PROJECT SUMMARY The overall goal of this project is to investigate the etiology of cognitive deficits and neuropsychiatric symptoms associated with phenylketonuria (PKU), one of the most common inborn errors of metabolism detected through neonatal screening. Many adolescents and adults with PKU struggle with adherence to recommended dietary therapy. Chronic hyperphenylalaninemia is frequently associated with anxiety, depression, and impaired executive function such as difficulties with concentration and short-term memory. Deficiencies of the monoamine neurotransmitters, dopamine and serotonin, in brain have been implicated as probable proximal causes of neurobehavioral difficulties in PKU. In our recent work with the Pahenu2 mouse, a model of human PKU that exhibits both a memory deficit and anxiety, we have found that brain dopamine content is relatively undisturbed but that serotonin is severely deficient in untreated adult mice. The most likely mechanism causing brain serotonin deficiency in Pahenu2 mice is phenylalanine-mediated competitive inhibition of TPH activity leading to impaired serotonin synthesis. Correction of hyperphenylalaninemia following administration of either a phenylalanine restricted diet or liver-directed gene therapy leads to increased brain serotonin content and improved anxiety behaviors but no change in memory. These outcomes are similar to those seen in adult humans with PKU who were late treated and suffered irreversible damage to the brain early in life prior to the initiation of dietary therapy. Our hypotheses are that anxiety in hyperphenylalaninemic mice is caused by CNS serotonin deficiency but that the memory deficit is a result of other more permanent abnormalities of brain development resulting from long standing hyperphenylalaninemia beginning in infancy. In the first aim of our project, we will continue to explore the relationship between brain serotonin and anxiety in Pahenu2/enu2 mice and to evaluate novel therapeutic approaches directed at this complication. In the second aim of our project, we will develop and characterize a novel model of PKU, more analogous to the status of contemporary adults with PKU who were detected by newborn screening, received dietary treatment early in life, but subsequently lost dietary control, by treating Pahenu2/enu2 neonates with recombinant phenylalanine ammonia lyase (PAL, Pegvaliase) to control blood phenylalanine throughout infancy and into adulthood. We will compare behavioral outcomes, neuropathology and neurochemistry between PAL-treated and untreated Pahenu2/enu2 mice and furthermore will study the consequences of remerging hyperphenylalaninemia upon discontinuation of PAL treatment. We propose that PAL-treated Pahenu2/enu2 mice will more effectively model the current status of contemporary PKU treatment and will provide novel insights into the nature of PKU- associated neurocognitive deficits.