A novel GTPCH deficiency mouse model exhibiting tetrahydrobiopterin-related metabolic disturbance and infancy-onset motor impairments

Evaluation of efficacy and safety of AAV8-ΔC4ATP7B gene therapy in a mutant mouse model of Wilson's disease

Wilson's disease (WD) is an autosomal recessive disorder caused by pathogenic variants in the ATP7B gene, resulting in the toxic accumulation of copper (Cu). Impaired Cu homeostasis in WD is characterized by low serum ceruloplasmin, excess hepatic Cu, and elevated urinary Cu. WD often presents with hepatic and/or neurological diseases and is fatal if untreated. Adeno-associated virus (AAV)-mediated gene therapy holds promise for WD, but challenges remain in efficacy and safety. Here, we established an Atp7b R780L knockin (KI) mouse model corresponding to the human ATP7B R778L variant and investigated the therapeutic efficacy and safety of liver-targeted AAV8-mediated ATP7B (AAV8-ΔC4ATP7B) gene therapy in this model. The results demonstrated the Atp7b KI/KI mice recapitulated key features of impaired Cu metabolism in WD but had mild liver disease. Ten-week-old Atp7b KI/KI mice received a single-dose of AAV8-ΔC4ATP7B and were sacrificed at 8 or 30 weeks after treatment. Treated Atp7b KI/KI mice showed normalization of serum ceruloplasmin, reduced hepatic Cu, decreased urinary Cu, and reversed liver histopathology. Serum transaminases had a transient increase at 8 weeks after treatment but returned to normal at 30 weeks after treatment. These data provide evidence for the efficacy and safety of AAV8-ΔC4ATP7B in animals, supporting clinical translation to patients with WD.

Mouse models for inherited monoamine neurotransmitter disorders

Several mouse models have been developed to study human defects of primary and secondary inherited monoamine neurotransmitter disorders (iMND). As the field continues to expand, current defects in corresponding mouse models include enzymes and a molecular co-chaperone involved in monoamine synthesis and metabolism (PAH, TH, PITX3, AADC, DBH, MAOA, DNAJC6), tetrahydrobiopterin (BH4) cofactor synthesis and recycling (adGTPCH1/DRD, arGTPCH1, PTPS, SR, DHPR), and vitamin B6 cofactor deficiency (ALDH7A1), as well as defective monoamine neurotransmitter packaging (VMAT1, VMAT2) and reuptake (DAT). No mouse models are available for human DNAJC12 co-chaperone and PNPO-B6 deficiencies, disorders associated with recessive variants that result in decreased stability and function of the aromatic amino acid hydroxylases and decreased neurotransmitter synthesis, respectively. More than one mutant mouse is available for some of these defects, which is invaluable as different variant-specific (knock-in) models may provide more insights into underlying mechanisms of disorders, while complete gene inactivation (knock-out) models often have limitations in terms of recapitulating complex human diseases. While these mouse models have common phenotypic traits also observed in patients, reflecting the defective homeostasis of the monoamine neurotransmitter pathways, they also present with disease-specific manifestations with toxic accumulation or deficiency of specific metabolites related to the specific gene affected. This review provides an overview of the currently available models and may give directions toward selecting existing models or generating new ones to investigate novel pathogenic mechanisms and precision therapies.

Mechanisms underlying the efficacy and limitation of dopa and tetrahydrobiopterin therapies for the deficiency of GTP cyclohydrolase 1 revealed in a novel mouse model

Dopa and tetrahydrobiopterin (BH4) supplementation are recommended therapies for the dopa-responsive dystonia caused by GTP cyclohydrolase 1 (GCH1, also known as GTPCH) deficits. However, the efficacy and mechanisms of these therapies have not been intensively studied yet. In this study, we tested the efficacy of dopa and BH4 therapies by using a novel GTPCH deficiency mouse model, Gch1KI/KI, which manifested infancy-onset motor deficits and growth retardation similar to the patients. First, dopa supplementation supported Gch1KI/KI mouse survival to adulthood, but residual motor deficits and dwarfism remained. Interestingly, RNAseq analysis indicated that while the genes participating in BH4 biosynthesis and regeneration were significantly increased in the liver, no significant changes were observed in the brain. Second, BH4 supplementation alone restored the growth of Gch1KI/KI pups only in early postnatal developmental stage. High doses of BH4 supplementation indeed restored the total brain BH4 levels, but brain dopamine deficiency remained. While total brain TH levels were relatively increased in the BH4 treated Gch1KI/KI mice, the TH in the striatum were still almost undetectable, suggesting differential BH4 requirements among brain regions. Last, the growth of Gch1KI/KI mice under combined therapy outperformed dopa or BH4 therapy alone. Notably, dopamine was abnormally high in more than half, but not all, of the treated Gch1KI/KI mice, suggesting the existence of variable synergetic effects of dopa and BH4 supplementation. Our results provide not only experimental evidence but also novel mechanistic insights into the efficacy and limitations of dopa and BH4 therapies for GTPCH deficiency.

