Genetic control of platelet activation in inbred mouse strains

Platelet activation in inbred mouse strains was studied using expression of P-selectin as a marker of activated platelets. P-selectin expression in response to no added stimulus (spontaneous activation) or in response to adenosine diphosphate (ADP) and epinephrine or thrombin, was assessed using a flow cytometric assay. Wide variation in the responsiveness of different strains was observed with strains SJL and AKR in particular showing very high levels of spontaneous activation. Genetic studies suggest that this phenomenon is under control of a small number of genes and that the same loci are probably responsible for the high activation of both SJL and AKR. Bone marrow transplant experiments show that the trait is expressed in the platelet itself. Screening of SWXJ and AKXD recombinant inbred lines suggests that one of the responsible genes is located on chromosome 3.

Gender- and compartment-specific bone loss in C57BL/6J mice: correlation to season?

Seasonal variation in bone mineral density (BMD) has been documented in humans, and has been attributed to changes in 25-hydroxyvitamin D [25(OH)D] synthesis. To test the hypothesis that seasonal changes in bone mass occur in laboratory mice, we measured body composition, femoral bone phenotypes, and serum bone markers in 16-wk-old male and female C57BL/6 (B6) mice during the summer (June-August) and winter (December-February) months at The Jackson Laboratory in Bar Harbor, Maine. Both male and female B6 mice had higher volumetric BMD in the summer than winter. Females showed reduced trabecular bone, whereas males showed changes in bone volume. Males, but not females, had higher insulin-like growth factor 1 in summer than in winter, and only males showed an increase in body weight during the winter. No seasonal differences in serum TRAP5b, osteocalcin, or 25(OH)D were noted for either sex. We conclude that seasonal variation in skeletal and body composition parameters in B6 mice is significant and must be considered when performing longitudinal phenotyping of the skeleton. Further studies are needed to determine the environmental factors that cue seasonal changes in body composition and the mechanisms that produce these changes.

Gender-specific changes in bone turnover and skeletal architecture in igfbp-2-null mice

IGF-binding protein-2 (IGFBP-2) is a 36-kDa protein that binds to the IGFs with high affinity. To determine its role in bone turnover, we compared Igfbp2(-/-) mice with Igfbp2(+/+) colony controls. Igfbp2(-/-) males had shorter femurs and were heavier than controls but were not insulin resistant. Serum IGF-I levels in Igfbp2(-/-) mice were 10% higher than Igfbp2(+/+) controls at 8 wk of age; in males, this was accompanied by a 3-fold increase in hepatic Igfbp3 and Igfbp5 mRNA transcripts compared with Igfbp2(+/+) controls. The skeletal phenotype of the Igfbp2(-/-) mice was gender and compartment specific; Igfbp2(-/-) females had increased cortical thickness with a greater periosteal circumference compared with controls, whereas male Igfbp2(-/-) males had reduced cortical bone area and a 20% reduction in the trabecular bone volume fraction due to thinner trabeculae than Igfbp2(+/+) controls. Serum osteocalcin levels were reduced by nearly 40% in Igfbp2(-/-) males, and in vitro, both CFU-ALP(+) preosteoblasts, and tartrate-resistant acid phosphatase-positive osteoclasts were significantly less abundant than in Igfbp2(+/+) male mice. Histomorphometry confirmed fewer osteoblasts and osteoclasts per bone perimeter and reduced bone formation in the Igfbp2(-/-) males. Lysates from both osteoblasts and osteoclasts in the Igfbp2(-/-) males had phosphatase and tensin homolog (PTEN) levels that were significantly higher than Igfbp2(+/+) controls and were suppressed by addition of exogenous IGFBP-2. In summary, there are gender- and compartment-specific changes in Igfbp2(-/-) mice. IGFBP-2 may regulate bone turnover in both an IGF-I-dependent and -independent manner.

Genetic regulation of femoral bone mineral density: complexity of sex effect in chromosome 1 revealed by congenic sublines of mice

The findings that sex-specific effects on femoral structure and peak bone mineral density (BMD) are linked to quantitative trait loci (QTL) provide evidence for the involvement of specific genes that contribute to gender variation in skeletal phenotype. Based on previous findings that the BMD QTL in chromosome 1 (Chr 1) exerts a sex-specific effect on femoral structure, we predicted that congenic sublines of mice that carry one or more of the Chr 1 BMD loci would exhibit gender difference in the volumetric BMD (vBMD) phenotype. To test this hypothesis, we compared skeletal parameters of male and female of five C57BL/6J (B6).CAST/EiJ (CAST)-1 congenic sublines of mice that carry overlapping CAST chromosomal segments from the vBMD loci in Chr 1. Femur vBMD measurements were performed by the peripheral quantitative computed tomography in male and female mice at 16 weeks of age. The skeletal phenotype of the C175-185 and C178-185 congenic sublines of mice provided evidence for the presence of the BMD1-4 locus at 178-180 Mb from the centromere. This QTL affects femur vBMD only in female mice. In contrast, CAST chromosomal region carrying BMD1-1 locus increased femur vBMD both in male and female mice. Furthermore, a gender specific effect on BMD of femur mid-shaft region (mid-BMD) was identified at 168-176 Mb in Chr 1 (F=16.49, P=0.0002), while no significant effect was found on total femur BMD (F=2.67, P=0.11). Moreover, this study allowed us to locate a body weight QTL at 168-172 Mb of Chr 1, the effect of this locus was altered in female mice that carry CAST chromosomal segment 168-176 Mb of Chr 1. Based on this study, we conclude that Chr 1 carries at least two vBMD gender-dependent loci; one genetic locus at 178-180 Mb (BMD1-4 locus) which affects both mid-shaft and total femur vBMD in female mice only, and another gender-dependent locus at 168-176 Mb (BMD1-2 locus) which affects femur mid-shaft vBMD in female but not male mice.

