A new FRDA mouse model [Fxn null:YG8s(GAA) > 800] with more than 800 GAA repeats

Robust behavioral assessment of the inducible Friedreich's ataxia mouse does not show improvement with NRF2 induction

Friedreich's ataxia, a recessive disorder caused by a mutation in the frataxin (FXN) gene, has few mouse models that demonstrate a progressive behavioral decline paralleling that of patients. A mouse model of systemic frataxin deficiency, the FXNKD, was recently developed using a doxycycline-inducible method; it is thought to mimic the patient phenotype seen when frataxin levels are decreased, but it has not been determined whether it is reliable for assessment of therapeutics. FXNKD mice underwent testing for 12 weeks alongside littermates, undergoing tests of motor function, gait and sensation. Additionally, a subset underwent treatment with omaveloxolone or dimethyl fumarate, both NRF2 inducers. We identified multiple techniques that sensitively detect decline in the mice, including open field, gait analysis and Von Frey tests. Furthermore, we developed a novel Salinas-Montgomery ataxia scale, which allows for more comprehensive assessment than a four-part cerebellar ataxia scale. Despite validating multiple sensitive techniques, we did not see any benefits of NRF2-inducing therapies in any tests. This was exacerbated by the discovery of a sexual dimorphism in FXNKD mice, in which males show more significant decline and better responsiveness to NRF2-inducing therapeutics.

Friedreich Ataxia: An (Almost) 30-Year History After Gene Discovery

In the late 1800s, Nikolaus Friedreich first described "degenerative atrophy of the posterior columns of the spinal cord," noting its connection to progressive ataxia, sensory loss, and muscle weakness, now recognized as Friedreich ataxia (FRDA). Renewed interest in the disease in the 1970s and 80s by the Quebec Cooperative Group and by Anita Harding led to the development of clinical diagnostic criteria and insights into associated biochemical abnormalities, although the primary defect remained unknown. In 1988, Susan Chamberlain mapped FRDA's location on chromosome 9. In the early 90s, collaborative research, including work by the author's team, identified a gene, later named FXN, containing an expanded GAA repeat-confirming it as the FRDA mutation. This discovery established a diagnostic foundation for FRDA, advancing genetic testing and opening new research avenues. These new areas of study included the characteristics, origin, and pathogenicity of the GAA repeat expansion; the characterization of frataxin, the encoded protein, including its subcellular localization, structure, and function; the identification of cellular pathways disrupted by frataxin deficiency; and the redefinition of FRDA phenotypes based on genetic testing, along with the study of FRDA's natural history. In addition, efforts focused on the search for biomarkers to reflect diagnosis, disease severity, and progression and, most importantly, the identification and development of therapeutic approaches in both preclinical and clinical settings. The creation of cellular and animal models was crucial to this progress, as was the formation of consortia to collaboratively drive basic and clinical research forward. Now, 28 years after the gene discovery, although much remains to be understood about the disease's mechanisms and the development of effective therapies, the progress is undeniable. A thriving community has emerged, uniting researchers, health care providers, industry professionals, individuals with FRDA, their families, and dedicated volunteers. With this collective effort, a cure is within reach.

A multiple animal and cellular models approach to study frataxin deficiency in Friedreich Ataxia

Friedreich's ataxia (FA) is one of the most frequent inherited recessive ataxias characterized by a progressive sensory and spinocerebellar ataxia. The main causative mutation is a GAA repeat expansion in the first intron of the frataxin (FXN) gene which leads to a transcriptional silencing of the gene resulting in a deficit in FXN protein. The nature of the mutation (an unstable GAA expansion), as well as the multi-systemic nature of the disease (with neural and non-neural sites affected) make the generation of models for Friedreich's ataxia quite challenging. Over the years, several cellular and animal models for FA have been developed. These models are all complementary and possess their own strengths to investigate different aspects of the disease, such as the epigenetics of the locus or the pathophysiology of the disease, as well as being used to developed novel therapeutic approaches. This review will explore the recent advancements in the different mammalian models developed for FA.

