Common variants in the region around Osterix are associated with bone mineral density and growth in childhood

A homozygous SP7/OSX mutation causes osteogenesis and dentinogenesis imperfecta with craniofacial anomalies

Osteogenesis imperfecta (OI) is a heterogeneous spectrum of hereditary genetic disorders that cause bone fragility, through various quantitative and qualitative defects of type 1 collagen, a triple helix composed of two α1 and one α2 chains encoded by COL1A1 and COL1A2, respectively. The main extra-skeletal manifestations of OI include blue sclerae, opalescent teeth, and hearing impairment. Moreover, multiple genes involved in osteoblast maturation and type 1 collagen biosynthesis are now known to cause recessive forms of OI. In this study a multiplex consanguineous family of two affected males with OI was recruited for genetic screening. To determine the causative, pathogenic variant(s), genomic DNA from two affected family members were analyzed using whole exome sequencing, autozygosity mapping, and then validated with Sanger sequencing. The analysis led to the mapping of a homozygous variant previously reported in SP7/OSX, a gene encoding for Osterix, a transcription factor that activates a repertoire of genes involved in osteoblast and osteocyte differentiation and function. The identified variant (c.946C > T; p.Arg316Cys) in exon 2 of SP7/OSX results in a pathogenic amino acid change in two affected male siblings and develops OI, dentinogenesis imperfecta, and craniofacial anomaly. On the basis of the findings of the present study, SP7/OSX:c. 946C > T is a rare homozygous variant causing OI with extra-skeletal features in inbred Arab populations.

Endocrine Health and Health Care Disparities in the Pediatric and Sexual and Gender Minority Populations: An Endocrine Society Scientific Statement

Endocrine care of pediatric and adult patients continues to be plagued by health and health care disparities that are perpetuated by the basic structures of our health systems and research modalities, as well as policies that impact access to care and social determinants of health. This scientific statement expands the Society's 2012 statement by focusing on endocrine disease disparities in the pediatric population and sexual and gender minority populations. These include pediatric and adult lesbian, gay, bisexual, transgender, queer, intersex, and asexual (LGBTQIA) persons. The writing group focused on highly prevalent conditions-growth disorders, puberty, metabolic bone disease, type 1 (T1D) and type 2 (T2D) diabetes mellitus, prediabetes, and obesity. Several important findings emerged. Compared with females and non-White children, non-Hispanic White males are more likely to come to medical attention for short stature. Racially and ethnically diverse populations and males are underrepresented in studies of pubertal development and attainment of peak bone mass, with current norms based on European populations. Like adults, racial and ethnic minority youth suffer a higher burden of disease from obesity, T1D and T2D, and have less access to diabetes treatment technologies and bariatric surgery. LGBTQIA youth and adults also face discrimination and multiple barriers to endocrine care due to pathologizing sexual orientation and gender identity, lack of culturally competent care providers, and policies. Multilevel interventions to address these disparities are required. Inclusion of racial, ethnic, and LGBTQIA populations in longitudinal life course studies is needed to assess growth, puberty, and attainment of peak bone mass. Growth and development charts may need to be adapted to non-European populations. In addition, extension of these studies will be required to understand the clinical and physiologic consequences of interventions to address abnormal development in these populations. Health policies should be recrafted to remove barriers in care for children with obesity and/or diabetes and for LGBTQIA children and adults to facilitate comprehensive access to care, therapeutics, and technological advances. Public health interventions encompassing collection of accurate demographic and social needs data, including the intersection of social determinants of health with health outcomes, and enactment of population health level interventions will be essential tools.

SP7: from Bone Development to Skeletal Disease

PURPOSE OF REVIEW

The purpose of this review is to summarize the different roles of the transcription factor SP7 in regulating bone formation and remodeling, discuss current studies in investigating the causal relationship between SP7 mutations and human skeletal disease, and highlight potential therapeutic treatments that targeting SP7 and the gene networks that it controls.

RECENT FINDINGS

Cell-type and stage-specific functions of SP7 have been identified during bone formation and remodeling. Normal bone development regulated by SP7 is strongly associated with human bone health. Dysfunction of SP7 results in common or rare skeletal diseases, including osteoporosis and osteogenesis imperfecta with different inheritance patterns. SP7-associated signaling pathways, SP7-dependent target genes, and epigenetic regulations of SP7 serve as new therapeutic targets in the treatment of skeletal disorders. This review addresses the importance of SP7-regulated bone development in studying bone health and skeletal disease. Recent advances in whole genome and exome sequencing, GWAS, multi-omics, and CRISPR-mediated activation and inhibition have provided the approaches to investigate the gene-regulatory networks controlled by SP7 in bone and the therapeutic targets to treat skeletal disease.

Pathways Controlling Formation and Maintenance of the Osteocyte Dendrite Network

PURPOSE OF REVIEW

The purpose of this review is to discuss the molecular mechanisms involved in osteocyte dendrite formation, summarize the similarities between osteocytic and neuronal projections, and highlight the importance of osteocyte dendrite maintenance in human skeletal disease.

RECENT FINDINGS

It is suggested that there is a causal relationship between the loss of osteocyte dendrites and the increased osteocyte apoptosis during conditions including aging, microdamage, and skeletal disease. A few mechanisms are proposed to control dendrite formation and outgrowth, such as via the regulation of actin polymerization dynamics. This review addresses the impact of osteocyte dendrites in bone health and disease. Recent advances in multi-omics, in vivo and in vitro models, and microscopy-based imaging have provided novel approaches to reveal the underlying mechanisms that regulate dendrite development. Future therapeutic approaches are needed to target the process of osteocyte dendrite formation.

Sp7 Action in the Skeleton: Its Mode of Action, Functions, and Relevance to Skeletal Diseases

Osteoblast differentiation is a tightly regulated process in which key transcription factors (TFs) and their target genes constitute gene regulatory networks (GRNs) under the control of osteogenic signaling pathways. Among these TFs, Sp7 works as an osteoblast determinant critical for osteoblast differentiation. Following the identification of Sp7 and a large number of its functional studies, recent genome-scale analyses have made a major contribution to the identification of a "non-canonical" mode of Sp7 action as well as "canonical" ones. The analyses have not only confirmed known Sp7 targets but have also uncovered its additional targets and upstream factors. In addition, biochemical analyses have demonstrated that Sp7 actions are regulated by chemical modifications and protein-protein interaction with other transcriptional regulators. Sp7 is also involved in chondrocyte differentiation and osteocyte biology as well as postnatal bone metabolism. The critical role of SP7 in the skeleton is supported by its relevance to human skeletal diseases. This review aims to overview the Sp7 actions in skeletal development and maintenance, particularly focusing on recent advances in our understanding of how Sp7 functions in the skeleton under physiological and pathological conditions.

Cytoskeletal Protein 4.1G Is Essential for the Primary Ciliogenesis and Osteoblast Differentiation in Bone Formation

The primary cilium is a hair-like immotile organelle with specific membrane receptors, including the receptor of Hedgehog signaling, smoothened. The cilium organized in preosteoblasts promotes differentiation of the cells into osteoblasts (osteoblast differentiation) by mediating Hedgehog signaling to achieve bone formation. Notably, 4.1G is a plasma membrane-associated cytoskeletal protein that plays essential roles in various tissues, including the peripheral nervous system, testis, and retina. However, its function in the bone remains unexplored. In this study, we identified 4.1G expression in the bone. We found that, in the 4.1G-knockout mice, calcium deposits and primary cilium formation were suppressed in the trabecular bone, which is preosteoblast-rich region of the newborn tibia, indicating that 4.1G is a prerequisite for osteoblast differentiation by organizing the primary cilia in preosteoblasts. Next, we found that the primary cilium was elongated in the differentiating mouse preosteoblast cell line MC3T3-E1, whereas the knockdown of 4.1G suppressed its elongation. Moreover, 4.1G-knockdown suppressed the induction of the cilia-mediated Hedgehog signaling and subsequent osteoblast differentiation. These results demonstrate a new regulatory mechanism of 4.1G in bone formation that promotes the primary ciliogenesis in the differentiating preosteoblasts and induction of cilia-mediated osteoblast differentiation, resulting in bone formation at the newborn stage.

A neomorphic variant in SP7 alters sequence specificity and causes a high-turnover bone disorder

SP7/Osterix is a transcription factor critical for osteoblast maturation and bone formation. Homozygous loss-of-function mutations in SP7 cause osteogenesis imperfecta type XII, but neomorphic (gain-of-new-function) mutations of SP7 have not been reported in humans. Here we describe a de novo dominant neomorphic missense variant (c.926 C > G:p.S309W) in SP7 in a patient with craniosynostosis, cranial hyperostosis, and long bone fragility. Histomorphometry shows increased osteoblasts but decreased bone mineralization. Mice with the corresponding variant also show a complex skeletal phenotype distinct from that of Sp7-null mice. The mutation alters the binding specificity of SP7 from AT-rich motifs to a GC-consensus sequence (typical of other SP family members) and produces an aberrant gene expression profile, including increased expression of Col1a1 and endogenous Sp7, but decreased expression of genes involved in matrix mineralization. Our study identifies a pathogenic mechanism in which a mutation in a transcription factor shifts DNA binding specificity and provides important in vivo evidence that the affinity of SP7 for AT-rich motifs, unique among SP proteins, is critical for normal osteoblast differentiation.

