1. Top of page
  2. Abstract
  3. Biological Mechanisms in ALS
  4. Currently Available Therapies
  5. Therapeutic Pipeline in 2013
  6. Unmet Needs
  7. New Research Directions
  8. Acknowledgment
  9. Potential Conflicts of Interest
  10. References

Amyotrophic lateral sclerosis (ALS) is a lethal degenerative disorder of motoneurons, which may occur concurrently with frontotemporal dementia. Genetic analyses of the ∼10% of ALS cases that are dominantly inherited provide insight into ALS pathobiology. Two broad themes are evident. One, prompted by investigations of the SOD1 gene, is that conformational instability of proteins triggers downstream neurotoxic processes. The second, from studies of the TDP43, FUS, and C9orf72 genes, is that perturbations of RNA processing can be highly adverse in motoneurons. Several investigations support the concept that non-neuronal cells (microglia, astroglia, oligodendroglia) participate in the degenerative process in ALS. Recent data also emphasize the importance of molecular events in the axon and distal motoneuron terminals. Only 1 compound, riluzole, is approved by the US Food and Drug Administration for ALS; several therapies are in clinical trials, including 2 mesenchymal stem cell trials. The challenges and unmet needs in ALS emphasize the importance of new research directions: high-throughput sequencing of large DNA sets of familial and sporadic ALS, which will define scores of candidate ALS genes and pathways and facilitate studies of epistasis and epigenetics; infrastructures for candidate gene validation, including in vitro and in vivo modeling; valid biomarkers that elucidate causative molecular events and accelerate clinical trials; and in the long term, methods to identify environmental toxins. The unprecedented intensity of research in ALS and the advent of extraordinary technologies (rapid, inexpensive DNA sequencing; stem cell production from skin-derived fibroblasts; silencing of miscreant mutant genes) bode well for discovery of innovative ALS therapies. Ann Neurol 2013;74:309–316

Amyotrophic lateral sclerosis (ALS) is a lethal disorder characterized by subtle onset of focal weakness, typically in the limbs but sometimes in bulbar muscles, which progresses to paralysis of almost all skeletal muscles. There is significant clinicopathological and genetic overlap between ALS and frontotemporal lobar dementia (FTLD). In ALS, death from respiratory paralysis is typically within 5 years. The cellular pathology is focal at onset and spreads in a pattern suggesting successive involvement of contiguous neuronal populations. Death of motoneurons occurs in conjunction with deposition of aggregated proteins in motoneurons and oligodendrocytes, and neuroinflammation. Whereas most cases of ALS are sporadic (SALS), about 10% are inherited, usually dominantly (familial ALS [FALS]). ALS is designated as an orphan disease, with 1 to 2 per 100,000 new cases and a total of ∼5 per 100,000 total cases each year. In the United States and the United Kingdom, ALS accounts for about 1 in 500 to 1 in 1,000 adult deaths.[1] Strikingly, this implies that approximately 500,000 people now alive in the United States will die of ALS. These parameters are largely constant across the globe.

Biological Mechanisms in ALS

  1. Top of page
  2. Abstract
  3. Biological Mechanisms in ALS
  4. Currently Available Therapies
  5. Therapeutic Pipeline in 2013
  6. Unmet Needs
  7. New Research Directions
  8. Acknowledgment
  9. Potential Conflicts of Interest
  10. References

Diverse genetic approaches have enabled rapid dissection of the complex genetic and cellular events that underlie initiation and progression of motoneuron death in ALS.[2] Clinicopathological characterization has helped drive these advances. Overall, progress in understanding ALS can usefully be divided into 2 eras encompassing investigations before and after the discovery of the role of mutations in the 43kd TAR DNA binding protein (TDP-43) in ALS.

ALS in the SOD1 Era: Protein Toxicity

The identification of mutations in the superoxide dismutase 1 (SOD1) gene in 1993 triggered the first major wave of molecular research in ALS.[3] SOD1 is a ubiquitously expressed protein that catalyzes the detoxification of superoxide. More than 160 mutations in SOD1 associate exclusively with ALS. Nearly all are dominant missense mutations. Transgenic mice overexpressing mutant SOD1 display an ALS-like phenotype and remain a cornerstone of ALS research.[4] Mutant SOD1 toxicity is mediated through several mechanisms, prominently including protein misfolding and oligomerization. Downstream effects of this gain-of-function toxicity include impaired mitochondrial metabolism, axonal degeneration, axonal transport failure, excitotoxicity, proteasomal disruption, and endoplasmic reticulum stress.

These insights notwithstanding, SOD1-based research has not yet led to clinically applicable therapeutic advances. That the mouse model has facilitated the definition of the pathobiology of ALS is clear. However, its very strength, the consistency of mutant SOD1-induced motoneuron degeneration and lethality, is a potential limitation in therapeutic trials because the overwhelming toxicity of SOD1 overexpression creates a nearly insurmountable therapeutic barrier. A corollary view is that using survival as the endpoint in these mouse studies may have vitiated any opportunity to see interventional benefit. Outcomes more directly related to the primary toxicities of mutant SOD1 (eg, motor endplate denervation, axonal transport failure) might be more sensitive measures of therapeutic efficacy in these mice.

