Screening of the TARDBP gene in familial and sporadic amyotrophic lateral sclerosis patients of Chinese origin

An Atypical Presentation of Upper Motor Neuron Predominant Juvenile Amyotrophic Lateral Sclerosis Associated with TARDBP Gene: A Case Report and Review of the Literature

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease that can rarely affect young individuals. Juvenile ALS (JALS) is defined for individuals with an onset of the disease before the age of 25. The contribution of genetics to ALS pathology is a field of growing interest. One of the differences between adult-onset ALS and JALS is their genetic background, with a higher contribution of genetic causes in JALS. We report a patient with JALS and a pathogenic variant in the TARDBP gene (c.1035C > G; p.Asn345Lys), previously reported only in adult-onset ALS, and with an atypical phenotype of marked upper motor neuron predominance. In addition, the proband presented an additional variant in the NEK1 gene, c.2961C > G (p.Phe987Leu), which is classified as a variant of unknown significance. Segregation studies showed a paternal origin of the TARDBP variant, while the variant in NEK1 was inherited from the mother. We hypothesize that the NEK1 variant acts as a disease modifier and suggests the possibility of a functional interaction between both genes in our case. This hypothesis could explain the peculiarities of the phenotype, penetrance, and the age of onset. This report highlights the heterogeneity of the phenotypic presentation of ALS associated with diverse pathogenic genetic variants.

Frequency of frontotemporal dementia-related gene variants in Turkey

Just as its clinical heterogeneity, genetic basis of Frontotemporal dementia (FTD) is also diverse and multiple molecular pathways are thought to be involved in disease pathogenesis. In the present study, FTD- related genes were evaluated in a Turkish cohort of 175 index FTD patients with a gene panel including GRN, MAPT, TARDBP, FUS, CHMP2B and VCP genes. Potential genetic associations were prospected in 16 patients (9.1%); five variants (p.(Gly35Glufs) and p.(Cys253Ter) in GRN; p.(Arg95Cys) in VCP; p.(Met405Val) in TARDBP and p.(Pro636Leu) in MAPT) were classified as pathogenic (P) or likely pathogenic (LP), in four familial and one sporadic patients. Three novel variants in MAPT, CHMP2B and FUS were also identified in familial cases. The most common pathogenic variants were observed in the GRN gene with a frequency of 1.14% (2/175) and this rate was 4.57% (8/175), including variants of uncertain significance (VUS). In this study with the largest cohort of Turkish FTD patients, GRN and MAPT variants were identified as the most common genetic associations; and rare causes like VCP, TARDBP, CHMP2B and FUS variants are recommended to be considered in patients with compatible clinical findings.

Identification of novel FUS and TARDBP gene mutations in Chinese amyotrophic lateral sclerosis patients with HRM analysis

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by progressive loss of motor neurons. More than 30 genes have been linked to ALS to date, including FUS and TARDBP, which exhibit similar roles in RNA metabolism. This study explored the use of high-resolution melting (HRM) analysis to screen for FUS and TARDBP mutation hotspot regions in 146 Chinese ALS patients, which achieved 100% detection. Two FUS mutations were observed in two different familial ALS probands, a missense mutation (p.R521H) and a novel splicing mutation (c.1541+1G>A). Five TARDBP mutations were identified in six ALS patients, including a novel 3'UTR mutation (c.*731A>G) and four missense mutations (p.G294V, p.M337V, p.G348V, and p.I383V). We found that FUS mutations were present in 1.4% of Chinese ALS patients, whereas TARDBP mutations were responsible for 4.1% of Chinese ALS cases. Here, we describe the accuracy of using highly sensitive HRM analysis to identify two novel FUS and TARDBP mutations in Chinese sporadic and familial ALS cases. Our study contributes to the further understanding of the genetic and phenotypic diversity of ALS.

