A genome-wide association study implicates the BMP7 locus as a risk factor for nonsyndromic metopic craniosynostosis

Reassessing the association: Evaluation of a polyalanine deletion variant of RUNX2 in non-syndromic sagittal and metopic craniosynostosis

The RUNT-related transcription factor RUNX2 plays a critical role in osteoblast differentiation, and alterations to gene dosage cause distinct craniofacial anomalies. Uniquely amongst the RUNT-related family, vertebrate RUNX2 encodes a polyglutamine/polyalanine repeat (Gln23-Glu-Ala17 in humans), with the length of the polyalanine component completely conserved in great apes. Surprisingly, a frequent 6-amino acid deletion polymorphism, p.(Ala84_Ala89)del, occurs in humans (termed 11A allele), and a previous association study (Cuellar et al. Bone 137:115395;2020) reported that the 11A variant was significantly more frequent in non-syndromic sagittal craniosynostosis (nsSag; allele frequency [AF] = 0.156; 95% confidence interval [CI] 0.126-0.189) compared to non-syndromic metopic craniosynostosis (nsMet; AF = 0.068; 95% CI 0.045-0.098). However, the gnomAD v.2.1.1 control population used by Cuellar et al. did not display Hardy-Weinberg equilibrium, hampering interpretation. To re-examine this association, we genotyped the RUNX2 11A polymorphism in 225 individuals with sporadic nsSag as parent-child trios and 164 singletons with sporadic nsMet, restricting our analysis to individuals of European ancestry. We compared observed allele frequencies to the non-transmitted alleles in the parent-child trios, and to the genome sequencing data from gnomAD v.4, which display Hardy-Weinberg equilibrium. Observed AFs (and 95% CI) were 0.076 (0.053-0.104) in nsSag and 0.082 (0.055-0.118) in nsMet, compared with 0.062 (0.042-0.089) in non-transmitted parental alleles and 0.065 (0.063-0.067) in gnomAD v.4.0.0 non-Finnish European control genomes. In summary, we observed a non-significant excess, compared to gnomAD data, of 11A alleles in both nsSag (relative risk 1.18, 95% CI 0.83-1.67) and nsMet (relative risk 1.29, 95% CI 0.87-1.92), but we did not replicate the much higher excess of RUNX2 11A alleles in nsSag previously reported (p = 0.0001).

Exploring Different Management Modalities of Nonsyndromic Craniosynostosis

Craniosynostosis is an atypical skull shape characterized by the premature fusion of cranial sutures. It is one of the most common congenital anomalies encountered by craniofacial surgeons, with a prevalence of one in every 2000-2500 births. It is classified into two main types: syndromic and nonsyndromic. In syndromic, the patient presents with other abnormalities involving the trunk, face, or extremities. While in nonsyndromic the only anomy is the premature fusion, which usually involves one suture; the most common subtypes are unicoronal, sagittal, bicoronal, metopic, and lambdoid. As a consequence, premature fusion before its natural time restricts the space for the brain to grow, increases intracranial pressure, causes damage to the brain tissue, and affects the development of the child. This review comprehensively provides a detailed overview of nonsyndromic craniosynostosis and aims to highlight the importance of early and accurate diagnosis, and determining the most suitable intervention, whether surgical or conservative modalities. The optimal treatment approach produces the most favorable aesthetic and functional outcomes.

Whole genome sequencing identifies associations for nonsyndromic sagittal craniosynostosis with the intergenic region of BMP2 and noncoding RNA gene LINC01428

