Osteoporosis and Bone Mass Disorders: From Gene Pathways to Treatments

SNX10 regulates osteoclastogenic cell fusion and osteoclast size in mice

Bone-resorbing osteoclasts (OCLs) are formed by differentiation and fusion of monocyte precursor cells, generating large multinucleated cells. Tightly regulated cell fusion during osteoclastogenesis leads to formation of resorption-competent OCLs, whose sizes fall within a predictable physiological range. The molecular mechanisms that regulate the onset of OCL fusion and its subsequent arrest are, however, largely unknown. We have previously shown that OCLs cultured from mice homozygous for the R51Q mutation in the vesicle trafficking-associated protein sorting nexin 10, a mutation that induces autosomal recessive osteopetrosis in humans and in mice, display deregulated and continuous fusion that generates gigantic, inactive OCLs. Fusion of mature OCLs is therefore arrested by an active, genetically encoded, cell-autonomous, and SNX10-dependent mechanism. To directly examine whether SNX10 performs a similar role in vivo, we generated SNX10-deficient (SKO) mice and demonstrated that they display massive osteopetrosis and that their OCLs fuse uncontrollably in culture, as do homozygous R51Q SNX10 (RQ/RQ) mice. OCLs that lack SNX10 exhibit persistent presence of DC-STAMP protein at their periphery, which may contribute to their uncontrolled fusion. To visualize endogenous SNX10-mutant OCLs in their native bone environment, we genetically labeled the OCLs of WT, SKO, and RQ/RQ mice with enhanced Green Fluorescent Protein (EGFP), and then visualized the 3D organization of resident OCLs and the pericellular bone matrix by 2-photon, confocal, and second harmonics generation microscopy. We show that the volumes, surface areas and, in particular, the numbers of nuclei in the OCLs of both mutant strains were on average 2-6-fold larger than those of OCLs from WT mice, indicating that deregulated, excessive fusion occurs in the mutant mice. We conclude that the fusion of OCLs, and consequently their size, is regulated in vivo by SNX10-dependent arrest of fusion of mature OCLs.

A Dyadic Nosology for Osteogenesis Imperfecta and Bone Fragility Syndromes 2024

In 2023 following extensive consultation with key stakeholders, the expert Nosology Working Group of the International Skeletal Dysplasia Society (ISDS) published the new Dyadic Nosology for Genetic Disorders of the Skeleton. Some 770 entities were delineated associated with 552 genes. From these entities, over 40 genes resulting in distinct forms of Osteogenesis Imperfecta (OI) and Bone Fragility and/or Familial Osteoporosis were identified. To assist clinicians and lay stake holders and bring the considerable body of knowledge of the matrix biology and genomics to people with OI as well as to clinicians and scientists, a dyadic nosology has been recommended. This combines a genomic co-descriptor with a phenotypic naming based on the widely used Sillence nosology for the OI syndromes and the many other syndromes characterized in part by bone fragility.This review recapitulates and explains the evolution from the simple Congenita and Tarda subclassification of OI in the 1970 nosology, which was replaced by the Sillence types I-IV nosology which was again replaced in 2009 with 5 clinical groups, type 1 to 5. Qualitative and quantitative defects in type I collagen polypeptides were postulated to account for the genetic heterogeneity in OI for nearly 30 years, when OI type 5, a non-collagen disorder was recognized. Advances in matrix biology and genomics since that time have confirmed a surprising complexity both in transcriptional as well as post-translational mechanisms of collagens as well as in the many mechanisms of calcified tissue homeostasis and integrity.

Onion (Allium cepa L.) Flavonoid Extract Ameliorates Osteoporosis in Rats Facilitating Osteoblast Proliferation and Differentiation in MG-63 Cells and Inhibiting RANKL-Induced Osteoclastogenesis in RAW 264.7 Cells

Osteoporosis, a prevalent chronic health issue among the elderly, is a global bone metabolic disease. Flavonoids, natural active compounds widely present in vegetables, fruits, beans, and cereals, have been reported for their anti-osteoporotic properties. Onion is a commonly consumed vegetable rich in flavonoids with diverse pharmacological activities. In this study, the trabecular structure was enhanced and bone mineral density (BMD) exhibited a twofold increase following oral administration of onion flavonoid extract (OFE). The levels of estradiol (E2), calcium (Ca), and phosphorus (P) in serum were significantly increased in ovariectomized (OVX) rats, with effects equal to alendronate sodium (ALN). Alkaline phosphatase (ALP) and tartrate-resistant acid phosphatase (TRAP) levels in rat serum were reduced by 35.7% and 36.9%, respectively, compared to the OVX group. In addition, the effects of OFE on bone health were assessed using human osteoblast-like cells MG-63 and osteoclast precursor RAW 264.7 cells in vitro as well. Proliferation and mineralization of MG-63 cells were promoted by OFE treatment, along with increased ALP activity and mRNA expression of osteoprotegerin (OPG)/receptor activator of nuclear factor-kappaB ligand (RANKL). Additionally, RANKL-induced osteoclastogenesis and osteoclast activity were inhibited by OFE treatment through decreased TRAP activity and down-regulation of mRNA expression-related enzymes in RAW 264.7 cells. Overall findings suggest that OFE holds promise as a natural functional component for alleviating osteoporosis.

Evolutionary and functional analyses of LRP5 in archaic and extant modern humans

BACKGROUND

The human lineage has undergone a postcranial skeleton gracilization (i.e. lower bone mass and strength relative to body size) compared to other primates and archaic populations such as the Neanderthals. This gracilization has been traditionally explained by differences in the mechanical load that our ancestors exercised. However, there is growing evidence that gracilization could also be genetically influenced.

RESULTS

We have analyzed the LRP5 gene, which is known to be associated with high bone mineral density conditions, from an evolutionary and functional point of view. Taking advantage of the published genomes of archaic Homo populations, our results suggest that this gene has a complex evolutionary history both between archaic and living humans and within living human populations. In particular, we identified the presence of different selective pressures in archaics and extant modern humans, as well as evidence of positive selection in the African and South East Asian populations from the 1000 Genomes Project. Furthermore, we observed a very limited evidence of archaic introgression in this gene (only at three haplotypes of East Asian ancestry out of the 1000 Genomes), compatible with a general erasing of the fingerprint of archaic introgression due to functional differences in archaics compared to extant modern humans. In agreement with this hypothesis, we observed private mutations in the archaic genomes that we experimentally validated as putatively increasing bone mineral density. In particular, four of five archaic missense mutations affecting the first β-propeller of LRP5 displayed enhanced Wnt pathway activation, of which two also displayed reduced negative regulation.

CONCLUSIONS

In summary, these data suggest a genetic component contributing to the understanding of skeletal differences between extant modern humans and archaic Homo populations.

Farnesyl Diphosphate Synthase Gene Associated with Loss of Bone Mass Density and Alendronate Treatment Failure in Patients with Primary Osteoporosis

Aminobisphosphonates (NBPs) are the first-choice medication for osteoporosis (OP); NBP treatment aims at increasing bone mineral density (BMD) by inhibiting the activity of farnesyl diphosphate synthase (FDPS) enzyme in osteoclasts. Despite its efficacy, inadequate response to the drug and side effects have been reported. The A allele of the rs2297480 (A > C) SNP, found in the regulatory region of the FDPS gene, is associated with reduced gene transcription. This study evaluates the FDPS variant rs2297480 (A > C) association with OP patients' response to alendronate sodium treatment. A total of 304 OP patients and 112 controls were enrolled; patients treated with alendronate sodium for two years were classified, according to BMD variations at specific regions (lumbar spine (L1-L4), femoral neck (FN) and total hip (TH), as responders (OP-R) (n = 20) and non-responders (OP-NR) (n = 40). We observed an association of CC genotype with treatment failure (p = 0.045), followed by a BMD decrease in the regions L1-L4 (CC = -2.21% ± 2.56; p = 0.026) and TH (CC = -2.06% ± 1.84; p = 0.015) after two years of alendronate sodium treatment. Relative expression of the FDPS gene was also evaluated in OP-R and OP-NR patients. Higher expression of the FDPS gene was also observed in OP-NR group (FC = 1.84 ± 0.77; p = 0.006) when compared to OP-R. In conclusion, the influence observed of FDPS expression and the rs2897480 variant on alendronate treatment highlights the importance of a genetic approach to improve the efficacy of treatment for primary osteoporosis.

Identification of an novel genetic variant associated with osteoporosis: insights from the Taiwan Biobank Study

Purpose

The purpose of this study was to identify new independent significant SNPs associated with osteoporosis using data from the Taiwan Biobank (TWBB).

