The occurrence of mutations in FUS in a Belgian cohort of patients with familial ALS

A microfluidic approach to rescue ALS motor neuron degeneration using rapamycin

TAR DNA-binding protein-43 (TDP-43) is known to accumulate in ubiquitinated inclusions of amyotrophic lateral sclerosis affected motor neurons, resulting in motor neuron degeneration, loss of motor functions, and eventually death. Rapamycin, an mTOR inhibitor and a commonly used immunosuppressive drug, has been shown to increase the survivability of Amyotrophic Lateral Sclerosis (ALS) affected motor neurons. Here we present a transgenic, TDP-43-A315T, mouse model expressing an ALS phenotype and demonstrate the presence of ubiquitinated cytoplasmic TDP-43 aggregates with > 80% cell death by 28 days post differentiation in vitro. Embryonic stem cells from this mouse model were used to study the onset, progression, and therapeutic remediation of TDP-43 aggregates using a novel microfluidic rapamycin concentration gradient generator. Results using a microfluidic device show that ALS affected motor neuron survival can be increased by 40.44% in a rapamycin dosage range between 0.4-1.0 µM.

The multifaceted role of kinases in amyotrophic lateral sclerosis: genetic, pathological and therapeutic implications

Amyotrophic lateral sclerosis is the most common degenerative disorder of motor neurons in adults. As there is no cure, thousands of individuals who are alive at present will succumb to the disease. In recent years, numerous causative genes and risk factors for amyotrophic lateral sclerosis have been identified. Several of the recently identified genes encode kinases. In addition, the hypothesis that (de)phosphorylation processes drive the disease process resulting in selective motor neuron degeneration in different disease variants has been postulated. We re-evaluate the evidence for this hypothesis based on recent findings and discuss the multiple roles of kinases in amyotrophic lateral sclerosis pathogenesis. We propose that kinases could represent promising therapeutic targets. Mainly due to the comprehensive regulation of kinases, however, a better understanding of the disturbances in the kinome network in amyotrophic lateral sclerosis is needed to properly target specific kinases in the clinic.

A Systematic Review of Genotype-Phenotype Correlation across Cohorts Having Causal Mutations of Different Genes in ALS

Amyotrophic lateral sclerosis is a rare and fatal neurodegenerative disease characterised by progressive deterioration of upper and lower motor neurons that eventually culminates in severe muscle atrophy, respiratory failure and death. There is a concerning lack of understanding regarding the mechanisms that lead to the onset of ALS and as a result there are no reliable biomarkers that aid in the early detection of the disease nor is there an effective treatment. This review first considers the clinical phenotypes associated with ALS, and discusses the broad categorisation of ALS and ALS-mimic diseases into upper and lower motor neuron diseases, before focusing on the genetic aetiology of ALS and considering the potential relationship of mutations of different genes to variations in phenotype. For this purpose, a systematic review is conducted collating data from 107 original published clinical studies on monogenic forms of the disease, surveying the age and site of onset, disease duration and motor neuron involvement. The collected data highlight the complexity of the disease's genotype-phenotype relationship, and thus the need for a nuanced approach to the development of clinical assays and therapeutics.

Phenotypes and malignancy risk of different FUS mutations in genetic amyotrophic lateral sclerosis

OBJECTIVE

Mutations in Fused in Sarcoma (FUS or TLS) are the fourth most prevalent in Western European familial amyotrophic lateral sclerosis (ALS) populations and have been associated with causing both early and very late disease onset. FUS aggregation, DNA repair deficiency, and genomic instability are contributors to the pathophysiology of FUS-ALS, but their clinical significance per se and their influence on the clinical variability have yet to be sufficiently investigated. The aim of this study was to analyze genotype-phenotype correlations and malignancy rates in a newly compiled FUS-ALS cohort.

METHODS

We cross-sectionally reviewed FUS-ALS patient histories in a multicenter cohort with 36 novel cases and did a meta-analysis of published FUS-ALS cases reporting the largest genotype-phenotype correlation of FUS-ALS.

RESULTS

The age of onset (median 39 years, range 11-80) was positively correlated with the disease duration. C-terminal domain mutations were found in 90%. Among all, P525L and truncating/ frameshift mutations most frequently caused juvenile onset, rapid disease progression, and atypical ALS often associated with negative family history while the R521 mutation site was associated with late disease onset and pure spinal phenotype. Malignancies were found in one of 40 patients.

