Genetic determinants of bone mineral content at the spine and radius: a twin study

Meta-analysis of genome-wide linkage studies for bone mineral density

Genome-wide linkage studies have shown several chromosome loci that may harbor genes that regulate bone mineral density (BMD), but results have been inconsistent. A meta-analysis was performed to assess evidence for linkage of BMD across whole genome scan studies. Eleven whole-genome scans of BMD or osteoporosis containing 3,097 families with 12,685 individuals were included in this genome scan meta-analysis (GSMA). For each study, 120 genomic bins of approximately 30 cM were defined and ranked according to maximum evidence for linkage within each bin. Bin ranks were weighted and summed across all studies. The summed rank for each bin was assessed empirically for significance using permutation methods. A total of seven bins lie above the 95% confidence level (P=0.05) and one bin was above the 99% confidence level (P=0.01) in the GSMA of eleven linkage studies: bins 16.1 (16pter-16p12.3, Psumrnk <0.01), 3.3 (3p22.2-3p14.1), 1.1 (1pter-1p36.22), 18.2 (18p11.23-18q12.2), 6.3 (6p21.1-6q15), 20.1 (20pter-20p12.3), and 18.1 (18pter-18p11.23). GSMA was performed with seven studies with linkage scores of LOD >1-1.85 for sensitivity test, confirming the linkage on chromosome 16p and 3p and revealing evidence of new linkage in bins 10.2 (10p14-10q11.21) and 22.2 (22q12.3-22pter). In conclusion, the meta-analysis of whole-genome linkage studies of BMD has shown chromosome 16pter-16p12.3 to have the greatest evidence of linkage as well as revealing evidence of linkage in chromosomes 1p, 3p, 6, 10, 18, 20p, and 22q across studies. This data may provide a basis with which to carry out targeted linkage and candidate gene studies particularly in these regions.

CA repeat polymorphism of the TNFR2 gene is not associated with bone mineral density in two independent Caucasian populations

Osteoporosis has a strong genetic component, but the genes involved are poorly defined. Genome-wide scans in multiple populations have identified chromosome 1p36 as one region linked to bone mineral density (BMD). The tumor necrosis factor receptor 2 (TNFR2) at 1p36 is a positional and functional candidate gene in osteoporosis. In this study, we conducted linkage and association tests between the CA repeat polymorphism of the TNFR2 gene and BMD in two large independent samples using the quantitative transmission disequilibrium test (QTDT) program. The first group of subjects was composed of 1836 individuals from 79 multigeneration pedigrees. The second group was a randomly ascertained set of 636 individuals from 157 nuclear families. We found no evidence of association or linkage for spine or hip BMD in the samples of the multigenerational pedigrees or nuclear families. Through testing for association and for linkage, our data do not support the TNFR2 gene as a QTL underlying hip or spine BMD variation in our Caucasian populations.

Familial aggregation of bone mineral density and bone mineral content in a Chinese population

Familial aggregation of bone mineral density (BMD) and bone mineral content (BMC) has been shown in twin and familial studies, but most sample sizes were small. We here report a large familial aggregation study in a Chinese population. A total of 13,973 siblings aged 25-64 years from 3,882 families were enrolled from Anhui, China. We assessed the whole-body, hip and lumbar spine BMD and BMC by dual-energy X-ray absorptiometry (DXA). Intra-class correlation coefficients of BMD and BMC between siblings varied among different skeletal sites and between different age groups of male sib-pairs and premenopausal and postmenopausal female sib-pairs, with a range of 0.228 to 0.397. The sibling recurrence risk ratio (lambdas) of osteoporosis was 2.6 in our population. We also evaluated the joint association of the BMD values of the first siblings and the second siblings with the risk of low BMD (defined as less than the 10th percentile of the same group population) of their younger siblings. If both the first and second siblings' BMDs were in the lowest tertile, the odd ratios (ORs) of low BMD in their subsequent siblings were 8.32 [95% confidence interval (CI) 5.59-12.39)], 8.71 (95% CI 5.74-13.22) and 5.90 (95% CI 3.57-9.76) for total body, total hip and lumbar spine, respectively. This study demonstrates a significant familial aggregation of BMD and BMC in a large sample of rural Chinese adults.

Is there overlap between the genetic determinants of mammographic density and bone mineral density?

Mammographic density and bone mineral density, risk factors for breast cancer and osteoporotic fractures, respectively, are both thought to reflect cumulative exposure to estrogen and are highly heritable. We asked if there was overlap between the genes that explain their variances. We studied 63 monozygous and 71 dizygous female twin pairs ages 38 to 71 years (mean, 50 years). Absolute and percent mammographic densities were measured by a computer-assisted method, and bone mineral density was measured at the lumbar spine, femoral neck, and forearm by dual energy X-ray absorptiometry. After adjusting for age, height, and weight, the within-person and cross-trait cross-twin correlations between the mammographic density and bone mineral density measures were between -0.09 and 0.16 (SEs, 0.07-0.09) and independent of zygosity (all P > 0.05). We conclude that there is little, if any, overlap between the genetic or environmental determinants of disease risk associated with these traits.

Genetic and environmental correlations between bone phenotypes and anthropometric indices in Chinese

Height, weight, bone mineral density (BMD), and bone size are all influenced by genetic and environmental factors as well as interactions between them. Height and weight are often used in population studies to adjust the bone phenotypes. However, it is still unknown what proportion of genetic and environmental variability is shared between these anthropometric characteristics and the bone phenotypes. The genetic and environmental correlations between the bone phenotypes and anthropometric indices in Chinese subjects were studied by bivariate quantitative genetic analysis on a sample of 931 healthy subjects from 292 Chinese nuclear families aged from 19 to 79 years. BMD and bone size at the lumbar spine (L1-L4) and the hip of all subjects were measured by dual-energy X-ray absorptiometry. We found significant genetic correlations between weight and spine BMD, hip BMD, spine bone size and hip bone size, which were 0.50 (P<0.01), 0.45 (P<0.01), 0.36 (P=0.02), and 0.38 (P<0.01), respectively. Likewise, significant genetic correlations between height and spine BMD, spine bone size, and hip bone size were 0.30 (P=0.02), 0.54 (P<0.01), and 0.58 (P<0.01), respectively. The environmental correlations were found to be significant only between height and spine bone size (P<0.001) and weight and hip BMD (P=0.02). These results suggest the probability that the same genetic and environmental factors contribute to these different phenotypes. Moreover, when a candidate gene or genomic region is responsible for the variation of both bone phenotypes and anthropometric indices, its true genetic effect on the bone phenotypes may be lost after one has adjusted the phenotypic values with weight and height as random environmental factors. It may have implications for population studies of candidate genes that underlie the complex bone phenotypes and for the development of strategies for therapeutic application.

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.

[Genetics of osteoporosis]

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

Adjusting data to body size: a comparison of methods as applied to quantitative trait loci analysis of musculoskeletal phenotypes

The aim of this study was to compare three methods of adjusting skeletal data for body size and examine their use in QTL analyses. It was found that dividing skeletal phenotypes by body mass index induced erroneous QTL results. The preferred method of body size adjustment was multiple regression.

INTRODUCTION

Many skeletal studies have reported strong correlations between phenotypes for muscle, bone, and body size, and these correlations add to the difficulty in identifying genetic influence on skeletal traits that are not mediated through overall body size. Quantitative trait loci (QTL) identified for skeletal phenotypes often map to the same chromosome regions as QTLs for body size. The actions of a QTL identified as influencing BMD could therefore be mediated through the generalized actions of growth on body size or muscle mass.

MATERIALS AND METHODS

Three methods of adjusting skeletal phenotypes to body size were performed on morphologic, structural, and compositional measurements of the femur and tibia in 200-day-old C57BL/6J x DBA/2 (BXD) second generation (F(2)) mice (n = 400). A common method of removing the size effect has been through the use of ratios. This technique and two alternative techniques using simple and multiple regression were performed on muscle and skeletal data before QTL analyses, and the differences in QTL results were examined.

RESULTS AND CONCLUSIONS

The use of ratios to remove the size effect was shown to increase the size effect by inducing spurious correlations, thereby leading to inaccurate QTL results. Adjustments for body size using multiple regression eliminated these problems. Multiple regression should be used to remove the variance of co-factors related to skeletal phenotypes to allow for the study of genetic influence independent of correlated phenotypes. However, to better understand the genetic influence, adjusted and unadjusted skeletal QTL results should be compared. Additional insight can be gained by observing the difference in LOD score between the adjusted and nonadjusted phenotypes. Identifying QTLs that exert their effects on skeletal phenotypes through body size-related pathways as well as those having a more direct and independent influence on bone are equally important in deciphering the complex physiologic pathways responsible for the maintenance of bone health.

