Evidence of mitochondrial dysfunction in fragile X-associated tremor/ataxia syndrome

Role of fragile X messenger ribonucleoprotein 1 in the pathophysiology of brain disorders: a glia perspective

Fragile X messenger ribonucleoprotein 1 (FMRP) is a widely expressed RNA binding protein involved in several steps of mRNA metabolism. Mutations in the FMR1 gene encoding FMRP are responsible for fragile X syndrome (FXS), a leading genetic cause of intellectual disability and autism spectrum disorder, and fragile X-associated tremor-ataxia syndrome (FXTAS), a neurodegenerative disorder in aging men. Although FMRP is mainly expressed in neurons, it is also present in glial cells and its deficiency or altered expression can affect functions of glial cells with implications for the pathophysiology of brain disorders. The present review focuses on recent advances on the role of glial subtypes, astrocytes, oligodendrocytes and microglia, in the pathophysiology of FXS and FXTAS, and describes how the absence or reduced expression of FMRP in these cells can impact on glial and neuronal functions. We will also briefly address the role of FMRP in radial glial cells and its effects on neural development, and gliomas and will speculate on the role of glial FMRP in other brain disorders.

Mitochondrial dysfunction in brain tissues and Extracellular Vesicles Fragile X-associated tremor/ataxia syndrome

OBJECTIVE

Mitochondrial impairments have been implicated in the pathogenesis of Fragile X-associated tremor/ataxia syndrome (FXTAS) based on analysis of mitochondria in peripheral tissues and cultured cells. We sought to assess whether mitochondrial abnormalities present in postmortem brain tissues of patients with FXTAS are also present in plasma neuron-derived extracellular vesicles (NDEVs) from living carriers of fragile X messenger ribonucleoprotein1 (FMR1) gene premutations at an early asymptomatic stage of the disease continuum.

METHODS

We utilized postmortem frozen cerebellar and frontal cortex samples from a cohort of eight patients with FXTAS and nine controls and measured the quantity and activity of the mitochondrial proteins complex IV and complex V. In addition, we evaluated the same measures in isolated plasma NDEVs by selective immunoaffinity capture targeting L1CAM from a separate cohort of eight FMR1 premutation carriers and four age-matched controls.

RESULTS

Lower complex IV and V quantity and activity were observed in the cerebellum of FXTAS patients compared to controls, without any differences in total mitochondrial content. No patient-control differences were observed in the frontal cortex. In NDEVs, FMR1 premutation carriers compared to controls had lower activity of Complex IV and Complex V, but higher Complex V quantity.

INTERPRETATION

Quantitative and functional abnormalities in mitochondrial electron transport chain complexes IV and V seen in the cerebellum of patients with FXTAS are also manifest in plasma NDEVs of FMR1 premutation carriers. Plasma NDEVs may provide further insights into mitochondrial pathologies in this syndrome and could potentially lead to the development of biomarkers for predicting symptomatic FXTAS among premutation carriers and disease monitoring.

The Role of NRF2 in Trinucleotide Repeat Expansion Disorders

Trinucleotide repeat expansion disorders, a diverse group of neurodegenerative diseases, are caused by abnormal expansions within specific genes. These expansions trigger a cascade of cellular damage, including protein aggregation and abnormal RNA binding. A key contributor to this damage is oxidative stress, an imbalance of reactive oxygen species that harms cellular components. This review explores the interplay between oxidative stress and the NRF2 pathway in these disorders. NRF2 acts as the master regulator of the cellular antioxidant response, orchestrating the expression of enzymes that combat oxidative stress. Trinucleotide repeat expansion disorders often exhibit impaired NRF2 signaling, resulting in inadequate responses to excessive ROS production. NRF2 activation has been shown to upregulate antioxidative gene expression, effectively alleviating oxidative stress damage. NRF2 activators, such as omaveloxolone, vatiquinone, curcumin, sulforaphane, dimethyl fumarate, and resveratrol, demonstrate neuroprotective effects by reducing oxidative stress in experimental cell and animal models of these diseases. However, translating these findings into successful clinical applications requires further research. In this article, we review the literature supporting the role of NRF2 in the pathogenesis of these diseases and the potential therapeutics of NRF2 activators.

Advances on the Mechanisms and Therapeutic Strategies in Non-coding CGG Repeat Expansion Diseases

Non-coding CGG repeat expansions within the 5' untranslated region are implicated in a range of neurological disorders, including fragile X-associated tremor/ataxia syndrome, oculopharyngeal myopathy with leukodystrophy, and oculopharyngodistal myopathy. This review outlined the general characteristics of diseases associated with non-coding CGG repeat expansions, detailing their clinical manifestations and neuroimaging patterns, which often overlap and indicate shared pathophysiological traits. We summarized the underlying molecular mechanisms of these disorders, providing new insights into the roles that DNA, RNA, and toxic proteins play. Understanding these mechanisms is crucial for the development of targeted therapeutic strategies. These strategies include a range of approaches, such as antisense oligonucleotides, RNA interference, genomic DNA editing, small molecule interventions, and other treatments aimed at correcting the dysregulated processes inherent in these disorders. A deeper understanding of the shared mechanisms among non-coding CGG repeat expansion disorders may hold the potential to catalyze the development of innovative therapies, ultimately offering relief to individuals grappling with these debilitating neurological conditions.

Mitochondrial dysfunction in Fragile X syndrome and Fragile X-associated tremor/ataxia syndrome: prospect use of antioxidants and mitochondrial nutrients

Fragile X syndrome (FXS) is a genetic disorder characterized by mutation in the FMR1 gene, leading to the absence or reduced levels of fragile X Messenger Ribonucleoprotein 1 (FMRP). This results in neurodevelopmental deficits, including autistic spectrum conditions. On the other hand, Fragile X-associated tremor/ataxia syndrome (FXTAS) is a distinct disorder caused by the premutation in the FMR1 gene. FXTAS is associated with elevated levels of FMR1 mRNA, leading to neurodegenerative manifestations such as tremors and ataxia.Mounting evidence suggests a link between both syndromes and mitochondrial dysfunction (MDF). In this minireview, we critically examine the intricate relationship between FXS, FXTAS, and MDF, focusing on potential therapeutic avenues to counteract or mitigate their adverse effects. Specifically, we explore the role of mitochondrial cofactors and antioxidants, with a particular emphasis on alpha-lipoic acid (ALA), carnitine (CARN) and Coenzyme Q10 (CoQ10). Findings from this review will contribute to a deeper understanding of these disorders and foster novel therapeutic strategies to enhance patient outcomes.

Bridging the gap: R-loop mediated genomic instability and its implications in neurological diseases

R-loops, intricate three-stranded structures formed by RNA-DNA hybrids and an exposed non-template DNA strand, are fundamental to various biological phenomena. They carry out essential and contrasting functions within cellular mechanisms, underlining their critical role in maintaining cellular homeostasis. The specific cellular context that dictates R-loop formation determines their function, particularly emphasizing the necessity for their meticulous genomic regulation. Notably, the aberrant formation or misregulation of R-loops is implicated in numerous neurological disorders. This review focuses on the complex interactions between R-loops and double-strand DNA breaks, exploring how R-loop dysregulation potentially contributes to the pathogenesis of various brain disorders, which could provide novel insights into the molecular mechanisms underpinning neurological disease progression and identify potential therapeutic targets by highlighting these aspects.

FXTAS Neuropathology Includes Widespread Reactive Astrogliosis and White Matter Specific Astrocyte Degeneration

OBJECTIVE

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a late-onset progressive genetic neurodegenerative disorder that occurs in FMR1 premutation carriers. The temporal, spatial, and cell-type specific patterns of neurodegeneration in the FXTAS brain remain incompletely characterized. Intranuclear inclusion bodies are the neuropathological hallmark of FXTAS, which are largest and occur most frequently in astrocytes, glial cells that maintain brain homeostasis. Here, we characterized neuropathological alterations in astrocytes in multiple regions of the FXTAS brain.