The genetic overlap between osteoporosis and craniosynostosis

Osteoporosis is the most prevalent bone condition in the ageing population. This systemic disease is characterized by microarchitectural deterioration of bone, leading to increased fracture risk. In the past 15 years, genome-wide association studies (GWAS), have pinpointed hundreds of loci associated with bone mineral density (BMD), helping elucidate the underlying molecular mechanisms and genetic architecture of fracture risk. However, the challenge remains in pinpointing causative genes driving GWAS signals as a pivotal step to drawing the translational therapeutic roadmap. Recently, a skull BMD-GWAS uncovered an intriguing intersection with craniosynostosis, a congenital anomaly due to premature suture fusion in the skull. Here, we recapitulate the genetic contribution to both osteoporosis and craniosynostosis, describing the biological underpinnings of this overlap and using zebrafish models to leverage the functional investigation of genes associated with skull development and systemic skeletal homeostasis.

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.

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.

3D assessment of intervertebral disc degeneration in zebrafish identifies changes in bone density that prime disc disease

Back pain is a common condition with a high social impact and represents a global health burden. Intervertebral disc disease (IVDD) is one of the major causes of back pain; no therapeutics are currently available to reverse this disease. The impact of bone mineral density (BMD) on IVDD has been controversial, with some studies suggesting osteoporosis as causative for IVDD and others suggesting it as protective for IVDD. Functional studies to evaluate the influence of genetic components of BMD in IVDD could highlight opportunities for drug development and repurposing. By taking a holistic 3D approach, we established an aging zebrafish model for spontaneous IVDD. Increased BMD in aging, detected by automated computational analysis, is caused by bone deformities at the endplates. However, aged zebrafish spines showed changes in bone morphology, microstructure, mineral heterogeneity, and increased fragility that resembled osteoporosis. Elements of the discs recapitulated IVDD symptoms found in humans: the intervertebral ligament (equivalent to the annulus fibrosus) showed disorganized collagen fibers and herniation, while the disc center (nucleus pulposus equivalent) showed dehydration and cellular abnormalities. We manipulated BMD in young zebrafish by mutating sp7 and cathepsin K, leading to low and high BMD, respectively. Remarkably, we detected IVDD in both groups, demonstrating that low BMD does not protect against IVDD, and we found a strong correlation between high BMD and IVDD. Deep learning was applied to high-resolution synchrotron µCT image data to analyze osteocyte 3D lacunar distribution and morphology, revealing a role of sp7 in controlling the osteocyte lacunar 3D profile. Our findings suggest potential avenues through which bone quality can be targeted to identify beneficial therapeutics for IVDD.

Approaches to osteoporosis in paleopathology: How did methodology shape bone loss research?

OBJECTIVE

This paper will review how different methods employed to study bone loss in the past were used to explore different questions and aspects of bone loss, how methodology has changed over time, and how these different approaches have informed our understanding of bone loss in the past.

MATERIALS AND METHODS

A review and discussion is conducted on research protocols and results of 84 paleopathology publications on bone loss in archaeological skeletal collections published between 1969 and 2021.

CONCLUSIONS

The variety in research protocols confounds accurate meta-analysis of previously published research; however, more recent publications incorporate a combination of bone mass and bone quality based methods. Biased sample selection has resulted in a predominance of European and Medieval publications, limiting more general observations on bone loss in the past. Collection of dietary or paleopathological covariables is underemployed in the effort to interpret bone loss patterns.

SIGNIFICANCE

Paleopathology publications have demonstrated differences in bone loss between distinct archaeological populations, between sex and age groups, and have suggested factors underlying observed differences. However, a lack of a gold standard has encouraged the use of a wide range of methods. Understanding how this array of methods effects results is crucial in contextualizing our knowledge of bone loss in the past.

LIMITATIONS

The development of a research protocol is also influenced by available expertise, available equipment, restrictions imposed by the curator, and site-specific taphonomic aspects. These factors will likely continue to cause (minor) biases even if a best practice can be established.

SUGGESTIONS FOR FUTURE RESEARCH

Greater effort to develop uniform terminology and operational definitions of osteoporosis in skeletal remains, as well as the expansion of time scale and geographical areas studied. The Next-Generation Sequencing revolution has also opened up the possibility of ancient DNA analyses to study genetic predisposition to bone loss in the past.

Twelve years of GWAS discoveries for osteoporosis and related traits: advances, challenges and applications

Osteoporosis is a common skeletal disease, affecting ~200 million people around the world. As a complex disease, osteoporosis is influenced by many factors, including diet (e.g. calcium and protein intake), physical activity, endocrine status, coexisting diseases and genetic factors. In this review, we first summarize the discovery from genome-wide association studies (GWASs) in the bone field in the last 12 years. To date, GWASs and meta-analyses have discovered hundreds of loci that are associated with bone mineral density (BMD), osteoporosis, and osteoporotic fractures. However, the GWAS approach has sometimes been criticized because of the small effect size of the discovered variants and the mystery of missing heritability, these two questions could be partially explained by the newly raised conceptual models, such as omnigenic model and natural selection. Finally, we introduce the clinical use of GWAS findings in the bone field, such as the identification of causal clinical risk factors, the development of drug targets and disease prediction. Despite the fruitful GWAS discoveries in the bone field, most of these GWAS participants were of European descent, and more genetic studies should be carried out in other ethnic populations to benefit disease prediction in the corresponding population.

Factors influencing peak bone mass gain

Bone mass is a key determinant of osteoporosis and fragility fractures. Epidemiologic studies have shown that a 10% increase in peak bone mass (PBM) at the population level reduces the risk of fracture later in life by 50%. Low PBM is possibly due to the bone loss caused by various conditions or processes that occur during adolescence and young adulthood. Race, gender, and family history (genetics) are responsible for the majority of PBM, but other factors, such as physical activity, calcium and vitamin D intake, weight, smoking and alcohol consumption, socioeconomic status, age at menarche, and other secondary causes (diseases and medications), play important roles in PBM gain during childhood and adolescence. Hence, the optimization of lifestyle factors that affect PBM and bone strength is an important strategy to maximize PBM among adolescents and young people, and thus to reduce the low bone mass or osteoporosis risk in later life. This review aims to summarize the available evidence for the common but important factors that influence bone mass gain during growth and development and discuss the advances of developing high PBM.

Genome-Wide Association Analysis of Longitudinal Bone Mineral Content Data From the Iowa Bone Development Study

The foundation for osteoporosis risk is, in part, established during childhood, adolescence, and young adulthood, all periods of development when bone mass is acquired rapidly. The relative quantity of bone mass accrued is influenced by both lifestyle and genetic factors, although the genetic component is not yet well understood. The purpose of this study was to use a genome-wide association (GWA) analysis to discover single nucleotide polymorphisms (SNPs) associated with: (1) the sex-specific hip bone mineral content at approximately the age of 19 when the amount of bone accrued is near its peak; and (2) the sex-specific rate of hip bone mineral content accrual during the adolescent growth spurt. The Iowa Bone Development Study, a longitudinal cohort study exploring bone health in children, adolescents, and young adults was the source of data. From this cohort, n = 364 (190 females, 174 males) participants were included in GWA analyses to address (1) and n = 258 participants (125 females and 133 males) were included in GWA analyses to address (2). Twenty SNPS were detected having p < 1.0 × 10-5. Of most biologic relevance were 2 suggestive SNPs (rs2051756 and rs2866908) detected in an intron of the DKK2 gene through the GWA analysis that explored peak bone mass in females.

Comprehensive Genetic Analysis of 128 Candidate Genes in a Cohort With Idiopathic, Severe, or Familial Osteoporosis

Context

The genetic bases of osteoporosis (OP), a disorder with high heritability, are poorly understood at an individual level. Cases of idiopathic or familial OP have long puzzled clinicians as to whether an actionable genetic cause could be identified.

Objective

We performed a genetic analysis of 28 cases of idiopathic, severe, or familial osteoporosis using targeted massively parallel sequencing.

Design

Targeted sequencing of 128 candidate genes was performed using Illumina NextSeq. Variants of interest were confirmed by Sanger sequencing or SNP array.

Patients and Setting

Thirty-seven patients in an academic tertiary hospital participated (54% male; median age, 44 years; 86% with fractures), corresponding to 28 sporadic or familial cases.

Main Outcome Measure

The identification of rare stop-gain, indel, splice site, copy-number, or nonsynonymous variants altering protein function.

