Conditional Isolation of Fgf23 Activity

描述（由申请方提供）：需要适当控制血清磷酸盐浓度以保持骨骼完整性。我们以前确定成纤维细胞生长因子-23（FGF 23）的错义突变是常染色体显性低磷血症性佝偻病（ADHR）的原因，其特征是继发于孤立性肾性磷酸盐消耗和代谢性骨病的低磷血症。我们随后证明，失活FGF 23突变导致ADHR的镜像障碍，家族性肿瘤性钙质沉着症（TC），表现为血清磷酸盐显著升高，通常伴有严重的异位和血管钙化。此外，FGF 23在X连锁低磷酸盐血症佝偻病（XLH）中升高，并且循环FGF 23升高也与慢性肾病（CKD）患者的死亡风险高5-6倍相关。对于上述综合征，目前没有治愈方法，只有维持治疗。虽然在了解磷酸盐代谢的基础和临床方面已经取得了很大进展，但在正常条件下和疾病中在骨骼水平调节Fgf 23的基本机制尚不清楚。FGF 23在成骨细胞和骨细胞中表达，然而，“感知”血清磷酸盐变化以控制这种激素的产生和分泌的细胞类型也是未知的。重要的是，我们最近确定成骨细胞/骨细胞的原代培养物增加Fgf 23对PTH和磷酸盐的反应，这代表了当前解释Fgf 23调节的模型的创新转变。与新提出的体内系统组合，培养物提供了用于测试Fgf 23的细胞特异性控制的新方法。 可用的Fgf 23敲除和转基因小鼠模型已经提供了关于FGF 23生物活性的基本体内数据，包括与其共受体α-Klotho（KL）的相互作用，然而存在若干限制，包括Fgf 23整体敲除在断奶时严重受损并在8-10周死亡，并且转基因小鼠不受控制地过表达FGF 23。因此，Fgf 23条件无效模型是探索Fgf 23在非致死状态下的病理生理学作用和分离其在人类疾病中的作用的必要工具。这些提出的研究将通过以下具体目标来解决中心假设：FGF 23以细胞特异性方式调节，其在疾病中改变：1）开发将携带在Cre重组酶存在下可以特异性中断的Fgf 23等位基因的小鼠;和2）利用条件性无效Fgf 23 flox/flox小鼠测试Fgf 23的细胞特异性调节。Fgf 23条件无效动物的成功开发将为我们和其他研究人员提供独特而重要的资源。在正常情况下和矿物质代谢紊乱中，大多数指导Fgf 23产生和调节的分子机制是未知的，因此该模型将允许测试否则不可能的假设。 公共卫生相关性：血清磷酸盐浓度的调节对正常骨骼形成和细胞功能至关重要。涉及成纤维细胞生长因子-23（FGF 23）的磷酸盐稳态的病原性紊乱，例如孟德尔疾病常染色体显性低磷酸盐血症性佝偻病（ADHR）、X连锁低磷酸盐血症性佝偻病（XLH）、常染色体隐性低磷酸盐血症性佝偻病（ARHR）和家族性肿瘤性钙质沉着症（TC）或常见疾病如肾衰竭中的那些，导致严重的内分泌和骨骼疾病。这些疾病目前没有足够的治疗。我们期望开发Fgf 23条件性缺失动物将揭示磷酸盐稳态的新机制，并将提供新的治疗靶点。

DESCRIPTION (provided by applicant): Proper control of serum phosphate concentrations is required to maintain skeletal integrity. We previously identified missense mutations in Fibroblast growth factor-23 (FGF23) as the cause of autosomal dominant hypophosphatemic rickets (ADHR), characterized by hypophosphatemia secondary to isolated renal phosphate wasting and metabolic bone disease. We subsequently demonstrated that inactivating FGF23 mutations result in the mirror-image disorder to ADHR, familial tumoral calcinosis (TC), which is manifested by markedly elevated serum phosphate and often severe ectopic and vascular calcifications. Further, FGF23 is elevated in X-linked hypophosphatemic rickets (XLH) and increased circulating FGF23 is also associated with a 5-6 fold higher mortality risk in patients with chronic kidney disease (CKD). There are currently no cures, only maintenance treatments, for the aforementioned syndromes. Although much progress has been made towards understanding both basic and clinical aspects of phosphate metabolism, the fundamental mechanisms regulating Fgf23 at the level of the skeleton under normal conditions and in disease are unknown. FGF23 is expressed in osteoblasts and osteocytes, however the cell type(s) that 'sense' changes in serum phosphate to control production and secretion of this hormone are also unknown. Importantly, we recently determined that primary cultures of osteoblasts/osteocytes increase Fgf23 in response to PTH and to phosphate, which represents an innovative shift in the current models explaining Fgf23 regulation. In combination with the newly- proposed in vivo systems, the cultures provide a novel approach for testing cell-specific control of Fgf23. The available Fgf23 knock-out and transgenic mouse models have provided essential in vivo data regarding FGF23 bioactivity, including interactions with its co-receptor a-Klotho (KL), however there are several limitations, including that the Fgf23 global knock-out is severely compromised at weaning and dies at 8-10 weeks, and the transgenic mice uncontrollably over-express FGF23. Therefore, an Fgf23 conditional-null model is a necessary tool to explore the pathophysiologic effects of Fgf23 in a non-lethal state and for isolating its role in human disorders. These proposed studies will address the central hypothesis that: FGF23 is regulated in a cell-specific manner, which is altered in disease, through undertaking the following specific aims: 1) To develop a mouse that will carry an Fgf23 allele that can be specifically interrupted in the presence of Cre recombinase; and 2) To test for cell-specific regulation of Fgf23 utilizing the conditional-null Fgf23flox/flox mice. Successful development of an Fgf23 conditional-null animal will provide a unique and important resource for us and for other investigators. Most of the molecular mechanisms guiding Fgf23 production and regulation under normal circumstances and in disorders of mineral metabolism are unknown, therefore this model will permit testing of hypotheses that are otherwise not possible. PUBLIC HEALTH RELEVANCE: The regulation of serum phosphate concentrations is critical for normal skeletal formation and cellular function. Pathogenic disturbances in phosphate homeostasis involving Fibroblast growth factor-23 (FGF23), such as those in the Mendelian disorders autosomal dominant hypophosphatemic rickets (ADHR), X-linked hypophosphatemic rickets (XLH), autosomal recessive hypophosphatemic rickets (ARHR), and familial tumoral calcinosis (TC), or common disorders such as renal failure, lead to severe endocrine and skeletal disease. These disorders currently have inadequate treatments. We expect that development of an Fgf23 conditional-null animal will reveal new mechanisms involved in phosphate homeostasis, and will provide novel therapeutic targets.