The effects of bone marrow transplantation on X-linked hypophosphatemic mice

The genes responsible for X-linked hypophosphatemic (XLH) vitamin D-resistant rickets and the murine homolog, hypophosphatemic mice (Hyp), were identified as PHEX and Phex (phosphate-regulating gene with homology to endopeptidases on the X chromosome), respectively. However, the mechanism by which inactivating mutations of PHEX cause XLH remains unknown. We investigated the mechanisms by syngeneic bone marrow transplantation (BMT) from wild mice to Hyp mice. The expression of the Phex gene was detected in mouse BM cells. BMT introduced a chimerism in recipient Hyp mice and a significant increase in the serum phosphorus level. The renal sodium phosphate cotransporter gene expression was significantly increased. The effect of BMT on the serum phosphorus level depended on engraftment efficiencies, which represent the dosage of normal gene. Similarly, the serum alkaline phosphatase (ALP) activity was decreased and bone mineral density was increased. Furthermore, the renal expression of 25-hydroxyvitamin D3 24-hydroxylase, which is a key enzyme in the catabolic pathway and is increased in XLH/Hyp, was improved. From these results, we conclude that transplantation of normal BM cells improved abnormal bone mineral metabolism and deranged vitamin D metabolism in Hyp by replacing defective gene product(s) with normal gene product(s). This result may provide strong evidence for clinical application of BMT in metabolic bone disorders.