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Biomedical subjects

L R Donahue

Publications and source records attributed to L R Donahue.

At least 19 recordsLinked to original sources
PubMed2006

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.

Animals↗
PubMed2003

Congenic mice reveal sex-specific genetic regulation of femoral structure and strength.

Genetic linkage studies in C3H/HeJ (C3H) and C57BL/6J (B6) mice identified several chromosomal locations or quantitative trait loci (QTL) linked to femoral volumetric bone mineral density (vBMD). From QTL identified on chromosomes (chr) 1, 4, 6, 13, and 18, five congenic mouse strains were developed. In each of these mice, genomic DNA from the QTL region of the donor C3H strain was transferred into the recipient B6 strain. Here we report the effects of donated C3H QTL on femoral structure, cortical vBMD and bending strength. Femoral structure was quantified by the polar moment of inertia (Ip) at the mid-diaphysis, which reflects the bending or torsional rigidity of the femur. Although the C3H progenitor mice have a smaller Ip than B6 progenitor mice, the congenic mice carrying the C3H segment at Chr 4 had significantly increased Ip in both males and females, giving these mice stronger femora. In female mice from the congenic Chr 1 strain, Ip was increased whereas male mice from the Chr 1 strain had smaller femoral cross-sections and significantly reduced Ip. This sex-specific effect on femoral structure was seen to a lesser extent in Chr 18 congenic mice. In addition, cortical vBMD was measured using peripheral quantitative computed tomography. Cortical vBMD was similar among most congenic strains except in Chr 6 congenic mice, where cortical vBMD was significantly less in females, but not in males. We conclude that (1) chromosomal QTL from C3H mice, which are genetically linked to total femoral vBMD, also regulate femoral structure; (2) the QTL on Chr 4 improves femoral structure and strength; (3) QTL on Chr 1 and 18 impart sex-specific effects on femoral structure; and (4) the QTL on Chr 6 imparts a sex-specific effect on cortical vBMD and femoral strength.

Animals↗
PubMed2002

Regulation of bone volume is different in the metaphyses of the femur and vertebra of C3H/HeJ and C57BL/6J mice.

The C3H/HeJ (C3H) mice exhibited a greater bone formation rate (BFR) and a greater mineral apposition rate (MAR) in the cortical bone of the midshafts of the femur and tibia than did C57BL/6J (B6) mice. This study sought to determine if these strain-related differences would also be observed in cancellous bone. Metaphyses of the femur and lumbar vertebra (L5-6) from C3H and B6 mice, 6 and 12 weeks of age, were analyzed by histomorphometry. Similar to cortical bone, the bone volume in the femoral metaphysis of C3H mice was greater (by 54% and 65%, respectively) than that of B6 mice at both 6 and 12 weeks of age. Higher BFR and mineral apposition rate (MAR) contributed to the higher bone volume in the C3H mice compared with the B6 mice. In contrast, bone volume (by 59% and 13%, respectively, p < 0.001) and trabecular number (by 55% and 35%, respectively, p < 0.001) in the vertebrae were lower in the C3H mice than in B6 mice at 6 and 12 weeks of age. At 6 weeks of age, MAR was higher (by 43%, p = 0.004) in C3H mice, but because of a low trabecular number, the BFR (by 37%, p = 0.026) and tetracycline-labeled bone surface (by 52%, p < 0.001) per tissue were lower in the vertebrae of C3H mice than B6 mice. The low bone volume in vertebrae of C3H mice was probably not due to a higher bone resorption, because the osteoclast number (by 55%, p < 0.001) and eroded surface (by 61%, p <0.001) per tissue area in the C3H mice were also lower in B6 mice. At 12 weeks, the trabecular thickness had increased (by 36%, p < 0.001) in the C3H mice and the difference in bone volume between strains was less than that at 6 weeks. These contrasting and apparently opposing strain-related differences in trabecular bone parameters between femur and vertebra in these two mouse strains suggest that the genetic regulation of bone volume in the metaphyses of different skeletal sites is different between C3H and B6 mice.

