Linkage of a QTL contributing to normal variation in bone mineral density to chromosome 11q12-13

Low density lipoprotein receptor-related protein 5 gene polymorphisms and osteoporosis in Thai menopausal women

Background

Osteoporosis, characterized by low bone mineral density (BMD) and high bone fracture risk, is prevalent in Thai menopausal women. Genetic factors are known to play a key role in BMD. Low density lipoprotein receptor-related protein 5 (LRP5), a co-receptor in the Wnt/beta-catenin pathway, is involved in many aspects of bone biology. As coding single nucleotide polymorphisms (cSNPs) of LRP5, including A1330V (rs3736228), and Asian-related Q89R (rs41494349) and N740N (rs2306862), are associated with lowered BMD, this study aimed to determine the relationship between these LRP5 polymorphisms and BMD in 277 Thai menopausal women.

Results

Only rs3736228 deviated from the Hardy–Weinberg equilibrium of allele frequency (p = 0.022). The median, range and p value for the BMD related to each SNP parameter were compared (Mann–Whitney U test). Significant differences were observed between wild-type and risk alleles for both rs3736228 (total radial, p = 0.011; and radial 33, p = 0.001) and rs2306862 (radial 33: p = 0.015) SNPs, with no significant difference for rs41494349 SNP. Linkage disequilibrium was strong for both rs3736228 and rs2306862 SNPs. Haplotype analysis identified high CC frequency in both normal and osteopenia/osteoporosis groups, with a significant odds ratio for carrying the TT haplotype; however, this was non-significant after adjusting for age. Multivariate binary logistic regression analysis performed for rs3736228 showed that individuals with a body mass index <25 kg/m² had an increased risk of osteoporosis for each decade, but the polymorphism had no effect.

Conclusions

This study did not identify LRP5 polymorphisms as a risk factor for osteoporosis in Thai menopausal women. Further studies with larger sample sizes are needed to further clarify the role of LRP5 as a genetic determinant of osteoporosis.

Electronic supplementary material

The online version of this article (doi:10.1186/s12952-016-0059-7) contains supplementary material, which is available to authorized users.

Genetic predictors of skeletal outcomes in healthy fertile women: the Bonturno study

Skeletal traits as height (Ht) or bone mineral density (BMD) are strongly inherited. Low-density lipoprotein receptor-related protein 5 (LRP5) and farnesyl diphosphonate synthase (FDPS) are candidate genes for bone phenotypes. From Bonturno study, we genotyped 570 healthy Caucasian women aged 20 to 50 years (yrs) for LRP5 rs4988321 (A/G) and rs3736228 (C/T) and FDPS rs2297480 (A/C) single nucleotide polymorphisms. Serum C-telopeptide of type I collagen (CTX), osteocalcin (OC), and N-terminal propeptide of type I procollagen (P1NP) were measured in BMD-evaluated subjects at lumbar spine (LS), total hip (TH) and femoral neck (FN) sites. LRP5 rs4988321 locus correlated with FN-BMD (P = 0.0230), while LRP5 rs3736228 genotypes differed in LS-BMD (P = 0.0428). When clustered by age, lower FN-BMD was detected in LRP5 GG (P = 0.030) subjects of 41 to 50 years but not in younger. Both LRP5 GG and CC genotypes showed higher age-adjusted values of OC, CTX and P1NP. Increased CTX values were in LRP5 GGCC subjects than in those having at least one LRP5 A plus T alleles (P = 0.0190). LRP5 CC, GG or GGCC subjects with at least one FDPS C allele showed higher levels of CTX and OC in 31 to 40 yrs or older subjects. In conclusion, LRP5 and FDPS loci age-specifically affect skeletal traits in healthy fertile women.

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

OBJECTIVE

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

METHODS

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

RESULTS

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

DISCUSSION/CONCLUSIONS

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

LRP5 sequence and polymorphisms in the baboon

BACKGROUND

LRP5 is known to have an important relationship with bone density and a variety of other biological processes. Mapping to human chromosome 11q13.2, LRP5 shows considerable evolutionary conservation. Orthologs of this gene exist in many species, although comparison of human LRP5 with other non-human primates has not been performed until now.

METHODS

We reported the complementary DNA (cDNA) sequence and deduced amino acid sequence for baboon LRP5, and compared the baboon and human sequences. cDNA sequences for 21 baboons were examined to identify single-nucleotide polymorphisms (SNPs).

RESULTS

Sequences of coding regions in human and baboon LRP5 showed 97- 99% homology. Twenty-five SNPs were identified in the coding region of baboon LRP5.

CONCLUSION

The observed degree of coding sequence homology in LRP5 led us to expect that the baboon may serve as a useful model for future research into the role(s) of this gene in primate metabolic diseases.

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

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

INTRODUCTION

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

MATERIALS AND METHODS

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

RESULTS

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

CONCLUSIONS

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

Association analysis of WNT10B with bone mass and structure among individuals of African ancestry

Wnts comprise a family of secreted growth factors that regulate the development and maintenance of many organs. Recently, Wnt10b was shown to stimulate osteoblastogenesis and bone formation in mice. To evaluate further the role of Wnt10b in bone health in humans, we performed bidirectional sequencing of approximately 8 kb of the WNT10B gene region in 192 individuals (96 African, 96 white) to identify single nucleotide polymorphisms (SNPs). We identified 19 SNPs with minor allele frequency (MAF) > or =0.01. Ten of these SNPs were not present in the NCBI dbSNP database (build 127), whereas 10 of the 20 SNPs (50%) reported in dbSNP were not verified. We initially genotyped seven tagging SNPs that captured common (MAF > or = 0.05) variation in the region with r (2) > 0.80 and a potentially functional SNP in exon 5 in 1035 Afro-Caribbean men > or =40 yr of age. Association analysis showed three SNPs in a 3' region of linkage disequilibrium that were associated with DXA measures of hip BMD. Associations between two of these three SNPs (rs1051886, rs3741627) with hip BMD were replicated in an additional 980 Afro-Caribbean men (p < 0.05), in the combined sample of 2015 men (p < or = 0.006), and in 416 individuals > or =18 yr of age (mean, 44 yr) belonging to eight extended, multigenerational Afro-Caribbean families with mean family size >50 (3535 relative pairs; p < 0.05). Further analysis showed that rs1051886 and rs3741627 were associated with cortical cross-sectional area, periosteal circumference, and BMC in the radius, such that individuals with the minor alleles had lower biomechanical indices of long-bone bending strength. This analysis implicates the WNT10B locus as a genetic element in the regulation of bone mass and structural geometry.