Intriguingly, recent studies have implicated wild-type SOD1 toxicity in the genesis of SALS.[5] Wild-type SOD1 in SALS can assume aberrant conformations that resemble those of mutant ALS; misfolded wild-type SOD1 can reproduce some forms of cytotoxicity induced by mutant SOD1. It has also been postulated that wild-type SOD1, once misfolded, can propagate intercellular pathology in a prion-like manner in which there is induction of toxic misfolding in otherwise normal SOD1 upon exposure to misfolded SOD1.[6] An important implication of these studies is that therapies that mitigate the neurotoxicity of mutant SOD1 may also be effective in SALS in the absence of SOD1 gene mutations.

ALS after TDP-43: RNA Processing

Following the SOD1 discovery, genetic linkage analyses and candidate gene screening identified rare ALS-related mutations in the alsin, senataxin, dynactin, and VAPB genes.[7-11] However, the next major milestone came in 2006 with the identification of TDP-43 inclusions in ALS brains and spinal cords.[12, 13] In 2008, the discovery of TDP-43 gene mutations in ∼4% of FALS confirmed a mechanistic link between TDP-43 and ALS pathogenesis.[14, 15]

TDP-43 is a ubiquitously expressed DNA/RNA binding protein with diverse roles including gene transcription, RNA splicing, RNA shuttling and translation, and microRNA biogenesis.[16] Although most abundant in the nucleus, TDP-43 shuttles between nuclear and cytoplasmic compartments and is transported along axons.[17] In disease, TDP-43 is ubiquitinated, hyperphosphorylated, and cleaved to form intranuclear and cytosolic aggregates. There is an overall shift in its localization from the nucleus to the cytoplasm and axons. More than 40 dominant missense mutations have been defined in TDP-43, all except 1 in the C-terminal glycine-rich domain. Mutant TDP-43 may have an increased propensity to cleavage[15, 18] and may be more resistant to degradation than wild-type protein.[19, 20] TDP-43 tightly regulates its own expression through negative feedback exerted by binding its own 3′ untranslated region (3′-UTR).[21] Variants in the 3′-UTR have been found in some ALS patients that predict failure of this feedback loop, suggesting that an excess of TDP-43 could contribute to disease.[22]

Evidence from transgenic worms, flies, zebrafish, and rodents indicates that both loss and gain-of-function mechanisms may underlie TDP-43–mediated neurodegeneration.[23-27] Mouse models of TDP-43–derived ALS have been challenging. Forcing high expression of wild-type or mutant TDP-43 generates variable phenotypes including tremor and death due to gastrointestinal stasis. However, bacterial artificial chromosome transgenic mice, conditional expression approaches, and rat models more accurately recapitulate the human disease course and pathology.[28-30] Overall, it is clear that TDP-43 plays vital roles both during development and in the survival of adult motoneurons, and that subtle perturbations in TDP-43 levels are poorly tolerated.

In 2009, mutations in FUS/TLS (fused in sarcoma/translocated in liposarcoma) were identified as a cause of around 4% of FALS.[31, 32] FUS shares similar functional domains to TDP-43 and is also predominantly intranuclear. Under neuronal stress, FUS can exit and subsequently re-enter the nucleus.[33] ALS-related FUS mutations may impair nuclear import, leading to loss of nuclear function and intracytosolic aggregation of FUS. Both TDP-43 and FUS possess prion-like domains.[34] Insoluble TDP-43 has been shown to seed TDP-43 aggregation.[35] This has underscored the hypothesis that, as argued for SOD1, propagation of misfolding between mutant and wild-type TDP-43 or FUS proteins may be critical in the pathogenesis, and spatial spreading, of motor neuron dysfunction in ALS.

C9orf72: Repeat Expansion Pathology

The concept that defective RNA processing represents an Achilles heel for motor neurons was further strengthened by investigations of C9orf72, a gene of unknown function. In 2006, linkage analysis of families with ALS-FTLD implicated a chromosome 9p locus.[36, 37] The same locus was significantly associated with sporadic ALS in genome-wide association studies.[38, 39] In 2011, the genetic culprit was finally identified as a massive intronic hexanucleotide expansion in C9orf72.[40, 41] C9orf72 expansions are the commonest genetic cause of ALS, accounting for up to 50% of FALS in populations of European descent, up to 25% of familial FTLD, and ∼5% of apparently sporadic ALS and FTLD; expansions are also seen in ∼0.5% of controls.[42] Haplotype analysis indicates that a common European founder appears to be responsible for all cases.[43] The high population frequency of the expansions has led to frenzied work to develop C9orf72 models of disease, which could provide valuable insight into sporadic disease, given that C9orf72 ALS shares 2 major features with SALS: co-occurrence of ALS and FTLD, and TDP-43 pathology, neither of which are seen in SOD1 ALS.[44]

Four mechanistic models of C9orf72-mediated ALS have been proposed. First, haploinsufficiency is suggested by the finding of reduced levels of C9orf72 transcripts in ALS brains.[45] Zebrafish C9orf72 knockdown also causes motoneuron degeneration.[46] Haploinsufficiency might be anticipated if an aberrant conformation of the hexanucleotide-expanded genomic DNA impaired C9orf72 transcription and/or if expanded RNA transcripts were abnormally spliced or translated.

Second, neuropathological studies indicate that the transcribed expansion forms nuclear RNA foci.[41] Biochemical studies and in silico modeling predict that the C9orf72 expansion can form highly stable G-quadruplex structures[47] and hairpin loops. DNA and RNA G-quadruplexes have physiological roles, such as transcriptional and translational regulation, RNA transport, and telomere stability, which are highly relevant to neuronal biology and ageing and could be disrupted if pathological C9orf72 expansions sequester the proteins to which physiological G-quadruplexes normally bind.

