Cellular mechanism of decreased bone in Brtl mouse model of OI: imbalance of decreased osteoblast function and increased osteoclasts and their precursors

Effect of sclerostin antibody treatment in a mouse model of severe osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a heritable bone fragility disorder that is usually caused by mutations affecting collagen type I production in osteoblasts. Stimulation of bone formation through sclerostin antibody treatment (Sost-ab) has shown promising results in mouse models of relatively mild OI. We assessed the effect of once-weekly intravenous Sost-ab injections for 4weeks in male Col1a1(Jrt)/+mice, a model of severe dominant OI, starting either at 4weeks (growing mice) or at 20weeks (adult mice) of age. Sost-ab had no effect on weight or femur length. In OI mice, no significant treatment-associated differences in serum markers of bone formation (alkaline phosphatase activity, procollagen type I N-propeptide) or resorption (C-telopeptide of collagen type I) were found. Micro-CT analyses at the femur showed that Sost-ab treatment was associated with higher trabecular bone volume and higher cortical thickness in wild type mice at both ages and in growing OI mice, but not in adult OI mice. Three-point bending tests of the femur showed that in wild type but not in OI mice, Sost-ab was associated with higher ultimate load and work to failure. Quantitative backscattered electron imaging of the femur did not show any effect of Sost-ab on CaPeak (the most frequently occurring calcium concentration in the bone mineral density distribution), regardless of genotype, age or measurement location. Thus, Sost-ab had a larger effect in wild type than in Col1a1(Jrt)/+mice. Previous studies had found marked improvements of Sost-ab on bone mass and strength in an OI mouse model with a milder phenotype. Our data therefore suggest that Sost-ab is less effective in a more severely affected OI mouse model.

Adult Brtl/+ mouse model of osteogenesis imperfecta demonstrates anabolic response to sclerostin antibody treatment with increased bone mass and strength

Treatments to reduce fracture rates in adults with osteogenesis imperfecta are limited. Sclerostin antibody, developed for treating osteoporosis, has not been explored in adults with OI. This study demonstrates that treatment of adult OI mice respond favorably to sclerostin antibody therapy despite retention of the OI-causing defect.

INTRODUCTION

Osteogenesis imperfecta (OI) is a heritable collagen-related bone dysplasia, characterized by brittle bones with increased fracture risk. Although OI fracture risk is greatest before puberty, adults with OI remain at risk of fracture. Antiresorptive bisphosphonates are commonly used to treat adult OI, but have shown mixed efficacy. New treatments which consistently improve bone mass throughout the skeleton may improve patient outcomes. Neutralizing antibodies to sclerostin (Scl-Ab) are a novel anabolic therapy that have shown efficacy in preclinical studies by stimulating bone formation via the canonical wnt signaling pathway. The purpose of this study was to evaluate Scl-Ab in an adult 6 month old Brtl/+ model of OI that harbors a typical heterozygous OI-causing Gly > Cys substitution on Col1a1.

METHODS

Six-month-old WT and Brtl/+ mice were treated with Scl-Ab (25 mg/kg, 2×/week) or Veh for 5 weeks. OCN and TRACP5b serum assays, dynamic histomorphometry, microCT and mechanical testing were performed.

RESULTS

Adult Brtl/+ mice demonstrated a strong anabolic response to Scl-Ab with increased serum osteocalcin and bone formation rate. This anabolic response led to improved trabecular and cortical bone mass in the femur. Mechanical testing revealed Scl-Ab increased Brtl/+ femoral stiffness and strength.

CONCLUSION

Scl-Ab was successfully anabolic in an adult Brtl/+ model of OI.

First mouse model for combined osteogenesis imperfecta and Ehlers-Danlos syndrome

