Phenylketonuria (PKU) (FAQ)

Answers to questions families often have about caring for their child with phenylketonuria

Phenylketonuria (PKU) is a rare, inherited disorder that causes an amino acid (a protein) called phenylalanine to build up in the body. This build-up is toxic for the growth and functioning of the central nervous system and leads to intellectual disability. To avoid this, phenylalanine, which is found in almost all foods containing protein, must be avoided. Many food items that are not thought of as sources of protein (such as fruits and vegetables) still have small traces of phenylalanine and must be avoided to maintain brain health in the patient with PKU.

PKU is caused by a defect in a gene that helps create phenylalanine hydroxylase, the enzyme that changes phenylalanine to tyrosine, leading to a toxic build-up of phenylalanine.

With early introduction and maintenance of a special diet that avoids phenylalanine, normal development and IQ can be expected. Without treatment, symptoms in classic PKU begin by about 6 months of age. The first symptoms may include:

• a musty or "mousy" odor of the body and urine
• developmental delays in sitting, crawling, and standing
• microcephaly (small head size due to decreased brain growth)

If children remain untreated, they may develop:

• decreased skin and hair pigmentation (due to lack of tyrosine)
• eczema
• seizures
• anxiety, depression, problems with concentration and other aspects of executive function
• profound intellectual disability

Most children with PKU are found by the newborn screening test (a heel prick done to all babies born in the United States). If suspected after the newborn period, diagnosis is made by a blood test. DNA testing can be used to confirm the diagnosis and determine the specific gene mutation. Children with different forms of PKU – benign hyperphenylalaninemia, mild PKU, and classic PKU – vary in their blood levels of phenylalanine and their tolerance of protein in their diet. Age is also important because infants and young children require more protein, including phenylalanine, for growth. Protein intake should be guided by blood levels of phenylalanine. In general, those with classic PKU have the lowest tolerance of protein; those with mild PKU can tolerate more, and those with hyperphenylalaninemia can tolerate the most .

The outcomes for people with PKU who stay on the diet for life are good. Relaxation of diet is linked to executive function problems, an increased risk of attention deficit disorder, and problems in school. Since 2007, there have been many new treatments available to make life with PKU more manageable. While there is not yet a cure, many improvements continue to be made and research is promising to continue improving life with PKU.

PKU is not contagious. It is an inherited disorder caused by an autosomal recessive gene from both the mother and the father. The only gene associated with PKU is phenylalanine hydroxylase (PAH) which is located on chromosome 12. If both parents carry the gene for PKU, each pregnancy will have a 1 in 4 chance of the child having PKU.

Phenylalanine is in almost all foods containing protein. Treatment for PKU involves a restrictive, low protein diet with reduction of phenylalanine and the addition of vitamins and minerals that are necessary for body metabolism. This diet requires special medical formula, which has been modified to remove phenylalanine. In addition, special low-protein medical foods may need to be used to satisfy hunger and caloric needs.

Saproterin (brand name Kuvan), a medication approved by the FDA in 2007, has been shown to increase phenylalanine tolerance in about 25% of patients. Pegvaliase-pqpz (brand name Palynziq), an enzyme therapy drug approved by the FDA in 2018, is used to aid adults with PKU with uncontrolled blood levels. Many other therapies are in the trial phase.

Although treatment with the low protein diet is linked to excellent outcomes, strict adherence to the diet can be difficult and expensive for families. Regular daily tracking of food and phenylalanine intake is necessary and can be time consuming. Blood tests, anywhere from weekly to monthly, are required to properly monitor blood phenylalanine levels, along with having regular appointments and conversations with a metabolic dietitian, who can help make adjustments when needed.

Getting immediate, and extended, family members involved in understanding and preparation of low protein foods can help a lot by supporting provision of low protein food at family meals and events and in understanding the limitations/tolerance of the PKU diet.

Getting support from other families who are experiencing or have experienced issues with PKU may be helpful—see information below for support groups. Your metabolic genetics clinic may also have resources that may be helpful.

It is often possible to breastfeed an infant with PKU, but this should be discussed in detail with the metabolic genetics team. Many mothers with PKU babies have successfully breastfed in tandem with metabolic formula and close monitoring of blood phenylalanine levels, making adjustments as necessary. Nursing mothers with PKU do NOT need to adhere to a low protein diet to accommodate their child with PKU.

Insurance will usually provide formula and supplements for children with PKU if they are prescribed by your doctor. The special formula alone can cost $500/month or more.

In some states, insurance companies are required to cover a portion of medically necessary food and formula, upon paid premiums, meeting deductibles and out of pocket max. However, such regulations may apply only to certain types of insurance plans. If your state or your insurance plan does not cover such formulas, talk to your employer’s Human Resources department to ask for coverage for low protein medical foods and formula. Some public health departments provide medical formula, particularly for young children.