The molecular pathology of syndromic craniosynostosis

Talocalcaneal coalition in Muenke syndrome: report of a patient, review of the literature in FGFR-related craniosynostoses, and consideration of mechanism

Muenke syndrome is an autosomal dominant craniosynostosis syndrome resulting from a defining point mutation in the Fibroblast Growth Factor Receptor3 (FGFR3) gene. Muenke syndrome is characterized by coronal craniosynostosis (bilateral more often than unilateral), hearing loss, developmental delay, and carpal and/or tarsal bone coalition. Tarsal coalition is a distinct feature of Muenke syndrome and has been reported since the initial description of the disorder in the 1990s. Although talocalcaneal coalition is the most common tarsal coalition in the general population, it has never previously been reported in a patient with Muenke syndrome. We present a 7-year-old female patient with Muenke syndrome and symptomatic talocalcaneal coalition. She presented at the age of 7 with limping, tenderness and pain in her right foot following a fall and strain of her right foot. She was treated with ibuprofen, shoe inserts, a CAM walker boot, and stretching exercises without much improvement in symptoms. A computed tomography (CT) scan revealed bilateral talocalcaneal coalitions involving the middle facet. She underwent resection of the talocalcaneal coalitions, remaining pain-free post-operatively with an improvement in her range of motion, gait, and mobility. This report expands the phenotype of tarsal coalition in Muenke syndrome to include talocalcaneal coalition. A literature review revealed a high incidence of tarsal coalition in all FGFR related craniosynostosis syndromes when compared to the general population, a difference that is statistically significant. The most common articulation involved in all syndromic craniosynostoses associated with FGFR mutations is the calcaneocuboid articulation.

Antley-Bixler-syndrome--staged management of craniofacial malformations from birth to adolescence--a case report

In 1975 Antley and Bixler described an unusual syndromal disorder consisting of complex craniosynostosis with midfacial hypoplasia, dysplasia of ears and nose, radiohumeral synostosis, congenital fractures of the femur and upper airway impairment in a newborn. Additional urogenital and cardiac malformations can be associated however diagnosis is based on a characteristic craniofacial deformity in association with humeroradial synostosis. Complex disturbance of craniofacial growth due to premature synostoses of the cranial base and vault results in a characteristic phenotype. Steroidogenesis due to intrinsic or extrinsic disturbance by maternal fluconazole ingestion during early pregnancy may be impaired. The mode of inheritance is supposed to be autosomal recessive. Mutations in the fibroblast growth factor receptor 2 (FGFR2) as well as mutations in the cytochrome P450 oxidoreductase (OR) gene have been verified. Like in other craniofacial dysostosis syndromes malformation of neuro- and viscerocranium is complex and requires a staged age- and growth-related interdisciplinary management with respect to the individual situation. This case report of a female patient born in 1994 suffering from that rare syndrome describes the interdisciplinary long-term management in one craniofacial centre over 16 years from birth to adolescence.

Advances in the molecular pathogenesis of craniofacial conditions

The impact that the understanding of fibroblast growth factor receptor (FGFR) biology and its relevance to the pathogenesis of the craniosynostoses has made cannot be underestimated. As the genetic and molecular pathology of other conditions become increasingly understood, there is much hope that robust and relevant animal models of these conditions may be generated. From these models-and in conjunction with laboratory studies in vitro-comes a real hope of improved therapeutic strategies. The future lies in increased cooperation between clinicians working in high-volume centers and basic scientists. This article decribes the results of a decade of research in which the molecular pathology of the craniosynostoses was unravelled. The understanding of the importance of FGFR mutations to the genetic etiology of craniosynostosis opened up novel studies in developmental biology in various tissues. Such studies describe the functional effects of FGFR mutations. Investigations of FGFR expression in human craniofacial development have related functional molecular studies to human craniosynostosis syndromes, which provides a link between the gene mutation and the affected child.

