Three DNA markers for hypophosphataemic rickets

The chicken or the egg: PHEX, FGF23 and SIBLINGs unscrambled

The eggshell is an ancient innovation that helped the vertebrates' transition from the oceans and gain dominion over the land. Coincident with this conquest, several new eggshell and noncollagenous bone-matrix proteins (NCPs) emerged. The protein ovocleidin-116 is one of these proteins with an ancestry stretching back to the Triassic. Ovocleidin-116 is an avian homolog of Matrix Extracellular Phosphoglycoprotein (MEPE) and belongs to a group of proteins called Small Integrin-Binding Ligand Interacting Glycoproteins (SIBLINGs). The genes for these NCPs are all clustered on chromosome 5q in mice and chromosome 4q in humans. A unifying feature of the SIBLING proteins is an Acidic Serine Aspartate-Rich MEPE (ASARM)-associated motif. The ASARM motif and the released ASARM peptide play roles in mineralization, bone turnover, mechanotransduction, phosphate regulation and energy metabolism. ASARM peptides and motifs are physiological substrates for phosphate-regulating gene with homologies to endopeptidases on the X chromosome (PHEX), a Zn metalloendopeptidase. Defects in PHEX are responsible for X-linked hypophosphatemic rickets. PHEX interacts with another ASARM motif containing SIBLING protein, Dentin Matrix Protein-1 (DMP1). DMP1 mutations cause bone-renal defects that are identical with the defects caused by loss of PHEX function. This results in autosomal recessive hypophosphatemic rickets (ARHR). In both X-linked hypophosphatemic rickets and ARHR, increased fibroblast growth factor 23 (FGF23) expression occurs, and activating mutations in FGF23 cause autosomal dominant hypophosphatemic rickets (ADHR). ASARM peptide administration in vitro and in vivo also induces increased FGF23 expression. This review will discuss the evidence for a new integrative pathway involved in bone formation, bone-renal mineralization, renal phosphate homeostasis and energy metabolism in disease and health.

Regulation of bone-renal mineral and energy metabolism: the PHEX, FGF23, DMP1, MEPE ASARM pathway

More than 300 million years ago, vertebrates emerged from the vast oceans to conquer gravity and the dry land. With this transition, new adaptations occurred that included ingenious changes in reproduction, waste secretion, and bone physiology. One new innovation, the egg shell, contained an ancestral protein (ovocleidin-116) that likely first appeared with the dinosaurs and was preserved through the theropod lineage in modern birds and reptiles. Ovocleidin-116 is an avian homolog of matrix extracellular phosphoglycoprotein (MEPE) and belongs to a group of proteins called short integrin-binding ligand-interacting glycoproteins (SIBLINGs). These proteins are all localized to a defined region on chromosome 5q in mice and chromosome 4q in humans. A unifying feature of SIBLING proteins is an acidic serine aspartate-rich MEPE-associated motif (ASARM). Recent research has shown that the ASARM motif and the released ASARM peptide have regulatory roles in mineralization (bone and teeth), phosphate regulation, vascularization, soft-tissue calcification, osteoclastogenesis, mechanotransduction, and fat energy metabolism. The MEPE ASARM motif and peptide are physiological substrates for PHEX, a zinc metalloendopeptidase. Defects in PHEX are responsible for X-linked hypophosphatemic rickets (HYP). There is evidence that PHEX interacts with another ASARM motif containing SIBLING protein, dentin matrix protein-1 (DMP1). DMP1 mutations cause bone and renal defects that are identical with the defects caused by a loss of PHEX function. This results in autosomal recessive hypophosphatemic rickets (ARHR). In both HYP and ARHR, increased FGF23 expression plays a major role in the disease and in autosomal dominant hypophosphatemic rickets (ADHR), FGF23 half-life is increased by activating mutations. ASARM peptide administration in vitro and in vivo also induces increased FGF23 expression. FGF23 is a member of the fibroblast growth factor (FGF) family of cytokines, which surfaced 500 million years ago with the boney fish (i.e., teleosts) that do not contain SIBLING proteins. In terrestrial vertebrates, FGF23, like SIBLING proteins, is expressed in the osteocyte. The boney fish, however, are an-osteocytic, so a physiological bone-renal link with FGF23 and the SIBLINGs was cemented when life ventured from the oceans to the land during the Triassic period, approximately 300 million years ago. This link has been revealed by recent research that indicates a competitive displacement of a PHEX-DMP1 interaction by an ASARM peptide that leads to increased FGF23 expression. This review discusses the new discoveries that reveal a novel PHEX, DMP1, MEPE, ASARM peptide, and FGF23 bone-renal pathway. This pathway impacts not only bone formation, bone-renal mineralization, and renal phosphate homeostasis but also energy metabolism. The study of this new pathway is relevant for developing therapies for several diseases: bone-teeth mineral loss disorders, renal osteodystrophy, chronic kidney disease and bone mineralization disorders (CKD-MBD), end-stage renal diseases, ectopic arterial-calcification, cardiovascular disease renal calcification, diabetes, and obesity.

