Identical mutations in three different fibroblast growth factor receptor genes in autosomal dominant craniosynostosis syndromes

Identification of rare variants in PTCH2 associated with non-syndromic orofacial clefts

Orofacial clefts (OFCs) represent the most prevalent congenital craniofacial anomalies, significantly impacting patients' appearance, oral function, and psychological well-being. Among these, non-syndromic OFCs (NSOFCs) are the most predominant type, with the etiology attributed to a combination of genetic and environmental factors. Rare variants of key genes involved in craniofacial development-related signaling pathway are crucial in the occurrence of NSOFCs, and our recent studies have identified PTCH1, a receptor-coding gene in the Hedgehog signaling pathway, as a causative gene for NSOFCs. However, the role of PTCH2, the paralog of PTCH1, in pathogenesis of NSOFCs remains unclear. Here, we perform whole-exome sequencing to explore the genetic basis of 144 sporadic NSOFC patients. We identify five heterozygous variants of PTCH2 in four patients: p.L104P, p.A131G, p.R557H, p.I927S, and p.V978D, with the latter two co-occurring in a single patient. These variants, all proven to be rare through multiple genomic databases, with p.I927S and p.V978D being novel variants and previously unreported. Sequence alignment suggests that these affected amino acids are evolutionarily conserved across vertebrates. Utilizing predictive structural modeling tools such as AlphaFold and SWISS-MODEL, we propose that these variants may disrupt the protein's structure and function. In summary, our findings suggest that PTCH2 may be a novel candidate gene predicted to be associated with NSOFCs, thereby broadening the spectrum of causative genes implicated in the craniofacial anomalies.

The value of genome-wide analysis in craniosynostosis

Background: This study assessed the diagnostic yield of high-throughput sequencing methods in a cohort of craniosynostosis (CS) patients not presenting causal variants identified through previous targeted analysis. Methods: Whole-genome or whole-exome sequencing (WGS/WES) was performed in a cohort of 59 patients (from 57 families) assessed by retrospective phenotyping as having syndromic or nonsyndromic CS. Results: A syndromic form was identified in 51% of the unrelated cases. A genetic cause was identified in 38% of syndromic cases, with novel variants detected in FGFR2 (a rare Alu insertion), TWIST1, TCF12, KIAA0586, HDAC9, FOXP1, and NSD2. Additionally, we report two patients with rare recurrent variants in KAT6A and YY1 as well as two patients with structural genomic aberrations: one with a 22q13 duplication and one with a complex rearrangement involving chromosome 2 (2p25 duplication including SOX11 and deletion of 2q22). Moreover, we identified potentially relevant variants in 87% of the remaining families with no previously detected causal variants, including novel variants in ADAMTSL4, ASH1L, ATRX, C2CD3, CHD5, ERF, H4C5, IFT122, IFT140, KDM6B, KMT2D, LTBP1, MAP3K7, NOTCH2, NSD1, SOS1, SPRY1, POLR2A, PRRX1, RECQL4, TAB2, TAOK1, TET3, TGFBR1, TCF20, and ZBTB20. Conclusion: These results confirm WGS/WES as a powerful diagnostic tool capable of either targeted in silico or broad genomic analysis depending on phenotypic presentation (e.g., classical or unusual forms of syndromic CS).

Automated three-dimensional analysis of facial asymmetry in patients with syndromic coronal synostosis: A retrospective study

Craniosynostosis, characterized by premature fusion of one or more cranial sutures, results in a distorted skull shape. Only three studies have assessed facial asymmetry manually in unicoronal synostosis patients. It is therefore important to understand how uni- and bicoronal synostosis affect facial asymmetry with a minimum risk of human bias. An automated algorithm was developed to quantify facial asymmetry from three-dimensional images, generating a mean facial asymmetry (MFA) value in millimeters to reflect the degree of asymmetry. The framework was applied to analyze postoperative 3D images of syndromic patients (N = 35) diagnosed with Muenke syndrome, Saethre-Chotzen syndrome, and TCF12-related craniosynostosis with respect to MFA values from a healthy control group (N = 89). Patients demonstrated substantially higher MFA values than controls: Muenke syndrome (unicoronal 1.74 ± 0.40 mm, bicoronal 0.77 ± 0.21 mm), Saethre-Chotzen syndrome (unicoronal 1.15 ± 0.20 mm, bicoronal 0.69 ± 0.16 mm), and TCF12-related craniosynostosis (unicoronal 1.40 ± 0.51 mm, bicoronal 0.66 ± 0.05 mm), compared with controls (0.49 ± 0.12 mm). Longitudinal analysis identified an increasing MFA trend in unicoronal synostosis patients. Our study revealed higher MFA in syndromic patients with uni- and bicoronal synostosis compared with controls, with the most pronounced MFA in Muenke syndrome patients with unilateral synostosis. Bicoronal synostosis patients demonstrated higher facial asymmetry than expected given the condition's symmetrical presentation.

Spaceflight-Associated Vascular Remodeling and Gene Expression in Mouse Calvaria

Astronauts suffer from a loss of bone mass at a rate of 1.5% per month from lower regions of the body during the course of long-duration (>30 days) spaceflight, a phenomenon that poses important risks for returning crew. Conversely, a gain in bone mass may occur in non-load bearing regions of the body as related to microgravity-induced cephalad fluid shift. Representing non-load bearing regions with mouse calvaria and leveraging the STS-131 (15-day) and BION-M1 (30-day) flights, we examined spatial and temporal calvarial vascular remodeling and gene expression related to microgravity exposure compared between spaceflight (SF) and ground control (GC) cohorts. We examined parasagittal capillary numbers and structures in calvaria from 16 to 23 week-old C57BL/6 female mice (GC, n = 4; SF, n = 5) from STS-131 and 19–20 week-old C57BL/6 male mice (GC, n = 6; SF, n = 6) from BION-M1 using a robust isolectin-IB4 vessel marker. We found that the vessel diameter reduces significantly in mice exposed to 15 days of spaceflight relative to control. Capillarization increases by 30% (SF vs. GC, p = 0.054) in SF mice compared to GC mice. The vessel numbers and diameter remain unchanged in BION-M1 mice calvarial section. We next analyzed the parietal pro-angiogenic (VEGFA) and pro-osteogenic gene (BMP-2, DMP1, RUNX2 and OCN) expression in BION-M1 mice using quantitative RT-PCR. VEGFA gene expression increased 15-fold while BMP-2 gene expression increased 11-fold in flight mice compared to GC. The linkage between vascular morphology and gene expression in the SF conditions suggests that angiogenesis may be important in the regulation of pathological bone growth in non-weight bearing regions of the body. Short-duration microgravity-mediated bone restructuring has implications in planning effective countermeasures for long-duration flights and extraterrestrial human habitation.

