Phenylketonuria (PKU) is an autosomal recessive disorder that results from phenylalanine hydroxylase (PAH) deficiency. If uncontrolled, PKU leads to mental retardation. The prevalence is approximately 1 in 10,000 in temperate climates and varies by race, with a frequency of 1 in 8,000 in U.S. Caucasians, and 1 in 50,000 in African Americans. Many mutant alleles are present in the population. The high frequency of defective genes suggests that there is an advantage to being a carrier, perhaps due to resistance to natural toxins. Newborn screening (with the Guthrie test) is mandated by law and is critical because the mental retardation is treatable by restricting dietary intake of phenylalanine and/or ingesting a form of phenylalanine ammonia lysase, a plant enzyme. Unfortunately, the offspring of affected mothers are typically mentally retarded, even when the child's endogenous PAH activity is adequate.

HARRY W. SCHROEDER, JR.

(SEE ALSO: Congenital Anomalies; Genetic Disorders; Genetics and Health; Newborn Screening)