Gene therapy for neurotransmitter-related disorders

Inborn errors of neurotransmitter (NT) metabolism are a group of rare, heterogenous diseases with predominant neurological features, such as movement disorders, autonomic dysfunction, and developmental delay. Clinical overlap with other disorders has led to delayed diagnosis and treatment, and some conditions are refractory to oral pharmacotherapies. Gene therapies have been developed and translated to clinics for paediatric inborn errors of metabolism, with 38 interventional clinical trials ongoing to date. Furthermore, efforts in restoring dopamine synthesis and neurotransmission through viral gene therapy have been developed for Parkinson's disease. Along with the recent European Medicines Agency (EMA) and Medicines and Healthcare Products Regulatory Agency (MHRA) approval of an AAV2 gene supplementation therapy for AADC deficiency, promising efficacy and safety profiles can be achieved in this group of diseases. In this review, we present preclinical and clinical advances to address NT-related diseases, and summarise potential challenges that require careful considerations for NT gene therapy studies.

GTP cyclohydroxylase1 (GCH1): Role in neurodegenerative diseases

GCH1 gene provides directions for the synthesis of GTP cyclohydrolase 1 which regulates the formation of Tetrahydrobiopterin (BH4). BH4 is a crucial cofactor for essential neurotransmitters synthesis such as dopamine, serotonin and nitric oxide synthases. Deficiency of GCH1 limits the synthesis of BH4 which is responsible for neuropsychiatric diseases such as dopa-responsive dystonia, hyperalaninemia, Parkinson's disease and depression. Few single nucleotide polymorphisms of GCH1 gene are also responsible for pain in sickle cell disease. Furthermore, GCH1 regulates NO activity which controls the blood pressure, vasodilatory functions and oxidative stress. Understanding the therapeutic implications of targeting GCH1 which holds promise for treating various diseases. Novel therapeutic strategies could involve small molecule drugs or gene therapy techniques that enhance GCH1 expression or activity.

Physiology of Dystonia: Animal Studies

Dystonia is currently ranked as the third most prevalent motor disorder. It is typically characterized by involuntary muscle over- or co-contractions that can cause painful abnormal postures and jerky movements. Dystonia is a heterogenous disorder-across patients, dystonic symptoms vary in their severity, body distribution, temporal pattern, onset, and progression. There are also a growing number of genes that are associated with hereditary dystonia. In addition, multiple brain regions are associated with dystonic symptoms in both genetic and sporadic forms of the disease. The heterogeneity of dystonia has made it difficult to fully understand its underlying pathophysiology. However, the use of animal models has been used to uncover the complex circuit mechanisms that lead to dystonic behaviors. Here, we summarize findings from animal models harboring mutations in dystonia-associated genes and phenotypic animal models with overt dystonic motor signs resulting from spontaneous mutations, neural circuit perturbations, or pharmacological manipulations. Taken together, an emerging picture depicts dystonia as a result of brain-wide network dysfunction driven by basal ganglia and cerebellar dysfunction. In the basal ganglia, changes in dopaminergic, serotonergic, noradrenergic, and cholinergic signaling are found across different animal models. In the cerebellum, abnormal burst firing activity is observed in multiple dystonia models. We are now beginning to unveil the extent to which these structures mechanistically interact with each other. Such mechanisms inspire the use of pre-clinical animal models that will be used to design new therapies including drug treatments and brain stimulation.

