Spatial regulation of gene expression in nonsyndromic sagittal craniosynostosis

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.

From mesenchymal niches to engineered in vitro model systems: Exploring and exploiting biomechanical regulation of vertebrate hedgehog signalling

Tissue patterning is the result of complex interactions between transcriptional programs and various mechanical cues that modulate cell behaviour and drive morphogenesis. Vertebrate Hedgehog signalling plays key roles in embryogenesis and adult tissue homeostasis, and is central to skeletal development and the osteogenic differentiation of mesenchymal stem cells. The expression of several components of the Hedgehog signalling pathway have been reported to be mechanically regulated in mesodermal tissue patterning and osteogenic differentiation in response to external stimulation. Since a number of bone developmental defects and skeletal diseases, such as osteoporosis, are directly linked to aberrant Hedgehog signalling, a better knowledge of the regulation of Hedgehog signalling in the mechanosensitive bone marrow-residing mesenchymal stromal cells will present novel avenues for modelling these diseases and uncover novel opportunities for extracellular matrix-targeted therapies. In this review, we present a brief overview of the key molecular players involved in Hedgehog signalling and the basic concepts of mechanobiology, with a focus on bone development and regeneration. We also highlight the correlation between the activation of the Hedgehog signalling pathway in response to mechanical cues and osteogenesis in bone marrow-derived mesenchymal stromal cells. Finally, we propose different tissue engineering strategies to apply the expanding knowledge of 3D material-cell interactions in the modulation of Hedgehog signalling in vitro for fundamental and translational research applications.

Differential Responsiveness to BMP9 between Patent and Fused Suture Progenitor Cells from Craniosynostosis Patients

BACKGROUND

Several studies have verified that bone morphogenetic proteins (BMPs) may be involved in the development of craniosynostosis; little attention has been focused on the role of BMP9 in cranial suture biology. The authors investigated the role of BMP9 in suture progenitor cells.

METHODS

The authors isolated and cultured prematurely fused and internal control patent suture progenitor cells from patients with nonsyndromic craniosynostosis. Overexpression of BMP9 was mediated by adenoviral vectors. Osteoblast and osteoclast differentiation-related markers were evaluated by staining techniques and touchdown quantitative polymerase chain reaction analysis. In vivo analysis of BMP9-induced suture progenitor cell osteogenesis was performed in an ectopic bone formation model.

RESULTS

The authors demonstrated that the prematurely fused sutures have a higher endogenous expression of the osteogenic differentiation-related genes than patent sutures, whereas the same pattern of gene expression exists between fused and patent suture progenitor cells. Importantly, both patent and fused suture progenitor cells undergo osteogenic differentiation and express multiple lineage regulators and NELL-1 on BMP9 stimulation, whereas fused suture progenitor cells have a higher basal osteogenic potential than patent suture progenitor cells. BMP9 regulates the expression of osteoclast differentiation-related genes in suture progenitor cells. Forced BMP9 expression enhances the mineralization and maturity of ectopic bone formation of suture progenitor cells implanted in vivo.

CONCLUSIONS

The authors' findings suggest that fused suture progenitor cells have elevated osteogenic potential. BMP9 could regulate the expression of multiple osteoblast and osteoclast differentiation-related genes, and NELL-1, in both suture progenitor cells, indicating that BMP9 may play a role in craniosynostosis.

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.