Results

Altogether, we identified 28 variants of interest, but only 3 were classified as pathogenic or likely pathogenic variants: COL1A2 p.(Arg708Gln), WNT1 p.(Gly169Asp), and IDUA p.(His82Gln). An association of variants in different genes was found in 21% of cases, including a young woman with severe OP bearing WNT1, PLS3, and NOTCH2 variants. Among genes of uncertain significance analyzed, a potential additional line of evidence has arisen for GWAS candidates GPR68 and NBR1, warranting further studies.

Conclusions

While we hope that continuing efforts to identify genetic predisposition to OP will lead to improved and personalized care in the future, the likelihood of identifying actionable pathogenic variants in intriguing cases of idiopathic or familial osteoporosis is seemingly low.

Biallelic variants in four genes underlying recessive osteogenesis imperfecta

Osteogenesis imperfecta (OI) is an inherited heterogeneous rare skeletal disorder characterized by increased bone fragility and low bone mass. The disorder mostly segregates in an autosomal dominant manner. However, several rare autosomal recessive and X-linked forms, caused by mutations in 18 different genes, have also been described in the literature. Here, we present five consanguineous families segregating OI in an autosomal recessive pattern. Affected individuals in the five families presented severe forms of skeletal deformities. It included frequent bone fractures with abnormal healing, short stature, facial dysmorphism, osteopenia, joint laxity, and severe scoliosis. In order to search for the causative variants, DNA of at least one affected individual in three families (A-C) were subjected to whole exome sequencing (WES). In two other families (D-E), linkage analysis using highly polymorphic microsatellite markers was followed by Sanger sequencing. Sequence analysis revealed two novels and three previously reported disease-causing variants. The two novel homozygous variants including [c.824G > A; p.(Cys275Tyr)] in the SP7 gene and [c.397C > T, p.(Gln133*)] in the SERPINF1 gene were identified in families A and B, respectively. The three previously reported homozygous variants including [c.497G > A; p.(Arg166His)] in the SPARC gene, (c.359-3C > G; intron 2) and [c.677C > T; p.(Ser226Leu)] in the WNT1 gene were identified in family C, D, and E. In conclusion, our findings provided additional evidence of involvement of homozygous sequence variants in the SP7, SERPINF1, SPARC and WNT1 genes causing severe OI. It also highlights the importance of extensive genetic investigations to search for the culprit gene in each case of skeletal deformity.

Juvenile Paget's Disease From Heterozygous Mutation of SP7 Encoding Osterix (Specificity Protein 7, Transcription Factor SP7)

Juvenile Paget's disease (JPD) became in 1974 the commonly used name for ultra-rare heritable occurrences of rapid bone remodeling throughout of the skeleton that present in infancy or early childhood as fractures and deformity hallmarked biochemically by marked elevation of serum alkaline phosphatase (ALP) activity (hyperphosphatasemia). Untreated, JPD can kill during childhood or young adult life. In 2002, we reported that homozygous deletion of the gene called tumor necrosis factor receptor superfamily, member 11B (TNFRSF11B) encoding osteoprotegerin (OPG) explained JPD in Navajos. Soon after, other bi-allelic loss-of-function TNFRSF11B defects were identified in JPD worldwide. OPG inhibits osteoclastogenesis and osteoclast activity by decoying receptor activator of nuclear factor κ-B (RANK) ligand (RANKL) away from its receptor RANK. Then, in 2014, we reported JPD in a Bolivian girl caused by a heterozygous activating duplication within TNFRSF11A encoding RANK. Herein, we identify mutation of a third gene underlying JPD. An infant girl began atraumatic fracturing of her lower extremity long-bones. Skull deformity and mild hearing loss followed. Our single investigation of the patient, when she was 15 years-of-age, showed generalized osteosclerosis and hyperostosis. DXA revealed a Z-score of +5.1 at her lumbar spine and T-score of +3.3 at her non-dominant wrist. Biochemical studies were consistent with positive mineral balance and several markers of bone turnover were elevated and included striking hyperphosphatasemia. Iliac crest histopathology was consistent with rapid skeletal remodeling. Measles virus transcripts, common in classic Paget's disease of bone, were not detected in circulating mononuclear cells. Then, reportedly, she responded to several months of alendronate therapy with less skeletal pain and correction of hyperphosphatasemia but had been lost to our follow-up. After we detected no defect in TNFRSF11A or B, trio exome sequencing revealed a de novo heterozygous missense mutation (c.926C>G; p.S309W) within SP7 encoding the osteoblast transcription factor osterix (specificity protein 7, transcription factor SP7). Thus, mutation of SP7 represents a third genetic cause of JPD.

Genetic variants affecting bone mineral density and bone mineral content at multiple skeletal sites in Hispanic children

CONTEXT

Osteoporosis is a major public health burden with significant economic costs. However, the correlates of bone health in Hispanic children are understudied.

OBJECTIVE

We aimed to identify genetic variants associated with bone mineral density (BMD) and bone mineral content (BMC) at multiple skeletal sites in Hispanic children.

METHODS

We conducted a cross-sectional genome-wide linkage analysis, genome-wide and exome-wide association analysis of BMD and BMC. The Viva La Familia Study is a family-based cohort with a total of 1030 Hispanic children (4-19 years old at baseline) conducted in Houston, TX. BMD and BMC were measured by Dual-energy X-ray absorptiometry.

RESULTS

Significant heritability were observed for BMC and BMD at multiple skeletal sites ranging between 44 and 68% (P < 2.8 × 10-9). Significant evidence for linkage was found for BMD of pelvis and left leg on chromosome 7p14, lumbar spine on 20q13 and left rib on 6p21, and BMC of pelvis on chromosome 20q12 and total body on 14q22-23 (logarithm of odds score > 3). We found genome-wide significant association between BMC of right arm and rs762920 at PVALB (P = 4.6 × 10-8), and between pelvis BMD and rs7000615 at PTK2B (P = 7.4 × 10-8). Exome-wide association analysis revealed novel association of variants at MEGF10 and ABRAXAS2 with left arm and lumber spine BMC, respectively (P < 9 × 10-7).

CONCLUSIONS

We identified novel loci associated with BMC and BMD in Hispanic children, with strongest evidence for PTK2B. These findings provide better understanding of bone genetics and shed light on biological mechanisms underlying BMD and BMC variation.

The genetic architecture of osteoporosis and fracture risk

Osteoporosis and fracture risk are common complex diseases, caused by an interaction of numerous disease susceptibility genes and environmental factors. With the advances in genomic technologies, large-scale genome-wide association studies (GWAS) have been performed which have broadened our understanding of the genetic architecture and biological mechanisms of complex disease. Currently, more than ~90 loci have been found associated with DXA derived bone mineral density (BMD), over ~500 loci with heel estimated BMD and several others with other less widely available bone parameters such as bone geometry, shape, and microarchitecture. Notably, several of the pathways identified by the GWAS efforts correspond to pathways that are currently targeted for the treatment of osteoporosis. Overall, tremendous progress in the field of the genetics of osteoporosis has been achieved with the discovery of WNT16, EN1, DAAM2, and GPC6 among others. Assessment of the function and biological mechanisms of the remaining genes may further untangle the complex genetic landscape of osteoporosis and fracture risk. With this review we aimed to provide a general overview of the existing GWAS studies on osteoporosis traits and fracture risk.

Advanced Genetic Approaches in Discovery and Characterization of Genes Involved With Osteoporosis in Mouse and Human

Osteoporosis is a complex condition with contributions from, and interactions between, multiple genetic loci and environmental factors. This review summarizes key advances in the application of genetic approaches for the identification of osteoporosis susceptibility genes. Genome-wide linkage analysis (GWLA) is the classical approach for identification of genes that cause monogenic diseases; however, it has shown limited success for complex diseases like osteoporosis. In contrast, genome-wide association studies (GWAS) have successfully identified over 200 osteoporosis susceptibility loci with genome-wide significance, and have provided most of the candidate genes identified to date. Phenome-wide association studies (PheWAS) apply a phenotype-to-genotype approach which can be used to complement GWAS. PheWAS is capable of characterizing the association between osteoporosis and uncommon and rare genetic variants. Another alternative approach, whole genome sequencing (WGS), will enable the discovery of uncommon and rare genetic variants in osteoporosis. Meta-analysis with increasing statistical power can offer greater confidence in gene searching through the analysis of combined results across genetic studies. Recently, new approaches to gene discovery include animal phenotype based models such as the Collaborative Cross and ENU mutagenesis. Site-directed mutagenesis and genome editing tools such as CRISPR/Cas9, TALENs and ZNFs have been used in functional analysis of candidate genes in vitro and in vivo. These resources are revolutionizing the identification of osteoporosis susceptibility genes through the use of genetically defined inbred mouse libraries, which are screened for bone phenotypes that are then correlated with known genetic variation. Identification of osteoporosis-related susceptibility genes by genetic approaches enables further characterization of gene function in animal models, with the ultimate aim being the identification of novel therapeutic targets for osteoporosis.

