Overview

Phenylketonuria (PKU) is an inherited metabolic condition in which the body cannot properly metabolize the amino acid phenylalanine. It is classically described as a genetic disorder present from birth. When phenylalanine accumulates it can cause neurological harm; untreated, PKU is associated with developmental delays, cognitive impairment and other systemic features.

Causes and genetic basis

Most cases result from loss of function in the gene that encodes phenylalanine hydroxylase (PAH), the enzyme that converts phenylalanine into tyrosine. The condition is usually inherited in an autosomal recessive pattern: a child must receive pathogenic variants from both parents to be affected. In some individuals, problems with the cofactor tetrahydrobiopterin (BH4) instead of PAH mutations produce a related biochemical picture and require different treatment approaches.

Clinical features

Signs range from subtle to severe and depend on how high phenylalanine levels become and for how long. Common findings include:

  • Developmental delay and intellectual disability if untreated
  • Seizures and behavioral problems
  • Pale skin and hair or light eye color in some people, due to reduced melanin synthesis
  • A characteristic musty odor in body fluids and urine

Diagnosis and screening

PKU is a leading example of effective newborn screening. A blood spot obtained in the first days of life is tested for elevated phenylalanine; positive screens are confirmed with repeat biochemistry and often genetic testing. Early detection permits prompt treatment and typically prevents the worst neurological outcomes.

Management and prognosis

The cornerstone of treatment is lifelong dietary management to restrict phenylalanine intake while providing adequate nutrition. Tyrosine becomes an essential amino acid and must be supplemented or provided by medical formulas. Pharmacological options include BH4 (sapropterin) for responsive patients and enzyme substitution therapies for some adults; all approaches require ongoing monitoring of blood phenylalanine. For women with PKU, maintaining target levels before and during pregnancy is critical because elevated maternal phenylalanine can harm the developing fetus.

History and significance

PKU was first recognized in the early 20th century and characterized biochemically in the 1930s. Its inclusion in mid-20th century newborn screening programs transformed outcomes and established a model for population screening for other metabolic diseases. Today PKU remains an important condition in pediatrics and medical genetics due to its treatability and the need for life-long care.