Meta-analysis of genome-wide scans for total body BMD in children and adults reveals allelic heterogeneity and age-specific effects at the WNT16 locus

High-throughput screening of mouse gene knockouts identifies established and novel skeletal phenotypes

Screening gene function in vivo is a powerful approach to discover novel drug targets. We present high-throughput screening (HTS) data for 3 762 distinct global gene knockout (KO) mouse lines with viable adult homozygous mice generated using either gene-trap or homologous recombination technologies. Bone mass was determined from DEXA scans of male and female mice at 14 weeks of age and by microCT analyses of bones from male mice at 16 weeks of age. Wild-type (WT) cagemates/littermates were examined for each gene KO. Lethality was observed in an additional 850 KO lines. Since primary HTS are susceptible to false positive findings, additional cohorts of mice from KO lines with intriguing HTS bone data were examined. Aging, ovariectomy, histomorphometry and bone strength studies were performed and possible non-skeletal phenotypes were explored. Together, these screens identified multiple genes affecting bone mass: 23 previously reported genes (Calcr, Cebpb, Crtap, Dcstamp, Dkk1, Duoxa2, Enpp1, Fgf23, Kiss1/Kiss1r, Kl (Klotho), Lrp5, Mstn, Neo1, Npr2, Ostm1, Postn, Sfrp4, Slc30a5, Slc39a13, Sost, Sumf1, Src, Wnt10b), five novel genes extensively characterized (Cldn18, Fam20c, Lrrk1, Sgpl1, Wnt16), five novel genes with preliminary characterization (Agpat2, Rassf5, Slc10a7, Slc26a7, Slc30a10) and three novel undisclosed genes coding for potential osteoporosis drug targets.

Osteoblast-derived WNT16 represses osteoclastogenesis and prevents cortical bone fragility fractures

The WNT16 locus is a major determinant of cortical bone thickness and nonvertebral fracture risk in humans. The disability, mortality and costs caused by osteoporosis-induced nonvertebral fractures are enormous. We demonstrate here that Wnt16-deficient mice develop spontaneous fractures as a result of low cortical thickness and high cortical porosity. In contrast, trabecular bone volume is not altered in these mice. Mechanistic studies revealed that WNT16 is osteoblast derived and inhibits human and mouse osteoclastogenesis both directly by acting on osteoclast progenitors and indirectly by increasing expression of osteoprotegerin (Opg) in osteoblasts. The signaling pathway activated by WNT16 in osteoclast progenitors is noncanonical, whereas the pathway activated in osteoblasts is both canonical and noncanonical. Conditional Wnt16 inactivation revealed that osteoblast-lineage cells are the principal source of WNT16, and its targeted deletion in osteoblasts increases fracture susceptibility. Thus, osteoblast-derived WNT16 is a previously unreported key regulator of osteoclastogenesis and fracture susceptibility. These findings open new avenues for the specific prevention or treatment of nonvertebral fractures, a substantial unmet medical need.

Unveiling the mysteries of the genetics of osteoporosis

INTRODUCTION

Osteoporosis is a common disease characterised by low bone mineral density and an increased risk of fragility fractures.

METHODS

We conducted a literature review of relevant studies relating to the genetics of osteoporosis.

RESULTS

Family studies have revealed that bone density and fractures have a strong heritable component but environmental factors also play an important role. This makes identification of the causative genetic variants challenging. Linkage analysis has been successful in identifying the genes responsible for rare inherited diseases associated with abnormalities of bone mass but has been of limited value in osteoporosis. In contrast, genome-wide association studies in large cohort studies have identified 56 loci with robust evidence of association with bone density and 14 loci that predispose to fractures. Although the effect size of the implicated variants is small, many of the loci contain genes known to be involved in regulating bone cell activity through the RANK and Wnt signalling pathways, whereas others contain novel genes not previously implicated in bone metabolism. In a few instances, whole genome and exome sequencing have been successfully used to identify rare variants of large effect size that influence susceptibility to osteoporosis.

