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 F1 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. © 2010 American Society for Bone and Mineral Research

Glycine (Gly) substitutions in collagen Gly-X-Y repeats disrupt folding of type I procollagen triple helix and cause severe bone fragility and malformations (osteogenesis imperfecta, aka OI). However, these mutations do not elicit the expected Endoplasmic Reticulum (ER) stress response, in contrast to other protein folding diseases. Thus, it has remained unclear whether cell stress and osteoblast malfunction contribute to the bone pathology caused by Gly substitutions. Here we used a mouse with a Gly610 to cysteine (Cys) substitution in the procollagen α2(I) chain to show that misfolded procollagen accumulation in the ER leads to an unusual form of cell stress, which is neither a conventional unfolded protein response stress nor ER overload. Despite pronounced ER dilation, there is no upregulation of BIP expected in the former and no activation of NFκB signaling expected in the latter. Altered expression of ER chaperones αB crystalline and HSP47, phosphorylation of EIF2α, activation of autophagy, upregulation of general stress response protein CHOP, and osteoblast malfunction reveal some other adaptive response to the ER disruption. We show how this response alters differentiation and function of osteoblasts in culture and in vivo. We demonstrate that bone matrix deposition by cultured osteoblasts is rescued by activation of misfolded procollagen autophagy, suggesting a new therapeutic strategy for OI.

IntroductionBone marrow transplantation (BMT) is an established therapeutic modality for both malignant and nonmalignant disorders of hematopoietic stem cells. After wide recognition that bone marrow also contains progenitors of bone, [1][2][3][4] we postulated that BMT should be applicable to the treatment of osteopoietic as well as hematopoietic disorders. 5 Nilsson et al demonstrated that transplantation of whole bone marrow leads to donor-derived osteopoiesis in mice, 6 while Pereira et al showed that systemically infused murine mesenchymal stromal cells (MSCs), which are plastic adherent in vitro, 7 engrafted in bone. 3 We showed that BMT in children with osteogenesis imperfecta (OI), a genetic disorder of collagen type I, the major structural protein in bone, leads to donor-derived osteopoiesis and consequent improvement in the microscopic structure of bone 5 and in the clinical manifestations of OI. 8 Recently, BMT in a murine model of OI has corroborated our early human studies. 9 Taken together, these data validate the functional competence of donor-derived osteopoietic cells, providing the necessary proof to move forward with the development of marrow cell-based treatments for disorders of bone.Despite this progress, the cellular mechanism(s) by which BMT gives rise to robust osteopoietic activity remains unproven. Pereira et al reported that systemically infused murine MSCs engrafted in the bone of a murine model of OI, and generated a small but statistically significant increase in collagen, 10 supporting the prevailing view that BMT-associated donor-derived osteopoiesis was attributable to the engraftment and differentiation of MSCs. Thus, we reasoned that a decrease in the rate of clinical improvement in our OI patients after BMT 8 might be corrected with a boost of donor-derived MSCs, which in fact led to a second wave of accelerated growth velocity in all 5 evaluable patients. 11 This result suggested that MSCs isolated on the basis of their adherence to plastic may provide adequate therapy for patients with OI or other bone disorders. However, the issue is complicated by work showing that so-called nonadherent bone marrow cells (NABMCs) have measurable osteoprogenitor activity, 12-14 raising questions as to the developmental origin of the transplantable marrow osteoprogenitors that give rise to donor-derived osteopoiesis and hence to the marrow population most likely to yield clinical improvement in patients.Here we show that NABMCs are significantly more robust transplantable osteoprogenitors than MSCs in mice, suggesting NABMC would be effective cell therapy for bone disorders. Translating this laboratory observation to a pilot clinical trial, T cell-depleted marrow mononuclear cells, comprising Ͻ 0.01% MSCs, engraft in bone after intravenous infusion and lead to a remarkable acceleration of growth in some OI patients, suggesting vigorous osteoprogenitor activity in humans as predicted by our animal model. Finally, we demonstrate that NABMCs produce their clinical activity by engrafting in bon...

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