METHODS

Striatal and cerebellar sections from FXTAS cases (n = 12) and controls (n = 12) were stained for the astrocyte markers glial fibrillary acidic protein (GFAP) and aldehyde dehydrogenase 1L1 (ALDH1L1) using immunohistochemistry. Reactive astrogliosis severity, the prevalence of GFAP+ fragments, and astrocyte density were scored. Double label immunofluorescence was utilized to detect co-localization of GFAP and cleaved caspase-3.

RESULTS

FXTAS cases showed widespread reactive gliosis in both grey and white matter. GFAP staining also revealed remarkably severe astrocyte pathology in FXTAS white matter - characterized by a significant and visible reduction in astrocyte density (-38.7% in striatum and - 32.2% in cerebellum) and the widespread presence of GFAP+ fragments reminiscent of apoptotic bodies. White matter specific reductions in astrocyte density were confirmed with ALDH1L1 staining. GFAP+ astrocytes and fragments in white matter were positive for cleaved caspase-3, suggesting that apoptosis-mediated degeneration is responsible for reduced astrocyte counts.

INTERPRETATION

We have established that FXTAS neuropathology includes robust degeneration of astrocytes, which is specific to white matter. Because astrocytes are essential for maintaining homeostasis within the central nervous system, a loss of astrocytes likely further exacerbates neuropathological progression of other cell types in the FXTAS brain. ANN NEUROL 2024;95:558-575.

CGG repeat expansion in LOC642361/NUTM2B-AS1 typically presents as oculopharyngodistal myopathy

CGG repeat expansions in LOC642361/NUTM2B-AS1 have recently been identified as a cause of oculopharyngeal myopathy with leukoencephalopathy. However, since only three patients from a single family were reported, it remains unknown whether their clinicopathological features are typical for CGG repeat expansions in LOC642361/NUTM2B-AS1. Here, using repeat-primed-polymerase chain reaction and long-read sequencing, we identify 12 individuals from 3 unrelated families with CGG repeat expansions in LOC642361/NUTM2B-AS1, typically presenting with oculopharyngodistal myopathy. The CGG repeat expansions range from 161 to 669 repeat units. Most of the patients present with ptosis, restricted eye movements, dysphagia, dysarthria, and diffuse limb muscle weakness. Only one patient shows T2-weighted hyperintensity in the cerebellar white matter surrounding the deep cerebellar nuclei on brain magnetic resonance imaging. Muscle biopsies from three patients show a myopathic pattern and rimmed vacuoles. Analyses of muscle biopsies suggest that CGG repeat expansions in LOC642361/NUTM2B-AS1 may deleteriously affect aggrephagic capacity, suggesting that RNA toxicity and mitochondrial dysfunction may contribute to pathogenesis. Our study thus expands the phenotypic spectrum for the CGG repeat expansion of LOC642361/NUTM2B-AS1 and indicates that this genetic variant typically manifests as oculopharyngodistal myopathy with chronic myopathic changes with rimmed vacuoles and filamentous intranuclear inclusions in muscle fibers.

Case Series: Vestibular Migraines in Fragile X Premutation Carriers

BACKGROUND

Vestibular migraine (VM) is one of the most common causes of recurrent vertigo and presents with a history of spontaneous or positional vertigo with a history of migraine headaches. While research has identified a high prevalence of migraine headaches and vestibular deficits among fragile X premutation carriers, there has been no discussion about VM within this population.

OBJECTIVE

This case series and review seeks to describe the clinical characteristics and pathophysiology of VM among individuals with the fragile X premutation. We also seek to discuss treatment and future steps in addressing VM in this population.

METHODS

A review of the literature regarding vestibular migraine and presentation of migraine headaches and vestibular deficits among premutation carriers was performed. A detailed clinical history of migraine headaches and vertigo was obtained from three patients with the fragile X premutation seen by the senior author (RJH).

RESULTS

All three cases first developed symptoms of migraine headaches earlier in life, with the development of VM near menopause. Two of the three cases developed progressive balance issues following the development of VM. All three cases found that their VM episodes were improved or resolved with pharmacological and/or lifestyle interventions.

CONCLUSIONS

It is important to recognize VM among premutation carriers because beneficial treatments are available. Future studies are needed regarding the prevalence of VM and the relationship to subsequent FXTAS. The pathophysiology of VM remains uncertain but possibilities include mitochondrial abnormalities, cranial nerve VIII toxicity secondary to neurotoxic protein accumulation, and calcitonin gene-related peptide (CGRP) signaling dysfunction due to altered levels of fragile X messenger ribonucleoprotein (FMRP).

Open-Label Sulforaphane Trial in FMR1 Premutation Carriers with Fragile-X-Associated Tremor and Ataxia Syndrome (FXTAS)

Fragile X (FMR1) premutation is a common mutation that affects about 1 in 200 females and 1 in 450 males and can lead to the development of fragile-X-associated tremor/ataxia syndrome (FXTAS). Although there is no targeted, proven treatment for FXTAS, research suggests that sulforaphane, an antioxidant present in cruciferous vegetables, can enhance mitochondrial function and maintain redox balance in the dermal fibroblasts of individuals with FXTAS, potentially leading to improved cognitive function. In a 24-week open-label trial involving 15 adults aged 60-88 with FXTAS, 11 participants successfully completed the study, demonstrating the safety and tolerability of sulforaphane. Clinical outcomes and biomarkers were measured to elucidate the effects of sulforaphane. While there were nominal improvements in multiple clinical measures, they were not significantly different after correction for multiple comparisons. PBMC energetic measures showed that the level of citrate synthase was higher after sulforaphane treatment, resulting in lower ATP production. The ratio of complex I to complex II showed positive correlations with the MoCA and BDS scores. Several mitochondrial biomarkers showed increased activity and quantity and were correlated with clinical improvements.

Exploration of SUMO2/3 Expression Levels and Autophagy Process in Fragile X-Associated Tremor/Ataxia Syndrome: Addressing Study Limitations and Insights for Future Research

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a late-onset neurodegenerative disorder that appears in adult FMR1 premutation carriers. The neuropathological hallmark of FXTAS is an intranuclear inclusion in neurons and astrocytes. Nearly 200 different proteins have been identified in FXTAS inclusions, being the small ubiquitin-related modifier 2 (SUMO2), ubiquitin and p62 the most highly abundant. These proteins are components of the protein degradation machinery. This study aimed to characterize SUMO2/3 expression levels and autophagy process in human postmortem brain samples and skin fibroblast cultures from FXTAS patients. Results revealed that FXTAS postmortem brain samples are positive for SUMO2/3 conjugates and supported the idea that SUMO2/3 accumulation is involved in inclusion formation. Insights from RNA-sequencing data indicated that SUMOylation processes are significantly upregulated in FXTAS samples. In addition, the analysis of the autophagy flux showed the accumulation of p62 protein levels and autophagosomes in skin fibroblasts from FXTAS patients. Similarly, gene set analysis evidenced a significant downregulation in gene ontology terms related to autophagy in FXTAS samples. Overall, this study provides new evidence supporting the role of SUMOylation and autophagic processes in the pathogenic mechanisms underlying FXTAS.

Insight and Recommendations for Fragile X-Premutation-Associated Conditions from the Fifth International Conference on FMR1 Premutation

The premutation of the fragile X messenger ribonucleoprotein 1 (FMR1) gene is characterized by an expansion of the CGG trinucleotide repeats (55 to 200 CGGs) in the 5' untranslated region and increased levels of FMR1 mRNA. Molecular mechanisms leading to fragile X-premutation-associated conditions (FXPAC) include cotranscriptional R-loop formations, FMR1 mRNA toxicity through both RNA gelation into nuclear foci and sequestration of various CGG-repeat-binding proteins, and the repeat-associated non-AUG (RAN)-initiated translation of potentially toxic proteins. Such molecular mechanisms contribute to subsequent consequences, including mitochondrial dysfunction and neuronal death. Clinically, premutation carriers may exhibit a wide range of symptoms and phenotypes. Any of the problems associated with the premutation can appropriately be called FXPAC. Fragile X-associated tremor/ataxia syndrome (FXTAS), fragile X-associated primary ovarian insufficiency (FXPOI), and fragile X-associated neuropsychiatric disorders (FXAND) can fall under FXPAC. Understanding the molecular and clinical aspects of the premutation of the FMR1 gene is crucial for the accurate diagnosis, genetic counseling, and appropriate management of affected individuals and families. This paper summarizes all the known problems associated with the premutation and documents the presentations and discussions that occurred at the International Premutation Conference, which took place in New Zealand in 2023.

