Craniosynostosis and related limb anomalies

Craniosynostosis as a clinical and diagnostic problem: molecular pathology and genetic counseling

Craniosynostosis (occurrence: 1/2500 live births) is a result of premature fusion of cranial sutures, leading to alterations of the pattern of cranial growth, resulting in abnormal shape of the head and dysmorphic facial features. In approximately 85% of cases, the disease is isolated and nonsyndromic and mainly involves only one suture. Syndromic craniosynostoses such as Crouzon, Apert, Pfeiffer, Muenke, and Saethre-Chotzen syndromes not only affect multiple sutures, but are also associated with the presence of additional clinical symptoms, including hand and feet malformations, skeletal and cardiac defects, developmental delay, and others. The etiology of craniosynostoses may involve genetic (also somatic mosaicism and regulatory mutations) and epigenetic factors, as well as environmental factors. According to the published data, chromosomal aberrations, mostly submicroscopic ones, account for about 6.7-40% of cases of syndromic craniosynostoses presenting with premature fusion of metopic or sagittal sutures. The best characterized is the deletion or translocation of the 7p21 region containing the TWIST1 gene. The deletions of 9p22 or 11q23-qter (Jacobsen syndrome) are both associated with trigonocephaly. The genes related to the pathogenesis of the craniosynostoses itself are those encoding transcription factors, e.g., TWIST1, MSX2, EN1, and ZIC1, and proteins involved in osteogenic proliferation, differentiation, and homeostasis, such as FGFR1, FGFR2, RUNX2, POR, and many others. In this review, we present the clinical and molecular features of selected craniosynostosis syndromes, genotype-phenotype correlation, family genetic counseling, and propose the most appropriate diagnostic algorithm.

Arterial Calcification in Diabetes Mellitus: Preclinical Models and Translational Implications

Diabetes mellitus increasingly afflicts our aging and dysmetabolic population. Type 2 diabetes mellitus and the antecedent metabolic syndrome represent the vast majority of the disease burden-increasingly prevalent in children and older adults. However, type 1 diabetes mellitus is also advancing in preadolescent children. As such, a crushing wave of cardiometabolic disease burden now faces our society. Arteriosclerotic calcification is increased in metabolic syndrome, type 2 diabetes mellitus, and type 1 diabetes mellitus-impairing conduit vessel compliance and function, thereby increasing the risk for dementia, stroke, heart attack, limb ischemia, renal insufficiency, and lower extremity amputation. Preclinical models of these dysmetabolic settings have provided insights into the pathobiology of arterial calcification. Osteochondrogenic morphogens in the BMP-Wnt signaling relay and transcriptional regulatory programs driven by Msx and Runx gene families are entrained to innate immune responses-responses activated by the dysmetabolic state-to direct arterial matrix deposition and mineralization. Recent studies implicate the endothelial-mesenchymal transition in contributing to the phenotypic drift of mineralizing vascular progenitors. In this brief overview, we discuss preclinical disease models that provide mechanistic insights-and point to challenges and opportunities to translate these insights into new therapeutic strategies for our patients afflicted with diabetes mellitus and its arteriosclerotic complications.

From Bench to Bedside and Back: Improving Diagnosis and Treatment of Craniofacial Malformations Utilizing Animal Models

Craniofacial anomalies are among the most common birth defects and are associated with increased mortality and, in many cases, the need for lifelong treatment. Over the past few decades, dramatic advances in the surgical and medical care of these patients have led to marked improvements in patient outcomes. However, none of the treatments currently in clinical use address the underlying molecular causes of these disorders. Fortunately, the field of craniofacial developmental biology provides a strong foundation for improved diagnosis and for therapies that target the genetic causes of birth defects. In this chapter, we discuss recent advances in our understanding of the embryology of craniofacial conditions, and we focus on the use of animal models to guide rational therapies anchored in genetics and biochemistry.

