Multiple Endocrine Neoplasia ... Health Article

Definition

The multiple endocrine neoplasia (MEN) syndromes are three related disorders in which two or more of the hormone-secreting (endocrine) glands of the body develop tumors. Commonly affected glands are the thyroid, parathyroids, pituitary, adrenals, and pancreas. Two common cancers are medullary thyroid cancer and gastrinomas. MEN is sometimes called familial multiple endocrine neoplasia (FMEN) and previously has been known as familial endocrine adenomatosis.

Description

The three forms of MEN are MEN1 (Wermer's syndrome), MEN2A (Sipple syndrome), and MEN2B (previously known as MEN3). Each form leads to excessive growth of normal cells (hyperplasia) and overactivity of a number of endocrine glands. Excessive growth can result in the formation of tumors (neoplasia) that are either benign (noncancerous) or malignant (cancerous). Overactive endocrine glands increase the secretion of hormones into the bloodstream. Hormones are important chemicals that control and instruct the functions of different organs. Their levels in the body are carefully balanced to maintain normal functioning of many vital processes, including metabolism, growth, timing of reproduction, and the composition of blood and other body fluids.

All three forms are genetic disorders. They result when an abnormal form of a gene is inherited from one parent. The gene causing MEN1, named the MEN1 gene, was isolated in 1997. Both types of MEN2 are caused by mutations of the RET (REarranged during Transfection) gene. MEN1 and MEN2 are both autosomal dominant genetic conditions, meaning that an individual needs only one defective copy of the MEN1 gene or the RET gene to develop the associated disorder. In all forms, the children of an affected individual have a 50% chance of inheriting the defective gene.

The three forms of MEN are further distinguished by the endocrine glands affected. MEN1 is characterized by conditions of the parathyroid glands, pancreas, and pituitary gland. Patients with MEN2 commonly experience a form of thyroid cancer and adrenal tumors.

MEN1

Enlarged and overactive parathyroid glands, a condition called hyperparathyroidism, is present in 90% to 97% of MEN1 gene carriers and is usually the first condition to develop. The four parathyroid glands are located in the neck region, with a pair of the glands on either side of the thyroid. They produce parathyroid hormone, which regulates calcium and phosphorus levels. Hyperparathyroidism leads to elevated levels of the hormone, resulting in high blood calcium levels (hypercalcemia), which can cause kidney stones and weakened bones. All four parathyroid glands tend to develop tumors, but most tumors are benign and parathyroid cancer is rare. Hyperparathyroidism may be present during the teenage years, but most individuals are affected by age 40.

Pancreatic tumors occur in 40% to 75% of individuals with the MEN1 gene. The pancreas, which sits behind the stomach, has two parts, an endocrine part and an exocrine part. Tumors in MEN1 occur only in the endocrine pancreas. Among the hormones secreted are ones that lower and raise blood sugar levels—insulin and glucagons—and the hormone gastrin, which is secreted into the stomach to aid in digestion. Thirty to 35% of pancreatic tumors are malignant, and they are the tumors most likely to cause cancer in MEN1 patients. Gastrinproducing tumors (gastrinomas) are the most common tumors that form, representing about 50% of the MEN1 pancreatic tumors. Other tumors that form are insulin-producing tumors (insulinomas), representing 25% to 30%, and glucagon-producing tumors (glucagonomas), representing 5% to 10%.

Gastrinomas can cause recurring upper gastrointestinal ulcers, a condition called Zollinger-Ellison syndrome. About half of MEN1 patients with a pancreatic condition develop this syndrome. Insulinomas raise the insulin level in the blood and can lead to hypoglycemia, or low blood sugar (glucose), resulting in glucose levels that are too low to fuel the body's activity. Glucagonomas can cause high blood sugar levels, or hyperglycemia.

Pituitary tumors are the third most common condition in MEN1, occurring in about 50% of MEN1 patients. Fewer than 5% of these tumors are malignant. The pituitary gland, located at the base of the brain, secretes many hormones that regulate the function of other endocrine glands. The most common tumors forming in MEN1 patients are prolactin-producing tumors (prolactinomas) and growth hormone-secreting tumors, which lead to a condition known as acromegaly.

MEN2

Patients with MEN2A and MEN2B experience two main symptoms, medullary thyroid cancer (MTC) and a medullary adrenal tumor known as pheochromocytoma. Additional symptoms distinguish the two forms of MEN2. Twenty percent of MEN2A patients develop parathyroid tumors, which have not been reported for MEN2B. As in MEN1, parathyroid tumors in MEN2A affect all four glands and are usually benign. MEN2B is further characterized by the occurrence of benign tumors of the tongue, nasal cavities, and other facial surfaces (mucosal neuromas) and by a condition known as marfanoid habitus. Marfanoid habitus features a characteristic appearance resulting from severe wasting of the proximal muscles. A distinct facial appearance—an elongated face with a thick forehead, wide-eyed look, and broad nose— is often noted at birth. Gastrointestinal, skeletal, and pigmentation abnormalities may also occur. Mucosal neuromas occur in all MEN2B patients, and marfanoid habitus occurs in 65%. About 5% of MEN2 cases are MEN2B.

Ninety-five percent of MEN2A patients and 90% of MEN2B patients develop medullary thyroid carcinoma (MTC). Medullary thyroid carcinoma forms from the C-cells of the thyroid. C-cells make the hormone calcitonin, which is involved in regulating the calcium levels in the blood and calcium absorption by the bones. The thyroid, which is located in the front of the neck between the Adam's apple and the collarbone, also secretes hormones that are essential for the regulation of body temperature, heart rate, and metabolism.

Medullary thyroid carcinoma causes high blood levels of calcitonin. In MEN2B, MTC develops earlier and is more aggressive than in MEN2A. It has been described in MEN2B patients younger than one year, whereas in MEN2A patients it is likely to occur between the ages of 20 and 40.

Pheochromocytoma is found in 50% of MEN2A patients and 45% of MEN2B patients. A tumor of the medulla portion of the adrenal gland, it is usually a slow-growing and benign adrenal tumor. The two flat adrenal glands, one situated above each kidney, secrete the hormones epinephrine and norepinephrine to increase heart rate and blood pressure, along with other effects. Excessive secretion of these adrenal hormones can cause life-threatening hypertension and cardiac arrhythmia. Tumors form on both adrenal glands in 50% of MEN2 patients diagnosed with a pheochromocytoma. Tumor malignancy is very rare.