Congenic strains of mice for verification and genetic decomposition of quantitative trait loci for femoral bone mineral density

Genetic analysis of osteoblast activity identifies Zbtb40 as a regulator of osteoblast activity and bone mass

Osteoporosis is a genetic disease characterized by progressive reductions in bone mineral density (BMD) leading to an increased risk of fracture. Over the last decade, genome-wide association studies (GWASs) have identified over 1000 associations for BMD. However, as a phenotype BMD is challenging as bone is a multicellular tissue affected by both local and systemic physiology. Here, we focused on a single component of BMD, osteoblast-mediated bone formation in mice, and identified associations influencing osteoblast activity on mouse Chromosomes (Chrs) 1, 4, and 17. The locus on Chr. 4 was in an intergenic region between Wnt4 and Zbtb40, homologous to a locus for BMD in humans. We tested both Wnt4 and Zbtb40 for a role in osteoblast activity and BMD. Knockdown of Zbtb40, but not Wnt4, in osteoblasts drastically reduced mineralization. Additionally, loss-of-function mouse models for both genes exhibited reduced BMD. Our results highlight that investigating the genetic basis of in vitro osteoblast mineralization can be used to identify genes impacting bone formation and BMD.

Mouse Models and Online Resources for Functional Analysis of Osteoporosis Genome-Wide Association Studies

Osteoporosis is a complex genetic disease in which the number of loci associated with the bone mineral density, a clinical risk factor for fracture, has increased at an exponential rate in the last decade. The identification of the causative variants and candidate genes underlying these loci has not been able to keep pace with the rate of locus discovery. A large number of tools and data resources have been built around the use of the mouse as model of human genetic disease. Herein, we describe resources available for functional validation of human Genome Wide Association Study (GWAS) loci using mouse models. We specifically focus on large-scale phenotyping efforts focused on bone relevant phenotypes and repositories of genotype-phenotype data that exist for transgenic and mutant mice, which can be readily mined as a first step toward more targeted efforts designed to deeply characterize the role of a gene in bone biology.

If genetic variation could talk: What genomic data may teach us about the importance of gene expression regulation in the genetics of autism

Autism spectrum disorder (ASD) has been long known to have substantial genetic etiology. Much research has attempted to identify specific genes contributing to ASD risk with the goal of tying gene function to a molecular pathological explanation for ASD. A unifying molecular pathology would potentially increase understanding of what is going wrong during development, and could lead to diagnostic biomarkers or targeted preventative or therapeutic directions. We review past and current genetic mapping approaches and discuss major results, leading to the hypothesis that global dysregulation of gene or protein expression may be implicated in ASD rather than disturbance of brain-specific functions. If substantiated, this hypothesis might indicate the need for novel experimental and analytical approaches in order to understand this neurodevelopmental disorder, develop biomarkers, or consider treatment approaches.

Genetics of aging bone

With aging, the skeleton experiences a number of changes, which include reductions in mass and changes in matrix composition, leading to fragility and ultimately an increase of fracture risk. A number of aspects of bone physiology are controlled by genetic factors, including peak bone mass, bone shape, and composition; however, forward genetic studies in humans have largely concentrated on clinically available measures such as bone mineral density (BMD). Forward genetic studies in rodents have also heavily focused on BMD; however, investigations of direct measures of bone strength, size, and shape have also been conducted. Overwhelmingly, these studies of the genetics of bone strength have identified loci that modulate strength via influencing bone size, and may not impact the matrix material properties of bone. Many of the rodent forward genetic studies lacked sufficient mapping resolution for candidate gene identification; however, newer studies using genetic mapping populations such as Advanced Intercrosses and the Collaborative Cross appear to have overcome this issue and show promise for future studies. The majority of the genetic mapping studies conducted to date have focused on younger animals and thus an understanding of the genetic control of age-related bone loss represents a key gap in knowledge.

Congenic Strains Confirm the Pleiotropic Effect of Chromosome 4 QTL on Mouse Femoral Geometry and Biomechanical Performance

A pleiotropic quantitative trait locus (QTL) for bone geometry and mechanical performance in mice was mapped to distal chromosome 4 via an intercross of recombinant congenic mice HcB-8 and HcB-23. To study the QTL in isolation, we have generated C3H.B10-(rs6355453-rs13478087) (C.B.4.3) and C3H.B10-(rs6369860-D4Mit170) (C.B.4.2) congenic strains that harbor ~20 Mb and ~3 Mb, respectively, of chromosome 4 overlapping segments from C57BL/10ScSnA (B10) within the locus on a C3H/DiSnA (C3H) background. Using 3-point bend testing and standard beam equations, we phenotyped these mice for femoral mid-diaphyseal geometry and biomechanical performance. We analyzed the results via 2-way ANOVA, using sex and genotype as factors. In the C.B.4.3 strain, we found that homozygous B10/B10 male mice had smaller cross sectional area (CSA) and reduced total displacement than homozygous C3H/C3H mice. Sex by genotype interaction was also observed for maximum load and stiffness for C3H/C3H and B10/B10 mice, respectively. In C.B.4.2 strain, we found that homozygous B10/B10 mice had lower total displacement, post-yield displacement (PYD), stiffness, yield load and maximum load than mice harboring C3H allele. Sex by genotype interaction was observed in B10/B10 mice for perimeter, outer minor axis (OMA) and CSA. There were no significant differences in tissue level mechanical performance, which suggest that the QTL acts primarily on circumferential bone size. These data confirm the prior QTL mapping data and support other work demonstrating the importance of chromosome 4 QTL on bone modeling and bone responses to mechanical loading.

Multimer Formation Explains Allelic Suppression of PRDM9 Recombination Hotspots

Genetic recombination during meiosis functions to increase genetic diversity, promotes elimination of deleterious alleles, and helps assure proper segregation of chromatids. Mammalian recombination events are concentrated at specialized sites, termed hotspots, whose locations are determined by PRDM9, a zinc finger DNA-binding histone methyltransferase. Prdm9 is highly polymorphic with most alleles activating their own set of hotspots. In populations exhibiting high frequencies of heterozygosity, questions remain about the influences different alleles have in heterozygous individuals where the two variant forms of PRDM9 typically do not activate equivalent populations of hotspots. We now find that, in addition to activating its own hotspots, the presence of one Prdm9 allele can modify the activity of hotspots activated by the other allele. PRDM9 function is also dosage sensitive; Prdm9^+/- heterozygous null mice have reduced numbers and less active hotspots and increased numbers of aberrant germ cells. In mice carrying two Prdm9 alleles, there is allelic competition; the stronger Prdm9 allele can partially or entirely suppress chromatin modification and recombination at hotspots of the weaker allele. In cell cultures, PRDM9 protein variants form functional heteromeric complexes which can bind hotspots sequences. When a heteromeric complex binds at a hotspot of one PRDM9 variant, the other PRDM9 variant, which would otherwise not bind, can still methylate hotspot nucleosomes. We propose that in heterozygous individuals the underlying molecular mechanism of allelic suppression results from formation of PRDM9 heteromers, where the DNA binding activity of one protein variant dominantly directs recombination initiation towards its own hotspots, effectively titrating down recombination by the other protein variant. In natural populations with many heterozygous individuals, allelic competition will influence the recombination landscape.

During formation of sperm and eggs chromosomes exchange DNA in a process known as recombination, creating new combinations responsible for much of the enormous diversity in populations. In some mammals, including humans, the locations of recombination are chosen by a DNA-binding protein named PRDM9. Importantly, there are tens to hundreds of different variations of the Prdm9 gene (termed alleles), many of which are predicted to bind a unique DNA sequence. This high frequency of variation results in many individuals having two different copies of Prdm9, and several lines of evidence indicate that alleles compete to initiate recombination. In seeking to understand the mechanism of this competition we found that Prdm9 activity is sensitive to the number of gene copies present, suggesting that availability of this protein is a limiting factor during recombination. Moreover, we found that variant forms of PRDM9 protein can physically interact suggesting that when this happens one variant can influence which hotspots will become activated. Genetic crosses in mice support these observations; the presence of a dominant Prdm9 allele can completely suppress recombination at some locations. We conclude that allele-dominance of PRDM9 is a consequence of protein-protein interaction and competition for DNA binding in a limited pool of molecules, thus shaping the recombination landscape in natural populations.