Craniosynostosis (CS) is a major birth defect resulting from premature fusion of cranial sutures. Nonsyndromic CS occurs more frequently than syndromic CS, with sagittal nonsyndromic craniosynostosis (sNCS) presenting as the most common CS phenotype. Previous genome-wide association and targeted sequencing analyses of sNCS have identified multiple associated loci, with the strongest association on chromosome 20. Herein, we report the first whole-genome sequencing study of sNCS using 63 proband-parent trios. Sequencing data for these trios were analyzed using the transmission disequilibrium test (TDT) and rare variant TDT (rvTDT) to identify high-risk rare gene variants. Sequencing data were also examined for copy number variants (CNVs) and de novo variants. TDT analysis identified a highly significant locus at 20p12.3, localized to the intergenic region between BMP2 and the noncoding RNA gene LINC01428. Three variants (rs6054763, rs6054764, rs932517) were identified as potential causal variants due to their probability of being transcription factor binding sites, deleterious combined annotation dependent depletion scores, and high minor allele enrichment in probands. Morphometric analysis of cranial vault shape in an unaffected cohort validated the effect of these three single nucleotide variants (SNVs) on dolichocephaly. No genome-wide significant rare variants, de novo loci, or CNVs were identified. Future efforts to identify risk variants for sNCS should include sequencing of larger and more diverse population samples and increased omics analyses, such as RNA-seq and ATAC-seq.

Transforming growth factor beta signaling and craniofacial development: modeling human diseases in zebrafish

Humans and other jawed vertebrates rely heavily on their craniofacial skeleton for eating, breathing, and communicating. As such, it is vital that the elements of the craniofacial skeleton develop properly during embryogenesis to ensure a high quality of life and evolutionary fitness. Indeed, craniofacial abnormalities, including cleft palate and craniosynostosis, represent some of the most common congenital abnormalities in newborns. Like many other organ systems, the development of the craniofacial skeleton is complex, relying on specification and migration of the neural crest, patterning of the pharyngeal arches, and morphogenesis of each skeletal element into its final form. These processes must be carefully coordinated and integrated. One way this is achieved is through the spatial and temporal deployment of cell signaling pathways. Recent studies conducted using the zebrafish model underscore the importance of the Transforming Growth Factor Beta (TGF-β) and Bone Morphogenetic Protein (BMP) pathways in craniofacial development. Although both pathways contain similar components, each pathway results in unique outcomes on a cellular level. In this review, we will cover studies conducted using zebrafish that show the necessity of these pathways in each stage of craniofacial development, starting with the induction of the neural crest, and ending with the morphogenesis of craniofacial elements. We will also cover human skeletal and craniofacial diseases and malformations caused by mutations in the components of these pathways (e.g., cleft palate, craniosynostosis, etc.) and the potential utility of zebrafish in studying the etiology of these diseases. We will also briefly cover the utility of the zebrafish model in joint development and biology and discuss the role of TGF-β/BMP signaling in these processes and the diseases that result from aberrancies in these pathways, including osteoarthritis and multiple synostoses syndrome. Overall, this review will demonstrate the critical roles of TGF-β/BMP signaling in craniofacial development and show the utility of the zebrafish model in development and disease.

Systematic Review of Nonsyndromic Craniosynostosis: Genomic Alterations and Impacted Signaling Pathways

BACKGROUND

Genetic research in nonsyndromic craniosynostosis remains limited compared with syndromic craniosynostosis. This systematic review aimed to comprehensively summarize the genetic literature of nonsyndromic craniosynostosis and highlight key signaling pathways.

METHODS

The authors performed a systematic literature search of PubMed, Ovid, and Google Scholar databases from inception until December of 2021 using search terms related to nonsyndromic craniosynostosis and genetics. Two reviewers screened titles and abstract for relevance, and three reviewers independently extracted study characteristics and genetic data. Gene networks were constructed using Search Tool for Retrieval of Interacting Genes/Proteins (version 11) analysis.

RESULTS

Thirty-three articles published between 2001 and 2020 met inclusion criteria. Studies were further classified into candidate gene screening and variant identification studies ( n = 16), genetic expression studies ( n = 13), and common and rare variant association studies ( n = 4). Most studies were good quality. Using our curated list of 116 genes extracted from the studies, two main networks were constructed.

CONCLUSIONS

This systematic review concerns the genetics of nonsyndromic craniosynostosis, with network construction revealing TGF-β/BMP, Wnt, and NF-κB/RANKL as important signaling pathways. Future studies should focus on rare rather than common variants to examine the missing heritability in this defect and, going forward, adopt a standard definition.