Spatial transcriptomics reveals a role for sensory nerves in preserving cranial suture patency through modulation of BMP/TGF-β signaling

The patterning and ossification of the mammalian skeleton requires the coordinated actions of both intrinsic bone morphogens and extrinsic neurovascular signals, which function in a temporal and spatial fashion to control mesenchymal progenitor cell (MPC) fate. Here, we show the genetic inhibition of tropomyosin receptor kinase A (TrkA) sensory nerve innervation of the developing cranium results in premature calvarial suture closure, associated with a decrease in suture MPC proliferation and increased mineralization. In vitro, axons from peripheral afferent neurons derived from dorsal root ganglions (DRGs) of wild-type mice induce MPC proliferation in a spatially restricted manner via a soluble factor when cocultured in microfluidic chambers. Comparative spatial transcriptomic analysis of the cranial sutures in vivo confirmed a positive association between sensory axons and proliferative MPCs. SpatialTime analysis across the developing suture revealed regional-specific alterations in bone morphogenetic protein (BMP) and TGF-β signaling pathway transcripts in response to TrkA inhibition. RNA sequencing of DRG cell bodies, following direct, axonal coculture with MPCs, confirmed the alterations in BMP/TGF-β signaling pathway transcripts. Among these, the BMP inhibitor follistatin-like 1 (FSTL1) replicated key features of the neural-to-bone influence, including mitogenic and anti-osteogenic effects via the inhibition of BMP/TGF-β signaling. Taken together, our results demonstrate that sensory nerve-derived signals, including FSTL1, function to coordinate cranial bone patterning by regulating MPC proliferation and differentiation in the suture mesenchyme.

Skeletal stem and progenitor cells maintain cranial suture patency and prevent craniosynostosis

Cranial sutures are major growth centers for the calvarial vault, and their premature fusion leads to a pathologic condition called craniosynostosis. This study investigates whether skeletal stem/progenitor cells are resident in the cranial sutures. Prospective isolation by FACS identifies this population with a significant difference in spatio-temporal representation between fusing versus patent sutures. Transcriptomic analysis highlights a distinct signature in cells derived from the physiological closing PF suture, and scRNA sequencing identifies transcriptional heterogeneity among sutures. Wnt-signaling activation increases skeletal stem/progenitor cells in sutures, whereas its inhibition decreases. Crossing Axin2LacZ/+ mouse, endowing enhanced Wnt activation, to a Twist1+/- mouse model of coronal craniosynostosis enriches skeletal stem/progenitor cells in sutures restoring patency. Co-transplantation of these cells with Wnt3a prevents resynostosis following suturectomy in Twist1+/- mice. Our study reveals that decrease and/or imbalance of skeletal stem/progenitor cells representation within sutures may underlie craniosynostosis. These findings have translational implications toward therapeutic approaches for craniosynostosis.

Mechanical properties of fused sagittal sutures in scaphocephaly

BACKGROUND

Craniosynostosis in newborns is caused by the premature closure of the cranial sutures leading to cranial vault deformity. It results in aesthetic imbalance and developmental disabilities and surgery is frequent during the first months of growth. Our study focused on scaphocephaly defined as the premature closure of the sagittal suture. We hypothesised that the effective mechanical properties of sutures were altered as compared to those of the parietal adjacent tissue considered as control.

METHODS

The population consisted of seven males and four females (mean age 4.9 months). Sixteen suture samples and thirty-four parietal tissue samples were harvested during corrective surgery and investigated by using three-point bending tests to obtain the structure-stiffness of specimens. An energy model was used to derive the effective Young's modulus. A histological study complemented the experimental protocol.

FINDINGS

Fused sutures were thicker than adjacent bone and the natural curvature of sutures did not influence the static mechanical response. The stiffness of stenotic sutures was significantly higher than that of the parietal bone. The effective Young's modulus of stenotic sutures was significantly lower than that of the parietal adjacent tissue. The parietal tissue showed a parallel bone architecture whereas the central stenotic tissue was disorganised with more vascularisation.

INTERPRETATION

The stenotic suture differed in structural and mechanical terms from the adjacent bone during calvarial growth in the first year of life. Our study emphasised the alteration of effective tissue properties in craniosynostosis.

Craniosynostosis: current conceptions and misconceptions

Cranial bones articulate in areas called sutures that must remain patent until skull growth is complete. Craniosynostosis is the condition that results from premature closure of one or more of the cranial vault sutures, generating facial deformities and more importantly, skull growth restrictions with the ability to severely affect brain growth. Typically, craniosynostosis can be expressed as an isolated event, or as part of syndromic phenotypes. Multiple signaling mechanisms interact during developmental stages to ensure proper and timely suture fusion. Clinical outcome is often a product of craniosynostosis subtypes, number of affected sutures and timing of premature suture fusion. The present work aimed to review the different aspects involved in the establishment of craniosynostosis, providing a close view of the cellular, molecular and genetic background of these malformations.

A bibliometric overview of craniosynostosis research development

This article reviews the development of research in the field of craniosynostosis from a bibliometric standpoint. Craniosynostosis is a malformation occurring during the early development of the skull, when one or more of the sutures close too early, causing problems with normal brain and skull growth. Research in this field has developed from early clinical case descriptions, to genetic discoveries responsible for the occurring malformations and onwards to developing sophisticated surgical treatment. In this article we describe these developments, zoom in on publication trends and characteristics and visualize developing networks and topic shifts in this research field.

Clinical study and some molecular features of Mexican patients with syndromic craniosynostosis

BACKGROUND

Craniosynostosis is one of the major genetic disorders affecting 1 in 2,100-2,500 live newborn children. Environmental and genetic factors are involved in the manifestation of this disease. The suggested genetic causes of craniosynostosis are pathogenic variants in FGFR1, FGFR2, FGFR3, and TWIST1 genes.

METHODS

In order to describe their major clinical characteristics and the presence of pathogenic variants, a sample of 36 Mexican patients with craniosynostosis diagnosed as: Crouzon (OMIM 123,500), Pfeiffer (OMIM 101,600), Apert (OMIM 101,200), Saethre-Chotzen (OMIM 101,400), and Muenke (OMIM 602,849) was analyzed.

RESULTS

In addition to craniosynostosis, most of the patients presented hypertelorism, midface hypoplasia, and abnormalities in hands and feet. To detect the pathogenic variants p.Pro252Arg FGFR1 (OMIM 136,350), p.Ser252Trp, p.Pro253Arg FGFR2 (OMIM 176,943), p.Pro250Arg, FGFR3 (OMIM 134,934), and p.Gln119Pro TWIST1 (OMIM 601,622), PCR amplification and restriction enzyme digestion were performed. Four and two patients with Apert presented the pathogenic variants p.Ser252Trp and p.Pro253Arg in FGFR2, respectively (with a frequency of 11.1% and 5.5%). The p.Pro250Arg pathogenic variant of FGFR3 was found in a patient with Muenke (with a frequency of 2.8%). The above percentages were calculated with the total number of patients.