INTERPRETATION

We report the largest genotype-phenotype correlation of FUS-ALS, which enables a careful prediction of the clinical course in newly diagnosed patients. In this cohort, FUS-ALS patients did not have an increased risk for malignant diseases.

Fused in Sarcoma: Properties, Self-Assembly and Correlation with Neurodegenerative Diseases

Fused in sarcoma (FUS) is a DNA/RNA binding protein that is involved in RNA metabolism and DNA repair. Numerous reports have demonstrated by pathological and genetic analysis that FUS is associated with a variety of neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS), frontotemporal lobar degeneration (FTLD), and polyglutamine diseases. Traditionally, the fibrillar aggregation of FUS was considered to be the cause of those diseases, especially via its prion-like domains (PrLDs), which are rich in glutamine and asparagine residues. Lately, a nonfibrillar self-assembling phenomenon, liquid–liquid phase separation (LLPS), was observed in FUS, and studies of its functions, mechanism, and mutual transformation with pathogenic amyloid have been emerging. This review summarizes recent studies on FUS self-assembling, including both aggregation and LLPS as well as their relationship with the pathology of ALS, FTLD, and other neurodegenerative diseases.

FUS-induced neurotoxicity in Drosophila is prevented by downregulating nucleocytoplasmic transport proteins

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are neurodegenerative diseases characterized by the progressive loss of specific groups of neurons. Due to clinical, genetic and pathological overlap, both diseases are considered as the extremes of one disease spectrum and in a number of ALS and FTD patients, fused in sarcoma (FUS) aggregates are present. Even in families with a monogenetic disease cause, a striking variability is observed in disease presentation. This suggests the presence of important modifying genes. The identification of disease-modifying genes will contribute to defining clear therapeutic targets and to understanding the pathways involved in motor neuron death. In this study, we established a novel in vivo screening platform in which new modifying genes of FUS toxicity can be identified. Expression of human FUS induced the selective apoptosis of crustacean cardioactive peptide (CCAP) neurons from the ventral nerve cord of fruit flies. No defects in the development of these neurons were observed nor were the regulatory CCAP neurons from the brain affected. We used the number of CCAP neurons from the ventral nerve cord as an in vivo read-out for FUS toxicity in neurons. Via a targeted screen, we discovered a potent modifying role of proteins involved in nucleocytoplasmic transport. Downregulation of Nucleoporin 154 and Exportin1 (XPO1) prevented FUS-induced neurotoxicity. Moreover, we show that XPO1 interacted with FUS. Silencing XPO1 significantly reduced the propensity of FUS to form inclusions upon stress. Taken together, our findings point to an important role of nucleocytoplasmic transport proteins in FUS-induced ALS/FTD.

Screening of SOD1, FUS and TARDBP genes in patients with amyotrophic lateral sclerosis in central-southern China

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease affecting motor neurons of the brain, brainstem and spinal cord. To date, mutations in more than 30 genes have been linked to the pathogenesis of ALS. Among them, SOD1, FUS and TARDBP are ranked as the three most common genes associated with ALS. However, no mutation analysis has been reported in central-southern China. In this study, we sequenced SOD1, FUS and TARDBP in a central-southern Chinese cohort of 173 patients with ALS (15 familial ALS and 158 sporadic ALS) to detect mutations. As a result, five missense mutations in SOD1, namely, p.D101N, p.D101G, p.C111Y, p.N86S and p.V87A, were identified in three unrelated familial probands and three sporadic cases; two mutations in FUS were found in two unrelated familial probands, including an insertion mutation (p.P525_Y526insY) and a missense mutation (p.R521H); no variants of TARDBP were observed in patients. Therefore, SOD1 mutations were present in 20.0% of familial ALS patients and 1.9% of sporadic ALS patients, while FUS mutations were responsible for 13.3% of familial ALS cases, and TARDBP mutations were rare in either familial or sporadic ALS cases. This study broadens the known mutational spectrum in patients with ALS and further demonstrates the necessity for genetic screening in ALS patients from central-southern China.

