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

LGR4 Gene Polymorphisms Are Associated With Bone and Obesity Phenotypes in Chinese Female Nuclear Families

OBJECTIVE

The current study was conducted to determine whether peak bone mineral density (BMD) and obesity phenotypes are associated with certain LGR4 gene polymorphisms found in Chinese nuclear families with female children.

METHODS

A total of 22 single nucleotide polymorphisms (SNPs) located in and around the LGR4 gene were identified in 1,300 subjects who were members of 390 Chinese nuclear families with female children. Then, BMD readings of the femoral neck, total hip, and lumbar spine as well as measurements of the total lean mass (TLM), total fat mass (TFM), and trunk fat mass were obtained via dual-energy X-ray absorptiometry. The quantitative transmission disequilibrium test was used to analyze the associations between specific SNPs and LGR4 haplotypes and peak BMD as well as between LGR4 haplotypes and TLM, percent lean mass, TFM, percent fat mass, trunk fat mass, and body mass index (BMI).

RESULTS

Here, rs7936621 was significantly associated with the BMD values for the total hip and lumbar spine, while rs10835171 and rs6484295 were associated with the trunk fat mass and BMI, respectively. Regarding the haplotypes, we found significant associations between GAA in block 2 and trunk fat mass and BMI, between AGCGT in block 3 and total hip BMD, between TGCTCC in block 5 and femoral neck BMD, and between TACTTC in block 5 and both lumbar spine and femoral neck BMD (all P-values < 0.05).

CONCLUSION

Genetic variations of the LGR4 gene are related to peak BMD, BMI, and trunk fat mass.

GREB1 genetic variants are associated with bone mineral density in Caucasians

Gaining an understanding of factors contributing to bone quality is key to the development of effective preventative treatments for osteoporosis and reduction in osteoporotic fractures. Oestrogen is a strong regulator of bone remodelling which maintains skeletal structural integrity. The growth regulation by oestrogen in breast cancer 1 (GREB1) gene, with an as yet undefined function, is an early response gene in the oestrogen-regulated pathway. Suggestive evidence of linkage with bone mineral density (BMD) variation has been reported with D2S168, located telomeric of GREB1. The aim of this study was to determine if genetic variation within GREB1 was associated with BMD variation at two sites with high fracture rates-the lumbar spine (LS) and the femoral neck (FN). Informative GREB1 single-nucleotide polymorphisms (SNPs) (n = 12) were selected for genotyping and tested for association in a family-based dataset (n = 508 individuals from 229 families). Significantly associated SNPs were tested further in a postmenopausal dataset from the same geographic region (n = 477 individuals). One intronic SNP, rs5020877, was significantly associated with LS and FN BMD in the family-based dataset (P ≤ 0.005). The association was not observed in the postmenopausal dataset (P > 0.017); however, rs10929757 was significantly associated with FN BMD (P = 0.006). Markers, rs5020877 and rs10929757, were constituent SNPs in one GREB1 linkage disequilibrium block, although not historically correlated (r ² = 0.07). Our findings suggest that GREB1 is a novel gene target for osteoporosis genetics and needs to be investigated further.

Association between SNPs and haplotypes in the METTL21C gene and peak bone mineral density and body composition in Chinese male nuclear families

The methyltransferase-like 21C gene (METTL21C), which is mainly expressed in muscle, can promote the differentiation of myoblasts to myotubes and reduce glucocorticoid-induced apoptosis of osteocytes. The purpose of this study was to explore the association between single nucleotide polymorphisms of METTL21C and peak bone mineral density (BMD), body mass index, total fat mass (TFM), and total lean mass (TLM) in Chinese young men. Fifteen tagging single nucleotide polymorphisms were genotyped, and haplotype blocks were derived in 400 Chinese male nuclear families. The peak BMD of the lumbar and hip, TFM, and TLM were measured by dual-energy X-ray absorptiometry. The association analyses were performed by a quantitative transmission disequilibrium test. Both TLM and TFM had a significant positive effect on peak BMD, but the positive regulation of TLM was stronger than that of TFM. After 1000 permutations, significant within-family associations were found between rs9585961 and lumbar spine BMD and femoral neck BMD, rs9518810 and femoral neck BMD, and rs599976 and body mass index, TFM, and percentage fat mass (all P < 0.05). The association analyses with haplotypes showed that haplotype AG in block 1 was significantly associated with TFM (P = 0.031) and haplotype CAG in block 2 was significantly associated with lumbar spine BMD (P = 0.020). Our study, for the first time, demonstrates that the polymorphisms and haplotypes of METTL21C contribute to the peak BMD and TFM in Chinese males, which suggests that as a quantitative trait locus with potential pleiotropy it may have an influence on osteoporosis and obesity.

