Suggestive linkage of the parathyroid receptor type 1 to osteoporosis

PTHR1 Genetic Polymorphisms Are Associated with Osteoporosis among Postmenopausal Arab Women

The parathyroid hormone 1 receptor (PTHR1) plays a crucial role in calcium homeostasis and bone metabolism. However, its genetic role in regulating bone turnover markers (BTMs) in postmenopausal osteoporosis (PMO) remains unclear. Herein, we explored parathyroid hormone (PTH) and PTHR gene variant susceptibility to osteoporosis and their association with various circulating BTM and inflammatory markers in postmenopausal women of Arab ethnicity. In total, 600 postmenopausal Arab women (300-PMO and 300-control) were genotyped for selected SNPs in PTH (rs1459015, rs307253, rs6054, rs307247, rs10500783 and rs10500784), PTHR1 (rs6442037, rs1138518, and rs724449 SNPs) and PTHR2 (rs9288393, rs10497900, and rs897083). Anthropometrics, BTMs, and inflammatory markers were measured. Bone mineral density (BMD) was measured at the lumbar spine L1–L4 and the femoral neck using dual-energy X-ray absorptiometry (DXA). PTHR1 rs1138518 genotype C/T was found to be a significant risk factor for PMO (OR = 1.49, 95% CI 1.0-2.1, P = 0.03). The genotypes C/T and T/T of PTHR1 rs1138518 were associated with 25-hydroxy-vitamin D (25(OH)D) regulation. In the PMO group, carriers of the C/T genotype had significantly lower 25(OH)D levels than carriers of the same genotypes in the control group (59.9 (36.7-92.4) nmol/l and 66.4 (43.5-87.8) nmol/l, respectively; P = 0.048]. Our study concludes that the PTHR1 rs1138518 genotype could be a potential risk factor for osteoporosis and 25(OH)D regulation in Arab women with PMO.

Omics analysis of human bone to identify genes and molecular networks regulating skeletal remodeling in health and disease

The skeleton is a metabolically active organ throughout life where specific bone cell activity and paracrine/endocrine factors regulate its morphogenesis and remodeling. In recent years, an increasing number of reports have used multi-omics technologies to characterize subsets of bone biological molecular networks. The skeleton is affected by primary and secondary disease, lifestyle and many drugs. Therefore, to obtain relevant and reliable data from well characterized patient and control cohorts are vital. Here we provide a brief overview of omics studies performed on human bone, of which our own studies performed on trans-iliacal bone biopsies from postmenopausal women with osteoporosis (OP) and healthy controls are among the first and largest. Most other studies have been performed on smaller groups of patients, undergoing hip replacement for osteoarthritis (OA) or fracture, and without healthy controls. The major findings emerging from the combined studies are: 1. Unstressed and stressed bone show profoundly different gene expression reflecting differences in bone turnover and remodeling and 2. Omics analyses comparing healthy/OP and control/OA cohorts reveal characteristic changes in transcriptomics, epigenomics (DNA methylation), proteomics and metabolomics. These studies, together with genome-wide association studies, in vitro observations and transgenic animal models have identified a number of genes and gene products that act via Wnt and other signaling systems and are highly associated to bone density and fracture. Future challenge is to understand the functional interactions between bone-related molecular networks and their significance in OP and OA pathogenesis, and also how the genomic architecture is affected in health and disease.

Parathyroid hormone and its receptor gene polymorphisms: implications in osteoporosis and in fracture healing

Parathyroid glands secrete parathyroid hormone (PTH) which plays multiple roles in calcium homeostasis and in bone remodeling. Secretion of PTH is regulated by extracellular calcium levels and other humoral factors including 1α,25(OH)2D3. PTH regulates gene expression and induces biological effects directly and indirectly. The human gene encoding PTH is located on chromosome 11. In this review, we study the diverse PTH along with its receptor gene polymorphisms and their association with osteoporosis and fracture healing. Genetic factors are associated with osteoporosis by influencing bone mineral density (BMD), bone turnover, calcium homeostasis, and susceptibility to osteoporotic fractures. Polymorphisms in genes encoding PTH may contribute to genetic regulation of BMD and thus susceptibility to fracture risk. PTH stimulates the proliferation of osteoprogenitor cells, production of alkaline phosphatise, and bone matrix proteins that contribute to hard callus formation and increases strength at the site of fractured bone. During remodeling, PTH promotes osteoclastogenesis restoring the original shape, structure, and mechanical strength of the bone. Some PTH polymorphisms have shown an association with fracture risk. Further research is needed to elucidate the relative importance of PTH genetics and the mechanisms of genetic contributions to gene-gene interactions in the pathogenesis of osteoporosis and in fracture healing.

Conditional testing of multiple variants associated with bone mineral density in the FLNB gene region suggests that they represent a single association signal

BACKGROUND

Low bone mineral density (BMD) is a primary risk factor for osteoporosis and is a highly heritable trait, but appears to be influenced by many genes. Genome-wide linkage studies have highlighted the chromosomal region 3p14-p22 as a quantitative trait locus for BMD (LOD 1.1 - 3.5). The FLNB gene, which is thought to have a role in cytoskeletal actin dynamics, is located within this chromosomal region and presents as a strong candidate for BMD regulation. We have previously identified significant associations between four SNPs in the FLNB gene and BMD in women. We have also previously identified associations between five SNPs located 5' of the transcription start site (TSS) and in intron 1 of the FLNB gene and expression of FLNB mRNA in osteoblasts in vitro. The latter five SNPs were genotyped in this study to test for association with BMD parameters in a family-based population of 769 Caucasian women.

RESULTS

Using FBAT, significant associations were seen for femoral neck BMD Z-score with the SNPs rs11720285, rs11130605 and rs9809315 (P = 0.004 - 0.043). These three SNPs were also found to be significantly associated with total hip BMD Z-score (P = 0.014 - 0.026). We then combined the genotype data for these three SNPs with the four SNPs we previously identified as associated with BMD and performed a conditional analysis to determine whether they represent multiple independent associations with BMD. The results from this analysis suggested that these variants represent a single association signal.

CONCLUSIONS

The SNPs identified in our studies as associated with BMD appear to be part of a single association signal between the FLNB gene and BMD in our data. FLNB is one of several genes located in 3p14-p22 that has been identified as significantly associated with BMD in Caucasian women.

Molecular disease map of bone characterizing the postmenopausal osteoporosis phenotype

Genome-wide gene expressions in bone biopsies from patients with postmenopausal osteoporosis and healthy controls were profiled, to identify osteoporosis candidate genes. All osteoporotic patients (n = 27) in an unbiased cohort of Norwegian women presented with bone mineral density (BMD) T-scores of less than -2.5 SD and one or more confirmed low-energy fracture(s). A validation group (n = 18) had clinical and laboratory parameters intermediate to the control (n = 39) and osteoporosis groups. RNA from iliac crest bone biopsies were analyzed by Affymetrix microarrays and real-time reverse-transcriptase polymerase chain reaction (RT-PCR). Differentially expressed genes in osteoporosis versus control groups were identified using the Bayesian ANOVA for microarrays (BAMarray) method, whereas the R-package Limma (Linear Models for Microarray Data) was used to determine whether these transcripts were explained by disease, age, body mass index (BMI), or combinations thereof. Laboratory tests showed normal ranges for the cohort. A total of 609 transcripts were differentially expressed in osteoporotic patients relative to controls; 256 transcripts were confirmed for disease when controlling for age or BMI. Most of the osteoporosis susceptibility genes (80%) also were confirmed to be regulated in the same direction in the validation group. Furthermore, 217 of 256 transcripts were correlated with BMD (adjusted for age and BMI) at various skeletal sites (|r| > 0.2, p < .05). Among the most distinctly expressed genes were Wnt antagonists DKK1 and SOST, the transcription factor SOX4, and the bone matrix proteins MMP13 and MEPE, all reduced in osteoporosis versus control groups. Our results identify potential osteoporosis susceptibility candidate genes adjusted for confounding factors (ie, age and BMI) with or without a significant correlation with BMD.

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.

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.

Common variants in FLNB/CRTAP, not ARHGEF3 at 3p, are associated with osteoporosis in southern Chinese women

SUMMARY

We performed an association study of five candidate genes within chromosome 3p14-25 in 1,080 Chinese female subjects. Polymorphisms in FLNB/CRTAP are associated with bone mineral density (BMD) in Chinese.

INTRODUCTION

Chromosomal region 3p14-25 has shown strong evidence of linkage to BMD in genome-wide linkage scans. The variants responsible for this linkage signal, nonetheless, remain obscure.

