Proximal hip geometry is linked to several chromosomal regions: genome-wide linkage results from the Framingham Osteoporosis Study

Genetic and environmental determinants of bone quality: a cross-sectional analysis of the Hungarian Twin Registry

There is abundant evidence that bone mineral content is highly heritable, while the heritability of bone quality (i.e. trabecular bone score [TBS] and quantitative ultrasound index [QUI]) is rarely investigated. We aimed to disentangle the role of genetic, shared and unique environmental factors on TBS and QUI among Hungarian twins. Our study includes 82 twin (48 monozygotic, 33 same-sex dizygotic) pairs from the Hungarian Twin Registry. TBS was determined by DXA, QUI by calcaneal bone ultrasound. To estimate the genetic and environmental effects, we utilized ACE-variance decomposition. For the unadjusted model of TBS, an AE model provided the best fit with > 80% additive genetic heritability. Adjustment for age, sex, BMI and smoking status improved model fit with 48.0% of total variance explained by independent variables. Furthermore, there was a strong dominant genetic effect (73.7%). In contrast, unadjusted and adjusted models for QUI showed an AE structure. Adjustments improved model fit and 25.7% of the total variance was explained by independent variables. Altogether 70-90% of the variance in QUI was related to additive genetic influences. We found a strong genetic heritability of bone quality in unadjusted models. Half of the variance of TBS was explained by age, sex and BMI. Furthermore, the adjusted model suggested that the genetic component of TBS could be dominant or an epistasis could be present. In contrast, independent variables explained only a quarter of the variance of QUI and the additive heritability explained more than half of all the variance.

Endophenotype trait domains for advancing gene discovery in autism spectrum disorder

Autism spectrum disorder (ASD) is associated with a diverse range of etiological processes, including both genetic and non-genetic causes. For a plurality of individuals with ASD, it is likely that the primary causes involve multiple common inherited variants that individually account for only small levels of variation in phenotypic outcomes. This genetic landscape creates a major challenge for detecting small but important pathogenic effects associated with ASD. To address similar challenges, separate fields of medicine have identified endophenotypes, or discrete, quantitative traits that reflect genetic likelihood for a particular clinical condition and leveraged the study of these traits to map polygenic mechanisms and advance more personalized therapeutic strategies for complex diseases. Endophenotypes represent a distinct class of biomarkers useful for understanding genetic contributions to psychiatric and developmental disorders because they are embedded within the causal chain between genotype and clinical phenotype, and they are more proximal to the action of the gene(s) than behavioral traits. Despite their demonstrated power for guiding new understanding of complex genetic structures of clinical conditions, few endophenotypes associated with ASD have been identified and integrated into family genetic studies. In this review, we argue that advancing knowledge of the complex pathogenic processes that contribute to ASD can be accelerated by refocusing attention toward identifying endophenotypic traits reflective of inherited mechanisms. This pivot requires renewed emphasis on study designs with measurement of familial co-variation including infant sibling studies, family trio and quad designs, and analysis of monozygotic and dizygotic twin concordance for select trait dimensions. We also emphasize that clarification of endophenotypic traits necessarily will involve integration of transdiagnostic approaches as candidate traits likely reflect liability for multiple clinical conditions and often are agnostic to diagnostic boundaries. Multiple candidate endophenotypes associated with ASD likelihood are described, and we propose a new focus on the analysis of "endophenotype trait domains" (ETDs), or traits measured across multiple levels (e.g., molecular, cellular, neural system, neuropsychological) along the causal pathway from genes to behavior. To inform our central argument for research efforts toward ETD discovery, we first provide a brief review of the concept of endophenotypes and their application to psychiatry. Next, we highlight key criteria for determining the value of candidate endophenotypes, including unique considerations for the study of ASD. Descriptions of different study designs for assessing endophenotypes in ASD research then are offered, including analysis of how select patterns of results may help prioritize candidate traits in future research. We also present multiple candidate ETDs that collectively cover a breadth of clinical phenomena associated with ASD, including social, language/communication, cognitive control, and sensorimotor processes. These ETDs are described because they represent promising targets for gene discovery related to clinical autistic traits, and they serve as models for analysis of separate candidate domains that may inform understanding of inherited etiological processes associated with ASD as well as overlapping neurodevelopmental disorders.

Trabecular organization of the proximal femur in Paranthropus robustus: Implications for the assessment of its hip joint loading conditions

Reconstruction of the locomotor repertoire of the australopiths (Australopithecus and Paranthropus) has progressively integrated information from the mechanosensitive internal structure of the appendicular skeleton. Recent investigations showed that the arrangement of the trabecular network at the femoral head center is biomechanically compatible with the pattern of cortical bone distribution across the neck, both suggesting a full commitment to bipedalism in australopiths, but associated with a slightly altered gait kinematics compared to Homo involving more lateral deviation of the body center of mass over the stance limb. To provide a global picture in Paranthropus robustus of the trabecular architecture of the proximal femur across the head, neck and greater trochanter compartments, we applied techniques of virtual imaging to the variably preserved Early Pleistocene specimens SK 82, SK 97, SK 3121, SKW 19 and SWT1/LB-2 from the cave site of Swartkrans, South Africa. We also assessed the coherence between the structural signals from the center of the head and those from the trabecular network of the inferolateral portion of the head and the inferior margin of the neck, sampling the so-called vertical bundle, which in humans represents the principal compressive system of the joint. Our analyses show a functionally related trabecular organization in Pa. robustus that closely resembles the extant human condition, but which also includes some specificities in local textural arrangement. The network of the inferolateral portion of the head shows a humanlike degree of anisotropy and a bone volume fraction intermediate between the extant human and the African ape patterns. These results suggest slight differences in gait kinematics between Pa. robustus and extant humans. The neck portion of the vertical bundle revealed a less biomechanically sensitive signal. Future investigations on the australopith hip joint loading environment should more carefully investigate the trabecular structure of the trochanteric region and possible structural covariation between cortical bone distribution across the neck and site-specific trabecular properties of the arcuate bundle.

