ABSTRACT:Introduction: Sclerosteosis is a rare high bone mass genetic disorder in humans caused by inactivating mutations in SOST, the gene encoding sclerostin. Based on these data, sclerostin has emerged as a key negative regulator of bone mass. We generated SOST knockout (KO) mice to gain a more detailed understanding of the effects of sclerostin deficiency on bone. Materials and Methods: Gene targeting was used to inactivate SOST and generate a line of SOST KO mice. Radiography, densitometry, CT, histomorphometry, and mechanical testing were used to characterize the impact of sclerostin deficiency on bone in male and female mice. Comparisons were made between same sex KO and wildtype (WT) mice. Results:The results for male and female SOST KO mice were similar, with differences only in the magnitude of some effects. SOST KO mice had increased radiodensity throughout the skeleton, with general skeletal morphology being normal in appearance. DXA analysis of lumbar vertebrae and whole leg showed that there was a significant increase in BMD (>50%) at both sites. CT analysis of femur showed that bone volume was significantly increased in both the trabecular and cortical compartments. Histomorphometry of trabecular bone revealed a significant increase in osteoblast surface and no significant change in osteoclast surface in SOST KO mice. The bone formation rate in SOST KO mice was significantly increased for trabecular bone (>9-fold) at the distal femur, as well as for the endocortical and periosteal surfaces of the femur midshaft. Mechanical testing of lumbar vertebrae and femur showed that bone strength was significantly increased at both sites in SOST KO mice. Conclusions: SOST KO mice have a high bone mass phenotype characterized by marked increases in BMD, bone volume, bone formation, and bone strength. These results show that sclerostin is a key negative regulator of a powerful, evolutionarily conserved bone formation pathway that acts on both trabecular and cortical bone.

The development of bone-rebuilding anabolic agents for potential use in the treatment of bone loss conditions, such as osteoporosis, has been a long-standing goal. Genetic studies in humans and mice have shown that the secreted protein sclerostin is a key negative regulator of bone formation, although the magnitude and extent of sclerostin's role in the control of bone formation in the aging skeleton is still unclear. To study this unexplored area of sclerostin biology and to assess the pharmacologic effects of sclerostin inhibition, we used a cell culture model of bone formation to identify a sclerostin neutralizing monoclonal antibody (Scl-AbII) for testing in an aged ovariectomized rat model of postmenopausal osteoporosis. Sixmonth-old female rats were ovariectomized and left untreated for 1 yr to allow for significant estrogen deficiency-induced bone loss, at which point Scl-AbII was administered for 5 wk. Scl-AbII treatment in these animals had robust anabolic effects, with marked increases in bone formation on trabecular, periosteal, endocortical, and intracortical surfaces. This not only resulted in complete reversal, at several skeletal sites, of the 1 yr of estrogen deficiency-induced bone loss, but also further increased bone mass and bone strength to levels greater than those found in non-ovariectomized control rats. Taken together, these preclinical results establish sclerostin's role as a pivotal negative regulator of bone formation in the aging skeleton and, furthermore, suggest that antibody-mediated inhibition of sclerostin represents a promising new therapeutic approach for the anabolic treatment of bone-related disorders, such as postmenopausal osteoporosis.

To create mice expressing exclusively human sickle hemoglobin (HbS), transgenic mice expressing human alpha-, gamma-, and betaS-globin were generated and bred with knockout mice that had deletions of the murine alpha- and beta-globin genes. These sickle cell mice have the major features (irreversibly sickled red cells, anemia, multiorgan pathology) found in humans with sickle cell disease and, as such, represent a useful in vivo system to accelerate the development of improved therapies for this common genetic disease.

The development of bone-rebuilding anabolic agents for treating bone-related conditions has been a long-standing goal. Genetic studies in humans and mice have shown that the secreted protein sclerostin is a key negative regulator of bone formation. More recently, administration of sclerostin-neutralizing monoclonal antibodies in rodent studies has shown that pharmacologic inhibition of sclerostin results in increased bone formation, bone mass, and bone strength. To explore the effects of sclerostin inhibition in primates, we administered a humanized sclerostin-neutralizing monoclonal antibody (Scl-AbIV) to gonad-intact female cynomolgus monkeys. Two once-monthly subcutaneous injections of Scl-AbIV were administered at three dose levels (3, 10, and 30 mg/kg), with study termination at 2 months. Scl-AbIV treatment had clear anabolic effects, with marked dose-dependent increases in bone formation on trabecular, periosteal, endocortical, and intracortical surfaces. Bone densitometry showed that the increases in bone formation with Scl-AbIV treatment resulted in significant increases in bone mineral content (BMC) and/or bone mineral density (BMD) at several skeletal sites (ie, femoral neck, radial metaphysis, and tibial metaphysis). These increases, expressed as percent changes from baseline were 11 to 29 percentage points higher than those found in the vehicle-treated group. Additionally, significant increases in trabecular thickness and bone strength were found at the lumbar vertebrae in the highest-dose group. Taken together, the marked bone-building effects achieved in this short-term monkey study suggest that sclerostin inhibition represents a promising new therapeutic approach for medical conditions where increases in bone formation might be desirable, such as in fracture healing and osteoporosis. ß