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

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

A haplotype-based analysis of the LRP5 gene in relation to osteoporosis phenotypes in Spanish postmenopausal women

LRP5 encodes the low-density lipoprotein receptor-related protein 5, a transmembrane protein involved in Wnt signaling. LRP5 is an important regulator of osteoblast growth and differentiation, affecting bone mass in vertebrates. Whether common variations in LRP5 are associated with normal BMD variation or osteoporotic phenotypes is of great relevance. We used a haplotype-based approach to search for common disease-associated variants in LRP5 in a cohort of 964 Spanish postmenopausal women. Twenty-four SNPs were selected, covering the LRP5 region, including the missense changes p.V667M and p.A1330V. The SNPs were genotyped and evaluated for association with BMD at the lumbar spine (LS) or femoral neck (FN) and with osteoporotic fracture, at single SNP and haplotype levels, by regression methods. Association with LS BMD was found for SNP 1, rs312009, located in the 5'-flanking region (p = 0.011, recessive model). SNP 6, rs2508836, in intron 1, was also associated with BMD, both at LS (p = 0.025, additive model) and FN (p = 0.031, recessive model). Two polymorphisms were associated with fracture: SNP 11, rs729635, in intron 1, and SNP 15, rs643892, in intron 5 (p = 0.007 additive model and p = 0.019 recessive model, respectively). Haplotype analyses did not provide additional information, except for haplotype "GC" of the block located at the 3'end of the gene. This haplotype spans intron 22 and the 3' untranslated region and was associated with FN BMD (p = 0.029, one copy of the haplotype versus none). In silico analyses showed that SNP 1 (rs312009) lies in a putative RUNX2 binding site. Electro-mobility shift assays confirmed RUNX2 binding to this site.

Association between the A1330V polymorphism of the low-density lipoprotein receptor-related protein 5 gene and bone mineral density: a meta-analysis

The association between the A1330V polymorphism of the low-density lipoprotein receptor-related protein 5 gene (LRP5) with bone mineral density (BMD) has been studied, but results have been mixed. Accordingly, the authors performed a meta-analysis on studies on the association between the A1330V LRP5 polymorphism and BMD. Appropriate studies were identified using MEDLINE and by manual searching. A total of 7 separate comparisons were considered in this meta-analysis. Individuals with the AA genotype showed significantly higher lumbar BMD than those with the AV/VV or VV genotype. Weighted mean differences (WMDs) were 0.147 g/cm(2) (95% confidence interval [CI] 0.069-0.224, P < 0.001) and 0.182 g/cm(2) (95% CI 0.024-0.340, P = 0.024) without between-study heterogeneity for AA versus AV/VV and AA versus VV, respectively. Six studies analyzed femur neck (FN) BMD. Individuals with the AA genotype had a significantly higher FN BMD than those with the AV/VV genotype (WMD = 0.165 g/cm(2), 95% CI = 0.087-0.215, P < 0.001), without between-study heterogeneity. Trochanter BMD was measured in three studies. Results showed that subjects with the AA genotype tended to have higher BMD than patients with the AV or VV genotype (WMD = 0.136 g/cm(2), 95% CI = -0.002 to 0.274, P = 0.053). In conclusion, this meta-analysis shows that the LRP5 A1330V polymorphism is associated with BMD, and that individuals with the AA genotype have a higher BMD than those with the AV/VV or VV genotype.

Sex-specific genetic loci for femoral neck bone mass and strength identified in inbred COP and DA rats

INTRODUCTION

Hip fracture is the most devastating osteoporotic fracture type with significant morbidity and mortality. Several studies in humans identified chromosomal regions linked to hip size and bone mass. Animal models, particularly the inbred rat, serve as complementary approaches for studying the genetic influence on hip fragility. The purpose of this study is to identify sex-independent and sex-specific quantitative trait loci (QTLs) for femoral neck density, structure, and strength in inbred Copenhagen 2331 (COP) and Dark Agouti (DA) rats.

MATERIALS AND METHODS

A total of 828 (405 males and 423 females) F(2) progeny derived from the inbred COP and DA strains of rats were phenotyped for femoral neck volumetric BMD (vBMD), cross-sectional area, polar moment of inertia (Ip), neck width, ultimate force, and energy to break. A whole genome screen was performed using 93 microsatellite markers with an average intermarker distance of 20 cM. Recombination-based marker maps were generated using MAPMAKER/EXP from the COP x DA F(2) data and compared with published Rat Genome Database (RGD) maps. These maps were used for genome-wide linkage analyses to detect sex-independent and sex-specific QTLs.

RESULTS

Significant evidence of linkage (p < 0.01) for sex-independent QTLs were detected for (1) femoral neck vBMD on chromosomes (Chrs) 1, 6, 10, and 12, (2) femoral neck structure on Chrs 5, 7, 10, and 18, and (3) biomechanical properties on Chrs 1 and 4. Male-specific QTLs were discovered on Chrs 2, 9, and 18 for total vBMD, on Chr 17 for trabecular vBMD, on Chr 9 for total bone area, and on Chr 15 for ultimate force. A female-specific QTL was discovered on Chr 2 for ultimate force. The effect size of the individual QTL varied between 1% and 4%.

CONCLUSIONS

We detected evidence that sex-independent and sex-specific QTLs contribute to hip fragility in the inbred rat. Several QTLs regions identified in this study are homologous to human chromosomal regions previously linked to QTLs contributing to femoral neck and related phenotypes.

Polymorphisms in the low-density lipoprotein receptor-related protein 5 (LRP5) gene are associated with peak bone mass in non-sedentary men: results from the Odense androgen study

PURPOSE

To investigate the impact of the Ala1330Val (rs3736228, exon 18) and Val667Met (rs4988321, exon 9) polymorphisms of the low-density lipoprotein receptor-related protein 5 (LRP5) gene on peak bone mass in young men.

METHODS

The Odense Androgen Study (OAS) is a population-based study comprising 783 Caucasian men aged 20-30 years. Genotyping was performed using real-time polymerase chain reaction (PCR) or fluorescence polarization. Bone mineral density (BMD) measurements were performed using dual-energy X-ray absorptiometry.

RESULTS

The CC, CT, and TT genotypes in Ala1330Val were found in 75.6%, 21.8%, and 2.6% of the participants, respectively. Similarly, the GG, GA, and AA genotypes of Val667Met were found in 89.7%, 9.8%, and 0.5%, respectively. For the Ala1330Val polymorphism, no significant differences between the genotypes were found regarding BMD in the overall study population. However, when analysis was restricted to non-sedentary men (n = 589), a significant association between the number of T-alleles and BMD in the spine and whole body were found. Each copy of the T-allele changed the Z-score of the spine by (median and 95% confidence interval) -0.21 [95% CI: -0.40; -0.03] (p < 0.02). Analysis suggested an association between the AA genotype in the Val667Met polymorphism and increased body height and decreased BMD of the femoral neck; however, no significant gene-dose effect of the A-allele could be demonstrated in the whole population. When the analysis was restricted to non-sedentary subjects, however, each number of A-alleles was associated with a change in Z-score of -0.26 [95% CI: -0.51; -0.01] (p = 0.04). No further significant results emerged with haplotype analysis.

CONCLUSION

The Ala1330Val and Val667Met polymorphisms in the LRP5 gene are significantly associated with peak bone mass in physically active men.

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.

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.