Phenotypes and Genotypes of Inherited Disorders of Biogenic Amine Neurotransmitter Metabolism

Inherited disorders of biogenic amine metabolism are genetically determined conditions resulting in dysfunctions or lack of enzymes involved in the synthesis, degradation, or transport of dopamine, serotonin, adrenaline/noradrenaline, and their metabolites or defects of their cofactor or chaperone biosynthesis. They represent a group of treatable diseases presenting with complex patterns of movement disorders (dystonia, oculogyric crises, severe/hypokinetic syndrome, myoclonic jerks, and tremors) associated with a delay in the emergence of postural reactions, global development delay, and autonomic dysregulation. The earlier the disease manifests, the more severe and widespread the impaired motor functions. Diagnosis mainly depends on measuring neurotransmitter metabolites in cerebrospinal fluid that may address the genetic confirmation. Correlations between the severity of phenotypes and genotypes may vary remarkably among the different diseases. Traditional pharmacological strategies are not disease-modifying in most cases. Gene therapy has provided promising results in patients with DYT-DDC and in vitro models of DYT/PARK-SLC6A3. The rarity of these diseases, combined with limited knowledge of their clinical, biochemical, and molecular genetic features, frequently leads to misdiagnosis or significant diagnostic delays. This review provides updates on these aspects with a final outlook on future perspectives.

Synaptic Dysfunction in Dystonia: Update From Experimental Models

Dystonia, the third most common movement disorder, refers to a heterogeneous group of neurological diseases characterized by involuntary, sustained or intermittent muscle contractions resulting in repetitive twisting movements and abnormal postures. In the last few years, several studies on animal models helped expand our knowledge of the molecular mechanisms underlying dystonia. These findings have reinforced the notion that the synaptic alterations found mainly in the basal ganglia and cerebellum, including the abnormal neurotransmitters signalling, receptor trafficking and synaptic plasticity, are a common hallmark of different forms of dystonia. In this review, we focus on the major contribution provided by rodent models of DYT-TOR1A, DYT-THAP1, DYT-GNAL, DYT/ PARK-GCH1, DYT/PARK-TH and DYT-SGCE dystonia, which reveal that an abnormal motor network and synaptic dysfunction represent key elements in the pathophysiology of dystonia.

A plant-based mutant huntingtin model-driven discovery of impaired expression of GTPCH and DHFR

Pathophysiology associated with Huntington's disease (HD) has been studied extensively in various cell and animal models since the 1993 discovery of the mutant huntingtin (mHtt) with abnormally expanded polyglutamine (polyQ) tracts as the causative factor. However, the sequence of early pathophysiological events leading to HD still remains elusive. To gain new insights into the early polyQ-induced pathogenic events, we expressed Htt exon1 (Htt_ex1) with a normal (21), or an extended (42 or 63) number of polyQ in tobacco plants. Here, we show that transgenic plants accumulated Htt_ex1 proteins with corresponding polyQ tracts, and mHtt_ex1 induced protein aggregation and affected plant growth, especially root and root hair development, in a polyQ length-dependent manner. Quantitative proteomic analysis of young roots from severely affected Htt_ex1Q63 and unaffected Htt_ex1Q21 plants showed that the most reduced protein by polyQ63 is a GTP cyclohydrolase I (GTPCH) along with many of its related one-carbon (C₁) metabolic pathway enzymes. GTPCH is a key enzyme involved in folate biosynthesis in plants and tetrahydrobiopterin (BH₄) biosynthesis in mammals. Validating studies in 4-week-old R6/2 HD mice expressing a mHtt_ex1 showed reduced levels of GTPCH and dihydrofolate reductase (DHFR, a key folate utilization/alternate BH₄ biosynthesis enzyme), and impaired C₁ and BH₄ metabolism. Our findings from mHtt_ex1 plants and mice reveal impaired expressions of GTPCH and DHFR and may contribute to a better understanding of mHtt-altered C₁ and BH₄ metabolism, and their roles in the pathogenesis of HD.

Insights into the expanding phenotypic spectrum of inherited disorders of biogenic amines

Inherited disorders of neurotransmitter metabolism are rare neurodevelopmental diseases presenting with movement disorders and global developmental delay. This study presents the results of the first standardized deep phenotyping approach and describes the clinical and biochemical presentation at disease onset as well as diagnostic approaches of 275 patients from the registry of the International Working Group on Neurotransmitter related Disorders. The results reveal an increased rate of prematurity, a high risk for being small for gestational age and for congenital microcephaly in some disorders. Age at diagnosis and the diagnostic delay are influenced by the diagnostic methods applied and by disease-specific symptoms. The timepoint of investigation was also a significant factor: delay to diagnosis has decreased in recent years, possibly due to novel diagnostic approaches or raised awareness. Although each disorder has a specific biochemical pattern, we observed confounding exceptions to the rule. The data provide comprehensive insights into the phenotypic spectrum of neurotransmitter disorders.