By using a genome-wide N-ethyl-N-nitrosourea (ENU)-induced dominant mutagenesis screen in mice, a founder with low bone mineral density (BMD) was identified. Mapping and sequencing revealed a T to C transition in a splice donor of the collagen alpha1 type I (Col1a1) gene, resulting in the skipping of exon 9 and a predicted 18-amino acid deletion within the N-terminal region of the triple helical domain of Col1a1. Col1a1(Jrt) /+ mice were smaller in size, had lower BMD associated with decreased bone volume/tissue volume (BV/TV) and reduced trabecular number, and furthermore exhibited mechanically weak, brittle, fracture-prone bones, a hallmark of osteogenesis imperfecta (OI). Several markers of osteoblast differentiation were upregulated in mutant bone, and histomorphometry showed that the proportion of trabecular bone surfaces covered by activated osteoblasts (Ob.S/BS and N.Ob/BS) was elevated, but bone surfaces undergoing resorption (Oc.S/BS and N.Oc/BS) were not. The number of bone marrow stromal osteoprogenitors (CFU-ALP) was unaffected, but mineralization was decreased in cultures from young Col1a1(Jrt) /+ versus +/+ mice. Total collagen and type I collagen content of matrices deposited by Col1a1(Jrt) /+ dermal fibroblasts in culture was ∼40% and 30%, respectively, that of +/+ cells, suggesting that mutant collagen chains exerted a dominant negative effect on type I collagen biosynthesis. Mutant collagen fibrils were also markedly smaller in diameter than +/+ fibrils in bone, tendon, and extracellular matrices deposited by dermal fibroblasts in vitro. Col1a1(Jrt) /+ mice also exhibited traits associated with Ehlers-Danlos syndrome (EDS): Their skin had reduced tensile properties, tail tendon appeared more frayed, and a third of the young adult mice had noticeable curvature of the spine. Col1a1(Jrt) /+ is the first reported model of combined OI/EDS and will be useful for exploring aspects of OI and EDS pathophysiology and treatment.

Serum sclerostin levels are decreased in adult patients with different types of osteogenesis imperfecta

CONTEXT

There are no specific biochemical bone markers available for osteogenesis imperfecta (OI), and the role of sclerostin as a key regulator of bone formation in OI is unknown.

OBJECTIVES

We aimed to evaluate the role of sclerostin and its association with bone turnover markers as well as body composition parameters in adult patients with different types of OI.

DESIGN, SETTING, AND PARTICIPANTS

This was a case-control study in 27 adult patients and 50 healthy age- and gender-matched controls.

MAIN OUTCOME MEASURES

Serum sclerostin levels and bone turnover markers including serum osteocalcin, amino terminal propeptide of type I procollagen, and CrossLaps as well as body composition parameters were determined in mild OI stage I (OI-I) and moderate-severe OI stages III-IV (OI-III-IV), according to Sillence classification. Data were compared with healthy controls.

RESULTS

Sclerostin levels were significantly lower in OI-I (19.9 ± 10.9 pmol/L; P < .001) and OI-III-IV (13.3 ± 10.0 pmol/L; P < .001) compared with healthy adults (45.3 ± 14.9 pmol/L), even after adjustment for age, sex, bone mineral content, and body mass index. CrossLaps and PTH were significantly lower in OI-I (0.197 ± 0.15 ng/L; P = .007 and 33.7 ± 19.1 pg/L; P = .033, respectively) and OI-III-IV (0.221 ± 0.18 ng/L; P = .039, and 27.9 ± 14.7 pg/L; P = .001, respectively) than in healthy controls (0.322 ± 0.15 ng/L and 45.0 ± 16.6 pg/L). Amino-terminal propeptide of type I procollagen was below the reference range for OI-I and OI-III-IV. Patients with OI were shorter and lighter and had a decreased bone mineral content (P < .001) but similar fat distribution and lean body mass, compared with controls. Serum sclerostin levels were not related to any bone marker except osteocalcin, the number of prevalent fractures, or body composition readings.

CONCLUSION

Decreased sclerostin levels in OI might reflect a down-regulation or negative feedback mechanism to prevent further bone loss.