Association between vitamin D receptor gene polymorphisms (Fok1 and Bsm1) and osteoporosis: a systematic review

Osteoporosis is a health concern characterized by reduced bone mineral density (BMD) and increased risk of fragility fractures. Many studies have investigated the association between genetic variants and osteoporosis. Polymorphism and allelic variations in the vitamin D receptor gene (VDR) have been found to be associated with bone mineral density. However, many studies have not been able to find this association. Literature review was conducted in several databases, including MEDLINE/Pubmed, Scopus, EMBASE, Ebsco, Science Citation Index Expanded, Ovid, Google Scholar, Iran Medex, Magiran and Scientific Information Database (SID) for papers published between 2000 and 2013 describing the association between Fok1 and Bsm1 polymorphisms of the VDR gene and osteoporosis risk. The majority of the revealed papers were conducted on postmenopausal women. Also, more than 50% studies reported significant relation between Fok1, Bsm1 and osteoporosis. Larger and more rigorous analytical studies with consideration of gene-gene and gene-environment interactions are needed to further dissect the mechanisms by which VDR polymorphisms influence osteoporosis.

Pregnancy-associated osteoporosis with a heterozygous deactivating LDL receptor-related protein 5 (LRP5) mutation and a homozygous methylenetetrahydrofolate reductase (MTHFR) polymorphism

Pregnancy-associated osteoporosis (PAO) is a rare, idiopathic disorder that usually presents with vertebral compression fractures (VCFs) within 6 months of a first pregnancy and delivery. Spontaneous improvement is typical. There is no known genetic basis for PAO. A 26-year-old primagravida with a neonatal history of unilateral blindness attributable to hyperplastic primary vitreous sustained postpartum VCFs consistent with PAO. Her low bone mineral density (BMD) seemed to respond to vitamin D and calcium therapy, with no fractures after her next successful pregnancy. Investigation of subsequent fetal losses revealed homozygosity for the methylenetetrahydrofolate reductase (MTHFR) C677T polymorphism associated both with fetal loss and with osteoporosis (OP). Because her neonatal unilateral blindness and OP were suggestive of loss-of-function mutation(s) in the gene that encodes LDL receptor-related protein 5 (LRP5), LRP5 exon and splice site sequencing was also performed. This revealed a unique heterozygous 12-bp deletion in exon 21 (c.4454_4465del, p.1485_1488del SSSS) in the patient, her mother and sons, but not her father or brother. Her mother had a normal BMD, no history of fractures, PAO, ophthalmopathy, or fetal loss. Her two sons had no ophthalmopathy and no skeletal issues. Her osteoporotic father (with a family history of blindness) and brother had low BMDs first documented at ages ∼40 and 32 years, respectively. Serum biochemical and bone turnover studies were unremarkable in all subjects. We postulate that our patient's heterozygous LRP5 mutation together with her homozygous MTHFR polymorphism likely predisposed her to low peak BMD. However, OP did not cosegregate in her family with the LRP5 mutation, the homozygous MTHFR polymorphism, or even the combination of the two, implicating additional genetic or nongenetic factors in her PAO. Nevertheless, exploration for potential genetic contributions to PAO may explain part of the pathogenesis of this enigmatic disorder and identify some at-risk women.

Polymorphisms in the human ALOX12 and ALOX15 genes are associated with peak bone mineral density in Chinese nuclear families

SUMMARY

Association between ten single-nucleotide polymorphisms (SNPs) in the human ALOX12 and ALOX15 genes and variations in peak bone mineral density (BMD) in a large sample of Chinese nuclear families with female offspring using the quantitative transmission disequilibrium test (QTDT). Our results suggest that the genetic polymorphisms in both human ALOX12 and ALOX15 may contribute to variations in the peak BMD of Chinese women.

INTRODUCTION

The aim of this study was to investigate whether polymorphisms in the human ALOX12 and ALOX15 genes are associated with variations in peak BMD in Chinese nuclear families with female offspring.