METHODS

Thirty SNPs in five positional and functional candidate genes within 3p14-25 (PPARG, CRTAP, TDGF1, PTHR1, and FLNB) and rs7646054 in the ARHGEF3 gene were genotyped in a case-control cohort of 1,080 Chinese females. Allelic and haplotypic association were tested using logistic regression analysis implemented in PLINK software. Potential transcription factor binding sites were predicted with MatInspector.

RESULTS

Multiple SNPs and haplotypes in FLNB were significantly associated with BMDs, with the strongest association between lumbar spine BMD and rs9828717 (p = 0.005). SNP rs7623768 and the haplotype G-C of rs4076086-rs7623768 in CRTAP were associated with femoral neck BMD (p = 0.009 and p = 0.003, respectively). PTHR1 showed haplotypic associations with lumbar spine and femoral neck BMD (p = 0.02 and p = 0.044, respectively). Nevertheless, the association between rs7646054 in ARHGEF3 and BMD observed in Caucasians was not replicated in our samples. Comparative genomics analysis indicated that rs9828717 is located within a highly conserved region. The minor T allele at rs9828717 may lead to loss of binding site for nuclear factor of activated T cells which binds and triggers the transcriptional program of osteoblasts.

CONCLUSIONS

Our data suggest that variants in FLNB and CRTAP at 3p are involved in BMD regulation in southern Chinese.

Genetic determinants of osteoporosis: common bases to cardiovascular diseases?

Osteoporosis is the most common and serious age-related skeletal disorder, characterized by a low bone mass and bone microarchitectural deterioration, with a consequent increase in bone fragility and susceptibility to spontaneous fractures, and it represents a major worldwide health care problem with important implications for health care costs, morbidity and mortality. Today is well accepted that osteoporosis is a multifactorial disorder caused by the interaction between environment and genes that singularly exert modest effects on bone mass and other aspects of bone strength and fracture risk. The individuation of genetic factors responsible for osteoporosis predisposition and development is fundamental for the disease prevention and for the setting of novel therapies, before fracture occurrence. In the last decades the interest of the Scientific Community has been concentrated in the understanding the genetic bases of this disease but with controversial and/or inconclusive results. This review tries to summarize data on the most representative osteoporosis candidate genes. Moreover, since recently osteoporosis and cardiovascular diseases have shown to share common physiopathological mechanisms, this review also provides information on the current understanding of osteoporosis and cardiovascular diseases common genetic bases.

Genes influencing spinal bone mineral density in inbred F344, LEW, COP, and DA rats

Previously, we identified the regions of chromosomes 10q12-q31 and 15p16-q21 harbor quantitative trait loci (QTLs) for lumbar volumetric bone mineral density (vBMD) in female F2 rats derived from Fischer 344 (F344) x Lewis (LEW) and Copenhagen 2331 (COP) x Dark Agouti (DA) crosses. The purpose of this study is to identify the candidate genes within these QTL regions contributing to the variation in lumbar vBMD. RNA was extracted from bone tissue of F344, LEW, COP, and DA rats. Microarray analysis was performed using Affymetrix Rat Genome 230 2.0 Arrays. Genes differentially expressed among the rat strains were then ranked based on the strength of the correlation with lumbar vBMD in F2 animals derived from these rats. Quantitative PCR (qPCR) analysis was performed to confirm the prioritized candidate genes. A total of 285 genes were differentially expressed among all strains of rats with a false discovery rate less than 10%. Among these genes, 18 candidate genes were prioritized based on their strong correlation (r (2) > 0.90) with lumbar vBMD. Of these, 14 genes (Akap1, Asgr2, Esd, Fam101b, Irf1, Lcp1, Ltc4s, Mdp-1, Pdhb, Plxdc1, Rabep1, Rhot1, Slc2a4, Xpo4) were confirmed by qPCR. We identified several novel candidate genes influencing spinal vBMD in rats.

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.

Genetics of osteoporosis: accelerating pace in gene identification and validation

Osteoporosis is characterized by low bone mineral density and structural deterioration of bone tissue, leading to an increased risk of fractures. It is the most common metabolic bone disorder worldwide, affecting one in three women and one in eight men over the age of 50. In the past 15 years, a large number of genes have been reported as being associated with osteoporosis. However, only in the past 4 years we have witnessed an accelerated pace in identifying and validating osteoporosis susceptibility loci. This increase in pace is mostly due to large-scale association studies, meta-analyses, and genome-wide association studies of both single nucleotide polymorphisms and copy number variations. A comprehensive review of these developments revealed that, to date, at least 15 genes (VDR, ESR1, ESR2, LRP5, LRP4, SOST, GRP177, OPG, RANK, RANKL, COLIA1, SPP1, ITGA1, SP7, and SOX6) can be reasonably assigned as confirmed osteoporosis susceptibility genes, whereas, another >30 genes are promising candidate genes. Notably, confirmed and promising genes are clustered in three biological pathways, the estrogen endocrine pathway, the Wnt/beta-catenin signaling pathway, and the RANKL/RANK/OPG pathway. New biological pathways will certainly emerge when more osteoporosis genes are identified and validated. These genetic findings may provide new routes toward improved therapeutic and preventive interventions of this complex disease.

The bone mass density in postmenopausal women using hormonal replacement therapy in relation to polymorphism in vitamin D receptor and estrogen receptor genes

The aims of the study were as follows: (1) To identify the differences in spinal body mass density (BMD) in relation to polymorphism in vitamin D receptor (VDR) and estrogen receptor-alpha (ERalpha) genes in untreated women with postmenopausal osteoporosis. (2) To assess the efficacy of treatment in women with postmenopausal osteoporosis in relation to polymorphism in VDR and ERalpha genes. (3) To find the estradiol concentration necessary to protect bone tissue in patients with a given polymorphism in VDR and ERalpha genes.

METHODS

The study included 44 postmenopausal women with primary osteoporosis who used cyclic hormonal replacement therapy (HRT) for a year. The polymorphism of ERalpha and VDR genes were evaluated. We also determined the age, body mass index and spinal BMD before and after 12 months of administration the HRT.

RESULTS

We found a significant spinal BMD increase, what is connected with ERalpha genotype and both VDR and ERalpha genes. There is no such a correlation observed in polymorphism of VDR gene.

CONCLUSIONS

(1) There is no relationship between VDR and ERalpha genes polymorphism and the stage of osteoporosis related to the spinal BMD value before treatment. (2) The XX, PP or Bb markers or only X, P, B alleles are connected with a significant decrease of treatment efficacy. (3) Estradiol serum concentration before and during HRT is not dependent on the polymorphism of VDR and ERalpha genes.

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

Peak bone mass achieved in adolescence is a determinant of bone mass in later life. In order to identify genetic variants affecting bone mineral density (BMD), we performed a genome-wide association study of BMD and related traits in 1518 children from the Avon Longitudinal Study of Parents and Children (ALSPAC). We compared results with a scan of 134 adults with high or low hip BMD. We identified associations with BMD in an area of chromosome 12 containing the Osterix (SP7) locus, a transcription factor responsible for regulating osteoblast differentiation (ALSPAC: P = 5.8 x 10(-4); Australia: P = 3.7 x 10(-4)). This region has previously shown evidence of association with adult hip and lumbar spine BMD in an Icelandic population, as well as nominal association in a UK population. A meta-analysis of these existing studies revealed strong association between SNPs in the Osterix region and adult lumbar spine BMD (P = 9.9 x 10(-11)). In light of these findings, we genotyped a further 3692 individuals from ALSPAC who had whole body BMD and confirmed the association in children as well (P = 5.4 x 10(-5)). Moreover, all SNPs were related to height in ALSPAC children, but not weight or body mass index, and when height was included as a covariate in the regression equation, the association with total body BMD was attenuated. We conclude that genetic variants in the region of Osterix are associated with BMD in children and adults probably through primary effects on growth.

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.

Linkage screen for BMD phenotypes in male and female COP and DA rat strains

Because particular inbred strains of experimental animals are informative for only a subset of the genes underlying variability in BMD, we undertook a genome screen to identify quantitative trait loci (QTLs) in 828 F(2) progeny (405 males and 423 females) derived from the Copenhagen 2331 (COP) and dark agouti (DA) strains of rats. This screen was performed to complement our study in female Fischer 344 (F344) and Lewis (LEW) rats and to further delineate the factors underlying the complex genetic architecture of BMD in the rat model. Microsatellite genotyping was performed using markers at an average density of 20 cM. BMD was measured by pQCT and DXA. These data were analyzed in the R/qtl software to detect QTLs acting in both sexes as well as those having sex-specific effects. A QTL was detected in both sexes on chromosome 18 for midfemur volumetric BMD (vBMD; genome-wide, p < 0.01). On distal chromosome 1, a QTL was found for femur and vertebral aBMD as well as distal femur vBMD, and this QTL appears distinct from the proximal chromosome 1 QTL impacting BMD in our F344/LEW cross. Additional aBMD and vBMD QTLs and several sex-specific QTLs were also detected. These included a male-specific QTL (p < 0.01) on chromosome 8 and a female-specific QTL on chromosomes 7 and 14 (p < 0.01). Few of the QTLs identified showed overlap with the significant QTLs from the F344/LEW cross. These results confirm that the genetic influence on BMD in the rat model is quite complex and would seem to be influenced by a number of different genes, some of which have sex-specific effects.

