Designing phase II clinical trials in Friedreich ataxia

NRF2 activation suppresses motor neuron ferroptosis induced by the SOD1G93A mutation and exerts neuroprotection in amyotrophic lateral sclerosis

The progressive neurodegenerative disease amyotrophic lateral sclerosis (ALS) is caused by a decline in motor neuron function, resulting in worsened motor impairments, malnutrition, respiratory failure and mortality, and there is a lack of effective clinical treatments. The exact mechanism of motor neuronal degeneration remains unclear. Previously, we reported that ferroptosis, which is characterized by the accumulation of lipid peroxide and glutathione depletion in an iron-dependent manner, contributed to motor neuronal death in ALS cell models with the hSOD1^G93A (human Cu/Zn-superoxide dismutase) gene mutation. In this study, we further explored the role of ferroptosis in motor neurons and its regulation in mutant hSOD1^G93A cell and mouse models. Our results showed that ferroptosis was activated in hSOD1^G93A NSC-34 cells and mouse models, which was accompanied by decreased nuclear retention of nuclear factor erythroid 2-related factor 2 (NRF2) and downregulation of solute carrier family 7 member 11 (SLC7A11) and glutathione peroxidase 4 (GPX4) levels. Moreover, RTA-408, an NRF2 activator, inhibited ferroptosis in hSOD1^G93A NSC-34 cells by upregulating the protein expression of SLC7A11 and GPX4. Moreover, hSOD1^G93A mice treated with RTA-408 showed obvious improvements in body weight and motor function. Our study demonstrated that ferroptosis contributed to the toxicity of motor neurons and that activating NRF2 could alleviate neuronal degeneration in ALS with the hSOD1^G93A mutation.

Perspectives on current models of Friedreich’s ataxia

Friedreich’s ataxia (FRDA, OMIM#229300) is the most common hereditary ataxia, resulting from the reduction of frataxin protein levels due to the expansion of GAA repeats in the first intron of the FXN gene. Why the triplet repeat expansion causes a decrease in Frataxin protein levels is not entirely known. Generation of effective FRDA disease models is crucial for answering questions regarding the pathophysiology of this disease. There have been considerable efforts to generate in vitro and in vivo models of FRDA. In this perspective article, we highlight studies conducted using FRDA animal models, patient-derived materials, and particularly induced pluripotent stem cell (iPSC)-derived models. We discuss the current challenges in using FRDA animal models and patient-derived cells. Additionally, we provide a brief overview of how iPSC-based models of FRDA were used to investigate the main pathways involved in disease progression and to screen for potential therapeutic agents for FRDA. The specific focus of this perspective article is to discuss the outlook and the remaining challenges in the context of FRDA iPSC-based models.

Hyperactivation of mTOR and AKT in a cardiac hypertrophy animal model of Friedreich ataxia

Cardiomyopathy is a primary cause of death in Friedreich ataxia (FRDA) patients with defective iron-sulfur cluster (ISC) biogenesis due to loss of functional frataxin and in rare patients with functional loss of other ISC biogenesis factors. The mechanistic target of rapamycin (mTOR) and AKT signaling cascades that coordinate eukaryotic cell growth and metabolism with environmental inputs, including nutrients and growth factors, are crucial regulators of cardiovascular growth and homeostasis. We observed increased phosphorylation of AKT and dysregulation of multiple downstream effectors of mTORC1, including S6K1, S6, ULK1 and 4EBP1, in a cardiac/skeletal muscle specific FRDA conditional knockout (cKO) mouse model and in human cell lines depleted of ISC biogenesis factors. Knockdown of several mitochondrial metabolic proteins that are downstream targets of ISC biogenesis, including lipoyl synthase and subunit B of succinate dehydrogenase, also resulted in activation of mTOR and AKT signaling, suggesting that mTOR and AKT hyperactivations are part of the metabolic stress response to ISC deficiencies. Administration of rapamycin, a specific inhibitor of mTOR signaling, enhanced the survival of the Fxn cKO mice, providing proof of concept for the potential of mTOR inhibition to ameliorate cardiac disease in patients with defective ISC biogenesis. However, AKT phosphorylation remained high in rapamycin-treated Fxn cKO hearts, suggesting that parallel mTOR and AKT inhibition might be necessary to further improve the lifespan and healthspan of ISC deficient individuals.

Drug Repositioning in Friedreich Ataxia

Friedreich ataxia is a rare neurodegenerative disorder caused by insufficient levels of the essential mitochondrial protein frataxin. It is a severely debilitating disease that significantly impacts the quality of life of affected patients and reduces their life expectancy, however, an adequate cure is not yet available for patients. Frataxin function, although not thoroughly elucidated, is associated with assembly of iron-sulfur cluster and iron metabolism, therefore insufficient frataxin levels lead to reduced activity of many mitochondrial enzymes involved in the electron transport chain, impaired mitochondrial metabolism, reduced ATP production and inefficient anti-oxidant response. As a consequence, neurons progressively die and patients progressively lose their ability to coordinate movement and perform daily activities. Therapeutic strategies aim at restoring sufficient frataxin levels or at correcting some of the downstream consequences of frataxin deficiency. However, the classical pathways of drug discovery are challenging, require a significant amount of resources and time to reach the final approval, and present a high failure rate. Drug repositioning represents a viable alternative to boost the identification of a therapy, particularly for rare diseases where resources are often limited. In this review we will describe recent efforts aimed at the identification of a therapy for Friedreich ataxia through drug repositioning, and discuss the limitation of such strategies.