METHODS

Each five SNPs in the ALOX12 and ALOX15 genes were genotyped in a total of 1,260 individuals from 401 Chinese nuclear families. The BMD of the lumbar spine, femoral neck and total hip was measured by dual-energy X-ray absorptiometry. We tested whether a single SNP or a haplotype was associated with peak BMD variations using the QTDT.

RESULTS

Using QTDT to measure within-family associations in ALOX15, we observed a significant association between rs916055 and BMD in the lumbar spine (p = 0.027 in the permutation 1,000 test). However, in ALOX12, rs312470 was significantly associated with BMD in the femoral neck (p = 0.029 and p = 0.036 in the permutation 1,000 test). The results of a haplotype analysis supported the findings of the single locus test for ALOX15.

CONCLUSIONS

Our results suggest that the genetic polymorphisms in both human ALOX12 and ALOX15 may contribute to variations in the peak BMD of Chinese women.

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

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

European bone mineral density loci are also associated with BMD in East-Asian populations

Most genome-wide association (GWA) studies have focused on populations of European ancestry with limited assessment of the influence of the sequence variants on populations of other ethnicities. To determine whether markers that we have recently shown to associate with Bone Mineral Density (BMD) in Europeans also associate with BMD in East-Asians we analysed 50 markers from 23 genomic loci in samples from Korea (n = 1,397) and two Chinese Hong Kong sample sets (n = 3,869 and n = 785). Through this effort we identified fourteen loci that associated with BMD in East-Asian samples using a false discovery rate (FDR) of 0.05; 1p36 (ZBTB40, P = 4.3×10⁻⁹), 1p31 (GPR177, P = 0.00012), 3p22 (CTNNB1, P = 0.00013), 4q22 (MEPE, P = 0.0026), 5q14 (MEF2C, P = 1.3×10⁻⁵), 6q25 (ESR1, P = 0.0011), 7p14 (STARD3NL, P = 0.00025), 7q21 (FLJ42280, P = 0.00017), 8q24 (TNFRSF11B, P = 3.4×10⁻⁵), 11p15 (SOX6, P = 0.00033), 11q13 (LRP5, P = 0.0033), 13q14 (TNFSF11, P = 7.5×10⁻⁵), 16q24 (FOXL1, P = 0.0010) and 17q21 (SOST, P = 0.015). Our study marks an early effort towards the challenge of cataloguing bone density variants shared by many ethnicities by testing BMD variants that have been established in Europeans, in East-Asians.

No association of the polymorphisms of the frizzled-related protein gene with peak bone mineral density in Chinese nuclear families

Background

The Wnt/beta-catenin signaling pathway plays an important role in skeletal development. Polymorphisms of frizzled-related protein (FRZB), an antagonist of this pathway, may generate variations in bone mineral density (BMD). In this study, we analyzed the association between FRZB genotypes and peak BMD variation in the spines and hips of two relatively large samples of Chinese female-offspring and male-offspring nuclear families.

Methods

We recruited 1,260 subjects from 401 female-offspring nuclear families and 1,296 subjects from 427 male-offspring nuclear families and genotyped four tagging single nucleotide polymorphisms (tagSNPs) (rs6433993, rs409238, rs288324, and rs4666865) spanning the entire FRZB gene. The SNPs rs288326 and rs7775, which are associated with hip osteoarthritis, were not selected in this study because of their low minor allele frequencies (MAFs) in Chinese people. The quantitative transmission disequilibrium test (QTDT) was used to analyze the association between each SNP and haplotype with peak BMD in female- and male-offspring nuclear families.

Results

In the female-offspring nuclear families, we found no evidence of an association between either single SNPs or haplotypes and peak BMD in the spine or hip. In the male-offspring nuclear families, no within-family association was observed for either SNPs or haplotypes, although a significant total association was found between rs4666865 and spine BMD (P = 0.0299).

Conclusion

Our results suggest that natural variation in FRZB is not a major contributor to the observed variability in peak BMD in either Chinese females or males. Because ethnic differences in the FRZB genotypes may exist, other studies in different population are required to confirm such results.

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.

Association between VDR and ESR1 gene polymorphisms with bone and obesity phenotypes in Chinese male nuclear families

AIM

The goal of this study was to determine whether polymorphisms in the vitamin D receptor (VDR) and estrogen receptor alpha (ESR1) genes are associated with variations of peak bone mineral density (BMD) and obesity phenotypes in young Chinese men.