Joint multi-ancestry and admixed GWAS reveals the complex genetics behind human cranial vault shape

The cranial vault in humans is highly variable, clinically relevant, and heritable, yet its genetic architecture remains poorly understood. Here, we conduct a joint multi-ancestry and admixed multivariate genome-wide association study on 3D cranial vault shape extracted from magnetic resonance images of 6772 children from the ABCD study cohort yielding 30 genome-wide significant loci. Follow-up analyses indicate that these loci overlap with genomic risk loci for sagittal craniosynostosis, show elevated activity cranial neural crest cells, are enriched for processes related to skeletal development, and are shared with the face and brain. We present supporting evidence of regional localization for several of the identified genes based on expression patterns in the cranial vault bones of E15.5 mice. Overall, our study provides a comprehensive overview of the genetics underlying normal-range cranial vault shape and its relevance for understanding modern human craniofacial diversity and the etiology of congenital malformations.

Craniofacial developmental biology in the single-cell era

The evolution of a unique craniofacial complex in vertebrates made possible new ways of breathing, eating, communicating and sensing the environment. The head and face develop through interactions of all three germ layers, the endoderm, ectoderm and mesoderm, as well as the so-called fourth germ layer, the cranial neural crest. Over a century of experimental embryology and genetics have revealed an incredible diversity of cell types derived from each germ layer, signaling pathways and genes that coordinate craniofacial development, and how changes to these underlie human disease and vertebrate evolution. Yet for many diseases and congenital anomalies, we have an incomplete picture of the causative genomic changes, in particular how alterations to the non-coding genome might affect craniofacial gene expression. Emerging genomics and single-cell technologies provide an opportunity to obtain a more holistic view of the genes and gene regulatory elements orchestrating craniofacial development across vertebrates. These single-cell studies generate novel hypotheses that can be experimentally validated in vivo. In this Review, we highlight recent advances in single-cell studies of diverse craniofacial structures, as well as potential pitfalls and the need for extensive in vivo validation. We discuss how these studies inform the developmental sources and regulation of head structures, bringing new insights into the etiology of structural birth anomalies that affect the vertebrate head.

Review of Recurrently Mutated Genes in Craniosynostosis Supports Expansion of Diagnostic Gene Panels

Craniosynostosis, the premature fusion of the cranial sutures, affects ~1 in 2000 children. Although many patients with a genetically determined cause harbor a variant in one of just seven genes or have a chromosomal abnormality, over 60 genes are known to be recurrently mutated, thus comprising a long tail of rarer diagnoses. Genome sequencing for the diagnosis of rare diseases is increasingly used in clinical settings, but analysis of the data is labor intensive and involves a trade-off between achieving high sensitivity or high precision. PanelApp, a crowd-sourced disease-focused set of gene panels, was designed to enable prioritization of variants in known disease genes for a given pathology, allowing enhanced identification of true-positives. For heterogeneous disorders like craniosynostosis, these panels must be regularly updated to ensure that diagnoses are not being missed. We provide a systematic review of genetic literature on craniosynostosis over the last 5 years, including additional results from resequencing a 42-gene panel in 617 affected individuals. We identify 16 genes (representing a 25% uplift) that should be added to the list of bona fide craniosynostosis disease genes and discuss the insights that these new genes provide into pathophysiological mechanisms of craniosynostosis.

Privacy-Preserving Statistical Analysis of Genomic Data Using Compressive Mechanism with Haar Wavelet Transform

To promote the use of personal genome information in medicine, it is important to analyze the relationship between diseases and the human genomes. Therefore, statistical analysis using genomic data is often conducted, but there is a privacy concern with respect to releasing the statistics as they are. Existing methods to address this problem using the concept of differential privacy cannot provide accurate outputs under strong privacy guarantees, making them less practical. In this study, for the first time, we investigate the application of a compressive mechanism to genomic statistical data and propose two approaches. The first is to apply the normal compressive mechanism to the statistics vector along with an algorithm to determine the number of nonzero entries in a sparse representation. The second is to alter the mechanism based on the data, aiming to release significant single nucleotide polymorphisms with a high probability. In this algorithm, we apply the compressive mechanism with the input as a sparse vector for significant data and the Laplace mechanism for nonsignificant data. By using the Haar wavelet transform for the compressive mechanism, we can determine the number of nonzero elements and the amount of noise. In addition, we give theoretical guarantees that our proposed methods achieve ϵ-differential privacy. We evaluated our methods in terms of accuracy and rank error compared with the Laplace and exponential mechanisms. The results show that our second method in particular can guarantee high privacy assurance as well as utility.

