Evaluation of Tetrahydrobiopterin Therapy with Large Neutral Amino Acid Supplementation in Phenylketonuria: Effects on Potential Peripheral Biomarkers, Melatonin and Dopamine, for Brain Monoamine Neurotransmitters

Abstract

Background: 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.

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.

Methods: Nine 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).

Conclusion: Blood 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.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Fig 1

Overnight serum melatonin AUC (A) and Plasma phenylalanine concentrations (B) over the 4 phases. Phase 1 (LNAA), Phase 2 (Washout), Phase 3 (BH4) and Phase 4 (BH4 + LNAA). A1 and B1: BH4 responders. A2 and B2: BH4 non-responders. All BH4 responders showing significant decrease in plasma Phe levels, increased serum melatonin (Phase 3 vs. Phase 2 and Phase 4 vs. Phase 1). BH4 non-responders did not show increases in serum melatonin levels (Phase 3 vs Phase 2 and Phase 4 vs. Phase 1). All BH4 responders and S2, S5, S6, and S9 (4 out of 6 subjects) increased serum melatonin with LNAA (Phase 2 vs. Phase 1). All BH4 responders and S2, S5, S6, S7, and S9 (5 out of 6 subjects) increased serum melatonin with BH4 and LNAA over BH4 only (Phase 4 vs. Phase 3).

Fig 2. Overnight (7 pm to 7 am) serum melatonin levels.

Phase 1: LNAA supplement, Phase 2: Washout, Phase 3: BH4 therapy, Phase 4: BH4 and LNAA. Left: melatonin responder to BH4 (S1), Right: melatonin non-responder to BH4 (S6). Serum melatonin (pg/ml).

Fig 3. Urine 6-sulfatoxymelatonin (C) and Urine dopamine concentrations (D) over the 4 phases.

Phase 1 (LNAA), Phase 2 (Washout), Phase 3 (BH4) and Phase 4 (BH4+LNAA). C1 and D1: BH4 responders. C2 and D2: BH4 non-responders. The data point at Phase 4 in S3 is missing due to sample collection failure.

Fig 4. Serum Melatonin (AUC) vs. Plasma Phenylalanine.

The number of each data point indicates the study phase. Negative correlations with plasma phenylalanine concentrations and serum melatonin AUC (p = 0.0005) is shown. Except for S8, a linear negative regression line is drawn for each subject. The slope of linear regression line differs among the 9 subjects (p = 0.066).

Fig 5. Urine 6-Sulfatoxymelatonin vs. Plasma Phenylalanine.

The number of each data point indicates the study phase. Negative correlations with plasma phenylalanine concentrations and urine 6-sulfatoxymelatonin (p = 0.04) is shown. Except for S8 and S9, a linear negative regression line is drawn for each subject.

Fig 6. Urine dopamine vs. Plasma phenylalanine.

The number of each data point indicates the study phase. Negative correlations with plasma phenylalanine concentrations and urine dopamine (p = 0.047) are shown. Except for S7, a linear negative regression line is drawn for each subject.