Several genomic regions potentially containing QTLs for bone size variation were identified in a whole-genome linkage scan

Linkage and association analyses using families identified a locus affecting an osteoporosis-related trait

Osteoporosis is a common disorder characterized by low bone mass and microarchitectural deterioration of bone tissue, resulting in an increase in bone fragility and in susceptibility to fractures. The genetic basis of osteoporosis is complex and involves multiple genes and environmental factors. Here we introduce a family-based study of the genetics of osteoporosis - the Genetic Analysis of Osteoporosis (GAO) Project - to discover genetic variants affecting osteoporosis-related phenotypes. The GAO Project involved 11 extended families from Barcelona, Spain selected through a proband with osteoporosis (N=367). We performed spine, femur and whole body densitometry for all participants and also analyzed strength and geometrical properties of the hip. Our study focused on 23 densitometric phenotypes that we considered of high clinical relevance and four definitions of low bone mass and fracture status. Pedigree validation was carried out through microsatellite genotyping. The same microsatellites were used to interrogate our data (i) for the replication of previous linkage signals and (ii) for the potential discovery of new linkage signals. The linkage analysis identified one region marked by microsatellite D17S787 showing a strong and significant signal of linkage with femoral shaft cross-sectional moment of inertia (CSMI; LOD=3.18; p=6.5×10(-5)). The chromosomal location marked by microsatellite D17S787 includes several genes, among which two are of particular interest: COL1A1 and SOST, coding for collagen alpha-1 (I) chain and sclerostin, respectively. Follow-up association analysis resulted in only one significant result for rs4792909 from the SOST genomic region (p=0.00248). As a result, we provide strong and significant evidence from both linkage and association analyses that the SOST gene may affect the strength of the femoral shaft. Future investigations should study the relationship between bone mass formation and strength properties of the bones.

Genetics of osteoporosis

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

Sexual dimorphism of the arm bones in a modern greek population

Several studies have shown that sex determination methods based on measurements of the skeleton are population specific. Metric traits of the long bones of the arm have been reported as reliable indicators of sex. This study was designed to determine whether the three long bones of the arm can be used for sex determination on a skeletal population from Greece. The material used consists of the arm bones of 204 adult individuals (111 males and 93 females) coming from the Modern Human Skeletal Collection of the University of Athens. The age range is 19-96 years for males and 20-99 years for females. The maximum lengths and epiphyseal widths were measured in the long bones of the arm (humerus, radius, and ulna). The discriminant analysis of the metrical data of each long bone gave very high discrimination accuracies. The rate of correct sex discrimination based on different long bones ranges from 90.30% (ulna) to 95.70% (humerus). In addition, intra- and inter-observer error tests were performed. These indicated that replication of measurements was satisfactory for the same observer over time and between observers. The results of this study show that metric characteristics of the arm bones can be used for the determination of sex in skeletal remains from Greece and that bone dimensions are population specific.

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

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

Bivariate genome linkage analysis suggests pleiotropic effects on chromosomes 20p and 3p for body fat mass and lean mass

Total body fat mass (TBFM) and total body lean mass (TBLM) are the major components of the human body. Although these highly correlated phenotypic traits are frequently used to characterize obesity, the specific shared genetic factors that influence both traits remain largely unknown. Our study was aimed at identifying common quantitative trait loci (QTLs) contributing to both TBFM and TBLM. We performed a whole genome-linkage scan study in a large sample of 3255 subjects from 420 Caucasian pedigrees. Bivariate linkage analysis was carried out in both the entire sample and gender-specific subsamples. Several potentially important genomic regions that may harbour QTLs important for TBFM and TBLM were identified. For example, 20p12-11 achieved a LOD score of 2.04 in the entire sample and, in the male subsample, two genomic regions, 20p12 (LOD=2.08) and 3p26-25 (LOD=1.92), showed suggestive linkage. In addition, two-point linkage analyses for chromosome X showed suggestive linkages on Xp22 in the entire sample (LOD=2.14) and significant linkage on Xp22 in the female subsample (LOD=3.05). Complete pleiotropy was suggested for 20p12 and 3p26-25 in males. Our results suggest that QTLs on chromosomes 20p12, 3p26-25 and Xp22 may jointly influence TBFM and TBLM. Further fine mapping and gene identification studies for these pleiotropic effects are needed.

