A GENETIC ABNORMALITY

FD is caused by a mutation (in fact two mutations are possible on the same gene) which inhibits the cells that make bone tissue (osteoblasts) from developing properly.

As the mutation occurs after fertilization it is therefore called somatic. Consequently, in the same person, both normal and mutated cells exist, in varying proportions, which explains the different levels of severity from person to person. Above all, this mutation is not transmissible and thus the disease is not hereditary.

The cause of the mutation has not been identified. It is likely that the mutation happens by chance, like many other genetic abnormalities.

BONE ABNORMALITIES CAUSED BY THE MUTATION

As a result of the mutation, these abnormal cells create a disorganised bone network that is poorly mineralised. In diseased bone tissue, blood vessels that are too large can be observed (the cause of intra-bone haemorrhaging, which may explain some of the pain experienced), as well as over or poorly calcified cartilage.

Surrounding dysplasic tissue the presence of a large number of bone destroying cells (osteoclasts) is often noted. These cells are attached by substances made by the mutated cells. These osteoclasts will destroy both the dysplasic and health bone tissue, thus contributing to the development of dysplasic surface lesions and the weakening of the bone. It is probable that this destruction of bone tissue is partly responsible for skeletal weakness.

ENDOCRINE ABNORMALITIES CAUSED BY THE MUTATION