Phenylketonuria (PKU)

PAH deficiency presents on a clinical severity spectrum that correlates with residual enzyme activity and is classified by the untreated blood phenylalanine concentration. Classic PKU is defined by an untreated concentration above 1,200 µmol/L, mild PKU by 600 to 1,200 µmol/L, and mild (non-PKU) hyperphenylalaninaemia by 120 to 600 µmol/L. Untreated, persistent severe hyperphenylalaninaemia produces irreversible intellectual disability, neurobehavioural and psychological disturbances, and motor features including movement disorders, seizures and microcephaly. Hypopigmentation of skin and hair reflects reduced melanin synthesis, because tyrosine, the precursor of melanin, is the downstream product of the blocked phenylalanine hydroxylase reaction. Excretion of phenylalanine metabolites produces a musty body odour and eczematous skin changes. The condition is typically identified in infancy through newborn screening. Elevated maternal phenylalanine concentrations are teratogenic and cause maternal PKU syndrome in the foetus of an inadequately treated affected woman.

Early diagnosis through newborn screening and prompt treatment can prevent the severe neurocognitive complications of untreated PKU.

The estimated worldwide incidence is approximately 1 in 24,000 live births. Reported figures vary regionally, with higher incidences in parts of Europe (around 1 in 4,500 in Italy) and substantially lower rates elsewhere (approximately 1 in 125,000 in Japan).

PKU is caused by biallelic germline pathogenic variants in PAH (12q23.2). The gene encodes phenylalanine hydroxylase, the rate-limiting enzyme that converts phenylalanine to tyrosine in a BH4-dependent reaction. More than 1,000 variants have been reported in PAH, and the genotype broadly predicts residual enzyme activity and metabolic phenotype. Loss-of-function variants reduce hepatic enzyme activity, leading to accumulation of phenylalanine and toxic alternative metabolites (phenylacetic, phenylpyruvic and phenyllactic acid), reduced tyrosine, and secondary deficiency of catecholamine neurotransmitters and melanin.

PKU is inherited in an autosomal recessive manner. At conception, each sibling of an affected individual has a 25% risk of being affected, a 50% risk of being a heterozygous carrier, and a 25% risk of being unaffected and not a carrier. Carrier testing of at-risk relatives and prenatal molecular testing are both technically possible once the familial pathogenic variants have been identified.

Diagnosis is initiated by newborn screening and confirmed biochemically. An out-of-range dried blood spot is followed by quantitative plasma amino-acid analysis showing blood phenylalanine above 120 µmol/L and a phenylalanine-to-tyrosine ratio of 3 or higher. Tetrahydrobiopterin (BH4) synthesis and recycling defects are excluded by quantitative pterin analysis. Identification of biallelic pathogenic PAH variants establishes the molecular diagnosis and confirms phenylalanine hydroxylase deficiency as the cause.

Main differential diagnoses include:

• 6-pyruvoyl-tetrahydropterin synthase deficiency: BH4 synthesis defect with hyperphenylalaninaemia, intellectual disability, seizures, dystonic movements and microcephaly; PTS.
• Dihydropteridine reductase deficiency: BH4 recycling defect with progressive neurological deterioration, microcephaly and movement disorder; QDPR.
• DNAJC12 deficiency: co-chaperone defect ranging from mild features to dystonia and parkinsonism; DNAJC12.

• Genetic testing supports establishing the diagnosis by identifying the biallelic pathogenic PAH variants, clarifying the genetic aetiology and providing a basis for genetic counselling.
• A confirmed result supports condition-specific surveillance and management options, including lifelong dietary phenylalanine control to maintain blood phenylalanine within recommended target ranges, which the treating physicians decide together with the affected individual.
• A known familial variant supports predictive and carrier testing of at-risk relatives, so they can clarify their personal and reproductive risk and access counselling where indicated.

• Arnold G, Vockley J. Phenylalanine hydroxylase deficiency. In: Adam MP, Feldman J, Mirzaa GM, Pagon RA, Wallace SE, Bean LJH, et al., editors. GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2024. PMID:.
• Hillert A, Anikster Y, Belanger-Quintana A, Burlina A, Burton BK, Carducci C, et al. The genetic landscape and epidemiology of phenylketonuria. Am J Hum Genet. 2020;107(2):234-50. doi:10.1016/j.ajhg.2020.06.006. PMID:; PMCID:PMC7413859.
• Vockley J, Andersson HC, Antshel KM, Braverman NE, Burton BK, Frazier DM, et al. Phenylalanine hydroxylase deficiency: diagnosis and management guideline. Genet Med. 2014;16(2):188-200. doi:10.1038/gim.2013.157. PMID:; PMCID:PMC3894190.
• van Wegberg AMJ, MacDonald A, Ahring K, Bélanger-Quintana A, Blau N, Bosch AM, et al. The complete European guidelines on phenylketonuria: diagnosis and treatment. Orphanet J Rare Dis. 2017;12(1):162. doi:10.1186/s13023-017-0685-2. PMID:; PMCID:PMC5639803.