Animals↗
PubMed2002

Construction of a BAC contig for a 3 cM biologically significant region of mouse chromosome 1.

One QTL and genes and phenotypes have been localized in the region between 92 cM and 95cM of mouse chromosome 1. The QTL locus contributes to approximately 40% of the variation of the peak bone density between C57BL/6J (B6) and CAST/EiJ (CAST) strains. Other loci located in this chromosomal region include a neural tube defect mutant loop-tail (Lp), a lymphocyte-stimulating determinant (Lsd), and the Transgelin 2 (Tagln 2). The human chromosome region homologous to this region is 1q21-23, which also contains a QTL locus for high bone mineral density (BMD). Furthermore, it has been reported that this region may have duplicated several times in the mouse genome. Therefore, genomic sequencing of this region will provide important information for mouse genome structure, for positional cloning of mouse genes, and for the study of human homologous genes. In order to provide a suitable template for genomic sequencing by the NIH-sponsored genomic centers, we have constructed a BAC contig of this region using the RPCI-23 library. We have also identified the currently available mouse genomic sequences localized in our BAC contig. Further analysis of these sequences and BAC clones indicated a high frequency of repetitive sequences within this chromosomal area. This region also contains L1 retrotransposon sequences, providing a potential mechanism for the repetitive sequences described in the literature.

Animals↗
PubMed2002

Genetics and bone. Using the mouse to understand man.

The rationale for use of inbred strains of mice in bone research is well recognized and includes: a) practical factors (economics of scale, rapid development of adult status, pre-existing knowledge, down-sized technologies) and b) proven methodologies for genetic studies (polygenic trait analyses, mapping tools, genomic sequencing, methods for gene manipulation). Initial investigations of inbred strains of mice showed that femoral and lumbar vertebral volumetric bone mineral density (BMD, mg/mm(3)) by pQCT varied in excess of 50% for femurs and 9% in vertebral BMD. Two strains - low BMD C57BL/6J (B6) mice and high BMD C3H/HeJ (C3H) - were investigated for insights to their BMD diversity. B6C3F2 females derived from intercrossing B6C3F1s were raised to adult skeletal status at 4 months, then necropsied for phenotyping of bone and genotyping of genomic DNA. 1000 F2 females were genotyped for PCR product polymorphisms on all 19 autosomes at approximately 15 cM. Genome wide analyses for genotype-phenotype correlations showed 10 chromosomes (Chrs) carried genes for femoral and 7 Chrs for vertebral BMD. LOD scores ranged from 2.90 to 24.4, and percent of F2 variance accounted for ranged from 1 to 10%. Analyses of main effects revealed both dominant-recessive and additive inheritance patterns. Both progenitor strains carried alleles with positive and negative effects on BMD of each bone sites. A remarkable array of additonal skeletal phenotypes (femur and vertebral geometry, strength measures, serum markers) also proved polygenic in nature, with complex segregation patterns. Verification of BMD quantitative trait loci (QTLs) was undertaken by creating congenic B6 strains carrying individual QTL regions from C3H. Following 6 cycles of backcrossing a QTL-containing region from C3H to the B6 strain, N6F2 congenic strain mice were aged to 4 months, then genotyped for the QTL region and phenotyped for skeletal traits. Comparison of mice homozygous for C3H alleles versus homozygous for B6 alleles in the QTL regions showed that femoral BMD increased or decreased significantly in congenic strains, as was predicted from F2 data. Gender differences specific to BMD QTLs have been revealed, as have more than 30 additional phenotypes associated with cortical and trabecular structural parameters and biomechanical properties.

Journal Article↗
PubMed2001

Defining the genetics of osteoporosis: using the mouse to understand man.

