MRNA expression of Phex in mice and rats: the effect of low phosphate diet

Phex is the gene whose mutation is the cause of X-linked hypophosphatemia in humans and mice. The organs expressing Phex in normal animals, and their possible sensitivity to stimulation by low phosphate diets, are unknown. In this study, Phex expression was measured in 6-wk-old normal B6C3H male and female mice and in 135 g Sprague-Dawley rats fed a normal phosphate diet or a low phosphate diet with deionized water ad libitum for 7 d. The animals were then anesthetized, and a variety of organs were collected and frozen in liquid nitrogen. Phex mRNA expression was measured in each organ by reverse transcription-polymerase chain reaction (RT-PCR) with primers specific for both Phex and glyceraldehyde-3-phosphate dehydrogenase (G3PDH). Southern blots were prepared, hybridized with 32P-labeled internal oligonucleotides, and quantified with a phosphor imager. The Phex/G3PDH ratio was computed, and the data were compiled as the mean +/- SEM. In these growing animals, the highest Phexexpression levels were found in the gonads, brain, and lung. In contrast, Phex expression in calvaria and femur was markedly less. Two significant changes were found in animals that were fed a low phosphate diet. Spleen showed a significant decrease in Phex mRNA levels on low phosphate diet (60+/-10% of normal P diet, n = 12/group, p = 0.002). The pituitary gland showed a significant increase in Phex expression with low phosphate diet (851+/-127% of G3PDH) over normal P diet (569+/-78%, n = 24 - 25/group, p = 0.03). No significant change was found in femur, calvaria, or a variety of soft tissues. In summary, Phex mRNA was found in most tissues examined. Expression levels varied by two orders of magnitude from highest to lowest with more in gonads, brain, and lung and with less in bone. Increased Phex mRNA was found in the pituitary gland of animals that were fed a low phosphate diet.