X-linked Hypophosphatemia Rickets Research Articles

CRISPR/Cas9-mediated mutation of PHEX in rabbit recapitulates human X-linked hypophosphatemia (XLH).

X-linked hypophosphatemia (XLH) is the most common cause of inheritable rickets, with an incidence of 1/20 000 in humans. Inactivation or mutation of the gene PHEX, a phosphate-regulating endopeptidase, leads to hypophosphatemia and defective bone mineralization in XLH patients. Presently, there is no adequate animal model for safety assessments of physiotherapies and drug screening for XLH rickets. In this study, an XLH model was generated via PHEX gene knockout (KO) through coinjection of clustered regularly interspaced short palindromic repeats (CRISPR)-associated protein 9 (Cas9)/sgRNA mRNA into rabbit zygotes. The typical phenotypes of growth retardation, hypophosphatemia, elevated serum FGF23 and bone mineralization were observed in the PHEX KO rabbits but not in normal controls. In summary, for the first time, we have successfully obtained PHEX KO rabbits and recapitulated human XLH using the CRISPR/Cas9 system. This novel XLH rabbit model could be utilized as a drug screening model for XLH prevention and preclinical therapy.

FGF23-FGF Receptor/Klotho Pathway as a New Drug Target for Disorders of Bone and Mineral Metabolism.

Fibroblast growth factor 23 (FGF23) is a phosphaturic hormone produced by bone and works by binding to Klotho-FGF receptor complex. Excessive and deficient actions of FGF23 result in hypophosphatemic and hyperphosphatemic diseases, respectively. Therefore, it is reasonable to think that modulating FGF23 activities may be a novel therapeutic measure for these diseases. Several preclinical reports indicate that the inhibition of FGF23 activities ameliorates hypophosphatemic rickets/osteomalacia caused by excessive actions of FGF23. In addition, phase I-II clinical trials of anti-FGF23 antibody in adult patients with X-linked hypophosphatemia rickets, the most prevalent cause of genetic FGF23-related hypophosphatemic rickets, indicated that the antibody enhances renal tubular phosphate reabsorption and increases serum phosphate. However, it is not known whether the inhibition of FGF23 activities actually brings clinical improvement of rickets and osteomalacia. Available data indicate that FGF23-FGF receptor/Klotho pathway can be a new drug target for disorders of phosphate and bone metabolism.

Prenatal Diagnosis for a Novel Splice Mutation of PHEX Gene in a Large Han Chinese Family Affected with X-Linked Hypophosphatemic Rickets

X-linked hypophosphatemia (XLH) is the most common form of heritable rickets characterized by X-linked dominant inheritance, renal phosphate wasting, hypophosphatemia, and defective bone mineralization. Inactivating mutations of the PHEX gene located at Xp22.1 have been linked with this disease. Ethnic distribution of such mutations seems widespread but only a few mutations in the Chinese population have been reported to date. We report on a large Han Chinese family affected with XLH rickets, which included 13 patients from four generations. Polymerase chain reaction and direct sequencing were performed for all exons and intron-exon boundaries of the PHEX gene. The effect of nucleotide changes was analyzed using bioinformatic software. Prenatal diagnosis was performed on umbilical cord blood at the 20th gestational week. A novel G-->A splice mutation in intron 7 (c.849+1G>A) was identified in all patients from the family. As confirmed by reverse-transcription (RT)-polymerase chain reaction (PCR), the mutation has rendered loss of a normal splice donor site (c.849+1G) while activating a cryptic one at c.849+519G, which resulted in addition of 518 nucleotides to the mature RNA. Prenatal diagnosis had excluded the fetus for carrying the same mutation. A healthy boy was born later. A novel splice mutation c.849+1G>A in the PHEX gene is responsible for XLH in the studied family. Further studies may enhance our understanding of the role of this mutation in the pathogenesis of XLH.

X-Linked Hypophosphatemia: New Horizons

X-linked hypophosphatemia (XLH) is a hypophosphatemic rachitic disorder, which occurs in one of 20,000 live births. The disease phenotype is quite variable, and many patients do not have profound signs or symptoms including rickets. Thus, XLH is sometimes called "XLH rickets without rickets". Such phenotypic variability makes a genetic examination of potentially affected patients inevitable for definite diagnosis. In most of the patients, hypophosphatemia, elevated serum alkaline phosphatase, and a lowered level of active vitamin D, as well as abnormalities in the bone mineralization, are seen. In hyp-mice, the model mouse of XLH, a hormonal abnormality that affects the Npt2 protein mediates the abnormal Na-dependent Pi transport in the renal tubules. The defect in vitamin D metabolism is the result of diminished 25(OH)D-1α-hydroxylase activity due to a post-transcriptional abnormality. There is a genetic abnormality of the PHEX gene causal of XLH. PHEX codes a protein from the cell membrane-bound endopeptidase family, which causes hypophosphatemia by decreasing the Npt2 in the kidneys; however, the localization of PHEX is limited to the bones and teeth. To clarify the pathophysiology of XLH, therefore, identification of the mechanism by which the diseased gene functions is urgently required.