Therapeutic enhancement of fracture healing would help to prevent the occurrence of orthopedic complications such as nonunion and revision surgery. Sclerostin is a negative regulator of bone formation, and treatment with a sclerostin monoclonal antibody (Scl-Ab) results in increased bone formation and bone mass in animal models. Our objective was to investigate the effects of systemic administration of Scl-Ab in two models of fracture healing. In both a closed femoral fracture model in rats and a fibular osteotomy model in cynomolgus monkeys, Scl-Ab significantly increased bone mass and bone strength at the site of fracture. After 10 weeks of healing in nonhuman primates, the fractures in the Scl-Ab group had less callus cartilage and smaller fracture gaps containing more bone and less fibrovascular tissue. These improvements at the fracture site corresponded with improvements in bone formation, bone mass, and bone strength at nonfractured cortical and trabecular sites in both studies. Thus the potent anabolic activity of Scl-Ab throughout the skeleton also was associated with an anabolic effect at the site of fracture. These results support the potential for systemic Scl-Ab administration to enhance fracture healing in patients. ß

The purpose of this study was to evaluate the effects of sclerostin inhibition by treatment with a sclerostin antibody (Scl-AbII) on bone formation, bone mass, and bone strength in an aged, gonad-intact male rat model. Sixteen-month-old male Sprague-Dawley rats were injected subcutaneously with vehicle or Scl-AbII at 5 or 25 mg/kg twice per week for 5 weeks (9-10/group). In vivo dual-energy X-ray absorptiometry (DXA) analysis showed that there was a marked increase in areal bone mineral density of the lumbar vertebrae (L 1 to L 5 ) and long bones (femur and tibia) in both the 5 and 25 mg/kg Scl-AbII-treated groups compared with baseline or vehicle controls at 3 and 5 weeks after treatment. Ex vivo micro-computed tomographic (mCT) analysis demonstrated improved trabecular and cortical architecture at the fifth lumbar vertebral body (L 5 ), femoral diaphysis (FD), and femoral neck (FN) in both Scl-AbII dose groups compared with vehicle controls. The increased cortical and trabecular bone mass was associated with a significantly higher maximal load of L 5 , FD, and FN in the high-dose group. Bone-formation parameters (ie, mineralizing surface, mineral apposition rate, and boneformation rate) at the proximal tibial metaphysis and tibial shaft were markedly greater on trabecular, periosteal, and endocortical surfaces in both Scl-AbII dose groups compared with controls. These results indicate that sclerostin inhibition by treatment with a sclerostin antibody increased bone formation, bone mass, and bone strength in aged male rats and, furthermore, suggest that pharmacologic inhibition of sclerostin may represent a promising anabolic therapy for low bone mass in aged men. ß

Apolipoprotein E (apo E)-deficient mice are severely hypercholesterolemic and develop advanced atheromas independent of diet. The C57BL/6 strain differs from most inbred strains by having lower HDL concentrations and a high risk of developing early atherosclerotic lesions when fed an atherogenic diet. The relative HDL deficiency and atherosclerosis susceptibility of the C57BL/6 strain are corrected with the expression of a human apolipoprotein Al (apo Al) transgene in this genetic background. To examine if increases in apo Al and HDL are also effective in minimizing apo E deficiency-induced atherosclerosis, we introduced the human apo AI transgene into the hypercholesterolemic apo E knockout background. Similar elevations of total plasma cholesterol occurred in both the apo E knockout and apo E knockout mice also expressing the human apo Al transgene. The latter animals, however, also showed a two-to threefold increase in HDL and a sixfold decrease in susceptibility to atherosclerosis. This study demonstrates that elevating the concentration of apo Al reduces atherosclerosis in apo E deficient-mice and suggests that elevation of apo Al and HDL may prove to be a useful approach for treating unrelated causes of heightened atherosclerosis susceptibility. (J. Clin. Invest. 1994. 94:899-903.)

No abstract