Association between single nucleotide polymorphisms of estrogen receptor alpha gene and efficacy of HRT on bone mineral density in post-menopausal Japanese women

BACKGROUND

Although HRT for post-menopausal women can protect against bone loss, variations in bone responses exist. We studied whether single nucleotide polymorphisms (SNP) of the estrogen receptor-alpha (ERalpha) gene contribute to the effect of HRT on lumbar spine bone mineral density (BMD).

METHODS

Subjects were 84 post-menopausal women who had been taking HRT for 3 years to treat osteopenia or osteoporosis. Eighteen SNP in the ERalpha gene were characterized by a single nucleotide primer extension assay.

RESULTS

Genotyping of the 84 individuals revealed that all SNP were quite common, the minor allele frequency being > or = 20%. A SNP in intron 6 (IVS6+14144) was significantly associated with the response to HRT for the first 3 years after starting treatment (P = 0.043, 0.025 and 0.032 for the first, second and third years respectively). Haplotype analysis revealed that a combination of SNP IVS6+14144 and IVS4+4238 was significantly correlated with the response to HRT; women with haplotype G-G (IVS6 14144-IVS4 4238) showed a significantly higher response (P = 0.014, 0.043 and 0.010 for the first second and third year respectively).

CONCLUSIONS

These results suggest that a specific SNP and the haplotype of the selected SNP could be used to predict the effect of HRT on lumbar BMD.

The -1997 G/T polymorphism in the COLIA1 upstream regulatory region is associated with hip bone mineral density (BMD) in Chinese nuclear families

Type I collagen is the most abundant protein of bone matrix, and the collagen type I alpha 1(COLIA1) gene has been considered one of the most important candidate genes for osteoporosis. In this study, we simultaneously tested linkage and/or association of the -1997 G/T polymorphism in the COLIA1 upstream regulatory region with the variation of bone mineral density (BMD) in 1263 subjects from 402 Chinese nuclear families, consisted of both parents and at least one healthy female offspring from 20 to 45 years of age. All the subjects were genotyped by using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP). BMD of the lumbar spine (L1-L4) and hip (respective and combined phenotype of the femoral neck, trochanter, and intertrochanter) was measured by dual-energy X-ray absorptiometry (DXA). By using the tests implemented in program QTDT (quantitative transmission disequilibrium test), we found significant within-family association (via TDT) between the -1997 G/T polymorphism with BMD variation at all the hip sites (respective and combined phenotypes, P < 0.05). The amount of BMD variation explained by the -1997G/T polymorphism was 1.6%, 2.0%, 1.2%, and 1.3% at the total hip, femoral neck, trochanter, and intertrochanter, respectively. Because of the limited number of sib pairs in this sample, we did not find evidence of linkage. In summary, the -1997 G/T polymorphism in the COLIA1 gene is likely to be in linkage disequilibrium with a nearby functional polymorphism affecting hip BMD, or the -1997 G/T polymorphism itself may have an important effect on the variation of hip BMD in our Chinese sample.

Quantitative trait loci analysis of structural and material skeletal phenotypes in C57BL/6J and DBA/2 second-generation and recombinant inbred mice

QTL analyses identified several chromosomal regions influencing skeletal phenotypes of the femur and tibia in BXD F2 and BXD RI populations of mice. QTLs for skeletal traits co-located with each other and with correlated traits such as body weight and length, adipose mass, and serum alkaline phosphatase.

INTRODUCTION

Past research has shown substantial genetic influence on bone quality, and the impact of reduced bone mass on our aging population has heightened the interest in skeletal genetic research.

MATERIALS AND METHODS

Quantitative trait loci (QTL) analyses were performed on morphologic measures and structural and material properties of the femur and tibia in 200-day-old C57BL/6J x DBA/2 (BXD) F2 (second filial generation; n = 400) and BXD recombinant inbred (RI; n = 23 strains) populations of mice. Body weight, body length, adipose mass, and serum alkaline phosphatase were correlated phenotypes included in the analyses.

RESULTS

Skeletal QTLs for morphologic bone measures such as length, width, cortical thickness, and cross-sectional area mapped to nearly every chromosome. QTLs for both structural properties (ultimate load, yield load, or stiffness) and material properties (stress and straincharacteristics and elastic modulus) mapped to chromosomes 4, 6, 9, 12, 13, 15, and 18. QTLs that were specific to structural properties were identified on chromosomes 1, 2, 3, 7, 8, and 17, and QTLs that were specific to skeletal material properties were identified on chromosomes 5, 11, 16, and 19. QTLs for body size (body weight, body length, and adipose mass) often mapped to the same chromosomal regions as those identified for skeletal traits, suggesting that several QTLs identified as influencing bone could be mediated through body size.

CONCLUSION

New QTLs, not previously reported in the literature, were identified for structural and material properties and morphological measures of the mouse femur and tibia. Body weight and length, adipose mass, and serum alkaline phosphatase were correlated phenotypes that mapped in close proximity of skeletal chromosomal loci. The more specific measures of bone quality included in this investigation enhance our understanding of the functional significance of previously identified QTLs.

A family history of fracture and fracture risk: a meta-analysis

The aims of the present study were to determine whether a parental history of any fracture or hip fracture specifically are significant risk factors for future fracture in an international setting, and to explore the effects of age, sex and bone mineral density (BMD) on this risk. We studied 34,928 men and women from seven prospectively studied cohorts followed for 134,374 person-years. The cohorts comprised the EPOS/EVOS study, CaMos, the Rotterdam Study, DOES and cohorts at Sheffield, Rochester and Gothenburg. The effect of family history of osteoporotic fracture or of hip fracture in first-degree relatives, BMD and age on all clinical fracture, osteoporotic fracture and hip fracture risk alone was examined using Poisson regression in each cohort and for each sex. The results of the different studies were merged from the weighted beta coefficients. A parental history of fracture was associated with a modest but significantly increased risk of any fracture, osteoporotic fracture and hip fracture in men and women combined. The risk ratio (RR) for any fracture was 1.17 (95% CI=1.07-1.28), for any osteoporotic fracture was 1.18 (95% CI=1.06-1.31), and for hip fracture was 1.49 (95% CI=1.17-1.89). The risk ratio was higher at younger ages but not significantly so. No significant difference in risk was seen between men and women with a parental history for any fracture (RR=1.17 and 1.17, respectively) or for an osteoporotic fracture (RR=1.17 and 1.18, respectively). For hip fracture, the risk ratios were somewhat higher, but not significantly higher, in men than in women (RR=2.02 and 1.38, respectively). A family history of hip fracture in parents was associated with a significant risk both of all osteoporotic fracture (RR 1.54; 95CI=1.25-1.88) and of hip fracture (RR=2.27; 95% CI=1.47-3.49). The risk was not significantly changed when BMD was added to the model. We conclude that a parental history of fracture (particularly a family history of hip fracture) confers an increased risk of fracture that is independent of BMD. Its identification on an international basis supports the use of this risk factor in case-finding strategies.

Bone intrinsic material properties in three inbred mouse strains

This study assessed genetically based differences in intrinsic material properties of both cortical and cancellous bone in adult females of three inbred mouse strains [C57BL/6J (B6), DBA/2J (D2), C3H/HeJ (C3)]. These mouse strains have previously been shown to differ in bone mineral content (BMC) and density (BMD). Distal femoral cancellous bone and midshaft cortical bone in femurs and tibias were assessed for intrinsic material properties using nanoindentation technique. The intrinsic material properties tested were modulus (E(b)) and hardness (H) of the midshaft femoral and tibial cortical bone cross sections and of cancellous bone in the distal femur. Both femoral and tibial cortical bone intrinsic material properties were different among the three inbred mouse strains. Femoral modulus and tibial hardness in cortical bone and hardness in cancellous bone were either greatest or showed greater trends in C3 mice as compared to both D2 and B6. Cancellous bone modulus was similar among the three mouse strains. With the exception of the D2 mice, the femoral and tibial cortical modulus were similar within each mouse strain. The tibial cortical modulus was smaller than the femoral cortical modulus for D2 mouse strain. The cortical hardness was greater in tibiae compared with that in femora within each mouse strain. The nanoindentation data suggest that cortical and cancellous intrinsic material properties are influenced by the genetic background of the inbred mouse strains. The inbred mouse strain-related intrinsic material property phenotype can be used to locate responsible quantitative trait loci (QTLs) in future studies of recombinant inbred mouse strains.