Material and Methods

The dataset was divided into discovery (60%) and replication (40%) subsets. Following data quality control, genome-wide association study (GWAS) analysis was performed, adjusting for sex, age, and the top 5 principal components, employing the Scalable and Accurate Implementation of the Generalized mixed model approach. This was followed by a meta-analysis of TWBB1 and TWBB2. The Functional Mapping and Annotation (FUMA) platform was used to identify osteoporosis-associated loci. Manhattan and quantile-quantile plots were generated using the FUMA platform to visualize the results. Independent significant SNPs were selected based on genome-wide significance (P < 5 × 10-8) and independence from each other (r2 < 0.6) within a 1 Mb window. Positional, eQTL(expression quantitative trait locus), and Chromatin interaction mapping were used to map SNPs to genes.

Results

A total of 29 084 individuals (3154 osteoporosis cases and 25 930 controls) were used for GWAS analysis (TWBB1 data), and 18 918 individuals (1917 cases and 17 001 controls) were utilized for replication studies (TWBB2 data). We identified a new independent significant SNP for osteoporosis in TWBB1, with the lead SNP rs76140829 (minor allele frequency = 0.055, P-value = 1.15 × 10-08). Replication of the association was performed in TWBB2, yielding a P-value of 6.56 × 10-3. The meta-analysis of TWBB1 and TWBB2 data demonstrated a highly significant association for SNP rs76140829 (P-value = 7.52 × 10-10). In the positional mapping of rs76140829, 6 genes (HABP2, RP11-481H12.1, RNU7-165P, RP11-139 K1.2, RP11-57H14.3, and RP11-214 N15.5) were identified through chromatin interaction mapping in mesenchymal stem cells.

Conclusions

Our GWAS analysis using the Taiwan Biobank dataset unveils rs76140829 in the VTI1A gene as a key risk variant associated with osteoporosis. This finding expands our understanding of the genetic basis of osteoporosis and highlights the potential regulatory role of this SNP in mesenchymal stem cells.

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.

Advances in the roles of ATF4 in osteoporosis

Osteoporosis (OP) is characterized by reduced bone mass, decreased strength, and enhanced bone fragility fracture risk. Activating transcription factor 4 (ATF4) plays a role in cell differentiation, proliferation, apoptosis, redox balance, amino acid uptake, and glycolipid metabolism. ATF4 induces the differentiation of bone marrow mesenchymal stem cells (BM-MSCs) into osteoblasts, increases osteoblast activity, and inhibits osteoclast formation, promoting bone formation and remodeling. In addition, ATF4 mediates the energy metabolism in osteoblasts and promotes angiogenesis. ATF4 is also involved in the mediation of adipogenesis. ATF4 can selectively accumulate in osteoblasts. ATF4 can directly interact with RUNT-related transcription factor 2 (RUNX2) and up-regulate the expression of osteocalcin (OCN) and osterix (Osx). Several upstream factors, such as Wnt/β-catenin and BMP2/Smad signaling pathways, have been involved in ATF4-mediated osteoblast differentiation. ATF4 promotes osteoclastogenesis by mediating the receptor activator of nuclear factor κ-B (NF-κB) ligand (RANKL) signaling. Several agents, such as parathyroid (PTH), melatonin, and natural compounds, have been reported to regulate ATF4 expression and mediate bone metabolism. In this review, we comprehensively discuss the biological activities of ATF4 in maintaining bone homeostasis and inhibiting OP development. ATF4 has become a therapeutic target for OP treatment.

Generic Platform for the Multiplexed Targeted Electrochemical Detection of Osteoporosis-Associated Single Nucleotide Polymorphisms Using Recombinase Polymerase Solid-Phase Primer Elongation and Ferrocene-Modified Nucleoside Triphosphates

Osteoporosis is a multifactorial disease influenced by genetic and environmental factors, which contributes to an increased risk of bone fracture, but early diagnosis of this disease cannot be achieved using current techniques. We describe a generic platform for the targeted electrochemical genotyping of SNPs identified by genome-wide association studies to be associated with a genetic predisposition to osteoporosis. The platform exploits isothermal solid-phase primer elongation with ferrocene-labeled nucleoside triphosphates. Thiolated reverse primers designed for each SNP were immobilized on individual gold electrodes of an array. These primers are designed to hybridize to the SNP site at their 3'OH terminal, and primer elongation occurs only where there is 100% complementarity, facilitating the identification and heterozygosity of each SNP under interrogation. The platform was applied to real blood samples, which were thermally lysed and directly used without the need for DNA extraction or purification. The results were validated using Taqman SNP genotyping assays and Sanger sequencing. The assay is complete in just 15 min with a total cost of 0.3€ per electrode. The platform is completely generic and has immense potential for deployment at the point of need in an automated device for targeted SNP genotyping with the only required end-user intervention being sample addition.

Converging evidence from exome sequencing and common variants implicates target genes for osteoporosis

In this study, we leveraged the combined evidence of rare coding variants and common alleles to identify therapeutic targets for osteoporosis. We undertook a large-scale multiancestry exome-wide association study for estimated bone mineral density, which showed that the burden of rare coding alleles in 19 genes was associated with estimated bone mineral density (P < 3.6 × 10-7). These genes were highly enriched for a set of known causal genes for osteoporosis (65-fold; P = 2.5 × 10-5). Exome-wide significant genes had 96-fold increased odds of being the top ranked effector gene at a given GWAS locus (P = 1.8 × 10-10). By integrating proteomics Mendelian randomization evidence, we prioritized CD109 (cluster of differentiation 109) as a gene for which heterozygous loss of function is associated with higher bone density. CRISPR-Cas9 editing of CD109 in SaOS-2 osteoblast-like cell lines showed that partial CD109 knockdown led to increased mineralization. This study demonstrates that the convergence of common and rare variants, proteomics and CRISPR can highlight new bone biology to guide therapeutic development.

Oxidative-Stress-Related Genes in Osteoporosis: A Systematic Review

Osteoporosis is characterized by a decline in bone mineral density (BMD) and increased fracture risk. Free radicals and antioxidant systems play a central role in bone remodeling. This study was conducted to illustrate the role of oxidative-stress-related genes in BMD and osteoporosis. A systematic review was performed following the PRISMA guidelines. The search was computed in PubMed, Web of Sciences, Scopus, EBSCO, and BVS from inception to November 1st, 2022. The risk of bias was evaluated using the Joanna Briggs Institute Critical Appraisal Checklist tool. A total of 427 potentially eligible articles exploring this search question were detected. After removing duplicates (n = 112) and excluding irrelevant manuscripts based on screenings of their titles and abstracts (n = 317), 19 articles were selected for full-text review. Finally, 14 original articles were included in this systematic review after we applied the exclusion and inclusion criteria. Data analyzed in this systematic review indicated that oxidative-stress-related genetic polymorphisms are associated with BMD at different skeletal sites in diverse populations, influencing the risk of osteoporosis or osteoporotic fracture. However, it is necessary to look deep into their association with bone metabolism to determine if the findings can be translated into the clinical management of osteoporosis and its progression.

MiR-218-5p-dependent SOCS3 downregulation increases osteoblast differentiation inpostmenopausal osteoporosis

BACKGROUND

Postmenopausal osteoporosis (POP) is a prevalent skeletal disease among elderly women. Previous study indicated that suppressor of cytokine signaling 3 (SOCS3) participates in the regulation of bone marrow stromal cell (BMSC) osteogenesis. Here, we further investigated the exact function and mechanism of SOCS3 in POP progression.

METHODS

BMSCs were isolated from Sprague-Dawley rats and treated with Dexamethasone (Dex). Alizarin Red staining and ALP activity assays were applied to assess the osteogenic differentiation of rat BMSCs under the indicated conditions. Osteogenic genes (ALP, OPN, OCN, COL1) mRNA levels were determined using quantitative RT-PCR. Luciferase reporter assay verified the interaction between SOCS3 and miR-218-5p. Rat models of POP were established in ovariectomized (OVX) rats to detect the in vivo effects of SOCS3 and miR-218-5p.

RESULTS

We found that silencing SOCS3 antagonized the suppressive effects of Dex on the osteogenic differentiation of BMSCs. SOCS3 was found to be targeted by miR-218-5p in BMSCs. The SOCS3 levels were negatively modulated by miR-218-5p in femurs of POP rats. MiR-218-5p upregulation promoted the BMSC osteogenic differentiation, while SOCS3 overexpression reversed the effects of miR-218-5p. Moreover, SOCS3 was highly expressed and miR-218-5p was downregulated in the OVX rat models, and silencing SOCS3 or overexpressing miR-218-5p alleviated POP in OVX rats by promoting osteogenesis.

CONCLUSION

SOCS3 downregulation mediated by miR-218-5p increases osteoblast differentiation to alleviate POP.

