Phenylketonuria (PKU; OMIM #261600) is an inborn error of amino acid metabolism due to the defect of the phenylalanine hydroxylase (PAH) enzyme that converts phenylalanine (Phe) into tyrosine. The introduction of newborn screening programs and the early start of dietary treatment radically changed the natural history of PKU. However, clinical outcome of early treated PKU patients is suboptimal because patients still disclose lower intelligence quotient, minor neuropsychological and psychiatric problems which remain challenging aspects of the disease and the possible targets for future improvement of the treatment.
Although several mechanisms, such as impairment in synaptic plasticity, neurotransmission, myelination, protein synthesis, energy metabolism, oxidative stress, and amyloid fibrils have been hypothesized, the pathophysiology of the neurological damage in PKU is still unknown. A big help in elucidating those mechanisms was given from the development of a genetic mouse model for PKU, the BTBR-ENU2 mouse (ENU2). Previous studies on ENU2 mouse showed that hyperphenylalaninemia interfere with brain tryptophan hydroxylase activity by reducing the availability of 5-HT, and that cortical spine maturation, and consequently cognitive deficits, are affected in ENU2 mice through a 5-HT-dependent pathway.
To date, the role of specific 5-HT receptor subtypes in the cognitive and behavioral profile of ENU2 mice has not been thoroughly investigated. Therefore, part I of present project will evaluate the cellular and molecular components that are altered during ENU2 mice development and adult life, with particular attention to serotoninergic pathway. This will allow finding suitable therapeutic windows to counteract cognitive delay in PKU adult affected patients.
In the second part of the project, which has a translational value, peripheral serotonin will be investigated in PKU patients with the aim of finding a predictive biomarker of neurocognitive dysfunction.
While the intellectual outcome is the result of a complex constellation of genetic and epigenetic events in normal as well in pathological conditions, American and European guidelines (Vockley et al, 2014; vanSpronsen et al, 2017) adopted blood Phe, which is a surrogate of brain Phe, as the only variable predicting neurological outcome in early treated PKU subjects. As consequence, the blood Phe level is the only factor affecting the treatment options. Despite it is well known that almost 10% of untreated PKU patient may have a normal clinical outcome (Ramus et al. Pediatric Research, 1999, 45), factors affecting this resilience to the disease or factors predicting it are not known and could not be included in decision trees suggested by the two guidelines. However, the awareness of the extreme variability in the outcome of early treated subjects, which suggests the occurrence of other environmental and biological factors affecting the prognosis of the disease, is expressed in both the guidelines.
Present project consists of two different part. The first part is aimed at elucidating pathophysiologic mechanism underling intellectual disability in the ENU2 mice, with special focus on the consequences of biogenic amines deficit in untreated PKU mice. The second one has a translational value. Recently, different therapeutic approaches have become available for adult PKU patients: enzyme substitution therapy with phenylalanine ammonia lyase (which converts Phe into nontoxic metabolites) has been approved by FDA and EMA and it is approaching the market; and gene therapy will soon be experimented in clinical trials. Those therapies, differently from dietary treatment, have a high impact on PKU patients¿ lives because of the management of therapy itself, and the reported adverse events, sometimes serious. The impact of these therapies will also be economic, because of the high cost of these new therapies. Choosing the right patient who will benefit from these alternative therapies is the real challenge in PKU patients¿ care. In this context the need for prognostic markers become even more essential. Indeed, present research project follows the recent interest in precision medicine, more specifically in personalized medicine, which take into account the biological individuality of the patients as criterion for treatment tailoring.