Glycine N-Methyltransferase Deficiency

What else is it called?


  • GNMT deficiency 
  • Hypermethioninemia due to GNMT deficiency 
  • Hypermethioninemia due to glycine N-methyltransferase deficiency 
  • Glycine N-methyltransferase deficiency 


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What causes it?

Glycine N-Methyltransferase Deficiency is caused by mutations (changes) in the GNMT gene. This leads to excess levels of methionine (a type of amino acid) in the blood. This causes an increase in levels of the enzyme, transaminase in the blood which causes a mild increase in liver size  

How common is it?

GNMT deficiency is a very rare inherited condition. To date only five individuals from four different families have been diagnosed.  

What are the signs and symptoms?

The only symptoms associated with this condition are mild hepatomegaly (enlargement of the liver) and chronic (long-term) elevation of serum transaminases (an enzyme).  

Some individuals with this condition may show no symptoms at all.  

Three of the individuals that have been diagnosed also presented with the symptoms:  

  •  Upper respiratory tract infection 
  • Failure to thrive  
  • Febrile convulsions  

However, it is unclear whether these symptoms are related to the GNMT deficiency.

How is it diagnosed?

A diagnosis of the condition usually starts with a full medical history, physical exam and laboratory tests. Specialised blood and urine tests can be beneficial in the diagnosis.  

A diagnosis of this condition is confirmed through molecular genetic testing which will show mutations in the GNMT gene.  

 In the USA, newborn screening is available for this condition. This condition is currently not one of the metabolic disorders that is included in the newborn screening in the UK.  

Can it be treated?

As there are usually no symptoms, treatment may not be necessary. However, a low methionine diet may be used to correct the biochemical abnormalities. Please be noted that any dietary changes require specialist dietitian’s advice and monitoring. 

Regular measurements of aminotransferases, liver function and methionine levels may be useful.  

Do my family need to be tested?

GNMT deficiency is an inherited condition. Humans have chromosomes made up of DNA. Genes are pieces of DNA that carry the genetic information. Each chromosome may have several thousand genes. We inherit chromosomes from the egg of the mother and sperm of the father. The genes on those chromosomes carry the instructions that determine a person’s characteristics, which are a combination of the parents. 

The pattern of inheritance of GNMT deficiency is autosomal recessive. This means that carriers of the condition do not have the disorder because the other gene of this pair is working normally. Parents of children with GNMT deficiency are carriers.  

When both parents are carriers, the risk to the baby in each pregnancy is 

  • 25% chance (1 in 4) of developing the condition 
  •  50% chance (1 in 2) for the baby to be a carrier of the condition 
  •  25% chance (1 in 4) for the baby to have two working genes and neither have the condition nor be a carrier 

Genetic counselling can be requested to get a full explanation.  

Relevant Organisations


References are available on request. Please contact us by phoning 0845 241 2173 or emailing [GCM060].

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