The genetic architecture of osteoporosis and fracture risk

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

Meta-Analysis of Genomewide Association Studies Reveals Genetic Variants for Hip Bone Geometry

Hip geometry is an important predictor of fracture. We performed a meta-analysis of GWAS studies in adults to identify genetic variants that are associated with proximal femur geometry phenotypes. We analyzed four phenotypes: (i) femoral neck length; (ii) neck-shaft angle; (iii) femoral neck width, and (iv) femoral neck section modulus, estimated from DXA scans using algorithms of hip structure analysis. In the Discovery stage, 10 cohort studies were included in the fixed-effect meta-analysis, with up to 18,719 men and women ages 16 to 93 years. Association analyses were performed with ∼2.5 million polymorphisms under an additive model adjusted for age, body mass index, and height. Replication analyses of meta-GWAS significant loci (at adjusted genomewide significance [GWS], threshold p ≤ 2.6 × 10-8 ) were performed in seven additional cohorts in silico. We looked up SNPs associated in our analysis, for association with height, bone mineral density (BMD), and fracture. In meta-analysis (combined Discovery and Replication stages), GWS associations were found at 5p15 (IRX1 and ADAMTS16); 5q35 near FGFR4; at 12p11 (in CCDC91); 11q13 (near LRP5 and PPP6R3 (rs7102273)). Several hip geometry signals overlapped with BMD, including LRP5 (chr. 11). Chr. 11 SNP rs7102273 was associated with any-type fracture (p = 7.5 × 10-5 ). We used bone transcriptome data and discovered several significant eQTLs, including rs7102273 and PPP6R3 expression (p = 0.0007), and rs6556301 (intergenic, chr.5 near FGFR4) and PDLIM7 expression (p = 0.005). In conclusion, we found associations between several genes and hip geometry measures that explained 12% to 22% of heritability at different sites. The results provide a defined set of genes related to biological pathways relevant to BMD and etiology of bone fragility. © 2019 American Society for Bone and Mineral Research.

Bone Strength Estimated by Micro-Finite Element Analysis (µFEA) Is Heritable and Shares Genetic Predisposition With Areal BMD: The Framingham Study

Genetic factors contribute to the risk of bone fractures, partly because of effects on bone strength. High-resolution peripheral quantitative computed tomography (HR-pQCT) estimates bone strength using micro-finite element analysis (µFEA). The goal of this study was to investigate if the bone failure load estimated by HR-pQCT-based µFEA is heritable and to what extent it shares genetic regulation with areal bone mineral density (aBMD). Bone microarchitecture was measured by HR-pQCT at the ultradistal tibia and ultradistal radius in adults from the Framingham Heart Study (n = 1087, mean age 72 years; 57% women). Radial and tibial failure load in compression were estimated by µFEA. Femoral neck (FN) and ultradistal forearm (UD) aBMD were measured by dual-energy X-ray absorptiometry (DXA). Heritability (h2 ) of failure load and aBMD and genetic correlations between them was estimated adjusting for covariates (age and sex). Failure load values at the non-weight-bearing ultradistal radius and at the weight-bearing ultradistal tibia were highly correlated (r = 0.906; p < 0.001). Estimates of h2 adjusted for covariates were 0.522 for the radius and 0.497 for the tibia. Additional adjustment for height did not impact on the h2 results, but adjustment for aBMD at the UD and FN somewhat decreased h2 point estimates: 0.222 and 0.380 for radius and tibia, respectively. In bivariate analysis, there was a high phenotypic and genetic correlation between covariate-adjusted failure load at the radius and UD aBMD (ρP  = 0.826, ρG  = 0.954, respectively), whereas environmental correlations were lower (ρE  = 0.696), all highly significant (p < 0.001). Similar correlations were observed between tibial failure load and femoral neck aBMD (ρP  = 0.577, ρG  = 0.703, both p < 0.001; ρE  = 0.432, p < 0.05). These data from adult members of families from a population-based cohort suggest that bone strength of distal extremities estimated by micro-finite element analysis is heritable and shares some genetic composition with areal BMD, regardless of the skeletal site. © 2017 American Society for Bone and Mineral Research.

The influence of acetabular morphology on prediction of proximal femur fractures types in an elderly population

INTRODUCTION

The role of proximal femur morphology to the development of certain proximal femur fracture types both femoral neck and trochanteric fractures has been observed. However, the relavance of acetabular morphology to the development of proximal femur fractures is not extensively questioned. Therefore the aim of the study was to determine whether there is a correlation between acetabular morphology and pathogenesis of 2 different hip fracture types after low energy trauma.

METHODS

This retrospective study includes 60 cases (41 women, 19 men) with a proximal femoral fracture after a low energy trauma between July 2012 and December 2014. Acetabular depth and acetabular index were measured on pelvic radiographs. Neck shaft angle, hip axis length and cortical index were measured on pelvic computed tomography scans. All measurements were performed on the contralateral hip.

RESULTS

Mean age was 77.56 ± 8.99 years (range 61-92 years). No statistically significant difference was found with regard to neck shaft angle, acetabular depth or cortical index measurements between patients with femoral neck fracture and patients with trochanteric femoral fractures (p>0.05). Acetabular index measurement was higher (p = 0.001) and hip axis length measurement was lower (p = 0.001) in trochanteric fracture group as compared to femoral neck fracture.

CONCLUSIONS

The rate of trochanteric femur fractures is higher in patients with high acetabular index, whereas the rate of femoral neck fractures is higher in patients with increased hip axis length.

The characteristic mid-shaft cross-sectional shape of the ulna in Jomon hunter-gatherers

OBJECTIVES

The Jomon people were hunter-gatherers who inhabited the Japanese archipelago between 10,000 BC and 300 BC. Here, we focus on the mid-shaft cross-sectional shape of the ulna in the Jomon population and compare it with modern Japanese people.

MATERIALS AND METHODS

Jomon specimens, including 32 males and 22 females, were excavated from shell mound sites in the Pacific and Seto inland coastal area of Honshu island in the Japanese archipelago dated to the Late-to-Final Jomon phase (between 2,000 BC and 300 BC). Mid-shaft ulna cross-sectional shapes were compared with modern Japanese specimens (25 males, 21 females) using standard linear measurements and elliptic Fourier analysis (EFA). Differences from both sides of this element were compared using predicted handedness.

RESULTS

Linear measurements and EFA results show that ulna shape of both sexes within the Jomon population are relatively larger in the antero-posterior direction and have more developed posterior borders than modern Japanese males and females. No significant differences were observed between Jomon sexes based on the predicted dominant side, but differences were evident in the predicted nondominant side. At the same time, bilateral differences were recognized in Jomon females, because of a lower level of posterior border development in the predicted nondominant side compared to the dominant side.

DISCUSSION

Shape differences between Jomon people and modern Japanese can be explained by variation in the habitual loading of the ulna. Sexual dimorphism in ulna shape within the Jomon population suggests division of labor differences.