Evidence for major pleiotropic effects on bone size variation from a principal component analysis of 451 Caucasian families

AIM

To identify pleiotropic quantitative trait loci (QTL) influencing bone size (BS) at different skeletal sites in Caucasians.

METHODS

In a sample containing 3899 Caucasians from 451 pedigrees, 410 microsatellite markers spaced approximately 8.9 cM apart across the human genome were genotyped. Phenotypical and genetic correlations of BS at lumbar spine, hip (femoral neck, trochanter, and intertrochanter regions), and wrist (ultradistal, mid-distal, and one-third distal sites) were determined using bivariate quantitative genetic analysis. A principal component analysis (PCA) was performed to obtain principal component (PC) factors that were then subjected to variance components linkage analysis to identify regions linked to the PC.

RESULTS

Genetic correlations of BS at different skeletal sites ranged from 0.40 to 0.79 (P<0.001). The PCA yielded a PC named PCtotal, which explained up to 76% of the total (co)variation of all the BS at the 7 skeletal sites for the whole sample. We identified a QTL influencing the BS of multiple skeletal sites on chromosome 7 at 140 cM [logarithm of odds (LOD)=2.85] in the overall sample. Sex-specific evidence for linkage was observed on chromosome 11 at 53 cM (LOD =2.82) in the male-only data subset.

CONCLUSION

Our study identified several genomic regions that may have pleiotropic effects on different skeletal sites. These regions may contain genes that play a critical role in overall bone development and osteoporosis at multiple skeletal sites, hence are biologically and clinically important.

Genetic dissection of mouse distal chromosome 1 reveals three linked BMD QTLs with sex-dependent regulation of bone phenotypes

Genetic analyses with mouse congenic strains for distal Chr1 have identified three closely linked QTLs regulating femoral vBMD, mid-diaphyseal cortical thickness, and trabecular microstructure in a sex-dependent fashion. The homologous relationship between distal mouse Chr 1 and human 1q21-24 offers the possibility of finding common regulatory genes for cortical and trabecular bone.

INTRODUCTION

The distal third of mouse chromosome 1 (Chr 1) has been shown to carry a major quantitative trait locus (QTL) for BMD from several inbred mouse strain crosses. Genetic and functional analyses are essential to identify genes and cellular mechanisms for acquisition of peak bone mass.

MATERIALS AND METHODS

Nested congenic sublines of mice were developed with a C57BL/6J (B6) background carrying <1- to 9-Mbp-sized segments donated from C3H/HeJ (C3H). Isolated femurs from 16-wk-old female and male mice were measured by pQCT and microCT40 for volumetric (v)BMD, mid-diaphyseal cortical thickness, and distal trabecular phenotypes. Static and dynamic histomorphologic data were obtained on selected females and males at 16 wk.

RESULTS AND CONCLUSIONS

We found that the original BMD QTL, Bmd5, mapped to distal Chr 1 consists of three QTLs with different effects on vBMD and trabecular bone in both sexes. Compared with B6 controls, femoral vBMD, BMD, and cortical thickness (p < 0.0001) were significantly increased in congenic subline females, but not in males, carrying C3H alleles at QTL-1. Both females and males carrying C3H alleles at QTL-1 showed marked increases in BV/TV by microCT compared with B6 mice (p < 0.0001). Females increased BV/TV by increasing trabecular thickness, whereas males increased trabecular number. In addition, the microCT40 data showed two unique QTLs for male trabecular bone, QTL-2 and QTL-3, which may interact to regulate trabecular thickness and number. These QTLs are closely linked with and proximal to QTL-1. The histomorphometric data revealed sex-specific differences in cellular and bone formation parameters. Mice and humans share genetic homology between distal mouse Chr 1 and human Chr 1q20-24 that is associated with adult human skeletal regulation. Sex- and compartment-specific regulatory QTLs in the mouse suggest the need to partition human data by sex to improve accuracy of mapping and genetic loci identification.

The genetic, environmental and phenotypic correlations of bone phenotypes at the spine and hip in Chinese

BACKGROUND

Bone mineral density (BMD), bone mineral content (BMC), and bone size have been widely studied individually as important risk factors for osteoporotic fracture, but little is known about the correlation and the degree of sharing genetic and environmental factors between the pairs of the three phenotypes.

