The Cellular and Molecular Basis of FOP Lesions

描述（由申请人提供）：进行性骨化纤维发育不良（FOP）是一种严重致残的人类疾病，发作性和广泛性异位（骨骼外）骨形成。这种异位软骨内成骨形成定性正常骨；这种畸变存在于调节骨形成途径的细胞信号中。我们最近发现，在所有典型临床表现为FOP的患者中，FOP是由激活素a受体(I型/激活素样激酶2 （ACVR1/ALK2)，一种I型骨形态发生蛋白（BMP）受体）的反复激活突变引起的。我们的数据支持这种突变（ACVR1 c.617G>A; R206H）是一种激活突变，部分通过bmp独立机制发出信号。虽然这种基因突变的发现迅速导致对FOP的遗传和分子原因的理解，但对FOP病变的细胞起源或支持偶发性病变形成的组织微环境知之甚少。我们对FOP患者的临床观察和FOP动物模型的初步数据表明，软组织损伤引起的炎症微环境调动血管来源的Tie2+结缔组织祖细胞的疾病模型。此外，我们的初步体外研究和蛋白同源性模型预测，突变ACVR1/ALK2受体通过突变受体中缺氧调节的ph敏感开关上调BMP信号。我们的中心假设是，激活的免疫系统与软结缔组织微环境和居住的软骨/骨祖细胞相互作用，与高度特异性的FOP ACVR1基因突变相关，诱导FOP异位骨化。我们建议通过三个特定的目的来研究诱导FOP病变形成的细胞和微环境条件。目的1：确定在BMP信号增强的背景下激活异位骨化的炎症细胞（及相关因子）。目的2：确定在异位骨化过程中，表达Tie2和成熟内皮标志物的细胞是否分化为软骨和骨。目的3：确定a)缺氧是否会增加FOP细胞中突变ACVR1/ALK2受体的BMP信号，b)这种影响是否部分归因于激活突变ACVR1/ALK2受体的酸性细胞内微环境。这些研究将为了解异位骨形成的过程提供关键信息，异位骨形成是一种严重的临床并发症，不仅与FOP有关，而且与头部损伤、机动车事故、髋关节置换术、多重创伤和战争创伤后形成的更常见的异位骨化形式的患者有关。这些知识将有助于我们开发更有效的治疗FOP和其他异位骨化疾病的长期目标。

DESCRIPTION (provided by applicant): Fibrodysplasia ossificans progressiva (FOP) is a severely disabling human disorder of episodic and extensive heterotopic (extraskeletal) bone formation. This ectopic endochondral ossification forms qualitatively normal bone; the aberration resides within the cellular signals that regulate commitment to the bone formation pathway. We recently discovered that FOP is caused by a recurrent activating mutation in Activin A receptor, type I/Activin-like kinase2 (ACVR1/ALK2), a type I bone morphogenetic protein (BMP) receptor, in all patients with a classic clinical presentation of FOP. Our data support that this mutation (ACVR1 c.617G>A; R206H) is an activating mutation that signals in part through a BMP-independent mechanism. While the discovery of this genetic mutation is rapidly leading to an understanding of the genetic and molecular cause of FOP, little is known about the cellular origins of FOP lesions or the tissue microenvironment that supports episodic lesion formation. Clinical observations in our FOP patients and preliminary data in our FOP animal models suggest a disease model in which an inflammatory microenvironment caused by soft tissue injury mobilizes resident Tie2+ connective tissue progenitor cells of vascular origin. Further, our preliminary in vitro studies and protein homology modeling predict that the mutant ACVR1/ALK2 receptor up-regulates BMP signaling through a hypoxia-regulated pH-sensitive switch in the mutant receptor. Our central hypothesis is that an activated immune system interacts with the soft connective tissue microenvironment and resident chondro/osseous progenitor cells, in association with the highly specific FOP ACVR1 gene mutation, to induce heterotopic ossification in FOP. We propose to investigate the cellular and microenvironmental conditions that induce the formation of FOP lesions through three specific aims. Aim 1: Identify the inflammatory cells (and associated factors) that activate heterotopic ossification in a background of enhanced BMP signaling. Aim 2: Determine if cells expressing both Tie2 and mature endothelial markers differentiate to cartilage and bone during heterotopic ossification. Aim 3: Determine whether a) hypoxia increases BMP signaling by the mutant ACVR1/ALK2 receptor in FOP cells, and b) whether this effect is due, in part, to an acidic intracellular microenvironment that activates the mutant ACVR1/ALK2 receptor. These investigations will provide critical information for understanding the process of heterotopic bone formation, a serious clinical complication that is relevant not only to FOP but also to patients with more common forms of heterotopic ossification that form after head injuries, motor vehicle accidents, hip replacements, multiple trauma, and war wounds. This knowledge will contribute to our long-term goal of developing more effective treatments for FOP and other disorders of heterotopic ossification.