Heritability and Genetic Correlations for Bone Microarchitecture: The Framingham Study Families

High-resolution peripheral quantitative computed tomography (HR-pQCT) measures bone microarchitecture and volumetric bone mineral density (vBMD), important risk factors for osteoporotic fractures. We estimated the heritability (h² ) of bone microstructure indices and vBMD, measured by HR-pQCT, and genetic correlations (ρ_G ) among them and between them and regional aBMD measured by dual-energy X-ray absorptiometry (DXA), in adult relatives from the Framingham Heart Study. Cortical (Ct) and trabecular (Tb) traits were measured at the distal radius and tibia in up to 1047 participants, and ultradistal radius (UD) aBMD was obtained by DXA. Heritability estimates, adjusted for age, sex, and estrogenic status (in women), ranged from 19.3% (trabecular number) to 82.8% (p < 0.01, Ct.vBMD) in the radius and from 51.9% (trabecular thickness) to 98.3% (cortical cross-sectional area fraction) in the tibia. Additional adjustments for height, weight, and radial aBMD had no major effect on h² estimates. In bivariate analyses, moderate to high genetic correlations were found between radial total vBMD and microarchitecture traits (ρ_G from 0.227 to 0.913), except for cortical porosity. At the tibia, a similar pattern of genetic correlations was observed (ρ_G from 0.274 to 0.948), except for cortical porosity. Environmental correlations between the microarchitecture traits were also substantial. There were high genetic correlations between UD aBMD and multivariable-adjusted total and trabecular vBMD at the radius (ρ_G  = 0.811 and 0.917, respectively). In summary, in related men and women from a population-based cohort, cortical and trabecular microarchitecture and vBMD at the radius and tibia were heritable and shared some h² with regional aBMD measured by DXA. These findings of high heritability of HR-pQCT traits, with a slight attenuation when adjusting for aBMD, supports further work to identify the specific variants underlying volumetric bone density and fine structure of long bones. Knowledge that some of these traits are genetically correlated can serve to reduce the number of traits for genetic association studies. © 2016 American Society for Bone and Mineral Research.

Limb Bone Structural Proportions and Locomotor Behavior in A.L. 288-1 ("Lucy")

While there is broad agreement that early hominins practiced some form of terrestrial bipedality, there is also evidence that arboreal behavior remained a part of the locomotor repertoire in some taxa, and that bipedal locomotion may not have been identical to that of modern humans. It has been difficult to evaluate such evidence, however, because of the possibility that early hominins retained primitive traits (such as relatively long upper limbs) of little contemporaneous adaptive significance. Here we examine bone structural properties of the femur and humerus in the Australopithecus afarensis A.L. 288-1 ("Lucy", 3.2 Myr) that are known to be developmentally plastic, and compare them with other early hominins, modern humans, and modern chimpanzees. Cross-sectional images were obtained from micro-CT scans of the original specimens and used to derive section properties of the diaphyses, as well as superior and inferior cortical thicknesses of the femoral neck. A.L. 288-1 shows femoral/humeral diaphyseal strength proportions that are intermediate between those of modern humans and chimpanzees, indicating more mechanical loading of the forelimb than in modern humans, and by implication, a significant arboreal locomotor component. Several features of the proximal femur in A.L. 288-1 and other australopiths, including relative femoral head size, distribution of cortical bone in the femoral neck, and cross-sectional shape of the proximal shaft, support the inference of a bipedal gait pattern that differed slightly from that of modern humans, involving more lateral deviation of the body center of mass over the support limb, which would have entailed increased cost of terrestrial locomotion. There is also evidence consistent with increased muscular strength among australopiths in both the forelimb and hind limb, possibly reflecting metabolic trade-offs between muscle and brain development during hominin evolution. Together these findings imply significant differences in both locomotor behavior and ecology between australopiths and later Homo.

Genetic Contribution of Femoral Neck Bone Geometry to the Risk of Developing Osteoporosis: A Family-Based Study

Femoral neck geometry parameters are believed to be as good as bone mineral density as independent factors in predicting hip fracture risk. This study was conducted to analyze the roles of genetic and environmental factors in femoral properties measured in a sample of Spanish families with osteoporotic fractures and extended genealogy. The "Genetic Analysis of Osteoporosis (GAO) Project" involved 11 extended families with a total number of 376 individuals. We studied three categorical phenotypes of particular clinical interest and we used a Hip structural analysis based on DXA to analyze 17 strength and geometrical phenotypes of the hip. All the femoral properties had highly significant heritability, ranging from 0.252 to 0.586. The most significant correlations were observed at the genetic level (ρG). Osteoporotic fracture status (Affected 2) and, particularly, low bone mass and osteoporotic condition (Affected 3) had the highest number of significant genetic correlations with diverse femoral properties. In conclusion, our findings suggest that a relatively simple and easy to use method based on DXA studies can provide useful data on properties of the Hip in clinical practice. Furthermore, our results provide a strong motivation for further studies in order to improve the understanding of the pathophysiological mechanism underlying bone architecture and the genetics of osteoporosis.

Bivariate genome-wide association analyses identified genes with pleiotropic effects for femoral neck bone geometry and age at menarche

Femoral neck geometric parameters (FNGPs), which include cortical thickness (CT), periosteal diameter (W), buckling ratio (BR), cross-sectional area (CSA), and section modulus (Z), contribute to bone strength and may predict hip fracture risk. Age at menarche (AAM) is an important risk factor for osteoporosis and bone fractures in women. Some FNGPs are genetically correlated with AAM. In this study, we performed a bivariate genome-wide association study (GWAS) to identify new candidate genes responsible for both FNGPs and AAM. In the discovery stage, we tested 760,794 SNPs in 1,728 unrelated Caucasian subject, followed by replication analyses in independent samples of US Caucasians (with 501 subjects) and Chinese (with 826 subjects). We found six SNPs that were associated with FNGPs and AAM. These SNPs are located in three genes (i.e. NRCAM, IDS and LOC148145), suggesting these three genes may co-regulate FNGPs and AAM. Our findings may help improve the understanding of genetic architecture and pathophysiological mechanisms underlying both osteoporosis and AAM.

Genome-wide association of an integrated osteoporosis-related phenotype: is there evidence for pleiotropic genes?

Multiple musculoskeletal traits assessed by various methods at different skeletal sites serve as surrogates for osteoporosis risk. However, it is a challenge to select the most relevant phenotypes for genetic study of fractures. Principal component analyses (PCA) were conducted in participants of the Framingham Osteoporosis Study on 17 measures including bond mineral density (BMD) (hip and spine), heel ultrasound, leg lean mass (LLM), and hip geometric indices, adjusting for covariates (age, height, body mass index [BMI]), in a combined sample of 1180 men and 1758 women, as well as in each sex. Four principal components (PCs) jointly explained ~69% of the total variability of musculoskeletal traits. PC1, explaining ~33% of the total variance, was referred to as the component of "Bone strength," because it included the hip and spine BMD as well as several hip cross-sectional properties. PC2 (20.5% variance) was labeled as "Femoral cross-sectional geometry;" PC3 (~8% variance) captured only ultrasound measures; PC4, explaining ~7% variance, was correlated with LLM and hip geometry. We then evaluated ~2.5 mil SNPs for association with PCs 1, 2, and 4. There were genome-wide significant associations (p < 5 × 10⁻⁸) between PC2 and HTR1E (that codes for one of the serotonin receptors) and PC4 with COL4A2 in women. In the sexes-combined sample, AKAP6 was associated with PC2 (p = 1.40 × 10⁻⁷). A single nucleotide polymorphism (SNP) in HTR1E was also associated with the risk of nonvertebral fractures in women (p = 0.005). Functions of top associated genes were enriched for the skeletal and muscular system development (p < 0.05). In conclusion, multivariate combination provides genetic associations not identified in the analysis of primary phenotypes. Genome-wide screening for the linear combinations of multiple osteoporosis-related phenotypes suggests that there are variants with potentially pleiotropic effects in established and novel pathways to be followed up to provide further evidence of their functions.

