What is HR ( Hypophosphatemic Rickets ) ?HR including …Hypo : means low or less and a few amount ofsomethingPhosphate (Pi) : it is one of vital elements in the bodyRickets : is a softening of bones in children due todeficiency or impaired metabolism of vitaminD, Phosphorus or Calcium, potentially leading tofractures and deformity. Rickets is among the mostfrequent childhood diseases in many developingcountries.
Then :HR means lack of Phosphate (Pi) thatleads to Rickets.Phosphate is the major stimulus for1 α Hydroxylase.
Phosphate metabolismPhosphates are absorbed from foodstuff. Specialized channel proteins called sodium-phosphate transporters 2b (or NaPi2b) located at the surface of the epithelial cells of thesmall intestine are performing the task. About 1.5 g of phosphate are captured daily bythis process by a normal adult.
Once in the bloodstream, phosphates can be absorbed by organs and tissues. Apart of it is stored in bones.Then, phosphates reach the kidneys where most of it is filtered out of the blood.But before being eliminated in the urine, another channel protein (NaPi2a)similar to the one that capture phosphate from foodstuff bring it back tobloodstream in a process called reabsorption. This steps of filtration andreabsorption taking place in the kidney are crucial for the maintenance ofphosphate levels.
Regulation of phosphate metabolism - where it occursBasically, regulation of bodys phosphate level can occur at different levels:-at the site of entry (absorption): it is accomplished by modulating the numberof NaPi2b channels. this mechanism is under regulation by calcitriol (1,25)-OHvitamin D3 (or 1,25-D3).-at different sites of use inside the body. Organs and tissue will fishphosphates in the bloodstream to fulfill their needs. Bone is a heavy consumerof phosphates both for maintenance of the structure and for the activity of theircells. Parathyroid is a special organ sensitive to the calcium (and phosphorus)levels. When calcium rises in the blood, parathyroid will release the hormonePTH.-at the site of elimination. In the kidneys, phosphate are first filtered out of thesystem (filtration). Then, a specialized mechanism called reabsorption occurs:phosphates are again filtered out of the urine and reinjected into thebloodstream. this is a major source of regulation of the blood phopshate.NaPi2a channels of the proximal tubules are doing this job. Napi2a is under theregulation of calcitriol (1,25-D3) and of FGF23.
the main regulators of phosphate are 1,25-D3, PTH and FGF23. FGF23 is synthesized bybone cells called Osteocytes and released in the bloodstream. It serves as a signal that isunderstood only by kidney cells (those posessing the appropriate receptor calledFGFR1-Klotho) . By an unknown mechanism, FGF 23 is inhibited by PHEX. We will seemore of the regulatory mechanism in the next section below.
Regulation of phosphate metabolism - how it occurs?
We will look at little bit a the interplay between the different actors of thephosphate regulation. In the figure , red arrows means a positive effect (upregulation, stimulation) and obviously blue arrows means a downregulation(inhibition).Starting from bone cells: the protein PHEX (mutated in XLH) exerts adowregulation on the synthesis of FGF23. So, the level of FGF23 in the blood ismoderate. FGF23 is sensed by kidney cells posessing the co-receptorfor FGF23. This triggers a signal resulting in downregualtion of 1,25-D3 andNaPi2a. 1,25-D3 is stimulating intestinal absorption of phosphate (via NaPi2b)and NaPi2a is responsible for the re-aborption of phosphate in the kidney. Butsince the FGF23 level is moderate, so is the phosphate wasting signal andhence, phosphate levels stay balanced.In XLH, PHEX is mutated and cannot exert its influence on FGF23. As aconsequence, there is an overproduction of FGF23. So, the phosphate wastingsignal is enhanced and this results in the main symptoms ofHypophosphatemia.
Vitamin D Metabolism
Phosphate is the major stimulus for 1αhydroxylase. Reduces 1 α hydroxylation of 25 hydroxy vitamin Dleads to low concentration of 1,25-di hydroxy vitamin D
Hypophosphatemic Rickets• X-linked dominant / Autosomal dominant• Males affected more than females• Commonest inherited form of Rickets• Prevalence 1: 25000• Phosphate wasting by renal tubules leads to:– Low serum phosphate– Normal calcium• In-appropriate low or normal 1,25-di hydroxyvitamin D• Severe rickets and short stature by 1-2 years
Which Genes are responsible of HR and I am going to explore them?1. PHEX: Phosphate regulating Endopeptidase Homolog, X-linked is located on Xp22.2-p22.1 and it has 22 exons, total annotated spliced Exon length 2861 bp. Mutations in PHEX lead to increased serum levels of FGF23.2. FGF23: Fibroblast Growth Factor 23 is located on 12p13.3 and it has 3 exons, total annotated spliced exon length 3018 bp. FGF23 causes downregulation of the renal 1 α hydroxylase enzyme and upregulation of the 24 hydroxylase enzyme leading to impaired 1,25(OH)2 vitamin D synthesis and increased degradation. This dual defect in phosphate metabolism leads to poor bone mineralization and fractures.3. DMP1: Dentin Matrix Acidic Phosphoprotein 1 is located on 4q21 and it has 6 exons, total annotated spliced Exon length 2687 bp. DMP1 is an extracellular matrix protein and a member of the small integrin binding ligand N-linked glycoprotein family. Mutations in the gene are known to cause autosomal recessive hypophosphatemia, a disease that manifests as rickets and osteomalacia.
Using a DNA extraction kit (Qiagen DNAMicro, Qiagen, Germany). All 22 exons and theirexon-intron boundaries of the PHEX gene and 3exons of the FGF23 gene and their correspondingintron-exon boundaries and 6 exones of DMP1 andtheir exon-intron boundaries will be amplified byPCR and direct sequencing will be carried out tofind the mutations.
After sequencing mutations listed in the database includemissense and nonsense mutations, deletions, insertions,frameshift, and splice site mutations as well as polymor-phisms. The ethnic distribution of the mutations is verywidespread and includes German, Belgian, Swiss, Spanish,Italian, French, French-Scottish, Balkan, Polish, SaudiArabian, Southeast Asian, African-American, Indian, andFinnish populations. Only a few mutations have beenreported in Chinese.
1. Xia W, Meng X, Jiang Y, Li M, Xing X, Pang L, Wang O, Pei Y, Yu Ly, Sun Y, Hu Y, Zhou X. Three novel mutations of the PHEX gene in three Chinese families with X-linked dominant hypophosphatemic rickets. 2007;81:415-4202. XLH Research 2012 Phosphate Metabolism (http://www.xlhresearch.net)3. Harpers illustrated Biochemistry -28th Edition (2000)