Ataxin-2 repeat-length variation and neurodegeneration

Yeast genetic screen reveals novel therapeutic strategy for ALS

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease caused by a selective loss of motor neurons. There is no cure and few effective treatments. The RNA-binding protein TDP-43 contributes to the pathogenesis of ALS. TDP-43 is depleted from the nucleus and accumulates in cytoplasmic aggregates in the degenerating neurons and glia of most ALS patients. Furthermore, mutations in the TDP-43 gene cause rare familial and sporadic forms of the disease. Thus, therapeutic strategies targeting TDP-43 may be efficacious. We have used the yeast model system to identify the mechanisms by which TDP-43 aggregation contributes to ALS and to identify approaches to protect cells from the toxic effects of TDP-43 aggregation. Using an unbiased yeast genetic screen we discovered Dbr1 as a potent suppressor of TDP-43 toxicity. Yeast cells in which Dbr1 is deleted are resistant to TDP-43 toxicity. Dbr1 inhibition in mammalian cells is also sufficient to protect against TDP-43 cytotoxicity. Here, we review this recent discovery, highlighting future approaches aimed at extending these studies and pursuing Dbr1 as a novel therapeutic target for ALS.

Selective positive modulator of calcium-activated potassium channels exerts beneficial effects in a mouse model of spinocerebellar ataxia type 2

Spinocerebellar ataxia type 2 (SCA2) is a neurodegenerative disorder caused by a polyglutamine expansion within the Ataxin-2 (Atxn2) protein. Purkinje cells (PC) of the cerebellum fire irregularly and eventually die in SCA2. We show here that the type 2 small conductance calcium-activated potassium channel (SK2) play a key role in control of normal PC activity. Using cerebellar slices from transgenic SCA2 mice we demonstrate that SK channel modulators restore regular pacemaker activity of SCA2 PCs. Furthermore, we also show that oral delivery of a more selective positive modulator of SK2/3 channels (NS13001) alleviates behavioral and neuropathological phenotypes of aging SCA2 transgenic mice. We conclude that SK2 channels constitute a therapeutic target for SCA2 treatment and that the developed selective SK2/3 modulator NS13001 holds promise as a potential therapeutic agent for treatment of SCA2 and possibly other cerebellar ataxias.

Clinical features, neurogenetics and neuropathology of the polyglutamine spinocerebellar ataxias type 1, 2, 3, 6 and 7

The spinocerebellar ataxias type 1 (SCA1), 2 (SCA2), 3 (SCA3), 6 (SCA6) and 7 (SCA7) are genetically defined autosomal dominantly inherited progressive cerebellar ataxias (ADCAs). They belong to the group of CAG-repeat or polyglutamine diseases and share pathologically expanded and meiotically unstable glutamine-encoding CAG-repeats at distinct gene loci encoding elongated polyglutamine stretches in the disease proteins. In recent years, progress has been made in the understanding of the pathogenesis of these currently incurable diseases: Identification of underlying genetic mechanisms made it possible to classify the different ADCAs and to define their clinical and pathological features. Furthermore, advances in molecular biology yielded new insights into the physiological and pathophysiological role of the gene products of SCA1, SCA2, SCA3, SCA6 and SCA7 (i.e. ataxin-1, ataxin-2, ataxin-3, α-1A subunit of the P/Q type voltage-dependent calcium channel, ataxin-7). In the present review we summarize our current knowledge about the polyglutamine ataxias SCA1, SCA2, SCA3, SCA6 and SCA7 and compare their clinical and electrophysiological features, genetic and molecular biological background, as well as their brain pathologies. Furthermore, we provide an overview of the structure, interactions and functions of the different disease proteins. On the basis of these comprehensive data, similarities, differences and possible disease mechanisms are discussed.

