Dosage Effect of a Phex Mutation in a Murine Model of X-Linked Hypophosphatemia

Abstract

X-linked hypophosphatemia (XLH) is caused by mutations in the PHEX gene, which increase circulating levels of the phosphaturic hormone, fibroblast growth factor 23 (FGF23). Since XLH is a dominant disease, one mutant allele is sufficient for manifestation of the disease. However, dosage effect of a PHEX mutation in XLH is not completely understood. To examine the effect of Phex genotypes, we compared serum biochemistries and skeletal measures between all five possible genotypes of a new murine model of XLH (PhexK496X or PhexJrt). Compared to sex-matched littermate controls, all Phex mutant mice had hypophosphatemia, mild hypocalcemia, and increased parathyroid hormone and alkaline phosphatase levels. Furthermore, mutant mice had markedly elevated serum Fgf23 levels due to increased Fgf23 expression and reduced cleavage of Fgf23. Although females with a homozygous Phex mutation were slightly more hypocalcemic and hypophosphatemic than heterozygous females, the two groups had comparable intact Fgf23 levels. Similarly, there was no difference in intact Fgf23 or phosphorus concentrations between hemizygous males and heterozygous females. Compared to heterozygous females, homozygous counterparts were significantly smaller and had shorter femurs with reduced bone mineral density, suggesting the existence of dosage effect in the skeletal phenotype of XLH. However, overall phenotypic trends in regards to mineral ion homeostasis were mostly unaffected by the presence of one or two mutant Phex allele(s). The lack of gene dosage effect on circulating Fgf23 (and thus, phosphorus) levels suggests that a Phex mutation may create the lower set point for extracellular phosphate concentrations.

PHEX 3′-UTR c.*231A>G Near The Polyadenylation Signal Is a Relatively Common, Mild, American Mutation That Masquerades as Sporadic or X-Linked Recessive Hypophosphatemic Rickets

“…Accordingly, these mothers with the pA defect did not always manifest even this biochemical hallmark of renal Pi wasting in XLH. Therefore, the pA mutation seems to manifest a gene dosage effect, including on biochemical parameters of Pi homeostasis, that may be unusual for other PHEX mutations as reported in 2013 by Ichikawa et al (28) Studies of large kindreds with the c. Ã 231A > G defect should clarify its gender effects.…”

Hypophosphatemic Rickets: Revealing Novel Control Points for Phosphate Homeostasis

“…Surprisingly, in the presence of only a slightly improved serum phosphate concentration, the double mutant mice markedly up-regulated FGF23 mRNA, well above the Hyp mouse with WT GALNT3 alleles (>20 fold in 12 week old Hyp mice) (55). In contrast to patients with ADHR, XLH patients and controls showed similar correlations between serum iron and iFGF23/cFGF23 (albeit XLH was on an increased scale) supporting that iron responsiveness is likely not a central factor in the pathogenesis of XLH (51). These results indicate that the PHEX mutation, in addition to potentially resulting in altered osteocyte cell differentiation, alters biological sensing for phosphate.…”

“…Inactivating PHEX mutations have previously been associated with a differentiation defect in osteocytes in the Hyp mouse model of XLH (47) which appears to be similar to that found in the Dmp1 -null mouse (see ARHR-1), and Hyp bone FGF23 mRNA expression is elevated (Figure 1) (48,49). The increased FGF23 mRNA levels indicate that the increase in serum FGF23 in XLH is due to both over production by skeletal cells, as well as potentially a decrease in secondary iFGF23 proteolysis by SPC family members (50,51). The interactions between FGF23 and PHEX are indirect as FGF23 is not a PHEX substrate (52).…”

“…To further examine a phosphate ‘set point’ in XLH, a study compared female mice homozygous for PHEX inactivating mutations to females heterozygous for PHEX mutations as well as male hemizygotes (51). Both models had similar iFGF23, and the iFGF23 and serum phosphate were also similar between heterozygous females and hemizygous males (51). This lack of a gene dosage effect on circulating FGF23 and phosphate further suggests that inactivating PHEX mutations may create a lower set point for maintaining blood phosphate concentrations.…”

Regulation of Hormone-Sensitive Renal Phosphate Transport

“…Dosage effects of the PHEX gene were also examined by comparing heterozygous and homozygous female Hyp mice. It was demonstrated that the skeletal phenotype was worse in homozygous female Hyp mice, but both groups of mice had comparable serum FGF23 levels (Ichikawa, Gray, Bikorimana, & Econs, 2013).…”