AKAP11 gene polymorphism is associated with bone mass measured by quantitative ultrasound in young adults

Background: Due to the increased prevalence of osteoporosis and direct health care cost of osteoporosis-related fractures, there is a growing interest in identifying genetic markers associated with osteoporosis phenotypes in order to develop genetic screening strategies. We aimed to analyze the possible associations between calcaneal Quantitative ultrasound (QUS), a valuable screening tool for assessing bone status in clinical practice, and ZBTB40 (rs7524102, rs6426749), SP7 (rs2016266) and AKAP11 (rs9533090) genes. Methods: A cross-sectional study was conducted on 550 healthy individuals of Caucasian ancestry (381 females and 169 males, median age 20.46±2,69). Bone mass was assessed through QUS to determine broadband ultrasound attenuation (BUA, dB/MHz). Single-nucleotide polymorphisms (SNPs) in ZBTB40 (rs7524102, rs6426749), SP7 (rs2016266) and AKAP11 (rs9533090) were selected as genetic markers and genotyped using TaqMan OpenArray® technology. Results: Linear regression analysis revealed that rs7524102 and rs6426749 in ZBTB40, and rs9533090 in AKAP11 were significantly associated with the calcaneal QUS parameter after adjustments for age, sex, weight, height, physical activity, and calcium intake (p=0.038, p=0.012 and p=0.008, respectively). After applying the Bonferroni correction for multiple testing (p=0.012), only the association of rs9533090 in AKAP11 remained significant. Conclusion:AKAP11 gene (rs9533090) influences QUS trait in a population of Caucasian young adults. The rs9533090 SNP may be considered a factor affecting peak bone mass acquisition.

miR-27b-3p Suppressed Osteogenic Differentiation of Maxillary Sinus Membrane Stem Cells by Targeting Sp7

OBJECTIVE

To explore the critical role and function of miRNAs in the regulation of development and physiology of maxillary sinus membrane stem cell (MSMSC) osteogenesis.

METHODS

Microarray analysis was performed to screen the miRNAs expression profiles during the process of MSMSC osteogenic differentiation. Quantitative real-time polymerase chain reaction was applied to verify the miRNAs expression profiles. Gain- and loss-of-function experiments were used to demonstrate that miR-27b-3p inhibited MSMSC osteoblastic differentiation. Bioinformatic analysis was performed to predict the potential target of miR-27b-3p and then demonstrated by luciferase reporter assay and western blot. The negative regulation between miR-27b-3p and Sp7 was further confirmed using mimic and inhibitor of miR-27b-3p in vitro. Xenograft mice model was generated to confirm the relationship between miR-27b-3p and Sp7 using recombinant adenoviruses in vivo.

RESULTS

MiR-27b-3p was downregulated during osteogenic differentiation of MSMSCs. The expression of Sp7, alkaline phosphatase, and osteocalcin decreased when transfected with miR-27b-3p-mimic in MSMSCs after osteogenic differentiation. MiR-27b-3p directly targeted Sp7 and inhibited the MSMSC osteogenesis in vivo.

CONCLUSION

MiR-27b-3p suppressed the osteogenic differentiation of MSMSCs by directly inhibiting Sp7.

Multidimensional Bone Density Phenotyping Reveals New Insights Into Genetic Regulation of the Pediatric Skeleton

Osteoporosis is a complex disease with developmental origins. It is therefore important to understand the genetic contribution to pediatric areal bone mineral density (aBMD). Individual skeletal site phenotyping has been primarily used to identify pediatric aBMD loci. However, this approach is limited because there is a degree of aBMD discordance across skeletal sites. We therefore applied a novel multidimensional phenotyping approach to further understand the genetic regulation of pediatric aBMD. Our sample comprised a prospective, longitudinal cohort of 1293 children of European ancestry (52% female; up to seven annual measurements). Principal components analysis was applied to dual-energy X-ray absorptiometry-derived aBMD Z-scores for total hip, femoral neck, spine, and distal radius to generate multidimensional aBMD phenotypes (ie, principal component scores). We tested the association between a genetic score (percentage of bone lowering alleles at 63 loci) and each principal component. We also performed a genomewide association study (GWAS) using the multiethnic baseline data (n = 1885) to identify novel loci associated with these principal components. The first component (PC1) reflected a concordant phenotypic model of the skeleton (eg, higher loading score = higher BMD across all sites). In contrast, PC2 was discordant for distal radius versus spine and hip aBMD, and PC3 was discordant for spine versus distal radius and hip aBMD. The genetic score was associated with PC1 (beta = -0.05, p = 3.9 × 10^-10 ), but was not associated with discordant PC2 or PC3. Our GWAS discovered variation near CPED1 that associated with PC2 (rs67991850, p = 2.5 × 10^-11 ) and near RAB11FIP5 (rs58649746, p = 4.8 × 10^-9 ) that associated with PC3. In conclusion, an established bone fragility genetic summary score was associated with a concordant skeletal phenotype, but not discordant skeletal phenotypes. Novel associations were observed for the discordant multidimensional skeletal phenotypes that provide new biological insights into the developing skeleton. © 2017 American Society for Bone and Mineral Research.

Novel variant in Sp7/Osx associated with recessive osteogenesis imperfecta with bone fragility and hearing impairment

Osteogenesis imperfecta (OI) is a connective tissue disorder characterized by low bone density and recurrent fractures with a wide genotypic and phenotypic spectrum. Common features include short stature, opalescent teeth, blue sclerae and hearing impairment. The majority (>90%) of patients with OI have autosomal dominant variants in COL1A1/COL1A2, which lead to defects in type 1 collagen. More recently, numerous recessive variants involving other genes have also been identified. Sp7/Osx gene, is a protein coding gene that encodes a zinc finger transcription factor, osterix, which is a member of the Sp subfamily of sequence-specific DNA-binding proteins. Osterix is expressed primarily by osteoblasts and has been shown to be vital for bone formation and bone homeostasis by promoting osteoblast differentiation and maturation. In animal models, Sp7/Osx has also been shown to regulate biomineralization of otoliths, calcium carbonate structures found in the inner ear of vertebrates. Until recently, only one report of a boy with an Sp7/Osx pathogenic variant presenting with bone fragility, limb deformities and normal hearing has been described in the literature. We have identified a novel Sp7/Osx variant in another sibship that presented with osteoporosis, low-trauma fractures and short stature. Progressive moderate-to-severe and severe-to-profound hearing loss secondary to otospongiosis and poor mineralization of ossicles and petrous temporal bone was also noted in two of the siblings. A homozygous pathogenic variant in exon 2 of the Sp7/Osx gene was found in all affected relatives; c.946C>T (p.Arg316Cys). Bone biopsies in the proband and his male sibling revealed significant cortical porosity and high trabecular bone turnover. This is the second report to describe children with OI associated with an Sp7/Osx variant. However, it is the first to describe the bone histomorphometry associated with this disorder and identifies a significant hearing loss as a potential feature in this OI subtype. Early audiology screening in these children is therefore warranted.

Genome-wide search for higher order epistasis as modifiers of treatment effects on bone mineral density in childhood cancer survivors

Single-nucleotide polymorphisms (SNPs) contributing to interactions between regulatory elements that modulate gene transcription may explain some of the uncharacterized variation for complex traits. We explored this hypothesis among 856 adult survivors of pediatric cancer exposed to curative treatments that adversely affect bone mineral density (BMD). To restrict our search to interactions among SNPs in regulatory elements, our analysis considered 75523 SNPs mapped to putative promoter or enhancer regions. In anticipation that power to detect higher order epistasis would be low using an exhaustive search and a Bonferroni-corrected threshold for genome-wide significance (e.g., P < 5.6 × 10^-14), a novel non-exhaustive statistical algorithm was implemented to detect chromosome-wide three-way regulatory interactions. We used a permutation-based evaluation statistic to identify candidate SNP interactions with stronger associations with BMD than expected. Of the six regulatory 3-SNP interactions identified as candidate interactions (P < 3.5 × 10^-11) among cancer survivors exposed to treatments, five were replicated in an independent cohort of survivors (N = 1428) as modifiers of treatment effects on BMD (P < 0.05). Analyses with publicly available bioinformatics data revealed that SNPs contributing to replicated interactions were enriched for gene expressions (P = 3.6 × 10^-4) and enhancer states (P < 0.05) in cells relevant for bone biology. For each replicated interaction, implicated SNPs were within or directly adjacent to 100-kb windows of genomic regions that plausibly physically interact in lymphoblastoid cells. Our study demonstrates the utility of a hypothesis-driven approach in revealing epistasis associated with complex traits.

p53 inhibits SP7/Osterix activity in the transcriptional program of osteoblast differentiation

Osteoblast differentiation is achieved by activating a transcriptional network in which Dlx5, Runx2 and Osx/SP7 have fundamental roles. The tumour suppressor p53 exerts a repressive effect on bone development and remodelling through an unknown mechanism that inhibits the osteoblast differentiation programme. Here we report a physical and functional interaction between Osx and p53 gene products. Physical interaction was found between overexpressed proteins and involved a region adjacent to the OSX zinc fingers and the DNA-binding domain of p53. This interaction results in a p53-mediated repression of OSX transcriptional activity leading to a downregulation of the osteogenic programme. Moreover, we show that p53 is also able to repress key osteoblastic genes in Runx2-deficient osteoblasts. The ability of p53 to suppress osteogenesis is independent of its DNA recognition ability but requires a native conformation of p53, as a conformational missense mutant failed to inhibit OSX. Our data further demonstrates that p53 inhibits OSX binding to their responsive Sp1/GC-rich sites in the promoters of their osteogenic target genes, such as IBSP or COL1A1. Moreover, p53 interaction to OSX sequesters OSX from binding to DLX5. This competition blocks the ability of OSX to act as a cofactor of DLX5 to activate homeodomain-containing promoters. Altogether, our data support a model wherein p53 represses OSX-DNA binding and DLX5-OSX interaction, and thereby deregulates the osteogenic transcriptional network. This mechanism might have relevant roles in bone pathologies associated to osteosarcomas and ageing.

