Phenylketonuria

Other Names: PKU; Phenylalanine hydroxylase deficiency; Oligophrenia phenylpyruvica; Folling disease

Phenylketonuria (PKU) is a genetic metabolic disorder that increases the body's levels of phenylalanine. Phenylalanine is one of the building blocks (amino acids) of proteins. Humans cannot make phenyalanine, but it is a natural part of the foods we eat. However, people do not need all the phenyalanine they eat, so the body converts extra phenylalanine to another harmless amino acid, tyrosine. People with PKU cannot properly break down the extra phenylalanine to convert it to tyrosine. This means phenylalanine builds up in the person's blood, urine, and body. If PKU is not treated, phenylalanine can build up to harmful levels in the body.

Types

PKU varies from mild to severe. The most severe form is known as classic PKU. Without treatment, children with classic PKU develop permanent intellectual disability. Light skin and hair, seizures, developmental delays, behavioral problems, and psychiatric disorders are also common. Less severe forms, sometimes called "mild PKU", "variant PKU" and "non-PKU hyperphenylalaninemia", have a smaller risk of brain damage. Mothers who have PKU and no longer follow a phenylalanine-restricted diet have an increased risk of having children with an intellectual disability, because their children may be exposed to very high levels of phenylalanine before birth.

Cause

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 health 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.

Inheritance

Autosomal recessive inheritance, a 25% chance

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.

Symptoms

Phenylalanine plays a role in the body's production of melanin. The pigment is responsible for skin and hair color. Therefore, infants with the condition often have lighter skin, hair, and eyes than brothers or sisters without the disease.

Other symptoms may include:

• Delayed mental and social skills
• Head size much smaller than normal
• Hyperactivity
• Jerking movements of the arms or legs
• Mental disability
• Seizures
• Skin rashes
• Tremors

If PKU is untreated, or if foods containing phenylalanine are eaten, the breath, skin, ear wax, and urine may have a "mousy" or "musty" odor. This odor is due to a buildup of phenylalanine substances in the body.

Diagnosis

PKU can be easily detected with a simple blood test. All states in the United States require a PKU screening test for all newborns as part of the newborn screening panel. The test is generally done by taking a few drops of blood from the baby before the baby leaves the hospital.

If the screening test is positive, further blood and urine tests are required to confirm the diagnosis. Genetic testing is also done. Screening for PKU is done with bacterial inhibition assay (Guthrie test), immunoassays using fluorometric or photometric detection, or amino acid measurement using tandem mass spectrometry (MS/MS). Measurements done using MS/MS determine the concentration of Phe and the ratio of Phe to tyrosine, the ratio will be elevated in PKU.

Treatment

PKU is a treatable disease. Treatment involves a diet that is very low in phenylalanine, particularly when the child is growing. The diet must be strictly followed. This requires close supervision by a registered dietitian or doctor, and cooperation of the parent and child. Those who continue the diet into adulthood have better physical and mental health than those who don't stay on it. "Diet for life" has become the standard most experts recommend. Women who have PKU need to follow the diet before conception and throughout pregnancy.

There are large amounts of phenylalanine in milk, eggs, and other common foods. The artificial sweetener NutraSweet (aspartame) also contains phenylalanine. Any products containing aspartame should be avoided.

There are several special formulas made for infants with PKU. These can be used as a protein source that is extremely low in phenylalanine and balanced for the remaining essential amino acids. Older children and adults use a different formula that provides protein in the amounts they need. People with PKU need to take formula every day for their entire life.

The medication(s) listed below have been approved by the Food and Drug Administration (FDA) as orphan products for treatment of this condition.

• Sapropterin (Brand name: Kuvan) approved to reduce blood phenylalanine (Phe) levels in patients with hyperphenylalaninemia (HPA) due to tetrahydrobiopterin- (BH4-) responsive Phenylketonuria (PKU). Kuvan is to be used in conjunction with a Phe-restricted diet.
• Pegvaliase (Brand name: Palynziq) approved to reduce blood phenylalanine concentrations in adult patients with phenylketonuria (PKU) who have uncontrolled blood phenylalanine concentrations greater than 600 micromol/L on existing management.

In 2018, the FDA approved an enzyme substitute called pegvaliase which metabolizes phenylalanine.It is for adults who are poorly managed on other treatments.

Tetrahydrobiopterin (BH4) (a cofactor for the oxidation of phenylalanine) when taken by mouth can reduce blood levels of this amino acid in some people. Most people, however, with the "classical" sequence of mutations, will have little or no benefit.

Prognosis

The outcome is expected to be very good if the diet is closely followed, starting shortly after the child's birth. If treatment is delayed or the condition remains untreated, brain damage will occur. School functioning may be mildly impaired.

If proteins containing phenylalanine are not avoided, PKU can lead to mental disability by the end of the first year of life.

Epidemiology

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.

NIH genetic and rare disease info

• NIH info on Phenylketonuria

Phenylketonuria is a rare disease.

Phenylketonuria Resources
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