Advances in understanding the molecular basis of frontotemporal dementia

Clinicopathologic assessment and imaging of tauopathies in neurodegenerative dementias

Microtubule-associated protein tau encoded by the MAPT gene binds to microtubules and is important for maintaining neuronal morphology and function. Alternative splicing of MAPT pre-mRNA generates six major tau isoforms in the adult central nervous system resulting in tau proteins with three or four microtubule-binding repeat domains. In a group of neurodegenerative disorders called tauopathies, tau becomes aberrantly hyperphosphorylated and dissociates from microtubules, resulting in a progressive accumulation of intracellular tau aggregates. The spectrum of sporadic frontotemporal lobar degeneration associated with tau pathology includes progressive supranuclear palsy, corticobasal degeneration, and Pick’s disease. Alzheimer’s disease is considered the most prevalent tauopathy. This review is divided into two broad sections. In the first section we discuss the molecular classification of sporadic tauopathies, with a focus on describing clinicopathologic relationships. In the second section we discuss the neuroimaging methodologies that are available for measuring tau pathology (directly using tau positron emission tomography ligands) and tau-mediated neuronal injury (magnetic resonance imaging and fluorodeoxyglucose positron emission tomography). Both sections have detailed descriptions of the following neurodegenerative dementias – Alzheimer’s disease, progressive supranuclear palsy, corticobasal degeneration and Pick’s disease.

Neuropsychiatric symptom profile differs based on pathology in patients with clinically diagnosed behavioral variant frontotemporal dementia

BACKGROUND

Behavioral variant frontotemporal dementia (bvFTD) is pathologically heterogeneous. With emerging therapeutics, determining underlying pathology during life is increasingly important. Neuropsychiatric symptoms are prevalent and diagnostic in bvFTD.

METHODS

We assessed the neuropsychiatric profile of patients with clinically diagnosed bvFTD as a function of pathology at autopsy. Patients with a clinical diagnosis of bvFTD at the initial visit were selected from the National Alzheimer's Coordinating Center (NACC) database. Neuropsychiatric symptoms endorsed on the Neuropsychiatric Inventory Questionnaire (NPI-Q) were analyzed.

RESULTS

Of 149 patients with clinically diagnosed bvFTD, pathology was primarily Alzheimer's disease (AD) in 20.5%. These patients differed from those with underlying frontotemporal lobar degeneration: patients with AD pathology (plaques and tangles) were more likely to have hallucinations, delusions, or agitation. Patients were further differentiated into tau-positive (30% of cases, including Pick's disease, FTD and parkinsonism with tau-positive or argyrophilic inclusions, and other tauopathies) or tau-negative cases (70% of cases, including bvFTD tau-negative ubiquitin-positive inclusions). These patients also differed in some of the neuropsychiatric symptoms seen. Tau-negative cases were more likely to demonstrate depression, delusions, and changes in appetite and eating.

CONCLUSIONS

These preliminary findings contribute to our increasing ability to predict, using simple clinical tools, the neuropathological underpinnings of bvFTD during life.

RNA-binding protein misregulation in microsatellite expansion disorders

RNA-binding proteins (RBPs) play pivotal roles in multiple cellular pathways from transcription to RNA turnover by interacting with RNA sequence and/or structural elements to form distinct RNA-protein complexes. Since these complexes are required for the normal regulation of gene expression, mutations that alter RBP functions may result in a cascade of deleterious events that lead to severe disease. Here, we focus on a group of hereditary disorders, the microsatellite expansion diseases, which alter RBP activities and result in abnormal neurological and neuromuscular phenotypes. While many of these diseases are classified as adult-onset disorders, mounting evidence indicates that disruption of normal RNA-protein interaction networks during embryogenesis modifies developmental pathways, which ultimately leads to disease manifestations later in life. Efforts to understand the molecular basis of these disorders has already uncovered novel pathogenic mechanisms, including RNA toxicity and repeat-associated non-ATG (RAN) translation, and current studies suggest that additional surprising insights into cellular regulatory pathways will emerge in the future.

Validation of next-generation sequencing technologies in genetic diagnosis of dementia

Identification of a specific genetic cause of early onset dementia (EOD) is important but can be difficult because of pleiotropy, locus heterogeneity and accessibility of gene tests. Here we assess the use of next generation sequencing (NGS) technologies as a quick, accurate and cost effective method to determine genetic diagnosis in EOD. We developed gene panel based technologies to assess 16 genes known to harbour mutations causal of dementia and combined these with PCR based assessments of the C9orf72 hexanucleotide repeat expansion and the octapeptide repeat region of PRNP. In a blinded study of 95 samples we show very high sensitivity and specificity are achievable using either Ion Torrent or MiSeq sequencing platforms. Modifications to the gene panel permit accurate detection of structural variation in APP. In 2/10 samples which had been selected because they possess a variant of uncertain significance the new technology discovered a causal mutation in genes not previously sequenced. A large proportion (23/85) of samples showed genetic variants of uncertain significance in addition to known mutations. The MRC Dementia Gene Panel and similar technologies are likely to be transformational in EOD diagnosis with a significant impact on the proportion of patients in whom a genetic cause is identified.

The FTLD risk factor TMEM106B and MAP6 control dendritic trafficking of lysosomes

TMEM106B is a major risk factor for frontotemporal lobar degeneration with TDP-43 pathology. TMEM106B localizes to lysosomes, but its function remains unclear. We show that TMEM106B knockdown in primary neurons affects lysosomal trafficking and blunts dendritic arborization. We identify microtubule-associated protein 6 (MAP6) as novel interacting protein for TMEM106B. MAP6 over-expression inhibits dendritic branching similar to TMEM106B knockdown. MAP6 knockdown fully rescues the dendritic phenotype of TMEM106B knockdown, supporting a functional interaction between TMEM106B and MAP6. Live imaging reveals that TMEM106B knockdown and MAP6 overexpression strongly increase retrograde transport of lysosomes in dendrites. Downregulation of MAP6 in TMEM106B knockdown neurons restores the balance of anterograde and retrograde lysosomal transport and thereby prevents loss of dendrites. To strengthen the link, we enhanced anterograde lysosomal transport by expressing dominant-negative Rab7-interacting lysosomal protein (RILP), which also rescues the dendrite loss in TMEM106B knockdown neurons. Thus, TMEM106B/MAP6 interaction is crucial for controlling dendritic trafficking of lysosomes, presumably by acting as a molecular brake for retrograde transport. Lysosomal misrouting may promote neurodegeneration in patients with TMEM106B risk variants.