Induced ablation of Bmp1 and Tll1 produces osteogenesis imperfecta in mice

Osteogenesis imperfecta (OI), or brittle bone disease, is most often caused by dominant mutations in the collagen I genes COL1A1/COL1A2, whereas rarer recessive OI is often caused by mutations in genes encoding collagen I-interacting proteins. Recently, mutations in the gene for the proteinase bone morphogenetic 1 (BMP1) were reported in two recessive OI families. BMP1 and the closely related proteinase mammalian tolloid-like 1 (mTLL1) are co-expressed in various tissues, including bone, and have overlapping activities that include biosynthetic processing of procollagen precursors into mature collagen monomers. However, early lethality of Bmp1- and Tll1-null mice has precluded use of such models for careful study of in vivo roles of their protein products. Here we employ novel mouse strains with floxed Bmp1 and Tll1 alleles to induce postnatal, simultaneous ablation of the two genes, thus avoiding barriers of Bmp1(-/-) and Tll1(-/-) lethality and issues of functional redundancy. Bones of the conditionally null mice are dramatically weakened and brittle, with spontaneous fractures-defining features of OI. Additional skeletal features include osteomalacia, thinned/porous cortical bone, reduced processing of procollagen and dentin matrix protein 1, remarkably high bone turnover and defective osteocyte maturation that is accompanied by decreased expression of the osteocyte marker and Wnt-signaling inhibitor sclerostin, and by marked induction of canonical Wnt signaling. The novel animal model presented here provides new opportunities for in-depth analyses of in vivo roles of BMP1-like proteinases in bone and other tissues, and for their roles, and for possible therapeutic interventions, in OI.

The long bone deformity of osteogenesis imperfecta III: analysis of structural changes carried out with scanning electron microscopic morphometry

The wedges of the mid-diaphyseal osteotomies carried out to correct the femoral and/or tibial native deformity in type III osteogenesis imperfecta (OI III) were used to study the remodeling patterns and lamellar organization at the level of the major deformity. Histology and scanning electron microscopy (SEM) morphology showed abnormal cortical remodeling characterized by the failure to form a cylinder of compact bone with a regular marrow canal. Atypical, flattened, and large resorption lacunae with a wide resorption front on one side and systems of parallel lamellae on the opposite side were observed, resembling those formerly reported as drifting osteons. SEM morphometry documented a higher percentage of nonossified vascular/resorption area (44.3 %) in OI than in controls (13.6 %), a lower density of secondary osteons, and lower values for the parameters expressing the individual osteon size. The mean osteon total area, the mean central canal area, and the mean osteon bone area of two selected, randomized populations of secondary osteons were significantly higher (p < 0.001, p = 0.028, and p < 0.001, respectively) in control bones than in OI. The mean ossified matrix area was not significantly different, but the mean secondary osteon number and mean density were higher in controls (both p < 0.001). Osteon wedges were carried out to correct the native deformity of OI III and morphologic analysis suggested that the abnormal remodeling pattern (with "drifting osteons") may result from the altered load and tensile stresses on the deformed tubular bones.

Fracture healing with alendronate treatment in the Brtl/+ mouse model of osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a heritable bone dysplasia characterized by increased skeletal fragility. Patients are often treated with bisphosphonates to attempt to reduce fracture risk. However, bisphosphonates reside in the skeleton for many years and long-term administration may impact bone material quality. Acutely, there is concern about risk of non-union of fractures that occur near the time of bisphosphonate administration. This study investigated the effect of alendronate, a potent aminobisphosphonate, on fracture healing. Using the Brtl/+ murine model of type IV OI, tibial fractures were generated in 8-week-old mice that were untreated, treated with alendronate before fracture, or treated before and after fracture. After 2, 3, or 5 weeks of healing, tibiae were assessed using microcomputed tomography (μCT), torsion testing, quantitative histomorphometry, and Raman microspectroscopy. There were no morphologic, biomechanical or histomorphometric differences in callus between untreated mice and mice that received alendronate before fracture. Alendronate treatment before fracture did not cause a significant increase in cartilage retention in fracture callus. Both Brtl/+ and WT mice that received alendronate before and after fracture had increases in the callus volume, bone volume fraction and torque at failure after 5 weeks of healing. Raman microspectroscopy results did not show any effects of alendronate in wild-type mice, but calluses from Brtl/+ mice treated with alendronate during healing had a decreased mineral-to-matrix ratio, decreased crystallinity and an increased carbonate-to-phosphate ratio. Treatment with alendronate altered the dynamics of healing by preventing callus volume decreases later in the healing process. Fracture healing in Brtl/+ untreated animals was not significantly different from animals in which alendronate was halted at the time of fracture.

