Genetic variation at the low-density lipoprotein receptor-related protein 5 (LRP5) locus modulates Wnt signaling and the relationship of physical activity with bone mineral density in men

Effects of Mechanical Stress Stimulation on Function and Expression Mechanism of Osteoblasts

Osteoclasts and osteoblasts play a major role in bone tissue homeostasis. The homeostasis and integrity of bone tissue are maintained by ensuring a balance between osteoclastic and osteogenic activities. The remodeling of bone tissue is a continuous ongoing process. Osteoclasts mainly play a role in bone resorption, whereas osteoblasts are mainly involved in bone remodeling processes, such as bone cell formation, mineralization, and secretion. These cell types balance and restrict each other to maintain bone tissue metabolism. Bone tissue is very sensitive to mechanical stress stimulation. Unloading and loading of mechanical stress are closely related to the differentiation and formation of osteoclasts and bone resorption function as well as the differentiation and formation of osteoblasts and bone formation function. Consequently, mechanical stress exerts an important influence on the bone microenvironment and bone metabolism. This review focuses on the effects of different forms of mechanical stress stimulation (including gravity, continuously compressive pressure, tensile strain, and fluid shear stress) on osteoclast and osteoblast function and expression mechanism. This article highlights the involvement of osteoclasts and osteoblasts in activating different mechanical transduction pathways and reports changings in their differentiation, formation, and functional mechanism induced by the application of different types of mechanical stress to bone tissue. This review could provide new ideas for further microscopic studies of bone health, disease, and tissue damage reconstruction.

Diagnostic yield of bone fragility gene panel sequencing in children and young adults referred for idiopathic primary osteoporosis at a single regional reference centre

Aim

To describe the presenting features, bone characteristics and molecular genetics in a large monocentric cohort of children and young adults with idiopathic primary osteoporosis.

Methods

Sixty-six patients (19 children, 47 adults; 28 males, 38 females; age at referral: 3.8 to 65 years) diagnosed with primary osteoporosis were included in this study; patients with features of osteogenesis imperfecta or other known syndromes associated with osteoporosis were excluded. For each patient, the following data were collected by retrospective chart review: family and personal history of fracture and osteoporosis, mineral homeostasis parameters and markers of bone formation and resorption, bone mineral density (BMD) of the lumbar spine (LS-BMD), the total body less head (TB-BMD), and total hip levels (TH-BMD) measured by DXA. As part of the initial assessment process, a bone fragility gene panel sequencing was performed in all of these patients.

Results

There was a higher predominance of males in the children (63%) and of females in the adults (66%) (p = 0.030). Compared to the adults, the children had a significantly lower frequency of vertebral fractures (26 vs 57%, p = 0.022) and a higher frequency of peripheral fractures (84 vs 53%; p = 0.019). Bone fragility gene panel sequencing allowed the identification of the heterozygous pathogenic variant in 27% of patients (most frequently in LRP5, WNT1 and COL1A1 or 2 genes) and the heterozygous p.(Val667Met) LRP5 variant in 11% of them. The frequency of pathogenic variants tended to be higher in the children compared to the adults without reaching statistical significance (42 vs 19%; p = 0.053). The frequency of the p.(Val667Met) LRP5 variant was similar in children and adults. No significant differences were found regarding the various clinical, biological and radiological characteristics of the patients according to genotype.

Conclusion

In this study, we reported the presenting features and bone characteristics in a large cohort of children and young adults with idiopathic primary osteoporosis. Bone fragility gene panel sequencing allowed the identification of genetic variants in a significant proportion of these patients. Molecular diagnosis in these patients is important in order to be able to offer genetic counselling and organise patient management.

The Wnt pathway: An important control mechanism in bone's response to mechanical loading

The conversion of mechanical energy into biochemical changes within living cells is process known as mechanotransduction. Bone is a quintessential tissue for studying the molecular mechanisms of mechanotransduction, as the skeleton's mechanical competence is crucial for vertebrate movement. Bone cell mechanotransduction is facilitated by a number of cell biological pathways, one of the most prominent of which is the Wnt signaling cascade. The Wnt co-receptor Lrp5 has been identified as a crucial protein for mechanical signaling in bone, and modifiers of Lrp5 activity play important roles in mediating signaling efficiency through Lrp5, including sclerostin, Dkk1, and the co-receptor Lrp4. Mechanical regulation of sclerostin is mediated by certain members of the Hdac family. Other mechanisms that influence Wnt signaling-some of which are mechanoresponsive-are coming to light, including R-spondins and their role in organizing the Rnf43/Znrf3 and Lgr4/5/6 complex that liberates Lrp5. While the identity of the key Wnt proteins involved in bone cell mechanical signaling are elusive, the likely pool of key players is narrowing. Identification of Wnt-based molecular targets that can be modulated pharmacologically to make mechanical stimulation (e.g., exercise) more beneficial is an emerging approach to improving skeletal integrity and reducing fracture risk.

Bone mineral density in high-level endurance runners: Part B-genotype-dependent characteristics

Purpose

Inter-individual variability in bone mineral density (BMD) exists within and between endurance runners and non-athletes, probably in part due to differing genetic profiles. Certainty is lacking, however, regarding which genetic variants may contribute to BMD in endurance runners and if specific genotypes are sensitive to environmental factors, such as mechanical loading via training.

Method

Ten single-nucleotide polymorphisms (SNPs) were identified from previous genome-wide and/or candidate gene association studies that have a functional effect on bone physiology. The aims of this study were to investigate (1) associations between genotype at those 10 SNPs and bone phenotypes in high-level endurance runners, and (2) interactions between genotype and athlete status on bone phenotypes.

Results

Female runners with P2RX7 rs3751143 AA genotype had 4% higher total-body BMD and 5% higher leg BMD than AC + CC genotypes. Male runners with WNT16 rs3801387 AA genotype had 14% lower lumbar spine BMD than AA genotype non-athletes, whilst AG + GG genotype runners also had 5% higher leg BMD than AG + GG genotype non-athletes.

Conclusion

We report novel associations between P2RX7 rs3751143 genotype and BMD in female runners, whilst differences in BMD between male runners and non-athletes with the same WNT16 rs3801387 genotype existed, highlighting a potential genetic interaction with factors common in endurance runners, such as high levels of mechanical loading. These findings contribute to our knowledge of the genetic associations with BMD and improve our understanding of why some runners have lower BMD than others.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00421-021-04789-z.

Association of regular aerobic exercises and neuromuscular junction variants with incidence of frailty: an analysis of the Chinese Longitudinal Health and Longevity Survey

BACKGROUND

Candidate genes of neuromuscular junction (NMJ) pathway increased risk of frailty, but the extent and whether can be offset by exercises was unclear. The aim of this study was to investigate the association between aerobic exercises and incident frailty regardless of NMJ pathway-related genetic risk.

METHODS

A cohort study on participants from Chinese Longitudinal Healthy Longevity Survey was conducted from 2008 to 2011. A total of 7006 participants (mean age of 80.6 ± 10.3 years) without frailty at baseline were interviewed to record aerobic exercise status, and 4053 individuals among them submitted saliva samples. NMJ pathway-related genes were genotyped and weighted genetic risk scores were constructed.