Genetic variation at the low-density lipoprotein receptor-related protein 5 (LRP5) locus modulates Wnt signaling and the relationship of physical activity with bone mineral density in men

Polymorphisms in the LRP5 gene have been associated with bone mineral density (BMD) in men and/or women. However, the functional basis for this association remains obscure. We hypothesized that LRP5 alleles could modulate Wnt signaling and the relationship between physical activity and BMD. This genetic association study was performed in the population-based Framingham Study Offspring Cohort, and included a subset of 1797 unrelated individuals who provided blood samples for DNA and who had BMD measurements of the hip and spine. Ten single-nucleotide polymorphisms (SNPs) spanning the LRP5 gene were genotyped and used for association and interaction analyses with BMD by regression methods. LRP5 haplotypes were transiently co-expressed with Wnt3a, MesD and Dkk1 in HEK293 cells and their activity evaluated by the TCF-Lef reporter assay. Six out of ten SNPs in LRP5 were associated with one or more of the femur or spine BMDs in men or women after adjustment for covariates, and these associations differed between genders. In men< or =age 60 years, 3 SNPs were significantly associated with BMD: rs2306862 on Exon 10 with femoral neck BMD (p=0.01) and Ward's BMD (p=0.01); rs4988321/p. V667M with Ward's BMD (p=0.02); and intronic rs901825 with trochanter BMD (p=0.03). In women, 3 SNPs in intron 2 were significantly associated with BMD: rs4988330 for trochanter (p=0.01) and spine BMD (p=0.003); rs312778 with femoral neck BMD (p=0.05); and rs4988331 with spine BMD (p=0.04). For each additional rare allele, BMD changed by 3-5% in males and 2-4% in females. Moreover, there was a significant interaction between physical activity and rs2306862 in exon 10 (p for interaction=0.02) and rs3736228/p. A1330V in exon 18 (p for interaction=0.05) on spine BMD in men. In both cases, the TT genotype was associated with lower BMD in men with higher physical activity scores, conversely with higher BMD in men with lower physical activity scores. In vitro, TCF-Lef activity in presence of Wnt3a was significantly reduced in cells expressing LRP5 haplotypes carrying the T allele of exon 10 and 18 compared to the wild-type allele, whereas co-expression of Dkk1 completely inhibited Wnt3a response through all LRP5 haplotypes. In summary, genetic variation in exons 10 and 18 of the LRP5 gene modulates Wnt signaling and the relationship between physical activity and BMD in men. These observations suggest that Wnt-LRP5 may play a role in the adaptation of bone to mechanical load in humans, and may explain some gender-related differences in bone mass.

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.

Identification of two sex-specific quantitative trait loci in chromosome 11q for hip bone mineral density in Chinese

BACKGROUND

Chromosome 11q has not only been found to contain mutations responsible for the several Mendelian disorders of the skeleton, but it has also been linked to bone mineral density (BMD) variation in several genome-wide linkage studies. Furthermore, quantitative trait loci (QTL) affecting BMD in inbred mice and baboons have been mapped to a region syntenic to human chromosome 11q. The aim of the present study is to determine whether there is a QTL for BMD variation on chromosome 11q in the Chinese population.

METHODS

Nineteen microsatellite markers were genotyped for a 75 cM region on 11q13-25 in 306 Chinese families with 1,459 subjects. BMD (g/cm(2)) was measured by DXA. Linkage analyses were performed using the variance component linkage analysis method implemented in Merlin software.

RESULTS

For women, a maximum LOD score of 1.62 was achieved at 90.8 cM on 11q21 near the marker D11S4175 for femoral neck BMD; LOD scores greater than 1.0 were observed on 11q13 for trochanter BMD. For men, a maximum LOD score of 1.57 was achieved at 135.8 cM on 11q24 near the marker D11S4126 for total hip BMD.

CONCLUSION

We have not only replicated the previous linkage finding on chromosome 11q but also identified two sex-specific QTL that contribute to BMD variation in Chinese women and men.

QTL analysis of trabecular bone in BXD F2 and RI mice

A sample of 693 mice was used to identify regions of the mouse genome associated with trabecular bone architecture as measured using microCT. QTLs for bone in the proximal tibial metaphysis were identified on several chromosomes indicating regions containing genes that regulate properties of trabecular bone.

INTRODUCTION

Age-related osteoporosis is a condition of major concern because of the morbidity and mortality associated with osteoporotic fractures in humans. Osteoporosis is characterized by reduced bone density, strength, and altered trabecular architecture, all of which are quantitative traits resulting from the actions of many genes working in concert with each other and the environment over the lifespan. microCT gives accurate measures of trabecular bone architecture providing phenotypic data related to bone volume and trabecular morphology. The primary objective of this research was to identify chromosomal regions called quantitative trait loci (QTLs) that contain genes influencing trabecular architecture as measured by microCT.

MATERIALS AND METHODS

The study used crosses between C57BL/6J (B6) and DBA/2J (D2) as progenitor strains of a second filial (F2) generation (n = 141 males and 148 females) and 23 BXD recombinant inbred (RI) strains (n approximately 9 of each sex per strain). The proximal tibial metaphyses of the 200-day-old mice were analyzed by microCT to assess phenotypic traits characterizing trabecular bone, including bone volume fraction, trabecular connectivity, and quantitative measures of trabecular orientation and anisotropy. Heritabilities were calculated and QTLs were identified using composite interval mapping.

RESULTS

A number of phenotypes were found to be highly heritable. Heritability values for measured phenotypes using RI strains ranged from 0.15 for degree of anisotropy in females to 0.51 for connectivity density in females and total volume in males. Significant and confirmed QTLs, with LOD scores 4.3 in the F2 cohort and 1.5 in the corresponding RI cohort were found on chromosomes 1 (43 cM), 5 (44 cM), 6 (20 cM), and 8 (49 cM). Other QTLs with LOD scores ranging from 2.8 to 6.9 in the F2 analyses were found on chromosomes 1, 5, 6, 8, 9, and 12. QTLs were identified using data sets comprised of both male and female quantitative traits, suggesting similar genetic action in both sexes, whereas others seemed to be associated exclusively with one sex or the other, suggesting the possibility of sex-dependent effects.

CONCLUSIONS

Identification of the genes underlying these QTLs may lead to improvements in recognizing individuals most at risk for developing osteoporosis and in the design of new therapeutic interventions.

Molecular bases of the regulation of bone remodeling by the canonical Wnt signaling pathway

Osteoporosis is a common, prevalent, and debilitating condition, particularly in postmenopausal women. Genetics play a major role in determining peak bone mass and fracture risk, but few genes have been demonstrated conclusively to be involved, much less the signaling pathways with which they are affiliated. The identification of mutations in the gene Lrp5, a Wnt coreceptor, as the cause for both osteoporotic and high-bone mass disorders implicated the canonical Wnt signaling pathway in bone mass regulation. Since Lrp5, other Wnt components have been identified as being regulators of bone mass, and Wnt target genes affecting bone homeostasis have begun to be elucidated. This chapter looks at the various components of the canonical Wnt signaling pathway and the data indicating that this pathway plays a major role in the control of both bone formation and bone resorption, the two key aspects of bone remodeling.

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.

Association of polymorphisms in low-density lipoprotein receptor-related protein 5 gene with bone mineral density in postmenopausal Chinese women

AIM

To investigate the possible association of Q89R, N740N and A1330V polymorphisms in low-density lipoprotein receptor-related protein 5 (LRP5) gene with bone mineral density (BMD) in postmenopausal Chinese women.

METHODS

Q89R, N740N and A1330V genotypes were determined by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) in 647 unrelated healthy postmenopausal Han Chinese women aged 43-76 years in Shanghai. BMD at lumbar spine 1-4 and the left proximal femur including the femoral neck, trochanter and Ward's triangle were measured by dual-energy X-ray absorptionmetry in all subjects.

