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

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.

An expressed fgf4 retrogene is associated with breed-defining chondrodysplasia in domestic dogs

Retrotransposition of processed mRNAs is a common source of novel sequence acquired during the evolution of genomes. Although the vast majority of retroposed gene copies, or retrogenes, rapidly accumulate debilitating mutations that disrupt the reading frame, a small percentage become new genes that encode functional proteins. By using a multibreed association analysis in the domestic dog, we demonstrate that expression of a recently acquired retrogene encoding fibroblast growth factor 4 (fgf4) is strongly associated with chondrodysplasia, a short-legged phenotype that defines at least 19 dog breeds including dachshund, corgi, and basset hound. These results illustrate the important role of a single evolutionary event in constraining and directing phenotypic diversity in the domestic dog.

Thanatophoric dysplasia caused by double missense FGFR3 mutations

Thanatophoric dysplasia is a lethal chondrodysplasia caused by heterozygous fibroblast growth factor receptor 3 (FGFR3) missense mutations. Mutations have been identified in several domains of the receptor. The most frequent mutations (p.R248C, p.S249C, p.Y373C) create a cysteine residue within the extracellular domain, whereas the others eliminate the termination codon (p.X807R, p.X807C, p.X807G, p.X807S, p.X807W). Here, we report a unique patient with thanatophoric dysplasia and a double de novo FGFR3 mutation, located on the same allele, (c.[1620C>A;1454A>G]), which corresponds to p.[N540K;Q485R]. The p.N540K mutation is associated with 60% of patients with hypochondroplasia and the p.Q485R mutation is a novel mutation located in a highly conserved domain of FGFRs. Evidence for the structural impact of the two concurrent missense mutations was achieved using protein alignments and three-dimensional structural prediction, in agreement with our modeling of the FGFR3 structure. In this patient with thanatophoric dysplasia, we conclude that the presence of the double FGFR3 missense mutation on the same allele alters the receptor structure, holding the receptor in its fully activated state, thus leading to lethal chondrodysplasia.

Implications of a vertex bulge following modified strip craniectomy for sagittal synostosis

BACKGROUND

Modified strip craniectomy is a common treatment for early isolated sagittal synostosis. The authors assessed the significance of the development of a progressive vertex bulge following strip craniectomy as a predictor of raised intracranial pressure or multiple suture synostosis.

METHODS

All cases of sagittal synostosis treated by modified strip craniectomy (removal of the sagittal suture with lateral barrel staving) at the authors' institution were reviewed. Eighty-nine patients with isolated sagittal synostosis were treated by modified strip craniectomy, usually before 6 months of age, between 1995 and 2005. Seven patients were noted to have developed a progressive vertex bulge. The vertex bulge was noted an average of 8 months postoperatively (range, 2 to 25 months). The clinical records of these seven patients were evaluated with regard to their clinical course, radiologic investigations, genetics testing, intracranial pressure monitoring, and the need for further surgery.

RESULTS

Computed tomographic scanning demonstrated new synostosis involving other calvarial sutures in five patients. Five patients underwent intracranial pressure monitoring, and this was elevated in four patients. One patient required a ventriculoperitoneal shunt for hydrocephalus. All patients underwent genetic screening, and two were found to have fibroblast growth factor receptor (FGFR) mutations (one FGFR2 and one FGFR3 mutation). All patients required reoperation (calvarial remodeling) for either raised intracranial pressure, deteriorating head shape, or both.

CONCLUSIONS

A progressive vertex bulge after modified strip craniectomy is a sign of possible raised intracranial pressure, the development of progressive multiple suture synostosis, or both. It is an indication for genetic testing for FGFR mutations.

Human immortalized chondrocytes carrying heterozygous FGFR3 mutations: an in vitro model to study chondrodysplasias

Achondroplasia and thanatophoric dysplasia are human chondrodysplasias caused by mutations in the fibroblast growth factor receptor 3 (FGFR3) gene. We have developed an immortalized human chondrocyte culture model to study the regulation of chondrocyte functions. One control and eight mutant chondrocytic lines expressing different FGFR3 heterozygous mutations were obtained. FGFR3 signaling pathways were modified in the mutant lines as revealed by the constitutive activation of the STAT pathway and an increased level of P21(WAF1/CIP1) protein. This model will be useful for the study of FGFR3 function in cartilage studies and future therapeutic approaches in chondrodysplasias.