CONCLUSION

A future challenge will be to conduct fine mapping and functional analysis of the loci implicated in osteoporosis in order to identify the causal genetic variants and examine the mechanisms by which they influence bone cell function and bone mass. Ultimately this may lead to the identification of biomarkers for susceptibility to osteoporosis and fractures or new therapeutic targets.

Heritability of bone mineral density in a multivariate family-based study

There is evidence for a genetic contribution to bone mineral density (BMD×). Different loci affecting BMD have been identified by diverse linkage and genome-wide association studies. We studied the heritability of and the correlations among six densitometric phenotypes and four bone mass/fracture phenotypes. For this purpose, we used a family-based study of the genetics of osteoporosis, the Genetic Analysis of Osteoporosis Project. The primary aim of our study was to examine the roles of genetic and environmental factors in determining osteoporosis-related phenotypes. The project consisted of 11 extended families from Spain. All of them were selected through a proband with osteoporosis. BMD was measured using dual-energy X-ray absorptiometry. The proportion of variance of BMD attributable to significant covariates ranged from 25% (for femoral neck BMD) to 48% (for whole-body total BMD). The vast majority of the densitometric phenotypes had highly significant heritability, ranging from 0.252 (whole-body total BMD) to 0.537 (trochanteric BMD) after correcting for covariate effects. All of the densitometric phenotypes showed high and significant genetic correlations (from -0.772 to -1.000) with a low bone mass/osteopenia condition (Affected 3). Our findings provide additional evidence on the heritability of BMD and a strong genetic correlation between BMD and bone mass/fracture phenotypes in a Spanish population. Our results emphasize the importance of detecting genetic risk factors and the benefit of early diagnosis and especially therapeutic and preventive strategies.

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.

Genome-wide Association Studies for Osteoporosis: A 2013 Update

In the past few years, the bone field has witnessed great advances in genome-wide association studies (GWASs) of osteoporosis, with a number of promising genes identified. In particular, meta-analysis of GWASs, aimed at increasing the power of studies by combining the results from different study populations, have led to the identification of novel associations that would not otherwise have been identified in individual GWASs. Recently, the first whole genome sequencing study for osteoporosis and fractures was published, reporting a novel rare nonsense mutation. This review summarizes the important and representative findings published by December 2013. Comments are made on the notable findings and representative studies for their potential influence and implications on our present understanding of the genetics of osteoporosis. Potential limitations of GWASs and their meta-analyses are evaluated, with an emphasis on understanding the reasons for inconsistent results between different studies and clarification of misinterpretation of GWAS meta-analysis results. Implications and challenges of GWAS are also discussed, including the need for multi- and inter-disciplinary studies.

THRA and DIO2 mutations are unlikely to be a common cause of increased bone mineral density in euthyroid post-menopausal women

OBJECTIVE

A new autosomal dominant disorder due to mutation of THRA, which encodes thyroid hormone receptor α, is characterised by severely delayed skeletal development but only slightly abnormal thyroid status. Adult mice with disrupted thyroid hormone action in bone due to a mutation of Thra or deletion of Dio2, encoding the type 2 deiodinase, have high bone mass and mineralisation despite essentially euthyroid status. No individuals with DIO2 mutations have been described and the adult phenotype of patients with THRA mutations is largely unknown. We hypothesised that screening euthyroid adults with high bone mineral density (BMD) could be used to identify individuals with mutations of THRA or DIO2.

DESIGN

The Osteoporosis and Ultrasound Study (OPUS) is a 6-year prospective study of fracture-related factors from five European centres.

METHODS

A cohort of 100 healthy euthyroid post-menopausal women with the highest BMD was selected from the OPUS population. We sequenced the intron-exon boundaries and critical exons of THRA and DIO2 in these subjects. TSH, free 3,5,3'-l-triiodothyronine, free thyroxine, vitamin D, parathyroid hormone and bone turnover marker concentrations, and BMD measurements were available in all OPUS participants.

RESULTS

No coding sequence or splice site mutations affecting THRA or DIO2 were identified.

CONCLUSIONS

Mutations affecting THRA or DIO2 are not a common cause of high BMD in healthy euthyroid post-menopausal women.