RESULTS

The distribution of the Q89R, N740N and A1330V genotypes in this population was as follows: QQ 80.5%, QR 18.7%, and RR 0.8%; TT 66.9%, TC 31.1%, and CC 2.0%; AA 68.0%, AV 29.7%, and VV 2.3%. The frequencies of the Q89R, N740N and A1330V genotypes and alleles did not deviate from the Hardy-Weinberg equilibrium. We found that the Q89R and A1330V polymorphisms were in linkage disequilibrium in our population (kappa2=13.50, P<0.01). Both before and after adjusting for age, years since menopause, height, and weight, the Q89R or N740N genotypes were significantly associated with BMD at the femoral neck (P<0.05). Subjects with the Q89R QQ genotype or the N740N TT genotype had a significantly higher BMD at the femoral neck, compared with those with the QR/RR or TC/CC genotypes, respectively. No significant association was found between A1330V polymorphism and BMD at any site.

CONCLUSION

Our findings suggest that the LRP5 gene is a candidate for the genetic determination of BMD in postmenopausal Chinese women.

Genetic disorders of the LRP5-Wnt signalling pathway affecting the skeleton

Osteoporosis is a common, increasingly prevalent and potentially debilitating condition of men and women. Genetic factors are major determinants of bone mass and the risk of fracture, but few genes have been definitively demonstrated to be involved. The identification of these factors will provide novel insights into the processes of bone formation and loss and thus the pathogenesis of osteoporosis, enabling the rational development of novel therapies. In this article, we present the extensive genetic and functional data indicating that the LRP5 gene and the Wnt signalling pathway are key players in bone formation and the risk of osteoporosis, and that LRP5 signalling is essential for normal morphology, developmental processes and bone health.

Haplotypes of the low-density lipoprotein receptor-related protein 5 (LRP5) gene: are they a risk factor in osteoarthritis?

OBJECTIVE

Several genome-wide scans have revealed an osteoarthritis (OA)-susceptibility locus on chromosome 11q in close proximity to the low-density lipoprotein receptor-related protein 5 (LRP5) gene. The regulation of bone mass is under the control of LRP5 and since increased bone mass is thought to play a role in the pathology of OA we examined LRP5 polymorphisms and haplotypes to determine if variants of this locus may predispose to OA.

METHODS

A UK control population of 187 individuals was examined for five commonly occurring polymorphisms against a cohort of 158 DNAs from patients with knee OA. An additional UK cohort was also examined to confirm the findings of the first study; this second group consisted of 110 knee OA patients. Haplotype analysis was also performed on patient and control DNAs.

RESULTS

A study of individual polymorphisms revealed no association with disease. However, haplotype analysis of the initial two populations revealed a common haplotype (C-G-C-C-A) that provided a 1.6-fold increased risk of OA (P(c)=0.021). The data obtained from the second cohort confirmed the initial findings, with a 1.6-fold increased risk observed within this cohort for the risk haplotype (P=0.012).

CONCLUSIONS

A closer investigation of LRP5 and associated Wnt signalling molecules in OA will help determine disease aetiology and the development of novel treatment strategies that specifically target the bone compartment.

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.

[Physiopathology of osteoporosis]

PURPOSE

The pathophysiology of osteoporosis has seen many recent progress especially with the use of genetically modified animal models.

CURRENT KNOWLEDGE AND KEY POINTS

Among many discoveries, one can notice the crucial role of LRP5, GH, IGF-1 and the sex hormones receptors in the acquisition of the peak bone mass, the control of bone remodeling by the sympathetic nervous system and his implication as a transmitter of mechanical loading in bone. Also, the role of estrogen and androgen receptors as well as the aromatase is specified according to sexes. The role of growth plate's chondrocytes in the installation of the trabecular bone network is better and better demonstrated. The greater periosteal apposition in men, mediated by androgens receptor, seems to explain the greatest radial growth and so the greatest bone resistance to mechanical strains like a lower fracture rate in men compared to women. The bone microarchitecture and quality explain an important part of the mechanical properties of bones and why considering the same bone mass one bone is breaking and another one not.

FUTURE PROSPECTS AND PROJECTS

Many therapeutic applications should finalize the discovery of these new bone cells signalisation pathways.

LRP5 mutations in osteoporosis-pseudoglioma syndrome and high-bone-mass disorders

The LDL receptor-related protein 5 (LRP5) is a member of the LDL receptor family, which also includes the VLDL receptor and the apolipoprotein E receptor 2. The LRP5 is a co-receptor of Wnt located on the osteoblast membrane between two other receptors, Frizzled and Kremen. Frizzled and LRP5 bind to Wnt, thereby stabilizing beta-catenin and activating bone formation. When the dickkopf protein (Dkk) binds to Kremen and LRP5, this last undergoes internalization and therefore becomes unable to bind Wnt; this leads to degradation of beta-catenin and to inhibition of bone formation. In humans, loss of LRP5 function causes osteoporosis-pseudoglioma syndrome, which is characterized by congenital blindness and extremely severe childhood-onset osteoporosis (lumbar spine Z-score often < -4) with fractures. The G171V mutation prevents Dkk from binding to LRP5, thereby increasing LRP5 function; the result is high bone mass due to uncoupling of bone formation and resorption. The Z-scores in this condition can exceed +6 at the hip and spine. The LRP5 and Wnt/beta-catenin reflect the level of bone formation and play a central role in bone mass accrual and normal distribution. Furthermore, LRP5 may contribute to mediate mechanical loads within bone tissue. Identification of the Wnt/beta-catenin pathway is a breakthrough in the elucidation of pathophysiological mechanisms affecting bone tissue and suggests new treatment targets for patients with osteoporosis or specific malignant conditions such as myeloma and sclerotic bone metastases.

Pathogenic mutations and polymorphisms in the lipoprotein receptor-related protein 5 reveal a new biological pathway for the control of bone mass

PURPOSE OF REVIEW

This review summarizes recent findings concerning the genomic variations of the lipoprotein receptor-related protein 5 (LPR5) in relation to bone biology.

RECENT FINDINGS

Mutations in the LRP5 gene causing high bone mass (HBM) and osteoporosis-pseudoglioma (OPPG) underscored the role of the Wnt-LRP5 canonical signaling on bone formation. Additional LRP5 activating mutations have been identified in a variety of sclerosing bone dysplasias, improving the diagnostic classification of these disorders. Association of polymorphisms in LRP5 with bone mineral density indicate that LRP5 genetic variation contribute to the risk of osteoporosis. Transgenic mice carrying the LRP5 HBM mutation have improved bone biomechanical properties, and the molecular mechanisms by which this mutation exerts its effects have been clarified. A number of KO mice have shown the complex effects of the Wnt-LRP5 pathway on bone mass and skeletal morphology. In vitro studies indicate that osteoblasts produce a variety of Wnts, the LRP5 co-receptor frizzled (Fzd), as well as LRP5 and Wnt inhibitors, i.e. dickkopf (Dkk1) and frizzled-related proteins (Sfrps), respectively, and delineate the role of these molecules in regulating the commitment of mesenchymal stem cells along the osteoblastic lineage.