Ontogeny of Phex/PHEX protein expression in mouse embryo and subcellular localization in osteoblasts

PHEX, a phosphate-regulating gene with homologies to endopeptidases on the X chromosome, is mutated in X-linked hypophosphatemia (XLH) in humans and mice (Hyp). Although recent observations indicate that Phex protein is expressed primarily in bone and may play an important role in osteoblast function and bone mineralization, the pattern of the Phex protein expression in the developing skeleton and its subcellular localization in osteoblasts remain unknown. We examined the ontogeny of the Phex protein in the developing mouse embryo and its subcellular localization in osteoblasts using a specific antibody to the protein. Immunohistochemical staining of mouse embryos revealed expression of Phex in osteogenic precursors in developing vertebral bodies and developing long bones on day 16 postcoitum (pc) and thereafter. Calvaria from day 18 pc mice showed Phex epitopes in osteoblasts. No Phex immunoreactivity was detected in lung, heart, hepatocytes, kidney, intestine, skeletal muscle, or adipose tissue of mouse embryos. Interestingly, embryonic mouse skin showed moderate amounts of Phex immunostaining. In postnatal mice, Phex expression was observed in osteoblasts and osteocytes. Moderate expression of Phex was seen in odontoblasts and slight immunoreactivity was observed in ameloblasts. Confocal microscopy revealed the presence of immunoreactive PHEX protein in the Golgi apparatus and endoplasmic reticulum of osteoblasts from normal mice and in osteoblasts from Hyp mice transduced with a human PHEX viral expression vector. PHEX protein was not detected in untransduced Hyp osteoblasts. These data indicate that Phex protein is expressed in osteoblasts and osteocytes during the embryonic and postnatal periods and that within bone, Phex may be a unique marker for cells of the osteoblast/osteocyte lineage.

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.

Autosomal dominant hypophosphataemic rickets is associated with mutations in FGF23

Proper serum phosphate concentrations are maintained by a complex and poorly understood process. Identification of genes responsible for inherited disorders involving disturbances in phosphate homeostasis may provide insight into the pathways that regulate phosphate balance. Several hereditary disorders of isolated phosphate wasting have been described, including X-linked hypophosphataemic rickets (XLH), hypophosphataemic bone disease (HBD), hereditary hypophosphataemic rickets with hypercalciuria (HHRH) and autosomal dominant hypophosphataemic rickets (ADHR). Inactivating mutations of the gene PHEX, encoding a member of the neutral endopeptidase family of proteins, are responsible for XLH (refs 6,7). ADHR (MIM 193100) is characterized by low serum phosphorus concentrations, rickets, osteomalacia, lower extremity deformities, short stature, bone pain and dental abscesses. Here we describe a positional cloning approach used to identify the ADHR gene which included the annotation of 37 genes within 4 Mb of genomic sequence. We identified missense mutations in a gene encoding a new member of the fibroblast growth factor (FGF) family, FGF23. These mutations in patients with ADHR represent the first mutations found in a human FGF gene.

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.

