Hypermethioninemia

What it is, symptoms and effects

From: MedlinePlus Genetics, National Library of Medicine

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. The condition is called primary hypermethioninemia when it is not associated with other metabolic disorders or excess methionine in the diet.

Causes and biological mechanisms

From: MedlinePlus Genetics, National Library of Medicine

Primary hypermethioninemia that is not associated with other metabolic disorders can be caused by variants (also known as mutations) in the MAT1A, GNMT, or AHCY gene. These genes provide instructions for making enzymes that each carry out one step of the multistep process to break down methionine. The reactions involved 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 (also known as AdoHcy).

The AHCY gene provides instructions for producing the enzyme S-adenosylhomocysteine hydrolase. This enzyme converts AdoHcy into the compound homocysteine. Homocysteine may be converted back to methionine or into another amino acid, cysteine.

Variants in one of these genes result in a shortage (deficiency) of an enzyme involved in breaking down methionine. A deficiency of any of these enzymes leads to a buildup of methionine in the body, which may cause signs and symptoms related to hypermethioninemia.

Inheritance and family implications

From: MedlinePlus Genetics, National Library of Medicine

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 variants. 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.

How common is it?

From: MedlinePlus Genetics, National Library of Medicine

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.

Which doctor should you see?

The suggested department for discussing Hypermethioninemia is Clinical Genetics, with a clinical geneticist as the relevant type of clinician. Paediatrician (children) or physician (adults), with clinical geneticist referral.

This is an editorial referral starting point. The appropriate clinic depends on the person’s age, symptoms, previous diagnosis and local services. The first clinician can decide whether another specialty or a team is needed; a department label does not confirm the diagnosis.

How to prepare for an assessment

Bring a short timeline of the main symptoms: when they first appeared, whether they are constant or episodic, what seems to change them, and how they affect daily activities. Include previous reports, discharge summaries, current medicines and supplements, allergies, and any relevant family history. A dated record is more useful than trying to match every feature in an online article.

Ask the clinician what is already established and what remains uncertain. If a test is suggested, ask what question it answers, what its limitations are and how the result would change the next step. The information here is not an instruction to arrange every possible test. In children, bring growth, developmental and school information if it is relevant to the concern.

• Does the exact genetic or chromosome finding explain the observed features?
• Would a genetic counsellor help the family understand the result?
• Which organ-specific assessments are appropriate for this particular diagnosis?

Treatment discussions and follow-up

The material gathered for this draft does not provide a complete condition-specific treatment pathway for Hypermethioninemia. That gap does not mean that treatment is unavailable. A clinician needs to establish the diagnosis and review current guidance before recommending medicines, procedures, rehabilitation or other support.

Before leaving the appointment, clarify the next review date, who will communicate results, and whom to contact if the situation changes. Discuss difficulties with sleep, work, school, mobility, eating or emotional wellbeing when these are relevant. Practical support may require coordination between the treating clinician and other services.

The collected references do not establish a complete prevention or long-term outlook section for this entry. Missing information should not be interpreted as proof that prevention is impossible or that a particular outcome is inevitable. Ask what is known for the exact subtype, stage and personal circumstances, and which uncertainties remain.

When to seek emergency help

Severe breathing difficulty, collapse, new stroke-like symptoms, a seizure that is prolonged or repeated without recovery, uncontrolled major bleeding, or an immediate risk of self-harm require emergency help. In India, call 112 or reach the nearest emergency department. This is a general, non-exhaustive warning list; it is not a condition-specific triage tool.