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MMAB gene
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Gene

MMAB gene

The MMAB gene provides instructions for an enzyme that converts vitamin B12 into adenosylcobalamin, a vital cofactor for breaking down proteins, fats, and cholesterol.

Normal Function

The MMAB gene provides instructions for making an enzyme involved in the formation of adenosylcobalamin (AdoCbl). AdoCbl, derived from vitamin B12 (cobalamin), is necessary for the normal function of the enzyme methylmalonyl CoA mutase, which helps break down certain proteins, fats (lipids), and cholesterol.

The MMAB enzyme is active in mitochondria. Once vitamin B12 is transported into the mitochondria, the MMAB enzyme converts a form of the vitamin called cob(I)alamin to AdoCbl. Studies suggest this enzyme may also deliver AdoCbl to methylmalonyl CoA mutase.

Health Conditions Related to Genetic Changes

Methylmalonic acidemia

At least 25 mutations in the MMAB gene have been found to cause methylmalonic acidemia, a condition characterized by feeding difficulties, developmental delay, and long-term health problems. These mutations generally lead to the production of a nonfunctional version of the enzyme.

As a result, AdoCbl cannot be made properly. A lack of AdoCbl impairs the function of methylmalonyl CoA mutase, resulting in the incomplete breakdown of certain proteins and lipids. This defect allows toxic compounds to build up in the body's organs and tissues, causing the signs and symptoms of the disorder.

Other Names for This Gene

  • ATP:Cob(I)alamin Adenosyltransferase
  • ATR
  • cblB
  • CFAP23
  • cob(I)alamin adenosyltransferase
  • methylmalonic aciduria (cobalamin deficiency) cblB type
  • methylmalonic aciduria (cobalamin deficiency) type B
  • MMAB_HUMAN

Frequently Asked Questions

protein‑restricted diet limiting propiogenic amino acids (isoleucine, valine, methionine, threonine) is recommended, often guided by a metabolic dietitian. Supplementation with a medical food low in these amino acids, along with adequate calories from carbohydrates and fats, helps prevent catabolism while supporting growth.
Sequencing of the MMAB gene (next‑generation panel or whole‑exome sequencing) detects point mutations and small deletions. Once pathogenic variants are identified, carrier testing of parents and at‑risk relatives can be offered, and couples can consider options such as prenatal diagnosis or preimplantation genetic testing to avoid recurrence.
MMAB-related disease (cblB type) primarily results in isolated methylmalonic acidemia without homocystinuria, so patients usually have normal homocysteine and methionine levels. Neurologic symptoms and growth failure are similar, but eye abnormalities and vascular complications seen in cblC are less common.
Early initiation of hydroxocobalamin therapy (often 1 mg IM weekly) combined with dietary management can normalize metabolic markers and improve neurodevelopmental outcomes. Many patients achieve normal growth and avoid severe neurologic deficits, though some may still experience mild learning difficulties or intermittent metabolic crises.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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