McCune-Albright syndrome (MAS) is an extremely rare disorder that classically affects the bones, skin, and endocrine system. MAS is characterized by fibrous dysplasia of bone that occurs with at least two additional findings – patches of abnormal skin pigmentation (i.e., areas of light-brown skin [cafe-au-lait spots] with jagged borders) and dysfunction of certain glands that regulate the body’s rate of growth, its sexual development, and certain other metabolic functions (multiple endocrine dysfunction). Fibrous dysplasia refers to bone that is replaced by abnormal scar-like (fibrous) connective tissue. This abnormal fibrous tissue weakens the bone, making it abnormally fragile and prone to fracture. Pain may occur in the affected areas. Malfunctioning endocrine glands can result in the development of secondary sexual characteristics at an age younger than normal (gonadotropin independent precocious puberty). MAS is the result of a genetic change (mutation) in the GNAS1 gene that occurs randomly, for no apparent reason (sporadic). In individuals with the disorder, this sporadic genetic mutation is present in only some of the body’s cells (mosaic pattern). The symptoms and physical characteristics associated with the disorder vary greatly from case to case, depending upon the specific body cells and tissues that are affected by the genetic mutation. This mutation occurs after fertilization (postzygotic somatic mutation). It is not inherited from the parents.
The range of severity of McCune-Albright syndrome is broad: some children are diagnosed in early infancy with obvious anomalies of bone and increased hormone production by one or more of the endocrine glands; others show no evidence of bone, skin or endocrine malfunction in childhood and may enter puberty at an appropriate age. The degree of severity of individual symptoms may also vary greatly. Therefore, it is important to note that affected individuals may not have all of the symptoms discussed below and that every individual case is unique. Parents should talk to their child’s physician and medical team about their specific case, associated symptoms and overall prognosis.
The parts of the body most commonly affected by MAS are the bone, skin and endocrine system.
Polyostotic fibrous dysplasia is a distinctive finding in affected individuals. Fibrous dysplasia refers to bone that is replaced by abnormal scar-like (fibrous) connective tissue. This abnormal fibrous tissue weakens the bone, making it abnormally fragile and prone to fracture. Fracture may be the presenting symptom in some cases. Pain may occur in the affected areas. Polyostotic refers to cases in which multiple skeletal sites are affected, but in some cases only one skeletal site is affected (monostotic fibrous dysplasia). Fibrous dysplasia often predominates on one side of the body (unilateral).
Specific symptoms associated with polyostotic fibrous dysplasia depend upon the specific bones involved. Any part of the skeleton can potentially be affected, but the long bones of the arms and legs, the bones of the face and skull (craniofacial area), and the ribs are most often affected. Polyostotic fibrous dysplasia often presents as a painless swellings on the ribs. Fibrous dysplasia affecting the spine can cause abnormal curvature of the spine (scoliosis), which is often progressive. When the long bones of the legs are affected, this can lead to frequent fractures due to weight bearing when walking or standing. Additionally, the long bones can eventually become bowed. In children, their legs may not be of equal length (limb length discrepancy). Eventually, this can affect a person’s ability to walk, causing an abnormal gait (e.g. walking with a limp).
Some affected individual develop polyostotic fibrous dysplasia of the craniofacial region, which can cause a variety of symptoms depending on the type and specific location of the lesions(s). Such symptoms can include pain, nasal congestion, misaligned or displaced teeth, uneven jaws, and facial asymmetry, in which one side of the face does not match the other side. Polyostotic fibrous dysplasia in the craniofacial region can alter the facial features resulting in an abnormally prominent forehead (frontal bossing), bulging eyes (proptosis), and difference in the vertical positions of the eyes so that the eyes are uneven (vertical dystopia). The degree of facial abnormality can vary greatly from one person to another. The shape of the skull may be altered in certain cases.
Fibrous dysplasia lesions can potentially cause a variety of neurological symptoms as areas of abnormal tissue development can compress nearby nerves. Specific symptoms are related to the specific nerves involved. For example, vision loss or hearing impairment can occur because of compression of optic and auditory nerves in the skull. However, vision loss and hearing impairment only occur in rare instances.
