3-Methylcrotonyl CoA Carboxylase Deficiency

What else is it called?

  • 3MMC deficiency
  • 3-MCC deficiency
  • 3-methylcrotonyl-coenzyme A carboxylase deficiency
  • 3-methylcrotonylglycinuria
  • BMCC deficiency
  • Deficiency of methylcrotonoyl-CoA carboxylase
  • MCC deficiency
  • Methylcrotonyl-CoA carboxylase deficiency

Get in touch

Contact our caring team on 08452 412 173 for help and support. Our phone lines open 10am-4pm, Monday to Friday.

Prefer to email? Our email address is contact@metabolicsupportuk.org.

What causes it?

This condition belongs to a group called the organic acid disorders (or organic acidaemias). It is caused by faults in the MCCC1 gene or the MCCC2 gene. These genes are needed to provide instructions to form part of an enzyme called 3-methylcrotonyl-coenzyme A carboxylase (3-MCC). This enzyme helps your body to break down specific proteins into amino acids for the body to use for energy or to replenish depleted energy stores. If you have this condition you are unable to break down the amino acid leucine which is part of many different proteins. Failure to break down leucine subsequently leads to a build up of the by-products of leucine, which cause the symptoms of this condition.

How common is it?

Current research suggests that this condition affects approximately 1 in 36,000 newborns worldwide. Screening for this condition in some countries has identified this as one of the most common organic acid disorders.

What are the signs and symptoms?

Signs and symptoms are highly variable. Symptoms can begin at any age and can range from mild to severe. However, studies have suggested that symptoms only occur in around 10% of cases so it is most likely that you will have no symptoms at all. Symptoms generally occur in the form of metabolic crises which can be triggered by episodes of fasting, eating protein-rich foods, or a mild infection. These are severe episodes have a sudden onset and require immediate medical treatment. Symptoms of a metabolic crisis include:

  • Lethargy
  • Changes in behaviour
  • Vomiting
  • Muscle spasms
  • Low/weak muscle tone
  • Involuntary movements
  • Seizures
  • Breathing difficulties
  • Coma

In most cases, there are no symptoms between the crises. In some cases this disorder may not occur until adulthood with some symptoms such as weakness or fatigue.

If symptoms are present, they may include:

  • Seizures
  • Delays in reaching developmental milestones
  • Recurrent vomiting and diarrhoea
  • Lethargy
  • Low/weak muscle tone (hypotonia)

How is it diagnosed?

This condition is diagnosed through a clinical examination and specialised blood and urine tests. In some countries this condition is screened for routinely at birth. This is not currently available in the UK.

The characteristic features of this condition are like those of Reye syndrome.

Can it be treated?

In most cases, where there are no symptoms treatment is not usually needed. Avoidance of triggers will help to prevent metabolic crises occurring and in some cases a leucine-restricted (or low-protein) diet may be recommended. Always follow the advice of your consultant and attend any follow-up appointments.

You will be given an emergency regimen to follow should you feel unwell. Metabolic crises require immediate treatment. During illness, you should follow your emergency regimen without delay, phone your metabolic team, and ensure you always take full amounts as prescribed. If symptoms continue and/or you are worried, go immediately to the hospital. Regularly update your metabolic team during your illness.

Do my family need to be tested?

3-MCC deficiency can only be passed on to a child if both parents have a copy of the faulty gene. This is called autosomal recessive inheritance. A person who has a copy of the faulty gene is known as a carrier.

If both parents are carriers, their child has a one in four (25%) chance of inheriting the disorder, and a one in two chance (50%) of being a carrier. This is the same for each child the parents have.

If the child only inherits one copy of the faulty gene, they will be a carrier but will not have the disorder. In some rare cases, carriers have had mild symptoms of the disorder for which they carry the faulty gene.

Once you are diagnosed, you can speak to a genetic counsellor. They can explain how you may have inherited 3-MCC deficiency. They can also tell you about genetic testing for the rest of your family. They can provide advice and support if you go on to have children of your own.

Diagram showing the autosomal recessive inheritance pattern.

Relevant Organisations


References are available on request. Please contact us by phoning 0845 241 2173 or emailing contact@metabolicsupportuk.org [Resource Library No: MAP029].

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