Healing of bone lesions with 1,25-dihydroxyvitamin D3 in the young X-linked hypophosphatemic male mouse.

The X-linked hypophosphatemic (Hyp) mouse presents with biochemical and skeletal abnormalities similar to those of human vitamin D-resistant rickets and hence is considered as a model of the human disease. In an attempt to correct osteomalacia, young (21-day-old) mutant male mice were infused continuously for 4 weeks with 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3; 0.05--0.25 microgram/kg . day]. Mineral and skeletal changes were assessed by serum, urinary, and bone ash concentrations of calcium, phosphorus, and magnesium and by histomorphometric analysis of bone formation measured on histological sections of tetracycline dual labeled undecalcified caudal vertebrae. Treatment with 1,25-(OH)2D3 produced a dose-dependent elevation of serum phosphorous that could be assigned to increased intestinal phosphate absorption. Concomitantly, epiphyseal, endosteal, and periosteal bone mineralization were improved in correlation with both the dosage of 1,25-(OH)2D3 and the serum phosphorus level. Normalization of serum calcium and phosphorus but not of urinary phosphate excretion were achieved together with complete healing of bone mineralization when the highest doses of 1,25-(OH)2D3 (0.175--0.35 microgram/kg . day) were given. The data show that rickets and osteomalacia, which characterize the young Hyp mouse, can be healed by 1,25-(OH)2D3 in doses high enough to normalize serum mineral concentrations. Unlike the renal phosphate leak, the phenotypic expression of the Hyp gene pertaining to bone mineralization is then corrected by 1,25-(OH)2D3 supplementation.