Novel FGF9 variant contributes to multiple synostoses syndrome 3.

Multiple synostoses syndromes (SYNS) are autosomal dominant syndromes characterized by multiple joint fusions commonly involving the carpal-tarsal, interphalangeal, humeroradial, and cervical spine joints. They display genetic heterogeneity with pathogenic variants reported in four separate genes (NOG, GDF5, FGF9, and GDF6) defining four different SYNS forms. FGF9 variants have been reported in SYNS3, a SYNS with multiple synostoses, normal cognition, normal hearing, and craniosynostosis. Here, we report a novel FGF9 c.569G > C p.(Arg190Thr) variant identified by whole-exome sequencing in a patient with multiple bony abnormalities. The patient initially presented with elbow instability and decreased range of motion. Imaging revealed bilateral radial head deformities, carpal-tarsal fusions, brachydactyly, and osteoarthritis of the sacroiliac joints. In silico protein modeling of the identified FGF9 variant predicts decreased stability of ligand-receptor binding supporting the pathogenicity of this finding. This finding expands the repertoire of FGF9 variants and phenotypic information reported for SYNS3 and suggest that genotype phenotype correlations due to localization seem less likely and more so due to the consequence of the pathogenic variant on the receptor. This is useful in the counseling in families as more de novo variants emerge.

siRNA Mediate RNA Interference Concordant with Early On-Target Transient Transcriptional Interference.

Exogenous siRNAs are commonly used to regulate endogenous gene expression levels for gene function analysis, genotype-phenotype association studies and for gene therapy. Exogenous siRNAs can target mRNAs within the cytosol as well as nascent RNA transcripts within the nucleus, thus complicating siRNA targeting specificity. To highlight challenges in achieving siRNA target specificity, we targeted an overlapping gene set that we found associated with a familial form of multiple synostosis syndrome type 4 (SYSN4). In the affected family, we found that a previously unknown non-coding gene TOSPEAK/C8orf37AS1 was disrupted and the adjacent gene GDF6 was downregulated. Moreover, a conserved long-range enhancer for GDF6 was found located within TOSPEAK which in turn overlapped another gene which we named SMALLTALK/C8orf37. In fibroblast cell lines, SMALLTALK is transcribed at much higher levels in the opposite (convergent) direction to TOSPEAK. siRNA targeting of SMALLTALK resulted in post transcriptional gene silencing (PTGS/RNAi) of SMALLTALK that peaked at 72 h together with a rapid early increase in the level of both TOSPEAK and GDF6 that peaked and waned after 24 h. These findings indicated the following sequence of events: Firstly, the siRNA designed to target SMALLTALK mRNA for RNAi in the cytosol had also caused an early and transient transcriptional interference of SMALLTALK in the nucleus; Secondly, the resulting interference of SMALLTALK transcription increased the transcription of TOSPEAK; Thirdly, the increased transcription of TOSPEAK increased the transcription of GDF6. These findings have implications for the design and application of RNA and DNA targeting technologies including siRNA and CRISPR. For example, we used siRNA targeting of SMALLTALK to successfully restore GDF6 levels in the gene therapy of SYNS4 family fibroblasts in culture. To confidently apply gene targeting technologies, it is important to first determine the transcriptional interference effects of the targeting reagent and the targeted gene.

Nonlethal presentations of CYP26B1-related skeletal anomalies and multiple synostoses syndrome.

Retinoic acid exposures as well as defects in the retinoic acid-degrading enzyme CYP26B1 have teratogenic effects on both limb and craniofacial skeleton. An initial report of four individuals described a syndrome of fetal and infantile lethality with craniosynostosis and skeletal anomalies caused by homozygous pathogenic missense variants in CYP26B1. In contrast, a 22-year-old female was reported with a homozygous missense pathogenic variant in CYP26B1 with complex multisuture craniosynostosis and intellectual disability, suggesting that in some cases, biallelic pathogenic variants of CYP26B1 may be compatible with life. Here we describe four additional living individuals from two families with compound heterozygous pathogenic missense variants in CYP26B1. Structural assessment of these additional missense variants places them further from the catalytic site and supports a model consistent with milder nonlethal disease. In addition to previously reported findings of multisuture craniosynostosis, conductive hearing loss, joint contractures, long slender fingers, camptodactly, broad fingertips, and developmental delay/intellectual disability, skeletal imaging in our cases also revealed gracile long bones, gracile ribs, radioulnar synostosis, and carpal and/or tarsal fusions. These individuals broaden the phenotypic range of biallelic pathogenic variants in CYPB26B1 and most significantly clarify that mortality can range from perinatal lethality to survival into adulthood.