Heritability of spinal trabecular volumetric bone mineral density measured by QCT in the Diabetes Heart Study

The heritability of trabecular volumetric bone mineral density (BMD) determined by quantitative computed tomography (QCT) has not yet been reported. The purpose of this study was to investigate the heritability of BMD as determined by QCT and DXA in 124 women and 120 men (age 39-83 years, BMI 17-75, 84% type 2 diabetics) from 101 families (232 sibling pairs) in the Diabetes Heart Study. Volumetric BMD had a heritability (h2) estimate of 0.73 (SE = 0.15, P < 0.0001) at the lumbar spine and 0.71 (SE = 0.15, P < 0.0001) at the thoracic spine. Areal BMD heritability estimates were 0.56 for PA spine, 0.43 for total hip, 0.43 for femoral neck, 0.45 for distal radius, 0.42 for mid-radius, and 0.52 for whole body (all P < 0.01). After accounting for familial correlation using generalized estimating equations, volumetric BMD was inversely associated with age (r = -0.52, P < 0.0001) and duration of diabetes (r = -0.24, P < 0.01) and positively associated with body weight (r = 0.25, P < 0.01). In multivariate analysis, adjustment for age, sex, and race lowered the h2 estimates for volumetric BMD at the lumbar (h2 = 0.41, P < 0.01) and thoracic (h2 = 0.48, P < 0.001) spine, increased the h2 estimate for areal BMD at the mid radius (h2 = 0.58, P < 0.0001), and had little effect on the h2 estimate for areal BMD at other sites (h2 = 0.41-0.55, all P < 0.01). Additional adjustment for BMI, duration of diabetes, and physical activity had little effect on the h2 estimates for volumetric BMD or areal BMD except at the hip where they were lowered (h2 = 0.31-0.33, all P < 0.05). These data suggest that, like areal BMD, volumetric BMD is highly heritable and may be used in designing linkage studies to locate genes governing bone metabolism.

A follow-up linkage study for bone size variation in an extended sample

Bone size, which has strong genetic determination, is an important determinant of bone strength and a risk factor of osteoporotic fractures. We previously reported an approximately 10-cm genome-wide linkage scan in 630 subjects from 53 US Caucasian pedigrees. The strongest evidence of linkage was obtained on chromosome 17q22 near the marker D17S787, with a two-point LOD score of 3.98 and a multipoint maximum LOD score (MLS) of 3.01. Additionally, suggestive linkages (1.54 < MLS < 2.83) were found at the other four chromosomal regions. In the present study, with an attempt to further examine our previous findings, we perform a follow-up linkage analysis in an expanded sample of 79 pedigrees with 1816 subjects. The total sample contains >80,000 informative relative pairs for linkage analyses, including 3846 sib pairs. Fifteen markers covering the above five promising regions are genotyped, narrowing the average genomic distance from approximately 10 to 5 cm. In the total 79 pedigrees, support of linkage was achieved for the wrist bone size at 17q22 with a two-point LOD score of 2.27 (P = 0.0006) and MLS of 1.78 (P = 0.002). The genomic region 17q22 includes COL1A1, a strong candidate gene that is significantly associated with osteoporotic fracture risk. Our data suggest that this region is promising for further exploratory studies.

Vitamin D receptor alleles, bone mineral density and turnover in postmenopausal osteoporotic and healthy women

OBJECTIVE

Vitamin D receptor (VDR) gene polymorphisms and bone metabolic markers were investigated as potential genetic markers for osteoporosis in postmenopausal Turkish women. The relationship between their VDR gene polymorphisms and bone states was determined.

MATERIALS AND METHODS

Restriction fragment length polymorphisms at the VDR gene locus (i.e., for BsmI, ApaI, and TaqI) was investigated in 75 postmenopausal osteoporotic (53.16 +/- 1.31 years) and 66 healthy (52.62 +/- 1.69 years) Turkish women and the genotypes were related to bone mineral density (BMD) at femoral neck (FN), lumbar spine (L1-4), trochanter, Ward's triangle (Ward's) and metabolic parameters of bone turnover.

RESULTS

In osteoporotic women, TaqI genotype-related differences of the VDR gene were found to be significant at all BMD sites; TT genotype had higher L1-4 BMD values than Tt and tt (p < 0.05); tt genotype had significantly lower BMD at FN (p < 0.05), trochanter (p < 0.01), and Ward's (p < 0.05) compared to TT genotype. The tt genotype was found to be associated with higher (p < 0.05) serum osteocalcin levels compared to Tt and TT genotypes in the osteoporotic women, whereas no such association was found for the healthy women.

CONCLUSION

Our data showed an association between VDR TaqI genotype and BMD at the FN, L1-4, trochanter and Ward's triangle in nonobese postmenopausal osteoporotic women. Thus the VDR gene Taql polymorphism modulates differences in BMD in the postmenopausal osteoporotic women.

The relationship between COLI A1 polymorphisms (Sp 1) and COLI A2 polymorphisms (Eco R1 and Puv II) with bone mineral density in Chinese men and women

Polymerase chain reaction was used to amplify across variable restriction sites of the COLI A1 and COLI A2 genes that encode the alpha 1 and 2 subunits of type I collagen. The relationship between these polymorphisms and bone mineral density (BMD) was studied in 683 Chinese men and women. In 100 men and women, COLI A1 Sp1 polymorphism was not found, which was consistent with other previous studies in Asian populations. However a statistically significant relationship was observed between COLI A2 Eco R1 and Puv II genotypes among the Chinese men studied. The mean BMD was consistently lower in men of the EE and PP genotype (P < 0.05 by analysis of variance [ANOVA]) than in men of the ee and pp genotypes. However, no association between BMD and the Eco R1 or Puv II genotypes was observed in Chinese women (P > 0.05 by ANOVA). We conclude that the COLI Al Sp1 binding site is absent in Hong Kong Chinese, whereas the COLI A2 Eco R1 and Puv II genetic polymorphisms may be associated with the BMD of elderly Chinese men.

A second-stage genome scan for QTLs influencing BMD variation

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

Genetic determinants of osteoporosis susceptibility in a female Ashkenazi Jewish population

PURPOSE

To determine the heritability of low bone mineral density (BMD) at the hip in Ashkenazi Jewish families.

METHODS

BMD at hip was accessed by dual x-ray absorptiometry (DEXA) in 166 female subjects from 61 families. Variance component analysis was used to estimate genetic contributions.

RESULTS

We observed significant genetic contributions to age-adjusted BMD at the femoral neck as measured by heritability 0.67 (P < 0.0001).

CONCLUSION

There is significant genetic determination in decreased BMD at the femoral neck in an Ashkenazi Jewish female population. These results warrant further gene mapping studies in this population to identify osteoporosis susceptibility loci.

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.

Bone densitometry in premenopausal women: synthesis and review

Bone loss prior to menopause is being increasingly identified in women. Clearly, low bone mineral density (BMD) is a significant risk factor for fracture in the estrogen-deficient female postmenopause. The significance of low bone density prior to menopause needs to be addressed. Low bone density in the premenopausal female may reflect attainment of a lower peak bone mass. It may also be secondary to progressive bone loss following achievement of peak bone density. The etiology of low bone density in the premenopausal female needs to be clarified with meticulous exclusion of secondary causes of bone loss. Menstrual status is an important determinant of peak bone mass as well as the development of bone loss in women prior to the onset of menopause. Subclinical decreases in circulating gonadal steroids may be associated with a lower peak bone mass as well as progressive bone loss in otherwise reproductively normal women. Elevations of follicle-stimulating hormone (FSH) of greater than 20 miu/L are associated with evidence of increased bone turnover marker activity and correlate with progressive bone loss in perimenopausal women. This transitional period requires further study with respect to the magnitude of bone loss experienced and the potential benefits of antiresorptive therapy. Detailed assessment of menstrual status is necessary in the evaluation of low bone density in premenopausal women. The majority of the cross-sectional and longitudinal studies completed evaluating BMD in the premenopausal years suggest that minimal bone loss does occur prior to menopause after attainment of peak bone mass. The magnitude of premenopausal bone loss, however, is controversial and may be site-dependent. More rapid rates of bone loss are seen in the transitional period beginning 2-3 yr prior to the onset of menopause. Prospective data are needed to understand further the relationship between BMD and fracture in the premenopausal period. Women with steroid-induced bone loss as well as other secondary causes of osteoporosis respond to antiresorptive therapy with documented improvements in BMD. Biomarkers can identify perimenopausal women with increased bone turnover. Lifestyle modification can improve BMD in the pre- and the perimenopausal period. Antiresorptive therapy has not been evaluated in pre- or perimenopausal women with low BMD in the absence of secondary causes of osteoporosis. As new treatment options are evaluated and become available, biomarker assessment may be of value in identifying women at risk of fracture.

