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Genetic and Rare Diseases Information Center (GARD)

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Hypophosphatemic rickets


Other Names for this Disease
  • HYP
  • Hypophosphatemia, X-linked
  • Hypophosphatemia, vitamin D-resistant rickets
  • Vitamin D-resistant rickets, X-linked
  • XLH
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Overview



What is hypophosphatemic rickets?

What causes hypophosphatemic rickets?

How might hypophosphatemic rickets be treated?


What is hypophosphatemic rickets?

Hypophosphatemic rickets (previously called vitamin D-resistant rickets) is a disorder in which the bones become painfully soft and bend easily because the blood contains low levels of phosphate and has inadequate amounts of the active form of vitamin D.[1] The condition can be caused by mutations in the phosphate-regulating endopeptidase gene, also known as the PHEX gene and is nearly always inherited.[2] The aim of treatment is to raise phosphate levels in the blood, which will promote normal bone formation. Phosphate can be taken by mouth and should be combined with calcitriol, the activated form of vitamin D.[1]
Last updated: 9/17/2009

What causes hypophosphatemic rickets?

Hypophosphatemic rickets can result from changes (mutations) in several different genes. Mutations in the PHEX gene, which are responsible for X-linked hypophosphatemic rickets, are the most common. Mutations in other genes cause the less common forms of the condition. Hypophosphatemic rickets is characterized by a phosphate imbalance in the body. Phosphate has many functions, but it plays a very important role in the formation and growth of bones in childhood and helps maintain bone strength in adults. Phosphate levels are mainly controlled by the kidneys. The kidneys normally take excess phosphate out of the body in the urine, and they reabsorb phosphate into the bloodstream when more is needed by the body. However, in people with hypophosphatemic rickets, the kidneys cannot reabsorb the phosphate effectively, and too much of the phosphate leaves the body in the urine. As a result, not enough phosphate is available in the bloodstream for normal bone development and maintenance.[3]

The genes associated with hypophosphatemic rickets are involved in maintaining the proper balance of phosphate in the body. Many of these genes, including the PHEX gene, may directly or indirectly regulate a protein called fibroblast growth factor 23 (produced from the FGF23 gene). This protein normally inhibits the kidneys' ability to reabsorb phosphate into the bloodstream. Gene mutations increase the production, or reduce the breakdown, of fibroblast growth factor 23 and the resulting overactivity of this protein reduces phosphate reabsorption by the kidneys, leading to hypophosphatemia and the other features of hypophosphatemic rickets. To see a list of genes associated with hypophosphatemic rickets, click here.[3]
Last updated: 1/10/2011

How might hypophosphatemic rickets be treated?

The aim of treatment for hypophosphatemic rickets is to raise phosphate levels in the blood, which promotes normal bone growth. Phosphate is typically combined with calcitriol, the activated form of vitamin D. Calcitriol increases calcium levels by promoting calcium absorption in the intestines, and calcium retention in kidneys.[4] The amounts of phosphate and calcitriol are usually adjusted carefully, because this treatment can lead to high levels of calcium in the blood, the accumulation of calcium in kidney tissue, or kidney stones. These effects can harm the kidneys and other tissues.[1] Other treatment options may include growth hormone and anticalciurics to promote healthy bone growth and lessen mineral loss. Osteotomy (a type of bone surgery) to fix leg curvatures may be necessary for children whose diagnosis was delayed, or whose initial treatment was not adequate. Skull abnormalities may require treatment for synostosis (premature closing of sutures in the brain). Spontaneous abscesses in the mouth may require dental procedures periodically. Apart from the short stature of most affected adults, the prognosis for a normal lifespan and normal health is typically good.[4]
Last updated: 1/4/2012

References
  1. Brazy PC. Hypophosphatemic Rickets. The Merck Manual of Medical Information - 2nd edition. 2006; http://www.merck.com/mmhe/print/sec11/ch146/ch146g.html. Accessed 1/3/2012.
  2. Hypophosphatemic Rickets, X-Linked Dominant. Online Mendelian Inheritance in Man (OMIM). 2006; http://www.ncbi.nlm.nih.gov/entrez/dispomim.cgi?id=307800. Accessed 9/13/2009.
  3. Hereditary hypophosphatemic rickets. Genetics Home Reference. September 2010; http://ghr.nlm.nih.gov/condition/hereditary-hypophosphatemic-rickets. Accessed 1/10/2011.
  4. Karl S Roth, James CM Chan. Hypophosphatemic Rickets: Treatment & Medication. eMedicine. February 6, 2009; http://emedicine.medscape.com/article/922305-treatment. Accessed 1/3/2012.