RESULTS

During a median follow-up of 3.1 years (19 634 person-years), there were 1345 cases (19.2%) of incident frailty. Persistent aerobic exercises were associated with a 26% lesser frailty risk [adjusted hazard ratio (HR) = 0.74, 95% confidence interval (CI) = 0.64-0.85]. This association was stronger in a subgroup of 1552 longevous participants (age between 90 and 111 years, adjusted HR = 0.72, 95% CI = 0.60-0.87). High genetic risk was associated with a 35% increased risk of frailty (adjusted HR = 1.35, 95% CI = 1.16-1.58). Of the participants with high genetic risk and no persistent aerobic exercises, there was a 59% increased risk of frailty (adjusted HR = 1.59, 95% CI = 1.20-2.09). HRs for the risk of frailty increased from the low genetic risk with persistent aerobic exercise to high genetic risk without persistent aerobic exercise (P trend <0.001).

CONCLUSIONS

Both aerobic exercises and NMJ pathway-related genetic risk were significantly associated with frailty. Persistent aerobic exercises can partly offset NMJ pathway-related genetic risk to frailty in elderly people.

Bifidobacterium pseudocatenulatum CECT 7765 reverses the adverse effects of diet-induced obesity through the gut-bone axis

Obesity and the associated chronic metabolic diseases (e.g., type-2 diabetes) adversely affect bone metabolism and health. Gut microbiota is considered to be involved in the pathophysiology of obesity and also represents a therapeutic target. This study has investigated the contribution of diet-induced obesity to alterations in bone health and metabolism and whether these could be restored by oral administration of Bifidobacterium pseudocatenulatum CECT 7765. To do so, adult male wild-type C57BL-6 mice were fed either a standard or high-fat diet (HFD), supplemented or not with B. pseudocatenulatum CECT 7765 (10⁹ CFU/day) for 14 weeks. Effects on bone mass density (BMD), bone mineral content, bone remodeling, bone structure and gene expression were assessed. In HFD-fed mice, bone microstructural properties at the distal femur showed deteriorated trabecular architecture in bone volumetric fraction, trabecular number and trabecular pattern factor. Besides, the HFD reduced the volumetric bone mineral density in the trabecular bone, but not in the cortical bone. All these bone microstructural alterations found in obese mice were reversed by B. pseudocatenulatum CECT 7765. Administration of the bacterium increased (p < .05) the Wnt/β-catenin pathway gene expression, which could mediate effects on BMD. Bifidobacterium pseudocatenulatum CECT 7765 supplementation increased (p < .05) serum osteocalcin (OC, bone formation parameter), and decreased serum C-terminal telopeptide (CTX) (p < .01) and parathormone (PTH) (p < .05) (both bone resorption parameters). It also altered the microstructure of the femur. In summary, HFD interfered with the normal bone homeostasis leading to increased bone loss. In obese mice, B. pseudocatenulatum CECT 7765 lowered bone mass loss and enhanced BMD by decreasing bone resorption and increasing bone formation.

Using zebrafish to study skeletal genomics

While genome-wide association studies (GWAS) have revolutionized our understanding of the genetic architecture of skeletal diseases, animal models are required to identify causal mechanisms and to translate underlying biology into new therapies. Despite large-scale knockout mouse phenotyping efforts, the skeletal functions of most genes residing at GWAS-identified loci remain unknown, highlighting a need for complementary model systems to accelerate gene discovery. Over the past several decades, zebrafish (Danio rerio) has emerged as a powerful system for modeling the genetics of human diseases. In this review, our goal is to outline evidence supporting the utility of zebrafish for accelerating our understanding of human skeletal genomics, as well as gaps in knowledge that need to be filled for this purpose. We do this by providing a basic foundation of the zebrafish skeletal morphophysiology and phenotypes, and surveying evidence of skeletal gene homology and the use of zebrafish for post-GWAS analysis in other tissues and organs. We also outline challenges in translating zebrafish mutant phenotypes. Finally, we conclude with recommendations of future directions and how to leverage the large body of tools and knowledge of skeletal genetics in zebrafish for the needs of human skeletal genomic exploration. Due to their amenability to rapid genetic approaches, as well as the large number of conserved genetic and phenotypic features, there is a strong rationale supporting the use of zebrafish for human skeletal genomic studies.

Regulation of Wnt signaling by physical exercise in the cell biological processes of the locomotor system

In the past decade, researches on Wnt signaling in cell biology have made remarkable progress regarding our understanding of embryonic development, bone formation, muscle injury and repair, neurogenesis, and tumorigenesis. The study also showed that physical activity can reverse age-dependent decline in skeletal muscle, preventing osteoporosis, regenerative neurogenesis, hippocampal function, cognitive ability, and neuromuscular junction formation, and the age-dependent recession is highly correlated with Wnt signaling pathways. However, how the biological processes in cell and physical activity during/following exercise affect the Wnt signaling path of the locomotor system is largely unknown. In this study, we first briefly introduce the important features of the cellular biological processes of exercise in the locomotor system. Then, we discuss Wnt signaling and review the very few studies that have examined Wnt signaling pathways in cellular biological processes of the locomotor system during physical exercise.

LRP5, Bone Density, and Mechanical Stress: A Case Report and Literature Review

The Wnt-β-catenin pathway receptor, low-density lipoprotein receptor-related protein 5 (LRP5), is a known regulator of bone mineral density. It has been hypothesized that specific human polymorphisms in LRP5 impact bone density, in part, by altering the anabolic response of bone to mechanical loading. Although experiments in animal models support this hypothesis, there is limited evidence that LRP5 polymorphisms can alter the anabolic response of bone to mechanical loading in humans. Herein, we report a young male who harbors a rare LRP5 missense mutation (A745V) and who provides potential proof of principle for this mechanotransduction hypothesis for low bone density. The subject had no history of fractures until age 18, a year into a career in competitive distance running. As he continued to run over the following 2 years, his mileage threshold to fracture steadily and rapidly decreased until he was diagnosed with severe osteoporosis (lumbar spine BMD Z-score of -3.2). By contextualizing this case within the existing LRP5 and mechanical stress literature, we speculate that this represents the first documented case of an individual in whom a genetic mutation altered the anabolic response of bone to mechanical stress in a manner sufficient to contribute to osteoporosis.

The interactions of physical activity, exercise and genetics and their associations with bone mineral density: implications for injury risk in elite athletes

Low bone mineral density (BMD) is established as a primary predictor of osteoporotic risk and can also have substantial implications for athlete health and injury risk in the elite sporting environment. BMD is a highly multi-factorial phenotype influenced by diet, hormonal characteristics and physical activity. The interrelationships between such factors, and a strong genetic component, suggested to be around 50-85% at various anatomical sites, determine skeletal health throughout life. Genome-wide association studies and case-control designs have revealed many loci associated with variation in BMD. However, a number of the candidate genes identified at these loci have no known associated biological function or have yet to be replicated in subsequent investigations. Furthermore, few investigations have considered gene-environment interactions-in particular, whether specific genes may be sensitive to mechanical loading from physical activity and the outcome of such an interaction for BMD and potential injury risk. Therefore, this review considers the importance of physical activity on BMD, genetic associations with BMD and how subsequent investigation requires consideration of the interaction between these determinants. Future research using well-defined independent cohorts such as elite athletes, who experience much greater mechanical stress than most, to study such phenotypes, can provide a greater understanding of these factors as well as the biological underpinnings of such a physiologically "extreme" population. Subsequently, modification of training, exercise or rehabilitation programmes based on genetic characteristics could have substantial implications in both the sporting and public health domains once the fundamental research has been conducted successfully.

LRP5 gene polymorphisms and radiographic joint damage in rheumatoid arthritis patients

Rheumatoid arthritis (RA) is characterized by increased bone resorption and impaired bone formation. Osteoblast function is regulated by the canonical LRP5/Wnt/β-catenin pathway. Bone mineral density and RA joint destruction are partially inherited. In line with this, we found significant associations between LRP5 SNPs (p.A1330V, p.N740N, p.V667M) and RA radiographic damage severity.

