Confirmation and fine mapping of chromosomal regions influencing peak bone mass in mice

Susceptibility to Low Vitamin B6 Diet-induced Gestational Diabetes Is Modulated by Strain Differences in Mice

Gestational diabetes is a common pregnancy complication that adversely influences the health and survival of mother and child. Pancreatic islet serotonin signaling plays an important role in β-cell proliferation in pregnancy, and environmental and genetic factors that disrupt serotonin signaling are associated with gestational diabetes in mice. Our previous studies show that pregnant C57BL/6J mice fed a diet that is low in vitamin B6, a critical co-factor in serotonin synthesis, develop hyperglycemia and glucose intolerance, phenotypes that are consistent with gestational diabetes in humans. The current study shows that, unlike in the C57BL/6J mice, low vitamin B6 diet does not alter glucose tolerance and insulin secretion in pregnant DBA/2J mice. The hypothesis to be tested in the current study is that pregnant DBA/2J mice are protected against low vitamin B6-induced gestational diabetes due to their higher expression and enzymatic activities of tissue nonspecific alkaline phosphatase (ALPL) relative to C57BL/6J. ALPL is a rate-limiting enzyme that regulates vitamin B6 bioavailability. Interestingly, treating pregnant DBA/2J mice with 7.5 mg/kg/day of the ALPL inhibitor SBI-425 is associated with glucose intolerance in low vitamin B6-fed mice, implying that inhibition of ALPL activity is sufficient to modulate resilience to low vitamin B6-induced metabolic impairment.

Loss of osteoblast Runx3 produces severe congenital osteopenia

Congenital osteopenia is a bone demineralization condition that is associated with elevated fracture risk in human infants. Here we show that Runx3, like Runx2, is expressed in precommitted embryonic osteoblasts and that Runx3-deficient mice develop severe congenital osteopenia. Runx3-deficient osteoblast-specific (Runx3(fl/fl)/Col1α1-cre), but not chondrocyte-specific (Runx3(fl/fl)/Col1α2-cre), mice are osteopenic. This demonstrates that an osteoblastic cell-autonomous function of Runx3 is required for proper osteogenesis. Bone histomorphometry revealed that decreased osteoblast numbers and reduced mineral deposition capacity in Runx3-deficient mice cause this bone formation deficiency. Neonatal bone and cultured primary osteoblast analyses revealed a Runx3-deficiency-associated decrease in the number of active osteoblasts resulting from diminished proliferation and not from enhanced osteoblast apoptosis. These findings are supported by Runx3-null culture transcriptome analyses showing significant decreases in the levels of osteoblastic markers and increases in the levels of Notch signaling components. Thus, while Runx2 is mandatory for the osteoblastic lineage commitment, Runx3 is nonredundantly required for the proliferation of these precommitted cells, to generate adequate numbers of active osteoblasts. Human RUNX3 resides on chromosome 1p36, a region that is associated with osteoporosis. Therefore, RUNX3 might also be involved in human bone mineralization.

Quantitative trait loci that modulate trabecular bone's risk of failure during unloading and reloading

Genetic makeup of an individual is a strong determinant of the morphologic and mechanical properties of bone. Here, in an effort to identify quantitative trait loci (QTLs) for changes in the simulated mechanical parameters of trabecular bone during altered mechanical demand, we subjected 352 second generation female adult (16 weeks old) BALBxC3H mice to 3 weeks of hindlimb unloading followed by 3 weeks of reambulation. Longitudinal in vivo microcomputed tomography (μCT) scans tracked trabecular changes in the distal femur. Tomographies were directly translated into finite element (FE) models and subjected to a uniaxial compression test. Apparent trabecular stiffness and components of the Von Mises (VM) stress distributions were computed for the distal metaphysis and associated with QTLs. At baseline, five QTLs explained 20% of the variation in trabecular peak stresses across the mouse population. During unloading, three QTLs accounted for 14% of the variability in peak stresses. During reambulation, one QTL accounted for 5% of the variability in peak stresses. QTLs were also identified for mechanically induced changes in stiffness, median stress values and skewness of stress distributions. There was little overlap between QTLs identified for baseline and QTLs for longitudinal changes in mechanical properties, suggesting that distinct genes may be responsible for the mechanical response of trabecular bone. Unloading related QTLs were also different from reambulation related QTLs. Further, QTLs identified here for mechanical properties differed from previously identified QTLs for trabecular morphology, perhaps revealing novel gene targets for reducing fracture risk in individuals exposed to unloading and for maximizing the recovery of trabecular bone's mechanical properties during reambulation.

Quantitative trait loci for energy balance traits in an advanced intercross line derived from mice divergently selected for heat loss

Obesity in human populations, currently a serious health concern, is considered to be the consequence of an energy imbalance in which more energy in calories is consumed than is expended. We used interval mapping techniques to investigate the genetic basis of a number of energy balance traits in an F₁₁ advanced intercross population of mice created from an original intercross of lines selected for increased and decreased heat loss. We uncovered a total of 137 quantitative trait loci (QTLs) for these traits at 41 unique sites on 18 of the 20 chromosomes in the mouse genome, with X-linked QTLs being most prevalent. Two QTLs were found for the selection target of heat loss, one on distal chromosome 1 and another on proximal chromosome 2. The number of QTLs affecting the various traits generally was consistent with previous estimates of heritabilities in the same population, with the most found for two bone mineral traits and the least for feed intake and several body composition traits. QTLs were generally additive in their effects, and some, especially those affecting the body weight traits, were sex-specific. Pleiotropy was extensive within trait groups (body weights, adiposity and organ weight traits, bone traits) and especially between body composition traits adjusted and not adjusted for body weight at sacrifice. Nine QTLs were found for one or more of the adiposity traits, five of which appeared to be unique. The confidence intervals among all QTLs averaged 13.3 Mb, much smaller than usually observed in an F₂ cross, and in some cases this allowed us to make reasonable inferences about candidate genes underlying these QTLs. This study combined QTL mapping with genetic parameter analysis in a large segregating population, and has advanced our understanding of the genetic architecture of complex traits related to obesity.

