Fibroblast growth factor 23 (FGF-23) is a protein and member of the fibroblast growth factor (FGF) family which participates in the regulation of phosphate in plasma and vitamin D metabolism. In humans it is encoded by the FGF23 gene. FGF-23 decreases reabsorption of phosphate in the kidney. Mutations in FGF23 can lead to its increased activity, resulting in autosomal dominant hypophosphatemic rickets.
Fibroblast growth factor 23 (FGF23) is a protein which in humans is encoded by the FGF23 gene. [5] FGF23 is a member of the fibroblast growth factor (FGF) family which participates in phosphate and vitamin D metabolism and regulation. [6] [7]
FGF23´s main function is to regulate the phosphate concentration in plasma. It does this by decreasing reabsorption of phosphate in the kidney, which means phosphate is excreted in urine. FGF23 is secreted by osteocytes in response to increased calcitriol and phosphate. [8] [9] [10] [11] FGF23 acts on the kidneys by decreasing the expression of NPT2, a sodium-phosphate cotransporter in the proximal tubule. [12]
FGF23 may also suppress 1-alpha-hydroxylase, reducing its ability to activate vitamin D and subsequently impairing calcium absorption. [7] [13]
In humans FGF23 is encoded by the FGF23 gene, which is located on chromosome 12 and is composed of three exons. The gene was identified by its mutations associated with autosomal dominant hypophosphatemic rickets. [14]
Mutations in FGF23, which render the protein resistant to proteolytic cleavage, lead to its increased activity and to renal phosphate loss, in the human disease autosomal dominant hypophosphatemic rickets.
FGF23 can also be overproduced by some types of tumors, such as the benign mesenchymal neoplasm phosphaturic mesenchymal tumor causing tumor-induced osteomalacia, a paraneoplastic syndrome. [15] [16]
Loss of FGF23 activity is thought to lead to increased phosphate levels and the clinical syndrome of familial tumor calcinosis. Mice lacking either FGF23 or the klotho enzyme age prematurely due to hyperphosphatemia. [17]
Over-expression of FGF23 has been associated with cardiovascular disease in chronic kidney disease including cardiomyocyte hypertrophy, vascular calcification, stroke, and endothelial dysfunction. [18]
FGF23 expression and cleavage is promoted by iron deficiency and inflammation. [19]
FGF23 is associated with at least 7 non-nutritional diseases of hypophosphatemia: aside from autosomal dominant hypophosphatemic rickets, X-linked hypophosphatemia, autosomal recessive hypophosphatemic rickets type 1, 2, and 3, Tumor-induced osteomalacia and Hypophosphatemic rickets with hypercalciuria. [18]
Prior to its discovery in 2000, it was hypothesized that a protein existed which performed the functions subsequently shown for FGF23. This putative protein was known as phosphatonin. [20] Several types of effects were described including impairment of sodium dependent phosphate transport in both intestinal and renal brush border membrane vesicles, inhibition of production of calcitriol, stimulation of breakdown of calcitriol, and inhibition of production/secretion of parathyroid hormone.
Osteomalacia is a disease characterized by the softening of the bones caused by impaired bone metabolism primarily due to inadequate levels of available phosphate, calcium, and vitamin D, or because of resorption of calcium. The impairment of bone metabolism causes inadequate bone mineralization.
Hypophosphatemia is an electrolyte disorder in which there is a low level of phosphate in the blood. Symptoms may include weakness, trouble breathing, and loss of appetite. Complications may include seizures, coma, rhabdomyolysis, or softening of the bones.
Calcitriol is a hormone and the active form of vitamin D, normally made in the kidney. It is also known as 1,25-dihydroxycholecalciferol. It binds to and activates the vitamin D receptor in the nucleus of the cell, which then increases the expression of many genes. Calcitriol increases blood calcium mainly by increasing the uptake of calcium from the intestines.
