C9orf72-Related Neurodegenerative Diseases: From Clinical Diagnosis to Therapeutic Strategies

Endosomal traffic disorders: a driving force behind neurodegenerative diseases

Endosomes are crucial sites for intracellular material sorting and transportation. Endosomal transport is a critical process involved in the selective uptake, processing, and intracellular transport of substances. The equilibrium between endocytosis and circulation mediated by the endosome-centered transport pathway plays a significant role in cell homeostasis, signal transduction, and immune response. In recent years, there have been hints linking endosomal transport abnormalities to neurodegenerative diseases, including Alzheimer's disease. Nonetheless, the related mechanisms remain unclear. Here, we provide an overview of endosomal-centered transport pathways and highlight potential physiological processes regulated by these pathways, with a particular focus on the correlation of endosomal trafficking disorders with common pathological features of neurodegenerative diseases. Additionally, we summarize potential therapeutic agents targeting endosomal trafficking for the treatment of neurodegenerative diseases.

Complex Genetic Framework in Familial Amyotrophic Lateral Sclerosis With a C9ORF72 Mutation: A Case Report

A significantly diverse clinical presentation of amyotrophic lateral sclerosis (ALS), even in its best-studied familial form, continues to hinder current efforts to develop effective disease-modifying drugs for the cure of this rapidly progressive, fatal neuromuscular disease. We have previously shown that clinical heterogeneity of sporadic ALS (sALS) could be explained, at least in part, by its polygenic nature as well as by the presence of mutated genes linked to non-ALS neurological diseases and genes known to mediate ALS-related pathologies. We hypothesized that a similar genetic framework could also be present in patients with familial ALS (fALS). To test this hypothesis, we conducted post-mortem genetic screening of an individual with fALS and a mutation in the C9ORF72 gene. C9ORF72 mutations are highly penetrant and are present in the majority of fALS patients. Genetic screening by whole exome sequencing (WES) on the next generation sequencing (NGS) Illumina platform (San Diego, CA, USA) followed by examination of the respective rare (minor allele frequency (MAF) ≤ 0.01) pathological/deleterious genetic variants yielded results consistent with our hypothesis of the presence of a complex genetic framework in fALS. Additional members of this genetic framework were identified when the low-frequency (0.01 < MAF < 0.05) pathological/deleterious genetic variants were analyzed with the low-frequency biallelic AHNAK2, GLI3, PTIRM1, and ZNF254 variants, warranting a closer look at their potentially important role in fALS as C9ORF72 genetic modifiers as well as their link to both neuromuscular disorders/ALS and cancer. Therefore, in addition to the current genetic screening using a standard panel of ALS-related genes, a supplementary screening by WES could be very beneficial for the development of personalized treatment of ALS patients as well as in search of the respective efficient disease-modifying drugs.

Cognitive reserve as a modulator of cognitive decline and of behavioral symptoms in patients with amyotrophic lateral sclerosis

Introduction: Amyotrophic lateral sclerosis (ALS) has heterogeneous manifestations ranging from motor neuron degeneration to cognitive and behavioral impairment. This study aims to clarify the interactions between cognition and behavioral symptoms with relevant disease predictors and with cognitive reserve (CR), quantified through education, physical activity, and occupation proxies. Methods: A prospective sample of 162 ALS patients and 61 controls were evaluated with the Edinburgh Cognitive and Behavioral ALS Screen (ECAS) (dependent variable), a Cognitive Reserve Index questionnaire (CRIq) and demographic data (age and sex), and, for patients, clinical variables: disease duration, site of onset, the ALS Functional Rating Scale (ALSFRS), forced vital capacity (FVC), and gene mutation chromosome 9 open reading frame 72 (C9orf72) (independent variables). Multiple regression and mediation analyses were performed to predict cognitive and behavioral symptoms. Results: For the ALS group, the statistical model explained 38.8% of variance in ECAS total (p < 0.001), 59.4% of executive functions (p < 0.001), and 55% of behavioral symptoms (p < 0.001). For controls, it accounted for 52.8% of variance in ECAS total (p < 0.001). Interaction effects and mediation analysis showed CR is an ECAS total modulator, with a differential effect within groups (p < 0.001). Verbal fluency was the single best cognitive score to differentiate patients from controls (p = 0.004), and the gene mutation C9orf72 was found to be a behavioral symptom' predictor in patients (p = 0.009). Conclusion: This study supports the proposed concept that CR acts as a cognitive modulator in ALS patients and healthy individuals. Moreover, CR also modulates behavioral manifestations in ALS.

