Phenylketonuria, PKU Causes, Symptoms, Testing, Diet & Stats

Phenylketonuria (PKU) definition and facts*

*Phenylketonuria (PKU) medically edited by:
Charles Patrick Davis, MD, PhD

  • Phenylketonuria (PKU) is a disease
    that’s inherited that increases the levels of phenylalanine in the blood. If
    left untreated, high phenylalanine levels can cause intellectual disability and
    other problems. Phenylketonuria disease was discovered in 1934 by Dr. A. Folling.
  • Signs and symptoms of PKU vary from
    mild to severe depending upon phenylalanine levels. Infants appear normal until
    a few months old when they start to show

    • intellectual disability,
    • seizures,
    • delayed development,
    • behavior problems, and
    • psychiatric disorders.
  • Children
    with untreated PKU also may have lighter skin and hair than other family members,
    and also may develop a musty or mouse-like odor due to excess phenylalanine. These are classic PKU symptoms.
  • Less severe forms of PKU (sometimes
    termed variant PKU and non-PKU hyperphenylalaninemia may develop less severe
    symptoms with a smaller risk of brain damage.
  • Women with PKU that have uncontrolled
    phenylalanine levels can have babies that are at significant risk for
    intellectual disability, low birth weight, heart defects, microcephaly, behavior
    problems, and slow growth.
  • PKU occurs in about 1/10,000 to15,000
    births, but classic PKU symptoms are rarely seen because of newborn screening
    tests for the disease.
  • Mutations in the PAH gene cause
    phenylketonuria. Some mutations in this gene allow the enzyme to retain some
    activity resulting in variant PKU and/or non-PKU hyperphenylalaninemia.
  • PKU is inherited in an autosomal
    recessive pattern so that both copies of the gene in the cell have mutations. Consequently, the autosomal recessive condition in both parents carry one copy
    of the mutated gene and parents typically do not show symptoms. Symptoms
    of PKU develop in their children when both parents donate one autosomal recessive
    condition to the newborn.
  • PKU has many different names, for example,
    phenylketonuria, PKU, Folling Disease, Folling’s Disease, and phenylalanine
    hydroxylase deficiency disease.
  • PKU does not shorten a person's life expectancy,
    with or without treatment.
  • Blood tests for PKU is required for
    infants (newborns) in all 50 states.
  • There is a 1 in 4 chance of having a
    PKU infant when both parents are genetic carriers. The approximate incident rate
    of PKU in the US is 0.01%. This means about 74 infants every day are diagnosed
    with PKU.
  • PKU patients usually are treated in a
    special metabolic disease clinic with restrictions on foods that contain
    phenylalanine and are given amino acids, vitamins, minerals, and other
    micronutrients along with close monitoring of phenylalanine levels.
  • A PKU diet bans meat, fish, chicken,
    eggs, nuts, beans, milk, and other dairy products. People with PKU also need to
    avoid aspartame, a type of peptide composed of aspartic acid and phenylalanine.

What is phenylketonuria?

Phenylketonuria (commonly known as PKU) is an inherited disorder that increases the levels of a substance called phenylalanine in the blood. Phenylalanine is a building block of proteins (an amino acid) that is obtained through the diet. It is found in all proteins and in some artificial sweeteners. If PKU is not treated, phenylalanine can build up to harmful levels in the body, causing intellectual disability and other serious health problems.

How common is phenylketonuria?

The occurrence of PKU varies among ethnic groups and geographic regions worldwide. In the United States, PKU occurs in 1 in 10,000 to 15,000 (newborns. Most cases of PKU are detected shortly after birth by newborn screening, and treatment is started promptly. As a result, the severe signs and symptoms of classic PKU are rarely seen.

What genes are related to phenylketonuria?

Mutations in the PAH gene cause phenylketonuria.

The PAH gene provides instructions for making an enzyme called phenylalanine hydroxylase. This enzyme converts the amino acid phenylalanine to other important compounds in the body. If gene mutations reduce the activity of phenylalanine hydroxylase, phenylalanine from the diet is not processed effectively. As a result, this amino acid can build up to toxic levels in the blood and other tissues. Because nerve cells in the brain are particularly sensitive to phenylalanine levels, excessive amounts of this substance can cause brain damage.

Classic PKU, the most severe form of the disorder, occurs when phenylalanine hydroxylase activity is severely reduced or absent. People with untreated classic PKU have levels of phenylalanine high enough to cause severe brain damage and other serious medical problems. Mutations in the PAH gene that allow the enzyme to retain some activity result in milder versions of this condition, such as variant PKU or non-PKU hyperphenylalaninemia.

Changes in other genes may influence the severity of PKU, but little is known about these additional genetic factors.

How do people inherit phenylketonuria?

This condition is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations. The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition.

What other names do people use for phenylketonuria?

  • Deficiency Disease, Phenylalanine Hydroxylase
  • Folling Disease
  • Folling’s Disease
  • PAH Deficiency
  • Phenylalanine Hydroxylase Deficiency Disease
  • PKU

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