Phenylketonuria (PKU) is an inborn error of metabolism caused by deficiency of the enzyme, phenylalanine hydroxylase (PAH), that converts phenylalanine (PHE) to tyrosine (TYR). If untreated, PHE accumulates in the blood and other body tissues and, in the severe form, results in intellectual disability, seizures and tremors, as well as other neurocognitive and behavioral problems ( TABLE #1, Laboratory and Clinical Findings in PKU). Characteristics of the different PKU phenotypes are in TABLE #2, Classification of PKU.
The traditional treatment for PKU is to restrict dietary PHE to the extent necessary to control blood PHE withn a treatment range of 120-360 μmol/L, while providing sufficient dietary PHE to promote normal growth and development. This dietary therapy also requires consumption of medical foods that provide PHE-free protein in the form of L-amino acids, as well as other necessary micro- and macronutrients. Other treatment components include the use of glycomacropeptide as a more intact protein source in medical foods, and large neutral amino acid therapy. Pharmacotherapy modalities now include adjunct treatment with sapropterin dihydrochloride, a form of tetrahydrobiopterin (BH4) that is the co-factor for PAH, and enzyme substitute therapy with pegvaliase (PAL) which metabolizes PHE in the blood.
Further information about PKU can be found at: http://www.ncbi.nlm.nih.gov/books/NBK1504/
The analytes used as markers for mass spectrometry (MS/MS) newborn screening for PKU are PHE and TYR. Most newborn screening laboratories determine their own cutoff levels, above which a test is considered to be presumptive positive and require further evaluation. An international database of 133 contributing laboratories reports a mean cutoff for PHE of 130 μmol/L (with a range of 65-234 μmol/L) and a PHE:TYR ratio >3 as abnormal (L.198).
Infants with newborn screening results consistent with PKU are referred to a metabolic clinic for confirmatory testing. Blood is tested for plasma amino acids, or at a minimum plasma PHE and TYR. PKU is diagnosed if blood PHE concentration is greater than 360 μmol/L and TYR is low or low normal, in an individual with an otherwise normal amino acid profile. About 2% of individuals detected to have hyperphenylalaninemia do not have PKU but have elevated blood PHE as a result of enzyme deficiencies in the recycling or synthesis of BH4. In order to rule out these possible pterin defects, urine pterins and blood dihdropteridine reductase activity are measured in all individuals with elevated PHE and PHE:TYR ratio. If screening is positive for pterin defects, further confirmatory testing to determine the cause of the resultant BH4 deficiency is done.
The ACMG algorithm for confirmatory testing is available at https://www.acmg.net/StaticContent/ACT/Phenylalanine.pdf
Mutation analysis, although not necessary for diagnosis, is recommended and may help predict severity of the disorder, or sapropterin dihydrochloride or pegvaliase responsiveness.
PKU is an autosomal recessive disorder that results from mutations in both copies of the PAH gene. See: http://omim.org/entry/261600 .
The human PAH gene is located on chromosome 12 in the band region q23.2 and contains approximately 100 kb of genomic DNA. There are hundreds of disease-causing mutations, of which more than 60% are missense alleles. Approximately 6 mutant alleles are responsible for the majority of European or Asian PAH mutations.
Carriers of PKU are not reported to exhibit symptoms caused by impaired PHE catabolism.