SMAD6-deficiency in human genetic disorders

SMAD6 encodes an intracellular inhibitor of the bone morphogenetic protein (BMP) signalling pathway. Until now, SMAD6-deficiency has been associated with three distinctive human congenital conditions, i.e., congenital heart diseases, including left ventricular obstruction and conotruncal defects, craniosynostosis and radioulnar synostosis. Intriguingly, a similar spectrum of heterozygous loss-of-function variants has been reported to cause these clinically distinct disorders without a genotype–phenotype correlation. Even identical nucleotide changes have been described in patients with either a cardiovascular phenotype, craniosynostosis or radioulnar synostosis. These findings suggest that the primary pathogenic variant alone cannot explain the resultant patient phenotype. In this review, we summarise clinical and (patho)genetic (dis)similarities between these three SMAD6-related conditions, compare published Madh6 mouse models, in which the importance and impact of the genetic background with respect to the observed phenotype is highlighted, and elaborate on the cellular key mechanisms orchestrated by SMAD6 in the development of these three discrete inherited disorders. In addition, we discuss future research needed to elucidate the pathogenetic mechanisms underlying these diseases in order to improve their molecular diagnosis, advance therapeutic strategies and facilitate counselling of patients and their families.

Targeted Sequencing of Candidate Regions Associated with Sagittal and Metopic Nonsyndromic Craniosynostosis

Craniosynostosis (CS) is a major birth defect in which one or more skull sutures fuse prematurely. We previously performed a genome-wide association study (GWAS) for sagittal non-syndromic CS (sNCS), identifying associations downstream from BMP2 on 20p12.3 and intronic to BBS9 on 7p14.3; analyses of imputed variants in DLG1 on 3q29 were also genome-wide significant. We followed this work with a GWAS for metopic non-syndromic NCS (mNCS), discovering a significant association intronic to BMP7 on 20q13.31. In the current study, we sequenced the associated regions on 3q29, 7p14.3, and 20p12.3, including two candidate genes (BMP2 and BMPER) near some of these regions in 83 sNCS child-parent trios, and sequenced regions on 7p14.3 and 20q13.2-q13.32 in 80 mNCS child-parent trios. These child-parent trios were selected from the original GWAS cohorts if the probands carried at least one copy of the top associated GWAS variant (rs1884302 C allele for sNCS; rs6127972 T allele for mNCS). Many of the variants sequenced in these targeted regions are strongly predicted to be within binding sites for transcription factors involved in craniofacial development or bone morphogenesis. Variants enriched in more than one trio and predicted to be damaging to gene function are prioritized for functional studies.

Major sex differences in allele frequencies for X chromosomal variants in both the 1000 Genomes Project and gnomAD

An unexpectedly high proportion of SNPs on the X chromosome in the 1000 Genomes Project phase 3 data were identified with significant sex differences in minor allele frequencies (sdMAF). sdMAF persisted for many of these SNPs in the recently released high coverage whole genome sequence of the 1000 Genomes Project that was aligned to GRCh38, and it was consistent between the five super-populations. Among the 245,825 common (MAF>5%) biallelic X-chromosomal SNPs in the phase 3 data presumed to be of high quality, 2,039 have genome-wide significant sdMAF (p-value <5e-8). sdMAF varied by location: non-pseudo-autosomal region (NPR) = 0.83%, pseudo-autosomal regions (PAR1) = 0.29%, PAR2 = 13.1%, and X-transposed region (XTR)/PAR3 = 0.85% of SNPs had sdMAF, and they were clustered at the NPR-PAR boundaries, among others. sdMAF at the NPR-PAR boundaries are biologically expected due to sex-linkage, but have generally been ignored in association studies. For comparison, similar analyses found only 6, 1 and 0 SNPs with significant sdMAF on chromosomes 1, 7 and 22, respectively. Similar sdMAF results for the X chromosome were obtained from the high coverage whole genome sequence data from gnomAD V 3.1.2 for both the non-Finnish European and African/African American samples. Future X chromosome analyses need to take sdMAF into account.