Morinda officinalis Polysaccharides Ameliorates Bone Growth by Attenuating Oxidative Stress and Regulating the Gut Microbiota in Thiram-Induced Tibial Dyschondroplasia Chickens

Tibial dyschondroplasia (TD) occurs in chickens and other fast-growing birds, affecting their cartilage growth and leading to reduced meat quality in broilers. Morinda officinalis polysaccharide (MOP) is one of the chief active components of Morinda officinalis, which promotes bone formation, inhibiting bone loss and having anti-oxidant and anti-inflammatory properties. A total of 120 AA chickens were randomly divided into the CON group (basal diet), TD group (100 mg/kg thiram + basal diet), and MOP group (100 mg/kg thiram + basal diet + water with 500 mg/kg MOP). The experiment lasted 21 days. The results showed that MOP could alleviates broiler lameness caused by TD, restore the morphological structure of tibial growth plate (TGP), increase tibial weight (p < 0.05), balance the disorder of calcium and phosphorus metabolism, and promote bone formation by increasing the expression of BMP-2, Smad4, and Runx2 genes In addition, MOP supplementation stimulated the secretion of plasma antioxidant enzymes (T-SOD and GSH-Px) by regulating the expression of SOD and GPX-1 genes, thereby enhancing the antioxidant capacity of TD broilers. Interestingly, we observed MOP can also improve gut microbiota by increasing the beneficial bacteria count and decreasing the harmful bacteria count. These findings indicated that MOP can regulate bone formation through the BMP/Smads signaling pathway, attenuating oxidative stress and regulating the gut microbiota of TD broilers, so as to achieve the effect of treating TD. This suggests that MOP might be a potential novel drug in the treatment of TD in chickens.

Development of Clinical Weekly-Dose Teriparatide Acetate Encapsulated Dissolving Microneedle Patch for Efficient Treatment of Osteoporosis

Teriparatide acetate (TA), which directly promotes bone formation, is subcutaneously injected to treat osteoporosis. In this study, TA with a once-weekly administration regimen was loaded on dissolving microneedles (DMNs) to effectively deliver it to the systemic circulation via the transdermal route. TA activity reduction during the drying process of various TA polymer solutions formulated with hyaluronic acid and trehalose was monitored and homogeneities were assessed. TA-DMN patches fabricated using centrifugal lithography in a two-layered structure with dried pure hyaluronic acid on the base layer and dried TA polymer solution on the top layer were evaluated for their physical properties. Rhodamine-B-loaded TA-DMNs were found to form perforations when inserted into porcine skin using a shooting device. In addition, 87.6% of TA was delivered to the porcine skin after a 5-min TA-DMN patch application. The relative bioavailability of TA via subcutaneous injection was 66.9% in rats treated with TA-DMN patches. The maximal TA concentration in rat plasma was proportional to the number of patches used. Therefore, the TA-DMN patch fabricated in this study may aid in the effective delivery of TA in a patient-friendly manner and enhance medical efficacy in osteoporosis treatment.

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).

Genetic variation in WNT16 and its association with bone mineral density, fractures and osteoporosis in children with bone fragility

Several genome-wide association studies (GWAS), GWAS meta-analyses, and mouse studies have demonstrated that wingless-related integration site 16 (WNT16) gene is associated with bone mineral density (BMD), cortical bone thickness, bone strength and fracture risk. Practically no data exist regarding the significance of WNT16 in childhood-onset osteoporosis and related fractures. We hypothesized that pathogenic variants and genetic variations in WNT16 could explain skeletal fragility in affected children. We screened the WNT16 gene by Sanger sequencing in three pediatric cohorts: 35 with primary osteoporosis, 59 with multiple fractures, and in 95 healthy controls. Altogether, we identified 12 variants in WNT16. Of them one was a rare 5′UTR variant rs1386898215 in genome aggregate and medical trans-omic databases (GnomAD, TOPMED; minor allele frequency (MAF) 0.00 and 0.000008, respectively). One variant rs1554366753, overrepresented in children with osteoporosis (MAF = 0.06 vs healthy controls MAF = 0.01), was significantly associated with lower BMD. This variant was found associated with increased WNT16 gene expression at mRNA level in fibroblast cultures. None of the other identified variants were rare (MAF < 0.001) or deemed pathogenic by predictor programs. WNT16 may play a role in childhood osteoporosis but genetic WNT16 variation is not a common cause of skeletal fragility in childhood.

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No pathogenic WNT16 variants were found associated with pediatric osteoporosis or fracture-prone patients

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Altogether, twelve WNT16 variants were found in pediatric osteoporosis or fracture-prone patients

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The genetic variation rs1554366753 in the WNT16 gene is associated with bone mineral density and primary osteoporosis

Biological Deciphering of the "Kidney Governing Bones" Theory in Traditional Chinese Medicine

The description of the "kidney" was entirely different from modern medicine. In traditional Chinese medicine (TCM), the kidney was a functional concept regulating water metabolism, which was closely related to the urinary system, reproductive system, nervous system, endocrine, skeleton, hearing, metabolism, immunity, etc. In particular, the kidney in TCM plays an important regulatory role in the processes of growth, development, prime, aging, and reproduction. Hence, "Kidney Governing Bone" (KGB) was a classical theory in TCM, which hypothesized that the function of the kidney was responsible for bone health. However, the related modern physiological mechanisms of this TCM theory are unclear. This present paper proposed a new understanding and explored the biological basis of the KGB theory. After searching through plenty of reported literature, we discovered that the functions of the kidney in TCM were closely associated with the hypothalamic-pituitary-gonadal (HPG) axis in modern science. The physiological mechanism of the KGB was regulated by sex hormones and their receptors. This review deciphered the connotation of the KGB theory in modern medicine and further verified the scientificity of the basic TCM theory.

Metabolic Bone Diseases and New Drug Developments

Metabolic bone diseases are serious health issues worldwide, since several million individuals over the age of 50 are at risk of bone damage and should be worried about their bone health. One in every two women and one in every four men will break a bone during their lifetime due to a metabolic bone disease. Early detection, raising bone health awareness, and maintaining a balanced healthy diet may reduce the risk of skeletal fractures caused by metabolic bone diseases. This review compiles information on the most common metabolic bone diseases (osteoporosis, primary hyperparathyroidism, osteomalacia, and fluorosis disease) seen in the global population, including their symptoms, mechanisms, and causes, as well as discussing their prevention and the development of new drugs for treatment. A large amount of research literature suggests that balanced nutrition and balanced periodic supplementation of calcium, phosphate, and vitamin D can improve re-absorption and the regrowth of bones, and inhibit the formation of skeletal fractures, except in the case of hereditary bone diseases. Meanwhile, new and improved drug formulations, such as raloxifene, teriparatide, sclerostin, denosumab, and abaloparatide, have been successfully developed and administered as treatments for metabolic bone diseases, while others (romososumab and odanacatib) are in various stages of clinical trials.

The Mechanosensory Role of Osteocytes and Implications for Bone Health and Disease States

Bone homeostasis is a dynamic equilibrium between bone-forming osteoblasts and bone-resorbing osteoclasts. This process is primarily controlled by the most abundant and mechanosensitive bone cells, osteocytes, that reside individually, within chambers of porous hydroxyapatite bone matrix. Recent studies have unveiled additional functional roles for osteocytes in directly contributing to local matrix regulation as well as systemic roles through endocrine functions by communicating with distant organs such as the kidney. Osteocyte function is governed largely by both biochemical signaling and the mechanical stimuli exerted on bone. Mechanical stimulation is required to maintain bone health whilst aging and reduced level of loading are known to result in bone loss. To date, both in vivo and in vitro approaches have been established to answer important questions such as the effect of mechanical stimuli, the mechanosensors involved, and the mechanosensitive signaling pathways in osteocytes. However, our understanding of osteocyte mechanotransduction has been limited due to the technical challenges of working with these cells since they are individually embedded within the hard hydroxyapatite bone matrix. This review highlights the current knowledge of the osteocyte functional role in maintaining bone health and the key regulatory pathways of these mechanosensitive cells. Finally, we elaborate on the current therapeutic opportunities offered by existing treatments and the potential for targeting osteocyte-directed signaling.

Association between genetic variants in oxidative stress-related genes and osteoporotic bone fracture. The Hortega follow-up study

The most widely accepted etiopathogenesis hypothesis of the origin of osteoporosis and its complications is that they are a consequence of bone aging and other environmental factors, together with a genetic predisposition. Evidence suggests that oxidative stress is crucial in bone pathologies associated with aging. The aim of this study was to determine whether genetic variants in oxidative stress-related genes modified the risk of osteoporotic fracture. We analysed 221 patients and 354 controls from the HORTEGA sample after 12-14 years of follow up. We studied the genotypic and allelic distribution of 53 SNPs in 24 genes involved in oxidative stress. The results showed that being a carrier of the variant allele of the SNP rs4077561 within TXNRD1 was the principal genetic risk factor associated with osteoporotic fracture and that variant allele of the rs1805754 M6PR, rs4964779 TXNRD1, rs406113 GPX6, rs2281082 TXN2 and rs974334 GPX6 polymorphisms are important genetic risk factors for fracture. This study provides information on the genetic factors associated with oxidative stress which are involved in the risk of osteoporotic fracture and reinforces the hypothesis that genetic factors are crucial in the etiopathogenesis of osteoporosis and its complications.

