Loci for regulation of bone mineral density in men and women identified by genome wide linkage scan: the FAMOS study

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

CONTEXT

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Differentially expressed circulating miRNAs in postmenopausal osteoporosis: a meta-analysis

MicroRNAs (miRNAs) have been proven to play a crucial role in postmenopausal osteoporosis (PMO), and studies on their diagnostic value have been increasing. In our study, we aim to identify the key miRNAs in the PMO that might be potential biomarkers. A comprehensive systematic literature search was conducted by searching PubMed, Web of Science, Embase and Cochrane Library databases. In the total of 16 independent miRNA expression studies which contained 327 PMO patients and 328 postmenopausal (PM) healthy control samples, miRNAs were evaluated by using robust rank aggregation (RRA) method. A statistically significant meta-signature of up-regulated hsa-miR-133a-3p (P = 1.38e-03) was determined. Then bioinformatics analysis to recruit putative target genes prediction of hsa-miR-133a-3p and pathway enrichment analysis to reveal what biological processes this miRNA may affect were conducted. It was indicated that pathways were commonly associated with adrenergic signaling in cardiomyocytes, adherens junction, PI3K-Akt signaling pathway and AMPK signaling pathway. Furthermore, STRING and Cytoscape tools were used to visualize the interactions between target genes of hsa-miR-133a-3p. Six genes were detected as hub genes among 576 targets which were CDC42, RHOA, EGFR, VAMP2, PIK3R2 and FN1. After Kyoto Encyclopedia of Genes and Genomes pathway analysis, it was detected that these hub genes were mostly enriched in signaling pathways and cancer. In this meta-analysis, it is stated that circulating hsa-miR-133a-3p may serve as a potential non-invasive biomarker and therapeutic target in PMO.

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

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

Regulation of Bone Metabolism by microRNAs

PURPOSE OF REVIEW

The small non-coding microRNAs (miRNAs) have emerged as important post-transcriptional regulators of various physiological and pathological processes. The purpose of this article is to review the important recent advances on the role of miRNAs in bone remodeling and metabolic bone disorders.

RECENT FINDINGS

In a physiological context, miRNAs regulate bone formation and bone resorption, thereby contributing to the maintenance of bone homeostasis. Under pathological conditions, an aberrant miRNA signaling contributes to the onset and progression of skeletal disorders, such as osteoporosis. Furthermore, miRNAs can be secreted to circulation and have clinical potential as non-invasive biomarkers. In a therapeutic setting, miRNA delivery or antagonism has been reported to affect several diseases under pre-clinical conditions thereby emerging as novel pharmacological tools. miRNAs are key regulators of bone remodeling in health and disease. The future perspectives in the field include the role of secreted miRNAs in cell-cell communication in the bone environment. Furthermore, the clinical potential of using miRNAs as diagnostic tools and therapeutic targets to treat metabolic bone diseases provides an attractive future direction.

MicroRNAs in bone diseases

MicroRNAs are small, noncoding single-stranded RNAs that have emerged as important posttranscriptional regulators of gene expression, with an essential role in vertebrate development and different biological processes. This review highlights the recent advances in the function of miRNAs and their roles in bone remodeling and bone diseases. MicroRNAs (miRNAs) are a class of small (∼22 nt), noncoding single-stranded RNAs that have emerged as important posttranscriptional regulators of gene expression. They are essential for vertebrate development and play critical roles in different biological processes related to cell differentiation, activity, metabolism, and apoptosis. A rising number of experimental reports now indicate that miRNAs contribute to every step of osteogenesis and bone homeostasis, from embryonic skeletal development to maintenance of adult bone tissue, by regulating the growth, differentiation, and activity of different cell systems inside and outside the skeleton. Importantly, emerging information from animal studies suggests that targeting miRNAs might become an attractive and new therapeutic approach for osteoporosis or other skeletal diseases, even though there are still major concerns related to potential off target effects and the need of efficient delivery methods in vivo. Moreover, besides their recognized effects at the cellular level, evidence is also gathering that miRNAs are excreted and can circulate in the blood or other body fluids with potential paracrine or endocrine functions. Thus, they could represent suitable candidates for becoming sensitive disease biomarkers in different pathologic conditions, including skeletal disorders. Despite these promising perspectives more work remains to be done until miRNAs can serve as robust therapeutic targets or established diagnostic tools for precision medicine in skeletal disorders.

Genetics of pediatric bone strength

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

Genome-Wide Association Study of Bone Mineral Density in Korean Men

Osteoporosis is a medical condition of global concern, with increasing incidence in both sexes. Bone mineral density (BMD), a highly heritable trait, has been proven a useful diagnostic factor in predicting fracture. Because medical information is lacking about male osteoporotic genetics, we conducted a genome-wide association study of BMD in Korean men. With 1,176 participants, we analyzed 4,414,664 single nucleotide polymorphisms (SNPs) after genomic imputation, and identified five SNPs and three loci correlated with bone density and strength. Multivariate linear regression models were applied to adjust for age and body mass index interference. Rs17124500 (p = 6.42 × 10^-7), rs34594869 (p = 6.53 × 10^-7) and rs17124504 (p = 6.53 × 10^-7) in 14q31.3 and rs140155614 (p = 8.64 × 10^-7) in 15q25.1 were significantly associated with lumbar spine BMD (LS-BMD), while rs111822233 (p = 6.35 × 10^-7) was linked with the femur total BMD (FT-BMD). Additionally, we analyzed the relationship between BMD and five genes previously identified in Korean men. Rs61382873 (p = 0.0009) in LRP5, rs9567003 (p = 0.0033) in TNFSF11 and rs9935828 (p = 0.0248) in FOXL1 were observed for LS-BMD. Furthermore, rs33997547 (p = 0.0057) in ZBTB and rs1664496 (p = 0.0012) in MEF2C were found to influence FT-BMD and rs61769193 (p = 0.0114) in ZBTB to influence femur neck BMD. We identified five SNPs and three genomic regions, associated with BMD. The significance of our results lies in the discovery of new loci, while also affirming a previously significant locus, as potential osteoporotic factors in the Korean male population.

Quantitative proteomics and integrative network analysis identified novel genes and pathways related to osteoporosis

Osteoporosis is mainly characterized by low bone mineral density (BMD), and can be attributed to excessive bone resorption by osteoclasts. Migration of circulating monocytes from blood to bone is important for subsequent osteoclast differentiation and bone resorption. Identification of those genes and pathways related to osteoclastogenesis and BMD will contribute to a better understanding of the pathophysiological mechanisms of osteoporosis. In this study, we applied the LC-nano-ESI-MS(E) (Liquid Chromatograph-nano-Electrospray Ionization-Mass Spectrometry) for quantitative proteomic profiling in 33 female Caucasians with discordant BMD levels, with 16 high vs. 17 low BMD subjects. Protein quantitation was accomplished by label-free measurement of total ion currents collected from MS(E) data. Comparison of protein expression in high vs. low BMD subjects showed that ITGA2B (p=0.0063) and GSN (p=0.019) were up-regulated in the high BMD group. Additionally, our protein-RNA integrative analysis showed that RHOA (p=0.00062) differentially expressed between high vs. low BMD groups. Network analysis based on multiple tools revealed two pathways: "regulation of actin cytoskeleton" (p=1.13E-5, FDR=3.34E-4) and "leukocyte transendothelial migration" (p=2.76E-4, FDR=4.71E-3) that are functionally relevant to osteoporosis. Consistently, ITGA2B, GSN and RHOA played crucial roles in these two pathways respectively. All together, our study strongly supported the contribution of the genes ITGA2B, GSN and RHOA and the two pathways to osteoporosis risk.