METHODS

A total of 1215 subjects from 400 Chinese nuclear families were genotyped by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) and allele-specific multiple PCR (ASM-PCR) analysis at the ApaI, FokI, and CDX2 sites in the VDR gene and the PvuII and XbaI sites in the ESR1 gene. BMD at the lumbar spine and hip, total fat mass, and total lean mass were measured using dual energy X-ray absorptiometry. The associations between VDR and ESR1 gene polymorphisms with peak BMD, body mass index (BMI), total fat mass, total lean mass, and percentage fat mass (PFM) were determined using quantitative transmission disequilibrium tests (QTDTs).

RESULTS

Using QTDTs, no significant within-family associations were obtained between genotypes or haplotypes of the VDR and ESR1 genes and peak BMD. For the obesity phenotypes, the within-family associations were significant between CDX2 genotypes and BMI (P=0.046), fat mass (P=0.004), and PFM (P=0.020). Further, PvuII was significantly associated with the variation of fat mass and PFM (P=0.002 and P=0.039, respectively). A subsequent 1000 permutations were in agreement with these within-family association results.

CONCLUSION

Our findings showed that VDR and ESR1 polymorphisms were associated with total fat mass in young Chinese men, but we failed to find a significant association between VDR and ESR1 genotypes and peak BMD. These findings suggested that the VDR and ESR1 genes are quantitative trait loci (QTL) underlying fat mass variation in young Chinese men.

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.

A1330V variant of the low-density lipoprotein receptor-related protein 5 (LRP5) gene decreases Wnt signaling and affects the total body bone mineral density in Japanese women

Wnt signaling is an important regulator of bone homeostasis. The Wnt co-receptor, namely, low-density lipoprotein receptor-related protein 5 (LRP5), initiates Wnt signal transduction. Recently, we and several other groups have shown that there is a single nucleotide polymorphism (SNP) located in the exon 18 of the LRP5 gene that leads to an amino acid change (3989C > T, A1330V), and is associated with lumbar spine, femoral neck, and radial bone mineral density (BMD), and incidence of fracture. These data suggest that the A1330V variation in the LRP5 gene may affect the pathogenesis of osteoporosis. However, the functional basis of the A1330V variation remains unclear. In the present study, we analyzed the effect of the A1330V variation on Wnt activity. We also investigated the association between this LRP5 SNP and total body BMD using 739 postmenopausal women. LRP5 with the A1330V SNP were transiently coexpressed with Wnt3a in 293T cells and their activity was evaluated by the TCF-Lef reporter assay. In vitro, the TCF-Lef activity in presence of Wnt3a in cells expressing LRP5 and carrying the T allele (Valine at 1330 (V1330)) of exon 18 was significantly reduced as compared to the wild-type allele. The association between the A1330V SNP and total body BMD were replicated in 739 postmenopausal Japanese women (AA vs. VV; P = 0.0026). These data suggest that the V1330 variant in the LRP5 gene decreases Wnt activity, which in turn decreases the BMD.

Estrogen receptor alpha CA dinucleotide repeat polymorphism is associated with rate of bone loss in perimenopausal women and bone mineral density and risk of osteoporotic fractures in postmenopausal women

The association between a newly identified CA repeat polymorphism of the estrogen receptor alpha gene (ESR1) with osteoporosis was investigated. Postmenopausal women with <18 CA repeats had low BMD, increased rate of bone loss and increased fracture risk.

INTRODUCTION

Studies have shown that intronic dinucleotide repeat polymorphisms in some genes are associated with disease risk by modulating mRNA splicing efficiency. D6S440 is a newly identified intronic CA repeat polymorphism located downstream of the 5'-splicing site of exon 5 of ESR1.

METHODS

The associations of D6S440 with bone mineral density (BMD), rate of bone loss and fracture risk were evaluated in 452 pre-, 110 peri- and 622 postmenopausal southern Chinese women using regression models.

RESULTS

Post- but not premenopausal women with less CA repeats had lower spine and hip BMD. The number of CA repeats was linearly related to hip BMD in postmenopausal women (beta=0.008; p=0.004). Postmenopausal women with CA repeats <18 had higher risks of having osteoporosis (BMD T-score< -2.5 at the spine: OR 2.46, 95% CI 1.30-4.65; at the hip: OR 3.79(1.64-8.74)) and low trauma fractures (OR 2.31(1.29-4.14)) than those with >or= 18 repeats. Perimenopausal women with <18 CA repeats had significantly greater bone loss in 18 months at the hip than those with >or= 18 repeats (-1.96% vs. -1.61%, p = 0.029).