Bone quality and bone strength in BXH recombinant inbred mice

The relationship between bone quality and strength was studied in 11 BXH recombinant inbred (RI) strains of mice. The bone quality parameters studied were bone mineralization, microhardness, architecture, and connectivity. Previous studies have demonstrated considerable variability in bone density, biomechanical properties, and microstructure among inbred strains of mice. In particular, C3H/HeJ (C3H) mice exhibit thicker femoral and vertebral cortices and fewer trabeculae in the vertebral body compared with C57BL/6J (B6) mice, despite having similar vertebral bone strength. A set of RI mouse strains has been generated from B6 and C3H (denoted BXH) in an attempt to isolate genetic regulation of numerous traits, including bone. The objective of this study was to investigate relationships among bone quality and bone strength in femurs and vertebrae among BXH RI mice. The study involved 11 BXH RI strains of female mice (n = 5-7) as well as the B6 and C3H progenitor strains. Parameters contributing to bone quality were evaluated, including BMD, bone mineralization, microhardness, architecture, and connectivity. There was a strong correlation between femoral and vertebral BMD in all strains (P < 0.001) except in BXH-9 and -10 (P < 0.001). Within the vertebrae, cortical bone was more mineralized than trabecular bone, and a strong correlation existed between the two (P < 0.001). However, cortical microhardness did not differ from trabecular microhardness. Cortical bone was more mineralized in the femur than in the vertebrae and significantly harder, by 30%. There was a wide range in trabecular connectivity, architecture, and femur geometry among BXH RI strains. BMD explained 43% of vertebral bone strength but only 11% of femoral bone strength. Trabecular connectivity explained an additional 8% of vertebral strength, while mineralization and femur geometry explained 7% and 50% of femoral strength, respectively. Different bone quality parameters had varying influences on bone mechanical properties, depending on bone site. BMD may play a larger role in explaining bone strength in the vertebrae than in the femur. Moreover, cortical bone in the femur is harder than in vertebrae. The control of cortical bone material properties may be site-dependent.

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.

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.

Early neoplastic and metastatic mammary tumours of transgenic mice detected by 5-aminolevulinic acid-stimulated protoporphyrin IX accumulation

A photodynamic technique for human breast cancer detection founded upon the ability of tumour cells to rapidly accumulate the fluorescent product protoporphyrin IX (PpIX) has been applied to transgenic mouse models of mammary tumorigenesis. A major goal of this investigation was to determine whether mouse mammary tumours are reliable models of human disease in terms of PpIX accumulation, for future mechanistic and therapeutic studies. The haeme substrate 5-aminolevulinic acid (5-ALA) (200 mg kg⁻¹) was administered to mouse strains that develop mammary tumours of various histological subtypes upon expression of the transgenic oncogenes HRAS, Polyoma Virus middle T antigen, or Simian Virus 40 large T antigen in the mammary gland. Early neoplastic lesions, primary tumours and metastases showed consistent and rapid PpIX accumulation compared to the normal surrounding tissues, as evidenced by red fluorescence (635 nm) when the tumours were directly illuminated with blue light (380–440 nm). Detection of mouse mammary tumours at the stage of ductal carcinomain situ by red fluorescence emissions suggests that enhanced PpIX synthesis is a good marker for early tumorigenic processes in the mammary gland. We propose the mouse models provide an ideal experimental system for further investigation of the early diagnostic and therapeutic potential of 5-ALA-stimulated PpIX accumulation in human breast cancer patients.

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.

Mouse genetic model for bone strength and size phenotypes: NZB/B1NJ and RF/J inbred strains

The relationships of bone size, bone strength, and bone formation were investigated in two strains of mice, NZB/B1NJ and RF/J. Measurement of the femur midshaft size by peripheral quantitative computed tomography (pQCT) showed that the RF/J mice had a 32% greater cross-sectional area than NZB/B1NJ mice at 10 weeks of age, and a 38% greater cross-sectional area at 22 weeks of age. Body weight in the RF/J mice was 10% higher at 10 weeks but 9% lower at 22 weeks. Bone strength was determined by a three-point bending method. In agreement with the difference in bone cross-sectional area, the femurs of the RF/J mice were stronger (80% greater) and stiffer (80% greater) than the bones of the NZB/B1NJ mice. To determine whether periosteal bone formation played a role in the greater size of the RF/J mice, the mice were injected with tetracycline to label areas of new bone formation. Histomorphometrical analysis of the femur diaphysis demonstrated higher rates of periosteal bone formation (131% greater) and of periosteal forming surface (81% greater) in RF/J than in NZB/B1NJ mice. We conclude that a high rate of periosteal bone formation increases bone size and strength in RF/J mice when compared with NZB/B1NJ mice. The NZB/B1NJ and RF/J mice should be an excellent model to investigate the genes that regulate femur size and strength.

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.

IGF-I regulates osteoprotegerin (OPG) and receptor activator of nuclear factor-kappaB ligand in vitro and OPG in vivo

IGF-I, a ubiquitous polypeptide, plays a key role in longitudinal bone growth and acquisition. The most predominant effect of skeletal IGF-I is acceleration of the differentiation program for osteoblasts. However, in vivo studies using recombinant human (rh) IGF-I and/or rhGH have demonstrated stimulation of both bone formation and resorption, thereby potentially limiting the usefulness of these peptides in the treatment of osteoporosis. In this study, we hypothesized that IGF-I modulates bone resorption by regulating expression of osteoprotegerin (OPG) and receptor activator of nuclear factor-kappaB (RANK) ligand (RANKL) in bone cells. Using Northern analysis in ST2 cells, we found that human IGF-I suppressed OPG mRNA in a time- and dose-dependent manner: 100 micro g/LIGF-I (13 nM) decreased OPG expression by 37.0 +/- 1.8% (P < 0.002). The half maximal inhibitory dose of IGF-I was reached at 50 micro g/liter ( approximately 6.5 nM) with no effect of IGF-I on OPG message stability. Conditioned media from ST2 cells confirmed that IGF-I decreased secreted OPG, reducing levels by 42%, from 12.1-7 ng/ml at 48 h (P < 0.05). Similarly, IGF-I at 100 micro g/liter (13 nM) increased RANKL mRNA expression to 353 +/- 74% above untreated cells as assessed by real-time PCR. In vivo, low doses of rhGH when administered to elderly postmenopausal women only modestly raised serum IGF-I (to concentrations of 18-26 nM) and did not affect circulating OPG concentrations; however, administration of rhIGF-I (30 micro g/kg.d) for 1 yr to older women resulted in a significant increase in serum IGF-I (to concentrations of 39-45 nM) and a 20% reduction in serum OPG (P < 0.05). In summary, we conclude that IGF-I in a dose- and time-dependent manner regulates OPG and RANKL in vitro and in vivo. These data suggest IGF-I may act as a coupling factor in bone remodeling by activating both bone formation and bone resorption; the latter effect appears to be mediated through the OPG/RANKL system in bone.