The human X chromosome contains over 800 genes and is the 8^th largest human chromosome. Genome-wide associations studies have generally failed to examine variants on the X chromosome for association with diseases and traits, partly due to complexities of the data analysis, and challenges with genotype imputation. We examined X chromosomal variants from the 1000 Genomes Project for sex differences in allele frequency and found that many variants showed significant differences. These variants cluster at the centromeric parts of the pseudoautosomal regions 1 and 2, as well as the putative pseudo-autosomal region 3 (also termed X-transposed region). This pattern was observed in high coverage whole genome sequence data from the same subjects that was aligned to GRCh38, suggesting that is not an artefact of low coverage sequencing or problems specific to GRCh37. In addition, we replicated this phenomenon in high coverage whole genome sequence aligned to GRCh38 from the gnomAD database in both the non-Finnish European and African/African American samples. These findings have implications for the analysis of X chromosomal variants for disease and trait associations.

Decoding the Human Face: Challenges and Progress in Understanding the Genetics of Craniofacial Morphology

Variations in the form of the human face, which plays a role in our individual identities and societal interactions, have fascinated scientists and artists alike. Here, we review our current understanding of the genetics underlying variation in craniofacial morphology and disease-associated dysmorphology, synthesizing decades of progress on Mendelian syndromes in addition to more recent results from genome-wide association studies of human facial shape and disease risk. We also discuss the various approaches used to phenotype and quantify facial shape, which are of particular importance due to the complex, multipartite nature of the craniofacial form. We close by discussing how experimental studies have contributed and will further contribute to our understanding of human genetic variation and then proposing future directions and applications for the field.

The clinical manifestations, molecular mechanisms and treatment of craniosynostosis

Craniosynostosis is a major congenital craniofacial disorder characterized by the premature fusion of cranial suture(s). Patients with severe craniosynostosis often have impairments in hearing, vision, intracranial pressure and/or neurocognitive functions. Craniosynostosis can result from mutations, chromosomal abnormalities or adverse environmental effects, and can occur in isolation or in association with numerous syndromes. To date, surgical correction remains the primary treatment for craniosynostosis, but it is associated with complications and with the potential for re-synostosis. There is, therefore, a strong unmet need for new therapies. Here, we provide a comprehensive review of our current understanding of craniosynostosis, including typical craniosynostosis types, their clinical manifestations, cranial suture development, and genetic and environmental causes. Based on studies from animal models, we present a framework for understanding the pathogenesis of craniosynostosis, with an emphasis on the loss of postnatal suture mesenchymal stem cells as an emerging disease-driving mechanism. We evaluate emerging treatment options and highlight the potential of mesenchymal stem cell-based suture regeneration as a therapeutic approach for craniosynostosis.

The genetic overlap between osteoporosis and craniosynostosis

Osteoporosis is the most prevalent bone condition in the ageing population. This systemic disease is characterized by microarchitectural deterioration of bone, leading to increased fracture risk. In the past 15 years, genome-wide association studies (GWAS), have pinpointed hundreds of loci associated with bone mineral density (BMD), helping elucidate the underlying molecular mechanisms and genetic architecture of fracture risk. However, the challenge remains in pinpointing causative genes driving GWAS signals as a pivotal step to drawing the translational therapeutic roadmap. Recently, a skull BMD-GWAS uncovered an intriguing intersection with craniosynostosis, a congenital anomaly due to premature suture fusion in the skull. Here, we recapitulate the genetic contribution to both osteoporosis and craniosynostosis, describing the biological underpinnings of this overlap and using zebrafish models to leverage the functional investigation of genes associated with skull development and systemic skeletal homeostasis.

Evaluating the performance of a clinical genome sequencing program for diagnosis of rare genetic disease, seen through the lens of craniosynostosis

PURPOSE

Genome sequencing (GS) for diagnosis of rare genetic disease is being introduced into the clinic, but the complexity of the data poses challenges for developing pipelines with high diagnostic sensitivity. We evaluated the performance of the Genomics England 100,000 Genomes Project (100kGP) panel-based pipelines, using craniosynostosis as a test disease.