BIOLOGICAL SIGNIFICANCE

Mass spectrometry based quantitative proteomics study integrated with network analysis identified novel genes and pathways related to osteoporosis. The results were further verified in multiple level studies including protein-RNA integrative analysis and genome wide association studies.

Genetic polymorphism in extracellular regulators of Wnt signaling pathway

The Wnt signaling pathway is mediated by a family of secreted glycoproteins through canonical and noncanonical mechanism. The signaling pathways are regulated by various modulators, which are classified into two classes on the basis of their interaction with either Wnt or its receptors. Secreted frizzled-related proteins (sFRPs) are the member of class that binds to Wnt protein and antagonizes Wnt signaling pathway. The other class consists of Dickkopf (DKK) proteins family that binds to Wnt receptor complex. The present review discusses the disease related association of various polymorphisms in Wnt signaling modulators. Furthermore, this review also highlights that some of the sFRPs and DKKs are unable to act as an antagonist for Wnt signaling pathway and thus their function needs to be explored more extensively.

Linkage and association analyses using families identified a locus affecting an osteoporosis-related trait

Osteoporosis is a common disorder characterized by low bone mass and microarchitectural deterioration of bone tissue, resulting in an increase in bone fragility and in susceptibility to fractures. The genetic basis of osteoporosis is complex and involves multiple genes and environmental factors. Here we introduce a family-based study of the genetics of osteoporosis - the Genetic Analysis of Osteoporosis (GAO) Project - to discover genetic variants affecting osteoporosis-related phenotypes. The GAO Project involved 11 extended families from Barcelona, Spain selected through a proband with osteoporosis (N=367). We performed spine, femur and whole body densitometry for all participants and also analyzed strength and geometrical properties of the hip. Our study focused on 23 densitometric phenotypes that we considered of high clinical relevance and four definitions of low bone mass and fracture status. Pedigree validation was carried out through microsatellite genotyping. The same microsatellites were used to interrogate our data (i) for the replication of previous linkage signals and (ii) for the potential discovery of new linkage signals. The linkage analysis identified one region marked by microsatellite D17S787 showing a strong and significant signal of linkage with femoral shaft cross-sectional moment of inertia (CSMI; LOD=3.18; p=6.5×10(-5)). The chromosomal location marked by microsatellite D17S787 includes several genes, among which two are of particular interest: COL1A1 and SOST, coding for collagen alpha-1 (I) chain and sclerostin, respectively. Follow-up association analysis resulted in only one significant result for rs4792909 from the SOST genomic region (p=0.00248). As a result, we provide strong and significant evidence from both linkage and association analyses that the SOST gene may affect the strength of the femoral shaft. Future investigations should study the relationship between bone mass formation and strength properties of the bones.

CER1 gene variations associated with bone mineral density, bone markers, and early menopause in postmenopausal women

Background

Osteoporosis has a multifactorial pathogenesis characterized by a combination of low bone mass and increased fragility. In our study, we focused on the effects of polymorphisms in CER1 and DKK1 genes, recently reported as important susceptibility genes for osteoporosis, on bone mineral density (BMD) and bone markers in osteoporotic women. Our objective was to evaluate the effect of CER1 and DKK1 variations in 607 postmenopausal women. The entire DKK1 gene sequence and five selected CER1 SNPs were amplified and resequenced to assess whether there is a correlation between these genes and BMD, early menopause, and bone turnover markers in osteoporotic patients.

Results

Osteoporotic women seem to suffer menopause 2 years earlier than the control group. The entire DKK1 gene sequence analysis revealed six variations. There was no correlation between the six DKK1 variations and osteoporosis, in contrast to the five common CER1 variations that were significantly associated with BMD. Additionally, osteoporotic patients with rs3747532 and rs7022304 CER1 variations had significantly higher serum levels of parathyroid hormone and calcitonin and lower serum levels of osteocalcin and IGF-1.

Conclusions

No significant association between the studied DKK1 variations and osteoporosis was found, while CER1 variations seem to play a significant role in the determination of osteoporosis and a potential predictive role, combined with bone markers, in postmenopausal osteoporotic women.

Densidade mineral óssea associada a características físicas e estilo de vida em adolescentes

INTRODUÇÃO: No Brasil, ainda são poucos os estudos que apresentam dados relevantes sobre fatores relacionados às características físicas ou estilo de vida e a densidade mineral óssea (DMO) de adolescentes do sexo feminino. OBJETIVO: Identificar e verificar a contribuição das características físicas e de estilo de vida relacionadas à DMO de adolescentes do sexo feminino. MÉTODOS: A amostra deste estudo foi composta por 329 meninas com idades entre 10 e 20 anos. Como características físicas, foram avaliados: peso corporal, estatura, índice de massa corporal, estágio de maturação sexual, raça e pigmentação cutânea. Já para o estilo de vida, os seguintes fatores foram avaliados: consumo diário de cálcio, nível de atividade física (NAF) e nível socioeconômico (NSE). A densidade mineral óssea (DMO) do corpo inteiro, da coluna lombar e do colo do fêmur foram avaliados pela densitometria óssea. As relações existentes entre variáveis dependentes e independentes foram avaliadas pela correlação de Pearson (r) e regressão múltipla Stepwise (p < 0,05). RESULTADOS: A DMO dos três sítios ósseos tende a aumentar conforme o aumento do peso corporal, estatura, IMC, idade e estágio de maturação sexual (r ≥ 0,43; p < 0,01). Por outro lado, somente o NAF (r = 0,12; p < 0,05) e o NSE (r = 0,14; p < 0,05) correlacionaram-se positivamente com a DMO. O peso corporal, estágio de maturação sexual, idade, consumo de cálcio, NSE e NAF explicaram de 48 a 68% da variação da DMO das adolescentes. CONCLUSÃO: Os resultados sugerem que a utilização de critérios como peso corporal, idade e maturação sexual sejam os mais indicados para controlar as variações da DMO de adolescentes do sexo feminino. Além disso, o NSE, o NAF e o consumo diário de cálcio possuem uma pequena participação na variação da DMO das adolescentes quando comparados com as características físicas.

Vitamin D receptor gene polymorphisms, bone mineral density and fractures in postmenopausal women with osteoporosis

The goal of the study was to investigate the possibility of an association between polymorphisms and single alleles of BsmI, ApaI, TaqI of the vitamin D receptor (VDR) gene with bone mineral density (BMD) and prevalence of vertebral/non-vertebral fractures in a group of postmenopausal Polish women with osteoporosis. The study group comprised of 501 postmenopausal females with osteoporosis (mean age 66.4 ± 8.9), who were diagnosed on the basis of either the WHO criteria or self-reported history of low-energy fractures. The three polymorphisms were determined by PCR (polymerase chain reaction) and RFLP (restriction fragment length polymorphism). BMD at the lumbar spine and femoral neck was assessed by dual energy X-ray absorptiometry (DXA). 285 fractures were reported in the whole group (168 vertebral and 117 non-vertebral). Incidence of non-vertebral fractures was significantly higher in the carriers of single alleles a of ApaI, b of BsmI and T of TaqI VDR gene polymorphisms (p = 0.021, 0.032, 0.020, respectively). No significant associations between allelic variants of the studied polymorphisms and BMD or fracture incidence were found. (1).The presence of single alleles a,b and T of ApaI, BsmI, TaqI VDR gene polymorphisms respectively, might serve as an indicator of non-vertebral fractures. (2). Lack of association between the VDR gene polymorphisms and BMD suggests that VDR contributes to low-energy fractures also through other ways.

