The TYMP gene (previously known as ECGF1) provides instructions for making an enzyme called thymidine phosphorylase. Thymidine is a molecule known as a nucleoside, which (after a chemical modification) is used as a building block of DNA. Thymidine phosphorylase converts thymidine into two smaller molecules, 2-deoxyribose 1-phosphate and thymine. This chemical reaction is an important step in the breakdown of thymidine, which helps regulate the level of nucleosides in cells.
Thymidine phosphorylase plays an important role in maintaining the appropriate amount of thymidine in cell structures called mitochondria. Mitochondria convert the energy from food into a form that cells can use. Although most DNA is packaged in chromosomes within the nucleus, mitochondria also have a small amount of their own DNA (called mitochondrial DNA or mtDNA). Mitochondria use nucleosides, including thymidine, to build new molecules of mtDNA as needed.
About 50 mutations in the TYMP gene have been identified in people with mitochondrial neurogastrointestinal encephalopathy (MNGIE) disease. TYMP mutations greatly reduce or eliminate the activity of thymidine phosphorylase. A shortage of this enzyme allows thymidine to build up to very high levels in the body. An excess of thymidine appears to be damaging to mtDNA, disrupting its usual maintenance and repair. As a result, mutations can accumulate in mtDNA, causing it to become unstable. Mitochondria may also have less mtDNA than usual (mtDNA depletion). These genetic changes impair the normal function of mitochondria. Although mtDNA abnormalities underlie the digestive and neurological problems characteristic of MNGIE disease, it is unclear how defective mitochondria cause the specific features of the disorder.
- endothelial cell growth factor 1 (platelet-derived)