SUMMARY

Identification of pathogenic mutations and allelic variations in LRP5 has improved our understanding of the physiology of bone mass acquisition and the pathophysiology of several bone diseases, including osteoporosis. Understanding how complex interactions between agonistic and inhibitory factors in the Wnt-LRP5 canonical pathway influence osteoblast functions has the potential of providing new anabolic treatments for osteoporosis.

Nonreplication in genetic studies of complex diseases--lessons learned from studies of osteoporosis and tentative remedies

Inconsistent results have accumulated in genetic studies of complex diseases/traits over the past decade. Using osteoporosis as an example, we address major potential factors for the nonreplication results and propose some potential remedies. Over the past decade, numerous linkage and association studies have been performed to search for genes predisposing to complex human diseases. However, relatively little success has been achieved, and inconsistent results have accumulated. We argue that those nonreplication results are not unexpected, given the complicated nature of complex diseases and a number of confounding factors. In this article, based on our experience in genetic studies of osteoporosis, we discuss major potential factors for the inconsistent results and propose some potential remedies. We believe that one of the main reasons for this lack of reproducibility is overinterpretation of nominally significant results from studies with insufficient statistical power. We indicate that the power of a study is not only influenced by the sample size, but also by genetic heterogeneity, the extent and degree of linkage disequilibrium (LD) between the markers tested and the causal variants, and the allele frequency differences between them. We also discuss the effects of other confounding factors, including population stratification, phenotype difference, genotype and phenotype quality control, multiple testing, and genuine biological differences. In addition, we note that with low statistical power, even a "replicated" finding is still likely to be a false positive. We believe that with rigorous control of study design and interpretation of different outcomes, inconsistency will be largely reduced, and the chances of successfully revealing genetic components of complex diseases will be greatly improved.

Wnt signaling in osteoblasts and bone diseases

Recent revelations that the canonical Wnt signaling pathway promotes postnatal bone accrual are major advances in our understanding of skeletal biology and bring tremendous promise for new therapeutic treatments for osteoporosis and other diseases of altered bone mass. Wnts are soluble glycoproteins that engage receptor complexes composed of Lrp5/6 and Frizzled proteins. A subgroup of Wnts induces a cascade of intracellular events that stabilize beta-catenin, facilitating its transport to nuclei where it binds Lef1/Tcf transcription factors and alters gene expression to promote osteoblast expansion and function. Natural extracellular Wnt antagonists, Dickkopfs and secreted frizzled-related proteins, impair osteoblast function and block bone formation. In several genetic disorders of altered skeletal mass, mutations in LRP5 create gain-of-function or loss-of-function receptors that are resistant to normal regulatory mechanisms and cause higher or lower bone density, respectively. In this review, we summarize the available molecular, cellular, and genetic data that demonstrate how Lrp5 and other components of the Wnt signaling pathway influence osteoblast proliferation, function, and survival. We also discuss regulatory mechanisms discovered in developmental and tumor models that may provide insights into novel therapies for bone diseases.

Risk of wrist fracture in women is heritable and is influenced by genes that are largely independent of those influencing BMD

Using a classical twin design study, we estimated the genetic contribution to liability of wrist fracture in women to be statistically and clinically significant. BMD is highly heritable, but statistical models showed very little overlap of shared genes between the two traits.

INTRODUCTION

Studies have observed contradictory evidence for genetic effects influencing the outcome of osteoporotic fracture, in part because of the methodological problems involved in analyzing age-related "censored" outcomes. Although a shared genetic etiology is often assumed between fracture and low BMD, this has not been shown to be the case.

MATERIALS AND METHODS

In a study of 6570 white healthy female volunteer twins between 18 and 80 years of age, we identified and validated 220 nontraumatic wrist fracture cases. From this we estimated the population prevalence, case-wise twin concordance, heritability in liability to wrist fracture (WF), and the genetic contribution to WFs controlling for age by analyzing the survival outcome using generalized linear mixed models implemented in Winbugs software. We included forearm BMD as a co-variate in some of the models to test whether there is a shared genetic etiology between WFs and BMD.

RESULTS

The prevalence of WFs in women was estimated to be 3.3% with a case-wise concordance in monozygotic twins of 0.28 and 0.11 in dizygotic twins. The additive polygenic heritability in liability was approximately 54%, and a significant genetic etiology was confirmed by analyzing WFs as a survival outcome. The magnitude of the genetic influence on risk of WFs reduced very little when BMD was included as a co-variate in the survival analysis model.

CONCLUSIONS

There is an important genetic contribution to the risk of WFs, but for the most part, these genes are unlikely to play a direct etiological role in the development of low BMD. If these results are confirmed for other sites, fracture and low BMD will have their own specific genetic risk factors that are unlikely to be shared between the two traits. This has important clinical and research implications.

Contribution of the LRP5 gene to normal variation in peak BMD in women

The role of the LRP5 gene in rare BMD-related traits has recently been shown. We tested whether variation in this gene might play a role in normal variation in peak BMD. Association between SNPs in LRP5 and hip and spine BMD was measured in 1301 premenopausal women. Only a small proportion of the BMD variation was attributable to LRP5 in our sample.

INTRODUCTION

Mutations in the low-density lipoprotein receptor-related protein 5 (LRP5) gene have been implicated as the cause of multiple distinct BMD-related rare Mendelian phenotypes. We sought to examine whether the LRP5 gene contributes to the observed variation in peak BMD in the normal population.

MATERIALS AND METHODS

We genotyped 12 single nucleotide polymorphisms (SNPs) in LRP5 using allele-specific PCR and mass spectrometry methods. Linkage disequilibrium between the genotyped LRP5 SNPs was measured. We tested for association between these SNPs and both hip and spine BMD (adjusted for age and body weight) in 1301 healthy premenopausal women who took part in a sibling pair study aimed at identifying the genes underlying peak bone mass. Our study used both population-based (ANOVA) and family-based (quantitative transmission disequilibrium test) association methodology.

RESULTS AND CONCLUSIONS

The linkage disequilibrium pattern and haplotype block structure within the LRP5 gene were consistent with that observed in other studies. Although significant evidence of association was found between LRP5 SNPs and both hip and spine BMD, only a small proportion of the total variation in these phenotypes was accounted for. The genotyped SNPs accounted for approximately 0.8% of the variation in femoral neck BMD and 1.1% of the variation in spine BMD. Results from our sample suggest that natural variation in and around LRP5 is not a major contributor to the observed variability in peak BMD at either the femoral neck or lumbar spine in white women.

Influence of LRP5 polymorphisms on normal variation in BMD

Genetic studies based on cohorts with rare and extreme bone phenotypes have shown that the LRP5 gene is an important genetic modulator of BMD. Using family-based and case-control approaches, this study examines the role of the LRP5 gene in determining normal population variation of BMD and describes significant association and suggestive linkage between LRP5 gene polymorphisms and BMD in >900 individuals with a broad range of BMD.

INTRODUCTION

Osteoporosis is a common, highly heritable condition determined by complex interactions of genetic and environmental etiologies. Genetic factors alone can account for 50-80% of the interindividual variation in BMD. Mutations in the LRP5 gene on chromosome 11q12-13 have been associated with rare syndromes characterized by extremely low or high BMD, but little is known about the contribution of this gene to the development of osteoporosis and determination of BMD in a normal population.