Modulation of strain-specific differences in gene expression by cannabinoid type 2 receptor deficiency

Previous studies have shown that the skeletal consequences of cannabinoid receptor deficiency differ in different strains of mice. In order to explore the mechanisms responsible, we analysed global gene expression in bone from wild-type CD1 mice and littermates with targeted inactivation of the type 2 cannabinoid receptor (Cnr2 (-/-)) and compared the results with those obtained from a similar analysis of wild-type and Cnr2 (-/-) C57BL/6 mice. Trabecular bone volume was increased in Cnr2 (-/-) CD1 mice compared with wild-type littermates but decreased in Cnr2 (-/-) C57BL/6 mice. Microarray analysis identified 354 genes in which substantial differences in gene expression (>1.5-fold) were observed that were specifically affected by Cnr2 deficiency. Bioinformatic analysis of data from wild-type mice of each strain revealed Cnr2-dependent differences in expression of genes clustering within the gene ontology (GO) terms immune response (p < 0.0001), positive regulation of response to stimulus (p < 0.0001), nucleotide binding (p = 0.002), and ribonucleotide binding (p = 0.003). Bioinformatic analysis of data from Cnr2 (-/-) mice of each strain revealed associations between GO terms corresponding to the extracellular region (p = 0.002), the cell surface (p = 0.02), antigen binding (p = 0.03), external side of plasma membrane (p = 0.04), and regulation of the force of heart contraction (p = 0.04). We conclude that Cnr2 deficiency affects expression of a large number of genes in different strains of mice, and that these differences are likely to be responsible in part for the differences in skeletal phenotype that we and others have observed in mice with defective cannabinoid receptor signalling in different genetic backgrounds.

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.

Congenic mice provide in vivo evidence for a genetic locus that modulates intrinsic transforming growth factor β1-mediated signaling and bone acquisition

Osteoporosis, the most common skeletal disorder, is characterized by low bone mineral density (BMD) and an increased risk of fragility fractures. BMD is the best clinical predictor of future osteoporotic fracture risk, but is a complex trait controlled by multiple environmental and genetic determinants with individually modest effects. Quantitative trait locus (QTL) mapping is a powerful method for identifying chromosomal regions encompassing genes involved in shaping complex phenotypes, such as BMD. Here we have applied QTL analysis to male and female genetically-heterogeneous F(2) mice derived from a cross between C57BL/6 and DBA/2 strains, and have identified 11 loci contributing to femoral BMD. Further analysis of a QTL on mouse chromosome 7 following the generation of reciprocal congenic strains has allowed us to determine that the high BMD trait, which tracks with the DBA/2 chromosome and exerts equivalent effects on male and female mice, is manifested by enhanced osteogenic differentiation of mesenchymal stem cells (MSCs) in vitro and by increased growth of metatarsal bones in short-term primary culture. An insertion/deletion DNA polymorphism in Ltbp4 exon 12 that causes the in-frame removal of 12 codons in the DBA/2-derived gene maps within 0.6 Mb of the marker most tightly linked to the QTL. LTBP4, one of four paralogous mouse proteins that modify the bioavailability of the transforming growth factor β (TGF-β) family of growth factors, is expressed in differentiating MSC-derived osteoblasts and in long bones, and reduced responsiveness to TGF-β1 is observed in MSCs of mice homozygous for the DBA/2 chromosome 7. Taken together, our results identify a potential genetic and biochemical relationship between decreased TGF-β1-mediated signaling and enhanced femoral BMD that may be regulated by a variant LTBP4 molecule.

Rare coding variants in ALPL are associated with low serum alkaline phosphatase and low bone mineral density

Alkaline phosphatase (ALP) plays an essential role in the regulation of tissue mineralization, and its activity is highly heritable. Guided by genetic associations discovered in a murine model, we hypothesized a role for rare coding variants in determining serum ALP level and bone mineral density (BMD) in humans. We sequenced the coding regions of the ALP gene (ALPL) in men with low and normal serum ALP activity levels. Single-nucleotide ALPL variants, including 19 rare nonsynonymous variants (minor allele frequency <1%), were much more frequent among the low ALP group (33.8%) than the normal group (1.4%, p = 1 × 10(-11)). Within the low ALP group, men with a rare, nonsynonymous variant had 11.2% lower mean serum ALP (p = 3.9 × 10(-4)), 6.7% lower BMD (p = 0.03), and 11.1% higher serum phosphate (p = 0.002) than those without. In contrast, common nonsynonymous variants had no association with serum ALP, phosphate, or BMD. Multiple rare ALPL coding variants are present in the general population, and nonsynonymous coding variants may be responsible for heritable differences in mineralization and thus BMD.

Whole bone mechanics and bone quality

BACKGROUND

The skeleton plays a critical structural role in bearing functional loads, and failure to do so results in fracture. As we evaluate new therapeutics and consider treatments to prevent skeletal fractures, understanding the basic mechanics underlying whole bone testing and the key principles and characteristics contributing to the structural strength of a bone is critical.

QUESTIONS/PURPOSES

We therefore asked: (1) How are whole bone mechanical tests performed and what are the key outcomes measured? (2) How do the intrinsic characteristics of bone tissue contribute to the mechanical properties of a whole bone? (3) What are the effects of extrinsic characteristics on whole bone mechanical behavior? (4) Do environmental factors affect whole bone mechanical properties?

METHODS

We conducted a PubMed search using specific search terms and limiting our included articles to those related to in vitro testing of whole bones. Basic solid mechanics concepts are summarized in the context of whole bone testing and the determinants of whole bone behavior.

RESULTS

Whole bone mechanical tests measure structural stiffness and strength from load-deformation data. Whole bone stiffness and strength are a function of total bone mass and the tissue geometric distribution and material properties. Age, sex, genetics, diet, and activity contribute to bone structural performance and affect the incidence of skeletal fractures.

CONCLUSIONS

Understanding and preventing skeletal fractures is clinically important. Laboratory tests of whole bone strength are currently the only measures for in vivo fracture prediction. In the future, combined imaging and engineering models may be able to predict whole bone strength noninvasively.

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.

Serious limitations of the QTL/microarray approach for QTL gene discovery

BACKGROUND

It has been proposed that the use of gene expression microarrays in nonrecombinant parental or congenic strains can accelerate the process of isolating individual genes underlying quantitative trait loci (QTL). However, the effectiveness of this approach has not been assessed.