The molecular basis of the frontotemporal lobar degeneration-amyotrophic lateral sclerosis spectrum

There is increasing evidence that frontotemporal lobar degeneration (FTLD) and amyotrophic lateral sclerosis (ALS) represent a continuum of neurodegenerative diseases. FTLD is complicated by ALS in a significant proportion of patients, and neuropsychological studies have demonstrated frontotemporal dysfunction in up to 50% of ALS patients. More recently, advances in neuropathology and molecular genetics have started to disclose the biological basis for the observed clinical concurrence. TDP-43 and FUS have been discovered as key pathological proteins in both FTLD and ALS. The most recent discovery of a pathological hexanucleotide repeat expansion in the gene C9orf72 as a frequent cause of both FTLD and ALS has eventually confirmed the association of these two at first sight distinct neurodegenerative diseases. Mutations in the TARDBP, FUS, and VCP genes had previously been associated with different phenotypes of the FTLD-ALS spectrum, although in these cases one end of the spectrum predominates. Whilst on the one hand providing evidence for overlap, these discoveries have also highlighted that FTLD and ALS are etiologically diverse. In this review, we review the recent advances that support the existence of an FTLD-ALS spectrum, with particular emphasis on the molecular genetic aspect.

Ataxin-2 interacts with FUS and intermediate-length polyglutamine expansions enhance FUS-related pathology in amyotrophic lateral sclerosis

Fused in sarcoma (FUS) is mutated in both sporadic amyotrophic lateral sclerosis (ALS) and familial ALS patients. The mechanisms underlying neurodegeneration are not fully understood, but FUS redistributes from the nucleus to the cytoplasm in affected motor neurons, where it triggers endoplasmic reticulum (ER) stress. Ataxin-2 is a polyglutamine protein which normally contains 22 repeats, but expanded repeats (>34) are found in Spinocerebellar Ataxia type 2. Recently ataxin-2 with intermediate length repeats (27-33) was found to increase the risk of ALS. Here we show that ataxin-2 with an ALS-linked intermediate length repeat (Q31) is a potent modifier of FUS pathology in cellular disease models. Translocation of FUS to the cytoplasm and ER stress were significantly enhanced by co-expression of mutant FUS with ataxin-2 Q31. Ataxin-2 also co-localized with FUS in sporadic and FUS-linked familial ALS patient motor neurons, co-precipitated with FUS in ALS spinal cord lysates, and co-localized with FUS in the ER-Golgi compartments in neuronal cell lines. Fragmentation of the Golgi apparatus is linked to neurodegeneration in ALS and here we show that Golgi fragmentation is induced in cells expressing mutant FUS. Moreover, Golgi fragmentation was enhanced, and the early stages of apoptosis were triggered, when ataxin-2 Q31 was co-expressed with mutant FUS. These findings describe new cellular mechanisms linking ALS with ataxin-2 intermediate length polyQ expansions and provide further evidence linking disruption to ER-Golgi compartments and FUS pathology in ALS.

Coexistence of Huntington's disease and amyotrophic lateral sclerosis: a clinicopathologic study

We report a retrospective case series of four patients with genetically confirmed Huntington's disease (HD) and sporadic amyotrophic lateral sclerosis (ALS), examining the brain and spinal cord in two cases. Neuropathological assessment included a polyglutamine recruitment method to detect sites of active polyglutamine aggregation, and biochemical and immunohistochemical assessment of TDP-43 pathology. The clinical sequence of HD and ALS varied, with the onset of ALS occurring after the mid-50's in all cases. Neuropathologic features of HD and ALS coexisted in both cases examined pathologically: neuronal loss and gliosis in the neostriatum and upper and lower motor neurons, with Bunina bodies and ubiquitin-immunoreactive skein-like inclusions in remaining lower motor neurons. One case showed relatively early HD pathology while the other was advanced. Expanded polyglutamine-immunoreactive inclusions and TDP-43-immunoreactive inclusions were widespread in many regions of the CNS, including the motor cortex and spinal anterior horn. Although these two different proteinaceous inclusions coexisted in a small number of neurons, the two proteins did not co-localize within inclusions. The regional distribution of TDP-43-immunoreactive inclusions in the cerebral cortex partly overlapped with that of expanded polyglutamine-immunoreactive inclusions. In the one case examined by TDP-43 immunoblotting, similar TDP-43 isoforms were observed as in ALS. Our findings suggest the possibility that a rare subset of older HD patients is prone to develop features of ALS with an atypical TDP-43 distribution that resembles that of aggregated mutant huntingtin. Age-dependent neuronal dysfunction induced by mutant polyglutamine protein expression may contribute to later-life development of TDP-43 associated motor neuron disease in a small subset of patients with HD.

