IDENTIFICATION OF NOVEL PHOSPHATE REGULATORS

描述（由申请人提供）：尽管磷酸盐稳态的调节已被证明具有广泛的重要性，但仍不完全清楚。除了编码不同的磷酸钠共转运蛋白的cDNA之外，迄今为止已知的参与调节磷酸盐稳态的所有分子都已通过定义人类和小鼠中以磷酸盐稳态的异常调节为特征的不同遗传性疾病的分子基础来鉴定。这些由不同小组进行的研究已经导致了关键调节因子的鉴定，包括1）PHEX，其在X连锁低磷酸盐血症（XLH）中突变，2）FGF 23，其功能获得性突变在常染色体显性低磷酸盐血症佝偻病（ADHR）中发现，功能丧失性突变在家族性肿瘤性钙质沉着症（FTC-2）中发现，和3）GALNT 3，一种对启动O-连接糖基化很重要的酶，在另一种家族性肿瘤性钙质沉着症（FTC-1）中发现了该酶的功能缺失突变。使用我们实验室开发的测定，显示FGF 23或GALNT 3中的功能缺失突变与C-末端FGF 23的血清浓度的显著增加相关，但正常的完整FGF 23水平。这些发现表明，FGF 23需要翻译后修饰以进行正常的细胞内加工，并可能通过一些已知的FGF受体介导其作用，这些受体可能招募Klotho作为共受体。使用定位克隆策略，我们最近确定了NaPi-llc突变为遗传性低磷酸盐血症佝偻病伴高钙尿症（HHRH）的原因，从而为NaPi-llc在肾磷酸盐处理中具有重要作用的结论提供了第一个证据。然而，尽管在鉴定磷酸盐稳态的关键调节剂方面取得了这些进展，但在很大程度上仍然不确定所鉴定的蛋白质如何以及在多大程度上有助于磷酸盐稳态的调节，并且很可能其他蛋白质参与这些调节机制。为了进一步探索磷酸盐稳态的生物学，我们现在建议通过寻找导致常染色体隐性磷酸盐消耗性疾病的分子缺陷来确定一种新的磷酸盐稳态调节剂，该疾病与骨质疏松症相关。我们将这种疾病定位于染色体4 q21，一旦致病基因突变已知，我们将开始探索导致磷酸盐消耗和骨质疏松的潜在病理生理机制。此外，我们将确定HPOS基因产物是否在健康个体的磷酸盐稳态调节中发挥作用。除了定义一种罕见的遗传性疾病外，这些努力还可能为治疗更常见的低磷血症和高磷血症提供新的治疗方法，这些疾病也会影响骨代谢，如X连锁低磷血症（XLH）和慢性肾病（CDK）。

DESCRIPTION (provided by applicant): Despite its demonstrated broad importance, the regulation of phosphate homeostasis remains incompletely understood. With the exception of cDNAs encoding different sodium-phosphate co-transporters, all molecules thus far known to be involved in regulating phosphate homeostasis have been identified by defining the molecular basis of different hereditary disorders in humans and mice that are characterized by an abnormal regulation of phosphate homeostasis. These studies by different groups have led to the identification of key regulators, including 1) PHEX, which is mutated in X-linked hypophosphatemia (XLH), 2) FGF23, for which gain-of-function mutations were found in autosomal dominant hypophosphatemic rickets (ADHR) and loss-of-function mutations in a form of familial tumoral calcinosis (FTC-2), and 3) GALNT3, an enzyme important for the initiation of O-linked glycosylation, for which loss-of-function mutations were identified in another familial form of tumoral calcinosis (FTC-1). Using the assay that was developed in our laboratory, loss of function mutations in either FGF23 or GALNT3 were shown to be associated with a dramatic increase in the serum concentration of C-terminal FGF23, yet normal intact FGF23 levels. These findings indicated that FGF23 requires post-translational modifications for normal intracellular processing and possibly for mediating its actions through some of the known FGF receptors that may recruit Klotho as a co-receptor. Using a positional cloning strategy, we recently identified NaPi-llc mutations as the cause of hereditary hypophosphatemic rickets with hypercalciuria (HHRH), thereby providing first evidence for the conclusion that NaPi-llc has a major role in renal phosphate handling. However, despite these advances in the identification of key regulators of phosphate homeostasis, it remains largely uncertain how and to what extent the identified proteins contribute to the regulation of phosphate homeostasis, and it is very likely that additional proteins are involved in these regulatory mechanisms. To explore the biology of phosphate homeostasis further, we now propose to identify a novel regulator of phosphate homeostasis by searching for the molecular defect leading to an autosomal recessive phosphate-wasting disorder that is associated with osteosclerosis. We mapped this disorder to chromosome 4q21, and once the disease-causing genetic mutation is known, we will begin exploring the underlying pathophysiological mechanisms leading to phosphate-wasting and osteosclerosis. Furthermore, we will determine whether the HPOS gene product has a role in the regulation of phosphate homeostasis in healthy individuals. In addition to defining a rare genetic disorder, these efforts may provide novel therapeutic approaches for the treatment of more common hypo- and hyperphosphatemic conditions that also affect bone metabolism, such as X-linked hypophosphatemia (XLH) and chronic kidney disease (CDK), respectively.