The name of this condition, Fibrous Dysplasia/McCune-Albright syndrome (FD/MAS), can sometimes be confusing. Some people only have affected bones, in which case it might be called FD. When bones and other tissues are affected, it is called MAS. Since both FD and MAS are caused by the exact same gene mutation, and since many people with FD also have other tissues involved, the FD/MAS Alliance and doctors have decided to call it FD/MAS. It is a rare condition caused by a mutation in the gene GNAS. The mutated gene produces a protein, Gs alpha, that is dysfunctional. Gs alpha is a sort of “on/off” switch in cells, and the mutated protein is stuck in the “on” position. Depending on the cell or tissue affected, the “on” switch has a different effect.
The tissue most commonly involved is the bone (FD). Here the mutated protein makes it, so bone cells aren’t able to mature into normal bone. When endocrine organs, such as the thyroid or ovary, are affected, the effect of the “on” switch is to make excess hormones. In certain skin cells, it produces pigment, leading to darkened skin, sometimes called café-au-lait macules. Since the Gs alpha protein is produced in almost every cell in the body, many organs can be involved.
Individuals may have one or many features of the condition, which can occur in any combination. There is no cure for FD/MAS. However, medications or surgery can usually alleviate symptoms.