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Urea Cycle Disorders

Group of inborn errors of metabolism


Updated April 03, 2006

In 1999, 18-year-old Jesse Gelsinger volunteered to be part of a clinical trial testing a new approach to treating ornithine transcarbamylase deficiency, a urea cycle disorder he was diagnosed with at age 2 years. Sadly, Jesse died as a result of the clinical trial, but research continues into treating, and possibly curing, urea cycle disorders.

What are urea cycle disorders?
The urea cycle is the metabolic process by which the body gets rid of nitrogen. There are six enzymes that take part in this process. A deficiency of any one of them upsets the process and causes excess nitrogen, in the form of ammonia, to accumulate in the body. The six urea cycle disorders are:

  • carbamyl phosphate synthetase deficiency
  • n-acetylglutamate synthetase deficiency
  • ornithine transcarbamylase deficiency (most common type)
  • argininosuccinic acid synthetase deficiency (also called citrullinemia)
  • argininosuccinase acid lyase deficiency
  • arginase deficiency

How is it diagnosed?
Urea cycle disorders are inherited genetic disorders. If the enzyme deficiency is severe, symptoms will be present at birth. The baby will have irritability and vomiting, followed by lethargy, seizures, poor muscle tone, and if left untreated, respiratory distress, coma, and death. These symptoms are due to the rising level of ammonia in the blood.

If the enzyme deficiency is partial (the body make some enzyme, but not enough) the symptoms may not start until childhood or adulthood. The child might be hyperactive, refuse to eat high-protein foods (which cause more nitrogen to be produced), and if the condition worsens due to illness or high-protein meals, vomiting, lethargy, delirium, coma, and death may occur.

How is it treated?
There is as yet no cure for the urea cycle disorders. However, if diagnosed promptly, measures can be taken to reduce harm to the body. A protein-restricted diet with special supplements is essential. In addition, several medications including sodium benzoate, sodium phenylacetate (Ucephan), and sodium phenylbutyrate (Buphenyl) are given to bind with ammonia and take it out of the body. Frequent blood tests to monitor the condition are essential.

Research continues into solving the puzzle of the urea cycle disorders. Liver transplants have been successful in a few cases. Correcting the defective gene responsible for a disorder--the clinical trial Jesse Gelsinger was involved in was exploring this--appeals to many people as a possible cure. Research is also being done on the connection between urea cycle disorders and other major diseases such as cancer, sickle cell anemia, and AIDS.

Information for this article was taken from:
National Urea Cycle Disorders Foundation. Understanding the Basics.

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