Some individuals with MAS have abnormal skin coloring (pigmentation) known as café-au-lait spots. Café-au-lait spots are abnormal patches of light-brown skin that have irregularly-shaped, jagged borders. They may be present at birth or shortly thereafter (neonatal period) and may become more pronounced with age. Individual lesions often abruptly stop at the midline of the body, which is the imaginary line that divides the body into right and left halves. The lesions can be of varying size and often affect only one side of the body. Some individuals do not develop café-au-lait spots.
The endocrine system is the system of glands that regulate the body’s rate of growth, its sexual development, and certain other metabolic functions. Individuals with MAS often experience an abnormally early onset of puberty (gonadotropin independent precocious puberty). A sequence of events occurs during which a child develops adult secondary sexual characteristics beginning at an unexpectedly early age. In MAS, this occurs because glands that secrete sex hormones are inappropriately active abnormally early in life.
In females, there may be vaginal bleeding or early development of breast tissue. Onset can be within the first few months of life or later during childhood around 6 or 7 years of age. Some females may have frequent episodes of vaginal bleeding, while others may only have a few sporadic episodes. Precocious puberty is often associated with the development of benign ovarian cysts. Precocious puberty may result in an advanced bone age, which could potentially limit growth potential and final adult height.
Precocious puberty is more common in females than males with more than 50% of females experiencing precocious puberty. In males, there may be enlargement of the penis and one or both testicles. The scrotum may thicken and gain wrinkles (rugae). Males may also grow pubic hair, hair under the armpits, and body odor.
Children with precocious puberty are often tall and have an increased growth velocity while they are still growing. Individuals end up short because they finish growing earlier than other people.
Other endocrine disorders may occur in individuals with MAS including those affecting the thyroid, the pituitary, and adrenal glands.
The thyroid is a butterfly-shaped gland at the base of the neck. Involvement of the thyroid may range from no obvious clinical symptoms to enlargement of the thyroid (goiter) and overproduction of thyroid hormone (hyperthyroidism). Hyperthyroidism can cause anxiety, fatigue, prominence of the eyes, sweating, heart palpitations, unintended weight loss, and heat intolerance. Osteoporosis may be caused by or worsened by hyperthyroidism. In some cases, hyperthyroidism cannot be controlled by medication.
The pituitary gland is a small gland located near the base of the skull that stores several hormones including growth hormone and releases them into the bloodstream as needed by the body. Some individuals with MAS experience excessive levels of growth hormone. Growth hormone has several functions in the body such as affecting growth and muscle mass. Increased growth velocity is the most common sign of growth hormone excess, however, in individuals with precocious puberty this may be masked (because precocious puberty has already increased the growth spurt). Growth hormone excess can lead to an abnormally large head (macrocephaly) and can potentially cause vision problems. Some individuals with MAS develop a disorder known as acromegaly, which is due to growth hormone excess after the growth plates have fused. The development of acromegaly can be associated with fibrous dysplasia affecting the skull base. (For more information on this disorder, choose “acromegaly” as your search term in the Rare Disease Database.)
The adrenal glands are located above the kidneys near the lower back and produce several hormones including cortisol. Cortisol is a glucocorticoid, a class of steroid hormone that is important in regulating metabolism of glucose and modulating stress. Individuals with MAS may have elevated levels of cortisol and can development a disorder known as Cushing syndrome. Symptoms can include upper body obesity, a round face, thin purple streaks (striae) that occur on the skin, increased fat around the neck, and thin, slender arms and legs. Children with Cushing syndrome are typically obese with slowed growth rates. (For more information on this disorder, choose “Cushing” as your search term in the Rare Disease Database.)
Some individuals with MAS develop elevated levels of phosphate in the blood (hypophosphatemia) because the kidneys cannot properly reabsorb phosphate. This occurs because fibrous dysplasia tissue produces a protein known as fibroblast growth factor 23 (FGF23). The amount of FGF23 correlates to the inability of the kidneys to metabolize phosphate (renal phosphate wasting). Therefore, individuals with significant fibrous dysplasia are more likely to develop hypophosphatemia. Hypophosphatemia can cause severe rickets or osteomalacia. Rickets is a childhood bone disease with characteristic bowing deformity of the legs and can contribute to short stature. Individuals with hypophosphatemia generally experience their first fracture at a younger age than individuals with MAS who do not have hypophosphatemia. Individuals with hypophosphatemia also develop more fractures and bone pain. In adults, hypophosphatemia can cause osteomalacia, a softening of the bones.
