Npt2 encodes a renal-specific, brush-border membrane Na ؉ -phosphate (P i ) cotransporter that is expressed in the proximal tubule where the bulk of filtered P i is reabsorbed. Mice deficient in the Npt2 gene were generated by targeted mutagenesis to define the role of Npt2 in the overall maintenance of P i homeostasis, determine its impact on skeletal development, and clarify its relationship to autosomal disorders of renal P i reabsorption in humans. Homozygous mutants (Npt2 ؊/؊ ) exhibit increased urinary P i excretion, hypophosphatemia, an appropriate elevation in the serum concentration of 1,25-dihydroxyvitamin D with attendant hypercalcemia, hypercalciuria and decreased serum parathyroid hormone levels, and increased serum alkaline phosphatase activity. These biochemical features are typical of patients with hereditary hypophosphatemic rickets with hypercalciuria (HHRH), a Mendelian disorder of renal P i reabsorption. However, unlike HHRH patients, Npt2؊/؊ mice do not have rickets or osteomalacia. At weaning, Npt2 ؊/؊ mice have poorly developed trabecular bone and retarded secondary ossification, but, with increasing age, there is a dramatic reversal and eventual overcompensation of the skeletal phenotype. Our findings demonstrate that Npt2 is a major regulator of P i homeostasis and necessary for normal skeletal development.Inorganic phosphate (P i ) is essential for a variety of cellular processes including skeletal mineralization. Because a disturbance in P i availability can affect the functional integrity of many organ systems, specialized tissues have evolved to maintain the extracellular P i concentration. In mammals, the regulation of P i homeostasis is largely determined by the kidney, with 60-70% of the filtered P i load reclaimed in the proximal segment of the nephron (1). Transepithelial transport of P i from the renal lumen to the blood compartment involves uptake across the brushborder membrane (BBM), translocation across the cell, and efflux across the basolateral membrane (2). Evidence suggests that P i transport across the BBM is the rate-limiting step in the overall P i reabsorptive process and the major site for its regulation (2, 3). Kinetics studies demonstrated that the transport is mediated by high capacity, low affinity, and low capacity, high affinity Na ϩ -P i cotransport systems (4).Recently, cDNAs encoding two distinct low capacity, high affinity, renal BBM Na ϩ -P i cotransporters (NPT1 and NPT2) that share only 20% identity have been identified in several mammalian species by expression and homology cloning (5-13). NPT2 is expressed exclusively in the proximal convoluted tubule and is regulated by P i intake (14) and parathyroid hormone (PTH) (15), factors known to regulate renal BBM Na ϩ -P i cotransport. In contrast, NPT1 is expressed throughout the proximal tubule and is not subject to regulation by dietary P i (10).X-chromosome-linked and autosomal disorders of renal P i reabsorption have been described in humans (16). Both are characterized by growth retardation, ...

PEX , a phosphate-regulating gene with homology to endopeptidases on the X chromosome, was recently identified as the candidate gene for X-linked hypophosphatemia. In the present study, we cloned mouse and human Pex/PEX cDNAs encoding part of the 5 Ј untranslated region, the protein coding region, and the entire 3 Ј untranslated region, determined the tissue distribution of Pex/PEX mRNA, and characterized the Pex mutation in the murine Hyp homologue of the human disease. Using the reverse transcriptase/polymerase chain reaction (RT/PCR) and ribonuclease protection assays, we found that Pex/PEX mRNA is expressed predominantly in human fetal and adult mouse calvaria and long bone. With RNA from Hyp mouse bone, an RT/PCR product was generated with 5 Ј but not 3 Ј Pex primer pairs and a protected Pex mRNA fragment was detected with 5 Ј but not 3 Ј Pex riboprobes by ribonuclease protection assay. Analysis of the RT/ PCR product derived from Hyp bone RNA revealed an aberrant Pex transcript with retention of intron sequence downstream from nucleotide 1302 of the Pex cDNA. Pex mRNA was not detected on Northern blots of poly (A) ϩ RNA from Hyp bone, while a low-abundance Pex transcript of Ϸ 7 kb was apparent in normal bone. Southern analysis of genomic DNA from Hyp mice revealed the absence of hybridizing bands with cDNA probes from the 3 Ј region of the Pex cDNA. We conclude that Pex/PEX is a low-abundance transcript that is expressed predominantly in bone of mice and humans and that a large deletion in the 3 Ј region of the Pex gene is present in the murine Hyp homologue of X-linked hypophosphatemia. ( J. Clin. Invest . 1997. 99:1200-1209.)