Phenylalanine Hydroxylase Deficiency - Phenylketonuria

Phenylketonuria, which is also called PKU, is an inherited condition in which the body cannot process a substance called phenylalanine. PKU is an inborn error of metabolism that can lead to severe communication\ \education, including reading, writing, and basic math\ \motor function\ \personal care, such as bathing, dressing, eating, and toileting\ \social skills\ \thinking skills, such as decision making, problem solving, and self-direction\ \working\ ',CAPTION,'Mental Retardation');" onmouseout="return nd();">mental retardation if it is not treated.

What is going on in the body?

Phenylketonuria is caused by an enzyme defect in the liver. Normally, the liver produces an enzyme that breaks down phenylalanine. Phenylalanine is a compound found in many foods. The liver of a person with phenylketonuria does not produce this enzyme. When this happens, phenylalanine and its by-products build up in the body. This leads to communication\ \education, including reading, writing, and basic math\ \motor function\ \personal care, such as bathing, dressing, eating, and toileting\ \social skills\ \thinking skills, such as decision making, problem solving, and self-direction\ \working\ ',CAPTION,'Mental Retardation');" onmouseout="return nd();">mental retardation.

What are the causes and risks of the disease?

A problem in a gene causes the enzyme defect that leads to phenylketonuria. In order to have PKU, a person must inherit the abnormal gene from both parents. Phenylketonuria occurs in 1 of 15,000 live births. It is much more common in whites than in other racial groups.

What are the signs and symptoms of the disease?

A newborn who has PKU will appear normal. By the age of four months, symptoms of arrested brain development will begin to appear. If the PKU is untreated, severe, irreversible communication\ \education, including reading, writing, and basic math\ \motor function\ \personal care, such as bathing, dressing, eating, and toileting\ \social skills\ \thinking skills, such as decision making, problem solving, and self-direction\ \working\ ',CAPTION,'Mental Retardation');" onmouseout="return nd();">mental retardation will occur.

Signs and symptoms of PKU may include the following:

How is the disease diagnosed?

Phenylketonuria is diagnosed when high levels of phenylalanine are found in the blood. Testing for PKU is generally done with a screening blood test within 48 hours of birth. Babies who test positive on the initial screening test will be evaluated again with a more specific test for PKU.

Phenylalanine levels may be normal at birth but go up once the baby is fed. It is important to evaluate babies after they have received dietary protein for 24 to 48 hours. The practice of discharging babies from the hospital within 24 hours of delivery has resulted in failure to detect some infants with PKU.

What can be done to prevent the disease?

The genetic defect causing PKU cannot be prevented. If the defect is detected early in life, an individual can follow a special diet low in phenylalanine. This will prevent the communication\ \education, including reading, writing, and basic math\ \motor function\ \personal care, such as bathing, dressing, eating, and toileting\ \social skills\ \thinking skills, such as decision making, problem solving, and self-direction\ \working\ ',CAPTION,'Mental Retardation');" onmouseout="return nd();">mental retardation and learning disabilities.

What are the long-term effects of the disease?

If treatment is started early in life, there are few long-term effects. Otherwise, affected persons may have serious learning disabilities. They may also die at a young age.

What are the risks to others?

Phenylketonuria is not contagious. However, a person with PKU will pass on an abnormal gene to his or her children. The children will not be affected, however, unless the other parent also has the abnormal gene. Parents who have a child with phenylketonuria are at risk for having other affected children. Genetic counseling can be helpful in this situation.

A woman with phenylketonuria must take special care during pregnancy. If she is on a regular diet, the enzyme defect may cause her unborn child to be severely affected. She will need to follow a special diet low in phenylalanine during pregnancy.

What are the treatments for the disease?

Treatment consists of limiting phenylalanine in the diet. Some healthcare providers are willing to relax the diet after several years of life. Others suggest limiting phenylalanine throughout life. The diet is difficult to follow because phenylalanine is present in many foods. Foods that replace phenylalanine with another substance often do not taste very good.

What are the side effects of the treatments?

There are no side effects to limiting phenylalanine in the diet. A person may find it challenging to avoid such foods, however.

What happens after treatment for the disease?

Pregnant women with phenylketonuria need to avoid phenylalanine. Women with PKU who do not limit phenylalanine intake during pregnancy are at high risk of having a baby with defects, including the following:

How is the disease monitored?

Regular blood and urine tests will be done to monitor PKU. Any new or worsening symptoms should be reported to the healthcare provider.

Author:Ronald J. Jorgenson, DDS, PhD, FACMG
Date Written:
Editor:Ballenberg, Sally, BS
Edit Date:03/30/01
Reviewer:Barbara Mallari, RN, BSN, PHN
Date Reviewed:08/07/01

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