Fibrous Dysplasia

This chapter focuses on fibrous dysplasia (FD) of bone, which is a genetic, noninherited disease of bone and bone marrow caused by activating mutations of the GNAS1 gene that encodes the α subunit of the stimulatory G protein, G_s. The mutation occurs postzygotically and results in a somatic mosaic state. Mutated cells are exposed to the effects of excess endogenous cAMP production, which results from the inappropriate stimulation of adenylyl cyclase by the mutated G_sα. In bone and marrow, the mutation affects cells of the osteogenic lineage at various stages of maturation, causing different types of dysfunction. FD is a disease of excess, abnormal, and imperfect bone growth. The disease produces excess bone growth by causing a localized increase in bone tissue within bone (or local bone mass). The disease causes abnormal bone growth because bone formation does not adhere to the architectural design of the affected, growing bone segments.