Comparative effectiveness of therapeutic modalities in X-linked hypophosphatemia

描述（由申请人提供）：x连锁低磷血症（XLH）的特征是FGF23水平升高，导致低磷血症和1,25-二羟基维生素D生成减少。目前的治疗包括每日补充磷酸盐和1,25-二羟基维生素D，这两种药物耐受性不好，也不能完全预防佝偻病。我们的目的是比较慢性低剂量或间歇性高剂量1,25-二羟基维生素D或FGF23阻断抗体对hyp小鼠（XLH小鼠模型）矿物质离子正常化，矿物质调节激素和预防佝偻病的能力。我们之前已经证明，低磷血症损害肥厚性软骨细胞凋亡并导致佝偻病。与亢进小鼠不同，低磷Npt2a缺失小鼠与1,25-二羟基维生素D水平升高有正常的生长板。我们的初步结果表明，1,25-二羟基维生素D对增生性软骨细胞的处理可诱导基础和磷酸盐诱导的ERK1/2磷酸化，这是磷酸盐介导的增生性软骨细胞凋亡所必需的。这些研究表明1,25-二羟基维生素D在预防低磷状态下受损的肥厚软骨细胞凋亡中的作用。我们假设1,25-二羟基维生素D治疗可以减轻低磷血症并促进肥厚性软骨细胞凋亡。抗fgf23抗体可提高1,25-二羟基维生素D的产生，减少尿磷酸盐排泄，从而增加血清磷酸盐，促进生长板成熟。对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小鼠生长板中PTHrP表达升高。我们已经证明，用PTH治疗肥大软骨细胞可抑制ERK1/2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已被证明能有效抑制PTH水平。我们假设PTH/PTHrP的降低可能通过增加p-ERK1/2和磷酸盐直接或间接地改善肥厚性软骨细胞凋亡。在特定的目的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fgf23抗体对削弱矿物质离子异常和促进hyp小鼠生长的能力。将评估骨矿物质密度，以检查这三种治疗对矿化的影响。我们还建议通过组织学、原位杂交和免疫组织化学分析来比较三种处理的疗效，并确定改善生长板表型的分子基础。

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.