Phenylketonuria: reduced tyrosine brain influx relates to reduced cerebral protein synthesis

Background: In phenylketonuria (PKU), elevated blood phenylalanine (Phe) concentrations are considered to impair transport of large neutral amino acids (LNAAs) from blood to brain. This impairment is believed to underlie cognitive deficits in PKU via different mechanisms, including reduced cerebral protein synthesis. In this study, we investigated the hypothesis that impaired LNAA influx relates to reduced cerebral protein synthesis. Methods: Using positron emission tomography, L-[1-C-11]-tyrosine (C-11-Tyr) brain influx and incorporation into cerebral protein were studied in 16 PKU patients (median age 24, range 16 - 47 years), most of whom were early and continuously treated. Data were analyzed by regression analyses, using either C-11-Tyr brain influx or 11C-Tyr cerebral protein incorporation as outcome variable. Predictor variables were baseline plasma Phe concentration, Phe tolerance, age, and C-11-Tyr brain efflux. For the modelling of cerebral protein incorporation, C-11-Tyr brain influx was added as a predictor variable. Results: C-11-Tyr brain influx was inversely associated with plasma Phe concentrations (median 512, range 233 - 1362 mu mol/L; delta adjusted R-2=0.571, p=0.013). In addition, C-11-Tyr brain influx was positively associated with C-11-Tyr brain efflux (delta adjusted R-2=0.098, p=0.041). Cerebral protein incorporation was positively associated with C-11-Tyr brain influx (adjusted R-2=0.567, p<0.001). All additional associations between predictor and outcome variables were statistically nonsignificant. Conclusions: Our data favour the hypothesis that an elevated concentration of Phe in blood reduces cerebral protein synthesis by impairing LNAA transport from blood to brain. Considering the importance of cerebral protein synthesis for adequate brain development and functioning, our results support the notion that PKU treatment be continued in adulthood. Future studies investigating the effects of impaired LNAA transport on cerebral protein synthesis in more detail are indicated.