R521C and R521H mutations in FUS result in weak binding with Karyopherinβ2 leading to Amyotrophic lateral sclerosis: a molecular docking and dynamics study

Fused in sarcoma (FUS) gene encodes the RNA binding protein FUS. This gene is mapped to chromosome 16p11.2. The FUS protein binds with karyopherineβ2 (Kapβ2) through its proline/tyrosine nuclear localization signal (PY-NLS) that helps in the localization of FUS protein within the nucleus. Arginine residue in 521 position (R521) of PY-NLS plays a vital role in the binding of FUS protein with Kapβ2. Mutations in this position (R521C and R521H) are the most predominant mutations associated with amyotrophic lateral sclerosis (ALS). However, the mechanism by which these mutations lead to ALS is poorly understood. We examined the binding behaviour of the mutants FUS (R521C) and FUS (R521H) with Kapβ2 through protein-protein docking and molecular dynamics simulation. The binding patterns of mutants were compared with the binding behaviour of wild FUS-Kapβ2. Our results suggest that these mutants have relatively weak binding affinity with Kapβ2 when compared with wild FUS-Kapβ2 as indicated by the lesser number of interactions found between the mutant FUS and Kapβ2. Hence, these mutations weakens the binding and this results in the cytoplasmic mislocalization of mutant FUS; and thereby it increases the severity of ALS.

Genotype-phenotype relationships in familial amyotrophic lateral sclerosis with FUS/TLS mutations in Japan

INTRODUCTION

We investigated possible genotype-phenotype correlations in Japanese patients with familial amyotrophic lateral sclerosis (FALS) carrying fused in sarcoma/translated in liposarcoma (FUS/TLS) gene mutations.

METHODS

A consecutive series of 111 Japanese FALS pedigrees were screened for copper/zinc superoxide dismutase 1 (SOD1) and FUS/TLS gene mutations. Clinical data, including onset age, onset site, disease duration, and extramotor symptoms, were collected.

RESULTS

Nine different FUS/TLS mutations were found in 12 pedigrees. Most of the patients with FUS/TLS-linked FALS demonstrated early onset in the brainstem/upper cervical region, and relatively short disease duration. A few mutations exhibited phenotypes that were distinct from typical cases. Frontotemporal dementia was present in 1 patient.

CONCLUSIONS

This study revealed a characteristic phenotype in FUS/TLS-linked FALS patients in Japan. FUS/TLS screening is recommended in patients with FALS with this phenotype. Muscle Nerve 54: 398-404, 2016.

Super-Resolution Microscopy Reveals Presynaptic Localization of the ALS/FTD Related Protein FUS in Hippocampal Neurons

Fused in Sarcoma (FUS) is a multifunctional RNA-/DNA-binding protein, which is involved in the pathogenesis of the neurodegenerative disorders amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). A common hallmark of these disorders is the abnormal accumulation of mutated FUS protein in the cytoplasm. Under normal conditions FUS is confined to the nuclear compartment, in neurons, however, additional somatodendritic localization can be observed. In this study, we carefully analyzed the subcellular localization of endogenous FUS at synaptic sites of hippocampal neurons which are among the most affected cell types in FTD with FUS pathology. We could confirm a strong nuclear localization of FUS as well as its prominent and widespread neuronal expression throughout the adult and developing rat brain, particularly in the hippocampus, the cerebellum and the outer layers of the cortex. Intriguingly, FUS was also consistently observed at synaptic sites as detected by neuronal subcellular fractionation as well as by immunolabeling. To define a pre- and/or postsynaptic localization of FUS, we employed super-resolution fluorescence localization microscopy. FUS was found to be localized within the axon terminal in close proximity to the presynaptic vesicle protein Synaptophysin1 and adjacent to the active zone protein Bassoon, but well separated from the postsynaptic protein PSD-95. Having shown the presynaptic localization of FUS in the nervous system, a novel extranuclear role of FUS at neuronal contact sites has to be considered. Since there is growing evidence that local presynaptic translation might also be an important mechanism for plasticity, FUS - like the fragile X mental retardation protein FMRP - might act as one of the presynaptic RNA-binding proteins regulating this machinery. Our observation of presynaptic FUS should foster further investigations to determine its role in neurodegenerative diseases such as ALS and FTD.

Aberrant RNA homeostasis in amyotrophic lateral sclerosis: potential for new therapeutic targets?

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder characterized by progressive motor neuron degeneration. The disease pathogenesis is multifaceted in that multiple cellular and molecular pathways have been identified as contributors to the disease progression. Consequently, numerous therapeutic targets have been pursued for clinical development, unfortunately with little success. The recent discovery of mutations in RNA modulating genes such as TARDBP/TDP-43, FUS/TLS or C9ORF72 changed our understanding of neurodegenerative mechanisms in ALS and introduced the role of dysfunctional RNA processing as a significant contributor to disease pathogenesis. This article discusses the latest findings on such RNA toxicity pathways in ALS and potential novel therapeutic approaches.

