Phosphate diabetes explained

Phosphate diabetes is a rare, congenital, hereditary disorder associated with inadequate tubular reabsorption that affects the way the body processes and absorbs phosphate.[1] Also named as X-linked dominant hypophosphatemic rickets (XLH),[2] this disease is caused by a mutation in the X-linked PHEX (phosphate regulating endopeptidase X-linked) gene, which encodes for a protein that regulates phosphate levels in the human body.[3] 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.[4] phosphate diabetes is a condition that falls under the category of tubulopathies, which refers to the pathologies of the renal tubules.[5] The mutated PHEX gene causes pathological elevations in fibroblast growth factor 23 (FGF23),[1] a hormone that regulates phosphate homeostasis by decreasing the reabsorption of phosphate in the kidneys.[6]

Elevated levels of FGF23 in phosphate diabetes lead to an increase in phosphate excretion through urine, thus reducing the phosphate levels in blood. However, due to impaired activation of vitamin D, which plays a crucial role in increasing intestinal calcium and phosphate absorption,[7] patients with this disorder are unable to replenish the lost phosphate. This results in low absorption of phosphate from the gastrointestinal system, leading to a deficiency of phosphate in the body and disrupting the full calcium-phosphate metabolism process.

Signs and symptoms

Short stature

A common symptom of phosphate diabetes is short stature.[8]

Delayed growth and development are common symptoms of phosphate diabetes in children, resulting in stunted growth and a shorter stature compared to their peers. This symptom is typically one of the earliest indicators of the disorder and may require treatment with growth hormone therapy to promote normal growth and development.

Delayed walking

Children with phosphate diabetes may start to walk late (at the age of one and a half years and later) due to impaired bone development.[9]

Craniosynostosis

Children with phosphate diabetes may have a birth defect in which the bones in a baby's skull fuse together too early before the brain is fully formed. This is known as craniosynostosis that may lead to head deformities.

Dental problems

As phosphate is essential for the formation and maintenance of healthy teeth, phosphate diabetes can lead to a wide range of dental problems, including the formation of cavities, abscesses, and tooth decay.[10]

Muscle weakness

The deficiency of phosphate may affect muscles, resulting in muscle weakness and fatigue. Patients may have difficulties in performing physical activities and may require physical therapy to improve muscle strength and function.

Bowed legs

Due to phosphate deficiency, patients' bones in the legs may become fragile and brittle, which leads to a characteristic bowing of the legs.

Bone pain

In phosphate diabetes, the softening of the bones can lead to bone pain, especially in the knees, hips, and lower back.

Deformities of the bones (rickets)

In severe cases of phosphate diabetes, the deficiency of phosphate can lead to deformities of the bones, resulting in conditions like rickets and osteomalacia (softening of the bones which leads to frequent fractures) and kyphoscoliosis (curvature of the spine).

Pathophysiology

phosphate diabetes is caused by a genetic mutation in the PHEX gene located on the X chromosome. The PHEX gene encodes for an enzyme called PHEX – phosphate regulating endopeptidase X-linked, which is involved in the regulation of phosphate metabolism in the body.[11]

An occurrence of PHEX gene mutation can lead to an increase in levels of fibroblast growth factor 23 (FGF23),[12] which is a growth factor that regulates phosphate and vitamin D metabolism. Increased levels of FGF23 leads to increase renal phosphate excretion and decrease intestinal phosphate absorption:[13]

Renal phosphate Excretion

FGF23 acts on the kidneys to reduce the expression of sodium/phosphate co-transporters (NaPi-2a and NaPi-2c) in the proximal tubules.[14] As these co-transporters are responsible for reabsorbing phosphate from urine back into the bloodstream, a decrease in their expression would reduce the amount of phosphate being reabsorbed back to blood, hence increasing the phosphate concentration in the urine being excreted (hypophosphatemia).

Intestinal phosphate absorption

FGF23 acts on the intestines to reduce the expression of the sodium-phosphate co-transporter (NaPi-2b) in the brush border membrane of enterocytes,[15] which is an important site for nutrient absorption. This transporter facilitates the absorption of phosphate from digested food in the small intestines into the bloodstream. Therefore, reduced activity of the transporter would lower the amount of phosphate being absorbed into the blood, which in turn increases the amount of phosphate excreted in the faeces.

