Frameshift and novel mutations in FUS in familial amyotrophic lateral sclerosis and ALS/dementia

From nucleation to widespread propagation: A prion-like concept for ALS

Propagation of pathological protein assemblies via a prion-like mechanism has been suggested to drive neurodegenerative diseases, such as Parkinson's and Alzheimer's. Recently, amyotrophic lateral sclerosis (ALS)-linked proteins, such as SOD1, TDP-43 and FUS were shown to follow self-perpetuating seeded aggregation, thereby adding ALS to the group of prion-like disorders. The cell-to-cell spread of these pathological protein assemblies and their pathogenic mechanism is poorly understood. However, as ALS is a non-cell autonomous disease and pathology in glial cells was shown to contribute to motor neuron damage, spreading mechanisms are likely to underlie disease progression via the interplay between affected neurons and their neighboring glial cells.

Frontotemporal dementia: a bridge between dementia and neuromuscular disease

The concept that frontotemporal dementia (FTD) is a purely cortical dementia has largely been refuted by the recognition of its close association with motor neuron disease, and the identification of transactive response DNA-binding protein 43 (TDP-43) as a major pathological substrate underlying both diseases. Genetic findings have transformed this field and revealed connections between disorders that were previous thought clinically unrelated. The discovery that the C9ORF72 locus is responsible for the majority of hereditary FTD, amyotrophic lateral sclerosis (ALS), and FTD-ALS cases and the understanding that repeat-containing RNA plays a crucial role in pathogenesis of both disorders has paved the way for the development of potential biomarkers and therapeutic targets for these devastating diseases. In this review, we summarize the historical aspects leading up to our current understanding of the genetic, clinical, and neuropathological overlap between FTD and ALS, and include brief discussions on chronic traumatic encephalopathy (CTE), given its association with TDP-43 pathology, its associated increased dementia risk, and reports of ALS in CTE patients. In addition, we describe other genetic associations between dementia and neuromuscular disease, such as inclusion body myositis with Paget's disease and FTD.

Autosomal dominant inheritance of rapidly progressive amyotrophic lateral sclerosis due to a truncation mutation in the fused in sarcoma (FUS) gene

Mutations in the gene encoding the RNA-binding protein fused in sarcoma (FUS) account for 4 - 5% of familial cases of amyotrophic lateral sclerosis (ALS). We describe the identification and in vitro cellular characterization of a genetic mutation in a family in which the index case, and subsequently her two children, each developed rapidly progressive ALS at a young age and died within a year of onset. Exome capture and sequencing revealed a mutation in the FUS gene consisting of a 2-bp deletion, c.1509_1510delAG, resulting in a predicted truncated protein, p.G504Wfs * 12, lacking the nuclear localization signal. Expression of this mutation in HEK293 and NSC-34 cells demonstrated severe cytoplasmic mislocalization of mutant FUS, and colocalization with stress granules when compared to wild-type, R521C and P525L mutant FUS. This study provides further evidence of a broad correlation between clinical severity of FUS-related ALS and mislocalization of the protein to the cytoplasm.

De novo FUS mutations in 2 Korean patients with sporadic amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is an adult-onset neurodegenerative disorder. Approximately 5% of ALS patients are familial (fALS) cases, and the remaining 95% are apparently sporadic (sALS) cases. To date, a number of genes have been discovered as associated with ALS, but the genetic background of sALS is not yet fully understood. The occurrence of de novo mutations in ALS genes might be an explanation for sALS, but reduced penetrance could be an alternative theory. Previously, we screened mutations in 5 ALS genes including SOD1 and FUS in 9 fALS and 249 sALS patients and found a total of 15 patients with either SOD1 (7 fALS and 3 sALS) or FUS (1 fALS and 4 sALS) mutations. Interestingly, only 1 fALS patient had the FUS mutation, whereas 4 sALS patients had mutations in this gene. To determine if the FUS mutations in sALS were de novo, we performed genetic analysis on 2 sALS patients with living parents. Genetic analysis confirmed that 2 FUS mutations, including the c.1483C>T (p.Arg495*) and the c.1509_1510delAG (p.Gly504Trpfs*12) mutations, were found only in the patients and not in their parents, confirming the de novo occurrence of these mutations. These findings support the notion that de novo mutations are responsible for a certain proportion of sALS.