Intragenic Deletions in FLNB Are Part of the Mutational Spectrum Causing Spondylocarpotarsal Synostosis Syndrome.

Spondylocarpotarsal synostosis syndrome (SCT) is characterized by vertebral fusions, a disproportionately short stature, and synostosis of carpal and tarsal bones. Pathogenic variants in FLNB, MYH3, and possibly in RFLNA, have been reported to be responsible for this condition. Here, we present two unrelated individuals presenting with features typical of SCT in which Sanger sequencing combined with whole genome sequencing identified novel, homozygous intragenic deletions in FLNB (c.1346-1372_1941+389del and c.3127-353_4223-1836del). Both deletions remove several consecutive exons and are predicted to result in a frameshift. To our knowledge, this is the first time that large structural variants in FLNB have been reported in SCT, and thus our findings add to the classes of variation that can lead to this disorder. These cases highlight the need for copy number sensitive methods to be utilized in order to be comprehensive in the search for a molecular diagnosis in individuals with a clinical diagnosis of SCT.

A novel heterozygous variant in FGF9 associated with previously unreported features of multiple synostosis syndrome 3.

Human multiple synostoses syndrome 3 is an autosomal dominant disorder caused by pathogenic variants in FGF9. Only two variants have been described in FGF9 in humans so far, and one in mice. Here we report a novel missense variant c.566C > G, p.(Pro189Arg) in FGF9. Functional studies showed this variant impairs FGF9 homodimerization, but not FGFR3c binding. We also review the findings of cases reported previously and report on additional features not described previously.

Roberts syndrome in an Indian patient with humeroradial synostosis, congenital elbow contractures and a novel homozygous splice variant in ESCO2.

Roberts syndrome (also known as Roberts-SC phocomelia syndrome) is an autosomal recessive developmental disorder, characterized by pre- and postnatal growth retardation, limb malformations including bilateral symmetric tetraphocomelia or mesomelia, and craniofacial dysmorphism. Biallelic loss-of-function variants in ESCO2, which codes for establishment of sister chromatid cohesion N-acetyltransferase 2, cause Roberts syndrome. Phenotypic spectrum among patients is broad, challenging clinical diagnosis in mildly affected individuals. Here we report a 3-year-old boy with a mild phenotype of Roberts syndrome with bilateral elbow contractures, humeroradial synostosis, mild lower limb disparity, and facial dysmorphism. Trio whole-exome sequencing identified the novel biallelic splice variant c.1673+1G>A in ESCO2 in the patient. Aberrant ESCO2 pre-mRNA splicing, reduced relative ESCO2 mRNA amount, and characteristic cytogenetic defects, such as premature centromere separation, heterochromatin repulsion, and chromosome breaks, in patient cells strongly supported pathogenicity of the ESCO2 variant affecting one of the highly conserved guanine-thymine dinucleotide of the donor splice site. Our case highlights the difficulty in establishing a clinical diagnosis in individuals with minor clinical features of Roberts syndrome and normal intellectual and social development. However, next-generation sequencing tools allow for molecular diagnosis in cases presenting with mild developmental defects.

Identification of a novel pathogenic variant in the MYH3 gene in a five-generation family with CPSFS1A (Contractures, Pterygia, and Spondylocarpotarsal Fusion Syndrome 1A).

BACKGROUND: Distal arthrogryposis (DA) is a group of rare Mendelian conditions that demonstrate heterogeneity with respect to genetics and phenotypes. Ten types of DAs, which collectively involve six genes, have been reported. Among them, the MYH3 gene causes several types of arthrogryposis conditions and therefore has a pivotal role in the skeletal and muscle development of the fetus. For this study, we recruited a five-generation Chinese family with members presenting DA features and phenotypic variability. Further clinical study characterized it as CPSFS1A (Contractures, Pterygia, and Spondylocarpotarsal Fusion Syndrome 1A). METHODS: Genomic DNA was extracted from eight family members, including one fetus. Whole-exome sequencing (WES) was then conducted on the proband's sample, followed by Sanger sequencing as validation for each of the participants. In silico analysis was performed. Western blotting (WB) detection and pathological staining were conducted on skeletal muscle tissue of the induced fetus after prenatal diagnosis. RESULTS: A novel heterozygous pathogenic variant, namely NM_002470.3: c.3044_3047delinsTCAATTTGTT: p.E1015_D1016delinsVNLF in the MYH3 gene, was identified and shown to be cosegregated with the condition in the subject family. This variant resulted in the replacement of amino-acid residues E1015 and D1016 by a string of VNLFs. The pregnancy was selectively terminated because the fetus was genetically affected. However, the WB and pathological results did not indicate a significant change in the norm. CONCLUSIONS: Our study expanded the variant spectrum of CPSFS1A, in addition to which it provided solid evidence for the appropriateness of genetic counseling and pregnancy management for the family. The results may also provide further insight into the molecular mechanism of MYH3.