Analysis of the C9orf72 repeat in Parkinson's disease, essential tremor and restless legs syndrome

The hexanucleotide expanded repeat (GGGGCC) in intron 1 of the C9orf72 gene is recognized as the most common genetic form of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). However, as part of the clinical phenotype, some patients present with parkinsonism. The present study investigated the potential expansion or association of the C9orf72 repeat length with susceptibility to Parkinson's disease and related disorders, essential tremor and restless legs syndrome. One restless legs syndrome patient was shown to harbor a repeat expansion, however on clinical follow-up this patient was observed to have developed frontotemporal dementia. There was no evidence of association of repeat length on disease risk or age-at-onset for any of the three disorders. Therefore the C9orf72 hexanucleotide repeat expansion appears to be specific to TDP-43 driven amyotrophic lateral sclerosis and dementia.

Contrasting pathology of the stress granule proteins TIA-1 and G3BP in tauopathies

Stress induces aggregation of RNA-binding proteins to form inclusions, termed stress granules (SGs). Recent evidence suggests that SG proteins also colocalize with neuropathological structures, but whether this occurs in Alzheimer's disease is unknown. We examined the relationship between SG proteins and neuropathology in brain tissue from P301L Tau transgenic mice, as well as in cases of Alzheimer's disease and FTDP-17. The pattern of SG pathology differs dramatically based on the RNA-binding protein examined. SGs positive for T-cell intracellular antigen-1 (TIA-1) or tristetraprolin (TTP) initially do not colocalize with tau pathology, but then merge with tau inclusions as disease severity increases. In contrast, G3BP (ras GAP-binding protein) identifies a novel type of molecular pathology that shows increasing accumulation in neurons with increasing disease severity, but often is not associated with classic markers of tau pathology. TIA-1 and TTP both bind phospho-tau, and TIA-1 overexpression induces formation of inclusions containing phospho-tau. These data suggest that SG formation might stimulate tau pathophysiology. Thus, study of RNA-binding proteins and SG biology highlights novel pathways interacting with the pathophysiology of AD, providing potentially new avenues for identifying diseased neurons and potentially novel mechanisms regulating tau biology.

ALS-associated ataxin 2 polyQ expansions enhance stress-induced caspase 3 activation and increase TDP-43 pathological modifications

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease caused by the loss of motor neurons. The degenerating motor neurons of ALS patients are characterized by the accumulation of cytoplasmic inclusions containing phosphorylated and truncated forms of the RNA-binding protein TDP-43. Ataxin 2 intermediate-length polyglutamine (polyQ) expansions were recently identified as a risk factor for ALS; however, the mechanism by which they contribute to disease is unknown. Here, we show that intermediate-length ataxin 2 polyQ expansions enhance stress-induced TDP-43 C-terminal cleavage and phosphorylation in human cells. We also connect intermediate-length ataxin 2 polyQ expansions to the stress-dependent activation of multiple caspases, including caspase 3. Caspase activation is upstream of TDP-43 cleavage and phosphorylation since caspase inhibitors block these pathological modifications. Analysis of the accumulation of activated caspase 3 in motor neurons revealed a striking association with ALS cases harboring ataxin 2 polyQ expansions. These findings indicate that activated caspase 3 defines a new pathological feature of ALS with intermediate-length ataxin 2 polyQ expansions. These results provide mechanistic insight into how ataxin 2 intermediate-length polyQ expansions could contribute to ALS--by enhancing stress-induced TDP-43 pathological modifications via caspase activation. Because longer ataxin 2 polyQ expansions are associated with a different disease, spinocerebellar ataxia 2, these findings help explain how different polyQ expansions in the same protein can have distinct cellular consequences, ultimately resulting in different clinical features. Finally, since caspase inhibitors are effective at reducing TDP-43 pathological modifications, this pathway could be pursued as a therapeutic target in ALS.