Mineral and matrix changes in Brtl/+ teeth provide insights into mineralization mechanisms

The Brtl/+ mouse is a knock-in model for osteogenesis imperfecta type IV in which a Gly349Cys substitution was introduced into one COL1A1 allele. To gain insight into the changes in dentin structure and mineral composition in these transgenic mice, the objective of this study was to use microcomputed tomography (micro-CT), scanning electron microscopy (SEM), and Fourier transform infrared imaging (FTIRI) to analyze these structures at 2 and 6 months of age. Results, consistent with the dental phenotype in humans with type IV OI, showed decreased molar volume and reduced mineralized tissue volume in the teeth without changes in enamel properties. Increased acid phosphate content was noted at 2 and 6 months by FTIRI, and a trend towards altered collagen structure was noted at 2 but not 6 months in the Brtl/+ teeth. The increase in acid phosphate content suggests a delay in the mineralization process, most likely associated with the defect in the collagen structure. It appears that in the Brtl/+ teeth slow maturation of the mineralized structures allows correction of altered mineral content and acid phosphate distribution.

Sclerostin antibody improves skeletal parameters in a Brtl/+ mouse model of osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a genetic bone dysplasia characterized by osteopenia and easy susceptibility to fracture. Symptoms are most prominent during childhood. Although antiresorptive bisphosphonates have been widely used to treat pediatric OI, controlled trials show improved vertebral parameters but equivocal effects on long-bone fracture rates. New treatments for OI are needed to increase bone mass throughout the skeleton. Sclerostin antibody (Scl-Ab) therapy is potently anabolic in the skeleton by stimulating osteoblasts via the canonical wnt signaling pathway, and may be beneficial for treating OI. In this study, Scl-Ab therapy was investigated in mice heterozygous for a typical OI-causing Gly→Cys substitution in col1a1. Two weeks of Scl-Ab successfully stimulated osteoblast bone formation in a knock-in model for moderately severe OI (Brtl/+) and in WT mice, leading to improved bone mass and reduced long-bone fragility. Image-guided nanoindentation revealed no alteration in local tissue mineralization dynamics with Scl-Ab. These results contrast with previous findings of antiresorptive efficacy in OI both in mechanism and potency of effects on fragility. In conclusion, short-term Scl-Ab was successfully anabolic in osteoblasts harboring a typical OI-causing collagen mutation and represents a potential new therapy to improve bone mass and reduce fractures in pediatric OI.

Impaired osteoblastogenesis in a murine model of dominant osteogenesis imperfecta: a new target for osteogenesis imperfecta pharmacological therapy

The molecular basis underlying the clinical phenotype in bone diseases is customarily associated with abnormal extracellular matrix structure and/or properties. More recently, cellular malfunction has been identified as a concomitant causative factor and increased attention has focused on stem cells differentiation. Classic osteogenesis imperfecta (OI) is a prototype for heritable bone dysplasias: it has dominant genetic transmission and is caused by mutations in the genes coding for collagen I, the most abundant protein in bone. Using the Brtl mouse, a well-characterized knockin model for moderately severe dominant OI, we demonstrated an impairment in the differentiation of bone marrow progenitor cells toward osteoblasts. In mutant mesenchymal stem cells (MSCs), the expression of early (Runx2 and Sp7) and late (Col1a1 and Ibsp) osteoblastic markers was significantly reduced with respect to wild type (WT). Conversely, mutant MSCs generated more colony-forming unit-adipocytes compared to WT, with more adipocytes per colony, and increased number and size of triglyceride drops per cell. Autophagy upregulation was also demonstrated in mutant adult MSCs differentiating toward osteogenic lineage as consequence of endoplasmic reticulum stress due to mutant collagen retention. Treatment of the Brtl mice with the proteasome inhibitor Bortezomib ameliorated both osteoblast differentiation in vitro and bone properties in vivo as demonstrated by colony-forming unit-osteoblasts assay and peripheral quantitative computed tomography analysis on long bones, respectively. This is the first report of impaired MSC differentiation to osteoblasts in OI, and it identifies a new potential target for the pharmacological treatment of the disorder.