RESULTS

Thirty-seven studies that have implemented the QTL/microarray approach in rodents were reviewed. About 30% of studies showed enrichment for QTL candidates, mostly in comparisons between congenic and background strains. Three studies led to the identification of an underlying QTL gene. To complement the literature results, a microarray experiment was performed using three mouse congenic strains isolating the effects of at least 25 biometric QTL. Results show that genes in the congenic donor regions were preferentially selected. However, within donor regions, the distribution of differentially expressed genes was homogeneous once gene density was accounted for. Genes within identical-by-descent (IBD) regions were less likely to be differentially expressed in chromosome 2, but not in chromosomes 11 and 17. Furthermore, expression of QTL regulated in cis (cis eQTL) showed higher expression in the background genotype, which was partially explained by the presence of single nucleotide polymorphisms (SNP).

CONCLUSIONS

The literature shows limited successes from the QTL/microarray approach to identify QTL genes. Our own results from microarray profiling of three congenic strains revealed a strong tendency to select cis-eQTL over trans-eQTL. IBD regions had little effect on rate of differential expression, and we provide several reasons why IBD should not be used to discard eQTL candidates. In addition, mismatch probes produced false cis-eQTL that could not be completely removed with the current strains genotypes and low probe density microarrays. The reviewed studies did not account for lack of coverage from the platforms used and therefore removed genes that were not tested. Together, our results explain the tendency to report QTL candidates as differentially expressed and indicate that the utility of the QTL/microarray as currently implemented is limited. Alternatives are proposed that make use of microarray data from multiple experiments to overcome the outlined limitations.

Identification of genes influencing skeletal phenotypes in congenic P/NP rats

We previously showed that alcohol-preferring (P) rats have higher bone density than alcohol-nonpreferring (NP) rats. Genetic mapping in P and NP rats identified a major quantitative trait locus (QTL) between 4q22 and 4q34 for alcohol preference. At the same location, several QTLs linked to bone density and structure were detected in Fischer 344 (F344) and Lewis (LEW) rats, suggesting that bone mass and strength genes might cosegregate with genes that regulate alcohol preference. The aim of this study was to identify the genes segregating for skeletal phenotypes in congenic P and NP rats. Transfer of the NP chromosome 4 QTL into the P background (P.NP) significantly decreased areal bone mineral density (aBMD) and volumetric bone mineral density (vBMD) at several skeletal sites, whereas transfer of the P chromosome 4 QTL into the NP background (NP.P) significantly increased bone mineral content (BMC) and aBMD in the same skeletal sites. Microarray analysis from the femurs using Affymetrix Rat Genome arrays revealed 53 genes that were differentially expressed among the rat strains with a false discovery rate (FDR) of less than 10%. Nine candidate genes were found to be strongly correlated (r(2) > 0.50) with bone mass at multiple skeletal sites. The top three candidate genes, neuropeptide Y (Npy), alpha synuclein (Snca), and sepiapterin reductase (Spr), were confirmed using real-time quantitative PCR (qPCR). Ingenuity pathway analysis revealed relationships among the candidate genes related to bone metabolism involving beta-estradiol, interferon-gamma, and a voltage-gated calcium channel. We identified several candidate genes, including some novel genes on chromosome 4 segregating for skeletal phenotypes in reciprocal congenic P and NP rats.

Quantitative trait loci for tibial bone strength in C57BL/6J and C3H/HeJ inbred strains of mice

Three-point bending technology has been widely used in the measurement of bone strength. Quantitative trait loci (QTLs) for bone strength have been identified using mouse femurs. In this study, we investigate the use of mouse tibiae in identification of QTLs that regulate bone strength. Mouse tibiae were from a F(2) population derived from C57BL/6J (B6) and C3H/HeJ (C3H). Three-point bending was measured using ISO 4049, with the support width adjustable to accommodate specimen sizes outside the scope of ISO 4049. The strain rate is selectable from 0.05 to 500 mm per min. All stress strain diagrams are recorded and retrieved in digital electronic form. Genome scan was performed in The Jackson Laboratory (TJL). QTL mapping was conducted using Map Manager QTX software. Data show that (i) both elastic modulus (stiffness) and maximum loading (strength) value appear as normal distributions, suggesting that multiple genetic factors control the bone strength; (ii) 11 QTLs, accounting for 90% of variation for strength, have been detected. More than half QTLs of three-point bending are located on the same locations of bone density earlier identified from mouse femurs; (iii) a major QTL of femoral and vertebral bone mineral density (BMD) was not detected for bone strength of tibiae; (iv) the QTL on chromosome 4 has extremely high LOD score of 31.8 and represents 60% of the variation of bone strength; and (v) four QTLs of stiffness (chromosomes 2, 11, 15 and 19) have been identified.

Association of adenylate cyclase 10 (ADCY10) polymorphisms and bone mineral density in healthy adults

Phenotypic variation in bone mineral density (BMD) among healthy adults is influenced by both genetic and environmental factors. Sequence variations in the adenylate cyclase 10 (ADCY10) gene, which is also called soluble adenylate cyclase, have previously been associated with low spinal BMD in hypercalciuric patients. Since ADCY10 is located in the region linked to spinal BMD in our previous linkage analysis, we tested whether polymorphisms in this gene are also associated with normal BMD variation in healthy adults. Sixteen single-nucleotide polymorphisms (SNPs) distributed throughout ADCY10 were genotyped in two healthy groups of American whites: 1692 premenopausal women and 715 men. Statistical analyses were performed in the two groups to test for association between these SNPs and the femoral neck and lumbar spine areal BMD. We observed significant evidence of association (p < 0.01), with one SNP each in men and women. Genotypes at these SNPs accounted for <1% of hip BMD variation in men but 1.5% of spinal BMD in women. However, adjacent SNPs did not corroborate the association in either men or women. In conclusion, we found a modest association between an ADCY10 polymorphism and the spinal areal BMD in premenopausal white women.

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.