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.

Genetic analyses in a sample of individuals with high or low BMD shows association with multiple Wnt pathway genes

Using a moderate-sized cohort selected with extreme BMD (n = 344; absolute value BMD, 1.5-4.0), significant association of several members of the Wnt signaling pathway with bone densitometry measures was shown. This confirms that extreme truncate selection is a powerful design for quantitative trait association studies of bone phenotypes.

INTRODUCTION

Although the high heritability of BMD variation has long been established, few genes have been conclusively shown to affect the variation of BMD in the general population. Extreme truncate selection has been proposed as a more powerful alternative to unselected cohort designs in quantitative trait association studies. We sought to test these theoretical predictions in studies of the bone densitometry measures BMD, BMC, and femoral neck area, by investigating their association with members of the Wnt pathway, some of which have previously been shown to be associated with BMD in much larger cohorts, in a moderate-sized extreme truncate selected cohort (absolute value BMD Z-scores = 1.5-4.0; n = 344).

MATERIALS AND METHODS

Ninety-six tag-single nucleotide polymorphism (SNPs) lying in 13 Wnt signaling pathway genes were selected to tag common genetic variation (minor allele frequency [MAF] > 5% with an r(2) > 0.8) within 5 kb of all exons of 13 Wnt signaling pathway genes. The genes studied included LRP1, LRP5, LRP6, Wnt3a, Wnt7b, Wnt10b, SFRP1, SFRP2, DKK1, DKK2, FZD7, WISP3, and SOST. Three hundred forty-four cases with either high or low BMD were genotyped by Illumina Goldengate microarray SNP genotyping methods. Association was tested either by Cochrane-Armitage test for dichotomous variables or by linear regression for quantitative traits.

RESULTS

Strong association was shown with LRP5, polymorphisms of which have previously been shown to influence total hip BMD (minimum p = 0.0006). In addition, polymorphisms of the Wnt antagonist, SFRP1, were significantly associated with BMD and BMC (minimum p = 0.00042). Previously reported associations of LRP1, LRP6, and SOST with BMD were confirmed. Two other Wnt pathway genes, Wnt3a and DKK2, also showed nominal association with BMD.

CONCLUSIONS

This study shows that polymorphisms of multiple members of the Wnt pathway are associated with BMD variation. Furthermore, this study shows in a practical trial that study designs involving extreme truncate selection and moderate sample sizes can robustly identify genes of relevant effect sizes involved in BMD variation in the general population. This has implications for the design of future genome-wide studies of quantitative bone phenotypes relevant to osteoporosis.

Genetic loci affecting bone structure and strength in inbred COP and DA rats

Previous studies have shown that the Copenhagen 2331 (COP) and Dark Agouti (DA) rats have significant differences in bone structure and strength despite their similar body mass. Thus, these inbred rat strains may provide a unique resource to identify the genetics underlying the phenotypic variation in bone fragility. A sample of 828 (405 males and 423 females) COPxDA F2 progeny had extensive phenotyping for bone structure measures including cortical bone area and polar moment of inertia at the femur midshaft and total, cortical and trabecular bone areas, for the lumbar vertebra 5 (L5). Bone strength phenotypes included ultimate force, stiffness and work to failure of femur and L5. These skeletal phenotypes were measured using peripheral quantitative computed tomography (pQCT) and mechanical testing. A whole-genome screen was conducted in the F2 rats, using microsatellite markers spaced at approximately 20 cM intervals. Genetic marker maps were generated from the F2 data and used for genome-wide linkage analyses to detect linkage to the bone structure and strength phenotypes. Permutation testing was employed to obtain the thresholds for genome-wide significance (p<0.01). Significant QTL for femur structure and strength were identified on chromosome (Chr) 1 with a maximum LOD score of 33.5; evidence of linkage was found in both the male and female rats. In addition, Chrs 6, 7, 10, 13, 15 and 18 were linked to femur midshaft structure. QTL linked to femur strength were identified on Chrs 5 and 10. For L5 vertebrae, Chrs 2, 16, and 18 harbored QTL for cortical structure and trabecular structure for L5 was linked to Chrs 1, 7, 12, and 18. One female-specific QTL for femur ultimate force was identified on Chr 5, and two male-specific QTL for L5 cortical area were found on Chrs 2 and 18. Our study demonstrates strong evidence of linkage for bone structure and strength to multiple rat chromosomes.

Studies on the Small Body Size Mouse Developed by Mutagen N-Ethyl-N-nitrosourea

Mutant mouse which show dwarfism has been developed by N-ethyl-N-nitrosourea (ENU) mutagenesis using BALB/c mice. The mutant mouse was inherited as autosomal recessive trait and named Small Body Size (SBS) mouse. The phenotype of SBS mouse was not apparent at birth, but it was possible to distinguish mutant phenotype from normal mice 1 week after birth. In this study, we examined body weight changes and bone mineral density (BMD), and we also carried out genetic linkage analysis to map the causative gene(s) of SBS mouse. Body weight changes were observed from birth to 14 weeks of age in both affected (n = 30) and normal mice (n = 24). BMD was examined in each five SBS and normal mice between 3 and 6 weeks of age, respectively. For the linkage analysis, we produced backcross progeny [(SBS × C57BL/6J) F₁ × SBS] N₂ mice (n = 142), and seventy-four microsatellite markers were used for primary linkage analysis. Body weight of affected mice was consistently lower than that of the normal mice, and was 43.7% less than that of normal mice at 3 weeks of age (P < 0.001). As compared with normal mice at 3 and 6 weeks of age, BMD of the SBS mice was significantly low. The results showed 15.5% and 14.1% lower in total body BMD, 15.3% and 8.7% lower in forearm BMD, and 29.7% and 20.1% lower in femur BMD, respectively. The causative gene was mapped on chromosome 10. The map order and the distance between markers were D10Mit248 - 2.1 cM - D10Mit51 - 4.2 cM - sbs - 0.7 cM - D10Mit283 - 1.4 cM - D10Mit106 - 11.2 cM - D10Mit170.

Chromosome 2q32 may harbor a QTL affecting BMD variation at different skeletal sites

BMDs at different skeletal sites share some common genetic determinants. Using PCA and bivariate linkage analysis, we identified a QTL on chromosome 2q32 with significant pleiotropic effects on BMDs at different skeletal sites.

INTRODUCTION

BMDs at the hip, spine, and forearm are genetically correlated, suggesting the existence of quantitative trait loci (QTLs) with concurrent effects on BMDs at these three skeletal sites. Consequently, it is important to identify these QTLs in the human genome and, for those implicated QTLs, it is important to differentiate between pleiotropic effects, caused by a single gene that concurrently effects these traits, and co-incident linkage, caused by multiple, closely linked, genes that independently effect these traits.

MATERIALS AND METHODS

For a sample of 451 American white pedigrees made up of 4,498 individuals, we evaluated the correlations between BMDs at the three skeletal sites. We carried out principal component analysis (PCA) for the three correlated traits and obtained a major component, PC1, which accounts for >75% of the co-variation of BMDs at the three sites. We subsequently conducted a whole genome linkage scan for PC1 and performed bivariate linkage analysis for pairs of the three traits (i.e., forearm/spine BMD, hip/forearm BMD, and hip/spine BMD).

RESULTS

Chromosome region 2q32, near the marker GATA65C03M, showed strong linkage to PC1 (LOD = 3.35). Subsequent bivariate linkage analysis substantiated linkage at 2q32 for each trait pair (LOD scores were 2.65, 2.42, and 2.13 for forearm/spine BMD, hip/forearm BMD, and hip/spine BMD, respectively). Further analyses rejected the hypothesis of co-incident linkage (p(0)[forearm/spine] = 0.0005, p(0)[hip/forearm] = 0.004, p(0)(hip/spine] = 0.001) but failed to reject the hypothesis of pleiotropy (p(1)[forearm/spine] = 0.35, p(1)[hip/forearm] = 0.07, p(1)[hip/spine] = 0.15).

CONCLUSIONS

Our results strongly support the conclusion that chromosome region 2q32 may harbor a QTL with pleiotropic effects on BMDs at different skeletal sites.