Increased susceptibility to microdamage in Brtl/+ mouse model for osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a genetic disease of collagen or collagen-related proteins that adversely impacts bone mass and fracture resistance. Little is known regarding the role that microdamage plays in OI and whether or not OI bone is more prone to damage accumulation than bone with unaffected collagen. The Brtl/+ mouse is a heterozygous model for OI which contains a Gly349Cys substitution in one COL1A1 allele, and demonstrates a low ductility phenotype. At 8 weeks of age, Brtl/+ demonstrates an increase in osteoclast number, which mimics the upregulated bone turnover often found in OI patients. We hypothesize that upregulated osteoclast activity in Brtl/+ is due, in part, to increased remodeling associated with microdamage repair. In the present study, we used Brtl/+ to investigate the susceptibility of OI bone to microdamage. The mouse ulnar loading model was used to induce microdamage and to test the hypothesis that Brtl/+ is more susceptible to damage accumulation than age-matched wild type (WT) counterparts. Linear elastic fracture mechanics (LEFM) was used to investigate the fracture toughness properties of both Brtl/+ and WT bones to determine if there is any correlation with toughness and the degree of microdamage. Results show that Brtl/+ ulnae subject to normal cage activity demonstrate an inherently larger amount of microdamage than WT controls. Following axial compressive loading, Brtl/+ ulnae are more prone to damage than WT counterparts despite demonstrating a greater resistance to whole-bone deformation. Fracture toughness results demonstrate that Brtl/+ specimens, despite not exhibiting a significant difference, display a trend toward lower fracture toughness values than their WT counterparts. Correlations show that microdamage levels tend to increase as fracture toughness decreases. Together, these findings may have strong clinical implications for explaining increased fragility and remodeling activity in OI patients.

New perspectives on osteogenesis imperfecta

A new paradigm has emerged for osteogenesis imperfecta as a collagen-related disorder. The more prevalent autosomal dominant forms of osteogenesis imperfecta are caused by primary defects in type I collagen, whereas autosomal recessive forms are caused by deficiency of proteins which interact with type I procollagen for post-translational modification and/or folding. Factors that contribute to the mechanism of dominant osteogenesis imperfecta include intracellular stress, disruption of interactions between collagen and noncollagenous proteins, compromised matrix structure, abnormal cell-cell and cell-matrix interactions and tissue mineralization. Recessive osteogenesis imperfecta is caused by deficiency of any of the three components of the collagen prolyl 3-hydroxylation complex. Absence of 3-hydroxylation is associated with increased modification of the collagen helix, consistent with delayed collagen folding. Other causes of recessive osteogenesis imperfecta include deficiency of the collagen chaperones FKBP10 or Serpin H1. Murine models are crucial to uncovering the common pathways in dominant and recessive osteogenesis imperfecta bone dysplasia. Clinical management of osteogenesis imperfecta is multidisciplinary, encompassing substantial progress in physical rehabilitation and surgical procedures, management of hearing, dental and pulmonary abnormalities, as well as drugs, such as bisphosphonates and recombinant human growth hormone. Novel treatments using cell therapy or new drug regimens hold promise for the future.

Chaperoning osteogenesis: new protein-folding disease paradigms

Recent discoveries of severe bone disorders in patients with deficiencies in several endoplasmic reticulum chaperones are reshaping the discussion of type I collagen folding and related diseases. Type I collagen is the most abundant protein in all vertebrates and a crucial structural molecule for bone and other connective tissues. Its misfolding causes bone fragility, skeletal deformity and other tissue failures. Studies of newly discovered bone disorders indicate that collagen folding, chaperones involved in the folding process, cellular responses to misfolding and related bone pathologies might not follow conventional protein folding paradigms. In this review, we examine the features that distinguish collagen folding from that of other proteins and describe the findings that are beginning to reveal how cells manage collagen folding and misfolding. We discuss implications of these studies for general protein folding paradigms, unfolded protein response in cells and protein folding diseases.

Near-infrared fluorescent probe traces bisphosphonate delivery and retention in vivo

Bisphosphonate use has expanded beyond traditional applications to include treatment of a variety of low-bone-mass conditions. Complications associated with long-term bisphosphonate treatment have been noted, generating a critical need for information describing the local bisphosphonate-cell interactions responsible for these observations. This study demonstrates that a fluorescent bisphosphonate analogue, far-red fluorescent pamidronate (FRFP), is an accurate biomarker of bisphosphonate deposition and retention in vivo and can be used to monitor site-specific local drug concentration. In vitro, FRFP is competitively inhibited from the surface of homogenized rat cortical bone by traditional bisphosphonates. In vivo, FRFP delivery to the skeleton is rapid, with fluorescence linearly correlated with bone surface area. Limb fluorescence increases linearly with injected dose of FRFP; injected FRFP does not interfere with binding of standard bisphosphonates at the doses used in this study. Long-term FRFP retention studies demonstrated that FRFP fluorescence decreases in conditions of normal bone turnover, whereas fluorescence was retained in conditions of reduced bone turnover, demonstrating preservation of local FRFP concentration. In the mandible, FRFP localized to the alveolar bone and bone surrounding the periodontal ligament and molar roots, consistent with findings of osteonecrosis of the jaw. These findings support a role for FRFP as an effective in vivo marker for bisphosphonate site-specific deposition, turnover, and long-term retention in the skeleton.

