Hypophosphatemic rickets/osteomalacia in linear sebaceous nevus syndrome: a variant of tumor-induced osteomalacia.

A severe form of vitamin D-resistant rickets is associated with the linear sebaceous nevus syndrome. We investigated the pathophysiology underlying defective bone mineralization in two individuals and examined the effects of 1,25-dihydroxyvitamin D (1,25(OH)2D, calcitriol) therapy on the clinical and biochemical abnormalities. Both patients had fasting hypophosphatemia, markedly diminished TmP/GFR, and elevated alkaline phosphase activity in the presence of normocalcemia. Before treatment with calcitriol, serum 1,25(OH)2D concentrations were reduced but serum 25-hydroxyvitamin D (25(OH)D) concentrations were normal. Administration of calcitriol increased serum 1,25(OH)2D concentrations and led to an increase in TmP/GFR and serum phosphorus levels and to a decrease in alkaline phosphatase activity. However, the renal tubular maximum for reabsorption of inorganic phosphate, normalized according to glomerular filtration rate, and serum phosphorus levels remained abnormally low even in the patient who also received phosphate supplementation. Bone histomorphologic studies in the adult patient showed extreme osteomalacia, which partially improved with calcitriol. These data demonstrate that the putative skin lesion-derived factor results in both a renal tubular defect in phosphate reabsorption and in 1,25-(OH)2 D deficiency. The vitamin D-resistant rickets of linear sebaceous nevus syndrome is a variant of tumor-induced osteomalacia.