Renal osteodystrophy is currently defined as an alteration of bone morphology in patients with chronic kidney disease (CKD). It is one measure of the skeletal component of the systemic disorder of chronic kidney disease-mineral and bone disorder (CKD-MBD). The term "renal osteodystrophy" was coined in 1943, 60 years after an association was identified between bone disease and kidney failure.
Klotho is an enzyme that in humans is encoded by the KL gene. The three subfamilies of klotho are α-klotho, β-klotho, and γ-klotho. α-klotho activates FGF23, and β-klotho activates FGF19 and FGF21. When the subfamily is not specified, the word "klotho" typically refers to the α-klotho subfamily, because α-klotho was discovered before the other members.
Renal tubular acidosis (RTA) is a medical condition that involves an accumulation of acid in the body due to a failure of the kidneys to appropriately acidify the urine. In renal physiology, when blood is filtered by the kidney, the filtrate passes through the tubules of the nephron, allowing for exchange of salts, acid equivalents, and other solutes before it drains into the bladder as urine. The metabolic acidosis that results from RTA may be caused either by insufficient secretion of hydrogen ions into the latter portions of the nephron or by failure to reabsorb sufficient bicarbonate ions from the filtrate in the early portion of the nephron. Although a metabolic acidosis also occurs in those with chronic kidney disease, the term RTA is reserved for individuals with poor urinary acidification in otherwise well-functioning kidneys. Several different types of RTA exist, which all have different syndromes and different causes. RTA is usually an incidental finding based on routine blood draws that show abnormal results. Clinically, patients may present with vague symptoms such as dehydration, mental status changes, or delayed growth in adolescents.
Tertiary hyperparathyroidism is a condition involving the overproduction of the hormone, parathyroid hormone, produced by the parathyroid glands. The parathyroid glands are involved in monitoring and regulating blood calcium levels and respond by either producing or ceasing to produce parathyroid hormone.
Fibroblast growth factors (FGF) are a family of cell signalling proteins produced by macrophages; they are involved in a wide variety of processes, most notably as crucial elements for normal development in animal cells. Any irregularities in their function lead to a range of developmental defects. These growth factors typically act as systemic or locally circulating molecules of extracellular origin that activate cell surface receptors. A defining property of FGFs is that they bind to heparin and to heparan sulfate. Thus, some are sequestered in the extracellular matrix of tissues that contains heparan sulfate proteoglycans and are released locally upon injury or tissue remodeling.
Autosomal dominant hypophosphatemic rickets (ADHR) is a rare hereditary disease in which excessive loss of phosphate in the urine leads to poorly formed bones (rickets), bone pain, and tooth abscesses. ADHR is caused by a mutation in the fibroblast growth factor 23 (FGF23). ADHR affects men and women equally; symptoms may become apparent at any point from childhood through early adulthood. Blood tests reveal low levels of phosphate (hypophosphatemia) and inappropriately normal levels of vitamin D. Occasionally, hypophosphatemia may improve over time as urine losses of phosphate partially correct.
X-linked hypophosphatemia (XLH) is an X-linked dominant form of rickets that differs from most cases of dietary deficiency rickets in that vitamin D supplementation does not cure it. It can cause bone deformity including short stature and genu varum (bow-leggedness). It is associated with a mutation in the PHEX gene sequence (Xp.22) and subsequent inactivity of the PHEX protein. PHEX mutations lead to an elevated circulating (systemic) level of the hormone FGF23 which results in renal phosphate wasting, and local elevations of the mineralization/calcification-inhibiting protein osteopontin in the extracellular matrix of bones and teeth. An inactivating mutation in the PHEX gene results in an increase in systemic circulating FGF23, and a decrease in the enzymatic activity of the PHEX enzyme which normally removes (degrades) mineralization-inhibiting osteopontin protein; in XLH, the decreased PHEX enzyme activity leads to an accumulation of inhibitory osteopontin locally in bones and teeth to block mineralization which, along with renal phosphate wasting, both cause osteomalacia and odontomalacia.