A novel mutation in the cleavage site N291 of TDP-43 protein in a familial case of amyotrophic lateral sclerosis

Cytoplasmic aggregation of TAR-DNA binding protein (TDP-43) in Amyotrophic Lateral Sclerosis (ALS) and fronto-temporal lobar dementia (FTLD) is associated with post-translational modifications (PTM) and delocalization. Studies on postmortem brains of ALS and FTLD patients showed the existence of TDP-43 fragments that end at position N291. We report a new heterozygous mutation p.N291H in a familial case of ALS. Expression of the mutant protein in cell lines and primary motor neurons induces aggregate formation in the cytoplasm and reduces cell viability. The discovery of mutations at cleavage sites in TDP-43 in patients, which we reviewed here, is valuable for understanding the true role of the various TDP-43 fragments identified in patients and thus, for developing effective targeted therapies for ALS and FTLD treatment.

High frequency of the TARDBP p.M337 V mutation among south-eastern Chinese patients with familial amyotrophic lateral sclerosis

Background

Amyotrophic lateral sclerosis (ALS) is a devastating motor neuron disease characterized by substantial clinical and genetic heterogeneity. Thus far, only a few TARDBP-ALS families have been reported in China, and no mutation analysis has been reported in south-eastern China.

Methods

Seven index cases from ALS families negative for SOD1 and FUS mutations were screened by Sanger sequencing for TARDBP gene exons 2-6. TARDBP exon 6 was analysed in 215 sporadic ALS patients.

Results

Two TARDBP mutations in exon 6 (p.M337 V and p.G348C) were identified in 5 unrelated families. Four of these 5 families carried the same p.M337 V mutation (family 1II3, family 2II6, family 3II4, and family 4II4), and the p.G348C mutation was identified in family 5 (II5). Among the 215 sporadic patients, only a single nucleotide polymorphism (p.A366A) was detected in 5 patients, and no responsible mutation was identified. Among the TARDBP-linked familial ALS patients, the average age of onset was 57.0 ± 4.7 years, and a trend towards higher rates of bulbar (50.0%, 6/12) onset and upper limb (41.7%, 5/12) onset than lower rates of limb onset (8.3%, 1/12) was observed. Furthermore, ALS patients with TARDBP mutations showed a benign disease course, and the average survival was 106.5 ± 41.8 months (n = 8).

Conclusions

We found a high frequency of the TARDBP p.M337 V mutation in familial ALS in south-eastern China. The TARDBP-linked ALS patients showed a benign disease course and prolonged survival.

Electronic supplementary material

The online version of this article (10.1186/s12883-018-1028-1) contains supplementary material, which is available to authorized users.

Genetic mutations in RNA-binding proteins and their roles in ALS

Mutations in genes that encode RNA-binding proteins (RBPs) have emerged as critical determinants of neurological diseases, especially motor neuron disorders such as amyotrophic lateral sclerosis (ALS). RBPs are involved in all aspects of RNA processing, controlling the life cycle of RNAs from synthesis to degradation. Hallmark features of RBPs in neuron dysfunction include misregulation of RNA processing, mislocalization of RBPs to the cytoplasm, and abnormal aggregation of RBPs. Much progress has been made in understanding how ALS-associated mutations in RBPs drive pathogenesis. Here, we focus on several key RBPs involved in ALS—TDP-43, HNRNP A2/B1, HNRNP A1, FUS, EWSR1, and TAF15—and review our current understanding of how mutations in these proteins cause disease.

Cytoplasmic Relocalization of TAR DNA-Binding Protein 43 Is Not Sufficient to Reproduce Cellular Pathologies Associated with ALS In vitro