INTRODUCTION

Increased bone resorption and impaired bone formation characterize rheumatoid arthritis (RA). Canonical Wnt/β-catenin pathway, signalled by lipoprotein receptor-related protein-5 (LRP5), regulates osteoblast function. Since bone mineral density (BMD) and RA joint destruction are partially inherited, we studied their association with LRP5 single nucleotide polymorphisms (SNPs).

METHODS

Clinical data and peripheral blood for biomarkers assessment and LRP5 genotyping were collected from 208 RA patients. Hands and feet X-rays were scored [modified Sharp/van der Heijde Score (SHS), joint space narrowing (JSN), and erosion scores]. Lumbar spine, total left proximal femur, and left hand BMD were assessed by dual-energy X-ray absorptiometry (DXA).

RESULTS

TT genotypes for p.A1330V and p.N740N LRP5 SNPs associated with total SHS, erosion score, and hands erosion score; the same for p.A1330V with feet JSN score and p.N740N with hands total score. AG genotype for p.V667M associated with sclerostin and hands JSN score. Femoral BMD associated with TC genotype for p.N740N. Multiple test correction precluded a few of these associations. Among V667M-N740N-A1330V haplotypes: GTT associated with higher feet JSN score (OR = 3.80; p = 0.016) and ATT with higher JSN score (OR = 4.60; p = 0.032), hands total score (OR = 5.65; p = 0.022), and total SHS (OR = 6.74; p = 0.024).

CONCLUSION

Significant associations between LRP5 SNPs (p.A1330V, p.N740N, and p.V667M) and the severity of radiographic damage reinforce the evidence of bone destruction heritability in RA.

cGMP-dependent protein kinase-2 regulates bone mass and prevents diabetic bone loss

NO/cGMP signaling is important for bone remodeling in response to mechanical and hormonal stimuli, but the downstream mediator(s) regulating skeletal homeostasis are incompletely defined. We generated transgenic mice expressing a partly-activated, mutant cGMP-dependent protein kinase type 2 (PKG2R242Q) under control of the osteoblast-specific Col1a1 promoter to characterize the role of PKG2 in post-natal bone formation. Primary osteoblasts from these mice showed a two- to three-fold increase in basal and total PKG2 activity; they proliferated faster and were resistant to apoptosis compared to cells from WT mice. Male Col1a1-Prkg2R242Q transgenic mice had increased osteoblast numbers, bone formation rates and Wnt/β-catenin-related gene expression in bone and a higher trabecular bone mass compared to their WT littermates. Streptozotocin-induced type 1 diabetes suppressed bone formation and caused rapid bone loss in WT mice, but male transgenic mice were protected from these effects. Surprisingly, we found no significant difference in bone micro-architecture or Wnt/β-catenin-related gene expression between female WT and transgenic mice; female mice of both genotypes showed higher systemic and osteoblastic NO/cGMP generation compared to their male counterparts, and a higher level of endogenous PKG2 activity may be responsible for masking effects of the PKG2R242Q transgene in females. Our data support sexual dimorphism in Wnt/β-catenin signaling and PKG2 regulation of this crucial pathway in bone homeostasis. This work establishes PKG2 as a key regulator of osteoblast proliferation and post-natal bone formation.

Association between gene polymorphisms and obesity and physical fitness in Korean children

Obesity is affected by genetic factors and environmental influences. This research was undertaken to identify single nucleotide polymorphisms (SNPs) related to obesity and physical fitness and then to analyse and compare interactions between physical fitness and obesity-associated genotypes. To investigate relationships between physical fitness and major SNPs previously reported to be related to obesity, 68 SNPs in 32 genes were genotyped in 71 Korean children. Tests were conducted to evaluate five elements of physical fitness (speed, aerobic endurance, muscular endurance, muscular strength, and flexibility). The results obtained showed significant (P<0.02) differences in physical fitness scores for the following genotypes: CNR1 (rs1049353; GG), LEP (rs7799039; AA+AG), HHEX (rs1111875; TT), GC (rs16847015; TG+GG), LRP5 (rs4988300; GG+GT), NPY2R (rs2880415; CT+CC), PPY (rs231472; GG), UCP2 (rs660339; CT+TT), CDKN2B (rs10811661; AA+AG), and ADIPOQ (rs266729; CG+GG). Ten physical fitness-related genotypes were newly identified during the present study. This study suggests that classification of genotypes by physical fitness level could be used as an index for predicting the risk of obesity and for selecting individuals for intervention programmes. Furthermore, the study shows that even children participating in the same physical fitness improvement programme can exhibit different genotype dependencies.

Endothelin Signaling in Bone

The endothelin (ET) system includes 3 small peptide hormones and a pair of G-protein-coupled receptors. This review first outlines the ET signaling pathway and ET metabolism. Next, it summarizes the role of ET1 signaling in craniofacial development. Then, it discusses observations relating ET signaling to osteoblastic and other osteosclerotic processes in cancer. Finally, it describes recent work in our laboratory that points to endothelin signaling as an upstream mediator of WNT signaling, promoting bone matrix synthesis and mineralization. It concludes with a statement of some remaining gaps in knowledge and proposals for future research.

Low density lipoprotein receptor-related protein 5 gene polymorphisms and osteoporosis in Thai menopausal women

Background

Osteoporosis, characterized by low bone mineral density (BMD) and high bone fracture risk, is prevalent in Thai menopausal women. Genetic factors are known to play a key role in BMD. Low density lipoprotein receptor-related protein 5 (LRP5), a co-receptor in the Wnt/beta-catenin pathway, is involved in many aspects of bone biology. As coding single nucleotide polymorphisms (cSNPs) of LRP5, including A1330V (rs3736228), and Asian-related Q89R (rs41494349) and N740N (rs2306862), are associated with lowered BMD, this study aimed to determine the relationship between these LRP5 polymorphisms and BMD in 277 Thai menopausal women.

Results

Only rs3736228 deviated from the Hardy–Weinberg equilibrium of allele frequency (p = 0.022). The median, range and p value for the BMD related to each SNP parameter were compared (Mann–Whitney U test). Significant differences were observed between wild-type and risk alleles for both rs3736228 (total radial, p = 0.011; and radial 33, p = 0.001) and rs2306862 (radial 33: p = 0.015) SNPs, with no significant difference for rs41494349 SNP. Linkage disequilibrium was strong for both rs3736228 and rs2306862 SNPs. Haplotype analysis identified high CC frequency in both normal and osteopenia/osteoporosis groups, with a significant odds ratio for carrying the TT haplotype; however, this was non-significant after adjusting for age. Multivariate binary logistic regression analysis performed for rs3736228 showed that individuals with a body mass index <25 kg/m² had an increased risk of osteoporosis for each decade, but the polymorphism had no effect.

Conclusions

This study did not identify LRP5 polymorphisms as a risk factor for osteoporosis in Thai menopausal women. Further studies with larger sample sizes are needed to further clarify the role of LRP5 as a genetic determinant of osteoporosis.

Electronic supplementary material

The online version of this article (doi:10.1186/s12952-016-0059-7) contains supplementary material, which is available to authorized users.

