Fibrodysplasia ossificans progressiva

Fibrodysplasia ossificans progressiva (/ˌfaɪbroʊdɪˈspleɪʒ(i)ə ɒˈsɪfɪkænz prəˈɡrɛsɪvə/;^[1] abbr. FOP), also called Münchmeyer disease or formerly myositis ossificans progressiva, is an extremely rare connective tissue disease. Fibrous connective tissue such as muscle, tendons, and ligaments ossify into bone tissue. The condition ultimately immobilises sufferers as new bone replaces musculature and fuses with the existing skeleton. This has earned FOP the nickname "stone man disease".^[2]

FOP is caused by a mutation of the gene ACVR1, affecting the body's repair mechanism. Fibrous tissue including muscle, tendons, and ligaments ossify, either spontaneously or when damaged by trauma. In many cases, otherwise minor injuries can cause joints to permanently fuse as new bone forms, replacing the damaged muscle tissue. This new bone formation (known as "heterotopic ossification") eventually forms a secondary skeleton progressively restricting the patient's ability to move. Circumstantial evidence suggests that the disease can cause joint degradation separate from its characteristic bone growth.^[3] It is a severe, disabling disorder.

Bone formed as a result of ossification is identical to "normal" bone, but in improper locations. The rate of ossified bone growth varies by patient. It is the only known medical condition in which tissue of one organ system changes into that of another.^[4]

Surgical removal of ossified bone causes the body to "repair" the affected area with additional bone. FOP has no current known cure. There are though intermittent treatments such as anti-inflammatory drugs. Promising breakthroughs include the approved treatment, Sohonos (palovarotene). Another promising treatment is Antisense-mediated therapy using allele-selective LNA gapmers.