Tet-Mediated DNA Demethylation Is Required for SWI/SNF-Dependent Chromatin Remodeling and Histone-Modifying Activities That Trigger Expression of the Sp7 Osteoblast Master Gene during Mesenchymal Lineage Commitment

Here we assess histone modification, chromatin remodeling, and DNA methylation processes that coordinately control the expression of the bone master transcription factor Sp7 (osterix) during mesenchymal lineage commitment in mammalian cells. We find that Sp7 gene silencing is mediated by DNA methyltransferase1/3 (DNMT1/3)-, histone deacetylase 1/2/4 (HDAC1/2/4)-, Setdb1/Suv39h1-, and Ezh1/2-containing complexes. In contrast, Sp7 gene activation involves changes in histone modifications, accompanied by decreased nucleosome enrichment and DNA demethylation mediated by SWI/SNF- and Tet1/Tet2-containing complexes, respectively. Inhibition of DNA methylation triggers changes in the histone modification profile and chromatin-remodeling events leading to Sp7 gene expression. Tet1/Tet2 silencing prevents Sp7 expression during osteoblast differentiation as it impairs DNA demethylation and alters the recruitment of histone methylase (COMPASS)-, histone demethylase (Jmjd2a/Jmjd3)-, and SWI/SNF-containing complexes to the Sp7 promoter. The dissection of these interconnected epigenetic mechanisms that govern Sp7 gene activation reveals a hierarchical process where regulatory components mediating DNA demethylation play a leading role.

Genetic Dissection of Trabecular Bone Structure with Mouse Intersubspecific Consomic Strains

Trabecular bone structure has an important influence on bone strength, but little is known about its genetic regulation. To elucidate the genetic factor(s) regulating trabecular bone structure, we compared the trabecular bone structures of two genetically remote mouse strains, C57BL/6J and Japanese wild mouse-derived MSM/Ms. Phenotyping by X-ray micro-CT revealed that MSM/Ms has structurally more fragile trabecular bone than C57BL/6J. Toward identification of genetic determinants for the difference in fragility of trabecular bone between the two mouse strains, we employed phenotype screening of consomic mouse strains in which each C57BL/6J chromosome is substituted by its counterpart from MSM/Ms. The results showed that many chromosomes affect trabecular bone structure, and that the consomic strain B6-Chr15^MSM, carrying MSM/Ms-derived chromosome 15 (Chr15), has the lowest values for the parameters BV/TV, Tb.N, and Conn.D, and the highest values for the parameters Tb.Sp and SMI. Subsequent phenotyping of subconsomic strains for Chr15 mapped four novel trabecular bone structure-related QTL (Tbsq1-4) on mouse Chr15. These results collectively indicate that genetic regulation of trabecular bone structure is highly complex, and that even in the single Chr15, the combined action of the four Tbsqs controls the fragility of trabecular bone. Given that Tbsq4 is syntenic to human Chr 12q12-13.3, where several bone-related SNPs are assigned, further study of Tbsq4 should facilitate our understanding of the genetic regulation of bone formation in humans.

Genetics of osteoporosis: searching for candidate genes for bone fragility

The pathogenesis of osteoporosis, a common disease with great morbidity and mortality, comprises environmental and genetic factors. As with other complex disorders, the genetic basis of osteoporosis has been difficult to identify. Nevertheless, several approaches have been undertaken in the past decades in order to identify candidate genes for bone fragility, including the study of rare monogenic syndromes with striking bone phenotypes (e.g. osteogenesis imperfecta and osteopetroses), the analysis of individuals or families with extreme osteoporotic phenotypes (e.g. idiopathic juvenile and pregnancy-related osteoporosis), and, chiefly, genome-wide association studies (GWAS) in large populations. Altogether, these efforts have greatly increased the understanding of molecular mechanisms behind bone remodelling, which has rapidly translated into the development of novel therapeutic strategies, exemplified by the tales of cathepsin K (CTSK) and sclerostin (SOST). Additional biological evidence of involvement in bone physiology still lacks for several candidate genes arisen from GWAS, opening an opportunity for the discovery of new mechanisms regulating bone strength, particularly with the advent of high-throughput genomic technologies. In this review, candidate genes for bone fragility will be presented in comprehensive tables and discussed with regard to how their association with osteoporosis emerged, highlighting key players such as LRP5, WNT1 and PLS3. Current limitations in our understanding of the genetic contribution to osteoporosis, such as yet unidentified genetic modifiers, may be overcome in the near future with better genotypic and phenotypic characterisation of large populations and the detailed study of candidate genes in informative individuals with marked phenotype.

Regulatory element-based prediction identifies new susceptibility regulatory variants for osteoporosis

Despite genome-wide association studies (GWASs) have identified many susceptibility genes for osteoporosis, it still leaves a large part of missing heritability to be discovered. Integrating regulatory information and GWASs could offer new insights into the biological link between the susceptibility SNPs and osteoporosis. We generated five machine learning classifiers with osteoporosis-associated variants and regulatory features data. We gained the optimal classifier and predicted genome-wide SNPs to discover susceptibility regulatory variants. We further utilized Genetic Factors for Osteoporosis Consortium (GEFOS) and three in-house GWASs samples to validate the associations for predicted positive SNPs. The random forest classifier performed best among all machine learning methods with the F1 score of 0.8871. Using the optimized model, we predicted 37,584 candidate SNPs for osteoporosis. According to the meta-analysis results, a list of regulatory variants was significantly associated with osteoporosis after multiple testing corrections and contributed to the expression of known osteoporosis-associated protein-coding genes. In summary, combining GWASs and regulatory elements through machine learning could provide additional information for understanding the mechanism of osteoporosis. The regulatory variants we predicted will provide novel targets for etiology research and treatment of osteoporosis.

A Genomewide Association Study Identifies Two Sex-Specific Loci, at SPTB and IZUMO3, Influencing Pediatric Bone Mineral Density at Multiple Skeletal Sites

Failure to achieve optimal bone mineral accretion during childhood and adolescence results in subsequent suboptimal peak bone mass, contributing to osteoporosis risk later in life. To identify novel genetic factors that influence pediatric bone mass at discrete skeletal sites, we performed a sex-stratified genomewide association study of areal bone mineral density (BMD) measured by dual-energy X-ray absorptiometry at the 1/3 distal radius, spine, total hip, and femoral neck in a cohort of 933 healthy European American children. We took forward signals with p < 5 × 10-5 and minor allele frequency (MAF) >5% into an independent cohort of 486 European American children in search of replication. In doing so, we identified five loci that achieved genome wide significance in the combined cohorts (nearest genes: CPED1, IZUMO3, RBFOX1, SPBT, and TBPL2), of which the last four were novel and two were sex-specific (SPTB in females and IZUMO3 in males), with all of them yielding associations that were particularly strong at a specific skeletal site. Annotation of potential regulatory function, expression quantitative trait loci (eQTL) effects and pathway analyses identified several potential target genes at these associated loci. This study highlights the importance of sex-stratified analyses at discrete skeletal sites during the critical period of bone accrual, and identifies novel loci for further functional follow-up to pinpoint key genes and better understand the regulation of bone development in children. © 2017 American Society for Bone and Mineral Research.

Genetic variation and bone mineral density in long-term adult survivors of childhood cancer

PURPOSE

Despite similarities in upfront treatment of childhood cancer, not every adult survivor of childhood cancer (CCS) has an impaired bone mineral density (BMD). No data are available on the role of genetic variation on impairment of BMD in CCS.