Third, the hexanucleotide repeat may promiscuously bind and sequester transcription factors, by analogy with sequestration of the transcription factor muscleblind by tri- and tetranucleotide expansions in myotonic dystrophy types 1 (DM1) and 2, respectively. Multiple investigators are avidly identifying RNA binding proteins and other factors that may be sequestered in C9orf72 foci.[48]

The fourth potential mechanism for neurotoxicity of C9orf72 expansions is a form of illegitimate protein translation termed repeat-associated, non-ATG (RAN) translation. This mechanism was first identified in DM1 and spinocerebellar ataxia 8.[49] Two groups have recently demonstrated that RAN translation arises from C9orf72 expansions yielding RAN polypeptides that are sequestered in insoluble aggregates.[50, 51] It remains to be established whether these proteins are cytotoxic.

Axon Biology in ALS and Modifiers of the ALS Phenotype

ALS research has generally focused on pathological processes affecting the cell body, but it is clear that critical events in ALS pathogenesis implicate the neuronal periphery. The earliest pathological changes in ALS appear to occur in axons, dendrites, and synapses. Pathological studies indicate early peripheral denervation before ventral nerve root or cell body degeneration.[52] In the central nervous system, spinal cords from ALS patients demonstrate distal corticospinal tract inflammatory changes[53, 54] and giant axonal swellings suggesting early distal axonal degeneration,[55] findings supported by antemortem diffusion tensor imaging.[56] TDP-43 aggregates form early within motor axons.[57, 58] Similarly, mutant-SOD1 mice demonstrate presymptomatic muscle denervation and terminal axonal degeneration before anterior horn cell loss.[52, 59]

Genetic findings also strongly implicate the axon in ALS. For example, FALS can be caused in some pedigrees by missense mutations in the gene profilin-1 (PFN1), which mediates actin polymerization.[60] PFN1 mutations impair axonal extension and growth cone elongation, leading to an adult onset, predominantly lower motor neuron deterioration.

Genetic variants that reduce function of EphA4, an ephrin receptor, have been shown to be beneficial in ALS. Lower expression of EphA4 in peripheral lymphocytes correlated with delayed onset of ALS and a more protracted disease course.[61] Among many functions, EphA4 signals cessation of axonal extension of distal neuronal terminals during synaptogenesis. These data strongly point to EphA4 as an important target for therapy development in ALS.

That axonal transport may be implicated in ALS has long been intimated on general grounds (ie, that normal transport is indispensable for a motor neuron, whose axon may be up to 20,000× larger than the cell body and extend to 1m in length). A role for altered axonal transport in ALS was also suggested by early studies of isolated human motor nerve axons from ALS cases.[62] Recent evidence shows that mutant SOD1 impairs axonal transport in transgenic ALS mice[63] and in isolated axoplasm from squid giant axons.[64]

Given the above, it is intriguing that we now know that axons have a self-destruct mechanism, independent of apoptosis, which can be significantly delayed in vivo given the correct molecular environment. The slow Wallerian degeneration (WldS) mouse first demonstrated that axons can survive independent of the cell body for weeks after nerve transection.[65, 66] Recent discoveries that loss-of-function mutations in the Sarm1 and highwire genes have similarly potent axon-protective effects clearly suggest that axon degeneration is an active process.[67, 68] Elucidating the molecular cascade responsible for axon killing holds promise for eventually identifying therapeutic targets for ALS.

In addition to EphA4, other genes have been reported to modify the ALS phenotype (Table 1). Genetic studies implicate UNC13A[38] (which is also a susceptibility factor) and KIFAP3[69] as modifiers of survival in some populations, whereas the single nucleotide polymorphism rs3011225 modifies age of onset,[70] as does the P413L allele of the chromogranin B gene.[71] Elegant studies by Henderson and colleagues indicate that the metalloprotease MMP9 is a determinant of ALS susceptibility; it is expressed more abundantly in motor neurons that are ALS susceptible (eg, lumbosacral motoneurons) than in those that are ALS resistant (eg, oculomotor and sphincteric motoneurons; C. Henderson, personal communication).

Table 1. Genes Implicated in Familial and Sporadic ALS
ALS TypeLocusFamilial, n = 24Sporadic, n = 6
  1. Thirty two genes (26 in FALS, 6 in SALS).

  2. ALS = amyotrophic lateral sclerosis.

ALS9qUbiquilin 1 
ALSXUbiquilin 2 

That non-neuronal cells can modulate the phenotype in animal models of ALS is now clear; mutant SOD1 in microglia and astrocytes accelerates the disease course after onset, and mutant SOD1 in oligodendroglial precursor cells hastens death in transgenic mice.[72-75] Similar findings were recently reported for astrocytes in a TDP-43 rat.[76] The mechanisms for these non–cell-autonomous influences are not well defined, although it is intriguing that oligodendrocytes are known to provide significant metabolic support to axons through lactate transport.[77]

Currently Available Therapies

  1. Top of page
  2. Abstract
  3. Biological Mechanisms in ALS
  4. Currently Available Therapies
  5. Therapeutic Pipeline in 2013
  6. Unmet Needs
  7. New Research Directions
  8. Acknowledgment
  9. Potential Conflicts of Interest
  10. References

The only therapy approved by the US Food and Drug Administration for ALS is riluzole, a small molecule with multiple mechanisms of action, including inhibition of excessive motoneuron excitation. Although modest, the benefit of riluzole (an increase in survival of perhaps 10–20%) has been seen in multiple studies.[78] Symptomatic/palliative measures are essential for ALS patients, not only at the end-stage of ALS, but during the entire course of the disease to maximize quality of life. Early placement of feeding tubes and the use of noninvasive, positive pressure respiratory assistance devices are important measures that improve quality of life and can prolong survival.

