Chapter Twelve - Mechanistic insights into skeletal development gained from genetic disorders
Introduction
Congenital skeletal dysplasia arises when genetic alterations result in the disruption of the pattern, structure and growth of the skeleton. Such perturbations manifest as one or more phenotypes affecting the shape and size of individual skeletal elements, such as short, stubby fingers, duplications of fingers or toes, clubfeet, missing bones, fragile bones or curved spines. Normal skeletogenesis requires spatial and temporal control and integration of various transcription factors and signaling pathways to coordinate precisely the initial condensation of mesenchymal cells, specification of osteo-chondroprogenitors and the sequential phases of chondrocyte differentiation, proliferation, cell cycle exit and maturation, hypertrophy and the transition to the osteoblast lineage. Several of these factors are described in detail in other chapters. Many genes and pathways were discovered through identifying causative mutations associated with human skeletal syndromes. Advances and affordable technologies for sequencing whole genomes have accelerated the discovery of new genes and genetic loci in skeletal dysplasias (Bonafe et al., 2015; Geister & Camper, 2015). In this chapter, we highlight the contribution of recent discoveries of causative mutations in human skeletal dysplasias, combined with functional genomics, to the identification of key genes and pathways and gene regulatory mechanisms that govern different phases of skeletal development (Fig. 1). We also briefly illustrate how knowledge of the underlying molecular pathogenesis is being exploited for clinical translation, leading to human trials on chondrodysplasias.
Section snippets
Genetic control of patterning the appendicular skeleton
Human limbs consist of bones and soft tissues of particular size and shape arranged in a precise pattern. Structural abnormalities are often unique and diagnostic. One of the most recognizable limb phenotypes is polydactyly (Greek for “many fingers”). Digits in human hands and feet are formed in a highly conserved pentadactyl pattern, but individuals with polydactyly have additional digits arising on the side of the thumb (preaxial), the little finger (postaxial), or the central fingers
Skeletal morphogenesis: Integrated control of chondrocyte differentiation
The cascade of differentiation steps in endochondral bone development is controlled by a combination of key transcription factors and the integrated action of signaling pathways, as exemplified by a number of skeletal dysplasias. The discovery of SOX9 as a master regulator of chondrogenesis came from the identification of its causative role in Campomelic dysplasia (CD, OMIM#114290), a rare, semi-lethal autosomal dominant congenital skeletal disorder that affects approximately 1 in 40,000 to
Integrated signaling control of osteoblast differentiation and activity
The mechanisms underlying several bone disorders highlight the integration of signaling pathways in controlling bone formation. Progressive osseous heteroplasia (POH, OMIM#166350) is an autosomal dominant disorder characterized by widespread and disabling heterotopic ossification of skeletal muscle and deep connective tissues. It is caused by a null mutation of GNAS, which encodes Gαs, a protein that transduces signals from G protein-coupled receptors (Shore et al., 2002). In contrast,
Ciliopathies and the primary cilia in skeletal development
In the past decade, primary cilia emerged as important modulators of vertebrate HH signaling and their dysfunction has been linked to a spectrum of human diseases, collectively termed ciliopathies (Huber & Cormier-Daire, 2012). As cilia are a component of almost all cells, ciliary dysfunction often affects multiple organs and the phenotypic outcome is characteristic of aberrant HH signaling (Waters & Beales, 2011). For example, Meckel's syndrome (MES, OMIM#249000), Bardet-Biedl syndrome (BBS,
Planar cell polarity in the development of growth plate
The Planar Cell Polarity (PCP) pathway controls the process of convergent extension and collective cell migration and thereby the elongation of the body axis and shapes of many organs (Henderson, Long, & Dean, 2018). In endochondral ossification, the growth plate architecture of organized columns of cells requires PCP activity and its defects predispose humans to various skeletal dysplasias (Wang, Sinha, Jiao, Serra, & Wang, 2011). Robinow syndrome (RS) is a genetically and phenotypically
The impact of ER stress signaling on chondrocyte differentiation
The different cell types in the mammalian skeleton are embedded in tissue-characteristic complex extracellular matrix (ECM) networks, composed of collagens, proteoglycans, glycosaminoglycans, and glycoproteins. The ECM is important not only in providing structural support but by influencing cell adhesion, proliferation, migration, survival, differentiation and control of cell fate and morphogenesis. The pivotal role of the ECM is reflected in the major contribution of disruption in genes
Non-coding mutations and regulatory control of skeletal development
The discovery of mutations in non-coding genomic regions that cause skeletal dysplasia has brought a new dimension to our understanding of the regulatory control of skeletogenesis (Fig. 2). An outstanding example is the identification of a highly conserved cis-regulatory element within the preaxial polydactyly (PPD) transcription-associated region, called the ZPA regulatory sequence (ZRS). This enhancer is responsible for the initiation and spatially restricted expression of Shh in the ZPA,
Impacting 3D genome folding in skeletal disorders
Genomic DNA in the nucleus is organized as topologically associating domains (TADs), which are fundamental structural units that guide the physical interaction between cis-regulatory elements and promoters while insulating adjacent domains from inappropriate contacts (Dixon et al., 2012; Rao et al., 2014). Structural variations caused by chromosomal rearrangements or insertions or deletions affecting TADs can have profound effects on gene regulation and thereby disease outcomes (Kragesteen et
Mechanistic insights from skeletal disorders: Impacting the path to therapy
The ultimate hope for skeletal dysplasia patients is the availability of therapies that can ameliorate or prevent the dysmorphology. In recent years some progress has been made toward the development of therapeutic approaches for certain types of congenital dwarfism, based on the underlying mechanistic insights gained from fundamental research. Here, we highlight two examples for which mechanistic insights have been exploited and have entered human clinical trials: Achondrodysplasia and Schmid
Future directions and perspectives
Development of therapeutic approaches for congenital skeletal dysplasias requires knowledge of the lineage origins of skeletal cells and the properties of resident stem/progenitor populations in development, growth and disease. In recent years, lineage tracing experiments have revealed that a substantial fraction of hypertrophic chondrocytes survive and differentiate into osteoblastic cells during endochondral bone development and repair (Park et al., 2015; Yang, Tsang, Tang, Chan, & Cheah, 2014
Acknowledgments
K.C. and D.C. are supported by the Hong Kong Research Grants Council T12-708/12 N. We thank Tiong Tan for valuable comments on the manuscript and Wilson Chan for assistance with figure drawing.
