X-linked hypophosphatemia: molecular biology and treatment controversies

X-linked hypophosphatemia, currently one of the most prevalent varieties of familiar rickets, is attributed to renal phosphate wasting secondary to a gene defect localized to X p22 chromosomal region. The proximal tubular phosphate reabsorption defect is associated with blunted 1,25-dihydroxyvitamin D synthesis to hypophosphatemia or parathyroid hormone administration. It is characterized clinically by hypophosphatemia, growth retardation, and rickets especially in male patients. As the affected patients mature, pseudofractures, skeletal deformities, osteomalacic bone pain, progressive ankylosis, and dental caries occur, which may be alleviated and even prevented with adequate medical therapy. Long term treatment combines phosphate supplementation with calcitriol, augmented occasionally by diuretics. Hypercalcemia, hyperparathyroidism, and nephrocalcinosis are potential complications which require careful monitoring and continued investigations. The use of recombinant human growth hormone to augment renal tubular reabsorption of phosphate and to promote linear growth remains to be examined in well controlled, clinical trials.