Variants of the heavy neurofilament subunit are associated with the development of amyotrophic lateral sclerosis

Genetic overlap between ALS and other neurodegenerative or neuromuscular disorders

OBJECTIVE

In Norway, 89% of patients with Amyotrophic lateral sclerosis (ALS) lacks a genetic diagnose. ALS genes and genes that cause other neuromuscular or neurodegenerative disorders extensively overlap. This population-based study examined whether patients with ALS have a family history of neurological disorders and explored the occurrence of rare genetic variants associated with other neurodegenerative or neuromuscular disorders.

METHODS

During a two-year period, blood samples and clinical data from patients with ALS were collected from all 17 neurological departments in Norway. Our genetic analysis involved exome sequencing and bioinformatics filtering of 510 genes associated with neurodegenerative and neuromuscular disorders. The variants were interpreted using genotype-phenotype correlations and bioinformatics tools.

RESULTS

A total of 279 patients from a Norwegian population-based ALS cohort participated in this study. Thirty-one percent of the patients had first- or second-degree relatives with other neurodegenerative disorders, most commonly dementia and Parkinson's disease. The genetic analysis identified 20 possible pathogenic variants, in ATL3, AFG3L2, ATP7A, BICD2, HARS1, KIF1A, LRRK2, MSTO1, NEK1, NEFH, and SORL1, in 25 patients. NEK1 risk variants were present in 2.5% of this ALS cohort. Only four of the 25 patients reported relatives with other neurodegenerative or neuromuscular disorders.

CONCLUSION

Gene variants known to cause other neurodegenerative or neuromuscular disorders, most frequently in NEK1, were identified in 9% of the patients with ALS. Most of these patients had no family history of other neurodegenerative or neuromuscular disorders. Our findings indicated that AFG3L2, ATP7A, BICD2, KIF1A, and MSTO1 should be further explored as potential ALS-causing genes.

Protein biomarkers for the diagnosis and prognosis of Amyotrophic Lateral Sclerosis

Amyotrophic Lateral Sclerosis (ALS) is the most common motor neuron disease, still incurable. The disease is highly heterogenous both genetically and phenotypically. Therefore, developing efficacious treatments is challenging in many aspects because it is difficult to predict the rate of disease progression and stratify the patients to minimize statistical variability in clinical studies. Moreover, there is a lack of sensitive measures of therapeutic effect to assess whether a pharmacological intervention ameliorates the disease. There is also urgency of markers that reflect a molecular mechanism dysregulated by ALS pathology and can be rescued when a treatment relieves the condition. Here, we summarize and discuss biomarkers tested in multicentered studies and across different laboratories like neurofilaments, the most used marker in ALS clinical studies, neuroinflammatory-related proteins, p75ECD, p-Tau/t-Tau, and UCHL1. We also explore the applicability of muscle proteins and extracellular vesicles as potential biomarkers.

Amyotrophic lateral sclerosis: translating genetic discoveries into therapies

Recent advances in sequencing technologies and collaborative efforts have led to substantial progress in identifying the genetic causes of amyotrophic lateral sclerosis (ALS). This momentum has, in turn, fostered the development of putative molecular therapies. In this Review, we outline the current genetic knowledge, emphasizing recent discoveries and emerging concepts such as the implication of distinct types of mutation, variability in mutated genes in diverse genetic ancestries and gene-environment interactions. We also propose a high-level model to synthesize the interdependent effects of genetics, environmental and lifestyle factors, and ageing into a unified theory of ALS. Furthermore, we summarize the current status of therapies developed on the basis of genetic knowledge established for ALS over the past 30 years, and we discuss how developing treatments for ALS will advance our understanding of targeting other neurological diseases.

Loss of function of the ALS-associated NEK1 kinase disrupts microtubule homeostasis and nuclear import

Loss-of-function variants in NIMA-related kinase 1 (NEK1) constitute a major genetic cause of amyotrophic lateral sclerosis (ALS), accounting for 2 to 3% of all cases. However, how NEK1 mutations cause motor neuron (MN) dysfunction is unknown. Using mass spectrometry analyses for NEK1 interactors and NEK1-dependent expression changes, we find functional enrichment for proteins involved in the microtubule cytoskeleton and nucleocytoplasmic transport. We show that α-tubulin and importin-β1, two key proteins involved in these processes, are phosphorylated by NEK1 in vitro. NEK1 is essential for motor control and survival in Drosophila models in vivo, while using several induced pluripotent stem cell (iPSC)-MN models, including NEK1 knockdown, kinase inhibition, and a patient mutation, we find evidence for disruptions in microtubule homeostasis and nuclear import. Notably, stabilizing microtubules with two distinct classes of drugs restored NEK1-dependent deficits in both pathways. The capacity of NEK1 to modulate these processes that are critically involved in ALS pathophysiology renders this kinase a formidable therapeutic candidate.

Fluid Biomarkers of Neuro-Glial Injury in Human Status Epilepticus: A Systematic Review

As per the latest ILAE definition, status epilepticus (SE) may lead to long-term irreversible consequences, such as neuronal death, neuronal injury, and alterations in neuronal networks. Consequently, there is growing interest in identifying biomarkers that can demonstrate and quantify the extent of neuronal and glial injury. Despite numerous studies conducted on animal models of status epilepticus, which clearly indicate seizure-induced neuronal and glial injury, as well as signs of atrophy and gliosis, evidence in humans remains limited to case reports and small case series. The implications of identifying such biomarkers in clinical practice are significant, including improved prognostic stratification of patients and the early identification of those at high risk of developing irreversible complications. Moreover, the clinical validation of these biomarkers could be crucial in promoting neuroprotective strategies in addition to antiseizure medications. In this study, we present a systematic review of research on biomarkers of neuro-glial injury in patients with status epilepticus.

Oral edaravone demonstrated a favorable safety profile in patients with amyotrophic lateral sclerosis after 48 weeks of treatment

INTRODUCTION/AIMS

An intravenous (IV) formulation of edaravone has been shown to slow the rate of physical functional decline in amyotrophic lateral sclerosis (ALS). An oral suspension formulation of edaravone was recently approved by the United States Food and Drug Administration for use in patients with ALS. This study assessed the safety and tolerability of oral edaravone.

METHODS

This global, open-label, phase 3 study evaluated the long-term safety and tolerability of oral edaravone in adults with ALS who had a baseline forced vital capacity ≥70% of predicted and disease duration ≤3 y. The primary safety analysis was assessed at weeks 24 and 48. Patients received a 105-mg dose of oral edaravone in treatment cycles replicating the dosing of IV edaravone.