PTHR1 polymorphisms influence BMD variation through effects on the growing skeleton

We investigated whether polymorphisms in PTHR1 are associated with bone mineral density (BMD), to determine whether the association of this gene with BMD was due to effects on attainment of peak bone mass or effects on subsequent bone loss. The PTHR1 gene, including its 14 exons, their exon-intron boundaries, and 1,500 bp of its promoter region, was screened for polymorphisms by denaturing high-performance liquid chromatography (dHPLC) and sequencing in 36 osteoporotic cases. Eleven single-nucleotide polymorphisms (SNPs), one tetranucleotide repeat, and one tetranucleotide deletion were identified. A cohort of 634 families, including 1,236 men (39%) and 1,926 women (61%) ascertained with probands with low BMD (Z< -2.0) and the Children in Focus subset of the Avon Longitudinal Study of Parents and Children (ALSPAC) cohort (785 unrelated individuals, mean age 118 months), were genotyped for the five most informative SNPs (minor allele frequency >5%) and the tetranucleotide repeat. In our osteoporosis families, association was noted between lumbar spine BMD and alleles of a known functional tetranucleotide repeat (U4) in the PTHR1 promoter region (P = 0.042) and between two and three marker haplotypes of PTHR1 polymorphisms with lumbar spine, femoral neck, and total hip BMD (P = 0.021-0.047). This association was restricted to the youngest tertile of the population (age 16-39 years, P = 0.013-0.048). A similar association was found for the ALSPAC cohort: two marker haplotypes of SNPs A48609T and C52813T were associated with height (P = 0.006) and total body less head BMD (P = 0.02), corrected for age and gender, confirming the family findings. These findings suggest a role for PTHR1 variation in determining peak BMD.

Quantitative trait locus that determines the cross-sectional shape of the femur in SAMP6 and SAMP2 mice

We segregated a QTL on chromosome 11 that affects femoral cross-sectional shape during growth by generating a congenic strain and an additional 16 subcongenic strains of the senescence-accelerated mouse strain, SAMP6. The QTL region was narrowed down to a 10.0-Mbp region.

INTRODUCTION

Genetic background is known to affect bone characteristics. However, little is known about how polymorphic genes modulate bone shape. In a previous study using SAMP2 and SAMP6 mice, we reported a quantitative trait locus (QTL) on chromosome (Chr) 11 that had significant linkage to peak relative bone mass in terms of cortical thickness index (CTI) in male mice. We named it Pbd1. Here we aimed to clarify the effects of Pbd1 on skeletal phenotype in male mice and to narrow down the QTL region.

MATERIALS AND METHODS

We generated a congenic strain named P6.P2-Pbd1(b), carrying a 39-cM SAMP2-derived Chr11 interval on a SAMP6 genetic background. Sixteen subcongenic strains with smaller overlapping intervals on the SAMP6 background were generated from P6.P2-Pbd1(b) to narrow the region of interest. The effects of Pbd1 on bone properties were determined. Gene expression analysis of all candidate genes in Pbd1 was performed using real-time RT-PCR.

RESULTS

The CTI of strain P6.P2-Pbd1(b) at 16 wk was higher than that of SAMP6. This was not caused by differences in cortical thickness but by cross-sectional shape. Morphological analysis by microCT revealed that the femoral cross-sectional shape of P6.P2-Pbd1(b) (and the other subcongenic strains with higher CTI or bone area fraction [BA/TA]) was more compressed anteroposteriorly than that of SAMP6, which was associated with superior mechanical properties. This feature was formed during bone modeling up to 16 wk of age. Subcongenic strains with a higher CTI showed significant increases in endocortical mineral apposition rate and significant reductions in periosteal mineral apposition rate at 8 wk compared with those of the SAMP6. The Pbd1 locus was successfully narrowed down to a 10.0-Mbp region, and the expression analysis suggested a candidate gene, Cacng4.

CONCLUSIONS

The Pbd1 affects femoral cross-sectional shape by regulating the rate of endocortical and periosteal bone formation of the femur during postnatal growth.

Linkage to chromosome 11p12 in two Maltese families with a highly penetrant form of osteoporosis

Osteoporosis is a metabolic bone disease with a strong genetic component. Family-based linkage studies were performed by a number of investigators to try to identify loci that might contain genes responsible for an increased susceptibility to osteoporosis. A whole-genome linkage scan using 400 microsatellite markers was performed in 27 members from two Maltese families with a highly penetrant form of osteoporosis. The phenotype was defined by lumbar and femoral z-scores calculated after measurement of bone mineral density by DEXA. Both males and females were among the affected individuals. Multipoint parametric and non-parametric linkage analyses were performed by EasyLinkage v4.01 using GENEHUNTER v2.1, assuming dominant and recessive modes of inheritance with variable penetrance. Evidence of linkage was observed to a marker at 11p12 where a non-parametric LOD score of 5.77 (P=0.0006) was obtained. A maximum heterogeneity LOD score of 2.55 for this region was obtained for the dominant mode of inheritance with 90% penetrance and a phenocopy rate of 1%. Following fine mapping, the critical interval was narrowed to a region that is 52.94 cM from 11p-telomere. In this region, the gene for tumour necrosis factor receptor-associated factor 6 (TRAF6) is located approximately 1 cM away from the indicated marker. Sequencing of the promoter region and exons of the TRAF6 gene revealed three sequence variants, one of which was found in three affected members within one family.

Meta-analysis of genome-wide scans provides evidence for sex- and site-specific regulation of bone mass

Several genome-wide scans have been performed to detect loci that regulate BMD, but these have yielded inconsistent results, with limited replication of linkage peaks in different studies. In an effort to improve statistical power for detection of these loci, we performed a meta-analysis of genome-wide scans in which spine or hip BMD were studied. Evidence was gained to suggest that several chromosomal loci regulate BMD in a site-specific and sex-specific manner.

INTRODUCTION

BMD is a heritable trait and an important predictor of osteoporotic fracture risk. Several genome-wide scans have been performed in an attempt to detect loci that regulate BMD, but there has been limited replication of linkage peaks between studies. In an attempt to resolve these inconsistencies, we conducted a collaborative meta-analysis of genome-wide linkage scans in which femoral neck BMD (FN-BMD) or lumbar spine BMD (LS-BMD) had been studied.

MATERIALS AND METHODS

Data were accumulated from nine genome-wide scans involving 11,842 subjects. Data were analyzed separately for LS-BMD and FN-BMD and by sex. For each study, genomic bins of 30 cM were defined and ranked according to the maximum LOD score they contained. While various densitometers were used in different studies, the ranking approach that we used means that the results are not confounded by the fact that different measurement devices were used. Significance for high average rank and heterogeneity was obtained through Monte Carlo testing.

RESULTS

For LS-BMD, the quantitative trait locus (QTL) with greatest significance was on chromosome 1p13.3-q23.3 (p = 0.004), but this exhibited high heterogeneity and the effect was specific for women. Other significant LS-BMD QTLs were on chromosomes 12q24.31-qter, 3p25.3-p22.1, 11p12-q13.3, and 1q32-q42.3, including one on 18p11-q12.3 that had not been detected by individual studies. For FN-BMD, the strongest QTL was on chromosome 9q31.1-q33.3 (p = 0.002). Other significant QTLs were identified on chromosomes 17p12-q21.33, 14q13.1-q24.1, 9q21.32-q31.1, and 5q14.3-q23.2. There was no correlation in average ranks of bins between men and women and the loci that regulated BMD in men and women and at different sites were largely distinct.

CONCLUSIONS

This large-scale meta-analysis provided evidence for replication of several QTLs identified in previous studies and also identified a QTL on chromosome 18p11-q12.3, which had not been detected by individual studies. However, despite the large sample size, none of the individual loci identified reached genome-wide significance.