Therapeutic Pipeline in 2013

  1. Top of page
  2. Abstract
  3. Biological Mechanisms in ALS
  4. Currently Available Therapies
  5. Therapeutic Pipeline in 2013
  6. Unmet Needs
  7. New Research Directions
  8. Acknowledgment
  9. Potential Conflicts of Interest
  10. References

Fortunately, there are several treatment modalities in the ALS pipeline. Small molecules in trial, or soon to be in trial, include Neuraltus's NP001 (modifies activation of macrophages), Cytokinetics's CK2017357 (activates troponin to enhance muscle contractility), GlaxoSmithKline's ozanezumab (blocks inhibition of axonal outgrowth), University of Miami's arimoclomol (tested only in SOD1-mediated ALS; improves cellular stress responses), mexiletine (reduces excessive neuronal firing, in trial via the Northeast ALS Consortium), and rasagiline (whose neuroprotective properties are being tested in ALS at the University of Kansas).

At least 2 trials of stem cell therapy are underway. A consortium involving the University of Massachusetts Medical School, Massachusetts General Hospital, Mayo Clinic, and Brainstorm, Inc (Jerusalem, Israel) have devised a study of autologous, marrow-derived mesenchymal stem cells prepared ex vivo using a proprietary protocol. A team of investigators from Emory University and the University of Michigan are testing fetal human stem cells (produced by Neural Stem) administered via direct intraspinal injection to individuals who are receiving a full immunosuppression protocol as used in organ transplantation. Of considerable interest is the observation that 1 individual with early ALS treated with this Emory protocol showed clear improvement over several months, followed by regression. To discern whether this unprecedented response was a consequence of the intraspinal stem cells or the immunosuppression protocol, a team from Emory, Massachusetts General Hospital, and University of Massachusetts Medical School are performing a small study of the immunosuppression regimen alone.

A third exciting treatment modality that has been pioneered by Isis Pharmaceuticals jointly with R. Smith, T. Miller, and D. Cleveland is the use of modified antisense oligonucleotides to silence the SOD1 gene as a therapy for SOD1-mediated ALS. This treatment showed benefit in transgenic ALS rats; an initial pilot, single-dose study in humans found it safe.[79] Further studies are planned to determine the efficacy of SOD1 gene silencing. Alternate approaches using inhibitory RNA, delivered either as a standalone drug, or via viral-mediated delivery to the central nervous system, are under development in several laboratories.

Unmet Needs

  1. Top of page
  2. Abstract
  3. Biological Mechanisms in ALS
  4. Currently Available Therapies
  5. Therapeutic Pipeline in 2013
  6. Unmet Needs
  7. New Research Directions
  8. Acknowledgment
  9. Potential Conflicts of Interest
  10. References

In our view, several challenges loom in the quest for meaningful ALS therapies; these suggest new, potentially promising directions for ALS research. A partial list of the central questions includes the following:

  1. The daunting heterogeneity of ALS is highlighted by the growing list of genes linked to ALS. Can we find the resources to support research for all genetic causes of ALS, or should we prioritize certain genes for study, and if so how should we prioritize? Will we have to tailor treatments according to individual genotypes, or does a final common pathway link these different forms of ALS? Furthermore, to what degree will studies of genetic defects in FALS illuminate the molecular pathology of SALS?
  2. RNA processing is emerging as a common theme in neurodegeneration, but what is the correct approach to this complex field? Our current understanding of TDP-43 and C9orf72 suggests that both loss and gain of function contribute to disease. Is it correct to compare RNA transcripts between transgenic rodent models and human postmortem tissues? There remain major differences in the way different laboratories conduct these studies, with divergent opinions on many stages of the investigations, from clinical phenotyping and accurate genotyping of C9orf72 expansions, to postmortem brain and spinal cord sampling and RNA sequencing approaches.
  3. Can we develop biomarkers that meaningfully gauge disease activity and therefore allow trials to be conducted more efficiently?
  4. Can we improve on trial design using newer concepts that facilitate earlier detection of failure and accept more risk?
  5. Fiscal pressures together with the increasing need for expensive technologies are already forcing us to collaborate in new ways. Can new models for collaborations with pharma beginning in early discovery phases be mutually beneficial, using academics to derisk the lengthy phases of basic investigation while leveraging pharma to facilitate drug screening and the costly process of dealing with regulatory hurdles? As a corollary, can we more fully draw upon the efforts of the National Institutes of Health? The National Institute of Neurological Diseases and Stroke has now created state-of-the-art facilities for drug testing and development that embrace all the expertise required to move from a high-throughput assay to a compound for clinical trial. This centralized facility offers the community of academic neuroscientists economies of scale that are otherwise very difficult to achieve.

