Hyperphosphatemic Familial Tumoral Calcinosis: Genetic Models of Deficient FGF23 Action

Hyperphosphatemic familial tumoral calcinosis (hFTC) is a rare disorder of phosphate metabolism defined by hyperphosphatemia and ectopic calcifications in various locations. To date, recessive mutations have been described in three genes involving phosphate metabolism: FGF23, GALNT3, and α-Klotho, all of which result in the phenotypic presentation of hFTC. These mutations result in either inadequate intact fibroblast growth factor-23 (FGF23) secretion (FGF23 or GALNT3) or resistance to FGF23 activity at the fibroblast growth factor receptor/α-Klotho complex (α-Klotho). The biochemical consequence of limitations in FGF23 activity includes increased renal tubular reabsorption of phosphate, hyperphosphatemia, and increased production of 1,25-dihydroxyvitamin D. The resultant ectopic calcifications can be painful and debilitating. Medical treatments are targeted toward decreasing intestinal phosphate absorption or increasing phosphate excretion; however, results have been variable and generally limited. Treatments that would increase FGF23 levels or signaling would more appropriately target the genetic etiologies of this disease and perhaps be more effective.