Clinical neurogenetics: amyotrophic lateral sclerosis

Our understanding of amyotrophic lateral sclerosis (ALS), a fatal neurodegenerative disease, is expanding rapidly as its genetic causes are uncovered. The pace of new gene discovery over the last 5 years has accelerated, providing new insights into the pathogenesis of disease and highlighting biological pathways as targets for therapeutic development. This article reviews our current understanding of the heritability of ALS and provides an overview of each of the major ALS genes, highlighting their phenotypic characteristics and frequencies as a guide for clinicians evaluating patients with ALS.

Tar DNA-binding protein-43 (TDP-43) regulates axon growth in vitro and in vivo

Intracellular inclusions of the TAR-DNA binding protein 43 (TDP-43) have been reported in amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD-TDP). Rare mutations in TARDBP have been linked to both ALS and FTD-TDP suggesting that TDP-43 dysfunction is mechanistic in causing disease. TDP-43 is a predominantly nuclear protein with roles in regulating RNA transcription, splicing, stability and transport. In ALS, TDP-43 aberrantly accumulates in the cytoplasm of motor neurons where it forms aggregates. However it has until recently been unclear whether the toxic effects of TDP-43 involve recruitment to motor axons, and what effects this might have on axonal growth and integrity. Here we use chick embryonic motor neurons, in vivo and in vitro, to model the acute effects of TDP-43. We show that wild-type and two TDP-43 mutant proteins cause toxicity in chick embryonic motor neurons in vivo. Moreover, TDP-43 is increasingly mislocalised to axons over time in vivo, axon growth to peripheral targets is truncated, and expression of neurofilament-associated antigen is reduced relative to control motor neurons. In primary spinal motor neurons in vitro, a progressive translocation of TDP-43 to the cytoplasm occurs over time, similar to that observed in vivo. This coincides with the appearance of cytoplasmic aggregates, a reduction in the axonal length, and cellular toxicity, which was most striking for neurons expressing TDP-43 mutant forms. These observations suggest that the capacity of spinal motor neurons to produce and maintain an axon is compromised by dysregulation of TDP-43 and that the disruption of cytoskeletal integrity may play a role in the pathogenesis of ALS and FTD-TDP.

Frequency of C9orf72 repeat expansions in amyotrophic lateral sclerosis: a Belgian cohort study

We determined the frequency of C9orf72 repeat expansions in a large cohort of Belgian patients with familial (fALS) and sporadic (sALS) amyotrophic lateral sclerosis (ALS). In total, 119 patients with fALS from 62 kindreds, 471 patients with sALS, and 384 control subjects were included. A C9orf72 repeat expansion was found in 32 of 62 fALS pedigrees (51.6%), in 45 of 471 patients with sALS (9.6%), but in none of the control subjects. Compared with fALS of unknown etiology or fALS caused by mutations in other ALS-causing genes, C9orf72 repeat expansion carriers had a later age at onset (57.3 vs. 51.4 years; p = 0.0061), a higher proportion of bulbar onset (31.9% vs. 12.5%, p < 0.0001), and a reduced survival (29.4 vs. 67.7 months, p = 0.0003). In the sALS cohort, there were no significant differences in these disease characteristics between the C9orf72 repeat expansion carriers and the noncarriers. C9orf72 repeat expansions are a frequent cause of ALS in Belgium, and also in sALS patients. These results might justify genetic testing of C9orf72 in all ALS patients.

ALS mutant FUS disrupts nuclear localization and sequesters wild-type FUS within cytoplasmic stress granules