Self-propagation of pathogenic protein aggregates in neurodegenerative diseases

For several decades scientists have speculated that the key to understanding age-related neurodegenerative disorders may be found in the unusual biology of the prion diseases. Recently, owing largely to the advent of new disease models, this hypothesis has gained experimental momentum. In a remarkable variety of diseases, specific proteins have been found to misfold and aggregate into seeds that structurally corrupt like proteins, causing them to aggregate and form pathogenic assemblies ranging from small oligomers to large masses of amyloid. Proteinaceous seeds can therefore serve as self-propagating agents for the instigation and progression of disease. Alzheimer's disease and other cerebral proteopathies seem to arise from the de novo misfolding and sustained corruption of endogenous proteins, whereas prion diseases can also be infectious in origin. However, the outcome in all cases is the functional compromise of the nervous system, because the aggregated proteins gain a toxic function and/or lose their normal function. As a unifying pathogenic principle, the prion paradigm suggests broadly relevant therapeutic directions for a large class of currently intractable diseases.

Bidirectional transcripts of the expanded C9orf72 hexanucleotide repeat are translated into aggregating dipeptide repeat proteins

Massive GGGGCC repeat expansion in the first intron of the gene C9orf72 is the most common known cause of familial frontotemporal lobar degeneration (FTLD) and amyotrophic lateral sclerosis (ALS). Despite its intronic localization and lack of an ATG start codon, the repeat region is translated in all three reading frames into aggregating dipeptide-repeat (DPR) proteins, poly-(Gly-Ala), poly-(Gly-Pro) and poly-(Gly-Arg). We took an antibody-based approach to further validate the translation of DPR proteins. To test whether the antisense repeat RNA transcript is also translated, we raised antibodies against the predicted products, poly-(Ala-Pro) and poly-(Pro-Arg). Both antibodies stained p62-positive neuronal cytoplasmic inclusions throughout the cerebellum and hippocampus indicating that not only sense but also antisense strand repeats are translated into DPR proteins in the absence of ATG start codons. Protein products of both strands co-aggregate suggesting concurrent translation of both strands. Moreover, an antibody targeting the putative carboxyl terminus of DPR proteins can detect inclusion pathology in C9orf72 repeat expansion carriers suggesting that the non-ATG translation continues through the entire repeat and beyond. A highly sensitive monoclonal antibody against poly-(Gly-Arg), visualized abundant inclusion pathology in all cortical regions and some inclusions also in motoneurons. Together, our data show that the GGGGCC repeat is bidirectionally translated into five distinct DPR proteins that co-aggregate in the characteristic p62-positive TDP-43 negative inclusions found in FTLD/ALS cases with C9orf72 repeat expansion. Novel monoclonal antibodies against poly-(Gly-Arg) will facilitate pathological diagnosis of C9orf72 FTLD/ALS.

Progranulin: at the interface of neurodegenerative and metabolic diseases

Progranulin is a widely expressed, cysteine-rich, secreted glycoprotein originally discovered for its growth factor-like properties. Its subsequent identification as a causative gene for frontotemporal dementia (FTD), a devastating early-onset neurodegenerative disease, has catalyzed a surge of new discoveries about progranulin function in the brain. More recently, progranulin was recognized as an adipokine involved in diet-induced obesity and insulin resistance, revealing its metabolic function. We review here progranulin biology in both neurodegenerative and metabolic diseases. In particular, we highlight the growth factor-like, trophic, and anti-inflammatory properties of progranulin as potential unifying themes in these seemingly divergent conditions. We also discuss potential therapeutic options for raising progranulin levels to treat progranulin-deficient FTD, as well as the possible consequences of such treatment.

Learning and memory with neuropathic pain: impact of old age and progranulin deficiency

Persistent neuropathic pain is a frequent consequence of peripheral nerve injuries, particularly in the elderly. Using the IntelliCage we studied if sciatic nerve injury obstructed learning and memory in young and aged mice, each in wild type and progranulin deficient mice, which develop premature signs of brain aging. Both young and aged mice developed long-term nerve injury-evoked hyperalgesia and allodynia. In both genotypes, aged mice with neuropathic pain showed high error rates in place avoidance acquisition tasks. However, once learnt, these aged mice with neuropathic pain showed a significantly stronger maintenance of the aversive memory. Nerve injury did not affect place preference behavior in neither genotype, neither in young nor aged mice. However, nerve injury in progranulin deficient mice impaired the learning of spatial sequences of awarded places, particularly in the aged mice. This task required a discrimination of clockwise and anti-clockwise sequences. The chaining failure occurred only in progranulin deficient mice after nerve injury, but not in sham operated or wildtype mice, suggesting that progranulin was particularly important for compensatory adaptations after nerve injury. In contrast, all aged mice with neuropathic pain, irrespective of the genotype, had a long maintenance of aversive memory suggesting a negative alliance and possibly mutual aggravation of chronic neuropathic pain and aversive memory at old age.

The mouse C9ORF72 ortholog is enriched in neurons known to degenerate in ALS and FTD

Using transgenic animals harboring a targeted LacZ insertion, we studied the expression pattern of the C9ORF72 mouse ortholog. Unlike most genes mutated in ALS, which are ubiquitously expressed, the C9ORF72-ortholog was most highly transcribed in the neuronal populations sensitive to degeneration in ALS and FTD. Thus, our study provides a potential explanation for the cell type specificity of neuronal degeneration caused by C9ORF72 mutations.

