Mutations in Osteoprotegerin Cause Calcium Pyrophosphate Deposition Disease

焦磷酸钙沉积病(CPDD)是一种常见的关节炎类型，由 关节软骨中存在焦磷酸钙(CPP)晶体。它特别是 在美国退伍军人中常见，作为慢性阻塞性肺疾病的危险因素，包括高龄和 先前的关节创伤在美国退伍军人中非常普遍。我们最近发表了一项关于 美国退伍军人使用ICD-9代码识别29,000名患有这种疾病的美国退伍军人 单一的5年期。虽然慢性阻塞性肺疾病以零星的方式影响老年患者，但它也会发生 在家族模式中过早地发生。对这种疾病的家族形式的研究令人兴奋 为这一目前无法治愈的关节炎寻找新的治疗目标的机会。我们 最近证实，TNFRSF11B终止密码子突变导致早发性CPDD。 TNFRSF11B编码护骨素(OPG)。引起慢性阻塞性肺疾病的OPG突变(MtOPG) 结果在靠近肝素的蛋白质的C末端增加了另外19个氨基酸 结合结构域。OPG是核因子-kappaB配体受体激活剂的诱骗受体 (RANKL)。RANKL促进破骨细胞的形成，因为OPG阻断了它的功能， OPG活性降低会导致破骨细胞生成增加。受影响的表型 这种突变的患者包括CPP晶体形成旺盛，轻度骨量减少和 严重的关节退行性变，一种与OPG功能丧失一致的表型。的确， 初步数据表明，重组mtOPG显示出对 膜结合的RANKL在体外导致过度的破骨细胞生成。有趣的是， 破骨细胞条件培养液有效刺激CPP晶体形成的生物标志物 软骨细胞。这项工作的目的是研究概念上的创新 MtOPG通过增加破骨细胞生成产生CPDD的假说 软骨下骨。在特定的目标1中，使用体外和体内模型，我们将证明 与野生型OPG(WtOPG)相比，mtOPG不能有效结合成骨细胞 薄膜。导致过量的非对偶膜结合RANKL和破骨细胞增加 队形。软骨下骨中过多的破骨细胞会造成关节损伤和释放 CPP晶体促进因子进入关节软骨。在第二个目标中，结构研究使用 层析、表面等离子体共振和小角x射线散射将证明 MtOPG与硫酸乙酰肝素结合的亲和力降低，二聚反应减少。科学的 这项工作的前提是仔细观察OPG患者的表型。 突变，并得到mtOPG的初步体外功能研究的支持。这些研究是 首次发现慢性阻塞性肺疾病中OPG/RANKL/RANK通路并与软骨下骨有关 作为这种疾病的靶组织。CCAL1患者与非CCAL1患者的共同临床特征 对于零星的CPDD以及针对这一途径的可用药物的存在 这项工作特别令人振奋。

Calcium pyrophosphate deposition disease (CPDD) is a common type of arthritis defined by the presence of calcium pyrophosphate (CPP) crystals in articular cartilage. It is particularly common in the US Veteran population as risk factors for CPDD including advanced age and prior joint trauma are highly prevalent in American Veterans. We recently published a study of US veterans using ICD-9 codes to identify 29,000 American Veterans with the disease in a single 5-year period. While CPDD affects elderly patients in a sporadic fashion, it also occurs prematurely in familial patterns. Studies of familial forms of this disease present exciting opportunities to identify novel therapeutic targets for this currently untreatable arthritis. We recently confirmed that a mutation in the stop codon of TNFRSF11B causes early onset CPDD. TNFRSF11B codes for osteoprotegerin (OPG). The OPG mutation causing CPDD (mtOPG) results in an additional 19 amino acids added to the protein's C terminus near the heparin binding domain. OPG is decoy receptor for receptor activator of nuclear factor kappa B ligand (RANKL). RANKL promotes osteoclast formation, and because OPG blocks its function, reduction of OPG activity results in increased osteoclastogenesis. The phenotype of affected patients with this mutation includes exuberant CPP crystal formation, mild osteopenia and severe joint degeneration, a phenotype consistent with a loss of function of OPG. Indeed, preliminary data demonstrate that recombinant mtOPG displays inefficient inhibition of membrane-bound RANKL resulting in excess osteoclastogenesis in vitro. Interestingly, osteoclast conditioned media potently stimulates biomarkers of CPP crystal formation in chondrocytes. The purpose of this work is to investigate the conceptually innovative hypothesis that mtOPG produces CPDD by increasing osteoclastogenesis in subchondral bone. In specific aim 1, using in vitro and in vivo models, we will show that compared to wild type OPG (wtOPG), mtOPG ineffectively binds to the osteoblast cell membrane. resulting in excess un-opposed membrane-bound RANKL and increased osteoclast formation. Overabundant osteoclasts in subchondral bone produce joint damage and release CPP crystal-promoting factors into articular cartilage. In the second aim, structural studies using chromatography, surface plasmon resonance and small angle x-ray scattering, will demonstrate reduced affinity of mtOPG to bind heparan sulfate and decreased dimerization. The scientific premise of this work stems from careful observations of the phenotype of patients with the OPG mutation and is supported by preliminary in vitro functional studies of mtOPG. These studies are the first to identify the OPG/RANKL/RANK pathway in CPDD and to implicate subchondral bone as a target tissue in this disease. The shared clinical features of patients with CCAL1 and those with sporadic CPDD as well as the existence of available drugs which target this pathway make this work particularly exciting.