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Phenylketonuria (PKU)
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Phenylketonuria (PKU)
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PKU, or phenylketonuria (pronounced fen-il-Key-to-New-ree-uh), is a serious but manageable condition. The incidence of PKU
is about 1 in 15,000. Currently there is no cure for PKU. The only way to manage the illness is through a lifelong strict diet.
Understanding the nature of this condition is essential for managing a child's condition. If left untreated PKU can cause
problems with brain development, leading to mental retardation and seizures.

PKU belongs to a class of diseases referred to as “inherited metabolic diseases,” or “inborn errors of metabolism.” Metabolic
diseases affect the body’s ability to execute certain chemical actions required for normal growth and development. For people
with PKU this means that a certain enzyme called phenylalanine hydroxylase (PAH) is flawed. This enzyme is required for the
breakdown of phenylalanine (Phe), an amino acid found in foods containing protein. Even though they produce some working
PAH, most of the PAH they produce is faulty and doesn’t function the way it’s supposed to, and isn’t able to properly break the
Phe down.

PAH is a molecule produced in the liver, which sets in motion a chemical reaction that converts Phe into another amino acid
called tyrosine, or Tyr. For individuals who don’t have PKU, the liver produces enough working PAH to convert Phe into Tyr,
and as a result, all the Phe is utilized. For an individual with PKU, Phe cannot be turned into Tyr and the excessive Phe builds
up, passes into the bloodstream, and travels throughout the body, eventually ending up in the brain. While a certain amount of
Phe is essential for normal growth and development, too much Phe in the brain causes damage, particularly in very young
children.

Phenylketonuria (PKU) Diagnosis

PKU is normally detected using the HPLC test, but some clinics still use the Guthrie test, part of national biochemical screening
programs. Most babies in the U.S. are screened for PKU soon after birth. When a child first tests positive for PKU, the results
are sent to a lab at your local metabolic clinic to confirm a diagnosis. The staff there is now committed to helping you and your
child with the management of PKU, including periodic blood testing, formula prescriptions, and dietary consultations based on
feedback from you and the blood tests. As part of managing your child’s condition, you will be required to stay in close contact
with the clinic.

Your child’s doctor will determine the frequency of Phe blood tests. In the beginning, your child may be tested rather
frequently while the clinic determines the appropriate diet. Testing can occur once or twice a week until your child’s Phe levels
become stable at an appropriate level. Once the levels are stable testing may be required once a month until the child reaches
adolescence. During adolescence through young adulthood, testing may be conducted twice a year. In adulthood, yearly testing
will likely continue for life.

Types of Phenylketonuria (PKU)

PKU is separated into two types: classic PKU and moderate PKU. These two types refer to different levels of blood Phe. A
person with moderate PKU has lower blood Phe levels than a person with classic PKU. Both classic and moderate PKU patients
can suffer significant neurological damage, if untreated, although this damage is more significant in classic PKU.

The key to PKU management is keeping the amount of Phe in the blood within a safe range. To achieve the proper blood Phe
level, PKU management involves a three-step approach: Record, Restrict, and Replace. A person with PKU (or the parent of a
young child with PKU) must record the amount of Phe consumed; restrict the diet to low-Phe foods; and replace those high-
Phe foods with low-Phe alternatives most commonly, a regimen of Phe-free medical protein supplement.

Causes for Phenylketonuria (PKU)

PKU is inherited. Both parents have to carry the PKU gene in order for a child to be born with PKU. You don’t have to actually
have PKU to carry the PKU gene. If you and your partner carry the gene but don’t have PKU, there is a 25% chance that any
future children you have together will have PKU. If one partner has PKU, and the other is a carrier of the gene, the chance of
having a child with PKU increases to 50%; if both partners have PKU, the likelihood increases to 100.

Over 500 different genetic mutations have been identified that result in defective functioning of the phenylalanine hydroxylase
enzyme, resulting in elevated phenylalanine levels. Each different mutation or combination of mutations results in more or less
enzyme activity in the effected person.

