|Classification and external resources|
Café-au-lait skin pigmentation.
A) A typical lesion on the face, chest, and arm of a 5-year-old girl with McCune-Albright syndrome which demonstrates jagged "coast of Maine" borders, and the tendency for the lesions to both respect the midline and follow the developmental lines of Blaschko.
B) Typical lesions that are often found on the nape of the neck and crease of the buttocks are shown (arrows).
McCune–Albright syndrome is suspected when two of the three following features are present:
Within the syndrome there are bone fractures and deformity of the legs, arms and skull, different pigment patches on the skin, and early puberty with increased rate of growth.
Approximately 20-30% of fibrous dysplasias are polyostotic and two thirds of patients are polyostotic before the age of ten.
The disease frequently involves the skull and facial bones, pelvis, spine and shoulder girdle. The sites of involvement are the femur (91%), tibia (81%), pelvis (78%), ribs, skull and facial bones (50%), upper extremities, lumbar spine, clavicle, and cervical spine, in decreasing order of frequency. The craniofacial pattern of the disease occurs in 50% of patients with the polyostotic form of fibrous dysplasia.
McCune–Albright syndrome has different levels of severity. For example, one child with McCune–Albright syndrome may be entirely healthy, with no outward evidence of bone or endocrine problems, enter puberty at close to the normal age, and have no unusual skin pigmentation. Alternatively, in other cases, the children are diagnosed in early infancy, show obvious bone disease, and obvious increased endocrine secretions from several glands.
Genetically, there is a post-zygotic mutation of the gene GNAS1, on the long (q) arm of chromosome 20 at position 13.3, which is involved in G-protein signalling. This mutation, often a mosaicism, prevents downregulation of cAMP signalling.
Polyostotic fibrous dysplasia is usually caused by mosaicism for a mutation in a gene called GNAS1 (Guanine Nucleotide binding protein, Alpha Stimulating activity polypeptide 1).
The disease made headlines in December, 2005 when a Haitian teen afflicted with the disease, Marlie Casseus, underwent a 17-hour emergency surgical procedure to remove a 7 kg (16 pound) tumour-like growth of bone from her face. A series of operations at Holtz Children's Hospital in Miami, Florida restored the child's face to a more normal proportion.