Mutation of the MTHFR gene causes hyperhomocysteinemia, a build-up of homocysteine in the blood, explains the U.S. National Library of Medicine. It can also cause neural tube birth defects like anencephaly, in which parts of the brain, skull or spinal cord are missing or malformed; spina bifida, in which defective vertebra expose the spinal cord; and cleft lip or palate.
Adults with the defect suffer elevated risks of a range of cardiovascular diseases, such as atherosclerosis, the hardening of the arteries caused by elevated homocysteine levels; stroke caused by clotting in veins and arteries; hypertension; and preeclampsia, or hypertension during pregnancy; as well as glaucoma and the incidence of certain cancers and psychiatric disorders. The mutation inhibits synthesis of methylenetetrahydrofolate, an enzyme that regulates the conversion of amino acids associated with vitamin B9, or folate, into proteins, according to the U.S. National Library of Medicine.
Specifically, the defect interferes with the conversion of the amino acid homocysteine into the amino acid methionine, and doctors first studied it in patients suffering from imbalances of these two chemicals. Isolation of the mutation's DNA sequence in 1994 made it easier to study its common variants and the diseases associated with them, but the mutation alone cannot predict their occurrence, notes the National Center for Biotechnology Information. Each has multiple risk factors, and researchers do not perfectly understand the role the mutation plays in them, as of 2015.