Studies on the function of responsible gene, PHEX, for X-linked hypophosphatemic vitamin D resistant rickets - Search for novel phosphate regulating systems

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X-linked hypophosphatemic vitamin D resistant rickets (XLH) is a relatively common vitamin D resistant hereditary rickets caused by remarkable renal phosphate leak. Its responsible gene was identified as PHEX, which has homology to neutral endopeptidase, from linkage analysis, and the mutations causes loss of its function. The pathogenetic mechanism has been hypothesized as follows. Humoral phosphaturic factor(s), which is degraded immediately by PHEX in normal condition, may escape this degradation mechanism by the loss of function of PHEX and may exert its effects on renal phosphate resorption. However, the phosphaturic factor is still hypothetical. Therefore we have been performing experiments to identify the phosphaturic factor by using Hyp mice, which have the same mutation in murine counterpart of PHEX gene.In this study project, we address this issue during two following approaches 1) murine bone marrow transplantation 2) search for PHEX inhibitors.1) Hyp mice received normal bo … More ne marrow showed improvements all disease parameters, such as hypophosphatemia, abnormal vitamin D metabolism and hypomineralization. These effects were evident under the conditions, in which 5% of bone marrow cells were normal donor cells. Moreover, there was positive correlation between serum phosphorus and implanted normal cell mass. From this result, natural substrate of PHEX may be expressed near the cells expressing PHEX or PHEX expressing cells themselves.2) Among NEP inhibitors, thiorphan could cause transient hypophosphatemia in wild type mice. However, FGF23 was identified as the causative factor for oncogenic osteomalacia and autosomal dominant hypophosphatemic rickets. And this information led us to explore its role in the pathogenesis of XLH. To this aid, we developed polyclonal antibody against FGF23, and examined protein expression. Moreover by RT-PCR we examined mRNA expression in Hyp mice. Even by RT-PCR, it was hard to detect FGF23 signal in adult Hyp mice. Combined with our bone marrow transplantation study, we could not exclude the other factor than FGF23 as a phosphaturic factor. For further experiment, we developed PHEX expression system and more sensitive methodology to detect the molecules which interact with recombinant PHEX protein. Less