Assessment of cell-free levels of iron and copper in patients with Friedreich's ataxia

Genetic Determined Iron Starvation Signature in Friedreich's Ataxia

BACKGROUND

Early studies in cellular models suggested an iron accumulation in Friedreich's ataxia (FA), yet findings from patients are lacking.

OBJECTIVES

The objective is to characterize systemic iron metabolism, body iron storages, and intracellular iron regulation in FA patients.

METHODS

In FA patients and matched healthy controls, we assessed serum iron parameters, regulatory hormones as well as the expression of regulatory proteins and iron distribution in peripheral blood mononuclear cells (PBMCs). We applied magnetic resonance imaging with R2*-relaxometry to quantify iron storages in the liver, spleen, and pancreas. Across all evaluations, we assessed the influence of the genetic severity as expressed by the length of the shorter GAA-expansion (GAA1).

RESULTS

We recruited 40 FA patients (19 women). Compared to controls, FA patients displayed lower serum iron and transferrin saturation. Serum ferritin, hepcidin, mean corpuscular hemoglobin and mean corpuscular volume in FA inversely correlated with the GAA1-repeat length, indicating iron deficiency and restricted availability for erythropoiesis with increasing genetic severity. R2*-relaxometry revealed a reduction of splenic and hepatic iron stores in FA. Liver and spleen R2* values inversely correlated with the GAA1-repeat length. FA PBMCs displayed downregulation of ferritin and upregulation of transferrin receptor and divalent metal transporter-1 mRNA, particularly in patients with >500 GAA1-repeats. In FA PBMCs, intracellular iron was not increased, but shifted toward mitochondria.

CONCLUSIONS

We provide evidence for a previously unrecognized iron starvation signature at systemic and cellular levels in FA patients, which is related to the underlying genetic severity. These findings challenge the use of systemic iron lowering therapies in FA. © 2024 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Long non-coding RNA TUG1 is downregulated in Friedreich's ataxia

Friedreich's ataxia is a neurodegenerative disorder caused by reduced frataxin levels. It leads to motor and sensory impairments and has a median life expectancy of around 35 years. As the most common inherited form of ataxia, Friedreich's ataxia lacks reliable, non-invasive biomarkers, prolonging and inflating the cost of clinical trials. This study proposes TUG1, a long non-coding RNA, as a promising blood-based biomarker for Friedreich's ataxia, which is known to regulate various cellular processes. In a previous study using a frataxin knockdown mouse model, we observed several hallmark Friedreich's ataxia symptoms. Building on this, we hypothesized that a dual-source approach-comparing the data from peripheral blood samples from Friedreich's ataxia patients with tissue samples from affected areas in Friedreich's ataxia knockdown mice, tissues usually unattainable from patients-would effectively identify robust biomarkers. A comprehensive reanalysis was conducted on gene expression data from 183 age- and sex-matched peripheral blood samples of Friedreich's ataxia patients, carriers and controls and 192 tissue data sets from Friedreich's ataxia knockdown mice. Blood and tissue samples underwent RNA isolation and quantitative reverse transcription polymerase chain reaction, and frataxin knockdown was confirmed through enzyme-linked immunosorbent assays. Tug1 RNA interaction was explored via RNA pull-down assays. Validation was performed in serum samples on an independent set of 45 controls and 45 Friedreich's ataxia patients and in blood samples from 66 heterozygous carriers and 72 Friedreich's ataxia patients. Tug1 and Slc40a1 emerged as potential blood-based biomarkers, confirmed in the Friedreich's ataxia knockdown mouse model (one-way ANOVA, P ≤ 0.05). Tug1 was consistently downregulated after Fxn knockdown and correlated strongly with Fxn levels (R 2 = 0.71 during depletion, R 2 = 0.74 during rescue). Slc40a1 showed a similar but tissue-specific pattern. Further validation of Tug1's downstream targets strengthened its biomarker candidacy. In additional human samples, TUG1 levels were significantly downregulated in both whole blood and serum of Friedreich's ataxia patients compared with controls (Wilcoxon signed-rank test, P < 0.05). Regression analyses revealed a negative correlation between TUG1 fold-change and disease onset (P < 0.0037) and positive correlations with disease duration and functional disability stage score (P < 0.04). This suggests that elevated TUG1 levels correlate with earlier onset and more severe cases. This study identifies TUG1 as a potential blood-based biomarker for Friedreich's ataxia, showing consistent expression variance in human and mouse tissues related to disease severity and key Friedreich's ataxia pathways. It correlates with frataxin levels, indicating its promise as an early, non-invasive marker. TUG1 holds potential for Friedreich's ataxia monitoring and therapeutic development, meriting additional research.