CONCLUSION

The contribution of this work is discreet, since only 4 genes were analyzed and sample size is small. However, this strategy could be improved by sequencing the FGFR1, FGFR2, FGFR3, and TWIST1 genes, to determine different pathogenic variants. On the other hand, it would be important to include other genes, such as TCF12 (OMIM 600,480), MSX2 (OMIM 123,101), RAB23 (OMIM 606,144), and EFNB1 (OMIM 300,035), to determine their participation in craniosynostosis in the Mexican population.

Muenke syndrome: long-term outcome of a syndrome-specific treatment protocol

OBJECTIVE

The authors evaluated the long-term outcome of their treatment protocol for Muenke syndrome, which includes a single craniofacial procedure.

METHODS

This was a prospective observational cohort study of Muenke syndrome patients who underwent surgery for craniosynostosis within the first year of life. Symptoms and determinants of intracranial hypertension were evaluated by longitudinal monitoring of the presence of papilledema (fundoscopy), obstructive sleep apnea (OSA; with polysomnography), cerebellar tonsillar herniation (MRI studies), ventricular size (MRI and CT studies), and skull growth (occipital frontal head circumference [OFC]). Other evaluated factors included hearing, speech, and ophthalmological outcomes.

RESULTS

The study included 38 patients; 36 patients underwent fronto-supraorbital advancement. The median age at last follow-up was 13.2 years (range 1.3-24.4 years). Three patients had papilledema, which was related to ophthalmological disorders in 2 patients. Three patients had mild OSA. Three patients had a Chiari I malformation, and tonsillar descent < 5 mm was present in 6 patients. Tonsillar position was unrelated to papilledema, ventricular size, or restricted skull growth. Ten patients had ventriculomegaly, and the OFC growth curve deflected in 3 patients. Twenty-two patients had hearing loss. Refraction anomalies were diagnosed in 14/15 patients measured at ≥ 8 years of age.

CONCLUSIONS

Patients with Muenke syndrome treated with a single fronto-supraorbital advancement in their first year of life rarely develop signs of intracranial hypertension, in accordance with the very low prevalence of its causative factors (OSA, hydrocephalus, and restricted skull growth). This illustrates that there is no need for a routine second craniofacial procedure. Patient follow-up should focus on visual assessment and speech and hearing outcomes.

What's retinoic acid got to do with it? Retinoic acid regulation of the neural crest in craniofacial and ocular development

Retinoic acid (RA), the active derivative of vitamin A (retinol), is an essential morphogen signaling molecule and major regulator of embryonic development. The dysregulation of RA levels during embryogenesis has been associated with numerous congenital anomalies, including craniofacial, auditory, and ocular defects. These anomalies result from disruptions in the cranial neural crest, a vertebrate-specific transient population of stem cells that contribute to the formation of diverse cell lineages and embryonic structures during development. In this review, we summarize our current knowledge of the RA-mediated regulation of cranial neural crest induction at the edge of the neural tube and the migration of these cells into the craniofacial region. Further, we discuss the role of RA in the regulation of cranial neural crest cells found within the frontonasal process, periocular mesenchyme, and pharyngeal arches, which eventually form the bones and connective tissues of the head and neck and contribute to structures in the anterior segment of the eye. We then review our understanding of the mechanisms underlying congenital craniofacial and ocular diseases caused by either the genetic or toxic disruption of RA signaling. Finally, we discuss the role of RA in maintaining neural crest-derived structures in postembryonic tissues and the implications of these studies in creating new treatments for degenerative craniofacial and ocular diseases.

Muenke syndrome: Medical and surgical comorbidities and long-term management

Muenke syndrome (MIM #602849), the most common syndromic craniosynostosis, results from the recurrent pathogenic p.P250R variant in FGFR3. Affected patients exhibit wide phenotypic variability. Common features include coronal craniosynostosis, hearing loss, carpal and tarsal anomalies, and developmental/behavioral issues. Our study examined the phenotypic findings, medical management, and surgical outcomes in a cohort of 26 probands with Muenke syndrome identified at the Children's Hospital of Philadelphia. All probands had craniosynostosis; 69.7% had bicoronal synostosis only, or bicoronal and additional suture synostosis. Three male patients had autism spectrum disorder. Recurrent ear infections were the most common comorbidity, and myringotomy tube placement the most common extracranial surgical procedure. Most patients (76%) required only one fronto-orbital advancement. de novo mutations were confirmed in 33% of the families in which proband and both parents were genetically tested, while in the remaining 66% one of the parents was a mutation carrier. In affected parents, 40% had craniosynostosis, including 71% of mothers and 13% of fathers. We additionally analyzed the medical resource utilization of probands with Muenke syndrome. To our knowledge, these data represent the first comprehensive examination of long-term management in a large cohort of patients with Muenke syndrome. Our study adds valuable information regarding neuropsychiatric and medical comorbidities, and highlights findings in affected relatives.

Expanding the phenotype for the recurrent p.Ala391Glu variant in FGFR3: Beyond crouzon syndrome and acanthosis nigricans

Background

Craniosynostosis, or premature fusion of the skull sutures, is a group of disorders that can present in isolation (nonsyndromic) or be associated with other anomalies (syndromic). Delineation of syndromic craniosynostosis is confounded due to phenotypic overlap, variable expression as well as molecular heterogeneity. We report on an infant who presented at birth with multisuture synostosis, turribrachycephaly, midface hypoplasia, beaked nose, low set ears, a high palate and short squat appearing thumbs, and great toes without deviation. The additional MRI findings of choanal stenosis and a Chiari I malformation suggested a diagnosis of Pfeiffer syndrome. First tier molecular testing did not reveal a pathogenic variant.

Methods

Whole exome sequencing on DNA samples from the proband and her unaffected parents was utilized to delineate the variant causative for the Pfeiffer syndrome diagnosis.

Results

On whole exome sequencing, a de novo NM_000142.4:c.1428C>A missense variant causing a p.Ala391Glu amino acid change in FGFR3 has been identified. The p.Ala391Glu change has been predominantly identified in patients with Crouzon syndrome with acanthosis nigricans.

Conclusions

This finding illustrates the first reported case of a child with an overlap with Pfeiffer syndrome to have the p.Ala391Glu variant.

Cdc42 regulates cranial suture morphogenesis and ossification

Cdc42 (cell division cycle 42) is ubiquitously expressed small GTPases belonging to the Rho family of proteins. Previously, we generated limb bud mesenchyme-specific Cdc42 inactivated mice (Cdc42 conditional knockout mice; Cdc42^fl/fl; Prx1-Cre), which showed short limbs and cranial bone deformities, though the mechanism related to the cranium phenotype was unclear. In the present study, we investigated the role of Cdc42 in cranial bone development. Our results showed that loss of Cdc42 caused a defect of intramembranous ossification in cranial bone tissues which is related to decreased expressions of cranial suture morphogenesis genes, including Indian hedgehog (Ihh) and bone morphogenetic proteins (BMPs). These findings demonstrate that Cdc42 plays a crucial role in cranial osteogenesis, and is controlled by Ihh- and BMP-mediated signaling during cranium development.

