Musculoskeletal phenotype in two unrelated individuals with a recurrent nonsense variant in SGMS2

Human genetic defects of sphingolipid synthesis

Sphingolipids are ubiquitous lipids, present in the membranes of all cell types, the stratum corneum and the circulating lipoproteins. Autosomal recessive as well as dominant diseases due to disturbed sphingolipid biosynthesis have been identified, including defects in the synthesis of ceramides, sphingomyelins and glycosphingolipids. In many instances, these gene variants result in the loss of catalytic function of the mutated enzymes. Additional gene defects implicate the subcellular localization of the sphingolipid-synthesizing enzyme, the regulation of its activity, or even the function of a sphingolipid-transporter protein. The resulting metabolic alterations lead to two major, non-exclusive types of clinical manifestations: a neurological disease, more or less rapidly progressive, associated or not with intellectual disability, and an ichthyotic-type skin disorder. These phenotypes highlight the critical importance of sphingolipids in brain and skin development and homeostasis. The present article reviews the clinical symptoms, genetic and biochemical alterations, pathophysiological mechanisms and therapeutic options of this relatively novel group of metabolic diseases.

Bone fragility and osteoporosis in children and young adults

Osteoporosis is a metabolic bone disorder which increases fragility fracture risk. Elderly individuals, especially postmenopausal women, are particularly susceptible to osteoporosis. Although rare, osteoporosis in children and young adults is becoming increasingly evident, highlighting the need for timely diagnosis, management and follow-up. Early-onset osteoporosis is defined as the presence of a low BMD (Z-score of ≤ -2.0 in individuals aged < 20 years; T-score of ≤ -2.5 in those aged between 20 to 50 years) accompanied by a clinically significant fracture history, or the presence of low-energy vertebral compression fractures even in the absence of osteoporosis. Affected children and young adults should undergo a thorough diagnostic workup, including collection of clinical history, radiography, biochemical investigation and possibly bone biopsy. Once secondary factors and comorbidities are excluded, genetic testing should be considered to determine the possibility of an underlying monogenic cause. Defects in genes related to type I collagen biosynthesis are the commonest contributors of primary osteoporosis, followed by loss-of-function variants in genes encoding key regulatory proteins of canonical WNT signalling (specifically LRP5 and WNT1), the actin-binding plastin-3 protein (encoded by PLS3) resulting in X-linked osteoporosis, and the more recent sphingomyelin synthase 2 (encoded by SGMS2) which is critical for signal transduction affecting sphingomyelin metabolism. Despite these discoveries, genetic causes and underlying mechanisms in early-onset osteoporosis remain largely unknown, and if no causal gene is identified, early-onset osteoporosis is deemed idiopathic. This calls for further research to unravel the molecular mechanisms driving early-onset osteoporosis that consequently will aid in patient management and individualised targeted therapy.

Hsa_circ_0000129 knockdown attenuates proliferation and migration in keloid fibroblasts by targeting miR-485-3p/SGMS2 pathway

BACKGROUND

Aberrant biofunction of circular RNAs (circRNAs) is potently implicated in keloid formation. However, their roles have been underinvestigated. Recent evidence has demonstrated the pro-tumor role of circ_0000129 in cancers, and yet its role in keloid remains elusive.

METHODS

RT-qPCR analysis and or western blotting of miR-485-3p, circ_0000129, and SGMS2 in keloid tissues and keloid fibroblasts was implemented. CCK8, EdU, scratch wound healing, and Transwell migration assays were perfomed to determine the keloid fibroblast proliferation and migration. Luciferase reporter and RIP assays were adopted to analyze the interaction among circ_0000129, miR-485-3p and SGMS2.

RESULTS

In keloid tissues and keloid fibroblasts, circ_0000129 and SGMS2 were amplified, although miR-485-3p expression was downregulated. Furthermore, siRNAs-targeting endogenous circ_0000129 resulted in proliferation and migration defect of keloid fibroblasts. MiR-485-3p was simultaneously recognized by circ_0000129 and SGMS2 3'UTR. Rescued functional assays also illustrated that miR-485-3p loss was beneficial to the proliferation and migration of keloid fibroblasts, and these promoting changes were nullified by accompanied silence circ_0000129 or SGMS2.

CONCLUSION

Circ_0000129 sponges miR-485-3p and releases expression of SGMS2 from the miR-485-3p suppression, promoting migration and proliferation of keloid fibroblasts, suggesting targeting circ_0000129/miR-485-3p/SGMS2 might be a promising strategy against keloid fibroblasts.