A single nucleotide polymorphism in osteonectin 3' untranslated region regulates bone volume and is targeted by miR-433

Osteonectin/SPARC is one of the most abundant noncollagenous extracellular matrix proteins in bone, regulating collagen fiber assembly and promoting osteoblast differentiation. Osteonectin-null and haploinsufficient mice have low-turnover osteopenia, indicating that osteonectin contributes to normal bone formation. In male idiopathic osteoporosis patients, osteonectin 3' untranslated region (UTR) single-nucleotide polymorphism (SNP) haplotypes that differed only at SNP1599 (rs1054204) were previously associated with bone mass. Haplotype A (containing SNP1599G) was more frequent in severely affected patients, whereas haplotype B (containing SNP1599C) was more frequent in less affected patients and healthy controls. We hypothesized that SNP1599 contributes to variability in bone mass by modulating osteonectin levels. Osteonectin 3' UTR reporter constructs demonstrated that haplotype A has a repressive effect on gene expression compared with B. We found that SNP1599G contributed to an miR-433 binding site, and miR-433 inhibitor relieved repression of the haplotype A, but not B, 3' UTR reporter construct. We tested our hypothesis in vivo, using a knock-in approach to replace the mouse osteonectin 3' UTR with human haplotype A or B 3' UTR. Compared with haplotype A mice, bone osteonectin levels were higher in haplotype B mice. B mice displayed higher bone formation rate and gained more trabecular bone with age. When parathyroid hormone was administered intermittently, haplotype B mice gained more cortical bone area than A mice. Cultured marrow stromal cells from B mice deposited more mineralized matrix and had higher osteocalcin mRNA compared with A mice, demonstrating a cell-autonomous effect on differentiation. Altogether, SNP1599 differentially regulates osteonectin expression and contributes to variability in bone mass, by a mechanism that may involve differential targeting by miR-433. This work validates the findings of the previous candidate gene study, and it assigns a physiological function to a common osteonectin allele, providing support for its role in the complex trait of skeletal phenotype. © 2014 American Society for Bone and Mineral Research.

Weak genetic relationship between trabecular bone morphology and obesity in mice

Obesity, in addition to being associated with metabolic diseases, such as diabetes, has also been found to lower the risk of osteoporotic fractures. The relationship between obesity and bone trabecular structure is complex, involving responses to mechanical loading and the effects of adipocyte-derived hormones, both directly interacting with bone tissue and indirectly through central nervous system signaling. Here we examine the effects of sex, a high fat diet, and genetics on the trabecular density and structure of the lumbar and caudal vertebra and the proximal tibia along with body weight, fat pad weight, and serum leptin levels in a murine obesity model, the LGXSM recombinant inbred (RI) mouse strains. The sample included 481 mice from 16 RI strains. We found that vertebral trabecular density was higher in males while the females had higher tibial trabecular density. The high fat diet led to only slightly higher trabecular density in both sexes despite its extreme effects on obesity and serum leptin levels. Trait heritabilities are moderate to strong and genetic correlations among trabecular features are high. Most genetic variation contrasts strains with large numbers of thick, closely-spaced, highly interconnected, plate-like trabeculae with a high bone volume to total volume ratio against strains displaying small numbers of thin, widely-spaced, sparsely connected, rod-like trabeculae with a low bone volume to total volume ratio. Genetic correlations between trabecular and obesity-related traits were low and not statistically significant. We mapped trabecular properties to 20 genomic locations. Only one-quarter of these locations also had effects on obesity. In this population obesity has a relatively minor effect on trabecular bone morphology.

Multiple quantitative trait loci for cortical and trabecular bone regulation map to mid-distal mouse chromosome 4 that shares linkage homology to human chromosome 1p36

The mid-distal region of mouse chromosome 4 (Chr 4) is homologous with human Chr 1p36. Previously, we reported that mouse Chr 4 carries a quantitative trait locus (QTL) with strong regulatory effect on volumetric bone mineral density (vBMD). The intent of this study is to utilize nested congenic strains to decompose the genetic complexity of this gene-rich region. Adult females and males from 18 nested congenic strains carrying discrete C3H sequences were phenotyped for femoral mineral and volume by pQCT and for trabecular bone volume (BV), tissue volume (TV), trabecular number (Trab.no), and trabecular thickness (Trab.thk) by MicroCT 40. Our data show that the mouse Chr 4 region consists of at least 10 regulatory QTL regions that affected either or both pQCT and MicroCT 40 phenotypes. The pQCT phenotypes were typically similar between sexes, whereas the MicroCT 40 phenotypes were divergent. Individual congenic strains contained one to seven QTL regions. These regions conferred large positive or negative effects in some congenic strains, depending on the particular bone phenotype. The QTL regions II to X are syntenic with human 1p36, containing from 1 to 102 known genes. We identified 13 candidate genes that can be linked to bone within these regions. Six of these genes were linked to osteoblasts, three linked to osteoclasts, and two linked to skeletal development. Three of these genes have been identified in Genome Wide Association Studies (GWAS) linked to 1p36. In region III, there is only one gene, Lck, which conferred negative pQCT and MicroCT 40 phenotypes in both sexes. This gene is important to development and functioning of T cells, has been associated with osteoclast activity, and represents a novel bone regulatory gene that merits further experimental evaluation. In summary, congenic strains are powerful tools for identifying regulatory regions that influence bone biology and offer models for testing hypotheses about gene-gene and gene-environment interactions that are not available to experimental work in humans.

Identification of quantitative trait loci influencing skeletal architecture in mice: emergence of Cdh11 as a primary candidate gene regulating femoral morphology

Bone strength is influenced by many properties intrinsic to bone, including its mass, geometry, and mineralization. To further advance our understanding of the genetic basis of bone-strength-related traits, we used a large (n = 815), moderately (G(4) ) advanced intercross line (AIL) of mice derived from a high-runner selection line (HR) and the C57BL/6J inbred strain. In total, 16 quantitative trait loci (QTLs) were identified that affected areal bone mineral density (aBMD) and femoral length and width. Four significant (p < .05) and one suggestive (p < .10) QTLs were identified for three aBMD measurements: total body, vertebral, and femoral. A QTL on chromosome (Chr.) 3 influenced all three aBMD measures, whereas the other four QTLs were unique to a single measure. A total of 10 significant and one suggestive QTLs were identified for femoral length (FL) and two measures of femoral width, anteroposterior (AP) and mediolateral (ML). FL QTLs were distinct from loci affecting AP and ML width, and of the 7 AP QTLs, only three affected ML. A QTL on Chr. 8 that explained 7.1% and 4.0% of the variance in AP and ML, respectively, was mapped to a 6-Mb region harboring 12 protein-coding genes. The pattern of haplotype diversity across the QTL region and expression profiles of QTL genes suggested that of the 12, cadherin 11 (Cdh11) was most likely the causal gene. These findings, when combined with existing data from gene knockouts, identify Cdh11 as a strong candidate gene within which genetic variation may affect bone morphology.

BMD regulation on mouse distal chromosome 1, candidate genes, and response to ovariectomy or dietary fat

The distal end of mouse chromosome 1 (Chr 1) harbors quantitative trait loci (QTLs) that regulate bone mineral density (BMD) and share conserved synteny with human chromosome 1q. The objective of this article was to map this mouse distal Chr 1 region and identify gene(s) responsible for BMD regulation in females. We used X-ray densitometry [ie, dual-energy X-ray Absorptiometry (DXA), micro-computed tomography (µCT), and peripheral quantitative computed tomography (pQCT)] to phenotype a set of nested congenic strains constructed from C57BL/6BmJ (B6/Bm) and C3H/HeJ (C3H) mice to map the region associated with the BMD QTL. The critical region has been reduced to an interval of 0.152 Mb that contributes to increased BMD when C3H alleles are present. Histomorphometry and osteoblast cultures indicated that increased osteoblast activity was associated with increased BMD in mouse strains with C3H alleles in this critical region. This region contains two genes, Aim2, which binds with cytoplasmic dsDNA and results in apoptosis, and AC084073.22, a predicted gene of unknown function. Ovariectomy induced bone loss in the B6/Bm progenitor and the three congenic strains regardless of the alleles present in the critical BMD region. High dietary fat treatment (thought to suppress distal Chr 1 QTL for BMD in mice) did not induce bone loss in the congenics carrying C3H alleles in the critical 0.152 Mb carrying the AIM2 and AC084073.22 genes. Gene expression studies in whole bone of key congenics showed differential expression of AC084073.22 for strains carrying B6/Bm versus C3H alleles but not for Aim2. In conclusion, our data suggest that osteoblasts are the cellular target of gene action and that AC084073.22 is the best candidate for female BMD regulation in the distal region of mouse Chr 1.

