A decade ago, only two hormones, parathyroid hormone and 1,25(OH)2D, were widely recognized to directly affect phosphate homeostasis. Since the discovery of fibroblast growth factor 23 (FGF23) in 2000 (1), our understanding of the mechanisms of phosphate homeostasis and of bone mineralization has grown exponentially. FGF23 is the link between intestine, bone, and kidney together in phosphate regulation. However, we still do not know the complex mechanism of phosphate homeostasis and bone mineralization. The physiological role of FGF23 is to regulate serum phosphate. Secreted mainly by osteocytes and osteoblasts in the skeleton (2-3), it modulates kidney handling of phosphate reabsorption and calcitriol production. Genetic and acquired abnormalities in FGF23 structure and metabolism cause conditions of either hyper-FGF23 or hypo-FGF23. Hyper-FGF23 is related to hypophosphatemia, while hypo-FGF23 is related to hyperphosphatemia. Both hyper-FGF23 and hypo-FGF23 are detrimentalto humans. In this review, we will discuss the pathophysiology of FGF23 and hyper-FGF23 related renal phosphate wasting disorders (4 IntroductionAccording to pathogenesis, ricketsis classified into calicopenic rickets, phosphopenicrickets, and a miscellaneous group associated with direct inhibitors of mineralization ( 5). In general, most instances of nutritional rickets are calicopenic, whereas heritable causes are usually phosphopenic. In this review we focus on hypophosphatemic rickets or osteomalacia. The causes of hypophosphatemia are various, of whichthe most prominent one is decreased reabsorption of phosphate in the proximal tubule.Although renal tubular disease can result in excessive renal phosphate wasting, in most hypophosphatemic disordersno abnormalities are found in the proximal tubule (6-7). It is speculated that an unknown factor is responsible for this phenomenon. The discovery of fibroblast growth factor 23 (FGF23), a member of the Phosphate homeostasisPhosphate comprises about 1% of total body weight. About 85% of total body phosphate resides in the bone, 14% in the cells, and only 1% in the serum and extracellular fluids. Maintenance of serum phosphate within its normal range allows for optimal mineralization of bone without deposition in vascular and other soft tissues. Serum phosphate concentration is determined Xianglan Huang et al. www.boneresearch.org | Bone Research 121by the balance among intestinal absorption of phosphate from the diet, its storage in bone, and its excretion in the urine. The proximal tubule is responsible for the reabsorption of phosphate filtered at the glomerulus and is the primary regulator of phosphate balance in the body. Transportation in the proximal tubule is driven primarily by sodium-potassium ATPase, which is located in the baso-lateral membrane of the cell (8-11). Under normal conditions, about 85% of the filtered phosphate is reabsorbed via the sodium-phosphate co-transporter (NaPi2a and NaPi2c) in the proximal tubule (9, 11). Parathyroid hormone (PTH) is one of the most potent horm...

Background: Familial hypomagnesemia with secondary hypocalcemia (HSH) is a rare autosomal recessive disease characterized by severe hypomagnesemia and hypocalcemia associated with neurological symptoms, including generalized seizures, tetany and muscle spasms, which are refractory to anticonvulsant treatment. The pathophysiological hallmarks of HSH are the impaired intestinal absorption of magnesium accompanied by renal magnesium wasting as a result of a reabsorption defect in the distal convoluted tubule. Mutations in TRPM6, the gene encoding the transient receptor potential cation channel subfamily member 6, have been found to be responsible for this disease. In the present study, we report a Chinese family with 2 sisters affected with severe HSH, and elucidate the characteristics of TRPM6 gene mutations in these 2 patients. Methods: We evaluated the clinical, laboratory, and radiographic findings. All 39 TRPM6 exons and flanking exon-intron junctions from genomic DNA were amplified and sequenced in 2 affected members suffering from HSH and their family. Results: We found two novel mutations in the family, one frameshift mutation (c.1196delC) and one non-sense mutation (c.4577G>A). These mutations were predicted to result in a complete loss of function of TRPM6. Both of the sisters were compound heterozygotes for these mutations. Conclusion: Our results suggested that the compound heterozygous mutations in TRPM6 were responsible for HSH in the Chinese family.