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Does early treatment of PKU patients with sapropterin dihydrochloride affect brain development?

ABSTRACT: Background: Phenylketonuria (PKU) is an inborn error of metabolism caused by phenylalanine hydroxylase (PAH) deficiency. Patients with mild or moderate PKU can be successfully treated with sapropterin dihydrochloride (SD) which since recently is registered for usage from the age of 4 months. SD is a pharmaceutical version of tetrahydrobiopterin (BH4), a cofactor of PAH. Similar BH4 concentrations to those observed in plasma have been found in CSF of patients after a single oral administration thus proving that SD crosses the BBB. Hyperactivity has recently been reported as a post-marketing observation in some PKU patients treated with SD. Methods: 60 or 120 ng/ml sepiapterin, a stable precursor of BH4, were applied to study the effects of BH4 on developing brain cells in 3D organotypic rat brain cell cultures at two developmental stages. Immunohistochemistry, western blotting, metabolomics and RNA sequencing were performed. Findings: BH4 and BH2 measurements confirmed a successful conversion of sepiapterin to the active form BH4. In the earlier developmental stage, brain cell specific markers showed swollen astrocytes and diminished astrocytic fibres, delayed differentiation of oligodendrocytes and perturbation of axonal elongation. Immunofluorescence for activated caspase-3 revealed an increased apoptosis rate. We also found signs of perturbated GABAergic neurotransmission. RNAseq analyses revealed a number of significantly affected genes. GO enrichment allowed to identify key biological processes. Interestingly, none of these effects was observed in the later developmental stage. We show deleterious effects of SD on developing brain cells in a rat in vitro model. This observation raises the question whether the use of SD can be recommended in very young PKU patients as currently licensed.

ORGANISM(S): Rattus norvegicus

PROVIDER: GSE112108 | GEO | 2021/03/20

REPOSITORIES: GEO

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Project description:Tetrahydrobiopterin (BH4) is an essential cofactor for several metabolic enzymes, including the aromatic amino acid hydroxylases, alkylglycerol mono-oxygenase and NO synthases. BH4 deficiency due to an autosomal recessive defect in its biosynthetic enzyme 6-pyruvoyltetrahydropterin synthase (PTPS, encoded by the PTS gene) leads to a variant form of hyperphenylalaninemia concomitant with severe deficiency of brain monoamine neurotransmitters. In contrast, augmentation of BH4 by pharmacological supplementation or stimulation of its biosynthesis is thought to correct eNOS dysfunction, to protect from (cardio) vascular disease and/or to prevent from abdominal obesity and development of the metabolic syndrome. We have previously reported that complete Pts knock-out (ko) mice die after birth (Elzaouk et al JBC 2003). Here we generated a murine Pts-knock-in (ki) allele expressing a PTPS-p.Arg15Cys mutant enzyme with low residual activity (12% of wild-type in vitro) and investigated heterozygous Pts-ko/wt, homozygous Pts-ki/ki and compound heterozygous Pts-ki/ko mutant mice. All mice were viable and, depending on the severity of the Pts alleles, exhibited up to 90% reduction of PTPS activity in liver and brain tissues concomitant with high neopterin, but neither an elevation of blood L-Phe, nor a decrease in brain monoamine neurotransmitters dopamine or serotonin. Upon a standard systemic and comprehensive phenotyping of Pts-ki/ki mice, we found alterations in energy metabolites with reduced body mass, higher fat content, lower lean mass, and increased blood glucose and cholesterol in mutant animals. Furthermore, heterozygous Pts-ko/wt and/or homozygous Pts-ki/ki mice exhibited increased body weight and elevated intra-abdominal fat tissue when fed with normal chow or high fat diet. We conclude that a reduced BH4-biosynthetic activity in mice leads to abnormal body fat distribution and abdominal obesity potentially through a mildly compromised eNOS function.

Project description:The nitric oxide synthase (NOS) co-factor, tetrahydrobiopterin (BH4), is a redox-active molecule that regulates nitric oxide (NO) and reactive oxygen species (ROS) production by NOS, and is an example of redox-dependent signalling in the endothelium that underlies both the maintenance of vascular homeostasis and the development of cardiovascular disease (CVD). Loss of endothelial BH4 is observed in CVD states and results in decreased NO production and increased ROS generation by endothelial NO synthase (uncoupling). Genetic mouse models of augmented endothelial BH4 synthesis have shown proof of concept that endothelial BH4 can alter CVD pathogenesis. However, clinical trials of BH4 therapy in vascular disease have been limited by systemic oxidation and limited endothelial uptake of BH4, highlighting the need to explore the wider roles of BH4 in order to find novel therapeutic targets. In this study we aim to elucidate the effects of BH4 depletion on mitochondrial function and bioenergetics using targeted knockdown of the BH4 synthetic enzyme GTPCH in vitro. Knockdown of GTPCH (and, therefore, intracellular BH4 levels) by >90% lead to a striking induction of ROS generation in the mitochondria of murine endothelial cells. This effect was likewise observed in BH4 depleted GCH fibroblasts devoid of NOS, indicating a novel NOS-independent role for BH4 in mitochondrial redox signalling. Furthermore, this BH4-dependent, mitochondrial-derived ROS oxidized mitochondrial BH4 and is seen alongside distinct changes in the thioredoxin and glutathione antioxidant pathways, revealed by mass spectrometry using an unbiased proteomic approach. These changes are accompanied by a modest increase in mitochondrial size, attenuated respiratory function, and marked changes in the cellular metabolic profile; including succinate accumulation. Taken together, these data reveal a novel NOS-independent role for BH4 in the regulation of mitochondrial redox signalling and metabolism.

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Does early treatment of PKU patients with sapropterin dihydrochloride affect brain development?

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