Two novel susceptibility loci for non-small cell lung cancer map to low-density lipoprotein receptor-related protein 5

This study investigated the effect of single-nucleotide polymorphisms (SNPs) of low-density lipoprotein receptor-related protein 5 (LRP5) on the risk of developing non-small cell lung cancer (NSCLC). A total of 500 NSCLC patients and 500 healthy controls were recruited for genotyping of 11 SNPs of LRP5. The association between genotype and NSCLC risk was evaluated by computing the odds ratio (OR) and 95% confidence interval (CI) from multivariate unconditional logistic regression analyses. Eleven Tag SNPs were detected. The frequency of the LRP5 rs3736228 T allele (18.9% in male NSCLC cases and 23.9% in male controls) was statistically different between male NSCLCs and male controls (P=0.03), and the T allele was associated with a lower risk of NSCLC (OR=0.74; 95% CI, 0.56–0.67), whereas the C/C homozygous genotype and the LRP5 rs64843 T/T genotype were associated with an increased risk of NSCLC and squamous cell carcinoma (SCC), respectively (OR=1.43 and 1.77, respectively). Using Haploview software, the frequency of the haplotypes of rs312009/rs3120015/rs3120014 CCC was was significantly higher in female SCC cases compared with female controls (0.064 vs. 0.009, P=0.04). LRP5 rs3736228 and rs64843 SNPs were significantly associated with an increased risk of NSCLC and SCC, respectively. Further studies are required to investigate the functional changes in LRP5 expression and activity in NSCLC in vitro.

Physical Activity Benefits the Skeleton of Children Genetically Predisposed to Lower Bone Density in Adulthood

Both genetics and physical activity (PA) contribute to bone mineral density (BMD), but it is unknown if the benefits of physical activity on childhood bone accretion depend on genetic risk. We, therefore, aimed to determine if PA influenced the effect of bone fragility genetic variants on BMD in childhood. Our sample comprised US children of European ancestry enrolled in the Bone Mineral Density in Childhood Study (N = 918, aged 5 to 19 years, and 52.4% female). We used a questionnaire to estimate hours per day spent in total, high-, and low-impact PA. We calculated a BMD genetic score (% BMD lowering alleles) using adult genome-wide association study (GWAS)-implicated BMD variants. We used dual-energy X-ray absorptiometry to estimate femoral neck, total hip, and spine areal-BMD and total body less head (TBLH) bone mineral content (BMC) Z-scores. The BMD genetic score was negatively associated with each bone Z-score (eg, TBLH-BMC: estimate = -0.03, p = 1.3 × 10(-6) ). Total PA was positively associated with bone Z-scores; these associations were driven by time spent in high-impact PA (eg, TBLH-BMC: estimate = 0.05, p = 4.0 × 10(-10) ) and were observed even for children with lower than average bone Z-scores. We found no evidence of PA-adult genetic score interactions (p interaction > 0.05) at any skeletal site, and there was no evidence of PA-genetic score-Tanner stage interactions at any skeletal site (p interaction > 0.05). However, exploratory analyses at the individual variant level revealed that PA statistically interacted with rs2887571 (ERC1/WNT5B) to influence TBLH-BMC in males (p interaction = 7.1 × 10(-5) ), where PA was associated with higher TBLH-BMC Z-score among the BMD-lowering allele carriers (rs2887571 AA homozygotes: estimate = 0.08 [95% CI 0.06, 0.11], p = 2.7 × 10(-9) ). In conclusion, the beneficial effect of PA on bone, especially high-impact PA, applies to the average child and those genetically predisposed to lower adult BMD (based on GWAS-implicated BMD variants). Independent replication of our exploratory individual variant findings is warranted. © 2016 American Society for Bone and Mineral Research.

Effects of Exercise on Low Density Lipoprotein Receptor Related Protein 5 Gene Expression in Patients With Postmenopausal Osteoporosis

Objectives

This study aims to investigate the effects of aerobic exercise on low density lipoprotein receptor related protein 5 (LRP5) gene messenger ribonucleic acid expression and evaluate the relationship between the clinical parameters and gene expression in patients with postmenopausal osteoporosis (OP).

Patients and methods

Seven patients with postmenopausal OP (mean age 60.0±5.3 years; range 51 to 66 years) were included in the study. An exercise protocol/program consisting of treadmill exercising for 30 minutes three days a week for six weeks was performed at a moderate intensity. LRP5 gene expression levels were evaluated before the onset of the exercise program and then four hours after the end of the first session and 12th (fourth week) and 18th (sixth week) sessions of exercise.

Results

Our results demonstrated variable changes in the LRP5 gene expression after the aerobic exercise sessions. Excluding one patient, the LRP5 gene expression levels showed a slight tendency to increase. In spite of this tendency, gene expression differences during the exercise sessions were not significant.

Conclusion

Our results suggest that interindividual variations of LRP5 gene expression exist after moderate intensity aerobic exercises in patients with postmenopausal OP. Despite of this variability, LRP5 gene expression levels increased slightly, except in peripheral blood in one patient. Future studies with larger sample sizes and different sampling time/tissues are required to shed more light on the impact of exercise at molecular level in OP.

A Candidate Gene Association Study of Bone Mineral Density in an Iranian Population

The genetic epidemiology of variation in bone mineral density (BMD) and osteoporosis is not well studied in Iranian populations and needs more research. We report a candidate gene association study of BMD variation in a healthy cross-sectional study of 501 males and females sampled from the Iranian Multi-Centre Osteoporosis Study, Shiraz, Iran. We selected to study the association with 21 single nucleotide polymorphisms (SNPs) located in the 7 candidate genes LRP5, RANK, RANKL, OPG, P2RX7, VDR, and ESR1. BMD was measured at the three sites L2-L4, neck of femur, and total hip. Association between BMD and each SNP was assessed using multiple linear regression assuming an allele dose (additive effect) on BMD (adjusted for age and sex). Statistically significant (at the unadjusted 5% level) associations were seen with seven SNPs in five of the candidate genes. Two SNPs showed statistically significant association with more than one BMD site. Significant association was seen between BMD at all the three sites with the VDR SNP rs731246 (L2-L4 p = 0.038; neck of femur p = 0.001; and total hip p < 0.001). The T allele was consistently associated with lower BMD than the C allele. Significant association was also seen for the P2RX7 SNP rs3751143, where the G allele was consistently associated with lower BMD than the T allele (L2-L4 p = 0.069; neck of femur p = 0.024; and total hip p = 0.045).

LRP5 polymorphisms and response to alendronate treatment in Chinese postmenopausal women with osteoporosis

AIM

To investigate the association between LRP5 gene polymorphisms and response to alendronate in Chinese osteoporotic women.

MATERIALS & METHODS

Six hundred and thirty nine Chinese postmenopausal women with osteopenia or osteoporosis were included and received alendronate treatment. The A1330V polymorphism of LRP5 was investigated. Bone mineral density (BMD) and bone turnover markers (ALP and β-isomerized carboxy-telopeptide of type I collagen [β-CTX]) were measured before and after treatment. The correlation of LRP5 polymorphisms with changes in BMD and bone turnover biomarkers were analyzed after treatment.

RESULTS

After 12 months of treatment, participants with CC and CT genotypes had a larger increase in lumbar spine BMD and a larger decrease in serum β-CTX and ALP levels than those with TT genotype (all p < 0.001). No significant genotype-treatment interaction was found in hip BMD.

CONCLUSION

The A1330V polymorphism of LRP5 is possibly correlated with response to alendronate treatment in Chinese women with osteoporosis, and the TT genotype could possibly predict a weak response to alendronate.

