FOP is a rare genetic disease that affects the ACVR1 gene on chromosome 2. One in two million people have FOP. The disease is an inherited dominant trait from the mutation. The ACVR1 gene provides instructions for producing a protein called bone morphogenetic protein(BMP) type 1 receptors. The mutation of this gene doesn’t allow the correct formation of BMPs, one amino acid in the chain is instead substituted for another. BMPs are found in many tissues of the body including the skeletal system and cartilage. They control the growth and development of the bones and muscles. One of their main features is to gradually replace cartilage by bone during skeletal maturation from birth to young adulthood.
The main symptom of FOP is muscle and connective tissue in the body are gradually replaced by bone. Trauma to muscles leading to muscle swelling and inflammation can increase bone growth in the area affected. The growing of bone in these areas outside of the skeletal system is painful and can cause serious discomfort. The bones can take six to eight weeks to develop. This growth of bone causes fusion of joints which constrains movement and those who have the disease lose their mobility. In some cases, the jaw becomes fused leading to difficulty speaking and eating which can lead to malnutrition. Difficulty breathing can occur due to bone formation around the rib cage restriction the expansion of lungs.
Diagnosis of the disease is quite difficult. The rate of misdiagnosis is estimated at eighty percent or higher. This misdiagnosis has led to unnecessary invasive procedures due to the belief that the disease is cancer, aggressive juvenile fibromatosis(desmoid tumors), or progressive osseous heteroplasia(another disease characterized by the abnormal growth of bone). At birth FOP is commonly diagnosed by a malformation of the big toe. At birth the big toe is short and deviated. In early childhood FOP is diagnosed by painful swellings on the head, neck, or back often after some sort of trauma to the body such as a bump or fall.
There is no effective treatment for FOP. Surgery is not an option to remove the excess bone because the trauma to that area will lead to even more bone growth. Researchers are currently developing a drug that may help control bone growth. For now, there are medications to relieve some of the symptoms such as pain and inflammation.
Since FOP is a progressive disease, it typically gets worse over time as the patient ages, but the rate of new bone formation differs for each person and the disease's progression is generally unpredictable. The disease normally progresses to the trunk and limbs of the body leading to most patients being confined to a wheelchair by the third decade of life.