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.

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.

Is skeletal mechanotransduction under genetic control?

Studies of twins have established that peak bone mass is about 70% heritable. The skeletal response to exercise contributes to peak bone mass, as mechanical loading increases skeletal mass during growth and development. It is possible that the skeletal responsiveness to mechanical loading is under genetic control, so that some individuals will build stronger bones with exercise. This appears to be the case in mice. Long bones in mice of the C3H/He strain are largely unresponsive to mechanical loading. Ironically, this strain of mice has very high bone density. Perhaps the genes that regulate BMD are not the same as those that regulate mechanical loading response. Studies of recombinant inbred and congenic strains derived from C3H mice will help to identify genes influencing bone size, density and responsiveness to mechanical loading.

Melanocyte and gonad activity as potential severity modifying factors in C3H/HeJ mouse alopecia areata

Circumstantial evidence has previously suggested gonad derived steroid hormones and melanogenesis related antigens may modify human alopecia areata (AA). AA-like hair loss can be induced in C3H/HeJ mice after skin allografts from spontaneous AA-affected mice. This inducible model was used to evaluate hormones and hair follicle melanocyte presence as disease-severity modifiers. Ten females and 9 males were gonadectomized and received AA-affected allografts. All gonadectomized mice had 2-4 weeks delay in AA onset relative to non-gonadectomized controls. Two females and 4 males failed to develop any AA by 25 weeks after grafting. The experiment was repeated with gonadectomized female and male mice plus non-gonadectomized mice subcutaneously implanted with silastic capsules containing 80 microg 17beta estradiol or 10 mg 5alpha dihydrotestosterone, respectively. Five of 11 ovariectomized and 9 of 11 non-ovariectomized, estradiol supplemented females developed AA with extremely rapid progression. Three of 8 castrated, but none of 11 non-castrated, dihydrotestosterone-supplemented males expressed AA. In a separate study, 14 mice were freeze-branded, producing white hair on the dorsal lumbar region, and later received full-thickness allografts. Thirteen mice developed patchy pigmented and non-pigmented hair loss. One mouse developed diffuse, pigmented hair loss, but with white hair survival persisting 25 weeks after grafting. The results suggest that gonadal steroid hormones can modulate C3H/HeJ mouse AA where estradiol promoted rapid progression of AA while dihydrotestosterone increased resistance to AA onset. In general, both pigmented and non-pigmented C3H/HeJ mouse hair is susceptible to AA. Murine AA susceptibility and severity clearly involves an interplay between genetic and epigenetic factors.

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.

Effects of dietary restriction on appendicular bone in the SENCAR mouse

Peptide hormones, cytokines, and growth factors regulate cellular metabolism by stimulating second messenger signal transduction cascades in target tissues. A mutation in the regulatory domain of protein kinase C (PKC) in SENCAR (sensitive to carcinogenesis) mice renders them extremely sensitive to diacylglycerol and phorbol esters, resulting in rapid growth, high free radical generation, carcinogenesis, and metabolic bone disease. Dietary restriction (DR) normalizes PKC and ameliorates adverse downstream effects, including carcinogenesis, in SENCAR mice. We hypothesized that DR sufficient to ameliorate carcinogenesis would prevent or delay the early onset of metabolic bone disease in SENCAR mice. Male mice were assigned to 1 of 4 feeding groups from 10 to 16 weeks of age (the critical period when metabolic bone disease develops): ad libitum (AL)-fed; AL antioxidant (0.07% thioproline)-fed; 40% DR; or 40% DR antioxidant-fed. Femoral bone mass was determined gravimetrically. Tibial total, cortical, and trabecular bone mineral density (BMD) were determined by quantitative computed tomography. Body weight, femoral bone mass, and tibial cortical BMD were lower in DR than in AL mice. However, tibial total and trabecular BMD were higher in DR than in AL mice. Serum calcitonin, the hormone that inhibits the osteoclastic bone resorption that is most notable in trabecular bone, was 2-fold higher in DR than in AL-fed mice. Dietary thioproline had no major effects. Thus, DR sufficient to ameliorate carcinogenesis in SENCAR mice did not prevent early-onset metabolic bone disease, but it had a beneficial effect on tibial trabecular BMD that occurred at the apparent expense of cortical BMD. DR in SENCAR mice was also associated with elevated serum calcitonin, which may inhibit osteoclastic resorption and account for trabecular bone conservation in this model. In conclusion, PKC or the downstream metabolic processes regulated by it appear to play previously unrecognized roles in the regulation of tibial trabecular BMD and serum calcitonin in SENCAR mice.

Nonablative neonatal marrow transplantation attenuates functional and physical defects of beta-glucuronidase deficiency

The toxicity of preparative regimens render neonatal bone marrow transplantation (BMT) for progressive childhood diseases a controversial treatment. Ablative BMT in neonatal mice with or without the lysosomal storage disease mucopolysaccharidosis type VII (MPS VII) show high morbidity and developmental disruption of both brain and bone structure. In this investigation, BMT was performed with a high dose of congenic, normal bone marrow into nonablated newborn mice. Recipients had lifelong, multilineage, peripheral blood chimerism with the donor beta-glucuronidase-positive (GUS(+)) cells that was both well tolerated and therapeutic. Three daily injections of normal adult marrow increased the average life span by at least 6 months and corrected the functional breeding deficits typical of the MPS VII mice. Twelve months after injection, several structural features of femurs were more like that of normal mice than of untreated MPS VII mice. Periosteal circumference and bone cortical thickness were significantly improved in males and cortical density did not differ significantly from values in normal females. Significant reduction of lysosomal glycosaminoglycan storage corresponded directly with GUS enzyme activity and percentage of histochemically GUS(+) cells in visceral organs and hematopoietic tissues such as thymus, spleen, peripheral blood, and bone marrow. By all criteria tested, BMT into neonatal MPS VII mice in the absence of any preparative regimen is a successful therapy.

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.

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.

