X-linked hypophosphataemia: a homologous phenotype in humans and mice with unusual organ-specific gene dosage.

XLH (X-linked hypophosphataemia, gene symbol HYP, McKusick 307800, 307810) and its murine counterparts (Hyp and Gy) map to a conserved segment on the X-chromosome (Xp 22.31-p.21.3, human; distal X, mouse). Gene dosage has received relatively little attention in the long history of research on this disease, which began over 50 years ago. Bone and teeth are sites of the principal disease manifestations in XLH (rickets, osteomalacia, interglobular dentin). Newer measures of quantitative XLH phenotypes reveal gene dose effects in bone and teeth with heterozygous values distributed between those in mutant hemizygotes and normal homozygotes. On the other hand, serum phosphate concentrations (which are low in the mutant phenotype and thereby contribute to bone and tooth phenotypes) do not show gene dosage. In Hyp mice serum values in mutant hemizygotes, mutant homozygotes and heterozygotes are similar. Phosphate homeostasis reflects its renal conservation. Renal absorption of phosphate on a high-affinity, Na+ ion-gradient coupled system in renal brush border membrane is impaired and gene dosage is absent at this level; the mutant phenotype is fully dominant. Synthesis and degradation of 1,25(OH)2D are also abnormal in XLH (and Hyp), but gene dosage in these parameters has not yet been measured. An (unidentified) inhibitory trans-acting product of the X-linked locus, affecting phosphate transport and vitamin D metabolism, acting perhaps through cytosolic protein kinase C, could explain the renal phenotype. But why would it have a normal gene dose effect in bone and teeth? Since the locus may have duplicated (to form Hyp and Gy), and shows evidence of variable expression in different organs (inner ear, bone/teeth, kidney), it may have been recruited during evolution to multiple functions.