Selective vulnerability of motoneuron and perturbed mitochondrial calcium homeostasis in amyotrophic lateral sclerosis: implications for motoneurons specific calcium dysregulation

Amyotrophic lateral sclerosis (ALS) is a lethal neurodegenerative disorder characterized by the selective degeneration of defined subgroups of motoneuron in the brainstem, spinal cord and motor cortex with signature hallmarks of mitochondrial Ca(2+) overload, free radical damage, excitotoxicity and impaired axonal transport. Although intracellular disruptions of cytosolic and mitochondrial calcium, and in particular low cytosolic calcium ([Ca(2+)]c) buffering and a strong interaction between metabolic mechanisms and [Ca(2+)]i have been identified predominantly in motoneuron impairment, the causes of these disruptions are unknown. The existing evidence suggests that the mutant superoxide dismutase1 (mtSOD1)-mediated toxicity in ALS acts through mitochondria, and that alteration in cytosolic and mitochondria-ER microdomain calcium accumulation are critical to the neurodegenerative process. Furthermore, chronic excitotoxcity mediated by Ca(2+)-permeable AMPA and NMDA receptors seems to initiate vicious cycle of intracellular calcium dysregulation which leads to toxic Ca(2+) overload and thereby selective neurodegeneration. Recent advancement in the experimental analysis of calcium signals with high spatiotemporal precision has allowed investigations of calcium regulation in-vivo and in-vitro in different cell types, in particular selectively vulnerable/resistant cell types in different animal models of this motoneuron disease. This review provides an overview of latest advances in this field, and focuses on details of what has been learned about disrupted Ca(2+) homeostasis and mitochondrial degeneration. It further emphasizes the critical role of mitochondria in preventing apoptosis by acting as a Ca(2+) buffers, especially in motoneurons, in pathophysiological conditions such as ALS.

Selective vulnerability of motoneuron and perturbed mitochondrial calcium homeostasis in amyotrophic lateral sclerosis: implications for motoneurons specific calcium dysregulation

Amyotrophic lateral sclerosis (ALS) is a lethal neurodegenerative disorder characterized by the selective degeneration of defined subgroups of motoneuron in the brainstem, spinal cord and motor cortex with signature hallmarks of mitochondrial Ca²⁺ overload, free radical damage, excitotoxicity and impaired axonal transport. Although intracellular disruptions of cytosolic and mitochondrial calcium, and in particular low cytosolic calcium ([Ca²⁺]_c) buffering and a strong interaction between metabolic mechanisms and [Ca²⁺]_i have been identified predominantly in motoneuron impairment, the causes of these disruptions are unknown. The existing evidence suggests that the mutant superoxide dismutase1 (mtSOD1)-mediated toxicity in ALS acts through mitochondria, and that alteration in cytosolic and mitochondria-ER microdomain calcium accumulation are critical to the neurodegenerative process. Furthermore, chronic excitotoxcity mediated by Ca²⁺-permeable AMPA and NMDA receptors seems to initiate vicious cycle of intracellular calcium dysregulation which leads to toxic Ca²⁺ overload and thereby selective neurodegeneration. Recent advancement in the experimental analysis of calcium signals with high spatiotemporal precision has allowed investigations of calcium regulation in-vivo and in-vitro in different cell types, in particular selectively vulnerable/resistant cell types in different animal models of this motoneuron disease. This review provides an overview of latest advances in this field, and focuses on details of what has been learned about disrupted Ca²⁺ homeostasis and mitochondrial degeneration. It further emphasizes the critical role of mitochondria in preventing apoptosis by acting as a Ca²⁺ buffers, especially in motoneurons, in pathophysiological conditions such as ALS.

De novo nonsense mutation of the FUS gene in an apparently familial amyotrophic lateral sclerosis case

Mutations in C9ORF72, SOD1, TARDBP, and FUS genes account for approximately two-third of familial cases and 5% of sporadic amyotrophic lateral sclerosis (ALS) cases. We present the first case of an ALS patient carrying a de novo nonsense mutation in exon 14 of the FUS gene (c.1483c>t; p.R495X) with an apparently familial ALS. This mutation causes a phenotype characterized by a young age at onset, a rapid course (<24 months), and a bulbar onset with early respiratory involvement with a predominant lower motor neuron disease. De novo mutations could account for a sizable number of apparently sporadic ALS patients carrying mutations of ALS-related genes.

Intranuclear aggregation of mutant FUS/TLS as a molecular pathomechanism of amyotrophic lateral sclerosis

Dominant mutations in FUS/TLS cause a familial form of amyotrophic lateral sclerosis (fALS), where abnormal accumulation of mutant FUS proteins in cytoplasm has been observed as a major pathological change. Many of pathogenic mutations have been shown to deteriorate the nuclear localization signal in FUS and thereby facilitate cytoplasmic mislocalization of mutant proteins. Several other mutations, however, exhibit no effects on the nuclear localization of FUS in cultured cells, and their roles in the pathomechanism of fALS remain obscure. Here, we show that a pathogenic mutation, G156E, significantly increases the propensities for aggregation of FUS in vitro and in vivo. Spontaneous in vitro formation of amyloid-like fibrillar aggregates was observed in mutant but not wild-type FUS, and notably, those fibrils functioned as efficient seeds to trigger the aggregation of wild-type protein. In addition, the G156E mutation did not disturb the nuclear localization of FUS but facilitated the formation of intranuclear inclusions in rat hippocampal neurons with significant cytotoxicity. We thus propose that intranuclear aggregation of FUS triggered by a subset of pathogenic mutations is an alternative pathomechanism of FUS-related fALS diseases.