The impact of reduced frequency of cage changes on the health of mice housed in ventilated cages

Our purpose in this investigation was to determine if we could reduce cage changing frequency without adversely affecting the health of mice. We housed mice at three different cage changing frequencies: 7, 14, and 21 days, each at three different cage ventilation rates: 30, 60 and 100 air changes per hour (ACH), for a total of nine experimental conditions. For each condition, we evaluated the health of 12 breeding pairs and 12 breeding trios of C57BL/6J mice for 7 months. Health was assessed by breeding performance, weanling weight and growth, plasma corticosterone levels, immune function, and histological examination of selected organs. Over a period of 4 months, we monitored the cage microenvironment for ammonia and carbon dioxide concentrations, relative humidity, and temperature one day prior to changing the cage. The relative humidity, carbon dioxide concentrations, and temperature of the cages at all conditions were within acceptable levels. Ammonia concentrations remained below 25 ppm (parts per million) in most cages, but, even at higher concentrations, did not adversely affect the health of mice. Frequency of cage changing had only one significant effect; pup mortality with pair matings was greater at the cage changing frequency of 7 days compared with 14 or 21 days. In addition, pup mortality with pair matings was higher at 30 ACH compared with other ventilation rates. In conclusion, under the conditions of this study, cage changes once every 14 days and ventilation rates of 60 ACH provide optimum conditions for animal health and practical husbandry.

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.

Atherogenic high-fat diet reduces bone mineralization in mice

The epidemiological correlation between osteoporosis and cardiovascular disease is independent of age, but the basis for this correlation is unknown. We previously found that atherogenic oxidized lipids inhibit osteoblastic differentiation in vitro and ex vivo, suggesting that an atherogenic diet may contribute to both diseases. In this study, effects of an atherogenic high-fat diet versus control chow diet on bone were tested in two strains of mice with genetically different susceptibility to atherosclerosis and lipid oxidation. After 4 months and 7 months on the diets, mineral content and density were measured in excised femurs and lumbar vertebrae using peripheral quantitative computed tomographic (pQCT) scanning. In addition, expression of osteocalcin in marrow isolated from the mice after 4 months on the diets was examined. After 7 months, femoral mineral content in C57BL/6 atherosclerosis-susceptible mice on the high-fat diet was 43% lower (0.73 +/- 0.09 mg vs. 1.28 +/- 0.42 mg; p = 0.008), and mineral density was 15% lower compared with mice on the chow diet. Smaller deficits were observed after 4 months. Vertebral mineral content also was lower in the fat-fed C57BL/6 mice. These changes in the atherosclerosis-resistant, C3H/HeJ mice were smaller and mostly not significant. Osteocalcin expression was reduced in the marrow of high fat-fed C57BL/6 mice. These findings suggest that an atherogenic diet inhibits bone formation by blocking differentiation of osteoblast progenitor cells.

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.

Comparison of bone formation responses to parathyroid hormone(1-34), (1-31), and (2-34) in mice

In this study we used a mouse model system to compare the in vivo effects of parathyroid hormone(1-34) [PTH(1-34)] with that of PTH(1-31) or PTH(2-34) analogs. Daily subcutaneous administration of PTH(1-34) for 15 days caused a dose-dependent increase in the serum osteocalcin level and bone extract alkaline phosphatase activity, markers of bone formation. PTH(2-34) was much less potent, whereas PTH(1-31) was equipotent in stimulating bone formation parameters in mice. PTH(1-34) caused significant increases in serum calcium (after 4 h) and tartrate-resistant acid phosphatase activity in bone extract (after 4 h), whereas PTH(2-34) and PTH(1-31) were less potent. Because PTH(1-31) caused a smaller increase in bone resorption parameters compared to PTH(1-34), despite similar effects on bone formation parameters, we evaluated the long-term anabolic effects of PTH(1-31) and PTH(1-34) in mice. Weekly evaluations of serum osteocalcin levels demonstrated that daily injections of PTH(1-34) and PTH(1-31) at 80 microg/kg body weight increased serum osteocalcin levels within 1 week of the start of treatment, which were maintained during the entire 22 week treatment. Assessment of bone density at the end of the treatment period with peripheral quantitated computed tomography (pQCT) revealed that PTH(1-34) caused a significantly greater increase in femoral bone density compared to PTH(1-31) at the middiaphysis (18% vs. 9% over vehicle control; p < 0.001). Both PTH(1-34) and PTH(1-31) increased periosteal circumference compared to vehicle (p < 0.01) without a significant difference between the two treatments. In contrast, PTH(1-34) caused a significantly greater reduction in endosteal circumference than PTH(1-31) (p < 0.001). Both analogs significantly increased maximum load and area of moment of inertia over the vehicle group. In conclusion, our findings suggest that PTH(1-34) and PTH(1-31) may exhibit different anabolic effects at the periosteum vs. endosteum in the long bones of mice.

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.

Genetic regulation of cortical and trabecular bone strength and microstructure in inbred strains of mice

The inbred strains of mice C57BL/6J (B6) and C3H/HeJ (C3H) have very different femoral peak bone densities and may serve as models for studying the genetic regulation of bone mass. Our objective was to further define the bone biomechanics and microstructure of these two inbred strains. Microarchitecture of the proximal femur, femoral midshaft, and lumbar vertebrae were evaluated in three dimensions using microcomputed tomography (microCT) with an isotropic voxel size of 17 microm. Mineralization of the distal femur was determined using quantitative back-scatter electron (BSE) imaging. MicroCT images suggested that C3H mice had thicker femoral and vertebral cortices compared with B6. The C3H bone tissue also was more highly mineralized. However, C3H mice had few trabeculae in the vertebral bodies, femoral neck, and greater trochanter. The trabecular number (Tb.N) in the C3H vertebral bodies was about half of that in B6 vertebrae (2.8(-1) +/- 0.1 mm(-1) vs. 5.1(-1) +/- 0.2 mm(-1); p < 0.0001). The thick, more highly mineralized femoral cortex of C3H mice resulted in greater bending strength of the femoral diaphysis (62.1 +/- 1.2N vs. 27.4 +/- 0.5N, p < 0.0001). In contrast, strengths of the lumbar vertebra were not significantly different between inbred strains (p = 0.5), presumably because the thicker cortices were combined with inferior trabecular structure in the vertebrae of C3H mice. These results indicate that C3H mice benefit from alleles that enhance femoral strength but paradoxically are deficient in trabecular bone structure in the lumbar vertebrae.

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.

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.