Less common symptoms sometimes associated with MAS include gastroesophageal reflux, gastrointestinal polyps, inflammation of the pancreas (pancreatitis), and several abnormalities of the heart (cardiac abnormalities). Such abnormalities include a faster than normal heart rate (tachycardia), high output heart failure, and aortic root dilatation.
Although the term tumor may be used to describe fibrous dysplasia lesions, these growths are benign (non-cancerous). Only in extremely rare cases, likely less than 1% of cases, have these lesions become cancerous (malignant transformation). These malignant tumors (osteosarcomas) developed in individuals who had been radiated for bone pain; a treatment option that has been abandoned.
Individuals may also be at an increased risk of developing breast cancer. Some reports suggest that this increased risk is more likely in individuals with growth hormone excess. Thyroid cancer and testicular cancer have also been reported in individuals with MAS in extremely rare instances.
McCune-Albright syndrome is caused by a mutation in a gene called GNAS1. This gene mutation occurs after fertilization of the embryo (somatic mutation) and is therefore not inherited, nor will affected individuals pass the mutation on to their children. Affected individuals have some cells with a normal copy of this gene and some cells with the abnormal gene (mosaic pattern). The variability of symptoms of MAS is due, in part, to the ratio of healthy cells to abnormal cells. Researchers do not know why these somatic mutations occur; they appear to develop randomly for unknown reasons (sporadically).
The GNAS1 gene is located on the long arm (q) of chromosome 20 (20q13.2) Chromosomes, which are present in the nucleus of human cells, carry the genetic information for each individual. Human body cells normally have 46 chromosomes. Pairs of human chromosomes are numbered from 1 through 22 and the sex chromosomes are designated X and Y. Males have one X and one Y chromosome and females have two X chromosomes. Each chromosome has a short arm designated “p” and a long arm designated “q”. Chromosomes are further sub-divided into many bands that are numbered. For example, “chromosome 20q13.2” refers to band 13.2 on the long arm of chromosome 20. The numbered bands specify the location of the thousands of genes that are present on each chromosome.
The GNAS1 gene creates (encodes) a subunit of a protein known as a G-protein. In MAS, a gain-of-function mutation in the GNAS1 gene results in continuous activation of this G-protein. In turn, there is an overproduction of a molecule known as cyclic adenosine monophosphate (cAMP), which is involved various chemical processes of the body.
Overproduction of cAMP contributes to the development of symptoms. For example, cAMP is involved in the change (differentiation) of osteoblasts in bone. Osteoblasts are immature bone-forming cells that form new bone. The human skeleton is living tissue that is constantly changing (remodeling). It is believed that MAS involves increased bone turnover. Bone turnover is a normal process in which bone gradually breaks down (bone resorption) and then reforms. Bone turnover involves osteoblasts and cells that control bone resorption (osteoclasts). The interaction between osteoclasts and osteoblasts determines how bone reforms. The interaction is a complex process that involves many factors. Improper differentiation of osteoblasts due to mutation of the GNAS1 gene is believed to contribute to the development of fibrous dysplasia in individuals with MAS.
When a GNAS1 mutation affects skin or endocrine cells, the additional characteristic symptoms of MAS can develop.
McCune-Albright syndrome affects males and females in equal numbers. Precocious puberty is more common in females. The disorder is estimated to affect 1 in 100,000 to 1 in 1,000,000 individuals in the general population. Because the disorder is difficult to diagnose, cases may go undiagnosed or misdiagnosed, making it difficult to determine the true frequency of MAS in the general population.