Amyotrophic lateral sclerosis: an update on recent genetic insights

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease affecting both upper and lower motor neurons. The prognosis for ALS is extremely poor, but there is a limited course of treatment with only one approved medication. A most striking recent discovery is that TDP-43 is identified as a key molecule that is associated with both sporadic and familial forms of ALS. TDP-43 is not only a pathological hallmark, but also a genetic cause for ALS. Subsequently, a number of ALS-causative genes have been found. Above all, the RNA-binding protein, such as FUS, TAF15, EWSR1 and hnRNPA1, have structural and functional similarities to TDP-43, and physiological functions of some molecules, including VCP, UBQLN2, OPTN, FIG4 and SQSTM1, are involved in a protein degradation system. These discoveries provide valuable insight into the pathogenesis of ALS, and open doors for developing an effective disease-modifying therapy.

Genetic analysis of the fused in sarcoma gene in Chinese Han patients with Parkinson's disease

BACKGROUND AND PURPOSE

Exome sequencing in a large essential tremor (ET) family identified a novel nonsense mutation (p.Q290X) in the fused in sarcoma gene (FUS) as the cause of this family. Because of the clinical overlap between ET and Parkinson's disease (PD), the role of FUS in an independent cohort of PD patients from China mainland was evaluated.

METHODS

The entire coding region of FUS in 508 Chinese Han patients with PD and the identified variants in 633 normal controls were evaluated. A variant was further screened in an additional 382 controls for the frequency in our population.

RESULTS

A novel variant c.696C > T (p.Y232Y) in 2 sporadic patients with PD and six variants (c.52C > A, p.P18T; c.52C > T, p.P18S; c.147C > A, p.G49G; c.291C > T, p.Y97Y; c.684C > T, p.G228G; c.1176G > A, p.M392I) without significant difference in genotypic and allelic distributions in our PD cohort were identified.

CONCLUSION

The FUS gene is not a genetic risk factor for PD in the population of Chinese Han ethnicity.

Aberrant splicing in neurological diseases

Splicing of precursor messenger RNA (pre-mRNA) removes the intervening sequences (introns) and joins the expressed regions (exons) in the nucleus, before an intron-containing eukaryotic mRNA transcript can be exported and translated into proteins in the cytoplasm. While some sequences are always included or excluded (constitutive splicing), others can be selectively used (alternative splicing) in this process. Particularly by alternative splicing, up to tens of thousands of variant transcripts can be produced from a single gene, which contributes greatly to the proteomic diversity for such complex cellular functions as 'wiring' neurons in the nervous system. Disruption of this process leads to aberrant splicing, which accounts for the defects of up to 50% of mutations that cause certain human genetic diseases. In this review, we describe the different mechanisms of aberrant splicing that cause or have been associated with neurological diseases.

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.

New perspective on parkinsonism in frontotemporal lobar degeneration

Frontotemporal dementia (FTD) is the second most common type of presenile dementia. Three clinical prototypes have been defined; behavioral variant FTD, semantic dementia, and progressive nonfluent aphasia. Progressive supranuclear palsy, corticobasal degeneration, and motor neuron disease may possess clinical and pathological characteristics that overlap with FTD, and it is possible that they may all belong to the same clinicopathological spectrum. Frontotemporal lobar degeneration (FTLD) is a clinicopathological syndrome that encompasses a heterogenous group of neurodegenerative disorders. Owing to the advancement in the field of molecular genetics, diagnostic imaging, and pathology, FTLD has been the focus of great interest. Nevertheless, parkinsonism in FTLD has received relatively less attention. Parkinsonism is found in approximately 20–30% of patients in FTLD. Furthermore, parkinsonism can be seen in all FTLD subtypes, and some patients with familial and sporadic FTLD can present with prominent parkinsonism. Therefore, there is a need to understand parkinsonism in FTLD in order to obtain a better understanding of the disease. With regard to the clinical characteristics, the akinetic rigid type of parkinsonism has predominantly been described. Parkinsonism is frequently observed in familial FTD, more specifically, in FTD with parkinsonism linked to chromosome 17q (FTDP-17). The genes associated with parkinsonism are microtubule associated protein tau (MAPT), progranulin (GRN or PGRN), and chromosome 9 open reading frame 72 (C9ORF72) repeat expansion. The neural substrate of parkinsonism remains to be unveiled. Dopamine transporter (DAT) imaging revealed decreased uptake of DAT, and imaging findings indicated atrophic changes of the basal ganglia. Parkinsonism can be an important feature in FTLD and, therefore, increased attention is needed on the subject.

