Ectopic secretion of peptides of the proopiomelanocortin family.

A wide spectrum of clinical and biochemical presentations characterize ectopic POMC syndrome. It is presently postulated that ectopic POMC production results from increased expression of the activity of a POMC gene normally occurring in a variety of tissues outside the pituitary gland. The syndrome is rapidly progressive and is characterized by severe clinical manifestations in patients with a more aggressively developing oat cell carcinoma of the lung. However, in patients with slower growing nonpituitary tumors, the presentation may overlap that seen in patients with pituitary ACTH-dependent Cushing's syndrome. In cases in which the biochemical testing results overlap with those seen in pituitary ACTH-dependent disease, a combination of lack of suppression with high-dose dexamethasone and absent response to CRH stimulation greatly increases the diagnostic accuracy. Abnormal alternative processing of POMC in nonpituitary neoplasms may render unusual POMC-derived peptides that could be used as markers in the diagnosis and follow-up of patients with this syndrome. The prognosis of patients afflicted with ectopic POMC syndrome is largely determined by the nature of the underlying tumor. However, the clinical course can be greatly modified by control of the metabolic manifestations of hypercortisolemia. A variety of surgical and pharmacologic options are available, including adrenalectomy and the use of adrenal inhibitors. Cushing's syndrome of long duration, the finding of ectopic pituitary adenomas, and the combined secretion of CRH and POMC by nonpituitary tumors constitute interesting variants of the classic picture.