Mutations in the gene encoding Fused in Sarcoma (FUS) cause amyotrophic lateral sclerosis (ALS), a fatal neurodegenerative disorder. FUS is a predominantly nuclear DNA- and RNA-binding protein that is involved in RNA processing. Large FUS-immunoreactive inclusions fill the perikaryon of surviving motor neurons of ALS patients carrying mutations at post-mortem. This sequestration of FUS is predicted to disrupt RNA processing and initiate neurodegeneration. Here, we demonstrate that C-terminal ALS mutations disrupt the nuclear localizing signal (NLS) of FUS resulting in cytoplasmic accumulation in transfected cells and patient fibroblasts. FUS mislocalization is rescued by the addition of the wild-type FUS NLS to mutant proteins. We also show that oxidative stress recruits mutant FUS to cytoplasmic stress granules where it is able to bind and sequester wild-type FUS. While FUS interacts with itself directly by protein-protein interaction, the recruitment of FUS to stress granules and interaction with PABP are RNA dependent. These findings support a two-hit hypothesis, whereby cytoplasmic mislocalization of FUS protein, followed by cellular stress, contributes to the formation of cytoplasmic aggregates that may sequester FUS, disrupt RNA processing and initiate motor neuron degeneration.

De novo FUS gene mutations are associated with juvenile-onset sporadic amyotrophic lateral sclerosis in China

Juvenile amyotrophic lateral sclerosis (ALS) is a rare form of motor neuron disease and occurs before 25 years of age. Only very few sporadic cases of juvenile-onset ALS have been reported. Rare SOD1 mutations and several FUS mutations have been identified in juvenile-onset ALS patients. To define the genetics of juvenile-onset sporadic ALS (SALS) of Chinese origin, we sequenced all 5 exons of SOD1, exons 3-6 and 12-15 of FUS in 11 juvenile-onset SALS patients, 105 adult-onset ALS patients (including 6 familial ALS [FALS] pedigrees), and 245 healthy controls. For the 11 juvenile-onset SALS and 6 FALS cases, the other 7 exons of FUS were also screened. A heterozygous de novo missense mutation c.1574C>T (p.P525L), a heterozygous de novo 2-base pair deletion c.1509_1510delAG (p.G504Wfs*12), and a nonsense mutation c.1483C>T (p.R495X) was each identified in 1 juvenile SALS patient. A heterozygous missense mutation c.1561C>G (p.R521G) was identified in a FALS proband. In the Chinese population, the frequency of FUS mutation in FALS is 11.4% (95% confidence interval [CI], 0.9%-22.0%), higher than the Japanese (10%; 95% CI, 0.7%-19.3%), and Caucasians (4.9%; 95% CI, 3.9%-6.0%). The frequency of FUS mutation in SALS patients is 1.5% (95% CI, 0.2%-2.9%), which is similar to Koreans (1.6%; 95% CI, 0%-3.2%), but higher than in Caucasians (0.6%; 95% CI, 0.4%-0.8%). Our findings suggest that de novo FUS mutations are associated with juvenile-onset SALS of Chinese origin and that this gene should be screened in ALS patients with a young age of onset, aggressive progression, and sporadic occurrence.

The complex molecular biology of amyotrophic lateral sclerosis (ALS)

Amyotrophic lateral sclerosis (ALS) is an adult-onset neurodegenerative disorder that causes selective death of motor neurons followed by paralysis and death. A subset of ALS cases is caused by mutations in the gene for Cu, Zn superoxide dismutase (SOD1), which impart a toxic gain of function to this antioxidant enzyme. This neurotoxic property is widely believed to stem from an increased propensity to misfold and aggregate caused by decreased stability of the native homodimer or a tendency to lose stabilizing posttranslational modifications. Study of the molecular mechanisms of SOD1-related ALS has revealed a complex array of interconnected pathological processes, including glutamate excitotoxicity, dysregulation of neurotrophic factors and axon guidance proteins, axonal transport defects, mitochondrial dysfunction, deficient protein quality control, and aberrant RNA processing. Many of these pathologies are directly exacerbated by misfolded and aggregated SOD1 and/or cytosolic calcium overload, suggesting the primacy of these events in disease etiology and their potential as targets for therapeutic intervention.

Mutant FUS induces endoplasmic reticulum stress in amyotrophic lateral sclerosis and interacts with protein disulfide-isomerase

Mutations in the gene encoding fused in sarcoma (FUS) are linked to amyotrophic lateral sclerosis (ALS), but the mechanisms by which these mutants trigger neurodegeneration remain unknown. Endoplasmic reticulum (ER) stress is increasingly recognized as an important and early pathway to motor neuron death in ALS. FUS is normally located in the nucleus but in ALS, FUS redistributes to the cytoplasm and forms inclusions. In this study, we investigated whether FUS induces ER stress in a motor neuron like cell line (NSC-34). We demonstrate that ER stress is triggered in cells expressing mutant FUS, and this is closely associated with redistribution of mutant FUS to the cytoplasm. Mutant FUS also colocalized with protein disulfide-isomerase (PDI), an important ER chaperone, in NSC-34 cells and PDI was colocalized with FUS inclusions in human ALS lumbar spinal cords, in both sporadic ALS and mutant FUS-linked familial ALS tissues. These findings implicate ER stress in the pathophysiology of FUS, and provide evidence for common pathogenic pathways in ALS linked to the ER.