Identification of a linkage disequilibrium block in chromosome 1q associated with BMD in premenopausal white women

Osteoporosis is a complex disease with both genetic and environmental risk factors. A major determinant of osteoporotic fractures is peak BMD obtained during young adulthood. We previously reported linkage of chromosome 1q (LOD = 4.3) with variation in spinal areal BMD in healthy premenopausal white women. In this study, we used a two-stage genotyping approach to identify genes in the linked region that contributed to the variation of femoral neck and lumbar spine areal BMD. In the first stage, 654 SNPs across the linked region were genotyped in a sample of 1309 premenopausal white women. The most significant evidence of association for lumbar spine (p = 1.3 x 10(-6)) was found with rs1127091 in the GATAD2B gene. In the second stage, 52 SNPs around this candidate gene were genotyped in an expanded sample of 1692 white women. Significant evidence of association with spinal BMD (p < 10(-5)), and to a lesser extent with femoral neck BMD, was observed with eight SNPs within a single 230-kb linkage disequilibrium (LD) block. The most significant SNP (p = 3.4 x 10(-7)) accounted for >2.5% of the variation in spinal BMD in these women. The 230-kb LD block contains 11 genes, but because of the extensive LD, the specific gene(s) contributing to the variation in BMD could not be determined. In conclusion, the significant association between spinal BMD and SNPs in the 230-kb LD block in chromosome 1q indicates that genetic factor(s) in this block plays an important role in peak spinal BMD in healthy premenopausal white women.

Pleiotropic patterns of quantitative trait loci for 70 murine skeletal traits

Quantitative trait locus (QTL) studies of a skeletal trait or a few related skeletal components are becoming commonplace, but as yet there has been no investigation of pleiotropic patterns throughout the skeleton. We present a comprehensive survey of pleiotropic patterns affecting mouse skeletal morphology in an intercross of LG/J and SM/J inbred strains (N = 1040), using QTL analysis on 70 skeletal traits. We identify 798 single-trait QTL, coalescing to 105 loci that affect on average 7-8 traits each. The number of traits affected per locus ranges from only 1 trait to 30 traits. Individual traits average 11 QTL each, ranging from 4 to 20. Skeletal traits are affected by many, small-effect loci. Significant additive genotypic values average 0.23 standard deviation (SD) units. Fifty percent of loci show codominance with heterozygotes having intermediate phenotypic values. When dominance does occur, the LG/J allele tends to be dominant to the SM/J allele (30% vs. 8%). Over- and underdominance are relatively rare (12%). Approximately one-fifth of QTL are sex specific, including many for pelvic traits. Evaluating the pleiotropic relationships of skeletal traits is important in understanding the role of genetic variation in the growth and development of the skeleton.

Genetics of the musculoskeletal system: a pleiotropic approach

The risk of osteoporotic fracture can be viewed as a function of loading conditions and the ability of the bone to withstand the load. Skeletal loads are dominated by muscle action. Recently, it has become clear that bone and muscle share genetic determinants. Involution of the musculoskeletal system manifests as bone loss (osteoporosis) and muscle wasting (sarcopenia). Therefore, the consideration of pleiotropy is an important aspect in the study of the genetics of osteoporosis and sarcopenia. This Perspective will provide the evidence for a shared genetic influence on bone and muscle. We will start with an overview of accumulating evidence that physical exercise produces effects on the adult skeleton, seeking to unravel some of the contradictory findings published thus far. We will provide indications that there are pleiotropic relationships between bone structure/mass and muscle mass/function. Finally, we will offer some insights and practical recommendations as to the value of studying shared genetic factors and will explore possible directions for future research. We consider several related questions that together comprise the general paradigm of bone responses to mechanical loading and the relationship between muscle strength and bone parameters, including the genetic factors that modulate these responses. We believe that further progress in understanding the common genetic etiology of osteoporosis and sarcopenia will provide valuable insight into important biological underpinnings for both conditions and may translate into new approaches to reduce the burdens of both conditions through improved diagnosis, prevention, and early targeted treatment.

Mapping of the chromosome 17 BMD QTL in the F(2) male mice of MRL/MpJ x SJL/J

Developing treatment strategies for osteoporosis would be facilitated by identifying genes regulating bone mineral density (BMD). One way to do so is through quantitative trait locus (QTL) mapping. However, there are sex differences in terms of the presence/absence and locations of BMD QTLs. In a previous study, our group identified a BMD QTL on chromosome 17 in the F(2) female mice of the MRL/MpJ x SJL/J cross. Here, we determined whether it was also present in the male mice of the same cross. Furthermore, we also intended to reduce the QTL region by increasing marker density. Interval mapping showed that the same QTL based on chromosomal positions was present in the male mice, with logarithmic odds (LOD) scores of 4.0 for femur BMD and 5.2 for total body BMD. Although there was a body weight QTL at the same location, the BMD QTL was not affected by the adjustment for body weight. Mapping with increased marker density indicated a most likely region of 35-55 Mb for this QTL. There were also co-localized QTLs for femur length, femur periosteal circumference (PC) and total body bone area, suggesting possibility of pleiotropy. Runx2 and VEGFA are strong candidate genes located within this QTL region.

Studies on the Small Body Size Mouse Developed by Mutagen N-Ethyl-N-nitrosourea

Mutant mouse which show dwarfism has been developed by N-ethyl-N-nitrosourea (ENU) mutagenesis using BALB/c mice. The mutant mouse was inherited as autosomal recessive trait and named Small Body Size (SBS) mouse. The phenotype of SBS mouse was not apparent at birth, but it was possible to distinguish mutant phenotype from normal mice 1 week after birth. In this study, we examined body weight changes and bone mineral density (BMD), and we also carried out genetic linkage analysis to map the causative gene(s) of SBS mouse. Body weight changes were observed from birth to 14 weeks of age in both affected (n = 30) and normal mice (n = 24). BMD was examined in each five SBS and normal mice between 3 and 6 weeks of age, respectively. For the linkage analysis, we produced backcross progeny [(SBS × C57BL/6J) F₁ × SBS] N₂ mice (n = 142), and seventy-four microsatellite markers were used for primary linkage analysis. Body weight of affected mice was consistently lower than that of the normal mice, and was 43.7% less than that of normal mice at 3 weeks of age (P < 0.001). As compared with normal mice at 3 and 6 weeks of age, BMD of the SBS mice was significantly low. The results showed 15.5% and 14.1% lower in total body BMD, 15.3% and 8.7% lower in forearm BMD, and 29.7% and 20.1% lower in femur BMD, respectively. The causative gene was mapped on chromosome 10. The map order and the distance between markers were D10Mit248 - 2.1 cM - D10Mit51 - 4.2 cM - sbs - 0.7 cM - D10Mit283 - 1.4 cM - D10Mit106 - 11.2 cM - D10Mit170.