Phenotype profile of a genetic mouse model for Muenke syndrome

PURPOSE

The Muenke syndrome mutation (FGFR3 (P250R)), which was discovered 15 years ago, represents the single most common craniosynostosis mutation. Muenke syndrome is characterized by coronal suture synostosis, midface hypoplasia, subtle limb anomalies, and hearing loss. However, the spectrum of clinical presentation continues to expand. To better understand the pathophysiology of the Muenke syndrome, we present collective findings from several recent studies that have characterized a genetically equivalent mouse model for Muenke syndrome (FgfR3 (P244R)) and compare them with human phenotypes.

CONCLUSIONS

FgfR3 (P244R) mutant mice show premature fusion of facial sutures, premaxillary and/or zygomatic sutures, but rarely the coronal suture. The mice also lack the typical limb phenotype. On the other hand, the mutant mice display maxillary retrusion in association with a shortening of the anterior cranial base and a premature closure of intersphenoidal and spheno-occipital synchondroses, resembling human midface hypoplasia. In addition, sensorineural hearing loss is detected in all FgfR3 (P244R) mutant mice as in the majority of Muenke syndrome patients. It is caused by a defect in the mechanism of cell fate determination in the organ of Corti. The mice also express phenotypes that have not been previously described in humans, such as reduced cortical bone thickness, hypoplastic trabecular bone, and defective temporomandibular joint structure. Therefore, the FgfR3 (P244R) mouse provides an excellent opportunity to study disease mechanisms of some classical phenotypes of Muenke syndrome and to test novel therapeutic strategies. The mouse model can also be further explored to discover previously unreported yet potentially significant phenotypes of Muenke syndrome.

Development of an efficient, non-viral transfection method for studying gene function and bone growth in human primary cranial suture mesenchymal cells reveals that the cells respond to BMP2 and BMP3

Background

Achieving efficient introduction of plasmid DNA into primary cultures of mammalian cells is a common problem in biomedical research. Human primary cranial suture cells are derived from the connective mesenchymal tissue between the bone forming regions at the edges of the calvarial plates of the skull. Typically they are referred to as suture mesenchymal cells and are a heterogeneous population responsible for driving the rapid skull growth that occurs in utero and postnatally. To better understand the molecular mechanisms involved in skull growth, and in abnormal growth conditions, such as craniosynostosis, caused by premature bony fusion, it is essential to be able to easily introduce genes into primary bone forming cells to study their function.

Results

A comparison of several lipid-based techniques with two electroporation-based techniques demonstrated that the electroporation method known as nucleofection produced the best transfection efficiency. The parameters of nucleofection, including cell number, amount of DNA and nucleofection program, were optimized for transfection efficiency and cell survival. Two different genes and two promoter reporter vectors were used to validate the nucleofection method and the responses of human primary suture mesenchymal cells by fluorescence microscopy, RT-PCR and the dual luciferase assay. Quantification of bone morphogenetic protein (BMP) signalling using luciferase reporters demonstrated robust responses of the cells to both osteogenic BMP2 and to the anti-osteogenic BMP3.

Conclusions

A nucleofection protocol has been developed that provides a simple and efficient, non-viral alternative method for in vitro studies of gene and protein function in human skull growth. Human primary suture mesenchymal cells exhibit robust responses to BMP2 and BMP3, and thus nucleofection can be a valuable method for studying the potential competing action of these two bone growth factors in a model system of cranial bone growth.

The Muenke syndrome mutation (FgfR3P244R) causes cranial base shortening associated with growth plate dysfunction and premature perichondrial ossification in murine basicranial synchondroses

Muenke syndrome caused by the FGFR3(P250R) mutation is an autosomal dominant disorder mostly identified with coronal suture synostosis, but it also presents with other craniofacial phenotypes that include mild to moderate midface hypoplasia. The Muenke syndrome mutation is thought to dysregulate intramembranous ossification at the cranial suture without disturbing endochondral bone formation in the skull. We show in this study that knock-in mice harboring the mutation responsible for the Muenke syndrome (FgfR3(P244R)) display postnatal shortening of the cranial base along with synchondrosis growth plate dysfunction characterized by loss of resting, proliferating and hypertrophic chondrocyte zones and decreased Ihh expression. Furthermore, premature conversion of resting chondrocytes along the perichondrium into prehypertrophic chondrocytes leads to perichondrial bony bridge formation, effectively terminating the postnatal growth of the cranial base. Thus, we conclude that the Muenke syndrome mutation disturbs endochondral and perichondrial ossification in the cranial base, explaining the midface hypoplasia in patients.