Linkage and association of the CA repeat polymorphism of the IL6 gene, obesity-related phenotypes, and bone mineral density (BMD) in two independent Caucasian populations

Genetic factors play an important role in osteoporosis and obesity, two serious public health problems in the world. We investigated the relationships between obesity-related phenotypes, bone mineral density (BMD) and the CA repeat polymorphism of the IL6 gene in two large independent samples using the quantitative transmission disequilibrium test (QTDT). The first sample consisted of 1,816 individuals from 79 multigenerational pedigrees. Each pedigree was identified through a proband with BMD Z-scores </=-1.28 at the hip or spine. The second sample was a randomly ascertained set of 636 individuals from 157 nuclear families. Ten alleles containing 9-18 CA repeats were identified in our Caucasian populations. For body mass index (BMI), fat mass and percentage fat mass (PFM), highly significant (P<0.01) or significant (P<0.05) results were found for linkage in our sample of nuclear families and for association in the multigenerational pedigrees. We also observed weak evidence for linkage (P=0.069) with spine BMD and for association with hip BMD in the sample of multigenerational pedigrees. Our results suggest that genetic variation in or near the IL6 locus may be involved in the etiology of obesity and osteoporosis.

Genetic and environmental determinants of bone mineral density in Mexican Americans: results from the San Antonio Family Osteoporosis Study

Osteoporosis is a major cause of disability in the United States. Numerous factors contribute to the decline in bone mineral density (BMD) that characterizes this disease, and the importance of heredity is now widely appreciated. We evaluated the joint contributions of genes and environmental factors on variation in BMD in 895 participants of the San Antonio Family Osteoporosis Study (SAFOS). Participants of the SAFOS ranged in age from 18 to 96 years and were members of 34 large families of Mexican American ancestry. BMD was measured at the spine, hip, and forearm by dual-energy X-ray absorptiometry. Information about medical history, lifestyle habits, dietary intake, and physical activity patterns was obtained by questionnaire. Age and body mass index were strongly associated with BMD at nearly every site; these and other measured risk factors accounted in aggregate for up to 46% of the total variation in BMD. In general, the environmental risk factors accounted for proportionately more of the total variation in BMD in men than in women. Genes accounted for 65-80% of the residual variation in spine and hip BMD, and 25-55% of the residual variability in forearm BMD. Although residual heritabilities were generally comparable between men and women across all ages combined, heritabilities at all sites tended to be higher in premenopausal women than in men younger than 50 years of age. Identifying the individual genes involved will shed insights into the processes that govern bone remodeling and may suggest strategies for the prevention of osteoporosis.

Determinants of bone density in 30- to 65-year-old women: a co-twin study

Reported effects of body composition and lifestyle on bone mineral density in pre-elderly adult women have been inconsistent. In a co-twin study, we measured bone mineral density, lean and fat mass, and lifestyle factors. Analyzing within pair differences, we found negative associations between bone mineral density and tobacco use (2.3-3.3% per 10 pack-years) and positive associations with sporting activity and lean and fat mass.

INTRODUCTION

Reported effects of body composition and lifestyle of bone mineral density in pre-elderly adult women have been inconsistent.

METHODS

In a co-twin study of 146 female twin pairs aged 30 to 65 years, DXA was used to measure bone mineral density at the lumbar spine, total hip, and forearm, total body bone mineral content, and lean and fat mass. Height and weight were measured. Menopausal status, dietary calcium intake, physical activity, current tobacco use, and alcohol consumption were determined by questionnaire. Within-pair differences in bone measures were regressed through the origin against within-pair differences in putative determinants.

RESULTS

Lean mass and fat mass were associated with greater bone mass at all sites. A discordance of 10 pack-years smoking was related to a 2.3-3.3% (SE, 0.8-1.0) decrease in bone density at all sites except the forearm, with the effects more evident in postmenopausal women. In all women, a 0.8% (SE, 0.3) difference in hip bone mineral density was associated with each hour per week difference in sporting activity, with effects more evident in premenopausal women. Daily dietary calcium intake was related to total body bone mineral content and forearm bone mineral density (1.4 +/- 0.7% increase for every 1000 mg). Lifetime alcohol consumption and walking were not consistently related to bone mass.

CONCLUSION

Several lifestyle and dietary factors, in particular tobacco use, were related to bone mineral density. Effect sizes varied by site. Characterization of determinants of bone mineral density in midlife and thereafter may lead to interventions that could minimize postmenopausal bone loss and reduce osteoporotic fracture risk.

An investigation of unique and shared gene effects on speed of sound and bone density using axial transmission quantitative ultrasound and DXA in twins

The genetic influences of speed of sound (SOS) and BMD were evaluated using 215 pairs of healthy female twins. Genetic influences were found for all SOS and BMD measurements. A combination of shared and unique genetic influences was found to control BMD and SOS at the radius.

INTRODUCTION

The aim of this study was to investigate to what extent axial transmission speed of sound (SOS) measurements in cortical bone at multiple, peripheral skeletal sites will be influenced by genetic factors and to estimate the proportion of shared and unique genetic influences controlling bone mineral density (BMD) and SOS at a single site, the radius.

MATERIALS AND METHODS

The study population consisted of 215 pairs of healthy female twins. Of these, 85 pairs were monozygotic (MZ) and 130 pairs were dizygotic (DZ). The twins had measurements of the nondominant third proximal phalanx, one-third radius, midshaft tibia, and fifth metatarsal using the Sunlight Omnisense and DXA measurements of the lumbar spine, nondominant proximal femur, nondominant radius, and whole body using Hologic QDR-4500W densitometers. Calcaneal quantitative ultrasound (QUS) measurements were performed using the McCue CUBA clinical. Intraclass correlations were calculated, and heritability was estimated using multiple regression analysis. Bivariate analysis of site-matched SOS and BMD measurements at the radius was performed using a variance components analysis program.

RESULTS

Age- and body mass index-adjusted heritability estimates ranged from 0.51 (95% CI, 0.32-0.70) to 0.56 (0.37-0.76) for SOS measurements, 0.58 (0.41-0.75) for broadband ultrasound attenuation (BUA), 0.72 (0.58-0.86) to 0.77 (0.63-0.91) for axial BMD, and 0.53 (0.16-0.90) to 0.63 (0.26-1.00) for radius and whole body BMD. The correlation between SOS and DXA at the radius was r = 0.34 (0.29-0.47). Thirty-eight percent (16-57%) of the genetic variance explained by SOS at the radius was also explained by BMD (one-third radius region of interest), with 62% being unique.

CONCLUSION

In conclusion, genetic influences were demonstrated for SOS measurements in cortical bone at multiple sites, axial BMD, calcaneal BUA, radius, and whole body BMD. At the radius, up to 38% of the genetic influence is shared by genes controlling BMD and SOS. Clarifying the site specificity and pleiotropic effects of bone genes should help our understanding of these complex pathways.

Estrogen receptor alpha gene polymorphisms and peak bone density in Chinese nuclear families

PBD is an important determinant of osteoporotic fractures. Few studies were performed to search for genes underlying PBD variation in Chinese populations. We tested linkage and/or association of the estrogen receptor alpha gene polymorphism with PBD in 401 Chinese nuclear families. This study suggests the ER-alpha gene may have some minor effects on PBM variation in the Chinese population. Low peak bone density (PBD) in adulthood is an important determinant of osteoporotic fractures in the elderly. PBD variation is mainly regulated by genetic factors. Extensive molecular genetics studies have been performed to search for genes underlying PBD variation, largely in whites. Few studies were performed in Chinese populations. In this study, we simultaneously test linkage and/or association of the estrogen receptor alpha (ER-alpha) gene polymorphism with PBD in 401 Chinese nuclear families (both parents plus their female children) of 1260 subjects, with the 458 children generally between 20 and 40 years of age. All the subjects were genotyped by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) at polymorphic PvuII and XbaI sites inside the ER-alpha gene. Bone mineral density was measured at the lumbar spine (L1-L4) and hip (femoral neck, trochanter, and intertrochanteric region). Raw bone mineral density values were adjusted by age, height, and weight as covariates. We detected marginally significant results for within-family association (transmission disequilibrium; p = 0.054) between the spine bone mineral density variation and the ER-alpha XbaI genotypes. For the hip bone mineral density variation, significant (p < 0.05) linkage results were generally found for the two intragenic markers. Analyses of the haplotypes defined by the two markers confer further evidence for linkage of the ER-alpha with the hip PBD variation. In conclusion, this study suggests that the ER-alpha gene may have minor effects on PBD variation in our Chinese population.