AIM

The study investigated genetic correlation (rhoG), environmental correlation (rhoE) and phenotypic correlation (rhoP) between BMD, BMC and bone size.

SUBJECTS AND METHODS

Bivariate variance decomposition analyses were performed in 904 subjects from 287 Chinese nuclear families.

RESULTS

Significant rhoE, rhoG and rhoP were detected between BMD, BMC and bone size, except for rhoE between BMD and bone size at the hip (rhoE = 0.121, p = 0.361). Common shared genetic factors explained 86.1% and 60% of BMD and BMC genetic variations at the spine and hip, respectively. However, the genetic and environmental correlations between BMD and bone size were limited. rhoE and rhoG at the spine were 0.392 and 0.381, and at the hip were 0.121 and -0.205, respectively. Only 14.5% and 4.2% of variations between BMD and bone size at the spine and hip may be due to the shared genetic factors.

CONCLUSION

The obtained results suggested that bone size may be used as another surrogate phenotype independently of BMD for eventual elucidation of the pathogenesis of osteoporosis because of the limited correlations between BMD and bone size.

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.

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.

A genomewide scan for quantitative trait loci underlying areal bone size variation in 451 Caucasian families

BACKGROUND

Bone size is an important determinant of bone strength and is under strong genetic control.

OBJECTIVE

To identify quantitative trait loci (QTL) for areal bone size variation, a large-scale genomewide linkage scan was carried out in 451 Caucasian families.

PARTICIPANTS AND METHODS

Of 4124 people with phenotypes, 3899 were genotyped with 410 microsatellite markers. Multipoint linkage analyses were carried out in the entire sample, as well as in men and women separately. Potential epistatic interactions between identified genomic regions were also assessed.

RESULTS

Several potentially important genomic regions were identified, such as 8q24 for hip bone size (logarithm of the ratio of the odds that two loci are linked (LOD) 3.27) and 2p24 (LOD 2.04) for spine bone size. 8q24 may also interact with 19p13 to affect hip bone size. Several sex-specific QTL were also detected, such as 14q21 (LOD 2.94) for wrist bone size in women and 16q12 (LOD 2.19) for hip bone size in men.

CONCLUSIONS

Together with previous findings, this study has further delineated the genetic basis of bone size and laid a foundation for future studies to eventually elucidate the mechanisms of bone size regulation and associated fracture risks.

Strong association between polymorphisms in ANKH locus and skeletal size traits

Loss of bone strength is the main determinant of bone fragility. Bone strength is directly dependent on bone size (BS). A substantial portion of BS variation is attributable to genetic effects. However, the list of genes and allelic variants involved in the determination of BS variation is far from being complete. Polymorphisms in the ANKH gene have been shown to be associated with radiographic hand BS-related phenotypes. The present study examined the possible association of the ANKH gene with skeletal size and shape in order to test the universality of the ANKH effect on BS traits. Our sample consisted of a total of 212 ethnically homogeneous nuclear families (743 individuals) of European origin. Each individual was measured for body height, weight, and several other anthropometrical measurements, and genotyped for nine polymorphic markers (the average heterozygosity level was 0.4). We observed significant associations with practically all the anthropometrical phenotypes studied. More specifically, we found associations with body weight and height, limb length (P</=0.001; promoter region). After adjustment for body height, we demonstrated the substantial association increase for biacromial breadth (P=0.0012; some 140 kb downstream from ANKH) and vertebral column length (P=0.0008; exons 2-7 region). The majority of the observed associations persisted even after correction for multiple testing. For the first time the reliable evidence in support of universality of ANKH gene polymorphisms effect on bone size was provided.

Linkage exclusion mapping with bone size in 79 Caucasian pedigrees

Bone size is an important risk factor of osteoporotic fractures and has strong genetic determination. However, genetic studies on bone size variation are relatively rare. In the present study, we conducted a linkage exclusion mapping for bone size variation on chromosomes 1, 4, 6, and 17 in 79 Caucasian pedigrees. For hip bone size variation, several genomic regions were excluded at effect sizes of 10% or greater, including regions of 61-77cM at 1p35-p34, 43-59cM at 4p15-p13, 1-59cM at 6p25-p21, 82-88cM at 17q23-q24, and 113-114cM at 17q25. For spine bone size, at effect sizes of 10% or greater, we excluded regions of 115-122cM at 1p31-p22, 136-141cM at 1p21, 207-260cM at 1q31-q42, 20-89cM at 4p16-q21, 11-21cM at 6p24-p23, and 1-6cM at 17p13. These results suggested that a number of candidate genes located in the excluded regions, such as the growth hormone (GH) gene, tumor necrosis factor-alpha (TNF-alpha) gene, and bone morphogenetic protein-3 (BMP3) gene, are unlikely to have a substantial effect on bone size variation in this Caucasian population.

