S-Adenosylhomocysteine Hydrolase Deficiency
What else is it called?
- Hypermethioninemia due to S-adenosylhomocysteine hydrolase deficiency
- Psychomotor delay due to S-adenosylhomocysteine hydrolase deficiency
- Psychomotor retardation due to S-adenosylhomocysteine hydrolase deficiency
- SAHH Deficiency
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What causes it?
S-adenosylhomocysteine hydrolase deficiency is caused by a mutation (change) in the AHCY gene. This gene is responsible for encoding S-adenosylhomocysteine hydrolase which is an enzyme that takes part in methionine metabolism (absorption). A problem in the AHCY gene leads to high blood levels of methionine and creatine kinase (CK).
How common is it?
S-Adenosylhomocysteine Hydrolase Deficiency is a very rare disorder. It has currently been reported in 3 unrelated patients. The prevalence worldwide is less than 1 in 1000,000.
What are the signs and symptoms?
Some patients may not have any symptoms. Age of onset is neonatal (newborn) or during infancy, however, in one patient the disorder was established only at age 26 years. The signs and symptoms that have been reported in patients include:
- Myopathy (muscle tissue disease)
- Hypotonia (abnormally low level of muscle tone)
- Developmental delay
- Behavioural disorders
- Microcephaly (Very small head)
- Myelination delay (Myelination is when the axon of each neuron gets coated with a fatty coating called myelin to protect the neuron and help it conduct signals more effectively).
- Strabismus (abnormal alignment of the eyes/the condition of having a squint)
- Problems with blood coagulation (blood clotting)
- Liver disease
Symptoms can vary from person to person and people with the disorder may not have all the symptoms listed.
30%-79% of people show these symptoms:
- Abnormal face shape
- Small/underdeveloped cerebellum (part of the brain that is responsible for regulating motor movements)
80%-99% of people show these symptoms:
- High liver enzyme (hepatic transaminase)
- Raised blood homocysteine
- Low blood albumin
How is it diagnosed?
This disorder can be diagnosed through laboratory tests including complete blood count, basic urine and sediment tests, screening for Duchenne muscular dystrophy, ECG and a heart ultrasound.
Can it be treated?
This disorder may be difficult to treat as it can affect many organs and can start before birth.
Possible treatments include:
- A low methionine diet (please note that any dietary changes require specialist dietitian’s advice and monitoring).
This can decrease or even normalize the abnormalities in the blood levels.
- Supplements of phosphatidylcholine and creatine (however, there is currently no evidence that this is effective)
- Liver transplantation
A successful liver transplantation was recently carried out in one patient who was 40 months. The outcome of this treatment depends on how severe the case is.
- Regular examination of all body systems (including: The nervous system, psychomotor development, muscles, liver and blood).
Early treatment may improve the outcome
Do my family need to be tested?
S-adenosylhomocysteine hydrolase 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.
Carriers of the condition do not have the disorder because the other gene of this pair is working normally. Parents of children with S-adenosylhomocysteine hydrolase deficiency are carriers.
The pattern of inheritance in this condition is autosomal recessive. This means:
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