[Establishment of assay system for fibroblast growth factor (FGF)-23 and pathophysiological roles of FGF-23 in the development of hypophosphatemic diseases].

Rickets/osteomalacia is characterized by impaired mineralization of bone matrix. X-linked hypophosphatemic rickets/osteomalacia(XLH), autosomal dominant hypophosphatemic rickets/osteomalacia(ADHR) and tumor-induced rickets/osteomalacia(TIO) share common clinical features including impaired proximal tubular phosphate reabsorption. Fibroblast growth factor(FGF)-23 was positionally cloned as a responsible gene for ADHR. We have also cloned FGF-23 as a causative factor for TIO. FGF-23 is a polypeptide with 251 amino acids. FGF-23 is proteolytically cleaved between Arg179 and Ser180 and only full-length FGF-23 showed biological activity to induce hypophosphatemia. Using two monoclonal antibodies that recognize N-terminal and C-terminal portion of the processing site of FGF-23, sandwich enzyme-linked immunosorbent assay for FGF-23 was established. This assay detects only full-length FGF-23. Circulatory level of FGF-23 in healthy controls distributed between 10 to 50 pg/ml suggesting that FGF-23 is a physiological humoral factor. FGF-23 levels in patients with TIO were elevated and rapidly decreased after removal of responsible tumors for TIO. These results confirm that FGF-23 is a causative factor of TIO and indicate that measurement of FGF-23 is useful for diagnosis and management of TIO. Moreover, FGF-23 levels were elevated in most patients with XLH suggesting FGF-23 is also involved in the pathogenesis of XLH. Further analysis is required to clarify more detailed actions of FGF-23 and its roles in the development of other diseases with disordered phosphate metabolism.