The effect of interleukin-1alpha polymorphisms on bone mineral density and the risk of vertebral fractures

Interleukin-1alpha (IL-1alpha) stimulates bone resorption via osteoclasts. Mononuclear cells from patients with osteoporosis show increased IL-1alpha production, and IL-1alpha mRNA is more often detected in bone biopsies from osteoporotic compared to normal postmenopausal women. Polymorphisms have been identified in the IL-1alpha gene; however, none of these has been examined for an effect on bone phenotypes in Caucasians. We investigated if the polymorphisms in the IL-1alpha gene affect the risk of osteoporotic fractures, bone mineral density (BMD), and bone turnover in 462 osteoporotic patients and 336 normal controls. Based on previous studies of polymorphisms in the gene and data from the International Hap-Map Project, four polymorphisms needed examination in order to investigate the effect of known polymorphisms in the IL-1alpha gene. We examined C(-1202)-T(rs1800794), C(-889)-T(rs1800587), T(155 + 209)-C(rs2071373), C(155 + 320)-T(rs2856838), and G(398)-T(rs 17561) by Taqman and restriction fragment-length polymorphism assays. BMD was examined by dual-energy X-ray absorptiometry. Bone turnover was evaluated by serum osteocalcin, serum carboxy-terminal propeptide of human type I procollagen, serum bone-specific alkaline phosphatase, serum carboxy-terminal telopeptide of type I collagen, and urinary hydroxyproline/creatinine. Genotype distributions were in Hardy-Weinberg equilibrium. All polymorphisms were in strong linkage disequilibrium. The C allele of the C(155 + 320)-T polymorphism tended to be more common among patients with vertebral fractures (P = 0.06) and patients with BMD T score <-2.5 (P = 0.05). Furthermore, haplotype 1 was associated with reduced risk of having BMD T score <-2.5 (P = 0.02). None of the other polymorphisms or haplotypes was associated with fracture risk or BMD T score <-2.5. BMD and bone turnover were not associated with any of the genetic variants. In conclusion, all the polymorphisms within the IL-1alpha gene are in strong linkage disequilibrium and not convincingly associated with fracture risk, BMD, or bone turnover.

Genomic regions identified for BMD in a large sample including epistatic interactions and gender-specific effects

A genome-wide linkage scan was conducted using a large white sample to identify QTLs for BMD. We found QTLs in the total sample and the gender-specific subgroups, as well as significant epistatic interactions underlying BMD variations.

INTRODUCTION

Low BMD is an important risk factor for osteoporosis and under strong genetic control.

MATERIALS AND METHODS

To identify quantitative trait loci (QTLs) for regulation of BMD, we performed a large-scale whole genome linkage scan (WGS) involving 4126 individuals from 451 families. In addition to the conventional linkage analyses in the total combined sample of males and females, we conducted epistatic interaction analyses and gender-specific linkage analyses.

RESULTS

Significant linkage was detected on 5q23 for wrist BMD (LOD = 3.39) and 15q13 for female spine BMD (LOD = 4.49). For spine BMD, we revealed significant epistatic interactions between 3p25 and 2q32 (p = 0.0022) and between 3p25 and 11q23 (p = 0.0007). We replicated several genomic regions that showed linkage with BMD in previous studies by others and ours, such as 3p21, 1p36, and Xq27.

CONCLUSIONS

This study highlights the importance of large sample size, incorporation of epistatic interaction, and consideration of gender-specific effects in identifying QTLs for BMD variation. The results of this study provide a foundation for the future fine mapping and gene identification in our population.

Molecular genetic studies of gene identification for osteoporosis: a 2004 update

This review summarizes comprehensively the most important and representative molecular genetics studies of gene identification for osteoporosis published up to the end of December 2004. It is intended to constitute a sequential update of our previously published review covering the available data up to the end of 2002. Evidence from candidate gene association studies and genome-wide linkage studies in humans, as well as quantitative trait locus mapping animal models are reviewed separately. Studies of transgenic and knockout mice models relevant to osteoporosis are summarized. An important extension of this update is incorporation of functional genomic studies (including DNA microarrays and proteomics) on osteogenesis and osteoporosis, in light of the rapid advances and the promising prospects of the field. 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.

Genetics of osteoporosis

Osteoporosis is a common disease with a strong genetic component. In recent years, some progress has been made in understanding the genetic basis of osteoporosis. Genetic factors contribute to osteoporosis by influencing not only bone mineral density but also bone size, bone quality, and bone turnover. Meta-analysis has been used to define the role of several candidate genes in osteoporosis. Some quantitative trait loci that regulate bone mass identified by linkage studies in humans and experimental animals have been replicated in multiple populations. Genes that cause monogenic bone diseases also contribute to regulation of bone mass in the normal population. Genome-wide association studies and functional genomics approaches have recently begun to apply to genetic studies of osteoporosis. In the future, not only single gene but also the entire gene networks involved in osteoporosis and regulation of bone mass will systematically be discovered through integrative genomics.

Identification of a quantitative trait locus on rat chromosome 4 that is strongly linked to femoral neck structure and strength

Risk factors for osteoporotic hip fracture include reduced bone mineral density and poor structure of the femoral neck, both of which are heritable traits. Previously, we showed that despite similar body size, Fischer 344 (F344) rats have significantly different skeletal traits compared with Lewis (LEW) rats. To identify a gene or genes regulating fracture risk at the femoral neck, we mapped quantitative trait loci (QTL) for femoral neck density and structure phenotypes using a 595 F2 progeny derived from the inbred F344 and LEW strains of rats. Femoral neck phenotypes included volumetric bone mineral density (vBMD), neck width, femoral neck cross-sectional area and polar moment of inertia (Ip). A 20-cM genome-wide scan was performed using 118 microsatellite markers and linkage analysis was conducted to identify chromosomal regions harbor QTL for femoral neck phenotypes. Strong evidence of linkage (P<0.01) to femoral neck vBMD was observed on chromosomes (Chrs) 1, 2, 4, 5, 7, 10 and 15. QTL affecting femoral neck structure and biomechanical properties were detected only on Chr 4 where the F344 alleles were shown to improve femoral neck structure, whereas these alleles had no effect on bone measurements at the lumbar spine and only modest effects at the femoral midshaft. In contrast, QTL on Chrs 1, 2 and 10 affected multiple skeletal sites. Several QTL regions in this study are homologous to human chromosomal regions, where linkage to femoral neck and related phenotypes has been reported previously. These findings represent an important first step in localizing and identifying genes that influence hip fragility.

Genetic and environmental correlations of bone mineral density at different skeletal sites in females and males

Bone mineral density (BMD) is a complex trait having genetic and environmental determination. There are gender-specific differences in BMD measurements, and the rate of BMD changes with age and lifestyle. Previous studies have shown that the genetic loci underlying BMD variation are gender-specific in mice and humans. Our study aimed to investigate correlations between BMD at the spine, hip, and ultradistal radius (UD) and degree of shared genetic and environmental factors among them in females and males, separately. For a large sample of 4,489 subjects containing 2,667 females and 1,822 males from 512 Caucasian pedigrees, we performed bivariate variance decomposition analyses. Our results showed that the genetic correlations (rhoG), environmental correlations (rhoE), and phenotypical correlations (rhoP) were all significant and positive. Strong genetic correlations were observed in both female and male groups, ranging 0.590-0.738 and 0.583-0.773, respectively. Genetic correlations of BMD at the spine, hip, and UD were generally higher than environmental correlations. In summary, we are the first to test the genetic and environmental correlations in females and males, separately. It is suggested that the phenotypic correlations of BMDs at the three different sites may have more genetic than environmental components. BMDs at the spine and hip may share more environmental components in females than males. We did not detect gender-specific difference in spine/UD and hip/UD. It is also indicated that the environmental factors that preserve or increase BMD at one skeletal site may have similar beneficial effects on some other skeletal sites and vice versa.

A functional polymorphism in the PTHR1 promoter region is associated with adult height and BMD measured at the femoral neck in a large cohort of young caucasian women

The parathyroid hormone type 1 receptor (PTHR1) mediates the actions of parathyroid hormone (PTH) and parathyroid hormone-related protein (PTHRP). Interacting with this receptor, PTHRP contributes to skeletal development through the regulation of chondrocyte proliferation and differentiation. Recently, a tetranucleotide repeat-(AAAG)( n )-in the P3 promoter of the PTHR1 gene has been shown to have functional activity in vitro, and homozygosity for (AAAG)(6), or the 6/6 genotype, has been associated with greater adult height compared to the 5/5 genotype. In this study, we evaluated the association of (AAAG)( n ) with height and bone mineral density (BMD) measured at lumbar spine (LS) and femoral neck (FN) in a cohort of 677 young caucasian women 18-35 years of age. Genomic DNA was amplified and genotyped by comparison with sequenced controls following electrophoretic separation through high-resolution polyacrylamide gels. Allele frequencies for (AAAG)( n ) were: 76.8% (n=5); 20.9% (n=6); 1.8% (n=7); 0.18% (n=8); 0.27% (n=9); 0.08% (n=2), and there was no evidence for Hardy-Weinberg disequilibrium. Analysis of variance showed that subjects bearing one or two (AAAG)(6) alleles (6/X & 6/6) were significantly taller (165.7+/-0.5 cm) than the others (X/X, 164.5+/-0.3 cm; P=0.034). This association was significant after adjusting for multiple covariates-current age, age at menarche, physical activity, smoking status, and intakes of caffeine and calcium. Comparison of genotype groups for BMD was not significant at LS, but BMD was significantly higher at FN in the group with at least one (AAAG)(6) allele (adjusted means: 1.021+/-0.008 vs. 0.999+/-0.006 g/cm(2), P=0.032). In conclusion, our data show that subjects bearing one or two (AAAG)(6) alleles are taller than subjects without, reinforcing the notion that in vivo variation in promoter activity of the PTHR1 gene may be a relevant genetic influence on final adult height and BMD.