Bone mass and strength: phenotypic and genetic relationship to alcohol preference in P/NP and HAD/LAD rats

BACKGROUND

The association between moderate alcohol intake and elevated bone mineral density observed in several epidemiologic studies might result from common genetic pathway regulating both phenotypes. In this study, we determined whether there is a relationship between alcohol preference and high bone mass or strength and whether bone mass-regulating genes segregate during selective breeding of alcohol preferring rats.

METHODS

Six different lines of male rats with high or low preference for alcohol consumption were used in this study. The high alcohol preference lines are alcohol-preferring (P), high-alcohol-drinking 1 (HAD1), and high-alcohol-drinking 2 (HAD2), and their corresponding low alcohol preference lines are alcohol-nonpreferring (NP), low-alcohol-drinking 1 (LAD1), and low-alcohol-drinking 2 (LAD2). Bone mass phenotypes were determined using dual energy x-ray absorptiometry (DXA), peripheral quantitative computed tomography (pQCT), and biomechanics in long bones and lumbar vertebrae from rats at 3 and 6 months of age.

RESULTS

P rats had significantly higher bone mass and strength compared with NP rats, mainly due to higher cortical bone in long bones and lumbar vertebrae. HAD2 rats also had significantly higher bone mass compared with LAD2 rats, but mostly due to increased trabecular bone leading to increased strength only in lumbar vertebra. Conversely, HAD1 rats had significantly lower bone mass and strength compared with LAD1 rats in long bones. The vertebral bone mass and strength did not differ between HAD1 and LAD1 rats.

CONCLUSIONS

This study demonstrated that preference for alcohol consumption had no consistent relationship with high bone mass or strength, as each alcohol-preferring rat line had their unique bone mass phenotypes. However, genes regulating bone mass and strength appear to segregate with alcohol preference genes in P and HAD rat lines, suggesting that alcohol preferring rat lines may be useful for identifying genes that regulate bone mass and structure.

Mapping quantitative trait loci for cross-sectional geometry at the femoral neck

A genome-wide linkage scan was performed in a sample of 79 multiplex pedigrees to identify genomic regions linked to femoral neck cross-sectional geometry. Potential quantitative trait loci were detected at several genomic regions, such as 10q26, 20p12-q12, and chromosome X.

INTRODUCTION

Bone geometry is an important determinant of bone strength and osteoporotic fractures. Previous studies have shown that femoral neck cross-sectional geometric variables are under genetic controls. To identify genetic loci underlying variation in femoral neck cross-sectional geometry, we conducted a whole genome linkage scan for four femoral neck cross-sectional geometric variables in 79 multiplex white pedigrees.

MATERIALS AND METHODS

A total of 1816 subjects from 79 pedigrees were genotyped with 451 microsatellite markers across the human genome. We performed linkage analyses on the entire data, as well as on men and women separately.

RESULTS

Significant linkage evidence was identified at 10q26 for buckling ratio (LOD = 3.27) and Xp11 (LOD = 3.45) for cortical thickness. Chromosome region 20p12-q12 showed suggestive linkage with cross-sectional area (LOD = 2.33), cortical thickness (LOD = 2.09), and buckling ratio (LOD = 1.94). Sex-specific linkage analyses further supported the importance of 20p12-q12 for cortical thickness (LOD = 2.74 in females and LOD = 1.88 in males) and buckling ratio (LOD = 5.00 in females and LOD = 3.18 in males).

CONCLUSIONS

This study is the first genome-wide linkage scan searching for quantitative trait loci underlying femoral neck cross-sectional geometry in humans. The identification of the genes responsible for bone geometric variation will improve our knowledge of bone strength and aid in development of diagnostic approaches and interventions for osteoporotic fractures.

