Malonyl CoA Decarboxylase Deficiency

What else is it called?

  • Deficiency of malonyl-CoA decarboxylase 
  • Malonic aciduria 
  • Malonyl-coenzyme A decarboxylase deficiency 
  • MCD deficiency 

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What causes it?

This disorder is caused by a problem in the MLYCD gene. This gene produces the malonyl CoA decarboxylase enzyme which helps to manage and breakdown a group of fats called fatty acids. Breaking down the fatty acids gives the body energy. Many parts of the body, including the heart, use fatty acids as their main source of energy. Fatty acids also help to produce hormones and to carry out a number of other important processes in the body. 

If there is a problem in the MLYCD gene, you will not produce enough of the malonyl CoA decarboxylase enzyme. Therefore, you will be unable to break down fatty acids properly. This causes low blood sugar levels, cardiomyopathy (a disease of the heart that makes pumping blood around the body more difficult) and can even lead to the build-up of fatty acid byproducts, which can be toxic. 


How common is it?

This disorder is extremely rare, with less than 30 cases reported worldwide. 

What are the signs and symptoms?

Symptoms for this disorder usually present in early childhood.  

With Malonyl CoA Decarboxylase Deficiency, your child may not have any clear symptoms at first (asymptomatic). Other symptoms could be confused for a different medical issue. Typical symptoms for this disorder include: 

  • Delayed development 
  • Weak muscle tone (hypotonia) 
  • Seizures 
  • Diarrhoea and/or vomiting 
  • Low blood sugar (hypoglycaemia) 
  • Disease of the heart (cardiomyopathy) 

Symptoms vary from person to person, so your child may have all of these or just one or two. Also, their symptoms may remain stable for long periods of time, or they may get progressively worse.  

Getting an early diagnosis is important to prevent any symptoms getting worse. Your child may be required to follow a diet plan and it is important that this is adhered to in order to prevent symptoms getting worse.   

Due to the rarity of the disorder, it is difficult to say what the average life expectancy is for people with Malonyl CoA Decarboxylase Deficiency. However, there is an increased risk to life for newborn babies, so getting an early diagnosis is essential. 

How is it diagnosed?

Malonyl CoA Decarboxylase Deficiency can be diagnosed with a specialised urine and blood test. A genetic test may also be required. 

This disorder is not currently detected as part of the newborn screening process in the UK or in other countries. However, if you reasonably suspect that your child may have Malonyl CoA Decarboxylase Deficiency, medical professionals may offer something called chorionic villus sampling during your pregnancy. This involves taking a small sample of cells from the placenta using a needle.

Can it be treated?

Treatment for Malonyl CoA Decarboxylase Deficiency includes a tailored diet plan that is low in fat and high in carbohydrates and may include supplements. It is extremely important that you follow your medical team’s advice and that you do not change your diet without the advice of a specialist dietician. 

Your plan will ensure that your child is not fasting for long periods of time in order to avoid metabolic crisis. This is where the body does not get the nutrients it requires and struggles to function properly. Typical symptoms include vomiting and diarrhoea, extreme sleepiness, and behavioural changes such as increased irritability. Metabolic crisis can lead to hospitalisation and, if left untreated, death. 

Your medical team will give you an emergency plan to follow in case of illness or refusal to feed. It is important to follow this and to go to your nearest A&E department if the symptoms do not improve. 

If diagnosed early and treated properly, there is an increased likelihood that your child’s symptoms will be less severe. However, many children do not get diagnosed until the symptoms have begun, and there may have already been irreversible damage. As a result, life expectancy is varied, with newborn babies being most at risk of death. 

Do my family need to be tested?

Malonyl CoA Decarboxylase Deficiency is inherited from both parents in an autosomal recessive inheritance pattern. This means that both parents are carriers of the disorder through their genes.  

All genes come in pairs. Carriers of Malonyl CoA Decarboxylase Deficiency have one working MLYCD gene and one faulty gene. Children inherit one gene from each parent to make their pair. Whether they have Malonyl CoA Decarboxylase Deficiency depends on what genes inherit: 

  • There is a 50% chance that the child will inherit one faulty and one working gene, which means they will also be a carrier. 
  • There is a 25% chance that the child will inherit two working genes and not inherit or carry the faulty gene. 
  • There is a 25% chance that the child will inherit two faulty genes, which means they will have Malonyl CoA Decarboxylase Deficiency. 

If you think somebody in your family may have Malonyl CoA Decarboxylase Deficiency, or that you may be a carrier, contact your medical team who may refer you for genetic testing. 

Relevant Organisations


References are available on request. Please contact Helen Morris by phoning 0845 241 2173 or emailing [Resource Library No: AAP002]. 

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