Defects in Metabolism of Amino Acids: PKU
One condition that all newborns in the U.S. are screened for is phenylketonuria (PKU),
a metabolic disorder passed down from both parents. Before the simple blood test that
checks for PKU became routine, the condition could go undetected long enough to cause
severe intellectual disability. Now the disorder can be diagnosed before it leads
to any damage.
What is PKU?
PKU is caused by a defect in a gene known as the PAH gene. This defect changes the
way that the amino acid phenylalanine is broken down by the body during digestion.
Phenylalanine comes from commonly eaten foods. It helps the body make protein. It
is also important for brain development. It is normally changed to tyrosine, which
helps create all of the body's proteins. PKU affects 1 out of every 10,000 to 15,000
newborns born in the U.S.
Because phenylalanine can't be properly processed when you have PKU, the amino acid
can build up in the bloodstream and cause harm.
Causes of PKU
PKU is passed on to children when both parents have only 1 mutated gene. This means
that neither parent has any symptoms of PKU but both are carriers of this gene. This
gene mutation is recessive. This means that a person needs to inherit 1 gene from
each parent in order to show signs of the disorder.
A baby born to a woman who has PKU that is not well-controlled through a special diet
is at high risk for serious complications. In this case, the fetus can be exposed
to extremely high levels of phenylalanine. This can cause low birth weight, slow growth,
small head, behavior difficulties, and heart disorders. These mothers are also at
risk for pregnancy loss.
In the U.S., all newborns are tested for PKU and placed on a special formula as soon
as possible. It is unusual, therefore, for babies to develop symptoms of the disease.
But if a baby isn't tested and has undiagnosed PKU, the child won't show any signs
or symptoms until he or she is several months old. Sometimes babies with PKU may seem
more drowsy and listless than normal, and they may have feeding difficulties. But
as they continue to take in protein (and phenylalanine) through their diets, they
may show other symptoms.
If PKU is not diagnosed and treated, phenylalanine will build up in the bloodstream
until it reaches levels that are toxic to brain tissue. It can cause permanent damage
to organs and tissues throughout the body.
PKU can lead to:
Heart defects, especially in infants born to mothers with PKU and uncontrolled phenylalanine
levels during pregnancy
Microcephaly, or very small head size
Eczema, a skin condition marked by an itchy red rash or blisters
Pale hair and skin compared with that of other family members
Delayed physical growth
Low birth weight in babies
Musty body odor
Symptoms can range from mild to severe based on how high the levels of phenylalanine
are in the blood.
Today, PKU rarely causes severe symptoms and damage in the U.S. because newborn screening
and treatment is so widespread. Newborns are screened for PKU using a blood test.
If you adopt children from another country, they may need to be screened for PKU,
along with other genetic illnesses.
Children born with PKU will need to follow a special low-protein diet throughout their
life. If they don't, they may suffer from mood disorders, poor memory and problem-solving
skills, tremors, and depression.
Phenylalanine-free formulas are available for infants. As infants start to eat solid
food, their diet will need to be restricted because phenylalanine is found in many
typical protein-rich foods. For example, a child with PKU should not eat milk, fish,
cheese, nuts, beans, or meat. Foods low in protein, such as vegetables, fruits, and
some cereals, can usually be eaten in certain quantities. Specific mineral and vitamin
supplements may be needed to make up for missing nutrients in the diet.
Children and adults with PKU cannot tolerate the sugar substitute called aspartame
and must read food labels to avoid sodas or any other products that contain it.
PKU can't be prevented, because the condition is inherited. Following the recommended
low-protein diet can help prevent serious complications and permanent damage caused
by PKU. Women with PKU who are of childbearing age should be particularly careful
to follow a strict low-protein diet.