Autosomal-recessive hypophosphatemic rickets is associated with an inactivation mutation in the ENPP1 gene.

Human disorders of phosphate (Pi) handling and hypophosphatemic rickets have been shown to result from mutations in PHEX, FGF23, and DMP1, presenting as X-linked recessive, autosomal-dominant, and autosomal-recessive patterns, respectively. We present the identification of an inactivating mutation in the ecto-nucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1) gene causing autosomal-recessive hypophosphatemic rickets (ARHR) with phosphaturia by positional cloning. ENPP1 generates inorganic pyrophosphate (PPi), an essential physiologic inhibitor of calcification, and previously described inactivating mutations in this gene were shown to cause aberrant ectopic calcification disorders, whereas no aberrant calcifications were present in our patients. Our surprising result suggests a different pathway involved in the generation of ARHR and possible additional functions for ENPP1.

Loss-of-function ENPP1 mutations cause both generalized arterial calcification of infancy and autosomal-recessive hypophosphatemic rickets.

The analysis of rare genetic disorders affecting phosphate homeostasis led to the identification of several proteins that are essential for the renal regulation of phosphate homeostasis; for example, fibroblast growth factor 23 (FGF23), which inhibits renal phosphate reabsorption and 1,25-dihydroxyvitamin D synthesis. Here, we report presumable loss-of-function mutations in the ENPP1 gene (ectonucleotide pyrophosphatase/phosphodiesterase) in members of four families affected with hypophosphatemic rickets. We provide evidence for the conclusion that ENPP1 is the fourth gene-in addition to PHEX, FGF23, and DMP1-that, if mutated, causes hypophosphatemic rickets resulting from elevated FGF23 levels. Surprisingly, ENPP1 loss-of-function mutations have previously been described in generalized arterial calcification of infancy, suggesting an as yet elusive mechanism that balances arterial calcification with bone mineralization.

Aberrant Phex function in osteoblasts and osteocytes alone underlies murine X-linked hypophosphatemia.

Patients with X-linked hypophosphatemia (XLH) and the hyp-mouse, a model of XLH characterized by a deletion in the Phex gene, manifest hypophosphatemia, renal phosphate wasting, and rickets/osteomalacia. Cloning of the PHEX/Phex gene and mutations in affected patients and hyp-mice established that alterations in PHEX/Phex expression underlie XLH. Although PHEX/Phex expression occurs primarily in osteoblast lineage cells, transgenic Phex expression in hyp-mouse osteoblasts fails to rescue the phenotype, suggesting that Phex expression at other sites underlies XLH. To establish whether abnormal Phex in osteoblasts and/or osteocytes alone generates the HYP phenotype, we created mice with a global Phex knockout (Cre-PhexDeltaflox/y mice) and conditional osteocalcin-promoted (OC-promoted) Phex inactivation in osteoblasts and osteocytes (OC-Cre-PhexDeltaflox/y). Serum phosphorus levels in Cre-PhexDeltaflox/y, OC-Cre-PhexDeltaflox/y, and hyp-mice were lower than those in normal mice. Kidney cell membrane phosphate transport in Cre-PhexDeltaflox/y, OC-Cre-PhexDeltaflox/y, and hyp-mice was likewise reduced compared with that in normal mice. Abnormal renal phosphate transport in Cre-PhexDeltaflox/y and OC-Cre-PhexDeltaflox/y mice was associated with increased bone production and serum FGF-23 levels and decreased kidney membrane type IIa sodium phosphate cotransporter protein, as was the case in hyp-mice. In addition, Cre-PhexDeltaflox/y, OC-Cre-PhexDeltaflox/y, and hyp-mice manifested comparable osteomalacia. These data provide evidence that aberrant Phex function in osteoblasts and/or osteocytes alone is sufficient to underlie the hyp-mouse phenotype.

Novel PHEX gene mutation associated with X linked hypophosphatemic rickets.

INTRODUCTION: X-linked hypophosphatemia (XLH) is characterized by renal phosphate wasting with hypophosphatemia, short stature, and rachitic manifestations. CLINICAL PICTURE: We describe a novel nonsense mutation in exon 3 of the PHEX gene (Glu(96)X (c.286G>T) causing XLH in a mother and daughter of Indian ancestry. The mother was noted to have concomitant vitamin D insufficiency. CONCLUSION: Our report identifies a novel nonsense mutation in the PHEX gene causing XLH. It also highlights the fact that the presence of concomitant vitamin D insufficiency should not preclude the diagnosis of familial forms of hypophosphatemic rickets, especially if more than one family member is affected.