Increased frequency of repeat expansion mutations across different populations

Repeat expansion disorders (REDs) are a devastating group of predominantly neurological diseases. Together they are common, affecting 1 in 3,000 people worldwide with population-specific differences. However, prevalence estimates of REDs are hampered by heterogeneous clinical presentation, variable geographic distributions and technological limitations leading to underascertainment. Here, leveraging whole-genome sequencing data from 82,176 individuals from different populations, we found an overall disease allele frequency of REDs of 1 in 283 individuals. Modeling disease prevalence using genetic data, age at onset and survival, we show that the expected number of people with REDs would be two to three times higher than currently reported figures, indicating underdiagnosis and/or incomplete penetrance. While some REDs are population specific, for example, Huntington disease-like 2 in Africans, most REDs are represented in all broad genetic ancestries (that is, Europeans, Africans, Americans, East Asians and South Asians), challenging the notion that some REDs are found only in specific populations. These results have worldwide implications for local and global health communities in the diagnosis and counseling of REDs.

Increased frequency of repeat expansion mutations across different populations

Repeat expansion disorders (REDs) are a devastating group of predominantly neurological diseases. Together they are common, affecting 1 in 3,000 people worldwide with population-specific differences. However, prevalence estimates of REDs are hampered by heterogeneous clinical presentation, variable geographic distributions, and technological limitations leading to under-ascertainment. Here, leveraging whole genome sequencing data from 82,176 individuals from different populations, we found an overall disease allele frequency of REDs of 1 in 283 individuals. Modelling disease prevalence using genetic data, age at onset and survival, we show that the expected number of people with REDs would be two to three times higher than currently reported figures, indicating under-diagnosis and/or incomplete penetrance. While some REDs are population-specific, e.g. Huntington disease-like 2 in Africans, most REDs are represented in all broad genetic ancestries (i.e. Europeans, Africans, Americans, East Asians, and South Asians), challenging the notion that some REDs are found only in specific populations. These results have worldwide implications for local and global health communities in the diagnosis and counselling of REDs.

Updates on Disease Mechanisms and Therapeutics for Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS), or Lou Gehrig's disease, is a motor neuron disease. In ALS, upper and lower motor neurons in the brain and spinal cord progressively degenerate during the course of the disease, leading to the loss of the voluntary movement of the arms and legs. Since its first description in 1869 by a French neurologist Jean-Martin Charcot, the scientific discoveries on ALS have increased our understanding of ALS genetics, pathology and mechanisms and provided novel therapeutic strategies. The goal of this review article is to provide a comprehensive summary of the recent findings on ALS mechanisms and related therapeutic strategies to the scientific audience. Several highlighted ALS research topics discussed in this article include the 2023 FDA approved drug for SOD1 ALS, the updated C9orf72 GGGGCC repeat-expansion-related mechanisms and therapeutic targets, TDP-43-mediated cryptic splicing and disease markers and diagnostic and therapeutic options offered by these recent discoveries.

C9ORF72 hexanucleotide repeat expansion: From ALS and FTD to a broader pathogenic role?

The major gene underlying monogenic forms of amyotrophic lateral sclerosis (ALS) and fronto-temporal dementia (FTD) is C9ORF72. The causative mutation in C9ORF72 is an abnormal hexanucleotide (G4C2) repeat expansion (HRE) located in the first intron of the gene. The aim of this review is to propose a comprehensive update on recent developments on clinical, biological and therapeutics aspects related to C9ORF72 in order to highlight the current understanding of genotype-phenotype correlations, and also on biological machinery leading to neuronal death. We will particularly focus on the broad phenotypic presentation of C9ORF72-related diseases, that goes well beyond the classical phenotypes observed in ALS and FTD patients. Last, we will comment the possible therapeutical hopes for patients carrying a C9ORF72 HRE.

Atypical Neuropsychiatric Presentation of FTD-ALS Caused by a Pathogenic Repeat Expansion in C9orf72: A Case Report

The case report describes the presentation of a 42-year-old male ultimately diagnosed with FTD-ALS caused by a genetic mutation, who initially presented with atypical psychiatric symptoms. Given that the initial clinical manifestations of FTD-ALS can be quite variable, the diagnosis is often challenging; the case report aims to highlight several key considerations in the diagnostic assessment, including genetic testing in order to guide clinicians in more timely diagnosis and ultimately improve patient care.