Very low bone mineral density (BMD) is now considered as diagnostic of osteoporosis. Moreover, many women who are osteopenic eventually develop osteoporotic fractures. Hence, bone density testing has occupied center stage in the diagnosis and treatment of this disorder. In addition, over the last several years, BMD has been utilized as the phenotype of choice for defining heritable markers for osteoporotic fractures. However, genetic studies in humans have been limited to some degree by the tremendous heterogeneity among populations, as well as multiple genetic, heritable and environmental determinants of the BMD phenotype. Recent advances in technology have afforded investigators the opportunity to study acquisition and maintenance of BMD in small animals. Along with newer knockout and transgenic strategies, quantification of mouse bone mass has advanced our understanding of both the biologic and genetic determinants of bone density. In this review, we will examine the use of the mouse to map the heritable factors that regulate bone acquisition. We will also examine the role of newer technology to decompose the bone density phenotype into components that are amenable to genetic studies. This review will focus on three models: (1) healthy inbred (2) recombinant inbred, and (3) congenic strains of mice. Progress in this area with these strains has been rapid, and a summary of several quantitative trait loci (QTLs) is provided. The future of the mouse as a tool to map the genes that define the osteoporosis syndrome is extremely promising.

Animals↗
PubMed2001

Mice with mutations in the mahogany gene Atrn have cerebral spongiform changes.

A new mutation characterized by mahogany coat color, sprawling gait, tremors, and severe vacuolization of cerebrum, brainstem, granular layer of cerebellum and spinal cord was discovered in a stock of Mus castaneus mice. Tests for allelism using mice homozygous for 2 known mahogany attractin (Atrnmg) mutants showed that the new mutation was an allele of Atrnmg. Northern analysis showed no expression of Atrn in the new mutants. Southern analysis strongly suggested that the new mutation deleted most of the Atrn gene, but was not large enough to affect flanking genes including the prion gene, Prnp, located 1.1 cM from Atrn on Chromosome 2. Histopathological analysis of brains from each of the 2 known Atrnmg mutants showed that they also have severe spongiform changes. This finding was surprising and raises questions about the mechanism by which mahogany controls appetite and metabolic rate, as recently reported.

Alleles↗
PubMed2001

Variation in bone biomechanical properties, microstructure, and density in BXH recombinant inbred mice.

To test the hypothesis that factors associated with bone strength (i.e., volumetric bone mineral density [vBMD], geometry, and microstructure) have heritable components, we exploited the 12 BXH recombinant inbred (RI) strains of mice derived from C57BL/6J (B6; low bone mass) and C3H/HeJ (C3H; high bone mass) progenitor strains. The femurs and lumbar vertebrae from each BXH RI strain were characterized for phenotypes of vBMD, microstructural, biomechanical, and geometrical properties. Methods included bending (femur) and compression (vertebra) testing, peripheral quantitative computed tomography (pQCT), and microcomputed tomography (microCT). Segregation patterns of femoral and vertebral biomechanical properties among the BXH RI strains suggested polygenic regulation. Femoral biomechanical properties were strongly associated with femoral width in the anteroposterior (AP) direction and cortical thickness--geometric properties with complex genetic regulation. Vertebral vBMD and biomechanical properties measured in BXH RI strains showed a greater variability than either B6 or C3H progenitors, suggesting both progenitor strains have independent subsets of genes that yield similar vBMD and strength. The microCT and pQCT data suggested that the distribution of vertebral mineral into cortical and trabecular compartments is regulated genetically. Although the B6 and C3H progenitors had similar vertebral strength, their vertebral structures were markedly different: B6 had good trabecular bone structure and modest cortical bone mineral content (BMC), whereas C3H had high cortical BMC combined with a deficiency in trabecular structure. These structural traits segregated independently in the BXH RI strains. Finally, vertebral strength was not correlated consistently with femoral strength among the BXH RI strains, suggesting genetic regulation of bone strength is site specific.

Animals↗
PubMed2001

Postnatal and pubertal skeletal changes contribute predominantly to the differences in peak bone density between C3H/HeJ and C57BL/6J mice.