Exclusion mapping of chromosomes 1, 4, 6 and 14 with bone mineral density in 79 Caucasian pedigrees

Low bone mineral density (BMD) is a major determinant of osteoporosis and is under strong genetic control. A large number of linkage and association studies for BMD variation have been conducted, with the results being largely inconsistent. Linkage exclusion analysis is a useful tool for gene mapping but has never been used on BMD. In the present study, we conducted a linkage exclusion mapping for BMD variation on chromosomes 1, 4, 6 and 17 in 79 Caucasian pedigrees. For hip BMD variation, several genomic regions were excluded for effect sizes of 10% or greater, including regions of 61-77 cM at 1p35-p34, 167-196 cM at 1q21-q23 and 261-291 cM at 1q42-q44; 85-112 cM at 4q21-q25 and 146-150 cM at 4q31; and 77-85 cM at 6p12-q13. For spine BMD, we were able to exclude the regions of 168-189 cM at 1q21-q23, 92-94 cM at 4q21 and 106-107 cM at 4q24 and 56-103 cM at 17q12-q25, as having effect sizes of 10% or greater. These results suggest that a number of candidate genes located in the excluded regions, such as interleukin 6 receptor (IL6R) gene, type I collagen alpha 1 (COL1A1) gene and bone morphogenetic protein-3 (BMP3) gene are unlikely to have a substantial effect on BMD variation in this Caucasian population. Along with previous studies searching for genes underlying BMD variation, the current study has further delineated the genetic basis of BMD variation and provided valuable information for future genetic studies.

Tests of linkage and association of PTH/PTHrP receptor type 1 gene with bone mineral density and height in Caucasians

Parathyroid hormone/parathyroid hormone-related peptide receptor type 1 (PTHR1) plays an important role in calcium metabolism. It was previously shown to influence variation in bone mineral density (BMD). To investigate its importance in a typical U.S. Caucasian population, we tested linkage or association of the PTHR1 gene with BMD and height. Altogether, 1873 subjects from 405 Caucasian nuclear families were studied. BMD was measured at the lumbar spine (L1-L4) and total hip (femoral neck, trochanter, and intertrochanter regions). Four single nucleotide polymorphisms (SNPs) in the PTHR1 gene were genotyped. Sixteen haplotypes were reconstructed. Only two major haplotypes had frequencies >3% and were thus used for the analysis. Analyses were performed for BMD and height in the total sample and for peak BMD (PBMD) achieved in offspring subjects aged 20-50 in a subsample of 387 nuclear families. We found suggestive evidence for total association between haplotype 13 (AATG) and hip PBMD (P = 0.031). For height, evidence of within-family association was suggested for SNP1, SNP2, and haplotype 4 (GGCA) (P < or = 0.05). Our findings suggest that the PTHR1 gene may be important for PBMD, height variation, or both, although the significance is dampened by correction for multiple testing.

CD38 is associated with premenopausal and postmenopausal bone mineral density and postmenopausal bone loss

One goal of osteoporosis research is to identify the genes and environmental factors that contribute to low bone mineral density (BMD) and fracture. Linkage analyses have identified quantitative trait loci (QTLs), however, the genes contributing to low BMD are largely unknown. We examined the potential association of an intronic polymorphism in CD38 with BMD and postmenopausal bone loss. CD38 resides in 4p15, where a QTL for BMD has been described. CD38-/- mice display an osteoporotic phenotype at 3 months, with normalization of BMD by 5 months. The CD38 polymorphism was identified by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) analysis in 457 postmenopausal and 173 premenopausal Caucasian women whose spine and hip BMD was measured by dual energy X-ray absorptiometry (DXA). Influence of the CD38 polymorphism on bone loss was analyzed in 273 postmenopausal women over a follow-up of 2.94 +/- 1.50 years. The CD38-PvuII polymorphism was significantly associated with premenopausal and postmenopausal (P = 0.001) lumbar spine BMD. Women homozygous for the G allele had >14% lower spinal BMD than women with GC/CC genotypes. An allele dose effect was observed at the spine in premenopausal (P = 0.002) and postmenopausal (P < 0.001) cohorts. The CD38-PvuII polymorphism was significantly associated with femoral neck BMD in pre- and postmenopausal women (P = 0.002 and P = 0.011, respectively). However, significance was lost following adjustment of hip BMD for covariates in the postmenopausal cohort (P = 0.081). The CD38-PvuII polymorphism was weakly associated with bone loss at the spine (P = 0.024), in postmenopausal women not taking hormone replacement therapy. We suggest that the CD38-PvuII polymorphism may influence the attainment and maintenance of peak BMD and postmenopausal bone loss.

Assessment of linkage and association of 13 genetic loci with bone mineral density

Bone mineral density (BMD), an important risk factor for osteoporosis, is a complex trait likely affected by multiple genes. The linkage and/or association of 13 polymorphic loci of seven candidate genes (estrogen receptor alpha [ERalpha] and beta [ERbeta], calcium-sensing receptor, vitamin D receptor, collagen type 1alpha1, low-density lipoprotein [LDL] receptor-related protein 5 [LRPS], and transforming growth factor beta1) were evaluated in 177 southern Chinese pedigrees of 674 subjects, with each pedigree identified through a proband having a BMD Z score of -1.28 or less at the hip or spine. A suggestive linkage was detected between the IVS1-351A/G polymorphism of ERalpha and spine BMD, and between the 1082G/A, 1730G/A, and D14S1026 polymorphisms of ERbeta and BMD at both spine and hip. The quantitative transmission disequilibrium test (QTDT) detected total family association between 1730G/A of ERbeta and BMD at spine and hip; between D14S1026 of ERbeta and hip BMD; and between the 266A/G and 2220C/T polymorphisms of LRP5 and hip BMD. Similar total family associations were detected when only the females were analyzed. In addition, the IVS1-397T/C polymorphism of ERalpha was associated with spine BMD, and the 266A/G and 2220C/T polymorphisms of LRP5 were associated with femoral neck BMD in the females. A within-family association was detected with the IVS1-397T/C polymorphism of ERalpha, and the 266A/G and 2220C/T polymorphisms of LRP5 in the females. The effect of each polymorphism on BMD variance ranged from 1% to 4%. In conclusion, ERalpha, ERbeta and LRP5 are important candidate genes determining BMD variation, especially in females.

Haplotype frequencies and linkage disequilibrium analysis of four frequent polymorphisms at the PTH/PTH-related peptide receptor gene locus

The parathyroid hormone (PTH)/PTH-related peptide receptor is a critical component in the control of mineral ion metabolism and in bone development. This receptor is encoded by a single gene (PTHR1) on chromosome 3p21.1-p24.2, and mutations in this gene have been found in several clinical disorders of bone and mineral metabolism. To facilitate future genetic studies of this important gene, we determined haplotype frequencies and performed linkage disequilibrium (LD) analysis of four different polymorphisms at the PTHR1 locus. Combined analysis of Caucasian, African-American and Asian individuals indicated that LD exists between all but one pair of the four polymorphisms. However, the pattern of LD differed substantially among the three subpopulations; for example, LD between two closely spaced (154-bp apart) single nucleotide polymorphisms appeared to be present only in Asians. Depending on the population under study, genetic association studies may need to test even more closely spaced polymorphic markers when screening the PTHR1 locus. These findings may thus affect the design and interpretation of future genetic studies involving PTHR1.