Whole-genome sequencing reveals a coding non-pathogenic variant tagging a non-coding pathogenic hexanucleotide repeat expansion in C9orf72 as cause of amyotrophic lateral sclerosis

Motor neuron degeneration in amyotrophic lateral sclerosis (ALS) has a familial cause in 10% of patients. Despite significant advances in the genetics of the disease, many families remain unexplained. We performed whole-genome sequencing in five family members from a pedigree with autosomal-dominant classical ALS. A family-based elimination approach was used to identify novel coding variants segregating with the disease. This list of variants was effectively shortened by genotyping these variants in 2 additional unaffected family members and 1500 unrelated population-specific controls. A novel rare coding variant in SPAG8 on chromosome 9p13.3 segregated with the disease and was not observed in controls. Mutations in SPAG8 were not encountered in 34 other unexplained ALS pedigrees, including 1 with linkage to chromosome 9p13.2–23.3. The shared haplotype containing the SPAG8 variant in this small pedigree was 22.7 Mb and overlapped with the core 9p21 linkage locus for ALS and frontotemporal dementia. Based on differences in coverage depth of known variable tandem repeat regions between affected and non-affected family members, the shared haplotype was found to contain an expanded hexanucleotide (GGGGCC)_n repeat in C9orf72 in the affected members. Our results demonstrate that rare coding variants identified by whole-genome sequencing can tag a shared haplotype containing a non-coding pathogenic mutation and that changes in coverage depth can be used to reveal tandem repeat expansions. It also confirms (GGGGCC)n repeat expansions in C9orf72 as a cause of familial ALS.

From El Escorial to Awaji: where do we go next with the amyotrophic lateral sclerosis criteria?

SUMMARY Making an early and accurate diagnosis in amyotrophic lateral sclerosis is important for patients and their families and for entry in clinical trials. Amyotrophic lateral sclerosis remains a clinical diagnosis, requiring the presence of upper and lower motor neuron symptoms and signs in multiple body regions, in patients with a progressive disease course and after exclusion of other diseases that can mimic the clinical presentation. Research criteria have been developed to allow uniform diagnosis. The original El Escorial criteria have been revised twice to improve the sensitivity. In this report, the current scientific status of these criteria is reviewed and suggestions for further adaptations are made.

FUS-related proteinopathies: lessons from animal models

The recent identification of ALS-linked mutations in FUS and TDP-43 has led to a major shift in our thinking in regard to the potential molecular mechanisms of amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). RNA-mediated proteinopathy is increasingly being recognized as a potential cause of neurodegenerative disorders. FUS and TDP-43 are structurally and functionally similar proteins. FUS is a DNA/RNA binding protein that may regulate aspects of RNA metabolism, including splicing, mRNA processing, and micro RNA biogenesis. It is unclear how ALS-linked mutations perturb the functions of FUS. This review highlights recent advances in understanding the functions of FUS and discusses findings from FUS animal models that provide several key insights into understanding the molecular mechanisms that might contribute to ALS pathogenesis.

Proteostasis and movement disorders: Parkinson's disease and amyotrophic lateral sclerosis

Parkinson's disease (PD) is a movement disorder that afflicts over one million in the U.S.; amyotrophic lateral sclerosis (ALS or Lou Gehrig's disease) is less prevalent but also has a high incidence. The two disorders sometimes present together, making a comparative study of interest. Both ALS and PD are neurodegenerative diseases, and are characterized by the presence of intraneuronal inclusions; however, different classes of neurons are affected and the primary protein in the inclusions differs between the diseases, and in some cases is different in distinct forms of the same disease. These observations might suggest that the more general approach of proteostasis pathway alteration would be a powerful one in treating these disorders. Examining results from human genetics and studies in model organisms, as well as from biochemical and biophysical characterization of the proteins involved in both diseases, we find that most instances of PD can be considered as arising from the misfolding, and self-association to a toxic species, of the small neuronal protein α-synuclein, and that proteostasis strategies are likely to be of value for this disorder. For ALS, the situation is much more complex and less clear-cut; the available data are most consistent with a view that ALS may actually be a family of disorders, presenting similarly but arising from distinct and nonoverlapping causes, including mislocalization of some properly folded proteins and derangement of RNA quality control pathways. Applying proteostasis approaches to this disease may require rethinking or broadening the concept of what proteostasis means.