Exome sequencing reveals a novel splice site variant in HUWE1 gene in patients with suspected Say-Meyer syndrome

Say-Meyer syndrome is a rare and clinically heterogeneous syndrome characterized by trigonocephaly, short stature, developmental delay and hypotelorism. Nine patients with this syndrome have been reported thus far although no causative gene has yet been identified. Here, we report two siblings with clinical phenotypes of Say-Meyer syndrome with moderate to severe intellectual disability and autism spectrum disorder. Cytogenetics and array-based comparative genomic hybridization did not reveal any chromosome abnormalities or copy number alterations. Exome sequencing of the patients revealed a novel X-linked recessive splice acceptor site variant c.145-2A > G in intron 5 of HUWE1 gene in both affected siblings. RT-PCR and sequencing revealed the use of an alternate cryptic splice acceptor site downstream, which led to deletion of six nucleotides resulting loss of two amino acids p.(Cys49-Glu50del) in HUWE1 protein. Deletion of these two amino acids, which are located in a highly conserved region, is predicted to be deleterious and quite likely to affect the function of HUWE1 protein. This is the first report of a potential candidate gene mutation for Say-Meyer syndrome, which was initially described four decades ago.

Genetics of Nonsyndromic Craniosynostosis

Occurring once in every 2000 live births, craniosynostosis is one of the most frequent congenital anomalies encountered by the craniofacial surgeon. Syndromic craniosynostoses account for approximately 15 percent of cases and demonstrate Mendelian patterns of inheritance with well-established genetic causes; however, nonsyndromic craniosynostoses, which account for approximately 85 percent of cases, are genetically heterogeneous and largely unexplored. Nonsyndromic craniosynostosis is sporadic in more than 95 percent of affected families; thus, surgeons have suggested for decades that nonsyndromic craniosynostosis is likely a fluke occurrence. Contrary to this, recent studies have established that genetics underlie a substantial fraction of nonsyndromic craniosynostosis risk. Given the predominantly sporadic occurrence of disease, parents are often bewildered by the primary occurrence of nonsyndromic craniosynostosis or even recurrence in their own families and request genetic testing. Existing genetic testing panels are useful when the phenotype strongly resembles a known syndrome, wherein the risk of disease recurrence can be accurately predicted for future offspring of the parents and the future offspring of the affected child. The diagnostic utility of existing panels for nonsyndromic craniosynostosis, however, is extremely low, and these tests are quite costly. Recent genetic studies have identified several novel genes and pathways that cause nonsyndromic craniosynostosis, providing genetic evidence linking the causes of syndromic and nonsyndromic craniosynostoses, and allowing for genotype-based prediction of risk of recurrence in some nonsyndromic families. Based on analysis of exome sequence data from 384 families, the authors provide recommendations for a new genetic testing protocol for children with nonsyndromic craniosynostosis, which include testing nonsyndromic cases of sagittal, metopic, and coronal craniosynostosis.

The utility of molecular genetic techniques in craniosynostosis cases associated with intellectual disability

Molecular genetic testing in craniosynostosis leads to the detection of the mutations in the genes encoding fibroblast growth factor receptors (FGFR), providing information about the etiology of the genetic disorder. Muenke syndrome is produced by p.Pro250Arg mutation in FGFR3 gene with evidence of variable expressivity, representing 8% of the syndromic craniosynostoses.

Here, we present the identification of a p.Pro250Arg pathogenic mutation (c.749C>G) in the FGFR3 gene using Multiplex Ligation-dependent Probes Amplification (MLPA) analysis in conjunction with Sanger sequencing in a patient with craniosynostosis and mild intellectual disability. The MLPA analysis detected a reduced signal of the probe, at the site of the c.749C>G mutation, defined by the presence of one allele of C749>G mutation in the FGFR3 gene, exon 7. Sanger sequencing was performed for confirmation and identified heterozygous p.Pro250Arg pathogenic variant (c.749C>G) in exon 7 of the FGFR3.

In conclusion, we assessed the validity and clinical utility of the combined molecular genetic techniques, MLPA analysis, and Sanger sequencing, for craniosynostosis and intellectual disability, improving not only the diagnostic testing but also the genetic counseling and management of the disorder.

Genetic Causes of Craniosynostosis: An Update

In 1993, Jabs et al. were the first to describe a genetic origin of craniosynostosis. Since this discovery, the genetic causes of the most common syndromes have been described. In 2015, a total of 57 human genes were reported for which there had been evidence that mutations were causally related to craniosynostosis. Facilitated by rapid technological developments, many others have been identified since then. Reviewing the literature, we characterize the most common craniosynostosis syndromes followed by a description of the novel causes that were identified between January 2015 and December 2017.

Q289P Mutation in FGFR2 Gene Causes Saethre-Chotzen Syndrome: Some Considerations About Familial Heterogeneity

Objective

To describe the first report on a three-generation family presenting typical features of Saethre-Chotzen syndrome, in which the Q289P mutation in the FGFR2 gene was detected.

Design

Dysmorphological evaluation was performed by a clinical geneticist. Direct sequencing of the polymerase chain reaction-amplified coding region of TWIST and screening for the P250R mutation in the FGFR3 gene were performed. Exons IIIa and IIIc of FGFR2 were sequenced also. The mutation was confirmed by both restriction-enzyme digestion and allelic-specific polymerase chain reaction.

Results

Neither TWIST gene analysis nor analysis of the P250R mutation on gene FGFR3 showed mutation within the coding sequence. A nucleotide change from CAG to CCG in exon IIIa of the FGFR2 gene that caused a Q289P mutation was detected, although exon IIIc in the propositus was normal. These same results were detected in his mother, but no other members of the kindred presented clinical features consistent with Saethre-Chotzen syndrome.

Conclusions

This mutation was previously reported in individuals with Crouzon and Jackson-Weiss syndromes. The FGFR2 mutation in the family with Saethre-Chotzen syndrome herein reported reinforces the idea of an interaction among TWIST and FGFR genes during development. Absence of the Q289P mutation in some affected individuals in this family is discussed.

Achondroplasia: Development, pathogenesis, and therapy

Autosomal dominant mutations in fibroblast growth factor receptor 3 (FGFR3) cause achondroplasia (Ach), the most common form of dwarfism in humans, and related chondrodysplasia syndromes that include hypochondroplasia (Hch), severe achondroplasia with developmental delay and acanthosis nigricans (SADDAN), and thanatophoric dysplasia (TD). FGFR3 is expressed in chondrocytes and mature osteoblasts where it functions to regulate bone growth. Analysis of the mutations in FGFR3 revealed increased signaling through a combination of mechanisms that include stabilization of the receptor, enhanced dimerization, and enhanced tyrosine kinase activity. Paradoxically, increased FGFR3 signaling profoundly suppresses proliferation and maturation of growth plate chondrocytes resulting in decreased growth plate size, reduced trabecular bone volume, and resulting decreased bone elongation. In this review, we discuss the molecular mechanisms that regulate growth plate chondrocytes, the pathogenesis of Ach, and therapeutic approaches that are being evaluated to improve endochondral bone growth in people with Ach and related conditions. Developmental Dynamics 246:291-309, 2017. © 2016 Wiley Periodicals, Inc.