Polymorphism of the low-density lipoprotein receptor-related protein 5 gene and fracture risk

Several molecular epidemiological studies have been conducted to examine the association between low-density lipoprotein receptor-related proteins (LRP5) Ala1330Val polymorphism and fracture; however, the conclusions remained controversial. We therefore performed an extensive meta-analysis on 10 published studies with 184479 subjects. Electronic databases, including PubMed, Excerpta Medica Database (EMBASE), Cochrane, Elsevier Science Direct and China National Knowledge Infrastructure (CNKI) databases were searched. Summary odds ratios (ORs) and corresponding 95% confidence intervals (CIs) were estimated using random-effects models. LRP5 Ala1330Val polymorphism was associated with a significantly increased risk of fracture (OR = 1.10; 95% CI, 1.06-1.14; I(2) = 29%). We also found that this polymorphism increased fracture risk in Caucasians. In the subgroup analysis according to gender, women was significantly associated with risk of fracture. In the subgroup analysis by type of fracture, LRP5 Ala1330Val polymorphism showed increased osteoporotic fracture risk. In conclusion, this meta-analysis suggested that an increased risk of fracture was associated with the LRP5 Ala1330Val polymorphism.

Bone and Muscle Pleiotropy: The Genetics of Associated Traits

Bone and muscle mass are highly correlated. In part, this is a consequence of both tissues sharing common genetic determinants. In addition, both tissues are responsive to their mechanical environments. New genetic tools in mice will allow genes of interest to be inactivated in experimentally defined contexts, thus allowing investigators to distinguish direct effects on each tissue from physiological responses to a primary phenotype in the other.

Association of LRP5 genotypes with osteoporosis in Tunisian post-menopausal women

Background

Osteoporosis is a highly heritable trait. Among the genes associated with bone mineral density (BMD), the low-density lipoprotein receptor-related protein 5 gene (LRP5) has been consistently identified in Caucasians. However LRP5 contribution to osteoporosis in populations of other ethnicities remains poorly known.

Methods

To determine whether LRP5 polymorphisms Ala1330Val and Val667Met are associated with BMD in North Africans, these genotypes were analyzed in 566 post-menopausal Tunisian women with mean age of 59.5 ± 7.7 years, of which 59.1% have low bone mass (T-score < −1 at spine or hip).

Results

In post-menopausal Tunisian women, 1330Val was weakly associated with reduced BMD T-score at lumbar spine (p = 0.047) but not femur neck. Moreover, the TT/TC genotypes tended to be more frequent in women with osteopenia and osteoporosis than in women with normal BMD (p = 0.066). Adjusting for body size and other potential confounders, LRP5 genotypes were no longer significantly associated with aBMD at any site.

Conclusions

The less common Val667Met polymorphism showed no association with osteoporosis. The Ala1330Val polymorphism is weakly associated with lower lumbar spine bone density and osteopenia/osteoporosis in postmenopausal Tunisian women. These observations expand our knowledge about the contribution of LRP5 genetic variation to osteoporosis risk in populations of diverse ethnic origin.

Whole-exome sequencing reveals LRP5 mutations and canonical Wnt signaling associated with hepatic cystogenesis

Polycystic livers are seen in the rare inherited disorder isolated polycystic liver disease (PCLD) and are recognized as the most common extrarenal manifestation in autosomal dominant polycystic kidney disease. Hepatic cystogenesis is characterized by progressive proliferation of cholangiocytes, ultimately causing hepatomegaly. Genetically, polycystic liver disease is a heterogeneous disorder with incomplete penetrance and caused by mutations in PRKCSH, SEC63, PKD1, or PKD2. Genome-wide SNP typing and Sanger sequencing revealed no pathogenic variants in hitherto genes in an extended PCLD family. We performed whole-exome sequencing of DNA samples from two members. A heterozygous variant c.3562C > T located at a highly conserved amino acid position (p.R1188W) in the low density lipoprotein receptor-related protein 5 (LRP5) gene segregated with the disease (logarithm of odds score, 4.62) but was not observed in more than 1,000 unaffected individuals. Screening of LRP5 in a PCLD cohort identified three additional mutations in three unrelated families with polycystic livers (p.V454M, p.R1529S, and p.D1551N), again all undetected in controls. All variants were predicted to be damaging with profound structural effects on LRP5 protein domains. Liver cyst tissue and normal hepatic tissue samples from patients and controls showed abundant LRP5 expression by immunohistochemistry. Functional activity analyses indicated that mutant LRP5 led to reduced wingless signal activation. In conclusion, we demonstrate that germ-line LRP5 missense mutations are associated with hepatic cystogenesis. The findings presented in this study link the pathophysiology of PCLD to deregulation of the canonical wingless signaling pathway.

Whole-exome sequencing reveals a heterozygous LRP5 mutation in a 6-year-old boy with vertebral compression fractures and low trabecular bone density

Juvenile osteoporosis (JO) is characterized by bone fragility during development, low bone mass and absence of extraskeletal features. Heterozygous loss-of-function mutations in LRP5 have been found in a few patients, but bone tissue and bone material abnormalities associated with such mutations have not been determined. Here we report on a 6-year-old boy who presented with a history of seven low-energy long-bone fractures starting at 19months of age and absence of extraskeletal involvement. Spine radiographs revealed multiple vertebral compression fractures. Despite tall stature (95th percentile), lumbar spine areal bone mineral density was low (z-score=-3.2). Trabecular volumetric bone mineral density, measured by peripheral quantitative computed tomography at the distal radius, was low (z-score=-5.1), but cortical thickness at the radial diaphysis was normal. Iliac bone histomorphometry demonstrated low bone formation activity in trabecular but not in cortical bone. Quantitative backscattered electron imaging showed normal material bone density in trabecular bone, but elevated results in the cortex. Whole-exome sequencing revealed a heterozygous insertion of a nucleotide in exon 12 of LRP5. This mutation had previously been reported in another JO patient and had been shown to lead to nonsense-mediated decay. Thus, heterozygous loss-of-function mutations in LRP5 can be associated with a bone formation deficit that affects mostly the trabecular compartment and can result in bone fragility during the first years of life.

Polymorphism of LRP5, but not of TNFRSF11B, is associated with a decrease in bone mineral density in postmenopausal Maya-Mestizo women

OBJECTIVE

Osteoporosis is a complex disease characterized principally by low bone mineral density (BMD), which is determined by an interaction of genetic, metabolic, and environmental factors. The aim of this study was to analyze the possible association among one polymorphism of LRP5 and three polymorphisms of TNFRSF11B as well as their haplotypes with BMD variations in Maya-Mestizo postmenopausal women.

METHODS

We studied 583 postmenopausal women of Maya-Mestizo ethnic origin. A structured questionnaire for risk factors was applied and BMD was measured in lumbar spine (LS), total hip (TH), and femoral neck (FN) by dual-energy X-ray absorptiometry. DNA was obtained from blood leukocytes. One single-nucleotide polymorphism of LRP5 (rs3736228, p.A1330V) and three of TNFRSF11B (rs4355801, rs2073618, and rs6993813) were studied using real-time PCR allelic discrimination for genotyping. Differences between the means of the BMDs according to the genotype were analyzed with covariance. Deviations from Hardy-Weinberg equilibrium were tested. Pairwise linkage disequilibrium between single nucleotide polymorphisms was calculated by direct correlation r(2), and haplotype analysis of TNFRSF11B was conducted.

RESULTS

The Val genotype of the rs3736228 (p.A1330V) of LRP5 was significantly associated with BMD variations at the LS, TH, and FN. None of the three polymorphisms of TNFRSF11B was associated with BMD variations.

CONCLUSIONS

Our results show that p.A1330V was significantly associated with BMD variations at all three skeletal sites analyzed; the Val allele and the Val/Val genotype were those most frequently found in our population.