Identification of homogeneous genetic architecture of multiple genetically correlated traits by block clustering of genome-wide associations

Genome-wide association studies (GWAS) using high-density genotyping platforms offer an unbiased strategy to identify new candidate genes for osteoporosis. It is imperative to be able to clearly distinguish signal from noise by focusing on the best phenotype in a genetic study. We performed GWAS of multiple phenotypes associated with fractures [bone mineral density (BMD), bone quantitative ultrasound (QUS), bone geometry, and muscle mass] with approximately 433,000 single-nucleotide polymorphisms (SNPs) and created a database of resulting associations. We performed analysis of GWAS data from 23 phenotypes by a novel modification of a block clustering algorithm followed by gene-set enrichment analysis. A data matrix of standardized regression coefficients was partitioned along both axes--SNPs and phenotypes. Each partition represents a distinct cluster of SNPs that have similar effects over a particular set of phenotypes. Application of this method to our data shows several SNP-phenotype connections. We found a strong cluster of association coefficients of high magnitude for 10 traits (BMD at several skeletal sites, ultrasound measures, cross-sectional bone area, and section modulus of femoral neck and shaft). These clustered traits were highly genetically correlated. Gene-set enrichment analyses indicated the augmentation of genes that cluster with the 10 osteoporosis-related traits in pathways such as aldosterone signaling in epithelial cells, role of osteoblasts, osteoclasts, and chondrocytes in rheumatoid arthritis, and Parkinson signaling. In addition to several known candidate genes, we also identified PRKCH and SCNN1B as potential candidate genes for multiple bone traits. In conclusion, our mining of GWAS results revealed the similarity of association results between bone strength phenotypes that may be attributed to pleiotropic effects of genes. This knowledge may prove helpful in identifying novel genes and pathways that underlie several correlated phenotypes, as well as in deciphering genetic and phenotypic modularity underlying osteoporosis risk.

Proximal hip geometry and hip fracture risk assessment in a Korean population

The association between proximal femoral geometry and hip fracture risk were investigated. The risk of intertrochanteric fractures increased 1.64-fold and 2.32-fold with 1 standard deviation (sd) increase of hip axis length and neck-shaft angle, respectively, while the risk of femur neck fracture 2.03-fold with 1 sd decrease in femoral head offset.

INTRODUCTION

The purpose of this study was to determine the association between proximal femoral geometry (PFG) and the risk of hip fracture in femur neck (FN) and intertrochanteric (IT) fractures in a Korean population.

METHODS

The study included 151 patients (57 patients with IT fractures, 43 patients with FN fractures, and 51 control patients). Data on BMD, PFG parameters (hip axis length [HAL], neck-shaft angle [NSA], neck length, femoral head offset, neck diameter, shaft diameter (SD), and demographics [age, gender, height, and body weight]) were collected. Descriptive statistics and odds ratios of PFG parameters corrected with demographic variables were obtained using logistic regressions.

RESULTS

HAL (p = 0.046) and NSA (p = 0.003) were significantly greater in the patients with IT fracture than in the control patients, while neither parameter was significantly greater in patients with FN fractures than the control patients. The femoral head offset was significantly shorter in the patients with FN fractures (p = 0.003) compared with the control patients. In patients with IT fractures, the fracture risk increased 1.64-fold (p = 0.048) with a 1 sd increase of the HAL, while it increased 2.32-fold (p = 0.003) with a 1 sd increase of the NSA. In FN fractures, the fracture risk increased 2.03-fold (p = 0.012) with a 1 sd decrease in femoral head offset.

CONCLUSIONS

Our study showed that some PFG parameters as well as BMD values predict hip fractures in a Korean population, and their evaluation may be useful in the understanding of the biomechanics of hip fractures.

Functional significance of genetic variation underlying limb bone diaphyseal structure

Limb bone diaphyseal structure is frequently used to infer hominin activity levels from skeletal remains, an approach based on the well-documented ability of bone to adjust to its loading environment during life. However, diaphyseal structure is also determined in part by genetic factors. This study investigates the possibility that genetic variation underlying diaphyseal structure is influenced by the activity levels of ancestral populations and might also have functional significance in an evolutionary context. We adopted an experimental evolution approach and tested for differences in femoral diaphyseal structure in 1-week-old mice from a line that had been artificially selected (45 generations) for high voluntary wheel running and non-selected controls. As adults, selected mice are significantly more active on wheels and in home cages, and have thicker diaphyses. Structural differences at 1 week can be assumed to primarily reflect the effects of selective breeding rather than direct mechanical stimuli, given that the onset of locomotion in mice is shortly after Day 7. We hypothesized that if genetically determined diaphyseal structure reflects the activity patterns of members of a lineage, then selected animals will have relatively larger diaphyseal dimensions at 1 week compared to controls. The results provide strong support for this hypothesis and suggest that limb bone cross sections may not always only reflect the activity levels of particular fossil individuals, but also convey an evolutionary signal providing information about hominin activity in the past.

Genetics of osteoporosis

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

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.

Genome-wide pleiotropy of osteoporosis-related phenotypes: the Framingham Study

Genome-wide association studies offer an unbiased approach to identify new candidate genes for osteoporosis. We examined the Affymetrix 500K + 50K SNP GeneChip marker sets for associations with multiple osteoporosis-related traits at various skeletal sites, including bone mineral density (BMD, hip and spine), heel ultrasound, and hip geometric indices in the Framingham Osteoporosis Study. We evaluated 433,510 single-nucleotide polymorphisms (SNPs) in 2073 women (mean age 65 years), members of two-generational families. Variance components analysis was performed to estimate phenotypic, genetic, and environmental correlations (rho(P), rho(G), and rho(E)) among bone traits. Linear mixed-effects models were used to test associations between SNPs and multivariable-adjusted trait values. We evaluated the proportion of SNPs associated with pairs of the traits at a nominal significance threshold alpha = 0.01. We found substantial correlation between the proportion of associated SNPs and the rho(P) and rho(G) (r = 0.91 and 0.84, respectively) but much lower with rho(E) (r = 0.38). Thus, for example, hip and spine BMD had 6.8% associated SNPs in common, corresponding to rho(P) = 0.55 and rho(G) = 0.66 between them. Fewer SNPs were associated with both BMD and any of the hip geometric traits (eg, femoral neck and shaft width, section moduli, neck shaft angle, and neck length); rho(G) between BMD and geometric traits ranged from -0.24 to +0.40. In conclusion, we examined relationships between osteoporosis-related traits based on genome-wide associations. Most of the similarity between the quantitative bone phenotypes may be attributed to pleiotropic effects of genes. This knowledge may prove helpful in defining the best phenotypes to be used in genetic studies of osteoporosis.