Functional Validation of Osteoporosis Genetic Findings Using Small Fish Models

The advancement of human genomics has revolutionized our understanding of the genetic architecture of many skeletal diseases, including osteoporosis. However, interpreting results from human association studies remains a challenge, since index variants often reside in non-coding regions of the genome and do not possess an obvious regulatory function. To bridge the gap between genetic association and causality, a systematic functional investigation is necessary, such as the one offered by animal models. These models enable us to identify causal mechanisms, clarify the underlying biology, and apply interventions. Over the past several decades, small teleost fishes, mostly zebrafish and medaka, have emerged as powerful systems for modeling the genetics of human diseases. Due to their amenability to genetic intervention and the highly conserved genetic and physiological features, fish have become indispensable for skeletal genomic studies. The goal of this review is to summarize the evidence supporting the utility of Zebrafish (Danio rerio) for accelerating our understanding of human skeletal genomics and outlining the remaining gaps in knowledge. We provide an overview of zebrafish skeletal morphophysiology and gene homology, shedding light on the advantages of human skeletal genomic exploration and validation. Knowledge of the biology underlying osteoporosis through animal models will lead to the translation into new, better and more effective therapeutic approaches.

Perspective of the GEMSTONE Consortium on Current and Future Approaches to Functional Validation for Skeletal Genetic Disease Using Cellular, Molecular and Animal-Modeling Techniques

The availability of large human datasets for genome-wide association studies (GWAS) and the advancement of sequencing technologies have boosted the identification of genetic variants in complex and rare diseases in the skeletal field. Yet, interpreting results from human association studies remains a challenge. To bridge the gap between genetic association and causality, a systematic functional investigation is necessary. Multiple unknowns exist for putative causal genes, including cellular localization of the molecular function. Intermediate traits ("endophenotypes"), e.g. molecular quantitative trait loci (molQTLs), are needed to identify mechanisms of underlying associations. Furthermore, index variants often reside in non-coding regions of the genome, therefore challenging for interpretation. Knowledge of non-coding variance (e.g. ncRNAs), repetitive sequences, and regulatory interactions between enhancers and their target genes is central for understanding causal genes in skeletal conditions. Animal models with deep skeletal phenotyping and cell culture models have already facilitated fine mapping of some association signals, elucidated gene mechanisms, and revealed disease-relevant biology. However, to accelerate research towards bridging the current gap between association and causality in skeletal diseases, alternative in vivo platforms need to be used and developed in parallel with the current -omics and traditional in vivo resources. Therefore, we argue that as a field we need to establish resource-sharing standards to collectively address complex research questions. These standards will promote data integration from various -omics technologies and functional dissection of human complex traits. In this mission statement, we review the current available resources and as a group propose a consensus to facilitate resource sharing using existing and future resources. Such coordination efforts will maximize the acquisition of knowledge from different approaches and thus reduce redundancy and duplication of resources. These measures will help to understand the pathogenesis of osteoporosis and other skeletal diseases towards defining new and more efficient therapeutic targets.

Identification of Rare LRP5 Variants in a Cohort of Males with Impaired Bone Mass

Osteoporosis is the most common bone disease characterized by reduced bone mass and increased bone fragility. Genetic contribution is one of the main causes of primary osteoporosis; therefore, both genders are affected by this skeletal disorder. Nonetheless, osteoporosis in men has received little attention, thus being underestimated and undertreated. The aim of this study was to identify novel genetic variants in a cohort of 128 males with idiopathic low bone mass using a next-generation sequencing (NGS) panel including genes whose mutations could result in reduced bone mineral density (BMD). Genetic analysis detected in eleven patients ten rare heterozygous variants within the LRP5 gene, which were categorized as VUS (variant of uncertain significance), likely pathogenic and benign variants according to American College of Medical Genetics and Genomics (ACMG) guidelines. Protein structural and Bayesian analysis performed on identified LRP5 variants pointed out p.R1036Q and p.R1135C as pathogenic, therefore suggesting the likely association of these two variants with the low bone mass phenotype. In conclusion, this study expands our understanding on the importance of a functional LRP5 protein in bone formation and highlights the necessity to sequence this gene in subjects with idiopathic low BMD.

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.

The "GEnomics of Musculo Skeletal Traits TranslatiOnal NEtwork": Origins, Rationale, Organization, and Prospects

Musculoskeletal research has been enriched in the past ten years with a great wealth of new discoveries arising from genome wide association studies (GWAS). In addition to the novel factors identified by GWAS, the advent of whole-genome and whole-exome sequencing efforts in family based studies has also identified new genes and pathways. However, the function and the mechanisms by which such genes influence clinical traits remain largely unknown. There is imperative need to bring multidisciplinary expertise together that will allow translating these genomic discoveries into useful clinical applications with the potential of improving patient care. Therefore "GEnomics of MusculoSkeletal traits TranslatiOnal NEtwork" (GEMSTONE) aims to set the ground for the: 1) functional characterization of discovered genes and pathways; 2) understanding of the correspondence between molecular and clinical assessments; and 3) implementation of novel methodological approaches. This research network is funded by The European Cooperation in Science and Technology (COST). GEMSTONE includes six working groups (WG), each with specific objectives: WG1-Study populations and expertise groups: creating, maintaining and updating an inventory of experts and resources (studies and datasets) participating in the network, helping to assemble focus groups defined by phenotype, functional and methodological expertise. WG2-Phenotyping: describe ways to decompose the phenotypes of the different functional studies into meaningful components that will aid the interpretation of identified biological pathways. WG3 Monogenic conditions - human KO models: makes an inventory of genes underlying musculoskeletal monogenic conditions that aids the assignment of genes to GWAS signals and prioritizing GWAS genes as candidates responsible for monogenic presentations, through biological plausibility. WG4 Functional investigations: creating a roadmap of genes and pathways to be prioritized for functional assessment in cell and organism models of the musculoskeletal system. WG5 Bioinformatics seeks the integration of the knowledge derived from the distinct efforts, with particular emphasis on systems biology and artificial intelligence applications. Finally, WG6 Translational outreach: makes a synopsis of the knowledge derived from the distinct efforts, allowing to prioritize factors within biological pathways, use refined disease trait definitions and/or improve study design of future investigations in a potential therapeutic context (e.g. clinical trials) for musculoskeletal diseases.

A Roadmap to Gene Discoveries and Novel Therapies in Monogenic Low and High Bone Mass Disorders

Genetic disorders of the skeleton encompass a diverse group of bone diseases differing in clinical characteristics, severity, incidence and molecular etiology. Of particular interest are the monogenic rare bone mass disorders, with the underlying genetic defect contributing to either low or high bone mass phenotype. Extensive, deep phenotyping coupled with high-throughput, cost-effective genotyping is crucial in the characterization and diagnosis of affected individuals. Massive parallel sequencing efforts have been instrumental in the discovery of novel causal genes that merit functional validation using in vitro and ex vivo cell-based techniques, and in vivo models, mainly mice and zebrafish. These translational models also serve as an excellent platform for therapeutic discovery, bridging the gap between basic science research and the clinic. Altogether, genetic studies of monogenic rare bone mass disorders have broadened our knowledge on molecular signaling pathways coordinating bone development and metabolism, disease inheritance patterns, development of new and improved bone biomarkers, and identification of novel drug targets. In this comprehensive review we describe approaches to further enhance the innovative processes taking discoveries from clinic to bench, and then back to clinic in rare bone mass disorders. We highlight the importance of cross laboratory collaboration to perform functional validation in multiple model systems after identification of a novel disease gene. We describe the monogenic forms of rare low and high rare bone mass disorders known to date, provide a roadmap to unravel the genetic determinants of monogenic rare bone mass disorders using proper phenotyping and genotyping methods, and describe different genetic validation approaches paving the way for future treatments.

