succinate dehydrogenase complex subunit C
The SDHC gene provides instructions for making one of four subunits of the succinate dehydrogenase (SDH) enzyme. The SDH enzyme plays a critical role in mitochondria, which are structures inside cells that convert the energy from food into a form that cells can use. The SDHC protein helps anchor the SDH enzyme in the mitochondrial membrane.
Within mitochondria, the SDH enzyme links two important cellular pathways in energy conversion: the citric acid cycle (or Krebs cycle) and oxidative phosphorylation. As part of the citric acid cycle, the SDH enzyme converts a compound called succinate to another compound called fumarate. Negatively charged particles called electrons are released during this reaction. The electrons are transferred through the SDH subunits, including the SDHC protein, to the oxidative phosphorylation pathway. In oxidative phosphorylation, the electrons help create an electrical charge that provides energy for the production of adenosine triphosphate (ATP), the cell's main energy source.
Succinate, the compound on which the SDH enzyme acts, is an oxygen sensor in the cell and can help turn on specific pathways that stimulate cells to grow in a low-oxygen environment (hypoxia). In particular, succinate stabilizes a protein called hypoxia-inducible factor (HIF) by preventing a reaction that would allow HIF to be broken down. HIF controls several important genes involved in cell division and the formation of new blood vessels in a hypoxic environment.
The SDHC gene is a tumor suppressor, which means it prevents cells from growing and dividing in an uncontrolled way.
More than 30 mutations in the SDHC gene have been found to increase the risk of hereditary paraganglioma-pheochromocytoma type 3. People with this condition have paragangliomas, pheochromocytomas, or both. An inherited SDHC gene mutation predisposes an individual to the condition, and a somatic mutation that deletes the normal copy of the SDHC gene is needed to cause hereditary paraganglioma-pheochromocytoma type 3.
Most of the inherited SDHC gene mutations change single protein building blocks (amino acids) in the SDHC protein sequence or result in a shortened protein. As a result, there is little or no SDH enzyme activity. Because the mutated SDH enzyme cannot convert succinate to fumarate, succinate accumulates in the cell. The excess succinate abnormally stabilizes HIF, which also builds up in cells. Excess HIF stimulates cells to divide and triggers the production of blood vessels when they are not needed. Rapid and uncontrolled cell division, along with the formation of new blood vessels, can lead to the development of tumors in people with hereditary paraganglioma-pheochromocytoma.
Mutations in the SDHC gene have been found in a small number of people with gastrointestinal stromal tumor (GIST), which is a cancer of the gastrointestinal tract. SDHC gene mutations have also been identified in people with noncancerous tumors associated with the nervous system called paragangliomas or pheochromocytomas (a type of paraganglioma). Some affected individuals have both paraganglioma and GIST, which is called Carney-Stratakis syndrome. An inherited SDHC gene mutation predisposes an individual to cancer formation. An additional mutation that deletes the normal copy of the gene is needed to cause these forms of GIST and paraganglioma. This second mutation, called a somatic mutation, is acquired during a person's lifetime and is present only in tumor cells.
Mutations of the SDHC gene lead to loss of SDH enzyme activity, which results in abnormal hypoxia signaling and formation of tumors.