans. Recent evidence suggests that the phenotypic differences may be due to specific alleles with varying degrees of ligand-independent activation, allowing the receptor to be constitutively active. Since the Gly380Arg achondroplasia mutation was recognized, similar observations regarding the conserved nature of FGFR mutations and resulting phenotype have been made regarding other skeletal phenotypes, including hypochondroplasia, thanatophoric dysplasia, and Muenke coronal craniosynostosis. These specific genotype-phenotype correlations in the FGFR disorders seem to be unprecedented in the study of human disease. The explanation for this high degree of mutability at specific bases remains an intriguing question.

Expression of the components of the insulin-like growth factor axis across the growth-plate

Linear bone growth occurs as the result of proliferation and differentiation of growth-plate chondrocytes. These two phases of chondrocyte growth are regulated separately, with insulin-like growth factor I (IGF-I) being the primary stimulator of proliferation. We studied the expression of the components of the growth hormone GH/IGF system to learn if this proliferative signal is altered as chondrocytes undergo differentiation. Growth-plate chondrocytes were isolated from fetal cows and fractionated on discontinuous Percoll gradients. Five populations were recovered, ranging from high density cells (proliferative chondrocytes) to low density cells (hypertrophic chondrocytes). Messenger RNAs (mRNAs) were analyzed by a reverse transcriptase/quantitative polymerase chain reaction (RT/qPCR) technique. Results showed that mRNA of IGF-I and IGF-II in proliferative chondrocytes was 32 and five fold more abundant, respectively, than in hypertrophic chondrocytes. Of the four major IGF-I mRNA transcripts, the class 1-Ea transcript was predominant. Messenger RNA levels for IGFBP-3, -4, and -5 were also reduced in hypertrophic chondrocytes. Levels of GH receptor, the type 1 IGF receptor, and IGF binding protein-2 (IGFBP-2) mRNAs were unchanged across the growth-plate. Since IGF-I and -II are potent stimulators of proliferation, the down-regulation of these genes may be necessary in order for hypertrophy to proceed.

The gene for cherubism maps to chromosome 4p16

Cherubism is an autosomal dominant disorder that may be related to tooth development and eruption. It is a disorder of age-related bone remodeling, mostly limited to the maxilla and the mandible, with loss of bone in the jaws and its replacement with large amounts of fibrous tissue. We have used a genomewide search with a three-generation family and have established linkage to chromosome 4p16. Three other families affected with cherubism were also genotyped and were mapped to the same locus. The combined LOD score is 4.21 at a recombination fraction of 0, and the locus spans an interval of approximately 22 cM.

Mutation in the gene encoding the fibroblast growth factor receptor-3 in Korean children with achondroplasia

BACKGROUND

Achondroplasia (ACH) is the most common form of osteochondrodysplasia, and is mostly associated with a point mutation in the gene on the transmembrane domain of fibroblast growth factor receptor-3 (FGFR-3) on chromosome 4p.

METHODS

We investigated the mutations in the gene encoding FGFR-3 in 15 Korean children with ACH, using polymerase chain reaction (PCR) coupled with direct sequencing.

RESULTS

In this study, all children with ACH showed the same mutation as those reported in France, USA and Japan; a G-->A transition at position 1138 of the coding sequence, resulting in the substitution of arginine for glycine at position 380 of the mature protein.

CONCLUSIONS

This consistent point mutation of Korean children with ACH indicates there is no significant racial difference in the pathogenesis of ACH, compared with data from Caucasian and Japanese children with ACH.