The genetics and neuropathology of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease of motor neurons leading to death from respiratory failure within about 3 years of symptom onset. A family history of ALS is obtained in about 5 % but the distinction between familial and apparently sporadic ALS is artificial and genetic factors play a role in all types. For several years, only one gene was known to have a role in ALS pathogenesis, SOD1. In the last few years there has been a rapid advance in our genetic knowledge of the causes of ALS, and the relationship of the genetic subtypes with pathological subtypes and clinical phenotype. Mutations in the gene for TDP-43 protein, TARDBP, highlight this, with pathology mimicking closely that found in other types of ALS, and a phenotypic spectrum that includes frontotemporal dementia. Mutations in the FUS gene, closely related to TDP-43, lead to a similar clinical phenotype but distinct pathology, so that the three pathological groups represented by SOD1, TARDBP, and FUS are distinct. In this review, we explore the genetic architecture of ALS, highlight some of the genes implicated in pathogenesis, and describe their phenotypic range and overlap with other diseases.

ATXN2-CAG42 sequesters PABPC1 into insolubility and induces FBXW8 in cerebellum of old ataxic knock-in mice

Spinocerebellar Ataxia Type 2 (SCA2) is caused by expansion of a polyglutamine encoding triplet repeat in the human ATXN2 gene beyond (CAG)₃₁. This is thought to mediate toxic gain-of-function by protein aggregation and to affect RNA processing, resulting in degenerative processes affecting preferentially cerebellar neurons. As a faithful animal model, we generated a knock-in mouse replacing the single CAG of murine Atxn2 with CAG42, a frequent patient genotype. This expansion size was inherited stably. The mice showed phenotypes with reduced weight and later motor incoordination. Although brain Atxn2 mRNA became elevated, soluble ATXN2 protein levels diminished over time, which might explain partial loss-of-function effects. Deficits in soluble ATXN2 protein correlated with the appearance of insoluble ATXN2, a progressive feature in cerebellum possibly reflecting toxic gains-of-function. Since in vitro ATXN2 overexpression was known to reduce levels of its protein interactor PABPC1, we studied expansion effects on PABPC1. In cortex, PABPC1 transcript and soluble and insoluble protein levels were increased. In the more vulnerable cerebellum, the progressive insolubility of PABPC1 was accompanied by decreased soluble protein levels, with PABPC1 mRNA showing no compensatory increase. The sequestration of PABPC1 into insolubility by ATXN2 function gains was validated in human cell culture. To understand consequences on mRNA processing, transcriptome profiles at medium and old age in three different tissues were studied and demonstrated a selective induction of Fbxw8 in the old cerebellum. Fbxw8 is encoded next to the Atxn2 locus and was shown in vitro to decrease the level of expanded insoluble ATXN2 protein. In conclusion, our data support the concept that expanded ATXN2 undergoes progressive insolubility and affects PABPC1 by a toxic gain-of-function mechanism with tissue-specific effects, which may be partially alleviated by the induction of FBXW8.