Quantitative trait locus on chromosome X affects bone loss after maturation in mice

Genetic programming is known to affect the peak bone mass and bone loss after maturation. However, little is known about how polymorphic genes on chromosome X (Chr X) modulate bone loss after maturation. We previously reported a quantitative trait locus (QTL) on Chr X, designated Pbd3, which had a suggestive linkage to bone mass, in male SAMP2 and SAMP6 mice. In this study, we aimed to clarify the effects of Pbd3 on the skeletal phenotype. We generated a congenic strain, P2.P6-X, carrying a 45.6-cM SAMP6-derived Chr X interval on a SAMP2 genetic background. The effects of Pbd3 on the bone phenotype were determined by microcomputed tomography (microCT), whole-body dual-energy X-ray absorptiometry (DXA), serum bone turnover markers, and histomorphometric parameters. Both the bone area fraction (BA/TA) on microCT and whole-body DXA revealed reduced bone loss in P2.P6-X compared with that in SAMP2. The serum concentrations of bone turnover markers at 4 months of age were significantly lower in P2.P6-X than in SAMP2, but did not differ at 8 months of age. These results were observed in female mice, but not in male mice. In conclusion, a QTL within a segregated 45.6-cM interval on Chr X is sex-specifically related to the rate of bone loss after maturation.

Mouse BMD quantitative trait loci show improved concordance with human genome-wide association loci when recalculated on a new, common mouse genetic map

Bone mineral density (BMD) is a heritable trait, and in mice, over 100 quantitative trait loci (QTLs) have been reported, but candidate genes have been identified for only a small percentage. Persistent errors in the mouse genetic map have negatively affected QTL localization, spurring the development of a new, corrected map. In this study, QTLs for BMD were remapped in 11 archival mouse data sets using this new genetic map. Since these QTLs all were mapped in a comparable way, direct comparisons of QTLs for concordance would be valid. We then compared human genome-wide association study (GWAS) BMD loci with the mouse QTLs. We found that 26 of the 28 human GWAS loci examined were located within the confidence interval of a mouse QTL. Furthermore, 14 of the GWAS loci mapped to within 3 cM of a mouse QTL peak. Lastly, we demonstrated that these newly remapped mouse QTLs can substantiate a candidate gene for a human GWAS locus, for which the peak single-nucleotide polymorphism (SNP) fell in an intergenic region. Specifically, we suggest that MEF2C (human chromosome 5, mouse chromosome 13) should be considered a candidate gene for the genetic regulation of BMD. In conclusion, use of the new mouse genetic map has improved the localization of mouse BMD QTLs, and these remapped QTLs show high concordance with human GWAS loci. We believe that this is an opportune time for a renewed effort by the genetics community to identify the causal variants regulating BMD using a synergistic mouse-human approach.

Analyzing complex traits with congenic strains

Congenic strains continue to be a fundamental resource for dissecting the genetic basis of complex traits. Traditionally, genetic variants (QTLs) that account for phenotypic variation in a panel of congenic strains are sought first by comparing phenotypes for each strain to the host (reference) strain, and then by examining the results to identify a common chromosome segment that provides the best match between genotype and phenotype across the panel. However, this "common-segment" method has significant limitations, including the subjective nature of the genetic model and an inability to deal formally with strain phenotypes that do not fit the model. We propose an alternative that we call "sequential" analysis and that is based on a unique principle of QTL analysis where each strain, corresponding to a single genotype, is tested individually for QTL effects rather than testing the congenic panel collectively for common effects across heterogeneous backgrounds. A minimum spanning tree, based on principles of graph theory, is used to determine the optimal sequence of strain comparisons. For two traits in two panels of congenic strains in mice, we compared results for the sequential method with the common-segment method as well as with two standard methods of QTL analysis, namely, interval mapping and multiple linear regression. The general utility of the sequential method was demonstrated with analysis of five additional traits in congenic panels from mice and rats. Sequential analysis rigorously resolved phenotypic heterogeneity among strains in the congenic panels and found QTLs that other methods failed to detect.

Trans-regulation of mouse meiotic recombination hotspots by Rcr1

Meiotic recombination is required for the orderly segregation of chromosomes during meiosis and for providing genetic diversity among offspring. Among mammals, as well as yeast and higher plants, recombination preferentially occurs at highly delimited chromosomal sites 1–2 kb long known as hotspots. Although considerable progress has been made in understanding the roles various proteins play in carrying out the molecular events of the recombination process, relatively little is understood about the factors controlling the location and relative activity of mammalian recombination hotspots. To search for trans-acting factors controlling the positioning of recombination events, we compared the locations of crossovers arising in an 8-Mb segment of a 100-Mb region of mouse Chromosome 1 (Chr 1) when the longer region was heterozygous C57BL/6J (B6) × CAST/EiJ (CAST) and the remainder of the genome was either similarly heterozygous or entirely homozygous B6. The lack of CAST alleles in the remainder of the genome resulted in profound changes in hotspot activity in both females and males. Recombination activity was lost at several hotspots; new, previously undetected hotspots appeared; and still other hotspots remained unaffected, indicating the presence of distant trans-acting gene(s) whose CAST allele(s) activate or suppress the activity of specific hotspots. Testing the activity of three activated hotspots in sperm samples from individual male progeny of two genetic crosses, we identified a single trans-acting regulator of hotspot activity, designated Rcr1, that is located in a 5.30-Mb interval (11.74–17.04 Mb) on Chr 17. Using an Escherichia coli cloning assay to characterize the molecular products of recombination at two of these hotspots, we found that Rcr1 controls the appearance of both crossover and noncrossover gene conversion events, indicating that it likely controls the sites of the double-strand DNA breaks that initiate the recombination process.

Recombination is an essential aspect of meiosis, ensuring proper contact and exchange of genetic material between homologous parental chromosomes, as well as their subsequent segregation to produce haploid gametes. In humans and mice, recombination events are located at preferential sites termed hotspots, whose placement and activity are tightly regulated. We have now identified a hotspot-regulating locus in mammals, Rcr1, that simultaneously controls the locations of multiple hotspots. The discovery of Rcr1 indicates the existence of a newly emerging class of genes important in the recombination processes. Gaining further insights into their function may contribute to a better understanding of genetic factors underlying human fertility and evolution.

Rcr1 is identified as atrans-regulator of meiotic recombination hotspots that appears to act at the initiation of recombination and that maps to a 5.3-megabase region on mouse Chromosome 17.

Replication of long-bone length QTL in the F9-F10 LG,SM advanced intercross

Quantitative trait locus (QTL) mapping techniques are frequently used to identify genomic regions associated with variation in phenotypes of interest. However, the F(2) intercross and congenic strain populations usually employed have limited genetic resolution resulting in relatively large confidence intervals that greatly inhibit functional confirmation of statistical results. Here we use the increased resolution of the combined F(9) and F(10) generations (n = 1455) of the LG,SM advanced intercross to fine-map previously identified QTL associated with the lengths of the humerus, ulna, femur, and tibia. We detected 81 QTL affecting long-bone lengths. Of these, 49 were previously identified in the combined F(2)-F(3) population of this intercross, while 32 represent novel contributors to trait variance. Pleiotropy analysis suggests that most QTL affect three to four long bones or serially homologous limb segments. We also identified 72 epistatic interactions involving 38 QTL and 88 novel regions. This analysis shows that using later generations of an advanced intercross greatly facilitates fine-mapping of confidence intervals, resolving three F(2)-F(3) QTL into multiple linked loci and narrowing confidence intervals of other loci, as well as allowing identification of additional QTL. Further characterization of the biological bases of these QTL will help provide a better understanding of the genetics of small variations in long-bone length.