Intradiploic Hematoma Associated With Synostosis in an Infant.

Intradiploic hematomas are extremely rare, particularly in newborns. Caused by bleeding between the inner and outer tables of the calvarium, they manifest with bony swelling of the skull. The authors present the first case of an intraosseous hematoma associated with synostosis, and the first report in a female patient. The clinical, radiological, surgical, and pathological characteristics of this lesion are discussed.

Multiple synostoses syndrome: Clinical report and retrospective analysis.

Multiple synostoses syndrome (SYNS1; OMIM# 186500) is a rare autosomal dominant disorder reported in a few cases worldwide. We report a Chinese pedigree characterized by proximal symphalangism, conductive hearing loss, and distinctive facies. We examined the genetic cause and reviewed the literature to discuss the pathogeny, treatment, and prevention of SYNS1. Audiological, ophthalmological, and radiological examinations were evaluated. Whole-exome sequencing (WES) was performed to identify mutations in the proband and her parents. Sanger sequencing was used to verify the results for the proband, parents, and grandmother. The literature on the genotype-phenotype correlation was reviewed. The patient was diagnosed with multiple synostoses syndrome clinically. WES and bioinformatic analysis revealed a novel missense mutation in the NOG gene, c.554C>G (p.Ser185Cys), cosegregated in this family. The literature review showed that the phenotype varies widely, but the typical facies, conductive hearing loss, and proximal symphalangism occurred frequently. All reported mutations are highly conserved in mammals based on conservation analysis, and there are regional hot spots for these mutations. However, no distinct genotype-phenotype correlations have been identified for mutations in NOG in different races. Regular systematic examinations and hearing aids are beneficial for this syndrome. However, the outcomes of otomicrosurgery are not encouraging owing to the regrowth of bone. This study expanded the mutation spectrum of NOG and is the first report of SYNS1 in a Chinese family. Genetic testing is recommended as part of the diagnosis of syndromic deafness. A clinical genetic evaluation is essential to guide prevention, such as preimplantation genetic diagnosis.

Cervical Rib Synostosis to the First Rib: A Rare Anatomic Variation.

BACKGROUND: Congenital anatomic variations exist in human anatomy, which create both diagnostic and treatment challenges. Understanding the osteologic and radiographic anatomy of supernumerary ribs arising from the cervical spine and recognizing the morphologic variations thereof is of great importance to clinicians, radiologists, and surgeons alike. CASE DESCRIPTION: This case study describes osteologic morphology and radiologic characteristics of a rare anatomic variant of a cervical rib (CR): a unilateral, right-sided CR synostosis to the first thoracic rib of a 50-year-old South African man of African ancestry. The characteristic features included increased angulation, widening of the body, and shortening of the length of the right-sided first thoracic rib. The synostosis of the CR shaft was at the level of the angle of the first thoracic rib. The widest aspect of the first thoracic rib was close to the site of fusion, namely the angle, with the mediolateral length approximately 34.51 mm. This is in contrast to the contralateral first thoracic rib measuring, at its widest, 26.39 mm. The CR was located approximately 3.34 mm superiorly to the first thoracic rib at the cervical articular facet. The CR presented with a well-defined head, which is small and rounded with the inclusion of an articular facet. Thereafter, it presented with a short neck, just over half the length of the inferiorly placed first thoracic rib, and a similar sized articulating facet at the tubercle. The appearance of the trabecular bone pattern on radiographs is in keeping with the contralateral left first rib, although altered in accordance with the gross osteologic appearance described earlier. Furthermore, the radiographs highlight an elliptical lucent-zone within the trabecular bone demonstrating decreased density centrally with a thin rim of sclerotic cortical bone peripherally. This is consistent with classical rib architecture in cross-section representing the CR shaft site of fusion to the first thoracic rib. The CR synostosis to the first thoracic rib represents a novel complex, termed by the authors as a cervicothoracic rib complex. CONCLUSION: The present report refers to the osteologic and radiographic description and comparison of a unilateral, right-sided CR synostosis to a first thoracic rib. The clinical implications of CRs may consist of neurologic, vascular complications, and functional deficits of the involved limb associated with thoracic outlet syndrome (TOS). A CR synostosis to the first thoracic rib represents an associated increased risk of vascular injury, with poorer operative outcomes associated with TOS. This case study is of particular importance to vascular surgeons and neurosurgeons involved with surgical planning and intervention strategies relating to CRs and TOS.

