Mitochondrial chain respiratory complex III

What else is it called?

Mitochondrial chain respiratory complex III (MCIIID) is also known as: 

  • Complex 3 mitochondrial respiratory chain deficiency  
  • Isolated CoQ-cytochrome C reductase deficiency  
  • Isolated coenzyme Q-cytochrome C reductase deficiency  
  • Isolated mitochondrial respiratory chain complex III deficiency  
  • Isolated ubiquinone-cytochrome C reductase deficiency  


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What causes it?

MCIIID is commonly caused by a problem in MT-CYB and BCS1L genes which provide the instructions to make a protein known as complex III. If you have a problem with these genes, then the protein complex III will not be made. Therefore, you will be unable to produce energy for your body and MCIIID will develop.

How common is it?

The prevalence of MCIIID is currently unknown as it is a very rare disease.  

What are the signs and symptoms?

The signs and symptoms of MCIIID vary dramatically between each affected person, this is because there will be unique distributions of healthy and not working mitochondria around the body.  There are several forms of MCIIID that have been grouped together based on the age of onset, the severity of symptoms and its nature. These forms are: 

  • Fatal infantile encephalomyopathy, characterised by lactic acidosis, low muscle tone, seizures and coma. 
  • Infantile histocytoid cardiomyopathy 
  • Childhood to adult onset, characterised by weakness, short stature, hearing loss, dementia and sensory neuropathy 
  • Myopathy with exercise intolerance, which leads into fixed weakness  

MCIIID can develop during infancy and in adulthood, the younger the person is when MCIIID begins to develop the more severe the disorder will be. 

The general signs and symptoms of MCIIID are:  

  • High levels of amino acids in urine (aminoaciduria)  
  • Brittle hair  
  • Decreased liver function 
  • Blocked bile flow from liver (Cholestasis)  
  • Failure to thrive  
  • Decreased muscle tone (hypotonia)  
  • Hypoglycaemia (low blood sugar)  
  • Increased lactate in body (lactic acidosis) 
  • Mitochondrial encephalopathy  
  • Seizures  
  • Muscle weakness 

How is it diagnosed?

Since MCIIID is an inherited metabolic disorder a doctor can diagnose individual using their medical history, molecular genetic testing, a physical examination, symptoms and bodily samples in order to run a laboratory test. There are a number of countries that are currently running diagnostic tests for this disease, these countries are:  

  • Austria  
  • Belgium  
  • Canada  
  • Estonia  
  • France 
  • Germany  
  • Israel 
  • Italy  
  • Netherlands 
  • Portugal 
  • Spain 
  • Sweden  
  • Switzerland 
  • United Kingdom 

Can it be treated?

Just like the other forms of this disease, MCIIID can be treated using the orphan drug known as Alpha-tocotrienol quinone which is also known as vatiquinone. This orphan drug is currently available in Europe and the USA. In the USA this drug is currently being used to treat Mitochondrial membrane transport disorders.  

Do my family need to be tested?

MCIIID is an autosomal recessive inherited metabolic disorder which means that for a person to inherit this disorder, they will need to inherit it from their parents. Humans are made up of 46 chromosomes and we inherit 23 chromosomes from each parent, in the form of the egg and sperm which combine to determine the characteristics of the child. Every human being has two copies of the MT-CYB and BCS1L genes which are needed in order to produce the complex III protein. If both of these genes are faulty then MCIIID will develop. The parents do not necessarily need to suffer from MCIIID they can be carriers, carriers are people who carry one faulty gene for a particular protein, and they do not develop the disease because the other gene works correctly.  

In the event that both of the parents are carriers of MCIIID. There is a: 

  • 25% chance that one of the children will be unaffected by MCIIID and will not be a carrier. 
  • 25% chance that the child will inherit MCIIID.  
  • 50% chance that the child will be unaffected by MCIIID and will be a carrier.     

MCIIID can also be inherited via Mitochondrial inheritance. Each mitochondrion (a single celled mitochondria) is made up of DNA which is known as mitochondrial DNA which has genes that provide information to make proteins. If these genes are faulty then the mitochondrial proteins will not be made correctly, or possibly not at all. For a child to inherit a disease via mitochondria inheritance they will need to inherit faulty genes from their mother. If the father has been affected with the mitochondrial disorder, he will be unable to pass the disease onto the children.  

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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