Emerging drugs for amyotrophic lateral sclerosis

INTRODUCTION

Amyotrophic lateral sclerosis (ALS) is a rapidly progressive neurodegenerative disease that results in increasing disability and that is uniformly fatal. Since its approval in the 1990s, riluzole remains the sole treatment for ALS offering modest survival benefit. While significant advances have been made in the symptomatic management of the disease, more effective drug therapy targeting disease progression is sorely needed.

AREAS COVERED

Advances in the understanding of pathogenic mechanisms involved in disease development and progression have provided multiple avenues for developing effective treatment strategies. This review highlights recent discoveries relating to these diverse mechanisms and their implications for the development of drug therapy. Previous human clinical trials that have targeted these pathways are mentioned and ongoing drug trials are discussed.

EXPERT OPINION

The search for effective drug therapy faces important challenges in the areas of basic science and animal research, translation of these results into human clinical trials, inherent bias in human studies and issues related to delays in clinical diagnosis. How these issues may be addressed and why ALS research constitutes fertile grounds for drug development not only for this devastating disease, but also for other more prevalent neurodegenerative diseases, is discussed in this review.

TDP-43 and FUS in amyotrophic lateral sclerosis and frontotemporal dementia

Abnormal intracellular protein aggregates comprise a key characteristic in most neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). The seminal discoveries of accumulation of TDP-43 in most cases of ALS and the most frequent form of FTD, frontotemporal lobar degeneration with ubiquitinated inclusions, followed by identification of FUS as the novel pathological protein in a small subset of patients with ALS and various FTD subtypes provide clear evidence that these disorders are related. The creation of a novel molecular classification of ALS and FTD based on the identity of the predominant protein abnormality has, therefore, been possible. The striking functional and structural similarities of TDP-43 and FUS, which are both DNA/RNA binding proteins, imply that abnormal RNA metabolism is a pivotal event, but the mechanisms leading to TDP-43 and FUS accumulation and the resulting neurodegeneration are currently unknown. Nonetheless, TDP-43 and FUS are promising candidates for the development of novel biomarker assays and targeted therapies.

Fus gene mutations in familial and sporadic amyotrophic lateral sclerosis

Mutations in the fused in sarcoma (FUS) gene have recently been found to cause familial amyotrophic lateral sclerosis (FALS). We screened FUS in a cohort of 200 ALS patients [32 FALS and 168 sporadic ALS (SALS)]. In one FALS proband, we identified a mutation (p.R521C) that was also present in her affected daughter. Their clinical phenotype was remarkably similar and atypical of classic ALS, with symmetric proximal pelvic and pectoral weakness. Distal weakness and upper motor neuron features only developed late. Neuropathological examination demonstrated FUS-immunoreactive neuronal and glial inclusions in the spinal cord and many extramotor regions, but no TDP-43 pathology. We also identified a novel mutation (p.G187S) in one SALS patient. Overall, FUS mutations accounted for 3% of our non-SOD1, non-TARDBP FALS cases and 0.6% of SALS. This study demonstrates that the phenotype with FUS mutations extends beyond classical ALS cases. Our findings suggest there are specific clinicogenetic correlations and provide the first detailed neuropathological description.

Mutant FUS proteins that cause amyotrophic lateral sclerosis incorporate into stress granules

Mutations in the RNA-binding protein FUS (fused in sarcoma) are linked to amyotrophic lateral sclerosis (ALS), but the mechanism by which these mutants cause motor neuron degeneration is not known. We report a novel ALS truncation mutant (R495X) that leads to a relatively severe ALS clinical phenotype compared with FUS missense mutations. Expression of R495X FUS, which abrogates a putative nuclear localization signal at the C-terminus of FUS, in HEK-293 cells and in the zebrafish spinal cord caused a striking cytoplasmic accumulation of the protein to a greater extent than that observed for recessive (H517Q) and dominant (R521G) missense mutants. Furthermore, in response to oxidative stress or heat shock conditions in cultures and in vivo, the ALS-linked FUS mutants, but not wild-type FUS, assembled into perinuclear stress granules in proportion to their cytoplasmic expression levels. These findings demonstrate a potential link between FUS mutations and cellular pathways involved in stress responses that may be relevant to altered motor neuron homeostasis in ALS.