RESULTS

The study enrolled 185 patients (64.3% male; mean age, 59.9 y; mean disease duration, 1.56 y). The most common treatment-emergent adverse events (TEAEs) at week 48 were fall (22.2%), muscular weakness (21.1%) and constipation (17.8%). Serious TEAEs were reported by 25.9% of patients; the most common were worsening ALS symptoms, dysphagia, dyspnea, and respiratory failure. Twelve TEAEs leading to death were reported. Forty-six (24.9%) patients reported TEAEs that were considered related to study drug; the most common were fatigue, dizziness, headache, and constipation. Sixteen (8.6%) patients discontinued study drug due to TEAEs. No serious TEAEs were related to study drug.

DISCUSSION

This study indicated that oral edaravone was well tolerated during 48 wk of treatment, with no new safety concerns identified.

Intronic NEFH variant is associated with reduced risk for sporadic ALS and later age of disease onset

Neurofilament heavy (NEFH) is one of the critical proteins required for the formation of the neuronal cytoskeleton and polymorphisms in NEFH are reported as a rare cause of sporadic ALS (sALS). In the current study, a candidate tetranucleotide (TTTA) repeat variant in NEFH was selected using an in-silico short structural variant (SSV) evaluation algorithm and investigated in two cohorts of North American sALS patients, both separately and combined (Duke cohort n = 138, Coriell cohort n = 333; combined cohort n = 471), compared to a group of healthy controls from the Coriell Institute biobank (n = 496). Stratification according to site of disease onset revealed that the 9 TTTA allele was associated with reduced disease risk, specifically confined to spinal-onset sALS patients in the Duke cohort (p = 0.001). Furthermore, carriage of the 10 TTTA allele was associated with a 2.7 year later age of disease onset in the larger combined sALS cohort (p = 0.02). These results suggest that the 9 and 10 TTTA motif length may have a protective advantage for potentially lowering the risk of sALS and delaying the age of disease onset, however, these results need to be replicated in larger multicenter and multi-ethnic cohorts.

Axon Biology in ALS: Mechanisms of Axon Degeneration and Prospects for Therapy

This review addresses the longstanding debate over whether amyotrophic lateral sclerosis (ALS) is a 'dying back' or 'dying forward' disorder in the light of new gene identifications and the increased understanding of mechanisms of action for previously identified ALS genes. While the topological pattern of pathology in animal models, and more anecdotally in patients is indeed 'dying back', this review discusses how this fits with the fact that many of the major initiating events are thought to occur within the soma. It also discusses how widely varying ALS risk factors, including some impacting axons directly, may combine to drive a common pathway involving TAR DNA binding protein 43 (TDP-43) and neuromuscular junction (NMJ) denervation. The emerging association between sterile alpha and TIR motif-containing 1 (SARM1), a protein so far mostly associated with axon degeneration, and sporadic ALS is another major theme. The strengths and limitations of the current evidence supporting an association are considered, along with ways in which SARM1 could become activated in ALS. The final section addresses SARM1-based therapies along with the prospects for targeting other axonal steps in ALS pathogenesis.

Thr124Met myelin protein zero mutation mimicking motor neuron disease

Mutations in myelin protein zero (MPZ) are associated with heterogeneous manifestations. In this study, we report clinical, electrophysiological, pathological, and muscle MRI findings from two relatives with MPZ Thr124Met variants, disclosing different phenotypes. The proband was a 73-year-old female with a 12-year-story of atrophy, weakness, and fasciculations in her proximal and distal lower limbs. EMG examination showed neurogenic signs with active denervation together with reduced sensory action potentials, without sensory symptoms. The initial diagnosis was of a slowly progressive lower motor neuron disease (MND) with subclinical sensory axonal neuropathy. Two years later, the observation of her 60-year-old nephew, who had a distal sensory-motor neuropathy, prompted the analysis of inherited neuropathies-related genes and revealed a MPZ Thr124Met mutation in both cases. Our findings expand the clinical spectrum of MPZ-related neuropathy and highlight that Thr124Met mutation may cause a syndrome mimicking MND. The challenging issue to detect sensory features in the diagnostic MND work up is discussed.

Novel TBK1 variant associated with Frontotemporal Dementia overlap syndrome

BACKGROUND

Recently, TANK binding kinase 1 (TBK1) mutation has been reported as a causative gene for overlap frontotemporal dementia (FTD)-amyotrophic lateral sclerosis (ALS) syndrome. However, there are no reports from families of South Asian ethnicity.

OBJECTIVE

To report a case study of a family with the proband having overlap FTD-ALS syndrome caused by a novel TBK1 variant.

MATERIALS AND METHODS

Clinical, brain imaging, genetic analysis and laboratory data of the patient with FTD-ALS were performed. In addition, family-based segregation analysis of identified novel variants was also done.

RESULTS

This study pertains to genetic analysis in 11 members in a family with only one member affected with overlap FTD-ALS syndrome. The whole-exome sequencing analysis in the symptomatic member showed a novel loss-of-function (LoF) variant c.1810G>T(p.E604X) in the TBK1 gene. Neuroimaging showed a pattern of asymmetric frontotemporal atrophy and hypometabolism. Segregation analysis of the variation demonstrated its presence in several family members, although none of the other members was symptomatic. Further, we observed another missense variation in the NEFH gene (p.Pro683Leu) which was seen in the symptomatic and two asymptomatic family members, the pathogenicity of which is unclear.

CONCLUSION

This is the first study of a rare novel TBK1 variant associated with FTD-ALS from India. Asymptomatic family members with the variant have important clinical implications and necessitate the genetic evaluation and long-term follow-up of family members of patients detected with TBK1 mutations. Therefore, although infrequent, genetic screening for the TBK1 gene should be considered when encountering overlap FTD syndromes.

Microglia Influence Neurofilament Deposition in ALS iPSC-Derived Motor Neurons

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease in which upper and lower motor neuron loss is the primary phenotype, leading to muscle weakness and wasting, respiratory failure, and death. Although a portion of ALS cases are linked to one of over 50 unique genes, the vast majority of cases are sporadic in nature. However, the mechanisms underlying the motor neuron loss in either familial or sporadic ALS are not entirely clear. Here, we used induced pluripotent stem cells derived from a set of identical twin brothers discordant for ALS to assess the role of astrocytes and microglia on the expression and accumulation of neurofilament proteins in motor neurons. We found that motor neurons derived from the affected twin which exhibited increased transcript levels of all three neurofilament isoforms and increased expression of phosphorylated neurofilament puncta. We further found that treatment of the motor neurons with astrocyte-conditioned medium and microglial-conditioned medium significantly impacted neurofilament deposition. Together, these data suggest that glial-secreted factors can alter neurofilament pathology in ALS iPSC-derived motor neurons.

Charcot-Marie-Tooth disease type 2CC due to NEFH variants causes a progressive, non-length-dependent, motor-predominant phenotype

OBJECTIVE

Neurofilaments are the major scaffolding proteins for the neuronal cytoskeleton, and variants in NEFH have recently been described to cause axonal Charcot-Marie-Tooth disease type 2CC (CMT2CC).

METHODS

In this large observational study, we present phenotype-genotype correlations on 30 affected and 3 asymptomatic mutation carriers from eight families.