METHODS

GS data from 114 probands with craniosynostosis and their relatives (314 samples), negative on routine genetic testing, were scrutinized by a specialized research team, and diagnoses compared with those made by 100kGP.

RESULTS

Sixteen likely pathogenic/pathogenic variants were identified by 100kGP. Eighteen additional likely pathogenic/pathogenic variants were identified by the research team, indicating that for craniosynostosis, 100kGP panels had a diagnostic sensitivity of only 47%. Measures that could have augmented diagnoses were improved calling of existing panel genes (+18% sensitivity), review of updated panels (+12%), comprehensive analysis of de novo small variants (+29%), and copy-number/structural variants (+9%). Recent NHS England recommendations that partially incorporate these measures should achieve 85% overall sensitivity (+38%).

CONCLUSION

GS identified likely pathogenic/pathogenic variants in 29.8% of previously undiagnosed patients with craniosynostosis. This demonstrates the value of research analysis and the importance of continually improving algorithms to maximize the potential of clinical GS.

Identification of differentially expressed proteins between fused and open sutures in sagittal nonsyndromic craniosynostosis during suture development by quantitative proteomic analysis

PURPOSE

Nonsyndromic craniosynostosis (NCS), the premature fusion of cranial sutures, results in an abnormal skull shape and is associated with a significant morbidity. Proteomics is a promising tool for disease characterization and biomarker discovery; we aimed to identify biologically relevant differentially expressed proteins for NCS.

EXPERIMENTAL DESIGN

Label-based quantitative proteomic profiling using TMT was performed on protein extracted from mesenchymal stem cells, osteoblasts and bone tissue of five open and five fused sutures of sagittal NCS (sNCS) and analyzed using quantitative LC-MS/MS based bottom-up proteomics. Differential protein abundance between open and fused sutures was determined to identify biologically relevant proteins of interest. Proteins were validated in an independent sample set by western blot and immunohistochemistry.

RESULTS

We observed 838 differentially expressed proteins between open and fused sutures of sNCS. Decorin, lumican, and asporin were significantly downregulated while COL4A1 and TGFβ1|1 were upregulated in fused compared to open sutures.

CONCLUSIONS AND CLINICAL RELEVANCE

The majority of significantly differentially expressed proteins between open and fused sutures were observed in the proteomes of osteoblasts suggesting that protein changes contributing to premature sagittal suture fusion occur predominantly at the osteoblast level. Our findings suggest a possible ineffective ECM deposition at the osteoblast cell stage.

Serum nickel is associated with craniosynostosis risk: Evidence from humans and mice

BACKGROUND

To date, few studies have explored the effects of exposure to metal mixtures on adverse developmental outcomes, and no reported studies have linked metal exposure to craniosynostosis (CS). The purpose of this study is to investigate the association between metal exposure and the risk of CS by conducting epidemiological and experimental studies.

METHODS

Inductively coupled plasma mass spectrometry (ICP-MS) was used to measure the concentrations of 6 metals (chromium [Cr], nickel [Ni], tin [Sn], arsenic [As], thallium [Tl], and lead [Pb]) in serum samples from 174 CS patients and 85 control individuals. Non-syndromic patients with isolated sagittal suture closure were selected as the case group, and healthy children matched by sex and age were selected as controls. Bayesian kernel machine regression (BKMR) models were used to account for joint metal effects. Multiple logistic regression analysis was used to explore the association between metal concentration and CS occurrence, with adjustment for potential confounders. During pregnancy, mice were exposed to Ni (0, 0.05, or 0.1 g/kg/day) until weaning, and the widths of the sutures and shapes of the skull were analysed by micro-CT 3D imaging and histological analysis. MC3T3-E1 cells were treated with Ni (0, 0.005, or 0.05 μg/mL) for 72 h. Alkaline phosphate (ALP) staining and Alizarin red staining were performed to observe the development of osteoblasts. The expression levels of osteoblast-related genes were also detected.