Immature osteoblast lineage cells increase osteoclastogenesis in osteogenesis imperfecta murine

This study addressed the role of impairment of osteoblastic differentiation as a mechanism underlying pathophysiology of the osteogenesis imperfecta (OI). We hypothesized that combination of impaired osteogenic differentiation with increased bone resorption leads to diminished bone mass. By introducing visual markers of distinct stages of osteoblast differentiation, pOBCol3.6GFP (3.6GFP; preosteoblast) and pOBCol2.3GFP (2.3GFP; osteoblast/osteocytes), into the OIM model, we assessed osteoblast maturation and the mechanism of increased osteoclastogenesis. Cultures from oim/oim;2.3GFP mice showed a marked reduction of cells expressing GFP relative to +/+;2.3GFP littermates. No significant difference in expression of 3.6GFP between the +/+ and oim/oim mice was observed. Histological analysis of the oim/oim;3.6GFP mice showed an increased area of GFP-positive cells lining the endocortical surface compared with +/+;3.6GFP mice. In contrast GFP expression was similar between oim/oim;2.3GFP and +/+;2.3GFP mice. These data indicate that the osteoblastic lineage is under continuous stimulation; however, only a proportion of cells attain the mature osteoblast stage. Indeed, immature osteoblasts exhibit a stronger potential to support osteoclast formation and differentiation. We detected a higher Rankl/Opg ratio and higher expression of TNF-alpha in sorted immature osteoblasts. In addition, increased osteoclast formation was observed when osteoclast progenitors were cocultured with oim/oim-derived osteoblasts compared with osteoblasts derived from +/+ mice. Taken together, our data indicate that osteoblast lineage maturation is a critical aspect underlying the pathophysiology of OI.

Variable bone fragility associated with an Amish COL1A2 variant and a knock-in mouse model

Osteogenesis imperfecta (OI) is a heritable form of bone fragility typically associated with a dominant COL1A1 or COL1A2 mutation. Variable phenotype for OI patients with identical collagen mutations is well established, but phenotype variability is described using the qualitative Sillence classification. Patterning a new OI mouse model on a specific collagen mutation therefore has been hindered by the absence of an appropriate kindred with extensive quantitative phenotype data. We benefited from the large sibships of the Old Order Amish (OOA) to define a wide range of OI phenotypes in 64 individuals with the identical COL1A2 mutation. Stratification of carrier spine (L1-4) areal bone mineral density (aBMD) Z-scores demonstrated that 73% had moderate to severe disease (less than -2), 23% had mild disease (-1 to -2), and 4% were in the unaffected range (greater than -1). A line of knock-in mice was patterned on the OOA mutation. Bone phenotype was evaluated in four F(1) lines of knock-in mice that each shared approximately 50% of their genetic background. Consistent with the human pedigree, these mice had reduced body mass, aBMD, and bone strength. Whole-bone fracture susceptibility was influenced by individual genomic factors that were reflected in size, shape, and possibly bone metabolic regulation. The results indicate that the G610C OI (Amish) knock-in mouse is a novel translational model to identify modifying genes that influence phenotype and for testing potential therapies for OI.

RANKL inhibition for the management of patients with benign metabolic bone disorders

The receptor activator of NF-kappaB ligand (RANKL) is a member of the TNF receptor superfamily, essential for osteoclastogenesis. It binds to its receptor activator of NF-kappaB on the surface of osteoclast precursors and enhances their differentiation, survival and fusion, while it activates mature osteoclasts and inhibits their apoptosis. The effects of RANKL are counteracted by osteoprotegerin (OPG), a neutralizing decoy receptor. Derangement of the balance in RANKL/OPG action is implicated in the pathophysiology of metabolic bone diseases, including osteoporosis. Current therapies used to prevent or treat metabolic bone diseases are thought to act, at least in part, through modification of the RANKL/OPG dipole. The idea of using a molecule that could specifically bind and neutralize RANKL to decrease bone resorption and subsequent bone loss is appealing. Recombinant OPG was initially tested. Denosumab, a fully human monoclonal antibody against RANKL, is a promising antiresorptive agent under investigation. It rapidly decreases bone turnover markers resulting in a significant increase in bone mineral density and reduction in fracture risk. However, because receptor activator of NF-kappaB activation by RANKL is also essential for T-cell growth and dendritic-cell function, inhibition of its action could simultaneously affect the immune system, leading to susceptibility in infections or malignancies.