Explaining additional genetic variation in complex traits

Genome-wide association studies (GWAS) have provided valuable insights into the genetic basis of complex traits, discovering >6000 variants associated with >500 quantitative traits and common complex diseases in humans. The associations identified so far represent only a fraction of those that influence phenotype, because there are likely to be many variants across the entire frequency spectrum, each of which influences multiple traits, with only a small average contribution to the phenotypic variance. This presents a considerable challenge to further dissection of the remaining unexplained genetic variance within populations, which limits our ability to predict disease risk, identify new drug targets, improve and maintain food sources, and understand natural diversity. This challenge will be met within the current framework through larger sample size, better phenotyping, including recording of nongenetic risk factors, focused study designs, and an integration of multiple sources of phenotypic and genetic information. The current evidence supports the application of quantitative genetic approaches, and we argue that one should retain simpler theories until simplicity can be traded for greater explanatory power.

Supplementation with calcium and short-chain fructo-oligosaccharides affects markers of bone turnover but not bone mineral density in postmenopausal women

This 24-mo randomized, double-blind, controlled trial aimed to examine whether supplementation with a natural marine-derived multi-mineral supplement rich in calcium (Ca) taken alone and in conjunction with short-chain fructo-oligosaccharide (scFOSs) has a beneficial effect on bone mineral density (BMD) and bone turnover markers (BTMs) in postmenopausal women. A total of 300 non-osteoporotic postmenopausal women were randomly assigned to daily supplements of 800 mg of Ca, 800 mg of Ca with 3.6 g of scFOS (CaFOS), or 9 g of maltodextrin. BMD was measured before and after intervention along with BTMs, which were also measured at 12 mo. Intention-to-treat ANCOVA identified that the change in BMD in the Ca and CaFOS groups did not differ from that in the maltodextrin group. Secondary analysis of changes to BTMs over time identified a greater decline in osteocalcin and C-telopeptide of type I collagen (CTX) in the Ca group compared with the maltodextrin group at 12 mo. A greater decline in CTX was observed at 12 mo and a greater decline in osteocalcin was observed at 24 mo in the CaFOS group compared with the maltodextrin group. In exploratory subanalyses of each treatment group against the maltodextrin group, women classified with osteopenia and taking CaFOS had a smaller decline in total-body (P = 0.03) and spinal (P = 0.03) BMD compared with the maltodextrin group, although this effect was restricted to those with higher total-body and mean spinal BMD at baseline, respectively. Although the change in BMD observed did not differ between the groups, the greater decline in BTMs in the Ca and CaFOS groups compared with the maltodextrin group suggests a more favorable bone health profile after supplementation with Ca and CaFOS. Supplementation with CaFOS slowed the rate of total-body and spinal bone loss in postmenopausal women with osteopenia-an effect that warrants additional investigation. This trial was registered at www.controlled-trials.com as ISRCTN63118444.

Variation in the Kozak sequence of WNT16 results in an increased translation and is associated with osteoporosis related parameters

The importance of WNT16 in the regulation of bone metabolism was recently confirmed by several genome-wide association studies and by a Wnt16 (Wnt16(-/-)) knockout mouse model. The aim of this study was thus to replicate and further elucidate the effect of common genetic variation in WNT16 on osteoporosis related parameters. Hereto, we performed a WNT16 candidate gene association study in a population of healthy Caucasian men from the Odense Androgen Study (OAS). Using HapMap, five tagSNPs and one multimarker test were selected for genotyping to cover most of the common genetic variation in and around WNT16 (MAF>5%). This study confirmed previously reported associations for rs3801387 and rs2707466 with bone mineral density (BMD) at several sites. Furthermore, we additionally demonstrated that rs2908007 is strongly associated with BMD at several sites in the young, elderly and complete OAS population. The observed effect of these three associated SNPs on the respective phenotypes is comparable and we can conclude that the presence of the minor allele results in an increase in BMD. Additionally, we performed re-sequencing of WNT16 on two cohorts selected from the young OAS cohort, based on their extreme BMD values. On this basis, rs55710688 was selected for an in vitro translation experiment since it is located in the Kozak sequence of WNT16a. We observed an increased translation efficiency and thus a higher amount of WNT16a for the Kozak sequence that was significantly more prevalent in the high BMD cohort. This observation is in line with the results of the Wnt16(-/-) mice. Finally, a WNT luciferase reporter assay was performed and showed no activation of the β-catenin dependent pathway by Wnt16. We did detect a dose-dependent inhibitory effect of Wnt16 on WNT1 activation of this canonical WNT pathway. Increased translation of WNT16 can thus lead to an increased inhibitory action of WNT16 on canonical WNT signaling. This statement is in contrast with the known activating effect of canonical WNT signaling on bone formation and suggests a stimulatory effect on bone metabolism via noncanonical WNT signaling. More research is required to not only confirm this hypothesis, but also to further elucidate the role of non-canonical WNT pathways in bone metabolism and the general mechanisms of interplay between the different WNT signaling pathways.

