Conditions Methylmalonic Acidemia (Methymalonyl-CoA Mutase Deficiency)

Methylmalonic acidemia is a condition with many different forms, all of which have different causes and treatments. Methylmalonic acidemia caused by methylmalonyl-CoA mutase deficiency is just one type of methylmalonic acidemia. To learn about other types of methylmalonic acidemia, read about methylmalonic acidemia caused by cobalamin disorders A and B or aboutmethylmalonic acidemia with homocystinuria, which is caused by cobalamin disorders C, D, and F.

Condition Type

Organic Acid Conditions


Methylmalonic acidemia is estimated to affect one out of every 50,000 to 100,000 babies born in the United States. Methylmalonic acidemia is caused by methylmalonyl-CoA mutase deficiency in about half of all cases of methylmalonic acidemia.

Also known as

  • MUT
  • Methylmalonyl-CoA mutase deficiency
  • MMA
  • MCM deficiency
  • Mut0
  • Mut-
  • Methylmalonic acidemia (mutase deficiency)

Follow-Up Testing

Your baby’s doctor may ask you if your baby is showing any of the signs of MUT (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 methylmalonyl-CoA mutase deficiency (MUT) was out of the normal range, your baby’s doctor or the state screening program will contact you to arrange for your child 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, a few babies do have the condition, so it is very important that you go to your follow-up appointment for a confirmatory test. Because the harmful effects of untreated MUT can occur within days 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 harmful levels of acids and toxins. Certain acids and toxins build up in the body when a child has an organic acid condition, so studying the levels of these substances in your baby’s body can help doctors determine if your child has a condition. High levels of acylcarnitine in the blood and organic acids in the urine might indicate that the child has methylmalonic acidemia. Sometimes follow-up testing may also include testing a very small sample of skin.

About Methylmalonic Acidemia (Methymalonyl-CoA Mutase Deficiency)

Early Signs

Signs of methylmalonic acidemia (MUT) can start at any time from birth to adulthood. In most cases, the signs begin during infancy (either in the first few days or in the first few months of life).

For babies, signs of MUT can include:

  • Sleeping longer or more often
  • Tiredness
  • Vomiting
  • Weak muscle tone (also called hypotonia)
  • Fever
  • Breathing trouble
  • Increased number of illnesses and infections
  • Increased bleeding and bruising

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.


There are two types of methylmalonic acidemia: cobalamin disorders and methylmalonic-CoA mutase (MUT) deficiencies. Cobalamin disorders are considered vitamin B12 responsive. MUT deficiencies are non-vitamin B12 responsive. You may hear about other babies with methylmalonic acidemia receiving vitamin B12 injections. This treatment will not help a baby with methylmalonyl-CoA mutase deficiency.

Supplements and Medications

Your baby’s doctor might recommend L-carnitine supplements. These supplements help the body break down fats, and they can remove harmful substances from the body. Your baby’s doctor will need to write a prescription for these supplements.

Antibiotics can also help reduce methylmalonic acid levels.

Dietary Treatments

Your baby will need a very carefully monitored diet. Children with methylmalonic acidemia need to avoid certain fats and proteins because their bodies cannot break down these substances, causing a build up of toxic substances. Your doctor can recommend special formulas and foods made for children with organic acid conditions. These formulas will likely need to be continued through adulthood.

It is also important for your baby to eat frequently. Long periods of time without food, illness, or infections may trigger many of the signs mentioned in the Early Signs section.

Expected Outcomes

Babies who receive treatment early on can have typical development and healthy lives. The earlier you start treatment, the better the outcome will be for your child.

It is possible to have some long-term effects even with treatment for methylmalonic acidemia (MUT) such as learning disabilities and developmental delays.

It is important to screen for and to treat MUT. If MUT is left untreated, children can develop breathing problems or permanent brain damage.


When we eat food, enzymes help break it down. Some enzymes break down proteins into their building blocks, amino acids. Other enzymes break down fats into their building blocks, fatty acids. More enzymes break down these amino acids and fatty acids.

In methylmalonic acidemia, the enzyme methylmalonyl-CoA mutase is not working correctly. This enzyme helps break down odd-chain fatty acids and the amino acids isoleucine, valine, methionine, and threonine. If your baby is affected with MUT, then their body is either not making enough or making non-working methylmalonyl-CoA mutase enzymes.

In “Mut 0” forms of MUT, this enzyme is completely deficient. That means that there are no working methylmalonyl-CoA mutase enzymes in the body. In “Mut –“ forms, some methylmalonyl-CoA mutase enzymes work correctly, but there are not enough. Without enough working enzymes, your baby’s body has trouble using fats and proteins for energy.

MUT is an autosomal recessive genetic condition. This means that a child must inherit two copies of the non-working gene for MUT, 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 MUT is rare, when both parents are carriers, they can have more than one child with the condition. Learn more about autosomal recessive inheritance.

Support for Methylmalonic Acidemia (Methymalonyl-CoA Mutase Deficiency)

Support Services

Support groups can help connect families who have a child or other family member affected with methylmalonic acidemia with a supportive community of people who have experience and expertise in living with the condition. These organizations offer resources for families, affected individuals, health care providers, and advocates.

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 who can help plan your child’s specialized diet, or other medical resources in your community. Some children with methylmalonic acidemia have developmental delays. If you think that your baby is not meeting their developmental milestones, ask your baby's health care provider about the next steps in accessing a developmental evaluation and care.

Because methylmalonic acidemia 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 methylmalonic acidemia, 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

The Organic Acidemia Association (OAA) publishes the stories of people who are affected with methylmalonic acidemia.

Johnny Tate Jr. is a young boy with methylmalonic acidemia (MUT). You can read about his challenges and successes on the website. You can also read other stories of families affected by methylmalonic acidemia on this page of website by clicking on the photographs.

References & Sources

Visit the National Human Genome Research Institute for more condition information

Visit GeneReviews for more information on methylmalonic acidemia

Visit MedlinePlus Genetics for more condition information

Visit the Screening, Technology And Research in Genetics (STAR-G) Project for more information on MUT

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.

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