Normal and disease-related biological functions of Twist1 and underlying molecular mechanisms

This article reviews the molecular structure, expression pattern, physiological function, pathological roles and molecular mechanisms of Twist1 in development, genetic disease and cancer. Twist1 is a basic helix-loop-helix domain-containing transcription factor. It forms homo- or hetero-dimers in order to bind the Nde1 E-box element and activate or repress its target genes. During development, Twist1 is essential for mesoderm specification and differentiation. Heterozygous loss-of-function mutations of the human Twist1 gene cause several diseases including the Saethre-Chotzen syndrome. The Twist1-null mouse embryos die with unclosed cranial neural tubes and defective head mesenchyme, somites and limb buds. Twist1 is expressed in breast, liver, prostate, gastric and other types of cancers, and its expression is usually associated with invasive and metastatic cancer phenotypes. In cancer cells, Twist1 is upregulated by multiple factors including SRC-1, STAT3, MSX2, HIF-1α, integrin-linked kinase and NF-κB. Twist1 significantly enhances epithelial-mesenchymal transition (EMT) and cancer cell migration and invasion, hence promoting cancer metastasis. Twist1 promotes EMT in part by directly repressing E-cadherin expression by recruiting the nucleosome remodeling and deacetylase complex for gene repression and by upregulating Bmi1, AKT2, YB-1, etc. Emerging evidence also suggests that Twist1 plays a role in expansion and chemotherapeutic resistance of cancer stem cells. Further understanding of the mechanisms by which Twist1 promotes metastasis and identification of Twist1 functional modulators may hold promise for developing new strategies to inhibit EMT and cancer metastasis.

Q289P mutation in the FGFR2 gene: first report in a patient with type 1 Pfeiffer syndrome

When normal development and growth of the calvarial sutures is disrupted, craniosynostosis (premature calvarial suture fusion) may result. Classical craniosynostosis syndromes are autosomal dominant traits and include Apert, Pfeiffer, Crouzon, Jackson-Weiss, and Saethre-Chotzen syndromes. In these conditions, there is premature fusion of skull bones leading to an abnormal head shape, ocular hypertelorism with proptosis, and midface hypoplasia. It is known that mutations in the fibroblast growth factor receptors 1, 2, and 3 cause craniosynostosis. We report on a child with a clinically diagnosed Pfeiffer syndrome that shows the missense point mutation Q289P in exon 8 of the FGFR2 gene. This is a mutation not previously described in the Pfeiffer syndrome but reported in the Crouzon, Jackson-Weiss, and Saethre-Chotzen syndromes. In this paper, we propose the concept that these disorders may represent one genetic condition with phenotypic variability.

Pfeiffer syndrome

Pfeiffer syndrome is a rare autosomal dominantly inherited disorder that associates craniosynostosis, broad and deviated thumbs and big toes, and partial syndactyly on hands and feet. Hydrocephaly may be found occasionally, along with severe ocular proptosis, ankylosed elbows, abnormal viscera, and slow development. Based on the severity of the phenotype, Pfeiffer syndrome is divided into three clinical subtypes. Type 1 "classic" Pfeiffer syndrome involves individuals with mild manifestations including brachycephaly, midface hypoplasia and finger and toe abnormalities; it is associated with normal intelligence and generally good outcome. Type 2 consists of cloverleaf skull, extreme proptosis, finger and toe abnormalities, elbow ankylosis or synostosis, developmental delay and neurological complications. Type 3 is similar to type 2 but without a cloverleaf skull. Clinical overlap between the three types may occur. Pfeiffer syndrome affects about 1 in 100,000 individuals. The disorder can be caused by mutations in the fibroblast growth factor receptor genes FGFR-1 or FGFR-2. Pfeiffer syndrome can be diagnosed prenatally by sonography showing craniosynostosis, hypertelorism with proptosis, and broad thumb, or molecularly if it concerns a recurrence and the causative mutation was found. Molecular genetic testing is important to confirm the diagnosis. Management includes multiple-staged surgery of craniosynostosis. Midfacial surgery is performed to reduce the exophthalmos and the midfacial hypoplasia.