A Postmortem MRI Study of Cerebrovascular Disease and Iron Content at End-Stage of Fragile X-Associated Tremor/Ataxia Syndrome

Brain changes at the end-stage of fragile X-associated tremor/ataxia syndrome (FXTAS) are largely unknown due to mobility impairment. We conducted a postmortem MRI study of FXTAS to quantify cerebrovascular disease, brain atrophy and iron content, and examined their relationships using principal component analysis (PCA). Intracranial hemorrhage (ICH) was observed in 4/17 FXTAS cases, among which one was confirmed by histologic staining. Compared with seven control brains, FXTAS cases showed higher ratings of T2-hyperintensities (indicating cerebral small vessel disease) in the cerebellum, globus pallidus and frontoparietal white matter, and significant atrophy in the cerebellar white matter, red nucleus and dentate nucleus. PCA of FXTAS cases revealed negative associations of T2-hyperintensity ratings with anatomic volumes and iron content in the white matter, hippocampus and amygdala, that were independent from a highly correlated number of regions with ICH and iron content in subcortical nuclei. Post-hoc analysis confirmed PCA findings and further revealed increased iron content in the white matter, hippocampus and amygdala in FXTAS cases compared to controls, after adjusting for T2-hyperintensity ratings. These findings indicate that both ischemic and hemorrhagic brain damage may occur in FXTAS, with the former being marked by demyelination/iron depletion and atrophy, and the latter by ICH and iron accumulation in basal ganglia.

A postmortem MRI study of cerebrovascular disease and iron content at end-stage of fragile X-associated tremor/ataxia syndrome

Brain changes at end-stage of fragile X-associated tremor/ataxia syndrome (FXTAS) are largely unknown due to mobility impairment. We conducted a postmortem MRI study of FXTAS to quantify cerebrovascular disease, brain atrophy, and iron content and examined their relationships using principal component analysis (PCA). Intracranial hemorrhage (ICH) was observed in 4/17 FXTAS cases among which one was confirmed by histologic staining. Compared with seven control brains, FXTAS cases showed higher ratings of T2-hyperintensities (indicating cerebral small vessel disease) in the cerebellum, globus pallidus, and frontoparietal white matter and significant atrophy in cerebellar white matter, red nucleus, and dentate nucleus. PCA of FXTAS cases revealed negative associations of T2-hyperintensity ratings with anatomic volumes and iron content in the white matter, hippocampus, and amygdala, that were independent from highly correlated number of regions with ICH and iron content in subcortical nuclei. Post hoc analysis confirmed PCA findings and further revealed increased iron content in the white matter, hippocampus, and amygdala in FXTAS cases than controls after adjusting for T2-hyperintensity ratings. These findings indicate that both ischemic and hemorrhagic brain damage may occur in FXTAS, with the former marked by demyelination/iron depletion and atrophy and the latter, ICH and iron accumulation in basal ganglia.

Artificial neural network applied to fragile X-associated tremor/ataxia syndrome stage diagnosis based on peripheral mitochondrial bioenergetics and brain imaging outcomes

No proven prognosis is available for the neurodegenerative disorder fragile X-associated tremor/ataxia syndrome (FXTAS). Artificial neural network analyses (ANN) were used to predict FXTAS progression using data from 127 adults (noncarriers and FMR1 premutation carriers with and without FXTAS) with five outcomes from brain MRI imaging and 22 peripheral bioenergetic outcomes from two cell types. Diagnosis accuracy by ANN predictions ranged from 41.7 to 86.3% (depending on the algorithm used), and those misclassified usually presented a higher FXTAS stage. ANN prediction of FXTAS stages was based on a combination of two imaging findings (white matter hyperintensity and whole-brain volumes adjusted for intracranial volume) and four bioenergetic outcomes. Those at Stage 3 vs. 0-2 showed lower mitochondrial mass, higher oxidative stress, and an altered electron transfer consistent with mitochondrial unfolded protein response activation. Those at Stages 4-5 vs. 3 had higher oxidative stress and glycerol-3-phosphate-linked ATP production, suggesting that targeting mGPDH activity may prevent a worse prognosis. This was confirmed by the bioenergetic improvement of inhibiting mGPDH with metformin in affected fibroblasts. ANN supports the prospect of an unbiased molecular definition in diagnosing FXTAS stages while identifying potential targets for personalized medicine.

The International Fragile X Premutation Registry: building a resource for research and clinical trial readiness

FMR1 premutation cytosine-guanine-guanine repeat expansion alleles are relatively common mutations in the general population that are associated with a neurodegenerative disease (fragile X-associated tremor/ataxia syndrome), reproductive health problems and potentially a wide range of additional mental and general health conditions that are not yet well-characterised. The International Fragile X Premutation Registry (IFXPR) was developed to facilitate and encourage research to better understand the FMR1 premutation and its impact on human health, to facilitate clinical trial readiness by identifying and characterising diverse cohorts of individuals interested in study participation, and to build community and collaboration among carriers, family members, researchers and clinicians around the world. Here, we describe the development and content of the IFXPR, characterise its first 747 registrants from 32 countries and invite investigators to apply for recruitment support for their project(s). With larger numbers, increased diversity and potentially the future clinical characterisation of registrants, the IFXPR will contribute to a more comprehensive and accurate understanding of the fragile X premutation in human health and support treatment studies.

Tophaceous gout of the nose in a male FMR1 premutation carrier

Premutation alleles with 55-200 CGG repeats in FMR1 can lead to fragile X-associated tremor/ataxia syndrome (FXTAS). In this case study, we report uncontrolled gout in a 68-year-old male with FXTAS with multiple sites of involvement including a rare gouty tophus in the nasal region.

CGG repeat expansion in NOTCH2NLC causes mitochondrial dysfunction and progressive neurodegeneration in Drosophila model

Neuronal intranuclear inclusion disease (NIID) is a neuromuscular/neurodegenerative disease caused by the expansion of CGG repeats in the 5' untranslated region (UTR) of the NOTCH2NLC gene. These repeats can be translated into a polyglycine-containing protein, uN2CpolyG, which forms protein inclusions and is toxic in cell models, albeit through an unknown mechanism. Here, we established a transgenic Drosophila model expressing uN2CpolyG in multiple systems, which resulted in progressive neuronal cell loss, locomotor deficiency, and shortened lifespan. Interestingly, electron microscopy revealed mitochondrial swelling both in transgenic flies and in muscle biopsies of individuals with NIID. Immunofluorescence and immunoelectron microscopy showed colocalization of uN2CpolyG with mitochondria in cell and patient samples, while biochemical analysis revealed that uN2CpolyG interacted with a mitochondrial RNA binding protein, LRPPRC (leucine-rich pentatricopeptide repeat motif-containing protein). Furthermore, RNA sequencing (RNA-seq) analysis and functional assays showed down-regulated mitochondrial oxidative phosphorylation in uN2CpolyG-expressing flies and NIID muscle biopsies. Finally, idebenone treatment restored mitochondrial function and alleviated neurodegenerative phenotypes in transgenic flies. Overall, these results indicate that transgenic flies expressing uN2CpolyG recapitulate key features of NIID and that reversing mitochondrial dysfunction might provide a potential therapeutic approach for this disorder.