Interaction of the C9orf72-Amyotrophic Lateral Sclerosis-Related Proline-Arginine Dipeptide Repeat Protein with the RNA-Binding Protein NOVA1 Causes Decreased Expression of UNC13A Due to Enhanced Inclusion of Cryptic Exons, Which Is Reversed by Betulin Treatment

C9orf72 mutations are the most common form of familial amyotrophic lateral sclerosis (C9-ALS). It causes the production of proline-arginine dipeptide repeat proteins (PR-DPRs) in motor neurons (MNs), leading to the molecular pathology characteristic of ALS. UNC13A is critical for maintaining the synaptic function of MNs. Most ALS patients have nuclear deletion of the splicing repressor TDP-43 in MNs, which causes inclusion of the cryptic exon (CE) of UNC13A mRNA, resulting in nonsense-mediated mRNA decay and reduced protein expression. Therefore, in this study, we explored the role of PR-DPR in CE inclusion of UNC13A mRNA. Our results showed that PR-DPR (PR50) induced CE inclusion and decreased the protein expression of UNC13A in human neuronal cell lines. We also identified an interaction between the RNA-binding protein NOVA1 and PR50 by yeast two-hybrid screening. NOVA1 expression is known to be reduced in patients with ALS. We found that knockdown of NOVA1 enhanced CE inclusion of UNC13A mRNA. Furthermore, the naturally occurring triterpene betulin can inhibit the interaction between NOVA1 and PR50, thus preventing CE inclusion of UNC13A mRNA and protein reduction in human neuronal cell lines. This study linked PR-DPR with CE inclusion of UNC13A mRNA and developed candidate therapeutic strategies for C9-ALS using betulin.

The HNRNPF/H RNA binding proteins and disease

The members of the HNRNPF/H family of heterogeneous nuclear RNA proteins-HNRNPF, HNRNPH1, HNRNPH2, HNRNPH3, and GRSF1, are critical regulators of RNA maturation. Documented functions of these proteins include regulating splicing, particularly alternative splicing, 5' capping and 3' polyadenylation of RNAs, and RNA export. The assignment of these proteins to the HNRNPF/H protein family members relates to differences in the amino acid composition of their RNA recognition motifs, which differ from those of other RNA binding proteins (RBPs). HNRNPF/H proteins typically bind RNA sequences enriched with guanine (G) residues, including sequences that, in the presence of a cation, have the potential to form higher-order G-quadruplex structures. The need to further investigate members of the HNRNPF/H family of RBPs has intensified with the recent descriptions of their involvement in several disease states, including the pediatric tumor Ewing sarcoma and the hematological malignancy mantle cell lymphoma; newly described groups of developmental syndromes; and neuronal-related disorders, including addictive behavior. Here, to foster the study of the HNRNPF/H family of RBPs, we discuss features of the genes encoding these proteins, their structures and functions, and emerging contributions to disease. This article is categorized under: RNA in Disease and Development > RNA in Disease RNA Processing > Splicing Regulation/Alternative Splicing RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications.

A narrative review on mild behavioural impairment: an exploration into its scientific perspectives

In clinical practice, the admission of patients with late-onset psychological and behavioural symptoms is frequent, regardless of the presence or absence of cognitive decline. These symptoms commonly occur in the prodromal stage of dementia and can precede the onset of dementia. While the concept of Mild Cognitive Impairment (MCI) -which is defined as a level of cognitive impairment insufficient to impact daily functioning- is well established, the notion of Mild Behavioural Impairment (MBI) is not yet widely recognized. However, studies have demonstrated that the presence of MBI in both cognitively normal patients and individuals with MCI is associated with an increased risk of dementia progression. Thus, MBI may serve as a neurobehavioral indicator of pre-dementia risk states. This narrative review aims to discuss the evolution of the term, the relevant clinical aspects, and potential biomarkers that may contribute to the clinical definition of MBI. The objective is to assist clinicians in recognizing the diagnosis and differentiating it from psychiatric syndromes, as well as identifying possible etiologies of neurodegeneration.

Reduced C9orf72 expression exacerbates polyGR toxicity in patient iPSC-derived motor neurons and a Type I protein arginine methyltransferase inhibitor reduces that toxicity

Introduction

Intronic repeat expansions in the C9orf72 gene are the most frequent known single genetic causes of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). These repeat expansions are believed to result in both loss-of-function and toxic gain-of-function. Gain-of-function results in the production of toxic arginine-rich dipeptide repeat proteins (DPRs), namely polyGR and polyPR. Small-molecule inhibition of Type I protein arginine methyltransferases (PRMTs) has been shown to protect against toxicity resulting from polyGR and polyPR challenge in NSC-34 cells and primary mouse-derived spinal neurons, but the effect in human motor neurons (MNs) has not yet been explored.

Methods

To study this, we generated a panel of C9orf72 homozygous and hemizygous knockout iPSCs to examine the contribution of C9orf72 loss-of-function toward disease pathogenesis. We differentiated these iPSCs into spinal motor neurons (sMNs).

Results

We found that reduced levels of C9orf72 exacerbate polyGR15 toxicity in a dose-dependent manner. Type I PRMT inhibition was able to partially rescue polyGR15 toxicity in both wild-type and C9orf72-expanded sMNs.

Discussion

This study explores the interplay of loss-of-function and gain-of-function toxicity in C9orf72 ALS. It also implicates type I PRMT inhibitors as a possible modulator of polyGR toxicity.