Long-term culture in dexamethasone unmasks an abnormal phenotype in osteoblasts isolated from osteoporotic subjects

We have shown that osteoblastic cells derived from trabecular bone explants of osteoporotic subjects (OP cells) exhibited an altered alkaline phosphatase (ALP) response to 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] compared to control (CON) cells. Our hypothesis that OP cells have other intrinsic abnormalities was investigated using our cell models representing two different stages of differentiation. OP and CON cells were cultured in the absence (-DEX) or presence (+DEX) of 10 nM dexamethasone (DEX) in 10% fetal calf serum (FCS) prior to exposure to serum-free medium containing 1 nM of PTH and/or 17-beta estradiol (E2). Both OP and CON cells responded to DEX with a two-fold increase in basal ALP activity. While E2 or PTH+E2 had no effect on OP cells, both treatments inhibited ALP activity in CON cells (p<0.05). OP and CON cells grown in DEX also expressed PTH-stimulated adenylate cyclase (AC) activities higher than those of (-DEX) cells. OP+DEX cells, however, exhibited activities which were 8-fold higher than those of CON+DEX cells (p<0.001). In OP+DEX cells, E2 stimulated basal AC activity (p<0.05) but did not affect PTH-stimulated activity. In contrast, in CON+DEX cells, E2 had no effect on basal activity but inhibited PTH-stimulated AC activity (p<0.001). Osteocalcin production was 4-fold lower in OP+DEX cells compared to OP-DEX and CON cells (p<0.05) while osteocalcin mRNA levels were significantly lower in OP+DEX and CON+/-DEX cells compared to OP-DEX cells (p<0.05). E2 did not affect osteocalcin protein or mRNA levels in either OP or CON cells. No differences in mRNA levels were found for estrogen receptor-alpha (ER-a) in OP+/-DEX cells whereas these levels were significantly higher in CON+DEX compared to CON-DEX cells (p<0.05). These results indicate that DEX amplified the differences between OP and CON cells and confirm the presence of intrinsic osteoblastic abnormalities in patients with osteoporosis that persist in culture.

Whole-genome scan for linkage to bone strength and structure in inbred Fischer 344 and Lewis rats

A genome-wide genetic linkage analysis identified several chromosomal regions influencing bone strength and structure in F2 progeny of Fischer 344 x Lewis inbred rats.

INTRODUCTION

Inbred Fischer 344 (F344) and Lewis (LEW) rats are similar in body size, but the F344 rats have significantly lower BMD and biomechanical strength of the femur and spine compared with LEW rats. The goal of this study was to identify quantitative trait loci (QTL) linked to bone strength and structure in adult female F2 rats from F344 and LEW progenitors.

MATERIALS AND METHODS

The 595 F2 progeny from F344 x LEW rats were phenotyped for measures of bone strength (ultimate force [Fu]; energy to break [U]; stiffness [S]) of the femur and lumbar vertebra and structure (femur midshaft polar moment of inertia [Ip]; femur midshaft cortical area; vertebral area). A genome-wide scan was completed in the F2 rats using 118 microsatellite markers at an average interval of 20 cM. Multipoint quantitative linkage analysis was performed to identify chromosomal regions that harbor QTL for bone strength and structure phenotypes.

RESULTS

Evidence of linkage for femur and lumbar strength was observed on chromosomes (Chrs) 1, 2, 5, 10, and 19. Significant linkage for femoral structure was detected on Chrs 2, 4, 5, 7, and 15. QTLs affecting femoral strength on Chrs 2 and 5 were also found to influence femur structure. Unique QTLs on Chrs 1, 10, and 19 were found that contributed to variability in bone strength but had no significant effect on structure. Also, unique QTLs were observed on Chrs 4, 7, and 15 that affected only bone structure without any effect on biomechanics.

CONCLUSION

We showed multiple genetic loci influencing bone strength and structure in F344 x LEW F2 rats. Some of these loci are homologous to mouse and human chromosomes previously linked to related bone phenotypes.

Gene variants for osteoporosis and their pleiotropic effects in aging

The prevalence of osteoporosis is raising worldwide as improving conditions of living and treatment of other common diseases continuously increases life expectancy. Thus, osteoporosis affects most women above 80 years of age and, at the age of 50, the lifetime risk of suffering an osteoporosis-related fracture approaches 50% in women and 20% in men. Numerous genetic, hormonal, nutritional and life-style factors contribute to the acquisition and maintenance of bone mass. Among them, genetic variations explain as much as 70% of the variance for bone mineral density (BMD) in the population. Dozens of quantitative trait loci (QTLs) for BMD have been identified by genome screening and linkage approaches in humans and mice, and more than 100 candidate gene polymorphisms tested for association with BMD and/or fracture. Sequence variants in the vitamin D receptor (VDR), collagen 1 alpha 1 chain (Col1A1), estrogen receptor alpha (ESR1), interleukin-6 (IL-6) and LDL receptor-related protein 5 (LRP5) genes were all found to be significantly associated with differences in BMD and/or fracture risk in multiple replication studies. Moreover, some genes, such as VDR and IL-6, were shown to interact with non-genetic factors, i.e. calcium intake and estrogens, to modulate BMD. Since these gene variants have also been associated with other complex disorders, including cancer and coronary heart disease, they may represent common genetic susceptibility factors exerting pleiotropic effects during the aging process.

Association tests of interleukin-6 (IL-6) and type II tumor necrosis factor receptor (TNFR2) genes with bone mineral density in Caucasians using a re-sampling approach

Interleukin 6 (IL-6) and tumor necrosis factor (TNF) are important cytokines for bone turnover. In this study, a promoter C-174G single-nucleotide polymorphism (SNP) within the IL-6 gene affecting the transcription rate of IL-6 and an exon 6 T676G SNP of the TNF receptor 2 (TNFR2) gene causing an M196R amino-acid change were examined for their relationship with bone mineral density (BMD). Four hundred and five multi-offspring Caucasian families, including 389 male children and 744 female children, were used. One thousand re-samplings were conducted and in each data set, one child was randomly chosen from each family. For each data set, one-way analysis of variance (ANOVA) test was independently implemented using age, age2, sex, height and weight as covariates. There were 523, 288, 204 and 369 significant results out of 1,000-replicate re-samplings of the data of the IL-6 SNP (P<0.05) for one-third, mid-distal, ultradistal radius BMD, and the first principal component (PC1) extracted from the three radial BMDs, respectively, which means that the confidences for associations of the C-174G SNP in the IL-6 gene with one-third, mid-distal, ultradistal radius (totally called distal forearm) BMDs, and PC1, were 52.3, 28.8, 20.4 and 36.9%, respectively. For this SNP with BMD at other skeletal sites and the TNFR2 T676G SNP with BMD at any site, significant results were far less than 200 times out of 1,000 re-sampling replicates. The exceedingly consistent permutation results further improved the confidence of the associations. It may imply that the IL-6 C-174G SNP is associated with distal forearm BMD, but there is no evidence that the TNFR2 T676G SNP is related with BMD in US Caucasians. This is the first attempt to conduct association test utilizing a re-sampling approach. Our results may be more informative than other association analyses that were only based on one sampling result. The results also suggest that different samplings could produce significantly diverse results even for the same population and the results from one sampling are unlikely to be conclusive. Our results have significant implications for association studies and interpretation of non-reproducible association findings.

[Genetics of osteoporosis]

Osteoporosis is a multifactorial disease involving genetic component and several environmental factors. Some rare diseases that are associated with osteoporosis such as Lobstein disease or the "pseudoglial osteoporosis" syndrom are monogenetic. Nevertheless common osteoporosis is a polygenic affection resulting from the interaction between the polymorphism of different genes and the environmental factors. The genetic component of osteoporosis encompasses roughly 60 to 70% of bone mineral density, whereas the effect on fracture risk seems lower because of the importance of other environmental factors as falls. Many polymorphisms of candidate genes involved in the regulation of bone mass have been correlated to bone density. It is likely that many genes participate to the regulation of bone density although the existence of a major gene is highly suspected. Moreover linkage analysis after genome-wide search in populations with severe osteoporosis has focused on some regions of interest (QTL) on the chromosomes. This will allow to localize one or more specific genes. The current genetic studies on different populations affected by osteoporosis or not will be useful in order to better predict the fracture risk in association with bone density and biochemical markers of bone turnover. Moreover, this will lead to the development of new treatments of osfeoporosis and will help to adapt the therapy for individual patients.

The (GT)n polymorphism and haplotype of the COL1A2 gene, but not the (AAAG)n polymorphism of the PTHR1 gene, are associated with bone mineral density in Chinese

Collagen type I alpha2 (COL1A2) and parathyroid hormone (PTH)/PTH-related peptide receptor (PTHR1) are two prominent candidate genes for bone mineral density (BMD). To test their importance for BMD variation in Chinese, we recruited 388 nuclear families composed of both parents and at least one healthy daughter with a total of 1,220 individuals, and simultaneously analyzed population stratification, total-family association, and within-family association between BMD at the spine and hip and the (GT)n marker in the intron 1 of the COL1A2 gene and the (AAAG)n marker in the P3 promoter of PTHR1 gene. We also performed these association analyses with haplotypes of the MspI and (GT)n polymorphisms in the COL1A2 gene. Significant within-family association was found between the M(GT)12 haplotype and trochanter BMD (P<0.001). Individuals with this haplotype have, on average, 9.53% lower trochanter BMD than the non-carriers. Suggestive evidence of the within-family association was detected between the (GT)17 allele and BMD at the spine (P=0.012), hip (P=0.011), femoral neck (P=0.032), trochanter (P=0.023), and intertrochanter (P=0.034). The association was confirmed by subsequent permutation tests. For the association, the proportion of phenotypic variance explained by the detected markers ranged from 1.2 to 3.9%, with the highest 3.9% at the trochanter for the M(GT)12 haplotype. This association indicates that there is strong linkage disequilibrium between the polymorphisms (MspI and GT repeat polymorphism) in the COL1A2 gene and a nearby quantitative trait locus (QTL) underlying BMD variation in Chinese, or the markers themselves may have an important effect on the variation of BMD. On the other hand, no significant within-family association, population stratification and total-family association between the PTHR1 polymorphism and BMD were found in our Chinese population.