RESULTS

A positive association between the metal mixture and CS risk was observed based on population data; the Ni group had the highest conditional posterior inclusion probability (PIP), at 0.8416, and in the fully adjusted model, the highest Ni exposure level had a more significant association with CS (coefficient = 2.65, 95% CI: 0.29, 5.02) than the lowest Ni exposure level. The mean widths of the sagittal sutures in mice were 8.8 ± 0.6 mm in the control group, 8.0 ± 0.8 mm in the 0.05 g/kg/day group and 6.8 ± 0.4 mm in the 0.1 g/kg/day group. After Ni exposure, ALP gene expression in skull tissue was increased, and ALP activity was increased in MC3T3-E1 cells. Moreover, increased collagen content in mouse skull sections and elevated osteocalcin (OCN) expression in MC3T3-E1 cells were observed in the Ni-treated groups compared to the control group.

CONCLUSIONS

This study is the first to provide evidence that increased serum Ni was associated with an increased risk of CS. Early life exposure to Ni promoted osteogenesis during skull growth, which may contribute to the development of CS.

SMAD6 variants in craniosynostosis: genotype and phenotype evaluation

PURPOSE

Enrichment of heterozygous missense and truncating SMAD6 variants was previously reported in nonsyndromic sagittal and metopic synostosis, and interaction of SMAD6 variants with a common polymorphism nearBMP2 (rs1884302) was proposed to contribute to inconsistent penetrance. We determined the occurrence of SMAD6 variants in all types of craniosynostosis, evaluated the impact of different missense variants on SMAD6 function, and tested independently whether rs1884302 genotype significantly modifies the phenotype.

METHODS

We performed resequencing of SMAD6 in 795 unsolved patients with any type of craniosynostosis and genotyped rs1884302 in SMAD6-positive individuals and relatives. We examined the inhibitory activity and stability of SMAD6 missense variants.

RESULTS

We found 18 (2.3%) different rare damaging SMAD6 variants, with the highest prevalence in metopic synostosis (5.8%) and an 18.3-fold enrichment of loss-of-function variants comparedwith gnomAD data (P < 10-7). Combined with eight additional variants, ≥20/26 were transmitted from an unaffected parent but rs1884302 genotype did not predict phenotype.

CONCLUSION

Pathogenic SMAD6 variants substantially increase the risk of both nonsyndromic and syndromic presentations of craniosynostosis, especially metopic synostosis. Functional analysis is important to evaluate missense variants. Genotyping of rs1884302 is not clinically useful. Mechanisms to explain the remarkable diversity of phenotypes associated with SMAD6 variants remain obscure.

GLI1 and AXIN2 Are Distinctive Markers of Human Calvarial Mesenchymal Stromal Cells in Nonsyndromic Craniosynostosis

All skeletal bones house osteogenic stem cell niches, in which mesenchymal stromal cells (MSC) provide progenitors for tissue growth and regeneration. They have been widely studied in long bones formed through endochondral ossification. Limited information is available on the composition of the osteogenic niche in flat bones (i.e., skull vault bones) that develop through direct membranous ossification. Craniosynostosis (CS) is a congenital craniofacial defect due to the excessive and premature ossification of skull vault sutures. This study aimed at analysing the expression of GLI1, AXIN2 and THY1 in the context of the human skull vault, using nonsyndromic forms of CS (NCS) as a model to test their functional implication in the aberrant osteogenic process. The expression of selected markers was studied in NCS patients' calvarial bone specimens, to assess the in vivo location of cells, and in MSC isolated thereof. The marker expression profile was analysed during in vitro osteogenic differentiation to validate the functional implication. Our results show that GLI1 and AXIN2 are expressed in periosteal and endosteal locations within the osteogenic niche of human calvarial bones. Their expression is higher in MSC isolated from calvarial bones than in those isolated from long bones and tends to decrease upon osteogenic commitment and differentiation. In particular, AXIN2 expression was lower in cells isolated from prematurely fused sutures than in those derived from patent sutures of NCS patients. This suggests that AXIN2 could reasonably represent a marker for the stem cell population that undergoes depletion during the premature ossification process occurring in CS.