Further expansion of the mutational spectrum of spondylo-meta-epiphyseal dysplasia with abnormal calcification

Discoidin domain receptors; an ancient family of collagen receptors has major roles in bone development, regeneration and metabolism

The extracellular matrix (ECM) niche plays a critical role in determining cellular behavior during bone development including the differentiation and lineage allocation of skeletal progenitor cells to chondrocytes, osteoblasts, or marrow adipocytes. As the major ECM component in mineralized tissues, collagen has instructive as well as structural roles during bone development and is required for bone cell differentiation. Cells sense their extracellular environment using specific cell surface receptors. For many years, specific β1 integrins were considered the main collagen receptors in bone, but, more recently, the important role of a second, more primordial collagen receptor family, the discoidin domain receptors, has become apparent. This review will specifically focus on the roles of discoidin domain receptors in mineralized tissue development as well as related functions in abnormal bone formation, regeneration and metabolism.

Spondylo-meta-epiphyseal dysplasia (SMED), short limb-hand abnormal calcification type: Further expanding the mutational spectrum and dental findings of three new patients

Genetic skeletal disorders are clinically and genetically heterogeneous group of disorders that affect the normal development, growth, and maintenance of the human skeleton. Spondylo-meta-epiphyseal dysplasia, short limb-abnormal calcification type (SMED-SL/AC; MIM# 271665) is a rare autosomal recessive genetic skeletal disorder characterized by distinctive facial features, disproportionate short stature, vertebral, metaphyseal, and epiphyseal abnormalities. This unique phenotype is caused by biallelic loss-of-function variants in Discoidin domain receptor 2 gene (DDR2, MIM# 191311). To date, only 10 pathogenic variants (six missense, two nonsense, one deletion, and one splice site) in DDR2 have been reported in patients with SMED-SL/AC. Dental anomalies related to skeletal dysplasia can include various abnormalities in the number, shape, and position of teeth in the jaw, as well as enamel hypoplasia and dentinogenesis imperfecta. Although abnormal dentition has previously been reported, orodental findings were described in only six patients with SMED-SL/AC. This study aimed to define the clinical, dental, radiological, and molecular findings of three new SMED-SL/AC patients from three unrelated families. Three DDR2 variants, two of which were novel, were detected with the aid of Sanger sequencing. Interestingly, one of the patients was diagnosed with Wilson's disease (WD) during the follow-up, a co-occurrence that has never been reported in patients with SMED-SL/AC so far.

Control of craniofacial development by the collagen receptor, discoidin domain receptor 2

Development of the craniofacial skeleton requires interactions between progenitor cells and the collagen-rich extracellular matrix (ECM). The mediators of these interactions are not well-defined. Mutations in the discoidin domain receptor 2 gene (DDR2), which encodes a non-integrin collagen receptor, are associated with human craniofacial abnormalities, such as midface hypoplasia and open fontanels. However, the exact role of this gene in craniofacial morphogenesis is not known. As will be shown, Ddr2-deficient mice exhibit defects in craniofacial bones including impaired calvarial growth and frontal suture formation, cranial base hypoplasia due to aberrant chondrogenesis and delayed ossification at growth plate synchondroses. These defects were associated with abnormal collagen fibril organization, chondrocyte proliferation and polarization. As established by localization and lineage-tracing studies, Ddr2 is expressed in progenitor cell-enriched craniofacial regions including sutures and synchondrosis resting zone cartilage, overlapping with GLI1 + cells, and contributing to chondrogenic and osteogenic lineages during skull growth. Tissue-specific knockouts further established the requirement for Ddr2 in GLI +skeletal progenitors and chondrocytes. These studies establish a cellular basis for regulation of craniofacial morphogenesis by this understudied collagen receptor and suggest that DDR2 is necessary for proper collagen organization, chondrocyte proliferation, and orientation.

Cranial Base Synchondrosis: Chondrocytes at the Hub

The cranial base is formed by endochondral ossification and functions as a driver of anteroposterior cranial elongation and overall craniofacial growth. The cranial base contains the synchondroses that are composed of opposite-facing layers of resting, proliferating and hypertrophic chondrocytes with unique developmental origins, both in the neural crest and mesoderm. In humans, premature ossification of the synchondroses causes midfacial hypoplasia, which commonly presents in patients with syndromic craniosynostoses and skeletal Class III malocclusion. Major signaling pathways and transcription factors that regulate the long bone growth plate-PTHrP-Ihh, FGF, Wnt, BMP signaling and Runx2-are also involved in the cranial base synchondrosis. Here, we provide an updated overview of the cranial base synchondrosis and the cell population within, as well as its molecular regulation, and further discuss future research opportunities to understand the unique function of this craniofacial skeletal structure.

The collagen receptor, discoidin domain receptor 2, functions in Gli1-positive skeletal progenitors and chondrocytes to control bone development

Discoidin Domain Receptor 2 (DDR2) is a collagen-activated receptor kinase that, together with integrins, is required for cells to respond to the extracellular matrix. Ddr2 loss-of-function mutations in humans and mice cause severe defects in skeletal growth and development. However, the cellular functions of Ddr2 in bone are not understood. Expression and lineage analysis showed selective expression of Ddr2 at early stages of bone formation in the resting zone and proliferating chondrocytes and periosteum. Consistent with these findings, Ddr2⁺ cells could differentiate into hypertrophic chondrocytes, osteoblasts, and osteocytes and showed a high degree of colocalization with the skeletal progenitor marker, Gli1. A conditional deletion approach showed a requirement for Ddr2 in Gli1-positive skeletal progenitors and chondrocytes but not mature osteoblasts. Furthermore, Ddr2 knockout in limb bud chondroprogenitors or purified marrow-derived skeletal progenitors inhibited chondrogenic or osteogenic differentiation, respectively. This work establishes a cell-autonomous function for Ddr2 in skeletal progenitors and cartilage and emphasizes the critical role of this collagen receptor in bone development.

Spondylometaepiphyseal Dysplasia Short Limb-Abnormal Calcification Type in Turkish Patients Reveals a Novel Mutation and New Features

Spondylometaepiphyseal dysplasia short limb-abnormal calcification type (SMED-SL/AC) is a rare autosomal recessive disorder. It is a severe dwarfism syndrome with a characteristic feature of progressive calcification of epiphyseal and other cartilaginous tissues. It is caused by pathogenic variants in the DDR2 gene encoding the discoidin domain receptor tyrosine kinase 2. Thus far, 37 cases and 8 pathogenic variants have been reported. Most of the reported cases are of Middle Eastern and Puerto Rican origins. Only one Turkish case has been reported previously with a novel truncating variant p.(R489*). Here, we report 2 new cases, 1 with a novel variant p.(S311G) and 1 with a splice site variant c.2283+1G>A. In addition, we reviewed a previously reported case, and sequencing of stored DNA revealed the recently reported nonsense variant p.(R489*) as the underlying cause. Therefore, our data increase the number of SMED-SL/AC Turkish patients with molecular results to 4. Furthermore, we compared the features of Turkish patients with other reported cases and expanded the characteristics of the disorder with new features such as triventricular hydrocephalus, intracranial hemorrhage, hypopigmentation of hair, dry and scaly skin, arthralgia, and hypocalcemia. We also compared the pathogenic variants of Turkish patients with other variants, aiming to explain the mechanism leading to a more severe and early fatal course in Turkish patients.