Diagnostic value of exome and whole genome sequencing in craniosynostosis

Background

Craniosynostosis, the premature fusion of one or more cranial sutures, occurs in ∼1 in 2250 births, either in isolation or as part of a syndrome. Mutations in at least 57 genes have been associated with craniosynostosis, but only a minority of these are included in routine laboratory genetic testing.

Methods

We used exome or whole genome sequencing to seek a genetic cause in a cohort of 40 subjects with craniosynostosis, selected by clinical or molecular geneticists as being high-priority cases, and in whom prior clinically driven genetic testing had been negative.

Results

We identified likely associated mutations in 15 patients (37.5%), involving 14 different genes. All genes were mutated in single families, except for IL11RA (two families). We classified the other positive diagnoses as follows: commonly mutated craniosynostosis genes with atypical presentation (EFNB1, TWIST1); other core craniosynostosis genes (CDC45, MSX2, ZIC1); genes for which mutations are only rarely associated with craniosynostosis (FBN1, HUWE1, KRAS, STAT3); and known disease genes for which a causal relationship with craniosynostosis is currently unknown (AHDC1, NTRK2). In two further families, likely novel disease genes are currently undergoing functional validation. In 5 of the 15 positive cases, the (previously unanticipated) molecular diagnosis had immediate, actionable consequences for either genetic or medical management (mutations in EFNB1, FBN1, KRAS, NTRK2, STAT3).

Conclusions

This substantial genetic heterogeneity, and the multiple actionable mutations identified, emphasises the benefits of exome/whole genome sequencing to identify causal mutations in craniosynostosis cases for which routine clinical testing has yielded negative results.

Association of achondroplasia with sagittal synostosis and scaphocephaly in two patients, an underestimated condition?

We report on two patients with an unusual combination of achondroplasia and surgically treated sagittal synostosis and scaphocephaly. The most common achondroplasia mutation, p.Gly380Arg in fibroblast growth factor receptor 3 (FGFR3), was detected in both patients. Molecular genetic testing of FGFR1, FGFR2, FGFR3 and TWIST1 genes failed to detect any additional mutations. There are several reports of achondroplasia with associated craniosynostosis, but no other cases of scaphocephaly in children with achondroplasia have been described. Recently it has been demonstrated that FGFR3 mutations affect not only endochondral ossification but also membranous ossification, providing new explanations for the craniofacial hallmarks in achondroplasia. Our report suggests that the association of isolated scaphocephaly and other craniosynostoses with achondroplasia may be under recognized.

Mosaic Activating Mutations in FGFR1 Cause Encephalocraniocutaneous Lipomatosis

Encephalocraniocutaneous lipomatosis (ECCL) is a sporadic condition characterized by ocular, cutaneous, and central nervous system anomalies. Key clinical features include a well-demarcated hairless fatty nevus on the scalp, benign ocular tumors, and central nervous system lipomas. Seizures, spasticity, and intellectual disability can be present, although affected individuals without seizures and with normal intellect have also been reported. Given the patchy and asymmetric nature of the malformations, ECCL has been hypothesized to be due to a post-zygotic, mosaic mutation. Despite phenotypic overlap with several other disorders associated with mutations in the RAS-MAPK and PI3K-AKT pathways, the molecular etiology of ECCL remains unknown. Using exome sequencing of DNA from multiple affected tissues from five unrelated individuals with ECCL, we identified two mosaic mutations, c.1638C>A (p.Asn546Lys) and c.1966A>G (p.Lys656Glu) within the tyrosine kinase domain of FGFR1, in two affected individuals each. These two residues are the most commonly mutated residues in FGFR1 in human cancers and are associated primarily with CNS tumors. Targeted resequencing of FGFR1 in multiple tissues from an independent cohort of individuals with ECCL identified one additional individual with a c.1638C>A (p.Asn546Lys) mutation in FGFR1. Functional studies of ECCL fibroblast cell lines show increased levels of phosphorylated FGFRs and phosphorylated FRS2, a direct substrate of FGFR1, as well as constitutive activation of RAS-MAPK signaling. In addition to identifying the molecular etiology of ECCL, our results support the emerging overlap between mosaic developmental disorders and tumorigenesis.

Articular cartilage tissue engineering: the role of signaling molecules

Effective early disease modifying options for osteoarthritis remain lacking. Tissue engineering approach to generate cartilage in vitro has emerged as a promising option for articular cartilage repair and regeneration. Signaling molecules and matrix modifying agents, derived from knowledge of cartilage development and homeostasis, have been used as biochemical stimuli toward cartilage tissue engineering and have led to improvements in the functionality of engineered cartilage. Clinical translation of neocartilage faces challenges, such as phenotypic instability of the engineered cartilage, poor integration, inflammation, and catabolic factors in the arthritic environment; these can all contribute to failure of implanted neocartilage. A comprehensive understanding of signaling molecules involved in osteoarthritis pathogenesis and their actions on engineered cartilage will be crucial. Thus, while it is important to continue deriving inspiration from cartilage development and homeostasis, it has become increasingly necessary to incorporate knowledge from osteoarthritis pathogenesis into cartilage tissue engineering.

Muenke syndrome: An international multicenter natural history study

Muenke syndrome is an autosomal dominant disorder characterized by coronal suture craniosynostosis, hearing loss, developmental delay, carpal, and calcaneal fusions, and behavioral differences. Reduced penetrance and variable expressivity contribute to the wide spectrum of clinical findings. Muenke syndrome constitutes the most common syndromic form of craniosynostosis, with an incidence of 1 in 30,000 births and is defined by the presence of the p.Pro250Arg mutation in FGFR3. Participants were recruited from international craniofacial surgery and genetic clinics. Affected individuals, parents, and their siblings, if available, were enrolled in the study if they had a p.Pro250Arg mutation in FGFR3. One hundred and six patients from 71 families participated in this study. In 51 informative probands, 33 cases (64.7%) were inherited. Eighty-five percent of the participants had craniosynostosis (16 of 103 did not have craniosynostosis), with 47.5% having bilateral and 28.2% with unilateral synostosis. Females and males were similarly affected with bicoronal craniosynostosis, 50% versus 44.4% (P = 0.84), respectively. Clefting was rare (1.1%). Hearing loss was identified in 70.8%, developmental delay in 66.3%, intellectual disability in 35.6%, attention deficit/hyperactivity disorder in 23.7%, and seizures in 20.2%. In patients with complete skeletal surveys (upper and lower extremity x-rays), 75% of individuals were found to have at least a single abnormal radiographical finding in addition to skull findings. This is the largest study of the natural history of Muenke syndrome, adding valuable clinical information to the care of these individuals including behavioral and cognitive impairment data, vision changes, and hearing loss.

