In an integrated program of laboratory and clinical investigation, we study the molecular biology of the heritable connective tissue disorders osteogenesis imperfecta (OI) and Ehlers-Danlos syndrome (EDS). 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 Branch has generated a knock-in murine model for OI with a classical collagen mutation. A fundamental insight into the mechanism of OI derived from Brtl has involved the role of ER stress. Combined microarray and proteomic investigations of Brtl showed a two fold increase in Gadd153/CHOP expression and protein in lethal pups shortly before death. The increase in Gadd 153 expression was bone specific. Gadd153 is a member of the C/EBP family activated by cell stress, in particular, by ER retention of misfolded protein. These studies suggest that relief of ER stress with chemical chaperones, such as SPB, may have a beneficial effect on the OI skeleton. Using Brtl, we completed a major theraeputic trial of the effect of bisphosphonate, which complements our pediatric trial. We treated Brtl and wild-type littermates with alendonate and compared treated and untreated femora of each genotype. Alendronate treatment increased femoral DXA and cortical volumetric BMD, but did not improve Brtl weight curves or femoral length. Brtl trabecular number and diaphyseal cortical thickness were improved, as was femoral stiffness and load to fracture. However, detrimental changes were also detected in material and cellular parameters of bone. Predicted material strength and elastic modulus of both Brtl and wild-type bone were deceased;brittleness of Brtl femora were unchanged, while that of wild-type was increased. Furthermore, dramatic retention of mineralized cartilage disrupts matrix continuity and may contribute to the weakening of bone material. In addition, the function osteoblasts was impaired, with severe reductions in mineral apposition rate and bone formation rate. Osteoblast morphology was altered, making Brtl Treated osteoblasts flattened, similar to lining cells. These studies contribute to the increased cautionary notes in the literature concerning avoidance of an elevated cummulative bisphosphonate dose. We have collaborated in a study demonstrating that a fluoresecent bisphosphonate analog is an accurate biomarker of deposition and retention in vivo is currently being used for trails of two non-traditional therapies. In a collabaorative study. Brtl was sued for an utero cell tansplantation trailof GFP expression stem cells. Despite low levels of engraftment, the perinatal lethality and femoral geometry and biomechanics of the of the engrafted Brtl mice were improved. The result are encouraging for translational trails. Second, we are modelling a lesson from type I OI to suppress mutantcollagen expression. Specific suprression of transcripts of the mutant collagen allele can biochemically transform individual with severe OI into mild type I OI. We have introduced a R7 target site into the BRTL mutant allele: we have henerated transgenic mice expressing ribozymes tragated to the Brtl mutation. Preliminary dataon Brtl/RZ mice is encouraging for improvement of Brtl biomechanical properties in female mice.
We have identified a novel """"""""high bone density"""""""" form of OI caused by mutations in the C-proteinase cleavage site of type I procollagen. The Asp-Ala dipeptide between the telopeptide and the C-propeptide of each chain is cleaved by C-proteinase/BMP1 to release mature collagen. We have identified children with substitutions at two of these 4 peptides. They 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, and in vitro cleavage by purified BMP1 is impaired. FTIR imaging of corticicol and trabecular bone confirms elevated mineral/matrix ratios in affected children, compared to normal controls and classical OI samples, as well significantly inreased collagen maturity in trabecular bone BBD (Bone mineralization density distribution) reveals a marked shift toward increased mineralizaton compared to controls but difeerent patterns for each patient, with heterogenity of minerlization density (area of both higher and lower mineralization than normal or classical OI bone)for the COL1A1 mutations and uniformaly high mineralization exceeding even classical OI for the COL1A2 bone. The data not only reveal a novel form of OI but also provide new fundamental on roles of procollagen processing and the mechanism of tissue mineralizations. We are currently generating a murine model for high bone density OI, in order to study the molecular and biochemical mechanism of the mineralization, and its developmental progression. To better understand the relationship of genotype and phenotype in human OI, the BEMB led and international constortium of connective tissue laboratories to assemble an BSEM are being 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. To better understand the relationship of genotype and phenotype in human OI, the BEMB led international consortium of connective tissue laboratories to assemble and analyze a mutation database. The initial database published in 2007 containing over 830 mutations;currently the database under analysis contains over 1300 mutations. Genotype-phenotype modeling revealed different functional relationships for each chain of type I collagen. Lethal mutations in alpha 1 (I) coincide with the Major Ligand Binding Regions. Lethal regions in alpha 2(I) continue to support the Regional Model first proposed by the BEMB, with lethal mutations in regularly-spaced clusters along the chain that coincide with proteoglycan binding regions. This model correctly predicts clinical outcome in 86% of alpha 2(I) mutations. .
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.
|Blouin, Stéphane; Fratzl-Zelman, Nadja; Glorieux, Francis H et al. (2017) Hypermineralization and High Osteocyte Lacunar Density in Osteogenesis Imperfecta Type V Bone Indicate Exuberant Primary Bone Formation. J Bone Miner Res 32:1884-1892|
|Perosky, Joseph E; Khoury, Basma M; Jenks, Terese N et al. (2016) Single dose of bisphosphonate preserves gains in bone mass following cessation of sclerostin antibody in Brtl/+ osteogenesis imperfecta model. Bone 93:79-85|
|Masci, Marco; Wang, Min; Imbert, Laurianne et al. (2016) Bone mineral properties in growing Col1a2(+/G610C) mice, an animal model of osteogenesis imperfecta. Bone 87:120-9|
|Marini, Joan C; Reich, Adi; Smith, Simone M (2014) Osteogenesis imperfecta due to mutations in non-collagenous genes: lessons in the biology of bone formation. Curr Opin Pediatr 26:500-7|
|Marini, Joan C; Blissett, Angela R (2013) New Genes in Bone Development: What's New in Osteogenesis Imperfecta. J Clin Endocrinol Metab 98:3095-103|
|Sinder, Benjamin P; Eddy, Mary M; Ominsky, Michael S et al. (2013) Sclerostin antibody improves skeletal parameters in a Brtl/+ mouse model of osteogenesis imperfecta. J Bone Miner Res 28:73-80|
|Gioia, Roberta; Panaroni, Cristina; Besio, Roberta et al. (2012) Impaired osteoblastogenesis in a murine model of dominant osteogenesis imperfecta: a new target for osteogenesis imperfecta pharmacological therapy. Stem Cells 30:1465-76|
|Bianchi, Laura; Gagliardi, Assunta; Gioia, Roberta et al. (2012) Differential response to intracellular stress in the skin from osteogenesis imperfecta Brtl mice with lethal and non lethal phenotype: a proteomic approach. J Proteomics 75:4717-33|
|Davis, Mathieu S; Kovacic, Bethany L; Marini, Joan C et al. (2012) Increased susceptibility to microdamage in Brtl/+ mouse model for osteogenesis imperfecta. Bone 50:784-91|
|Thiele, Frank; Cohrs, Christian M; Flor, Armando et al. (2012) Cardiopulmonary dysfunction in the Osteogenesis imperfecta mouse model Aga2 and human patients are caused by bone-independent mechanisms. Hum Mol Genet 21:3535-45|
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