Mitochondrial respiratory chain complex II

What else is it called?

Mitochondrial respiratory chain complex II may also be known as:  

  • Mitochondrial complex II deficiency  
  • Isolated mitochondrial respiratory chain complex II deficiency  
  • Complex 2 mitochondrial respiratory chain deficiency 
  • Succinate coQ reductase deficiency  
  • Mitochondrial respiratory chain complex II deficiency  
  • Succinate dehydrogenase deficiency  

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What causes it?

Mitochondrial complex II deficiency (MCIID) can be caused by a problem in multiple genes, these genes are: SDHA, SDHB, SDHD or SDHAF1. If you have a problem in any one of these genes then succinate dehydrogenase which is made by these genes will not be made and this leads to MCIID.  

How common is it?

MCIID is extremely uncommon, the prevalence of MCIID is less than 1:1,000,000 

What are the signs and symptoms?

The signs and symptoms of MCIID usually present during the neonatal and infancy periods. However, the symptoms presented in each individual may be different to depending on which gene out of the four has been mutated, it could range from severe symptoms which are presented in infancy to less severe symptoms presented in adulthood. 

The signs and symptoms of MCIID are:  

  • Developmental regression 
  • Distal muscle wasting (distal amyotrophy) 
  • Easy fatigability  
  • Hyperactive deep tendon reflexes  
  • Motor deterioration  
  • Weight loss 
  • Feeding difficulties in infancy  
  • Decreased muscle tone  
  • Prenatal growth deficiency (intrauterine growth deficiency)  

How is it diagnosed?

MCIID can be diagnosed using a combination of molecular genetic testing, their symptoms a physical examination and samples of their urine and blood. In addition to this a foetus can be diagnosed neonatally. There are a few countries currently running diagnostic tests for this condition, these countries are: 

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

Can it be treated?

There is an orphan drug that is being used to treat MCIID currently available in two locations which are: Europe and the USA. This orphan drug is known as alpha-tocotrienol quinone also known as vatiquinone. This drug is currently being used to treat mitochondrial membrane transport disorders in the USA. 

Do my family need to be tested?

MCIID 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 SDHA, SDHB, SDHD or SDHAF1 genes which are needed in order to produce the succinate dehydrogenase protein. If both of these genes are faulty then MCIID will develop. The parents do not necessarily need to suffer from MCIID 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 MCIID. There is a: 

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

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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