Quantitative trait loci for bone density in C57BL/6J and CAST/EiJ inbred mice

Genetic analyses for loci regulating bone mineral density have been conducted in a cohort of F(2) mice derived from intercross matings of (C57BL/6J x CAST/EiJ)F(1) parents. Femurs were isolated from 714 4-month-old females when peak adult bone density had been achieved. Bone mineral density (BMD) data were obtained by peripheral quantitative computed tomography (pQCT), and genotype data were obtained by Polymerase Chain Reaction (PCR) assays for polymorphic markers carried in genomic DNA of each mouse. Genome-wide scans for co-segregation of genetic marker data with high or low BMD revealed loci on eight different chromosomes, four of which (Chrs 1, 5, 13, and 15) achieved conservative statistical criteria for suggestive, significant, or highly significant linkage with BMD. These four quantitative trait loci (QTLs) were confirmed by a linear regression model developed to describe the main effects; none of the loci exhibited significant interaction effects by ANOVA. The four QTLs have been named Bmd1 (Chr 1), Bmd2 (Chr 5), Bmd3 (Chr 13), and Bmd4 (Chr 15). Additive effects were observed for Bmd1, recessive for Bmd3, and dominant effects for Bmd2 and Bmd4. The current large size of the QTL regions (6-->31 cM) renders premature any discussion of candidate genes at this time. Fine mapping of these QTLs is in progress to refine their genetic positions and to evaluate human homologies.

Histomorphometric studies show that bone formation and bone mineral apposition rates are greater in C3H/HeJ (high-density) than C57BL/6J (low-density) mice during growth

High-density C3H/HeJ (C3H) and low-density C57BL/6J (B6) mice, with femoral bone density differing by 50%, were chosen as a model to investigate the mechanisms controlling peak bone density and to map peak bone density genes. The present longitudinal study was undertaken to further establish the bone biologic phenotypes of these two inbred strains of mice. To evaluate phenotypic differences in bone formation parameters in C3H and B6 mice between the ages of 6 and 26 weeks, undecalcified ground sections from the diaphyses of the tibia and femur were prepared from mice receiving two injections of tetracycline. Histomorphometric analyses revealed that the cortical bone area was significantly greater (16%-56%, p < 0.001) in both the femur and tibia of the C3H mice than in the B6 mice at all timepoints. This difference in cortical bone area was due to significantly smaller medullary areas in the C3H mice than in the B6 mice. The bone formation rates (BFR) at the endosteum in both the femur and tibia were significantly greater (28%-117%,p < 0.001) in the young C3H mice (6-12 weeks old) than in B6 mice. The higher bone formation in C3H mice was associated with higher values of the bone mineral apposition rate (25%-94%, p < 0.001), and was not associated with higher values of the forming surface length as measured by tetracycline label length. Similar interstrain differences in mineral apposition and bone formation rates were observed in the periosteum of the femur and tibia. In conclusion, the greater bone area in the high-density C3H mice vs. the low-density B6 mice was, in part, due to the greater periosteal and endosteal bone formation rates during growth in the C3H mice. Because the C3H and B6 mice were maintained under identical environmental conditions (diet, lighting, etc.), the observed interstrain differences in bone parameters were the result of the action of genetic factors. Consequently, these two inbred strains of mice are suitable as a model to identify genetic factors responsible for high bone formation rates.

Androgenetic alopecia: in vivo models

Androgenetic alopecia is the most common form of balding in humans. There is great interest in finding a reliable animal model to study the pathogenesis and treatment of this abnormality. The sump-tailed macaque (Macaca artoides) has been the standard model and appears to be useful homologue. These primates are reasonably good predictors of compound efficacy. Due to reduced size and expense, rodent models have been sought. Testosterone inducible models require more development but offer potential. Xenografts of human skin to immunodeficient mice, notably nude or severe combined immunodeficiency, are small, relatively inexpensive, and easy to work with if a source of human tissue is available. Xenografts to double mutant mice for severe combined immunodeficiency and a number of hormone receptor null mutations offer new refinements to these xenograft models.

Physiologic and endocrinologic characterization of male sex-biased diabetes in C57BLKS/J mice congenic for the fat mutation at the carboxypeptidease E locus

The fat gene in mice represents a recessive mutation at the carboxypeptidase E (Cpe) locus. The mutant allele (Cpe(fat)) encodes a highly unstable enzyme and produces an obesity phenotype characterized by attenuated processing of prohormones such as proinsulin that require this exopeptidase for full maturation. This article presents a preliminary physiologic and endocrinologic characterization of the stock of C57BLKS/LtJ-Cpe(fat)/Cpe(fat) mice at the backcross generation (N10) currently distributed by The Jackson Laboratory. Although previously reported not to be diabetogenic at N5, an additional five backcrosses to the C57BLKS/J background resulted in a male-biased development of both obesity and diabetes. Major differences distinguishing this mutant stock from the phenotypes produced by either the diabetes (Lepr(db)) or obese (Lep(ob)) mutations on the same inbred strain background are lack of hyperphagia and hypercorticism, sensitivity of diabetic males to exogenous insulin, and a milder and male-biased diabetes syndrome that is not associated with widespread beta-cell necrosis and islet atrophy, and that often remits with age.

Osteoclast formation in bone marrow cultures from two inbred strains of mice with different bone densities

For the purpose of identifying genes that affect bone volume, we previously identified two inbred mouse strains (C57BL/6J and C3H/HeJ) with large differences in femoral bone density and medullary cavity volume. The lower density and larger medullary cavity volume in C57BL/6J mice could result from either decreased formation or increased resorption or both. We recently reported evidence suggesting that bone formation was increased in vivo and that osteoblast progenitor cells are more numerous in the bone marrow of C3H/HeJ compared with C57BL/6J mice. In the present study, we determined whether osteoclast numbers in vivo and osteoclast formation from bone marrow cells in vitro might also differ between the two mouse strains. We have found that the number of osteoclasts on bone surfaces of distal humerus secondary spongiosa was 2-fold higher in 5.5-week-old C57BL/6J mice than in C3H/HeJ mice of the same age (p < 0.001). Bone marrow cells of C57BL/6J mice cocultured with Swiss/Webster mouse osteoblasts consistently produced more osteoclasts than did C3H/HeJ bone marrow cells at all ages tested from 3.5-14 weeks of age (p < 0.001). Osteoclast formation was also greater from spleen cells of 3.5-week-old C57BL/6J mice than C3H/HeJ mice. The distribution of nuclei per osteoclast and the 1, 25-dihydroxyvitamin D3 dose dependence of osteoclast production from bone marrow cells were similar. Osteoclasts that developed from both C57BL/6J and C3H/HeJ marrow cells formed pits in dentin slices. Cultures from C57BL/6J marrow cells formed 2.5-fold more pits than cultures from C3H/HeJ marrow cells (p < 0.02). We compared the abilities of C57BL/6J and C3H/HeJ osteoblasts to support osteoclast formation. When bone marrow cells from either C57BL/6J or C3H/HeJ mice were cocultured with osteoblasts from either C57BL/6J or C3H/HeJ newborn calvaria, the strain from which osteoblasts were derived did not affect the number of osteoclasts formed from marrow cells of either strain. Together, these observations suggest that genes affecting the bone marrow osteoclast precursor population may contribute to the relative differences in bone density that occur between C3H/HeJ and C57BL/6J mouse strains.

