Newborn Screening: Detecting Phenylketonuria

One of the six conditions currently on the South Carolina Newborn Screening panel is phenylketonuria (PKU). In its “classic” form, PKU is a genetic disorder characterized by an inability of the body to break down phenylalanine, one of the eight essential amino acids. Amino acids are the building blocks for proteins and essential amino acids can only be obtained from foods we eat. They cannot be produced by the body.

Classic PKU results from the absence of a single enzyme, phenylalanine hydroxylase, which normally converts phenylalanine to another amino acid called tyrosine. Without this enzyme, phenylalanine accumulates in the blood and body tissues causing damage to the brain and nervous system. The mechanism of this damage is not fully understood.

PKU is inherited in an autosomal recessive manner meaning that the affected individual inherited a mutation from each parent involving a gene on chromosome 12. Classic PKU and other forms of elevated phenylalanine affect about one in every 10,000 to 20,000 Caucasian or Oriental births. Incidence in African-Americans is significantly less and these disorders are equally frequent in males and females.

Infants with PKU appear normal at birth. Many have fair skin, light hair color and blue eyes because phenylalanine is also involved in the production of melanin pigment. PKU is a treatable disease, easily detected by a simple blood test. Newborn screening for PKU began in the mid 1960’s and is conducted in all 50 states prior to the infant’s discharge from the hospital.

PKU treatment includes a diet that is low in phenylalanine. High protein foods such as meat, fish, poultry, eggs and milk are avoided and replaced with cereals, fruits, starches and vegetables. A synthetic formula and special low protein foods are added as nutritional substitutes for the restricted foods. Diet drinks containing aspartame must also be avoided because of the presence of phenylalanine.

Dietary maintenance should be a lifelong endeavor. Recent studies indicate that loss of dietary control can hinder cognitive development including drops in IQ, learning disabilities, behavioral problems, neurological symptoms, and personality disorders. It is imperative that females of child-bearing age adhere to their low phenylalanine diet before conception and throughout the pregnancy. The risk for spontaneous abortion, mental retardation, and microcephaly (small head size) is significantly increased with mom’s high phenylalanine levels.

With good dietary control, the potential negative effects of the condition can be greatly minimized. Children with the disease are now growing and developing normally and living productive lives.