Functional and association analysis of frizzled 1 (FZD1) promoter haplotypes with femoral neck geometry

Frizzleds are receptors for Wnt signaling and are involved in skeletal morphogenesis. Little is known about the transcriptional regulation of frizzleds in bone cells. In the current study, we determined if two common and potentially functional genetic variants (rs2232157, rs2232158) in the frizzled-1 (FZD1) promoter region and their haplotypes influence FZD1 promoter activity in human osteoblast-like cells. We also determined if these variants are associated with femoral neck bone mineral density (BMD) and geometry in 1319 African ancestry men aged > or =40 years. Real-time quantitative PCR and western blot analysis demonstrated FZD1 mRNA and protein expression in the human osteoblast-like cell lines, MG63 and SaOS-2. Promoter activity was next assessed by transient expression of haplotype specific FZD1 promoter reporter plasmids in these cells. In comparison to the common GG haplotype, promoter activity was 3-fold higher for the TC haplotype in both cell lines (p<0.05). We previously demonstrated that rs2232158 is associated with differential FZD1 promoter activity and Egr1 binding and thus focused further functional analyses on the rs2232157 G-to-T polymorphism. Electrophoretic mobility shift assay demonstrated that distinct nuclear protein complexes were associated with rs2232157 in an allele specific manner. Bioinformatics analysis predicted that the G to T transversion creates an E2F1 binding site, further supporting the functional significance of rs2232157 in FZD1 promoter regulation. Individual SNPs and haplotypes were not associated with femoral neck BMD. The TC haplotype was associated with larger subperiosteal width and greater CSMI (p<0.05). These results suggest that FZD1 expression is regulated in a haplotype-dependent manner in osteoblasts and that these same haplotypes may be associated with biomechanical indices of bone strength.

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 copy number variation association study suggested VPS13B gene for osteoporosis in Caucasians

Osteoporotic fracture (OF) is a serious outcome of osteoporosis. Important risk factors for OF include reduced bone mineral density and unstable bone structure. This genome-wide copy number variation association study suggested VPS13B gene for osteoporosis in Caucasians.

INTRODUCTION

Bone mineral density (BMD) and femoral neck cross-sectional geometric parameters (FNCSGPs) are under strong genetic control. DNA copy number variation (CNV) is an important source of genetic diversity for human diseases. This study aims to identify CNVs associated with BMD and FNCSGPs.

METHODS

Genome-wide CNV association analyses were conducted in 1,000 unrelated Caucasian subjects for BMD at the spine, hip, femoral neck, and for three FNCSGPs -cortical thickness (CT), cross-section area (CSA), and buckling ratio (BR). BMD was measured by dual energy X-ray absorptiometry (DEXA). CT, CSA, and BR were estimated using DEXA measurements. Affymetrix 500K arrays and copy number analysis tool was used to identify CNVs.

RESULTS

A CNV in VPS13B gene was significantly associated with spine, hip and FN BMDs, and CT, CSA, and BR (p < 0.05). Compared to subjects with two copies of the CNV, carriers of one copy had an average of 14.6%, 12.4%, and 13.6% higher spine, hip, and FN BMD, 20.0% thicker CT, 10.6% larger CSA, and 12.4% lower BR. Thus, a decrease of the CNV consistently produced stronger bone, thereby reducing osteoporotic fracture risk.

CONCLUSIONS

VPS13B gene, via affecting BMD and FNCSGPs, is a novel osteoporosis risk gene.

Quantifying the material and structural determinants of bone strength

The ability of a bone to resist fracture depends on the amount of bone present, the spatial distribution of the bone mass as cortical and trabecular bone and the intrinsic properties of the bone material. Whereas low areal bone mineral density (aBMD) predicts fractures, its sensitivity and specificity is low, as over 50% of fractures occur in persons without osteoporosis by BMD testing and most women with osteoporosis do not sustain a fracture. New non-invasive imaging techniques, including three-dimensional (3D) assessments of bone density and geometry, microarchitecture and integrated measurements of bone strength such as finite element analysis (FEA), provide estimates of bone strength that can be used to increase the sensitivity and specificity of fracture risk assessment. Initial observations have shown that these techniques provide information that will improve our understanding of the pathophysiology of skeletal fragility and suggest that these techniques are likely to have a role in the clinical management of individuals at risk for fracture.

Genome-wide association study for femoral neck bone geometry

Poor femoral neck bone geometry at the femur is an important risk factor for hip fracture. We conducted a genome-wide association study (GWAS) of femoral neck bone geometry, examining approximately 379,000 eligible single-nucleotide polymorphisms (SNPs) in 1000 Caucasians. A common genetic variant, rs7430431 in the receptor transporting protein 3 (RTP3) gene, was identified in strong association with the buckling ratio (BR, P = 1.6 x 10(-7)), an index of bone structural instability, and with femoral cortical thickness (CT, P = 1.9 x 10(-6)). The RTP3 gene is located in 3p21.31, a region that we found to be linked with CT (LOD = 2.19, P = 6.0 x 10(-4)) in 3998 individuals from 434 pedigrees. The replication analyses in 1488 independent Caucasians and 2118 Chinese confirmed the association of rs7430431 to BR and CT (combined P = 7.0 x 10(-3) for BR and P = 1.4 x 10(-2) for CT). In addition, 350 hip fracture patients and 350 healthy control individuals were genotyped to assess the association of the RTP3 gene with the risk of hip fracture. Significant association between a nearby common SNP, rs10514713 of the RTP3 gene, and hip fracture (P = 1.0 x 10(-3)) was found. Our observations suggest that RTP3 may be a novel candidate gene for femoral neck bone geometry.

