In an integrated program of laboratory and clinical investigation, we study the molecular biology of the heritable connective tissue disorder osteogenesis imperfecta (OI). Our objective is to elucidate the mechanisms by which the primary gene defect causes skeletal fragility and other connective tissue symptoms and then apply the knowledge gained from our studies to the treatment of children with these conditions. Our Section has generated a knock-in murine model for OI with a COL1A1 collagen mutation. Using Brtl, we conducted a therapeutic trial of bisphosphonate, which complements our pediatric trial. Detrimental changes were detected in bone material strength and brittleness. Furthermore, retention of mineralized cartilage disrupts matrix continuity and may contribute to crack propagation. Bone cell function declined and osteoblasts were altered to a flattened morphology, similar to lining cells. These studies contribute to the increased cautionary notes in the literature regarding elevated cumulative bisphosphonate dose. Brtl is also being used as the model for testing an anabolic therapy for OI, anti-sclerostin antibody, which works by stimulating bone formation along the canonical wnt pathway. When growing young Brtl mice were treated with Scl-Ab for 5 weeks, Brtl femora increased cortical bone formation, which improved mechanical strength to WT levels, without further exacerbating the underlying brittleness of OI bone material. Trabecular bone formation was also stimulated in Brtl mice by SclAb treatment, although to a lesser extent that in WT mice. In contrast, administration of Scl-Ab to adult Brtl mice resulted in improved trabecular and cortical bone mass and serum osteocalcin, leading to improved bone stiffness and mechanical strength. These data suggest Scl-AB alters the matrix chemistry of newly formed bone while not impairing material properties. They present the encouraging prospect of the potential for new bone formation in adults with OI. Although Scl-AB is a short-acting drug, this collaboration has demonstrated that a single dose of bisphosphonate will preserve the gains to trabecular bone mass following cessation of antibody in a dose-dependent manner. Cortical bone mass was less susceptible to cessation effects and showed no function deficits in the absence of bisphosphonate. We have identified a novel high bone density form of OI caused by mutations in the C-proteinase cleavage site. The Asp-Ala dipeptide between the telopeptide and the C-propeptide of each chain is cleaved by C-proteinase/BMP1 to release mature collagen. Children with substitutions at these residues present with fractures and a high DEXA z-score. Interestingly, despite the high DEXA, radiographs and histomorphometry are similar to type I OI and point to matrix deficiency. Pericellular processing of procollagen C-propeptide is delayed. FTIR and BSEM were used to study the amount and crystallinity of bone samples from our two probands. These data not only reveal a novel form of OI but also provide new fundamental information on roles of procollagen processing and the mechanism of tissue mineralization. We have now generated a mouse model for HBM OI, to investigate the role of type I procollagen C-propeptide cleavage in the mechanism of increased bone mineralization, both at the matrix and intracellular levels. In bench studies aimed at understanding the basis of the phenotypic variability of patients with the identical OI-causing mutation, we collaborated on investigations of cellular cytoskeleton in Brtl lethal and surviving mice. Components of intermediate filaments, microtubules and actin filaments were all shown to be abnormal only in tissues from lethal mice. The aberrant cytoskeleton affects TGF-b and integrin signaling. This data was extended to cells from patients with lethal and non-lethal mutations caused by identical glycine substitutions. They point to the cytoskeleton as a phenotypic modulator and potential novel target for OI treatment. We are also continuing our clinical studies of children with types III and IV OI. The BEMB undertook the first randomized controlled trial of bisphosphonate in children with types III and IV OI.
The aim was to test both the primary skeletal gains and secondary gains (improved functional level and muscle strength and decreased pain) reported in observational trials. The treatment group experienced improvement in vertebral parameters, including BMD z-scores, central vertebral height and vertebral area. However, the increment in vertebral BMD in the treatment group tapered off after one to two years of treatment. There was no significant change in ambulation level, lower-extremity strength or pain in children with OI treated with pamidronate. Hence the changes previously reported appear to have been a placebo effect in uncontrolled trials. We are recommending that treatment of children with types III and IV OI with pamidronate be limited to at most three years, with subsequent follow-up of bone status. Furthermore, we are currently engaged in a dose comparison trial. We are also focusing on the variability of response to treatment in each group. The improvements in vertebral height and area do not correlate with changes in DXA z-score, nor did the improvement in vertebral height and area correlate for individual children. These differences may be related to important individual variation in ability to synthesize new bone or to remodel bone. They also highlight the inadequacy of DXA as a surrogate for bone strength.
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