Single nucleotide polymorphisms in sFRP4 are associated with bone and body composition related parameters in Danish but not in Belgian men

The senescence accelerated mouse P6 (SAMP6) has a low bone mass and has previously shown to be a good model for senile osteoporosis in humans. In addition to a reduced bone mass, SAMP6 mice are obese and have hyperlipidemia. Using positional cloning and expression studies, an increased expression of sfrp4 was found in these mice. SFRP4 is a modulator of the Wnt signalling pathway. This pathway has been previously shown to be involved in regulating bone mass. Additional evidence that sFRP4 has an influence on BMD was delivered by linkage and association studies mostly performed in Asian populations. Based on these data we decided to perform an association study between common variants in sFRP4, BMD, hip geometry parameters and body composition parameters in a population consisting of 1383 Danish men (783 aged 20-29 years; 600 aged 60-74 years). Afterwards we tried to replicate the significant results in a population of 994 Belgian men. In the Danish population we found 6 SNPs associated with BMD at the hip and/or femoral neck. Furthermore, all 6 SNPs were associated with several hip geometry parameters. The homozygous presence of the minor allele resulted for all SNPs (except rs4720265) in a decrease in bone density and bone strength. Finally, we observed in the Danish population age specific associations with height and fat mass. In the Belgian population we tried to replicate the results of three SNPs with BMD and body composition parameters. Unfortunately, we were not able to replicate the results found in the Danish cohort but we found one SNP (rs2598116) associated with height. In conclusion, genetic variation in sFRP4 has an influence on hip fracture risk, percentage body fat and height in a Danish male population. However, we were unable to replicate these results in an independent Belgian population.

Contribution of myostatin gene polymorphisms to normal variation in lean mass, fat mass and peak BMD in Chinese male offspring

AIM

Myostatin gene is a member of the transforming growth factor-β (TGF-β) family that negatively regulates skeletal muscle growth. Genetic polymorphisms in Myostatin were found to be associated with the peak bone mineral density (BMD) in Chinese women. The purpose of this study was to investigate whether myostatin played a role in the normal variation in peak BMD, lean mass (LM), and fat mass (FM) of Chinese men.

METHODS

Four hundred male-offspring nuclear families of Chinese Han ethnic group were recruited. Anthropometric measurements, including the peak BMD, body LM and FM were measured using dual-energy X-ray absorptiometry (DXA). The single nucleotide polymorphisms (SNPs) studied were tag-SNPs selected by sequencing. Both rs2293284 and +2278GA were genotyped using TaqMan assay, and rs3791783 was genotyped with PCR-restriction fragment length polymorphism (RFLP) analysis. The associations of the SNPs with anthropometric variations were analyzed using the quantitative transmission disequilibrium test (QTDT).

RESULTS

Using QTDT to detect within-family associations, neither single SNP nor haplotype was found to be associated with peak BMD at any bone site. However, rs3791783 was found to be significantly associated with fat mass of the trunk (P<0.001). Moreover, for within-family associations, haplotypes AGG, AAA, and TGG were found to be significantly associated with the trunk fat mass (all P<0.001).

CONCLUSION

Our results suggest that genetic variation within myostatin may play a role in regulating the variation in fat mass in Chinese males. Additionally, the myostatin gene may be a candidate that determines body fat mass in Chinese men.

MiR-133a in human circulating monocytes: a potential biomarker associated with postmenopausal osteoporosis

Background

Osteoporosis mainly occurs in postmenopausal women, which is characterized by low bone mineral density (BMD) due to unbalanced bone resorption by osteoclasts and formation by osteoblasts. Circulating monocytes play important roles in osteoclastogenesis by acting as osteoclast precursors and secreting osteoclastogenic factors, such as IL-1, IL-6 and TNF-α. MicroRNAs (miRNAs) have been implicated as important biomarkers in various diseases. The present study aimed to find significant miRNA biomarkers in human circulating monocytes underlying postmenopausal osteoporosis.

Methodology/Principal Findings

We used ABI TaqMan® miRNA array followed by qRT-PCR validation in circulating monocytes to identify miRNA biomarkers in 10 high and 10 low BMD postmenopausal Caucasian women. MiR-133a was upregulated (P=0.007) in the low compared with the high BMD groups in the array analyses, which was also validated by qRT-PCR (P=0.044). We performed bioinformatic target gene analysis and found three potential osteoclast-related target genes, CXCL11, CXCR3 and SLC39A1. In addition, we performed Pearson correlation analyses between the expression levels of miR-133a and the three potential target genes in the 20 postmenopausal women. We did find negative correlations between miR-133a and all the three genes though not significant.

Conclusions/Significance

This is the first in vivo miRNA expression analysis in human circulating monocytes to identify novel miRNA biomarkers underlying postmenopausal osteoporosis. Our results suggest that miR-133a in circulating monocytes is a potential biomarker for postmenopausal osteoporosis.

Genome-wide linkage scan for quantitative trait loci underlying normal variation in heel bone ultrasound measures

Quantitative ultrasound (QUS) traits are correlated with bone mineral density (BMD), but predict risk for future fracture independent of BMD. Only a few studies, however, have sought to identify specific genes influencing calcaneal QUS measures. The aim of this study was to conduct a genome-wide linkage scan to identify quantitative trait loci (QTL) influencing normal variation in QUS traits. QUS measures were collected from a total of 719 individuals (336 males and 383 females) from the Fels Longitudinal Study who have been genotyped and have at least one set of QUS measurements. Participants ranged in age from 18.0 to 96.6 years and were distributed across 110 nuclear and extended families. Using the Sahara ® bone sonometer, broadband ultrasound attenuation (BUA), speed of sound (SOS) and stiffness index (QUI) were collected from the right heel. Variance components based linkage analysis was performed on the three traits using 400 polymorphic short tandem repeat (STR) markers spaced approximately 10 cM apart across the autosomes to identify QTL influencing the QUS traits. Age, sex, and other significant covariates were simultaneously adjusted. Heritability estimates (h²) for the QUS traits ranged from 0.42 to 0.57. Significant evidence for a QTL influencing BUA was found on chromosome 11p15 near marker D11S902 (LOD = 3.11). Our results provide additional evidence for a QTL on chromosome 11p that harbors a potential candidate gene(s) related to BUA and bone metabolism.

Inositol polyphosphate 4-phosphatase B as a regulator of bone mass in mice and humans

Osteoporosis is a multifactorial genetic disease characterized by reduction of bone mass due to dysregulation of osteoclast differentiation or maturation. Herein, we identified a regulator of osteoclastogenesis, the murine homolog of inositol polyphosphate 4-phosphatase type IIα (Inpp4bα). Expression of Inpp4bα is detected from early osteoclast differentiation to activation stage. Targeted expression of native Inpp4bα ex vivo repressed whereas phosphatase-inactive Inpp4bα stimulated osteoclast differentiation. Inpp4bα acts on intracellular calcium level that modulates NFATc1 nuclear translocation and activation. In vivo mice deficient in Inpp4b displayed increased osteoclast differentiation rate and potential resulting in decreased bone mass and osteoporosis. Importantly, INPP4B in human was identified as a susceptibility locus for osteoporosis. This study defined Inpp4b as a major modulator of the osteoclast differentiation and as a gene linked to variability of bone mineral density in mice and humans.