First case of compound heterozygous BHLHA9 variants in mesoaxial synostotic syndactyly with phalangeal reduction.

Mesoaxial synostotic syndactyly with phalangeal reduction (MSSD) is an extremely rare autosomal recessive limb abnormality characterized by the fusion of third and fourth fingers. To date, only homozygous missense and frameshift mutations have been reported in BHLHA9 associated to MSSD. In this study, we report a patient who presented with clinical and radiological features of MSSD. A customized skeletal dysplasia NGS panel revealed the presence of two novel compounds heterozygous variants in BHLHA9: NM_001164405.1: c.[226A>T][269G>C]; p.[(Lys76*)][(Arg90Pro)]. Thus, this is the first case of MSSD in a nonconsanguineous family.

Bilateral congenital radioulnar synostosis in an Early Horizon subadult burial from the site of Atalla, Peru.

OBJECTIVE: This study was undertaken to identify pathological conditions within the population living at Atalla (1000-500 BCE), an important early village site and ritual center located in Huancavelica, Peru. MATERIALS: Articulated burials (N = 3) and commingled human remains excavated during the 2015 and 2016 field seasons. METHODS: Osteological remains were analyzed for macroscopic evidence of pathological changes. RESULTS: A case of bilateral proximal radioulnar fusion was observed in an Early Horizon (ca. 800 BCE) subadult skeleton (Individual 1). A differential diagnosis of this pathology supports congenital radioulnar synostosis (CRUS), a rare developmental condition. Enamel hypoplasia was also identified in the same individual. CONCLUSIONS: Burial treatment of Individual 1 does not provide any indication that CRUS was afforded an exceptional social significance. CONTRIBUTION TO PALEOPATHOLOGY: This example of CRUS is notable as it represents the second published archaeological case of CRUS from Peru and the earliest reported case globally. LIMITATIONS OF THIS STUDY: The osteological sample currently available from this site is limited. SUGGESTIONS FOR FUTURE RESEARCH: Increased fieldwork in this region is recommended to further clarify the distribution and social significance of CRUS in the prehistoric Andes.

Identification of a homozygous frameshift variant in RFLNA in a patient with a typical phenotype of spondylocarpotarsal synostosis syndrome.

Spondylocarpotarsal synostosis syndrome, a rare syndromic skeletal disorder characterized by disrupted vertebral segmentation with vertebral fusion, scoliosis, short stature, and carpal/tarsal synostosis, has been associated with biallelic truncating mutations in the filamin B gene or monoallelic mutations in the myosin heavy chain 3 gene. We herein report the case of a patient with a typical phenotype of spondylocarpotarsal synostosis syndrome who had a homozygous frameshift mutation in the refilin A gene (RFLNA) [c.241delC, p.(Leu81Cysfs*111)], which encodes one of the filamin-binding proteins. Refilins, filamins, and myosins play critical roles in forming perinuclear actin caps, which change the nuclear morphology during cell migration and differentiation. The present study implies that RFLNA is an additional causative gene for spondylocarpotarsal synostosis syndrome in humans and a defect in forming actin bundles and perinuclear actin caps may be a critical mechanism for the development of spondylocarpotarsal synostosis syndrome.

A novel truncating mutation in MYH3 causes spondylocarpotarsal synostosis syndrome with basilar invagination.