CONCLUSIONS

ESR1 CA repeat polymorphism is associated with BMD variation, rate of bone loss and fracture risk, and this may be a useful genetic marker for fracture risk assessment.

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.

Association between myostatin gene polymorphisms and peak BMD variation in Chinese nuclear families

We identified 17 polymorphisms in myostatin by sequencing, and three informative single nucleotide polymorphisms (SNPs) were selected for further observation for their association with peak BMD of women in 401 Chinese nuclear families. Our results suggest that genetic polymorphisms in myostatin likely play a role in attainment of peak BMD in Chinese women.

INTRODUCTION

Myostatin is a TGF-beta family member that is a negative regulator of skeletal muscle growth.

MATERIALS AND METHODS

We identified SNPs in myostatin by direct sequencing. Furthermore, using a quantitative transmission disequilibrium test (QTDT). we tested and further test whether SNPs were associated with peak bone mineral density (BMD) variation at the spines and hips of 401 Chinese nuclear families. We identified 17 polymorphisms in myostatin by sequencing. Next, we selected three informative SNPs for further observation of an association with peak BMD of premenopausal women in 401 Chinese nuclear families.

RESULTS

Using QTDT for the within-family association, we found significant association between rs2293284 and total hip, femoral neck, and trochanter BMD (all p < 0.05), while rs7570532 was associated with total hip and trochanter BMD (p = 0.034 and p = 0.035, respectively). The within-family association was significant between BMI and +2278G > A (p = 0.022). Subsequent permutations were in agreement with these significant within-family association results. Moreover, analyses of the haplotypes confer further evidence for association of rs2293284 and rs7570532 with hip peak BMD variation.

CONCLUSIONS

These results suggest, for the first time, the genetic polymorphisms in myostatin likely play a role in attainment of peak BMD in Chinese women.

Association of low-density lipoprotein receptor-related protein 5 (LRP5) promoter SNP with peak bone mineral density in Chinese women

OBJECTIVE

Low-density lipoprotein receptor-related protein 5 (LRP5) is important for osteoblast differentiation and mutations of the gene are associated with both low and high bone mass syndromes. Our study aimed to evaluate the importance of LRP5 in the determination of peak bone mass acquisition in Chinese females in the general population.

METHODS

A total of 286 young southern Chinese females (aged 22-44 years) with low bone mineral density (BMD) (defined by a BMD Z score < or =-1.28 at either the hip or spine) or high BMD (Z score > or =+1) were studied. The LRP5 gene was sequenced for single nucleotide polymorphisms (SNPs) and 4 SNPs were tagged from 8 genotyped SNPs for this study.

RESULTS

Single locus allele association tests revealed significant associations of rs682429 and rs686921 with BMD variation (p < 0.05). Omnibus test (likelihood ratio test) revealed overall significant association between LRP5 gene locus and total hip BMD, with rs682429 being most predictive. rs682429 is located in 5'UTR, 2 bases adjacent to a consensus recognition site for the Elk-1 binding element.

CONCLUSION

Common variations of the LRP5 promoter are associated with BMD in young women. These significant associations appear to be driven by rs682429. Functional studies are necessary to elucidate the role of this SNP on the effect of Elk-1 binding element transcriptional activity of LRP5 gene.

Racial and ethnic differences in bone turnover markers in men

CONTEXT

Whereas racial and ethnic differences in fracture risk and bone mineral density (BMD) in men have been well described, the influence of race and ethnicity on biochemical markers of bone turnover is less clear.

METHODS

To examine the relationship between bone turnover, BMD, and race and ethnicity in men, we measured BMD, serum intact osteocalcin (OC), and serum C-terminal telopeptides of type 1 collagen (CTx) in 1029 men (aged 30-79 yr) enrolled in the Boston Area Community Health/Bone Survey, a population-based random sample of Black, Hispanic, and White. Men with diseases or on medications known to affect bone metabolism were excluded from the analysis. Mean serum levels of OC and CTx were adjusted for age, month and time of blood sample, and 25-hydroxyvitamin D.