Frequent age-associated neurodegenerative disorders like Alzheimer's, Parkinson's, and Lou Gehrig's disease are being elucidated molecularly by studying rare heritable variants. Various hereditary neurodegenerative disorders are caused by polyglutamine expansions in different proteins. In spite of this common pathogenesis and the pathological aggregation of most affected proteins, investigators were puzzled that the pattern of affected neuron population varies and that molecular mechanisms seem different between such disorders. The polyglutamine expansions in the Ataxin-2 (ATXN2) protein are exceptional in view of the lack of aggregate clumps in nuclei of affected Purkinje neurons and well documented alterations of RNA processing in the resulting disorders SCA2 and ALS. Here, as a faithful disease model and to overcome the unavailability of autopsied patient brain tissues, we generated and characterized an ATXN2-CAG42-knock-in mouse mutant. Our data show that the unspecific, chronically present mutation leads to progressive insolubility and to reduced soluble levels of the disease protein and of an interactor protein, which modulates RNA processing. Compensatory efforts are particularly weak in vulnerable tissue. They appear to include the increased degradation of the toxic disease protein by FBXW8. Thus the link between protein and RNA pathology becomes clear, and crucial molecular targets for preventive therapy are identified.

ETS1 regulates the expression of ATXN2

Spinocerebellar ataxia type 2 (SCA2) is an autosomal dominant disorder caused by the expansion of a CAG tract in the ATXN2 gene. The SCA2 phenotype is characterized by cerebellar ataxia, neuropathy and slow saccades. SCA2 foreshortens life span and is currently without symptomatic or disease-modifying treatments. Identifying function-specific therapeutics for SCA2 is problematic due to the limited knowledge of ATXN2 function. As SCA2 is likely caused by a gain-of-toxic or gain-of-normal function like other polyglutamine disorders, targeting ATXN2 expression may represent a valid therapeutic approach. This study characterized aspects of ATXN2 expression control using an ATXN2 promoter-luciferase (luc) reporter construct. We verified the fidelity of construct expression by generating transgenic mice expressing the reporter construct. High reporter expression was seen in the cerebellum and olfactory bulb in vivo but there was relatively low expression in other tissues, similar to the expression of endogenous ataxin-2. We verified the second of two possible start codons as the functional start codon in ATXN2. By evaluating deletions in the ATXN2 promoter, we identified an E-twenty six (ETS)-binding site required for ATXN2 expression. We verified that endogenous ETS1 interacted with the ATXN2 promoter by an electromobility supershift assay and chromatin immunoprecipitation polymerase chain reaction. ETS1 overexpression increased ATXN2-luc (ATXN2-luciferase) as well as endogenous ATXN2 expression. Deletion of the putative ETS1-binding site abrogated the effects on the expression of ATXN2-luc. A dominant negative ETS1 and an ETS1 short-hairpin RNA both reduced ATXN2-luc expression. Our study broadens the understanding on the transcriptional control of ATXN2 and reveals specific regulatory features of the ATXN2 promoter that can be exploited therapeutically.

ATXN2 and its neighbouring gene SH2B3 are associated with increased ALS risk in the Turkish population

Expansions of the polyglutamine (polyQ) domain (≥ 34) in Ataxin-2 (ATXN2) are the primary cause of spinocerebellar ataxia type 2 (SCA2). Recent studies reported that intermediate-length (27-33) expansions increase the risk of Amyotrophic Lateral Sclerosis (ALS) in 1-4% of cases in diverse populations. This study investigates the Turkish population with respect to ALS risk, genotyping 158 sporadic, 78 familial patients and 420 neurologically healthy controls. We re-assessed the effect of ATXN2 expansions and extended the analysis for the first time to cover the ATXN2 locus with 18 Single Nucleotide Polymorphisms (SNPs) and their haplotypes. In accordance with other studies, our results confirmed that 31-32 polyQ repeats in the ATXN2 gene are associated with risk of developing ALS in 1.7% of the Turkish ALS cohort (p=0.0172). Additionally, a significant association of a 136 kb haplotype block across the ATXN2 and SH2B3 genes was found in 19.4% of a subset of our ALS cohort and in 10.1% of the controls (p=0.0057, OR: 2.23). ATXN2 and SH2B3 encode proteins that both interact with growth receptor tyrosine kinases. Our novel observations suggest that genotyping of SNPs at this locus may be useful for the study of ALS risk in a high percentage of individuals and that ATXN2 and SH2B3 variants may interact in modulating the disease pathway.