Nonassociation of interleukin-1 receptor antagonist genotypes with bone mineral density, bone turnover status, and estrogen responsiveness in Korean postmenopausal women

Interleukin-1 receptor antagonist (IL-1ra), a natural inhibitor of interleukin-1 (IL-1), completely inhibits the stimulatory effects of IL-1 on bone resorption. Bioactivity of IL-1 increases in the estrogen-deficient state with an increased IL-1:IL-1ra ratio and decreases after estrogen replacement therapy with a decreased IL-1:IL-1ra ratio. An association was found between an 86 basepair variable number tandem-repeat (VNTR) polymorphism of the IL-1ra gene and an increased production of IL-1ra in a cultured monocyte system. The IL-1ra VNTR polymorphism, therefore, is an attractive candidate gene for osteoporosis susceptibility as well as hormone responsiveness after estrogen replacement. We examined the association of this VNTR polymorphism with bone mass, bone turnover, and the change of bone mineral density (BMD) after 1 year of hormone replacement therapy (HRT). The frequencies of the five alleles were as follows: A1, 90.8% (410 bp, four repeats); A2, 7.2% (240 bp, two repeats); A3, 1.6% (500 bp, five repeats); A4, 0.4% (326 bp, three repeats); and A5, 0% (595 bp, six repeats), in 714 healthy ethnically Korean postmenopausal women, aged 41-74 years (55.2 +/- 6.3 years mean +/- SD). Spine (L2-4) and femoral neck BMD were not significantly different among IL-1ra genotypes, and no significant genotypic differences were found in bone markers. There were no differences in genotypic proportions when we categorized the subjects into a high-loss group and a normal-loss group with regard to levels of bone marker. No significant genotypic differences were found in changes in lumbar and femoral neck BMD and those in bone markers before and after 1 year of HRT in 312 women. Our data suggest that these IL-1ra polymorphisms are not associated with BMD, bone turnover, or the change of BMD after 1 year of HRT in Korean women.

A whole-genome linkage scan suggests several genomic regions potentially containing quantitative trait Loci for osteoporosis

Osteoporosis is an important health problem, particularly in the elderly women. Bone mineral density (BMD) is a major determinant of osteoporosis. For a sample of 53 pedigrees that contain 1249 sibling pairs, 1098 grandparent-grandchildren pairs, and 2589 first cousin pairs, we performed a whole- genome linkage scan using 380 microsatellite markers to identify genomic regions that may contain quantitative trait loci (QTL) of BMD. Each pedigree was ascertained through a proband with BMD values belonging to the bottom 10% of the population. We conducted two-point and multipoint linkage analyses. Several potentially important genomic regions were suggested. For example, the genomic region near the marker D10S1651 may contain a QTL for hip BMD variation (with two-point analysis LOD score of 1.97 and multipoint analysis LOD score of 2.29). The genomic regions near the markers D4S413 and D12S1723 may contain QTLs for spine BMD variation (with two-point analysis LOD score of 2.12 and 2.17 and multipoint analysis LOD score of 3.08 and 2.96, respectively). The genomic regions identified in this and some earlier reports are compared for exploration in extension studies with larger samples and/or denser markers for confirmation and fine mapping to eventually identify major functional genes involved in osteoporosis.

Candidate genes for osteoporosis. Therapeutic implications

Osteoporosis, which afflicts 10 million Americans, is a complex disease characterized by decreased bone mass, microarchitectural deterioration of bone tissue, and an increase in fracture risk. Family and twin studies have established a genetic contribution to the etiology of osteoporosis. The biological candidate genes of osteoporosis can be ordered into 5 categories: (i) calcium homeostasis; (ii) hormonal dysfunction; (iii) osteoblast and osteoclast development and regulation; (vi) cartilage matrix metabolism; and (v) lipoprotein metabolism. In addition, genome-wide scans have identified a number of chromosomal regions harboring genes that influence bone mineral density. Moreover, the drug responses to various treatments of osteoporosis are reported to be modulated by DNA polymorphisms of the vitamin D receptor gene, the estrogen receptor 1 gene, and the transforming growth factor beta 1 gene. With the rapid advancement of the Human Genome Project and biotechnology, it will be possible to carry out parallel analyses of large numbers of candidate genes for osteoporosis and to calculate a patient's individual fracture risk in the context of specific environmental influences. This will eventually lead to more advanced diagnostic methods and more efficacious drugs targeting osteoporosis.

Sibling pair linkage and association studies between peak bone mineral density and the gene locus for the osteoclast-specific subunit (OC116) of the vacuolar proton pump on chromosome 11p12-13

A major determinant of the risk of osteoporosis is peak bone mineral density (BMD), which has been shown to have substantial heritability. The genes for 3 BMD-related phenotypes (autosomal dominant high bone mass, autosomal recessive osteoporosis-pseudoglioma, and autosomal recessives osteopetrosis) are all in the chromosome 11q12-13 region. We reported linkage of peak BMD in a large sample of healthy premenopausal sister pairs to this same chromosomal region, suggesting that the genes underlying these 3 disorders may also play a role in determining peak BMD within the normal population. To test this hypothesis, we examined the gene responsible for 1 form of autosomal recessive osteopetrosis, TCIRG1, which encodes an osteoclast-specific subunit (OC116) of the vacuolar proton pump. We identified 3 variants in the sequence of TCIRG1, but only one, single nuclear polymorphism 906713, had sufficient heterozygosity for use in genetic analyses. Our findings were consistent with linkage to femoral neck BMD, but not to spine BMD, in a sample of 995 healthy premenopausal sister pairs. However, further analysis, using both population and family-based disequilibrium approaches, did not demonstrate any evidence of association between TCIRG1 and the spine or femoral neck BMD. Therefore, our linkage data suggest that the chromosomal region that contains OC116 harbors a gene that affects peak BMD, but our association results indicate that polymorphisms in the OC116 gene do not affect peak BMD.

Association between a functional interleukin-6 gene polymorphism and peak bone mineral density and postmenopausal bone loss in women: the OFELY study

Genetic factors play an important role in determining bone mass and several genes are involved in this process. Interleukin-6 (IL-6) is a candidate gene for regulation of bone mineral density (BMD) and it has been suggested recently that novel IL-6 -174 G/C allelic variants may be associated with peak BMD in young men and with bone resorption in elderly women. In this study, we assessed the relationships between IL-6 gene polymorphism, peak BMD, rate of postmenopausal BMD loss, and bone turnover in women. BMD was measured by dual-energy X-ray absorptiometry in 255 healthy premenopausal women, aged 31-57 years. BMD loss at the forearm was measured over 4 years in 298 healthy untreated postmenopausal women, 50-88 years (mean 64 years). We also measured levels of serum osteocalcin, bone alkaline phosphatase, and N-propeptide of type I collagen for bone formation and three markers of bone resorption, including urinary and serum C-terminal cross-linking telopeptide of type I collagen and urinary N-terminal telopeptide of type I collagen, in both pre- and postmenopausal women at baseline. In premenopausal women we found a significant association between IL-6 genotypes and BMD at the whole body (analysis of variance [ANOVA], p = 0.03), femoral neck (p = 0.03), trochanter (p = 0.014), Ward's triangle (p = 0.03), and total hip (p = 0.006), with subjects having the CC genotype showing 3%-7% higher BMD levels than their GG counterparts. However, after matching women with CC and GG genotypes for body height the differences decreased (2%-4%), and were no longer significant (p = 0.10-0.23). In postmenopausal women the mean rate of loss at the ultradistal radius was significantly associated with IL-6 genotypes (ANOVA, p = 0.049), with women having the CC genotype showing a significantly greater rate of bone loss (p < 0.05) compared with their GC and GG counterparts. After adjustment for weight changes, the difference in the rate of ultradistal radius bone loss between genotypes decreased and was not significant (p = 0.06 for CC vs. GG). A similar trend was observed for distal radius bone loss (p = 0.10, ANOVA), but not for the middle radius. We found no significant association between genotypes, bone turnover markers in premenopausal women, and either bone turnover or BMD in postmenopausal women. We conclude that this new functional IL-6 polymorphism was weakly associated with level of peak BMD and the rate of forearm trabecular postmenopausal bone loss in this cohort of healthy French women. IL-6 genotypes accounted only for a small proportion of the interindividual variation of both peak BMD and rate of bone loss and were not significant after adjustment for height and changes in body weight, respectively, suggesting that part of the effect may have been due to the differences in body size. Larger long-term studies are necessary to assess adequately the relationships between IL-6 genotype, rate of bone loss, and risk of fracture.