Autism spectrum disorder in the fragile X premutation state: possible mechanisms and implications

There is increasing recognition of the heterogeneity of origin of cases of autism spectrum disorder (ASD) with multiple forms of ASD having been identified over the decades. Among these, a genetic etiology can be identified in 20-40% of cases when a full genetic work-up is completed. The Fragile X premutation state (characterized by the presence of 55-200 CGG repeats in the FMR1 gene) is a relatively newly identified disease state that has since been associated with several disorders including fragile X-associated tremor ataxia syndrome (FXTAS), fragile X-associated primary ovarian insufficiency (FXPOI) and most recently, fragile X-associated neurodevelopmental disorders (FXAND) which commonly includes anxiety and depression. In addition to these associated disorders, extant literature and clinical observations have suggested an association between the premutation state and ASD. In this paper, we review the literature pertinent to this and discuss possible molecular mechanisms that may explain this association. This includes lowered levels of the FMR1 Protein (FMRP), GABA deficits, mitochondrial dysfunction and secondary genetic abnormalities that is seen in premutation carriers as well as their increased vulnerability to environmental stressors. Understanding these mechanisms can facilitate development of targeted treatment for specific sub-groups of ASD and premutation disorders in future.

Neurodegenerative diseases associated with non-coding CGG tandem repeat expansions

Non-coding CGG repeat expansions cause multiple neurodegenerative disorders, including fragile X-associated tremor/ataxia syndrome, neuronal intranuclear inclusion disease, oculopharyngeal myopathy with leukodystrophy, and oculopharyngodistal myopathy. The underlying genetic causes of several of these diseases have been identified only in the past 2-3 years. These expansion disorders have substantial overlapping clinical, neuroimaging and histopathological features. The shared features suggest common mechanisms that could have implications for the development of therapies for this group of diseases - similar therapeutic strategies or drugs may be effective for various neurodegenerative disorders induced by non-coding CGG expansions. In this Review, we provide an overview of clinical and pathological features of these CGG repeat expansion diseases and consider the likely pathological mechanisms, including RNA toxicity, CGG repeat-associated non-AUG-initiated translation, protein aggregation and mitochondrial impairment. We then discuss future research needed to improve the identification and diagnosis of CGG repeat expansion diseases, to improve modelling of these diseases and to understand their pathogenesis. We also consider possible therapeutic strategies. Finally, we propose that CGG repeat expansion diseases may represent manifestations of a single underlying neuromyodegenerative syndrome in which different organs are affected to different extents depending on the gene location of the repeat expansion.

Fragile X Premutation: Medications, Therapy and Lifestyle Advice

The fragile X premutation is characterized by 55–200 CGG repeats in the 5ʹ untranslated region of FMR1, whereas full fragile X mutation has greater than 200 repeats and full methylation, which manifests as fragile X syndrome (FXS). The premutation spectrum of clinical involvement includes fragile X-associated tremor/ataxia syndrome (FXTAS), fragile X-associated primary ovarian insufficiency (FXPOI), and fragile X-associated neuropsychiatric disorders (FXAND). In addition, premutation carriers also suffer from various other health problems such as endocrine abnormalities and autoimmune problems. In this paper, we have discussed different health issues faced by the carriers and interventions including medications, therapy and lifestyle changes that could improve their health.

Association of Oxidative Stress with Neurological Disorders

BACKGORUND

Oxidative stress is one of the main contributing factors involved in cerebral biochemical impairment. The higher susceptibility of the central nervous system to reactive oxygen species mediated damage could be attributed to several factors. For example, neurons use a greater quantity of oxygen, many parts of the brain have higher concentraton of iron, and neuronal mitochondria produce huge content of hydrogen peroxide. In addition, neuronal membranes have polyunsaturated fatty acids, which are predominantly vulnerable to oxidative stress (OS). OS is the imbalance between reactive oxygen species generation and cellular antioxidant potential. This may lead to various pathological conditions and diseases, especially neurodegenerative diseases such as, Parkinson's, Alzheimer's, and Huntington's diseases.

OBJECTIVES

In this study, we explored the involvement of OS in neurodegenerative diseases.

METHODS

We used different search terms like "oxidative stress and neurological disorders" "free radicals and neurodegenerative disorders" "oxidative stress, free radicals, and neurological disorders" and "association of oxidative stress with the name of disorders taken from the list of neurological disorders. We tried to summarize the source, biological effects, and physiologic functions of ROS.

RESULTS

Finally, it was noted that more than 190 neurological disorders are associated with oxidative stress.

CONCLUSION

More elaborated studies in the future will certainly help in understanding the exact mechanism involved in neurological diseases and provide insight into revelation of therapeutic targets.

Relationships between Mitochondrial Function, AMPK, and TORC1 Signaling in Lymphoblasts with Premutation Alleles of the FMR1 Gene

The X-linked FMR1 gene contains a non-coding trinucleotide repeat in its 5' region that, in normal, healthy individuals contains 20-44 copies. Large expansions of this region (>200 copies) cause fragile X syndrome (FXS), but expansions of 55-199 copies (referred to as premutation alleles) predispose carriers to a neurodegenerative disease called fragile X-associated tremor/ataxia syndrome (FXTAS). The cytopathological mechanisms underlying FXTAS are poorly understood, but abnormalities in mitochondrial function are believed to play a role. We previously reported that lymphoblastoid cell lines (LCLs, or lymphoblasts) of premutation carriers have elevated mitochondrial respiratory activities. In the carriers, especially those not clinically affected with FXTAS, AMP-activated protein kinase (AMPK) activity was shown to be elevated. In the FXTAS patients, however, it was negatively correlated with brain white matter lesions, suggesting a protective role in the molecular mechanisms. Here, we report an enlarged and extended study of mitochondrial function and associated cellular stress-signaling pathways in lymphoblasts isolated from male and female premutation carriers, regardless of their clinical status, and healthy controls. The results confirmed the elevation of AMPK and mitochondrial respiratory activities and reduction in reactive O2 species (ROS) levels in premutation cells and revealed for the first time that target of rapamycin complex I (TORC1) activities are reduced. Extensive correlation, multiple regression, and principal components analysis revealed the best fitting statistical explanations of these changes in terms of the other variables measured. These suggested which variables might be the most "proximal" regulators of the others in the extensive network of known causal interactions amongst the measured parameters of mitochondrial function and cellular stress signaling. In the resulting model, the premutation alleles activate AMPK and inhibit both TORC1 and ROS production, the reduced TORC1 activity contributes to activation of AMPK and of nonmitochondrial metabolism, and the higher AMPK activity results in elevated catabolic metabolism, mitochondrial respiration, and ATP steady state levels. In addition, the results suggest a separate CGG repeat number-dependent elevation of TORC1 activity that is insufficient to overcome the inhibition of TORC1 in premutation cells but may presage the previously reported activation of TORC1 in FXS cells.

Sulforaphane improves mitochondrial metabolism in fibroblasts from patients with fragile X-associated tremor and ataxia syndrome

CGG expansions between 55 and 200 in the 5'-untranslated region of the fragile-X mental retardation gene (FMR1) increase the risk of developing the late-onset debilitating neuromuscular disease Fragile X-Associated Tremor/Ataxia Syndrome (FXTAS). While the science behind this mutation, as a paradigm for RNA-mediated nucleotide triplet repeat expansion diseases, has progressed rapidly, no treatment has proven effective at delaying the onset or decreasing morbidity, especially at later stages of the disease. Here, we demonstrated the beneficial effect of the phytochemical sulforaphane (SFN), exerted through NRF2-dependent and independent manner, on pathways relevant to brain function, bioenergetics, unfolded protein response, proteosome, antioxidant defenses, and iron metabolism in fibroblasts from FXTAS-affected subjects at all disease stages. This study paves the way for future clinical studies with SFN in the treatment of FXTAS, substantiated by the established use of this agent in clinical trials of diseases with NRF2 dysregulation and in which age is the leading risk factor.

