X-linked hypophosphatemia (XLH) is an X-linked dominant form of rickets (or osteomalacia) 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 locally in the extracellular matrix of bones and teeth an elevated level of the mineralization/calcification-inhibiting protein osteopontin. 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. For both XLH and hypophosphatasia, inhibitor-enzyme pair relationships function to regulate mineralization in the extracellular matrix through a double-negative (inhibiting the inhibitors) activation effect in a manner described as the Stenciling Principle. Both these underlying mechanisms (renal phosphate wasting systemically, and mineralization inhibitor accumulation locally) contribute to the pathophysiology of XLH that leads to soft bones and teeth (hypomineralization, osteomalacia/odontomalacia). The prevalence of the disease is 1 in 20,000.X-linked hypophosphatemia may be lumped in with autosomal dominant hypophosphatemic rickets under general terms such as hypophosphatemic rickets.
Hypophosphatemic rickets are associated with at least nine other genetic mutations. Clinical management of hypophosphatemic rickets may differ depending on the specific mutations associated with an individual case, but treatments are aimed at raising phosphate levels to promote normal bone formation. Symptoms and signs
The most common symptoms of XLH affect the bones and teeth, causing pain, abnormalities, and osteoarthritis. Symptoms and signs can vary between children and adults and can include:
Children

Adults

Osteomalacia
Dental abscesses
Limited range of movement (enthesopathy)
Short stature
Fatigue
Fractures / pseudofracture
Bone pain
Craniostenosis
Osteoarthritis
Spinal stenosis
Hearing loss
Depression

Genetics
XLH is associated with a mutation in the PHEX gene sequence, located on the human X chromosome at location Xp22.2-p22.1. The PHEX protein regulates another protein called fibroblast growth factor 23 (produced from the FGF23 gene). Fibroblast growth factor 23 normally inhibits the kidneys ability to reabsorb phosphate into the bloodstream. Gene mutations in PHEX prevent it from correctly regulating fibroblast growth factor 23. The resulting overactivity of FGF-23 reduces vitamin D 1α-hydroxylation and phosphate reabsorption by the kidneys, leading to hypophosphatemia and the related features of hereditary hypophosphatemic rickets. Also in XLH, where PHEX enzymatic activity is absent or reduced, osteopontin—a mineralization-inhibiting secreted substrate protein found in the extracellular matrix of bone—accumulates in bone (and teeth) to contribute to the osteomalacia (and odontomalacia) as shown in the mouse homolog (Hyp) of XLH and in XLH patients. Biochemically in blood, XLH is recognized by hypophosphatemia and an inappropriately low level of calcitriol (1,25-(OH)2 vitamin D3).