Contribution of the sclerostin domain-containing protein 1 (SOSTDC1) gene to normal variation of peak bone mineral density in Chinese women and men

A genome-wide linkage analysis in Chinese families revealed a significant quantitative trait loci on chromosome 7p21.1 for femoral neck bone mineral density (BMD) (LOD = 3.68), and a potential candidate gene, sclerostin domain-containing protein 1 (SOSTDC1), is located in this region. SOSTDC1 belongs to a class of bone morphogenetic protein (BMP) antagonists that bind BMPs and regulate their signaling. We therefore genotyped 6 tag single nucleotide polymorphisms (tag-SNPs) in SOSTDC1 gene using allele-specific PCR method and investigated the association between SOSTDC1 gene polymorphisms and peak BMD variation in 401 Chinese female-offspring nuclear families (including 1260 subjects) and 400 Chinese male-offspring nuclear families (including 1215 subjects), respectively. Using both family-based (quantitative transmission disequilibrium test) and population-based (ANOVA) methods of analyses, BMD values were adjusted for age, height and weight. In female-offspring nuclear families, we found a significant within family association between rs16878759 and the lumbar spine peak BMD (P = 0.003) and rs16878759 accounted for 1.4% of the lumbar spine peak BMD variation. Moreover, haplotype CCC (containing rs12699800, rs16878759, and rs17619769) had a significant within family association with the lumbar spine peak BMD (P = 0.001) and accounted for 1.9% of the peak BMD variation at this bone site. However, in the male-offspring nuclear families, we failed to detect any significant association between any SNP or haplotype and peak BMD at any bone site. In conclusion, our results indicate for the first time that the genetic polymorphisms in SOSTDC1 have an effect on attainment and maintenance of peak bone mass in Chinese women.

Suggestive linkage to chromosome 1q for bone mineral apparent density in Brazilian sister adolescents

OBJECTIVE

To investigate linkage to chromosome 1q and 11q region for lumbar spine, femoral neck and total body BMD and volumetric BMD in Brazilian sister adolescents aged 10-20-year-old and 57 mothers.

METHODS

We evaluated 161 sister pairs (n=329) aged 10-20 years old and 57 of their mothers in this study. Physical traits and lifestyle factors were collected as covariates for lumbar spine (LS), femoral neck (FN) and total body (TB) BMD and bone mineral apparent density (BMAD). We selected nine microsatellite markers in chromosome 1q region (spanning nearly 33cM) and eight in chromosome 11q region (spanning nearly 34cM) to perform linkage analysis.

RESULTS

The highest LOD score values obtained from our data were in sister pairs LS BMAD analysis. Their values were: 1.32 (P<0.006), 2.61 (P<0.0002) and 2.44 (P<0.0004) in D1S218, D1S2640 and D1S2623 markers, respectively. No significant LOD score was found with LS and FN BMD/BMAD in chromosome 11q region. Only TB BMD showed significant linkage higher than 1.0 for chromosome 11q region in the markers D11S4191 and D11S937.

DISCUSSION/CONCLUSIONS

Our results provided suggestive linkage for LS BMAD at D1S2640 marker in adolescent sister pairs and suggest a possible candidate gene (LHX4) related to adolescent LS BMAD in this region. These results reinforce chromosome 1q21-23 as a candidate region to harbor one or more bone formation/maintenance gene. In the other hand, it did not repeat for chromosome 11q12-13 in our population.

Polymorphisms in the 5' flank of COL1A1 gene and osteoporosis: meta-analysis of published studies

A meta-analysis of studies was conducted involving 24,511 participants with 7,864 fractures in which polymorphisms in the 5' flank of COL1A1 (rs1107946, rs2412298, and rs1800012) were related to osteoporosis phenotypes. Polymorphisms of all three sites were associated with BMD, and rs1800012 was associated with fracture but effect sizes were modest.

INTRODUCTION AND HYPOTHESIS

Polymorphisms in the 5' flank of COL1A1 gene have been implicated as genetic markers for susceptibility to osteoporosis, but previous studies have yielded conflicting results.

METHODS

We conducted a meta-analysis of 32 studies including 24,511 participants and 7,864 fractures in which alleles at the -1997G/T (rs1107946), -1663in/delT (rs2412298), and Sp1 binding site polymorphisms (rs1800012) of COL1A1 had been related to bone mineral density (BMD) or fracture.

RESULTS

For the Sp1 polymorphism, BMD values in TT homozygotes were 0.13 units [95% CI, 0.03 to 0.24] lower at the spine (p = 0.01) and 0.16 units [0.10 to 0.23] lower at the hip (p = 1 x 10⁻⁶) than GG homozygotes. Clinical fractures were 1.31-fold [1.04-1.65] increased in TT homozygotes (p = 0.02) and vertebral fractures were 1.34-fold [1.01-1.77] increased (p = 0.04). We also observed associations between spine BMD and allelic variants at the -1997G/T (p = 0.05) and the -1663indelT (p = 0.009) sites. We found no association between alleles at the -1997G/T or -1663indelT sites and fracture but power was limited.

CONCLUSIONS

The COL1A1 Sp1 polymorphism is associated with a modest reduction in BMD and an increased risk of fracture, although we cannot fully exclude the possibility that the results may have been influenced by publication bias. Further studies are required to fully evaluate the contribution of the -1997G/T and -1663in/delT sites to these phenotypes and to determine if they interact with the Sp1 polymorphism to regulate susceptibility to osteoporosis.

Bone mineral density is linked to 1p36 and 7p15-13 in a southern Chinese population

Genome-wide linkage scans have identified a number of quantitative trait loci (QTLs) affecting bone mineral density (BMD), mainly in the Caucasian population. In this study, we aim to determine whether seven well-replicated QTLs also contribute to BMD variation in the southern Han Chinese population. Thirty-three microsatellite markers in the proximity of seven QTLs were genotyped in 1,459 subjects from 306 families ascertained through a proband with BMD Z-score equal to or less than -1.3 at either the lumbar spine or hip. Regression-based multipoint linkage analysis was performed. In the entire study population, good linkage evidence of total hip BMD to 7p14 [maximum log of odds (LOD) score (MLS) = 2.75; nominal P = 0.0002] and 1p36 (MLS = 1.6, P = 0.003) was revealed. In the subgroup analysis of 1,166 female subjects, MLS of 3.42, 2.65, 2.42, and 1.54 were obtained on 7p12 for total hip, lumbar spine, trochanter, and femoral neck BMD, respectively. A suggestive linkage signal was achieved at 7p14-15 with a MLS of 3.38 and 3.15 for trochanter and total hip BMD in the 678 premenopausal women, and at 7p12 for femoral neck and total hip BMD with MLS of 2.22 and 3.04 in postmenopausal women. Subgroup analysis of premenopausal women also provided additional evidence of suggestive linkage of total hip BMD to 1p36, with a MLS of 2.84 at 17.07 cM. Thus, linkage of BMD to 1p36 and 7p15-13 is confirmed in southern Chinese. Computational prioritization strategy and published genome-wide association studies suggested RERE and SFRP4 as two promising candidate genes in which variants responsible for the linkage signal may be identified by follow-up gene-wide association studies.

Genetic loci for bone architecture determined by three-dimensional CT in crosses with the diabetic GK rat

The F344 rat carries alleles contributing to bone fragility while the GK rat spontaneously develops type-2 diabetes. These characteristics make F344×GK crosses well suited for the identification of genes related to bone size and allow for future investigation on the association with type-2 diabetes. The aim of this study was to identify quantitative trait loci (QTLs) for bone size phenotypes measured by a new application of three-dimensional computed tomography (3DCT) and to investigate the effects of sex- and reciprocal cross. Tibia from male and female GK and F344 rats, representing the parental, F1 and F2 generations, were examined with 3DCT and analyzed for: total and cortical volumetric BMD, straight and curved length, peri- and endosteal area at mid-shaft. F2 progeny (108 male and 98 female) were genotyped with 192 genome-wide microsatellite markers (average distance 10 cM). Sex- and reciprocal cross-separated QTL analyses were performed for the identification of QTLs linked to 3DCT phenotypes and true interactions were confirmed by likelihood ratio analysis in all F2 animals. Several genome-wide significant QTLs were found in the sex- and reciprocal cross-separated progeny on chromosomes (chr) 1, 3, 4, 9, 10, 14, and 17. Overlapping QTLs for both males and females in the (GK×F344)F2 progeny were located on chr 1 (39-67 cM). This region confirms previously reported pQCT QTLs and overlaps loci for fasting glucose. Sex separated linkage analysis confirmed a male specific QTL on chr 9 (67-82 cM) for endosteal area at the fibula site. Analyses separating the F2 population both by sex and reciprocal cross identified cross specific QTLs on chr 14 (males) and chr 3 and 4 (females). Two loci, chr 4 and 6, are unique to 3DCT and separate from pQCT generated loci. The 3DCT method was highly reproducible and provided high precision measurements of bone size in the rat enabling identification of new sex- and cross-specific loci. The QTLs on chr 1 indicate potential genetic association between bone-related phenotypes and traits affecting type-2 diabetes. The results illustrate the complexity of the genetic architecture of bone size phenotypes and demonstrate the importance of complementary methods for bone analysis.