Multigenic and imprinting control of ovarian granulosa cell tumorigenesis in mice

Spontaneous juvenile ovarian granulosa cell (GC) tumors that occur in young girls are similar to GC carcinomas that develop in SWR-derived inbred mice. We analyzed female offspring from a series of matings among SWR and SJL inbred mice for chromosomal loci underlying tumor susceptibility. Intercross F2 female mice were produced by reciprocal matings of (SWR x SJL)F1 and (SJL x SWR)F1 parents. Tumorigenesis in these F2 mice as well as in SWXJ recombinant inbred and congenic strains of mice derived from SWR and SJL showed significant (P < 0.001) association with Gct1, a dominant susceptibility locus on chromosome (CHR) 4 and with Gct2 on CHR 12. Suggestive (P < 0.01) association was found with Gct3 on CHR 15. A fourth susceptibility locus, Gct4 on CHR X, was demonstrated with a strong parent-of-origin effect associated with the paternal genotype. Imprinting and complex interactions among these four loci combine to establish the probability for GC tumorigenesis in this mouse model.

Lymphadenopathy, elevated serum IgE levels, autoimmunity, and mast cell accumulation in flaky skin mutant mice

The autosomal recessive mutation "flaky skin" (fsn) causes pleiotropic abnormalities in the immune and hematopoietic systems accompanied by pathologic changes in the skin. Homozygotes (fsn/fsn) showed increased size and histological alterations in the spleen and lymph nodes. Abnormalities in lymphoid architecture of the spleen in fsn/fsn mice were accompanied by marked increases in total numbers of B cells, macrophages, and immature erythroid cells. Splenic B cells displayed elevated MHC class II expression. Serum IgE levels were greater than 100 microg/ml by 10 weeks of age, representing > 7000-fold increase compared with normal littermates. This increased IgE level was associated with elevated IL-4 production by spleen cells and with increased amounts of serum IL-4. Serum IgM, IgG1, and IgG2b levels were also increased in fsn/fsn mice while IgG3 was decreased. Autoimmunity in fsn/fsn mice was evidenced by glomerulonephritis accompanied by immune complex deposition in the kidneys, increased serum blood urea nitrogen levels, and the presence of circulating anti-double-stranded DNA autoantibodies. Pathological changes in the skin of fsn/fsn mice were characterized by epidermal hyperplasia and mixed dermal inflammation. Increased numbers of mast cells were also observed in the dermis of the truncal skin as well as in the epithelial stomach. These marked immunological abnormalities suggest that the fsn locus encodes a major immunoregulatory molecule important in multiple immune and hematopoietic functions.

Alkaline phosphatase levels and osteoprogenitor cell numbers suggest bone formation may contribute to peak bone density differences between two inbred strains of mice

Previous studies have shown that C3H/HeJ (C3H) mice have higher peak bone density than C57BL/6J (B6) mice, at least in part because of differences in rates of bone resorption. The current studies were intended to examine the alternative, additional hypothesis that the greater bone density in C3H mice might also be a consequence of increased bone formation. To that end, we measured two presumptive, indirect indices of bone formation and osteoblast number in these inbred strains of mice: alkaline phosphatase (ALP) activity in serum, bones, and bone cells; and the number of ALP-positive colony-forming units (CFU) in bone marrow stromal cell cultures. We found that C3H mice had higher serum levels of ALP activity than B6 mice at 6 (118 vs. 100 U/L, p < 0.03) and 32 weeks of age (22.2 vs. 17.2 U/L, p < 0.001). Tibiae from C3H mice also contained higher levels of ALP activity than tibiae from B6 mice at 6 (417 vs. 254 mU/mg protein, p < 0.02) and 14 weeks of age (132 vs. 79 mU/mg protein, p < 0.001), as did monolayer cultures of bone-derived cells from explants of 7.5-week-old C3H calvariae and femora (8.2 times more, p < 0.02, and 4.6 times more, p < 0.001, respectively). Monolayer cell cultures prepared by collagenase digestion of calvariae from newborn and 6-week-old mice also showed similar strain-dependent differences in ALP-specific activity (p < 0.001 for each). Our studies also showed more ALP-positive CFU in bone marrow stromal cell cultures from 8-week-old C3H mice, compared with B6 mice (72.3 vs. 26.1 ALP-positive CFU/culture dish, p < 0.001). A similar result was seen for ALP-positive CFU production at 6 and 14 weeks of age, and the difference was greatest for the CFU that contained the greatest numbers of ALP-positive cells. Because skeletal ALP activity is a product of osteoblasts and has been shown to correlate with rates of bone formation, and because the number of ALP-positive CFU is believed to reflect the number of osteoprogenitor cells, the current data are consistent with the general hypothesis that bone formation may be greater in C3H than B6 mice because of a difference in osteoblast number. Our data further suggest that peak bone density may be greater in C3H mice than B6 mice due to a combination of decreased bone resorption and increased bone formation.