Tests of linkage and/or association of TGF-beta1 and COL1A1 genes with bone mass

Transforming growth factor beta 1 (TGF-beta1) is involved in bone metabolism and collagen type I alpha 1 (COL1A1) is the most abundant protein of bone matrix. Both have been considered as candidate genes for osteoporosis. In this study, we employed the transmission disequilibrium test (TDT) to examine the relationship between each of the two genes with bone mineral density (BMD) and bone mineral content (BMC) at the spine and hip in a sample of 1668 subjects from 387 Caucasian nuclear families. For the TGF-beta1 gene, three SNPs, SNP1, SNP2, and SNP4 (located in exon 1, intron 4 and intron 5, respectively) were tested and the minor allele frequencies were 30.9%, 2.1% and 27.0%, respectively. All eight possible haplotypes (TGF1-8) were observed. For the COL1A1 gene, the minor allele frequencies of SNP5, SNP6 and SNP8 (located in exon 1, intron 1, and exon 45, respectively) were 15.2%, 18.7%, 2.0%, respectively, and only six of eight potential haplotypes (COL1-6) were obtained. In the whole sample, total associations were observed between haplotype COL5 with spine BMD (P=0.027), haplotypes COL3 and TGF4 with hip BMC (P=0.002, 0.003, respectively). Within-family associations were found for spine BMD at haplotypes TGF4 (P=0.027) in female offspring families and TGF3 (P=0.021) in male offspring families. Further studies with denser markers and larger sample size are required to eventually define the relationship between these two genes with bone mass at the spine and hip.

Association and linkage analyses of interleukin-6 gene 634C/G polymorphism and bone phenotypes in Chinese

In this study, we tested the interleukin-6 (IL-6) gene as an important candidate gene for its linkage and association with the variation of bone phenotypes (bone mineral density [BMD] and bone size) in young Chinese female subjects. We genotyped the IL-6 gene at the -634C/G restriction fragment length polymorphism (RFLP) site (ID, RS1800796) in 1263 individuals from 402 Chinese nuclear families, composed of both parents and at least one healthy daughter (mean age +/- SD, 31.4 +/- 5.8 years). Using the daughters' bone phenotypes, we tested total-family association, within-family association (via transmission disequilibrium test, [TDT]), and linkage, between the -634C/G marker and bone phenotypes at the spine and the hip. No significant association or linkage was found for bone size and BMD, although a trend was observed for linkage between the IL-6 gene -634C/G marker and L1-4 spinal BMD (adjusted for age, weight, and height). Our results, together with the findings from other studies, indicate that the IL-6 gene, although important for postmenopausal bone loss, may have a limited impact on peak bone mass variation in a Chinese population.

Polymorphisms in the sclerosteosis/van Buchem disease gene (SOST) region are associated with bone-mineral density in elderly whites

Osteoporosis has a strong genetic component, but the genes involved are poorly defined. We studied whether the sclerosteosis/van Buchem disease gene (SOST) is an osteoporosis-risk gene by examining its association with bone-mineral density (BMD). Mutations in SOST result in sclerosteosis, and alterations in the SOST gene expression may be causal in the closely related van Buchem disease. We used a set of eight polymorphisms from the SOST gene region to genotype 1,939 elderly men and women from a large population-based prospective-cohort study of Dutch whites. A 3-bp insertion (f=0.38) in the presumed SOST promoter region (SRP3) was associated with decreased BMD in women at the femoral neck (FN) (P=.05) and lumbar spine (LS) (P=.01), with evidence of an allele-dose effect in the oldest age group (P=.006). Similarly, a G variant (f=0.40) in the van Buchem deletion region (SRP9) was associated with increased BMD in men at the FN (P=.007) and LS (P=.02). In both cases, differences between extreme genotypes reached 0.2 SD. We observed no genotype effects on fracture risk, for the 234 osteoporotic fractures validated during 8.2 years of follow-up and for the 146 vertebral prevalent fractures analyzed. We did not find association between any of several frequent haplotypes across the SOST gene region and BMD. We did find evidence of additive effects of SRP3 with the COLIA1 Sp1 polymorphism but not with haplotypes of 3' polymorphisms in the vitamin-D receptor gene. The SOST-COLIA1 additive effect increased with age and reached 0.5 SD difference in BMD at LS in the oldest age group (P=.02). The molecular mechanism whereby these moderate SOST genotype effects are mediated remains to be elucidated, but it is likely to involve differences in regulation of SOST gene expression.

APOE haplotypes influence bone mineral density in Caucasian males but not females

Low bone mineral density (BMD) is one of the most important risk factors for osteoporosis. Apolipoprotein E (APOE) has been considered as a candidate gene for osteoporosis because of its influence on osteoblast uptake of lipoprotein-borne vitamin K. Using the quantitative transmission disequilibrium test QTDT, we examined linkage and/or association of APOE and BMD at the lumbar spine and the total hip in a sample of 387 Caucasian nuclear families with 715 parents and 953 children. The children were aged 20-50 years and female offspring were premenopausal as well. Four single nucleotide polymorphisms (SNP1-4) in the APOE gene, 4-locus haplotypes and 2-locus haplotypes (epsilon1, epsilon2, epsilon3, epsilon4 isoforms, reconstructed by SNP3 and SNP4) were analyzed. In the whole sample and the female offspring families we found no evidence of linkage or association for either single SNP or haplotype with BMD at the two studied skeletal sites. In the male offspring families, within-family associations were observed at the haplotypes CGTC (P = 0.001), GGTT (P = 0.002), and GATC (P = 0.006) for the lumbar spine BMD, and GATC (P = 0.008) for the total hip BMD. These data suggested that in our studied Caucasian population, APOE may have effects on BMD variation in males but not females. Further studies with a larger sample size are required to confirm such results.

A second-stage genome scan for QTLs influencing BMD variation

Low bone mineral density (BMD) is a major risk factor for osteoporotic fracture. To identify genomic regions harboring quantitative trait loci (QTLs) contributing to BMD variation, we performed a two-stage genome screen. The first stage involved genotyping of a sample of 53 pedigrees with 630 individuals using 400 microsatellite markers spaced at approximately 10-cM intervals throughout the genome. Ten genomic regions with multi- and/or two-point LOD scores greater than 1.5 were observed. In the present second-stage study, 60 microsatellite markers, with a mean spacing of about 5 cM, were genotyped in these regions in an expanded sample of 79 pedigrees that contained 1816 subjects. Each pedigree was ascertained through a proband with extreme BMD at the hip or spine. BMD at the spine (L1-4), hip (the femoral neck, trochanter, and intertrochanteric region), and wrist (the ultradistal region) was measured by dual-energy X-ray absorptiometry (DXA) and was adjusted for age, sex, height, and weight. Two-point and multipoint linkage analyses were performed for each BMD site using statistical genetic methods that are implemented in the computer package SOLAR. Several regions (7q11, 10q26, 12q13, and 12q24) achieved LOD scores in excess of 1 in the second-stage followup study. The current results replicate some of our previous linkage findings and also highlight some of the difficulties facing microsatellite linkage mapping for complex human diseases.

Genome scan for QTLs underlying bone size variation at 10 refined skeletal sites: genetic heterogeneity and the significance of phenotype refinement

To identify quantitative trait loci (QTLs) underlying variation in bone size, we conducted a whole-genome linkage scan in 53 pedigrees with 630 subjects using 380 microsatellite markers. Lumbar area 1, 2, 3, and 4 at the spine, femoral neck, trochanter, intertrochanter areas at the hip, ultradistal, mid-distal, and one-third distal areas at the wrist were measured by dual-energy X-ray absorptiometry (DXA), and adjusted for age, height, weight, and sex. Two-point and multipoint linkage analyses were performed for skeletal bone size at each site and their composite measurements using the SOLAR package. Two chromosomal regions (1q22 and 10q21) were identified with significant evidence of linkage (LOD > 4.32) to one-third distal area, and three were identified with suggestive evidence of linkage (LOD > 2.93) to bone size in one skeletal site. Our results indicated that the low power of QTLs mapping for composite phenotypic measurements may result from genetic heterogeneity of complex traits.