Previous studies have shown that 60-70% of variance in peak bone density is determined genetically. The higher the peak bone density, the less likely an individual is to eventually develop osteoporosis. Therefore, the amount of bone accrued during postnatal and pubertal growth is an important determining factor in the development of osteoporosis. We evaluated the contribution of skeletal changes before, during, and after puberty to the development of peak bone density in C3H/HeJ (C3H) and C57BL/6J (B6) mice. Volumetric bone density and geometric parameters at the middiaphysis of femora were measured by peripheral quantitative computed tomography (pQCT) from days 7 to 56. Additionally, biochemical markers of bone remodeling in serum and bone extracts were quantified. Both B6 and C3H mice showed similar body and femoral weights. B6 mice had greater middiaphyseal total bone area and thinner cortices than did C3H mice. Within strains, males had thicker cortices than did females. C3H mice accumulated more minerals throughout the study, with the most rapid accumulation occurring postnatally (days 7-23) and during pubertal maturation (days 23-31). C3H mice had higher volumetric bone density as early as day 7, compared with B6 mice. Higher serum insulin-like growth factor I (IGF-I) was present in C3H mice postnatally at day 7 and day 14. Until day 31, B6 male and female mice had significantly higher serum osteocalcin than C3H male and female mice, respectively. Alkaline phosphatase (ALP) was found to be significantly higher in the bone extract of C3H mice compared with B6 mice at day 14. These data are consistent with and support the hypothesis that the greater amount of bone accrued during postnatal and pubertal growth in C3H mice compared with B6 mice may be caused by increased cortical thickness, increased endosteal bone formation, and decreased endosteal bone resorption.

Animals↗
PubMed2001

Quantitative trait loci for femoral and lumbar vertebral bone mineral density in C57BL/6J and C3H/HeJ inbred strains of mice.

Significant differences in vertebral (9%) and femoral (50%) adult bone mineral density (BMD) between the C57BL/6J (B6) and C3H/HeJ (C3H) inbred strains of mice have been subjected to genetic analyses for quantitative trait loci (QTL). Nine hundred eighty-six B6C3F2 females were analyzed to gain insight into the number of genes that regulate peak BMD and their locations. Femurs and lumbar vertebrae were isolated from 4-month-old B6C3F2 females at skeletal maturity and then BMD was determined by peripheral quantitative computed tomography (pQCT). Estimates of BMD heritability were 83% for femurs and 72% for vertebrae. Genomic DNA from F2 progeny was screened for 107 polymerase chain reaction (PCR)-based markers discriminating B6 and C3H alleles on all 19 autosomes. The regression analyses of markers on BMD revealed ten chromosomes (1, 2, 4, 6, 11, 12, 13, 14, 16, and 18) carrying QTLs for femurs and seven chromosomes (1, 4, 7, 9, 11, 14, and 18) carrying QTLs for vertebrae, each with log10 of the odds ratio (LOD) scores of 2.8 or better. The QTLs on chromosomes (Chrs) 2, 6, 12, 13, and 16 were unique to femurs, whereas the QTLs on Chrs 7 and 9 were unique to vertebrae. When the two bone sites had a QTL on the same chromosome, the same marker had the highest, although different, LOD score. A pairwise comparison by analysis of variance (ANOVA) did not reveal significant gene x gene interactions between QTLs for either bone site. BMD variance accounted for by individual QTLs ranged from 1% to 10%. Collectively, the BMD QTLs for femurs accounted for 35.1% and for vertebrae accounted for 23.7 % of the F2 population variances in these bones. When mice were homozygous c3/c3 in the QTL region, 8 of the 10 QTLs increased, while the remaining two QTLs on Chrs 6 and 12 decreased, femoral BMD. Similarly, when mice were homozygous c3/c3 in the QTL region for the vertebrae, five of the seven QTLs increased, while two QTLs on Chrs 7 and 9 decreased, BMD. These findings show the genetic complexity of BMD with multiple genes participating in its regulation. Although 5 of the 12 QTLs are considered to be skeleton-wide loci and commonly affect both femurs and vertebrae, each of the bone sites also exhibited unique QTLs. Thus, the BMD phenotype can be partitioned into its genetic components and the effects of these loci on normal bone biology can be determined. Importantly, the BMD QTLs that we have identified are in regions of the mouse genome that have known human homology, and the QTLs will become useful experimental tools for mechanistic and therapeutic analyses of bone regulatory genes.

Alleles↗
PubMed2000

Exercise and mechanical loading increase periosteal bone formation and whole bone strength in C57BL/6J mice but not in C3H/Hej mice.