RESULTS

Compared with Black men, adjusted mean OC levels were 17.6 and 20.5% higher in Hispanic (P = 0.02) and White men (P < 0.01), respectively. There was no significant difference between White and Hispanic men. Adjusted mean CTx levels were 14.3% higher in White men, compared with Black men (P = 0.04), but no other differences were significant. OC declined by 0.4%/yr from age 30 to 65 yr and increased thereafter by 2.1%/yr. The age trend in CTx appeared to follow a pattern consistent with a quadratic function of age. Model-estimated annual percent changes within age decade were as follows: 30-39 yr, -2.5%; 40-49 yr, -1.4%; 50-59 yr, -0.3%; 60-69 yr, +0.9%; 70-79 yr, +1.7%. There was no variation in the shape of the age trend in OC or CTx by race or ethnic group. Correlations between bone turnover markers and BMD (adjusted for age, height, weight, serum 25-hydroxyvitamin D, and PTH and month and time of blood sample) were generally weak.

CONCLUSIONS

Bone turnover markers are lower in Black men, compared with White and Hispanic men. Age trends in bone turnover markers are not influenced by race or ethnicity. Future studies in this cohort and others are needed to explore further these reported differences in bone metabolism among Black, Hispanic, and White men.

The effect of LRP5 polymorphisms on bone mineral density is apparent in childhood

Bone mass acquired during childhood is the primary determinant of adult bone mineral density (BMD) and osteoporosis risk. Bone accrual is subject to genetic influences. Activating and inactivating LRP5 gene mutations elicit extreme bone phenotypes, while more common LRP5 polymorphisms are associated with normal variation of BMD. Our aim was to test the hypothesis that LRP5 gene polymorphisms influence bone mass acquisition during childhood. The association between LRP5 gene polymorphisms and bone size and mineralization was examined in 819 unrelated British Caucasian children (n = 429 boys) aged 9 years. Height, weight, pubertal status (where available), total-body and spinal bone area, bone mineral content (BMC), BMD, and area-adjusted BMC (aBMC) were assessed. Dual-energy X-ray absorptiometry (DXA)-gene associations were assessed by linear regression, with adjustment for age, gender, pubertal status, and body size parameters. There were 140, 79, 12, and 2 girls who achieved Tanner stages I-IV, respectively, and 179 and 32 boys who achieved Tanner stages I and II, respectively. The rs2,306,862 (N740N) coding polymorphism in exon 10 of the LRP5 gene was associated with spinal BMD and aBMC (each P = 0.01) and total-body BMD and aBMC (P = 0.04 and 0.03, respectively). Adjusting for pubertal stage strengthened associations between this polymorphism and spinal BMD and aBMC (P = 0.01 and 0.002, respectively). Individuals homozygous for the T allele had greater spinal BMD and aBMC scores than those homozygous for the C allele. A dose effect was apparent as the mean spinal BMD and aBMC of heterozygous TC individuals were intermediate between those of their TT and CC counterparts. The N740N polymorphism in exon 10 of LRP5 was associated with spinal BMD and aBMC in pre- and early pubertal children. These results indicate that LRP5 influences volumetric bone density in childhood, possibly through effects on trabecular bone formation.

The genetics of low-density lipoprotein receptor-related protein 5 in bone: a story of extremes

A few years ago, human genetic studies provided compelling evidence that the low-density lipoprotein receptor-related protein 5 (LRP5) is involved in the regulation of bone homeostasis because pathogenic LRP5 mutations were found in monogenic conditions with abnormal bone density. On the one hand, the osteoporosis pseudoglioma syndrome results from loss of function of LRP5, whereas on the other hand, gain-of-function mutations in LRP5 cause conditions with an increased bone density. On the molecular level, these types of mutations result in disturbed (respectively, decreased and increased) canonical Wnt signaling, an important metabolic pathway in osteoblasts during embryonic and postnatal osteogenesis. This signaling cascade is activated by binding of Wnt ligand to the Frizzled/LRP5 receptor complex. In addition to the involvement of LRP5 in conditions with extreme bone phenotypes, the genetic profile of this gene has also been shown to contribute to the determination of bone density in the general population. Quite a number of studies already demonstrated that common polymorphic variants in LRP5 are associated with bone mineral density and consequently osteoporosis, a multifactorial trait with low bone mass and porous bone structure. These genetic studies together with results obtained from in vitro and in vivo studies emphasize the importance of LRP5 and canonical Wnt signaling in the regulation of bone homeostasis. Therefore, unraveling the exact mechanisms of this signaling cascade has become an important area in bone research. This review focuses on the genetics of LRP5 and summarizes the findings on monogenic bone conditions as well as the current knowledge of its involvement in the pathogenesis of osteoporosis.