Brain Atrophy and White Matter Damage Linked to Peripheral Bioenergetic Deficits in the Neurodegenerative Disease FXTAS

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a neurodegenerative disorder affecting subjects (premutation carriers) with a 55-200 CGG-trinucleotide expansion in the 5'UTR of the fragile X mental retardation 1 gene (FMR1) typically after age 50. As both the presence of white matter hyperintensities (WMHs) and atrophied gray matter on magnetic resonance imaging (MRI) are linked to age-dependent decline in cognition, here we tested whether MRI outcomes (WMH volume (WMHV) and brain volume) were correlated with mitochondrial bioenergetics from peripheral blood monocytic cells in 87 carriers with and without FXTAS. As a parameter assessing cumulative damage, WMHV was correlated to both FXTAS stages and age, and brain volume discriminated between carriers and non-carriers. Similarly, mitochondrial mass and ATP production showed an age-dependent decline across all participants, but in contrast to WMHV, only FADH2-linked ATP production was significantly reduced in carriers vs. non-carriers. In carriers, WMHV negatively correlated with ATP production sustained by glucose-glutamine and FADH2-linked substrates, whereas brain volume was positively associated with the latter and mitochondrial mass. The observed correlations between peripheral mitochondrial bioenergetics and MRI findings-and the lack of correlations with FXTAS diagnosis/stages-may stem from early brain bioenergetic deficits even before overt FXTAS symptoms and/or imaging findings.

Molecular Pathogenesis and Peripheral Monitoring of Adult Fragile X-Associated Syndromes

Abnormal trinucleotide expansions cause rare disorders that compromise quality of life and, in some cases, lifespan. In particular, the expansions of the CGG-repeats stretch at the 5’-UTR of the Fragile X Mental Retardation 1 (FMR1) gene have pleiotropic effects that lead to a variety of Fragile X-associated syndromes: the neurodevelopmental Fragile X syndrome (FXS) in children, the late-onset neurodegenerative disorder Fragile X-associated tremor-ataxia syndrome (FXTAS) that mainly affects adult men, the Fragile X-associated primary ovarian insufficiency (FXPOI) in adult women, and a variety of psychiatric and affective disorders that are under the term of Fragile X-associated neuropsychiatric disorders (FXAND). In this review, we will describe the pathological mechanisms of the adult “gain-of-function” syndromes that are mainly caused by the toxic actions of CGG RNA and FMRpolyG peptide. There have been intensive attempts to identify reliable peripheral biomarkers to assess disease progression and onset of specific pathological traits. Mitochondrial dysfunction, altered miRNA expression, endocrine system failure, and impairment of the GABAergic transmission are some of the affectations that are susceptible to be tracked using peripheral blood for monitoring of the motor, cognitive, psychiatric and reproductive impairment of the CGG-expansion carriers. We provided some illustrative examples from our own cohort. Understanding the association between molecular pathogenesis and biomarkers dynamics will improve effective prognosis and clinical management of CGG-expansion carriers.

Cerebral Microbleeds in Fragile X-Associated Tremor/Ataxia Syndrome

BACKGROUND

Fragile X-associated tremor/ataxia syndrome is a neurodegenerative disease of late onset developed by carriers of the premutation in the fragile x mental retardation 1 (FMR1) gene. Pathological features of neurodegeneration in fragile X-associated tremor/ataxia syndrome include toxic levels of FMR1 mRNA, ubiquitin-positive intranuclear inclusions, white matter disease, iron accumulation, and a proinflammatory state.

OBJECTIVE

The objective of this study was to analyze the presence of cerebral microbleeds in the brains of patients with fragile X-associated tremor/ataxia syndrome and investigate plausible causes for cerebral microbleeds in fragile X-associated tremor/ataxia syndrome.

METHODS

We collected cerebral and cerebellar tissue from 15 fragile X-associated tremor/ataxia syndrome cases and 15 control cases carrying FMR1 normal alleles. We performed hematoxylin and eosin, Perls and Congo red stains, ubiquitin, and amyloid β protein immunostaining. We quantified the number of cerebral microbleeds, amount of iron, presence of amyloid β within the capillaries, and number of endothelial cells containing intranuclear inclusions. We evaluated the relationships between pathological findings using correlation analysis.

RESULTS

We found intranuclear inclusions in the endothelial cells of capillaries and an increased number of cerebral microbleeds in the brains of those with fragile X-associated tremor/ataxia syndrome, both of which are indicators of cerebrovascular dysfunction. We also found a suggestive association between the amount of capillaries that contain amyloid β in the cerebral cortex and the rate of disease progression.

CONCLUSION

We propose microangiopathy as a pathologic feature of fragile X-associated tremor/ataxia syndrome. © 2021 International Parkinson and Movement Disorder Society.

X-Linked Parkinsonism: Phenotypic and Genetic Heterogeneity

X-linked parkinsonism encompasses rare heterogeneous disorders mainly inherited as a recessive trait, therefore being more prevalent in males. Recent developments have revealed a complex underlying panorama, including a spectrum of disorders in which parkinsonism is variably associated with additional neurological and non-neurological signs. In particular, a childhood-onset encephalopathy with epilepsy and/or cognitive disability is the most common feature. Their genetic basis is also heterogeneous, with many causative genes and different mutation types ranging from "classical" coding variants to intronic repeat expansions. In this review, we provide an updated overview of the phenotypic and genetic spectrum of the most relevant X-linked parkinsonian syndromes, namely X-linked dystonia-parkinsonism (XDP, Lubag disease), fragile X-associated tremor/ataxia syndrome (FXTAS), beta-propeller protein-associated neurodegeneration (BPAN, NBIA/PARK-WDR45), Fabry disease, Waisman syndrome, methyl CpG-binding protein 2 (MeCP2) spectrum disorder, phosphoglycerate kinase-1 deficiency syndrome (PGK1) and X-linked parkinsonism and spasticity (XPDS). All clinical and radiological features reported in the literature have been reviewed. Epilepsy occasionally represents the symptom of onset, predating parkinsonism even by a few years; action tremor is another common feature along with akinetic-rigid parkinsonism. A focus on the genetic background and its pathophysiological implications is provided. The pathogenesis of these disorders ranges from well-defined metabolic alterations (PGK1) to non-specific lysosomal dysfunctions (XPDS) and vesicular trafficking alterations (Waisman syndrome). However, in other cases it still remains poorly defined. Recognition of the phenotypic and genetic heterogeneity of X-linked parkinsonism has important implications for diagnosis, management, and genetic counseling. © 2021 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Deficits in Prenatal Serine Biosynthesis Underlie the Mitochondrial Dysfunction Associated with the Autism-Linked FMR1 Gene

Fifty-five to two hundred CGG repeats (called a premutation, or PM) in the 5′-UTR of the FMR1 gene are generally unstable, often expanding to a full mutation (>200) in one generation through maternal inheritance, leading to fragile X syndrome, a condition associated with autism and other intellectual disabilities. To uncover the early mechanisms of pathogenesis, we performed metabolomics and proteomics on amniotic fluids from PM carriers, pregnant with male fetuses, who had undergone amniocentesis for fragile X prenatal diagnosis. The prenatal metabolic footprint identified mitochondrial deficits, which were further validated by using internal and external cohorts. Deficits in the anaplerosis of the Krebs cycle were noted at the level of serine biosynthesis, which was confirmed by rescuing the mitochondrial dysfunction in the carriers’ umbilical cord fibroblasts using alpha-ketoglutarate precursors. Maternal administration of serine and its precursors has the potential to decrease the risk of developing energy shortages associated with mitochondrial dysfunction and linked comorbidities.

FMRP attenuates activity dependent modifications in the mitochondrial proteome

Homeostatic plasticity is necessary for the construction and maintenance of functional neuronal networks, but principal molecular mechanisms required for or modified by homeostatic plasticity are not well understood. We recently reported that homeostatic plasticity induced by activity deprivation is dysregulated in cortical neurons from Fragile X Mental Retardation protein (FMRP) knockout mice (Bulow et al. in Cell Rep 26: 1378-1388 e1373, 2019). These findings led us to hypothesize that identifying proteins sensitive to activity deprivation and/or FMRP expression could reveal pathways required for or modified by homeostatic plasticity. Here, we report an unbiased quantitative mass spectrometry used to quantify steady-state proteome changes following chronic activity deprivation in wild type and Fmr1-/y cortical neurons. Proteome hits responsive to both activity deprivation and the Fmr1-/y genotype were significantly annotated to mitochondria. We found an increased number of mitochondria annotated proteins whose expression was sensitive to activity deprivation in Fmr1-/y cortical neurons as compared to wild type neurons. These findings support a novel role of FMRP in attenuating mitochondrial proteome modifications induced by activity deprivation.

