THE genetic trigger for “stone man syndrome”, a rare but devastating condition that turns muscle into bone to imprison its victims in a second skeleton, has been identified.

The discovery of a single gene that causes fibrodysplasia ossificans progressiva (FOP) offers new hope of a first effective therapy for the disorder, which begins early in childhood and has no cure.

By providing insights into the genetic signals that govern bone growth, the research should also improve understanding and treatment of a wide range of more common skeletal conditions. These include osteoporosis, spinal injuries and sports injuries such as the bone spurs that have hampered the cricketers Andrew Flintoff and Glenn McGrath.

In the longer term, it could also allow scientists to make bone in the laboratory, for treating fractures that fail to heal and skeletal malformations.

“The discovery of the FOP gene is relevant to every condition that affects the formation of bone and every condition that effects the formation of the skeleton,” said Frederick Kaplan, of the University of Pennsylvania, who led the research.

FOP is one of the rarest diseases caused by a genetic mutation, affecting about one in two million individuals, or an estimated 2,500 people.

In the new study, which is published today in the journal Nature Genetics, scientists first established that FOP is likely to be caused by a mutated gene that affects bone morphogenetic proteins (BMPs), which control the formation and repair of the skeleton. This insight led them to a gene called ACVR1, which controls one of the three main receptors for BMP that determine how cells respond to its signals. In patients with FOP, a tiny mutation in one of the two copies of the ACVR1 gene changes the meaning of its genetic message, so a faulty protein is made.

“The substitution of one genetic letter for another out of six billion genetic letters in the human genome is like a molecular terrorist that 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,” Dr Kaplan said.

VICTIMS’ PLIGHT

• FOP is an incurable genetic disease that, from birth, causes muscle and soft tissues gradually to turn to bone
• There is no association with any particular race or ethnic group
• Babies appear normal except for a malformation of the big toe
• Extra bone usually forms first in the neck, spine, and shoulders
• The disease does not affect intelligence or cognitive abilities

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