RESULTS

The majority of patients presented in adulthood with motor-predominant and lower limb-predominant symptoms and the average age of onset was 31.0±15.1 years. A prominent feature was the development of proximal weakness early in the course of the disease. The disease progressed rapidly, unlike other Charcot-Marie-Tooth disease (CMT) subtypes, and half of the patients (53%) needed to use a wheelchair on average 24.1 years after symptom onset. Furthermore, 40% of patients had evidence of early ankle plantarflexion weakness, a feature which is observed in only a handful of CMT subtypes. Neurophysiological studies and MRI of the lower limbs confirmed the presence of a non-length-dependent neuropathy in the majority of patients.All families harboured heterozygous frameshift variants in the last exon of NEFH, resulting in a reading frameshift to an alternate open reading frame and the translation of approximately 42 additional amino acids from the 3' untranslated region (3'-UTR).

CONCLUSIONS

This phenotype-genotype study highlights the unusual phenotype of CMT2CC, which is more akin to spinal muscular atrophy rather than classic CMT. Furthermore, the study will enable more informative discussions on the natural history of the disease and will aid in NEFH variant interpretation in the context of the disease's unique molecular genetics.

New perspectives on cytoskeletal dysregulation and mitochondrial mislocalization in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease characterized by selective, early degeneration of motor neurons in the brain and spinal cord. Motor neurons have long axonal projections, which rely on the integrity of neuronal cytoskeleton and mitochondria to regulate energy requirements for maintaining axonal stability, anterograde and retrograde transport, and signaling between neurons. The formation of protein aggregates which contain cytoskeletal proteins, and mitochondrial dysfunction both have devastating effects on the function of neurons and are shared pathological features across several neurodegenerative conditions, including ALS, Alzheimer's disease, Parkinson's disease, Huntington's disease and Charcot-Marie-Tooth disease. Furthermore, it is becoming increasingly clear that cytoskeletal integrity and mitochondrial function are intricately linked. Therefore, dysregulations of the cytoskeletal network and mitochondrial homeostasis and localization, may be common pathways in the initial steps of neurodegeneration. Here we review and discuss known contributors, including variants in genetic loci and aberrant protein activities, which modify cytoskeletal integrity, axonal transport and mitochondrial localization in ALS and have overlapping features with other neurodegenerative diseases. Additionally, we explore some emerging pathways that may contribute to this disruption in ALS.

Advancing mechanistic understanding and biomarker development in amyotrophic lateral sclerosis

INTRODUCTION

Proteomic analysis has contributed significantly to the study of the neurodegenerative disease amyotrophic lateral sclerosis (ALS). It has helped to define the pathological change common to nearly all cases, namely intracellular aggregates of phosphorylated TDP-43, shifting the focus of pathogenesis in ALS toward RNA biology. Proteomics has also uniquely underpinned the delineation of disease mechanisms in model systems and has been central to recent advances in human ALS biomarker development.

AREAS COVERED

The contribution of proteomics to understanding the cellular pathological changes, disease mechanisms, and biomarker development in ALS are covered.

EXPERT OPINION

Proteomics has delivered unique insights into the pathogenesis of ALS and advanced the goal of objective measurements of disease activity to improve therapeutic trials. Further developments in sensitivity and quantification are expected, with application to the presymptomatic phase of human disease offering the hope of prevention strategies.

Sequencing of neurofilament genes identified NEFH Ser787Arg as a novel risk variant of sporadic amyotrophic lateral sclerosis in Chinese subjects

BACKGROUND

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease with neuronal cell inclusions composed of neurofilaments and other abnormal aggregative proteins as pathological hallmarks. Approximately 90% of patients have sporadic cases (sALS), and at least 4 genes, i.e. C9orf72, SOD1, FUS and TARDBP, have been identified as the main causative genes, while many others have been proposed as potential risk genes. However, these mutations could explain only ~ 10% of sALS cases. The neurofilament polypeptides encoded by NEFH, NEFM, and NEFL are promising protein biomarkers for ALS and other degenerative diseases. However, whether the genetic variants of these genes were associated with ALS remain ambiguous.

METHODS

Here, we used PCR-Sanger to sequence the exons of these three genes in a cohort of 371 sALS patients and 711 healthy controls (Phase I) and validated the risk variant in another 300 sALS patients and 1076 controls (Phase II).

RESULTS

A total of 92 variants were identified, including 36 rare heterozygous variants in NEFH, 27 in NEFM, and 16 in NEFL, and only rs568759161 (p.Ser787Arg) in NEFH reached nominal statistical power (P = 0.02 at Phase I, P = 0.009 at Phase II) in the case-control comparison. Together, the Phase I and II studies showed the significantly higher frequency of the variant in cases (9/1342, 0.67%) than in controls (2/3574, 0.07%) (OR 12.06; 95% CI 2.60-55.88; P = 0.0003). No variants passed multiple testing in the discovery cohort, but rs568759161 was associated with ALS in a replication cohort.

CONCLUSIONS

Our results confirmed that NEFH Ser787Arg is a novel sALS risk variant in Chinese subjects, but NEFM and NEFL were not associated with sALS. These data may have implications for genetic counselling and for understanding the pathogenesis of sALS.

Radicava/Edaravone Findings in Biomarkers From Amyotrophic Lateral Sclerosis (REFINE-ALS): Protocol and Study Design

Objectives

To identify putative biomarkers that may serve as quantifiable, biological, nonclinical measures of the pharmacodynamic effect of edaravone in amyotrophic lateral sclerosis (ALS) and to report real-world treatment outcomes.

Methods

This is a prospective, observational, longitudinal, multicenter (up to 40 sites) US study (Clinicaltrials.gov; NCT04259255) with at least 200 patients with ALS who will receive edaravone for 24 weeks (6 cycles; Food and Drug Administration-approved regimen). All participants must either be treatment naive for edaravone or be more than 1 month without receiving any edaravone dose before screening. Biomarker quantification and other assessments will be performed at baseline (before cycle 1) and during cycles 1, 3, and 6. Selected biomarkers of oxidative stress, inflammation, neuronal injury and death, and muscle injury, as well as biomarker discovery panels (EpiSwitch and SOMAscan), will be evaluated and, when feasible, compared with biobanked samples. Clinical efficacy assessments will include the ALS Functional Rating Scale-Revised, King's clinical staging, ALS Assessment Questionnaire-40, Appel ALS Score (Rating Scale), slow vital capacity, hand-held dynamometry and grip strength, and time to specified states of disease progression or death. DNA samples will also be collected for potential genomic evaluation. The predicted rates of progression and survival, and their potential correlations with biomarkers, will be evaluated. Adverse events related to the study will be reported.

Results

The study is estimated to be completed in 2022 with an interim analysis planned.

Conclusions

Findings may help to further the understanding of the pharmacodynamic effect of edaravone, including changes in biomarkers, in response to treatment.