Pathway-based genome-wide association analysis identified the importance of EphrinA-EphR pathway for femoral neck bone geometry

Femoral neck (FN) bone geometry is an important predictor of bone strength with high heritability. Previous studies have revealed certain candidate genes for FN bone geometry. However, the majority of the underlying genetic factors remain to be discovered. In this study, pathway-based genome-wide association analysis was performed to explore the joint effects of genes within biological pathways on FN bone geometry variations in a cohort of 1000 unrelated US whites. Nominal significant associations (nominal p value<0.05) were observed between 76 pathways and a key FN bone geometry variable-section modulus (Z), biomechanically indicative of bone strength subject to bending. Among them, EphrinA-EphR pathway was most significantly associated with FN Z even after multiple testing adjustments (p(FWER) value=0.035). The association of EphrinA-EphR pathway with FN Z was also observed in an independent sample from Framingham Osteoporosis Study. Overall, these results suggest the significant genetic contribution of EphrinA-EphR pathway to femoral neck bone geometry.

Association analyses of RANKL/RANK/OPG gene polymorphisms with femoral neck compression strength index variation in Caucasians

Femoral neck compression strength index (fCSI), a novel phenotypic parameter that integrates bone density, bone size, and body size, has significant potential to improve hip fracture risk assessment. The genetic factors underlying variations in fCSI, however, remain largely unknown. Given the important roles of the receptor activator of the nuclear factor-kappaB ligand/receptor activator of the nuclear factor-kappaB/osteoprotegerin (RANKL/RANK/OPG) pathway in the regulation of bone remodeling, we tested the associations between RANKL/RANK/OPG polymorphisms and variations in fCSI as well as its components (femoral neck bone mineral density [fBMD], femoral neck width [FNW], and weight). This was accomplished with a sample comprising 1873 subjects from 405 Caucasian nuclear families. Of the 37 total SNPs studied in these three genes, 3 SNPs, namely, rs12585014, rs7988338, and rs2148073, of RANKL were significantly associated with fCSI (P = 0.0007, 0.0007, and 0.0005, respectively) after conservative Bonferroni correction. Moreover, the three SNPs were approximately in complete linkage disequilibrium. Haplotype-based association tests corroborated the single-SNP results since haplotype 1 of block 1 of the RANKL gene achieved an even more significant association with fCSI (P = 0.0003) than any of the individual SNPs. However, we did not detect any significant associations of these genes with fBMD, FNW, or weight. In summary, our findings suggest that the RANKL gene may play an important role in variation in fCSI, independent of fBMD and non-fBMD components.

No association between hip geometry and four common polymorphisms associated with fracture: the Danish osteoporosis prevention study

Both osteoporosis and hip geometry are independently associated with fracture risk. There is a significant genetic contribution to the risk of osteoporosis, and evidence provided by twin studies has suggested that hip geometry may also in part be genetically programmed. Polymorphisms in a number of genes, including those coding for methylene-tetrahydrofolate reductase (MTHFR c.677C > T), the purinergic P2X(7) receptor (Glu496Ala and Ile568Asn), and the low-density lipoprotein receptor-related protein 5 (LRP5 exon 9 [c.266A > G]), have been associated with an increased fracture incidence and/or reduced bone mineral density (BMD). The aim of the present study was to test whether these polymorphisms influence hip structural geometry in perimenopausal women. The four polymorphisms were genotyped in 800 healthy recently perimenopausal women never using hormone replacement therapy. BMD of the femoral neck was measured using a Hologic QDR-2000 densitometer and femoral neck axis length, neck width, neck shaft angle, and femoral head diameter were measured from the screen images. Genotype frequencies were compatible with Hardy-Weinberg equilibrium. No significant differences between homozygotes for the minor allele and carriers of the common allele regarding parameters of hip geometry were demonstrated. According to the anthropometric characteristics of the subjects, only body height in the MTHFR TT genotype group was significantly different from the combined CT/CC genotype group (P < 0.05). The geometric dimensions of the proximal femur in perimenopausal women are not associated with the MTHFR c.677C > T, P2X(7) (Glu496Ala), P2X(7) (Ile568Asn), and LRP5 exon 9 (c.266A > G) polymorphisms.

Bivariate genome-wide linkage analysis of femoral bone traits and leg lean mass: Framingham study

The risk of osteoporotic fracture is a function of both applied muscle mass and bone tissue distribution. Leg lean mass (LLM) and femoral bone geometry are both known to have substantial genetic components. Therefore, we estimated shared heritability (h(2)) and performed linkage analysis to identify chromosomal regions governing both LLM and bone geometry. A genome-wide scan (using 636 microsatellite markers) for linkage analyses was performed on 1346 adults from 327 extended families of the Framingham study. DXA measures were LLM, femoral neck length, neck-shaft angle (NSA), subperiosteal width, cross-sectional area (CSA), and section modulus (Z) at the femoral narrow neck and shaft (S) regions. Variance component linkage analysis was performed on normalized residuals (adjusted for age, height, BMI, and estrogen status in women). The results indicated substantial h(2) for LLM (0.42 +/- 0.07) that was comparable to bone geometry traits. Phenotypic correlations between LLM and bone geometry phenotypes ranged from 0.033 with NSA (p > 0.05) to 0.251 with S_Z (p < 0.001); genetic correlations ranged from 0.087 (NSA, p > 0.05) to 0.454 (S_Z, p < 0.001). Univariate linkage analysis of covariate-adjusted LLM identified no chromosomal regions with LOD scores >or=2.0; however, bivariate analysis identified two loci with LOD scores >3.0, shared by LLM with S_CSA on chromosome 12p12.3-12p13.2, and with NSA, on 14q21.3-22.1. In conclusion, we identified chromosomal regions potentially linked to both LLM and femoral bone geometry. Identification and subsequent characterization of these shared loci may further elucidate the genetic contributions to both osteoporosis and sarcopenia.

A genome wide linkage scan of metacarpal size and geometry in the Framingham Study

Bone geometry is a significant component of bone strength, and has a clinical utility in predicting fractures and quantifying bone loss. Bone geometry is known to have a substantial genetic component. We performed linkage analysis to identify chromosomal regions governing metacarpal bone geometry. A genome-wide scan (with a set of 615 markers with spacing of approximately 5.7 cM) was performed on 1,702 individuals from 330 extended families of the Framingham Study. Midshaft width was measured and several indices calculated, namely Metacarpal Cortical Thickness (MCT), Cortical Index (MCI), and Section Modulus (MZ), using digitized X-rays of 1,380 participants (men, n = 666, mean age 55.2 yr, women, n = 714, 55.5 yr). Metacarpals 2, 3, and 4 were averaged. Heritability was significant for all indices, ranging from 0.51 to 0.72. Linkage analysis of indices adjusted for age, age(2), and estrogen status in women, identified chromosomal regions 6p21, 9p21, 11q21-q22, and Xq26-Xq27, with LOD scores >2.0. Additional adjustment for smoking, height, and BMI, generally reduced the LOD scores. Finally, bivariate linkage analysis confirmed that a QTL on chr. 6 (51 cM) was shared by midshaft width and MZ (LOD = 2.40, adjusted for all covariates). Neither MCT nor MCI shared linkage loci with width or MZ. In conclusion, we have identified chromosomal regions potentially linked to bone geometry. Genes in these regions may regulate bone geometry via effects on body size. Identification and subsequent characterization of loci for bone geometry can further elucidate the genetic contributions to bone's resistance to stress.