In addition, increased levels of FGF23 would affect vitamin D metabolism and inhibit the action of vitamin D.[16] Vitamin D needs to be converted into its activated form, 1,25-dihydroxyvitamin D, to perform its role of regulating calcium and phosphate absorption in the intestines.[17] A series of enzymatic reactions are required for the activation of vitamin D, and enzymes like 25-hydroxyvitamin D-1α-hydroxylase (CYP27B1) and 1,25-dihydroxyvitamin D-24-hydroxylase (CYP24A1) play an active role in these reactions.[18] However, high levels of FGF23 in blood hinders the activation of vitamin D:[19]

Inhibition of CYP27B1 activity

FGF23 inhibits the catalytic activity of CYP27B1 in activating vitamin D in the kidneys through a signalling pathway that involves the FGF receptor and downstream intracellular signalling molecules (e.g. FGFRs, MAPK, PI3K etc.).[20] This leads to a decreased levels of activated vitamin D (1,25-dihydroxyvitamin D), which lowers the activity of vitamin D and slows down the absorption of calcium and phosphate in the small intestines.

Stimulation of CYP24A1 activity

FGF23 stimulates the activity of CYP24A1 in breaking down the activated form of vitamin D.[21] As the availability of activated vitamin D in blood is decreased, the absorption of phosphate into bloodstream is hindered, which further intensifies the systemic phosphate deficiency in the patient's body.

Due to the increased phosphate loss through the excretion of urine and faeces, as well as the reduced absorption of phosphate into blood due to the reduced activity of vitamin D, patients' plasma phosphate levels become lower than normal. This results in a chronic systemic phosphate deficiency that may cause a variety of symptoms with varying degrees of intensity.

Genetics

Phosphate diabetes that results from mutations in the PHEX gene is an X-linked dominant disorder,[22] where the mutated gene is located on the X chromosome (one of the sex chromosomes). This inheritance trait is dominant, a single copy of the mutation from the parent is sufficient to cause the disorder in the child.[23]

As males have only one X chromosome (and one Y chromosome), while females have two X chromosomes, the inheritance of phosphate diabetes largely depends on the gender of the parent who carries the mutated gene. Affected fathers with phosphate diabetes are unable to pass the disease to their sons, but all of their daughters will be affected. In contrast, affected mothers with phosphate diabetes will pass the disease to half of their sons and half of their daughters statistically.[24] Thus, this disorder most often occurs in females.[25]

While phosphate diabetes is typically inherited through X-linked dominant inheritance, in some rare cases, the disorder may occur sporadically, meaning that there is no family history of the diseased condition.[26] This may happen due to a new mutation in the PHEX gene which arises during fetal development or due to other genetic factors.

Epidemiology

phosphate diabetes is a rare condition that affects approximately 1 in 20000-25000 individuals,[27] making it relatively difficult to study epidemiologically. However, advances in genetic testing and improved awareness of the condition have led to increased diagnosis rates in recent years.

While phosphate diabetes can affect individuals of any race or ethnicity,[28] it is more common in certain populations, such as those of European and Middle Eastern descent.[29]

Diagnosis

Consultation with doctors

When the patients' body appear symptoms of phosphate diabetes, they are recommended to go to the hospital for consultation and body check. Doctors specialised in endocrinology and orthopaedics can examine the patient's health condition, and prescribe suitable medicine or arrange referral for further checking.

Blood test

In phosphate diabetes patients' blood, the phosphate levels are level while calcium and parathyroid hormone (PTH) levels remain to be normal. Blood tests can be performed to measure if there are any abnormalities with the phosphate levels in blood.[30]

Urine Test

In the urine of phosphate diabetes patients, excess amount of phosphate can be detected due to the impaired reabsorption of phosphate in the kidneys. By testing for the concentration of phosphate in urine, whether the patient is suffering from phosphate diabetes can be determined.

X-ray scan

X-ray scans of bones can be useful for doctors to assess abnormalities in bone density and detect bone deformities,[31] such as the bowing of the legs, curvature of spines, which are the symptoms of phosphate diabetes.

Genetic Analysis

Patients with mutations in the PHEX gene usually possess phosphate diabetes. Through the genetic analysis of X chromosome(s) of patients, it can confirm a diagnosis of phosphate diabetes.[32] At the same time, other family members who are at risk of the disease can be identified.

Treatment

Prevention

Genetic screening test

Since phosphate diabetes is an inheritable condition, immediate genetic analysis should be performed on a child after birth if one of the parents has been diagnosed with the disorder during childhood. Earlier diagnosis of the disease can facilitate more effective treatments, hence minimising its impact on the child.