Bone microstructure and its associated genetic variability in 12 inbred mouse strains: microCT study and in silico genome scan

MicroCT analysis of 12 inbred strains of mice identified 5 novel chromosomal regions influencing skeletal phenotype. Bone morphology varied in a compartment- and site-specific fashion across strains and genetic influences contributed to the morphometric similarities observed in femoral and vertebral bone within the trabecular bone compartment.

INTRODUCTION

Skeletal development is known to be regulated by both heritable and environmental factors, but whether genetic influence on peak bone mass is site- or compartment-specific is unknown. This study examined the genetic variation of cortical and trabecular bone microarchitecture across 12 strains of mice.

MATERIALS AND METHODS

MicroCT scanning was used to measure trabecular and cortical bone morphometry in the femur and vertebra of 12 strains of 4-month-old inbred male mice. A computational genome mapping technique was used to identify chromosomal intervals associated with skeletal traits.

RESULTS

Skeletal microarchitecture varied in a compartment- and site-specific fashion across strains. Genome mapping identified 13 chromosomal intervals associated with skeletal traits and 5 of these intervals were novel. Trabecular microarchitecture in different bone sites correlated across strains and most of the chromosomal intervals associated with these trabecular traits were shared between skeletal sites. Conversely, no chromosomal intervals were shared between the trabecular and cortical bone compartments in the femur, even though there was a strong correlation for these different bone compartments across strains, suggesting site-specific regulation by environmental or intrinsic factors.

CONCLUSION

In summary, these data confirm that there are distinct genetic determinants that define the skeletal phenotype at the time when peak bone mass is being acquired, and that genomic regulation of bone morphology is specific for skeletal compartment.

Automated compartmental analysis for high-throughput skeletal phenotyping in femora of genetic mouse models

Mouse models are widely used in genomic studies for quantitative trait loci (QTL) analyses. In the field of skeletal micro-structure, microCT has proven to be an invaluable imaging tool for the characterization of structural bone traits. However, the definition of analysis compartments requires a lot of user interaction, and therefore is not applicable as a standard way to analyze genetic linkage studies with several hundreds of animals. Here, we developed an automated three-dimensional based algorithm for a high-throughput regional analysis of three compartments in murine femora, including whole bone, cortical bone in the diaphysis and trabecular bone in the metaphysis. The algorithm relies on basic image processing concepts using morphological operators as well as a new approach of separating cortical from trabecular bone. Reproducibility of the automatic approach was investigated with respect to precision errors (PE(%CV)) of micro-structural indices analyzed in these automatically defined compartments. The developed algorithm was then used to perform a high-throughput analysis of over 2000 femora in a genetic linkage study for further examination of stability and performance. Precision errors were 3.5% or less for all micro-structural indices. The analysis of one femur (mask generation and parameter evaluation) took 7 min on an AlphaServer DS25. The algorithm showed a very high reliability and worked successfully for 99.64% of all femora. Investigations of correlations amongst the assessed micro-structural indices together with heritability and polygene estimates revealed apparent volume density (AVD), trabecular thickness (Tb.Th), trabecular separation (Tb.Sp) and cortical thickness (Ct.Th) as candidates for a successful QTL analysis. The presented automatic analysis allows for standardized high-throughput phenotypic screening in mice femora for large genetic linkage studies with very high reliability and good precision.

P518/Qrfp sequence polymorphisms in SAMP6 osteopenic mouse

Mice lacking GPR103A expression display osteopenia. Analysis of mouse quantitative trait loci literature associated with bone mineral density suggested GPR103A ligand P518/Qrfp (chromosome 2qB) as a candidate osteoporosis gene. Promoter and coding regions of mouse P518/Qrfp were sequenced from genomic DNA obtained from the osteoporosis-prone strain SAMP6 and control strains SAMR1, A/J, AKR/J, BALB/c, C3H/HeJ, C57BL/6J, and DBA/2J. Four single-nucleotide polymorphisms (SNPs) were identified in only SAMP6 genomic DNA, g.-1773 T-->C, g.110 A-->G (N37S), g.188 G-->A (R63K), and g.135 T-->C (H45H). The promoter SNP generated a novel neuron-restrictive silencing factor binding site, a repressor that decreases gene expression in nonneuronal tissues. TaqMan analysis demonstrated fivefold lower P518/Qrfp liver expression in SAMP6 versus SAMR1 or C57BL/6J control strains. Tissue distribution of human, mouse, and rat P518/Qrfp and its receptors showed expression in bone and spinal cord. A direct role for P518/Qrfp function in maintaining bone mineral density is suggested.

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.

Genetics and osteoporosis

Over the past 10 years, many advances have been made in understanding the mechanisms by which genetic factors regulate susceptibility to osteoporosis. It has become clear from studies in man and experimental animals that different genes regulate BMD at different skeletal sites and in men and women. Linkage studies have identified several chromosomal regions that regulate BMD, but only a few causative genes have been discovered so far using this approach. In contrast, significant advances have been made in identifying the genes that cause monogenic bone diseases, and polymorphic variation is some of these genes has been found to contribute to the genetic regulation of BMD in the normal population. Other genes that have been investigated as possible candidates for susceptibility to osteoporosis because of their role in bone biology, such as vitamin D, have yielded mixed results. Many candidate gene association studies have been underpowered, and meta-analysis has been used to try to confirm or refute potential associations and gain a better estimate of their true effect size in the population. Most of the genetic variants that confer susceptibility to osteoporosis remain to be discovered. It is likely that new techniques such as whole-genome association will provide new insights into the genetic determinants of osteoporosis and will help to identify genes of modest effect size. From a clinical standpoint, genetic variants that are found to predispose to osteoporosis will advance our understanding of the pathophysiology of the disease. They could be developed as diagnostic genetic tests or form molecular targets for design of new drugs for the prevention and treatment of osteoporosis and other bone diseases.

