No abstract

Hereditary hypophosphatemic rickets with hypercalciuria (HHRH) is a rare disorder of autosomal recessive inheritance that was first described in a large consanguineous Bedouin kindred. HHRH is characterized by the presence of hypophosphatemia secondary to renal phosphate wasting, radiographic and/or histological evidence of rickets, limb deformities, muscle weakness, and bone pain. HHRH is distinct from other forms of hypophosphatemic rickets in that affected individuals present with hypercalciuria due to increased serum 1,25-dihydroxyvitamin D levels and increased intestinal calcium absorption. We performed a genomewide linkage scan combined with homozygosity mapping, using genomic DNA from a large consanguineous Bedouin kindred that included 10 patients who received the diagnosis of HHRH. The disease mapped to a 1.6-Mbp region on chromosome 9q34, which contains SLC34A3, the gene encoding the renal sodium-phosphate cotransporter NaP(i)-IIc. Nucleotide sequence analysis revealed a homozygous single-nucleotide deletion (c.228delC) in this candidate gene in all individuals affected by HHRH. This mutation is predicted to truncate the NaP(i)-IIc protein in the first membrane-spanning domain and thus likely results in a complete loss of function of this protein in individuals homozygous for c.228delC. In addition, compound heterozygous missense and deletion mutations were found in three additional unrelated HHRH kindreds, which supports the conclusion that this disease is caused by SLC34A3 mutations affecting both alleles. Individuals of the investigated kindreds who were heterozygous for a SLC34A3 mutation frequently showed hypercalciuria, often in association with mild hypophosphatemia and/or elevations in 1,25-dihydroxyvitamin D levels. We conclude that NaP(i)-IIc has a key role in the regulation of phosphate homeostasis.

Thyroparathyroidectomy of rats on a diet low in calcium reduces production of 1,25-dihydroxycholecalciferol from 25-hydroxycholecalciferol to negligible levels within 40 hr, and increases production of another metabolite, called Va. Parathyroid extract, at a dose of 20 units per day, prevents these changes. When 40 units per day of parathyroid extract is given 48 hr after thyroparathyroidectomy, 1,25-dihydroxycholecalciferol production is restored almost to control levels within 36 hr. The change brought about by parathyroid extract cannot be attributed to resulting changes in serum calcium or phosphorus concentration. It appears that the parathyroid hormone serves as a tropin for production of 1,25-dihydroxycholecalciferol, the hormonal form of vitamin D responsible for calcium mobilization from intestinal contents and bone.Recently, a hormonal system involved in the regulation of calcium concentration in blood plasma was discovered. This system, which derives its active component from vitamin D, is located in kidney tissue. Vitamin D must first be metabolized in the liver to its 25-hydroxy derivative [25-hydroxycholecalciferol (25-OHD3) ] (1-3). This metabolite is further converted in the kidney to either 1,25-dihydroxycholecalciferol [1,2D.] (4-7) or to an unidentified metabolite designated as Va (8-10). The Va metabolite has not been assigned a function, but evidence is convincing that 1,25-(OH)2D3 is the metabolically active form of vitamin D that induces intestinal calcium absorption (11)(12)(13) and mobilization of calcium from bone (14-16). The synthesis of 1,25-(OH)2D3 is regulated by negative feedback, thus establishing 1,25-(OH)2D3 as a true hormone involved in the regulation of the plasma calcium concentration (17,18). The nature of the feedback regulation, i.e., the sharp shut-off of 1,25-(OH)2D3 synthesis at serum calcium concentrations very close to normal, suggested that the regulation involves secretion of parathyroid hormone. We considered it possible that the parathyroid glands might actually detect low serum calcium concentrations, and that the hormone might actually regulate the synthesis of 1,25-(OH)2D3 in kidney. It is the purpose of this communication to demonstrate that parathyroid hormone is a major factor in the regulation cf 25-OHD3 metabolism that was previously attributed to serum calcium concentration. MATERIALS AND METHODSHormones. Parathyroid extract was a gift from the Eli Lilly Co., Indianapolis, Ind., while highly purified parathyroid hormone was purchased from Wilson Laboratories, Chicago, Ill. The hormone was dissolved in 1 mM acetic acid, to which was added gelatin (15%) and phenol (0.4%). The final concentration was 5 units of hormone per 0.05 ml.The [26,27-3H] Tap water was added to the remaining serum to bring the volume to 5 ml, then 15 ml of methanol-chloroform 2:1 was added. The mixture was allowed to stand overnight at 4°. 5 ml of chloroform was added, the mixture was shaken, and the two phases were allowed to separate. The chloroform phase was drawn off and ...