G protein subunit alpha q
The GNAQ gene provides instructions for making a protein called guanine nucleotide-binding protein G(q) subunit alpha (Gαq). The Gαq protein is part of a group of proteins called the trimeric G protein complex. This complex attaches (binds) to other proteins called G protein coupled receptors. When the protein complex is bound to a receptor, the Gαq protein binds to a molecule called GTP and is turned on (activated). The activated Gαq protein then separates from the protein complex and activates signaling pathways that help to regulate the development and function of blood vessels. The Gαq protein converts GTP to a similar molecule called GDP, which turns off (inactivates) the protein. It then reattaches to the trimeric G protein complex, turning off the signaling pathways.
At least one mutation in the GNAQ gene has been found to cause Sturge-Weber syndrome. Sturge-Weber syndrome is a condition that affects the development of certain blood vessels and often leads to three major features: a red or pink birthmark called a port-wine birthmark, brain abnormalities, and increased pressure in the eye (glaucoma) or other eye problems. The GNAQ gene mutation associated with Sturge-Weber syndrome changes a single building block (amino acid) in the Gαq protein. It replaces the amino acid arginine with the amino acid glutamine at position 183 in the Gαq protein (written as Arg183Gln or R183Q). This mutation is not inherited but occurs after conception. This alteration is called a somatic mutation and is present only in certain cells, specifically cells in the brain, eyes, and skin that are involved in blood vessel formation.
Following its activation, the altered Gαq protein cannot convert GTP to GDP. As a result, the protein is always active, and signaling pathways controlled by it are constantly turned on. This increased signaling likely disrupts the regulation of blood vessel development, causing abnormal and excessive formation of vessels before birth in people with Sturge-Weber syndrome.
The R183Q mutation in the GNAQ gene can also cause port-wine birthmarks without the brain or eye abnormalities that are often associated with Sturge-Weber syndrome (described above). As in Sturge-Weber syndrome, isolated port-wine birthmarks caused by a GNAQ gene mutation are usually on one side of the face but can be on both sides. It is thought that somatic GNAQ gene mutations that cause isolated port-wine birthmarks occur later in fetal development than those that cause Sturge-Weber syndrome and so affect fewer cells and tissues.
Somatic mutations in the GNAQ gene have also been found in an eye cancer called uveal melanoma. This cancer occurs in the middle layer of the eye called the uvea. The uvea includes the colored portion of the eye (the iris) and related tissues that underlie the white part of the eye (the sclera). The GNAQ gene mutations in uveal melanoma result in an overactive protein, which leads to excessive signaling. This abnormal signaling likely contributes to the overgrowth of cells and to the formation of a cancerous tumor. While the R183Q mutation has been found in uveal melanoma, individuals with Sturge-Weber syndrome or isolated port-wine birthmark do not have an increased risk of this form of cancer. GNAQ gene mutations that lead to uveal melanoma usually occur later in a person's life, typically in adulthood, and are limited to the cells that give rise to the tumor.
- guanine nucleotide binding protein (G protein), q polypeptide
- guanine nucleotide-binding protein alpha-q
- guanine nucleotide-binding protein G(q) subunit alpha
- Basic Neurochemistry (sixth edition, 1999): The Functional Activity of G Proteins Involves Their Dissociation and Reassociation in Response to Extracellular Signals
- Biochemistry (fifth edition, 2002): G Proteins Cycle Between GDP- and GTP-Bound Forms
- Molecular Biology of the Cell (fourth edition, 2002): Trimeric G Proteins Disassemble to Relay Signals from G-Protein-linked Receptors
- National Cancer Institute: Intraocular (Uveal) Melanoma Treatment PDQ