The Development of the Calvarial Bones and Sutures and the Pathophysiology of Craniosynostosis

The skull vault is a complex, exquisitely patterned structure that plays a variety of key roles in vertebrate life, ranging from the acquisition of food to the support of the sense organs for hearing, smell, sight, and taste. During its development, it must meet the dual challenges of protecting the brain and accommodating its growth. The bones and sutures of the skull vault are derived from cranial neural crest and head mesoderm. The frontal and parietal bones develop from osteogenic rudiments in the supraorbital ridge. The coronal suture develops from a group of Shh-responsive cells in the head mesoderm that are collocated, with the osteogenic precursors, in the supraorbital ridge. The osteogenic rudiments and the prospective coronal suture expand apically by cell migration. A number of congenital disorders affect the skull vault. Prominent among these is craniosynostosis, the fusion of the bones at the sutures. Analysis of the pathophysiology underling craniosynostosis has identified a variety of cellular mechanisms, mediated by a range of signaling pathways and effector transcription factors. These cellular mechanisms include loss of boundary integrity, altered sutural cell specification in embryos, and loss of a suture stem cell population in adults. Future work making use of genome-wide transcriptomic approaches will address the deep structure of regulatory interactions and cellular processes that unify these seemingly diverse mechanisms.

A Genetic-Pathophysiological Framework for Craniosynostosis

Craniosynostosis, the premature fusion of one or more cranial sutures of the skull, provides a paradigm for investigating the interplay of genetic and environmental factors leading to malformation. Over the past 20 years molecular genetic techniques have provided a new approach to dissect the underlying causes; success has mostly come from investigation of clinical samples, and recent advances in high-throughput DNA sequencing have dramatically enhanced the study of the human as the preferred "model organism." In parallel, however, we need a pathogenetic classification to describe the pathways and processes that lead to cranial suture fusion. Given the prenatal onset of most craniosynostosis, investigation of mechanisms requires more conventional model organisms; principally the mouse, because of similarities in cranial suture development. We present a framework for classifying genetic causes of craniosynostosis based on current understanding of cranial suture biology and molecular and developmental pathogenesis. Of note, few pathologies result from complete loss of gene function. Instead, biochemical mechanisms involving haploinsufficiency, dominant gain-of-function and recessive hypomorphic mutations, and an unusual X-linked cellular interference process have all been implicated. Although few of the genes involved could have been predicted based on expression patterns alone (because the genes play much wider roles in embryonic development or cellular homeostasis), we argue that they fit into a limited number of functional modules active at different stages of cranial suture development. This provides a useful approach both when defining the potential role of new candidate genes in craniosynostosis and, potentially, for devising pharmacological approaches to therapy.

Executive Function and Adaptive Behavior in Muenke Syndrome

OBJECTIVE

To investigate executive function and adaptive behavior in individuals with Muenke syndrome using validated instruments with a normative population and unaffected siblings as controls.

STUDY DESIGN

Participants in this cross-sectional study included individuals with Muenke syndrome (P250R mutation in FGFR3) and their mutation-negative siblings. Participants completed validated assessments of executive functioning (Behavior Rating Inventory of Executive Function [BRIEF]) and adaptive behavior skills (Adaptive Behavior Assessment System, Second Edition [ABAS-II]).

RESULTS

Forty-four with a positive FGFR3 mutation, median age 9 years, range 7 months to 52 years were enrolled. In addition, 10 unaffected siblings served as controls (5 males, 5 females; median age, 13 years; range, 3-18 years). For the General Executive Composite scale of the BRIEF, 32.1% of the cohort had scores greater than +1.5 SD, signifying potential clinical significance. For the General Adaptive Composite of the ABAS-II, 28.2% of affected individuals scored in the 3rd-8th percentile of the normative population, and 56.4% were below the average category (<25th percentile). Multiple regression analysis did not identify craniosynostosis as a predictor of BRIEF (P = .70) or ABAS-II scores (P = .70). In the sibling pair analysis, affected siblings performed significantly poorer on the BRIEF General Executive Composite and the ABAS-II General Adaptive Composite.

CONCLUSION

Individuals with Muenke syndrome are at an increased risk for developing adaptive and executive function behavioral changes compared with a normative population and unaffected siblings.

Functions of Fibroblast Growth Factor Receptors in cancer defined by novel translocations and mutations

The four receptor tyrosine kinases (RTKs) within the family of Fibroblast Growth Factor Receptors (FGFRs) are critical for normal development but also play an enormous role in oncogenesis. Mutations and/or abnormal expression often lead to constitutive dimerization and kinase activation of FGFRs, and represent the primary mechanism for aberrant signaling. Sequencing of human tumors has revealed a plethora of somatic mutations in FGFRs that are frequently identical to germline mutations in developmental syndromes, and has also identified novel FGFR fusion proteins arising from chromosomal rearrangements that contribute to malignancy. This review details approximately 200 specific point mutations in FGFRs and 40 different fusion proteins created by translocations involving FGFRs that have been identified in human cancer. This review discusses the effects of these genetic alterations on downstream signaling cascades, and the challenge of drug resistance in cancer treatment with antagonists of FGFRs.

DSDs: genetics, underlying pathologies and psychosexual differentiation

Mammalian sex determination is the unique process whereby a single organ, the bipotential gonad, undergoes a developmental switch that promotes its differentiation into either a testis or an ovary. Disruptions of this complex genetic process during human development can manifest as disorders of sex development (DSDs). Sex development can be divided into two distinct processes: sex determination, in which the bipotential gonads form either testes or ovaries, and sex differentiation, in which the fully formed testes or ovaries secrete local and hormonal factors to drive differentiation of internal and external genitals, as well as extragonadal tissues such as the brain. DSDs can arise from a number of genetic lesions, which manifest as a spectrum of gonadal (gonadal dysgenesis to ovotestis) and genital (mild hypospadias or clitoromegaly to ambiguous genitalia) phenotypes. The physical attributes and medical implications associated with DSDs confront families of affected newborns with decisions, such as gender of rearing or genital surgery, and additional concerns, such as uncertainty over the child's psychosexual development and personal wishes later in life. In this Review, we discuss the underlying genetics of human sex determination and focus on emerging data, genetic classification of DSDs and other considerations that surround gender development and identity in individuals with DSDs.

Unilateral coronal craniosynostosis and Down syndrome

There is no known correlation between Down syndrome and craniosynostosis. The authors report 2 infants with trisomy 21 and right unilateral coronal craniosynostosis. Both patients were clinically asymptomatic but displayed characteristic craniofacial features associated with each disorder. One patient underwent a bilateral fronto-orbital advancement and the other underwent an endoscopically assisted strip craniectomy with postoperative helmet therapy. Both patients demonstrated good cosmesis at follow-up.