Maternal Phenylketonuria (PKU)

For women affected with PKU, it is crucial for the health of their child to maintain low phenylalanine levels before and during
pregnancy. Though the developing fetus may only be a carrier of the PKU gene, the intrauterine (location of the uterus)
environment can have very high levels of phenylalanine, which can cross the placenta. The result is that the child may develop
congenital heart disease, growth retardation, microcephaly (an abnormally small head and underdeveloped brain) and mental
retardation. PKU affected women themselves are not at risk from additional complications during pregnancy.

In most countries, women with PKU who wish to have children are advised to lower their blood phenylalanine levels before
they become pregnant and carefully control their phenylalanine levels throughout the pregnancy. This is achieved by performing
regular blood tests and adhering very strictly to a diet, generally monitored on a day-to-day basis by a specialist metabolic
dietitian. When low phenylalanine levels are maintained for the duration of pregnancy there are no elevated levels of risk of birth
defects compared with a baby born to a non-PKU mother. Babies with PKU may drink breast milk, while also taking their
special metabolic formula. Some research has indicated that an exclusive diet of breast milk for PKU babies may alter the
effects of the deficiency, though during breastfeeding the mother must maintain a strict diet to keep their phenylalanine levels
low. More research is needed.

Phenylketonuria (PKU) Diet

The good news is that brain damage can be prevented. Currently, this is accomplished by restricting the amount of Phe
ingested. Foods that are very high in Phe, such as meat, eggs, dairy, and nuts are avoided completely. These foods are replaced
by a medical protein formula that has no Phe. Foods with moderate amounts of Phe can be eaten but in limited quantities. These
foods include certain fruits and starches. Most foods have some Phe in them, meaning people with PKU must always be very
careful about what they eat. Therefore, it is important for individuals with PKU to track daily intake of Phe. With proper diet
management, newborns with PKU can look forward to healthy and fulfilling lives.

The PKU diet replaces meat, dairy, and eggs with a special medical formula that does not contain Phe. This formula is
prescribed by a doctor at a metabolic clinic. This formula is actually the centerpiece of the PKU diet. For people with PKU,
most of the calories they consume come from this formula, which may take the form of a powder mixed with drinks or as a
premixed drink. The remaining calories, and all of the Phe that a person with PKU needs, come mostly from relatively low-Phe
foods such as grains, fruits, and vegetables. Since most food contains some Phe, people with PKU must be cautious about
what—and how much—they eat. Remember, it’s not that your child can’t consume protein, but rather your child can’t have
too much of a certain amino acid, Phe, that’s found in proteins. Individuals with PKU requiring dietary restriction may never be
taken off the special diet. This is often referred to as “diet for life.”

Phenylketonuria (PKU) Treatment

If PKU is diagnosed early enough, an affected newborn can grow up with normal brain development, but only by eating a
special diet low in phenylalanine for the rest of his or her life. This requires strictly restricting or eliminating foods high in
phenylalanine, such as breast milk, meat, chicken, fish, nuts, cheese, legumes and other dairy products. Starchy foods such as
potatoes, bread, pasta, and corn must be monitored. Infants may still be breastfed to provide all of the benefits of breast milk,
though the quantity must be monitored and supplementation will be required. Many diet foods and diet soft drinks that contain
the sweetener aspartame must also be avoided, as aspartame consists of two amino acids: phenylalanine and aspartic acid. A
diet low in phenylalanine and high in tyrosine can be a very effective treatment.

Supplementary infant formulas are used in these patients to provide the amino acids and other necessary nutrients that would
otherwise be lacking in a protein free diet. These can continue in other forms as the child grows up such as pills, formulas, and
specially formulated foods. (Since phenylalanine is necessary for the synthesis of many proteins, it is required but levels must
be strictly controlled, usually being limited to 10 grams of protein. More severe forms of PKU, such as CPKU, require patients
to be restricted to less than 5. In addition, tyrosine, which is normally derived from phenylalanine, must be supplemented).

There are a number of other therapies currently under investigation, including gene therapy. There is at least one specialized
PKU treatment program in most US states. In some less-populated states, families may need to travel to another nearby state, or
the treatment may be coordinated through a state health department. All children with PKU ideally should be followed by a
specialized PKU program with adequate monitoring facilities. Information about the nearest PKU treatment program can be
obtained through the state newborn screening program.