How do the RNA-binding proteins TDP-43 and FUS relate to amyotrophic lateral sclerosis and frontotemporal degeneration, and to each other?

PURPOSE OF REVIEW

This review examines the recent research developments aimed at defining the role of RNA-binding proteins (TDP-43 and FUS) in amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD).

RECENT FINDINGS

TAR DNA-binding protein 43 kDa (TDP-43) and fused in sarcoma (FUS) are RNA-binding proteins that form aggregates in ALS and FTLD, and when mutated can drive the pathogenesis of these disorders. However, fundamental questions remain as to the relationship between TDP-43 and FUS aggregation and disease, their normal and pathologic function, and where they converge on the same cellular pathways. Autopsy series point to distinct molecular actions as TDP-43 and FUS neuronal inclusions do not overlap, with FUS inclusions being present in only a small subgroup of patients. By contrast, modeling experiments in lower organisms support a genetic interaction between TDP-43 and FUS, although it is likely indirect. Regardless, the recent finding that additional RNA-binding proteins may also cause ALS, and the observation that TDP-43 aggregation remains a core feature in all of the recently identified genetic forms of ALS (C9ORF72, VCP, UBQLN2, and PFN1), underscores the central role of TDP-43 and RNA metabolism in ALS and FTLD.

SUMMARY

Recent discoveries point to an unprecedented convergence of molecular pathways in ALS and FTLD involving RNA metabolism. Defining the exact points of convergence will likely be key to advancing therapeutics development in the coming years.

Fused in sarcoma (FUS): an oncogene goes awry in neurodegeneration

Fused in sarcoma (FUS) is a nuclear DNA/RNA binding protein that regulates different steps of gene expression, including transcription, splicing and mRNA transport. FUS has been implicated in neurodegeneration, since mutations in FUS cause familial amyotrophic lateral sclerosis (ALS-FUS) and lead to the cytosolic deposition of FUS in the brain and spinal cord of ALS-FUS patients. Moreover, FUS and two related proteins of the same protein family (FET family) are co-deposited in cytoplasmic inclusions in a subset of patients with frontotemporal lobar degeneration (FTLD-FUS). Cytosolic deposition of these otherwise nuclear proteins most likely causes the loss of a yet unknown essential nuclear function and/or the gain of a toxic function in the cytosol. Here we summarize what is known about the physiological functions of the FET proteins in the nucleus and cytoplasm and review the distinctive pathomechanisms that lead to the deposition of only FUS in ALS-FUS, but all three FET proteins in FTLD-FUS. We suggest that ALS-FUS is caused by a selective dysfunction of FUS, while FTLD-FUS may be caused by a dysfunction of the entire FET family. This article is part of a Special Issue entitled 'RNA and splicing regulation in neurodegeneration'.

Mislocalised FUS mutants stall spliceosomal snRNPs in the cytoplasm

Genes encoding RNA-binding proteins have frequently been implicated in various motor neuron diseases, but the particular step in RNA metabolism that is vulnerable in motor neurons remains unknown. FUS, a nuclear protein, forms cytoplasmic aggregates in cells affected by amyotrophic lateral sclerosis (ALS), and mutations disturbing the nuclear import of FUS cause the disease. It is extremely likely that the cytoplasmic aggregates are cytotoxic because they trap important factors; the nature of these factors, however, remains to be elucidated. Here we show that FUS associates in a neuronal cell line with SMN, the causative factor in spinal muscular atrophy (SMA). The two genes work on the same pathway, as FUS binds to spliceosomal snRNPs downstream of the SMN function. Pathogenic FUS mutations do not disturb snRNP binding. Instead, cytoplasmic mislocalisation of FUS causes partial mis-localisation of snRNAs to the cytoplasm, which in turn causes a change in the behaviour of the alternative splicing machinery. FUS, and especially its mutations, thus have a similar effect as SMN1 deletion in SMA, suggesting that motor neurons could indeed be particularly sensitive to changes in alternative splicing.

