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McCune-Albright syndrome is a rare genetic disorder caused by an activating mutation of the GNAS gene resulting in various phenotypic presentations. McCune-Albright syndrome typically presents with the triad of fibrous dysplasia, precocious puberty and café au lait spots in both genders. Other manifestations include acromegaly, Cushing’s syndrome and thyroid hyperplasia or toxic nodular goiters.
- McCune-Albright syndrome was first discovered by physicians Donovan McCune and Fuller Albright in 1937. 
There is no established system for the classification of McCune-Albright syndrome.
- The pathogenesis of McCune-Albright syndrome is characterized by increased cAMP signaling in bone, skin and endocrine tissues. In bone, osteoblasts differentiation results in fibrous dysplasia. In the skin, there is stimulation of melanin production resulting in café au lait macules with irregular borders. In endocrine tissues, increased cAMP results in increased production of hormones depending on which tissue is affected including the gonads, thyroid, parathyroid, pituitary and adrenal glands. 
- The GNAS gene mutation has been associated with the development of McCune-Albright syndrome, involving the cAMP pathway-associated G-protein, Gsα. 
McCune-Albright syndrome is caused by a missense mutation of the GNAS gene alpha subunit which becomes constitutively activated. This increases intracellular cAMP which activates downstream hormones resulting in multiple tissue types being affected and mosaicism presented in its patients. 
Differentiating McCune-Albright syndrome from other Diseases
- Neurofibromatosis and McCune-Albright syndrome both have café au lait macules. However, neurofibromatosis macules have smooth borders with McCune-Albright being irregular. 
- Neurfibromatosis would also have the additional sign of neurofibromas, optic gliomas, iris hamartomas, and axillary freckling.
- The absence of endocrinopathies and café au lait macules would differentiate MAS from isolated fibrous dysplasia.
Epidemiology and Demographics
- The prevalence of McCune-Albright syndrome is between 1:100,000 and 1:1,000,000 making it very rare. 
- McCune-Albright syndrome is more commonly observed among infants and young children. 
- Girls are more commonly affected with McCune-Albright syndrome than boys. 
- There is no racial predilection for McCune-Albright syndrome. 
There are no risk factors for McCune-Albright syndrome.
Natural History, Complications and Prognosis
- Early clinical features include café-au-lait macules often at birth. 
- If left untreated, patients may progress to develop decreased adult stature as a result of precocious puberty.
- Common complications of McCune-Albright syndrome include rickets because of phosphate wasting and fractures at fibrous dysplastic sites. 
- The diagnosis of McCune-Albright syndrome is a clinical diagnosis. 
- Though there are genetic tests available for the GNAS gene mutation with new PCR techniques for those who show clinical signs of the syndrome. 
- Patients who present with monostotic fibrous dysplasia are required to have evidence of the pathogenic GNAS gene by molecular testing. 
- Symptoms of McCune-Albright syndrome include the following:
- Precocious puberty being the most common presentation. 
- Fibrous dysplasia leading to pathologic fractures or pain  because of fibrous tissue replacing the normal bone.
- Café au lait spots with “coast of Maine” irregular borders.  are often the first presenting sign  and may increase over time. 
- Possible hyperthyroidism, Cushing syndrome, acromegaly or prolactin secretion due to increased thyroid, cortisol, growth hormone or prolactin secretion respectively depending on which tissues are affected. 
- Phosphate wasting with or without hypophosphatemia mediated by Fibroblast growth factor 23 (FGF23). 
- Possible hepatitis, cholestasis and cardiac arrhythmias. 
- Patients with McCune-Albright syndrome usually present with bone pain or limping.
- Physical examination may be remarkable for:
- Facial asymmetry due to craniofacial fibrous dysplasia.
- Hearing impairment or visual disturbances due to craniofacial fibrous dysplasia.
- Café au lait macules.
- Goiter due to hyperthyroidism.
- Macrocephaly due to excess growth hormone.
- Vaginal bleeding in females due to ovarian cysts.
- Pubic and or axillary hair and penile enlargement in males.
- Growth acceleration in both genders.
- Cushingoid features due to hypercortisolism.
- Girls will have raised estradiol levels indicating an activated hypothalamic-pituitary axis.
- Other laboratory findings consistent with the diagnosis of McCune-Albright syndrome are elevated growth hormone which can be deduced by an oral glucose tolerance test, serum GH and prolactin measurements.
- Evaluation of hyperthyroidism by measuring TSH, free and bound thyroxine and T3. 
There are no ECG findings associated with McCune-Albright syndrome.
The fibrous dysplasia of bone is present at multiple sites (polyostotic) and xrays would give a ground glass appearance. 
Echocardiography or Ultrasound
US in boys may be helpful in the diagnosis of McCune-Albright syndrome. Findings on an US suggestive of McCune-Albright syndrome include testicular ultrasounds to detect hormonally active tumors.
CT scan may be helpful in the diagnosis of McCune-Albright syndrome. Findings on CT scan suggestive of/diagnostic of McCune-Albright syndrome include fibroblastic lesions. It is helpful to identify these early in children to prevent permanent deformities. 
There are no MRI findings associated with McCune-Albright syndrome.
Other Imaging Findings
There are no other imaging findings associated with McCune-Albright syndrome.
Other Diagnostic Studies
- McCune Albright may be diagnosed using total body bone scintigraphy to identify the fibrous dysplastic lesions. This should then be followed up with radiographs and CT to clearly determine the extent of the lesions.
- It is helpful to perform baseline ophthalmic and audiologic testing for those patients whose fibrous dysplastic lesions involve the craniofacial area. 
- The treatment for McCune Albright syndrome includes:
- Precocious puberty- treatment is aimed to prevent bone age advancement. 
- Fibrous dysplasia – treatment is aimed to minimize pain and fractures:
- Thyroid disease – Methimazole is effective though surgery is preferred.
- Increased growth hormone:
- Hypercortisolism - metyparone is effective.
- FGF23-mediated phosphate wasting with oral phosphorus and calcitriol. 
- Café-au-lait macules:
- There is no effective treatment 
- Surgery should be used in extreme caution for female patients with ovarian cysts because of the high risk of recurrence and resultant decrease in ovarian reserve.
- Patients with hyperthyroidism are preferred to have a thyroidectomy with total gland resection. 
- Once diagnosed, patients with McCune-Albright syndrome are followed-up with:
- Females – early breast cancer screening.
- Males – testicular lesions by physical examination and US. 
- Both genders – signs of precocious puberty, growth hormone excess or growth acceleration (IGF-1 levels), monitoring thyroid function (TSH, free T4, and T3), routine radiographs for fibrous dysplasia lesions particularly to check for scoliosis, serum phosphorous to monitor for development of hypophosphatemia  and annual vision and hearing testing to monitor fibrotic lesions affecting these areas. 
- Parents should be counseled that McCune-Albright syndrome is not inherited. 
- There is an increased risk for malignancies. 
- Patients should also be advised to avoid contact sports and optic nerve decompressions. 
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