FGF23 Gene

Hyperphosphatemic familial tumoral calcinosis (HFTC) is a condition characterized by an increase in the levels of phosphate in the blood (hyperphosphatemia) and abnormal deposits of phosphate and calcium (calcinosis) in the body's tissues. Calcinosis typically develops in early childhood to early adulthood, although in some people the deposits first appear in infancy or in late adulthood. Calcinosis usually occurs in and just under skin tissue around the joints, most often the hips, shoulders, and elbows. Calcinosis may also develop in the soft tissue of the feet, legs, and hands. Rarely, calcinosis occurs in blood vessels or in the brain and can cause serious health problems. The deposits develop over time and vary in size. Larger deposits form masses that are noticeable under the skin and can interfere with the function of joints and impair movement. These large deposits may appear tumor-like (tumoral), but they are not tumors or cancerous. The number and frequency of deposits varies among affected individuals; some develop few deposits during their lifetime, while others may develop many in a short period of time.

Other features of HFTC include eye abnormalities such as calcium buildup in the clear front covering of the eye (corneal calcification) or angioid streaks that occur when tiny breaks form in the layer of tissue at the back of the eye called Bruch's membrane. Inflammation of the long bones (diaphysis) or excessive bone growth (hyperostosis) may occur. Some affected individuals have dental abnormalities. In males, small crystals of cholesterol can accumulate (microlithiasis) in the testicles, which usually causes no health problems.

A similar condition called hyperphosphatemia-hyperostosis syndrome (HHS) results in increased levels of phosphate in the blood, excessive bone growth, and bone lesions. This condition used to be considered a separate disorder, but it is now thought to be a mild variant of HFTC.

Hereditary hypophosphatemic rickets is a disorder related to low levels of phosphate in the blood (hypophosphatemia). Phosphate is a mineral that is essential for the normal formation of bones and teeth.

In most cases, the signs and symptoms of hereditary hypophosphatemic rickets begin in early childhood. The features of the disorder vary widely, even among affected members of the same family. Mildly affected individuals may have hypophosphatemia without other signs and symptoms. More severely affected children experience slow growth and are shorter than their peers. They develop bone abnormalities that can interfere with movement and cause bone pain. The most noticeable of these abnormalities are bowed legs or knock knees. These abnormalities become apparent with weight-bearing activities such as walking. If untreated, they tend to worsen with time.

Other signs and symptoms of hereditary hypophosphatemic rickets can include premature fusion of the skull bones (craniosynostosis) and dental abnormalities. The disorder may also cause abnormal bone growth where ligaments and tendons attach to joints (enthesopathy). In adults, hypophosphatemia is characterized by a softening of the bones known as osteomalacia.

Researchers have described several forms of hereditary hypophosphatemic rickets, which are distinguished by their pattern of inheritance and genetic cause. The most common form of the disorder is known as X-linked hypophosphatemic rickets (XLH). It has an X-linked dominant pattern of inheritance. X-linked recessive, autosomal dominant, and autosomal recessive forms of the disorder are much rarer.

Another rare type of the disorder is known as hereditary hypophosphatemic rickets with hypercalciuria (HHRH). In addition to hypophosphatemia, this condition is characterized by the excretion of high levels of calcium in the urine (hypercalciuria).