Impact of the environment on the skeleton: is it modulated by genetic factors?

The etiology of skeletal disease is driven by genetic and environmental factors. Genome-wide association studies (GWAS) of osteoporotic phenotypes have identified novel candidate genes, but have only uncovered a small proportion of the trait variance explained. This "missing heritability" is caused by several factors, including the failure to consider gene-by-environmental (G*E) interactions. Some G*E interactions have been investigated, but new approaches to integrate environmental data into genomic studies are needed. Advances in genotyping and meta-analysis techniques now allow combining genotype data from multiple studies, but the measurement of key environmental factors in large human cohorts still lags behind, as do the statistical tools needed to incorporate these measures in genome-wide association meta-studies. This review focuses on discussing ways to enhance G*E interaction studies in humans and how the use of rodent models can inform genetic studies. Understanding G*E interactions will provide opportunities to effectively target intervention strategies for individualized therapy.

Genetic predictors of skeletal outcomes in healthy fertile women: the Bonturno study

Skeletal traits as height (Ht) or bone mineral density (BMD) are strongly inherited. Low-density lipoprotein receptor-related protein 5 (LRP5) and farnesyl diphosphonate synthase (FDPS) are candidate genes for bone phenotypes. From Bonturno study, we genotyped 570 healthy Caucasian women aged 20 to 50 years (yrs) for LRP5 rs4988321 (A/G) and rs3736228 (C/T) and FDPS rs2297480 (A/C) single nucleotide polymorphisms. Serum C-telopeptide of type I collagen (CTX), osteocalcin (OC), and N-terminal propeptide of type I procollagen (P1NP) were measured in BMD-evaluated subjects at lumbar spine (LS), total hip (TH) and femoral neck (FN) sites. LRP5 rs4988321 locus correlated with FN-BMD (P = 0.0230), while LRP5 rs3736228 genotypes differed in LS-BMD (P = 0.0428). When clustered by age, lower FN-BMD was detected in LRP5 GG (P = 0.030) subjects of 41 to 50 years but not in younger. Both LRP5 GG and CC genotypes showed higher age-adjusted values of OC, CTX and P1NP. Increased CTX values were in LRP5 GGCC subjects than in those having at least one LRP5 A plus T alleles (P = 0.0190). LRP5 CC, GG or GGCC subjects with at least one FDPS C allele showed higher levels of CTX and OC in 31 to 40 yrs or older subjects. In conclusion, LRP5 and FDPS loci age-specifically affect skeletal traits in healthy fertile women.

Sclerostin and Dickkopf-1 as therapeutic targets in bone diseases

The processes of bone growth, modeling, and remodeling determine the structure, mass, and biomechanical properties of the skeleton. Dysregulated bone resorption or bone formation may lead to metabolic bone diseases. The Wnt pathway plays an important role in bone formation and regeneration, and expression of two Wnt pathway inhibitors, sclerostin and Dickkopf-1 (DKK1), appears to be associated with changes in bone mass. Inactivation of sclerostin leads to substantially increased bone mass in humans and in genetically manipulated animals. Studies in various animal models of bone disease have shown that inhibition of sclerostin using a monoclonal antibody (Scl-Ab) increases bone formation, density, and strength. Additional studies show that Scl-Ab improves bone healing in models of bone repair. Inhibition of DKK1 by monoclonal antibody (DKK1-Ab) stimulates bone formation in younger animals and to a lesser extent in adult animals and enhances fracture healing. Thus, sclerostin and DKK1 are emerging as the leading new targets for anabolic therapies to treat bone diseases such as osteoporosis and for bone repair. Clinical trials are ongoing to evaluate the effects of Scl-Ab and DKK1-Ab in humans for the treatment of bone loss and for bone repair.

Mechanotransduction in bone tissue: The A214V and G171V mutations in Lrp5 enhance load-induced osteogenesis in a surface-selective manner

Mechanotransduction in bone requires components of the Wnt signaling pathway to produce structurally adapted bone elements. In particular, the Wnt co-receptor LDL-receptor-related protein 5 (LRP5) appears to be a crucial protein in the mechanotransduction cascades that translate physical tissue deformation into new bone formation. Recently discovered missense mutations in LRP5 are associated with high bone mass (HBM), and the altered function of these proteins provide insight into LRP5 function in many skeletal processes, including mechanotransduction. We further investigated the role of LRP5 in bone cell mechanotransduction by applying mechanical stimulation in vivo to two different mutant mouse lines, which harbor HBM-causing missense mutations in Lrp5. Axial tibia loading was applied to mature male Lrp5 G171V and Lrp5 A214V knock-in mice, and to their wild type controls. Fluorochrome labeling revealed that 3 days of loading resulted in a significantly enhanced periosteal response in the A214V knock in mice, whereas the G171V mice exhibited a lowered osteogenic threshold on the endocortical surface. In summary, our data further highlight the importance of Lrp5 in bone cell mechanotransduction, and indicate that the HBM-causing mutations in Lrp5 can alter the anabolic response to mechanical stimulation in favor of increased bone gain.

The genetic pleiotropy of musculoskeletal aging

Musculoskeletal aging is detrimental to multiple bodily functions and starts early, probably in the fourth decade of an individual's life. Sarcopenia is a health problem that is expected to only increase as a greater portion of the population lives longer; prevalence of the related musculoskeletal diseases is similarly expected to increase. Unraveling the biological and biomechanical associations and molecular mechanisms underlying these diseases represents a formidable challenge. There are two major problems making disentangling the biological complexity of musculoskeletal aging difficult: (a) it is a systemic, rather than "compartmental," problem, which should be approached accordingly, and (b) the aging per se is neither well defined nor reliably measurable. A unique challenge of studying any age-related condition is a need of distinguishing between the "norm" and "pathology," which are interwoven throughout the aging organism. We argue that detecting genes with pleiotropic functions in musculoskeletal aging is needed to provide insights into the potential biological mechanisms underlying inter-individual differences insusceptibility to the musculoskeletal diseases. However, exploring pleiotropic relationships among the system's components is challenging both methodologically and conceptually. We aimed to focus on genetic aspects of the cross-talk between muscle and its "neighboring" tissues and organs (tendon, bone, and cartilage), and to explore the role of genetics to find the new molecular links between skeletal muscle and other parts of the "musculoskeleton." Identification of significant genetic variants underlying the musculoskeletal system's aging is now possible more than ever due to the currently available advanced genomic technologies. In summary, a "holistic" genetic approach is needed to study the systems's normal functioning and the disease predisposition in order to improve musculoskeletal health.

Primary osteoporosis without features of OI in children and adolescents: clinical and genetic characteristics

Our aim was to characterize clinical findings and familial associations, and to examine candidate genes for disease-causing mutations in a cohort of children suffering from primary osteoporosis without features of osteogenesis imperfecta. Patients with osteoporosis and their nuclear families were studied. Medical history was reviewed. Calcium homeostasis parameters were measured and spinal radiographs obtained. BMD was determined by DXA for patients, parents and siblings. LRP5, LRP6, and PTHLH genes were sequenced. Twenty-seven patients (14 males) from 24 families were recruited. Median age at presentation was 10.1 years (range 3.3-15.6 years). One-third of the children had at least one parent with a BMD below the expected range for age. LRP5, LRP6, and PTHLH showed no causative mutations. Four polymorphisms in LRP5 were overrepresented in patients; the minor allele frequency of Q89R, V667M, N740N, and A1330V was significantly higher than in controls. Age of onset, clinical severity, and inheritance patterns are variable in children with primary osteoporosis. Several patients had evidence suggestive of familial transmission. The underlying genetic factors remain to be elucidated.