Glial cell activation precedes neurodegeneration in the cerebellar cortex of the YG8-800 murine model of Friedreich ataxia

Friedreich ataxia is a hereditary neurodegenerative disorder resulting from reduced levels of the protein frataxin due to an expanded GAA repeat in the FXN gene. This deficiency causes progressive degeneration of specific neuronal populations in the cerebellum and the consequent loss of movement coordination and equilibrium, which are some of the main symptoms observed in affected individuals. Like in other neurodegenerative diseases, previous studies suggest that glial cells could be involved in the neurodegenerative process and disease progression in patients with Friedreich ataxia. In this work, we followed and characterized the progression of changes in the cerebellar cortex in the latest version of Friedreich ataxia humanized mouse model, YG8-800 (Fxnnull:YG8s(GAA)>800), which carries a human FXN transgene containing >800 GAA repeats. Comparative analyses of behavioral, histopathological, and biochemical parameters were conducted between the control strain Y47R and YG8-800 mice at different time points. Our findings revealed that YG8-800 mice exhibit an ataxic phenotype characterized by poor motor coordination, decreased body weight, cerebellar atrophy, neuronal loss, and changes in synaptic proteins. Additionally, early activation of glial cells, predominantly astrocytes and microglia, was observed preceding neuronal degeneration, as was increased expression of key proinflammatory cytokines and downregulation of neurotrophic factors. Together, our results show that the YG8-800 mouse model exhibits a stronger phenotype than previous experimental murine models, reliably recapitulating some of the features observed in humans. Accordingly, this humanized model could represent a valuable tool for studying Friedreich ataxia molecular disease mechanisms and for preclinical evaluation of possible therapies.

Anatomical and functional analysis of the corticospinal tract in an FRDA mouse model

Friedreich's ataxia (FRDA) is one of the most common hereditary ataxias. It is caused by a GAA repeat in the first intron of the FXN gene, which encodes an essential mitochondrial protein. Patients suffer from progressive motor dysfunction due to the degeneration of mechanoreceptive and proprioceptive neurons in dorsal root ganglia (DRG) and cerebellar dentate nucleus neurons, especially at early disease stages. Postmortem analyses of FRDA patients also indicate pathological changes in motor cortex including in the projection neurons that give rise to the cortical spinal tract (CST). Yet, it remains poorly understood how early in the disease cortical spinal neurons (CSNs) show these alterations, or whether CSN/CST pathology resembles the abnormalities observed in other tissues affected by FXN loss. To address these questions, we examined CSN driven motor behaviors and pathology in the YG8JR FRDA mouse model. We find that FRDA mice show impaired motor skills, exhibit significant reductions in CSN functional output, and, among other pathological changes, show abnormal mitochondrial distributions in CSN neurons and CST axonal tracts. Moreover, some of these alterations were observed as early as two months of age, suggesting that CSN/CST pathology may be an earlier event in FRDA disease than previously appreciated. These studies warrant a detailed mechanistic understanding of how FXN loss impacts CSN health and functionality.

Somatic and intergenerational G4C2 hexanucleotide repeat instability in a human C9orf72 knock-in mouse model

Expansion of a G4C2 repeat in the C9orf72 gene is associated with familial Amyotrophic Lateral Sclerosis (ALS) and Frontotemporal Dementia (FTD). To investigate the underlying mechanisms of repeat instability, which occurs both somatically and intergenerationally, we created a novel mouse model of familial ALS/FTD that harbors 96 copies of G4C2 repeats at a humanized C9orf72 locus. In mouse embryonic stem cells, we observed two modes of repeat expansion. First, we noted minor increases in repeat length per expansion event, which was dependent on a mismatch repair pathway protein Msh2. Second, we found major increases in repeat length per event when a DNA double- or single-strand break (DSB/SSB) was artificially introduced proximal to the repeats, and which was dependent on the homology-directed repair (HDR) pathway. In mice, the first mode primarily drove somatic repeat expansion. Major changes in repeat length, including expansion, were observed when SSB was introduced in one-cell embryos, or intergenerationally without DSB/SSB introduction if G4C2 repeats exceeded 400 copies, although spontaneous HDR-mediated expansion has yet to be identified. These findings provide a novel strategy to model repeat expansion in a non-human genome and offer insights into the mechanism behind C9orf72 G4C2 repeat instability.

