Parental origin of mutant allele does not explain absence of gene dose in X-linked Hyp mice

Seven novel and six de novo PHEX gene mutations in patients with hypophosphatemic rickets

Inactivating mutations in phosphate-regulating gene with homologies to endopeptidase on the X chromosome (PHEX) have been identified as a cause of X-linked hypophosphatemic rickets (XLH; OMIM 307800). In the present study, we enrolled 43 patients from 18 unrelated families clinically diagnosed with hypophosphatemic rickets and 250 healthy controls. For each available individual, all 22 exons with their exon-intron boundaries of the PHEX gene were directly sequenced. The levels of serum fibroblast growth factor 23 (FGF23) were measured as well. Sequencing analysis detected 17 different PHEX gene mutations, and 7 of these were identified as novel: 3 missense mutations, including c.304G>A (p.Gly102Arg) in exon 3, c.229T>C (p.Cys77Arg) in exon 3 and c.824T>C (p.Leu275Pro) in exon 7; 2 deletion mutations, including c.528delT (p.Glu177LysfsX44) in exon 5 and c.1234delA (p.Ser412ValfsX12) in exon 11; and 2 alternative splicing mutations, including c.436_436+1delAG in intron 4 at splicing donor sites and c.1483-1G>C in intron 13 at splicing acceptor sites. Moreover, 6 mutations were proven to be de novo in 6 sporadic cases and the probands were all females. No mutations were found in the 250 healthy controls. The serum levels of FGF23 varied widely among the patients with XLH, and no significant difference was found when compared with those of the healthy controls. On the whole, the findings of this study provide new insight into the spectrum of PHEX mutations and provide potential evidence of a critical domain in PHEX protein. In addition, the finding of an overlap of the serum FGF23 levels between the patients with XLH and the healthy controls indicates its limited diagnostic value in XLH.

Dosage effect of a Phex mutation in a murine model of X-linked hypophosphatemia

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). Because XLH is a dominant disease, one mutant allele is sufficient for manifestation of the disease. However, the 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 (Phex (K496X) or Phex (Jrt) ). 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 a 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.

Somatic and germline mosaicism for a mutation of the PHEX gene can lead to genetic transmission of X-linked hypophosphatemic rickets that mimics an autosomal dominant trait

CONTEXT

Familial hypophosphatemic rickets is usually transmitted as an X-linked dominant disorder (XLH), although autosomal dominant forms have also been observed. Genetic studies of these disorders have identified mutations in PHEX and FGF23 as the causes of X-linked dominant disorder and autosomal dominant forms, respectively.

OBJECTIVE

The objective of the study was to describe the molecular genetic findings in a family affected by hypophosphatemic rickets with presumed autosomal dominant inheritance.

PATIENTS

We studied a family in which the father and the elder of his two daughters, but not the second daughter, were affected by hypophosphatemic rickets. The pedigree interpretation of the family suggested that genetic transmission of the disorder occurred as an autosomal dominant trait.

METHODS AND RESULTS

Direct nucleotide sequencing of FGF23 and PHEX revealed that the elder daughter was heterozygous for an R567X mutation in PHEX, rather than FGF23, suggesting that the genetic transmission occurred as an X-linked dominant trait. Unexpectedly, the father was heterozygous for this mutation. Single-nucleotide primer extension and denaturing HPLC analysis of the father using DNA from single hair roots revealed that he was a somatic mosaic for the mutation. Haplotype analysis confirmed that the father transmitted the genotypes for 18 markers on the X chromosome equally to his two daughters. The fact that the father transmitted the mutation to only one of his two daughters indicated that he was a germline mosaic for the mutation.

CONCLUSIONS

Somatic and germline mosaicism for an X-linked dominant mutation in PHEX may mimic autosomal dominant inheritance.

