New findings about neuropathological outcomes in the PKU mouse throughout lifespan

Phenylketonuria (PKU, OMIM 261600) is a genetic disorder caused by a deficiency of the hepatic enzyme phenylalanine hydroxylase (PAH). If left untreated, PKU leads to systemic phenylalanine (Phe) accumulation, which can result in irreversible brain damage and intellectual disabilities. In the last 60 years, early and strict dietary restriction of phenylalanine (Phe) intake proved to prevent the severe clinical phenotype of untreated PKU. While the specific mechanisms through which phenylalanine causes brain damage are still poorly understood, preclinical models have been deeply explored to characterize the neurotoxic effect of Phe on neurodevelopmental processes. At the same time, that on the aging brain still needs to be explored. In the brain of untreated PAH^Enu2(-/-) mouse, we previously reported a reduction of myelin basic protein (MBP) during postnatal development up to 60 PND. Later in the diseased mouse's life, a spontaneous and persistent restoration of MBP was detected. In this present longitudinal study, ranging from 14 to 540 post-natal days (PND) of untreated PAH^Enu2(-/-) mice, we further investigated: a) the long-life consistency of two Phe-related brain metabolic alterations, such as large neutral amino acids (LNAA) and biogenic amine neurotransmitters' depletion; b) the outcome of locomotor functions during the same life span; c) the integrity of myelin as assessed ex vivo by central (hippocampus) and peripheral (extensor digitorum longus-sciatic nerve) action potential conduction velocities. In contrast with the results of other studies, brain Leu, Ile, and Val concentrations were not significantly altered in the brain PAH^Enu2(-/-) mouse. On the other hand, 3-O-Methyldopa (3-OMD, a biomarker of L-DOPA), serotonin, and its associated metabolites were reduced throughout most of the considered time points, with consistent reductions observed prevalently from 14 to 60 PND. Normal saltatory conduction was restored after 60 PND and remained normal at the last examination at 360 PND, resulting nonetheless in a persistent locomotor impairment throughout a lifetime. These new findings contribute to laying the foundations for the preclinical characterization of aging in PKU, confirming neurotransmitter defects as consistent metabolic traits. LNAAs have a minor role, if any, in brain damage pathogenesis. Transient myelin synthesis failure may impact brain connectivity during postnatal development but not nervous signal conduction.