Detecting novel bone density and bone size quantitative trait loci using a cross of MRL/MpJ and CAST/EiJ inbred mice

Most previous studies to identify loci involved in bone mineral density (BMD) regulation have used inbred strains with high and low BMD in generating F(2) mice. However, differences in BMD may not be a requirement in selecting parental strains for BMD quantitative trait loci (QTL) studies. In this study, we intended to identify novel QTL using a cross of two strains, MRL/MpJ (MRL) and CAST/EiJ (CAST), both of which exhibit relatively high BMD when compared to previously used strains. In addition, CAST was genetically distinct. We generated 328 MRL x CAST F(2) mice of both sexes and measured femur BMD and periosteal circumference (PC) using peripheral quantitative computed tomography. Whole-genome genotyping was performed with 86 microsatellite markers. A new BMD QTL on chromosome 10 and another suggestive one on chromosome 15 were identified. A significant femur PC QTL identified on chromosome 9 and a suggestive one on chromosome 2 were similar to those detected in MRL x SJL. QTL were also identified for other femur and forearm bone density and bone size phenotypes, some of which were colocalized within the same chromosomal positions as those for femur BMD and femur PC. This study demonstrates the utility of crosses involving inbred strains of mice which exhibit a similar phenotype in QTL identification.

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.

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.

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.

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.

Quantitative trait loci that determine BMD in C57BL/6J and 129S1/SvImJ inbred mice

BMD is highly heritable; however, little is known about the genes. To identify loci controlling BMD, we conducted a QTL analysis in a (B6 x 129) F2 population of mice. We report on additional QTLs and also narrow one QTL by combining the data from multiple crosses and through haplotype analysis.

INTRODUCTION

Previous studies have identified quantitative trait loci (QTL) that determine BMD in mice; however, identification of genes underlying QTLs is impeded by the large size of QTL regions.

MATERIALS AND METHODS

To identify loci controlling BMD, we performed a QTL analysis of 291 (B6 x 129) F2 females. Total body and vertebral areal BMD (aBMD) were determined by peripheral DXA when mice were 20 weeks old and had consumed a high-fat diet for 14 weeks.

RESULTS AND CONCLUSIONS

Two QTLs were common for both total body and vertebral aBMD: Bmd20 on chromosome (Chr) 6 (total aBMD; peak cM 26, logarithm of odds [LOD] 3.8, and vertebral aBMD; cM 32, LOD 3.6) and Bmd22 on Chr 1 (total aBMD; cM 104, LOD 2.5, and vertebral aBMD; cM 98, LOD 2.6). A QTL on Chr 10 (Bmd21, cM 68, LOD 3.0) affected total body aBMD and a QTL on Chr 7 (Bmd9, cM 44, LOD 2.7) affected vertebral aBMD. A pairwise genome-wide search did not reveal significant gene-gene interactions. Collectively, the QTLs accounted for 21.6% of total aBMD and 17.3% of vertebral aBMD of the F(2) population variances. Bmd9 was previously identified in a cross between C57BL/6J and C3H/HeJ mice, and we narrowed this QTL from 34 to 22 cM by combining the data from these crosses. By examining the Bmd9 region for conservation of ancestral alleles among the low allele strains (129S1/SvImJ and C3H/HeJ) that differed from the high allele strain (C57BL/6J), we further narrowed the region to approximately 9.9 cM, where the low allele strains share a common haplotype. Identifying the genes for these QTLs will enhance our understanding of skeletal biology.

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.

Interspecies synteny mapping identifies a quantitative trait locus for bone mineral density on human chromosome Xp22

Bone mineral density (BMD) is a complex trait with a strong genetic component and an important predictor of osteoporotic fracture risk. Here we report the use of a cross-species strategy to identify genes that regulate BMD, proceeding from quantitative trait mapping in mice to association mapping of the syntenic region in the human genome. We identified a quantitative trait locus (QTL) on the mouse X-chromosome for post-maturity change in spine BMD in a cross of SAMP6 and AKR/J mice and conducted association mapping of the syntenic region on human chromosome Xp22. We studied 76 single nucleotide polymorphisms (SNP) from the human region in two sets of DNA pools prepared from individuals with lumbar spine-BMD (LS-BMD) values falling into the top and bottom 13th percentiles of a population-based study of 3100 post-menopausal women. This procedure identified a region of significant association for two adjacent SNP (rs234494 and rs234495) within the Xp22 locus (P<0.001). Individual genotyping for rs234494 in the BMD pools confirmed the presence of an association for alleles (P=0.018) and genotypes (P=0.008). Analysis of rs234494 and rs234495 in 1053 women derived from the same population who were not selected for BMD values showed an association with LS-BMD for rs234495 (P=0.01) and for haplotypes defined by both SNP (P=0.002). Our study illustrates that interspecies synteny can be used to identify and refine QTL for complex traits and represents the first example where a human QTL for BMD regulation has been mapped using this approach.

Whole-genome scan for linkage to bone strength and structure in inbred Fischer 344 and Lewis rats

A genome-wide genetic linkage analysis identified several chromosomal regions influencing bone strength and structure in F2 progeny of Fischer 344 x Lewis inbred rats.

INTRODUCTION

Inbred Fischer 344 (F344) and Lewis (LEW) rats are similar in body size, but the F344 rats have significantly lower BMD and biomechanical strength of the femur and spine compared with LEW rats. The goal of this study was to identify quantitative trait loci (QTL) linked to bone strength and structure in adult female F2 rats from F344 and LEW progenitors.

MATERIALS AND METHODS

The 595 F2 progeny from F344 x LEW rats were phenotyped for measures of bone strength (ultimate force [Fu]; energy to break [U]; stiffness [S]) of the femur and lumbar vertebra and structure (femur midshaft polar moment of inertia [Ip]; femur midshaft cortical area; vertebral area). A genome-wide scan was completed in the F2 rats using 118 microsatellite markers at an average interval of 20 cM. Multipoint quantitative linkage analysis was performed to identify chromosomal regions that harbor QTL for bone strength and structure phenotypes.