Fox-2 mediates epithelial cell-specific fibroblast growth factor receptor 2 exon choice

Alternative splicing of fibroblast growth factor receptor 2 (FGFR2) transcripts occurs in a cell-type-specific manner leading to the mutually exclusive use of exon IIIb in epithelia or exon IIIc in mesenchyme. Epithelial cell-specific exon choice is dependent on (U)GCAUG elements, which have been shown to bind Fox protein family members. In this paper we show that FGFR2 exon choice is regulated by (U)GCAUG elements and Fox protein family members. Fox-2 isoforms are differentially expressed in IIIb+ cells in comparison to IIIc+ cells, and expression of Fox-1 or Fox-2 in the latter led to a striking alteration in FGFR2 splice choice from IIIc to IIIb. This switch was absolutely dependent on the (U)GCAUG elements present in the FGFR2 pre-mRNA and required critical residues in the C-terminal region of Fox-2. Interestingly, Fox-2 expression led to skipping of exon 6 among endogenous Fox-2 transcripts and formation of an inactive Fox-2 isoform, which suggests that Fox-2 can regulate its own activity. Moreover, the repression of exon IIIc in IIIb+ cells was abrogated by interfering RNA-mediated knockdown of Fox-2. We also show that Fox-2 is critical for the FGFR2(IIIb)-to-FGFR2(IIIc) switch observed in T Rex-293 cells grown to overconfluency. Overconfluent T Rex-293 cells show molecular and morphological changes consistent with a mesenchymal-to-epithelial transition. If overconfluent cells are depleted of Fox-2, the switch from IIIc to IIIb is abrogated. The data in this paper place Fox-2 among critical regulators of gene expression during mesenchymal-epithelial transitions and demonstrate that this action of Fox-2 is mediated by mechanisms distinct from those described for other cases of Fox activity.

Disruption of Fgf10/Fgfr2b-coordinated epithelial-mesenchymal interactions causes cleft palate

Classical research has suggested that early palate formation develops via epithelial-mesenchymal interactions, and in this study we reveal which signals control this process. Using Fgf10-/-, FGF receptor 2b-/- (Fgfr2b-/-), and Sonic hedgehog (Shh) mutant mice, which all exhibit cleft palate, we show that Shh is a downstream target of Fgf10/Fgfr2b signaling. Our results demonstrate that mesenchymal Fgf10 regulates the epithelial expression of Shh, which in turn signals back to the mesenchyme. This was confirmed by demonstrating that cell proliferation is decreased not only in the palatal epithelium but also in the mesenchyme of Fgfr2b-/- mice. These results reveal a new role for Fgf signaling in mammalian palate development. We show that coordinated epithelial-mesenchymal interactions are essential during the initial stages of palate development and require an Fgf-Shh signaling network.

Msx2 and Twist cooperatively control the development of the neural crest-derived skeletogenic mesenchyme of the murine skull vault