Wnt16 is involved in intramembranous ossification and suppresses osteoblast differentiation through the Wnt/β-catenin pathway

In the course of embryonic development skeletal elements form either through intramembranous or endochondral ossification. Wnt proteins play diverse roles during vertebrate skeletal development. Wnt16 is a key factor in developing long bones, but its exact role in craniofacial bone formation remains unclear. This study was initially undertaken to investigate the expression of Wnt16 during craniofacial bone development in mouse embryos. Wnt16 expression in the osteoid of calvaria, maxilla, and mandible started later than that of ALP and osteocalcin (OCN), but before mineralization of the craniofacial bones, suggesting that Wnt16 is involved in intramembranous ossification in the head. To confirm this, MC3T3-E1 cells were transfected with an adenovirus containing Wnt16 (Ad-Wnt16). Ad-Wnt16 cells showed decreased ALP activity and less mineralized nodule formations compared with control cells. In addition, the mRNA levels of osteogenic markers were reduced. Moreover, Wnt16 activated β-catenin signaling in MC3T3-E1 cells at both transcription and protein levels as shown by a TOPflash luciferase reporter gene assay and western blot analysis. On the other hand, Wnt/β-catenin pathway blockade by Dickkopf 1 abrogated the suppression of mineralization by Wnt16. Our findings suggest that Wnt16 is involved in intramembranous ossification and suppresses osteoblast differentiation through the Wnt/β-catenin pathway.

Contribution of genetic and epigenetic mechanisms to Wnt pathway activity in prevalent skeletal disorders

We reported previously that the expression of Wnt-related genes is lower in osteoporotic hip fractures than in osteoarthritis. We aimed to confirm those results by analyzing β-catenin levels and explored potential genetic and epigenetic mechanisms involved. β-Catenin gene expression and nuclear levels were analyzed by real time PCR and confocal immunofluorescence. Increased nuclear β-catenin was found in osteoblasts isolated from patients with osteoarthritis (99 ± 4 units vs. 76 ± 12, p=0.01, n=10), without differences in gene transcription, which is consistent with a post-translational down-regulation of β-catenin and decreased Wnt pathway activity. Twenty four single nucleotide polymorphisms (SNPs) of genes showing differential expression between fractures and osteoarthritis (WNT4, WNT10A, WNT16 and SFRP1) were analyzed in DNA isolated from blood of 853 patients. The genotypic frequencies were similar in both groups of patients, with no significant differences. Methylation of Wnt pathway genes was analyzed in bone tissue samples (15 with fractures and 15 with osteoarthritis) by interrogating a CpG-based methylation array. Six genes showed significant methylation differences between both groups of patients: FZD10, TBL1X, CSNK1E, WNT8A, CSNK1A1L and SFRP4. The DNA demethylating agent 5-deoxycytidine up-regulated 8 genes, including FZD10, in an osteoblast-like cell line, whereas it down-regulated other 16 genes. In conclusion, Wnt activity is reduced in patients with hip fractures, in comparison with those with osteoarthritis. It does not appear to be related to differences in the allele frequencies of the Wnt genes studied. On the other hand, methylation differences between both groups could contribute to explain the differences in Wnt activity.

