Homocystinuria

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HCU is an inherited metabolic disorder. Homocystinuria means too much homocysteine in the blood and urine. 

HCU affects the way your baby breaks down protein. Many foods contain protein. The body needs protein for growth and repair. 

Protein is broken down into amino acids (building blocks of protein) by enzymes (which are like chemical scissors). Enzymes then further break the amino acids into smaller parts. 

Metabolism refers to the chemical processes that occur inside the cells of the body. 

In HCU the body lacks an enzyme called cystathionine beta-synthase. This means that the body is unable to break down an amino acid called methionine. As a result, a harmful chemical called homocysteine builds up in the blood. 

According to data collected from countries that have screened over 200,000 newborns, the cumulative detection rate of HCU is 1 in 344,000. In some areas, the reported incidence based on clinical cases is approximately 1 in 65,000. More recently, screening has led to reported incidences as high as 1 in 20,000.

The build-up of homocysteine can cause many problems: 

• Short sightedness and dislocated eye lenses 
• Long and thin bones 
• Difficulties with learning and behaviour 
• Blood clots and strokes 

HCU is diagnosed by newborn screening. High levels of methionine and homocysteine are found in the blood. 

In some people with HCU, the enzyme does not work without the help of a vitamin called vitamin B6 

(together with folic acid). This vitamin helps the enzyme work better and if successful, is the only management needed in about 10% of patients. 

 HCU is managed with the following special diet: 

• Limited high protein foods 
• Measured amounts of methionine (protein) containing foods 
• A protein substitute. Sometimes extra cystine is needed 
• Low protein foods 

Other forms of management may include a drug called betaine and folic acid and vitamin B12 supplements. Betaine can help lower the amount of homocysteine in the blood. 

These foods are high in methionine (protein) and must be avoided:  

• meat 
• fish 
• eggs 
• cheese 
• bread 
• pasta 
• nuts 
• seeds 
• soya 
• Quorn  
• tofu 

In babies, a restricted amount of methionine (protein) is given from breast milk or measured amounts of infant formula. The amount given will be monitored regularly by your specialist metabolic dietitian. Protein substitute is essential for metabolic control. 

It will help to meet your protein, energy, vitamin and mineral requirements. It is available on prescription. There are many low protein foods. These include fruit, many vegetables and low protein foods such as bread and pasta which can be prescribed. They provide a source of energy and variety in the diet. 

During any illness, catabolism or protein breakdown occurs, causing blood homocysteine levels to increase. It is important to continue with the usual diet as much as possible. 

• Frequent blood tests to check homocysteine, methionine and cystine 
• Height and weight 
• Developmental checks 
• Diet and medications are adjusted according to age, weight and blood chemical levels 

HCU is a serious inherited metabolic disorder. 

Damage can be prevented with a protein restricted diet and a protein substitute. 

With early management normal development is achievable. This should also prevent long term complications such as osteoporosis (thin bones), blood clots and strokes. 

Hints and Tips: 

• Always ensure you have a good supply of your dietary products and medicines and that they are in date. 
• Your dietary products are prescribed by your GP. These are obtained via a pharmacy or home delivery. 
• Always ensure you have sufficient blood testing equipment and send samples on a regular basis. 
• Medications to control fever should be given as normally recommended – always keep supplies available. 

Humans have chromosomes composed of DNA. Genes are pieces of DNA that carry the genetic instruction. Each chromosome may have several thousand genes.  

The word mutation means a change or error in the genetic instruction. We inherit particular chromosomes from the egg of the mother and sperm of the father. 

The genes on those chromosomes carry the instruction that determines characteristics, which are a combination of the parents. 

HCU is an inherited condition. There is nothing that could have been done to prevent your baby from having HCU. Everyone has a pair of genes that make the cystathionine beta-synthase enzyme. In children with HCU neither of these genes work correctly. These children inherit one non-working HCU gene from each parent. Parents of children with HCU are carriers of the condition. 

Carriers do not have HCU because the other gene of this pair is working correctly. 

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

• 25% chance (1 in 4) of HCU 
• 50% chance (1 in 2) for the baby to be a carrier of HCU 
• 25% chance (1 in 4) for the baby to have two working genes and neither have HCU or be a carrier 

References are available on request. Please contact us on 0800 652 3181 or by emailing contact@metabolicsupport.org.

Please note calls to 0800 numbers are free from UK landlines and mobiles. Calls from business lines may be charged – please check with your provider.

This information about metabolic diseases is provided by Metabolic Support UK and is intended for educational purposes only. It should not be used for diagnostic or treatment purposes.

Should you require more detailed information please contact Metabolic Support UK by email (contact@metabolicsupport.org) or by telephone ( 0800 652 3181). For specific medical information regarding a disorder or individual please contact your GP or Paediatrician. Metabolic Support UK accepts no responsibility for any errors or omissions nor does Metabolic Support UK assume any liability of any kind for the content of any information contained within this summary or any use that you may make of it.