AVAILABILITY OF DATA AND MATERIAL

All data generated or analyzed during this study are included in this article.

SGMS2 in primary osteoporosis with facial nerve palsy

Pathogenic heterozygous variants in SGMS2 cause a rare monogenic form of osteoporosis known as calvarial doughnut lesions with bone fragility (CDL). The clinical presentations of SGMS2-related bone pathology range from childhood-onset osteoporosis with low bone mineral density and sclerotic doughnut-shaped lesions in the skull to a severe spondylometaphyseal dysplasia with neonatal fractures, long-bone deformities, and short stature. In addition, neurological manifestations occur in some patients. SGMS2 encodes sphingomyelin synthase 2 (SMS2), an enzyme involved in the production of sphingomyelin (SM). This review describes the biochemical structure of SM, SM metabolism, and their molecular actions in skeletal and neural tissue. We postulate how disrupted SM gradient can influence bone formation and how animal models may facilitate a better understanding of SGMS2-related osteoporosis.

Clinical and Genetic Characteristics of Calvarial Doughnut Lesions with Bone Fragility in Three Families with a Reccurent SGMS2 Gene Variant

Calvarial doughnut lesions (CDL) with bone fragility with or without spondylometaphyseal dysplasia (MIM: #126550) is a rare autosomal dominant skeletal disorder characterized by low bone mineral density, spinal and peripheral fractures, and specific sclerotic lesions of the cranial bones. In the current classification of skeletal disorders, the disease is included in the group of bone fragility disorders along with osteogenesis imperfecta. The disease is caused by pathogenic variants in the SGMS2 gene, the protein product of which is sphingomyelin synthase 2, which primarily contributes to sphingomyelin (SM) synthesis-the main lipid component of the plasma membrane essential for bone mineralization. To date, 15 patients from eight families with CDL with bone fragility have been described in the literature, and a recurrent variant c.148C>T (p.Arg50Ter) in the SGMS2 gene has been identified, which was found in patients from six families. We diagnosed the disease in 11 more patients from three unrelated families, caused by the same heterozygous nonsense variant c.148C>T (p.Arg50Ter) in the SGMS2 gene. Our results show wide interfamilial and intrafamilial phenotypic variability in patients with a detected recurrent variant in the SGMS2 gene, the presence of which must be taken into consideration in the diagnosis of the disease. The primary analysis of this variant will contribute to optimal molecular genetic diagnostics, which can reduce diagnostic costs and time.

Early-Onset Osteoporosis: Rare Monogenic Forms Elucidate the Complexity of Disease Pathogenesis Beyond Type I Collagen

Early-onset osteoporosis (EOOP), characterized by low bone mineral density (BMD) and fractures, affects children, premenopausal women and men aged <50 years. EOOP may be secondary to a chronic illness, long-term medication, nutritional deficiencies, etc. If no such cause is identified, EOOP is regarded primary and may then be related to rare variants in genes playing a pivotal role in bone homeostasis. If the cause remains unknown, EOOP is considered idiopathic. The scope of this review is to guide through clinical and genetic diagnostics of EOOP, summarize the present knowledge on rare monogenic forms of EOOP, and describe how analysis of bone biopsy samples can lead to a better understanding of the disease pathogenesis. The diagnostic pathway of EOOP is often complicated and extensive assessments may be needed to reliably exclude secondary causes. Due to the genetic heterogeneity and overlapping features in the various genetic forms of EOOP and other bone fragility disorders, the genetic diagnosis usually requires the use of next-generation sequencing to investigate several genes simultaneously. Recent discoveries have elucidated the complexity of disease pathogenesis both regarding genetic architecture and bone tissue-level pathology. Two rare monogenic forms of EOOP are due to defects in genes partaking in the canonical WNT pathway: LRP5 and WNT1. Variants in the genes encoding plastin-3 (PLS3) and sphingomyelin synthase 2 (SGMS2) have also been found in children and young adults with skeletal fragility. The molecular mechanisms leading from gene defects to clinical manifestations are often not fully understood. Detailed analysis of patient-derived transiliac bone biopsies gives valuable information to understand disease pathogenesis, distinguishes EOOP from other bone fragility disorders, and guides in patient management, but is not widely available in clinical settings. Despite the great advances in this field, EOOP remains an insufficiently explored entity and further research is needed to optimize diagnostic and therapeutic approaches. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Early-Onset Osteoporosis