Missense polymorphisms of the WNT16 gene are associated with bone mass, hip geometry and fractures

Two missense polymorphisms of WNT16 were associated with hip bone mineral density (BMD), the buckling ratio of the femoral neck, calcaneal ultrasound and hip fractures in individuals under 80 years of age. These results confirm the association of the WNT16 gene with bone mass and osteoporotic fractures.

INTRODUCTION

Osteoporosis has a strong genetic component. Wnt ligands stimulate the differentiation of osteoblast precursors and play a major role in skeletal homeostasis. Therefore, the aim of this study was to explore the association of allelic variants of the WNT16 gene with BMD, other structural parameters of bone and osteoporotic hip fractures.

METHODS

Six single nucleotide polymorphisms were analysed in 1,083 Caucasian individuals over 49 years of age.

RESULTS

Two missense polymorphisms (rs2908004 and rs2707466) were associated with femoral neck BMD, with average differences across genotypes of 35 mg/cm(2) (p = 0.00037 and 0.0015, respectively). Likewise, the polymorphisms were associated with calcaneal quantitative ultrasound parameters (p = 0.00004 and 0.0014, respectively) and the buckling ratio, an index of cortical instability of the femoral neck (p = 0.0007 and 0.0029, respectively). Although there were no significant differences in the genotype frequency distributions between 294 patients with hip fractures and 670 controls, among the subgroup under 80 years of age, TT genotypes were underrepresented in patients with fractures (odds ratio 0.50; CI 0.27-0.94).

CONCLUSION

Common missense polymorphisms of the WNT16 gene are associated with BMD at the hip, calcaneal ultrasound and the buckling ratio of the femoral neck, as well as with hip fractures in individuals under 80 years of age. Overall, these results confirm the association of the WNT16 locus with BMD identified in genome-wide association studies and support its role in determining the risk of osteoporotic fractures.

Structural maintenance of chromosome complexes and bone development: the beginning of a wonderful relationship?

Bone development depends on environmental, nutritional and hormonal factors. Yet, an ordered and timed activation of genes and their associated molecular pathways are central for the growth and development of healthy bones. The correct expression of genes depends on both cis- and trans-regulatory elements. Of these, the elusive role of chromatin ultrastructure is just beginning to become appreciated. Changes in the higher-order structure of chromatin are affecting the expression of genes in response to intrinsic and environmental signals. Cohesin and condensin are members of the structural maintenance of chromosome (SMC) family of protein complexes, which mediate higher-order chromatin structure by tethering distinct regions of chromatin either inter- or intra-molecularly. In recent years, SMCs had been identified for their function in the regulation of gene expression and developmental processes, whereas malfunction of cohesin or condensin has an impact on human health. However, little is known about the specific roles of SMC complexes in bone development and their possible effect on bone health. Here, we review studies that suggest an intimate link between SMCs and bone development, as well as a plausible effect, direct or indirect, on the bone health. We describe genetic syndromes associated with SMCs with distinctive bone phenotypes and identify links between SMCs and bone-related molecular pathways. Future studies of the relationship between SMCs and bone development will reveal new understandings of both the cellular and molecular roles of SMC complexes and provide new insights into the growth and developmental processes in the bone.

Involvement of WNT/β-catenin signaling in the treatment of osteoporosis

Osteoblast differentiation is predominantly regulated by the WNT/β-catenin signaling (canonical WNT pathway), which, together with bone morphogenetic proteins, acts as the master regulator of osteogenesis. The recent characterization of the canonical WNT pathway in the regulation of bone modeling and remodeling provided important insights for our understanding of the pathophysiology of a number of conditions and of the mechanism of action of hormones or drugs with important effect on bone metabolism. This review is mainly focused on the growing therapeutic implications of these new findings. WNT/β-catenin signaling plays a key role in bone tissue by determining the differentiation of stem cells into mature osteoblasts rather than into chondrocytes and adipocytes. Its regulation is predominantly driven by the production of two WNT signaling antagonists: sclerostin (SOST) and Dickkopf-related protein 1 (DKK1). The most proximate regulator of SOST expression by osteocytes and its serum levels is bone mechanical load. SOST expression is increased with advancing age, by glucocorticoid treatment and during treatment with antiresorptive agents such as bisphosphonates and denosumab, while it is decreased by parathyroid hormone excess or administration of estrogens. Correlation between DKK1 serum levels and bone formation in various pathological conditions or during osteoporosis treatment has been reported. Inhibitors of the negative regulators of WNT/β-catenin signaling ("inhibiting the endogenous inhibitors") are potential candidates for the prevention and treatment of bone loss. Inactivating monoclonal antibodies against SOST appears to be the most attractive strategy because SOST is the only component of the WNT pathway expressed almost exclusively by osteocytes.