Mitochondrial impairment, decreased sirtuin activity and protein acetylation in dorsal root ganglia in Friedreich Ataxia models

Friedreich ataxia (FA) is a rare, recessive neuro-cardiodegenerative disease caused by deficiency of the mitochondrial protein frataxin. Mitochondrial dysfunction, a reduction in the activity of iron-sulfur enzymes, iron accumulation, and increased oxidative stress have been described. Dorsal root ganglion (DRG) sensory neurons are among the cellular types most affected in the early stages of this disease. However, its effect on mitochondrial function remains to be elucidated. In the present study, we found that in primary cultures of DRG neurons as well as in DRGs from the FXNI151F mouse model, frataxin deficiency resulted in lower activity and levels of the electron transport complexes, mainly complexes I and II. In addition, altered mitochondrial morphology, indicative of degeneration was observed in DRGs from FXNI151F mice. Moreover, the NAD+/NADH ratio was reduced and sirtuin activity was impaired. We identified alpha tubulin as the major acetylated protein from DRG homogenates whose levels were increased in FXNI151F mice compared to WT mice. In the mitochondria, superoxide dismutase (SOD2), a SirT3 substrate, displayed increased acetylation in frataxin-deficient DRG neurons. Since SOD2 acetylation inactivates the enzyme, and higher levels of mitochondrial superoxide anion were detected, oxidative stress markers were analyzed. Elevated levels of hydroxynonenal bound to proteins and mitochondrial Fe2+ accumulation was detected when frataxin decreased. Honokiol, a SirT3 activator, restores mitochondrial respiration, decreases SOD2 acetylation and reduces mitochondrial superoxide levels. Altogether, these results provide data at the molecular level of the consequences of electron transport chain dysfunction, which starts negative feedback, contributing to neuron lethality. This is especially important in sensory neurons which have greater susceptibility to frataxin deficiency compared to other tissues.

Humanized Mice as a Model to Assess the Response of Human Hematopoietic Stem Cells to Irradiation

NOD SCID mice were humanized by transplanting human hematopoietic cells isolated from umbilical cord blood. A dose-dependent death of hematopoietic cells and their subsequent recovery were shown after acute external γ-irradiation in the model of humanized mice. The proposed approach can be used for preclinical studies of radioprotective agents and for assessment of the impact of adverse factors on the survival rate and functional properties of human hematopoietic stem cells in vivo.

Finding an Appropriate Mouse Model to Study the Impact of a Treatment for Friedreich Ataxia on the Behavioral Phenotype

Friedreich ataxia (FRDA) is a progressive neurodegenerative disease caused by a GAA repeat in the intron 1 of the frataxin gene (FXN) leading to a lower expression of the frataxin protein. The YG8sR mice are Knock-Out (KO) for their murine frataxin gene but contain a human frataxin transgene derived from an FRDA patient with 300 GAA repeats. These mice are used as a FRDA model but even with a low frataxin concentration, their phenotype is mild. We aimed to find an optimized mouse model with a phenotype comparable to the human patients to study the impact of therapy on the phenotype. We compared two mouse models: the YG8sR injected with an AAV. PHP.B coding for a shRNA targeting the human frataxin gene and the YG8-800, a new mouse model with a human transgene containing 800 GAA repeats. Both mouse models were compared to Y47R mice containing nine GAA repeats that were considered healthy mice. Behavior tests (parallel rod floor apparatus, hanging test, inverted T beam, and notched beam test) were carried out from 2 to 11 months and significant differences were noticed for both YG8sR mice injected with an anti-FXN shRNA and the YG8-800 mice compared to healthy mice. In conclusion, YG8sR mice have a slight phenotype, and injecting them with an AAV-PHP.B expressing an shRNA targeting frataxin does increase their phenotype. The YG8-800 mice have a phenotype comparable to the human ataxic phenotype.