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McCune-Albright syndrome
A rare mosaic syndrome characterized by the combination of two or more of the following: fibrous dysplasia of bone (FD), hyperpigmented macules, and hyperfunctioning endocrinopathies (precocious puberty, hyperthyroidism, growth hormone excess, endogenous Cushing syndrome).
ORPHA:562
It is a rare disease with an estimated prevalence of between 1/100,000 and 1/1,000,000.
FD can involve a single or multiple skeletal sites and can present with a limp, pain, pathologic fracture, or craniofacial asymmetry. Scoliosis is common and may be progressive. In addition to precocious puberty (vaginal bleeding or spotting and early development of breast tissue in girls, testicular and penile enlargement and precocious sexual behavior in boys), other hyperfunctioning endocrinopathies may occur including hyperthyroidism, growth hormone excess, Cushing syndrome, and hypophosphatemia due to renal phosphate loss. Hyperpigmented macules usually appear in the neonatal period.
The disease results from somatic variants of the GNAS gene, specifically in the cAMP-regulating protein, Gs alpha. The extent of the disease is determined by the proliferation, migration and survival of the cell in which the mutation spontaneously occurred during embryonic development.
Diagnosis of McCune-Albright syndrome (MAS) is usually established on clinical grounds, based on the presence of two or more characteristic features. The evaluation of patients with MAS should be guided by knowledge of the spectrum of tissues that may be involved, with specific testing for each. Genetic testing is possible, but is not routinely available.
Differential diagnoses include neurofibromatosis, osteofibrous dysplasia, non-ossifying fibromas, idiopathic central precocious puberty, and ovarian neoplasm.
Treatment is dictated by the tissues affected, and the extent to which they are affected. Some forms of surgical interventions may be indicated for treatment of craniofacial and skeletal abnormalities associated with FD (progressive visual disturbance, severe pain, severe disfigurement), as well as in the management of MAS-associated endocrinopathies and malignancies. Bisphosphonates may be helpful for treatment of bone pain. Strengthening exercises are recommended to help maintain the musculature around the FD bone and minimize the risk of fracture. Treatment of all endocrinopathies is required.
Morbidity is proportional to the number and extent of tissues involved. Longevity is generally not affected. MAS is rarely associated with malignancy. Malignant transformation of FD lesions occurs in probably less than 1% MAS patients.
Last update: June 2024 - Expert reviewer(s): Dr Michael COLLINS
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