Genomic editing opens new avenues for zebrafish as a model for neurodegeneration

Zebrafish has become a popular model organism to study human diseases. We will highlight the advantages and limitations of zebrafish as a model organism to study neurodegenerative diseases and introduce zinc finger nucleases, transcription activator-like effector nucleases, and the recently established clustered regularly interspaced short palindromic repeats/clustered regularly interspaced short palindromic repeat-associated system for genome editing. The efficiency of the novel genome editing tools now greatly facilitates knock-out and, importantly, also makes knock-in approaches feasible in zebrafish. Genome editing in zebrafish avoids unspecific phenotypes caused by off-target effects and toxicity as frequently seen in conventional knock-down approaches.

Heterozygous TREM2 mutations in frontotemporal dementia

A causative association was recently demonstrated between homozygous TREM2 mutations and frontotemporal dementia (FTD)-like syndrome and between heterozygous TREM2 exon2 genetic variations and late-onset Alzheimer's disease (AD). The objective of this study was to evaluate whether heterozygous TREM2 genetic variations might be associated to the risk of FTD. TREM2 exon 2 was sequenced in a group of 1030 subjects-namely, 352 patients fulfilling clinical criteria for FTD, 484 healthy control subjects (HCs), and 194 patients with AD. The mutation frequency and the associated clinical characteristics were analyzed. We identified 8 missense and nonsense mutations in TREM2 exon 2 in 24 subjects. These mutations were more frequent in patients with FTD than in HCs (4.0% vs. 1.0%, p = 0.005). In particular, TREM2 Q33X, R47H, T66M, and S116C mutations were found in FTD and were absent in HCs. These mutations were associated with either the semantic variant of primary progressive aphasia or the behavioral variant FTD phenotypes. The FTD and AD groups were not significantly different with regard to TREM2 genetic variation frequency (AD: 2.6%, p = 0.39). Heterozygous TREM2 mutations modulate the risk of FTD in addition to increasing susceptibility to AD. Additional studies are warranted to investigate the possible role of these mutations in the pathogenesis of neurodegenerative disorders.

Modeling neurodegenerative diseases in Caenorhabditis elegans

Neurodegenerative diseases which include Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), Huntington disease (HD), and others are becoming an increasing threat to human health worldwide. The degeneration and death of certain specific groups of neurons are the hallmarks of these diseases. Despite the research progress in identification of several disease-related genes, the mechanisms underlying the neurodegeneration in these diseases remain unclear. Given the molecular conservation in neuronal signaling between Caenorhabditis elegans and vertebrates, an increasing number of research scientists have used the nematode to study this group of diseases. This review paper will focus on the model system that has been established in C. elegans to investigate the pathogenetic roles of those reported disease-related genes in AD, PD, ALS, HD and others. The progress in C. elegans provides useful information of the genetic interactions and molecular pathways that are critical in the disease process, and may help better our understanding of the disease mechanisms and search for new therapeutics for these devastating diseases.

Epigenetics of Alzheimer's disease and frontotemporal dementia

This article will review the recent advances in the understanding of the role of epigenetic modifications and the promise of future epigenetic therapy in neurodegenerative dementias, including Alzheimer's disease and frontotemporal dementia.

A novel MAPT mutation, G55R, in a frontotemporal dementia patient leads to altered Tau function

Over two dozen mutations in the gene encoding the microtubule associated protein tau cause a variety of neurodegenerative dementias known as tauopathies, including frontotemporal dementia (FTD), PSP, CBD and Pick's disease. The vast majority of these mutations map to the C-terminal region of tau possessing microtubule assembly and microtubule dynamics regulatory activities as well as the ability to promote pathological tau aggregation. Here, we describe a novel and non-conservative tau mutation (G55R) mapping to an alternatively spliced exon encoding part of the N-terminal region of the protein in a patient with the behavioral variant of FTD. Although less well understood than the C-terminal region of tau, the N-terminal region can influence both MT mediated effects as well as tau aggregation. The mutation changes an uncharged glycine to a basic arginine in the midst of a highly conserved and very acidic region. In vitro, 4-repeat G55R tau nucleates microtubule assembly more effectively than wild-type 4-repeat tau; surprisingly, this effect is tau isoform specific and is not observed in a 3-repeat G55R tau versus 3-repeat wild-type tau comparison. In contrast, the G55R mutation has no effect upon the abilities of tau to regulate MT growing and shortening dynamics or to aggregate. Additionally, the mutation has no effect upon kinesin translocation in a microtubule gliding assay. Together, (i) we have identified a novel tau mutation mapping to a mutation deficient region of the protein in a bvFTD patient, and (ii) the G55R mutation affects the ability of tau to nucleate microtubule assembly in vitro in a 4-repeat tau isoform specific manner. This altered capability could markedly affect in vivo microtubule function and neuronal cell biology. We consider G55R to be a candidate mutation for bvFTD since additional criteria required to establish causality are not yet available for assessment.

Pathways to neurodegeneration: mechanistic insights from GWAS in Alzheimer's disease, Parkinson's disease, and related disorders

