Conditions Hypermethioninemia

Hypermethioninemia (MET) is a condition that occurs when there is a high amount of methionine in the body. It is considered an amino acid condition because people with MET are unable to break down an amino acid, a building block of proteins, known as methionine. Many people with MET do not show signs of the condition. However, if MET is untreated, it can cause learning delays, muscle weakness, and other health problems in some affected individuals.

Condition Type

Amino Acid Disorders


Hypermethioninemia is a rare condition. The exact number of individuals affected by this condition is currently unknown.

Also known as

  • MET
  • Deficiency of methionine adenosyltransferase
  • Glycine N-methyltransferase deficiency
  • GNMT deficiency
  • Hepatic methionine adenosyltransferase deficiency
  • MAT deficiency
  • MET, methionine adenosyltransferase deficiency
  • Methioninemia
  • S-adenosylhomocysteine hydrolase deficiency

Follow-Up Testing

Your baby’s doctor may ask you if your baby is showing any of the signs of MET (see Early Signs below). If your baby has certain signs, your baby’s doctor may suggest starting immediate treatment.

If your baby’s newborn screening result for hypermethioninemia (MET) was out of the normal range, your baby’s doctor or the state screening program will contact you to arrange for your baby to have additional testing. It is important to remember that an out-of-range screening result does not necessarily mean that your child has the condition. An out-of-range result may occur because the initial blood sample was too small or the test was performed too early. However, as a few babies do have the condition, it is very important that you go to your follow-up appointment for a confirmatory test. Because the harmful effects of untreated MET can occur soon after birth, follow-up testing must be completed as soon as possible to determine whether or not your baby has the condition.

Follow-up testing will involve checking your baby’s urine and blood samples for signs of MET. Harmful amounts of an amino acid called methionine builds up in the body when a child has MET. Your baby’s doctor will measure the amount of methionine in your baby’s blood to determine if your child has the condition. Sometimes follow-up testing may also include testing a very small sample of skin.

About Hypermethioninemia

Early Signs

Most individuals with hypermethioninemia (MET) do not show any signs. If your baby does show signs of MET, you may notice:

  • Developmental delays
  • Sleeping longer or more often
  • Weak muscle tone (known as hypotonia)
  • A “cabbage-like” smell

Many of these signs may occur when your baby eats foods that their body cannot break down. They can be triggered by long periods of time without eating, illnesses, and infections.

If your baby shows any of these signs, be sure to contact your baby’s doctor immediately.


Dietary Treatments

Babies with hypermethioninemia (MET) may need to be on a restricted diet to avoid the amino acid methionine. A nutritionist or dietician can help you plan a healthy diet for your child.

Your baby’s doctor may also recommend special foods or formulas for children with MET. These formulas will likely need to continue through adulthood.

Expected Outcomes

Many babies with hypermethioninemia (MET) will never develop any signs or symptoms of the condition. Babies who receive treatment typically have no health complications. They can have healthy growth and development.

Babies who show signs of MET and do not receive treatment are at risk for intellectual disabilities. This is why newborn screening for MET is important.


When we eat food, enzymes help to break it down. Some enzymes break down proteins into their building blocks, called amino acids. Other enzymes break down these amino acids. In hypermethioninemia (MET), the enzymes involved in breaking down the amino acid methionine are not working correctly.

MET may affect the enzyme methionine adenosyltransferase, glycine N-methyltransferase, or S-adenosylhomocysteine hydrolase. Individuals with MET either do not make enough of one enzyme or make a non-working enzyme. If any of these enzymes are not working correctly, then the body cannot break down methionine. This causes high levels of methionine in the blood. Everyone has some methionine in their blood, but high levels can be toxic. In most cases of MET, the methionine levels are not high enough to be dangerous.

In most cases, MET is an autosomal recessive genetic condition. This means that a child must inherit two copies of the non-working gene for MET, one from each parent, in order to have the condition. The parents of a child with an autosomal recessive condition each carry one copy of the non-working gene, but they typically do not show signs and symptoms of the condition. While having a child with MET is rare, when both parents are carriers, they can have more than one child with the condition. Less commonly, MET can be inherited in an autosomal dominant pattern. Learn more about autosomal recessive inheritance and autosomal dominant inheritance.

Support for Hypermethioninemia

Support Services

At this time we have not located a condition-specific support group for hypermethioninemia (MET). However, there are resources that will be helpful in answering your questions about this condition.

Established by the National Human Genome Research Institute and the Office of Rare Diseases Research, the Genetic and Rare Diseases Information Center (GARD) is staffed by experienced information specialists to answer questions in English and Spanish from the general public, including patients and their families, health care professionals, and biomedical researchers.

The GARD website can help you contact the organization with questions about the condition.

Mums National Parent-to-Parent Network provides support to parents of children with health conditions by linking them to other parents who have a child with the same or similar condition.

Your baby’s doctor may also be familiar with support organizations in your area. Ask your baby’s doctor for more information about accessing support in your community.

If you are part of an organization that provides support for individuals with MET, please contact us, so that more families affected by this condition may learn about the services your organization provides.

Accessing Care

Work with your baby’s doctor to determine the next steps for your baby’s care. Your baby’s doctor may help you coordinate care with a physician who specializes in metabolism, a dietician, and other medical resources in your community. Some children with hypermethioninemia (MET) have developmental delays. If you think that your baby is not meeting their developmental milestones, ask your baby’s doctor about the next steps in accessing a developmental evaluation and care.

Because MET is a genetic condition, you may want to talk with a genetics specialist. A genetic counselor or geneticist can help you understand the causes of the condition, discuss genetic testing for MET, and understand what this diagnosis means for other family members and future pregnancies. Speak with your baby’s doctor about getting a referral. The Clinic Services Search Engine offered by the American College of Medical Genetics and Genomics (ACMG) and the Find a Genetic Counselor tool on the National Society of Genetic Counselors (NSGC) website are two good resources for you or your baby's health care provider to use to identify local specialists.

Families' Experiences

At this time we have not located a family story for this particular condition. If your family is affected by hypermethioninemia and you would like to share your story, please contact us so other families may benefit from your experience.

References & Sources

Visit MedlinePlus Genetics for more condition information

Read this fact sheet prepared by the Missouri Department of Health for more information

ACT Sheets

Healthcare professionals can learn more about confirmatory testing by reading the American College of Medical Genetics and Genomics' Algorithm for Diagnosis and ACT Sheet, a guide for follow-up after newborn screening. You can visit this page of the ACMG website here.

Was this Helpful?

Your input helps us improve the site for parents and practitioners. Leave us feedback about this page.

Was this page helpful?

Was this Helpful? - Feedback

Your input helps us improve the site for parents and practitioners. Leave us feedback about this page.

We're sorry to hear that. How can we do to improve it?

This question is for testing whether or not you are a human visitor and to prevent automated spam submissions.