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From mesenchymal niches to engineered in vitro model systems: Exploring and exploiting biomechanical regulation of vertebrate hedgehog signalling
2022, Materials Today BioCitation Excerpt :Mechanical loading is known to regulate bone development, homeostasis, remodelling and adaptation [169]. Loss of mechanical stimulation can affect various stages of skeletal development including patterning, differentiation, growth and morphogenesis, thereby leading to bone weakening and increasing risk of bone fracture [170]. Furthermore, bones are shaped according to the mechanical force exerted by muscle contraction and gravity during embryonic development [171].
The extended chondrocyte lineage: implications for skeletal homeostasis and disorders
2019, Current Opinion in Cell BiologyCitation Excerpt :Within the growth plate the proliferating chondrocytes organise into a columnar structure, exit the cell cycle and undergo hypertrophy to become hypertrophic chondrocytes (HCs). The hypertrophic cartilage zone becomes calcified and is subsequently resorbed and replaced with bone and vasculature (reviewed in Refs. [1,2]). During hypertrophy, HCs undergo phasic enlargement which is crucial in the growth of long bone [3].
SOX9 in cartilage development and disease
2019, Current Opinion in Cell BiologyCitation Excerpt :SOX9 belongs to a family of twenty SRY-related HMG box-containing (SOX) proteins, most of which contribute to cell type specification and differentiation in discrete lineages [5]. The first clue that SOX9 is essential in development, including chondrogenesis, came with the discovery that heterozygous mutations occurring within and around SOX9 cause Campomelic Dysplasia (CMPD), a severe skeleton malformation syndrome often associated with XY sex reversal, and milder skeletal dysplasias, namely acampomelic campomelic dysplasia (ACMPD) and Pierre Robin Sequence (PRS) [6–9]. Studies in animal models and molecular studies have uncovered many aspects of SOX9’s-specific roles and modes of actions and regulation in development and diseases, but many questions remain.
A Randomized, Controlled Trial of the Analytic and Diagnostic Performance of Singleton and Trio, Rapid Genome and Exome Sequencing in Ill Infants
2019, American Journal of Human GeneticsCitation Excerpt :Four previous within-cohort comparisons of WES and WGS reported 4%–7% increased diagnostic yield with WGS.42–45 Conditions that resulted in diagnosis by WGS but not WES included non-coding variants in neurodevelopmental and skeletal disorders, pseudogenes in polycystic kidney disease, and structural variants.45–50 Herein, one diagnosis (6%) by WGS, renal hypodysplasia/aplasia 3 (GREB1L c.3194C>T) would have been missed had that infant been randomized to WES, since that gene lacks sequence coverage by WES.
A combination insecticide at sub-lethal dose debilitated the expression pattern of crucial signalling molecules that facilitate craniofacial patterning in domestic chick Gallus domesticus
2019, Neurotoxicology and TeratologyCitation Excerpt :Both the signals together indicate their expression to be a powerful activator of BMP2 expression required for normal osteoblast differentiation. Gli3, a transcriptional repressor of SHH pathway, also has been reported to trigger skeletal defects in mice and humans (Yip et al., 2019). Downregulated levels of GLI3 observed in the current study support the concept of FGF8 overexpression, causing abnormal apoptosis of the skeletal tissues.
Application of Single-Cell and Spatial Omics in Musculoskeletal Disorder Research
2023, International Journal of Molecular Sciences
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Current address: ACRF Stem Cells and Cancer Division, The Walter and Eliza Hall Institute of Medical Research, Parkville, VIC, Australia; Department of Medical Biology, University of Melbourne, Parkville, VIC, Australia.