Comparative effectiveness of therapeutic modalities in X-linked hypophosphatemia

描述（由申请人提供）：X连锁低磷酸盐血症（XLH）的特征是FGF 23水平升高，导致低磷酸盐血症和1，25-二羟维生素D产生减少。目前的治疗方法包括每天补充磷酸盐和1，25-二羟维生素D，这些药物的耐受性不好，也不能均匀地预防佝偻病。我们的目的是比较慢性低剂量或间歇性高剂量的1，25-二羟维生素D或FGF 23阻断抗体使矿物质离子正常化、调节矿物质调节激素和预防hypmice（XLH小鼠模型）佝偻病的能力。我们以前已经证明，低磷酸盐血症损害肥大软骨细胞凋亡，导致佝偻病。与hyp小鼠不同，具有升高的1，25-二羟维生素D水平的低磷酸盐血症Npt 2a缺失小鼠具有正常的生长板。我们的初步结果表明，治疗肥大的软骨细胞与1，25-二羟维生素D诱导基础和磷酸诱导的ERK 1/2磷酸化，这是磷酸介导的肥大软骨细胞凋亡所需的。这些研究表明，1，25-二羟维生素D在预防低磷酸盐血症状态下受损肥大软骨细胞凋亡中的作用。我们推测，治疗与1，25-二羟维生素D将减轻低磷酸盐血症，促进肥大软骨细胞凋亡。施用抗FGF 23抗体将改善1，25-二羟基维生素D的产生并减少尿磷酸盐排泄，从而增加血清磷酸盐并促进生长板成熟。PTHrP缺失小鼠的研究表明，PTH/PTHrP是生长板成熟的抑制剂。此外，hyp小鼠的生长板具有升高的PTHrP表达。我们已经证明，用PTH处理肥大软骨细胞可抑制ERK 1/2磷酸化对磷酸盐的反应。我们提出的所有三种治疗策略都应该通过增加1，25-二羟维生素D来降低PTH/PTHrP水平。特别是，间歇性高剂量的1，25-二羟维生素D已被证明可以有效抑制PTH水平。我们推测PTH/PTHrP的降低可能通过增加p-ERK 1/2和磷酸盐直接或间接地改善肥大软骨细胞的凋亡。在具体目标1中，我们将比较慢性低剂量或间歇性高剂量1，25-二羟基维生素D或抗FGF 23抗体减弱矿物质离子异常和促进hyp-mice生长的能力。将评价骨矿物质密度，以检查这三种治疗对矿化的影响。我们还建议比较三种治疗方法的疗效，并通过组织学、原位杂交和免疫组化分析确定生长板表型改善的分子基础。

DESCRIPTION (provided by applicant): X-linked hypophosphatemia (XLH) is characterized by elevated FGF23 levels, which leads to hypophosphatemia and decreased 1,25-dihydroxyvitamin D production. The current treatment includes daily supplementation with phosphate and 1,25-dihydroxyvitamin D, which are not well tolerated and do not uniformly prevent rickets. We aim to compare the ability of chronic low dose or intermittent high dose 1,25-dihydroxyvitamin D or FGF23 blocking antibodies to normalize mineral ions, modulate mineral regulatory hormones, and prevent rickets in hyp mice (murine model of XLH). We have previously shown that hypophosphatemia impairs hypertrophic chondrocyte apoptosis and leads to rickets. Unlike hyp mice, hypophosphatemic Npt2a null mice with elevated 1,25-dihydroxyvitamin D levels have normal growth plates. Our preliminary results demonstrate that treatment of hypertrophic chondrocytes with 1,25-dihydroxyvitamin D induces basal and phosphate-induced ERK1/2 phosphorylation, which is required for phosphate-mediated hypertrophic chondrocyte apoptosis. These studies suggest a role for 1,25-dihydroxyvitamin D in preventing impaired hypertrophic chondrocyte apoptosis in hypophosphatemic states. We hypothesize that treatment with 1,25-dihydroxyvitamin D will attenuate hypophosphatemia and promote hypertrophic chondrocyte apoptosis. Administration of anti-FGF23 antibodies will improve 1,25- dihydroxyvitamin D production and decrease urinary phosphate excretion, thereby increasing serum phosphate and promoting growth plate maturation. Studies with PTHrP null mice have shown PTH/PTHrP to be an inhibitor of growth plate maturation. Moreover, the growth plates in hyp mice have elevated PTHrP expression. We have demonstrated that treatment of hypertrophic chondrocytes with PTH inhibits ERK1/2 phosphorylation in response to phosphate. All three treatment strategies that we propose should decrease PTH/PTHrP levels by increasing 1,25-dihydroxyvitamin D. In particular, intermittent high dosing of 1,25-dihydroxyvitamin D has been shown to effectively suppress PTH levels. We hypothesize that this decrease in PTH/PTHrP may directly and indirectly improve hypertrophic chondrocyte apoptosis by increasing p-ERK1/2 and phosphate. In specific aim 1, we will compare the ability of chronic low dose or intermittent high dose 1,25-dihydroxyvitamin D or anti-FGF23 antibodies to attenuate mineral ion abnormalities and promote growth in hyp mice. Bone mineral density will be evaluated to examine the effects of these three treatments on mineralization. We also propose to compare the efficacy of the three treatments and identify the molecular basis for the improvement of the growth plate phenotype through histological, in situ hybridization, and immunohistochemical analyses.