Genetics of osteoporosis

Genetic factors play an important role in regulating bone mineral density and other phenotypes relevant to the pathogenesis of osteoporosis such as ultrasound properties of bone, skeletal geometry, and bone turnover. Progress has been made in identifying quantitative traits for regulation of bone mineral density by linkage studies in man and mouse, but relatively few causal genes have been identified. Dramatic progress has been made in identifying the genes responsible for monogenic bone diseases and it appears that polymorphisms in many of these genes also play a role in regulating bone mineral density in the general population. Advances in knowledge about the genetic basis of osteoporosis and other bone diseases offer the prospect of developing new markers for assessment of fracture risk and the identification of novel molecular targets for the design of new drug treatments for osteoporosis.

Identification of novel genetic loci for bone size and mechanosensitivity in an ENU mutant exhibiting decreased bone size

Using a dominant ENU mutagenesis screen in C57BL/6J (B6) mice to reveal gene function, we identified a mutant, 917M, with a reduced bone size phenotype, which is expressed only in males. We show that mutation results in osteoblasts with reduced proliferation, increased apoptosis, and an impaired response to in vitro mechanical load. The mutation is mapped to a novel locus (LOD score of 7.9 at 10.5 cM) on chromosome 4.

INTRODUCTION

Using a dominant ENU mutagenesis screen in C57BL/6J (B6) mice to reveal gene function, we identified a mutant, 917M, with a reduced bone size phenotype, which is expressed only in males. In this report, we show the chromosomal location of this mutation using linkage analysis and cellular characterization of the mutant phenotype.

MATERIALS AND METHODS

The mutant mouse was bred to wildtype B6 to produce progeny for characterization of the bone size phenotype. Periosteal osteoblasts isolated from the tibia and femur of mutant and wildtype mice were studied for proliferation, differentiation, and apoptosis potential. To determine the chromosomal location of the mutation, a low-resolution linkage map was established by completing a genome-wide scan in B6C3H F2 male mice generated from intercross breeding of mutant mice.

RESULTS AND CONCLUSIONS

Mutant progeny (16 weeks old) displayed a total body bone area that was 10-13% lower and a periosteal circumference that was 5-8% lower at the femur and tibia midshaft compared with wildtype B6 mice. Periosteal osteoblasts from mutant mice showed 17-27% reduced cell proliferation and 23% increased apoptosis compared with wildtype controls. In addition, osteoblasts from mutant mice showed an impaired response to shear stress-induced proliferation rate, an in vitro model for mechanical loading. Interval mapping in B6C3H F2 males (n = 69) indicated two major loci affecting bone size on chromosome 1 at 45 cM (LOD 4.9) and chromosome 4 at 10.5 cM (LOD 7.9, genome-wide p < 0.01). Interval mapping using body weight as covariate revealed only one significant interval at chromosome 4 (LOD 6.8). Alleles of the chromosome 4 interval inherited from the B6 mutant strain contributed to a significantly lower bone size than those inherited from C3H. A pairwise interaction analysis showed evidence for a significant interaction between loci on chromosome 1 with the chromosome 4 quantitative trait loci. The 917M locus on chromosome 4 seems to be novel because it does not correspond with those loci previously associated with bone size on chromosome 4 in B6 and C3H/HeJ mice or other crosses.

A gene(s) for all-trans-retinoic acid-induced forelimb defects mapped and confirmed to murine chromosome 11

All-trans-retinoic acid (RA) induces various anatomical limb dysmorphologies in mice dependent on the time of exposure. During early limb development, RA induces forelimb ectrodactyly (digital absence) with varying susceptibilities for different inbred mouse strains; C57BL/6N are highly susceptible while SWV are resistant. To isolate the genetic basis of this defect, a full-genome scan was performed in 406 backcross fetuses of F(1) males to C57BL/6N females. Fetuses were exposed via a maternal injection of 75 mg of RA per kilogram of body weight on gestational day 9.25. The genome-wide analysis revealed significant linkage to a chromosome 11 locus near D11Mit39 with a maximum LOD score of 9.0 and to a chromosome 4 locus near D4Mit170. An epistatic interaction was detected between loci on chromosome 11 (D11Mit39) and chromosome 18 (D18Mit64). Linkage to the chromosome 11 locus (D11Mit39) was confirmed in RA-treated backcross fetuses of F(1) females to C57BL/6N males. Loci associated with bone density/mass in both human and mouse were previously detected in the same region, suggesting a mechanistic linkage with bone homeostasis. The human syntenic region of this locus has been previously linked to Meckel syndrome; the phenotype includes postaxial polydactyly, an ectopic digital defect hypothesized to be induced by a common molecular pathway with ectrodactyly.