The discovery of causative genetic mutations in affected family members has historically dominated our understanding of neurodegenerative diseases such as Alzheimer's disease (AD), Parkinson's disease (PD), frontotemporal dementia (FTD), and amyotrophic lateral sclerosis (ALS). Nevertheless, most cases of neurodegenerative disease are not explained by Mendelian inheritance of known genetic variants, but instead are thought to have a complex etiology with numerous genetic and environmental factors contributing to susceptibility. Although unbiased genome-wide association studies (GWAS) have identified novel associations to neurodegenerative diseases, most of these hits explain only modest fractions of disease heritability. In addition, despite the substantial overlap of clinical and pathologic features among major neurodegenerative diseases, surprisingly few GWAS-implicated variants appear to exhibit cross-disease association. These realities suggest limitations of the focus on individual genetic variants and create challenges for the development of diagnostic and therapeutic strategies, which traditionally target an isolated molecule or mechanistic step. Recently, GWAS of complex diseases and traits have focused less on individual susceptibility variants and instead have emphasized the biological pathways and networks revealed by genetic associations. This new paradigm draws on the hypothesis that fundamental disease processes may be influenced on a personalized basis by a combination of variants - some common and others rare, some protective and others deleterious - in key genes and pathways. Here, we review and synthesize the major pathways implicated in neurodegeneration, focusing on GWAS from the most prevalent neurodegenerative disorders, AD and PD. Using literature mining, we also discover a novel regulatory network that is enriched with AD- and PD-associated genes and centered on the SP1 and AP-1 (Jun/Fos) transcription factors. Overall, this pathway- and network-driven model highlights several potential shared mechanisms in AD and PD that will inform future studies of these and other neurodegenerative disorders. These insights also suggest that biomarker and treatment strategies may require simultaneous targeting of multiple components, including some specific to disease stage, in order to assess and modulate neurodegeneration. Pathways and networks will provide ideal vehicles for integrating relevant findings from GWAS and other modalities to enhance clinical translation.

C9ORF72 repeat expansions in cases with previously identified pathogenic mutations

OBJECTIVE

To identify potential genetic modifiers contributing to the phenotypic variability that is detected in patients with repeat expansions in chromosome 9 open reading frame 72 (C9ORF72), we investigated the frequency of these expansions in a cohort of 334 subjects previously found to carry mutations in genes known to be associated with a spectrum of neurodegenerative diseases.

METHODS

A 2-step protocol, with a fluorescent PCR and a repeat-primed PCR, was used to determine the presence of hexanucleotide expansions in C9ORF72. For one double mutant, we performed Southern blots to assess expansion sizes, and immunohistochemistry to characterize neuropathology.

RESULTS

We detected C9ORF72 repeat expansions in 4 of 334 subjects (1.2% [or 1.8% of 217 families]). All these subjects had behavioral phenotypes and also harbored well-known pathogenic mutations in either progranulin (GRN: p.C466LfsX46, p.R493X, p.C31LfsX35) or microtubule-associated protein tau (MAPT: p.P301L). Southern blotting of one double mutant with a p.C466LfsX46 GRN mutation demonstrated a long repeat expansion in brain (>3,000 repeats), and immunohistochemistry showed mixed neuropathology with characteristics of both C9ORF72 expansions and GRN mutations.

CONCLUSIONS

Our findings indicate that co-occurrence of 2 evidently pathogenic mutations could contribute to the pleiotropy that is detected in patients with C9ORF72 repeat expansions. These findings suggest that patients with known mutations should not be excluded from further studies, and that genetic counselors should be aware of this phenomenon when advising patients and their family members.

Cerebrospinal fluid tau levels predict prognosis in non-inherited frontotemporal dementia

BACKGROUND

The course of frontotemporal dementia (FTD) is heterogeneous and no predictors of survival are currently available. Cerebrospinal fluid (CSF) tau dosage has been demonstrated to be useful in predicting outcome over time in a number of neurological disorders.

OBJECTIVE

To assess CSF tau levels in FTD and to evaluate their prognostic value.

METHODS

Seventy-seven FTD patients with no mutations in known causative genes were consecutively enrolled, and CSF tau and phospho-tau levels analysed. Each patient was reassessed over time, and survival (i.e. death/bedridden and otherwise) was evaluated. The survival analysis was carried out by Cox proportional hazards regression models.

RESULTS

Patients with high CSF tau levels (≥400 pg/ml) had shorter survival than those with low CSF tau levels [hazard ratio (HR) = 3.406; 95% CI: 1.151-10.077; Wald χ(2) = 4.902; d.f. = 1; p = 0.027]. The association between tau levels and survival probability was confirmed after adjusting for age, gender, clinical phenotype and FTD clinical dementia rating at enrolment (HR = 3.769; 95% CI: 1.143-12.433; Wald χ(2) = 4.748; d.f. = 1; p = 0.029). Neither demographic or clinical characteristics nor CSF phospho-tau levels or apolipoprotein E genotype were significantly associated with prognosis.

CONCLUSIONS

This study argues that CSF tau levels may be considered in FTD to predict patients' outcome. Establishing in vivo prognostic biomarkers is mandatory to define homogeneous groups for inclusion in future clinical trials and to monitor the effectiveness of future therapeutic approaches.

Unravelling the mysteries of frontotemporal dementia

Frontotemporal dementia (FTD) is a clinical term that encompasses the neurodegenerative diseases that selectively affect the frontal and anterior temporal lobes of the brain. FTD, which is underdiagnosed in clinical settings, presents with behavioral changes or deficits in language. The last two decades have seen tremendous advances in the appreciation of the clinical assessment, genetics, and molecular pathology of this group of enigmatic diseases, thus offering hope for the development of rational therapeutic strategies.

FUS-mediated alternative splicing in the nervous system: consequences for ALS and FTLD

Mutations in fused in sarcoma (FUS) in a subset of patients with amyotrophic lateral sclerosis (ALS) linked this DNA/RNA-binding protein to neurodegeneration. Most of the mutations disrupt the nuclear localization signal which strongly suggests a loss-of-function pathomechanism, supported by cytoplasmic inclusions. FUS-positive neuronal cytoplasmic inclusions are also found in a subset of patients with frontotemporal lobar degeneration (FTLD). Here, we discuss recent data on the role of alternative splicing in FUS-mediated pathology in the central nervous system. Several groups have shown that FUS binds broadly to many transcripts in the brain and have also identified a plethora of putative splice targets; however, only ABLIM1, BRAF, Ewing sarcoma protein R1 (EWSR1), microtubule-associated protein tau (MAPT), NgCAM cell adhesion molecule (NRCAM), and netrin G1 (NTNG1) have been identified in at least three of four studies. Gene ontology analysis of all putative targets unanimously suggests a role in axon growth and cytoskeletal organization, consistent with the altered morphology of dendritic spines and axonal growth cones reported upon loss of FUS. Among the axonal targets, MAPT/tau and NTNG1 have been further validated in biochemical studies. The next challenge will be to confirm changes of FUS-mediated alternative splicing in patients and define their precise role in the pathophysiology of ALS and FTLD.