The Skeletal Muscle Emerges as a New Disease Target in Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder that leads to progressive degeneration of motor neurons (MNs) and severe muscle atrophy without effective treatment. Most research on ALS has been focused on the study of MNs and supporting cells of the central nervous system. Strikingly, the recent observations of pathological changes in muscle occurring before disease onset and independent from MN degeneration have bolstered the interest for the study of muscle tissue as a potential target for delivery of therapies for ALS. Skeletal muscle has just been described as a tissue with an important secretory function that is toxic to MNs in the context of ALS. Moreover, a fine-tuning balance between biosynthetic and atrophic pathways is necessary to induce myogenesis for muscle tissue repair. Compromising this response due to primary metabolic abnormalities in the muscle could trigger defective muscle regeneration and neuromuscular junction restoration, with deleterious consequences for MNs and thereby hastening the development of ALS. However, it remains puzzling how backward signaling from the muscle could impinge on MN death. This review provides a comprehensive analysis on the current state-of-the-art of the role of the skeletal muscle in ALS, highlighting its contribution to the neurodegeneration in ALS through backward-signaling processes as a newly uncovered mechanism for a peripheral etiopathogenesis of the disease.

Neurofilament Light Chain as Biomarker for Amyotrophic Lateral Sclerosis and Frontotemporal Dementia

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are two related currently incurable neurodegenerative diseases. ALS is characterized by degeneration of upper and lower motor neurons causing relentless paralysis of voluntary muscles, whereas in FTD, progressive atrophy of the frontal and temporal lobes of the brain results in deterioration of cognitive functions, language, personality, and behavior. In contrast to Alzheimer's disease (AD), ALS and FTD still lack a specific neurochemical biomarker reflecting neuropathology ex vivo. However, in the past 10 years, considerable progress has been made in the characterization of neurofilament light chain (NFL) as cerebrospinal fluid (CSF) and blood biomarker for both diseases. NFL is a structural component of the axonal cytoskeleton and is released into the CSF as a consequence of axonal damage or degeneration, thus behaving in general as a relatively non-specific marker of neuroaxonal pathology. However, in ALS, the elevation of its CSF levels exceeds that observed in most other neurological diseases, making it useful for the discrimination from mimic conditions and potentially worthy of consideration for introduction into diagnostic criteria. Moreover, NFL correlates with disease progression rate and is negatively associated with survival, thus providing prognostic information. In FTD patients, CSF NFL is elevated compared with healthy individuals and, to a lesser extent, patients with other forms of dementia, but the latter difference is not sufficient to enable a satisfying diagnostic performance at individual patient level. However, also in FTD, CSF NFL correlates with several measures of disease severity. Due to technological progress, NFL can now be quantified also in peripheral blood, where it is present at much lower concentrations compared with CSF, thus allowing less invasive sampling, scalability, and longitudinal measurements. The latter has promoted innovative studies demonstrating longitudinal kinetics of NFL in presymptomatic individuals harboring gene mutations causing ALS and FTD. Especially in ALS, NFL levels are generally stable over time, which, together with their correlation with progression rate, makes NFL an ideal pharmacodynamic biomarker for therapeutic trials. In this review, we illustrate the significance of NFL as biomarker for ALS and FTD and discuss unsolved issues and potential for future developments.

Proximal weakness involvement in the first Italian case of Charcot-Marie-Tooth 2CC harboring a novel frameshift variant in NEFH

Charcot-Marie-Tooth (CMT) diseases are a clinically and genetically heterogeneous group of disorders. Different variants in the neurofilament heavy chain (NEFH) gene have been described to cause the CMT2CC subtype. Here we report the first Italian patient affected by CMT2CC, harboring a novel variant in NEFH. In describing our patient, we also reviewed previously CMT2CC individuals, and suggested to consider NEFH variant if patients have an axonal sensory-motor neuropathy with a prominent proximal muscles involvement with early requirement of walking aids or wheelchair, remembering a motor neuron disorder.

Retinal Spheroids and Axon Pathology Identified in Amyotrophic Lateral Sclerosis

Purpose

To determine whether patients with amyotrophic lateral sclerosis (ALS) show retinal axon pathology.

Methods

Postmortem eyes from 10 patients with ALS were sectioned and compared with 10 age-matched controls. Retinal sections were evaluated with periodic acid Schiff and phosphorylated (P-NF) and nonphosphorylated (NP-NF) forms of neurofilament with SMI 31 and 32 antibodies. Spheroids identified in the retinal nerve fiber layer were counted and their overall density was calculated in central, peripheral, and peripapillary regions. P-NF intensity was quantified. Morphometric features of ALS cases were compared with age-matched controls using the exact Wilcoxon matched-pairs signed-rank test.

Results

Distinct periodic acid Schiff–positive round profiles were identified in the retinal nerve fiber layer of patients with ALS and were most commonly observed in the peripapillary and peripheral retina. The density of periodic acid Schiff–positive spheroids was significantly greater in patients with ALS compared with controls (P = 0.027), with increased density in the peripapillary region (P = 0.047). Spheroids positive for P-NF and NP-NF were detected. P-NF–positive spheroid density was significantly increased in patients with ALS (P = 0.004), while the density of NP-NF spheroids did not differ significantly between ALS and control groups (P > 0.05). P-NF immunoreactivity in the retinal nerve fiber layer was significantly greater in patients with ALS than in controls (P = 0.002).

Conclusions

Retinal spheroids and axon pathology discovered in patients with ALS, similar to hallmark findings in spinal cord motor neurons, point to disrupted axon transport as a shared pathogenesis. Retinal manifestations detected in ALS suggest a novel biomarker detectable by noninvasive retinal imaging to help to diagnose and monitor ALS disease.

New advances in Amyotrophic Lateral Sclerosis genetics: Towards gene therapy opportunities for familial and young cases

Due to novel gene therapy opportunities, genetic screening is no longer restricted to familial cases of ALS (FALS) cases but also aplies to the sporadic populations (SALS). Screening of four main genes (C9orf72, SOD1, TARDBP and FUS) identified the causes in 15% of Amyotrophic Lateral Sclerosis (ALS) patients (two third of the familial cases and 8% of the sporadic ones) but their respective contribution to ALS phenotype varies according the age of disease onset. The genetic overlap between ALS and other diseases is expanding and includes frontotemporal dementia, Paget's Disease of Bone, myopathy for adult cases, HSP and CMT for young cases highlighing the importance of retrieving the exhaustive familial history for each indivdual with ALS. Incomplete disease penetrance, diversity of the possible phenotypes, as well as the lack of confidence concerning the pathogenicity of most identified variants and/or possible oligogenic inheritance are burdens of ALS genetic counseling to be delivered to patients and at risk individuals. The multitude of rare ALS genetic causes identifed seems to converge to similar cellular pathways leading to inapropriate response to stress emphacising new potential therapeutic options for the disease.