Spondylocarpotarsal synostosis syndrome (SCT) is a rare group of skeletal dysplasias, characterized by disproportionate short stature with a short trunk, abnormal segmentation of the spine with vertebral fusion, scoliosis and lordosis, carpal and tarsal synostosis, and mild facial dysmorphisms. While the majority of the cases show autosomal recessive inheritance, only a few cases of vertical transmissions, with MYH3 mutations, have been reported. Here we report a case with typical SCT, carrying a novel heterozygous mutation in MYH3. This observation supports the hypothesis of a pathogenic link between autosomal dominant SCT and heterozygous mutations in MYH3. Of note, our case showed basilar invagination on brain magnetic resonance imaging at the age of 10 years. Basilar invagination could be a rare complication of both autosomal recessive and dominant SCT, indicating that prompt investigation are warranted for SCT patients.

Recombinant mouse periostin ameliorates coronal sutures fusion in Twist1+/- mice.

BACKGROUND: Saethre-Chotzen syndrome is an autosomal dominantly inherited disorder caused by mutations in the twist family basic helix-loop-helix transcription factor 1 (TWIST1) gene. Surgical procedures are frequently required to reduce morphological and functional defects in patients with Saethre-Chotzen syndrome. Therefore, the development of noninvasive procedures to treat Saethre-Chotzen syndrome is critical. We identified that periostin, which is an extracellular matrix protein that plays an important role in both bone and connective tissues, is downregulated in craniosynostosis patients. METHODS: We aimed to verify the effects of different concentrations (0, 50, 100, and 200 µg/l) of recombinant mouse periostin in Twist1+/- mice (a mouse model of Saethre-Chotzen syndrome) coronal suture cells in vitro and in vivo. Cell proliferation, migration, and osteogenic differentiation were observed and detected. Twist1+/- mice were also injected with recombinant mouse periostin to verify the treatment effects. RESULTS: Cell Counting Kit-8 results showed that recombinant mouse periostin inhibited the proliferation of suture-derived cells in a time- and concentration-dependent manner. Cell migration was also suppressed when treated with recombinant mouse periostin. Real-time quantitative PCR and Western blotting results suggested that messenger ribonucleic acid and protein expression of alkaline phosphatase, bone sialoprotein, collagen type I, and osteocalcin were all downregulated after treatment with recombinant mouse periostin. However, the expression of Wnt-3a, Wnt-1, and ß-catenin were upregulated. The in vivo results demonstrated that periostin-treated Twist1+/- mice showed patent coronal sutures in comparison with non-treated Twist1+/- mice which have coronal craniosynostosis. CONCLUSION: Our results suggest that recombinant mouse periostin can inhibit coronal suture cell proliferation and migration and suppress osteogenic differentiation of suture-derived cells via Wnt canonical signaling, as well as ameliorate coronal suture fusion in Twist1+/- mice.

A 590 kb deletion caused by non-allelic homologous recombination between two LINE-1 elements in a patient with mesomelia-synostosis syndrome.

Mesomelia-synostoses syndrome (MSS) is a rare, autosomal-dominant, syndromal osteochondrodysplasia characterized by mesomelic limb shortening, acral synostoses, and multiple congenital malformations due to a non-recurrent deletion at 8q13 that always encompasses two coding-genes, SULF1 and SLCO5A1. To date, five unrelated patients have been reported worldwide, and MMS was previously proposed to not be a genomic disorder associated with deletions recurring from non-allelic homologous recombination (NAHR) in at least two analyzed cases. We conducted targeted gene panel sequencing and subsequent array-based copy number analysis in an 11-year-old undiagnosed Japanese female patient with multiple congenital anomalies that included mesomelic limb shortening and detected a novel 590 Kb deletion at 8q13 encompassing the same gene set as reported previously, resulting in the diagnosis of MSS. Breakpoint sequences of the deleted region in our case demonstrated the first LINE-1s (L1s)-mediated unequal NAHR event utilizing two distant L1 elements as homology substrates in this disease, which may represent a novel causative mechanism of the 8q13 deletion, expanding the range of mechanisms involved in the chromosomal rearrangements responsible for MSS.

Capitate-trapezoid synostosis: analysis of an Early Bronze Age case and review of the literature.