Quantitative trait loci for biomechanical performance and femoral geometry in an intercross of recombinant congenic mice: restriction of the Bmd7 candidate interval

Despite steady progress in identifying quantitative trait loci (QTLs) for bone phenotypes, relatively little progress has been made in moving from QTLs to identifying the relevant gene. We exploited the genetic structure of recombinant congenic mouse strains by performing a reciprocal intercross of the strains HcB-8 and HcB-23, phenotyped for body size, femoral biomechanical performance, and femoral diaphyseal geometry and mapped with R/qtl and QTL Cartographer. Significant QTLs are present on chromosomes 1, 2, 3, 4, 6, and 10. We found significant sex x QTL and cross-direction x QTL interactions. The chromosome 4 QTL affects multiple femoral anatomic features and biomechanical properties. The known segregating segment of chromosome 4 contains only 18 genes, among which Ece1, encoding endothelin-converting enzyme 1, stands out as a candidate. Endothelin signaling has been shown to promote the growth of osteoblastic metastases and to potentiate signaling via the Wnt pathway. The colocalizing chromosome 4 QTL Bmd7 (for bone mineral density 7) increases responsiveness to mechanical loading. By exploiting the short informative segment of chromosome 4 and the known biology, we propose that Ece1 is the gene responsible for Bmd7 and that it acts by increasing responsiveness to mechanical loading through modulation of Wnt signaling.

Quantitative trait loci, genes, and polymorphisms that regulate bone mineral density in mouse

This is an in silico analysis of data available from genome-wide scans. Through analysis of QTL, genes and polymorphisms that regulate BMD, we identified 82 BMD QTL, 191 BMD-associated (BMDA) genes, and 83 genes containing known BMD-associated polymorphisms (BMDAP). The catalogue of all BMDA/BMDAP genes and relevant literatures are provided. In total, there are substantially more BMDA/BMDAP genes in regions of the genome where QTL have been identified than in non-QTL regions. Among 191 BMDA genes and 83 BMDAP genes, 133 and 58 are localized in QTL regions, respectively. The difference was still noticeable for the chromosome distribution of these genes between QTL and non-QTL regions. These results have allowed us to generate an integrative profile of QTL, genes, polymorphisms that determine BMD. These data could facilitate more rapid and comprehensive identification of causal genes underlying the determination of BMD in mouse and provide new insights into how BMD is regulated in humans.

An integrative genetics approach to identify candidate genes regulating BMD: combining linkage, gene expression, and association

Numerous quantitative trait loci (QTLs) affecting bone traits have been identified in the mouse; however, few of the underlying genes have been discovered. To improve the process of transitioning from QTL to gene, we describe an integrative genetics approach, which combines linkage analysis, expression QTL (eQTL) mapping, causality modeling, and genetic association in outbred mice. In C57BL/6J x C3H/HeJ (BXH) F(2) mice, nine QTLs regulating femoral BMD were identified. To select candidate genes from within each QTL region, microarray gene expression profiles from individual F(2) mice were used to identify 148 genes whose expression was correlated with BMD and regulated by local eQTLs. Many of the genes that were the most highly correlated with BMD have been previously shown to modulate bone mass or skeletal development. Candidates were further prioritized by determining whether their expression was predicted to underlie variation in BMD. Using network edge orienting (NEO), a causality modeling algorithm, 18 of the 148 candidates were predicted to be causally related to differences in BMD. To fine-map QTLs, markers in outbred MF1 mice were tested for association with BMD. Three chromosome 11 SNPs were identified that were associated with BMD within the Bmd11 QTL. Finally, our approach provides strong support for Wnt9a, Rasd1, or both underlying Bmd11. Integration of multiple genetic and genomic data sets can substantially improve the efficiency of QTL fine-mapping and candidate gene identification.

Vitamin D-binding protein gene microsatellite polymorphism influences BMD and risk of fractures in men

Here we report the results of a vitamin D-binding protein gene microsatellite polymorphism study in 170 men, comprising healthy male subjects and men with osteoporosis-related symptomatic vertebral fractures. We confirm the results of an earlier study in a different cohort, showing relationship between certain genotypes of (TAAAn)-Alu repeats and reduced BMD and vertebral fractures.

INTRODUCTION

Vitamin D-binding protein (DBP) plays a critical role in the transport and metabolism of metabolites of vitamin D, including the key calciotropic hormone 1alpha,25-dihydroxyvitamin D3 (1,25(OH)2D3).

METHODS

We have investigated intra-intronic variable tandem (TAAA)n-Alu repeat expansion in the DBP gene in 170 men, comprising healthy male subjects and men with idiopathic osteoporosis and low trauma fractures.

RESULTS AND CONCLUSIONS

The predominant DBP-Alu genotype in the control subjects was 10/10 (frequency 0.421), whereas the frequency of this genotype in men with osteoporosis was 0.089. DBP-Alu alleles *10, *8 and *9, respectively, were the three commonest in both healthy subjects and men with osteoporosis. Allele *10 was associated with a lower risk of osteoporosis (OR 0.39, 95% CI 0.25-0.64; p < 0.0005), as was allele *11 (odds ratio 0.09, 95% CI 0.01-0.67; p < 0.007). Logistic regression gave similar results, showing that individuals with genotype 10/10 and 19-20 repeats (genotypes 9/10, 9/11, 10/10,) are protected from fracture or osteoporosis. Overall, there was a relationship between DBP Alu genotype and BMD, suggesting that DBP-Alu genotype may influence fracture risk. This effect may be mediated by changes in the circulating concentrations of DBP which influences free concentrations of vitamin D.

Identification of quantitative trait loci affecting murine long bone length in a two-generation intercross of LG/J and SM/J Mice

INTRODUCTION

Study of mutations with large phenotypic effects has allowed the identification of key players in skeletal development. However, the molecular nature of variation in large, phenotypically normal populations tends to be characterized by smaller phenotypic effects that remain undefined.

MATERIALS AND METHODS

We use interval mapping and quantitative trait locus (QTL) mapping techniques in the combined F2-F3 populations (n = 2111) of an LG/J x SM/J mouse intercross to detect QTLs associated with the lengths of the humerus, ulna, femur, and tibia.

RESULTS

Seventy individual trait QTLs affecting long bone lengths were detected, with several chromosomes harboring multiple QTLs. The genetic architecture suggests mainly small, additive effects on long bone length, with roughly one third of the QTLs displaying dominance. Sex interactions were common, and four sex-specific QTLs were observed. Pleiotropy could not be rejected for most of the QTLs identified. Thirty-one epistatic interactions were detected, almost all affecting regions including or immediately adjacent to QTLs.

CONCLUSIONS

A complex regulatory network with many gene interactions modulates bone growth, possibly with integrated skeletal modules that allow fine-tuning of developmental processes present. Candidate genes in the QTL CIs include many genes known to affect endochondral bone growth and genes that have not yet been associated with bone growth or body size but have a strong potential to influence these traits.

Forty mouse strain survey of voluntary calcium intake, blood calcium, and bone mineral content

We measured voluntary calcium intake, blood calcium, and bone mineral content of male and female mice from 40 inbred strains. Calcium intakes were assessed using 48-h two-bottle tests with a choice between water and one of the following: water, 7.5, 25, and 75 mM CaCl(2), then 7.5, 25, and 75 mM calcium lactate (CaLa). Intakes were affected by strain, sex, anion, and concentration. In 11 strains females consumed more calcium than did males and in the remaining 29 strains there were no sex differences. Nine strains drank more CaLa than CaCl(2) whereas only one strain (JF1/Ms) drank more CaCl(2) than CaLa. Some strains had consistently high calcium intakes and preferred all calcium solutions relative to water (e.g., PWK/PhJ, BTBR T(+)tf/J, JF1/Ms). Others had consistently low calcium intakes and avoided all calcium solutions relative to water (e.g., KK/H1J, C57BL/10J, CE/J, C58/J). After behavioral tests, blood was sampled and assayed for pH, ionized calcium concentration, and plasma total calcium concentration. Bone mineral density and content were assessed by DEXA. There were no significant correlations between any of these physiological measures and calcium intake. However, strains of mice that had the highest calcium intakes generally fell at the extremes of the physiological distributions. We conclude that the avidity for calcium is determined by different genetic architecture and thus different physiological mechanisms in different strains.

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.

Genetic effects on bone mechanotransduction in congenic mice harboring bone size and strength quantitative trait loci

The degree to which bone tissue responds to mechanical loading events is partially under genetic control. We assess the contribution of three genetic loci (QTLs linked to bone geometry and strength)--located on mouse Chrs. 1, 8, and 13--to mechanically stimulated bone formation, through in vivo skeletal loading of congenic strains. Bone size was not consistently associated with mechano-responsiveness, indicating that the genetic regulation of mechanotransduction is a complex process that involves a number of genes and is sex-specific.