The diagnosis of McCune-Albright syndrome may be suspected at birth based upon identification of the characteristic skin pigmentations (cafe-au-lait spots). However, in many cases, the disorder may not be suspected until late infancy or childhood when precocious puberty develops or when bone deformities become obvious. A diagnosis may be confirmed based upon characteristic physical findings (i.e., association of characteristic skin, bone, and endocrine abnormalities), a detailed patient history, thorough clinical evaluation, and specialized tests including x-ray studies and blood tests.
Clinical Testing and Workup
A complete body survey should be performed for the characteristic cafe-au-lait spots, and x-ray studies should be combined with bone scans to evaluate the presence and extent of fibrous dysplasia. Blood tests may reveal elevated hormone levels (e.g., estrogen, testosterone, cortisol, thyroid hormone, growth hormone, prolactin, somatomedin C) and evidence of abnormally increased bone activity (elevated alkaline phosphatase).
Specialized imaging techniques may be used to evaluate bone. Such imaging techniques include computerized tomography (CT) scanning and magnetic resonance imaging (MRI). During CT scanning, a computer and x-rays are used to create a film showing cross-sectional images of certain tissue structures. An MRI uses a magnetic field and radio waves to produce cross-sectional images of particular organs and bodily tissues. The abnormal tissue in FD resembles ground glass when seen on x-ray. These tests may be used to determine how extensively bones are affected.
A bone scan, also known as bone scintigraphy, is used to determine the extent of bone disease. During this test, a harmless radioactive dye is injected into the affected bone. A special camera that can track the dye as it travels through bone is used to create a picture of the skeleton and determine all affected areas. Bone biopsy is the surgical removal and microscopic examination of a small sample of affected tissue. A bone biopsy can reveal characteristic changes to bone that occur in individuals with FD and may be necessary to distinguish a FD lesion from other types of growths or tumors if it is unclear after an x-ray.
A highly sensitive, specific form of polymerase chain reaction (PCR) has been used to detect somatic mutations of the GNAS1 gene that characterize MAS. PCR is a laboratory test that has been described as a form of “photocopying.” It enables researchers to enlarge and repeatedly copy sequences of DNA. As a result, they are able to closely analyze DNA and more easily identify genes and genetic changes (mutations). In MAS, a specific form of PCR testing can detect activating mutations of GNAS1 in peripheral blood cells. However, because only some cells in the body are affected by the mutation, a normal test would not rule out MAS, and so this test is not frequently used in clinical diagnosis.
The treatment of McCune-Albright syndrome is directed toward the specific symptoms that are apparent in each individual. Treatment may require the coordinated efforts of a team of specialists. Pediatricians, orthopedists, orthopedic surgeons, endocrinologists, dermatologists, and other healthcare professionals may need to systematically and comprehensively plan an affected child’s treatment. Psychosocial support for the entire family is essential as well. Although MAS is not inherited, genetic counseling may be of benefit for affected individuals and their families.
Specific therapeutic procedures and interventions may vary, depending upon numerous factors, such as extent of the disease; the presence or absence of certain symptoms; an individual’s age and general health; and/or other elements. Decisions concerning the use of particular drug regimens and/or other treatments should be made by physicians and other members of the health care team in careful consultation with the patient based upon the specifics of his or her case; a thorough discussion of the potential benefits and risks, including possible side effects and long-term effects; patient preference; and other appropriate factors.
Fibrous dysplasia associated with MAS has been treated with drugs known as bisphosphonates such as pamidronate or alendronate. These drugs reduce bone turnover by inhibiting bone resorption. Calcium and vitamin D may be given along with the drug. Some affected individuals respond favorably to such therapy with the main benefit being decreased bone pain. Other affected individuals do not respond to therapy with bisphosphonates or relapse after an initial period of improvement. Relapse of bone pain is more common. Stronger bisphosphonate medications such as zoledronic acid may be used in such cases and may be most effective in improving bone pain.
Surgery is sometimes used to treat fibrous dysplasia, although most physicians recommend a conservative strategy. Surgery should be undertaken only for lesions that causing difficulty in some way. Surgery may be undertaken to correct disfigurement or deformity, to correct limb length discrepancy, to eradicate symptomatic lesions (e.g. those causing pain and/or compressing a nerve), to treat specific complications such as scoliosis, or to prevent fracture.