METHODS

This cross-sectional single-center cohort study included 334 adult CCSs (median follow-up time after cessation of treatment: 15 years; median age at follow-up: 26 years). Total body BMD (BMD_TB ) and lumbar spine BMD (BMD_LS ) were measured by dual x-ray absorptiometry. We selected 12 candidate single-nucleotide polymorphisms (SNPs) in 11 genes (COL1A1, TNFSF11, TNFRSF11, TNRFSA11B, VDR, ESR1, WLS, LRP5, MTHFR, MTRR, IL-6).

RESULTS

Multivariate analyses revealed that lower BMD was associated with lower weight and height at follow-up, male sex, and previously administered radiotherapy. Survivors with the homozygous minor allele (GG) genotype of rs2504063 (ESR1: estrogen receptor type 1) had a lower BMD_TB values (-1.16 vs. -0.82; P = 0.01) than those with the AG/AA genotype; however, BMD_LS was not different. Carriers of two minor alleles (GG) of rs599083 (LRP5: low-density lipoprotein receptor) revealed lower BMD_TB (-1.20 vs. -0.78; P = 0.02) and lower BMD_LS (-0.95 vs. -0.46; P = 0.01) values than those with the TT/TG genotype.

CONCLUSION

CCSs who are carriers of candidate SNPs in the ESR1 or LRP5 genes seem to have an impaired bone mass at an early adult age. Information on genetic variation, in addition to patient- and treatment-related factors, may be helpful in identifying survivors who are at risk for low bone density after childhood cancer treatment.

From the Cover: Embryonic Exposure to TCDD Impacts Osteogenesis of the Axial Skeleton in Japanese medaka, Oryzias latipes

Recent studies from mammalian, fish, and in vitro models have identified bone and cartilage development as sensitive targets for dioxins and other aryl hydrocarbon receptor ligands. In this study, we assess how embryonic 2,3,7,8-tetrachlorochlorodibenzo-p-dioxin (TCDD) exposure impacts axial osteogenesis in Japanese medaka (Oryzias latipes), a vertebrate model of human bone development. Embryos from inbred wild-type Orange-red Hd-dR and 3 transgenic medaka lines (twist:EGFP, osx/sp7:mCherry, col10a1:nlGFP) were exposed to 0.15 nM and 0.3 nM TCDD and reared until 20 dpf. Individuals were stained for mineralized bone and imaged using confocal microscopy to assess skeletal alterations in medial vertebrae in combination with a qualitative spatial analysis of osteoblast and osteoblast progenitor cell populations. Exposure to TCDD resulted in an overall attenuation of vertebral ossification characterized by truncated centra, and reduced neural and hemal arch lengths. Effects on mineralization were consistent with modifications in cell number and cell localization of transgene-labeled osteoblast and osteoblast progenitor cells. Endogenous expression of osteogenic regulators runt-related transcription factor 2 (runx2) and osterix (osx/sp7), and extracellular matrix genes osteopontin (spp1), collagen type I alpha I (col1), collagen type X alpha I (col10a1), and osteocalcin (bglap/osc) was significantly diminished at 20 dpf following TCDD exposure as compared with controls. Through global transcriptomic analysis more than 590 differentially expressed genes were identified and mapped to select pathological states including inflammatory disease, connective tissue disorders, and skeletal and muscular disorders. Taken together, results from this study suggest that TCDD exposure inhibits axial bone formation through dysregulation of osteoblast differentiation. This approach highlights the advantages and sensitivity of using small fish models to investigate how xenobiotic exposure may impact skeletal development.

Osterix and RUNX2 are Transcriptional Regulators of Sclerostin in Human Bone

Sclerostin, encoded by the SOST gene, works as an inhibitor of the Wnt pathway and therefore is an important regulator of bone homeostasis. Due to its potent action as an inhibitor of bone formation, blocking sclerostin activity is the purpose of recently developed anti-osteoporotic treatments. Two bone-specific transcription factors, RUNX2 and OSX, have been shown to interact and co-ordinately regulate the expression of bone-specific genes. Although it has been recently shown that sclerostin is targeted by OSX in mice, there is currently no information of whether this is also the case in human cells. We have identified SP-protein family and AML1 consensus binding sequences at the human SOST promoter and have shown that OSX, together with RUNX2, binds to a specific region close to the transcription start site. Furthermore, we show that OSX and RUNX2 activate SOST expression in a co-ordinated manner in vitro and that SOST expression levels show a significant positive correlation with OSX/RUNX2 expression levels in human bone. We also confirmed previous results showing an association of several SOST/RUNX2 polymorphisms with bone mineral density.

Genetics of pediatric bone strength

Osteoporosis is one of the most common chronic forms of disability in postmenopausal women and represents a major health burden around the world. Bone fragility is affected by bone mineral density (BMD), and, one of the most important factors in preventing osteoporosis is optimizing peak bone mass, which is achieved during growth in childhood and adolescence. BMD is a complex trait resulting from environmental and genetic factors. Genome-wide association studies have discovered robust genetic signals influencing BMD in adults, and similar studies have also been conducted to investigate the genetics of BMD in the pediatric setting. These latter studies have revealed that many adult osteoporosis-related loci also regulate BMD during growth. These investigations have the potential to profoundly impact public health and will allow for the eventual development of effective interventions for the prevention of osteoporosis.

Osterix acetylation at K307 and K312 enhances its transcriptional activity and is required for osteoblast differentiation

Osterix (Osx) is an essential transcription factor involved in osteoblast differentiation and bone formation. The precise molecular mechanisms of the regulation of Osx expression are not fully understood. In the present study, we found that in cells, both endogenous and exogenous Osx protein increased after treatment with histone deacetylase inhibitors Trichostatin A and hydroxamic acid. Meanwhile, the results of immunoprecipitation indicated that Osx was an acetylated protein and that the CREB binding protein (CBP), and less efficiently p300, acetylated Osx. The interaction and colocalization of CBP and Osx were demonstrated by Co-immunoprecipitation and immunofluorescence, respectively. In addition, K307 and K312 were identified as the acetylated sites of Osx. By contrast, HDAC4, a histone deacetylase (HDAC), was observed to interact and co-localize with Osx. HDAC4 was demonstrated to mediate the deacetylation of Osx. Moreover, we found that acetylation of Osx enhanced its stability, DNA binding ability and transcriptional activity. Finally, we demonstrated that acetylation of Osx was required for the osteogenic differentiation of C2C12 cells. Taken together, our results provide evidence that CBP-mediated acetylation and HDAC4-mediated deacetylation have critical roles in the modification of Osx, and thus are important in osteoblast differentiation.

Transcriptional regulation of bone sialoprotein gene expression by Osx

Osteoporosis is the most common metabolic bone disease characterized by decreased bone mass, decreased bone strength, and increased risk of fracture. It is due to unbalance between bone formation and bone resorption. Bone formation is a complex process which involves the differentiation of mesenchymal stem cells to osteoblasts. Osteoblasts produce a characteristic extracellular collagenous matrix that subsequently becomes mineralized. Osterix (Osx) is an osteoblast-specific transcription factor required for osteoblast differentiation. Bone sialoprotein (Bsp) is a member of the SIBLING gene family. Expression of Bsp correlates with the differentiation of osteoblasts and the onset of mineralization. Our preliminary data showed that Bsp was abolished in Osx-null mice; however, the detailed mechanism of Osx regulation on Bsp is not fully understood. In this study, regulation of Bsp expression by Osx was further characterized. It was shown that overexpression of Osx led to Bsp upregulation. Inhibition of Osx by small interfering RNA resulted in Bsp downregulation in osteoblast. Transfection assay demonstrated that Osx was able to activate Bsp promoter reporter in a dose-dependent manner. To define minimal region of Bsp promoter activated by Osx, a series of deletion mutants of Bsp promoter were generated, and the minimal region was narrowed down to the proximal 100 bp. Point-mutagenesis studies showed that one GC-rich site was required for Bsp promoter activation by Osx. ChIP assays demonstrated that endogenous Osx associated with native Bsp promoter in primary osteoblasts. Our observations provide evidence that Osx targets Bsp expression directly.

The case for genome-wide association studies of bone acquisition in paediatric and adolescent populations

Peak bone mass, the maximum amount of bone accrued at the end of the growth period, is an important predictor of future risk of osteoporosis and fracture. Hence, the contribution of genetic factors influencing bone accrual is of considerable interest to the osteoporosis research community. In this article, we review evidence that genetic factors play an important role in bone growth, describe the genetic loci implicated so far and briefly discuss lessons learned from the application of genome-wide association studies. Moreover, we attempt to make the case for genetic investigations of bone mineral density in paediatric and young adult populations, describing their potential to increase our knowledge of the process of bone metabolism throughout the life course, and in turn, identify novel targets for the pharmacological treatment of osteoporosis.