INTRODUCTION

We showed previously that C57BL/6J (B6) mice are more responsive to mechanical stimulation than C3H/HeJ (C3H) mice and that B6 mice harboring a 40-Mb region of distal C3H Chromosome (Chr.) 4 are more responsive to mechanical stimulation than are fully B6 mice. Here, we assess the contribution of three more genetic loci--located on mouse Chrs. 1, 8, and 1--to mechanically stimulated bone formation.

MATERIALS AND METHODS

Three congenic mouse strains were created in which a region of mouse Chr. 1 (approximately 64 cM; 150 Mb), Chr. 8 (approximately 45 cM; 86 Mb), or Chr. 13 (approximately 24 cM; 42 Mb) was moved from C3H stock to a B6 background through selective breeding over nine generations. The regions moved to the B6 background correspond to three of several quantitative trait loci (QTLs) identified for bone size and strength. The resulting congenic mice were 99% B6, with the remaining genomic DNA comprised of the Chr. 1, 8, or 13 QTLs of interest. Male and female congenic (1T, 8T, and 13B) and B6 control mice were subjected to in vivo loading of the right ulna at one of three different load magnitudes. A separate set of animals from each group had strain gauges applied at the ulnar midshaft to estimate strain at each loading level. Loading was conducted once per day for 3 days (60 cycles/d; 2 Hz). Fluorochrome labels were injected intraperitoneally 4 and 11 days after loading began. Using quantitative histomorphometry, bone formation rates were measured in loaded (right) and control (left) ulnas.

RESULTS

All male congenic mice exhibited significantly reduced mechano-responsiveness compared with male B6 controls, but the same comparison among females yielded no difference from controls, with the exception of the 1T congenics, which showed increased responsiveness to loading. Among the congenic strains, smaller bone size was not consistently associated with reduced mechano-responsiveness.

CONCLUSIONS

Our results indicate that the genetic regulation of mechanotransduction is a complex process that involves a number of genes and is sex-specific. Our data might explain why different individuals can engage in similar exercise protocols yet experience different results in terms of bone mass accrual.

A genome-wide set of congenic mouse strains derived from CAST/Ei on a C57BL/6 background

We previously reported the construction of two sets of heterozygous congenic strains spanning the mouse genome. For both sets, C57BL/6J was employed as the background strain while DNA from either DBA/2 or CAST/Ei was introgressed to form the congenic region. We have subsequently bred most of these strains to produce homozygous breeding stocks. Here, we report the characterization of the strain set based on CAST/Ei. CAST/Ei is the most genetically distant strain within the Mus mus species and many trait variations relevant to common diseases have been identified in CAST/Ei mice. Despite breeding difficulties for some congenic regions, presumably due to incompatible allelic variations between CAST/Ei and C57BL/6, the resulting congenic strains cover about 80% of the autosomal chromosomes and will be useful as a resource for the further analysis of quantitative trait loci between the strains.

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

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

INTRODUCTION

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

MATERIALS AND METHODS

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

RESULTS

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

CONCLUSIONS

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

Secreted frizzled-related protein 4 is a negative regulator of peak BMD in SAMP6 mice

We segregated a QTL for peak BMD on Chr 13 by generating congenic sublines of the senescence-accelerated mouse SAMP6. Sfrp 4 within this locus was responsible for lower BMD of SAMP6.

INTRODUCTION

Our genome-wide linkage study using SAMP6 and SAMP2 showed a significant quantitative trait locus (QTL) for peak BMD on chromosome (Chr) 13. To verify the gene that regulates peak BMD, we generated a congenic strain, P6.P2-Pbd2(b), which carried a 15-cM SAMP2 interval on an osteoporotic SAMP6 background, and showed that this Pbd2 locus increased peak BMD in SAMP6.

MATERIALS AND METHODS

To narrow down this interval, we generated a new congenic subline P6.P2-13. We studied the effect of this locus on morphological and histomorphological features in vivo and on osteoblasts in vitro. The levels of expression of all genes in the segregated interval were examined, and we clarified the effect of the candidate gene, secreted frizzled-related protein (Sfrp4), on osteoblasts in vitro.

RESULTS

The new congenic strain, P6.P2-13, retained the 2.4-Mb SAMP2 interval on the SAMP6 background, and 11 genes existed in this interval. In morphometrical analysis, P6.P2-13 increased the bone area fraction (BA/TA) by 6.6% at the diaphysial cortex (p < 0.001) and increased the trabecular bone volume (BV/TV) by 54.2% at the distal metaphysis (p < 0.05) in the femora compared with those of SAMP6. The bone formation rate of P6.P2-13 was markedly increased at the periosteal surface of femoral cortex and that was caused by a higher proliferation rate of osteoblasts in P6.P2-13 compared with those in SAMP6. Quantitative RT-PCR analysis of calvaria tissue showed approximately 40-fold higher levels of expression of Sfrp4 in SAMP6 than in P6.P2-13. Taken together with the result that recombinant Sfrp4 suppressed the proliferation of osteoblasts, we hypothesized that Sfrp4 inhibited the proliferation of osteoblasts through its antagonistic effect on Wnt signaling. TCF/beta-catenin-dependent reporter activity in osteoblasts derived from SAMP6 showed lower responsiveness for the Wnt ligand, Wnt3A, than that in osteoblasts from P6.P2-13.

CONCLUSIONS

In SAMP6 mice, Sfrp4 negatively regulates bone formation and decreases BMD through the inhibition of Wnt signaling.

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

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

Identification of genetic loci that regulate bone adaptive response to mechanical loading in C57BL/6J and C3H/HeJ mice intercross

Strain-dependent differences in bone adaptive responses to loading among inbred mouse strains suggest that genetic background contributes significantly to adaptation to exercise. To explore the genetic regulation of response to loading, we performed a genome-wide search for linkage in a cross between two strains, a good responder, C57BL6/J (B6), and a poor responder, C3H/HeJ (C3H). Using a four-point bending model, the right tibia was loaded by applying 9 N force for 36 cycles for 12 days in 10-week-old female B6xC3H F2 mice. Changes in bone density (BMD) and bone size were evaluated in vivo by pQCT. Measurements from non-loaded left tibia were used as an internal control to calculate loading-induced percent increase in BMD and bone size, thus excluding the possibility of identifying background QTL(s) due to natural allelic variation in mapping strains. A genome-wide scan was performed using 111 microsatellite markers in DNA samples collected from 329 F2 mice. Heritability of bone adaptive response to loading was between 70 and 80%. The mean increase, expressed as percent of unloaded tibia, was 5% for BMD, 9% for periosteal circumference (PC), and 14% for cortical thickness in F2 mice (n = 329). All these phenotypes showed normal distributions. Absence of significant correlation between BMD response to four-point bending and body weight or bone size suggested that the bone adaptive response was independent of bone size. Interval mapping revealed that BMD response to four-point bending was influenced by three significant loci on Chrs 1 (log-of-odds ratio score (LOD) 3.4, 91.8 cM), 3 (LOD 3.6, 50.3 cM), and 8 (LOD 4.2, 60.1 cM) and one suggestive QTL on Chr 9 (LOD 2.5, 33.9 cM). Loading-induced increases in PC and Cth were influenced by four significant loci on Chrs 8 (LOD 3.0, 68.9 cM), 9 (LOD 3.0, 13.1 cM), 17 (LOD 3.0, 39.3 cM), and 18 (LOD 3.0, 0 cM) and two suggestive loci on Chr 9 (LOD 2.2, 24 cM) and 11 (LOD 2.1, 69.9 cM). Pairwise analysis showed the presence of several significant and suggestive interactions between loci on Chrs 1, 3, 8, and 13 for BMD trait. This is the first study that provides evidence for the presence of multiple genetic loci regulating bone anabolic responses to loading in the B6xC3H intercross. Knowledge of the genes underlying these loci could provide novel approaches to improve skeletal mass.

