Disease gene that turns muscle into bone identified

US researchers have identified the faulty gene that causes a rare condition in which the muscles, ligaments and tendons turn into bone. People born with Fibrodysplasia Ossificans Progressiva (FOP) gradually become encased in a 'second skeleton', and cannot move. A team based at the University of Pennsylvania has discovered that mutations in a bone growth gene cause the disorder, which usually begins in childhood. The findings, published in the journal Nature Genetics, should shed light on other conditions that affect bone growth, say the scientists.

FOP is a devastating but rare condition affecting about one in every two million babies born. To carry out the study, the researchers collected DNA samples from 600 patients living around the world. They discovered that affected people have mutations in a gene called ACVR1, which controls how cells respond to bone growth-promoting proteins. The mutated gene acts as a 'molecular terrorist', according to team leader Frederick Kaplan, explaining that it 'short-circuits a functioning set of muscles and connective tissues and transforms them into a second skeleton, in essence turning a light bulb into an atom bomb'.

Babies with FOP appear healthy when they are first born, except for a a malformed big toe that affects some patients. But during the first 10-20 years of their life, affected individuals develop painful growths in their neck, back and shoulders. As these gradually turn into bone, they increasingly restrict the patient's movement until eventually they become encased in bony tissue. Any attempts to surgically remove the extra bone triggers the growth of even more affected tissue.

There is currently no cure for FOP, but the authors of the latest study hope their discovery will lead to new treatments for the condition. Kaplan also says that the finding is relevant to 'every condition that affects the formation of bone and every condition that affects the formation of the skeleton' - such as osteoporosis, spinal injuries and sports injuries.