The wrickkened pathways of FGF23, MEPE and PHEX

The last 350 years since the publication of the first medical monograph on rickets (old English term wrickken) (Glisson et al., 1651) have seen spectacular advances in our understanding of mineral-homeostasis. Seminal and exciting discoveries have revealed the roles of PTH, vitamin D, and calcitonin in regulating calcium and phosphate, and maintaining healthy teeth and skeleton. However, it is clear that the PTH/Vitamin D axis does not account for the entire picture, and a new bone-renal metabolic milieu has emerged, implicating a novel set of matrix proteins, hormones, and Zn-metallopeptidases. The primary defects in X-linked hypophosphatemic rickets (HYP) and autosomal-dominant hypophosphatemic rickets (ADHR) are now identified as inactivating mutations in a Zn-metalloendopeptidase (PHEX) and activating mutations in fibroblast-growth-factor-23 (FGF23), respectively. In oncogenic hypophosphatemic osteomalacia (OHO), several tumor-expressed proteins (MEPE, FGF23, and FRP-4) have emerged as candidate mediators of the bone-renal pathophysiology. This has stimulated the proposal of a global model that takes into account the remarkable similarities between the inherited diseases (HYP and ADHR) and the tumor-acquired disease OHO. In HYP, loss of PHEX function is proposed to result in an increase in uncleaved full-length FGF23 and/or inappropriate processing of MEPE. In ADHR, a mutation in FGF23 results in resistance to proteolysis by PHEX or other proteases and an increase in half-life of full-length phosphaturic FGF23. In OHO, over-expression of FGF23 and/or MEPE is proposed to result in abnormal renal-phosphate handling and mineralization. Although this model is attractive, many questions remain unanswered, suggesting a more complex picture. The following review will present a global hypothesis that attempts to explain the experimental and clinical observations in HYP, ADHR, and OHO, plus diverse mouse models that include the MEPE null mutant, HYP-PHEX transgenic mouse, and MEPE-PHEX double-null-mutant.

Differential regulation of PHEX expression in bone and parathyroid gland by chronic renal insufficiency and 1,25-dihydroxyvitamin D3

Mutations in the PHEX gene are responsible for X-linked hypophosphatemia, a renal phosphate-wasting disorder associated with defective skeletal mineralization. PHEX is predominantly expressed in bones and teeth and in the parathyroid gland of patients with chronic renal failure and tertiary hyperparathyroidism. The purpose of the present study was to examine the effects of renal insufficiency and 1,25-dihydroxyvitamin D3[1,25(OH)2D3] on the regulation of PHEX expression in rat tibia and parathyroid gland. In rats fed a high-phosphate (Pi) diet, ⅚ nephrectomy elicited a significant increase in the serum parathyroid hormone (PTH) concentration that was associated with a significant increase in the abundance of PHEX mRNA and protein in the tibia and a significant increase in PHEX mRNA in the parathyroid gland. In contrast, 1,25(OH)2D3administration to intact rats fed a control diet elicited a significant decrease in the serum PTH concentration that was accompanied by a significant decrease in PHEX mRNA and protein abundance in the tibia and a significant decrease in PHEX mRNA in the parathyroid gland. In addition, the increases in serum PTH levels and PHEX mRNA in the tibia and parathyroid gland in ⅚ nephrectomized rats fed a high-Pidiet were blunted by 1,25(OH)2D3. Serum PTH concentration was positively and significantly correlated with tibial PHEX mRNA and protein abundance. In summary, we demonstrate that PHEX expression in the tibia and parathyroid gland is increased by chronic renal insufficiency and decreased by 1,25(OH)2D3administration and suggest that PTH status may play an important role in mediating these changes in PHEX expression.

Effect of gene dose and parental origin on bone histomorphometry in X-linked Hyp mice

X-linked hypophosphatemia (XLH) is characterized by rickets and osteomalacia and arises from mutations in the Phex and PHEX genes in mice (Hyp) and humans, respectively. The present study was undertaken to examine the effect of gene dose on the skeletal phenotype using a histomorphometric approach. Metrical traits (vertebral length, growth plate thickness, cancellous osteoid volume per bone volume, and cancellous, endocortical, and periosteal osteoid thickness) were compared in caudal vertebrae of mutant female (Hyp/+, Hyp/Hyp) and male (Hyp/Y) mice and their normal female (+/+) and male (+/Y) littermates. Mutant animals had trait values that differed significantly from those of normal animals. However, with the exception of vertebral length and cancellous osteoid thickness, values were not significantly different between the three mutant genotypes. We also examined the effect of gamete-of-origin on histomorphometric parameters in obligate Hyp/+ females derived from male or female transmitting parents. The metrical trait values in both groups of Hyp/+ mice were similar, with the exception of vertebral length and cancellous osteoid volume per bone volume. In summary, we demonstrate that the amount of osteoid per bone volume is similar in the three mutant genotypes and conclude that the extent and magnitude of the mineralization defect is fully dominant and likely not affected by gene dose. The differences in vertebral length in the mutants suggest that rickets and osteomalacia are not the only causes of decreased vertebral growth in Hyp mice and that Phex protein may influence bone growth and mineralization by distinct pathways.