SOD1-related ALS with anticipation in a large family from Martinique

Amyotrophic Lateral Sclerosis (ALS) is a rare neurological disorder that causes degeneration of upper and lower motor neurons and their axons. ALS is mostly sporadic, but there are familial forms. In more than half of the familial forms, a pathogenic variant is found in one of the following genes: C9ORF72, SOD1, TDP-43, FUS, and VCP. SOD1 is the 2nd most common gene involved in genetic forms of ALS. Genotype-phenotype relationships are occasionally established in genetic forms of ALS associated with SOD1 mutations pathogenic variants. The c.281G > T (p.[G93V]) variant in SOD1 is associated with a rarely described and unexplained anticipation phenomenon. We report a large family from Martinique in whom ALS is associated with a c.281G > T (p.[G93V]) pathogenic variant in SOD1 and a statistically suggested anticipation. A whole-exome study and detection of CNVs (CoDESeq) from 3 affected members of this family revealed the presence of variants of uncertain signification (VUS) in other ALS genes. VUS in DCTN1 and NEFH were present in patients of the 2nd generation, and CNVs involving UBQLN2 and C21orf2 were found in the youngest case of the family.

Emerging Evidence Highlighting the Importance of Redox Dysregulation in the Pathogenesis of Amyotrophic Lateral Sclerosis (ALS)

The cellular redox state, or balance between cellular oxidation and reduction reactions, serves as a vital antioxidant defence system that is linked to all important cellular activities. Redox regulation is therefore a fundamental cellular process for aerobic organisms. Whilst oxidative stress is well described in neurodegenerative disorders including amyotrophic lateral sclerosis (ALS), other aspects of redox dysfunction and their contributions to pathophysiology are only just emerging. ALS is a fatal neurodegenerative disease affecting motor neurons, with few useful treatments. Hence there is an urgent need to develop more effective therapeutics in the future. Here, we discuss the increasing evidence for redox dysregulation as an important and primary contributor to ALS pathogenesis, which is associated with multiple disease mechanisms. Understanding the connection between redox homeostasis, proteins that mediate redox regulation, and disease pathophysiology in ALS, may facilitate a better understanding of disease mechanisms, and lead to the design of better therapeutic strategies.

Potential roles of stem cell marker genes in axon regeneration

Axon regeneration is orchestrated by many genes that are differentially expressed in response to injury. Through a comparative analysis of gene expression profiling, injury-responsive genes that are potential targets for understanding the mechanisms underlying regeneration have been revealed. As the efficiency of axon regeneration in both the peripheral and central nervous systems can be manipulated, we suggest that identifying regeneration-associated genes is a promising approach for developing therapeutic applications in vivo. Here, we review the possible roles of stem cell marker- or stemness-related genes in axon regeneration to gain a better understanding of the regeneration mechanism and to identify targets that can enhance regenerative capacity.

Neurofilaments: neurobiological foundations for biomarker applications

Interest in neurofilaments has risen sharply in recent years with recognition of their potential as biomarkers of brain injury or neurodegeneration in CSF and blood. This is in the context of a growing appreciation for the complexity of the neurobiology of neurofilaments, new recognition of specialized roles for neurofilaments in synapses and a developing understanding of mechanisms responsible for their turnover. Here we will review the neurobiology of neurofilament proteins, describing current understanding of their structure and function, including recently discovered evidence for their roles in synapses. We will explore emerging understanding of the mechanisms of neurofilament degradation and clearance and review new methods for future elucidation of the kinetics of their turnover in humans. Primary roles of neurofilaments in the pathogenesis of human diseases will be described. With this background, we then will review critically evidence supporting use of neurofilament concentration measures as biomarkers of neuronal injury or degeneration. Finally, we will reflect on major challenges for studies of the neurobiology of intermediate filaments with specific attention to identifying what needs to be learned for more precise use and confident interpretation of neurofilament measures as biomarkers of neurodegeneration.

Canadian Pioneers of Amyotrophic Lateral Sclerosis: A Brief History

This is an historical account of Canadian pioneers working in amyotrophic lateral sclerosis (ALS) in the 1970s and 1980s. Key contributions included the development of specialized clinics, the ALS Society of Canada, human motor unit estimates in vivo, use of transcranial magnetic stimulation (TMS), the dementias of ALS, the importance of neurofilaments and axonal flow, neuroinflammation and immunity related to ALS, use of tissue culture to study pathogenesis, and the story of ALS in Guam. Their work set the stage for future generations of ALS physicians and scientists to bring about meaningful therapies and hopefully a cure for ALS.

Pathogenic Genome Signatures That Damage Motor Neurons in Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is the most frequent motor neuron disease and a neurodegenerative disorder, affecting the upper and/or lower motor neurons. Notably, it invariably leads to death within a few years of onset. Although most ALS cases are sporadic, familial amyotrophic lateral sclerosis (fALS) forms 10% of the cases. In 1993, the first causative gene (SOD1) of fALS was identified. With rapid advances in genetics, over fifty potentially causative or disease-modifying genes have been found in ALS so far. Accordingly, routine diagnostic tests should encompass the oldest and most frequently mutated ALS genes as well as several new important genetic variants in ALS. Herein, we discuss current literatures on the four newly identified ALS-associated genes (CYLD, S1R, GLT8D1, and KIF5A) and the previously well-known ALS genes including SOD1, TARDBP, FUS, and C9orf72. Moreover, we review the pathogenic implications and disease mechanisms of these genes. Elucidation of the cellular and molecular functions of the mutated genes will bring substantial insights for the development of therapeutic approaches to treat ALS.

Blood Neurofilament Light Chain: The Neurologist's Troponin?

Blood neurofilament light chain (NfL) is a marker of neuro-axonal injury showing promising associations with outcomes of interest in several neurological conditions. Although initially discovered and investigated in the cerebrospinal fluid (CSF), the recent development of ultrasensitive digital immunoassay technologies has enabled reliable detection in serum/plasma, obviating the need for invasive lumbar punctures for longitudinal assessment. The most evidence for utility relates to multiple sclerosis (MS) where it serves as an objective measure of both the inflammatory and degenerative pathologies that characterise this disease. In this review, we summarise the physiology and pathophysiology of neurofilaments before focusing on the technological advancements that have enabled reliable quantification of NfL in blood. As the test case for clinical translation, we then highlight important recent developments linking blood NfL levels to outcomes in MS and the next steps to be overcome before this test is adopted on a routine clinical basis.

