Background: Phenylketonuria (PKU) is an inherited metabolic disease of amino acid metabolism characterized by the deficiency of activity in phenylalanine (Phe) hydroxylase enzyme, leading to the accumulation of Phe and its metabolites in the blood and tissues of affected patients. PKU, caused by various mutations leading to misfolded or nonfunctional proteins in the cells, still has an incompletely understood pathophysiology. This study aims to investigate the role of endoplasmic reticulum (ER) stress and ER stress response in PKU pathophysiology.

Results: Peripheral blood mononuclear cells (PBMCs) isolated from PKU patients carrying distinct protein misfolding mutations were analyzed to investigate the involvement of ER stress-related gene expression and cell death pathways. A comprehensive evaluation was conducted using quantitative real-time PCR and flow cytometry under both physiological (basal) conditions and following Phe administration, with the aim of elucidating the molecular mechanisms underlying Phe-induced cellular stress. Our results demonstrates that PKU patients exhibit a response to ER stress mediated by the IRE1 and ATF6 pathways of the UPR. We also observe disruptions in autophagic flux, as shown by the expression results of LC3B and p62 proteins. Importantly, the PBMCs do not appear to undergo immediate apoptotic cell death. The avoidance of apoptosis induction, even in the context of acute Phe treatment in the PBMCs of PKU patients who have been chronically exposed to ER stress, indicates an adaptive mechanism that facilitates cellular survival in these patients.

Conclusions: By focusing on the ER stress response in PKU patients with different type of mutations leading to protein misfolding, this study has the potential to provide new insights into PKU pathophysiology.