Comparison of clinical-radiological and molecular findings in hypochondroplasia

Hypochondroplasia is an autosomal dominant skeletal dysplasia characterized by disproportionate short stature. A mutation (N540K) in the fibroblast growth factor receptor 3 (FGFR3) gene was described in some patients with this condition. The aims of the study were to identify the frequency of the FGFR3 gene mutation, to define the salient clinical and radiological abnormalities of the affected subjects, and to verify the contribution of molecular findings to the clinical and radiological definition of hypochondroplasia. Based on the most common radiological criteria, we selected 18 patients with a phenotype compatible with hypochondroplasia. Height, sitting height, and cranial circumference were measured in all patients. Radiographs of the lumbar spine, left leg, pelvis, and left hand were also obtained. The presence of the N540K mutation was verified by restriction enzyme digestions. Half of our patients carried the N540K mutation. Although similar in phenotype to the patients without the mutation, they showed in addition relative macrocephaly. The association of the unchanged/narrow interpedicular distance with the fibula longer than the tibia was more common in patients with gene mutation. Although we did not find a firm correlation between genotype and phenotype, in our study the N540K mutation was most often associated with disproportionate short stature, macrocephaly, and with radiological findings of unchanged/narrow interpedicular distance and fibula longer than tibia.

Functions of fibroblast growth factors in vertebrate development

Fibroblast growth factors (FGFs) are a class of secreted polypeptide ligands which mediate diverse cellular responses during embryonic, fetal, and postnatal vertebrate development. The purposes of this review are to provide a condensed overview of FGFs and their receptors, to catalog and categorize the functions of FGFs in vertebrate development, to present recent discoveries relating to the interplay of FGFs with other secreted ligands in the control of tissue growth and patterning, and to discuss several potential directions for future research in the field.

Involvement of FGF-8 in initiation, outgrowth and patterning of the vertebrate limb

Fibroblast Growth Factors (FGFs) are signaling molecules that are important in patterning and growth control during vertebrate limb development. Beads soaked in FGF-1, FGF-2 and FGF-4 are able to induce additional limbs when applied to the flank of young chick embryos (Cohn, M.J., Izpisua-Belmonte, J-C., Abud, H., Heath, J. K., Tickle, C. (1995) Cell 80, 739–746). However, biochemical and expression studies suggest that none of these FGFs is the endogenous signal that initiates limb development. During chick limb development, Fgf-8 transcripts are detected in the intermediate mesoderm and subsequently in the prelimb field ectoderm prior to the formation of the apical ectodermal ridge, structures required for limb initiation and outgrowth, respectively. Later on, Fgf-8 expression is restricted to the ridge cells and expression disappears when the ridge regresses. Application of FGF-8 protein to the flank induces the development of additional limbs. Moreover, we show that FGF-8 can replace the apical ectodermal ridge to maintain Shh expression and outgrowth and patterning of the developing chick limb. Furthermore, continuous and widespread misexpression of FGF-8 causes limb truncations and skeletal alterations with phocomelic or achondroplasia phenotype. Thus, FGF-8 appears to be a key signal involved in initiation, outgrowth and patterning of the developing vertebrate limb.

In vitro studies on clonal growth of chondrocytes in thanatophoric dysplasia

Thanatophoric dysplasia (TD) is characterized by a disorganized growth plate with markedly reduced proliferative and hypertrophic cartilage zones. Therefore, we studied in vitro the proliferation rates of articular chondrocytes from five TD patients and age-matched controls in response to bFGF, IGF-I, IGF-II, and TGF-beta 1. In human fetal controls bFGF was the most potent growth factor. Clonal growth the articular chondrocytes in response to bFGF was reduced in two of five TD patients and slightly below the range of controls in a third case. Stimulation of chondrocyte proliferation by IGF I and II was reduced in the patient whose response to bFGF was most markedly impaired. The effect of TGF-beta 1 ranged from normal to slightly elevated values in TD fetuses. These results indicate heterogeneity of the underlying defects in TD. Low proliferative responses of chondrocytes to bFGF and IGF-I/II are likely to play a key role in the pathogenesis of some cases. In two of five patients studied, the mechanisms of bFGF and IGF-signal transduction are candidates for the primary molecular defect.

