Pex/PEX tissue distribution and evidence for a deletion in the 3' region of the Pex gene in X-linked hypophosphatemic mice.

Abstract

PEX , a phosphate-regulating gene with homology to endopeptidases on the X chromosome, was recently identified as the candidate gene for X-linked hypophosphatemia. In the present study, we cloned mouse and human Pex/PEX cDNAs encoding part of the 5 Ј untranslated region, the protein coding region, and the entire 3 Ј untranslated region, determined the tissue distribution of Pex/PEX mRNA, and characterized the Pex mutation in the murine Hyp homologue of the human disease. Using the reverse transcriptase/polymerase chain reaction (RT/PCR) and ribonuclease protection assays, we found that Pex/PEX mRNA is expressed predominantly in human fetal and adult mouse calvaria and long bone. With RNA from Hyp mouse bone, an RT/PCR product was generated with 5 Ј but not 3 Ј Pex primer pairs and a protected Pex mRNA fragment was detected with 5 Ј but not 3 Ј Pex riboprobes by ribonuclease protection assay. Analysis of the RT/ PCR product derived from Hyp bone RNA revealed an aberrant Pex transcript with retention of intron sequence downstream from nucleotide 1302 of the Pex cDNA. Pex mRNA was not detected on Northern blots of poly (A) ϩ RNA from Hyp bone, while a low-abundance Pex transcript of Ϸ 7 kb was apparent in normal bone. Southern analysis of genomic DNA from Hyp mice revealed the absence of hybridizing bands with cDNA probes from the 3 Ј region of the Pex cDNA. We conclude that Pex/PEX is a low-abundance transcript that is expressed predominantly in bone of mice and humans and that a large deletion in the 3 Ј region of the Pex gene is present in the murine Hyp homologue of X-linked hypophosphatemia. ( J. Clin. Invest . 1997. 99:1200-1209.)

Skin fibroblasts from spermine synthase-deficient hemizygous gyro male (Gy/Y) mice overproduce spermidine and exhibit increased resistance to oxidative stress but decreased resistance to UV irradiation

“…With the same method we demonstrated the presence of spermine synthase mRNA in X\Y cells but not in Gy\Y cells (Figure 1). In agreement with the results of others [24], the Phex gene was found not to be expressed in fibroblasts (Gy\Y and X\Y cells) but to be expressed in osteoblasts (UMR-106 cells) (Figure 1).…”

“…We therefore chose to isolate skin fibroblasts, which exhibit no significant Phex gene expression [24]. X\Y littermates were used as controls.…”

“…The mutant mice have rickets\ osteomalacia, reduced viability and a circling behaviour. The Hyp mouse, which likewise has a deletion in the Phex gene, exhibits a less complex phenotype than does the Gy\Y mouse [23,24]. This appears to be due to the fact that the Gy\Y mouse carries a more extensive deletion, including the neighbouring spermine synthase gene [25][26][27].…”

Oncogenic osteomalacia: is there a new phosphate regulating hormone?

“…Lack of functional PEX, leading to an increased level of the hormone in bone, could lead to local effects on osteoblast metabolism and bone formation. This is consistent with the intrinsic defects observed in osteoblasts in Hyp and with the demonstration that conditioned medium from Hyp mouse primary osteoblastlike cultures inhibited phosphate uptake by mouse proximal tubular cells (Lajeunesse et al, 1996) mRNA has been reported in a range of human foetal and adult mouse tissues (Beck et al, 1997). The expression appeared to be at low levels and to be predominantly in bone.…”

Disorders of Renal Tubular Phosphate Transport

“…The pattern of PHEX expression is consistent with the skeletal and dental abnormalities evident in XLH and Hyp, and the notion that the renal P i leak in Hyp mice is dependent on a circulating factor (54 -57). Although it is not immediately apparent how loss of PHEX function leads to a decrease in renal P i reabsorption, it has been suggested that PHEX is involved in the inactivation of a phosphaturic hormone or the activation of a P i conserving hormone (58,62,64) and that loss of PHEX function is associated with either an excess of phosphaturic hormone or a deficiency in a P i conserving hormone. In either case, renal type IIa, type IIc, and perhaps other Na/P i cotransporters would be downregulated and P i wasting would ensue.…”