A genome-wide association analysis implicates SOX6 as a candidate gene for wrist bone mass

Osteoporosis is a highly heritable common bone disease leading to fractures that severely impair the life quality of patients. Wrist fractures caused by osteoporosis are largely due to the scarcity of wrist bone mass. Here we report the results of a genome-wide association study (GWAS) of wrist bone mineral density (BMD). We examined ∼500000 SNP markers in 1000 unrelated homogeneous Caucasian subjects and found a novel allelic association with wrist BMD at rs11023787 in the SOX6 (SRY (sex determining region Y)-box 6) gene (P=9.00×10(-5)). Subjects carrying the C allele of rs11023787 in SOX6 had significantly higher mean wrist BMD values than those with the T allele (0.485:0.462 g cm(-2) for C allele vs. T allele carriers). For validation, we performed SOX6 association for BMD in an independent Chinese sample and found that SNP rs11023787 was significantly associated with wrist BMD in the Chinese sample (P=6.41×10(-3)). Meta-analyses of the GWAS scan and the replication studies yielded P-values of 5.20×10(-6) for rs11023787. Results of this study, together with the functional relevance of SOX6 in cartilage formation, support the SOX6 gene as an important gene for BMD variation.

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.

Polymorphisms in the HOXD4 gene are not associated with peak bone mineral density in Chinese nuclear families

AIM

To determine the associations between HOXD4 gene polymorphisms with peak bone mineral density (BMD) throughing measuring three tagging single nucleotide polymorphisms (tagSNPs), including rs1867863, rs13418078, and rs4972504, in HOXD4.

METHODS

Four hundred Chinese nuclear families with male offspring (1215 subjects) and 401 Chinese nuclear families with female offspring (1260 subjects) were recruited. BMD of the lumbar spine 1-4 (L1-4) and left proximal femur including total hip and femoral neck were measured by dual-energy X-ray absorptiometry. The quantitative transmission disequilibrium test (QTDT) was performed to investigate the association among the tagging SNPs, haplotypes and peak BMD.

RESULTS

Only the CC genotype was identified in rs13418078 in the Chinese population, unlike other populations. We failed to find significant within-family association among these SNPs, haplotypes and peak BMD at any bone site in either male- or female-offspring nuclear families.

CONCLUSION

The results suggest that genetic polymorphisms in HOXD4 may not be a major contributor to the observed variability in peak BMD in the lumbar spine and the hip in Chinese men and women.

Hip geometry variation is associated with bone mineralization pathway gene variants: The Framingham Study

Mineralization of bone matrix is an important process in bone formation; thus defects in mineralization have been implicated in bone mineral density (BMD) and bone structure alterations. Three central regulators of phosphate balance, ALPL, ANKH, and ENPP1, are central in the matrix mineralization process; therefore, the genes encoding them are considered important candidates genes for BMD and bone geometry. To test for an association between these three candidate genes and BMD and bone geometry traits, 124 informative singlenucleotide polymorphisms (SNPs) were selected and genotyped in 1513 unrelated subjects from the Framingham offspring cohort. Initial results showed that SNP rs1974201 in the gene ENPP1 was a susceptibility variant associated with several hip geometric indices, with the strongest p value of 3.8 × 10(7) being observed for femoral neck width. A few modest associations were observed between SNPs in or near ALPL and several bone traits, but no association was observed with ANKH. The association signals observed for SNPs around rs1974201 were attenuated after conditional analysis on rs1974201. Transcription factor binding-site prediction revealed that the HOXA7 binding site was present in the reference sequence with the major allele, whereas this potential binding site is lost in the sequence with the minor allele of rs1974201. In conclusion, we found evidence for association of bone geometry variation with an SNP in ENPP1, a gene in the mineralization pathway. The alteration of a binding site of the deregulator of extracellular matrix HOXA7 warrants further investigation.

miR-29 modulates Wnt signaling in human osteoblasts through a positive feedback loop

Differentiation of human mesenchymal stem cells into osteoblasts is controlled by extracellular cues. Canonical Wnt signaling is particularly important for maintenance of bone mass in humans. Post-transcriptional regulation of gene expression, mediated by microRNAs, plays an essential role in the control of osteoblast differentiation. Here, we find that miR-29a is necessary for human osteoblast differentiation, and miR-29a is increased during differentiation in the mesenchymal precursor cell line hFOB1.19 and in primary cultures of human osteoblasts. Furthermore, the promoter of the expressed sequence tag containing the human miR-29a gene is induced by canonical Wnt signaling. This effect is mediated, at least in part, by two T-cell factor/LEF-binding sites within the proximal promoter. Furthermore, we show that the negative regulators of Wnt signaling, Dikkopf-1 (Dkk1), Kremen2, and secreted frizzled related protein 2 (sFRP2), are direct targets of miR-29a. Endogenous protein levels for these Wnt antagonists are increased in cells transfected with synthetic miR-29a inhibitor. In contrast, transfection with miR-29a mimic decreases expression of these antagonists and potentiates Wnt signaling. Overall, we demonstrate that miR-29 and Wnt signaling are involved in a regulatory circuit that can modulate osteoblast differentiation. Specifically, canonical Wnt signaling induces miR-29a transcription. The subsequent down-regulation of key Wnt signaling antagonists, Dkk1, Kremen2, and sFRP2, by miR-29a potentiates Wnt signaling, contributing to a gene expression program important for osteoblast differentiation. This novel regulatory circuit provides additional insight into how microRNAs interact with signaling molecules during osteoblast differentiation, allowing for fine-tuning of intricate cellular processes.

Clinical review 2: Genetic determinants of bone density and fracture risk--state of the art and future directions

CONTEXT

Osteoporosis is a common, highly heritable condition that causes substantial morbidity and mortality, the etiopathogenesis of which is poorly understood. Genetic studies are making increasingly rapid progress in identifying the genes involved.

EVIDENCE ACQUISITION AND SYNTHESIS

In this review, we will summarize the current understanding of the genetics of osteoporosis based on publications from PubMed from the year 1987 onward.

CONCLUSIONS

Most genes involved in osteoporosis identified to date encode components of known pathways involved in bone synthesis or resorption, but as the field progresses, new pathways are being identified. Only a small proportion of the total genetic variation involved in osteoporosis has been identified, and new approaches will be required to identify most of the remaining genes.