Genetics of osteoporosis

Osteoporosis is a common multifactorial disorder of reduced bone mass. The disorder in its most common form is generalized, affecting the elderly, both sexes, and all racial groups. Multiple environmental factors are involved in the pathogenesis. Genes also play a major role as reflected by heritability of many components of bone strength. Quantitative phenotypes in bone strength in the normal population do not conform to a monogenetic mode of inheritance. The common form of osteoporosis is generally considered to be a polygenic disorder arising from the interaction of common polymorphic alleles at quantitative trait loci, with multiple environmental factors. Finding the susceptibility genes underlying osteoporosis requires identifying specific alleles that coinherit with key heritable phenotypes in bone strength. Because of the close correspondence among mammalian genomes, identification of the genes underlying bone strength in mammals such as the mouse is likely to be of major assistance in human studies. Identification of susceptibility genes for osteoporosis is one of several important approaches toward the long-term goal of understanding the molecular biology of the normal variation in bone strength and how it may be modified to prevent osteoporosis. As with all genetic studies in humans, these scientific advances will need to be made in an environment of legal and ethical safeguards that are acceptable to the general public.

Genetic and environmental factors affect bone density variances of families of men and women with osteoporosis

Our aim was to assess the relative impacts of genetics and environment in the families of osteoporotic patients and identify the best subgroup of patients to investigate the genes associated with osteoporosis. We recruited 36 men and 47 women with osteoporosis (probands), median age of 52 and 68 yr, and all their siblings (90) and offspring (83). The families were classified as young or old on the basis of the median age of the probands. We measured the bone mineral density at the femoral neck (FN) and lumbar spine (LS) adjusted for age and weight and standardized (Z-score). Physical activity, nutritional calcium, and alcohol and tobacco consumption were investigated. We compared the mean Z-score using linear mixed model and assessed the familial resemblance using intraclass correlation. The mean Z-scores of the families of osteoporotic patients were significantly negative at FN and LS, with no intergeneration or intergender differences. At FN, but not at LS, the mean Z-score was independently lower in the families of male probands (mean +/- SD: -0.57 +/- 0.96, female: -0.18 +/- 0.85, P = 0.012) and in young families (-0.58 +/- 0.94, old families: -0.11 +/- 0.83, P = 0.006). This suggested that the lower Z-score in the families of men with osteoporosis was related to their younger age. There was significant phenotypic resemblance among members in the families. In the families of female probands, the correlation between the probands and her siblings was weak and disappeared after adjustment on environment, and a resemblance appeared within their children (FN: r = 0.61) suggesting that different environment had masked the resemblance in this subgroup. In the families of male probands, a strong resemblance persisted after adjusting for environment, (proband-offspring at FN: r = 0.46 and within offspring at FN: r = 0.66, at LS: r = 0.61). This showed that resemblance was independent of a common measurable environment in these families of men with osteoporosis. In conclusion, mainly young osteoporotic patients, most of whom were male in our study, are affected by the genetic component.

Protein intake and bone growth

Among osteotrophic nutrients, proteins play an important role in bone development, thereby influencing peak bone mass. Consequently, protein malnutrition during development can increase the risk of osteoporosis and of fragility fracture later in life. Both animal and human studies indicate that low protein intake can be detrimental for both the acquisition of bone mass during growth and its conservation during adulthood. Low protein intake impairs both the production and action of IGF-I (Insulin-like growth factor-I). IGF-I is an essential factor for bone longitudinal growth, as it stimulates proliferation and differentiation of chondrocytes in the epiphyseal plate, and also for bone formation. It can be considered as a key factor in the adjustments of calcium-phosphate metabolism required for normal skeletal development and bone mineralization during growth. In healthy children and adolescents, a positive association between the amount of ingested proteins and bone mass gain was observed in both sexes at the level of the lumbar spine, the proximal femur and the midfemoral shaft. This association appears to be particularly significant in prepubertal children. This suggests that, like for the bone response to either the intake of calcium or weight-bearing exercise, the skeleton would be particularly responsive to the protein intake during the years preceding the onset of pubertal maturation.

Gender differences in the genetic factors responsible for variation in bone density and ultrasound

Although genetic factors are thought to explain a large proportion of the variation in bone density in women, few studies have been conducted in men. Therefore, it is unclear whether the individual differences in bone strength between men and women are a reflection of gender differences in the relative influence of genetic and environmental factors on bone density variance. The aim of this study was to determine if there were gender differences in the genetic components of variance for bone density and ultrasound. In addition, the study aimed to explore the hypothesis that there are unique gender-specific genetic determinants of these traits. Bone mineral density (BMD) of the hip, distal forearm, and lumbar spine were measured by dual-energy X-ray absorptiometry (DXA) as well as quantitative ultrasound (QUS) at the calcaneus in healthy female twin pairs (286 identical [MZ] and 265 nonidentical [DZ]), male twin pairs (72 MZ and 65 DZ), and 82 opposite-sex (OS) pairs aged between 18 and 80 years. For hip BMD, distal forearm, and QUS measurements, the differences between MZ correlations and like-sex DZ correlations were similar for both sexes, suggesting little difference in the component of total variance explained by genetic factors between male and female twin pairs. However, correlations between OS twin pairs were lower than that of like-sex twin pairs, suggesting the possibility of unique gender-specific genetic effects. At the forearm, model fitting suggested a small gender difference in the magnitude of genetic variance as well as the presence of a unique gender-specific genetic variance component. Hip, lumbar spine, and QUS measurements were better explained by models that assumed no gender differences in genetic variance between the sexes, but the study had insufficient power to detect small differences in the genetic components of variance. The results of this study suggest that the proportion of bone strength variance explained by genetic factors is similar for men and women. However, at some regions there is evidence to suggest a gender-specific genetic component to the overall genetic variance.

Tests of linkage and/or association of genes for vitamin D receptor, osteocalcin, and parathyroid hormone with bone mineral density

Bone mineral density (BMD) is a major determinant of osteoporotic fractures (OFs). The heritability of BMD ranges from 50% to 90% in human populations. Extensive molecular genetic analyses have been performed through traditional linkage or association approaches to test and identify genes or genomic regions underlying BMD variation. The results, particularly those concerning the vitamin D receptor (VDR) gene, have been inconsistent and controversial. In this study, we simultaneously test linkage and/or association of the genes for VDR, osteocalcin (also known as bone Gla protein [BGP]), and parathyroid hormone (PTH) with BMD in 630 subjects from 53 human pedigrees. Each of these pedigrees was ascertained through a proband with an extreme BMD value at the hip or spine (Z score < or = -1.28). For the raw BMD values, adjusting for significant covariate effects of age, sex, and weight, we performed tests for linkage alone, association alone, and then both linkage and association. For the spine BMD, at the two markers (ApaI and FokI) inside the VDR gene we found evidence for linkage (p < 0.05) and for both linkage and association by the transmission disequilibrium test (TDT; p < 0.05); association was detected (p < 0.07) with regular statistical testing by analyses of variance (ANOVA). In addition, significant results were found for association alone (p < 0.05), linkage alone (p = 0.0005), and for linkage and association (p = 0.0019) for the intragenic marker HindIII of the BGP gene for the hip BMD. Through testing for association, linkage, and linkage and association simultaneously, our data support the VDR gene as a quantitative trait locus (QTL) underlying spine BMD variation and the BGP gene as a QTL underlying hip BMD variation. However, our data do not support the PTH gene as a QTL underlying hip or spine BMD variation. This is the first study in the broad field of bone genetics that tests candidate genes as QTLs for BMD by testing simultaneously for association alone, for linkage alone, and for association and linkage (via the TDT).

