Hypophosphatemia : cDNA cloning of a Na^+-dependent phosphate co-transporter from human kidney

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Hypophosphatemic vitamin D-resistant rickets is the most common form of hypophosphatemic rickets in humans. It is characterized by low renal tubular reabsorption of phosphate, low plasma phosphate, absence of elevated 1,25-dihydroxy-vitamine D despite the presence of hypophosphatemia and osteomalacic bone disease. In the present study, a cDNA (NPT-1) encoding a protein 69% identical in amino acid sequence to that of the Na/Pi cotransporter NaPi-1 was isolated from a human kidney library. Injected of RNA transcribed from this clone, NPT-1, into Xenopus oocuytes results in expression of a Na/Pi cotransport activity with high affinity for transport (Km=0.29mM). Kinetic characterization ([Pi], [Na]) showed that expressed transport activity has properties similar to that of oocytes injected juman kidney poly (A)^+RNA.Northern blotting showes that NPT-1 mRNA is expressed in kidney cortex, liver and brain but not in other tissues. Hybrid depletion with antisense oligonucleotide of NaPi-3 and NPT-1 completely inhibited poly(A)^+RNA-induced Na^+-dependent phosphate uptake in oocytes. These finding indicate that two high affinity Na/Pi cotransporter (NaPi-3 and NPT-1) are present in human kidney cortex.NPT-1 maps the location to human chromosome 6. NaPi-3 maps the location to human chromosome 5. The role of high affinity Na/Pi cotransporter in Hereditary Hypophosphatemic Rickets with Hypercarciuria (HHRH) which is autosomally inherited disorder and caused by defective reabsorption of Pi in the proximal tubule of the kidney is under study.