PFN1 mutations are rare in Han Chinese populations with amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease with unknown pathophysiological mechanisms. Profilin 1 gene (PFN1) has been identified as a causative gene, which accounts for 1% to 2% of familial ALS. In this study, we investigated the mutation spectrum of PFN1 in Chinese patients with ALS. A total of 550 ALS patients (including 540 sporadic ALS [SALS] and 10 familial ALS) from the Department of Neurology, West China Hospital of Sichuan University, were recruited for the study. From the same region, 545 healthy control individuals (HC) were recruited as a control group. The encoding regions of the PFN1 gene were screened by direct sequencing. Novel candidate mutations or variations were confirmed by polymerase chain reaction-restriction fragment length polymorphism. A novel nonsynonymous p.R136W mutation was identified in an early-onset SALS female patient. A novel synonymous mutation p.L88L detected in a late-onset SALS female patient was considered nonpathogenic, as it was also detected in a control subject. No mutations were found in 10 familial ALS patients. Moreover, we found a significant difference in the genotype distribution of reported rs13204 (p.L112L) between SALS patients and HC (p = 0.0030). The frequency of minor allele 'T' of rs13204 in the SALS group was significantly lower than that in HC (p = 0.0040, OR = 0.7270, 95% CI = 0.5848-0.9039). Our results suggest that PFN1 mutation is an uncommon cause of ALS in the Han Chinese population. The SNP rs13204 of the PFN1 gene may have an important function in ALS development. The phenotype of ALS patients with mutantPFN1 gene varies among different genetic backgrounds.

Truncating mutations in FUS/TLS give rise to a more aggressive ALS-phenotype than missense mutations: a clinico-genetic study in Germany

BACKGROUND AND PURPOSE

Mutations in the FUS/TLS have been associated with amyotrophic lateral sclerosis (ALS) in a few percent of patients.

METHODS

We screened 184 familial (FALS) and 200 sporadic German patients with ALS for FUS/TLS mutations by sequence analysis of exons 5, 6 and 13-15. We compared the phenotypes of patients with different FUS/TLS mutations.

RESULTS

We identified three missense mutations p.K510R, p.R514G, p.R521H, and the two truncating mutations p.R495X and p.G478LfsX23 in samples from eight pedigrees. Both truncating mutations were associated with young onset and very aggressive disease courses, whereas the p.R521H, p.R514G and in particular the p.K510R mutation showed a milder phenotype with disease durations ranging from 3 years to more than 26 years, the longest reported for a patient with a FUS/TLS mutation. Also, in a pair of monozygous twins with the p.K510R mutation, a remarkable similar disease course was observed.

CONCLUSIONS

Mutations in FUS/TLS account for 8.7% (16 of 184) of FALS in Germany. This is a higher prevalence than reported from other countries. Truncating FUS/TLS mutations result in a more severe phenotype than most missense mutations. The wide phenotypic differences have implications for genetic counselling.

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.

Genetic overlap between apparently sporadic motor neuron diseases

Progressive muscular atrophy (PMA) and amyotrophic lateral sclerosis (ALS) are devastating motor neuron diseases (MNDs), which result in muscle weakness and/or spasticity. We compared mutation frequencies in genes known to be associated with MNDs between patients with apparently sporadic PMA and ALS. A total of 261 patients with adult-onset sporadic PMA, patients with sporadic ALS, and control subjects of Dutch descent were obtained at national referral centers for neuromuscular diseases in The Netherlands. Sanger sequencing was used to screen these subjects for mutations in the coding regions of superoxide dismutase-1 (SOD1), angiogenin (ANG), fused in sarcoma/translated in liposarcoma (FUS/TLS), TAR DNA-binding protein 43 (TARDBP), and multivesicular body protein 2B (CHMP2B). In our cohort of PMA patients we identified two SOD1 mutations (p.D90A, p.I113T), one ANG mutation (p.K17I), one FUS/TLS mutation (p.R521H), one TARDBP mutation (p.N352S), and one novel CHMP2B mutation (p.R69Q). The mutation frequency of these genes was similar in sporadic PMA (2.7%) and ALS (2.0%) patients, and therefore, our findings demonstrate a genetic overlap between apparently sporadic PMA and ALS.

FUS-NLS/Transportin 1 complex structure provides insights into the nuclear targeting mechanism of FUS and the implications in ALS

The C-terminal nuclear localization sequence of FUsed in Sarcoma (FUS-NLS) is critical for its nuclear import mediated by transportin (Trn1). Familial amyotrophic lateral sclerosis (ALS) related mutations are clustered in FUS-NLS. We report here the structural, biochemical and cell biological characterization of the FUS-NLS and its clinical implications. The crystal structure of the FUS-NLS/Trn1 complex shows extensive contacts between the two proteins and a unique α-helical structure in the FUS-NLS. The binding affinity between Trn1 and FUS-NLS (wide-type and 12 ALS-associated mutants) was determined. As compared to the wide-type FUS-NLS (K(D) = 1.7 nM), each ALS-associated mutation caused a decreased affinity and the range of this reduction varied widely from 1.4-fold over 700-fold. The affinity of the mutants correlated with the extent of impaired nuclear localization, and more importantly, with the duration of disease progression in ALS patients. This study provides a comprehensive understanding of the nuclear targeting mechanism of FUS and illustrates the significance of FUS-NLS in ALS.