Meta-analysis of genome-wide studies identifies WNT16 and ESR1 SNPs associated with bone mineral density in premenopausal women

Previous genome-wide association studies (GWAS) have identified common variants in genes associated with variation in bone mineral density (BMD), although most have been carried out in combined samples of older women and men. Meta-analyses of these results have identified numerous single-nucleotide polymorphisms (SNPs) of modest effect at genome-wide significance levels in genes involved in both bone formation and resorption, as well as other pathways. We performed a meta-analysis restricted to premenopausal white women from four cohorts (n = 4061 women, aged 20 to 45 years) to identify genes influencing peak bone mass at the lumbar spine and femoral neck. After imputation, age- and weight-adjusted bone-mineral density (BMD) values were tested for association with each SNP. Association of an SNP in the WNT16 gene (rs3801387; p = 1.7 × 10(-9) ) and multiple SNPs in the ESR1/C6orf97 region (rs4870044; p = 1.3 × 10(-8) ) achieved genome-wide significance levels for lumbar spine BMD. These SNPs, along with others demonstrating suggestive evidence of association, were then tested for association in seven replication cohorts that included premenopausal women of European, Hispanic-American, and African-American descent (combined n = 5597 for femoral neck; n = 4744 for lumbar spine). When the data from the discovery and replication cohorts were analyzed jointly, the evidence was more significant (WNT16 joint p = 1.3 × 10(-11) ; ESR1/C6orf97 joint p = 1.4 × 10(-10) ). Multiple independent association signals were observed with spine BMD at the ESR1 region after conditioning on the primary signal. Analyses of femoral neck BMD also supported association with SNPs in WNT16 and ESR1/C6orf97 (p < 1 × 10(-5) ). Our results confirm that several of the genes contributing to BMD variation across a broad age range in both sexes have effects of similar magnitude on BMD of the spine in premenopausal women. These data support the hypothesis that variants in these genes of known skeletal function also affect BMD during the premenopausal period.

Assessing bone health in children and adolescents

During normal childhood and adolescence, the skeleton undergoes tremendous change. Utilizing the processes of modeling and remodeling, the skeleton acquires its adult configuration and ultimately achieves peak bone mass. Optimization of peak bone mass requires the proper interaction of environmental, dietary, hormonal, and genetic influences. A variety of acute and chronic conditions, as well as genetic polymorphisms, are associated with reduced bone density, which can lead to an increased risk of fracture both in childhood and later during adulthood. Bone densitometry has an established role in the evaluation of adults with bone disorders, and the development of suitable reference ranges for children now permits the application of this technology to younger individuals. We present a brief overview of the factors that determine bone density and the emerging role of bone densitometry in the assessment of bone mass in growing children and adolescents.

Genetics of osteoporosis from genome-wide association studies: advances and challenges

Osteoporosis is among the most common and costly diseases and is increasing in prevalence owing to the ageing of our global population. Clinically defined largely through bone mineral density, osteoporosis and osteoporotic fractures have reasonably high heritabilities, prompting much effort to identify the genetic determinants of this disease. Genome-wide association studies have recently provided rapid insights into the allelic architecture of this condition, identifying 62 genome-wide-significant loci. Here, we review how these new loci provide an opportunity to explore how the genetics of osteoporosis can elucidate its pathophysiology, provide drug targets and allow for prediction of future fracture risk.