The genetics of bone mass and susceptibility to bone diseases

Osteoporosis is characterized by low bone mass and an increased risk of fracture. Genetic factors, environmental factors and gene-environment interactions all contribute to a person's lifetime risk of developing an osteoporotic fracture. This Review summarizes key advances in understanding of the genetics of bone traits and their role in osteoporosis. Candidate-gene approaches dominated this field 20 years ago, but clinical and preclinical genetic studies published in the past 5 years generally utilize more-sophisticated and better-powered genome-wide association studies (GWAS). High-throughput DNA sequencing, large genomic databases and improved methods of data analysis have greatly accelerated the gene-discovery process. Linkage analyses of single-gene traits that segregate in families with extreme phenotypes have led to the elucidation of critical pathways controlling bone mass. For example, components of the Wnt-β-catenin signalling pathway have been validated (in both GWAS and functional studies) as contributing to various bone phenotypes. These notable advances in gene discovery suggest that the next decade will witness cataloguing of the hundreds of genes that influence bone mass and osteoporosis, which in turn will provide a roadmap for the development of new drugs that target diseases of low bone mass, including osteoporosis.

The National Osteoporosis Foundation's position statement on peak bone mass development and lifestyle factors: a systematic review and implementation recommendations

Lifestyle choices influence 20-40 % of adult peak bone mass. Therefore, optimization of lifestyle factors known to influence peak bone mass and strength is an important strategy aimed at reducing risk of osteoporosis or low bone mass later in life. The National Osteoporosis Foundation has issued this scientific statement to provide evidence-based guidance and a national implementation strategy for the purpose of helping individuals achieve maximal peak bone mass early in life. In this scientific statement, we (1) report the results of an evidence-based review of the literature since 2000 on factors that influence achieving the full genetic potential for skeletal mass; (2) recommend lifestyle choices that promote maximal bone health throughout the lifespan; (3) outline a research agenda to address current gaps; and (4) identify implementation strategies. We conducted a systematic review of the role of individual nutrients, food patterns, special issues, contraceptives, and physical activity on bone mass and strength development in youth. An evidence grading system was applied to describe the strength of available evidence on these individual modifiable lifestyle factors that may (or may not) influence the development of peak bone mass (Table 1). A summary of the grades for each of these factors is given below. We describe the underpinning biology of these relationships as well as other factors for which a systematic review approach was not possible. Articles published since 2000, all of which followed the report by Heaney et al. [1] published in that year, were considered for this scientific statement. This current review is a systematic update of the previous review conducted by the National Osteoporosis Foundation [1]. [Table: see text] Considering the evidence-based literature review, we recommend lifestyle choices that promote maximal bone health from childhood through young to late adolescence and outline a research agenda to address current gaps in knowledge. The best evidence (grade A) is available for positive effects of calcium intake and physical activity, especially during the late childhood and peripubertal years-a critical period for bone accretion. Good evidence is also available for a role of vitamin D and dairy consumption and a detriment of DMPA injections. However, more rigorous trial data on many other lifestyle choices are needed and this need is outlined in our research agenda. Implementation strategies for lifestyle modifications to promote development of peak bone mass and strength within one's genetic potential require a multisectored (i.e., family, schools, healthcare systems) approach.

Osterix/Sp7 limits cranial bone initiation sites and is required for formation of sutures

During growth, individual skull bones overlap at sutures, where osteoblast differentiation and bone deposition occur. Mutations causing skull malformations have revealed some required genes, but many aspects of suture regulation remain poorly understood. We describe a zebrafish mutation in osterix/sp7, which causes a generalized delay in osteoblast maturation. While most of the skeleton is patterned normally, mutants have specific defects in the anterior skull and upper jaw, and the top of the skull comprises a random mosaic of bones derived from individual initiation sites. Osteoblasts at the edges of the bones are highly proliferative and fail to differentiate, consistent with global changes in gene expression. We propose that signals from the bone itself are required for orderly recruitment of precursor cells and growth along the edges. The delay in bone maturation caused by loss of Sp7 leads to unregulated bone formation, revealing a new mechanism for patterning the skull and sutures.

Polymorphisms of the WNT16 gene are associated with the heel ultrasound parameter in young adults

SUMMARY

Bone mineral content is influenced by genetic factors. We investigated the role of WNT16 in bone properties determined using quantitative ultrasound (QUS) on young adults. Three WNT16 genetic markers (rs2908007, rs2908004, and rs2707466) were found to have a significant association with the broadband ultrasound attenuation (BUA) measurement, suggesting that WNT16 influences bone mass in young adults.

INTRODUCTION

The aim of this study was to investigate whether genetic markers on the WNT16 gene are associated with bone mass, as assessed using QUS in a population of healthy young Spanish adults.

METHODS

A cross-sectional study was conducted on 575 individuals (mean age 20.41 ± 2.69). Bone quality was assessed using BUA measurements (dB/MHz) on the right calcaneus. Six single nucleotide polymorphisms (SNPs) (rs2908007, rs2908004, rs3801387, rs3801385, rs2707466, and rs2536184) covering the WNT16 gene were selected as genetic markers and genotyped to test their association with BUA variations.

RESULTS

The rs2908007, rs2908004, and rs2707466 SNPs were found to have a significant association with BUA (p = 0.004, p = 0.001, and p = 0.004, respectively).

CONCLUSION

We demonstrate for the first time that WNT16 genetic polymorphisms influence QUS traits in a population of young adults. This finding suggests that WNT16 might be an important genetic factor in determining peak bone mass acquisition.

β-catenin signaling induces the osteoblastogenic differentiation of human pre-osteoblastic and bone marrow stromal cells mainly through the upregulation of osterix expression

Both β-catenin (β-cat) and osterix (OSX) are known to be essential for embryonic and postnatal osteoblast differentiation and bone growth. In the present study, we explored the crosstalk between β-cat signaling and OSX, and assessed its effect on the osteoblastogenic differentiation of human pre-osteoblastic cells (MG-63) and bone marrow stromal cells (HS-27A). In the HS-27A and MG-63 cells, the selective β-cat signaling inhibitor, CCT031374, and the stable overexpression of a constitutively active β-cat mutant respectively decreased and increased the cytoplasmic/soluble β-cat levels, and respectively decreased and increased TOPflash reporter activity, the mRNA levels of β-cat signaling target genes c-Myc and c-Jun, as well as the mRNA and protein expression levels of OSX. Mutational analyses and electrophoretic mobility shift assays revealed that the increased binding activity of c-Jun at a putative c-Jun binding site (-858/-852 relative to the translation start codon, which was designated as +1) in the human OSX gene promoter was required for teh β-cat signaling-induced expression of OSX in the HS-27A and MG-63 cells. During osteoblastogenic culture, stimulating β-cat signaling activity by the stable overexpression of the active β-cat mutant markedly increased alkaline phosphatase (ALP) activity and calcium deposition in the HS-27A and MG-63 cells, which was abolished by knocking down OSX using shRNA. On the other hand, the inhibition of β-cat signaling activity with CCT031374 decreased the ALP activity and calcium deposition, which was completely reversed by the overexpression of OSX. On the whole, the findings of our study suggest that β-cat signaling upregulates the expression of OSX in human pre-osteoblastic and bone marrow stromal cells by trans-activating the OSX gene promoter mainly through increased c-Jun binding at a putative c-Jun binding site; OSX largely mediates β-cat signaling-induced osteoblastogenic differentiation. The present study provides new insight into the molecular mechanisms underlying osteoblast differentiation.

Contributions of Caucasian-associated bone mass loci to the variation in bone mineral density in Vietnamese population

BACKGROUND AND AIM

Bone mineral density (BMD) is under strong genetic regulation, but it is not clear which genes are involved in the regulation, particularly in Asian populations. This study sought to determine the association between 29 genes discovered by Caucasian-based genome-wide association studies and BMD in a Vietnamese population.

METHODS

The study involved 564 Vietnamese men and women aged 18 years and over (average age: 47 years) who were randomly sampled from the Ho Chi Minh City. BMD at the femoral neck, lumbar spine, total hip and whole body was measured by DXA (Hologic QDR4500, Bedford, MA, USA). Thirty-two single nucleotide polymorphisms (SNPs) in 29 genes were genotyped using Sequenom MassARRAY technology. The magnitude of association between SNPs and BMD was analyzed by the linear regression model. The Bayesian model average method was used to identify SNPs that are independently associated with BMD.

RESULTS

The distribution of genotypes of all, but two, SNPs was consistent with the Hardy-Weinberg equilibrium law. After adjusting for age, gender and weight, 3 SNPs were associated with BMD: rs2016266 (SP7 gene), rs7543680 (ZBTB40 gene), and rs1373004 (MBL2/DKK1 gene). Among the three genetic variants, the SNP rs2016266 had the strongest association, with each minor allele being associated with ~0.02 g/cm(2) increase in BMD at the femoral neck and whole body. Each of these genetic variant explained about 0.2 to 1.1% variance of BMD. All other SNPs were not significantly associated with BMD.