MATERIALS AND METHODS

To examine the entire spectrum of low to high BMD, 152 osteoporotic probands, their families (597 individuals), and 160 women with elevated BMD (T score > 2.5) were recruited. BMD at the lumbar spine, femoral neck, and hip were measured in each subject using DXA.

RESULTS

PAGE sequencing of the LRP5 gene revealed 10 single nucleotide polymorphisms (SNPs), 8 of which had allele frequencies of >5%, in exons 8, 9, 10, 15, and 18 and in introns 6, 7, and 21. Within families, a strong association was observed between an SNP at nucleotide C171346A in intron 21 and total hip BMD (p < 1 x 10(-5) in men only, p = 0.0019 in both men and women). This association was also observed in comparisons of osteoporotic probands and unrelated elevated BMD in women (p = 0.03), along with associations with markers in exons 8 (C135242T, p = 0.007) and 9 (C141759T, p = 0.02). Haplotypes composed of two to three of the SNPs G121513A, C135242T, G138351A, and C141759T were strongly associated with BMD when comparing osteoporotic probands and high BMD cases (p < 0.003). An SNP at nucleotide C165215T in exon 18 was linked to BMD at the lumbar spine, femoral neck, and total hip (parametric LOD scores = 2.8, 2.5, and 2.2 and nonparametric LOD scores = 0.3, 1.1, and 2.2, respectively) but was not genetically associated with BMD variation.

CONCLUSION

These results show that common LRP5 polymorphisms contribute to the determination of BMD in the general population.

Genes and osteoporosis

Genes play an important role in the development of osteoporosis. Twin and family studies have consistently shown that peak bone mass, ultrasound properties of bone, skeletal geometry, bone turnover, and fracture are heritable. Yet, as we report in this paper, few candidate genes have been implicated without ambiguity. Osteoporosis is thought to be a polygenic disorder, determined by multiple genes and environmental risk factors, each with small to modest effect on bone mass and fracture. Here we argue that future success in finding genes is only possible with improved study design and the use of more rigorous analytic approaches that are now becoming available.

Association between bone mineral density and LDL receptor-related protein 5 gene polymorphisms in young Korean men

Recently, It has been reported that the LDL receptor-related protein 5 (LRP5) regulates bone formation, and that mutations of the gene cause osteoporosis-pseudoglioma syndrome or high bone mass phenotypes. However, the mutations cannot explain a genetic trait for osteoporosis in the general population because of their rarity. From 219 Korean men aged 20-34 yr, we looked for six known polymorphisms causing amino acid changes in the LRP5 coding region, and investigated their association with bone mineral density (BMD) at the following anatomical sites: lumbar spine (L2-L4) and the left proximal femur (femoral neck, Ward's triangle, trochanter and shaft). We found that the Q89R polymorphism was significantly associated with BMD at the femoral neck and Ward's triangle (p=0.004 and <0.001, respectively). However, after adjusting for age, weight and height, a statistically significant association only occurred at the Ward's triangle (p=0.043), and a marginal association was observed at the femoral neck (p=0.098). No A400V, V667M, R1036Q and A1525V polymorphisms were found, and no statistically significant association was found between the A1330V polymorphism and BMD at any sites. Although we failed to demonstrate a clear association between the LRP5 polymorphism and peak bone mass in young men, the present study suggests that larger-scale studies on the Q89R polymorphism need to be performed.

Polymorphisms in the low-density lipoprotein receptor-related protein 5 (LRP5) gene are associated with variation in vertebral bone mass, vertebral bone size, and stature in whites

Stature, bone size, and bone mass are interrelated traits with high heritability, but the major genes that govern these phenotypes remain unknown. Independent genomewide quantitative-trait locus studies have suggested a locus for bone-mineral density and stature at chromosome 11q12-13, a region harboring the low-density lipoprotein receptor-related protein 5 (LRP5) gene. Mutations in the LRP5 gene were recently implicated in osteoporosis-pseudoglioma and "high-bone-mass" syndromes. To test whether polymorphisms in the LRP5 gene contribute to bone-mass determination in the general population, we studied a cross-sectional cohort of 889 healthy whites of both sexes. Significant associations were found for a missense substitution in exon 9 (c.2047G-->A) with lumbar spine (LS)-bone-mineral content (BMC) (P=.0032), with bone area (P=.0014), and with stature (P=.0062). The associations were observed mainly in adult men, in whom LRP5 polymorphisms accounted for <or=15% of the traits' variances. Results of haplotype analysis of five single-nucleotide polymorphisms in the LRP5 region suggest that additional genetic variation within the locus might also contribute to bone-mass and size determination. To confirm our results, we investigated whether LRP5 haplotypes were associated with 1-year gain in vertebral bone mass and size in 386 prepubertal children. Significant associations were observed for changes in BMC (P=.0348) and bone area (P=.0286) in males but not females, independently supporting our observations of a mostly male-specific effect, as seen in the adults. Together, these results suggest that LRP5 variants significantly contribute to LS-bone-mass and size determination in men by influencing vertebral bone growth during childhood.

Complex segregation analyses of bone mineral density in Chinese

China has the largest population in the world; approximately 7% of the total population suffers from primary osteoporosis. Osteoporosis is mainly characterized by low bone mineral density (BMD). In the present study, familial correlation and segregation analyses for spine and hip BMDs have been undertaken for the first time in a Chinese sample composed of 401 nuclear families with a total of 1260 individuals. The results indicate a major gene of additive inheritance for hip BMD, whereas there is no evidence of a major gene influencing spine BMD. Significant familial residual effects are found for both traits, and heritability estimates (+/-SE) for spine and hip BMDs are 0.807(0.099) and 0.897(0.101), respectively. Sex and age differences in genotype-specific average BMD are also observed. This study provides the first evidence quantifying the high degree of genetic determination of BMD variation in the Chinese.

LRP5, low-density-lipoprotein-receptor-related protein 5, is a determinant for bone mineral density

Osteoporosis is a multifactorial trait with low bone mineral density (BMD). We report results of an association study between BMD and nine candidate genes ( TGFB1, TGFBR2, SMAD2, SMAD3, SMAD4, IFNB1, IFNAR1, FOS and LRP5), as well as of a case-control study of osteoporosis. Samples for the former association study included 481 general Japanese women. Among the nine candidate genes examined, only LRP5 showed a significant association with BMD. We identified a strong linkage disequilibrium (LD) block within LRP5. Of five LPR5 single nucleotide polymorphisms (SNPs) that are located in the LD block, three gave relatively significant results: Women with the C/C genotype at the c.2220C>T SNP site had higher adjusted BMD (AdjBMD) value compared to those with C/T and T/T (p=0.022); and likewise, G/G at IVS17-30G>A and C/C women at c.3989C>T showed higher AdjBMD than those with G/A or A/A (p=0.039) and with C/T or T/T ( p=0.053), respectively. The case-control study in another series of samples consisting of 126 osteoporotic patients and 131 normal controls also gave a significant difference in allele frequency at c.2220C>T (kappa2=6.737, p=0.009). These results suggest that LRP5 is a BMD determinant and also contributes to a risk of osteoporosis.