Targeting RNA binding proteins involved in neurodegeneration

Dysfunctions at the level of RNA processing have recently been shown to play a fundamental role in the pathogenesis of many neurodegenerative diseases. Several proteins responsible for these dysfunctions (TDP-43, FUS/TLS, and hnRNP A/Bs) belong to the nuclear class of heterogeneous ribonucleoproteins (hnRNPs) that predominantly function as general regulators of both coding and noncoding RNA metabolism. The discovery of the importance of these factors in mediating neuronal death has represented a major paradigmatic shift in our understanding of neurodegenerative processes. As a result, these discoveries have also opened the way toward novel biomolecular screening approaches in our search for therapeutic options. One of the major hurdles in this search is represented by the correct identification of the most promising targets to be prioritized. These may include aberrant aggregation processes, protein-protein interactions, RNA-protein interactions, or specific cellular pathways altered by disease. In this review, we discuss these four major options together with their various advantages and drawbacks.

Genetics of amyotrophic lateral sclerosis: an update

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disorder involving both upper motor neurons (UMN) and lower motor neurons (LMN). Enormous research has been done in the past few decades in unveiling the genetics of ALS, successfully identifying at least fifteen candidate genes associated with familial and sporadic ALS. Numerous studies attempting to define the pathogenesis of ALS have identified several plausible determinants and molecular pathways leading to motor neuron degeneration, which include oxidative stress, glutamate excitotoxicity, apoptosis, abnormal neurofilament function, protein misfolding and subsequent aggregation, impairment of RNA processing, defects in axonal transport, changes in endosomal trafficking, increased inflammation, and mitochondrial dysfunction. This review is to update the recent discoveries in genetics of ALS, which may provide insight information to help us better understanding of the disease neuropathogenesis.

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

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

Between new genetic discoveries and large randomized trials--neurological research in the era of systems medicine

The 2012 Eibsee meeting on 'Cellular Mechanisms of Neurodegeneration' addressed the need to integrate research on classical neurodegenerative mechanisms with investigations that relate to the immunological, glial and vascular sequels that accompany and often propagate neuronal injury. We report on the central topics that were addressed and discuss future directions towards establishing 'systems neurology' as a new integrated research field.

How do C9ORF72 repeat expansions cause amyotrophic lateral sclerosis and frontotemporal dementia: can we learn from other noncoding repeat expansion disorders?

PURPOSE OF REVIEW

The aim of this review is to describe disease mechanisms by which chromosome 9 open reading frame 72 (C9ORF72) repeat expansions could lead to amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) and to discuss these diseases in relation to other noncoding repeat expansion disorders.

RECENT FINDINGS

ALS and FTD are complex neurodegenerative disorders with a considerable clinical and pathological overlap, and this overlap is further substantiated by the recent discovery of C9ORF72 repeat expansions. These repeat expansions are currently the most important genetic cause of familial ALS and FTD, accounting for approximately 34.2 and 25.9% of the cases. Clinical phenotypes associated with these repeat expansions are highly variable, and combinations with mutations in other ALS-associated and/or FTD-associated genes may contribute to this pleiotropy. It is challenging, however, to diagnose patients with C9ORF72 expansions, not only because of large repeat sizes, but also due to somatic heterogeneity. Most other noncoding repeat expansion disorders share an RNA gain-of-function disease mechanism, a mechanism that could underlie the development of ALS and/or FTD as well.

SUMMARY

The discovery of C9ORF72 repeat expansions provides novel insights into the pathogenesis of ALS and FTD and highlights the importance of noncoding repeat expansions and RNA toxicity in neurodegenerative diseases.

Emotional empathy in amyotrophic lateral sclerosis: a behavioural and voxel-based morphometry study

Amyotrophic lateral sclerosis (ALS) is a multisystem condition, in which executive and/or behavioural symptoms can occur. Deficits of social cognition, including defective cognitive and emotional empathy, have been recently reported in ALS subjects. The neurostructural correlates of these disorders in ALS are still unknown. The aims of this study were to evaluate two components of empathy in non-demented ALS subjects, and to associate performance with regional grey-matter density using voxel-based morphometry (VBM). Twenty non-demented sporadic probable or definite ALS patients and 56 matched healthy controls (HC) participated in a non-verbal task requiring the attribution of emotional versus cognitive states to identify the correct ending of comic strips, compared with a control condition requiring identifying causal relationships devoid of social components. A subgroup of 14 ALS and 20 HC joined the VBM study. Results demonstrated that, compared with controls, ALS patients showed defective emotional empathy attribution, related with reduced grey-matter density in the anterior cingulate cortex and right inferior frontal gyrus. Our study provided evidence of a specific impairment of emotional empathy in ALS patients, reflecting neural damage in a limbic prefrontal network involved in emotional processing. Social cognition disorders may represent a marker of cognitive dysfunction in ALS.

Lack of C9ORF72 coding mutations supports a gain of function for repeat expansions in amyotrophic lateral sclerosis

Hexanucleotide repeat expansions in C9ORF72 are a common cause of familial and apparently sporadic amyotrophic lateral sclerosis (ALS) and frontal temporal dementia (FTD). The mechanism by which expansions cause neurodegeneration is unknown, but current evidence supports both loss-of-function and gain-of-function mechanisms. We used pooled next-generation sequencing of the C9ORF72 gene in 389 ALS patients to look for traditional loss-of-function mutations. Although rare variants were identified, none were likely to be pathogenic, suggesting that mutations other than the repeat expansion are not a common cause of ALS, and providing supportive evidence for a gain-of-function mechanism. We also show by repeat-primed PCR genotyping that the C9ORF72 expansion frequency varies by geographical region within the United States, with an unexpectedly high frequency in the Mid-West. Finally we also show evidence of somatic instability of the expansion size by Southern blot, with the largest expansions occurring in brain tissue.

