This common variant results in a form of methylenetetrahydrofolate reductase that has reduced activity at higher temperatures (the enzyme is thermolabile). Specifically, it replaces the nucleotide cytosine with the nucleotide thymine at position 677 (written as 677C>T). The most well-studied MTHFR polymorphism changes a single DNA building block (nucleotide) in the MTHFR gene. Affected individuals are missing large parts of the brain and have missing or incompletely formed skull bones. Anencephaly is one of the most common types of neural tube defect. Some studies have found that variations (polymorphisms) in the MTHFR gene have been associated with a small increased risk of neural tube defects, a group of birth defects that occur during the development of the brain and spinal cord. Researchers have not determined how altered levels of homocysteine and methionine lead to the various health problems affecting multiple parts of the body in people with homocystinuria. Some of the excess homocysteine is excreted in urine (homocystinuria). As a result, homocysteine builds up in the bloodstream, and the amount of methionine is reduced. Without functional methylenetetrahydrofolate reductase, homocysteine cannot be converted to methionine. Other mutations lead to the production of an abnormally small, nonfunctional version of the enzyme. ![]() These changes impair the function of the enzyme, and some cause the enzyme to be turned off (inactivated). Most of the mutations that cause homocystinuria change single amino acids in methylenetetrahydrofolate reductase. ![]() People with this condition often develop eye problems, abnormal blood clotting, skeletal abnormalities, and learning problems. At least 40 mutations in the MTHFR gene have been identified in people with homocystinuria, a disorder in which the body is unable to process homocysteine and methionine properly.
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