An ARHGAP25 variant links aberrant Rac1 function to early-onset skeletal fragility

Ras homologous guanosine triphosphatases (RhoGTPases) control several cellular functions, including cytoskeletal actin remodeling and cell migration. Their activities are downregulated by GTPase-activating proteins (GAPs). Although RhoGTPases are implicated in bone remodeling and osteoclast and osteoblast function, their significance in human bone health and disease remains elusive. Here, we report defective RhoGTPase regulation as a cause of severe, early-onset, autosomal-dominant skeletal fragility in a three-generation Finnish family. Affected individuals (n = 13) presented with multiple low-energy peripheral and vertebral fractures despite normal bone mineral density (BMD). Bone histomorphometry suggested reduced bone volume, low surface area covered by osteoblasts and osteoclasts, and low bone turnover. Exome sequencing identified a novel heterozygous missense variant c.652G>A (p.G218R) in ARHGAP25, encoding a GAP for Rho-family GTPase Rac1. Variants in the ARHGAP25 5' untranslated region (UTR) also associated with BMD and fracture risk in the general population, across multiple genomewide association study (GWAS) meta-analyses (lead variant rs10048745). ARHGAP25 messenger RNA (mRNA) was expressed in macrophage colony-stimulating factor (M-CSF)-stimulated human monocytes and mouse osteoblasts, indicating a possible role for ARHGAP25 in osteoclast and osteoblast differentiation and activity. Studies on subject-derived osteoclasts from peripheral blood mononuclear cells did not reveal robust defects in mature osteoclast formation or resorptive activity. However, analysis of osteosarcoma cells overexpressing the ARHGAP25 G218R-mutant, combined with structural modeling, confirmed that the mutant protein had decreased GAP-activity against Rac1, resulting in elevated Rac1 activity, increased cell spreading, and membrane ruffling. Our findings indicate that mutated ARHGAP25 causes aberrant Rac1 function and consequently abnormal bone metabolism, highlighting the importance of RhoGAP signaling in bone metabolism in familial forms of skeletal fragility and in the general population, and expanding our understanding of the molecular pathways underlying skeletal fragility. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

PAX8-AS1 knockdown facilitates cell growth and inactivates autophagy in osteoblasts via the miR-1252-5p/GNB1 axis in osteoporosis

Osteoporosis (OP) is the most common systematic bone disorder among elderly individuals worldwide. Long noncoding RNAs (lncRNAs) are involved in biological processes in various human diseases. It has been previously revealed that PAX8 antisense RNA 1 (PAX8-AS1) is upregulated in OP. However, its molecular mechanism in OP remains unclear. Therefore, we specifically designed this study to determine the specific role of PAX8-AS1 in OP. We first established a rat model of OP and then detected PAX8-AS1 expression in the rats with RT-qPCR. Next, to explore the biological function of PAX8-AS1 in osteoblasts, in vitro experiments, such as Cell Counting Kit-8 (CCK-8) assays, flow cytometry, western blotting and immunofluorescence (IF) staining, were conducted. Subsequently, we performed bioinformatic analysis and luciferase reporter assays to predict and identify the relationships between microRNA 1252-5p (miR-1252-5p) and both PAX8-AS1 and G protein subunit beta 1 (GNB1). Additionally, rescue assays in osteoblasts clarified the regulatory network of the PAX8-AS1/miR-1252-5p/GNB1 axis. Finally, in vivo loss-of-function studies verified the role of PAX8-AS1 in OP progression. The results illustrated that PAX8-AS1 was upregulated in the proximal tibia of OP rats. PAX8-AS1 silencing promoted the viability and inhibited the apoptosis and autophagy of osteoblasts. PAX8-AS1 interacted with miR-1252-5p. GNB1 was negatively regulated by miR-1252-5p. In addition, the impacts of PAX8-AS1 knockdown on osteoblasts were counteracted by GNB1 overexpression. PAX8-AS1 depletion suppressed OP progression by inhibiting apoptosis and autophagy in osteoblasts. In summary, PAX8-AS1 suppressed the viability and activated the autophagy of osteoblasts via the miR-1252-5p/GNB1 axis in OP.

Osteoporosis in Men: A Review of an Underestimated Bone Condition

Osteoporosis is called the 'silent disease' because, although it does not give significant symptoms when it is not complicated, can cause fragility fractures, with serious consequences and death. Furthermore, the consequences of osteoporosis have been calculated to weigh heavily on the costs of health systems in all the countries. Osteoporosis is considered a female disease. Actually, the hormonal changes that occur after menopause certainly determine a significant increase in osteoporosis and the risk of fractures in women. However, while there is no doubt that women are more exposed to osteoporosis and fragility fractures, the literature clearly indicates that physicians tend to underestimate the osteoporosis in men. The review of the literature done by the authors shows that osteoporosis and fragility fractures have a high incidence also in men; and, furthermore, the risk of fatal complications in hip fractured men is higher than that for women. The authors report the evidence of the literature on male osteoporosis, dwelling on epidemiology, causes of osteoporosis in men, diagnosis, and treatment. The analysis of the literature shows that male osteoporosis is underscreened, underdiagnosed, and undertreated, both in primary and secondary prevention of fragility fractures.

Opportunities and Challenges in Functional Genomics Research in Osteoporosis: Report From a Workshop Held by the Causes Working Group of the Osteoporosis and Bone Research Academy of the Royal Osteoporosis Society on October 5th 2020

The discovery that sclerostin is the defective protein underlying the rare heritable bone mass disorder, sclerosteosis, ultimately led to development of anti-sclerostin antibodies as a new treatment for osteoporosis. In the era of large scale GWAS, many additional genetic signals associated with bone mass and related traits have since been reported. However, how best to interrogate these signals in order to identify the underlying gene responsible for these genetic associations, a prerequisite for identifying drug targets for further treatments, remains a challenge. The resources available for supporting functional genomics research continues to expand, exemplified by "multi-omics" database resources, with improved availability of datasets derived from bone tissues. These databases provide information about potential molecular mediators such as mRNA expression, protein expression, and DNA methylation levels, which can be interrogated to map genetic signals to specific genes based on identification of causal pathways between the genetic signal and the phenotype being studied. Functional evaluation of potential causative genes has been facilitated by characterization of the "osteocyte signature", by broad phenotyping of knockout mice with deletions of over 7,000 genes, in which more detailed skeletal phenotyping is currently being undertaken, and by development of zebrafish as a highly efficient additional in vivo model for functional studies of the skeleton. Looking to the future, this expanding repertoire of tools offers the hope of accurately defining the major genetic signals which contribute to osteoporosis. This may in turn lead to the identification of additional therapeutic targets, and ultimately new treatments for osteoporosis.

MiR-23b-3p functions as a positive factor for osteoporosis progression by targeting CCND1 in MC3T3-E1 cells

MiRNAs have gained tremendous attention as studies have shown that miRNAs play important roles in osteoporosis (OP) progression. This study attempted to explore whether miR-23b-3p is involved in the pathogenesis of OP. We detected the miR-23b-3p and Cyclin D1 (CCND1) expressional patterns in the bone of patients with or without OP relying on the GEO database. β-Glycerophosphate disodium salt and L-ascorbic acid were utilized to stimulate differentiation of MC3T3-E1 cells. Cell proliferative, apoptotic abilities, and cell cycle distribution were determined by CCK-8 and flow cytometry experiments. TargetScan and dual-luciferase reporter analysis were employed to predict and verify the targets of miR-23b-3p. Western blot was implemented to detect the expression of CCND1, apoptosis-related proteins, and cell osteogenesis-related proteins. ALP activity of MC3T3-E1 cells was measured using ALP kit. MiR-23b-3p was increased in OP specimens. Gain-/loss-of-function analysis indicated that the miR-23b-3p inhibited proliferation and differentiation and promoted apoptosis of MC3T3-E1 cells. The levels of Bax and cleaved caspase-3 were increased while those of Bcl-2 were decreased. ALP activity was reduced, and the levels of ALP, Runx2, Osterix, and OPN were declined in MC3T3-E1 cells relative to control. Further analyses demonstrated that CCND1 was a putative target gene of miR-23b-3p. Moreover, knockdown of CCND1 could reverse the impacts of miR-23b-3p inhibitor in MC3T3-E1 cells. MiR-23b-3p functioned as an O-positive factor through regulating cell cycle, proliferation, apoptosis, and differentiation via targeting CCND1.