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.

Novel FUS mutation in patients with sporadic amyotrophic lateral sclerosis and corticobasal degeneration

We report a patient with sporadic amyotrophic lateral sclerosis (ALS) with a novel fusion in malignant liposarcoma (FUS) gene mutation whose neurological signs were conspicuous left-sided rigidity and apraxia. A novel heterozygous guanine (G)-to-thymine (T) transition at position 1392, c.1392G>T, leading to a methionine-to-isoleucine substitution (p.Met464Ile), was found in exon13 of FUS. Re-sequencing of the genes for superoxide dismutase 1 (SOD1) and transactive response-DNA binding protein (TARDBP) revealed no mutations. The present findings suggest that this novel FUS mutation (p.Met464Ile) is related to manifestations of ALS as well as clinical features of corticobasal degeneration.

No GGGGCC-hexanucleotide repeat expansion in C9ORF72 in parkinsonism patients in Sweden

An intronic GGGGCC-hexanucleotide repeat expansion in C9ORF72 was recently identified as a major cause of amyotrophic lateral sclerosis and frontotemporal dementia. Some amyotrophic lateral sclerosis patients have signs of parkinsonism, and many parkinsonism patients develop dementia. In this study we examined if the hexanucleotide repeat expansion was present in parkinsonism patients, to clarify if there could be a relationship between the repeat expansion and disease. We studied the size of the hexanucleotide repeat expansion in a well defined population-based cohort of 135 Parkinson's disease patients and 39 patients with atypical parkinsonism and compared with 645 Swedish control subjects. We found no correlation between Parkinson's disease or atypical parkinsonism and the size of the GGGGCC repeat expansion in C9ORF72. In conclusion, this GGGGCC-repeat expansion in C9ORF72 is not a cause of parkinsonism in the Swedish population.

Arginine methylation next to the PY-NLS modulates Transportin binding and nuclear import of FUS

Fused in sarcoma (FUS) is a nuclear protein that carries a proline-tyrosine nuclear localization signal (PY-NLS) and is imported into the nucleus via Transportin (TRN). Defects in nuclear import of FUS have been implicated in neurodegeneration, since mutations in the PY-NLS of FUS cause amyotrophic lateral sclerosis (ALS). Moreover, FUS is deposited in the cytosol in a subset of frontotemporal lobar degeneration (FTLD) patients. Here, we show that arginine methylation modulates nuclear import of FUS via a novel TRN-binding epitope. Chemical or genetic inhibition of arginine methylation restores TRN-mediated nuclear import of ALS-associated FUS mutants. The unmethylated arginine-glycine-glycine domain preceding the PY-NLS interacts with TRN and arginine methylation in this domain reduces TRN binding. Inclusions in ALS-FUS patients contain methylated FUS, while inclusions in FTLD-FUS patients are not methylated. Together with recent findings that FUS co-aggregates with two related proteins of the FET family and TRN in FTLD-FUS but not in ALS-FUS, our study provides evidence that these two diseases may be initiated by distinct pathomechanisms and implicates alterations in arginine methylation in pathogenesis.

Genetics of dementia: update and guidelines for the clinician

With increased frequency, clinical geneticists are asked for genetic advice on the heredity of dementia in families. Alzheimer's disease is in most cases a complex disease, but may be autosomal dominant inherited. Mutations in the PSEN1 gene are the most common genetic cause of early onset Alzheimer's disease, whereas APP and PSEN2 gene mutations are less frequent. Familial frontotemporal dementia may be associated with a mutation in the MAPT or GRN gene, or with a repeat expansion in the C9orf72 gene. All these genes show autosomal dominant inheritance with a high penetrance. Although Alzheimer's disease and frontotemporal dementia are clinically distinguishable entities, phenotypical overlap may occur. Rarely, dementia is caused by mutations in other autosomal dominant genes or by genetic defects with autosomal recessive, X-linked dominant or mitochondrial inheritance. The inherited forms of frontotemporal dementia and Alzheimer's disease show a large phenotypic variability also within families, resulting in many remaining uncertainties for mutation carriers. Therefore, genetic counseling before performing genetic testing is essential in both symptomatic individuals and healthy at risk relatives. This review provides an overview of the genetic causes of dementia and discusses all aspects relevant for genetic counseling and testing. Furthermore, based on current knowledge, we provide algorithms for genetic testing in patients with early onset Alzheimer's disease or frontotemporal dementia.