See also

Notes and References

  1. Laroche M, Boyer JF . Phosphate diabetes, tubular phosphate reabsorption and phosphatonins . Joint Bone Spine . 72 . 5 . 376–381 . October 2005 . 16214071 . 10.1016/j.jbspin.2004.07.013 .
  2. Web site: Schnabel D, Haffner D . Kirchhoff M . 18 May 2018 . What is phosphate diabetes(XLH) . Phosphatdiabetes e.V. . 2023-04-13.
  3. Web site: PHEX phosphate regulating endopeptidase X-linked . Entrez Gene . U.S. National Library of Medicine .
  4. Encyclopedia: Phosphorus in diet . MedlinePlus Medical Encyclopedia . U.S. National Library of Medicine . 2023-04-13 . en.
  5. Web site: Viktorovich VV . 29 May 2020 . Phosphate diabetes: symptoms, diagnosis, treatment blog . 2023-04-13 . Ladisten . en-US.
  6. Book: Fukumoto S . Fibroblast growth factor 23. . Principles of Bone Biology . January 2020 . 1529–1538 . Academic Press . 10.1016/B978-0-12-814841-9.00063-4 . 978-0-12-814841-9 . 202038125 .
  7. Book: Akimbekov NS, Digel I, Sherelkhan DK, Razzaque MS . Phosphate Metabolism . Vitamin D and Phosphate Interactions in Health and Disease . Advances in Experimental Medicine and Biology . 1362 . 37–46 . 2022 . 35288871 . 10.1007/978-3-030-91623-7_5 . 978-3-030-91621-3 .
  8. Klatka M, Partyka M, Polak A, Terpiłowska B, Terpiłowski M, Chałas R . Vitamin D, calcium and phosphorus status in children with short stature - effect of growth hormone therapy . english . Annals of Agricultural and Environmental Medicine . 28 . 4 . 686–691 . December 2021 . 34969230 . 10.26444/aaem/139569 . 237851541 . free .
  9. Web site: Rickets and Osteomalacia Are the Underlying Sources of Symptoms That Will Progress Throughout Adulthood . https://web.archive.org/web/20200715182303/https://www.xlhlink.com/hcp/clinical-presentation-and-disease-progression-children/ . 15 July 2020 . Ultragenyx Pharmaceutical Inc. .
  10. Nguyen C, Celestin E, Chambolle D, Linglart A, Biosse Duplan M, Chaussain C, Friedlander L . Oral health-related quality of life in patients with X-linked hypophosphatemia: a qualitative exploration . Endocrine Connections . 11 . 1 . e210564 . January 2022 . 34941571 . 8859955 . 10.1530/EC-21-0564 .
  11. Web site: PHEX phosphate regulating endopeptidase X-linked [Homo sapiens (human)] ]. Gene - NCBI . 2023-03-28 . U.S. National Library of Medicine .
  12. Beck-Nielsen SS, Mughal Z, Haffner D, Nilsson O, Levtchenko E, Ariceta G, de Lucas Collantes C, Schnabel D, Jandhyala R, Mäkitie O . 6 . FGF23 and its role in X-linked hypophosphatemia-related morbidity . Orphanet Journal of Rare Diseases . 14 . 1 . 58 . February 2019 . 30808384 . 6390548 . 10.1186/s13023-019-1014-8 . free .
  13. Jüppner H . Phosphate and FGF-23 . Kidney International. Supplement . 79 . 121 . S24–S27 . April 2011 . 21346724 . 3257051 . 10.1038/ki.2011.27 .
  14. Gattineni J, Bates C, Twombley K, Dwarakanath V, Robinson ML, Goetz R, Mohammadi M, Baum M . 6 . FGF23 decreases renal NaPi-2a and NaPi-2c expression and induces hypophosphatemia in vivo predominantly via FGF receptor 1 . American Journal of Physiology. Renal Physiology . 297 . 2 . F282–F291 . August 2009 . 19515808 . 2724258 . 10.1152/ajprenal.90742.2008 .
  15. Tang X, Liu X, Liu H . Mechanisms of Epidermal Growth Factor Effect on Animal Intestinal Phosphate Absorption: A Review . Frontiers in Veterinary Science . 8 . 670140 . 2021-06-14 . 34195248 . 8236626 . 10.3389/fvets.2021.670140 . free .
  16. Quarles LD . Role of FGF23 in vitamin D and phosphate metabolism: implications in chronic kidney disease . Experimental Cell Research . 318 . 9 . 1040–1048 . May 2012 . 22421513 . 3336874 . 10.1016/j.yexcr.2012.02.027 .
  17. Christakos S, Dhawan P, Porta A, Mady LJ, Seth T . Vitamin D and intestinal calcium absorption . Molecular and Cellular Endocrinology . 347 . 1–2 . 25–29 . December 2011 . 21664413 . 3405161 . 10.1016/j.mce.2011.05.038 .