PHEX analysis in 118 pedigrees reveals new genetic clues in hypophosphatemic rickets.

Familial hypophosphatemic rickets is a rare disease, which is mostly transmitted as an X-linked dominant trait, and mutations on the phosphate regulating gene with homologies to endopeptidases on the X-chromosome (PHEX) gene are responsible for the disease in most familial cases. In this study we analyzed PHEX in a large cohort of 118 pedigrees representing 56 familial cases and 62 sporadic cases. The high-resolution melting curves technique was tested as a screening method, along with classical sequencing. PHEX mutations have been found in 87% of familial cases but also in 72% of sporadic cases. Missense mutations were found in 16 probands, two of which being associated with other PHEX mutations resulting into truncated proteins. By plotting missense mutations described so far on a 3D model of PHEX we observed that these mutations focus on two regions located in the inner part of the PHEX protein. Family members of 13 sporadic cases were analyzed and a PHEX mutation was detected in one of the apparently healthy mother. These results highlight the major role of PHEX in X-linked dominant hypophosphatemic rickets, and give new clues regarding the genetic analysis of the disease. A screening of the different family members should be mandatory when a PHEX mutation is assessed in a sporadic case and the search for another PHEX mutation should be systematically proceed when facing a missense mutation.

A Case of Vitamin-D-Dependent Rickets Type 1A with Normal 1,25-Dihydroxyvitamin D Caused by Two Novel Mutations of the CYP27B1 Gene.

BACKGROUND: Vitamin-D-dependent rickets 1A (VDDR-1A) is caused by mutations of the renal CYP27B1 gene and is a rare form of rickets. Herein, we report a 20-month-old toddler who presented with inability to walk and failure to thrive. The clinical, biochemical and radiological findings were consistent with a diagnosis of rickets, specifically of the genetic type due to increased 25-OH vitamin D stores. METHODS AND RESULTS: Our patient was a compound heterozygote with 2 novel mutations: c.242G>A(p.Gly81Glu) and c.1144C>G (p.Pro382Ala) in the CYP27B1 gene. Analysis of both mutations with in silico models predicted a deleterious effect on 25-OH vitamin D 1α-hydroxylase function. Interestingly, the levels of 1,25-(OH)2 vitamin D were within normal limits. Our patient was initiated on 1α-hydroxyvitamin D (alfacalcidol) and supplemental calcium. Monitoring of bone metabolism showed a normalization of all bone metabolism serum indices after 3 months of therapy and, thereafter, only alfacalcidol was given at a maintenance dose. The clinical follow-up showed a dramatic improvement in musculoskeletal activity, and the patient regained acceleration in height and weight appropriate for his age. CONCLUSION: This rare case report of VDDR-1A with normal levels of 1,25-(OH)2 vitamin D enhances our awareness for this type of rickets in clinical practice.

A novel pathogenic mutation of the CYP27B1 gene in a patient with vitamin D-dependent rickets type 1: a case report.

BACKGROUND: Rickets can occur due to Vitamin D deficiency or defects in its metabolism. Three rare genetic types of rickets with different alterations of genes have been reported, including: Vitamin D dependent rickets type 1, Vitamin D dependent rickets type 2 or also known as Vitamin D resistant rickets and 25 hydroxylase deficiency rickets. Vitamin D dependent rickets type 1 is inherited in an autosomal recessive pattern, and is caused by mutations in the CYP27B1 gene encoding the 1α-hydroxylase enzyme. We report here a new mutation in CYP27B1, which lead to Vitamin D dependent rickets type 1. CASE PRESENTATION: We report on a 13-month-old Arabic Saudi girl with Vitamin D dependent rickets type 1 presented with multiple fractures and classic features of rickets. A whole exome sequencing identified a novel pathogenic missense mutation (CYP27B1:Homozygous c.1510C > T(p.Q504X)) which results in a protein truncating alteration. Both parents are heterozygous carriers of the mutation. Based on data search in Human Gene Mutation Database, 63 CYP27B1 alterations were reported: only 28.6% are protein truncating (5 nonsense, 13 frameshift insertions/deletions, 0 gross deletions), while 61.9% are non-truncating (38 missense, 1 small in-frame insertions/deletion), and 9.5% are possible protein-truncating (5 splice, 1 regulatory). CONCLUSION: The deleterious effect of this alteration, which was the only mutation detected in the CYP27B1 common gene of Vitamin D dependent rickets type 1 in the proband, and its autosomal recessive inheritance fashion, both support a pathogenic nature of this mutation as the cause of Vitamin D dependent rickets type 1.