Genome-wide meta-analysis identifies 56 bone mineral density loci and reveals 14 loci associated with risk of fracture

Bone mineral density (BMD) is the most widely used predictor of fracture risk. We performed the largest meta-analysis to date on lumbar spine and femoral neck BMD, including 17 genome-wide association studies and 32,961 individuals of European and east Asian ancestry. We tested the top BMD-associated markers for replication in 50,933 independent subjects and for association with risk of low-trauma fracture in 31,016 individuals with a history of fracture (cases) and 102,444 controls. We identified 56 loci (32 new) associated with BMD at genome-wide significance (P < 5 × 10(-8)). Several of these factors cluster within the RANK-RANKL-OPG, mesenchymal stem cell differentiation, endochondral ossification and Wnt signaling pathways. However, we also discovered loci that were localized to genes not known to have a role in bone biology. Fourteen BMD-associated loci were also associated with fracture risk (P < 5 × 10(-4), Bonferroni corrected), of which six reached P < 5 × 10(-8), including at 18p11.21 (FAM210A), 7q21.3 (SLC25A13), 11q13.2 (LRP5), 4q22.1 (MEPE), 2p16.2 (SPTBN1) and 10q21.1 (DKK1). These findings shed light on the genetic architecture and pathophysiological mechanisms underlying BMD variation and fracture susceptibility.

Association study of common variants in the sFRP1 gene region and parameters of bone strength and body composition in two independent healthy Caucasian male cohorts

Bone mineral density (BMD) and bone strength are predictive parameters for the development of osteoporosis and related fracture later in life. Although it is well known that BMD and bone strength have a high heritability, not much of the variation is already explained. Mice models showed that sFRP1 has an influence on bone formation. Therefore this study aimed to investigate the effect of common genetic variation on BMD and bone strength in Caucasian men of different ages. Using HapMap we selected 13 tagSNPs which tag most common genetic variation in and around sFRP1 and we genotyped these SNPs in the young cohort of the Odense Androgen Study (OAS). The OAS includes a total of 1383 Danish men from two different age groups ([20-29 years]: N=783; [60-74 years]: N=600) and is well characterised. The subjects were phenotyped for BMD at several sites, and additionally for body composition and hip geometry parameters. Based on the results of the young cohort we selected three SNPs for further analysis in the complete OAS population. To conclude we tried to replicate the results of two SNPs in an independent population of 994 Belgian men. We found a strong association for rs9694405 with BMI as well in both cohorts separately as in the whole OAS population. Further we found rs4736965 associated with several hip geometry parameters in the same population. However we were not able to replicate those results in the Belgian population. At last we found in the OAS population age specific effects for rs10106678 with whole body BMD and waist to hip ratio.

Update on Wnt signaling in bone cell biology and bone disease

For more than a decade, Wnt signaling pathways have been the focus of intense research activity in bone biology laboratories because of their importance in skeletal development, bone mass maintenance, and therapeutic potential for regenerative medicine. It is evident that even subtle alterations in the intensity, amplitude, location, and duration of Wnt signaling pathways affects skeletal development, as well as bone remodeling, regeneration, and repair during a lifespan. Here we review recent advances and discrepancies in how Wnt/Lrp5 signaling regulates osteoblasts and osteocytes, introduce new players in Wnt signaling pathways that have important roles in bone development, discuss emerging areas such as the role of Wnt signaling in osteoclastogenesis, and summarize progress made in translating basic studies to clinical therapeutics and diagnostics centered around inhibiting Wnt pathway antagonists, such as sclerostin, Dkk1 and Sfrp1. Emphasis is placed on the plethora of genetic studies in mouse models and genome wide association studies that reveal the requirement for and crucial roles of Wnt pathway components during skeletal development and disease.

Levels of serotonin, sclerostin, bone turnover markers as well as bone density and microarchitecture in patients with high-bone-mass phenotype due to a mutation in Lrp5

Patients with an activation mutation of the Lrp5 gene exhibit high bone mass (HBM). Limited information is available regarding compartment-specific changes in bone. The relationship between the phenotype and serum serotonin is not well documented. To evaluate bone, serotonin, and bone turnover markers (BTM) in Lrp5-HBM patients, we studied 19 Lrp5-HBM patients (T253I) and 19 age- and sex-matched controls. DXA and HR-pQCT were used to assess BMD and bone structure. Serum serotonin, sclerostin, dickkopf-related protein 1 (DKK1), and BTM were evaluated. Z-scores for the forearm, total hip, lumbar spine, forearm, and whole body were significantly increased (mean ± SD) between 4.94 ± 1.45 and 7.52 ± 1.99 in cases versus -0.19 ± 1.19 to 0.58 ± 0.84 in controls. Tibial and radial cortical areas, thicknesses, and BMD were significantly higher in cases. In cases, BMD at the lumbar spine and forearm and cortical thickness were positively associated and trabecular area negatively associated with age (r = 0.49, 0.57, 0.74, and -0.61, respectively, p < .05). Serotonin was lowest in cases (69.5 [29.9-110.4] ng/mL versus 119.4 [62.3-231.0] ng/mL, p < .001) and inversely associated with tibial cortical density (r = -0.49, p < .05) and directly with osteocalcin (OC), bone-specific alkaline phosphatase (B-ALP), and procollagen type 1 amino-terminal propeptide (PINP) (r = 0.52-0.65, p < .05) in controls only. OC and S-CTX were lower and sclerostin higher in cases, whereas B-ALP, PINP, tartrate-resistant acid phosphatase (TRAP), and dickkopf-related protein 1 (DKK1) were similar in cases and controls. In conclusion, increased bone mass in Lrp5-HBM patients seems to be caused primarily by changes in trabecular and cortical bone mass and structure. The phenotype appeared to progress with age, but BTM did not suggest increased bone formation.

An Akt-dependent increase in canonical Wnt signaling and a decrease in sclerostin protein levels are involved in strontium ranelate-induced osteogenic effects in human osteoblasts

Sclerostin is an important regulator of bone homeostasis and canonical Wnt signaling is a key regulator of osteogenesis. Strontium ranelate is a treatment for osteoporosis that has been shown to reduce fracture risk, in part, by increasing bone formation. Here we show that exposure of human osteoblasts in primary culture to strontium increased mineralization and decreased the expression of sclerostin, an osteocyte-specific secreted protein that acts as a negative regulator of bone formation by inhibiting canonical Wnt signaling. Strontium also activated, in an apparently separate process, an Akt-dependent signaling cascade via the calcium-sensing receptor that promoted the nuclear translocation of β-catenin. We propose that two discrete pathways linked to canonical Wnt signaling contribute to strontium-induced osteogenic effects in osteoblasts.

Genetics of osteoporosis: perspectives for personalized medicine

Osteoporosis is the most common metabolic bone disorder worldwide. At least 15 genes (e.g., ESR1, LRP5, SOST, OPG, RANK and RANKL) have been confirmed as osteoporosis susceptibility genes, and another 30 have been highlighted as promising susceptibility genes. Notably, these genes are clustered in three biological pathways: the estrogen endocrine pathway, the Wnt/β-catenin signaling pathway and the RANK/RANKL/osteoprotegerin (OPG) pathway. In this article, using data pertaining to these three biological pathways as examples, we illustrate possible principles of personalized therapy for osteoporosis. In particular, we propose to use inhibitors (e.g., denosumab) of the RANK/RANKL/OPG signaling pathway to circumvent resistance to estrogen-replacement therapy: a novel idea resulting from the consideration of a mechanistic link between the estrogen endocrine pathway and the RANK/RANKL/OPG signaling pathway. In addition, we call for more attention to be focused on rare variants of major effects in future studies.