Osteoporosis is a skeletal disorder with enhanced bone fragility, usually affecting the elderly. It is very rare in children and young adults and the definition is not only based on a low BMD (a Z-score < - 2.0 in growing children and a Z-score ≤ - 2.0 or a T-score ≤ - 2.5 in young adults) but also on the occurrence of fragility fractures and/or the existence of underlying chronic diseases or secondary factors such as use of glucocorticoids. In the absence of a known chronic disease, fragility fractures and low BMD should prompt extensive screening for secondary causes, which can be found in up to 90% of cases. When fragility fractures occur in childhood or young adulthood without an evident secondary cause, investigations should explore the possibility of an underlying monogenetic bone disease, where bone fragility is caused by a single variant in a gene that has a major role in the skeleton. Several monogenic forms relate to type I collagen, but other forms also exist. Loss-of-function variants in LRP5 and WNT1 may lead to early-onset osteoporosis. The X-chromosomal osteoporosis caused by PLS3 gene mutations affects especially males. Another recently discovered form relates to disturbed sphingolipid metabolism due to SGMS2 mutations, underscoring the complexity of molecular pathology in monogenic early-onset osteoporosis. Management of young patients consists of treatment of secondary factors, optimizing lifestyle factors including calcium and vitamin D and physical exercise. Treatment with bone-active medication should be discussed on a personalized basis, considering the severity of osteoporosis and underlying disease versus the absence of evidence on anti-fracture efficacy and potential harmful effects in pregnancy.

Abnormal Bone Tissue Organization and Osteocyte Lacunocanalicular Network in Early-Onset Osteoporosis Due to SGMS2 Mutations

Pathological variants in SGMS2, encoding sphingomyelin synthase 2 (SMS2), result in a rare autosomal dominant skeletal disorder with cranial doughnut lesions. The disease manifests as early-onset osteoporosis or a more severe skeletal dysplasia with low bone mineral density, frequent fractures, long-bone deformities, and multiple sclerotic cranial lesions. The exact underlying molecular features and skeletal consequences, however, remain elusive. This study investigated bone tissue characteristics in two adult males with a heterozygous SGMS2 mutation p.Arg50* and significant bone fragility. Transiliac bone biopsy samples from both (patient 1: 61 years; patient 2: 29 years) were analyzed by bone histomorphometry, confocal laser scanning microscopy, and quantitative backscattered electron imaging (qBEI). Bone histomorphometry portrayed largely normal values for structural and turnover parameters, but in both patient 1 and patient 2, respectively, osteoid thickness (-1.80 SD, -1.37 SD) and mineralizing surface (-1.03 SD, -2.73 SD) were reduced and osteoid surface increased (+9.03 SD, +0.98 SD), leading to elevated mineralization lag time (+8.16 SD, +4.10 SD). qBEI showed low and heterogeneous matrix mineralization (CaPeak -2.41 SD, -3.72 SD; CaWidth +7.47 SD, +4.41 SD) with a chaotic arrangement of collagenous fibrils under polarized light. Last, osteocyte lacunae appeared abnormally large and round in shape and the canalicular network severely disturbed with short-spanned canaliculi lacking any orderliness or continuity. Taken together, these data underline a central role for functional SMS2 in bone matrix organization and mineralization, lacunocanalicular network, and in maintaining skeletal strength and integrity. These data bring new knowledge on changes in bone histology resulting from abnormal sphingomyelin metabolism and aid en route to better understanding of sphingolipid-related skeletal disorders. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

The Polygenic and Monogenic Basis of Paediatric Fractures

PURPOSE OF REVIEW

Fractures are frequently encountered in paediatric practice. Although recurrent fractures in children usually unveil a monogenic syndrome, paediatric fracture risk could be shaped by the individual genetic background influencing the acquisition of bone mineral density, and therefore, the skeletal fragility as shown in adults. Here, we examine paediatric fractures from the perspective of monogenic and complex trait genetics.

RECENT FINDINGS

Large-scale genome-wide studies in children have identified ~44 genetic loci associated with fracture or bone traits whereas ~35 monogenic diseases characterized by paediatric fractures have been described. Genetic variation can predispose to paediatric fractures through monogenic risk variants with a large effect and polygenic risk involving many variants of small effects. Studying genetic factors influencing peak bone attainment might help in identifying individuals at higher risk of developing early-onset osteoporosis and discovering drug targets to be used as bone restorative pharmacotherapies to prevent, or even reverse, bone loss later in life.