In utero transplantation of adult bone marrow decreases perinatal lethality and rescues the bone phenotype in the knockin murine model for classical, dominant osteogenesis imperfecta

Autosomal dominant osteogenesis imperfecta (OI) caused by glycine substitutions in type I collagen is a paradigmatic disorder for stem cell therapy. Bone marrow transplantation in OI children has produced a low engraftment rate, but surprisingly encouraging symptomatic improvements. In utero transplantation (IUT) may hold even more promise. However, systematic studies of both methods have so far been limited to a recessive mouse model. In this study, we evaluated intrauterine transplantation of adult bone marrow into heterozygous BrtlIV mice. Brtl is a knockin mouse with a classical glycine substitution in type I collagen [alpha1(I)-Gly349Cys], dominant trait transmission, and a phenotype resembling moderately severe and lethal OI. Adult bone marrow donor cells from enhanced green fluorescent protein (eGFP) transgenic mice engrafted in hematopoietic and nonhematopoietic tissues differentiated to trabecular and cortical bone cells and synthesized up to 20% of all type I collagen in the host bone. The transplantation eliminated the perinatal lethality of heterozygous BrtlIV mice. At 2 months of age, femora of treated Brtl mice had significant improvement in geometric parameters (P < .05) versus untreated Brtl mice, and their mechanical properties attained wild-type values. Our results suggest that the engrafted cells form bone with higher efficiency than the endogenous cells, supporting IUT as a promising approach for the treatment of genetic bone diseases.

Alendronate treatment of the brtl osteogenesis imperfecta mouse improves femoral geometry and load response before fracture but decreases predicted material properties and has detrimental effects on osteoblasts and bone formation

Long courses of bisphosphonates are widely administered to children with osteogenesis imperfecta (OI), although bisphosphonates do not block mutant collagen secretion and may affect bone matrix composition or structure. The Brtl mouse has a glycine substitution in col1a1 and is ideal for modeling the effects of bisphosphonate in classical OI. We treated Brtl and wildtype mice with alendronate (Aln; 0.219 mg/kg/wk, SC) for 6 or 12 wk and compared treated and untreated femora of both genotypes. Mutant and wildtype bone had similar responses to Aln treatment. Femoral areal BMD and cortical volumetric BMD increased significantly after 12 wk, but femoral length and growth curves were unaltered. Aln improved Brtl diaphyseal cortical thickness and trabecular number after 6 wk and cross-sectional shape after 12 wk. Mechanically, Aln significantly increased stiffness in wildtype femora and load to fracture in both genotypes after 12 wk. However, predicted material strength and elastic modulus were negatively impacted by 12 wk of Aln in both genotypes, and metaphyseal remnants of mineralized cartilage also increased. Brtl femoral brittleness was unimproved. Brtl osteoclast and osteoblast surface were unchanged by treatment. However, decreased mineral apposition rate and bone formation rate/bone surface and the flattened morphology of Brtl osteoblasts suggested that Aln impaired osteoblast function and matrix synthesis. We conclude that Aln treatment improves Brtl femoral geometry and load to fracture but decreases bone matrix synthesis and predicted material modulus and strength, with striking retention of mineralized cartilage. Beneficial and detrimental changes appear concomitantly. Limiting cumulative bisphosphonate exposure of OI bone will minimize detrimental effects.

Potential implications of cell therapy for osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a brittle-bone disease whose hallmark is bone fragility. Since the disease is genetic, there is currently no available cure. Several pharmacological agents have been tried with not much success, except the recent use of bisphosphonates. Stem cells have been suggested as an alternative OI treatment, but many hurdles remain before this technology can be applied for treating patients with OI. This review summarizes what is known at present regarding the application of stem cells to treat OI using animal models, clinical trials using mesenchymal stem cells to treat patients with OI and the knowledge gained from the clinical trials. Application of gene therapy in combination with stem cells is also discussed. The hurdles to be overcome to bring stem cells close to the clinic and future perspectives are discussed.