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Inherited Disorders and Birth Defects

PKU

11/06/2007

Question:

How do you tell if you carry the gene that causes PKU.

Answer:

PKU stands for a genetic disease known as phenylalanine hydroxylase (PAH) deficiency. The body is unable to process or break down the essential amino acid phenylalanine which is found in protein and leads to phenylketonuria (PKU).

Classic PKU is due to a complete or near-complete lack of the PAH enzyme. If phenylalanine is not restricted (cut down) by using a special diet low in phenylalanine, children with PKU develop profound and irreversible mental retardation. It affects about 1 in 10,000 newborns.

All 50 states require that all newborns are screened for PKU at birth - this is the heel stick blood test that is done after the baby is born. If the screening test is positive for PKU, then the baby has an additional test to make the diagnosis. If this test is positive, then treatment is started with a special diet low in phenylalanine. The sooner treatment is started the less likely there will be serious complications. Babies that are started on the special diet usually develop normally. It is recommended that someone with PKU stay on the diet for life, but at least through their teen years.

PKU is inherited as an autosomal recessive disorder - that means that you need to have 2 changed (mutated) genes to have the disease. Parents are carriers. Each parent has one normal gene and one changed gene - you only need one normal gene to make enough of the enzyme needed to break down phenylalanine. Parents who each carry a mutated gene for PKU have a 25% chance with each pregnancy of both passing on the gene and having a baby affected with PKU.

A carrier has no physical or mental problems. There is no way to tell if someone is a carrier for PKU except by doing genetic tests. The best approach is to look at the DNA of the child who has PKU and find the specific mutation in the gene that caused the disease. Once this is known, then that specific mutation can be looked for in the parents or other family members, such as brothers and sisters of the affected child. If DNA testing is not possible, biochemical testing may be done. Because these are expensive tests, they usually are not done unless there is a family history of PKU.

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Response by:

Anne   Matthews, RN, PhD Anne Matthews, RN, PhD
Associate Professor of Genetics
School of Medicine
Case Western Reserve University