New Research Directions

  1. Top of page
  2. Abstract
  3. Biological Mechanisms in ALS
  4. Currently Available Therapies
  5. Therapeutic Pipeline in 2013
  6. Unmet Needs
  7. New Research Directions
  8. Acknowledgment
  9. Potential Conflicts of Interest
  10. References

In our view, the unmet needs described above and other considerations point to several important research directions.

Enhanced Candidate Gene Identification

Dramatic improvements in nucleotide sequencing and drops in sequencing costs will permit large-scale exome/genome resequencing (Fig 1). At least 3 projects are currently underway that purport to fully sequence the exomes of >1,000 ALS cases. This will generate hundreds of new candidate ALS genes, each pointing to potential pathways that are targets for therapy development.


Figure 1. The rate of discovery of genes whose mutations cause amyotrophic lateral sclerosis has accelerated, reflecting in part the impact of accessible high-throughput DNA sequencing technology. The total number is depicted on the y-axis versus year on the x-axis. [Color figure can be viewed in the online issue, which is available at]

Download figure to PowerPoint

New Wave Genetics

A corollary to (1) will be the opportunity to perform analyses of epistatic interactions between multiple genetic variants. Data suggest the possibility that seemingly sporadic ALS and some instances of FALS may be caused by multiple interacting genetic variants. Additionally, the advent of high-throughput, low-cost sequencing, including RNA sequencing for analysis of levels and complexity of gene expression, will also greatly facilitate epigenetic studies that characterize changes in gene expression and cellular function that are not inherited.

Candidate Gene and Pathway Validation

It will be essential to have new pipelines of cellular and animal models for validation of candidate pathways. In vitro validation studies will almost surely include the use of human motoneurons generated from induced pluripotential stem cells. In vivo validation will require the efficient creation of new genetically engineered animal models, which will be greatly facilitated by technologies such as TALENs and CRISPR/CAS9.

Biomarker Analysis

The search for biologically relevant biomarkers will be enhanced by improved technologies for detection of signature small molecules (metabolomics, lipidomics) and proteins (proteomics).

Environmental Factors

The paucity of studies of environmental factors as precipitants of neurodegenerative diseases reflects many limitations, including the inherent difficulties in screening for toxic substances that may be present at low levels, or that may have only been present transiently, triggering disease in the past. Epidemiological studies have provided an avenue to assessing environmental risk factors. It is conceivable that large-scale studies that prospectively collect DNA and body fluids will eventually have the appropriate sample sizes to detect significant distinctions between affected and unaffected individuals.


  1. Top of page
  2. Abstract
  3. Biological Mechanisms in ALS
  4. Currently Available Therapies
  5. Therapeutic Pipeline in 2013
  6. Unmet Needs
  7. New Research Directions
  8. Acknowledgment
  9. Potential Conflicts of Interest
  10. References

R.B.H. is supported by NIH grants 5RC2NS070342-02, 5R01NS065847-04, 5R01NS67206-04, 1RO1NS073873-03, 5R01NS079836-02, 1R01FD004127-01, ALSA #2003, BiogenIdec, Project ALS, Angel Fund, and ALS Therapy Alliance.

Potential Conflicts of Interest

  1. Top of page
  2. Abstract
  3. Biological Mechanisms in ALS
  4. Currently Available Therapies
  5. Therapeutic Pipeline in 2013
  6. Unmet Needs
  7. New Research Directions
  8. Acknowledgment
  9. Potential Conflicts of Interest
  10. References

R.H.B.: science advisory board membership, Biogen Idec; consultancy, Kirac Foundation; grants/grants pending, DOD; patents, patented SOD1 as an ALS gene in 1993; royalties, McGraw-Hill; stock/stock options, AviTx; travel expenses, UMass Medical School.