Mutations in the gene TARDBP, which encodes TAR DNA-binding protein 43 (TDP-43), are a rare cause of familial forms of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). While the majority of mutations are found in the C-terminal glycine-rich domain, an alanine to valine amino acid change at position 90 (A90V) in the bipartite nuclear localization signal (NLS) of TDP-43 has been described. This sequence variant has previously been shown to cause cytoplasmic mislocalization of TDP-43 and decrease protein solubility, leading to the formation of insoluble aggregates. Since the A90V mutation has been described both in patients as well as healthy controls, its pathogenic potential in ALS and FTD remains unclear. Here we compare properties of overexpressed A90V to the highly pathogenic M337V mutation. Though both mutations drive mislocalization of the protein to the cytoplasm to the same extent, M337V produces more significant damage in terms of protein solubility, levels of pathogenic phosphorylation, and formation of C-terminal truncated protein species. Furthermore, the M337V, but not the A90V mutant, leads to a downregulation of histone deacetylase 6 and Ras GTPase-activating protein-binding protein. We conclude that in the absence of another genetic or environmental ‘hit’ the A90V variant is not sufficient to cause the deleterious phenotypes associated with ALS and FTD, despite prominent cytoplasmic protein relocalization of TDP-43.

Phase to Phase with TDP-43

TDP-43 is a dimeric nuclear protein that plays a central role in RNA metabolism. In recent years, this protein has become a focal point of research in the amyotrophic lateral sclerosis and frontotemporal dementia (ALS/FTD) disease spectrum, as pathognomonic inclusions within affected neurons contain post-translationally modified TDP-43. A key question in TDP-43 research involves determining the mechanisms and triggers that cause TDP-43 to form pathological aggregates. This review gives a brief overview of the physiological and pathological roles of TDP-43 and focuses on the structural features of its protein domains and how they may contribute to normal protein function and to disease. A special emphasis is placed on the C-terminal prion-like region thought to be implicated in pathology, as it is where nearly all ALS/FTD-associated mutations reside. Recent structural studies of this domain revealed its crucial role in the formation of phase-separated liquid droplets through a partially populated α-helix. This new discovery provides further support for the theory that liquid droplets such as stress granules may be precursors to pathological aggregates, linking environmental effects such as stress to the potential etiology of the disease. The transition of TDP-43 among soluble, droplet, and aggregate phases and the implications of these transitions for pathological aggregation are summarized and discussed.

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

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

Structural analysis of disease-related TDP-43 D169G mutation: linking enhanced stability and caspase cleavage efficiency to protein accumulation

The RNA-binding protein TDP-43 forms intracellular inclusions in amyotrophic lateral sclerosis (ALS). While TDP-43 mutations have been identified in ALS patients, how these mutations are linked to ALS remains unclear. Here we examined the biophysical properties of six ALS-linked TDP-43 mutants and found that one of the mutants, D169G, had higher thermal stability than wild-type TDP-43 and that it was cleaved by caspase 3 more efficiently, producing increased levels of the C-terminal 35 kD fragments (TDP-35) in vitro and in neuroblastoma cells. The crystal structure of the TDP-43 RRM1 domain containing the D169G mutation in complex with DNA along with molecular dynamics simulations reveal that the D169G mutation induces a local conformational change in a β turn and increases the hydrophobic interactions in the RRM1 core, thus enhancing the thermal stability of the RRM1 domain. Our results provide the first crystal structure of TDP-43 containing a disease-linked D169G mutation and a disease-related mechanism showing that D169G mutant is more susceptible to proteolytic cleavage by caspase 3 into the pathogenic C-terminal 35-kD fragments due to its increased stability in the RRM1 domain. Modulation of TDP-43 stability and caspase cleavage efficiency could present an avenue for prevention and treatment of TDP-43-linked neurodegeneration.