Cerebral Iron Deposition in Neurodegeneration

Disruption of cerebral iron regulation appears to have a role in aging and in the pathogenesis of various neurodegenerative disorders. Possible unfavorable impacts of iron accumulation include reactive oxygen species generation, induction of ferroptosis, and acceleration of inflammatory changes. Whole-brain iron-sensitive magnetic resonance imaging (MRI) techniques allow the examination of macroscopic patterns of brain iron deposits in vivo, while modern analytical methods ex vivo enable the determination of metal-specific content inside individual cell-types, sometimes also within specific cellular compartments. The present review summarizes the whole brain, cellular, and subcellular patterns of iron accumulation in neurodegenerative diseases of genetic and sporadic origin. We also provide an update on mechanisms, biomarkers, and effects of brain iron accumulation in these disorders, focusing on recent publications. In Parkinson’s disease, Friedreich’s disease, and several disorders within the neurodegeneration with brain iron accumulation group, there is a focal siderosis, typically in regions with the most pronounced neuropathological changes. The second group of disorders including multiple sclerosis, Alzheimer’s disease, and amyotrophic lateral sclerosis shows iron accumulation in the globus pallidus, caudate, and putamen, and in specific cortical regions. Yet, other disorders such as aceruloplasminemia, neuroferritinopathy, or Wilson disease manifest with diffuse iron accumulation in the deep gray matter in a pattern comparable to or even more extensive than that observed during normal aging. On the microscopic level, brain iron deposits are present mostly in dystrophic microglia variably accompanied by iron-laden macrophages and in astrocytes, implicating a role of inflammatory changes and blood–brain barrier disturbance in iron accumulation. Options and potential benefits of iron reducing strategies in neurodegeneration are discussed. Future research investigating whether genetic predispositions play a role in brain Fe accumulation is necessary. If confirmed, the prevention of further brain Fe uptake in individuals at risk may be key for preventing neurodegenerative disorders.

Molecular Defects in Friedreich's Ataxia: Convergence of Oxidative Stress and Cytoskeletal Abnormalities

Friedreich’s ataxia (FRDA) is a multi-faceted disease characterized by progressive sensory–motor loss, neurodegeneration, brain iron accumulation, and eventual death by hypertrophic cardiomyopathy. FRDA follows loss of frataxin (FXN), a mitochondrial chaperone protein required for incorporation of iron into iron–sulfur cluster and heme precursors. After the discovery of the molecular basis of FRDA in 1996, over two decades of research have been dedicated to understanding the temporal manifestations of disease both at the whole body and molecular level. Early research indicated strong cellular iron dysregulation in both human and yeast models followed by onset of oxidative stress. Since then, the pathophysiology due to dysregulation of intracellular iron chaperoning has become central in FRDA relative to antioxidant defense and run-down in energy metabolism. At the same time, limited consideration has been given to changes in cytoskeletal organization, which was one of the first molecular defects noted. These alterations include both post-translational oxidative glutathionylation of actin monomers and differential DNA processing of a cytoskeletal regulator PIP5K1β. Currently unknown in respect to FRDA but well understood in the context of FXN-deficient cell physiology is the resulting impact on the cytoskeleton; this disassembly of actin filaments has a particularly profound effect on cell–cell junctions characteristic of barrier cells. With respect to a neurodegenerative disorder such as FRDA, this cytoskeletal and tight junction breakdown in the brain microvascular endothelial cells of the blood–brain barrier is likely a component of disease etiology. This review serves to outline a brief history of this research and hones in on pathway dysregulation downstream of iron-related pathology in FRDA related to actin dynamics. The review presented here was not written with the intent of being exhaustive, but to instead urge the reader to consider the essentiality of the cytoskeleton and appreciate the limited knowledge on FRDA-related cytoskeletal dysfunction as a result of oxidative stress. The review examines previous hypotheses of neurodegeneration with brain iron accumulation (NBIA) in FRDA with a specific biochemical focus.