Structure and function of disease-causing missense mutations in the PHEX gene

The PHEX gene that is mutated in patients with X-linked hypophosphatemia (XLH) encodes a protein homologous to the M13 family of zinc metallopeptidases. The present study was undertaken to assess the impact of nine PHEX missense mutations on cellular trafficking, endopeptidase activity, and protein conformation. Secreted forms of wild-type and mutant PHEX proteins were generated by PCR mutagenesis; these included C85R, D237G, Y317F, G579R, G579V, S711R, A720T, and F731Y identified in XLH patients, and E581V, which in neutral endopeptidase 24.11 abolishes catalytic activity but not plasma membrane localization. The wild-type and D237G, Y317F, E581V, and F731Y proteins were terminally glycosylated and secreted into the medium, whereas the C85R, G579R, G579V, S711R, and A720T proteins were trapped inside the transfected cells. Growing the cells at 26 C permitted the secretion of G579V, S711R, and A720T proteins, although the yield of rescued G579V was insufficient for further analysis. Endopeptidase activity of secreted and rescued PHEX proteins, assessed using a novel internally quenched fluorogenic peptide substrate, revealed that E581V and S711R are completely inactive; D237G and Y317F exhibit 50-60% of wild-type activity; and A720T and F731Y retain full catalytic activity. Conformational analysis by limited proteolysis demonstrated that F731Y is more sensitive to trypsin and D237G is more resistant to endoproteinase Glu-c than the wild-type protein. Thus, defects in protein trafficking, endopeptidase activity, and protein conformation account for loss of PHEX function in XLH patients harboring these missense mutations.

Mutational analysis and genotype-phenotype correlation of the PHEX gene in X-linked hypophosphatemic rickets

PHEX is the gene defective in X-linked hypophosphatemic rickets. In this study, analysis of PHEX revealed mutations in 22 hypophosphatemic rickets patients, including 16 of 28 patients in whom all 22 PHEX exons were studied. In 13 patients, in whom no PHEX mutation had been previously detected in 17 exons, the remaining 5 PHEX exons were analyzed and mutations found in 6 patients. Twenty different mutations were identified, including 16 mutations predicted to truncate PHEX and 4 missense mutations. Phenotype analysis was performed on 31 hypophosphatemic rickets patients with PHEX mutations, including the 22 patients identified in this study, 9 patients previously identified, and affected family members. No correlation was found between the severity of disease and the type or location of the mutation. However, among patients with a family history of hypophosphatemic rickets, there was a trend toward more severe skeletal disease in patients with truncating mutations. Family members in more recent generations had a milder phenotype. Postpubertal males had a more severe dental phenotype. In conclusion, although identifying mutations in PHEX may have limited prognostic value, genetic testing may be useful for the early identification and treatment of affected individuals. Furthermore, this study suggests that other genes and environmental factors affect the severity of hypophosphatemic rickets.

Disorders of phosphate metabolism

Correct identification of the disorders of hypophosphatemia and hyperphosphatemia is important for determining therapy. Further research will provide insights into normal phosphate homeostasis, a complex and fascinating process.

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.

The role of the PHEX gene (PEX) in families with X-linked hypophosphataemic rickets

For over a hundred years, the bane of rickets (a disease of bone), has been prominent in those countries that have participated in, and seeded, the industrial revolution. Industrialisation had major effects of the demography of populations, and many people moved to dark, heavily industrialised cities to find work. It soon became apparent that rickets could be cured by supplementing the diet with cod liver oil and exposure to sunlight. This in turn led to the discovery that photoactivation of 7-dehydrocholesterol was required to produce vitamin D, an indispensable regulator of bone mineral metabolism. Although inadequate exposure to light and poor dietary intake are the main causes of rickets and osteomalacia, recent research has confirmed the role of familial, and tumour forms of the disease. This review will describe the recent advances in our knowledge of the molecular defects in X-linked hypophosphataemic rickets (HYP), and oncogenic hypophosphataemic osteomalacia (OHO). Although HYP and OHO have different primary defects, both diseases have similarities that suggest a linked or overlapping pathophysiology. Also, without doubt, the recent cloning of the gene defective in HYP (the PHEX gene), has given researchers a new reagent to explore the molecular regulation of bone and its links to kidney endocrine function. The fact that the PHEX gene codes for a Zn metallopeptidase raises new and intriguing questions, and adds new momentum to the research on diseases of bone mineral metabolism.