Quantitative trait loci on chromosomes 2p, 4p, and 13q influence bone mineral density of the forearm and hip in Mexican Americans

We performed a genome scan using BMD data of the forearm and hip on 664 individuals in 29 Mexican-American families. We obtained evidence for QTL on chromosome 4p, affecting forearm BMD overall, and on chromosomes 2p and 13q, affecting hip BMD in men.

INTRODUCTION

The San Antonio Family Osteoporosis Study (SAFOS) was designed to identify genes and environmental factors that influence bone mineral density (BMD) using data from large Mexican-American families.

MATERIALS AND METHODS

We performed a genome-wide linkage analysis using 416 highly polymorphic microsatellite markers spaced approximately 9.5 cM apart to locate and identify quantitative trait loci (QTL) that affect BMD of the forearm and hip. Multipoint variance components linkage analyses were done using data on all 664 subjects, as well as two subgroups of 259 men and 261 premenopausal women, from 29 families for which genotypic and phenotypic data were available.

RESULTS

We obtained significant evidence for a QTL affecting forearm (radius midpoint) BMD in men and women combined on chromosome 4p near D4S2639 (maximum LOD = 4.33, genomic p = 0.006) and suggestive evidence for a QTL on chromosome 12q near locus D12S2070 (maximum conditional LOD = 2.35). We found suggestive evidence for a QTL influencing trochanter BMD on chromosome 6 (maximum LOD = 2.27), but no evidence for QTL affecting the femoral neck in men and women combined. In men, we obtained evidence for QTL affecting neck and trochanter BMD on chromosomes 2p near D2S1780 (maximum LOD = 3.98, genomic p = 0.013) and 13q near D13S788 (maximum LOD = 3.46, genomic p = 0.039), respectively. We found no evidence for QTL affecting forearm or hip BMD in premenopausal women.

CONCLUSION

These results provide strong evidence that a QTL on chromosome 4p affects radius BMD in Mexican-American men and women, as well as evidence that QTL on chromosomes 2p and 13q affect hip BMD in men. Our results are consistent with some reports in humans and mice. J Bone Miner Res 2003;18:2245-2252

Linkage of osteoporosis to chromosome 20p12 and association to BMP2

Osteoporotic fractures are a major cause of morbidity and mortality in ageing populations. Osteoporosis, defined as low bone mineral density (BMD) and associated fractures, have significant genetic components that are largely unknown. Linkage analysis in a large number of extended osteoporosis families in Iceland, using a phenotype that combines osteoporotic fractures and BMD measurements, showed linkage to Chromosome 20p12.3 (multipoint allele-sharing LOD, 5.10; p value, 6.3 x 10(-7)), results that are statistically significant after adjusting for the number of phenotypes tested and the genome-wide search. A follow-up association analysis using closely spaced polymorphic markers was performed. Three variants in the bone morphogenetic protein 2 (BMP2) gene, a missense polymorphism and two anonymous single nucleotide polymorphism haplotypes, were determined to be associated with osteoporosis in the Icelandic patients. The association is seen with many definitions of an osteoporotic phenotype, including osteoporotic fractures as well as low BMD, both before and after menopause. A replication study with a Danish cohort of postmenopausal women was conducted to confirm the contribution of the three identified variants. In conclusion, we find that a region on the short arm of Chromosome 20 contains a gene or genes that appear to be a major risk factor for osteoporosis and osteoporotic fractures, and our evidence supports the view that BMP2 is at least one of these genes.

Bone density in sheep: genetic variation and quantitative trait loci localisation

Bone density (BD) is an important factor in osteoporotic fracture risk in humans. However, BD is a complex trait confounded by environmental influences and polygenic inheritance. Sheep provide a potentially useful model for studying differences in BD, as they provide a means of circumventing complex environmental factors and are a similar weight to humans. The aims of this study were to establish whether there is genetic variation in BD in sheep and then to localise quantitative trait loci (QTLs) associated with this variation. We also aimed to evaluate the relationship between fat and muscle body components and BD in sheep. Results showed that there was significant (P < 0.01) genetic variation among Coopworth sheep sires for BD. This genetic difference was correlated (P < 0.01) with body weight and muscle mass. A number of QTLs exceeding the suggestive threshold were identified (nine in total). Of these, two (chromosomes 1, P < 0.05; chromosome 24, P < 0.01) were significant using genome-wide permutation significance thresholds (2000 iterations). The position of the QTL on chromosome 24 coincided with a number of other body composition QTLs, indicating possible pleiotropic effects or the presence of multiple genes affecting body composition at that site. This study shows that sheep are potentially a useful model for studying the genetics of BD.

A genomewide screening of N-ethyl-N-nitrosourea-mutagenized mice for musculoskeletal phenotypes

Chemical mutagenesis followed by screening for abnormal phenotypes in the mouse holds much promise as a method for revealing gene function. We describe a mouse N-ethyl-N-nitrosourea (ENU) mutagenesis program incorporating a genomewide screen of dominant as well as recessive mutations affecting musculoskeletal disorders in C3H/HeJ mice. In a primary screen, progeny of one-generation dominant mutations (F(1)) and three-generation recessive (F(3)) mutations were screened at 10 weeks of age for musculoskeletal disorders using dual-energy X-ray absorptiometery (DEXA) and biochemical markers affecting bone metabolism, such as osteocalcin, type I collagen breakdown product, skeletal alkaline phosphatase, and insulin-like growth factor I (IGF-I). Abnormal phenotypes were identified as +/-3SD units different from baseline data collected from age- and sex-matched nonmutagenized control mice. A secondary screen at 16 weeks of age, which included peripheral quantitative computed tomography (pQCT) in addition to those parameters described in our primary screen, was used to confirm the abnormal phenotypes observed in the primary screen. The phenodeviant or outlier mice were progeny tested to determine whether their abnormality segregates bimodally in their offspring with the expected 1:1 or 1:3 Mendelian ratio, in dominant and recessive screens, respectively. With the above screening strategy, we were able to identify several mice with quantitative abnormalities in BMD, BMC, bone size, and bone metabolism. We have progeny tested and confirmed four outliers with low BMD, low bone size, and growth-related abnormality. Our results indicate that the magnitude of change in quantitative phenotypes in the ENU-mutagenized progeny was between 10 and 15%, and hence, the yield of outliers was dependent on the precision of the methods. So far, this ENU mutagenesis program has identified four outliers that can undergo positional cloning.

Site and gender specificity of inheritance of bone mineral density

Differences in genetic control of BMD by skeletal sites and genders were examined by complex segregation analysis in 816 members of 147 families with probands with extreme low BMD. Spine BMD correlated more strongly in male-male comparisons and hip BMD in female-female comparisons, consistent with gender- and site-specificity of BMD heritability.

INTRODUCTION

Evidence from studies in animals and humans suggests that the genetic control of bone mineral density (BMD) may differ at different skeletal sites and between genders. This question has important implications for the design and interpretation of genetic studies of osteoporosis.

METHODS

We examined the genetic profile of 147 families with 816 individuals recruited through probands with extreme low BMD (T-score < -2.5, Z-score < -2.0). Complex segregation analysis was performed using the Pedigree Analysis Package. BMD was measured by DXA at both lumbar spine (L1-L4) and femoral neck.