Congenic mice provide in vivo evidence for a genetic locus that modulates serum insulin-like growth factor-I and bone acquisition

We identified quantitative trait loci (QTL) that determined the genetic variance in serum IGF-I through genome-wide scanning of mice derived from C57BL/6J(B6) x C3H/HeJ(C3H) intercrosses. One QTL (Igf1s2), on mouse chromosome 10 (Chr10), produces a 15% increase in serum IGF-I in B6C3 F2 mice carrying c3 alleles at that position. We constructed a congenic mouse, B6.C3H-10 (10T), by backcrossing c3 alleles from this 57-Mb region into B6 for 10 generations. 10T mice have higher serum and skeletal IGF-I, greater trabecular bone volume fraction, more trabeculae, and a higher number of osteoclasts at 16 wk, compared with B6 (P < 0.05). Nested congenic sublines generated from further backcrossing of 10T allowed for recombination and produced four smaller sublines with significantly increased serum IGF-I at 16 wk (i.e. 10-4, 10-7, 10-10, and 10-13), compared with B6 (P < 0.0003), and three smaller sublines that showed no differences in IGF-I vs. age- and gender-matched B6 mice. Like 10T, the 10-4 nested sublines at 16 wk had higher femoral mineral (P < 0.0001) and greater trabecular connectivity density with significantly more trabeculae than B6 (P < 0.01). Thus, by comprehensive phenotyping, we were able to narrow the QTL to an 18.3-Mb region containing approximately 148 genes, including Igf1 and Elk-3(ETS domain protein). Allelic differences in the Igf1s2 QTL produce a phenotype characterized by increased serum IGF-I and greater peak bone density. Congenic mice establish proof of concept of shared genetic determinants for both circulating IGF-I and bone acquisition.

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.

Confirmation of linkage to chromosome 1q for spine bone mineral density in southern Chinese

Chromosome 1q has previously been linked to bone mineral density (BMD) variation in the general population in several genome-wide linkage studies in both humans and mouse model. The aim of present study is to replicate and fine map the QTL influencing BMD in chromosome 1q in southern Chinese. Twelve microsatellite markers were genotyped for a 57 cMu region in the chromosome 1q in 306 southern Chinese families with 1,459 subjects. Each of these families was ascertained through a proband with BMD Z-scores less than -1.3 at the hip or spine. BMD (g/cm2) at the L1-4 lumbar spine, femoral neck (FN), trochanter and total hip was measured by dual-energy X-ray absortiometry. Linkage analyses were performed using the variance component linkage analysis method implemented in Merlin software. Four markers (D1S2878, D1S196, D1S452, and D1S218) achieved a LOD score greater than 1.0 with spine BMD, with the maximum multipoint LOD score of 2.36 at the marker D1S196. We did not detect a LOD score greater than 1.0 for BMD at the FN, trochanter, or total hip in multipoint linkage analyses. Our results present the first evidence for the presence of an osteoporosis susceptibility gene on chromosome 1q in non-Caucasian subjects. Further analyses of candidate genes are warranted to identify QTL genes and variants underlying the variations of BMD in this region.

The mechanical phenotype of biglycan-deficient mice is bone- and gender-specific

Biglycan (bgn) is a small leucine-rich proteoglycan (SLRP) enriched in the extracellular matrix of skeletal tissues. While bgn is known to be involved in the growth and differentiation of osteoblast precursor cells and regulation of collagen fibril formation, it is unclear how these functions impact bone's geometric and mechanical properties, properties which are integral to the structural function of bone. Because the genetic control of bone structure and function is both local- and gender-specific and because there is evidence of gender-specific effects associated with genetic deficiencies, it was hypothesized that the engineered deletion of the gene encoding bgn would result in a cortical bone mechanical phenotype that was bone- and gender-specific. In 11-week-old C57BL6/129 mice, the cortical bone in the mid-diaphyses of the femora and tibiae of both genders was examined. Phenotypic changes in bgn-deficient mice relative to wild type controls were assayed by four-point bending tests to determine mechanical properties at the whole bone (structural) and tissue levels, as well as analyses of bone geometry and bone formation using histomorphometry. Of the bones examined, bgn deficiency most strongly affected the male tibiae, where enhanced cross-sectional geometric properties and bone mineral density were accompanied by decreased tissue-level yield strength and pre-yield structural deformation and energy dissipation. Because pre-yield properties alone were impacted, this implies that the gene deletion causes important alterations in mineral and/or the matrix/mineral ultrastructure and suggests a new understanding of the functional role of bgn in regulating bone mineralization in vivo.

PAPPA2, an enzyme that cleaves an insulin-like growth-factor-binding protein, is a candidate gene for a quantitative trait locus affecting body size in mice

Identifying genes responsible for quantitative variation remains a major challenge. We previously identified a quantitative trait locus (QTL) affecting body size that segregated between two inbred strains of mice. By fine mapping, we have refined the location of this QTL to a genomic region containing only four protein-coding genes. One of these genes, PAPPA2, is a strong candidate because it codes for an enzyme that cleaves insulin-like growth-factor-binding protein 5 (IGFBP-5), an important stimulator of bone formation. Among littermates that segregate only for the four-gene region, we show that the QTL has a significant effect on the circulating levels of IGFBP-5 and IGFBP-3 (the latter subject to limited degradation by PAPPA2), but not on levels of IGFBP-2 and IGFBP-4, which are not cleaved by PAPPA2. There are 14 nonsynonymous SNPs among QTL alleles, which may affect the activity of the translated protein. The refinement of the target region to four genes and the finding that the QTL affects IGFBP-5 levels suggest that PAPPA2 may be involved with normal postnatal growth. Our mapping results also illustrate the potentially fractal nature of QTL: as we mapped our QTL with increasing resolution, what appeared to be a single QTL resolved into three closely linked QTL (previous work), and then one of these was further dissected into two in this study.

Allelic variation on chromosome 5 controls beta-cell mass expansion during hyperglycemia in leptin receptor-deficient diabetes mice

Leptin signaling is a critical component of normal insulin sensitivity. Overt hyperglycemia and type 2 diabetes mellitus can be manifested in states of leptin signaling deficiencies by the additional effects of other genetic factors. We have previously described the contrasting insulin sensitivities and glycemic states of two congenic diabetes (db/db) mouse strains. C57BL/6J db/db mice have mild insulin resistance and achieve euglycemia with mild hyperinsulinemia. FVB db/db mice have severe insulin resistance and are hyperglycemic despite escalating hyperinsulinemia with expanded pancreatic beta-cell mass. Analysis of obese progeny from the two reciprocal backcrosses suggests that genetic modifiers for insulin sensitivity are separable from loci that modulate beta-cell mass. A genome scan of the backcross to FVB suggests that one or more modifier genes are present on chromosome 5. This evidence is supported by the phenotypes of multiple incipient congenic strains wherein the hyperglycemia observed in obese FVB mice is reproduced. With similar degrees of hyperglycemia in obese mice of these strains, the haplotype at chromosome 5 is associated with beta-cell mass and circulating insulin concentrations. Finally, we offer arguments that production of multiple incipient congenic lines is an economical alternative to the production of speed congenic strains.

Genome-wide scan identified QTLs underlying femoral neck cross-sectional geometry that are novel studied risk factors of osteoporosis

A genome-wide screen was conducted using a large white sample to identify QTLs for FNCS geometry. We found significant linkage of FNCS parameters to 20q12 and Xq25, plus significant epistatic interactions and sex-specific QTLs influencing FNCS geometry variation.

INTRODUCTION

Bone geometry, a highly heritable trait, is a critical component of bone strength that significantly determines osteoporotic fracture risk. Specifically, femoral neck cross-sectional (FNCS) geometry is significantly associated with hip fracture risk as well as genetic factors. However, genetic research in this respect is still in its infancy.

MATERIALS AND METHODS

To identify the underlying genomic regions influencing FNCS variables, we performed a remarkably large-scale whole genome linkage scan involving 3998 individuals from 434 pedigrees for four FNCS geometry parameters, namely buckling ratio (BR), cross-sectional area (CSA), cortical thickness (CT), and section modulus (Z). The major statistical approach adopted is the variance component method implemented in SOLAR.