Positional cloning of the PEX gene: new insights into the pathophysiology of X-linked hypophosphatemic rickets

X-linked hypophosphatemic rickets (HYP) is the most common form of hereditary renal phosphate wasting. The hallmarks of this disease are isolated renal phosphate wasting with inappropriately normal calcitriol concentrations and a mineralization defect in bone. Studies in the Hyp mouse, one of the murine models of the human disease, suggest that there is an approximately 50% decrease in both message and protein of NPT-2, the predominant sodium-phosphate cotransporter in the proximal tubule. However, human NPT-2 maps to chromosome 5q35, indicating that it is not the disease gene. Positional cloning studies have led to the identification of a gene, PEX, which is responsible for the disorder. Further studies have led to identification of the murine Pex gene, which is mutated in the murine models of the disorder. These studies, in concert with other studies, have led to improved understanding of the pathophysiology of HYP and a new appreciation for the complexity of normal phosphate homeostasis.

Expression and cloning of the human X-linked hypophosphatemia gene cDNA

X-linked hypophosphatemia (XLH), which is a heritable metabolic bone disease characterized biochemically by selective renal phosphate (Pi) wasting, is associated with mutations in the PEX (Phosphate-regulating gene with homologies to Endopeptidases on the X-chromosome) gene. To further explore the physiologic role of PEX and define its effect in XLH we have determined the expression and tissue distribution. Northern analysis found abundant PEX mRNA in a restricted pattern, predominantly in adult ovary and fetal lung. In addition, PEX expression was also found in adult lung and fetal liver. A PEX cDNA of 2550 basepairs, which contains the full PEX coding region, was isolated from a human ovary cDNA library. The PEX cDNA shows high homology to other membrane-bound zinc metallopeptidases. The presence of PEX in nonosseous tissues strongly suggests features of a systemic role, rather than a unique function in bone development.

Renal Na(+)-phosphate cotransporter gene expression in X-linked Hyp and Gy mice

The X-linked Hyp and Gy mutations are murine homologues of X-linked hypophosphatemia (XLH), a dominant disorder of phosphate (Pi) homeostasis characterized by growth retardation, rickets, hypophosphatemia and decreased renal tubular maximum for Pi reabsorption relative to glomerular filtration rate (Tmp/GFR). In Hyp and Gy mice, the decrease in Tmp/GFR is associated with a reduction in renal brush-border membrane (BBM) Na(+)-Pi cotransport that can be ascribed to a decrease in renal-specific, Na(+)-Pi cotransporter (NPT2) mRNA and protein abundance. Although renal NPT2 gene expression is reduced in Hyp and Gy mice, the NPT2 gene does not map to the X chromosome. These findings exclude NPT2 as a candidate gene for murine and human X-linked hypophosphatemias and suggest that genes at the Hyp, Gy and XLH (HYP) loci are involved in regulation of NPT2 gene expression. Both Hyp and Gy mice respond to low Pi diet with an increase in BBM Na(+)-Pi cotransport, NPT2 mRNA and protein. The increase in NPT2 protein in Pi-depleted mice far exceeds the increase in NPT2 mRNA, suggesting that translational or post-translational mechanisms are involved in the adaptive process. NPT2 protein is localized to the apical surface of the proximal tubule, where immunostaining in both normal and Hyp mice is increased in response to low Pi diet. Pi-deprived Hyp and Gy mice fail to show an increase in Tmp/GFR, indicating that adaptation at the BBM is not sufficient for the overall increase in Tmp/GFR in response to low Pi diet.

Renal expression of Na+-phosphate cotransporter mRNA and protein: effect of the Gy mutation and low phosphate diet

The X-linked Gy mutation is closely linked, but not allelic, to Hyp and is characterized by rickets, hypophosphatemia, decreased renal tubular maximum for phosphate (Pi) reabsorption (TmP) and a specific reduction in renal brush-border membrane (BBM) Na+-Pi cotransport. Gy mice, like their normal littermates, respond to a low-Pi diet with an increase in BBM Na+-Pi cotransport, but fail to show an adaptive increase in Tmp. Using an antibody raised against the NH2 terminal peptide of the rat renal-specific Na+-Pi cotransporter (NaPi-2) and a NaPi-2 cDNA probe, we examined the effect of the Gy mutation and low-Pi diet (0.03% Pi) on NaPi-2 protein and mRNA abundance. The reduction in BBM Na+-Pi cotransport in Gy mice (51 +/- 5% of normal, P < 0.05) was associated with a decrease in NaPi-2 protein (46 +/- 12% of normal, P < 0.05) and mRNA abundance (76 +/- 5%, P < 0.05). The low-Pi diet elicited a two- to three-fold increase in Na+-Pi cotransport in both normal and Gy mice that was accompanied by a large increase in NaPi-2 protein (10.2-fold in normal and 16.9-fold in Gy mice) and a modest increase in NaPi-2 mRNA (1.3-fold in both mouse strains, P < 0.05). The present data demonstrate that (1) the renal defect in BBM Pi transport in Gy mice can be ascribed to a deficit in NaPi-2 protein and mRNA abundance, (2) both normal and Gy mice respond to low Pi with an adaptive increase in NaPi-2 protein that exceeds the increase in Na+-Pi cotransport activity and NaPi-2 mRNA, (3) the adaptive increase in NaPi-2 protein and mRNA are not sufficient for the overall increase in TmP following Pi restriction.

