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Fibrodysplasia ossificans progressiva (FOP) is a disorder in which muscle tissue and connective tissue such as tendons and ligaments are gradually replaced by bone (ossified), forming bone outside the skeleton (extra-skeletal or heterotopic bone) that constrains movement. This process generally becomes noticeable in early childhood, starting with the neck and shoulders and proceeding down the body and into the limbs.
Extra-skeletal bone formation causes progressive loss of mobility as the joints become affected. Inability to fully open the mouth may cause difficulty in speaking and eating. Over time, people with this disorder may experience malnutrition due to their eating problems. They may also have breathing difficulties as a result of extra bone formation around the rib cage that restricts expansion of the lungs.
Any trauma to the muscles of an individual with fibrodysplasia ossificans progressiva, such as a fall or invasive medical procedures, may trigger episodes of muscle swelling and inflammation (myositis) followed by more rapid ossification in the injured area. Flare-ups may also be caused by viral illnesses such as influenza.
People with fibrodysplasia ossificans progressiva are generally born with malformed big toes. This abnormality of the big toes is a characteristic feature that helps to distinguish this disorder from other bone and muscle problems. Affected individuals may also have short thumbs and other skeletal abnormalities.
Fibrodysplasia ossificans progressiva is a very rare disorder, believed to occur in approximately 1 in 2 million people worldwide. Several hundred cases have been reported.
Mutations in the ACVR1 gene cause fibrodysplasia ossificans progressiva.
The ACVR1 gene provides instructions for producing a member of a protein family called bone morphogenetic protein (BMP) type I receptors. The ACVR1 protein is found in many tissues of the body including skeletal muscle and cartilage. It helps to control the growth and development of the bones and muscles, including the gradual replacement of cartilage by bone (ossification) that occurs in normal skeletal maturation from birth to young adulthood.
Researchers believe that a mutation in the ACVR1 gene may change the shape of the receptor under certain conditions and disrupt mechanisms that control the receptor's activity. As a result, the receptor may be constantly turned on (constitutive activation). Constitutive activation of the receptor causes overgrowth of bone and cartilage and fusion of joints, resulting in the signs and symptoms of fibrodysplasia ossificans progressiva.
This condition is inherited in an autosomal dominant pattern, which means one copy of the altered gene in each cell is sufficient to cause the disorder.
Most cases of fibrodysplasia ossificans progressiva result from new mutations in the gene. These cases occur in people with no history of the disorder in their family. In a small number of cases, an affected person has inherited the mutation from one affected parent.
'''Fibrodysplasia ossificans progressiva''' (FOP) is a rare disorder of the connective tissue, in which fibrous tissue (including bone, muscle, tendon, and ligament) is ossified and slowly turned into bone. This occurs when the fibrous tissue is damaged and is replaced with bone tissue, because the body's repair mechanism is mutated. These bony growths occur painfully and slowly over a period of weeks or months, and usually begin in the upper back and shoulders. FOP bones are not usually removed with surgery because that causes the body to "repair" the area of surgery with more bones. Over time, as more FOP bones grow and joints get stiffer, it becomes more difficult to walk, eat, and even breathe. Children born with FOP have a characteristic shortening of the great toe. Their first "flare-up" that leads to the formation of FOP bones is usually before the age of 10. Sometimes after a flare-up is over there is no new noticeable FOP bone. It is important that people with FOP do not participate in strenuous activity, and they should try to avoid falling or getting bruises, because those things can cause more FOP bones to grow. People with FOP should never allow anyone to try to stretch out their joints or bend them more than they can go on their own. However, sometimes flare-ups happen for no apparent reason, so being careful is not a guarantee of health. FOP is caused by an autosomal dominant allele on chromosome 4. There have been fewer than 200 cases reported, but a study found that it affects approximately 1 in 1.64 million people. Most cases are caused by spontaneous mutation in the gametes, because most people with FOP cannot have children. However, the allele has varying expressivity, but complete penetrance (i.e. it always affects the bearer, but its effects are variable). A similar, but less catastrophic disease is Fibrous dysplasia, which is caused by a postzygotic mutation.
Weldon FOP Research Fund - Raises awareness and funding for the University of Pennsylvania and Dr. Fred Kaplan, the world's authority on Fibrodysplasia Ossificans Progressiva. Find info about the fund, news and events and support the fund.
IFOPA - International Fibrodysplasia Ossificans Progressiva Association - Support group for families. Information about the condition, treatment, and research efforts.