Fragile X Premutation rCGG Repeats Impair Synaptic Growth and Synaptic Transmission at Drosophila larval Neuromuscular Junction

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a late-onset neurodegenerative disease that develops in some premutation (PM) carriers of the FMR1 gene with alleles bearing 55-200 CGG repeats. The discovery of a broad spectrum of clinical and cell developmental abnormalities among PM carriers with or without FXTAS and in model systems suggests that neurodegeneration seen in FXTAS could be the inevitable end-result of pathophysiological processes set during early development. Hence, it is imperative to trace early PM-induced pathological abnormalities. Previous studies have shown that transgenic Drosophila carrying PM-length CGG repeats are sufficient to cause neurodegeneration. Here, we used the same transgenic model to understand the effect of CGG repeats on the structure and function of the developing nervous system. We show that presynaptic expression of CGG repeats restricts synaptic growth, reduces the number of synaptic boutons, leads to aberrant presynaptic varicosities, and impairs synaptic transmission at the larval neuromuscular junctions. The postsynaptic analysis shows that both glutamate receptors and subsynaptic reticulum proteins were normal. However, a high percentage of boutons show a reduced density of Bruchpilot protein, a key component of presynaptic active zones required for vesicle release. The electrophysiological analysis shows a significant reduction in quantal content, a measure of total synaptic vesicles released per excitation potential. Together, these findings suggest that synapse perturbation caused by rCGG repeats mediates presynaptically during larval NMJ development. We also suggest that the stress-activated c-Jun N-terminal kinase protein Basket and CIDE-N protein Drep-2 positively mediate Bruchpilot active zone defects caused by rCGG repeats.

Expression of expanded FMR1-CGG repeats alters mitochondrial miRNAs and modulates mitochondrial functions and cell death in cellular model of FXTAS

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a progressive neurodegenerative disorder caused by an expansion of 55 to 200 CGG repeats located within 5'UTR of FMR1.These CGG repeats are transcribed into RNAs, which sequester several RNA binding proteins and alter the processing of miRNAs. CGG repeats are also translated into a toxic polyglycine-containing protein, FMRpolyG, that affects mitochondrial and nuclear functions reported in cell and animal models and patient studies. Nuclear-encoded small non-coding RNAs, including miRNAs, are transported to mitochondria; however, the role of mitochondrial miRNAs in FXTAS pathogenesis is not understood. Here, we analyzed mitochondrial miRNAs from HEK293 cells expressing expanded CGG repeats and their implication in the regulation of mitochondrial functions. The analysis of next generation sequencing (NGS) data of small RNAs from HEK293 cells expressing CGG premutation showed decreased level of cellular miRNAs and an altered pattern of association of miRNAs with mitochondria (mito-miRs). Among such mito-miRs, miR-320a was highly enriched in mitoplast and RNA immunoprecipitation of Ago2 (Argonaute-2) followed by Droplet digital PCR (ddPCR)suggested that miR-320a may form a complex with Ago2 and mitotranscripts. Finally, transfection of miR-320a mimic in cells expressing CGG permutation recovers mitochondrial functions and rescues cell death. Overall, this work reveals an altered translocation of miRNAs to mitochondria and the role of miR-320a in FXTAS pathology.

Human Cerebral Cortex Proteome of Fragile X-Associated Tremor/Ataxia Syndrome

Background: Fragile X-associated tremor/ataxia syndrome (FXTAS) is an adult-onset neurodegenerative disorder associated with premutation CGG-repeat expansions (55–200 repeats) in the 5′ non-coding portion of the fragile X mental retardation 1 (FMR1) gene. Core features of FXTAS include progressive tremor/ataxia, cognitive decline, variable brain volume loss, and white matter disease. The principal histopathological feature of FXTAS is the presence of central nervous system (CNS) and non-CNS intranuclear inclusions.

Objective: To further elucidate the molecular underpinnings of FXTAS through the proteomic characterization of human FXTAS cortexes.

Results: Proteomic analysis of FXTAS brain cortical tissue (n = 8) identified minor differences in protein abundance compared to control brains (n = 6). Significant differences in FXTAS relative to control brain predominantly involved decreased abundance of proteins, with the greatest decreases observed for tenascin-C (TNC), cluster of differentiation 38 (CD38), and phosphoserine aminotransferase 1 (PSAT1); proteins typically increased in other neurodegenerative diseases. Proteins with the greatest increased abundance include potentially novel neurodegeneration-related proteins and small ubiquitin-like modifier 1/2 (SUMO1/2). The FMRpolyG peptide, proposed in models of FXTAS pathogenesis but only identified in trace amounts in the earlier study of FXTAS inclusions, was not identified in any of the FXTAS or control brains in the current study.

Discussion: The observed proteomic shifts, while generally relatively modest, do show a bias toward decreased protein abundance with FXTAS. Such shifts in protein abundance also suggest altered RNA binding as well as loss of cell–cell adhesion/structural integrity. Unlike other neurodegenerative diseases, the proteome of end-stage FXTAS does not suggest a strong inflammation-mediated degenerative response.

Fragile X premutation and associated health conditions: A review

Fragile X syndrome (FXS) is the most common single gene disorder, which causes autism and intellectual disability. The fragile X mental retardation 1 (FMR1) gene is silenced when cytosine-guanine-guanine (CGG) triplet repeats exceed 200, which is the full mutation that causes FXS. Carriers of FXS have a CGG repeat between 55 and 200, which is defined as a premutation and transcription of the gene is overactive with high levels of the FMR1 mRNA. Most carriers of the premutation have normal levels of fragile X mental retardation protein (FMRP) and a normal intelligence, but in the upper range of the premutation (120-200) the FMRP level may be lower than normal. The clinical problems associated with the premutation are caused by the RNA toxicity associated with increased FMR1 mRNA levels, although for some mildly lowered FMRP can cause problems associated with FXS. The RNA toxicity causes various health problems in the carriers including but not limited to fragile X-associated tremor/ataxia syndrome, fragile X-associated primary ovarian insufficiency, and fragile X-associated neuropsychiatric disorders. Since some individuals with neuropsychiatric problems do not meet the severity for a diagnosis of a "disorder" then the condition can be labeled as fragile X premutation associated condition (FXPAC). Physicians must be able to recognize these health problems in the carriers and provide appropriate management.

Cellular Bioenergetics and AMPK and TORC1 Signalling in Blood Lymphoblasts Are Biomarkers of Clinical Status in FMR1 Premutation Carriers

Fragile X Associated Tremor/Ataxia Syndrome (FXTAS) is a neurodegenerative disorder affecting carriers of premutation alleles (PM) of the X-linked FMR1 gene, which contain CGG repeat expansions of 55-200 range in a non-coding region. This late-onset disorder is characterised by the presence of tremor/ataxia and cognitive decline, associated with the white matter lesions throughout the brain, especially involving the middle cerebellar peduncles. Nearly half of older male and ~ 20% of female PM carriers develop FXTAS. While there is evidence for mitochondrial dysfunction in neural and some peripheral tissues from FXTAS patients (though less obvious in the non-FXTAS PM carriers), the results from peripheral blood mononuclear cells (PBMC) are still controversial. Motor, cognitive, and neuropsychiatric impairments were correlated with measures of mitochondrial and non-mitochondrial respiratory activity, AMPK, and TORC1 cellular stress-sensing protein kinases, and CGG repeat size, in a sample of adult FXTAS male and female carriers. Moreover, the levels of these cellular measures, all derived from Epstein- Barr virus (EBV)- transformed and easily accessible blood lymphoblasts, were compared between the FXTAS (N = 23) and non-FXTAS (n = 30) subgroups, and with baseline data from 33 healthy non-carriers. A significant hyperactivity of cellular bioenergetics components as compared with the baseline data, more marked in the non-FXTAS PMs, was negatively correlated with repeat numbers at the lower end of the CGG-PM distribution. Significant associations of these components with motor impairment measures, including tremor-ataxia and parkinsonism, and neuropsychiatric changes, were prevalent in the FXTAS subgroup. Moreover, a striking elevation of AMPK activity, and a decrease in TORC1 levels, especially in the non-FXTAS carriers, were related to the size of CGG expansion. The bioenergetics changes in blood lymphoblasts are biomarkers of the clinical status of FMR1 carriers. The relationship between these changes and neurological involvement in the affected carriers suggests that brain bioenergetic alterations are reflected in this peripheral tissue. A possible neuroprotective role of stress sensing kinase, AMPK, in PM carriers, should be addressed in future longitudinal studies. A decreased level of TORC1-the mechanistic target of the rapamycin complex, suggests a possible future approach to therapy in FXTAS.