Association and haplotype analyses of the COL1A2 and ER-alpha gene polymorphisms with bone size and height in Chinese

Bone size (BS) is another risk factor of fracture independent of BMD in determining bone strength, and height is highly related with BS. To test the effect of the estrogen receptor-alpha (ER-alpha) and collagen type I alpha 2 (COL1A2) genes on the variation of BS and height, we genotyped the PvuII and XbaI polymorphisms in the intron 1 of the ER-alpha gene and the MspI and (GT)n markers in the intron 47 and intron 1 of the COL1A2 gene in 400 Chinese nuclear families with a total of 1256 individuals. The BS at the hip and spine was measured using a Hologic QDR 2000 dual-energy X-ray absorptiometry (DXA) scanner. Population stratification, total-family association, and within-family association were used to test the relationship of BS (at the spine and hip) and height with the four polymorphisms. We also performed these association analyses with the haplotypes of the MspI and (GT)n polymorphisms in the COL1A2 gene, and with the haplotypes of the PvuII and XbaI markers in the ER-alpha gene. Weak within-family association was found between the COL1A2-MspI (P = 0.05) and the femoral neck BS, between the ER-alpha-PX (P = 0.04) and the intertrochanter BS, and between the COL1A2-(GT)(17) (P = 0.02), COL1A2-m(GT)(17) (P = 0.009) and height. Subsequent permutation tests generally confirmed the suggestive within-family association. For the weak within-family association, the proportions of phenotypic variance accounted by the COL1A2-MspI, ER-alpha-PX, COL1A2-(GT)(17), COL1A2-m(GT)(17) markers were 1.50%, 1.51%, 2.15%, and 2.43% for the corresponding phenotypes. The association results indicate that the (GT)n and MspI markers of COL1A2 gene may have some influence on the variation of both BS and height, and the XbaI and PvuII markers of ER-alpha gene may have some effect on the variation of height in Chinese but not on the variation of BS.

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

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

Race and sex differences and contribution of height: A study on bone size in healthy Caucasians and Chinese

Osteoporosis is characterized by a loss of bone strength, of which bone size (BS) is an important determinant. However, studies on the factors determining BS are relatively few. The present study evaluated the independent effects of height, age, weight, sex, and race on areal BS at the hip and spine, measured by dual-energy X-ray absorptiometry, while focusing on the differential contributions of height to BS across sex, race, and skeletal site. The subjects were aged 40 years or older, including 763 Chinese (384 males and 379 females) from Shanghai, People's Republic of China, and 424 Caucasians (188 males and 236 females) from Omaha, Nebraska. Basically, Caucasians had significantly larger BS than Chinese. After adjusting for height, age, and weight, the Chinese had similar spine BS, but significantly larger intertrochanter BS in both sexes and larger total hip BS in females compared with Caucasians. Males had significantly larger BS than females before and after adjustment in both ethnic groups. The effects of age, weight, and race varied, depending on skeletal site. As expected, height had major effects on BS variation in both sexes and races. Height tended to account for larger BS variation at the spine than at the hip (except for Chinese females), and larger BS variation in Caucasians than in Chinese of the same sex (except for the trochanter in females). We conclude that height is a major predictor for BS, and its contributions vary across sex, race, and skeletal site.

Genetic variation in femur extrinsic strength in 29 different inbred strains of mice is dependent on variations in femur cross-sectional geometry and bone density

The femurs from groups of mice from 29 different inbred strains were characterized to study the genetic variations in bone parameters. For these analyses, we used peripheral quantitative computed tomography to assess bone size and density in addition to three-point bend testing to assess bone mechanical properties. Highly significant differences between inbred strains were found for all size, density, and mechanical parameters measured (P < 0.0001). Correcting femoral cross-sectional geometry values or bone mechanical properties values for body weight or femur length reduced but did not eliminate the variations in bone geometry or bone mechanical properties. Mice of similar body size had as much as a 40% difference in the midshaft total area of the femur. Regression analysis suggested that 50.9% of the variation in maximum load among strains was related to variations in section modulus, i.e., cross-sectional geometry, 21.5% was related to variations in material bone density, and 27.7% to variations in quality. These components were further analyzed to show that 3.9-27.8% of the variation in maximum load was related to adaptation to mechanical stress. These findings indicate that there is a significant genetic variation in the femur cross-sectional area, density, and mechanical properties between inbred mouse strains. These studies identify inbred mouse strains suitable for future studies identifying genes regulating bone geometry and mechanical properties.