De novo truncating FUS gene mutation as a cause of sporadic amyotrophic lateral sclerosis

Mutations in the gene encoding fused in sarcoma (FUS) were recently identified as a novel cause of amyotrophic lateral sclerosis (ALS), emphasizing the genetic heterogeneity of ALS. We sequenced the genes encoding superoxide dismutase (SOD1), TAR DNA-binding protein 43 (TARDBP) and FUS in 99 sporadic and 17 familial ALS patients ascertained at Mayo Clinic. We identified two novel mutations in FUS in two out of 99 (2.0%) sporadic ALS patients and established the de novo occurrence of one FUS mutation. In familial patients, we identified three (17.6%) SOD1 mutations, while FUS and TARDBP mutations were excluded. The de novo FUS mutation (g.10747A>G; IVS13-2A>G) affects the splice-acceptor site of FUS intron 13 and was shown to induce skipping of FUS exon 14 leading to the C-terminal truncation of FUS (p.G466VfsX14). Subcellular localization studies showed a dramatic increase in the cytoplasmic localization of FUS and a reduction of normal nuclear expression in cells transfected with truncated compared to wild-type FUS. We further identified a novel in-frame insertion/deletion mutation in FUS exon 12 (p.S402_P411delinsGGGG) which is predicted to expand a conserved poly-glycine motif. Our findings extend the mutation spectrum in FUS leading to ALS and describe the first de novo mutation in FUS.

Sustained expression of TDP-43 and FUS in motor neurons in rodent's lifetime

TAR DNA-binding protein (TDP-43) and fused in sarcoma (FUS) are two highly conserved ribonucleoproteins. Pathogenic mutations of the TDP-43 or the FUS gene are all linked to amyotrophic lateral sclerosis (ALS) that is characterized by progressive degeneration of motor neurons. To better understand the correlation of ALS disease genes with the selectivity of chronic motor neuron degeneration, we examined the longitudinal expression of the TDP-43 and the FUS genes in C57BL6 mice and in Sprague-Dawley rats. TDP-43 and FUS were robustly and ubiquitously expressed in the postnatal mice and rats, but were markedly decreased in the adult rodents. In adulthood, TDP-43 and FUS proteins were even undetectable in peripheral organs including skeletal muscles, liver, and kidney, but were constantly expressed at substantial levels in the central nervous system. Motor neurons expressed the TDP-43 and the FUS genes at robust levels throughout rodent's lifetime. Moreover, TDP-43 and FUS were accumulated in the cytoplasm of motor neurons in aged animals. Our findings suggest that TDP-43 and FUS play an important role in development and that constant and robust expression of the genes in motor neurons may render the neurons vulnerable to pathogenic mutation of the TDP-43 or the FUS gene. To faithfully model the pathology of TDP-43- or FUS gene mutations in rodents, we must replicate the expression patterns of the TDP-43 and the FUS gene in animals.

The neurobiology of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis is a degenerative disease affecting the motor neurons. In spite of our growing insights into its biology, it remains a lethal condition. The identification of the cause of several of the familial forms of ALS allowed generation of models to study this disease both in vitro and in vivo. Here, we summarize what is known about the pathogenic mechanisms of ALS induced by hereditary mutations, and attempt to identify the relevance of these findings for understanding the pathogenic mechanisms of the sporadic form of this disease.

TDP-43 and FUS/TLS: emerging roles in RNA processing and neurodegeneration

Amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD) are neurodegenerative diseases with clinical and pathological overlap. Landmark discoveries of mutations in the transactive response DNA-binding protein (TDP-43) and fused in sarcoma/translocated in liposarcoma (FUS/TLS) as causative of ALS and FTLD, combined with the abnormal aggregation of these proteins, have initiated a shifting paradigm for the underlying pathogenesis of multiple neurodegenerative diseases. TDP-43 and FUS/TLS are both RNA/DNA-binding proteins with striking structural and functional similarities. Their association with ALS and other neurodegenerative diseases is redirecting research efforts toward understanding the role of RNA processing regulation in neurodegeneration.