Distinct TDP-43 pathology in ALS patients with ataxin 2 intermediate-length polyQ expansions

Amyotrophic lateral sclerosis (ALS) is a progressive, adult-onset neurodegenerative disease characterized by degeneration of motor neurons, resulting in paralysis and death. A pathological hallmark of the degenerating motor neurons in most ALS patients is the presence of cytoplasmic inclusions containing the protein TDP-43. The morphology and type of TDP-43 pathological inclusions is variable and can range from large round Lewy body-like inclusions to filamentous skein-like inclusions. The clinical significance of this variable pathology is unclear. Intermediate-length polyglutamine (polyQ) expansions in ataxin 2 were recently identified as a genetic risk factor for ALS. Here we have analyzed TDP-43 pathology in a series of ALS cases with or without ataxin 2 intermediate-length polyQ expansions. The motor neurons of ALS cases harboring ataxin 2 polyQ expansions (n = 6) contained primarily skein-like or filamentous TDP-43 pathology and only rarely, if ever, contained large round inclusions, whereas the ALS cases without ataxin 2 polyQ expansions (n = 13) contained abundant large round and skein-like TDP-43 pathology. The paucity of large round TDP-43 inclusions in ALS cases with ataxin 2 polyQ expansions suggests a distinct pathological subtype of ALS and highlights the possibility for distinct pathogenic mechanisms.

Angiogenin variants in Parkinson disease and amyotrophic lateral sclerosis

OBJECTIVE

Several studies have suggested an increased frequency of variants in the gene encoding angiogenin (ANG) in patients with amyotrophic lateral sclerosis (ALS). Interestingly, a few ALS patients carrying ANG variants also showed signs of Parkinson disease (PD). Furthermore, relatives of ALS patients have an increased risk to develop PD, and the prevalence of concomitant motor neuron disease in PD is higher than expected based on chance occurrence. We therefore investigated whether ANG variants could predispose to both ALS and PD.

METHODS

We reviewed all previous studies on ANG in ALS and performed sequence experiments on additional samples, which allowed us to analyze data from 6,471 ALS patients and 7,668 controls from 15 centers (13 from Europe and 2 from the USA). We sequenced DNA samples from 3,146 PD patients from 6 centers (5 from Europe and 1 from the USA). Statistical analysis was performed using the variable threshold test, and the Mantel-Haenszel procedure was used to estimate odds ratios.

RESULTS

Analysis of sequence data from 17,258 individuals demonstrated a significantly higher frequency of ANG variants in both ALS and PD patients compared to control subjects (p = 9.3 × 10(-6) for ALS and p = 4.3 × 10(-5) for PD). The odds ratio for any ANG variant in patients versus controls was 9.2 for ALS and 6.7 for PD.

INTERPRETATION

The data from this multicenter study demonstrate that there is a strong association between PD, ALS, and ANG variants. ANG is a genetic link between ALS and PD.

Local RNA translation at the synapse and in disease

Local regulation of protein synthesis in neurons has emerged as a leading research focus because of its importance in synaptic plasticity and neurological diseases. The complexity of neuronal subcellular domains and their distance from the soma demand local spatial and temporal control of protein synthesis. Synthesis of many synaptic proteins, such as GluR and PSD-95, is under local control. mRNA binding proteins (RBPs), such as FMRP, function as key regulators of local RNA translation, and the mTORC1 pathway acts as a primary signaling cascade for regulation of these proteins. Much of the regulation occurs through structures termed RNA granules, which are based on reversible aggregation of the RBPs, some of which have aggregation prone domains with sequence features similar to yeast prion proteins. Mutations in many of these RBPs are associated with neurological diseases, including FMRP in fragile X syndrome; TDP-43, FUS (fused in sarcoma), angiogenin, and ataxin-2 in amyotrophic lateral sclerosis; ataxin-2 in spinocerebellar ataxia; and SMN (survival of motor neuron protein) in spinal muscular atrophy.