Development and progression of psoriasiform dermatitis and systemic lesions in the flaky skin (fsn) mouse mutant

Flaky skin (fsn) mutant mice were originally described as a mouse model for psoriasis accompanied by hematological abnormalities. However, homozygous (fsn/fsn) mice develop a number of other pathological changes. Systematic evaluation of over 300 fsn/fsn and normal littermate control (+/+ or +/fsn) mice was carried out to characterize these changes. Psoriasiform skin lesions were first evident as focal epidermal hyperplasia and inflammation at 2 weeks of age. These lesions became confluent and diffuse by 3-4 weeks of age and were associated with marked dermal infiltration of lymphocytes and small numbers of neutrophils and macrophages. Mast cell numbers increased significantly in the dermis from 2 weeks of age onward. Diffuse dermal neovascularization accompanied these cutaneous changes. Systemic lesions included progressive and massive papillomatosis of the stratified squamous epithelium of the forestomach, hyperplasia and dysplasia of the glandular stomach, increased apoptosis of cecal enterocytes, renal glomerulopathy associated with immune complex and complement deposition, testicular degeneration, mixed inflammatory cell infiltrates and fibrosis around portal triads in the liver, splenomegaly due to massive erythropoiesis, and granulomatous lymphadenitis. This spontaneous mouse mutation provides a useful model for modulating neovascularization and keratinocyte hyperproliferation, especially since the cutaneous changes resemble some forms of psoriasis in humans.

Comparison of chemical carcinogen skin tumor induction efficacy in inbred, mutant, and hybrid strains of mice: morphologic variations of induced tumors and absence of a papillomavirus cocarcinogen

Chemical carcinogen induction of skin tumors in mice was investigated to determine (i) if tumor induction efficacy was modified by single gene mutations, (ii) if the histologic types of the tumors varied with these mutations, and (iii) if a novel papillomavirus was involved as a cocarcinogen. A two-stage carcinogenesis protocol (7,12-dimethylbenz[a]anthracene followed by 12-O-tetradecanoylphorbol-13-acetate) was used to induce papillomas in 14 inbred, two hybrid, and 15 other genetic stocks of mice with inherited, single-gene mutations causing skin abnormalities. Histopathological, immunohistochemical, and Southern blot analyses were performed to determine tumor type and to detect the presence of papillomaviruses. The histologic types of tumors induced included early follicular papillomas, mixed papillomas, exophytic papillomas, hyperplastic papillomas, fibropapillomas, squamous cell carcinomas, and mast cell tumors. The efficacy of tumor induction was influenced by strain background, as seen by the clustering of mice into high-, intermediate-, and nonresponding groups. Similarly, tumor induction efficacy was affected by specific mutant genes that cause skin abnormalities. No evidence of papillomavirus structural antigens or viral genomic DNA was identified in 547 induced tumors. These observations indicate that numerous modifier genes but not papillomaviruses are involved in cutaneous chemical carcinogenesis.

Circulating and skeletal insulin-like growth factor-I (IGF-I) concentrations in two inbred strains of mice with different bone mineral densities

Recent work has demonstrated differences in femoral bone mineral density between two common inbred strains of mice, C3H/HeJ (C3H) and C57BL/6J (B6), across a wide age range. To investigate one possible mechanism that could affect acquisition and maintenance of bone mass in mice, we studied circulatory and skeletal insulin-like growth factor-I (IGF-I) and femoral bone mineral density (F-BMD) by pQCT in C3H and B6 progenitor strains, as well as serum IGF-I obtained from matings between these two strains and mice bred from subsequent F1 intercrosses (F2). Serum IGF-I measured by radioimmunoassay was more than 35% higher in virgin progenitor C3H than virgin B6 at 1, 4, 8, and 10 months of age, and in 8-month-old C3H compared with B6 retired breeders (p < 0.001). In the progenitors, there was also a strong correlation between serum IGF-I and serum alkaline phosphatase (r = 0.51, p = 0.001). In the 4 month F1 females IGF-I levels and F-BMD were intermediate between C3H and B6 progenitors. In contrast, groups of F2 mice with the highest or lowest BMD also had the highest or lowest serum IGF-I (p = 0.0001). IGF-I accounted for > 35% of the variance in F-BMD among the F2 mice. Conditioned media from newborn C3H calvarial cultures had higher concentrations of IGF-I than media from B6 cultures, and cell layer extracts from C3H calvariae exhibited greater alkaline phosphatase activity than cultures from B6 calvarial cells (p < 0.0001). The skeletal content of IGF-I in C3H tibiae, femorae, and calvariae (6-14 weeks of age) was also significantly higher than IGF-I content in the same bones of the B6 mice (p < 0.05). These data suggest that a possible mechanism for the difference in acquisition and maintenance of bone mass between these two inbred strains is related to systemic and skeletal IGF-I synthesis.

Hormone synthesis and responsiveness of spontaneous granulosa cell tumors in (SWR x SWXJ-9) F1 mice

Granulosa cell tumors spontaneously occur in approximately 10-25% of female (SWR x SWXJ-9) F1 mice. The present studies were designed to test whether tumor-bearing mice produce a distinct hormonal profile by which they could be identified and determine whether cultured tumor cells are responsive to hormones and growth factors that regulate normal granulosa cells. Samples of female mouse blood taken from age 3 to 10 weeks allowed estimation of serum FSH, 17beta-estradiol, and inhibin levels for normal mice and for mice destined to develop tumors. These studies indicated that FSH and 17beta-estradiol values differed between normal and tumor-bearing animals, but overlapped sufficiently that such values could not accurately predict the tumor-bearing state. Inhibin concentrations did differentiate normal from tumor-bearing animals in all cases. Increased levels of inhibin were observed coincident in time with visibly detectable tumors within the ovaries. Compared to inhibin synthesis in vivo, hormonal responsiveness in vitro was much more variable. Steroidogenesis was stimulated in all tumors by dibutyryl-cAMP and low-density lipoprotein (LDL). Some, but not all, tumors responded to IGF1, EGF, FSH, and hCG. In about one-half of the tumors tested, FSH could induce hCG or dibutyryl-cAMP responsiveness. IGF1 pretreatment consistently increased the responsiveness of tumor cells stimulated by dibutyryl-cAMP and LDL. Production of inhibin by isolated tumor cells was detectable and decreased by EGF or dibutyryl-cAMP treatments. We conclude that granulosa tumor cell secretion of inhibin may be under different control than secretion from normal granulosa cells and acts as an excellent marker for these tumors.

Pituitary lineage determination by the Prophet of Pit-1 homeodomain factor defective in Ames dwarfism

The gene apparently responsible for a heritable form of murine pituitary-dependent dwarfism (Ames dwarf, df) has been positionally cloned, identifying a novel, tissue-specific, paired-like homeodomain transcription factor, termed Prophet of Pit-1 (Prop-1). The df phenotype results from an apparent failure of initial determination of the Pit-1 lineage required for production of growth hormone, prolactin or thyroid-stimulating hormone, resulting in dysmorphogenesis and failure to activate Pit-1 gene expression. These results imply that a cascade of tissue-specific regulators is responsible for the determination and differentiation of specific cell lineages in pituitary organogenesis.