Exome sequencing identifies FUS mutations as a cause of essential tremor

Essential tremor (ET) is a common neurodegenerative disorder that is characterized by a postural or motion tremor. Despite a strong genetic basis, a gene with rare pathogenic mutations that cause ET has not yet been reported. We used exome sequencing to implement a simple approach to control for misdiagnosis of ET, as well as phenocopies involving sporadic and senile ET cases. We studied a large ET-affected family and identified a FUS p.Gln290(∗) mutation as the cause of ET in this family. Further screening of 270 ET cases identified two additional rare missense FUS variants. Functional considerations suggest that the pathogenic effects of ET-specific FUS mutations are different from the effects observed when FUS is mutated in amyotrophic lateral sclerosis cases; we have shown that the ET FUS nonsense mutation is degraded by the nonsense-mediated-decay pathway, whereas amyotrophic lateral sclerosis FUS mutant transcripts are not.

Entorhinal cortical neurons are the primary targets of FUS mislocalization and ubiquitin aggregation in FUS transgenic rats

Ubiquitin-positive inclusion containing Fused in Sarcoma (FUS) defines a new subtype of frontotemporal lobar degeneration (FTLD). FTLD is characterized by progressive alteration in cognitions and it preferentially affects the superficial layers of frontotemporal cortex. Mutation of FUS is linked to amyotrophic lateral sclerosis and to motor neuron disease with FTLD. To examine FUS pathology in FTLD, we developed the first mammalian animal model expressing human FUS with pathogenic mutation and developing progressive loss of memory. In FUS transgenic rats, ubiquitin aggregation and FUS mislocalization were developed primarily in the entorhinal cortex of temporal lobe, particularly in the superficial layers of affected cortex. Overexpression of mutant FUS led to Golgi fragmentation and mitochondrion aggregation. Intriguingly, aggregated ubiquitin was not colocalized with either fragmented Golgi or aggregated mitochondria, and neurons with ubiquitin aggregates were deprived of endogenous TDP-43. Agonists of peroxisome proliferator-activated receptor gamma (PPAR-γ) possess anti-glial inflammation effects and are also shown to preserve the dendrite and dendritic spines of cortical neurons in culture. Here we show that rosiglitazone, a PPAR-γ agonist, rescued the dendrites and dendritic spines of neurons from FUS toxicity and preserved rats' spatial memory. Our FUS transgenic rats would be useful to the mechanistic study of cortical dementia in FTLD. As rosiglitazone is clinically used to treat diabetes, our results would encourage immediate application of PPAR-γ agonists in treating patients with cortical dementia.

SOD1, ANG, TARDBP and FUS mutations in amyotrophic lateral sclerosis: a United States clinical testing lab experience

SOD1, ANG, TARDBP and FUS mutations have been associated with amyotrophic lateral sclerosis (ALS). Our goal was to extend molecular genetic analysis to newly identified ALS genetic loci and to determine the frequency of mutations, distribution of disease genes, and variant spectrum of these genes in a large United States ALS-phenotype cohort. We screened 1220 probands with an ALS phenotype, referred originally for SOD1 molecular genetic analysis. 1128 SOD1-negative probands were screened for ANG, and 277 and 223 SOD1- and ANG-negative samples were screened for TARDBP and FUS, respectively. One hundred additional probands were specifically screened only for FUS exon 15. We identified a total of 36 different SOD1 mutations, including three novel mutations, in 92 probands. ANG screening identified three mutations, including two novel mutations, and TARDBP screening identified two previously reported TARDBP mutations. We also identified four mutations in FUS, including the reported FUS in-frame deletion, c.430_447del, p.Gly144_Tyr149del, in a patient with inclusion body myositis, and two known FUS missense mutations. From this study, we estimate frequencies for SOD1, ANG, TARDBP and FUS mutations, in this United States cohort, to be 7.5%, 0.71%, 0.72% and 1.9%, respectively. In conclusion, we identify novel variants in SOD1, ANG, TARDBP and FUS, and expand the FUS-associated clinicopathologic phenotype.

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.

Early onset Parkinsonism associated with an intronic SOD1 mutation

We report on a patient belonging to a large family with autosomal-dominant amyotrophic lateral sclerosis, who developed asymmetrical akineto-rigid symptoms at 33 years of age. He had no signs of lower motor neuron disease after four years of follow-up. All seven ALS patients from this family harboured a mutation located in the fourth intron of the SOD1 gene. The proband also harboured the same mutation, associated with a 40% decrease in SOD1 erythrocyte activity. This case report suggests that SOD1 mutations might be associated with marked phenotypic variability (ALS or early onset Parkinsonism in this family).

