Abstract
BackgroundPhenylketonuria (PKU) is due to a defective hepatic enzyme, phenylalanine (Phe) hydroxylase. Transport of the precursor amino acids from blood into the brain for serotonin and dopamine synthesis is reported to be inhibited by high blood Phe concentrations. Deficiencies of serotonin and dopamine are involved in neurocognitive dysfunction in PKU.Objective(1) To evaluate the effects of sapropterin (BH4) and concurrent use of large neutral amino acids (LNAA) on the peripheral biomarkers, melatonin and dopamine with the hypothesis they reflect brain serotonin and dopamine metabolism. (2) To evaluate synergistic effects with BH4 and LNAA. (3) To determine the effects of blood Phe concentrations on the peripheral biomarkers concentrations.MethodsNine adults with PKU completed our study consisting of four 4-week phases: (1) LNAA supplementation, (2) Washout, (3) BH4 therapy, and (4) LNAA with BH4 therapy. An overnight protocol measured plasma amino acids, serum melatonin, and 6-sulfatoxymelatonin and dopamine in first void urine after each phase.Results(1) Three out of nine subjects responded to BH4. A significant increase of serum melatonin levels was observed in BH4 responders with decreased blood Phe concentration. No significant change in melatonin, dopamine or Phe levels was observed with BH4 in the subjects as a whole. (2) Synergistic effects with BH4 and LNAA were observed in serum melatonin in BH4 responders. (3) The relationship between serum melatonin and Phe showed a significant negative slope (p = 0.0005) with a trend toward differing slopes among individual subjects (p = 0.066). There was also a negative association overall between blood Phe and urine 6-sulfatoxymelatonin and dopamine (P = 0.040 and 0.047).ConclusionBlood Phe concentrations affected peripheral monoamine neurotransmitter biomarker concentrations differently in each individual with PKU. Melatonin levels increased with BH4 therapy only when blood Phe decreased. Monitoring peripheral neurotransmitter metabolites may assist in optimizing individualized treatment in PKU.
Highlights
Since newborn screening has been introduced, early diagnosis and dietary management based on blood phenylalanine (Phe) concentrations successfully prevented individuals with Phenylketonuria (PKU; OMIM 261600) from developing significant intellectual disability
Tryptophan (Trp) and tyrosine (Tyr) are the precursor amino acids for serotonin and dopamine, respectively. They are transported into the central nervous system (CNS) by the large neutral amino acid (LNAA) transporter, LAT1 (SLC7A5) which transports the LNAA including branched chain, aromatic (Phe, Tyr, and Trp), and other amino acids [9]
Unique environment in the CNS with high blood Phe and relatively low concentrations of the other amino acids. This may contribute to fetal microcephaly in maternal PKU syndrome through decreased protein synthesis in the rapidly growing stage and to neurocognitive and neuropsychological symptoms after CNS development and growth are completed, possibly due to abnormal neurotransmitter metabolism [3, 19]
Summary
Phenylketonuria (PKU) is due to a defective hepatic enzyme, phenylalanine (Phe) hydroxylase. Transport of the precursor amino acids from blood into the brain for serotonin and dopamine synthesis is reported to be inhibited by high blood Phe concentrations. Deficiencies of serotonin and dopamine are involved in neurocognitive dysfunction in PKU
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