Genotype and clinical care correlations in craniosynostosis: findings from a cohort of 630 Australian and New Zealand patients

Craniosynostosis is one of the most common craniofacial disorders encountered in clinical genetics practice, with an overall incidence of 1 in 2,500. Between 30% and 70% of syndromic craniosynostoses are caused by mutations in hotspots in the fibroblast growth factor receptor (FGFR) genes or in the TWIST1 gene with the difference in detection rates likely to be related to different study populations within craniofacial centers. Here we present results from molecular testing of an Australia and New Zealand cohort of 630 individuals with a diagnosis of craniosynostosis. Data were obtained by Sanger sequencing of FGFR1, FGFR2, and FGFR3 hotspot exons and the TWIST1 gene, as well as copy number detection of TWIST1. Of the 630 probands, there were 231 who had one of 80 distinct mutations (36%). Among the 80 mutations, 17 novel sequence variants were detected in three of the four genes screened. In addition to the proband cohort there were 96 individuals who underwent predictive or prenatal testing as part of family studies. Dysmorphic features consistent with the known FGFR1-3/TWIST1-associated syndromes were predictive for mutation detection. We also show a statistically significant association between splice site mutations in FGFR2 and a clinical diagnosis of Pfeiffer syndrome, more severe clinical phenotypes associated with FGFR2 exon 10 versus exon 8 mutations, and more frequent surgical procedures in the presence of a pathogenic mutation. Targeting gene hot spot areas for mutation analysis is a useful strategy to maximize the success of molecular diagnosis for individuals with craniosynostosis.

Achondroplasia with multiple-suture craniosynostosis: a report of a new case of this rare association

We report on a female patient with an exceedingly rare combination of achondroplasia and multiple-suture craniosynostosis. Besides the specific features of achondroplasia, synostosis of the metopic, coronal, lambdoid, and squamosal sutures was found. Series of neurosurgical interventions were carried out, principally for acrocephaly and posterior plagiocephaly. The most common achondroplasia mutation, a p.Gly380Arg in the fibroblast growth factor receptor 3 (FGFR3) gene, was detected. Cytogenetic and array CGH analyses, as well as molecular genetic testing of FGFR1, 2, 3 and TWIST1 genes failed to identify any additional genetic alteration. It is suggested that this unusual phenotype is a result of variable expressivity of the common achondroplasia mutation.

Familial nonsyndromic craniosynostosis with specific deformity of the cranium

An otherwise healthy, developmentally normal 3-week-old male infant presented with complex multisuture craniosynostosis involving the metopic suture and bilateral coronal sutures with frontal prominence and hypotelorism. Frontal craniectomy and bilateral frontoorbital advancement remodeling were performed at the age of 5 months. The postoperative course was uneventful. The child's development was normal up to 8 months after the operation. His father and grandfather had similar specific deformities of the cranium, but no anomaly of the extremities was found, and conversation suggested that their intelligence was normal, excluding the possibility of syndromic craniosynostosis. A DNA analysis revealed large-scale copy number polymorphism of chromosome 4 in the patient and his family, which may include the phenotype of the cranium. Neither FGFR mutation nor absence of a TWIST1 mutation in the sequence from 291 to 1087, which includes DNA binding, Helix1, Loop, and Helix2, was identified. The patient apparently had a rare case of familial nonsyndromic craniosynostosis. The authors plan further genomic analysis of this family and long-term observation of the craniofacial deformity of this patient.

Epilepsy in Muenke syndrome: FGFR3-related craniosynostosis

Epilepsy, a neurologic disorder characterized by the predisposition to recurrent unprovoked seizures, is reported in more than 300 genetic syndromes. Muenke syndrome is an autosomal-dominant craniosynostosis syndrome characterized by unilateral or bilateral coronal craniosynostosis, hearing loss, intellectual disability, and relatively subtle limb findings such as carpal bone fusion and tarsal bone fusion. Muenke syndrome is caused by a single defining point mutation in the fibroblast growth factor receptor 3 (FGFR3) gene. Epilepsy rarely occurs in individuals with Muenke syndrome, and little detail is reported on types of epilepsy, patient characteristics, and long-term outcomes. We present seven patients with Muenke syndrome and seizures. A review of 789 published cases of Muenke syndrome, with a focus on epilepsy and intracranial anomalies in Muenke syndrome, revealed epilepsy in six patients, with intracranial anomalies in five. The occurrence of epilepsy in Muenke syndrome within our cohort of 58 patients, of whom seven manifested epilepsy, and the intracranial anomalies and epilepsy reported in the literature, suggest that patients with Muenke syndrome may be at risk for epilepsy and intracranial anomalies. Furthermore, the impact of Muenke syndrome on the central nervous system may be greater than previously thought.

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.

Impact of genetics on the diagnosis and clinical management of syndromic craniosynostoses

PURPOSE

More than 60 different mutations have been identified to be causal in syndromic forms of craniosynostosis. The majority of these mutations occur in the fibroblast growth factor receptor 2 gene (FGFR2). The clinical management of syndromic craniosynostosis varies based on the particular causal mutation. Additionally, the diagnosis of a patient with syndromic craniosynostosis is based on the clinical presentation, signs, and symptoms. The understanding of the hallmark features of particular syndromic forms of craniosynostosis leads to efficient diagnosis, management, and long-term prognosis of patients with syndromic craniosynostoses.

METHODS

A comprehensive literature review was done with respect to the major forms of syndromic craniosynostosis and additional less common FGFR-related forms of syndromic craniosynostosis. Additionally, information and data gathered from studies performed in our own investigative lab (lab of Dr. Muenke) were further analyzed and reviewed. A literature review was also performed with regard to the genetic workup and diagnosis of patients with craniosynostosis.

RESULTS

Patients with Apert syndrome (craniosynostosis syndrome due to mutations in FGFR2) are most severely affected in terms of intellectual disability, developmental delay, central nervous system anomalies, and limb anomalies. All patients with FGFR-related syndromic craniosynostosis have some degree of hearing loss that requires thorough initial evaluations and subsequent follow-up.

CONCLUSIONS

Patients with syndromic craniosynostosis require management and treatment of issues involving multiple organ systems which span beyond craniosynostosis. Thus, effective care of these patients requires a multidisciplinary approach.