The Neglected Genes of ALS: Cytoskeletal Dynamics Impact Synaptic Degeneration in ALS

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease that selectively affects motor neurons (MNs) of the cortex, brainstem, and spinal cord. Several genes have been linked to both familial (fALS) and sporadic (sALS) cases of ALS. Among all the ALS-related genes, a group of genes known to directly affect cytoskeletal dynamics (ALS2, DCTN1, PFN1, KIF5A, NF-L, NF-H, PRPH, SPAST, and TUBA4A) is of high importance for MN health and survival, considering that MNs are large polarized cells with axons that can reach up to 1 m in length. In particular, cytoskeletal dynamics facilitate the transport of organelles and molecules across the long axonal distances within the cell, playing a key role in synapse maintenance. The majority of ALS-related genes affecting cytoskeletal dynamics were identified within the past two decades, making it a new area to explore for ALS. The purpose of this review is to provide insights into ALS-associated cytoskeletal genes and outline how recent studies have pointed towards novel pathways that might be impacted in ALS. Further studies making use of extensive analysis models to look for true hits, the newest technologies such as CRIPSR/Cas9, human induced pluripotent stem cells (iPSCs) and axon sequencing, as well as the development of more transgenic animal models could potentially help to: differentiate the variants that truly act as a primary cause of the disease from the ones that act as risk factors or disease modifiers, identify potential interactions between two or more ALS-related genes in disease onset and progression and increase our understanding of the molecular mechanisms leading to cytoskeletal defects. Altogether, this information will give us a hint on the real contribution of the cytoskeletal ALS-related genes during this lethal disease.

Blood neurofilament light: a critical review of its application to neurologic disease

Neuronal injury is a universal event that occurs in disease processes that affect both the central and peripheral nervous systems. A blood biomarker linked to neuronal injury would provide a critical measure to understand and treat neurologic diseases. Neurofilament light chain (NfL), a cytoskeletal protein expressed only in neurons, has emerged as such a biomarker. With the ability to quantify neuronal damage in blood, NfL is being applied to a wide range of neurologic conditions to investigate and monitor disease including assessment of treatment efficacy. Blood NfL is not specific for one disease and its release can also be induced by physiological processes. Longitudinal studies in multiple sclerosis, traumatic brain injury, and stroke show accumulation of NfL over days followed by elevated levels over months. Therefore, it may be hard to determine with a single measurement when the peak of NfL is reached and when the levels are normalized. Nonetheless, measurement of blood NfL provides a new blood biomarker for neurologic diseases overcoming the invasiveness of CSF sampling that restricted NfL clinical application. In this review, we examine the use of blood NfL as a biologic test for neurologic disease.

High-Throughput Genetic Testing in ALS: The Challenging Path of Variant Classification Considering the ACMG Guidelines

The development of high-throughput sequencing technologies and screening of big patient cohorts with familial and sporadic amyotrophic lateral sclerosis (ALS) led to the identification of a significant number of genetic variants, which are sometimes difficult to interpret. The American College of Medical Genetics and Genomics (ACMG) provided guidelines to help molecular geneticists and pathologists to interpret variants found in laboratory testing. We assessed the application of the ACMG criteria to ALS-related variants, combining data from literature with our experience. We analyzed a cohort of 498 ALS patients using massive parallel sequencing of ALS-associated genes and identified 280 variants with a minor allele frequency < 1%. Examining all variants using the ACMG criteria, thus considering the type of variant, inheritance, familial segregation, and possible functional studies, we classified 20 variants as “pathogenic”. In conclusion, ALS’s genetic complexity, such as oligogenic inheritance, presence of genes acting as risk factors, and reduced penetrance, needs to be considered when interpreting variants. The goal of this work is to provide helpful suggestions to geneticists and clinicians dealing with ALS.

Anterograde Axonal Transport in Neuronal Homeostasis and Disease

Neurons are highly polarized cells with an elongated axon that extends far away from the cell body. To maintain their homeostasis, neurons rely extensively on axonal transport of membranous organelles and other molecular complexes. Axonal transport allows for spatio-temporal activation and modulation of numerous molecular cascades, thus playing a central role in the establishment of neuronal polarity, axonal growth and stabilization, and synapses formation. Anterograde and retrograde axonal transport are supported by various molecular motors, such as kinesins and dynein, and a complex microtubule network. In this review article, we will primarily discuss the molecular mechanisms underlying anterograde axonal transport and its role in neuronal development and maturation, including the establishment of functional synaptic connections. We will then provide an overview of the molecular and cellular perturbations that affect axonal transport and are often associated with axonal degeneration. Lastly, we will relate our current understanding of the role of axonal trafficking concerning anterograde trafficking of mRNA and its involvement in the maintenance of the axonal compartment and disease.

Molecular and Cellular Mechanisms Affected in ALS

Amyotrophic lateral sclerosis (ALS) is a terminal late-onset condition characterized by the loss of upper and lower motor neurons. Mutations in more than 30 genes are associated to the disease, but these explain only ~20% of cases. The molecular functions of these genes implicate a wide range of cellular processes in ALS pathology, a cohesive understanding of which may provide clues to common molecular mechanisms across both familial (inherited) and sporadic cases and could be key to the development of effective therapeutic approaches. Here, the different pathways that have been investigated in ALS are summarized, discussing in detail: mitochondrial dysfunction, oxidative stress, axonal transport dysregulation, glutamate excitotoxicity, endosomal and vesicular transport impairment, impaired protein homeostasis, and aberrant RNA metabolism. This review considers the mechanistic roles of ALS-associated genes in pathology, viewed through the prism of shared molecular pathways.

The Role of Sex and Sex Hormones in Neurodegenerative Diseases

Neurodegenerative diseases (NDs) are a wide class of disorders of the central nervous system (CNS) with unknown etiology. Several factors were hypothesized to be involved in the pathogenesis of these diseases, including genetic and environmental factors. Many of these diseases show a sex prevalence and sex steroids were shown to have a role in the progression of specific forms of neurodegeneration. Estrogens were reported to be neuroprotective through their action on cognate nuclear and membrane receptors, while adverse effects of male hormones have been described on neuronal cells, although some data also suggest neuroprotective activities. The response of the CNS to sex steroids is a complex and integrated process that depends on (i) the type and amount of the cognate steroid receptor and (ii) the target cell type-either neurons, glia, or microglia. Moreover, the levels of sex steroids in the CNS fluctuate due to gonadal activities and to local metabolism and synthesis. Importantly, biochemical processes involved in the pathogenesis of NDs are increasingly being recognized as different between the two sexes and as influenced by sex steroids. The aim of this review is to present current state-of-the-art understanding on the potential role of sex steroids and their receptors on the onset and progression of major neurodegenerative disorders, namely, Alzheimer's disease, Parkinson's diseases, amyotrophic lateral sclerosis, and the peculiar motoneuron disease spinal and bulbar muscular atrophy, in which hormonal therapy is potentially useful as disease modifier.