Two new single-nucleotide polymorphisms in the COL1A1 upstream regulatory region and their relationship to bone mineral density

Single-nucleotide polymorphisms (SNPs) in regulatory regions of candidate genes may determine variability in bone mineral density (BMD) because they may be responsible for differences in levels of a gene product in response to external signals. Under this hypothesis, we scanned an 800-base pair (bp) region within the COL1A1 promoter, known to harbor cis elements important for in vivo expression, and we found two new polymorphisms: -1663indelT and -1997 G/T. The G to T transversion at -1997 was associated with lumbar spine BMD (p = 0.015) when tested in a cohort of 256 postmenopausal women after adjusting by age, body weight, and years since menopause; a lower degree of association was detected also for femoral neck BMD in a subgroup of 146 women in univariate analysis and after adjusting by age (p = 0.044). The polymorphism -1663indelT, which corresponds to a deletion of a T in a tract of eight T residues (-1670 to -1663), did not show significant association with BMD. Interestingly, -1663indelT is in strong linkage disequilibrium (LD) with the previously described Sp1 polymorphism of intron 1, which in this study did not show association with BMD either. Significant interaction between -1997 G/T and -1663indelT (p = 0.019), and between -1997 G/T and Sp1 (p = 0.045) was observed also. Individuals heterozygous for the three polymorphisms showed the highest mean BMD value. Gel retardation assays showed that oligonucleotides containing either the -1663 or the -1997 polymorphic sites specifically bind primary osteoblast nuclear proteins. We named these binding sites as PCOL1 and PCOL2, respectively. In summary, this study describes two new SNPs in the COL1A1 promoter, which may affect bone mass determination.

Role of genetics in osteoporosis

Osteoporosis is a disease characterized by fragile bones and high susceptibility to low-trauma fractures. It is a serious health problem, especially in elderly women. Bone mineral density (BMD) has been employed most commonly as the index for defining and studying osteoporosis. BMD has high genetic determination, with heritability ranging from 50 to 90%. Various gene-mapping approaches have been applied to identify specific genes underlying osteoporosis, largely using BMD as the study phenotype. We review here the genetic determination of osteoporosis as defined by BMD and discuss a fundamental issue we encounter in genetic research in osteoporosis: the choice of phenotype(s) to study. We briefly summarize and discuss advantages and disadvantages of various approaches used in genetic studies of osteoporosis. Finally, we review and discuss the current status for mapping and identification of genes for osteoporosis. We focus on linkage studies in humans and quantitative trait loci mapping in mice to supplement the already extensive reviews of association studies made by many investigators for candidate genes.

Relevance of the genes for bone mass variation to susceptibility to osteoporotic fractures and its implications to gene search for complex human diseases

We investigate the relevance of the genetic determination of bone mineral density (BMD) variation to that of differential risk to osteoporotic fractures (OF). The high heritability (h(2)) of BMD and the significant phenotypic correlations between high BMD and low risk to OF are well known. Little is reported on h(2) for OF. Extensive molecular genetic studies aimed at uncovering genes for differential risks to OF have focussed on BMD as a surrogate phenotype. However, the relevance of the genetic determination of BMD to that of OF is unknown. This relevance can be characterized by genetic correlation between BMD and OF. For 50 Caucasian pedigrees, we estimated that h(2) at the hip is 0.65 (P < 0.0001) for BMD and 0.53 (P < 0.05) for OF; however, the genetic correlation between BMD and OF is nonsignificant (P > 0.45) and less than 1% of additive genetic variance is shared between them. Hence, most genes found important for BMD may not be relevant to OF at the hip. The phenotypic correlation between high BMD and low risk to OF at the hip (approximately -0.30) is largely due to an environmental correlation (rho(E) = -0.73, P < 0.0001). The search for genes for OF should start with a significant h(2) for OF and should include risk factors (besides BMD) that are genetically correlated with OF. All genes found important for various risk factors must be tested for their relevance to OF. Ideally, employing OF per se as a direct phenotype for gene hunting and testing can ensure the importance and direct relevance of the genes found for the risk of OF. This study may have significant implications for the common practice of gene search for complex diseases through underlying risk factors (usually quantitative traits).

Bone adaptation response to sham and bending stimuli in mice

This study presents inbred-strain-related differences in tibial bone adaptation response to low-force loading in four-point bending and sham (pad pressure) arrangements in mice. Our previous work in mice has shown that at relatively high but equal bending forces (9 N or a bending moment of 16.88 N-mm), C57BL/6J mice respond with significantly greater bone formation than C3H/HeJ mice. Because of high tibial strains, the majority of the bone response in our previous study was woven bone. In this, study, we reduced the loading forces to 5 N or a bending moment of 9.38 N-mm (to decrease the woven-bone formation response) and investigated inbred-strain-related bone adaptation differences resulting from bending and sham loading (reported here for the first time in C57BL/6J) in these mice. Twenty-four female mice within each inbred mouse strain (C3H/HeJ [C3H] and C57BL/6J [B6]) were randomly divided into the two loading groups (12 per group sham and bending, total of 48 mice). All of the external loading was done for 36 cycles at 2 Hz, 3 d/wk for 3 wk. The bone adaptation response at lower forces exhibited a pattern similar to that seen for the higher forces in the previous study, suggesting that the patterns of bone adaptation were inbred strain related and independent of bending force magnitude. The bending-related periosteal mineral apposition surface (pMS) and mineral apposition rate (MAR) were respectively 40% and 45% greater in B6 than in C3H. The cortical bone adaptation response to bending was greater when compared to sham or pad pressure for each inbred strain of mice, suggesting that the majority of the bone adaptation response was the result of bending stimulus and not local pressure from pad contact. In addition, regardless of loading arrangement (sham or bending), the bone adaptation response in C57BL/6J mice was greater than C3H/HeJ.

Complex segregation analysis of the radiographic phalanges bone mineral density and their age-related changes

The complex segregation analyses performed in our previous studies revealed a significant major gene (MG) effect on the age-adjusted cortical and cancellous bone mineral density (BMD) in two ethnically different populations, Chuvasha and Turkmenians. The aim of the present study was to test the hypothesis of pleiotropic MG control of three components of bone aging, that is, the baseline level of BMD (mu(gs)), the age at onset of the bone mass loss (T(gs)), and the rate of this loss over the years (alpha(gs)). Nuclear and more complex pedigrees from the same two ethnic samples were assessed for hand phalangeal BMD (Chuvasha, 1208 individuals, and Turkmenians, 643 individuals), and complex segregational analysis incorporating age and sex effects directly into MG penetrance function was carried out. The results of the present analysis clearly confirmed the existence of the putative MG and showed that the proportion of BMD variation attributable to this MG effect within the sex was remarkably similar in both populations and ranged between 34.7% and 35.2%. The most parsimonious model for BMD transmission in Chuvasha pedigrees additionally indicated significant residual correlation between siblings and clear sex differences in the annual rates of bone loss alpha(gs). The latter was more than twice as high in females than that in males (0.086 SD vs. 0.033 SD per year). In Turkmenian pedigrees the most parsimonious model presented obvious evidence of the MG control of BMD baseline levels in both sexes with significantly lower baseline levels and younger age at onset (T(gs)) in females. No clear MG effects were inferred on T(gs) and/or alpha(gs) in either sample, either in males or in females. That is, the present study does not suggest MG x SEX x AGE interaction. We suppose that if the rate of age-related changes in phalangeal BMD is genetically determined, then these are not the same genes as those affecting the BMD baseline levels.

Familial resemblance of bone mineral density between females 18 years and older and their mothers

Potential determinants of bone mass were investigated in a group of 70 young females (mean age 26.6 years), daughters of women studied in premenopause. Nutritional data, leisure physical activity level, lifestyle habits as well as familial similarities were assessed. The daughters' bone mineral density (BMD), measured by dual-energy absorptiometry, was significantly correlated with their body mass index (BMI) (r = 0.22), dietary vitamin D intake (r = 0.19) and their mothers' BMD (r = 0.44). Multiple regression analysis indicated that only the mothers' BMD remained an independent predictor of bone mass. Mother-daughter correlations were also observed for body weight (r = 0.24), height (r = 0.39), BMI (r = 0.29), dietary calcium intake (r = 0.20), and calcium (r = 0.20) or vitamin D (r = 0.25) intakes from dairy products. Hence, these observations support the evidence that mothers' BMD is the strongest predictor of bone mass of young women in their third decade.

