Hypophosphatemic rickets and osteomalacia

Abstract

The hypophosphatemic conditions that interfere in bone mineralization comprise many hereditary or acquired diseases, all of them sharing the same pathophysiologic mechanism: reduction in the phosphate reabsorption by the renal tubuli. This process leads to chronic hyperphosphaturia and hypophosphatemia, associated with inappropriately normal or low levels of calcitriol, causing osteomalacia or rickets in children and osteomalacia in adults. X-linked hypophosphatemic rickets, autosomal-dominant hypophosphatemic rickets, and tumor-induced osteomalacia are the main syndromes involved in the hypophosphatemic rickets. Although these conditions exhibit different etiologies, there is a common link among them: increased activity of a phosphaturic factor, being the fibroblast growth factor 23 (FGF-23) the most studied one and to which is attributed a central role in the pathophysiology of the hyperphosphaturic disturbances. Activating mutations of FGF-23 and inactivating mutations in the PHEX gene (a gene on the X chromosome that codes for a Zn-metaloendopeptidase proteolytic enzyme which regulates the phosphate) involved in the regulation of FGF-23 have been identified and have been implicated in the pathogenesis of these disturbances. Genetic studies tend to show that the phosphorus homeostasis depends on a complex osteo-renal metabolic axis, whose mechanisms of interaction have been poorly understood so far. This paper reviews the current knowledge status concerning the pathophysiology of phosphate metabolism regulation and the pathophysiologic basis of hypophosphatemic rickets. It also analyzes the clinical picture and the therapeutic aspects of these conditions as well.