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Reviewed April 2007
What is hypermethioninemia?
Hypermethioninemia is an excess of a particular protein building block (amino acid), called methionine, in the blood. This condition can occur when methionine is not broken down (metabolized) properly in the body.
People with hypermethioninemia often do not show any symptoms. Some individuals with hypermethioninemia exhibit intellectual disability and other neurological problems; delays in motor skills such as standing or walking; sluggishness; muscle weakness; liver problems; unusual facial features; and their breath, sweat, or urine may have a smell resembling boiled cabbage.
Hypermethioninemia can occur with other metabolic disorders, such as homocystinuria, tyrosinemia and galactosemia, which also involve the faulty breakdown of particular molecules. It can also result from liver disease or excessive dietary intake of methionine from consuming large amounts of protein or a methionine-enriched infant formula.
How common is hypermethioninemia?
Primary hypermethioninemia that is not caused by other disorders or excess methionine intake appears to be rare; only a small number of cases have been reported. The actual incidence is difficult to determine, however, since many individuals with hypermethioninemia have no symptoms.
What genes are related to hypermethioninemia?
Mutations in the AHCY, GNMT, and MAT1A genes cause hypermethioninemia.
Inherited hypermethioninemia that is not associated with other metabolic disorders can be caused by shortages (deficiencies) in the enzymes that break down methionine. These enzymes are produced from the MAT1A, GNMT and AHCY genes. The reactions involved in metabolizing methionine help supply some of the amino acids needed for protein production. These reactions are also involved in transferring methyl groups, consisting of a carbon atom and three hydrogen atoms, from one molecule to another (transmethylation), which is important in many cellular processes.
The MAT1A gene provides instructions for producing the enzyme methionine adenosyltransferase. This enzyme converts methionine into a compound called S-adenosylmethionine, also known as AdoMet or SAMe. The GNMT gene provides instructions for making the enzyme glycine N-methyltransferase. This enzyme starts the next step in the process, converting AdoMet to a compound called S-adenosyl homocysteine, or AdoHcy. The AHCY gene provides instructions for producing the enzyme S-adenosylhomocysteine hydrolase. This enzyme converts the AdoHcy into the compound homocysteine. Homocysteine may be converted back to methionine or into another amino acid, cysteine.
A deficiency of any of these enzymes results in a buildup of methionine in the body, and may cause signs and symptoms related to hypermethioninemia.
How do people inherit hypermethioninemia?
Hypermethioninemia can have different inheritance patterns. This condition is usually inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations. Most often, the parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but do not show signs and symptoms of the condition.
Hypermethioninemia is occasionally inherited in an autosomal dominant pattern, which means one copy of the altered gene in each cell is sufficient to cause the disorder. In these cases, an affected person usually has one parent with the condition.
Where can I find information about diagnosis or management of hypermethioninemia?
These resources address the diagnosis or management of hypermethioninemia and may include treatment providers.
General information about the diagnosis and management of genetic conditions is available in the Handbook. Read more about genetic testing, particularly the difference between clinical tests and research tests.
To locate a healthcare provider, see How can I find a genetics professional in my area? in the Handbook.
Where can I find additional information about hypermethioninemia?
You may find the following resources about hypermethioninemia helpful. These materials are written for the general public.
You may also be interested in these resources, which are designed for healthcare professionals and researchers.
What other names do people use for hypermethioninemia?
What if I still have specific questions about hypermethioninemia?
Where can I find general information about genetic conditions?
The Handbook provides basic information about genetics in clear language.
These links provide additional genetics resources that may be useful.
What glossary definitions help with understanding hypermethioninemia?
acids ; amino acid ; atom ; autosomal ; autosomal dominant ; autosomal recessive ; cell ; compound ; deficiency ; enzyme ; gene ; glycine ; hepatic ; hydrolase ; incidence ; inheritance ; methyl ; methyltransferase ; molecule ; motor ; neurological ; newborn screening ; protein ; recessive ; screening
You may find definitions for these and many other terms in the Genetics Home Reference Glossary.
See also Understanding Medical Terminology.
References (18 links)
The resources on this site should not be used as a substitute for professional medical care or advice. Users seeking information about a personal genetic disease, syndrome, or condition should consult with a qualified healthcare professional. See How can I find a genetics professional in my area? in the Handbook.