Expanded GGGGCC repeat RNA associated with amyotrophic lateral sclerosis and frontotemporal dementia causes neurodegeneration

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) share phenotypic and pathologic overlap. Recently, an expansion of GGGGCC repeats in the first intron of C9orf72 was found to be a common cause of both illnesses; however, the molecular pathogenesis of this expanded repeat is unknown. Here we developed both Drosophila and mammalian models of this expanded hexanucleotide repeat and showed that expression of the expanded GGGGCC repeat RNA (rGGGGCC) is sufficient to cause neurodegeneration. We further identified Pur α as the RNA-binding protein of rGGGGCC repeats and discovered that Pur α and rGGGGCC repeats interact in vitro and in vivo in a sequence-specific fashion that is conserved between mammals and Drosophila. Furthermore, overexpression of Pur α in mouse neuronal cells and Drosophila mitigates rGGGGCC repeat-mediated neurodegeneration, and Pur α forms inclusions in the fly eye expressing expanded rGGGGCC repeats, as well as in cerebellum of human carriers of expanded GGGGCC repeats. These data suggest that expanded rGGGGCC repeats could sequester specific RNA-binding protein from their normal functions, ultimately leading to cell death. Taken together, these findings suggest that the expanded rGGGGCC repeats could cause neurodegeneration, and that Pur α may play a role in the pathogenesis of amyotrophic lateral sclerosis and frontotemporal dementia.

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'.

The epidemiology of frontotemporal dementia

Frontotemporal dementia, a heterogeneous neurodegenerative disorder, is a common cause of young onset dementia (i.e. dementia developing in midlife or earlier). The estimated point prevalence is 15-22/100,000, and incidence 2.7-4.1/100,000. Some 25% are late-life onset cases. Population studies show nearly equal distribution by gender, which contrasts with myriad clinical and neuropathology reports. FTD is frequently familial and hereditary; five genetic loci for causal mutations have been identified, all showing 100% penetrance. Non-genetic risk factors are yet to be identified. FTD shows poor life expectancy but with survival comparable to that of Alzheimer's disease. Recent progress includes the formulation of up-to-date diagnostic criteria for the behavioural and language variants, and the development of new and urgently needed instruments for monitoring and staging the illness. There is still need for descriptive population studies to fill gaps in our knowledge about minority groups and developing regions. More pressing, however, is the need for reliable physiological markers for disease. There is a present imperative to develop a translational science to form the conduit for transferring neurobiological discoveries and insights from bench to bedside.

Characterizing TDP-43 interaction with its RNA targets

One of the most important functional features of nuclear factor TDP-43 is its ability to bind UG-repeats with high efficiency. Several cross-linking and immunoprecipitation (CLIP) and RNA immunoprecipitation-sequencing (RIP-seq) analyses have indicated that TDP-43 in vivo can also specifically bind loosely conserved UG/GU-rich repeats interspersed by other nucleotides. These sequences are predominantly localized within long introns and in the 3′UTR of various genes. Most importantly, some of these sequences have been found to exist in the 3′UTR region of TDP-43 itself. In the TDP-43 3′UTR context, the presence of these UG-like sequences is essential for TDP-43 to autoregulate its own levels through a negative feedback loop. In this work, we have compared the binding of TDP-43 with these types of sequences as opposed to perfect UG-stretches. We show that the binding affinity to the UG-like sequences has a dissociation constant (K_d) of ∼110 nM compared with a K_d of 8 nM for straight UGs, and have mapped the region of contact between protein and RNA. In addition, our results indicate that the local concentration of UG dinucleotides in the CLIP sequences is one of the major factors influencing the interaction of these RNA sequences with TDP-43.

RNA toxicity in human disease and animal models: from the uncovering of a new mechanism to the development of promising therapies

Mutant ribonucleic acid (RNA) molecules can be toxic to the cell, causing human disease through trans-acting dominant mechanisms. RNA toxicity was first described in myotonic dystrophy type 1, a multisystemic disorder caused by the abnormal expansion of a non-coding trinucleotide repeat sequence. The development of multiple and complementary animal models of disease has greatly contributed to clarifying the complex disease pathways mediated by toxic RNA molecules. RNA toxicity is not limited to myotonic dystrophy and spreads to an increasing number of human conditions, which share some unifying pathogenic events mediated by toxic RNA accumulation and disruption of RNA-binding proteins. The remarkable progress in the dissection of disease pathobiology resulted in the rational design of molecular therapies, which have been successfully tested in animal models. Toxic RNA diseases, and in particular myotonic dystrophy, clearly illustrate the critical contribution of animal models of disease in translational research: from gene mutation to disease mechanisms, and ultimately to therapy development. This article is part of a Special Issue entitled: Animal Models of Disease.

TDP-43 high throughput screening analyses in neurodegeneration: advantages and pitfalls

Dysfunctions in RNA processing and in particular the aberrant regulation of RNA binding proteins (RBPs) have recently been shown to play a fundamental role in the pathogenesis of neurodegenerative diseases. Understanding the pathogenic mechanisms involved will require the elucidation of the role(s) played by these RBPs in the general cell metabolism and neuronal survival in particular. In the past, the preferred approach has been to determine first of all the functional properties of the factor(s) of interest and then use this knowledge to determine targets in biologically relevant events. More recently, novel experimental approaches such as microarrays, RNA-seq and CLIP-seq have also become very popular to study RBPs. The advantage of these approaches, collectively known as high throughput screening (HTS), is their ability to determine gene expression changes or RNA/protein targets at a global cellular level. In theory, HTS strategies should be ideal for uncovering novel functional roles/targets of any RBP inside the cell. In practice, however, there are still difficulties in getting a coherent picture from all the huge amount of data they generate, frequently not validated experimentally and thus of unknown value. They may even act unfavorably towards a specific increase of knowledge of RBP functions, as the incomplete results are taken as solid data. In this work we will illustrate as an example the use of the HTS methodologies to characterize the interactions of a specific RBP: TDP-43. The multiple functions of this protein in RNA processing and its involvement in the pathogenesis of several forms of amyotrophic lateral sclerosis, frontotemporal lobar degeneration and other neurodegenerative diseases make it an excellent substrate for our analysis of the various advantages and limitations of different HTS experimental approaches.