Identification of genes influencing skeletal phenotypes in congenic P/NP rats

We previously showed that alcohol-preferring (P) rats have higher bone density than alcohol-nonpreferring (NP) rats. Genetic mapping in P and NP rats identified a major quantitative trait locus (QTL) between 4q22 and 4q34 for alcohol preference. At the same location, several QTLs linked to bone density and structure were detected in Fischer 344 (F344) and Lewis (LEW) rats, suggesting that bone mass and strength genes might cosegregate with genes that regulate alcohol preference. The aim of this study was to identify the genes segregating for skeletal phenotypes in congenic P and NP rats. Transfer of the NP chromosome 4 QTL into the P background (P.NP) significantly decreased areal bone mineral density (aBMD) and volumetric bone mineral density (vBMD) at several skeletal sites, whereas transfer of the P chromosome 4 QTL into the NP background (NP.P) significantly increased bone mineral content (BMC) and aBMD in the same skeletal sites. Microarray analysis from the femurs using Affymetrix Rat Genome arrays revealed 53 genes that were differentially expressed among the rat strains with a false discovery rate (FDR) of less than 10%. Nine candidate genes were found to be strongly correlated (r(2) > 0.50) with bone mass at multiple skeletal sites. The top three candidate genes, neuropeptide Y (Npy), alpha synuclein (Snca), and sepiapterin reductase (Spr), were confirmed using real-time quantitative PCR (qPCR). Ingenuity pathway analysis revealed relationships among the candidate genes related to bone metabolism involving beta-estradiol, interferon-gamma, and a voltage-gated calcium channel. We identified several candidate genes, including some novel genes on chromosome 4 segregating for skeletal phenotypes in reciprocal congenic P and NP rats.

Skeletal site-related variation in human trabecular bone transcriptome and signaling

Background

The skeletal site-specific influence of multiple genes on bone morphology is recognised, but the question as to how these influences may be exerted at the molecular and cellular level has not been explored.

Methodology

To address this question, we have compared global gene expression profiles of human trabecular bone from two different skeletal sites that experience vastly different degrees of mechanical loading, namely biopsies from iliac crest and lumbar spinal lamina.

Principal Findings

In the lumbar spine, compared to the iliac crest, the majority of the differentially expressed genes showed significantly increased levels of expression; 3406 transcripts were up- whilst 838 were down-regulated. Interestingly, all gene transcripts that have been recently demonstrated to be markers of osteocyte, as well as osteoblast and osteoclast-related genes, were markedly up-regulated in the spine. The transcriptome data is consistent with osteocyte numbers being almost identical at the two anatomical sites, but suggesting a relatively low osteocyte functional activity in the iliac crest. Similarly, osteoblast and osteoclast expression data suggested similar numbers of the cells, but presented with higher activity in the spine than iliac crest. This analysis has also led to the identification of expression of a number of transcripts, previously known and novel, which to our knowledge have never earlier been associated with bone growth and remodelling.

Conclusions and Significance

This study provides molecular evidence explaining anatomical and micro-architectural site-related changes in bone cell function, which is predominantly attributable to alteration in cell transcriptional activity. A number of novel signaling molecules in critical pathways, which have been hitherto not known to be expressed in bone cells of mature vertebrates, were identified.

Common genetic variation in the DKK1 gene is associated with hip axis length but not with bone mineral density and bone turnover markers in young adult men: results from the Odense Androgen Study

LRP5 was recently confirmed as an important susceptibility gene for osteoporosis. Our objective was to evaluate the effect of DKK1 polymorphisms on bone mineral density (BMD), hip geometry, and bone turnover. DKK1 is a secreted protein that binds to LRP5/6 receptors and inhibits canonical Wnt signaling. Using HapMap, we selected three SNPs covering the genetic variation in a 13.53-kb region comprising DKK1. The Odense Androgen Study is a population-based study comprising 783 Caucasian men aged 20-29 years. BMD and hip structural parameters were available for study. Bone turnover markers were used as a secondary end point. All analyses were repeated after adjusting for covariables and in subgroups according to physical activity. We found no significant association between DKK1 and BMD or markers of bone turnover; however, a significant association (P = 0.012) was found for rs1569198 with hip axis length (HAL), independent of BMD and height. Moreover, the association seemed to be driven by the non-sedentary subgroup (P = 0.004). Haplotype analysis further confirmed the association of rs1569198 with HAL. Furthermore, we obtained indications for interaction between DKK1 and LRP5 genotypes for different hip geometry parameters. As almost all variance within the DKK1 gene was covered, we conclude that common variation in this gene does not markedly influence BMD or bone turnover markers in young men. In this population, however, a common SNP in DKK1 does have a significant effect on HAL, implying a possible effect on hip fracture risk in the general population. This finding could be of interest but needs replication in independent populations.

Refined QTLs of osteoporosis-related traits by linkage analysis with genome-wide SNPs: Framingham SHARe

Genome-wide association studies (GWAS) using high-density array of single-nucleotide polymorphisms (SNPs) offer an unbiased strategy to identify new candidate genes for osteoporosis. We used a subset of autosomal SNPs from the Affymetrix 500K+50K SNP GeneChip marker set to examine genetic linkage with multiple highly heritable osteoporosis-related traits, including BMD of the hip and spine, heel ultrasound (attenuation and speed of sound), and geometric indices of the hip, in two generations from the Framingham Osteoporosis Study. Variance component linkage analysis was performed using normalized residuals (adjusted for age, height, BMI, and estrogen status in women). Multipoint linkage analyses produced LOD scores > or =3.0 for BMD on chromosomes (chr.) 9 and 11 and for ultrasound speed of sound on chr. 5. Hip geometric traits were linked with higher LOD scores, such as with shaft width on chr. 4 (LOD=3.9) and chr. 16 (LOD=3.8) and with shaft section modulus on chr. 22 (LOD=4.0). LOD score > or =5.0 was obtained for femoral neck width on chr. 7. In conclusion, with an SNP-based linkage approach, we identified several novel potential QTLs and confirmed previously identified chromosomal regions linked to bone mass and geometry. Subsequent focus on the spectrum of genetic polymorphisms in these refined regions may contribute to finding variants predisposing to osteoporosis.

Genome-wide analyses of gene expression during mouse endochondral ossification

Background

Endochondral ossification is a complex process involving a series of events that are initiated by the establishment of a chondrogenic template and culminate in its replacement through the coordinated activity of osteoblasts, osteoclasts and endothelial cells. Comprehensive analyses of in vivo gene expression profiles during these processes are essential to obtain a complete understanding of the regulatory mechanisms involved.

Methodology/Principal Findings

To address these issues, we completed a microarray screen of three zones derived from manually segmented embryonic mouse tibiae. Classification of genes differentially expressed between each respective zone, functional categorization as well as characterization of gene expression patterns, cytogenetic loci, signaling pathways and functional motifs both confirmed reported data and provided novel insights into endochondral ossification. Parallel comparisons of the microdissected tibiae data set with our previously completed micromass culture screen further corroborated the suitability of micromass cultures for modeling gene expression in chondrocyte development. The micromass culture system demonstrated striking similarities to the in vivo microdissected tibiae screen; however, the micromass system was unable to accurately distinguish gene expression differences in the hypertrophic and mineralized zones of the tibia.

Conclusions/Significance

These studies allow us to better understand gene expression patterns in the growth plate and endochondral bones and provide an important technical resource for comparison of gene expression in diseased or experimentally-manipulated cartilages. Ultimately, this work will help to define the genomic context in which genes are expressed in long bones and to understand physiological and pathological ossification.

