The clinical relevance of novel biomarkers as outcome parameter in adults with phenylketonuria.

Recent studies in PKU patients identified alternative biomarkers in blood using untargeted metabolomics. To test the added clinical value of these novel biomarkers, targeted metabolomics of 11 PKU biomarkers (phenylalanine, glutamyl-phenylalanine, glutamyl-glutamyl-phenylalanine, N-lactoyl-phenylalanine, N-acetyl-phenylalanine, the dipeptides phenylalanyl-phenylalanine and phenylalanyl-leucine, phenylalanine-hexose conjugate, phenyllactate, phenylpyruvate, and phenylacetate) was performed in stored serum samples of the well-defined PKU patient-COBESO cohort and a healthy control group. Serum samples of 35 PKU adults and 20 healthy age- and sex-matched controls were analyzed using ultra-high performance liquid chromatography quadrupole time-of-flight mass spectrometry. Group differences were tested using the Mann-Whitney U test. Multiple linear regression analyses were performed with these biomarkers as predictors of (neuro-)cognitive functions working memory, sustained attention, inhibitory control, and mental health. Compared to healthy controls, phenylalanine, glutamyl-phenylalanine, N-lactoyl-phenylalanine, N-acetyl-phenylalanine, phenylalanine-hexose conjugate, phenyllactate, phenylpyruvate, and phenylacetate were significant elevated in PKU adults (p < 0.001). The remaining three were below limit of detection in PKU and controls. Both phenylalanine and N-lactoyl-phenylalanine were associated with DSM-VI Attention deficit/hyperactivity (R2 = 0.195, p = 0.039 and R2 = 0.335, p = 0.002, respectively) of the ASR questionnaire. In addition, N-lactoyl-phenylalanine showed significant associations with ASR DSM-VI avoidant personality (R2 = 0.265, p = 0.010), internalizing (R2 = 0.192, p = 0.046) and externalizing problems (R2 = 0.217, p = 0.029) of the ASR questionnaire and multiple aspects of the MS2D and FI tests, reflecting working memory with R2 between 0.178 (p = 0.048) and 0.204 (p = 0.033). Even though the strength of the models was not considered strong, N-lactoyl-phenylalanine outperformed phenylalanine in its association with working memory and mental health outcomes.

Cerebral dopamine deficiency, plasma monoamine alterations and neurocognitive deficits in adults with phenylketonuria.

BACKGROUND: Phenylketonuria (PKU), a genetic metabolic disorder that is characterized by the inability to convert phenylalanine to tyrosine, leads to severe intellectual disability and other cerebral complications if left untreated. Dietary treatment, initiated soon after birth, prevents most brain-related complications. A leading hypothesis postulates that a shortage of brain monoamines may be associated with neurocognitive deficits that are observable even in early-treated PKU. However, there is a paucity of evidence as yet for this hypothesis. METHODS: We therefore assessed in vivo striatal dopamine D2/3 receptor (D2/3R) availability and plasma monoamine metabolite levels together with measures of impulsivity and executive functioning in 18 adults with PKU and average intellect (31.2 ± 7.4 years, nine females), most of whom were early and continuously treated. Comparison data from 12 healthy controls that did not differ in gender and age were available. RESULTS: Mean D2/3R availability was significantly higher (13%; p = 0.032) in the PKU group (n = 15) than in the controls, which may reflect reduced synaptic brain dopamine levels in PKU. The PKU group had lower plasma levels of homovanillic acid (p < 0.001) and 3-methoxy-4-hydroxy-phenylglycol (p < 0.0001), the predominant metabolites of dopamine and norepinephrine, respectively. Self-reported impulsivity levels were significantly higher in the PKU group compared with healthy controls (p = 0.033). Within the PKU group, D2/3R availability showed a positive correlation with both impulsivity (r = 0.72, p = 0.003) and the error rate during a cognitive flexibility task (r = 0.59, p = 0.020). CONCLUSIONS: These findings provide further support for the hypothesis that executive functioning deficits in treated adult PKU may be associated with cerebral dopamine deficiency.