To identify the genes, and the mechanisms that account for the 53% higher peak bone density in C3H/HeJ (C3H) mice compared with C57BL/6J (B6) mice, we are performing quantitative trait locus and phenotypic analyses. The phenotypic studies revealed differences in bone formation and resorption, and showed that hindlimb immobilization (by sciatic neurectomy) caused a greater increase in endosteal resorption in the tibiae of B6 compared with C3H mice. The current studies were intended to examine the hypothesis that the bones of C3H mice are less sensitive to mechanical loading than the bones of B6 mice. To increase mechanical loading, 9-week-old female B6 and C3H mice (n = 10-13 mice/group) were subjected to a jumping exercise (20 jumps/day, 5 days/week, to heights of 20-30 cm) for a total of 4 weeks. Control mice did not jump. Osteocalcin, alkaline phosphatase (ALP) activity, and IGF-I were measured in serum. The left tibiae were used for histomorphometry (ground cross-sections prepared at the tibiofibular junction) and the right tibiae and femora were used for determinations of bone breaking strength (3-point bending). The results of these studies revealed (1) significant effects of both mouse strain (B6 and C3H) and the jumping exercise on tibial strength; (2) an exercise-dependent increase in serum IGF-I in C3H, but not B6 mice; and (3) no effects on serum ALP or osteocalcin. The histomorphometric analyses showed no effect of exercise on C3H tibiae, but significant exercise-dependent increases in total bone area, periosteal perimeter, periosteal mineral apposition rate (MAR), and periosteal bone formation (P < 0.02 for each) in B6 tibiae. There were no effects of exercise on periosteal resorption or any endosteal measurement in either C3H or B6 mice. Since the jumping exercise was designed to cause a two-three fold increase in muscular-skeletal loading at the tibio-fibular junction, and the calculated stress (g/mm2) at this sampling site was only 16% greater for B6 compared with C3H mice, we had anticipated that both strains of mice would show exercise-dependent increases in periosteal bone formation, with a greater response in the B6 mice. The lack of a response in the C3H tibiae demonstrates that the bones of C3H mice are less sensitive to mechanical loading (and unloading) than the bones of B6 mice.

Alkaline Phosphatase↗
PubMed2000

Neuromuscular ataxia: a new spontaneous mutation in the mouse.

Neuromuscular ataxia, nma, is a new autosomal recessive mutation that arose spontaneously in CBA/J inbred mice at The Jackson Laboratory. The mutation, now maintained on the B6C3FeF(1) hybrid background, when homozygous, causes small size, uncoordinated gait, dysmetria, dystonia, general weakness, and death shortly after weaning. No biochemical or morphological abnormalities have been detected. We used an intercross between the B6C3FeF(1) mutant and CAST/Ei to map the nma mutation to the proximal end of Chr 12. The most likely gene order places the mutation between D12Mit270 and D12Mit54, non-recombinant with D12Mit2 in 96 tested meioses.

Animals↗
PubMed2000

Insulin-like growth factor I and bone: from mouse to man.

Serum insulin-like growth factor I (IGF-I) is regulated by numerous variables, including growth hormone (GH), nutritional status, gonadal steroids and other hormones. However, the circulating IGF-I phenotype is also under heritable regulation, and several genetic determinants may be important in defining tissue-specific expression of the gene encoding this peptide. A very strong correlation has been found between serum IGF-I concentration and bone acquisition in both mice and humans. Based on previous studies as well as ongoing work with mice, it has been hypothesized that regulation of the serum IGF-I phenotype includes non-GH-dependent factors and, furthermore, that these determinants are also involved in the acquisition of bone mass. This paper reports that, by performing intercrosses between two inbred strains of mice of similar age, size and length, but with different serum levels of IGF-I, we have identified regulatory loci for serum IGF-I and established their relationship to putative quantitative trait loci for bone mineral density. Mapping these quantitative trait loci will help refine our understanding of disorders related to IGF-I.

Animals↗
PubMed2000

Effects of dietary calcium depletion and repletion on dynamic determinants of tibial bone volume in two inbred strains of mice.