Neurodegeneration the RNA way

The expression, processing, transport and activities of both coding and non-coding RNAs play critical roles in normal neuronal function and differentiation. Over the past decade, these same pathways have come under scrutiny as potential contributors to neurodegenerative disease. Here we focus broadly on the roles of RNA and RNA processing in neurodegeneration. We first discuss a set of "RNAopathies", where non-coding repeat expansions drive pathogenesis through a surprisingly diverse set of mechanisms. We next explore an emerging class of "RNA binding proteinopathies" where redistribution and aggregation of the RNA binding proteins TDP-43 or FUS contribute to a potentially broad range of neurodegenerative disorders. Lastly, we delve into the potential contributions of alterations in both short and long non-coding RNAs to neurodegenerative illness.

RNA-binding proteins in neurodegenerative disease: TDP-43 and beyond

Neurodegenerative diseases are a diverse group of disorders that affect different neuron populations, differ in onset and severity, and can be either inherited or sporadic. One common pathological feature of most of these diseases is the presence of insoluble inclusions in and around neurons, which largely consist of misfolded and aggregated protein. For this reason, neurodegenerative diseases are typically thought to be disorders of aberrant protein processing, in which the cumulative effects of misfolded protein aggregates overwhelm the neuron's proteostatic capacity. However, a growing body of evidence suggests a role for abnormal RNA processing in neurodegenerative disease. The importance of RNA metabolism in disease was highlighted by the discovery of TDP-43 (TAR DNA-binding protein of 43 kDa), an RNA-binding protein (RBP), as a primary component of insoluble aggregates in patients with sporadic amyotrophic lateral sclerosis (ALS). Subsequently, inherited mutations in TDP-43 and the structurally related RBP, FUS/TLS (fused in sarcoma/translated in liposarcoma), were found to cause ALS. These exciting findings have ushered in a new era of ALS research in which the deregulation of RNA metabolism is viewed as a central cause of motor neuron deterioration. In addition, the fact that neuropathologically and anatomically distinct neurodegenerative diseases display altered RNA metabolism suggests that common pathologic mechanisms may underlie many of these disorders.

Clinical genetics of amyotrophic lateral sclerosis: what do we really know?

Hereditary amyotrophic lateral sclerosis (ALS) encompasses a group of genetic disorders characterized by adult-onset loss of the lower and upper motor neuron systems, often with involvement of other parts of the nervous system. Cases of hereditary ALS have been attributed to mutations in 12 different genes, the most common being SOD1, FUS and TARDBP-mutations in the other genes are rare. The identified genes explain 25-35% of cases of familial ALS, but identifying the remaining genes has proved difficult. Only a few genes seem to account for significant numbers of ALS cases, with many others causing a few cases each. Hereditary ALS can be inherited in an autosomal dominant, autosomal recessive or X-linked manner, and families with low disease penetrance are frequently observed. In such families, the genetic predisposition may remain unnoticed, so many patients carry a diagnosis of isolated or sporadic ALS. The only clinical feature that distinguishes recognized hereditary from apparently sporadic ALS is a lower mean age of onset in the former. All the clinical features reported in hereditary cases (including signs of extrapyramidal, cerebellar or cognitive involvement) have also been observed in sporadic cases. Genetic counseling and risk assessment in relatives depend on establishing the specific gene defect and the disease penetrance in the particular family.

On the development of markers for pathological TDP-43 in amyotrophic lateral sclerosis with and without dementia