Femoral geometry as a risk factor for osteoporotic hip fracture in men and women

Osteoporotic hip fracture is associated with high mortality and morbidity and often results in a loss of mobility and independence. Osteoporosis is diagnosed by measuring Bone Mineral Density (BMD), a measure of the amount of mineral in a bone. Although BMD continues to serve well it does not fully account for bone strength and only partially accounts for the risk of hip fracture. The shape and structure of the proximal femur also help to determine how forces act in the hip in a fall and their measurement can aid the prediction of hip fracture. This review examines the link between simple geometrical measures of the proximal femur and hip fracture, or bone strength. It will explore how they relate to each other and to anthropometric factors such as sex, height, weight and age. Limitations in these measures will be identified and new methods of analysis reviewed that encompass many different aspects of the shape of the femur. These new methods show great promise for improving the prediction of fracture risk in the future.

Autosome-wide linkage analysis of hip structural phenotypes in the Old Order Amish

INTRODUCTION

Fracture risk is associated with bone mineral density (BMD) and with other indices of bone strength, including hip geometry. While the heritability and associated fracture risk of BMD are well described, less is known about genetic influences of bone geometry. We derived hip structural phenotypes using the Hip Structural Analysis program (HSA) and performed autosome-wide linkage analysis of hip geometric structural phenotypes.

MATERIALS AND METHODS

The Amish Family Osteoporosis Study was designed to identify genes affecting bone health. BMD was measured at the hip using dual X-ray absorptiometry (DXA) in 879 participants (mean age+/-SD=49.8+/-16.1 years, range 18-91 years) from large multigenerational families. From DXA scans, we computed structural measures of hip geometry at the femoral neck (NN) and shaft (S) by HSA, including cross-sectional area (CSA), endocortical or inner diameter (ID), outer diameter (OD) buckling ratio (BR) and section modulus (Z). Genotyping of 731 highly polymorphic microsatellite markers (average spacing of 5.4 cM) and autosome-wide multipoint linkage analysis was performed.

RESULTS

The heritability of HSA-derived hip phenotypes ranged from 40 to 84%. In the group as a whole, autosome-wide linkage analysis suggested evidence of linkage for QTLs related to NN_Z on chromosome 1p36 (LOD=2.36). In subgroup analysis, ten additional suggestive regions of linkage were found on chromosomes 1, 2, 5, 6, 11, 12, 14, 15 and 17, all with LOD>2.3 except for our linkage at 17q11.2-13 for men and women age 50 and under for NN_CSA, which had a lower LOD of 2.16, but confirmed a previous linkage report.

CONCLUSIONS

We found HSA-derived measures of hip structure to be highly heritable independent of BMD. No strong evidence of linkage was found for any phenotype. Confirmatory evidence of linkage was found on chromosome 17q11.2-12 for NN_CSA. Modest evidence was found for genes affecting hip structural phenotypes at ten other chromosomal locations.

Genetics of the musculoskeletal system: a pleiotropic approach

The risk of osteoporotic fracture can be viewed as a function of loading conditions and the ability of the bone to withstand the load. Skeletal loads are dominated by muscle action. Recently, it has become clear that bone and muscle share genetic determinants. Involution of the musculoskeletal system manifests as bone loss (osteoporosis) and muscle wasting (sarcopenia). Therefore, the consideration of pleiotropy is an important aspect in the study of the genetics of osteoporosis and sarcopenia. This Perspective will provide the evidence for a shared genetic influence on bone and muscle. We will start with an overview of accumulating evidence that physical exercise produces effects on the adult skeleton, seeking to unravel some of the contradictory findings published thus far. We will provide indications that there are pleiotropic relationships between bone structure/mass and muscle mass/function. Finally, we will offer some insights and practical recommendations as to the value of studying shared genetic factors and will explore possible directions for future research. We consider several related questions that together comprise the general paradigm of bone responses to mechanical loading and the relationship between muscle strength and bone parameters, including the genetic factors that modulate these responses. We believe that further progress in understanding the common genetic etiology of osteoporosis and sarcopenia will provide valuable insight into important biological underpinnings for both conditions and may translate into new approaches to reduce the burdens of both conditions through improved diagnosis, prevention, and early targeted treatment.

Contribution of gender-specific genetic factors to osteoporosis risk

Common diseases result from the complex relationship between genetic and environmental factors. The aim of this review is to provide perspective for a conceptual framework aimed at studying the interplay of gender-specific genetic and environmental factors in the etiology of complex disease, using osteoporosis as an example. In recent years, gender differences in the heritability of the osteoporosis-related phenotypes have been reported and sex-specific quantitative-trait loci were discovered by linkage studies in humans and mice. Results of numerous allelic association studies also differed by gender. In most cases, it was not clear whether or not this phenomenon should be attributed to the effect of sex-chromosomes, sex hormones, or other intrinsic or extrinsic differences between the genders, such as the level of bioavailable estrogen and of physical activity. We conclude that there is need to consider gender-specific genetic and environmental factors in the planning of future association studies on the etiology of osteoporosis and other complex diseases prevalent in the general population.

Increased plasma osteoprotegerin concentrations are associated with indices of bone strength of the hip

OBJECTIVE

Osteoprotegerin (OPG) is an important regulator of bone turnover through its effects on osteoclastogenesis, yet findings from previous studies of circulating OPG and commonly measured bone indices in humans have been conflicting. We conducted a cross-sectional study to evaluate the association between plasma OPG and femoral neck (FN) bone density (BMD) and geometry in a large cohort of women and men.

DESIGN

Participants included 1379 postmenopausal women and 1165 men, aged 50-89 yr (mean, 64 yr), in the Framingham Offspring Study. Dual x-ray absorptiometry was used to evaluate FN BMD and geometry (bone width, section modulus, and cross-sectional area at the narrow neck region). Plasma OPG concentrations were measured by ELISA. Sex-specific analysis of covariance was used to calculate means and assess linear trend in BMD and geometry values across OPG quartiles, adjusted for confounders.

RESULTS

OPG concentrations were greater in women than men, increased with age, and were greater in smokers and those with diabetes and heart disease. Multivariable-adjusted mean FN BMD in women increased from the lowest to the highest OPG quartile (trend, P < 0.01). However, no linear trend between FN BMD and OPG was observed in men (trend, P = 0.34). Section modulus and bone width increased with OPG in men (trend, P < 0.01), whereas no association between hip geometry indices and OPG was observed in women.