Mitochondria in Ovarian Aging and Reproductive Longevity

Mitochondrial dysfunction is one of the hallmarks of aging. Consistently mitochondrial DNA (mtDNA) copy number and function decline with age in various tissues. There is increasing evidence to support that mitochondrial dysfunction drives ovarian aging. A decreased mtDNA copy number is also reported during ovarian aging. However, the mitochondrial mechanisms contributing to ovarian aging and infertility are not fully understood. Additionally, investigations into mitochondrial therapies to rejuvenate oocyte quality, select viable embryos and improve mitochondrial function may help enhance fertility or extend reproductive longevity in the future. These therapies include the use of mitochondrial replacement techniques, quantification of mtDNA copy number, and various pharmacologic and lifestyle measures. This review aims to describe the key evidence and current knowledge of the role of mitochondria in ovarian aging and identify the emerging potential options for therapy to extend reproductive longevity and improve fertility.

Cortical gyrification and its relationships with molecular measures and cognition in children with the FMR1 premutation

Neurobiological basis for cognitive development and psychiatric conditions remains unexplored in children with the FMR1 premutation (PM). Knock-in mouse models of PM revealed defects in embryonic cortical development that may affect cortical folding. Cortical-folding complexity quantified using local gyrification index (LGI) was examined in 61 children (age 8–12 years, 19/14 male/female PM carriers, 15/13 male/female controls). Whole-brain vertex-wise analysis of LGI was performed for group comparisons and correlations with IQ. Individuals with aberrant gyrification in 68 cortical areas were identified using Z-scores of LGI (hyper: Z ≥ 2.58, hypo: Z ≤ − 2.58). Significant group-by-sex-by-age interaction in LGI was detected in right inferior temporal and fusiform cortices, which correlated negatively with CGG repeat length in the PM carriers. Sixteen PM boys (hyper/hypo: 7/9) and 10 PM girls (hyper/hypo: 2/5, 3 both) displayed aberrant LGI in 1–17 regions/person while 2 control boys (hyper/hypo: 0/2) and 2 control girls (hyper/hypo: 1/1) met the same criteria in only 1 region/person. LGI in the precuneus and cingulate cortices correlated positively with IQ scores in PM and control boys while negatively in PM girls and no significant correlation in control girls. These findings reveal aberrant gyrification, which may underlie cognitive performance in children with the PM.

The emerging molecular mechanisms for mitochondrial dysfunctions in FXTAS

Fragile X-associated tremor/ataxia syndrome (FXTAS) is an inherited neurodegenerative disorder caused by an expansion of 55-200 CGG repeats at 5UTR of FMR1 gene, known as premutation. The main clinical and neuropathological features of FXTAS include progressive intention tremor, gait ataxia, neuronal cell loss and presence of ubiquitin-positive intranuclear inclusions in neurons and astrocytes. Various mitochondrial dysfunctions are reported in in vitro/vivo models of FXTAS; however, the molecular mechanisms underlying such mitochondrial dysfunctions are unclear. CGG expansions are pathogenic through distinct mechanisms involving RNA gain of function, impaired DNA damage repair and FMRpolyG toxicity. Here, we have systematically reviewed the reports of mitochondrial dysfunctions under premutation condition. We have also focused on potential emerging mechanisms to understand mitochondrial associated pathology in FXTAS. This review highlights the important role of mitochondria in FXTAS and other related disorders; and suggests focus of future studies on mitochondrial dysfunction along with other prevailing mechanisms to alleviate neurodegeneration.

CGG Repeat Expansion, and Elevated Fmr1 Transcription and Mitochondrial Copy Number in a New Fragile X PM Mouse Embryonic Stem Cell Model

The Fragile-X related disorders (FXDs) are Repeat Expansion Diseases (REDs) that result from expansion of a CGG-repeat tract located at the 5′ end of the FMR1 gene. While expansion affects transmission risk and can also affect disease risk and severity, the underlying molecular mechanism responsible is unknown. Despite the fact that expanded alleles can be seen both in humans and mouse models in vivo, existing patient-derived cells do not show significant repeat expansions even after extended periods in culture. In order to develop a good tissue culture model for studying expansions we tested whether mouse embryonic stem cells (mESCs) carrying an expanded CGG repeat tract in the endogenous Fmr1 gene are permissive for expansion. We show here that these mESCs have a very high frequency of expansion that allows changes in the repeat number to be seen within a matter of days. CRISPR-Cas9 gene editing of these cells suggests that this may be due in part to the fact that non-homologous end-joining (NHEJ), which is able to protect against expansions in some cell types, is not effective in mESCs. CRISPR-Cas9 gene editing also shows that these expansions are MSH2-dependent, consistent with those seen in vivo. While comparable human Genome Wide Association (GWA) studies are not available for the FXDs, such studies have implicated MSH2 in expansion in other REDs. The shared unusual requirement for MSH2 for this type of microsatellite instability suggests that this new cell-based system is relevant for understanding the mechanism responsible for this peculiar type of mutation in humans. The high frequency of expansions and the ease of gene editing these cells should expedite the identification of factors that affect expansion risk. Additionally, we found that, as with cells from human premutation (PM) carriers, these cell lines have elevated mitochondrial copy numbers and Fmr1 hyperexpression, that we show here is O₂-sensitive. Thus, this new stem cell model should facilitate studies of both repeat expansion and the consequences of expansion during early embryonic development.

Fragile X-Associated Tremor/Ataxia Syndrome (FXTAS): Pathophysiology and Clinical Implications

The fragile X-associated tremor/ataxia syndrome (FXTAS) is a neurodegenerative disorder seen in older premutation (55-200 CGG repeats) carriers of FMR1. The premutation has excessive levels of FMR1 mRNA that lead to toxicity and mitochondrial dysfunction. The clinical features usually begin in the 60 s with an action or intention tremor followed by cerebellar ataxia, although 20% have only ataxia. MRI features include brain atrophy and white matter disease, especially in the middle cerebellar peduncles, periventricular areas, and splenium of the corpus callosum. Neurocognitive problems include memory and executive function deficits, although 50% of males can develop dementia. Females can be less affected by FXTAS because of a second X chromosome that does not carry the premutation. Approximately 40% of males and 16% of female carriers develop FXTAS. Since the premutation can occur in less than 1 in 200 women and 1 in 400 men, the FXTAS diagnosis should be considered in patients that present with tremor, ataxia, parkinsonian symptoms, neuropathy, and psychiatric problems. If a family history of a fragile X mutation is known, then FMR1 DNA testing is essential in patients with these symptoms.

Oxidative Stress in DNA Repeat Expansion Disorders: A Focus on NRF2 Signaling Involvement

DNA repeat expansion disorders are a group of neuromuscular and neurodegenerative diseases that arise from the inheritance of long tracts of nucleotide repetitions, located in the regulatory region, introns, or inside the coding sequence of a gene. Although loss of protein expression and/or the gain of function of its transcribed mRNA or translated product represent the major pathogenic effect of these pathologies, mitochondrial dysfunction and imbalance in redox homeostasis are reported as common features in these disorders, deeply affecting their severity and progression. In this review, we examine the role that the redox imbalance plays in the pathological mechanisms of DNA expansion disorders and the recent advances on antioxidant treatments, particularly focusing on the expression and the activity of the transcription factor NRF2, the main cellular regulator of the antioxidant response.