Calvarial doughnut lesions with bone fragility in a French-Canadian family; case report and review of the literature

Calvarial Doughnut Lesions with Bone Fragility (CDL) is an autosomal dominant genetic disease, characterized by low bone mineral density, multiple fractures starting in childhood, and sclerotic doughnut-shaped lesions in the cranial bones. Aubé and colleagues described in 1988 a French-Canadian family of 12 affected members who had a clinical diagnosis of doughnut lesions of the skull, with pathological fractures, osteopenia, "bone in bone" in the vertebral bodies and squaring of metatarsal and metacarpal bones. Herein we study new members of this family. Sequential genetic testing identified a nonsense variant c.148C>T, p. Arg50^⁎ in SGMS2 previously reported in other families. SGMS2 encodes Sphingomyelin Synthase 2, which produces Sphingomyelin (SM), a major lipid component of the plasma membrane that plays a role in bone mineralization. The nonsense variant is associated with milder phenotype. The proband presents with bone in bone vertebral appearance that had been defined uniquely in the first cases described in the same family. The proband's son was identified to carry the same variant, which makes him the sixth generation with the diagnosis of CDL. We also report that the same pathogenic variant was identified in another previously described family, from France. These reports further confirm the genetic basis of CDL, the recurrence of the same variant (p.Arg50*) in individuals of the same ancestry, and the variable penetrance of some of the clinical findings.

Multicentric Osteolysis, Nodulosis, and Arthropathy in two unrelated children with matrix metalloproteinase 2 variants: Genetic-skeletal correlations

Multicentric Osteolysis, Nodulosis, and Arthropathy (MONA) syndrome is a rare genetic skeletal dysplasia. Its diagnosis can be deceptively similar to childhood-onset genetic skeletal dysplasias and juvenile idiopathic arthritis. We aimed to report the syndrome’s clinical and radiologic features with emphasis on skeletal manifestations. And establish relevant phenotype-genotype correlations. We evaluated two boys, 4-and-7-years-old with MONA syndrome. Both patients had consanguineous parents. We verified the diagnosis by correlating the outcomes of clinical, radiologic and molecular analysis. We specifically evaluated the craniofacial morphology and clinical and radiographic skeletal abnormalities. We contextualized the resultant phenotype-genotype correlations to publications on MONA and its differential diagnosis. Skeletal manifestations were the presenting symptoms and mostly restricted to hands and feet in terms of fixed extension deformity of the metacarpophalangeal and flexion deformity of the interphalangeal joints with extension deformity of big toes. There were arthritic symptoms in the older patient especially of the wrists and minute pathologic fractures. The skeletal radiographs showed osteopenia/dysplastic changes of hands and feet. Both patients had variants in the matrix metalloproteinase2 gene which conformed to phenotype of previously reported literature in one patient while the other had a novel variant which conformed to MONA phenotype. Craniofacial abnormalities were present. However, minimal extra-skeletal manifestations. Overall, there is an emerging distinctive skeletal pattern of involvement in terms of both clinical and radiographic features. This includes age of onset and location of presenting skeletal manifestations, chronological order of joint affection, longitudinal disease progression, specifics of skeletal radiographic pathology and craniofacial features. Nevertheless, physicians are cautioned against differential diagnosis of similar genetic skeletal dysplasias and juvenile idiopathic arthritis.

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Presenting manifestations erupt simultaneously in the hands and feet.

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Skeletal manifestations proceed fairly rapidly in a distal-to-proximal fashion.

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Radiographic features are a mixture of osteopenia, joint destruction and fractures.

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Major disability may ensue in late childhood/adolescence, arthrogenic dysplasia

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Non-skeletal manifestations are variable in terms of age of appearance and frequency

Home-Based Telehealth Exercise Intervention in Early-On Survivors of Childhood Acute Lymphoblastic Leukemia: Feasibility Study

Background

Acute lymphoblastic leukemia is the most common type of pediatric cancer. Acute lymphoblastic leukemia causes an altered bone mineral homeostasis state, which can contribute to osteopenia, and bone fractures, most commonly vertebral fractures. With the increasing number of childhood cancer survivors, late adverse effects such as musculoskeletal comorbidities are often reported and are further influenced by inactive lifestyle habits. Physical activity has been shown to increase the mechanical workload of the bone, mitigating bone impairment in other cancer-specific populations.

Objective

This interventional pilot study aims to investigate the use of telehealth to deliver a home-based exercise intervention for early-on survivors of bone marrow–related hematological malignancies and to assess its impact on survivors’ musculoskeletal and functional health.

