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  1. Paragangliomas are mostly benign, slow growing tumors in the head and neck region originating from the parasympathetic paraganglion system.

  2. Between 10 and 50 percent of the cases are caused by an inherited gene-defect. The inheritance pattern seen in most families displays a parent-of-origin effect. In these families, children of affected mothers never develop the disease; only paternal transmission of the gene-defect confers susceptibility.

  3. Mutations in SDHD gene are the major cause of this inherited tumor. SDHD is a subunit of the mitochondrial respiratory chain complex II. Two families have been identified in which the disease mutation resides in other genes, namely in SDHC in a family of German origin, and in an as yet unidentified gene (PGL2) in a Dutch family. Mutations in SDHB have been found in a small number of families with adrenal pheochromocytoma, with or without head and neck paraganglioma. SDHB and SDHC are two other subunits of the mitochondrial complex II.

  4. Tumors developing in carriers of an SDHC or SDHD mutation have invariably lost the wild-type allele, suggesting that both genes function as a tumor-suppressor gene in parasympathetic paraganglion tissues.

  5. SDHC and SDHD are the first tumor suppressor genes whose normal cellular function are exerted in the mitochondria.

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