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.

HSV-1 amplicon-mediated transfer of 128-kb BMP-2 genomic locus stimulates osteoblast differentiation in vitro

In previous studies, we developed mouse genetic models and discovered genetic components of quantitative trait loci on mouse chromosomes that contribute to phenotypes such as bone size, bone density, and fracture healing. However, these regions contain dozens of genes in several overlapping bacterial artificial chromosomes (BACs) and are difficult to clone by physical cloning strategies. A feasible and efficient approach of identifying candidate genes is to transfer the genomic loci in BAC clones into mammalian cells for functional studies. In this study, we retrofitted a BAC construct into herpes simplex virus-1 amplicon and packaged it into an infectious BAC (iBAC) to test gene function in a cell-based system, using a 128-kb clone containing the complete bone morphogenetic protein-2 (BMP-2) gene. We transduced MC3T3-E1 cells with the iBAC bearing BMP-2 gene and examined transgene expression and function. Our results have demonstrated that an iBAC can efficiently deliver a BMP-2 genomic locus into preosteoblast cells and express functional BMP-2 protein, inducing a phenotype of cell differentiation, as indicated by an increase in alkaline phosphatase activity. Therefore, this experimental system provides a rapid, efficient cell-based model of high-throughput phenotypic screening to identify the BAC clones from physically mapped regions that are important for osteoblast differentiation. It also illustrates the potential of iBAC technology in functional testing of single nucleotide polymorphisms located in the distal promoter or/and intron regions responsible for low bone density.

Association between COL1A1 gene polymorphisms and bone size in Caucasians

Bone size is an important determinant of bone strength and a risk factor of osteoporotic fracture. Several studies indicate that bone size has a high heritability. Thus, a better understanding of genetic factors regulating bone size might have important clinical implications. In the present study, we examined the relationship between the collagen type I alpha 1 (COL1A1) gene and bone size at the spine, hip and wrist in a sample of 1873 subjects of Caucasian origin from 405 nuclear families. Three single-nucleotide polymorphisms (SNPs) in the COL1A1 gene were analyzed. The minor allele frequencies were 15.4, 18.8, and 1.9% for SNP1, SNP2, and SNP3, respectively. Haplotypes were reconstructed based on the family information as well as marker genotypes using the program Genehunter. We did not find evidence of population stratification, within-family association, or linkage for either single SNPs or haplotypes at any skeletal site. Suggestive evidence of total association was observed for the wrist size at SNP2 (P=0.011). After adjusting age, sex, height, and weight, subjects with the T allele of SNP2 had, on average, 3.05% smaller wrist size than noncarriers. When the subjects were divided into families with only female offspring and families with male offspring only, similar total associations were found at the wrist size for SNP2 with P-values of 0.011 and 0.010, respectively. In conclusion, the COL1A1 gene may have some effects on bone size variation at the wrist, but not at the spine or hip in our Caucasian nuclear families.

Evidence for a major gene underlying bone size variation in the Chinese

Osteoporosis is a major public health problem defined as a loss of bone strength, of which bone size is an important determinant. In the present study, familial correlation and segregation analyses for the spine and hip bone sizes were performed for the first time in a Chinese sample composed of 393 nuclear families with a total of 1,193 individuals. The results indicate a major gene of codominant inheritance for spine bone size; however, there is no evidence of a major gene influencing hip bone size. Significant familial residual effects are found for both traits, suggesting their polygenic inheritance. Heritability estimates (+/-SE) for spine and hip bone size were 0.62 (0.13) and 0.59 (0.12), respectively. Sex and age differences in genotype-specific average bone size were observed. Compared with our previous study on bone mineral density (BMD) in the same population, this study suggests that genetic determination of bone size may be different from that of BMD, and thus studying bone size as one surrogate phenotype for osteoporotic fractures may be necessary.