RESULTS

Complex segregation analysis excluded purely monogenic and environmental models of segregation of lumbar spine and femoral neck BMD in these families. Pure polygenic models were excluded at the lumbar spine when menopausal status was considered as a covariate, but not at the femoral neck. Mendelian models with a residual polygenic component were not excluded. These models were consistent with the presence of a rare Mendelian genotype of prevalence 3-19%, causing high BMD at the hip and spine in these families, with additional polygenic effects. Total heritability range at the lumbar spine was 61-67% and at the femoral neck was 44-67%. Significant differences in correlation of femoral neck and lumbar spine BMD were observed between male and female relative pairs, with male-male comparisons exhibiting stronger lumbar spine BMD correlation than femoral neck, and female-female comparisons having greater femoral neck BMD correlation than lumbar spine. These findings remained true for parent-offspring correlations when menopausal status was taken into account. The recurrence risk ratio for siblings of probands of a Z-score < -2.0 was 5.4 at the lumbar spine and 5.9 at the femoral neck.

CONCLUSIONS

These findings support gender- and site-specificity of the inheritance of BMD. These results should be considered in the design and interpretation of genetic studies of osteoporosis.

Interpreting the molecular biology and clinical behavior of multiple myeloma in the context of global gene expression profiling

Multiple myeloma (MM) is a rare but uniformly fatal malignancy of antibody-secreting plasma cells (PCs). Although several key molecular events in disease initiation or progression have been confirmed (e.g. FGFR3/MMSET activation) or implicated (e.g. chromosome 13 deletion), the mechanisms of MM development remain enigmatic. Importantly, although generally being indistinguishable morphologically, MM exhibits a tremendous degree of variability in clinical course, with some patients surviving only months and others many years. However, current laboratory parameters can account for no more than 20% of this outcome variability. Furthermore, the means by which current drugs impart their anti-MM effect are also mostly unknown. In addition, the mechanisms by which MM cells contribute to serious comorbidities, such as osteopenia and/or focal lytic lesions of bone, are also poorly understood. Finally, very little knowledge exists concerning the molecular events leading to benign hyperplasia and/or overt malignancy of PCs. Given that abnormal gene expression lies at the heart of most, if not all, cancers, high-throughput global gene expression profiling has become a powerful tool for investigating molecular biology and clinical behavior of diseases. Here, we discuss recent progress made in addressing many of the above issues through the molecular dissection of the transcriptome of normal PCs and MM.

High bone mass in mice expressing a mutant LRP5 gene

A unique mutation in LRP5 is associated with high bone mass in man. Transgenic mice expressing this LRP5 mutation have a similar phenotype with high bone mass and enhanced strength. These results underscore the importance of LRP5 in skeletal regulation and suggest targets for therapies for bone disease. A mutation (G171V) in the low-density lipoprotein receptor related protein 5 (LRP5) has been associated with high bone mass (HBM) in two independent human kindreds. To validate the role of the mutation, several lines of transgenic mice were created expressing either the human LRP5 G171V substitution or the wildtype LRP5 gene in bone. Volumetric bone mineral density (vBMD) analysis by pQCT showed dramatic increases in both total vBMD (30-55%) and trabecular vBMD (103-250%) of the distal femoral metaphysis and increased cortical size of the femoral diaphysis in mutant G171V transgenics at 5, 9, 17, 26, and 52 weeks of age (p < 0.01 for all). In addition, high-resolution microcomputed tomography (microCT) analysis of the distal femorae and lumbar vertebrae revealed an increase (110-232%) in trabecular bone volume fraction caused by both increased trabecular number (41-74%) and increased trabecular thickness (34-46%; p < 0.01 for all) in the mutant G171V mice. The increased bone mass was associated with significant increases in vertebral compressive strength (80-140%) and the increased cortical size with significant increases in femoral bending strength (50-130%). There were no differences in osteoclast number at 17 weeks of age. However, compared with littermate controls, the mutant G171V transgenic mice showed an increase in actively mineralizing bone surface, enhanced alkaline phosphatase staining in osteoblasts, and a significant reduction in the number of TUNEL-positive osteoblasts and osteocytes. These results suggest that the increased bone mineral density in mutant G171V mice was caused by increased numbers of active osteoblasts, which could in part be because of their increased functional lifespan. While slight bone anabolic activity was observed from overexpression of the wildtype LRP5 gene, it is clear that the G171V mutation, rather than overexpression of the receptor itself, is primarily responsible for the dramatic HBM bone effects. Together, these findings establish the importance of this novel and unexpected role of a lipoprotein receptor in regulating bone mass and afford a new model to explore LRP5 and its recent association with Wnt signaling in bone biology.

Linkage of structure at the proximal femur to chromosomes 3, 7, 8, and 19

Risk for osteoporotic fracture is determined in part by femoral structure, which is under genetic control. We conducted a genome scan in 638 sister-pairs for structure phenotypes. Significant evidence of linkage was detected with several chromosomal regions, including confirmation of our prior linkage findings. Bone strength and resistance to fracture at the proximal femur is determined in part by structural variables. We previously reported that several structural variables, including pelvic axis length, femur axis length, femur head width, and femur midshaft width, had significant or suggestive linkage to regions of chromosomes 3, 4, 5, 7, 9, 17, and 19 in a sample of 309 white premenopausal sister pairs. We now report the results of a genome-wide linkage analysis of femoral structure variables in 437 white and 201 black healthy premenopausal sister pairs, of which 191 white pairs overlapped with our previously published sample. Multipoint quantitative linkage analysis was performed using microsatellite markers genotyped throughout the genome. In the current sample, linkage of femoral structure to chromosomes 3, 7, and 19 was confirmed in the white sister pairs, and a new linkage to chromosome 8 was identified. There was linkage at chromosome 3 to femoral head width (logarithm of the odds [LOD] = 5.0) and femur shaft width (LOD = 3.6). On chromosome 19, there was linkage to femoral neck axis length (LOD = 3.2); on chromosome 7, to femoral head width (LOD = 5.0); and on chromosome 8, to femoral head width (LOD = 6.0). The current findings emphasize the importance of increasing sample size to replicate linkage findings and identify new regions of linkage.

Genetic determinants of susceptibility to osteoporosis

Osteoporosis has a strong genetic component, and clinical studies have shown that heritable factors play a key role in regulating bone mineral density, ultrasound properties of bone, skeletal geometry, and bone turnover and contribute to the pathogenesis of osteoporotic fracture. In most cases, osteoporosis is caused by the combined effects of several different genes and their interaction with environmental influences, but it can occasionally occur as the result of mutations in a single gene. Genes that have been implicated in the regulation of bone mass in humans include the genes encoding lipoprotein receptor-related protein 5, sclerostin, transforming growth factor beta-1, collagen Ialpha1, vitamin D receptor, tumor necrosis factor receptor 2, and the estrogen receptor alpha. From a clinical standpoint, advances in knowledge about the genetic basis of osteoporosis are important because they offer the prospect of developing genetic markers for the assessment of fracture risk and the opportunity to identify molecules that will be used as targets for the design of new drugs for the prevention and treatment of bone disease.