As an adjunct to our efforts to identify the genes that determine peak bone density, we examined phenotypic differences between two inbred strains of mice, C3H/HeJ (C3H) and C57BL/6J (B6), which are of similar size but differ with respect to peak bone density (e.g., C3H mice have 53% higher femoral bone density than B6 mice). The current studies were intended to compare the skeletal responses of C3H and B6 mice to 2 weeks of dietary calcium (Ca) depletion, followed by 2 weeks of Ca repletion. Initial studies showed that: (a) femur dry weight decreased during Ca depletion in both C3H and B6 mice (by 25% and 19%, respectively, p < 0.001) and most of this loss was recovered during Ca repletion; and (b) serum alkaline phosphatase (ALP) activity increased during Ca depletion, in both strains of mice (p < 0.001), and returned to normal after Ca repletion. Histological analyses of ground cross sections prepared at the tibiofibular junction showed that Ca-depletion increased medullary area in both C3H and B6 mice (indicating endosteal bone loss, p < 0.01), with reversal during Ca repletion. There were no effects of Ca depletion or repletion on periosteal bone growth. Endosteal bone forming surface and endosteal mineral apposition decreased during Ca depletion and increased during repletion in both C3H and B6 mice (p < 0.05). Net bone formation decreased during Ca depletion in C3H mice, but not B6 mice (p < 0.01), and was normal during Ca repletion in both strains. Endosteal bone resorbing surface and net bone resorption increased during Ca depletion and decreased during repletion in both strains (p < 0.01). A supplemental study (of Ca depletion without repletion) confirmed the effects of Ca depletion on femoral dry weight and serum ALP activity (p < 0.001 for each). This supplemental study also showed that Ca deficiency increased serum parathyroid hormone (PTH) (p < 0.05) and decreased (tibial) cortical bone area and cortical mineral content (p < 0.05 to p < 0.001) in both strains of mice. Together, these data demonstrate that the skeletal responses to Ca depletion and repletion are, qualitatively, similar in C3H and B6 mice.

Alkaline Phosphatase↗
PubMed2000

Mapping quantitative trait loci for serum insulin-like growth factor-1 levels in mice.

Serum insulin-like growth factor-1 (IGF-1) and femoral bone mineral density (BMD) differ between two inbred strains of mice, C3H/HeJ (C3H) and C57BL/6J (B6), by approximately 30% and 50%, respectively. Similarly, skeletal IGF-1 content, bone formation, mineral apposition, and marrow stromal cell numbers are higher in C3H than in B6 mice. Because IGF-1 and several bone parameters cosegregate, we hypothesize that the serum IGF-1 phenotype has a strong heritable component and that genetic determinants for serum IGF-1 are involved in the regulation of bone mass. We intercrossed (B6 x C3H)F1 hybrids and analyzed 682 F2 female offspring at 4 months of age for serum IGF-1 by radioimmunoassay and femoral BMD by peripheral quantitative computerized tomography (pQCT). Genomic DNA was assayed by polymerase chain reaction (PCR) to determine alleles for 114 Mit markers inherited in F2 mice at average distances of 14 centimorgans (cM) along each chromosome (Chr). Serum IGF-1 levels in the F2 progeny were relatively normal in distribution, but showed a greater range than either progenitor, indicating that serum IGF-1 level is a polygenic trait with an estimated heritability of 52%. Serum IGF-1 correlated with femoral length (r = 0.266, p < 0.0001) and femoral BMD (r = 0.267, p < 0.0001). Whole genome scans for main effects associated with serum IGF-1 levels revealed three significant QTLs (in order of significance) on mouse Chrs 6, 15, and 10. The QTL on Chr 6 showed a significant reduction in IGF-1 associated with increasing C3H allele number, whereas the Chr 15 and Chr 10 loci showed additive effects with increasing C3H allele number. A genome-wide search for interacting marker pairs identified a significant interaction between the Chr 6 QTL and a locus on Chr 11. This interactive effect suggested that when the Chr 11 locus was homozygous for C3H, there was no effect of the Chr 6 locus on serum IGF-1; however, the combination of C3H alleles on Chr 6 with B6 alleles on Chr 11 was associated with reduced serum IGF-1 concentrations. To test this in vivo, we tested congenic mice carrying the Chr 6 QTL region from C3H on a B6 background (B6.C3H-6). Both serum IGF-1 and femoral BMD were significantly lower in female congenic than progenitor B6 mice. In summary, we identified three major QTLs on mouse Chrs 6, 10, and 15, and noted a major locus-locus interaction between Chrs 6 and 11. We named these QTLs IGF-1 serum levels (Igf1sl1 to Igf1sl4). Functional isolation of the Igf1sl1 QTL on Chr 6 for IGF-1 in B6.C3H-6 congenic mice demonstrated effects on both the IGF-1 and BMD phenotypes. The genetic determinants of these Igf1sl QTLs will provide much insight into the regulation of IGF-1 and the subsequent acquisition of peak bone mass.