RESULTS

Evidence of linkage for femur and lumbar strength was observed on chromosomes (Chrs) 1, 2, 5, 10, and 19. Significant linkage for femoral structure was detected on Chrs 2, 4, 5, 7, and 15. QTLs affecting femoral strength on Chrs 2 and 5 were also found to influence femur structure. Unique QTLs on Chrs 1, 10, and 19 were found that contributed to variability in bone strength but had no significant effect on structure. Also, unique QTLs were observed on Chrs 4, 7, and 15 that affected only bone structure without any effect on biomechanics.

CONCLUSION

We showed multiple genetic loci influencing bone strength and structure in F344 x LEW F2 rats. Some of these loci are homologous to mouse and human chromosomes previously linked to related bone phenotypes.

Univariate and bivariate variance component linkage analysis of a whole-genome scan for loci contributing to bone mineral density

Osteoporosis is a common condition characterized by reduced skeletal strength and increased susceptibility to fracture. The single major risk factor for osteoporosis is low bone mineral density (BMD) and strong evidence exists that genetic factors are in part responsible for an individual's BMD. A cohort of 40 multiplex Caucasian families selected through a proband with osteoporosis was genotyped for microsatellite markers spaced at an average of 10 cM, and linkage to femoral neck (FN), lumbar spine (LS) and trochanter (TR) BMD was analyzed using univariate and bivariate variance component linkage analysis. Maximum univariate multipoint lod-scores were 2.87 on chromosome 1p36 for FN BMD, 1.89 on 6q27 for TR BMD, and 2.15 on 7p15 for LS BMD. Results of bivariate linkage analysis were highly correlated with those of the univariate analysis, although generally less significant, suggesting the possibility that some of these susceptibility loci may exert pleiotropic effects on multiple skeletal sites.

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.

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.

Bone biomechanical properties in LRP5 mutant mice

The mutation responsible for the high bone mass (HBM) phenotype has been postulated to act through the adaptive response of bone to mechanical load resulting in denser and stronger skeletons in humans and animals. The bone phenotype of members of a HBM family is characterized by normally shaped bones that are exceptionally dense, particularly at load bearing sites [Cancer Res. 59 (1999) 1572]. The high bone mass (HBM) mutation was identified as a glycine to valine substitution at amino acid residue 171 in the gene coding for low-density lipoprotein receptor-related protein 5 (LRP5) [Bone Miner. Res. 16(4) (2001) 758]. Thus, efforts have focused on the examination of the role of LRP5 and the G171V mutation in bone mechanotransduction responses [J. Bone Miner. Res 18 (2002) 960]. Transgenic mice expressing the human G171V mutation have been shown to have skeletal phenotypes remarkably similar to those seen in affected individuals. In this study, we have identified differences in biomechanical (structural and apparent material) properties, bone mass/ash, and bone stiffness of cortical and cancellous bone driven by the G171V mutation in LRP5. As in humans, the LRP5 G171V plays an important role in regulating bone structural phenotypes in mice. These bone phenotypes include greater structural and apparent material properties in HBM HET as compared to non-transgenic littermates (NTG) mice. Body size and weight in HBM HET were similar to that in NTG control mice. However, the LRP5 G171V mutation in HET mice results in a skeleton that has greater structural (femoral shaft, femoral neck, tibiae, vertebral body) and apparent material (vertebral body) strength, percent bone ash weight (ulnae), and tibial stiffness. Despite similar body weight to NTG mice, the denser and stiffer bones in G171V mice may represent greater bone formation sensitivity to normal mechanical stimuli resulting in an overadaptation of skeleton to weight-related forces.

Genome scan for QTLs underlying bone size variation at 10 refined skeletal sites: genetic heterogeneity and the significance of phenotype refinement

To identify quantitative trait loci (QTLs) underlying variation in bone size, we conducted a whole-genome linkage scan in 53 pedigrees with 630 subjects using 380 microsatellite markers. Lumbar area 1, 2, 3, and 4 at the spine, femoral neck, trochanter, intertrochanter areas at the hip, ultradistal, mid-distal, and one-third distal areas at the wrist were measured by dual-energy X-ray absorptiometry (DXA), and adjusted for age, height, weight, and sex. Two-point and multipoint linkage analyses were performed for skeletal bone size at each site and their composite measurements using the SOLAR package. Two chromosomal regions (1q22 and 10q21) were identified with significant evidence of linkage (LOD > 4.32) to one-third distal area, and three were identified with suggestive evidence of linkage (LOD > 2.93) to bone size in one skeletal site. Our results indicated that the low power of QTLs mapping for composite phenotypic measurements may result from genetic heterogeneity of complex traits.

Genetically based influences on the site-specific regulation of trabecular and cortical bone morphology

The degree of site-specificity by which genes influence bone quantity and architecture was investigated in the femur of three strains of mice. Morphological indices were highly dependent on both genetic makeup as well as anatomical location showing that the assessment of bone structure from a single site cannot be extrapolated to other sites even within a single bone.

INTRODUCTION

The identification of genes responsible for establishing peak BMD will yield critical information on the regulation of bone quantity and quality. Whereas such knowledge may eventually uncover novel molecular drug targets or enable the identification of individuals at risk of osteoporosis, the site-specificity by which putative genotypes cause low or high bone mass (and effective bone morphology) is essentially unknown.

MATERIALS AND METHODS

microCT was used to determine morphological and microarchitectural features of the femora harvested from three genetically distinct strains of 4-month-old female mice, each with distinct skeletal mass (low: C57BL/6J [B6], medium: BALB/cByJ [BALB], high: C3H/HeJ [C3H]). Two trabecular regions (distal epiphysis and metaphysis) were considered in addition to four cortical regions within the metaphysis and diaphysis.

RESULTS AND CONCLUSIONS

Comparing morphological properties of the different trabecular and cortical femoral regions between the three strains of mice, it was apparent that high or low values of specific parameters of bone morphology could not be consistently attributed to the same genetic strain. Trabecular metaphyseal bone volume, for instance, was 385% larger in C3H mice than in B6 mice, yet the two strains displayed similar bone volume fractions in the epiphysis. Similarly, BALB mice had 48% more trabecular bone than C3H mice in the epiphysis, but there were no strain-specific differences in cortical bone area at the diaphysis. These data suggest that the genetic control of bone mass and morphology, even within a given bone, is highly site-specific and that a comprehensive search for genes that are indicative of bone quantity and quality may also have to occur on a very site-specific basis.

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.