Genetic pathways disrupted by ENPP1 deficiency provide insight into mechanisms of osteoporosis, osteomalacia, and paradoxical mineralization

Ectonucleotide phosphatase/phosphodiesterase 1 (ENPP1) deficiency results in either lethal arterial calcifications ('Generalized Arterial Calcification of Infancy' - GACI), phosphate wasting rickets ('Autosomal Recessive Hypophosphatemic Rickets type 2' - ARHR2), early onset osteoporosis, or progressive spinal rigidity ('Ossification of the Posterior Longitudinal Ligament' - OPLL). As ENPP1 generates a strong endogenous mineralization inhibitor - extracellular pyrophosphate (PPi) - ENPP1 deficiency should not result in reduced bone volume, and therefore the mechanism ENPP1 associated osteoporosis is not apparent given current understanding of the enzyme's function. To investigate genetic pathways driving the skeletal phenotype of ENPP1 deficiency we compared gene expression in Enpp1^asj/asj mice and WT sibling pairs by RNAseq and qPCR in whole bones, and in the liver and kidney by qPCR, directly correlating gene expression with measures of bone microarchitectural and biomechanical phenotypes. Unbiased analysis of the differentially expressed genes compared to relevant human disease phenotypes revealed that Enpp1^asj/asj mice exhibit strong signatures of osteoporosis, ARHR2 and OPLL. We found that ENPP1 deficient mice exhibited reduced gene transcription of Wnt ligands in whole bone and increased transcription of soluble Wnt inhibitors in the liver and kidney, suggestive of multiorgan inhibition of Wnt activity. Consistent with Wnt suppression in bone, Collagen gene pathways in bone were significantly decreased and Fgf23 was significantly increased, all of which directly correlated with bone microarchitectural defects and fracture risk in Enpp1^asj/asj mice. Moreover, the bone findings in 10-week old mice correlated with Enpp1 transcript counts but not plasma [PPi], suggesting that the skeletal phenotype at 10 weeks is driven by catalytically independent ENPP1 function. In contrast, the bone findings in 23-week Enpp1^asj/asj mice strongly correlated with plasma PPi, suggesting that chronically low PPi drives the skeletal phenotype in older mice. Finally, correlation between Enpp1 and Fgf23 transcription suggested ENPP1 regulation of Fgf23, which we confirmed by dosing Enpp1^asj/asj mice with soluble ENPP1-Fc and observing suppression of intact plasma FGF23 and ALP. In summary, our findings suggest that osteoporosis associated with ENPP1 deficiency involves the suppression of Wnt via catalytically independent Enpp1 pathways, and validates Enpp1^asj/asj mice as tools to better understand OPLL and Paradoxical Mineralization Disorders.

Genomic Medicine: Lessons Learned From Monogenic and Complex Bone Disorders

Current genetic studies of monogenic and complex bone diseases have broadened our understanding of disease pathophysiology, highlighting the need for medical interventions and treatments tailored to the characteristics of patients. As genomic research progresses, novel insights into the molecular mechanisms are starting to provide support to clinical decision-making; now offering ample opportunities for disease screening, diagnosis, prognosis and treatment. Drug targets holding mechanisms with genetic support are more likely to be successful. Therefore, implementing genetic information to the drug development process and a molecular redefinition of skeletal disease can help overcoming current shortcomings in pharmaceutical research, including failed attempts and appalling costs. This review summarizes the achievements of genetic studies in the bone field and their application to clinical care, illustrating the imminent advent of the genomic medicine era.

Unique, Gender-Dependent Serum microRNA Profile in PLS3 Gene-Related Osteoporosis

Plastin 3 (PLS3), encoded by PLS3, is a newly recognized regulator of bone metabolism, and mutations in the encoding gene result in severe childhood-onset osteoporosis. Because it is an X chromosomal gene, PLS3 mutation-positive males are typically more severely affected whereas females portray normal to increased skeletal fragility. Despite the severe skeletal pathology, conventional metabolic bone markers tend to be normal and are thus insufficient for diagnosing or monitoring patients. Our study aimed to explore serum microRNA (miRNA) concentrations in subjects with defective PLS3 function to identify novel markers that could differentiate subjects according to mutation status and give insight into the molecular mechanisms by which PLS3 regulates skeletal health. We analyzed fasting serum samples for a custom-designed panel comprising 192 miRNAs in 15 mutation-positive (five males, age range 8-76 years, median 41 years) and 14 mutation-negative (six males, age range 8-69 years, median 40 years) subjects from four Finnish families with different PLS3 mutations. We identified a unique miRNA expression profile in the mutation-positive subjects with seven significantly upregulated or downregulated miRNAs (miR-93-3p, miR-532-3p, miR-133a-3p, miR-301b-3p, miR-181c-5p, miR-203a-3p, and miR-590-3p; p values, range .004-.044). Surprisingly, gender subgroup analysis revealed the difference to be even more distinct in female mutation-positive subjects (congruent p values, range .007-.086) than in males (p values, range .127-.843) in comparison to corresponding mutation-negative subjects. Although the seven identified miRNAs have all been linked to bone metabolism and two of them (miR-181c-5p and miR-203a-3p) have bioinformatically predicted targets in the PLS3 3' untranslated region (3'-UTR), none have previously been reported to associate with PLS3. Our results indicate that PLS3 mutations are reflected in altered serum miRNA levels and suggest there is crosstalk between PLS3 and these miRNAs in bone metabolism. These provide new understanding of the pathomechanisms by which mutations in PLS3 lead to skeletal disease and may provide novel avenues for exploring miRNAs as biomarkers in PLS3 osteoporosis or as target molecules in future therapeutic applications. © 2020 The Authors. Journal of Bone and Mineral Research published by American Society for Bone and Mineral Research.

Polymorphisms in genes involved in inflammation, the NF-kB pathway and the renin-angiotensin-aldosterone system are associated with the risk of osteoporotic fracture. The Hortega Follow-up Study

Osteoporosis is the most common bone disorder worldwide and is associated with a reduced quality of life with important clinical and economic consequences. The most widely accepted etiopathogenic hypothesis on the origin of osteoporosis and its complications is that they are a consequence of the synergic action of environmental and genetic factors. Bone is constantly being remodelled through anabolic and catabolic pathways in which inflammation, the NF-kB pathway and the renin-angiotensin-aldosterone system (RAAS) are crucial. The aim of our study was to determine whether polymorphisms in genes implicated in inflammation, the NF-kB pathway and RAAS modified the risk of osteoporotic fracture. We analysed 221 patients with osteoporotic fracture and 354 controls without fracture from the HORTEGA sample after 12-14 years of follow up. In addition, we studied the genotypic distribution of 230 single nucleotide polymorphisms (SNPs) in genes involved in inflammation, NF-kB pathway and RAAS. Our results showed that be carrier of the C allele of the rs2228145 IL6R polymorphism was the principal genetic risk factor associated with osteoporotic fracture. The results also showed that variant genotypes of the rs4762 AGT, rs4073 IL8, rs2070699 END1 and rs4291 ACE polymorphisms were important genetic risk factors for fracture. The study provides information about the genetic factors associated with inflammation, the NF-kB pathway and RAAS, which are involved in the risk of osteoporotic fracture and reinforces the hypothesis that genetic factors are crucial in the etiopathogenesis of osteoporosis and its complications.

The Musculoskeletal Knowledge Portal: Making Omics Data Useful to the Broader Scientific Community

The development of high-throughput genotyping technologies and large biobank collections, complemented with rapid methodological advances in statistical genetics, has enabled hypothesis-free genome-wide association studies (GWAS), which have identified hundreds of genetic variants across many loci associated with musculoskeletal conditions. Similarly, basic scientists have valuable molecular cellular and animal data based on musculoskeletal disease that would be enhanced by being able to determine the human translation of their findings. By integrating these large-scale human genomic musculoskeletal datasets with complementary evidence from model organisms, new and existing genetic loci can be statistically fine-mapped to plausibly causal variants, candidate genes, and biological pathways. Genes and pathways identified using this approach can be further prioritized as drug targets, including side-effect profiling and the potential for new indications. To bring together these big data, and to realize the vision of creating a knowledge portal, the International Federation of Musculoskeletal Research Societies (IFMRS) established a working group to collaborate with scientists from the Broad Institute to create the Musculoskeletal Knowledge Portal (MSK-KP)(http://mskkp.org/). The MSK consolidates omics datasets from humans, cellular experiments, and model organisms into a central repository that can be accessed by researchers. The vision of the MSK-KP is to enable better understanding of the biological mechanisms underlying musculoskeletal disease and apply this knowledge to identify and develop new disease interventions. © 2020 American Society for Bone and Mineral Research (ASBMR).

Gallium ions promote osteoinduction of human and mouse osteoblasts via the TRPM7/Akt signaling pathway

Gallium (Ga) ions have been widely utilized for biomedical applications; however, their role in osteoblast regulation is not completely understood. The aim of the present study was to investigate the potential effect of Ga ions on osteoinduction in two osteoblast cell lines and to explore the underlying mechanisms. Human hFOB1.19 and mouse MC3T3‑E1 osteoblasts were treated with Ga nitride (GaN) at different concentrations, and the degree of osteoinduction was assessed. Ga ion treatment was found to increase alkaline phosphatase activity and accelerate calcium nodule formation, as assessed using ALP activity assay and Alizarin red S staining. Moreover, upregulated expression levels of osteogenic proteins in osteoblasts were identified using western blotting and reverse transcription‑quantitative PCR. Western blotting was also performed to demonstrate that the biological action of Ga ions was closely associated with the transient receptor potential melastatin 7/Akt signaling pathway. Furthermore, it was found that Ga ions did not cause osteoblast apoptosis, as indicated via flow cytometry, but promoted osteoclast proliferation, migration or invasion. The present study investigated the potential role of Ga ions in regulating osteoinduction of osteoblasts, thereby providing a promising strategy for the treatment of osteoporosis.