CONCLUSION

These results suggest that genetic variants in the SP7, ZBTB40 and MBL2/DKK1 genes are associated with BMD in the Vietnamese population, and that the effect of these genes on BMD is likely to be modest.

A trans-ethnic genome-wide association study identifies gender-specific loci influencing pediatric aBMD and BMC at the distal radius

Childhood fractures are common, with the forearm being the most common site. Genome-wide association studies (GWAS) have identified more than 60 loci associated with bone mineral density (BMD) in adults but less is known about genetic influences specific to bone in childhood. To identify novel genetic factors that influence pediatric bone strength at a common site for childhood fractures, we performed a sex-stratified trans-ethnic genome-wide association study of areal BMD (aBMD) and bone mineral content (BMC) Z-scores measured by dual energy X-ray absorptiometry at the one-third distal radius, in a cohort of 1399 children without clinical abnormalities in bone health. We tested signals with P < 5 × 10(-6) for replication in an independent, same-age cohort of 486 Caucasian children. Two loci yielded a genome-wide significant combined P-value: rs7797976 within CPED1 in females [P = 2.4 × 10(-11), β =- 0.30 standard deviations (SD) per T allele; aBMD-Z] and rs7035284 at 9p21.3 in males (P = 1.2 × 10(-8), β = 0.28 SD per G allele; BMC-Z). Signals at the CPED1-WNT16-FAM3C locus have been previously associated with BMD at other skeletal sites in adults and children. Our result at the distal radius underscores the importance of this locus at multiple skeletal sites. The 9p21.3 locus is within a gene desert, with the nearest gene flanking each side being MIR31HG and MTAP, neither of which has been implicated in BMD or BMC previously. These findings suggest that genetic determinants of childhood bone accretion at the radius, a skeletal site that is primarily cortical bone, exist and also differ by sex.

Genetic regulation of bone metabolism in the chicken: similarities and differences to Mammalian systems

Birds have a unique bone physiology, due to the demands placed on them through egg production. In particular their medullary bone serves as a source of calcium for eggshell production during lay and undergoes continuous and rapid remodelling. We take advantage of the fact that bone traits have diverged massively during chicken domestication to map the genetic basis of bone metabolism in the chicken. We performed a quantitative trait locus (QTL) and expression QTL (eQTL) mapping study in an advanced intercross based on Red Junglefowl (the wild progenitor of the modern domestic chicken) and White Leghorn chickens. We measured femoral bone traits in 456 chickens by peripheral computerised tomography and femoral gene expression in a subset of 125 females from the cross with microarrays. This resulted in 25 loci for female bone traits, 26 loci for male bone traits and 6318 local eQTL loci. We then overlapped bone and gene expression loci, before checking for an association between gene expression and trait values to identify candidate quantitative trait genes for bone traits. A handful of our candidates have been previously associated with bone traits in mice, but our results also implicate unexpected and largely unknown genes in bone metabolism. In summary, by utilising the unique bone metabolism of an avian species, we have identified a number of candidate genes affecting bone allocation and metabolism. These findings can have ramifications not only for the understanding of bone metabolism genetics in general, but could also be used as a potential model for osteoporosis as well as revealing new aspects of vertebrate bone regulation or features that distinguish avian and mammalian bone.

In this work we seek to further the understanding of bone genetics by mapping bone traits and gene expression in the chicken. Bone in female birds is special due to egg production. In this study, we combine the genetic mapping of bone traits with bone gene expression to find candidate quantitative trait genes that explain the differences between wild and domestic chickens in terms of bone production. The concept of combining genetic mapping and gene expression mapping is not new, and has already been successful in isolating bone-related genes in mammals, however this is the first time it has been applied to an avian system with such unique bone modelling processes. We aim to reveal new molecular mechanisms of bone regulation, and many of the candidates we find are new, highlighting the potential this technique has to identify the potential differences between avian and mammalian bone biology.

Limited clinical utility of a genetic risk score for the prediction of fracture risk in elderly subjects

It is important to identify the patients at highest risk of fractures. A recent large-scale meta-analysis identified 63 autosomal single-nucleotide polymorphisms (SNPs) associated with bone mineral density (BMD), of which 16 were also associated with fracture risk. Based on these findings, two genetic risk scores (GRS63 and GRS16) were developed. Our aim was to determine the clinical usefulness of these GRSs for the prediction of BMD, BMD change, and fracture risk in elderly subjects. We studied two male (Osteoporotic Fractures in Men Study [MrOS] US, MrOS Sweden) and one female (Study of Osteoporotic Fractures [SOF]) large prospective cohorts of older subjects, looking at BMD, BMD change, and radiographically and/or medically confirmed incident fractures (8067 subjects, 2185 incident nonvertebral or vertebral fractures). GRS63 was associated with BMD (≅3% of the variation explained) but not with BMD change. Both GRS63 and GRS16 were associated with fractures. After BMD adjustment, the effect sizes for these associations were substantially reduced. Similar results were found using an unweighted GRS63 and an unweighted GRS16 compared with those found using the corresponding weighted risk scores. Only minor improvements in C-statistics (AUC) for fractures were found when the GRSs were added to a base model (age, weight, and height), and no significant improvements in C-statistics were found when they were added to a model further adjusted for BMD. Net reclassification improvements with the addition of the GRSs to a base model were modest and substantially attenuated in BMD-adjusted models. GRS63 is associated with BMD, but not BMD change, suggesting that the genetic determinants of BMD differ from those of BMD change. When BMD is known, the clinical utility of the two GRSs for fracture prediction is limited in elderly subjects.

Mitogen-activated protein kinase (MAPK)-regulated interactions between Osterix and Runx2 are critical for the transcriptional osteogenic program

The transcription factors Runx2 and Osx (Osterix) are required for osteoblast differentiation and bone formation. Runx2 expression occurs at early stages of osteochondroprogenitor determination, followed by Osx induction during osteoblast maturation. We demonstrate that coexpression of Osx and Runx2 leads to cooperative induction of expression of the osteogenic genes Col1a1, Fmod, and Ibsp. Functional interaction of Osx and Runx2 in the regulation of these promoters is mediated by enhancer regions with adjacent Sp1 and Runx2 DNA-binding sites. These enhancers allow formation of a cooperative transcriptional complex, mediated by the binding of Osx and Runx2 to their specific DNA promoter sequences and by the protein-protein interactions between them. We also identified the domains involved in the interaction between Osx and Runx2. These regions contain the amino acids in Osx and Runx2 known to be phosphorylated by p38 and ERK MAPKs. Inhibition of p38 and ERK kinase activities or mutation of their known phosphorylation sites in Osx or Runx2 strongly disrupts their physical interaction and cooperative transcriptional effects. Altogether, our results provide a molecular description of a mechanism for Osx and Runx2 transcriptional cooperation that is subject to further regulation by MAPK-activating signals during osteogenesis.

Phenotypic dissection of bone mineral density reveals skeletal site specificity and facilitates the identification of novel loci in the genetic regulation of bone mass attainment

Heritability of bone mineral density (BMD) varies across skeletal sites, reflecting different relative contributions of genetic and environmental influences. To quantify the degree to which common genetic variants tag and environmental factors influence BMD, at different sites, we estimated the genetic (rg) and residual (re) correlations between BMD measured at the upper limbs (UL-BMD), lower limbs (LL-BMD) and skull (SK-BMD), using total-body DXA scans of ∼ 4,890 participants recruited by the Avon Longitudinal Study of Parents and their Children (ALSPAC). Point estimates of rg indicated that appendicular sites have a greater proportion of shared genetic architecture (LL-/UL-BMD rg = 0.78) between them, than with the skull (UL-/SK-BMD rg = 0.58 and LL-/SK-BMD rg = 0.43). Likewise, the residual correlation between BMD at appendicular sites (r(e) = 0.55) was higher than the residual correlation between SK-BMD and BMD at appendicular sites (r(e) = 0.20-0.24). To explore the basis for the observed differences in rg and re, genome-wide association meta-analyses were performed (n ∼ 9,395), combining data from ALSPAC and the Generation R Study identifying 15 independent signals from 13 loci associated at genome-wide significant level across different skeletal regions. Results suggested that previously identified BMD-associated variants may exert site-specific effects (i.e. differ in the strength of their association and magnitude of effect across different skeletal sites). In particular, variants at CPED1 exerted a larger influence on SK-BMD and UL-BMD when compared to LL-BMD (P = 2.01 × 10(-37)), whilst variants at WNT16 influenced UL-BMD to a greater degree when compared to SK- and LL-BMD (P = 2.31 × 10(-14)). In addition, we report a novel association between RIN3 (previously associated with Paget's disease) and LL-BMD (rs754388: β = 0.13, SE = 0.02, P = 1.4 × 10(-10)). Our results suggest that BMD at different skeletal sites is under a mixture of shared and specific genetic and environmental influences. Allowing for these differences by performing genome-wide association at different skeletal sites may help uncover new genetic influences on BMD.