Identification of novel target genes of CeTwist and CeE/DA

Twist, a basic helix-loop-helix (bHLH) transcription factor, plays an important role in mesoderm development in many organisms, including C. elegans where CeTwist is required to direct cell fate specifications of a subset of mesodermal cells. Although several target genes of CeTwist have been identified, how this protein accomplishes its function is unclear. In addition, several human genes whose mutations cause different syndromes of craniosynostosis (premature fusion of cranial sutures) have homologues in the CeTwist pathway. Identification of novel target genes of CeTwist will shed more light on the functions of CeTwist in mesoderm development, and the corresponding human homologues will be good candidates for related syndromes with unidentified mutated genes. In our study, both CeTwist and its heterodimeric partner, CeE/DA, were overexpressed from the inducible heat-shock promoter, and potential target genes were detected with Affymetrix oligonucleotide microarrays. Using transcriptional GFP reporters, we found 11 genes were expressed in cells coincident with known CeTwist target gene products. Based on subsequent validation experiments, 9 genes were defined as novel CeTwist and CeE/DA targets. Human homologues of two of these genes might be involved in craniofacial diseases, which further validates C. elegans as a good model organism for providing insights into these disorders.

Characterization of a dominant negativeC. elegansTwist mutant protein with implications for human Saethre-Chotzen syndrome

Twist is a transcription factor that is required for mesodermal cell fates in all animals studied to date. Mutations of this locus in humans have been identified as the cause of the craniofacial disorder Saethre-Chotzen syndrome. The Caenorhabditis elegans Twist homolog is required for the development of a subset of the mesoderm. A semidominant allele of the gene that codes for CeTwist, hlh-8, has defects that occur earlier in the mesodermal lineage than a previously studied null allele of the gene. The semidominant allele has a charge change (E29K) in the basic DNA-binding domain of CeTwist. Surprisingly, the mutant protein retains DNA-binding activity as both a homodimer and a heterodimer with its partner E/Daughterless (CeE/DA). However, the mutant protein blocks the activation of the promoter of a target gene. Therefore, the mutant CeTwist may cause cellular defects as a dominant negative protein by binding to target promoters as a homo- or heterodimer and then blocking transcription. Similar phenotypes as those caused by the E29K mutation were observed when amino acid substitutions in the DNA-binding domain that are associated with the human Saethre-Chotzen syndrome were engineered into the C. elegans protein. These data suggest that Saethre-Chotzen syndrome may be caused, in some cases, by dominant negative proteins, rather than by haploinsufficiency of the locus.

Blocking endogenous FGF-2 activity prevents cranial osteogenesis

Normal growth and morphogenesis of the cranial vault reflect a balance between cell proliferation in the sutures and osteogenesis at the margins of the cranial bones. In the clinical condition craniosynostosis, the sutures fuse prematurely as a result of precocious osteogenic differentiation and craniofacial malformation results. Mutations in several fibroblast growth factor receptor (FGFR) genes have now been identified as being responsible for the major craniosynostotic syndromes. We have used a grafting technique to manipulate the levels of endogenous FGF-2 ligand in embryonic chick cranial vaults and thereby perturb morphogenesis. Implantation of beads loaded with FGF-2 did not affect normal cranial development at physiological concentrations, although they elicited a morphogenetic response in the limb. Implantation of beads loaded with a neutralising antibody to FGF-2 generated a concentration-dependent response. When a single bead was implanted, the grafts grew to a massive size as a result of increased cell division in the tissue. With greater inactivation of FGF-2 protein (two to three beads implanted), all further bone differentiation and cell proliferation was blocked. These data further support the emerging idea that the intensity of FGF-mediated signalling determines the developmental fate of the skeletogenic cells in the cranial vault. High and low levels correlate with differentiation and proliferation, respectively. A balance between the two ensures normal cranial vault morphogenesis. This is consistent with the observation that several FGFR mutations causing craniosynostosis result in constitutive activation of the receptor.

In vivo modulation of FGF biological activity alters cranial suture fate

Gain-of-function mutations in fibroblast growth factor receptors have been identified in numerous syndromes associated with premature cranial suture fusion. Murine models in which the posterior frontal suture undergoes programmed fusion after birth while all other sutures remain patent provide an ideal model to study the biomolecular mechanisms that govern cranial suture fusion. Using adenoviral vectors and targeted in utero injections in rats, we demonstrate that physiological posterior frontal suture fusion is inhibited using a dominant-negative fibroblast growth factor receptor-1 construct, whereas the normally patent coronal suture fuses when infected with a construct that increases basic fibroblast growth factor biological activity. Our data may facilitate the development of novel, less invasive treatment options for children with craniosynostosis.