BACKGROUND: Carpal synostoses are congenital defects characterised by complete or incomplete coalition of two or more carpal bones. Although most of these defects are discovered only incidentally, sometimes they become clinically manifest. Among the different types of carpal coalition, the synostosis between capitate and trapezoid bones is quite rare, with only sparse data available in the literature. The aim of this report was to describe a case of capitate-trapezoid synostosis (CTS) observed in an ancient human skeleton, as well as to scrutinise the pertinent literature in order to assess for the characteristics of this type of defect, including its potential relevance to clinical practice. MATERIALS AND METHODS: We studied the skeletal remains of an Early Bronze Age male warrior affected by incomplete CTS. Macroscopic and radiological examination of the defect was carried out. We also performed a comprehensive PubMed search in the Medline and other specialty literature databases to retrieve and analyse data relevant to the subject under consideration. RESULTS AND CONCLUSIONS: The present case is the most ancient CTS ever found. In those literature-reported cases accompanied by careful anatomical description, such as the present one, incomplete coalition invariably occurs between the dorsal surfaces of the two bones, this characteristic emerging as a distinctive morphological trait. Literature analysis further suggests that the true prevalence of CTS is likely to be higher than estimates based on data gathered from radiology series, and that this defect may be associated with pain and carpal bossing more frequently than generally thought.

Stapedectomy in Teunissen-Cremers Syndrome: Intraoperative Findings and Hearing Outcomes.

OBJECTIVE: To describe the intraoperative findings and outcomes of stapedectomy surgery in Teunissen-Cremers syndrome. PATIENTS: A family of three patients with bilateral conductive hearing loss because of Teunissen-Cremers syndrome. INTERVENTION: Six exploratory tympanotomies and stapedectomies, including one revision operation. MAIN OUTCOME MEASURES: Intraoperative findings and postoperative hearing results. RESULTS: There was an increased distance between the incus and the vestibule, a thicker long process of the incus, and slight variation in the relative position of the ossicles in all patients. Surgery resulted in closure of the air-bone gap to less than 10 dB in all five operated ears. CONCLUSION: We describe anatomic abnormalities of the ossicular chain in Teunissen-Cremers syndrome. Prosthesis availability should include prostheses that can adapt to these potential anatomic abnormalities. Stapedectomy resulted in good long-term hearing outcomes in this series.

Protein-altering MYH3 variants are associated with a spectrum of phenotypes extending to spondylocarpotarsal synostosis syndrome.

Spondylocarpotarsal synostosis syndrome (SCT) is a rare Mendelian disorder (OMIM #272460) characterized by prenatal vertebral fusion, scoliosis, short stature and carpal and tarsal synostosis. SCT is typically known as an autosomal recessive disease caused by variants in the FLNB gene. The genetic basis of the rarer cases of vertical transmissions remains unknown. In two independent families with symptoms related to autosomal dominant SCT, we identified - by exome sequencing - two protein-altering variants in the embryonic myosin heavy chain 3 (MYH3) gene. As MYH3 variants are also associated with distal arthrogryposis (DA1, DA2A, DA2B) and autosomal dominant multiple pterygium syndromes (MPS), the present study expands the phenotypic spectrum of MYH3 variants to autosomal dominant SCT. Vertebral, carpal and tarsal fusions observed in both families further confirm that MYH3 plays a key role in skeletal development.

FGFR2c-mediated ERK-MAPK activity regulates coronal suture development.

Fibroblast growth factor receptor 2 (FGFR2) signaling is critical for proper craniofacial development. A gain-of-function mutation in the 2c splice variant of the receptor's gene is associated with Crouzon syndrome, which is characterized by craniosynostosis, the premature fusion of one or more of the cranial vault sutures, leading to craniofacial maldevelopment. Insight into the molecular mechanism of craniosynostosis has identified the ERK-MAPK signaling cascade as a critical regulator of suture patency. The aim of this study is to investigate the role of FGFR2c-induced ERK-MAPK activation in the regulation of coronal suture development. Loss-of-function and gain-of-function Fgfr2c mutant mice have overlapping phenotypes, including coronal synostosis and craniofacial dysmorphia. In vivo analysis of coronal sutures in loss-of-function and gain-of-function models demonstrated fundamentally different pathogenesis underlying coronal suture synostosis. Calvarial osteoblasts from gain-of-function mice demonstrated enhanced osteoblastic function and maturation with concomitant increase in ERK-MAPK activation. In vitro inhibition with the ERK protein inhibitor U0126 mitigated ERK protein activation levels with a concomitant reduction in alkaline phosphatase activity. This study identifies FGFR2c-mediated ERK-MAPK signaling as a key mediator of craniofacial growth and coronal suture development. Furthermore, our results solve the apparent paradox between loss-of-function and gain-of-function FGFR2c mutants with respect to coronal suture synostosis.