Comparative profiles of DNA methylation and differential gene expression in osteocytic areas from aged and young mice

Altered DNA methylation upon ageing may result in many age-related diseases such as osteoporosis. However, the changes in DNA methylation that occur in cortical bones, the major osteocytic areas, remain unknown. In our study, we extracted total DNA and RNA from the cortical bones of 6-month-old and 24-month-old mice and systematically analysed the differentially methylated regions (DMRs), differentially methylated promoters (DMPs) and differentially expressed genes (DEGs) between the mouse groups. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis of the DMR-related genes revealed that they were mainly associated with metabolic signalling pathways, including glycolysis, fatty acid and amino acid metabolism. Other genes with DMRs were related to signalling pathways that regulate the growth and development of cells, including the PI3K-AKT, Ras and Rap1 signalling pathways. The gene expression profiles indicated that the DEGs were mainly involved in metabolic pathways and the PI3K-AKT signalling pathway, and the profiles were verified through real-time quantitative PCR (RT-qPCR). Due to the pivotal roles of the affected genes in maintaining bone homeostasis, we suspect that these changes may be key factors in age-related bone loss, either together or individually. Our study may provide a novel perspective for understanding the osteocyte and its relationship with osteoporosis during ageing. SIGNIFICANCE OF THE STUDY: Our study identified age-related changes in gene expressions in osteocytic areas through whole-genome bisulfite sequencing (WGBS) and RNA-seq, providing new theoretical foundations for the targeted treatment of senile osteoporosis.

Biomarkers in WNT1 and PLS3 Osteoporosis: Altered Concentrations of DKK1 and FGF23

Recent advancements in genetic research have uncovered new forms of monogenic osteoporosis, expanding our understanding of the molecular pathways regulating bone health. Despite active research, knowledge on the pathomechanisms, disease-specific biomarkers, and optimal treatment in these disorders is still limited. Mutations in WNT1, encoding a WNT/β-catenin pathway ligand WNT1, and PLS3, encoding X chromosomally inherited plastin 3 (PLS3), both result in early-onset osteoporosis with prevalent fractures and disrupted bone metabolism. However, despite marked skeletal pathology, conventional bone markers are usually normal in both diseases. Our study aimed to identify novel bone markers in PLS3 and WNT1 osteoporosis that could offer diagnostic potential and shed light on the mechanisms behind these skeletal pathologies. We measured several parameters of bone metabolism, including serum dickkopf-1 (DKK1), sclerostin, and intact and C-terminal fibroblast growth factor 23 (FGF23) concentrations in 17 WNT1 and 14 PLS3 mutation-positive subjects. Findings were compared with 34 healthy mutation-negative subjects from the same families. Results confirmed normal concentrations of conventional metabolic bone markers in both groups. DKK1 concentrations were significantly elevated in PLS3 mutation-positive subjects compared with WNT1 mutation-positive subjects (p < .001) or the mutation-negative subjects (p = .002). Similar differences were not seen in WNT1 subjects. Sclerostin concentrations did not differ between any groups. Both intact and C-terminal FGF23 were significantly elevated in WNT1 mutation-positive subjects (p = .039 and p = .027, respectively) and normal in PLS3 subjects. Our results indicate a link between PLS3 and DKK1 and WNT1 and FGF23 in bone metabolism. The normal sclerostin and DKK1 levels in patients with impaired WNT signaling suggest another parallel regulatory mechanism. These findings provide novel information on the molecular networks in bone. Extended studies are needed to investigate whether these biomarkers offer diagnostic value or potential as treatment targets in osteoporosis. © 2020 American Society for Bone and Mineral Research.

The potential function of miR-135b-mediated JAK2/STAT3 signaling pathway during osteoblast differentiation

MC3T3-E1 cells were divided into Blank, miR-135b mimics, miR-135b inhibitors, AG490, and miR-135b inhibitors + AG490 groups. Cell viability was determined by MTT, alkaline phosphatase (ALP) activity by the corresponding kit, and mineralization by alizarin red staining. Furthermore, miR-135b, osteoblast-specific genes, and JAK2/STAT3 were detected through quantitative real-time polymerase chain reaction and Western blotting. MiR-135b downregulation was identified with increased JAK2 during osteoblast differentiation. JAK2 was confirmed as a target gene of miR-135b by dual-luciferase reporter assay. MC3T3-E1 cells in both miR-135b mimics and AG490 groups manifested decrease in cell viability, ALP activity, and mineralized nodes, as well as reductions in osteoblast-specific genes and proteins of JAK2, p-JAK2, and p-STAT3, but increase in cell apoptosis. However, opposite changes of the above factors were shown in cells from miR-135b inhibitors group. Notably, AG490 could reverse promotion effects of miR-135b inhibitors on osteoblast differentiation. Inhibiting miR-135b could activate the JAK2/STAT3 signaling pathway, thereby improving the cell viability and promoting the osteoblast differentiation.

Galectin-3 and TRIM16 coregulate osteogenic differentiation of human bone marrow-derived mesenchymal stem cells at least partly via enhancing autophagy

BACKGROUND

The osteogenic differentiation of human bone marrow-derived mesenchymal stem cells (hBMSCs) is critical for bone homeostasis. Here, we investigated the regulation of Galectin-3 and tripartite motif protein 16 (TRIM16) on osteogenic differentiation of hBMSCs through autophagy.

METHODS

Quantitative PCR (qPCR) and western blot were performed to determine the expression of osteogenic markers, autophagic markers, Galectin-3 and TRIM16. Short-hairpin RNAs (shRNAs) and overexpression plasmids were used to manipulate the expression of Galectin-3, TRIM16 and Unc-51 like autophagy activating kinase 1 (ULK1). Alkaline phosphatase (ALP) activity was measured by ALP staining assay. Calcium deposition in differentiated hBMSCs was assessed by Alizarin Red S staining. LC3 puncta formation was monitored by immunofluorescence staining. The interaction between indicated proteins was confirmed by co-immunoprecipitation (Co-IP) assay.

RESULTS

Either Galectin-3 or TRIM16 knockdown led to impaired ALP activity, reduced calcium deposition, down-regulation of pro-osteogenic markers as well as restrained autophagy in osteogenic-induced hBMSCs. However, overexpression of Galectin-3 or TRIM16 promoted osteogenic differentiation of hBMSCs, which was then compromised by autophagy inhibition. Co-IP experiment demonstrated that TRIM16 associated with Galectin-3 through ULK1. Meanwhile, osteogenic induction enhanced the association between TRIM16 and ULK1 or coiled-coil myosin-like BCL2-interacting protein (Beclin1), and TRIM16 increased the stability of ULK1 and Beclin1. Moreover, either TRIM16 or ULK1 knockdown dampened the pro-osteogenic effect of Galectin-3, which elucidated that Galectin-3 mediated osteogenic differentiation was at least partly dependent on TRIM16 and ULK1.

CONCLUSION

In summary, the present study revealed Galectin-3 and TRIM16 co-regulated osteogenic differentiation of hBMSCs at least partly via enhancing autophagy, which might provide a promising approach for osteoporosis treatment in future.

Genetics of Skeletal Disorders

Bone and mineral diseases encompass a variety of conditions that involve altered skeletal homeostasis and are frequently associated with changes in circulating calcium, phosphate, or vitamin D metabolites. These disorders often have a genetic etiology and comprise monogenic disorders caused by a single-gene mutation, which may be germline or somatic, or an oligogenic or polygenic condition involving multiple genetic variants. Single-gene mutations causing Mendelian diseases are usually highly penetrant, whereas the gene variants contributing to oligogenic or polygenic disorders are each associated with smaller effects with additional contributions from environmental factors. The detection of monogenic disorders is clinically important and facilitates timely assessment and management of the patient and their affected relatives. The diagnosis of monogenic metabolic bone disorders requires detailed clinical assessment of the wide variety of symptoms and signs associated with these diseases. Thus, clinicians should undertake a systematic approach commencing with careful history taking and physical examination, followed by appropriate laboratory and skeletal imaging investigations. Finally, clinicians should be familiar with the range of molecular genetic tests available to ensure their appropriate use and interpretation. These considerations are reviewed in this chapter.