CONCLUSION

Higher OPG concentration may indicate greater skeletal strength in women and men, possibly through reducing bone loss in women and increasing periosteal apposition in men.

Molecular genetic studies of gene identification for osteoporosis

This review comprehensively summarizes the most important and representative molecular genetics studies of gene identification for osteoporosis published up to the end of September 2007. It is intended to constitute a sequential update of our previously published reviews covering the available data up to the end of 2004. Evidence from candidate gene-association studies, genome-wide linkage and association studies, as well as functional genomic studies (including gene-expression microarray and proteomics) on osteogenesis and osteoporosis, are reviewed separately. Studies of transgenic and knockout mice models relevant to osteoporosis are summarized. The major results of all studies are tabulated for comparison and ease of reference. Comments are made on the most notable findings and representative studies for their potential influence and implications on our present understanding of genetics of osteoporosis. The format adopted by this review should be ideal for accommodating future new advances and studies.

Age trends in proximal femur geometry in men: variation by race and ethnicity

Data on bone architecture in diverse male populations are limited. We examined proximal femur geometry in 1,190 black, Hispanic, and white men. Cross-sectional analyses indicate greatest bone strength among black men, and greater age-related differences in bone strength among Hispanic men than other subjects at the narrow neck and intertrochanter regions of the proximal femur.

INTRODUCTION

Although race/ethnic differences in bone mass are well-documented, less is known about differences in bone architecture. We examined proximal femur geometry in a diverse, randomly-sampled population of 1,190 community-dwelling men (age 30-79 y).

METHODS

Dual X-ray absorptiometry scans were obtained for 355 black, 394 Hispanic, and 441 white subjects. Measures were obtained for the narrow neck (NN), intertrochanter (IT) and shaft regions of the proximal femur via hip structural analysis. Analyses considered bone mineral density (BMD, g/cm2), outer diameter (cm), cross-sectional area (CSA, cm2), section modulus (Z, cm3), and buckling ratio (BR). Results were adjusted for height, weight and physical activity level.

RESULTS

Black subjects exhibited greater age-specific BMD, CSA and Z, than their white counterparts. For instance, at age 50 y, NN BMD was approximately 11% higher among black men (p < 0.001). Hispanic men exhibited sharper age-related differences in NN and IT BMD than did others. IT BMD, for instance, decreased by 2.4% with 10 y age among Hispanic subjects, but had virtually no age trend in others (p < 0.001).

CONCLUSIONS

These results imply greater bone strength among black American men than among their white counterparts, and may indicate elevated fracture risk among older Hispanic American subpopulations.

Genome-wide association with bone mass and geometry in the Framingham Heart Study

BACKGROUND

Osteoporosis is characterized by low bone mass and compromised bone structure, heritable traits that contribute to fracture risk. There have been no genome-wide association and linkage studies for these traits using high-density genotyping platforms.

METHODS

We used the Affymetrix 100K SNP GeneChip marker set in the Framingham Heart Study (FHS) to examine genetic associations with ten primary quantitative traits: bone mineral density (BMD), calcaneal ultrasound, and geometric indices of the hip. To test associations with multivariable-adjusted residual trait values, we used additive generalized estimating equation (GEE) and family-based association tests (FBAT) models within each sex as well as sexes combined. We evaluated 70,987 autosomal SNPs with genotypic call rates > or =80%, HWE p > or = 0.001, and MAF > or =10% in up to 1141 phenotyped individuals (495 men and 646 women, mean age 62.5 yrs). Variance component linkage analysis was performed using 11,200 markers.

RESULTS

Heritability estimates for all bone phenotypes were 30-66%. LOD scores > or =3.0 were found on chromosomes 15 (1.5 LOD confidence interval: 51,336,679-58,934,236 bp) and 22 (35,890,398-48,603,847 bp) for femoral shaft section modulus. The ten primary phenotypes had 12 associations with 100K SNPs in GEE models at p < 0.000001 and 2 associations in FBAT models at p < 0.000001. The 25 most significant p-values for GEE and FBAT were all less than 3.5 x 10(-6) and 2.5 x 10(-5), respectively. Of the 40 top SNPs with the greatest numbers of significantly associated BMD traits (including femoral neck, trochanter, and lumbar spine), one half to two-thirds were in or near genes that have not previously been studied for osteoporosis. Notably, pleiotropic associations between BMD and bone geometric traits were uncommon. Evidence for association (FBAT or GEE p < 0.05) was observed for several SNPs in candidate genes for osteoporosis, such as rs1801133 in MTHFR; rs1884052 and rs3778099 in ESR1; rs4988300 in LRP5; rs2189480 in VDR; rs2075555 in COLIA1; rs10519297 and rs2008691 in CYP19, as well as SNPs in PPARG (rs10510418 and rs2938392) and ANKH (rs2454873 and rs379016). All GEE, FBAT and linkage results are provided as an open-access results resource at http://www.ncbi.nlm.nih.gov/projects/gap/cgi-bin/study.cgi?id=phs000007 webcite.

CONCLUSION

The FHS 100K SNP project offers an unbiased genome-wide strategy to identify new candidate loci and to replicate previously suggested candidate genes for osteoporosis.

Bivariate linkage study of proximal hip geometry and body size indices: the Framingham study

Femoral geometry and body size are both characterized by substantial heritability. The purpose of this study was to discern whether hip geometry and body size (height and body mass index, BMI) share quantitative trait loci (QTL). Dual-energy X-ray absorptiometric scans of the proximal femur from 1,473 members in 323 pedigrees (ages 31-96 years) from the Framingham Osteoporosis Study were studied. We measured femoral neck length, neck-shaft angle, subperiosteal width (outer diameter), cross-sectional bone area, and section modulus, at the narrowest section of the femoral neck (NN), intertrochanteric (IT), and femoral shaft (S) regions. In variance component analyses, genetic correlations (rho ( G )) between hip geometry traits and height ranged 0.30-0.59 and between hip geometry and BMI ranged 0.11-0.47. In a genomewide linkage scan with 636 markers, we obtained nominally suggestive linkages (bivariate LOD scores > or =1.9) for geometric traits and either height or BMI at several chromosomes (4, 6, 9, 15, and 21). Two loci, on chr. 2 (80 cM, BMI/shaft section modulus) and chr. X (height/shaft outer diameter), yielded bivariate LOD scores > or =3.0; although these loci were linked in univariate analyses with a geometric trait, neither was linked with either height or BMI. In conclusion, substantial genetic correlations were found between the femoral geometric traits, height and BMI. Linkage signals from bivariate linkage analyses of bone geometric indices and body size were similar to those obtained in univariate linkage analyses of femoral geometric traits, suggesting that most of the detected QTL primarily influence geometry of the hip.