A Phex mutation in a murine model of X-linked hypophosphatemia alters phosphate responsiveness of bone cells.

Mutations in the PHEX gene cause X-linked hypophosphatemia (XLH). Hypophosphatemia in XLH results from increased circulating levels of a phosphaturic hormone, fibroblast growth factor 23 (FGF23), which inhibits renal phosphate reabsorption and 1,25-dihydroxyvitamin D (calcitriol) synthesis. The current standard therapy for XLH--high-dose phosphate and calcitriol--further increases FGF23 concentrations, suggesting that patients with XLH may have an altered response to extracellular phosphate. To test for the presence of abnormal phosphate responsiveness, we compared serum biochemistries and femoral Fgf23 mRNA expression between wild-type mice, murine models of XLH (Phex(K496X)) and hyperphosphatemic tumoral calcinosis (Galnt3(-/-)), and Galnt3/Phex double-mutant mice. Phex mutant mice had not only increased Fgf23 expression but also reduced proteolytic cleavage of intact Fgf23 protein, resulting in markedly elevated intact Fgf23 levels and consequent hypophosphatemia. In contrast, despite markedly increased Fgf23 expression, Galnt3 knockout mice had significantly high proteolytic cleavage of Fgf23 protein, leading to low intact Fgf23 concentrations and hyperphosphatemia. Galnt3/Phex double-mutant mice had an intermediate biochemical phenotype between wild-type and Phex mutant mice, including slightly elevated intact Fgf23 concentrations with milder hypophosphatemia. Despite the hypophosphatemia, double-mutant mice attempted to reduce serum phosphate back to the level of Phex mutant mice by upregulating Fgf23 expression as much as 24-fold higher than Phex mutant mice. These data suggest that Phex mutations alter the responsiveness of bone cells to extracellular phosphate concentrations and may create a lower set point for "normal" phosphate levels.

Novel PHEX nonsense mutation in a patient with X-linked hypophosphatemic rickets and review of current therapeutic regimens.

INTRODUCTION: The most common form of familial hypophosphatemic rickets is X-linked. PHEX has been identified as the gene defective in this phosphate wasting disorder leading to decreased renal phosphate reabsorption, hypophosphatemia and inappropriate concentrations of 1,25-dihydroxyvitamin D in regard to hypophosphatemia. Clinical manifestation are skeletal deformities, short stature, osteomalacia, dental abscesses, bone pain, and loss of hearing. SUBJECTS AND METHODS: We report 3 cases of hypophosphatemic rickets with genetic mutational analysis of the PHEX gene. In 1 male patient an unknown nonsense mutation was found in exon 7, codon 245 (c.735T>G, Tyr245Term, Y245X). In both female patients known mutations were found: c.682delTC (exon 6, codon 228) and c.1952G>C (exon 19, codon 651, R651P). Age at diagnosis ranged from early childhood to the age of 35 years. Clinical complications were hip replacement in 1 patient, mild nephrocalcinosis in 2 patients and loss of hearing in 1 patient. All 3 patients have been treated with phosphate supplements and receive 1,25-dihydroxyvitamin D. Under this regimen all patients show stable biochemical markers with slight hyperparathyreoidism. In all patients at least one family member is affected by rickets, as well. CONCLUSIONS: We report a novel nonsense mutation of PHEX that has not been identified so far. The recent discovery of FGF23 and MEPE has changed our understanding of the kidney-bone metabolism, but also raises concerns about the efficacy of current therapeutic regimens that are reviewed in this context.

A patient with hypophosphatemic rickets and ossification of posterior longitudinal ligament caused by a novel homozygous mutation in ENPP1 gene.