Genetically linked site-specificity of disuse osteoporosis

The genetic influence on bone loss in response to mechanical unloading was investigated within diaphyseal and distal femoral regions in three genetically distinct strains of mice. One mouse strain failed to lose bone after removal of function, whereas osteopenia was evident in multiple regions of the remaining two strains but in different areas of the bone.

INTRODUCTION

It is well recognized that susceptibility to osteoporosis is, in large measure, determined by the genome, but whether this influence is systemic or site-specific is not yet known. Here, the extent to which genetic variations influence regional bone loss caused by disuse was studied in the femora of adult female mice from three inbred strains.

MATERIALS AND METHODS

Adult C57BL/6J (B6), C3H/HeJ (C3H), and BALB/cByJ (BALB) mice were subjected to 15-21 days of disuse, achieved by hindlimb suspension, and six distinct anatomical regions of the femur were analyzed by high-resolution microCT.

RESULTS AND CONCLUSIONS

In B6 mice, the amount of disuse stimulated bone loss was relatively uniform across all regions, with 20% loss of trabecular bone and 10% loss of cortical bone. The degree of bone loss in BALB mice varied greatly, ranging from 59% in the metaphysis to 3% in the proximal diaphysis. In this strain, the nonuniformity of bone loss was directly related to the nonuniform distribution of baseline bone morphology (r2 = 0.94). In direct contrast with BALB and B6, disuse failed to produce significant losses of bone in any of the analyzed regions of the C3H mice. Instead, these animals displayed a unique compensatory mechanism to disuse, where the large loss of calcified tissue from the endocortical surface (-24%) was compensated for by an expansion of the periosteal envelope (10%). These data indicate a strong, yet complex, genetic dependence of the site-specific regulation of bone remodeling in response to a powerful catabolic signal. Consequently, the skeletal region of interest and the genetic make-up of the individual may have to be considered interdependently when considering the pathogenesis of osteoporosis or the efficacy of an intervention to prevent or recover bone loss.

Detection of different quantitative trait loci for antibody responses to keyhole lympet hemocyanin and Mycobacterium butyricum in two unrelated populations of laying hens

Quantitative trait loci involved in the primary antibody response to keyhole lympet hemocyanin (KLH) and Mycobacterium butyricum were detected in two independent populations of laying hens. The first population was an F2 cross (H/L) of lines divergently selected for either high or low primary antibody responses to SRBC, and the second population was an F2 cross between 2 commercial layer lines displaying differences in feather pecking behavior (FP). Both populations were typed with microsatellite markers widely distributed over the genome with similar intervals between markers. Titers of antibodies binding KLH and M. butyricum were measured for all individuals by ELISA. Two genetic models were applied to detect QTL involved in the humoral immune response: a half-sib model and a line-cross model, both using the regression interval method. In the half-sib analysis, 2 QTL (on GGA14 and GGA27) were detected for the antibody response to KLH for the H/L population, and 2 QTL (on GGA14 and GGA18) were detected for the FP population. Only 1 QTL was detected for M. butyricum on GGA14 in the FP population using the half-sib analysis model. Two QTL were detected for the FP population on GGA2 and GGA3 using the line-cross analysis model. A QTL for the primary antibody response to KLH detected on GGA14 was validated in both populations under the half-sib analysis model. The present data suggest differences in the genetic regulation of antibody responses to two different T-cell dependent antigens.

Regulation of bone mass in mice by the lipoxygenase gene Alox15

The development of osteoporosis involves the interaction of multiple environmental and genetic factors. Through combined genetic and genomic approaches, we identified the lipoxygenase gene Alox15 as a negative regulator of peak bone mineral density in mice. Crossbreeding experiments with Alox15 knockout mice confirmed that 12/15-lipoxygenase plays a role in skeletal development. Pharmacologic inhibitors of this enzyme improved bone density and strength in two rodent models of osteoporosis. These results suggest that drugs targeting the 12/15-lipoxygenase pathway merit investigation as a therapy for osteoporosis.

Confirmation of linkage to chromosome 1q for peak vertebral bone mineral density in premenopausal white women

Peak bone mineral density (BMD) is a highly heritable trait and is a good predictor of the risk of osteoporosis and fracture in later life. Recent studies have sought to identify the genes underlying peak BMD. Linkage analysis in a sample of 464 premenopausal white sister pairs detected linkage of spine BMD to chromosome 1q (LOD 3.6). An independent sample of 254 white sister pairs has now been genotyped, and it also provides evidence of linkage to chromosome 1q (LOD 2.5) for spine BMD. Microsatellite markers were subsequently genotyped for a 4-cM map in the chromosome 1q region in all available white sister pairs (n=938), and a LOD score of 4.3 was obtained near the marker D1S445. Studies in the mouse have also detected evidence of linkage to BMD phenotypes in the region syntenic to our linkage finding on chromosome 1q. Thus, we have replicated a locus on 1q contributing to BMD at the spine and have found further support for the region in analyses employing an enlarged sample. Studies are now ongoing to identify the gene(s) contributing to peak spine BMD in women.

ENU large-scale mutagenesis and quantitative trait linkage (QTL) analysis in mice: novel technologies for searching polygenetic determinants of craniofacial abnormalities

Discrepancies in size and shape of the jaws are the underlying etiology in many orthodontic and orthognathic surgery patients. Genetic factors combined with environmental interactions have been postulated to play a causal or contributory role in these craniofacial abnormalities. Along with the soon-to-be-available complete human and mouse genomic sequence data, mouse mutants have become a valuable tool in the functional mapping of genes involved in the development of human maxillofacial dysmorphologies. We review two powerful methods in such efforts: N-ethyl-N-nitrosourea (ENU) large-scale mutagenesis and quantitative trait linkage (QTL) analysis. The former aims at producing a plethora of novel variants of particular trait(s), and ultimately mapping the point mutations responsible for the appearance of these new traits. In contrast, the latter applies intensive breeding and mapping techniques to identify multiple loci (and, subsequently, genes) contributing to the phenotypic difference between the tested strains. A prerequisite for either approach to studying variations in the traits of interest is the application of effective mouse cephalometric phenotype analysis and rapid DNA mapping techniques. These approaches will produce a wealth of new data on critical genes that influence the size and shape of the human face.