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.

Novel mutation and atlantoaxial dislocation in two siblings from India with Dyggve-Melchior-Clausen syndrome.

Dyggve-Melchior-Clausen syndrome is a rare variety of spondyloepimetaphyseal dysplasia which often resembles Morquio syndrome. We describe two siblings from India with the condition and report a novel homozygous mutation in them (c.1172_1173insC). One of them had atlantoaxial dislocation.

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.

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.

Mutation screening in patients with syndromic craniosynostoses indicates that a limited number of recurrent FGFR2 mutations accounts for severe forms of Pfeiffer syndrome.

Crouzon Syndrome (CS), Pfeiffer syndrome (PS) and the phenotypically related Jackson-Weiss (JW) variant are three craniosynostotic conditions caused by heterozygous mutations in Fibroblast Growth Factor Receptor (FGFR) genes. Screening a large cohort of 84 patients with clinical features of CS, PS or JW by direct sequencing of genomic DNA, enabled FGFR1, 2 or 3 mutation detection in 79 cases. Mutations preferentially occurred in exons 8 and 10 of FGFR2 encoding the third Ig loop of the receptor. Among the 74 FGFR2 mutations that we identified, four were novel including three missense substitutions causing CS and a 2 bp deletion creating a premature stop codon and producing JW phenotype. Five FGFR2 mutations were found in one of the two tyrosine kinase subdomains and one in the Ig I loop. Interestingly, two FGFR2 mutations creating cysteine residues (W290C and Y340C) caused severe forms of PS while conversion of the same residues into another amino-acid (W290G/R, Y340H) resulted in Crouzon phenotype exclusively. Our data provide conclusive evidence that the mutational spectrum of FGFR2 mutations in CS and PS is wider than originally thought. Genotype-phenotype analyses based on our cohort and previous studies further indicate that in spite of some overlap, PS and CS are preferentially accounted for by two distinct sets of FGFR2 mutations. A limited number of recurrent amino-acid changes (W290C, Y340C, C342R and S351C) is commonly associated with the most severe Pfeiffer phenotypes of poor prognosis.

X-linked spondyloepiphyseal dysplasia tarda: Novel and recurrent mutations in 13 European families.

X-linked spondyloepiphyseal dysplasia tarda is a skeletal dysplasia mainly affecting the vertebrae and epiphyses and commonly associated with the early development of degenerative joint disease. Radiographically the disorder is characterized by a typical hump-shaped deformity of the vertebral bodies. SEDT is caused by mutations in SEDL located on Xp22.12-p22.31. To further elucidate the spectrum of underlying variations we performed a screening of all 6 exons of SEDL within 13 European SEDT families and identified 6 new (c.99delC, c.183_184delGA, c.236-5_236-8delATTA, c.325delT, c.345_346delTG, c.94-?_423+?del) and 9 previously reported mutations (c.1-?_93+?del, c.93+5G>A, c.157_158delAT, c.210G>A, c.236-9_236-12delTTAA, c.267_275delAAGAC, c.324-4_324-10delTCTTTCCinsAA). The recurrent splice site alteration c.93+5G>A (formerly described as IVS3+5G>A) was detected in 3 unrelated families. Two patients were carrying 2 changes in the allele. In one case, a novel variation in exon 4 (c.99delC) was associated with several nucleotide deletions in intron 4 (c.236-5_236-8delATTA), and in the second case we identified a previously reported transition c.210G>A and a novel deletion in exon 6 (c.325delT). All sequence variations identified are either deletions of complete exons or predicted to result in a premature stop codon or to lead into splicing defects and are associated with a loss of considerable parts of the sedlin protein.