Regional differentiation of cranial suture-associated dura mater in vivo and in vitro: implications for suture fusion and patency

Despite its prevalence, the etiopathogenesis of craniosynostosis is poorly understood. To better understand the biomolecular events that occur when normal craniofacial growth development goes awry, we must first investigate the mechanisms of normal suture fusion. Murine models in which the posterior frontal (PF) suture undergoes programmed sutural fusion shortly after birth provide an ideal model to study these mechanisms. In previous studies, our group and others have shown that sutural fate (i.e., fusion vs. patency) is regulated by the dura mater (DM) directly underlying a cranial suture. These studies have led to the hypothesis that calvarial DM is regionally differentiated and that this differentiation guides the development of the overlying suture. To test this hypothesis, we evaluated the messenger RNA (mRNA) expression of osteogenic cytokines (transforming growth factor beta1 [TGF-beta1] and TGF-beta3) and bone-associated extracellular matrix (ECM) molecules (collagen I, collagen III, osteocalcin, and alkaline phosphatase) in freshly isolated, rat dural tissues associated with the PF (programmed to fuse) or sagittal (SAG; remains patent) sutures before histological evidence of sutural fusion (postnatal day 6 [N6]). In addition, osteocalcin protein expression and cellular proliferation were localized using immunohistochemical staining and 5-bromo-2'deoxyuridine (BrdU) incorporation, respectively. We showed that the expression of osteogenic cytokines and bone-associated ECM molecules is potently up-regulated in the DM associated with the PF suture. In addition, we showed that cellular proliferation in the DM associated with the fusing PF suture is significantly less than that found in the patent SAG suture just before the initiation of sutural fusion N6. Interestingly, no differences in cellular proliferation rates were noted in younger animals (embryonic day 18 [E18] and N2). To further analyze regional differentiation of cranial suture-associated dural cells, we established dural cell cultures from fusing and patent rat cranial sutures in N6 rats and evaluated the expression of osteogenic cytokines (TGF-beta1 and fibroblast growth factor 2 [FGF-2]) and collagen I. In addition, we analyzed cellular production of proliferating cell nuclear antigen (PCNA). These studies confirmed our in vivo findings and showed that dural cell cultures derived from the fusing PF suture expressed significantly greater amounts of TGF-beta1, FGF-2, and collagen I. In addition, similar to our in vivo findings, we showed that PF suture-derived dural cells produced significantly less PCNA than SAG suture-derived dural cells. Finally, coculture of dural cells with fetal rat calvarial osteoblastic cells (FRCs) revealed a statistically significant increase in proliferation (*p < 0.001) in FRCs cocultured with SAG suture-derived dural cells as compared with FRCs cocultured alone or with PF suture-derived dural cells. Taken together, these data strongly support the hypothesis that the calvarial DM is regionally differentiated resulting in the up-regulation of osteogenic cytokines and bone ECM molecules in the dural tissues underlying fusing but not patent cranial sutures. Alterations in cytokine expression may govern osteoblastic differentiation and ECM molecule deposition, thus regulating sutural fate. Elucidation of the biomolecular events that occur before normal cranial suture fusion in the rat may increase our understanding of the events that lead to premature cranial suture fusion.

Evidence for digenic inheritance in some cases of Antley-Bixler syndrome?

The Antley-Bixler syndrome has been thought to be caused by an autosomal recessive gene. However, patients with this phenotype have been reported with a new dominant mutation at the FGFR2 locus as well as in the offspring of mothers taking the antifungal agent fluconazole during early pregnancy. In addition to the craniosynostosis and joint ankylosis which are the clinical hallmarks of the condition, many patients, especially females, have genital abnormalities. We now report abnormalities of steroid biogenesis in seven of 16 patients with an Antley-Bixler phenotype. Additionally, we identify FGFR2 mutations in seven of these 16 patients, including one patient with abnormal steroidogenesis. These findings, suggesting that some cases of Antley-Bixler syndrome are the outcome of two distinct genetic events, allow a hypothesis to be formulated under which we may explain all the differing and seemingly contradictory circumstances in which the Antley-Bixler phenotype has been recognised.