Variation in within-bone stiffness measured by nanoindentation in mice bred for high levels of voluntary wheel running

The hierarchical structure of bone, involving micro-scale organization and interaction of material components, is a critical determinant of macro-scale mechanics. Changes in whole-bone morphology in response to the actions of individual genes, physiological loading during life, or evolutionary processes, may be accompanied by alterations in underlying mineralization or architecture. Here, we used nanoindentation to precisely measure compressive stiffness in the femoral mid-diaphysis of mice that had experienced 37 generations of selective breeding for high levels of voluntary wheel running (HR). Mice (n = 48 total), half from HR lines and half from non-selected control (C) lines, were divided into two experimental groups, one with 13-14 weeks of access to a running wheel and one housed without wheels (n = 12 in each group). At the end of the experiment, gross and micro-computed tomography (microCT)-based morphometric traits were measured, and reduced elastic modulus (E(r)) was estimated separately for four anatomical quadrants of the femoral cortex: anterior, posterior, lateral, and medial. Two-way, mixed-model analysis of covariance (ancova) showed that body mass was a highly significant predictor of all morphometric traits and that structural change is more apparent at the microCT level than in conventional morphometrics of whole bones. Both line type (HR vs. C) and presence of the mini-muscle phenotype (caused by a Mendelian recessive allele and characterized by a approximately 50% reduction in mass of the gastrocnemius muscle complex) were significant predictors of femoral cortical cross-sectional anatomy. Measurement of reduced modulus obtained by nanoindentation was repeatable within a single quadrant and sensitive enough to detect inter-individual differences. Although we found no significant effects of line type (HR vs. C) or physical activity (wheel vs. no wheel) on mean stiffness, anterior and posterior quadrants were significantly stiffer (P < 0.0001) than medial and lateral quadrants (32.67 and 33.09 GPa vs. 29.78 and 30.46 GPa, respectively). Our findings of no significant difference in compressive stiffness in the anterior and posterior quadrants agree with previous results for mice, but differ from those for large mammals. Integrating these results with others from ongoing research on these mice, we hypothesize that the skeletons of female HR mice may be less sensitive to the effects of chronic exercise, due to decreased circulating leptin levels and potentially altered endocannabinoid signaling.

How genomics has informed our understanding of the pathogenesis of osteoporosis

Osteoporosis is a skeletal disorder characterized by compromised bone strength that predisposes a person to an increased risk of fracture. Osteoporosis is a complex trait that involves multiple genes, environmental factors, and gene-gene and gene-environment interactions. Twin and family studies have indicated that between 25% and 85% of the variation in bone mass and other skeletal phenotypes is heritable, but our knowledge of the underlying genes is limited. Bone mineral density is the most common assessment for diagnosing osteoporosis and is the most often used quantitative value in the design of genetic studies. In recent years, our understanding of the pathophysiology of osteoporosis has been greatly facilitated by advances brought about by the Human Genome Project. Genetic approaches ranging from family studies of monogenic traits to association studies with candidate genes, to whole-genome scans in both humans and animals have identified a small number of genes that contribute to the heritability of bone mass. Studies with transgenic and knockout mouse models have revealed major new insights into the biology of many of these identified genes, but much more needs to be learned. Ultimately, we hope that by revealing the underlying genetics and biology driving the pathophysiology of osteoporosis, new and effective treatment can be developed to combat and possibly cure this devastating disease. Here we review the rapidly evolving field of the genomics of osteoporosis with a focus on important gene discoveries, new biological/physiological paradigms that are emerging, and many of the unanswered questions and hurdles yet to be overcome in the field.

Bone mineral density variation in men is influenced by sex-specific and non sex-specific quantitative trait loci

INTRODUCTION

A major predictor of age-related osteoporotic fracture is peak areal bone mineral density (aBMD) which is a highly heritable trait. However, few linkage and association studies have been performed in men to identify the genes contributing to normal variation in aBMD. The aim of this study was to perform a genome wide scan in healthy men to identify quantitative trait loci (QTL) that were significantly linked to aBMD and to test whether any of these might be sex-specific.

METHODS

aBMD at the spine and hip were measured in 515 pairs of brothers, aged 18-61 (405 white pairs, 110 black pairs). Linkage analysis in the brother sample was compared with results in a previously published sample of 774 sister pairs to identify sex-specific quantitative trait loci (QTL).

RESULTS

A genome wide scan identified significant QTL (LOD>3.6) for aBMD on chromosomes 4q21 (hip), 7q34 (spine), 14q32 (hip), 19p13 (hip), 21q21 (hip), and 22q13 (hip). Analysis suggested that the QTL on chromosomes 7q34, 14q32, and 21q21 were male-specific whereas the others were not sex-specific.

CONCLUSIONS

This study demonstrates that six QTL were significantly linked with aBMD in men. One was linked to the spine and five were linked to the hip. When compared to published data in women from the same geographical region, the QTL on chromosomes 7, 14 and 21 were male-specific. The occurrence of sex-specific genes in humans for aBMD has important implications for the pathogenesis and treatment of osteoporosis.

Further genetic evidence suggesting a role for the RhoGTPase-RhoGEF pathway in osteoporosis

Osteoporosis is a highly heritable trait that appears to be influenced by multiple genes. Genome-wide linkage studies have highlighted the chromosomal region 3p14-p21 as a quantitative trait locus for BMD. We have previously published evidence suggesting that the ARHGEF3 gene from this region is associated with BMD in women. The product of this gene activates the RHOA GTPase, the gene for which is also located within this region. The aim of this study was to evaluate the influence of genetic polymorphism in RHOA on bone density in women. Sequence variation within the RHOA gene region was determined using 9 single nucleotide polymorphisms (SNPs) in a discovery cohort of 769 female sibs. Of the 9 SNPs, one was found to be monomorphic with the others representing 3 distinct linkage disequilibrium (LD) blocks. Using FBAT software, significant associations were found between two of these LD blocks and BMD Z-score of the spine and hip (P=0.001-0.036). The LD block tagged by the SNP rs17595772 showed maximal association, with the more common G allele at rs17595772 associated with decreased BMD Z-score. Genotyping for rs17595772 in a replication cohort of 780 postmenopausal women confirmed an association with BMD Z-score (P=0.002-0.036). Again, the G allele was found to be associated with a reduced hip and spine BMD Z-score. These results support the implication of the RhoGTPase-RhoGEF pathway in osteoporosis, and suggest that one or more genes in this pathway may be responsible for the linkage observed between 3p14-p21 and BMD.

Comparison of whole genome linkage scans in premenopausal and postmenopausal women: no bone-loss-specific QTLs were implicated

This study was conducted to investigate if there exist bone-loss-specific quantitative trait loci (QTLs) for females. Genome-wide linkage scans were conducted in total, premenopausal, and postmenopausal women, respectively. No QTLs exclusively were found in postmenopausal women, suggesting that no bone-loss-specific QTL was implicated independent of BMD in our sample.

INTRODUCTION

Bone mineral density (BMD) in elderly women is determined jointly by peak bone mass achieved before menopause and by subsequent bone loss upon and after menopause. Peak bone mass is under strong genetic control, but whether bone loss has genetic determination independent of peak BMD is unknown.

MATERIALS AND METHODS

To investigate if there exist bone-loss-specific quantitative trait loci (QTLs) for females, we conducted genome-wide linkage scans in 2,582 Caucasian females from 451 pedigrees including 1,486 premenopausal and 1,096 postmenopausal women. Linkage analyses were performed in the total sample and premenopausal and postmenopausal women subgroups, respectively, and the results were compared.

RESULTS

No linkage evidence was found exclusively in postmenopausal women. Linkage signals identified are largely consistent in the total, premenopausal, and postmenopausal samples. For example, for spine BMD, for the total sample, a significant linkage was obtained on 15q13 (LOD = 3.67), and LOD scores of 1.52 and 2.49 were achieved on 15q13 in premenopausal and postmenopausal women, respectively.

CONCLUSIONS

We did not find any QTLs exclusively in postmenopausal women; hence, no specific QTL for bone loss was implicated independent of BMD in our female sample.