  1. Top of page
  2. Abstract
  3. Biological Mechanisms in ALS
  4. Currently Available Therapies
  5. Therapeutic Pipeline in 2013
  6. Unmet Needs
  7. New Research Directions
  8. Acknowledgment
  9. Potential Conflicts of Interest
  10. References
  • 1
    Johnston CA, Stanton BR, Turner MR, et al. Amyotrophic lateral sclerosis in an urban setting: a population based study of inner city London. J Neurol 2006;253:16421643.
  • 2
    Abel O, Powell JF, Andersen PM, Al-Chalabi A. ALSoD: a user-friendly online bioinformatics tool for amyotrophic lateral sclerosis genetics. Hum Mutat 2012;33:13451351.
  • 3
    Rosen DR, Siddique T, Patterson D, et al. Mutations in Cu/Zn superoxide dismutase gene are associated with familial amyotrophic lateral sclerosis. Nature 1993;362:5962.
  • 4
    Gurney ME, Pu H, Chiu AY, et al. Motor neuron degeneration in mice that express a human Cu,Zn superoxide dismutase mutation. Science 1994;264:17721775.
  • 5
    Synofzik M, Ronchi D, Keskin I, et al. Mutant superoxide dismutase-1 indistinguishable from wild-type causes ALS. Hum Mol Genet 2012;21:35683574.
  • 6
    Munch C, O'Brien J, Bertolotti A. Prion-like propagation of mutant superoxide dismutase-1 misfolding in neuronal cells. Proc Natl Acad Sci U S A 2011;108:35483553.
  • 7
    Hadano S, Hand CK, Osuga H, et al. A gene encoding a putative GTPase regulator is mutated in familial amyotrophic lateral sclerosis 2. Nat Genet 2001;29:166173.
  • 8
    Yang Y, Hentati A, Deng HX, et al. The gene encoding alsin, a protein with three guanine-nucleotide exchange factor domains, is mutated in a form of recessive amyotrophic lateral sclerosis. Nat Genet 2001;29:160165.
  • 9
    Chance PF, Rabin BA, Ryan SG, et al. Linkage of the gene for an autosomal dominant form of juvenile amyotrophic lateral sclerosis to chromosome 9q34. Am J Hum Genet 1998;62:633640.
  • 10
    Puls I, Jonnakuty C, LaMonte BH, et al. Mutant dynactin in motor neuron disease. Nat Genet 2003;33:455456.
  • 11
    Nishimura AL, Mitne-Neto M, Silva HC, et al. A mutation in the vesicle-trafficking protein VAPB causes late-onset spinal muscular atrophy and amyotrophic lateral sclerosis. Am J Hum Genet 2004;75:822831.
  • 12
    Neumann M, Sampathu DM, Kwong LK, et al. Ubiquitinated TDP-43 in frontotemporal lobar degeneration and amyotrophic lateral sclerosis. Science 2006;314:130133.
  • 13
    Arai T, Hasegawa M, Akiyama H, et al. TDP-43 is a component of ubiquitin-positive tau-negative inclusions in frontotemporal lobar degeneration and amyotrophic lateral sclerosis. Biochem Biophys Res Commun 2006;351:602611.
  • 14
    Sreedharan J, Blair IP, Tripathi VB, et al. TDP-43 mutations in familial and sporadic amyotrophic lateral sclerosis. Science 2008;319:16681672.
  • 15
    Kabashi E, Valdmanis PN, Dion P, et al. TARDBP mutations in individuals with sporadic and familial amyotrophic lateral sclerosis. Nat Genet 2008;40:572574.
  • 16
    Buratti E, Baralle FE. Multiple roles of TDP-43 in gene expression, splicing regulation, and human disease. Front Biosci 2008;13:867878.
  • 17
    Fallini C, Bassell GJ, Rossoll W. The ALS disease protein TDP-43 is actively transported in motor neuron axons and regulates axon outgrowth. Hum Mol Genet 2012;21:37033718.
  • 18
    Rutherford NJ, Zhang YJ, Baker M, et al. Novel mutations in TARDBP (TDP-43) in patients with familial amyotrophic lateral sclerosis. PLoS Genet 2008;4:e1000193.
  • 19
    Ling SC, Albuquerque CP, Han JS, et al. ALS-associated mutations in TDP-43 increase its stability and promote TDP-43 complexes with FUS/TLS. Proc Natl Acad Sci U S A 2010;107:1331813323.
  • 20
    Watanabe S, Kaneko K, Yamanaka K. Accelerated disease onset with stabilized familial amyotrophic lateral sclerosis (ALS)-linked mutant TDP-43 proteins. J Biol Chem 2013;288:36413654.
  • 21
    Ayala YM, De Conti L, Avendano-Vazquez SE, et al. TDP-43 regulates its mRNA levels through a negative feedback loop. EMBO J 2011;30:277288.
  • 22
    Gitcho MA, Bigio EH, Mishra M, et al. TARDBP 3′-UTR variant in autopsy-confirmed frontotemporal lobar degeneration with TDP-43 proteinopathy. Acta Neuropathol 2009;118:633645.
  • 23
    Ash PE, Zhang YJ, Roberts CM, et al. Neurotoxic effects of TDP-43 overexpression in C. elegans. Hum Mol Genet 2010;19:32063218.
  • 24
    Voigt A, Herholz D, Fiesel FC, et al. TDP-43-mediated neuron loss in vivo requires RNA-binding activity. PLoS One 2010;5:e12247.
  • 25
    Diaper DC, Adachi Y, Sutcliffe B, et al. Loss and gain of Drosophila TDP-43 impair synaptic efficacy and motor control leading to age-related neurodegeneration by loss-of-function phenotypes. Hum Mol Genet 2013;22:15391557.
  • 26
    Kabashi E, Lin L, Tradewell ML, et al. Gain and loss of function of ALS-related mutations of TARDBP (TDP-43) cause motor deficits in vivo. Hum Mol Genet 2010;19:671683.
  • 27