The Membrane Transporter OAT7 (SLC22A9) Is Not a Susceptibility Factor for Osteoporosis in Europeans

Bone production, maintenance, and modeling are a well-balanced process involving mineralization by osteoblasts and resorption by osteoclasts. Sex steroid hormones, including their conjugated forms, contribute majorly to maintaining this balance. Recently, variants in the SLC22A9 gene have been associated with osteoporosis in Korean females. We had recently shown that SLC22A9, encoding organic anion transporter 7 (OAT7), is an uptake transporter of estrone sulfate and identified several genetic variants in Europeans leading to functional consequences in vitro. We therefore hypothesized that SLC22A9 genetic variants may contribute to the pathophysiology of osteoporosis in Europeans. To test this hypothesis, we examined the associations of SLC22A9 variants with bone quality, fractures, and bone turnover markers. We genotyped SLC22A9 variants in 5,701 (2,930 female) subjects (age range, 20-93 years) extracted from the population-based Study of Health in Pomerania (SHIP and SHIP-TREND) covered by the Illumina Infinium HumanExome BeadChip version v1.0 (Exome Chip). Descriptive data (e.g., history of fractures), ultrasonography of the calcaneus, as well as serum concentrations of carboxy-terminal telopeptide of type I collagen, amino-terminal propeptide of type I procollagen, and vitamin D were determined. Comprehensive statistical analyses revealed no association between low-frequency and rare SLC22A9 variants and bone quality, fractures, and bone turnover markers. Our results indicate that single genetic SLC22A9 variants do not have a major impact on osteoporosis risk prediction in Europeans, yet findings need to be replicated in larger-scale studies.

Comparison of bone biomechanical properties after bone marrow mesenchymal stem cell or alendronate treatment in an osteoporotic animal model

The aim of this study was to explore the effects of bone marrow mesenchymal stem cells (BMMSCs) and alendronate sodium (ALN) intervention on osteoporosis (OP). Sixty-eight 6-month-old healthy female Sprague Dawley (SD) rats were used to generate an OP model by removal of the ovaries. After 12 weeks, rats were treated with BMMSCs (BMMSC group) or ALN (ALN group) for 5 weeks. Serum type I collagen C terminal peptide (CTX_1), procollagen type I N-terminal propeptide (PINP), and bone alkaline phosphatase (BALP) were tested along with the femur bone density and other properties, including bone mineral density (BMD), BALP, percent trabecular area (BV/TV), trabecular thickness (Tb.Th), trabecular number (TbN), maximum load, maximum stress, maximum strain, and elastic modulus. BMD, BALP, BV/TV, Tb.Th, TbN, maximum load, maximum stress, maximum strain, and elastic modulus values were higher in the BMMSC group versus the ALN group relative to the control group (p < 0.05); CTX_1, PINP, trabecular separation (Tb.Sp), and osteoclast number (OC.N) were lowest in the BMMSC group versus the ALN group relative to the control group (p < 0.05). Both BMMSCs and ALN could improve the metabolic function and bone quality in osteoporotic mice while restoring the strength and toughness of bones. The intervention effects of BMMSCs are better than ALN in this model.

Monogene frühmanifeste Osteoporose und Altersosteoporose – ein Kontinuum

Das Risiko für atraumatische/osteoporotische Frakturen nimmt ab einem Alter von 55 Jahren zu und wird zu einem großen Teil durch die individuelle Knochenmineraldichte und -struktur bestimmt. Durch Modeling während des Wachstums und anschließendes Remodeling passen Osteoblasten und Osteoklasten als Teil der sog. „basic multicellular unit“ das Knochengewebe kontinuierlich an die Erfordernisse an. Angeborene Störungen ihrer Funktion und/oder ihres Zusammenspiels durch häufige oder seltene Genvarianten können durch verzögerten Knochenaufbau oder beschleunigten Knochenabbau zu einer pathologisch niedrigen Knochenmineraldichte (BMD) führen. Häufige Varianten in über 500 Genloci erklären zusammen derzeit ca. 20 % der BMD-Varianz und beeinflussen das Risiko der Altersosteoporose. In einem signifikanten Teil der erwachsenen Patienten mit frühmanifester Osteoporose (vor dem 55. Lebensjahr) finden sich hingegen seltene Varianten als monogene Krankheitsursache. Aufgrund der mitunter sehr milden und variablen Manifestation dieser monogenen Krankheiten ist die genetische Diagnostik die zuverlässigste Möglichkeit der molekularen Zuordnung. Die bei der früh- und spätmanifesten Osteoporose involvierten Gene zeigen eine deutliche Überlappung, besonders bei Genen mit Funktion im Wnt-Signalweg. Die Einbeziehung genetischer Varianten in den diagnostischen Prozess erlaubt eine genauere Prognose und möglicherweise auch eine spezifischere Therapie. Auf die Altersosteoporose lässt sich dieser personalisierte Ansatz unter Umständen in einem nächsten Schritt mithilfe polygener Risiko-Scores übertragen.

Polymorphisms of FDPS, LRP5, SOST and VKORC1 genes and their relation with osteoporosis in postmenopausal Romanian women

OBJECTIVES

This study aimed to assess the relationship between bone mineral density and genotypes of four polymorphisms in previously detected osteoporosis-candidate genes (FDPS rs2297480, LRP5 rs3736228, SOST rs1234612, VKORC1 rs9934438) in postmenopausal Romanian women with primary osteoporosis.

METHODS

An analytical, prospective, transversal, observational, case-control study on 364 postmenopausal Romanian women was carried out between June 2016 and August 2017 in Cluj Napoca, Romania. Clinical data and blood samples were collected from all study participants. Four polymorphisms were genotyped using TaqMan SNP Genotyping assays, run on a QuantStudio 3 real-time PCR machine.

RESULTS

Women with TT genotype in FDPS rs2297480 had significantly lower bone mineral density values in the lumbar spine and total hip, and the presence of the T allele was significantly associated with the osteoporosis. Women carrying the CC genotype in LRP5 rs3736228 tend to have lower bone mineral density values in the femoral neck and total hip. No significant association was found for the genotypes of SOST rs1234612 or VKORC1 rs9934438.

CONCLUSIONS

Our study showed a strong association between bone mineral density and polymorphisms in the FDPS gene, and a borderline association with LRP5 and SOST polymorphisms in postmenopausal Romanian women with osteoporosis. No association was found for VKORC1.

Pleiotropic mapping and annotation selection in genome-wide association studies with penalized Gaussian mixture models

Motivation

Genome-wide association studies (GWASs) have identified many genetic loci associated with complex traits. A substantial fraction of these identified loci is associated with multiple traits-a phenomena known as pleiotropy. Identification of pleiotropic associations can help characterize the genetic relationship among complex traits and can facilitate our understanding of disease etiology. Effective pleiotropic association mapping requires the development of statistical methods that can jointly model multiple traits with genome-wide single nucleic polymorphisms (SNPs) together.

Results

We develop a joint modeling method, which we refer to as the integrative MApping of Pleiotropic association (iMAP). iMAP models summary statistics from GWASs, uses a multivariate Gaussian distribution to account for phenotypic correlation, simultaneously infers genome-wide SNP association pattern using mixture modeling and has the potential to reveal causal relationship between traits. Importantly, iMAP integrates a large number of SNP functional annotations to substantially improve association mapping power, and, with a sparsity-inducing penalty, is capable of selecting informative annotations from a large, potentially non-informative set. To enable scalable inference of iMAP to association studies with hundreds of thousands of individuals and millions of SNPs, we develop an efficient expectation maximization algorithm based on an approximate penalized regression algorithm. With simulations and comparisons to existing methods, we illustrate the benefits of iMAP in terms of both high association mapping power and accurate estimation of genome-wide SNP association patterns. Finally, we apply iMAP to perform a joint analysis of 48 traits from 31 GWAS consortia together with 40 tissue-specific SNP annotations generated from the Roadmap Project.

Availability and implementation

iMAP is freely available at http://www.xzlab.org/software.html.

Supplementary information

Supplementary data are available at Bioinformatics online.

Mendelian bone fragility disorders

Mendelian bone fragility disorders are caused by genetic variants that can be inherited in an autosomal dominant, autosomal recessive or X-linked manner and have a large detrimental effect on bone strength. As a rule, the more damaging the genetic defect is, the earlier the first fracture will occur, typically during bone development. This review focusses on conditions where bone fragility is the most conspicuous characteristic, of which osteogenesis imperfecta (OI) is the best-known disorder. The large majority of individuals with an OI phenotype have disease-causing dominant variants in COL1A1 or COL1A2, the genes coding for collagen type I. Interestingly, large sequencing databases indicate that there are about 10 times more carriers of COL1A1/COL1A2 variants that should lead to OI than there are individuals with a diagnosis of OI. It is possible that at least some of these variants lead to incomplete OI phenotypes and are diagnosed as osteoporosis during adulthood. Apart from mutations affecting collagen type I production, biallelic mutations in LRP5 and WNT1 can cause very rare and severe bone fragility disorders. Heterozygous pathogenic variants in these genes are much more common and can cause the clinical picture of primary osteoporosis. As sequencing studies are more widely performed in adults with bone fragility disorders, evidence is emerging that what appears as primary osteoporosis in fact can be due to mutations in bona fide OI genes. The distinction between OI and primary osteoporosis is therefore likely to blur in future.