Inhibition of TFG function causes hereditary axon degeneration by impairing endoplasmic reticulum structure

Hereditary spastic paraplegias are a clinically and genetically heterogeneous group of gait disorders. Their pathological hallmark is a length-dependent distal axonopathy of nerve fibers in the corticospinal tract. Involvement of other neurons can cause additional neurological symptoms, which define a diverse set of complex hereditary spastic paraplegias. We present two siblings who have the unusual combination of early-onset spastic paraplegia, optic atrophy, and neuropathy. Genome-wide SNP-typing, linkage analysis, and exome sequencing revealed a homozygous c.316C>T (p.R106C) variant in the Trk-fused gene (TFG) as the only plausible mutation. Biochemical characterization of the mutant protein demonstrated a defect in its ability to self-assemble into an oligomeric complex, which is critical for normal TFG function. In cell lines, TFG inhibition slows protein secretion from the endoplasmic reticulum (ER) and alters ER morphology, disrupting organization of peripheral ER tubules and causing collapse of the ER network onto the underlying microtubule cytoskeleton. The present study provides a unique link between altered ER architecture and neurodegeneration.

hnRNP A3 binds to GGGGCC repeats and is a constituent of p62-positive/TDP43-negative inclusions in the hippocampus of patients with C9orf72 mutations

Genetic analysis revealed the hexanucleotide repeat expansion GGGGCC within the regulatory region of the gene C9orf72 as the most common cause of familial amyotrophic lateral sclerosis and the second most common cause of frontotemporal lobar degeneration. Since repeat expansions might cause RNA toxicity via sequestration of RNA-binding proteins, we searched for proteins capable of binding to GGGGCC repeats. In vitro-transcribed biotinylated RNA containing hexanucleotide GGGGCC or, as control, AAAACC repeats were incubated with nuclear protein extracts. Using stringent filtering protocols 20 RNA-binding proteins with a variety of different functions in RNA metabolism, translation and transport were identified. A subset of these proteins was further investigated by immunohistochemistry in human autopsy brains. This revealed that hnRNP A3 formed neuronal cytoplasmic and intranuclear inclusions in the hippocampus of patients with C9orf72 repeat extensions. Confocal microcopy showed that these inclusions belong to the group of the so far enigmatic p62-positive/TDP-43 negative inclusions characteristically seen in autopsy cases of diseased C9orf72 repeat expansion carriers. Thus, we have identified one protein component of these pathognomonic inclusions.

Evolution of neurodegeneration

A number of neurodegenerative diseases principally affect humans as they age and are characterized by the loss of specific groups of neurons in different brain regions. Although these disorders are generally sporadic, it is now clear that many of them have a substantial genetic component. As genes are the raw material with which evolution works, we might benefit from understanding these genes in an evolutionary framework. Here, I will discuss how we can understand whether evolution has shaped genes involved in neurodegeneration and the implications for practical issues, such as our choice of model systems for studying these diseases, and more theoretical concerns, such as the level of selection against these phenotypes.

The C9orf72 GGGGCC repeat is translated into aggregating dipeptide-repeat proteins in FTLD/ALS

Expansion of a GGGGCC hexanucleotide repeat upstream of the C9orf72 coding region is the most common cause of familial frontotemporal lobar degeneration and amyotrophic lateral sclerosis (FTLD/ALS), but the pathomechanisms involved are unknown. As in other FTLD/ALS variants, characteristic intracellular inclusions of misfolded proteins define C9orf72 pathology, but the core proteins of the majority of inclusions are still unknown. Here, we found that most of these characteristic inclusions contain poly-(Gly-Ala) and, to a lesser extent, poly-(Gly-Pro) and poly-(Gly-Arg) dipeptide-repeat proteins presumably generated by non-ATG-initiated translation from the expanded GGGGCC repeat in three reading frames. These findings directly link the FTLD/ALS-associated genetic mutation to the predominant pathology in patients with C9orf72 hexanucleotide expansion.

Apolipoprotein Ε ε4 frequency is increased among Chinese patients with frontotemporal dementia and Alzheimer's disease

The relationship between the apolipoprotein E (ApoE) ε4 genotype and an increased risk of developing Alzheimer's disease (AD) has been well established in Caucasians but is less established among other ethnicities. ApoE ε4 has also been associated with several other neurological disorders. Whether ApoΕ4 ε4 is a risk factor for frontotemporal dementia (FTD) remains controversial. This study examined 432 patients with AD, 62 with FTD, and 381 sex- and age-matched controls. The ApoE ε4 allele frequency was significantly increased among patients in the AD and FTD groups compared with controls. The frequency of the ApoΕ ε4 allele was 24.86% in late-onset AD (p < 0.01), 18.02% in early-onset AD (p < 0.01), 16.13% in FTD (p < 0.01), and 7.34% in controls. ApoΕ ε4 prevalence was similar in the FTD and AD groups. The present study suggests that the ApoE ε4 allele is a risk factor for both disorders.