Methods

We aimed to recruit a group of 12 early-on survivors of acute lymphoblastic leukemia, within 6 months to 5 years of treatment, to participate in and complete the proposed telehealth intervention with a parent. The 16-week intervention included 40 potential home-based physical activity interventions supervised by a kinesiologist through a telehealth internet platform, with monthly progression. Patients were recruited to the cohort if they were able to participate in the intervention during the first month (minimum 12 weeks of intervention). Evaluation before and after the intervention protocol highlighted differences in functional capacities and musculoskeletal health of patients using mechanography, peripheral quantitative computed tomography, 6-minute walk test, and grip force test.

Results

The recruitment rate for the intervention was low (12/57, 21% of contacted patients). Of 12 patients, 3 were excluded (1=relapse, 1=failure to meet technical requirements, and 1=abandoned). The 9 patients who completed the intervention (6 girls; mean age 10.93, SD 2.83 years; mean BMI 21.58, SD 6.55 kg/m²; mean time since treatment completion 36.67, SD 16.37 months) had a mean adherence of 89% and a completion rate of 75%. In addition, these patients showed functional improvements in lower limb muscle force and power as well as in the 6-minute walk test distance. Participants also showed improved bone health after the intervention on the following parameters: bone mineral content, stress-strain index, total and cortical cross-sectional area at the 14% site (P=.03, P=.01, P=.01, and P=.001, respectively) and 38% site of the tibia (P=.003, P=.04, P=.001, and P=.003, respectively).

Conclusions

High adherence and participation rates suggest that telehealth is a feasible method to deliver exercise interventions to young early-on survivors of acute lymphoblastic leukemia. The proposed intervention seems promising in providing benefits to patients’ functional performance and bone health, but a large-scale study is needed to confirm this assumption.

Single-Cell RNA-Sequencing Reveals the Breadth of Osteoblast Heterogeneity

The current paradigm of osteoblast fate is that the majority undergo apoptosis, while some further differentiate into osteocytes and others flatten and cover bone surfaces as bone lining cells. Osteoblasts have been described to exhibit heterogeneous expression of a variety of osteoblast markers at both transcriptional and protein levels. To explore further this heterogeneity and its biological significance, Venus‐positive (Venus⁺) cells expressing the fluorescent protein Venus under the control of the 2.3‐kb Col1a1 promoter were isolated from newborn mouse calvariae and subjected to single‐cell RNA sequencing. Functional annotation of the genes expressed in 272 Venus⁺ single cells indicated that Venus⁺ cells are osteoblasts that can be categorized into four clusters. Of these, three clusters (clusters 1 to 3) exhibited similarities in their expression of osteoblast markers, while one (cluster 4) was distinctly different. We identified a total of 1920 cluster‐specific genes and pseudotime ordering analyses based on established concepts and known markers showed that clusters 1 to 3 captured osteoblasts at different maturational stages. Analysis of gene co‐expression networks showed that genes involved in protein synthesis and protein trafficking between endoplasmic reticulum (ER) and Golgi are active in these clusters. However, the cells in these clusters were also defined by extensive heterogeneity of gene expression, independently of maturational stage. Cells of cluster 4 expressed Cd34 and Cxcl12 with relatively lower levels of osteoblast markers, suggesting that this cell type differs from actively bone‐forming osteoblasts and retain or reacquire progenitor properties. Based on expression and machine learning analyses of the transcriptomes of individual osteoblasts, we also identified genes that may be useful as new markers of osteoblast maturational stages. Taken together, our data show much more extensive heterogeneity of osteoblasts than previously documented, with gene profiles supporting diversity of osteoblast functional activities and developmental fates. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

New gene discoveries in skeletal diseases with short stature

In the last decade, the widespread use of massively parallel sequencing has considerably boosted the number of novel gene discoveries in monogenic skeletal diseases with short stature. Defects in genes playing a role in the maintenance and function of the growth plate, the site of longitudinal bone growth, are a well-known cause of skeletal diseases with short stature. However, several genes involved in extracellular matrix composition or maintenance as well as genes partaking in various biological processes have also been characterized. This review aims to describe the latest genetic findings in spondyloepiphyseal dysplasias, spondyloepimetaphyseal dysplasias, and some monogenic forms of isolated short stature. Some examples of novel genetic mechanisms leading to skeletal conditions with short stature will be described. Strategies on how to successfully characterize novel skeletal phenotypes with short stature and genetic approaches to detect and validate novel gene-disease correlations will be discussed in detail. In summary, we review the latest gene discoveries underlying skeletal diseases with short stature and emphasize the importance of characterizing novel molecular mechanisms for genetic counseling, for an optimal management of the disease, and for therapeutic innovations.