Individual-Level Identification of Gene Expression Associated with Volume Differences among Neocortical Areas

The human cerebral cortex is the source of many complex behaviors and is a vulnerable target of various neuropsychiatric disorders, but transcriptional profiles linked to cerebral cortical volume (CCV) differences across brain areas remain unknown. Here, we screened CCV-related genes using an across-sample spatial correlation analysis in 6 postmortem brains and then individually validated these correlations in 1091 subjects with different ages and ethnicities. We identified 62 genes whose transcriptional profiles were repeatedly associated with CCV in more than 90% of individuals. CCV-related genes were specifically expressed in neurons and in developmental periods from middle childhood to young adulthood, were enriched in ion channels and developmental processes, and showed significant overlap with genes linked to brain functional activity and mental disorders. The identified genes represent the conserved transcriptional architecture of the human cerebral cortex, suggesting a link between conserved gene transcription and neocortical structural properties.

Overlapping spectrums: The clinicogenetic commonalities between Charcot-Marie-Tooth and other neurodegenerative diseases

Charcot-Marie-Tooth (CMT) disease is a progressive and heterogeneous inherited peripheral neuropathy. A myriad of genetic factors have been identified that contribute to the degeneration of motor and sensory axons in a length-dependent manner. Emerging biological themes underlying disease include defects in axonal trafficking, dysfunction in RNA metabolism and protein homeostasis, as well deficits in the cellular stress response. Moreover, genetic contributions to CMT can have overlap with other neuropathies, motor neuron diseases (MNDs) and neurodegenerative disorders. Recent progress in understanding the molecular biology of CMT and overlapping syndromes aids in the search for necessary therapeutic targets.

ALS Genetics, Mechanisms, and Therapeutics: Where Are We Now?

The scientific landscape surrounding amyotrophic lateral sclerosis (ALS) continues to shift as the number of genes associated with the disease risk and pathogenesis, and the cellular processes involved, continues to grow. Despite decades of intense research and over 50 potentially causative or disease-modifying genes identified, etiology remains unexplained and treatment options remain limited for the majority of ALS patients. Various factors have contributed to the slow progress in understanding and developing therapeutics for this disease. Here, we review the genetic basis of ALS, highlighting factors that have contributed to the elusiveness of genetic heritability. The most commonly mutated ALS-linked genes are reviewed with an emphasis on disease-causing mechanisms. The cellular processes involved in ALS pathogenesis are discussed, with evidence implicating their involvement in ALS summarized. Past and present therapeutic strategies and the benefits and limitations of the model systems available to ALS researchers are discussed with future directions for research that may lead to effective treatment strategies outlined.

Comprehensive Genetic Analysis of a Hungarian Amyotrophic Lateral Sclerosis Cohort

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease characterized by the degeneration of motor neurons. Genetic factors play a key role in ALS, and identifying variants that contribute to ALS susceptibility is an important step toward understanding the etiology of the disease. The frequency of protein altering variants in ALS patients has been extensively investigated in populations of different ethnic origin. To further delineate the genetic architecture of the Hungarian ALS patients, we aimed to detect potentially damaging variants in major and minor ALS genes and in genes related to other neurogenetic disorders. A combination of repeat-sizing of C9orf72 and next-generation sequencing (NGS) was used to comprehensively assess genetic variations in 107 Hungarian patients with ALS. Variants in major ALS genes were detected in 36.45% of patients. As a result of repeat sizing, pathogenic repeat expansions in the C9orf72 gene were detected in 10 patients (9.3%). According to the NGS results, the most frequently mutated genes were NEK1 (5.6%), NEFH, SQSTM1 (3.7%), KIF5A, SPG11 (2.8%), ALS2, CCNF, FUS, MATR3, TBK1, and UBQLN2 (1.9%). Furthermore, potentially pathogenic variants were found in GRN and SIGMAR1 genes in single patients. Additional 33 novel or rare known variants were detected in minor ALS genes, as well as 48 variants in genes previously linked to other neurogenetic disorders. The latter finding supports the hypothesis that common pathways in different neurodegenerative diseases may contribute to the development of ALS. While the disease-causing role of several variants identified in this study has previously been established, other variants may show reduced penetrance or may be rare benign variants. Our findings highlight the necessity for large-scale multicenter studies on ALS patients to gain a more accurate view of the genetic pattern of ALS.

Antigenic Targets of Patient and Maternal Autoantibodies in Autism Spectrum Disorder

Autism spectrum disorder (ASD) is a neurodevelopmental disorder whose behavioral symptoms become apparent in early childhood. The underlying pathophysiological mechanisms are only partially understood and the clinical manifestations are heterogeneous in nature, which poses a major challenge for diagnosis, prognosis and intervention. In the last years, an important role of a dysregulated immune system in ASD has emerged, but the mechanisms connecting this to a disruption of brain development are still largely unknown. Although ASD is not considered as a typical autoimmune disease, self-reactive antibodies or autoantibodies against a wide variety of targets have been found in a subset of ASD patients. In addition, autoantibodies reactive to fetal brain proteins have also been described in the prenatal stage of neurodevelopment, where they can be transferred from the mother to the fetus by transplacental transport. In this review, we give an extensive overview of the antibodies described in ASD according to their target antigens, their different origins, and timing of exposure during neurodevelopment.

Hot-spot KIF5A mutations cause familial ALS

Heterozygous missense mutations in the N-terminal motor or coiled-coil domains of the kinesin family member 5A (KIF5A) gene cause monogenic spastic paraplegia (HSP10) and Charcot-Marie-Tooth disease type 2 (CMT2). Moreover, heterozygous de novo frame-shift mutations in the C-terminal domain of KIF5A are associated with neonatal intractable myoclonus, a neurodevelopmental syndrome. These findings, together with the observation that many of the disease genes associated with amyotrophic lateral sclerosis disrupt cytoskeletal function and intracellular transport, led us to hypothesize that mutations in KIF5A are also a cause of amyotrophic lateral sclerosis. Using whole exome sequencing followed by rare variant analysis of 426 patients with familial amyotrophic lateral sclerosis and 6137 control subjects, we detected an enrichment of KIF5A splice-site mutations in amyotrophic lateral sclerosis (2/426 compared to 0/6137 in controls; P = 4.2 × 10⁻³), both located in a hot-spot in the C-terminus of the protein and predicted to affect splicing exon 27. We additionally show co-segregation with amyotrophic lateral sclerosis of two canonical splice-site mutations in two families. Investigation of lymphoblast cell lines from patients with KIF5A splice-site mutations revealed the loss of mutant RNA expression and suggested haploinsufficiency as the most probable underlying molecular mechanism. Furthermore, mRNA sequencing of a rare non-synonymous missense mutation (predicting p.Arg1007Gly) located in the C-terminus of the protein shortly upstream of the splice donor of exon 27 revealed defective KIF5A pre-mRNA splicing in respective patient-derived cell lines owing to abrogation of the donor site. Finally, the non-synonymous single nucleotide variant rs113247976 (minor allele frequency = 1.00% in controls, n = 6137), also located in the C-terminal region [p.(Pro986Leu) in exon 26], was significantly enriched in familial amyotrophic lateral sclerosis patients (minor allele frequency = 3.40%; P = 1.28 × 10⁻⁷). Our study demonstrates that mutations located specifically in a C-terminal hotspot of KIF5A can cause a classical amyotrophic lateral sclerosis phenotype, and underline the involvement of intracellular transport processes in amyotrophic lateral sclerosis pathogenesis.