RESULTS

Significant linkage evidence (threshold LOD = 3.72 after correction for tests of multiple phenotypes) was found in the regions of 20q12 and Xq25 for CT (LOD = 4.28 and 3.90, respectively). We also identified eight suggestive linkage signals (threshold LOD = 2.31 after correction for multiple tests) for the respective geometry traits. The above findings were supported by principal component linkage analysis. Of them, 20q12 was of particular interest because it was linked to multiple FNCS geometry traits and significantly interacted with five other genomic loci to influence CSA variation. The effects of 20q12 on FNCS geometry were present in both male and female subgroups. Subgroup analysis also revealed the presence of sex-specific quantitative trait loci (QTLs) for FNCS traits in the regions such as 2p14, 3q26, 7q21 and 15q21.

CONCLUSIONS

Our findings laid a foundation for further replication and fine-mapping studies as well as for positional and functional candidate gene studies, aiming at eventually finding the causal genetic variants and hidden mechanisms concerning FNCS geometry variation and the associated hip fractures.

Multiple genetic loci from CAST/EiJ chromosome 1 affect vBMD either positively or negatively in a C57BL/6J background

Skeletal phenotype analyses of 10 B6.CAST-1 congenic sublines of mice have revealed evidence for the presence of three closely linked QTLs in Chr 1 that influence femoral vBMD both positively and negatively.

INTRODUCTION

BMD is an important component of bone strength and a recognized predictor of risk for osteoporotic fracture. Our goal in this study was to fine map the chromosomal location of volumetric BMD (vBMD) quantitative trait loci (QTLs) in mouse distal chromosome 1 (Chr 1).

MATERIALS AND METHODS

After several backcrosses of the B6.CAST-1T congenic strain, which carried the initial BMD QTL in Chr 1 with B6 mice, the N10F1 generation mice were intercrossed to obtain recombinations that yielded different regions of the QTL. Thirty-eight polymorphic markers were used to fine map the initial 1T QTL region (100-192 Mb). Different skeletal parameters were compared between the 10 sublines and B6 female mice at 16 weeks of age. A t-test was used to determine the significant difference between sublines and B6 control mice, whereas one-way ANOVA and posthoc (Newman-Keuls) tests were performed to compare the phenotype between the sublines.

RESULTS

Significantly higher femur vBMD was found in sublines that carried cast alleles from 100 to 169 and 172 to 185 Mb of the centromere compared with the B6 control mice (10-12%, p < 0.001). However, sublines that carried cast alleles from 185 to 192 Mb showed significantly lower femur vBMD compared with the control mice (-6%, p < 0.05). Furthermore, femur vBMD phenotype showed a negative correlation with endosteal circumference (r = -0.8, p = 0.003), and a strong correlation with cortical thickness for combined data from the 10 sublines (r = 0.97, p < 0.001). Moreover, a high correlation was found between body weight and both periosteal and endosteal circumferences for sublines carrying cast alleles from 167 to 175, 168 to 185, and 169 to 185 Mb, whereas no significant correlation was found between these parameters for sublines carrying cast alleles from 172 to 185 Mb.

CONCLUSIONS

Genetic analysis using congenic sublines revealed that the initial BMD QTL on Chr 1 is a complex site with multiple loci affecting bone phenotypes, showing the value of the congenic approach in clearly identifying loci that control specific traits.

A comparison of mechanical properties derived from multiple skeletal sites in mice

Laboratory mice provide a versatile experimental model for studies of skeletal biomechanics. In order to determine the strength of the mouse skeleton, mechanical testing has been performed on a variety of bones using several procedures. Because of differences in testing methods, the data from previous studies are not comparable. The purpose of this study was to determine which long bone provides the values closest to the published material properties of bone, while also providing reliable and reproducible results. To do this, the femur, humerus, third metatarsal, radius, and tibia of both the low bone mass C57BL/6H (B6) and high bone mass C3H/HeJ (C3H) mice were mechanically tested under three-point bending. The biomechanical tests showed significant differences between the bones and between mouse strains for the five bones tested (p < 0.05). Computational models of the femur, metatarsal, and radius were developed to visualize the types of measurement error inherent in the three-point bending tests. The models demonstrated that measurement error arose from local deformation at the loading point, shear deformation and ring-type deformation of the cylindrical cross-section. Increasing the aspect ratio (bone length/width) improved the measurement of Young's modulus of the bone for both mouse strains (p < 0.01). Bones with the highest aspect ratio and largest cortical thickness to radius ratio were better for bending tests since less measurement error was observed in the computational models. Of the bones tested, the radius was preferred for mechanical testing because of its high aspect ratio, minimal measurement error, and low variability.

Met proto-oncogene juxtamembrane rare variations in mouse and humans: differential effects of Arg and Cys alleles on mouse lung tumorigenesis

Analysis of seven candidate genes mapping in the 1-Mb region of the mouse pulmonary adenoma resistance 4 (Par4) locus revealed a single amino-acid change, consisting in a nonconservative Arg968Cys variation in the juxtamembrane domain of the Met proto-oncogene-encoded protein. The BALB/c strain (resistant allele) carried the Arg allele, whereas the SWR/J mouse strain (Par4-susceptible allele) carried the Cys variation, recently proven to functionally modulate tumorigenesis. Seven genetic linkage crosses herein analysed and six crosses reported in the literature pointed to the candidacy of the Met gene for Par4. Analysis of genomic DNA of 126 lung adenocarcinoma patients for the Met juxtamembrane domain revealed the same Arg/Cys variation at the mouse homologous position in one patient; two other patients carried additional variants in the same domain, suggesting a potential role for rare MET juxtamembrane variants in human lung cancer.

Fine mapping of a murine growth locus to a 1.4-cM region and resolution of linked QTL

Previous work identified a QTL affecting murine size (particularly tail length) in a cross between C57BL/6J and DBA/2J mice and refined its location to an 8-cM region between D1Mit30 and D1Mit57. The present study used recombinant progeny testing to fine map this QTL. Individuals from a partially congenic strain carrying chromosomes recombinant between D1Mit30 and D1Mit57 were mated to DBA/2J, generating 942 progeny. Two QTL affecting 10-week tail length were identified in this population: one at 9.7 cM distal to D1Mit30 (the position estimated in previous work), and another of smaller effect near D1Mit30. A second population ( n = 787) was generated by mating siblings from the progeny test population that were heterozygous for the same segment of chromosome, including only recombinants between D1Mit265 and D1Mit57. In the latter population, two QTL were also identified: one at 10.2 cM distal to D1Mit30, and another of smaller effect at the distal end of the mapped region (at D1Mit150). When the two populations were analyzed together, the estimated location of the central QTL was 10.2 cM distal to D1Mit30 and there was marginally significant evidence of the distal QTL. The central QTL explained approximately 7% of the phenotypic variance, and the 95% confidence interval for its position (determined by bootstrapping) was a 1.4-cM region, approximately the region from D1Mit451 to D1Mit219. The central QTL also affected tail length and body mass at 3 and 6 weeks of age, but to a lesser degree than 10-week tail length.

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.

Congenic mice with low serum IGF-I have increased body fat, reduced bone mineral density, and an altered osteoblast differentiation program

Targeted gene studies have demonstrated the importance of insulin-like growth factor-I (IGF-I) for osteoblast (OB) differentiation and the acquisition of peak bone mineral density (BMD). The skeletal response to allelic differences in IGF-I expression can also be measured in vivo, using congenic mice. We created a congenic strain with reduced (approximately 20%) circulating IGF-I (C3H.B6-6T [6T]) by backcrossing a small genomic region (30 cM) of Chromosome 6 (Chr6) from C3H/HeJ (C3H) onto a C57Bl/6J (B6) background. 6T female mice have lower serum IGF-I (P<0.001 vs. B6) but similar growth hormone (GH) and serum IGF binding protein (IGFBP) concentrations as B6. At 16 weeks of age, congenics have greater body fat (P<0.02 vs. B6) despite less total body weight, and exhibit smaller femoral cross-sectional size (P=0.001), reduced cortical thickness (P<0.001) and lower trabecular BV/TV (P<0.05) than B6. 6T mice also have suppressed serum leptin (P<0.01), but compared to B6 have similar markers of bone resorption (i.e., urine CTx and serum TRAP 5B). At 8 weeks of age, skeletal IGF-I mRNA from long bones was reduced by 40% (P<0.05) as were liver mRNA transcripts (i.e., 50%, P<0.01). Osteoblast progenitors from the bone marrow of 6T mice formed less colony forming unit fibroblasts by crystal violet staining than B6 (P<0.007) and had significantly reduced alkaline phosphatase-positive colonies than B6(P<0.0001). In addition, staining of bone marrow with oil red O revealed greater numbers of adipocytes in 6T than B6. Several candidate genes in the Chr6 QTL were excluded by lack of strain-related expression differences in bone, but genes positively regulating adipocyte differentiation including Alox 5 and PPAR-gamma require further study as either "pathway" or candidate genes. In summary, allelic differences in a QTL on Chr6 result in altered IGF-I gene expression, changes in OB lineage allocation, and reduced peak bone mass. Congenic mice are useful models not only for mapping genes related to bone mass but also for elucidating the biology underlying various skeletal phenotypes associated with more subtle manipulation of the mouse genome.