Inherited disorders of neurotransmitter metabolism represent a group of rare neurometabolic diseases characterized by movement disorders and developmental delay. Here, the authors report a standardized evaluation of a registry of 275 patients from 42 countries, and highlight an evolving phenotypic spectrum of this disease group and factors influencing diagnostic processes.

Apolipoprotein A-I mimetic peptide inhibits atherosclerosis by increasing tetrahydrobiopterin via regulation of GTP-cyclohydrolase 1 and reducing uncoupled endothelial nitric oxide synthase activity

BACKGROUND AND AIMS

The apolipoprotein A-I mimetic peptide D-4F, among its anti-atherosclerotic effects, improves vasodilation through mechanisms not fully elucidated yet.

METHODS

Low-density lipoprotein (LDL) receptor null (LDLr-/-) mice were fed Western diet with or without D-4F. We then measured atherosclerotic lesion formation, endothelial nitric oxide synthase (eNOS) phosphorylation and its association with heat shock protein 90 (HSP90), nitric oxide (NO) and superoxide anion (O2•-) production, and tetrahydrobiopterin (BH4) and GTP-cyclohydrolase 1 (GCH-1) concentration in the aorta. Human umbilical vein endothelial cells (HUVECs) and aortas were treated with oxidized LDL (oxLDL) with or without D-4F; subsequently, BH4 and GCH-1 concentration, NO and O2•- production, eNOS association with HSP90, and endothelium-dependent vasodilation were measured.

RESULTS

Unexpectedly, eNOS phosphorylation, eNOS-HSP90 association, and O2•- production were increased, whereas BH4 and GCH-1 concentration and NO production were reduced in atherosclerosis. D-4F significantly inhibited atherosclerosis, eNOS phosphorylation, eNOS-HSP90 association, and O2•- generation but increased NO production and BH4 and GCH-1 concentration. OxLDL reduced NO production and BH4 and GCH-1 concentration but enhanced O2•- generation and eNOS association with HSP90, and impaired endothelium-dependent vasodilation. D-4F inhibited the overall effects of oxLDL.

CONCLUSIONS

Hypercholesterolemia enhanced uncoupled eNOS activity by decreasing GCH-1 concentration, thereby reducing BH4 levels. D-4F reduced uncoupled eNOS activity by increasing BH4 levels through GCH-1 expression and decreasing eNOS phosphorylation and eNOS-HSP90 association. Our findings elucidate a novel mechanism by which hypercholesterolemia induces atherosclerosis and D-4F inhibits it, providing a potential therapeutic approach.

Sepiapterin reductase promotes hepatocellular carcinoma progression via FoxO3a/Bim signaling in a nonenzymatic manner

Sepiapterin reductase plays an enzymatic role in the biosynthesis of tetrahydrobiopterin, which is reported in limited studies to regulate the progression of several tumors. However, the role of sepiapterin reductase in hepatocellular carcinoma remains largely unknown. Here, we found that sepiapterin reductase was frequently highly expressed in human hepatocellular carcinoma, which was significantly associated with higher T stage, higher tumor node metastasis stage, and even shorter survival of hepatocellular carcinoma patients. Furthermore, cell and animal experiments showed that sepiapterin reductase depletion inhibited cancer cell proliferation and promoted cancer cell apoptosis. Importantly, the results suggested that sepiapterin reductase enzymatic activity was not necessary for the progression of hepatocellular carcinoma, based on the comparison between SMMC-7721 and SMMC-7721 containing sepiapterin reductase mutant. Moreover, we showed that sepiapterin reductase regulated the development of hepatocellular carcinoma via the FoxO3a/Bim-signaling pathway. Collectively, our study suggests that sepiapterin reductase controls hepatocellular carcinoma progression via FoxO3a/Bim signaling in a nonenzymatic manner, which provides a potential prognostic factor and therapeutic strategy for hepatocellular carcinoma.