Animals↗
PubMed2000

Spontaneous fracture (sfx): a mouse genetic model of defective peripubertal bone formation.

A new mouse model of stage-specific bone growth failure and fracture has been recovered as an autosomal recessive mutation, designated spontaneous fracture (sfx). The sfx/sfx mice are phenotypically normal until shortly after weaning, when reduced mobility and impaired somatic growth are first noted. By 6 weeks of age, body, spleen, and thymus weights, as well as hematocrits and serum calcium, inorganic phosphate, total alkaline phosphatase, insulin-like growth factor-I, and osteocalcin levels are decreased. The sfx/sfx mice also show reduced femoral cortical density and diaphyseal circumference, as well as a paucity of mature osteoblasts on bone surfaces. Histological analyses of the femur and tibia in the mutants show subtle reduction of chondrocyte numbers in epiphyseal-plate columns, reduction of matrix, and near absence of osteoid below the differentiated chondrocytes. Trabeculae in proximal tibiae, iliacs, and vertebral bodies are sparse and thin. Cortical bone thickness of mutants is markedly thinned in all sites examined. By 7-8 weeks, radiographic films routinely show spontaneous impact fractures of the distal femur accompanied by callus formation, whereas complete fractures are less commonly observed. Volumetric bone mineral density (BMD) of mutant femurs is similar to +/? littermates in the center of the femoral diaphysis, but BMD declines as either end of the femoral diaphysis is approached. We have mapped the gene responsible for this phenotype to central Chromosome 14. Reduced bone mass, impaired bone formation, abnormalities of bone architecture, and a disposition to spontaneous fracture identify sfx/sfx mice as a useful model for understanding the mechanisms responsible for peripubertal bone formation.

Animals↗
PubMed2000

Targeted overexpression of insulin-like growth factor I to osteoblasts of transgenic mice: increased trabecular bone volume without increased osteoblast proliferation.

Insulin-like growth factor I (IGF-I) is an important growth factor for bone, yet the mechanisms that mediate its anabolic activity in the skeleton are poorly understood. To examine the effects of locally produced IGF-I in bone in vivo, we targeted expression IGF-I to osteoblasts of transgenic mice using a human osteocalcin promoter. The IGF-I transgene was expressed in bone osteoblasts in OC-IGF-I transgenic mice at high levels in the absence of any change in serum IGF-I levels, or of total body growth. Bone formation rate at the distal femur in 3-week-old OC-IGF-I transgenic mice was approximately twice that of controls. By 6 weeks, bone mineral density as measured by dual energy x-ray, and quantitative computed tomography was significantly greater in OC-IGF-I transgenic mice compared with controls. Histomorphometric measurements revealed a marked (30%) increase femoral cancellous bone volume in the OC-IGF-I transgenic mice, but no change in the total number of osteoblasts or osteoclasts. Transgenic mice also demonstrated an increase in the osteocyte lacunea occupancy, suggesting that IGF-I may extend the osteocyte life span. We conclude that IGF-I produced locally in bone osteoblasts exerts its anabolic effect primarily by increasing the activity of resident osteoblasts.

Animals↗