Strain distribution pattern for SSLP markers in the SWXJ recombinant inbred strain set: chromosomes 7 to X

The SWXJ recombinant inbred (RI) set was developed for genetic analysis of heritable ovarian tumors. In this report we present data for 223 simple sequence length polymorphisms spanning Chromosomes (Chrs) 7-X to complete the genetic marking of this RI set. The strain distribution patterns (SDP) for these loci were combined with data from 19 other polymorphic genes, resulting in densely marked maps for Chrs 7-X. Combined with the 165 loci for Chr 1-6 reported previously (Svenson et al., Mamm. Genome 6, 867, 1995), the SWXJ RI set represents a powerful tool for mapping genes in neoplastic as well as other heritable disorders.

Genetic variability in adult bone density among inbred strains of mice

More than 70% of the variability in human bone density has been attributed to genetic factors as a result of studies with twins, osteoporotic families, and individuals with rare heritable bone disorders. We have applied the Stratec XCT 960M pQCT, specifically modified for small skeletal specimens, to analyses of bones from 11 inbred strains (AKR/J, BALB/cByJ, C3H/HeJ, C57BL/6J, C57L/J, DBA/2J, NZB/B1NJ, SM/J, SJL/BmJ, SWR/BmJ, and 129/J) of female mice to determine the extent of heritable differences in peak bone density, pQCT scans were taken of femurs from (a) 12-month-old inbred strain females and (b) a subset of four strains (C3H/HeJ, DBA/2J, BALB/cByJ, C57BL/6J) at 2, 4, and 8 months. In addition, pQCT scans were also obtained from L5-L6 vertebrae and proximal phalanges from the same subset of four inbred strains at 12 months of age. Comparison of bone parameters among inbred strains revealed significant differences at each of the three sites investigated. Femoral and phalangeal bones differed among strains with respect to total and cortical density, mineral, and volume. Only cortical bone parameters were significantly different among strains at the vertebral site. With respect to strain differences, the highest value for any given bone parameter was found in the C3H/HeJ strain, whereas C57BL/6J values were absolutely, or statistically, the lowest. Similarly, with respect to bone sites, cortical bone density was significantly correlated among strains. On the other hand, we found that none of the femur, vertebral, or phalangeal parameters correlated with body weight, even though body weight varied by 86% among those inbred strains. The developmental studies of femurs conducted at 2, 4, and 8 months of age with C3H/HeJ, DBA/2J, BALB/cByJ, and C57BL/6J females showed differences in total density among strains at 2 months and thereafter. Adult peak bone density was typically achieved by 4 months, whereas femurs continued to lengthen for 4 to 8 months thereafter. We conclude that (1) major genetic effects on femoral, vertebral, and phalangeal bone density are detectable among inbred strains of mice; (2) cortical bone density shares common genetic regulation at the three measured sites; and (3) within the femur, genes that regulate length and density are different.

Reduced growth of human breast cancer xenografts in hosts homozygous for the lit mutation

Insulin-like growth factor I (IGF-I) is a potent breast cancer mitogen. Growth hormone (GH) up-regulates hepatic IGF-I gene expression and circulating IGF-I level. Tissue IGF bioactivity is influenced not only by circulating IGF-I and IGF-II levels but also by autocrine and paracrine production of these growth factors and by IGF binding proteins. There is considerable person-to-person variability in GH-IGF-I physiology. Both laboratory and epidemiological data are consistent with the hypothesis that the host GH-IGF-I axis influences breast cancer behavior, but such an effect has not been directly demonstrated. To determine whether breast cancer growth in an in vivo model is influenced by the host GH-IGF-I axis, we compared the growth of human MCF-7 breast cancer cells in control mice to that in mice homozygous for lit, a missense mutation resulting in loss of function of the pituitary GH-releasing hormone receptor and secondary suppression of GH and IGF-I. Breast cancer growth was significantly reduced in lit/lit animals compared to control hosts [tumor size (mean +/- SD) on day 39,444 +/- 82 versus 845 +/- 444 mm3, respectively; P < 0.001, Mann-Whitney U test]. These data demonstrate that in our model, host GH-IGF-I axis physiology plays a role in determining breast cancer behavior. The results a) suggest that patient-to-patient variability in GH-IGF-I physiology may contribute to the large variability between patients regarding breast cancer behavior, and b) motivate clinical trials of novel hormonal treatment strategies that target the GH-IGF-I axis.

Mechanisms of hormone-mediated carcinogenesis of the ovary in mice

Experimental ovarian carcinogenesis has been investigated in inbred and hybrid strains of mice and induced by a diversity of mechanisms including X-irradiation, oocytotoxic xenobiotic chemicals, ovarian grafting to ectopic or orthotopic sites, neonatal thymectomy, mutant genes reducing germ cell populations, and aging. The mechanisms are briefly reviewed whereby disruptions in the function of graafian follicles results in a spectrum of ovarian proliferative lesions including tumors. The findings in mutant mice support the concept of a secondary (hormonally-mediated) mechanism of ovarian carcinogenesis in mice associated with sterility. Multiple pathogenetic factors that either destroy or diminish the numbers of graafian follicles in the ovary result in decreased sex hormone secretion (especially estradiol-17 beta) leading to a compensatory over-production of pituitary gonadotrophins (particularly luteinizing hormone), which places the mouse ovary at an increased risk to develop tumors. The intense proliferation of ovarian surface epithelium and stromal (interstitial) cells with the development of unique tubular adenomas in response to sterility does not appear to have a counterpart in the ovaries of women.

Strain distribution pattern for SSLP markers in the SWXJ recombinant inbred strain set: chromosomes 1 to 6

We typed 147 simple sequence length polymorphisms in the SWXJ recombinant inbred (RI) strain set spanning Chromosomes (Chrs) 1-6. The strain distribution pattern for these loci was combined with data from 18 previously typed loci for SWXJ, resulting in new chromosome maps for this RI set, with an average density of 3.5 cM between loci. This is the first systematic effort to develop a more highly resolved genetic map for the SWXJ RI set and thereby improves the usefulness of this genetic tool for mapping genes underlying both simple and complex genetic disorders.