Translational Nephrology More on the bone-kidney axis—lessons from hypophosphataemia*

Summary There are several hypophosphataemic disorders attrib-uted to the phosphaturic effect of high fibroblast growth factor (FGF23) levels (Table 1). X-linked hypophosphatemia (XLH) is caused by inactivating mutations in a gene encoding a putative endopeptidase (PHEX) which normally cleaves the phosphaturic hormone FGF23 [1]. Both PHEX and FGF23 are abundantly expressed in bone. Inactivation of PHEX leads to excess of FGF23 which in turn causes phosphaturia, hypophosphataemia and inappropri-ately normal levels of 1,25(OH)2 D3 that should be high due to the hypophosphataemic stimulus of renal 1a hydroxylase activity. Autosomal dominant hypophos-phataemic rickets (ADHR) are caused by mutation