TDP-43 Proteinopathy and ALS: Insights into Disease Mechanisms and Therapeutic Targets

Therapeutic options for patients with amyotrophic lateral sclerosis (ALS) are currently limited. However, recent studies show that almost all cases of ALS, as well as tau-negative frontotemporal dementia (FTD), share a common neuropathology characterized by the deposition of TAR-DNA binding protein (TDP)-43-positive protein inclusions, offering an attractive target for the design and testing of novel therapeutics. Here we demonstrate how diverse environmental stressors linked to stress granule formation, as well as mutations in genes encoding RNA processing proteins and protein degradation adaptors, initiate ALS pathogenesis via TDP-43. We review the progressive development of TDP-43 proteinopathy from cytoplasmic mislocalization and misfolding through to macroaggregation and the addition of phosphate and ubiquitin moieties. Drawing from cellular and animal studies, we explore the feasibility of therapeutics that act at each point in pathogenesis, from mitigating genetic risk using antisense oligonucleotides to modulating TDP-43 proteinopathy itself using small molecule activators of autophagy, the ubiquitin-proteasome system, or the chaperone network. We present the case that preventing the misfolding of TDP-43 and/or enhancing its clearance represents the most important target for effectively treating ALS and frontotemporal dementia.

Novel TARDBP sequence variant and C9ORF72 repeat expansion in a family with frontotemporal dementia

Frontotemporal lobar degeneration (FTLD) is a genetically heterogenous syndrome and has been associated most recently with a hexanucleotide repeat expansion within the C9ORF72 gene. Pathogenic TDP-43 gene (TARDBP) mutations have been identified in amyotrophic lateral sclerosis, but the role of TARDBP mutations in FTLD is more contradictory. To investigate the role of TARDBP mutations in a clinical series of Finnish FTLD patients, we sequenced TARDBP exons 1 to 6 in 77 FTLD patients. No evident pathogenic mutations were found. We identified a novel heterozygous c.876_878delCAG sequence variant in 2 related patients with behavioral variant frontotemporal dementia without amyotrophic lateral sclerosis. The variant is predicted to cause an amino acid deletion of serine at position 292 (p.Ser292del). However, p.Ser292del was also found in 1 healthy middle-aged control. Interestingly, both patients carried the C9ORF72 expansion. Therefore, the TARDBP variant p.Ser292del might be considered a rare polymorphism and the C9ORF72 repeat expansion the actual disease-causing mutation in the family. Our results suggest that TARDBP mutations are a rare cause of FTLD. However, the interaction of several genetic factors needs to be taken into account when investigating neurodegenerative diseases.

Heterogeneous nuclear ribonucleoproteins in amyotrophic lateral sclerosis: what do we know?

ABSTRACT: Amyotrophic lateral sclerosis (ALS) is a fatal adult-onset motor neuron disease that results from the progressive loss of motor neurons in the brainstem and spinal cord, and of upper motor neurons in the motor cortex. TDP-43 was the first protein identified in ALS. It is present in cytoplasmic inclusions in motor neurons of affected patient brains and spinal cords, a hallmark feature of this disease. Successive studies have identified missense mutations in TARDBP, and, to date, more than 40 mutations have been identified. Recent studies have indicated that altered RNA metabolism is a key feature of ALS. This article reviews an emerging role of heterogeneous nuclear ribonucleoproteins driving disease pathogenesis that include TDP-43, FUS, hnRNPA1, hnRNPA2/B1 and hnRNPA3. Determining the molecular pathways involved may provide a promising prospect for heterogeneous nuclear ribonucleoproteins being potential biomarkers in ALS in order to develop therapeutic strategies for mitigating this disease, for which there is currently no cure.

Absence of mutations in exon 6 of the TARDBP gene in 207 Chinese patients with sporadic amyotrohic lateral sclerosis

Mutations in the TARDBP gene, which encodes the Tar DNA binding protein, have been shown to causes of both familial amyotrophic lateral sclerosis (FALS) and sporadic ALS (SALS). Recently, several novel TARDBP exon 6 mutants have been reported in patients with ALS in Europe and America but not in Asia. To further examine the spectrum and frequency of TARDBP exon 6 mutations, we investigated their frequency in ethnic Chinese patients with sporadic ALS. TARDBP exon 6 was screened by direct sequencing in 207 non-SOD1 SALS patients and 230 unrelated healthy controls but no mutations were identified. Our data indicate that exon 6 mutations in TARDBP are not a common cause of SALS in Han Chinese population from Southern Mainland China.

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.