How the Stone Man Syndrome turns muscle into bone

What is it?

Fibrodysplasia ossificans progressiva (FOP), also known as “Stone Man Syndrome”, is a connective tissue disease. Tissues like muscles, joints and ligaments are gradually replaced by bone. This process typically begins in the upper parts of the body and moves downwards; although all those born with the disease have malformed big toes. People with FOP suffer from progressive loss of mobility, as their joints can be “frozen” into place. Significant problems with eating and breathing are not uncommon, because the formed bone can, for example, make it impossible for them to properly open their mouths and expand their lungs. Bone formation is often triggered by damage to the tissue, and surgical removal of the extra bone often causes it to grow back even quicker.

FOP only occurs in about 1 in 2 million people worldwide. The median age of survival is 40 years.

What causes it?

FOP is caused by one faulty gene. We all have two sets of genes, one from each of our parents, and they are the instructions for basically everything that happens in our bodies. Since we have a pair of each one gene, these two can either work together or one can dominate the other. The FOP gene is dominant: it is much louder than the other, healthy gene, and because of this only one copy is enough to be affected.

The child of someone with FOP has therefore a 50% chance of having it themselves. Most cases of FOP occur, however, because of a mutation in the sex cell: most FOP sufferers can’t or choose not to have children.

What happens during it?

All of our cells have the same genetic material, so they rely on messages from the outside to find out what exactly they are supposed to do. To receive these messages, our cells have proteins call receptors in their membranes. They have many functions, and one of these is to determine what type of cell a certain unspecialized cell should become.

In FOP, a mutation has occurred in the gene that codes a receptor protein whose activation encourages the cell to become a bone cell. It is thought that this mutation disrupts the normal regulation of the activity of this receptor. This gene is normally deactivated right after the bones of the fetus are formed in the womb, but in FOP, it continues to function. When cells are injured they normally produce enzymes that help the repair of that tissue. Connective tissue injury probably triggers bone formation because the cells inappropriately start to produce an enzyme that helps the growth of bone tissue.

How can it be treated?

Unfortunately, an effective treatment for FOP doesn’t exist. Injury to soft tissue should be avoided as this can provoke bone formation, and when the disease progresses, help with everyday things like eating and breathing can become necessary. The discovery that FOP is caused by one gene offers, however, a well defined target for researchers, and the advances in gene technology (if you’re interested, google the fascinating new thing called CRISPR, for example) offer hope that FOP may some day be defeated.





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