The flat bones of the vertebrate skull vault develop from two migratory mesenchymal cell populations, the cranial neural crest and paraxial mesoderm. At the onset of skull vault development, these mesenchymal cells emigrate from their sites of origin to positions between the ectoderm and the developing cerebral hemispheres. There they combine, proliferate and differentiate along an osteogenic pathway. Anomalies in skull vault development are relatively common in humans. One such anomaly is familial calvarial foramina, persistent unossified areas within the skull vault. Mutations in MSX2 and TWIST are known to cause calvarial foramina in humans. Little is known of the cellular and developmental processes underlying this defect. Neither is it known whether MSX2 and TWIST function in the same or distinct pathways. We trace the origin of the calvarial foramen defect in Msx2 mutant mice to a group of skeletogenic mesenchyme cells that compose the frontal bone rudiment. We show that this cell population is reduced not because of apoptosis or deficient migration of neural crest-derived precursor cells, but because of defects in its differentiation and proliferation. We demonstrate, in addition, that heterozygous loss of Twist function causes a foramen in the skull vault similar to that caused by loss of Msx2 function. Both the quantity and proliferation of the frontal bone skeletogenic mesenchyme are reduced in Msx2-Twist double mutants compared with individual mutants. Thus Msx2 and Twist cooperate in the control of the differentiation and proliferation of skeletogenic mesenchyme. Molecular epistasis analysis suggests that Msx2 and Twist do not act in tandem to control osteoblast differentiation, but function at the same epistatic level.

Foster-type modification of the Knapp procedure for anomalous superior rectus muscles in syndromic craniosynostoses

PURPOSE

To describe the surgical management of anomalous superior rectus muscles in patients with syndromic craniosynostoses.

METHODS

Retrospectively reviewed were case notes of 3 patients with vertical deviations that were thought to have anomalous superior rectus muscles.

RESULTS

All 3 patients had hypotropia preoperatively, and 2 had coexisting exotropia. Two patients exhibited massive subconjunctival fibrosis intraoperatively, but none had undergone previous strabismus surgery, although they had undergone craniofacial procedures. Orbital imaging (either computed tomographic or magnetic resonance imaging scans) confirmed an absent or thinned superior rectus muscle in all 3 patients. All 3 underwent a Knapp procedure with appropriate recession and resection of the transposed horizontal rectus muscles if indicated. A nonabsorbable suture was placed in the sclera at the upper border of each horizontal rectus muscle to draw this border closer to the vertical midline, approximately 16 to 18 mm from the limbus (Foster-type modification). In each case, the hypotropia and upgaze were improved but not completely normalized.

CONCLUSIONS

A Foster-type modification of the Knapp procedure satisfactorily corrected the hypotropia in these patients. Orbital imaging can confirm the presence of an anomalous superior rectus muscle. The massive subconjunctival fibrosis may be explained by the type of previous craniofacial surgery the patients had undergone.

Fgfr mRNA isoforms in craniofacial bone development

Mutations in genes encoding for fibroblast growth factor receptors (FGFRs) have been identified as causes of both chondrodysplasias and craniosynostoses, both of which cause abnormalities in the growth and development of the craniofacial region. FGFRs form mRNA splicing isoforms, each with distinct ligand binding specificity and tissue distribution. These confer specific biological functions on these isoforms. Although it is known that FGFRs are expressed at numerous locations during early mouse development, including the craniofacial area, relatively little is known about the expression of the splicing isoforms during craniofacial bone development. To address this, we have performed a detailed survey to detect these genes in the developing mouse craniofacial region. We have analyzed the developing mouse mandible, calvaria, and cranial base, in particular the spheno-occipital synchondrosis, a key centre of craniofacial growth. Fgfr1c was detected weakly in osteoblastic cells in both the developing calvarial and mandibular bones. Fgfr3b and Fgfr3c were found chiefly in proliferating chondrocytes of the cranial base synchondroses and the mandibular condyle. Fgfr2b transcripts were most notably detected in the perichondria of the mandibular condyle and the cranial base. Fgfr2c transcripts were detected with high intensity in differentiating osteoblasts at the sutural osteogenic fronts of the calvarial bones. In addition, Fgfr2c was also expressed in the perichondria of the mandibular condyle and the cranial base. These expression patterns suggest both differing and similar functions for -b and -c isoforms. The former is exemplified by Fgfr1 transcripts, which show distinct differences in their distribution, being mutually exclusive. Similar functions are suggested by the overlapping expression patterns of the -b and -c isoforms of both Fgfr2 and Fgfr3. Fgfr4 transcripts were found in developing muscles. These data help to explain the disturbances in craniofacial growth exhibited by both patients and the growing number of transgenic mice carrying mutations in genes encoding FGFRs/Fgfrs.