Ferrous iron binding to epinephrine promotes the oxidation of iron and impedes activation of adrenergic receptors

Upon release in response to stress, epinephrine (Epi) may interact with labile iron pool in human plasma with potentially important (patho)physiological consequences. We have shown that Epi and Fe³⁺ build stable 1:1 high-spin bidentate complex at physiological pH, and that Epi does not undergo degradation in the presence of iron. However, the interactions of Epi with the more soluble Fe²⁺, and the impact of iron on biological activity of Epi are still not known. Herein we showed that Epi and Fe²⁺ build colorless complex which is stable under anaerobic conditions. In the presence of O₂, Epi promoted the oxidation of Fe²⁺ and the formation of Epi-Fe³⁺ complex. Cyclic voltammetry showed that mid-point potential of Epi-Fe²⁺ complex is very low (-582 mV vs. standard hydrogen electrode), which explains catalyzed oxidation of Fe²⁺. Next, we examined the impact of iron binding on biological performance of Epi using patch clamping in cell culture with constitutive expression of adrenergic receptors. Epi alone evoked an increase of outward currents, whereas Epi in the complex with Fe³⁺ did not. This implies that the binding of Epi to adrenergic receptors and their activation is prevented by the formation of complex with iron. Pro-oxidative activity of Epi-Fe²⁺ complex may represent a link between chronic stress and cardiovascular problems. On the other hand, labile iron could serve as a modulator of biological activity of ligands. Such interactions may be important in human pathologies that are related to iron overload or deficiency.

Quantitative Proteomic and Network Analysis of Differentially Expressed Proteins in PBMC of Friedreich's Ataxia (FRDA) Patients

Friedreich’s ataxia (FRDA) is an autosomal recessive neurodegenerative disorder caused by an expanded (GAA) trinucleotide repeat in the FXN gene. The extended repeats expansion results in reduced transcription and, thereby, decreased expression of the mitochondrial protein, frataxin. Given the ongoing drug trials, identification of reliable and easily accessible biomarkers for monitoring disease progression and therapeutic intervention is a foremost requirement. In this study, comparative proteomic profiling of PBMC proteins from FRDA patients and age- and gender-matched healthy controls was done using 2D-Differential in-Gel Electrophoresis (2D-DIGE). Protein–protein interaction (PPI) was analyzed using BioGRID and STRING pathway analysis tools. Using biological variance analysis (BVA) and LC/MS, we found eight differentially expressed proteins with fold change ≥1.5; p ≤ 0.05. Based on their cellular function, the identified proteins showed a strong pathological role in neuroinflammation, cardiomyopathy, compromised glucose metabolism, and iron transport, which are the major clinical manifestations of FRDA. Protein–protein network analysis of differentially expressed proteins with frataxin further supports their involvement in the pathophysiology of FRDA. Considering their crucial role in the cardiac and neurological complications, respectively, the two down-regulated proteins, actin α cardiac muscle 1 (ACTC1) and pyruvate dehydrogenase E1 component subunit β (PDHE1), are suggested as potential prognostic markers for FRDA.