  18. Bikle DD . Vitamin D metabolism, mechanism of action, and clinical applications . Chemistry & Biology . 21 . 3 . 319–329 . March 2014 . 24529992 . 3968073 . 10.1016/j.chembiol.2013.12.016 .
  19. Latic N, Erben RG . FGF23 and Vitamin D Metabolism . JBMR Plus . 5 . 12 . e10558 . December 2021 . 34950827 . 8674776 . 10.1002/jbm4.10558 .
  20. Chanakul A, Zhang MY, Louw A, Armbrecht HJ, Miller WL, Portale AA, Perwad F . FGF-23 regulates CYP27B1 transcription in the kidney and in extra-renal tissues . PLOS ONE . 8 . 9 . e72816 . 2013-09-03 . 24019880 . 3760837 . 10.1371/journal.pone.0072816 . 2013PLoSO...872816C . Dussaule JC . free .
  21. Jones G, Prosser DE, Kaufmann M . Cytochrome P450-mediated metabolism of vitamin D . English . Journal of Lipid Research . 55 . 1 . 13–31 . January 2014 . 23564710 . 3927478 . 10.1194/jlr.R031534 . free .
  22. Laroche M . Phosphate, the renal tubule, and the musculoskeletal system . Joint Bone Spine . 68 . 3 . 211–5 . May 2001 . 11394620 . 10.1016/s1297-319x(01)00274-3 .
  23. Web site: 2012-07-20 . Definition of X-linked dominant inheritance . 2023-04-13 . National Cancer Institute . U.S. Department of Health and Human Services . en.
  24. Web site: Padiath QS . June 2023 . Inheritance of Single-Gene Disorders - Fundamentals . 2023-04-13 . MSD Manual Consumer Version . en.
  25. Web site: Causes of XLH . 2023-04-13 . XLH Link . en-us.
  26. Al Juraibah F, Al Amiri E, Al Dubayee M, Al Jubeh J, Al Kandari H, Al Sagheir A, Al Shaikh A, Beshyah SA, Deeb A, Habeb A, Mustafa M, Zidan H, Mughal MZ . 6 . Diagnosis and management of X-linked hypophosphatemia in children and adolescent in the Gulf Cooperation Council countries . Archives of Osteoporosis . 16 . 1 . 52 . March 2021 . 33660084 . 7929956 . 10.1007/s11657-021-00879-9 .
  27. Skrinar A, Dvorak-Ewell M, Evins A, Macica C, Linglart A, Imel EA, Theodore-Oklota C, San Martin J . 6 . The Lifelong Impact of X-Linked Hypophosphatemia: Results From a Burden of Disease Survey . Journal of the Endocrine Society . 3 . 7 . 1321–1334 . July 2019 . 31259293 . 6595532 . 10.1210/js.2018-00365 .
  28. Jagga S, Venkat S, Sorsby M, Liu ES . March 2023 . Insights into the Molecular and Hormonal Regulation of Complications of X-Linked Hypophosphatemia . Endocrines . en . 4 . 1 . 151–168 . 10.3390/endocrines4010014 . 2673-396X . free .
  29. Rafaelsen S, Johansson S, Ræder H, Bjerknes R . Hereditary hypophosphatemia in Norway: a retrospective population-based study of genotypes, phenotypes, and treatment complications . European Journal of Endocrinology . 174 . 2 . 125–136 . February 2016 . 26543054 . 4674593 . 10.1530/EJE-15-0515 .
  30. Web site: 2021-11-05 . High Phosphorus (hyperphosphatemia) . 2023-04-12 . American Kidney Fund, Inc. . en.
  31. Web site: 2017-10-19 . Bone density scan (DEXA scan) . 2023-04-12 . National Health Service . United Kingdom . en.
  32. Book: Ruppe MD . X-Linked Hypophosphatemia . 1993 . http://www.ncbi.nlm.nih.gov/books/NBK83985/ . GeneReviews . Seattle (WA) . University of Washington, Seattle . Adam MP, Mirzaa GM, Pagon RA, Wallace SE, Bean LJ, Gripp KW, Amemiya A .
  33. Padidela R, Cheung MS, Saraff V, Dharmaraj P . Clinical guidelines for burosumab in the treatment of XLH in children and adolescents: British paediatric and adolescent bone group recommendations . Endocrine Connections . 9 . 10 . 1051–1056 . October 2020 . 33112809 . 7707830 . 10.1530/EC-20-0291 .
  34. Malberti F . Hyperphosphataemia: treatment options . Drugs . 73 . 7 . 673–688 . May 2013 . 23625273 . 10.1007/s40265-013-0054-y . 26266988 .
  35. Web site: Qasımov E . 9 January 2022 . Knee joint deformities in children (leg curvature) . 2023-04-12 . en-US.