Quantitative trait loci that modulate femoral mechanical properties in a genetically heterogeneous mouse population

The goal of this study was to investigate genetic effects on mechanical properties of the mouse femur. We found evidence for QTL on eight chromosomes that affect mechanical traits. Some of these QTL may have primary effects on body weight or femoral geometry, and others seem to affect bone quality directly.

INTRODUCTION

Previous studies have shown a dependence of fragility-related fracture risk on genetic background. Although many of these studies investigated the effect of genetics on BMD, basic measures of bone geometry and mechanical integrity may provide a more comprehensive characterization of the genetic effects on bone fragility. The purpose of this study was to identify quantitative trait loci (QTL) that affect mechanical and material properties of cortical bone in a genetically heterogeneous mouse population.

MATERIALS AND METHODS

A total of 486 female UM-HET3 mice was used for this study. UM-HET3 mice are produced as the offspring of (BALB/cJ x C57BL/6J) F(1) females and (C3H/HeJ x DBA/2J) F(1) males. Femurs from 18-month-old mice were tested to failure in four-point bending to assess mechanical properties of cortical bone; these properties were compared with genotype data from 185 biallelic loci. A permutation-based test was used to detect significant associations between genetic markers and mechanical traits. This test generates p values that account for the effect of testing multiple hypotheses. Throughout the experiment, p < or = 0.05 was considered statistically significant. Analysis of covariance was used to examine possible effects of body weight and femoral geometry.

RESULTS

We found evidence for genes on maternal chromosomes 11 and 13 and paternal chromosomes 2, 4, 7, 10, 11, and 17 that affect mechanical and material properties of femoral bone. The total variance explained by genetic effects on each mechanical trait ranges from 2.9% to 15.4%. Most of the identified polymorphisms influence mechanical traits even after adjustment for body weight. Adjustment for femoral geometry reduces the effects of some of the QTL, but those on chromosomes 2 and 10 do not seem to be influenced by femoral geometry.

CONCLUSIONS

Many genes and chromosomes are involved in the genetic control over mechanical integrity of cortical bone. QTL on paternal chromosomes 4 and 11 may mediate mechanical properties, at least in part, by modulation of femoral geometry. Other QTL identified here may directly affect bone tissue quality.

Transgenic mice expressing selected insulin-like growth factor-binding protein-5 fragments do not exhibit enhanced bone formation

Skeletal cells synthesize insulin like growth factors (IGF) and six IGF binding proteins (IGFBP). IGFBP-5 and fragments were reported to stimulate bone cell growth and parameters of osteoblastic function. We investigated the effects of IGFBP-5 1-162 and 1-193 on bone remodeling in transgenic mice overexpressing these fragments under the control of the osteocalcin promoter. Transgenic mice had normal appearance, weight, and bone mineral density. Static and dynamic histomorphometry revealed that transgenic mice overexpressing IGFBP-5 1-162 or 1-193 had normal trabecular bone volume, osteoblast and osteoclast number, and normal bone formation rate. MC3T3 cells transduced with retroviral vectors overexpressing IGFBP-5 1-235, 1-193, and 1-162 fragments displayed normal cell growth and maturation, and failed to enhance the expression of alkaline phosphatase, osteocalcin, and type I collagen mRNA when compared to cells transduced with vector alone. In conclusion, transgenic mice expressing IGFBP-5 1-162 and 1-193 in the bone microenvironment do not exhibit an obvious skeletal phenotype.

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.

Mapping quantitative trait loci for vertebral trabecular bone volume fraction and microarchitecture in mice

BMD, which reflects both cortical and cancellous bone, has been shown to be highly heritable; however, little is known about the specific genetic factors regulating trabecular bone. Genome-wide linkage analysis of vertebral trabecular bone traits in 914 adult female mice from the F2 intercross of C57BL/6J and C3H/HeJ inbred strains revealed a pattern of genetic regulation derived from 13 autosomes, with 5-13 QTLs associated with each of the traits. Ultimately, identification of genes that regulate trabecular bone traits may yield important information regarding mechanisms that regulate mechanical integrity of the skeleton.

INTRODUCTION

Both cortical and cancellous bone influence the mechanical integrity of the skeleton, with the relative contribution of each varying with skeletal site. Whereas areal BMD, which reflects both cortical and cancellous bone, has been shown to be highly heritable, little is known about the genetic determinants of trabecular bone density and architecture.

MATERIALS AND METHODS

To identify heritable determinants of vertebral trabecular bone traits, we evaluated the fifth lumbar vertebra from 914 adult female mice from the F2 intercross of C57BL/6J (B6) and C3H/HeJ (C3H) progenitor strains. High-resolution microCT was used to assess total volume (TV), bone volume (BV), bone volume fraction (BV/TV), trabecular thickness (Tb.Th), separation (Tb.Sp), and number (Tb.N) of the trabecular bone in the vertebral body in the progenitors (n = 8/strain) and female B6C3H-F2 progeny (n = 914). Genomic DNA from F2 progeny was screened for 118 PCR-based markers discriminating B6 and C3H alleles on all 19 autosomes.

RESULTS AND CONCLUSIONS

Despite having a slightly larger trabecular bone compartment, C3H progenitors had dramatically lower vertebral trabecular BV/TV (-53%) and Tb.N (-40%) and higher Tb.Sp (71%) compared with B6 progenitors (p < 0.001 for all). Genome-wide quantitative trait analysis revealed a pattern of genetic regulation derived from 13 autosomes, with 5-13 quantitative trait loci (QTLs) associated with each of the vertebral trabecular bone traits, exhibiting adjusted LOD scores ranging from 3.1 to 14.4. The variance explained in the F2 population by each of the individual QTL after adjusting for contributions from other QTLs ranged from 0.8% to 5.9%. Taken together, the QTLs explained 22-33% of the variance of the vertebral traits in the F2 population. In conclusion, we observed a complex pattern of genetic regulation for vertebral trabecular bone volume fraction and microarchitecture using the F2 intercross of the C57BL/6J and C3H/HeJ inbred mouse strains and identified a number of QTLs, some of which are distinct from those that were previously identified for total femoral and vertebral BMD. Identification of genes that regulate trabecular bone traits may ultimately yield important information regarding the mechanisms that regulate the acquisition and maintenance of mechanical integrity of the skeleton.

Rat strain differences in the ectopic osteogenic potential of recombinant human BMP adenoviruses

Different animal strains have different genetic backgrounds that influence their physiological function and pathological process. The differences in genetic background may affect the efficiency of adenoviral infection and target gene expression and further cause different gene therapy results when target genes are delivered with adenoviral vectors. In this study, ectopic bone was not seen in ADCMVBMP4 injection sites, but was formed in ADCMVBMP9 injection sites in all rat strains. The mean volumes of bone induced with ADCMVBMP9 were 0.87 +/- 0.2 cm3 in Wistar, 0.26 +/- 0.1 cm3 in Long-Evans, 0.34 +/- 0.2 cm3 in Sprague-Dawley, 0.44 +/- 0.1 cm3 in ACI, 0.66 +/- 0.2 cm3 in PVG, and 0.58 +/- 0.1 cm3 in Fischer 344 rats. This indicates that ADCMVBMP9 has different bone formation potentials in different immunocompetent rat strains (P = 0.02). The basic levels of CD4+ and CD8+ T cells in blood before viral infection and titers of adenoviral neutralizing antibodies 30 days post-viral infection were significantly different among rat strains (P < 0.01). The efficiencies of target gene expression delivered with adenovirus were also significantly different in primary muscle cell cultures from different rat strains (P < 0.01). The different osteogenic potentials of ADCMVBMP9 among rat strains may be, in part, due to the differences in immune factors and target gene expression efficiency in muscle tissue.