Functional characterization of a novel FGFR2 mutation, E731K, in craniosynostosis

Craniosynostosis is a condition in which some or all of the sutures in the skull of an infant close prematurely. Fibroblast growth factor receptor 2 (FGFR2) mutations are a well-known cause of craniosynostosis. Many syndromes that comprise craniosynostosis, such as Apert syndrome, Crouzon syndrome, and Pfeiffer syndrome, have one of the phenotypes that have been reported in FGFR2 mutant patients. FGFRs have been reported in four types (FGFR1-4), and upon binding with FGF ligands, signal transduction occurs inside of cells. Activated FGFR stimulates an osteogenic master transcription factor, Runx2, through the MAP kinase and PKC pathways. We obtained a genetic analysis of six Korean patients who have craniosynostosis as a phenotype. All of the patients had at least one mutation in the FGFR2 gene; five of those mutations have already been reported elsewhere, while one mutation is novel and was hypothesized to lead to Apert syndrome. In this study, we reported and functionally analyzed a novel mutation of the FGFR2 gene found in a craniosynostosis patient, E731K. The mutation is in the 2nd tyrosine kinase domain in the C-terminal cytoplasmic region of the molecule. The mutation caused an enhanced phosphorylation of the FGFR2(E731K) and ERK-MAP kinase, the stimulation of transcriptional activity of Runx2, and consequently, the enhancement of osteogenic marker gene expression. We conclude that the substitution of E731K in FGFR2 is a novel mutation that resulted in a constitutive activation of the receptor and ultimately resulted in premature suture obliteration.

FGF and ERK signaling coordinately regulate mineralization-related genes and play essential roles in osteocyte differentiation

To examine the roles of FGF and ERK MAPK signaling in osteocyte differentiation and function, we performed microarray analyses using the osteocyte cell line MLO-Y4. This experiment identified a number of mineralization-related genes that were regulated by FGF2 in an ERK MAPK-dependent manner. Real-time PCR analysis indicated that FGF2 upregulates Ank, Enpp1, Mgp, Slc20a1, and Dmp1 in MLO-Y4 cells. Consistent with this observation, the selective FGF receptor inhibitor PD173074 decreased Ank, Enpp1, Slc20a1, and Dmp1 mRNA expression in mouse calvaria in organ culture. Since Dmp1 plays a central role in osteocyte differentiation and mineral homeostasis, we further analyzed FGF regulation of Dmp1. Similar to FGF2, FGF23 upregulated Dmp1 expression in MLO-Y4 cells in the presence of Klotho. Furthermore, increased extracellular phosphate levels partially inhibited FGF2-induced upregulation of Dmp1 mRNA expression, suggesting a coordinated regulation of Dmp1 expression by FGF signaling and extracellular phosphate. In MLO-Y4 osteocytes and in MC3T3E1 and primary calvaria osteoblasts, U0126 strongly inhibited both basal expression of Dmp1 mRNA and FGF2-induced upregulation. Consistent with the in vitro observations, real-time PCR and immunohistochemical analysis showed a strong decrease in Dmp1 expression in the skeletal elements of ERK1(-/-); ERK2(flox/flox); Prx1-Cre mice. Furthermore, scanning electron microscopic analysis revealed that no osteocytes with characteristic dendritic processes develop in the limbs of ERK1(-/-); ERK2 (flox/flox); Prx1-Cre mice. Collectively, our observations indicate that FGF signaling coordinately regulates mineralization-related genes in the osteoblast lineage and that ERK signaling is essential for Dmp1 expression and osteocyte differentiation.

Additional phenotypic features of Muenke syndrome in 2 Dutch families

In about 30% of the patients with syndromal craniosynostosis, a genetic mutation can be traced. For the purpose of adequate genetic counseling and treatment of these patients, the full spectrum of clinical findings for each specific mutation needs to be appreciated. The Pro250Arg mutation in the FGFR3 gene is found in patients with Muenke syndrome and is one of the most frequently encountered mutations in craniosynostosis syndromes. A number of studies on the relationship between genotype and phenotype concerning this specific mutation have been published. Two Dutch families with Muenke syndrome were screened for the reported characteristics of this syndrome and for additional features. New phenotypical findings were hypoplasia of the frontal sinus, ptosis of the upper eyelids, dysplastic elbow joints with restricted elbow motion, and mild cutaneous syndactyly. Incidentally, polydactyly, severe ankylosis of the elbow, fusion of cervical vertebrae, and epilepsy were found. Upper eyelid ptosis is thought to be pathognomonic for Saethre-Chotzen syndrome but was also observed in our series of patients with Muenke syndrome. Because Muenke and Saethre-Chotzen syndrome can have similar phenotypes, DNA analysis is needed to distinguish between these syndromes, even when a syndrome diagnosis is already made in a family member.

Fgf-18 is required for osteogenesis but not angiogenesis during long bone repair

Bone regeneration is a complex event that requires the interaction of numerous growth factors. Fibroblast growth factor (Fgf)-ligands have been previously described for their importance in osteogenesis during development. In the current study, we investigated the role of Fgf-18 during bone regeneration. By utilizing a unicortical tibial defect model, we revealed that mice haploinsufficient for Fgf-18 have a markedly reduced healing capacity as compared with wild-type mice. Reduced levels of Runx2 and Osteocalcin but not Vegfa accompanied the impaired bone regeneration. Interestingly, our data indicated that upon injury angiogenesis was not impaired in Fgf-18(+/-) mice. Moreover, other Fgf-ligands and Bmp-2 could not compensate for the loss of Fgf-18. Finally, application of FGF-18 protein was able to rescue the impaired healing in Fgf-18(+/-) mice. Thus, we identified Fgf-18 as an important mediator of bone regeneration, which is required during later stages of bone regeneration. This study provides hints on how to engineering efficiently programmed bony tissue for long bone repair.

Craniosynostosis of coronal suture in twist1 mice occurs through endochondral ossification recapitulating the physiological closure of posterior frontal suture

Craniosynostosis, the premature closure of cranial suture, is a pathologic condition that affects 1/2000 live births. Saethre-Chotzen syndrome is a genetic condition characterized by craniosynostosis. The Saethre-Chotzen syndrome, which is defined by loss-of-function mutations in the TWIST gene, is the second most prevalent craniosynostosis. Although much of the genetics and phenotypes in craniosynostosis syndromes is understood, less is known about the underlying ossification mechanism during suture closure. We have previously demonstrated that physiological closure of the posterior frontal suture occurs through endochondral ossification. Moreover, we revealed that antagonizing canonical Wnt-signaling in the sagittal suture leads to endochondral ossification of the suture mesenchyme and sagittal synostosis, presumably by inhibiting Twist1. Classic Saethre-Chotzen syndrome is characterized by coronal synostosis, and the haploinsufficient Twist1^+/− mice represents a suitable model for studying this syndrome. Thus, we seeked to understand the underlying ossification process in coronal craniosynostosis in Twist1^+/− mice. Our data indicate that coronal suture closure in Twist1^+/− mice occurs between postnatal day 9 and 13 by endochondral ossification, as shown by histology, gene expression analysis, and immunohistochemistry. In conclusion, this study reveals that coronal craniosynostosis in Twist1^+/− mice occurs through endochondral ossification. Moreover, it suggests that haploinsufficiency of Twist1 gene, a target of canonical Wnt-signaling, and inhibitor of chondrogenesis, mimics conditions of inactive canonical Wnt-signaling leading to craniosynostosis.