Bone mass and lifetime physical activity in Flemish males: a 27-year follow-up study

PURPOSE

The present population study is conducted to examine the extent to which lifetime physical activity and lifestyle parameters contribute to bone mass.

METHODS

The design of the project is a 27-yr prospective follow-up study. Subjects are 126 males gathered from the Leuven Longitudinal Study on Lifestyle, Physical Fitness and Health, and aged 13 yr at the onset of the study and 40 at the end of the follow-up. Physical activity and lifestyle parameters are obtained with questionnaires. Bone mass is measured by means of dual-energy x-ray absorptiometry (DXA).

RESULTS

Results from correlation and regression analyses show that the body mass index (BMI) is the most important parameter in relation to cortical and trabecular bone mass at every examination period. Longitudinally, static arm strength, running speed, and upper muscular endurance contribute significantly to the prediction of adult bone mass. The parameter "change in motor fitness" between 18 and 13 yr old was used to control for hereditary influences. The score for static arm strength and trunk muscle strength demonstrates a significant correlation with adult total bone mineral content (BMC) and lumbar bone mineral density (BMD), respectively. At the age of 40, the Baecke sports index is almost equally important as BMI in explaining the variance in BMD, and static arm strength is the most important parameter (after BMI) for BMC.

CONCLUSION

Lifetime physical activity, physical fitness, and BMI all contribute to adult bone mass. The clinical relevance of these findings is emphasized by the fact that the observed patterns of physical activity and motor fitness pertain to customary lifestyle and are thus feasible targets.

Leisure-time physical activity and rate of bone loss among peri- and postmenopausal women: a longitudinal study

We examined the association between continuous leisure-time physical activity and the change in bone mineral density (BMD) and bone mineral content (BMC) in a population-based random sample of 1873 peri- and postmenopausal women. Leisure-time physical activities were registered with self-administered questionnaires in 1989 and 1994, and with an assisted questionnaire in 1995-1997. BMD and BMC were measured from lumbar vertebrae L2-4 and left femoral neck using dual-energy X-ray absorptiometry (DXA) in 1989-1991 and 1994-1997. During the average 5.6 year follow-up, annual loss of lumbar BMC was 124 mg (311 vs. 435 mg, p = 0.036) and annual loss of lumbar BMD was 1.22 mg/cm(2) (4.15 vs. 5.37 mg/cm(2), p = 0.21) smaller among women with regular (at least 1 h each week) weight-bearing leisure-time exercise compared with sedentary women. The advantage was even larger in women with walking or jogging as their only regular weight-bearing leisure-time exercise; that is, their annual loss of lumbar BMC was 180 mg (272 vs. 452 mg, p = 0.022), and annual loss of lumbar BMD was 2.78 mg/cm(2) (2.96 vs. 5.74 mg/cm(2), p = 0.029) smaller than in sedentary women. Continuous leisure-time physical activity did not have any association with loss of BMC or BMD in the femoral neck Physical activity during 12 months before the last bone densitometry was not associated with loss of BMC or BMD at any site. Our results suggest that regular weight-bearing exercise diminishes lumbar bone loss, but might be ineffective in the prevention of femoral osteoporosis among peri- and early postmenopausal women.

Evidence for common controls over inheritance of bone quantity and body size from segregation analysis in a pedigreed colony of nonhuman primates (Macaca nemestrina)

The genetic determinants of bone mineral quantity and body size and their postulated interaction are just beginning to be elucidated. The heritability of bone quantity and its relationship to components of body size were therefore investigated using segregation analysis applied to a large pedigreed nonhuman primate (Macaca nemestrina) breeding colony. The colony consisted of 216 females and 16 males with uniform dietary histories, environmental conditions, and rearing of offspring apart from the mother to minimize familial aggregation. Bone quantity (bone mineral content and spinal areal density) was measured by dual-energy X-ray absorptiometry (DXA). Size included measures of body mass, length, breadth, and a composite index. Body mass was determined from both body weight and lean body mass by DXA. Length was assessed by measuring trunk and thigh lengths, and breadth by measuring chest circumference and bitrochanteric width. A composite index of size was also calculated from a linear function of trunk and thigh lengths, chest circumference and bitrochanteric width, and lean body mass. Traits of bone quantity and size were highly correlated (r = 0.56-0.96, p < 0.001). Significant (p < or = 0.03) univariate heritabilities were found for spine bone mineral density (SPBMD; h(2) = 0.66) and whole body bone mineral content (WBBMC; h(2) = 0.40) and size measures of length (trunk h(2) = 0.71, thigh h(2) = 0.65), breadth (bitrochanteric width h(2) = 0.31), lean body mass (LEAN; h(2) = 0.37), and the composite index of size (SIZE-PC, h(2) = 0.49) adjusted for demographic variables. The data were also subjected to an analysis of bivariate genetic correlations and factor analysis, both of which suggested a robust interaction between body size and bone quantity. Bivariate genetic correlations between body size and the bone quantities WBBMC, SBMD, and spine bone mineral content (SPBMC) were high (e.g., using LEAN as a measure of size, r = 0.57, 0.41, and 0.57, respectively). Factor analysis showed that 80% of the phenotypic and 72% of the genetic variances of all traits were accounted for by a single factor, suggesting common genetic controls operative over bone quantity and size.

Quantitative ultrasound at the calcaneus in premenopausal women and their postmenopausal mothers

The aim of the study was to establish a relationship between mothers' and daughters' bone status. Forty-eight postmenopausal women and their 48 premenopausal daughters were evaluated. The analysis was made for the whole group and for two subgroups: 27 healthy mothers and their 27 daughters; and 21 fractured mothers and their 21 daughters. The subgroups were matched for age and years since menopause (YSM), and height, weight, and body mass index (BMI) did not differ significantly. Bone status was evaluated by ultrasound measurement at the heel using the Achilles system (Lunar, Madison, WI), which measures speed of sound (SOS [m/sec]) and broadband ultrasound attenuation (BUA [dB/MHz]). The Achilles software also calculates a stiffness index (SI [%]). Ultrasound values for BUA, SOS, stiffness index, and Z score were significantly lower both in mothers with previous fractures and in their daughters, compared with respective values in mothers without fractures and their daughters. Future values in daughters were predicted using a stepwise, multiple regression analysis separately in the whole group and in the two subgroups. Future values were predicted in two models taking into consideration mothers' present SOS, BUA, and age or present SOS, BUA, and YSM. In both models, daughters' present SOS, BUA, age, height, and weight were taken into consideration. Predictive values were found to be high for daughters of women having had fractures (r = 0.72-0.87, p = 0.015-0.00007, SEE = 6.0-15.8) and lower for all daughters studied (r = 0.38-0.62, p = 0.03-0.0001, SEE = 8.8-21.5). In daughters of mothers without past fractures, prediction was not possible. Heritability of ultrasound values in daughters of women with past fractures ranged between 52% and 76%, whereas in the whole group the range was 14%-40%. In conclusion, the data indicate that, as a group, the daughters of women with osteoporotic fracture are likely to be at an increased risk for fractures because they have relatively low ultrasound values. Their future ultrasound values can be predicted on the basis of a single ultrasound evaluation with the condition that there is a history of maternal past fracture.

Bone strength and related traits in HcB/Dem recombinant congenic mice

Fracture susceptibility depends jointly on bone mineral content (BMC), gross bone anatomy, and bone microarchitecture and quality. Overall, it has been estimated that 50-70% of bone strength is determined genetically. Because of the difficulty of performing studies of the genetics of bone strength in humans, we have used the HcB/Dem series of recombinant congenic (RC) mice to investigate this phenotype. We performed a comprehensive phenotypic analysis of the HcB/Dem strains including morphological analysis of long bones, measurement of ash percentage, and biomechanical testing. Body mass, ash percentage, and moment of inertia each correlated moderately but imperfectly with biomechanical performance. Several chromosome regions, on chromosomes 1, 2, 8, 10, 11, and 12, show sufficient evidence of linkage to warrant closer examination in further crosses. These studies support the view that mineral content, diaphyseal diameter, and additional nonmineral material properties contributing to overall bone strength are controlled by distinct sets of genes. Moreover, the mapping data are consistent with the existence of pleiotropic loci for bone strength-related phenotypes. These findings show the importance of factors other than mineral content in determining skeletal performance and that these factors can be dissected genetically.