Pleiotropic features of syndromic craniosynostoses correlate with differential expression of fibroblast growth factor receptors 1 and 2 during human craniofacial development

Mutations in FGFR1, -2, and -3 are linked to five human craniosynostosis syndromes. In addition to premature fusion of cranial sutures, nonskeletal manifestations in skin, and teeth together with CNS abnormalities, reflect widespread effects of these mutations. To understand this pleiotropy, we have assessed craniofacial FGFR1 and -2 expression in the human embryo from 6 wk postfertilization. We found that both genes are expressed in sheets of condensed mesenchyme before overt chondrogenic differentiation and that distinct patterns of expression are established by 8 wk. Thus, FGFR2(BEK) is expressed evenly throughout developing cartilage and bone, whereas FGFR1 transcripts predominate in perichondria and periostea. Complementary patterns of FGFR1 and FGFR2(BEK and KGFR) expression are also observed in the enamel epithelium and papilla mesenchyme of the tooth germ, at a stage when morphogenetic tissue interactions ensue. Both genes are expressed in the cortical layer of the brain, but expression levels vary significantly within the choroid plexus and wall of the fourth ventricle. Similarly, tissue-specific differences in receptor expression are found in both the skin and salivary glands. These expression data are consistent with the pleiotropic manifestations of syndromic craniosynostoses and provide the basis for a new paradigm to explain the associated CNS problems.

Maxillary distraction osteogenesis in Pfeiffer's syndrome: urgent ocular protection by gradual midfacial skeletal advancement

Distraction osteogenesis is increasingly recognised as a potentially useful technique to achieve the co-ordinated augmentation of craniofacial skeletal and soft tissue. A case is presented where bilateral maxillary distraction was successfully used to advance the midface in the treatment of recurrent ocular dislocation, in a 10-month-old boy with Pfeiffer's syndrome.

Craniofacial developmental abnormalities

Major advances have been made in the elucidation of the molecular basis of a number of human dysmorphic syndromes involving abnormalities of craniofacial development. This will lead, in turn, to a greater understanding of the mechanisms that underlie normal craniofacial development.

Craniosynostosis associated with FGFR3 pro250arg mutation results in a range of clinical presentations including unisutural sporadic craniosynostosis

Several mutations involving the fibroblast growth factor receptor (FGFR) gene family have been identified in association with phenotypically distinct forms of craniosynostosis. One such point mutation, resulting in the substitution of proline by arginine in a critical region of the linker region between the first and second immunoglobulin-like domains, is associated with highly specific phenotypic consequences in that mutation at this point in FGFR1 results in Pfeiffer syndrome and analogous mutation in FGFR2 results in Apert syndrome. We now show that a much more variable clinical presentation accompanies analogous mutation in the FGFR3 gene. Specifically, mental retardation, apparently unrelated to the management of the craniosynostosis, appears to be a variable clinical consequence of this FGFR3 mutation.

Skeletal dysplasias detectable by DNA analysis

The emerging data of the last few years outlining the molecular basis of skeletal dysplasias has been instructive in several respects. The number of genetic loci involved appears to be much fewer than anticipated. This is offset by the identification of several instances where phenotypically distinct entities are found to be allelic variants. With respect to diagnosis by DNA, most of the conditions recognized have several different mutations described. Consequently, while mutation analysis may be possible in a given case, close liaison with the investigating laboratory is essential if optimal results are to be obtained. Achondroplasia is unusual in that there is a common mutation and the other mutations related to the phenotype appear to cluster to a few codons. This review highlights the relationship between phenotypes of skeletal malformation, their underlying loci and mutations. These mutations appear to mediate their phenotypic effects through a diverse range of genetic mechanisms.

Analysing human developmental abnormalities

About one in forty babies is born with a recognisable congenital anomaly at birth. Rapid progress is being made in recognising the genetic contribution to these defects. From over 2000 likely single gene malformation syndromes in humans the gene has been isolated or mapped in about 10%. Despite the availability of animal models, the study of malformations in humans continues to reveal novel genes and unpredicted functions for known genes. The importance of the study of clinical malformations to the understanding of embryological development in humans and other organisms is discussed and reviewed.

A recurrent mutation, ala391glu, in the transmembrane region of FGFR3 causes Crouzon syndrome and acanthosis nigricans

Mutations in the fibroblast growth factor receptor 2 (FGFR2) gene have previously been identified in Crouzon syndrome, an autosomal dominant condition involving premature fusion of the cranial sutures. Several different missense and other mutations have been identified in Crouzon syndrome patients, clustering around the third immunoglobulin-like domain. We report here the identification of a mutation in the transmembrane region of FGFR3, common to three unrelated patients with classical Crouzon syndrome and acanthosis nigricans, a dermatological condition associated with thickening and abnormal pigmentation of the skin. The mutation within the FGFR3 transcript was determined by direct sequencing as a specific gcg to gag transversion, resulting in an amino acid substitution ala391glu within the transmembrane region.