X-linked hypophosphatemic rickets/osteomalacia (XLH), autosomal dominant hypophosphatemic rickets/osteomalacia (ADHR) and autosomal recessive hypophosphatemic rickets/osteomalacia (ARHR1 or ARHR2) are hereditary fibroblast growth factor 23 (FGF23)-related hypophosphatemic rickets showing similar clinical features. We here show a patient with hypophosphatemic rickets and widespread ossification of posterior longitudinal ligament (OPLL). The proband is a 62-year-old female. Her parents are first cousins and showed no signs of rickets or osteomalacia. She showed hypophosphatemic rickets with elevated FGF23 level and had been clinically considered to be suffering from XLH. However, direct sequencing of all coding exons and exon-intron junctions of phosphate regulating gene with homologies to endopeptidases on the X chromosome (PHEX), FGF23 and dentin matrix protein 1 (DMP1) genes, responsible genes for XLH, ADHR and ARHR1, respectively, showed no mutation. A novel homozygous splice donor site mutation was found at the exon-intron junction of exon 21 of ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1) gene responsible for ARHR2 (IVS21+1_3(GTA>CACC)). Subsequent analysis of mRNA revealed that this mutation caused skipping of exon 21 which created a premature stop codon in exon 22. These results indicate that genetic analysis is mandatory for the correct diagnosis of hereditary FGF23-related hypophosphatemic rickets. Because Enpp1 knockout mouse is a model of OPLL, this case also suggests that OPLL is associated with ARHR2.

Hereditary vitamin D-resistant rickets (HVDRR) owing to a heterozygous mutation in the vitamin D receptor.

Hereditary vitamin D-resistant rickets (HVDRR) is a rare autosomal recessive disease caused by mutations in the vitamin D receptor (VDR). Patients exhibit severe rickets and hypocalcemia. Heterozygous parents and siblings appear normal and exhibit no symptoms of the disease. We analyzed the VDR gene of a young girl who exhibited the clinical features of HVDRR without alopecia. The patient had clinical and radiographic features of rickets, hypocalcemia, and elevated serum concentrations of 1,25-dihydroxyvitamin D [1,25(OH)(2)D]. A single heterozygous missense mutation was found in the VDR gene that substituted glutamic acid with alanine at amino acid 420 (E420A). Sequencing of the girl's VDR cDNAs showed that the f/M1 allele contained the E420A mutation, whereas the F/M4 allele was completely normal. The girl's father, who was also heterozygous for the E420A mutation on the f/M1 allele, exhibited minor symptoms of vitamin D resistance. In contrast, the mother had no signs of the disease and had no mutations in her VDR gene. Both the girl and the father's skin fibroblasts showed resistance to 1,25(OH)(2)D(3) by their severely reduced induction of CYP24A1 gene expression. In transactivation assays, the E420A mutant VDR showed dominant-negative activity towards the wild-type VDR. This is the first report that we are aware of describing a patient with HVDRR caused by a single heterozygous missense mutation in the VDR gene. The E420A mutant appears to act in a dominant-negative fashion, silencing the wild-type VDR and resulting in an attenuated response to 1,25(OH)(2)D(3).

A novel Phex mutation with defective glycosylation causes hypophosphatemia and rickets in mice.

N-ethyl-N-nitrosourea (ENU) mutagenesis is a phenotype-driven approach with potential to assign function to every locus in the mouse genome. In this article, we describe a new mutation, Pug, as a mouse model for X-linked hypophosphatemic rickets (XLH) in human. Mice carrying the Pug mutation exhibit abnormal phenotypes including growth retardation, hypophosphatemia and decreased bone mineral density (BMD). The new mutation was mapped to X-chromosome between 65.4 cM and 66.6 cM, where Phex gene resides. Sequence analysis revealed a unique T-to-C transition mutation resulting in Phe-to-Ser substitution at amino acid 80 of PHEX protein. In vitro studies of Pug mutation demonstrated that PHEX(pug) was incompletely glycosylated and sequestrated in the endoplasmic reticulum region of cell, whereas wild-type PHEX could be fully glycosylated and transported to the plasma membrane to exert its function as an endopeptidase. Taken together, the Pug mutant directly confirms the role of Phex in phosphate homeostasis and normal skeletal development and may serves as a new disease model of human hypophosphatemic rickets.