A 6-Mb YAC contig in Xp22.1-p22.2 spanning the DXS69E, XE59, GLRA2, PIGA, GRPR, CALB3, and PHKA2 genes

We report the generation of an approximately 6-Mb contig of 70 overlapping yeast artificial chromosomes (YAC) covering the interval between DXS16 and DXS1229 in Xp22.1-p22.2. Within this region lie the genes for calbindin (CALB3), gastrin-releasing peptide receptor (GRPR), phosphatidyl-inositol glycan-class A protein (PIGA), glycine receptor alpha-2 (GLRA2), phosphorylase kinase alpha (PHKA2), XE59 (a gene escaping X chromosome inactivation), and DXS69E (71-7A). YACs were isolated initially from four libraries either by hybridization or using sequence tagged sites (STSs) for DXS16, DXS9, GLRA2, DXS207, DXS43, DXS1416, DXS1317, DXS1195, and DXS418. Additional STSs were obtained from the end fragments of the original YACs studied, thus allowing us to cover the contig with a series of 73 STSs, approximately 1 per 100 kb. YAC contig construction allowed the following locus order to be established: Xpter-DXS16-DXS69E-DXS414-XE59 - DXS9 - (GLRA2, DXS987) - (PIGA, DXS207) - DXS1053-DXS197-(GRPR,DXS43)-CALB3-DXS14 16- DXS1317 - DXS1195 - DXS418 - DXS257 - (PHKA2, DXS999)-DXS443-DXS1229-Xcen. Restriction mapping of the DXS16-DXS43 interval predicted the existence of several CpG islands, suggesting the presence of other genes in the region. This work provides a starting point for further mapping and positional cloning of several X-linked disease genes.

Flanking markers define the X-linked hypophosphatemic rickets gene locus

X-linked hypophosphatemic rickets (HYP) is an X-linked dominant disorder characterized by decreased renal tubular phosphate reabsorption and consequent hypophosphatemia. The defect in tubular phosphate reabsorption is probably secondary to an unidentified humoral factor. Identification of the humoral factor and a full understanding of the pathophysiology of the disease await the identification of the HYP gene. Previously we demonstrated that DXS257 and DXS41 are flanking markers for the HYP gene. Two markers, DXS365 and DXS274, are tightly linked to the HYP gene, but investigators have been unable to determine whether they are centromeric or telomeric to the disease gene. Since tightly linked flanking markers are necessary prerequisites to obtain the gene by positional cloning techniques, we sought to determine the relative positions of these markers to the HYP gene by expanding our data base for linkage studies. We also investigated a new polymorphic probe for linkage to HYP to construct a more detailed genetic map around the HYP locus. Our data indicate that the markers DXS365, DXS274, and DXS92 are tightly linked to the HYP locus and suggest a locus order of Xtel-(DXS444/DXS315)-DXS43-(DXS257/DXS3 65)-HYP-(DXS274/DXS41/DXS92)-DXS-451- DXS319-Xeen. These results will facilitate attempts further to localize and clone the HYP gene.

Use of Alu-PCR to characterize hybrids containing multiple fragments and to generate new Xp21.3-p22.2 markers

Irradiation fragment hybrids potentially provide highly enriched sources of region-specific human DNA. However, such hybrids often contain multiple human pieces, not all of which can be easily detected. To develop specific resources for rapidly generating markers from Xp21.3-p22.2, we have single cell cloned two previously constructed irradiation hybrids that contain markers in this region and have achieved segregation of the different known fragments originally retained. Alu-PCR products were generated from subclones positive or negative for Xp21.3-p22.2 markers, and comparison of the ethidium bromide patterns between sister subclones facilitated identification of bands likely to map to particular regions; in contrast, subclones that shared markers but were derived from independent lines showed no overlap in ethidium bromide pattern. All Alu-PCR products from one subclone, 50K-19E, in which only three closely linked markers were detected (DXS41, DXS208, DXS274) were mapped back to their region of origin. Of 28 products, 15 mapped to Xp21.2-p22.2, and these make up a new set of regionally assigned markers. However, the mapping data identified four separate Xp fragments in 50K-19E, only one of which had been picked up by marker analysis. Mapping back gel-isolated Alu-PCR products from an irradiation hybrid prior to any cloning or screening generates a comprehensive profile of the human DNA retained and permits rapid selection of sequences derived only from the region of interest.