Pre-B-cell leukemia homeobox 1 (PBX1) shows functional and possible genetic association with bone mineral density variation

Bone mineral density (BMD) is one of the major determinants of risk for osteoporotic fracture. Multiple studies reveal that peak bone mass is under strong genetic influence. One of the major susceptibility loci for peak spine BMD has been mapped to chromosome 1q21-q23 in the Caucasian population. We have previously replicated this finding in Southern Chinese pedigrees and detected a maximum multipoint log of odds (LOD) score of 2.36 in this region. To further fine-map this region, 380 single-nucleotide polymorphic (SNP) markers were genotyped in 610 sibpairs from 231 families. Several markers were identified in the association analysis as important candidates underlying BMD variation. Among them, successful replication was demonstrated for SNPs in pre-B-cell leukemia homeobox 1 (PBX1) gene in two other unrelated case-control cohorts. The functional role of PBX1 in bone metabolism was examined in vitro using human bone-derived cells (HBDC) and murine MC3T3-E1 pre-osteoblasts. PBX1 mRNA was constitutively expressed in both HBDC and MC3T3-E1 cells. Immunostaining revealed that PBX1 is localized in the nucleus compartment. Silencing of PBX1 by RNAi in MC3T3-E1 cells decreased the expression of Runx2 and Osterix, the critical transcription factors for osteogenesis, but accelerated cell proliferation and bone nodule formation. Overall, our data suggest a genetic and functional association of PBX1 with BMD.

Quantitative trait locus on chromosome 1q influences bone loss in young Mexican American adults

Bone loss occurs as early as the third decade and its cumulative effect throughout adulthood may impact risk for osteoporosis in later life, however, the genes and environmental factors influencing early bone loss are largely unknown. We investigated the role of genes in the change in bone mineral density (BMD) in participants in the San Antonio Family Osteoporosis Study. BMD change in 327 Mexican Americans (ages 25-45 years) from 32 extended pedigrees was calculated from DXA measurements at baseline and follow-up (3.5 to 8.9 years later). Family-based likelihood methods were used to estimate heritability (h(2)) and perform autosome-wide linkage analysis for BMD change of the proximal femur and forearm and to estimate heritability for BMD change of lumbar spine. BMD change was significantly heritable for total hip, ultradistal radius, and 33% radius (h(2) = 0.34, 0.34, and 0.27, respectively; p < 0.03 for all), modestly heritable for femoral neck (h(2) = 0.22; p = 0.06) and not heritable for spine BMD. Covariates associated with BMD change included age, sex, baseline BMD, menopause, body mass index, and interim BMI change, and accounted for 6% to 24% of phenotype variation. A significant quantitative trait locus (LOD = 3.6) for femoral neck BMD change was observed on chromosome 1q23. In conclusion, we observed that change in BMD in young adults is heritable and performed one of the first linkage studies for BMD change. Linkage to chromosome 1q23 suggests that this region may harbor one or more genes involved in regulating early BMD change of the femoral neck.

Quantitative trait loci, genes, and polymorphisms that regulate bone mineral density in mouse

This is an in silico analysis of data available from genome-wide scans. Through analysis of QTL, genes and polymorphisms that regulate BMD, we identified 82 BMD QTL, 191 BMD-associated (BMDA) genes, and 83 genes containing known BMD-associated polymorphisms (BMDAP). The catalogue of all BMDA/BMDAP genes and relevant literatures are provided. In total, there are substantially more BMDA/BMDAP genes in regions of the genome where QTL have been identified than in non-QTL regions. Among 191 BMDA genes and 83 BMDAP genes, 133 and 58 are localized in QTL regions, respectively. The difference was still noticeable for the chromosome distribution of these genes between QTL and non-QTL regions. These results have allowed us to generate an integrative profile of QTL, genes, polymorphisms that determine BMD. These data could facilitate more rapid and comprehensive identification of causal genes underlying the determination of BMD in mouse and provide new insights into how BMD is regulated in humans.

Human genetics of osteoporosis

A family history of hip fracture carries a twofold increased risk of fracture among descendants. Genetic factors indeed play a major role in the determination of bone mineral density (BMD) and osteoporosis risk. Multiple chromosomal loci have been mapped by linkage approaches which potentially carry hundreds of genes involved in the determination of bone mass and quality. Association studies based on candidate gene polymorphisms and subsequent meta-analyses, and the more recent genome-wide association studies (GWAS), have clearly identified a handful of genes associated with BMD and/or fragility fractures. Among them are genes coding for the LDL-receptor related protein 5 (LRP5), estrogen receptor alpha (ESR1) and osteoprotegerin, OPG (TNFRSf11b). However, the percentage of osteoporosis risk explained by any of these polymorphisms is small, indicating that most genetic risk factors remain to be discovered and/or that interaction with environmental factors needs further consideration.

Genomic expression analysis of rat chromosome 4 for skeletal traits at femoral neck

Hip fracture is the most devastating osteoporotic fracture type with significant morbidity and mortality. Several studies in humans and animal models identified chromosomal regions linked to hip size and bone mass. Previously, we identified that the region of 4q21-q41 on rat chromosome (Chr) 4 harbors multiple femoral neck quantitative trait loci (QTLs) in inbred Fischer 344 (F344) and Lewis (LEW) rats. The purpose of this study is to identify the candidate genes for femoral neck structure and density by correlating gene expression in the proximal femur with the femoral neck phenotypes linked to the QTLs on Chr 4. RNA was extracted from proximal femora of 4-wk-old rats from F344 and LEW strains, and two other strains, Copenhagen 2331 and Dark Agouti, were used as a negative control. Microarray analysis was performed using Affymetrix Rat Genome 230 2.0 arrays. A total of 99 genes in the 4q21-q41 region were differentially expressed (P < 0.05) among all strains of rats with a false discovery rate <10%. These 99 genes were then ranked based on the strength of correlation between femoral neck phenotypes measured in F2 animals, homozygous for a particular strain's allele at the Chr 4 QTL and the expression level of the gene in that strain. A total of 18 candidate genes were strongly correlated (r(2) > 0.50) with femoral neck width and prioritized for further analysis. Quantitative PCR analysis confirmed 14 of 18 of the candidate genes. Ingenuity pathway analysis revealed several direct or indirect relationships among the candidate genes related to angiogenesis (VEGF), bone growth (FGF2), bone formation (IGF2 and IGF2BP3), and resorption (TNF). This study provides a shortened list of genetic determinants of skeletal traits at the hip and may lead to novel approaches for prevention and treatment of hip fracture.

Complex genetic regulation of bone mineral density and insulin-like growth factor-I in C57BL/6J-Chr #A/J/NaJ chromosome substitution strains

Low bone mineral density (BMD) is a phenotype associated with osteoporosis and increased risk of fracture. Since 60-80% of variation in BMD is associated with genetic factors, we used the novel approach of chromosome substitution strains (CSS) to identify chromosomes that harbor genes that regulate BMD. Specifically, we evaluated 24 wk old C57BL/6J-Chr #(A/J)/NaJ CSS (n = 7 to 18) in which each chromosome in the host C57BL/6J strain is replaced by a corresponding chromosome from the donor A/J strain (19 autosomes, X, Y). We determined several A/J chromosomes contribute to body weight (BW), percent body fat (BF), whole body areal BMD (aBMD), and serum insulin-like growth factor (IGF)-I in both a positive and negative manner when compared with C57BL/6J. Specifically, C57BL/6J-Chr 5(A/J)/NaJ (B.A-5) (males) and B.A-13 (females) contributed to increased BW, whereas B.A-3, 4, 8, 9, 12, and 18 (males) and B.A-3, 4, and 11 (females) contributed to reduced BW. B.A-5 (males) and B.A-13 (females) contributed to increased BF, whereas B.A-12 (males) and B.A-3, 4, 10, and 11 (females) contributed to reduced BF. B.A-14 (females) contributed to increased aBMD and B.A-1, 2, 6, 9, 10, and 18 (males) and B.A-8, 9, and 10 (females) contributed to reduced aBMD. To determine if similar chromosomes regulate aBMD and IGF-I, we determined serum concentrations of IGF-I. B.A-14 and Y (males) and B.A-6 (females) contributed to increased IGF-I and male B.A-3 and female B.A-8 contributed to reduced IGF-I. Overall, we identified several sex-dependent and -independent chromosomes that regulate aBMD and IGF-I in adult mice.