    Swarup V, Julien JP. ALS pathogenesis: recent insights from genetics and mouse models. Prog Neuropsychopharmacol Biol Psychiatry 2011;35:363369.
  • 28
    Swarup V, Phaneuf D, Bareil C, et al. Pathological hallmarks of amyotrophic lateral sclerosis/frontotemporal lobar degeneration in transgenic mice produced with TDP-43 genomic fragments. Brain 2011;134(pt 9):26102626.
  • 29
    Cannon A, Yang B, Knight J, et al. Neuronal sensitivity to TDP-43 overexpression is dependent on timing of induction. Acta Neuropathol 2012;123:807823.
  • 30
    Zhou H, Huang C, Chen H, et al. Transgenic rat model of neurodegeneration caused by mutation in the TDP gene. PLoS Genet 2010;6:e1000887.
  • 31
    Kwiatkowski TJ Jr, Bosco DA, Leclerc AL, et al. Mutations in the FUS/TLS gene on chromosome 16 cause familial amyotrophic lateral sclerosis. Science 2009;323:12051208.
  • 32
    Vance C, Rogelj B, Hortobagyi T, et al. Mutations in FUS, an RNA processing protein, cause familial amyotrophic lateral sclerosis type 6. Science 2009;323:12081211.
  • 33
    Bosco DA, Lemay N, Ko HK, et al. Mutant FUS proteins that cause amyotrophic lateral sclerosis incorporate into stress granules. Hum Mol Genet 2010;19:41604175.
  • 34
    King OD, Gitler AD, Shorter J. The tip of the iceberg: RNA-binding proteins with prion-like domains in neurodegenerative disease. Brain Res 2012;1462:6180.
  • 35
    Nonaka T, Masuda-Suzukake M, Arai T, et al. Prion-like properties of pathological TDP-43 aggregates from diseased brains. Cell Rep 2013;4:124134.
  • 36
    Vance C, Al-Chalabi A, Ruddy D, et al. Familial amyotrophic lateral sclerosis with frontotemporal dementia is linked to a locus on chromosome 9p13.2-21.3. Brain 2006;129:868876.
  • 37
    Morita M, Al-Chalabi A, Andersen PM, et al. A locus on chromosome 9p confers susceptibility to ALS and frontotemporal dementia. Neurology 2006;66:839844.
  • 38
    van Es MA, Veldink JH, Saris CG, et al. Genome-wide association study identifies 19p13.3 (UNC13A) and 9p21.2 as susceptibility loci for sporadic amyotrophic lateral sclerosis. Nat Genet 2009;41:10831087.
  • 39
    Shatunov A, Mok K, Newhouse S, et al. Chromosome 9p21 in sporadic amyotrophic lateral sclerosis in the UK and seven other countries: a genome-wide association study. Lancet neurol 2010;9:986994.
  • 40
    Renton AE, Majounie E, Waite A, et al. A hexanucleotide repeat expansion in C9ORF72 is the cause of chromosome 9p21-linked ALS-FTD. Neuron 2011;72:257268.
  • 41
    DeJesus-Hernandez M, Mackenzie IR, Boeve BF, et al. Expanded GGGGCC hexanucleotide repeat in noncoding region of C9ORF72 causes chromosome 9p-linked FTD and ALS. Neuron 2011;72:245256.
  • 42
    Beck J, Poulter M, Hensman D, et al. Large C9orf72 hexanucleotide repeat expansions are seen in multiple neurodegenerative syndromes and are more frequent than expected in the UK population. Am J Hum Genet 2013;92:345353.
  • 43
    Smith BN, Newhouse S, Shatunov A, et al. The C9ORF72 expansion mutation is a common cause of ALS+/-FTD in Europe and has a single founder. Eur J Hum Genet 2013;21:102108.
  • 44
    Robertson J, Sanelli T, Xiao S, et al. Lack of TDP-43 abnormalities in mutant SOD1 transgenic mice shows disparity with ALS. Neurosci Lett 2007;420:128132.
  • 45
    Couthouis J, Hart MP, Shorter J, et al. A yeast functional screen predicts new candidate ALS disease genes. Proc Natl Acad Sci U S A 2011;108:2088120890.
  • 46
    Ciura S, Lattante S, Le Ber I, et al. Loss of function of C9orf72 causes motor deficits in a zebrafish model of Amyotrophic Lateral Sclerosis. Ann Neurol 2013 May 30. doi: 10.1002/ana.23946. [Epub ahead of print].
  • 47
    Fratta P, Mizielinska S, Nicoll AJ, et al. C9orf72 hexanucleotide repeat associated with amyotrophic lateral sclerosis and frontotemporal dementia forms RNA G-quadruplexes. Sci Rep 2012;2:1016.
  • 48
    Mori K, Lammich S, Mackenzie IR, et al. hnRNP A3 binds to GGGGCC repeats and is a constituent of p62-positive/TDP43-negative inclusions in the hippocampus of patients with C9orf72 mutations. Acta Neuropathol 2013;125:413423.
  • 49
    Zu T, Gibbens B, Doty NS, et al. Non-ATG-initiated translation directed by microsatellite expansions. Proc Natl Acad Sci U S A 2011;108:260265.
  • 50
    Mori K, Weng SM, Arzberger T, et al. The C9orf72 GGGGCC repeat is translated into aggregating dipeptide-repeat proteins in FTLD/ALS. Science 2013;339:13351338.
  • 51
    Ash PE, Bieniek KF, Gendron TF, et al. Unconventional translation of C9ORF72 GGGGCC expansion generates insoluble polypeptides specific to c9FTD/ALS. Neuron 2013;77:639646.
  • 52
    Fischer LR, Culver DG, Tennant P, et al. Amyotrophic lateral sclerosis is a distal axonopathy: evidence in mice and man. Exp Neurol 2004;185:232240.
  • 53
    Ince PG, ShawPJ, Slade JY, et al. Familial amyotrophic lateral sclerosis with a mutation in exon 4 of the Cu/Zn superoxide dismutase gene: pathological and immunocytochemical changes. Acta Neuropathol 1996;92:395403.