Phosphate-regulating endopeptidase homolog X-linked also known as phosphate-regulating gene with homologies to endopeptidases on the X chromosome or metalloendopeptidase homolog PEX is an enzyme that in humans is encoded by the PHEX gene. This gene contains 18 exons and is located on the X chromosome.
Fibroblast growth factor receptor 1 (FGFR-1), also known as basic fibroblast growth factor receptor 1, fms-related tyrosine kinase-2 / Pfeiffer syndrome, and CD331, is a receptor tyrosine kinase whose ligands are specific members of the fibroblast growth factor family. FGFR-1 has been shown to be associated with Pfeiffer syndrome, and clonal eosinophilias.
Matrix extracellular phosphoglycoprotein is a protein that in humans is encoded by the MEPE gene. A conserved RGD motif is found in this protein, and this is potentially involved in integrin recognition.
Sodium-dependent phosphate transport protein 2C is a protein that in humans is encoded by the SLC34A3 gene.
Oncogenic osteomalacia, also known as tumor-induced osteomalacia or oncogenic hypophosphatemic osteomalacia, is an uncommon disorder resulting in increased renal phosphate excretion, hypophosphatemia and osteomalacia. It may be caused by a phosphaturic mesenchymal tumor. Symptoms typically include crushing fatigue, severe muscle weakness and brain fog due to the low circulating levels of serum phosphate.
Family with sequence similarity 20, member C also known as FAM20C or DMP4 is a protein which in humans is encoded by the FAM20C gene. Fam20C, a Golgi localized protein kinase, is a serine kinase that phosphorylates both casein and other highly acidic proteins and members of the small integrin-binding ligand, the N-linked glycoproteins (SIBLING) family at the target motif SerXGlu.
Tumoral calcinosis is a rare condition in which there is calcium deposition in the soft tissue in periarticular location, around joints, outside the joint capsule. They are frequently (0.5–3%) seen in patients undergoing renal dialysis. Clinically also known as hyperphosphatemic familial tumoral calcinosis (HFTC), is often caused by genetic mutations in genes that regulate phosphate physiology in the body. Best described genes that harbour mutations in humans are FGF-23, Klotho (KL), or GALNT3. A zebrafish animal model with reduced GALNT3 expression also showed HFTC-like phenotype, indicating an evolutionary conserved mechanism that is involved in developing tumoral calcinosis.
Fanconi syndrome or Fanconi's syndrome is a syndrome of inadequate reabsorption in the proximal renal tubules of the kidney. The syndrome can be caused by various underlying congenital or acquired diseases, by toxicity, or by adverse drug reactions. It results in various small molecules of metabolism being passed into the urine instead of being reabsorbed from the tubular fluid. Fanconi syndrome affects the proximal tubules, namely, the proximal convoluted tubule (PCT), which is the first part of the tubule to process fluid after it is filtered through the glomerulus, and the proximal straight tubule, which leads to the descending limb of loop of Henle.
Burosumab, sold under the brand name Crysvita, is a human monoclonal antibody medication approved 2018 for the treatment of X-linked hypophosphatemia and tumor-induced osteomalacia.
Phosphate diabetes is a rare, congenital, hereditary disorder associated with inadequate tubular reabsorption that affects the way the body processes and absorbs phosphate. Also named as X-linked dominant hypophosphatemic rickets (XLH), this disease is caused by a mutation in the X-linked PHEX gene, which encodes for a protein that regulates phosphate levels in the human body. phosphate is an essential mineral which plays a significant role in the formation and maintenance of bones and teeth, energy production and other important cellular processes. phosphate diabetes is a condition that falls under the category of tubulopathies, which refers to the pathologies of the renal tubules. The mutated PHEX gene causes pathological elevations in fibroblast growth factor 23 (FGF23), a hormone that regulates phosphate homeostasis by decreasing the reabsorption of phosphate in the kidneys.
This article incorporates text from the United States National Library of Medicine, which is in the public domain.
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