Definition of McCune-Albright syndrome
McCune-Albright syndrome: A genetic disorder of bones, skin pigmentation and hormonal problems with premature sexual development. Also called Albright syndrome or polyostotic fibrous dysplasia. In the syndrome, there is bone disease with fractures and deformity of the legs, arms and skull; pigment patches of the skin; and endocrine (hormonal) disease with early puberty (early menstrual bleeding, development of breasts and pubic hair) and an increased rate of growth. The McCune-Albright syndrome is usually caused by mosaicism for a mutation in a gene called GNAS1 (Guanine Nucleotide binding protein, Alpha Stimulating activity polypeptide 1).
The syndrome shows a broad spectrum of severity. Sometimes, children are diagnosed in early infancy with obvious bone disease and markedly increased endocrine secretions from several glands. At the opposite end of the spectrum, many children are entirely healthy, and have little or no outward evidence of bone or endocrine involvement. They may enter puberty close to the normal age, and have no unusual skin pigment at all.
- Precocious Puberty -- When the signs of puberty (development of breasts, testes, pubic and underarm hair, body odor, menstrual bleeding, and increased growth rate) appear before the age of 8 years in a girl or 9 1/2 years in a boy, it is termed "precocious puberty." In the most common form of precocious puberty, there is early activation of the regions in the brain which control the maturation of the gonads (ovaries in a girl and testes in a boy). One brain center, the hypothalamus, secretes a substance called gonadotropin-releasing-hormone or "GnRH." This acts, in turn, on another part of the brain, the pituitary gland, to cause increased secretion of hormones called "gonadotropins" (LH and FSH) that travel through the bloodstream, and act on the ovaries or testes to stimulate secretion of estrogen or testosterone.
The precocious puberty in McCune-Albright girls is caused by estrogens which are secreted into the bloodstream by ovarian cysts, which enlarge, and then decrease in size over periods of weeks to days. The cysts can be visualized and measured by ultrasonography, in which sound waves are used to outline the dimensions of the ovaries. The cysts may become quite big, occasionally over 50 cc in volume (about the size of a golf ball). Frequently, menstrual bleeding and breast enlargement accompany the growth of a cyst. In fact, menstrual bleeding under 2 years of age has been the first symptom of McCune-Albright syndrome in 85% of patients. Although ovarian cysts and irregular menstrual bleeding may continue into adolescence and adulthood, many adult women with McCune-Albright syndrome are fertile, and can bear normal children.
The precocious puberty in McCune-Albright syndrome has been difficult to treat. After surgical removal of the cyst or of the entire affected ovary, cysts usually recur in the remaining ovary. A progesterone-like hormone called Provera can be given to suppress the menstrual bleeding, but does not appear to slow the rapid rates of growth and bone development, and may have unwanted effects on adrenal functioning. The synthetic forms of GnRH (Deslorelin, Histerelin, and Lupron) which suppress LH and FSH, and are used to treat the common, gonadotropin-dependent form of precocious puberty, are not effective in most girls with McCune-Albright syndrome.
- Thyroid Function -- Almost 50% of patients with McCune-Albright syndrome have thyroid gland abnormalities. These include generalized enlargement called goiter and irregular masses called nodules and cysts. Some patients have subtle structural changes detected only by ultrasonography. Pituitary thyroid-stimulating-hormone (TSH) levels are low in these patients, and thyroid hormone levels may be normal or elevated. Therapy with drugs which block thyroid hormone synthesis (Propylthiouracil or Methimazole), can be given if thyroid hormone levels are excessively high.
- Growth Hormone -- Excessive secretion of pituitary growth hormone is sometimes seen in McCune-Albright syndrome. This may be diagnosed in young adults, when they developed coarsening of facial features, enlargement of hands and feet, and arthritis characteristic of the condition termed "acromegaly." Therapy has included surgical removal of the area of the pituitary which is secreting the hormone, and use of new, synthetic analogs of the hormone somatostatin, which suppress growth hormone secretion.
- Other Endocrine Abnormalities -- Although rare, adrenal enlargement and excessive secretion of the adrenal hormone cortisol is seen in McCune-Albright syndrome. This may cause obesity of the face and trunk, weight gain, skin fragility and cessation of growth in childhood. These symptoms are called "Cushing's syndrome." Treatment is removal of the affected adrenal glands, or use of drugs which block cortisol synthesis.
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