  • 54
    Blair IP, Williams KL, Warraich ST, et al. FUS mutations in amyotrophic lateral sclerosis: clinical, pathological, neurophysiological and genetic analysis. J Neurol Neurosurg Psychiatry 2010;81:639645.
  • 55
    Okamoto K, Hirai S, Shoji M, et al. Axonal swellings in the corticospinal tracts in amyotrophic lateral sclerosis. Acta Neuropath 1990;80:222226.
  • 56
    Blain CR, Brunton S, Williams VC, et al. Differential corticospinal tract degeneration in homozygous 'D90A' SOD-1 ALS and sporadic ALS. J Neurol Neurosurg Psychiatry 2011;82:843849.
  • 57
    Braak H, Ludolph A, Thal DR, Del Tredici K. Amyotrophic lateral sclerosis: dash-like accumulation of phosphorylated TDP-43 in somatodendritic and axonal compartments of somatomotor neurons of the lower brainstem and spinal cord. Acta Neuropathol 2010;120:6774.
  • 58
    Brettschneider J, Del Tredici K, Toledo JB, et al. Stages of pTDP-43 pathology in amyotrophic lateral sclerosis. Ann Neurol 2013;74:2038.
  • 59
    Pun S, Santos AF, Saxena S, et al. Selective vulnerability and pruning of phasic motoneuron axons in motoneuron disease alleviated by CNTF. Nat Neurosci 2006;9:408419.
  • 60
    Wu CH, Fallini C, Ticozzi N, et al. Mutations in the profilin 1 gene cause familial amyotrophic lateral sclerosis. Nature 2012;488:499503.
  • 61
    Van Hoecke A, Schoonaert L, Lemmens R, et al. EPHA4 is a disease modifier of amyotrophic lateral sclerosis in animal models and in humans. Nat Med 2012;18:14181422.
  • 62
    Breuer AC, Lynn MP, Atkinson MB, et al. Fast axonal transport in amyotrophic lateral sclerosis: an intra-axonal organelle traffic analysis. Neurology 1987;37:738748.
  • 63
    De Vos KJ, Chapman AL, Tennant ME, et al. Familial amyotrophic lateral sclerosis-linked SOD1 mutants perturb fast axonal transport to reduce axonal mitochondria content. Hum Mol Genet 2007;16:27202728.
  • 64
    Song Y, Nagy M, Ni W, et al. Molecular chaperone Hsp110 rescues a vesicle transport defect produced by an ALS-associated mutant SOD1 protein in squid axoplasm. Proc Natl Acad Sci U S A 2013;110:54285433.
  • 65
    Coleman MP, Freeman MR. Wallerian degeneration, wld(s), and nmnat. Ann Rev Neurosci 2010;33:245267.
  • 66
    Milde S, Gilley J, Coleman MP. Subcellular localization determines the stability and axon protective capacity of axon survival factor Nmnat2. PLoS Biol 2013;11:e1001539.
  • 67
    Osterloh JM, Yang J, Rooney TM, et al. dSarm/Sarm1 is required for activation of an injury-induced axon death pathway. Science 2012;337:481484.
  • 68
    Xiong X, Hao Y, Sun K, et al. The Highwire ubiquitin ligase promotes axonal degeneration by tuning levels of Nmnat protein. PLoS Biol 2012;10:e1001440.
  • 69
    Landers JE, Melki J, Meininger V, et al. Reduced expression of the Kinesin-Associated Protein 3 (KIFAP3) gene increases survival in sporadic amyotrophic lateral sclerosis. Proc Natl Acad Sci U S A 2009;106:90049009.
  • 70
    ALSGEN Consortium; Ahmeti KB, Ajroud-Driss S, Al-Chalabi A, et al. Age of onset of amyotrophic lateral sclerosis is modulated by a locus on 1p34.1. Neurobiol Aging 2013;34:357.e7e9.
  • 71
    Gros-Louis F, Andersen PM, Dupre N, et al. Chromogranin B P413L variant as risk factor and modifier of disease onset for amyotrophic lateral sclerosis. Proc Natl Acad Sci U S A 2009;106:2177721782.
  • 72
    Boillee S, Yamanaka K, Lobsiger CS, et al. Onset and progression in inherited ALS determined by motor neurons and microglia. Science 2006;312:13891392.
  • 73
    Nagai M, Re DB, Nagata T, et al. Astrocytes expressing ALS-linked mutated SOD1 release factors selectively toxic to motor neurons. Nat Neurosci 2007;10:615622.
  • 74
    Yamanaka K, Boillee S, Roberts EA, et al. Mutant SOD1 in cell types other than motor neurons and oligodendrocytes accelerates onset of disease in ALS mice. Proc Natl Acad Sci U S A 2008;105:75947599.
  • 75
    Kang SH, Li Y, Fukaya M, et al. Degeneration and impaired regeneration of gray matter oligodendrocytes in amyotrophic lateral sclerosis. Nat Neurosci 2013;16:571579.
  • 76
    Tong J, Huang C, Bi F, et al. Expression of ALS-linked TDP-43 mutant in astrocytes causes non-cell-autonomous motor neuron death in rats. EMBO J 2013;32:19171926.
  • 77
    Lee Y, Morrison BM, Li Y, et al. Oligodendroglia metabolically support axons and contribute to neurodegeneration. Nature 2012;487:443448.
  • 78
    Miller RG, Mitchell JD, Moore DH. Riluzole for amyotrophic lateral sclerosis (ALS)/motor neuron disease (MND). Cochrane Database Syst Rev 2012;3:CD001447.
  • 79
    Miller TM, Pestronk A, David W, et al. An antisense oligonucleotide against SOD1 delivered intrathecally for patients with SOD1 familial amyotrophic lateral sclerosis: a phase 1, randomised, first-in-man study. Lancet Neurol 2013;12:435442.