Limb anomalies: Developmental and evolutionary aspects

In this review we describe the developmental mechanisms involved in the making of a limb, by focusing on the nature and types of interactions of the molecules that play a part in the regulation of limb patterning and characterizing clinical conditions that are known to result from the abnormal function of these molecules. The latter subject is divided into sections dealing with syndromal and nonsyndromal deficiencies, polydactylies, and brachydactylies. Conditions caused by mutations in homeobox genes and fibroblast growth factors and their receptor genes are listed separately. Since the process of limb development has been conserved for more than 300 millions years, with all the necessary adaptive modifications occurring throughout evolution, we also take into consideration the evolutionary aspects of limb development in terms of genetic repertoire, molecular pathways, and morphogenetic events.

Mouse Twist is required for fibroblast growth factor-mediated epithelial-mesenchymal signalling and cell survival during limb morphogenesis

Mouse Twist is essential for cranial neural tube, limb and somite development. [Genes Dev. 9 (1995) 686]. To identify the molecular defects disrupting limb morphogenesis, we have analysed expression of mesenchymal transcription factors involved in patterning and the cell-cell signalling cascades controlling limb bud development. These studies establish that Twist is essential for maintenance and progression of limb bud morphogenesis. In particular, the SHH/FGF signalling feedback loop operating between the polarizing region and the apical ectodermal ridge (AER) is disrupted. These defects in epithelial-mesenchymal signalling are most likely a direct consequence of disrupted fibroblast growth factor (FGF) signalling in Twist-deficient limb buds. In early limb buds, down-regulation of Fgf receptor 1 and Fgf10 expression in the mesenchyme occurs concurrent with loss of Fgf4 and Fgf8 expression in the AER. Finally, Twist function, most likely by regulating FGF signalling, is required for cell survival as apoptotic cells are detected in posterior and distal limb bud mesenchyme.

Expression patterns of fibroblast growth factors-18 and -20 in mouse embryos is suggestive of novel roles in calvarial and limb development

The normal development of calvarial bones and sutures critically relies on proper signalling through Fgf receptors, but the source and identity of cognate ligands have remained unknown. Reverse transcriptase polymerase chain reaction analysis in this study shows that a broad range of Fgf ligands are expressed in the coronal sutures separating the parietal and frontal bones. Analysis by whole mount in situ hybridization further reveals distinct expression patterns for Fgf-18, Fgf-20, and by comparison, Fgf-9, in the calvaria, and Fgfs-20 and -9 in the developing limbs, suggestive of their role in proliferation, differentiation and apoptosis.

The life cycle of chondrocytes in the developing skeleton

Cartilage serves multiple functions in the developing embryo and in postnatal life. Genetic mutations affecting cartilage development are relatively common and lead to skeletal malformations, dysfunction or increased susceptibility to disease or injury. Characterization of these mutations and investigation of the molecular pathways in which these genes function have contributed to an understanding of the mechanisms regulating skeletal patterning, chondrogenesis, endochondral ossification and joint formation. Extracellular growth and differentiation factors including bone morphogenetic proteins, fibroblast growth factors, parathyroid hormone-related peptide, extracellular matrix components, and members of the hedgehog and Wnt families provide important signals for the regulation of cell proliferation, differentiation and apoptosis. Transduction of these signals within the developing mesenchymal cells and chondrocytes results in changes in gene expression mediated by transcription factors including Smads, Msx2, Sox9, signal transducer and activator of transcription (STAT), and core-binding factor alpha 1. Further investigation of the interactions of these signaling pathways will contribute to an understanding of cartilage growth and development, and will allow for the development of strategies for the early detection, prevention and treatment of diseases and disorders affecting the skeleton.