Pathological 43-kDa transactive response sequence DNA-binding protein (TDP-43) has been recognized as the major disease protein in amyotrophic lateral sclerosis (ALS), frontotemporal lobar degeneration with ubiquitin positive, tau and α-synuclein negative inclusions (FTLD-U) and the transitional forms between these multisystem conditions. In order to develop TDP-43 into a successful ALS biomarker, the natural history of TDP-43 pathology needs to be characterized and the underlying pathophysiology established. Here we propose a spatial and temporal "two-axes" model of central nervous system vulnerability for TDP-43 linked degeneration and review recent studies on potential biomarkers related to pathological TDP-43 in the cerebrospinal fluid (CSF), blood, and skeletal muscle. The model includes the following two arms: Firstly, a "motor neuron disease" or "spinal cord/brainstem to motor cortex" axis (with degeneration possibly ascending from the lower motor neurons to the upper motor neurons); and secondly, a "dementia" or "corticoid/allocortex to neocortex" axis (with a probable spread of TDP-43 linked degeneration from the mediotemporal lobe to wider mesocortical and neocortical brain areas). At the cellular level, there is a gradual disappearance of normal TDP-43 in the nucleus in combination with the formation of pathological aggregates in the cell body and cellular processes, which can also be used to identify the stage of the disease process. Moreover, TDP-43 lesions in subpial/subependymal or perivascular localizations have been noted, and this might account for increased CSF and blood TDP-43 levels through mechanisms that remain to be elucidated.

Amyotrophic lateral sclerosis: new genes, new models, and new mechanisms

Research aimed at understanding amyotrophic lateral sclerosis (ALS) has seen exceptional growth in the past few years. New genes, new models, and new mechanisms have not only improved our understanding, but also contributed to the increasing complexity of ALS pathogenesis. The focus of this piece is to highlight some of the more notable developments in the field and to encourage a re-appreciation for the superoxide dismutase 1 (SOD1) mouse models.

The modulation of Amyotrophic Lateral Sclerosis risk by ataxin-2 intermediate polyglutamine expansions is a specific effect

Full expansions of the polyglutamine domain (polyQ≥34) within the polysome-associated protein ataxin-2 (ATXN2) are the cause of a multi-system neurodegenerative disorder, which usually presents as a Spino-Cerebellar Ataxia and is therefore known as SCA2, but may rarely manifest as Levodopa-responsive Parkinson syndrome or as motor neuron disease. Intermediate expansions (27≤polyQ≤33) were reported to modify the risk of Amyotrophic Lateral Sclerosis (ALS). We have now tested the reproducibility and the specificity of this observation. In 559 independent ALS patients from Central Europe, the association of ATXN2 expansions (30≤polyQ≤35) with ALS was highly significant. The study of 1490 patients with Parkinson's disease (PD) showed an enrichment of ATXN2 alleles 27/28 in a subgroup with familial cases, but the overall risk of sporadic PD was unchanged. No association was found between polyQ expansions in Ataxin-3 (ATXN3) and ALS risk. These data indicate a specific interaction between ATXN2 expansions and the causes of ALS, possibly through altered RNA-processing as a common pathogenic factor.

RNA-binding proteins with prion-like domains in ALS and FTLD-U

Amyotrophic lateral sclerosis (ALS, also known as Lou Gehrig's disease) is a debilitating, and universally fatal, neurodegenerative disease that devastates upper and lower motor neurons. The causes of ALS are poorly understood. A central role for RNA-binding proteins and RNA metabolism in ALS has recently emerged. The RNA-binding proteins, TDP-43 and FUS, are principal components of cytoplasmic inclusions found in motor neurons of ALS patients and mutations in TDP-43 and FUS are linked to familial and sporadic ALS. Pathology and genetics also connect TDP-43 and FUS with frontotemporal lobar degeneration with ubiquitin-positive inclusions (FTLD-U). It was unknown whether mechanisms of FUS aggregation and toxicity were similar or different to those of TDP-43. To address this issue, we have employed yeast models and pure protein biochemistry to define mechanisms underlying TDP-43 and FUS aggregation and toxicity, and to identify genetic modifiers relevant for human disease. We have identified prion-like domains in FUS and TDP-43 and provide evidence that these domains are required for aggregation. Our studies have defined key similarities as well as important differences between the two proteins. Collectively, however, our findings lead us to suggest that FUS and TDP-43, though similar RNA-binding proteins, likely aggregate and confer disease phenotypes via distinct mechanisms.