Exercises designed to strengthen the muscles surrounding fibrous dysplasia lesions may be recommended and may help to reduce the risk of fracture.
In mild cases, females experiencing precious puberty may not require treatment, but only observation. Drug therapy may be required if females experience progressive precious puberty. Such drugs include letrozole, which is an aromatase inhibitor and has a long history of safety for the treatment of precocious puberty in MAS. These drugs block the conversion of androgens to estrogen. No drugs have been shown to be completely effective to-date. In some cases, males experiencing precious puberty may be treated with aromatase inhibitors. Additional drugs are being studied for the treatment of precious puberty in individuals with MAS.
Precocious puberty in MAS is known as gonadotropin-independent. Gonadotropins are hormones such as follicle stimulating hormone and luteinizing hormone that are produced by the pituitary gland during puberty and regulate various actions involved in puberty. Most cases of precocious puberty (e.g. those not associated with MAS) are known as central precocious puberty or gonadotropin-dependent precious puberty and can be successfully treated with gonadotropin-releasing hormone, a hormone that, when used regularly, decreases the amount of these hormones released by the pituitary gland. This drug is not effective in most cases of MAS. However, in some affected females, central precocious puberty can develop as a secondary condition and can be successfully treated by long-acting gonadotropin-releasing hormone analogues.
Hyperthyroidism may also be treated with drug therapy, specifically thionamides, which inhibit the production of thyroid hormones. Most individuals with MAS respond favorably to this therapy. However, hyperthyroidism in MAS is often persistent and some physicians recommend surgical removal of the thyroid (thyroidectomy) followed by radioactive iodine ablation. Iodine is a chemical element used by the thyroid to synthesize thyroid hormones. Nearly all of the iodine in a person’s blood is absorbed by thyroid tissue. Radioactive iodine therapy destroys any thyroid tissue that remains after a near-total thyroidectomy. After these procedures, individuals must take hormone replacement therapy for the remainder of their lives to replace the hormones normally produced by the thyroid.
Growth hormone excess may be treated by drugs known as long-acting somatostatin analogues such as octreotide or bromocriptine. This class of drugs inhibits the production of growth hormone. A growth hormone receptor antagonist, pegvisomant, has also been used to treat growth hormone excess, although somatostatins have generally proven more effective, particularly in children. If medication does not work, surgical removal of the pituitary gland and the destruction of pituitary tissue using radiation (radiotherapy) may be necessary.
In some cases Cushing’s syndrome can resolve on its own. Drugs that suppress the production of cortisol may be used and have been effective even in severe cases. However, Cushing’s syndrome can potentially be a severe complication of MAS and may not respond to drug therapy and some physicians consider the surgical removal of the adrenal glands (adrenalectomy) the treatment of choice. Individuals who undergo an adrenalectomy will receive hormone replacement therapy.
Individuals with rickets or osteomalacia due to hypophosphatemia may require treatment with oral phosphorous supplementation and calcitriol, an activated vitamin-D metabolite. Whether children who have hypophosphatemia, but do not have signs of rickets require treatment is debated. Some physicians recommend that individuals with markedly low serum phosphate levels should be treated.
Additional drugs are being studied to treat precious puberty in individuals with MAS including estrogen receptor antagonists such as tamoxifen, raloxifene, and fulvestrant. These drugs block or hamper the effects of estrogen in the body. More research is necessary to determine the long-term safety and effectiveness of these drugs for treating precious puberty in individuals with MAS.
The drug tocilizumab is being studied as a potential therapy for fibrous dysplasia. Tocilizumab is a drug that works by blocking the activity of interleukin-6 (IL-6), a specialized protein (cytokine) that stimulates bone resorption. Affected bone cells in fibrous dysplasia release excess levels of interleukin-6 and researchers believe inhibiting IL-6 will decrease bone resorption. Additional drugs are being study as potential therapies. Such drugs include denosumab, a monoclonal antibody currently approved to treat osteoporosis and pregabalin, which is effective at treating neuropathic pain. More research is necessary to determine the long-term safety and effectiveness of experimental medications for the treatment of individuals with fibrous dysplasia.
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