Genetics of osteoporosis

Osteoporosis is a common disease with a strong genetic component characterized by reduced bone mass, defects in the microarchitecture of bone tissue, and an increased risk of fragility fractures. Twin and family studies have shown high heritability of bone mineral density (BMD) and other determinants of fracture risk such as ultrasound properties of bone, skeletal geometry, and bone turnover. Osteoporotic fractures also have a heritable component, but this reduces with age as environmental factors such as risk of falling come into play. Susceptibility to osteoporosis is governed by many different genetic variants and their interaction with environmental factors such as diet and exercise. Notable successes in identification of genes that regulate BMD have come from the study of rare Mendelian bone diseases characterized by major abnormalities of bone mass where variants of large effect size are operative. Genome-wide association studies have also identified common genetic variants of small effect size that contribute to regulation of BMD and fracture risk in the general population. In many cases, the loci and genes identified by these studies had not previously been suspected to play a role in bone metabolism. Although there has been extensive progress in identifying the genes and loci that contribute to the regulation of BMD and fracture over the past 15 yr, most of the genetic variants that regulate these phenotypes remain to be discovered.

Genetic epidemiology of age-related osteoporosis and its clinical applications

Osteoporosis is an important and complex disorder that is highly prevalent worldwide. This disease poses a major challenge to modern medicine and its treatment is associated with high costs. Numerous studies have endeavored to decipher the pathogenesis of this disease. The clinical assessment of patients often incorporates information about a family history of osteoporotic fractures. Indeed, the observation of an increased risk of fracture in an individual with a positive parental history of hip fracture provides strong evidence for the heritability of osteoporosis. The onset and progression of osteoporosis are generally controlled by multiple genetic and environmental factors, as well as interactions between them, with rare cases determined by a single gene. In an attempt to identify the genetic markers of complex diseases such as osteoporosis, there has been a move away from traditional linkage mapping studies and candidate gene association studies to higher-density genome-wide association studies. The advent of high-throughput technology enables genotyping of millions of DNA markers in the human genome, and consequently the identification and characterization of causal variants and loci that underlie osteoporosis. This Review presents an overview of the major findings since 2007 and clinical applications of these genome-wide linkage and association studies.

A polymorphism near osteoprotegerin gene confer risk of obesity in Uyghurs

To investigate the association of single nucleotide polymorphism (SNP) rs4355801 near osteoprotegerin (OPG) gene and rs3736228 in low-density lipoprotein receptor-related protein 5 (LRP5) gene with metabolic phenotypes [body mass index (BMI), waist-hip ratio, glucose, total cholesterol (CHO), and triglyceride], we carried out a population-based association study in Uyghur population living in Xinjiang Uyghur Autonomous Region of China. We observed a significant higher level of BMI in AG/AA carriers than in GG carriers (P = 0.022) for rs4355801. Subjects with the AG/GG genotype significantly increased the risk of BMI related obesity than subjects with the AA genotype, with an odds ratio of 1.31 (95% CI 1.09-1.56, P = 0.005). The association remained significant after controlling for covariates of age and gender. In addition, we observed a significant higher level of CHO in CT/TT carriers than in CC carriers (P = 0.021) for rs3736228. Our observations provide the first evidence that rs4355801 near OPG gene may confer susceptibility to obesity. In addition, SNP rs3736228 in LPR5 gene may affects the level of CHO in Uyghur population.

Genetics of osteoporosis

Osteoporosis is a common disease with a strong genetic component characterized by reduced bone mass and an increased risk of fragility fractures. Twin and family studies have shown that the heritability of bone mineral density and other determinants of fracture risk, such as ultrasound properties of bone, skeletal geometry, and bone turnover, is high, although heritability of fracture is modest. Many different genetic variants contribute to the regulation of these phenotypes. Most are common variants of small effect size, but there is evidence that rare variants of large effect size also contribute in some individuals. Many of the genes that regulate susceptibility to osteoporosis have been identified through studies of rare bone diseases, but genome-wide association studies have also been successful in identifying genes that predispose to osteoporosis. Although there has been extensive progress in this area over the past 10 years, most of the genetic variants that regulate susceptibility to osteoporosis remain to be discovered.

Replication study of candidate genes/loci associated with osteoporosis based on genome-wide screening

Osteoporosis is a major public health problem characterized by low bone mineral density (BMD). This replication study confirmed 38 single-nucleotide polymorphisms (SNPs) out of 139 SNPs previously reported in three recent genome-wide association studies (GWASs) in an independent US white sample. Ten SNPs achieved combined p < 3.6 x 10(-4).

INTRODUCTION

BMD is under strong genetic control. This study aims to verify the potential associations between BMD and candidate genes/loci reported by GWAS of FHS100K, Icelandic deCODE, and UK-NL.

METHODS

Eight promising (at the genome-wide significant level after Bonferroni correction) and 131 available sub-promising (at the most stringent p value, p < 5.5 x 10(-5) in the three GWASs reports) SNPs were selected. By using genotypic information from Affymetrix 500 K SNP arrays, we tested their associations with BMD in 1,000 unrelated US whites. Fisher's combined probability method was used to quantify the overall evidence of association. BMD was measured by dual energy X-ray absorptiometry.

RESULTS

Two promising SNPs, rs3762397 and rs3736228, were replicated in the current study with p < 0.05. Besides, 36 sub-promising SNPs were replicated at the same significant level. Ten SNPs achieved significant combined p < 3.6 x 10(-4) (0.05/139 SNPs, corrected for multiple testing).

CONCLUSIONS

Osteoporosis susceptibility of 38 SNPs was replicated in 1,000 unrelated US whites. This study showed promise for replication of some initial genome-wide association signals.

Evidence for pleiotropic factors in genetics of the musculoskeletal system

There are both theoretical and empirical underpinnings that provide evidence that the musculoskeletal system develops, functions, and ages as a whole. Thus, the risk of osteoporotic fracture can be viewed as a function of loading conditions and the ability of the bone to withstand the load. Both bone loss (osteoporosis) and muscle wasting (sarcopenia) are the two sides of the same coin, an involution of the musculoskeletal system. Skeletal loads are dominated by muscle action; both bone and muscle share environmental, endocrine and paracrine influences. Muscle also has an endocrine function by producing bioactive molecules, which can contribute to homeostatic regulation of both bone and muscle. It also becomes clear that bone and muscle share genetic determinants; therefore the consideration of pleiotropy is an important aspect in the study of the genetics of osteoporosis and sarcopenia. The aim of this review is to provide an additional evidence for existence of the tight genetic co-regulation of muscles and bones, starting early in development and still evident in aging. Recently, important papers were published, including those dealing with the cellular mechanisms and anatomic substrate of bone mechanosensitivity. Further evidence has emerged suggesting that the relationship between skeletal muscle and bone parameters extends beyond the general paradigm of bone responses to mechanical loading. We provide insights into several pathways and single genes, which apparently have a biologically plausible pleiotropic effect on both bones and muscles; the list is continuing to grow. Understanding the crosstalk between muscles and bones will translate into a conceptual framework aimed at studying the pleiotropic genetic relationships in the etiology of complex musculoskeletal disease. We believe that further progress in understanding the common genetic etiology of osteoporosis and sarcopenia will provide valuable insight into important biological underpinnings for both musculoskeletal conditions. This may translate into new approaches to reduce the burden of both conditions, which are prevalent in the elderly population.