Motor Neuron Susceptibility in ALS/FTD

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by the death of both upper and lower motor neurons (MNs) in the brain, brainstem and spinal cord. The neurodegenerative mechanisms leading to MN loss in ALS are not fully understood. Importantly, the reasons why MNs are specifically targeted in this disorder are unclear, when the proteins associated genetically or pathologically with ALS are expressed ubiquitously. Furthermore, MNs themselves are not affected equally; specific MNs subpopulations are more susceptible than others in both animal models and human patients. Corticospinal MNs and lower somatic MNs, which innervate voluntary muscles, degenerate more readily than specific subgroups of lower MNs, which remain resistant to degeneration, reflecting the clinical manifestations of ALS. In this review, we discuss the possible factors intrinsic to MNs that render them uniquely susceptible to neurodegeneration in ALS. We also speculate why some MN subpopulations are more vulnerable than others, focusing on both their molecular and physiological properties. Finally, we review the anatomical network and neuronal microenvironment as determinants of MN subtype vulnerability and hence the progression of ALS.

The Peripheral Nervous System in Amyotrophic Lateral Sclerosis: Opportunities for Translational Research

Although amyotrophic lateral sclerosis (ALS) has been considered as a disorder of the motor neuron (MN) cell body, recent evidences show the non-cell-autonomous pathogenic nature of the disease. Axonal degeneration, loss of peripheral axons and destruction of nerve terminals are early events in the disease pathogenic cascade, anticipating MN degeneration, and the onset of clinical symptoms. Therefore, although ALS and peripheral axonal neuropathies should be differentiated in clinical practice, they also share damage to common molecular pathways, including axonal transport, RNA metabolism and proteostasis. Thus, an extensive evaluation of the molecular events occurring in the peripheral nervous system (PNS) could be fundamental to understand the pathogenic mechanisms of ALS, favoring the discovery of potential disease biomarkers, and new therapeutic targets.

A Systematic Review of Suggested Molecular Strata, Biomarkers and Their Tissue Sources in ALS

Amyotrophic lateral sclerosis (ALS), also known as motor neuron disease, is an incurable neurodegenerative condition, characterized by the loss of upper and lower motor neurons. It affects 1–1.8/100,000 individuals worldwide, and the number of cases is projected to increase as the population ages. Thus, there is an urgent need to identify both therapeutic targets and disease-specific biomarkers–biomarkers that would be useful to diagnose and stratify patients into different sub-groups for therapeutic strategies, as well as biomarkers to follow the efficacy of any treatment tested during clinical trials. There is a lack of knowledge about pathogenesis and many hypotheses. Numerous “omics” studies have been conducted on ALS in the past decade to identify a disease-signature in tissues and circulating biomarkers. The first goal of the present review was to group the molecular pathways that have been implicated in monogenic forms of ALS, to enable the description of patient strata corresponding to each pathway grouping. This strategy allowed us to suggest 14 strata, each potentially targetable by different pharmacological strategies. The second goal of this review was to identify diagnostic/prognostic biomarker candidates consistently observed across the literature. For this purpose, we explore previous biomarker-relevant “omics” studies of ALS and summarize their findings, focusing on potential circulating biomarker candidates. We systematically review 118 papers on biomarkers published during the last decade. Several candidate markers were consistently shared across the results of different studies in either cerebrospinal fluid (CSF) or blood (leukocyte or serum/plasma). Although these candidates still need to be validated in a systematic manner, we suggest the use of combinations of biomarkers that would likely reflect the “health status” of different tissues, including motor neuron health (e.g., pNFH and NF-L, cystatin C, Transthyretin), inflammation status (e.g., MCP-1, miR451), muscle health (miR-338-3p, miR-206) and metabolism (homocysteine, glutamate, cholesterol). In light of these studies and because ALS is increasingly perceived as a multi-system disease, the identification of a panel of biomarkers that accurately reflect features of pathology is a priority, not only for diagnostic purposes but also for prognostic or predictive applications.

Rab-dependent cellular trafficking and amyotrophic lateral sclerosis

Rab GTPases are becoming increasingly implicated in neurodegenerative disorders, although their role in amyotrophic lateral sclerosis (ALS) has been somewhat overlooked. However, dysfunction of intracellular transport is gaining increasing attention as a pathogenic mechanism in ALS. Many previous studies have focused axonal trafficking, and the extreme length of axons in motor neurons may contribute to their unique susceptibility in this disorder. In contrast, the role of transport defects within the cell body has been relatively neglected. Similarly, whilst Rab GTPases control all intracellular membrane trafficking events, their role in ALS is poorly understood. Emerging evidence now highlights this family of proteins in ALS, particularly the discovery that C9orf72 functions in intra transport in conjunction with several Rab GTPases. Here, we summarize recent updates on cellular transport defects in ALS, with a focus on Rab GTPases and how their dysfunction may specifically target neurons and contribute to pathophysiology. We discuss the molecular mechanisms associated with dysfunction of Rab proteins in ALS. Finally, we also discuss dysfunction in other modes of transport recently implicated in ALS, including nucleocytoplasmic transport and the ER-mitochondrial contact regions (MAM compartment), and speculate whether these may also involve Rab GTPases.

Meta-analysis of Genetic Modifiers Reveals Candidate Dysregulated Pathways in Amyotrophic Lateral Sclerosis

Amyotrophic Lateral Sclerosis (ALS) is a neurodegenerative disease that has significant overlap with frontotemporal dementia (FTD). Mutations in specific genes have been identified that can cause and/or predispose patients to ALS. However, the clinical variability seen in ALS patients suggests that additional genes impact pathology, susceptibility, severity, and/or progression of the disease. To identify molecular pathways involved in ALS, we undertook a meta-analysis of published genetic modifiers both in patients and in model organisms, and undertook bioinformatic pathway analysis. From 72 published studies, we generated a list of 946 genes whose perturbation (1) impacted ALS in patient populations, (2) altered defects in laboratory models, or (3) modified defects caused by ALS gene ortholog loss of function. Herein, these are all called modifier genes. We found 727 modifier genes that encode proteins with human orthologs. Of these, 43 modifier genes were identified as modifiers of more than one ALS gene/model, consistent with the hypothesis that shared genes and pathways may underlie ALS. Further, we used a gene ontology-based bioinformatic analysis to identify pathways and associated genes that may be important in ALS. To our knowledge this is the first comprehensive survey of ALS modifier genes. This work suggests that shared molecular mechanisms may underlie pathology caused by different ALS disease genes. Surprisingly, few ALS modifier genes have been tested in more than one disease model. Understanding genes that modify ALS-associated defects will help to elucidate the molecular pathways that underlie ALS and provide additional targets for therapeutic intervention.