Candidate 56 and 58 kDa protein(s) responsible for mediating the renal defects in oncogenic hypophosphatemic osteomalacia

The effects of tumor-conditioned media (TCM) derived from cultured cells from an oncogenic hypophosphatemic osteomalacia (OHO) tumor on transformed human kidney cells were investigated. Dose-dependent cell detachment and aggregation occurred in kidney cells cultured in serum-free medium supplemented with TCM, but not in skin fibroblast controls, or in kidney cells cultured in the presence of serum. Kidney cells exposed to TCM in the presence of serum (0.5%) had reduced Na(+)-dependent phosphate cotransport (36%, p < 0.04) and increased 1alpha-hydroxylase activity (48%, p < 0.05). In contrast, TCM had no significant effect on Na(+)-dependent alpha-methyl-glucose transport. To investigate these effects further, serum from an OHO patient, before and after tumor resection, was used to raise polyclonal antiserum to tumor-derived products (preoperative and postoperative antiserum, respectively). Changes in Na(+)-dependent phosphate cotransport and vitamin D metabolism induced by TCM were prevented by the addition of preoperative but not postoperative antisera. Furthermore, Western analysis revealed the presence of two proteins (56-58 kDa) in TCM media screened with preoperative antisera. These proteins were not detected by postoperative antisera and were absent in skin fibroblast control media. Direct inhibition of Na(+)-dependent phosphate cotransport by phosphonoformic acid did not affect 1,25-dihydroxy vitamin D(3) synthesis. These studies provide support for a circulating component affecting phosphate handling and vitamin D metabolism in OHO.

A gene (PEX) with homologies to endopeptidases is mutated in patients with X-linked hypophosphatemic rickets. The HYP Consortium

X-linked hypophosphatemic rickets (HYP) is a dominant disorder characterised by impaired phosphate uptake in the kidney, which is likely to be caused by abnormal regulation of sodium phosphate cotransport in the proximal tubules. By positional cloning, we have isolated a candidate gene from the HYP region in Xp22.1. This gene exhibits homology to a family of endopeptidase genes, members of which are involved in the degradation or activation of a variety of peptide hormones. This gene (which we have called PEX) is composed of multiple exons which span at least five cosmids. Intragenic non-overlapping deletions from four different families and three mutations (two splice sites and one frameshift) have been detected in HYP patients, which suggest that the PEX gene is involved in the HYP disorder.

Renal Na(+)-phosphate cotransport in murine X-linked hypophosphatemic rickets. Molecular characterization

The X-linked Hyp mouse is characterized by a specific defect in proximal tubular phosphate (Pi) reabsorption that is associated with a decrease in Vmax of the high affinity Na(+)-Pi cotransport system in the renal brush border membrane. To understand the mechanism for Vmax reduction, we examined the effect of the Hyp mutation on renal expression of Na(+)-Pi cotransporter mRNA and protein. Northern hybridization of renal RNA with a rat, renal-specific Na(+)-Pi cotransporter cDNA probe (NaPi-2) (Magagnin et al. 1993. Proc. Natl. Acad. Sci. USA. 90:5979-5983.) demonstrated a reduction in a 2.6-kb transcript in kidneys of Hyp mice relative to normal littermates (NaPi-2/beta-actin mRNA = 57 +/- 6% of normal in Hyp mice, n = 6, P < 0.01). Na(+)-Pi cotransport, but not Na(+)-sulfate cotransport, was approximately 50% lower in Xenopus oocytes injected with renal mRNA extracted from Hyp mice when compared with that from normal mice. Hybrid depletion experiments documented that the mRNA-dependent expression of Na(+)-Pi cotransport in oocytes was related to NaPi-2. Western analysis demonstrated that NaPi-2 protein is also significantly reduced in brush border membranes of Hyp mice when compared to normals. The present data demonstrate that the specific reduction in renal Na(+)-Pi cotransport in brush border membranes of Hyp mice can be ascribed to a proportionate decrease in the abundance of Na(+)-Pi cotransporter mRNA and protein.