Sporadic juvenile amyotrophic lateral sclerosis caused by mutant FUS/TLS: possible association of mental retardation with this mutation

We present two cases of patients with juvenile amyotrophic lateral sclerosis (ALS), who had no history of familial ALS. The symptoms of both patients started as weakness of the unilateral upper limb and neck, and extended to bulbar and respiratory weakness in a relatively short period. Of note, the first patient was mentally retarded before the onset of weakness. Fused in sarcoma/translocated in liposarcoma (FUS/TLS) gene analyses revealed mutations of p. G492EfsX527 (c. 1475delG), which is a novel deletion/frameshift mutation, in the first patient and p. R514S mutation (c. 1542G > T) in the second patient. Molecular analysis revealed that the mutant FUS/TLS, especially the deletion/frameshift mutation, showed significant cytoplasmic localization in transfected motor neuron-like cells. Our findings suggest the association of mental retardation with the FUS/TLS mutation. Further investigation, including the effect of FUS/TLS on cognitive function, would aid better understanding of FUS/TLS proteinopathies.

Novel types of frontotemporal lobar degeneration: beyond tau and TDP-43

Most cases of frontotemporal lobar degeneration (FTLD) are characterized by the abnormal accumulation of either the microtubule-associated protein tau or the transactive response DNA-binding protein with M(r) 43 kDa, TDP-43 (FTLD-tau and FTLD-TDP, respectively). However, there remain ∼10% of cases, composed of a heterogenous collection of uncommon disorders, for which the molecular basis remains uncertain. In this review, we describe the characteristic genetic, clinical, and pathological features of the major tau/TDP-negative FTLD subtypes, with focus on recent advances in our understanding of their molecular basis. This includes the discovery that the pathological changes in atypical FTLD with ubiquitinated inclusions, neuronal intermediate filament inclusion disease, and basophilic inclusion body disease are immunoreactive for the fused in sarcoma (FUS) protein, resulting in the creation of a new molecular subgroup (FTLD-FUS), and studies clarifying the functional consequences of pathogenic CHMP2B mutations.

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.

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.

Familial amyotrophic lateral sclerosis, a historical perspective

Amyotrophic lateral sclerosis is a fatal neurodegenerative disease of the upper and lower motor neuron of unknown etiology. Although a familial cause for this disease has been suspected early one, it is only in the past two decades that advances in modern genetics led to the identification of more than 10 genes linked to familial ALS and helped us understand some of the complex genetic and environmental interactions that may contribute to sporadic ALS. In this article, we chronologically summarize the genetic breakthroughs in familial and sporadic ALS and depict how it shaped our understanding of disease pathogenesis and our quest for rational therapies.

P525L FUS mutation is consistently associated with a severe form of juvenile amyotrophic lateral sclerosis

Some FUS mutations have been observed in patients with the juvenile form of Amyotrophic Lateral Sclerosis starting before 25 years. We report an 11-year-old girl affected by sporadic juvenile ALS with a rapid course resulting in tracheostomy after 14 months from the onset. Sequencing FUS gene revealed a de novo P525L mutation. Our findings, together with literature data, indicate that this mutation is consistently associated with a specific phenotype characterized by juvenile onset, severe course and high proportion of de novo mutations in sporadic cases.

Making connections: pathology and genetics link amyotrophic lateral sclerosis with frontotemporal lobe dementia

Over the last couple of decades, there has been a growing body of clinical, genetic, and histopathological evidence that similar pathological processes underlie amyotrophic lateral sclerosis (ALS) and some types of frontotemporal lobe dementia (FTD). Even though there is great diversity in the genetic causes of these disorders, there is a high degree of overlap in their histopathology. Genes linked to rare cases of familial ALS and/or FTD, like FUS, TARDBP, OPTN, and UBQLN2 may converge onto a unifying pathogenic pathway and thereby provide novel therapeutic targets common to a spectrum of etiologically diverse forms of ALS and ALS-FTD. Additionally, there are major loci for ALS-FTD on chromosomes 9p and 15q. Identification of causative genetic alterations at those loci will be an important step in understanding the pathogenesis of juvenile- and adult-onset ALS and ALS-FTD. Interactions between TDP-43, FUS, optineurin, and ubiquilin 2 need to be studied to understand their common molecular pathways. Future efforts should also be directed towards generation and characterization of in vivo models to dissect the pathogenic mechanisms of these diseases. Such efforts will rapidly accelerate the discovery of new drugs that regulate accumulation of pathogenic proteins and their downstream consequences.