Around the world, there are 800 confirmed cases (although the estimated total is higher) of a rare and debilitating disease called fibrodysplasia ossificans progressiva (FOP). Better known as “stone man syndrome,” it causes muscle to turn into bone, locking limbs in place, and is often fatal. It is currently untreatable, but scientists think they may now be a step closer to a cure.

The research, which included Regeneron Pharmaceuticals in New York, was published in the journal Science Translational Medicine and describes how the lethal disorder is caused by mutations in a gene called ACVR1, which produces a cell surface protein called a transmembrane receptor. The mutation affects the way the receptor responds to a protein called activin A, causing it to go into overdrive when normally it would be repressed. This can ultimately lead to the replacement of muscle and soft tissue with bone, causing – for example – joints in elbows or ankles to freeze.

"Imagine you are driving your car down the road and you come to a red light. You press on the brakes, and the car stops," Aris Economides, senior author on the study and executive director at Regeneron, told HealthDay. With the disorder, "not only does your car not stop, actually the brakes are hot-wired to the accelerator."

The researchers studying the disease, though, found that the protein activin A can be prevented from triggering the gene using an antibody, developed by scientists at Regeneron. Tests on mice with FOP showed that the treatment successfully stopped the growth of new bone.

Speaking to HealthDay, Betsy Bogard from the International FOP Association said the finding was “extraordinary.”

The tests showed that the antibody was effective in mice for up to six weeks, with no obvious side effects. Of course, it remains to be seen if it is as effective in humans, but these results are promising.

Existing remedies are few and far between for FOP. A steroid drug called prednisone can be used to manage the pain, but nothing so far can stop the bone formation. Even trying to cut away the bone in surgery makes it worse, as bone development subsequently intensifies.

Regeneron is now beginning procedures to start clinical tests in humans, although there is no guarantee the same antibody will work in people. But at least there is some hope for treating the disease.