Role of mitochondrial DNA variants in the development of fragile X-associated tremor/ataxia syndrome

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a late-onset neurodegenerative disorder that appears in at least one-third of adult carriers of FMR1 premutation. Several studies have shown that mitochondrial dysfunction may play a role in neurodegenerative disorders. In order to assess whether mitochondrial DNA variants are involved in the risk of developing FXTAS we evaluated the frequency of mitochondrial haplogroups in 132 unrelated Spanish FMR1 premutation carriers. In addition, the entire mitogenome of 26 FMR1 premutation carriers was sequenced using massively parallel sequencing technologies to analyze mitochondrial DNA variants. Statistical analyses reveal a significant difference in the frequency of T haplogroup. Data analysis of mitochondrial DNA sequences evidence an association between FXTAS and the burden of heteroplasmic variants as well as their distribution. Our results suggest that haplogroup T might be a potential protective factor for FXTAS and that FXTAS individuals accumulate higher rates of heteroplasmic variants in compromised regions of the mitochondrial genome. These results may explain, in part, the role of mitochondrial DNA in the development of FXTAS.

FMRP ribonucleoprotein complexes and RNA homeostasis

The Fragile Mental Retardation 1 gene (FMR1), at Xq27.3, encodes the fragile mental retardation protein (FMRP), and displays in its 5'-untranslated region a series of polymorphic CGG triplet repeats that may undergo dynamic mutation. Fragile X syndrome (FXS) is the leading cause of inherited intellectual disability among men, and is most frequently due to FMR1 full mutation and consequent transcription repression. FMR1 premutations may associate with at least two other clinical conditions, named fragile X-associated primary ovarian insufficiency (FXPOI) and tremor and ataxia syndrome (FXTAS). While FXPOI and FXTAS appear to be mediated by FMR1 mRNA accumulation, relative reduction of FMRP, and triplet repeat translation, FXS is due to the lack of the RNA-binding protein FMRP. Besides its function as mRNA translation repressor in neuronal and stem/progenitor cells, RNA editing roles have been assigned to FMRP. In this review, we provide a brief description of FMR1 transcribed microsatellite and associated clinical disorders, and discuss FMRP molecular roles in ribonucleoprotein complex assembly and trafficking, as well as aspects of RNA homeostasis affected in FXS cells.

A patent review of histone deacetylase 6 inhibitors in neurodegenerative diseases (2014-2019)

Introduction: Histone deacetylase 6 (HDAC6) is unique in comparison with other zinc-dependent HDAC family members. An increasing amount of evidence from clinical and preclinical research demonstrates the potential of HDAC6 inhibition as an effective therapeutic approach for the treatment of cancer, autoimmune diseases, as well as neurological disorders. The recently disclosed crystal structures of HDAC6-ligand complexes offer further means for achieving pharmacophore refinement, thus further accelerating the pace of HDAC6 inhibitor discovery in the last few years.Area covered: This review summarizes the latest clinical status of HDAC6 inhibitors, discusses pharmacological applications of selective HDAC6 inhibitors in neurodegenerative diseases, and describes the patent applications dealing with HDAC6 inhibitors from 2014-2019 that have not been reported in research articles.Expert opinion: Phenylhydroxamate has proven a very useful scaffold in the discovery of potent and selective HDAC6 inhibitors. However, weaknesses of the hydroxamate function such as metabolic instability and mutagenic potential limit its application in the neurological field, where long-term administration is required. The recent invention of oxadiazole-based ligands by pharmaceutical companies may provide a new opportunity to optimize the druglike properties of HDAC6 inhibitors for the treatment of neurodegenerative diseases.

Altered mitochondrial function in cells carrying a premutation or unmethylated full mutation of the FMR1 gene

Fragile X-related disorders are due to a dynamic mutation of the CGG repeat at the 5' UTR of the FMR1 gene, coding for the RNA-binding protein FMRP. As the CGG sequence expands from premutation (PM, 56-200 CGGs) to full mutation (> 200 CGGs), FMRP synthesis decreases until it is practically abolished in fragile X syndrome (FXS) patients, mainly due to FMR1 methylation. Cells from rare individuals with no intellectual disability and carriers of an unmethylated full mutation (UFM) produce slightly elevated levels of FMR1-mRNA and relatively low levels of FMRP, like in PM carriers. With the aim of clarifying how UFM cells differ from CTRL and FXS cells, a comparative proteomic approach was undertaken, from which emerged an overexpression of SOD2 in UFM cells, also confirmed in PM but not in FXS. The SOD2-mRNA bound to FMRP in UFM more than in the other cell types. The high SOD2 levels in UFM and PM cells correlated with lower levels of superoxide and reactive oxygen species (ROS), and with morphological anomalies and depolarization of the mitochondrial membrane detected through confocal microscopy. The same effect was observed in CTRL and FXS after treatment with MC2791, causing SOD2 overexpression. These mitochondrial phenotypes reverted after knock-down with siRNA against SOD2-mRNA and FMR1-mRNA in UFM and PM. Overall, these data suggest that in PM and UFM carriers, which have high levels of FMR1 transcription and may develop FXTAS, SOD2 overexpression helps to maintain low levels of both superoxide and ROS with signs of mitochondrial degradation.

Composition of the Intranuclear Inclusions of Fragile X-associated Tremor/Ataxia Syndrome

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a neurodegenerative disorder associated with a premutation repeat expansion (55-200 CGG repeats) in the 5' noncoding region of the FMR1 gene. Solitary intranuclear inclusions within FXTAS neurons and astrocytes constitute a hallmark of the disorder, yet our understanding of how and why these bodies form is limited. Here, we have discovered that FXTAS inclusions emit a distinct autofluorescence spectrum, which forms the basis of a novel, unbiased method for isolating FXTAS inclusions by preparative fluorescence-activated cell sorting (FACS). Using a combination of autofluorescence-based FACS and liquid chromatography/tandem mass spectrometry (LC-MS/MS)-based proteomics, we have identified more than two hundred proteins that are enriched within the inclusions relative to FXTAS whole nuclei. Whereas no single protein species dominates inclusion composition, highly enriched levels of conjugated small ubiquitin-related modifier 2 (SUMO 2) protein and p62/sequestosome-1 (p62/SQSTM1) protein were found within the inclusions. Many additional proteins involved with RNA binding, protein turnover, and DNA damage repair were enriched within inclusions relative to total nuclear protein. The current analysis has also allowed the first direct detection, through peptide sequencing, of endogenous FMRpolyG peptide, the product of repeat-associated non-ATG (RAN) translation of the FMR1 mRNA. However, this peptide was found only at extremely low levels and not within whole FXTAS nuclear preparations, raising the question whether endogenous RAN products exist at quantities sufficient to contribute to FXTAS pathogenesis. The abundance of the inclusion-associated ubiquitin- and SUMO-based modifiers supports a model for inclusion formation as the result of increased protein loads and elevated oxidative stress leading to maladaptive autophagy. These results highlight the need to further investigate FXTAS pathogenesis in the context of endogenous systems.

General Anesthetic Use in Fragile X Spectrum Disorders

The fragile X premutation is characterized by a repeat expansion mutation (between 55 to 200 CGG repeats) in the fragile X mental retardation 1 (FMR1) gene, which leads to RNA toxicity at the cellular level. This may cause patients with the premutation to be particularly susceptible to environmental toxins, which could manifest clinically as new or worsening ataxia and memory loss. Multiple published case reports have also suggested general anesthetics as a potential toxin leading to negative side effects when used in patients with fragile X-associated disorders. However, at this time, there have been no formal research studies regarding cellular changes or long-term clinical manifestations after general anesthetic use in this population. This review aims to highlight previous case reports regarding sequelae related to general anesthetic use in fragile X-associated disorders. New case reports related to this phenomenon are also included.