Human growth hormone treatment in prepubertal children with achondroplasia

We studied the effects of recombinant human growth hormone (GH) treatment in 6 prepubertal children with achondroplasia. The patients' age ranged from 2 11/12 to 8 5/12 years and the GH dose was of 0.1 IU/kg/day subcutaneously. Auxological assessments and bone age determinations were performed 6 months before, at the beginning, and after 6 and 12 months of therapy. The growth velocity increase during the whole year of treatment ranged from 1.1 to 2.6 cm/year in 3 patients while in the others no variation was detected. No side effects were observed during the trial apart from a slight advancement of bone age in two patients. MRI at the cervicomedullary junction and CT scan of the base of the skull did not show any variation of the dimensions of the foramen magnum at the end of the trial compared to baseline. Our study shows that r-hGH can safely increase short-term growth velocity in some but not all prepubertal children with achondroplasia. Our data confirm the individual variability in the response to the GH treatment.

Predominance of the mutation at 1138 of the cDNA for the fibroblast growth factor receptor 3 in Japanese patients with achondroplasia

Fibroblast growth factor receptor 3 (FGFR3) has recently been identified as a putative gene for achondroplasia. Since a guanine to adenine mutation at 1138 of the cDNA for FGFR3 had been identified in most of the patients in Western population, we examined 13 Japanese patients to see if they also share the same mutation. Specific endonuclease digestion of the amplified coding sequence for the transmembrane domain of the FGFR3 revealed that the 12 patients have the G to A change at 1138, while the other had the G to C substitution at the same point, both of which result in G380A substitution. As far as we studied, the homogeneity of the point mutation at 1138 is also authentic to Japanese patient as well as Western patients.

Fibroblast growth factor receptor 3 (FGFR3) transmembrane mutation in Crouzon syndrome with acanthosis nigricans

Crouzon syndrome, an autosomal dominant condition characterized by craniosynostosis, ocular proptosis and midface hypoplasia, is associated with mutations in fibroblast growth factor receptor 2 (FGFR2) (refs 1-3). For example, we have identified 10 different mutations in the FGFR2 extracellular immunoglobulin III (IgIII) domain in 50% (16/32) of our Crouzon syndrome patients. All mutations described so far for other craniosynostotic syndromes with associated limb anomalies--Jackson-Weiss, Pfeiffer, and Apert--also occur in the extracellular domain of FGFR2, as well as FGFR1 for Pfeiffer syndrome. In contrast, only FGFR3 mutations have been reported in dwarfing conditions--achondroplasia, thanatophoric dysplasia, and hypochondroplasia. For achondroplasia, greater than 99% of mutations occur in the FGFR3 transmembrane domain. We now report the unexpected observation of a FGFR3 transmembrane domain mutation, Ala391Glu, in three unrelated families with Crouzon syndrome and acanthosis nigricans, a specific skin disorder of hyperkeratosis and hyperpigmentation. The association of non-dwarfing and even non-skeletal conditions with FGFR3 mutations reveals the potential for a wide range of FGFR pleiotropic effects as well as locus heterogeneity in Crouzon syndrome. Our study underscores the biologic complexity of the FGFR gene family.

A recurrent mutation in the tyrosine kinase domain of fibroblast growth factor receptor 3 causes hypochondroplasia

Hypochondroplasia (MIM 146000) is an autosomal dominant skeletal dysplasia with skeletal features similar to but milder than those seen in achondroplasia. Within the past year, the achondroplasia locus has been mapped to 4p 16.3 (refs 5-7) and mutations in the fibroblast growth factor receptor 3 (FGFR3) gene have been identified in patients with the disorder. More than 95% of 242 cases reported so far are accounted for by a single Gly380Arg mutation. McKusick et al. proposed that achondroplasia and hypochondroplasia are allelic based on the similarities in phenotype between the two disorders and the identification of a severely dwarfed individual whose father had achondroplasia and whose mother had hypochondroplasia. There is also genetic linkage evidence that hypochondroplasia and achondroplasia map to the same locus. We therefore began a systematic screening of FGFR3 to detect mutations in patients with hypochondroplasia. We now report a single FGFR3 mutation found in 8 out of 14 unrelated patients with hypochondroplasia. This mutation causes a C to A transversion at nucleotide 1620, resulting in an Asn540Lys substitution in the proximal tyrosine kinase domain.