RNA-mediated toxicity in neurodegenerative disease

Cellular viability depends upon the well-orchestrated functions carried out by numerous protein-coding and non-coding RNAs, as well as RNA-binding proteins. During the last decade, it has become increasingly evident that abnormalities in RNA processing represent a common feature among many neurodegenerative diseases. In "RNAopathies", which include diseases caused by non-coding repeat expansions, RNAs exert toxicity via diverse mechanisms: RNA foci formation, bidirectional transcription, and the production of toxic RNAs and proteins by repeat associated non-ATG translation. The mechanisms of toxicity in "RNA-binding proteinopathies", diseases in which RNA-binding proteins like TDP-43 and FUS play a prominent role, have yet to be fully elucidated. Nonetheless, both loss of function of the RNA binding protein, and a toxic gain of function resulting from its aggregation, are thought to be involved in disease pathogenesis. As part of the special issue on RNA and Splicing Regulation in Neurodegeneration, this review intends to explore the diverse RNA-related mechanisms contributing to neurodegeneration, with a special emphasis on findings emerging from animal models.

Discovery of Novel DENN Proteins: Implications for the Evolution of Eukaryotic Intracellular Membrane Structures and Human Disease

The tripartite DENN module, comprised of a N-terminal longin domain, followed by DENN, and d-DENN domains, is a GDP-GTP exchange factor (GEFs) for Rab GTPases, which are regulators of practically all membrane trafficking events in eukaryotes. Using sequence and structure analysis we identify multiple novel homologs of the DENN module, many of which can be traced back to the ancestral eukaryote. These findings provide unexpected leads regarding key cellular processes such as autophagy, vesicle-vacuole interactions, chromosome segregation, and human disease. Of these, SMCR8, the folliculin interacting protein-1 and 2 (FNIP1 and FNIP2), nitrogen permease regulator 2 (NPR2), and NPR3 are proposed to function in recruiting Rab GTPases during different steps of autophagy, fusion of autophagosomes with the vacuole and regulation of cellular metabolism. Another novel DENN protein identified in this study is C9ORF72; expansions of the hexanucleotide GGGGCC in its first intron have been recently implicated in amyotrophic lateral sclerosis (ALS) and fronto-temporal dementia (FTD). While this mutation is proposed to cause a RNA-level defect, the identification of C9ORF72 as a potential DENN-type GEF raises the possibility that at least part of the pathology might relate to a specific Rab-dependent vesicular trafficking process, as has been observed in the case of some other neurological conditions with similar phenotypes. We present evidence that the longin domain, such as those found in the DENN module, are likely to have been ultimately derived from the related domains found in prokaryotic GTPase-activating proteins of MglA-like GTPases. Thus, the origin of the longin domains from this ancient GTPase-interacting domain, concomitant with the radiation of GTPases, especially of the Rab clade, played an important role in the dynamics of eukaryotic intracellular membrane systems.

Co-aggregation of RNA binding proteins in ALS spinal motor neurons: evidence of a common pathogenic mechanism

While the pathogenesis of amyotrophic lateral sclerosis (ALS) remains to be clearly delineated, there is mounting evidence that altered RNA metabolism is a commonality amongst several of the known genetic variants of the disease. In this study, we evaluated the expression of 10 ALS-associated proteins in spinal motor neurons (MNs) in ALS patients with mutations in C9orf72 (C9orf72(GGGGCC)-ALS; n = 5), SOD1 (mtSOD1-ALS; n = 9), FUS/TLS (mtFUS/TLS-ALS; n = 2), or TARDBP (mtTDP-43-ALS; n = 2) and contrasted these to cases of sporadic ALS (sALS; n = 4) and familial ALS without known mutations (fALS; n = 2). We performed colorimetric immunohistochemistry (IHC) using antibodies against TDP-43, FUS/TLS, SOD1, C9orf72, ubiquitin, sequestosome 1 (p62), optineurin, phosphorylated high molecular weight neurofilament, peripherin, and Rho-guanine nucleotide exchange factor (RGNEF). We observed that RGNEF-immunoreactive neuronal cytoplasmic inclusions (NCIs) can co-localize with TDP-43, FUS/TLS and p62 within spinal MNs. We confirmed their capacity to interact by co-immunoprecipitations. We also found that mtSOD1-ALS cases possess a unique IHC signature, including the presence of C9orf72-immunoreactive diffuse NCIs, which allows them to be distinguished from other variants of ALS at the level of light microscopy. These findings support the hypothesis that alterations in RNA metabolism are a core pathogenic pathway in ALS. We also conclude that routine IHC-based analysis of spinal MNs may aid in the identification of families not previously suspected to harbor SOD1 mutations.

Parkinson disease is not associated with C9ORF72 repeat expansions

Hexanucleotide expansions in the C9ORF72 gene are frequently found in patients with amyotrophic lateral sclerosis, frontotemporal dementia or both, some of whom exhibit concurrent extrapyramidal symptoms. To determine if repeat expansions are a cause of Parkinson's disease (PD), we used repeat-primed polymerase chain reaction to investigate the frequency of C9ORF72 repeat expansions in a cohort of 478 patients with PD and 662 control subjects. Three control subjects were found to be expansion carriers, and no expansions were found among patients, suggesting that C9ORF72 expansions are not a common cause of PD.

The JNK/c-Jun signaling axis contributes to the TDP-43-induced cell death

Dysregulation of transactive response DNA-binding protein-43 (TDP-43) is closely linked to the pathogenesis of amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration with ubiquitin-positive inclusions (FTLD-U). The contribution of the upregulation of TDP-43 expression to the pathogenesis has been strongly suggested by the observation that the level of TDP-43 expression is increased in both ALS and FTLD-U patients. We previously found that the low-grade (twice to five times more than the endogenous level) overexpression of TDP-43 induces neuronal cell death through the upregulation of Bim and CHOP expression and the downregulation of Bcl-xL expression. In this study, we further show that the low-grade overexpression of TDP-43 increases the level of phosphorylated c-Jun N-terminal kinase (JNK) and the co-incubation with a JNK inhibitor, the expression of a dominant-negative JNK, or the expression of a dominant-negative c-Jun inhibited the TDP-43-induced death in NSC34 motor neuronal cells. These data together suggest that the JNK/c-Jun signaling axis contributes to the TDP-43-induced cell death.