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Homocystinuria

(HOH-moh-SIS-tin-UR-ee-uh)

General Condition Information

Other Names

  • Homocystinuria (cystathionine beta synthase)
  • Homocystinuria (cystathionine beta synthetase)
  • Homocystinuria (cystathionine beta-synthase deficiency)
  • Homocystinuria (cystathoinine beta synthetase)
  • Homocystinuria or variant forms of hypermethioninemia

Condition Type

Birth Prevalence

  • The true prevalence of this condition is unknown, but it is estimated that less than 20 babies are born with this condition in the United States each year.
  • It is more common in babies of Irish, German, Norwegian, or Qatari descent.
  • Visit GeneReviews to learn more about how often this condition occurs.

Screening Finding

Increased methionine and/or increased homocysteine

What is Homocystinuria

Homocystinuria is an inherited (genetic) condition that prevents the body from processing proteins correctly. Your body breaks down the protein that you eat into parts called amino acids. Your body then uses those amino acids to make other proteins that it needs to function.

There are different forms of homocystinuria. All the forms result in high levels of amino acids called methionine or homocysteine in the body. 

In the most common form of homocystinuria, you cannot make an enzyme called cystathionine beta-synthase (CBS) correctly. This enzyme uses vitamin B-6 to help change homocysteine into other amino acids that your body needs. This form of the condition is also called “homocystinuria due to CBS deficiency.” There are other, less-common forms of homocystinuria, but they are not well-understood.

There are two types of homocystinuria due to CBS deficiency. 

  • B-6 responsive homocystinuria: The CBS enzyme is damaged but can still use vitamin B-6 to work.
  • B-6 nonresponsive homocystinuria: The CBS enzyme is damaged and cannot use vitamin B-6.

If your baby has any form of homocystinuria, their body has trouble getting rid of homocysteine. This results in a buildup of homocysteine and also methionine, which is made from homocysteine.

High levels of homocysteine and methionine can cause blood clots and damage to the brain and eyes. If untreated, this damage leads to the signs and symptoms of the condition.

Newborn Screening and Follow-Up

Condition Details

Treatment and Management

It is important to talk to your health care provider about which treatment(s) are best for your baby. The goal of treatment is to prevent the health problems caused by this condition.

Treatments may include the following:

  • Diet low in protein 
  • Special formulas and foods
  • Vitamin B-6, betaine, folic acid, or other vitamin supplements in some cases of homocystinuria

Children who receive early and ongoing treatment for homocystinuria can have healthy growth and development. Treatment is very important for lowering the chance of blood clots and eye and heart problems. Even with treatment, some children will still have eye problems, but these can be corrected.

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