The AHCY gene provides instructions for producing the enzyme S-adenosylhomocysteine hydrolase. This enzyme is involved in a multistep process that breaks down the protein building block (amino acid) methionine. Specifically, S-adenosylhomocysteine hydrolase controls the step that converts the compound S-adenosylhomocysteine to the compounds adenosine and homocysteine. This reaction also plays an important role in regulating the addition of methyl groups, consisting of one carbon atom and three hydrogen atoms, to other compounds (methylation). Methylation is important in many cellular processes. These include determining whether the instructions in a particular segment of DNA are carried out, regulating reactions involving proteins and lipids, and controlling the processing of chemicals that relay signals in the nervous system (neurotransmitters).
Three mutations in the AHCY gene have been described in people with hypermethioninemia. In a Croatian family, one mutation substitutes the amino acid cysteine for the amino acid tyrosine at protein position 143 (written as Tyr143Cys or Y143C). Another mutation replaces the amino acid tryptophan with a premature stop signal at position 112 (written as Trp112X or W112X), resulting in an enzyme that is abnormally short. A U.S. patient was found to have, in addition to the Y143C mutation, a mutation that substitutes the amino acid valine for the amino acid alanine at position 89 (written as Ala89Val or A89V). These mutations reduce the activity of the S-adenosylhomocysteine hydrolase enzyme, resulting in the signs and symptoms of hypermethioninemia.